Ethanol steam reforming for hydrogen production_

Renewable and Sustainable Energy Reviews 74 (2017) 89–103

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Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser

Ethanol steam reforming for hydrogen production: Latest and effective catalyst modification strategies to minimize carbonaceous deactivation

MARK

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Yogesh Chandra Sharmaa, , Ashutosh Kumara, Ram Prasadb, Siddh Nath Upadhyayb,1 a b

Department of Chemistry, Indian Institute of Technology (BHU) Varanasi, Varanasi 221005, India Department of Chemical Engineering & Technology, Indian Institute of Technology (BHU) Varanasi, Varanasi 221005, India

A R T I C L E I N F O

A BS T RAC T

Keywords: Carbon deposit Catalyst deactivation Ethanol steam reforming Hydrogen

Hydrogen is being contemplated as the future fuel in view of the abundant availability of hydrogen bearing substances in nature, its high energy content (120.7 kJ/g), and its combustion without creating any environmental pollution. Pollution free sources for hydrogen generation and efficient conversion to useful energy are the two important factors controlling the development of hydrogen economy. Out of various liquid hydrogen sources, ethanol is a sustainable candidate because of its renewable nature, increasing availability, biodegradable nature, low toxicity, and ease of transport. It can be easily converted to a hydrogen rich mixture through catalytic steam reforming process. Further, ethanol steam reforming (ESR) is thermodynamically feasible and does not cause catalyst poisoning due to complete absence of S-impurities. However, the carbonaceous deposition during ESR is still an issue to make it sustainable for hydrogen generation. This review contains all parallel possible reactions besides the desired reactions, which can promote carbonaceous deposition over catalyst surface with respect to temperature. The role of operating conditions such as water and ethanol feed ratio and temperature with carbon generation were interrelated. The characterization of different carbon forms synthesized during ESR and the possible role of active catalyst into carbon synthesis mechanism was also considered. The contribution of precursor used for catalyst preparation, the role of active metals, the interaction between active metals for bimetallic catalyst, different kind of support prominently studied for ESR and their structural behaviors were also correlated. This review makes an attempt to critically summarize the recent strategies used to reduce the carbonaceous deactivation of catalyst during ESR on the basis of available literature survey. The focus of the review is catalyst deactivation due to carbonaceous deposition during reforming and possible strategies used to control the deactivation process during ESR.

1. Introduction Energy crisis, global warming, climate change and need for sustainable development have aroused global interest in developing renewable energy sources [1]. Under such a scenario researchers look forward to hydrogen as the future energy for transportation, fuel cells and power stations [2–7]. The comparative study of technical and environmental issues between fossils fuels and hydrogen energy reveals hydrogen economy as an absolute solution for our planet from environmental challenges [8]. There are several pathways of hydrogen production such as electrolysis, photolysis and thermo-lysis of water, biological reactions, gasification and pyrolysis of biomass, steam reforming and partial oxidation of hydrocarbons [9,10]. Recently, photocatalytic production of hydrogen from ethanol has also gained impetus [11,12]. However, excessive requirement of electricity makes

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electrochemical process expensive and also, photolysis of water is highly expensive owing to requirement of expensive electrodes. Biological methods are also reported but they suffer from a lower rate of hydrogen generation. These drawbacks make thermo chemical method a feasible pathway for hydrogen generation sustainably [13,14]. All over the world, 50% feedstocks of natural gases are used for hydrogen production via steam reforming which is not a renewable source [4]. Other feeds such as glycerol; methanol, and ethanol have also been used for the steam reforming to produce hydrogen. Among these ethanol steam reforming (ESR) using an appropriate catalyst shows the most efficient way of renewable hydrogen production [15] as represented by the following overall stoichiometric equation: C2H5OH+3H2O→2CO2+6H2

Corresponding author. E-mail address: [email protected] (Y.C. Sharma). Raja Ramanna Fellow.

http://dx.doi.org/10.1016/j.rser.2017.02.049 Received 14 April 2016; Received in revised form 8 November 2016; Accepted 8 February 2017 1364-0321/ © 2017 Elsevier Ltd. All rights reserved.

(1)

Various types of catalysts such as noble metals [16–23] (Pt, Pd, Rh,


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C2H5OH+2H2→2CH4+H2O

Au and Ru), transition metals (Cu, Co and Ni) as well as the combinations of both [24–27] have been used to catalyze the ESR reaction. However, these catalysts get deactivated due to carbonaceous deposits formed as the byproduct during ESR reaction (described later under reaction mechanism). Up to 1990s only a few papers discussed the catalytic steam reforming of ethanol [28–30], whereas, in the last two decades a lot of work has been done on the modification strategies to minimize carbon formation during ESR. In catalytic steam reforming of ethanol, noble metal catalysts show high activity and selectivity for hydrogen production with negligible or no coke formation [16,17,21,27,31–35]. However, noble metal catalysts are not cost effective [36]. In periodic table, among non-noble metal catalyst Cu, Ni and Co are reported to be most active metals for ESR [37]. Therefore, these metals have been widely studied on several supports such as Al2O3, SiO2, ZrO2 and CeO2 [38]. Large volumes of work have been reported on the deactivation of non-noble metal catalysts by carbonaceous deposition during ESR [39–45]. The deposition of carbonaceous materials on their surface is still a major challenge. In this regard, the oxidative steam reforming of ethanol on non-noble catalyst [46] is a good approach for hydrogen production though the carbonaceous deposition is comparatively lower than ESR, but the overall yield of hydrogen per mole of ethanol is also lower [47]. Modification of non-noble catalysts using an appropriate approach to minimize the catalyst deactivation by carbonaceous deposition is being projected as a useful way to produce hydrogen. The growth of carbon on the surface of catalyst is determined by the structure of hydrocarbons [48] (CH4/C2H6/C2H4) resulting as byproducts during ESR. Thus, the carbon source as well as catalyst composition play a major role in the rate of carbon deposition and its development. The rate of carbon deposition gets accelerated at high temperature [49]. The support as well as metal and support interface are the major sites for carbon deposition [50]. Recently, in reviews on hydrogen production via ESR, a very brief account of catalyst deactivation has been reported [51–53]. Although, there is no review paper solely devoted to noble metal or non-noble metal catalysts as modification strategies to minimize carbon formation during ESR. Therefore, on the basis of a critical review of the previous literature, an attempt has been made for the first time to summarize sustainable catalyst formulation in reference to their stability as well as ESR efficiency for renewable hydrogen production emphasizing non noble cost effective catalysts.

Other reactions:

C2H5OH+H2O→CH3COCH3+CO2+4H2

(3)

C2H5OH→CH4+CO+H2

(4)

C2H5OH→C2H4+H2O

(8)

(9)

C2H4→Coke→2C+H2

(10)

C2H4↔2H2+2C

(11)

CH4↔2H2+C

(12)

2CO↔CO2+C

(13)

CO+H2↔H2O+C

(14)

CO2+2H2↔2H2O+C

(15)

2CH3COCH3→CH2COHCH3+CH3COCH3 → (CH3)2 C (OH) CH2COCH3 → (CH3)2C=CHCOCH3 (Mesityl Oxide)+H2O

(16)

