Mixed Metal Oxide Catalysts Research Articles

Solar photocatalytic degradation of methylene blue by mixed metal oxide catalysts derived from ZnAlTi layered double hydroxides

ZnAlTi layered double hydroxides (LDHs) with different Zn:Al:Ti ratios were synthesized by homogeneous precipitation method utilizing urea hydrolysis. The synthesized samples were calcined at 873K in order to obtain mixed metal oxides with high photocatalytic activity. Multiple characterizations such as XRD, FT-IR, TG–DSC, XPS, XRF, N2 adsorption–desorption isotherms and UV–vis DRS were carried out. The photocatalytic activity was evaluated by degradation of methylene blue (MB) dye under solar light irradiation. The experimental parameters such as the catalyst with different molar ratios, catalyst dose, initial MB concentration, and initial pH were studied in detail. A maximum of 99% photodegradation of MB was achieved with the catalyst loading of 1.0gL−1 and initial MB concentration of 10mgL−1, at the condition of neutral pH. The kinetic study indicated that the photocatalytic process was well fitted to the Langmuir–Hinshelwood model. Furthermore, after three cycles, the degradation of MB was still close to 90%, indicating that the photocatalyst had high stability for recycle.

Comparison of CuO-MO x (M=Ce, Zn, Cr and Zr) catalysts in various water-gas shift reactions

The water-gas shift (WGS) reaction in the temperature range of 100–350 °C for various feed compositions simulating forward, reverse and real WGS conditions was studied for a series of coprecipitated mixed metal oxide catalysts of 30 wt% of CuO and 70 wt% of metal oxide (CeO2, ZnO, Cr2O3, and ZrO2) as well as for a commercial WGS catalyst (ICI 83-3). The catalysts were characterized using BET, XRD, H2-TPR and N2O dissociation studies. Among the tested catalysts, CuO-Cr2O3 showed the best activity in the forward WGS, while the commercial catalyst was the best catalyst in the real and reverse WGS reactions. The effect of Cu content in the catalyst was also studied and, in the case of the real WGS, 50 wt% CuO-Cr2O3 was more active than 30 wt% CuO-Cr2O3. H2 and CO2 were found to inhibit the forward WGS, decreasing the reaction rate substantially, particularly at temperatures below 200 °C. The inhibition effect varied depending on the tested catalyst and increased with increasing H2 or CO2 concentration. As the inhibition effect was reversible, the competitive adsorption of H2 or CO2 on the active sites has been suggested to be responsible for the effect. The high activity of the commercial catalyst in the H2 rich real WGS could be described by the difference in the H2 inhibition between the catalysts. An easily reducible copper species was found in CuO-Cr2O3 and could be attributed to the high activity in the forward WGS. Keywords:Mixed Metal Oxide Catalysts, Water Gas Shift Reaction, H2 Inhibition, CO2 Inhibition, Copper Chromate Catalyst

Heterogeneous base catalysts for edible palm and non-edible Jatropha-based biodiesel production.

BackgroundTransesterification catalyzed by solid base catalyst is a brilliant technology for the noble process featuring the fast reaction under mild reacting condition in biodiesel production. Heterogeneous base catalysts are generally more reactive than solid acid catalysts which require extreme operating condition for high conversion and biodiesel yield. In the present study, synthesis of biodiesel was studied by using edible (palm) or non-edible (Jatropha) feedstock catalyzed by heterogeneous base catalysts such as supported alkali metal (NaOH/Al2O3), alkaline-earth metal oxide (MgO, CaO and SrO) and mixed metal oxides catalysts (CaMgO and CaZnO).ResultsThe chemical characteristic, textural properties, basicity profile and leaching test of synthesized catalysts were studied by using X-ray diffraction, BET measurement, TPD-CO2 and ICP-AES analysis, respectively. Transesterification activity of solid base catalysts showed that > 90% of palm biodiesel and > 80% of Jatropha biodiesel yield under 3 wt.% of catalyst, 3 h reaction time, methanol to oil ratio of 15:1 under 65°C. This indicated that other than physicochemical characteristic of catalysts; different types of natural oil greatly influence the catalytic reaction due to the presence of free fatty acids (FFAs).ConclusionsAmong the solid base catalysts, calcium based mixed metal oxides catalysts with binary metal system (CaMgO and CaZnO) showed capability to maintain the transesterification activity for 3 continuous runs at ~ 80% yield. These catalysts render high durability characteristic in transesterification with low active metal leaching for several cycles.

