(21c) Fundamental Mechanistic Studies of Formic Acid Decomposition on Pd Catalysts

Obtained as a byproduct in biomass reforming, formic acid has been widely considered as a promising material for chemical storage of hydrogen for on-demand production in fuel cells^1,2. Formic acid decomposition can occur through the dehydrogenation reaction to form CO₂ and H₂, or through the dehydration reaction to produce CO and H₂O, the two routes coupled together through the water gas shift reaction (WGSR). Successful deployment of vapor phase technologies depend strongly on identification of catalysts that can preferentially dehydrogenate formic acid at mild temperatures.

Despite numerous experimental and theoretical studies, the mechanism of formic acid decomposition remains largely unresolved on Pd, one of the most active and selective monometallic catalyst for this reaction. While formate (HCOO) has strong spectroscopic presence on the surface of the catalyst³, selectivity toward CO implies the participation of spectroscopically elusive carboxyl (COOH) species⁴. In this study, we utilize density functional theory (DFT) calculations, reaction kinetics experiments, and mean field microkinetic modeling to identify the nature of the active site. Microkinetic models formulated using the energetics derived on clean extended surface Pd(111)⁵ and Pd(100) though capable of capturing the experiment, yielded solutions with coverage predictions inconsistent with the DFT model assumptions. Using insights from operando FTIR spectroscopy, we revised our DFT models and reinvestigated the reaction network under realistic surface conditions, followed by microkinetic modeling to iteratively identify a coverage self-consistent description of the active site.

The results yield mechanistic insights into reactive intermediate(s), rate-controlling steps and the surface environment under reaction conditions. These models can serve as the basis for design of catalytic materials with improved activity and selectivity for on-demand hydrogen production.

References

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3. Solymosi, F., Koós, Á., Liliom, N. & Ugrai, I., J. Catal. 279, 213–219 (2011).
4. Gokhale, A. A., Dumesic, J. A. & Mavrikakis, M., J. Am. Chem. Soc. 130, 1402–1414 (2008).
5. Scaranto, J. & Mavrikakis, M., Surf. Sci. 650, 111–120 (2016).