Effect of calcination atmospheres on the catalytic performance of nano-CeO2 in direct synthesis of DMC from methanol and CO2

Polarized Active Pairs at Grain Boundary Boost CO2 Chemical Fixation

The chemical fixation of CO2 as a C1 feedstock is considered one of the most promising ways to obtain long-chain chemicals, but its efficiency was limited by the ineffective activation of CO2. Herein, we propose a grain boundary engineering strategy to construct polarized active pairs with electron poor-rich character for effective CO2 activation. By taking CeO2 as a model system, we illustrate that the polarized "Ce4+-Ce3+-Ce4+" pairs at the grain boundary can simultaneously accept and donate electrons to coordinate with O and C, respectively, in CO2. By the combination of synchrotron radiation in situ technique and density functional theory calculations, the mechanism of the catalytic reaction has been systematically investigated. As a result, the CeO2 nanosheets with a rich grain boundary show a high DMC yield of 60.3 mmol/gcat with 100% atomic economy. This study provides a practical way for the chemical fixation of CO2 to high-value-added chemicals via grain boundary engineering.

Elucidating the Role of Surface Ce4+ and Oxygen Vacancies of CeO2 in the Direct Synthesis of Dimethyl Carbonate from CO2 and Methanol

Cerium dioxide (CeO₂) was pretreated with reduction and reoxidation under different conditions in order to elucidate the role of surface Ce⁴⁺ and oxygen vacancies in the catalytic activity for direct synthesis of dimethyl carbonate (DMC) from CO₂ and methanol. The corresponding catalysts were comprehensively characterized using N₂ physisorption, XRD, TEM, XPS, TPD, and CO₂-FTIR. The results indicated that reduction treatment promotes the conversion of Ce⁴⁺ to Ce³⁺ and improves the concentration of surface oxygen vacancies, while reoxidation treatment facilitates the conversion of Ce³⁺ to Ce⁴⁺ and decreases the concentration of surface oxygen vacancies. The catalytic activity was linear with the number of moderate acidic/basic sites. The surface Ce⁴⁺ rather than oxygen vacancies, as Lewis acid sites, promoted the adsorption of CO₂ and the formation of active bidentate carbonates. The number of moderate basic sites and the catalytic activity were positively correlated with the surface concentration of Ce⁴⁺ but negatively correlated with the surface concentration of oxygen vacancies. The surface Ce⁴⁺ and lattice oxygen were active Lewis acid and base sites respectively for CeO₂ catalyst, while surface oxygen vacancy and lattice oxygen were active Lewis acid and base sites, respectively, for metal-doped CeO₂ catalysts. This may result from the different natures of oxygen vacancies in CeO₂ and metal-doped CeO₂ catalysts.

Biochar-loaded Ce3+-enriched ultra-fine ceria nanoparticles for phosphate adsorption

Biochar-loaded Ce³⁺-enriched ultra-fine ceria nanoparticles (Ce-BC) was prepared by a facile impregnation-precipitation-pyrolysis process and applied as adsorbents to adsorb phosphate from water. The crystal size of ceria nanoparticles in the Ce-BC was as small as 2-5 nm and the concentration of Ce³⁺ was high to 59.6 %, which was benefited from the rapid precipitation, N₂ pyrolysis atmosphere and the presence of the biochar during preparation. Ce-BC exhibited a fast adsorption kinetics for phosphate and the adsorption equilibrium could be reached within 10 min. The maximum phosphate adsorption capacity was up to 77.7 mg P g^-1 at pH 3.0. Based on Fourier transform infrared (FTIR), X-ray powder diffraction (XRD), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS) analysis, Ce³⁺ of ceria was demonstrated playing the vital role on phosphate removal and the formation of CePO₄ nanocrystals was the main adsorption mechanism. This work provides a facile strategy for preparing high Ce³⁺ contenting materials and shows a great potential application for the phosphate removal for its high-effective and high stability.