N doping of TiO2(110): photoemission and density-functional studies

The electronic properties of N-doped rutile TiO2(110) have been investigated using synchrotron-based photoemission and density-functional calculations. The doping via N2+ ion bombardment leads to the implantation of N atoms (approximately 5% saturation concentration) that coexist with O vacancies. Ti 2p core level spectra show the formation of Ti3+ and a second partially reduced Ti species with oxidation states between +4 and +3. The valence region of the TiO(2-x)N(y)(110) systems exhibits a broad peak for Ti3+ near the Fermi level and N-induced features above the O 2p valence band that shift the edge up by approximately 0.5 eV. The magnitude of this shift is consistent with the "redshift" observed in the ultraviolet spectrum of N-doped TiO2. The experimental and theoretical results show the existence of attractive interactions between the dopant and O vacancies. First, the presence of N embedded in the surface layer reduces the formation energy of O vacancies. Second, the existence of O vacancies stabilizes the N impurities with respect to N2(g) formation. When oxygen vacancies and N impurities are together there is an electron transfer from the higher energy 3d band of Ti3+ to the lower energy 2p band of the N(2-) impurities.     

SCF MO pseudopotential studies of chemisorption. The Pd(111) oxygen system

Following previous work on Pt(111)/hydrogen system, SCF MO pseudopotential calculations have been done for the case of atomic oxygen chemisorption on Pd(111), using clusters of up to four metal atoms. No site preference is found for oxygen chemisorption, and the chemisorption bond appears to involve only nearest-neighbor metal atoms. The use of a double-zeta 4d function for the metal results in significant changes in the bonding pattern with respect to single-zeta.     

Dissociative chemisorption of H2 on the Cu(110) surface: a quantum and quasiclassical dynamical study

Six-dimensional quantum dynamical and quasiclassical trajectory (QCT) calculations are reported for the reaction and vibrationally inelastic scattering of (v = 0,1,j = 0) H(2) scattering from Cu(110), and for the reaction and rovibrationally elastic and inelastic scattering of (v = 1,j = 1) H(2) scattering from Cu(110). The dynamics results were obtained using a potential energy surface obtained with density functional theory using the PW91 functional. The reaction probabilities computed with quantum dynamics for (v = 0,1,j = 0) were in excellent agreement with the QCT results obtained earlier for these states, thereby validating the QCT approach to sticking of hydrogen on Cu(110). The vibrational de-excitation probability P(v=1,j = 0 --> v = 0) computed with the QCT method is in remarkably good agreement with the quantum dynamical results for normal incidence energies E(n) between 0.2 and 0.6 eV. The QCT result for the vibrational excitation probability P(v = 0,j = 0 --> v = 1) is likewise accurate for E(n) between 0.8 and 1 eV, but the QCT method overestimates vibrational excitation for lower E(n). The QCT method gives probabilities for rovibrationally (in)elastic scattering, P(v = 1,j = 1 --> v('),j(')), which are in remarkably good agreement with quantum dynamical results. The rotationally averaged, initial vibrational state-selective reaction probability obtained with QCT agrees well with the initial vibrational state-selective reaction probability extracted from molecular beam experiments for v = 1, for the range of collision energies for which the v=1 contribution to the measured total sticking probability dominates. The quantum dynamical probabilities for rovibrationally elastic scattering of (v = 1,j = 1) H(2) from Cu(110) are in good agreement with experiment for E(n) between 0.08 and 0.25 eV.     

N2 emission in NO and N2O reduction on Rh(100) and Rh(110)

The angular distribution of desorbing N(2) was studied in both the thermal decomposition of N(2)O(a) on Rh(100) at 60-140 K and the steady-state NO (or N(2)O) + D(2) reaction on Rh(100) and Rh(110) at 280-900 K. In the former, N(2) desorption shows two peaks at around 85 and 110 K. At low N(2)O coverage, the desorption at 85 K collimates at about 66 degrees off normal towards the [001] direction, whereas at high coverage, it sharply collimates along the surface normal. In the NO reduction on Rh(100), the N(2) desorption preferentially collimates at around 71 degrees off normal towards the [001] direction below about 700 K, whereas it collimates predominantly along the surface normal at higher temperatures. At lower temperatures, the surface nitrogen removal in the NO reduction is due to the process of NO(a) + N(a) --> N(2)O(a) --> N(2)(g) + O(a). On the other hand, in the steady-state N(2)O + D(2) reaction on Rh(110), the N(2) desorption collimates closely along the [001] direction (close to the surface parallel) below 340 K and shifts to ca. 65 degrees off normal at higher temperatures. In the reduction with CO, the N(2) desorption collimates along around 65 degrees off normal towards the [001] direction above 520 K, and shifts to 45 degrees at 445 K with decreasing surface temperature. It is proposed that N(2)O is oriented along the [001] direction on both surfaces before dissociation and the emitted N(2) is not scattered by adsorbed hydrogen.     

