J.A. Rodriguez

Physical and chemical properties of bimetallic surfaces

Recent studies dealing with the structural, electronic, chemical and catalytic properties of well-defined bimetallic surfaces are reviewed. LEED and STM show that two metals interacting on a surface can form compounds with structures not seen in bulk alloys. Many novel phenomena related to the kinetics of growth of metals on metals have been discovered. The knowledge gathered in this area provides a solid basis for the synthesis of new materials with applications in areas of catalysis, electro-chemistry and microelectronics. In many cases, the formation of a surface bimetallic bond induces large changes in the band structure of the metals. For surfaces that contain transition or s,p metals, the strongest metal-metal interactions occur in systems that combine a metal with a valence band almost fully occupied and a metal in which the valence band is almost empty. A very good correlation is found between the electronic perturbations in a bimetallic system and its cohesive energy. Bimetallic bonds that display a large stability usually involve a significant redistribution of charge around the metal centers. The electronic perturbations affect the reactivity of the bonded metals toward small molecules (CO, NO, H2, O2, S2, C2H4, CH3OH, etc.). For supported monolayers of Ni, Pd, Pt and Cu a correlation is observed between the shifts in surface core-level binding energies and changes in the desorption temperature of CO from the metal adlayers. Examples are provided which demonstrate the utility of single-crystal studies for understanding the role of “ensemble” and “ligand” effects in bimetallic catalysts.

A systematic density functional theory study of the electronic structure of bulk and (001) surface of transition-metals carbides

A systematic study of the bulk and surface geometrical and electronic properties of a series of transition-metal carbides (TMC with TM = Ti, V, Zr, Nb, Mo, Hf, Ta, and W) by first-principles methods is presented. It is shown that in these materials the chemical bonding is strongly covalent, the cohesive energies being directly related to the bonding-antibonding gap although the shift of the center of the C(2s) band related peak in the density of states with respect to diamond indicates that some metal to carbon charge transfer does also take place. The (001) face of these metal carbides exhibits a noticeable surface rumpling which grows along the series. It is shown that neglecting surface relaxation results in very large errors on the surface energy and work function. The surface formation induces a significant shift of electronic energy levels with respect to the corresponding values in the bulk. The extent and nature of the shift can be understood from simple bonding-antibonding arguments and is enhanced by the structural rippling of this surface.

Electronic and chemical properties of Pt, Pd and Ni in bimetallic surfaces

Abstract The electronic properties of a series of bimetallic surfaces that combine Group-10 elements (Pt, Pd or Ni) with transition and s,p metals have been examined using ab initio self-consistent-field (SCF) calculations and cluster models. By analyzing the results of these theoretical studies together with the results of experimental techniques (photoemission, L-edge X-ray absorption fine structure, work function measurements, CO chemisorption, etc.), one can obtain a general idea of the nature of the bimetallic bond in these systems. A Group-10 adatom in contact with the surface of a s,p or early-transition metal exhibits large perturbations in its electronic and chemical properties. In this type of system, there is an important redistribution of charge that shifts d electrons from around the Group-10 metal into the interface region between the admetal and substrate, producing an accumulation of electrons around the bimetallic bonds. This redistribution of charge affects the stability of the core levels and valence d band of the Group-10 metal. The larger the movement of d electrons from the Group-10 metal toward the admetal-substrate interface, the stronger the bimetallic bond, and the lower the ability of the Group-10 metal to bond CO through π-backdonation. Among the Group-10 metals, Pd shows the strongest electronic and chemical perturbations, while Ni exhibits the weakest (Ni

Reaction of SO2 with Au/CeO2(111) : Importance of O vacancies in the activation of gold

Synchrotron-based high-resolution photoemission was used to study the adsorption and chemistry of SO(2) on AuCeO(2)(111) and AuO(x)CeO(2) surfaces. The heat of adsorption of the molecule on Au nanoparticles supported on stoichiometric CeO(2)(111) was 4-7 kcalmol larger than on Au(111). However, there was negligible dissociation of SO(2) on the AuCeO(2)(111) surfaces. The full decomposition of SO(2) was observed only after introducing O vacancies in the ceria support. AuO(x)CeO(2) surfaces were found to be much less chemically active than AuCeO(2)(111) or AuCeO(2-x)(111) surfaces. The active sites in {Au + AuO(x)}ceria catalysts should involve pure gold nanoparticles in contact with O vacancies.

