Adsorption and activation of CO over flat and stepped Co surfaces: a first principles analysis

The adsorption and activation of CO over flat Co{0001}, corrugated Co{1120}, and stepped Co{1012} and Co{1124} surfaces have been analyzed using periodic density functional theory calculations. CO strongly chemisorbs on all these surfaces but does not show a strong dependence on the surface structure. The calculated structure of adsorbed CO on Co{0001} at 1/3 monolayer (ML) of coverage was found to be in good agreement with the experiment. The barrier for CO dissociation over Co{0001} was found to decrease with decreasing CO coverage, taking on a value of 232 kJ/mol at 1/4 ML and 218 kJ/mol at 1/9 ML. The presence of the "zigzag" channel on Co{1120} enhances the reactivity slightly by reducing the barrier for CO dissociation to 195 kJ/mol. In contrast, the stepped Co{1012} and Co{1124} surfaces are much more active than the flat and corrugated surfaces. Both stepped surfaces provide direct channels for CO dissociation that do not have barriers with respect to gas-phase CO. In general the activation barriers lower as the reaction energies become more exothermic. Reconstruction of the step edges that occur in the product state, however, prevents a linear correlation between the reaction energy and the activation energy.     

Structure of Pt deposited on TiO2 surfaces modified by high-energy electron-beams under ambient pressure

Synchrotron-radiation based photoemission spectroscopy (SRPES) was used to examine the changes in the TiO₂ surface structures as a result of high-energy e-beam exposure (under atmospheric pressure). The influence of the e-beam pre-treatment on the structure of the post-deposited Pt films on TiO₂ was also investigated. High-energy e-beam treatment increased concentrations of OH species and C–O bonds on the surface. The interface reaction between Ti-oxide and Pt was reduced by the e-beam treatment. Consequently, different Pt film surface morphologies could be found on TiO₂ with and without e-beam treatment.     

Electrochemical and spectroscopic studies of ethanol oxidation on Pt stepped surfaces modified by tin adatoms

Ethanol oxidation on platinum stepped surfaces vicinal to the (111) pole modified by tin has been studied to determine the role of this adatom in the oxidation mechanism. Tin has been slowly deposited so that the initial stages of the deposition take place on the step, and deposition on the terrace only occurs when the step has been completely decorated. Voltammetric and chronoamperometric experiments demonstrate that tin on the step catalyzes the oxidation. The maximum enhancement is found when the step is completely decorated by tin. FTIR experiments using normal and isotopically labeled ethanol have been used to elucidate the effect of the tin adatoms in the mechanism. The obtained results indicate that the role of tin is double: (i) when the surface has sites capable of breaking the C-C bond of the molecule, that is, when the step sites are not completely covered by tin, it promotes the oxidation of CO formed from the molecular fragments to CO(2) through a bifunctional mechanism and (ii) it catalyzes the oxidation of ethanol to acetic acid.     

Isobaric-isothermal monte carlo simulations from first principles: application to liquid water at ambient conditions.

A series of first-principles Monte Carlo simulations in the isobaric-isothermal ensemble were carried out for liquid water at ambient conditions (T=298 K and p=1 atm). The Becke-Lee-Yang-Parr (BLYP) exchange and correlation energy functionals and norm-conserving Goedecker-Teter-Hutter (GTH) pseudopotentials were employed with the CP2 K simulation package to examine systems consisting of 64 water molecules. The fluctuations in the system volume encountered in simulations in the isobaric-isothermal ensemble require a reconsideration of the suitability of the typical charge-density cutoff and the regular grid-generation method previously used for the computation of the electrostatic energy in first-principles simulations in the microcanonical or canonical ensembles. In particular, it is noted that a much higher cutoff is needed and that the most computationally efficient method of creating grids can result in poor simulations. Analysis of the simulation trajectories using a very large charge-density cutoff at 1200 Ry and four different grid-generation methods point to a significantly underestimated liquid density of about 0.8 g cm-3 resulting in a somewhat understructured liquid (with a value of about 2.7 for the height of the first peak in the oxygen-oxygen radial distribution function) for BLYP-GTH water at ambient conditions. In addition, a simulation using a charge-density cutoff at 280 Ry yields a higher density of 0.9 g cm-3, showing the sensitivity of the simulation outcome to this parameter.     

