Coadsorption Of CO Research Articles

CO catalytic oxidation on Pt-doped single wall boron nitride nanotube: first-principles investigations

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 O2 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 O2 binds stronger than CO molecules, thus alleviating the CO poisoning of the platinum catalysts. Coadsorption of CO and O2 on Pt(N)–BNNT results in additional charge transfer to O2. 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.

Surprising Coordination Chemistry of Cu+Cations in Zeolites: FTIR Study of Adsorption and Coadsorption of CO, NO, N2, and H2O on Cu–ZSM-5

Cations exchanged in zeolites are generally characterized by a low coordination number and can thus attach simultaneously more than one small guest molecule. For instance, Cu+ ions in ZSM-5 can accept, at low temperature, up to three CO and up to two NO molecules. However, only one N2 molecule can be coordinated to such sites. Although mixed aqua-carbonyl and aqua-dinitrogen complexes are formed, no mixed carbonyl-nitrosyl, carbonyl-dinitrogen, or nitrosyl-dinitrogen species can be produced. Thus, adsorption of NO on CO precovered sample results in segregation of the CO adsorption layer according to the reaction: 2Cu+–CO + 2NO → Cu+(CO)2 + Cu+(NO)2. Adsorption of N2 on NO precovered sample leads to a similar process: 2Cu+–NO + N2 → Cu+(NO)2 + Cu+–N2. No carbonyl-dinitrogen complexes are produced during CO–N2 coadsorption. The role of the ligand and the nature of the bond in the formation of geminal and mixed-ligand complexes are discussed.

Structure sensitivity of the oxidative activation of methane over MgO model catalysts: II. Nature of active sites and reaction mechanism

A series of pure, nanostructured magnesium oxides prepared by different synthesis techniques that show different initial, but similar steady-state activity in the oxidative coupling of methane (OCM) (Schwach et al., submitted for publication) has been studied by infrared and photoluminescence spectroscopy in the dehydroxylated state before the reaction and after catalysis. The abundance of structural defects, in particular mono-atomic steps, on the dehydroxylated MgO surface characterized by a band in the FTIR spectrum of adsorbed CO at 2146cm−1 and Lewis acid/base pairs probed by co-adsorption of CO and CH4 correlate with the initial rates of both methane consumption and C2+ hydrocarbon formation. Infrared spectroscopy evidences strong polarization of C–H bonds due to adsorption of methane on dehydroxylated MgO surfaces that contain a high number of mono-atomic steps. It is postulated that these sites effectively promote intermolecular charge transfer between adsorbed methane and weakly adsorbed oxygen that leads to the dissociation of one C–H bond in the methane molecule and simultaneous formation of a superoxide species. Heterolytic splitting of C–H bonds in the presence of oxygen at the surface of dehydroxylated MgO already at room temperature has been proven by the appearance of an EPR signal associated with superoxide species that are located in close vicinity to a proton. With time on stream, MgO sinters and loses activity. The deactivation process involves the depletion of mono-atomic steps and the reconstruction of the MgO termination under formation of polar and faceted surfaces.

Desorption Temperature Control of Palladium-Dissolved Hydrogen through Surface Structural Manipulation

To assess the possibility of controlling the desorption temperature of palladium-absorbed hydrogen (Habs) through surface structural manipulation, we investigated coadsorption systems of H and CO on Habs-charged Pd(110) surfaces through temperature-programmed desorption, low-energy electron diffraction, and H-depth profiling by nuclear reaction analysis (NRA). A CO coverage of 0.5 ML lifts the H-induced (1 × 2) pairing-row (PR) reconstruction on Habs precharged Pd(110), and, as on clean Pd(110), heating Pd(110) (1 × 1) holding 0.3–1.0 ML of CO gives rise to a missing-row (MR) structure. Whereas Habs desorbs through surface defects of clean, PR-reconstructed Pd(110) at 160 K, CO coadsorption onto Habs-loaded Pd(110) gives rise to three new high-temperature shifted Habs desorption modes at 200, 270, and 375 K that are assigned to different exit sites for resurfacing Habs atoms at regular terraces of the individual Pd(110) structures, i.e., the (1 × 2) PR, bulk-terminated (1 × 1), and (1 × 2) MR reconstructi...

