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• Density functional theory calculations of adsorption energies of CO on gold surfaces and surface edges. • Minimum energy paths show that adsorption energies don't capture the nuances of mechanisms. • The lowest barriers are seen on the (311)/(311) edge. Localized surface plasmons (LSP) on faceted surfaces of gold nanoparticles enable carbon monoxide disproportionation to be driven at room temperature. In order to expand the known surfaces that catalyze this reaction, we explore the adsorption of carbon monoxide at top, long bridge, short bridge, and hole sites on gold (100), (110), (111), (211), and (311) faceted surfaces, as well as the reaction barriers for disproportionation at the lowest energy adsorption site on each surface and edges between two (311) surfaces and (100) and (110) surfaces. Generally, the less atomically dense, higher index facets promote both good adsorption and reactivity, and the edges show lower barriers for disproportionation. For most of the explored surfaces, adsorption directly on top of a gold atom is most favorable. The lowest activation energy for carbon monoxide disproportionation to amorphous carbon and carbon dioxide is predicted for two carbon monoxides adsorbed on top of atoms on the (311)/(311) edge. [Display omitted] [ABSTRACT FROM AUTHOR]

• Surface species classified based on height analysis using the Watershed algorithm. • Scanning tunneling microscopy for understanding the water gas shift reaction. • H 2 O and CO coadsorpion on magnetite at room temperature progresses the reaction. Understanding the structure of catalyst surfaces with adsorbed molecules is key to improving catalyst design. Scanning tunneling microscopy (STM) allows the observation of adsorption states and sites and provides insights into diffusion and desorption processes; however, the presence of multiple types of molecules on the surface presents challenges such as the identification of species and verification of reaction progress, particularly at room temperature or higher. In this study, we develop a protocol for the height classification analysis of STM images using the Watershed algorithm. This method is applied to a system involving the co-adsorption of H 2 O and CO on the Fe 3 O 4 (111) surface, which represents the beginning of the water-gas shift reaction. Water molecules and dissociated OH species were identified in STM images of the Fe 3 O 4 (111) surface following the adsorption of water. Furthermore, gradual changes in the types of surface species were observed upon exposure of the surface to CO, indicating reaction progression. Our observations suggest that CO may react with molecular water rather than with dissociated OH on Fe sites. Despite its simplicity, the height classification analysis effectively identifies changes in the adsorbates on the catalyst surface. This method can be extended to other catalyst surfaces with adsorbed gasses. [Display omitted] [ABSTRACT FROM AUTHOR]

Highlights • CO molecule is stably adsorbed on top of a Pt atom on the (111) Pt surface according to our DFT calculation. • The experimentally observed order of site preference for CO adsorption is reproduced in this DFT calculation. • Inclusion of van der Waals interaction is necessary for obtaining the correct order of adsorption site preference. • The calculated adsorption energy agrees very well with experimentally measured one when CO is adsorbed on top of a Pt atom. • The vdW-DF and vdW-DF2 functionals give the best results for CO adsorption on Pt(111), in terms of adsorption energy and site preference. ABSTRACT Adsorption of CO on Pt (111) surface is a well known problem due to the fact that theoretical calculations based on standard Density functional theory (DFT) fail to predict the adsorption site and energy in agreement with the experimental results. Here, we investigate the role of van der Waals interaction on the CO/Pt (111) system, using different functionals of the vdW-DF family. Our calculations show that the optimized vdW functionals such as optPBE-vdW, optB88-vdW and optB86b-vdW give a wrong order of site preference and tend to overestimate adsorption energies. We find that vdW-DF and vdW-DF2 functionals predict stable adsorption at atop site. Both these functionals give adsorption energy values and order of site preference that agree well with the experimental results. Our results demonstrate that the nature of vdW functional plays an important role in obtaining a correct description of adsorption in the CO/Pt (111) system. Graphical abstract Image, graphical abstract [ABSTRACT FROM AUTHOR]

Highlights • The hexagonal reconstructions of Au surfaces in the electrochemical environment exhibit a potential dependent compressibility. • Both Au(111) and Au(001) surface exhibit similar behaviour despite the differences in the underlying crystal symmetry. • Analysis of the data within a free electron model suggests that the compression is driven by the charge on the surface Au atoms. • The adsorption of carbon monoxide completely suppresses the potential-dependent compression. • Adsorbed CO enhances both the electrocatalytic activity and the stability of the reconstructed surfaces. Abstract The surfaces of gold exhibit a rich physical behaviour that is interesting not only from a structural perspective but also for applications in areas such as heterogeneous catalysis and electrocatalysis. In this paper we show that the hexagonal reconstructions of both the Au(111) and the cubic Au(001) surfaces in alkaline electrolyte exhibit a potential-dependent in-plane compression that is remarkably similar despite the substantial difference in the geometry of the underlying substrate. The compressibility is linked to the charge on the surface Au atoms within a simple free electron model. The interplay between surface charge and the adsorption of hydroxide species determines both the surface compression and the reversible lifting of the reconstructions. In the presence of adsorbed carbon monoxide both the potential-induced changes in the surface compression and the lifting of the reconstruction are suppressed leading to the promotion of electrocatalytic reactivity. Graphical abstract Image, graphical abstract [ABSTRACT FROM AUTHOR]

