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ACTIVE CENTERS OF CATALYSTS FOR HIGHER ALCOHOL SYNTHESIS FROM SYNGAS: A REVIEW Min Ao, Gia Hung Pham, Jaka Sunarso, Moses O. Tade, and Shaomin Liu ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b01391 • Publication Date (Web): 13 Jun 2018 Downloaded from http://pubs.acs.org on June 13, 2018

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ACTIVE CENTERS OF CATALYSTS FOR HIGHER ALCOHOL SYNTHESIS FROM SYNGAS: A REVIEW Min Ao,a Gia Hung Pham,a,* Jaka Sunarso,b Moses O. Tade,a Shaomin Liua,* a

Department of Chemical Engineering, Curtin University, Perth, Western Australia 6845,

Australia b

Research Centre for Sustainable Technologies, Faculty of Engineering, Computing and

Science, Swinburne University of Technology, Jalan Simpang Tiga, 93350 Kuching, Sarawak, Malaysia * Corresponding authors email addresses: [email protected] (G. H. Pham) and [email protected] (S. Liu)

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Abstract The gradual depletion of oil resources and the necessity to reduce greenhouse gas emissions portray a concerning image of our contemporary security of liquid transportation fuels in the event of a global crisis. Despite a vast amount of natural gas resources that we have and the huge economic incentive, the conversion for gas-to-liquid fuels or chemicals is still very limited due to the high technological complexity and capital cost for facilities. However, with the anticipated depletion of liquid petroleum and the soaring price of crude oil, the conversion of natural gas to liquid feedstock or fuels will become more and more important. Higher alcohols are important feedstocks for chemical and pharmaceutical industries and have wide applications as the potential fuel additives or hydrogen carriers for fuel cells for clean energy delivery. There is a long-standing interest for higher alcohols synthesis from syngas, an important Fischer-Tropsch technology for natural gas conversion. The purpose of this article is to provide readers with an extensive account on catalytic higher alcohols synthesis from syngas using various catalysts; reviewed from a unique perspective— clarification of the active centers and reaction pathways. In light of the different sources to provide the active centers, three major classes of catalysts in terms of monometallic, bimetallic, and trimetallic/multimetallic catalysts have been systematically reviewed and their respective performances are carefully compared. Finally, future works proposed to improve the catalyst design are described.

Keywords: natural gas, higher alcohol synthesis from syngas, mechanism, active center, synergistic effect


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1. Introduction The gradual depletion of crude oil reserves and increasing environmental issues have driven a worldwide research on alternative processes for the production of fuels and commodity chemicals. Natural gas is the third largest global energy source at present, behind oil and coal. This fossil fuel currently accounts for about one-fifth of the worldwide energy consumption and this ratio is expected to increase substantially in coming decades.1 The fastgrowing consumption of natural gas is attributed to its abundant resources, robust production, and somewhat cleaner output than its counterparts of coal and oil.2 Forty to sixty percent of the world’s proven natural gas reservoirs are remote or stranded, so transporting natural gas from the reservoir to the end users by pipeline is difficult and expensive.3 Liquefied natural gas (LNG) is a current solution for natural gas transportation, when pipelines are not an option (e.g. overseas export), but the relatively high cost of production process and the expensive infrastructures (e.g. cryogenic tanks) have limited its current commercial application to large natural gas resources, rather than the small/stranded ones. Strict transportation requirement, energy security of supply and environmental concerns create a need to look at alternative ways to utilize natural gas resources. Chemically converting this methane-rich feedstock into transport fuels or chemicals, via the Gas-to-Liquids (GTL) technology, has been proven to be a promising way for natural gas utilization. In the GTL process, methane from the natural gas is first transformed into syngas (a mixture of CO and H2), which is then catalytically converted into liquid synthetic fuels or chemicals in a second step via the Fischer-Tropsch (F-T) synthesis. Prior to the syngas generation process, mercury, sulphur, and nitrogen are removed, so the products produced by syngas are virtually free of contaminants such as sulphur and metals. In view of environmental protection and resources utilization, the catalytic conversion of syngas is generally recognized as a promising route for providing clean fuels and chemicals for the 3 ACS Paragon Plus Environment

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chemical industry. The F-T synthesis produces a wide range of products including high value hydrocarbons and related oxygenated compounds (fuels), methanol, and higher alcohols (HA). For the past 30 years approximately, there have been intensive researches focused on shifting the synthesis away from hydrocarbons and methanol toward HA. Higher alcohols contain two or more carbon atoms –– (C2+OH) ––including primary and secondary alcohols of both linear and branched carbon chains. The production of HA from natural gas through GTL process is expected to become an attractive pathway to reduce the crude oil dependence due to their broad range of applications. The most useful HA are C2 to C5 alcohols (linear or branched), some of which can also be used directly as transportation fuels, either alone or in gasoline blends, to extend gasoline supplies and enhance octane number, thus improving engine performance.4 These short-chain alcohols are also regarded as specialty solvents used in a wide variety of formulated products, from paints and coatings to cleaning agents and printing inks, to adhesives and lacquer thinners. Long-chain alcohols (C6-C22) are mainly employed as intermediates and have been widely used in the production of detergents and surfactants.5 All these facts highlight the widespread potential market for HA. Thus, the higher alcohol synthesis (HAS) from syngas through F-T process has been intensively investigated and reported in the literature by industrial and academic research organizations. However, none of the HAS catalysts or processes developed to date have been sufficiently active and/or selective to motivate industrial commercialization of the F-T process for HAS. Consequently, there are no higher alcohol F-T production plants in operation today. Most of the reported results of this process are based on experimental works at laboratory bench or pilot scale. Several good review papers have been published in the literature on higher alcohol synthesis from syngas covering the catalyst synthesis, catalyst types, and catalytic reaction 4 ACS Paragon Plus Environment

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performances and mechanisms.6-20 For example, Luk et al. comprehensively overviewed the development of different HAS catalyst systems, reactors and the kinetics of HAS in 2017.16 However, the active centers, especially the synergistic effect between the different active centers have been rather overlooked in HAS studies despite their significance to enhance the catalyst activity and HA selectivity. Consequently, this review aims to address this gap by summarizing the development and current status of HAS catalysts based on different types of active centers. The overview starts with the introduction of HAS from syngas and then moves on to review the development and the performance of catalysts with different active centers used for HAS. The emphasis has been placed on the clarification of roles of active centers and their effects on catalyst performance. 2. Higher alcohol synthesis from syngas Alcohols are formed from syngas according to Equation (1)11, wherein selective hydrogenation of CO occurs on the catalyst surface to produce alcohols directly. 2n H2 +n CO → CnH2n+1 OH + (n-1) H2 O

(1)

Since the early development of HAS from syngas, various theories about the underlying reaction mechanisms have been proposed. The CO insertion mechanism proposed by Xu et al. is widely accepted for the HAS process.15 In general, the above alcohol reaction includes CO dissociation, carbon chain growth, CO insertion and step-wise hydrogenation to the alcohol, as shown in Figure 1. The reaction scheme starts with the CO adsorption and dissociation to form surface carbon and subsequently hydrogenated to produce adsorbed C1Hx* group, then the carbon chain growth of this alkyl group is propagated via the CHx* addition. For HA formation, the adsorbed CO* molecule inserts between the metal site and the alkyl group to give an acyl intermediate (CnHzCO*) which can further form Cn+1 alcohol by hydrogenation. Alternatively, adsorbed CO* molecule can be partly hydrogenated to form formyl species (CHO*), which acts as an intermediate and then it is inserted into the alkyl 5 ACS Paragon Plus Environment

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species, thereby leading to HA via a step-wise hydrogenation. In parallel, the alkyl groups can be directly hydrogenated to form paraffin products. Note that the methanol formation via CO insertion is different from the mechanism mentioned above as the adsorbed CO* molecule is directly hydrogenated to form methanol. However, CO2 produced from CO through the water gas shift (WGS) reaction is considered as the other carbon source of methanol.10, 21 HAS is a complicated process because both carbon chain growth and CO insertion occur simultaneously and in proximity.16 However, these reaction steps usually occur on different active sites. As a result, the formation of HA needs the synergism of two active sites of the catalysts with one site catalyzing the carbon chain growth, and the other facilitating the CO insertion. Here, the overview of the HAS catalyst development will be given based on the following three aspects: monometallic, bimetallic, and trimetallic/multimetallic catalysts. Noted that the classification is based on the active site. The evolution of the number of publications for each catalyst type since 1980 is provided in the Supporting Information (Figure S1). Different catalysts are evaluated for HAS from syngas in terms of CO conversion and alcohol selectivity. To make the consistent and accurate comparison, carbon atom percentage (C atom %) is used to indicate selectivity in this work. C atom % of a carbon-containing product (for example C2H5OH) is defined as the selectivity of this product based on the carbon atom of the total carbon-containing products formed from consumed CO. A side reaction in F-T synthesis frequently encountered is the Water Gas Shift (WGS) reaction. Thus, C atom % on a CO2-free basis, to exclude the effect of the WGS reaction, has often been used for alcohol and hydrocarbon selectivity calculations in the literature. For comparison purposes, the alcohol selectivity in the present review is reported as C atom% on a CO2-free basis. Necessary recalculations were applied in some cases where reported selectivity included CO2. Instead of HA selectivity, some authors reported the total alcohol 6 ACS Paragon Plus Environment

