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The chemisorption of water vapour on activated charcoal McGear, J. P. 1946

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THE OHEMISORPTION OP WATER VAPOUR ON ACTIVATED CHARCOAL BY J. P. Mo&eer Under the direction of Dr. M. J. Marshall A Thesis Submitted in Partial Fulfillment of the Requirements for the Degree of MASTER OP ARTS in the department of CHEMISTRY. THE UOTVERSITY OP BRITISH COLUMBIA April, 1946. A0M01LEDSMENT The author wishes to thank Dr. M. J. Marshall, without whose assistance the work would not have "been ac-complished* TABLE OF CX3WPENTS Page Introduction. 1 Apparatus 2 Experimental. Activation of the Charcoal 2 Water Vapour - P u r i f i c a t i o n and admission. 5 Methods of GAs Analysis 3 Hydrogen 4 Carbon Dioxide 4 Acetylene ............. 5 y Residue ............... 5 Equilibrium Pressure over the Charcoal 6 Results TCLTDI© I • • • • • • • • • • • • • • • • • • • * * • * • • * • • • • 6 Table I I 7 Discussion 7 Summary * . .. 9 Bibliography 10 - 11 Appendix 12 Diagram I 13 J THE CHEfflSORPTION OP WATER VAPOUR ON ACTIVATED CHARCOAL INTRODUCTION The faot that oxygen chemi-sorbs on charcoal is well known, and i recent work in this laboratory has shown that the oxides of carbon combine similarly with an activated surface. This investigation was undertaken in the hope that water vapour might preform similarly on charcoal, for i t would seem that i f the active carbon has such a high affinity for oxygen as is indicated, that the water vapour would he decomposed by the surface j: with the formation of hydrogen* Literature on the absorption of water vapour by charcoal^ is complex, and there i s disagreement regarding chemisorption in this case. J. ¥. McBain^ reported that a hydrogen spectrum was obtainable in the gases produced when water vapour was left in contact with pure activated charcoal over a period of eighteen months. Other workers^ have demonstrated that charcoal, aotivated in vacuoat 1100 C for twelve to twenty four hours, will fix steam at 300 C. On heating this complex will break down, and the quantity of hydrogen and the oxides of carbon coming off is equivalent to the amount of steam fixed* On the other hand chemisorption of water at room temperature i s not indicated by the heat of adsorption. Measurements show5»^ that the dif-ferential heat of adsorption closely approaches the heat of condensation of water. The role played by impurities in the charcoal in this chemisorption has been discussed. Coolidge states? that water absorbs only slightly on pure charcoal and that inorganic impurities greatly increase the affinity between the carbon and the vapour. Lawson^ claims that ash-free charcoal does not chemi-sorb water. On the other hand, Burrage^ believes that the ash plays no part in the retention of water at zero pressure. He assumes that the water is held in a quasi-chemical manner by the surface oxide, and by active centers. APPARATUS The main features of the apparatus have been described in the l i t -erature and will not be reproduced here. The charcoal system may be found in Findlay* s theses^} the apparatus for the purification and admission of the water vapour has been described by Keyes and Marshall5 and i s shown in diagram 1 and the set up for gas analysis by McMahon^  and others. The picnometer in which the water was measured out was separated from the charcoal by a mercury cut-off. This cut-off was placed as close to the charcoal as was feasible. During the course of the experiment one slight modification was made in the gas analysis apparatus. Prom one of the analysis tips a Line was lead immediately through a small stopcock, and from there to a mercury dif-fusion pump. This alteration permitted a given sample in the gauge to be analysed for more than two constituents, for the sample could be held In the main volume while the tips were replaced and outgassed. EXPERIMENTAL Activation of the Charcoal; The charcoal used for the investigation was the same sample as was used by McMahon^ . This was outgassed for sixty hours at 1000 C and was then cooled to room temperature with the pumps running. -3-It was then pumped intermittently over a period of three days, the total pumping time being about 15 hours* The gas was collected and ad-mitted to the analysis system, but its amount was insufficient to permit a determination of the constituents. WAter Vapour - Purification and Admission.: The water was first purified in the system described by Keyes and Marshall. It was freed of dissolved gases by vacuum distillation, small amounts of the gas phase being pumped off from time to time. The absence of non-oondensible vapours in the apparatus was detected by means of the inverted cup of glass, which could be raised magnetically and allowed to fall back into the liquid* After all foreign gas had been removed a small amount of water was condensed out in the picnometer, and the height of the meniscus was read at equilibrium. The admission to the system was carried out by loweriag the mercury cut-off between the picnometer and the charcoal for a few seconds. This was raised again, and when the picnometer had attained a steady state the meniscus was read once again. Since some of the water admitted will not reaah the charcoal but will disappear onto the walls, a more accurate system of measurement i f the amount admitted is unnecessary. Methods of Gas Analysis? The samples for analysis were drawn directly