Complex Metal Oxide Catalysts Research Articles

Synergistic Effect between Fe<sup>4+</sup> and Co<sup>4+</sup> on Oxygen Evolution Reaction Catalysis for CaFe<sub>1−</sub><i><sub>x</sub></i>Co<i><sub>x</sub></i>O<sub>3</sub>

Chemical substitution is an effective way to improve electrocatalytic properties in transition metal oxides. We investigate the synergistic effect between Fe4+ and Co4+ ions on the catalytic activity for oxygen evolution reaction (OER) in the Fe-Co-mixed perovskite oxide CaFe1–xCoxO3. The OER activity of CaFe1–xCoxO3 is substantially increased by small amounts of Co (Fe) doping into CaFeO3 (CaCoO3), leading to the superiority compared to the pure Fe and Co perovskite oxides. The x dependences of the OER overpotential and specific activity for CaFe1–xCoxO3 (0.05 ≦ x ≦ 0.95) are expressed by constant offset from the weighted average between CaFeO3 and CaCoO3, which can be interpreted to be the synergistic effect between Fe4+ and Co4+ ions on OER activity. The absence of the optimum x for the highest activity for CaFe1–xCoxO3 contrasts with the volcano-like plots reported in various mixed-metal oxides. First-principle calculations using the special quasirandom structure models on CaFe1–xCoxO3 (x = 0.03–0.5) demonstrate that about half the amount of Fe4+ is electronically activated to possess smaller charge-transfer energies, corroborating the enhancement of catalytic activity in CaFe1–xCoxO3. These findings provide new insight into the synergistic effects in complex transition metal oxide catalysts.

Graphene-Supported Sc 2 O 3 /TiO 2 for Superior Catalysis on Hydrogen Sorption Kinetics of MgH 2

Complex metal oxide catalysts greatly accelerate the hydrogen sorption rates in the magnesium hydride system. In this study, the graphene-supported Sc 2 O 3 /TiO 2 catalyst is synthesized by means of a simple method, and a surprisingly synergetic effect of the Sc 2 O 3 -TiO 2 cocatalyst on the hydrogen storage performance of MgH 2 is observed. The MgH 2 -Sc 2 O 3 /TiO 2 @Gn composite starts to release hydrogen at 140 °C and reaches the peak dehydrogenation temperature at 239.9 °C. It absorbs 6.55 wt% of H 2 in 1 min and desorbs 5.71 wt% of H 2 in 10 min at 300 °C, showing excellent hydrogen absorption and desorption performance. Furthermore, with the decrease of the grain size and changes in the structure, the activity of the catalyst is greatly improved. The low-valent titanium and scandium and oxygen vacancies formed in the process of dehydrogenation facilitate hydrogen diffusion and electron transfer, and further improve the kinetic performance of the Mg/MgH 2 -Sc 2 O 3 /TiO 2 @Gn system. This study aims to provide insights into studying complex metal oxides as catalysts to improve hydrogen storage performance, and shed light on other catalysis-related research.

Ethanol dry reforming over Ni supported on modified ceria-zirconia catalysts– the effect of Ti and Nb dopants

In this work complex metal-oxide catalysts with the general formula 5 wt% Ni/Ce0·75Zr0.25-x(Nb,Ti)xO2-δ were synthesized by the solvothermal method using supercritical alcohols followed by nickel deposition. The catalysts were characterized and studied in ethanol dry reforming reaction (EDR) in the temperature range 600–750 °C. XRD and TEM showed that the synthesis method provides incorporation of doping cations into the ceria-zirconia fluorite structure, leading to mixed oxides formation. The effect of doping cations on structural and surface properties of 5 wt% Ni/Ce0·75Zr0.25-x(Nb,Ti)xO2-δ and activity in the EDR reaction was investigated. Oxygen deficiency δ increases with the introduction of titanium and niobium cations, which contributes to the bifunctional reforming mechanism implementation with rapid oxidation of coke precursors and activation of СО2 at surface oxygen vacancies. TPO-O2 analysis after reaction showed no carbon formation above 700 °C, and a few carbon deposits (not exceeding 4%) even after significant catalyst deactivation at 600 °C.

