Separation of Geometric and Electronic Effects of the Support on the CO and H2 Chemisorption Properties of Supported Pt Particles: The Effect of Ionicity in Modified Alumina Supports

Abstract

The geometric and electronic properties of supported Pt particles have been altered by modifying the ionicity (acid base properties) of the Al2O3 support via the sol−gel method. The Si modifier resulted in the most acidic and Cs in the most basic Al2O3 support. Application of the new Delta XANES technique shows that, above 373K in vacuum, the Pt surface is covered with hydrogen chemisorbed in an atop site for Pt particles dispersed on an acidic Cl−Al2O3 and mostly in the n-fold sites on Pt particles dispersed on a basic Rb−Al2O3. Further, FTIR data shows a significant bridged CO coverage in the Rb−Al2O3 case but not in the Cl−Al2O3. At low temperatures, when the coverage of both CO and H should be nearly complete, the Delta XANES results show that the coverage of H on Pt/Cl−Al2O3 is about twice that of Pt/Rb−Al2O3, consistent with the FTIR data which shows a similar reduction of linear CO adsorption on Pt/Rb−Al2O3. This is attributed to the different dispersions of the particles. EXAFS analysis makes clear that this difference in dispersion is mostly due to different particle morphologies, almost flat (for Pt/Cl−Al2O3) versus (hemi)spherical (for Pt/Rb−Al2O3), although the sizes are also different. The observed changes in CO and H2 chemisorption properties at high temperature and in Pt particle morphology are due to a shift of the Pt valence band to higher binding energy with decreasing ionicity (increasing acidity) of the support, as indicated by the atomic XAFS results. These atomic XAFS results can be directly correlated, assuming the oxygen Madelung potential model, with the XPS shift of the O 1s BE of about 2 eV, showing a decrease of the net electron charge on the support oxygen atoms with decreasing ionicity of the support. The hydrogen Delta XANES results are combined with a three-site (atop, 2- or 3-fold, and ontop H, in order of decreasing bond strength) Langmuir adsorption model for hydrogen chemisorption. This combination accounts for the variation in hydrogen coverage with change in T, P, and support ionicity as described above. The consequences of these results for Pt-catalyzed CO oxidation and hydrogenolysis/hydrogenation reactions are discussed.