(481b) Controlling Formic Acid Decomposition through Alloy and Ensemble Effects on PdCu Catalysts

Formic acid (FA) is a promising hydrogen carrier, because it can be formed through CO₂ hydrogenation and subsequently release hydrogen under moderate conditions. To effectively use FA as hydrogen carrier, however, a catalyst with high selectivity to CO₂ and H₂ is needed. The periodic table contains just over 10 transition metals that have practical uses as supported metal catalysts. With very few exceptions, these pure metals rarely provide active sites with both high activity and selectivity towards the desired product, but designer alloys can often circumvent these limitations.

For FA decomposition, Pd is known for its activity but lacks selectivity and is easily poisoned by the by-product CO. Cu, on the other hand, exhibits good selectivity but with much lower turnover frequencies (TOFs). Using density functional theory and microkinetic modeling, we show that the selectivity of Cu can be combined with the activity of Pd over dilute PdCu alloys. An upshift of Cu’s d-band center upon alloying greatly increases the reactivity of Cu (Fig. 1), without compromising its selectivity.^[1] Isolated and unselective Pd sites at low Pd/Cu ratio can be fully poisoned by trace amount of CO, without negatively affecting surrounding Cu sites. Increasing the Pd content, however, causes the formation of larger Pd ensembles, which cannot be passivated by CO and provide active sites for unselective reaction pathways. Our results and their interpretation are consistent with TPD, TEM, and EXAFS characterization, and reaction studies.

Our detailed structure-function relationship for FA decomposition over PdCu alloy catalysts suggest that by carefully balancing alloy (electronic) and ensemble (geometric) effects it may be possible to rationally design metal alloy catalysts with exceptional activity and selectivity.

[1] Bathena, Phung, Svadlenak, Lu, Grabow, Goulas, Oxygenate Reactions over PdCu and PdAg Catalysts: Distinguishing Electronic and Geometric Effects on Reactivity and Selectivity. ChemRxiv (2021), DOI: 10.26434/chemrxiv.12950336.v1.