Solar photothermochemical alkane reverse combustion

A one-step, gas-phase photothermocatalytic process for the synthesis of hydrocarbons, including liquid alkanes, aromatics, and oxygenates, with carbon numbers (C_n) up to C₁₃, from CO₂ and water is demonstrated in a flow photoreactor operating at elevated temperatures (180–200 °C) and pressures (1–6 bar) using a 5% cobalt on TiO₂ catalyst and under UV irradiation. A parametric study of temperature, pressure, and partial pressure ratio revealed that temperatures in excess of 160 °C are needed to obtain the higher C_n products in quantity and that the product distribution shifts toward higher C_n products with increasing pressure. In the best run so far, over 13% by mass of the products were C₅+ hydrocarbons and some of these, i.e., octane, are drop-in replacements for existing liquid hydrocarbons fuels. Dioxygen was detected in yields ranging between 64% and 150%. In principle, this tandem photochemical–thermochemical process, fitted with a photocatalyst better matched to the solar spectrum, could provide a cheap and direct method to produce liquid hydrocarbons from CO₂ and water via a solar process which uses concentrated sunlight for both photochemical excitation to generate high-energy intermediates and heat to drive important thermochemical carbon-chain-forming reactions.Oil is essential for sustaining the current global population and economy because it is the primary source of transportation fuels. Diesel, jet, and gasoline hydrocarbon fuels are unrivaled in terms of energy density and ease of use and storage; however, as fossil fuels their combustion leads to a significant anthropogenic contribution of CO₂ to the atmosphere, estimated at 40 × 10⁹ metric tons of CO₂ in 2012 alone (1, 2). The eventual replacement of oil with fuels generated from sustainable and carbon-neutral sources is necessary if we are to avoid harmful climate change due to the buildup of greenhouse gases in the atmosphere (3). Advances in solar-based technologies are the most promising (4); however, these technologies generally produce either electricity or hydrogen, neither of which is an ideal replacement for liquid hydrocarbons. The least disruptive technology would replace oil-derived hydrocarbons with liquid hydrocarbon fuels derived from CO₂, water, and a clean energy source, such as the sun, leading to a carbon-neutral fuel cycle (5–7).Currently, there are a number of promising strategies to harness solar energy to generate high-energy molecules (fuels) from water and/or carbon dioxide, including (i) high-temperature thermochemical cycles (8), (ii) coupling photovoltaics to water electrolysis (PV-EC) (9, 10), (iii) developing single or tandem photoelectrochemical cells (PEC) (11–13), or (iv) direct photochemical methods (PC) using semiconductor materials, often modified by added cocatalysts or nanostructuring techniques (12, 14, 15). Hydrogen, carbon monoxide, C₁ hydrocarbons, and syngas are the most commonly produced fuels and are derived from water or water and CO₂ (6, 16, 17). Hydrogen produced via the water-splitting reaction (WSR, reaction 1) is arguably the easiest to produce and stores the most energy on a mass basis (kJ/kg); however, it is not a particularly attractive replacement fuel for transportation, due to technological issues with low-volume energy density, safe storage, and transportation (18). Moreover, switching to a hydrogen-based transportation fuel would also require a considerable investment in upgrading the existing automotive fleet and fuel distribution infrastructure.One commonly proposed solution to this dilemma is to use the H₂ generated via the WSR, reaction 1, in combination with CO₂ to synthesize liquid hydrocarbon fuels, using the reverse water–gas shift (RWGS), reaction 2, and Fischer–Tropsch synthesis (FTS), reaction 3. The combination of reactions 1–3 is the reverse ofH₂O → H₂ + 1/2O₂?ΔG° = 237.3kJ/mol?(WSR), [1]CO₂+H₂ ? CO + H₂O?ΔG° = 25.2kJ/mol?(RWGS), [2](2n + 1)H₂ + n?CO → C_nH_(2n+2) + n?H₂O?ΔG° ～ ‐99n?kJ/mol?(FTS), [3](n + 1)H₂O + n?CO₂ → C_nH_2n+2 + (3/2n+1/2)O₂?ΔG°～665n?kJ/mol?(ARC)[4]combustion and, as generally proposed, would be carried out as separate unit operations, each with its attendant efficiency losses and capital and operating costs (19, 20). We report here a photothermochemical process for driving the alkane reverse combustion (ARC) reaction (reaction 4) to produce C₁ to C₁₃ hydrocarbons in a single operation unit. If the process was driven by the sun to provide both photons and heat, a solar photothermochemical alkane reverse combustion (SPARC) process could be achieved in one step. If the SPARC reaction could be optimized to predominantly produce liquid hydrocarbons, and these products were derived from atmospheric CO₂, a sustainable and carbon-neutral liquid fuel cycle could be realized.The direct production of C₁ hydrocarbons such as methane, methanol, formic acid, and CO from CO₂ and water in a photoelectrochemical reactor was realized as early as the mid 1970s when Halmann (21) and then Inoue et al. (22) showed that irradiation of TiO₂-coated electrode suspended in CO₂-saturated aqueous solutions yielded a number of C₁ products. Since this time, research has largely focused on the exploration and development of new semiconductor photocatalysts (23), modifications of the semiconductor catalyst in the form of added cocatalysts, e.g., Ni, Cu, Ag, and Pt (23–25), new methods to nanostructure the catalyst (26), and the coupling of molecular dyes and cocatalysts with the heterogeneous catalyst (27, 28). Whereas these advances have led to improvements in catalytic efficiency and quantum yields, little progress has been realized in extending the selectivity of the reaction to favor the more desirable, higher carbon number liquid hydrocarbons (29). Although reports of trace amounts of products, such as ethane, ethanol, acetic acid, propanol, and butanol, are not unknown (30–34), they are the exception and frequently the underlying cause for their generation is unknown. Of these reports, the work of Roy et al. (35) and Varghese et al. (36) stands out. They reported that high-temperature-annealed TiO₂ nanotubes modified with Pt, Pd, or Cu not only gave methane, but also traces of ethane, propane, butane, pentane, and hexane as well as olefins and branched paraffins, although the details regarding product quantification and characterization were omitted. Whereas this work yielded the highest C_n products yet reported, the results contributed little toward a mechanistic understanding as to how to deliberately target these products.Thus, despite over 40 y of research on the photochemical CO₂ reduction with semiconductors, no clear method for directly producing C₅+ liquid hydrocarbon products exists. Herein, we demonstrate that operation of the photochemical ARC reaction at elevated temperatures in the presence of a hybrid FTS-like photocatalyst (cobalt on TiO₂ support) directly yields C₂+ hydrocarbons as the dominant products, including a significant portion of C₅+ liquid hydrocarbons. The key insights are that: (i) even though the FTS reaction is exothermic, a minimum temperature of ∼180 °C is required to obtain reasonable kinetics for the carbon-chain-forming process (37, 38) and (ii) the majority of metals (i.e., Ni, Pd, Pt, Cu) used for CO₂ photocatalytic hydrogenation are poor FTS catalysts as they favor C₁ products (39). Fe, Co, and Ru are commonly used as FTS catalysts because they exhibit high C_n selectivity (39), but they have not been commonly explored in the context of CO₂ photoreduction/hydrogenation. By operating the SPARC reaction at elevated temperature and pressure over a hybrid FTS photocatalyst, the products of the photocatalytic reaction can be consumed in thermal reactions which favor higher C_n products.