INTRODUCTION:

Metal Oxides:

Metal oxides play a very important role in many areas of chemistry, physical and materials science [1-6]. Metal oxides are formed as a consequence of co-ordination tendency of metal ions so that oxide ions form co-ordination sphere around metal ions and give rise to close packed structure. The different physical, magnetic, optical and chemical properties of metal oxides are of great interest to chemists because these are extremely sensitive to change in composition and structure. Extensive studies of this relationship leads to a better understanding of the chemical bond in crystal. The metal oxides are attracting special attention of scientists due to their easy mode of formation and multifunctional behavior. Various metal/mixed metal oxides have been synthesized and further studied for their applications in diverse field [7-29] The transition metals and their compounds are used as catalysts is chemical industry and in battery industries.

Besides, these compounds can be used in formation of interstitial compounds and alloy formation. The transition metals have the special properties of formation of coloured compounds and show magnetic properties. Metals of d-block elements are used for many industrial applications. They behave as catalysts, super conducting materials, sensors, ceramics, phosphors, crystalline lasers etc. Besides these they are excellent photoactive materials and work as photosensitizer. Mixed metal oxide (MMO) electrodes are devices with useful properties for chemical electrolysis.

Applications of Metal/Mixed Metal Oxides as Catalyst:

Metal/Mixed metal oxides have wide applications as catalyst because of their high surface area and reactive sites. Number of scientists and academicians are using metal/mixed metal oxides as catalyst in various organic reactions. Metal oxides are used as heterogeneous catalyst in various reactions such as Fischer–Tropsch process, alkylation, and transesterification and environmental applications such as the oxidation of volatile organic compounds and the reduction of NO_x[30].

The Fischer–Tropsch process is a collection of chemical reactions that converts a mixture of carbon monoxide and hydrogen into liquid hydrocarbons. These reactions occur in the presence of certain metal catalysts, typically at temperatures of 150–300 °C (302–572 °F) and pressures of one to several tens of atmospheres. The process was first developed by Franz Fischer and Hans Tropsch at the Kaiser-Wilhelm-InstitutfürKohlenforschung in Mülheim an der Ruhr, Germany, in 1925. It serves as an important reaction in both coal liquefaction and gas to liquids technology as well as many other chemical processes aimed at producing compounds based on hydrocarbon chains. It works by combining carbon monoxide and hydrogen that are produced from coal, natural gas, or biomass in a process known as gasification, and the Fischer–Tropsch process then turns these gases into a synthetic lubrication oil and synthetic fuel. The Fischer–Tropsch process has received intermittent attention as a source of low-sulfur diesel fuel and to address the supply or cost of petroleum-derived hydrocarbons. A Fischer–Tropsch-type process has also been suggested to have produced a few of the building blocks of DNA and RNA within asteroids. Similarly, naturally occurring FT processes have also been described as important for the formation of abiogenic petroleum [31]. ZSM-5 nanoparticles (around 180 nm) supported cobalt catalyst have been used in Fischer – TropschSynthesus. The hierarchical ZSM-5 support and Co-based catalyst were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and N₂ sorption. The results indicate that the hierarchical ZSM-5 supported cobalt catalyst performs excellently in FTS, since the mesoporous structure in the hierarchical ZSM-5 support can enhance the mass transfer of the reaction products and the acid sites on the microporous ZSM-5 framework can promote the hydrocracking of long chain hydrocarbon products [32]. The Fischer–Tropsch synthesis (FTS) performance of the catalysts with or without copper was studied in a slurry-phase continuously stirred tank reactor. The characterization results indicate that several kinds of metal oxide–silica interactions are present on Fe–Mn–K/SiO₂ catalysts with or without copper, which include iron–silica, copper–silica, and potassium–silica interactions.

In combustion of alcohols and tar:

Ethanol is used as fuel blended with gasoline. Combustion of Ethanol produces carbon dioxide and water along with a amount of heat. Metal oxides work as catalyst for combustion of Ethanol.

