Preparation Methods and Applications of CuO-CeO 2 Catalysts : A Short Review

The CuO-CeO2 catalytic systems are getting popular for catalyzing very actively the various reactions of environmental, commercial and other importance. In recent years, many methods have been in use for the preparation of versatile CuO-CeO2 catalysts. Reviewing the useful preparation methods of such catalysts is thus the need of the time in view of the globally increasing interest towards all the low temperature redox reactions. This article presents a short review on seventeen different preparation methods of the copperceria catalysts, followed by critical discussions on the related redox properties and advancements accomplished with respect to their application aspect, including a systematic compilation of the concerned newer literature in a well-concievable tabular form. © 2010 BCREC UNDIP. All rights reserved. .

Reactions of other importance are concerned with the selective oxidation of ammonia to nitrogen [68] and lower hydrocarbon reforming into synthetic gas [69].Further, Cu/CeO2 composite catalytic electrodes are used for direct oxidation of various fuels in fuel cells [70][71][72][73].In addition, these binary oxides are also useful for H2O2 decomposition [74] as well as methanol synthesis [75], etc To date, there has been frequent most use of CuO-CeO2 systems as additives to reduce the cost of noble metals in the three-way catalysts [23], for the purification of automotive exhaust gas.An excellent activity of the CuO-CeO2 systems is on record in the oxidation of CO [1], which is significantly higher than that of the commercially precious forms of metal catalysts [76].These have been thus widely studied with the aim to possibly replacing the expensive noble metals [1,2,53,[77][78][79][80][81][82].
Abundant availability of Cu and Ce, coupled with their lower costs compared to precious metals, make them strongly competitive.This type of composite catalyst also shows remarkably higher resistance to carbon dioxide, water poisoning, and sulphur compounds [83].Development of solid oxide fuel cells (SOFC) for operation in the intermediate temperature regime of 600-800 0 C with hydrocarbon/methanol/ ethanol fuel(s) requires a cathode and an anode possessing high electro-catalytic activity to facilitate O2 reduction and direct oxidation of fuels, respectively.San Ping Jiang [71] reviewed and discussed the progress concerning application of the wet impregnation technique in the development of Cu/CeO2 composite electrodes for direct oxidation of hydrocarbon in SOFC.
It is a long recognized fact that the properties of the catalysts often depend on their preparation methods.The dispersion and size distribution of metal crystallites, their spatial distribution on the support, the homogeneity of components in a multi-component catalyst, the porosity, surface area, and pore size distribution are the examples of sensitive functions.This sensitivity is with respect to the precursors used and the treatment temperature and atmosphere, as well as other preparation variables, such as pH of the preparation solution or the use of aqueous or organic medium [94], which in turn strongly affect the catalyst activity.In recent years, there has been a significant progress towards understanding the relationship between the preparation method and the final properties of catalysts and catalytic supports.Several methods have been used for the preparation of the versatile CuO-CeO2 catalysts.However, preparation methods of such catalysts have hardly been reviwed so far.Owing to the recurrently expanding interest the world over on the application of copper-ceria catalysts, this brief article is an attempt to review the various useful preparation methods of these copper-ceria catalytic systems, and discuss the redox properties related catalytic activities of the CuO-CeO2 catalysts in light of the preparation methods.

Methods of Catalyst Preparation
The choice of a laboratory method for preparing a given catalyst depends on the physico-chemical characteristics desired in its final composition.Nevertheless, the preparation methods are dependent on the choice of basic materials, and earlier experiences support considering support towardss diverse ways of preparation, even for a given selection of the basic material.Following methods for the preparation of CuO-CeO2 catalysts are on record:

Wet Impregnation Method
The wet impregnation method is a common procedure to prepare the CuO-CeO2 catalysts.It involves three steps: (i) contacting the support (CeO2) with the impregnating solution of copper precursors (nitrate, sulphate, acetate, etc.) [80,93] for a certain period of time, (ii) drying the support to remove the imbibed liquid, and (iii) thermal decomposition, followed by activating the catalyst by reduction or other appropriate treatment.But the Cu(II) ions tend to segregate from CeO2 supports in the form of non-active Tenorite (CuO) particles, even for low Cu(II) loadings [2,94].A typical procedure followed by Zheng et al. [80] is described here.CeO2 is prepared by serial thermal decomposition of cerous nitrate [Ce(NO3)3.6H2O] in air for 4 h at 400, 500, 600, or 700 0 C. The prepared buff particles are in turn denoted as CeO2-A, CeO2-B, CeO2-C and CeO2-D.The prepared CeO2 supports are impregnated with an aqueous solution of Cu(NO3)2.3H2Ofor 5 h, where after the excess of water is vaporized by heating at 600C.The materials, thus obtained, are dried overnight in an oven at 800 C and serially calcined in air for 3.5 h at 300, 400, 500, 600, or 800 0 C. The prepared catalysts are denoted as CuO/CeO2-A, CuO/CeO2-B, CuO/CeO2-C and CuO/CeO2-D.The final loading of CuO for all the samples is 6.3-wt percentage.

Co-impregnation Method
Co-impregnation is a general technique for the synthesis of supported heterogeneous catalyst, containing active metal, promoter, stabiliser, etc.In this method, typically, copper and cerium precursors (nitrate, sulphate, acetate, etc.) are dissolved in water solution.Afterwards, this solution is added to a catalyst support up to the time required for total impregnation.The catalyst can then be dried and calcined to drive off the volatile components within the solution, depositing the metal, promoter, stabiliser, etc. on the surface of the support.Gonzalo Aguila [9] prepared bimetallic CuO-CeO2 catalysts by co-impregnation of the support Al2O3/ZrO2/SiO2 for the oxidation of CO at low temperature.They inferred that the support has a strong influence on the activity of the different bimetallic catalysts.Interestingly, the SiO2 supported catalyst showed a higher activity.The bimetallic supported catalysts followed the activity sequence: CuO-CeO2/SiO2 > CuO-CeO2/ ZrO2 > CuO-CeO2/Al2O3.The co-impregnation process of ceria and Cu appears to play an important role in the performance of the Cu-based composite anodes for direct oxidation of hydrocarbon in SOFC [70].

