Recent developments in sulphur-resilient catalytic systems for syngas production

Renewable and Sustainable Energy Reviews 100 (2019) 52–70

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

Recent developments in sulphur-resilient catalytic systems for syngas production Tze Yuen Yeo, Jangam Ashok, Sibudjing Kawi

T

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Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive, 117576 Singapore

ARTICLE INFO

ABSTRACT

Keywords: Sulphur resistance Syngas production Catalyst design Process improvements Sulphur utilisation

The importance of sulphur resilience in hydrocarbon reforming systems cannot be overstated. Sulphur compounds can deactivate the reforming catalysts and complicate downstream separations, as well as lead to undesired side reactions that affect the overall performance of reforming. In this article, we review recent efforts in the development of sulphur-resistant catalysts, as well as process enhancements that help to prolong the operating lifetimes of conventional reforming catalysts. Here, we briefly look at the sulphur content of reforming feedstock materials, and also analyse the typical poisoning mechanisms to try to understand how to better prevent them. We then move on to consider various strategies that have been developed recently to impart sulphur resilience, including changing catalyst compositions, engineering catalyst designs, feedstock pre-treatment, and reaction design and integration to improve reforming performance. Finally, we look at some possible directions moving forward, where sulphur compounds are treated not as a nuisance to be dealt with, but as a valuable reactant that can help to produce valuable materials for clean energy generation.

1. Introduction Reforming reactions involving hydrocarbon-based feedstocks into syngas (a mixture of hydrogen gas and carbon monoxide, in variable compositions) represent an important set of technologies that help to pave the way for clean energy production [1]. Reforming reactions enable complex feedstocks to be converted into fuels that are compatible with hydrogen fuel cells, which are higher in efficiency and produce next to no CO2 in the course of electricity generation. These reactions also enable customisable production of carbon-rich gases that act as a feedstock for various synthesis reactions such as the Fischer-Tropsch process. The raw materials for reforming reactions can come from a variety of sources; these include biomass, petroleum derivatives, tars, natural gas and even municipal waste. A common feature of these materials is that they contain a non-negligible amount of sulphur, which quickly deactivates reforming catalysts by poisoning the active sites of the catalysts and thus inhibiting reactions [2]. Sulphur-poisoned catalysts can usually be regenerated at high temperatures, but the regeneration steps entail an additional energy cost and potentially increases the downtime of the plant for catalyst replacement [3]. In addition, sulphur poisoning can also alter catalyst functions by inhibiting the desired reforming reactions and channelling the actual reaction pathways towards other unintended side products [4,5].

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Even if the catalysts themselves are somehow resistant to sulphur poisoning during the reforming process, unprocessed sulphur compounds in the product gas can also often complicate downstream operations. Sulphur compounds in a syngas stream can seriously foul and deactivate the membranes used to recover hydrogen gas as well as the electrodes in the fuel cells [6–8]. These issues caused by sulphur poisoning often necessitate the incorporation of sulphur removal systems (usually upstream prior to the reforming reactor), which incurs additional capital and operational costs to a plant [9]. Thus, the engineering of a sulphur-resistant reaction system that is tolerant to the sulphur content in as-is feedstock materials is of the utmost economic importance. In this paper, we review a selection of recent literature (2013 – present) related to the development of sulphur-resistant catalysts, as well as the broader aspects relating to sulphur tolerance and removal in reforming reaction systems. Before studying these new developments, we attempt to summarise the consensus relating to sulphur poisoning mechanisms in order to better understand the effects of sulphur on reforming. Hopefully this will also encourage researchers to develop rational catalyst designs and countermeasures against sulphur poisoning. Topics such as feedstock properties and pre-treatment, catalyst design, operating conditions, process integration and approximate costs related to desulphurisation are briefly discussed in this review. In addition to that, we also consider some unconventional and interesting

Corresponding author. E-mail address: [email protected] (S. Kawi).

https://doi.org/10.1016/j.rser.2018.10.016 Received 28 July 2018; Received in revised form 12 September 2018; Accepted 13 October 2018 1364-0321/ © 2018 Elsevier Ltd. All rights reserved.

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strategies to improve sulphur resistance in hydrocarbon reforming reactions.

Table 1 Approximate sulphur content of various potential feedstocks for reforming reactions.

2. Feedstock types and sulphur content

Feedstock Type

A wide range of hydrocarbon-based materials can be reformed into syngas. From a renewable energy perspective, biomass (and subsequently bioethanol) is an ideal source of materials for reforming reactions [10]. In general, virgin biomass is not used for reforming reactions directly. Typically, biomass is first partially converted to syngas via gasification reactions, which typically occur at lower temperatures. Volatiles and easily-decomposed fractions in biomass are converted into hydrogen gas, carbon monoxide and other impurities. In regions where low temperature regimes occur (i.e. when T < 800 °C), some heavy residues with lower calorific value (tars) are generated. These tars are created as a result of condensation and polymerisation reactions, which lead to the formation of polycyclic aromatic hydrocarbons with H:C ratios that are less than 1. These tars can be carried forward together with the syngas, or separated from the gasifier as a heavy product at the bottom. These biomass-derived tars are the raw materials for tar reforming reactions to ensure full utilisation and conversion of biomass into green fuels [11]. The sulphur content of the biomass tar depends on many factors such as the source of biomass (species/type of plants or algae etc.), catalyst types, gasification conditions and gasifier design etc. According to Asadullah, woody biomass contains less sulphur than herbaceous biomass (0.1% compared with up to 0.4%), which in turn is relatively cleaner than anthracite coal and waste-derived fuels (up to 1% content) [12]. Panisko et al found that while the hydrotreatment of a variety of lignocellulosic biomass pyrolysis oils generally yielded lower amounts of inorganic residues, the leachable sulphur content was relatively high. They attribute the high sulphur content to potential release of the element from the sulphided noble metal catalyst, in addition to the natural content of sulphur in the biomass [13]. Biodiesel obtained from transesterification of waste oil and methanol is typically substantially sulphur-free, as the starting materials are usually quite low in sulphur. An alternative biological-based hydrocarbon source is biogas, which is a mixture of methane and carbon dioxide. Biogas is produced via anaerobic digestion of waste feeds by bacteria or algae. The quality of biogas obviously varies based on the species of microorganisms, fermentation conditions and feed composition. Compared with natural gas, biogas is relatively rich in CO2 and other contaminants. The sulphur content of biogas is very high, being at least a few orders of magnitude higher than that of typical natural gas compositions (up to 1% H2S compared with < 0.1 ppm in natural gas) [14]. Non-renewable hydrocarbons can also be used as raw materials for reforming reactions. However, these feedstocks (for example crude oil prior to refining) can contain a lot of natural sulphur. An assay of crude oil from Kurdistan by Karim et al shows that the feedstock can contain up to 2% sulphur, which is clearly unacceptable for direct use for reforming into syngas [15]. As a result, fractionation and desulphurisation is often required before the petroleum products (i.e. diesel and naphtha) can be used in other applications [16]. Due to the extensive desulphurisation processes used to upgrade the products to environmentally acceptable standards, processed diesels and gasoline typically have much lower sulphur content (usually less than 10 ppm) [17,18]. However, this still has a significant impact on the downstream reforming reactions, as the catalyst performances are incredibly sensitive to sulphur contamination, denaturation and deactivation. A recent surge in interest to “close the waste loop” has driven research into studying the possibilities of utilising plastic and municipal waste as sources of hydrocarbons to be reformed into syngas. Wu et al has studied the possibility of utilising various plastic waste materials such as polyvinyl chloride (PVC), high density polyethylene (HDPE) and contaminated plastic materials (described in their paper as motor oil containers, MOC) as hydrocarbon sources for pyrolysis/reforming reactions. While the washed and pelleted PVC and HDPE did not

Biomass and renewables Ethanol/gasoline blend (85/ 15) Woody biomass Herbaceous biomass Hydrotreated biomass residue (leachates) Pine Poplar Switchgrass Biomass blend Corn stover Biogas from various feeds Wastewater Food waste Animal waste Landfill Non-renewables Anthracite coal Natural gas Crude oil Various diesel blend stocks Diesel fuel Bituminous coal Waste-based Refuse-derived fuel Waste Plastics Polyvinyl chloride (PVC) High density polyethylene (HDPE) Motor oil container (MOC) Municipal solid waste (MSW)

Approximate sulphur content (ppm)

Reference

5

Simson et al [10]

< 1000 3000 – 4000

Asadullah [12]

10 – 45 60 – 90 180 65 – 140 570

Panisko et al [13]

< < < <

Pieta et al [14]

400 11,000 330 230

10,000 < 10 6000 – 21,000 85 – 5300 0–8 25,100

Asadullah [12] Pieta et al [14] Karim et al [15] Nabgan et al [16] Gonzalez & Pettersson [17] Connell et al [21]

10,000

Asadullah [12]

0 0

Wu et al [19]

460 Highly variable

Pieta et al [20]

contain any significant amount of sulphur, the MOC had some residual sulphur compounds from the container contents in the feed [19]. Municipal solid waste (MSW) was also considered as a source of organic matter for gasification and reforming reactions. Pieta et al reviewed the possibility of converting the organic fractions in MSW into syngas, and concluded that due to the large variations in waste composition, there is a huge challenge in proper conditioning and decontamination of the syngas [20]. These results suggest that the use of waste plastics and MSW as sources of hydrocarbon materials for reforming is an incredibly complicated affair, and proper modelling of the performance of reforming catalysts and desulphurisation systems needs to take into account the additional effects of other contaminants such as chlorides and nitrates etc. A summary of recent surveys of feedstocks for various reforming reactions is shown in Table 1. 3. Sulphur poisoning mechanisms “Sulphur” refers to a wide variety of compounds in the feedstock materials prior to their treatment and removal. The most common forms are hydrogen sulphide, sulphur dioxide and thiophene and its derivatives. All of them contribute significantly to catalyst poisoning; however it is generally thought that their poisoning mechanisms are quite different, and they bring about the reductions in catalyst performance through different deactivation pathways. A brief look through recent literature seems to suggest a general consensus that sulphur compounds affect the intended performance of catalysts through three major types of mechanisms. These are the sulphidation, alteration and coke formation mechanisms. The question of which mechanism will predominate in the system depends significantly on the actual reaction conditions [22,23]. Fig. 1 illustrates the three 53

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Fig. 1. An illustration of the three major sulphur deactivation mechanisms of catalysts. 1) Sulphidation, 2) Alteration, and 3) Coke formation. These mechanisms are summarised from a survey of recent literature that have provided insight on possible sulphur poisoning pathways [24–48].