C₂H₅OH (l) + 3 O₂ (g) → 2 CO₂ (g) + 3 H₂O (l);−ΔHc = 1371

Morales et al. [33] synthesized manganese iron or nickel mixed oxide catalysts (MnFe or MnNi, respectively). A detailed description of the bulk and surface structure of each system was achieved by means of measurements of specific surface area, XRD, XPS, FT-IR, and Mo¨ssbauer spectroscopies. The characterisation results show that MnNi catalysts are formed as NiMnO₃ and Ni₆MnO₈ mixed oxides besides a little amount of Mn₂O₃. In contrast, MnFe catalysts consist of an oxide mixture (Fe₂O₃, Mn₂O₃ and Mn₅O₈) forming incipiently a solid solution. The catalytic activity was evaluated in the combustion of propane and ethanol, selected as model volatile organic compounds. Syngas, or synthesis gas, is a fuel gas mixture consisting primarily of hydrogen, carbon monoxide, and very often some carbon dioxide. The cleaning of syngas is one of the most important challenges in the development of technologies based on gasification of biomass. Tar is an undesired byproduct because, once condensed, it can cause fouling and plugging and damage the downstream equipment. Metal oxide/Mixed Metal Oxides have been used as catalyst for combustion by many researchers. Luca and coworkers [34] used the tar reforming catalytic activity of iron and nickel based catalyst supported on alkaline-earth oxides CaO, MgO and calcined dolomite [a (CaMg)O solid solution] investigated in a fixed bed reactor operating at temperatures ranging from 650 to 850 °C; Toluene and 1-methyl naphthalene were used as model compounds for tar generated during biomassgasification. Mostafa et al. [35] synthesized the iron nickel oxide catalysts using co-precipitation procedure and studied for the conversion of synthesis gas to light olefins. In particular, the effects of a range of preparation variables such as [Fe]/[Ni] molar ratios of the precipitation solution, precipitate aging times, calcination conditions, different supports and loading of optimum support on the structure of catalysts and their catalytic performance for the tested reaction were investigated. It was found that the catalyst containing 40%Fe/60%Ni/40wt%Al₂O₃, which was aged for 180 min and calcined at 600 °C for 6 h was the optimum modified catalyst.

In oxidation of alcohols:

Oxidation of alcohols gives different type of ketones which are synthetically important. Metal oxides worked as catalyst in various oxidation reactions of alcohols. AYen –Chun Liu et al. [36] worked on copper ferrite nanopowders, synthesized by a microwave-induced combustion process using copper nitrate, iron nitrate, and urea. The process only took a few minutes to obtain CuFe₂O₄nanopowders. The CuFe₂O₄ powders specific surface area was 5.60 m²/g. Moreover, these copper ferrite magnetic nanopowders also acted as a catalyst for the oxidation of 2,3,6-trimethylphenol to synthesize 2,3,5-trimethylhydrogenquinone and 2,3,5-trimethyl-1,4-benzoquinone for the first time. On the basis of experimental evidence, a rational reaction mechanism is proposed to explain the results satisfactorily. Tsubokawaet al. [37] worked on the oxidations of alcohols with copper (II) salts mediated by 2,2,6,6-tetramethylpiperidinyloxy radical (TEMPO) moieties immobilized on ultrafine silica and ferrite surface were investigated. Based on the above results, the oxidations of alcohols were considered to proceed as follows: alcohols are oxidized with oxoaminium moieties on silica and ferrite surface, which were formed by the reaction of surface TEMPO moieties with copper(II) salts, and oxoaminium moieties on the surface itself are reduced to the corresponding hydroxylamine moieties after the oxidation. Then the hydroxylamine moieties are oxidized with copper(II) salts to regenerate TEMPO moieties on the surface. For example, in the case of the oxidation of benzyl alcohol, Silica-TEMPO was recycled about 45 times. Silica-TEMPO and Ferrite-TEMPO were readily recovered from reaction mixture by centrifugation.