Tuning surface V5+ concentration in M1 phase MoVSbOx catalysts for ethylene production from ethane through oxidative dehydrogenation reaction

MoVSbOx complex metal oxides are efficient catalysts for converting ethane to ethylene through oxidative dehydrogenation (ODE) reaction. Active sites containing V5+ are crucial to the catalytic performance of molybdenum-vanadium based catalysts. One of the main challenges of these catalysts lies in controlling active sites containing V5+ on the surface. In this work, a series of pure M1 phase MoVSbOx catalysts with different surface contents of V5+ sites were prepared by hydrothermal and self-assembly methods. Their catalytic ODE performance was found to have a strong correlation with surface V5+, showing firstly an increase and then a substantial drop as a function of surface V5+ concentration on M1 phase MoVSbOx catalysts. This may be attributable to the presence of different catalytic sites on the catalysts. The best catalyst synthesized by hydrothermal method has excellent reaction stability and gives 77.6 % ethane conversion and 64.1 % ethylene selectivity. The results of this study may be useful in developing highly efficient complex metal oxide catalysts for ODE.

Performance of Catalytic Fast Pyrolysis Using a γ-Al2O3 Catalyst with Compound Modification of ZrO2 and CeO2

To investigate the catalytic pyrolysis performance of complex metal oxide catalysts for biomass, γ-Al2O3 was prepared through the precipitation method, and then ZrO2 and γ-Al2O3 were blended in the proportion of 2:8 using the co-precipitation method. Next, CeO2 was loaded on the surface of the catalyst for further modification. The three catalysts, A, ZA and CZA, were obtained. The specific surface and acidity of the catalysts were characterized by nitrogen adsorption–desorption and NH3-Temperature Programmed Desorption (NH3-TPD) respectively. The catalytic pyrolysis performance of catalysts for bamboo residues was investigated by Pyrolysis gas chromatography mass spectrometry (Py-GC/MS). Chromatograms were analyzed for identification of the pyrolysis products and the relative amounts of each component were calculated. Experimental results indicated that catalyst A had a good catalytic activity for the fast pyrolysis of bamboo residues. The addition of ZrO2 and CeO2 could continuously enhance the acidity of the catalyst and further promote the pyrolysis of macromolecular compounds and deoxidation of oxygen-containing compounds. Finally, catalyst CZA, obtained by compound modification, could not only dramatically reduce the relative content of phenol, acid and aldehyde and other oxygen-containing compounds, but also achieved the maximum hydrocarbon yield of 23.38%. The catalytic performance of catalyst CZA improved significantly compared with catalyst A.

Single-Site Vanadyl Species Isolated within Molybdenum Oxide Monolayers in Propane Oxidation

The cooperation of metal oxide subunits in complex mixed metal oxide catalysts for selective oxidation of alkanes still needs deeper understanding to allow for a rational tuning of catalyst perform...

Multi-dimensional Crystal Structuring of Complex Metal Oxide Catalysts of Group V and VI Elements by Unit-Assembling

Four new porous crystalline complex metal oxide families based on group V and VI elements, all of which are synthesized through unit-assembling, are introduced along with crystal structure formation mechanism. Polyoxometalates (POMs) are utilized as building units and assembled for constructing microporous complex metal oxides. Assembling of MoVO-POMs having pentagonal units of [Mo6O21] with {VO} linkers under hydrothermal conditions forms microporous orthorhombic (NH4)4[Mo30V4O106] {VO}6 oxide and trigonal (NH4)3[Mo19.5V1.5O69] {VO}6 oxide. Assembling of e-Keggin Mo-POMs with bismuth ions as a linker under a hydrothermal condition produces a cubic (NH4)4[Mo9.4V3.6O40] {Bi}2 crystal with cages and channels with the diameter sizes of 0.77 and 0.34 nm, respectively. One dimensional anionic tungstosellenate molecular wire building block, [SeW6O21]2−n, is first formed by linear connection of hexagonal tungstosellenate POM units [SeW6O27] and then linked with the CoII ion to form a crystalline microporous materials, (NH4)0.4 [SeW6O21] {Co(OH)}1.3. [W4O16] building blocks are orderly connected with {VO} linkers to form a microporous framework (K1.5(NH4)0.2H0.3[W4O16]{VO}3) with a pore diameter of 0.43 nm which is fully opened and is accessible to small molecules. These new porous crystalline complex metal oxides showed high catalytic performance for alkane oxidation, aldehyde oxidation, alcohol oxidation, H2O2 oxidation, NH3-SCR, and so on.

