Jason F. Weaver

The adsorption and reaction of low molecular weight alkanes on metallic single crystal surfaces

The adsorption and reaction of alkanes on/with metal single crystal surfaces are reviewed in this report. A comprehensive treatment of the structures and binding of molecular alkanes on metals is first presented. A detailed discussion of the current state of understanding of the dynamics of alkane trapping at surfaces then follows, including the most recent treatments of the roles of both intrinsic and extrinsic precursor states and the influence of preadsorbed species on trapping. The progress in the use of molecular dynamics simulations to predict trapping probabilities is thoroughly discussed. The current states of both the dynamics and the kinetics of dissociative adsorption on a variety of metal surfaces are elaborated.

Determination of Surface Exchange Coefficients of LSM, LSCF, YSZ, GDC Constituent Materials in Composite SOFC Cathodes

A novel approach to processing and modeling isothermal isotope exchange (IIE) data was developed to extract kinetic rate coefficients for the oxygen reduction reaction (ORR) on cathode materials used for solid oxide fuel cells (SOFC). IIE is capable of testing powders with particle sizes on the nano-scale, where the effective sample thickness (particle size) was shown to be below the characteristic thickness (L c ) for ionically conducting materials. This allows for accurate kinetic measurements in surface exchange controlled regimes, in contrast to secondary ion mass spectrometry depth profiling and electrical conductivity relaxation techniques where sample thicknesses are typically in the diffusion limited or mixed regimes. Surface exchange coefficients (k * ) were extracted and L c values calculated from cathode (La 0.6 Sr 0.4 )(Co 0.2 Fe 0.8 )O 3-δ (LSCF) and (La 0.8 Sr 0.2 )MnO 3 (LSM), and electrolyte (Ce 0.9 Gd 0.1 )O 1-δ (GDC) and (Zr 0.8 Y 0.2 )O 2 (YSZ) materials. Additionally, diffusion coefficients (D * ) were extracted for LSM. In a surface exchange controlled regime LSCF exhibits a low activation energy for k * , while for LSM k * was observed to increase with decreasing temperature consistent with a precursor-mediated mechanism in which there is a negative apparent activation energy for the dissociative chemisorption of O 2 . GDC was shown to exhibit a low activation energy for k * , lower than YSZ, which is attributed to higher concentration of electrons in GDC than YSZ.

Molecular adsorption of small alkanes on a PdO(101) thin film: Evidence of σ-complex formation

We investigated the molecular adsorption of methane, ethane, and propane on a PdO(101) thin film using temperature programmed desorption (TPD) and density functional theory (DFT) calculations. The TPD data reveal that each of the alkanes adsorbs into a low-coverage molecular state on PdO(101) in which the binding is stronger than that for alkanes physically adsorbed on Pd(111). Analysis of the TPD data using limiting values of the desorption prefactors predicts that the alkane binding energies on PdO(101) increase linearly with increasing chain length, but that the resulting line extrapolates to a nonzero value between about 22 and 26 kJ/mol at zero chain length. This constant offset implies that a roughly molecule-independent interaction contributes to the alkane binding energies for the molecules studied. DFT calculations predict that the small alkanes bind on PdO(101) by forming dative bonds with coordinatively unsaturated Pd atoms. The resulting adsorbed species are analogous to alkane sigma-complexes in that the bonding involves electron donation from C-H sigma bonds to the Pd center as well as backdonation from the metal, which weakens the C-H bonds. The binding energies predicted by DFT lie in a range from 16 to 24 kJ/mol, in good agreement with the constant offsets estimated from the TPD data. We conclude that both the dispersion interaction and the formation of sigma-complexes contribute to the binding of small alkanes on PdO(101), and estimate that sigma-complex formation accounts for between 30% and 50% of the total binding energy for the molecules studied. The predicted weakening of C-H bonds resulting from sigma-complex formation may help to explain the high activity of PdO surfaces toward alkane activation.

Dispersion-corrected density functional theory calculations of the molecular binding of n-alkanes on Pd(111) and PdO(101)

We investigated the molecular binding of n-alkanes on Pd(111) and PdO(101) using conventional density functional theory (DFT) and the dispersion-corrected DFT-D3 method. In agreement with experimental findings, DFT-D3 predicts that the n-alkane desorption energies scale linearly with the molecule chain length on both surfaces, and that n-alkanes bind more strongly on PdO(101) than on Pd(111). The desorption energies computed using DFT-D3 are slightly higher than the measured values for n-alkanes on Pd(111), though the agreement between computation and experiment is a significant improvement over conventional DFT. The measured desorption energies of n-alkanes on PdO(101) and the energies computed using DFT-D3 agree to within better than 2.5 kJ/mol (< 5%) for chain lengths up to n-butane. The DFT-D3 calculations predict that the molecule-surface dispersion energy for a given n-alkane is similar in magnitude on Pd(111) and PdO(101), and that dative bonding between the alkanes and coordinatively unsaturated Pd atoms is primarily responsible for the enhanced binding of n-alkanes on PdO(101). From analysis of the DFT-D3 results, we estimate that the strength of an alkane η(2)(H, H) interaction on PdO(101) is ~16 kJ/mol, while a single η(1) H-Pd dative bond is worth about 10 kJ/mol.