Dynamics of CO at the solid/liquid interface studied by modeling and simulation of CO electro-oxidation on Pt and PtRu electrodes

We consider theoretical models for CO monolayer oxidation on stepped Pt single-crystal electrodes and Ru-modified Pt(111) electrodes. For both systems, our aim is to assess the importance of CO surface diffusion in reproducing the experimental chronoamperometry or voltammetry. By comparing the simulations with the experimental chronoamperometric transients for CO oxidation on a series of stepped Pt surfaces, it was concluded that mixing of CO on the Pt(111) terrace is good, implying rapid diffusion (N. P. Lebedeva, M. T. M. Koper, J. M. Feliu and R. A. van Santen, J. Phys. Chem. B, submitted). We discuss here a more detailed model in which the CO adsorbed on steps is converted into CO adsorbed on terraces as the oxygen-containing species occupy the steps (as observed experimentally on stepped Pt in UHV), followed by a subsequent oxidation of the latter, to reproduce the observed chronoamperometry on stepped surfaces with a higher step density. On Ru-modified Pt(111), the experimentally observed splitting of the CO stripping voltammetry into two stripping peaks, may suggest a slow diffusion of CO on Pt(111). This apparent contradiction with the conclusions of the experiments on stepped surfaces, is resolved by assuming a weaker CO binding to a Pt atom which has Ru neighbors than to "bulk" Pt(111), in agreement with recent quantum-chemical calculations. This makes the effective diffusion from the uncovered Pt(111) surface to the perimeter of the Ru islands, which are considered to be the active sites in CO oxidation electrocatalysis on PtRu surfaces, very slow. Different models for the reaction are considered, and discussed in terms of their ability to explain experimental observations.     

CO hydrogenation on stepped Cu and CuZn alloy surfaces: Competition between methanol synthesis and methanation pathways

Comprehensive fundamental understanding of CO hydrogenation reactions over Cu and ZnCu alloy surfaces is of great importance. Herein, we report a comparative DFT calculation study of elementary surface reaction network of CO hydrogenation reactions on stepped Cu(211), Cu(611), ZnCu(211) and ZnCu(611) surfaces. On ZnCu(211) and ZnCu(611) surfaces, the energetic favorable reaction path of CO hydrogenation reaction follows CO* → HCO* → H₂CO* → H₃CO* → CH₃OH* → CH₃OH with H₃CO* hydrogenation as the rate-limiting step and proceeds more facilely on ZnCu(611) surface than on ZnCu(211) surface. On Cu(211) and Cu(611) surfaces, the energetic favorable reaction path of CO hydrogenation reaction follows CO* → HCO* → HCOH* → H₂COH* → H₃COH* → CH₃* → CH₄* → CH₄ with H₂COH* hydrogenation as the rate-limiting step and proceeds more facilely on Cu(611) than on Cu(211). The key difference of CO hydrogenation reaction on ZnCu alloy surface and Cu is that the resulting CH₃OH* species desorbs to produce CH₃OH on ZnCu alloy but undergoes H*-assisted decomposition to CH₃* and eventually to CH₄ on Cu surface. These results successfully unveil elementary surface reaction networks and structure sensitivity of Cu and ZnCu alloy-catalyzed CO hydrogenation reactions.     

