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Abstract:
Uncovering the role of reaction intermediates is crucial to developing an understanding of heterogeneous catalysis because catalytic reactions often involve complex networks of elementary steps. Unfortunately, the short lifetimes and low concentrations of intermediates present during reaction often makes observing and identifying them with surface sensitive spectroscopies experimentally challenging. In this paper we report a different approach to identifying intermediates based on isotopologue specific thermal reaction rates of formic acid decomposition on Pd(111) and Pd(332). At low surface temperatures (~ 400 K) CO2 formation is the major reaction pathway on both facets. The kinetic data shows this occurs via two temporally resolved reaction processes indicating there must be two parallel pathways strongly suggesting two intermediate species are involved. Isotopic substitution reveals large and isotopologue specific kinetic isotope effects which allow us identify the two intermediates as bidentate formate and carboxyl. We demonstrate that the bidentate formate intermediate decomposes slowly whereas the carboxyl intermediate decomposes rapidly. At high surface temperatures (643 K to 693 K) we observe the formation of CO on Pd(332). This observation is consistent with the density functional theory based proposal that the carboxyl intermediate plays a major role in the water-gas shift reaction (WGSR) where it bridges CO2 formation from CO and H2O.
