Supported Copper Oxide Catalysts Research Articles

Improved Catalytic Partial Oxidation of Methane via Lattice Oxygen Modification on Supported Copper Oxide Catalyst System

Improved Catalytic Partial Oxidation of Methane via Lattice Oxygen Modification on Supported Copper Oxide Catalyst System

CH3 radical-mediated direct methane to methanol conversion over CuO supported on rutile oxides

Direct conversion of methane into methanol is an attractive strategy for the production of manifold value-added chemicals. Herein, we investigated the conversion of methane to methanol over CuO supported on the rutile metal oxides (TiO2, SnO2 and RuO2) based on results of density functional theory (DFT) calculations. The results show that the oxygen site of the supported CuO exhibits the characteristics of a radical anion. This radical anionic oxygen site enables homolytic C-H cleavage by abstracting the hydrogen atom, resulting in a CH3 radical. The CH3 radical captured by the Cu site next to the radical anionic oxygen enables the coupling of CH3 and OH to form a C-O bond, resulting in methanol. The free energy of activation for C-H activation and C-O formation were found to correlate linearly with the p band center of the radical anionic oxygen, but the slope has the opposite signs, i.e., a lower free energy of activation for C-H scission corresponds to a higher free energy formation for C-O formation. Among rutile oxides studied, SnO2 offers a balanced reactivity for C-H activation and C-O formation. These findings demonstrate the presence of the radical anionic oxygen on rutile oxide-supported CuO catalyst and their crucial role in regulating the reactivity for direct methane to methanol conversion. The mechanistic insights from this study will benefit the development of supported copper oxide catalysts for effective methane conversion.

Kinetic and thermodynamic analysis of ammonia electro-oxidation over alumina supported copper oxide (CuO/Al2O3) catalysts for direct ammonia fuel cells

In this work, ammonia electrooxidation (AEO) is studied to probe the electrochemical behavior of aqueous NH3 as direct fuel by implication of gamma alumina supported copper oxide catalysts (CuO/Al2O3). Precipitation and impregnation techniques are adapted to synthesize the substrate Al2O3, and loading of different ratios (1%, 2%, 3%, 4%) of active precursor i.e., CuO, respectively. Small average crystallite sizes (DAvg) in range of 1.46–6.76 nm and smaller particle sizes from micrographs proposed a superb activity of the catalysts. Modified GCE exhibited the excellent conductive properties towards standard redox probe K4[Fe(CN)6] and KCl, displaying a high active electrochemical surface area of up to 0.0021 cm2. Current profiles in response to increase in scan rates, concentrations of ammonia and temperature have been observed in 0.1 M KOH, thereby estimating the thermodynamic and kinetic constraints of the AEO process. Attributed to the electroactive properties towards AEO, CuO/Al2O3 is found to exhibit the desirable physiochemical properties owing to large oxidation current, large diffusion coefficient “D°” (3.6 × 10-9 cm2 s-1), large rate constant “ko” (1.2 × 10-5 cm s-1), large system entropy “ΔS” (-108 J K-1 mol-1), high change in enthalpy “ΔH” (72.3 J mol-1) and low activation energy “ΔG” (32.8 kJ mol-1). Resultingly, the oxidation of ammonia is found to be facile and robust by incorporation of CuO/Al2O3 catalysts owing to large ko, ΔH and ΔH. This study opened a gateway towards eco-benign and economical efficient energy generation and viable market entry of direct ammonia fuel cells.

