Publications

• A combination of an aprotic solvent (DMSO) and proton source (phenol) creates a unique environment for the ORR on Au electrodes.
• In situ SERS and DFT identify O₂^– as the first reaction intermediate in 2.2~3.0 V vs. Li/Li⁺, and H₂O₂ as a product below 2.5 V.
• First-principles ORR phase diagram on Au(111) is in good agreement with observed threshold potentials and explains why O₂^– and H₂O₂ appear together in this system.
• The predominance of the O₂^– in DMSO offers insights into the ORR mechanism at the non-aqueous Li-air cathode.

• TPD, sXPS, NEXAFS, RAIRS, and DFT were used to probe adsorbates resulting from acetic acid adsorption on CeO₂(111).
• Desorption products depend strongly on extent of surface reduction prior to adsorption.
• Acetic acid adsorption on stoichiometric CeO₂(111) leads to water and vacancy formation, vacancy-stabilized acetate as a major surface species, and ketene and acetone as a major and a minor desorption product.
• Dehydrogenation and carbon deposition dominate on CeO_2-x(111).

• Addition of Na or K disperses Au atomically on zeolites and mesoporous silica, without which Au agglomerates on the same supports.
• Intrinsic activity of the new single-site Au species is the same as other previously reported single-site Au species on reducible oxides for WGS.
• Au-O(OH)_x-Na_y clusters are systematically investigated using DFT; several clusters consistent with experiment in character are likely WGS-active.
• Novel atomic-scale active site configuration suggests ways to maximize atomic efficiency of precious metals in heterogenous catalysis.

• Adsorption of CH₂I₂ on Pt(111) is studied using single-crystal adsorption calorimetry and DFT to directly measure the Pt-CH₂ bond energy.
• CH₂I₂ is expected to dissociate to yield CH₂ at 100 K but rapidly decompose to CH at 210 K temperature.
• Pt-CH₂ and Pt-CH bond energies are measured to be 197 and 663 kJ/mol respectively, with the latter in line with DFT but the former suggesting destabilization by co-adsorbed iodine atoms at 100 K.
• An alternate model supported by DFT is molecular adsorption at 100 K; in either case CH₂ as an isolated species is not produced in experiment.

• NH_x prefers bonding directly to metal cations with open coordination rather than oxo groups on the cations.
• Initial H abstraction from NH₃ involves surface-stabilized NH₂; binding sites for H and NH₂ strongly affect activation barrier.
• NH insertion into the π-allyl intermediate is very facile.
• Selectivity between C-N and C-O coupling likely depends on relative abundance of N vs. O surface groups.

Objectives and accomplishments of the Center for Atomic-Level Catalyst Design, an Energy Frontier Research Center funded by the U.S. Department of Energy and led by Louisiana State University.

• Activation barrier for initial H abstraction on V⁵⁺=O is consistent with this being the rate-limiting step in propane ODH on the M1 ab plane.
• V⁵⁺=O can activate both propane and propene.
• Mo⁵⁺=O may be involved in propane ODH as an H acceptor from the isopropyl radical.
• Activation barriers for H abstraction from propaned linearly correlate with H adsorption energies on various metal oxo groups.

• Au deposited at 115 K can form a dense super-lattice of Au clusters of φ~3 nm on BN/Rh(111) at near micron scale.
• Au is confined to the pores even after warming to room temperature.
• Monoatomic-high clusters preferentially form at low coverage and temperature; bi- and multilayer Au clusters appear upon annealing.
• DFT suggests that Au clusters transitions from monolayer to bilayer between 30 and 37 atoms and are negatively charged, both consistent with our experiment.

• Dissociative adsorption of t-butyl iodide on Pt(111) is used to measure the bond energy of t-butyl on Pt(111).
• Adsorption is dissociative, molecular, and multi-layer from 0 to 0.07 ML, 0.15 ML and beyond based on RAIRS and calculated IR spectra and calculated activation barriers.
• Bond energy and molecular adsorption energy of t-butyI on Pt(111) is 224 kJ/mol and 100~134 kJ/mol, respectively.
• OptB86b vdW-DF yields predictions (225, 140 kJ/mol) in much closer agreement with experiment than GGA-PBE.