In ozonation of dyes:

Ozone is used as a strong oxidant for decolorization of solution containing organic dyes e.g. azo dyes, CI acid black 1 (AB1), CI acid yellow 19 (AY19) and CI acid orange 7 (AO7) which are widely used colorants in leather dyeing and finishing processes [38]. Metal Oxides have been used as catalyst in ozonation of dyes for their decomposition. M. M. Rashad et al. [39] discussed application of cubic copper ferrite CuFe₂O₄nanopowders for dye decomposition. Copper - ferrites have been synthesized via a hydrothermal route using industrial wastes. The synthesis conditions were systematically studied using statistical design (Box–Behnken Program) and the optimum conditions were determined. The results revealed that single phase of cubic copper ferrite powders can be obtained at different temperatures from 100 to 200 °C for times from 12 to 36 h with pH values 8–12. The crystallite size of the produced powders was in the range between 24.6 and 51.5 nm. The produced copper ferrite powders were appeared as a homogeneous pseudo-cubic-like structure. A high saturation magnetization (M_s 83.7 emu/g) was achieved at hydrothermal temperature 200 °C for 24 h and pH 8. Photocatalytic degradation of the methylene blue dye using copper ferrite powders produced at different conditions was investigated. A good catalytic efficiency was 95.9% at hydrothermal temperature 200 °C for hydrothermal time 24 h at pH 12 due to high surface area (118.4 m²/g). Shou-Quing Liu et al. [40] worked on a magnetic species, synthesized in a 100 mL Teflon-lined stainless steel autoclave at 180 °C for 10 h. The synthesized species was characterized by powder X-ray diffraction, transmission electron microscopy, scanning electronic microscopy, Fourier-transform infrared spectroscopy, X-ray photoelectron spectroscopy and vibrating sample magnetometry at room temperature. The results showed that the synthesized species was nickel ferrite nanoparticles with diameters of approximately 10 nm. The nanoparticles exhibited a photo-Fenton catalytic feature for the degradation of rhodamine B in the presence of oxalic acid. The effects of pH, oxalic acid concentration, and dosage of the catalyst, on the degradation rates of the dyes were examined.Mahmoodi et al. [41] discussed photocatalytic ozonation of dyes with copper ferrite (CuFe₂O₄) nanoparticle (CF nanoparticle) prepared by co-precipitation method was investigated. Reactive Red 198 (RR198) and Reactive Red 120 (RR120) were used as dye models. The characteristics of CF nanoparticle were studied using Fourier transform infrared (FTIR) and scanning electron microscopy (SEM). UV–Vis and ion chromatography (IC) analyses were employed to study of dye degradation. The effect of operational parameters on dye degradation such as CF nanoparticle dosage, pH, dye concentration and salt (inorganic anions) was studied. Formate, acetate and oxalate anions were detected as dominant aliphatic intermediates.

In hydrogen production:

Hydrogen Gas has many applications. Most of this hydrogen is used for industrial applications such as refining, treating metals, and food processing. Liquid hydrogen is the fuel that once propelled the space shuttle and other rockets.Hydrogen can be combined with compressed natural gas (CNG) to increase performance and reduce pollution.

