Dipped Adcluster Model Research Articles

XPS of oxygen atoms on Ag(111) and Ag(110) surfaces: Accurate study with SAC/SAC‐CI combined with dipped adcluster model

O1s core-electron binding energies (CEBE) of the atomic oxygens on different Ag surfaces were investigated by the symmetry adapted cluster-configuration interaction (SAC-CI) method combined with the dipped adcluster model, in which the electron exchange between bulk metal and adsorbate is taken into account properly. Electrophilic and nucleophilic oxygens (O(elec) and O(nuc)) that might be important for olefin epoxidation in a low-oxygen coverage condition were focused here. We consider the O1s CEBE as a key property to distinguish the surface oxygen states, and series of calculation was carried out by the Hartree-Fock, Density functional theory, and SAC/SAC-CI methods. The experimental information and our SAC/SAC-CI results indicate that O(elec) is the atomic oxygen adsorbed on the fcc site of Ag(111) and that O(nuc) is the one on the reconstructed added-row site of Ag(110) and that one- and two-electron transfers occur, respectively, to the O(elec) and O(nuc) adclusters from the silver surface.

Possible reaction pathway in methanol dehydrogenation on Pt and Ag surfaces/clusters starting from O–H scission: Dipped adcluster model study

The mechanism of the methanol dehydrogenation reaction on a Pt surface has been investigated using the dipped adcluster model (DAM) combined with density-functional theory (DFT) calculations. Starting from O–H bond scission, methanol decomposes to form CO exothermically on the Pt surface, where the Pt-dσ orbital effectively interacts with the O–H antibonding orbital. The donative interaction of the Pt-dσ orbitals was found to be important for catalytic activation on the Pt surface. The reaction pathway starting from C–H bond scission has a larger activation barrier and, therefore, is less kinetically favorable. Electron transfer from the bulk, which is included in the present DAM calculation, plays an important role in the reaction pathway from O–H bond scission, in particular for the dehydrogenation of formaldehyde. On the other hand, the Ag surface has been shown to be effective for formaldehyde synthesis, because formaldehyde desorbs spontaneously from the Ag surface. The present reaction has also been examined and discussed in view of the nanoscale clusters and nanorods.

Deepening and Extending the Quantum Principles in Chemistry

Abstract A brief summary of the author’s research projects for deepening and extending the quantum principles in chemistry is given. First, the structure of the exact wave function that is the solution of the Schrödinger equation was clarified and a method of calculating it starting from an approximate wave function is given. The singularity problem intrinsic to atoms and molecules was overcome, and a general method of solving the Schrödinger equation in an analytical form has been established. Quantum chemistry of excited states and ionized states is essential since it is difficult with experiments alone to explore this field due to the short lives of these species. The SAC-CI theory developed in the author’s groups offers a powerful method for investigating the chemistry of excited and ionized states. The energy gradient method incorporated into this program is particularly useful for studying geometries and reactions involving these states. The catalysis on a metal surface is a magic process that must be clarified with the help of quantum theory. The author studied it using his dipped adcluster model (DAM) that includes the effects of the free electrons of the bulk metal. The epoxidation reactions of ethylene and propylene were studied with this method. The NMR chemical shifts include much information about the valence electrons of molecular systems. The author clarified that the major electronic mechanisms of the chemical shifts could be attributed to the intrinsic atomic properties that are related to the position of the element in the periodic table. For the chemical shifts induced by heavy ligands, the relativistic effect is sometimes dominant. The spin-orbit effect is most important for light resonant nuclei. For heavier resonant nuclei, other spin-free relativistic effects and the electron correlation effects are also important and they strongly couple with each other, implying an existence of something “unexpected” in the chemistry of heavy elements. Finally, the strategy of the author’s study in quantum chemistry is shortly described.

Theoretical Surface Spectroscopy of NO on the Pt(111) Surface with the DAM (Dipped Adcluster Model) and the SAC-CI (Symmetry-Adapted-Cluster Configuration-Interaction) Method.

