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the surface chemistry of H2S and S2 on metallic Cs and Cs ZnO surfaces has been investigated using high-resolution synchrotron based photoemission and ab initio self-consistent-field calculations. Metallic Cs is very reactive toward H2S and S2 at temperatures between 100 and 300 K. Pure cesium decomposes H2S to form Cs2S compounds. After dosing S2 to Cs, one obtains Cs2S and Cs2S2m (m ≥1 ) compounds. The formation of cesium sulfides induces an increase in the intensity of the Cs 3d levels and large negative shifts (0.8–1.3 eV) in their peak positions. Cesium atoms supported on ZnO are in an ionic state (Csδ+), but they are still able to interact with H2S and S2 more strongly than Zn and O sites of the oxide support. A correlation is found between the electron density on the Cs adatoms and their reactivity: Cs atoms supported on Zn sites of the oxide bond S-containing species (H2S, HS, S2, S) are stronger than Cs atoms supported on O sites. H2S dissociates into HS and atomic S upon adsorption on Cs ZnO surfaces at 300 K. The HS species decompose at temperatures below 450 K leaving S atoms that are bonded to Cs and Zn. The adsorption of S2 on Cs ZnO surfaces at 300 K leads to the formation of Cs2S and Cs2S2m (m ≥ 1) compounds. Cs↔S interactions increase the thermal stability of cesium on the ZnO surface. The poisoning of Cs/Cu/ZnO catalysts is discussed in light of these results and those previously reported for the S2/Cu/ZnO system.

The interaction of O2 with Zn, Cs Zn and Cs CuZn surfaces was investigated using photoemission and ab initio self-consistent-field (SCF) calculations. On zinc films, the sticking probability of O2 is extremely low (10−3–10−2), and O2 exposures in the range of 103 to 104 langmuirs are necessary to produce a significant adsorption of oxygen and the transformation of metallic zinc into zinc oxide. The presence of sub monolayer coverages of cesium enhances the oxidation rate of zinc by 2–3 orders of magnitude. In the Cs Zn system, the alkali atom donates electrons to zinc. This charge transfer facilitates the formation of Zn→O2 dative bonds and breaking of the OO bond. For the coadsorption of Cs and O2 on Zn(001), the larger the electron transfer from Zn into the O2 (1πg) orbitals, the bigger the adsorption energy of the molecule and the elongation of the OO bond. In general, cesium does not promote the oxidation of copper. In the Cs CuZn system, copper withdraws electrons from zinc. The presence of copper in the Cs CuZn system inhibits the oxidation of the Zn component compared with the Cs Zn system by lowering the electron density on the Zn atoms. After exposing the Cs CuZn system to O2, zinc is oxidized at a rate that is larger than that found for clean CuZn surfaces and smaller than seen in Cs Zn surfaces. Molecular hydrogen is found to have no effect on oxidized Cu, Zn and CuZn films. However, atomic hydrogen reduces ZnO to metallic zinc and CuO to Cu2O. In the oxidized CuZn alloy, CuO is reduced first followed by the reduction of ZnO. A comparison of the behavior of O2/Cs/Zn and H2O/Cs/Zn systems shows that while O2 causes severe oxidation of Cs promoted Zn surfaces, H2O has little or no effect.

The bonding interactions between Cu and Al are much stronger than those between Cu and Al2O3. Cu atoms supported on alumina show a narrow 3d band with a centroid shifted ∼0.35 eV with respect to that of the 3d band in bulk metallic Cu. In contrast, Cu atoms deposited on aluminum exhibit shifts of 1.3–1.6 eV in the centroid of the 3d band. Similar differences are observed when comparing the behavior of Ag and Pt overlayers on alumina and aluminum. The d band shifts on the oxide substrate are in the order of 0.3–0.4 eV, whereas on the metal substrate they vary from 0.8 to 2.0 eV. These trends are explained in terms of a simple model that takes into account changes in the energy of the Al(3s, 3p) bands when going from metallic aluminum to alumina. The sticking coefficient of S2 on alumina surfaces is at least one order of magnitude smaller than on aluminum, a difference that also reflects variations in the position of the Al(3s,3p) bands. Submonolayer coverages of Cu do not produce significant changes in the electronic properties of Al2O3. In contrast, the deposition of small amounts of sulfur (∼0.1 ML) induces a substantial reduction (0.4–0.5 eV) in the binding energies of the O KVV, O 1s and Al 2p features of alumina. This is consistent with a transfer of electrons from alumina into the S atoms that produces a transformation similar to a change from n-type to p-type semiconductors. The reactivity of Cu Al 2 O 3 surfaces toward sulfur is much larger than that of pure Al2O3 surfaces. Cu clusters supported on alumina react with S2 to form CuSx compounds that decompose at temperatures between 850 and 1100 K.

