Peer-Reviewed Journal Articles

78. Chemical engineering at crossroads, K. Nandakumar,* M. Tyagi, Y. Xu, K.T. Valsaraj, J.B. Joshi, Canadian Journal of Chemical Engineering 100 (2022) 2011-2027.

77. Hydrolysis of acetamide on low-index CeO2 surfaces: Ceria as a deamination and general de-esterification catalyst, S. Bhasker-Ranganath, Y. Xu*, ACS Catalysis 12 (2022) 10222-10234. {open access}

76. Thermal stability and protective properties of phenylphosphonic acid on Cu(111), V. Kalinovych, Md. Saeedur Rahman, L. Piliai, Y. Kosto, S.L. Mehl, T. Skála, I. Matolínová, V. Matolín, K.C. Prince, Y. Xu, N. Tsud*, Applied Surface Science 600 (2022) 154036. {access before 8/23/2022}

  • Molecular adlayer of phenylphosphonic acid on Cu(111) is stable up to ca. 300 °C, in which phenylphosphonate groups adsorb in a tridentate geometry.
  • A PPA adlayer protects Cu(111) from oxidation when exposed to ambient atmosphere.
  • Thermal treatment of a PPA adlayer at 375 °C initiates decomposition through P-O, C-P and C-H bond scission channels all having activation barriers of 1.7–1.8 eV, promoted by condensation and formation of pyrophosphonates or metaphosphates.
  • Reduced phosphorus species (with P-P and C-P bonds) remain on Cu(111) after 500 °C treatment.

75. Oxygen electrochemistry in Li-O2 batteries probed by in situ surface-enhanced Raman spectroscopy, J. Wang*, L. Ma, J. Xu, Y. Xu, K. Sun*, Z. Peng*, SusMat 1 (2021) 345-358. {open access}

74. CoCrFeNi high entropy alloy as enhanced hydrogen evolution catalyst in an acidic solution, F. McKay, Y. Fang, O. Kizilkaya, P. Singh, D.D. Johnson, A. Roy, D.P. Young, P.T. Sprunger, J.C. Flake, W.A. Shelton, Y. Xu*, Journal of Physical Chemistry C, 125 (2021) 17008-17018. {open access}

  • The high entropy alloy CoCrFeNi is predicted computationally to be more active for electrocatalytic hydrogen evolution reaction (HER) than the individual base metals.
  • Theory suggests the active state of the HEA surface to be partially oxidized under typical HER conditions in an acidic solution.
  • The HEA exhibits an overpotential of only 60 mV relative to Pt for HER at a current density of 1 mA/cm2 in 0.5 M H2SO4.
  • X-ray spectroscopies show Ni to be most resistant to oxidation among the four components in the HEA.

73. Elucidating the mechanism of ambient-temperature aldol condensation of acetaldehyde on ceria, S. Bhasker-Ranganath, Md.S. Rahman, C. Zhao, F. Calaza, Z. Wu, Y. Xu*, ACS Catalysis, 11 (2021) 8621-8634. {open access}

  • Based on DRIFTS for ceria nano-octahedra under flow conditions and DFT-based reaction energy profiles and simulated IR spectra, trans-crotonaldehyde is identified as the primary product of aldol condensation of acetaldehyde on CeO2(111).
  • Reaction proceeds via enolization of acetaldehyde and formation of 3-hydroxybutanal, and is limited by the desorption of crotonaldehyde at ambient temperature.
  • Our proposed mechanism implies different kinetic behavior under different conditions that is consistent with experimental observations in the literature.
  • A 1620 cm-1 band is assigned to the enolate of 3-hydroxybutanal stabilized in an oxygen vacancy, which is unable to react further to crotonaldehyde.

72. Theoretical analysis of the adsorption of phosphoric acid and model phosphate monoesters on CeO2(111), S. Bhasker-Ranganath, C. Zhao, Y. Xu*, Surface Science, 705 (2021) 121776.

  • Adsorption of phosphoric acid, methyl phosphate, and para-nitrophenyl phosphate on CeO2(111) is studied theoretically at the GGA+U level.
  • Non-dissociative molecular adsorption occurs with P bonded to lattice O, and the P-O bond strength is affected by entities that reduce Ce4+ to Ce3+, not by Ce3+ alone.
  • Deprotonation of the phosphate species is exothermic, and more so if the molecules are adsorbed in an oxygen vacancy than on an stoichiometric surface.
  • Electronic structure and simulated infrared spectra are analyzed for the adsorbed species.

71. Hydrogen adsorption on ordered and disordered Pt-Ni alloys, S. Zhang, D.D. Johnson, W.A. Shelton, Y. Xu*, Topics in Catalysis, 63 (2020) 714-727.