Highlights • Complex adsorption mechanism of CO on Ca 3 Ru 2 O 7 (001). • Initial physisorption followed by formation of surface carboxylate. • High stability of resulting carboxylate. • High-coverage phase determined by patterns of the bare Ca 3 Ru 2 O 7 (001) surface. Graphical abstract Abstract The adsorption of CO molecules at the Ca 3 Ru 2 O 7 (001) surface was studied using low-temperature scanning tunneling microscopy (STM) and density functional theory (DFT). Ca 3 Ru 2 O 7 can be easily cleaved along the (001) plane, yielding a smooth, CaO-terminated surface. The STM shows a characteristic pattern with alternating dark and bright stripes, resulting from the tilting of the RuO 6 octahedra. At 78 K, CO adsorbs at an apical surface O at the channel edge with a predicted binding energy of E ads = − 0.85 eV. After annealing at room temperature, the CO forms a strong bond (E ads = − 2.04 eV) with the apical O and the resulting carboxylate takes the place of the former surface O. This carboxylate can be decomposed by scanning the surface with a high sample bias voltage of +2.7 V, restoring the original surface. [ABSTRACT FROM AUTHOR]

Highlights • Highly oriented pyrolytic graphite is a versatile model substrate for IRAS studies. • HOPG modification by Ar+ or O+ bombardment was studied by in-situ IRAS and STM. • HOPG modified by Ar+ or O+ bombardment efficiently anchor very small Pd particles. • The anchored Pd shows modified adsorption properties. • Anchored Pd nanoparticles are stable against sintering up to 550 K in O 2 and CO. Abstract Carbon is a common support material for noble metal nanoparticles in heterogeneous catalysis and electrocatalysis. Very few surface-science-based model studies have been performed, however, with noble metal nanoparticles on carbon supports. In this work, we present the first model study on carbon-supported noble metal nanoparticles using infrared reflection absorption spectroscopy (IRAS). The model catalyst is prepared on highly oriented pyrolytic graphite (HOPG) by physical vapor deposition (PVD) of Pd metal. We characterize the particle growth and morphology by scanning tunneling microscopy (STM) and atomic force microscopy (AFM) and probe the adsorption properties by IRAS of adsorbed CO. PVD of Pd on clean HOPG leads to the formation of very large, weakly interacting particles. In order to increase the nucleation density upon Pd deposition, the HOPG surface was modified by Ar+ and O+ bombardment. The defect structures formed were characterized by STM and by IRAS during the ion bombardment. On the pretreated HOPG, Pd nucleates homogeneously and stable Pd nanoparticles (NPs) are formed. We investigated the adsorption properties of the Pd NPs as a function of the particle size and their thermal stability upon annealing. It is shown that carbon species migrate from the HOPG modified by ion bombardment onto the Pd NPs leading to modification of the CO adsorption properties. The work shows that Pd/HOPG is a very well-suited model system for in-situ adsorption and reaction studies by IRAS, in spite of the non-metallicity of the support. Graphical abstract Image, graphical abstract [ABSTRACT FROM AUTHOR]

The intact adsorption and decomposition of furan (C 4 H 4 O) on vicinal palladium surfaces with (111)-oriented terraces has been studied by scanning tunneling microscopy (STM) over a range of temperatures. STM images at 225 K show that furan molecules lie flat and prefer to adsorb at upper step edges. At 225 K, furan molecules adsorbed on “narrow” terraces of 20 to 45 Å in width appear to diffuse more readily than those adsorbed on “wide” terraces of 160 to 220 Å. A distinct population of smaller features appears in STM images on “narrow” terraces at 288 K and on “wide” terraces at 415 K and is identified with the C 3 H 3 decomposition product, agreeing with prior studies which demonstrated that furan dissociates on Pd(111) to yield carbon monoxide (CO) and a C 3 H 3 moiety in the 280 to 320 K range. Based on our direct visualization of this reaction using STM, we propose a spatial mechanism in which adsorption of furan at upper step edges allows catalysis of the dissociation, followed by diffusion of the product to lower step edges. [ABSTRACT FROM AUTHOR]

First principle based density functional theory (DFT) was used to calculate the step-by-step hydrogenation and dissociation reaction network of carbon monoxide (CO) over Co-doped Cu(111) surface as a model for understanding the lateral interaction of surface hydroxyl species (OH) on these reactions. We discussed the C O bond length and the adsorption energy changes of reaction intermediates under different adsorption circumstances for purpose of making out the effect of surface hydroxyl on the reaction selectivity. Reaction intermediates co-adsorbed with H atom and hydroxyl could undergo H-assisted or OH-assisted routes. The calculations show that the OH-assisted route prefers with the formation of COH, CHOH and CH 2 OH while general H-assisted route prefers with the formation of HCO, CH 2 O and CH 3 O. Considering the rather low activation barrier of COH, CHOH and CH 2 OH to form CH X , the existence of hydroxyl on the surface is in favor of boosting the CH X and suppressing the methanol. [ABSTRACT FROM AUTHOR]

Electrical charging of an FeN 4 cluster embedded in graphene (FeN 4 G) is proposed as an approach for electrocatalytically switchable carbon monoxide (CO) adsorption. Using density functional theory (DFT), we found that the CO molecule is strongly adsorbed on the uncharged FeN 4 G cluster. Our results show that the adsorption energy of a CO molecule on the FeN 4 G cluster is dramatically decreased by introducing extra electrons into the cluster. Once the charges are removed, the CO molecule is spontaneously adsorbed on the FeN 4 G absorbent. In the framework of frontier molecular orbital (FMO) analysis, the enhanced sensitivity and reactivity of the FeN 4 G cluster towards the CO molecule can be interpreted in terms of interaction between the HOMO of CO molecule and the LUMO of FeN 4 G cluster. Therefore, this approach promises both facile reversibility and tunable kinetics without the need of specific catalysts. Our study indicates that the FeN 4 G nanomaterial is an excellent absorbent for controllable and reversible capture and release of the CO. [ABSTRACT FROM AUTHOR]