The nature of the metal-metal interactions in a series of Pd Rh (111) surfaces has been examined using thermal desorption mass spectroscopy, core- and valence-level photoemission, CO chemisorption, and ab initio self-consistent field calculations. A monolayer of Pd supported on Rh(111) exhibits a Pd 3 d 5 2 binding energy ∼0.2 eV higher than that of the surface atoms of Pd(100), and desorbs at ∼1390 K. The desorption temperature of CO from this Pd Rh (111) system is 50–70 K lower than those seen on Pd(111) and Rh(111), indicating a weakening of 3–5 kcal mol−1 in the strength of the CO adsorption bond. These results are compared with data reported in the literature for bimetallic surfaces that contain Pd and Rh, and general trends are explained in terms of a simple model.

Exposure of a clean Mo(110) surface to S2 at room temperature or at 700 K results in a chemisorbed layer of sulfur only, without any evidence for the formation of molybdenum sulfides. The effects of Fe, Pt, and Al on the reactivity of Mo(110) toward sulfur have been studied using thermal desorption spectroscopy (TDS), X-ray photoelectron spectroscopy (XPS) and X-ray excited Auger electron spectroscopy (XAES). It is found that at room temperature, the reactivity of S2 with multilayers of the admetal follows the order Fe > Al ⪢ Pt. While most of the Fe becomes sulfidized, only a few layers of the Al become sulfidized before passivation, and the least reactive Pt surface is covered by a chemisorbed layer of sulfur. Upon exposure of the FeMo(110) (θFe

The properties of a series of X Mo (110) and S/X/Mo(110) surfaces (X = Co or Ni) have been examined using photoemission, thermal desorption spectroscopy and ab initio SCF calculations. The bimetallic bonds in the Co Mo (110) and Ni Mo (110) systems are complex, involving Co(3d,4s)→Mo(5s,5p) and Ni(3d,4s)→Mo(5s,5p) electron transfers and a Mo(4d)→Mo(5s,5p) rehybridization. These redistributions of charge lead to positive core-level shifts for all the metals. The exposure of Mo(110) to large amounts of S 2 gas produces only a chemisorbed layer of sulfur, without forming molybdenum sulfides. The sulfidation of Mo occurs after exposing Co Mo (110) and Ni Mo (110) surfaces to S 2 . Co and Ni promote the formation of molybdenum sulfides by transferring charge to Mo (favoring in this way an electrophilic attack of S on Mo), and by changing the structure of the surface (making it easier for the penetration of S into the bulk of the sample). Co and Ni exhibit a unique ability to enhance the Mo↔S interactions. A comparison of the behavior of several admetals on S Mo (110) surfaces indicates that the “promotional effect” of an admetal on the sulfidation of Mo increases in the following order: Ag ≈ Zn TMS y MoS 2 catalysts (TM = Zn, Cu, Co or Ni) in hydrodesulfurization (HDS) reactions and the trends found for the sulfidation of Mo in S/TM/Mo(110) surfaces. Systems that contain Co and Ni display the largest HDS activity and the strongest Mo↔S interactions.

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.

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

The coadsorption of Zn and S on Mo(110) has been investigated using TDS, XPS and XAES. Zn atoms supported on clean Mo(110) desorb at 670 (first layer), 510 (second layer) and 480 K (multilayer). The MoZn bond is considerably stronger (∼ 10 kcal/mol) than the ZnZn bond. The strong Mo ↔ Zn interaction leads to shifts of ∼ 0.35 eV toward lower binding energy in the Zn 3d and 2p levels, and in the Zn L3M45M45 Auger transition. At submonolayer coverages (θS + θZn 1 ML) at 300 K produces zinc sulfides and chemisorbed sulfur, without forming molybdenum sulfides. At 600–700 K, zinc promotes the formation of molybdenum sulfides by favoring the migration of sulfur from the surface into the lattice of the Mo substrate. In the ZnxS/MoyS/Mo(110) systems, the zinc sulfides decompose from 750–950 K, whereas the molybdenum sulfides dissociate at much higher temperatures (1200–1350 K).

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.

The interaction of Pd and Au with Al(111) has been studied using ab initio SCF calculations and cluster models. The bonding mechanism ofPd and Au on Al(111) involves electron transfer from the valence d orbitals of the admetals toward the substrate, and a compensating charge transfer from the substrate into the valence (s,p) orbitals of the admetals. Pd behaves as a net electron donor (Pdδ+, δ≈0.25e), while Au acts as a net electron acceptor (Auδ−, δ≈0.10e). In spite of this difference in the net direction of charge transfer, both admetals show shifts toward higher binding energy in their d bands, as a consequence of losses in the d-electron population. The electronic perturbations observed after bonding Pd to Al are as large as those found for Pd bonded to early-transition metals, and much bigger than those found when Pd is bonded to late-transition metals. Similar trends are seen for Au adatoms. In general, the reduction in the Pd(4d) or Au(5d) population increases when the fraction of empty states in the valence band of the substrate rises. Changes in the electronic and chemical properties of Pd overlayers (positive binding-energy shifts in the core and valence levels of the admetal, decrease in the work function of the substrate, reduction in the CO-desorption temperature from Pd) can be explained in terms of a simple model that involves Pd(4d) → substrate charge transfer and Pd(4d) → Pd (5s,5p) rehybridization.

