[14C] Radiotracer Studies of the Hydrogenation Reactions Over Supported Platinum Catalysts

The adsorption of acetylene, ethylene and carbon monoxide; the hydrogenation of acetylene and buta-1,3-diene; the effects of sulphur poisoning on the adsorption, and the hydrogenation reactions, have been studied at ambient temperature in a static system using a variety of supported platinum catalysts. The catalysts used were EUROPT-1 (6% Pt/SiO2), EUROPT-3 (0.3% Pt/Al2O3), 0.5% Pt/MoO3, 0.8% Pt/SiO2 and 0.8% Pt/Al2O3. Using the [14-C] radiotracer technique, the shape of acetylene adsorption isotherms over each catalyst showed two distinct adsorption regions, a steep primary region followed (except Pt/MoO3) by a linear secondary region. The Pt/MoO3 catalyst showed a nonlinear secondary region. The adsorption of ethylene also occurred in two distinct stages, but the primary region was less steep than was found with acetylene. The adsorptive capacity of the catalysts for ethylene was approximately one quarter that for acetylene. In contrast, the adsorption isotherms of carbon monoxide showed distinct dissimilarities between catalysts. On EUROPT-1, 0.8% Pt/SiO2 and 0.8% Pt/Al2O3 catalysts, the isotherms displayed different behaviour from that observed with the hydrocarbons in that they showed a prolonged secondary region with a positive gradient. The adsorption isotherm of CO on EUROPT-3 was of a similar shape to the isotherms observed with the hydrocarbons. The Pt/MoO3 catalyst exhibited the expected Langmuir-type CO adsorption isotherm. On steady state catalysts the amount of each adsorbate that can be adsorbed was substantially less than that on freshly reduced catalysts. This has been suggested as being due to the permanently retained hydrocarbon species located at the primary adsorption region. With each catalyst the experimental observations are consistent with the dissociative adsorption of the hydrocarbons on the primary region, whilst the species involved in the actual catalysis process are located on the secondary region and are associatively adsorbed. From the poisoning experiments, evidence has been obtained to show that a surface reconstruction process accompanies the initial adsorption. The results are interpreted in terms of a model involving the migration of metal atoms into the adsorbate ad-layer. The adsorption of acetylene in the presence of gas phase ethylene and vice versa indicates that the adsorption of the two hydrocarbons takes place on the same adsorption sites with acetylene being more strongly adsorbed than ethylene. However, during the hydrogenation of acetylene, hydrogen was found to create sites which were active for the non-competitive adsorption of ethylene. IR evidence has been obtained for the formation of an ethylidyne (-C-CH3) species upon the exposure of a freshly reduced EUROPT-1 to C2H2 or C2H4, while bands characteristic of hydrocarbon polymeric species were detected with C2H2/H2 deactivated catalysts. On each catalyst, the adsorbed carbon monoxide was found to exist in the linear form. From the experimental observations regarding the hydrogenation of acetylene in the presence of various pressures of [14-C] -ethylene and determination of the kinetics and product distribution, it has been established that the major route to ethane formation is via a route involving direct hydrogenation of acetylene, rather than via ethylene as an intermediate. The results have been interpreted in terms of the presence of separate surface sites for the hydrogenation of acetylene to ethylene, the hydrogenation of acetylene to ethane and for the hydrogenation of ethylene to ethane. From the kinetics and variations of the product compositions with the experimental variables for buta-1,3-diene hydrogenation, it has been proposed that, whilst 1:2 addition of hydrogen to the buta-1,3-diene was responsible for but-l-ene formation, 1:4 addition of hydrogen to buta-1,3-diene, rather than the isomerisation of but-l-ene, was responsible for the production of trans-but-2-ene and cis-but-2-ene