The interaction of Ni and Fe with sulfur and molybdenum-sulfide surfaces: a TDS, XPS and hydrogen-chemisorption study

Abstract Sulfur multilayers, containing Sn species (n = 2, 4, and 8), are very reactive toward admetals like nickel and iron. Ni and Fe atoms supported on sulfur films at 200–300 K exhibit core-level binding energies and band structures very similar to those of nickel and iron sulfides. In contrast, Ni atoms supported on molybdenum-sulfide surfaces remain in a metallic state. NiMoS and FeMoS films can be generated by heating Ni/Sfilm/Mo(110) and Fe/Sfilm/Mo(110) systems to high temperature. The behavior of the Ni/Sfilm/Mo(110) and Fe/Sfilm/Mo(110) systems indicates that Ni and Fe promote Mo↔S interactions and the subsequent formation of molybdenum sulfides. On TM MoS x and TM/S/Mo(110) surfaces (TM = Ni or Fe), the slow step in the D2,gas + Ssolid → D2Sgas reaction is the dissociation of molecular hydrogen. Ni MoS x and Fe MoS x surfaces interact strongly with atomic hydrogen (D), sorbing this element and forming gaseous hydrogen sulfide. The sorption of D produces uniform changes in the electronic properties of the MoSx substrate, with positive binding energy shifts (0.3–0.4 eV) in the core levels of molybdenum and sulfur. Most of the sorbed hydrogen evolves into gas phase as D2 at temperatures between 350 and 500 K. Trends seen in the hydrodesulfurization activity of NiMoS and FeMoS catalysts are analyzed following our results for the sulfidation of Mo and the hydrogenation of S in NiMoS and FeMoS films.

Reaction of hydrogen with SMo(110) and MoSx films: formation of hydrogen sulfide

Abstract The reaction of hydrogen (H 2 , D 2 , or D) with sulfur multilayers, S Mo (110) , surfaces and MoS x films has been investigated at temperatures between 100 and 400 K. All the surfaces were unreactive toward molecular hydrogen under UHV conditions. However, these systems showed a large reactivity toward atomic hydrogen. As gas-phase hydrogen atoms impinged on the surfaces, gaseous hydrogen sulfide was formed. This reaction was very effective for the removal of sulfur atoms from sulfur multilayers and MoS x films. On MoS x films the 2D(gas) + S(solid) → D 2 S(gas) reaction was 3–4 times slower than on sulfur multilayers, and at least 6 times faster than on S Mo (110) surfaces. A good correlation was found between the rate of formation of gaseous hydrogen sulfide and the stability of the SS or SMo bonds in a surface. The bonding interactions between hydrogen sulfide and S 0.9−0.6 Mo (110) or MoS x were negligible at temperatures above 200 K. Rough MoS x films that exposed unsaturated molybdenum sites were more reactive toward hydrogen sulfide than the sulfur-basal plane of MoS 2 or S 0.9−0.6 Mo (110) surfaces. The behavior of molybdenum sulfide catalysts in hydrogenation and hydrodesulfurization processes is discussed in light of these results.

The interaction of oxygen with TiC(001): photoemission and first-principles studies.

High-resolution photoemission and first-principles density-functional slab calculations were used to study the interaction of oxygen with a TiC(001) surface. Atomic oxygen is present on the TiC(001) substrate after small doses of O(2) at room temperature. A big positive shift (1.5-1.8 eV) was detected for the C 1s core level. These photoemission studies suggest the existence of strong O<-->C interactions. A phenomenon corroborated by the results of first-principles calculations, which show a CTiTi hollow as the most stable site for the adsorption of O. Ti and C atoms are involved in the adsorption and dissociation of the O(2) molecule. In general, the bond between O and the TiC(001) surface contains a large degree of ionic character. The carbide-->O charge transfer is substantial even at high coverages (>0.5 ML) of oxygen. At 500 K and large doses of O(2), oxidation of the carbide surface occurs with the removal of C and formation of titanium oxides. There is an activation barrier for the exchange of Ti-C and Ti-O bonds which is overcome only by the formation of C-C or C-O bonds on the surface. The mechanism for the removal of a C atom as CO gas involves a minimum of two O adatoms, and three O adatoms are required for the formation of CO(2) gas. Due to the high stability of TiC, an O adatom alone cannot induce the generation of a C vacancy in a flat TiC(001) surface.