  • KKR-CPA and plane-wave DFT using large explicit bulk cells agree on lattice constant and electronic structure of disordered PtxNi1-y alloys, which compare well with limited available experimental evidence.
  • Based on ∆Eads of H on (111) facets, ordered vs. disordered PtNi3, PtNi, and Pt3Ni in the L12 structure have similar chemical reactivity.
  • On the bulk-terminated surfaces, ∆Eads is more exothermic with decreasing xPt, while a reverse trend is found for Pt-terminated surfaces due to strain.
  • Eads of atomic H on Pt-Ni alloys spans ~ 0.7 eV; any value in this range may be dialed into via a disordered alloy.

70. Adsorption structure of adenine on cerium oxide, S. Bercha, S. Bhasker-Ranganath, X. Zheng, K. Beranová, M. Vorokhta, R.G. Acres, T. Skála, V. Matolín, K.C. Prince, Y. Xu, N. Tsud*, Applied Surface Science,  530 (2020) 147257.

  • The adsorption geometry of adenine on CeO2(111) is studied by PES, RPES, and NEXAFS, complemented by DFT calculated energetics and geometries.
  • At < 0.1 ML, adenine predominantly adsorbs with its purine ring parallel to surface in hydrogen bonded chains, which desorb at ca. 250 °C. 
  • Beyond 0.1 ML, an additional upright state is detected in which the adenine adsorbs through N9H and N3, and is stable up to 120 °C. 
  • Some type of surface reaction may take place after annealing above 150 °C.

69. Strain effect in palladium nanostructures as nanozymes, Z. Xi, X, Cheng, Z. Gao, M. Wang, T. Cai, M. Muzzio, E. Davidson, O. Chen, Y. Jung, S. Sun, Y. Xu*, X. Xia*, Nano Letters, 20 (2020) 272-277.

  • Pd nanoparticles exhibit peroxidase-like activity toward oxidation of TMB.
  • Pd icosahedra are 2x more active than Pd octahedra with higher levels of twin defects amplifying surface strain field.
  • A reverse Eley-Rideal mechanism is proposed for formation of OH radicals in solution.
  • Microkinetic analysis suggests tensile strain enhances OH formation, and confirms higher surface-averaged activity of Pd icosahedra than Pd octahedra.

68. Bioalcohol production from acidogenic products via a two-step process: A case study of butyric acid to butanol, S.H. Cho, J. Kim, J. Han*, D. Lee, H.J. Kim, Y.T. Kim, X. Cheng, Y. Xu, J. Lee*, E.E. Kwon*, Applied Energy, 252 (2019) 113482.

  • A two-step process for production of butanol from organic waste based on bimetallic Pt alloy catalysts is demonstrated.
  • Butanol can be produced with a 0.231 g/g yield via the two-step process.
  • Minimum selling price of butanol is US$ 3.388 per gallon of gasoline equivalent.

67. CO2 electrochemical reduction at thiolate-modified Au electrodes, Y. Fang, X. Cheng, J. Flake*, Y. Xu*, Catalysis Science & Technology, 9 (2019) 2689-2701. {a 2019 CST HOT Article}

  • Polycrystalline Au modified with 2-phenylethanethiol exhibits 2-fold increase in faradaic efficiency and current density for CO2 reduction to CO.
  • 2-mercaptopropionic acid enhances HER with up to 100% faradaic efficiency.
  • Adsorption of the two thiols does not have a significant impact on total current density vs. blank Au.
  • Ligand-induced reconstruction of Au surfaces is proposed to be the source of altered activity and selectivity for CO2RR.

66. Coupling of acetaldehyde to crotonaldehyde on CeO2-x(111): Bifunctional mechanism and role of oxygen vacancies, C. Zhao, C.L. Watt, P.R.C. Kent, S.H. Overbury, D.R. Mullins, F.C. Calaza*, A. Savara*, Y. Xu*, Journal of Physical Chemistry C, 123 (2019) 8273-8286.

  • First-principles modeling determines that C-C coupling occurs via an enolate intermediate in an oxygen vacancy dimer.
  • A surface oxygen vacancy dimer is disfavored by energetics vs. point vacancies on a blank surface, but the enolate stabilizes it.
  • Partially desorbing the enolate would allow oxygen vacancies to diffuse and form dimers.
  • A double-ramp procedure is devised to realize the self-coupling of acetaldehyde to crotonaldehyde in UHV on CeO2-x(111).

65. Efficiency enhancements of a restricted stochastic search algorithm for locating local and global minima, W.C. McKee*, S.H. Rawal, Y. Xu, Chemical Physics Letters, 725 (2019) 1-7.

  • Filtering candidate structures by minimum bond lengths and bond connectivity significantly improves convergence rates over the Kick method in stochastic search for minima on PES.
  • Instead of kick-and-filter, the Random Assembly Stochastic Search (RASS) method is proposed to assemble random candidate structures that automatically satisfy the new filters.
  • Computational cost for creating a given number of valid candidate structures is far less for RASS than Kick, especially for larger clusters.
  • RASS may be used with more sophisticated global minimum search algorithms that require random structures as input.

64. Reaction pathways for HCN on transition metal surfaces, M. Abdel-Rahman, X. Feng, M. Muir, K. Ghale, Y. Xu, M. Trenary*, Physical Chemistry Chemical Physics, 21 (2019) 5274-5284.

