Dry reforming of ethanol and glycerol: Mini-review

catalysts Review Dry Reforming of Ethanol and Glycerol: Mini-Review Jie Yu 1,*, José A. Odriozola 2,3 and Tomas R. Reina 2 1State Key Laboratory of Coal Combustion, Huazhong University of Science and Technology, Wuhan 430074, China 2Department of Chemical and Process Engineering Department, University of Surrey, Guildford GU2 7XH, UK; [email protected] (J.A.O.); [email protected] (T.R.R.) 3 Departamento de Qu í mica Inorg á nica, Universidad de Sevilla, Instituto de Ciencias de Materiales de Sevilla Centro mixto US-CSIC Avda. Américo Vespucio 49, 41092 Seville, Spain *Correspondence: [email protected] Received: 12 November 2019; Accepted: 28 November 2019; Published: 2 December 2019   Abstract: Dry reforming of ethanol and glycerol using CO 2 are promising technologies for H 2 production while mitigating CO 2 emission. Current studies mainly focused on steam reforming technology, while dry reforming has been typically less studied. Nevertheless, the urgent problem of CO 2 emissions directly linked to global warming has sparked a renewed interest on the catalysis community to pursue dry reforming routes. Indeed, dry reforming represents a straightforward route to utilize CO 2 while producing added value products such as syngas or hydrogen. In the absence of catalysts, the direct decomposition for H 2 production is less efficient. In this mini-review, ethanol and glycerol dry reforming processes have been discussed including their mechanistic aspects and strategies for catalysts successful design. The effect of support and promoters is addressed for better elucidating the catalytic mechanism of dry reforming of ethanol and glycerol. Activity and stability of state-of-the-art catalysts are comprehensively discussed in this review along with challenges and future opportunities to further develop the dry reforming routes as viable CO 2 utilization alternatives. Keywords: CO2utilization; dry reforming; ethanol; glycerol; catalyst 1. Introduction Carbon dioxide is one of the key greenhouse gases causing the observed global warming. Therefore, more attention has been given to the utilization of renewable energy and CO 2 recycling technologies. It is mainly from the combustion of fossil fuels since the Industrial Revolution age. In 1750, the CO 2 concentration in the atmosphere was 277 ppm according to Joos et al. and reached concentrations as high as 406.7 ppm in July 2017 [ 1 ]. However, CO 2 emissions have been found to be constant for the last three years as shown Figure 1a according to the International Energy Agency, proving the effect of the global effort regarding environmental policies. Carbon capture and storage is regarded as a main method to limit CO 2 release in the atmosphere; nevertheless, this technology has not demonstrated commercial viability so far. A more challenging alternative from the chemical and engineering perspective is the carbon capture and recycling (CCR) approach, where CO2is converted and recycled back to fuels and added value chemicals. Catalysts 2019,9, 1015; doi:10.3390/catal9121015 www.mdpi.com/journal/catalysts Catalysts 2019,9, 1015 2 of 18 Catalysts 2019, 9, 1015 2 of 20 Figure 1. CO 2 emissions from fossil fuels and industry for (a) the globe (grey shading); (b) global emissions by fuel type; (c) territorial CO 2 emissions for USA, China, India, and for the EU— reproduced from Le Quéré et al. [2]. CO 2 can react with methane following the reaction of dry reforming of methane reaction (DRM) forming syngas. Syngas contained a mixture of carbon monoxide and hydrogen, which is a crucial raw compound in the chemical industry. Applications of synthesis gas are summarized in Figure 2. It can be used as base material for methanol synthesis (Equation (1)) that can later be upgraded to higher commercial value chemicals such as dimethyl ether (DME), formaldehyde or acetic acid. When introduced into a Fisher–Tropsch unit, syngas can produce fuels or waxes (Equation (2)). Finally, it can be rearranged into methane (Equation (3)) or used as a hydrogen source: CO + 2H2 ⇌ CH3OH, (1) nCO + (2n + 1) H2 ⇌ CnH2n+2 + H2O, (2) CO + 3H2 ⇌ CH4 + H2O. (3) Figure 2. Applications of syngas. Figure 1. CO 2 emissions from fossil fuels and industry for ( a ) the globe (grey shading); ( b ) global emissions by fuel type; ( c ) territorial CO 2 emissions for USA, China, India, and for the EU—reproduced from Le Quéréet al. [2]. CO2can react with methane following the reaction of dry reforming of methane reaction (DRM) forming syngas. Syngas contained a mixture of carbon monoxide and hydrogen, which is a crucial raw compound in the chemical industry. Applications of synthesis gas are summarized in Figure 2. It can be used as base material for methanol synthesis (Equation (1)) that can later be upgraded to higher commercial value chemicals such as dimethyl ether (DME), formaldehyde or acetic acid. When introduced into a Fisher–Tropsch unit, syngas can produce fuels or waxes (Equation (2)). Finally, it can be rearranged into methane (Equation (3)) or used as a hydrogen source: CO +2H2CH3OH, (1) nCO +(2n +1) H2CnH2n+2+H2O, (2) CO +3H2CH4+H2O. (3) Catalysts 2019, 9, 1015 2 of 20 Figure 1. CO 2 emissions from fossil fuels and industry for (a) the globe (grey shading); (b) global emissions by fuel type; (c) territorial CO 2 emissions for USA, China, India, and for the EU— reproduced from Le Quéré et al. [2]. CO 2 can react with methane following the reaction of dry reforming of methane reaction (DRM) forming syngas. Syngas contained a mixture of carbon monoxide and hydrogen, which is a crucial raw compound in the chemical industry. Applications of synthesis gas are summarized in Figure 2. It can be used as base material for methanol synthesis (Equation (1)) that can later be upgraded to higher commercial value chemicals such as dimethyl ether (DME), formaldehyde or acetic acid. When introduced into a Fisher–Tropsch unit, syngas can produce fuels or waxes (Equation (2)). Finally, it can be rearranged into methane (Equation (3)) or used as a hydrogen source: CO + 2H2 ⇌ CH3OH, (1) nCO + (2n + 1) H2 ⇌ CnH2n+2 + H2O, (2) CO + 3H2 ⇌ CH4 + H2O. (3) Figure 2. Applications of syngas. Figure 2. Applications of syngas. Catalysts 2019,9, 1015 3 of 18 Bio-ethanol, as a renewable biofuel, has been proposed as a substitute for fossil fuels. First generation of bio-ethanol was produced from food and was also heavily criticized since it opened the dilemma “food or fuel”. Currently, the second generation of bio-ethanol is mainly from lignocellulosic biomass residues and organic municipal solid waste with more complicated methods in order to avoid the consumption of food as shown in Figure 3. Therefore, bio-ethanol can be regarded as nearly CO 2 neutral. In 2018, the total production of bio-ethanol reached 28,700 million gallons and 83.7% is from USA and Brazil [3]. Catalysts 2019, 9, 1015 3 of 20 Bio-ethanol, as a renewable biofuel, has been proposed as a substitute for fossil fuels. First generation of bio-ethanol was produced from food and was also heavily criticized since it opened the dilemma “food or fuel”. Currently, the second generation of bio-ethanol is mainly from lignocellulosic biomass residues and organic municipal solid waste with more complicated methods in order to avoid the consumption of food as shown in Figure 3. Therefore, bio-ethanol can be regarded as nearly CO2 neutral. In 2018, the total production of bio-ethanol reached 28,700 million gallons and 83.7% is from USA and Brazil [3]. Biodiesel, the second most used biofuels after bio-ethanol, also attracted a wide amount of attention, which is mainly from the transesterification reaction of triglycerides. During the process, 1 kg of glycerol, the main by-product, was produced for 9 kg of biodiesel [4]. It was estimated that the production of glycerol each year can reach 4.2 million tons up to 2020 globally, while less than 3.5 million tons is needed each year [5]. Therefore, it is necessary to explore new market opportunities for glycerol. Figure 3. The pathway of production of bio-ethanol and glycerol. A number of processes, such as pyrolysis and gasification with and without catalysts, have been developed to utilize bio-ethanol and glycerol [6–9]. To effectively close the carbon cycle, dry reforming of bio-ethanol and glycerol to produce syngas or H2, which can be used directly or converted into high value chemical compounds were proposed [10–13]. Dry reforming normally occurs at relatively high temperatures (900–1200 K) in the existence of CO2. The process involved several parallel reactions, including direct cracking, water gas shift reaction (WGSR), and methanation reactions, which may affect the final products. Therefore, extensive studies have been conducted to understand the dry reforming pathways of bio-ethanol and glycerol and try to maximize the production of H2 [14,15]. Various catalysts, such as Ni, Rh, and Cu-based, were also proposed to optimize the reaction pathways to avoid the formation of undesired by-products and hinder the formation of carbon on the catalyst. 2. Dry Reforming of Ethanol The simulation of dry reforming of ethanol by thermodynamic analysis has been studied to determine optimum conditions for H2 production [16–18]. Figure 4 showed the variation of gas composition with temperature. For the studied temperature ranges, H2 and CO increase linearly with temperature from 823 to 1123 K reflecting the endothermic character of the reforming reaction. According to Equation (4), an equal amount of H2 and CO can be formed from the ethanol dry reforming. However, the thermodynamic analysis predicted that ethanol can be completely converted to H2, CO, CH4, CO2, H2O, and C. In addition, CH3CHO may be formed by a dehydrogenation reaction following Equation (5). The formed CH3CHO can be further decomposed into CH4 and CO (Equation (6)) or reformed into H2 and CO (Equation (7)). The CH3CHO can also be dehydrated into C2H4 (Equation (8)) or ether Equation (9). Carbon deposition was predicted to happen especially in the low temperature window with a slight decrease of solid carbon formation Lignocellulosic Biomass Pretreatment Hydrolysis Fermentation Purification Glycerol Figure 