Unit Synthesis Approach for Creating High Dimensionally Structured Complex Metal Oxides as Catalysts for Selective Oxidations

Solid-state catalysts for selective oxidations have been prepared by simple traditional protocols including impregnation, precipitation, solid-state reaction, and so on. Although these catalyst preparation protocols are convenient to form new or improved catalysts, a huge number of practical experiments have been inevitably necessary. This situation will be even more difficult because solid-state catalysts become increasingly complex in order to introduce multifunction. It is time that catalyst researchers should devote their efforts to establish new synthetic methodology to improve the situation and ultimately create new catalysts. Here, we propose a new catalyst synthesis approach, that is, unit synthesis, to produce high dimensionally structured crystalline complex metal oxide catalysts. By applying this synthesis methodology, we have been able to synthesize several new crystalline complex metal oxides active for catalytic selective oxidations. In this Perspective, crystalline Mo3VOx oxides and crystal...

Characterization of an Iron-Oxide-Based Catalyst Used for Catalytic Cracking of Heavy Oil with Steam

Characterization of a complex metal oxide catalyst (Fe, Zr, and Al) used for catalytic cracking of petroleum residual oil with steam was examined. Heavy oil fractions were oxidatively cracked on th...

Direct oxidative transformation of glycerol to acrylic acid over Nb-based complex metal oxide catalysts

W–V–Nb–O complex metal oxides having a structure like that of orthorhombic Mo3VOx were found to be an efficient catalyst for the gas-phase direct oxidative transformation of glycerol to acrylic acid. The catalysts with various compositions were facilely synthesized by hydrothermal method. The W–Nb–O catalyst without V component, mainly promoted the glycerol dehydration, giving acrolein selectively. Since Nb–O catalyst showed poor activity for the reaction, the introduction of W into the Nb–O catalysts was found effective for improving the activity for the dehydration of glycerol to acrolein. This is mainly due to the increase of Brönsted acidity by the combination of W and Nb. When V was incorporated into the framework of W–Nb–O, the resulting catalyst was found to prominently promote the formation of acrylic acid in the glycerol transformation in the presence of O2, while showed no effect on the dehydration of glycerol to acrolein. It was also found that V acts as an oxidation site only when glycerol is completely reacted. The optimum elemental composition for the reaction was W2.2V0.4Nb2.4O14 giving ca. 46% one path yield of acrylic acid and this catalytic performance was further improved by the surface modification with phosphoric acid, achieving ca. 60% one path yield of acrylic acid.

Investigation of Various Calcium-Based Transition Metal Oxides Compounds for the Oxygen Evolution Reaction in Alkaline Media

Electrocatalysts for oxygen evolution reaction in alkaline medium are critical for the successful realization of various kinds of aqueous metal-air rechargeable batteries as well as in the electrolysis of water. The seminal work of Trasatti, Buckris, and other researchers has unfolded the importance of metal oxides for the evolution of oxygen in alkaline medium. [1,2] Many of the state-of-the-art electrocatalysts based on metal oxides for the oxygen evolution reaction contain rare earth element as for example in the perovskites. [3] However, use of materials containing rare earth elements will be constraint for large scale deployment of systems due to its relatively high price and limited availability. Apart from rare earth element, some of the transition metals like cobalt also add significantly to the cost. Therefore, it would be very useful to maximize the activity of cobalt based complex metal oxide catalysts without the use of rare earth metals. In this work, we have explored calcium-based oxides of various complex transition metals such as cobalt and manganese for oxygen evolution reaction, with the goal of avoiding rare earth metals and minimizing the use of transition metal in the electrocatalyst.The sol-gel method using the citrate route was used to synthesize all calcium-based transition metal oxides catalysts.[3] The compounds of calcium and cobalt as well as calcium and manganese with the following formulae have been synthesized: CaCo2O4 ; Ca2Co2O5 ; Ca3Co4O9 ; Ca9Co12O28 ; CaMn2O4 ; Ca2Mn3O8 ; A simple two-step based sintering method was used to complete the synthesis of all these compounds.[3]. All catalysts samples were prepared by this method were sintered at temperatures in the range of 750 to 900 degrees Celsius. Powder X-ray diffraction method was used to identify the crystalline phases.The kinetics of oxygen evolution reaction was studied using thin film catalyst layers of the synthesized powder catalysts on a glassy carbon electrode. The steady state polarization method was used to evaluate oxygen evolution activity in 1 M potassium hydroxide solution for all the catalysts. Figure 1 shows the results of steady state current from polarization measurements for oxygen evolution experiments at 615 mV (vs. Hg/HgO 20% KOH) for all catalysts. The activity data has been normalized to the total mass of the catalyst.Fig. 1: Mass-normalized steady-state current for oxygen evolution reaction on various complex oxides of calcium, cobalt, and manganese at 615 mV (vs. reference Hg/HgO 20 % KOH) in 1M potassium hydroxide solution). The normalization is for unit mass of the transition metal.We find that the activity per milligram of cobalt varies considerably among the various types of cobalt-containing catalysts. Further, the manganese based catalysts have about one-fifth the activity of the best cobalt based catalysts. The presentation will discuss these findings in more elaborately and offer some insights for the design of new electrocatalysts. Acknowledgement The work presented here was funded by the ARPA-E Grids Program, the University of Southern California and the Loker Hydrocarbon Research Institute. References S. Trassati, J. Electroanal. Chem., 111, 125 (1980).J. O’M Bockris and T. Otagawa, J. Electrochem. Soc.,131, 290 (1984).S. Malkhandi, B. Yang, A. K. Manohar, A. Manivannan, G. K. S. Prakash and S. R. Narayanan, J. Phys. Chem. Lett., 3, 967 (2012).

