(683a) Achieving High Selectivity to H2 Production from Formic Acid Decomposition on Pd-Ag Alloys: The Roles of Geometric and Electronic Structure.

Formic acid decomposition is of interest as a source of H₂ for fuel cell applications. One of the most active catalysts for this reaction is Pd, however, the formation of CO as a byproduct irreversibly binds to active sites precluding further turnover. Recent studies have shown that modification of Pd by Ag reduces CO poisoning and increases the selectivity of H₂ formation. However, an atomic-level understanding of structure-reactivity relationship for this reaction on Pd-Ag alloys has remained elusive. To identify the role of Ag in selectivity and also the atomic distribution of alloy surfaces, we examined the reaction of formic acid on Pd_xAg_1-x surface alloys on Pd(111) catalyst in UHV conditions. Using a combination of infrared reflection absorption spectroscopy (Fig. 1a), X-ray photoelectron spectroscopy (Fig.1b), and density functional theory (Fig. 1c), we established that the Ag atoms that are in contact with Pd in the alloy are electronically modified compared to bulk Ag. While bulk Ag is inert to formic acid, the electronically modified Ag domains are activated and generate an alternative reaction pathway, which yields only H₂ and CO₂. Presence of Ag d-states between the fermi level and -3 eV due to mixing of Ag and P d-states is key to make Ag reactive (Fig. 1c). Furthermore, using temperature-programmed reaction spectroscopy (Fig. 1d), the Pd_0.2Ag_0.8 surface alloy that contains only isolated Pd atoms exhibited reactivity similar to Pd(111) with much higher selectivity to H₂ formation. Taken together, these results indicate that Pd serves as an active site and as a promoter for neighboring Ag atoms that exhibit enhanced selectivity towards formic acid. To achieve high selectivity while maintaining high reactivity, atomic-scale control of Pd distribution is needed in Pd-Ag alloys.