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(including methanol) selectivity in their work. Because of the lack of necessary information for HA selectivity recalculation, these reports were not included for discussion on selectivity in this review (marked as N/A in the Figures in this review). 3. Monometallic catalysts In general, monometallic catalysts have been recognized as the atom efficient system. However, such a system is not appealing for HAS owing to the requirement of the bifunctionality of the HAS catalysts. Nevertheless, four different metals were reported as monometallic catalysts for HAS from syngas: Mo,22-59 Rh,60-71 Co72-79 and Fe80-83. These transition metals are usually exhibited two or more oxidation states, which provide the opportunity to form the dual active site (e.g. metallic-oxide pair) for HAS. The typical monometallic catalysts with different active centers reported for HAS are given in the Supporting Information (Table S1) 3.1. Mo monometallic catalysts Though molybdenum has a relatively low activity for syngas conversion and mainly produces hydrocarbons, Mo catalysts bonded to various ligands have been considered to be the most attractive monometallic catalysts for HAS. According to the literature, Mo monometallic catalysts for HAS mainly exist in three different forms: oxide,22-23 carbide,24-35 and sulfide36-56. Besides, Mo phosphide57-58 and nitride59 have also been reported as catalysts for HAS. Although the electronic structure and/or the surface geometry are different over Mo bound ligands (e.g. O, C, S, P and N), the mechanism of the HAS over Mo-based catalysts have mostly been described by the CO insertion mechanism.13 In general, there are two types of Mo species that involve in the HA formation: the low valence Mo0-2+ and the high valence Mo4+, which form the dual active site (Mo0-2+─Mo4+). The dissociation of CO on Mo0-2+ sites promotes the formation of CHx* species, followed by the carbon chain growth to form CnHz*. Then the CnHz* group migrates to the high valence Mo4+ sites, which is inserted by the non7 ACS Paragon Plus Environment

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dissociative adsorbed CO to form alcohols. The relative concentration of these two different Mo species on the catalyst surface is crucial for the catalyst performance for HAS: the higher concentration of Mo4+ increase the selectivity to alcohols, while the corresponding lower Mo0-2+ would lead to the diminished CO conversion and the carbon chain growth ability, and vice versa. To enhance the HA formation, it is necessary to get the balance of the two Mo species. Alkali metals, especially potassium is commonly used in Mo monometallic catalysts as the promoter to change the surface concentration of the active Mo species. Several studies have reported that the changes of Mo surface composition take place because of its interaction with K under the reaction conditions. For example, Xiang et al. found that the K promoter played different roles over Mo carbide catalysts (β-Mo2C and α-MoC1−x) for HAS.26-27 Based on XPS results, the strong electronic effect of K promoter in β-Mo2C based catalyst leads to a higher content of Mo4+ species on the catalyst surface, which is responsible for adsorbed CO insertion and HA formation. On the other hand, there was weak K-Mo interactions and less electronic effect of K to the α-MoC1−x phase. As a result, lower valence Mo0 and Mo2+ species become dominant, which are active sites for hydrocarbon formation. Thus, the maximum HA yield of 7.9% was observed at the unsupported K-β-Mo2C catalyst with K/Mo ratio =0.2, with the balance of the two types of Mo species in higher and lower valences.28 The addition of K was also reported to stabilize the Mo δ+ (including Mo2+ and Mo3+) state in Mo2N catalysts by back donation of electrons, which suppressed the hydrocarbon formation by increasing the CO insertion to CHx intermediates.59 Indeed, the role of K in Mo catalysts was suggested to not only suppress the hydrogenation of surface CnHz* species to form hydrocarbons but also increase the active sites for alcohol formation by retarding the reduction of high valence Mo species. Different potassium compounds as promoters of unsupported MoS2 catalysts also been investigated. The results showed that the HA yield 8 ACS Paragon Plus Environment

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increased with the pKa of the potassium counter anion: KOH> K2CO3>K2SO4>KCl.38 For the promoters with high pKa values, it is much easier to remove their counter anions under the reaction conditions, resulting in highly dispersed basic alkali species which modified the surface reactivity of MoS2. 3.2. Rh monometallic catalysts Rh is generally recognized as the catalyst to directly convert syngas to ethanol and HA since 1978. Despite some minor differences, the formation of HA from CO hydrogenation over Rh monometallic catalysts can still be recognized to follow the CO insertion mechanism.6, 12, 16 It is reported that Rh catalyzes both CO dissociation and CO insertion by forming the atomically adjacent Rh0─Rhn+ species, resulting in the formation of C2 oxygenates including ethanol, acetaldehyde, and acetic acid from syngas conversion.60 The CO molecules are firstly adsorbed on Rh0 sites in the linear form while on Rhn+ sites in the germinal form.84 The adsorbed CO on Rh0 sites then dissociates to form the CHx* intermediate, which can either be hydrogenated to form hydrocarbons, or an adsorbed CO can be inserted into the CHx* on ionic Rhn+ species to form a CHxCO* intermediate which can then be hydrogenated to produce ethanol or reacted with adsorbed CO to form higher oxygenates.62 A comparison of the surface oxidation state of the Rh species has revealed that the relative proportion of atomically adjacent Rh+/Rh0 species determines the ethanol selectivity.63 However, this kind of catalysts are not attractive because of the limited availability and high cost of rhodium. To minimize the Rh metal content in the catalysts, different kinds of supports are commonly used in Rh monometallic catalysts. Furthermore, it has been found that the nature of the support is critical to achieving high C2+OH yield. The neutral or acidic supports (SiO2 and ZrO2) usually promote the formation of C2-oxygenates such as acetaldehyde and ethanol, while the basic supports (MgO and CeO2) are active for mixed 9 ACS Paragon Plus Environment

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alcohols including methanol and ethanol.64 The same study further proved that with the proper co-existence of acidity and basicity in the Ce0.8Zr0.2O2 support, both CO conversion (27.3% vs. 23.7%) and HA selectivity (44.6% vs. 35.3%) of Rh catalyst were enhanced compared to that of Rh/CeO2 catalysts. On the other hand, the mesoporous materials such as MCM-41 and MXM-41 were used as the support of Rh catalysts owing to their high surface area.65-66 3.3. Co monometallic catalysts The highly dispersed metallic-oxide Co pair (Co0-Co2+ pair) has been regarded as the active site for HA formation. Pei et al.72 tested the cobalt metal-carbide (Co─Co2C) catalysts free from promoters and supports for HAS. The results in agreement with the widely accepted reaction pathway over Co monometallic catalysts that the synergetic effect of CoCo2C is responsible for HA formation, with metallic cobalt for CO dissociative adsorption and subsequent carbon chain growth, and Co2C active for CO non-dissociative adsorption and subsequent CO insertion into the linear hydrocarbon intermediates. The interface between cobalt and its carbide phase provided efficient dual sites for HAS. Unfortunately, it is known that Co monometallic catalysts during the HAS reaction suffer from the low thermal stability due to the sintering of the Co nanoparticles (NPs).85 The agglomeration could also lead to the deactivation of the catalyst, resulting in the change of catalyst performance. In the case of the sintering, ordered mesoporous carbon (OMC)73 and activated carbon (AC)74-75 were used as supports to improve the cobalt dispersion due to their high surface area (above 1000 m2 g-1). For OMC supported Co catalyst, the confinement effect not only enables an accurate control the cobalt particle size (< 4 nm) but also inhibits the conglomeration and sintering of active species, resulting in the superior stability during a period of 96 h.73 Besides the support, promoters can readily affect the catalyst properties and catalytic performances. Both SiO274 and Al2O375 were introduced to AC supported Co 10 ACS Paragon Plus Environment

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catalysts and was found to behave similarly for HAS. Although the catalyst reducibility is significantly reduced owing to the promotion of the refractory oxides, the dispersion of Co active centers is further improved, leading to the inhabitation of cobalt aggregation during the reaction. Moreover, the appropriate amounts of SiO2 and Al2O3 facilitate the formation of Co2C, which increase the HA selectivity. More recently, the metal organic frameworks (MOFs)-derived Co monometallic catalysts has been studied for HAS.78 The MOF zeolitic imidazolate (ZIF-67) is a novel material with a unique highly ordered structure and rich carbon environment, leading to the formation of the highly dispersed Co NPs. According to the characterization results, the Co2C phase formed during the reaction, which associates to the increased higher oxygenates selectivity. It is known that the cobalt carbide that formed during the reaction is usually unstable, which can be decomposed to metallic cobalt and polymeric carbon above 300 °C. However, for the ZIF67 derived Co catalysts, the Co2C phase could be maintained after reaction at 325 °C with the C2+ oxygenates selectivity up to 36%.78 This observation can be related to the ZIF-67 precursor, which provides the highly-dispersed and stable cobalt skeleton. Besides cobalt metal-carbide catalysts, a series of cobalt phosphide catalysts supported on silica has also been studied for HAS.79 The addition of P changes the electronic and adoption properties of metallic Co, resulting in the Coδ+ species in the form of Co2P. The Co-Co2P species are hypothesized to play the similar role as the Co-Co2C in HAS, where metallic cobalt is responsible for CO dissociation and carbon chain growth, while Coδ+ species in Co2P active for CO non-dissociative adsorption and subsequent insertion to form HA. 3.4. Fe monometallic catalysts Iron is attractive for HAS owing to its inexpensiveness and strong ability for hydrocarbon formation. In 1985, Pijolat et al.80 explored the Fe/Al2O3 catalyst for HAS using a fixed bed reactor. The authors found that the catalyst follows the CO insertion mechanism, with 11 ACS Paragon Plus Environment