Reaction mechanisms for CO catalytic oxidation on monodisperse Mo atom-embedded graphene

The geometric stability, electronic structure and catalytic activity of the Mo-embedded graphene (Mo/SV-graphene) are investigated by using the first-principle calculations. Compared with a Mo atom on pristine graphene, the Mo dopant in defective graphene exhibits more positively charged, which helps to weaken the CO adsorption and facilitates the O2 adsorption. Besides, the two mechanisms (Langmuir–Hinshelwood, LH and Eley–Rideal, ER) for the sequential CO oxidation reactions are investigated comparison. Among the reaction processes, the coadsorption of O2 and CO exists at the Mo/SV-graphene surface, the first step (CO + O2 → OOCO) with energy barrier is 0.60 eV and then form a CO2 molecule through the reaction (OOCO → CO2 + Oads) without any energy barrier, and thus the formation of OOCO complex is viewed as rate-controlling step. In the ER reaction, although the CO molecule reacts with the preadsorbed O2 by the low-energy barrier (CO + O2 → CO3, 0.13 eV), the formation of CO3 is more stable than the generating CO2 and Oads (0.84 eV). Hence, the LH mechanism as the starting step is energetically more favorable. The results provide the valuable guidance to fabricate graphene-based catalysis and validate the reactivity of atomic-scale catalyst.

Combined experiment and theory approach in surface chemistry: Stairway to heaven?

In this perspective we discuss how an intimate interaction of experiments with theory is able to deepen our insight into the catalytic reaction system on the molecular level. This strategy is illustrated by discussing various examples from our own research of surface chemistry and model catalysis. The particular examples were carefully chosen to balance the specific strength of both approaches – theory and experiment – and emphasize the benefit of this combined approach. We start with the determination of complex surface structures, where diffraction techniques in combination with theory are clear-cut. The promoter action of alkali metals in heterogeneous catalysis is rationalized with theory and experiment for the case of CO coadsorption. Predictive power of theory is limited as demonstrated with the apparent activity of chlorinated TiO2(110) in the oxidation of HCl: Even if we know all elementary reaction steps of a catalytic reaction mechanism, the overall kinetics may remain elusive and require the application kinetic Monte Carlo simulations. Catalysts are not always stable under reaction conditions and may chemically transform as discussed for the CO oxidation reaction over ruthenium. Under oxidizing reaction conditions ruthenium transforms into RuO2, a process which is hardly understood on the molecular level. Lastly we focus on electrochemical reactions. Here theory is clearly ahead since spectroscopic methods are not available to resolve the processes at the electrode surface.

High coverage adsorption and co-adsorption of CO and H2 on Ru(0001) from DFT and thermodynamics.

The adsorption and co-adsorption of CO and H2 at different coverages on p(4 × 4) Ru(0001) have been computed using periodic density functional theory (GGA-RPBE) and atomistic thermodynamics. Only molecular CO adsorption is possible and the saturation coverage is 0.75 ML (nCO = 12) with CO molecules co-adsorbed at different sites and has a hexagonal adsorption pattern as found by low energy electron diffraction. Only dissociative H2 adsorption is possible and the saturation coverage is 1 ML (nH = 16) with H atoms at face-centered cubic sites. The computed CO and H2 desorption patterns and temperatures agree reasonably with the experiments under ultrahigh vacuum conditions. For CO and H2 co-adsorption (nCO + mH2; n = 1-6 and m = 7, 6, 5, 5, 3, 1), CO pre-coverage affects H adsorption strongly, and each pre-adsorbed CO molecule blocks 2H adsorption sites and H2 does not adsorb on the surface with CO pre-coverage larger than 0.44 ML (nCO = 7); all these are in full agreement with the experiments under ultrahigh vacuum conditions. Our results provide the basis for exploring the mechanisms of catalytic conversion of synthesis gas.

Coverage dependent adsorption and co-adsorption of CO and H₂ on the CdI₂-antitype metallic Mo₂C(001) surface.