  • HCN adsorption on Pd(111) and Ru(001) is characterized by RAIRS.
  • Upright HCN, aminocarbyne (CNH2) are demonstrated by IR and DFT calculations to be present on Pd(111).
  • HCN is proposed to convert to CNH2 below room temperature on Pd(111) via a novel bi-molecular mechanism that bypasses high-barrier direct C-H bond scission and formation steps.
  • On Ru(001) HCN is decomposed to atomic species on heating without any stable intermediate.

63. Defect engineering activating (boosting) zinc storage capacity of MoS2, W. Xu, C. Sun, K. Zhao, X. Cheng, S.H. Rawal, Y. Xu, Y. Wang*, Energy Storage Materials, 16 (2019) 527-534.

  • MoS2 nanosheets with low and high concentrations of defects are synthesized and investigated as cathodes for Zn ion cells.
  • Theoretical modeling suggests bare edges and S vacancy clusters are active sites for Zn adsorption/intercalation on MoS2-x.
  • Highly defective MoS2-x nanosheets indeed exhibit much greater charge storage capacity.
  • The MoS2-x cathode maintains nearly 90% of the initial capacity after 1,000 cycles at 1,000 mA/g.

62. Theoretical investigation of dephosphorylation of phosphate monoesters on CeO2(111), C. Zhao, Y. Xu*, Catalysis Today, 312 (2018) 141-148.

  • Dephosphorylation of phosphate monoesters on CeO2(111) was studied using DFT.
  • P-O ester bond scission is much more facile than in aqueous-phase hydrolysis.
  • Hydration and desorption of phosphate group are rate-limiting on CeO2(111) in vacuo.
  • Nature of alkoxy group predicted not to affect overall catalytic activity.

61. An experimental and theoretical study of adenine adsorption on Au(111), R.G. Acres, X. Cheng, K. Beranová, S. Bercha, S. Skála, V. Matolín, Y. Xu, K.C. Prince, N. Tsud*, Physical Chemistry Chemical Physics, 20 (2018) 4688-4698. {a 2018 PCCP HOT Article}

  • Adenine adsorption on Au(111) as deposited in vacuum and from aqueous solution was studied using XPS, NEXAFS, and modeled using DFT.
  • When deposited in vacuum, adenine lies parallel to the surface and is bound predominantly by vdW forces without clearly preferred sites, and desorption is mostly complete by 125 °C.
  • When deposited from aqueous solution, adenine adlayer is disordered with tilted molecular plane and bonding through N atoms.
  • We propose that interfacial effects stabilize adenine in upright geometry and allows molecules to assemble into base-stacked chains that persist even after sample is dried.

60. Estimation of electric field effects on the adsorption of molecular superoxide species on Au based on density functional theory, S.H. Rawal, W.C. McKee, Y. Xu*, Physical Chemistry Chemical Physics, 19 (2017) 32626-32635.

  • Stability of molecular superoxide species, key metal-air battery intermediates, is investigated on Au under electric fields.
  • Step edges and explicit DMSO solvent molecules stabilize alkali superoxides (MO2) compared to the Au(111) | vacuum interface.
  • Contribution of electric field to free energy can be significant but readily estimated using a first-order approximation to Stark effect based on dipole moments at zero fields.
  • Anionic superoxide (O2) has a large polarizability although its stability varies slightly in the range of potentials relevant to the discharge of metal-air batteries.

59. Simulated temperature programmed desorption of acetaldehyde on CeO2(111): Evidence for the role of oxygen vacancy and hydrogen transfer, C. Zhao, Y. Xu*, Topics in Catalysis, 60 (2017) 446-458.

  • Simulated TPD based on DFT energetics and microkinetic modeling closely captures major desorption features.
  • Minority intermediates, including vacancy-stabilized ethoxy, ethyleneoxy, and formylmethylene, are proposed that have not been observed experimentally.
  • Surface oxygen vacancies activate the carbonyl bond, thereby facilitating beta C–H bond scission in acetaldehyde.
  • This leads to enolization, intermolecular hydrogen transfer, deoxygenation, and potentially C–C coupling.

58. Mechanistic origin of low polarization in aprotic Na-O2 batteries, S. Ma, W.C. McKee, J. Wang, L. Guo, M. Jansen, Y. Xu*, Z. Peng*, Physical Chemistry Chemical Physics, 19 (2017) 12375-12383.

  • Reactions underlying discharge and charge in a model Na-O2 cell comprised of an Au electrode and aprotic DMSO electrolyte are investigated in detail.
  • SERS identifies only O2 and NaO2 down to 1.5 V vs. Na/Na+ before Na2O2 is formed.
  • Onset of ORR is due to O2 formation, whereas NaO2 formation on Au is controlled by stability of molecular NaO2.
  • Solid NaO2 is much more soluble than Li2O2 in DMSO and be oxidized in bulk or following dissolution into Na+ and O2.

