Gold, still a surprising catalyst: Selective hydrogenation of acetylene to ethylene over Au nanoparticles

The hydrogenation of C2H2 in the presence and the absence of C2H4 has been studied over un-promoted and promoted Au/Al2O3. A number of parameters have been varied: the Au particle size, the pre-treatment conditions (hydrogen versus oxygen) and the nature of the promoters. Promoters include ceria, lithium and barium oxides. Our results show that hydrogenation of C2H2 proceeds with 100% selectivity towards C2H4, both in the presence and the absence of C2H4. Moreover, there is a strong dependence of the catalytic performance on the size of the Au particles: Au particles below 3 nm enhance the C2H2 conversion both in the absence and the presence of C2H4, without decreasing the selectivity to C2H4. Furthermore, metallic Au and Ce4+ appear to be more effective than Au3+ and Ce3+. Our findings also indicate that Li2O has a beneficial effect on C2H2 conversion, while BaO has a slight detrimental effect, both having no influence on the selectivity to C2H4. The key to 100% selectivity to C2H4 resides in non-competitive adsorption of C2H2 and C2H4 on the Au surface when both hydrocarbons are present in the feed. The deactivation during C2H2 hydrogenation is a reversible process and is due to accumulation of C deposits on the catalyst surface, as result of C2H2 adsorption on different active Au sites. These deposits can be easily burned off by a thermal treatment in oxygen.