Mixed Metal Oxide Catalysts Research Articles

Preparation of mesoporous Fe-Cu mixed metal oxide nanopowder as active and stable catalyst for low-temperature CO oxidation

A series of mesoporous Fe-Cu mixed metal oxide nanopowders with different Cu/Fe molar ratios and high specific surface areas were synthesized via a simple, inexpensive, surfactant-free sol-gel route using propylene oxide as the gelation agent. The catalytic behavior of the nanopowders in low-temperature CO oxidation was investigated using a microreactor-gas chromatography system. The prepared materials were characterized by X-ray diffraction, N2 adsorption-desorption, thermogravimetric-differential thermal analysis, temperature-programmed reduction, Fourier transform infrared spectroscopy, and transmission electron microscopy. These mesoporous Fe-Cu mixed metal oxide catalysts had nanocrystalline structures, narrow pore size distributions, and high surface areas; they showed high catalytic activities and stabilities in low-temperature CO oxidation. The addition of CuO to iron oxide affected the structure and catalytic performance of the iron oxide. The catalyst containing 15 mol% CuO had the highest specific surface area and catalytic activity, and showed high catalytic stability in low-temperature CO oxidation.

Preparation, characterization, and catalytic activity of Ca2CuO3/CaCu2O3/CaO nanocomposite as a novel and bio-derived mixed metal oxide catalyst in the green synthesis of 2H-indazolo[2,1-b]phthalazine-triones

As a novel and heterogeneous catalyst, the Ca2CuO3/CaCu2O3/CaO nanocomposite was synthesized via coprecipitation and thermal decomposition methods using CuSO4 and eggshell wastes as inexpensive starting materials. The catalytic activity of the mesoporous mixed metal oxide in the synthesis of 2H-indazolo[2,1-b]phthalazine-triones was investigated. The reaction proceeds to completion within 5–20min with yields of 80–94%. The suggested strategy for synthesizing 2H-indazolo[2,1-b]phthalazine-triones is of great interest because a novel, green, and low-cost mixed metal oxide is used as a heterogeneous catalyst, on account of its convenient preparation, short reaction time, and high reusability without any loss of catalytic activity.

Catalytic Removal of Volatile Organic Compounds over Porous Catalysts

In this review, we summarize the recent research progress on the preparation and catalytic performance of meso- and macroporous metal oxide or mixed metal oxide (including manganese oxides, cobalt oxides, iron oxides, chromium oxides, and perovskite-type oxides) catalysts and their supported transition metal and noble metal catalysts for the oxidative removal of typical volatile organic compounds (VOCs), which were prepared using the hard-templating and polyvinyl alcohol-protected reduction methods, respectively. Most of these porous catalysts performed well for the addressed reactions, which was associated with their surface areas, adsorbed oxygen species concentrations, low-temperature reducibility, interactions between noble metal or metal oxide and support as well as porous structures. In addition, the perspectives for developing high-performance catalytic materials and novel VOCs removal technologies are also proposed.

Biodiesel production from Jatropha curcas L. oil with Ca and La mixed oxide catalyst in near supercritical methanol conditions

