High Coordination Sites Research Articles

Angle-Resolved Photoemission Extended Fine Structure Study Of Chemisorbed c(2×2)P/Fe(100): Comparison With Self-Consistent-Field Xα Scattered Wave Calculations

AbstractAngle-resolved photoemission extended fine structure (ARPEFS) was used to determine the structure of c(2×2)P/Fe(100) for the first time. P 1s core-level photoemission data were collected normal to the (100) surface and 45° off-normal along the [011] direction at room temperature. A close analysis of the auto-regressive linear prediction based Fourier transform and multiple-scattering spherical-wave calculations indicate that the P atoms adsorb in the high-coordination four-fold hollow sites. The P atoms bond 1.02 Å above the first layer of Fe atoms and the Fe-P-Fe bond angle is 140.6°. Additionally, it was determined that there is no expansion of the Fe surface. Self-consistent-field Xα scattered wave calculations were performed for the c(2×2)P/Fe(100) and the c(2×2)S/Fe(100) systems. These independent results are in excellent agreement with this P/Fe structure and the S/Fe structure previously published, confirming the ARPEFS determination that the Fe1-Fe2 interlayer spacing is contracted from the bulk value for S/Fe but not for P/Fe.

An ab initio molecular orbital study on adsorption at the MgO surface. III. Cooperativity of ionic adsorbates

The cooperativity between coadsorbed molecules on the MgO surface was investigated with the ab initio molecular orbital (MO) method using (MgO)4 and (MgO)6 clusters as models of the catalytic surface. On the pair of three-coordinate sites, the H2, NH3, or H2O molecule is chemisorbed heterolytically, like H−–Mg–O–H+, H2N−–Mg–O–H+, or HO−–Mg–O–H+. When the two adsorbate molecules are cochemisorbed on the adjacent set of four three-coordinate sites, the total chemisorption energy is much larger than the sum of chemisorption energies of each molecule. The extra stability, i.e., the energy cooperativity, is almost the same, around 20 kcal/mol, regardless of the adsorbed species, as far as both of them are heterolytically chemisorbed. The H2–CO coadsorbed system, in which CO is adsorbed nondissociatively, shows no cooperativity. The cooperativity above is partly due to the electrostatic attraction between the coadsorbed ionic species and partly due to the enhanced charge transfer (CT) interaction. Based on these results, it is suggested that higher coordination sites, inactive for chemisorption by themselves, may be activated by preadsorption at the neighboring pair of three-coordinate sites and may actually be ‘‘the active catalytic site’’ on the MgO surface.

An ab initio molecular orbital study on adsorption at the MgO surface. II. Site dependence of hydrogen chemisorption on the (MgO)4,6,8 clusters

Using (MgO)4, (MgO)6, and (MgO)8 clusters as models of the MgO surface, the structures and energies of H2 chemisorption on various pair sites are studied with ab initio molecular orbital calculations. The effect of Madelung potential on the chemisorption energetics is also studied. The chemisorption energy is not very sensitive to the cluster size used in the calculation. It however depends strongly on the coordination numbers of Mg and O at the chemisorption site, and decreases in the following order: the valley site ≫Mg3c–O3c≫Mg4c–O3c≳Mg3c–O4c site. The valley and the Mg3c–O3c pair sites are strongly bonding, whereas the Mg4c–O3c and the Mg3c–O4c sites are only marginally bonding. All other higher coordination sites are nonbonding. The coordination number of the nearest neighbor atoms only secondarily affects the stability of hydrogen chemisorption. The contribution of electron correlation to the chemisorption energy is essentially determined only by the localized electrons near the chemisorption site.

Sulfur adsorption near a step on Al.

Local density-functional calculations show that on Al(331), a S adatom binds preferentially in a step-edge geometry, where it can passivate two relatively undercoordinated surface Al atoms, where it can simultaneously satisfy its own need to complete a closed-shell electronic configuration, and where its nonbonding electrons are minimally repelled by electrons of the substrate. In contrast, on the close-packed Al(111) surface, S optimally resides in a high-coordination site, specifically, a fcc threefold hollow.

