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Platinum-based catalysts are used in several commercial applications and a fundamental understanding of their reactivity is important to the design of next-generation catalysts. CO, NO, and hydrocarbon oxidation on metal surfaces has been of particular interest for their importance in various downstream applications in the petroleum industry. Earlier work on transition metal (TM) surfaces were performed at low oxygen coverages, but reactive conditions involve oxygen-rich environments. Experimentally, it has been established that under oxygen-rich conditions a dramatic range of complex oxygen phases form and are critical to the reactivity of the surfaces. Currently, our understanding of the evolution of these oxygen phases is limited; in particular the atomic-scale processes that dictate the large-scale changes that occur in the transition from a bare metal surface to metal oxide development.

In the past year we have examined mechanisms for the initiation of oxidation on Pt(111). Recent ultrahigh vacuum (UHV) experiments suggest that a transition from ordered chemisorbed surface oxygen to a oxide-like phase occurs at coverages around 0.5 monolayers (ML). Scanning tunneling microscopy (STM) further show that the precursor mechanism to oxidation occurs on a p(2x1) oxygen domain on the Pt(111) surface. We have used density functional theory (DFT) to probe subsurface oxygen as a possible precursor mechanism for oxidation. The expectation is that with increasing surface O coverage, the O-O repulsions will lead some surface O atoms to diffuse subsurface. Subsurface oxygen has been suggested as the precursor mechanism for oxidation for several late TM surfaces. Our DFT calculations indicate that subsurface oxygen only becomes favorable after coverages of 0.75 ML, which conflicts with the recent STM experiments. These results suggested the possibility of another mechanism for oxidation on Pt(111). Motivated by the STM images we probed the role of O atom clustering beyond 0.5 ML coverages using DFT. This search led us to the discovery of a novel precursor mechanism for oxidation on Pt(111). Instead of oxygen going subsurface to relieve the O-O repulsive interactions, O atoms on the surface induce Pt atoms to dramatically buckle upwards to form a PtOx compound. The buckling in these structures is around 1.4 angstroms, which is very close to the value of 1.6 angstroms identified in the STM experiments. DFT derived STM images also qualitatively match the experimental STM images, which suggests that we have indeed identified the correct structure for the Pt oxide compound. We then used DFT to further probe the growth of the PtOx compound. We find that the oxide grows as a chain running parallel to the p(2x1) rows and that these chains are far more stable that surface O arrangements on Pt(111) beyond 0.5 ML. We are currently probing both the chemistry of the 1-D Pt oxide chains and the experimentally observed formation of the superstructures that form on Pt(111), which are expected to be intimately related to strain relief due to the lattice mismatch between Pt oxide and the underlying Pt(111) surface.  

Our work on Pt(111) has identified an entirely new mechanism for oxidation on TM surfaces. The buckled PtOx structures show tremendous charge transfer and the charges in these compounds are very close to bulk PtO₂ values suggesting that these structures will have dramatically different reactivity than chemisorbed oxygen. Our findings have important consequences to understanding NO and CO oxidation on Pt(111) surfaces, but also suggest that future studies of bimetallic surfaces must incorporate more complex mechanisms for oxidation. It is critical to identify the transition from chemisorbed oxygen to oxide formation since the reactivity might be very sensitive to this transition. In our study, close collaboration with the experimental UHV group of Jason Weaver allowed us to identify this unexpected oxidation mechanism, but motivate the need to develop tractable modeling methods to probe these mechanisms a priori without experimental input. Such methods will especially be important for understanding oxidation of bimetallic surfaces, which have too large a parameter space to be tractable with experiments alone.

While not a primary focus of the ACS-PRF grant we have also collaborated with Jason Weaver’s group to examine the reactivity of several molecules (CO, H₂, H₂O, O₂) on the oxides of Pd(111). In a recent UHV-DFT study we identified the presence of coadsorbed H₂O-OH complexes on the bulk PdO surfaces. Interestingly, our attempts to identify the precursor to oxidation on Pd(111) indicate that it does not share the mechanism identified in Pt(111) but instead oxygen goes subsurface at lower coverages (0.25 to 0.5 ML). The differences between Pd and Pt can be attributed to a lower strain cost for subsurface oxygen on Pd(111). The two TM surfaces also have very different surface and bulk oxide phases, which lead to very different reactivity of the oxide phases in the two TM surfaces. Such dramatic differences between TM surfaces suggest that it will be difficult to predict a priori the 1D and 2D oxide phases that develop on these surfaces. This difficulty again motivates the need to develop methods that will allow us to probe large range of configurations/models for oxide formation. We are still working on the development of charge transfer potentials for these systems and these efforts will be a major focus of my group in the next decade.