An XPS and ups study of chloride chemisorption on Ag(110)

Chlorine chemisorbs rapidly on Ag(110) at 300 K to a saturation coverage of about one monolayer and with a sticking probability of order unity. The corresponding work function shift is + 1.7 eV. XP and UP spectra are consistent with the presence of a single king of chemisorbed species. It appears that Ag behaves as a single s-band solid and that the adsorbate 3p level is shifted so as to lie in the metal s-band; these observations are in agreement with the recent theoretical predictions of Anderson.     

The chemisorption of carbon monoxide on Ni(110) studied by electron energy loss spectroscopy

The vibrational spectrum of carbon monoxide chemisorbed on Ni(110) at 300K has been recorded as a function of surface coverage. At low and intermediate coverage the adsorbate is bonded either to single nickel atoms (linear site) or to two nickel atoms in contact (B₂ site). As the coverage approaches unity the spectrum changes rapidly until at saturation virtually all adsorbed molecules are of B₂ type.     

Nondissociative chemisorption of methanethiol on Ag(110): a critical result for self-assembled monolayers

Three definitive experiments have been performed to investigate the possibility of dissociative adsorption of methanethiol (CH3SH) on clean Ag(110). On the clean Ag(110) surface, the adsorption in the first layer occurs to 0.5 ML, producing a (2 x 1) low-energy electron diffraction (LEED) structure. The undissociated molecule desorbs starting at approximately 140 K, and only tiny quantities of other gaseous products are desorbed, and only tiny quantities of S-containing species remain. Using a 50:50% mixture of CH3SD and CD3SH, we find no evidence of S-H or S-D bond scission between these molecules upon desorption. And finally, when the CH3SH molecule is incident on the clean Ag(110) surface in the temperature range of 230-400 K, less than 1% of the incident molecules dissociate to produce adsorbed sulfur-containing species. The results influence our thinking about the surface bonding of alkanethiol-based self-assembled monolayers (SAMs) on noble metals.     

MIES and UPS(HeI) studies on reduced TiO2(110)

We have studied reduced TiO₂(110) surfaces by combining metastable impact electron spectroscopy (MIES) and UPS(HeI). The reduced Ti species were preparation‐induced: their number density was modified either by adsorption of K atoms or by a combined annealing/oxygen exposure procedure. The emission from the bandgap state (binding energy 0.9 eV), caused by reduced Ti³⁺ 3d species, was monitored. Bandgap emission is seen clearly with UPS(HeI) and thus can be used to monitor the number density of the near‐surface reduced species. A corresponding spectral structure cannot be seen with MIES. We propose that the excess charge density introduced either by preparation‐induced oxygen vacancies or by K adsorption is delocalized over several surface and subsurface Ti sites; this, together with the partial shielding of the reduced Ti species, prevents detection of the reduced Ti species with MIES. The re‐oxidation and restructuring of the reduced TiO₂(110) surface, caused by simultaneous oxygen exposure and annealing, was studied at temperatures between 400 and 770 K, again by recording the Ti³⁺ 3d emission (0.9 eV bandgap state) with UPS(HeI). The surface can be completely re‐oxidized by oxygen exposure at any selected annealing temperature in the range given above. Morphology changes, leading to a partially reduced surface, take place when the re‐oxidized surface is further annealed at T > 600 K under reducing conditions. The results give support to the assumption that the re‐oxidation is caused by the growth of additional titania whereby the Ti stems from the bulk and the oxygen originates from the gas. The restructuring of the re‐oxidized surface upon annealing under reducing conditions appears to be due to diffusion of Ti interstitials to the surface. Copyright © 2004 John Wiley & Sons, Ltd.     

N2O decomposition on TiO2 (110) from dynamic first-principles calculations

We have carried out a systematic study of N(2)O dissociation on a TiO(2) (110) surface by means of plane-wave pseudopotential density-functional theory calculations. We have made use of both static and dynamic calculations in order to elucidate N(2)O decomposition mechanisms. We find that dissociation is not favorable on the stoichiometric surface. On the other hand, the presence of oxygen bridging vacancies make the N(2)O decomposition possible. The role of the defective surface is to provide electrons to the adsorbed molecule. We find two channels for decomposition, depending on whether the molecule is adsorbed with the O or the N end of the molecule on a vacancy. The first case is energetically downhill and proceeds spontaneously, leading to N(2) ejection from the surface and vacancy oxidation. The second case relies on the formation of an intermediate bridging configuration of the adsorbed molecule and is hindered by a small energy barrier. In this case, molecule breaking produces N(2) in the gas phase and leaves oxygen adatoms on the surface. We relate our results to recent experimental findings.     