Structural and electronic properties of PbTiO3, PbZrO3, and PbZr0.5Ti0.5O3: First-principles density-functional studies

Perovskites of the PbZr1−xTixO3 type are among the most important ferroelectric materials and highly active catalysts. The structural and electronic properties of PbTiO3, PbZrO3, and PbZr0.5Ti0.5O3 were examined using first-principles density-functional (DF) calculations with the local-density-approximation (LDA) or the generalized-gradient approximation (GGA, Perdew–Wang and Perdew–Burke–Ernzerhoff functionals). A series of crystal structures were considered for each compound. In several cases, the structural parameters predicted by the GGA functionals were clearly in better agreement with experimental results than the LDA-predicted values, but in qualitative terms the LDA and GGA approaches always predicted similar trends for crystal geometries and differences in thermochemical stability. DF calculations at the LDA level could underestimate the ferroelectric character of PbTiO3 and PbZr1−xTixO3. In the perovskites, the most stable structures belong to tetragonal (PbTiO3), orthorhombic (PbZrO3), and mono...

Reaction of SO2 with pure and metal-doped MgO: Basic principles for the cleavage of S-O bonds

Synchrotron-based high-resolution photoemission, x-ray absorption near-edge spectroscopy, and first-principles density-functional calculations are used to examine the interaction of SO2 with pure and modified surfaces of magnesium oxide. On a MgO(100) single crystal, SO2 reacts with O centers to form SO3 and SO4 species. The bonding interactions with the Mg cations are weak and do not lead to cleavage of S–O bonds. An identical result is found after adsorbing SO2 on pure stoichiometric powders of MgO and other oxides (TiO2, Cr2O3, Fe2O3, NiO, CuO, ZnO, V2O5, CeO2, BaO). In these systems, the occupied cations bands are too stable for effective bonding interactions with the LUMO of SO2. To activate an oxide for S–O bond cleavage, one has to create occupied metal states above the valence band of the oxide. DF calculations predict that in the presence of these “extra” electronic states the adsorption energy of SO2 should increase, and there should be a significant oxide→SO2(LUMO) charge transfer that facilitates the cleavage of the S–O bonds. In this article, we explore three different approaches (formation of O vacancies, promotion with alkali metals, and doping with transition metals) that lead to the activation of SO2 and S–O bond breaking on MgO and oxides in general. Basic principles for a rational design of catalysts with a high efficiency for the destruction of SO2 are presented.Synchrotron-based high-resolution photoemission, x-ray absorption near-edge spectroscopy, and first-principles density-functional calculations are used to examine the interaction of SO2 with pure and modified surfaces of magnesium oxide. On a MgO(100) single crystal, SO2 reacts with O centers to form SO3 and SO4 species. The bonding interactions with the Mg cations are weak and do not lead to cleavage of S–O bonds. An identical result is found after adsorbing SO2 on pure stoichiometric powders of MgO and other oxides (TiO2, Cr2O3, Fe2O3, NiO, CuO, ZnO, V2O5, CeO2, BaO). In these systems, the occupied cations bands are too stable for effective bonding interactions with the LUMO of SO2. To activate an oxide for S–O bond cleavage, one has to create occupied metal states above the valence band of the oxide. DF calculations predict that in the presence of these “extra” electronic states the adsorption energy of SO2 should increase, and there should be a significant oxide→SO2(LUMO) charge transfer that facilita...

Electronic properties of gold on Mo(110): d → s,p charge redistribution and valence band shifts

Abstract The interaction between Au atoms and Mo(110) has been investigated using photoelectron spectroscopy and ab initio self-consistent field calculations. The formation of AuMo bonds induces shifts toward higher binding energy (0.3–0.7 eV) in the core levels and valence d band of gold. This is accompanied by an important redistribution of charge, in which Au loses 5d electrons and gains (6s,6p) electrons. The positive binding-energy shifts in the Au 4f levels and 5d band reflect the effects of a Mo-induced reduction in the Au 5d electron population.