The properties of Ag-Zn alloy films on Ru(001) have been examined using thermal desorption spectroscopy and photoemission. The results were compared to those previously reported for Cu-Zn and Au-Zn alloys on Ru(001). For the Zn-noble-metal alloys, the mechanism of decomposition was always the same: evolution of Zn into gas phase, with the noble metal remaining solid. In thick alloys with a low concentration of Zn, the decomposition temperature of Ag-Zn (600 K) was lower than that of Cu-Zn (640 K) or Au-Zn (670 K). Alloy formation induced only small shifts ( −0.2 to −0.3 eV) in the position of the Zn 2p, 3s and 3d levels. In contrast, the core and valence levels of the noble metals showed large shifts toward higher binding energy. For small amounts of Cu, Ag and Au dissolved in Zn multilayers, the shifts in the noble-metal core levels follow the sequence: Ag(3d52), 0.72 eV < Cu(2p32), 0.85 eV < Au(4f72), 1.40 eV. These shifts reflect a nd → (n + l)s,p rehybridization in the valence levels of the noble metal induced by the formation of bonds with Zn. Two-dimensional (2D) alloys of Zn and a noble metal in contact with Ru(001), θZn + θNM ⩽1, show Zn-Ru and Zn-NM bonds more stable than the corresponding bonds in ZnRu(001) or thick three-dimensional (3D) alloys. This phenomenon is caused by synergistic interactions that involve the Ru-Zn, Ru-NM and Zn-NM bonds. The strength of these interactions depends on the ability of the noble metal to form strong metal-metal bonds: Ag < Cu < Au. The supported 2D alloys show noble-metal core levels shifted toward lower binding energy with respect to those of thick 3D alloys. These shifts and the large thermal stability of the supported 2D alloys can be attributed to a cooperative charge transfer from Ru to Zn to the noble metal.

The interaction of a Pd atom with Ti(001), Ru(001) and Pd(111) surfaces has been studied using semiempirical MO-SCF calculations (INDO/1) and cluster models. In addition, the electronic properties of the diatomic PdTi, PdRu and Pd 2 molecules have been examined using ab initio SCF calculations. The results of the calculations indicate that the charge transfer in the Pd-Ti and Pd-Ru bonds is small. For supported Pd monolayers, the Pd-substrate bonds can be described as mainly metallic, with a small degree of ionic character. Adsorption of Pd on an early transition metal induces a reduction in the electron population of the Pd(4d) orbitals by: (1) charge transfer from Pd to the metal substrate, and (2) rehybridization of the Pd(4d,5s,5p) levels. The magnitude of both phenomena increases when the fraction of empty orbitals in the valence band of the metal substrate rises. The Pd(4d) → Pd(5s,5p) electron transfer plays an important role in the strength of the bimetallic bonds: the larger this rehybridization, the stronger the Pd-substrate bond. Electronic perturbations induced by Ti or Ru on Pd reduce the CO-chemisorption ability of Pd by weakening simultaneously the Pd(4d)-CO(2π ∗ ) and Pd(5s,5p)-CO(5σ) bonding interactions. For adsorption of CO on supported Pd, the π back-donation and strength of the Pd-CO bond decrease when the fraction of empty states in the valence band of the support increases.

The interaction between Ag and Ru(001) has been studied using TDS, XPS and XAES. Submonolayer coverages of Ag desorbed in the temperature range from 900 to 1050 K. The desorption of the Ag monolayer followed zero-order kinetics with an activation energy of 65 kcal/mol. By comparing the desorption temperatures of Cu, Ag and Au from W(110) and Ru(001), a direct relation was found between the bulk cohesive energy of a noble metal and its adsorption energy on a transition metal. A monolayer of Ag on Ru(001) shows Ag 3d levels and Auger MVV transitions that are shifted ∼ 0.1 eV toward lower binding energy with respect to those of the bulk atoms in pure Ag. In CO/Ag/Ru(001) surfaces, the interaction between Ag and Ru enhances the strength of the AgCO bond and weakens the RuCO bond. The CO-desorption temperature from one monolayer of Ag supported on Ru(001) is ∼ 90 K higher than that found on Ag(111). This corresponds to an increase of ∼ 5 kcal/mol in the strength of the AgCO bond. The results of CO-TDS and O(1s)-XPS show that the presence of Ag induces a reduction in the charge transfer from Ru into the CO (2π ∗ ) orbitals, which weakens the RuCO bond by ∼ 6 kcal/mol and shifts the O(1s) peak position 0.4 eV toward higher binding energy. Oxygen molecules adsorbed on Ag atoms bonded to Ru(001) exhibit non-equivalent oxygen atoms (probably a “superoxo-like” species), a desorption temperature of 230–240 K and a very low probability for dissociation (0.05–0.15 at T