3. The pathway of production of bio-ethanol and glycerol. Biodiesel, the second most used biofuels after bio-ethanol, also attracted a wide amount of attention, which is mainly from the transesterification reaction of triglycerides. During the process, 1 kg of glycerol, the main by-product, was produced for 9 kg of biodiesel [ 4 ]. It was estimated that the production of glycerol each year can reach 4.2 million tons up to 2020 globally, while less than 3.5 million tons is needed each year [ 5 ]. Therefore, it is necessary to explore new market opportunities for glycerol. A number of processes, such as pyrolysis and gasification with and without catalysts, have been developed to utilize bio-ethanol and glycerol [ 6 – 9 ]. To effectively close the carbon cycle, dry reforming of bio-ethanol and glycerol to produce syngas or H 2 , which can be used directly or converted into high value chemical compounds were proposed [ 10 – 13 ]. Dry reforming normally occurs at relatively high temperatures (900–1200 K) in the existence of CO 2 . The process involved several parallel reactions, including direct cracking, water gas shift reaction (WGSR), and methanation reactions, which may affect the final products. Therefore, extensive studies have been conducted to understand the dry reforming pathways of bio-ethanol and glycerol and try to maximize the production of H 2 [ 14 , 15 ]. Various catalysts, such as Ni, Rh, and Cu-based, were also proposed to optimize the reaction pathways to avoid the formation of undesired by-products and hinder the formation of carbon on the catalyst. 2. Dry Reforming of Ethanol The simulation of dry reforming of ethanol by thermodynamic analysis has been studied to determine optimum conditions for H 2 production [ 16 – 18 ]. Figure 4showed the variation of gas composition with temperature. For the studied temperature ranges, H 2 and CO increase linearly with temperature from 823 to 1123 K reflecting the endothermic character of the reforming reaction. According to Equation (4), an equal amount of H 2 and CO can be formed from the ethanol dry reforming. However, the thermodynamic analysis predicted that ethanol can be completely converted to H 2 , CO, CH 4 , CO 2 , H 2 O, and C. In addition, CH 3 CHO may be formed by a dehydrogenation reaction following Equation (5). The formed CH 3 CHO can be further decomposed into CH 4 and CO (Equation (6)) or reformed into H 2 and CO (Equation (7)). The CH 3 CHO can also be dehydrated into C 2 H 4 (Equation (8)) or ether Equation (9). Carbon deposition was predicted to happen especially in the low temperature window with a slight decrease of solid carbon formation when temperature is increased. Above 1100 Catalysts 2019,9, 1015 4 of 18 K, ethanol was completely transformed into H 2 , CO, and C. According to Equation (10), ethylene polymerization can partially form the carbon. The ethanol can also decompose into CO, H 2 , CH 4 , and acetone directly (Equations (11)–(14)). The formed CO and H 2 can be transformed into methane according to Equations (15) and (16). The DRM is strongly endothermic and can produce hydrogen and carbon monoxide (Equation (17)). At lower temperatures, WGSR, an exothermic and reversible reaction, can favor the production of hydrogen and carbon oxide (Equation (18)). The carbon residues can also be formed following decomposition of methane, Boudouard reaction, and reduction of CO (Equations (19)–(22)). According to the thermodynamics, a two-step reaction mechanism was then proposed for the dry reforming of ethanol. Initially, ethanol was directly decomposed into methane and acetaldehyde intermediates, which can be subsequently reformed to syngas [ 19 , 20 ]. Moreover, H 2 O was predicted due to the reversed WGSR. Therefore, it is evident that ethanol reforming by CO 2 forming dominated for the whole temperatures ranges and other side reactions may also occur such as direct decomposition into methane, H 2 /CO/C mixtures and C 2 H 4 O, methane reforming, and reverse WGSR: cC2H5OH +CO2→3H2+3CO∆Ho 298 =296.7 KJ mol−1, (4) C2H5OH →CH3CHO +H2∆Ho 298 =68.4 KJ mol−1, (5) CH3CHO →CH4+CO ∆Ho 298 =−18.785 KJ mol−1, (6) CH3CHO +CO2→3CO +2H2∆Ho 298 =228.2 KJ mol−1, (7) C2H5OH →H2O+C2H4∆Ho 298 =45.4 KJ mol−1, (8) C2H5OH →1/2H2O+1/2C2H5OC2H5∆Ho 298 =−12.0 KJ mol−1, (9) C2H4→polymer →2C +2H2, (10) C2H5OH →3H2+CO +C∆Ho 298 =124.3 KJ mol−1, (11) C2H5OH →CH4+H2+CO ∆Ho 298 =49.7 KJ mol−1, (12) C2H5OH →3/2CH4+1/2CO2∆Ho 298 =−73.9 KJ mol−1, (13) C2H5OH →0.6CH3COCH3+1.2H2+0.2CO2∆Ho 298 =1.2 KJ mol−1, (14) 2H2+2CO →CH4+CO2∆Ho 298 =−247.0 KJ mol−1, (15) 4H2+CO2→CH4+2H2O∆Ho 298 =−146.7 KJ mol−1, (16) CH4+CO2→2H2+2CO ∆Ho 298 =247.0 KJ mol−1, (17) H2O+CO →H2+CO2∆Ho 298 =−41.1 KJ mol−1, (18) CH4→C+2H2∆Ho 298 =74.6 KJ mol−1, (19) CO →1/2C +1/2CO2∆Ho 298 =−86.2 KJ mol−1, (20) H2+CO →C+H2O∆Ho 298 =−131.3 KJ mol−1, (21) 2H2+CO2→C+2H2O∆Ho 298 =−90.1 KJ mol−1. (22) Catalysts 2019,9, 1015 5 of 18 Catalysts 2019, 9, 1015 4 of 20 when temperature is increased. Above 1100 K, ethanol was completely transformed into H2, CO, and C. According to Equation (10), ethylene polymerization can partially form the carbon. The