Yasuo Iizuka

Adsorption of CO on gold supported on TiO2

Abstract The adsorption of CO on TiO2 supported gold has been investigated both under a constant pressure static system and under a closed recirculation system with liquid nitrogen cooled trap. Au TiO 2 with 3.3 wt% loading and 3.5 nm mean particle diameter of Au was prepared by deposition-precipitation. Adsorption of CO on Au TiO 2 was 90% reversible and satisfied the Langmuir isotherm. The amount of CO2 produced during CO adsorption agreed well with the amount of irreversible CO adsorption. TiO2 powder produced only 1/185th the amount of CO2 at 273 K compared to gold powder, even though the amount of CO adsorbed per unit surface area was similar to that of gold powder. The results indicate that a reaction between oxygen adsorbed on the surface of small gold particles with CO is one of the major reaction pathways in forming CO2.

The catalysis of carbon monoxide oxidation with oxygen on molybdenum trioxide

The mechanism of the catalytic oxidation of CO with O2 on MoO3 was studied kinetically over a wide pressure range in a closed circulation and static system, by measuring the electric conductivity of the catalyst during the reaction and by using the tracer technique with isotopic oxygen 18O2. The rate-determining step is the reaction between the active oxygen of the catalyst and CO from the gas phase. During the reaction, oxygen molecule is chemisorbed dissociatively and irreversibly on such sites which are composed of two adjacent defects of active oxygen. From the measurements of the electric conductivity, the catalyst was found to be in a slightly reduced state during the catalysis of CO oxidation. The deficient amount of active oxygen in the steady state of the catalysis was determined by reducing the oxidized catalyst by CO of low pressure until the conductivity became the same value as that in the steady state of the catalysis. The deficient ratio of active oxygen, 1-θ, was calculated from the CO2 produced. The following relation was obtained between the rate constant of reduction of MoO3 by CO, kCO, and that of the oxidation of the reduced MoO3 by O2, kO2, in the steady state of the reaction in a mixed gas with a composition of CO/O2 = 21: kCO · PCO · θ = 2 kO2 · PO2 · (1-θ)2.

Mechanism of the catalytic oxidation of CO with O2 on an Mo catalyst supported on silica and on bulk MoO3

The mechanism of the catalytic oxidation of CO with O2 on an Mo catalyst supported on silica has been studied by a tracer technique using 18O2. The supported Mo catalyst was prepared by using the ready reaction between Mo(η3-C3H5)4 and the OH groups on silica. Extended X-ray absorption fine structure (EXAFS) spectroscopy showed that the Mo species attached to silica are dispersed atomically and have a dioxostructure. At steady state during oxidation with CO/O2= 2/1, 32% of the attached Mo species were estimated to be present as Mo6+ and 68% as Mo4+. The catalytic oxidation was accompanied by oxygen isotope exchange between O2 molecules. The oxidation of the oxostructure (Mo4+) to the dioxostructure (Mo6+) by O2 molecules during the catalytic oxidation produces O atoms on the silica surface. Recombination of two migrating O atoms leads to oxygen isotope exchange between O2 molecules. The mechanism of the catalytic oxidation of CO with O2 on bulk MoO3 was re-examined with reference to that elucidated on the SiO2-supported Mo catalyst.

18 O tracer studies of CO oxidation with O2 on MoO3. Part 1.—Diffusion of 18O atoms from active sites during the catalysis and the determination of the number of active sites

The catalytic oxidation of CO with 18O2 has been carried out on MoO3 at 753 K. The isotopic composition of the product CO2 was examined as a function of time. The amount of C 18O2 was always negligible. The percentage of C 18O 16O increased rapidly at the beginning of the reaction then gradually reached a plateau value. Interruption of the oxidation did not significantly affect the percentage of C 18O 16O in the CO2 produced. Oxygen vacancies produced by the reduction of MoO3 with CO apparently remained unchanged on the surface after evacuation for 120 min at 718 K. The incorporation of O2 into oxygen vacancies took place rapidly. A small fraction of 18O atoms taken up into oxygen vacancies were recovered in subsequent CO reduction processes, while the greater part of the 18O diffused into the bulk of the MoO3. The catalytic oxidation of CO with 18O2 on MoO3 could be explained in terms of mobile and immobile sites with regard to 18O atoms taken up into vacancies of active oxygen. This model enables calculation of the time dependence of the yield of C 18O 16O in the product CO2 and estimation of the number of immobile sites.

