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The photochemical oxidation of formaldehyde in the gaseous phase Sharp, James Harry 1960

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i TEE PHOTOCHEMICAL OKIDATION OP FORMALDEHYDE IN THE GASEOUS PHASE by JAMES HARRY SHARP B. A., University of B r i t i s h Columbia, 1957 A THESIS SUBMITTED IN PARTIAL FULFILMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE i n the Department of CHEMISTRY We ..accept this thesis as conforming to the required standard THE UNIVERSITY OF BRITISH COLUMBIA AUGUST, I960 In presenting t h i s thesis i n p a r t i a l fulfilment of the requirements for an advanced degree at the University of B r i t i s h Columbia, I agree that the Library s h a l l make i t f r e e l y available f o r reference and study. I. further agree that permission f o r extensive copying of t h i s thesis f o r scholarly purposes may be granted by the Head of my Department or by his representatives. It i s understood that copying or publication of t h i s thesis f o r f i n a n c i a l gain s h a l l not be allowed without my written permission. Department of Chemistry  TLo University of B r i t i s h Columbia, Vancouver Canada, Date August 26, 1960 i i ABSTRACT The object of t h i s work was the i n v e s t i g a t i o n of the photochemical o x i d a t i o n of formaldehyde in.the gaseous phase at 110°C. Reaction mixtures, where the 02: CH2O r a t i o was approximately 1:10, were i r r a d i a t e d w i t h u l t r a v i o l e t l i g h t at a wavelength of 3130A0 and the r e a c t i o n products analyzed. The major products were found to be CO, H2 and HCOOH. CO2 was a minor product. Wb: peroxides were found and the reaction was oxygen independent at low 02:CE"20 r a t i o s . The formation of the major products was found to be d i r e c t l y p r o p o r t i o n a l to the i n i t i a l formaldehyde pressure and to the i n t e n s i t y of the absorbed l i g h t . A s a t i s f a c t o r y mechanism i s proposed to e x p l a i n the formation of the r e a c t i o n products, and the f o l l o w i n g k i n e t i c equations were derived: (1) d CCOJ dt (2) dt K 8 -XIX a [HCOOH] = 2 J l a [HCHO] d t Z~ I d [C02J = 2K2 $ l a ( K 8 + K 3 [HCHO] ) A b s t r a c t A p p r o v e d i v C O N T E N T S Page CHAPTER I * INTRODUCTION 1. The Thermal Oxidation 2. The Photolysis 3. The Photochemical Oxidation 17 1 2 CHAPTER II PREPARATION A. Materials 1. Formaldehyde 23 2. Oxygen 26 3. Actinometer Solutions 26 4. Solutions f o r Peroxide Determination 29 5. Light F i l t e r Solutions 30 6. Gas Chromatographic Columns 31 7. Sol'ns. f o r Formic Acid Det'n 32 B. Calibration 1. Actinometry 33 2. Calibration f o r Reaction Products 40 CHAPTER III APPARATUS 1. The Vacuum System 45 2. The Furnace 51 3. The Optical System 53 4. The Lamp 53 5. The Photometer Unit 56 6. The Gas Admission System to the Perkin-Elmer Vapour Fractometer 59 7. The Thermal Couple 61 CHAPTER IV THE MECHANISM OF THE PHOTOCHEMICAL OXIDATION OF GASEOUS FORMALDEHYDE AT 110° C 1. Object of the Investigation 2. Experimental Procedure 3. Determination of Formic Acid 66 67 68 V C O N T E N T S Page CHAPTER IV (CON1D) 4. Determination of Hydrogen and Carbon Monoxide 79 5. Determination of Carbon Dioxide 83 6. The Influence of Oxygen 86 7. Peroxide Test 86 8. The Photolysis of Formaldehyde 89 9. A Summary of the Experimental Results 90 10. The Proposed Mechanism 90 REFERENCES 100 v i ACKNOWLEDGEMENT I wish to express my sincere gratitude and thanks to Professor C.A. McDowell for his generous encouragement, supervision and enlightening discussions throughout the course of the work. I also wish to ex-tend my appreciation and thanks to Dr. K.M. B e l l f o r his many helpful suggestions, his kindness and his interest i n thi s work. Appreciation and thanks are also expressed to Dr. G. Porter and Dr. A.D. Kirk f o r t h e i r interest and many encouraging discussions. I am grateful to the University of B r i t i s h Columbia fo r Student Assistantships during the 1958-59 and 1959-60 sessions. F i n a l l y , I wish to thank the glassblowing, electronic and workshop s t a f f f o r t h e i r assistance i n the construction of parts of the apparatus. CHAPTER I INTRODUCTION The Thermal Oxidation The Photolysis The Photochemical Oxidation -1-CHAPTER I - INTRODUCTION 1. The Thermal O x i d a t i o n The o x i d a t i o n of many organic compounds, p a r t i c u l a r l y hydrocarbons, produces aldehydes as inte r m e d i a t e s . Hence, a knowledge of the mechanism by which aldehydes are them-s e l v e s o x i d i z e d i s of importance i n the understanding of the g e n e r a l mechanism of hydrocarbon o x i d a t i o n . Although, formaldehyde has been found to p l a y an im-po r t a n t r o l e i n the o x i d a t i o n o f many of the l i g h t e r hy-drocarbons (1-9), and would appear to be the l o g i c a l choice of aldehyde f o r the study of such mechanisms, i t has not proved to be a very popular one. S e v e r a l mechanisms and a number o f f r e e r a d i c a l s and a c t i v a t e d molecules have been p o s t u l a t e d f o r the thermal o x i d a t i o n of formaldehyde where-as there i s gen e r a l agreement as to the cha i n c a r r i e r s i n the slow combustion of acetaldehyde. Askey (10), i n 1930, was the f i r s t t o study the o x i d a t i o n of formaldehyde i n the gaseous phase, Although h i s experiments were f a r from exhaustive, they did i n d i c a t e t hat the o x i d a t i o n of formaldehyde at 321°C was a cha i n mechanism and that the main products were H 2 O , CO, H2 and C O 2 . No mechanism was proposed. In the same year, F o r t and Hinshelwood ( l l ) were able to show t h a t the r a t e of pressure r i s e was independent of the oxygen c o n c e n t r a t i o n and s t r o n g l y dependent upon the formaldehyde c o n c e n t r a t i o n . -2-They also found that the activation energy was 20 k i l o c alories per mole.as determined from the times of half reaction at di f f e r e n t temperatures. Once again t h e i r re-sults indicated a chain mechanism, but no mechanism was proposed. In 193.6, Bone and Gardner (12) working at consider-ably higher pressures than the previous workers ( 1 atm.) studied the slow combustion between 250Q'and 290° C. Their experiments included 2:1 and 1:1 mixtures of formaldehyde and oxygen and they found that the former mixture was much more reactive at a l l temperatures. Although there was no observable induction period, they found that an increase i n the surface to volume ra t i o of the s i l i c a reaction vessel from o.8 to 1.8 decidedly retarded the reaction. By an extensive analysis of the reaction products, they found that CO and H^ O were the major products whereas CO2, H2, HCOOH and peroxides were formed i n smaller amounts. There was evidence that the peroxides were performic acid and dioxymethy! peroxide, CH2OH-Q-O-CH2OH. These peroxidic substances were formed during the early stages of the re-action and were shown to disappear with time. They con-cluded that formic acid, performic acid and dioxymethyl peroxide were always intermediately formed and envisioned the following successive reations: -3-( i ) 2 C:0 + 02-H0N 2 0:0-H •2C0 + 2 H20 OH HO a) 2 N C : 0 ^ 2 C 0 2 + 2H20 ( i i ) 2 0:0 + 0 2 — H O H H . b) 2 0-^-200 + H 20 2 ¥0° ( i i i ) 2 0:0 + H 20 2 — ( C H 2 0 H ) ? 02^ >- 200 + 2H20 + H 2 Later, i n the same year, Spence (13) showed that i n the presence of an extensive surface of powdered glass oxidation proceeded according to the following equation: CH20 + 0 2 > H20 + C0 2 In unpacked pyrex vessels, he showed that an increase i n the diameter of the reaction vessel resulted i n an increase i n the rate of formation of carbon monoxide. From the var i a t i o n of i n i t i a l rates of pressure r i s e with temper-ature, Spence calculated an ove r a l l activation energy of 17.6 k i l o calories per mole. Although oxygen had l i t t l e influence on the rate of reaction, he found that the rate was strongly dependant on the formaldehyde concentration. In agreement with Bone and Gardner he found HCOOH, H 2, H20, CO and C0 2 as reaction products. -4-He suggested a reaction chain which i s analogous to that given by Backstrom (14) f o r the oxidation of aldehydes i n the l i q u i d phase. The chain included HCOjH, activated H2CO and activated HCOOH. With the assumption of steady state for these molecules the following equation f o r the i n i t i a l rate of disappearance of formaldehyde was obtained. - dE = K 1 ( H 2 C 0 ) 2 - K i j -( 'H2C0) + K 1 1 1 dt Although this equation i s i n f a i r agreement with his experimental re s u l t s , l i t t l e weight can be placed upon his mechanism. Some time l a t e r , i n 1939, Snowden and Style (15) reinvestigated formaldehyde oxidation at 344°G and using both a n a l y t i c a l and monometric techniques along with a spectrophotometric method of determining formaldehyde,they found that CO, H2O, H2 and CO2 were the main products. Their analyses c l e a r l y showed that CO and CO2 were not the only carbon containing products, especially i n the e a r l i e r stages of the reaction, and since only traces of peroxides were detected, they obtained evidence f o r the presence of HCOOH. They found that the rate of oxidation could be ex-pressed by an equation of the form, - d j = KF(P-C) dt where F i s the formaldehyde concentration and where K and C are constants. -5-Both these constants varied e r r a t i c a l l y , This formula was also shown to f i t Spence's results s a t i s f a c t o r i l y . The above expression can be formally deduced from the following scheme. X + E ^ 2X Ka X + X ^ end products Kb X end products Kc and, since they found evidence that CO2 formation was, i n part at l e a s t , connected with a chain ending reaction, they postulated the following tentative reaction scheme. CH2O3 + CH 2 0 > 2 C H 2 0 2 K i GH 202 + CH 2 02 °\ GO + H2.O + CH2O3 K 2 2 CH2O2 2 C 0 + 2 H 2 0 K3 CH2O2 + ? ^ CO + ? K 4 They also roughly examined the effect of various surface treatments and found that concentrated n i t r i c acid produced the least changeable surface and, at the same time, one which gave a reasonable rate of reaction. Hydrofluoric acid and caustic soda solutions both gave slow and unsteady rates and small yields of formic acid. The presence of mercury vapour increased the rate of pressure r i s e . - 6 -Axford and Norrish ( 16) , i n 1948, were the next investigators to study the oxidation of gaseous form-aldehyde. They also followed the reaction