Turnover rate enhancement of reforming reactions on polycrystalline Pt-Ir foils

The reactions of n-hexane over bimetallic Pt-It polycrystalline foils were studied at 720 K, H-2/n-hexane = 67, and a total pressure of 0.9 MPa. The addition of Ir to Pt increases the hydrogenolysis turnover rate by up to two orders of magnitude and decreases the dehydrogenation turnover rate to l-hexene by up to one order of magnitude. The rates of isomerization to 2- and 3-methylpentane and cyclization to methylcyclopentane show a volcano-type correlation when plotted against the surface Ir composition. The maximum rate occurs at a surface composition of Pt2Ir and the rate is about three times higher than the rate for;pure Pt. Addition of sulfur to the surface decreases the rates for isomerization, cyclization, and especially the rate for hydrogenolysis, resulting in a substantial decrease in the initial selectivity for hydrogenolysis. However, sulfur addition increases the dehydrogenation rates for the monometallic and bimetallic surfaces. It is observed that Pt-Ir shows a surface chemistry for the reactions of n-hexane similar to the surface chemistry of Pt when the bimetallic surface is sulfided. The turnover rates, however, are higher for Pt2Ir than for Pt in the presence and absence of sulfur. (C) 1996 Academic Press, Inc.