Weaker CO Adsorption Research Articles

Enhanced activity for CO oxidation over WO3 nanolamella supported Pt catalyst.

WO3 nanolamella supported Pt catalyst has been prepared and applied for CO oxidation in this work. A significantly enhanced activity has been achieved, compared to that of the Pt catalyst supported by the WO3 nanoparticle. The catalyst characterization using X-ray diffraction (XRD), scanning electronic microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), and N2 adsorption-desorption confirms that the WO3 nanolamella supported Pt catalyst possesses higher Pt dispersion, improved metal-support interaction with a higher electron density of Pt, and a weak adsorption of CO, leading to the significantly enhanced activity for CO oxidation.

Ferric Oxide-Supported Pt Subnano Clusters for Preferential Oxidation of CO in H2-Rich Gas at Room Temperature

Pt single atoms and small clusters were dispersed on iron oxides by a facile coprecipitation method. These catalysts, with or without calcination at elevated temperatures, show excellent activity and selectivity for preferential oxidation of CO in the H2-rich gas. They can completely remove CO from H2-rich gas at a wide temperature range of 20–70 °C, which renders them suitable for low-temperature applications. The reaction followed a mixture of competitive mechanism and a noncompetitive/redox mechanism. The weakened CO adsorption on small Pt clusters and atoms makes the competitive adsorption of O2 feasible, which ensures a high activity of Pt/Fe catalysts, even calcined at elevated temperature.

Interfacial oxygen under TiO2 supported Au clusters revealed by a genetic algorithm search

We present a density functional theory study of the oxidation of 1D periodic rods supported along the [001] direction on the rutile TiO2(110) surface. The study shows evidence for an oxidation of the interface between the supported Au and the TiO2 crystal. The added O atoms adsorb at the 5f-Ti atoms in the through under the Au rod and are stabilized by charge transfer from the nearest Au atoms. Despite an extensive search, we find no low energy barrier pathways for CO oxidation involving CO adsorbed on Au and O at the perimeter of the Au/TiO2 interface. This is in part attributed the weak adsorption of CO on cationic Au at the perimeter.

CO adsorption on Pt clusters supported on graphite

Density functional theory calculations are used to characterize the influence of the nanoparticle–substrate interaction on CO adsorption in various Pt cluster sizes. Adsorption energies are evaluated on different sites of clusters supported on graphite and on unsupported clusters. Charge transfer and density of states are examined in order to determine the influence of the support on the cluster electronic properties. In comparison with adsorption on substrate-free cluster, we find weaker CO adsorption energy when CO is adsorbed on top of the cluster and stronger adsorption energy when CO is adsorbed on sites near the surface. The substrate has a larger influence on CO adsorption in sub-nanometer clusters; however some effect is detected in a 1nm-diameter cluster. The graphite support has the capacity to accept electrons from the deposited cluster, but can donate electrons when the cluster adsorbs CO. The density of states of the CO molecule when it is adsorbed on clusters with different carbon substrates shows particular features that reveal the capacity of the support to influence CO adsorption.

Electroreduction of Carbon Dioxide to Methane on Copper, Copper–Silver, and Copper–Gold Catalysts: A DFT Study

The electrochemical reduction of CO2 is a promising process capable of efficiently recycling CO2 waste and converting it into hydrocarbon fuel. To date, copper is the best metal catalyst; however the overpotential to achieve this reaction on Cu is excessively high. It follows that the development of a catalyst to efficiently catalyze the conversion with a low overpotential at a reasonable current density is needed. Many aspects of the molecular details of the reaction are still unclear. In this work, DFT calculations are applied to investigate CO2 electroreduction to CH4 over Cu3Ag and Cu3Au stepped surfaces (211) compared to that over Cu(211). In the resulting analysis, the Cu3Ag surface shows a slightly lower overpotential and suppresses OH poisoning compared to the Cu surface, yet the selectivity toward H2 increases. The Cu3Au is not a good candidate due to higher overpotential and a relatively weak CO adsorption resulting in CO desorption rather than further reduction. The CO desorption can also be pr...

