CO2/H2 methanation technology of strontia based catalyst: physicochemical and optimisation studies by Box–Behnken design
Introduction
A great demand for energy has led to the increase of carbon dioxide (CO2) emissions into the environment. It has been proven to be a major factor for the greenhouse effect, contributing to the increase of the earth's surface temperature (Xu and Lin, 2016). In the industrial developing country, Kyoto Protocol had limited the greenhouse gas emissions into the environment (Quader et al., 2015). Carbon credit will be awarded for the countries that can reduce the emission of greenhouse gases. To date, the most widely employed mitigation method to treat the CO2 gas is by capturing the CO2 (Luu et al., 2016) produced from power generation sources due to their large contribution in total CO2 emission (about 78%) (EIA, 2013). The captured CO2 can be either sequestered in depleted gas/oil fields (Li et al., 2015) or can be utilized for production of valuable intermediate chemicals and products such as methanol (Taghdisian et al., 2015), olefin, formic acid (Dominguez-Ramos et al., 2015) and methane (Davis and Martin, 2014).
The other alternative and preferable technology to remove and utilize the CO2 gas is by catalytic conversion in which the emitted CO2 gas will be converted to valuable product of methane (CH4). The developed catalysts will not only contribute to the boost of the nation's economy but also create a green and sustainable environment which will catch the eye of the world. The utilization of CO2 was promoted vigorously all over the world, aiming at cutting the CO2 emissions to alleviate the greenhouse effect in the short term (Yang and Wang, 2015). Through this technique, a large amount of CO2 can also be treated in a short time.
A large variety of catalysts have been studied for CO2 hydrogenation for over the years. Most of them are typically bimetallic or trimetallic catalysts which are made from transition metal (Razzaq et al., 2015) and rare earth metal oxides (Razzaq et al., 2013a). However, the alkali and alkaline earth metals have been largely used as promoter or support material for the reaction. The addition of such modifier is an effective way to improve the catalytic reaction (Hoost and Goodwin, 1991), promote the CO dissociation (Borodko and Somorjai, 1999), stabilise the catalyst structure (Yan et al., 2007) or impart thermal stability of the catalyst support (Lakshmi et al., 1997). Shen et al. (1994) who studied on support modification with additives such as K2O, MgO, La2O3, and SnO2 found that these additives influence the number and nature of basic or acid sites on the support. Yan et al. (2007) found that the strontium doping would modify the bulk and the surface structure of the catalyst, resulting in various states of the surface species.
As alkali earth metals, magnesium had been usually investigated as a composite catalyst with other catalytic materials. Park and McFarland (2009) and Kim et al. (2010) found that the addition of Mg into Pd/SiO2 may induce the production of CH4 by initiating the reaction with binding a CO2 molecule and forming a magnesium carbonate species. The addition of MgO into the catalyst had an optimum level as said by Hu et al. (2012) in which the incorporating of this alkaline earth metal oxide beyond the optimal level will reduce the activity of the catalyst. Zhang et al. (2012) had investigated the ion-exchanged of Mn+-SAPO-34 materials which being incorporated with Sr and Na cations. This material acts as the adsorbents for the selective removal of CO2 at ambient conditions.
From the CO2 adsorption data at low pressure, it is clearly indicated that a larger adsorption capacity is obtained in the presence of Sr2+ cation (0.9 mmol/g) compared to the Na+ cation (0.1 mmol/g). They found that Sr2+ occupies the sites within the structure of SAPO-34 that favours the interaction between the alkaline earth metal and the CO2 molecules. Matsuzaki et al. (1997) found that the addition of 169 mmol Sr into Co (3.4 wt%)/SiO2 catalyst achieved 5.5% of CO conversion and 18% of selectivity towards CH4 at reaction temperature of 250 °C. The selectivity of methane was slightly decreased to 17% by a small amount of Sr (84 mmol) while the CO conversion kept rather constant.
Recently, Mignard et al. (2014) had reported the production of methane from the electroreduction of CO2 at a gas diffusion electrode loaded with a strontium-doped lanthanum cuprate perovskite, La1.8Sr0.2CuO4 as electrocatalyst in 0.5 M KOH. They found that there are others products (ethylene and CO) that were also detected from the CO2 electroreduction. The maximum 20% of methane was produced along with 5.3% of ethylene and 0.2% of carbon monoxide after 20 min electrolysis at 10 barg and temperature of 2 °C as well as current density of 300 mA/cm2. From the previous research therefore, the most active catalyst from alkaline earth metals was found to be MgO while the strength of basic sites is in the order of MgO < CaO < SrO < BaO (Hattori, 2004). Although the MgO is less basic than SrO, the used of SrO catalyst is still not been widely investigated in CO2 methanation reaction whether as promoter, support material or based catalyst.
Response Surface Methodology (RSM) is one of the optimisation processes that were used extensively by many researchers in a wide variety of situations, especially in the fields of chemistry and chemical engineering. The main objective of the RSM is to design the experiment to attain the best optimal operating conditions. According to Kumar and Bansal (2013), the method of RSM was proven as an effective statistical tool for optimization of processes in many studies especially in photocatalytic field. In terms of CO2 methanation reaction, however, there is no research was done on RSM in this field. The method was applied in order to check the suitability of RSM on the CO2 methanation reaction. The main objective of this research is to investigate the effect of the strontium loading, calcination temperature and catalyst dosage towards Ru/Mn/Sr/Al2O3 catalyst on the catalytic activity and its physicochemical properties towards CO2 methanation reaction.
Section snippets
Experimental
This research was conducted in four stages. The first stage was the preparation of strontium based catalyst. The second stage involved the catalytic testing using micro reactor coupled with Fourier Transform Infra Red (FTIR) and Gas Chromatography (GC). The third stage was the characterisation using various instrumental analyses techniques and then optimisation of the potential catalyst via RSM.
Results and discussion
Prior to the start of the testing, the Ru/Mn/Sr/Al2O3 catalyst calcined at 1000 °C for 5 h was subjected to pretreatment at 300 °C for 30 min in the presence of H2 (Tada et al., 2011). The reason for preheating the catalyst is to activate the catalyst as well as to remove the moisture in the catalyst. Fig. 1 shows the catalytic activity towards CO2 methanation over various parameters.
Conclusion
The operating condition of 65 wt% of strontia, calcined at 1000 °C for 5 h with 10 g of catalyst dosage would be the ideal conditions for the catalytic methanation of Ru/Mn/Sr (5:30:65)/Al2O3. This optimisation is having similar reaction conditions as has been suggested by Box–Behnken Design. The experimental result gave higher CO2 conversion of 73.10% at reaction temperature 210 °C with 43.58% methane formation. The value was closely agreed with the predicted result obtained by RSM which
Acknowledgement
The authors gratefully acknowledge Universiti Teknologi Malaysia (UTM), Ministry of Education (MOE) for MyBrain 15 scholarship to Susilawati Toemen and Ministry of Science, Technology and Innovation (MOSTI) for the financial support given under E-Science Fund Vote no 4S082.
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