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The development of efficient and metal-free heterogeneous catalysts for the chemical fixation of CO2 into value-added products is still a challenge. Herein, we reported two kinds of polar group (−COOH, −OH)-functionalized porous ionic polymers (PIPs) that were constructed from the corresponding phosphonium salt monomers (v-PBC and v-PBH) using a solvothermal radical polymerization method. The resulting PIPs (POP-PBC and POP-PBH) can be used as efficient bifunctional heterogeneous catalysts in the cycloaddition reaction of CO2 with epoxides under relatively low temperature, ambient pressure, and metal-free conditions without any additives. It was found that the catalytic activities of the POP-PBC and POP-PBH were comparable with the homogeneous catalysts of Me-PBC and PBH and were higher than that of the POP-PPh3-COOH that was synthesized through a post-modification method, indicating the importance of the high concentration catalytic active sites in the heterogeneous catalysts. Reaction under low CO2 concentration conditions showed that the activity of the POP-PBC (with a conversion of 53.8% and a selectivity of 99.0%) was higher than that of the POP-PBH (with a conversion of 32.3% and a selectivity of 99.0%), verifying the promoting effect of the polar group (−COOH group) in the porous framework. The POP-PBC can also be recycled at least five times without a significant loss of catalytic activity, indicating the high stability and robustness of the PIPs-based heterogeneous catalysts. [ABSTRACT FROM AUTHOR]

Pd/ZnO catalysts with ca. 2 wt.% Pd loading employing ZnO nanocrystals with various morphologies as supports, including ZnO nanoplates (p−ZnO) predominantly exposing polar {002} facets, ZnO nanorods (r−ZnO) predominantly exposing nonpolar {100} and {101} facets, and commercial ZnO (c−ZnO) exposing random facets, have been prepared as catalysts for the catalytic reaction of CO2 hydrogenation into formic acid (FA) without the addition of base under mild conditions. A remarkable morphology‐dependence of zinc oxide in the Pd/ZnO catalysts was observed. Catalytic activities of various Pd/ZnO catalysts in CO2 hydrogenation into FA follow an order of Pd/r−ZnO>Pd/p−ZnO>Pd/c−ZnO, and all the catalysts are stable. Kinetic study, CO2‐TPD analysis, and in situ DRIFTS spectra provide evidence that all the Pd/ZnO catalysts follow a similar catalytic mechanism that CO2 activation is the rate‐determining step. Consequently, r−ZnO, possessing a higher density of weak basic sites than p−ZnO and c−ZnO, can act as a superb support to prepare Pd/r−ZnO catalyst, which is more active than Pd/p−ZnO and Pd/c−ZnO catalysts for the production of FA. These findings nicely demonstrate the crystal‐plane engineering of the metal oxide support in tuning the catalytic performance of Pd‐based catalysts for CO2 hydrogenation into FA. [ABSTRACT FROM AUTHOR]

[Display omitted] • Ultrathin BiNS decorated with monodispersed Pd NPs by two-step reconstruction was synthesized. • Near-unity formate selectivity over a broadscope low-potential window. • Breaking the limitation that Pd-site is easily poisoned by the generated CO at low overpotentials. • The capability of Pd-Bi interface for stabilizing and adsorbing *OCHO intermediate was revealed. • A new CO 2 RR mechanism with HCO 3 − participating as pre-proton donor for formate generation. A novel electrocatalyst for CO 2 -to-formate conversion at low overpotentials with abundant bimetallic interfaces aroused from Pd nanoparticles anchoring in the underlying Bi nanosheet (Pd-BiNS) was developed. This unique synthesis procedure undergoes a continuous reconstruction of Pd-BiOINS to Pd-Bi 2 O 2 CO 3 NS to Pd-BiNS, which was confirmed by in situ Raman spectra combined with quasi- in - situ electron microscopy. The optimized Pd 1 -BiNS exhibits a high Faradaic efficiency of 95 % for formate at only − 0.4 V versus reversible hydrogen electrode over 30 h electrolysis, breaking the limitation that metallic Pd-site is easily poisoned by the generated CO at low overpotentials. The in situ infrared spectroscopy indicates that Pd-BiNS has much suppressed *CO binding and enhanced *OCHO adsorption to inhibit the competing hydrogen evolution reaction by repelling surface water molecules, moreover, a systematic CO 2 RR mechanism with HCO 3 − participating as pre-proton donor for formate generation is unveiled. This work provides new insight into the multisite synergistic regulation at the atomic scale. [ABSTRACT FROM AUTHOR]

