CuO CeO2 Catalysts Research Articles

Structure and catalytic behaviour of CuO-CeO2 prepared by high-energy ball milling.

The aim of this study is to prepare CuO–CeO2 composite by means of mechanical milling and to investigate its characteristics as a catalyst. The structural and morphological features of milled samples are observed by X-ray diffractometry and scanning electron microscopy. The redox property and total OSC (oxygen storage capacity) of the milled sample were measured by using GC-TCD and TG-DTA, which are important parameters to indicate the effectiveness of catalysts. Interestingly, reduction of CuO is repeatedly observed when milling of CuO–CeO2 powder mixture is processed in air. The redox property of milled CuO–CeO2 sample is investigated by H2-TPR, where three reduction peaks are observed for 0 h milling and only one broad peak for various other milling times. The total OSC of mechanically driven CuO–CeO2 catalyst is much higher than that of the CeO2–ZrO2 traditional catalyst system at low temperatures.

CeO2-CuO bimetal oxides derived from Ce-based MOF and their difference in catalytic activities for CO oxidation

Abstract Bimetal oxides derived from metal-organic-frameworks, by means of either one-step or post-treated method, have been extensively studied so far. However, for the same kind of bimetal oxides prepared from different methods stated above, their comparison in catalytic reactions has rarely reported. In this work, two kinds of CeO2-CuO catalysts derived from Ce-MOF are prepared, and their catalytic activities toward CO oxidation are compared as well. Based on the X-ray diffraction, X-ray photoelectron spectroscopy, Brunaue-Emmett-Teller, temperature programmed reduction, UV-Raman, and in-situ diffuse reflectance infrared Fourier transform spectroscopy characterizations, we found MOF-derived CeO2-CuO catalysts obtained from one-step and post-treated methods exhibit different synergistic interactions between components, thus resulting in different catalytic activities toward CO oxidation.

CuO and CeO2-doped catalytic material synthesized from red mud and rice husk ash for p-xylene deep oxidation

CuO–CeO2 catalysts supported on material synthesized from red mud and rice husk ash (CuO-CeO2/ZRM) were prepared by co-impregnation method. The role of CeO2 additive in the improvement of physicochemical properties and catalytic activity of CuO-CeO2/ZRM catalysts were emphasized. Several techniques, including Brunauer–Emmett–Teller Nitrogen physisorption measurements, X-ray powder diffraction, hydrogen temperature programed reduction, scanning electron microscopy and transmission electron microscopy (TEM) were used to investigate the properties of catalysts. Crystallite size calculated by Scherrer’ equation was 17.4 – 21.8 nm. Modification of 5 wt% CuO/ZRM catalyst with CeO2 had reduced the size of the nanoparticles leading to a significant enhancement of the catalytic activity in p-xylene deep oxidation at temperature range of 275 – 400 °C. The 5 wt% CuO/ZRM sample promoted by 3 wt% of nanoparticle CeO2 with the average size of 17.5 nm and BET surface area of 31.3 m2 g−1 exhibited the best activity for p-xylene deep oxidation. In this sample, the conversion of p-xylene reaches to 90% at 350 °C.

Preparation and Characterization of UiO-66-Supported Cu–Ce Bimetal Catalysts for Low-Temperature CO Oxidation

A series of Zr-based metal–organic frameworks (UiO-66) supported CuO–CeO2 catalysts with various total metal loadings and various Cu/Ce ratios were prepared by using a co-impregnation method. The catalytic activities of the catalysts for low temperature oxidation of carbon monoxide were compared with that supported on the ZrO2 and mesostructured cellular foam silica. The results show that the catalysts supported by UiO-66 are expected to give the best catalytic activity, which should be attributed to the special construction of UiO-66. The samples were further characterized by XRD, TEM, N2 adsorption–desorption, H2-TPR, XPS, and in situ DRIFT. And the catalyst with a bimetallic loading of 50 wt% and Cu/Ce ratio of 3:7 (wt/wt) has the highest catalytic activity, which can be attributed to the larger amount of finely dispersed Cu species strongly interacting with CeO2, the highest amounts of oxygen vacancies and Cu+ species on the catalyst surface.