Selective hydrogenolysis of glycerol to propylene glycol by using Cu:Zn:Cr:Zr mixed metal oxides catalyst

Hydrogeneolysis of glycerol to propylene glycol has high interest due to cost and ease of availability of glycerol. In this study, Cu:Zn:Cr:Zr based catalyst was used for selective hydrogeneolysis of glycerol to propylene glycol. It was observed that incorporation of zinc and zirconium in Cu:Cr catalyst matrix improved glycerol conversion and propylene glycol selectivity. This improvement is due to increases in acidity which is confirmed by NH3-TPD and Cu dispersion in the catalyst matrix which is measured by CO-chemisorption. XRD results confirmed the presence of active metallic Cu in the reduced catalyst. The liquid phase reaction was carried out with 80wt% of glycerol solution at 240°C, with 4MPa of hydrogen pressure and 3wt% catalyst loading. The novel catalyst Cu:Zn:Cr:Zr with the elemental molar ratio of 3:2:1:3 gave 100% of pure glycerol conversion and 97% of propylene glycol selectivity. The effects of temperature, hydrogen pressure, glycerol concentration and speed of agitation were studied for reaction optimization. A hydrogenolysis of crude glycerol obtained from biodiesel process, was also carried out under similar reaction condition to yield 90% of propylene glycol. Kinetic study was delineated and reaction followed the pseudo first order with respect to glycerol. The apparent activation energy was found to be 31.7kcal/mol. The catalyst has good reusability up to four runs.

Simultaneous Removal of Toluene (Model Tar), NH3, and H2S, from Biomass-Generated Producer Gas Using Biochar-Based and Mixed-Metal Oxide Catalysts

Effectiveness of four catalysts (biochar, activated carbon, acidic surface activated carbon, and mixed metal oxide) was studied for simultaneous removal of toluene, NH3, and H2S from biomass-generated producer gas. NH3 (0.03%), H2S (0.015%), and toluene at a flow rate of 2 mL/h were mixed with a synthetic producer gas composition (H2: 8.5%, N2: 58%, CO: 17%, CH4: 2%, and CO2: 11%) and passed over a catalyst bed in a fixed-bed reactor tube maintained at 800 °C. Results indicate that simultaneous removal of contaminants from producer gas is feasible using biochar-based catalysts. High surface area mixed metal oxides synthesized using microwave and ultrasonication were also effective for simultaneous removal of contaminants. Among the four catalysts, acidic surface activated carbon resulted in the highest toluene removal efficiency of 97.5% and highest breakthrough time of 145 min for NH3. For H2S removal, mixed metal oxides resulted in the highest breakthrough time of 105 min. For simultaneous removal of toluene, H2S and NH3, activated carbon showed good removal capacity (91% toluene removal efficiency; NH3 adsorption capacity of 0.03g-NH3/g-activated carbon; H2S adsorption capacity of 0.008 g-H2S/g-activated carbon), whereas biochar had moderate removal capacity (86% toluene removal efficiency; NH3 adsorption capacity of 0.008 g-NH3/g-biochar; H2S adsorption capacity of 0.008 g-H2S/g-biochar).

Transesterification of Jatropha curcas crude oil to biodiesel on calcium lanthanum mixed oxide catalyst: Effect of stoichiometric composition

Heterogeneous solid mixed oxide (CaO–La2O3) catalysts with different molar ratios of calcium to lanthanum (Ca-to-La) were synthesized by co-precipitation method. The synthesized solid CaO–La2O3 mixed metal oxide catalysts were utilized in transesterification of Jatropha curcus oil as feedstock to produce biodiesel. Under the optimized conditions at 65°C, 4% catalyst dose with 24:1 MeOH to Jatropha oil molar ratio, the transesterification reaction exhibited 86.51% of biodiesel yield. The prepared catalysts were characterized using various techniques such as X-ray diffraction (XRD), nitrogen sorption with Brunauer–Emmer–Teller (BET) method, temperature-programmed desorption of CO2 (CO2-TPD) and scanning electron microscopy (SEM). Influence of Ca-to-La atomic ratio in the mixed metal oxide catalyst, catalyst amount, methanol to oil molar ratio, reaction time, different oils on the fatty acid methyl ester (FAME) yield were appraised. Different catalyst regeneration procedures were also performed to investigate the reusability of the CaO–La2O3 catalyst.

