The Role of Different Types of CuO in CuO–CeO2/Al2O3 for Total Oxidation (original) (raw)
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Nature of the active sites for the total oxidation of toluene by CuOCeO2/Al2O3
Journal of Catalysis, 2012
The binary metal oxide, CuOACeO 2 /c-Al 2 O 3 , has been compared with the single oxide components CuO/ c-Al 2 O 3 and CeO 2 /c-Al 2 O 3 for toluene total oxidation. The nature of the active sites was determined by means of several spectroscopic techniques, while the transient response technique TAP (Temporal Analysis of Products) was used to investigate the catalytic performance.
Nature of the active sites for the total oxidation of toluene by CuOsingle bondCeO2/Al2O3
Journal of …, 2012
"The binary metal oxide, CuOsingle bondCeO2/γ-Al2O3, has been compared with the single oxide components CuO/γ-Al2O3 and CeO2/γ-Al2O3 for toluene total oxidation. The nature of the active sites was determined by means of several spectroscopic techniques, while the transient response technique TAP (Temporal Analysis of Products) was used to investigate the catalytic performance. The improved performance of the CuOsingle bondCeO2/γ-Al2O3 catalyst compared to CuO/γ-Al2O3 is attributed to the formation of a Ce1−xCuxO2−x solid solution with a crystallite size of 6 nm. Within this phase, oxidation of toluene occurs at Cu2+ sites and reduction of oxygen at Ce3+ sites. Similar to Wacker chemistry, two redox couples, Ce4+/Ce3+ and Cu2+/Cu1+, are operational. Apart from the solid solution, a copper oxide phase with a crystallite size of 100 nm shows significantly lower catalytic activity. X-ray absorption near-edge structure (XANES) experiments at the copper and cerium edge indicate that Ce4+ is reduced at lower temperature than Cu2+. Upon re-oxidation with CO2 or H2O, Ce3+ is partly re-oxidized, while Cu0 is not. This explains an activity increase in the CuOsingle bondCeO2/γ-Al2O3 in the presence of H2O or CO2. CuO/γ-Al2O3 shows loss of activity in the presence of H2O as site blocking is not compensated by an increase in the re-oxidation rate."
Applied Catalysis A: General, 2008
CuO/CeO 2 catalysts with CuO content ranging from 0.5 wt.% to 8 wt.%, prepared by wet impregnation of commercial ceria, have been tested for the preferential oxidation of CO (CO-PROX) under H 2 -rich conditions at 70-210 8C. Catalytic activity increases up to 4 wt.% CuO content, with less concentrated catalysts showing higher intrinsic activity. Catalysts have been characterized by means of XRD, BET analysis and UV spectroscopy. Formation of segregated CuO clusters has been detected for Cu richest CuO/CeO 2 sample. Redox properties have been deeply investigated using TP analysis (H 2 TPR, CO TPR, TPO) of fresh or pre-treated samples. Participation of surface ceria, induced by the strong interaction with copper, to reduction/oxidation reactions in the temperature range explored (up to 430 8C) has been demonstrated. Different copper species and their reactivity towards H 2 and CO have been individuated by comparing TPR of fully oxidized catalysts with those of partially oxidized catalysts. Active species have been identified as copper-ceria sites able to oxidize CO even at room temperature and to be re-oxidized by O 2 at the same temperature. Transient experiments have been carried out at different temperature using a diluted mixture starting from oxidized or reduced catalysts and followed by a H 2 TPR of the used samples. The results of these tests have showed that active centres for CO oxidation contain copper in the +2 oxidation state. At T > 100 8C some reduced copper sites are stabilized which promote H 2 oxidation thus lowering the selectivity of the CO-PROX process.
Applied Catalysis B-environmental, 2006
A catalyst of copper oxide supported on nanostructured ceria has been examined with the aim of exploring deactivating effects produced by CO 2 or H 2 O presence on its activity for preferential oxidation of CO in a H 2 -rich stream. For this purpose, the catalyst is explored by means of operando-DRIFTS experiments. The results allow determining most relevant deactivating effects induced by CO 2 and H 2 O. These are mainly related to modifications of interfacial sites upon formation of specific carbonates and a blocking effect induced by the presence of adsorbed molecular water, respectively, which limit redox/catalytic activity of the interfacial zone of the catalyst active for CO oxidation. Such modifications are directly evidenced by the difficulties of ceria to promote the generation of partially reduced states at interfacial sites of the dispersed copper oxide particles or to propagate the reduction over such particles, which affects to the hydrogen oxidation activity of the catalyst, leading on the whole to a general decrease of the CO-PROX performance of the system.
