Strategies for overcoming water inhibition of methane oxidation over palladium-based catalysts
Patrick Lott, Karlsruhe Institute of Technology
Patrick Lotta, Kyle A. Karinshakb, Maria Casapua, Michael P. Haroldb, Jan-Dierk Grunwaldta, Olaf Deutschmanna
a Institute for Chemical Technology and Polymer Chemistry, Karlsruhe Institute of Technology (KIT), Germany
b Department of Chemical and Biomolecular Engineering, University of Houston (UH), USA
Compared to conventional diesel and gasoline engines, their high fuel efficiency and advantageous carbon dioxide emissions make lean-burn natural gas engines (NGE) attractive for meeting tightening environmental legislation. However, incomplete combustion in NGEs results in emission of small amounts of unburned methane, which poses a more than 20 times stronger greenhouse gas than carbon dioxide. Therefore, a highly active catalytic converter that converts methane under typically low exhaust gas temperatures is imperative for an efficient exhaust gas after-treatment. In this regard, palladium-based catalysts show the highest methane conversion activity . Further improvement of their low-temperature performance has been in the focus of scientific interest for decades, particularly since the inevitable exhaust gas component water strongly inhibits methane oxidation over palladium. Although Pt-doping substantially improves both catalytic activity and catalyst stability , the development of novel catalyst formulations exhibiting long-lasting and high catalytic activity proceeds only slowly.
Rationally designed process control is an alternative approach that can ensure high methane oxidation activity. Fundamental insights in monometallic palladium-based catalysts via in situ X-ray absorption spectroscopy revealed PdO as the active species present under typical lean conditions . However, prereduction prior to a light-off test was found to increase the catalytic activity, pointing to the great potential of utilizing the complex Pd-PdO equilibrium and surface roughening phenomena on the one hand and removal of surface adsorbates on the other hand for enhanced activity. Short reductive pulses during lean catalyst operation do not only reactivate the catalyst on a regular basis, but also serve as an in situ activation procedure, either because the noble metal particles contain not only PdO, but also small amounts of metallic palladium that is less prone to water poisoning or due to an increased activity via the Mars-van-Krevelen mechanism . Combining the application of such a procedure with an optimized catalyst formulation, i.e. using ceria-based support materials that reduce the effect of water and benefit (re-)activation due to their high oxygen mobility, results in synergies that further increase the catalytic activity. Our findings represent first steps towards a more efficient exhaust gas after-treatment system for lean-burn gas engines, in which optimized engine operation and catalyst operation parameters allow permanently high methane conversion in the low temperature regime.
 P. Gelin, M. Primet, Complete oxidation of methane at low temperature over noble metal based catalysts: a review, Appl. Catal. B 39 (2002) 1-37.
 G. Lapisardi, L. Urfels, P. Gelin, M. Primet, A. Kaddouri, E. Garbowski, S. Toppi, E. Tena, Superior catalytic behavior of Pt-doped Pd catalysts in the complete oxidation of methane at low temperature, Catal. Today 117 (2006) 564-568.
 P. Lott, P. Dolcet, M. Casapu, J.-D. Grunwaldt, O. Deutschmann, The Effect of Prereduction on the Performance of Pd/Al2O3 and Pd/CeO2 Catalysts during Methane Oxidation, Ind. Eng. Chem. Res. 58 (2019) 12561-12570.
 K. A. Karinshak, P. Lott, M. P. Harold, O. Deutschmann, In situ activation of bimetallic Pd-Pt methane oxidation catalysts, ChemCatChem (2020). DOI: https://doi.org/10.1002/cctc.202000603