Synthesis of fatty acid methyl esters via the methanolysis of palm oil over Ca3.5xZr0.5yAlxO3 mixed oxide catalyst

Novel mixed metal oxide catalyst Ca3.5xZr0.5yAlxO3 was synthesized through the coprecipitation of metal hydroxides. The textural, morphological, and surface properties of the synthesized catalysts were characterized via Brunauer–Emmett–Teller method, X-ray diffraction, scanning electron microscopy, Fourier transform infrared spectroscopy, and energy-dispersive X-ray spectroscopy. The catalytic performance of the as-synthesized catalyst series was evaluated during the transesterification of cooking palm oil with methanol to produce fatty acid methyl esters (FAME). The influence of different parameters, including the calcination temperature (300–700 °C), methanol to oil molar ratio (6:1–25:1), catalyst amount (0.5–6.5 wt%), reaction time (0.5–12 h) and temperature (70–180 °C), on the process was thoroughly investigated. The metal oxide composite catalyst with a Ca:Zr ratio of 7:1 showed good catalytic activity toward methyl esters. Over 87% of FAME content was obtained when the methanol to oil molar ratio was 12:1, reaction temperature 150 °C, reaction time 5 h and 2.5 wt% of catalyst loading. The catalyst could also be reused for over four cycles.

Water-Gas Shift Reaction on Ni–W–Ce Catalysts: Catalytic Activity and Structural Characterization

The water-gas shift reaction (WGS, CO + H2O → H2 + CO2) was studied over a series of W–Ce, Ni–Ce, and Ni–W–Ce mixed-metal oxide catalysts. The structure of the catalysts and the WGS reaction intermediates were characterized using in situ techniques including X-ray diffraction (XRD), X-ray absorption near edge structure (XANES), scanning transmission electron microscopy (STEM), and diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS). XANES showed the existence of Ni2+ and W6+ inside the ceria lattices. The coexistence of Ni and W inside of ceria led to a large lattice strain, not seen for Ni–Ce and W–Ce, that facilitated the reduction of Ni–W–Ce and gave this oxide special catalytic properties. A Ni0.2W0.1Ce0.7O2 catalyst displayed the highest catalytic activity among all the mixed oxides, followed by a Ni0.2W0.2Ce0.6O2 catalyst. Besides high activity, the Ni–W–Ce catalysts also displayed the effective suppression of the methanation reaction (CO + 3H2 → CH4 + H2O) under WGS conditions compared to W-free Ni–Ce catalysts. The introduction of W in the lattice of Ni–Ce favored the formation of O vacancies that facilitated the dissociation of water, preventing the dissociation of CO and the formation of methane. Because of the special chemical properties of Ni–W–Ce, monodentate formates and carbonates, which could be chemically active species for the WGS reaction, appear on the surface of these catalysts. Synergistic interactions between Ni and W give Ni–W–Ce unique structural and chemical properties not seen for W–Ce or Ni–Ce mixed-metal oxides.

Preparation and Characterization of High-Surface-Area Bi(1–x)/3V1–xMoxO4Catalysts

We report the successful application of a templating approach employing ordered mesoporous carbon to the synthesis of BiVO4, Bi2Mo3O12, and Bi0.85V0.55Mo0.45O4 and the performance of these materials as catalysts for the oxidation of propene to acrolein. Ordered mesoporous carbon templates were used to control the nucleation and growth of the mixed metal oxide crystals, allowing higher final surface areas to be obtained. The resulting materials were characterized by X-ray diffraction, Raman spectroscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, and BET surface area analysis. The surface area of the mixed metal oxide catalysts was found to depend on the type of mesoporous silica used to prepare the carbon template and on the conditions under which the carbon template was formed. Through an appropriate choice of template, the surface areas of the mixed metal oxides exceeded 15 m(2)/g. Catalytic testing revealed that materials produced via templating in ordered mesoporous carbon had per-gram activities that were up to 85 times higher than those produced by a conventional hydrothermal synthesis and exhibited stable catalytic activities over 24 h.

