Dynamic restructuring drives catalytic activity on nanoporous gold–silver alloy catalysts

Abstract

Bimetallic, nanostructured materials hold promise for improving catalyst activity and selectivity, yet little is known about the dynamic compositional and structural changes that these systems undergo during pretreatment that leads to efficient catalyst function. Here we use ozone-activated silver-gold alloys in the form of nanoporous gold as a case study to demonstrate the dynamic behaviour of bimetallic systems during activation to produce a functioning catalyst. We show that it is these dynamic changes that give rise to the observed catalytic activity. Advanced in situ electron microscopy and X-ray photoelectron spectroscopy are used to demonstrate that major restructuring and compositional changes occur along the path to catalytic function for selective alcohol oxidation. Transient kinetic measurements correlate the restructuring to three types of oxygen on the surface. The direct influence of changes in surface silver concentration and restructuring at the nanoscale on oxidation activity is demonstrated. Our results demonstrate that characterization of these dynamic changes is necessary to unlock the full potential of bimetallic catalytic materials.

Dynamic Processes on Gold-Based Catalysts Followed by Environmental Microscopies

“…This distinct oxygen is inactive for CO oxidation, but active for selective methanol oxidation into formaldehyde. Ambient-pressure XPS and the temporal analysis of products (TAP) method corroborate these observations [217]. It can be concluded that the surface undergoes structural and compositional changes during activation by ozone and that the Ag segregation plays an important role in the formation of oxygen species for selective oxidation of methanol.…”

“…This distinct oxygen is inactive for CO oxidation, but active for selective methanol oxidation into formaldehyde. Ambient-pressure XPS and the temporal analysis of products (TAP) method corroborate these observations [217].…”

“…The conclusions of this work are: (1) an increase in Au content decreases the sticking probability of oxygen, (2) there is no oxygen detected on pure Au at 300 K, and (3) there is a decrease of the heat of adsorption with added gold. It is interesting to note that 40 years after this study, Au-Ag systems are still studied to understand the behaviour of Ag within an Au backbone and its influence on the catalytic behaviour [217,218], and that relatively simple studies by FEM enable the nanoscale effect of Au-Ag alloying on the energies of adsorption to be measured.…”

“…The catalytic activity of nanoporous gold catalysts with a ligament size of ≈50 nm ( Figure 24a) and a residual Ag content of ≈3 at% were studied for the selective oxidation of methanol [217]. Both the ligament size and the Ag content are expected to influence the catalytic activity as compared to the previous example.…”

Toward digitally controlled catalyst architectures: Hierarchical nanoporous gold via 3D printing

“…2). Energy-dispersive x-ray spectroscopy (EDS) analysis of 3DP-hnp-Au reveals ~2 atomic % (at %) of residual Ag that is similar to the residual Ag content of bulk np-Au obtained through free corrosion ( 30 ). Figure 1 (S and T) shows that the nanoscale feature size increases from ~40 to 500 nm by reannealing at 500°C for 1 hour.…”