CO Oxidation Behavior of Copper and Copper Oxides

Carbon monoxide oxidation activities over Cu, Cu₂O, and CuO were studied to seek insight into the role of the copper species in the oxidation reaction. The activity of copper oxide species can be elucidated in terms of species transformation and change in the number of surface lattice oxygen ions. The propensity of Cu₂O toward valence variations and thus its ability to seize or release surface lattice oxygen more readily enables Cu₂O to exhibit higher activities than the other two copper species. The non-stoichiometric metastable copper oxide species formed during reduction are very active in the course of CO oxidation because of its excellent ability to transport surface lattice oxygen. Consequently, the metastable cluster of CuO is more active than CuO, and the activity will be significantly enhanced when non-stoichiometric copper oxides are formed. In addition, the light-off behaviors were observed over both Cu and Cu₂O powders. CO oxidation over metallic Cu powders was lighted-off because of a synergistic effect of temperature rises due to heat generation from Cu oxidation as well as CO oxidation over the partially oxidized copper species.     

Preferential oxidation of CO in rich H2 over CuO/CeO2: Details of selectivity and deactivation under the reactant stream

A CuO/CeO₂ catalyst is examined with respect to its performance for preferential oxidation of CO in H₂-rich streams. Catalytic activity results are explained on the basis of characterization by operando-DRIFTS and complemented with the analysis of redox properties by electron paramagnetic resonance (EPR) and X-ray photoelectron spectra (XPS). General catalytic activity features are accounted for by comparative analysis of the activities for individual CO and H₂ oxidation, for which similar CuO and CeO₂ interfacial active sites appear to be involved. An interesting particularity is related to observation of a low temperature hydrogen oxidation process in which CO apparently acts as gaseous promoter. A deactivation process taking place rapidly under the reactant stream is evidenced and attributed to accumulation of hydroxyls on the interfacial active sites and/or to copper sintering in the course of the run.     

COMPARATIVE STUDY OF Ag2O/CO3O4 CATALYSTS PREPARED BY THE SOL-GEL AND CO-PRECIPITATION TECHNIQUES: CHARACTERIZATION AND CO ACTIVITY STUDIES

In this study effects of the preparation method on the characteristic properties and CO oxidation activities of Ag₂O/Co₃O₄ catalysts were investigated. Catalysts were prepared by two different methods: sol gel and co-precipitation. N₂ physisorption measurements, X-ray diffraction, and scanning electron microscopy measurements were used to characterize the catalysts. CO oxidation activity tests were carried out under 1% CO, 21% O₂, and the remainder He feed condition between 20° and 200°C. According to the N₂ physisorption measurements, catalysts prepared by the co-precipitation method have a higher surface area than the catalysts prepared by the sol-gel method. Co₃O₄ and AgCoO₂ phases were obtained from catalysts prepared by both techniques. In addition to these phases, metallic silver peaks were obtained by increasing calcination temperature. SEM micrographs of the catalysts showed that catalysts have uniform particles. Increasing the calcination temperature caused the formation of different-sized agglomerates and an increase in the gaps between agglomerates. The best activity was obtained from the Ag₂ O/Co₃ O₄ catalyst calcined at 200°C and prepared by the co-precipitation method. This catalyst gave 50% CO conversion at 106°C. The other two catalysts gave 100% CO conversion at a higher temperature of 200°C.     

The catalytic activity for CO oxidation of CuO supported on Ce0.8Zr0.2O2 prepared via citrate method

CuO/Ce_0.8Zr_0.2O₂ catalysts were prepared by citrate method and used for carbon monoxide oxidation. The samples were characterized by XRD, XPS, BET and ICP-AES techniques. The catalytic properties of the catalysts were studied by using a microreactor-GC system. XRD analysis showed Ce_0.8Zr_0.2O₂ was cubic, fluorite structure for all the catalysts. The XPS indicated the valence of Ce atom was +4 and there were reduced copper species presented in the CuO/Ce_0.8Zr_0.2O₂ catalyst. The results showed that the CuO loadings, calcination temperature and calcination time affected the catalytic activity of the catalysts for low-temperature CO oxidation. For comparison, the catalytic activities of CuO/CeO₂ catalysts calcined at different temperatures were also studied. The results indicated that CuO/Ce_0.8Zr_0.2O₂ catalyst had better thermal resistance than CuO/CeO₂ catalyst and had inferior activity than the CuO/CeO₂ catalyst when they were both calcined at 600 °C.     