a n a l y t i c a l l y and monometrieally i n the region of 340° C. They found that CO and H^ O were the major products and that CO2 and i n almost equal amounts, were the minor products. No evidence f o r the presence of HCOOH or peroxides was found. The effect of varying the oxygen concentration on the rate of the reaction was negligible "but they found that the i n i t i a l rate was nearly proportional to the square of the formaldehyde concentration. They determined the activation energy of the reaction from v a r i a t i o n of i n i t i a l rates with temperature, and found i t to be equal to 21.0 k i l o c a l o r i e s per mole. Small quantities of oxygen were found to induce a slow decomposition of formaldehyde where i t was normally quite stable. No observable diameter effect upon the rate of pressure r i s e was noifced i n any of t h e i r experiments. Hence several discrepancies were noted between t h e i r work and the work of previous i n -vestigators,/ They c r i t i c i z e d the mechanism of Snowden and Style on two points. F i r s t , although oxygen i s present i n t h e i r reaction scheme, i t does not enter into t h e i r kinetics to obtain the r e s u l t i n g equation and hence i t s presence i s obscure. Secondly, they made no suggestion as to the i n i t i a l process, except to state that i t probably occurred on the surface. -7-In order to explain t h e i r r e s u l t s , Axford and Norrish suggested a mechanism based on the oxidation of hydrogen and hydrogen - carbon monoxide mixtures and involving Hydrogen and oxygen atoms and hydroxyl and HO2 r a d i c a l s . Work on the photolysis and the photo oxidation of formaldehyde had led to the suggestion that the reactions H + HCHO = H 2 + CHO 0 + HCHO = OH + CHO were involved. There was some doubt as to the sta-b i l i t y of the formyl r a d i c a l , but since Akeroyd and Norrish (17) i n 1936 and leermakers (18) i n 1934 had concluded from t h e i r results that the formyl r a d i c a l decom-0 posed to CO and H atoms above 100 , Axford and Norrish assumed that, at elevated temperatures, the formyl r a d i c a l would, s p l i t to give CO and H. Hence, the preceding equation can be written. H + HCHO = H 2 + CO + H 0 + HCHO = OH.+ 00 + H Prom the i r results and these assumptions, they postulated the following reaction (P= CH 20). ) -8-1) 0 2 + F -J_>. H 2G0 2 + 0 K± i n i t i a t i o n 2) 0 + F OH + H + CO K 2 3) H + F > H 2 + CO + H propagation (decomposition) 4) OH + F -> H20 + CO + H K 4 5) H + 0 2 + F H 20 + CO + OH K 5 Propagation (oxidation) 6) H + 0 2 + F -> H 20 + COg + H Kg 7) H.+ 0 2+ 0 2 p. H0 2 + 0 2 K 7 8) H * 0 2+ X H0 2 + X K 8 termination 9) H + 0 2+ Wall—> H0 2 Kg With reference to ( l ) they point out that Bowen and Tietz (19) found that oxygen reacts rapidly with formaldehyde to give performic acid at low temperatures and hence i t was reasonable to suppose, at higher tem-peratures, that the normal acid i s formed. They found that this scheme f i t t e d t h e i r experimental results well and were able to derive the following equation: -dF =_Ki 2K5 ( F ) 2 dt K 7 -9-However, i t should be pointed out, that although they form formic acid i n the i r proposed scheme they did not detect i t i n their experimental r e s u l t s . Although there seemed to be a difference of opinion between the kin e t i c results of Axford and Norrish on the one hand and Spence, Snowden and Style on the other, lewis and von Elbe (20) i n 1951 attempted to reconcile these two views. They noted two important points. The f i r s t was that Snowden and Style had found that mercury vapour greatly increased the rate of pressure r i s e and that Axford and Norrish had used heated mercury seals leading to the reaction vessel. Secondly, Axford and Norrish obtained no induction period whereas Snowden and Style found i n -duction periods varying with the type of surface. Lewis and von Elbe concluded that the experimental arrangement of Axford and Norrish may have introduced s u f f i c i e n t mer-cury vapour into the reaction vessel to cause rapid removal of peroxide by decomposition, thus simplifying the k i n e t i c s . In reference to G-orin's (21) work, they give evidence that the formyl r a d i c a l was much more stable than supposed by Axford and Norrish. Style and Summers (22) had also coime to the conclusion that the formyl r a d i c a l was more stable than had been anticipated. With these facts, Lewis and von Elbe suggested a mechanism i n which HCO^H was formed by a chain reaction where the formyl r a d i c a l was the predominent chain c a r r i e r . - 1 0 -The performic acid then diffused to the wall and decomposed into CO, B" 20, C 0 2 , H 2 and HC00H. Further gas phase reactions of the intermediate performic acid with HCO were neglected i f the destruction was rapid. I f however, the vessel surface was inactive toward peroxide destruction, gas phase reactions of the performic acid were considered important and the kinetics became more complex. Their reaction scheme may be written as follows where F = HCHO, P.A. = HC000H. F + 0 2 = CHOO + OH (or CHO + H 0 2 ) Free r a d i c a l + F = saturated compound + CHO CHO + 0 2 + F = P.A + CHO CHO + 0 2 + M = CHO3 + M surface CHO^ > destruction P.A. + CHO = F + C H 0 5 P.A. + F = CHOO + H 2 0 + CHO P.A. >. C 0 2 , H 2, H 2 C 0 0 , H 2 0 , CO Hence, the simple k i n e t i c s of Axford and Morrish were attributed to a system which rapidly destroyed per-oxides, whereas the more complicated k i n e t i c data of Spence, -11-Snowden and Style, Bone and Gardner was attributed to a system where the intermediate P.A. was not rapidly destroyed. More recently, Scheer (23) i n 1954 also attempted to reconcile the difference between the kinetics of previous workers. CO, C0 2, 0 2 , H 2, HCOOH and HCHO were analysed mass spectroscopically. Products, condensable at-110°C from a reaction i n an aged vessel showed the presence of formic acid plus some unidentified peaks which were assigned to performic acid. In systems containing mercury vapour, he found that the formic acid concentration was much reduced. He envisioned the same o v e r a l l reaction as Bone and Gardner to account for the products of the oxidation and proposed a rather complicated re a c t i o n scheme. Scheer states that although his mechanism provides a sat i s f a c t o r y explantation for most of the observations encountered for thi s reaction, there are several results reported by prevous investigators which do not f i t into his scheme. The history of the thermal oxidation of formaldehyde has been traced from the o r i g i n a l work of Askey to the more recent work of Scheer and i t i s obvious that the mechanism i s very complex and that the reaction i s very dependent upon the area and type of surface. Photochemical i n i t i a t i o n has been used by several workers i n non thermal oxidation studies of formaldehyde. -12-However, before d i s c u s s i n g the work which has been accomplished i n t h i s f i e l d , i t i s advantageous to r e -view some of the work on the p h o t o l y s i s of formaldehyde. 2. The P h o t o l y s i s Formaldehyde was f i r s t i n v e s t i g a t e d photochemically by B e r t h e l o t and Gaudechon (24) i n 1910. They found that i n the form of trioxymethylene i t i s mainly decomposed i n t o H 2 and CO w i t h small q u a n t i t i e s of C0 2 and CH^.hy u l t r a v i o l e t l i g h t . I n 1924, Bredig and Goldberg (25) studied the r e a c t i o n i n the vapour phase and found that at 80° the r e a c t i o n proceeded almost q u a n t i t a t i v e l y according to CH20 = H 2 + CO but found that at 195° considerable q u a n t i t i e s of C0 2 and CH4 were found. These authors did not attempt to measure the quantum efficiency of the r e a c t i o n or to c h a r a c t e r i z e the wavelength of the a c t i v e r a d i a t i o n . Worrish and K i r k b r i d e (26) i n 1932 were the f i r s t i n v e s t i g a t o r s to measure the quantum y i e l d of form-aldehyde at i s o l a t e d wave lengths. They measured the quantum e f f i c i e n c y f o r three s p e c t r a l regions, 2540-2800A 0, 3030 - 3130A 0 and 3340 - 3650A 0 at 100°C and -13-found that the mean quantum e f f i c i e n c i e s were o.9, 1.1 and 0.7 respectively. Since the predissociation l i m i t l i e s .at 2800A0 (27) they found no evidence for a photochemical thres-hold associated with i t . In the predissociation range, they concluded that the reaction ::: hv + C H 2 O = H 2 + eo was spontaneous while i n the region of fine structure i t occurs as a r e s u l t of a c o l l i s i o n between an excited molecule and a second body, such a c o l l i s i o n bringing about dissociation by a r e d i s t r i b u t i o n of the energy of activation. hV + H 2C0 = H 2CO* X + H 2CO* = H 2 + CO + X Akeroyd and Norrish (17) i n 1936 i n studying the chain photolysis of formaldehyde at higher temperatures came to the conclusion that CO was formed d i r e c t l y i n the process of disruption and that the formyl r a d i c a l and hydrogen atom are only set free i n small amounts (lOfo of the decomposition). -14-They found that the quantum yei l d s were 1.0 at 100°, 2.9 at 200°, 8.0 at 250° with increasing quantum yeilds up to 100 at 350°. Their modified scheme for the photo-l y s i s of formaldehyde may he written as follows: 1),2) hV + ECHO = H 2 + CO p r i f f i a r y = H + HCO = H + H + CO P r o c e s s 3) H + HCHO = H 2 + HCO 4) HCO = H + CO 5) H + H = Ho Go,rin (21) i n 1939 studied the photdysis i n the presence of iodine vapour which resulted i n a contradiction to the above mechanism. At 100° and a wavelength of 3130 A 0 he found that the permanent gases consisted a l -most en t i r e l y of CO. In addition, the r a t i o of HI/CO was nearly two. These facts showed that the main reaction was the s p l i t t i n g off of one hydrogen atom since i t takes 102 k i l o c a l o r i e s to p u l l off the two hydrogen atom from formaldehyde and i t i s obvious thdfc a quantum of l i g h t at 3130A° possesses s u f f i c i e n t energy todetatch only one hydrogen atom. -15-Hence he postulated the primary process at 3130 A0' as hY + HCHO = H. + HCO In the presence of iodine vapour this* would he followed by H + I 2 > HI + I HCO + HCO > H2C0 + CO The quantum ef f i c i e n c y f o r the formation of hydrogen iodide was unity and the rate of formation of CO i n the presence of I 2 was \ as large as i t was i n the absence of iodine as required by the above mechanism. From his results using l i g h t i r r a