Adsorption and reaction of methanethiol on ruthenium(0001)

Methanethiol (CH[sub 3]SH) adsorbs dissociatively on Ru(0001) at 100 K forming two molecular intermediates. These species have been identified by S 2p soft X-ray photoelectron spectroscopy (SXPS) as methylthiolate moieties adsorbed in different bonding sites. At low coverages only a high coordination adsorption site is occupied. The high coordination site is saturated at about 0.20 monolayer of CH[sub 3]SH, and then a low coordination site is populated. Upon heating, the thiolates totally decompose into atomic C, S, and gaseous H[sub 2] with a competing pathway producing atomic S and gaseous CH[sub 4] at high coverage. The methylthiolate in the low coordination site decomposes more readily than the methylthiolate in the high coordination site. Some surface hydrocarbon fragments are formed following C-S bond scission which totally decompose at higher temperatures. Isotopic labeling indicates that the desorption of the sulfhydryl hydrogen precedes the decomposition of the thiolate at high coverage and that there is exchange of hydrogen between the surface and the methyl group. 32 refs., 5 figs.

Novel reaction path induced by selective blocking of surface atoms: methanol dehydrogenation on Mo(112)-(1 × 2)-O

The reaction of methanol on clean and oxygen-modified Mo(112) has been investigated by low-energy electron diffraction (LEED), Auger electron spectroscopy (AES) and temperature-programmed reaction (TPR). Irrespective of oxygen precoverage. CH 3OH is dissociated into CH 3O(a) and H(a) at low temperature. The CH 3O(a) species is decomposed completely on clean Mod(112), resulting in the reaction-limited desorption of H 2 at 410 K and the formation of C(a) and O(a). which undergo recombinative desorption above 800 K. A small amount of methoxy species is decomposed around 500 K to form CH 4, H 2, C(a). and O(a). On the p(2 × 1)-O (θ 0 = 0.5) surface, the main products are the same as those on the clean surface, while the selectivity to CH 4 increases. On the p(1 × 2)-O (θ 0 = 1.0) surface, however, a novel dehydrogenation reaction was observed to occur. Except for the H 2 desorption below 400 K, due to hydroxyl-hydrogen recombination, almost all desorption species, including H 2CO, CH 4, H 2, and CO, were observed to desorb simultaneously at 560 K. No H 2O desorption was observed at any oxygen coverage. The comprehensive data, including isotopic scrambling experiments using CH 3OH/CD 3OD mixture, reveal the stabilization of the C-O bond in CH 3O(a) species on the p(1 × 2)-O surface. This is ascribed to the isolation of one-dimensional Mo rows due to the selective blocking, by oxygen adatoms, of the second-layer Mo atoms, which serve high-coordination metal sites and are proposed to play an essential role in the C-O bond scission.

Molecular orbital studies of the adsorption of CH 3, CH 2, and ch on Rh(111) and Ni(111) surfaces

The adsorption of methyl (CH 3), methylene (CH 2), and methyne (CH) is studied on Rh(111) and Ni(III) with the atom superposition and electron delocalization (ASED) and extended Hückel molecular orbital methods. Results are analyzed by calculating the local density of states (LDOS) and bond order overlap populations. On Rh(111) CH is adsorbed on threefold sites, CH2 on twofold sites and CH 3 on onefold sites in order to restore missing CH bonds. The height of CH x to the metal surface decreases with decreasing hydrogen content x, while the adsorption energy increases. Adsorption of CH 3 on Rh(111) is studied in detail. CH 3 bonds on the metal surface mainly via σ type interactions of the n CH 3 orbital with surface metal atoms of the same symmetry. In case of the onefold adsorption, the highest occupied molecular orbital ( n CH 3) has a large interaction with metal s, ν ζ and d 2 z orbitals. For twofold adsorption it interacts mainly with the symmetric s, p x , and d zx metal group orbitals. Interactions of surface metal orbitals with the π and π ∗ orbitals are weak. Preliminary results are presented for the coupling reaction of coadsorbed CH 2 and CH 3. A strong repulsion due to steric interaction of the hydrogen atoms is found when the carbon-carbon distance is decreased. As a result the direct coupling reaction of CH 3 and CH 2 does not seem to be a suitable reaction path for the CC coupling reaction. In a final section we have analyzed CH 3 adsorption on Ni(111). With parameters implying a large spatial extension of the d orbitals, CH 3 is found to adsorb on onefold sites. Decreasing the spatial extension of the Ni d orbitals causes a shift to multiply bonding adsorption sites. This illustrates the subtle balance between the interaction with the s valence electrons that favor high coordination sites and the interaction with the d valence electrons favoring low coordination sites.

