Effects of Alkali Metals on Nickel/Alumina Catalyzed Ethanol Dry Reforming

catalysts

Article Effects of Alkali Metals on Nickel/Alumina Catalyzed Ethanol Dry Reforming

Se-Won Park †, Dongseok Lee †, Seung-Ik Kim , Young Jin Kim, Ji Hoon Park, Iljeong Heo, Tae Sun Chang * and Jin Hee Lee *

Center for Environment and Sustainable Resources, Korea Research Institute of Chemical Technology, Daejeon 34114, Korea; [email protected] (S.-W.P.); [email protected] (D.L.); [email protected] (S.-I.K.); [email protected] (Y.J.K.); [email protected] (J.H.P.); [email protected] (I.H.) * Correspondence: [email protected] (T.S.C.); [email protected] (J.H.L.); Tel.: +82-42-860-7598 (J.H.L.); Fax: +82-42-860-7593 (J.H.L.) † Se-Won Park and Dongseok Lee contributed equally to this work.

Abstract: Although ethanol dry reforming is an attractive carbon utilization technology, problems of severe coke formation and low catalytic activity should be solved for realization of the technology. We demonstrate the effects of alkali metal additives (lithium, sodium, and potassium) on nickel catalyzed

ethanol dry reforming. Potassium doped nickel catalyst (Ni/K2O-Al2O3) showed enhanced catalytic activity and durability in ethanol dry reforming. Thermogravimetric analysis (TGA) showed that

Ni/K2O-Al2O3 had a high resistance to coke formation. The amounts of coke formed on Ni/K2O- Al2O3 were 1/3 lower than the amounts of coke formed on Ni/Al2O3. The total coke quantities were closely correlated to the number of basic sites of the nickel catalysts. Raman spectroscopy and

transmission electron microscopy analyses revealed that the alkali metals control the coke formation on the catalysts.

Citation: Park, S.-W.; Lee, D.; Kim, S.-I.; Kim, Y.J.; Park, J.H.; Heo, I.; Keywords: carbon capture and utilization; ethanol dry reforming; alkali metal; nickel catalyst Chang, T.S.; Lee, J.H. Effects of Alkali Metals on Nickel/Alumina Catalyzed Ethanol Dry Reforming. Catalysts 2021, 11, 260. https://doi.org/ 1. Introduction 10.3390/catal11020260 Greenhouse gases (GHGs) have been cited as a main cause of global warming. Carbon dioxide especially is the main GHG, accounting for 88% of global warming [1–4]. Carbon Academic Editor: Leonarda capture and utilization (CCU) is of enormous interest to mitigate global warming [5,6]. Francesca Liotta As one type of CCU technology, methane dry reforming has been widely studied due to Received: 12 January 2021 its high carbon dioxide conversion and production of a useful gas mixture, synthesis gas Accepted: 12 February 2021 (Equation (1)). Synthesis gas is a key substance that can be converted into a number of Published: 15 February 2021 platform chemicals such as methanol, dimethyl ether, oleﬁns, synthetic fuel, and ammonia.

Publisher’s Note: MDPI stays neutral −1 CH4 + CO2 → 2CO + 2H2 (4H298K = + 247 kJ mol ) (1) with regard to jurisdictional claims in published maps and institutional afﬁl- Although methane dry reforming is a valuable CCU technology, it is necessary to iations. replace petroleum-derived methane with non-petroleum derived chemicals for effective carbon reduction and termination of additional carbon emission. The utilization of alternative carbon sources derived from waste plastic, biomass and/or sewage sludge is an attractive challenge. The thermal decomposition of alternative carbon sources produces Copyright: © 2021 by the authors. useful chemicals including ethanol, acetic acid, ethylene glycol, propionic acid, phenol, Licensee MDPI, Basel, Switzerland. etc. [7]. In this study, for CCU, we take advantage of ethanol, which is obtained by thermal This article is an open access article decomposition of numerous types of biomass, and is highly stable at ambient conditions, distributed under the terms and non-toxic, and readily stored and transported [8–10]. conditions of the Creative Commons One ethanol molecule reacts with one carbon dioxide molecule to produce a one to one Attribution (CC BY) license (https:// ratio of carbon monoxide and hydrogen, similar to methane dry reforming (Equation (2)). creativecommons.org/licenses/by/ For ethanol dry reforming, nickel catalysts have most widely been studied. A number 4.0/).

Catalysts 2021, 11, 260. https://doi.org/10.3390/catal11020260 https://www.mdpi.com/journal/catalysts Catalysts 2021, 11, 260 2 of 13

of transition metal catalysts were used for reforming reactions. Among these catalysts, compared to novel metal catalysts, the nickel based catalysts have been commercialized and the most widely used due to high catalyst activity, a low price, and favorable C-C bond-breaking capability [11,12]. However, existing nickel-based catalysts have some problems such as carbon deposition, sintering, sulfur poisoning, and the formation of toxic nickel compound [13]. For example, Assaf and co-workers [14] reported nickel supported on Y2O3-ZrO2 for ethanol dry reforming and found that the oxygen vacancy in Y2O3-ZrO2 strongly interacted with nickel oxide to hamper catalyst agglomeration. Hu and co-workers [11] investigated the effects of temperature and carbon dioxide/ethanol ratios on coke formation during ethanol dry reforming. Their study pointed out that high temperature and high carbon dioxide concentration are required to reduce coke formation. Vo’s research group [15] reported that performance of nickel catalysts can be enhanced by promotion with cobalt and lanthanum. They also employed lanthanum promoted copper alumina catalyst for the ethanol dry reforming [16]. Cao et al., utilized reducible oxide support for ethanol dry reforming. CuCeZr showed the best activity and durability among the tested catalysts [17]. Lastly, Aziz and co-workers [18] reported that the coke could be removed by the gasiﬁcation, and it could be enhanced by the basic site of the catalyst.

−1 C2H5OH + CO2 → 3CO + 3H2 (4H298K = + 296.7 kJ mol ) (2)

Numerous studies were conducted to improve the catalytic activity and stability. However, the coke formation mechanism is not clear and the catalyst stability is still needed to be improved. In this study, we report a coke-resistant ethanol dry reforming catalyst via the addition of alkali metals to Ni/Al2O3. The alkali metals successfully reduced the coke formation and consequently enhanced catalyst stability. Detailed analyses of deposited coke revealed that the basic sites of the alkali metal promoted catalytic performance and hampered the coke deposition as well. The roles of alkali metals both on suppressing coke formation and accelerating coke removal were studied.

2. Results and Discussion 2.1. Ethanol Dry Reforming Catalyzed by Ni/Al2O3

The catalytic activity of Ni/Al2O3 on ethanol dry reforming was tested under a ﬂow of 1:1 mixture of ethanol and carbon dioxide at 700 ◦C (Figure1). The initial carbon dioxide and ethanol conversion was higher; however, carbon dioxide conversion sharply decreased after 1 h and ethanol conversion decreased after 2 h. These catalytic activity decreases were closely related to product gas composition. The time on stream concentrations of desired products, carbon monoxide and hydrogen, were correspondingly decreased with ethanol and carbon dioxide conversion. Interestingly, methane and ethylene appeared until 1 h and 2 h of the reaction, respectively, and then disappeared. This might be due to the rapid alteration of catalytic active sites. Because methane and ethylene, which can be produced via ethanol dehydrogenation and dehydration, respectively (Scheme1), are by-products of ethanol dry reforming [19,20], we can infer that ethanol dry reforming initially includes ethanol dehydrogenation and dehydration as side reactions. Ethanol and carbon dioxide conversion without nickel are shown in Figure S1. Lower carbon dioxide conversion on Al2O3 indicates that nickel is pivotal to activate carbon dioxide molecule. It is well known that methane and ethylene generate coke on the catalyst surface via decomposition reaction [21]. Thus, the catalytic activity decrease observed in the initial 2 h might originate from the decomposition of methane and ethylene, and the ensuing coke formation. The simultaneous termination of intense catalyst deactivation and formation of methane and ethylene indicate that catalyst deactivation is associated with methane and ethylene. In addition, the initial coke likely formed on the catalyst active site related to the ethanol dehydrogenation and dehydration rather than the carbon dioxide activation. Catalysts 2021, 11, x FOR PEER REVIEW 3 of 14 Catalysts 2021, 11, x FOR PEER REVIEW 3 of 14 Catalysts 2021, 11, 260 3 of 13

