Reducible support effects in the gas phase hydrogenation of p-Chloronitrobenzene over gold

The use of nonreducible (Al2O3) and reducible (Ce0.62Zr0.38O2, CZ) carriers to support nanoscale Au has been studied in gas phase p-chloronitrobenzene hydrogenation. Reaction over Au/Al2O3 generated p-chloroaniline as the sole product, whereas Au/CZ catalyzed nitro-group reduction and dechlorination to aniline. A parallel/consecutive kinetic model has been applied to quantify selectivity for Au/CZ. Catalyst characterization has included temperature programmed reduction (TPR)/desorption (TPD), XPS, HAADF-STEM, CO adsorption-FTIR, and oxygen storage capacity (OSC) measurements. The incorporation of Au with CZ promoted reduction of the support with the generation of surface hydrogen and oxygen vacancies, where the latter was facilitated at higher reduction temperature (from 393 to 973 K). Strong Au-CZ interactions enhanced Au dispersion with a narrow size distribution (mean = 1.8-1.9 nm) and influenced adsorptive and catalytic properties. Sintering of Au (from 5.7 to 8.8 nm mean) on Al2O3 was observed with increasing reduction temperature (473-673 K). A higher H-2 content in the reacting gas elevated hydrogenation (action of supported Au), whereas dechlorination (action of oxygen vacancies) over Au/CZ was favored under H-2 lean conditions. The contribution of spillover hydrogen to increase selective hydrogenation rate is demonstrated. A temporal irreversible loss of activity is established and linked to Cl poisoning of oxygen vacancies.