Modeling the growth of dendrite-like gold islands on graphite substrates

Experimental images of dendrite-like gold islands obtained by transmission electron microscopy were digitized by image analysis methods, and the fractal dimensions of the digital images were calculated. These fractal dimensions were then compared with those obtained from a computer simulation model of island growth which used the diffusion limited aggregation (DLA) scheme as framework. The computer model developed in this study differed from conventional DLA in that site-specific sticking coefficients were allowed to differ from unity. Good agreement between experimental and simulated results were found when the sticking probability at low coordination edge sites was reduced to the range 0.02 to 0.05, while sticking probabilities at higher coordination sites were held at unity. An estimate of sticking probabilities by means of a physical model confirmed the plausibility of low sticking coefficients at low coordination sites, thus supporting the hypothesis of site-selective sticking probabilities in the growth mechanism.

Reactivity of CO on stepped and non-stepped surfaces of transition metals

Results of extended Huckel calculations show that coordination of CO to Group VIII transition-metal surfaces depends on a subtle balance of the interaction with the CO 5σ orbital, that tends to direct the CO molecule to the atop position and the CO 2 π* orbital, that directs the molecule to higher coordination sites. In the atop position, the changes in bonding to different surfaces of the same metal can be mainly attributed to differences in the interaction with the CO 5σ molecular orbital. The favoured dissociation path is such that carbon and oxygen atoms end in high coordination sites, sharing as few surface metal atoms as possible. The CO bond is activated by the metal atoms that are crossed upon dissociation.

Infrared and Mössbauer spectroscopy studies of the interaction of NO with iron-exchanged zeolites

Nitric oxide adsorption on iron-exchanged, silicon-substituted Y-zeolite, iron-exchanged mordenite, and iron-exchanged ZSM-5 has been studied by both infrared and Mössbauer spectroscopies. In addition, the catalytic activities of these zeolites for nitrous oxide decomposition have been determined. The results show that NO adsorption is qualitatively the same for all the zeolites. However, the framework differences and the locations of the exchangeable cations control the relative concentrations of the various nitrosyl species formed upon adsorption. Zeolites that have a large fraction of the iron in accessible, low-coordination sites form predominantly mononitrosyls. Those zeolites that have a large fraction of iron in inaccessible, high-coordination sites form predominantly dinitrosyls and form them slowly. Measurements of catalytic activity and NO adsorption capacities correlate well with the relative accessibilities of the cations to reactant molecules. This indicates that the active sites for reaction are associated with those sites which form mononitrosyl species with NO. Thus, in Y-zeolite differences in activities can be accounted for by the changes in cation siting brought about by changes in the framework aluminum content.

The cluster approach in the study of atomic and molecular chemisorption on silicon

Ab initio and MNDO methods have been used in the framework of the cluster model for a systematic study of the chemisorption of atoms and molecules on low-index silicon surfaces. Chemisorption of hydrogen and halogen atoms occurs preferentially at on-top positions for the (111) surface, but (except hydrogen) at bridge positions on the (100) surface. Chemisorption of other species (O, OH, S, SH, H 2 O, H 2 S) always occurs at bridge positions. When large adsorbates are involved, high coordination sites become to assume an increasing importance, although the binding energy at the preferred site generally decreases with the dimensions of the adsorbate.

