Hydrogen generation via methanol steam reforming is a promising method for producing renewable energy. In this work, a series of Mn 2+ -substituted CuAl 2 O 4 spinels were prepared by solution combustion method, and the crystal structure, micromorphology, chemical constitution, reduction behavior, acidity, specific surface area and surface chemical state of the spinels were comprehensively characterized by various equipments. The obtained spinels were washcoated on Cu foams to prepare monolithic catalysts, and the catalytic performance of the catalysts was evaluated in a methanol steam reforming microreactor. Compared with the binary Cu–Al spinel, the Mn 2+ substitution led to a decrease in the particle size, a change in the chemical composition and reduction behavior, a decline in the acidity, an increase in the specific surface area, and an improvement in the surface chemical state. As a result, the release rate of active Cu from the Mn-containing CuAl 2 O 4 spinel was significantly slowed down and the formed nanoparticles were fine, which was believed to be in favor of maintaining a stable catalytic performance longer. Among the prepared catalysts, the monolithic catalyst loaded with Cu 0.4 Mn 0.6 Al 2 O 4 exhibited the highest activity and stability. The findings of this work suggested that introducing Mn 2+ might be a promising way to regulate the Cu releasing property for obtaining a better sustained release catalyst system.
Wolframite-structured Zn 1−x Mn x ZrNb 2 O 8 (x=0.02, 0.04, 0.06, 0.08, 0.10) ceramics were synthesized by the conventional solid-state method. The effects of Mn 2+ substitution on the crystal structure, crystal chemistry and microwave dielectric properties of ZnZrNb 2 O 8 were investigated. Bond ionicity, lattice energy and thermal expansion of the ceramics were calculated by the complex chemical bond theory, which were used to investigate the correlation between intrinsic factors and microwave dielectric properties. The ε r values were dependent on the electronic oxide polarizabilities of Zn 1−x Mn x ZrNb 2 O 8 ceramics. The Q·f values decreased from 51,300 to 44,900 GHz, which could be explained in terms of packing fraction and lattice energy. The τ f values were shifted to positive direction with the increase of Mn 2+ contents, which indicated that the distortion of the [BO6] octahedral was closely related to τ f values. In addition, thermal expansion coefficient also affected the temperature stability of ceramics. Typically, an excellent microwave dielectric property was obtained in the Zn 0.9 Mn 0.1 ZrNb 2 O 8 with a ε r of 29.43, an appropriate Q·f of 44,900 GHz and a near zero τ f of −5.61 ppm/°C.
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