CO adsorption, CO dissociation, and C-C coupling on Cu monolayer-covered Fe(100)

CO adsorption, CO dissociation and C-H and C-C bond-making reactions on Cu-covered Fe(100) have been computed within the framework of periodic density functional theory. Compared to the clean Fe(100) surface with strong CO activation, Cu monolayer-covered Fe(100) surface has weaker CO adsorption energy, lower CO activation degree; and ultra higher CO dissociation barrier (2.4 eV). Under H co-adsorption, CO dissociation is favored kinetically on Fe(100). On the Cu-monolayer covered Fe(100), co-adsorbed H largely facilitates CO dissociation via the formation of CHO. The energy barrier decreases to as low as 0.92 eV. It is also found that C-H and C-C bond-making reactions of Cu monolayer-covered Fe(100) surface have lower activation barriers and are more exothermic than on the clean Fe(100) surface.

Photocatalytic H 2 production on hybrid catalyst system composed of inorganic semiconductor and cobaloximes catalysts

An artificial photocatalytic system mimicking photosystem I (PSI) has been assembled using semiconductor (CdS) as photosensitizer, cobaloximes (Co III complexes) as H 2 evolution catalysts, and triethanolamine (TEOA) as sacrificial electron donor. This artificial photocatalytic system shows high hydrogen evolution activity (turnover number up to 171 based on Co III(dmgH) 2pyCl 1) under visible light irradiation. The apparent quantum efficiency (QE) for 1/CdS hybrid photocatalytic system in acetonitrile solution at 420 nm is calculated to be 9.1%. The interfacial electron transfer from photoexcited CdS to Co III complexes is very efficient through the weak adsorption of Co III complexes on CdS. The adsorption of 1 on CdS in acetonitrile fits Langmuir equation, the maximum monolayer adsorption capacity is 3 × 10 −3 mmol g −1, which means most of 1 are in the solution. The rate of hydrogen production exhibits a quadratic dependence on the total concentration of 1. Therefore, a bimetallic catalysis pathway is proposed. The efficient electron transfer, the broad electronic absorption character of CdS photosensitizer as well as the H 2 evolution ability of Co III complexes, account for the high photocatalytic activity of this hybrid photocatalytic system.

Basic catalysis and catalysis assisted by basicity: FT-IR and TPD characterization of potassium-doped alumina

Adsorbents and catalyst supports produced by loading different amounts of potassium acetate on alumina followed by calcination are characterized using IR spectroscopy and TPD. CO, CO 2 and NO x adsorption is studied. Two different ranges of potassium loading on alumina are found, namely light and heavy doping corresponding to 1% and more than 3% K wt/wt, respectively. Light doping results in the weak adsorption of CO 2 as bicarbonate species and NO 2 as bidentate nitrates. Heavy doping results in the adsorption of CO 2 as bidentate carbonates and NO 2 as polydentate nitrate species. Three families of exposed K + ions are observed at increasing loading.

A highly active PtNi/C electrocatalyst for methanol electro-oxidation in alkaline media

A carbon supported PtNi/C electrocatalyst synthesized via a polyol process was characterized by XRD, TEM and voltammetry techniques. The XRD analysis showed a decreased lattice constant of Pt for the PtNi/C catalyst compared with the Pt/C. indicating partial PtNi alloy formed. The PtNi/C exhibited enhanced mass activity towards methanol oxidation reaction as well as decreased CO stripping currents in alkaline media compared with the Pt/C. Density functional theory studies revealed the electronic structure modification of Pt by Ni atoms and weakened CO adsorption energy on the Pt(3)Ni than on the Pt(4) clusters, which may account for the increased MOR activity. Crown Copyright (C) 2010 Published by Elsevier By. All rights reserved.