The adsorption and co-adsorption of CO and H2 at different coverage on the CdI2-antitype metallic Mo2C(001) surface termination have been systematically computed at the level of periodic density functional theory. Only molecular CO adsorption is possible and the monolayer coverage (1 ML) can have 16CO adsorbed at the top sites. Dissociative H2 adsorption is favored thermodynamically and the monolayer coverage (1 ML) can have 16H adsorbed at the hollow sites. Since CO has much stronger adsorption energy than H2, pre-adsorption of CO is possible. CO pre-adsorption strongly affects atomic hydrogen co-adsorption at a high CO/H2 ratio, while hardly affects that at a low CO/H2 ratio. Under ultra-high vacuum conditions (200 K, 10(-12) atm and CO/H2 = 1/1), the most stable adsorbed surface state has CO/H2 = 15/1. Comparison among the metallic terminations of the CdI2-antitype Mo2C(001), eclipsed Mo2C(001) and orthorhombic Mo2C(100) surfaces shows their different CO and hydrogen adsorption as well as activation properties, which reveals that the CdI2-antitype Mo2C(001) surface is least active. These differences come from their surface bonding properties; the CdI2-antitype Mo2C(001) surface is saturated and less metallic, while the eclipsed Mo2C(001) and orthorhombic Mo2C(100) surfaces are unsaturated and more metallic.

A novel Pd3O9@α-Al2O3 catalyst under a hydroxylated effect: high activity in the CO oxidation reaction.

Considering the importance of palladium-based and doped metal-oxide catalysts in CO oxidation, we design a new Pd3O9@α-Al2O3 catalyst and simulate its efficiency under a hydroxylated effect. The structure, electronic structure and oxidation activity of the hydroxylated Pd3O9@α-Al2O3(0001) surface are investigated by density functional theory. Under the O-rich growth conditions, Pd preferentially replaces Al. The lowest formation energy of the Pd-doped α-Al2O3(0001) surface is 0.21 eV under conditions wherein the coverage of the Pd-doped α-Al2O3 is 0.75 on a pre-hydroxylated surface and the water coverage is 0.25, which leads to formation of a Pd3O9 cluster embedded in the Al2O3(0001) surface. The reaction mechanisms of CO oxidization have been elucidated first by CO adsorption and migration, second by O(v) formation with the first CO2 release, then by the first foreign O2 filling and CO co-adsorption, and finally by the second CO2 desorption and restoration of the hydroxylated Pd3O9@α-Al2O3(0001) surface. The rate-determining step is the formation of the first CO2 in the whole catalytic cycle. The results also indicate that the energy barrier for CO oxidization is obviously reduced compared to that of the undoped surface, which implies that the introduction of Pd can efficiently improve the oxidation reactivity of the α-Al2O3(0001) surface. Compared to the synthesized Ir1/FeO(x) (1.41 eV) and Pt1/FeO(x) (0.79 eV) catalysts, the reaction activation barrier of CO oxidation is lowered by 0.65 eV and 0.03 eV, respectively. Therefore, the Pd3O9@α-Al2O3 catalyst shows superior catalytic activity in CO oxidation. The present results enrich the understanding of the catalytic oxidation of CO by palladium-based catalysts and provide a clue for fabricating palladium-based catalysts with low cost and high activity.

The density functional theory studies on the promoting effect of the Cu-modified Fe 3 O 4 catalysts

Abstract The adsorption of carbon monoxide and water on the Cux/Fe3O4(111) surface (the Fe3O4(111) surface modified by the small Cux ( x = 1 – 4 ) clusters) is investigated. It is found that the addition of Cu to the Fe3O4(111) plays a critical role in activating the surface for CO adsorption. The presence of Cux clusters on the surface enhances the adsorption of CO, which would provide the CO with more stable adsorption sites and harmonize the coadsorption of CO and H2O on the Cux/Fe3O4 surface. The results shed light on the promoting effect of the Cu-modified Fe3O4 catalysts.

Effect of Amine Surface Coverage on the Co-Adsorption of CO2 and Water: Spectral Deconvolution of Adsorbed Species.