• Acetic acid hydrogenation on Ru/C in two- and three-phase regimes is studied using kinetic flow reactor experiments.
• Microkinetic analysis using DFT results captures apparent activation energy, ethanol selectivity, and reaction order w.r.t. hydrogen.
• Hydrogen partial pressure is shown by experiment and theory to determine ethanol selectivity in this reaction.
• Aqueous phase is beneficial, but not necessary for high ethanol selectivity.

• Oxygen functional groups on few-layered graphene are characterized with TPD and catalytic testing of isobutane ODH.
• Stability of different oxygen functional groups are estimated based on DFT and compared with CO and CO₂ desorption profiles.
• Activation barriers and reaction energy profiles for isobutane ODH are calculated on representative groups.
• Comparison of theory and experiments suggest dicarbonyls at zigzag edges and quinones at armchair edges are responsible for observed ODH activity.

• Global hopping barriers between moiré cells are estimated for 4d (Y-Ag) and 5d (La-Au) transition metal adatoms on g/Ru(0001).
• Earlier 4d and 5d adatoms have stronger adsorption energies and higher diffusion barriers than later ones.
• Necessary conditions to achieve super-lattices of monodisperse nanoclusters for a given metal are suggested.

• Coarse-grained potential energy surfaces for Rh₁ and Au₁ on full g/Ru(0001) surface are calculated from DFT.
• Diffusion properties of Rh₁ and Au₁ are calculated in detail on different regions of g/Ru(0001) using smaller models.
• Combination of results reveals global diffusion paths and barriers of Rh₁ and Au₁ on full g/Ru(0001).
• Au₁ is predicted to be more mobile than Rh₁, which is inconsistent with observed nucleation behavior and suggests diffusing species other than adatoms for Au.

• The aqueous phase hydrogenation and hydrogenolysis activities of several different metals have been systematically tested and compared.
• Ru is most active for APH of non-furanic C=O groups, whereas Pd is most active for APH of furanic C=O and C=C bonds.
• Initial rates of aqueous-phase hydrogenolysis of THFA and xylitol are much lower than APH of C=O groups.
• APH activity of furfural is more sensitive to metal-C bonding than that of acetaldehyde, suggesting different nature of reaction intermediates.

• Method is demonstrated to prepare a new class of metal-supported graphene material, using Ru(0001) as example.
• G/Ru coupling can be tuned by forming a pseudomorphic overlayer of another metal on Ru, while preserving moiré perodicity.
• Corrugation and adsorption energy of graphene moiré on Ru(0001), Co/Ru(0001), and Pd/Ru(0001) are calculated using DFT.
• Corrugation correlates with adsorption energy and is in good agreement with STM measurements.

• Acetaldehyde adsorption on thin-film CeO₂(111) surfaces is studied using RAIRS and DFT calculations.
• Acetaldehyde adsorbs weakly on CeO₂(111) but produces new states on CeO_2-x(111) at 300 and 400 K without the carbonyl C=O stretch.
• Experimental and calculated IR spectra identify the two states as a C-O coupled dimer and the enolate of acetaldehyde.
• DFT-calculated reaction energy profile based on the two intermediates is consistent with our previous TPD results.

• Location of Nb in the Mo-V-Te-Ta mixed metal oxide cannot be directly observed in electron microscopy due to similar atomic masses of Nb and Mo.
• M1 phase with uniform Ta distribution is produced via hydrothermal synthesis.
• Ta is predominantly located in the S9 site according to HAADF STEM and statistical thermodynamic analysis based on DFT energetics.
• The theoretical approach further predicts that Ta and Nb are chemically similar and that Nb is also predominantly located in the S9 site.

• Minimum-energy decomposition pathway for furan on clean Pd(111) is mapped.
• Furan decomposition begins with ring opening at C-O position (E_a = 0.82 eV) followed by dehydrogenation.
• Redhead analysis indicates that initial ring opening begins at 240-270 K, in agreement with previous experiment.
• CO does not form immediately after ring opening because C₄H₂O decarbonylation begins around 350 K.

• ORR by Li is studied on six metals (Au, Ag, Pt, Pd, Ir, and Ru).
• Intrinsic activity is limited by initial electron transfer step, and forms volcano trend with respect to ΔE_O.
• Step edge sites are more active than flat terrace sites.
• Solution-phase O₂ reduction to superoxide can mask activity of less reactive metals.