Emission Abatement at Small-Scale Biomass Combustion Unit with High-Temperature Catalysts

A newly designed downdraft wood stove achieved low-emission heating by integrating an alumina-supported mixed metal oxide catalyst in the combustion chamber operated under high temperature conditions. In the first step, a catalyst screening has been carried out with a lab-scale plug flow reactor in order to identify the potentially active mixed metal oxide catalysts. Mixed metal oxide catalysts have been the center of attention because of their expected high temperature stability and activity. The catalyst has been synthesized through two novel routes, and it has been integrated into a downdraft wood stove. The alumina-supported mixed metal oxide catalyst reduced the volatile hydrocarbons, carbon monoxide and carbonaceous aerosols by more than 60%.

Enhanced oxygen evolution activity by NiOx and Ni(OH)2 nanoparticles.

Small nanoparticles of nickel oxide and nickel hydroxide were investigated for the oxygen evolution reaction (OER) in alkaline solutions. These nanoparticles showed excellent catalytic activity, giving a current density of 10 mA cm(-2) at overpotentials (η) of 300 and 330 mV for Ni(OH)(2) and NiO(x), respectively. The catalytic activity was comparable to that of the best mixed metal oxide catalysts under the same conditions. The activity was significantly higher than that of electrochemically formed Ni(OH)(2) and NiO(x) films, demonstrating the benefit of nanostructuring. Pretreatment of the working electrode was important for the activity and stability of the catalyst layer on the electrode.

On the faradaic selectivity and the role of surface inhomogeneity during the chlorine evolution reaction on ternary Ti-Ru-Ir mixed metal oxide electrocatalysts.

The faradaic selectivity of the chlorine evolution reaction (CER) and oxygen evolution reaction (OER) on the industrially important Ti-Ru-Ir mixed metal oxide is discussed. Absolute evolution rates as well as volume fractions of Cl2 and O2 were quantified using differential electrochemical mass spectrometry (DEMS), while the catalyst surface redox behavior was analyzed using cyclic voltammetry. The spatial inhomogeneity of the surface catalytic reaction rate was probed using Scanning Electrochemical Microscopy (SECM). Although the nature of the competition between electrochemical discharging of chloride ions and water molecules remains elusive on a molecular scale, new insights into the spatial reactivity distribution of the CER and OER were obtained. Oxidation of water is the initial step in corrosion and concomitant deactivation of the oxide electrodes; however, at the same time the nature of interaction between the oxide surface and water is used as a rational indicator of selectivity and catalytic activity. An experimental procedure was established that would allow the study of selectivity of a variety of different catalyst materials using polycrystalline electrode surfaces.

Synthetic Cluster Models of Biological and Heterogeneous Manganese Catalysts for O2Evolution

Artificial photosynthesis has emerged as an important strategy toward clean and renewable fuels. Catalytic oxidation of water to O2 remains a significant challenge in this context. A mechanistic understanding of currently known heterogeneous and biological catalysts at a molecular level is highly desirable for fundamental reasons as well as for the rational design of practical catalysts. This Award Article discusses recent efforts in synthesizing structural models of the oxygen-evolving complex of photosystem II. These structural motifs are also related to heterogeneous mixed-metal oxide catalysts. A stepwise synthetic methodology was developed toward achieving the structural complexity of the targeted active sites. A geometrically restricted multinucleating ligand, but with labile coordination modes, was employed for the synthesis of low-oxidation-state trimetallic species. These precursors were elaborated to site-differentiated tetrametallic complexes in high oxidation states. This methodology has allowed for structure-reactivity studies that have offered insight into the effects of different components of the clusters. Mechanistic aspects of oxygen-atom transfer and incorporation from water have been interrogated. Significantly, a large and systematic effect of redox-inactive metals on the redox properties of these clusters was discovered. With the pKa value of the redox-inactive metal-aqua complex as a measure of the Lewis acidity, structurally analogous clusters display a linear dependence between the reduction potential and acidity; each pKa unit shifts the potential by ca. 90 mV. Implications for the function of the biological and heterogeneous catalysts are discussed.