Inclined N2 desorption in N2O reduction by D2 and CO on Pd(110)

Inclined N2 desorption was examined in the course of a catalyzed N2O + D2 (or CO) reaction on Pd(110) by angle-resolved mass spectroscopy combined with cross-correlation time-of-flight techniques. N2 desorption collimated at around 45 degrees off the normal toward the [001] direction in the temperature range of 400-800 K. Its collimation angle and kinetic energy were insensitive to both the surface temperature and surface conditions throughout the kinetic transition. It is proposed that this peculiar N2 desorption is induced by the decomposition of N2O oriented along the [001] direction.     

The reactive chemisorption of alkyl iodides at Cu(110) and Ag(111) surfaces: a combined STM and XPS study

The chemisorption of methyl and phenyl iodide has been studied at Cu(110) and Ag(111) surfaces at 290 K with STM and XPS. At both surfaces dissociative adsorption of both molecules leads to chemisorbed iodine, with the STM showing c(2 x 2) and (square root 3 x square root 3)R30 structures at the Cu(110) and Ag(111) surfaces, respectively. At the Cu(110) surface a comparison of coexisting c(2 x 2) I(a) and p(2 x 1) O(a) domains shows the iodine adatoms to be chemisorbed in hollow sites with evidence at low coverage for diffusion in the (110) direction. In the case of methyl iodide no carbon adsorption is observed at either the silver or the copper surfaces, but chemisorbed phenyl groups are imaged at the Cu(110) surface after exposure to phenyl iodide. The STM images show the phenyl groups as bright features approximately 0.7 nm in diameter and 0.11 nm above the iodine adlayer, reaching a maximum surface concentration after approximately 6 Langmuir exposure. However, the phenyl coverage decreases with subsequent exposures to PhI and is negligible by approximately 1000 L exposure, consistent with the formation and desorption of biphenyl. The adsorbed phenyls are located above hollow sites in the substrate, they are stabilized at the top and bottom of step edges and in paired chains (1.1 nm apart) on the terraces with a regular interphenyl spacing within the chains of 1.0 nm in the (110) direction. The interphenyl ring spacing and diffusion of individual phenyls from within the chains shows that the chains do not consist of biphenyl species but may be a precursor to their formation. Although the XPS data shows carbon present at the Ag(111) surface after exposure to PhI, no features attributable to phenyl groups were observed by STM.     

The chemisorption of hydrogen on Cu(111): A dynamical study

Ab initio band-structure calculations within a density functional formalism were performed to compute the binding energy curves of atomic hydrogen with the high-symmetry adsorption sites of the (111) surface of copper. For a two-layer slab of Cu atoms and H coverage equal to 0.25, the binding energies are 2.25, 3.12, and 3.24 eV, for on-top, bridge, and threefold sites, so that the chemisorption of H₂ on Cu(111) is exothermic for threefold and bridge sites, but endothermic for on-top sites. Starting from these results, an LEPS potential for the interaction of H₂ with the Cu(111) surface was built. In this model potential, the most favored approaches correspond to a H₂ molecule parallel to the Cu surface, and for them, the activation barrier is located at the corner between the entrance and the exit channels of the reaction, and its lowest value is 0.6 eV. The LEPS potential was used in quasi-classical trajectories calculations to simulate the adsorption of a beam of H₂ molecules on Cu(111). The results show that (a) when H₂ is in the ground vibrational state the dissociative adsorption probability P_a increases from 0 to .90 along a roughly sigmoidal curve by increasing the collision kinetic energy from 0.4 to 1.3 eV, and (b) the vibrational energy can be as effective as the translational one in promoting dissociative chemisorption, in agreement with the experimental results. © 1994 John Wiley & Sons, Inc.     

Inclined N2 desorption in a steady-state N2O + CO reaction on Pd(110)

The angular and velocity distributions of desorbing products were analyzed in the course of a catalyzed N2O + CO reaction on Pd(110). The reaction proceeded steadily above 450 K, and the N2 desorption merely collimated sharply along 45 degrees off the surface normal toward the [001] direction. It is proposed that this peculiar N2 desorption is induced by the decomposition of adsorbed N2O oriented along the [001] direction. On the basis of the observation of similar inclined N2 desorption in both NO + CO and N2O + CO reactions, the N2 formation via the intermediate N2Oa dissociation was confirmed in catalytic NO reduction.     

