Catalysts for chemoselective hydrogenation are of vital importance for the synthesis of various important chemicals and intermediates. Herein we developed a simple method for preparing a highly efficient Ni-MoC_x nanocomposite catalyst via temperature-programmed carburization of a polyoxometalate precursor. X-ray diffraction (XRD), scanning transmission electron microscopy (STEM), X-ray photoelectron spectroscopy (XPS), and X-ray absorption spectroscopy (XAS) analyses indicate that the resulting mesoporous nanocomposite catalyst is made up of well-dispersed metallic nickel particles embedded in a MoC_x matrix. This catalyst exhibits high activity and selectivity (> 99%) in the hydrogenation of various substituted nitroaromatics to corresponding anilines. The high efficiency is attributed to the intimate contact of the constituents favoring electron transfer and hydrogen adsorption. Dihydrogen is physisorbed on the carbide support and dissociates on the nickel particles, as evidenced by Mo K-edge X-ray absorption near-edge structure (XANES) spectra, density functional theory (DFT), and hydrogen–deuterium exchange. The remarkable catalytic performance of the catalyst could be traced back to the synergistic interaction between the Ni particles and the carbide support. In-situ infrared spectroscopy and DFT simulations indicated that the adsorption/activation of the nitro group is favored compared to that of other substituents at the aromatic ring. In recyclability tests, the Ni-MoC_x nanocomposite showed no significant loss of catalytic performance in seven consecutive runs, indicating its robust nature.

Morphological effects of nanoparticles are crucial in many solid-catalyzed chemical transformations. We herein prepared two manganese-ceria solid solutions, well-defined MnCeO_x nanorods and MnCeO_x-nanocubes, exposing preferentially (111) and (100) facets of ceria, respectively. The incorporation of Mn dopant into ceria lattice strongly enhanced the catalytic performance in the NO reduction with CO. MnCeO_x (111) catalyst outperformed MnCeO_x (100) counterpart due to its higher population density of oxygen vacancy defects. In-situ infrared spectroscopy investigations indicated that the reaction pathway over MnCeO_x and pristine CeO₂ is similar and that besides the direct pathway, an indirect pathway via adsorbed hyponitrite as an intermediate cannot be ruled out. X-ray photoelectron and Raman spectroscopies as well as first-principles density functional theory (DFT) calculations indicate that the enhanced catalytic performance of MnCeO_x can be traced back to its “Mn–O_L(VÖ)–Mn–O_L(VÖ)–Ce” connectivities. The Mn dopant strongly facilitates the formation of surface oxygen vacancies (VÖ) by liberating surface lattice oxygen (O_L) via CO* + O_L → CO₂* + VÖ and promotes the reduction of NO, according to NO* + VÖ → N* + O_L and 2N* → N₂. The Mn dopant impact on both the adsorption of CO and activation of O_L reveals that a balance between these two effects is critical for facilitating all reaction steps.

We evaluated bismuth doped cerium oxide catalysts for the continuous synthesis of dimethyl carbonate (DMC) from methanol and carbon dioxide in the absence of a dehydrating agent. Bi_xCe_1-xO_δ nanocomposites of various compositions (x = 0.06-0.24) were coated on a ceramic honeycomb and their structural and catalytic properties were examined. The incorporation of Bi species into the CeO₂ lattice facilitated controlling of the surface population of oxygen vacancies, which is shown to play a crucial role in the mechanism of this reaction and is an important parameter for the design of ceria-based catalysts. The DMC production rate of the Bi_xCe_1-xO_δ catalysts was found to be strongly enhanced with increasing O_V concentration. The concentration of oxygen vacancies exhibited a maximum for Bi_0.12Ce_0.88O_δ, which afforded the highest DMC production rate. Long-term tests showed stable activity and selectivity of this catalyst over 45 h on-stream at 140 ℃ and a gas-hourly space velocity of 2,880 mL·g_cat^-1·h^-1. In-situ modulation excitation diffuse reflection Fourier transform infrared spectroscopy and first-principle calculations indicate that the DMC synthesis occurs through reaction of a bidentate carbonate intermediate with the activated methoxy (-OCH₃) species. The activation of CO₂ to form the bidentate carbonate intermediate on the oxygen vacancy sites is identified as highest energy barrier in the reaction pathway and thus is likely the rate-determining step.