FigureFigure 1. Plots 1. Plots of Ni/Al of Ni/Al2O3 catalyzedO catalyzed ethanol ethanol dry reforming dry reforming performed performed at 700 at °C, 700 40,000◦C, 40,000 h−1 gas h −1 gas Figure 1. Plots of Ni/Al2O23 catalyzed3 ethanol dry reforming performed at 700 °C, 40,000 h−1 gas hourly space velocity (GHSV), and ethanol/carbon dioxide ratio of 1. (a) Carbon dioxide and etha- hourlyhourly space space velocity velocity (GHSV), (GHSV), and ethanol/carbon dioxide ratio of 1. (a)) CarbonCarbon dioxidedioxide andand ethanol ethanol conversion, and (b) product gas composition. nolconversion, conversion, and and (b) ( productb) product gas gas composition. composition.

Scheme 1. Ethanol dehydrogenation and dehydration pathway. SchemeScheme 1. 1. EthanolEthanol dehydrogenation dehydrogenation and and dehydration dehydration pathway. pathway. It is well known that methane and ethylene generate coke on the catalyst surface via ItThe is well catalyst known durability that methane is improved and ethylene by additional generate carbon coke dioxideon the catalyst stream. surface There wasvia decomposition reaction [21]. Thus, the catalytic activity decrease observed in the initial 2 decompositionno critical catalyst reaction deactivation [21]. Thus, when the thecatalytic carbon activity dioxide/ethanol decrease observed ratio was in 3the(Figure initial2a). 2 h might originate from the decomposition of methane and ethylene, and the ensuing coke hThe might monitoring originate offrom the the product decomposition gases showed of methane that theand additional ethylene, and carbon the ensuing dioxide coke flow formation. The simultaneous termination of intense catalyst deactivation and formation formation.inhibited the The formation simultaneous of methane termination and ethylene of intense (Figure catalyst2b).These deactivation results furtherand formation support of methane and ethylene indicate that catalyst deactivation is associated with methane ofthat methane the rapid and catalystethylene deactivation indicate that and catalyst coke deactivation formation at is the associated low ethanol with tomethane carbon and ethylene. In addition, the initial coke likely formed on the catalyst active site related anddioxide ethylene. ratio In originates addition, from the initial the formation coke likely of formed unwanted on the byproducts, catalyst active such assite methane related to the ethanol dehydrogenation and dehydration rather than the carbon dioxide activa- toand the ethylene. ethanol dehydrogenation Reduction of methane and dehydration and ethylene rather concentrations than the signifiescarbon dioxide that the activa- carbon tion. tion.dioxide might promote the direct reaction of ethanol and carbon dioxide by preventing ethanolThe catalyst dehydrogenation durability is and improved dehydration. by additional In addition, carbon carbon dioxide dioxide stream. can There hinder was coke The catalyst durability is improved by additional carbon dioxide stream. There was noformation critical catalyst via reverse deactivation Boudouard when reaction the carbon [22,23 dioxide/ethanol]. ratio was 3 (Figure 2a). no critical catalyst deactivation when the carbon dioxide/ethanol ratio was 3 (Figure 2a). The monitoring of the product gases showed that the additional carbon dioxide flow in- The monitoring of the product gases showed that the additional carbon dioxide flow in- hibited2.2. Characterizations the formation of of methane Alkali Metal and Promoted ethylene Ni/Al(Figure2O 32b). These results further support hibited the formation of methane and ethylene (Figure 2b). These results further support that theOur rapid initial catalyst results deactivation show that and the coke minimization formation of at cokethe low formation ethanol by to carboncarbon dioxidediox- that the rapid catalyst deactivation and coke formation at the low ethanol to carbon diox- ideaddition ratio originates is crucial from for stablethe formation catalytic of performance unwanted byproducts, of ethanol drysuch reforming. as methane We and thus ide ratio originates from the formation of unwanted byproducts, such as methane and ethylene.utilized Reduction alkali metal of methane additives and to theethylene nickel co catalystncentrations to increase signifies the that local the carbon carbon dioxide di- ethylene. Reduction of methane and ethylene concentrations signifies that the carbon di- oxideconcentration might promote adjacent the direct to active reaction sites. of The ethanol utilization and carbon of alkali dioxide or alkali-earth by preventing metals eth- for oxide might promote the direct reaction of ethanol and carbon dioxide by preventing eth- anolcarbon dehydrogenation dioxide adsorption and dehydration. has widely In been addition, studied; carbon this process dioxide exploits can hinder the high coke affinity for- anol dehydrogenation and dehydration. In addition, carbon dioxide can hinder coke for- mationof basic via alkalireverse or Boudouard alkali metal reaction sites toward [22,23]. carbon dioxide, which attributed to the highly mation via reverse Boudouard reaction [22,23]. oxidized and electron deficient characteristic of carbon dioxide [24–26].

Catalysts 2021, 11, x FOR PEER REVIEW 4 of 14 Catalysts 2021, 11, 260 4 of 13

◦ −1−1 FigureFigure 2. Plots 2. Plots of ofNi/Al Ni/Al2O32 Ocatalyzed3 catalyzed ethanol ethanol dry dry reforming reforming process process performed performed at at 700 700 °C,C, 40,00040,000 h h gasgas hourly hourly space space velocity velocity (GHSV), (GHSV), and and carbon carbon dioxide/ethanol dioxide/ethanol ratio ratio of 3. of(a) 3. Carbon (a) Carbon dioxide dioxide and and ethanolethanol conversion, conversion, and and (b) ( bproduct) product gas gas composition composition..

2.2. CharacterizationsThe crystal structures of Alkali ofMetal the Promoted prepared Ni/Al catalysts2O3 were determined by X-ray diffraction (XRD)Our analysesinitial results (Figure show3). that Sodium the minimization and potassium of didcoke not formation alter the by original carbon structuredioxide additionof γ-Al is2O crucial3. The for XRD stable patterns catalytic of Na performance2O-Al2O3 and of ethanol K2O-Al dry2O 3reforming.were almost We identical thus uti- to lizedthat alkali of γ-Al metal2O3 additives[27]. However, to the lithiumnickel catalyst produced to increase lithium-aluminum the local carbon mixed dioxide oxide con- phase. centrationMoreover, adjacent the sharp to active peaks sites. of LiThe2O-Al utilization2O3 signify of alkali that or the alkali-earth crystallinity metals of the for resultingcarbon dioxidematerial adsorption increased has after widely lithium been incorporation. studied; this process The XRD exploits patterns the ofhigh Ni/Al affinity2O3 ,of Ni/K basic2O- alkaliAl2 Oor3 ,alkali and Ni/Nametal sites2O-Al toward2O3 showed carbon di nooxide, speciﬁc which diffraction attributed patterns to the highly related oxidized to nickel andoxide. electron The deficient absence characteristic of any diffractions of carbon corresponding dioxide [24–26]. to nickel oxide indicates that the impregnatedThe crystal nickel structures oxide of is the either prepared well dispersed catalysts overwere the determined supports orby forms X-ray adiffraction new crystal (XRD)phase analyses such as (Figure NiAl2O 3).4 [ Sodium28]. The and XRD potassi patternsum ofdid Ni/Li not alter2O-Al the2O original3 are almost structure identical of γ- to the XRD patterns of Li O-Al O . Nickel did not affect the crystal structure of Li O-Al O Al2O3. The XRD patterns 2of Na22O-Al3 2O3 and K2O-Al2O3 were almost identical to that2 of γ2- 3 and likely forms small nickel oxide nanoparticles. The existence of both small nickel oxide Al2O3 [27]. However, lithium produced lithium-aluminum mixed oxide phase. Moreover, particles and NiAl2O4 was conﬁrmed by H2-temperature programmed reduction (H2-TPR) the sharp peaks of Li2O-Al2O3 signify that the crystallinity of the resulting material in- experiments (Figure S2). The reduction peak at lower temperatures (400 ◦C) corresponds creased after lithium incorporation. The XRD patterns of Ni/Al2O3, Ni/K2O-Al2O3, and to the reduction of the well dispersed nickel oxide to metallic nickel and large reduction Ni/Na2O-Al2O3 showed no specific diffraction patterns related to nickel oxide. The ab- peak at high temperature (850 ◦C) corresponds to the reduction of NiAl O [29,30]. sence of any diffractions corresponding to nickel oxide indicates that the2 4impregnated N -adsorption and desorption isotherm analysis was implemented to elucidate the nickel oxide2 is either well dispersed over the supports or forms a new crystal phase such pore structure of the prepared catalysts. All the prepared catalysts showed type IV isotherm as NiAl2O4 [28]. The XRD patterns of Ni/Li2O-Al2O3 are almost identical to the XRD pat- curves with hysteresis at p/po > 0.6. These nitrogen isotherm curves indicate that the terns of Li2O-Al2O3. Nickel did not affect the crystal structure of Li2O-Al2O3 and likely mesoporous nature of the Al O based materials is maintained after alkali metal and nickel forms small nickel oxide nanoparticles.2 3 The existence of both small nickel oxide particles incorporation (Figure S3). Brunauer–Emmett–Teller (BET) surface area and total pore and NiAl2O4 was confirmed by H2-temperature programmed reduction (H2-TPR) experi- volume of Li O-Al O were lower than those of other M O-Al O supports (Table1, entries ments (Figure 2S2). The2 3reduction peak at lower temperatures2 (4002 3 °C) corresponds to the 1–4). The low total pore volume and large average pore size of Li2O-Al2O3 compared to