Precipitation deposition method
In precipitation, the objective is to achieve a reaction of the type: Metal Salt (nitrate/sulphate) Solution + NaOH/ KOH/ Na2CO3 + Support (powder) → Metal hydroxide or carbonate on support Two processes are involved in the deposition: (i) precipitation of solution in bulk and pore fluid, and (ii) interaction with the support surface.Rapid nucleation and growth in the bulk solution is ought to be avoided, as it produces a deposition exclusively outside the support porosity.Use of urea rather than conventional alkalis has proved to be an effective method to obtain a uniform precipitation.Urea dissolves in water but decomposes quite slow at 90°C, giving thereby a uniform concentration of OH¯ both in the bulk and pores.So precipitation takes place homogeneously over the support surface [95] and happens to be the preferred deposition route for loading higher than 10-20%.Below this value, impregnation is usually practised.
Kebin Zhou and coworkers [5] prepared the CeO2 supported CuO catalysts by the precipitation deposition method as follows: The ceria are suspended in water.To this suspension, an aqueous solution of Cu(NO3)2 (0.1 M) is added while stirring.During this process, the suspension is kept constant at a pH of about 9.0 by adding 0.25 M NaOH solution.After an additional 60 min of continuous stirring, the precipitate is filtered and washed.The filtrate is then dried overnight at 80 0 C in air and calcined at 400 0 C for 4 h.The loading of CuO is 1 wt percentage for both of the catalysts.They claimed that the high-energy, more reactive {001} and {110} planes of CeO2 nanorods were found to generate synergetic effects between CuO and ceria, resulting in significant enhancement of the copper catalyst performance for CO oxidation.

Co-precipitation method
The synthesis of the mixed Ce(III) and Cu(II) precursors is generally achieved by heterogeneous co-precipitation in basic media [37,[96][97][98], the inhomogeneities during the formation of solids being an inherent vice of this procedure [3].Petar Djinovic et al. [99] prepared the CuO-CeO2 precursor by co-precipitation [by adding water solution of Na2CO3 drop-wise to the required amount of aqueous solutions of Cu(NO3)2 and Ce (NO3)3 with concurrent vigorous stirring].In this process, the pH of mixed solution is maintained Copyright © 2010, BCREC, ISSN 1978-2993 below 6.0.The formed precipitate is thoroughly washed with hot distilled water in order to remove undesired sodium ions, and dried overnight in an oven at 110 0 C. Final CuO-CeO2 catalyst emerges after decomposing and calcining the precursor at 650 0 C.
It is reported that the catalytic performance of these non-noble metal-containing catalysts is comparable with that of other selective CO oxidation catalysts and for water gas shift reaction catalysts respectively, as reported in the literature.Liu and Flytzani-Stephanopoulos [1] prepared CuO-CeO2 via co-precipitation methods for the total oxidation of CO and CH4.The Cuceria catalysts exhibit substantially high activity and stability for CO oxidation at a space velocity of 45,000 v/v h -1 and complete CO conversion occurs at around 80 0 C. The authors explained the increase in activity of these catalysts owing to the stabilization of Cu +1 in catalysts, prepared via coprecipitation methods, originated from the interaction between copper clusters and cerium oxide, and addressed to ceria for performing the role of oxygen source.

Urea gelation method
The urea method provides a highly reproducible homogeneous precipitation process, which makes use of the thermal hydrolysis of urea into ammonium carbonate [100].Matias Jobbagy et al. [101] explored the urea method for a high-yield of CuO-CeO2 catalyst precursors.To start with, solutions containing urea, Ce(NO3)3, and Cu(NO3)2 are aged at 363 K for 5 h, achieving a quantitative co-precipitation in the form of amorphous Cu(II)-Ce(III) basic carbonates, with Cu(II) contents up to 40%.They observed no Tenorite (CuO) segregation after annealing at 873 K and evaluated the possibilities and limitations of the urea method in the synthesis of mixed Cu (II)-Ce(III) particles-as precursors for copperpromoted-CeO2 catalysts.The samples containing around 20% in copper atoms for preferential oxidation of carbon monoxide (CO-PROX) performed as the best.
Liu et al. [84] quantitatively described this method, in which the precursor salts are metal nitrates and the cerium salt is (NH4)2Ce(NO3)6.The preparation procedure consists of mixing the aqueous metal nitrate solutions with urea (NH2-CO-NH2); heating the solution to 100 0 C under vigorous stirring and addition of de-ionized water; boiling the resulting gel for 8 h at 100 0 C; filtering and washing the precipitate twice with de-ionized water at 50-70 0 C; drying the cake in a vacuum oven at 80-100 0 C for 10-12 h; crushing the dried lump into smaller particles and calcining the powder in a muffle furnace in air at 650 0 C for 4 h.A heating rate of 2 0 C/min is used in the calcination step.The BET surface areas of the thus prepared catalysts are in the range of 90-100 m 2 /g after calcinations at 650 0 C. The copper content in CuO-CeO2 or in the doped catalysts is 10 wt%.

