Effects of oxidation-reduction and oxychlorination-reduction cycles on Pt-Ge/Al2O3 catalysts

Two Pt(0.3 %)-Ge/Al2O3 catalysts containing 0.15 and 0.45 % Ge have been ch aracterized by CO chemisorption and FTIR spectra of adsorbed CO after serie s of oxidation/reduction and oxychlorination/reduction cycles and have also been used for the catalytic hydroreforming of heptane. Catalysts after oxy chlorination contained chloro- and oxychloro-Pt complexes which were respon sible for the efficient spreading of Pt over the support surface and hence for the maintenance of a good Pt dispersion after reduction. XRD study of a more highly loaded catalyst gave no evidence for Pt-Ge alloy formation or for Ge-0 species. Reduced Cl-containing catalysts contained small clustered arrays of Pt atoms dispersed possibly as mats rather than particles over t he GeOx-modified alumina such that all the exposed Pt atoms were influenced by an electron withdrawing effect of both Ge2+ ions and Cl ions. Germanium did not block low-coordination Pt sites but may have partially decorated a rrays of high coordination sites. Germanium partially stabilized Pt/Al2O3 a gainst loss of activity through coking. The dominant mutual effect of Ge an d Cl in catalysts was a significant loss in hydrogenolysis selectivity, but gains in the selectivities of aromatization, isomerization, and cyclizatio n reactions. The addition of Ge to Pt/Al2O3 did not compromise the availabi lity of Pt adsorption sites and only reduced by a small amount the turnover frequencies for heptane reforming reactions. Good activities were therefor e maintained in the absence of GeO or alloy formation during catalyst reduc tion, (C) 2000 Academic Press.