Catalysts 2021, 11, 260 5 of 13

other types of M2O-Al2O3 indicate that the added lithium blocks small pores of γ-Al2O3 by formation of lithium-aluminum mixed oxide, which was observed by XRD (Figure3). The pore structures of Na2O-Al2O3 and K2O-Al2O3 were not severely changed. Only slight decreases of BET surface area and pore volume were observed (Table1, entries 3–4, and Figure S4). The nickel incorporation as well slightly reduced the BET surface area and pore volume of all M2O-Al2O3 samples, but there were no signiﬁcant alterations of pore structure Catalysts 2021, 11, x FOR PEER REVIEW 5 of 14 (Table1, entries 5–8, and Figure S4) [ 31]. Transmission electron microscopy (TEM) images of the reduced Ni catalysts are shown in Figure S5. At all images, nickel nanoparticles were dispersed on the Al2O3 support. The respective average nickel particle sizes of Ni/Al2O3, reductionNi/Li of the2O-Al well2 dispersedO3, Ni/Na nickel2O-Al oxide2O3 to, andmetallic Ni/K nickel2O-Al and2O large3 is 15.5reduction nm, 24.7peak nm, at 24.7 nm, and high temperature32.9 nm. The(850 °C) added corresp alkalionds metals to the reduction were located of NiAl near2O4 the[29,30]. nickel nanoparticles and these would make the particle size bigger.

Figure 3. XRD patterns of (a)M2O-Al2O3, and (b) Ni/M2O-Al2O3. Figure 3. XRD patterns of (a) M2O-Al2O3, and (b) Ni/M2O-Al2O3. 2.3. Ethanol Dry Reforming Catalyzed by Alkali Metal Modiﬁed Ni/Al2O3 N2-adsorption and desorption isotherm analysis was implemented to elucidate the pore structureTo of evaluate the prepared the inﬂuences catalysts. Al ofl the alkali prepared metal catalysts additives, showed Ni/M type2O-Al IV 2iso-O3 samples were tested for ethanol dry reforming. The reactions were performed at 700◦ stream conver- therm curves with hysteresis at p/po > 0.6. These nitrogen isotherm curves indicate that the mesoporoussion of ethanol nature of and the carbon Al2O3 based dioxide. materials The highestis maintained carbon after dioxide alkali metal andethanol and conversion nickel incorporationappeared with (Figure Ni/K S3).2O-Al Brunauer–Emmett–Teller2O3. Here as well, similar (BET) to surface the case area of and Ni/Al total2O 3, decreases of pore volumecarbon of dioxideLi2O-Al2 andO3 were ethanol lower conversion than those of were other observed M2O-Al2O after3 supports 1 h of (Table reaction, 1, which means entries the1–4). coke The formslow total on pore the catalystvolume and surface. large However,average pore the size deactivation of Li2O-Al2 ratesO3 com- after initial sharp pared todeactivation other types of were M2O-Al varied2O3 betweenindicate that catalysts. the added The lithi catalystum blocks deactivation small pores rates of were evaluated γ-Al2O3by bycalculating formation of slopes lithium-aluminum of the time mixed on stream oxide, carbon which dioxidewas observed conversion by XRD plots (Figure4). (FigureThe 3). The slopes pore werestructures calculated of Na2O-Al using2O3 carbon and K2O-Al dioxide2O3 were conversion not severely between changed. 2 h and 10 h. The Only slight decreases of BET surface area and pore volume were observed (Table 1, entries calculated slopes of time on stream plots are as follows; −0.96 for Ni/K2O- Al2O3 > −1.49 3–4, and Figure S4). The nickel incorporation as well slightly reduced the BET surface area for Ni/Na2O-Al2O3 > −2.77 for Ni/Al2O3 > −3.01 for Ni/Li2O-Al2O3. Potassium and and pore volume of all M2O-Al2O3 samples, but there were no significant alterations of sodium containing catalysts exhibited enhanced catalyst stability than Ni/Al O . On the pore structure (Table 1, entries 5–8, and Figure S4) [31]. Transmission electron microscopy 2 3 (TEM) images of the reduced Ni catalysts are shown in Figure S5. At all images, nickel nanoparticles were dispersed on the Al2O3 support. The respective average nickel particle sizes of Ni/Al2O3, Ni/Li2O-Al2O3, Ni/Na2O-Al2O3, and Ni/K2O-Al2O3 is 15.5 nm, 24.7 nm,

Catalysts 2021, 11, 260 6 of 13

other hand, the addition of lithium deteriorated the catalytic ethanol dry reforming in terms of both activity and durability. The lowest surface area and pore volume, attributed to the formation of lithium-aluminum mixed oxide, might inﬂuence the performance of the nickel catalyst. The time on stream methane and ethylene distributions were examined (Figure S6). The concentrations of methane and ethylene were increased and disappeared at the beginning of the reaction, and the concentration difference between four catalysts was lower than 0.5%. These results indicate that the rate of methane and ethylene formation is similar, but coke formation is suppressed by alkali metal sites. Catalytic activity at lower temperatures than 700 ◦C (550 ◦C, 600 ◦C, and 650 ◦C) were also evaluated (Figure S7). Obviously, Ni/K2O-Al2O3 exhibited the better catalytic activity for both carbon dioxide and ethanol conversion than Ni/Al2O3, at all temperatures. In addition, alkali metal added ◦ nickel catalysts showed carbon dioxide conversion at 550 C while the Ni/Al2O3 were still not activated.

Table 1. Brunauer–Emmett–Teller (BET) surface area and pore structure information of prepared

catalysts analyzed by N2-isothrem.

