Behavior of bimetallic Pd–Cr/Al2O3 and Pd–Cr/(Ce,Zr)Ox/Al2O3 catalysts for CO and NO elimination

A series of Pd–Cr bimetallic catalysts supported on a (Ce,Zr)Ox/Al2O3 mixed support or Al2O3 alone have been characterized using a combination of X-ray diffraction, electron paramagnetic resonance, and Raman spectroscopy and employing in situ diffuse reflectance infra-red Fourier transform and X-ray near-edge structure spectroscopy to analyze the redox and chemical processes taking place during the course of the CO+NO+O2 reaction. The catalytic behavior of these bimetallic systems was strongly affected by the nature of the support. In the case of the alumina support, evidence for interactions between Pd and Cr was observed in the calcined state. Under reaction conditions, formation of a mixed oxide phase containing Pd(I) and Cr(III) appears to be responsible for the improvement in CO oxidation and for the detrimental effect in the NO reduction process with respect to a monometallic, Pd reference system. In the case of the (Ce,Zr)Ox/Al2O3 mixed support, the addition of Cr was less influential in terms of catalytic activity. Results for the latter are rationalized mainly on the basis of the absence of interaction between the metal components in the calcined state and the presence of surface entities containing Cr alone when metallic-like Pd particles are formed during the course of reaction.