major sulphur deactivation mechanisms commonly encountered in reforming systems. First of all are the straightforward interactions between sulphur and the catalyst active sites. These include adsorption of sulphur compounds and the formation of metal sulphides. Hydrogen sulphide gas can react with the active metal sites in the catalyst to form metal sulphides, which typically do not have any significant catalytic properties for reforming hydrocarbons into syngas. Nickel-based catalysts are particularly susceptible to deactivation by sulphur compounds, regardless of the sulphur species [24]. Although the deactivation products would clearly be different based on the actual sulphur species, it has been reported that significant deactivation is observed in all cases [25,26]. In the case of catalyst deactivation by H2S gas in the feedstock, the actual deactivation mechanism depends primarily on the H2S concentration and operating temperature [27]. Catalysts exposed to low H2S concentrations typically are deactivated via the adsorption of sulphur onto the catalyst surfaces, thereby blocking off access of the reactants to the active metal sites. The adsorption is usually reversible, and catalytic activity can be regenerated by raising the temperature or lowering the feed sulphur content (i.e. H2S partial pressure) to desorb the sulphur from the poisoned surfaces [28–30]. At higher H2S concentrations, the chemical potential of sulphur increases, and this favours the chemisorption of sulphur into the catalyst material which leads to subsequent sulphidation reactions to give metal sulphides [31]. The sulphidation reactions of transition metal active sites are generally irreversible, and can result in permanent loss in activity of

the catalysts [32,33]. Theoretical studies by Ocsachoque et al also suggests that a slightly oxidising environment during reforming reactions can help to suppress the formation of tightly bound, stable sulphides by converting the sulphur atoms into labile SO2 species [34]. The oxygen required for SO2 formation may be supplied in the gas phase, or be derived from the lattice structures of the catalyst support itself. Of course, control over the degree and selectivity of oxidation reactions remains a challenge in these cases. The second mechanism is closely related to the first. Sulphur compounds can interact with the active metal sites in catalysts in different manners, and these differences are more pronounced in catalysts that contain heterogeneity in their material structure [35]. It is known that sulphur poisoning can alter the functionality of catalysts, by affecting the affinity, selectivity and reaction pathways of adsorbed reactants on the surface of the metal sites [36,37]. These phenomena have been widely reported. For example, Mihai et al reported that the aging of a Pt/Al2O3 catalyst with SO2 enhanced its selectivity and conversions for dimethyl ether (DME) and propane oxidation reactions, but its activity towards CO oxidation reactions was inhibited. In addition, they have shown that steam reforming of DME with the sulphur-aged catalyst yielded more methanol than green or thermally-aged catalysts. They attribute this phenomenon to the reduction in the number of sites available for water activation in the reaction system due to increased sulphur deposition [38]. Dreher et al conducted even deeper studies on the effects of sulphur poisoning on the mechanisms of methanation 54

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reactions using isotope labelling and in situ XAS monitoring. They concluded that sulphur poisoning not only reduces the activity of Ru/C catalysts for methanation reactions, but it also fundamentally alters the reaction mechanism by favouring CH* radicals instead of C* radicals in the reaction system. As a result, there was a shift from tetra-deuterated CD4 products to the triply deuterated CHD3 product upon addition of dimethyl sulphoxide (DMSO) into the reaction system [39]. Cimino et al. reported that the addition of SO2 to an ethane partial oxidation reaction system with Pt and Rh catalysts actually enhanced the production of ethylene by selectively shutting down the steam reforming capabilities of the catalysts [40]. To go even further, some parties have proposed the use of catalysts that are intentionally moderated via the addition of sulphur to have better control over the reforming reactions. Mortensen and Dybkjaer proposed a sulphur-passivated reforming (SPARG) process that precisely manipulates the amount and nature of sulphur depositions on the nickel active sites in the catalyst. This not only prevented any significant coke deposition for extended periods of operations, but also increased the CO ratio in the syngas to condition it for downstream applications [41]. The third mechanism involves the role of sulphur compounds in the growth and deposition of carbonaceous coke on the catalysts. Studies by various groups suggest that in alkane reforming reactions, coke formation is accelerated for sulphur-poisoned catalysts owing to preferential interactions of alkyl radicals with the sulphur atoms to form R-S species. The R-S species then rapidly dehydrogenate to give indirectly-bonded coke deposits on the surface of the catalyst active sites, which are very stable [42–44]. Some support for this idea can perhaps be found in other tangentially related works. Xu and Li proposed an alternative method (instead of conventional hydrotreatment) for the desulphurisation of C9 fractions without reducing their octane number. In their work, they proposed the use of Amberlyst 36, a sulphonic acid-based ion exchange resin, to alkylate thiophenes contained within C9 fractions. The alkylation reactions would attach bulky R-groups onto the thiophenes, thus increasing their molecular weight and boiling points. This would therefore enable easy separations in downstream processing. The reported conversions of the thiophene compounds were close to 100% [45]. Although the reaction system and conditions are notably different from those typically encountered in a reforming reactor, their work suggests that sulphur compounds may indirectly promote coke formation on catalyst surfaces (especially near acidic sites) by incorporating alkyl and aromatic groups onto the initial adsorbed thiophene molecule. Dehydrogenation of these attached groups would then rapidly lead to significant coke formation in the vicinity of the sulphur poisoning site. On the other hand, many studies have also shown that small amounts of sulphur poisoning can help to moderate the activity of catalyst active sites, and reduce the amount of carbon formation [46]. Neubert et al studied the effects of thiophene addition on methanation reactions and concluded that sulphur compounds deactivated some of the metal sites with higher activity in the catalysts, and this reduces the rates of coke formation [47]. Chattanathan et al also report similarly decreased yields and degrees of coke formation for biogas reforming in the presence of H2S, suggesting that sulphur poisoning in this case merely reduced activity but did not encourage carbon growth from alkylation and decomposition of the reactants [48]. These results suggest that reforming reactions based on small alkyl groups (C1-C2) tend to be less affected by sulphur-induced coke formation, since these groups are quite small and result in much slower rates of carbon growth. As such, it can be seen that the role of sulphur on coke formation is perhaps not as straightforward as initially believed and may have conflicting effects, and depends quite a bit on the reaction types and conditions.

Fig. 2. Recently reported elemental additives that increase the sulphur resistance of reforming catalysts [49–72].

conditions. To this end, many approaches have been developed to achieve sulphur resilience. These encompass both extrinsic (process integration, feed treatment etc.) and intrinsic (i.e. improved catalyst compositions and structures) enhancements to the reforming reactions. In this section we will look at some of these approaches and attempt to summarise the current trends in designing sulphur-resistant reforming systems. 4.1. Catalyst compositions One of the most fundamental approaches to enhance sulphur resistance in reforming systems is to modify the inherent properties of the catalyst material. The introduction or substitution of various chemical elements that are known to have counter-sulphur properties into existing catalyst designs is the most straightforward method to improve the catalysts. These elements either help to prevent sulphur deposits on the catalyst, have similar catalytic properties to existing materials but are much more sulphur-resistant, or have certain synergistic effects when combined with existing catalysts. A graphical summary and categorisation of these elements are given in Fig. 2. 4.1.1. Sulphur prevention (rare-earth metals and alkaline species) A few elements (especially ceria and alkaline metals) have been considered as potential additives to reforming catalysts to help prevent sulphur build-up on catalyst active sites. The function and operating mechanisms of these elements vary slightly. In the case of ceria, defects in the crystal structure surrounding the cerium atom introduces oxygen vacancies in the lattice, which increases oxygen mobility and enables the oxygen anions to interact with sulphur on the active metal sites. This leads to the oxidation of the sulphur deposit to form sulphur oxides, which are much more easily removed at the high temperatures under which reforming conditions usually take place and thus prevents sulphur from accumulating on the active sites [49–51]. Lo Faro et al prepared a nickel doped perovskite catalyst, which they then mixed and milled together with ceria doped gadolinia. The composite catalyst showed high activity in reforming propane to syngas, with 85% conversions at up to 80 ppm H2S content [52]. This suggests that even physical mixtures of catalysts with ceria-based oxide materials have a similar effect, indicating that the ceria did not necessarily had to have intimate interactions with, or be in the near vicinity of the active sites. Another interesting effect was also reported when ceria is present in the catalyst material. Postole et al reported that in a steam/methane feed which contained up to 220 ppm H2S, it was found that the sulphur actually promoted the steam reforming reaction especially when sulphur anions diffused into the oxygen vacancies that are formed when Ce4+ is reduced to Ce3+. The substitution of oxygen sites with sulphur disrupted the crystal structure and generated new catalytic sites, which increased methane conversions. However, XRD analysis of the catalyst material showed a gradual formation of a Ce2O2S phase, which had

4. Strategies for sulphur resistance It should have been made sufficiently clear by now that sulphur resilience in reforming reactions is a key aspect that has to be addressed, in order to ensure proper operation under industrial 55

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contained significant amounts of sulphur and aromatics. Their work clearly showed that the dolomite supported nickel catalyst had much lower carbon deposition, and sulphur deactivation in this catalyst was nearly non-existent [58]. Moud et al studied the effect of potassium coverage in a nickel based catalyst for tar reforming, and found that increased potassium loading correlated with a decrease in sulphur poisoning in the catalyst. The exact reason for this remains inconclusive, though they also suggested the possibility that potassium influenced the Ni-S bond and enabled easier dissociation to prevent permanent poisoning of the catalyst [59]. On the contrary, Garbarino et al reported that a Ni-Mg/Al2O3 catalyst in fact deactivated more quickly than a standard nickel on alumina catalyst in the presence of around 200 ppm thiophene, and they attributed this to the a modification of alumina support by diffusion of the magnesium ions into the material. This somehow resulted in the formation of larger nickel particles on the surface, which was more susceptible to sulphur poisoning [60].

Fig. 3. XRD evidence for the gradual formation of a Ce2O2S phase in ceria doped gadolinia catalysts under 220 ppm H2S during steam reforming of methane. The patterns are for a) Fresh catalyst, b) Reactions after 1 h, c) After 3 h, and d) After 9 h. Figure reprinted with permission (copyright 2018) from Postole et al [53].

