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We report the results from the first 5D tomographic diffraction imaging experiment of a complex Ni–Pd/CeO2–ZrO2/Al2O3 catalyst used for methane reforming. This five-dimensional (three spatial, one scattering and one dimension to denote time/imposed state) approach enabled us to track the chemical evolution of many particles across the catalyst bed and relate these changes to the gas environment that the particles experience. Rietveld analysis of some 2 × 106 diffraction patterns allowed us to extract heterogeneities in the catalyst from the Å to the nm and to the μm scale (3D maps corresponding to unit cell lattice parameters, crystallite sizes and phase distribution maps respectively) under different chemical environments. We are able to capture the evolution of the Ni-containing species and gain a more complete insight into the multiple roles of the CeO2-ZrO2 promoters and the reasons behind the partial deactivation of the catalyst during partial oxidation of methane.

Methane dehydroaromatization was studied over a series of impregnated Mo/ZSM-5 catalysts with different molybdenum contents and Si/Al atomic ratios in the parent H-ZSM-5 zeolites. The maximum catalytic activity (∼14% CH4 conversion) and benzene formation selectivity (∼70%) were observed for the samples with 2%−5% molybdenum. The activity and selectivity are improved when the Si/Al ratio is decreased from 45 to 17. After pretreatment in argon and reaction at 720 °C, the catalysts have been characterized by textural methods, X-ray diffractometry (XRD), differential thermal analysis (DTA), high-resolution transmission electron microscopy (HRTEM), and energy-dispersive X-ray spectroscopy (EDX). As determined by HRTEM, after the pretreatment, the MoO x phase is highly dispersed on the external surface of the zeolite. During the reaction, molybdenum carbide nanoparticles 2−15 nm in size are formed on the external surface, and the molybdenum-containing clusters are deposited in the zeolite channels. The carbonaceous deposits (CDs) are formed as graphite layers on the surface of Mo2C nanoparticles that were >2 nm in size, and as friable layers with a disordered structure on the external surface of the zeolite. According to EDX, XRD, and DTA studies, the content of the CD and the extent of their condensation (the C/H ratio) increase with the time-on-stream. For all the studied molybdenum contents (1%−10%) and time-on-stream values (0.5−6 h), the CDs formed on the catalysts with Si/Al = 17 are characterized by one maximum of the exothermic burn-out effect in DTA, whereas on the catalysts with Si/Al = 30 and 45, they are characterized by two such maxima. A correlation between the catalyst activity, the selectivity versus nanostructure, and the location of the molybdenum phases and CDs is discussed.