Faculty

Publications

R. B. Getman and W. F. Schneider. DFT-Based Characterization of the Multiple Adsorption Modes of Nitrogen Oxides on Pt(111). J. Phys. Chem. C, 111:389-397, 2007. view abstract // link Pt is the most common catalyst for NO oxidation to NO2, a key reaction in NOx remediation chemistry. In this work, density functional theory calculations and plane-wave supercell models are used to calculate the energies, charge distributions, and vibrational spectra of the stable and metastable states of adsorbed NO, NO2, and NO3 on Pt(111), the most likely active metal face for this catalytic oxidation. NO, NO2, and NO3 are all strong electron acceptors and bind to the Pt(111) surface via charge donation from the surface. NO and NO2, in particular, exhibit a variety of adsorption geometries, the most favorable at low coverage being those that maximize surface-adsorbate charge transfer through binding to multiple surface Pt. At low coverage, the order of binding energies is NO > NO3> NO2, and the oxidation of adsorbed NO to NO2 is endothermic by 0.78 eV. Higher surface coverages favor migration of NO and NO2 to lower-coordination surface sites due to competition for metal d charge density. These changes in surface binding configurations, along with the general decrease in surface-adsorbate bond energies associated with higher surface coverages, both tend to energetically promote NO conversion to NO2 and are important in describing this catalytic chemistry.

Y. Xu, W. A. Shelton, and W. F. Schneider. Effect of Particle Size on the Oxidizability of Platinum Clusters. J. Phys. Chem. A, 110:5839-5846, 2006. view abstract // link The catalytic properties of transition metal particles often depend crucially on their chemical environment, but so far, little is known about how the effects of the environment vary with particle size, especially for clusters consisting of only a few atoms. To gain insight into this topic, we have studied the oxygen affinity of free Ptx clusters as a function of cluster size (x = 1, 2, 3, 4, 5, and 10) using density functional theory (DFT) calculations (GGA-PW91). DFT-based Nosé-Hoover molecular dynamics has been used to explore the configuration space of the PtxOx and PtxO2x clusters, leading to the discovery of several novel Pt-oxide structures. The formation of small Pt-oxide clusters by oxidizing the corresponding Ptx clusters is found to be significantly more exothermic than the formation of bulk Pt-oxides from Pt metal. The exothermicity generally increases as cluster size decreases but exhibits strongly nonlinear dependence on the cluster size. The nanoclusters are also structurally distinct from the bulk oxides and prefer one- and two-dimensional chain and ringlike shapes. These findings help elucidate the oxidation behavior of Pt nanoclusters and lay the foundation for understanding the reactivity of Pt nanoclusters in oxidizing chemical environments.

Y. Xu, W. A. Shelton, and W. F. Schneider. Thermodynamic equilibrium compositions, structures, and reaction energies of PtxOy (x = 1–3) clusters predicted from first principles. J. Phys. Chem. B, 110:16591-16599, 2006. view abstract // link As synthetic nanocatalysis strives to create and apply well-defined catalytic centers containing as few as a handful of active metal atoms, it becomes particularly important to understand the structures, compositions, and reactivity of small metal clusters as a function of size and chemical environment. As a part of our effort to better understand the oxidation chemistry of Pt clusters, we present here a comprehensive set of density functional theory simulations combined with thermodynamic modeling that allow us to map out the T-pO2 phase diagrams and predict the oxygen affinity of PtxOy clusters, x = 1-3. We find that the Pt clusters have a much stronger tendency to form oxides than does the bulk metal, that these oxides persist over a wide range of oxygen chemical potentials, and that the most stable cluster stoichiometry varies with size and may differ from the stoichiometry of the stable bulk oxide in the same environment. Further, the facility with which the clusters are reduced depends both on size and on composition. These models provide a systematic framework for understanding the compositions and energies of redox reactions of discrete metal clusters of interest in supported and gas-phase nanocatalysis.

