Effects of Copper on the Catalytic Properties of Bimetallic Pd–Cu/(Ce,Zr)Ox/Al2O3 and Pd–Cu/(Ce,Zr)Ox Catalysts for CO and NO Elimination

Abstract

A series of Pd–Cu bimetallic catalysts supported on Ce–Zr mixed oxide or on (Ce,Zr)Ox/Al2O3 mixed supports (with loadings of 10 and 33 wt% Ce–Zr mixed oxide) have been examined and compared to reference monometallic catalysts to determine the influence of copper on the catalytic activity for CO oxidation and NO reduction using a mixture of either O2 or O2–NO as oxidant under stoichiometric conditions. The samples were characterized by X-ray diffraction, transmission electron microscopy–X-ray energy-dispersive spectrometry, and electron paramagnetic resonance, employing in situ diffuse reflectance Fourier transform spectroscopy and X-ray absorption near-edge spectroscopy techniques to characterize physicochemical processes taking place during the course of the reactions. All the catalysts show Ce–Zr mixed oxide nanoparticles with pseudocubic phases and a Ce/Zr atomic ratio close to 1. The main differences between the samples are attributed to changes in the distributions of the two metallic components over the supports. The beneficial effects of copper on the catalytic activity of alumina-containing samples are related to the formation of an active Pd–Cu alloy in contact with the Ce–Zr mixed oxide component. A preferential interaction between copper and alumina in these samples is proposed to optimize the properties of the alloy phase by decreasing the copper concentration in the latter. However, the destruction of the alloy under reaction conditions at high temperatures, required for NO activation, removes the copper-promoting effects for NO reduction. The catalytic behavior of the Pd–Cu/(Ce,Zr)Ox sample is governed by the Cu–(Ce,Zr)Ox character of its active sites, which induces both beneficial and detrimental effects, on its catalytic properties, depending on the type of reaction.