Model calculations of chemisorption on transition metal clusters

Alloying of a transition metal with a non-transition metal affects the nature of the chemical bonds between the transition metal atoms in the surface and hydrogen or other atoms chemisorbed on them. The resultant changes in strength of the chemisorption bonds are due to (1) changes in delocalization of the metal electrons involved in the chemisorption bonds, (2) changes in the number of electrons available for such bonds. Our calculations have shown that the effects of alloying on chemisorption are different for monocoordinated adsorbates (ad-atom on top of metal atom) and tricoordinated adsorbates (ad-atom equidistant to three transition metal atoms) respectively. With localized bonds, alloying increases the strength of the former bond, but weakens the latter. As a result the ratio of monocoordinated to tricoordinated adsorbates, present in equilibrium at high coverage, is drastically increased by this type of alloying. The increase is much larger than expected on a geometrical basis only.     

The influence of lateral interactions in the occurrence of ensemble effects in chemisorption and catalysis

Monte Carlo methods have been used to examine the influence of lateral interactions between adspecies at a metal surface on the poisoning of surface reactions. A simple model is employed in which the nearest neighbour interaction energy (B) between the poison species is allowed to vary between −5 and 10 kJ mol⁻¹. The variation in the maximum extent of chemisorption is evaluated for differing values of B and poison coverage. It is demonstrated that the common procedure used to determine critical ensemble sizes produces misleading results which are seriously in error when lateral forces of interaction are considered and the catalytic significance of the results is discussed.     

Chemisorption and surface barrier structure

The effects of chemisorption on the potential barrier at a metal surface are discussed. Simple physical considerations predict an outward shift in the barrier origin, an increased saturation of the barrier, and changes in the inner potential. However, an analysis of low-energy electron diffraction (LEED) data for p(2 × 1) O/W(110) and p(2 × 1) H/W(110) indicates that these changes are small, and that the observed modifications are due primarily to changes in the surface scattering properties accompanying chemisorption. LEED fine structure analysis should then be a useful technique for identifying adsorption sites on metal surfaces.     

Topological study of the chemisorption behavior of carbon monoxide on the (112) surface of Ni---Cu alloys

Chemisorption properties of bimetallic Ni---Cu systems are studied. Ni atoms in a Cu lattice are strongly negatively charged as compared with a pure Ni crystal. The insertion of Cu atoms into a Ni lattice causes the opposite effect. The chemisorption properties are practically not changed by insertion of other metal atoms into the bulk. However, the substitution of metal atoms on the surface leads to a substantial change in chemisorption properties which cannot be explained on the basis of the behavior of pure monometallic species.     

Oxygen consumption and chemisorption in low-temperature oxidation of sub-bituminous pulverized coal

Studying the effect of oxygen in coal oxidation is very important for understanding and controlling coal spontaneous combustion. However, the oxygen effect is not very easy to determine clearly due to the large effect of heat source on coal oxidation in temperature rising experiments. Here, focused on sub-bituminous coal, the oxygen effect was separated from coal oxidation by continuously measuring FTIR spectra of coal with respect to varying temperatures and under oxygen and nitrogen. The active groups’ real-time changes of coal oxidation, thermal treatment and oxygen effect were measured. The carboxylic ester and carboxyl units are the main functional groups that increase with temperatures increasing under oxygen and nitrogen, while the other functional groups decrease in quantity. The oxygen effect promoted the consumption of aliphatic hydrocarbons and hydroxyl groups and also promoted the formation of oxygen-containing groups (except hydroxyl). Four characteristic temperature stages involved in the oxygen effect and their key functional groups were identified. Simultaneously, the relationship of oxygen consumption and chemisorption in oxygen effect was analyzed. The starting temperature of oxygen chemisorption is between 50 and 60°C. The maximum contribution of oxygen effect was observed in methyl and methylene groups. These results are important for chemical control of coal spontaneous combustion. The oxidation of aliphatic hydrocarbon should be controlled before oxygen chemisorption. The value of oxygen consumption between 70 and 80°C can be measured accurately due to the constant chemisorption rate, which help to identify the tendency for spontaneous combustion. These results will help in better understanding of the reaction mechanism of coal oxidation, especially the oxygen effect.     