57. Covalent versus localized nature of 4f electrons in ceria: Resonant angle-resolved photoemission spectroscopy and density functional theory, T. Duchoň*, M. Aulická, E.F. Schwier, H. Iwasawa, C. Zhao, Y. Xu, K. Veltruská, K. Shimada, V. Matolín, Physical Review B, 95 (2017) 165124.

  • Whether CeO2 is a mixed valent or covalent compound remains debated.
  • Mixed valent CeO2 would have cation-cation hybridization with partially occupation of localized Ce 4f level, whereas covalent CeO2 would have cation-anion hybridization with electrons tied up in ligand states.
  • ARPES and DFT were used to study the 4f electrons in ceria by comparing in situprepared CeO2 and c-Ce2O3 model surfaces.
  • The 4f admixture into the O 2p levels in CeO2 is covalent in nature, unlike highly localized 4f electrons in c-Ce2O3.

56. A DFT-based method for more accurate adsorption energies: An adaptive sum of energies from RPBE and vdW density functionals, A.J.R. Hensley, K. Ghale, C. Rieg, T. Dang, E.S. Anderst, F. Studt, C.T. Campbell, J.-S. McEwen*, Y. Xu*, Journal of Physical Chemistry C, 121 (2017) 4937-4945. {open access}

  • Catalysis research using DFT needs more accurate adsorption energies for thermodynamic and kinetic modeling.
  • A simple method has been developed to more accurately predict adsorption energies by adaptively combining two DFT functionals developed for strong covalent bonding and vdW bonding, respectively.
  • This method has been tested against 39 accurate experimental adsorption energies on various single-crystal late transition-metal surfaces.
  • It significantly reduces errors of both parent functionals, and performs better than BEEF-vdW.

55. Adsorption of transition metal adatoms on h-BN/Rh(111): Implications for nanocluster self-assembly, W.C. McKee, M.C. Patterson, J.R. Frick, P.T. Sprunger, Y. Xu*, Catalysis Today, 280 (2017) 220-231.

  • Diffusion of Au, Pt, Ag, Pd, Cu, and Ni adatoms on h-BN/Rh(111) is examined via DFT to determine the ability of this 2D nanomesh surface to effect the self-assembly of monodisperse and size-controlled metal nanoparticles.
  • Au and Pt adatoms are predicted to have high diffusion barriers (>1.2 eV) limiting their mobility across the nanomesh surface, whereas Ag, Pd, Cu, and Ni do not.
  • STM verifies that Au and Pt deposited at room temperature form numerous monodisperse, size-controlled nanoparticles on h-BN/Rh(111) whereas Ag fails to do so, consistent with DFT predictions.
  • H-BN/Rh(111) nanomesh is shown to be an effective, though chemically inert surface for driving the self-assembly of Au and Pt nanoparticles that may have novel catalytic properties.

54. Spectroscopic identification of the Au-C bond formation upon electro-reduction of an aryl diazonium salt on gold, L. Guo, L. Ma, Y. Zhang, X. Cheng, Y. Xu, J. Wang, E. Wang*, Z. Peng*, Langmuir, 32 (2016) 11514-11519.

  • Electro-reduction of an aryl diazonium salt on gold can produce organic films more robust than self-assembled monolayers of organic thiols.
  • 4,4′-disulfanediyldibenzenediazonium forms a multilayer of Au−(Ar−S−S−Ar)n on Au, which can be reversibly degraded to a monolayer of Au−Ar−S− by electrochemical cleavage of the S−S bond.
  • In situ SERS and DFT calculations suggest an Au−C bond forms upon electro-reduction of the salt.

53. Amorphous Li2O2: Chemical synthesis and electrochemical properties, Y. Zhang, Q. Cui, X. Zhang, W.C. McKee, Y. Xu, S. Ling, H. Li, G. Zhong, Y. Yang, Z. Peng*, Angewandte Chemie International Edition, 55 (2016) 10717-10721.

  • Amorphous Li2O2 is synthesized via reacting TMAO2 and LiClO4 in acetonitrile, which promotes rapid LiO2 disproportionation.
  • Compared to crystalline Li2O2, amorphous Li2O2 exhibits an increase in both its electronic and ionic conductivities by ~2-3 orders of magnitude.
  • Oxidation potential for amorphous Li2O2 is lower by ~0.5 V than crystalline Li2O2.
  • Amorphous Li2O2 is identified as the desired discharge product of high theoretic energy density Li-air batteries.

52. CO adsorption on Au nanoparticles grown on hexagonal boron nitride/Rh(111), W.C. McKee, M.C. Patterson, D. Huang, J.R. Frick, R.L. Kurtz, P.T. Sprunger, L. Liu*, Y. Xu*, Journal of Physical Chemistry C, 120 (2016) 10909-10918.

  • Au nanoarticles formed on the inert h-BN/Rh(111) surface at liquid nitrogen temperature exhibit enhanced CO binding relative to bulk crystalline Au.
  • Resulting CO states correspond to those on catalytically active Au nanoparticles grown on reactive metal oxides.
  • High Au deposition amounts, or high substrate temperature, leads to bulk-like CO adsorption states.
  • Enhanced CO adsorption likely occurs on highly undercoordinated Au atoms on the perimeters of the nanoparticles.

