Activity For Low-temperature CO Oxidation Research Articles

Microwave Synthesis of Supported Au and Pd Nanoparticle Catalysts for CO Oxidation

We report the microwave synthesis and characterization of Au and Pd nanoparticle catalysts supported on CeO2, CuO, and ZnO nanoparticles for CO oxidation. The results indicate that supported Au/CeO2 catalysts exhibit excellent activity for low-temperature CO oxidation. The Pd/CeO2 catalyst shows a uniform dispersion of Pd nanoparticles with a narrow size distribution within the ceria support. A remarkable enhancement of the catalytic activity is observed and directly correlated with the change in the morphology of the supported catalyst and the efficient dispersion of the active metal on the support achieved by using capping agents during the microwave synthesis. The significance of the current method lies mainly in its simplicity, flexibility, and the control of the different factors that determine the activity of the nanoparticle catalysts.

Powder XRD analysis and catalysis characterization of ultra-small gold nanoparticles deposited on titania-modified SBA-15

Ultra-small gold nanoparticles (0.8–1 nm) are successfully deposited on titania-modified SBA-15 via a deposition–precipitation method. A comparison of experimental X-ray diffraction (XRD) patterns with theoretical ones shows that gold exists as Au 3+ and Au 0 in the as-synthesized and reduced catalyst, respectively. The XRD analyses also suggest that Au nanoparticles are more developed along the 〈1 1 1〉 direction forming a raft-type structure. Z-contrast transmission electron microscopy analyses indicate that the ultra-small gold nanoparticles are uniformly dispersed on the surface of the substrate. The material is found to possess high catalytic activity for low-temperature CO oxidation.

Highly Active Au/TiO2 Catalysts for Low-Temperature CO Oxidation: Preparation, Conditioning and Stability

Using a modified depositionprecipitation procedure and a new reductive conditioning method, Au/TiO2 catalysts with small metallic Au particles (<2 nm) and a very high activity for low-temperature CO oxidation were prepared. The particles are stable during reaction; a decreasing activity is caused by the accumulation of by-products on the catalyst.

Development and characterization of Pt–SnO 2/γ-Al 2O 3 catalysts

Effects of several preparation parameters on the physical properties and low-temperature CO oxidation activity of γ-Al 2O 3 supported Pt–SnO 2 catalysts were examined. Results obtained with different Pt and SnO 2 loadings and ratios are compared on the basis of 1 wt % Pt/6 wt % SnO 2/γ-Al 2O 3 which corresponds to an optimum atomic Pt:Sn ratio of 1:8. A space time of 15 mg min / ml gives the highest activity with no catalyst deactivation. Catalytic activity is enhanced in reaction mixtures containing excess oxygen (CO/O 2=1/2 or 1), while the stoichiometric feed ratio of CO/O 2=2 leads to catalyst deactivation and lower activity. Kinetic data show that low-temperature CO oxidation over 1 wt % Pt/6 wt % SnO 2/γ-Al 2O 3 is best described by a model based on oxygen adsorption on SnO 2 followed by reverse-spillover from the oxide onto Pt sites and surface reaction occurring on Pt sites between chemisorbed CO and oxygen, with dissociative adsorption of oxygen as the rate-determining step.

Low-Temperature CO Oxidation over Platinum and Cobalt Oxide Catalysts

The low-temperature CO oxidation activity of three monolith catalysts (Pt/Al2O3, CoOx/Al2O3, and Pt/CoOx/Al2O3) has been studied. The catalytic properties of the catalysts were investigated by using different temperature-programmed methods: CO-TPD (desorption), TPO (oxidation), TPR (reduction), TPReaction, and CO light-off. Preoxidized cobalt-oxide-containing catalysts were found to be highly active for CO oxidation at temperatures as low as 200 K, and at these low temperatures the activity is independent of the presence of platinum. The catalytic properties and how they relate to the surface reaction kinetics are discussed.

Supported gold catalysis derived from the interaction of a Au–phosphine complex with as-precipitated titanium hydroxide and titanium oxide