Surface Area Pore Size Pore Volume Entry Sample (m2/g) (nm) (cm3/g)

1 Al2O3 205 9 0.48 2 Li2O-Al2O3 89 16 0.41 3 Na2O-Al2O3 142 11 0.41 4 K2O-Al2O3 166 10 0.41 5 Ni/Al O 143 11 0.4 Catalysts 2021, 11, x FOR PEER REVIEW 2 3 7 of 14 6 Ni/Li2O-Al2O3 65 17 0.27 7 Ni/Na2O-Al2O3 127 9 0.3 8 Ni/K2O-Al2O3 137 9 0.31

◦ −1 FigureFigure 4. 4.PlotsPlots of of Ni/M Ni/M2O-Al2O-Al2O23O catalyzed3 catalyzed ethanol ethanol dry dry reforming reforming performed performed at 700 °C,C, 40,00040,000h h−1 GHSV,GHSV, and and carbon carbon dioxide dioxide/ethanol/ethanol ratio ratio of 1. of ( 1.a) Carbon (a) Carbon dioxide dioxide conversion, conversion, and and (b) ethanol (b) ethanol conversion.conversion.

2.4. Characterizations of Deposited Coke Since the catalytic activity and durability are closely related to the coke formation, we examined the cokes deposited on the catalysts after 10 h of ethanol dry reforming. As compared to fresh catalysts, coke formation, which caused catalytic activity decrease, was observed in TEM images of all the catalysts (Figure S5 and Figure 5). After the reforming reaction, amorphous carbon and nanotube shaped carbon were mainly observed. Energy- dispersive X-ray spectroscopy (EDS) analyses on spent catalysts showed the presence of carbon around the nickel catalysts more clearly (Figure S8).

Figure 5. TEM images of spent catalysts after 10 h of ethanol dry reforming. (a) Ni/Al2O3, (b) Ni/Li2O-Al2O3, (c) Ni/Na2O-Al2O3, and (d) Ni/K2O-Al2O3.

The structure of carbon was further studied by Raman spectroscopy. The D peak at around 1330 cm−1, which attributes to the disordered carbon structure, and the G peak at

Catalysts 2021, 11, x FOR PEER REVIEW 7 of 14

Catalysts 2021, 11, 260 Figure 4. Plots of Ni/M2O-Al2O3 catalyzed ethanol dry reforming performed at 700 °C, 40,0007 h of−1 13 GHSV, and carbon dioxide/ethanol ratio of 1. (a) Carbon dioxide conversion, and (b) ethanol conversion.

2.4.2.4. CharacterizationsCharacterizations ofof DepositedDeposited Coke Coke SinceSince thethe catalyticcatalytic activityactivity andand durabilitydurability areare closelyclosely relatedrelated toto thethe cokecoke formation,formation, wewe examined examined the the cokes cokes deposited deposited on on the the catalysts catalysts after after 10 10 h h of of ethanol ethanol dry dry reforming. reforming. As As comparedcompared to to fresh fresh catalysts, catalysts, coke coke formation, formation, which which caused caused catalytic catalytic activity activity decrease, decrease, was was observedobserved in in TEM TEM images images of of all all the the catalysts catalysts (Figure (Figure S5 S5 and and Figure Figure5 ).5). After After the the reforming reforming reaction,reaction, amorphous amorphous carbon carbon and and nanotube nanotube shaped shaped carbon carbon were were mainly mainly observed. observed. Energy-Energy- dispersivedispersive X-rayX-ray spectroscopyspectroscopy (EDS)(EDS) analyses analyseson on spent spent catalysts catalysts showed showed the the presence presence of of carboncarbon around around the the nickel nickel catalysts catalysts more more clearly clearly (Figure (Figure S8). S8).

Figure 5. TEM images of spent catalysts after 10 h of ethanol dry reforming. (a) Ni/Al2O3, (b) Figure 5. TEM images of spent catalysts after 10 h of ethanol dry reforming. (a) Ni/Al2O3,(b) Catalysts 2021, 11, x FOR PEER REVIEW Ni/Li2O-Al2O3, (c) Ni/Na2O-Al2O3, and (d) Ni/K2O-Al2O3. 8 of 14 Ni/Li2O-Al2O3,(c) Ni/Na2O-Al2O3, and (d) Ni/K2O-Al2O3.

TheThe structure structure of of carbon carbon was was further further studied studied by by Raman Raman spectroscopy. spectroscopy. The The D D peak peak at at around 1330 cm−−11, which attributes to the disordered carbon structure, and the G peak at aroundaround 1550 cm 1330−1, which cm , attributes which attributes to the graphitic to the disordered sp2 carbon carbon structure structure, were observed and the G in peak at −1 2 the Ramanaround spectra 1550 cmof the, whichfour catalysts attributes (Figure to the 6) graphitic [32–34]. sp Thecarbon ID/IG ratios structure of the were catalysts observed in were 1.5,the 0.98, Raman 0.90, spectra and 1.0 of for the Ni/Al four catalysts2O3, Ni/Li (Figure2O-Al26O)[3, 32Ni/Na–34].2-Al The2O I3D, /IandG ratiosNi/K2 ofO-Al the2O catalysts3,

respectively.were 1.5, Since 0.98, amorphous 0.90, and 1.0 carbon for Ni/Al consti2Otutes3, Ni/Li more2O-Al disordered2O3, Ni/Na structure2-Al2O than3, and gra- Ni/K2O- phitic Alcarbon,2O3, respectively. the ID/IG ratios Since in amorphousthe Raman carbonspectra constitutes of carbon morematerials disordered increase structure as the than amountsgraphitic of amorphous carbon, carbon the ID/I increases.G ratios inThus, the Ramanthe higher spectra ID/IG ratio of carbon of Ni/Al materials2O3 signifies increase as the amounts of amorphous carbon increases. Thus, the higher I /I ratio of Ni/Al O that the more amorphous carbon forms on Ni/Al2O3 than on the alkaliD metalG promoted 2 3 signiﬁes that the more amorphous carbon forms on Ni/Al O than on the alkali metal catalysts, and the alkali metal additives lead to the formation of different2 3 carbon structure. promoted catalysts, and the alkali metal additives lead to the formation of different carbon structure.

Figure Figure6. Raman 6. Raman spectra spectraof catalysts of catalysts after 10 after h of 10ethanol h of ethanol dry reforming. dry reforming.

Since the amounts of deposited coke were also examined by TGA (Figure 7), the weight loss, which is related to the oxidation of carbon species, mainly started after 370 °C. The beginning temperature of coke removal of Ni/Al2O3 was 460 °C, which is about 100 oC higher than that of other catalysts. The lower coke removal initiation temperature of alkali metal promoted catalysts indicates that the alkali metals oxidize the carbon more easily than Ni/Al2O3. As well as the coke oxidation rate, the total coke amounts are also controlled by alkali metal additives. The alkali metal promoted catalysts showed much smaller amounts of coke formation (Figure 7). The suppression of coke formation by alkali metal additives is more pronounced in the amounts of coke relative to the amounts of reacted ethanol (Figure 8). In general, the amounts of coke deposition increase as conversion increases. It is therefore more reasonable to compare the amounts of deposited coke relative to conversion of coke generation source, namely ethanol in this case. Figure 8 shows that all the alkali metal additives successfully control coke deposition. The amount of coke relative to converted ethanol catalyzed by Ni/K2O-Al2O3 is one-third that of Ni/Al2O3. These results show that not only the inhibition of coke formation but also the selective conversion of ethanol and carbon dioxide to synthesis gas are accomplished by alkali metal additives.