Copper mixed metal oxide catalysts in the hydrogenolysis of 5-methylfurfuryl alcohol

The hydrogenolysis of 5-methylfurfuryl alcohol to 2,5-dimethylfuran was investigated over CuCrO2·CuO, CuFe2O4·CuO, and CuMn2O4·CuO catalysts. The synthesized catalytic materials were extensively characterized. The Cu0 surface area of the catalysts varied from 0.3 to 20.4m2/g. CuCrO2·CuO demonstrated higher activity relative to surface Cu0 than any of the other synthesized materials and had the same activity as a commercially available copper chromite catalyst. The Cu–Fe catalysts showed lower activities with coinciding lower selectivities when compared to the Cu–Cr catalysts. This behavior of the Cu–Fe catalyst may have been caused by the presence of Fe0. The CuMn2O4·CuO catalysts gave a similar selectivity to the Cu–Cr catalysts, but showed a dramatically lower activity than either the Cu–Cr or Cu–Fe catalysts with respect to Cu0 surface area. While the CuCrO2·CuO, CuCr2O4·CuO, and most active CuFe2O4·CuO catalyst all exhibited very comparable activity relative to Cu0 surface concentrations, the other catalysts became markedly less active.

Mixed Alcohol Synthesis over a K Promoted Cu/ZnO/Al2O3Catalyst in Supercritical Hexanes

Mixed alcohol synthesis (MAS) from syngas involves an overall reduction in the number of moles (i.e., volume reduction) and is highly exothermic. As such, the high-pressure, dense-phase solvent conditions of a supercritical reaction medium afford opportunities to enhance the performance of these reactions. In this paper, the effect of supercritical hexanes (a mixture of hexane isomers) on the reaction performance of MAS was studied in a fixed bed reactor. Experiments were carried out over a 0.5 wt % K promoted Cu based mixed metal oxide catalyst in the temperature range of 200 °C–300 °C, a partial pressure of syngas of 4.5 MPa, and the hexanes/syngas molar ratio ranging from 0 to 3. The results of mixed alcohol synthesis under supercritical hexanes phase conditions demonstrated remarkable enhancement in CO conversion and methanol productivity, while the CH4 and CO2 selectivity was notably reduced. In addition, the effect of temperature has also been investigated on conversion, selectivity, and alcohol pro...

Ethanol steam reforming over NiLaZr and NiCuLaZr mixed metal oxide catalysts

NiLaZr and NiCuLaZr mixed metal oxide catalysts were prepared by co-precipitation with oxalic acid in alcoholic medium, followed by calcination. XRD analysis of the unreduced catalysts showed the formation of crystalline phases corresponding to the pyrochlore structure La2Zr2O7 and NiO following calcination at 850°C, 900°C and 950°C. TEM microscopy of the amorphous solids obtained by calcination at 700°C showed the formation of nanoparticles 20–30nm in size. TPR analysis showed a shift in the Ni and Cu reduction temperature towards lower values with increasing calcination temperature. All catalysts were active in runs of ethanol steam reforming, leading to the formation of gaseous mixtures containing hydrogen, carbon monoxide, carbon dioxide and methane. Deactivation due to carbon formation was observed at the lowest reaction temperature tested (500°C). XRD, FTIR and thermogravimetric analysis revealed differences in the textural properties of catalysts before and after reforming, which varied in degree according to the catalyst calcination temperature. The NiCuLaZr catalysts showed a lower activity than their NiLaZr analogues, a fact that was ascribed to the formation of a nickel–copper solution, with a lower catalytic activity in the cleavage of CC bonds.

Cu‐based mixed metal oxide catalysts for WGSR: Reduction kinetics and catalytic activity

AbstractThis communication reports the effects of Mn/Cr on the reducibility and catalytic activity of Cu–Fe–Mn and Cu–Fe–Cr mixed oxide catalysts for the water gas shift reaction (WGSR). The reduction kinetics of the mixed oxide catalysts is investigated using TPR data, nucleation/nuclei growth models, and a power law model. Based on the statistical indicators, it is concluded that a second‐order power law model describes the reduction of all catalysts adequately. The estimated activation energy for the reduction of the Cu–Fe–Mn catalyst is low compared to the Cu–Fe–Cr catalyst. The TPR analysis of the catalysts reveals that the addition of Mn significantly improved the reducibility of Cu‐oxide species, which is consistent with the low activation energy for reduction of the Cu–Fe–Mn catalyst. In a flow type reactor, the Cu–Fe–Mn catalyst showed highest CO conversion at around 220°C, achieving a high specific reaction rate compared to the Cu–Fe–Cr catalyst. The enhanced reducibility of Cu–Fe–Mn catalyst played the key role in the high conversion of CO. These results are comparable with the results obtained for a commercial Cu–ZnO/Al2O3 catalyst, which was evaluated under the same reaction conditions.