Theoretical surface spectroscopy for NO on the Pt(111) surface is carried out and combined with the experimentally known facts to elucidate the structures, absorption sites, absorption energies and K-shell binding energies of NO adsorbates on the surface. The electronic structures were studied by using the dipped adcluster model (DAM) for chemisorptions on a metal surface proposed previously and the symmetry-adapted-cluster configuration-interaction (SAC-CI) method, which is an established method for studying molecular spectroscopy. The natures of the two different adsorption species suggested experimentally have clearly been identified based on the studies on the geometries, vibrational frequencies, adsorption energies, and the N and O K-shell binding energies. The PES (potential energy surface) of NO on the metal surface was also calculated. The most stable adsorption species was on the fcc or the hcp hollow site, and the on-top one was less stable. The 2-fold bridge site did not have a minimum on the PES and therefore was only transient. The inter NO interactions at higher densities were shown to be rather weak. We examined the cluster model (CM) vs the DAM as a model of the surface adsorption on a metal surface. The CM was shown to be unable to describe the adsorption of NO on a metal surface, demonstrating the importance of the electron transfer between the NO and Pt surfaces included in the DAM. The DAM and the SAC-CI methods proved to be a useful tool for studying the nature, electronic structure, and the spectroscopic properties of the adsorbates on a metal surface.

Mechanism of methanol synthesis on Cu(100) and Zn/Cu(100) surfaces: Comparative dipped adcluster model study Dedicated to Professor Michael C. Zerner in celebration of his 60th birthday.

The mechanism of methanol synthesis from CO2 and H2 on Cu(100) and Zn/Cu(100) surfaces was studied using the dipped adcluster model (DAM) combined with ab initio Hartree–Fock (HF) and second-order Møller–Plesset (MP2) calculations. On clean Cu(100) surface, our calculations show that five successive hydrogenations are involved in the hydrogenation of adsorbed CO2 to methanol, and the intermediates are formate, dioxomethylene, formaldehyde, and methoxy. The rate-limiting step is the hydrogenation of formate to formaldehyde, and the Cu–Cu site is responsible for the reaction on Cu(100). The roles of Zn on Zn/Cu(100) catalyst are to modify the rate-limiting step of the reaction: to lower the activation energies of this step and to stabilize the dioxomethylene intermediate at the Cu–Zn site. The present comparative results indicate that the Cu–Zn site is the active site, which cooperates with the Cu–Cu site to catalyze methanol synthesis on a Cu-based catalyst. Electron transfer from surface to adsorbates is the most important factor in affecting the reactivity of these surface catalysts. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 77: 341–349, 2000

Mechanism of methanol synthesis on Cu(100) and Zn/Cu(100) surfaces: Comparative dipped adcluster model study

The mechanism of methanol synthesis from CO2 and H2 on Cu(100) and Zn/Cu(100) surfaces was studied using the dipped adcluster model (DAM) combined with ab initio Hartree-Fock (HF) and second-order Moller-Plesset (MP2) calculations. On clean Cu(100) surface, our calculations show that five successive hydrogenations are involved in the hydrogenation of adsorbed CO2 to methanol, and the intermediates are formate, dioxomethylene, formaldehyde, and methoxy. The rate-limiting step is the hydrogenation of formate to formaldehyde, and the Cu-Cu site is responsible for the reaction on Cu(100). The roles of Zn on Zn/Cu(100) catalyst are to modify the rate-limiting step of the reaction: to lower the activation energies of this step and to stabilize the dioxomethylene intermediate at the Cu-Zn site. The present comparative results indicate that the Cu-Zn site is the active site, which cooperates with the Cu-Cu site to catalyze methanol synthesis on a Cu-based catalyst. Electron transfer from surface to adsorbates is the most important factor in affecting the reactivity of these surface catalysts. c 2000 John Wiley & Sons, Inc. Int J Quant Chem 77: 341-349, 2000

Mechanism of the hydrogenation of CO2 to methanol on a Cu(100) surface: dipped adcluster model study

The mechanism of the hydrogenation of CO2 to methanol on a Cu(100) surface was studied using the dipped adcluster model (DAM) combined with ab initio Hartree–Fock (HF) and second-order Møller–Plesset (MP2) calculations. The Langmuir–Hinshelwood (LH) mode, which corresponds to the reaction between coadsorbed species on the surface, was adopted. Our calculations show that hydrogen and formate are adsorbed at short-bridge sites. The coadsorption of hydrogen and CO2, in which CO2 is chemisorbed in the bent anionic state, is described well by the DAM. Five successive hydrogenations are involved in the hydrogenation of adsorbed CO2 to methanol: the intermediates are formate, dioxomethylene, formaldehyde and methoxy. The geometries of these intermediates and the transition states, as well as the energy diagrams in the reaction process, are presented. The rate-limiting step is the hydrogenation of adsorbed formate. Subsequent steps occur relatively readily, and lead to the formation of methanol. Clearly, any factor that could enhance the hydrogenation of formate on copper should lead to enhanced activity in methanol synthesis.

