Methane Reforming with Carbon Dioxide. The Behavior of Pd/α-Al₂O₃ and Pd−CeO<i>_x</i>/α-Al₂O₃ Catalysts

Abstract

The catalytic reforming of methane with carbon dioxide on Pd (∼1%)/α-Al2O3 and Pd (∼1%)−CeOx/α-Al2O3 catalysts was studied at 627 °C using a stoichiometric feed mixture. Characterization of fresh and used samples was carried out by TEM and TG analyses. Preliminary experiments in the 600−730 °C temperature range demonstrated that, on Pd/α-Al2O3, the conversion of CO2 was larger than that of CH4 because of the reverse water−gas shift (RWGS) reaction, although the CO/H2 ratio was close to 1. This behavior is attributed to the predominant CH4 decomposition reaction that leads to carbon deposition and catalyst deactivation. The decline in activity was also due to palladium sintering, a process favored by the reducing atmosphere generated by the presence of CO and H2. Kinetic information obtained in experiments carried out under differential conditions was used to evaluate the importance of mass- and heat-transport limitations. It is shown that intraparticle mass transfer and interparticle heat resistances can be controlled by using small catalyst particles and diluting the catalyst bed with an inert solid, respectively. On the other hand, gas−solid temperature gradients are expected if the reforming reaction is carried out at >600 °C at low flow rates. The catalytic activity, selectivity, and stability of Pd/α-Al2O3 and Pd−CeOx/α-Al2O3 were tested in 24-h runs. It was found that a Pd/α-Al2O3 sample with an average particle size of 5 nm exhibits an initial activity comparable to that of Pt/ZrO2 catalysts and a high and constant selectivity to H2 (CO/H2 ≈ 1). However, the catalytic activity decreased by 50% after a reaction time of 24 h due, in part, to metal sintering but mainly to carbon formation. A presintered Pd/α-Al2O3 sample with an average particle size of 19 nm deactivates more rapidly because of a rapid accumulation of carbon deposits. The process of carbon deposition can be suppressed to a very large extent by Ce addition, although the CO/H2 ratio becomes larger (CO/H2 ≈ 1.3). In addition, the presence of Ce reduces the sintering process.