Synthesis of Propylene Carbonate from Urea and 1,2-Propylene Glycol in a Monolithic Stirrer Reactor

A monolithic stirrer reactor, in which the cordierite monolith-supported metal oxide and mixed-metal oxide catalysts are used as stirrer blades, is applied for the alcoholysis of urea with 1,2-propylene glycol to synthesize propylene carbonate. It is shown that the mixed-metal oxides give much higher urea alcoholysis performances than the metal oxides, arising from the existence of a strong synergetic effect in the mixed-metal oxides. The zinc–chromium mixed oxide not only shows an excellent catalytic performance but also has extremely strong adhesion strength on the monolithic substrate. For the monolith-supported zinc–chromium mixed oxide catalyst, the monolithic stirrer reactor performs very well, and the highest yield of propylene carbonate reaches 97.8%. The performance is comparable to that of a conventional mechanically agitated slurry reactor, revealing the monolithic stirrer reactor can be an attractive alternative to the slurry reactor for heterogeneously catalyzed liquid–liquid reactions.

Sol–gel auto-combustion synthesis of Ni–CexZr1−xO2 catalysts for carbon dioxide reforming of methane

Carbon dioxide reforming of methane (methane dry reforming) over Ni–Ce0.8Zr0.2O2 catalysts prepared by a sol–gel auto-combustion method and a conventional co-precipitation method were comparatively studied. We show that sol–gel auto-combustion is very promising for preparing thermal stable homogeneous mixed metal oxide catalysts. The auto-combustion synthesized catalyst exhibited higher initial activity and stability due to its smaller Ni crystalline size and intimate interaction between Ni and Ce0.8Zr0.2O2. In contrast, the co-precipitated catalyst showed poor activity and deactivated rapidly. The rapid deactivation was caused by a higher graphitization degree of the deposited carbon over co-precipitated catalyst with larger Ni crystalline size. We also found that the physico-chemical properties and catalytic activity of sol–gel auto-combustion synthesized catalysts were closely related to the metal nitrate (MN)/citric acid (CA) ratio. High MN/CA ratio led to more violent combustion behaviour and an accordingly higher degree of crystallization of the synthesized catalyst. In contrast, a low MN/CA ratio resulted in more carbon species residues and poor catalytic performance. The Ce/Zr ratio also had a profound influence on the phase structure, reducibility, oxygen vacancies and catalytic performance of Ni–Ce0.8Zr0.2O2 catalysts. Ni–Ce0.8Zr0.2O2 catalyst with cubic phase exhibited the best catalytic performance because of high reducibility, high Ni dispersion and strong Ni-CexZr1−xO2 interaction, and considerable amounts of oxygen vacancies.

Transesterification of jatropha oil with methanol over Mg–Zn mixed metal oxide catalysts

A design was developed for the transesterification reaction of non-edible Jatropha Curcas oil using a heterogeneous catalysis system to replace the use of a homogeneous catalytic reaction. Investigations were conducted on solid MgO–ZnO mixed metal oxide catalyst bases with different atomic ratios of magnesium to zinc (Mg/Zn). These catalysts were characterized by BET (Brunauer–Emmer–Teller) surface area analysis, X-ray Diffraction (XRD), and Scanning Electron Microscopy (SEM) with Energy Dispersive X-ray Spectroscopy (EDS), and the alkalinity of the catalysts was studied by Temperature Programmed Desorption of carbon dioxide (TPD-CO2). The physicochemical properties of the MgO–ZnO binary system were superior to those of the individual bulk oxides of MgO and ZnO. In addition, the formation of a binary system between MgO and ZnO established an effective method for transesterification processes. In this study, the effects of stoichiometric composition and surface characteristics on the transesterification activity of MgO–ZnO were investigated. The catalysts exhibited high catalytic activity (∼80%) with reliable reusability for biodiesel production.