Formic acid decomposition over supported Pd alloy catalysts: role of oxygen in hydrogen production

Aqueous formic acid (FA) catalytic dehydrogenation to hydrogen (H<inf>2</inf>) is a potential green H<inf>2</inf> source under ambient conditions (in air). We show that commonly studied Pd nanoparticles supported on CeO<inf>2</inf> provide low H<inf>2</inf> selectivity and TOF (10%; 20 h⁻¹) against water (H<inf>2</inf>O) formation from the aqueous FA reaction under ambient conditions. Further, this study explains how Ag alloying of Pd and interfacial sites affect the selective production of H<inf>2</inf> from FA with rates and apparent energy barrier measurements without promoters in the liquid phase under ambient conditions. The presence of a high surface coverage of oxygen on Pd-CeO<inf>2</inf> decreases the H<inf>2</inf> selectivity and promotes H<inf>2</inf>O formation from the aqueous FA reaction at a temperature as low as 298 K. However, 0.5 PdAg-CeO<inf>2</inf> (the atomic ratio of Pd and Ag is 0.5) catalysts provide a higher H<inf>2</inf> selectivity (22%) and TOF (178 h⁻¹) when compared to Pd-CeO<inf>2</inf> (10%; 20 h⁻¹) at 298 K. Despite the changes in H<inf>2</inf> TOF and selectivity, Pd-CeO<inf>2</inf> and 0.5 PdAg-CeO<inf>2</inf> present the C-H bond activation of formate as a kinetically relevant step for H<inf>2</inf> production from FA over a formate covered surface. Even in the presence of an oxidative environment, Ag increases the surface coverage of reduced Pd on 0.5 PdAg-CeO<inf>2</inf> compared to Pd-CeO<inf>2</inf>, and these electronic modifications of PdAg compared to Pd decrease the apparent activation barrier over 0.5 PdAg-CeO<inf>2</inf> (8 ± 4 kJ mol⁻¹) compared to Pd-CeO<inf>2</inf> (25 ± 3 kJ mol⁻¹) and increase the H<inf>2</inf> TOF/selectivity from the FA reaction. Similar-sized PdAg nanoparticles on CeO<inf>2</inf> and TiO<inf>2</inf> provide comparable rates of H<inf>2</inf> production from the FA reaction independent of the identity of the support due to negligible differences in the support basicity. The mechanistic insights of H<inf>2</inf> production from FA in the presence of surface oxygen on PdAg catalysts and the derived rate law will aid in the rational design of future catalysts for aerobic H<inf>2</inf> production.