Mechanism for CO oxidation catalyzed by Pd-substituted BaCeO 3, and the local structure of the active sites

Ba(Ce,Pd)O 3 perovskite is an unusual catalyst for CO oxidation by O 2, given its low surface area. The rate law for the catalytic reaction was evaluated under CO-lean and CO-rich conditions, as well as near-stoichiometric conditions. When O 2 is present in excess, the kinetics show CO inhibition, consistent with a Langmuir–Hinshelwood mechanism in which both reactants compete for the same adsorption sites. The Arrhenius activation energy for this mechanism is surprisingly low, (7.8 ± 0.3) kcal/mol. It is attributed to weak adsorption of CO on ionic surface Pd(II) sites. When O 2 is limiting, the reaction orders for both CO and O 2 show a strong dependence on P(CO)/ P(O 2), and eventually become independent of both P(CO) and P(O 2) at high P(CO). This suggests a new BaCeO 3-mediated mechanism which dominates the reaction at high P(CO)/ P(O 2). Its Arrhenius activation energy is (9.7 ± 0.3) kcal/mol, probably reflecting the barrier to oxygen migration in the oxide phase. Both mechanisms contribute significantly for P(CO)/ P(O 2) ≈ 1. The catalytic activity of Pd-substituted BaCeO 3 is attributed to the increased bulk oxygen mobility in the presence of square-planar Pd(II) ions that are located on the perovskite B-sites, each adjacent to an oxygen vacancy.

Effect of La doping on CO adsorption at ceria surfaces

In this paper we use density functional theory corrected for on-site Coulomb interactions (DFT+U) to study the adsorption of CO at La-doped ceria (111) and (110) surfaces. Doping of ceria with La is known to enhance oxidation of CO to CO(2) and this study investigates the atomic level details of this reaction. With La(3+) doping, an [La(3+)-O(-)] defect state with an oxygen hole is formed at both surfaces. The formation energy of an oxygen vacancy is reduced and vacancy formation results in the appearance of Ce(3+), instead of hole compensation. On the doped surfaces weak and strong adsorption of CO is found. In the former, the molecule remains intact. In the latter, the final adsorption species depends strongly on the surface and whether oxygen vacancies are present or not. On (111) a CO(2)-like species forms, while on the (110) surface, mono- or bidentate carbonates are present. La-doping of ceria surfaces shows enhanced reactivity over the undoped surfaces and we discuss the origin of the enhanced reactivity and the nature of the surface species upon CO adsorption.

Theoretical investigation of CO adsorption on Rhn(n= 3–13) clusters

The adsorption of CO onto Rhn(n = 3–13) clusters has been investigated using the density functional approach. Stable active sites for CO adsorption such as top, bridge, and hollow have been identified on these clusters. Our results show that CO mostly prefers the bridge or top site, except on the Rh4and Rh11clusters where it prefers hollow sites. Rh6demonstrates two different active sites of almost equal energies for CO adsorption. Highly stable clusters show weak CO adsorption behavior. We also observe that the magnetic moment of the clusters is usually reduced after the CO adsorption. The preference of the active sites for CO adsorption has been analyzed using the charge density difference plots.

Support effects and catalytic trends for water gas shift activity of transition metals

Water gas shift activity measurements for 12 transition metals (Fe, Co, Ni, Cu, Ru, Rh, Pd, Ag, Re, Ir, Pt, Au) supported on inert MgAl 2O 4 and Ce 0.75Zr 0.25O 2 are presented, to elucidate the influence of the active metal and the support. The activity is related to the adsorption energy of molecular CO and atomic oxygen on the metal; the latter is a good measure for the reactivity of the metal towards H 2O. Generally, the activity of the catalysts with the Ce 0.75Zr 0.25O 2 support is higher, compared to the corresponding MgAl 2O 4-supported catalysts. Exceptions are Cu and Au, which have a higher activity on the MgAl 2O 4 support and are both characterized by weak CO adsorption. For the MgAl 2O 4-supported catalysts a volcano-type relation between the activity and the adsorption energy of atomic oxygen on the metal is obtained. The maximum activity is found for metals with a binding energy of oxygen around −2.5 eV. No clear correlation exists with the adsorption energy of CO. In contrast, the activity for the Ce 0.75Zr 0.25O 2 support increases with increasing adsorption strength for CO, and based on a relatively low activity of Cu the activity does not seem to depend on the adsorption energy of oxygen. Such a change in activity-descriptor for the different supports can be rationalized by the possibility that water dissociation occurs on the redox-active Ce 0.75Zr 0.25O 2 support, whereas the MgAl 2O 4 support is inactive.