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metallic Fe not only active for CO dissociation but also responsible for CO non-dissociative adsorption. However, the active phase of the Fe catalyst for HAS has not been clarified. Later, the Fe2O3/Al2O3 catalyst was tested in a slurry reactor and the results proved that in this catalyst system, pure Al2O3 is inert for syngas conversion and FeO is the active phase of HA formation.82 Still, a controversy exists in explaining the active centers in monometallic Fe catalysts for HAS and further investigation is needed.86 Although several strategies have been applied to modify the monometallic catalysts, the catalyst performances still have not achieved a suitable yield for HAS. On virtually all monometallic catalysts following the trend that high HA selectivity with low CO conversion (e.g. Mn and Rh) or low HA selectivity with high CO conversion (e.g. Co and Fe). Their main disadvantage is that they suffer from the lack of sufficient active centers, including the restricted active center combination, difficulty in the formation of adjacent metal-oxide pair and less control of the balance between active sites with different functionalities. Accordingly, it is recommended that research efforts shift to the development of dual active sites with a various choice of active centers and different functionalities that are chemically and economically promising. 4. Bimetallic catalysts Bimetallic catalysts are usually composed of two different metal elements, with various structure and supports (with or without alkali promoters) and prepared via different methods. According to the CO insertion mechanism, two different types of active sites are identified for HAS from syngas, one for CO dissociation and carbon chain growth and the other actives for CO insertion and alcohol formation. Since the bimetallic catalysts have more opportunities to be tailored with the required bifunctionality as mentioned above, they have been mostly investigated for HAS since 1990 (Figure S1). In this subsection, different roles of active centers and their synergistic effects in HAS reaction have been further discussed. 12 ACS Paragon Plus Environment

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The roles of active centers in bimetallic catalysts and their performances for HAS are summarized in the Supporting Information (Table S2). 4.1. Mo bimetallic catalysts Mo species in monometallic catalyst exhibits excellent selectivity towards HA but relatively lower activity, thus the selection of the second metal with high CO dissociation ability is necessary to improve its performance in HAS. 4.1.1. Fischer-Tropsch element modified Mo bimetallic catalysts Fischer-Tropsch elements (Co, Ni and Fe) are recognized as the active centers for hydrocarbon formation, which facilitate the CO dissociation and carbon chain growth in CO hydrogenation reaction. As a result, much attention has been paid to the Mo-Co,87-115 Mo-Ni 116-137

and Mo-Fe138-141 bimetallic catalysts for HAS.

It has been claimed that the synergistic interaction between the F-T element and Mo is responsible for the high activity of C2+OH over different Mo catalyst systems. A series of Kpromoted Co-Mo bimetallic catalysts were prepared by sol-gel method and reported to be active for HA formation.89-91 The homogenous component distribution and small particle size on the MoOx-based catalysts enhanced the synergism between Co and Mo, resulting in the improved HA formation. Li et al. studied the effect of the Co-promoter on K-MoS2/AC catalysts and observed the enhanced activity and selectivity for C2+OH formation due to the synergism resulting from Co-S-Mo species and MoS2 phase.107 Noted that the amount of Co loading is critical for catalyst performance. The authors found that with higher Co loadings, the active Co-Mo-S species would decrease due to the formation of Co9S8 crystallites which favorable to the hydrocarbon formation. Similar results were also observed in K-promoted Ni-MoS2 catalysts, with the synergism of Mo and Ni is essential for HA formation.116-125 Moreover, Ni is reported to show a bifunctionality in Ni-MoS2 catalyst. Sun’s group found that Ni species not only promoted the carbon chain growth but also activated for CO 13 ACS Paragon Plus Environment

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insertion, resulting in a maximum ratio of C2+OH/C1OH of 8.75 when Ni/Mo ratio was 0.5.121 In addition, the same authors prepared a highly homogeneous and dispersed K-Ni-MoS2 catalyst using the ultrasonic technology under water-absent environment, which suppressed the formation of segregated NiSx at high Ni loading.122 Wang et al.140 examined the Fe-modified K-β-Mo2C catalyst for HAS from syngas and found a strong synergistic interaction between Fe and Mo which resulting from the Fe3Mo3C phase.

The Fe3Mo3C phase contributes the active site for CO insertion while Fe3C is

associated with the hydrocarbon production. Noteworthy that the addition of Fe facilities the carbon chain growth of both alcohols and hydrocarbons, in particular for the step C1 to C2. Studies over K-Co-Mo2C92-93 and K-Ni-Mo2C118 catalysts showed that the promoters Co and Ni play similar roles to Fe where the Co3Mo3C and Ni6Mo6C are the active centers for HA formation, respectively. And the highly dispersed Co and Ni species can also enhance the carbon chain propagation, particularly from C1 to C2 step. Xu et al.114 alternatively investigated K-Co-MoP/SiO2 catalyst for HAS. XRD analysis revealed that the addition of Co to MoP catalyst would form the CoMoP phase, accelerating the dispersion of catalytic components over the catalysts with improved CO conversion and HA selectivity. On the other hand, the surface concentration of the Mo and F-T elements also affects the HA formation. A representative example is the active carbon (AC) supported K-Co-Mo oxide catalysts, which shows enhanced reduction of Mo6+ species to Mo4+ species with higher Mo loading.94 This work confirmed that the Mo4+ provides the active site for HAS and the HA formation increased with the molar percentage of active site Mo4+ on the catalyst surface. The surface concentration of the active centers can further be optimized by introducing proper promoters. For example, the K-Co-Mo oxide catalyst showed the high CO conversion (34.5%) and high C2+OH selectivity (76.2%) with the promotion of Co-decorated h-type multiwall carbon nanotubes (MWCNT).95 The addition of a minor amount promoter 14 ACS Paragon Plus Environment

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increased the surface concentration of the two kinds of active centers, namely CoO(OH)/Co3O4 and Mo4+, both closely associated with the alcohol generation. 4.1.2. Noble metal modified Mo bimetallic catalysts In addition to the conventional F-T elements, noble metals such as Rh142-144 and Pd145 have also been used as the second metal in Mo bimetallic catalysts and showed effective promotion to the HA formation. Li et al.142 investigated the Rh-promoted K-MoS2/Al2O3 catalysts and found that the synergism between Rh and Mo species increased the appearance of catalytically active surfaces or sites. Such Mo-Rh interaction further stabilized the coexistence of Rh0/Rh+ species, leading to the increased CO conversion and HA selectivity. Besides, the same authors suggested the Pd-promoted K-MoS2/Al2O3 catalysts followed the same mechanism as the Rh-promoted catalysts for HAS.145 Besides, V in Group VIII has also been used as the second metal in Mo catalysts. Chiang et al.146 synthesized V-promoted K-Mo2C catalysts and their results showed relativity high CO conversion (35.7% vs. 25.8%) and C2+OH selectivity (63.5% vs. 45.5%) compared to βMo2C-based catalysts. Although V has a strong ability for hydrogenation, its addition further increased the chain propagation from C1OH to C2+OH, due to the synergistic effect of V and K. 4.2. Rh bimetallic catalysts As mentioned previously, monometallic Rh catalysts exhibit excellent selectivity towards HA, especially ethanol. However, the relatively low CO conversion (2-30%) and high methane selectivity have been long considered as the major restrictions. Thus the modification of this catalyst is highly necessary to reduce the consumption of Rh and increase the HA formation. 4.2.1. Rh-Fe bimetallic catalysts


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Among transition metals, Fe is considered as an effective promoter for Rh-based catalysts, owing to its capability of methane suppression.147 Consequently, numerous studies were conducted for Rh-Fe bimetallic catalysts.148-157 The addition Fe into Rh/ZrO2 catalysts was reported to enhance the activity of the catalysts and their selectivity towards C2+OH.149 The direct interaction between Rh and Fe species promotes the formation of double-bonded CO (gem-dicarbonyl, Rh-CO-Fe), which is paramount active for the formation of oxygenated products, especially ethanol. For catalyst after reduction, the presence of the Rh-Fe alloy is active to form ethanol and simultaneously reduces the methane formation. Besides, the presence of Fe stabilizes the adsorbed acetyl species to form ethanol rather than desorption of acetaldehyde, which further increased the C2+OH selectivity. To increase the dispersion of active centers, various supports have been extensively investigated over Rh-Fe bimetallic catalysts, including ZrO2,148 TiO2,149-150 SiO2,151 Al2O3,152153

SBA15154-155 and CeO2156. However, most of these catalysts were prepared by the

impregnation method, which restricted the formation of the Rh-Fe alloy. To address this issue, Han et al.157 prepared the ZrO2-supported perovskite catalyst YRh0.5Fe0.5O3 by using citrate complexing method. The uniformly mixed Rh and Fe species in perovskite structure led to a much closer interaction between Rh and Fe species, resulting in the formation of RhFe alloy after reduction (see Figure 2 (b)). The catalyst demonstrated a high CO conversion of 34% and the total alcohol selectivity reached the maximum at 59%, with C2+OH weight distribution up to 81.6%. However, the ethanol distribution decreased with reaction time while the catalyst particle size distribution based on TEM images increased from 2.2 (TOS=20 h) to 3.2 (TOS=130h) nm (see Figure 2 (e) and (f)). The increased Rh particle size mirrors the sintering of Rh species, resulting in the phase separation of Rh-Fe alloy, which led to the decrease of selectivity to ethanol. 4.2.2. Rh-Mn bimetallic catalysts 16 ACS Paragon Plus Environment

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In addition to Fe, Mn is widely used as the second metal for Rh-based bimetallic catalysts. 158-177

The role of the active centers in Rh-Mn bimetallic catalysts have been reported based

on the intrinsic kinetics study and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) analysis.161-162 The close interaction between Rh0 and MnOx species leads to the adsorbed CO species being altered from linear and germinal to the “tilted” form, which is more thermally stable and more difficult to desorb. These tilted-adsorbed CO species boost the CO dissociation on Rh0 site which increases CO conversion. On the other hand, MnOx species enhance the CO insertion by promoting linearly adsorbed CO species on Rhδ+ sites. This is in accordance with the findings over Rh-Mn/Al2O3 catalysts.173 Still, the majority of the products over Rh-Mn bimetallic catalysts are hydrocarbons and further improvement is necessary. Different approaches have been undertaken to enhance the HA formation, i.e. (1) addition of alkali metal Li in Rh-Mn bimetallic catalysts;167 (2) pretreatment of SiO2 support with nC1-C5 alcohols;163 (3) introduction of supports such as SiO2-TiO2164 and SiO2-ZrO2165. In doing these approaches, the dispersion of Rh species is enhanced and the Rh species become less reducible, thereby suppressing CO dissociation while increasing CO insertion which is thought to promote HA formation. In addition, by adjusting the loading of Mn species to 2%, the 2%Mn3%Rh/CNTs catalyst exhibited a higher ethanol selectivity of 37.2% with the higher degree of Rh-Mn interaction.171 4.3. Co bimetallic catalysts Co is a well-known F-T element and shows the potential for HAS as monometallic catalysts. However, hydrocarbons, especially methane is the major product in HAS, with a hydrocarbon/alcohol ratio of 1 or higher. This led researchers to explore the addition of the second metal to the Co catalysts, which can increase the formation of HA and at the same time reduce the formation of unwanted hydrocarbons. 4.3.1. Methanol formation element modified Co bimetallic catalysts 17 ACS Paragon Plus Environment

