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.