18 O tracer studies of CO oxidation with O2 on MoO3. Part 2.—Active sites for CO oxidation with O2 and for oxygen isotope exchange between CO2 and MoO3

Oxygen isotope exchange between C 18O2 and MoO3 has been examined in connection with the catalytic oxidation of C 16O with 18O2 on MoO3. Oxide ions at mobile and immobile sites on MoO3, which oxidize CO in the catalytic oxidation, were found to work also as active sites in the oxygen isotope exchange between C 18O2 and Mo 16O3. The observed time dependences of the yields of C 18O2, C 18O 16O and C 16O2 in the exchange agreed satisfactorily with those calculated from the mobile and immobile site model. In the calculation, the number of immobile sites was estimated to be 1.8% of the total surface lattice oxide ions. The exchange rate at immobile sites was 13% of the total exchange rate.

An examination of active sites on Au-Ag bimetallic catalysts based on CO oxidation over Au/Ag 2 O and a comparison to Ag-contaminated Au powder

There are two theories regarding the origin of the remarkable synergistic effect observed in Au-Ag bimetallic catalysts when applied to various oxidative reactions. One is based on the importance of the contact interfaces between AgO x regions and the surface of the bulk Au as active working sites, while the other holds that charge transfer from Ag to Au in a surface Au-Ag alloy causes the catalytic activity. One key point in examining these theories and determining the origin of the synergy involves determining whether or not Ag exists as an oxide or as a metallic alloy on the Au surface. To confirm that enhanced activity results from contact between Ag 2 O and Au nanoparticles (NPs), a comparative study of catalytic CO oxidation over Au/Ag 2 O and Ag 2 O was performed in the present work, using a closed recirculation reaction system. A reaction mixture consisting of a stoichiometric composition of CO and O 2 (CO/O 2 =2/1) was supplied to both catalysts and the resulting pressure decrease rates were tracked, from which the amounts of gas consumed as well as the quantity of CO 2 produced were determined. The steady state reactions of both Au/Ag 2 O and Ag 2 O did not lead to any meaningful difference in the rate of pressure decrease during the oxidation. The pressure decrease over both catalysts was attributed to the reduction of surface lattice O on Ag 2 O by CO. The results obtained for Au/Ag 2 O are in good agreement with previous data resulting from the use of Ag-contaminated Au powder (Ag/Au-b) having an oxidized surfaces. This finding suggests that the perimeters between AgO x zones and the bulk Au surface may not function as active sites during CO oxidation. A review of previous results obtained with Ag/Au-b specimens having so-called steady state surfaces indicates that AgO x species in such materials are reduced to the 0 state to form a Ag-Au alloy that provides the active sites.

18 O tracer studies of CO oxidation with O2 on MoO3. Part 3.—Reaction mechanism of oxygen isotope exchange between CO2 and MoO3

The kinetics and mechanism of the oxygen exchange between C18O2 and MoO3 have been examined in the temperature range 673–793 K and in the pressure range 24–146 Pa. The rate of oxygen exchange was proportional to the pressure of CO2 with an activation energy of 94.5 kJ mol–1, which was smaller than that for the catalytic oxidation of CO with O2 on MoO3 by 22.1 kJ mol–1. The rate constant of oxygen exchange between CO2 and MoO3, Rco2, was 51–25 times larger than that for the catalytic oxidation of CO with O2 on MoO3, kco2, at 673–793 K; both had similar frequency factors and fractions of effective collisions were estimated to be almost the same. The difference between Rco2 and kco2 originated from the difference in the activation energies between the two reactions. An active complex with a unidentate carbonate was proposed in the oxygen exchange mechanism.

Diffusion of oxygen atoms from active sites during the catalytic oxidation of carbon monoxide with oxygen on molybdenum trioxide

Abstract The diffusion of 18 O atoms from active sites during the catalytic CO oxidation with 18 O 2 on MoO 3 has been examined. There are two kinds of 18 O atoms adsorbed on vacancies of active oxygen. One kind is immobile and responsible for the formation of C 18 O 16 O in the course of the catalysis. The other kind is rapidly mobile from active sites by being replaced with 16 O oxide ions from the bulk.

Catalytic behavior of silicon carbide for hydrogen activation

The catalytic activities for hydrogen-deuterium exchange were examined at 473 K for three kinds of SiC powders with different particle sizes. Three SiC powders showed a high activity after outgassing at high temperatures above 873 K. The thermodesorbed gases from SiC were mainly H2 and CO, and they were produced from the thermal decomposition of adsorbed water on SiC surfaces. The extent of decomposition was found to be parallel to the catalytic activity at 473 K. Therefore, the sites of the décomposition are responsible for the catalytic activity.