Urea-nitrate combustion method
In context with the above-mentioned reactions, urea combustion with nitrates is an effective, onestep technique for the preparation of CuO-CeO2 catalysts with favorable characteristics and catalytic properties [17,83,86,102].The reactions describing the combustion of urea with copper and cerium nitrate salts can be written as follows: Avgouropoulos et al. [53] described the following urea-nitrate combustion method for the synthesis of CuO-CeO2 mixed oxide catalysts.Accordingly, cerium nitrate [Ce(NO3)3•6H2O], copper nitrate [Cu(NO3)2•3H2O], and urea [CO (NH2)2] are mixed in the appropriate molar ratios in a minimum volume of distilled water to obtain a transparent solution.The initial urea/nitrate molar ratio is adjusted according to the principle of propellant chemistry [103], taking into account that the urea/nitrate stoichiometric molar ratio is equal to 5(3 − x)/6, where x denotes the Cu/(Cu + Ce) molar ratio.The urea/nitrate ratio varies from stoichiometric (urea/nitrate = 2.38) to 5.5, while the Cu/(Cu + Ce) molar ratio is equal to 0.15.In order to determine the optimum copper loading, two additional catalytic samples with the optimum urea/nitrates molar ratio are prepared, with Cu/(Cu + Ce) molar ratio equal to 0.10 and 0.20.The mixed solutions are heated for a few minutes at 80 0 C and the resulting viscous gel is introduced in an open muffle furnace, preheated at 400-500 0 C, in a fuming cupboard.The gel starts boiling with frothing and foaming, and in a couple of minutes it is ignited spontaneously with rapid evolution of a large quantity of gases, yielding a foamy voluminous powder.The powder obtained after combustion contains small amounts of carbonaceous residues as well, because the autoignition lasts only for a few seconds.In order to burn-off carbon residues, the powder is heated further at 550 0 C for 1 h.

Solution combustion method
Bera et al. [18] described synthesis of fine particle and large surface area Cu-CeO2 catalysts of crystallite sizes in the range of 100-200 Ǻ by the solution combustion method, for NO reduction.In this method, ceric ammonium nitrate and copper nitrate are used as the sources of cerium and copper.Oxalyldihydrazide (ODH, C2H6N4O2) prepared from diethyl oxalate and hydrazine hydrate is used as the fuel.In a typical combustion synthesis, a Pyrex dish (300 cm 3 ), containing an aqueous redox mixture of stoichiometric amounts of ceric ammonium nitrate (5 g), copper nitrate (0.1419 g), and ODH (2.6444 g) in 100 cm 3 volume of H2O, is introduced into a muffle furnace preheated to 350 0 C. The solution boiled with foaming and frothing and ignited to burn with a flame yields about 1.5 g voluminous oxide product within 5 min.Similarly, Zr, Y, and Ca doped CeO2 and 10% Cu/CeO2 are prepared by this method from their respective metal nitrates and ODH fuel.These oxides are prepared in an open muffle furnace kept in a fuming cupboard.Exhaust is kept on during the firing.The reaction can be controlled by carrying out the combustion in an open atmosphere.By choosing proper sizes of the container and muffle furnace larger quantity of the catalysts (up to 500 g) can be prepared in a single batch.Since the oxides absorb the moisture, it is necessary to store them in a vacuum desiccator and heat them at 300 0 C for 12 h before using.

Citric acid sol-gel method
Sol-gel method has several promising advantages over precipitation.In general, sol-gel synthesis offers better control over surface area, pore volume and pore size distribution.Hydrophilic colloidal solutions are formed of micelles that remain separated because of electrical charges on their surfaces and in the surrounding solution.These charges create repelling forces which prohibit coagulation of the micelles.Such micelles are produced via chemical reactions of polymerization and poly-condensation.Following preparation details followed by two groups of authors have been illustrated.

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1/10.This was accomplished following the citric acid sol-gel method.This involves mixing of nitrates [Ce(NO3)3, ZrO(NO3)2 and/or Cu(NO3)2] in deionized water according to the desired molar ratio.Citric acid is added as the complexing agent with a 1.3:1 ratio of the acid to metal ions including Ce 3+ , Zr 4+ and Cu 2+ .Appropriate amount of polyglycol is followed in accordance with the weight of 10% citric acid added.
The blended solution is sufficiently mixed in a magnetic stirrer and heated at 80 0 C until transparent gel is formed.The resulting gel is dried at 110 0 C overnight.The powder received is subjected to decomposition at 300 0 C for 1 h and calcined at 500 0 C for 3 h under static air in a muffle.According to the authors, CuO-CeO2 mixed oxides, which behave as active and remarkably selective to CO2 while operating at significantly low reaction temperature, seem to be a promising candidate catalyst for the selective soot oxidation.CuO-CeO2-ZrO2 mixed oxides are less active but more thermo-stable.
Marban et al. [86] prepared solid dispersions of copper oxide in ceria following this method.In fact copper(II) nitrate, cerium(III) nitrate and citric acid are dissolved in 5 mL of deionised water in appropriate amounts to get solutions with the following characteristics: 1 M in total metals; s = Cu/(Cu + Ce) molar ratio = 0, 0.065, 0.15 or 0.25; z = citric acid/(Cu + Ce) molar ratio = 1.2.The solution is placed in an oven at 70 0 C and left to be dried for two days.A yellow-green rigid meringue is obtained that is heated under air flow at a given heating rate (h.r.= 1, 5 or 10 0 C/min) up to the calcination temperature (Tcalc = 450 or 550 0 C), at which it is maintained for 4 h.The calcined material has a cigarette ash consistence and is powdered by gentle dis-aggregation in a glass mortar.Following the same procedure, solid dispersions of cobalt oxide in ceria and manganese oxide in ceria too are prepared.Cobalt (II) nitrate and manganese (II) nitrate, respectively, are used in this process instead of copper (II) nitrate (s = [Co or Mn]/([Co or Mn] + Ce) molar ratio = 0.15; z = citric acid/([Co or Mn] + Ce) molar ratio = 1.2; h.r.= 1 o C/min, Tcalc = 550 o C).

