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

Unlike homogeneous catalysts, heterogeneous catalysts that have been optimized through decades are typically so complex and hard to characterize that the nature of the catalytically active site is not known. This is one of the main stumbling blocks in developing rational catalyst design strategies in heterogeneous catalysis. We show here how to identify the crucial atomic structure motif for the industrial Cu/ZnO/Al{sub 2}O{sub 3} methanol synthesis catalyst. Using a combination of experimental evidence from bulk-, surface-sensitive and imaging methods collected on real high-performance catalytic systems in combination with DFT calculations. We show that the active site consists of Cu steps peppered with Zn atoms, all stabilized by a series of well defined bulk defects and surface species that need jointly to be present for the system to work.

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The Active Site of Methanol Synthesis over Cu/ZnO/Al2O3 Industrial Catalysts

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

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

Ocean acidification and climate change are expected to be two of the most difficult scientific challenges of the 21st century. Converting CO2 into valuable chemicals and fuels is one of the most practical routes for reducing CO2 emissions while fossil fuels continue to dominate the energy sector. Reducing CO2 by H2 using heterogeneous catalysis has been studied extensively, but there are still significant challenges in developing active, selective and stable catalysts suitable for large-scale commercialization. The catalytic reduction of CO2 by H2 can lead to the formation of three types of products: CO through the reverse water–gas shift (RWGS) reaction, methanol via selective hydrogenation, and hydrocarbons through combination of CO2 reduction with Fischer–Tropsch (FT) reactions. Investigations into these routes reveal that the stabilization of key reaction intermediates is critically important for controlling catalytic selectivity. Furthermore, viability of these processes is contingent on the development of a CO2-free H2 source on a large enough scale to significantly reduce CO2 emissions.

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Catalytic reduction of CO2 by H2 for synthesis of CO, methanol and hydrocarbons: challenges and opportunities

The Cu/ZnO/ZrO2 catalyst has been modified by adding small amounts of B, Ga, In, Gd, Y, Mn and Mg oxides. Two series of catalysts were obtained: series A: by the co-precipitation of basic carbonates and series B: by complexing with citric acid. The oxide additives were found to influence the catalytic activity in the reaction of methanol synthesis from CO2, dispersion of Cu, surface composition of the catalyst, and the stability of catalysts during their operation. In both series of catalysts, the Ga2O3 additive was especially useful. The mechanism of copper sintering and the performance of the oxide additives are discussed.

Effect of metal oxide additives on the activity and stability of Cu/ZnO/ZrO2 catalysts in the synthesis of methanol from CO2 and H2

Chromium Oxide/Alumina Catalysts in Oxidative Dehydrogenation of Isobutane☆

Morphology, surface composition, dispersion of the metals were determined for a series of the M/(3ZnO·ZrO 2 ) catalysts (M = Cu, Ag, Au) with the use of the XPS, XRD and TEM techniques. Catalytic activity in the reaction of the synthesis of methanol from CO 2 was also determined. The catalysts contain crystallites of ZnO and amorphous ZrO 2 , and they show a considerable segregation of the components. The surface is depleted of the metal, metallic Cu is present mainly in the surrounding of ZnO. In the catalysts containing gold and silver, Au and Ag are distributed more uniformly. The dispersion of the metals in the catalysts depends on the nature of the metal and decreases in the series Au ≅ Cu > Ag, and for a given metal it decreases with the increase in the metal content. The catalysts containing copper exhibit by far higher catalytic activity when compared to the catalysts containing Ag and Au, due to a synergy between Cu and ZnO or ZrO 2 .

Catalytic activity of the M/(3ZnO·ZrO2) system (M = Cu, Ag, Au) in the hydrogenation of CO2 to methanol

Effect of addition of the Mg and Mn promoters on the activity of the Cu/ZnO/ZrO2 catalysts in the reaction of the synthesis of methanol from CO2 and H2, and the steam reforming of methanol, as well as on the catalysts’ adsorptive properties with respect to reactants (CO, CO2, H2O, methanol) was studied. Using the X-ray diffraction (XRD) and X-ray photoelectron spectra (XPS) techniques as well as investigating the reactive N2O adsorption, it was revealed that addition of the promoters leads to an increase of the copper dispersion in the reduced catalysts. The surface layers are depleted of copper and enriched in zinc and zirconium. The promoters introduced are accumulated preferentially on the catalysts’ surface. Correlation between the adsorptive properties and the catalytic activity was established. The overall factor combining the adsorptive properties favoring the synthesis of methanol (RAF) and the catalytic activity increase in the series CuZnZr<CuZnZrMg<CuZnZrMn.

Effect of Mg and Mn oxide additions on structural and adsorptive properties of Cu/ZnO/ZrO2 catalysts for the methanol synthesis from CO2

Reduction of copper oxide in the CuO/ZnO/ZrO2 system, which is an oxide precursor of the catalysts active in the synthesis of methanol from CO2 and H2, was investigated. Preparations with varying CuO contents were obtained by co-precipitation or complexing the components with citric acid; also modifying additives (MgO, MnO) were introduced. The prepared materials were characterised by determining their surface area (BET), the active surface of Cu using the method of reactive adsorption of N2O, the size of the CuO crystallites by XRD line broadening, the surface composition by XPS, and the kinetics of reduction using TPR, XRD and gravimetric methods. It was found that the reduction of CuO was an autocatalytic consecutive reaction and that the intermediate product was amorphous Cu2O. The autocatalytic effect is due to facilitated dissociation of H2 on the metallic copper formed. Water formed during the reaction hinders the reduction by blocking the hydrogen adsorption centres. Changes in preparation methods, CuO content, as well as introduction of additives, affect the rate of the CuO reduction mainly by morphological changes: content of the amorphous CuO phase, size of the CuO crystallites and its surface segregation.

A. Kozłowska

Papers

Effect of metal oxide additives on the activity and stability of Cu/ZnO/ZrO2 catalysts in the synthesis of methanol from CO2 and H2

Chromium Oxide/Alumina Catalysts in Oxidative Dehydrogenation of Isobutane☆

Catalytic activity of the M/(3ZnO·ZrO2) system (M = Cu, Ag, Au) in the hydrogenation of CO2 to methanol

Effect of Mg and Mn oxide additions on structural and adsorptive properties of Cu/ZnO/ZrO2 catalysts for the methanol synthesis from CO2

Reduction kinetics of CuO in CuO/ZnO/ZrO2 systems