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Elements known for methanol formation (Cu,178-220 Pd,221-222 Pt,223 etc.) are commonly used as the second metal of Co bimetallic catalyst with the aim of introducing the active site that contains an oxygenated functionality. (a) Co-Cu bimetallic catalysts Since 1978, the Institut Français du Pétrole (IFP) has patented and developed a series of Co-Cu bimetallic catalysts, which not only reached a high range of C2+OH selectivity (3050%) but also exhibited the low material cost.15, 19 The nature of the active centers has been extensively investigated in the Co-Cu bimetallic NPs: the metallic Co can dissociate the molecular CO and subsequently hydrogenate the resultant surface carbon to produce *CnHz groups while Cu is active for non-dissociative adsorption of molecular CO and CO insertion into surface *CnHz group to form alcohols.188, 192, 198 The combination of metallic Co and Cu shows pronounced synergistic effects and thereby leads to the higher C2+OH selectivity.198, 224 In addition, the investigation of the various Co/Cu molar ratio in CoCu/SiO2 catalysts demonstrates that with the proper amount of Cu addition, the HA selectivity increased twofold but the CO conversion dropped greatly.192 A further increment of Cu loading has no effect on the catalyst performance, while the Cu-only catalyst shows inferior activity and methanol is the dominate alcohol product. Based on the kinetic study, the role of Cu is identified to weaken the CO dissociation and reduce the formation of CHx* species. Thus, it is necessary to keep an appropriate balance between the Co and Cu active centers to meet the requirement of the effective HAS. In other words, a good balance between carbon chain growth and CO insertion is important for HAS. Co-Cu bimetallic catalysts with a variety of supports such as Al2O3,178-187 SiO2,188-192 TiO2,193 and carbon nanotubes (CNTs)194-197 have been widely investigated for HAS. It is reported that the higher surface area and strong interaction between active centers and support lead to the improved HA formation. In addition, both the active centers and the 18 ACS Paragon Plus Environment

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catalyst performance have a tight correlation with the catalyst precursor structure. Taking CoCu bimetallic catalysts as examples, we further discuss the effect of catalyst precursor structure on synergistic catalysis for HAS. •

Core-shell Co-Cu bimetallic catalysts

The core-shell structure is an attractive option for HAS because of the improved proximity of Co and Cu.198-201 Subramanian et al.199 studied the CO hydrogenation over Cu@Co3O4 (core@shell) bimetallic NPs. It is believed that the Cu@Co3O4 core-shell NPs are active for both associate and dissociate CO adsorption, resulting in a good balance between CO insertion and carbon chain growth. The surface properties of the core-shell structured catalysts can be affected by modulation of core and shell dimensions or application of different activation conditions. Xiang et al.201 reported that the H2-activated catalysts led to the simple Co@Cu core-shell structure, with Co-rich in the core and a Cu dominated shell (Figure 3 (a-b). On the contrary, the CO-activated Co2Cu1 bimetallic catalyst was transformed to an “onion-like” graphitic carbon shell, with a slightly larger particle size than the H2-activated catalyst (Figure 3(c-d)). The CO activation led to a higher Co/Cu surface ratio due to the surface segregation of Co, and the formation of cobalt carbide was also observed. Thus, the Co-rich catalyst activated by CO showed significantly higher CO conversion than that activated by H2 (27.1% and 5.7%, respectively), owing to its strong CO dissociation ability. Furthermore, the formation of cobalt carbide on CO-activated catalyst led to the enhancement of chain lengthening probability of alcohol products (0.48 vs. 0.25, respectively). •

Co-Cu bimetallic catalysts derived from perovskite structure

Perovskite oxides have an ABO3 type crystal structure, where A- and B-sites can be occupied by a board range of metallic cations of different sizes and oxidation states.202-203 19 ACS Paragon Plus Environment

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Owing to their flexibility in catalyst design and preparation, promoted Co-Cu bimetallic catalysts in the form of LaCo1-xCuxO3 perovskite precursors have been developed for HAS from syngas.204-209 In the perovskite structure, the Co and Cu ions are uniformly distributed at the atomic level in the crystal lattice. A noteworthy point is that the LaCo1-xCuxO3 perovskite structure is not stable in reducing atmosphere. After reduction, the perovskite structure collapsed, and NPs of bimetallic Co-Cu alloy with strong synergetic effect is formed, which acted as the active center for HA formation. Unfortunately, the Co-Cu bimetallic catalysts derived from perovskite structure suffer from the sintering during the reaction due to their low specific surface area. The introduction of the support such as SiO2210 and ZrO2211-212 have been proved to enhance the catalyst stability of LaCo1-xCuxO3 by improving the dispersion of the active centers. Nevertheless, there are still some disadvantages that cannot be ignored. On one hand, only limited amount of perovskites can be loaded on the support (up to 40 wt. %). On the other hand, the sintering of Co-Cu alloy is unavoidable owing to the insufficient interaction between the active metals and the support. To overcome these issues, Liu’s group213-215 bring up the idea using the perovskite LaFeO3 as the support to load bimetallic Cu-Co NPs. Co3O4/mesoporous-LaFe0.7Cu0.3O3 was used as the catalyst precursor.214 During the reduction, the highly dispersed Co3O4 and copper ions formed the Co-Cu alloy and the precursor transformed to the La2O3-doped Co-Cu/LaFeO3 after reduction. The mesoporous LaFeO3 with high surface area guarantees the good dispersion and uniform mixing of the Co-Cu alloy, and the strong interaction between Co-Cu alloy and LaFeO3 support restrains the sintering of the Co-Cu NPs.214 Among the perovskite derived Co-Cu bimetallic catalysts, the best catalyst performance was observed in the graphene-LaFeO3 supported Co-Cu NPs, with CO conversion reached 50%, HA selectivity of 57% and negligible deactivation was observed up to 160 h time on stream.215 The graphene


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sheets embedded into LaFeO3 not only enhanced the catalyst surface area but also produced the slit-like mesoporous which provide well passes for transmitting the reaction molecules. •

Hierarchical Co-Cu bimetallic catalysts

Layered double hydroxides (LDHs) have recently received special attention in HAS owing to their topological structure and uniform distribution of the metal species in the mixed metal oxides.216-220 Cao et al.217 prepared the Co-Cu bimetallic catalysts by using LDHs as the precursors. The authors found that for (CuxCoy)2Al-LDHs catalysts, when the Cu/Co ratio equal to 0.5, the catalyst exhibited the best HA selectivity. This is because the topological effect of LDHs leads to the uniform mixing of the Cu and Co species in the catalysts, which formed a Co-Cu alloy after reduction and acted as the active center for HA formation. Very recently, Yang et al.220 tested the (Cu1Co2)2Al-LDHs catalysts for HAS in a slurry reactor and investigated the active center variation with reaction times by using XRD, TEM, EDS, and ICP-MS techniques. The catalytic performance correlated to the change of active centers is shown in Figure 4. It was found that the Co-Cu alloy was formed at first as the active center after reduction, but started to decompose into Cu and Co species at the beginning of the reaction (less than 100 h). As the reaction continues (TOS = 100-300 h), smaller cobalt NPs started to be enriched on the surface of copper NPs, resulting in a Cu@Co core-shell status. After reaction for 300 h, the sintering of Cu led to the enlargement of the Cu@Co particles, and the carbonizing of Co resulted in the formation of Co2C. At this stage, the active centers were transformed to Cu@Co@Co2C. During the end of reaction period (TOS=500-750 h), the Cu and Co NPs were wrapped by Co2C. The high C2+OH selectivity during the whole reaction process is mainly due to the close contact of Cu and Co species, including Cu-Co alloy and Cu@Co NPs. Furthermore, the significant deactivation was also observed over the catalysts during the reaction. In addition to the Co-Cu phase separation and Co loss, the sintering of active metal Cu and Co NPs is considered as another factor that led 21 ACS Paragon Plus Environment

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to the deactivation of the catalysts, which was different from the above-mentioned perovskite structure derived Co-Cu bimetallic catalysts. (b) Co-Pt and Co-Pd bimetallic catalysts Noble metals such as Pd221-222 Pt223 are introduced to Co-based catalysts due to their activity to CO non-dissociative adsorption and oxygenate formation. Kumar et al.221 have devoted efforts to investigate the effect of Co/Pd ratio (2/2 and10/2) for HAS and concluded that the Co-rich catalyst is more active but less selectivity to HA, while the 2Co-2Pd catalyst is less active but more selective towards these compounds. Besides, the close contact between Co and Pd in 2Co-2Pd catalyst has played a key role in the stability of HAS, with no deactivation observed. These findings are in agreement with the Co-Pt bimetallic catalysts, where Co-Pt alloy is the active center and when a molar Co/Pt ratio at 1 was reached, the CoPt bimetallic catalyst was selective to HA.223 4.3.2. Fischer-Tropsch element modified Co bimetallic catalysts Co-Fe based bimetallic catalysts have been studied for HAS because Co and Fe are considered as the two most active F-T elements, which showed higher C2+OH selectivity than expected from either Co or Fe catalysts.225-227 The increased HA selectivity can be attributed to the formation of Co-Fe alloy, with Co processes CO dissociation and Fe active for CO insertion to produce oxygenates. The alcohol formation is reported to depend strongly on the Fe/Co ratio of the catalysts. The higher Fe/Co ratio, the more Co-Fe alloy is formed, and the larger size of the Co-Fe NPs, resulting in the enhanced HA formation.227 However, the excess of Fe would lead to the significantly increased CO2 selectivity due to its higher WGS activity. Another approach to improve the HA selectivity of Co-Fe bimetallic catalyst was proposed by Du et al.228-229, who used the active carbon as the catalyst support. Different from the aforementioned catalysts, the Co/Co2C species was observed in the Co-Fe/AC catalysts. Based on the XRD analysis, the activated carbon facilitated the Co2C formation and the 22 ACS Paragon Plus Environment

