Small CuO Particles Research Articles

Weak Metal-Support Interaction over CuO/TiO2 Catalyst Governed Low-Temperature Toluene Oxidation.

Regulating the metal-support interaction is essential for obtaining highly efficient catalysts for the catalytic oxidation of volatile organic compounds (VOCs). In this work, CuO-TiO2(coll) and CuO/TiO2(imp) with different metal-support interactions were prepared via colloidal and impregnation methods, respectively. The results demonstrated that CuO/TiO2(imp) has higher low-temperature catalytic activity, with a 50% removal of toluene at 170 °C compared to CuO-TiO2(coll). Additionally, the normalized reaction rate (6.4 × 10-6 mol·g-1·s-1) at 160 °C over CuO/TiO2(imp) was almost four-fold higher than that over CuO-TiO2(coll) (1.5 × 10-6 mol·g-1·s-1), and the apparent activation energy value (27.9 ± 2.9 kJ·mol-1) was lower. Systematic structure and surface analysis results disclosed that abundant Cu2+ active species and numerous small CuO particles were presented over CuO/TiO2(imp). Owing to the weak interaction of CuO and TiO2 in this optimized catalyst, the concentration of reducible oxygen species associated with the superior redox property could be enhanced, thus significantly contributing to its low-temperature catalytic activity for toluene oxidation. This work is helpful in exploring the influence of metal-support interaction on the catalytic oxidation of VOCs and developing low-temperature catalysts for VOCs catalytic oxidation.

Unravelling the properties of supported copper oxide: can the particle size induce acidic behaviour?

There is a renewed interest in designing solid acid catalysts particularly due to the significance of Lewis acid catalyzed processes such as Friedel-Crafts acylation and alkylation and cellulose hydrolysis for the development of sustainable chemistry. This paper reports a new focus point on the properties of supported CuO on silica, a material that up to now has been considered only as the precursor of an effective hydrogenation catalyst. Thus, it deals with a re-interpretation of some of our results with supported copper oxide aimed to unveil the root of acidic activity exhibited by this material, e.g. in alcoholysis reactions. Several techniques were used to highlight the very high dispersion of the oxide phase on the support allowing us to ascribe the acidic behavior to coordinative unsaturation of the very small CuO particles. In turn this unsaturation makes the CuO particles prone to coordinate surrounding molecules present in the reaction mixture and to exchange them according to their nucleophilicity.

Effect of titania structure on the properties of its supported copper oxide catalysts

Anatase and rutile have the same chemical composition as TiO2 but different crystalline structures (space group of D4h19-I41 for anatase and D4h14-P42 for rutile, respectively), which result in different surface structures and can be used ideally to investigate the effects of support and surface structure on the properties of their supported catalysts. In this work, anatase- or rutile-supported copper oxide catalysts (signed as Cu-A and Cu-R, thereafter) prepared by the impregnation method were characterized by X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET) surface area, X-ray photoelectron spectroscopy (XPS), and H2 temperature-programmed reduction (H2-TPR) to study the support effect of titania on the physicochemical properties and catalytic properties for the NO reduction by CO of these supported copper oxide catalysts. The results indicated that (1) copper oxide presented different dispersion capacities on anatase or rutile; (2) dispersed copper oxide species on anatase and rutile also showed different reduction behaviors; i.e., one-step reduction for dispersed copper oxide species in Cu-A and stepwise reduction for that in Cu-R; (3) the NO+CO activity test suggested dispersed copper oxide and small CuO particles on rutile are the main active species under the current reaction conditions and copper oxide supported on rutile is more active than that on anatase, which might result from Cu+ species in Cu-R sample formed during the reaction due to its stepwise reduction behavior.

CO as an IR probe molecule for characterization of copper ions in a basolite C300 MOF sample

The state of the accessible copper ions in a Cu-basolite C300 sample (copper benzene-1,3,5-tricarboxylate) was studied by CO as an IR probe molecule. Low temperature CO adsorption on a sample activated at 473 K results in a series of carbonyl bands. Three intense sharp bands at 2141, 2128 and 2123 cm(-1) are very sensitive to the CO equilibrium pressure and are associated with CO polarized by the aromatic ring. Three bands, at ca. 2192, 2178 and 2148 cm(-1) are assigned to three different kinds of monocarbonyl species. At high CO coverage, the principal carbonyls (2178 cm(-1)) are converted into polycarbonyls. A band at 2125 cm(-1) (resistant to evacuation) is associated with Cu(+) ions on small CuO particles. Experiments on (12)CO-(13)CO adsorption revealed lack of vibrational coupling and also showed that the bands at 2192 and 2148 cm(-1), although changing almost in concert, were due to individual species. Sample activated at 373 K does not contain copper oxide, which indicates that activation at higher temperatures, 473 K, leads to some irreversible changes.