Facile synthesis of the atomically dispersed hydrotalcite oxide supported copper catalysts for the selective hydrogenation of 5–hydroxymethylfurfural into 2,5-bis(hydroxymethyl)furan

The selective hydrogenation of 5-hydroxymethylfurfural (HMF) to 2,5-bis(hydroxymethyl)furan (BHMF) using the atomically dispersed supported copper catalyst is investigated. The hydrotalcite oxide supported copper materials (Cu(x)HTO) are facilely prepared by coprecipitating metal precursors in a methanolic solution under a tuned pH. The surface characterization involving PXRD, TEM, H2/N2O-TPR, and XAS reveals unequivocal evidence for the presence of the atomically dispersed copper on HTO surface. XAS specifically indicates the formation of mononuclear copper species, and H2/N2O-TPR strongly supports the copper atoms of Cu(5)HTO are evenly distributed in 99% dispersion. Moreover, the reduced Cu(5)HTO (r-Cu(5)HTO) enables to completely hydrogenate HMF to BHMF under mild conditions, in comparison to the poor reactivity catalyzed by the hydrotalcite oxide supported copper nanoparticles (r-Cu(4)@HTO). The dramatic enhancement of HMF hydrogenation catalyzed by r-Cu(5)HTO can be attributed to the fine distribution of copper atoms which are situated homogeneously over HTO surface as well as chemically reactive for the carbonyl group.

Catalytic activity of SBA-15 supported CuO for selective oxidation of veratryl alcohol to veratraldehyde

Selective oxidation of veratryl alcohol (VAlc) to veratraldehyde (VAld) under mild conditions using heterogeneous catalysts is advantageous for industrial applications. In the present study, newly developed SBA-15 supported copper oxide catalyst designated as CuO(5,10,15 wt.%)/SBA-15 was examined for the selective oxidation of VAlc to VAld using tert-butyl hydroperoxide (TBHP) as an oxidant between 40 and 100 °C. CuO(10 wt.%)/SBA-15 showed high conversion (82.5%) compared to others by forming VAld with 100% selectivity. We examined its selective oxidising power using a substrate possessing both the primary and secondary alcoholic functions, namely propylene glycol (PG). In this substrate, only the secondary alcoholic group was oxidized to hydroxyacetone (HA). It suggests preferential adsorption of the primary alcoholic group of PG on the catalyst surface close to the adsorbed TBHP, thus leaving only the secondary alcoholic group positionally favourable for oxidation. In addition, the catalyst effectively oxidized 2-butanol to 2-butanone. So, the catalyst is verified active against both primary and secondary alcohols, but when both of them are present on the adjacent carbons of the same compound, only the secondary alcoholic function is oxidized. So, the present catalyst could have tremendous applications for selective oxidation in organic synthesis.

Ceria-nano supported copper oxide catalysts for CO preferential oxidation: Importance of oxygen species and metal-support interaction

CeO2 with four different morphologies (sphere, rod, octahedron, cube) are used as the supports to synthesize the CuCe catalysts for the CO preferential oxidation in the H2-rich gas and characterized by various techniques (TEM, HR-TEM, SEM, XRD, N2 physisorption, XPS, Raman spectra, CO-TPD, CO-DRIFTS, O2-TPD, CO pulses, H2-TPR) to obtain an insight into the impact of the CeO2 morphology on the surface structure, composition and redox properties, especially the active species. The four supports expose {111}, {110} and {100} planes, and sphere-shape CeO2 has a higher ratio of {111}, which can form more reductive copper species and active oxygen species. And the catalytic performance followed by this order: spheres (28.5 kJ/mol) > rods (31.0 kJ/mol) > octahedrons (34.1 kJ/mol) > cubes (40.3 kJ/mol). By further analysis of the characterization results, a series of structure-activity relationships are established to confirm that the active oxygen species is the decisive factor of preferential oxidation of CO in the H2-rich stream.

Role of Support Lewis Acid Strength in Copper-Oxide-Catalyzed Oxidative Dehydrogenation of Cyclohexane

Alkane oxidative dehydrogenation (ODH) over supported redox active metal oxides is highly sensitive to support identity, but the underlying cause of support effects has not been well-established. Here, we provide evidence that charge transfer between the support and active oxide phase impacts the rates of C–H bond abstraction and COX formation pathways in the oxidative dehydrogenation of cyclohexane over supported copper oxide catalysts. The surface Lewis acid strength of nine metal oxide supports is quantified by alizarin dye intramolecular charge transfer shifts and compared with supported copper oxide d–d transition energies to determine the relationship between support Lewis acid strength and copper oxide electronic properties. Model cyclohexane ODH reaction studies show that selectivity to C6 products increases with increasing support Lewis acid strength, with selectivities to benzene and cyclohexene over combustion products at zero conversion increasing from 20% over nucleophilic Cu/MgO to over 90% ...

