Oxide Solid Solutions as Catalysts
TL;DR: In this article, a review of the development of oxide solid solutions as catalysts from their first use in the 1960s to their current application in basic and applied research is presented.
Abstract: This review traces the development of oxide solid solutions as catalysts from their first use in the 1960s to their current application in basic and applied research. Oxide solid solutions provide the means to control the properties of catalytically active ions in defined surface environments. When applied to transition metal (TM) ions, interaction with neighbors can be suppressed or progressively developed, depending on the concentration chosen for the active solute and the structure of the insulating matrix selected as solvent oxide. Simple examples are nickel, cobalt and chromium ions in MgO and MgAl 2 O 4 . The successful preparation of solid solutions demands a knowledge of the reactivity of solids and the behavior of crystal defects. This is exemplified in the methods described for preparing solid solutions of low and high specific surface area, respectively. Characterization receives detailed attention and the methods specific to oxide solid solutions are illustrated. Emphasis is placed on quantitative determination of surface composition for which X-ray photo-electron spectroscopy is the most widely applicable technique. The acidity and basicity of oxide solid solution surfaces is linked with coordinative unsaturation and this aspect of characterization involves adsorption calorimetry and infra-red spectroscopy. The account of oxide solid solutions as catalysts is divided into two parts. The first comprises studies where solid solutions have been used as model catalysts to identify and compare the catalytic properties of individual TM ions. For this purpose the catalysis of N 2 0 decomposition, CO oxidation and H 2 D 2 equilibration have long served as prototypical test reactions. These simple reactions enable issues such as the distinctive behavior of isolated ions, pairs and chains to be addressed and matrix effects to be explored. The motivation here is detailed understanding of catalysis on highly characterized microcrystalline oxides. The second catalytic part is broader in scope and focuses to a greater extent on the application of oxide solid solutions as catalysts for reactions of industrial interest. Combustion of hydrocarbons is a high-temperature reaction for which perovskite-structured solid solution catalysts are especially attractive since they accommodate a wide range of TM and main group ions in solid solution. A second sector covered is selective oxidation of hydrocarbons. Oxide solid solutions containing TM ions made an early entry as catalysts for alkene conversion and remained when interest switched to alkanes. The solid solution approach featured strongly in the search for methane coupling catalysts and currently contributes in a new guise in titanium silicalite. The acidity developed at solute sites is the source of activity for carbenium ion catalysis. SAPOs and McAPOs fall within the solid solution domain as covalently-bonded counterparts of ionic solid solutions. Finally, reduced solid solutions with phase-separated transition metal clusters are effective catalysts for reforming of alkanes.
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TL;DR: Experimental and theoretical results indicate that the synergetic effect between Zn and Zr sites results in the excellent performance of the ZnO-ZrO2 solid solution catalyst, which can achieve methanol selectivity of up to 86 to 91% with CO2 single-pass conversion of more than 10% under reaction conditions.
Abstract: Although methanol synthesis via CO hydrogenation has been industrialized, CO2 hydrogenation to methanol still confronts great obstacles of low methanol selectivity and poor stability, particularly for supported metal catalysts under industrial conditions. We report a binary metal oxide, ZnO-ZrO2 solid solution catalyst, which can achieve methanol selectivity of up to 86 to 91% with CO2 single-pass conversion of more than 10% under reaction conditions of 5.0 MPa, 24,000 ml/(g hour), H2/CO2 = 3:1 to 4:1, 320° to 315°C. Experimental and theoretical results indicate that the synergetic effect between Zn and Zr sites results in the excellent performance. The ZnO-ZrO2 solid solution catalyst shows high stability for at least 500 hours on stream and is also resistant to sintering at higher temperatures. Moreover, no deactivation is observed in the presence of 50 ppm SO2 or H2S in the reaction stream.
563 citations
Cites background from "Oxide Solid Solutions as Catalysts"
...These facts further affirm the conclusion that ZnO-ZrO2 is in a solid solution state, with Zn incorporated into the ZrO2 lattice matrix (27)....
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TL;DR: In this article, a study on the synthesis of LaCoO3 nanocrystals using a new ultrasonic process has been carried out by comparing the formation time with the same factor under conventional coprecipitation process.