Study of the influence of the Cu/Ce loading ratio in the formation of highly active species on ZrO2 supported copper-ceria catalysts

This work studies the effect of different Cu/Ce loading ratios in the formation of highly dispersed copper species being interacting with CeO2 on zirconia-supported CuOCeO2 catalysts. These species have a strong effect on the CO oxidation reaction and for this reason such reaction was selected to conduct the present study. Catalysts with different metal loads were prepared by the co-impregnation method of Cu and Ce nitrates on ZrO2 support. It was found that the influence of the two metals occurs cooperatively. The type of copper species formed on the ZrO2 support depends on the Cu and Ce loads, with the existence of an optimum in activity when the Cu/Ce loading ratio being around 0.5. The incorporation of CeO2 increases the dispersion capacity of ZrO2 and facilitates the reduction of surface CuO species. Maintaining the Cu/Ce loading ratio constant at the optimal ratio (0.5) and increasing the total metal load (Cu+Ce) gave no significant increase in the catalytic activity above 6%. The latter suggests that higher metal loadings (Cu+Ce) does not lead to an increment of highly dispersed copper species interacting with CeO2, which is the most active species on the surface of this catalysts.

Impact of acid treatment of CuO-CeO2 catalysts on the preferential oxidation of CO reaction

The effect of a post-synthesis treatment of 7.5 wt% CuO-CeO2 catalyst with acid solutions on its performance towards the preferential oxidation of CO reaction was investigated. The acidic environment was found to affect the redox and catalytic properties of this solid composition. In particular, modification at low pH (< 4) provided the required amount of acid which led to better dispersion of the active copper species, higher concentration of surface Ce3+ species, an increased concentration of oxygen vacancies and higher reducibility of the solid catalyst. These characteristics were found to be critical for the promotion of the catalytic activity and selectivity in the title process.

Metal-organic-framework derived controllable synthesis of mesoporous copper-cerium oxide composite catalysts for the preferential oxidation of carbon monoxide

Among currently studied catalysts, CuO-CeO2 based materials hold the greatest promise for the preferential oxidation of CO (CO-PROX). Recently, many efforts have been concentrated on developing the original nanostructures inherited from metal-organic-frameworks (MOFs), which are considered to be excellent sacrificial templates or precursors to achieve metal oxide (or metal) nanoparticles with unique structure. In this paper, we synthesized CuO-CeO2 catalysts using an efficient and general strategy derived from CuxCe1−x-BTC MOFs after high temperature treatment. The as-prepared CuO-CeO2 catalysts display variable morphologies, crystal structures, and specific surface areas based on different ratios of Cu/Ce and calcination temperature. The catalytic performance shows that all CuO-CeO2 composite catalysts derived from the CuxCe1−x-BTC MOFs via heat treatment exhibit excellent catalytic performance for the CO-PROX reaction, and the Cu0.3Ce0.7O2 is the most active catalyst obtained under high calcination temperature at 650 °C for 4 h, demonstrating that the increase of Cu content and high temperature treatment can create more highly dispersed CuO clusters, which is in favor of the CO-PROX reaction. Meanwhile, the in-situ DRIFTS results show that the Cu0.3Ce0.7O2 catalyst displays the super CO adsorption capability, which induces the difference of catalytic performance for the CO-PROX reaction.

Preparation and performances of nanorod-like inverse CeO2–CuO catalysts derived from Ce-1,3,5-Benzene tricarboxylic acid for CO preferential oxidation

A facile one-pot strategy of using CeBTC MOF as template and precursor was explored to prepare rod-like inverse CeO2–CuO catalysts. The inverse CeO2–CuO catalysts were characterized by using techniques of N2 adsorption and desorption, XRD, TEM, TPR and XPS. The results indicated that the rod-like inverse CeO2–CuO catalysts maintained the shapes of CeBTC MOF templates. The Cu0.71Ce0.29 catalysts exhibited excellent catalytic performance which was attributed to the strong synergistic effect between CuO and CeO2 and subsequent formation of reduced copper species.

RETRACTED ARTICLE: Wiener model and extremum seeking control for a CO preferential oxidation reactor with CuO-CeO2 catalyst

RETRACTED ARTICLE: Wiener model and extremum seeking control for a CO preferential oxidation reactor with CuO-CeO2 catalyst

A solvent-free method to rapidly synthesize CuO-CeO2 catalysts to enhance their CO preferential oxidation: Effects of Cu loading and calcination temperature