d i a t i o n of wavelength o 3650A he was able to show that the formyl r a d i c a l was: much more stable than was previously anticipated, about 26 k i l o calories being required to dissocia te i t . Gorin's scheme for the chain photolysis i s as follows: ~'"-f h y + HCHO ^ H + HCO HCO ^ H + CO K± : ; H + H2C0 >j H2+ H + CO K 2 H + HCO \ > H 2 + CO l f , K _ > * 2 K 4 Since, K-^  had a large temperature c o e f f i c i e n t , K 2 would be dominant at lower temperature s while K 3 -16-would be the largest at high temperatures. Style and Summers (22) i n 1945 concluded that since hydrogen atoms had been reported to react rapidly with formaldehyde (28) neither of the mechanisms as pro-posed by Norrish and Carruthers or by G-orin could be considered satisfactory. They suggested the following which was a s i m p l i f i c a t i o n of Gorin's scheme. CH20 + hY _> H + HCO A H + HCHO H 2 + HCO % ( l ) HCO + HCHO—*. H 2 + CO + HCO K 2 (2) 2HC0 _ > CO + HCHO K3 (3) Stea.eie and Calvert (29) deduced an activation emtergy of 13.5 k i l o calories for the decomposition of the formyl f a d i c a l from t h e i r results on the photo-l y s i s of formaldehyde. The mechanism they proposed i s : CH20 + bV H + HCO ( i ) H + H2C0 —>» H 2 + HCO ( i i ) HCO >» H + CO ( i i i ) wall HCO >- products (iv) More recently, i n 1954, Horner, Style and Summers (30) i n a discussion of the oxidation of formaldehyde suggested that at 100°C, Ki:^j_ was zero. -17-At higher temperatures, when (ii i . ) becomes sign-i f i c a n t , they suggest that i t should not be a f i r s t order decomposition, but would more l i k e l y be a second order process. Hence, ( i i i ) would become Prom these examples, we see that there was some doubt as to the s t a b i l i t y of the formyl r a d i c a l and as to the mechanism of the photolysis of formaldehyde. 3. The Photochemical Oxidation In 1936, Garruthers and Norrish (31) studied the photochemical oxidation of f ormaldehydte at 100°C. Upon i r r a d i a t i o n , they found a rapid decreaseJin pressure and were able to account f o r this decrease by an analysis of the reaction products. The products consisted of HCOOH, HgO and CO as major products and COg and Hg as minor products. No peroxides were detected. They explained t h e i r results by the primary ox-idation of formaldehyde to formic acid according to the equation, HCHO -18-The presence of C O 2 , CO, H2 and H2O was explained by the subsequent decomposition of the formic a c i d i n two ways. C 0 2 + H 2 H-COOH CO + H 2 0 Evidence f o r the above decomposition at lower wave-lengths had been noted by several workers (32). They found that i n c r e a s i n g the oxygen con-c e n t r a t i o n had l i t t l e e f f e c t on the course of the r e a c t i o n . The quantum y i e l d f o r a CB^O + 0 2 mixture was reported to be 12.6 whereas the quantum y i e l d f o r a 2:1 mixture was reported to be 9 . 0 . The next study of the photo-oxidation of formaldehyde was made i n 1945 by S t y l e and Summers (22). They found the same products as Carruthers and N o r r i s h but t h e i r quantum y i e l d s are considerably lower. Only t r a c e s of peroxides were detected. that the quantum y i e l d depended somewhat on the oxygen concentration. I t i s i n t e r e s t i n g to note that at low oxygen pressures, t h e i r quantum y i e l d s increased r a p i d l y . Using l i g h t of wavelength 2650A 0 and 2537A* o they found that -19-They concluded that the mechanism of the ox-idation depended to some extent upon the photolysis of formaldehyde. Since the formyl r a d i c a l was known to react rapidly with oxygen ( 3 3 ) , they reasoned that reactions ( 2 ) and ( 3 ) of t h e i r proposed mechanism of the photolysis of formaldehyde could be neglected when the pressure of oxygen was above a few tenths of a millimeter of mercury. This assumption had already been made by Lewis and von Elbe ( 3 4 ) . The v a r i a t i o n of ^>nA with increased oxygen concentration led them to postulate a competition for ( l) H + 0 2 + M —v H02 + M H02 would also be formed by the reaction of the formyl r a d i c a l with oxygen, HC6 + 02 H02 + CO and since at low oxygen c o n c e n t r a t i o n , ^ was greater than unity, they postulated a subsequent reaction of the H02 r a d i c a l with formaldehyde to regenerate hydrogen atom, H0o + CHo0 = CO + COo + HCOOH + H -20-The chain breaking reaction was given as • HO2 = ? The above mechanism only p a r t i a l l y , accounted fo r t h e i r experimental results and i t i s especially weak i n that formic acid, a major product, i s not s a t i s f a c t o r i l y accounted f o r . In t h e i r paper of 1954, Horner, Style and Summers (30) suggested the following mechanism at 100^0, H2C0 + h _ ^ H + H00 ( i ) M + H + 02__> H0 2 + M 1 H + CH20 —>- H 2 + H00 2 HCO + 0 2 H0 2 + CO 3 H0 2 + CH20 H + CO + ? 4 H0 2 _>. H 2 + OCO 5 and although i t is i n f a i r agreement with t h e i r previous results, i t does not include formic acid. At higher temperatures, ^ 150°C, t h e i r results had indicated that a new mechanism occurred and i n this region, they replaced reactions 4 and 5 by -21-H02 + CH20 P 4 P —>» R + end products 5"*" P + CH2O _ > H + CO + end products , 6 1 R + CH20 HCO + qH2 + nCO + mC02 + 1HC02H 7 1 W/>(.t. R H 2 + $ CO + - - - -where P = H 9 CT°* , R = HC*Q 0 2H °-This mechanism was not unambiguously established and the true mechanism i s obviously extremely complicated. It i s clear from the preceding discussions on the thermal oxidation, the photolysis and the photochemical oxidation of formaldehyde that there i s a wide discrepancy of results, that the mechanisms are very complex, and that no one mechanism has been decidedly established. The object of this investigation was to investigate the photochemical oxidation of formaldehyde at r e l a t i v e l y low oxygen concentration. With the aid . of gas chromatography i t was hoped that a more complete analysis of reaction, products could be made and that a satisfactory mechanism could be proposed for the photochemical oxidation at^*100°C. -22-CHAPTER II  PREPARATION A Materials B Calibration -23-CHAPTER II - PREPARATION  A Preparation of Materials 1. Formaldehyde Formaldehyde was prepared by the method of Spence and Wild (35) and stored at l i q u i d nitrogen temperature. The apparatus, which was connected to the vacuum system, i s shown i n F i g . 1. Dow Corning S i l i c o n high vacuum grease was used for the heated taps. The d i s t i l l a t i o n vessel was p a r t i a l l y f i l l e d with chemically pure paraformaldehyde which had been dried over granular anhydrous magnesium perchlorate i n a vacuum dess-icator for 48 hours. A piece of glass wool was then i n -troduced on top of the paraformaldehyde to prevent p a r t i c l e s of the s o l i d from being carried over into the condensing system during the d i s t i l l a t i o n . The system was then pumped down with the mechanical and d i f f u s i o n pumps. During this period, the separator and trap were heated \ith an e l e c t r i c gun to 150°C while the current i n the e l e c t r i c a l heating tapes (shaded area i n F i g . l ) was adjusted, by means of a o variac, to give a. uniform temperature of 110 C. D i s t i l l a t i o n was begun by heating the paraformaldehyde i n a beaker of glycerine which rested on a small hot plate. The i n i t i a l d i s t i l l a t i o n temperature during this time was llX)°C. -24--25-During the removal of the f i r s t f r a c t i o n hy the pumps, the condensing vessels were continually heated with the e l e c t r i c gun. After the f i r s t f r a c t i o n had d i s t i l l e d o f f , the bottom portion of the separator was cooled i n an ethanol -dry ice mixture, the trap was immersed i n l i q u i d nitrogen and the stopcock leading to the mechanical and d i f f u s i o n pumps was closed. As d i s t i l l a t i o n proceeded, the rate gradually became slower and slower and the temperature of the glycerine bath was raised to 120°C. When s u f f i c i e n t s o l i d had collected i n the trap, the glycerine bath was removed and the apparatus once more connected to the pumps. The cold bath was removed from the separator and the trap was isolated f rom the separator v i a the stopcock shown i n Pig. 1. It was found that pure monomeric formaldehyde could be prepared i n t h i s manner and that very l i t t l e polymerization occurred as long as the trap was kept at l i q u i d nitrogen temperature. -26- 3-2. Oxygen Oxygen, 99.6 per cent pure, was taken from a cylinder, passed through a s p i r a l trap immersed i n an ethanol - dry ice hath and led to the main l i n e of the vacuum system. The main l i n e was flushed out 3 or 4 times with the oxygen and then the oxygen was allowed to enter a large evacuated bulb u n t i l atmospheric pressure had been attained. The oxygen was stored there u n t i l ready f o r use. The admitting system i s shown i n F i g . 2. 3. Actinometer Solutions Actinometry was carried out according to the work of Hatchard and Parker (36). The following solutions were prepared. a.) Potassium Ferrioxalate Pure Fe (CgO^)^ was prepared by mixing 3 volumes of 1.5 molar A.R. potassium oxalate with 1 volume of 1.5 molar A.R. f e r r i c chloride with vigorous s t i r r i n g . The ferrioxalate was then r e c r y s t a l l i z e d 3 times from warm water and dried i n a current of a i r at 45 C. A 6 x 10 — molar solution was then made by dissolving 2.947 grams of the r e c r y s t a l l i z e d product i n 800 ml of water. 100 ml of l.OiJ" .'iHgSO^  was then added and the solution diluted to 1 l i t r e . This solution was stored i n a black bottle u n t i l required. -27--28-b. ) Potassium dichromate A 0 . 1 N solution was prepared by dissolving 2 . 5 0 6 2 grams of A.R. K 2 C r 2 0 7 i n 500 ml of d i s t i l l e d water. c. ) Ferrous Sulphate The 0 . 1 M FeSO^ solution was prepared by dissolving 1.4 grams of A.R. FeSO^ • 7H 2 0 i n 50 ml of 0 . 1 N sulphuric acid. This solution was then standardized with the standard potassium dichromate. The proceedure was as follows: A 1 0 ml sample of the ferrous sulphate solution was transferred to a 1 2 5 ml conical flask and 2 ml of cone. HOI and 3 ml of phosphoric acid ( 8 5 $ ) were added. Several drops of barium diphenyl amine sulphonate indicator were then added and the res u l t i n g solution t i t r a t e d with the standard dichromate u n t i l a permanent v i o l e t - blue colour was observed. Hence, ^(FeSO^.) =(0.l) X Volume KgCrQ/ytml.) 1 0 The oxidation - reduction equations involved are, 6e + 1 4 H + + C r 2 0 7 = > 2 C r + J + + 7 H 2 0 6Fe * + 6Fe + + ++ 6e = + + • + . +++• ++•+ G r 2 0 7 + 6Fe + I4H > 2Cr + 6Fe + 7H20 -29-d. ) Phenanthroline A 0.1$ phenanthroline solution was prepared by dissolving one gram of 1:10 phenanthroline monohydrate i n 100 grams of d i s t i l l e d water. e. ) Buffer Solution The buffer solution was prepared by mixing 600 ml of normal sodium acetate and 360 ml of normal sulphuric acid and d i l u t i n g the re s u l t i n g solution to one l i t r e . 