Catalytic properties of a CuO–CeO 2 sorbent-catalyst for de-SO x reaction

The preparation and characterization of a CuO–CeO 2 sorbent-catalyst was performed by an inorganic/organic synthesis route and the materials were tested in de-SO x type reactions. The preparation of mesostructured CeO 2 was made using hexadecylamine surfactant and cerium acetate as the inorganic precursor. This procedure was modified by inclusion of the CuO precursor during the synthesis. These sorbent-catalysts were characterized by transmission electronic microscopy (TEM), N 2 physisorption, X-ray diffraction (XRD) and temperature-programmed reduction (TPR). The adsorbent-catalysts were tested by means of sulfurization from room temperature to 760 °C using a combined SO 2/N 2 gas mixture (3600 ppm) and the gain of weight was recorded in a thermo-balance. The materials prepared with those surfactants showed high surface area due to mesopores and small CuO particles dispersed on the ceria support. The catalytic performance in the de-SO x reaction was higher than conventional sorbent-catalysts prepared by impregnation of commercial ceria. The improved SO 2 adsorption capacity of these materials is attributed to a better distribution and interaction of the copper species with the ceria support.

Thermal analysis and temperature-programmed reduction studies of copper–zirconium and copper–zirconium–yttrium compounds

Differential thermal analysis evidenced that introduction of copper or yttrium into zirconia matrix delays its crystallization temperature. This result was connected to the stabilization of the zirconia tetragonal phase observed by X-ray diffraction. In addition, it was observed a decomposition delay of supported copper nitrate with yttrium content in a zirconia support. Temperature-programmed reduction revealed the existence of different copper species: isolated copper ions, copper clusters, small particles of copper oxide and bulk copper oxide. The reduction temperature of these species was strongly influenced by the preparation method of Cu–Zr systems and by the presence of yttrium in ZrO 2. But in all the samples, isolated copper ions and clusters were postulated to be reduced at the same temperature since these species have the same electron paramagnetic resonance parameters and then occupy the same sites. A correlation was done between the reducibility of these copper species and their activity in propene and toluene oxidation reactions. In the absence of yttrium, the best activity measured was correlated to an easier reduction but also to a higher quantity of isolated copper ions and clusters. Addition of yttrium to the support modified the copper interaction. At low copper/zirconium atomic ratio (0.01), the reduction temperature of small CuO particles increased with the yttrium content in the support. At high copper content (Cu/Zr = 0.1), the easier copper species reduction was obtained for Y 2O 3 content of 5 wt.%, the corresponding solid providing the best activity in the propene oxidation.

Infrared spectroscopic studies of carbon monoxide adsorbed on a series of silica-supported copper catalysts in different oxidation states

Infrared spectroscopy has been used to study the adsorption of carbon monoxide (358–493 K, 0.1–20 kPa) on four copper-on-silica (2–10 wt% Cu) catalysts prepared by the “ion-exchange” technique. The measurements are made for each sample in three different states: unreduced (predominantly Cu 2+), reduced (Cu 0), and partially reoxidised in nitrous oxide (Cu +). On unreduced samples, a major absorption band between 2127 and 2132 cm −1 due to CO adsorbed on small CuO particles and a weak band at 2199 cm −1 due to CO on isolated Cu 2+ ions incorporated in the silica surface have been identified. The former adsorption obeys a Langmuir isotherm with a heat of adsorption of 29 kj/mol independent of CuO particle size and surface coverage. After catalyst reduction, the major absorption band lies between 2090 and 2113 cm −1 and arises from CO linearly bound to very small (1- to 5-nm) copper metal clusters. The observed frequency shifts indicate the presence of steps and terraces similar to low index Cu planes in very small particles (1- to 2-nm), and the presence of similar higher index Cu planes on larger clusters (2- to 5-nm). The absorption is described by a Freundlich isotherm with the heat of CO adsorption decreasing with coverage from 50 to 22 kJ/mol on bigger particles but more constant (27 to 22 kj/mol) on small particles. A surface copper/CO atomic ratio increasing from 5 to 12 is established at equilibrium saturation between 358 and 493 K using extinction coefficients determined in this study. In the reduced catalyst, a weakly adsorbed ( ΔH a = −20 kj/mol) species assigned to CO bound to isolated Cu + ions is also found and absorbs at 2175 cm −1. The frequency of this band does not vary with catalyst loading and is not affected by reoxidation of the catalyst in nitrous oxide.