Direct Oxidative Transformation of Glycerol into Acrylic Acid over Phosphoric Acid-added W–V–Nb Complex Metal Oxide Catalysts

Abstract The addition of phosphoric acid to W–Nb–O catalyst active for glycerol transformation to acrolein and to W–V–Nb–O catalyst active for direct transformation of glycerol to acrylic acid appreciably improved their catalytic performance. The phosphoric acid-added W–Nb–O catalyst gave acrolein yield of 81.8%, and the phosphoric acid-added W–V–Nb–O catalyst gave acrylic acid yield of 59.2% in the direct glycerol transformation. The improvement of the catalytic performance seems due to the increases of the acid amount and the Brønsted acidity.

Solution combustion synthesized cobalt oxide catalyst precursor for NaBH4 hydrolysis

In this paper we report the solution combustion synthesis of cobalt oxide nanofoam from solutions of cobalt nitrate and glycine and subsequent use as an effective catalyst precursor for NaBH4 hydrolysis. The catalytic activity results show that the hydrogen generation rate (HGR) at room temperature was much higher for the solution combustion synthesized material than for commercial Co3O4 nanopowder, though their specific surface areas were comparable (∼26–32 m2/g). Using a 0.6 wt.% aqueous solution of NaBH4 at 20 °C and a 5 wt.% catalyst precursor loading, a HGR of 1.93 L min−1 gcat−1 was achieved for solution combustion synthesized Co3O4. In contrast, at the same conditions, for commercial Co3O4 and elemental Co powders HGRs of 0.98 and 0.49 L min−1 gcat−1 were achieved respectively. This type of synthesis is amenable to many complex metal oxide catalysts as well, such as LiCoO2, which have also been shown to be good catalyst precursors for hydrolysis of NaBH4.

An orthorhombic Mo3VOxcatalyst most active for oxidative dehydrogenation of ethane among related complex metal oxides

Four distinct structural types (orthorhombic, trigonal, tetragonal and amorphous) of Mo3VOx catalyst were each synthesized by a hydrothermal method as a single phase, characterized structurally and tested for oxidative dehydrogenation of ethane. A common structural feature of the catalysts is that the materials are a layer-type structure and constructed with pentagonal {Mo6O21} units. The arrangement of the pentagonal units can form heptagonal channels to create different structural features. The orthorhombic Mo3VOx catalyst has microporosity due to the open heptagonal channels adsorbing nitrogen molecules and showed the highest activity for the reaction among four distinct catalysts. Furthermore, this phase appeared to be most active, currently, compared to other complex metal oxide catalysts reported. An observed positive relation between the microporosity and the oxidation activity suggests that the catalytic oxidation takes place at the heptagonal channels.

Atomic Resolution Imaging of Nanoscale Structural Ordering in a Complex Metal Oxide Catalyst

The determination of the atomic structure of a functional material is crucial to understanding its “structure-to-property” relationship (e.g., the active sites in a catalyst), which is however challenging if the structure possesses complex inhomogeneities. Here, we report an atomic structure study of an important MoVTeO complex metal oxide catalyst that is potentially useful for the industrially relevant propane-based BP/SOHIO process. We combined aberration-corrected scanning transmission electron microscopy with synchrotron powder X-ray crystallography to explore the structure at both nanoscopic and macroscopic scales. At the nanoscopic scale, this material exhibits structural and compositional order within nanosized “domains”, while the domains show disordered distribution at the macroscopic scale. We proposed that the intradomain compositional ordering and the interdomain electric dipolar interaction synergistically induce the displacement of Te atoms in the Mo–V–O channels, which determines the geometry of the multifunctional metal oxo-active sites.

