CO₂ conversion to hydrocarbons over catalysts has been known for several decades but has been shown very little research and development attention, as other technologies have been much cheaper and efficient in yielding cheap oil. However, with the increasing awareness of the impact CO₂ has on the environment more and more attention is being directed at how to mitigate the effects CO₂ has as a greenhouse gas. Most research to date however is focusing on the sequestration of CO₂ in underground reservoirs.

Our research proposes the utilization of CO₂ into fuel, recycling the gas and using it as a raw material rather than a waste product. In light of dwindling oil resources and the looming presence of peak oil, alternative fuels that are environmentally friendly and enhance energy security are of mounting importance. Our research is aiming at increasing productivity and selectivity of the desired products formed; thus reducing unwanted side-products and lowering costs, making this technology more economically feasible.

—Robert Dorner

Initial tests were performed on a Co/Pt/Al₂O₃ catalyst under several experimental conditions (varying CO₂:H₂ ratios and pressure). This catalyst converts the feed gas predominantly to methane under all conditions (ca. 95%). Iron-based catalysts however show a much improved water-gas-shift and CO₂ hydrogenation ability, mainly yielding short-chain hydrocarbons, and thus making them superior to the Co-based catalysts.

The in-situ reduction environment of the iron catalyst plays a pivotal role in the products formed, with chain-growth only achieved at higher activation temperatures. The product distribution over this iron catalyst shows a clear ability to convert CO₂ to longer chain hydrocarbons (especially olefins), with methane selectivity of only around 30%.

Fe-catalysts therefore lend themselves well to achieve the research objective—synthesizing unsaturated, short-chain hydrocarbons that can be oligomerized to jet fuel, with the help of a second solid acid catalyst, such as zeolites. The Fe-catalyst’s ability to form olefins is tailored by the addition of co-catalysts (such as Mn) at varying loadings, while alternative supports are also investigated to increase CO₂ conversion.

—Robert Dorner

The electrochemical reduction of carbon dioxide. The NRL work was followed by a presentation of work being done at the University of Liverpool (UK) on the electrochemical reduction of CO₂, focused on surface structures of copper electrodes and the role of solution-based copper species for their catalytic effect on the reaction.

The scientific community has known for several decades the ability of certain metals, particularly copper, to convert carbon dioxide into small organic molecules by using electricity as an energy source. This conversion of carbon dioxide occurs only at the interface between the metal surface and carbon dioxide gas. Studying such interfaces is challenging and presents novel research opportunities because the region where the chemistry occurs is of only nanometer dimensions, and therefore identifying specific reactions is like searching for a needle in a very large haystack.

Our work is unique in that we are creating highly controlled reaction environments and using advanced spectroscopic techniques that could, in the needle-in-haystack analogy, provide us an extremely powerful metal detector. This provides an excellent opportunity to study exactly how carbon dioxide transforms into useful, carbon-based, products.

—Scott Shaw

The University of Liverpool work received support from the European Union ELCAT (Electrocatalytic gas-phase conversion of CO₂ in confined catalysts) project. (Earlier post.)

Other papers presented in the symposium included:

• Methane-carbon dioxide reforming over Ni/CaO-ZrO₂ catalyst. Researchers from the Chinese Academy of Sciences are investigating the carbon dioxide reforming of methane over an Ni/CaO-ZrO₂ catalyst derived from co-precipitation method. The catalyst shows both high catalytic activity and stability at the methane and carbon dioxide ratio of 1:1. The characterization confirms that the nano-porous framework of as-prepared support together with the Ni-support interaction enhances the dispersion of Ni, and then promotes the resistance to sintering under reaction condition. As a result, carbon deposition is prevented, which is important for the catalyst stability.

• Ni-based nanocomposite catalysts for energy-saving syngas and hydrogen production from CH₄/CO₂ and CH₄/CO₂/H₂O. Researchers from Tsinghua University (China) are investigating energy-saving catalysts for natural gas conversion. They developed nanostructured Ni-oxide (oxide = ZrO₂, MgO and Al₂O₃) catalysts as nanocomposites consisting of comparably sized metallic Ni nanocrystals and nanoparticles of “support” oxides. Compared with the conventional oxide-supported Ni catalysts, the nanocomposite catalysts are found extremely stable in catalyzing the methane reforming reactions using stoichiometric CO₂ and methane as well as steam (H₂O) and methane.

  The nanocomposite catalysts also show stable catalysis for a combined steam and CO₂ reforming of methane under stoichiometric feed compositions, enabling modulation of product syngas molar ratios (H₂/CO = 1.0~3.0) by varying the feed H₂O/CO₂ ratio. Further tests of nanocomposite Ni-ZrO₂ and Ni-MgO catalysts for hydrogen generation by stepwised methane reforming processes, involving a catalytic methane decomposition to produce pure hydrogen and carbon deposits as the first step (step-I) and a volatilization of the carbon deposits by steam or CO₂ as the second step (step-II) demonstrate that the nanocomposite catalysts are optimistic for energy-saving in methane reforming technologies.