Three primary amine materials functionalized onto mesoporous silica with low, medium, and high surface amine coverages are prepared and evaluated for binary CO2/H2O adsorption under dilute conditions. Enhancement of amine efficiency due to humid adsorption is most pronounced for low surface amine coverage materials. In situ FT-IR spectra of adsorbed CO2 on these materials suggest this enhancement may be associated with the formation of bicarbonate species during adsorption on materials with low surface amine coverage, though such species are not observed on high surface coverage materials. On the materials with the lowest amine loading, bicarbonate is observed on longer time scales of adsorption, but only after spectral contributions from rapidly forming alkylammonium carbamate species are removed. This is the first time that direct evidence for bicarbonate formation, which is known to occur in liquid aqueous amine solutions, has been presented for CO2 adsorption on solid amine adsorbents.

Bismuth and CO Coadsorption on Platinum Nanoparticles

CO adsorption onto Pt- and Pt-based catalysts is a very relevant topic in electrocatalysis, and in particular, in fuel cells research. CO is known to be the main responsible of the poisoning of the catalysts and consequent decrease on performance of the fuel cells devices. In this paper, density functional theory (DFT) calculations and experiments were combined to access the effect of modifying Pt nanoparticles with Bi on CO adsorption. CO adsorption energies were calculated for Pt sites nearby Bi atoms in different type of structures: Pt clusters with Bi as surface adatom and Pt clusters with Bi as surface and subsurface dopant (alloys). The results show that, when compared with pure Pt, the adsorption energies for CO are lower on PtBi clusters in both adatom and surface alloy configurations. Subsurface PtBi alloys reveal higher adsorption energies for CO but these structures are energetically very unfavorable. On the basis of the calculations, a high degree of mobility of Bi on the surface was found in ...

High Catalytic Activity of Au Clusters Supported on ZnO Nanosheets

Catalytic activity of metal clusters supported on a two-dimensional (2d) nanosheet has attracted great interest recently. In this paper, via first-principles calculations, we investigate for the first time Au catalysis on recently proposed 2d one- atom-thick ZnO nanosheets. An Au8 cluster adsorbed on a 2d ZnO sheet (Au8/ZnO) is chosen as a model catalyst, and the CO oxidation is used as the probe reaction for the catalytic activity. When adsorbed on 2d one-atom-thick ZnO, the Au8 cluster is found to adopt a planar structure and exhibit high catalytic activities for the CO oxidation with a low reaction barrier around 0.3 eV. The catalyzed CO oxidation highly prefers the Eley− Rideal (ER) mechanism, and the coadsorption of CO and O2 on the Au8 cluster plays an essential role in the ER type of reaction in which an additional CO approaches O2 in a vacuum. The high catalytic activity (low reaction barrier) is due to the charge transfer from Au8/ZnO to the antibonding 2π* orbital of the O2 molecule that is facilitated by the coadsorption of a CO molecule. These findings provide new opportunities for the future development of ZnO-based catalysts.

Influence of Step Geometry on the Reconstruction of Stepped Platinum Surfaces under Coadsorption of Ethylene and CO.

We demonstrate the critical role of the specific atomic arrangement at step sites in the restructuring processes of low-coordinated surface atoms at high adsorbate coverage. By using high-pressure scanning tunneling microscopy (HP-STM) and ambient-pressure X-ray photoelectron spectroscopy (AP-XPS), we have investigated the reconstruction of Pt(332) (with (111)-oriented triangular steps) and Pt(557) surfaces (with (100)-oriented square steps) in the mixture of CO and C2H4 in the Torr pressure range at room temperature. CO creates Pt clusters at the step edges on both surfaces, although the clusters have different shapes and densities. A subsequent exposure to a similar partial pressure of C2H4 partially reverts the clusters on Pt(332). In contrast, the cluster structure is barely changed on Pt(557). These different reconstruction phenomena are attributed to the fact that the 3-fold (111)-step sites on Pt(332) allows for adsorption of ethylidyne-a strong adsorbate formed from ethylene-that does not form on the 4-fold (100)-step sites on Pt(557).

