Michael J. Janik

A first principles comparison of the mechanism and site requirements for the electrocatalytic oxidation of methanol and formic acid over Pt

First principles density functional theoretical calculations were carried out to examine and compare the reaction paths and ensembles for the electrocatalytic oxidation of methanol and formic acid in the presence of solution and applied electrochemical potential. Methanol proceeds via both direct and indirect pathways which are governed by the initial C-H and O-H bond activation, respectively. The primary path requires an ensemble size of between 3-4 Pt atoms, whereas the secondary path is much less structure sensitive, requiring only 1-2 metal atoms. The CO that forms inhibits the surface at potentials below 0.66 V NHE. The addition of Ru results in bifunctional as well as electronic effects that lower the onset potential for CO oxidation. In comparison, formic acid proceeds via direct, indirect and formate pathways. The direct path, which involves the activation of the C-H bond followed by the rapid activation of the O-H bond, was calculated to be the predominant path especially at potentials greater than 0.6 V. The activation of the O-H bond of formic acid has a very low barrier and readily proceeds to form surface formate intermediates as the first step of the indirect formate path. Adsorbed formate, however, was calculated to be very stable, and thus acts as a spectator species. At potentials below 0.6 V NHE, CO, which forms via the non-Faradaic hydrolytic splitting of the C-O bond over stepped or defect sites in the indirect path, can build up and poison the surface. The results indicate that the direct path only requires a single Pt atom whereas the indirect path requires a larger surface ensemble and stepped sites. This suggests that alloys will not have the same influence on formic acid oxidation as they do for methanol oxidation.

Selectivity of CO2 Reduction on Copper Electrodes: The Role of the Kinetics of Elementary Steps†

On the right path: Based on DFT calculations (incorporating the role of water solvation) of the activation barriers of elementary steps, a new path that leads to methane and ethylene for CO(2) electroreduction on Cu(111) was identified. Methane formation proceeds through reduction of CO to COH (path II, see picture), which leads to CH(x) species that can produce both methane and ethylene, as observed experimentally.

Mechanistic Consequences of Composition in Acid Catalysis by Polyoxometalate Keggin Clusters

The kinetics and mechanism of ether and alkanol cleavage reactions on Brønsted acid catalysts based on polyoxometalate (POM) clusters are described in terms of the identity and dynamics of elementary steps and the stability of the transition states involved. Measured rates and theoretical calculations show that the energies of cationic transition states and intermediates depend on the properties of reactants (proton affinity), POM clusters (deprotonation enthalpy), and ion-pairs in transition states or intermediates (stabilization energy). Rate equations and elementary steps were similar for dehydration of alkanols (2-propanol, 1- and 2-butanol, tert-butanol) and cleavage of sec-butyl-methyl ether on POM clusters with different central atoms (P, Si, Co, Al). Dehydration rates depend on the rate constant for elimination from adsorbed alkanols or ethers and on the equilibrium constant for the formation of unreactive reactant dimers. Elimination involves E1 pathways and late carbenium-ion transition states. This is consistent with small kinetic isotope effects for all deuterated alkanols, with strong effects of substituents on elimination rates, and with the similar alkene stereoselectivities measured for alkanol dehydration, ether cleavage, and alkene double-bond isomerization. n-Donor reactants (alkanols, ethers) and products (water) inhibit dehydration rates by forming stable dimers that do not undergo elimination; their stability is consistent with theoretical estimates, with the dynamics of homogeneous analogues, and with the structure and proton affinity of the n-donors. Elimination rate constants increased with increasing valence of the central POM atom, because of a concurrent decrease in deprotonation enthalpies (DPE), which leads to more stable anionic clusters and ion-pairs at transition states. The DPE of POM clusters influences catalytic rates less than the proton affinity of the alkene-like organic moiety at the late carbenium-ion-type transition states involved. These different sensitivities reflect the fact that weaker acids typically form anionic clusters with a higher charge density at the transition state; these clusters stabilize cationic fragments more effectively than those of stronger acids, which form more stable conjugate bases with lower charge densities. These compensation effects are ubiquitous in acid chemistry and also evident for mineral acids. The stabilization energy and the concomitant charge density and distribution in the anion, but not the acid strength (DPE), determine the kinetic tolerance of n-donors and the selectivity of reactions catalyzed by Brønsted acids.