4) Solutions f o r Peroxide Determination Tests f o r peroxides i n the reaction products were made by employing the method of Young et a l (37). The following solutions were prepared. N a.) Ammonium Thiocyanate i n Methanol A stock solution of ammonium thiocyanate was used for preparing a solution of ferrous thiocyanate. 5 grams of A.R. ammonium tfei-6cyanate was dissolved i n about 600 ml of absolute methanol, 05 ml of concentrated sulphuric acid was added, and the res u l t i n g solution diluted to one l i t r e . This solution invariably had a very f a i n t pink colouration, probably due to the presence of a minute amount of Fe"*"*"*" i n the sulphuric acid. -30-No precautions to use p e r f e c t l y dry methanol were taken since the presence of varying p r o p o r t i o n of water made no d i f f e r e n c e to the accuracy of the t e s t . The s o l u t i o n was normally q u i t e stable and was stored i n an amber coloured b o t t l e u n t i l required f o r use. b.) Ferrous Thiocyanate i n Methanol A s o l u t i o n of ferrous thiocyanate i s slowly o x i d i z e d by atmospheric oxygen w i t h the formation of a dark red s o l u t i o n of f e r r i c thiocyanate complex. For t h i s reason, the s o l u t i o n of ferrous thiocyanate was always made up j u s t p r i o r to being used. 50 ml of the stock s o l u t i o n of ammonium thiocyanate was shaken wi t h 0.1 grams of A.R. ferrous ammonium sulphate f o r one minute and allowed to stand f o r two minutes. The s o l u t i o n was then decanted from the undissolved ferrous s a l t . 5• l i g h t F i l t e r S olutions Two,light f i l t e r s o l u t i o n s and a glass f i l t e r were used i n t h i s work as recommended by Hunt and Davis (38) to i s o l a t e a f a i r l y narrow wavelength band close to the 3130A0 mercury l i n e . a.) Potassium Hydrogen Phthalate F i v e grams of A.R. potassium hydrogen phthalate was di s s o l v e d i n 500 ml of d i s t i l l e d water. This s o l u t i o n was always used i n conjunction w i t h the two centimeter f i l t e r c e l l . - 3 1 -Since this solution was not stable to the action of lighjb, i t was stored i n the dark and a new f i l t e r had to be prepared after each run. D») Potassium Chrornate 0.246 grams of potassium chromate was dissolved i n 500 ml of d i s t i l l e d water. This solution was alwa^rs used i n conjunction with the one centimeter f i l t e r c e l l . It was quite stable to the action of l i g h t and this f i l t e r could be used f o r several runs. c.) Glass F i l t e r A 2 mm Corning 9863 f i l t e r was used i n conjunction with the above solutions. 6. Gas Chromatographic Columns Es s e n t i a l l y , two columns were used throughout this work. The f i r s t , a molecular sieve column, was used for the separation of hydrogen, oxygen, nitrogen and carbon monoxide. The second, designated as column " J " by Perkin Elmer was successfLilly used f o r the analysis of carbon dioxide. a.) Molecular Sieve Column This column was prepared by grinding molecular sieve (aluminium calcium s i l i c a t e ) , obtained from the B r i t i s h Drug Houses Ltd., with a mortar and pestle so that i t passed through a #10 and was retained on a #20 Tyler standard screen sieve. -32-The screened material was then packed into a 10' length of copper tubing, - j " O.D., and neatly folded. Helium gas was then passed through the column at 100°C f o r 8 hours. It was found that t h i s column e f f e c t i v e l y separated hydrogen, oxygen, nitrogen and carbon monoxide both q u a l i t a t i v e l y and quantitatively. This column could not be used for the analysis of carbon dioxide since i t was i r r e v e r s i b l y absorbed. b.) Column " J " This column was obtained from the Perkin - Elmer Corporation and consisted of s i l i c a gel, type 15, packed i n a two meter length of A " stainless steel tubing. 7. Solutions for the Analysis of Formic Acid a. ) Indicator Solution This solution was prepared by dissolving 3 - 4 drops of phenolphthalien i n 100 ml of d i s t i l l e d water. b. ) N/1000 Sodium hydroxide This solution was prepared by d i l u t i n g *~ N/lO sodium hydroxide. - 3 3 -I t was s t a n d a r d i z e d w i t h N/lOO h y d r o c h l o r i c a c i d which had been s t a n d a r d i z e d a g a i n s t a known weight o f m e r c u r i c o x i d e i n p o t a s s i u m i o d i d e . The s t a n d a r d i z e d base was k e p t i n a s t o p p e r e d f l a s k and a l t h o u g h i t was r e s t a n d a r d -i z e d on s e v e r a l o c c a s i o n s , i t was found t o be q u i t e s t a b l e . B CALIBRATION . _ 1. A c t i n o m e t r y a.) C a l i b r a t i o n G-raph f o r F e r r o u s I r o n Four ml of the s t a n d a r d i z e d 0.1M F e r r o u s s u l p h a t e s o l u t i o n was d i l u t e d t o 500 ml w i t h 0.1N s u l p h u r i c a c i d . The r e s u l t i n g s o l u t i o n c o n t a i n e d 0.8 x 10"^ moles F e + + p e r c c . Next, 0, 1, 2, 4, and 6 ml a l i q u o t s o f t h i s s o l u t i o n were added t o i n d i v i d u a l 50 ml v o l u m e t r i c f l a s k s . A s u f f i c i e n t volume of 0.1N s u l p h u r i c a c i d was t h e n added t o each f l a s k to--make the t o t a l volume o f a c i d e q u a l t o 25 m l . F i v e ml o f the Q.lfo 1:10 p h e n a n t h r o l i n e s o l u t i o n and 12.5 ml o f the b u f f e r s o l u t i o n were t h e n added t o each fl a . s k . The f l a s k s were d i l u t e d t o volume w i t h d i s t i l l e d water and a l l o w e d to s t a n d f o r \ hour. At the end o f t h i s t i m e , the o p t i c a l d e n s i t i e s o f the developed s o l u t i o n s were measured a t 510 rsjlton a Unicam Sp. 600 s p e c t r o p h o t m e t e r . D i s t i l l e d w a ter was used as the r e f e r e n c e i n ea,ch cas e . A graph o f the r e s u l t i n g o p t i c a l d e n s i t i e s as o r d i n a t e -against the c o n c e n t r a t i o n o f f e r r o u s i o n i n m i c r o moles as a b s c i s s a e i s shown in ' F i g u r e 3. . The t a b u l a t e d r e s u l t s are shown " i n Table I . TABLE I ml. Fe s o l u t i o n M i c r o moles Fee C o r r e c t e d o p t i c a l D e n s i t y 0 0 0 1 0.8 0.164 2 1.6 0.330 4 3.2 0.670 6 4.8 1.062 A c t i n o m e t r i c C a l i b r a t i o n o f t h e l i g h t Source The d e s c r i p t i o n s o f the o p t i c a l system, the photo-meter u n i t and the l i g h t s o u r c e are g i v e n i n Chapter I I I . The lamp and the photometer u n i t were t u r n e d on and a l l o w e d to warm up f o r 15 minutes w i t h the s h u t t e r S ( F i g u r e 12) c l o s e d . D u r i n g t h i s t i m e , t h e r e a c t i o n v e s s e l was t h o r o u g h l y evacuated and f r e s h f i l t e r s o l u t i o n s were put i n c e l l s F - l and F-2 (Figure" 1.!?). -35-- 3 6 -When the lamp had a t t a i n e d a steady output, the s h u t t e r was opened and l i g h t was allowed to f a l l on the p h o t o c e l l . The p o t e n t i o m e t r i c reading was taken by means of the photometer u n i t and was designated as I 0 ( i n i t i a l ) . The s h u t t e r was now c l o s e d and the p h o t o c e l l e x a c t l y r e p l a c e d by a c e l l c o n t a i n i n g 10 ml of the f e r r i o x a l a t e actinometer f o r a s p e c i f i e d time i n t e r v a l . At the end of t h i s time, the actinometer was removed and the p h o t o c e l l r e p l a c e d . Another r e a d i n g I 0 ( f i n a l ) was then taken. ( I t was necessary to take both i n i t i a l and f i n a l readings s i n c e the potassium hydrogen p h t h a l a t e s o l u t i o n slowly photolyzed) . The average of these I 0 readings was used. A f t e r i r r a d i a t i o n , the actinometer s o l u t i o n was t r a n s f e r r e d to a 50 ml amber coloured f l a s k and 5 ml of the phenanthroline s o l u t i o n and 5 ml of the b u f f e r s o l u t i o n were added. The volume was then made up to 50 ml with d i s t i l l e d water and the f l a s k was allowed to stand f o r •j hour. The o p t i c a l d e n s i t y o f the r e s u l t i n g s o l u t i o n was then measured on the Unicam at a wavelength of 510 rn^. The r e s u l t i n g o p t i c a l d e n s i t y was then converted to micro moles' of ferrous, abn produced. The above procedure was a a r r i e d out i n d u p l i c a t e aid blanks were run along with each determination. Results and calculations are shown below. etermination / 'I o Cinit-i a l ) Io ( f i n a l ) lo Averg. Time of I r r a d i -ation O.D. Corr-ected O.D. Micro moles' Fe 1 9 0 8 0 J « - 8 8 0 0 ^ 8940J»- 1 5 min. 1 . 1 1 7 1 . 0 3 9 4 . 7 1 1-B 0 . 0 7 8 2 8 6 6 3 v « . 8 5 3 0 - n - 8 5 9 6 - * ' 1 5 min. 1 . 0 5 8 0 . 9 8 0 4 . 4 4 2-B 0 . 0 7 8 * The mean quantum y i e l d as given by Hatchard and Parker (36) f o r the f errioxailate actinometer at 3130A° i s 1 .23 . Determination # 1 Quanta per ohm per sec Determination #2 Quanta per ohm per sec ( 4 . 7 1 x 1 0 " 6 ) ( 6 . 0 2 x 1 0 ) ( 1 . 2 3 ) ( 1 5 x 6 0 ) ( 8 9 4 0 ) 2 . 8 7 x 1 0 " (4.44) ( 6 . 0 3 ) x l O 1 7 ( 1 . 2 3 ) ( 1 5 x 6 0 ) ( 8 5 9 6 ) 2 . 8 4 x 1 0 " Average Quanta per ohm per sec = 2 . 