51. Potential-dependent generation of O2 and LiO2 and their critical roles in O2 reduction to Li2O2 in aprotic Li-O2 batteries, Y. Zhang, X. Zhang, J. Wang, W.C. McKee, Y. Xu, Z. Peng*, Journal of Physical Chemistry C, 120 (2016) 3690-3698.

  • ORR mechanism on an Au electrode in a Li-DMSO electrolyte is investigated in detail.
  • SERS indicates only O2 on Au at the onset potential, followed by bulk LiO2 and Li2O2 at increasingly higher overpotentials.
  • Reaction between O2 and Li+ in DMSO is shown to readily yield Li2O2 solid and O2 gas.
  • Onset potential is controlled by O2 formation on Au, but solution vs. surface formation of Li2O2 is controlled by molecular LiO2 formation on Au.

50. Reversibility of noble metal-catalyzed aprotic Li-O2 batteries, S. Ma, Y. Wu*, J. Wang, Y. Zhang, Y. Zhang, X. Yan, Y. Wei, P. Liu, J. Wang, K. Jiang, S. Fan, Y. Xu, Z. Peng*, Nano Letters, 15 (2015) 8084-8090.

  • Magnetron sputtered Pd- and Ru-catalyzed carbon nanotube cathodes are tested in aprotic Li-O2 cell.
  • All CNT based Li-O2 cells exhibit prolonged cycle life, with the metal-catalyzed cells having high round-trip efficiency initially.
  • Pd- and Ru-CNT have somewhat higher discharge plateau than pure CNT.
  • Charging overpotentials are significantly reduced by the metals, but much more CO2 is produce than pure CNT.

49. Pd-Ir core-shell nanocubes: A type of highly efficient and versatile peroxidase mimic, X. Xia*, J. Zhang, N. Lu, M.J. Kim, K. Ghale, Y. Xu, E. McKenzie, J. Liu, H. Ye, ACS Nano, 9 (2015) 9994-10004.

  • Pd nanocubes clad in ultra-thin Ir skins are effective mimics of the enzyme peroxidase.
  • Activity of Pd@Ir nanocubes significantly exceeds horseradish peroxidase and pure Pd nanocubes.
  • Maximum activity corresponds to PdIr0.062.
  • 3 ML of Ir on Pd exhibits intermediate binding for reaction species between Pd and 1 ML of Ir/Pd.

48. Reconciling the electronic and geometric corrugations of the hexagonal boron nitride and graphene nanomeshes, W.C. McKee, V. Meunier, Y. Xu*, Surface Science, 642 (2015) L16-L19.

  • GGA and vdW functionals predict metal-supported graphene and boron nitride films to have geometric corrugations significantly larger than often thought to be the case.
  • STM simulation based on the highly corrugated structures yields small apparent corrugations that are observed experimentally over a wide range of bias voltages.
  • The disparity is proposed to be due to unequal contributions of low/high-lying atoms to local density of states in the vicinity of the Fermi level.
  • This phenomenon has important implications for potential novel electronic and material applications for certain supported 2D films.

47. Unlocking the energy capabilities of micron-sized LiFePO4, L. Guo, Y. Zhang, J. Wang, L. Ma, S. Ma, Y. Zhang, E. Wang*, Y. Bi, D. Wang, W.C. McKee, Y. Xu, J. Chen, Q.H. Zhang, C.W. Nan, L. Gu, P.G. Bruce*, Z. Peng*, Nature Communications, 6 (2015) 7898.

  • Micron-sized LiFePO4 particles have been successfully applied as a cathode material for Li-ion batteries by forming a uniform conducting polyphenylene coating.
  • A green diazonium chemistry is used without pyrolysis or emission of VOCs.
  • Improved electric and ionic conductivity, high rate capability, excellent cyclability are obtained vs. pristine or Super P-coated LiFePO4 particles.
  • DFT calculations suggest the bonding of phenyl to LixFePO4 to be covalent, and that it is subject to weakening when surface is fully lithiated.

46. Direct detection of the superoxide anion as a stable intermediate in the electro-reduction of oxygen in a non-aqueous electrolyte containing phenol as a proton source, Z. Peng*, Y. Chen, P.G. Bruce, Y. Xu*, Angewandte Chemie International Edition, 54 (2015) 8165-8168.

  • 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.

45. Reactivity and reaction intermediates for acetic acid adsorbed on CeO2(111), F.C. Calaza, T.L. Chen, D.R. Mullins, Y. Xu, S.H. Overbury*, Catalysis Today, 253 (2015) 65-76.

  • 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 CeO2-x(111).

44. Catalytically active Au-O(OH)x– species stabilized by alkali ions on zeolites and mesoporous oxides, M. Yang, S. Li, Y. Wang, J.A. Herron, Y. Xu, L.F. Allard, S. Lee, J. Huang, M. Mavrikakis, M. Flytzani-Stephanopoulos*, Science, 346 (2014) 1498-1501.

