Oxide Solid Solution Catalysts Research Articles

Difference in reaction mechanism between ZnZrOx and InZrOx for CO2 hydrogenation.

Oxide solid-solution catalysts, such as Zn-doped ZrO2 (ZnZrOx) and In-doped ZrO2 (InZrOx), exhibit distinctive catalytic capabilities for CH3OH synthesis via CO2 hydrogenation. We investigated the active site structures of these catalysts and their associated reaction mechanisms using both experimental and computational approaches. Electron microscopy and X-ray absorption spectroscopy reveal that the primary active sites are isolated cations, such as Zn2+ and In3+, dissolved in tetragonal ZrO2. Notably, for Zn2+, decomposition of the methoxy group, which is an essential intermediate in CH4 synthesis, is partially suppressed because of the relatively high stability of the methoxy group. Conversely, the methyl group strongly adsorbs on In3+, facilitating the conversion of the methoxy species into methyl groups. The decomposition of CH3OH is also suggested to contribute to CH4 synthesis. These results highlight the generation of CH4 as a byproduct of the InZrOx catalyst. Understanding the active site structure and elucidating the reaction mechanism at the atomic level are anticipated to contribute significantly to the future development of oxide solid-solution catalysts.

Temperature Dependence of Oxygen Evolution Reaction Activity in Alkaline Solution at Ni–Co Oxide Catalysts with Amorphous/Crystalline Surfaces

The development of high-performance oxygen evolution reaction (OER) catalysts based on earth-abundant transition metal oxides as alternatives to precious metal oxides is being pursued intensively for the large-scale application of alkaline water electrolysis for hydrogen production. Active OER electrocatalysts can be designed on the basis of fundamental insights into the catalytic phenomena occurring on surfaces. In particular, the temperature dependence of OER activity as a measure of reaction energies/barriers needs to be characterized by taking into account the operating temperature (60–80 °C) of a practical water electrolyzer. In this work, we synthesized Ni–Co oxide solid solution catalysts by the flame pyrolysis method, with particular emphasis on the temperature dependence of the OER catalysis, as a way to elucidate the catalytic reactions, from both experimental and theoretical perspectives. The as-prepared catalysts structured with conductive Ni–Co alloys were heat-treated at different temperatures in the presence of O2 to regulate the crystallinity of the oxide surfaces. The OER activity has been evaluated from 20 to 80 °C in 1 M KOH, and the OER kinetic current density was found to increase with increasing temperature for all catalysts obtained. Interestingly, the Ni–Co/oxide materials with amorphous or poorly crystalline oxide surfaces showed superior activity (3–8 times higher surface-specific activity at 1.55 V vs reversible hydrogen electrode (RHE)) than their crystalline counterparts at all temperatures. The OER activation energy also decreased with decrease of the crystalline oxide phases. It was demonstrated that the catalyst with amorphous Ni–Co oxide surfaces could be readily activated to form active oxyhydroxides under alkaline OER conditions. The amorphous or disordered structure of the surface oxide with low-coordinated sites is proposed to render the surface geometry energetically favorable for the rate-determining step O + OH → OOH, which accelerates the reaction kinetics and thus improves the OER activity, as explained by density functional theory calculations.

Highly dispersed metal doping to ZnZr oxide catalyst for CO2 hydrogenation to methanol: Insight into hydrogen spillover

Metallic oxide based catalysts have received much attention in CO2 hydrogenation to methanol because of their high methanol selectivity and long-term stability. But the relatively weak hydrogenation ability leads to low methanol activity. Here, we introduce a small amount of Cu (molar fraction < 2%) into ZnZrOx to form ternary metal oxide solid solution catalysts (x% CuZnZr) to promote methanol production. The optimized 0.5% CuZnZr catalyst performs CO2 conversion of 9.5% and methanol yield of 7.2%, both of which are about 4 times that of ZnZr catalyst at comparable methanol selectivity (76% versus 80%) at 290 °C. Chemisorption and in situ diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS) analyses reveal that the enhanced activity is attributed to the improved hydrogenation capacity and accelerated transformation of HCOO* to CH3O* caused by hydrogen spillover from well dispersed Cu. This can also be applied to trace amount of Pd or Pt doping to ZnZrOx.