Catalysts 2021, 11, 260 8 of 13

Since the amounts of deposited coke were also examined by TGA (Figure7), the weight loss, which is related to the oxidation of carbon species, mainly started after 370 ◦C. ◦ ◦ The beginning temperature of coke removal of Ni/Al2O3 was 460 C, which is about 100 C higher than that of other catalysts. The lower coke removal initiation temperature of alkali metal promoted catalysts indicates that the alkali metals oxidize the carbon more easily than Ni/Al2O3. As well as the coke oxidation rate, the total coke amounts are also controlled by alkali metal additives. The alkali metal promoted catalysts showed much smaller amounts of coke formation (Figure7). The suppression of coke formation by alkali metal additives is more pronounced in the amounts of coke relative to the amounts of reacted ethanol (Figure8). In general, the amounts of coke deposition increase as conversion increases. It is therefore more reasonable to compare the amounts of deposited coke relative to Catalysts 2021, 11, x FOR PEER REVIEWconversion of coke generation source, namely ethanol in this case. Figure8 shows that9 of all14

Catalysts 2021, 11, x FOR PEER REVIEW the alkali metal additives successfully control coke deposition. The amount of coke9 relativeof 14

to converted ethanol catalyzed by Ni/K2O-Al2O3 is one-third that of Ni/Al2O3. These results show that not only the inhibition of coke formation but also the selective conversion of ethanol and carbon dioxide to synthesis gas are accomplished by alkali metal additives. 105 105100 100 95 95 90 90 85 85 80 80 Ni/Al O 75 2 3

Weight (%) Ni/Al Ni/LiO O-Al O 75 70 2 32 2 3

Weight (%) Ni/Li O-Al O Ni/Na2 O-Al2 3 O 70 65 2 2 3 Ni/Na Ni/KO-AlO-AlOO 65 2 2 2 23 3 60 Ni/K O-Al O 2 2 3 60 100 200 300 400 500 600 700 800 900 100 200 300 400 500 600 700 800 900 Temperature (℃) Temperature (℃) Figure 7. Thermogravimetric analysis (TGA) curves of catalysts after 10 h of ethanol dry reform- Figureing.Figure 7. Thermogravimetric 7. Thermogravimetric analysis analysis (TGA) (TGA) curves curves of catalysts of catalysts after after 10 h 10 of h ethanol of ethanol dry dry reform- reforming. ing. ) )

) )

ethanol 1.4 ethanol /mg 1.4 coke /mg 1.2

coke 1.2 1.0 1.0 0.8 0.8

0.6 0.6 0.4 0.4 0.2 0.2

Amounts of deposited coke relative to to coke relative deposited of Amounts 0.0 Ni/Al O Ni/Na O-Al O Ni/K O-Al O 2 3 Ni/Li O-Al O 2 2 3 2 2 3 Amounts of deposited coke relative to to coke relative deposited of Amounts 0.0 2 2 3

amounts of converted ethanol (mg ethanol converted of amounts Ni/Al O Ni/Li O-Al O Ni/Na O-Al O Ni/K O-Al O 2 3 2 2 3 2 2 3 2 2 3

amounts of converted ethanol (mg ethanol converted of amounts Figure 8. Coke analysis relative to converted ethanol. Figure 8. Coke analysis relative to converted ethanol. Figure 8. BecauseCoke analysis alkali relative metals to areconverted known ethanol. to provide basic sites for catalysts, basic sites in eachBecause catalysts alkali were metals examined are known by CO to2-temperature provide basic programmed sites for catalysts, desorption basic sites (CO in2-TPD) each catalysts(FigureBecause9 were) alkali to elucidate examined metals theare by inﬂuence knownCO2-temperature to ofprovide basicity programmedba onsic cokesites for deposition. desorptioncatalysts, The basic (CO two 2sites-TPD) main in (Figureeach carbon catalysts9)dioxide to elucidate were desorption examined the influence peaks by CO appeared of2-temperature basicity at on 130 coke programmed◦C anddeposition. 250–330 desorption The◦C, two which (COmain correspond2-TPD) carbon (Figure dioxide to the 9) desorptionto elucidate peaks the influence appeared of at basicity 130 °C onand coke 250–330 deposition. °C, which The correspond two main carbonto the weak dioxide basic desorptionsite and thepeaks strong appeared basic siteat 130 [31,35]. °C and When 250–330 alkali °C, metals which (Ni/M correspond2O-Al2 Oto3) the were weak added, basic the sitelow and temperature the strong basic desorption site [31,35]. peak When was dominaalkali metalsnt for (Ni/M the alkali2O-Al 2metalO3) were doped added, catalysts, the lowwhile temperature the high desorptiontemperature peak desorption was domina peak ntwas for dominant the alkali for metal Ni/Al doped2O3. The catalysts, order of whilebasic the site high quantity temperature is as desorptionfollows: Ni/K peak2O-Al was2O dominant3 > Ni/Na for2O-Al Ni/Al2O3 2O> 3.Ni/Li The 2orderO-Al2 Oof3 > basicNi/Al site2O 3quantity. This result is showsas follows: that the Ni/K alkali2O-Al metals2O3 > provide Ni/Na 2weakO-Al2 basicO3 > sitesNi/Li for2O-Al nickel2O3 cata-> Ni/Allysts.2O In3. This addition, result the shows catalytic that activitythe alkali order metals is identical provide toweak the numberbasic sites of basicfor nickel sites, cata- except lysts.for InNi/Li addition,2O-Al2 theO3 (Figurecatalytic 4). activity Considering order is that identical the low to the catalytic number activity of basic of sites, Ni/L except2O-Al2 O3 fororiginated Ni/Li2O-Al from2O3 (Figure the low 4). surface Considering area, we that can the infer low that catalytic the increased activity catalyticof Ni/L2 O-Alactivity2O3 of originatedNi/K2O-Al from2O3 theand low Ni/Na surface2O-Al area,2O3 attributeswe can in ferto thethat basic the increased sites. The catalytic correlation activity between of Ni/Knumber2O-Al 2ofO3 basicand Ni/Nasites and2O-Al coke2O 3deposition attributes isto presentedthe basic sites.in Figure The 10.correlation The amounts between of de- numberposited of coke basic decreased sites and greatlycoke deposition on alkali metalis presented promoted in Figure catalysts. 10. TheThis amounts result shows of de- that positedcoke formationcoke decreased is inhibited greatly by on the alkali weak metal basi promotedc sites that catalysts.are offered This by resultalkali showsmetals. that coke formation is inhibited by the weak basic sites that are offered by alkali metals.

Catalysts 2021, 11, 260 9 of 13 Catalysts 2021, 11, x FOR PEER REVIEW 10 of 14

Catalysts 2021, 11, x FOR PEER REVIEW 10 of 14

weak basic site and the strong basic site [31,35]. When alkali metals (Ni/M O-Al O ) were Analyses of spent catalysts and deposited coke suggest that the basic2 sites of2 nickel3 added, the low temperature desorption peak was dominant for the alkali metal doped catalysts are critical to promote ethanol dry reforming and to reduce coke deposition. Analysescatalysts, of while spent the catalysts high temperature and deposited desorption coke suggest peak that was the dominant basic sites for Ni/Alof nickelO . The Given that coke forms by unwanted reactions, i.e., ethanol dehydrogenation and 2ethanol3 catalystsorder are ofcritical basic siteto promote quantity ethanol is as follows: dry reforming Ni/K O-Al and Oto >reduce Ni/Na cokeO-Al deposition.O > Ni/Li O- dehydration (Figure 1 and Scheme 1), the direct 2reaction2 3of ethanol 2and carbon2 3 dioxide2 Given AlthatO coke> Ni/Al forms Oby. unwanted This result reaction showss, that i.e., the ethanol alkali dehydr metalsogenation provide weak and ethanol basic sites for should2 3 be accelerated2 3 to facilitate the formation of synthesis gas rather than methane and dehydrationnickel catalysts.(Figure 1 Inand addition, Scheme the 1), catalyticthe direct activity reaction order of ethanol is identical and tocarbon the number dioxide of basic ethylene. This can be achieved by increasing the carbon dioxide concentration (Figure 2). shouldsites, be accelerated except for to Ni/Li facilitate2O-Al the2O format3 (Figureion4 of). synthesis Considering gas rather that the than low methane catalytic and activity In our catalytic system, the high carbon dioxide affinity of the basic sites induced by alkali ethylene.of Ni/LThis 2canO-Al be2 Oachieved3 originated by increasing from the the low carbon surface dioxide area,we concentration can infer that (Figure the increased 2). metals plays a role similar to that of the high carbon dioxide concentration by increasing In our catalytic activitysystem, the of Ni/K high2 carbonO-Al2O dioxide3 and Ni/Na affinity2O-Al of the2O basic3 attributes sites induced to the basicby alkali sites. The the local carbon dioxide concentration near the nickel active sites [36]. metalscorrelation plays a role between similar to number that of of the basic high sites carbon and dioxide coke deposition concentration is presented by increasing in Figure 10. the localThe carbon amounts dioxide of deposited concentration coke decreased near the nickel greatly active on alkali sites metal[36]. promoted catalysts. This result shows that coke formation is inhibited by the weak basic sites that are offered by alkali metals.