Supported gold catalysts derived from interaction of a Au–phosphine complex Au(PPh 3)(NO 3) ( 1) with conventional titanium oxide TiO 2 and as-precipitated titanium hydroxide Ti(OH) ∗ 4 (*, as-precipitated) have been characterized by means of XRD, XPS, EXAFS, and 31 P CP/MAS–NMR. The Au complex 1 was supported on TiO 2 and Ti(OH) ∗ 4 without loss of Au–P bonding at room temperature. The Au complex 1 on TiO 2 was readily and completely decomposed to form metallic gold particles by calcination at 473 K, whereas only a small part of the complex 1 on Ti(OH) ∗ 4 was transformed to metallic gold particles. By calcination of 1/ Ti(OH) ∗ 4 at 573 K the formation of both metallic gold particles and crystalline titanium oxides became notable as evidenced by XRD, XPS and 31 P CP/MAS–NMR. The mean diameter of Au particles in 1/ Ti(OH) ∗ 4 calcined at 673 K was less than 30 Å as estimated from Au(2 0 0) diffraction, which was about one-tenth of that for the corresponding 1/TiO 2. Thus the as-precipitated titanium hydroxide Ti(OH) ∗ 4 was able to stabilize the Au complex 1 to lead to the simultaneous decomposition of Au complex and Ti(OH) ∗ 4 . The catalyst 1/ Ti(OH) ∗ 4 calcined at 673 K afforded remarkably high catalytic activity for low-temperature CO oxidation at 273–373 K as compared to the catalyst 1/TiO 2.

The effect of the structure of Cu–Ti oxide systems obtained by sol–gel synthesis on the nature of catalytic centres and catalytic activity in low-temperature CO oxidation

It has been shown by X-ray diffraction analysis, EXAFS and XPS methods and in experiments on the low-temperature oxidation of CO that two types of catalytic oxidation centres are formed during sol–gel synthesis of single-phase Cu–Ti oxide systems (0.5–5.0 mass% Cu) based on anatase: the first type involves Cu + ions in the [111] crystal planes, while the second type involves Cu 2+ ions, which isomorphically substitute titanium ions in the [100] planes of TiO 2

Characterization study of CeO2 supported Pd catalyst for low-temperature carbon monoxide oxidation

Catalysts consisting of palladium supported on cerium dioxide (Pd/CeO2) were prepared and used for carbon monoxide oxidation in a stoichiometric mixture of carbon monoxide and oxygen. Pd/CeO2 exhibits high catalytic activity for the oxidation of CO, showing markedly enhanced catalytic activities due to the combined effect of palladium and cerium dioxide. The Pd/CeO2 catalyst is superior to Pd/ZrO2, Pd/Al2O3, Pd/TiO2, Pd/ZSM-5 and Pd/SiO2 catalysts with regard to the activity under the conditions examined. The catalysts were characterized by means of XRD and TPR. The position of the H2-TPR peak shifts to lower temperature with increasing Pd loading from 0.25 to 2.0%. CeO2 inhibits the hydrogen reduction of PdO. CO-TPR measurements have shown the existence of three peaks. The low-temperature peak (α) is due to the Pd hydroxide species. The β peak has been attributed to finely dispersed PdO. The high-temperature peak (γ) has been attributed to crystal phase PdO. Crystal phase PdO is more difficult to reduce by CO than finely dispersed PdO. On the basis of the catalytic activity and CO-TPR results, we conclude α species (Pd hydroxide) mainly contribute to the catalytic activity for low-temperature CO oxidation.

Supported Au Catalysts Prepared from Au Phosphine Complexes and As-Precipitated Metal Hydroxides: Characterization and Low-Temperature CO Oxidation

Supported Au catalysts were prepared by attaching Au phosphinecomplexes, Au(PPh3)(NO3) (1) and [Au9(PPh3)8](NO3)3(2), on as-precipitated metal hydroxidesM(OH)x*(x*, as-precipitated;M=Mn2+, Co2+, Fe3+, Ni2+, Zn2+, Mg2+, Cu2+, Ti4+, Ce4+, and La3+), followed by temperature-programmed calcination in a flow of dry air. The obtained Au catalysts showed high catalytic activities in low-temperature CO oxidation. Among the obtained Au catalysts1/Mn(OH)x* and1/Co(OH)x* were most highly active even at 203 K.1/Fe(OH)3* and1/Ti(OH)4* also catalyzed CO oxidation at low temperatures 203–273 K, whereas1/Fe2O3and 1/TiO2prepared by supporting1on conventional Fe2O3and TiO2showed negligible activity under the similar reactionconditions. It was estimated by TEM and XRD that the mean diameter of Au particles in1/Fe(OH)3* was about 2.9 nm, which was about 10 times smaller than that for1/Fe2O3. EXAFS for1/Ti(OH)4* revealed that the coordination number of Au–Au bond was 8–10, while that for1/TiO2was 11.0, which also indicates that Au particle size for1/Ti(OH)4* is smaller thanthat for1/TiO2. The catalysts obtained by attaching the Au complexes on commercially available metal hydroxides also showednegligible activity for the low-temperature CO oxidation underidentical conditions. These results demonstrate that supportedAu catalysts with small Au particles, tremendously active for thelow-temperature CO oxidation, can be prepared by attaching theAu phosphine complexes on the as-precipitated metal hydroxides.Sodium cations exhibited positive effect on the Au catalysis,whereas chloride anions drastically decreased the CO oxidation activity.