Photodesorption of diatomic molecules from surfaces: A theoretical approach based on first principles

Photodesorption of small molecules from surfaces is one of the most fundamental processes in surface photochemistry. Despite its apparent simplicity, a microscopic understanding beyond a qualitative picture still poses a true challenge for theory. While the dynamics of nuclear motion can be treated on various levels of sophistication, all approaches suffer from the lack of sufficiently accurate potential energy surfaces, in particular for electronically excited states involved in the desorption scenario.In the last decade, a systematic and accurate methodology has been developed which allows a reliable calculation of accurate ground and excited state potential energy surfaces (PES) for different adsorbate–substrate systems. These potential energy surfaces serve as a prerequisite for subsequent quantum dynamical wave packet calculations, which allow for a direct simulation of experimentally observable quantities such as quantum state resolved velocity distributions.In the first part of this review, we will focus on scalar properties of desorbing diatomic molecules from insulating surfaces, where we also present a recently developed strategy of obtaining accurate potential energy surfaces using quantum chemical approaches. In general, diatomic molecules on large band gap materials such as oxide surfaces are studied which allows the use of sufficiently large cluster models and accurate ab initio methods beyond density functional theory (DFT). In the second part, we will focus on the vectorial aspects of the dynamics of nuclear motion and present simulations of experimentally accessible observables such as velocity distributions, Doppler profiles and alignment parameters. For each system, the microscopic mechanism of photodesorption is elucidated. We will demonstrate that the driving force of surface photochemistry is strongly dependent on details of the electronic structure of the adsorbate–substrate systems. This implies that great caution is advisable if experimental results are interpreted using empirical or semi-empirical models.     

Vibrational energy relaxation at surfaces: O2 chemisorbed on Pt(111)

It is shown that the O-O stretch for O₂ chemisorbed on Pt(111) is likely to be damped via excitation of electron-hole pairs. Vibrational spectroscopic data for this system are analyzed using a model which assumes an adsorbate-induced resonance in the vicinity of the Fermi energy. In accordance with theory, the vibrational line profile is asymmetric and from the deduced linewidth γ, asymmetry parameter τ and vibrational polarizability α_v, we estimate the magnitude of the electron-phonon coupling parameter and derive information as to the nature of the adsorbate-induced resonance state.     

O2-coverage-dependent CO oxidation on reduced TiO2(110): A first principles study

First principles periodic slab calculations based on gradient-corrected density functional theory have been performed to investigate CO oxidation on rutile TiO2(110) at varying O2 coverages (theta = 1, 2, and 3, where theta is defined as the number of O2 per oxygen vacancy). For each coverage we only present the reaction of CO with oxygen species in the most stable configuration. Our results show a significant variation in the oxidation activation energy with O2 coverage.     

Perturbation of adsorbed CO by amine derivatives coadsorbed on the gamma-Al2O3 surface: FTIR and first principles studies

The coadsorption of CO and triethylenediamine (TEDA) (also called 1,4-diazabicyclo[2.2.2]octane, DABCO) on a high-area gamma-Al2O3 surface has been investigated with use of transmission FTIR spectroscopy. It has been found that TEDA binds more strongly to both Lewis acid sites and to Br?nsted Al-OH sites than does CO. Competition experiments indicate that TEDA displaces CO to less strong binding sites. Evidence for weak CO...TEDA interactions is found in which small nu(CO) redshifts are produced. Comparison between different amines such as triethylenemonoamine (TEMA) (also called 1-azabicyclo[2.2.2]octane, ABCO), trimethylamine (TMA), and ammonia indicates that the nu(CO) redshift increases with increasing amine polarizability, indicating that the redshift is mainly due to dipole image damping effects on the CO oscillator frequency. The direct bonding between the exposed N lone pair electrons of the TEDA molecule and CO does not occur. First principles theoretical studies have characterized the bonding of CO with gamma-Al2O3 Lewis acid sites of various types as well as TEDA bonding to both Lewis acid sites and to Al-OH groups. The theoretical studies also indicate that strong bonding of adsorbed CO with TEDA molecules does not occur, and that the observed decrease in the binding energy of CO when coadsorbed with TEDA on gamma-Al2O3 is expected.     