Direct Observation of Surface Reconstruction and Termination on a Complex Metal Oxide Catalyst by Electron Microscopy

This research was supported by the baseline research fund from King Abdullah University of Science and Technology.

Direct Observation of Surface Reconstruction and Termination on a Complex Metal Oxide Catalyst by Electron Microscopy

Es liegt an der Oberfläche: Die Oberflächenumstrukturierung eines MoVTeO-Mischoxidkatalysators wurde mit einer Reihe von Elektronenmikroskopietechniken direkt beobachtet. Die Ergebnisse erklären das erstaunliche Verhalten in Katalysen. Detailed facts of importance to specialist readers are published as ”Supporting Information”. Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

Synthesis of porous and acidic complex metal oxide catalyst based on group 5 and 6 elements

Complex metal oxides of transition metals using Mo, W, V, Nb and Ta were synthesized by hydrothermal method. The synthesized complex metal oxides were characterized and the solid acid catalytic activity was tested through the Friedel–Crafts alkylation of anisole and benzyl alcohol. X-ray diffraction patterns of the obtained samples showed two peaks at 2θ=22° and 46° indicating these materials have layer-type crystal structure, regardless of the combination of several metals. Moreover, Mo–V–O, Mo–Nb–O, W–Nb–O and W–Ta–O showed microporosity from N2 adsorption isotherm. Among of these combinations, Mo–Nb–O, W–Nb–O and W–Ta–O showed a higher catalytic performance for alkylation reaction.

Creation of Oxidation Catalysts Based on Molybdenum Oxides with Structural Variety

Complex metal oxide catalysts active for gas-phase selective oxidations of olefins and alkanes are usually constructed with molybdenum oxide as a main and key component with various metal elements. Two catalytic systems, multicomponent Mo-Bi oxide catalysts for propylene selective oxidation and crystalline Mo-V oxide-based catalysts for alkane selective oxidation have been described in details in terms of catalytic component collaboration sustained by high dimensional catalyst structures.

High-temperature sulfuric acid decomposition over complex metal oxide catalysts

Abstract Activity and stability of FeTiO 3 , MnTiO 3 , NiFe 2 O 4 , CuFe 2 O 4 , NiCr 2 O 4 , 2CuO·Cr 2 O 3 , CuO and Fe 2 O 3 for the atmospheric decomposition of concentrated sulfuric acid in sulfur-based thermochemical water splitting cycles are presented. Catalyst activity was determined at temperatures from 725 to 900 °C. Catalytic stability was examined at 850 °C for up to 1 week of continuous operation. The results were compared to a 1.0 wt% Pt/TiO 2 catalyst. Surface area by nitrogen physisorption, X-ray diffraction analyses, and temperature programmed desorption and oxidation were used to characterize fresh and spent catalyst samples. Over the temperature range, the catalyst activity of the complex oxides followed the general trend: 2CuO·Cr 2 O 3 > CuFe 2 O 4 > NiCr 2 O 4 ≈ NiFe 2 O 4 > MnTiO 3 ≈ FeTiO 3 . At temperatures less than 800 °C, the 1.0 wt% Pt/TiO 2 catalyst had higher activity than the complex oxides, but at temperatures above 850 °C, the 2CuO·Cr 2 O 3 and CuFe 2 O 4 samples had the highest activity. Surface area was found to decrease for all of the metal oxides after exposure to reaction conditions. In addition, the two complex metal oxides that contained chromium were not stable in the reaction environment; both leached chromium into the acid stream and decomposed into their individual oxides. The FeTiO 3 sample also produced a discoloration of the reactor due to minor leaching and converted to Fe 2 TiO 5 . Fe 2 O 3 , MnTiO 3 and NiFe 2 O 4 were relatively stable in the reaction environment. In addition, CuFe 2 O 4 catalyst appeared relatively promising due to its high activity and lack of any leaching issues; however it deactivated in week-long stability experiments. Complex metal oxides may provide an attractive alternative to platinum-based catalyst for the decomposition of sulfuric acid; however, the materials examined in this study all displayed shortcomings including material sintering, phase changes, low activity at moderated temperatures due to sulfate formation, and decomposition to their individual oxides. More effort is needed in this area to discover metal oxide materials that are less expensive, more active and more stable than platinum catalysts.