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.

Carbon nanotubes (CNTs) with three different inner diameters (2–5 nm, 5–10 nm and 20–50 nm) were employed as supports to prepare Cu-based catalysts for oxidative carbonylation of methanol to dimethyl carbonate (DMC). The catalysts were extensively characterized by N₂-physisorption, TEM, H₂-TPR, XRD, TPD-MS, in situ-XRD, XPS, N₂O chemisorption and AAS. The results indicated that CNTs with a small inner diameter were favorable for the entry of Cu species into the cavity and the formation of highly dispersed Cu particles. The highly dispersed Cu species presented strong interaction with CNTs, which facilitated the auto-reduction of CuO to active Cu₂O and Cu during the calcination. The dispersion and percentage of active Cu species for Cu/CNT (2–5) were maximum among all the catalysts, and hence it presented the optimal initial activity for oxidative carbonylation of methanol. The conversion of methanol and the space time yield of DMC were 4.2% and 2.2 g g⁻¹ h⁻¹), respectively. Moreover, the stability of Cu species was closely related to their particle size, the inner diameter of CNTs and the interaction between Cu species and CNTs. The Cu species of Cu/CNT (20–50) seriously aggregated after the reaction due to the weak interaction between Cu species and CNT (20–50) as well as the insufficient confinement effect of CNT (20–50). CNT (2–5) presented the strongest interaction with Cu species; however, it cannot effectively retard the migration and agglomeration of Cu species resulting from the extremely high surface energy of these small particles (1–3 nm). Unlike Cu/CNT (2–5) and Cu/CNT (20–50), the particles of Cu species for Cu/CNT (5–10) were more stable owing to the modest particle size and the moderate interaction between Cu species and CNT (5–10). Furthermore, the enhanced interaction between Cu species and CNTs can effectively suppress the oxidation of Cu species during the reaction. The deactivation of Cu/CNT (20–50) was due to the agglomeration and oxidation of Cu species, and the deactivation of Cu/CNT (2–5) was due to the agglomeration of Cu species, whereas that of Cu/CNT (5–10) was due to the oxidation of Cu species. Cu/CNT (5–10) exhibited the optimal stability.

In the mixture of coprecipitated precursors of Cu/ZnO/Al₂O₃ catalysts, only the fraction of Al³⁺ ions incorporated into a single-phase zinc malachite actually contributes to the dilution of Cu²⁺ and is responsible for the promotional action. However, the preparation of promoter ion doped phase pure zinc malachite is difficult via coprecipitation routes and further, the effect of such promotion on the microstructure properties and activity is poorly understood. The present isomorphous substitution method delivers a controlled and efficient incorporation of group 13 metal ions, viz., Al³⁺, Ga³⁺ and In³⁺ into the pure phase malachite structure. The formation of M³⁺-doped pure phase precursors ensured maximum dilution of Cu²⁺ ions that lead to a compelling improvement in the microstructure properties of the calcined catalysts. As compared to the undoped catalyst, the Al³⁺ and Ga³⁺-doped catalysts exhibited an ∼8 and ∼5.4-fold increase in the methanol formation rate, respectively. The best performing Al³⁺-doped catalyst exhibited superior microstructure properties and high (855 mg g_cat ⁻¹ h⁻¹) and stable STY_MeOH, which could be correlated with the promoter induced O vacancy sites. As a consequence of the promotion, the Cu–ZnO synergy was found to be improved resulting in the formation of a reduced ZnO_x overlayer as evidenced by XPS-AES and N₂O coupled H₂ TPD analyses. The Ga3d XPS and ²⁷Al MAS NMR results suggested that in the reduced catalysts, the major fractions of Al³⁺ and Ga³⁺ were found to be incorporated into the tetrahedral sites of the ZnO lattice and led to the contraction of unit cell volume eventually resulting in the modification of electronic properties of ZnO by inducing defects in its structure. Surprisingly, In³⁺ exhibited a detrimental effect on the catalytic performance as a result of its revoked dilution effect in the reduced catalyst owing to its reactive support–metal interaction resulting in the formation of a Cu–In bimetallic alloy.