The reactions (Eqs. (2)–(8)) suggest that saturated, unsaturated hydrocarbons or both can be produced simultaneously during ESR. The dehydrogenation, dehydration, polymerization and decomposition reactions of different byproducts (Eqs. (9)–(16)) are able to generate different forms of carbon. The unsaturated hydrocarbon, C2H4 formed after ethanol dehydration, gets polymerized to coke [61] on the surface of catalyst (Eq. (10)). But the presence of π- bonds in the structure makes them easily disruptive in nature. In a recent review [51], it was pointed out that the decomposition of C2H4 occurred at higher temperature but contradicting experimental results showed that even at the lower temperature ( > 200 °C), the decomposition of C2H4 (Eq. (11)) occurred efficiently [62]. Lower temperature hindered the carbon crystallization during deposition of carbon on the metal surface especially in presence of carburizing gases because of highly intense C–C bond. Saturated hydrocarbon such as CH4 is highly stable and its thermal decomposition (Eq. (12)) occurs at temperature > 1173 K [63]. On decomposition, CH4 is able to generate three kinds of surface carbonaceous species. Firstly, the carbidic species which is highly reactive (hydrogenable at temperature ≈323 K) and is formed by methane dissociation [48]. It acts as an intermediate in the filamentous carbon growth [54]. During ESR, Co and Cu (over TiO2 and SiO2) did not form carbide species but Ni and Fe may form this precursor [64,65]. The spectroscopic study during hydrogenation of carbidic carbon by He et al. found that carbon on Ni (100) needs lower decomposition temperature than Ni (111) surface. They suggested that the different form of Ni surfaces (100) and (111) may affect the local structure of CHx species growth and so thermal stability may be different [66]. However, nickel carbide is not stable and is easy to decompose into metallic as well as graphite form of nickel at high ( > 873 K) temperature [67]. Secondly, amorphous carbon that is less reactive is formed by polymerization of carbidic carbon. It helps to form carbon whisker. Thirdly, the graphite carbon is hydrogenable at temperature around 673 K and has different reactivity for oxidation and hydrogenation. The study over Ni (111) catalyst revealed that carbide formation either in the isolated or string form on the terrace site took place without island formation for C2H4. But the growth of carbide formation

Hydrogen Consuming: (C2H5)O+2H2→2C2H6+H2O

(7)

C2H6↔3H2+2C

Steam reforming of ethanol may comprise several other simultaneous reactions along with the hydrogen producing reactions. A few of these result in the generation of unwanted products [54–56]. These unwanted products are formed by dehydrogenation (Eq. (2)), decomposition (Eq. (4)), dehydration (Eq. (8)), hydrogenolysis (Eq. (6)), and aldolic condensation followed by dehydrogenation (Eq. (3)) of ethanol itself. Acetaldehyde on decarbonylation [57] also produces CH4 and CO (Eq. (7)). Ethane can be the major product over a selective catalyst which facilitates ethanol adsorption (leading to diethyl ether formation) followed by hydrogenation (Eq. (5)) [22,58,59]. The reaction network can be written as follows: Hydrogen producing: (2)

CH3CHO→CH4+CO

The intermediate reactions during ESR and liable for the carbon deposition on the catalyst surface are as follows [60].

2. Reaction mechanism of ESR and carbon formation

C2H5OH→CH3CHO+H2

(6)

(5) 90


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quency D band (1330–1360 cm−1) as compared to graphitic carbon (ordered) G band (1580–1596 cm−1). Here D band and G-band are attributed to the vibrations of carbon atoms with dangling bonds and the stretching mode of carbon sp2 bonds, respectively [42]. The single walled carbon nanotube was also suggested by asymmetric G band centered at 1596 cm−1.

during Boudouard reaction occurred with island formation. Carbon island consists of graphitic carbon which forms during dissociation of CO on Ni by the diffusion of carbon into the bulk between the metal particles and the catalyst support [68]. The different ways of growth of carbon are attributed to the dissimilarity in the C2H4 and CO dissociation sites [69]. Boudard reaction (Eq. (13)) is sensitive to the structure of metal in catalyst [70]. The activation energy for CO disproportionation reaction declines by step edge of metal (Ni (111)) but enhances for the associative desorption [71]. Therefore, step edge played a critical role leading to the development of carbide species due to dissociation of CO [70]. The hydrogenation reactions of CO (Eq. (14)) and CO2 (Eq. (15)) are favored at high temperature [72]. Thermo-gravimetric analysis of deactivated catalyst during steam reforming of acetic acid over ZrO2 support suggested that ketene and mesityl oxide is formed due to acetone undergo oligomerization followed by coke formation (Eq (16)) [73]. Therefore, the formation of CH3COCH3 as an intermediate byproduct during ESR may contribute the formation of coke deposition on catalyst surface. Best of the present literature survey, the coke formation due to mesityl oxide and ketene during ESR has not been reported in the literature so far.

2.2. Carbon synthesis mechanism involved during ESR The synthesis mechanism responsible for the filamentous carbon formation comprises tip growth phenomena mechanism [41]. Frusteri et al. however, suggested that ESR did not involve this phenomena [74]. This statement is irreconcilable since few workers had anticipated tip growth phenomena as a possible growth mechanism [84,85]. Among these, contradictory studies about carbon growth mechanism during ESR had been reported for catalyst Ni but over a different support. It emphasizes that support may play a major role in determining the growth mechanism pathway for carbon formation. In this regard, IR spectroscopy studies over growth mechanism during ESR over different catalyst having various supports can be helpful. Moreover, the proposed tip growth mechanism of filamentous carbon formation can be understood by the work of Jeong et al. The root of filament and a large nickel particle are indicated with black and white arrows respectively in Fig. 3(a). The filamentous carbon of different lengths enclosed with Ni (quasi spherical shape) at the tip are shown in Fig. 39(b) and (c). The surface of quasi-spherical shaped large nickel particles with octopus like carbon network is shown in Fig. 3(d). If the numbers of fibers grow from a single metal crystal, then it formed the octopus like structure. For the development of highly crystalline carbon on the surface of metal, the carbon atoms need to dissolve as well as diffuse among metal particles. The crystallization must proceed on an appropriate lattice plane help the epitaxial development of carbon crystal [62]. Study of the correlation between size of carbon filament and metal particle size during ESR depicted that larger Ni particle size leads to larger carbon filament. The comparison of the nature of carbon formed during ESR and the dry reforming of methane over similar catalyst indicates that the mechanism of the carbon formation in ESR is different from methane reforming [74]. The physical shape of the precipitated carbon depends on the particle size of catalyst as well as precipitation rate. The free energy of less crystalline carbon is comparatively lower than highly crystalline carbon [62]. The deposition of carbon on catalyst surface may or may not be the culprit for deactivation of the catalyst. It depends on the site of deposition on the catalyst. The direct deactivation of catalysts occurs principally through the covering of the active phases due to enclosing carbon. During filamentous growth of carbon, deposition occurs on the metal surface and the species made up of carbon move towards the bulk phase of metal which later on saturate and condense. It does not lead to direct deactivation of catalyst but its continuous accumulation blocks the beds [45,79].