R. B. Getman, Y. Xu, and W. F. Schneider. Thermodynamics of Environment Dependent Oxygen Adsorption on Pt(111). J. Phys. Chem. C, 112:9559-9572, 2008. view abstract // link The reactivity of heterogeneous metal catalysts can be a strong function of the coverage of adsorbates. For example, Pt-catalyzed NO oxidation to NO₂ requires high concentrations of chemisorbed (surface-bound) O, but the development of surface oxides is
detrimental to reaction kinetics. Quantifying the structures, properties, and especially the conditions that produce various adsorbate coverages is essential to developing qualitatively and quantitatively correct models of surface reactivity. In this work, we examine these ideas in the context of oxidation reactions on a Pt(111), the lowest energy face of Pt. We use extensive supercell density functional theory (DFT) calculations to catalog and characterize the stable binding sites and arrangements of chemisorbed O on Pt(111), as a function of O coverage. O atoms are found to uniformly prefer FCC binding sites and to arrange to minimize various destabilizing interactions with neighbor O. These destabilizing interactions are shown to have electronic and strain components that can either reinforce or oppose one another depending
upon O--O separation. Because of the nature and magnitudes of these lateral interactions, the thermodynamically stable O orderings partition into four coverage regimes of decreasing adsorption energy: 02(g) can produce coverages up to 1/2-ML; either NO₂ decomposition or "NO-assisted" O₂ dissociation can access coverages approaching 2/$-ML, as observed during NO oxidation catalysis, and equilibrium with a solid-oxygen storage material, like ceria-zirconia, can buffer equilibrium coverages at a constant
1/4-ML O. These various oxidation reaction energies can be summarized in a single "Ellingham" free energy diagram, providing a convenient representation of the relationship between surface coverage and reaction thermodynamics, and a useful guide towards relevant
coverage regimes for more detailed study of reaction kinetics.

A. D. Smeltz, R. B. Getman, W. F. Schneider, and F. H. Ribeiro. Coupled Theoretical and Experimental Analysis of Surface Coverage Effects in Pt-Catalyzed NO and O2 Reaction to NO2 on Pt(111). Catal. Today, 136:84-92, 2008. view abstract // link Batch reactor results and analysis are reported for the reaction of NO with O2 to form NO2 over a Pt(1 1 1) single crystal at atmospheric pressure. The apparent activation energy and NO, O2, and NO2 reaction orders are found to be 80 kJ mol−1, 1.3, 1, and −2 and are comparable to previous studies on supported Pt catalysts which take inhibition by the product NO2 into account. The absolute rates on a per Pt atom basis are the highest yet reported 0.34 ± 0.02 s−1, at 300 °C, 73 ppm NO, 27 ppm NO2 and 5% O2. Auger electron spectroscopy and X-ray photoelectron spectroscopy are used to show that the surface chemisorbed oxygen coverage under reaction conditions is 0.76 ± 0.06 ML, consistent with a coverage controlled by NO2 dissociation. DFT calculations are used to compare the stability of possible surface intermediates on a clean Pt(1 1 1) surface with those on a p(√3 × √3)-2O (2/3 ML) ordering surface. In contrast to the clean surface, O2 adsorption and dissociation are endothermic at 2/3 ML oxygen, but a peroxynitrite intermediate OONO* is slightly stable and may provide an alternative, associative pathway to NO2 formation that is consistent with the observed first order reaction kinetics in O2.

H. Wang and W. F. Schneider. The Effects of Coverage on the Structures, Energetics, and Electronics of Oxygen Adsorption on RuO2(110). J. Chem. Phys., 127:064706, 2007. view abstract // link Plane-wave supercell DFT calculations within the PW91 generalized gradient approximation are used to examine the influence of oxygen coverage on the structure, energetics, and electronics of the RuO2(110) surface. Filling of Obr and Ocus sites is exothermic with respect to molecular O2 at all coverages and causes changes in local Ru electronic structure consistent with the changing metal coordination. By fitting the surface energies of a large number of surface configurations to a two-body interaction model, an O atom is calculated to be bound by 2.55 eV within a filled Obr row and by 0.98 eV along an otherwise vacant Ocus row. Lateral interactions modify these binding
energies by up to 20%. Ocus – Ocus interactions are repulsive and diminish binding energy with increasing Ocus filling. Due to the favorable relief of local strain, Obr – Obr interactions are attractive and favor filling of neighbor br sites. These interaction effects are relatively modest in absolute magnitude but are large enough to influence the ability of the RuO2(110) surface to promote oxidation of relatively weak reductants, such as NO and C2H4.

Awards

Professional Growth and Development Award

Given on April 13, 2008 by University of Michigan-Dearborn Alumni Association

Technical Achievement Award

Given on May 1, 2000 by Ford Motor Company

50 Publications Award

Given on May 1, 2001 by Ford Motor Company

Henry Ford Technology Award

Given on September 1, 1996 by Ford Motor Company

NSF Predoctoral Fellowship

Given on August 1, 1986 by National Science Foundation

Arch T. Colwell Outstanding Publication Award

Given on March 1, 2001 by Society of Automotive Engineers

Courses

• CBE 40445 - Chemical Reaction Engineering - The basic concepts of chemical rate processes are applied to the theory of the design and operation of the various types of commercial reactors for both noncatalytic and catalytic reactions. Topics... more >

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