from above the charcoal, collected with the Toepher pump and admitted to the gauge. Unfortunately this method permitted no check on a given analysis. The inclusion of a storage bulb and the drawing of larger samples would be advisable in future work* The gases were determined in the following order; condsnsible in liquid air, hydrogen, carbon dioxide, acetylene, others* The reagents used for the gas analysis can be considered under the headings of the constituent for which they were employed* Hydrogen; This gas was removed by diffusion through a heated palladium tube* This tube was approximately 1 mm in diameter and l± centimeters long* A glass to metal seal was made to a short piece of soft glass tubing of approximately the same diameter. The seal was coated with Picein cement to prevent leakage* This assembly was then sealed in to the analysis ap-paratus with Picein cement* The palladium tip was heated by means of a small calibrated furnace* This furnace consisted of a few turns of nichrome wire about a hollow glass tube, and could be moved in so that the palladium tip was inside the tubing* The voltage across the furnace was controlled by a variac* It has been re-ported, however, that certain foreign gases cause irregularities* If present in sufficient amounts methone and carbon monoxide will interfere-^ , Experiments here have shown that acetylene is evidently decomposed by heated palladium, and i f sufficiently high temperatures are reached the oxides of carbon may be reduced to methane^. In this investigation, these errors have been minimized, as far as was possible, by removing a l l gases oondensible in liquid air fwsm mixture before admitting i t to the palladium tube* In addition, the mixture was heated in the tube over a short period only (l+ minutes). In cases where hydrogen is reported, the drop in pressure, which can be followed in the capillary of the gauge, was sudden, and this phenomena was taken as an indication that the gas being removed was hydrogen. Carbon Dioxide; This gas was determined by adsorption in a eutectic mixture of the -5-hydroxides of lithium, sodium and potassium16*. When properly outgassed this reagent was satisfactory* Acetylene; This constituent was unexpected and consequently time did not permit an intensive investigation to determine the reagent most suitable for its removal from the mixture in the apparatus used* The reagent employed to date is cuprous chloride^*-, and it was pre-pared in the following manner* Powdered cuprous chloride was moistened with two normal (2N) potassium hydroxide* The resultant mixture was placed in a glass tip, which was sealed into the analysis apparatus with Picein cement* The reagent was then thoroughly outgassed* The functioning of this reagent was uncertain. In certain cases i t absorbed all the acetylene present in the mixture, in others it failed to affect the gas mixture in any way. The only other reagent tried was mercuric cyanide-^. This was of no value, however, since it exhibited a vapour pressure greater than 10*3 millimetres* In the analysis, the only gases present which would condense in liquid air were carbon dioxide and acetylene* In cases where the reagents failed to adsorb the acetylene, it was determined as the difference between the amount condensed by the liquid air, and the quantity absorbed as carbon dioxide* The Residue; An attempt was made to discover the nature of this gas by oxidizing i t . The combustion was carried out by heating the gas mixture in a quartz tip containing a copper wire, which had previously been coated with an oxide layer by heating it in the air. During its oxidation the gas was also ex-posed to a tip immersed in liquid air, which removed the carbon dioxide as it was formed* The method is only semi-quantitative since when the tempera-ture is raised, both the quartz and the copper give off a certain amount of gas* A mixture of hope elite and silver oxide, recommended for the oxida-tion of carbon monoxide by Browning^ was also prepared* It was rejected however when lengthy outgassing failed to render its equilibrium pressure negligible. The Equilibrium Pressure ever the Charcoal: These measurements were carried out with a large McLeod gauge con-nected to the charcoal through a mercury cut-off. RESULTS. Table I shows the increments of water vapour, and the data regarding their general behaviour on the charcoal. Before each new charge was ad-mitted, the charcoal was pumped off until the amount of gas collected over five hours was negligible* TABLE I Run No*' Mieromols of water* Gas collected. 'Amount of gas' Time - Gram Admitted per gram "For analysis 'generated per* admission to of charcoal. *in Mieromols. •Mieromols per' removal of • •gram admitted' first sample •in Mieromols.* i t •in hours. 