2.1. Nature of carbon deposit The carbon deposited on the surface of catalyst during ESR is reported to be of amorphous as well as of filamentous forms [74]. The amorphous carbon contributes more severe deactivation as compared to filamentous carbon [75]. The nanosized carbon filaments such as carbon nanotubes and carbon nanofibers have interlayer spacing equivalent to bulk graphite [76]. Graphitic form of carbon is generated either through the direct deposition of carbon in the vapour phase or through heat treatment of amorphous carbon [76]. Espinal et al. reported the formation of carbon nanotubes on the surface of reacted cobalt hydrotalcite catalyst after performing the ESR [77]. De Bokx et al. studied production of filamentous carbon from CO and CH4 over iron and nickel catalyst in the temperature range of 650–1000 K and showed that carbon was deposited as a metastable carbon intermediate, leading to filamentous carbon on decomposition [78]. Filamentous carbon has comparatively high mechanical strength and so it has the capability to completely disintegrate the support of catalyst [79]. Typical forms of carbon are shown in Fig. The Fig. 1(a) and (b) represent the Ru particles with support and without carbon and encapsulated Ni particle with graphitic layers respectively. The octopus like carbon nanofiber around Ni crystallite (Fig. 1(c)) and in carbon nanofibre at the apex of Ni particle (Fig. 1(d)) was represented. The spent catalyst characterization with the help of XPS-TPO was unable to reveal the nature of carbon. However, the lower temperature peaks in TPO ascribed the carbon presence in vicinity to the metal, whereas; higher temperature peaks represent carbon existence on the surface of support. The higher and lower temperature oxidation of carbon during thermogravimetric analysis (TG-TPO) also ascribed for more ordered form (nanotubes and graphitic) and amorphous forms of carbon respectively [80]. Nevertheless, the CO2 released during TPO cannot be used to differentiate whether it is due to oxidation of carbon deposits or carbonate species [81]. FTIR Raman spectroscopy is best technique to characterize the nature of carbon [82]. Typical spectrum of single walled carbon nanotube suggested lower frequency ( < 200 cm−1) peak for radial breathing mode. It is absent in graphite carbon. The D band is assigned for a group of peak around 1340 cm−1 as measure of disorder in carbon.[83] The characterization of different form of deposited carbon by FTIR Raman spectra (Fig. 2) up to 2000 cm−1 frequency range was recently reported in few literatures during ESR. At the low frequency range (165 and 196 cm−1), the peak was reported for carbon nano-tube, whereas, the high frequency range (1200–1700 cm−1) was attributed to amorphous and graphitic forms of carbon. Amorphous (disordered) carbon was reported in lower fre-

2.3. Effect of reaction operating conditions The operating environment such as water to ethanol ratio (S/C) and temperature during ESR has the crucial role for the carbonaceous deposition. It occurs due to discriminating activities of intermediate hydrocarbon products (CH4, C2H4 and C2H6) with temperature and amount of water. These hydrocarbons lead to coke deposition at higher temperature but presence of water facilitates the increase in overall hydrogen yield. It can be clearly observed by the following reactions: [86]

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C2H4+2H2O↔2CO+4H2

(17)

C2H4+4H2O↔2CO2+6H2

(18)


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Fig. 1. Typical forms of carbon formed during ESR. Reprinted with permission from Refs. [182].

C2H6+2H2O↔2CO+5H2

(19)

C2H6+2H2O↔2CO2+7H2

(20)

CH4+H2O↔CO+3H2

(21)

CH4+2H2O↔2CO2+4H2

(22)

temperature, water gas shift reaction leads to increase in hydrogen productivity and decreasing carbon deposition although it requires high energy consumption [72]. The concentration of water higher than stoichiometric quantity was reported to have an inhibitory effect on coke formation [87,88]. This statement is also supported by the experiment of different molar ratio concentration over Rh catalyst for 100 h time on stream. Molar ratio of water and ethanol lower than stoichiometric ratio showed deactivation within 4 h of operation. The antagonistic effect of higher ethanol to steam ratio for ESR was experimentally approved over Ni catalyst on various support (La2O3,

Therefore, molar ratio of ethanol and water in the feed during ESR has significant role on the suppression of carbon deposition. It can influence the yield, selectivity and deactivation of catalyst. At high

Fig. 2. FTIR raman spectra of typical carbon over different catalyst. Reprinted with permission from Refs. [100] and [131] copyright year Elsevier.

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Fig. 3. Role of nickel nanoparticles in the growth of the filamentous carbon on the surface of the nickel foam. (a) overview of as-grown filamentous carbon, (b) size-dependent growth of a filamentous carbon from a small nickel nanoparticle ( < 50 nm), (c) size-independent growth of a filamentous carbon from a large nickel nanoparticle ( > 100 nm), and (d) octopus-like network formation of filamentous carbon. Reprinted with permission from Ref. [41]. Copyright 2008 Elsevier (Jeong [41]).

γ -Al2O3, La2O3/γ -Al2O3) [68]. Thermodynamic analysis depicted that rate of coking was very low for molar ratio of water and ethanol higher than 10:1 [89]. It is also noteworthy that the thermodynamic equilibrium calculations of ethanol dehydration at 873 K suggested that the product should be altered towards ethane and water despite of the water content [90]. It implies the importance of acidic, basic or neutral nature of support regarding the effect of S/C ratio during ESR. However, the effect of S/C ratio was studied till now mainly over acidic support. Therefore, the study of S/C optimization over different nature of support may lighten the information about its exact contribution in the carbonaceous deposition. Temperature plays an important role in carbon deposition but the available information is that the carbon deposition is governed by different kinds of reactions at different temperatures and hence is not so consistent. At lower temperature, Boudard reaction and hydrogenation of CO are the dominant reactions facilitating the carbon formation. Whereas, at high temperature boudard reaction reversed but carbon deposition facilitates through decomposition of hydrocarbons (Eqs. (9), (11), (12)) [28,91,92]. Therefore, the intermediate reactions responsible for deposition of carbon are different at different temperatures. It had been reported that usually for Ni catalysts, high temperature facilitated the carbon deposition, [93–95] whereas, in case of Rh catalysts, the rate of carbon formation is increased with lowering of temperature [88]. These two contradictory statements concerning temperature indicate that owing to the diverse reaction pathways depending on catalysts during ESR, the intermediate reaction might be different and hence, it showed atypical performance with respect to temperature. Recently the uniformity of heating temperature for catalytic bed was also reported as contributor for coke formation during ESR. Microwave heating was confirmed as a better option comparative to conventionally heated reactor [96]. Besides these parameters effect of inert atmosphere has also seen a role in coke deposition over catalyst surface. It acts as remover of less dense coke formed during ESR over catalyst surfaces [97]. However, its role is less vital as compared to temperature and S/C ratio.

2.4. Effect of catalyst preparation conditions (precursor and methods) The effect of nature of catalyst precursor used for the preparation of catalyst contributes to the carbon deposition. The work reported by Galetti et al. over nature of catalyst precursor revealed that the nitrate salt precursor used for catalyst preparation via impregnation method exhibited most carbon resistance characteristics. The authors supposed that in the inorganic precursor derived catalyst, deactivation occurs due to active carbon (C-C), whereas, the nickel acetate precursor used for catalyst preparation led to the formation of -CO-C- inactive species during ESR. Therefore, catalyst deactivation becomes more severe for the catalyst derived from organic precursors. The catalyst calcined under reductive atmosphere was also found to be more stable compared to the catalyst calcined under oxidative atmosphere [98]. Preparation methods govern the dispersion of active phase or stronger metal support instauration interaction. It confines the coke filaments formation during low temperature ESR [80]. Effect of support (SiO2) preparation method on carbon deposition was carried out by Rossetti et al. [50] The liquid phase synthesis of support only showed silanols responsible for deposition of carbon. Moreover, the Lewis acid sites were also detected in the support prepared by flame pyrolysis method and so were responsible for the dehydration of ethanol leading to coke deposition. The catalyst prepared by sol-gel method showed the effect of citric acid concentration used during catalyst preparation on coke formation. It was increased with the increasing citric acid content in the sol–gel preparation process. The optimization of citric acid during catalyst preparation is also noteworthy as lower concentration of citric acid leads to deactivation due to sintering [84]. Effect of citric acid concentration on coke deposition is associated with the surface area of catalyst as indicated by the work of Takahashi et al. [93] Citric acid makes hydrogen bond with silanol during preparation of silica supported catalyst via sol-gel method. Therefore, high citric acid concentration leads to collapse of space occupied on calcination in the catalyst precursor and ultimately resulted into broad pore size distribution. However, diversity of carbon 93


