The electrochemical reduction of CO2 (CO2RR) to produce valuable synthetic fuel like CH3OH not only mitigates the accumulated greenhouse gas from the environment but is also a promising direction toward attenuating our continuous reliance on fossil fuels. However, CO2RR to yield CH3OH suffers because of large overpotential, competitive H2 evolution reaction (HER), and poor product selectivity. In this regard, intermetallic alloy catalysts open up a wide possibility of fine-tuning the electronic property and attain appropriate structures that facilitate selective CO2RR. Here, we report for the first time the CO2RR over carbon-supported PtZn nano-alloys and probed the crucial role of structures and interfaces as active sites. PtZn/C, Pt3Zn/C, and Pt x Zn/C (1 < x < 3) synthesized from the metal–organic framework material were characterized structurally and morphologically. The catalysts demonstrated structure dependency toward CH3OH selectivity, as the mixed-phase Pt x Zn/C outperformed the phase-pure PtZn/C and Pt3Zn/C. The structure-dependent reaction mechanism and the kinetics were elucidated over the synthesized catalysts with the help of detail experiments and associated density functional theory calculations. Results showed that in spite of low electrochemically active surface area, Pt x Zn could not only have facilitated the single electron transfer to adsorbed CO2 but also showed better binding of the intermediate CO2 •– over its surface. Moreover, the lower bond energy between the mixed-phase surface and −OCH3 compared to the phase-pure catalysts has enabled higher CH3OH selectivity over Pt x Zn. This work opens a wide possibility of studying the role of interfaces between phase-pure nano-alloys toward CO2RR.

Fixing the greenhouse gas CO2 via cycloaddition reactions to value-added cyclic carbonates or photocatalytic reduction of CO2 to produce desirable fuels is the most coveted, although challenging, energy-efficient valorization technique owing to the chemical and thermodynamic stability of CO2. Therefore, catalytic materials with an optimum amount of surface porosity and Lewis acidity and basicity play an instrumental and crucial role in CO2 activation in the CO2 fixation reaction. On the other hand, the efficacy of the photocatalytic reduction of CO2 depends on the appropriate band alignment of the catalytic materials and the competitive H2 evolution reaction. Here, we have synthesized Ce-MOFs with and without amine functionalization and compared both MOFs for CO2 fixation with epoxides and photocatalytic reduction of CO2. The amine-functionalized MOF, Ce-BDC-NH2, not only exhibited efficient CO2 fixation to produce cyclic carbonate at room temperature and pressure but also demonstrated effective photoreduction of CO2 with high selectivity toward CH3OH and HCO2H. The probing of the physical properties of the MOFs unveiled that due to the optimum amount of Lewis acidic site Ce3+ and Ce4+, Ce-BDC-NH2 could effectively adsorb the epoxide for the CO2 cycloaddition reaction, whereas the amino functionalization highly influenced the band maxima and minima to facilitate the photoreduction of CO2 with minimized H2 evolution reaction.

Epoxidation of propylene into propylene oxide (PO) in the gas phase is a highly challenging reaction. Cu-based catalysts have been active for this reaction, but the state of Cu as an active species is still debatable. In this paper, we report the propylene epoxidation activity of solution combustion synthesized Cu/CeO2 catalysts with the CO + O2 mixture at low temperatures (50–100 °C) peaking at ∼80 °C. The highest PO yield was obtained with 20–25% Cu loading in CeO2. In contrast, the reaction over the catalyst containing nonreducible support such as Cu/SiO2 occurred above 170 °C. Detailed structural characterization indicated two types of Cu species such as Cu2+ partly (∼3%) dissolved in CeO2 forming a Cu x Ce1–x O2−δ phase and the remaining amount formed highly dispersed CuO as a separate phase. Thus, the highest activity relates to the optimum presence of CuO along with Ce1–x Cu x O2−δ. The reducibility of the Cu species in two phases was significantly shifted toward lower temperatures, indicating strong electronic interaction between the two phases. The substituted Cu2+ was reduced first, and then, the bulk CuO reduction was initiated. In situ spectroscopic studies showed Cu+ formation from Cu2+ over Cu/CeO2 catalysts even at room temperature unlike in CeO2 or CuO + CeO2 physical mixtures, indicating strong electronic interaction between Ce1–x Cu x O2−δ and CuO phases on CO adsorption in the Cu/CeO2 catalyst. It is proposed that substituted Cu2+ along with Ce4+ is reduced easily, and then, Ce3+ promotes the reduction of the interfacial CuO phase that might donate active oxygen species for epoxidation reaction.

A Ce/Ti-based bimetallic 2-aminoterephthalate metal–organic framework (MOF) was synthesized and evaluated for photocatalytic reduction of CO2 in comparison with an isoreticular pristine monometallic Ce-terephthalate MOF. Owing to highly selective CO2 adsorption capability, optimized band gaps, higher flux of photogenerated electron–hole pairs, and a lower rate of recombination, this material exhibited better photocatalytic reduction of CO2 and lower hydrogen evolution compared to Ce-terephthalate. Thorough probing of the surface and electronic structure inferred that the reducibility of Ce4+ to Ce3+ was due to the introduction of an amine functional group into the linker, and low-lying Ti­(3d) orbitals in Ce/Ti-2-aminoterephthalate facilitated the photoreduction reaction. Both the MOFs were calcined to their respective oxides of Ce1–x Ti x O2 and CeO2, and the electrocatalytic reduction of CO2 was performed over the oxidic materials. In contrast to the photocatalytic reaction mechanism, the lattice substitution of Ti in the CeO2 fluorite cubic structure showed a better hydrogen evolution reaction and consequently, poorer electroreduction of CO2 compared to pristine CeO2. Density functional theory calculations of the competitive hydrogen evolution reaction on the MOF and the oxide surfaces corroborated the experimental findings.