The bonding of acetate (CH 3 COO), methoxy (CH 3 O), thiomethoxy (CH 3 S) and pyridine (C 5 H 5 N) to copper surfaces has been examined employing semi-empirical MO-SCF calculations (INDO/S) and metal clusters of limited size (Cu n , n = 16 or 18 atoms). CH 3 COO, CH 3 O and CH 3 S behave as electron acceptors when adsorbed. For these species, the chemisorption bond is dominated by the interaction between the LUMO of the adsorbate and the Cu(4s, 4p) bands. The relatively weak CS bond in CH 3 S a makes decomposition to form sulfur adatoms and alkanes a very exothermic process (− ΔH = 20 to 30 kcal / mol ). In contrast, similar types of decomposition reactions for CH 3 O a are almost thermoneutral (as a consequence of a strong CO bond), and the molecule prefers to decompose forming H 2 CO a and H a species. The results of a thermochemical analysis indicate that reactions which involve the cleavage of SH and/or CS bonds of alkanethiols are very exothermic on copper. The bonding mechanism of pyridine involves a large charge transfer from the 7a 1 , and 2b 1 orbitals of the molecule into the Cu(4s, 4p) orbitals, and a very small electron transfer from the substrate into the CN antibonding 3b 1 orbital of the adsorbate (π-backbonding). The fact that Cu is poor at π-backdonation makes the metal inactive for pyridine decomposition. On the basis of these INDO/S results, the possible UPS spectra of CH 3 O and CH 3 S on Cu(111), and of CH 3 COO and C 5 H 5 N on Cu(110) are discussed and compared with experimental results.

Infrared reflection-absorption spectroscopy (IRAS) has been used to study the adsorption of carbon monoxide on a Cu(100) surface. Adsorption isotherms were determined at CO pressures from 10−6 to 10 Torr, and at temperatures from 115 to 340 K, and the isosteric heats of adsorption (δEads) evaluated as a function of CO coverage. For increasing CO coverages between 0-0.15 monolayers (ML), δEads decreases sharply from 16.7 to 12.7 kcal/mol. From 0.15 to 0.35 ML, δEads remains approximately 12.7 kcal/mol and exhibits little coverage dependence. These results are in excellent agreement with previously reported data for the CO/Cu(100) system acquired at much lower pressures ( 10−4, significant bathochromic shifts of the CO frequency to lower wavenumbers are observed.

The interaction between ammonia and Ru(0001) has been studied by means of Fourier-transform infrared reflection absorption spectroscopy (FT-IRAS). Chemisorption of NH 3 on Ru(0001) enhances the IR cross section of the umbrella mode of the molecule. For the first adsorption layer, changes in the IR intensity of the umbrella mode correlate with variations in the orientation of the molecules observed in ESDIAD and work function measurements. Formation of hydrogen bonds between first- and second-layer NH 3 molecules reduces drastically the IR cross section of the umbrella mode of chemisorbed ammonia.

Studies dealing with high-pressure catalytic reactions over single-crystal surfaces are reviewed. The coupling of an apparatus for the measurement of reaction kinetics at elevated pressures with an ultrahigh vacuum system for surface analysis allows detailed study of structure sensitivity, the effects of promoters and inhibitors on catalytic activity, and, in certain cases, identification of reaction intermediates by post-reaction surface analysis. Examples are provided which demonstrate the relevance of single-crystal studies for modeling the behaviour of high-surface-area supported catalysts. Studies of CO methanation and CO oxidation over single-crystal surfaces provide convincing evidence that these reactions are structure insensitive. For structure-sensitive reactions (ammonia synthesis, alkane hydrogenolysis, alkane isomerization, water-gas shift reaction, etc.) model single-crystal studies allow correlations to be established between surface structure and catalytic activity. The effects of both electronegative (S and P) and electropositive (alkali metals) impurities upon the catalytic activity of metal single crystals for ammonia synthesis, CO methanation, alkane hydrogenolysis, ethylene epoxidation and water-gas shift are discussed. The roles of “ensemble” and “ligand” effects in bimetallic catalysts are examined in light of data obtained using surfaces prepared by vapor-depositing one metal onto a crystal face of a dissimilar metal.