ethanol can also decompose into CO, H2, CH4, and acetone directly (Equations (11)–(14)). The formed CO and H2 can be transformed into methane according to Equations (15) and (16). The DRM is strongly endothermic and can produce hydrogen and carbon monoxide (Equation (17)). At lower temperatures, WGSR, an exothermic and reversible reaction, can favor the production of hydrogen and carbon oxide (Equation (18)). The carbon residues can also be formed following decomposition of methane, Boudouard reaction, and reduction of CO (Equations (19)–(22)). According to the thermodynamics, a two-step reaction mechanism was then proposed for the dry reforming of ethanol. Initially, ethanol was directly decomposed into methane and acetaldehyde intermediates, which can be subsequently reformed to syngas [19,20]. Moreover, H2O was predicted due to the reversed WGSR. Therefore, it is evident that ethanol reforming by CO2 forming dominated for the whole temperatures ranges and other side reactions may also occur such as direct decomposition into methane, H2/CO/C mixtures and C2H4O, methane reforming, and reverse WGSR: Figure 4. Equilibrium content of gas composition for the ethanol dry reforming process (Reproduced from [16]). CHOH + CO→3H +3CO ∆  = 296.7 KJ mol , (4) CHOH → CHCHO + H ∆ =68.4 KJ mol , (5) CHCHO → CH+CO ∆  = −18.785 KJ mol, (6) CHCHO + CO→3CO+2H  ∆ = 228.2 KJ mol, (7) CHOH → HO+C H ∆ =45.4 KJ mol , (8) CHOH → 1/2HO + 1/2CHOCH ∆ =−12.0 KJ mol , (9) CH→polymer→2C+2H , (10) CHOH → 3H+CO+C ∆  = 124.3 KJ mol , (11) CHOH → CH+H +CO ∆  =49.7 KJ mol , (12) Figure 4. Equilibrium content of gas composition for the ethanol dry reforming process (Reproduced from [16]). Thermodynamic analysis showed that the optimal conditions for dry reforming of ethanol are: reaction temperatures between 1200–1300 K, CO 2 /ethanol ratios of 1.2–1.3. At these conditions, ~45% H 2 , ~50% CO, and complete conversion of ethanol can be realized without remarkable solid carbon formation [ 21 ]. Ethanol conversion increased with the CO 2 /ethanol ratio increasing, but kept almost constant at higher CO 2 /ethanol ratios. A recent experimental study showed that ethanol conversion stabilized from CO 2 /ethanol ratio of 1.4 [ 22 ]. Moreover, the H 2 in syngas also decreased at CO 2 /ethanol higher than 1.4, while CO increased. This trend showcases the direct impact of the reversed WGSR during the dry reforming processes as reported elsewhere [23–25]. In order to avoid the carbon formation and undesired gas components, different catalysts were synthesized and utilized for the dry reforming of ethanol, such as Ni, Cu, Rh, and Co based catalysts. The Ni based catalyst is the most studied one. Al supported nano-NiO-SiO 2 catalysts with Ni amount of 5, 7, 10, and 15% have been synthesized and used for the dry reforming of ethanol [ 26 ]. Catalysts containing 10% Ni showed the best activity partially due to the clustering of Ni particles at higher Ni amounts. Less CO and higher CH 4 were also observed due to higher rate of WGSR and direct decomposition of ethanol into CH 4 promoted by the catalyst, respectively. The desired condition for dry reforming of ethanol was found at 1023 K with the ratio of CO 2 /Ethanol being 1.4. At this condition, the conversion of CO 2 was observed to be 75%, while 100% conversion of ethanol was nearly obtained. Wei et al. used the Ni/KIT-6 (mesoporous silica) and Ni/SiO 2 for the dry reforming of ethanol [ 27 ]. A comparatively better activity and stability was shown for Ni/KIT-6 catalyst; however, Ni/SiO 2 was deactivated quickly. Complete ethanol conversion was obtained at 823 K for Ni/KIT-6 without observing clear deactivation of the catalyst after 40 h tests. Smaller particle size of Ni was observed for Ni/KIT-6, which can be stabilized by the confinement effect of catalyst channels. The smaller particle size of Ni markedly hindered the carbon formation. The stronger interaction of metal-support also inhibited the sintering of active metals. Therefore, the catalyst of Ni/KIT-6 greatly inhibited the coke deposition and contributed to the superior activity. Catalysts of Ni/Y 2 O 3 –ZrO 2 using two different preparation methods of polymerization and wet impregnation were proposed for the dry reforming of ethanol at 600, 700 and 800 ◦ C [ 10 ]. At 600 ◦ C, the highest CH 4 was obtained using a 5%Ni/Y 2 O 3 –ZrO 2 catalyst, due to the direct cracking of ethanol into CH 4 and slow dry reforming rate of CH 4 at 600 ◦ C. Increasing temperature to 700 ◦ C accelerated the dry reforming rate of CH 4 , which was then enhanced further at 800 ◦ C. It was showed that stronger interaction between NiO and oxygen vacancies of support was responsible for its superior reforming ability. The preparation of Catalysts 2019,9, 1015 6 of 18 Ni/Y 2 O 3 –ZrO 2 catalyst by polymerization in one step was proven as an easier and superior way of loading Ni onto Y 2 O 3 –ZrO 2 support than by wet impregnation. In order to prevent the deactivation of Ni catalysts, Ni has also been incorporated into different supports of Al 2 O 3 , CeO 2 , MgO and ZrO 2 to study their activity in the dry