Dissociative and non-dissociative adsorption dynamics of N2 on Fe(110)

We study the adsorption dynamics of N(2) on the Fe(110) surface. Classical molecular dynamics calculations are performed on top of a six-dimensional potential energy surface calculated within density functional theory. Our results show that N(2) dissociation on this surface is a highly activated process that takes place along a very narrow reaction path with an energy barrier of around 1.1 eV, which explains the measured low reactivity of this system. By incorporating energy exchange with the lattice in the dynamics, we also study the non-dissociative molecular adsorption process. From the analysis of the potential energy surface, we observe the presence of two distinct N(2) adsorption wells. Our dynamics calculations show that the relative population of these adsorption sites varies with the incident energy of the molecule and the surface temperature. We find an activation energy of around 150 meV that prevents molecular adsorption under thermal and hypothermal N(2) gas exposure of the surface. This finding is also consistent with the available experimental information.     

Surface-mediated NH and N addition to styrene on Ag(110)

In this paper, we report the investigation of the reaction of adsorbed NH and N with styrene on Ag(110) using temperature-programmed reaction spectroscopy. Using O2 and NH3 as the starting reagents, NH and N species were deposited on a Ag surface. The reaction of styrene on NH- and N-covered Ag surface appears to yield 2-phenylaziridine and benzonitrile, with additional products HCN and NH3. The formation of aziridine, the nitrogenous analogue of styrene epoxide, is proposed to be due to the cycloaddition of adsorbed NH to the carbon-carbon double bond. These results suggest that Ag-based heterogeneous catalysts may be useful for the aziridination of olefins.     

The collimation angle shift of desorbing product N2 in a steady-state N2O+CO reaction on Rh(110)

The angular distribution of desorbing product N2 was studied in N2O decompositions on Rh(110) in the temperature range of 60-700 K. The N2 desorption collimates along 62 degrees -68 degrees off normal toward either the [001] or [001] direction in a transient N2O decomposition below ca. 470 K or in the steady-state N2O+CO reaction above 540 K. In the steady-state reaction at the temperature from ca. 470 to 540 K, however, the collimation angle shifts from 62 degrees to 45 degrees with decreasing surface temperature. This angle shift is ascribed to the steric hindrance by coadsorbed CO because the N2 collimation in transient N2O decomposition at around 65 degrees is recovered in the range of 380-500 K by an abrupt CO pressure drop followed by the decrease in CO coverage. N2O is oriented along the [001] direction before dissociation. A scattering model of the nascent N2 by adsorbed CO is proposed, yielding smaller collimation angles.     

A molecular orbital study of the difluorodiazine (N2F2) system

A semiempirical MO method (CNDO/2) with empirical constants adjusted to give agreement with ground state geometries has been used to study isomerization mechanism in the N₂F₂ system. Calculated transition states do not correspond to the simple postulated transition states.     

A tandem cross-coupling/S(N)Ar approach to functionalized carbazoles

A novel route to functionalized carbazoles utilizing a tandem Suzuki cross-coupling/SNAr protocol is described. This process was found to be compatible with a variety of electron-withdrawing groups including aldehydes, esters, and sulfones. Using this method, a concise total synthesis (four steps, 50% overall yield) of the carbazole alkaloid glycosinine was achieved.     

The chemisorption of coronene on Si(001)-2 x 1

Coronene (C24H12) adsorption on the clean Si(001)-2 x 1 surface was investigated by scanning tunneling microscopy and by density-functional calculations. The coronene adsorbed randomly at 25 degrees C on the surface and did not form two-dimensional islands. The scanning tunneling microscopy measurements revealed three adsorption sites for the coronene molecule on the Si(001) surface at low coverage. The major adsorption configuration involves coronene bonding to four underlying Si atoms spaced two lattice spacings apart in a dimer row. The two minor adsorption configurations involve asymmetrical bonding of a coronene molecule between Si dimer rows and form surface species with a mirror plane symmetry to their chiral neighbor species. The two minor bonding arrangements are stabilized by a type-C defect on the Si(001) surface.     

Abstractive chemisorption of O2 on Al(111)

We present experimental evidence that abstraction is a common mechanism (approximately 50%) in the dissociative chemisorption of oxygen on Al(111) at a translational energy of 0.5 eV. As a result of this mechanism, individual isolated O-atoms are observed in scanning tunneling microscopy (STM). At this translational energy ordinary dissociative chemisorption processes also occur, resulting in pairs of adatoms. The ejected O-atoms originating from the abstraction reaction are detected in the gas phase using laser spectrometry. Together, these observations provide compelling evidence for the abstraction mechanism.