Enhancement of catalytic activity of Au/TiO2 by thermal and plasma treatment

A significant enhancement in the catalytic activity of Au/TiO₂ in CO oxidation and preferential oxidation reaction by creating the active sites on the catalyst surface by thermal treatment as well as by producing small gold particles by plasma treatment has been studied. Au/TiO₂ catalyst (Au (1 wt%) supported on TiO₂) was prepared by conventional deposition-precipitation method with NaOH (DP NaOH) followed by washing, drying and calcination in air at 400 °C for 4 h. Thermal treatment of Au/TiO₂ was carried out at 550 °C under 0.05 mTorr. A small amount of Au/TiO₂ catalyst was taken from the untreated and thermally treated Au/TiO₂ and both kinds of catalysts were treated with plasma sputtering at room temperature. The activity of the catalysts has been examined in the reaction of CO oxidation and preferential oxidation (PROX) at 25–250 °C. Thermally treated Au/TiO₂ showed better catalytic activity as compared to the untreated catalyst. There is also an additional enhancement in the catalytic activity due to plasma sputtering on the both kinds of catalysts. Thermally treated Au/TiO₂ followed by plasma sputtering Au/TiO₂ showed higher conversion rates for CO oxidation reaction compared with untreated, thermally treated and plasma sputtered Au/TiO₂ catalysts. It may be concluded that the enhancement of catalytic activity of thermally treated Au/TiO₂ followed by plasma sputtering is owing to the generation of active sites such as oxygen vacancies/defects in TiO₂ support using thermal treatment as well as by producing small gold particles using plasma treatment.     

Mesoporous CuO/ZrO<Subscript>2</Subscript> nanocatalysts: synthesis,characterization and low-temperature CO oxidation activities

Mesoporous ZrO₂ was synthesized by a simple surfactant-assisted method in the presence of the cationic surfactant cetyltrimethylammonium bromide, and the CuO loaded catalysts CuO/ZrO₂ were prepared by a deposition–precipitation method. The obtained products were characterized by means of powder X-ray diffraction, N₂-sorption, transmission electron microscopy (TEM), hydrogen temperature-programmed reduction and X-ray Photoelectron Spectroscopy techniques. Their catalytic performance for low-temperature CO oxidation was studied by using a microreactor-GC system. TEM and N₂-sorption results showed that the obtained materials possess a mesoporous structure with high-surface area and uniform pore-size distribution. The results of catalytic activity tests revealed that these mesoporous nanostructured CuO/ZrO₂ catalysts were very active for low-temperature CO oxidation.     

Metal organic framework Cu/MIL-53(Ce)-mediated synthesis of highly active and stable CO oxidation catalysts

The highly dispersed CuO@CeO₂ catalysts were produced from Cu/MIL-53(Ce) through encapsulated copper in a matrix of metal organic framework MIL-53(Ce), serving as a catalyst for CO oxidation. Comparison on the catalytic performance between the CuO@CeO₂ and the (CuO or MIL-53(Ce)) catalytic material has been conducted to understand the catalytic behavior of the catalysts. The CuO@CeO₂ catalysts were characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), N₂ adsorption-desorption, and the hydrogen temperature-programmed reduction (H₂-TPR). The analyses indicated that the unique CuO@CeO₂ spatial confinement in the obtained solids minimize catalyst deactivation, resulting in high active and stable operation. Indeed, the heterogeneous catalytic composite materials CuO@CeO₂ catalyst exhibited excellent activity in CO oxidation test, with 100% conversion at 79–95 °C and the catalytic activity were stable after reaction 300 h.     

Synthesis and characterization of CuO/TiO2 catalysts for low-temperature CO oxidation

In this paper, the CuO/TiO₂ catalysts prepared by the deposition–precipitation (DP) method were extensively investigated for CO oxidation reaction. The structural characters of the CuO/TiO₂ catalysts were comparatively investigated by TG-DTA, XRD, and XPS measurements. It was shown that the catalytic behavior of CuO/TiO₂ catalysts greatly depended on the TiO₂-support calcination temperature, the CuO loading amount and the CuO/TiO₂ catalysts calcination temperature. CuO supported on the anatase phase of TiO₂-support calcined at 400 °C showed better catalytic activity than those supported on TiO₂ calcined at 500 and 700 °C. Among all our investigated catalysts with CuO loading from 2% to 12%, the catalyst with 8 wt% CuO loading exhibited the highest catalytic activity. The optimum calcination temperature of the CuO/TiO₂ catalysts was 300 °C. The XRD results indicated that the catalytic activity of the CuO/TiO₂ catalysts was related to the crystal phase and particle size of TiO₂ support and CuO active component.     