CuO/CeO2 catalysts for glycerol selective conversion to lactic acid.

Ceria supported copper oxide catalysts were produced by a deposition-precipitation method, at a high copper loading (up to >25 wt%). These materials demonstrated excellent properties for glycerol selective conversion to lactic acid, with a conversion reaching up to 87% with a selectivity to lactic acid of 74% (8 h reaction, 220 °C, under N2 pressure). These catalysts also exhibited high stability upon 5 successive reaction cycles. The formation of a crystalline CuO phase was demonstrated in the nanocomposites at a high Cu loading, with elongated shaped particles formed on the cerium oxide surface. Such particles were however, not observed at low Cu loadings. XPS analysis revealed that Cu(ii) was the main Cu species on the fresh catalyst, and that this species was reduced to Cu(i) during the reaction. Complementary characterization over the spent catalyst clearly showed the morphological modifications of the CuO phase, however, did not impact significantly either glycerol conversion or selectivity to lactic acid upon recycling. For instance, apparently, the catalytic activity of CuO largely depends on the Cu(ii) species.

Extremely enhanced generation of reactive oxygen species for oxidation of pollutants from peroxymonosulfate induced by a supported copper oxide catalyst

In this work, carbon fibers (CFs), were innovatively integrated with copper oxide (CuO) to construct a distinctive catalyst with strong catalytic activity, CuO-CFs, which can effectively activate peroxymonosulfate (PMS) to generate reactive oxygen species (ROS) for the decomposition of contaminants. The introduction of CFs significantly improved the catalytic performance of CuO, leading to a higher removal efficiency with AR1 as model compounds, which was attributed to the increased generation of ROS (OH, SO4− and 1O2). Futhermore, the TOC removal rate indicated that CuO-CFs/PMS system could mineralize contaminant. Also, other contaminants such as benzoic acid (BA), phenol, sulfamethoxazole (SMX) and 8-hydroxyquinoline (8-HQ) could be also removed to some extent in the system. An investigation into the reaction mechanism suggested that the persistent free radicals (PFRs) in CFs serve as an electron sink to offer electrons to CuO and O2, accelerating the key reaction step of Cu(II) to Cu(I) and O2 to O2−, thus enhancing the generation of ROS, which has been reported herein, for the first time, and offers a new and deeper insight into the positive effect of PFRs in CFs. Moreover, the addition of bicarbonate ion (HCO3−) further enhanced the oxidation performance of the CuO-CFs/PMS system, this observation contradicted previous reports stating the negative role of HCO3−. This discovery has major significance for recognizing the role of PFRs contained-carbonaceous materials, which not only provide a novel and simple strategy for PMS activation, but also initiated the latest study field of PFRs-enhanced catalysis.

Novel non-noble bimetallic Cu-Zn/TiO2 catalysts for selective hydrogenation of butadiene