reforming of ethanol [ 28 ]. Ni-CeO 2 catalyst showed the best performance at 1023 K than at 973 K, suggesting that CO 2 conversion can be enhanced at higher temperatures, while carbon deposition was hindered. It was found that the reducibility of Ni, which was influenced by the interaction between support and Ni, along with the redox property of CeO 2 can promote the selectivity for H 2 and inhibit the unfavored side reactions. The results revealed that specific surface area of supports can affect the particle size and reducibility of active metals. It can be observed that interaction between Ni and the supports affected the particle size strongly, reducibility of Ni. Oxygen vacancy of support also can inhibit the formation of carbon. To optimize the performance of Ni based catalysts, promoters such as La and Ce were also incorporated [ 11 , 19 , 29 ]. The advanced redox properties of Ce resulted in great benefits for a Ce-Ni/Al 2 O 3 tested in the dry reforming of ethanol. Compared with unpromoted ones, Ce-promoted catalyst can hinder the formation of solid coke due to its oxygen mobility ascribed to CeO 2 as a promoter. The proportion of amorphous carbon as opposed to graphitic carbon was also observed with Ce-Ni/Al 2 O 3 . Upon incorporation of Ce, the interaction between Ni and the supports was strengthened, which can promote the formation of smaller NiO clusters facilitating the reducibility of Ni. The incorporation of Ce lowered reduction temperatures by around 315 K. NiO and NiAl 2 O 4 were observed on the surface of the supports during calcination while CeO 2 was observed on the promoted one. The derivative weight of calcination and H 2 -temperature program reduction (H 2 -TPR) of two catalysts was shown in Figure 5, respectively. As shown in Figure 5a, the first peak, P1 (at 501–509 K), was caused by the direct decomposition of metal nitrates forming the corresponding metal oxides (Equations (23) and (24)). The second peak (P2) (591–598 K) was ascribed to NiAl 2 O 4 formation (Equation (25)). The third peaks (P3) of Ce-promoted catalyst (661 K) was due to the transformation of Ce 3+ to Ce 4+ during calcination (Equation (26)). In Figure 5b, two distinct peaks were observed for the H 2 -TPR analysis of catalysts. The P1 (557–599 K) was due to the formation of metallic Ni from the reduction of NiO (Equation (27)), while P2 peak (668–671 K) was due to the transformation of NiAl2O4to Ni (Equation (28)): Ni(NO3)2→NiO +N2O5, (23) 2Ce(NO3)3→3N2O5+Ce2O3, (24) NiO +Al2O3→NiAl2O4, (25) Ce2O3+0.5O2→2CeO2, (26) NiO +H2→Ni +H2O, (27) NiAl2O4+H2→Ni +Al2O3+H2O. (28) Similarly, La has also been incorporated into Ni/Al 2 O 3 [ 11 , 20 ]. La can enhance the dispersion of nickel particle and hinder the agglomeration of active phases. The best performance of dry reforming of ethanol was observed at 3% La loading due to the benefits of La 2 O 3 as promoter. The H 2 -TPR analysis of three catalysts were shown in Figure 6. Different from Ce promoted Ni/Al 2 O 3 , three peaks were observed for 3%La-10%Ni/Al 2 O 3 . The first one, P1, was due to the reduction of NiO to metallic Ni form. The second peak P2 was due to the reduction of NiO particles strongly absorbed by supports. The third peak P3 was due to the reduction of spinel NiLa 2 O 4 phase to elementary Ni. For 10%Ni/Al 2 O 3 and 3%La-10%Ni/Al 2 O 3 , the temperature for NiO reduction (P1 and P2) decreased to a lower value after La loading, which may be ascribed to the transferring electrons from La 2 O 3 to NiO particles. As a result, the enhanced electron density facilitated reduction of NiO. It was also found that dry reforming of ethanol may occur through direct decomposition of ethanol to methane intermediates. The formed intermediated were then reformed to a mixture of CO and H 2 following DRM. Similar to Ce promoted Catalysts 2019,9, 1015 7 of 18 catalyst, amorphous carbon dominated on the surface of used La-Ni/Al 2 O 3 catalyst. This is may be due to the enhanced basicity of the promoted catalysts and the multiple oxidation states of La, which can provide a shortcut of coke conversion by redox reactions (Equations (29) and (30)). LaαOβ+CxH1−x→LaαOβ−x+xCO +(1−x)/2H2, (29) LaαOβ−x+CO2→LaαOβ+xCO. (30) Catalysts 2019, 9, 1015 6 of 20 superior way of loading Ni onto Y2O3–ZrO2 support than by wet impregnation. In order to prevent the deactivation of Ni catalysts, Ni has also been incorporated into different supports of Al2O3, CeO2, MgO and ZrO2 to study their activity in the dry reforming of ethanol [28]. Ni-CeO2 catalyst showed the best performance at 1023 K than at 973 K, suggesting that CO2 conversion can be enhanced at higher temperatures, while carbon deposition was hindered. It was found that the reducibility of Ni, which was influenced by the interaction between support and Ni, along with the redox property of CeO2 can promote the selectivity for H2 and inhibit the unfavored side reactions. The results revealed that specific surface area of supports can affect the particle size and reducibility of active metals. It can be observed that interaction between Ni and the supports affected the particle size