Adsorption/oxidation of CO on highly dispersed Pt catalyst studied by combined electrochemical and ATR-FTIRAS methods: oxidation of CO adsorbed on carbon-supported Pt catalyst and unsupported Pt black

ATR-FTIRAS measurements combined with linear potential sweep voltammetry were conducted to investigate oxidation of CO adsorbed on a highly dispersed Pt catalyst supported on carbon black, Pt/C, and carbon-unsupported Pt black catalyst, Pt-B. Bands nu(CO) of atop- and bridge-bonded COs were resolved into those of COs adsorbed at terrace and step edge sites by curve-fitting analysis. At the high coverage near the saturation, a band around 1950-1960 cm(-1) assigned to asymmetric bridge-bonded CO, CO(B)(asym), was observed to develop on both Pt/C and Pt-B, which was the predominant type on the latter. Preferential oxidation of atop-CO adsorbed at the step edge site was commonly observed on both Pt/C and Pt-B during the potential sweep from 0.05 to 1.2 V. However, it has been found that CO(B)(asym) is the most reactive species. The high reactivity of the CO(B)(asym) on Pt/C and Pt-B is demonstrated for the first time in the present report. Adsorption of CO on the Pt/C and Pt-B resulted in growth of a sharp nu(OH) band around 3642-3645 cm(-1) which is assigned to non-hydrogen-bonded water molecules coadsorbed with CO. The nu(OH) band frequency exhibits a linear increase with potential with a Stark tuning rate of ca. 20 cm(-1)/V. Analysis of the potential dependence of this band in the CO oxidation potential region led us to conclude that this is the oxygen-containing species to oxidize adsorbed CO. Stark tuning rates of nu(CO) bands for the COs at the terrace and step edge sites on both Pt/C and Pt-B are almost independent of the adsorption sites for both atop- and bridge-bonded COs. However, CO(B)(asym) exhibits tuning rates of 41 cm-1/V and 37 cm-1/ V on Pt/C and Pt-B, respectively, which is in between the rates of atop and symmetric bridge-bonded COs.     

Real-time monitoring of the progress of polymerization reactions directly on surfaces at open atmosphere by ambient mass spectrometry

The progress of an on-surface polymerization process involving alkyl and perfluoroalkyl silanes and siloxanes was monitored in real-time via easy ambient sonic spray ionization mass spectrometry (EASI-MS). When sprayed on surfaces, the organosilicon compounds present in commercially available nanofilm products (NFPs) react by condensation to form a polymeric coating. A NFP for coating of floor materials (NFP-1) and a second NFP for coating tiles and ceramics (NFP-2) were applied to glass, filter paper or cotton surfaces and the progress of the polymerization was monitored by slowly scanning the surface. Via EASI(+)-MS monitoring, significant changes in the composition of hydrolysates and condensates of 1H,1H,2H,2H-perfluorooctyl triisopropoxysilane (NFP-1) and hexadecyl triethoxysilane (NFP-2) were observed over time. The abundances of the hydrolyzed species decreased compared with those of the non-hydrolysed species for both NFP-1 and NFP-2 and the heavier oligomers became relatively more abundant over a period of 15-20 min. A similar tendency favouring the heavier oligomers was observed via EASI(-)-MS. This work illustrates the potential of ambient mass spectrometry for the direct monitoring of polymerization reactions on surfaces.     