Oxide-supported copper catalysts are active for the selective hydrogenation of polyunsaturated hydrocarbons with high selectivity to alkenes formation, but a low catalytic stability limits its application. The goal of the work was to investigate whether alloying Cu with Zn will improve catalyst stability, and whether such a non-noble bimetallic catalyst could compete with noble metal-based catalysts for this type of reaction. TiO2-supported mono-metallic Cu and Zn and bimetallic Cu-Zn (Cu/Zn atomic ratio of 1 and 3) were prepared by deposition-precipitation with urea (DPU) and deposition-precipitation at pH≈8 (DP8). Small metal nanoparticles (<2nm) with uniform particle sizes were obtained from DP8 samples, while two different particle size ranges (∼3nm and >15nm) were found in the DPU samples after calcination at 400°C and then reduction at 350°C. The bimetallic character of the nanoparticles was attested by XRD and STEM-HAADF coupled with EDS analysis, as Cu3Zn alloy was found in DPU CuZn 1:1/TO2 sample and an “intermediate” Cu-Zn alloy (Cu0.9Zn0.1) was found in DPU CuZn 3:1/TiO2 sample. UV-visible spectroscopy showed that alloying of copper with zinc inhibits the reoxidation of copper by contact with air. The catalytic results showed that alloying Cu with Zn slightly decreased the activity in butadiene selective hydrogenation, but had almost no effect on the selectivity to alkenes. The important result is that the bimetallic Cu-Zn/TiO2 catalysts displayed much higher stability under isothermal reaction than Cu/TiO2, which deactivated rapidly in the first few hours. This higher stability was ascribed to the formation of a lower amount of carbonaceous species on the bimetallic catalysts, as revealed by TGA analyses. Moreover, the DP8 Cu-Zn/TiO2 samples showed lower T100% but higher stability than the corresponding DPU samples. Surprisingly, change in the composition of the Cu-Zn alloy was observed during the catalytic reaction at 105°C, with a transition from Cu3Zn to Cu0.7Zn0.3 alloy for the DPU CuZn 1:1/TiO2 sample.

Cyclohexane oxidative dehydrogenation over copper oxide catalysts

Here, we report on structure-reactivity trends for cyclohexane oxidative dehydrogenation (ODH) with silica supported copper oxide catalysts as a function of surface structure. Copper oxide was supported on mesostructured KIT-6 silica at low surface densities <0.2Cu/nm2 using copper (II) nitrate, ammonium and sodium copper (II) ethylenediaminetetraacetate, and a hexanuclear copper (I) siloxide complex. Copper oxide surface structures were characterized by X-ray absorption spectroscopy as well as ambient and in situ diffuse reflectance UV–visible (DRUV–vis) spectroscopy to determine trends in copper oxide nuclearity. DRUV–vis spectroscopy identifies three copper species based on Cu2+ ligand to metal transfer (LMCT) bands at 238, 266, and >300nm as well as Cu+ LMCT bands at 235, 296, and 312nm. Counterintuitively, EXAFS analysis shows that the multinuclear precursor leads to fewer average Cu–Cu interactions than syntheses with mononuclear copper salt precursors. Turnover frequency and selectivity to benzene increase with decreasing copper oxide nuclearity, and thus the multinuclear precursor leads to the highest turnover frequency and benzene production. This work shows the variety of surface species that exist even at extremely low copper surface densities, control of which can improve reactivity of an atypical ODH catalyst up to rates comparable to benchmark vanadia catalysts.

Catalytic ozonation of simulated textile dyeing wastewater using mesoporous carbon aerogel supported copper oxide catalyst

Textile dyeing uses large amount of freshwater and synthetic dyes. Its effluents have to be subjected to certain treatments to remove the contaminants, especially for residue dyestuffs, before discharging to water bodies for elimination of adverse impacts to the environment. The objective of this research is to fabricate a novel catalyst, carbon aerogel supported copper oxide, and study its catalytic performance for the first time in ozonation of simulated dyeing wastewater for dye degradation. The catalyst was prepared using sol–gel and impregnation method, and characterized by Transmission Electron Microscope and X-Ray Diffraction techniques which suggesting that the nano-sized copper oxide particles successfully embedded and well-dispersed in the amorphous carbon aerogel. The catalytic performance of the catalyst was evaluated by degradation of C.I. Reactive Black 5 in a semi-continuous reactor. The results show that the catalyst has promising potential in ozonation towards dye elimination in dyeing wastewater, in which the dye removal efficiency can be effectively enhanced by the catalyst. In the specific conditions, COD removal could reach 46% in catalytic ozonation system after 60-min reaction, while it was only 29% in ozonation alone without catalyst. In addition, a systematic parameter study was performed to investigate the effect of the temperature, pH, ozone dosage, catalyst amount and dye concentration on dye degradation in terms of color and COD removal.