The catalytic transesterification of crude Jatropha curcus oil (JCO) with supercritical methanol (scMeOH), in the presence of calcium lanthanum mixed oxide (CaLaO) heterogeneous base catalyst was carried out using a batch reactor at near critical temperatures and pressures. The performance of synthesized CaLaO mixed oxide catalysts was examined by characterizing it through instruments such as XRD, BET and CO2-TPD, revealed that the Ca/La atomic ratio strongly affects the phase structure, catalyst basic sites, and thus the catalytic reactivity. The reaction parameters including Ca/La atomic molar ratio in the mixed metal oxide catalyst, molar ratio of methanol to oil, catalyst concentration, reaction pressure, temperature and time were varied one at a time and optimized based on the content of fatty acid methyl esters (FAMEs). The highest FAME yield for supercritical methanolysis reached 93% under the optimum reaction conditions: 240°C, 8.2MPa, a molar ratio of methanol to oil of 21:1, and reaction time of 10min in the presence of 1wt.% catalyst. The results demonstrated that the presence of CaLaO mixed oxide catalyst in the reaction system effectively reduced reaction temperature, time and pressure of supercritical conditions. It required a very low concentration to mitigate the harsh operation conditions (290°C, 15MPa, 60min) of the scMeOH process. On the other hand, supercritical reaction compensated for low conversion rate of solid catalytic transesterification whereby, it takes one step further by improving the role of catalyst with supercritical conditions to achieve higher yield and shorter processing time. The reusability of CaLaO mixed oxide catalyst for repeated use was tested, the catalytic activity was >80% when the catalyst was employed for fourth time. The study concluded that slight leaching of Ca2+ (0.52–6.07ppm) and La3+ (0.34–2.33ppm) occurred during transesterification reaction, however it is below acceptable levels of metals as ASTM D6751 (United State) and in Europe, EN 14214 (Europe) standards. This proved that heterogeneous catalytic supercritical reaction process is more promising than non-catalytic processes and it can be turned to practical use in the near future.

Hydrodeoxygenation of Guaiacol over Ceria-Zirconia Catalysts.

The hydrodeoxygenation of guaiacol is investigated over bulk ceria and ceria-zirconia catalysts with different elemental compositions. The reactions are performed in a flow reactor at 1 atm and 275-400 °C. The primary products are phenol and catechol, whereas cresol and benzene are formed as secondary products. No products with hydrogenated rings are formed. The highest conversion of guaiacol is achieved over a catalyst containing 60 mol % CeO2 and 40 mol % ZrO2 . Pseudo-first-order activation energies of 97-114 kJ mol(-1) are observed over the mixed metal oxide catalysts. None of the catalysts show significant deactivation during 72 h on stream. The important physicochemical properties of the catalysts are characterized by X-ray diffraction (XRD), temperature-programmed reduction, titration of oxygen vacancies, and temperature-programmed desorption of ammonia. On the basis of these experimental results, the reasons for the observed reactivity trends are identified.

Ceria doped mixed metal oxide nanoparticles as oxidation catalysts: Synthesis and their characterization

Mixed metal nanoparticles (NPs) have attracted significant attention as catalysts for various organic transformations. In this study, we have demonstrated the preparation of nickel–manganese mixed metal oxide NPs doped with X% nano cerium oxide (X=1, 3, 5mol%) by a facile co-precipitation technique using surfactant and surfactant free methodologies. The as-synthesized materials were calcined at different temperatures (300°C, 400°C, and 500°C), and were characterized using various spectroscopic techniques, including, FTIR and XRD. SEM analysis, TEM analysis and TGA were employed to evaluate the structural properties of the as-prepared catalyst. These were evaluated for their catalytic behaviour towards the conversion of benzyl alcohol to benzaldehyde, which was used as a model reaction with molecular oxygen as oxidant. Furthermore, the effect of the variation of the percentage of nano ceria doping and the calcination temperature on the performance of as-prepared mixed metal catalysts was also evaluated. The kinetic studies of the reactions performed employing gas chromatographic technique have revealed that the mixed metal oxide catalyst doped with 5% nano ceria displayed excellent catalytc activity, among various catalysts synthesized.