4.1.2. Sulphur tolerance (noble metals) Noble metals such as platinum, ruthenium and rhodium have also been proposed as substitutes for transition metals in reforming reactions, as they have similar catalytic properties but are much more resistant against reactions with sulphur compounds. Rhyner et al report the use of a noble metal-based monolithic catalytic converter with 400 channels per square inch (CPSI) for the reforming of tars to syngas. While the authors do not explicitly state which noble metals were used, they have specified that the catalyst monolith consisted of a ceramic base, the active noble metal, alumina and ceria. Their results suggest that the reforming catalyst has excellent stability and activity against sulphur poisoning, and H2S added to the system passed through the reforming zone without interacting much with the noble metals. On the other hand, thiophenes were also decomposed by the catalyst [61]. Mota et al prepared a perovskite material containing ruthenium, and further enhanced its properties by partial substitution of lanthanum for strontium in the perovskite (LSCR). The material without Sr substitution (LaCr0.85Ru0.15O3) had fairly good performance for methane reforming. Addition of Sr had marginal improvement effects in terms of catalyst stability, suggesting that Ru by itself is already a fairly good sulphur-resistant catalyst for methane reforming reactions [62]. Fig. 4 shows a comparison of the performance of the synthesised catalysts, as well as the stability of La0.95Sr0.05Cr0.85Ru0.15 in the presence of 50 ppm H2S. In contrast, Jung et al reported that the addition of ceria to an Rh/ γ-Al2O3 catalyst prolonged the stable operating time of the catalyst for gasoline reforming, under 5 – 7 ppm H2S contamination of the feedstock. The ceria-enhanced catalyst was stable for at least 100 h of

inhibitory effects on the catalytic activity as well (Fig. 3) [53]. Lanthana also reportedly plays a similar role by reacting with sulphur in the feed to form La2O2S, but apparently lacks the capability of ceria for active in situ oxidation of sulphur deposits and prevention of poisoning in the catalyst [54,55]. Addition of alkaline components to reforming catalysts helps to preferentially adsorb or react with sulphur compounds present in the feedstock, before they can reach the active metal sites and poison the catalyst [56]. Da Silva and Heck reported that the incorporation of alkaline-earth metal oxides into a nickel based solid oxide fuel cell anode prolonged its useful operating time. They suggested that the alkaline-earth metal oxides helped to reduce the sulphur chemical potential in the system, thus preventing the formation of nickel sulphides which are inactive for reactions. In particular, barium oxide was found to be exceptionally effective in preventing sulphide formation on the catalyst, through an in situ self-regenerating mechanism where BaS is reverted to BaO in the presence of water. However, concerns were raised about the possibility of deactivation of the barium component itself, through the formation of stable barium carbonates [57]. The incorporation of natural minerals that contain alkaline-earth metals also increases the sulphur resistance of the catalysts. Elbaba and Williams compared the coke and sulphur resistances of nickel-based catalysts with different supports (alumina and dolomite) for tar reforming. The tar was derived from the gasification of waste tires, and hence

Fig. 4. Left: methane conversions and H2, CO and CO2 yields for the various LSCR catalysts. Letters on X-axis represent various LSCR compositions: a) LaCr0.85Ru0.15, b) La0.99Sr0.01 Cr0.85Ru0.15, c) La0.98Sr0.02Cr0.85Ru0.15, and d) La0.95Sr0.05Cr0.85Ru0.15. Top right: Stability tests of (d) in the absence and presence of 50 ppm H2S. Figures adapted with permission (copyright 2018) from Mota et al [62].

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deactivated via sulphide formation. Their results suggest that as the sulphur reacts with the borate species in the catalyst, boron oxide is formed and thus gradually reduces the sulphur tolerance of the catalyst [72]. A summary of the compositions of catalysts developed for syngas production, as reviewed in this section, is given in Table 2. 4.2. Catalyst structures In addition to incorporating elemental components into the catalyst material to modify its composition, it is also possible to enhance the sulphur resistance of the catalyst via customisation of its structural properties. Many studies have looked into the structural-activity relationships (SAR) for various catalysts, and have proposed various modifications and enhancements to impart sulphur resistance. These methods include the use of inherently resilient material frameworks such as alloys, perovskites and core-shell structures, advanced synthesis techniques to obtain catalysts with special properties, and material engineering techniques to generate favourable reaction interfaces that enhance catalyst performances [73]. Fig. 6 highlights some of the more recent routes in the development of sulphur resistant catalyst structures.

Fig. 5. Comparative stability of nickel GDC anodes modified with gold and molybdenum, under methane steam reforming conditions with 10 ppm H2S. Figure taken from Niakolas et al [69].

4.2.1. Perovskite structures and metallic alloys Perovskite materials have a general ABO3 composition, and are also a class of material structure upon themselves. The perovskite template is based on the calcium titanate mineral, CaTiO3. The metal cations in a perovskite structure can be substituted for various other metal components, which impart specific properties to the material depending on the nature of the dopant [74]. This functionalise them for a wide variety of applications, though in some cases the exchange of cations may lead to destabilisation and disintegration of the perovskite structure. Perovskites have been reported to have promising properties against sulphur compounds. Incorporation of an active catalytic metal site into the perovskite structure allows it to be used as a catalyst, while retaining the inherent sulphur resistance of the original material [75–78]. In addition, perovskite materials also improve sulphur resistance by providing labile lattice oxygen atoms that facilitate the oxidation and removal of sulphur from the catalysts, thus minimising its build-up and negative effects on the intended syngas production reactions. Because of this, a huge amount of interest has developed in this area, and a wealth of excellent work has been published. A summary of recent perovskite materials that are either sulphur resistant or have been shown to be effective for syngas production is given in Table 3. Alloys of nickel are also shown to have certain sulphur-resistant properties as well. Nickel can be alloyed with several metals, including iron, cobalt, manganese and others [121,122]. Hua et al reported that a nickel-tin alloy embedded in a foam structure had excellent resistance against sulphur poisoning in biogas reforming reactions. In the presence of 200 ppm H2S, their catalyst showed only a slight drop in the conversion of methane at higher temperatures (> 800 °C). The selectivity for CO was also close to 100% under these conditions [123]. Extended X-ray Absorption Fine Structure (EXAFS) studies by Stanley et al showed that (at least for Pt-Ru) alloys indeed do have resistance against sulphur poisoning, but this is a result of a self-regenerating mechanism instead of any inherent inertness of the material against reactions with sulphur. They report that for Pt-Ru alloys of various compositions, sulphur attack does indeed result in the dealloying of the material to form separate Pt and Ru-S species. Presence of hydrogen in the system (as is common in reforming conditions) quickly regenerates the material to reform the Pt-Ru alloy and desorbs the sulphur [124]. This is what gives the alloy its apparent sulphur resistance. The mechanism for self-regeneration of the Pt-Ru alloy, as described by Stanley et al, is shown graphically in Fig. 7. Similar studies were done by Lakhapatri and Abraham where they focused on Rh-Ni alloys for the steam reforming of diesels. They showed that it is possible to engineer the reaction such that the sulphur poisoning is reversed at the H2S adsorption stage (prior to irreversible

operation in the presence of H2S contamination. The as-is Rh catalyst had excellent reforming activity for simulated iso-octane reforming in the absence of sulphur compounds, but its stability dropped drastically (significant deactivation from 80% conversion to around 10% at the 8 h mark) when even small amounts of H2S was added [63]. Kantserova et al used nickel-based composites containing small amounts (0.1%) of palladium or platinum for the tri-reforming of methane. Their work suggested that while addition of the noble metals helped to delay the deactivation of the catalyst, it did not fully impart a permanent resistance against high levels of H2S (3500 ppm in their experiments). However, they reported that regeneration of the Pd-Ni composite allowed the catalyst to regain its original activity [64]. 4.1.3. Synergistic combinations (transition metals, precious metals and other elements) Synergies can also be observed when two separate components (both of which would be susceptible to sulphur poisoning if implemented individually) are combined and incorporated into the catalyst. Bimetallic formulations involving nickel and another complementary metal such as cobalt, iron or copper seem to impart variable degrees of sulphur resistance to the catalyst [65–67]. Molybdenum also appears to have certain supportive properties against sulphur deactivation when combined with nickel. Gaillard et al showed that molybdenum interacts with nickel to form a bimetallic species, which significantly improves their reducibility and assists in the removal of coke and sulphur from their surface [68]. Niakolas et al reviewed and studied the effects of gold incorporation on sulphur resistance in nickelbased anodes for solid oxide fuel cell applications. They reported that a ternary composite of gold, molybdenum and nickel on gadolinia doped ceria had higher tolerance for sulphur poisoning (as shown in Fig. 5). The doping of Au into the material changed the electronic properties of the catalyst and shifted the equilibrium of the sulphur reactions towards H2S formation, which effectively weakens the bonding of sulphur to the active nickel sites [69,70]. Cobalt also seems to play a similar role for nickel-based systems. Saha et al reported that co-impregnation of cobalt with nickel to obtain a bimetallic catalyst resulted in increased reducibility of the nickel, and also led to increased stability against sulphur poisoning. However they note that the cobalt also played a sacrificial role by intercepting some of the sulphur content in the feed and preventing it from deactivating the nickel sites [71]. Liu and Hong also report similar phenomena in the case of nickel borate catalysts, where the borate anion displays Lewis acid behaviour and preferentially reacts with Lewis basic sulphur compounds, thus prolonging the life of the nickel active sites from being 57

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Table 2 Summary of recently investigated sulphur resistant catalyst compositions developed for syngas production. Catalyst Composition

Reforming Reaction and Conditions

Remarks

Reference

Mn- and La-doped CeO2

Syngas tar reforming (650 – 725 °C, 40 ppm H2S) Syngas tar reforming (650 – 800 °C, 40 ppm H2S) Syngas tar reforming (800 °C, 15 ppm H2S) Steam reforming of tar (620 – 750 °C, 20 – 100 ppm H2S) Steam reforming of tar

C3H8 as tar model compound, no deactivation after 12 h on stream.

Lee et al [49]

Naphthalene as model compound.

Li et al [51]

Alkali present in system as KCl. High T, low GHSV and low steam favoured conversion.

Moud et al [59] Rhyner et al [61]

Review paper.

Steam reforming of tar (700 – 800 °C, 50 ppm C4H4S) Reforming of waste tire tar (800 °C, 1 g tire to 0.5 g catalyst) Tar reforming via chemical looping (750 – 850 °C) Propane reforming (600 – 800 °C, 20 – 80 ppm H2S) Biogas reforming for SOFC (750 °C, up to 1000 ppm H2S) Methane steam reforming for SOFC Methane steam reforming for SOFC (800 °C, 10 ppm H2S dosed) Methane steam reforming for SOFC (750 °C, up to 220 ppm H2S) Biogas reforming

Toluene and naphthalene as model compound.

Cavattoni and Garbarino [65] Savuto et al [50]

Mn or Fe on CeO2/La2O3 K added to Ni catalysts Unspecified commercial noble metal monolith Bimetallic NiCu, NiFe Ce added to Ni/Mayenite Nickel supported on dolomite Fe,Sr-doped La2Zr2O7 pyrochlore supported on ZrO2 Ni perovskite and doped CeO2 composite Bimetallic Cu-Co-ceria Au, Mo doping on Ni anodes Au-modified Ni/Gd doped ceria Gd-doped ceria Alkaline-earth oxides added to Ni catalysts Mo-Ni on γ-Al2O3 Series of Ni + Co, Mg, Ca, La, Y, Gd, Al and/or Zr on alumina Mg & B added to Ni catalysts Ni-La on γ-Al2O3 Ni-Cu, Ni-Pt, Ni-Pd on CeO2 Ru-, Sr- modified lanthanum chromites Rh, Ce, Zr on γ-Al2O3 Ni on La- doped CeO2 La, Ba added to Ni catalysts Ceria supported nickel borate

Biogas reforming (650 – 800 °C, 50 ppm H2S) Biogas reforming (900 °C, 100 ppm H2S) Steam reforming of ethanol and phenol (500 – 750 °C, 210 ppm C4H8S) Steam reforming of ethanol and phenol (250 – 750 °C, 0.011–0.033 molS/molNi) Tri-reforming of methane (700–800 °C, 3500 ppm H2S) Autothermal reforming of methane (650 – 950 °C, 50 ppm H2S) Autothermal reforming of iso-octane and gasoline (700 °C, 5 – 7 ppm sulphur) Autothermal reforming of diesel (750 °C, 100 ppm sulphur) Steam reforming of diesel (795 °C, 10 ppm sulphur) Jet fuel reforming (750 °C, 100 ppm sulphur)

Pyrolysis/gasification/reforming combined reaction system. Oxygen carrier is resistant to sulphidation, thus isolating the reforming system from sulphur content present in feedstock. Catalytic behaviour changed in presence of H2S.

