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

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

Methanation of coal-or biomass-derived carbon oxides for production of synthetic natural gas (SNG) is gaining considerable interest due to energy issues and the opportunity of reducing greenhouse gases by carbon dioxide conversion. The key component of the methanation process is the catalyst design. Ideally, the catalyst should show high activity at low temperatures (200-300 degrees C) and high stability at high temperatures (600-700 degrees C). In the past decades, various methanation catalysts have been investigated, among which transition metals including Ni, Fe, Co, Ru, Mo, etc. dispersed on metal oxide supports such as Al2O3, SiO2, TiO2, ZrO2, CeO2 etc. have received great attention due to their relatively high catalytic activity and selectivity. Furthermore, over the past few years, great efforts have been made both in methanation catalysts development and reaction mechanism investigation. Here we provide a comprehensive review to these most advancements, covering the reaction thermodynamics, mechanism and kinetics, the effects of catalyst active components, supports, promoters and preparation methods, hoping to outline the pathways for the future methanation catalysts design and development for SNG production.

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Recent advances in methanation catalysts for the production of synthetic natural gas

Current society is inherently based on liquid hydrocarbon fuel economies and seems to be so for the foreseeable future. Due to the low rates (photocatalysis) and high capital investments (solar-thermo-chemical cycles) of competing technologies, reverse water gas shift (rWGS) catalysis appears as the prominent technology for converting CO2 to CO, which can then be converted via CO hydrogenation to a liquid fuel of choice (diesel, gasoline, and alcohols). This approach has the advantage of high rates, selectivity, and technological readiness, but requires renewable hydrogen generation from direct (photocatalysis) or indirect (electricity and electrolysis) sources. The goal of this review is to examine the literature on rWGS catalyst types, catalyst mechanisms, and the implications of their use CO2 conversion processes in the future.

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CO2 conversion by reverse water gas shift catalysis: comparison of catalysts, mechanisms and their consequences for CO2 conversion to liquid fuels

A zirconia supported nickel catalyst for CO2 methanation has been prepared via a rapid and effective plasma decomposition of nickel precursor at atmospheric pressure and temperature around 150 °C, followed by hydrogen reduction at 500 °C. The obtained Ni/ZrO2 catalyst shows high Ni dispersion with principally exposed Ni(111) lattice plane. An enhanced cooperation between Ni and interfacial active sites is achieved as well, which leads to rapid dissociative adsorption of H2 and hydrogen spillover. Sufficient H atoms are thereby generated for CO2 hydrogenation and helpful to create oxygen vacancies on the ZrO2 surface. Higher basicity correlated to the oxygen vacancies further enhances CO2 adsorption and activation. Hence, a superior low temperature activity for CO2 methanation was achieved. For example, a CO2 conversion of 71.9% with a CH4 yield of 69.5% was achieved over the plasma decomposed catalyst at 300 °C with a feeding gas consisting of H2 (32 ml min−1), CO2 (8 ml min−1) and N2 (10 ml min−1) at a GHSV of 60,000 h-1. The corresponding TOF value towards CO2 conversion is 0.61 s-1. However, the CO2 conversion, CH4 yield and TOF value are only 32.9%, 30.3% and 0.39 s-1 at the same reaction conditions over the thermally prepared catalyst. The operando DRIFT analyses demonstrate that CO2 methanation over the plasma decomposed catalyst follows the CO-hydrogenation route. The exposed high-coordinate Ni(111) facets of the plasma decomposed catalyst facilitate the decomposition of CO2 and formates into adsorbed CO. The subsequent hydrogenation of adsorbed CO leads to the production of methane. However, the thermally decomposed catalyst with complex Ni crystal structure and more defects mainly takes the pathway of direct formate hydrogenation. The present study confirms the structure of Ni/ZrO2 has significant effect on the catalytic activity for CO2 methanation.

Structural effect of Ni/ZrO2 catalyst on CO2 methanation with enhanced activity

Methanation over Ni/SiO2: Effect of the catalyst preparation methodologies

Manipulating interfacial structure and interaction between metal and supports is important for many heterogenous catalysts with the aim at achieving high and stable activity and selectivity. In this work, two kinds of Ni/CeO2-SiO2 catalysts are fabricated and designed, including CeO2 close contact with Ni nanoparticles (Ni/CeO2-SiO2-P) or CeO2 away from Ni nanoparticles (Ni/CeO2-SiO2-C) for dry reforming of methane. Ni/CeO2-SiO2-P exhibits superior low-temperature activity and H2/CO ratio compared with Ni/CeO2-SiO2-C. CO2 and CH4 conversions on the former (87.3% and 78.5%) are higher than those of the later (80.5% and 67.8%) at 700 °C. Meanwhile, Ni/CeO2-SiO2-P is stable in the long-term study whereas Ni/CeO2-SiO2-C presents poor stability and the activity dramatically decreases in 10 h. The improved performance and stability on Ni/CeO2-SiO2-P originates from more reactive oxygen species and more accessible sites for the formate species on the metal–support interface. The reaction order and activation energy of both catalysts are also calculated in the kinetic studies. This work opens up new possibilities for exploring the effect of metal–support on designing highly efficient heterogeneous catalysts.

Binran Zhao

Papers

Methanation over Ni/SiO2: Effect of the catalyst preparation methodologies

Highly efficient and stable Ni/CeO2-SiO2 catalyst for dry reforming of methane: Effect of interfacial structure of Ni/CeO2 on SiO2

Improved activity of Ni/MgAl2O4 for CO2 methanation by the plasma decomposition

Dielectric barrier discharge plasma for preparation of Ni-based catalysts with enhanced coke resistance: Current status and perspective

The promotion effect of CeO2 on CO2 adsorption and hydrogenation over Ga2O3