Methane Catalytic Combustion over Hierarchical Pd@CeO₂/Si‐Al₂O₃: Effect of the Presence of Water

Abstract

The influence of water vapor on methane catalytic combustion was studied over a Pd@CeO2/Si‐Al2O3 catalyst, carefully designed to maximize Pd‐CeO2 interaction and prevent metal sintering and compared to a conventional impregnated catalyst with identical chemical composition. Although the nanostructured Pd@CeO2/Si‐Al2O3 catalyst is thermally stable, the addition of water to the reaction feed leads to a transient deactivation at low temperatures, consistent with the well documented competitive adsorption. In addition to this, the hierarchically structured catalyst exhibits an additional severe deactivation after methane oxidation in the presence of water vapor at 600 °C that can be reversed only by heating the catalyst above 700 °C. The presence of water in the reaction feed deactivates the conventional impregnated catalyst less severely and the activity largely returns upon water removal. Catalytic FTIR and CO‐chemisorption data indicate that this severe deactivation process in the hierarchical catalyst is due to the formation of stable OH groups on the surface of the ceria nanoparticles. These hydroxyl groups are suggested to significantly inhibit the oxygen spillover from the CeO2 nanoparticles to Pd, preventing its efficient re‐oxidation, as observed by operando X‐ray absorption near edge spectroscopy (XANES) experiments. At the same time, their presence can contribute to limit the gas phase accessibility of Pd, as indicated by the decrease of CO chemisorption capability. The presence of hydroxyls plays a minor role on the deactivation of the conventional catalyst at 600 °C.

A comparison of hierarchical Pt@CeO2/Si–Al2O3 and Pd@CeO2/Si–Al2O3

“…3. As reported previously [8,9], Pd@CeO 2 /Si-Al 2 O 3 catalysts calcined at 773 K deactivate rapidly in the WGS environment due to reduction of the ceria, which in turn causes a loss in the CO adsorption uptakes on Pd, probably due to the ceria shell closing up around the Pd [23]. This is shown in Fig.…”

“…Reduction of the Pd@CeO 2 /Si-Al 2 O 3 occurred gradually with decreasing P(O 2 ). Since spectroscopic data showed that the Pd in these catalysts is easily oxidized to PdO [23], the results in Fig. 4B imply that the PdO and CeO 2 reduction occur together.…”

Active and Stable Methane Oxidation Nano-Catalyst with Highly-Ionized Palladium Species Prepared by Solution Combustion Synthesis

“…Methane conversion over 5P5C-SCS increased with increasing operating temperature with full conversion attained at around 410 • C. It was worth noticing that under similar reaction conditions, complete combustion of methane over conventional 5P-I and 5P5C-I catalysts was never achieved. Secondly, contrary to the conventional 5P-I and 5P5C-I catalysts, the 5P5C-SCS nanocatalyst exhibited superior activity with no sign of deactivation in the temperature range betweeñ 400 and 800 • C. This behavior of the SCS catalysts resembled the core shell catalysts that have made a breakthrough in methane oxidation research [25,34]. On the other hand, the SCS catalysts were superior to the traditional catalysts which all showed significant deactivation at high temperature [15,19,20].…”

“…The adsorption of O 2 on oxygen vacancies, presumably followed by its dissociation is expected to play a major role in the oxidation reaction. It is generally accepted that the prevalent mechanism of the high temperature CH 4 catalytic oxidation involves a redox Mars-van Krevelen-type reaction [20,34,66]. In the present catalysts, the point of interaction between Pd and CeO 2 and/or the intrinsic oxygen vacancies presumably acted as centers for dissociating molecular oxygen into chemisorbed oxygen (O*) which were then consumed in the oxidation reaction.…”

“…Moreover, the transient deactivation was more evident during the cooling cycle. This was because with decrease in reaction temperature, the re-oxidation of Pd 0 to the more active PdO was reported to be kinetically less favorable [34].…”