Effect of calcination temperature on the physicochemical and catalytic properties of SZSBA-15 catalyst in the production of monopalmitin

Sulfated zirconia on SBA-15 catalysts with different calcination temperatures (450, 550, 600 and 650°C) were synthesized through urea hydrolysis method. The catalysts were characterized using N₂ adsorption–desorption analysis, scanning electron microscopy, energy dispersive X-ray, transmission electron microscopy, thermogravimetric analysis, temperature-programmed desorption of ammonia (NH₃–TPD), X-ray diffraction, and determination of surface acidity by HCl titration method. The catalyst’s characteristics and their correlation with the catalytic activity in the esterification of palmitic acid with glycerol were particularly investigated. The characterization results revealed that the morphology of all catalysts remained virtually unaffected with increasing calcination temperature but the surface area and pore volume of the catalysts showed some reduction. Pore diameter was not significantly affected by the calcination temperature which indicated the stability of the porous catalysts. Furthermore, the increase in the calcination temperature exceeding 600°C would reduce the catalytic activity toward monopalmitin due to a decrease in the active acid sites concentration and acidic strength of the catalyst as a result of sulfur decomposition.     

CO<Subscript>2</Subscript> hydrogenation to methanol over Cu/ZnO catalysts synthesized via a facile solid-phase grinding process using oxalic acid

Reduced Cu/ZnO catalyst was synthesized through solid phase grinding of the mixture of oxalic acid, copper nitrate and zinc nitrate, followed by subsequent calcination in N₂ atmosphere without further H₂ reduction. The catalysts were characterized by various techniques, such as XRD, TG-DTA, TPR and N₂O chemisorption. Characterization results suggested that during the calcination in N₂, as-ground precursor (oxalate complexes) decomposed to CuO and ZnO, releasing considerable amount of CO, which could be used for in situ reduction of CuO to Cu^o. The in situ reduced O/I-Cu/ZnO catalyst was evaluated in CO₂ hydrogenation to methanol, which exhibited superior catalytic performance to its counterpart O/H-Cu/ZnO catalyst obtained through conventional H₂ reduction. The decomposition of precursor and reduction of CuO happened simultaneously during the calcination in N₂, preventing the growth of active Cu⁰ species and aggregation of catalyst particles, which was inevitable during conventional H₂ reduction process. This method is simple and solvent-free, opening a new route to prepare metallic catalysts without further reduction.     

Fe-promoted CuO/CeO2 catalyst: Structural characterization and CO oxidation activity

In the present paper, effect of iron on activity of catalyst CuO/CeO₂ in selective CO oxidation in H₂-containing gases mixture was investigated. Catalysts were prepared by wet impregnation and calcination at 400 °C, characterized by ICP, BET, H₂-TPR, XRD and TEM. The addition of iron to Cu/CeO₂ catalyst improved the catalytic activity and selectivity for CO oxidation. Discussion of the results showed that the synergistic effect is correlated to better reducibility and dispersion of copper in the presence of the iron metal additive.     

Activity and Selectivity of a Nanostructured CuO/ZrO2 Catalyst in the Steam Reforming of Methanol

Steam reforming of methanol for production of hydrogen can be carried out over copper based catalyst. In the work presented here, the catalytic properties of a CuO/ZrO₂ catalyst (8.5wt%) synthesised by a templating technique were investigated with respect to activity, long term stability, CO formation, and response to oxygen addition to the feed. The results were obtained using a fixed bed reactor and compared to a commercial methanol synthesis catalyst CuO/ZnO/Al₂O₃. It is shown that, depending on the time on stream, the temporary addition of oxygen to the feed has a beneficial effect on the activity of the CuO/ZrO₂ catalyst. After activation, the CuO/ZrO₂ catalyst is found to be more active (per copper mass) than the CuO/ZnO/Al₂O₃ system, more stable during time on stream (measured up to 250h), and to produce less CO. Structural characterisation by means of X-ray powder diffraction (XRD) and X-ray absorption spectroscopy (XAS) reveals that the catalyst (as prepared) consists of crystalline, tetragonal zirconia with small domain sizes (about 60Å) and small/disordered crystallites of CuO.     