Direct borohydride oxidation: mechanism determination and design of alloy catalysts guided by density functional theory

Direct borohydride fuel cells (DBFCs) convert an aqueous soluble, high specific energy density borohydride fuel directly to electrical energy. The lack of effective anode electrocatalysts for the anodic oxidation of borohydride limits the efficiency and power density attainable in these devices. The complexity of the eight electron reaction makes experimental determination of the reaction mechanism extremely challenging, thereby hampering the development of a rationale for optimizing catalyst composition. Computational quantum mechanical methods provide a unique tool for evaluating elementary step reaction kinetics in this system, and can be applied to guide a rational catalyst design procedure. In this perspective, we review the experimental literature on borohydride oxidation catalysis and discuss the usefulness of quantum mechanical methods towards electrode design. Mechanistic insights provided by these computational methods are discussed as well as the prospects of applying a computationally guided design procedure towards developing novel catalyst compositions.

Experimental and Density Functional Theory Study of the Tribochemical Wear Behavior of SiO2 in Humid and Alcohol Vapor Environments

This paper investigates the reaction steps involved in tribochemical wear of SiO(2) surfaces in humid ambient conditions and the mechanism of wear prevention due to alcohol adsorption. The friction and wear behaviors of SiO(2) were tested in three distinct gaseous environments at room temperature: dry argon, argon with 50% relative humidity (RH), and argon with n-pentanol vapor pressure 50% relative to the saturation pressure (P/P(sat)). Adsorbed gas molecules have significant chemical influences on the wear of the surface. The SiO(2) surface wears more readily in humid ambient compared to the dry case; however, it does not show any measurable wear in 50% P/P(sat) n-pentanol vapor at the same nominal contact load tested in the dry and humid environments. The tribochemical wear of the SiO(2) surface can be considered the Si-O-Si bond cleavage upon reactions with the impinging vapor molecules under tribological stress. DFT calculations were used to estimate the apparent activation energy needed to cleave the Si-O-Si bond at beta-cristobalite (111) and alpha-quartz (001) surfaces by reactions with impinging water and alcohol vapor molecules. The alkoxide termination of the SiO(2) surfaces increases the energy barrier required to cleave the Si-O-Si bonds when compared to hydroxyl-terminated SiO(2) surfaces.

A first-principles study of molecular oxygen dissociation at an electrode surface: a comparison of potential variation and coadsorption effects

Influences of coadsorbed sodium and water, aqueous solvent, and electrode potential on the kinetics of O(2) dissociation over Pt(111) are systematically investigated using density functional theory models of vacuum and electrochemical interfaces. Na coadsorption alters the electronic states of Pt to stabilize the reactant (O(2)*), transition, and product (2O*) states by facilitating electron donation to oxygen, causing a more exothermic reaction energy (-0.84 eV for Na and O(2), -0.81 eV for isolated O(2)) and a decrease in dissociation barrier (0.39 eV for Na and O(2), 0.57 eV for isolated O(2)). Solvation decreases the reaction energy (-0.67 eV) due to enhanced hydrogen bond stabilization of O(2)* compared to 2O*. The influence of Na is less pronounced at the solvated interface (barrier decreases by only 0.11 eV) because H(2)O screens Na charge-donation. In the electrochemical model system, the dissociation energy becomes more exothermic and the barrier decreases toward more positive potentials. Potential-dependent behavior results from changes in interfacial dipole moment and polarizability between O(2)*, the dissociation transition state, and 2O*; each are influenced by changes in adsorption and hydrogen bonding. Coadsorption of Na in the solvated system dampens the dipole moment change between O(2)* and 2O* and significantly increases the polarizability at the dissociation transition state and for 2O*; the combination causes little change in the reaction energy but reduces the activation barrier by 0.08 eV at 0 V versus NHE. The potential-dependent behavior contrasts that determined at a constant surface charge or from an applied electric field, illustrating the importance of considering the electrochemical potential at the fully-solvated interface in determining reaction energetics, even for non-redox reactions.

Development of a ReaxFF potential for Pd/O and application to palladium oxide formation