[Display omitted] • Co 2 CrO 4 spinel oxide was more active than Co 3 O 4 and CoCr 2 O 4 for propane oxidation. • B-sites metallic cations in spinel oxide were essential in determining reaction behaviors. • B-sites equally occupied by Co and Cr cations enhanced O 2 adsorption capacity. • C 3 H 8 and O 2 non-competitively adsorbed on spinel oxide catalysts. The essential roles of octahedrally coordinated cations (B-site in AB 2 O 4 formula) in Co 3-x Cr x O 4 spinel oxides in the total oxidation of propane were demonstrated, using a series of Co 3-x Cr x O 4 spinel oxides with different Co/Cr molar ratios derived from layered double hydroxides. The catalytic activity was strongly related to the compositions of B-site cations. The Co 2 CrO 4 catalyst with B-sites equally occupied with Co and Cr cations was more active than the Co 3 O 4 (Co cations only in B-sites) and CoCr 2 O 4 (Cr cations only in B-sites), giving turnover frequencies of 0.86 × 10-3, 0.72 × 10-3 and 0.11 × 10-3 s−1 at 250 °C (under kinetically-controlled reaction), respectively. Kinetics and density functional theory (DFT) calculations on the Co 2 CrO 4 and CoCr 2 O 4 catalysts indicated that the reaction followed a Langmuir-Hinshelwood mechanism, and the adsorption of oxygen on the former was easier. Moreover, the cleavage of the first C–H bond in the propane as the rate-determining step was more favored on the Co 2 CrO 4 , which accounted for its higher activity. These findings thus provide valuable insights on the mechanistic roles of B-site cations of spinel oxides in total oxidation of hydrocarbon. [ABSTRACT FROM AUTHOR]

[Display omitted] • Pd/TiO 2 {100} is highly active and stable for CO 2 hydrogenation into formate. • The activity is benefit from the moderate basic site and metallic Pd species. • Higher density of moderate basicity over Pd/TiO 2 {100} favors for CO 2 activation. • More metallic Pd species over Pd/TiO 2 {100} contributes to H 2 activation. • The stability is benefit from strong Pd-TiO 2 interactions. • The oxygen vacancy in the TiO 2 {100} promote the Pd-TiO 2 interactions. Pd supported on different anatase TiO 2 nanocrystals predominantly exposing either {1 0 0}, {1 0 1}, or {0 0 1} facets were tested for CO 2 hydrogenation into formate. Remarkable morphology-dependent catalysis was observed. Compared to 2%Pd/TiO 2 {1 0 1} and 2%Pd/TiO 2 {0 0 1} catalysts, 2%Pd/TiO 2 {1 0 0} is highly active and stable, affording an unprecedented turnover frequency of ca. 1369 h−1 and keeping stable after 6 cycles at 313 K. This can be associated with, on the one hand, higher density of moderate basic site and relatively more Pd(0) species over 2%Pd/TiO 2 {1 0 0} contribute to the activations of CO 2 and H 2 , respectively, favorable for the activity; On the other hand, higher oxygen vacancy concentrations in the TiO 2 {1 0 0} promote the Pd-TiO 2 interactions and result in the formation and stability of flat Pd particles over 2%Pd/TiO 2 {1 0 0}, beneficial to the stability. These results highlight the importance of oxide morphology in formate formation and open up possibilities of oxide morphology engineering for developing efficient Pd-based catalysts for CO 2 hydrogenation. [ABSTRACT FROM AUTHOR]

