Effect of In and Sn on the adsorption behavior and hydrogenolysis activity of Pt/Al2O3 catalysts

The influence of Sn and In on the properties of Pt/Al2O3 catalysts was investitgated. Integral heats of adsorption, Q(ad), for H-2 at 300 K were determined on a group of Pt, Pt-Sn, and Pt-In/Al2O3 catalysts. Although H-2 chemisorption was significantly suppressed in the case of the bimetallic catalysts, the irreversible Q(ad) values were typically decreased to a lesser extent and fell between 10.6 and 13.3 kcal/mol, which is slightly lower than the range of values previously measured for H-2 adsorption on Pt. The presence of these two promoters also decreased the amount of CO adsorbed at 300 K; however, the irreversible heats of CO adsorption were essentially not altered by the presence of either Sn or In as the Q(ad) values were 23 +/- 2 kcal/mol and consistent with values previously measured for Al2O3-supported Pt. DRIFTS measurements at 300 K implied that surface Pt atoms were more homogeneously diluted or covered by In than by Sn. Thermal desorption experiments in the DRIFTS cell showed that the IR vibration for CO species adsorbed on on-top sites was 2042 +/- 3 cm(-1) at low coverage for all the samples investigated. The turnover frequency (TOF) for n-butane hydrogenolysis on Pt/Al2O3 was decreased by the addition of either Sn or In, but the largest variation occurred with Pt-In/Al2O3, which gave TOF values at 573 K up se, 40 times lower than those for Pt/Al2O3 catalysts, The results can be explained by a geometric effect in which the number of contiguous Pt atoms is decreased by dilution with either Sn or In atoms, i.e., ensemble size decreases, and any electronic effects appear to play only a minor role, The variations in butane hydrogenolysis are consistent with DRIFTS results, in that the dilution of Pt surface atoms by In atoms seems no be more homogeneous thus causing a moral effective suppression of hydrogenolysis activity. (C) 1996 Academic Press, Inc.