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conjunction of the interface between Co and Co2C species resulted in the synergistic effect, which is active for HA formation. Besides, the Co-Fe alloy was observed in the spent Co-Fe bimetallic catalysts, which provides the synergistic effect between Co and Fe species. Consequently, the authors claimed that the co-existence of Co/Co2C and Co-Fe alloy might be responsible for the rather high C2+OH selectivity in HAS. The bimetal Co-Ni alloy in La0.9Sr0.1Co1-xNixO3 perovskite catalysts has a positive effect on HA selectivity and yield, for low nickel substitution level (x=0.1).230 The synergistic effect of the two metals in the Co-Ni alloy played a crucial role in enhancing the HA selectivity, with Co preferentially catalyzing the formation of the hydrocarbon precursors by CO dissociation and carbon chain propagation while Ni promoting the succeeding CO insertion under hydrogenation conditions. 4.3.3. Other Co-based bimetallic catalysts Co-based bimetallic catalysts have gained increasing attention due to their relatively high HA selectivity, low cost and wide availability of the precursors. A wide selection of the second metal such as Ga,231-234 La,235-237 Ca,238 Ce,239 and Mn240-243 for Co bimetallic catalysts have been reported due to their strong interaction with Co which influences the electronic properties of the active sites. Supported Co-Ga bimetallic catalyst originating from the reduction of the CoZnGaAlLDHs/c-Al2O3 sample provides high selectivity to C2+OH (total alcohol selectivity 59% with C2+OH fraction reached 92.8 wt.%) while remains the high activity (43.5%) and excellent stability in HAS.231 As shown in Figure 5, the Co-Ga particles are uniformly dispersed in both fresh and used catalysts. The mechanism study of the Co-Ga in HAS indicated that the linkage of Ga and Co atoms leads to the isolating Co sites which is responsible for linearly CO non-dissociative adsorption and subsequent CO insertion to produce HA.232 In addition, the close contact of Ga and Co contributes to the electronic donation from Ga to Co, which 23 ACS Paragon Plus Environment

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promotes the CO dissociation and facilities the chain propagation. More significantly, with the trapped structure produced by the LDH precursor, no obvious aggregation was observed in the Co-Ga based catalysts after the reaction, resulting in a stable catalytic performance (Figure 5 (b)). Doping of La onto the active carbon supported Co catalyst is proved to enhance the formation of Co2C via the combination of in-situ and ex-situ XRD analysis.237 The metallic cobalt and Co2C particles are active centers for HAS. As the Co2C content increased, so did the selectivity of HA. However, the excess of Co2C phase would lead to the deactivation of the catalyst, which is mainly due to the inability of Co2C to catalyze CO dissociation along with the lowered amount of metallic cobalt. The catalyst 15Co0.5La/AC with the relative Co2C/CoO ratio of 1.6 exhibited the highest HA formation because it offers the best compromise between CO insertion and carbon chain growth. Similar effects were also reported by Co-Ca bimetallic catalysts.238 Very recently, a series of studies focused on the Co-Mn bimetallic catalysts for HAS.240-243 The metallic cobalt in Co-Mn bimetallic catalyst was found to transform into a Co2C phase during the syngas conversion with the addition of potassium.240 Such reconstruction results in the intimate contact between the Co2C and mixed-valence Mn species (Mn2+ and Mn4+), which acts synergistically to promote the formation of oxygenates, especially higher aldehydes and alcohols. Later on, Zhao et al.243 claimed that the Mn doping facilities the restructure of Co NPs to form the Co@Co2C dual active site over Co/AC catalyst under HAS reaction conditions. The synergistic effect between Co0 and Co2C from the Co@Co2C interface is believed to active for HA formation. However, only limited incensement of HA selectivity was observed in Mn promoted Co catalysts. This can be explained by the nature of Mn, which is active for the dissociation and disproportionation of CO and prohibits H2


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adsorption to some extent, leading to the relative C-rich and H-lean surface chemical environment for olefin production. 4.4. Fe bimetallic catalysts As discussed in Co bimetallic catalysts, HA formation over F-T element-based catalysts needs the synergistic function of copper, which active for methanol formation. Xiao et al. prepared the non-supported Fe-Cu bimetallic catalysts with different Cu/Fe ratio.244-247 It is noteworthy that the Fe-Cu bimetallic catalysts are in situ activated/reduced by syngas before the reaction. During the activation process, carburization of Fe species to iron carbides (FeCx) occurred, resulting in a FeCx-Cu dual site, which is considered as the active center for the HA formation.245-246 The synergistic effect of FeCx and Cu benefits HA formation, with FeCx active for carbon chain initiation and propagation while Cu active for CO non- dissociative adsorption and insertion. A major concern of the Fe-Cu bimetallic catalysts is the rapid catalyst deactivation. Due to the instability of Fe, the modification of the catalyst is generally required to improve the catalyst performance. These methods include (1) introduction of the higher surface area supports such as SiO2248-254 and Al2O3255-256; (2) using the special structure as the catalyst precursor such as LDH257-258 and three-dimensionally ordered macroporous (3DOM).259 These methods can provide a uniform and highly dispersed bimetal sites, which enhances the synergism between Fe and Cu species and helps to provide abundant, active and stable dual active sites on the catalyst surface, thus promoting the catalytic activity and HA selectivity. However, the sintering of Cu or the phase separation of the Cu and FeCx phases during the long-turn reaction would weaken the synergism and lead to a decrease in HA selectivity.246 4.5. Zn-Cr bimetallic catalysts Zn-Cr bimetallic catalysts have been widely used as methanol synthesis catalysts in the industry for years, with HA as side products during the reaction. To improve HA formation, 25 ACS Paragon Plus Environment

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alkali metals such as Li, Na, K and Cs with different loadings have been investigated.260-267 Due to the addition of alkali promoters, the catalyst reducibility and space electronic composition of active phases in Zn-Cr catalysts are changed, resulting in the shift of the alcohol distribution, with a mixture of methanol and branched alcohols (especially isobutanol) as major alcohol products.260 However, the real active sites in Zn-Cr bimetallic catalysts have been controversially debated over the last 30 years.261 Recently, Tan’s group266-270 studied the structure-activity relationship of active sites over Zn-Cr catalysts for isobutanol formation. The existence of excess Zn2+ ions inside the spinel structure would lead to the non-stoichiometric Zn-Cr spinel and enhance the level of cation disorder.267 The reaction tests showed that the more serious disorder of the cation distribution, the more defects of the catalyst and the more efficient for isobutanol generation. To be more specific, Zn2+ cations in octahedral vacancy are active sites for carbon chain growth to form isobutanol while the ZnO phase inspires the formation of methanol. Figure 6 shows the catalytic performances over the four typical active center-based (Mo, Rh, Co, and Fe) HAS catalysts, for which information about CO conversion (X axis) and C2+OH selectivity (Y axis) is provided. Compared to the catalyst performance over monometallic catalysts, the bimetallic catalysts with the dual active site and synergistic effect showed significant improvement both in CO conversion and C2+OH selectivity. The combination of the two active centers with different functions for HAS is the key to the enhanced catalyst performance. With the addition of F-T metals (Co, Ni and Fe), the CO conversion of the Mo bimetallic catalysts increased from ~30% up to ~80% while retaining the high HA selectivity, leading to a significant increase of HA yield. A positive change of CO conversion over the Rh bimetallic catalyst is also observed but it appears less markedly. For Co bimetallic catalysts, the HA selectivity increases in the order: Co-Mo>Co-Cu>Co26 ACS Paragon Plus Environment

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other metals>Co-Fe/Ni. The is probably due to the nature of Mo element, with the higher valences Mo4+ active for CO insertion and the lower valences Mo0 and Mo2+ for CO dissociation and carbon chain growth. While the metallic Cu only active for methanol formation in Co-Cu bimetallic catalysts. Furthermore, Fe-based catalysts with the combination of the second element such as Cu and Mo exhibit significant improvement in both CO conversion and C2+OH selectivity. 5. Trimetallic/multimetallic catalysts There

are

limited

publications

but

with

a

rapidly

growing

interest

in

trimetallic/multimetallic catalysts for HAS in the last decade. Trimetallic/multimetallic catalysts contain more variables which could be tuned according to the requirement of the HAS. Due to the variety of the catalyst composition in HAS process, only the most common investigated catalysts will be discussed. The details of the active centers in typical trimetallic/multimetallic catalysts for HAS are given in the Supporting Information (Table S3). 5.1. Co-Cu-based trimetallic/multimetallic catalysts Although the Cu-Co bimetallic catalysts display high yield towards C2+OH in HAS, methanol still remains the dominant alcohol, and the methane is the main byproduct with relatively high selectivity. There is still a limitation of carbon chain growth, especially the C1 to C2 step. Therefore, it is necessary to seek controlled tuning of the catalyst by introducing new active centers, such as Fe,271-277 Mo,278-282 and Ce283-285 to further improve the HA formation. Wang et al.271 reported that the addition of small amount of Fe to the Co-Cu/Al2O3 catalyst led to significant increase in HA yield from 11.6 to 46.4 mg/gcat h. The promotion of Fe not only improved the dispersion of Co-Cu bimetallic particles but also favoured the reducibility of the catalyst. Furthermore, the authors proposed that in addition to Co-Cu bimetallic 27 ACS Paragon Plus Environment