In a recent study we reported on the catalytic action of gold on the oxidative removal of carbon from carburized W(110) surfaces at high temperatures, shifting the desorption peak of the formed carbon monoxide by roughly 200 K from 1100 to 1150 K down to 900 K. Motivated by this observation we examined the structure/morphology of gold on these surfaces by scanning tunneling microscopy after annealing to various temperatures, both with and without the presence of adsorbed oxygen. With increasing temperature structural rearrangements of the gold clusters/islands are observed which become most pronounced in the temperature range where the catalytic reaction sets in. In the same temperature range gold-induced segregation of carbon to the surface occurs. As the strong structural changes of the gold clusters require the existence of mobile gold species, we suggest that mobile atomic gold is related to both the enhanced carbon segregation and the observed catalytic effect. [ABSTRACT FROM AUTHOR]

Unusual quantized beam fluctuations were found in the emission current from a cold-field emitter (CFE) operating in an extremely high vacuum of 10 − 10 Pa. To clarify the microscopic mechanism behind these fluctuations, we developed a new calculation method to evaluate the field emission from a heterogeneous surface under a strong electric field of 4 × 10 9 V/m by using the local potential distribution obtained by a first-principles calculation, instead of by using the work function. As a result of the first-principles calculations of a single molecule adsorbed on a tungsten surface, we found that dissociative adsorption of a carbon monoxide (CO) molecule enhances the emission current by changing the potential barrier in the area surrounding the C and O adatoms when these two atoms are placed at their most stable positions. It is also found that the migration of the O atom from the most stable position reduces the emission current. These types of enhancement and reduction of the emission current quantitatively explain the observed quantized fluctuations of the CFE emission current. [ABSTRACT FROM AUTHOR]

The co-adsorption of CO and H 2 O on a Co(0001) surface at 100 K has been systematically studied using temperature programmed desorption (TPD) and density functional theory (DFT) calculations. While the TPD spectra of CO is almost not affected by the presence of H 2 O, the stabilization of H 2 O by co-adsorbed CO is found for the first time in a large coverage range (0.15 ML < θ CO < 0.66 ML; 0.01 ML < θ H2O < 0.6 ML). When the coverage of predosed CO is lower than 0.27 ML, the formerly single desorption peak of H 2 O is gradually separated into three peaks at 0.6 ML coverage. Those at lower and higher temperatures may be attributed to the repulsive interaction between H 2 O molecules and the attractive interaction between H 2 O and CO molecules, respectively. With increasing the coverage of predosed CO, not only the position of the high temperature peak shifts toward higher temperature (by about 15 K), but the intensity is greatly strengthened until a maximum is achieved when θ CO = 0.36 ML. DFT calculations suggest that the attractive interaction between H 2 O and CO on Co(0001) originates from the formation of intermolecular hydrogen bonds. This work not only provides insights into water gas shift reactions with H 2 O and CO as reactants, but opens new avenues for a volume of catalytic process of technological importance. [ABSTRACT FROM AUTHOR]

We describe a previously unreported ordered phase of carbon monoxide adsorbed on the (111) facet of single crystal palladium at near-saturation coverage. The adlayer superstructure is identified from low energy electron diffraction to be c(16×2) with respect to the underlying Pd(111) surface net. The ideal coverage is determined to be 0.6875 ML, approximately 92% of the 0.75‐ML saturation coverage. Density functional theory calculations support a model for the molecular packing characterized by strips of locally-saturated (2×2) regions, with the CO bound near high-symmetry surface sites, separated by antiphase domain boundaries. The structure exists in a narrow coverage range and is prepared by heating the saturated adlayer to desorb a small fraction of the CO. Comparison of the c(16×2) domain-boundary structure with structural motifs at lower coverages suggests that between 0.6 and 0.6875 ML the adlayer order may be more strongly influenced by interadsorbate repulsion than by adsorption-site-specific interactions. The system is an example of the structural complexity that results from the compromise between adsorbate–substrate and adsorbate–adsorbate interactions. [ABSTRACT FROM AUTHOR]

A considerable promotional effect of MoO x species observed at high pressures on the catalytic activity of rhodium initiated the present UHV model study. The MoO x overlayers formed on Rh films (0.15–20.0 ML) supported by TiO 2 (110) substrate were characterized by AES, TPD, work function (WF) measurements and CO adsorption. On the mixed oxide support produced by depositing 1.2 ML Mo onto TiO 2 (110), a new recombinative CO desorption state was observed with T p =700 K, assigned as β-CO and related to the promotional effect of MoO x species diffused onto Rh particles of 1.0 ML coverage. The development of β-CO needs 0.5–0.7 ML threshold Rh coverage, attributable to particle size effect and geometric factors governing the CO adsorption. The β-CO state with T p =725–742 K could also be detected on Rh films covered by MoO x moiety formed by the oxidation of Mo overlayers in O 2 . Remarkably, recombinative CO desorption with T p =700 K could be observed on the Rh nanoparticles covered by MoO z C y produced from pure Mo deposits by CO adsorption, too. In harmony with the promotional effect of MoO x overlayer found at high pressures, it is established that the dissociation of CO is maximal at 0.2–0.3 ML Mo coverage, attributed to the presence of active sites at the oxide-metal interface. The low desorption peak temperature (700 K) of associative CO desorption observed in the presence of MoO x and MoO z C y overlayers indicates a low activation energy for the reactions of O a and C a atoms, allowing high reaction rates for these intermediates. The MoO x species exerted both promotion and inhibition effects on CO adsorption at sub-monolayer coverages, but above 1 ML it completely suppressed the reactivity of rhodium layers towards CO, suggesting that its surface concentration is a critical factor. [ABSTRACT FROM AUTHOR]

Periodic density functional theory calculations were performed to investigate the Lewis acidity of unsaturated surface cations of ZnF 2 , using CO as probe molecule at different coverages. We have calculated adsorption energies for CO on all low index ZnF 2 surfaces using DFT with the B3LYP functional and subsequent dispersion correction. Additionally local second order Møller-Plesset perturbation theory (LMP2) calculations were performed. In most of the cases, the adsorption of CO on different surfaces is described well using B3LYP. Dispersion correction to B3LYP is found to overestimate the adsorption energy. The interaction among adsorbed CO molecules appears to have a significant effect on the adsorption energies at full coverage. [ABSTRACT FROM AUTHOR]

