SHORT-RANGE AND LONG-RANGE PROMOTION BY SODIUM ADDITIVE IN D2 + CH2=CH2 REACTION OVER SILICA-SUPPORTED PLATINUM CATALYSTS

The mechanism of D2 + CH2=CH2 reaction on Na/Pt/SiO2 was investigated to characterize the reaction environment of Pt surface modified by Na additive in relation to SMSI and general promoter effects. The structural and electronic properties of Na/Pt/SiO2 catalysts were studied by TEM, XPS, and hydrogen adsorption (H/M). The H/M for Pt/SiO2 coincides with that calculated from the average particle size in TEM, whereas for Na/Pt/SiO2 it was always smaller than that expected from TEM. The peak intensity of Pt 4f 7 2 decreased by small amounts of Na additives and kept constant in the higher loadings. From these results, we estimated the saturation coverage of Na on Pt surface to be 0.2. The binding energy of Pt 4f 7 2 changed as a function of Na loading in the range of ζ(= Na/(Na + Pt)) from 0.16 to 0.86, due to a direct interaction (ζ < 0.43) and an indirect interaction (ζ ≥ 0.43) through SiO2 surface. The deuterogenation of ethene was enhanced by Na addition. The profile of the deuterogenation activity of Na/Pt/SiO2 is explained by the location of Na and the direct/indirect electron transfer. π-Ethene and di-σ-ethene at the Pt surfaces were observed by FT-IR. The rate of ethane formation and the νC-H of di-σ-ethene were correlated with Pt 4f 7 2 binding energy. On the other hand, the rate of ethene-hydrogen exchange reaction and νC-H of π-ethene were independent of Na loading. The dual site model for the deuterogenation and hydrogen exchange in the D2 + CH2=CH2 system is proposed; the former reaction proceeds at the bare metal sites through di-σ-ethene and the latter occurs at the periphery sites around Na2O island on Pt particles through-π-ethene. © 1991.