杂质对担载式催化剂化学吸附的影响

本文采用化学吸附的格林函数理论和复能量积分方法研究H在担载式催化剂ZnO/Ni上的化学吸附,使用由s轨道和p轨道交迭排列的有限原子链和由d轨道组成的半无限原子链,以描述半导体ZnO和金属Ni构成的复合衬底,用Koster-Slater模型来表示杂质原子,分别研究了化学吸附能随ZnO层厚度的变化和化学吸附能与掺杂的关系,结果表明:1)H的化学吸附能随ZnO层数的增加而单调地减小;2)在Ni中掺入杂质Cu和Pt会削弱H的化学吸附,而掺入Co和W将加强H的吸附;3)Ni中的杂质位于界面附近时对化学吸附能的影响最 关键词：     

Heat of molecular chemisorption from bond-order-conservation viewpoint: Why morse potentials are so efficient

Our analytic Morse-potential model of chemisorption based on bond-order conservation [Surface Sci. 150 (1985) L115; 163 (1985) L730] has been extended to calculate the heat of chemisorption Q_AZ of molecules AZ coordinated parallel to a metal surface, where A and Z may be either the same or different atoms or atomic groups (“quasiatoms”). Examples include O₂, CO, H₂CCH₂, HCCH, H₂CO, and (CH₃)₂CO, where comparisons with the perpendicular AZ coordinations, considered earlier, are also made. The projected qualitative trends and quantitative estimates are in agreement with (scarce) experiment. It is argued that, within the bond-order-conservation framework, the efficiency of Morse potentials to describe the energetics of various chemisorption phenomena ultimately originates from the zero-energy gap between the occupied and vacant parts of the metal band.     

Observation of nonuniform attenuation of d-band photoemission due to chemisorption: CO on Re(0001)

Photoelectron spectra of the Re(0001) surface show a strong angular dependence. After chemisorption of CO only a weak angular dependence remains. The resulting nonuniform attenuation of the photoemission from the Re 5d-bands is ascribed to the chemisorption bond. An attempt to assign the metal d-states involved in the chemisorption of CO on Re(0001) is presented.     

Influence de la chimisorption de l'oxygéne et l'azote sur l'émission ionique secondaire d'échantillons monocristallins de nickel et d'alliage nickel-chrome

The influence of an active gas adsorption on the secondary ion emission yields of a mono-crystalline target bombarded with a primary argon beam has been investigated. On the one hand, the chemisorption changes the electronic structure on the surface metal atoms and increases the positive ion emission yields, on the other hand, channeling conditions of the primary ions through the target lattice are modified according to the relation of the chemisorbed layer with the metal lattice. Two samples were examinated: pure nickel and a nickel chromium alloy in either an oxygen or a nitrogen partial pressure. Important modifications of the coverage are due to the sputtering yield variations. The experiments described here suggest that secondary ion emission could be used for chemisorption studies.     

Relaxation of transport properties in nanocomposites of ferromagnetic insulators

The effect heat treatment has on the transport properties of metal–insulator nanocomposites of amorphous structure is investigated. An increase in the electric resistance and magnetoresistance after the heat treatment of nanocomposites in which the concentration of the metal phase lies below the percolation threshold is established. The observed changes are due to structural relaxation of the amorphous dielectric matrix.     

Comments on chemisorption at a metal surface

The effect of the gradient correction to the exchange and correlation energies on chemisorption properties is examined using the evaluation of B_χc arrived at by Rasolt and Geldart. Quantities common to the study of hydrogen and alkali chemisorption on a high density metal are calculated for various forms of the energy functional within the linear response approach. It is found that the gradient correction is a quantitatively significant term, lowering binding energies by ≈ 10% and increasing the dipole sizably.     

Structure and electrical properties of thin-film nanoheterogeneous Pd9(Cu x (In31Y4O65)100 − x )91 composites

A heterophase Pd₉(Cu _x (In₃₁Y₄O₆₅)_{100 ? x} )₉₁ system is obtained in a wide range of concentrations via the ion-beam sputtering of a melted In-Y-O target with Pd and Cu plates. It is established that the change in the films’ electrical resistance during heat treatment is due to crystallization of the heterogeneous structure and oxidation of the metal phase. The effect of the copper concentration on the change in the films’ electrical resistance upon hydrogen chemisorption is demonstrated.     

Benzene adsorption on the Rh(111) metal surface: A theoretical study

The chemisorption of benzene on the Rh(111) surface is studied in the general framework of the extended Hückel theory, with optimized Rh parameters. MO arguments are used to discuss the main features of the electronic interaction of the aromatic system with the surface. Binding energy curves for adsorption on the most likely surface sites are computed, and the effect of the CH bond back-bending and of tilting of the benzene plane are examined. The most favorable chemisorption geometry is found when the C-atom ring is parallel to the surface and the center of the ring is above a threefold hollow site, but H atoms are farther away from the surface than C atoms. The optimum CRh distance compares well with those found in metal complex molecules. The on-top site is unfavorable for benzene chemisorption. The results of tight-binding calculations on infinite slabs agree with, and support, those of simple cluster calculations. A slight “Kekulé distortion” and a small activation barrier to chemisorption are predicted.     