Adsorption of CO molecules on Si(111)-(7×7) is investigated by using scanning tunneling microscopy (STM) and density-functional theory calculations. The most reactive site on the Si(111)-(7×7) surface is the corner adatom in the faulted half unit (FHU), followed by the center adatom in the FHU. The initial sticking probability of CO molecules on Si(111)-(7×7) at room temperature (RT) is estimated to be ∼ 4 × 10 10 molecules/( cm 2 ·Langmuir), which is comparable with that on Si(001)-(2×1). From the experiments and theoretical calculations, the adsorption of CO molecules are found to occur on Si adatoms either in the upright on-top configuration or in the back-bond inserted configuration, while the adsorption on the rest and the corner hole atoms (which are theoretically probable) are not observable using STM due to their low-lying geometries. Though the sticking probability is very low, every surface dangling bond on the surface can bind strongly with the C atom of a CO molecule even at temperatures higher than RT. [ABSTRACT FROM AUTHOR]

This computational study of carbon induced reconstruction of Co surfaces demonstrates that surface reconstruction is stable in the presence of a hydrogen at low coverage. These reconstructions can create new sites that allow for low activation energy CO dissociation. Carbon induced surface reconstruction of the edge of the fcc -Co(221) step surface will result in highly reactive step-edge sites. Such sites also provide a low activation energy for carbon to diffuse into the subsurface layer of cobalt. [ABSTRACT FROM AUTHOR]

The adsorption of CO on two different mixed-terminated, single-crystal surfaces of zinc oxide, ZnO(10 1 − 0) and ZnO(11 2 − 0), was investigated by employing infrared reflection absorption spectroscopy (IRRAS) using p- and s-polarized light. For both surfaces, one negative CO band was observed only for p-polarized light, indicating that CO is bound to the surface Zn 2 + sites in a nearly upright geometry. In addition, we observed a substantial coverage-induced frequency shift for CO adsorption on both ZnO surfaces. For ZnO(10 1 − 0), the ν (C-O) band shows a red shift of 16 cm − 1 when increasing the CO coverage from 0.5 ML (2185 cm − 1 ) to 1 ML (2169 cm − 1 ). On the ZnO(11 2 − 0) surface, for isolated CO molecules, a frequency of 2192 cm − 1 was detected. The CO band shifts to 2170 cm − 1 at full monolayer. On both surfaces, the coverage-dependent frequency shift is attributed to a combination of dynamic and substrate-mediated static adsorbate–adsorbate interactions. [ABSTRACT FROM AUTHOR]

The adsorption and reactions of CO adsorbed on Rh particles deposited on K-covered CeO 2 (111) were studied by temperature programmed desorption and photoelectron spectroscopy. K deposited on CeO 2 (111) forms a KO X over-layer by extracting O from the ceria and partially reducing some of the Ce 4 + to Ce 3 + . CO does not adsorb on the KO X / CeO 2 – X (111) surface in the absence of Rh particles. CO adsorbed on Rh/K/CeO 2 (111) adsorbs molecularly on the Rh at 200 K. As the surface is heated the CO spills-over and reacts with the KO X to form carbonate. The carbonate decomposes at elevated temperature to produce CO and CO 2 . The carbonate stabilizes the KO X so that K desorbs at a higher temperature than it would in the absence of CO. When the Rh and K deposition are reversed so that K is deposited on both the Rh and the CeO 2 (111), CO adsorbs as CO 2 − at 200 K. The CO 2 − decomposes below 350 K to produce gas phase CO and adsorbed CO 3 2 − and CO. The CO is stabilized by the K on the Rh and desorbs above 540 K. The carbonate decomposes into gas phase CO and CO 2 . [ABSTRACT FROM AUTHOR]

In this paper, we observed physically adsorbed CO molecules on C 60 surface being displaced by impinging noble gas atoms (He, Ne, Ar, Kr), either through a dynamic displacement process or an exothermic replacement process, depending on their adsorption energies. This displacement mechanism could shift from one to the other depending on the surface coverage and temperature. Furthermore, rotational energy of the impinging molecules may also contribute to the dynamic displacement process by supplying additional energy. [ABSTRACT FROM AUTHOR]

We have studied the catalytic carbon monoxide (CO) oxidation (CO + 0.5O 2 → CO 2 ) reaction using a powder catalyst composed of both copper (5 wt.% loading) and titania (CuO x –TiO 2 ). Our study was focused on revealing the role of Cu, and the interaction between Cu and TiO 2 , by systematic comparison between two nanocatalysts, CuO x –TiO 2 and pure CuO x . We interrogated these catalysts under in situ conditions using X-ray diffraction (XRD), X-ray absorption fine structure (XAFS) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) to probe the structure and electronic properties of the catalyst at all stages of the reaction and simultaneously probe the surface states or intermediates of this reaction. With the aid of several ex situ characterization techniques including transmission electron microscopy (TEM), the local catalyst morphology and structure were also studied. Our results show that a CuO x –TiO 2 system is more active than bulk CuO x for the CO oxidation reaction due to its lower onset temperature and better stability at higher temperatures. Our results also suggest that surface Cu + species observed in the CuO x –TiO 2 interface are likely to be a key player in the CO oxidation mechanism, while implicating that the stabilization of this species is probably associated with the oxide–oxide interface. Both in situ DRIFTS and XAFS measurements reveal that there is likely to be a Cu(Ti)–O mixed oxide at this interface. We discuss the nature of this Cu(Ti)–O interface and interpret its role on the CO oxidation reaction. [ABSTRACT FROM AUTHOR]

