Reaction Rate Of CO Oxidation Research Articles

Investigation of CO Oxidation Transient Kinetics on an Oxygen Pre-covered Au(211) Stepped Surface

The desire to explain the origin(s) of the unexpected catalytic activity of oxide-supported Au nanoparticles for CO oxidation discovered by Haruta and coworkers has stimulated numerous experimental and theoretical studies of Au nanoclusters in the gas phase and on metal oxide supports, and on Au single-crystal surfaces. In order to explore further the reactivity of low-coordination Au step sites, we have performed transient kinetics studies of CO oxidation on an O-precovered, stepped Au(211) single crystal surface. We found behavior similar to that observed previously on flat Au(111) and (110) surfaces; i.e., there is no evidence in these transient kinetics for any special reactivity associated with this stepped Au surface. The CO oxidation reaction rate was highly dependent on the initial oxygen coverage, and we determined an apparent activation energy for CO oxidation of −7.0 kJ mol−1 for θ O init = 0.9 ML. Within the Langmuir-Hinschelwood (LH) reaction scheme, we estimate an activation energy of E LH = 20–43 kJ mol−1 on this surface for CO oxidation via this pathway. This is somewhat below the value of 67 kJ mol−1 predicted by recent theoretical calculations.

Synthesis of Nanosized ZnO/MgO Solid and Its Catalytic Activity for CO Oxidation

Abstract ZnO/MgO solid samples containing ZnO nanoparticles of controllable size were prepared using colloidal technique. The catalytic performance of the ZnO/MgO samples for CO oxidation was measured. The reaction rate of CO oxidation on the ZnO nanoparticles with variable average radius (2.01−2.29 nm) shows nonmonotonic dependence caused by the quantum-confinement effect.

The Role of the Interface in CO Oxidation on Au/CeO2 Multilayer Nanotowers

AbstractThe CO oxidation reaction is studied in this work using a controlled structure consisting of Au/CeO2 multilayers, in the form of towers of 10 × 10 µm2 base, with layer thicknesses in the nanometer range. This structure is prepared on a 3″ Si wafer using reverse photolithography and vapor deposition in an e‐beam chamber. The thickness of each layer is of nanometer dimensions and only the edges of the Au and CeO2 layers are exposed to the reaction gas mixture. The CO oxidation reaction rate is found to scale with the total length of the Au/CeO2 interfaces for nanotowers with the same total Au and CeO2 surface areas. TEM and X‐ray diffraction (XRD) analyses reveal highly stressed gold films in the nanotowers, the lattice strain being temperature and film thickness dependent. Deactivation with time‐on‐stream is commensurate with relaxation of the gold films, as measured by a drop in their lattice strain.

The Relativistic Effect Brings Significant Results on Characteristic Properties of Platinum Surfaces

Reconstruction on open surfaces of Ir, Pt and Au, i.e. (001) and (110) surfaces of fcc metals, is caused by the relativistic effect through increase of the d-hole density of the valence state. The reconstruction of Pt(001) and Pt(110) surfaces is lifted by adsorption of CO and NO molecules and its lifting is caused by reduction of the d-hole density due to donation of lone-pair electrons of these molecules. Although oscillation in the reaction rate of CO oxidation and fantastic two-dimensional images corresponding to various oscillation patterns on Pt(001) and Pt(110) surfaces were observed by Ertl's group, these phenomena can be explained by the change of the d-hole density enhanced by the relativistic effect. Finally, laser-induced desortion of NO from Pt(111) and Pt(111)-Ge surface alloy is also discussed using the relativistic effect.

Research on unsupported nanoporous gold catalyst for CO oxidation

Nanoporous gold (NPG), a novel unsupported gold catalyst prepared by dealloying, exhibits exceptional catalytic activity for CO oxidation. Systematic studies were carried out on this new catalytic system, including the active sites of catalysts, the reaction kinetics, and activity dependence as functions of space velocity and temperature. Our results show strong evidence that metallic gold atoms on NPG are the intrinsic active sites at which the reaction of CO with O 2 occurs. The kinetic study found that the reaction rate of CO oxidation on unsupported NPG depends significantly on CO concentration but only slightly on O 2 concentration. We suggest that CO adsorption plays a decisive role in CO oxidation on NPG as the rate-limiting step. By completely ruling out the support influence, our findings provide considerable insight into the role of gold catalysts.

