Starting with coal, followed by petroleum oil and natural gas, the utilization of fossil fuels has allowed the fast and unprecedented development of human society. However, the burning of these resources in ever increasing pace is accompanied by large amounts of anthropogenic CO2 emissions, which are outpacing the natural carbon cycle, causing adverse global environmental changes, the full extent of which is still unclear. Even through fossil fuels are still abundant, they are nevertheless limited and will, in time, be depleted. Chemical recycling of CO2 to renewable fuels and materials, primarily methanol, offers a powerful alternative to tackle both issues, that is, global climate change and fossil fuel depletion. The energy needed for the reduction of CO2 can come from any renewable energy source such as solar and wind. Methanol, the simplest C1 liquid product that can be easily obtained from any carbon source, including biomass and CO2, has been proposed as a key component of such an anthropogenic carbon cycle in the framework of a “Methanol Economy”. Methanol itself is an excellent fuel for internal combustion engines, fuel cells, stoves, etc. It's dehydration product, dimethyl ether, is a diesel fuel and liquefied petroleum gas (LPG) substitute. Furthermore, methanol can be transformed to ethylene, propylene and most of the petrochemical products currently obtained from fossil fuels. The conversion of CO2 to methanol is discussed in detail in this review.

Recycling of carbon dioxide to methanol and derived products – closing the loop

Oxidation into CO2 is a major solution to CO abatement in air depollution treatments. The development of catalytic converters led to an extraordinary high number of publications on metal catalysts during the last fifty years. Due to the increasing price of noble metals and to remarkable progresses in oxide syntheses, catalytic oxidation of carbon monoxide over oxide catalysts has recently gained in interest, even if some oxides are known to present remarkable activity since the beginning of the 20th century. In this Review, the kinetics and mechanism of CO oxidation on single and mixed oxides are examined, alongside the catalyst structures

Catalytic Oxidation of Carbon Monoxide over Transition Metal Oxides

The chemical transformation of CO2 not only mitigates the anthropogenic CO2 emission into the Earth’s atmosphere but also produces carbon compounds that can be used as precursors for the production of chemicals and fuels. The activation and conversion of CO2 can be achieved on multifunctional catalytic sites available at the metal/oxide interface by taking advantage of the synergy between the metal nanoparticles and oxide support. Herein, we look at the recent progress in mechanistic studies of CO2 hydrogenation to C1 (CO, CH3OH, and CH4) compounds on metal/oxide catalysts. On this basis, we are able to provide a better understanding of the complex reaction network, grasp the capability of manipulating structure and combination of metal and oxide at the interface in tuning selectivity, and identify the key descriptors to control the activity and, in particular, the selectivity of catalysts. Finally, we also discuss challenges and future research opportunities for tuning the selective conversion of CO2 on ...

/pdf/tuning-selectivity-of-co2-hydrogenation-reactions-at-the-vsg702s3kj.pdf

Tuning Selectivity of CO2 Hydrogenation Reactions at the Metal/Oxide Interface

The utilization of fossil fuels has enabled an unprecedented era of prosperity and advancement of well-being for human society. However, the associated increase in anthropogenic carbon dioxide (CO2) emissions can negatively affect global temperatures and ocean acidity. Moreover, fossil fuels are a limited resource and their depletion will ultimately force one to seek alternative carbon sources to maintain a sustainable economy. Converting CO2 into value-added chemicals and fuels, using renewable energy, is one of the promising approaches in this regard. Major advances in energy-efficient CO2 conversion can potentially alleviate CO2 emissions, reduce the dependence on nonrenewable resources, and minimize the environmental impacts from the portions of fossil fuels displaced. Methanol (CH3OH) is an important chemical feedstock and can be used as a fuel for internal combustion engines and fuel cells, as well as a platform molecule for the production of chemicals and fuels. As one of the promising approaches, thermocatalytic CO2 hydrogenation to CH3OH via heterogeneous catalysis has attracted great attention in the past decades. Major progress has been made in the development of various catalysts including metals, metal oxides, and intermetallic compounds. In addition, efforts are also put forth to define catalyst structures in nanoscale by taking advantage of nanostructured materials, which enables the tuning of the catalyst composition and modulation of surface structures and potentially endows more promising catalytic performance in comparison to the bulk materials prepared by traditional methods. Despite these achievements, significant challenges still exist in developing robust catalysts with good catalytic performance and long-term stability. In this review, we will provide a comprehensive overview of the recent advances in this area, especially focusing on structure-activity relationship, as well as the importance of combining catalytic measurements, in situ characterization, and theoretical studies in understanding reaction mechanisms and identifying key descriptors for designing improved catalysts.

Recent Advances in Carbon Dioxide Hydrogenation to Methanol via Heterogeneous Catalysis.

Metal-organic frameworks (MOFs) are a class of distinctive porous crystalline materials constructed by metal ions/clusters and organic linkers. Owing to their structural diversity, functional adjustability, and high surface area, different types of MOF-based single metal sites are well exploited, including coordinately unsaturated metal sites from metal nodes and metallolinkers, as well as active metal species immobilized to MOFs. Furthermore, controllable thermal transformation of MOFs can upgrade them to nanomaterials functionalized with active single-atom catalysts (SACs). These unique features of MOFs and their derivatives enable them to serve as a highly versatile platform for catalysis, which has actually been becoming a rapidly developing interdisciplinary research area. In this review, we overview the recent developments of catalysis at single metal sites in MOF-based materials with emphasis on their structures and applications for thermocatalysis, electrocatalysis, and photocatalysis. We also compare the results and summarize the major insights gained from the works in this review, providing the challenges and prospects in this emerging field.