Preferential oxidation of CO in a hydrogen rich feed stream using Co–Fe mixed metal oxide catalysts prepared from hydrotalcite precursors

Mixed metal oxide catalysts of Co and Fe in the ratio of 1:1, 2:1 and 3:1 were derived from hydrotalcite-like precursors. The catalysts were characterized by XRD, BET, SEM, HR-TEM, H2-TPR, FTIR and ICP-OES. The solids which were polycrystalline in nature, showed mesoporous character and an average crystallite size of 15nm. Powder XRD of the calcined precursors showed the presence of CoFe2O4 and the surface area of the catalysts ranged between 73 and 83m2/g. H2-TPR studies revealed that the catalyst with a 3:1 Co:Fe ratio had a higher H2 consumption at the peak corresponding to the reduction of Co3O4 to CoO when compared to the catalysts with a Co:Fe ratio of 2:1 and 1:1. The catalysts prepared from hydrotalcite-like precursors were more active than the physical mixtures. Furthermore, a catalyst prepared by conventional co-precipitation resembling a ratio of 2:1 was less active than the catalyst of the same metal ratio prepared via hydrotalcite route in the preferential oxidation of CO over the temperature range of 25–180°C. CO conversions of 100% between 120 and 180°C with varied selectivity toward CO2 were obtained at a space velocity of 12,000h−1. Time on stream experiments showed that the catalysts were stable over 24h. The presence of CO2 and H2O in the feed-stream resulted in catalyst deactivation. However, the deactivated catalysts were regenerated when calcined in situ.

Copper–Manganese Mixed Metal Oxide Catalysts for the Direct Epoxidation of Propylene by Molecular Oxygen

Cu–Mn mixed metal oxide (MMO) catalysts doped with NaCl were synthesized using combustion synthesis technique with a variety of fuels for the direct gas-phase epoxidation of propylene with molecular oxygen. The catalyst structure consisted of Mn2O3 and Cu1+xMn2–xO4 crystals. The catalyst crystallinity, primarily influenced by the combustion synthesis fuel choice and calcination temperature, has been found to be of particular importance in affecting the selectivity of the epoxidation reaction. The optimum catalyst composition was 10–20 mol % Cu in Mn doped with 0.1–0.2 mol % NaCl. It exhibited 30–37% propylene oxide selectivity and 1.3–1.5% propylene conversion.

Ta and Ti Anti-passivation Interlayers for Oxygen-Evolving Anodes Produced by Cold Gas Spray

Conventional electrodes for electrochemical oxygen evolution (OER) comprise titanium substrate, an anti-passivation interlayer, and a mixed metal oxide catalyst layer. We report the use of cold gas spray to deposit titanium and tantalum anti-passivation interlayers, directly on titanium, for OER applications. Accelerated electrochemical life tests show that 100% tantalum interlayers exceeded the performance of commercial state-of-the-art electrodes. Microscopy reveals that electrode failure, in the case of 100% tantalum interlayers, is primarily due to catalyst loss; in contrast, the typical failure mode for conventional electrodes of this type is passivation of the titanium substrate. These results highlight the effectiveness of cold gas spray tantalum for extending electrode lifetimes while maximizing catalyst utilization.

Synthesis of Glycerol Carbonate by Transesterification of Glycerol with Urea Over Zn/Al Mixed Oxide.

Reactions of glycerol carbonate using glycerol and urea have been carried out previously using ZnSO4 and ZnO catalysts, and high yields have been reported using ZnSO4 as catalyst. However, this salt is soluble in glycerol, and recycling of catalyst is difficult after the reaction. In this study, we prepared a mixed metal oxide catalyst using Zn and Al, and this catalyst consisted of a mixture of ZnO and ZnAl2O4. We confirmed the conversion of glycerol and the yield of glycerol carbonate of the amount of Al. As a result, we obtained a yield of 82.3% and a conversion of 82.7%. In addition we obtained high yield in recycling of catalyst. The yield of the glycerol carbonate increases with an increase of acid and base site of catalysts and the highest catalytic activity was obtained when acid/base ratio was approx. 1. From this result, we may conclude that the acid and base site density and ratio of catalysts were very important parameters in the synthesis of glycerol carbonate from urea and glycerol.