Chemisorption of group IIA elements

Until now theoretical studies of chemisorbed species have been concentrated around a limited number of systems, the alkali metals, the elements in the second period of the periodic table and some molecules as well. The reason is of course the important role these adsorbates play in many catalytic reactions and also that many of them are simple to handle and that the results gained may be used for qualitative descriptions of more complex systems. However, there are many classes of interesting elements in the periodic table, both for theoretical and applied reasons: metallic and non-metallic, electropositive and electronegative, etc.The work presented here deals with the elements of group IIA, Be, Mg, Ca and Sr, and two neighbouring elements, Li and B, and is part of a systematic investigation of potential-energy curves, adsorbate-induced dipole moments and density of states and other chemisorption properties.It is shown that the adsorption of Be and Mg is characterized by the fact that these elements have filled s-subshells which result in weak chemisorption and strong physisorption on a jellium surface. The substrate lattice gives additional effects, such as strong electrostatic stabilization of Be on high-coordination sites. In the case of Mg, geometric considerations play an even stronger role than for Be, giving a weak chemisorption on some close-packed surfaces.The properties of chemisorbed Ca and Sr differ radically from those of Be and Mg, for instance the chemisorption energy is much larger. The origin of this lies in the fact that Ca and Sr are neighbours to the transition metals and have d-states that are empty in the free atom ground state, but get partially filled upon chemisorption. In the interaction with the substrate lattice, the directional properties of the d-states influence the adsorption site.Sub-shell closure, early p- and d-mixing and core-orthogonalization are identified as important adsorbate factors for the chemisorption bond. With respect to their chemisorption properties the group IIA elements can be classified as neutral atoms that are apt for polarization and charge transfer due to their low-lying p- and d-orbitals.

Activated chemisorption of hydrogen on supported ruthenium: II. Effects of crystallite size and adsorbed chlorine on accurate surface area measurements

A crystallite size effect has been observed during the activated adsorption of hydrogen on supported ruthenium catalysts. This effect does not occur in the absence of adsorbed chlorine and is increasingly more pronounced as metal dispersion is increased. Activation energies for adsorption, at constant hydrogen coverage, may be as high as 16 kcal/mole and have been measured utilizing (i) Al 2O 3 and SiO 2 support materials, (ii) crystallites which range in size from 2.6 to 22.0 nm, and (iii) surface chlorine coverages which vary from 0.01 to 0.35 of the number of surface ruthenium atoms. Results of these studies suggest that preferential adsorption of chlorine atoms at high coordination sites reduces electron density at adjacent low coordination sites, thereby increasing the activation energy barrier for electron donation to, and dissociative chemisorption of, incoming hydrogen molecules. While this effect may be viewed as primarily electronic in origin, a structure-sensitive (i.e., crystallite size-dependent) mechanism has been proposed to account for the observed adsorption behavior based on the number of high coordination-chlorine adsorption sites in close proximity to low coordination-hydrogen adsorption sites. Activated adsorption sites, attributed to the above-noted mechanism, can be responsible for a greater than two-fold underestimation of the number of surface ruthenium atoms measured by irreversible hydrogen adsorption at 298 K. Significant differences in adsorption behavior between silica- and alumina-supported crystallites of equal dispersion also suggest that deliberate addition of chlorine adatoms may provide a sensitive probe for discriminating differences in crystallite shape and surface texture.