Ni/K O-Al O 2 2 3 Ni/K O-Al O 2 2 3 Ni/Na O-Al O 2 2 3 Ni/Na O-Al O 2Ni/Li2O-Al3 O 2 2 3 Ni/Li O-Al O TCD Signal (a.u.) Signal TCD 2 2 3 Ni/Al O 2 3 TCD Signal (a.u.) Signal TCD Ni/Al O 2 3 100 200 300 400 500 600 700 800

100 200 300Temperature 400 500 600(℃) 700 800

2 Temperature2 2 3. (℃) Figure 9. CO -TPD plots of Ni/M O-Al O

Figure Figure9. CO2-TPD 9. CO plots2-TPD of plotsNi/M of2O-Al Ni/M2O23.O-Al 2O3.

) Ni/Al O

ethanol 1.2 2 3 )

/mg Ni/Al O

ethanol 1.2 2 3

coke 1.0 /mg mg (

coke 1.0 0.8 mg ( 0.8 0.6 Ni/Li O-Al O Ni/Na O-Al O 2 2 3 2 2 3 0.6 0.4 Ni/Li O-Al O Ni/Na O-Al O 2 2 3 2 2 3 Ni/K O-Al O 2 2 3 0.4 ethanol converted of 0.2 Ni/K O-Al O Amounts of deposited to relative of deposited coke Amounts 2 2 3 of converted ethanol ethanol converted of 0.2 amounts amounts Amounts of deposited to relative of deposited coke Amounts 0.0 200 300 400 500

amounts amounts 0.0 200 300Absorbed CO 400(μmol/g ) 500 2 cat

Figure 10. Amounts of depositedAbsorbed coke CO on (Ni/Mμmol/g2O-Al2)O3 as a function of amounts of adsorbed car- Figure 10. Amounts of deposited coke2 on Ni/Mca2O-Alt 2O3 as a function of amounts of adsorbed boncarbon dioxide. dioxide. Figure 10. Amounts of deposited coke on Ni/M2O-Al2O3 as a function of amounts of adsorbed carbon dioxide.3. MaterialsAnalyses and of Methods spent catalysts and deposited coke suggest that the basic sites of nickel catalysts are critical to promote ethanol dry reforming and to reduce coke deposition. 3. Materials3.1. Catalyst and MethodsSynthesis 3.1. Catalyst Synthesis

Catalysts 2021, 11, 260 10 of 13

Given that coke forms by unwanted reactions, i.e., ethanol dehydrogenation and ethanol dehydration (Figure1 and Scheme1), the direct reaction of ethanol and carbon dioxide should be accelerated to facilitate the formation of synthesis gas rather than methane and ethylene. This can be achieved by increasing the carbon dioxide concentration (Figure2). In our catalytic system, the high carbon dioxide afﬁnity of the basic sites induced by alkali metals plays a role similar to that of the high carbon dioxide concentration by increasing the local carbon dioxide concentration near the nickel active sites [36].

3. Materials and Methods 3.1. Catalyst Synthesis 2 Alkali metals were added to γ-Al2O3 (STREM, 97%, SBET = 205 m /g) by incipient wetness impregnation. Calculated amounts of alkali metal precursor LiNO3 (SAMCHUN, 99%), NaNO3 (SAMCHUN, 99%), or KNO3 (SHOWA, 99.5%) were dissolved into 4 mL of distilled water, and the resulting solution was added dropwise to 10 g of γ-Al2O3. The obtained solid was dried at 100 ◦C overnight, followed by calcination at 800 ◦C for 6 h which yielded M2O-Al2O3 (M = Li, Na, or K, 5 wt%). The nickel catalysts were prepared by incipient wetness impregnation of Ni(NO3)2·6H2O (Alfa Aesar) into the M2O-Al2O3 support. The amount of nickel was adjusted to 10 wt%. The Ni(NO3)2·6H2O aqueous solution (5.0 g Ni(NO3)2·6H2O dissolved in 4.0 mL of distilled water) was added dropwise o ◦ to 10 g of M2O-Al2O3. Drying at 100 C for 24 h and ensuing calcination at 800 C for 6 h yielded Ni/M2O-Al2O3 catalyst.

3.2. Catalyst Characterization

XRD analysis was implemented on a Rigaku Ultima IV with built-in a Cu Kα radiation (1.5418Å) to identify the crystal structures of the catalysts. Nitrogen adsorption desorption isotherm analysis was performed using a Micromerit- ics Tristar II to clarify the pore characteristics of the catalysts. The surface area of the catalyst was calculated by the BET method and the pore structure was analyzed using the Barrett–Joyner–Halenda (BJH) model. H2-TPR was measured using a Micromeritics AutoChem II 2920. After filling the U-shaped quartz reactor with 0.15 g of catalyst, the pre-treatment process was carried out ◦ at 300 C for 1 h under helium flow. Then, 10% H2/Ar gas was flowed while increasing the temperature from 50 ◦C to 1000 ◦C at a ramp rate of 10 ◦C·min−1. CO2-TPD was performed using a Micromeritics AutoChem II 2920. After filling the U-shaped quartz reactor with 0.13 g of catalyst, the pre-treatment process was carried out at 300 ◦C for 1 h under a helium flow, followed by reduction at 750 ◦C for 1 h using 5% H2/Ar. Subsequently, helium gas was flowed to remove absorbed H2 and 10% CO2 was absorbed for 60 min at 50 ◦C. The carbon dioxide desorption was achieved by increasing the temperature from 100 ◦C to 900 ◦C at a ramp rate of 5 ◦C·min−1 under helium flow. To confirm the amounts of deposited carbon, TGA was performed by TGA/DSC 1 STARe. The weight change was monitored by increasing the temperature from 25 ◦C to 930 ◦C at a ramp rate of 10 ◦C·min−1 in air. The amount of deposited carbon relative to amounts of reacted ethanol was calculated using the following Equation (3): Amounts of deposited carbon relative to amounts of reacted ethanol =

mgcarbon Amounts of carbon deposition for 10 h ( g ) catalyst . (3) Amounts of converted ethanol for 10 h ( mgethanol ) gcatalyst

Raman analysis was conducted to identify the type of carbon deposited on the catalysts using a Senterra (Bruker) device with a 532 nm laser at 20 mW. Transmission electron microscopy (TEM) images were obtained from a F200S (Talos) microscope operating at 200 kV. Catalysts 2021, 11, 260 11 of 13

3.3. Ethanol Dry Reforming Catalysts 2021, 11, x FOR PEER REVIEW 12 of 14 Figure 11 provides a schematic diagram of the ethanol dry reforming reactor system used in this study. A 1/2 inch quartz tube was used as a reactor and the temperature was controlled using a temperature controller inside the quartz tube. For ethanol dry reforming, 0.50ratio g of of 3. catalyst, The product shaped gases to have were a diameteranalyzed ofby 30–60 gas chromatographyµm, was inserted equipped and the temperature with a ther- ◦ wasmal increasedconductivity to 750detectorC under (Agilent nitrogen Micro-GC ﬂow. Then,490, Molsieve the catalyst 5A; wasH2, CO, reduced CH4, underPora PLOT ﬂow ofU; 5%CO H2, 2C/He2H4, forCP-Wax 1 h at 52 the CB; same C2H temperature.5OH) and infrared Ethanol spectrometer dry reforming (Nicolet was conductediS10, Thermo at ◦ −1 700Fisher).C with a GHSV of 40,000 h . The reaction gases were composed of 5 vol.% carbon dioxideEthanol and 5and vol.% carbon ethanol dioxide balanced conversions with helium were forcalculated a carbon using dioxide/ethanol the following ratio equa- of 1,tion, and Equation 15 vol.% (4). carbon dioxide and 5 vol.% of ethanol for carbon dioxide/ethanol ratio of 3. The product gases were analyzed by gas chromatography equipped with a thermal Conversion (%) conductivity detector (Agilent Micro-GC 490, Molsieve 5A; H2, CO, CH4, Pora PLOT U; Carbon dioxide (or Ethanol) Carbon dioxide (or Ethanol) (4) CO2,C2H4, CP-Wax 52 CB; C2H5OH) and infrared spectrometer (Nicolet100 iS10, Thermo Fisher). Carbon dioxide (or Ethanol)

Figure 11. Schematic diagram of ethanol dry reforming reactorreactor system.system.