Interaction of Coadsorbed CO and Deuterium on a Bimetallic, Pt Monolayer Island Modified Ru(0001) Surface

We have investigated the coadsorption of CO and deuterium on structurally well-defined bimetallic, Pt monolayer island modified Ru(0001) surfaces, focusing on the interactions between the coadsorbed species and their impact on the adsorption and desorption characteristics. Temperature-programmed desorption (TPD) and infrared reflection absorption spectroscopy (IRRAS) measurements after adsorption at 90 K reveal considerable differences between adlayers formed by preadsorption of deuterium and subsequent saturation by CO or in the reverse way. We demonstrate that these differences are caused by the limited mobility of adsorbed CO at low temperatures and spillover of COad from Pt monolayer areas to Ru(0001) areas upon heating, e.g., during a TPD measurement. The interplay between energetics, including the presence of weakly adsorbing Pt monolayer sites and strongly adsorbing Ru(0001) sites as well as interactions between coadsorbed species, and the onset of COad spillover on the adsorption and desorption behavior of deuterium, which results in complex deuterium desorption spectra, are illustrated and discussed.

CO Oxidation on TiO2(110) Supported Subnanometer Gold Clusters: Size and Shape Effects

We performed a comprehensive study of catalytic activities of subnanometer Au clusters supported on TiO2(110) surface (Aun/TiO2, n = 1-4, 7, 16-20) by means of density functional theory (DFT) calculations and microkinetics analysis. The creditability of the chosen DFT/microkienetics methodologies was demonstrated by the very good agreement between predicted catalytic activities with experimental measurement (J. Am. Chem. Soc, 2004, 126, 5682-5483) for the Au1-4/TiO2 and Au7/TiO2 benchmark systems. For the first time, the size- and shape-dependent catalytic activities of the subnanometer Au clusters (Au16-Au20) on TiO2 supports were systematically investigated. We found that catalytic activities of the Aun/TiO2 systems increase with the size n up to Au18, for which the hollow-cage Au18 isomer exhibits highest activity for the CO oxidation, with a reaction rate ∼30 times higher than that of Au7/TiO2 system. In stark contrast, the pyramidal isomer of Au18 exhibits much lower activity comparable to the Au3-4/TiO2 systems. Moreover, we found that the hollow-cage Au18 is robust upon the soft-landing with an impact velocity of 200 m/s to the TiO2 substrate, and also exhibits thermal stability upon CO and O2 co-adsorption. The larger pyramidal Au19 and Au20 clusters (on the TiO2 support) display much lower reaction rates than the pyramidal Au18. Results of rate of reactions for unsupported (gas-phase) and supported Au clusters can be correlated by a contour plot that illustrates the dependence of the reaction rates on the CO and O2 adsorption energies. With the TiO2 support, however, the catalytic activities can be greatly enhanced due to the weaker adsorption of CO on the TiO2 support than on the Au clusters, thereby not only the ratio of O2/CO adsorption energy and the probability for the O2 to occupy the Ti sites are increased but also the requirement for meeting the critical line becomes weaker. The obtained contour plot not only can provide guidance for the theoretical investigation of catalytic activity on other metal cluster/support systems, but also assist experimental design of optimal metal cluster/support systems to achieve higher catalytic efficiency.

DFT study of benzene and CO co-adsorption on PtCo(1 1 1)

Co-adsorption of benzene and CO on PtCo(111) surface at low coverage is studied using density functional theory calculations. We investigated the PtCo FCT alloy surface with a uniform distribution. The most favorable site for CO is top on a Pt atom whereas for benzene is an HCP hollow site (formed by 2 Pt atoms and 1 Co atom). The co-adsorption energy is −1.62eV. The calculations indicate a CO molecule with a ∼4° tilt angle with the normal to the surface. The most important bond is PtCCO, as revealed by overlap population analysis. A very small CO–benzene interaction is also detected. The vibrational frequencies of adsorbed benzene and CO were also computed.