Elbaba and Williams [58] Keller et al [67] Lo Faro et al [52]

Stable performance up to 740 h on stream.

Fuerte et al [66]

Short review paper. Annealing at 1100 °C results in dense layer of Au that hinders H2S penetration to Ni sites. Diffusion of S into oxygen vacancies results in Ce2O2S formation, which negates sulphur resistance. Thermodynamic modelling study.

Niakolas et al [69] Sapountzi et al [70]

Synergetic effect from Mo-Ni bimetallic catalyst; better than either metal alone. Found that the order of impregnation of metals affected the degree of sulphur resistance. Boron addition imparted slight sulphur resistance.

Postole et al [53] da Silva and Heck [57] Gaillard et al [68] Saha et al [71] Garbarino et al [60]

Lanthana does not protect reduced nickel sites in catalyst, but suggested addition of La-Al2O3 at inlet of reactor to pre-eliminate sulphur. Noble metals imparted high degree of sulphur resistance to catalyst.

Garbarino et al [54] Kantserova et al [64]

La0.95Sr0.05Cr0.15Ru0.05O3 had highest degree of sulphur resistance

Mota et al [62]

Ceria addition increased sulphur resistance, improving catalyst operation time from < 4 h to > 100 h. Optimum doping of La0.1 had the best effect, and higher ratios decreased surface area and activity. Combined doping of La + Ba increased catalyst lifetime to > 160 h.

Jung et al [63]

Nickel borate acts as sacrificial element to consume H2S, which degrades to give more Ni0 sites for reaction.

Liu and Hong [72]

Liu and Hong [55] Tribalis et al [56]

relatively inert material over the active catalyst sites, essentially forming a core-shell structure. This architecture selectively enables reactants to penetrate and enter the core side for reaction, while isolating the unwanted sulphur compounds outside in the feed stream. In theory, an infinite variety of combinations core and shell materials can be matched to form effective catalysts; and shells that function well as carbon resistant layers may well be sulphur resistant as well. This may be especially true for those that operate through oxidative removal mechanisms [126]. Laosiripojana et al synthesised a Ni-Fe/Mg-Al2O3 catalyst for biomass tar reforming, and then proceeded to coat the catalyst with a layer of ceria doped gadolinia. Their results clearly show that while the coated catalyst initially had slightly lower conversions compared to the non-coated catalyst (likely due to increased mass transfer resistance from the shell structure), it performed remarkably better under sulphur contamination conditions. Not only did the coated catalyst have higher activity in the presence of 500 ppm H2S (55% conversion vs 15% for non-coated catalyst), but it could also be regenerated to its initial activity when H2S was removed. The non-coated catalyst remained as partially deactivated [127]. Fig. 8 shows the effects of H2S presence and catalyst regeneration on the performance of the coated and uncoated

Fig. 6. Recently reported structural engineering strategies to obtain sulphur resistant catalysts [73–151].

formation of nickel sulphide phases), by supplying sufficiently high hydrogen partial pressures and temperatures [125]. This further lends evidence to the idea that alloys are more or less self-repairing catalysts, the ease of which depends on its elemental compositions and ratios. 4.2.2. Core-shell structures Sulphur resistance can also be introduced to a catalyst by coating a 58

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Table 3 List of recently developed perovskite catalysts for syngas production. Perovskite composition

Reaction and conditions

Remarks

Reference

La1−xCexNi1−yFeyO3 (x, y = 0–0.4) La2−xCexCoO4 ± y

Dry reforming of methane Dry reforming of methane (400 – 800 °C, 3:3:2 of CH4:CO2:N2, WHSV = 16,000 ml/h/gcat) Dry reforming of methane (800 °C, 1:0.8 of CH4:CO2, GHSV = 243,000/h)

Simulation and modelling study. Doping of ceria improves dispersion of active sites, and increased surface area t from 0.2 m2/g to 8.5 m2/g. Optimised perovskite composition of La0.9Ba0.1MnO3 allowed easy reducibility in Mn and enhanced oxygen donation to reduce coke formation. Pulse experiments show that lattice oxygen helps to remove deposited carbon, likely effective for sulphur removal as well. BaZrRhO3 shows the least amount of carbon formation

Abedini et al [79] Bai et al [80]

La1−xBaxMnO3 (x = 0.10 −0.50) Srn+1Tin-xNixO3n+1

Dry reforming of methane (600 – 800 °C, 1:1:1 of CH4:CO2:N2, GHSV = 20,000/h)

BaZr1-xMxO3 (M = Rh, Ru, Pt)

Dry reforming of methane (550 – 900 °C, 1:1 of CH4:CO2, GHSV = 353,000/h) Dry reforming of methane (650 – 800 °C, 1:0.8 of CH4:CO2, 6 ml/min to 0.18 g catalyst) Dry reforming of methane (800 °C, 0.8:1:0.2 of CO2:CH4:O2, GHSV = 24,000/h) Dry reforming of methane (700 – 800 °C, GHSV = 30,000 ml/h/gcat) Dry reforming of methane (800 °C, 1:1 of CH4:CO2) Dry reforming of methane (600 – 800 °C, 1:1 of CH4:CO2, GHSV = 10,000/h) Dry reforming of methane (500 – 700 °C, 1:1 of CH4:CO2, 20 ml/min to 100 mg catalyst) Dry reforming of methane (800 °C, 1:1 of CH4:CO2, WHSV = 24,000 ml/h/gcat) Dry reforming of methane (550 – 800 °C, 1:1 of CH4:CO2, GHSV = 36,000 ml/h/gcat) Dry reforming of methane (700 – 900 °C, 1:1 of CH4:CO2, GHSV = 3000 – 20,000/h) Dry reforming of methane/ Partial oxidation of methane (600 – 800 °C, 1:0.5:0.5 of CH4:CO2:O2, WHSV = 15,000 ml/h/gcat) Dry reforming of methane/ Partial oxidation of methane (600 – 800 °C, 1:0.5:0.5 of CH4:CO2:O2, WHSV = 15,000 ml/h/gcat) Partial oxidation of methane (900 °C, 2:1:2 CH4:O2:N2, GHSV = 36,600 ml/h/gcat) Partial oxidation of methane (400 – 1000 °C, 1:0.5 of CH4:O2, GHSV = 12,200/h) Partial oxidation of methane (850 – 960 °C, 1.8 – 4.5:1 of CH4:O2, GHSV = 15,000–75,000 ml/h/ gcat) Partial oxidation of methane (300 – 800 °C, 1:0.5 of CH4:O2, GHSV = 21,600 ml/h/gcat) Partial oxidation of methane (900 °C, 1% CH4, 0.6% O2, Argon balance, GHSV = 20,000/h) Partial oxidation of methane (850 °C, 5% CH4, 95% Ar, GHSV = 750/h) Partial oxidation of methane

La0.6Sr0.4Co0.8Ga0.2O3‑δ Membrane reactor LaNi1-xTixO3 SmCoO3 La0.75Sr0.25Cr0.5Mn0.5O3−δ La1-xCexNiO3 (x ≤ 0.5) LaCuO3, LaCu0.53Ni0.47O3 La1-xSrxCoO3 SBA− 15, MCM− 41 and silica supported LaNiO3 La0.9M0.1Ni0.5Fe0.5O3 (M = Sr, Ca) LaNi1–xCoxO3 LaNi1-xMgxO3-δ LaCoO3 LaCaMAlO (M = Co, Cr, Fe, Mn) NdCaCoO3.96 LaNiO3 LaNiO3 La0.7A0.3BO3 (A = Ba, Ca, Mg, Sr; B = Cr, Fe) NdCaCoO4

LaCr0.85Ru0.15O3

Partial oxidation of methane (850 °C, 1:1:0.75:2.5 of CH4:H2O:O2:He, GHSV = 10,660/h)

La0.3Sr0.7Fe0.7Cu0.2Mo0.1O3 Membrane reactor LaGa0.65Mg0.15Ni0.20O3–δ

Partial oxidation of methane (950 °C, ~2:1 of CH4:O2, RT = 0.4 min) Partial oxidation of methane (700 °C – 1000 °C, 48:20 of air: CH4, 40 ml/min to 0.2 – 0.5 g catalyst) Partial oxidation of methane (900 °C, 10% CH4 for reduction, 10% O2 for oxidation, 100 ml/min to 100 mg catalyst) Partial oxidation of methane (400 – 1000 °C, 2:1:4 of CH4:O2:N2, GHSV = 30,000 ml/h/gcat)

AMnxB1−xO3 (A = Ca, Ba; B = Fe, Ni) La0.5Sr0.5CoO3-δ La0.75Sr0.25(Fe0.80Co0.20)(1-x)AlxO3-δ (x = 0, 0.10, 0.25, 0.40, 0.60) LaCo1-xFexO3 (x = 0, 0.5, 1) LaNi1-xCoxO3 (x = 0, 0.2, 0.5, 1)

Partial oxidation of methane (600 °C, 10% CH4 in He for reduction, 10% O2 in He for oxidation, 0.1 g catalyst in pulsed studies) Partial oxidation of methane (700 °C, 2 – 4:1 of CH4:O2, 0.2 – 0.7 kgcat·s/mol) Partial oxidation of methane (800 °C, 2:1 of CH4:O2, 100 ml/min to 5 mg catalyst)

Oxygen delivery of perovskite membrane supported long operating life (> 160 h) of standard nickel catalyst. Ti substitution into B-site gave higher stability and activity when x = 0.4 – 0.6. Authors reported a lower activation energy (44 kJ/mol) compared to other data. Perovskite anode imparted significant stability to Ni-Cu catalyst in terms of carbon resistance. Authors showed that carbonate species formed in the catalyst inhibited sintering and improved stability. Composite of perovskite support and NiO/LaCuO3 gave better performance than simple perovskite alone. Suggests that good dispersion of Sr in La2O2CO3 matrix was crucial to inhibit carbon formation. Perovskite coated on high-surface area materials gave excellent activity in addition to stability. Improved CO2 adsorption facilitated carbon removal, likely through Boudouard reaction mechanisms. Authors reported that Co doping in B-site resulted in worse metal site reduction behaviours. Direct correlation was found between higher oxygen content in feed and lower carbon deposition, reaction temperatures and energy consumption. Preparation method using ammonium precipitation gave higher porosity in catalyst, which improved performance. Reduction of doped metal led to irreversible destruction of perovskite structure. Porous carbon was formed on the surface, which did not inhibit catalyst performance. This is likely not to be the case for sulphur. Only small amounts of carbon formed, due to good dispersion and lowered sintering of Ni in the catalyst. Authors reported that NiO de-mixes from perovskite structure twice to yield Ni0 which is the actual catalyst. A-site alkaline-earth metals helped with oxidative regeneration of the catalysts. Material characterisation analysis paper. Authors suggest that the oxygen deficiencies, structure defects, electrical conductivity and its activation energy play a key role in its activity. Kinetic studies were also done at high GHSV (~10,000,000/h). Authors also show that un-incorporated RuO2 on the catalyst surface contributed to its high activity and low fouling. Operation up to 86 h lifetime before membrane failed.