Catalytic oxidation of CO by O2 over nanosized CuO–ZnO system prepared under various conditions

A series of copper and zinc mixed oxides composed of equimolar ratio and treated with 0.05 Al₂O₃/mol catalyst has been prepared by coprecipitation method using their nitrates in presence of 1 M NaOH solution. The precipitation was carried out at a pH 9.5 and 50°C. The results revealed that the bulk of various solids consisted of CuO and ZnO having ratios varying between 0.84 and 0.98. On the other hand, the values of surface concentrations of copper and zinc were bigger than those present in the bulk. The values of surface Cu/Zn ratio varied between 1.04 and 1.46 depending on the mode of preparation and calcination conditions. The mixed solids existed as nanocrystalline CuO and ZnO phases. The surface characteristics are strongly dependent on mode of coprecipitation and calcination temperature. The computed S_BET values varied between 146 and 23 m²/g depending on mode of coprecipitation and calcination temperature. The increase of calcination temperature of different adsorbents within 300–400°C led to a significant progressive decrease in the BET‐surface area with subsequent increase in the values of mean pore radius. All adsorbents are mesoporous solids. All prepared solids showed a good catalytic activity in CO oxidation by O₂, which proceeds via first order kinetics in all cases. The activation energy of the catalysed reaction was determined for various solids and the computed values were very small indicating the big catalytic activity of the investigated solids.     

Low-temperature Oxidation of Carbon Monoxide on Co/ZrO2

A 10%Co/ZrO₂ catalyst prepared by impregnation was tested for its activity for the oxidation of CO to CO₂ in excess oxygen. Activity tests showed that conversion could be obtained at temperatures as low as 20 °C. Time-on-stream studies showed no loss of activity in these experiments, indicating that this catalyst is stable in the experimental oxidizing conditions. The activation energy for the CO to CO₂ oxidation reaction was calculated as E_a = 54 kJ/mol over this catalyst. Characterization of the material by thermogravimetric analysis, temperature-programmed techniques, X-ray photoelectron spectroscopy, and laser Raman spectroscopy indicate that Co₃O₄ is present on monoclinic ZrO₂ after the calcination of the catalyst.     

Active species of copper chromite catalyst in C–O hydrogenolysis of 5-methylfurfuryl alcohol

The active sites of copper chromite catalyst, CuCr₂O₄·CuO, were investigated for the condensed-phase hydrogenolysis of 5-methylfurfuryl alcohol to 2,5-dimethylfuran at 220 °C. The bulk and surface features of the catalyst were characterized by XRD, H₂-TPR, N₂ adsorption, CO chemisorption, N₂O titration, NH₃-TPD, XPS, and AES. Maxima of both of the potential active species, Cu⁰ and Cu⁺, occurred after reduction in H₂ at 300 °C compared to 240 and 360 °C. These Cu⁰ and Cu⁺ maxima also coincided with the highest specific rate of reaction based on the surface area of the reduced catalyst. The trends of Cu⁰ and Cu⁺ observed by N₂O titration and CO chemisorption were also observed qualitatively by AES. Correlations between activity and the possible active species suggested that Cu⁰ was primarily responsible for the activity of the catalysts.     

ZrO_2负载Cu催化剂对CO低温催化氧化研究

用溶胶-凝胶-超临界干燥法制备了纳米氧化锆。采用沉淀法制备氧化锆负载铜催化剂。制备的催化剂用X-ray射线衍射(XRD),透射电镜(TEM),比表面积(BET)和H2-TPR等进行了表征。研究了催化剂的焙烧温度和负载比例对CO转化效率的影响,其最佳焙烧温度为650℃,Zr与Cu的最佳物质的量比是10:8。获得催化剂在温度为68℃具有催化活性,176℃时CO的转化率达到50%,较好地实现了ZrO2负载Cu在较低温度下对CO的催化。     

Influence of calcination temperature on the hydrolysis of carbonyl sulfide over hydrotalcite-derived Zn–Ni–Al catalyst

A novel catalyst for low temperature hydrolysis of carbonyl sulfide (COS) was prepared by thermal decomposition of Zn–Ni–Al hydrotalcite-like compounds (HTLCs). As the key factors of catalyst activity, effects of calcination temperature have been studied. The samples were carefully characterized by XRD, FTIR, SEM, CO₂-TPD and N₂ adsorption/desorption. Results showed that HTLCs calcined at 350 °C exhibited excellent activity due to the production of more M–O pairs which are active sites of the hydrolysis of COS. However, calcination at 500 °C led to the destruction of pore structure and reduction of active sites, ultimately led to a lower COS conversion.     