A facile and solvent-free route has been developed to rapidly synthesize CuO-CeO2 catalysts within 30min. The effect of calcination temperature on catalytic performances is more significant than Cu loading, and the optimal calcination temperature and Cu loading are 700°C and 7.5wt.%, respectively. The optimal catalyst 7.5CuCe-SF-700 exhibits the highest CO conversion and the corresponding temperature window for full CO conversion is as wide as 50°C. According to the characterization results of XRD, N2 adsorption-desorption, H2-TPR, TEM, HR-TEM, STEM, EDS, Raman and XPS, it can be concluded that the high-performances of CuO-CeO2 catalyst should be attributed to synergistic effects of high dispersion Cu species and CeO2, and the presence of more reduced copper species as the active site in the catalyst. 7.5CuCe-SF-700 exhibits excellent stability for CO-PROX during the 200 time-on-stream test under actual reaction conditions (with 15% CO2 and 10% H2O). In addition, surfactant template, urea gelation/co-precipitation and sol-gel methods were used to synthesize CuO-CeO2 catalysts so as to compare them with 7.5CuCe-SF-700, and the results exhibit not only 7.5CuCe-SF-700 is superior to the former three catalysts but also our synthesis route is more simple, controllable, solvent-free, time-saving and energy-saving.

Promoted the reduction of Cu2+ to enhance CuO[sbnd]CeO2 catalysts for CO preferential oxidation in H2-rich streams: Effects of preparation methods and copper precursors

A series of CuOCeO2 catalyst samples synthesized by using various methods (CuCe-SF-N, CuCe-UGC-N, CuCe-SG-N and CuCe-ST-N) and copper precursors (CuCe-SF-N, CuCe-SF-C, CuCe-SF-A and CuCe-SF-S) were estimated for CO preferential oxidation in H2-rich streams. It was found that both synthesis routes and copper precursors have an important effect on catalytic behaviors of CuOCeO2 catalyst. Compared to CuCe-UGC-N, CuCe-SG-N and CuCe-ST-N, CuCe-SF-N exhibits the lowest temperature and the widest temperature window for 100% CO conversion (about 50 °C), which should be attributed to synergistic effects of smaller crystallite size, the formation of more Cu+ species together with the high ratio of Ce3+/(Ce3++Ce4+). Among the four catalysts prepared with different Cu precursors (CuCe-SF-N, CuCe-SF-C, CuCe-SF-A and CuCe-SF-S), the corresponding CO conversions of them are in the order of CuCe-SF-N > CuCe-SF-A > CuCe-SF-C » CuCe-SF-S. The lowest catalytic activity of CuCe-SF-S should be due to the presence of SO42− species covered on the surface of the catalyst, which not only results in the formation of the less Cu active species but inhibits the interaction between Cu species and CeO2. In addition, the optimal CuCe-SF-N catalyst displays relative stability during the 200 h time-on-stream test even in the presence of H2O and CO2.

Catalytic combustion of volatile aromatic compounds over CuO-CeO2 catalyst

Ce1−x Cu x O2 oxide solid solution catalysts with different Ce/Cu mole ratios were synthesized by the one-pot complex method. The prepared Ce1−x Cu x O2 catalysts were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and H2 temperature-programmed reduction (H2-TPR). Their catalytic properties were also investigated by catalytic combustion of phenyl volatile organic compounds (PVOCs: benzene, toluene, xylene, and ethylbenzene) in air. XRD analysis confirmed that the CuO species can fully dissolve into the CeO2 lattice to form CeCu oxide solid solutions. XPS and H2-TPR results indicated that the prepared Ce1−x Cu x O2 catalysts contain abundant reactive oxygen species and superior reducibility. Furthermore, the physicochemical properties of the prepared Ce1−x Cu x O2 catalysts are affected by the Ce/Cu mole ratio. The CeCu3 catalyst with Ce/Cu mole ratio of 3.0 contains abundant reactive oxygen species and exhibits superior catalytic combustion activity of PVOCs. Moreover, the ignitability of PVOCs is also affected by the respective physicochemical properties. The catalytic combustion conversions of ethylbenzene, xylene, toluene, and benzene are 99%, 98.9%, 94.3%, and 62.8% at 205, 220, 225, and 225 °C, respectively.

Efficient and selective transformation of biomass-derived furfural with aliphatic alcohols catalyzed by a binary Cu-Ce oxide

The efficient transformation of furfural (FUR) with aliphatic alcohols to achieve the carbon-chain growth has been developed using a binary Cu-Ce oxide as the catalyst. In the presence of molecular oxygen, the tandem oxidative condensation of FUR with n-propanol is successfully performed, in which an 85.4% conversion of FUR in 95.3% selectivity of 3-(furan-2-yl-)-2-methylacryaldehyde was obtained. The effects of different Cu/Ce ratios and base additives were investigated in detail. As a result, it is found that the CuO-CeO2 (1: 9) catalyst is optimal and potassium carbonate is a suitable additive. Next, the recycling of CuO-CeO2 catalyst was tested and there is no obvious activity loss after being reused five times. Moreover, the oxidative condensation of FUR with various aliphatic alcohols including ethanol, isopropanol, n-butanol and n-hexanol was studied where the long chain alcoholic molecule hinders the proceeding of reaction. Finally, based on the experimental results and reaction phenomena, a possible mechanism for the oxidative condensation of FUR with n-propanol-O2 is proposed.

