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Platinum and other transition metals are used in oxidation catalysis for industrially relevant processes such as the remediation of automobile exhaust, gas turbines, and selective oxidation of organics. The oxygen-rich conditions found in these applications can lead to the oxidation of the metal catalyst surface with important consequences to the reactive behavior. 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.

Through the funding of the ACS PRF fund we have examined mechanisms for the initiation of oxidation on Pt(111). 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 the atomic-level mechanism that leads to the formation of an oxide on the p(2x1) oxygen phase. In our study, we identified a novel precursor mechanism for oxidation on the Pt(111) surface. 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 PtO_x 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 PtO_x 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. Analysis of the charge on the PtOx chains shows a similarity to bulk a-PtO₂, which suggests that these features can be truly considered an oxide instead of a chemisorbed oxygen phase. This work was published in Physical Review B and was chosen as an Editor’s suggestion and highlighted in the Virtual Journal of Nanoscale Science & Technology [April 13^th 2009]).

We are currently probing the chemistry of the 1-D Pt oxide chains. Calculations of CO, NO, and H₂ adsorption on the Pt oxide chains show much different behavior than on the O-covered Pt(111) surface. Both CO and NO prefer to adsorb on the end of the oxide chains suggesting that the chain terminus might show enhanced reactivity. Future work will map out the kinetics of CO and NO oxidation of these oxide chains to determine if they are reactive in comparison to the O-covered Pt(111) surface. This work would be important in determining if the onset of oxidation has any role (retarding or enhancement) of the enhanced reactivity that has been observed for CO and NO oxidation experimentally. 

We also continue to use DFT to test models for changes to the oxide chain that has been observed in the STM images, but ultimately to probe the changes that have been observed at higher O coverages will require accurate charge-transfer potentials. We are collaborating with Prof. Sinnott and Phillpot from the Materials Science Engineering department, who have developed a charge optimized many body (COMB) methodology. The COMB potential shows great promise in accurately capturing the dramatic changes that occur in transition metal oxidation. We have tested the COMB potential that was developed for SiO₂/Si interfaces and shown that it qualitatively captures the initial adsorption site preference of oxygen on Si(001) predicted by DFT. Ultimately, our aim is to apply COMB potentials to the oxidation of Pt(111).

While not a primary focus of the ACS-PRF grant we have also collaborated with Jason Weaver group at the University of Florida 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.

All the work outlined in this report has been led by my student, Jeffery M. Hawkins, who received his Ph.D. in March 2010 and was supported primarily through the ACS-PRF grant. We have published three papers that were partially supported by the ACS-PRF grant and two papers are in preparation examining the chemistry of 1D Pt oxide chains and the difference in H₂ bonding and reactivity on the 2D and bulk oxide that forms on Pd(111). The ACS PRF grant has initiated research in my group on oxidation and reactivity of transition metal surfaces and we are continuing research in this area through funding from the Department of Energy’s Basic Energy Sciences division.