Preparation, characterization and kinetic behavior of supported copper oxide catalysts on almond shell-based activated carbon for oxidation of toluene in air

Dispersed copper oxide nano-catalysts supported on almond shell-based activated carbon were prepared for catalytic oxidation of toluene in air. The response surface methodology was used to express the catalyst removal efficiency in terms of catalyst metal loading and calcination temperature. Catalyst activity increased with both increasing calcination temperature and metal loading. Calcination temperature had a significant effect on the catalyst activity only at high metal loadings. Two different catalyst preparation methods were employed to investigate the effect of impregnation technique on the deposition–precipitation method. Well-dispersed nano catalysts with higher efficiency towards oxidation of toluene were prepared by the heterogeneous deposition–precipitation (HDP) as compared with the combined impregnation and deposition–precipitation method. The support and catalyst properties were determined by X-ray diffraction, Transmission electron microscopy, field-emission scanning electron microscope, Boehm test, Brunauer–Emmett–Teller surface area measurements, and energy-dispersive X-ray spectroscopy. Characterization analyses and reaction experiments indicated the antonym effect of impregnation method on the deposition–precipitation method for catalyst preparation. Two types of crystallite (large and small) were formed on the support as a consequence of using the combined catalyst preparation method. Kinetic models were proposed for oxidation of toluene using copper oxide catalysts prepared by the HDP method. The kinetic study indicated that an Eley–Rideal mechanism could adequately describe the kinetic behavior of toluene oxidation.

The Properties and Mechanism of CuO Modified Carbon Nanotube for NOx Removal

Carbon nanotube (CNT) supported copper oxide catalysts were prepared, and the morphology and structure of the catalyst were characterized by using scanning electron microscopy, transmission electron microscopy, X-ray diffraction and temperature-programmed desorption experiments. The CuO/CNT catalysts demonstrated high catalytic activities during the selective catalytic reduction (SCR) of NO with NH3 over a temperature range of 150–250 °C. The amount of NH3 adsorbed on the catalyst surface was greater than that of NO or NO + O2 adsorbed on the catalyst surface. These results suggest that the SCR reaction might proceed on the surface of the CuO/CNT catalysts and occur between the adsorbed ammonia and gas phase NO or weakly adsorbed NO. The CuO/CNT catalysts exhibited good stability at low temperatures, which makes them suitable for potential applications in industry.

Simultaneous removal of NO and SO2 from combustion fuel gases using supported copper oxide catalysts

Nitrogen oxide (NOx) and sulphur dioxide (SO2) emissions produced by the combustion of fuel in stationary sources, such as power stations, industrial heaters or cogeneration plants, cause significant environmental problems. Selective catalytic reduction (SCR) is a well established process for the control of NOx emissions mainly due to its efficiency, selectivity and economics. High removal efficiencies for both NOx and SO2 can be achieved with the use of copper oxide catalysts, as they act as sorbents for the latter - forming copper sulfate - and catalyze the reduction of the former - to N2, in the presence of NH3. An added advantage is that these catalysts are relatively easy to be regenerated under reducing conditions. This study examines the deactivation and regeneration procedures of copper oxide catalysts/sorbents that are supported on Al2O3, SiO2, CeO2-Al2O3, in the presence of SO2 and identifies the appropriate conditions for the simultaneous removal of NO and SO2. The results obtained indicate that copper oxide supported on alumina, ceria-alumina and silica carriers, can be used as effective catalysts for the simultaneous removal of NO and SO2. Furthermore, they can be easily regenerated with a gas mixture of 1 % NH3/Ar at an optimum temperature of 673 K and they retain their initial activity. Thus, the use of regenerative fixed bed catalytic reactor appears as one of the most promising technologies however, further research focusing in new catalytic materials performance is necessary.