Active sites for methanol synthesis on a Zn/Cu(100) catalyst

The active sites for methanol synthesis on a Zn/Cu(100) catalyst are clarified using the dipped adcluster model (DAM) combined with ab initio HF and MP2 calculations. In comparison with that on a Cu(100) catalyst studied previously, it is shown that the Cu–Zn site provides an easier reaction pathway for the rate-limiting step; the energy barriers are all lower and the dioxomethylene intermediate is more stable than that at the Cu–Cu site on a Cu(100) catalyst. The present results prove that the Cu–Zn site is the active site which cooperates with the Cu–Cu site to catalyze methanol synthesis. Electron transfer is the important factor in affecting the reactivity of the catalysts.

Mechanism of the oxidation of acetylene on a Ag surface: dipped adcluster model study

The mechanism of the oxidation of acetylene on a silver surface was studied by the dipped adcluster model (DAM) combined with ab initio Hartree–Fock and MP2 calculations. The reactions of the hydrogen and carbon of acetylene with both molecularly and atomically adsorbed oxygens were investigated. Our results show that the reaction path for acetylene via epoxidation is energetically unfavorable. On the other hand, the reaction path via hydrogen abstraction is much easier and leads to surface acetylide and hydroxyl intermediates, and is likely the initial reaction path responsible for the complete oxidation reaction of acetylene on a silver surface. The reaction mechanisms derived adequately explain the available experimental results.

Adsorption and disproportionation reaction of OH on Ag surfaces: dipped adcluster model study

The adsorption and surface disproportionation reactions of OH on various silver surfaces were studied by the dipped adcluster model (DAM) combined with ab initio HF and MP2 methods. Our studies show that OH binds strongly to Ag surfaces; the adsorption energies were calculated to be 118.3kcalmol−1 at the short bridge site on Ag(110), 108.6kcalmol−1 at the fourfold hollow site on Ag(100), and 97.3kcalmol−1 at the threefold hollow site on Ag(111). The H–O axis is preferentially perpendicular to the surface for OH on Ag(100) and Ag(111), but tilts about 50° along the [001] azimuthal orientation on Ag(110). The Ag4d orbital plays an important role in adsorbing OH, whose molecular orbital analysis is described. Coadsorption of OH at the nearest fourfold sites on Ag(100) is stable with the H–O axis perpendicular to the surface; an inclined OH structure is proposed for the coadsorption of OH at the bridge sites and it is shown to be very reactive regarding the surface disproportionation reaction of OH. The structures and energy surfaces for the disproportionation reaction of OH to form H2O on Ag(100) are presented. The present study provides clear information regarding the adsorption, coadsorption and disproportionation of OH on Ag surfaces.

Oxidation mechanism of propylene on an Ag surface: dipped adcluster model study

The mechanisms of the epoxidation and complete oxidation of propylene over a silver surface are studied by the dipped adcluster model (DAM) combined with the ab initio HF and MP2 methods. Several postulated reaction pathways involving both molecularly and atomically adsorbed oxygens are investigated. The energy diagrams of the epoxidation and complete oxidation of propylene on the silver surface are presented, and the detailed electronic mechanisms for the oxidation of propylene are clarified. The calculations show that the mechanism involving the single precursor intermediate common to both epoxidation and complete oxidation proposed in some literatures can be ruled out. Instead, the two competitive mechanisms are suggested: the epoxide formation is initiated by the reaction of the olefinic carbon with the adsorbed oxygen, while the combustion is initiated by the abstraction of the allylic hydrogen by the adsorbed oxygen. The former mechanism is essentially the same as that in the epoxidation of ethylene reported previously. The latter mechanism is energetically more favorable for propylene, which is a reason for the low selectivity of the epoxidation of propylene. The adsorbed allyl species, which is an intermediate in the latter mechanism, is demonstrated to exist in the form of chemisorbed anion. The mechanisms presented in this paper seem to be useful for explaining the epoxidation and complete oxidation of other olefins. A comparison with the previous studies is also presented.

Electron transfer and back-transfer in the partial oxidation of ethylene on an Ag surface: dipped adcluster model study

The crucial step in catalyst design to increase the overall selectivity of epoxidation of ethylene on a silver surface is the reaction of atomically adsorbed oxygen with ethylene to give ethylene oxide, as shown previously. The energy diagram is recalculated using the dipped adcluster model combined with MP2 geometry optimizations. The previous result is confirmed. At the key transition state (TS), one-electron is back-transferred from the admolecule to the bulk Ag surface. The energy barrier is calculated to be 41 kcal/mol, comparable with the energy barrier of 39 kcal/mol in the complete oxidation path. The energy barrier of this electron-transfer type TS must be lowered to obtain a higher overall selectivity of ethylene oxide.