Oxide formation on palladium surfaces impacts the activity and selectivity of Pd-based catalysts, which are widely employed under oxygen rich operating conditions. To investigate oxidation processes over Pd catalysts at time and length scales inaccessible to quantum based computational methods, we have developed a Pd∕O interaction potential for the ReaxFF reactive force field. The parameters of the ReaxFF potential were fit against an extensive set of quantum data for both bulk and surface properties. Using the resulting potential, we conducted molecular dynamics simulations of oxide formation on Pd(111), Pd(110), and Pd(100) surfaces. The results demonstrate good agreement with previous experimental observations; oxygen diffusion from the surface to the subsurface occurs faster on the Pd(110) surface than on the Pd(111) and Pd(100) surfaces under comparable conditions at high temperatures and pressures. Additionally, we developed a ReaxFF-based hybrid grand canonical Monte Carlo∕molecular dynamics (GC-MC∕MD) approach to assess the thermodynamic stability of oxide formations. This method is used to derive a theoretical phase diagram for the oxidation of Pd935 clusters in temperatures ranging from 300 K to 1300 K and oxygen pressures ranging from 10(-14) atm to 1 atm. We observe good agreement between experiment and ReaxFF, which validates the Pd∕O interaction potential and demonstrates the feasibility of the hybrid GC-MC∕MD method for deriving theoretical phase diagrams. This GC-MC∕MD method is novel to ReaxFF, and is well suited to studies of supported-metal-oxide catalysts, where the extent of oxidation in metal clusters can significantly influence catalytic activity, selectivity, and stability.

Ab initio thermodynamic evaluation of Pd atom interaction with CeO2 surfaces

Palladium supported on ceria is an effective catalytic material for three-way automotive catalysis, catalytic combustion, and solid-oxide fuel cell (SOFC) anodes. The morphology, oxidation state, and particle size of Pd on ceria affect catalytic activity and are a function of experimental conditions. This work utilizes ab initio thermodynamics using density functional theory (DFT) (DFT+U) methods to evaluate the stability of Pd atoms, PdO(x) species, and small Pd particles in varying configurations on CeO(2) (111), (110), and (100) single crystal surfaces. Over specific oxygen partial pressure and temperature ranges, palladium incorporation to form a mixed surface oxide is thermodynamically favorable versus other single Pd atom states on each ceria surface. For example, Pd atoms may incorporate into Ce fluorite lattice positions in a Pd(4+) oxidation state on the CeO(2) (111) surface. The ceria support shifts the transition between formal Pd oxidation states (Pd(0), Pd(2+), Pd(4+)) relative to bulk palladium and stabilizes certain oxidized palladium species on each surface. We show that temperature, oxygen pressure, and cell potential in a SOFC can influence the stable states of palladium supported on ceria surfaces, providing insight into structural stability during catalytic operation.

Density functional theory-based electrochemical models for the oxygen reduction reaction: Comparison of modeling approaches for electric field and solvent effects

A series of density functional theory (DFT) based electrochemical models are applied to systematically examine the effect of solvent, local electric field, and electrode potential on oxygen reduction reaction (ORR) kinetics. Specifically, the key elementary reaction steps of molecular oxygen dissociation, molecular oxygen protonation, and reduction of a hydroxyl adsorbate to water over the Pt(111) surface were considered. The local electric field has slight influence on reaction energetics at the vacuum interface. Solvent molecules stabilize surface adsorbates, assisting oxygen reduction. A collective solvation‐potential coupled effect is identified by including long range solvent‐solvent interactions in the DFT model. The dominant path of the ORR reaction varies with electrode potential and among the modeling approaches considered. The potential dependent reaction path determined from the solvated model qualitatively agrees with experiment ORR kinetics.

Influence of Hydroxyls on Pd Atom Mobility and Clustering on Rutile TiO2(011)-2 × 1

Understanding agglomeration of late transition metal atoms, such as Pd, on metal oxide supports, such as TiO2, is critical for designing heterogeneous catalysts as well as for controlling metal/oxide interfaces in general. One approach for reducing particle sintering is to modify the metal oxide surface with hydroxyls that decrease adatom mobility. We study by scanning tunneling microscopy experiments, density functional theory (DFT) calculations, and Monte Carlo (MC) computer simulations the atomistic processes of Pd sintering on a hydroxyl-modified TiO2(011)-2 × 1 surface. The formation of small 1-3 atom clusters that are stable at room temperature is achieved on the hydroxylated surface, while much larger clusters are formed under the same conditions on a hydroxyl-free surface. DFT shows that this is a consequence of stronger binding of Pd atoms adjacent to hydroxyls and increased surface diffusion barriers for Pd atoms on the hydroxylated surface. DFT, kinetic MC, and ReaxFF-based NVT-MC simulations show that Pd clusters larger than single Pd monomers can adsorb the hydrogen from the oxide surface and form Pd hydrides. This depletes the surface hydroxyl coverage, thus allowing Pd to more freely diffuse and agglomerate at room temperature. Experimentally, this causes a bimodal cluster size distribution with 1-3 atom clusters prevalent at low Pd coverage, while significantly larger clusters become dominant at higher Pd concentrations. This study demonstrates that hydroxylated oxide surfaces can significantly reduce Pd cluster sizes, thus enabling the preparation of surfaces populated with metal clusters composed of single to few atoms.