Recent Progress in the Removal of Volatile Organic Compounds by Mesoporous Silica Materials and Supported Catalysts

Mesoporous silica materials have attracted much attention because of their large surface area,uniform pore-size distribution, large pore size, and wide potential applications in the fields of separation,adsorption, and catalysis. The progress in the removal of volatile organic compounds(VOCs, mainly containing hydrocarbons, methanol, formaldehyde, acetone, benzene, toluene, naphthalene, and ethyl acetate) by mesoporous silica materials and supported catalysts in recent years is reviewed. The effect of the structure of mesoporous silica materials on the adsorption of VOCs is discussed. We also discuss the catalytic performance and reaction mechanism for catalytic VOC oxidation over supported catalysts. The recent developments in catalytic combustion of toluene are examined in detail. We found that the surface environment, pore structure, and morphology of mesoporous silica materials are the main factors influencing adsorption of VOC molecules. The application of noble metal catalyst focuses on improving poison resistance and reducing cost. The research on non-noble metal catalysts focuses on developing supported mixed-metal oxide catalysts with high activity. Future developments of mesoporous silica materials and supported catalysts are highlighted. The design of the catalyst can be carried out from two aspects: the silica support and the mesoporous channel. This review will be helpful in choosing an appropriate catalyst for the removal of VOCs with high activity and stability.

Superior performance of Ni–W–Ce mixed-metal oxide catalysts for ethanol steam reforming: Synergistic effects of W- and Ni-dopants

The ethanol steam reforming (ESR) reaction was studied over a series of Ni–W–Ce oxide catalysts. The structures of the catalysts were characterized using insitu techniques including X-ray diffraction, pair distribution function, X-ray absorption fine structure, and transmission electron microscopy; while possible surface intermediates for the ESR reaction were investigated by diffuse reflectance infrared Fourier transform spectroscopy. In these materials, all the W and part of the Ni were incorporated into the CeO2 lattice, with the remaining Ni forming highly dispersed nano-NiO (<2nm) outside the Ni–W–Ce oxide structure. The nano-NiO was reduced to Ni under ESR conditions. The Ni–W–Ce system exhibited a much larger lattice strain than those seen for Ni–Ce and W–Ce. Synergistic effects between Ni and W inside ceria produced a substantial amount of defects and O vacancies that led to high catalytic activity, selectivity, and stability (i.e., resistance to coke formation) during ethanol steam reforming.

Effect of Cu substitution on promoted benzene oxidation over porous CuCo-based catalysts derived from layered double hydroxide with resistance of water vapor

Porous and dispersed CuCo-based mixed metal oxides catalysts (denoted as CuxCo3−xAl-MMO) are obtained via the calcination of ternary CuxCo3−xAl-layered double hydroxide (LDH) precursors, which exhibit excellent catalytic activity towards complete oxidation of benzene. The Cu0.5Co2.5Al-MMO sample shows the maximum activity of 2.41 mmol gcat−1 h−1 with 90% benzene conversion at 290°C at a high space velocity (SV=60,000mLg−1h−1), comparable to that of only 8% conversion for Cu3Al-MMO sample. The significantly enhanced activity is correlated with higher surface area, narrower pore size, low-temperature reducibility and rich oxygen vacancies and lattice oxygen derived from the synergistic effect over porous and dispersive CuxCo3−xAl-MMO catalysts containing CuO and Co3O4 spinel mixed oxides verified by XRD, BET, H2-TPR, TEM and XPS measurements. Stability with prolonged time on benzene stream and the resistance to water vapor are further investigated. In addition, a monolithic CuCoAl-MMO film catalyst is fabricated by an in situ growth-calcination method, which displays comparable catalytic activity with CuxCo3−xAl-MMO powder. Therefore, this work provides a facile method for the preparation of CuCo-based catalysts with excellent characters responsible for better catalytic activities, which can be used as promising candidates for practical VOCs oxidation.