• Au deposited on graphene moiré/Ru(0001) at room temperature exclusively forms 2D islands with φ~a few nm.
• STM line scan and DFT calculations suggest the 2D Au consists of two atomic layers of Au.
• Model 2D Au structures are calculated to interact with the substrate weakly and to be negatively charged.
• Preliminary results suggest the 2D Au may be active for CO oxidation at cryogenic temperatures.

• O₂ reduction via LiO₂ formation on graphite basal plane is limited to 1.1-1.2 V vs. Li/Li⁺.
• Edges, defects are oxidized to various extents in equilibrium with an O₂ atmosphere.
• Oxygen functional groups, particular di-carbonyl moieties, stabilizes LiO₂ and promotes Li-ORR to 2.3 V.
• Substantial oxidation, as through OER, may cause carbon electrodes to disintegrate.

• Adsorption of C₃H_x (x=5-8) and H on the proposed active center in the ab plane of the M1 phase of Mo-V-Te-Nb mixed metal oxide is studied.
• Iso-propyl and allyl adsorb on V=O and Te=O much more strongly than on other oxygen sites; H strongly prefers Te=O.
• E_a for H abstarction from propane is linearly correlated with H adsorption energy on metal-oxo groups, with that on V⁵⁺=O being most consistent with reported apparent E_a for propane conversion on V-based oxides.

• Adsorptions of methyl acetate and its dissociation products (enolate, methylene acetate, acetate, acetyl, ketene, methoxy, formaldehyde, CO, C, O, and H) are enhanced by step edges vs. terrace sites on (111) and (100) facets of Pd.
• Step edges promote both C-H and C-O bond scission.
• Square symmetry of the (100) facet promotes C-O bond scission more than C-H bond scission.

• Aqueous-phase hydrogenation of acetic acid over Ru, Pt, Pd, Rh, Ir, Ni and Cu is investigated.
• Kinetic flow reactor experiments show the metals have significantly different activities for this reaction.
• Ru is the most active and selective of the seven metals tested.
• DFT calculations and kinetic modeling suggest activity likely to be controlled by initial C-O bond scission.

• O₂ reduction by hydrogen on the (111) facets of Rh, Ir, Ni, Pd, Pt, Cu, Ag, and Au is studied.
• Three selectivity regimes are identified based on opposing needs to catalyze O–O bond scission and O–H bond formation: dissociative adsorption; water and H₂O₂ formation via associative mechanisms.
• Associative mechanisms include peroxyl (OOH), peroxide (HOOH), and aquoxyl (OOHH) mechanisms.
• Reducing power of hydrogen and the selective regimes can be varied electrochemically on the metals.

• Initial O₂ reduction on Au(111) occurs via superoxide (LiO₂) formation at a low Eo=1.51 V vs. Li/Li⁺.
• O₂ can dissociate readily on Pt(111) and the lithiation of O occurs at E^o=1.97 V.
• Lithiation weakens the O-O bond resulting in (Li_xO) aggregates on these metals.
• Bulk Li₂O is the most stable phase up to 2.59 V, followed by bulk Li₂O₂ up to 2.68 V, before decomposition to O₂.

• Methyl acetate preferentially undergoes dehydrogenation of the methyl groups, instead of C-O bond scission, on Pd(111).
• The enolate pathway (CH₂COOCH₃) is slightly preferred over the methylene acetate (CH₃COOCH₂) pathway.
• The former leads to further dehydrogenation, whereas the latter yields acetyl.
• BEP-type linear energy relations exists for C-H and C-O bond scission steps in methyl acetate and its derivatives.

• The proposed active center in the ab plane of the M1 phase of Mo-V-Te-Nb mixed metal oxide for propane ammoxidation is studied using cluster models.
• A minimum of three ab planes are needed for energy convergence.
• Propane, isopropyl, and H adsorption is studied on various terminal and bridging O sites.
• Te=O sites exhibit high reactivity for radical species and interlayer interactions.

• Adsorption of formic acid on thin-film CeO₂(111) surfaces is studied using RAIRS and DFT calculations.
• On CeO₂(111), formic acid produces co-adsorbed bidentate formate and hydroxyl around 250 K and tilted formate at 400 K, which is identified to be vacancy-stabilized formate due to water desorption at 300 K.
• Additional formate species are seen on CeO_2-x(111) above 250 K due to formate interacting with different numbers of oxygen vacancies and hydroxyls.

