Plasma-photocatalytic conversion of CO2 at low temperatures: Understanding the synergistic effect of plasma-catalysis
Graphical abstract
Introduction
Recently, the abatement of carbon dioxide (CO2) has become a major global challenge as CO2 is the main greenhouse gas and its emissions lead to the problems of climate change and global warming. Different strategies are being developed to tackle the challenges associated with CO2 emissions, including carbon capture and storage (CCS), carbon capture and utilization (CCU), reducing fossil fuel consumption and boosting clean and renewable energy use. Direct conversion of CO2 into value-added fuels and chemicals (e.g. CO, CH4, and methanol) offers an attractive route for efficient utilization of low value CO2 whilst significantly reducing CO2 emissions [1]. However, CO2 is a highly stable and non-combustible molecule, requiring considerable energy for upgrading and activation. Various synthetic approaches for CO2 conversion have been explored, including solar driven photochemical reduction [2], electrochemical reduction [3] and thermal catalysis [4]. Despite their potential, further investigation into the development of cost-effective H2 production methods, novel multifunctional catalysts and new catalytic processes are essential to improve the overall energy efficiency of CO2 conversion processes and the product selectivity to practical and implementable levels.
Non-thermal plasma technology provides a promising alternative to the traditional catalytic route for the conversion of CO2 into value-added fuels and chemicals at ambient conditions [5]. In non-thermal plasmas, highly energetic electrons and chemically reactive species (e.g. free radicals, excited atoms, ions, and molecules) can be generated for the initiation of both physical and chemical reactions. Non-thermal plasma has a distinct non-equilibrium character, which means the gas temperature in the plasma can be close to room temperature, whilst the electrons are highly energetic with a typical mean energy of 1–10 eV [6]. As a result, non-thermal plasma can easily break most chemical bonds (e.g. CO bonds), and enable thermodynamically unfavourable chemical reactions (e.g. CO2 decomposition) to occur at ambient conditions. However, the use of plasma alone leads to low selectivity and yield towards the target end-products, and consequently causes low energy efficiency of the plasma processes. Recently, the combination of plasma with catalysis, known as plasma-catalysis, has attracted tremendous interest for environmental clean-up, greenhouse gas reforming, growth of carbon nanomaterials, ammonia synthesis and catalyst treatment [6], [7], [8], [9], [10], [11], [12], [13]. The integration of plasma and solid catalysts has great potential to generate a synergistic effect, which can activate the catalysts at low temperatures and improve their activity and stability, resulting in the remarkable enhancement of reactant conversion, selectivity and yield of target products, as well as the energy efficiency of the plasma process [6]. Direct conversion of CO2 into valuable CO and O2 has been explored using different non-thermal plasmas [5], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24], [25]. However, most previous works have mainly focused on the conversion of CO2 diluted with noble gases (e.g. He and Ar), which is not preferable from an industrial application point of view [14], [22], [25]. Further fundamental work is still required to optimize and improve the energy efficiency of the plasma process. In addition, finding a suitable and cost-effective catalyst for this reaction to enhance the overall efficiency of the process is a great challenge as very limited work has been focused on plasma-catalytic CO2 conversion. A detailed understanding of the synergistic effect resulting from the combination of plasma and photocatalysts at low temperatures is still required due to gaps in current knowledge resulting in only a vague idea of the interactions occurring. For example, it is not clear what the roles are of UV light and highly energetic electrons generated by the plasma in the plasma-photocatalytic chemical reactions.
In this work, a coaxial dielectric barrier discharge (DBD) has been developed for the plasma-photocatalytic conversion of pure CO2 into CO and O2 at low temperatures. The effect of photocatalysts (BaTiO3 and TiO2) on the temperatures (plasma gas temperature and the temperature on the catalyst surface) in the CO2 DBD has been evaluated. The synergistic effect resulting from the combination of plasma and photocatalysts (BaTiO3 and TiO2) for CO2 conversion has been investigated from both physical and chemical perspectives for the first time.
Section snippets
Experimental
In this study, a coaxial DBD reactor has been developed for the plasma-catalytic reduction of pure CO2 into CO and O2 at atmospheric pressure and low temperatures (<150 °C), as shown in Fig. 1. An Al foil (ground electrode) was wrapped around the outside of a quartz tube with an external diameter of 22 mm and an inner diameter of 19 mm. A stainless steel tube with an outer diameter of 14 mm was used as the inner electrode (high voltage electrode). The discharge gap was fixed at 2.5 mm, whilst the
Plasma-assisted conversion of CO2 without catalyst
Fig. 2 shows the effect of SED on the conversion of CO2 and the energy efficiency of the plasma reaction in the absence of a catalyst. Clearly, increasing the specific energy density significantly enhances CO2 conversion due to the increase in energy input to the discharge. The conversion of CO2 is increased by a factor of 3 (from 6.7 to 21.7%) as the SED rises from 8 to 80 kJ/L. Similar conversion trends have been reported either using plasma alone or using plasma-catalysis for chemical
Conclusions
In this study, plasma-photocatalytic conversion of CO2 into CO and O2 has been investigated using a DBD reactor combined with BaTiO3 and TiO2 photocatalysts. The combination of plasma with the BaTiO3 and TiO2 photocatalysts in the CO2 DBD slightly increases the gas temperature of the plasma by 6–11 °C compared to the CO2 discharge in the absence of a catalyst at a SED of 28 kJ/L, while the plasma gas temperature in the gas phase is almost the same as the temperature on the surface of the
Acknowledgement
Support of this work by the Engineering and Physical Sciences Research Council (EPSRC) of the UK is gratefully acknowledged.
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