The electronic properties of ultrathin films of Ni on Ru(OO01) and Mo(ll0) have been studied using X-ray photoelectron spectroscopy (XPS). The effects of Ni coverage (film thickness) are investigated for both substrates as is the effect of surface annealing temperature for the Ru(0001) substrate. In addition the effects of CO and H, chemisorption on Ni-covered Mo(ll0) and CO chemisorption on Ni-covered Ru(0001) are examined. The results indicate that the atoms in 1 ML of Ni on Mo(ll0) are electronically perturbed with respect to Ni{lOO) surface atoms, while the electronic perturbation for a monolayer of Ni on Ru(OOO1) is burns. There is qualitative agreement between the shifts measured in the core-level binding energies and the corresponding CO desorption temperatures. The shifts can be explained by: (1) variations in the Ni-Ni interactions caused by a change in geometry of Ni surface atoms on Ru(OOO1) or Mo(ll0) as compared to Ni(lOO), and (2) the effects of Ni-Ru and Ni-Mo interactions. The adsorption of CO and H, induces a decrease in the electron density of the Ni adlayers.

The electronic and chemical properties of ultrathin films of Cu on Re(0001) have been investigated by means of X-ray photoelectron spectroscopy (XPS) and chemisorption of CO, H 2 , O 2 and CO 2 . A very similar Cu(2p 3 2 ) binding energy is obtained for a monolayer of Cu on Re(0001) and the surface atoms of Cu(100). Measurements of the Cu(2p 3 2 ) XPS peak position of Cu/Re(0001) as a function of film thickness show convergence to bulk properties for films with 4–5 layers of Cu atoms. Chemisorption of CO induces a large decrease in the electron density of the Cu adlayers. This is a consequence of: (1) charge transfer from the Cu overlayer to the Re(0001) substrate (induced by a repulsive interaction between the Cu σ charge and the electrons in the 5σ orbital of CO) and (2) transfer of electrons from the Cu adatoms into the unoccupied 2π orbitais of the CO molecules (π back-bonding). The CuRe interaction enhances the electron donor capabilities of Cu atoms supported on Re(0001), making them more active for CO adsorption and CO 2 dissociation. Hydrogen adatoms shift the Cu(2p 3 2 ) XPS peak position of Cu/Re(0001) surfaces toward higher binding energies. The present results indicate that spillover of hydrogen from Re to Cu can occur in mixed Cu/Re catalysts.

The interaction of ultrathin films of Ni and Pd with W(110) has been examined using X-ray photoelectron spectroscopy (XPS) and the effects of annealing temperature and adsorbate coverage (film thickness) are investigated. The XPS data show that the atoms in a monolayer of Pd or Ni supported on W(110) are electronically perturbed with respect to the surface atoms of Pd(100) and Ni(100). The magnitude of the electronic perturbations is larger for Pd than for Ni adatoms. Our results indicate that the difference in Pd (3 d 5 2 ) XPS binding energies between a pseudomorphic monolayer of Pd on W(110) and the surface atoms of Pd(100) correlates with the variations observed for the desorption temperature of CO (i.e., the strength of the PdCO bond) on these surfaces. A similar correlation is seen for the Ni (2 p 3 2 ) XPS binding energies of Ni/W(110) and Ni(100) and the CO desorption temperatures from the surfaces. The shifts in XPS binding energies and CO desorption temperatures can be explained in terms of: (1) variations that occur in the NiNi and PdPd interactions when Ni and Pd adopt the lattice parameters of W(110) in a pseudomorphic adlayer; and (2) transfer of electron density from the metal overlayer to the W(110) substrate upon adsorption. Measurements of the Pd (3 d 5 2 ) XP binding energy of Pd/W(110) as a function of film thickness indicate that the PdW interaction affects the electronic properties of several layers of Pd atoms.

The adsorption and reaction of formic acid on a clean Cu(110) surface and Cu(110) surfaces covered with oxygen, cesium, and oxygen + cesium have been studied with thermal desorption mass spectroscopy (TDS) and X-ray photoelectron spectroscopy (XPS). Formic acid adsorbs molecularly on clean Cu(110) at 110 K, but largely decomposes on the surface to yield adsorbed formate (HCOOa) and Ha, at T ⩽ 270 K (possibly as low as 200 K). The Ha desorbs as H2 at ~ 300 K. The HCOOa decomposes to yield H2 and CO2 at 475 K. Preadsorbed oxygen abstracts the acid hydrogen of HCOOH more efficiently than clean Cu, so formate production increases. At high θo, evidence is seen for formate disproportionation (2HCOOa→ HCOOH + CO2), probably via OHa. Adsorbed cesium also increases formate production. At low coverages, cesium accelerates HCOOa decomposition. Above 1 2 monolayer, a new formate decomposition peak at 530 K is attributed to a new, more stable formate which is directly bonded to Cs. At high θCs, surface carbonate (Cs · CO3,a) is formed from the decomposition products of HCOOa. This carbonate decomposes at ~ 650 K. When oxygen and cesium are coadsorbed, their effects on HCOOHa and HCOOa are largely a sum of their separate effects, although Cs-stabilized surface hydroxyl species are also produced with the combination.

