Study of Gold Species in Iron-Oxide-Supported Gold Catalysts Derived from Gold-Phosphine Complex Au(PPh3)(NO3) and As-Precipitated Wet Fe(OH)3*

Abstract

Iron-oxide-supported gold catalysts were prepared by supporting a Au phosphine complex Au(PPh3)(NO3) on as-precipitated wet iron hydroxide Fe(OH)3*, followed by temperature-programmed calcination. The Au/Fe(OH)3*catalysts calcined at the temperatures 573–773 K showed extremely high catalytic performance for CO oxidation at temperatures as low as 203–253 K. Interaction of the Au(PPh3)(NO3) gold precursor with the Fe(OH)3*upon supporting, transformation of the precursor during the heat treatments, and state of the gold in the catalysts were studied by FT-IR, XRD, TEM, XPS, and EXAFS. The gold precursor dissociated on the Fe(OH)3*surface to produce [Au(PPh3)]+species which partially decomposed at 473 K and was transformed to small gold metallic particles with coordination numbers of 7.4–8.0 for Au-Au bond at calcination temperatures ≥573 K. In contrast, decomposition of the gold complex over crystalline Fe2O3*resulted in large gold particles. The Au/Fe2O3*sample was inactive at 203–253 K and exhibited very low activity for CO oxidation at room temperature. The efficiency of the as-precipitated wet Fe(OH)3*as a support is explained in terms of a higher stability of [Au(PPh3)]+on the Fe(OH)3*as compared to the Fe2O3*due to more effective interaction of the Au species with OH groups and defects of the amorphous Fe(OH)3*surface. The results demonstrate the importance of support–metal precursor interactions, both upon supporting and during calcination, in the formation of highly active catalysts with small Au particles for low-temperature CO oxidation.