Bhavani et al [81] Dama et al [82] de Caprariis et al [83] Kathiraser et al [84] Nuvula et al [85] Osazuwa et al [86,87] Qin et al [88] Su et al [89] Touahra et al [90] Valderrama et al [91] Wang et al [92] Yang et al [93] Jahangiri et al [94] Jahangiri et al [95] Alvarez-Galvan et al [96] Cihlar Jr et al [97] Dedov et al [98] Duan et al [99] Nguyen et al [100] Khine et al [101] Mazo et al [102]

Melchiori et al [103] Meng et al [104]

Results further reinforce the idea that La2O3 improves carbon tolerance.

Meng et al [105]

Redox studies of oxygen carrier materials for chemical looping partial oxidation of methane. Ba series of catalysts were highly coke resistant. Catalyst became more stable as the number of redox cycles increased. Result attributed to better dispersion from destruction and reconstruction of catalyst structure. Results suggest that product selectivity in this system is highly dependent on equilibrium factors of the gases.

Mishra et al [106]

Different reaction mechanisms were observed when the Co:Fe ratios were varied. Partial substitution of nickel with cobalt afforded better carbon resistance to the catalyst.

Roseno et al [109]

Morales et al [107] Mudu et al [108]

Santos et al [110]

(continued on next page) 59

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Table 3 (continued) Perovskite composition

Reaction and conditions

Remarks

Reference

Ba0.9Co0.7Fe0.2Nb0.1O3-δ Membrane reactor

Partial oxidation of methane (800 – 875 °C, 3:7 of CH4:Ar, 40 ml/min) Chemical looping reforming of methane (redox cycles with methane and syngas at 950 °C)

Oxygen supplied by air on the other side. Membrane has three layers, one dense and two porous catalytic layers. All catalyst compositions generally stable even at high temperatures (> 1000 °C), and able to perform for more than 50 cycles. Addition of Ni to the perovskite increased crystallinity and enhanced oxygen availability.

Song et al [111]

CaMnO3-δ + MgO NiFe2O4 LaNi0.5Mn0.5O3

Chemical looping reforming of methane with CO2 (redox cycles with methane and CO2 at 900 °C) Steam reforming of tar (800 °C, 50 ppm H2S)

LaxCe1−xNi0.5Cu0.5O3

Water gas shift (350 °C, 200 ppm H2S)

La0.75Sr0.25FeO3

Reverse water gas shift (redox cycles with 10% H2 in He and 10% CO2 in He at 950 °C)

La0.4Sr0.6Ti1-xMxO3 (M = Ru, Ni, Co)

Dodecane reforming (800 °C, 50 ppm sulphur)

Y0.08Sr0.88Ti0.9M0.1O3 (M = Ru, Ni, Co)

Dodecane reforming (700 – 900 °C, GHSV = 160,000/h) Steam reforming of toluene (500 – 800 °C, S/C ratios of 2:1 and 4:1, 2000 ml/min to 1 g catalyst) Partial oxidation of diesel (800 – 1000 °C, 1000 ppm dimethyl disulphide) Partial oxidation of glycerol (200 – 600 °C, spray nebuliser, RT = 16 – 44 ms) Steam reforming of ethanol (500 – 800 °C, 2.5% EtOH, 7.5% water, 90% N2, 30 ml/min to 20 – 67 mg catalyst) Water splitting coupled to syngas production (H2 and H2O redox cycles at 950 °C) Biomass steam gasification (600 – 800 °C, 0.768–3.702 g/min biomass, 1.704– 4.297 g/ min water to 3 g catalyst) Dry reforming of coke oven gas (800 °C, 1:1 of CH4:CO2, GHSV = 12,000 ml/g/h)

La1-xCexCoO3 La0.8Sr0.2Cr0.95Ru0.05O3-x Sm0.8Ba0.2Cr0.95Ru0.05O3-x LaMnO3, LaNiO3 LaNiO3/CeSiO2 BaMn0.5Fe0.5O3-δ LaCo1-xCuxO3 La0.6Sr0.4NixCo1−xO3

Fig. 7. Proposed mechanism for self-regeneration of Pt-Ru alloys under sulphur poisoning conditions. a) Thiophene attack on alloy, binding to Pt atom, b) Migration of thiophene from Pt to Ru atom, c) Thiophene attack on alloy, binding to Ru atom, d) Hydrogen adsorption on Pt atom, e) Desorption of reaction products and regeneration of Pt-Ru alloy. Figure reprinted with permission (copyright 2018) from Stanley et al [124].

Ni series were strongly deactivated, but regenerable. Mn series had lower activity but much more resistant to H2S. Demonstrated that a different reaction (methanation) was favoured at higher temperatures in the presence of H2S. SiO2 support for the perovskite catalyst significantly increased the oxygen diffusion rates, thus improving its fouling resistance. Only Ru series of catalysts were noticeably stable in presence of sulphur. Ru series of catalysts showed the best stability and resistance to carbon formation and sintering. Authors note that Ce addition to the catalyst did not enhance its activity, but merely increased its stability.

Jing et al [112] Lim et al [113] Quitete et al [77] Oemar et al [78] Hare et al [114] Hbaieb [75] Hbaieb et al [74] Soongprasit et al [115]

Sulphur tolerance of SBCR catalyst was found to be much better than LSCR at high temperatures. LaMnO3 was more active than LaNiO3 for autothermal operation, and vice versa for steam reforming. Authors suggest that high temperatures suppressed carbon formation, though likely not through oxidation of deposited carbon by lattice oxygen. Tenfold increase in water splitting efficiency reported compared with conventional solar methods. Authors note that higher temperatures, S/C ratios and WHSV all contributed to suppression of coke formation.

Lee et al [76]

Authors suggest that La2O2CO3 is a key intermediate species in the perovskite catalyst that helps to inhibit carbon formation.

Zhu et al [120]

Liu and Lin [116] Marinho et al [117] Haribal et al [118] Yao et al [119]

Fig. 8. Comparison of coated (core-shell structured) and uncoated catalyst performances under H2S poisoning conditions. Figure reprinted with permission (copyright 2018) from Laosiripojana et al [127].

It is also possible to engineer the shell side of the catalyst to actively participate in the sulphur removal mechanisms. Tsodikov et al prepared a core/shell catalyst for methane steam reforming that incorporated tiny superparamagnetic γ-Fe2O3 particles into the shell structure of the catalyst. They showed that the core/shell catalyst functioned relatively well under up to 30 ppm sulphur in the feed, and that the iron oxide particles had the ability to convert H2S into elemental sulphur, thus providing further negation of the impacts of sulphur on the reforming system [129,130]. A summary of recently reported core-shell combinations in catalysts for various syngas production reactions is given in Table 4.

catalysts. Hua et al also included a ceria based shell onto a nickel/ copper bimetallic cluster catalyst. The combination of a bimetallic catalyst with a ceria shell gave double benefits, which enabled higher conversions for methane dry reforming both in the presence and absence of H2S. For a 500 ppm H2S feed, the conversions of the coated bimetallic catalysts reached around 90% at 800 °C, while the coated monometallic nickel catalyst only had around 40% conversions under the same conditions. The basic uncoated monometallic nickel catalyst fared the worst, and merely had a conversion of around 10% [128]. 60

Shell material

Zirconia doped ceria Mesoporous silica

Mesoporous silica

Mesoporous silica

Silica

Silica

Ni

ZrO2

Superparamagnetic γ-Fe2O3 La-Sr-Fe perovskites

Silica

La-Sr-Fe perovskites

Pd

Mesoporous silica

Au

Core material

Ni, Cu bimetallic cluster Ni-Mg phyllosilicate nanotubes Lanthana-doped Ni

LaNiO3 nanocubes

LaNiO3

“Yolk” Ni

Silica

Ni, Fe2O3, ZrO2,

Fe-Ni alloy Fe2O3

61

Lanthana-doped Ni

Co3O4, Mn3O4

Ni

Ceria-promoted Ni

Cu nanowires

Partial oxidation of methane (650 – 800 °C, 2:1:3 ratio of CH4/O2/ N2, GHSV = 72,000 ml/h/gcat) Chemical looping partial oxidation of methane (800 – 900 °C, pulse studies) Partial oxidation of butane (700 – 800 °C, 4:1 of butane: air at 30 ml/ min to 0.2 g catalyst) Biogas reforming (600 – 750 °C, 45 ml/min CH4, 30 ml/min CO2, 35.72 ml/min air to 100 mg catalyst) Electrochemical reduction of CO2 (540 mV overpotential, 0.5 M KHCO3 electrolyte)

Methane steam reforming (800 °C, 5 – 30 ppm H2S) Partial oxidation of methane (700 – 900 °C, pulse studies)

Dry reforming of methane for SOFC (800 °C, 50 ppm H2S) Dry reforming of methane (750 °C, 1:1:1 of He: CO2:CH4, 10 ml/min to 30 mg catalyst) Dry reforming of methane (700 °C, 1:1:1 ratio of CH4/CO2/N2, 60 ml/min to 0.05 g catalyst) Dry reforming of methane (600 – 800 °C, 1:1 ratio of CH4:CO2, WHSV = 18,000 ml/h/gcat) Dry reforming of methane (Non-thermal plasma, 400 ml/min of Ar, CH4 and CO2) Dry reforming of methane (up to 800 °C, 1:1 ratio of CH4:CO2, GHSV = 12,000 ml/h/gcat) Dry reforming of syngas methane (500 – 800 °C, 5% H2, 5% CH4, 10% CO, 10% CO2 and balance N2, 250 ml/min to 0.2 g catalyst) Chemical looping dry reforming of methane (750 °C, 25 redox cycles)

Reaction and Conditions

Table 4 List of recent core-shell catalyst combinations developed for various syngas production reactions.

Likely first instance of core-shell electrodes for electrochemical reduction reactions.

Concluded that La2O3 must be well distributed in material to ensure good coke resistance. Suggested that the perovskite shell could be tuned for gains in selectivity or coke resistance. Coated Ni@Pd catalysts were mounted on alumina support for higher stability. Silica shell imparted high resistance to coke formation.