8 5 x 1 0 " -40-2. Calibr a t i o n Graphs f o r Reaction Products a. ) Carbon Monoxide Carbon monoxide, 99$ pure, was taken from a cylinder and admitted to an evacuated gas sampling bulb. The l u l b was then placed on the gas admission system f o r the Perkin Elmer Vapour Fractometer (Figure/6 ) and various pressures of the CO were admitted to the Fract-ometer. The molecular sieve column was used f o r analysis. C a l i b r a t i o n graphs were then prepared i n which the pressures of CO i n mm was plotted against the area of the peak i n cm f o r diff e r e n t s e n s i t i v i t i e s . These graphs appear i n Figure 4 and 5. The column temperature was maintained at 69 C, the column pressure was 0.2 #/in and the flow rate was approximately 150 cc He per minute. b. ) Hydrogen Hydrogen was taken from a cylinder and admitted to an evacuated gas sampling bulb. Calibration was carried out i n the same way that the CO c a l i b r a t i o n was done except that peak heights were used instead of peak areas. The results are plotted i n Figure 6. Since the same column was u t i l i z e d , the experimental conditions were maintained. - 4 2 -c.) Carbon Dioxide Carbon dioxide was taken from a cylinder attributed to contain 99$ COg. The same mdthod of c a l i b r a t i o n was employed as above except that column " J " was used. The experimental conditions f o r the COg analysis were: column temperature = 69°C; column pressure = 7^/in 2; flow rate = 68 cc He per minute. The results are plotted i n Figure 7. - 4 3 = i \ N V Q Q Q _44-CHAPTER III APPARATUS 1. The Vacuum System 2 . The Furnace 3. The Optical System 4. The Lamp 5. The Photometer Unit 6 . The the Gas Admission System f o r Perkin-Elmer Model 154-0 Vapour Fractometer 7. Temperature Measurement - The Thermal Couple - 4 5 -/ CHAPTER III - APPARATUS 1. The Vacuum System The vacuum system as shown i n Figures 8-A and 8-B was of conventional design and constructed of pyrex apart from the quartz reaction vessel. A l l the high vacuum stopcocks were lubricated with Apiezon N grease except those which were wound with e l e c t r i c a l heating tape. The l a t t e r were lubricated with Dow Corning high vacuum Silicone grease which was stable to 400°F. The evacuation system (not shown) comprised a one-stage mercury d i f f u s i o n pump of conventional design backed by a "Hyvac" rotary o i l pump. A P2O5 trap and cold trap were also included. The cold trap was detatchable and the refrigerant was always l i q u i d nitrogen. Using the l i q u i d nitrogen trap, the system could be readily evacuated to 10"" ^  mm Hg. The one piece quartz reaction vessel was c y l i n d r i c a l with f l a t ends and was connected to the vacuum system by a quartz-to-pyrex graded seal. It was 1 0 . 0 cms long and had an illuminated volume of 7 3 . 0 ml. The narrow gauge tubing' between the c e l l and the tap to the vacuum system had a t o t a l volume of 3.0 ml and was not illuminated. - 4 6 --48-Since formaldehyde rapidly polymerizes at room temperature, a l l tubing leading from the reaction vessel to the s p i r a l gauge, the " B e l l " trap ( 3 9 ) and the forma-ldehyde storage was wound with e l e c t r i c a l heating tapes whose temperature was maintained at 100°C by means of varices'* (Heating tapes are represented by the shaded areas i n Figures 8A, 8B and 9 ) Reactants were metered into the reaction vessel using a s p i r a l gauge. The s p i r a l gauge was calibrated as a dire c t reading manometer and was about 5 times as sensitive as a standard mercury manometer without a vernier scale. This gauge i s i l l u s t r a t e d In Figure 9 and the c a l i b r a t i o n graph i s shown i n Figure 10. After an experiment had been c a r r i e d out, the reaction products were led to the " B e l l " trap which was cooled i n l i q u i d nitrogen. The permanent gases were then pumped over into the gas sampling bulb with the Toeppler pump. Hydrogen and carbon monoxide could then be analyzed on the Vapour Fractometer. I f formic acid was to be determined, the lower portion of the " B e l l " trap was removed and the acid t i t r a t e d directly-I f carbon dioxide was to be determined, the condensible products were recondensed into the removable trap (Fig. 8-B) and analyzed on the Vapour Fractometer. -49--51-2. The Furnace The aluminum - block furnace (Figure 11) was used to keep the reaction vessel at a chosen temperature. The furnace core comprised two aluminum cylinders, with 1.5 cm thick walls, which f i t t e d snugly around each end of the reaction vessel, and which mated at the middle of the vessel. In th i s way the vessel could be easily re-moved. The mating aluminum cylinders were housed i n a brass box which ms adjustably mounted on a standard optical.bench f i t t i n g . Each aluminum cylinder was separately wound with an e l e c t r i c a l heating c o i l and the two c o i l s were wired i n series. With the aid of a variac transformer, voltages from 0 to 120 volts could be applied to the heating c o i l s . It was found that a voltage setting of 82 volts was s u f f i c i e n t to keep the furnace at a temperature of 110°C. This temperature could be held constant to within one degree during the course of an experiment. There WAS a conveniently placed hole i n the aluminum block to accommodate a thermocouple. The furnace was insulated by f i l l i n g the space between the cylinders and the brass box with vermiculite. -52-COU/=>L £ - 5 3 -3 . The O p t i c a l System The o p t i c a l system i s shown i n F i g u r e 12. The lamp was a G e n e r a l E l e c t r i c , water c o o l e d AH6 h i g h p r e s s u r e mercury a r c w i t h a h o u s i n g d e s i g n e d t o g i v e a 3 mm. d i a m e t e r s o u r c e . The p o s i t i o n o f the lamp and the q u a r t z l e n s e s a t A was a d j u s t e d so t h a t a v e r y n e a r l y p a r a l l e l l i g h t beam c o m p l e t e l y f i l l e d t he r e a c t i o n v e s s e l V. The q u a r t z l e n s B f o c u s s e d the l i g h t onto the p h o t o c e l l P. Very f i n e w i r e mesh screens.D, c o u l d be p l a c e d i n the l i g h t beam and t h e i r p o s i t i o n was a d j u s t e d so t h a t t h e s c r e e n s were out o f f o c u s a l o n g the l e n g t h o f the r e a c t i o n v e s s e l . S was a manual s h u t t e r . Fj_ was a 2 cm q u a r t z f i l t e r c e l l c o n t a i n i n g the p o t a s s i u m hydrogen p h t h a l a t e s o l u t i o n . • F 2 was a 1 cm q u a r t z f i l t e r c e l l c o n t a i n i n g the p o t a s s i u m dichromate s o l u t i o n . The spectrum o f the f i l t e r s o l u t i o n and g l a s s f i l t e r c o m b i n a t i o n was measured a g a i n s t no b l a n k on a Carey a u t -omatic - r e c o r d i n g s p e c t r o p h o t o m e t e r . The spectrum i s shown i n F i g u r e 13. An a b s o r p t i o n minimum i s shown a t 3130A 0. A l l experiments were c a r r i e d out u s i n g this f i l t e r c o m b i n a t i o n . 4. The Lamp The lamp, lamp h o u s i n g and a.c. s u p p l y are shown i n F i g u r e 14. The a r c was r a t e d a t 1000 watt s and was c o o l e d w i t h water f l o w i n g at more the n 3 l i t r e s p e r minute. -55-£SJV£L ore fac C SOLA - 5 6 -When the lamp was running, current was taken from a 1000 vol t transformer. The transformer primary was supplied from 110 v o l t a.c. mains stabalized by a Sola constant voltage transformer. The st a r t i n g current required by the lamp was 2.5 amps at 1000 v o l t s . The lamp quickly attained normal operating conditions and the illumination remained constant to ± 1$ over a period of half an hour. Over a period of time the intensity from any one arcv. decreased. It was estimated that on an average the intensity dropped to about 80$ of i t s o r i g i n a l value after 50 hours operation. Thev performance of individual arcs varied i n as much as some would start only a few times and then f a i l to s t a r t , whereas some started up to 50 times before f a i l i n g . 5. The Photometer Unit The c i r c u i t diagram i s shown i n Figure 15. a) Photocell - the photocell was a CinUel QYA39 con-taining a quartz envelope and designed for u l t r a v i o l e t l i g h t . It i s claimed to give an output which i s accurately proportional to the intensity of l i g h t shone into i t . b) Operation - the photocell current passed through the r e s i s t o r chain R]__4 and the required voltage was tapped of f with the selector switch S n. - 5 7 -s . s * K E X : -B l B2 G. P.C. S. S. V. »1 R 2 R 3 54 •5,6 ;7,8 2 v o l t accumulator. 1 2 0 v o l t h . t . b a t t e r y . Galvanometer. P h o t o c e l l : QVA 3 9 . Weston Sta ndard C e l l . 6SC7 v a l v e . h. s . c. : 1W. R 5M : 1.5M 500k 150k 10 k l k : 100 ohms h. s. c. h. s. c. h. s . c. dual 1W. 1W. 1W. decade *9',10 K l l , 1 2 10 ohras Muirhead, it R 1 4 n i l R 1 9 R 2 1 R 2 2 R 2 3 " 2 4 R 2 5 2 5 k : 2k : 5 0 k ; 5 0 k : 2. 2k 1M : 1 0 0 k 3 3 k : 1 0 k : 3.3k l k : w. w. w. w. w. w. : w. w. £w. : w. w. w. w. w. w. : w. w. w. w. ; 330 ohms : 1 0 k : w.w. 1W. 1W. : 1W. : 1W. 1W. 1W. : 1W. 1W. : 1W. 1W. P i g , 1 5 . Photometer U n i t . -58- X An o p p o s i n g v o l t a g e was then a p p l i e d by the p o t e n t i o m e t e r c i r c u i t c o n s i s t i n g o f the b a t t e r y B^ and the s e r i e s o f d u a l decades, RCJ_]_2. Bj_ was n o r m a l l y 2 V and the v o l t a g e tapped from the p o t e n t i o m e t e r was a d j u s t a b l e to any v a l u e between 0 and 2 v o l t s w i t h an a c c u r a c y o f 2 x 10"^ v o l t s . The d i f f e r e n c e between the v o l t a g e tapped from t h e r e s i s t o r c h a i n R^_4 and the opposing v o l t a g e from the p o t e n t i o m e t e r was a p p l i e d t o the double t r i o d e a m p l i f i e r v a l u e V. The second t r i o d e u n i t compensated f o r s u p p l y v o l t a g e v a r i a t i o n . The output o f the a m p l i f i e r was the n f e d i n t o the galvonometer G v i a the a t t e n u a t o r R]_o,_24» The h i g h t e n s i o n f o r the u n i t was d e r i v e d from a 120 v o l t b a t t e r y and the low t e n s i o n from a 6 v o l t mains-t r a n s f o r m e r . c) P h o t o m e t r i c measurements - the i n s t r u m e n t was t u r n e d on by c l o s i n g s w i t c h e s S 2, and and a l l o w i n g i t to warm up f o r 15 minutes. With no l i g h t f a l l i n g on t h e p h o t o c e l l , and the p o t -e n t i o m e t e r r e a d i n g z e r o , the dark c u r r e n t from t h e p h o t o c e l l was b a l a n c e d by a d j u s t i n g R-^ and R-^ f o r zero galvanometer c u r r e n t . The c i r c u i t had t o be r e b a l a n c e d f o r each p o s i t i o n o f the s e l e c t o r s w i t c h S n. -59-c. Sg was then closed, thus applying a standard voltage across the r e s i s t o r chain R-]__4» The potentiometer voltage was then increased u n t i l the galvonometer showed zero deflection. The potentiometer reading was taken. This operation was carried out to ensure that gave a constant voltage. Sg was then opened and l i g h t allowed to f a l l on the photocell. The potentiometer was adjusted so that the galvonometer showed zero def l e c t i o n and the potentiometer reading taken. Assuming that the photocell characteristic i s l i n e a r , then t h i s potentiometer reading was proportional to the l i g h t i n t e nsity. The photometer unit was used f o r monitoring the out-put of the mercury arc and mearuring the absorption of l i g h t by a sample of formaldehyde i n the reaction vessel. 