Surfactant-assisted method
Cao et al. [8] presented surfactant-assisted method for the preparation of CuO/Ce0.8Zr0.2O2catalysts with different CuO content of nanoparticle assembly.In this method, 6 mmol of cetyltrimethylammonium bromide (CTAB) is dissolved into 200 ml distilled water under Copyright © 2010, BCREC, ISSN 1978-2993 ultrasound irradiation for 15 min at room temperature.To this solution, 8 mmol of Ce(NO3)2•6H2O, 2 mmol of Zr(NO3)4.5H2Oand calculated amount of Cu(NO3)2.3H2Oare added under vigorous stirring.After stirring for 0.5 h, 0.2 mol/l sodium hydroxide solution is added slowly to the above solution until the pH value of the mixed solution reached 10.At this stage, the mixed solution is further stirred for about 12 h.The final suspended solution is aged at 90 0 C for 3 h, washed with hot water, dried in the oven at 110 0 C for 6 h, then milled and calcined at 400 0 C for 4 h.The content of CuO is 0, 5, 10, 15, 20, 25, 30, 40 mol%, and the corresponding catalysts are denoted as CeZrCu0, CeZrCu5, CeZrCu10, CeZrCu15, CeZrCu20, CeZrCu25, CeZrCu30, CeZrCu40, respectively.In order to make clear the influence of the calcination temperature on the catalyst property, a series of CeZrCu25 catalysts calcined at different temperatures are prepared in the similar manner.

Solvothermal synthesis
Xiucheng Zheng and co-workers [79] presented solvothermal synthesis combined with impregnation method.They synthesised CeO2 nano-particles via alcohothermal method and CuO/ CeO2 catalysts via impregnation method.The procedure is described as follows: CeO2 is prepared via alcohothermal synthesis method.In a Teflon bottle with an inner volume of 50 ml, 0.87 g Ce (NO3)2•6H2O is dissolved into 40 mL ethanol absolute (0.05 mol/l).Thereafter 0.45 g KOH is slowly added to the above solution under vigorous stirring (10 min).When the gray solution changes to yellow colour (30 min), the Teflon bottle is hold in a stainless steel vessel and the vessel is sealed tightly.The alcohothermal treatment is performed at 180 0 C for 5 h under auto-genous pressure in an oven.After the alcohothermal treatment, the autoclave is allowed to cool down to the room temperature.The precipitates are separated by centrifuging, washed with de-ionized water and ethanol absolute, and dried in vacuum at 75 0 C overnight to get yellow-white CeO2 nano-crystals.The CuO/CeO2 catalysts are prepared by impregnation of the obtained CeO2 with Cu(NO3)2 aqueous solutions.The prepared samples are dried at 80 0 C overnight and then calcined at 300, 400 and 500 0 C for 3.5 h in air.The CuO loading was 6 wt%.

Leaching method
Zhu et al. [104] prepared a series of mesoporous copper cerium bimetal oxides as follows: Stock mixed solution of Cu(II) nitrate and Ce(III) nitrate

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is prepared by dissolving copper nitrate and cerium nitrate in ethanol.Typically, 0.2 g of KIT-6 silica is dispersed in 3.0 ml of the above ethanol solution, containing stoichiometric amounts of corresponding metal salts.The same is stirred at room temperature for 1 h.Ethanol is removed by evaporation through heating the mixture overnight at 373 K. Afterwards, the resulting powder is heated in a ceramic crucible in an oven at 673 K for 6 h to completely decompose the nitrate species.The impregnation step is repeated with 2.0 mL of the metal salt solution in order to achieve higher loadings.After evaporation of the solvent, the resulting material is calcined at 823 K for 6 h.The silica template is then removed at 323 K through etching twice in 10 ml of 2.0 M NaOH aqueous solution.The meso-porous bimetal oxides are recovered by centrifugation, washed with water and finally dried at 323 K.In all cases, the concentrations of the total metal ions, i.e., [Cu(II)] + [Ce(III)] are kept constant at 0.7 M. The molar percentage ratio of Cu(II) to total metal, i.e., X = [{Cu(II)}/{Ce(III) + Cu(II)}] x 100, varies between 5 and 50.They claimed that the catalysts reported in this work showed comparable or even superior activities to literature data for catalytic CO oxidation.

Chelating method
Zhigang Liu et al. [61] studied different methods used to prepare CuO-CeO2 catalysts for preferential oxidation of CO in excess of hydrogen.They showed that the chelating method enhances the formation of defects of ceria and produces a synergistic effect between the cycle of Cu 1+ /Cu 2+ and that of Ce 3+ /Ce 4+ .The later is beneficial to the improvement of the performance of CuO-CeO2 catalysts for the preferential oxidation of CO.The preparation method is as follows: The CuO-CeO2 catalyst is prepared by chelating method and denoted as 5CuC-CH.The solution of cetyltrimethyl-ammonium bromide (C19H42BrN) is added in drops into the mixture of 0.055 mol/L Ce (NO3)2 and 0.008 mol/L Cu(NO3)2 solutions with vigorous stirring, and the sol-gel obtained is aged for 30 minutes at ambient temperatures.It is important to note that the solvent used in the experiment is ethanol and not water.The sol-gel is then dried at 100 0 C for about 5 h and then heated at 500 0 C for 2 h.As a reference, the CuO-CeO2 catalyst is synthesized by coprecipitation method according to the literature [55] and denoted as 5CuC-CP.KOH (0.362 mol/L) is used as precipitator and added to the mixture of 0.055 mol/ L Ce(NO3)2 and 0.008 mol/L Cu(NO3)2 solutions, Copyright © 2010, BCREC, ISSN 1978-2993 and the pH value of supernatant liquid is kept at 12.5.The two catalysts are crushed and sieved to 60−80 mesh.The loading of Cu in the catalysts is 5 wt%.

