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 heat evolved during the adsorption and oligomerization of CO on outgassed CaO and MgO has been determined with a heat-flow calorimeter. The chemisorbed complexes formed include highly-conjugated (CO)
 n
 2
 anions which impart colour to the oxides. The reaction of these complexes with oxygen has been quantified calorimetrically and correlated with changes observed spectroscopically.

The reaction of oxygen with CO complexes on calcium oxide and magnesium oxide

Oxygen adsorption has been used as a probe for manganese ions in the surface of CaO-MnO. By exploiting adsorption microcalorimetry, the energetics of the oxygen uptake have been determined for the pure solvent matrix CaO and for solid solutions of nominal bulk solute concentrations of 1 mol%, 25 mol% and 50 mol% Mn, designated CM 1, CM 25 and CM 50. Oxygen adsorption on Mn 2+ ions is characterised by a much higher heat than on CaO. In this way it is shown that the uptakes on CM 1, CM 25 and CM 50 are much in excess of those expected on the basis of the nominal (bulk) concentrations of Mn. The conclusion is that the solid solutions exhibit substantial surface enrichment in manganese, and this also affects their reactivity towards water vapour.

Oxygen Reactivity as a Probe for the Surface Composition of Cao-Mno Solid Solutions

The behaviour of MgO as a BrØnsted base in chemisorption and surface processes

The sections in this article are


Introduction
Principles, Preparation, and Characterization
Principles
Preparation
Characterization
Bulk Composition and Structure
Surface Composition




Catalysis in the Context of Activity
N2O Decomposition, CO Oxidation and H2D2 Equilibration
Combustion of Hydrocarbons
Perovskite Catalysts
Hexa-Aluminate Catalysts
Ceria-Containing Catalysts




Catalysis in the Context of Selectivity
Selective Oxidation
Reactions Involving Dehydrogenation, Hydrogenation, and Reforming


Catalysis in the Context of Surface Acidity and Basicity
Dehydration of Alcohols on Ionic Solid Solutions
Acid Catalysis by Aluminophosphates


Conclusions


Keywords:

oxide solid solutions;
transition metal ions;
perovskites;
scheelites

Frank S. Stone

Papers

The reaction of oxygen with CO complexes on calcium oxide and magnesium oxide

Oxygen Reactivity as a Probe for the Surface Composition of Cao-Mno Solid Solutions

The behaviour of MgO as a BrØnsted base in chemisorption and surface processes

Oxide Solid Solutions

Structural characterization of cadmium-copper gallium oxide spinels