Unlocking synergy in bimetallic catalysts by core-shell design.

Extending the toolbox from mono- to bimetallic catalysts is key in realizing efficient chemical processes1. Traditionally, the performance of bimetallic catalysts featuring one active and one selective metal is optimized by varying the metal composition1-3, often resulting in a compromise between the catalytic properties of the two metals4-6. Here we show that by designing the atomic distribution of bimetallic Au-Pd nanocatalysts, we obtain a synergistic catalytic performance in the industrially relevant selective hydrogenation of butadiene. Our single-crystalline Au-core Pd-shell nanorods were up to 50 times more active than their alloyed and monometallic counterparts, while retaining high selectivity. We find a shell-thickness-dependent catalytic activity, indicating that not only the nature of the surface but also several subsurface layers play a crucial role in the catalytic performance, and rationalize this finding using density functional theory calculations. Our results open up an alternative avenue for the structural design of bimetallic catalysts.

Insight into the Nature of the ZnO x Promoter during Methanol Synthesis.

Despite the great commercial relevance of zinc-promoted copper catalysts for methanol synthesis, the nature of the Cu-ZnO x synergy and the nature of the active Zn-based promoter species under industrially relevant conditions are still a topic of vivid debate. Detailed characterization of the chemical speciation of any promoter under high-pressure working conditions is challenging but specifically hampered by the large fraction of Zn spectator species bound to the oxidic catalyst support. We present the use of weakly interacting graphitic carbon supports as a tool to study the active speciation of the Zn promoter phase that is in close contact with the Cu nanoparticles using time-resolved X-ray absorption spectroscopy under working conditions. Without an oxidic support, much fewer Zn species need to be added for maximum catalyst activity. A 5-15 min exposure to 1 bar H2 at 543 K only slightly reduces the Zn(II), but exposure for several hours to 20 bar H2/CO and/or H2/CO/CO2 leads to an average Zn oxidation number of +(0.5-0.6), only slightly increasing to +0.8 in a 20 bar H2/CO2 feed. This means that most of the added Zn is in a zerovalent oxidation state during methanol synthesis conditions. The Zn average coordination number is 8, showing that this phase is not at the surface but surrounded by other metal atoms (whether Zn or Cu), and indicating that the Zn diffuses into the Cu nanoparticles under reaction conditions. The time scale of this process corresponds to that of the generally observed activation period for these catalysts. These results reveal the speciation of the relevant Zn promoter species under methanol synthesis conditions and, more generally, present the use of weakly interacting graphitic supports as an important strategy to avoid excessive spectator species, thereby allowing us to study the nature of relevant promoter species.

Supported Cu Nanoparticles as Selective and Stable Catalysts for the Gas Phase Hydrogenation of 1,3-Butadiene in Alkene-Rich Feeds.

Supported copper nanoparticles are a promising alternative to supported noble metal catalysts, in particular for the selective gas phase hydrogenation of polyunsaturated molecules. In this article, the catalytic performance of copper nanoparticles (3 and 7 nm) supported on either silica gel or graphitic carbon is discussed in the selective hydrogenation of 1,3-butadiene in the presence of a 100-fold excess of propene. We demonstrate that the routinely used temperature ramp-up method is not suitable in this case to reliably measure catalyst activity, and we present an alternative measurement method. The catalysts exhibited selectivity to butenes as high as 99% at nearly complete 1,3-butadiene conversion (95%). Kinetic analysis showed that the high selectivity can be explained by considering H2 activation as the rate-limiting step and the occurrence of a strong adsorption of 1,3-butadiene with respect to mono-olefins on the Cu surface. The 7 nm Cu nanoparticles on SiO2 were found to be a very stable catalyst, with almost full retention of its initial activity over 60 h of time on stream at 140 °C. This remarkable long-term stability and high selectivity toward alkenes indicate that Cu nanoparticles are a promising alternative to replace precious-metal-based catalysts in selective hydrogenation.