• Adsorption of O₂, O, CO, CO₂, NO, NO₂, and CO/NO with O₂/O on Pt_{1-5, 10} are markedly enhanced vs. Pt(111) and trend toward Pt(111) levels as cluster size increases.
• Deep energy sinks exist along the CO/NO oxidation processes, indicating worse energetics than bulk Pt with smaller size.
• A size threshold around Pt₁₀ may exist, above which CO oxidation proceeds entirely downhill in energy on Pt clusters.
• CO/NO oxidation on more oxidized phases of Pt clusters needs to be better understood.

• Surface phase diagram for O/Pt(111) as a function of temperature and pressure is established based on an extensive set of oxygen adsorption coverages and configurations on Pt(111) calculated using DFT and ab initio thermodynamics.
• This approach explains the maximum of 0.5 ML coverage ambient O₂ produces on Pt(111) and how other oxidants, such as NO₂ and O₃, produce higher O coverages.
• Same approach can be extended to quantify O coverage on Pt(111) in equilibrium with reducible oxides.
• An Ellingham diagram is used to summarize the relationship between surface O coverage and oxidation reaction thermodynamics.

• Pt monolayer and mixed metal Pt monolayer supported on different late transition metals show different oxygen reduction activity from pure Pt.
• When supported on Pd nanoparticles, Pt monolayer shows 2x mass activity (current per Pt+Pd mass) or 10x mass activity (current per Pt mass) for ORR.
• Mixed-metal Pt monolayer (e.g. (Ir_0.2Pt_0.8)ML/Pd) can further increase the mass activity for ORR.
• DFT calculations suggest that the oxophilic elements form surface O/OH species that further destabilizes OH on adjacent Pt sites.

• Adsorption and diffusion of atomic K and O on (1x1) and missing-row reconstructed Rh(110) were studied theoretically.
• K prefers to be located in the trough, while O prefers to be located on the ridge.
• O and K diffusion both favor the [110] direction, with K diffusion being more anisotropic.
• Diffusion barriers for O are in the range of 0.6-0.8 eV while those for K are much lower (ca. 0.1 eV).

• (T, p_O2) phase diagrams and oxygen affinity of Pt_1-3 nanoclusters is calculated theoretically.
• Nano Pt_xO_y oxides persist over a wider range of oxygen chemical potential than bulk Pt.
• Off-stoichiometric phases are predicted compared to stable bulk oxides (PtO and PtO₂).
• Reducibility of nano Pt_xO_y depends on cluster size and composition.

• Rate of CO oxidation on Pt(111) is analyzed using microkinetic modeling as a function of temperature and surface strain.
• Maximum rate occurs at a positive strain value which increases with temperature.
• O₂ dissociation or CO₂ formation is rate-limiting on either side of rate maximum.
• Helps explain periodic mechanical oscillation of a thin Pt crystal during CO oxidation.

• Structures and energies of free Pt_x clusters (x=1-5, 10) were investigated theoretically.
• Minimum energy structures of Pt_xO_x and Pt_xO_2x are one-dimensional and distinct from bulk Pt oxides.
• Oxidation of Pt_x is significantly more exothermic than the formation of bulk Pt oxides.
• Pt oxidation is thus more likely in few-atom clusters than in larger particles, potentially imparting disparate catalytic activities for such clusters.

• Effect of subsurface oxygen on the adsorption of several common atomic and molecular adsorbates on Ag(111) is investigated theoretically.
• 0, 1/4, 1/2, and 1 ML of subsurface oxygen are considered, with significant surface reconstruction found at the higher coverages.
• Subsurface oxygen induces upshift of surface Ag d-band center, which stabilizes adsorptions and enhances the kinetics of H₂, O₂, and NO dissociation.
• Subsurface oxygen may play an important role in industrial reactions catalyzed by Ag.