Inert gas condensation method
By employing the inert gas condensation (IGC) technique, almost any metal can be used to produce composites with a wide range of different compositions [105].It also provides the possibility to alter the nano-sized morphology, the crystallinity, and particle size [106].Yet another advantage of the IGC method is that, because of the nature of vapour-condensation growth process, a larger portion of internal interfaces and grain boundaries with a high degree of cleanliness between the metals can be obtained [107].These are the prerequisites for obtaining highly active catalysts.Skårman et al. [106] followed this method, which is described as follows: The catalyst powders of CuOx/CeO2 are synthesized by inert gas condensation (IGC) utilizing resistive heating evaporation.Pure metallic cerium and copper granules are used as source materials and evaporated simultaneously in two or three resistively heated tungsten crucibles.After pumping down to UHV conditions (<10-9 Torr), the chamber is filled with a low pressure of inert helium gas.The evaporated metallic monomers are cooled by collisions with the "cold" inert helium gas atoms and aggregated into clusters from collisions between monomers.The produced particle size can be manipulated by the gas pressure or by the evaporation rate.Helium pressures of 0.5, 1.0, 5.0, and 10.0 Torr were tested.The aerosol of particles is transported via self-induced thermal convective flux to a cylindrical liquid N2-cooled rotating coldfinger, where it is continuously collected [108].After the evaporation the UHV is restored and then slowly back-filled with oxygen to a final pressure of 1.0 Torr.The oxidized material is scraped off from the coldfinger and characterized in this as-prepared powder form.The treatment and storage in gastight glass cylinders are maintained identical for all samples.More information about the advantages and limitations of the IGC method can be obtained elsewhere [106,109].

Electroless method
Shiau et al. [90] prepared catalysts by the electroless plating process, which is decribed as follows: Before conducting electroless deposition, γ-Al2O3 support is pre-treated with nitric acid to remove any impurities, and activated by palladium

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chloride solution to provide palladium nucleating centres on surface of γ-Al2O3.The activated γ-Al2O3 is finally contacted with copper solution for copper plating.In the copper solution, formaldehyde is added as reducing agent for the oxidationreduction reaction.The plating bath is maintained at 70 0 C and the pH is adjusted to 12.5.The plated γ-Al2O3 is filtrated and washed with distilled water, where after it is dried at 110 0 C for 24 h.The first catalyst, namely EI, is prepared by electroless plating Cu onto γ-Al2O3 and then impregnating Ce followed by drying andcalcinations.The second one, namely IE, is prepared by impregnating Ce first, followed by electroless plating Cu onto the support.The third one, namely CI, is prepared by co-impregnation of Cu and Ce onto γ-Al2O3.Since the first two catalysts (EI and IE) contain a small amount of Pd, which is required to activate the substrate during the electroless-plating course, Pd (0.012 wt%) is also added into CI catalyst to get an equal basis.The fourth catalyst, namely pure Ecu, is prepared by electroless plating Cu onto γ-Al2O3.All the catalysts contain 5 wt% Cu and 10 wt% Ce (except pure Ecu).

Laser vaporization and controlled condensation (LVCC)
Sundar and Deevi [6] studied CO oxidation activity of Cu-CeO2 nano-composite catalysts prepared by laser vaporization and controlled condensation.They described the method as follows: Desired ratio of metallic copper (2 µm) and ceria particles (1 µm) is mixed and pressed in a mechanical press to form the targets for LVCC experiments.The LVCC process involves pulsed laser vaporization of a target in a chamber under a selected gas mixture.The chamber consists of two parallel plates separated by a quartz ring of 5 cm height.Before the start of experiment, the chamber is evacuated several times and finally filled with argon gas.The top plate is maintained at room temperature and the bottom plate is maintained at a desired higher temperature using an electrical heater.The temperature gradient between the bottom and top plates results in a steady convection current, which can be enhanced under high-pressure (103 Torr) and large temperature gradient (∆T~200 0 C) conditions.The metal vapour is generated by pulsed laser vaporization using the second harmonic (532 nm) of an Nd-YAG laser (100 mJ/pulse, 10 -8 s pulse).Ablation of the target material with the laser results in the formation of atomic, molecular and ionic species.These species react in the gas phase, in a region close to the Copyright © 2010, BCREC, ISSN 1978-2993 target, to form the corresponding nanosized materials.These species are carried by the convective flow generated due to the temperature gradient within the chamber, and deposited on the cold plate of the chamber.

Solvated metal atom impregnation method (SMAI)
The procedure for preparing Cu/CeO2, catalysts by SMAI method is described by Zhang et al. [4] as shown below: The dehydrated CeO2 is used as support and dehydrated and degassed toluene is used as the solvating medium.The preparation of the precursor solution of bis (toluene) copper (0) is carried out in the static metal atom reactor.In a typical experiment, approximately 1 g of copper chop (99.9%) is evacuated under a dynamic vacuum of less than 1.33x10 -2 Pa over aperiod of about1 h.After finishing the co-condensation, the co-condensation is warmed up to -78 0 C and melted down to the bottom of the reactor.The bis (toluene) copper (0) complex prepared in this way is extremely air sensitive and thermally unstable, which decomposes into copper (0) and toluene at about -100 0 C. The precursor solution is transferred to the pre-cooled (-78 0 C) CeO2 through a stainless steel tube.The CeO2 (20 g) is impregnated with solvated Cu atom (cluster) solution for 5 h at -78 0 C under stirring.Then the Cu-toluene /CeO2 slurry is gradually warmed to room temperature.A syringe removes the colourless excess toluene and the Cu/CeO2 catalyst is dried under vacuum at room temperature for several hours.The dry sample is stored and handled in a nitrogen-filled glove box.CuO/CeO2 catalyst is produced by oxidizing the CuO/CeO2 catalyst in the reaction cell (in oxygen) at 200 0 C for 3 h.The authors also prepared CuO/CeO2 catalyst via conventional impregnation method using Cu(NO3)2 solution (to give a copper loading the same as the sample prepared via SMAI).Prepared catalysts are comparatively studied for low temperature CO oxidation.