Yolk structure of core reduced carbon deposition, compared with conventional core structures. Interesting inversion of typical core-shell catalyst designs, but had reasonable coke inhibition effects. Chemical looping helps to combust and regenerate sulphur deactivated catalyst. Shell side ferric oxide helped to decompose adsorbed sulphur. Studied mechanism of O2- transport and reaction in perovskite shell.

Steady operation up to 48 h without degradation in performance. Catalysts stable in excess of 72 h, shell exhibited excellent carbon-resistant properties. Excellent coke resistance and stability, operating up to 100 h with only slight drop in performance. Reports that the mesoporous silica shell coupled with strong metal-support interactions between Ni and La2O3 improved coke resistance significantly. Plasma further improved core-shell catalyst stability and activity.

Remarks

Chen et al [146]

Kathiraser et al [145]

Mosayebi et al [144]

Neal et al [142,143]

Tsodikov et al [129,130] Shafiefarhood et al [139,140] Li et al [141]

Hu et al [138]

Yang et al [137]

Yang et al [136]

Zheng et al [134,135]

Zhang et al [133]

Mo et al [132]

Hua et al [128] Bian et al [131]

Reference

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and provide us with clues into future designs of catalysts. Misture et al highlight that defective spinel oxide sites may have similar functionalities as the oxygen vacancies in ceria or zirconia, possibly paving the way for other tailored materials to act as substitutes for the traditional oxygen ion conducting materials. Furthermore, they hypothesise that facets play an important role in providing oxygen access to the sulphur poisoned sites, which helps to remove deposited sulphur on the catalyst [151]. This is likely analogous to the observation that lightly calcined magnesia is much more reactive than dead-burned magnesia, which is in fact refractory and inert against most chemical attacks. For example, when magnesite (MgCO3) is heated, a series of changes occur to the material. Magnesite decomposes and gives off CO2 at temperatures between 300 °C and 600 °C. This relatively low calcination temperature leaves behind a porous structure. It was also observed that the unit cells of the crystal structures were slightly enlarged, resulting in the formation of disrupted lattices and uneven surfaces. As the calcination temperatures increase, a corresponding enlargement of the average particle sizes was also noted, and the previously porous surfaces were sintered and smoothed out. As a result, magnesia that is calcined at high temperatures is much less reactive than lightly calcined samples [152]. 4.3. Catalyst regeneration

Fig. 9. TEM images of iridium nanoparticles obtained via solution combustion methods. Inset picture shows a high resolution image of the iridium nanoparticle, highlighting (111) and (100) facets. Figure reprinted with permission (copyright 2018) from Postole et al [148].

If sulphur poisoning cannot be avoided, then the catalyst has to be regenerated in order to prolong its useful lifetime in the reforming system. Frequent and cyclic catalyst regeneration is commonly practised in many setups, for example in fluid catalytic crackers (FCC) and Fischer-Tropsch synthesis. However, catalyst regeneration is generally seen as a last resort strategy for maintaining the lifetime of a catalyst. Catalyst regeneration usually requires harsher conditions than the standard operating conditions in a reactor, and may risk degradation of the catalyst due to sintering, attrition and denaturation of the material. Furthermore, the regeneration step also often consumes significant amounts of energy, which can reduce the profitability of a reforming process. Even so, catalyst regeneration remains an interesting (though less explored) area of research in reforming systems. The regeneration procedures to remove sulphur poisoning in reforming catalysts can be as simple as eliminating the sulphur content in the feed, though in some cases the selectivity and yields of the catalyst may be permanently altered [153]. Yang further studied the regenerability of a nickel-based steam reforming catalyst under three conditions. They looked at the effects of removing H2S in the feed, increasing the temperature, and steam treatment. According to their results, removal of H2S from the feed gradually resulted in the recovery of catalyst activity, and it was found that the speed of recovery depended on both the regeneration temperature and original feed sulphur content. Additionally, by increasing the temperature without removing sulphur from the feed, it was found that the catalysts regained some activity, though whether this was a result of increased reaction rates or true sulphur removal was not conclusively proven. Finally, they have also showed that steam treatment for 3 h almost fully regenerated the catalyst (back to nearly 100% CH4 conversions) [154]. Sulphur deposited on the surface of the active sites can also be removed by first oxidising them into sulphate species, and then thermally decomposing the sulphate into SO2 gas and the residual oxide. Depending on the catalyst, this oxide may have to be reduced back to the metal to be recycled. Studies done by Izquierdo et al suggest that the incorporation of ceria helped with the oxidative regeneration of the reforming catalyst, likely because of the aforementioned enhanced oxygen mobility promoted by the defects induced via Ce3+/Ce4+ redox mechanisms [155]. Other less conventional methods for the regeneration of reforming catalysts have also been studied. Sadatshojaei et al attempted to use supercritical CO2 (sCO2) to regenerate a spent commercial Ni/Al2O3 catalyst (Midrex reformer catalyst) to remove sulphur and coke deposits. Rather than thermally decomposing the sulphide species, sCO2 dissolves them and carries the contaminants away from the catalyst

4.2.3. Unique catalyst structures from innovative syntheses In addition, fine tuning of the synthesis methods of common catalyst materials also allows researchers to control and enhance the sulphur resistant properties of catalysts. Relatively recent methods such as exsolvation and solution combustion allow for the synthesis of engineered nanoparticles that have certain desired behaviours in the presence of sulphur compounds. Cassidy et al prepared materials based on Ni- and Rh- doped perovskites. Upon their reduction, it was found that nanoparticles were exsolved from the perovskite structure onto the surface of the materials. Catalysts with these exsolved nanoparticles had higher activities than the basic perovskites. Although it was noted that these were still susceptible to sulphur poisoning to a certain degree, the catalysts were able to be regenerated to their original activities, making them temporarily sulphur tolerant. The Rh based material was reported to be more robust than the Ni based material [147]. Postole et al developed a sequential method for the one pot synthesis of iridium and ceria based catalysts for methane reforming. They found that oxidation of the iridium at 300 °C followed by reduction of the same catalyst at 500 °C resulted in the formation of iridium nanoparticles with diameters around 2 nm, and these were much more robust than catalysts with similar composition that were obtained from conventional incipient wetness impregnation methods (as can be seen in Fig. 9). In addition, they also found that while their catalyst deactivated rather significantly when exposed to H2S, it could recover its activity upon removal of sulphur from the feed stream [148]. Shiratori et al developed a catalyst synthesis method inspired by papermaking techniques, which dispersed hydrotalcite (Mg6Al2(OH)16CO3·4H2O) onto an inorganic fibre network. The hydrotalcite material was then modified by exchanging some of the magnesium in the hydrotalcite with nickel. They note that the amount of nickel doping had to be controlled precisely to avoid excessive carbon deposition, and also be active enough for reforming reactions to take place. In its application for methane dry reforming reactions, it was reported that a nickel loading of around 8.6% with a specific nickel area of 21 m2/g had the best resistance against sulphur poisoning [149]. In a follow up paper, they attribute the improved performance of the catalyst to the unique geometric structure of the paperlike material compared to a catalyst pellet, which increases accessibility of the reactants to the active sites [150]. Finally, in-depth understandings of the physical structures of catalyst materials allow us to have insights into the actual reaction mechanisms 62

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bed. Since the critical pressures and temperatures of CO2 are rather low (73 bar and 31 °C), its use allows the catalyst to be regenerated at a very low temperature. In their work, the authors used a working regeneration temperature of around 50 °C. The pressure, however, may be an issue in conventional reformers that operate at near atmospheric conditions. They also found that the reforming catalyst actually slightly improved in its mechanical properties, by being more flexible and resistant to deformation after sCO2 treatment [156]. The use of sCO2 to regenerate sulphur-poisoned catalysts is thus likely to find great use in high-pressure reforming plant setups with existing compression facilities, and where the reactors are designed to withstand significant amounts of pressure. A selection of recently reported schemes for the regeneration of sulphur poisoned catalysts is given in Table 5.

acidic gas mixtures. For example, H2S can be preferentially adsorbed onto the zeolite pores while leaving CO2 untouched for downstream dry reforming reactions. Papurello et al used a NaX zeolite bed to scrub H2S from a biogas feed prior to its reformation and use in a solid oxide fuel cell. Their results show that the zeolites were capable of reducing the H2S content in a biogas feed from 30 ppmv to zero, while leaving the CO2 content unchanged. The breakthrough time was estimated to be around 60 h in their setup, and the adsorber continued to function quite well for up to 250 h by lowering the cleaned gas sulphur content to 70 ppbv at that point [164]. Careful control of the upstream steps (for example during gasification and pyrolysis) can also help to mitigate the amount of sulphur being generated together in the feedstock stream. Aljbour and Kawamoto studied the effects of operation parameters of cedar wood gasification on the release of various catalyst poisons into the syngas stream. They found that higher equivalence ratios and steam/carbon ratios contributed most significantly to the conversion of biomass sulphur into gaseous sulphur compounds (primarily H2S). A similar trend is observed for chlorides and nitrates as well, reinforcing the notion that oxidative conditions strongly encourage the formation of non-desirable volatiles during gasification reactions [165]. Gall et al studied the effects of adding potassium into a biomass gasification process to improve its tar formation properties. They found that while the addition of the alkali metal had some beneficial effects on the gasification process, the potassium was vaporised at the relatively high temperatures in the reactor and may have detrimental effects on downstream operations. They also found that while the presence of sulphur in the system initially formed K2SO4 particles, this quickly decomposed in the presence of CO2 to give potassium carbonate and released the sulphur as H2S gas [166]. This suggests that it may be possible to tailor the gasification conditions to utilise the alkali metal content in the feed, to sequester sulphur as non-volatile residues that are easily separated. A graphical summary of some interesting feedstock pre-treatment processing methods is shown in Fig. 10. Other less common (and often situational) desulphurisation methods include feedstock dilution, biological treatment, as well as “self-cleaning” of the feedstock materials. Martin et al proposed a simple solution to overcome sulphur deactivation in reforming reactions. They blended commercial diesel, which may contain up to 10 ppm sulphur, with biodiesel which was practically sulphur free to obtain a low sulphur feed for reforming over a 600 CPSI monolithic platinum catalyst at 800 °C and 3 bars. The conversions were as high as 98.7% and the catalyst lifetime was satisfactory, being capable of operating for longer than 100 h without noticeable deactivation [167]. Navarro et al describe the Thiopaq process, which is a commercial technology that utilises chemotrophic bacteria to consume and convert the H2S gas in a biogas stream into elemental sulphur. The Thiopaq system is basically an anaerobic digestion process, where H2S gas is scrubbed by an alkaline liquid and fed into the bioreactor to be metabolised by the microorganisms. The process claims to be able to treat large amounts of biogas, and bringing the feed sulphur content down from up to 7000 ppmv H2S to less than 4 ppmv [168]. As with bioprocesses in general, the Thiopaq process appears to have relatively low energy requirements, though the bioreactor volumes may be quite significant and unsuitable for small scale operations. “Self-cleaning” refers to a class of methods that use components within the feedstock itself to interact with the sulphur content, thereby immobilising the sulphur and isolating them from the feedstock. Xu and Li proposed the use of sulphonic acid ion exchange resins to conduct in situ reactions within raw C9 resin fractions to remove thiophenes from the feedstock. The reactions involved the alkylation of the thiophene content with vinyltoluene contained in the feedstock. This increased the molecular weight of the sulphur compounds and made them less volatile, and hence easier to separate from the vaporised feedstock for reforming. This not only removed sulphur, but also increased the octane