• Photoreduction of CO₂ to CO in the presence of H₂ over various basic metal oxide photocatalysts. Researchers at Kyoto University (Japan) are exploring the chemical fixation of CO₂ in the presence of a heterogeneous photocatalyst as a method for converting it into other carbon sources such as carbon monoxide (CO), formaldehyde (HCHO), formic acid (HCOOH), methanol (CH₃OH), and methane (CH₄).

  The researchers have reported the photoreduction of CO₂ to CO as the product over Rh/TiO₂ and basic metal oxides such as ZrO₂, MgO and Ga₂O₃ in the presence of H₂ as the reductant. In addition, it has been found that CO is formed as a result of the photoreduction of CO₂ in the presence of CH₄ as a substitute for H₂ over ZrO₂ and MgO. In this study, they reported that various metal oxides exhibit the photocatalytic activity for the photoreduction of CO₂ in the presence of H₂.

• Synthesis and characterization of ferrite materials for thermochemical CO₂ splitting using concentrated solar energy. Researchers at Sandia National Laboratories are investigating the use of concentrated solar power to convert carbon dioxide and water to precursors for liquid hydrocarbon fuels (Sunshine to Petrol) using concentrated solar power. (Earlier post.)

  The researchers note that significant advances have been made in the field of solar thermochemical CO₂-splitting technologies utilizing yttria-stabilized zirconia (YSZ)-supported ferrite composites. Such materials work via the basic redox reactions:

  Fe₃O₄ → 3FeO + ½O₂      (Thermal reduction, >1350 °C)

  3FeO + CO₂ → Fe₃O₄ + CO     (CO₂-splitting oxidation, <1200 °C)

  The Sandia team has a systematic study underway of the ferrite-based materials at the high temperatures and conditions present in these cycles. At the ACS meeting, they discussed the synthesis, structural characterization (including scanning electron microscopy and room temperature and in-situ x-ray diffraction), and thermogravimetric analysis of YSZ-supported ferrites.

• CO₂ splitting via two-step solar thermochemical cycles via metal oxide redox reactions: Thermodynamic and kinetic analyses. Researchers from the Paul Scherrer Institute and ETH - Swiss Federal Institute of Technology are using concentrated solar energy as the source of high-temperature process heat in a two-step CO₂ splitting cycle based on Zn/ZnO redox reactions to produce renewable carbon-neutral fuels.

  The solar thermochemical cycle consists of:

  1. the solar endothermic dissociation of ZnO to Zn and O₂;
  2. a non-solar exothermic reduction of CO₂ with Zn to CO and ZnO; the latter is the recycled to first 1st solar step.
  The net reaction is CO₂ = CO + ½O₂, with products formed in different steps, thereby eliminating the need for their separation.

  A Second-Law thermodynamic analysis indicates a maximum solar-to-chemical energy conversion efficiency of 39% for a solar concentration ratio of 5000 suns. The technical feasibility of the first step of the cycle has been demonstrated in a solar furnace with a 10 kW solar reactor prototype. The team experimentally investigated the second step of the cycle in a vertical quartz aerosol reactor, designed for in-situ quenching of Zn vapor, formation of Zn nanoparticles, and oxidation with CO₂. They obtained CO₂ conversions of up to 45% are obtained for a residence time of ~ 1 s.

• Conversion of CO₂ into methanol in a novel two-stage catalyst bed concept. Researchers from Shiraz University (Iran) are investigating a two-stage catalyst bed concept for conversion of CO₂ to methanol.

  In the first catalyst bed, synthesis gas is partly converted to methanol in a conventional water-cooled reactor. This bed operates at higher than normal operating temperature and at high yield. In the second bed, the reaction heat is used to pre-heat the feed gas to the first bed. The continuously reduced temperature in this bed provides increasing thermodynamic equilibrium potential. In this bed, the reaction rate is much lower and, consequently, so is the amount of the reaction heat.

  This feature results in milder temperature profiles in the second bed because less heat is liberated compared to the first bed. In this way the catalysts are exposed to less extreme temperatures and, catalyst deactivation via sintering is circumvented.

  The researchers presented a one-dimensional dynamic plug flow dynamic used to analyze and compare the performance of two-stage bed and conventional single bed reactors. The results of this work show that the two-stage catalyst bed system can be operated with higher conversion and longer catalyst lifetime.

A number of other papers presented during the symposium focused on novel methods for carbon dioxide capture or adsorption of CO₂ on a catalyst as a key step of the catalytic conversion of CO₂ to liquid fuels.