Growth and properties of high-concentration phases of atomic oxygen on platinum single-crystal surfaces

We present results of our recent investigations detailing the growth and properties of oxygen phases prepared on Pt(111) and Pt(100) surfaces in ultrahigh vacuum using oxygen atom beams. Our studies reveal common features in the oxidation mechanisms of Pt(111) and Pt(100). On both surfaces, oxygen atoms initially populate a chemisorbed phase, and then incorporate into intermediate phases prior to the growth of bulk-like oxide. The bulk oxide grows on both surfaces as three-dimensional particles with properties similar to those of PtO2 and decomposes explosively during heating, exhibiting higher thermal stability than the intermediate oxygen phases. Our results suggest that kinetic barriers stabilize the oxide particles against decomposition, thereby producing explosive desorption, and hence also hinder Pt oxide growth at low coverages. We also find that the kinetics of bulk oxide formation on Pt(100) measured as a function of the O atom incident flux and surface temperature is quantitatively reproduced by a model based on a precursor-mediated mechanism. The model assumes that oxygen atoms adsorbed on top of a surface oxide phase act as a precursor species that can either associatively desorb or react with the surface oxide to produce a bulk oxide particle. Similarities in the development of intermediate oxygen phases on Pt(100), Pt(111) and other transition metal surfaces suggest that precursor-mediated kinetics may be a general feature in transition metal oxidation. Finally, we find that Pt oxide particles are less active than lower-coverage oxygen phases on Pt(111) and Pt(100) toward the oxidation of CO, and that the reaction exhibits autocatalytic kinetics that can be explained with a model that treats the reaction as occurring within a dilute oxygen phase that coexists with oxide particles.

Kinetics of CO oxidation on high-concentration phases of atomic oxygen on Pt(111).

Temperature-programmed reaction spectroscopy (TPRS) and direct, isothermal reaction-rate measurements were employed to investigate the oxidation of CO on Pt(111) covered with high concentrations of atomic oxygen. The TPRS results show that oxygen atoms chemisorbed on Pt(111) at coverages just above 0.25 ML (monolayers) are reactive toward coadsorbed CO, producing CO(2) at about 295 K. The uptake of CO on Pt(111) is found to decrease with increasing oxygen coverage beyond 0.25 ML and becomes immeasurable at a surface temperature of 100 K when Pt(111) is partially covered with Pt oxide domains at oxygen coverages above 1.5 ML. The rate of CO oxidation measured as a function of CO beam exposure to the surface exhibits a nearly linear increase toward a maximum for initial oxygen coverages between 0.25 and 0.50 ML and constant surface temperatures between 300 and 500 K. At a fixed CO incident flux, the time required to reach the maximum reaction rate increases as the initial oxygen coverage is increased to 0.50 ML. A time lag prior to the reaction-rate maximum is also observed when Pt oxide domains are present on the surface, but the reaction rate increases more slowly with CO exposure and much longer time lags are observed, indicating that the oxide phase is less reactive toward CO than are chemisorbed oxygen atoms on Pt(111). On the partially oxidized surface, the CO exposure needed to reach the rate maximum increases significantly with increases in both the initial oxygen coverage and the surface temperature. A kinetic model is developed that reproduces the qualitative dependence of the CO oxidation rate on the atomic oxygen coverage and the surface temperature. The model assumes that CO chemisorption and reaction occur only on regions of the surface covered by chemisorbed oxygen atoms and describes the CO chemisorption probability as a decreasing function of the atomic oxygen coverage in the chemisorbed phase. The model also takes into account the migration of oxygen atoms from oxide domains to domains with chemisorbed oxygen atoms. According to the model, the reaction rate initially increases with the CO exposure because the rate of CO chemisorption is enhanced as the coverage of chemisorbed oxygen atoms decreases during reaction. Longer rate delays are predicted for the partially oxidized surface because oxygen migration from the oxide phase maintains high oxygen coverages in the coexisting chemisorbed oxygen phase that hinder CO chemisorption. It is shown that the time evolution of the CO oxidation rate is determined by the relative rates of CO chemisorption and oxygen migration, R(ad) and R(m), respectively, with an increase in the relative rate of oxygen migration acting to inhibit the reaction. We find that the time lag in the reaction rate increases nearly exponentially with the initial oxygen coverage [O](i) (tot) when [O](i) (tot) exceeds a critical value, which is defined as the coverage above which R(ad)R(m) is less than unity at fixed CO incident flux and surface temperature. These results demonstrate that the kinetics for CO oxidation on oxidized Pt(111) is governed by the sensitivity of CO binding and chemisorption on the atomic oxygen coverage and the distribution of surface oxygen phases.