Mesoporous Co-Al oxide nanosheets as highly efficient catalysts for CO oxidation

We report mesoporous Co-Al oxide nanosheets (Co_xAl-Ns, where x denotes the Co/Al ratio in the samples) prepared by calcination of CoAl-hydrotalcite and subsequent alkaline treatment. X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy measurements show that the prepared Co-Al oxide nanosheets (Co_xAl-Ns) are very thin (10–15 nm) and exhibit high mesoporosity (3–5 nm). Catalytic CO oxidation tests reveal that the Co_xAl-Ns exhibit excellent catalytic performances at relatively low temperatures: for example, the Co_2.5Al-Ns catalyst could achieve 99% CO conversion at −98°C. Kinetic studies and experimental investigations indicate that the high activity of the Co_2.5Al-Ns sample is strongly related to the abundance of active sites associated with the large Brunauer–Emmett–Teller surface area. The Co_2.5Al-Ns catalyst also achieves full conversion of CO in tests performed with a gas mixture simulating automobile exhaust gas at 200°C. After loading the Co_2.5Al-Ns on a porous ceramic substrate, the obtained Co_2.5Al-Ns/PC shows high activity and stability in CO oxidation process. These features are potentially important for future industrial applications of these catalysts.     

Mesoporous copper–cerium–oxygen hybrid nanostructures for low temperature catalytic oxidation of carbon monoxide

Mesoporous copper–cerium–oxygen hybrid nanostructures were prepared by one-pot cetyltrimethylammonium bromide surfactant-assisted method, and were characterized by thermogravimetry, X-ray diffraction, transmission electron microscopy, nitrogen adsorption–desorption, X-ray photoelectron spectroscopy and temperature-programmed reduction techniques. Low temperature carbon monoxide oxidation was used as probe reaction to investigate the application of the prepared mesoporous copper–cerium–oxygen hybrid nanostructures in catalysis. The product calcined at 400 °C, with disordered wormlike mesoporous structure, high specific surface area (SSA) of 117.4 m²/g and small catalyst particle size of 8.3 nm, shows high catalytic activity with the 100 % CO conversion at 110 °C, indicating its potential application in catalysis. Catalytic activity results from the samples calcinied at different temperature suggested that high SSA, small catalyst particle size, finely dispersed CuO species and synergistic effect between CuO and CeO₂ were responsible for the high catalytic activity of the catalysts.     

Kinetic modeling of CO oxidation on Pt/CeO2 in a gradientless reactor

Cerium oxide is a major additive in three-way catalysts used in emission control of automobile exhaust. Pt/CeO₂ was studied in order to better understand the role of ceria in promoting CO oxidation reaction. The kinetics of carbon monoxide oxidation on Pt/cerium oxide catalyst, was studied over the temperature range 100–170°C. Steady state kinetic measurements of CO oxidation were obtained in a computer controlled micro-CSTR reactor. Activation energies were reported to vary between 39·5 and 51·2 kJ mol⁻¹. At low concentrations of either reactant (CO, O₂) and total conversion, the catalyst exhibited multiple steady states, similar to the multiplicity behavior of Pt/Al₂O₃. The total conversion was reached at 120°C. In comparison, the total conversion at low reactant concentrations was reached at a temperature of 148°C for the alumina-supported catalyst. Langmuir–Hinshelwood mechanisms gave a good fit to the data. However, no single rate expression could effectively describe the CO oxidation data over the whole concentration in the product of the CSTR reactor. The facts gathered indicate that oxygen adsorbed on interfacial Pt/Ce sites and ceria lattice oxygen provides oxygen for CO oxidation. Cerium oxide has been found to lower CO oxidation activation energy, enhance reaction activity and tends to suppress the usual CO inhibition effect.