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Three supported Ir/TiO2 catalysts, containing anatase TiO2 nanocrystals with predominantly exposed {101}, {100}, and {001} planes, were subjected to various pre-treatments (H2 reduction at different temperatures and O2 re-oxidation) and then tested in the vapor phase selective hydrogenation of crotonaldehyde. The pre-treatments significantly altered the Ir-TiOx interactions, including the morphologies and electronic properties of the Ir species and their surface acidity. These interactions were also closely related to the crystal planes of TiO2, which further supported the observed reaction behaviors of the various Ir/TiO2 catalysts. The best performance was obtained using the Ir/TiO2-{101} catalyst pre-reduced at 300 °C, owing to its higher Ir0 surface concentration and moderate surface acidity compared to the other catalysts. Moreover, these findings indicated the synergistic role of the Ir-TiOx interface in the reaction, as the interfacial sites were responsible for the adsorption/activation of H2 and the C=O bond in the crotonaldehyde molecule. However, pre-reduction at 400 °C resulted in partial encapsulation of the Ir particles by TiOx via strong metal-support interactions, which is unfavorable for the catalytic reaction owing to the loss of Ir-TiOx interfacial sites.

The Cu-based catalysts have been widely employed in the hydrogenation of CO2-derived ethylene carbonate (EC) to methanol (MeOH) and ethylene glycol (EG), but still suffered from some problems such as excess hydrogen/ester ratio, unsatisfactory MeOH selectivity and catalyst stability & deactivation. Herein, a MoOx-promoted Cu/SiO2 catalyst, which was prepared via a one-pot modified hydrothermal method and derived from copper phyllosilicate precursors, exhibited superior catalytic performance (89% MeOH yield and 99% EG yield) and good stability (>150 h) at a H2/EC ratio (20). Remarkably, the turnover frequency (TOF) and space time yield of MeOH (STYMeOH) reached up to 17.5 h−1 and 0.207 g⋅gcat−1⋅h−1, respectively, which are among the highest values in the literature data. The characterization results and kinetic studies revealed that modification of Cu species by MoOx obviously changed the electronic properties of Cu species, reduced the activation energy and enhanced the Cu+/(Cu0 + Cu+) ratios. Such modification may lead to possible formation of Cu-O-Mo bond between Cu species and MoOx species located on the periphery of Cu-MoOx interface, which might strengthen the adsorption and activation of EC, stabilize the Cu nanoparticles, and thus improve the selectivity to MeOH and catalyst stability. In addition, MoOx decoration had obvious effect on the strength and quantity of the surface acid sites.

Supported Ir/TiO2 catalysts with the anatase TiO2 nanocrystals exposing {1 0 1}, {1 0 0} and {0 0 1} planes were tested for selective hydrogenation of crotonaldehyde. The Ir/TiO2-{1 0 1} catalyst dominantly exposing {1 0 1} plane gave a initial reaction rate of 166.1 μmol gIr-1 s−1 at 80 °C and a turnover frequency of crotyl alcohol formation of 0.022 s−1, which is 5 and 7 times respectively higher than those obtained on the Ir/TiO2-{0 0 1} catalyst dominantly exposing {0 0 1} plane even though they have similar Ir particle sizes (ca. 1.3 nm). The reaction behaviors were related to the surface oxygen vacancy, which served as surface acid sites and provide adsorption sites for C O bond in the CRAL molecule. Therefore, the higher concentration of oxygen vacancy in the Ir/TiO2-{1 0 1} accounted for the improved performance compared to the Ir/TiO2-{0 0 1}. However, strong adsorption of crotonaldehyde and/or products on the catalyst surface, as well as the CO poisoning via decarbonylation of crotonaldehyde, were the main reasons for the catalyst deactivation.

Cyclohexanol (CHOL), as a value-added chemical, has attracted much attention due to its huge application market. The hydrogenation of cyclohexyl acetate (CHA), derived from the esterification of acetic acid and cyclohexene, not only provides a novel route to yield CHOL and ethanol (EtOH), but also rationally utilizes excess acetic acid. In this work, a series of Zn-promoted Cu/Al2O3 catalysts were prepared via a deposition–precipitation method for the liquid-phase hydrogenation of CHA to yield CHOL and EtOH. As a result, the addition of Zn species with an optimal amount greatly improved the activity and selectivity to CHOL and EtOH. A Cu2Zn1.25/Al2O3 catalyst, which contained 16.2 wt% Cu and 9.6 wt% Zn, exhibited superior catalytic performance with 93.9% conversion of CHA and 97.2% selectivity to EtOH along with 97.1% selectivity to CHOL in a batch reactor. The Cu2Zn1.25/Al2O3 catalyst also showed excellent stability and there was no deactivation after five runs. Based on detailed characterization, it was revealed that the addition of Zn species increased the dispersion of Cu particles, adjusted the strength and amount of acid sites, and changed the electronic properties of Cu species and thus the ratio of Cu+/(Cu0 + Cu+).