Catalytic Hydrogenation of Acetic Acid to Acetaldehyde: Synergistic Effect of Bifunctional Co/Ce‐Fe Oxide Solid Solution Catalysts

Summary of main observation and conclusionThe large‐scale industrial production of acetic acid (HAc) from carbonylation of methanol has enabled intense research interest from direct hydrogenation of HAc to acetaldehyde (AA). Herein, a series of cerium‐iron oxide solid solution supported metallic cobalt catalysts were prepared by modified sol‐gel method and were applied in gas‐phase hydrogenation of HAc to AA. A synergistic effect between the hydrogenation metal cobalt and Ce‐Fe oxide solid solution is revealed. Specifically, oxygen vacancies provide the active sites for adsorption of HAc, while highly uniformly dispersed metallic Co adsorbs H2 and activates the reduction of HAc into AA. Moreover, the metallic Co can also assist the cyclical conversion between Fe3+/Fe2+ and Ce3+/Ce4+ on the surface of Ce1‐xFexO2‐δ supports. The unique effect substantially enhances the ability of the support material to rapidly capture oxygen atoms from HAc. It is found that the catalyst of 5% Co/Ce0.8Fe0.2O2‐δ with the highest concentration of oxygen vacancy presents the best catalytic performance (i.e. acetaldehyde yield reaches 49.9%) under the optimal reaction conditions (i.e. 623 K and H2 flow rate = 10 mL/min). This work indicates that the Co/Ce‐Fe oxide solid solution catalyst can be potentially used for the selective hydrogenation from HAc to AA. The synergy between the metallic Co and Ce1‐xFexO2‐δ revealed can be extended to the design of other composite catalysts.

Catalytic combustion of volatile aromatic compounds over CuO-CeO2 catalyst

Ce1−x Cu x O2 oxide solid solution catalysts with different Ce/Cu mole ratios were synthesized by the one-pot complex method. The prepared Ce1−x Cu x O2 catalysts were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and H2 temperature-programmed reduction (H2-TPR). Their catalytic properties were also investigated by catalytic combustion of phenyl volatile organic compounds (PVOCs: benzene, toluene, xylene, and ethylbenzene) in air. XRD analysis confirmed that the CuO species can fully dissolve into the CeO2 lattice to form CeCu oxide solid solutions. XPS and H2-TPR results indicated that the prepared Ce1−x Cu x O2 catalysts contain abundant reactive oxygen species and superior reducibility. Furthermore, the physicochemical properties of the prepared Ce1−x Cu x O2 catalysts are affected by the Ce/Cu mole ratio. The CeCu3 catalyst with Ce/Cu mole ratio of 3.0 contains abundant reactive oxygen species and exhibits superior catalytic combustion activity of PVOCs. Moreover, the ignitability of PVOCs is also affected by the respective physicochemical properties. The catalytic combustion conversions of ethylbenzene, xylene, toluene, and benzene are 99%, 98.9%, 94.3%, and 62.8% at 205, 220, 225, and 225 °C, respectively.

Partial Oxidation of Methane over Ni/CeZrO Mixed Oxide Solid Solution Catalysts

In this study, nickel catalysts over the CeO 2 -ZrO 2 solid solution were prepared by the co-precipitation method for partial oxidation of methane. The structures of the catalysts were systematically examined by N 2 adsorption, X-ray diffraction (XRD) and H 2 -TPR techniques. The catalytic performance was investigated for partial oxidation of methane as well. The results showed that the Ni/CeO 2 -ZrO 2 catalysts had a large BET area. The Ni/CeO 2 -ZrO 2 catalysts showed high activity and 5%Ni/Ce 0.25 Zr 0.75 O 2 was reported to exhibit the highest BET surface area, activity, stability, and H 2 and CO selectivity for partial oxidation of methane.

XRD and XPS Study of Cu−Ni Interactions on Reduced Copper−Nickel−Aluminum Oxide Solid Solution Catalysts

Copper−nickel−aluminum oxide solid solutions were reduced in hydrogen to produce alumina-supported copper−nickel alloy catalysts. XRD patterns of reduced oxides showed that the type of active metals which emerged upon reduction were sensitive to the reduction temperature and the copper content. Variations from +0.8 to +1 eV were found in the experimental Ni 2p3/2 binding energy (BE) of nickel in the solid solutions compared to the experimental Ni 2p3/2 BE of bulk nickel, attributed to the Ni−Ni arrangements in the solid solution. Also, when the curve-fitted BE values of Ni 2p3/2 and Cu 2p3/2 of different reduced solid solutions were compared, it was found that copper and nickel were in different chemical states depending on reduction temperature and the amount of the copper. Changes in Ni 2p3/2 BE in the reduced solid solutions were also discussed in terms of the filling of nickel d-hole bands due to nickel−copper d−d band interactions. At both high copper content and reduction temperature, copper had a tendency to segregate in order to reduce the interfacial surface energies. This was postulated based on the observation that the Ni 2p3/2 curve-fitted BE values were restored to somewhat close to bulk nickel at a copper content of 11 mol % and reduction temperature of 700 °C.