CuO/CeO 2 catalysts: Redox features and catalytic behaviors

The reduction and oxidation features of nanostructured CuO/CeO 2 catalysts prepared by the deposition–precipitation method were extensively investigated by TPR, FT-IR and in situ XPS techniques. Both the chemical states of copper and the reduction degree of ceria could be well controlled during the reduction with hydrogen by adjusting the temperature. Noticeably, the fully reduced Cu 0 could be further oxidized into Cu + in hydrogen by increasing the reduction temperature through the interaction between Cu 0 and lattice oxygen of ceria immigrating to the surface. Structure–reactivity relationship was established between the structural features of CuO/CeO 2 formed during the pre-reduction with hydrogen and its catalytic activities for CO oxidation. It was observed that reduction with hydrogen at 473–573 K, which leads to the full presence of metallic copper in the catalyst, gives rise to higher CO conversion. These phenomena were interpreted in terms of the reduction degrees of ceria, the changes of surface morphology and the chemical states of copper species. The interface oxygen activation as well as its transfer from the interface to the adsorbed reactant was found to play decisive roles in determining the reaction rate of CO oxidation.

Acoustic excitation influence on the oscillatory CO oxidation reaction over a Pt{110} crystal

Kinetic oscillations in the CO oxidation reaction rate have been initiated when a thin Pt{110} crystal, bonded onto the surface of an interdigital transducer device, was excited with surface acoustic waves (SAWs – Rayleigh waves). The oscillations were square waves with periods between 500 and 1000 s and became regular after an induction period of the order of 40 min following the acoustic excitation of the catalyst. A very small influence of further acoustic excitation has been observed when it occurs at the maximum of the reactivity, conversely, a four-fold increase in the reaction rate was recorded if the SAW excitation was applied when the system exhibited minimum reactivity. It is therefore concluded that acoustic excitation is effective only when the reaction rate is limited by the rate of oxygen adsorption. Hence, the effect of SAW is to increase the sticking probability of oxygen. The SAW chemical effect is not associated with a purely thermal effect, since the catalyst temperature was accurately stabilised during the course of the experiments by means of a feedback controlled temperature regulator.

CO adsorption and CO oxidation on Rh(100)

The adsorption of CO on a Rh(100) surface and the interaction of CO with p(2 × 2) and (2 × 2)p4g oxygen layers on Rh(100) at different temperatures have been studied by means of Thermal Programmed Desorption (TPD), X-Ray Photoelectron Spectroscopy (XPS) and Low Energy Electron Diffraction (LEED). As a measure of the reaction rate the changes in the partial pressure of CO 2 (mass 44) during titration of the oxygen layers with CO were used. The composition of the surface layer and the surface structure were monitored at various stages of the reaction. The CO O 1s XPS spectra showed that CO adsorption occurs in two bonding configurations characterized by O 1s binding energies of 532.4 and 531.5 eV, assigned to occupation of on-top and bridge sites, respectively. The on-top sites are occupied first yielding an ordered c(2 × 2) layer at coverage of 0.5 ML. At higher coverages a fraction of CO adsorbs in bridge sites and conversion to compressed p(4√2 × √2) and ‘split’ (2 × 1) structures occurs. Three CO desorption states were observed in the CO TPD curves which correlate well with the development of the three ordered structures with increasing CO coverage. The desorption parameters of each ordered structure and their variations with CO coverage were evaluated. The rate of CO oxidation reaction during titration of p(2 × 2) and (2 × 2)p4g layers was studied at temperatures ranging from 290 to 670 K. The variation of the CO 2 production revealed dependence on the temperature, reactant surface structure and coverages. It has been found that the following factors are of importance for the reaction rate: (i) the mobility of CO which at the same reaction temperature is affected by the actual oxygen surface structure; (ii) the ordering of the reactants and the structural changes in the course of the titration reaction.

The influence of surface reconstructions on the CO oxidation reaction rate for the [formula omitted] surface

The influence of the oxygen induced surface reconstructions on the CO 2 production rate at the Ag(110)O system will be discussed, based on titration, reactive thermal desorption and LEED measurements. We present evidence that the reactivity of oxygen is reduced in the temperature range of 190–250 K by embedding oxygen in a reconstructed surface as compared to chemisorbed oxygen; above 250 K the coverage dependence of the oxygen induced reconstructions generates an autocatalytic component into the CO 2 formation rate.