Catalytic Behaviour of CuO-CeO2 Systems Prepared by Different Synthetic Methodologies in the CO-PROX Reaction under CO2-H2O Feed Stream

CuO-CeO2 catalysts, with 6 wt % of Cu, have been synthesised by different preparation methods (calcination of nitrate precursors, thermal urea-nitrate combustion, freeze-drying method, using polymethyl metacrylate PMMA microspheres as template and precipitation using NaOH or the decomposition of urea as precipitating agents). The obtained materials have been characterised by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), Raman spectroscopy, N2 adsorption-desorption at −196 °C, H2 thermoprogrammed reduction (H2-TPR) and X-ray photoelectron spectroscopy (XPS). The catalysts displayed high dispersion of copper oxide, obtaining CO conversion values of 90–100% at 115 °C in the CO preferential oxidation in excess of hydrogen (CO-PROX) and maintaining this activity even after 48 h of time on stream. The incorporation of CO2 and H2O in the feed stream (simulating a PROX unit) caused a decrease in the CO conversion, except for the catalyst synthesised using PMMA microspheres as a template which maintained a CO conversion of 95% at 115 °C. This catalyst exhibits an excellent catalytic performance, also under real operating conditions, thanks to many and concomitant factors, such as the very small CeO2 particle size (5.6 nm), the surface being rich in copper (atomic ratio Cu/Ce = 0.35) that is easily reducible, and the peculiar morphology and porosity of the material.

Facile hydrothermal procedure to synthesize sheet-on-sheet reduced graphene oxide (RGO)/CuxO[sbnd]CeO2 nanocomposites for preferential oxidation of carbon monoxide

The sheet-on-sheet reduced graphene oxide (RGO)/CuxOCeO2 catalysts were synthesized by a facile hydrothermal procedure and systematically characterized by X-ray powder diffraction, scanning electron microscopy, transmission electron microscopy, hydrogen temperature-programmed reduction, X-ray photoelectron spectroscopy and N2 adsorption–desorption techniques. It is found that there is a large amount of reduced copper species in the RGO/CuxOCeO2 catalysts, and the addition of RGO was beneficial for the dispersion of particles and the improvement of textural properties. The spherical nanoparticles of copper species can be served as spacers to prevent the stacking of RGO. Copper and cerium species are homogenously distributed on the surface of RGO sheets. The introduction of RGO improves the formation of high Ce3+ concentration and oxygen vacancies. Therefore, the RGO/CuxOCeO2 catalysts show higher catalytic activity than the CuOCeO2 catalyst at low temperature (below 135 °C). Moreover, good resistance to H2O and CO2 of the RGO/CuxOCeO2 may be from for the formation of hydroxyl groups by dissociative adsorption of H2O on the surface of RGO, which can improve the WGS reaction and the decomposition of carbonates.

Total Oxidation of Propane Using CeO2 and CuO-CeO2 Catalysts Prepared Using Templates of Different Nature

Several CeO2 and CuO-CeO2 catalysts were prepared using different methods, i.e., a homogeneous precipitation with urea, a nanocasting route using CMK-3 carbon as a hard template and a sol–gel process using Poly(methyl methacrylate) (PMMA) polymer as a soft template, and tested in the total oxidation of propane. The catalysts were characterized by a number of physicochemical techniques (XRD, N2 adsorption, TPR, XPS, Raman spectroscopy) showing distinct characteristics. For each series, Cu-Ce-O catalysts with low Cu-loadings (5 wt % CuO) showed the highest activity, higher than those samples either without copper or with high Cu-loading (13 wt % CuO). The incorporation of copper leads to an increase of the concentration of bulk defects but if the Cu-loading is too high the surface area drastically falls. The highest activity in the total oxidation of propane was achieved by Cu-containing ceria catalysts synthesized using a polymer as a template, as this method yields high surface area materials. The surface area and the number of bulk/sub-surface defects of the ceria seem to be the main properties determining the catalytic activity.