Hydrogen production from palm oil mill effluent (POME) was investigated with the incorporation of nanoparticles (NPs) comprising of nickel (NiO) and cobalt oxides (CoO). The NPs of NiO and CoO were prepared using hydrothermal method and were further applied to analyse, their effect on hydrogen production. The results demonstrated that, a maxima volumetric hydrogen production rate of 21 ml H₂/L-POME/h with the hydrogen yield of 0.563 L H₂/g-COD_removed was obtained with 1.5 mg/L concentration of NiO NPs. On the other hand, the addition of CoO NPs produced maximum volumetric hydrogen production rate of 18 ml H₂/L-POME/h with a hydrogen yield of 0.487 L H₂/g-COD_removed with 1.0 mg/L of CoO NPs. Results showed that addition of optimal concentration of NiO and CoO NPs to the POME enhances the hydrogen yield by 1.51 and 1.67 fold respectively. Besides, this addition of NiO and CoO enhanced the COD removal efficiency by 15 and 10% respectively as compared to an un-additive NPs POME. The toxicity of NPs was also tested using bacterial viability test, which revealed that application of 3.0 mg/L of NiO and CoO NPs to modified Luria-Bertani (LB) medium had 63% and 83% reduction in bacterial cell growth. The results concluded that supplementation of NiO and CoO NPs under an optimal range to the wastewater can improve the hydrogen productivity [42].Huang et al. [43] showed the iron-doped nickel oxide films application as oxygen evolution catalysts in the photoelectrochemical production of hydrogen from solar energy. The effects of processing parameters on the film properties, such as overpotential, composition, surface morphology and preferred orientation, were investigated. The electrochemical experiment, structural and compositional measurements indicate that the relative lower substrate temperature, higher RF power, higher working pressure and oxygen content are necessary to gain lower overpotential. Youn et al. [44] analysedmetal oxide-stabilized mesoporous zirconia supports (M–ZrO₂) with different metal oxide stabilizer (M = Zr, Y, La, Ca, and Mg) prepared by a templating sol–gel method. 20 wt% Ni catalysts supported on M–ZrO₂ (M = Zr, Y, La, Ca, and Mg) were then prepared by an incipient wetness impregnation method for use in hydrogen production by auto-thermal reforming of ethanol.The effect of metal oxide stabilizer (M = Zr, Y, La, Ca, and Mg) on the catalytic performance of supported nickel catalysts was investigated. Ni/M–ZrO₂ (M = Y, La, Ca, and Mg) catalysts exhibited a higher catalytic performance than Ni/Zr–ZrO₂, because surface oxygen vacancy of M–ZrO₂ (M = Y, La, Ca, and Mg) and reducibility of Ni/M–ZrO₂ (M = Y, La, Ca, and Mg) were enhanced by the addition of lower valent metal cation. Hydrogen yield over Ni/M–ZrO₂ (M = Zr, Y, La, Ca, and Mg) catalyst was monotonically increased with increasing both surface oxygen vacancy of M–ZrO₂ support and reducibility of Ni/M–ZrO₂ catalyst. Among the catalysts tested, Ni catalyst supported on yttria-stabilized mesoporous zirconia (Ni/Y–ZrO₂) showed the best catalytic performance. Tsonchevaet al. [45] worked on nanosized copper ferrites, prepared by thermal method from the corresponding hydroxide carbonate precursors varying the temperature of synthesis. The phase composition of the obtained materials was characterized by XRD, Mössbauer spectroscopy, DSC and TPR analysis. Their catalytic properties were tested in total oxidation of toluene and methanol decomposition to CO and hydrogen.

In reduction of NO:

NOx is a generic term for mono-nitrogen oxides, namely NO and NO₂, which are produced during combustion at high temperatures (above 1350 _C). NOx (x = 1, 2) emitted to air are largely responsible for the ozone decline in middle to high latitudes from spring to fall, and for the acid rain perturbing the ecosystems and the cause of biological death of lakes and rivers. Peroxyacetylene nitrates (PAN) canalso be formed from nitric oxide and contribute significantly to global photo-oxidation pollution. In addition, NOx species are also harmful to the human body, which can diffuse through the alveolar cells and the adjacent capillary vessels of the lungs and damage the alveolar structures and their functions throughout the lungs, provoking both lung infections and respiratory allergies like bronchitis, pneumonia, etc [46].Suarez et al. [47] worked on the influence of ammonia and nitric oxide oxidation on the selective catalytic reduction (SCR) of NO by ammonia with copper/nickel and vanadium oxide catalysts, supported on titania or alumina. In the SCR reaction, the VTi catalyst had a higher activity than VAI at low temperatures, while the CuNiAl catalyst had a higher activity that CuNiTi. A linear relationship between the reaction rate of ammonia oxidation and the initial reduction temperature of the catalysts obtained by H₂TPR showed that the formation rate of NH₃ species in copper/ nickel catalysts would be higher than in vanadia catalysts. Bianco et al. [48] discovered the selective reduction of NO_x, with ammonia on alumina-supported copper catalysts . It is shown to be effective when O₂ or NO₂ are present in the feed. Under steady state conditions, the presence of NO₂ in the feed stream increases the overall rate of reduction of NO_x and simultaneously reduces its dependence on the oxygen concentration. A maximum in activity is found for a molar inlet ratio NO₂/NO ≈ 1. It has also been observed that the stoichiometry of the process is a function of the reaction temperature, with a secondary NH₃ oxidation reaction appearing at temperatures above 500 K. Orsini and coworkers [49] utilized the reduction of NO with H₂ on copper nickel and chromium – nickel catalysts. Both CuNi and CrNi catalysts were prepared by impregnation on Al₂O₃pellets. Dry-pressed catalysts, CrNi with Al₂O₃ powder were formed with several catalysts containing the NiCr₂O₄ spinel. A pressed nickel oxide catalyst was highly active for the reduction of NO while impregnated nickel oxide was much less active. Copper oxide and chromium oxide were relatively inactive. Addition of small amounts of copper to impregnated nickel oxide improved the activity of the latter catalysts, but above about 24 atmoic percent copper, activity decreased with increase in copper content. The activity of the pressed chromium – nickel catalysts increased with increase in nickel content. Chakrabarti et al. [50] studied catalytic reduction of nitric oxide with ammonia by use of Girdler's G-22 (barium promoted copper chromite) and T-312 (nickel oxide and copper oxide on gamma alumina) catalysts inside a carberry type stirred tank reactor. Helium was used as the carrier gas. The nitric oxide and ammonia concentrations in the feed varied from 550 to 1400 ppm and 700 to 7900 ppm, respectively. Temperature levels were from 177 to 316°C.Gas flow rates ranged from 200 to 300 lh⁻¹. Empirical kinetic expressions for both catalysts were developed which adequately represent the experimental results.

In oxidation of CO:

Derazet al. [51] synthesized alumina-supported NiO catalysts, promoted with 0.14-3 wt.% ZnO prepared by impregnation and then calcined at 400, 600, and 800 °C for 4 and 40 h. The phase analysis, surface and catalytic properties were investigatedby using XRD technique, nitrogen adsorption at -196 °C, and oxidation of CO by O₂ at 200-300°C, respectively. The results obtained reveal that ZnO doping of Ni/Al mixed oxides followed by calcination at 400 or 600 °C for 4 h brought about slight increase in their specific surface area, which decreased progressively by increasing the calcination temperature of doped solids to 800 °C for 4 and 40 h. CO oxidation activity over NiO/Al₂O₃ mixed solids increased by treatment with ZnO followed by heating at 400 or 600°C for 4 h, and then decreased by increasing the calcination temperature to 800 °C for 4 and 40 h. Komatsu and coworkers[52] find out the catalytic activity of pure, doped nickel oxide, and mixtures of nickel oxides with different dopents were investigated by the reaction of carbon monoxide oxidation. The incorporation of lithium ions in the oxide enhanced the activity and the addition of indium lowered the activity. The activity of mixtures increased to several times greater than would be predicted by simple additive effect of single doped catalysts.

Ethanol steam-reforming reaction:

Homes et al.[53] discussed ZnO-supported Ni and Cu as well as bimetallic Co-Ni and Co-Cu catalysts containing 0.7 wt% sodium promoter in the ethanol steam-reforming reaction at low temperature (523–723 K), using a bioethanol-like mixture diluted in Ar. Monometallic ZnO-supported Cu or Ni samples do not exhibit good catalytic performance in the steam-reforming of ethanol for hydrogen production. Copper catalyst mainly dehydrogenates ethanol to acetaldehyde, whereas nickel catalyst favours ethanol decomposition.