[Display omitted] • The Pd-CeO 2 interaction is related to the oxygen vacancy concentration in the CeO 2. • Higher oxygen vacancy concentration in Pd/r-CeO 2 results in higher content of Pd2+ species. • The highest content of metallic Pd species in Pd/p-CeO 2 accounts for the best activity. • Hydrogenation of bidentate carbonate and bicarbonate species is the rate determining step. Several Pd catalysts supported on CeO 2 with different morphologies (i.e. CeO 2 rods (r-CeO 2), cubes (c-CeO 2) and polyhedra (p-CeO 2)) were tested for the CO 2 hydrogenation into formate. Remarkable morphology - dependent Pd-CeO 2 interaction was observed, which was related to the oxygen vacancies in the CeO 2. The r-CeO 2 with the highest concentration of oxygen vacancy resulted in smaller Pd particles and more positively-charged Pd species in the Pd/r-CeO 2 catalyst. The best performance was obtained over the Pd/p-CeO 2 catalyst, giving a turnover frequency of 746 h−1 at 40 °C. The enhanced activity was due to the highest content of metallic Pd species in the catalyst, which was responsible for the facile activation of H 2. Moreover, in situ spectroscopic results revealed that the hydrogenation of the carbonaceous intermediates was the rate - determining step. This work demonstrates oxide morphology engineering as a powerful strategy in developing efficient Pd - based catalysts for CO 2 hydrogenation. [ABSTRACT FROM AUTHOR]

A Pt catalyst supported on CuO-CrO x composite oxide (Pt/CuCrO x ) was prepared and tested for CO oxidation in the presence of CO 2 and H 2 O. It was found that the catalyst was stable in the realistic reaction conditions and the catalytic activity was improved in the presence of CO 2 and H 2 O compared to that in dry condition. Kinetic investigation and temperature – programmed desorption of CO results revealed that the addition of CO 2 in the feed resulted in the competitive adsorption of CO/CO 2 and the formation of surface carbonate species, which consequently deactivated the catalyst. In contrast, although the presence of H 2 O also inhibited the adsorption of CO, the possible formation of surface hydroxyl groups may trigger a new and more facile reaction route for CO oxidation, which could explain the promoting effect of H 2 O. Therefore, the current findings makes the catalyst promising in CO oxidation under realistic reaction conditions. [ABSTRACT FROM AUTHOR]

[Display omitted] • Surface active sites on Co 3 O 4 catalyst for CO and propane oxidation have been clearly differentiated. • CO oxidation activity of Co 3 O 4 catalyst depends on O 2 activation on oxygen vacancies. • Synergistic catalysis of Co 3 O 4 species and acid sites from ZSM-5 accelerates propane oxidation. • Increasing surface Co3+ specie of Co 3 O 4 catalyst is beneficial for both CO and propane oxidation. Identifying active sites is critical for rational catalyst design. We clarified the surface active sites for propane and CO oxidation over Co 3 O 4 -based catalysts using probe reactions of propane and CO oxidation, respectively. In comparison to Co 3 O 4 /ZSM-5, Co 3 O 4 /CeO 2 has superior CO oxidation activity but inferior propane oxidation activity. CeO 2 oxygen vacancies promote O 2 activation, the rate-determined step (RSD) for CO oxidation, and avoid competitive adsorption of O 2 and CO on Co 3 O 4 species, resulting in superior CO oxidation activity on Co 3 O 4 /CeO 2. Whereas the RSD for propane oxidation is synergistic catalysis of Co 3 O 4 species and acid sites, Co 3 O 4 /ZSM-5 exhibits improved propane oxidation activity as a result of the abundant acid sites from ZSM-5. Due to a lack of oxygen vacancies for O 2 activation, competitive adsorption of O 2 with propane or CO on Co 3 O 4 species is observed on Co 3 O 4 /ZSM-5. Furthermore, surface Co3+ species in Co 3 O 4 are discovered to be active sites for both CO and propane activation. As a result of the increased surface Co3+ species, the pre-reduced and post-oxidized Co 3 O 4 catalysts (Co 3 O 4 /ZSM-5-A and Co 3 O 4 /CeO 2 -A) exhibit improved activity for both propane and CO oxidation. [ABSTRACT FROM AUTHOR]