  • 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-Nay 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.

43. Energetics of adsorbed CH2 and CH on Pt(111) by calorimetry: The dissociative adsorption of diiodomethane, C.A. Wolcott, I.X. Green, T.L. Silbaugh, Y. Xu, C.T. Campbell*, Journal of Physical Chemistry C, 118 (2014) 29310-29321.

  • 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.

42. Propane ammoxidation over Mo–V–Te–Nb–O M1 phase investigated by DFT: Elementary steps of ammonia adsorption, activation and NH insertion into π-allyl intermediate, J. Yu, Y. Xu, V.V. Guliants*, Topics in Catalysis, 57 (2014) 1145-1151.

  • NHx 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.

41. Synthesis, characterization, and computation of catalysts at the Center for Atomic-Level Catalyst Design, J.J. Spivey*, K.S. Krishna, C.S.S.R. Kumar, K.M. Dooley, J.C. Flake, L.H. Haber, Y. Xu, M.J. Janik, S.B. Sinnott, Y.T. Cheng, T. Liang, D.S. Sholl, T.A. Manz, U. Diebold, G.S. Parkinson, D.A. Bruce, P.E. de Jongh, Journal of Physical Chemistry C, 118 (2014) 20043-20069. {cover image}

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.

40. Propane ammoxidation over Mo-V-Te-Nb-O M1 phase: Density functional theory study of propane oxidative dehydrogenation steps, J. Yu, Y. Xu*, and V.V. Guliants*, Catalysis Today, 238 (2014) 28–34.

  • 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.

39. Formation and stability of dense arrays of Au nanoclusters on hexagonal boron nitride/Rh(111), M.C. Patterson, B.F. Habenicht, R.L. Kurtz, L. Liu, Y. Xu, P.T. Sprunger*, Physical Review B, 89 (2014) 205423.

  • 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.

38. Adsorption energy of tert-butyl on Pt(111) by dissociation of tert-butyl iodide: Calorimetry and DFT, T.L. Silbaugh, J.B. Giorgi, Y. Xu, A. Tillekaratne, F. Zaera, C.T. Campbell*, Journal of Physical Chemistry C, 118 (2014) 427-438.

  • 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.

37. Effects of hydrogen and water on the activity and selectivity of acetic acid hydrogenation on ruthenium, H. Olcay, Y. Xu*, G.W. Huber*, Green Chemistry, 16 (2014) 911-924.

  • 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.

36. Identifying active functionalities on few-layered graphene catalysts for oxidative dehydrogenation of isobutane, G.K.P. Dathar, Y.-T. Tsai, K. Gierszal, Y. Xu, C. Liang, A. Rondinone, S.H. Overbury, V. Schwartz*, ChemSusChem, 7 (2014) 483-491.

  • 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.

35. Adsorption and diffusion of 4d and 5d metal adatoms on graphene moiré/Ru(0001) and the implications for cluster nucleation, B.F. Habenicht, D. Teng, L. Semidey-Flecha, D.S. Sholl, Y. Xu*, Topics in Catalysis, 57 (2014) 69-79.

  • 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.

34. Adsorption and diffusion of the Rh and Au adatom on graphene moiré/Ru(0001), L. Semidey-Flecha, D. Teng, B.F. Habenicht, D.S. Sholl, Y. Xu*, Journal of Chemical Physics, 138 (2013) 184710.

  • 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.

33. High-throughput screening of monometallic catalysts for aqueous-phase hydrogenation of biomass-derived oxygenates,J. Lee, Y. Xu, G.W. Huber*, Applied Catalysis B, 140-141 (2013) 98-107.

  • 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.

32. Graphene moiré structure grown on a pseudomorphic metal overlayer supported on Ru(0001), Z. Zhou, B.F. Habenicht, Q. Guo, Z. Yan, Y. Xu, L. Liu*, D.W. Goodman, Surface Science, 611 (2013) 67-73.

  • 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.

31. Oxygen vacancy-assisted coupling and enolization of acetaldehyde on CeO2(111), F.C. Calaza, Y. Xu, D.R. Mullins, S.H. Overbury*, Journal of the American Chemical Society, 134 (2012) 18034-18045.

  • 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 CeO2-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.

30. A combined HAADF STEM and DFT study of tantalum and niobium locations in the Mo-V-Te-Ta(Nb)-O M1 phases, J. Yu, J. Woo, A. Borisevich, Y. Xu*, V.V. Guliants*, Catalysis Communications, 29 (2012) 68-72.

  • 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.

29. Decomposition of furan on Pd(111), Y. Xu, Topics in Catalysis, 55 (2012) 290-299.

  • Minimum-energy decomposition pathway for furan on clean Pd(111) is mapped.
  • Furan decomposition begins with ring opening at C-O position (Ea = 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.