6. The G-as Admission System to the Perkin - Elmer  Model 154-C Vapour Fractometer.. The gas admission system as shown i n Figure 16 was constructed of pyrex and a l l stopcocks were lubricated with Apiezon "N" grease. The system was backed by a cold trap, a conventional mercury d i f f u s i o n pump and a rotary "Hyvac" mechanical pump. The refrigerant for the cold trap was always l i q u i d nitrogen. -61-This system was used for the analysis of hydrogen, carbon monoxide and carbon dioxide. The permanent gases (hydrogen and carbon monoxide) were admitted to the system by means of the removable gas sampling bulb as shoT/m i n Figure 8-B. The following procedure was employed f o r analysis. With helium from the Fractometer flowing through column A, the system was evacuated by opening a l l the taps except a. The manometer reading was then taken, tap e closed and tap a opened. This allowed the gas to expand into the large volume'V. Tap b was then closed and the mercury raised i n the Toepple-r pump to push most of the gas into column B of the gas admission system. The manometer reading was themtaken, tap c closed and taps 1 and 2 simultaneously turned to divert the helium stream through column B thus picking up the gas and carrying i t to the Fractometer. Since the volume of B had been previously calibrated ( 3 2 . 1 ml) i t was possible to calculate the number of moles of gas present i n any p a r t i c u l a r sample. 7• Temperature Measurement - The Thermocouple•„.. The thermal couple c i r c u i t i s shown i n Figure 18. The Itermal couples were used to measure the temperature of the furnace and the e l e c t r i c a l heating tapes. •62 -63-The potentiometer was a Students' Type "S" obtained from the Rubicon Company and was•comprised of calibrated • r e s i s t o r s . Two ranges were provided, 0 to 1.6 volts and 0 to 16 m i l l i v o l t s . . Each range was covered by two measuring d i a l s , the f i r s t of which was comprised of a 16 position switch c o n t r o l l i n g 15 fixed 10 - ohm r e s i s t o r s , and the second of which was comprised of a 14 - inch slidewire with 200 d i v i s i o n s . On the upper range 15 increments of 0.1 v o l t each were developed across the f i r s t d i a l r e s i s t o r s , the slidewire affording continuous va r i a t i o n throughout a 0.1 v o l t i n t e r v a l with 0.0005 v o l t each. On the lower range, the corresponding values were one-hundredth of the foregoing. Potentiometric measurements were made by connecting the potentiometer to the accessories as shown i n Figure 18. To adjust the current i n the potentiometric c i r c u i t y S ^ was set to the standard c e l l position, S 2 to the 1.6 v o l t range and d i a l s 1 and B set to standard c e l l value. KQ_ was then tapped and the wire wound variable resistances were adjusted u n t i l the galvanometer G showed no deflection. K 2 was then tapped and the resistance readjusted u n t i l no galvanometer deflection was observed. The thermo"-couple switch (T/C) was then turned to the thermos couple required, S 2 to the required potentiometer range, Kj_ tapped and d i a l s A and B adjusted to give no deflection on the galvanometer. -64-K 2 was then tapped and d i a l s A and B readjusted to give the f i n a l potentiometric reading. The thermP.ir- couples were comprised of copper-constantan wires and a reference junction of 0°C was used. The temperature corresponding to a measured E .M.P. was taken from the Handbook of Chemistry and Physics (40). - 6 5 -CHAPTER IV THE MECHANISM OF THE PHOTOCHEMICAL OXIDATION OE GASEOUS FORMALDEHYDE AT 110°C 1 . Object of the investigation 2. Experimental Procedure 3. Determination of Formic Acid 4. Determination of Hydrogen and carbon monoxide 5. Determination of Carbon Dioxide 6. The influence of oxygen 7. Tests f o r Peroxide 8 . The Photolysis of Formaldehyde 9 . A summary of the Experimental Results 1 0 . Proposed Mechanism -66-CHAPTER IV THE MECHANISM OF THE PHOTOCHEMICAL OXIDATION OF  FORMALDEHYDE AT llQ°C 1' Object of the Investigation I t i s well known that the products of the photochemical oxidation of formaldehyde are CO, H 2, HCOOH and C02. How-ever, there has been some disagreement on the r e l a t i v e amounts of these products, the resulting'quantum yie l d s and especially the mechanism of the photochemical oxidation. Other workers (22) have used l i g h t at a wavelength of 2537A0 and 2650A0, and reaction mixtures which contain a rather high 02/CH20 r a t i o . They found that the reaction was oxygen dependent to some extent. A l l workers agree that no peroxides or only traces of peroxides are found i n the reaction pro-ducts. One of the main problems of the kinetics i s to s a t i s f a c t o r i l y account for the formic acid produced. The object of this work was to study the photochemical oxidation at 110°C using monochromatic l i g h t at a wavelength of 3130A0. In most experiments, the O2/CH2O ra t i o was 1:10 which i s much less than that used by other workers. The reaction products and t h e i r quantum y i e l d were obtained as a function of the formaldehyde pressure. Tests f o r peroxides were made and the effect of an increased O2/CH2O ra t i o was studied. -67-Experiments were also carried out i n order to determine the effect of varying the absorbed l i g h t intensity, l a . The experimental results follow. 2. Experimental Prodedure The lamp and the photometer unit were turned on and allowed to warm up f o r 15 minutes. The potentiometer of the photometer unit was then balanced against the standard c e l l with no l i g h t f a l l i n g on the photocell, l i g h t was then allowed to f a l l on the photocell and the i n i t i a l incident intensity, ( l 0 ) i , as measured by the photometer unit was recorded. The temperature of the furnace had been adjusted to 110°C as registered by the thermal couple. During this period, the vacuum system was thoroughly pumped down. Oxygen and formaldehyde were then metered into the reaction vessel by means of the s p i r a l gauge. Oxygen was always admitted f i r s t , l i g h t was then shone onto the reaction mixture f o r an appropriate time as measured by a time clock. The transmitted l i g h t intensity, I t , was measured at convenient intervals during the reaction. The l i g h t was then cut off and the reaction products were led to the " B e l l " trap, which was cooled i n l i q u i d nitrogen. After 10 minutes, the non condensable gases were pumped off by means of the toeppler pump and admitted to the removable gas analysis bulb. -68-The f i n a l incident intensity, (Io)f was then measured. The average of ( l o ) i and ( l o ) f was taken as the incident i n t e n s i t y , I 0 . The difference between th i s value and the value of the averaged transmitted l i g h t i ntensity value, I t , was the value of the l i g h t absorbed during the reaction,I 3. Determination of Formic Acid After the non - condensible gases had been removed from the " B e l l " trap, a i r was admitted and the lower portion of the trap removed while s t i l l immersed i n the l i q u i d nitrogen dewar. The trap was then quickly re-moved from the dewar and 4 or 5 ml of the phenolphthalien indicator solution,were c a r e f u l l y added to the condensed products. The r e s u l t i n g clear solution was then quickly t i t r a t e d i n the trap with the standardized N/1000 sodium hydroixide to the pink end point. Hence, the number of micro moles of formic acid produced was given d i r e c t l y as the volume i n ml of the base used. In order to allow f o r impurities or the effect of increasing amounts of formaldehyde i n the acid t i t r a t i o n , 50 mm, 100 mm, 150 mm, 200 mm, 250 mm and 300 mm pressures of formaldehyde were photolyzed and the amount of formic acid determined. The results are shown i n Table II and i t i s evident that, i n the absence of oxygen, no appreciable - 6 9 -amount of formic acid was produced. For following de-terminations, a correction was applied to the t i t r a t i o n value depending on the concentration of formaldehyde used. TABLE II Blank T i t r a t i o n s f o r the determination of formic acid  Run Time of Run PCH?0(mm) ml N/1000 NaOH 1 30 min. 50 0.10 2 30 min. 100 0.15 3 30 min. 100 0.15 4 30 min. 150 0.20 5 30 min. 200 0.25 6 30'min. 200 0.25 7 30 min. 250 0.25 .8 30 min. 300 0 .25 a) Variation with time of i r r a d i a t i o n The v a r i a t i o n of formic acid produced during the photooxidation with respect to the time of i r r a d i a t i o n i s shown i n .Figure 19. In these runs, the i n i t i a l pressure of oxygen and formaldehyde were approximately 10 mm and 100 mm respectively. I 0 was approximately 8000 S L . The results are shown i n Table I I I . -71-TABLE III Run Time of run (min) ml N/1000 NaOH 9 5 4220 10 15 9.60 11 J) 17.60 12 40 27.50 13 60 33.0 14 80 44.50 b) Variation with the absorbed l i g h t The v a r i a t i o n of the formic acid produced with the intensity of the absorbed l i g h t , l a , i s shown i n Figure 20 where the l o g of the absorbed l i g h t , l a , i s plotted against the log of the number of micro, moles-ofxformic acid produced. Two sets of data are l i s t e d i n Table IV; one set f o r mixtures containing 100 mm Hg of formaldehyde, 10 mm Hg of 0 2, and the other set f o r mixtures containing 50 mm Hg of formaldehyde, 5 mm Hg02. TABLE IV Run PCH9O (mm) PO? (mm) ml N/1000 HaOH Ia(*i) 15 101.1 11.5 18.00 2850 16 104.7 13.5 14.10 2155 17 103.5 11.1 8 .32 1260 18 101.9 11.1 4.05 475 19 103.3 12.2 18.50 2840 20 49.5 5.4 3.05 695 21 52.7 5.3 9.15 1920 22 51.5 4.9 5.65 1305 - 7 2 --73-From the re s u l t i n g p l o t s , i t i s evident that formic , acid i s nearly d i r e c t l y proportional to the absorbed l i g h t i n t e nsity. c) Variation with the i n i t i a l formaldehyde pressure -It was found that the amount of formic acid produced was proportional to the i n i t i a l pressure of formaldehyde. Data f o r these runs aire shown i n Table V. In a l l cases, PO2 was approximately 10 per cent of PCB^O, the incident l i g h t intensity, I Q , was 8000 SX. and the time of each run was 30 minutes. TABLE V Run PCHoO (mm Hg) ml N/1000 NaOH l a (Jt) ^HCOOH 23 20 2.58 24 21 2.50 25 23 2.55 26 40 4.52 27 40 5.30 28 . 41 ' ' 5.05 29 44 5.50 30 48 7.50 31 48 6.80 32 48 6.95 33 ' 50 7.35 34 59 9.65 35 . 