4.4. Feedstock pre-treatment By preventing sulphur contaminants from entering the reactor in the first place, the entire reforming system can be made to become more robust. Pre-treatment of the feedstock enables a less tolerant but cheaper reforming catalyst to be used, and may reduce the reactor sizes and downstream separation costs as well. The most straightforward way of preventing sulphur penetration into the reactor is to absorb or consume the sulphur compounds in the feedstock prior to its entry. Conventional methods of doing so include absorption of H2S using zinc oxide, and hydrodesulphurisation (HDS) to convert thiophene compounds into elemental sulphur and remove them from the feed [157–159]. Chemical absorbents to remove sulphur contamination are usually fast-acting and non-discriminate (meaning that they can react with a wide range of sulphur species), and can be very simple to implement in a plant. Spies et al show that a zinc oxide absorbent can effectively remove not only H2S, but also carbonyl sulphide (COS). The absorbent undergoes sulphidation to form zinc sulphide, which loses its absorption capability and has to be regenerated prior to further reuse. Regeneration entails the addition of oxygen, which oxidises the sulphide into SO2 and is removed from the absorber. However, they report that the absorbent undergoes deactivation to a certain degree, most likely due to formation of zinc sulphate which is more difficult to decompose [160]. Zuber et al also showed that if the zinc oxide absorbent was allowed to form sulphates extensively, then the regeneration of the absorbent was practically impossible even at temperatures up to 800 °C. However, carbon deposition was fully removed at these conditions. Sintering and cracking were also observed in the absorbent after high temperature regeneration tests, suggesting that milder methods of regeneration are needed [161]. Tran synthesised a highly porous ZnO absorbent for sulphur absorption using agarose gel as a moulding template to generate high surface areas in the material. It was found that doping 4% of nickel into the synthesised absorbent somehow increased its sulphur capacity by 3 times compared to commercial absorbents. The new Ni/ZnO absorbent was also regenerable at 600 °C, and retained comparable absorption performances after two cycles [162]. Hydrodesulphurisation can also be used in combination with ZnO absorption for the removal of problematic sulphur compounds in the feedstock. Zuber et al recently studied a combined cleanup method to desulphurise the syngas from biomass gasification prior to its entry into a reforming reactor. Zinc oxide appears to be less effective in absorbing thiophenes compared with H2S. As such, a single ZnO absorber would likely result in sulphur slip into the reformer due to insufficient interception of the thiophene components. The authors proposed the use of cobalt and molybdenum catalysts for hydrodesulphurisation to fully convert thiophenes in the feed to H2S, which can then easily absorbed into the ZnO absorber. Their results suggest that this was probably a difficult process to implement, due to temperature mismatches and some inhibition of HDS activity due to chemical equilibrium in the reactor [163]. Zeolites have also been proposed as a potential adsorbent for the desulphurisation of reforming feedstocks. The use of zeolites confers certain advantages in the selective removal of sulphur compounds from 63

64

Water gas shift reaction

Steam reforming of methane Steam reforming of methane Steam reforming of ethanol/phenol mixture Methane reforming

LaxCe1−xNi0.5Cu0.5O3

Gd-doped ceria

Ni/Al2O3 (Midrex reformer catalyst)

Ni/Al2O3

Ir-CeO2

Ni/Co spinel catalysts

Chemical looping dry reforming Dry reforming of methane

Steam gasification of biooil/ biochar slurry Partial oxidation of methane Partial oxidation of methane Tri-reforming of methane

NiO/ZrO2, Fe2O3/ZrO2 core shell catalysts

Ni-Co-CeO2, Pt-Ni and Pd-Ni

La0.5Sr0.5CoO3-δ

Fe2O3 @LaxSr1-xFeO3 core–shell catalyst

LaCo0.9Cu0.1O3

Chemical looping tar reforming Biomass tar reforming

Steam reforming of methane Tri-reforming of biogas

Ni/Al2O3

Ni- and Rh-Ni supported on CeO2, ZrO2 and/or Al2O3 Fe,Sr-doped La2Zr2O7 pyrochlore supported on ZrO2 CGO-coated Ni Fe/MgO Al2O3

Reforming reaction

Catalyst system

Extraction with supercritical CO2 at 40 – 50 °C, 100 – 180bars

Removal of sulphur from feed.

Removal of sulphur from feed.

Removal of sulphur from feed.

In-situ hydrogen reduction.

800 °C in Ar for 2 h followed by steam oxidation (2.4% H2O in Ar) for 5–10 h and H2 reduction (10% H2 in Ar) for 1 h. H2 reduction (10% H2 in Ar) for 10 mins, autothermal conditions. Oxidation in air followed by reduction in H2 at 900 °C.

Calcination in air at 1000 °C for 12 h, or 300 °C for 72 h.

Calcination in air at 900 °C.

Calcination in air for 2 h at 850 °C.

High pressure setup; supercritical CO2 dissolved sulphur compounds from catalyst monolith surface.

Sintering observed and activity could not be fully recovered; noted that Fe catalyst recovered more than Ni catalyst. Ni0.375Co0.375Mg0.25Al2O4 catalyst showed marked increase in activity after regeneration, which was better than fresh material. Authors note that sulphate species in catalyst converted to sulphides, which adsorb weakly at high temperatures. Poisoning was fully reversible after regeneration; irreversible only when Ce2O2S was formed. Nearly full recovery of catalyst activity within 1 – 2 h of sulphur removal from feed. Alteration in selectivity and yields of catalyst was noted after regeneration.

Activity of catalyst fluctuates after regeneration cycles, but overall performance is not significantly affected. Slight increase in catalyst activity after several regeneration cycles, attributed to attrition (smaller particles) and increase in surface area. Steam oxidation prevented SO3 and SO4 formation, but catalyst activity could not be fully recovered.

No drop in performance after 3 regeneration cycles.

Oxygen helped to revert Ce2O2S to CeO2 phase.

Calcination in air at 800 °C.

Calcination in air at 800 °C.

Oxidation in air at 600 °C.

Heating to 900 °C recovered 80% of initial activity even at 150 ppm H2S, though inconclusive whether it’s due to desorption of S or increase in reaction rates. Addition of ceria allowed oxidative regeneration of catalyst at low temperatures. Fe species is oxygen carrier in redox reactions during looping.

Remarks

Raise temperature to 900 °C.

Regeneration method

Table 5 List of recently reported regeneration schemes to remove sulphur poisoning for various reforming catalysts.

Sadatshojaei et al [156]

Garbarino et al [153]

Postole et al [148]

Postole et al [53]

Oemar et al [78]

Misture et al [151]

Hu et al [138]

Kantserova et al [64]

Shafiefarhood et al [139] Morales et al [107]

Laosiripojana et al [127] Yao et al [119]

Keller et al [67]

Izquierdo et al [155]

Yang [154]

Reference

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reactor contained within a larger setup for biomass gasification. They showed that the competing reactions effected by each impregnated material complemented each other very well. The Ni/olivine catalyst cleaned up the steam and tar content in the syngas via steam reforming reactions, while the ZnO/olivine absorbent desulphurised the gas product. The explanation for the improved performance was that the consumption of steam by the reforming reaction resulted in more efficient desulphurisation by the absorbent, and this in turn inhibited sulphur poisoning of the catalyst and allowed a synergistic effect to take place [182]. In addition, the same authors also showed that both catalyst and absorbent worked well in the same temperature ranges (600–800 °C), and the absorbent was effective in removing not only H2S, but also thiophenes as well [183]. On the other hand, Sharifi et al proposed the use of a zeolite, HZSM-5, for the concurrent desulphurisation and aromatisation of heavy naphtha for gasoline production. Their process helped to cut down on capital costs as both aromatisation and desulphurisation took place in the same reactor, however the product was only partially reduced in sulphur (from 250 ppm in the feed to 64 ppm), suggesting that a (smaller) polishing step is still needed downstream before the gasoline can be used for reforming reactions [184]. Iranshahi et al combined naphtha reforming, which is an endothermic reaction, with an exothermic reaction (sulphur dioxide oxidation) in a chambered reactor to enable both reactions to benefit from each other [185]. Their results suggest the possibility of taking sulphur dioxide produced from a prior desulphurisation step for the production of sulphur trioxide (and subsequently sulphuric acid), which generates heat that can be used to drive reforming reactions. This serves as an alternative method to dispose of the sulphur, and also potentially helps to reduce energy costs. The design of the reforming reactor also helps to ensure that the effects of sulphur poisoning are kept to a minimum. Proper catalyst positioning in the reforming reactor can help to alleviate coking and sulphur poisoning to a certain degree. Based on their analysis of temperature and poisoning profiles in a fixed bed reforming reactor, Smith et al suggested that placement of a nickel-based combustion catalyst near the inlet helps to take advantage of the high temperatures in the zone to prevent sulphur deposition. Further down the reactor, the temperature drops and poisoning becomes more prevalent, hence they introduced an Rh-based catalyst near the outlet to ensure continued and sustained activity throughout the reactor for reforming reactions. The placement of the three catalysts in the reforming reactor, as proposed by Smith et al, is shown in Fig. 11. This tailored approach not only prevents significant sulphur poisoning of the overall catalyst bed, but also lowers the costs by only utilising expensive noble metal catalysts in certain regions of the reactor where fouling is most likely to occur [186].

