Determination of Metal Dispersion and Surface Composition in Supported Cu–Pt Catalysts

Abstract

A combination of adsorption and surface reaction probes has been used to characterize SiO2-supported Cu–Pt catalysts. CO chemisorption at 300 K on the reduced catalyst measured only surface Pt atoms (Pts). Dissociative N2O adsorption at 363 K produced a monolayer of oxygen on the Pts atoms and converted the Cu surface atoms (Cus) to a surface “Cu2O” phase. The hydrogen titration reaction on this O-covered surface at 210 K occurred only with oxygen chemisorbed on Pts sites with no H spillover to the Cu2O surface, thus providing a second measure of Pts atoms. In contrast, this titration reaction at 300 K provided evidence for H interaction with this Cus–O–Cus phase via a spillover process. CO adsorption on this O-covered surface created by N2O dissociation resulted in CO adsorption on the Cu+1 sites and titration of the chemisorbed oxygen on the Pts atoms according to the two equations Cus+1+CO(g)→Cus+1−CO,Pts−O+2CO(g)→Pts−CO+CO2(g). Thus the total number of surface metal atoms (Cus+Pts) can be measured if there is no CO2 adsorption. The difference between this number and that for Pts atoms gives the number of Cus atoms. The oxygen uptakes via N2O dissociation as well as the net weight changes during the CO titration reaction predicted by these surface compositions were in agreement with values measured gravimetrically and with information deduced from X-ray diffraction measurements. DRIFT spectra of the reduced bimetallic catalyst showed only one band at 2061 cm−1 for CO on reduced Pts atoms, whereas CO adsorbed at 300 K after exposure of this catalyst to N2O again produced this band plus another near 2125 cm−1, with the latter band indicative of CO adsorbed on Cu+1 sites. No evidence for residual oxidized Pts atoms was observed, in agreement with the latter equation above.