Achieving high catalytic performance with Cu-based catalysts is crucial for the hydrogenation of CO2-derived ethylene carbonate (EC) to simultaneously yield methanol (MeOH) and ethylene glycol (EG). Although Cu-based catalysts are widely studied for the target reaction, there are still some issues to be clarified. In this study, we prepared a 29.1 wt% Cu/SiO2-MHT catalyst by a modified hydrothermal method and successfully applied it to the hydrogenation of EC. In comparison with its three analogues having similar Cu loading and comparable Cu particle size but prepared by different methods, we found that the catalytic performance of the Cu/SiO2 catalysts, including the activity and the selectivity to MeOH, could be tuned by adjusting the Cu+/(Cu0 + Cu+) ratio via different preparation methods. As a result, the Cu/SiO2-MHT catalyst furnished 89% MeOH yield and 37.6 h−1 turnover frequency. To our best knowledge, these values are one of the highest among all heterogeneous Cu catalysts up to now. The results reveal that a cooperative effect exists between Cu0 and Cu+. When the Cu0 species are sufficient to activate H2, a more favorable formation of MeOH can be achieved with more Cu+ species and greater Cu+ surface areas. Furthermore, a greater amount of Lewis acid on the Cu/SiO2-MHT catalyst is also beneficial for MeOH formation.

Indirect CO2 conversion to ethylene glycol (EG) and methanol (ME) via CO2-derived ethylene carbonate (EC) as the intermediate is an attractive and promising approach. Cu-Based catalysts have been regarded as effective catalysts for the hydrogenation of EC to yield ME and EG. In this work, we prepared Cu/SiO2 catalysts through different methods for the hydrogenation of EC to yield ME and EG. The results show that Cu/SiO2-AE catalysts prepared by an ammonia evaporation (AE) method are superior to Cu/SiO2-DP catalysts prepared by a deposition–precipitation (DP) method. After modification using glucose with an optimal amount, the Cu8G1/SiO2-AE catalysts showed enhanced catalytic performance, furnishing 86% selectivity to ME and 99% selectivity to EG at almost full conversion of EC, with a TOF value for ME formation reaching about 39 h−1. A series of techniques such as N2 sorption, ICP-AES, FT-IR, H2-TPR, H2-TPD, TEM, and XPS were adopted to characterize the physicochemical properties of relevant catalysts. It was found that the Cu/SiO2-AE catalyst had a smaller Cu particle size. After modification with glucose, the Cu8G1/SiO2-AE catalyst had the smallest Cu particle size, the highest hydrogen adsorption ability, and the strongest interaction of Cu with SiO2 so that the highest Cu+/(Cu0 + Cu+) molar ratio was detected. In addition, upon investigation of catalyst stability and reusability, it was found that glucose modification for the Cu/SiO2-AE catalyst can also alleviate the deactivation distinctly.

Various supported Ir/TiO2 catalysts were prepared using different Ir precursors (i.e., H2IrCl6, (NH4)2IrCl6 and Ir(acac)3) and tested for vapor phase selective hydrogenation of crotonaldehyde. The choice of Ir precursor significantly altered the Ir-TiOx interaction in the catalyst, which thus had essential influences on the geometric and electronic properties of the Ir species, reducibility, and surface acidity, and, consequently, their reaction behaviors. The Ir/TiO2-N catalyst using (NH4)2IrCl6 as the precursor gave the highest initial reaction rates and turnover frequencies of crotyl alcohol formation. Such high performance was ascribed to the high Ir dispersion and high surface concentration of Ir0 species, as well as a higher surface acidity, in the Ir/TiO2-N catalyst compared to its counterparts, indicating the synergistic roles of the Ir-TiOx interface in the reaction, as the interfacial sites were responsible for the adsorption/activation of H2 and the C=O bond in the crotonaldehyde molecule.