The bonding of pyrazine (C 4 H 4 N 2 ), hydrogen sulfide (H 2 S) and thiophene (C 4 H 4 S) to copper surfaces has been examined employing semi-empirical MO-SCF calculations (INDO/S) and metal clusters of limited size (Cu n n = 14, 16, 17 and 18). The C 4 H 4 N 2 Cu, H 2 SCu and C 4 H 4 SCu adsorption bonds are mainly a product of the interaction of the 4s and 4p orbitals of the substrate with the LUMO and the sulfur or nitrogen lone pairs of the adsorbate. Based on these INDO/S results, the UPS spectra of pyrazine, H 2 S and thiophene on copper surfaces are discussed and compared with experimental results. For each adsorbed molecule, charge transfers important to the change in work function are presented. The effects of chemisorption upon the CN, CC and CH bonds of pyrazine, the CS, CC and CH bonds of thiophene, and the SH bonds of H 2 S are examined. The large reactivity of H 2 S on copper surfaces is explained in terms of a thermochemical analysis.

The bonding of nitrogen dioxide (NO2), nitrate (NO3) and sulfate (SO4) to Ag(110) has been examined employing semi-empirical MO-SCF calculations (INDO/S) and metal clusters of limited size (Agn, n = 17,18). NO2 and NO3 appear as net electron acceptors (π-donors, σ-acceptors) when adsorbed on a-top and bridge sites of Ag(110). The NO2Ag(110) bond is mainly a product of the interaction between the 6a1, 1a2 and 4b2 orbitals of nitrogen dioxide and the Ag(5s,5p) orbitals. The possible effects of coadsorbed electron-transferring species on the surface chemistry of NO2 are analyzed in light of the present description of the NO2-Ag(110) chemisorption bond. The bonding mechanism of NO3 to Ag(110) involves transfer of electrons from the NO3(4e′, 1e″) orbitals into the orbitals of the surface and charge transfer from the substrate into the NO3(1a'2) orbital. The bond order indices indicate that from the viewpoint of the substrate the Ag(5s,5p) orbitals are primarily responsible for the chemisorption of NO2, NO3 and SO4 on Ag(110). An increase in the work function of this surface upon adsorption of NO2, NO3 and SO4 is predicted. The influence of chemisorption effects on the NO bonds of NO2 and NO3, and on the SO bonds of SO4 is examined. On the basis of these INDO/S results, the possible UPS spectra for NO2, NO3 and SO4 adsorbed on Ag(110) are discussed.

The bonding of nitric oxide (NO), pyridine (C5H5N) and sulfur dioxide (SO2) to silver surfaces has been examined employing semi-empirical MOSCF calculations (INDO/S) and metal clusters of limited size (Agn, n = 10, 13, 14, 15). Pyridine is a σ- and π-donor of electrons when adsorbed on a-top and bridge sites of Ag(100) and Ag(111). The C5H5NAg adsorption bond is mainly a product of the interaction between the occupied 7a1 and 2b1 Orbitals of pyridine and the Ag(5s, 5p) orbitals. On silver surfaces, NO and SO2 are net electron acceptors (σ-donors, π-acceptors). The bonding mechanism of these molecules to Ag surfaces is dominated by the interaction between the LUMO of the adsorbate (2π orbitals in NO; 3b1 orbital in SO2) and the 5s and 5p orbitals of the substrate. For each adsorbed molecule, charge transfers and dipole moments important to the change in work function are presented. The effects of chemisorption upon the CN, CC and CH bonds of pyridine, the NO bond of NO and the SO bonds of SO examined. Based on these INDO/S results, the UPS spectra of NO, pyridine and SO2 on silver surfaces and the IPS spectrum of pyridine on Ag(111) are discussed and compared with experimental results. The possible effects of electronic changes in the metal surface upon the adsorbed NO and SO2 species are analyzed in light of the molecular-orbital description of the chemisorption bonds.

The bonding of thiophene (C 4 H 4 S), sulfhydryl (HS), thiomethoxy (CH 3 S) and phenyl thiolate (C 6 H 5 S) to molybdenum surfaces has been examined employing MO-SCF calculations (INDO/1) and metal clusters of limited size (Mo n , n =13−19 atoms ). The calculations indicate that the preferred binding sites for the RS species on Mo(100) are the four-fold hollows. The bonding mechanism of thiophene involves a large charge transfer from the S-lone pairs of the molecule (6a 1 and 2b 1 orbitals) into the surface, and electron transfer from the substrate into the C-S antibonding 3b 1 orbital of the adsorbate. Adsorption of thiophene on a hollow site leads to a large weakening in the strength of the C-S and C-H bonds, producing a precursor for the dissociation of the molecule. HS, CH 3 S and C 6 H 5 S behave as electron acceptors when bonded to Mo. For these species, the chemisorption bond is dominated by the interaction between the LUMO of the adsorbate and the Mo(4d,5s) bands. The results of a thermochemical analysis indicate that reactions which involve the cleavage of H-S and C-S bonds of alkanethiols on Mo (RSH g → S a + RH g ) are very exothermic (− ΔH = 40–50 kcal/mol). The thermodynamics suggests that C-S bond breaking should be the most difficult step in the desulfurization process. The hydrogenolysis of the C-S bond (RS a + H a → S a + RH g ) is ∼ 12 kcal/mol more exothermic for CH 3 S than for C 6 H 5 S.