28. Trends in the catalytic activity of transition metals for the oxygen reduction reaction by lithium, G.K.P. Dathar, W.A. Shelton, Y. Xu*, Journal of Physical Chemistry Letters, 3 (2012) 891-895.

  • 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 ΔEO.
  • Step edge sites are more active than flat terrace sites.
  • Solution-phase O2 reduction to superoxide can mask activity of less reactive metals.

27. Exploring the structure and chemical activity of 2-D gold islands on graphene moiré/Ru(0001), Y. Xu*, L. Semidey-Flecha, L. Liu, Z. Zhou, D.W. Goodman*, Faraday Discussions, 152 (2011) 267-276.

  • 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.

26. Oxygen reduction by lithium on model carbon and oxidized carbon structures, Y. Xu*, W.A. Shelton, Journal of the Electrochemical Society, 158 (2011) A1177-A1184.

  • 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.

25. Propane ammoxidation over the Mo-V-Te-Nb-O M1 phase: Reactivity of surface cations in hydrogen abstraction steps, K. Muthukumar, J. Yu, Y. Xu*, V.V. Guliants*, Topics in Catalysis, 54 (2011) 605-613.

  • Adsorption of C3Hx (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.
  • Ea 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 Ea for propane conversion on V-based oxides.

24. Effect of Pd surface structure on the activation of methyl acetate, L. Xu, Y. Xu*, Catalysis Today, 165 (2011) 96-105.

  • 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.

23. Aqueous-phase hydrogenation of acetic acid over transition metal catalysts, H. Olcay, L. Xu, Y. Xu*, G.W. Huber*, ChemCatChem, 2 (2010) 1420-1424. {cover image; 10 most accessed in 2011}

  • 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.

22. Partial and complete reduction of O2 by hydrogen on transition metal surfaces, D.C. Ford, A.U. Nilekar, Y. Xu*, M. Mavrikakis*, Surface Science, 604 (2010) 1565-1575. {top 10 most cited in Surf. Sci. 2009-2014}

  • 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.

21. O2 reduction by lithium on Au(111) and Pt(111), Y. Xu*, W.A. Shelton, Journal of Chemical Physics, 133 (2010) 024703. {open access}

  • 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 Eo=1.97 V.
  • Lithiation weakens the O-O bond resulting in (LixO) 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.

20. Activation of methyl acetate on Pd(111), L. Xu, Y. Xu*, Surface Science, 604 (2010) 887-892.

  • 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.

19. Adsorption of propane, isopropyl, and hydrogen on cluster models of the M1 phase of Mo-V-Te-Nb-O mixed metal oxide catalyst, A. Govindasamy, K. Muthukumar, J. Yu, Y. Xu*, V.V. Guliants*, Journal of Physical Chemistry C, 114 (2010) 4544-4549.

  • 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.

18. Temperature evolution of structure and bonding of formic acid and formate on fully oxidized and highly reduced CeO2(111), W.O. Gordon, Y. Xu, D.R. Mullins, S.H. Overbury*, Physical Chemistry Chemical Physics, 11 (2009) 11171-11183.

  • 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 CeO2-x(111) above 250 K due to formate interacting with different numbers of oxygen vacancies and hydroxyls.

17. A first-principles investigation of the effect of Pt cluster size on CO and NO oxidation intermediates and energetics, Y. Xu, R.B. Getman, W.A. Shelton, W.F. Schneider*, Physical Chemistry Chemical Physics, 10 (2008) 6009-6018.

  • Adsorption of O2, O, CO, CO2, NO, NO2, and CO/NO with O2/O on Pt1-5, 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.

16. Thermodynamics of environment dependent oxygen adsorption on Pt(111), R.B. Getman, Y. Xu, W.F. Schneider*, Journal of Physical Chemistry C, 112 (2008) 9559-9572. {cover image}

  • 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.

15. Bimetallic and ternary alloys for improved oxygen reduction catalysis, A.U. Nilekar, Y. Xu, J. Zhang, M.B. Vukmirovic, K. Sasaki, R.R. Adzic*, M. Mavrikakis*, Topics in Catalysis, 46 (2007) 276-284.

  • 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. (Ir0.2Pt0.8)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.

14. The effect of co-adsorbed oxygen on the adsorption and diffusion of potassium on Rh(110): A first-principles study, Y. Xu, H. Marbach, R. Imbihl, I.G. Kevrekidis, M. Mavrikakis*, Journal of Physical Chemistry C, 111 (2007) 7446-7455.

  • 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).

13. Thermodynamic equilibrium compositions, structures, and reaction energies of PtxOy (x=1-3) clusters predicted from first principles, Y. Xu, W.A. Shelton, W.F. Schneider*, Journal of Physical Chemistry B, 110 (2006) 16591-16599.

  • (T, pO2) phase diagrams and oxygen affinity of Pt1-3 nanoclusters is calculated theoretically.
  • Nano PtxOy 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 PtxOy depends on cluster size and composition.

12. Lattice strain effects on CO oxidation on Pt(111), L. Grabow, Y. Xu, M. Mavrikakis*, Physical Chemistry Chemical Physics, 8 (2006) 3369-3374. {cover image}

  • 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.