Combinatorial synthesis of mixed metal oxides
Combinatorial synthesis is a powerful approach for the study of advanced materials.It is based on high-throughput experimentation, where libraries of potential catalysts are prepared and investigated in a parallel or an automated sequential manner to speed up development and discovery of materials with desired properties.Several reviews of catalyst development provide indepth background into the technology [110,111,112].
One application of combinatorial synthesis of catalyst to soot oxidation has been reported by Reichenbach et al. [113].A polymerizable complex method (PCM) of powder processing is applied to the combinatorial synthesis of Cu1−x CexO3, for CO oxidation.In PCM, metal ions (nitrates) are dissolved in solution with a chelating agent (citric acid) and a polyhydroxyl alcohol (ethylene glycol).The metal ions are chelated by citric acid and are evenly distributed throughout the solution.Upon heating, the water or solvent evaporates, and the ethylene glycol undergoes polyesterification.Thus, a polymer resin is formed with the metal ions homogeneously distributed throughout.The resin is then essentially heated to higher temperatures for helping resin decomposition and formation of oxide powders.Because the PCM is a liquid mix process, metal ions are mixed on a molecular level, thus requiring lower processing temperatures and shorter processing times than comparable solidstate processes.The oxide powders produced in this manner have a higher surface area than those produced by solid-state methods.In this approach, inkjet dispensing technology is used to deposit PCM libraries of compositions combinatorially into "wells" on a metal plate.The solutions are then reacted in parallel in a furnace below 500 0 C, with the result being an array of oxide catalysts "caps" of varying compositions.

Redox properties of CuO-CeO2 catalysts
Copper catalyst has been found to be an excellent base metal catalyst for CO oxidation [114].However, pure copper catalyst is less active and stable than the precious metal catalysts.Carbon monoxide oxidation involves surface oxygen and oxygen vacancy participation.The oxygen mobility of metal oxide catalysts also has some thing to do with catalyst activity [115,116].Cerium oxide is an outstanding oxygen ion conduction material and possesses redox properties, capable of activating the metal-oxygen bond of the active phase and/or of releasing Copyright © 2010, BCREC, ISSN 1978-2993 nascent oxygen with high reactivity.It is well accepted that the high activity of CuO/CeO2 is attributed to the quick reversible Cu 2+ /Cu + redox couples of highly dispersed copper species [105,117].Meanwhile, the redox properties of ceria are generally regarded to play key roles in governing the catalytic behaviours by assisting the Cu 2+ /Cu + couples through Ce 4+ /Ce 3+ cycles [118].The ease of the Ce 4+ /Ce 3+ redox cycle leads to outstanding oxygen storage capacity (OSC), coupled with the high mobility of oxygen in the crystal structure are two important properties of CeO2.As a result of that, these oxides are capable of ''adsorbing'' oxygen reversibly [119], a property that is used in catalytic converters of automobiles as a source of oxygen when the effluent from the engine has a reducing nature [120].Under real operating conditions, where the engine regime depends on the traffic conditions, the exhaust gases can be either in oxidant or reducing condition.Hence the use of ceria is specifically advantageous, due to its capability to store oxygen under oxidant conditions and to give it back under reducing conditions.The high activity of the CuO-CeO2 system is attributed to the strong metal-support interactions (SMSI) between Cu species and the CeO2 support.This interaction causes the reduction of the support and of the small CuO clusters to occur at low temperature.In this way, adsorption of CO produces an easy reduction of the catalyst's surface with the generation of CO2 at low temperature.Enhanced catalytic activity in oxidation reactions is believed to be connected to synergistic redox effect of Cu on CeO2 and vice versa [121].
However, the use of ceria as an active catalyst support is limited by its strong tendency to sintering under high temperature conditions.It is shown that by doping ceria with zirconium oxide, the thermal stability of the material is significantly increased because of the formation of a solid solution which inhibits the sintering at high temperature [122][123][124].The enhanced reducibility of ceria-zirconia solutions has sometimes been associated with the ability of the solid solutions to maintain high surface areas; however, recent thermodynamic studies have shown that even bulk oxygen is released much more easily in the mixed oxide [125].The enthalpy change for oxidation of reduced oxides is found to be -520 kJ/mol O2 for ceria-zirconia solutions (having a range of compositions), compared to an enthalpy change of -760 kJ/mol O2 for pure ceria.The introduction of zirconium ions increases the formation of structural defects, which enhances the oxygen storage capacity [126][127][128][129]. Indeed, for pure ceria the oxygen exchange between the gas phase and the support is limited only to surface oxygen (homogeneous exchange), while the increased oxygen mobility obtained by doping ceria with zirconium extends the participation also to the bulk oxygen (heterogeneous exchange).Consequently, CuO/CeO2-ZrO2 catalysts present an increased reducibility in comparison to ceriasupported systems, due to the promotion also of the bulk ceria to the reduction at low temperature [76,130,131].

Activity of CuO-CeO2 catalysts
CuO-CeO2 catalytic systems have been examined for several processes as mentioned above.All these processes involve oxidationreduction steps, which are promoted by the presence of ceria in comparison to traditional copper-based systems.The catalysts' activity of CuO-CeO2 systems has been discussed for the following applications:

CO, HC and NOx pollution control
Carbon monoxide, usually emitted from many industrial process, transportation and domestic activities, is the major air pollutant.In addition to CO, hydrocarbons, and NOx are also vehicular exhaust pollutants.The three-way catalytic converter has effectively reduced emissions of these pollutants from automobiles.However, higher cost of noble metal catalyst used in current catalytic converters and recent regulations on emission control of vehicles, present a large challenge and opportunity for development of low cost alternatives such as transition metal oxide catalysts [132].Cu-CeO2 systems are identified as one of the promising catalysts to substitute for noble metal catalysts for vehicular emission control because of their high activities toward NO and CO and hydrocarbon oxidation [133].
The Cu-ceria catalysts showed substantially high activity and stability for CO oxidation [1], at a space velocity of 45,000 v/v h -1 .Complete CO conversion occurred at about 80 0 C. Liu and Flytzani-Stephanopoulos [1] explained the increased activity of these catalysts by the stabilization of Cu +1 in catalysts prepared via coprecipitation methods, originating from the interaction between copper clusters and cerium oxide, and addressing ceria to perform the role of oxygen source.
Oxidation of hydrocarbon, reduction of NO by NH3 as well as CO and hydrocarbon occurs over 5% The CuO-CrOx catalyst is regarded as the most active base metal catalyst for CO oxidation at high temperatures prior to the discovery of the present composite catalyst.Reaction kinetics analysis shows that relative reaction rate constants at 150 0 C for the composite catalyst 9.4x10 4 [1] is 5 times more than that for the CuOCrOx catalyst [134].The activation energies for the composite catalyst, 78 kJ.mol -1 is also lower than that of the CuO-CrOx catalyst, 91 kJ.mol -1 .In a recent work [135] the present authors reported the strong interaction between CuO and CeO2 closely related to the preparation routes and calcinations temperature playing a crucial role in the CO oxidation over the CuO-CeO2 catalysts.The ranking order of the preparation methods of the catalyst in CO oxidation activity followed: sol-gel > urea nitrate > wet impregnation > thermal decomposition > coprecipitation.

Emission control from diesel engines
Diesel engines are the workhorses for industrial, commercial and personal transportation and also play a vital role in power generation.However, they have a serious drawback of relatively high emissions of particulate matter (PM) or soot and nitrogen oxides (NOx).Reduction of emissions is of prime importance for both environmental [136] and health concerns [137].Concerns on health and environmental effects, and the tightening of diesel emissions regulations have driven research and development on control technologies for the control of harmful diesel exhaust pollutants.Among several technologies proposed to control emissions of soot, the catalytic combustion of soot is one of the most promising methods [138].Effluent gases of the engines have a temperature range of 150-400 0 C, and consequently it is necessary to develop catalysts active at those temperature levels [138][139][140].CuO-CeO2 composite catalysts show excellent performance for the combustion of soot and are cited in the bibliography [24][25][26][27][28].

Bulletin of Chemical Reaction Engineering & Catalysis, 5 (1), 2010, 16
oxides by depositing Al2O3 powders in Cu-Ce gel and treating at 800 0 C in air for 10 h.The modification of Al2O3 obviously increases the textural stability of the catalyst and improves the dispersion of CuO and CeO2, which induces a lower soot oxidation temperature and a higher simultaneous NO reduction.Reddy and Rao [28] evaluated bimetallic (CuO-CoO/CeO2-ZrO2 and CuO-NiO/CeO2-ZrO2) catalysts for soot oxidation.They reported that the CuO-CoO/CeO2-ZrO2 combination exhibited an excellent catalytic activity (T1/2 = 363 0 C) and nearly 100% selectivity towards CO2.The high activity and stability of the catalyst could be attributed to solid solution formation, facile reduction and oxygen vacancies as well as the size of the metal particles along with their specific surface area.

VOC oxidation
Volatile organic compounds (VOCs), emitted from a variety of industrial processes and transportation activities, are considered as an important class of air pollutants.Catalytic oxidation is one of the most developed techniques used for the elimination of VOCs.CuO-CeO2 mixed metal oxides comprise a promising family of catalysts and have been found effective catalysts in VOC oxidation.Larsson and Andersson [141] have studied the oxidation of ethanol and ethyl acetate over CuO/TiO2 and CuO-CeO2/TiO2 catalysts.Cu-Ce-Ti-O catalysts show good performance for feeds both without and with water vapour.Hu et al. [16] have worked on the high activity of a CuO-CeO2 (10 at.% Cu) catalyst in benzene oxidation.CuO-CeO2 mixed oxides prepared by a combustion method [17] have higher surface areas than the corresponding pure oxides and are efficient total oxidation catalysts allowing destruction of ethanol and ethylacetate with minimal formation of undesired byproducts (acetaldehyde) at all conversion levels.

SO2 reduction to elemental sulphur
Strong synergism of the composite catalyst has been demonstrated for SO2 reduction, which is another major air pollutant, emitted by several industrial processes, such as power plants and automobiles.Scrubbing with an adsorbent material and Claus process are commercial technologies for SO2 removal.The former generates solid waste, while the latter produces elemental sulphur that can be used to make sulphuric acid.However, Claus process involves a multi-step and complex reactor system.High throughput reactors for onestep, direct conversion of SO2 into elemental Copyright © 2010, BCREC, ISSN 1978-2993 sulphur is highly desirable for treatment of SO2laden industrial streams [142].Cu-or Ni-modified ceria are active and selective catalysts for SO2 reduction by CO to elemental sulphur [30].Reduction of SO2 to elemental sulphur is represented as follows:

Selective catalytic oxidation of ammonia
As is known, ammonia is a toxic inorganic gas with a pungent odor under ambient conditions, and is potentially harmful to public health [143,144].Therefore, the removal and the control and prevention, of the ammonia emission from air and waste streams are important in view of the environmental concerns.The selective catalytic oxidation (SCO) of ammonia in a stream to molecular nitrogen and water is one method for solving problems of ammonia pollution [145].The catalytic oxidation of ammonia has been reported to precede the exothermic reaction as follows: Chang-Mao Hung [33] has shown that SCO for ammonia by a bimetallic CuO/CeO2 nanoparticle catalyst promotes the oxidation of NH3.The SCO process was found to be more effective at lower temperatures.This work shows that the SCO process has the potential to treat highly concentrated streams of NH3, helping industrial plants to meet discharge regulations.