Electron energy loss spectroscopic investigation of palladium metal and palladium(II) oxide

Abstract Electron energy loss spectra (ELS) obtained from polycrystalline Pd metal and PdO powder using primary electron energies ranging from 100 to 1150 eV have been obtained and examined in an attempt to gain a better understanding of the origins of the loss features and to assess the utility of ELS in investigations of Pd catalysts. The two sets of ELS spectra differ significantly. The ELS spectra from Pd metal exhibit a predominant peak at 6.5 eV, shown to arise from a surface plasmon excitation, and two broad features at 25.1 and 31.9 eV, which originate from bulk loss processes. The broad features consist of several overlapping losses due mainly to interband transitions from the d-band, though a bulk plasmon excitation is believed to produce a feature near 24 eV. Two distinct peaks are present at 3.7 and 7.6 eV in the ELS spectra obtained from PdO, while a broad region of intensity appears over the range from 20 to 40 eV. The peak at 3.7 eV is attributed to a transition between the top of the valence band and the bottom of the conduction band. The feature at 7.6 eV is broad and arises from several overlapping features that are most likely caused by interband transitions rather than collective excitations. Furthermore, the ELS spectra obtained from PdO and oxidized Pd are also quite different indicating that ELS can provide useful information for determining the bonding states of oxygen on Pd-containing catalysts.

Hot precursor reactions during the collisions of gas-phase oxygen atoms with deuterium chemisorbed on Pt(100)

We utilized direct rate measurements and temperature programmed desorption to investigate reactions that occur during the collisions of gaseous oxygen atoms with deuterium-covered Pt(100). We find that both D2O and D2 desorb promptly when an oxygen atom beam impinges upon D-covered Pt(100) held at surface temperatures ranging from 90 to 150 K, and estimate effective cross sections of 12 and 1.8 A2, respectively, for the production of gaseous D2O and D2 at 90 K. The yields of D2O and D2 that desorb at 90 K are about 13% and 2%, respectively, of the initial D atom coverage, though most of the D2O product molecules (approximately 80%) thermalize to the surface rather than desorb at the surface temperatures studied. Increasing the surface temperature from 90 to 150 K causes the D2O desorption rate to decay more quickly during O atom exposures to the surface and results in lower yields of gaseous D2O. We attribute the production of D2O and D2 in these experiments to reactions involving intermediates that are not thermally accommodated to the surface, so-called hot precursors. The results are consistent with the production of hot D2O involving first the generation of hot OD groups from the reaction O*+D(a)-->OD*, where the asterisk denotes a hot precursor, followed by the parallel pathways OD*+D(a)-->D2O* and OD*+OD(a)-->D2O*+O(a). The final reaction contributes significantly to hot D2O production only later in the reaction period when thermalized OD groups have accumulated on the surface, and it becomes less important at higher temperature due to depletion of the OD(a) concentration by thermally activated D2O production.

Ag2O XPS Spectra

XPS data have been obtained from a pressed AgO powder sample obtained from AESAR. Obtaining high‐quality XPS data from a single oxide state is difficult because multiple oxide states, hydroxides, and other contaminants are usually present. The AgO sample was annealed in vacuum at 100 °C for 30 min to reduce the amounts of contaminants present. The primary contaminant on this sample is a bicarbonate species that was significantly reduced during the anneal. The anneal temperature is below that which results in decomposition of AgO and Ag2O. These spectra will be useful in XPS studies of O/Ag systems such as alumina‐supported silver epoxidation catalysts.

High Selectivity for Primary C–H Bond Cleavage of Propane σ-Complexes on the PdO(101) Surface

We investigated regioselectivity in the initial C-H bond activation of propane σ-complexes on the PdO(101) surface using temperature programmed reaction spectroscopy (TPRS) experiments. We observe a significant kinetic isotope effect (KIE) in the initial C-H(D) bond cleavage of propane on PdO(101) such that the dissociation yield of C(3)H(8) is 2.7 times higher than that of C(3)D(8) at temperatures between 150 and 200 K. Measurements of the reactivity of (CH(3))(2)CD(2) and (CD(3))(2)CH(2) show that deuteration of the methyl groups is primarily responsible for the lower reactivity of C(3)D(8) relative to C(3)H(8), and thus that 1° C-H bond cleavage is the preferred pathway for propane activation on PdO(101). By analyzing the rate data within the context of a kinetic model for precursor-mediated dissociation, we estimate that 90% of the propane σ-complexes which dissociate on PdO(101) during TPRS do so by 1° C-H bond cleavage.