Fig. 10. Some of the more common feedstock sulphur control mechanisms. 1) Absorption, 2) Feedstock self-cleaning, and 3) Upstream control. These mechanisms are summarised from recent studies as published in the literature [156–170].

number of the feed [169]. Wang et al utilised the hot char of a municipal solid waste pyrolysis process to condition the volatile products (bio oil and syngas) obtained from the pyrolysis process itself. The hot char self-cleaning process was shown to be able to improve overall syngas yields, as well as decrease the SO2 content in the syngas product as well [170]. These self-cleaning methods can potentially simplify the pre-treatment process, and achieve higher atom efficiencies by ensuring that the raw materials are utilised to the fullest extent possible. Last but not least, Materazzi et al showed that the use of a plasma gasifier helped to reduce tars and problematic sulphur content in the gas product, which in turn reduces complications associated with downstream reforming reactions. They showed that the plasma converted practically all of the organic sulphur into H2S, which was easier to remove using an absorbent. Sulphates in the system were converted to H2S and COS as well, which again were easier to handle in a downstream cleaning stage [171]. 4.5. Process integration and reaction conditions Optimisation of the reaction conditions and process integration goes a long way towards mitigating the effects of sulphur poisoning of the reforming system. Proper operating conditions in the reforming reactor can delay the poisoning rates and prolong catalyst lives, and coupling of complementary reactions to the reforming reaction allows it to benefit from synergies that are otherwise absent in individual and isolated operations. In the context of sulphur resistance, there are a few common operating parameters that can be varied in reforming systems to control or eliminate the degree of poisoning. These include operating temperatures, catalyst regeneration frequencies, steam and/or oxygen ratios etc. [172,173]. Multiple studies indicate that higher operating temperatures in the reforming reactor prevent (or at least retard) the poisoning of Ni-based catalysts by sulphur [174–178]. The presence of an oxidative component in the reforming reactor also helps to remove sulphur deposits; the oxidising agents that are most commonly used are steam or oxygen in small quantities [179–181]. Additionally, integration of the reforming section with various other supporting reactions may also indirectly help to increase sulphur resilience and reduce overall costs. Wang et al separately impregnated nickel and zinc oxide onto olivine, and then combined the Ni/olivine (catalyst) and ZnO/olivine (absorbent) materials into a syngas cleaning

5. Costs associated with strategies for sulphur resistance Ultimately, the biggest driving force for the development of sulphur resilient systems for hydrocarbon reforming is the monetary costs associated with catalyst deactivation and product cleaning. Since reforming catalysts may contain some semi-rare transition or heavy metals, the costs of frequent catalyst purchasing/replacement can be quite high. In addition, the costs for safe disposal of the materials can be a concern as well. Substitution of the easily-deactivated transition metals with rarer noble metals should always entail a proper cost/benefit analysis, to ensure that the increase in yields and reductions in costs (reactor sizes, separations, regeneration, pre-treatment etc.) should be economically worth the huge increase in catalyst prices. The same considerations also apply to any other countermeasures taken against sulphur resistance. Unfortunately, a lot of the operating and capital cost data associated with current industrial reforming plants is rather closely guarded commercial information. To make matters worse, geographical differences across industrial landscapes also add complexity to cost/benefit analyses, since a lot of preliminary studies of the economics of a process (as is normal for technologies in the early development stages) depend a 65

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proposed solutions have been multi-pronged, with different groups approaching the issue from unique angles to tackle the problem. These proposed methods can be broadly classified into several categories, including incorporation of various elements into the reforming catalyst compositions; engineering of catalyst structures to maximise beneficial sulphur resistant features into the materials; prediction of catalyst deactivation and their proper regeneration regimes; feedstock pre-treatment to prolong the lifetime of the catalysts; and optimisation of reaction conditions and rational designs of reforming reactors. By incorporating sulphur-resistant elements into a reforming catalyst, the operating lifetime and performance of the catalyst can be enhanced. These elements improve catalyst sulphur resistance through various mechanisms. They mostly help to prevent sulphur deposition, increase poisoning tolerance or enhance active site behaviours against sulphur deactivation. Rare-earth (La, Ce, Gd), alkaline (K) and alkalineearth (Mg, Ca, Ba) metals help to prevent sulphur deposition mainly through oxidation and scavenging actions. Noble metals (Ru, Rh, Pd, Pt) enhance the sulphur tolerance of the catalyst and enable them to operate under mild-to-medium sulphur poisoning conditions. Transition (Fe, Co, Cu, Mo) and precious (Au) metals interact with the active sites in the catalyst to give indirect sulphur resistance to the catalyst. The physical structure of reforming catalysts also plays a key role in determining its sulphur resistance properties. Perovskite materials, core-shell catalyst structures and alloys have been shown to impart sulphur resistance to reforming catalysts. Core-shell structures prevent sulphur contaminants from accessing the sensitive active site in the catalyst, while perovskite materials and alloys modify the electronic properties of the active site to facilitate sulphur desorption from the surface. Catalyst surface engineering and novel syntheses also allow catalysts with traditional compositions to gain sulphur-resistant properties that are otherwise absent in conventional formulations. Extrinsic improvements to reforming systems also play a key role in ensuring that the catalysts are tolerant towards poisoning conditions. Poisoned catalysts can be regenerated by further increasing the temperature; however the trend has been to design self-regenerable catalysts that recover their activity upon removal of sulphur species in the feed. The introduction of small quantities of oxygen or steam into the reactor can also help to convert deposited sulphur into sulphates, which are easier to remove. Some unconventional methods such as supercritical CO2 extraction of the sulphur species have also been proposed to treat poisoned catalysts as well. Feedstock cleaning can help to prolong the lifetime of reforming catalysts before a regeneration step is required. Several desulphurisation techniques are available. One of the most common cleaning technologies is absorption, where the sulphur components in the feed are removed by absorption or adsorption prior to entering the reactor. There are also studies into the use of components within the feedstock itself to react with and sequester the sulphur species before reforming takes place (self-cleaning technologies). Another feedstock cleaning method is to prevent the sulphur contaminants from existing in the feedstock in the first place. This is usually achieved by carefully controlling the gasification or digestion steps to ensure that volatile sulphur compounds are kept at a minimum within the feedstock. Last but not least, the importance of engineering approaches should not be overlooked in the design and operation of reforming systems. Heat integration, operating cycles, reactor design, catalyst placement and other techniques are also valid methods to enhance the tolerance of sulphur within the reforming system. Desulphurisation and regeneration can inflict a high economic cost on syngas production systems. While the actual costs associated with sulphur resistance in industrial settings are often not easy to obtain from literature, some have suggested that the capital cost component in reforming plants is usually the largest expenditure. In addition to those mentioned above, there is also an emerging new paradigm that proposes the concept of embracing sulphur as a reactant

Fig. 11. Tailored catalyst placement in reforming reactor to increase efficiency and minimise sulphur poisoning. Figure adapted from Smith et al [186].

lot on assumptions that may or may not be widely applicable. However, based on some scant economic data as reported by some of the industrial plants, we can infer, model and deduce how changes in the operating conditions, feedstock, catalyst compositions etc. can affect the economics of existing processes. ASPEN Plus and HYSYS are popular tools for the modelling and economic evaluation of hypothetical processes. Researchers obtain kinetic, thermodynamic and other operational data from experiments, and use them to simulate their process in different degrees of detail. These tools enable rapid screening of the effects of process variables on reforming performance, and can give rudimentary economic evaluation of a designed process based on information in inbuilt databases. For example, Tjaden et al studied the effects of various fuel reforming options on the performance of a small scale biogas SOFC plant. They showed that steam reforming gave the highest efficiency for electricity production, and autothermal reforming had less robustness and was unable to tolerate significant fluctuations in biogas compositions [187]. Lanzini et al also conducted preliminary analyses on the cleaning of biogas and how the feed contaminants affect costs for SOFC electricity production. They report that apart from HCl, all other poisoning components contributed to significant degradation of SOFC performance and had to be removed prior to the reforming step. They estimated the costs of removal for these contaminants (mainly H2S and siloxane) to be approximately $0.02 per kWh electricity generated, which was around 22% of the total price of each kilowatt-hour supplied to medium industrial users. This indicated that feed cleaning and sulphur resistance are major cost sinks for SOFC electricity generation from renewables [188]. Analyses like these can help to guide research directions and targets, by identifying the largest problems in chemical processes. Giarola et al gives an excellent summary of the current and target costs for SOFC electricity generation, and compared it against conventional internal combustion engines and microgeneration turbines. They showed that SOFC was by far the most expensive technology at the moment, with the fixed costs being the largest cost components in the process. Furthermore, the use of SOFC for electricity generation also required the installation of gas cleaning units, which added nearly $1000 in capital costs per kilowatt capacity to the overall costs [189]. It is envisioned that the introduction of effective sulphur resistant catalysts or setups in SOFC will not only eliminate operating (i.e. gas cleaning and catalyst regeneration) costs, but also reduce the overall capital costs by increasing the lifetime of nearly all components in the reforming process. 6. Summary and outlook To summarise, there have been many recent advances in mitigating of the effects of sulphur deactivation in reforming reactions. The 66

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Fig. 12. Potential future routes to utilising sulphur as a reactant. Steam reforming and sulphidation (either partial of full) may enable the production of green energy from a catalyst poison [189–196].

in itself, rather than as a nuisance or poison. This is termed as the “sulphur tempering” of catalysts, to increase their resilience under sulphur contamination as well as tweak their activities and selectivities towards certain favoured reactions. Based on a good understanding of the poisoning mechanisms of sulphur, it may be possible to selectively neuter some of the more active sites in a catalyst and avoid unwanted runaway reactions, or inhibit coke formation arising from the high activities of the catalyst. Weber et al. show the use of a sulphided catalyst (MoS2) for converting syngas to various alcohols. The catalyst was highly stable in the presence of various sulphur components, even at low temperatures. It was also shown that doping of alkali onto the catalyst promoted the conversion of H2S into various mercaptan species in the product [190]. In a similar vein, sulphur can also help to inhibit the deactivation of catalytic surfaces by other more destructive elements such as potassium. Moud et al. showed that the adsorption of a small amount of sulphur onto a Ni-based catalyst inhibited the uptake of gas phase potassium (originating from the prior gasification step) on the surface of the catalyst [191]. Perhaps more interestingly, there are efforts to utilise sulphur compounds, more specifically H2S, as a reactant analogous to H2O, as shown in Fig. 12. These typically resemble steam reforming or partial oxidation reactions, with the substitution of sulphur for oxygen in the reactants and products [192]. Baltrusaitis et al explored the possibility of partially sulphiding methane using H2S to obtain methanethiol (CH3SH), which is the sulphur analogue of methanol. They suggested that sulphided catalysts such as RuS or NiS, or sulphur resistant catalysts such as FeGaO3 can be used to conduct the partial sulphidation reaction [193]. Martinez-Salazar et al also propose something very similar, the H2S reforming of methane to obtain carbon disulphide (CS2) and hydrogen gas. The catalysts used were variations of molybdenum and/or chromium on lanthana, zirconia and SBA15, and operating in the temperature range of 800–1100 °C [194,195]. AuYeung and Yokochi also proposed the rather unconventional concept for the steam reforming of H2S to produce sulphur dioxide and hydrogen gas. The reactions take place at relatively high temperatures (around 900–1500 °C), catalysed by a molybdenum metal wire [196]. The high temperatures mean that the technology will likely ever only benefit from the utilisation of renewable energy sources such as solar power, though the concept itself is admittedly quite intriguing and may provide some inspiration for future research to consider sulphur compounds as a feedstock rather than as a poison. Acknowledgements This work was funded by National Research Foundation, Singapore under the Prime Minister’s Office, and National Environment Agency, Singapore under the Waste to Energy Competitive Research Program (project number WTE-CRP-1501-103). 67

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