We have performed a comparative study of the acid–base characteristics of the surfaces of anatase TiO 2 and tetragonal ZrO 2 . To this end we performed DFT + U calculations on CO and CO 2 probe molecules adsorbed both on terraces and steps of the two oxides. For titania, CO adsorption results in a moderate adsorption energy (about − 0.3 eV) and in a positive shift of the C O stretching frequency (about + 40 cm − 1 ), typical of Lewis acid sites, with no clear difference in the acidity between terraces or steps. For zirconia we found a similar CO binding energy as for titania, and a CO vibrational shift that depends on the location of the Zr cation: negligible on terraces, similar to TiO 2 on steps. We conclude that the acidic properties are similar in the two oxide surfaces. Things are different for CO 2 adsorption. On titania the interaction is weak and surface carbonates compete with physisorbed CO 2 , indicating a weak basic character. On the contrary, on zirconia three types of stable carbonates have been identified. Their vibrational frequencies are consistent with IR measurements reported in the literature. The most stable species forms on steps of the t-ZrO 2 surface and consists of a CO 3 2 − unit which lies flat on the surface with the O atoms pointing towards three Zr ions. The species forms spontaneously by extraction of a lattice oxygen by an incoming CO 2 molecule. The different reactivity points towards a much more pronounced basic character of zirconia compared to titania, at least if measured by CO 2 adsorption. [ABSTRACT FROM AUTHOR]

The bulk terminated Cu(100) surface becomes unstable in the presence of CO at room temperature when the pressure reaches the mbar range. Scanning tunneling microscopy images show that above 0.25mbar the surface forms nanoclusters with CO attached to peripheral Cu atoms. At 20mbar and above 3-atom wide one-dimensional nanoclusters parallel to < 001> directions cover the surface, with CO on every Cu atom, increasing in density up to 115mbar. Density functional theory explains the findings as a result of the detachment of Cu atoms from step edges caused by the stronger binding of CO relative to that on flat terraces. Graphical abstractImage 1Highlights· Cu(100) surfaces breaks up into rectangular nanoclusters in the presence of CO at 0.25mbar at RT. · At 20-115mBar of CO at RT, the surface consists of 3-atom wide linear clusters with CO attached to each Cu atom. · Clustering happens due to low cohesive energy of copper and gain in energy through CO adsorption on under-coordinated sites. [ABSTRACT FROM AUTHOR]

We have investigated CO adsorption on structurally well-defined Pt x Ag 1-x /Pt(111) surface alloys, combining temperature-programmed desorption (TPD) and infrared reflection absorption spectroscopy (IRRAS) as well as density functional theory (DFT) based calculations. This is part of a systematic approach including previous studies of CO adsorption on closely related Pt(111)- and Pd(111)-based surface alloys. Following changes in the adsorption properties with increasing Ag content and correlating them with structural changes allow us to assign desorption features to specific adsorption sites/ensembles identified in previous scanning tunneling microscopy (STM) measurements, and thus to identify and separate contributions from different effects such as geometric ensemble effects and electronic ligand/strain effects. DFT calculations give further insight into the nature of the metal–CO bond on these bimetallic sites. Most prominently, the growth of a new CO desorption feature at higher temperature (~ 550 K) in the TPD spectra of Ag-rich surface alloys, which is unique for the group of Pt(111)- and Pd(111)-based surface alloys, is attributed to CO adsorption on Pt atoms surrounded by a Ag-rich neighborhood. Adsorption on these sites manifests in an IR band down-shifted to significantly lower wave number. Systematic comparison of the present results with previous findings for CO adsorption on the related Pt(111)- and Pd(111)-based surface alloys gains a detailed insight into general trends in the adsorption behavior of bimetallic surfaces. [ABSTRACT FROM AUTHOR]

Collision-induced desorption of CO from Ru(0001) by hyperthermal (5–9 eV) effusive beams of Ar and N + N 2 has been studied at a sample temperature of 400 K. Prompt desorption occurs with cross sections on the order of 4 Å 2 . Based on post-exposure thermal desorption measurements, ~ 1/3 of the initial CO coverage cannot be desorbed by Ar on the time scale of the current experiments. In contrast, exposure to the mixed N + N 2 beam appears to remove all CO from the irradiated region. This is attributed to a lowering of the CO binding energy by adsorbed N-atoms. While there is no evidence of a large influence of surface diffusion on the time scale of these exposure, desorption simulations suggest that local diffusion in the periphery of the exposed region influences the measured decay. [ABSTRACT FROM AUTHOR]

The oxidation of carbon monoxide on oxygen-modified Au(111) surfaces is studied using a combination of reflection-absorption infrared spectroscopy (RAIRS) and temperature-programmed desorption (TPD). TPD reveals that CO desorbs in two states with the low-temperature state have a peak temperature between ~ 130 and 150 K, and the higher-temperature state having a peak temperature that varies from ~ 175 to ~ 220 K depending on the initial oxygen and CO coverages. Infrared spectroscopy indicates that the low-temperature CO desorption state is predominantly associated with CO adsorbed on Au δ + sites, while the higher-temperature states are due to CO on Au 0 sites. No additional vibrational features are detected indicating that CO reacts directly with adsorbed atomic oxygen on gold to form CO 2 . Estimates of the activation energy for CO 2 formation suggest that they are in the same range and found for supported gold catalysts at reaction temperature below ~ 300 K. [ABSTRACT FROM AUTHOR]

