Efficient chemical synthesis requires the use of selective catalysts. High selectivity can be obtained by using noble-metals catalysts. For example, Ag partially oxidizes ethylene and nano-structured Au can act as selective catalyst for many reactions. However, the activity of these noble metals can be challenging as the dissociation of O2 and H2 are usually highly activated. To improve activity, while retaining selectivity, a new class of diluted alloys have been developed that consist of an active metal diluted in a noble-metal host. Furthermore, these alloys are nanoporous (np) bulk materials with a high surface area without needing an oxide support. This reduces the material complexity and allows for single-crystal surfaces as model catalysts. This approach leads to detailed information on elementary reaction steps and catalyst stability under reaction conditions. The first studied reaction is the selective oxidation of methanol to form esters using np Au(Ag) as catalyst, in which Ag dissociates O2. The second reaction is the anhydrous dehydrogenation of ethanol producing acetaldehyde and H2. To perform this reaction selectively, np Cu is used, doped with small amounts of Ni to active the C-H bond. AP XPS measurements combined with E TEM observations show that the surface composition of both these alloy catalysts strongly dependents on the reaction conditions. For example, pre-oxidation of np Au(Ag) enriches the surface with Ag. This Ag is oxidized and non-selective, but transforms under reaction conditions into a metallic surface alloy surface with high chemical selectivity. For the np Cu(Ni) alloy, oxidation leads to enrichment of Cu on the surface, possibly due to overgrowing of Ni oxide by Cu oxide. Currently, this approach is being extended to study selective hydrogenation. Strong evidence of H2 activation was found after depositing a submonolayer Pd on an inert Ag model catalyst, the Ag(111) surface. The Pd3d peaks shift significantly to higher binding energy (0.2–0.35 eV) in the presence of the H2 atmosphere. The magnitude of this shift shows roughly a linear dependence on the logarithm of the H2 pressure. Furthermore, this shift is almost completely reversed by evacuating the sample environment. These results indicate the formation of strong Pd-H bonds and possibly a H-induced structural change in the PdAg alloy. To gain structural information, the AP XPS measurements are being supplemented by STM measurements both under UHV and under AP.

Diluted alloys based on noble metals as selective catalysts for oxidation and (de)hydrogenation

254th National Meeting (2017)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.