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particles, the possible formation of Fe0 could act as the active site for hydrogenation and carbon chain propagation. Guo et al.272 found that the low amount of Co loading to KCo3Cu25Fe22/SiO2 catalyst enhances the high surface concentration of the Cu-FeCx dual site, resulting in the improved synergism between Fe and Cu, which is active for HA formation. To further improve the catalytic activity, the authors deposited Co-Fe-Cu catalysts onto different supports such as palygorskite273 and H2SO4-activated attapulgite (ATP)274, resulting in higher CO conversion (62.8% and 70.5%, respectively) and C2+OH yield (103.7 g/ml h and 102.6 g/kg h, respectively). In the Co and Mo promoted CuLa2Zr2O7 catalyst, CuLa2Zr2O7 phase is active for methanol formation, the addition of Co favors the CO dissociation and carbon chain growth, while Mo enhances the hydrogenating capability of the catalyst.280 With the promotion of Co and Mo, the selectivity of HA increased drastically with the promoters, however, the CO conversion decreased compared with that of the CuLa2Zr2O7 catalyst. The yields of HA reached the maximum with an optimal loading of 5% Co and 3% Mo, owing to the reasonable compromise between catalyst activity and HA selectivity. Wang et al.283 compared the performance of a series of CNT supported Co-Cu-Ce trimetallic catalysts. HA selectivity of 23.1% was observed over the Co-Cu-Ce/CNT catalysts, which is much higher than the monometallic catalysts (Co/CNT and Cu/CNT) and bimetallic catalysts (Co-Ce/CNT, Cu-Ce/CNT, and Co-Cu/CNT). The addition of Ce promoted the reduction of Co-Cu catalyst and created the Co-CeO2 interface, which acted as a new active site in addition to the Cu-Co dual sites. According to the proposed mechanism, a “tilted” adsorption of CO occurs with the C atom binding to the Co species and the O atom to the adjacent partially reduced CeO2, which would enhance the CO dissociation and provide more surface hydrocarbon species. Therefore the carbon chain propagation, especially the formation of the first C-C bond would increase, accelerating the formation of HA. 28 ACS Paragon Plus Environment

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On the other hand, the additional element such as Mn,286-288 Rh,289 Nb,290 Zr,291 Cr292-293 and La294 in the Co-Cu-based catalysts could act as the structural promoter or synergetic promoter, which also improve the HA yield. In the active carbon supported Co-Cu-Mn trimetallic catalysts, Co-Cu dual sites are reported as the active centers for HA formation while Mn acted as the structural promoter.287 The addition of Mn led to the intimate contact between Co and Cu species, with the small spherical Co NPs decorated the surface of the large Cu NPs. Such a microstructure of the active centers in the catalyst results in the improved HA formation. With only 1 wt.% Rh addition, the trimetallic Co-Cu-Rh/Al2O3 catalyst exhibited a better performance than the Co-Cu bimetallic catalyst (CO conversion 36.7% vs. 26.7%, C2+OH selectivity 24.3% vs. 10%, respectively) for HAS.289 The promotion of Rh is reflected not only in the hydrogenation ability but also the surface concentration of Cu which could provide more active sites for CO insertion. The Co-Cu-Nb trimetallic catalysts are also targeted by researchers to form C2-C5 alcohols from syngas.290 As shown in Figure 7, the bimodal nanosized particle distribution was achieved in Co-Cu-Nb catalyst with smaller NbOx acting as a structural “spacer” to enfold the larger Co-Cu particles, which results in an increased specific surface area. Besides, the strong metal-support interaction effect between Nb4+ and metallic Co led to an enrichment of Co on the Co-Cu particle surface, which could increase the concentration of surface CHx* species for HA formation. For Co-Cu-La trimetallic catalyst, the Co2C phase is observed after reaction at 250 °C due to the existence of La.294 During the reaction, Co2C promotes the CO insertion into the alkyl species, which are formed on the metallic cobalt. Such acyl intermediates can be fully hydrogenated to form HA (especially ethanol) owing to the Cu in close proximity, which provides abundant hydrogen via H2 spillover. As a result, the trimetallic Co-Cu-La catalyst is 29 ACS Paragon Plus Environment

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superior to the Co-La counterpart for HAS. However, the formed Co2C phase is not stable and would be decomposed when the reaction temperature increased to 300 °C. Even when the temperature is decreased to 250 °C again, the Co2C phase cannot be restored. Thus, one main limitation of the above mentioned Co2C-contained catalysts is the narrow reaction temperature range. 5.2. Mo-Co-based trimetallic/multimetallic catalysts Among the Mo family, MoS2 is commonly attractive for HAS due to its high sulfur resistance and activity for WGS reaction.53, 119 As discussed previously, the addition of both F-T elements (Co, Fe, and Ni) and the noble elements (e.g. Rh and Pd) exhibits the improved HA formation due to the enhanced synergism between the dual active centers. Thus the alkali-promoted trimetallic Mo-Co-Rh catalyst system is attractive which could take advantages of the Co for CO dissociation and Rh for alcohol formation.295-301 Surisetty et al.295 promoted the K-MoS2/WMCNT catalysts with Co and Rh species. The MWCNT supported 9 wt.% K-4.5 wt.% Co-1.5 wt.% Rh-15 wt.% MoS2 catalyst displayed the highest metal dispersion, resulted in the highest C2+OH selectivity of 39.4 wt.% with alcohol yield of 0.29 g/g h. According to the XANES spectra, the co-existence of Co, Rh, and Mo species favored the formation of surface molybdenum phase Co(Rh)-Mo-S. This synergistic effect led to the increased active sites for HAS, in agreement with their improved catalytic performance. The same authors found that during the reaction, Co promoted the metal dispersion, increased CO conversion and decreased the WGS reaction while Rh enhanced the dispersion of smaller sized Mo particles.296 To reduce the Rh usage and lower the catalyst cost, inexpensive Ni is reported to replace Rh by the same group.301-303 Although the addition of Co and Ni promoters improved the catalyst reducibility by shifting the reduction temperature of Mo+6 to lower temperature, the Rh-free trimetallic catalysts exhibited lower catalytic performance compared to the Mo-Co-Rh catalyst. 30 ACS Paragon Plus Environment

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In K-promoted La-Co-MoS2 trimetallic catalysts, La acted as the promoter that significantly affected the catalyst structure and performance.304-305 With the addition of La, a homogeneously dispersed MoS2 was formed, which promoted the interaction between Co and Mo sulfide species to form the Mo-Co-S phase. Furthermore, La is found to inhibit the formation of unwanted CoSx, which is active for hydrocarbon formation, and thus increases the number of active centers for HA formation. Consequently, the catalytic activity and selectivity to HA has been improved compared to the Co-Mo bimetallic catalysts.305 Their study showed that other than adjusting the molar ratio of the active centers, the addition of appropriate promoter is also an option to get the balance between carbon chain growth and CO insertion to improve the HA formation.305 5.3. Rh-Mn-based trimetallic/multimetallic catalysts Rh-Mn bimetallic catalysts have been proven to be the most efficient Rh-based catalysts in HAS (Figure 6) owing to the close interaction between Rh and Mn species. On the other hand, Fe is an abundant element and has also been used as the promoter in Rh-based catalysts for HAS. Therefore, the combination of Rh, Fe, and Mn is expected to improve the HA formation and also reduce the usage of expensive Rh.306-317 For Rh-Mn-Fe tri-metallic catalysts, the catalyst properties and HAS performances are closely related to the Fe addition level. Yu et al.307-309 conducted a series of study to investigate the promoting effect of Fe over Rh-Mn-Li/SiO2 catalysts. With a low amount of Fe addition (0.1 wt.%), the reducibility of Rh increased thus more metallic Rh species available for hydrocarbon formation. In addition, the CO conversion was decreased due to the coverage of active sites by superfluous Fe species.308 In addition to the nature of the active centers, the catalytic performance is also affected by the textural of the support. Kim et al.312 used the CMK-5 mesoporous carbon nanoparticles (MCN) as the support for Li-promoted Rh-Mn-Fe catalysts. The low amount of Rh metal was highly dispersed in the nanosized MCN supports with a hollow framework configuration, leading to a faster CO insertion for HA formation while limiting the carbon chain growth for hydrocarbon formation. Other types of carbon-based materials are also used as supports for Li-promoted Rh-Mn-Fe catalysts to increase the catalyst performances, including OMC,311, 316

CNT,313 graphitic mesoporous carbon (GMC)314 and carbon sphere 315.