We have investigated the interaction of CO with Ir/Au{111} bimetallic surfaces, and the influence of morphology changes as Ir moves sub-surface into the Au bulk, using reflection–absorption infrared spectroscopy (RAIRS). The presence of Ir stabilises CO on exposed regions of the Au surface at temperatures up to around 200 K: we attribute this to low-coordinated Au sites, probably associated with lifting of the clean-surface ‘herringbone’ reconstruction by Ir deposition. The highest density of active Au sites is obtained after annealing the bimetallic surface to 500–600 K: we attribute this to morphology changes associated with the movement of Ir into bulk Au. [ABSTRACT FROM AUTHOR]

Segregation induced by the adsorption of gas phase species can strongly influence the composition of bimetallic surfaces and can therefore play an important role in influencing heterogeneous catalytic reactions. The addition of palladium to cobalt catalysts has been shown to promote Fischer Tropsch catalysis. We investigate the adsorption of CO onto bimetallic CoPd surfaces on Pd{111} using a combination of reflection absorption infrared spectroscopy and medium energy ion scattering. The vibrational frequency of adsorbed CO provides crucial information on the adsorption sites adopted by CO and medium energy ion scattering probes the surface composition before and after CO exposure. We show that cobalt segregation is induced by CO adsorption and rationalise these observations in terms of the strength of adsorption of CO in various surface adsorption sites. [ABSTRACT FROM AUTHOR]

CO vibrational spectra over catalytic nanoparticles under high coverages/pressures are discussed from a DFT perspective. Hybrid B3LYP and PBE DFT calculations of CO chemisorbed over Pd 4 and Pd 13 nanoclusters, and a 1.1 nm Pd 38 nanoparticle, have been performed in order to simulate the corresponding coverage dependent infrared (IR) absorption spectra, and hence provide a quantitative foundation for the interpretation of experimental IR spectra of CO over Pd nanocatalysts. B3LYP simulated IR intensities are used to quantify site occupation numbers through comparison with experimental DRIFTS spectra, allowing an atomistic model of CO surface coverage to be created. DFT adsorption energetics for low CO coverage (θ → 0) suggest the CO binding strength follows the order hollow > bridge > linear, even for dispersion-corrected functionals for sub-nanometre Pd nanoclusters. For a Pd 38 nanoparticle, hollow and bridge-bound are energetically similar (hollow ≈ bridge > atop). It is well known that this ordering has not been found at the high coverages used experimentally, wherein atop CO has a much higher population than observed over Pd(111), confirmed by our DRIFTS spectra for Pd nanoparticles supported on a KIT-6 silica, and hence site populations were calculated through a comparison of DFT and spectroscopic data. At high CO coverage (θ = 1), all three adsorbed CO species co-exist on Pd 38 , and their interdiffusion is thermally feasible at STP. Under such high surface coverages, DFT predicts that bridge-bound CO chains are thermodynamically stable and isoenergetic to an entirely hollow bound Pd/CO system. The Pd 38 nanoparticle undergoes a linear (3.5%), isotropic expansion with increasing CO coverage, accompanied by 63 and 30 cm − 1 blue-shifts of hollow and linear bound CO respectively. [ABSTRACT FROM AUTHOR]

Typical Fischer–Tropsch catalysts display different selectivity and activity in catalyzing CO hydrogenation to diverse products. In this work, the preferable routes for CH 3 OH formation on Cu, chain growth on Co and CH 4 formation on Ni are identified guided by the comprehensive reaction network that is mapped out by density function theory calculations. The difference in selectivity among catalysts is controlled delicately by several reactions, including CH 3 O + H ↔ CH 3 OH, CH 3 + H ↔ CH 4 and CH 2 + CO ↔ CH 2 CO. The equilibrium shifts of CH 2 O + H ↔ CH 3 O and CH 2 + H ↔ CH 3 also make an impact on selectivity. The distinct selectivity can be understood further with the activity of catalysts. Our results show that the ability of surface to absorb species increases in the order Cu < Ni < Co. Generally, Cu catalyzes the association reaction better than Co and Ni, while Co facilitates the dissociation reaction. Two key factors, thermodynamic effect and kinetic effect, are identified in determining the activity of catalyst. We proof that surface with strong binding capability promotes the dissociation reaction, meanwhile impedes the association reaction when the thermodynamic effect is dominant in determining the barrier height. The Brønsted–Evans–Polanyi relation is observed for C–O bond breaking reactions. In addition, kinetic effect also affects the barrier when special transition state exists. The tilt of CO at the transition state for COH formation and chain growth reactions introduces the interaction of atom O with surface. The stronger binding of atom O on Co is crucial to branch the selectivity of Co to chain growth rather than methane. Present study provides a comprehensive picture on the activity and selectivity of catalysts, which is the essential to develop novel catalyst for syngas conversion. [ABSTRACT FROM AUTHOR]

The catalytic oxidation of CO at Pt-doped BNNT (5,5) has been investigated theoretically using density functional theory. The electronic structures and thermochemical properties of CO and O 2 that adsorbed on Pt embedded at the B- and N-vacancy sites of BNNTs are analyzed. It is demonstrated that the different BNNT substrates can modify the electronic structure of the Pt catalysts and cause different effects in the catalytic activities. With the N-vacancy (Pt (N) –BNNT), the Pt behaves as a Lewis acid for accepting an electron from the substrate, thus O 2 binds stronger than CO molecules, thus alleviating the CO poisoning of the platinum catalysts. Coadsorption of CO and O 2 on Pt (N) –BNNT results in additional charge transfer to O 2 . CO oxidation proceeds via the Eley–Rideal (ER) mechanism entails lower activation barrier and higher reaction rate than that of Langmuir–Hinshelwood (LH) mechanism suggesting the superiority of the ER mechanism for CO oxidation at Pt (N) –BNNT. Therefore, Pt (N) –BNNT might be a good candidate for low-cost, highly active, and stable catalysts for CO oxidation. [ABSTRACT FROM AUTHOR]