Temporal Analysis of Products Reactor as a Complementary Tool to Study the Mechanism of Some Green Catalytic Reactions

A Temporal Analysis of Products (TAP) reactor was used to study the mechanism of two green catalytic reactions, namely low-temperature CO oxidation on supported gold catalysts and volatile organic compounds (VOCs) elimination, with propane as model molecule, on vanadia-based catalysts. Single-pulse TAP experiments evidenced a weak and reversible adsorption of CO on Au/Ti(OH)(4)* catalyst and of propane on vanadia-based catalytic materials obtained by DC magnetron sputter deposition, while O-2 adsorption was irreversible on both catalysts. Alternating TAP pulse experiments provided evidence for the nature of oxygen species involved in the oxidation reactions. If the adsorbed oxygen was involved in the CO oxidation, the lattice oxygen was responsible for the propane oxidation. Multi-pulse TAP experiments assessed the degree of reduction of the catalyst while its influence upon the mechanism is determined by single-pulse experiments. The obtained results will allow the optimal formulation of industrial scale catalysts with applicability to a wide range of industrial processes.

Promotional effects of Cu and K on precipitated iron-based catalysts for Fischer–Tropsch synthesis

The effects of Cu and K promoters on precipitated iron-based Fischer–Tropsch synthesis (FTS) catalysts were investigated by using N 2 physical adsorption, temperature-programmed reduction/desorption (TPR/TPD) and Mössbauer effect spectroscopy (MES). The FTS performances of the catalysts were tested in a slurry-phase continuously stirred tank reactor (CSTR). The characterization results indicated that Cu promoter facilitates the high dispersion of Fe 2O 3, significantly promotes the reduction and H 2 adsorption, but severely suppresses CO adsorption and the carburization. However, K promoter severely retards the reduction and suppresses the H 2 adsorption, facilitates the CO adsorption and promotes the carburization. In the FTS reaction, it was found that Cu promoter decreases the FTS initial activity and water gas shift (WGS) reaction activity, promotes the oxidation of iron carbides to Fe 3O 4 and accelerates the deactivation of iron-based catalyst. However, K promoter improves the FTS activity and WGS reaction activity, suppresses the oxidation of iron carbide to Fe 3O 4 and significantly improves the stability of iron-based catalyst. As compared with individual promotion of Cu or K, the double promotions of Cu and K significantly improve the FTS and WGS activities and keep excellent stability. Due to weaker CO adsorption and stronger H 2 adsorption than the catalysts without Cu, Cu promoted catalysts have higher selectivity to light hydrocarbons and methane and lower selectivity to heavy hydrocarbons. However, the opposite result is obtained on the catalyst incorporated with K promoter.

Adsorption of camphor and 2,2′-bipyridine on Bi(1 1 1) electrode surface

Impedance spectroscopy and in situ STM methods have been used for investigation of the camphor and 2,2′-bipyridine (2,2′-BP) adsorption at the electrochemically polished Bi(1 1 1) electrode from weakly acidified Na 2SO 4 supporting electrolyte solution. The influence of electrode potential on the adsorption kinetics of camphor and 2,2′-BP on Bi(1 1 1) has been demonstrated. In the region of maximal adsorption, i.e. capacitance pit in the differential capacitance versus electrode potential curve, the heterogeneous adsorption and diffusion steps are the rate determining stages for camphor and 2,2′-BP adsorption at the Bi(1 1 1) electrode. It was found that for camphor | Bi(1 1 1) interface the stable adsorbate adlayer detectable by using the in situ STM method has been observed only at the positively charged electrode surface, where the weak co-adsorption of SO 4 2− anions and camphor molecules is possible. At the weakly negatively charged Bi(1 1 1) electrode surface there are only physically adsorbed camphor molecules forming the compact adsorption layer. The in situ STM data in a good agreement with impedance data indicate that a very well detectable 2,2′-BP adsorption layer is formed at Bi(1 1 1) electrode in the wide region of charge densities around the zero charge potential.