78 12.15 36 100 .16.50 37 100 18.40 38 105 16.30 39 128 21.42 1100 1420 1575 1485 1580 1925 1800 1825 1840 1120 1160 1880 2295 2430 2760 2630 2810 2.70 2.68 2.58 3.74 3.98 4.1 4.07 4.57 4.44 4.47 4.78 5.74 6.27 8.02 7.88 7.40 9.00 -74- 7. TABLE V Run PCHpO (mm Hg) ml N/lOOO NaOH I a (it) jHCOOH 40 41 42 43 44 45 46 47 48 49 50 51 150 152 152 160 191 200 200 24.90 28.50 29.75 26.00 35.62 33.50 36.60 38.00 43.20 45.00 3.05 3.15 3030 3570 3610 3380 4010 3760 4240 9.68 9.38 9.60 9.05 10.30 10.40 10.10 220 4190 4320 4400 1200 10.75 11.80 240 250 30 31 1100 12.00 3.00 3.42 Some of the tabulated results are plotted i n Figure .. 21. In Figure 22, the logarxthmoof the formaldehyde pressure (mm Hg) i s plotted against the logarithm of the number of micro moles of formic acid produced. The slope of t h i s l i n e i s 1.17 and hence the formic acid produced i s nearly d i r e c t l y proportional to the i n i t i a l formaldehyde concentration. d) Quantum Y i e l d as a function of formaldehyde pressure Quantum yiel d s for HCOOH, H 2 and CO are plotted i n Figure 23. The quantum y i e l d f o r each of these products increases gradually with increasing formaldehyde pressure. The data f o r formic acid are found i n Table V. 77--78-Sample C a l c u l a t i o n s f o r ^ HCOOH From the a c t i n o m e t r i c c a l i b r a t i o n s (see p37 ) the number o f quanta p e r ohm p e r second i s 2.85 x 10"*""'". Hence, the t o t a l quanta absorbed w i l l be g i v e n by the f o l l o w i n g e x p r e s s i o n . Q T = (2.85 x 1 0 1 1 ) (time i n sec.) ( I a ( a ) ) The t o t a l number o f m o l e c u l e s o f f o r m i c a c i d produced w i l l be % = (moles HCOOH) (6.02 x 1 0 2 5 ) The r e s u l t i n g quantum y i e l d i s t h e n g i v e n by (£) = Nrp Example #1 - Run #20 Q T = (2.85 x 1 0 1 1 ) ( 3 0 x 60)(1485)=(2.85)(1.8)(1.485) x 1 0 1 7 N T = (5.05) (6.02) x 1 0 1 7 Hence, $HCOOH = (5.05)(6.02) = 4 . 0 (2.85Ttl.8lTlT485) Example #2 - Run #44 Q5, = (1.980) (2.85) (1.8) x 1 0 1 7 % = (9.65) (6.02) x 1 0 1 7 HCOOH = % = 5.7 0? -79-Z 4• Determination of Hydrogen and Carbon Monoxide The non-condensible gases were comprised of hydrogen, carbon monoxide and unreacted oxygen. By means of the Toeppler pump, these gases were pumped i n t o the c a l i b r a t e d burette of the Toeppler pump and the volume and pressure of the gases were recorded. They were then forced up i n t o the removeable gas a n a l y s i s bulb by means of compressed a i r . Analjrsis of these gases was c a r r i e d out on the P e r k i n - Elmer, Model 154-C Vapour Fractometer u t i l i z i n g the molecular sieve column. Since the pressure and volume of the gases being analyzed was known from the reading of the mercury manometer and the c a l i b r a t e d column 1 of the gas admission system, the number of micro moles of product could be c a l c u l a t e d from the c a l i b r a t i o n graphs f o r hy-drogen and carbon monoxide (see Figure*^-*) a) ' V a r i a t i o n w i t h the time of' i r r a d i a t i o n Both hydrogen and carbon monoxide were found to be p r o p o r t i o n a l to the time of i r r a d i a t i o n . The r e s u l t s shown i n Table VI are f o r mixtures c o n t a i n i n g 100 mm Hg of form-aldehyde and 10 mm H„ of Op and an i n c i d e n t i n t e n s i t y , Io, of 8 0 0 0 S L . -80-TABLE VI Run Time of run (min) micro moles Hp Micro moles CO 58 5 6.10 7.43 59 5 7.83 8.20 60 15 14.90 16.40 61 15 13.85 15.80 62 30 25.30 34.00 63 30 24.70 34.50 64 45 38.00 41.20 65 60 50.50 66.80 Variation with the absorbed l i g h t The v a r i a t i o n of hydrogen and carbon monoxide produced with the intensity of the absorbed l i g h t i s shown i n Figure 24 where the logarithm of the absorbed l i g h t , l a , i s plotted against the number of micro moles of product formed. The data i n Table VII corresponds to mixtures containing 100 mm Hg of formaldehyde, 10 mm H g of 0 2 and to a time of i r r a d i a t i o n of 30 minutes. TABLE VII Run micro moles Hp micro moles CO l a (St) 67 14.2 14.8 1480 68 15.2 16.5 1632 69 30.4 37.2 3345 70 21.4 24.2 2400 71 32.7 39.8 3185 72 9.8 12.2 1250 -82-Erom these p l o t s , i t i s e v i d e n t t h a t hydrogen and carbon monoxide are n e a r l y p r o p o r t i o n a l t o the absorbed l i g h t i n t e n s i t y , l a . c) V a r i a t i o n w i t h the i n i t i a l formaldehyde p r e s s u r e The amount o f hydrogen and carbon monoxide p r o -duced was a l s o found to be p r o p o r t i o n a l t o t h e i n i t i a l p r e s s u r e o f formaldehyde. D a t a f o r these runs i s shown i n Table V I I I . A l l d a t a r e f e r s t o m i x t u r e s where Pog = PCB^O, where I 0 = 8000A., and where the time o f i r r a d i a t i o n 10 was 30 minutes. TABLE V I I I Run 80 73 74 75 76 77 78 79 87 81 82 83 84: 85 86 88 18 4.2 5.3 1350 3.80 4.65 22 4 .8 6.2 1390 4.05 5.25 38 7.2 10.8 1640 5.10 7.70 48 11.0 15.2 2140 5.90 8.20 55 13.7 15.5 2000 8.05 9.10 68 15.1 18.4 2320 7.64 9.30 75 15.2 20.0 2570 6.9 9.10 82 16.9 20.2 2690 7.26 8.70 98 22.3 24.5 2720 9.52 10.50 102 21.3 2£.0 2740 9.05 10.70 107 25.3 . 27.2 2940 10.20 11.00 112 24.8 30.8 2980 9.65 12.00 133 29.0 35.0 3420 10.0 12.00 148 32.5 38.2 3665 10.30 12.20 172 36.2 43.5 4110 10.30 12.40 200 45.0 49.8 ' 4260 12.40 13.70 - 8 3 -TABLE V I I I  Run PCHoO(mm H g) l 4 o l e s Ho 89 209 4 9 . 2 90 230 5 0 . 3 91 235 5 2 . 1 (Cont..) ^moles CO IaC-n^E^ $C0_ 52.5 4390 13.20 14.10 57.0 4570 12.80 14.60 60.8 4790 12.80 14.80 Some of the tabulated r e s u l t s are p l o t t e d i n Figure 21. In Eigure 25, the logarithm of the number of micro moles of hydrogen and carbon monoxide produced i s p l o t t e d against the logarithm of the formaldehyde pressure. d) Quantum Y i e l d as a f u n c t i o n of formaldehyde pressure Quantum y i e l d s are p l o t t e d i n Figure 23. The data f o r hydrogen and carbon monoxide i s found i n Table V I I I . The c a l c u l a t i o n s are s i m i l a r to those shown f o r formic a c i d . 5. Determination of Carbon Dioxide A f t e r the non-condensible gases had been pumped o f f , the l i q u i d n i t r o g e n dewar was removed from the " B e l l " trap and the condensed products allowed to warm up. The dewar was then placed around the removable trap (Figure 8-B) and the products re-condensed. Carbon dioxide was then analyzed on the Vapour Fractometer u t i l i z i n g column " J " . Since C 0 2 was a minor product, i t was necessary to condense the CO2 i n t o column B. of the gas admission system. This was done by wrapping a piece of cotton wool around the top p o r t i o n of column B and s a t u r a t i n g i t with l i q u i d n i t r o g e n . -84-- 8 5 -13 I n t h i s way, a l l the C0 2 was a n a l y z e d . Another problem w i t h the CO2 a n a l y s i s was the p o l y m e r i z a t i o n o f u n r e a c t e d form-aldehyde. Each t i m e the condensed p r o d u c t s were allowed" t o warm up, some p o l y m e r i z a t i o n o c c u r r e d w i t h the r e s u l t t h a t some o f the COg was t r a p p e d i n the polymer. E o r t h i s r e a s o n , the CO2 a n a l y s i s are p r o b a b l y low and a r a t h e r l a r g e e r r o r i n t h e d e t e r m i n a t i o n r e s u l t e d . a) V a r i a t i o n w i t h formaldehyde P r e s s u r e COg was found t o v a r y p r o p o r t i o n a t e l y w i t h the i n i t i a l p r e s s u r e o f formaldehyde. Some r e s u l t s are l i s t e d i n Table IX. These runs correspond t o m i x t u r e s o f CH2O/0>2 i n a l O / l r a t i o , an i n i t i a l i n t e n s i t y Io of 8 0 0 0 , and a t i m e o f i r r a d i a t i o n o f 3 0 minutes.' TABLE IX Run PCHoO(mm H„) m i c r o moles COQ IaL-rO 9 2 30 0 . 3 4 1 3 5 0 0 . 3 1 93 51 0 . 4 5 1 8 4 0 0 . 3 1 94 56 0 . 5 5 2100 0 . 3 1 95 70 0 . 8 7 2490 0 . 4 0 5 96 82 0 . 8 0 2500 0 . 3 7 5 97 1 0 0 1 . 1 5 2780 0 . 4 8 98 1 0 4 1 . 0 5 2910 0 . 4 2 99 1 1 0 1 . 4 0 2900 0 . 5 7 1 0 0 1 3 0 1 . 4 6 3 4 2 0 0 . 5 0 5 1 0 1 1 5 5 1 . 7 0 3 6 8 0 0 . 5 4 1 0 2 1 6 0 1 . 7 8 3975 0 . 5 3 1 0 3 1 6 8 1 , 8 0 4080 0 . 5 2 - 8 6 -Some of the tabulated results are plotted i n Figure 26. b) Quantum Y i e l d as a function of Formaldehyde Pressure Quantum yiel d s f o r varying pressures of form-aldehyde are shorn i n Figure 21. The experimental results are shown i n Table IX 6. The influence of Oxygen Although most experimental results were obtained for reaction mixtures containing a 1:10 r a t i o of oxygen to formaldehyde, several runs were carried out i n which thi s r a t i o was increased. In general, an increased proportion of oxygen had l i t t l e effect on the amount of reaction products formed. However, at f a i r l y large oxygen concentrations, 3:2 and 2:1 mixtures of oxygen to formaldehyde, a noticable reduction i n the amounts of reaction products was apparent. This fact was also observed by Style and Summers (M) who did most of t h e i r work at r e l a t i v e l y high r a t i o s of oxygen to formaldehyde. 7. Test f o r Peroxides. Peroxides were tested f o r i n the condensed products by the ferrous thiocyanate method. About 5 x 10 mole of peroxide can be estimated with a f a i r degree of accuracy by t h i s method. -89-I t r e l i e s on measuring the i n t e n s i t y o f the r e d c o l o u r developed (due t o the f o r m a t i o n o f r e d f e r r i c t h i o c y a n a t e complex) when a s m a l l amount o f p e r o x i d e i s added t o a known volume o f f e r r o u s t h i o c y a n a t e s o l u t i o n . I n o r d e r t o make q u a l i t a t i v e t e s t s f o r p e r o x i d e , the " B e l l " t r a p was removed w h i l e s t i l l immersed i n l i q u i d n i t r o g e n . S e v e r a l ml o f the pr e p a r e d f e r r o u s t h i o c y a n a t e s o l u t i o n were th e n added and the r e s u l t i n g s o l u t i o n a l l o w e d t o warm up. The r e d c o l o u r was th e n a l l o w e d t o develop f o r 10 minutes a t room temperature. I n a l l the t e s t s c a r r i e d o u t , o n l y a v e r y p a l e p i n k c o l o u r r e s u l t e d i n d i c a t i n g t h a t o n l y t r a c e amounts o f p e r o x i d e were p r e s e n t i n t h e r e a c t i o n p r o d u c t s . 