Adsorption states and photoreactions of CO and D 2 O adsorbed on NiAl(110) at 90 K were studied by temperature-programmed desorption (TPD). Coverage dependence of TPD spectra showed growth behavior of adlayers of these molecules consistent with the literature, but a new path of thermal dissociation of D 2 O upon adsorption at 90 K was found. The photoreaction cross sections of CO and D 2 O at 4.7 eV were estimated in the order of 10 − 21 cm 2 and 10 − 20 cm 2 , respectively. This result suggests that photoexcitation of the NiAl(110) substrate itself, a typical substrate used in supported model catalyst studies, will induce only negligible photoreactions of molecules adsorbed at surfaces of metal nanoparticles supported on ultrathin alumina films formed on NiAl(110). [ABSTRACT FROM AUTHOR]

Monolayers of carbon monoxide (CO) on Pt(111) surfaces are one of the most studied adsorption systems. However, molecular models of this system still do not take into account the reliable potential of lateral interactions between adsorbed CO molecules. Recent advances in experimental technique have brought high-resolution real-space images of CO/Pt(111) monolayers. For example, Yang et al. ( J. Phys. Chem. C 117 (2013) 16429–16437) found island structures for coverages from 0.11 to 0.25 ML. In this study we have shown that these island structures can be explained with long-range oscillating lateral interactions. Parameters of the proposed potential were fitted to experimental scanning tunneling microscopy images with a series of Monte Carlo simulations. [ABSTRACT FROM AUTHOR]

Small metallic nanoparticles supported on transition metal carbides exhibit an unexpected high activity towards a series of chemical reactions. In particular, the Au/TiC system has proven to be an excellent catalyst for SO 2 decomposition, thiophene hydrodesulfurization, O 2 and H 2 dissociation and the water gas shift reaction. Recent studies have shown that Au/TiC is a very good catalyst for the reverse water–gas shift (CO 2 + H 2 → CO + H 2 O) and CO 2 hydrogenation to methanol. The present work further expands the range of applicability of this novel type of systems by exploring the catalytic activity of Au/TiC towards the hydrogenation of CO or CO 2 with periodic density functional theory (DFT) calculations on model systems. Hydrogen dissociates easily on Au/TiC but direct hydrogenation of CO to methanol is hindered by very high activation barriers implying that, on this model catalyst, methanol production from CO 2 involves the hydrogenation of a HOCO-like intermediate. When dealing with mixtures of syngas (CO/CO 2 /H 2 /H 2 O), CO could be transformed into CO 2 through the water gas shift reaction with subsequent hydrogenation of CO 2 to methanol. [ABSTRACT FROM AUTHOR]

A single layer, bi-metallic RuPt catalyst on Au(111) is synthesized using surface limited red-ox replacement of underpotentially deposited Cu and Pb monolayers though a two-step process. The resulting 2D RuPt monolayer nanoclusters have a unique core-edge structure with a Ru core and Pt at the edge along the perimeter. The activity of this catalyst is evaluated using CO monolayer oxidation as the probe reaction. Cyclic voltammetry demonstrates that the 2D RuPt core-edge catalyst morphology is significantly more active than either Pt or Ru monolayer catalysts. Density functional theory calculations in combination with infra-red spectroscopy data point towards oscillating variations (ripples) in the adsorption energy landscape along the radial direction of the Ru core as the origin of the observed behavior. Both, CO and OH experience a thermodynamic driving force for surface migration towards the Ru-Pt interface, where they adsorb most strongly and react rapidly. We propose that the complex interplay between epitaxial strain, ligand and finite size effects is responsible for the formation of the rippled RuPt monolayer cluster, which provides optimal conditions for a quasi-ideal bi-functional mechanism for CO oxidation, in which CO is adsorbed mainly on Pt, and Ru provides OH to the active Pt-Ru interface. [ABSTRACT FROM AUTHOR]

We present density functional theory modeling of time-resolved optical pump/X-ray spectroscopic probe data of CO desorption from Ru(0001). The BEEF van der Waals functional predicts a weakly bound state as a precursor to desorption. The optical pump leads to a near-instantaneous (< 100 fs) increase of the electronic temperature to nearly 7000 K. The temperature evolution and energy transfer between electrons, substrate phonons and adsorbate is described by the two-temperature model and found to equilibrate on a timescale of a few picoseconds to an elevated local temperature of ~ 2000 K. Estimating the free energy based on the computed potential of mean force along the desorption path, we find an entropic barrier to desorption (and by time-reversal also to adsorption). This entropic barrier separates the chemisorbed and precursor states, and becomes significant at the elevated temperature of the experiment (~ 1.4 eV at 2000 K). Experimental pump-probe X-ray absorption/X-ray emission spectroscopy indicates population of a precursor state to desorption upon laser-excitation of the system (Dell'Angela et al., 2013). Computing spectra along the desorption path confirms the picture of a weakly bound transient state arising from ultrafast heating of the metal substrate. [ABSTRACT FROM AUTHOR]

In this work we present a detailed investigation of the adsorption of CO on a well defined Pt(111) surface with low-temperature Single-Crystal-Adsorption-Calorimetry (SCAC) by measuring the coverage dependent adsorption enthalpies at 150 K, 250 K, 300 K and 350 K. Considerable changes in the coverage dependent differential and integral adsorption enthalpies with temperature indicate an order–disorder transition into the fluid phase which becomes apparent in a significant increase in the corresponding heat capacity of the CO adlayer. [ABSTRACT FROM AUTHOR]

We used density functional theory and molecular dynamics to explore the adsorption of CO, CO 2 , and CH 4 on a layer of h-BN with high titanium coverage. After optimization, we found that each titanium atom is located above each of the hexagons of the surface. We considered atmospheric pressure and 300 K. It is found that the first molecule is adsorbed and dissociated on the surface. The CO 2 molecule is broken into O and CO. The methane molecule is physisorbed, and not dissociated. It is desorbed at 370 K. [ABSTRACT FROM AUTHOR]