Metal-Organic Framework-Based Catalysts with Single Metal Sites.

Carbon monoxide oxidation activities over Cu, Cu2O, and CuO were studied to seek insight into the role of the copper species in the oxidation reaction. The activity of copper oxide species can be elucidated in terms of species transformation and change in the number of surface lattice oxygen ions. The propensity of Cu2O toward valence variations and thus its ability to seize or release surface lattice oxygen more readily enables Cu2O to exhibit higher activities than the other two copper species. The non-stoichiometric metastable copper oxide species formed during reduction are very active in the course of CO oxidation because of its excellent ability to transport surface lattice oxygen. Consequently, the metastable cluster of CuO is more active than CuO, and the activity will be significantly enhanced when non-stoichiometric copper oxides are formed. In addition, the light-off behaviors were observed over both Cu and Cu2O powders. CO oxidation over metallic Cu powders was lighted-off because of a synergistic effect of temperature rises due to heat generation from Cu oxidation as well as CO oxidation over the partially oxidized copper species.

CO Oxidation Behavior of Copper and Copper Oxides

A comparison of the activity behaviors of the mechanistically similar reactions of carbon dioxide reforming and steam reforming of methane was carried out at 400∼550 °C over nickel catalysts with samaria- and gadolinia-doped ceria and α-alumina as the supports. Results show that the activity behaviors of carbon dioxide reforming and steam reforming of methane are similar and very sensitive to the oxygen-vacancy properties of the support, with a drastic increase of the activities as the temperature increases from 450 to 500 °C. Nevertheless, a difference in the activity behaviors between carbon dioxide and steam reforming has been observed and is due to a difference in the reaction mechanisms of CO2 and H2O dissociations. Possible carbon deposition (coking) is lessened due to the reaction of the surface carbon species with the lattice oxygen or the surface O species as produced from CO2 or H2O. It was found that the samaria-doped ceria supported nickel catalyst has better de-coking ability than that of the gadolinia-doped ceria supported one. An oxygen-transport reaction mechanism for doped-ceria supported Ni catalyst has been proposed and shown to explain the activity behaviors successfully.

A Comparison of Oxygen-vacancy Effect on Activity Behaviors of Carbon Dioxide and Steam Reforming of Methane over Supported Nickel Catalysts

Effects of steam (H2O) and carbon dioxide (CO2) pretreatments on methane (CH4) decomposition and carbon gasification over doped-ceria supported nickel catalysts have been studied from 400 to 500 °C. The doped ceria employed were gadolinia-doped ceria and samaria-doped ceria. Results indicate that a drastic increase of both H2O and CO2 dissociation activities occurs as the temperature increases from 450 to 500 °C. The formation of the surface hydroxyl species during H2O treatment inhibits the followed CH4 decomposition. CO but no CO2 was formed during CH4 reaction after H2O treatment. Carbon deposition during CH4 decomposition is quite large but can be removed via gasification with afterward CO2 treatment. However, some of the deposited carbon species is in a form which can not be removed with CO2 treatment but can be removed with O2 treatment. And, higher values of the oxygen-ion conductivity and the density of the surface oxygen vacancies lead to higher activities for all dissociation and decomposition reactions.

Effect of steam and carbon dioxide pretreatments on methane decomposition and carbon gasification over doped-ceria supported nickel catalyst

Methanol synthesis from carbon dioxide hydrogenation was studied over ceria/γ-alumina- and yttria-doped ceria (YDC)/γ-alumina-supported copper oxide catalysts to seek insight into the catalysis at metal–support interfaces. It was found that, in comparison with Cu/γ-Al2O3, the Cu/CeO2/γ-Al2O3 and Cu/YDC/γ-Al2O3 catalysts exhibited substantial enhancement in activity and selectivity toward methanol formation. The extent of enhancement was augmented by increased ceria loading on γ-alumina and with increased yttria doping into ceria. The enhancement is inferred to result from the synergistic effect between copper oxide and surface oxygen vacancies of ceria.

Synergistic Catalysis of Carbon Dioxide Hydrogenation into Methanol by Yttria-Doped Ceria/γ-Alumina-Supported Copper Oxide Catalysts: Effect of Support and Dopant

Temperature-programmed reaction of methane and temperature-programmed reduction were performed over gadolinia-doped ceria (GDC). It was found that CO2 formation can occur at very much lower temperature than CO formation. The surface lattice oxygen acts as the active site for CH4 adsorption. This active site has a dynamic characteristic due to the mobility of the lattice oxygen. The rates of CO and CO2 formations can be controlled by the supply rate of the lattice oxygen from the GDC bulk; this supply rate depends on the mobility and the concentration of the bulk lattice oxygen. CO2 formation is associated with the existing surface lattice oxygen while CO formation depends on the oxygen species coming from the bulk lattice during methane reaction.

Ta-Jen Huang

Papers

CO Oxidation Behavior of Copper and Copper Oxides

A Comparison of Oxygen-vacancy Effect on Activity Behaviors of Carbon Dioxide and Steam Reforming of Methane over Supported Nickel Catalysts

Effect of steam and carbon dioxide pretreatments on methane decomposition and carbon gasification over doped-ceria supported nickel catalyst

Synergistic Catalysis of Carbon Dioxide Hydrogenation into Methanol by Yttria-Doped Ceria/γ-Alumina-Supported Copper Oxide Catalysts: Effect of Support and Dopant

Roles of Surface and Bulk Lattice Oxygen in Forming CO2 and CO During Methane Reaction over Gadolinia-Doped Ceria