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Effect of H₂O on CH₄ oxidation over PdO/Al₂O₃ and CeOx/PdO/Al₂O₃ catalysts Alyani, Mina


Natural gas is a promising alternative fuel for transportation systems because of reduced CO, CO₂, SO₂, and NOx emissions into the environment, and its abundance and low cost compared with gasoline and diesel. A significant obstacle in the use of NG for vehicle fuels is that CH₄ is difficult to oxidize in the presence of CO₂ and H₂O and at the low exhaust gas temperature (500-550°C) of natural gas vehicles (NGV). Although Pd is the most active catalyst for CH₄ oxidation, the presence of H₂O suppresses the catalyst activity. The effect of H₂O on the activity of Pd/Al₂O₃ catalysts with Pd loadings of 0.3, 2.6 and 6.5Pd (wt.%) and corresponding dispersions of 57%, 48%, and 33%, was well described by a kinetic model that accounted for the effect of H₂O. Langmuir adsorption was assumed to determine the amount of H₂O adsorbed on active sites for the catalysts with different Pd dispersions under wet and dry reaction conditions. The estimated kinetic parameters of apparent activation energy, Ea of 60.6±11.5 kJ.mol-¹ and heat of H₂O adsorption, ∆H(H₂O) of -81.5±9.1 kJ.mol-¹ indicate that CH₄ oxidation is independent of Pd dispersion. Using different preparation methods and varying Ce:Pd ratios, it was found that sequential impregnation of the Al₂O₃ support by Ce and Pd, with Ce:Pd ratio of 5, yielded a catalyst that had the least inhibition by H₂O. H₂O adsorption is the dominant mechanism for activity loss, although some sintering of the support may also occur. In a Time-on-Stream (TOS) study with extra H₂O added to the feed gas, the chemical and physical properties of the catalysts showed only small changes before and after use. The less negative effect of H₂O at higher temperature and at lower H₂O concentration was also confirmed by the kinetic study. The kinetic model is consistent with a Langmuir mechanism in which H₂O adsorption suppresses C-H bond activation on the active sites. The kinetic analysis shows that the Ce added to the PdO/Al₂O₃ catalyst suppresses the amount of H₂O adsorbed onto the catalyst, thereby reducing the H₂O inhibition effect in the presence of Ce.

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