4. ConclusionsEthanol and carbon dioxide conversions were calculated using the following equation, EquationThe coke (4). reduction effect of alkali metals on nickel catalyzed ethanol dry reforming was demonstrated. Moreover, the origin of catalyst deactivation was revealed to be coke Carbon dioxide (or Ethanol)input − Carbon dioxide (or Ethanol)output Conversion (%) =deposition via ethanol dehydrogenation and dehydration. The alkali metals× 100 successfully (4) Carbon dioxide (or Ethanol) inhibited coke formation and provided high catalyticinput activity and stability. To the best of our knowledge, this is the first report to employ alkali meals in ethanol dry reforming. 4. Conclusions The potassium modified nickel catalyst Ni/K2O-Al2O3 showed the best performance amongThe the coke investigated reduction catalysts. effect of alkali TGA, metals Raman, on and nickel CO catalyzed2-TPD analyses ethanol verified dry reforming that the washigh demonstrated.coke resistance Moreover, of the alkali the originmetal ofpromoted catalyst deactivationcatalysts attributed was revealed to the tobasic be cokesites depositionoffered by alkali via ethanol metals. dehydrogenation The alkali metals and reduced dehydration. coke formation The alkali and metals accelerated successfully coke inhibitedremoval rate coke by formation increasing and the provided local carbon high catalyticdioxide activityconcentration and stability. near catalytic To the best active of oursites. knowledge, Following thisthese is results, the first the report potassium to employ could alkali be mealsutilized in ethanolas a promoter dry reforming. for reducing The potassiumcoke formation modified in ethanol nickel dry catalyst reforming. Ni/K 2O-Al 2O3 showed the best performance among the investigated catalysts. TGA, Raman, and CO2-TPD analyses verified that the high coke resistanceSupplementary of the Materials: alkali metal The following promoted are catalystsavailable online attributed at www.mdpi.com/xxx/s1, to the basic sites offered Figure byS1: alkaliPlots of metals. γ-Al2O The3 catalyzed alkali metalsethanol reduceddry reforming coke formationperformed andat 700 accelerated °C, 40,000 h coke-1 GHSV, removal and etha- rate bynol/carbon increasing dioxide the localratio of carbon 1. Figure dioxide S2: H concentration2-TPR plots Ni/M near2O-Al catalytic2O3, Figure active S3: N sites.2-adsorption Following de- thesesorption results, isotherm the potassiumplots of Ni/M could2O-Al be2O utilized3, Figure asS4: a BJH promoter pore size for distributions reducing coke of (a) formation M2O-Al2 inO3 ethanoland (b) Ni/M dry reforming.2O-Al2O3, Figure S5: TEM images of reduced catalysts before ethanol dry reforming reaction. Figure S6: Time-dependent concentrations of methane and ethylene by ethanol dry reforming reaction, Figure S7: Plots of carbon dioxide and ethanol conversion by nickel catalysts at different temperatures of 550~700 °C, Figure S8: HAADF-STEM images and EDS analyses on spent catalysts after 10 h of ethanol dry reforming reaction. Author Contributions: Conceptualization: J.H.L. and I.H.; Methodology: S.-W.P., Y.J.K., and J.H.P.; Investigation: S.-W.P., D.L., and S.-I.K.; Resources: I.H., T.S.C.; Writing—original draft: S.W.P., D.L.; Writing—review and editing: Y. J.K., J.H.P., J.H.L., and T.S.C.; Supervision: J.H.L. and T.S.C. All authors have read and agreed to publish the manuscript.

Catalysts 2021, 11, 260 12 of 13

Supplementary Materials: The following are available online at https://www.mdpi.com/2073-434 ◦ 4/11/2/260/s1, Figure S1: Plots of γ-Al2O3 catalyzed ethanol dry reforming performed at 700 C, -1 40,000 h GHSV, and ethanol/carbon dioxide ratio of 1. Figure S2: H2-TPR plots Ni/M2O-Al2O3, Figure S3: N2-adsorption desorption isotherm plots of Ni/M2O-Al2O3, Figure S4: BJH pore size distributions of (a) M2O-Al2O3 and (b) Ni/M2O-Al2O3, Figure S5: TEM images of reduced catalysts before ethanol dry reforming reaction. Figure S6: Time-dependent concentrations of methane and ethylene by ethanol dry reforming reaction, Figure S7: Plots of carbon dioxide and ethanol conversion by nickel catalysts at different temperatures of 550~700 ◦C, Figure S8: HAADF-STEM images and EDS analyses on spent catalysts after 10 h of ethanol dry reforming reaction. Author Contributions: Conceptualization: J.H.L. and I.H.; Methodology: S.-W.P., Y.J.K., and J.H.P.; Investigation: S.-W.P., D.L., and S.-I.K.; Resources: I.H., T.S.C.; Writing—original draft: S.-W.P., D.L.; Writing—review and editing: Y.J.K., J.H.P., J.H.L., and T.S.C.; Supervision: J.H.L. and T.S.C. All authors have read and agreed to publish the manuscript. Funding: This research was performed as project No. SI2111-20, funded by the Korea Research Institute of Chemical Technology (KRICT), and supported by the Technology Innovation Program (20012409, CO2 upcycling cascade hydrogen production technology), funded by the Ministry of Trade, Industry & Energy (MOTIE, Korea), and supported by the National Research Council of Science & Technology (NST) grant by the Korea government (MSIP) No. CAP-16-05-KIMM. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: The data presented in this study are available on request from the corresponding author. The data are not publicly available due to privacy restrictions. Conﬂicts of Interest: The authors declare no conﬂict of interest.