11. Effect of particle size on the oxidizability of platinum clusters, Y. Xu, W.A. Shelton, W.F. Schneider*, Journal of Physical Chemistry A, 110 (2006) 5839-5846.

  • Structures and energies of free Ptx clusters (x=1-5, 10) were investigated theoretically.
  • Minimum energy structures of PtxOx and PtxO2x are one-dimensional and distinct from bulk Pt oxides.
  • Oxidation of Ptx 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.

10. Effect of subsurface oxygen on the reactivity of the Ag(111) surface, Y. Xu, J. Greeley, M. Mavrikakis*, Journal of the American Chemical Society, 127 (2005) 12823-12827.

  • 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.

9. Atomic and molecular adsorption on Pt(111), D.C. Ford, Y. Xu, M. Mavrikakis*, Surface Science, 587 (2005) 159-174.

  • 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, CH3, NH3, NOH, and HNO was analyzed.

8. Controlling the catalytic activity of platinum-monolayer electrocatalysts for oxygen reduction with different substrates, J. Zhang, M.B. Vukmirovic, Y. Xu, M. Mavrikakis*, R.R. Adzic*, Angewandte Chemie International Edition, 44 (2005) 2132-2135.

  • 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 PtML/Pd(111) were near the trend line crossing, which represented optimal compromise between the opposite demands.

7. Adsorption and dissociation of O2 on Pt-Co and Pt-Fe alloys, Y. Xu, A.V. Ruban, M. Mavrikakis*, Journal of the American Chemical Society, 126 (2004) 4717-4725.

  • 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.

6. On the origin of the catalytic activity of gold nanoparticles for low-temperature CO oxidation, N. Lopez, T.V.W. Janssens, B.S. Clausen, Y. Xu, M. Mavrikakis, T. Bligaard, J.K. Nørskov*, Journal of Catalysis, 223 (2004) 232-235.

  • 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.

5. Adsorption and dissociation of O2 on gold surfaces: Effect of steps and strain, Y. Xu, M. Mavrikakis*, Journal of Physical Chemistry B, 107 (2003) 9298-9307.

  • 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.

4. The adsorption and dissociation of O2 molecular precursors on Cu: The effect of steps, Y. Xu, M. Mavrikakis*, Surface Science, 538 (2003) 219-232.

  • 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.

3. Universality in heterogeneous catalysis, J.K. Nørskov*, T. Bligaard, A. Logadottir, S. Bahn, L.B. Hansen, M. Bollinger, H. Bengaard, B. Hammer, Z. Sljivancanin, M. Mavrikakis, Y. Xu, S. Dahl, C.J.H. Jacobsen, Journal of Catalysis, 209 (2002) 275-278.

  • 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.

2. Adsorption and dissociation of O2 on Ir(111), Y. Xu, M. Mavrikakis*, Journal of Chemical Physics, 116 (2002) 10846-10853.

  • 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.

1. Adsorption and dissociation of O2 on Cu(111): Thermochemistry, reaction barrier and the effect of strain, Y. Xu, M. Mavrikakis*, Surface Science, 494 (2001) 131-144.

  • 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.



8. Catalytic Encounters at the Molecular Level: Gabor A. Somorjai Award Symposium for Creative Research in Catalysis in Honor of Professor Manos Mavrikakis, Y. Xu, L.C. Grabow, Topics in Catalysis, 63 (2020) 617.

7. A decade of effort in addressing the grand challenges in catalysis, Y. Lei, Y. Xu, Catalysis Today, 280 (2017) 209.

6. Two-dimensional gold islands on graphene/Ru(0001) moiré structure, Y. Yue, Y. Xu, L. Liu, Annals of Materials Science and Engineering, 1 (2014).

5. Preface, Special Section on Current Topics in Computational Catalysis, Y. Xu, D.A. Bruce, J.J. Spivey, Catalysis Communications, 52 (2014) 59.

4. Graphene moiré supported metal clusters for model catalysis studies, B.F. Habenicht, Y. Xu, L. Liu, in Graphene Chemistry: Theoretical Perspectives, ed. by Z. Chen and D. Jiang. Wiley; 2013.

3. Recent advances in heterogeneous catalysis enabled by first-principles methods, Y. Xu, in RSC Catalysis series, ed. by J.J. Spivey, K.M. Dooley. RSC; v. 21, 2009.

2. Recent developments in the electrocatalysis of the O2 reduction reaction, Y. Xu, M. Shao, M. Mavrikakis, R.R. Adzic, in Fuel Cell Catalysis: A Surface Science Approach; ed. by M.T.M. Koper. Wiley; 2009.

1. Theoretical aspects of oxide particle stability and chemical reactivity, Y. Xu, W.A. Shelton, W.F. Schneider, in Synthesis, Properties, and Applications of Oxide Nanomaterials; ed. by J.A. Rodriguez, M. Fernández-Garcías. Wiley; 2007.

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