5.4. Fe-Cu-based trimetallic/multimetallic catalysts Similar to Co-Cu bimetallic catalysts, the Cu-Fe catalysts face the same problems in HAS, including high selectivity to hydrocarbons and poor catalyst stability during the long-turn reaction. To overcome these shortcomings, many promoters such as Zn,318-322 Mn,318-323 La,324-325 Zr323, 326 and Mo327 have been introduced to improve the catalyst system. Lu et al.318 introduced Zn and Mn into Cu-Fe catalysts (Cu0 and FeC as the active centers) and highlighted that these promoters promoted the Cu-Fe catalysts in a different manner. Zn is regarded as an electrical/chemical promoter which would foster the formation of ZnFe2O4 spinel phase to improve the CO conversion significantly. Mn has been recognized as a structural promoter which facilitates the dispersion of Fe and Cu species, resulting in the enhanced synergism between Fe and Cu. The synergistic effect of Zn and Mn over Cu-Fe catalysts led to a rise in the overall catalytic performance with high CO conversion of 72.6% 32 ACS Paragon Plus Environment

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and C2+OH yield up to 116 mg/mlcat. h. However, the majority of the products were C1 products—methane and methanol. Noted that there was no obvious deactivation observed over 120 h time on stream, which is probably due to the higher dispersion of the active centers. Other studies on the Zn-Mn-Cu-Fe multi-metallic catalysts showed the same effect.319-320 Han et al.323 proposed a triple-active-site model to evaluate the effect of metal promoters such as Mn, Zr, and Ce on Fe-Cu catalyst derived from LDHs. Based on XPS analysis, the addition of these additives promoted the formation of tetrahedrally coordinated Cu species (CuB2+). As shown in Figure 8, there is a linear relationship between alcohol selectivity and the CuB2+ contents for the promoted catalysts, suggesting that the highly dispersed CuB2+ species is active for alcohol formation, especially the CO insertion part. Although Ce and Zr exhibited higher alcohol selectivity, methanol remained to a greater extent in alcohols (49.2 wt. % and 38.5 wt. %, respectively).323 This can be explained based on the interaction among the active components: Ce and Zr additives generated strong interaction between Fe-Ce and Fe-Zr for CO insertion but decreased the Cu-Fe interaction for carbon chain growth. However, the addition of Mn not only promoted the Cu-Fe interaction but also contributed to a strong interaction of Cu-Mn, resulting in the increased chain growth sites for HAS. 5.5. Cu-Zn-based trimetallic/multimetallic catalysts Cu-Zn bimetallic catalysts are the typical methanol synthesis catalysts.6 Given the importance of HA, alkali promoters such as Li,328 K329-331 and Cs332-333 have been introduced to the Cu-Zn-Al catalysts to facilitate the carbon chain growth which further increase the formation of HA. Sun et al.332 compared the effect of K and Cs promotion on Cu-ZnO-Al2O3 trimetallic catalysts (Cu60Zn30Al10) for HAS. The results showed that Cs promotion was favorable for HA formation compared to K-promoted catalyst (CO conversion 38.8% vs.


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34.7%, C2+OH selectivity 18.5% vs. 12.3%, respectively), suggesting that the HA formation requires basic sites and increased basicity is needed. Nonetheless, Huang’s group reported a series of study over Cu-Zn-Al catalysts without alkali promoters for HAS using the slurry reactor.334-341 The authors first believed that the coexistence of Cu+ and Cu0 on ZnO surface are necessary for ethanol formation, in which Cu+ helps stabilize the methoxy and acyl species and the metallic Cu species activated H2.336 Later, the authors claimed that the weak acidic sites Al on the catalyst surface are beneficial for HA formation, which favor CO dissociation and carbon chain propagation.338 The role of Al in HAS was further proved by the investigation of the multimetallic Cu-Zn-Al-Zr,339 CuZn-Al-P340 and Cu-Zn-Al-La341 catalysts in a slurry reactor. With the addition of the promoters (Zr, P or La), the amount of weak acid and the copper content on catalyst surface were increased, leading to the enhanced HA formation. Although these findings are contradicted by the consensus that acidic sites suppress the HA selectivity, this work provides a new concept on HAS over Cu-Zn-Al catalysts. Co is the most commonly used transition metal in Cu-Zn-based catalysts for HAS.342-350 Anton et al.348 reported their works on the Co-modified Cu-ZnO/Al2O3 catalysts for HAS. The overall selectivity to HA increased with increasing Cu/Co ratio and reached the maximum at Cu/Co ratio of 2.5. There are three types of active sites existed in this Co-Cu-Zn tri-metallic catalyst system: Co0 is active for hydrocarbon formation, the interaction between Co0 and ZnOx promoted methanol formation, while the close interaction of Cu0 and Co0 species is active for HA formation with both dissociative and non-dissociate adsorption of CO occurred on this dual site. Later, the same authors investigated the effect of different Na addition on the Co-Cu-ZnO/Al2O3 catalysts.349 With low Na loadings (≤ 0.6 wt. %), the active sites as mentioned above were maintained on the catalyst surface. The selectivity to C2+OH alcohols gradually increased during the first 10 h TOS indicating enhanced Cu-Co surface 34 ACS Paragon Plus Environment

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alloy formation during the reaction (Figure 9(a)). However, probably due to the highly exothermic methanation (promoted by Co0), coking of metallic cobalt occurred and contributed to the strong initial deactivation of the catalyst (Figure 9(b)). With high Na loadings (≥ 0.8 wt. %), deactivation did not occur, and constant CO conversion and product distribution were observed. Moreover, the Co-Cu-ZnO/Al2O3 catalyst promoted by 1.2 wt.% Na showed the highest selectivity of 48.5% for HA, indicating the existence of different types of active centers from the catalysts with low sodium loadings. The higher amount of Na addition caused the collapse of the ZnO-Al2O3 matrix, leading to the sintering of metallic Cu particles and the loss of Cu-Co active sites (see Figure 9(d)). In parallel, the existence of high Na loadings favored the metallic Co carbonization to form the bulk Co2C (see Figure 9 (e)). The Co2C-Co0 interface is considered as an additional active site for HAS in this catalyst. Addition of Mn to Cu-Zn nanowires is reported to increase the HA selectivity from 5.4% to 15.7% along with the reduction in methane (from 55.5% to 30.7%) and methanol (from 13.8% to 3.8%) formation.351 This change can be explained by the nature of Mn. When Mn is deposited to the Cu-ZnO nanowire, it can be electrochemically co-reduced with Cu species to form Mn4+Mn2+2O4, with the Mn2+ species known for carbon chain growth. Besides, the addition of Mn provides the distribution effect, which increases the Cu compositional uniformity on the higher surface of the tube morphology, leading to the further improvement of the catalyst performance. With a small amount of Ce (4 mol %) as a promoter, the promoted Ce-Cu-Zn/Al2O3 based catalysts was found to show higher isobutanol selectivity.352 The co-existence of Cu, Zn and Ce ions is necessary for oxygenates formation. Besides, they reported that the location of Ce ions plays an important role in product selectivity: the Ce species located in bulk favors the isobutanol formation while those presented on the catalyst surface shifted the products to alkanes and CO2. 35 ACS Paragon Plus Environment

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5.6. Other trimetallic/multimetallic catalysts Apart from the above mentioned trimetallic/multimetallic catalysts, substantial research efforts have been diverted to investigate the different combination of active centers for HAS.353-383 Palladium is often reported as a promoter for Zn-Cr bimetallic catalysts to facilitate the catalyst reducibility and improve the activation of H2 and CO.353-357 In general, Zn-Cr bimetallic catalysts require high operating temperatures and pressures and methanol remains to be the dominant alcohol product. Minahan et al.353 found that the addition of Pd to Cspromoted Zn-Cr spinel catalyst contributed to a reduction in the optimum reaction pressure (from 10.8 to 6.9 MPa), which is economically favorable. In addition, Pd facilitates the catalyst reducibility and improves the activation of H2 and CO, leading to the enhanced isobutanol production rate.354-355 Sun’s group358 proved that the promotion of F-T elements such as Co, Fe, and Ni to CuMnZrO2 catalysts could shift the alcohol products from methanol to HA, owing to their strong ability to promote carbon chain propagation. Thereafter, the same group was focusing their investigation on the role of Fe on CuMnZrO2 catalysts for HAS.359-361 Based on XRD and XAFS analysis, the addition of Fe increases the catalyst stability by stabilizing the amorphous phase of Zr and then enhancing the Cu dispersion. However, the catalytic activity and selectivity towards alcohols were suppressed significantly. This is due to the strong interaction between Fe and Zr species, resulting in a decrease of Cu-Zr and Cu-Fe interactions. Although the Fe-free CuMnZrO2 catalysts exhibit a better performance for HAS, the Fe promoter increases the HA weight distribution. Goodwin and co-workers362-364 explored the use of La and V as promoters for Rh/SiO2 catalysts and found the higher activity and selectivity towards ethanol compared to the unprompted catalyst. Later, the authors continued the research with the addition of Fe to La36 ACS Paragon Plus Environment

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V-Rh/SiO2 catalyst, which showed the maximum ethanol selectivity of 34.6% under a very moderate reaction condition.365 The authors proposed that each promoter played different roles in HAS process: La increased the total CO adsorption and CO insertion, V decreased CO adsorption but facilitated the CO dissociation and carbon chain growth, while the main effect of Fe was to decrease CO adsorption and enhance hydrogenation. Furthermore, the synergistic effects of the promoters (Fe, La and V) and Rh lead to a balance between CO dissociation, hydrogenation, and CO insertion for HAS. Luo et al.366 used V and another lanthanide element Sm as promoters in Rh/SiO2 catalysts. Under the reduction condition, the Sm3+ ions are very difficult to reduce, resulting in the improvement of Rh and V species dispersion. On the contrary, V could be easily reduced and shows the high capability of hydrogenation. Consequently, the synergism between Sm, V and Rh species results in HA formation of 30.6%. The enhanced ethanol formation was also observed by Xue et al.367 over trimetallic Cr-FeRh/SiO2 (mass ratio Rh:Cr:Fe=1.5:x:y) catalysts. As shown in Figure 10, the promotion of Cr increases Rh dispersion and inclines towards forming the interface between Rh particles and the silica support. However, the addition of Fe has a negligible effect on Rh particle size but shows a tendency to cover the Rh particles. During the reaction, Cr exhibits the capability of CO dissociation and Fe is considered to increase hydrogenation. Moreover, the intimate interaction between Cr, Fe, and Rh suppresses the reduction of Rh, resulting in more Rh+ active sites for CO insertion. La-promoted K-Ni-MoS2 catalysts were investigated by Li et al.368 for the conversion of syngas to HA and reached a high C2+OH selectivity of 58.5%. The addition of La restrains the congregation of nickel and results in more highly dispersed nickel, thus suppressing the methanation reaction and improving the formation of HA. The congregated NiSx particles are