The chemisorption of CN on the (111), (100), and (110) surfaces of Cu has been studied with semi-empirical quantum-mechanical calculations (INDO/2 and INDO/S) of the interaction of CN with Cu clusters. Only perpendicular bonding geometries with the carbon atom down are considered here. The results indicate that the CN chemisorption bond is formed by net π-donation of electrons from CN to the metal and net σ-backdonation of electrons from the metal to CN. The bond between CN and the copper surface is very ionic, and energetically it is dominated by the transfer of electron density from the metal to CN. For Cu(100) and Cu(110), CN adsorption on bridge sites appears as more stable than on top sites. Upon CN adsorption, it is found that the C-N bond order increases on top sites and decreases on bridge sites (relative to the free CN radical). The total bond-order index between adsorbed CN and the clusters is larger on bridge sites than on top sites. Peak assignments for the ultraviolet photoemission spectrum of CN on Cu(110) and for the energy loss spectrum of CN on Cu(111) and on Cu(100) are discussed based on these results. The possible effects of electronic changes in the metal surface upon the adsorbed CN species are analysed in light of the molecular-orbital description of the chemisorption bond.

The chemisorption of H 2 O, H 2 CO and NH 3 on Cu surfaces and of H 2 O on ZnO(0001) is examined employing semiempirical quantum-chemical calculations (INDO/S) and clusters of limited size ( Cu n , n = 14, 16, 18; Zn m O m , m = 10). The present results combined with our previous results for H 2 CO and NH 3 on ZnO(0001) indicate that upon adsorption on Cu surfaces and on ZnO(0001), water, formaldehyde and ammonia are electron donors. Most of the transfer of charge from the molecules toward the surfaces is from the lone pairs: the 3a 1 MO in NH 3 , the 3a 1 and 1b 1 MO's in H 2 O, and the 2b 2 MO in H 2 CO. H 2 CO is a σ-donor and a π-acceptor when adsorbed on Cu surfaces. In the cases of H 2 O and NH 3 , the contribution of the LUMO of the free molecule to the chemisorption bond is negligible. In contrast, for H 2 CO, a contribution of the 2b 1 (π) orbital (LUMO) to the Cu surface-molecule bond is observed. The bond order indices indicate that the 4s and 4p orbitals of the metal (Cu or Zn) are primarily responsible for the chemisorption bond of H 2 O, H 2 CO and NH 3 on copper and on zinc oxide surfaces. The influence of chemisorption effects upon the NH bond orders of NH 3 , the CO and CH bond orders of H 2 CO, and the OH bond orders of H 2 O are discussed. Based on these INDO/S results, the UPS spectra of H 2 O, NH 3 and H 2 CO on Cu surfaces and of H 2 O on ZnO(0001) are discussed and compared with experimental results.

The adsorption of H2O and the coadsorption of oxygen and H2O on Cu(110) containing overlayers of Cs have been studied with thermal desorption mass spectroscopy (TDS), X-ray photoelectron spectroscopy, Auger electron spectroscopy, and work function measurements. On clean Cu(110), H2O adsorbs at 110 K and desorbs intact at 175 K, with no measurable dissociation probability (

The chemisorption of ammonia, pyridine, formaldehyde, formate and methoxy on the Zn-terminated (0001) surface of zinc oxide is examined employing semiempirical quantum-chemical calculations (INDO/S) and a ZnO cluster of limited size (Zn 10 O 10 ). The results indicate that upon adsorption on ZnO(0001), ammonia, pyridine and formaldehyde are moderate electron donors and that formate and methoxy are reasonably strong acceptors. H 2 CO and C 5 H 5 N can be essentially considered as σ-donors, and HCOO is a σ-acceptor and a τ-donor. In the cases of ammonia, pyridine and formaldehyde, the contribution of the LUMO of the free molecule to the chemisorption bond is neglible. For the adsorption geometries examined here, it was found that the CH bond orders in C 5 H 5 N, H 2 CO, HCOO and H 3 CO, and the NH bond order in NH 3 , are almost not affected upon adsorption. The chemisorption effects on the molecular orbitals and their energy positions are examined for all of the adsorbates studied. Based on these INDO/S results, the experimental UPS spectrum of NH 3 on ZnO(0001) is discussed. For each adsorbed molecule, charge transfers and dipole moments important to the change in work function are presented. Qualitative trends in the heat of adsorption of CO, H 2 CO, NH 3 and HCOO on ZnO(0001) are discussed.