Surface chemistry on bimetallic alloy surfaces: adsorption of anions and oxidation of CO on Pt3Sn(111)

The microscopic structure of the Pt(3)Sn(111) surface in an electrochemical environment has been studied by a combination of ex situ low-energy electron diffraction (LEED), Auger electron spectroscopy (AES), and low-energy ion scattering (LEIS) and in situ surface X-ray scattering (SXS) and Fourier transform infrared (FTIR) spectroscopy. In ultrahigh vacuum (UHV) the clean-annealed surface produces a p(2 x 2) LEED pattern consistent with the surface composition, determined by LEIS, of 25 at. % Sn. SXS results show that the p(2 x 2) structure can be "transferred" from UHV into 0.5 M H(2)SO(4) and that the surface structure remains stable from 0.05 to 0.8 V. At 0.05 V the expansion of Pt surface atoms, ca. +2% from the bulk lattice spacing, is induced by adsorption of underpotential-deposited (UPD) hydrogen. At 0.5 V, where Pt atoms are covered by (bi)sulfate anions, the topmost layer is contracted relative to 0.05 V, although Sn atoms expand significantly, ca. 8.5%. The p(2 x 2) structure is stable even in solutions containing CO. In contrast to the Pt(111)-CO system, no ordered structures of CO are formed on the Pt(3)Sn(111) surface and the topmost layer expands relatively little (ca. 1.5%) from the bulk lattice spacing upon the adsorption of CO. The binding site geometry of CO on Pt(3)Sn(111) is determined by FTIR. In contrast to the near invariant band shape of a-top CO on Pt(111), changes in band morphology (splitting of the band) and vibrational properties (increase in the frequency mode) are clearly visible on the Pt(3)Sn(111) surface. To explain the line shape of the CO bands, we suggest that in addition to alloying effects other factors, such as intermolecular repulsion between coadsorbed CO and OH species, are controlling segregation of CO into cluster domains where the local CO coverage is different from the coverage expected for the CO-CO interaction on an unmodified Pt(111) surface.     

Adsorbate-adsorbate interactions and chemisorption at different coverages studied by accurate ab initio calculations: CO on transition metal surfaces

We use density functional theory (DFT) with the generalized gradient approximation (GGA) and our first-principles extrapolation method for accurate chemisorption energies (Mason et al. Phys. Rev. B 2004, 69, 161401R) to calculate the chemisorption energy for CO on a variety of transition metal surfaces for various adsorbate densities and patterns. We identify adsorbate through-space repulsion, bonding competition, and substrate-mediated electron delocalization as key factors determining the preferred chemisorption patterns for different metal surfaces and adsorbate coverages. We discuss how the balance of these interactions, along with the inherent adsorption site preference on each metal surface, can explain the observed CO adsorbate patterns at different coverages.     

CO oxidation on nanostructured SnO(x)/Pt(111) surfaces: unique properties of reduced SnO(x)

We have investigated surface CO oxidation on "inverse catalysts" composed of SnO(x) nanostructures supported on Pt(111) using X-ray photoelectron spectroscopy (XPS), low-energy ion scattering spectroscopy (LEISS) and temperature-programmed desorption (TPD). Nanostructures of SnO(x) were prepared by depositing Sn on Pt(111) pre-covered by NO(2) layers at low temperatures. XPS data show that the SnO(x) nanoparticles are highly reduced with Sn(II)O being the dominant oxide species, but the relative concentration of Sn(II) in the SnO(x) nanoparticles decreases with increasing Sn coverage. We find that the most active SnO(x)/Pt(111) surface for CO oxidation has smallest SnO(x) coverage. Increasing the surface coverage of SnO(x) reduces CO oxidation activity and eventually suppresses it altogether. The study suggests that reduced Sn(II)O, rather than Sn(IV)O(2), is responsible for surface CO oxidation. The occurrence of a non-CO oxidation reaction path involving reduced Sn(II)O species at higher SnO(x) coverages accounts for the decreased CO oxidation activity. From these results, we conclude that the efficacy of CO oxidation is strongly dependent on the availability of reduced tin oxide sites at the Pt-SnO(x) interface, as well as unique chemical properties of the SnO(x) nanoparticles.     