Adsorption of camphor at Bi(111) electrode

Impedance spectroscopy and in situ STM methods have been used for investigation of the camphor adsorption at the electrochemically polished Bi(111) electrode from weakly acidified Na2SO4 base electrolyte aqueous solution. The influence of the electrode potential on the adsorption kinetics and camphor adsorption layer structure on Bi(111) has been demonstrated. In the region of the capacitance pit in the differential capacitance vs. electrode potential curve the so- called heterogeneous adsorption step is the rate determining stage for camphor adsorption at the electrochemically polished Bi(111) electrode, but not diffusion as usually assumed. It was found that the stable adsorbate adlayer detectable by using the in situ STM method has been observed only at the positively charged electrode surface, where the weak co-adsorption of SO42- anions and camphor molecules is possible. At the weakly negatively charged Bi(111) electrode surface there are only physically adsorbed camphor molecules forming the compact adsorption layer.

Temporal analysis of products (TAP) study on low temperature carbon monoxide oxidation on supported Au catalysts

TAP (temporal analysis of products) technique was used to clarify the controversial mechanism for low-temperature CO oxidation on supported Au catalysts involving unidentified moisture effects on the performances. The unique TAP transient technique, along the use of a specially prepared, highly active Au/Ti(OH) 4 * catalyst, provided the information and characterization of each elementary step involving weak and reversible CO adsorption, strong and molecular O2 adsorption, and their surface reaction, which are suppressed by the coexistence of water vapor.

Formation and catalytic activity of partially oxidized Pd nanoparticles

Combining multi molecular beam (MB) experiments and in-situ time-resolved infrared reflection absorption spectroscopy (TR-IRAS), we have studied the formation and catalytic activity of Pd oxide species on a well-defined Fe3O4 supported Pd model catalyst. It was found that for oxidation temperatures up to 450 K oxygen predominantly chemisorbs on metallic Pd whereas at 500 K and above (~10−6 mbar effective oxygen pressure) large amounts of Pd oxide are formed. These Pd oxide species preferentially form a thin layer at the particle/support interface. Their formation and reduction is fully reversible. As a consequence, the Pd interface oxide layer acts as an oxygen reservoir providing oxygen for catalytic surface reactions. In addition to the Pd interface oxide, the formation of surface oxides was also observed for temperatures above 500 K. The extent of surface oxide formation critically depends on the oxidation temperature resulting in partially oxidized Pd particles between 500 and 600 K. It is shown that the catalytic activity of the model catalyst for CO oxidation decreases significantly with increasing surface oxide coverage independent of the composition of the reactants. We address this deactivation of the catalyst to the weak CO adsorption on Pd surface oxides, leading to a very low reaction probability.

Effect of the Intrinsic Properties of Metals on the Adsorption Behavior of MoleculesCompetitive and Cooperative Adsorption of Benzene and Other Species on Pt and Rh Surfaces

Our previous study of benzene adsorption on Pt groups reveals the marked effect of the intrinsic properties of the metals. In the present paper, we further investigated the competitive and cooperative adsorption effects of halogen ions, SCN-, and pyridine on benzene adsorption on Pt and Rh surfaces using in situ electrochemical surface-enhanced Raman spectroscopy (SERS). The effect of the adsorption strength and the concentration of these species, the electrode potential, and the surface morphology on the adsorption behavior of benzene were systematically studied. Very different effects have been found on Pt and Rh surfaces. On Pt surfaces, Br- and I- competed with the chemisorbed benzene; meanwhile, the chemisorbed ions induced the physisorption of benzene. However, on Rh surfaces, a much weaker coadsorption was observed for I- and benzene. With the addition of a 1/100 amount of pyridine, the chemisorbed benzene was totally removed from the Pt surface. However, on the Rh surface, signals from both benzen...