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

Abstract

Aqueous formic acid (FA) catalytic dehydrogenation to hydrogen (H₂) is a potential green H₂ source under ambient conditions (in air). We show that commonly studied Pd nanoparticles supported on CeO₂ provide low H₂ selectivity and TOF (10%; 20 h⁻¹) against water (H₂O) 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 H₂ 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–CeO₂ decreases the H₂ selectivity and promotes H₂O formation from the aqueous FA reaction at a temperature as low as 298 K. However, 0.5 PdAg–CeO₂ (the atomic ratio of Pd and Ag is 0.5) catalysts provide a higher H₂ selectivity (22%) and TOF (178 h⁻¹) when compared to Pd–CeO₂ (10%; 20 h⁻¹) at 298 K. Despite the changes in H₂ TOF and selectivity, Pd–CeO₂ and 0.5 PdAg–CeO₂ present the C–H bond activation of formate as a kinetically relevant step for H₂ 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–CeO₂ compared to Pd–CeO₂, and these electronic modifications of PdAg compared to Pd decrease the apparent activation barrier over 0.5 PdAg–CeO₂ (8 ± 4 kJ mol⁻¹) compared to Pd–CeO₂ (25 ± 3 kJ mol⁻¹) and increase the H₂ TOF/selectivity from the FA reaction. Similar-sized PdAg nanoparticles on CeO₂ and TiO₂ provide comparable rates of H₂ production from the FA reaction independent of the identity of the support due to negligible differences in the support basicity. The mechanistic insights of H₂ 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 H₂ production.