Density functional theory studies of HCOOH decomposition on Pd(111)

Abstract

The investigation of formic acid (HCOOH) decomposition on transition metal surfaces is important to derive useful insights for vapor phase catalysis involving HCOOH and for the development of direct HCOOH fuel cells (DFAFC). Here we present the results obtained from periodic, self-consistent, density functional theory (DFT-GGA) calculations for the elementary steps involved in the gas-phase decomposition of HCOOH on Pd(111). Accordingly, we analyzed the minimum energy paths for HCOOH dehydrogenation to CO2 + H2 and dehydration to CO + H2O through the carboxyl (COOH) and formate (HCOO) intermediates. Our results suggest that HCOO formation is easier than COOH formation, but HCOO decomposition is more difficult than COOH decomposition, in particular in the presence of co-adsorbed O and OH species. Therefore, both paths may contribute to HCOOH decomposition. CO formation goes mainly through COOH decomposition.

Boron Nitride‐supported Sub‐nanometer Pd6 Clusters for Formic Acid Decomposition: A DFT Study

“…Indeed, platinum and palladium, the most active catalysts for formic acid decomposition and typical DFAFCs anode electrocatalysts, suffer considerably from CO poisoning. Hence, different experimental and computational studies, aimed at understanding the reaction mechanism and improving the catalyst selectivity of HCOOH decomposition, have been carried out . For example, Herron et al.…”

“…Metal‐catalyzed formic acid decomposition has been studied extensively (Scheme ) . To study the competition between HCOO and COOH–mediated paths, and also the different reactivity of top and perimetral sites of the cluster, the following procedure was used: 1) reaction intermediates and transition states (TS) associated with the first hydrogen abstraction in the HCOO and COOH–mediated paths were optimized on the top and on the lateral face of the cluster, following the convention illustrated in Figure ; 2) the final state (FS) of the first molecular event was re‐optimized varying the adsorption site of the lost hydrogen atom; 3) the most stable configuration found was then used as best initial state (BIS) for the abstraction of the second hydrogen atom.…”

“…The hydrogen ends up atop of a Pd atom, with a bond distance of 1.59 Å, whereas the COOH interacts with the same Pd via its carbon atom, with a Pd−C bond distance of 1.95 Å. This particular configuration, which is similar to what was previously found on a Pd(111) surface, allows for the interaction of the carbonylic oxygen with the Pd cluster; the Pd−O bond distance is 2.24 Å. All of these features contribute to an overall stabilization of the COOH+H co‐adsorbed state by 31.4 kJ mol −1 (see Figure ) with respect to the initial state (IS).…”

“…On the top face, the bidentate mode has an adsorption energy of −82.4 kJ mol −1 while the parallel mode has an adsorption energy of −46.3 kJ mol −1 . These two results can be compared with those of Scaranto et al., which were obtained on a Pd(111) surface. For the same bidentate and parallel configurations they obtained adsorption energies of −39.6 and −2.9 kJ mol −1 .…”

“…At this point, a coordination change should occur to break the C−H bond, as the C−H bond of HCOO in the bidentate adsorption mode is too far away from the cluster to be activated. As shown in Figure , this reaction step, which is common on metallic surfaces, requires 108.4 kJ mol −1 . The HCOO monodentate mode, characterized by close proximity of the remaining hydrogen to a Pd atom (Pd−‐H distance 1.89 Å), is 75.3 kJ mol −1 less stable than the bidentate mode.…”