Pt,In and Pd,In catalysts for the hydrogenation of nitrates and nitrites in water. FTIR characterization and reaction studies

Abstract Pt,In and Pd,In catalysts supported on alumina or silica are active for the reduction of nitrate to N2 in water, using H2 as reducing agent. The characterization of the catalysts using FTIR of adsorbed CO shows the formation of both linear and bridged CO–Pd and CO–Pt bonds. Besides, carbonate species are formed due to the presence of OH groups provided by the support. The presence of In produces a shift in CO–Pd and CO–Pt bands, suggesting the formation of intermetallic particles. After use in reaction, the amount of CO–Pd and CO–Pt species strongly decreases, the main signals being those associated with carbonate groups. The proportion between bridged and linear species also changes after reaction. These results indicate the agglomeration of metallic particles during reaction, which is in agreement with TEM results. Increasing the In loading from 0.25 to 0.5 wt.% results in a moderate decrease in the nitrate conversion for all of the catalysts, which could be associated with a decrease of isolated Pd sites which are responsible for the dissociation of hydrogen. The experimental evidence for this point is the decrease of bridged CO adsorbed observed via FTIR for the samples with higher In loading and for all the catalysts. TPR experiments show low-temperature reduction peaks due to the presence of oxychloride and hydroxychloride particles in monometallic Pd and Pt catalysts. When In is present, the formation of Pd–In and Pt–In compounds provoke the shift of the reduction peaks and the appearance of new reduction signals.