Owing to the increasing emissions of carbon dioxide (CO2), human life and the ecological environment have been affected by global warming and climate changes. To mitigate the concentration of CO2 in the atmosphere various strategies have been implemented such as separation, storage, and utilization of CO2. Although it has been explored for many years, hydrogenation reaction, an important representative among chemical conversions of CO2, offers challenging opportunities for sustainable development in energy and the environment. Indeed, the hydrogenation of CO2 not only reduces the increasing CO2 buildup but also produces fuels and chemicals. In this critical review we discuss recent developments in this area, with emphases on catalytic reactivity, reactor innovation, and reaction mechanism. We also provide an overview regarding the challenges and opportunities for future research in the field (319 references).

Recent advances in catalytic hydrogenation of carbon dioxide

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The surface chemistry of amorphous silica. Zhuravlev model

Different-sized CdSe quantum dots have been assembled on TiO2 films composed of particle and nanotube morphologies using a bifunctional linker molecule. Upon band-gap excitation, CdSe quantum dots inject electrons into TiO2 nanoparticles and nanotubes, thus enabling the generation of photocurrent in a photoelectrochemical solar cell. The results presented in this study highlight two major findings: (i) ability to tune the photoelectrochemical response and photoconversion efficiency via size control of CdSe quantum dots and (ii) improvement in the photoconversion efficiency by facilitating the charge transport through TiO2 nanotube architecture. The maximum IPCE (photon-to-charge carrier generation efficiency) obtained with 3 nm diameter CdSe nanoparticles was 35% for particulate TiO2 and 45% for tubular TiO2 morphology. The maximum IPCE observed at the excitonic band increases with decreasing particle size, whereas the shift in the conduction band to more negative potentials increases the driving force and favors fast electron injection. The maximum power-conversion efficiency </=1% obtained with CdSe-TiO2 nanotube film highlights the usefulness of tubular morphology in facilitating charge transport in nanostructure-based solar cells. Ways to further improve power-conversion efficiency and maximize light-harvesting capability through the construction of a rainbow solar cell are discussed.

Quantum Dot Solar Cells. Tuning Photoresponse through Size and Shape Control of CdSe−TiO2 Architecture

CO2 conversion covers a wide range of possible application areas from fuels to bulk and commodity chemicals and even to specialty products with biological activity such as pharmaceuticals. In the present review, we discuss selected examples in these areas in a combined analysis of the state-of-the-art of synthetic methodologies and processes with their life cycle assessment. Thereby, we attempted to assess the potential to reduce the environmental footprint in these application fields relative to the current petrochemical value chain. This analysis and discussion differs significantly from a viewpoint on CO2 utilization as a measure for global CO2 mitigation. Whereas the latter focuses on reducing the end-of-pipe problem “CO2 emissions” from todays’ industries, the approach taken here tries to identify opportunities by exploiting a novel feedstock that avoids the utilization of fossil resource in transition toward more sustainable future production. Thus, the motivation to develop CO2-based chemistry does...

Sustainable Conversion of Carbon Dioxide: An Integrated Review of Catalysis and Life Cycle Assessment

An overview is given on the ongoing activities in the area of the decomposition of nitrous oxide, N2O, over solid catalysts. These catalysts include metals, pure and mixed oxides, supported as well as unsupported, and zeolitic systems. The review covers aspects of the reaction mechanism and kinetics, focusing on the role of surface oxygen, the inhibition by molecular oxygen, water and other species, poisoning phenomena and practical developments.

Heterogeneous catalytic decomposition of nitrous oxide

The reactivity of oxides with water vapor Solid State Ionics

Reflectance spectra of SrO and BaO powders outgassed at temperatures up to 1073 K have been measured over the range from text-decoration:overlineν= 20 000 cm–1 to text-decoration:overlineν= 52 000 cm–1. Spectral features attributable to exciton states are described, and the effects of O2, N2O, CO2 and H2O on the spectra are reported. Fluorescence occurs when spectra are measured in vacuo. SrO-coated MgO and BaO-coated MgO have also been studied. The spectra show two regions separated by an absorption edge which is centred at 44 000 cm–1 for SrO and 31 500 cm–1 for BaO. On the low text-decoration:overlineν side of the edge the spectra are sensitive to the adsorption of the gases, but exposure to gases causes little or no change in the spectra on the high text-decoration:overlineν side (except for H2O vapour which reacts in depth). The absorption bands at low text-decoration:overlineν are ascribed to excitons bound at surface ions and those at high text-decoration:overlineν to excitons bound at ions in the bulk.The chemical interaction of the gases with the surface states increases in intensity in the order O2 < N2O < CO2 < H2O for SrO and N2O < O2 < CO2 < H2O for BaO. The results are correlated with those for MgO and CaO reported previously.

Reflectance spectra of surface states in strontium oxide and barium oxide

Thermal decomposition of managanese oxyhydroxide

Water desorption from a γ-phase manganese dioxide has been studied by gravimetric temperature programmed desorption (TPD) in static water vapor environments. Desorption peaks due to the presence of physisorbed and “structured” hydrogen-bonded water (peak I), chemisorbed molecular water (IIa) and hydroxyl groups (IIb) have been identified. Additional hydroxyl groups were introduced into the interior of the solid by reaction with hydrazine at room temperature. Their effect on the TPD spectra was studied.

Frank S. Stone

Papers

The reactivity of oxides with water vapor Solid State Ionics

Reflectance spectra of surface states in strontium oxide and barium oxide

Thermal decomposition of managanese oxyhydroxide

Temperature programmed desorption studies on γ-phase manganese dioxide in static water vapor environments

Surface properties of alkaline earth oxides as studied by UV-visible diffuse reflectance spectroscopy