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aldehyde decarbonylation and aldol condensation reactions compete with each other and basic properties of support lead to further generation of acetone [112]. Cobalt catalysts are predominantly active at low temperature and enhance the water gas shift reactions [64]. Existences of Co2+ species facilitate ESR, whereas the metallic cobalt leads to the deposition of carbon over catalyst surface [77]. Because the metallic form favors decarbonylation of adsorbed ethoxy species, Co2+ is efficient for dehydrogenation reactions [113]. It is highly active for C–C bond cleavage [114–116]. Even at low temperature (673 K), performance equivalent to noble metal catalyst in terms of C–C bond scission was reported for supported cobalt [117,118]. However, few authors had found Ni over γ-Al2O3 and ZrO2 support to be more active catalyst compared to Co [62]. The Cu based catalysts had poor catalytic activity for the steam reforming of ethanol. Cu has good catalytic property for dehydrogenation but is ineffective in the cleavage of C–C bonds. It formed high concentrations of intermediates such as acetaldehyde and ethylene [119–121]. However, at lower temperature (623 K) Cu showed more stability towards coke deposition as compared to Ni [62].

formation was not correlated with citric acid concentration. Wurzler et al. were recently found that during precipitation method of catalyst precipitation, aging has significant role to increase the basicity. The better distribution of NiO with MgO, highest reduction degree and basicity leads to decline in coke formation [99]. The homogenous dispersion and smaller size of metal clusters through redistribution of metals or lowering of metal valency can be induce by non-thermal plasma treatment. Redistribution of metals check the blockage of pores of catalyst and so coke deposition get reduced. Wu et al. used the dielectric barrier charge after calcinations of Ni/γ-Al2O3 catalyst and found relatively more catalytic activity with 97% carbon balance as compared to untreated one at 810 K [100]. 3. Catalyst modification strategy There are several approaches to minimize carbon deposition during ESR, such as increasing water to ethanol ratio; autothermal steam reforming, co-feeding of H2, co-feeding of CO2 and catalyst modification. Increase in water to ethanol ratio requires high temperature to facilitate the reaction. Autothermal steam reforming is an exothermic reaction but overall yield of hydrogen gets reduced compared to ESR. Co-feeding of CO2 at lower temperature reduces the carbon deposition up to certain extent but competition with ethanol adsorption at common site is major disadvantage of this approach. The hydrogenation of deposited carbon reduces the deactivation but it facilitates the hydrocarbon yield in spite of H2. Therefore, catalysts modification strategy has shown better possibility in minimizing the carbon deactivation during ESR in terms of yield as well as cost. Catalyst formulation must require knowledge of activity of metals, support and synergistic interactions between metals along with their support during ESR for the carbon generation and deposition.

3.2. Bimetallic interaction Monometallic catalysts had solitary catalytic nature while synergistic interaction between the two metal components of a bimetallic catalyst may have beneficial effect on carbon deactivation during ESR [122]. The noble metal is comparatively more active but due to cost issue, addition of smaller amount of these metals with non-noble metals are used to improve the stability. Therefore a lot of research work was carried out to find out the interaction between noble and non-noble metal catalysts. If Pt is segregated toward Ni particle surface through bimetallic interaction than rate of hydrogenation of the adsorbed carbon species at the surface increases relative to its (carbon) diffusion into bulk nickel [123]. The Co with Rh metal catalysts mutually over ZrO2 and CeO2 support were reported to be less prone to catalyst deactivation compared to monometallic cobalt catalyst [124,125]. Moreover, Rh and Ni bimetal over CeZr oxide support showed lower carbon filament formation due to increase in Ni reducibility and its dispersion [126]. The Cu and Ni is particularly efficient in catalyzing water gas shift reaction [127]. The study of Fierro et al. [60] suggested that on addition of Cu, the large ensembles of Ni metal atoms extinguished, melting temperature turn out to be average; existing state of Ni altered and the affinity with carbon gets changed. It formed a NiCu solution and resulted into lower deposition of carbon by CH4 decomposition [119,128]. Stability of catalyst with reference to activation of C–C and C–H bonds gets lowered by partial substitution of Ni with Cu. But, it favored the formation of low-volatility polymers owing to condensation reactions of acetaldehyde and other intermediates [112]. The synergistic effect of Ni and W was reported over SBA-15 and ceria. Inside CeO2 the structure due to the difference in preferable coordination no of Ni, W (6) and Ce (8), electronic and structural perturbation increased substantially. W and Ni promote the Ce3+ site formation and it increases the –OH species on the surface of catalyst due to the partial dissociation of water. Ultimately, it leads to oxidation of CHx species and lowers down the coke formation [52]. The suggested model states that CH4 and CO formation occurred over Ni sites and further CH4 reforming and CO-Water gas shift reaction occurred over NiO and Ni-W alloy sites respectively [129]. Easily combustible coke is easy to remove and its formation depends on the metal catalyst on which ethanol gets adsorbed or desorbed. Hence, different kind of coke in terms of reactivity towards combustion on metal was reported on Ni and Co monometallic as well as spinel catalysts. This reactivity was reported in order the 10%Co > 3.3%Ni 6.7%Co > 6.7%Ni 3.3%Co > 10%Ni and therefore, higher activity of Co catalyst as compared to Ni over γ-Al2O3 support. The amorphous and filamentous nature of carbon was also reported over

3.1. Active metals The noble metals are more resistant to carbon deactivation compared to non noble metals. Among noble metals, Rh is highly active for both C–C and C–H bond scission [22]. It induces hydrogenation reaction and leads to very low carbon deposition [101]. It also has the ability to evade carbonate formation that's why the oxygen vacancies are easily accessible for oxidizing carbon [102]. Carbon formation was reported insignificantly for Rh at stoichiometric molar ratio of water and ethanol [33]. Nevertheless, Pt catalyst over different oxide supports (CeO2, Al2O3, ZrO2) during ESR showed significant amount of carbon deposition [19]. Comparative study of ESR on Au and Pd catalyst over SiO2 support revealed Au to be more stable compared to Pd [103]. The catalyst Ir over CeO2 at low temperature (623 K) and PrOx doped CeO2 up to 923 K did not show deactivation during ESR [104–106]. Ru provokes the ethane formation via ethanol dehydration and ultimately results in coke formation. Though, study of nano sized Ru, Pd and Ag over mixed oxide (CeO2/YSZ) support revealed the order of stability towards carbon as Ru > Pd > > Ag [17]. Overall, the carbon depositions as compared to nonnoble metals are considerably less. Pure Ni has the ability to cleave the bond of ethanol in the order of O–H, –CH2–, C–C, and –CH3 [107,108] Coke formation is strongly depressed due to the benefits induced by the use of basic carrier which positively modify the electronic properties of supported Ni [109]. Skeletal Ni based catalyst has large exposed surface area and high dispersion of Ni, so it is also useful in reducing carbon deposition [110]. If catalyst is deficient of the support and metal interaction then it leads to significant amount of filamentous carbon whisker [111]. Study of temperature program reduction showed that reduction of NiO is very easy if the interaction between the support and metal is weak. But it is not only facile to be sintered but also resulted larger particles, vulnerable for carbon formation. Promotion of the C–C bond splitting imparted by Ni is not efficient at low temperature. Therefore, acet94


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noble metal over different kinds of oxide supports such as Al2O3, CeO2, ZnO, MgO, ZrO2 and SiO2 with variation of metal loading. Lot of literature is available on the study with ZnO and MgO but is less specific in terms of their carbon stability [61,74,109,113,115,120,135– 143]. Temperature program desorption study showed that coking was affected by supported metal. The low temperature study of Pt catalyst over different support had shown carbon deactivation in the order CeO2 > > Al2O3 > ZrO2 indicating that the nature of the metal oxide support has significant contribution in favor of carbon deposition.