1 « 27 ' 1.95 ' 0.072 « 0,5 2 « 10 » 1.72 « 0.173 * 1.2 3 | * 1.21 « 0*303 * 0.25* A- ' 6.79 3 A3 ' 0*504- ' 36 5 J 6.79 ! ! + Run not completed* * First sample drawn small* •7-The analyses completed to date have been chiefly qualitative* Most of the thirty five made have been only partial either because of lack of proper reagents, or because of errors in technique* Table U gives some of the more complete analyses in their chronoligical order. TABLE II Run No. • Amt. Analysed* Percentages - *Condensible Cu. mm. ' H2 CO2 C2H2 'in Liquid Air 'Residue * Remarks 3 5 5 1 1 1.43 • 0 24.O 66.0 * 90.0 * 1 J 1 6.36 • 17.0 25.0 47.5 ' 73.5 1 « 1 t 6.84 ' 17.0 12.0 61.5 * 74.5 1 t 1 1 8.37 ' 31.0 3.5 26.5 * 30.0 1 1 1 t 6*84 ' 17.6 12.1 62.5 ' 74*5 1 t i 1 10.0 ' 10.5 t 8.5 39.0 ' 13th and • last sample • of the run 4th sample of run 5th sample of run 1st sample of run 2nd sample of run DISCUSSION The following phenomena were noticeable during the experiment. As is briefly indicated in Table II, hydrogen is only present in the earlier samples taken after each additional increment of water* The residue appearing in the analysis was partially oxidisable, but results on this are not quantitative enough to merit inclusion. The reaction producing the gases has a measurable velocity. The pressure over the charcoal increases rapidly at first but does not attain a maximum until twenty hours or more after the addition of the water vapour. The evidence seems conclusive that the water is entering into a chemical reaction. It would also seem that there must be more than one such reaction occuring since it is doubtful that any one constituent could produce acetylene, carbon dioxide, hydrogen and another gases (or other gases) from water vapour. The following hypotheses have been suggested to account for the behavior. 1. The reaction occurs between the carbon surface and the water vapour. This investigation was originally undertaken with the intention of verifying this hypothesis. The reaction would be expected to take place as follows: C (surface) + HgO (gas) = R*2 (gas) + C^ Oy (surface) Since carbon dioxide appears, and the residue is very possible carbon monoxide, the behaviour in this case would be remarkably similar to that found by Muller and Cobb4. 2. The reactions are being caused by metals in -the charcoal. If certain residual, metals were present, it is possible that during the stringent activation process they could become converted to carbides. On the admission of water these carbides would react to produce acetylene and other hydrocarbons. Among these latter would be methane, and the behaviour of the residue indicates that it might be this gas. These residual metals might also produce hydrogen, for during the activation process the oxide would be reduced to metals, and i f these were not converted to carbides, they could reduce the water vapour to hydrogen. In all likelyhood the true picture of the reaction is a combination of the two postulated here. The fact that hydrogen is absent from the latter analyses in a run is fairly easily explained. It is known that both acetylene and carbon -9-dioxide exhibit an affinity for a carbon surface which is not characteristic of hydrogen. Therefore as soon as the pressure above the charcoal is reduced to zero all the hydrogen will have been removed, but this will not be true of the acetylene or carbon dioxide, as they will continue to desorb from the charcoal for some time. A comparison between the quantity of water vapour admitted, and the amount of gas generated indicates that only a small amount of it is reacting chemically. This is probably due to two factors. 1. Some of the gas adsorbs on the walls of the apparatus. 2. The reaction is proceeding toward equilibrium. The latter hypothesis is suggested for two reasons. First, while the method of analysis does not indicate the presence of water, it has been detected several times during the compression of the gases in the Toeplar pump. Second, because table I indicates some corelation between the amount of gas generated, and the time during which the reaction was allowed to proceed be-fore a sample was withdrawn. SUMMARY 1. Water reacts with activated charcoal, which has been thoroughly outgassed to produce small quantities of permanent gas. This may explain the inability to maintain a complete vacuum in an apparatus containing charcoal, since small amounts of water continually desorbs from the walls in an evacuated apparatus. 2. The gas generated consists chiefly of acetylene, hydrogen and carbon dioxide. 5. Further experimental work is indicated • -10-BIBLIOGRAPRY 1. - McMahon, H.O., Thesis, University of British Columbia, 1937. 2. - Bibliography of Solid Adsorbents, Dietz, V.R., Research Project of U.S. Cane Sugar Refiners and the National Bureau of Standards 1944. 3. - McBain, Porter and Sessions; J.A.C.S.. 55, 2294-2304 (1933). 4. - Muller and Cobb^  J. Chem. Soc. (1940) 177-83. 5. - Keyes and Marshall; J.A.C.S. 49, 156-73, 1927 6. - Allmond and King Proc. Roy. Soc. (London) A130, 210-7, 1930. 7. - Coolidge, A.S. J.A.C.S. 42, 1393-1408, 1920. 8. - Lawson, G.K. Trans Faraday Soc. 32,473-8, 1936. 9. - Bur rage, J.L. J. Phys. Chem. 37,1095-1101, 1933. 10.- Pindlay, R.A. Thesis, University of British Columbia 1934. BIBLIOGRAFHY (continued) 11.- Lombard and Eichner;- Gompte Rendu, 195 322-4 1932 12.- Fleigner, A.G. Ind. and Eng. Chem. (Anal. Ed.) 10 544 1938 13.- Sabatier and Senderens r Gompte Rendu j 134, 691, 1902. 14.- Blacet, McDonald and Leighton, Ind. Eng. Chem. Anal. Ed., 5, 272, 1933. 15.- Blacet, Sellers and Blaedel, Ind. Eng. Chem. Anal. Ed., 12, 356, 1940. 16.- Constaboris, G.C. Thesis, University of British Columbia 1945. 17.- Browning, G. Thesis, University of British Columbia 1942. -12-APPENDIX Calibration of the McLeod Gauge Volume of large bulb and capillary 136.3 cubic centimeters Radius of capillary tube 0.08967 centimeters Volume of dead space in analysis apparatus - 50 cubic centimeters. Radius of Picnometer Tube used for the Admission of Water Vapour - 0.1367 centimeters Weight of Charcoal Used - 24.55 grams. -o-o-o-o-


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