Oxidation of Methane over Nb-doped Ce0.75Zr0.25O2 Mixed Oxide Solid Solution Catalysts

In this study, Ce0.75Zr0.25O2 mixed oxide catalysts and with 1, 5 and 10 wt% Nb-doped were prepared via urea hydrolysis and tested for methane oxidation. The XRD patterns of Ce0.75Zr0.25O2 mixed oxide catalysts and Nb-doped indicate a cubic fluorite structure. The evidence for extra peaks due to non-incorporated Nb was not observed in any XRD patterns of Nb-doped Ce0.75Zr0.25O2. The Raman spectra of niobium oxide surface species were found in 1 wt% Nb loading. The incorporation of 1 wt% Nb was found to promote the reduction of Ce0.75Zr0.25O2 mixed oxide whereas increasing the amount of Nb (5 and 10 wt%) retarded surface reduction. The methane oxidation activity of the mixed oxides was apparently dependent on Nb loading, which relates to the degree of reducibility. In general the catalytic activity decreased with increasing Nb content (above 1 wt%). Nevertheless, it was found that 1 wt% Nb loading promotes the catalytic activity of Ce0.75Zr0.25O2 mixed oxides.

Methane partial oxidation over Ni/CeO 2–ZrO 2 mixed oxide solid solution catalysts

In this study, methane partial oxidation (MPO) to synthesis gas over Ni/Ce 1− x Zr x O 2 ( x=0, 0.25, and 1.0 with varying Ni loading of 5, 10 and 15 wt.%) was investigated over the temperature range of 400–800 °C. The experimental results showed that the catalysts prepared by impregnation method are more active than those prepared by gel impregnation method because of their higher degrees of metal dispersion and reducibility. Under the reaction conditions, MPO over Ni/CeO 2 and Ni/Ce 0.75Zr 0.25O 2 mixed oxide catalysts were active at temperatures above 550 °C whereas that of Ni/ZrO 2 took place at temperatures above 650 °C. The H 2/CO molar ratio of 2.0±0.05 was obtained. Generally, the CH 4 conversion slightly increased while the CO and H 2 selectivities remained unchanged with increasing Ni loading. The Ni/Ce 0.75Zr 0.25O 2 mixed oxide catalysts were found to resist to coke formation more than the other catalysts due to their high degrees of metal dispersion and surface oxygen mobility. The TPO results indicated that a major source of carbon deposition on these catalysts is due to the methane decomposition. Results also showed that the activity and selectivity of the catalysts can be regained after the regeneration under oxidizing atmosphere.

Catalytic oxidation of methane over CeO 2-ZrO 2 mixed oxide solid solution catalysts prepared via urea hydrolysis

In this study, CeO 2-ZrO 2 mixed oxide catalysts were prepared via urea hydrolysis and tested for methane oxidation. Highly uniform solid solution particles of ceria-zirconia were obtained under the conditions of this study. The incorporation of Zr into the CeO 2 lattice was found to promote the redox properties. The methane oxidation activity of the mixed oxides was found to be dependent on the Ce:Zr ratio, which relates to the degree of reducibility. It was postulated that the cubic phase, fluorite structure, which is mainly found in Ce 1 − x Zr x O 2 (where x<0.5) can be reduced more easily than the tetragonal phase found in Ce 1 − x Zr x O 2 (where x>0.5). The catalytic activity decreased with an increasing Zr content. The mixed oxide catalyst, Ce 0.75Zr 0.25O 2 solid solution, was reported to exhibit the highest activity for methane oxidation. Kinetic studies of methane oxidation over such a mixed oxide catalyst (Ce 0.75Zr 0.25O 2) showed that the methane oxidation rates strongly depend on methane concentration, but only slightly on the oxygen concentrations. The Langmuir–Hinshelwood mechanism (oxygen dissociative chemisorption on the active sites and non-dissociative chemisorption of methane) can satisfactorily fit the experimental results obtained from the kinetic studies for this catalyst. The activation energy of methane oxidation is calculated based on this surface reaction mechanism as being 100.8 kJ/mol.

