Ternary Ni-Ce-Mg-O Composites: In-Depth Optical Spectroscopy Study and Catalytic Performance in CO Oxidation

In the present work, ternary Ni-Ce-Mg-O composites containing various amounts of NiO and CeO2 were synthesized via a sol-gel approach. Aqueous solutions of cerium and nickel nitrates were introduced at the stage of hydrolysis of magnesium methoxide, which allowed for avoiding the use of expensive organic precursors. It was revealed that the properties of the composites were defined by the complex interactions between NiO, CeO2, and MgO components. In order to perform an in-depth characterization of the prepared samples, diffuse reflectance UV–vis and Raman spectroscopies were applied. According to the results of these methods, Mg2+ ions did not substitute Ce4+ ions in the CeO2 lattice. However, in the case of the Ni-containing samples, approximately 2–3% of the Ce4+ ions were substituted by Ni2+, thus resulting in the formation of vacancies in the CeO2. The strong interaction of NiO with MgO predictably resulted in the formation of NixMg1−xO solid solutions. When the NiO content in the sample was 20 wt%, the composition of the formed solid solution was estimated to be Ni0.60Mg0.40O. In addition, the presence of CeO2 affected the texture of the ternary composites, thus leading to a slight decrease in the specific surface area. The catalytic performance of the Ni-Ce-Mg-O composites was examined in the CO oxidation reaction under prompt thermal aging conditions. The choice of reaction conditions was due to a high sensitivity of the CO oxidation response toward the available metal surface area and possible metal-support interactions

Sol-Gel Synthesis and Characterization of the Cu-Mg-O System for Chemical Looping Application

A sol-gel technique was applied to prepare the two-component oxide system Cu-Mg-O, where MgO plays the role of oxide matrix, and CuO is an active chemical looping component. The prepared samples were characterized by scanning electron microscopy, low-temperature nitrogen adsorption, and X-ray diffraction analysis. The reduction behavior of the Cu-Mg-O system was examined in nine consecutive reduction/oxidation cycles. The presence of the MgO matrix was shown to affect the ability of CuO towards reduction and re-oxidation significantly. During the first reduction/oxidation cycle, the main characteristics of the oxide system (particle size, crystallization degree, etc.) undergo noticeable changes. Starting from the third cycle, the system exhibits a stable operation, providing the uptake of similar hydrogen amounts within the same temperature range. Based on the obtained results, the two-component Cu-Mg-O system can be considered as a prospective chemical looping agent

Preparation of the Nanostructured Ni-Mg-O Oxide System by a Sol–Gel Technique at Varied pH

In the present work, a series of two-component Ni-Mg-O oxide systems were prepared using a sol–gel technique at varied pH of hydrolysis procedure. The aqueous solutions of nitric acid or ammonia were added to control the pH values. The xerogel samples obtained after drying were analysed using a thermogravimetric approach. The oxide systems were characterized by a set of physicochemical methods (low-temperature nitrogen adsorption, X-ray diffraction analysis, scanning electron microscopy, UV-vis spectroscopy, and temperature-programmed reduction method). The thermal stability of the samples was examined in a testing reaction of CO oxidation in a prompt thermal aging regime. It was revealed that the pH value during the magnesium methoxide hydrolysis stage significantly affects the properties of the intermediate hydroxide and final oxide nanomaterials. The thermal decomposition of nitric acid or ammonia is accompanied by exothermal effects, which noticeably influence the textural characteristics. Moreover, the pH of the hydrolysing solution defines the strength of the nickel interaction with the MgO matrix. An increase in pH facilitates the formation of the NixMg1−xO solid solution with a higher amount of incorporated nickel, which is characterized by the reproducible broad temperature range of the hydrogen uptake and the enhanced thermal stability

Exploration of Optical, Redox, and Catalytic Properties of Vanadia-Mayenite Nanocomposites