A membrane electrode assembly for the electrochemical synthesis of hydrocarbons from CO2(g) and H2O(g)

We report the electrochemical reduction of CO2 into hydrocarbons using a new electrochemical membrane reactor holding a yet unreported membrane electrode assembly comprising a copper mesh cathode and a Ti felt coated with mixed metal oxide (MMO) catalyst anode separated by a proton conductive membrane. CO2(g) was supplied to the cathodic reduction compartment, whilst humidified N2 was supplied to the anodic oxidation compartment. The MMO anode produces protons transported across the proton exchange membrane and electrons transported via the external circuit to the copper cathode to reduce CO2(g). Production rates of methane, propane, propene, iso-butane and n-butane were determined as a function of cell potential at temperatures between 30 and 70°C and relative humidity between ca. 25% and 75%. Maximum methane concentration and the current efficiency for production of hydrocarbons were 3.29ppm and 0.12%, respectively. Whilst the observed product spectrum is desirable, such low current efficiencies require systematic optimization of the catalytic membrane system, in particular an improved cathode with an optimum contact between proton conducting membrane, electrode and catalyst is desired.

Selective oxidation of 5-hydroxymethyl-2-furfural to furan-2,5-dicarboxylic acid over spinel mixed metal oxide catalyst

Furan-2,5-dicarboxylic acid (FDCA) is a potential platform chemical in polymer industry for manufacturing polyester, polyamides and polyurethanes. Here, we introduced Li2CoMn3O8 as an efficient catalyst for the selective oxidation of 5-hydroxymethyl-2-furfural (HMF) to FDCA with 80% isolated yield in a gram scale reaction. The nanocrystalline spinel Li2CoMn3O8 catalyst was synthesized by gel pyrolysis method using urea and citric acid as complexing agent. The catalyst was characterized using PXRD, BET, ICP-MS and TEM. The catalyst is cheap and easy to prepare. The use of naturally abundant oxygen as oxidant results to high isolated product yield making this procedure scalable.

Band-gap energy as a descriptor of catalytic activity for propene oxidation over mixed metal oxide catalysts.

The development of a descriptor or descriptors that can relate the activity of catalysts to their physical properties is a major objective of catalysis research. In this study, we have found that the apparent activation energy for propene oxidation to acrolein over scheelite-structured, multicomponent, mixed metal oxides (Bi3FeMo2O12, Bi2Mo2.5W0.5O12, and Bi1-x/3V1-xMoxO4, where 0 ≤ x ≤ 1) correlates with the band gap of the catalyst measured at reaction temperature. We show through theoretical analysis of the energy components comprising the activation energy why the band-gap energy is the primary component dependent on catalyst composition and, hence, why one should expect the activation energy for propene oxidation to correlate with the band-gap energy. We also demonstrate that the change in band-gap energy with composition arises from the interplay between the sizes and energies of the V 3d, Fe 3d, Mo 4d, and W 5d orbitals, which give rise to the lowest unoccupied crystal orbitals. Both the utility of the band-gap energy as a descriptor for catalytic activity and the role of orbital overlap in determining the band gap are likely to be general features in mixed metal oxide oxidation catalysts, enabling the rational design of catalysts with greater activity for oxidation reactions.

Catalytic Etherification of Glycerol to Diglycerol Over Heterogeneous Calcium-Based Mixed-Oxide Catalyst: Reusability and Stability

The activities of different heterogeneous alkaline-earth metal oxide catalysts and mixed-metal oxide catalysts were investigated. Glycerol etherification was carried out at 250°C in a three-necked glass reactor vessel at atmospheric pressure. In a typical experiment, 50 g of anhydrous glycerol was loaded into the reactor. Then, 2 wt.% of catalyst was added to the reactor. The reactor was then heated to the appropriate reaction temperature in nitrogen atmosphere under continuous stirring. The heterogeneous CaO catalyst showed the highest catalytic conversion (72%) compared with other alkaline-earth metal oxides, with a diglycerol yield of 19%. The highest glycerol conversion of 96% and diglycerol yield of 52% were observed for the mixed-metal oxide catalyst (Ca1.6Al0.4La0.6O3). Reusability and stability of this catalyst were tested. The ICP-AES analysis was performed to confirm the leaching of the metal species in the liquid phase of the reaction mixture.