8. The P h o t o l y s i s o f Formaldehyde Three experiments were c a r r i e d out i n o r d e r to determine the quantum y i e l d s of hydrogen and carbon monoxide d u r i n g the p h o t o l y s i s o f formaldehyde a t 110°C. An i n c i d e n t i r r a d i a t i o n o f 8000 J1. was used i n each r u n . The r e s u l t s a r e summarized i n Table X. TABLE X ( A l l runs i r r a d i a t e d f o r 30 minutes) Run PCHoO(mm Hg) jjmoles CO p i n o l e s H? Ia ( n ) $ 0 0 $ S 2 104 166 4.62 4.70 3960 1.36 1.38 105 163 4.88 4.60 4130 1.38 1.29 106 121 4.02 3.90 3310 1.43 1.39 - 9 0 -9« A Summary o f the e x p e r i m e n t a l r e s u l t s B e f o r e d i s c u s s i n g the mechanism o f the p h o t o c h e m i c a l o x i d a t i o n o f formaldehyde a t 110°C, i t i s advantageous t o summarize the f a c t s o b t a i n e d from the p r e c e d i n g e x p e r i m e n t s . a) GO, H 2 and HCOOH are the major p r o d u c t s formed; C 0 2 i s a minor p r o d u c t . b) The major p r o d u c t s produced are d i r e c t l y p r o p o r t i o n a l t o t h e i n i t i a l formaldehyde p r e s s u r e . c) The major p r o d u c t s . p r o d u c e d are d i r e c t l y p r o p o r t i o n a l t o the i n t e n s i t y of t h e absorbed l i g h t , l a . d) The quantum y i e l d s o f thes e p r o d u c t s v a r y w i t h the i n i t i a l p r e s s u r e o f formaldehyde; ^) CO ^  <|)H2 ^  <J)HC00H e) No p e r o x i d e s were d e t e c t e d i n t h e r e a c t i o n p r o d u c t s . f ) Most of the e x p e r i m e n t a l r e s u l t s were o b t a i n e d w i t h 1:10 m i x t u r e o f 0 2 t o H2CO; t h e r e a c t i o n was independent o f O2 u n t i l f a i r l y h i g h r a t i o s o f 0 2/CH 20 were used. 10. The Proposed Mechanism The p r e c e d i n g r e s u l t s on the p h o t o c h e m i c a l o x i d a t i o n of formaldehyde a t 110°C, u s i n g l i g h t a t a wavelength o f 3130A 0 and r e a c t i o n m i x t u r e s where the CH2O: 02 r a t i o i s a p p r o x i m a t e l y 10:1, show t h a t t h e r a t e o f f o r m a t i o n o f the main p r o d u c t s i s governed by the f o l l o w i n g k i n e t i c e q u a t i o n s : -91-( i i i ) ( I D ( i ) d [CO] / d t d [H2] / d t d [HCOOH] = KIa [ C H 2 0 ] = K r l a [CH 20] / d t = K 1 1 l a [CH 2 0 ] M i x t u r e s o f formaldehyde and oxygen a t 110°C are known to be u n r e a c t i v e and hence the mechanism o f the ph o t o c h e m i c a l o x i d a t i o n must be dependent on the p r i m a r y p r o c e s s i n the p h o t o l y s i s o f formaldehyde. Most workers (21,22,29) agree t h a t the primarjr p r o c e s s i n the p h o t o l y s i s at 3130A0 i s the f o r m a t i o n of a hydrogen atom and the f o r m y l r a d i c a l . S e v e r a l p o s s i b i l i t i e s e x i s t f o r the f a t e o f the r e s u l t i n g f o r m y l r a d i c a l . There i s disagreement as t o whether o r no t the f o r m y l r a d i c a l w i l l decompose s i n c e a c t i v a t i o n e n e r g i e s f o r i t s d e c o m p o s i t i o n v a r y from 13.5 to 26 k i l o c a l o r i e s . However, most o f the evidence i n d i c a t e s t h a t the f o r m y l r a d i c a l w i l l be s t a b l e a t 110°C. S e v e r a l r e a c t i o n s o f the f o r m y l r a d i c a l w i t h oxygen are p o s s i b l e . These are o u t l i n e d below. Hence CH2O + h Y — H + HCO (1) CHO + 02. CO + H02 -92-T h i s r e a c t i o n has been p o s t u l a t e d by Chamberlain et a l (41) i n the o x i d a t i o n of methane and by S t y l e and Summers(22) i n the p h o t o o x i d a t i o n o f formaldehyde. ( I I ) CHO + 02~->• CO2 + OH M a r c o t t e and Noyes (42) have p o s t u l a t e d t h i s - r e a c t i o n i n the p h o t o l y s i s o f acetone i n the presence o f 0 2 and i t has a l s o been p o s t u l a t e d i n the o x i d a t i o n o f methane (43). ( I I I ) CHO + 0 2 —>• CH0 5 T h i s r e a c t i o n has been p o s t u l a t e d i n the o x i d a t i o n o f formaldehyde and methane.by Lewis and von E l b e (20) The f i r s t two o f thes e t h r e e r e a c t i o n s have been i n c o r p o r a t e d i n t o the proposed mechanism, and a l t h o u g h the t h i r d seems t o be q u i t e l i k e l y , no s a t i s f a c t o r y mechanism c o u l d be o b t a i n e d when i t was i n c l u d e d i n the r e a c t i o n mechanism. Horner, S t y l e and Summers (30) were a l s o u n a b le t o i n c o r p o r a t e t h i s r e a c t i o n i n t o t h e i r mechanism. Many workers (21,22,28,30) have proposed the f o l l o w i n g r e a c t i o n between hydrogen atoms and formaldehyde. H + HCHO — > H 2 + HCO I t seems r e a s o n a b l e t h a t t h i s r e a c t i o n i s the main source o f the hydrogen produced. I t might a l s o be ex p e c t e d t h a t hydrogen atoms w i l l r e a c t w i t h 02 t o form the H0 2 -33-r a d i c a l . Hence, the r e a c t i o n H + 02 + M —>. H 0 2 + M (Where M i s a formaldehyde or oxygen molecule) must a l s o he considered. The r e a c t i o n of the hydroxyl r a d i c a l with form-maldehyde has been p o s t u l a t e d i n the o x i d a t i o n o f methane and formaldehyde ( 2 0 , 4 4 , 4 5 ) . I t may he w r i t t e n as f o l l o w s , OH + HCHO — > OHO + H2O In order that the h i g h quantum y i e l d of hydrogen can he accounted f o r , i t i s necessary to reproduce hydrogen atoms. Since the decomposition of the formyl r a d i c a l i s u n l i k e l y at 110°C, hydrogen atoms must be produced by some other means. S e v e r a l r e a c t i o n s between formaldehyde and the HO2 r a d i c a l have been p o s t u l a t e d , some of which produce the r e q u i r e d hydrogen atoms. S t y l e and Summers (22) p o s t u l a t e d the f o l l o w i n g r e a c t i o n , H 0 2 + HCHO _ > CO, CO2, HCOOH, H . In order to e x p l a i n the experimental r e s u l t s i n t h i s work, i t was necessary to p o s t u l a t e the f o l l o w i n g r e a c t i o n between HO2 and formaldehyde H 0 2 + HCHO—>• HCO3H + H -94-S i n c e p e r a c e t i c a c i d i s found i n the ph o t o c h e m i c a l o x i d a t i o n o f ac e t a l d e h y d e (48), i t i s not unreasonable t o expect the p e r f o r m i c a c i d i n the p h o t o o x i d a t i o n o f formaldehyde. However, no p e r o x i d e s are d e t e c t e d i n the r e a c t i o n p r o d u c t s and i t must be assumed t h a t the p e r f o r m i c a c i d decomposes t o form the normal a c i d , HCOjH > HCOOH + | 0 2 I t has been found (47) t h a t p e r f o r m i c a c i d i s q u i t e u n s t a b l e i n comparison t o p e r a c e t i c a c i d and t h e r e f o r e i t seems q u i t e r e a s o n a b l e t o assume t h i s d e c o m p o s i t i o n . W i t h t h e s e p o s t u l a t e s and the e x p e r i m e n t a l f a c t s i n mind, i t has been p o s s i b l e t o propose a mechanism which s a t i s f a c t o r i l y e x p l a i n s the k i n e t i c r e s u l t s . The f o l l o w i n g r e a c t i o n s are e n v i s i o n e d . HCHO + hY — V H + HCO £'la HCO + 0 2 — H 0 2 + CO K i HCO + 0 2 >. OH + C 0 2 K 2 H 0 2 + HCHO ^ HCO^H + H HCO3H HCOOH + i 0 2 K4 H + 0 2 + M ^EOo + M K5 H + HCHO — > H 2 + HCO Kg OH + HCHO — > H 20 + HCO K ? H 0 2 ±Zt% K 8 - 9 5 -S i n c e the r a t i o of CH^OcC^was 1 0 : 1 i n the r e a c t i o n m i x t u r e s , i t might be argued t h a t K 5 w i l l be s m a l l compared t o Kg. I f we make t h i s assumption and appl y the s t a t i o n a r y s t a t e h y p o t h e s i s t o the hydrogen atoms, the OH, H O 2 , HCO r a d i c a l s and t o the p e r f o r m i c a c i d , then we may d e r i v e the f o l l o w i n g e q u a t i o n s . H Atoms 1 ) J ' l a + K 5 [ H 0 2 ] [ H C H 0 j = Kg [H] [HCHO] OH r a d i c a l s 2 ) K 2 [HCO] [ 0 2 ] = K 7[OH] [HCHO] H 0 2 R a d i c a l s 3) * a [HCO] [ 0 2 ] = K3 [ H 0 2 ] [HCHO] + K 8 [HO2 ] HCO r a d i c a l s 4 ) |>Ia + K 6 [H] [HCHO] + K ? [ 0H j JHCHO] = K-j^HCO] [ 0 2 ] + K 2 [ H C 0 ] [ O 2 ] HCO3H m o l e c u l e s 5) K^ [ H 0 2 ] [HCHO] = K 4 [HCO^H] S i n c e K 2 [HCO] [o2] = K 7 [OH] [HCHO] from 2 ) , r e a c t i o n 4 ) becomes l a + K 6 [ H ] [HCHO] = Ki[HC0] [ 0 2 ] S u b s t i t u t i n g f o r K i [ H C 0 ] [ 0 2 ] from 3), the above e x p r e s s i o n becomes, $ ) l a + K 5[H] [HCHO] = K3 [ H 0 2 ] [HCHO] + K 8 [H0O] -96-Adding l ) t o t h i s e x p r e s s i o n , we o b t a i n 2$ l a = Z 8 H02 Hence, H02 = 2 $ I a / K s I n o r d e r t o o b t a i n an e x p r e s s i o n f o r [H] , we can s u b s t i t u t e the r e s u l t f o r £H02] i n t o 1 ) . Upon d o i n g t h i s , we o b t a i n the f o l l o w i n g e x p r e s s i o n , [H] = (ft Ia(K 8 + 2K3 [HCHO] ) K6K8 [HCHO] From 3) we get the f o l l o w i n g e x p r e s s i o n f p r (HCo3 , [HCO] = 2 j>Ia(K 8 + K3 [HCHO] ) K i K 8 [0 2 ] and from 2) we o b t a i n an e x p r e s s i o n f o r £OHJ . [OH] = 2 j la(Kg + K3 [HCHO] ) Kg K.^K8K7 [HCHO] ¥e are now a b l e t o d e r i v e the e x p r e s s i o n s f o r the r e a c t i o n p r o d u c t s ; d [HCOOHjyut = K4JHC03H]= K3 {H0^(HCH01 d ^HCOOHj/dt = 2 ^ ' l a K^fHCHO] d { H 2 ] / d t = K6[H]JHCHO] d [H 2]yut = ^ I a ( K 8 + 2K 3[HCH0 ] ) K 8 S i m i l a r i l y , d [ c o j ^ d t = 2 $ I a ( K 8 + K3O1CHO] ) K 8 d ( 0 0 2 ] / d t = 2 K 2 $ I a ( K 8 + K3 [HCHo] ) KlK 8 -97-These e x p r e s s i o n s agree w e l l w i t h t h e e x p e r i m e n t a l r e s u l t s . S i n c e the r a t i o o f C0:CC>2 ^ s f 0 ^ 1 1 1 ^ D e about 30:1, t h e n K^iKi must a l s o be about 30:1. E x p r e s s i o n s f o r the quantum y i e l d s o f the major p r o d u c t s can be deduced by d i v i d i n g by l a , Hence, we can d e r i v e t h e f o l l o w i n g e x p r e s s i o n s f o r the quantum y i e l d s . ^ HCOOH = 2<§Kj [HCHO] ^ H 2 = (£'(K8 + 2Kj (HCHO] ) _ CO = 2 J fo + K 5 [HCHO] ) These e x p r e s s i o n s can be reduced t o the f o l l o w i n g forms i f we l e t 2^K^ [HCHo] = A K8 Hence, ^ HCOOH = A <£H 2 = ^ + A ^>C0 = 2 < § ' + A .*. ^ HCOOH = $H2 - $ = ^CO - 2 $ From F i g u r e 23, :.- where the quantum-yields o f thes e p r o d u c t s are shown, we see t h a t the above r e l a t i o n i s i n f a i r agreement w i t h the e x p e r i m e n t a l r e s u l t s . -98-When the r a t i o of oxygen to formaldehyde was increased, i t was found that the amounts of products formed was reduced. This would he expected from the proposed mechanism, since with increasing oxygen con-centration, K5 w i l l begin to compete with Kg. When we include this reaction, we f i n d that the rate equation? governing the formation of products become d LHC00B"]/dt = 2<£la K 3 [HCHO] d[H 2]/dt = Kg^Ia (K 8 + 2K5 [HCHOJ ) fciCHo] K Q (Kg [HGHOj * K 5 I O 2 K M ] ) d ( c o]/dt = 2 $ I a (K 8 + K 3 [ECHO] ) -<j)Ia K5 [M] [O 2] (K 8 + 2K3 [HCHO] ) KQ Ks (Kg [HCHOj . + K 5 to23 £M] ). d[C0 2l/dt = ( d [ C O ] / d t ) and we see that the products H 2, CO and C 0 2 w i l l become less when the oxygen concentration i s increased which i s i n agreement with the experimental results. 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