The Origin of Sulfur Tolerance in Supported Platinum Catalysts: The Relationship between Structural and Catalytic Properties in Acidic and Alkaline Pt/LTL.

Abstract

The reactivity, structure, and sulfur tolerance is compared for platinum supported on acidic and alkaline LTL zeolite. In the absence of sulfur, EXAFS spectroscopy indicates that small metallic platinum particles of approximately 6 to 14 atoms/cluster are present. The TOF for neopentane hydrogenolysis and isomerization is ca 100 times higher on the acidic LTL due to the metal-support interaction. Saturation of the platinum by H{2}S, results in the formation of surface Pt-S bonds with a bond length of 2.33 A. Comparison of the EXAFS results of the sulfur poisoned catalysts, indicates that the S to Pt ratio is lower for platinum on the acidic zeolite. Catalytically, the initial activity (per gram) of both catalysts is greatly reduced after sulfur poisoning due to the loss of exposed platinum, however, the initial neopentane TOFs are nearly unchanged. Because of its higher TOF, the catalytic activity (per gram) of sulfur poisoned, acidic Pt/LTL is comparable to that of nonsulfur poisoned alkaline Pt/LTL. In both sulfur poisoned catalysts the neopentane isomerization selectivity increases compared to the sulfur free catalyst. Although the initial TOFs of the nonsulfur poisoned and sulfur poisoned catalysts are the same, there is a rapid loss in activity due to coke formation in the sulfur poisoned, alkaline LTL, while rate of deactivation by coke in the sulfided, acidic LTL is much lower. The increased sulfur tolerance of acidic supported noble metal catalysts appears to result primarily from the higher intrinsic TOF. In addition, because of its resistance to coke deactivation, the acidic supported sulfur tolerant catalyst is able to maintain stable catalytic activity.