The chemisorption of HCOO on Cu(100) and Cu(110) has been studied with semi-empirical quantum-mechanical calculations (INDO/S) of Cu clusters. The results indicate that the HCOO chemisorption bond is formed on both surfaces by net π-donation of electrons from formate to the metal, and net σ-backdonation of electrons from the metal to HCOO. The total bond-order index between formate and the clusters is almost the same on both surfaces. Upon HCOO adsorption, it is found that the C-O bond-order index decreases and that the C-H bond is not affected (relative to the free formate radical). Peak assignments for the ultraviolet photoemission spectrum of HCOO on Cu(110) and of HCOO on Cu(100) are made based on these results. The possible effects of electronic changes in the Cu surface upon the formate species are analyzed in light of the molecular-orbital description of the chemisorption bond. Possible pathways for the dissociation of adsorbed formate are discussed.

The chemisorption of carbonate (CO 3 ) and ethylene epoxide (C 2 H 4 O) on silver has been examined employing semi-empirical quantum-chemical calculations (INDO/S) and clusters of limited size (Ag 18 ). The results indicate that ethylene epoxide is an electron donor upon adsorption on Ag surfaces, with no contribution of the empty orbitais of the free molecule to the surface-adsorbate bond. Most of the charge transfer from ethylene epoxide toward the surface is from the occupied frontier orbitals: 2b 1 and 6a 1 . The CO 3 group appears as a net electron acceptor (π-donor, σ-acceptor) when adsorbed on Ag(110). The influence of chemisorption effects upon the CO, CC and CH bonds of ethylene epoxide, and the CO bonds of carbonate are discussed. The bond order indices indicate that from the viewpoint of the substrate the Ag(5s, 5p) orbitals are primarily responsible for the chemisorption of ethylene epoxide and carbonate on silver surfaces. For each adsorbed molecule, charge transfers and dipole moments important to the change in work function are presented. Based on these INDO/S results, the UPS spectra of ethylene epoxide and carbonate on Ag(110) and the NEXAFS spectrum of carbonate on Ag(110) are discussed and compared with experimental results.

Molybdenum carbides are increasingly used in heterogeneously catalyzed hydrogenation reactions, which imply the adsorption and dissociation of molecular hydrogen. Here a systematic density functional theory based study, including or excluding dispersion terms, concerning the interaction and stability of H2 with cubic δ-MoC(001) and orthorhombic β-Mo2C(001) surfaces, is presented. In the latter case the two possible C or Mo terminations are considered. In addition, different situations for the H covered surfaces are examined. Computational results including dispersive forces predict an essentially spontaneous dissociation of H2 on β-Mo2C(001) independently of the surface termination, whereas on δ-MoC(001) molecular hydrogen dissociation implies a small but noticeable energy barrier. Furthermore, the ab initio thermodynamics formalism has been used to compare the stability of different H coverages. Finally, core level binding energies and vibrational frequencies are presented with the aim to assist the interpretation of yet unavailable data from X-ray photoelectron and infrared spectroscopies.

The chemisorptions of methyl (CH3), acetylide (H-CC), chlorine (Cl) and phosphorus trifluoride (PF3) on ZnO(0001) and of Cl on ZnO(1010) have been examined employing semi-em- pirical quantum-chemical calculations (INDO/S) and neutral clusters of limited size (Zn13O13). CH3, H-CC and Cl appear as strong electron acceptors when adsorbed on Zn sites of ZnO. The chemisorption bonds of these molecules are almost pure σ-bonds and are largely localized on the adsorption site. An increase in the work function of ZnO surfaces upon adsorption of CH3, H-CC and Cl is predicted. The PF3 molecule is a very weak acceptor of electrons when adsorbed on a-top sites of ZnO(0001). The bonding mechanism of CH3, H-CC, Cl and PF3 on the ZnO(0001) surface involves primarily the HOMO and LUMO of the adsorbate and the Zn(4s,4p) orbitals of the substrate. The effects of chemisorption on the C-H bonds of CH3 and H-CC, the C-C bond of H-CC, and the P-F bonds of PF3 are examined. On the basis of these INDO/S results, the possible UPS spectra for CH3, H-CC and PF3 adsorbed on ZnO(0001) are discussed and compared with results for adsorption on transition-metal surfaces. A general picture of the chemisorption bond of alkyls, acetylides, alkoxides, carboxylates and halogens on a-top sites of ZnO(0001) is obtained by comparing our results for adsorption of CH3, H-CC and Cl with those previously reported for adsorption of methoxy, OH and formate.