The Inhibition of the CO/H20/02Reaction by Trace Quantities of HCl

Abstract The inhibition of the CO/H20/02 reaction by small quantities or HCl was studied in a high temperature, atmospheric pressure flow reactor at temperatures near 1000K. The initial species concentrations of the CO/H30/02; reaction bath were kept constant at about l%/0·6%/0·5% respectively, while the initial concentration of HCl was varied from 0. to 200ppm. These mixtures provided a stoichiometry close to unity with a Cl/H ratio ranging from 0. to 0·016. Results indicated that these small Cl/H ratios can significantly inhibit both the induction time and the reaction rate of CO oxidation. The concentration of HCl was measured by an FTIR spectrometer and was observed to vary throughout the reaction. The mole fraction of HCl was observed to decrease until approximately 50% of the CO was consumed and to slowly rise afterwards. The inhibitory effect of HCl on the CO oxidation rate was observed to decrease after the minimum measured HCl concentration. The experimental results were compared with a reaction ...

Development of supported manganese oxides for partial oxidation: Adsorption of CO and CO2 in total oxidation reactions

The role of individual reactants and products in the mechanisms of CO oxidation on MnOx/TiO2. MnOx/CeO2 and MnOx/Al2O3 was investigated. Complementary adsorptive and spectroscopic techniques were used. It was found that carbon monoxide is a very weak adsorbate on supported MnOx catalysts, but forms intermediates which are bonded to the surface with variable strength. On MnOx/CeO2 and MnOx/Al2O3 the higher stability of the reaction intermediates poisons the catalytic surface and lowers the reaction rates. On MnOx/TiO2 the reaction rate of CO oxidation is independent of PCO2, and the amount of irreversibly adsorbed CO2 is small (≅ 20%). This indicates that the CO2 adsorption sites are different from reaction intermediates formation sites.

Development of supported manganese oxides for partial oxidation: Adsorption of CO and CO 2 in total oxidation reactions

The role of individual reactants and products in the mechanisms of CO oxidation on MnO x/TiO 2. MnO x/CeO 2 and MnO x/Al 2O 3 was investigated. Complementary adsorptive and spectroscopic techniques were used. It was found that carbon monoxide is a very weak adsorbate on supported MnO x catalysts, but forms intermediates which are bonded to the surface with variable strength. On MnO x/CeO 2 and MnO x/Al 2O 3 the higher stability of the reaction intermediates poisons the catalytic surface and lowers the reaction rates. On MnO x/TiO 2 the reaction rate of CO oxidation is independent of PCO 2, and the amount of irreversibly adsorbed CO 2 is small (≅ 20%). This indicates that the CO 2 adsorption sites are different from reaction intermediates formation sites.

Bistability of the reaction rate in the oxidation of carbon monoxide by nitrogen monoxide on polycrystalline platinum

Multiplicity of the stationary reaction rate of CO oxidation by NO is observed under reducing conditions at pressures below 4×10 −4 mbar in the temperature range 500< T<525 K. The two branches of the reaction rate coalesce at a point where sustained oscillations are observed.

Bistability of the reaction rate in the oxidation of carbon monoxide by nitrogen monoxide on polycrystalline platinum

Multiplicity of the stationary reaction rate of CO oxidation by NO is observed under reducing conditions at pressures below 4 x 10 -4 mbar in the temperature range 500< T<525 K. The two branches of the reaction rate coalesce at a point where sustained oscillations are observed.

CO/Air turbulent diffusion flame: Measurements and modeling

Measurements are presented for a turbulent diffusion flame jet of carbon monoxide in coflowing air. Detailed profiles of average velocity, turbulence intensity, average temperature, and species concentrations were measured at a Reynolds number of 1.14 × 10 4. Several checks on the data indicated good accuracy of the measurements. A model was also developed for turbulent diffusion flames on the basis of their local laminar flame structure. Laminar flame calculations, using a global reaction rate for CO oxidation, provided unique relationships between thermochemical variables and the mixture fraction. These relationships along with a K-ϵ-g model of turbulence in Favre-averaged form were used to make predictions in the CO/air diffusion flame jet. The model predictions showed good agreement for x D ≤ 50 , with some discrepancy for x D > 50 .