References 1. Grubb, M.; Vrolijk, C.; Brack, D. Kyoto Protocol: A Guide and Assessment, 1st ed.; Routledge: London, UK, 1999. 2. Climent, M.J.; Corma, A.; Iborra, S. Conversion of biomass platform molecules into fuel additives and liquid hydrocarbon fuels. Green Chem. 2014, 16, 516–547. [CrossRef] 3. Zhang, G.; Qu, J.; Su, A.; Zhang, Y.; Xu, Y. Towards understanding the carbon catalyzed CO2 reforming of methane to syngas. J. Ind. Eng. Chem. 2015, 21, 311–317. [CrossRef] 4. Diaz, J.H. The inﬂuence of global warming on natural disasters and their public health outcomes. Am. J. Disaster Med. 2007, 2, 33–43. [CrossRef][PubMed] 5. Gibbins, J.; Chalmers, H. Carbon capture and storage. Energy Policy 2008, 36, 4317–4322. [CrossRef] 6. Markewitz, P.; Kuckshinrichs, W.; Leitner, W.; Linssen, J.; Zapp, P.; Bongartz, R.; Schreiber, A.; Müller, T.E. Worldwide innovations in the development of carbon capture technologies and the utilization of CO2. Energy Environ. Sci. 2012, 5, 7281–7305. [CrossRef] 7. Wang, J.; Cui, H.; Wei, S.; Zhuo, S.; Wang, L.; Li, Z.; Yi, W. Separation of biomass pyrolysis oil by supercritical CO2 extraction. Smart Grid Renew. Energy 2010, 1, 98–107. [CrossRef] 8. Xu, W.; Liu, Z.; Johnston-Peck, A.C.; Senanayake, S.D.; Zhou, G.; Stacchiola, D.; Stach, E.A.; Rodriguez, J.A. Steam reforming of ethanol on Ni/CeO2: Reaction pathway and interaction between Ni and the CeO2 support. ACS Catal. 2013, 3, 975–984. [CrossRef] 9. Liu, C.-J.; Ye, J.; Jiang, J.; Pan, Y. Progresses in the preparation of coke resistant Ni-based catalyst for steam and CO2 reforming of methane. ChemCatChem 2011, 3, 529–541. [CrossRef] 10. Wang, S.; Lu, G.Q.M. CO2 reforming of methane on Ni catalysts: Effects of the support phase and preparation technique. Appl. Catal. B 1998, 16, 269–277. [CrossRef] 11. Hu, X.; Lu, G. Syngas production by CO2 reforming of ethanol over Ni/Al2O3 catalyst. Catal. Commun. 2009, 10, 1633–1637. [CrossRef] 12. Du, X.; Zhang, D.; Shi, L.; Gao, R.; Zhang, J. Morphology dependence of catalytic properties of Ni/CeO2 nanostructures for carbon dioxide reforming of methane. J. Phys. Chem. C 2012, 116, 10009–10016. [CrossRef] 13. Shi, Z. Nickel carbonyl: Toxicity and human health. Sci. Total. Environ. 1994, 148, 293–298. [CrossRef] 14. Bellido, J.D.A.; Tanabe, E.Y.; Assaf, E.M. Carbon dioxide reforming of ethanol over Ni/Y2O3–ZrO2 catalysts. Appl. Catal. B 2009, 90, 485–488. [CrossRef] 15. Bahari, M.B.; Phuc, N.H.H.; Alenazey, F.; Vu, K.B.; Ainirazali, N.; Vo, D.-V.N. Catalytic performance of La-Ni/Al2O3 catalyst for CO2 reforming of ethanol. Catal. Today 2017, 291, 67–75. [CrossRef] 16. Bahari, M.; Fayaz, F.; Ainirazali, N.; Huy, P.N.H.; Vo, D.-V.N. Evaluation of co-promoted Ni/Al2O3 catalyst for CO2 reforming of ethanol. J. Eng. Applied Sci. 2016, 11, 7249–7253. Catalysts 2021, 11, 260 13 of 13

17. Cao, D.; Zeng, F.; Zhao, Z.; Cai, W.; Li, Y.; Yu, H.; Zhang, S.; Qu, F. Cu based catalysts for syngas production from ethanol dry reforming: Effect of oxide supports. Fuel 2018, 219, 406–416. [CrossRef] 18. Aziz, M.A.A.; Jalil, A.A.; Wongsakulphasatch, S.; Vo, D.-V.N. Understanding the role of surface basic sites of catalysts in CO2 activation in dry reforming of methane: A short review. Catal. Sci. Technol. 2020, 10, 35–45. [CrossRef] 19. De Souza, G.; Balzaretti, N.; Marcilio, N.; Perez-Lopez, O. Decomposition of ethanol over Ni-Al catalysts: Effect of copper addition. Procedia Eng. 2012, 42, 335–345. [CrossRef] 20. Sivaramakrishnan, R.; Su, M.-C.; Michael, J.V.; Klippenstein, S.J.; Harding, L.B.; Ruscic, B. Rate constants for the thermal decomposition of ethanol and its bimolecular reactions with OH and D: Reflected shock tube and theoretical studies. J. Phys. Chem. A 2010, 114, 9425–9439. [CrossRef] 21. Xu, G.; Shi, K.; Gao, Y.; Xu, H.; Wei, Y. Studies of reforming natural gas with carbon dioxide to produce synthesis gas: X. The role of CeO2 and MgO promoters. J. Mol. Catal. A 1999, 147, 47–54. [CrossRef] 22. Nederlof, C.; Talay, G.; Kapteijn, F.; Makkee, M. The role of RWGS in the dehydrogenation of ethylbenzene to styrene in CO2. Appl. Catal. A 2012, 59–68. [CrossRef] 23. Jabbour, K.; Massiani, P.; Davidson, A.; Casale, S.; El Hassan, N. Ordered mesoporous “one-pot” synthesized Ni-Mg(Ca)-Al2O3 as effective and remarkably stable catalysts for combined steam and dry reforming of methane (CSDRM). Appl. Catal. B 2017, 201, 527–542. [CrossRef] 24. Feng, B.; An, H.; Tan, E. Screening of CO2 adsorbing materials for zero emission power generation systems. Energy Fuels 2007, 21, 426–434. [CrossRef] 25. Lu, H.; Reddy, E.P.; Smirniotis, P.G. Calcium oxide based sorbents for capture of carbon dioxide at high temperatures. Ind. Eng. Chem. Res. 2006, 45, 3944–3949. [CrossRef] 26. Wang, S.; Lu, G.Q. Effects of promoters on catalytic activity and carbon deposition of Ni/γ-Al2O3 catalysts in CO2 reforming of CH4. J. Chem. Tech. Biotech. 2000, 75, 589–595. [CrossRef] 27. Pegios, N.; Bliznuk, V.; Prünte, S.; Schneider, J.M.; Palkovits, R.; Simeonov, K. Comparative study on La-promoted Ni/γ-Al2O3 for methane dry reforming—Spray drying for enhanced nickel dispersion and strong metal—Support interactions. RSC Adv. 2018, 8, 606–618. [CrossRef] 28. Zangouei, M.; Moghaddam, A.Z.; Arasteh, M. The influence of nickel loading on reducibility of NiO/Al2O3 catalysts synthesized by sol-gel method. Chem. Eng. Res. Bull. 2010, 14, 97–102. [CrossRef] 29. Alberton, A.L.; Souza, M.M.; Schmal, M. Carbon formation and its influence on ethanol steam reforming over Ni/Al2O3 catalysts. Catal. Today 2007, 123, 257–264. [CrossRef] 30. Kim, P.; Joo, J.; Kim, H.; Kim, W.; Kim, Y.; Song, I.; Yi, J. Preparation of mesoporous Ni–alumina catalyst by one-step Sol–gel method: Control of textural properties and catalytic application to the hydrodechlorination of o-dichlorobenzene. Catal. Lett. 2005, 104, 181–189. [CrossRef] 31. Huang, L.Q.; Zhang, Z.H.; Guo, Z.L. The effect of MgO precoating on the perfomance of Ni/MgO-Al2O3 catalysts for carbon dioxide reforming of methane. Adv. Mater. Res. 2014, 997, 272–278. [CrossRef] 32. Chu, P.K.; Li, L. Characterization of amorphous and nanocrystalline carbon films. Mater. Chem. Phys. 2006, 96, 253–277. [CrossRef] 33. Marton, M.; Vojs, M.; Zdravecká, E.; Himmerlich, M.; Haensel, T.; Krischok, S.; Kotlar, M.; Michniak, P.; Vesely, M.; Redhammer, R. Raman spectroscopy of amorphous carbon prepared by pulsed arc discharge in various gas mixtures. J. Spectrosc. 2013, 2013, 1–6. [CrossRef] 34. Ferrari, A.C.; Robertson, J. Interpretation of Raman spectra of disordered and amorphous carbon. Phys. Rev. B 2000, 61, 14095–14107. [CrossRef] 35. Yu, C.; Ge, Q.; Xu, H.; Li, W. Effects of Ce addition on the Pt-Sn/γ-Al2O3 catalyst for propane dehydrogenation to propylene. Appl. Catal. A 2006, 315, 58–67. [CrossRef] 36. Álvarez, A.; Borges, M.; Corral-Pérez, J.J.; Olcina, J.G.; Hu, L.; Cornu, D.; Huang, R.; Stoian, D.; Urakawa, A. CO2 Activation over catalytic surfaces. Chem. Phys. Chem. 2017, 18, 3135–3141. [CrossRef]