Physico-chemical and catalytic properties of Mg–Al hydrotalcite and Mg–Al mixed oxide supported copper catalysts

The Mg–Al hydrotalcite (HT) and Mg–Al mixed oxide supported copper catalysts containing 3–3.5wt.% copper in finely dispersed form were synthesized and characterized. The effect of support nature on physico-chemical and catalytic properties of supported copper species were studied. The loading of copper on the supports was observed to be influencing the surface acidic, basic and reducibility properties, and catalytic behavior in dehydrogenation of benzyl alcohol. The high basicity and intercalated copper ions in Mg–Al hydrotalcite supported copper sample showed multifunctional activity in catalytic transformations of alcohols (primary, secondary and aromatic alcohols).

Enhanced catalytic performance of rare earth-doped Cu/H-Sep for the selective catalytic reduction of NO with C3H6

The undoped and rare earth oxide (REOx, RE=La, Ce, Pr, Sm)-doped acid-treated sepiolite (H-Sep)-supported copper oxide catalysts (Cu/H-Sep and RE-Cu/H-Sep; Cu and RE loading=15 and 4wt%, respectively) were prepared using the incipient wetness impregnation method. The catalysts were characterized by means of the XRD, BET, XPS, H2-TPR and NH3-TPD techniques, and their catalytic activities were evaluated for the selective catalytic reduction (SCR) of NO with propylene. It is shown that the doping of REOx led to an enhancement in catalytic performance of Cu/H-Sep in the SCR of NO with C3H6. Among the RE-Cu/H-Sep catalysts, Ce-Cu/H-Sep performed the best, with 62% NO conversion being achieved at 350°C in the (NO+C3H6+O2) reaction. We believe that the high catalytic performance of Ce-Cu/H-Sep for the SCR of NO with C3H6 was associated with the good low-temperature reducibility, high copper oxide dispersion, and strong metal–support interaction.

Effect of ZrO2 addition method on the activity of Al2O3-supported CuO for NO reduction with CO: Impregnation vs. coprecipitation

Supported copper oxide catalysts (CuO/Al2O3, CuO/ZrO2/Al2O3, CuO/ZrO2–Al2O3) were prepared from different procedures and tested in the catalytic reduction of NO with CO to investigate the effect of ZrO2 addition method. The catalysts were characterized by means of ICP, BET, XRD, Raman, H2-TPR, XPS, FTIR of NO or/and CO adsorption. Activity results demonstrated that the addition of ZrO2 could improve catalytic performance, which is dependent on the addition method. The sequence of activity is CuO/ZrO2–Al2O3>CuO/ZrO2/Al2O3>CuO/Al2O3. Based on the results of composition and texture, CuO/ZrO2–Al2O3 sample shows more surface zirconia species and larger surface area than CuO/ZrO2/Al2O3. XRD and Raman results suggest amorphous ZrO2 and highly dispersed copper oxide species are mainly present in the ZrO2-added catalysts except for a few CuO crystallites in CuO/ZrO2/Al2O3. Highly dispersed copper oxide species strongly interacting with amorphous zirconia (Cu⋯O⋯Zr species) was observed in H2-TPR and XPS, which was considered as the origin of the enhanced activity. In addition, the adsorption type and configuration of NO was similar over these catalysts. However, CuO/ZrO2–Al2O3 sample shows the greater capacity to activate the adsorbed NO species than CuO/ZrO2/Al2O3.

Catalytic combustion of benzene over CuO/CeO2 catalysts prepared using the precipitation–deposition method

Catalytic combustion of benzene over CeO2-supported copper oxides has been investigated. The supported copper oxide catalysts were prepared by use of the precipitation–deposition method at different solution pH, and characterized by XRD, FT-IR, and TPR. CuO/CeO2 catalysts prepared by use of this method contained highly dispersed copper oxide species at high loading ratio. The CuO supported on CeO2 prepared at pH 7 had the highest activity, with conversion reaching almost 100% at 210 °C. However, CuO supported on CeO2 prepared without NaOH had poor activity. In addition, catalytic activity increased with increasing CuO loading ratio, with the 10 wt% loaded CuO/CeO2 catalyst having the highest activity.

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.