Nov_Dec_AMP_Digital

A D V A N C E D M A T E R I A L S & P R O C E S S E S | N O V E M B E R / D E C E M B E R 2 0 1 7 2 6 fuel and value-added products. Cur- rently, CO 2 is mainly used in the pro- duction of chemicals such as salicylic acid, urea, and its derivatives. However, conversion of CO 2 to other long-chain hydrocarbons and potential fuels is difficult due to its thermodynamic sta- bility. Reduction of CO 2 requires high energy, which makes the process eco- nomically unviable. Many researchers have explored the electrochemical conversion of CO 2 to useful products. Metal-based cata- lysts, such as copper [2] , platinum [3] , iron, tin, silver, and gold [4] , together with car- bons such as g-C 3 N 4 [5] , are used to suc- cessfully reduce CO 2 . Copper, which can convert CO 2 into more than 30 prod- ucts, is the best metal catalyst known so far. However, efficiency and selec- tivity of these catalysts for any product with two or more carbons is quite low (often less than a tenth of a percent [6] ) and thus cannot be practically imple- mented. Recently, in two independent studies [7,8] , multifunctional catalysts were developed that convert CO 2 to gas- oline through direct and indirect hydro- genation, which comprises hydrocar- bons with five or more carbon atoms. In one of these studies, Wei et al. [8] developed a multifunctional cat- alyst comprising Na-Fe 3 O 4 and H-form Zeolite Socony Mobil-5 (HZSM-5) to catalyze the conversion of CO 2 to hy- drocarbons. They also used partially re- duced magnetite to catalyze the RWGS reaction, producing a high amount of carbon monoxide (CO). Subsequent- ly, some of the Fe 3 O 4 sites are convert- ed to Fe 5 C 2 sites, which induces the Fischer-Tropsch synthesis resulting in the conversion of CO to α -olefins. In the last step of the process, olefins re- act to form long-chain hydrocarbons when exposed to HZSM-5 zeolite. Their process shows a CO 2 conversion of 34% and a high selectivity of 73% to C 5 -C 11 hydrocarbons (gasoline range), which increased to 78% when the H 2 /CO 2 ra- tio of feed gas was decreased from 3 to 1. A schematic of the reaction scheme is shown in Fig. 2. By using such conversion methods, CO 2 emissions can be controlled by circulating the car- bon to other product forms. In the long term, it could reduce CO 2 concentration in the atmosphere, as it would be con- verted to other forms of carbon. Reduc- tion of atmospheric CO 2 would reduce global warming, leading to eventual stabilization of SLR. In another study, Gao et al. [7] de- veloped a bifunctional catalyst con- taining partially reduced In 2 O 3 and HZSM-5 for indirect hydrogenation of CO 2 to gasoline-range hydrocarbons. Cu-ZnO-Al 2 O 3 type catalysts are com- monly used for methanol production from CO 2 and hydrogen for eventual transformation to longer hydrocarbons, but these catalysts show high selectivi- ty toward formation of CO via the RWGS reaction, which reduces the yield of gasoline-range hydrocarbons. Indium oxide suppresses the RWGS reaction, resulting in higher selectivity towards CH 3 OH, which can then be converted to gasoline-range C 5+ hydrocarbons by the zeolite. The researchers showed a 78.6% selectivity for C 5+ with a 13.1% CO 2 conversion. Commercial-scale solutions have been developed in recent years as well. The National Aeronautics and Space Administration (NASA) developed a technology [9] that converts CO 2 into fuel using solar power. It uses metal-oxide thin films to produce a photoelectro- chemical cell powered by solar energy. NASA claims its proprietary technolo- gy provides a high-efficiency solution to CO 2 conversion. Carbon Clean Solu- tions Ltd., UK, developed CO 2 capturing solutions and is working with power generation companies in the U.S., UK, and India [10] . Current research and in- dustry trends show that this path could lead to a significant reduction in greenhouse gas emissions, which will help to stabilize rising sea levels. How- ever, these methods and technolo- gies are either at research scale or are too expensive for industrial applica- tions. Therefore, new cost- and perfor- mance-efficient methods of CO 2 capture and conversion—which can be com- bined with other industrial processes to significantly reduce CO 2 concentration in the atmosphere—need to be devel- oped to mitigate climate change and its associated effects, including SLR. FUTURE OUTLOOK Climate change and SLR are pro- jected to have a significant impact on the infrastructure and economy, es- pecially in coastal regions around the globe. Lack of necessary and appropri- ate preparedness could result in huge economic setbacks. A bifurcated ap- proach with short-term and long-term preparedness is needed and would be beneficial for an overall action plan. A more integrated approach is needed both in academia and industry among civil and environmental engineers, ma- terials engineers, and chemical engi- neers to develop solutions. The Netherlands and Japan, two low-lying countries, are leading de- velopment for SLR preparedness. For Fig. 2 — Reaction scheme of a process for direct conversion of CO 2 to gasoline-range hydrocarbons [8] .