• DFT calculations were used to investigate the adsorption properties of common atoms, molecules, and fragments on Pt(111), including binding energies, site preferences, geometries, surface deformation, and diffusion barriers, in comparison with experimental results.
• Binding strength follows the order: N₂ < NH₃ < HCN < NO < CO < CH₃ < OH < NH₂ < H < CN < NH < O < HNO < CH₂ < NOH < CNH₂ < N < S < CH < C.
• Atomic species generally prefer fcc threefold sites.
• Decomposition thermochemistry of CO, NO, CH_3, NH₃, NOH, and HNO was analyzed.

• Electrocatalytic ORR was tested on Pt monolayer supported on Au(111), Rh(111), Pd(111), Ru(0001) and Ir(111) in acidic solution.
• A volcano-like dependence of ORR activity on surface d-band center was obtained, with Pt/Pd(111) exhibiting the highest kinetic current density that was even higher than on Pt(111).
• O₂ dissociation and O hydrogenation were used to represent the opposite demands by ORR on the electrode both to dissociate the O-O bond and to remove O species.
• The activation barriers for these two steps formed linear trends with respect to O binding energy; only Pt(111) and Pt_ML/Pd(111) were near the trend line crossing, which represented optimal compromise between the opposite demands.

• To better understand why Pt-base metal alloys are more active for the ORR than pure Pt, adsorption and dissociation of O₂ are investigated on Pt(111), compressed Pt(111), and bulk- and Pt skin-terminated Pt₃Co(111) and Pt₃Fe(111).
• O and O₂ binding energies follow the order of Pt skin < compressed Pt < equilibrium Pt < bulk alloys, and O₂ dissociation is kinetically more hindered on the Pt skins than on Pt.
• These trends are well explained by the d-band model of Hammer and Nørskov, and fit linear energy relations between transition states and atomic oxygen.
• Alleviation of poisoning by O and enhanced rates for reactions involving O may be reasons why Pt skins are more active ORR catalysts in low-temperature fuel cells.

• We demonstrate that high concentration of low-coordinated surface atoms is the main reason for special catalytic properties of gold nanoparticles.
• Survey of literature shows low-temperature CO oxidation activity of Au on different supports follows a consistent trend with Au particle size.
• DFT calculations show that both CO and O adsorb more strongly on Au sites with lower coordination number.
• The underlying electronic factor is upshifted d-band center at low-coordinated Au sites.

• O₂ dissociation on stretched Au(111), un/stretched Au(211) is investigated using DFT.
• O₂ adsorbs weakly on these three surfaces, but not on unstretched Au(111).
• Tensile strain, step edge both facilitate O₂ activation; both upshift d band center.
• 10% stretched step edge can lower O₂ activation barrier from well over 1 eV to 0.6 eV.

• Adsorption of oxygen on stepped Cu(211) is enhanced compared to Cu(111).
• Binding energy of atomic oxygen is more exothermic by ca. 0.2 eV, while that of O₂ is enhanced from ca. -0.6 to ~-1.0 eV.
• Most stable adsorption sites for both O and O₂ are on upper edge of step.
• O₂ dissociation paths over edge of step have comparable activation energies to that on Cu(111), 0.20 eV, while O₂ spontaneously dissociates at foot of step.

• A relation exists between activation energy and stability of intermediates for a range of diatomic dissociation reactions.
• This is demonstrated with DFT calculations of CO, NO, O₂, N₂ dissociation on flat as well as stepped metal surfaces.
• The Brønsted–Evans–Polanyi lines are the result of late transition states and their analogous geometries.
• An optimal adsorption energy range is predicted for catalysts for reactions involving the dissociation of such molecules.

• Molecular O₂ adsorbs in several di-σ configurations on Ir(111), all of which are identified as peroxo states.
• The most stable precursor state is top-bridge-top with a binding energy of -1.27 eV.
• A negligible activation energy (<0.1 eV) is found using the nudged elastic band method.
• O₂ dissociation is highly exothermic.

• O₂ adsorbs in several high-symmetry precursor states on Cu(111) with binding energies of -0.4 ~ -0.6 eV, the most stable of which is  bridge-hollow-bridge.
• Activation activation for O₂ dissociation is ca. 0.2 eV.
• Compressive strain down-shifts Cu(111) d-band center and destabilizes atomic and molecular oxygen states, while tensile strain has the opposite effects.
• d-band center also down-shifts along the O₂ dissociation pathway due to increasing surface-adsorbate interaction from molecular to atomic oxygen state.