Activation of O 2 on Cu, Ag, and Au surfaces for the epoxidation of ethylene: dipped adcluster model study

Aiming to clarify why only silver is an effective catalyst for the partial oxidation of ethylene, we studied theoretically the reactivity and the stability of oxygen species on Cu, Ag, and Au surfaces. We used the dipped adcluster model (DAM), since electron transfer from metal to oxygen is important, and the SAC SAC-CI method, since several electronic states are involved. We found that if superoxide species exists on the surface, both Cu and Au surfaces show a reactivity similar to Ag surface, leading smoothly to ethylene oxide, and the barriers leading to complete oxidations should be very high. Therefore, the point is the relative stability of the various oxygen species and, in particular, the stability of the superoxide species on the metal surface. On Cu, superoxide is much less stable than peroxide, which again is less stable than the dissociated species, and no barrier exists for the conversion from superoxide to peroxide. On Au, our DAM calculations show that the electron flow from the bulk metal into the adcluster does not occur, so that the molecularly adsorbed oxygen species, as well as the dissociative ones, find it difficult to exist stably on the clean surface. On Ag, superoxide should certainly have some life time to react with ethylene to give ethylene oxide, which is considered to be the origin of the unique catalytic activity of silver for the epoxidation of ethylene. This is related to the ability of electron transfer and to the geometry of the Ag surface. We have proposed a basic idea for a new catalytic design of the epoxidation reaction.

Mechanism of the partial oxidation of ethylene on an Ag surface: dipped adcluster model study

The partial oxidation of ethylene to ethylene oxide catalyzed by silver is studied by the ab-initio Hartree-Fock and MP2 methods using the dipped adcluster model (DAM). The active species is the superoxide O − 2 which is molecularly adsorbed in a bent end-on geometry on the silver surface. Ethylene reacts with the terminal oxygen atom and the reaction proceeds smoothly without a large barrier to yield ethylene oxide. The complete oxidation of ethylene involving the superoxide is forbidden due to the existence of a large energy barrier. This is one origin of high selectivity. Without the silver surface, the epoxidation reaction is very unfavorable, showing the catalytic activity of the silver surface. The atomically adsorbed oxygen seems not to be selective: it gives both ethylene oxide and complete oxidation products. Therefore, adding to the selectivity due to the superoxide, an overall selectivity larger than 6 7 can be possible. In the process yielding ethylene oxide from atomic oxygen, electron transfer and back-transfer from/to the metal are important processes which can be promoted by both electron donor and acceptor. Detailed electronic mechanisms are clarified and discussed.

Theoretical studies on the catalytic activity of Ag surface for the oxidation of olefins

Systematic theoretical studies for the mechanisms of the epoxidation and complete oxidation of ethylene and propylene over silver surface as well as the reactivity and the stability of oxygen species on Cu, Ag, and Au surfaces have been presented. The . . dipped adcluster model DAM combined with the ab initio Hartree)Fock HF , second- . order Moller-Plesset MP2 , and SACrSAC-CI symmetry-adapted clusterrconfiguration . interaction methods are used. These studies clarify the origin of silver as a unique effective catalyst for the epoxidation of ethylene and the different mechanisms for the oxidation of olefins over silver surface. For the epoxidation of ethylene, the superoxide O y , which is molecularly adsorbed in bent end-on geometry on the silver surface, is the 2 active species. The origin of the unique catalytic activity of silver for the epoxidation of ethylene is due to its ability to adsorb oxygen as the superoxide species. Such an adsorbed species cannot be stable or exist on Cu and Au surfaces. For the oxidation of propylene, both reaction mechanisms initiated by the activation of olefinic carbon and by the activation of the allyl hydrogen exist. The activation of the allyl hydrogen is the origin of the complete oxidation of some olefins over silver surface. The present results not only let us have a better understanding of the reaction mechanisms of olefins over silver surface but also supply a basic idea for the new catalyst design of the epoxidation

Theoretical Analysis of Adsorption Properties and Vibrational Behavior of O2 on Ag (110) Surface with Ab Initio CM and DAM Calculations

Abstract Chemisorption properties and vibrational behaviors of molecular oxygen on Ag (110) surface have been studied by the cluster model (CM) and dipped adcluster model (DAM) with an Ag6 cluster. Two stable states, 1A1 [] and 3A2 [001], have been found and assigned to peroxide (O22−) and superoxide (O2−) species, respectively. Theoretically predicted adsorption geometries and vibrational frequencies for these states are similar to those measured in the experiments. With the DAM method, the calculated adsorption energies agree well with the experimental value. The O22− species is predicted to be more stable than the O2− species. Cluster size and basis set effects have also been investigated.