strongly, reducibility of Ni. Oxygen vacancy of support also can inhibit the formation of carbon. To optimize the performance of Ni based catalysts, promoters such as La and Ce were also incorporated [11,19,29]. The advanced redox properties of Ce resulted in great benefits for a Ce- Ni/Al2O3 tested in the dry reforming of ethanol. Compared with unpromoted ones, Ce-promoted catalyst can hinder the formation of solid coke due to its oxygen mobility ascribed to CeO2 as a promoter. The proportion of amorphous carbon as opposed to graphitic carbon was also observed with Ce-Ni/Al2O3. Upon incorporation of Ce, the interaction between Ni and the supports was strengthened, which can promote the formation of smaller NiO clusters facilitating the reducibility of Ni. The incorporation of Ce lowered reduction temperatures by around 315 K. NiO and NiAl2O4 were observed on the surface of the supports during calcination while CeO2 was observed on the promoted one. The derivative weight of calcination and H2-temperature program reduction (H2-TPR) of two catalysts was shown in Figure 5, respectively. As shown in Figure 5a, the first peak, P1 (at 501– 509 K), was caused by the direct decomposition of metal nitrates forming the corresponding metal oxides (Equations (23) and (24)). The second peak (P2) (591–598 K) was ascribed to NiAl2O4 formation (Equation (25)). The third peaks (P3) of Ce-promoted catalyst (661 K) was due to the transformation of Ce3+ to Ce4+ during calcination (Equation (26)). In Figure 5b, two distinct peaks were observed for the H2-TPR analysis of catalysts. The P1 (557–599 K) was due to the formation of metallic Ni from the reduction of NiO (Equation (27)), while P2 peak (668–671 K) was due to the transformation of NiAl2O4 to Ni (Equation (28)): Figure 5. Derivative weight for temperature-programmed calcination (a) and H2-TPR (b) 3%Ce– 10%Ni/Al2O3 and 10%Ni/Al2O3 catalysts, reproduced with permission from [19]. Ni(NO)→NiO+N O, (23) 2Ce(NO)→3N O+Ce O, (24) NiO + AlO→NiAl O , (25) CeO+0.5O →2CeO  , (26) NiO + H→Ni+H O , (27) NiAlO+H →Ni+Al O+H O. (28) a b Figure 5. Derivative weight for temperature-programmed calcination ( a ) and H 2 -TPR ( b ) 3%Ce–10%Ni/Al2O3and 10%Ni/Al2O3catalysts, reproduced with permission from [19]. Catalysts 2019, 9, 1015 7 of 20 Similarly, La has also been incorporated into Ni/Al2O3 [11,20]. La can enhance the dispersion of nickel particle and hinder the agglomeration of active phases. The best performance of dry reforming of ethanol was observed at 3% La loading due to the benefits of La2O3 as promoter. The H2-TPR analysis of three catalysts were shown in Figure 6. Different from Ce promoted Ni/Al2O3, three peaks were observed for 3%La-10%Ni/Al2O3. The first one, P1, was due to the reduction of NiO to metallic Ni form. The second peak P2 was due to the reduction of NiO particles strongly absorbed by supports. The third peak P3 was due to the reduction of spinel NiLa2O4 phase to elementary Ni. For 10%Ni/Al2O3 and 3%La-10%Ni/Al2O3, the temperature for NiO reduction (P1 and P2) decreased to a lower value after La loading, which may be ascribed to the transferring electrons from La2O3 to NiO particles. As a result, the enhanced electron density facilitated reduction of NiO. It was also found that dry reforming of ethanol may occur through direct decomposition of ethanol to methane intermediates. The formed intermediated were then reformed to a mixture of CO and H2 following DRM. Similar to Ce promoted catalyst, amorphous carbon dominated on the surface of used La- Ni/Al2O3 catalyst. This is may be due to the enhanced basicity of the promoted catalysts and the multiple oxidation states of La, which can provide a shortcut of coke conversion by redox reactions (Equations (29) and (30)). Figure 6. H2-TPR (a) Al2O3, (b) 10%Ni/Al2O3 and (c) 3%La-10%Ni/Al2O3 catalysts, reproduced with permission from [11]. + H →  +xCO+(1−x)/2H  , (29)  +CO → +xCO (30). (30) Co based catalysts promoted by Ce and La have also been proposed for the dry reforming of ethanol [30–32]. The addition of Ce and La facilitated the reduction of Co3O4 to CoO, which can be further transformed into Co. The reducibility of active metals in catalysts were analyzed using H2- TPR. The reduction ability followed the order of 10%Co/Al2O3 < 3%Ce-10%Co/Al2O3 < 3%La- 10%Co/Al2O3. Three peaks were observed. The first peak for all catalysts was due to the reduction of Co3O4 to CoO, while the second peak was ascribed to the formation of Co from CoO reduction. The third small shoulder was most likely ascribed to the reduction of pinel CoAl2O4 species strongly absorbed by support . Consistent with reducibility of catalysts, the dry reforming activity of ethanol followed the order of 3%La-10%Co/Al2O3 > 10%Co/Al2O3 > 3%Ce-10%Co/Al2O3. The relationship indicated that the La and Ce can promote the amount of Co active sites by accelerating the reduction of Co3O4 demonstrating a clear correlation redox properties—catalytic performance. The dry reforming of ethanol was proposed using Cu-based catalysts [33–36]. A catalyst of Cu/Ce0.8Zr0.2O2 was prepared using the co-precipitation