Surface science and catalysis

Modern surface science studies have explored a large number of metal catalyst systems. Three classes of catalytic reactions can be identified: (1) those that occur over the metal surface; (2) reactions that take place on top of a strongly adsorbed overlayer and (3) reactions that occur on coadsorbate modified surfaces. Case histories for each class are presented. The first class of reactions is structure sensitive, and rough surfaces are needed for high reactivity. Ammonia synthesis, H 2 /D 2 exchange and complex hydrocarbon reaction studies all indicate the importance of high coordination sites that are present on atomically rough, open crystal surfaces, mostly in the second layer. A model is presented that correlates increased charge fluctuations at these sites with enhanced catalytic activity. The second class of reactions includes ethylene hydrogenation. It occurs via hydrogen transfer through a tenaciously held ethylidyne overlayer to weakly adsorbed ethylene molecules on top of this layer. Most catalytic reactions occur on coadsorbate modified metal surfaces. Sulphur, potassium, carbonaceous fragments and other metal constituents are employed most frequently as surface modifiers. These additives block sites and alter the bonding of the reactants, thereby changing the reaction paths. Their effects will be demonstrated through discussions of the CO/benzene, S/thiophene and CO/K interactions.

Adsorption of carbon monoxide on the surfaces of polycrystalline transition metals and alloys: electron energy loss and ultraviolet photoelectron spectral studies

Adsorption of CO has been investigated on the surfaces of polycrystalline transition metals as well as alloys by employing electron energy loss spectroscopy (eels) and ultraviolet photoelectron spectroscopy (ups). CO adsorbs on polycrystalline transition metal surfaces with a multiplicity of sites, each being associated with a characteristic CO stretching frequency; the relative intensities vary with temperature as well as coverage. Whilst at low temperatures (80–120 K), low coordination sites are stabilized, the higher coordination sites are stabilized at higher temperatures (270–300 K). Adsorption on surfaces of polycrystalline alloys gives characteristic stretching frequencies due to the constituent metal sites. Alloying, however, causes a shift in the stretching frequencies, indicating the effect of the band structure on the nature of adsorption. The up spectra provide confirmatory evidence for the existence of separate metal sites in the alloys as well as for the high-temperature and low-temperature phases of adsorbed CO.

Structure sensitivity and its effects on product distribution in CO hydrogenation on cobalt/alumina

Activities and selectivities of cobalt/alumina catalysts for CO hydrogenation were determined as a function of metal loading, reduction temperature, and preparation. Specific activities and average carbon numbers of the hydrocarbon product were found to vary significantly with metal loading, reduction temperature, and preparation. These changes were also found to be linearly correlated with changes in dispersion indicating that CO hydrogenation on Co Al 2O 3 is structure-sensitive. The changes in specific activity with dispersion can be explained by variations in the distribution of low and high coordination sites and by changes in the nature of adsorbed CO species available for reaction. High specific activity is apparently favored on sites to which CO is strongly coordinated. Variations in the average carbon number of the product can be explained by changes in the rate of termination relative to the rate of propagation. High-molecular-weight products are favored by catalysts having high rates of propagation and low rates of termination.

Correlation between catalytic activity and bonding and coordination number of atoms and molecules on transition metal surfaces: Theory and experimental evidence

Correlation between catalytic activity and low-energy local electronic fluctuations in transition metals is proposed. A theory and calculations are presented which indicate that maximum electronic fluctuations take place at high-coordination metal sites. Either (i) atomically rough surfaces that expose to the reactant molecules atoms with large numbers of nonmagnetic or weakly magnetic neighbors in the first or second layer at the surface or (ii) stepped and kinked surfaces are the most active in carrying out structure-sensitive catalytic reactions. The synthesis of ammonia from N(2) and H(2) over iron and rhenium surfaces, (1)H(2)/(2)H(2) exchange over stepped platinum crystal surfaces at low pressures, and the hydrogenolysis (C-C bond breaking) of isobutane at kinked platinum crystal surfaces are presented as experimental evidence in support of the theory.

Crystallographic Dependence of Chemisorption Bonding for Sulfur on (001), (110), and (111) Nickel

The degree to which the surface atomic arrangement influences the bonding geometry of sulfur atoms on (001), (110), and (111) Ni surfaces is investigated by an analysis of experimental low-energy electron diffraction data. For all surfaces, the sulfur atoms reside in high-coordination sites---the atomic hollows of the surface; all nearest-neighbor Ni-S bond lengths are less than those of stable bulk compounds.