Co and Ni respectively on study of Ni-Co alloy nano particles supported over La2O3 [75]. Since Ni has strong capability to break C-C bond as compared to Co, carbon deposited on Ni was more crystalline and difficult to burn out compared to that on Co [62]. Ni is more efficient to break C–C bond compared to Co, whereas Co leads to less stable coke therefore optimum concentration of bimetallic catalyst formulation may result into highly efficient and stable catalyst. Moretti et al. had found the synergistic effect of Co and Ni over Ce/Zr (9:1) support as an increment of reducibility of both metal phases [130]. Recently, Zhao et al. studied the bimetallic Ni-Co alloy perovskite catalyst supported by ZrO2 and it had been found that bimetallic perovskite had better antisintering and anticoking ability as compared to their monometallic perovskite form [131]. Zhang et al. [40] had conducted the low temperature ESR over skeletal Ni based catalyst and reported that bimetallic catalyst Cu–Ni, Co–Ni, and Pt–Ni did not show any C2 or C3 species on their surface due to the promoting effect of these bimetallics. Low temperature steam reforming and less amount of amorphous carbon has been reported compared to that for supported nickel nanoparticle catalysts. The carbon deposited is considered to be formed by decomposition of methane [110]. The decomposition of methane, most apparently occurs if the high density of nickel d-state which is near to Fermi level, interacts with the vacant anti bonding orbitals of C-H bond. The element such as Au which is able to lower the d- band state reduced the interaction of antibonding orbital and d- state. So, the alloy formation with such element is beneficial reducing coking on Ni atom [132]. A model was proposed by using Ni and Mo bimetals over SBA-15 support. It states that conversion of ethanol into acetaldehyde and acetaldehyde into CH4 and CO over Ni active sites and CH4 reforming over Mo sites leads to deduction in carbonaceous deposition [133]. Kim et al. proposed the model of two bimetallic catalysts over common support SBA-15. They had stated that Mo has higher efficiency to adsorb CH4. Therefore, the parallel side reaction as a methane steam reforming will occur at the surface of Mo for NiMo bimetallic catalyst. Nevertheless, methane steam reforming will be prominently occurred over Ni in case of NiW bimetallic catalyst. Aforementioned both the proposed model mechanisms of ESR over bimetallic Ni based catalyst over SBA-15 is collectively illustrated in Fig. 4 below.

3.3.1. Alumina Alumina is a highly mechanical as well as thermally stable metal oxide. It acts as an acidic support in catalytic reactions due to the acidic centers present on its surface. Therefore, it promotes dehydration of ethanol and leads to ethylene formation [144]. It is responsible for coke deposition in highly active carbon form and is easy to oxidize. To reduce the coke deposition by increasing the surface area, the spherical catalyst Ni/Al2O3 was reported with the help of chitosan and Al solution [145]. However, during ESR on prepared catalyst CO was missing but other coke forming intermediate product C2H4 was detected. If ZnO is mixed with Al2O3, then it not only removes the acidic centers but also creates some Lewis acidity and thus promotes ethylene selectivity [74]. When Zn reacts with Al2O3, it forms zinc aluminate. Ultimately, it distorts the structure as well as surface chemistry of the catalyst. Catalytic properties of the catalyst's active phase can also be altered by the reduction of zinc oxide into metallic zinc. If the reduction temperature for nickel and zinc oxide is not sufficient to reduce all Ni (II) and Zn(II) in ZnAl2O4, then it promotes water gas shift reaction and so production of CO is lowered [146]. The temperature programed oxidation of ZrO2 and Ca-γ-Al2O3 supported catalyst reported by Chen et al. [62] is indicated that ZrO2 is more prone to the formation of C2H4 at any temperature compared to Ca-γ-Al2O3. The major drawback of Ca addition in Al2O3 supported Ni catalyst is that it decreases the strength of bond between Ni and Al2O3. On the other hand, the benefit of Ca is that its addition favors declining of acid sites owing to decrease in the surface area of Al2O3. It leads to the alteration in the reactivity and stability. Calcium has availability of electrons to enrich the Ni 3d-band electrons. Hence, the loading of Ca promotes electron enrichment on the Ni metal. The decomposition of methane got enhanced with Ca doped catalyst. Choong et al. [147] had reported the trend of coking rate with 10% loaded Ni catalyst on Al2O3 with varying Ca loading (wt%). The order sequence was Ni/7Ca- Al2O3 > Ni/3Ca- Al2O3 > Ni/Ca- Al2O3. The crystalline nature of carbon was found and so it was easily gasified. The optimum loading was reported as 3% under consideration that coke deposited by CH4 decomposition is reduced at the steady state of coke gasification deposits. Silver addition on Ni/Al2O3 was reported to have an inverse effect on the inhibition of carbon deposition. The reason behind this pessimistic action is the decrease in Ni sites resulting in less coordination between metal and ethanol. According to Liberatori et al. [148] activation of ethanol molecules on the surface of metal is relatively easier than that of methane. Therefore, the coordination of Ni sites is not responsible for the activation of ethanol.

3.3. Support Catalyst support plays significant role during ESR [29] and takes part in the generation of carbon over catalyst surface. The strong interaction between metal and support leads to lower generation of carbon [134]. A number of workers studied ESR on the noble and non-

3.3.2. Silica The novel work on cobalt-talc nano layers dispersed in silica aerogel for efficient hydrogen production via ESR was reported but not with reference to carbon deposition [149]. The comparative study of Co supported on SiO2 and Al2O3 revealed that the type of carbon formation may be controlled by the particle size [150]. This study was also supported by the work of Lee et al. [151] It had been reported that in case of Ni catalyst, smaller size particles facilitate the synthesis of filamentous form of carbon with a slower rate because of the low driving force for carbon diffusion through the small crystals. The maximum amount of filamentous carbon was reported for Co catalyst supported on Al2O3 compared to SiO2 support, owing to acid sites of

Fig. 4. The proposed model mechanism of ESR over bimettalic Ni based catalyst over SBA-15 support.

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3.4. Doping with other metals

alumina. Chen et al. [62] also found that the yield of carbon nanofiber (CNF) is extremely sensitive to Ni (NiO) crystal size and the yield of the CNF increased with Ni crystal size at a certain range. Optimum ratio of Cu/Ni≥1 on SiO2 support was suggested by Chen et al. [62] for the low tendency of carbon and scission of C–C bond energetically. Furthermore, addition of Ca and Mg generate defective disordered carbon leading to lower stability and easier thermodynamic removal during steam reforming of ethanol.

The effect of doping in catalyst may be inhibitory as well as facilitative in terms of carbon deposition [140,147,165]. The addition of basic solid into acidic support decreases the rate of coking [165,166]. This statement was contradicted by Moura et al. [167,168]. However, the basis of contradiction was not authentic as the effect of doping was compared with the catalysts used for the steam reforming of other source of hydrocarbon (CH4) in place of ethanol. The different source of hydrocarbon has different role in the growth of carbon [49]. Therefore, study about doping of metal effect over catalyst during ESR in terms of carbon deactivation for catalyst formulation is very essential.