Hydrogen production -related applications
Water-gas-shift reaction, steam reforming of methanol/ethanol and hydrocarbon, partial oxidation of hydrocarbons, preferential oxidation of CO in hydrogen rich gases, etc. is unit catalytic processes for hydrogen production.Although catalytic processes for hydrogen production from hydrocarbon fuels have been in commercial use for a long time, strong momentum propelled by high cost of energy and promise of fuel cells as a more efficient energy conversion process have recently rejuvenated and relegated research interest to hydrogen production technologies.Widespread use of fuel cells requires availability of hydrogen gas on demand at low costs.Hydrogen production-related reactions are as follows: Steam reforming of CH3OH: CH3OH + H2O → CO2 + H2 Partial oxidation of HC: CH4 + O2 → CO + H2

Bulletin of Chemical Reaction Engineering & Catalysis, 5 (1), 2010, 17
Water-gas-shift reaction: CO + H2O →CO2 + H2 Preferential oxidation of CO: CO + O2 →CO2, H2 +O2 ≠ H2O The Cu-ceria composite catalysts show strong synergism for all these above mentioned reactions.Sedmak et al. [83] demonstrated that it is possible to increase the CuO/CeO2 catalytic performances in CO-PROX by using a sol-gel preparation method.On these systems, characterization analysis evidenced the absence of interstitial copper ions, present in the samples prepared by coprecipitation, but CuO phase was finely dispersed on the surface of large CeO2 crystallites [98].These high dispersed copper clusters are believed to play a key role in the enhancement of activity of these catalysts.Thus, in contrast with Liu et al. [146] who supposed that the stabilizing effect of ceria on certain redox states of copper is the main reason for better catalytic performances, Martinez-Arias et al. [78] propounded that the facile redox interplay between the two components is the key factor of high catalyst performances.
In 2001 Avgouropoulos [147] tested CuO/CeO2 systems for the reaction of CO oxidation in H2 rich streams.A sample containing low copper concentration (6wt%) and relatively low surface area (16m 2 /g), prepared via coprecipitation method, showed catalytic performances higher than noble metals based catalysts traditionally used for this application with 95% conversion at 200°C and 60% selectivity.Moreover, these systems showed a very high selectivity (namely 80%) in a wide range of temperature (up to 150 °C corresponding to 80% selectivity).Sedmak [82] tested CuO/CeO2 systems prepared via sol-gel method for the CO-PROX reaction obtaining a significant increase of the catalyst activity with 100% conversion at 100°C and 40% selectivity.A higher activity of catalysts prepared via sol-gel is also observed for the catalytic wet oxidation of the phenol, application in which the system is four time more active and 25% more selective than those prepared via coprecipitation methods.Also CuO/CeO2 catalysts, prepared by urea-nitrate combustion method, have been tested in CO-PROX without showing remarkable improvements [53], but on the contrary a high dependence of the catalyst performances on the urea/nitrate ratio.Wang [148] proposed for CO-PROX systems based on copper and supported on ceria doped with samaria.The presence of a cation Sm +3 in the support generates in the ceria structure a charge defect which must be compensated with the vacancy formation.An increase of the vacancy concentration increases the 2000-6000ml/min, 500-3000ppp CO, 500-4500ppm O2, 2O2/CO = 0.33-3, 0-70%H2, 0-15%CO2, 0-2%CH4, 0-15% H2O, temp: 90-300 0 C. H2 in feed had little influence on the CO conversion, CO2 and H2O decreased the acti-vity, power-law expression gave a good fit in limited concn ranges, reaction orders depend on the reactant molar fractions.oxygen ion mobility in the structure, thus enhancing the redox properties of the system.In relation to the CO-PROX reaction, the catalyst activity increases compared to CuO/CeO2 catalysts prepared via coprecipitation methods but the selectivity decreases.
Table 1 provides a list of representative literature survey on composite catalysts for the above mentioned reactions.The Cu-ceria composite catalysts show strong synergism for all these reactions.These seemingly different reactions have two common fundamental attributes.First, all the reactions involve one oxidizing molecule and another reducing molecule.Second, all these reactions involve transfer of oxygen atom from one molecule to the other.

Conclusions
This review summarizes the advances in the preparation methods of CuO-CeO2 catalysts for many potential applications for various reactions of environmental, commercial and other importance.This catalyst system of particular pore structures, or compositions, crystal structures or high hydrothermal stability depends on the preparation methods.The choice of a laboratory method for preparing a given catalyst depends on the physicochemical characteristics desired in the final composition for specific application.The authors of the present paper reported ranking order of the preparation methods of the catalyst in CO oxidation activity as: sol-gel > urea nitrate > wet impregnation > thermal decomposition > coprecipitation.
The CuO-CeO2 catalysts exhibit versatile applications owing to the large interest in all the low temperature redox reactions.The high activity of CuO-CeO2 is attributed to the quick reversible Cu 2+ /Cu + redox couples assisted by Ce 4+ /Ce 3+ cycles.The CuO-CeO2 catalysts show excellent activities in comparision to traditional copper-based systems because of the following properties: (i) outstanding oxygen storage capacity (OSC), (ii) high mobility of oxygen in the crystal structure, release of nascent oxygen with high reactivity, (iii) the strong metalsupport interactions (SMSI), i.e., Cu species-CeO2 support, and (iv) the synergistic redox effect of Cu on CeO2 and vice versa.There is a significant increase in the thermal stability of the material by doping ceria with zirconium oxide, because of the formation of a solid solution inhibiting the sintering at high temperature.The introduction of zirconium ions also enhances the formation of structural defects, which supports an increase in the oxygen storage capacity.

Table 1 .
Recent literature review at a glance on CuO/CeO2 catalyst development for various reactions: (A) Reactions of environmental importance

Table 1 .
Recent literature review at a glance on CuO/CeO2 catalyst development for various reactions: (B) Reactions of commercial importance

Table 1 .
Recent literature review at a glance on CuO/CeO2 catalyst development for various reactions: (C) Reactions of other importance cm −2 and 408 mW cm −2 at 800 0 C in H2 and ethanol steam resp., and cell obtained stable output in ethanol steam over an operation period of 50 h.