method. The synergistic interaction between Figure 6. H 2 -TPR ( a ) Al 2 O 3 , ( b ) 10%Ni/Al 2 O 3 and ( c ) 3%La-10%Ni/Al 2 O 3 catalysts, reproduced with permission from [11]. Co based catalysts promoted by Ce and La have also been proposed for the dry reforming of ethanol [ 30 – 32 ]. The addition of Ce and La facilitated the reduction of Co 3 O 4 to CoO, which can be further transformed into Co. The reducibility of active metals in catalysts were analyzed using H 2 -TPR. The reduction ability followed the order of 10%Co/Al 2 O 3 <3%Ce-10%Co/Al 2 O 3 <3%La-10%Co/Al 2 O 3 . Three peaks were observed. The first peak for all catalysts was due to the reduction of Co 3 O 4 to CoO, while the second peak was ascribed to the formation of Co from CoO reduction. The third small shoulder was most likely ascribed to the reduction of pinel CoAl2O4species strongly absorbed by support. Consistent with reducibility of catalysts, the dry reforming activity of ethanol followed the order of 3%La-10%Co/Al 2 O 3 >10%Co/Al 2 O 3 >3%Ce-10%Co/Al 2 O 3 . The relationship indicated Catalysts 2019,9, 1015 8 of 18 that the La and Ce can promote the amount of Co active sites by accelerating the reduction of Co 3 O 4 demonstrating a clear correlation redox properties—catalytic performance. The dry reforming of ethanol was proposed using Cu-based catalysts [ 33 – 36 ]. A catalyst of Cu/Ce 0.8 Zr 0.2 O 2 was prepared using the co-precipitation method. The synergistic interaction between metal and support along with the redox characteristic of Ce caused a promising activity. A new preparation method of the ball-milling technique was then proposed by the same group for dry reforming of ethanol. The catalyst synthesized by the ball-milling technique showed a better performance than the co-precipitation method, which can be ascribed to the better dispersion of Cu and optimized interaction between metal and support. Moreover, no encapsulated graphite-type carbon was found for this catalyst. In a further study, Cu-based catalysts on various supports (CeO 2 , ZrO 2 , CeO 2 -ZrO 2 ) were synthesized. As revealed, the reducibility of Cu and oxygen vacancies decreased in the order of CuCeZr >CuCe >Cu/Zr. As expected, the CuCeZr catalyst showed the best performance due to higher metal dispersion, stronger interaction between metal and support, and greater concentration oxygen vacancies that are envisaged as reactive sites for molecules’ activation. To prove the assumption, all catalysts were exposed to H 2 -TPR analysis as shown in Figure 7. For the supports, the incorporation of Zr 4+ accelerated mobility of O and increased reducible Ce 4+ cations. For Cu-based catalysts, the reduction started at about 100 ◦ C with the highest peaks at 142 ◦ C, 154 ◦ C and 162 ◦ C for CuCe, CuCeZr and CuZr, respectively. This is due to the stronger interaction between Cu and CeZr supports that can accelerate oxygen mobility. As a result, catalyst reduction temperature can be reduced. Catalysts 2019, 9, 1015 8 of 20 metal and support along with the redox characteristic of Ce caused a promising activity. A new preparation method of the ball-milling technique was then proposed by the same group for dry reforming of ethanol. The catalyst synthesized by the ball-milling technique showed a better performance than the co-precipitation method, which can be ascribed to the better dispersion of Cu and optimized interaction between metal and support. Moreover, no encapsulated graphite-type carbon was found for this catalyst. In a further study, Cu-based catalysts on various supports (CeO2, ZrO2, CeO2-ZrO2) were synthesized. As revealed, the reducibility of Cu and oxygen vacancies decreased in the order of CuCeZr > CuCe > Cu/Zr. As expected, the CuCeZr catalyst showed the best performance due to higher metal dispersion, stronger interaction between metal and support, and greater concentration oxygen vacancies that are envisaged as reactive sites for molecules’ activation. To prove the assumption, all catalysts were exposed to H2-TPR analysis as shown in Figure 7. For the supports, the incorporation of Zr4+ accelerated mobility of O and increased reducible Ce4+ cations. For Cu-based catalysts, the reduction started at about 100 °C with the highest peaks at 142 °C, 154 °C and 162 °C for CuCe, CuCeZr and CuZr, respectively. This is due to the stronger interaction between Cu and CeZr supports that can accelerate oxygen mobility. As a result, catalyst reduction temperature can be reduced. Figure 7. H2-TPR profiles of the as-prepared catalysts (a) supports; (b) Cu-based catalysts, reproduced with permission from [33]. Due to the high activity for C–C bond cleavage and low coke formation, Rh has been extensively used for dry reforming of methane [37]. To validate its applicability on the dry reforming of ethanol by Rh based catalysts, various Rh based catalysts have been prepared. Figure 8 shows that ethanol decomposition may occur by two major routes [38]: (1) forming an oxametallacycle intermediate on Rh(1,1,1), which was directly decomposed to methane and CO; and (2) dehydrogenated intermediates (C2H4O and acetyl