3.3.3. Ceria The catalyst support having high oxygen storage capability leads to oxidation of deposited carbon on the surface of catalyst. CeO2 has efficient oxygen vacancies but its catalytic property is unstable at temperatures higher than 673 K. During ESR, high oxygen defects with mobility is prerequisite for better activity and stability of catalysts [152]. It has reversible change oxidation state (Ce4+/Ce3+) ability and so it is able to exchange oxygen much efficiently [153]. The effect of doping of Ca, Pr, Fe and Zr in CeO2 was studied for ESR [154–157]. Small addition of Fe, Pr, Zr and over 5% Ca addition in CeO2 not only promotes the reduction ability but also enhances the surface area and oxygen vacancies. Addition of Ca also minimizes the particle size of ceria in Co/CeO2 catalyst and Zirconium addition elevate the thermal stability to CeO2. Recently, Wang et al. were reported that small addition of Ce with Ni/SBA-15 facilitate the dispersion of active metal and strengthen the metal support interaction. It enhance the anticoking ability significantly [158]. Morphology and size of CeO2 has also a significant role in ESR, especially for anticarbon deposition. Because the potential of oxygen vacancies concerned to morphology and the different plane (100) as well as (110) of ceria [159]. Furthermore, study of ESR on nanorods and nanoparticles of CeO2 with Co suggest nanorods of CeO2 as a better option with respect to carbon deposition as well as oxygen vacancies. Soyakal et al. [160] used Co supported on nanoparticles and nanocubes of CeO2. The nanocubes of CeO2 support showed higher oxygen mobility and higher density of surface basic sites. The Ni catalyst over (10%) CeO2 modified ZnAl2O4 spinel had capability to cause 50% reduction in carbon formation on the catalyst [42]. When ceria is doped with nickel, it creates oxygen vacancies and Ce (III) sites which had comparatively higher atomic radii than Ce (IV) sites.

3.4.1. Zirconium addition ZrO2 itself has been used as a support in catalytic reactions but low surface area and weak mechanical stability make it inappropriate for ESR. Whereas, the stability of Al2O3 supported catalyst gets intensified on addition of ZrO2 because of its property to increase the adsorption as well as dissociation of water on the surface of nickel catalysts. The capability of support for the H2O adsorption decides the reaction pathway involved which leads to product selectivity followed by carbon deposition. Water-gas shift reaction on the catalyst surface increases with increase in acidity due to adsorption of higher number of CO species [169]. Ultimately, a surplus amount of ZrO2 lowers down water-gas shift reaction in the steam reforming of ethanol. Zirconium also reduces the surface area but enhances the reducibility and eventually decreasing the acid sites of catalyst. Effect of Zr/Al molar ratio in Ni-Al2O3-ZrO2xerogel catalyst revealed that surface area and interaction between support (Al2O3-ZrO2) and nickel oxide both were inversely related to molar ratio. Reduction of surface area occurred because Zr4+ ion has comparatively larger radius than Al3+ ion therefore incorporation of aluminum ions into ZrO2 leads to lattice contraction [170]. The lower interaction between metal and support occurred owing to formation of the composite structure of NiO-Al2O3ZrO2 [171]. It has been also reported that in the aforementioned catalyst, the coking culprit reactions i.e ethanol dehydration and ethylene formation reactions also get reduced [147]. 3.4.2. Lanthanum addition Lanthanum has been shown to reduce the carbon deposition over Ni catalysts [68,148,172]. It oxidized the adsorbed carbon to CO by means of lanthanum oxycarbonate and ultimately hampered the carbon island [121,173,174].

3.3.4. Titania Titanium oxide is typically used in photocatalytic hydrogen generation from ethanol or water [11,12,161]. However, carbonaceous deposition is not an issue in photocatalytic conversion of ethanol to hydrogen but C–C bond scission at ambient condition is still a challenge to increase the hydrogen yield [162]. Titanium oxide exists in three phases viz. rutile, anatase and brookite [163]. In photocatalytic activity, rutile phase is comparatively less active than anatase. Infrared spectroscopic study of ethanol on different temperature suggests that over rutile phase at 523 K, ethanol decomposition was favored. But the rise in temperature from 573 to 673 K leads ethane production trace to major product. Recently, the Ni catalyst over TiO2 dominant with nano crystalline anatase was reported for ESR. The coking was significantly occurred due to the lewis acidity of support [164]. Overall, the acidic property of support promotes the carbon deposition owing to dehydration of ethanol. So, the selectivity of ethylene increases with lewis acidity of support. The basic and amphoteric nature of support has lower carbonaceous deposition. To decompose or gasify the polymerized coke needs optimum metal and support interaction otherwise it hindered the ESR. Therefore, decreasing acid sites on support means reduction of coke formation but not at the cost of metal support interaction as it resulted into lower availability of active metal in the reaction. Furthermore, the size of particle also affects the nature of carbon because the smaller size of particle have slower rate of carbon deposition and so filamentous carbon facilitated during ESR.

La2O2CO3+2C→La2O3+2CO

(17)

Lanthanum forms perovskite structure with active non noble metals such as Ni, Co, Fe etc. The strong interaction of highly dispersed Ni-Fe alloy with disordered perovskite matrix compels perovskite derived catalyst as a better coking stability rather than perovskite supported one [175]. Lanthanum is also able to obstruct the graphitic carbon formation on the catalyst surface and impregnation of La2O3 over Al2O3 reduces the rate of coking [68]. The addition of lanthanum in acidic supports reduced the activity associated with their acidic sites. La2O3 contributes to the activity, supports the aldol condensation or dehydrogenation reactions during ESR. Moreover, La2O3 has the tendency to use the water and is liable to deform the structure [176]. Doping of lanthanum into Al2O3 shows incorporation of two dimensional well scattered entities on characterization of supports. Study of interaction between Ni and La atoms suggested that the close contact between these two atoms could enhance the obstruction of Ni sites, responsible for ethylene dehydrogenation. Sanchezsanchez et al. [177] has reported the alteration in surface of Al2O3 with lanthanum resulting in increase in the dispersion and stability of nickel metallic phases on addition of La. The contradictory result was also reported by Melchor-Hernandez et al. [173] that progressive incorporation of La 96


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on the catalyst surface and so ultimately reduce the carbon formation [187]. Potassium is also able to hamper the CH4 dehydrogenation and retard the deposition of coke over Ni catalyst supported on alumina [188]. It has been revealed from CO chemisorption followed by FTIR characterization that addition of Na might block the active sites of Co. Initially rate of dehydrogenation decreases for (0.98%) Na doped Co/ ZnO catalyst, but the deactivation of catalyst got reduced [143]. Oxide supports are capable of water adsorption which leads to the coke gasification especially at higher temperature and so activate the catalysts [79]. The addition of alkali and alkaline earth metals act as a promoter for generation of large number of adsorption sites for water [147].

apparently promotes the appearance of NiO species having weak interaction with the support. The study on Ag and La promoted Ni/Al2O3 catalyst depicted that Ag strongly deformed the Ni surface but was unable to suppress the coke formation whereas, La was able to decrease the coking. Lanthanum promoted Ni/Al2O3 catalyst has been shown to be highly susceptible to oxidation by water compared to unsupported catalyst [148]. The Co over MgAl2O4 doped with La, Ce and Pr shows similar quantity of carbon deposition. However TG analysis of Ce and Pr doped catalyst confirm lesser burning temperature than La doped catalyst. Authors suggested that it may be due to difference in the fiber structure that is parallel (a center hole and the metallic particle on the tip) or fishbone type. The region of support containing Co and La showed high amount of coke deposition [178] on the spent catalyst.

3.4.4. Yttria addition Different molar ratios of Y: Al were used to prepare Y2O3-Al2O3 support for Ni metal and used for ethanol steam reforming. The catalyst with 1:1 molar ratio of Y: Al showed comparatively less carbon deposition on the reactant catalyst. It occurs due to suitable interface between the composite support and NiO resulting in easy reduction of NiO. So, the active oxygen species and active sites form abundantly over the composite supported catalysts. The synergistic effect between support and active constituent could be persuaded by different mole ratio of Y and Al in the support [189].

