Session: 05-04: Methane Emissions Control

Paper Number: 110107

110107 - Catalyst Deactivation Modes of Pd/γ-Al2o3 for Lean Methane Oxidation 

Complete oxidation of methane is an effective way to reduce its environmental impact as the global warming potential of methane is about 28-36 times greater than that of CO₂ over a 100-year period. Catalytic oxidation operates at lower temperatures than thermal processes would require, but still needs above 550 ℃ to maintain high methane conversion efficiency. At temperatures below 550 ℃, a steady decline of methane oxidation activity can be observed on most of the catalysts, including some of the commercially available Pd/γ-Al₂O₃ based catalysts. A considerable amount of research work has been carried out to understand the catalyst deactivation mechanisms. Formation of surface hydroxyl groups on the alumina support and the PdO-Al₂O₃ interfacial regions which inhibit methane oxidation is one of the leading theories [1]. This, however, could not well explain the irreversible nature of the catalyst deactivation. Another leading theory is that the formation of Pd-OH leads to Pd sintering [2]. The fact that the catalysts show more pronounced deactivation at lower temperatures makes this questionable.

In this study, we established a testing protocol that clearly differentiated four different types of deactivation modes of Pd/γ-Al₂O₃ catalysts: (1) an acute loss of performance with a wet feed as water competes for the same adsorption sites where methane oxidation occurs; (2) a rapid decline of catalytic activity due to the inhibition effect caused by the formation of surface hydroxyl groups; (3) a gradual deactivation process under dry methane oxidation conditions; and (4) a steady but more pronounced process under wet methane oxidation conditions. The loss of performance from the first two modes was recoverable when water was removed from the feed, but the performance loss under methane oxidation conditions, either dry or wet, was irreversible. Once a catalyst was deactivated, calcining the catalyst at temperatures up to 650 ℃ was not able to fully restore its original activity. Pd dispersion measurement on catalysts before and after methane oxidation indicated no change in the average size of PdO nanoparticles on the catalysts. Interestingly, when a deactivated Pd/γ-Al₂O₃ catalyst was exposed to various dynamic reaction conditions, even in the presence of water and net oxidization atmosphere, the catalyst could recover its activity with the extent of recovery depending on the treatment conditions.   

Time-resolved CO chemisorption DRIFTS experiment was conducted to probe the surface adsorption sites of the Pd/γ-Al₂O₃ catalysts before and after methane oxidation. The results revealed that the catalysts became less accessible and reducible after methane oxidation, suggesting a morphological change of the PdO surface layer during methane oxidation. This is likely to be the leading cause of the irreversible deactivation of the catalysts.

Presenting Author: Haiying Chen Oak Ridge National Laboratory

Presenting Author Biography: Dr. Hai-Ying Chen is a Distinguished R&D Staff Member at Oak Ridge National Laboratory. His research interest is in advancing catalysis science and technologies to enable internal combustion engines to meet near-zero pollutant emission standards, and in developing energy-efficient catalytic processes to accelerate the decarbonization of the transportation sector. Before he joined ORNL in April 2021, Dr. Chen was a Global Technology Fellow at JM with more than 21 years of industrial catalysis experience in exhaust emission control technologies. Dr. Chen has published more than 60 technical papers and holds more than 380 granted patents in various jurisdictions around the world. Dr. Chen is a Fellow of the Society of Automotive Engineering. He received numerous awards, including the 2019 Eugene J. Houdry Award in Applied Catalysis by the North American Catalysis Society and the 2009 American Chemical Society Award for Team Innovation.