Low-temperature carbon monoxide oxidation over zirconia-supported CuO–CeO2 catalysts: Effect of zirconia support properties

A study was conducted to investigate the effect of the preparation route of ZrO2 in CuO–CeO2/ZrO2 catalysts for the oxidation of carbon monoxide at low temperature (COX). Four ZrO2 supports were synthetized via either type sol-gel methodology or precipitation. The final Cu-Ce-Zr oxide catalysts were prepared by incipient wetness co-impregnation with copper and cerium solutions (with a loading of 6wt% of CuO and 20wt% of CeO2). The catalyst crystalline phases, texture and active species reducibility were determined by XRD, N2 physisorption at −196°C and H2-TPR, respectively; meanwhile the surface composition and copper-cerium electronic states were studied by XPS. The catalytic activity was evaluated in the oxidation of CO to CO2, in the 40–215°C temperature range. Catalytic results evidenced that the samples prepared by a sol-gel methodology showed, after the impregnation, a severe decrease of specific surface area and pore volume attributable to a wide degree of pore blockage caused by the presence of metal oxide particles and a collapse of the structure partially burying the active sites. A simple co-impregnation of a zirconia support, obtained through facile and fast precipitation, provided instead a catalyst with very good redox properties and high dispersion of the active phases, which completely oxidizes CO in the range 115–215°C with T50 of 65°C. This higher observed activity was ascribed to the formation of a larger fraction of highly dispersed and easily reducible Cu species and ceria nanocrystallites, mainly present as Ce(IV), with an average size of 5nm.

Preparation and characterization of mesostructured cellular foam silica supported Cu–Ce mixed oxide catalysts for CO oxidation

A series of mesostructured cellular foam (MCF) silica supported CuO–CeO2 catalysts with various total metal loadings (10–40 wt%) and various Cu/Ce ratios (Cu/Ce = 1/9, 2/8, and 3/7 wt/wt) were prepared and tested for CO oxidation.

CO oxidation on mesoporous SBA-15 supported CuO–CeO2catalyst prepared by a surfactant-assisted impregnation method

A series of mesoporous SBA-15 supported CuO–CeO2catalysts were prepared by a surfactant-assisted impregnation method with PEG 200 as the surfactant.

Recent Advances in the Preferential Thermal-/Photo-Oxidation of Carbon Monoxide: Noble Versus Inexpensive Metals and Their Reaction Mechanisms

Preferential oxidation (PROX) of CO is applicable because of its low cost, ease of implementation, and low loss of H2 during purification. Mo–SiO2 and Cr–SiO2 photocatalysts utilized charge separation at metal=O bonds, and the PROX selectivity of CO was high. The CO PROX rates using semiconductor-based photocatalysts were comparable to those of photocatalysts (380 μmol h−1 g cat −1 ) and were also selective (100 %). These photocatalysts are advantageous because they do not activate H2 in comparison to noble metal catalysts. Below 473 K using noble metals, Ru, Rh, Pt, or Au supported on reducible metal oxides exhibited excellent CO thermal-PROX rates of ~4900 μmol h−1 g cat −1 ; however, the CO PROX selectivity of ~48 % was insufficient because of H2 activation on noble metals in nature. CO adsorbed onto TiO2 and O2 was stabilized at the interface between a Ti site and an Au atom. Weakly adsorbed water increased the effective number of active sites by stabilizing Au–OOH or Au–COOH. The PROX rates of CO using Au/TiO2-based catalysts under dark conditions increased under UV–visible light by the effect of charge separation and surface plasmon resonance and the promoted electron transfer to the adsorbed O2. In the case of CuO–CeO2 catalysts, CO adsorbed onto CuO and reacted with lattice O atoms at the boundary between CuO and CeO2 to form CO2 at an O vacancy, which was subsequently filled with an O2 molecule. The combination of Cu or Co with a reducible metal oxides also provided performance comparable to or higher than that of CuO–CeO2 owing to adequate standard reduction potential for Cu2+, Co3+, Mn3+/4+, and Ce4+. Finally, binary metal–organic framework consisting of oxyhydroxide Ti clusters interlinked by organic ligands and Cu oxyhydroxide ligands showed superior CO PROX performance (76 % conversion and 99 % selectivity) to that achieved using CuO–CeO2 owing to effective dispersion of Cu–O–Ti connection in microporous crystallites. Further progress is needed to alleviate the activity loss in the presence of moisture and/or CO2 based on suggestions that steric hindrance of some types of microporous crystallites would suppress the blocking of moisture or CO2.