Catalytic Combustion of Methane over Cobalt–Magnesium Oxide Solid Solution Catalysts

A series of cobalt–magnesium oxide solid solution catalysts (CoMgO) have been prepared using urea combustion methods, and characterised by X-ray diffraction (XRD) and laser Raman (LR). The catalytic activities for methane combustion have been tested in a continuous-flow microreactor. The Co content has a significant effect on the activity of the cobalt–magnesium oxide solid solution catalysts. The catalysts containing 5 and 10% Co have the lowest light-off temperature in methane combustion. In the preparation of cobalt–magnesium oxide solid solution catalysts, higher urea to metal ratio favors the formation of the catalysts with smaller crystal particles and leads to a better catalytic performance for methane combustion. Addition of lanthanum nitrate to the solution of Co and Mg nitrate depressed the formation of the cobalt–magnesium oxide solid solution and decreased the activity of the catalysts for methane combustion. The cobalt–magnesium oxide solid solution catalysts are very stable when the calcination or reaction temperature is no more than 900°C. However, the catalytic activity decreases rapidly after high temperature (>1000°C) calcination, possibly due to sintering of the catalyst and thus decrease of the surface area.

Methane Coupling Reaction on Oxide Solid Solution Catalysts

The methane coupling reaction to form C 2 hydrocarbons has been studied over perovskite-type oxides La 1− x Sr x B 1− y M y O 3−δ( B = Co, Cr, Fe, Mn, Y; M Nb, Ti), K 2NiF 4-type superconducting oxides A 2− x B x CuO 4−δ( A = La, Nd; B = Sr, Ce), and the solid solution La 0.9Sr 0.1O 1.45. The reaction was performed in a fixed bed flow reactor with a feed containing 30% methane, 5% oxygen, and balance helium at 795°C. The results indicated that the superconducting oxides and the perovskites with high p-type electronic conductivity were not active for the methane conversion to C 2 hydrocarbons. The compounds with n-type conductivity, electrolytes, and insulators were all found to proceed with selective oxidation to C 2 compounds. A combination of an insulator or electrolyte associated with increased oxygen vacancy concentration appears to increase the C 2 yield. Electrical conductivity, oxygen diffusion coefficient, and oxygen vacancy concentrations of the materials were considered to explain the observed variations in the methane coupling reactivity.

Carbon Monoxide and Methane Oxidation Properties of Oxide Solid Solution Catalysts

The CO and CH4 deep oxidation properties of the rare-earth-based perovskites, La1−xSrxM1−yByO3−δ (M = Co, Cr, Fe, Mn, Y; B = Nb, Ti) and the superconducting systems with the K2NiF4 structure, La2−xSrxCuO4−δ and Nd2−xCexCuO4−δ were investigated. For CO oxidation p-type conducting materials were found to exhibit the highest catalytic activity. For CH4 oxidation, the activity appears to depend on small electronic conductivity both for p-type and n-type conductors combined with a good oxide ion conduction.

An interpretation of the activity behavior of model oxide solid solution catalysts on the basis of a solid-state polaron hopping mechanism

The catalytic activity of doped oxide systems α-Al 2−xCr x O 3, MgAl 2−xCr x O 4, Mg 1−xCu x Al 2O 2, Mg 1−xNi x Al 2O 4, Mg 1−xNi x O, and Mg 1−xCo x O for reactions such as N 2O decomposition, CO oxidation, H 2-D 2 exchange, the decomposition of isopropanol, and the oxychlorination of ethylene is not a monotonie function of doping but appears to follow a regular pattern. On doping with transition metal cations from zero up to 10 to 15% (dependent on the host solid) the catalytic activity increases, then drops and thereafter increases again. This pattern of activity is explained by using the concept of polaron formation around the impurity center in the host solid. It is suggested that in the region of 10 to 15% doping each polaron will have at least one polaron nearest neighbor, which will greatly facilitate the electron movement through the solid. This in turn restricts the desorption step for reasons connected with the Pauli principle.