Palladium dispersion effects on wet methane oxidation kinetics

Abstract

The catalytic activity for dry and wet methane oxidation over a series of palladium-alumina catalysts with palladium loadings from 0.23 to 3.6 wt.-% Pd and systematically varied PdO dispersions from...

Hampered PdO Redox Dynamics by Water Suppresses Lean Methane Oxidation over Realistic Palladium Catalysts

“…We mention that for palladium-alumina systems, the Pd particle size should not exceed about 2 nm as to balance palladium utilization and water inhibition. [15] The different response to water is further visualized by the DRIFTS spectra for lean CH 4 oxidation in dry-wet-dry conditions shown in Figure 2 for the alumina (panel b and d) and zeolite (panel c and e) catalysts. On the alumina catalyst, spill-over of hydrogen-containing species form hydroxyl ad-species with various configurations presumably close to the PdO particles already at the start with dry conditions.…”

“…[3,4] Even when using the most active supported palladium-based catalysts known hitherto, [5][6][7] the desired practical methane conversion is often difficult to achieve because water inhibits the methane oxidation, effectively, at all temperatures from which methane starts to be converted to as high as 550°C. [8][9][10][11][12][13][14][15] The problem is obvious for gas combustion exhaust aftertreatment as water concentrations may reach well over 10 % by volume. [16] Over the last decades, the development of methane oxidation catalysts has faced a weak progress as compared to, e. g., catalysts for abatement of nitrogen oxides emissions.…”

“…t = 0-0.5 h in Figure 2a) primarily thanks to the presence of a larger number of active sites as compared to the low-dispersed counterparts. The water sensitivity of supported palladium catalysts depends on palladium particle size [15] but is also much dependent on the choice of support material. [16,32,33] As can be seen, when 10 % H 2 O is introduced (t = 0.5 h) to the lean CH 4 /O 2 mixture, the methane conversion drops for both γ-Al 2 O 3 and the hydrophobic ZSM-5 supported palladium, present as PdO particles under prevailing conditions.…”

“…We mention that hydroxyls on PdO cannot be resolved with certainty in this measurement, however, highly active sites, likely present at the rim of the PdO particles, are clearly inhibited. [14,15,43] The hydroxyls on the extended alumina surface may function as a reservoir that supply hydroxyl ad-species to otherwise active sites impeding the reaction for sustained times after water is removed from the feed. A deeper discussion of rim sites and routes for hydroxyl formation is presented in previous systematic studies.…”

“…A deeper discussion of rim sites and routes for hydroxyl formation is presented in previous systematic studies. [14,15] The zeolite catalyst responds in stark contrast to the alumina catalyst in that hydroxyl formation could hardly be detected neither in dry nor wet conditions. This is an important explanation for the higher water tolerance and rapid recovery from wet conditions of zeolite supported PdO catalysts.…”