Abstract: A study on the synthesis of LaCoO3 nanocrystals using a new ultrasonic process has been carried out by comparing the formation time with the same factor under conventional coprecipitation process. The samples are characterized by Fourier transform infrared spectroscopy (FTIR), X-ray powder diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive X-ray (EDX). Magnetic measurements are carried out by a vibrating sample magnetometer (VSM) on the product at room temperature. While both powder exhibited the same XRD patterns indexed as pure perovskite structure, their homogeneity is found to be strongly influenced by the preparation method.
27 citations
Cites background from "Oxide Solid Solutions as Catalysts"
..., CO, NOx, CH4) to interact selectively with only one transition metal surface ion.([2]) As the perovskite structure is capable of incorporating a large number of atoms, the perovskites show many useful properties, such as ionic conductivity,([3]) superconductivity, giant magnetoresistance,([4]) and ferroelectricity....
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TL;DR: In this article, a photodiode based on MgCrO4-MgCu2O3/p-Si was assembled and electrically characterized in dark and illumination conditions.
Abstract: Crystalline MgCrO4-MgCu2O3 layered films prepared by the sol–gel process were deposited on glass and p-Si substrates. The films were characterized by XRD, SEM/HR-TEM, and UV–Vis optical spectroscopy. A photodiode based on MgCrO4-MgCu2O3/p-Si was assembled and electrically characterized in dark and illumination conditions. Structure and surface morphology investigations demonstrated that the MgCrO4-MgCu2O3 layered films exhibit a higher crystalline structure with a high surface area and a crystallite size of 14 nm. The layered films are opaque in the UV region and transmit light in visible and IR regions. The determined optical band gap is about 3.5 eV independent of the transition types — direct or indirect. In dark conditions, the noted values of the ideality factor, the barrier height, the series resistance of the assembled diode were 1.875, 1.097 eV and 46 . 72 k Ω , respectively. The recognized photocurrent action is driven by traps that lie inside the bandgap. These improvements in structural and optical properties can be attributed to the excellent internal rearrangement and the lower concentration of oxygen vacancy connected with Cr and Cu.
26 citations
TL;DR: In this paper, the physical and chemical properties of NiO/ZnxZr1−x catalysts were studied using FTIR to determine the adsorbed species and intermediates and the results showed that CO2 methanation follows the formate pathway.
Abstract: The NiO/ZnxZr1−x (x represents the molar mass of Zn) catalyst was prepared by the impregnation method and tested in CO2 methanation. The activity results show that NiO/Zn0.3Zr0.7 has a higher CO2 conversion rate and methane selectivity than NiO/ZnO and NiO/ZnO–ZrO2. Combined with N2 adsorption–desorption, H2-TPR, CO2-TPD, H2-TPD, XRD, TEM, XPS and FTIR and other characterization methods, the physical and chemical properties of NiO/ZnO–ZrO2 were studied. The incorporation of ZnO into NiO/ZrO2 forms a ZnO–ZrO2 solid solution, and the combination of the solid solution weakens the interaction between NiO and the oxide support, thereby promoting the reduction and dispersion of NiO. The H2-TPR experiment results show that, because ZnO–ZrO2 forms a solid solution, NiO is better dispersed on the surface, resulting in a significant reduction in the reduction temperature of NiO. Using FTIR to conduct CO2 adsorption and methanation experiments on NiO/ZnxZr1−x to determine the adsorbed species and intermediates, the results show that CO2 methanation follows the formate pathway.
5 citations
TL;DR: In this article, the authors describe the successful development of a catalyst for the decomposition of nitrous oxide in the ammonia burner, from laboratory, pilot and plant-scale testing.
Abstract: Nitrous oxide is a powerful greenhouse gas with a global warming potential stated to be between 265 and 310. The production of nitric acid is the largest source of nitrous oxide from the chemical process industries, and it equates to circa 50% of the total greenhouse gas emissions from nitric acid production. This paper describes the successful development of a catalyst for the decomposition of nitrous oxide in the ammonia burner, from laboratory, pilot and plant-scale testing. This catalyst is capable of reducing nitrous oxide emissions by more than 90%, with no significant modifications to plant operation.
4 citations
Cites background from "Oxide Solid Solutions as Catalysts"
...Frank Stone, along with co-author Alessandro Cimino included N2O decomposition reactions on oxide surfaces in their extensive review of Oxide Solid Solutions As Catalysts [13]....
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