Low-temperature synthesis of Mn-based mixed metal oxides with novel fluffy structures as efficient catalysts for selective reduction of nitrogen oxides by ammonia.

A series of Mn-based mixed metal oxide catalysts (Co-Mn-O, Fe-Mn-O, Ni-Mn-O) with high surface areas were prepared via low temperature crystal splitting and exhibited extremely high catalytic activity for the low-temperature selective catalytic reduction of nitrogen oxides with ammonia.

Titanium and Copper Oxide Based Catalysts for the Insitu Reactions of Methanation and Desulfurization in the Removal of Sour Gases from Simulated Natural Gas

The objective of this novel catalyst development is to achieve both low temperature and high conversion of sour gases. Supported mixed metal oxide catalysts were prepared by impregnating the catalysts on alumina beads for the in-situ reactions of H2S desulfurization and CO2 methanation from room temperature up to 200oC. The results showed that the 100% conversion of H2S to elemental sulfur for all of the potential catalysts was achieved at 100oC. However, methanation of CO2 in the presence of H2S yielded 0.4% CH4 over Fe/Zn/Cu/Ti-Al2O3 catalyst and 0.7% CH4 over Fe/Zn/Cu-Al2O3 catalyst at maximum studied temperature of 200oC. XPS results indicated that spinel compounds of CuFe2O4 and Fe3O4 act as the active sites on the Fe/Zn/Cu-Al2O3 and Fe/Zn/Cu/Ti-Al2O3 catalysts. The appearance of Fe3+-OH on Fe/Zn/Cu/Ti-Al2O3 catalyst increased its H2S desulfurization activity. N2 adsorption-desorption analysis illustrated that 34% of the surface area of Fe/Zn/Cu-Al2O3 catalyst was reduced while Fe/Zn/Cu/Ti-Al2O3 catalyst showedreduction of 17% after catalytic testing, which indicated the deactivation of the catalysts resulted from sulfur poisoning.

The Reduction-Coupled Oxo Activation (ROA) Mechanism Responsible for the Catalytic Selective Activation and Functionalization of n-Butane to Maleic Anhydride by Vanadium Phosphate Oxide

We report here the results of density functional theory quantum mechanical (QM) studies of the detailed chemical mechanism underlying the n-butane selective oxidation to form maleic anhydride (MA) on vanadyl pyrophosphate [(VO)2P2O7] and vanadyl phosphate [VOPO4] surfaces. This QM-derived mechanism differs substantially from previous suggestions but is in excellent agreement with key experimental observations. We find that the O(1)=P bond of the oxidized X1 phase of the VOPO4 surface is the active site for initiating the VPO chemistry, by extracting the H from the n-butane C–H bond. This contrasts sharply with previous suggestions, all of which involved the V=O bonds. The ability of O(1)=P to cleave alkane C–H bonds arises from a new unique mechanism that decouples the proton transfer and electron transfer components of this H atom transfer reaction. We find that the juxtaposition of a highly reducible V+5 next to the P=O bond but coupled via a bridging oxygen dramatically enhances the activity of the P=O bond to extract the proton from an alkane, while simultaneously transferring the electron to the V to form V+4. This Reduction-Coupled Oxo Activation (ROA) mechanism had not been known prior to these QM studies, but we believe that it may lead to a new strategy in designing selective catalysts for alkane activation and functionalization, and indeed it may be responsible for the selective oxidation by a number of known mixed metal oxide catalysts. To demonstrate the viability of this new ROA mechanism, we examine step by step the full sequence of reactions from n-butane to MA via two independent pathways. We that find that every step is plausible, with a highest reaction barrier of 21.7 kcal/mol.