Electron correlations in the chemisorption theory: Self-consistent calculations of the adatom charge

The single hydrogen-like adatom chemisorption on transition metal surfaces is studied by using the generalized model Hamiltonian. This Hamiltonian includes the possibility of influence of the adatom orbital occupancy on the charge transfer between the adatom and the substrate metal. The correlation effects were included up to second order in V (the single particle coupling strength). The numerical calculations of the charge transfer between an adatom and a substrate metal, as well as the comparison with results obtained for the standard Newns-Anderson model indicate that this generalized Hamiltonian can be more efficient in describing the chemisorption process.     

Memory effects in the two-level model for glasses

We study an ensemble of two-level systems interacting with a thermal bath. This is a well-known model for glasses. The origin of memory effects in this model is a quasistationary but nonequilibrium state of a single two-level system, which is realized due to a finite-rate cooling and slow thermally activated relaxation. We show that single-particle memory effects, such as negativity of the specific heat under reheating, vanish for a sufficiently disordered ensemble. In contrast, a disordered ensemble displays a collective memory effect [similar to the Kovacs effect], where nonequilibrium features of the ensemble are monitored via a macroscopic observable. An experimental realization of the effect can be used to further assess the consistency of the model.     

Chemisorption on supported-metal catalysts

A Gree-function formalism is developed to describe the electronic and chemisorption properties of a supported-metal composite substrate. Within the framework of the tight-binding approximation, the metal catalyst is represented by a finite chain of d-orbitals, while the semi-infinite semiconductor support is characterized by a linear chain of alternating s- and p-orbitals. The Anderson-Newns model is used to calculate the chemisorption energy and adatom charge transfer for hydrogen chemisorption on the Ni/ZnO composite system.     

Plasmonic Dicke effect

We study cooperative emission by an ensemble of emitters, such as fluorescing molecules or semiconductor quantum dots, near a metal nanoparticle. The primary mechanism of cooperative emission is resonant energy transfer between emitters and plasmons rather than Dicke radiative coupling between emitters. The emission is dominated by three superradiant states with the same quantum yield as a single emitter, leading to a drastic reduction of ensemble radiated energy down to just thrice of that by a single emitter, the remaining energy being dissipated in the metal through subradiant states. We perform numerical calculations of system eigenstates and find that the plasmonic Dicke effect interactions affect is not impacted by the interactions between emitters or non-radiative losses in the metal.     

Oxygen adsorption on an alkali metal-covered Ni(100) surface

The oxygen chemisorption on an alkali (Na, K, Cs) covered Ni(100) surface and its initial oxidation were studied by Auger and electron energy loss spectroscopy (ELS). It was found that in the presence of an alkali metal, the sticking coefficient S remains unity up to a given oxygen coverage of θ_O^cwhose value depends on the alkali overlayer concentration and the ionicity of the Ni-alkali metal bond. At a given oxygen coverage, the line shapes of Auger and loss spectra are almost the same for alkali-covered and clean Ni(100), which suggests that alkali metals cause no change in the character of the Ni-O bond. The effect of alkali metals is associated with increasing electron charge in the surface region, which facilitates oxygen chemisorption. The enhanced surface oxygen concentration in the presence of an alkali metal results in the formation of an oxide phase at lower oxygen exposures than is the case of clean Ni surfaces.     

The reaction of oxygen with Ti,Cr, Fe and Ni studied by appearance potential spectroscopy

The interaction of oxygen with the surface of evaporated films of Ti, Cr, Fe and Ni has been studied with appearance potential spectroscopy (APS). Both oxygen Is and metal 2p spectra were recorded. Two adsorption regimes were clearly distinguishable: (i) a chemisorption regime characterized by a single peak in the oxygen 1s spectra and unchanged metal 2p spectra, (ii) an oxide regime with pronounced multipeaked structures in the oxygen 1s spectra and appreciable changes in the metal 2p spectra. The technique allows the determination of the exposure at which an oxide starts to grow on the surface.     

A method for measuring the proportion of different plane orientations in metal supported catalysts by gas chemisorption

A method is presented, which allows us to calculate the percentage of metal surface atoms with different coordination numbers, on metal supported catalysts. This method is based on gas-metal stoichiometry values reported for the chemisorption of H₂, O₂ and CO on well defined monocrystals, and on gas chemisorption measurements on metal supported catalysts. It has here been applied to a series of Pd/SiO₂ catalysts.