Dynamics of Surface Alloys: Rearrangement of Pd/Ag(111) Induced by CO and O2
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- Harvard Univ., Cambridge, MA (United States). Dept. of Chemistry and Chemical Biology; Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States). Materials Science Division
- Univ. of California, Los Angeles, CA (United States). Dept. of Chemical and Biomolecular Engineering
- Harvard Univ., Cambridge, MA (United States). Dept. of Chemistry and Chemical Biology
- Harvard Univ., Cambridge, MA (United States). Dept. of Chemistry and Chemical Biology; Harvard Univ., Cambridge, MA (United States). School of Engineering and Applied Sciences
- Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States). Materials Science Division
- Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States). Chemical Science Division
- Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States). Materials Science Division; Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States). Chemical Science Division; Univ. of California, Berkeley, CA (United States). Dept. of Materials Science and Engineering
- Harvard Univ., Cambridge, MA (United States). School of Engineering and Applied Sciences
- Univ. of California, Los Angeles, CA (United States). Dept. of Chemical and Biomolecular Engineering; Univ. of California, Los Angeles, CA (United States). Dept. of Chemistry and Biochemistry
- Harvard Univ., Cambridge, MA (United States). Dept. of Chemistry and Chemical Biology; Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States). Chemical Science Division
To begin, alloys of Ag and small amounts of Pd are promising as bifunctional catalysts, potentially combining the inherent selectivity of the noble Ag with that of the more reactive Pd. Stable PdAg surface alloys are prepared via evaporation of Pd onto Ag(111) at room temperature followed by annealing at 400 K to create a model system. Using this procedure, the most stable form of the surface alloy under vacuum was determined to be a Ag-capped PdAg surface alloy, on the basis of a combination of X-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy (STM), and density functional theory (DFT). Extensive roughening of the surface was apparent in STM images, characterized by islands of the Ag/PdAg/Ag(111) alloy of several layers thickness. The roughening is attributed to transport of Ag from the Ag(111) surface into the alloy islands. Within these islands, there is a driving force for Pd to be dispersed, surrounded by Ag, on the basis of DFT modeling. Exposure of these Ag/PdAg/Ag(111) islands to CO (0.5 Torr) at 300 K induces migration of Pd to the surface, driven by the energetic stabilization of the Pd–CO bond based on ambient-pressure XPS. Once the Pd is drawn to the surface by higher pressures of CO at room temperature, it remains stable even under very low CO partial pressures at temperatures of 300 K and below, on the basis of DFT-modeled phase behavior. Exposure to 1 Torr of O2 at 400 K also causes Pd to resurface, and the resulting structure persists even at low pressures and temperatures below 300 K. These results establish that the state of the PdAg catalyst surface depends strongly on pretreatment and operational conditions. Hence, exposure of an alloy catalyst to CO or O2 at moderate temperatures and pressures can lead to catalyst activation by bringing Pd to the surface. Furthermore, these results demonstrate that exposure to CO at room temperature, which is often used as a proxy for evaluating the Pd coordination sites available in a catalyst, changes the surface structure. Therefore, the CO vibrational frequencies measured with diffuse-reflectance infrared Fourier-transform spectroscopy (DRIFTS) on PdAg catalyst materials do not necessarily provide information about their working state, and fundamental understanding of the CO-PdAg alloy is crucial.
- Research Organization:
- Energy Frontier Research Centers (EFRC) (United States). Integrated Mesoscale Architectures for Sustainable Catalysis (IMASC); Lawrence Berkeley National Lab. (LBNL), Berkeley, CA (United States)
- Sponsoring Organization:
- USDOE Office of Science (SC), Basic Energy Sciences (BES); Extreme Science and Engineering Discovery Environment (XSEDE); National Science Foundation (NSF)
- Grant/Contract Number:
- AC02-05CH11231; SC0012573; CHE170060; ACI-1548562
- OSTI ID:
- 1566522
- Alternate ID(s):
- OSTI ID: 1656506
- Journal Information:
- Journal of Physical Chemistry. C, Vol. 123, Issue 13; ISSN 1932-7447
- Publisher:
- American Chemical SocietyCopyright Statement
- Country of Publication:
- United States
- Language:
- English
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