| AVS 65th International Symposium & Exhibition | |
| Fundamental Discoveries in Heterogeneous Catalysis Focus Topic | Wednesday Sessions |
| Session HC+SS-WeA |
| Session: | Theory and Dynamics of Heterogeneously Catalyzed Reactions |
| Presenter: | Matthijs van Spronsen, Lawrence Berkeley National Laboratory |
| Authors: | M. van Spronsen, Lawrence Berkeley National Laboratory K. Duanmu, UCLA R. Madix, Harvard University M.B. Salmeron, Lawrence Berkeley National Laboratory P. Sautet, UCLA C. Friend, Harvard University |
| Correspondent: | Click to Email |
Efficient chemical production requires the use of materials able to selectively catalyze complex chemical reactions. High selectivity can be obtained by using noble metals. To improve activity, while retaining selectivity, small amounts of an active metal can be added. These species can function as active sites for, e.g., O2 activation.
A promising alloy for selective hydrogenation is AgPd. The challenge using this material as a catalyst is the larger surface free energy of Pd, compared to Ag. This difference leads to Ag enrichment of the surface, hence, more Pd, both costly and scarce, is needed to ensure that Pd is present in the surface.
Here, the stability of Pd was probed in (sub)surface alloys formed by depositing sub-monolayer amounts of Pd on pure Ag single crystals, in order to evaluate the feasibility of using surface alloys as catalysts. These surface alloys would use much less Pd, but depend critically on understanding and control of the alloy composition and stability.
Mild annealing (~400 K) under ultra-high vacuum, leads to extensive migration of Ag and the formation of a Ag capping layer on the Pd. These measurements were based on scanning tunneling microscopy and X-ray photoelectron spectroscopy, supported by modeling (density functional theory). The etching, due to the loss of Ag, and the increase in island height, due to the capping, results in three-layer-high islands, covering both the terraces and the steps of the Ag(111) surface. The absence of significant amounts of Pd in the surface was confirmed by the lack of CO adsorption at 120 K.
The exposure of the Ag/Pd/Ag(111) surface alloy to gas atmospheres, either pure CO or O2 (0.5 & 1 Torr, respectively), reverses the energetics, thus making Pd resurface. This was concluded from large core-level shifts observed in AP XPS measurements. Due to the proximity of Pd to the surface, the required temperature for Pd to segregate was rather low, in fact, for CO it was detected for 300 K. Resurfacing, and subsequent oxidation, of Pd was much slower, requiring cycling the temperature between 300 and 400 K in the O2 atmosphere, several times. The difference stems from the fact that O2 needs to be dissociated before it can form strong bonds to Pd, while CO can adsorb directly to Pd. The initial O2-dissociation activity of the Ag-capped Pd is expected to be rather small.
To conclude, precise understanding of the stability of active metals with a high surface free energy is required to control the composition of surface alloys and its potential as active/selective catalyst.