Dipped adcluster model and SAC-CI method applied to harpooning, chemiluminescence and electron emission in halogen chemisorption on alkali metal surface

The methodology of our laboratory, the dipped adcluster model (DAM) and the symmetry adapted cluster configuration interaction (SAC-CI) method, are briefly reviewed and successfully applied to the electron transfer processes, harpooning, surface chemiluminescence, and surface electron emission, in the course of the chemisorption of halogen molecule on an alkali metal surface. This success encourages us to apply our methodology to surface photochemistry.

Dipped adcluster model study for molecular and dissociative chemisorptions of O2 on Ag surface

Chemisorption of an O2 molecule on a Ag surface is studied theoretically with the use of the dipped adcluster model (DAM). Electron correlations in low-lying surface states and electron-transferred states from bulk metal, which are shown to be very important, are described by the symmetry adapted cluster (SAC)/SAC-configuration interaction (SAC-CI) method. Side-on geometries are used, different from the (bent) end-on geometries studied previously. Potential curves for the O2 approaching and dissociating processes are investigated with the use of Ag2O2 and Ag4O2 adclusters. For the occurrence of chemisorption, the electron transfer from bulk metal to the adcluster, and the electrostatic image force are important, which cannot be treated by the conventional cluster model. Two different molecular adsorption states are obtained from the calculations for the Ag2O2 adcluster, namely superoxide (O2−) and peroxide (O22−) species and the corresponding adsorption energies are calculated to be 5.5 and 17.8 kcal/mol, respectively, which compare well with the experimental value 9.2–9.3 kcal/mol. The O–O stretching frequencies of these species are in good agreement with the experimental values. From the calculations for the Ag4O2 adcluster, the potential minima corresponding not only to the molecular adsorption, but also to the dissociative adsorption are obtained. The dissociative adsorption is shown to be led from the peroxide. The geometry of the dissociative O− is at the bridge site on a silver surface and the calculated Ag–O bond distance of 2.16 Å agrees well with the experimental value 2.06–2.17 Å. The dissociative adsorption energy is estimated to be 44.0–61.4 kcal/mol, which is compared with the experimental value 40.8–44.0 kcal/mol.

Theoretical model studies for surface-molecule interacting systems

Surface-molecule interactions and reactions are important elementary steps of catalytic reactions. Since they involve interactions between infinite and finite systems, modelings are necessary for theoretical investigations of catalytic reactions on a surface. The cluster model (CM) is most frequently used for quantum chemical calculations but neglects the effect of the bulk solid. For including such effect, ( 1 ) embedding the cluster onto a surface (actually into a larger cluster) is a method proposed by Grimley and Pisani, and (2) dipping the adcluster (admolecule + cluster) onto the electron bath of the solid and letting the system be at equilibrium for electron and spin exchanges is another model proposed by Nakatsuji. We show some applications of the embedding cluster model (ECM) and the dipped adcluster model (DAM). We will also report in the lecture on the study of the photochemical decomposition reaction of MnO into MnO + O2. © 1992 John Wiley & Sons, Inc.

Dipped adcluster model study for the end-on chemisorption of O2 on an Ag surface

Theoretical study for the end-on adsorption of an O2 molecule on an Ag surface is carried out with the use of the dipped adcluster model (DAM). The adcluster of AgO2 is taken and the highest spin coupling model is employed. Electron transfer from the bulk metal to the adcluster considered by DAM is important for the occurrence of the chemisorption, and this cannot be described by the small-size cluster model. Electron correlations are also quite important and are described by the SD-CI method based on the corresponding parent configurations. In the end-on geometry, the superoxide species is the ground state, and there are no peroxide species in the lower energy region. The calculated adsorption energy compares reasonably with the experimental value. The O—O axis of the superoxide is inclined by 70°–80° from the surface normal. The outside oxygen atom of the adsorbed species seems to be more reactive than the inside one, while the net charge on the former is smaller than that on the latter. Keywords: DAM (dipped adcluster model), silver, superoxide, chemisorption.