Photocatalytic conversion of CO2 and CH4 using ZnO coated mesh: Effect of operational parameters and optimization
Highlights
► Immobilized ZnO on mesh was used in photocatalytic conversion of CO2 and CH4. ► Experimental design technique was applied to determinate optimum conditions. ► Stainless steel meshes led to prepare large surface area and direct utilization of UV light. ► Effective parameters such as amount of ZnO, feed ratio, and UV power were investigated. ► The products of conversion were characterized to be formate and acetate derivatives.
Introduction
Greenhouse gases such as carbon dioxide, methane and chlorofluorocarbons (CFC) are the main reason of global warming. Current research shows that there is an excess of about 3.9% CO2 with respect to the natural carbon cycle [1], [2]. The natural carbon cycle is the carbon-flow between the atmosphere and oceans, and meanwhile, human activities produce an annual excess of CO2 to the carbon cycle [3]. There is an excess of 115 ppm of CO2 in the atmosphere with respect to the pre-industrial value of 270 ppm that amounts to approximately 900 Gt CO2. In order to bring the CO2 level back to where it was, we need to develop processes, techniques and applications capable of handling CO2 on the scale of 1000 Gt. Handling CO2 at this scale implies significant challenges, especially in terms of how we transform it and how we either use it or store it safely [2].
The reduction of carbon dioxide pollutant effect is one of the most important territories for researchers in environmental and chemistry science, not only for solving the global environmental problems but also for finding a new approach to support vital carbon resources [4]. Many efforts are performed to decrease CO2 emissions such as CO2 capture followed by compression and geological isolation [5]. These processes are energetically intensive and thus costly. In addition, there are many uncertainties with regard to long-term storage of CO2 in geological formations [6]. For permanence solving the CO2 problem, an alternative and more preferable direction is to convert CO2 as a fuel feedstock into other useful or non-toxic compounds.
CH4, the main component of natural gas, is one of the most abundant, low-cost, carbon-based feedstock [7], [8], [9]. The methane has increased in the atmosphere as a result of human activities related to agriculture, natural gas distribution and landfills. The concentration of CH4 in the atmosphere has almost tripled in the last 150 years. Current natural and man-made sources include coal mining but also fermentation sources from ruminant livestock, rice cultivation, landfill, wastewater, wetlands and marine sediments [10]. Also, methane is a dominant greenhouse gas responsible for nearly one-fifth of anthropogenic global warming. As a comparison with other greenhouse gases, methane is 25 times more powerful than CO2 over a 100-year time horizon (GWP100=25) and global warming is likely to enhance methane release from a number of sources (including permafrost and submarine methane hydrates). Consequently, the transforming atmospheric CH4 into equimolar amounts of CO2 can have a significant impact on reducing global warming [10].
Conversion and utilization of CO2 as well as CH4 are important subjects in the field of C1 chemistry, but there is no practical technique for such conversion [7], [8], [9]. Generally, the direct conversion of CO2 and CH4 to oxygenated compounds is not thermodynamically favorable [8], and catalysts for efficient and selective conversion have not been developed. Conversion of CO2 is a reduction reaction, while that of CH4 is an oxygenation process. The process, which realizes the conversion of CH4 and CO2 at the same time, is an ideal redox reaction.
Application of photocatalysts is one of the most promising methods for reduction of CO2 since UV or visible light irradiation at certain conditions can reduce it to useful compounds [6], [11], [12], [13]. Irradiation with distinct energy was utilized as the excitation source for semiconductor catalysts (such as TiO2, ZrO2, CdS, ZnO, CeO2, NbO5, etc.), and the photo-excited electrons reduce CO2 with another compound as a reductant such as H2, CH4 or H2O on the catalyst surface and create energy-bearing products such as carbon monoxide (CO), methanol (CH3OH), formaldehyde, acetic acid, etc. [13], [14], [15], [16]. Normally, the process requires some reductants. CH4, the main component of natural gas, is one of the most abundant, low-cost and carbon-based feedstock. Conversion of CO2 requires consumption of hydrogen, while that of CH4 requires oxygen. Therefore, the conversion of CO2 and CH4 together is an ideal combination of a reduction and an oxygenation reaction [8].
The direct conversion of CO2 and CH4 mixture into valuable products by catalysts is a pleasing process, as some papers have been reported for this purpose. Huang et al. [8] investigated the feasibility of the direct conversion of CH4 and CO2 to oxygenated compounds at low temperature by means of a two-stepped reaction sequence on Cu–Co-based catalysts. Direct synthesis of acetic acid by means of homogeneous [17] and heterogeneous [18] catalysts without irradiation were performed. Kim et al. [19] reported the conversion of the gas mixture CH4:CO2:He=1:1:1 into synthetic gas (H2/CO) at 500 °C, using catalysts containing two different amount of Ni supported on γ-Al2O3 under the electron beam radiation influence. Zhang et al. [9] used dielectric-barrier discharge plasma to converse methane in the presence of carbon dioxide and the product contained gaseous hydrocarbons, syngas and oxygenated compounds. A series of Ni/SiO2 catalysts containing different amounts of Gd2O3 as a promoter was prepared by Guo et al. [20], and used for carbon dioxide reforming of methane in a fluidized bed reactor. Fidalgo et al. [21] studied the microwave-assisted CO2 reforming of CH4 over mixtures of carbonaceous materials (activated carbon and metallurgical coke) and an in-lab prepared Ni/Al2O3 was studied. However, these reactions often require a severe condition of high temperature and/or high pressure. On the contrary, photocatalytical reduction of carbon dioxide assisted with photo-irradiation can be occurred even under mild conditions.