Adsorption of carbon dioxide-methane mixtures in porous carbons: effect of surface chemistry

A combined experimental and molecular simulation study of the coadsorption of CO2 and CH4 in porous carbons is reported. We address the effect of surface chemistry by considering a numerical model of disordered porous carbons which has been modified to include heterochemistry (with a chemical composition consistent with that of the experimental sample). We discuss how realistic the numerical sample is by comparing its pore size distribution (PSD), specific surface area, porous volume, and porosity with those for the experimental sample. We also discuss the different criteria used to estimate the latter properties from a geometrical analysis. We demonstrate the ability of the MP method to estimate PSD of porous carbons from nitrogen adsorption isotherms. Both the experimental and simulated coadsorption isotherms resemble those obtained for pure gases (type I in the IUPAC classification). On the other hand, only the porous carbon including the heterogroups allows simulating quantitatively the selectivity of the experimental adsorbent for different carbon dioxide/methane mixtures. This result shows that taking into account the heterochemistry present in porous carbons is crucial to represent correctly adsorption selectivities in such hydrophobic samples. We also show that the adsorbed solution theory describes quantitatively the simulated and experimental coadsorption isotherms without any parameter adjustment.

In Situ Ru K-Edge X-ray Absorption Spectroscopy Study of Methanol Oxidation Mechanisms on Model Submonolayer Ru on Pt Nanoparticle Electrocatalyst

In situ X-ray absorption spectroscopy (XAS) with electrochemical reaction control has enabled a detailed investigation of the mechanism of the methanol electrooxidation by a bimetallic PtRu catalyst. By the use of an original electrodeposition technique, ca. 0.3 monolayer of Ru was deposited on the surface of unsupported Pt nanoparticles (Ru@Pt). The presence of Ru atoms only at the surface of nanoparticles turns a bulk sensitive XAS technique into a surface methodology which permits correlation of the X-ray absorption fine structure at the Ru K-edge to the role of Ru atoms in the methanol oxidation process. In situ XAS spectra of the Ru@Pt nanoparticles were collected at various electrode potentials in background electrolyte, and then in 1 M solution of methanol (CH3OH) in the same electrolyte. Significant differences in the catalyst state have been revealed between these two environments. In the background electrolyte, Ru gradually oxidizes from mostly metallic to a Ru(III)/Ru(IV) mixture at the highest potentials. In the presence of CH3OH, the Ru oxidation state remains a mixture of metallic and Ru(III) even at the highest potentials. CO-type species were found adsorbed on Ru atoms at all potentials and coadsorbed with OH species at potentials 0.175 V vs Ag/AgCl and higher with steady number of near neighbors. The same potential correlates to the beginning of methanol oxidation reaction observed electrochemically. Therefore coadsorption of CO and OH groups on Ru atoms appears to be critical in the methanol oxidation process. The need for OH groups for CO removal from Pt sites was previously suggested by the bifunctional CH3OH oxidation mechanism; however, occupancy of Ru atoms with CO species and direct structural observations of the coadsorption of CO and OH on the same Ru atom is a novel finding resulting from this study. Other changes in the catalyst environment were observed, including decreased Ru–Ru and Ru–Pt bond distances, increased numbers of Ru–Pt near neighbors, and decreased number of Ru–Ru near neighbors.

Co-adsorption of hydrogen and CO on Pt film: An in-situ ATR-IR study combined with DFT calculations

The co-adsorption of hydrogen and CO on Pt film was studied by in-situ attenuated total reflection Infrared (ATR-IR) Spectroscopy. It was found that the adsorption of CO on Pt is much stronger than that of hydrogen, and the co-adsorbed hydrogen species lead to dramatic red shifts of both linear and bridged-adsorbed CO peaks. The more hydrogen species adsorbed on Pt surface, the more red shifts of CO frequencies were observed. Based on the results obtained from the well-designed experiments, it is deduced that H may act as an electron donor on Pt surface and negative charge is transferred from H to Pt. Thus, the surface charge properties of Pt film were changed, which enhances the d–π∗ backdonation effect between Pt and CO and consequently leads to dramatic red shifts of CO peaks. The density functional theory (DFT) calculations based on simple models confirmed the charge transfer effect between hydrogen and Pt. This study also shows that ATR-IR technique is a powerful tool to investigate the gas–solid interface.