Silicon carbide (SiC) nanostructures have attracted extensive researches regarding their unique properties. Surface modification of the SiC nanotubes and nanocages can be useful for developing their potential applications to the gas sensors. In this paper we have studied the adsorption properties of the CO and NO molecules at different situations on a zigzag SiCNT(10,0) and armchair SiCNT(6,6) nanotubes and Si 12 C 12 nanocage using density functional theory (DFT) calculations. The spin polarized DFT with PBE functional is employed in all calculations for the total energy calculation and optimization of the considered structures. Our results reveal that both zigzag and armchair nanotubes can adsorb CO molecule due to chemisorption and physisorption processes as its binding energy in its most stable configuration is about 0.95 and 0.89 eV, respectively. The binding energy of the NO molecule to the considered SiC nanotubes and nanocages is somewhat higher than CO binding energy in its most stable configuration. The most stable configuration for NO adsorption on SiC nanotubes is inside the nanotubes with a small different binding energy value for considered zigzag and armchair SiC nanotubes. The electronic structure of the SiC nanocage interacting with CO and NO is also discussed in this paper as our results show that the geometry structure of the nanocages is very important to the adsorption properties of the CO and NO molecules. Moreover, the present results may be useful for the design of the SiC based nanodevices for gas adsorption, storage and sensors. [ABSTRACT FROM AUTHOR]

We demonstrate that temperature-programmed vibrational sum-frequency generation (SFG) spectroscopy has a unique sensitivity to certain adsorbate overlayer structural transitions. In the CO stretching vibration of co-adsorbed CO/O(2× 1)/Ru(0001) we observe pronounced dips in the spectral intensity as the adsorbate overlayer undergoes structural transitions with temperature. Combining with temperature-programmed desorption (TPD) a more complete picture of temperature-dependent structural transitions is obtained. We extract kinetic parameters from the SFG data and obtain good agreement with TPD when both techniques see the same transition. Infrared-infrared visible SFG is used to determine changes in inter-adsorbate coupling that allow us to experimentally assign the structural transitions. Furthermore, density functional theory calculations of the proposed structures and energetics are performed to verify the experimental assignments. [ABSTRACT FROM AUTHOR]

Carbon mono-oxide (CO) adsorption on a regularly stepped Cu(410) surface was investigated with infrared reflection-absorption spectroscopy (IRAS) and temperature programmed desorption (TPD). CO was adsorbed on Cu(410) molecularly at 96 K and all species were desorbed below 230 K. There were three desorption peaks of CO; high temperature TPD peak (~ 210 K) corresponds to the adsorption on the step edge, while the lower temperature TPD peaks (~ 138 and ~ 170 K) correspond to the adsorption on the terrace. IRAS measurements suggest that CO molecules are adsorbed on the on-top sites. [ABSTRACT FROM AUTHOR]

Abstract: The weak adsorption state of CO molecules on a Si(001)-c(4×2) surface at the temperature of 75K has been investigated by means of scanning tunneling microscopy (STM). Lateral expansion of CO-adsorbed islands and preferential formation of a weakly adsorbed state near the CO adsorbed islands were directly observed with STM at low CO exposure. We found that the weakly adsorbed CO is desorbed from the surface when CO exposure is stopped. At high CO exposure, we observed multilayered CO islands, which disappeared after cessation of CO exposure. [Copyright &y& Elsevier]

Abstract: The adsorption of CO on bimetallic RhN/Ru(0001) layers with N=0–5 Rh layers has been investigated using Fourier transform infrared spectroscopy, thermal desorption, and low energy electron diffraction. It is found that the morphology as well as the thickness of these ultra thin Rh films lead to characteristic frequency shifts of the internal CO stretch modes as well as variations of the CO desorption energies. For N≥3ML differences with respect to CO adsorption on Rh(111) single crystal substrates are only very minor. Our findings confirm similar observations and conclusions associated with CO on bimetallic PtN/Ru(0001) layers. Dependent on the coverage of CO adsorbed on Rh/Ru(0001) monolayer films we identify a series of ordered overlayer structures which have been correlated with characteristic CO vibrational spectra. [Copyright &y& Elsevier]

Abstract: We investigated the interactions of CO and O2 with Ru( ) single crystal surfaces, and studied the in-situ catalytic oxidation reaction of CO on the surface under near realistic pressure conditions by using a combination of near-ambient-pressure x-ray photoelectron spectroscopy and differential pumping mass spectroscopy. At lower temperatures (T <190°C), most of the surface keeps metallic and is covered by both chemisorbed atomic oxygen and CO, and the CO2 formation rate is relatively slow. At higher temperatures, the reaction rate significantly increases and reaches the saturation, where the Ru surface is dominated by a bulk oxide (i.e. RuO2). [Copyright &y& Elsevier]

The reduction of the surface oxide on Pd(100) and Pd75Ag25(100) surfaces by CO has been investigated by high-resolution X-ray photoelectron spectroscopy in combination with kinetic analysis using the Avrami–Erofeev theory. For both surfaces we observed faster reduction with increasing temperature. Kinetic analysis indicates that the reduction process is phase boundary controlled for Pd(100) in the temperature range investigated, from 30°C to 120°C. On Pd75Ag25(100) the surface oxide reduction is significantly slower compared to Pd(100). Also in this case, a phase boundary controlled reduction of the surface oxide is observed at temperatures of 120°C and above, while at T ≤70°C the reduction is found to be diffusion limited. Density functional theory calculations show that the presence of silver in the outermost surface layer significantly increases the CO diffusion barriers on the reduced areas, supporting a diffusion limited reduction process for Pd75Ag25(100) at lower temperatures. [ABSTRACT FROM AUTHOR]