3.4.3. Alkali and alkaline earth metals As it is well known that acidic supports promote formation of coke precursors, hence the introduction of basic carrier such as alkali (Li, Na, K and Cs) and alkaline metals (Ca, Mg and Sr) to neutralize acid sites has been suggested as an effective approach [179]. Intermediate hydrocarbons (such as methane and ethylene formed during ESR) decomposition and Boudouard reaction also get supressed by alkali metals, because they are able to provoke an electronic enrichment on the active phase of catalyst surface. Integration of CaO and MgO has been shown to decrease the amount of coke deposition as well as alter the nature of coke [180,181]. The initial rate of coke synthesis got enhanced for the catalyst doped with promoters (Mg and Ca), whereas, over the long deactivation period, quantitatively the coke formed on the surface of catalyst with promoter was reported to be lower compared to the catalyst without promoter [182]. The Ni/Al2O3 catalyst synthesized with the lamellar double hydroxides (LDHs) as precursors contained extremely dispersed metal particles in an aluminum structure. Furthermore, addition of Mg with Mg/Ni ratio more than 0.1 to Ni(II)Al(III) precursor resulted in very high activity with low amount of coke deposition (reduced up to 25 wt%), because of hindrance in ethanol dehydration [183]. Among alkaline earth metals, Mg and Ca both promote the catalytic properties of Ni/α-Al2O3 but Mg only was reported to alter the Ni particles. Mg inhibits the synthesis of less reactive coke because it blocks the pathway essential for the nucleation of graphene. Moreover, the promotion characteristics of Ca were suggested due to its ability to increase gasification of adsorbed carbon [180,184]. The contrasting result was reported about the role of Ca and Mg for carbon deactivation on Cu-Ni bimetallic catalyst supported over SiO2. It was suggested that Ca modified catalyst generates lower carbon deposition as compared to Mg modified catalyst due to increase in basicity as well as a reduction in active phase particle size [39]. However, both of these contradictory results suggested that either acidity of support or synergistic effect of two metals might play a major role for the efficiency of these doped alkaline earth metals to reduce carbon deactivation during ESR. Moreover, the study of Mg and Ca addition with SBA-15 were studied with Ni and Co active metals. It was reported that Mg and Ca both had not common coke reduction with both of the active metals. The prominent reduction in coke formation was found for Mg with Co and Ca with Ni [185]. It indicates some active and doped metal interaction role in the dispersion or metal support interaction which effects the coking ability of catalysts. Among alkali metals, effect of Li, Na and K doping on Ni/MgO catalyst had been studied. It was revealed that morphology and dispersion is not significantly affected by K whereas, Li and Na inversely affected the dispersion of the Ni/MgO catalyst, but support the reduction of NiO. The rate of formation of carbon species for bare and alkali-doped Ni/MgO systems was reported to be relatively slower than Ni catalyst supported on acidic carrier [74]. Besides these, optimum doping [186] of K (0.5%) over Ni/MgAl2O4 leads to blocking of the step-site of metal catalyst as well as elevate the steam adsorption

3.5. Metal nanoparticles on supports The catalysts of smaller size used during ESR not only increase the conversion [190] but also decrease the carbonaceous deposition. But the temperature is limiting parameters as the nanoparticles are more prone to get sintered at high temperature and lead to coke deposition [164]. Hence, the roles of support with nano active metal particles become a very important matter of concern. The carbon filament formation is typically interrelated with dispersion of metal particles on support. The metal nanoparticles with support reduce the carbon filament formation during ESR. The similar dispersion of Ni particle reported as analogous coking behavior and an opposite trend of Ni crystal size with C balance [191,192]. The nickel crystal with larger size generates higher amount of nanotube formation during ESR [80]. Recently, a few works has been reported with active nano metal catalysts. The nano size Rh particles over SiO2 with 15% La2O3 doping have shown very low deposition of carbon [193]. The nano-NiO/SiO2 with optimum (10.28 wt%) loading over Al2O3 support remains stable for the long period [194]. The work over noble metals (Ru, Pd and Ag) nanocomposite was also reported over CeO2/YSZ support [17]. The Ru and Pd based catalyst shows very low coke deposition and the location and morphology of the coke deposition also depends on the metal. The nano composite formation did not affect the pores structure of support because it substitute the part of support matrix. Therefore, it ultimately increase the surface area and metal support interaction [195]. The cobalt and nickel nanoparticle over alumina support also have comparatively lower carbon deposition with respect to larger one [196,197]. The Table 1 presents a summary of the available literature compares the amount of carbon deposited during ESR. These were reported by earlier workers using different catalysts and supports and effecting operative parameters. It has been represented to find out the correlation among the different type of metals (noble and non-noble metal), bimetallic interaction, particular support and mixed oxide support for carbonaceous deposition during ESR. There are only few works reported on the catalyst deactivation and their characterization, especially in terms of the amount of carbon. Most of the authors had described about the stability on comparing the experimentally performed data of their catalyst in terms of relativity in stability of catalyst. The data was also found with different time on stream (TOS) over different temperature. The amount of water in feed mixture was also taken randomly or stoichiometric requisite basis by several authors. 97


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Table 1 Carbon deposition during ESR over different types of catalyst. Catalyst Active metal

Support

H2O /EtOH

Temperature (K)

Carbon (gCgcat−1 h−1)

Reference

Rh Rh Ni Ni Ni

Al2O3 Al2O3 Al2O3 SiO2 SiO2-0.5! SiO2-1.0! SiO2-1.5! SiO22.0! SiO2-3.0! CeO2 ZrO2 ZrO2 with Ca (9) ZrO2 SiO2 Al2O3 α- Al2O3

8.4 6 13 3.5

923 859 773 873

[101] [205] [189] [206] [84]

5.8 3

865 773

3 3 3 5

573 773 773 823

0.0027 10.57^ 0.103 0.37 (TOS:60 h) 0.007*(TOS:2 h) 0.009*(TOS:2 h) 0.032*(TOS:2 h) 0.053*(TOS:2 h) 0.075*(TOS:2 h) 0.018 0.0101 0.0075 0.018 0.008 0.29 0.418*(TOS:3 h)

CeO2 CeO2 Al2O3 Al2O3 Al2O3 SiO2 SiO2 SiO2 SiO2 SiO2

3 6 3

723 723 823

[16]

3.7 3.7

973 873

5

823

0.0067 0.0045 0.144*(TOS:6 h) 0.032*(TOS:6 h) 0.136*(TOS:6 h) 0.12*(TOS:6 h) 0.184*(TOS:6 h) 0.168* 0.1534 0.23 0.37 0.57 0.62 0.538*(TOS:3 h)

Ce0.8Zr0.2O2 Ce(70)Zr(30)O2 La2O3(15)- SiO2 La2O3(40)- SiO2 Ce0.6Zr0.4O2 Y2O3-Al2O3(1:1) ZnO/ Al2O3 Al2O3-CeO2 La2O3-Al2O3

8 6 5

723 859 773

[211] [205] [193]

5.8 13 5 3 4

865 773 873 773 873

0.011*(TOS:8.3 h) 6.75^(TOS:20 h) 0.00003 0.0002 0.0093 0.033* 0.097* 0.660* 0.82#

MgAl2O4

10

673

MgAl2O4 MgAl2O4 MgAl2O4 with La MgAl2O4 with Pr MgAl2O4 with Ce

3 4.8

773 923

MgAl2O4 MgAl2O4 MgAl2O4 MgAl2O4

3

773

10

673

MgAl2O4

6.2

865

Mg4Al2(OH)16]CO3·4H2O Co2Mg4Al2(OH)16]CO3·4H2O Co2Mg4Al2(OH)16]CO3·4H2O Co2Mg4Al2(OH)16]CO3·4H2O Co2Mg4Al2(OH)16]CO3·4H2O Co2Mg4Al2(OH)16]CO3·4H2O