The most widespread used photocatalyst is TiO2 [22], but in the recent years ZnO has attracted special attention owing to its low cost. Among various semiconductor metal oxides, ZnO presents itself as one of the promising photocatalyst for the photocatalytic degradation of organic dyes and chemicals due to its versatile properties such as direct and wide band gap (∼3.37 eV), large exciton binding energy (60 meV), semiconducting, piezoelectric and pyroelectric properties and so on. Moreover, ZnO exhibits almost similar band gap and degradation mechanism as of TiO2[23], [24], [25], [26], [27]. Despite the importance of ZnO in the photocatalytic processes, little work has been done on ZnO thin films and their photocatalytic properties, especially in gas phase reactions.
Only a few papers have used photocatalysts to converse CO2 and methane mixture into usable products. Tanaka et al. [28], [29] used zirconium oxide (ZrO2) for photoreduction of carbon dioxide to carbon monoxide by hydrogen and methane. The amount of CO formation improved with increasing the irradiation time, although the rate of CO formation was slightly decelerated. Shi et al. [7], [30] applied coupled Cu/CdS–TiO2/SiO2 semiconductor photocatalyst conversion of CH4 and CO2 to oxygenated compounds. Also, the mechanism of the CO2photocatalytical reduction to CO in the presence of H2 or CH4 over MgO was studied by Teramura et al. [31]. Yuliati et al. [32] showed that methane converted to only hydrocarbons and hydrogen in the presence of carbon dioxide on gallium oxide photocatalyst at room temperature, although CO was additionally produced photocatalytically at mild temperatures such as 200 °C. Also, it is reported that ZnO can reduce CO2 to oxygenated compounds using irradiation in the presence of reductants such as H2O under high pressures of 25–35 kg/cm2 of CO2 gas [33].
To establish better conditions by considering all the effective factors, plentiful experiments have to be carried out with all the possible parameter combinations, which is not practical [34]. The design of experiments (DOE) is one approach, by taking a large number of variables, to reduce number of experiments. Statistical experimental design using response surface methodology (RSM) was applied to optimize all of the effecting parameters [35]. RMS is an effective technique for the optimization of complicated systems such as chemical reactions and/or industrial processes, which enables the evaluation of effects of multiple parameters, alone or in combination with response variables [36]. RSM also quantifies the relationship between the controllable input parameters and the obtained response surfaces. Process optimization by RSM is faster for gathering experimental research results than the rather conventional, time consuming one-factor-at-a-time approach [37].
In this study, for the first time calcinated ZnO semiconductor was coated on stainless steel mesh and used for photocatalytic conversion of carbon dioxide in the presence of methane as the reductant under the mild conditions. The mesh structure provides a large surface area for ZnO film, also, suitable ventilation for gases passing and good utilization of UV light. The experiments were done under UV–A irradiation in an appropriate gas-phase self-designed batch reactor equipped with a temperature and pressure controller system. Statistical experimental design using the RSM was applied to optimize all of the effecting parameters such as amount of coated calcinated ZnO, reactor initial pressure (total pressure), CO2:CH4:He feed ratio and UV-lamp power, and better evaluation of the interaction among the factors.
Section snippets
Photocatalyst preparation
ZnO (Wurtzite hexagonal structure) was purchased from Merck Company, and used to coat on 120 mesh size stainless steel mesh. The ultra-high purity CO2, CH4 and He gases were used as feeds and neutral mediums. Ethanol (96%), nitric acid (65%), hydrochloric acid (37%) and acetone were obtained from Merck. The deionized water was used in catalyst preparation.
Immobilization of ZnO particles on stainless steel meshes was done using a simple and efficient method which is ideal for industrial
Characterization of calcinated ZnO catalyst
The ZnO particles were coated on mesh by a simple and inexpensive method. For suitable immobilization and increasing durability of ZnO on the support, particles were calcinated after coating on the mesh surface. The optimum calcination temperature (350 °C) was selected from literature [42], [43]. It is reported that size of the particles were reduced by calcination method [42], which can increase surface area of catalyst. The calcination leads to decreasing agglomeration and uniform distribution
Conclusion
This study showed the application of experimental design technique and response surface methodology for determining the optimal operation variables in photocatalytic conversion of CO2 and CH4 using calcinated immobilized ZnO coated on stainless steel meshes. The results generated from the statistical analysis and model evaluation revealed that the RSM method can be effectively adopted to optimize operation variables and the experimental values agreed with the predicted ones. The significant
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