Acetylene semi-hydrogenation over PdZn/CeO2: CO methanation as unequivocal descriptor of selectivation

The semi-hydrogenation of acetylene to ethylene can be achieved over bulk CeO21 and ZnO-supported PdZn2, the latter being active below 100°C. To our knowledge, PdZn/CeO2 has not yet been investigated for this reaction. The effect of Zn alloying with Pd and the selectivation of the sample with reduction temperature is presented here. PdZn samples free of Pd-Pd ensembles (showing no bridged CO) were prepared, since the most selective PdZn surfaces were proposed to be intermetallic that exhibited only Pd-Zn-Pd units2. Pd and Pd+Zn were impregnated over a custom-made ceria. Acetylene hydrogenation was carried out in a tubular stainless steel reactor using a feed composed of 1.2% C2H2 + 4.3% H2 + 32% C2H4 in He. A diffuse reflectance FT-IR (DRIFTS) cell was used to investigate CO hydrogenation to methane and determine CO adsorption mode and heat in operando conditions. All samples were highly active for acetylene hydrogenation (70°C < T50% < 85°C). Zn-free Pd/CeO2 led to negative selectivities, as acetylene and some of the ethylene fed into the reactor were hydrogenated to ethane (Fig. 1, left). PdZn/CeO2 reduced at 400 and 500°C exhibited only a moderate selectivity of ca. 50%, despite the fact that these samples did not exhibit any bridged CO (Fig. 1, top right). Bridged CO is thus not a satisfactory descriptor. PdZn/CeO2 reduced at 550 and 650°C led to near 100% selectivity up to full conversion, but were too dark to be investigated by IR. CO methanation (Fig. 1, bottom right) was shown to be a suitable selectivity descriptor, as only the highly selective PdZn/CeO2 reduced at 550 and 650°C led to no measurable methane formation.