species), which can be further decomposed to CO and CHx species. Figure 7. H 2 -TPR profiles of the as-prepared catalysts ( a ) supports; ( b ) Cu-based catalysts, reproduced with permission from [33]. Due to the high activity for C–C bond cleavage and low coke formation, Rh has been extensively used for dry reforming of methane [ 37 ]. To validate its applicability on the dry reforming of ethanol by Rh based catalysts, various Rh based catalysts have been prepared. Figure 8shows that ethanol decomposition may occur by two major routes [ 38 ]: (1) forming an oxametallacycle intermediate on Rh(1,1,1), which was directly decomposed to methane and CO; and (2) dehydrogenated intermediates (C2H4O and acetyl species), which can be further decomposed to CO and CHx species. Due to the proven redox property of CeO 2 , Rh/CeO 2 has been synthesized for the dry reforming of ethanol [ 14 , 39 ]. A relatively good activity and stability were observed for this catalyst due to the interaction between Rh and CeO 2 and again the advanced redox features of CeO 2 , which can accelerate CO2activation and prevent carbon deposition. The mechanism was illustrated in Figure 9. In further research, Rh/Ce-SBA-15 has been synthesized for the dry reforming of ethanol [ 40 ]. The proportion of Ce relative to Si affected markedly the H 2 production and selectivity. The introduction of Ce into the 1%Rh/SBA-15 increased reforming activity due to the strengthened mobility of oxygen species on the surface of catalysts. Rh-based catalysts were also prepared for the dry reforming of ethanol using MxOy–Al 2 O 3 oxides (M =Zr, Mg, Ni, Ce, La) as supports [ 41 ]. As shown in Figure 10, the production of H 2 followed the order: Rh/NiO–Al 2 O 3 >> Rh/Al 2 O 3≈ Rh/MgO–Al 2 O 3≈ Rh/CeO 2 –Al 2 O 3 Catalysts 2019,9, 1015 9 of 18 >Rh/ZrO 2 –Al 2 O 3≈ Rh/La 2 O 3 –Al 2 O 3 . The superior performance of Rh/NiO–Al 2 O 3 may be due to the existence of NiAl 2 O 4 spinel phase, hindering the rhodium deactivation, and the high dispersion of Rh favored by co-existence of nickel particles. Catalysts 2019, 9, 1015 9 of 20 Figure 8. Reaction pathways observed for ethanol decomposition on Rh surfaces, reproduced with permission from [38]. Due to the proven redox property of CeO2, Rh/CeO2 has been synthesized for the dry reforming of ethanol [14,39]. A relatively good activity and stability were observed for this catalyst due to the interaction between Rh and CeO2 and again the advanced redox features of CeO2, which can accelerate CO2 activation and prevent carbon deposition. The mechanism was illustrated in Figure 9. Figure 9. Reaction pathways observed for ethanol on Rh/CeO2 surface, reproduced with permission from [14]. In further research, Rh/Ce-SBA-15 has been synthesized for the dry reforming of ethanol [40]. The proportion of Ce relative to Si affected markedly the H2 production and selectivity. The introduction of Ce into the 1%Rh/SBA-15 increased reforming activity due to the strengthened Figure 8. Reaction pathways observed for ethanol decomposition on Rh surfaces, reproduced with permission from [38]. Catalysts 2019, 9, 1015 9 of 20 Figure 8. Reaction pathways observed for ethanol decomposition on Rh surfaces, reproduced with permission from [38]. Due to the proven redox property of CeO2, Rh/CeO2 has been synthesized for the dry reforming of ethanol [14,39]. A relatively good activity and stability were observed for this catalyst due to the interaction between Rh and CeO2 and again the advanced redox features of CeO2, which can accelerate CO2 activation and prevent carbon deposition. The mechanism was illustrated in Figure 9. Figure 9. Reaction pathways observed for ethanol on Rh/CeO2 surface, reproduced with permission from [14]. In further research, Rh/Ce-SBA-15 has been synthesized for the dry reforming of ethanol [40]. The proportion of Ce relative to Si affected markedly the H2 production and selectivity. The introduction of Ce into the 1%Rh/SBA-15 increased reforming activity due to the strengthened Figure 9. Reaction pathways observed for ethanol on Rh/CeO 2 surface, reproduced with permission from [14]. Catalysts 2019,9, 1015 16 of 18 academic community should embrace to ensure a solid design of scalable high-performing catalysts. In any case, there is a lot of room for research in this promising area of catalysis for CO2valorization, and this mini-review just intends to be an introductory quick-guide to selected dry reforming reactions showcasing their potential contribution in the next generation of zero-carbon catalytic processes. Author Contributions: Conceptualization, J.Y.; writing—original draft preparation, J.Y., T.R.R.; writing—review and editing, J.A.O. Funding: This research was funded by the National Natural Science Foundation of China (NSFC)(51806078), and the Foundation of State Key Laboratory of Coal Combustion(FSKLCCB1904), the Royal society with the Grants EP/R512904/1 and RSGR1180353. The APC was funded by 51806078. Conflicts of Interest: The authors declare no conflict of interest. References 1. Joos, F.; Spahni, R. 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