The present paper continues the exploration of the physicochemical and catalytic properties of vanadia-mayenite composites. The samples were prepared by an impregnation of calcium aluminate Ca12Al14O33 (mayenite, C12A7) with a solution of vanadium precursor. Pure mayenite and V/C12A7 nanocomposites were characterized by Raman and diffuse reflectance UV–Vis spectroscopies. The reducibility of the samples was examined in temperature-programmed reduction experiments performed in a hydrogen atmosphere. The catalytic performance of vanadium-containing systems was studied in the non-oxidative dehydrogenation of ethane. As found, the low-loaded sample (5%V/C12A7 sample) contains vanadium predominantly in the form of Ca3(VO4)2, while for the 10%V/C12A7 sample, two types of calcium vanadates (Ca2V2O7 and Ca3(VO4)2) are registered. The presence of these phases defines the spectroscopic characteristics and the redox properties of nanocomposites. Both the samples, 5%V/C12A7 and 10%V/C12A7, exhibit comparable catalytic activity, which is mainly connected with the amount of the Ca3(VO4)2 phase. The uniqueness of the studied catalysts is their excellent tolerance toward coke formation under the reaction conditions

Synthesis of Ni-Cu-CNF Composite Materials via Carbon Erosion of Ni-Cu Bulk Alloys Prepared by Mechanochemical Alloying

The unique physical and chemical properties of composite materials based on carbon nanofibers (CNFs) makes them attractive to scientists and manufacturers. One promising method to produce CNFs is catalytic chemical vapor deposition (CCVD). In the present work, a method based on carbon erosion (CE) of bulk microdispersed Ni-Cu alloys has been proposed to prepare efficient catalysts for the synthesis of CNF-based composites. The initial Ni-Cu alloys were obtained by mechanochemical alloying (MCA) of metallic powders in a planetary mill. The effect of MCA duration on the phase composition of Ni-Cu samples was studied by X-ray diffraction analysis and temperature-programmed reduction in hydrogen. It has been also revealed that, during such stages as heating, reduction, and short-term exposure to the reaction mixture (C2H4/H2/Ar) at 550 °C, the formation of a Ni-based solid solution from the initial Ni-Cu alloys takes place. The early stages of the CE process were monitored by transmission electron microscopy combined with energy-dispersive X-Ray analysis. It was found that the composition of the catalytic particles is identical to that of the initial alloy. The morphological and structural features of the prepared Ni-Cu-CNF composites were studied by scanning and transmission electron microscopies. The textural characteristics of the composites were found to be dependent on the reaction time

High-Temperature Behavior of Laser Electrodispersion-Prepared Pd/ZSM-5 Hydrocarbon Traps under CO Oxidation Conditions

Zeolites and metal-doped zeolites are now widely considered as low-temperature hydrocarbon traps to be a part of emission control systems in automobiles. However, due to the high temperature of exhaust gases, the thermal stability of such sorbent materials is of great concern. To avoid the thermal instability problem, in the present work, laser electrodispersion was used to deposit Pd particles on the surface of ZSM-5 zeolite grains (SiO2/Al2O3 = 55 and SiO2/Al2O3 = 30) to obtain Pd/ZSM-5 materials with a Pd loading as low as 0.03 wt.%. The thermal stability was evaluated in a prompt thermal aging regime involving thermal treatment at temperatures up to 1000 °C in a real reaction mixture (CO, hydrocarbons, NO, an excess of O2, and balance N2) and a model mixture of the same composition with the exception of hydrocarbons. Low-temperature nitrogen adsorption and X-ray diffraction analysis were used to examine the stability of the zeolite framework. Special attention was paid to the state of Pd after thermal aging at varied temperatures. By means of transmission electron microscopy, X-ray photoelectron spectroscopy, and diffuse reflectance UV–Vis spectroscopy, it was shown that palladium, having been initially located on the surface of zeolite, undergoes oxidation and migrates into the zeolite’s channels. This enhances the trapping of hydrocarbons and their subsequent oxidation at lower temperatures