Reaction network for the total oxidation of toluene over CuO–CeO2/Al2O3
Journal of Catalysis, 2011
"The total oxidation of toluene was studied over a CuO–CeO2/γ-Al2O3 catalyst in a Temporal Analysis of Products (TAP) setup in the temperature range 673–923 K in the presence and absence of dioxygen and at various degrees of reduction of the catalyst. The reaction rate significantly decreases over a mildly reduced catalyst. Under vacuum and at high-temperature, mild reduction also occurs in the absence of toluene. In the presence of dioxygen, the catalyst activity is determined by weakly bound surface lattice oxygen atoms and adsorbed oxygen species, the lifetime of which is close to 1 s. The weakly bound oxygen is highly reactive and is only found over a fully oxidized catalyst. The formation of products containing 18O during the isotope-exchange experiments with 18O2 indicates that both lattice and adsorbed oxygen are involved in (a) reoxidation of mildly reduced copper oxide and (b) abstraction of hydrogen atoms and scission of C–C bonds. Isotopic labeling with C6H5–13CH3 and C6H5–CD3 indicates the following reaction paths: adsorption of toluene on the active site, containing Cu2+ with 4–5 adjacent surface lattice oxygen atoms; the simultaneous abstraction of H from the methyl and the phenyl group followed by the abstraction of the methyl carbon and next the destruction of the aromatic ring."
Journal of Catalysis, 2009
A catalyst of copper oxide supported on nanostructured ceria has been examined with the aim of exploring deactivating effects produced by CO 2 or H 2 O presence on its activity for preferential oxidation of CO in a H 2 -rich stream. For this purpose, the catalyst is explored by means of operando-DRIFTS experiments. The results allow determining most relevant deactivating effects induced by CO 2 and H 2 O. These are mainly related to modifications of interfacial sites upon formation of specific carbonates and a blocking effect induced by the presence of adsorbed molecular water, respectively, which limit redox/catalytic activity of the interfacial zone of the catalyst active for CO oxidation. Such modifications are directly evidenced by the difficulties of ceria to promote the generation of partially reduced states at interfacial sites of the dispersed copper oxide particles or to propagate the reduction over such particles, which affects to the hydrogen oxidation activity of the catalyst, leading on the whole to a general decrease of the CO-PROX performance of the system.
Journal of Experimental Nanoscience, 2011
Three catalyst samples 10 wt% CuO/-Al 2 O 3 (CuAl), 10 wt%CuO þ 10 wt% Cr 2 O 3 /-Al 2 O 3 (CuCrAl) and 10 wt%CuO þ 20 wt% CeO 2 /-Al 2 O 3 (CuCeAl) have been taken for the study. Physico-chemical characteristics of the catalysts were determined by the methods of BET adsorption, X-ray diffraction and temperature-programmed reduction. Due to a strong interaction of CuO with Al 2 O 3 , resulting in the formation of copper aluminate the catalytic activity of CuO/-Al 2 O 3 has been reduced. The bi-oxide catalyst CuCrAl was more active than CuAl, thanks to the formation of high catalytically active spinel CuCr 2 O 4. The fact of very high activity of the CuCeAl sample can be explained by the presence of the catalytically active form CuO-CeO 2-Al 2 O 3. The kinetics of CO oxidation reaction on the given catalysts was studied at the temperature range between 200 C and 270 C. The values of initial partial pressures of carbon monoxide (P o CO), oxygen (P o O2) and carbon dioxide (P o CO2) were varied in ranges (hPa): 10 Ä 45; 33 Ä 100 and 0 Ä 30, respectively. It has been found that on all the catalysts, the common kinetic equation for the reaction was as follows: r ¼ kP CO P 0:5 O2 ð1 þ k 2 P 0:5 O2 þ k 3 P CO2 Þ , where r is the reaction rate, k is the reaction constant, and k 2 and k 3 are the O 2 and CO 2 adsorption constants, respectively. In the case of CuAl and CuCrAl, k 3 ¼ 0, and since k 2 P 0:5 o2) 1, the equation should become r ¼ k.P CO .
Kinetics of CO and H2 oxidation over CuO–CeO2 and CuO catalysts
Chemical Engineering Journal, 2011
The kinetics of CO and H 2 oxidation over CuO-CeO 2 and CuO catalysts have been investigated. Rate expressions derived from red-ox (Mars-van Krevelen) reaction mechanisms describe satisfactorily the observed kinetic behavior in both oxidation reactions. In the case of CuO-CeO 2 , both a standard redox expression (with P O 2 n , n ∼ 0.07) and a redox expression (with n = 1) including the CO adsorption term describe equally well the kinetic results. The promoting effect of CeO 2 is present only in CO oxidation and is due mostly to enhancement of the surface reduction rate constant via lowering of the activation energy for surface reduction. On the other hand, the CuO-CeO 2 catalyst is actually less active than CuO in H 2 oxidation. The calculated heat of adsorption of CO 2 , which inhibits both oxidation reactions, is twice as large on CuO compared to CuO-CeO 2 .
Egg-shell CuO/CeO2/Al2O3 catalysts for CO preferential oxidation
International Journal of Hydrogen Energy, 2015
Catalytic systems based on copper and cerium supported on g-Al 2 O 3 have shown to be extremely effective for CO preferential oxidation. In order to selectively oxidize carbon monoxide, it is desired that only CO can access to the active sites. Since the effective diffusion of H 2 is higher than that of CO, an eggshell type distribution is preferred. With the objective of modifying the radial distribution of the active phases in the catalyst particle, the effect of four variables of the impregnation process is analyzed: metal loading, support-solution contact time, impregnation temperature and drying time. Radial profiles of Cu and Ce show that the eggshell type distribution is favored by low metal loading, short contact and drying times and by high impregnation temperature. The effect of such variables is stronger on copper profile than on cerium profile. Catalytic performance on COPROX was enhanced by eggshell type distribution.