Rapid on-site detection of explosives on surfaces by ambient pressure laser desorption and direct inlet single photon ionization or chemical ionization mass spectrometry

Considering current security issues, powerful tools for detection of security-relevant substances such as traces of explosives and drugs/drug precursors related to clandestine laboratories are required. Especially in the field of detection of explosives and improvised explosive devices, several relevant compounds exhibit a very low vapor pressure. Ambient pressure laser desorption is proposed to make these substances available in the gas phase for the detection by adapted mass spectrometers or in the future with ion-mobility spectrometry as well. In contrast to the state-of-the-art thermal desorption approach, by which the sample surface is probed for explosive traces by a wipe pad being transferred to a thermal desorber unit, by the ambient pressure laser desorption approach presented here, the sample is directly shockwave ablated from the surface. The laser-dispersed molecules are sampled by a heated sniffing capillary located in the vicinity of the ablation spot into the mass analyzer. This approach has the advantage that the target molecules are dispersed more gently than in a thermal desorber unit where the analyte molecules may be decomposed by the thermal intake. In the technical realization, the sampling capillary as well as the laser desorption optics are integrated in the tip of an endoscopic probe or a handheld sampling module. Laboratory as well as field test scenarios were performed, partially in cooperation with the Federal Criminal Police Office (Bundeskriminalamt, BKA, Wiesbaden, Germany), in order to demonstrate the applicability for various explosives, drugs, and drug precursors. In this work, we concentrate on the detection of explosives. A wide range of samples and matrices have been investigated successfully.     

C-H activation of imidazolium salts by Pt(0) at ambient temperature: synthesis of hydrido platinum bis(carbene) compounds

A zerovalent platinum(carbene) complex with two monoalkene ligands, which is able to activate C-H bonds of imidazolium salts at room temperature to yield isolable hydrido platinum(II) bis(carbene) compounds, has been synthesised for the first time.     

Reaction-path switching induced by spatial-distribution change of reactants: CO oxidation on Pt(111)

We studied the mechanism of CO oxidation on O-covered Pt(111) surfaces during CO exposure by means of time-resolved near edge x-ray absorption fine structure spectroscopy. Two distinct reaction processes were found to occur sequentially; isolated O atoms and island-periphery O atoms contribute to each process. Combination of in situ monitoring of the reaction kinetics and Monte Carlo simulations revealed that CO coadsorption plays a role of inducing the dynamic change in spatial distribution of O atoms, which switches over the two reaction paths.     

The comparison of geometric and electronic properties of molecular surfaces by neural networks: Application to the analysis of corticosteroid-binding globulin activity of steroids

Summary It is shown how a self-organizing neural network such as the one introduced by Kohonen can be used to analyze features of molecular surfaces, such as shape and the molecular electrostatic potential. On the one hand, two-dimensional maps of molecular surface properties can be generated and used for the comparison of a set of molecules. On the other hand, the surface geometry of one molecule can be stored in a network and this network can be used as a template for the analysis of the shape of various other molecules. The application of these techniques to a series of steroids exhibiting a range of binding activities to the corticosteroid-binding globulin receptor allows one to pinpoint the essential features necessary for biological activity.     

Nucleation of chemical waves at defects: a mirror electron microscopy study of catalytic CO oxidation on Pt(110)

Using mirror electron microscopy (MEM) as spatially resolving method the nucleation of chemical waves in catalytic CO oxidation on a Pt(110) surface was investigated in the 10(-5) mbar range. The waves nucleated at an electrically insulating impurity of approximately 15 microm diameter (the "defect") which most likely represents a diamond particle left over from the polishing process. Nucleation events are initiated by a dynamic process in a boundary layer of approximately 1 microm width between the defect and the surrounding Pt(110) surface. Depending on the parameter choice the fronts/pulses do not escape from the vicinity of the defect and later on die out or, in a supercritical nucleation, propagate across the surface. Asymmetric nucleation leads to spiral waves which remain pinned to the defect. The defect has a kind of steering effect causing chemical waves to collide exactly at the defect. This steering effect is evidently due to a distortion of the substrate lattice in the vicinity of the defect.