4

823

Ni Ni Ni Co Co Co Bimetallic (3)Pt Ni(10) (3)Pt Ni(10) CoAl (MC) CoAl (M) CoAl CoSi (MC) CoSi (M) CoSi CuNi Ce/Ni Ni/Ce Zr/Ni Ni/Zr (0.055)Fe/Co (0.11)Fe/Co (0.22)Fe/Co (0.44)Fe/Co (0.88)Fe/Co (1.76)Fe/Co Mixed oxide Rh Rh Rh Ni Ni Cu Ni Ni Spinel Ru Ni Co Co

Bimetallic spinel Co Pt Co Ir Co Ru 1%Ag-Ni 1%K-Ni Ni Hydrotalcite Co 0.1 Pt 0.3Pt 0.5Pt 1 Pt 0.1Rh

SiO2 α- Al2O3

[90] [191] [207] [208] [209]

[150]

[210] [206]

[209]

[90] [189] [137] [212] [213]

0.01# 0.046# 0.26 0.302*(TOS:7 h) 0.226*(TOS:7 h) 0.225*(TOS:7 h) 0.225*(TOS:7 h)

[187]

0.20 0.25 0.19 0.41# 0.17# 0.028

[208]

0.002 0.017 0.040 0.031 0.096 0.018

[208] [178]

[187] [90]

[214]

(continued on next page)

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Table 1 (continued) Catalyst Active metal

Support

0.3Rh 0.5Rh

Co2Mg4Al2(OH)16]CO3·4H2O Co2Mg4Al2(OH)16]CO3·4H2O

Bimetal and mixed oxide Ni0.1Pd Ni0.3Pd Ni0.1Pt Ni0.3Pt Pt5Ni Pt10Ni Pt25Ni Co/Cu Ni/Cu 30Ni5La

H2O /EtOH

Temperature (K)

Carbon (gCgcat−1 h−1)

Reference

0.012 0.013

La2O3-Al2O3

4

873

Al2O3-CeO2

3

773

ZnO/Al2O3

5

873

Al2O3·SiO2

4

673

0.27# 0.32# 0.51# 0.49# 0.644*(TOS:24 h) 0.629*(TOS:24 h) 0.583*(TOS:24 h) 0.576*(TOS:75 h) 0.380*(TOS:75 h) 0.086

[213]

[212]

[137] [215]

MC methanolic solution and calcined, M methanolic solution, without this aqueous solution is used. * indicates unit gC gcat−1, # stands unit gc h−1,^ indicates unit mmol gcat−1,! indicates the molar ratio of Nickel and citric acid concentration.

support interaction are able to diminish carbon deposition. But as per the cost concern, utilization of perovskite and ZrO2 as supports having significant lattice oxygen is a better option. Mixed oxide support could also be used by mixing CeO2, ZnO and ZrO2 as these are able to increase the density of oxygen vacancies and redox properties on the support surface. Ethanol dehydration is favored at the expense of ethanol reforming and so acidic sites present on support lead to ethylene formation which gets polymerized to form coke. The reductions of relevant number of acid sites reduce dehydration up to certain extent. The additions of metal such as Mg in Ni/α-Al2O3 catalyst results into formation of easily oxidizable coke, by enhancing the reaction between steam and carbon and ultimately decrease the carbon polymerization. Thermodynamically, metal particles with smaller size favor oxidizing property while metal particles with larger size reduce the interaction with support and so favor carbon filament formation. Moreover the non-noble metals with smaller particle size require low driving force to diffuse the carbon and resulted into lower coking rate. Therefore, increasing the surface area with nano size particle formation will hinder the growth of carbon. Nano particles derived from cluster precursor were superior up to a large extent regarding this concern as it was reported much smaller a salt derived nano particles. However, the reduction of carbon deposition should not be a primary goal. Keeping all these as a testimony, the catalyst formulation for ESR becomes easier, not only with respect to productivity, but also with stability and it is well known that without stability of catalyst, productivity is not an area of concern especially when it is the matter of energy.

Therefore, a number of variations in operating parameters which has a major role in catalyst deactivation cannot lead to an absolute interrelated conclusion. The quantity of carbonaceous deposition was reported as gc gcat−1 with different time on stream. But few authors used the term deactivation rate (gc gcat h−1). The rate of carbon deposition may vary with TOS [193]. Since, if the carbonaceous deposition able to block the active sites of metal catalyst then it lead to higher rate of coke deposition with TOS compared to the unblocked active site containing catalyst surface. The work reported by authors especially in the recent past is significant and noteworthy regarding various aspects of catalysts deactivation [198–204]. Overall, Rh has been reported negligible carbon deposition even with Al2O3 based catalyst and among non- noble metals Co and Ni or its bimetallic effect with CeO2, mixed oxide or Special structural catalyst (spinel) has shown significant capabilities to reduce coke deposition during ESR.

4. Conclusion The best approach to reduce catalyst deactivation due to carbon formation is either by preventing carbon to form or if formed, then by converting it into the gaseous form for easy removal. Decreasing the rate of carbon deposition is also a way to increase the life of catalyst but not an eternal solution. Catalyst modification strategy by modifying metal, support or both is able to stop carbon formation prior to deposition as well as subsequent to deposition. The metal must cleave the C–C bond to stop polymerization of intermediate products during ESR. A number of intermediate reactions are involved during ESR which determines the pathway for hydrogen generation. The different kinds of metal favor distinct pathways which may or may not generate carbide species. Specific plane of metal crystal is highly liable to carbon formation. The flat terrace and kinked Ni sites are prone to carbon formation. During alloy formation with Cu these sites get blocked. Therefore, the proper and optimized ratio of different metal crystallite with appropriate planes for the bimetallic catalyst formulation may be effective in reducing carbon deposition even at lower water to ethanol ratio. If the alloy formation between two metals gets hindered due to structure and is incapable of reducing the carbon deactivation as in the case of Co and Cu then a two bed reactor is the possible alternative for simultaneous use of the catalytic properties. Oxygen facilitates the nucleophilic reaction of the acetaldehyde species in conversion to acetate. It is also able to oxidize the deposited carbon that is easy to remove. The feeding of oxygen during ESR leads to metal sintering, therefore it is better to utilize a support having oxygen vacancies. The CO2 formed as a product of ESR acts as a contributor of oxygen for the oxygen vacancies present in the support. The high oxygen carrying catalysts such as CeO2 with strong metal-

Acknowledgements The authors are thankful to the Institute to provide financial Assistance (T.A.) to Mr. Ashutosh Kumar and also to provide Institute Research Project (IRP) to Y.C.S. References [1] Florez-Orrego D, Silva JAM, de Oliveira S. Exergy and environmental comparison of the end use of vehicle fuels: the Brazilian case. Energ Convers Manag 2015;100:220–31. [2] Kirillov VA, Meshcheryakov VD, Sobyanin VA, Belyaev VD, Amosov YI, Kuzin NA, et al. Bioethanol as a promising fuel for fuel cell power plants. Theor Found Chem Eng 2008;42:1–11. [3] Momirlan M, Veziroglu T. The properties of hydrogen as fuel tomorrow in sustainable energy system for a cleaner planet. Int J Hydrog Energy 2005;30:795–802. [4] Haryanto A, Fernando S, Murali N, Adhikari S. Current status of hydrogen production techniques by steam reforming of ethanol: a review. Energy Fuels 2005;19:2098–106.

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