In hydrogenation:

Yadav and Kharkara [54] followed liquid phase hydrogenation of a series of nitriles, namely, benzonitrile, butyrotirle, cinnamonitrile and crotonontrile over and has been investigated and the activities of the catalysts have been correlated with the structure of the catalysts on the basis of frontier orbital energy levels. Bridieret al. [55] studied the partial hydrogenation of propyne over copper-based catalysts derived from Cu–Al hydrotalcite and malachite precursors and compared with supported systems (Cu/Al₂O₃ and Cu/SiO₂). The as-synthesized samples and the materials derived from calcination and reduction were characterized by XRF, XRD, TGA, TEM, N₂ adsorption, H₂-TPR, XPS, and N₂O pulse chemisorption. Catalytic tests were carried out in a continuous flow-reactor at ambient pressure and 423–523K using H₂:C₃H₄ ratios of 1–12 and were complemented by operando DRIFTS experiments. The propyne conversion and propene selectivity correlated with the copper dispersion, which varied with the type of precursor or support and the calcination and reduction temperatures. Ferraris and Rossi et al.[56] developed the behaviour of Cu-ZnO catalysts in propene hydrogenation at 323 K has been investigated in order to gain information as to whether or not a synergic effect due to ZnO on the activity of copper is present. With this aim, two different series of catalysts were prepared by coprecipitation at (A) variable or (B) constant (≈ 8) pH. The whole composition range from CuO/ZnO 100:0 to 0:100 was covered in preparation A, while only copper-rich samples (CuO/ZnO≥67:33) were prepared from method B. Chemisorption experiments of hydrogen and propene on samples reduced with hydrogen at 473 K point to the presence of adsorption sites in binary samples different from those existing in single components, that are influenced by the outgassing temperature.

Different applications of Metal oxides:

Kozlowski et al. [57] discoverdmetal ions, especially with high chemical activity (e.g. redox-active Cu and Fe) must be carefully managed in biological systems. The “uncontrolled” activity, e.g. catalysis of Fenton-like reactions by ions like Cu(I) or Fe(II), is so damaging for the biological milieu that right from their entry, metal ions need to be strictly controlled until they arrive at their storage site. Majumdar et al. [58] used the thermal decomposition of the oxalates of zinc; nickel and iron (II) have been re examined from a fresh experimental approach. Differential thermal analysis (DTA) and thermogravimetry (TG) of the individual oxalates and of mixtures of zinc oxalate with either nickel/iron oxalate or products of decomposition of the latter two, were carried out in air in sample cells made of different materials (Pt, Al, Al₂O₃, Ni). The information gathered from thermo analytical experiments, together with information derived from specific chemical tests for the evolution of carbon monoxide during decomposition, chemical analyses, XRD and stoichiometric and thermochemical considerations helped to specify some of the inadequately explained features of the courses and kinetics of decomposition of the metal oxalates. Buscaet al. [59] carried out the urea hydrolysis method to prepare well-crystallized Ni-Co-Zn-Al layered double hydroxides to be used as precursors of mixed oxide catalysts for the ethanol steam reforming (ESR) reaction. The calcination of the layered precursors gives rise to high surface area mixed oxides, being actually a mixture of a rock salt phase (NiO), a wurtzite phase (ZnO) and a spinel phase. The steam reforming of ethanol has been investigated over these catalysts after calcination at 973 K in flow reactor experiments. All these catalysts are active for ESR. At 820 K the selectivity to hydrogen increases with cobalt content. The most selective catalyst is the Ni-free Co-Zn-Al mixed oxide essentially constituted by a single spinel type phase Zn _0.55Co _0.45[Al _0.45Co _0.55]₂O₄. Zoo et al. [60] studied the pre-reforming of commercial liquefied petroleum gas (LPG) over Ni–CeO₂ catalysts at low steam to carbon (S/C) molar ratios less than 1.0. It was found that the catalytic activity and selectivity depends strongly on the nature of the support and the interaction between Ni and CeO₂. The Ni–CeO₂/Al₂O₃ catalysts, which were prepared by impregnating boehmite (AlOOH) with an aqueous solution of cerium and nickel nitrates, exhibited the optimal catalytic activity and remarkable stability for the steam reforming of LPG in the temperature range of 275–375 °C. The effects of CeO₂ loading, reaction temperature and S/C ratio on the catalytic behavior of the Ni–CeO₂/Al₂O₃catalysts were discussed in detail. Aseem et. al. [61] studied oxidative coupling steady state multiplicity and catalyst durability of methane over mixed metal oxide catalysts:

Exothermic heat effects are a crucial factor in determining the performance and stability of catalysts for the oxidative coupling of methane (OCM). Fixed bed temperature rise, steady state multiplicity, and catalyst durability are investigated over a range of feed conditions for the mixed metal oxides Cs/Sr/MgO, Cs/Ba/MgO, Cs/Sr/La₂O₃, and Na₂WO₄-Mn/SiO₂. A comparison with studies on doped metal oxides catalysts for OCM clearly indicates that doping not only improves the performance but also significantly improves the catalyst stability.Aaiet al. [62] followed the results of a leaching kinetics study of spent nickel oxide catalyst with sulfuric acid. The effects of spent catalyst particle size, sulfuric acid concentration, and reaction temperature on Ni extraction rate were determined. The results obtained show that extraction of about 94% is achieved using −200+270 mesh spent catalyst particle size at a reaction temperature of 85 °C for 150 min reaction time with 50% sulfuric acid concentration. The solid/liquid ratio was maintained constant at 1:20 g/ml. The leaching kinetics indicate that chemical reaction at the surface of the particles is the rate-controlling process during the reaction. The activation energy was determined as about 9.8 kcal/mol, which is characteristic for a surface-controlled process. Fuentes et al. [63] evaluated nickel or copper-based catalysts obtained from hydrotalcite-like precursors in order to find catalysts able to work at intermediates temperatures (200–350°C) in water gas shift reaction (WGSR). Samples based on nickel (or copper), aluminum and zinc were obtained by co-precipitation, characterized by several techniques and evaluated in WGSR. Zinc caused changes in the cell parameters of hydrotalcite-type structure, which determined the structural and textural properties of calcined samples. For all catalysts, zinc oxide was detected. In the case of nickel-based hydrotalcites, aluminum cations were incorporated into nickel oxide lattice, hindering reduction; however, the addition of zinc decreased this effect. For copper-based samples, aluminum entered into copper oxide lattice and the copper reduction decreased with the increase of zinc amount in solids. After calcination, copper catalysts showed lower specific surface areas than nickel ones. Shimomura et al. [64] investigated copper oxide-zinc oxide-alumina catalysts for methanol synthesis and kneading methods for comparison of catalytic activity, physical properties, and estimation of the effective surface area by carbon monoxide chemisorptions. The co precipitated catalysts, which are already industrially employed, have higher catalytic activities than the kneaded ones, the optimum chemical composition being around Cu:Zn:Al = 60:35:5 (atom%) in both cases. The total pore volume, the mean pore radius, and the porosity of the coprecipitated catalysts were three to four times larger than those of the kneaded ones. The pore-size distribution ranges from 50 to 5000 Å for the coprecipitated catalysts and from 40 to 10,000 Å for the kneaded ones. The former is equivalent to the copper and zinc oxide values in an individual state, while in the latter the values change in the composite form, with 80- and 20-Å mean particle sizes, respectively. The crystallite size of the composition with a higher catalytic activity after reduction was about 100 Å both for zinc oxide and copper in the coprecipitated catalysts and 170–180 Å for copper and 270–280 Å for zinc oxide in the kneaded ones. From these facts it was proposed that a finely mixed state in the oxidized precursor has its origin during the co precipitation process.

CONCLUSION:

Above literature mentioned shows wide application of metal/mixed metal oxides as catalyst in various reactions like combustion of gases, for the reforming of tar, dye degradation, catalysts for methanol synthesis, for carbon monoxide oxidation, liquid phase hydrogenation of a series of nitriles, namely, benzonitrile, butyrotirle, cinnamonitrile and crotonontrile, for the conversion of synthesis gas to ethylene and propylene etc. So metal/mixed metal oxides have broad applications as catalyst in various conversions.

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Received on 30.09.2018 Modified on 06.12.2018

Asian J. Research Chem. 2018; 11(6):893-899.

DOI:10.5958/0974-4150.2018.00155.4