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recognized as the active centers for hydrocarbon formation while the highly dispersed nickel species promote the HA formation. K-promoted Cu-Mg-Ce catalysts have been recognized as the selective catalysts for branched alcohols formation.369-374 Xu et al.369 reported that the trimetallic catalysts provide the compositional and synthesis flexibility required for improving isobutanol formation from syngas: Cu is active for methanol synthesis, CeO2 acts as structural promoter to increase Cu dispersion and catalyst surface area, the incorporation of CeO2 and K further increase the basic sites’ density and strength on the catalyst surface, and the bifunctional of Cu and basic sites are active for alcohol chain growth. The aforementioned trimetallic/multimetallic catalysts demonstrated that the introduction of a proper third/fourth metal with the different chemically function would lead to the improved HAS performance compared to their counterpart bimetallic catalysts. In addition, the third and/or fourth metal also act as the structural promoter which further influence the catalyst behaviors in HAS,

such as change of catalyst reducibility and structure,

enhancement of metal dispersion, inhabitation of catalyst deactivation, and improvement of synergism effect of active centers. Figure 11 summarizes the C2+OH selectivity as a function of CO conversion for selected bimetallic and trimetallic/multimetallic catalysts for HAS. Surprisingly, the modification with third/fourth elements displays the limited or even adverse effect on CO conversion and HA selectivity. The positive impact only exists in the Zn-Cu-based trimetallic/multimetallic catalysts. This observation proved the complexity of the trimetallic/multimetallic catalysts for HAS. Except for the balance of the active centers with different functionalities, the interfaces and structures of the active centers, the interaction between active sites and supports/promoters, and the reactive environment of the catalyst surface have to be


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considered. Thus, the HAS trimetallic/multimetallic catalysts need to be further studied with the precise catalyst design. 6. Conclusions, challenges, and perspectives Given the gradual depletion of oil resources and the necessity to reduce greenhouse gas emissions, there is a strong research enthusiasm to realize the production of higher alcohols from non-petroleum but more renewable resources such as natural gas via the syngas as the intermediate. This inspires a lot of studies using catalytic thermochemical conversion of syngas to HA, for which the effective catalyst design with excellent stability, high selectivity and activity is of vital importance. In this review, an extensive account on the performance of many different catalysts for HAS from syngas available in literature up to 2018 is provided. Different from other review articles in this area, more emphasis of this paper has been placed on the clarification of catalyst active centers. In light of the different sources to provide the active centers, three major classes of catalysts in terms of monometallic, bimetallic and trimetallic/multimetallic have been discussed and their respective reaction pathways have been explored. Some conclusions can be drawn from these studies: (1) The CO insertion mechanism is widely accepted as the predominant mechanism for HAS from syngas based on the combination of the experimental and theoretical studies. According to this mechanism, the HAS requires the pathway for carbon chain growth and CO insertion challenging the catalyst design to possess bifunctional activity. (2) Despite the presence of the metallic-oxide (M0-Mn+) pair to provide the active centers for carbon chain growth (M0) and CO insertion (Mn+), these Mo, Rh, or Co monometallic catalysts are not sufficient for HAS. All challenges including the lack of sufficient active centers, difficulties in forming the adjacent dual sites and adjusting the proper M0/Mn+ ratio on the catalyst surface restricts the further development of monometallic catalysts for HAS.


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(3) Compared to monometallic catalysts, the more widely developed bimetallic ones exhibit significantly improved catalytic activity and selectivity. The combination of the two active centers with the different functionality in the same catalyst system for HAS is the key strategy to the enhanced catalyst performance due to the presence of synergism effect. Among these reported bimetallic catalysts, Co-Cu and Co-Mo bimetallic catalysts display the most potential in HAS. (4) The trimetallic/multimetallic catalyst design provides flexibility of tuning the active centers and the catalyst properties/structures by introducing the third/fourth element. However, their catalytic performance for HAS only show limited or even no improvement compared to the bimetallic catalysts. The complexity of the trimetallic/multimetallic catalysts requires the precisely controlled active centers combination and catalyst properties to improve the HA formation. (5) In addition to the active centers, other factors should be considered as they also significantly affect the catalytic performance for HAS, such as promoters, supports, preparation methods, and the catalyst structures. For example, the promoters from alkali metals are reported to shift the product from hydrocarbon towards alcohols. On the other hand, a unique structure with high surface area that can help to provide good active site dispersion and integration would facilitate the HA formation. This review highlights an important criterion for effective HAS catalyst design. The catalyst should possess the dual or multi-active centers to facilitate CO dissociation and CO insertion. While some progress has been made, a number of challenges still need to be overcome to further advance the direct synthesis of HA from syngas. Firstly, most of the HAS catalysts exhibit a tradeoff between selectivity and conversion, leading to the low yield of HA. Secondly, HAS catalysts suffer the short catalyst lifetime due to the deactivation problem stemmed from metal sintering or phase separation. Thirdly, the high HA production 40 ACS Paragon Plus Environment

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cost including catalyst preparation and energy intensive operation at high temperature and pressure. Lastly, the nature of the active centers remains ambiguous due to the complexity of the reaction pathways and the limitations of the characterization methods. Thus, further improvements in the development of HAS catalysts are still urgently required to achieve a higher yield of C2+OH alcohols for industrial applications. Future works should focus on (i) deep understanding of the roles of active centers in HAS by means of cutting edge characterization techniques, such as Scanning Transmission Electron Microscopy (STEM), Atom Probe Tomography (APT), and the in-situ characterization methods to better identify and understand the structure evolution and properties of the active centers during the reaction; (ii) investigating the possible combination of different active centers to gain bifunctionality for HAS; (iii) precise design of the catalyst structure tuning at atomic level to get the highly dispersed, adjacent and stable active centers on the catalyst surface; (iv) reducing the usage of the expensive metals (i.e. Rh) in the catalysts to lower the production cost. 7. Supporting Information The supporting information for this work includes the number of publications related to different HAS catalysts published since 1980, details of the active centers in typical monometallic, bimetallic and tri/multimetallic catalysts and their performances for HAS. 8. Acknowledgements The authors acknowledge the financial support from Australian Research Council (ARC)Linkage Project-150101158 and EcoTechnol Pty. Co, Perth, Australia. 9. References (1) Wender, I. Reactions of Synthesis Gas. Fuel Process.Technol. 1996, 48, 189-297.


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List of Figures

Figure 1. Hydrocarbon and alcohol formation through CO insertion pathway.


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Figure 2. TEM images of (a) fresh YRh0.5Fe0.5O3/ZrO2, (b) YRh0.5Fe0.5O3/ZrO2 after TOS=20 h reaction, (c) YRh0.5Fe0.5O3/ZrO2 after TOS=130 h reaction, (d) the distribution to alcohols with time on stream, (e) and (f) are catalyst particle size distributions for (b) and (c), respectively. Reproduced from ref. 157, copyright 2017, with permission from Elsevier.


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Figure 3. TEM image of Co2Cu1 oxalate decomposed in 10% CO (a−b) and 10% H2 (c-d). The inset provides a model of the catalyst structure as mounted on the basis of the selected regions in TEM images. Reproduced from ref. 201, copyright 2014, with permission from American Chemical Society.


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Figure 4. TEM images and structure variation of (Cu1Co2)2Al-LDHs catalysts (a) after reduction, (b) after reaction of 100 h, (c) after reaction of 300 h, (d) after reduction of 500 h, and (e) after reaction of 750 h. The inset provides a model of the catalyst structure variation as mounted on the basis of the selected regions in TEM images. Reproduced from ref. 220, copyright 2017, with permission from American Chemical Society.


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Figure 5. Characterization of CoGa-ZnAl-LDO/γ-Al2O3 catalysts: (a) preparation process from LDHs precursors, HAADF images of fresh (b) and used (c) catalysts with Co (red) and Ga (green) element mapping. Reproduced from ref. 231, copyright 2016, with permission from Elsevier.


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Figure 6. C2+OH selectivity versus CO conversion over catalysts containing Mo (a), Co (b), Rh (c), and Fe (d) active centers.


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Figure 7. HRTEM images of the CO-Cu-Nb trimetallic catalysts (a-b). (c) particle size distribution for “Nb” spacer and “Co-Cu” particles. The inset of (a) provides a model of the bimodal catalyst structure as mounted on the basis of the selected regions in images (a). Reproduced from ref. 290, copyright 2015, with permission from American Chemical Society.


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Figure 8. Relationship between alcohol selectivity and the CuB2+ content, and the alcohol distribution over different catalysts. The inset provides a triple-active-site model of promoting effect. Reproduced from ref. 323, copyright 2015, with permission from The Royal Society of Chemistry.


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Figure 9. Performances of Na-doped Co-modified Cu/ZnO/Al2O3 methanol synthesis catalysts tested at 280 °C, 6 MPa, H2/CO = 1 and 9600 h-1: (a) initial product selectivity during 10 h TOS, (b) CO conversion during 40 h TOS, (c) productivities to linear alcohols, characterization of spent catalysts: (d) C 1s, Co 2p and Cu 2p XPS spectra, (e) XRD patterns. Reproduced from ref. 349, copyright 2016, with permission from Elsevier.


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Figure 10. TEM images of reduced Rh-Fe-Cr/SiO2 catalysts and the proposed contact mode between Rh and promoters over Rh-Fe-Cr/SiO2 catalysts (insert): (a) x=0, y=0, (b) x=0, y=0.2, (c) x=0.4, y=0, and (d) x=0.4, y=0.2. Reproduced from ref. 367, copyright 2016, with permission from The Royal Society of Chemistry.


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Figure 11. C2+OH selectivity versus CO conversion over bimetallic catalysts (●) and trimetallic/multimetallic catalysts (●): (a) Co-Cu-based catalysts, (b) Mo-Co-based catalysts, (c) Rh-Mn-based catalysts, (d) Cu-Fe-based catalysts, (e) Cu-Zn-based catalysts, and (f) overview graph of all catalysts.


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Table of Contents:


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