Cr-free Fe based catalysts for high-temperature water-gas shift reactions

Cr-free Fe based catalysts for high-temperature water-gas shift reactions Xúc tác Cr-Fe nhiệt độ cao cho phản ứng water-gas shift Các phản ứng thay đổi nước và khí (WGSR) mô tả phản ứng của carbon monoxide và hơi nước để tạo thành carbon dioxide và hydrogen (hỗn hợp của carbon monoxide và hydrogen được gọi là khí nước)Các phản ứng thay đổi khí nước được phát hiện bởi nhà vật lý người Ý Felice Fontana trong năm 1780. Mãi đến sau này nhiều mà giá trị công nghiệp của phản ứng này đã được thực hiện. Trước những năm đầu thế kỷ 20, hydro thu được từ phản ứng của hơi nước dưới áp lực cao với sắt để sản xuất sắt, oxit sắt và hydrogen. Với sự phát triển của quá trình công nghiệp mà yêu cầu hydro, ví dụ như các HaberBosch tổng hợp amoniac, nhu cầu về một phương pháp ít tốn kém và hiệu quả hơn trong sản xuất hydro là cần thiết. 1 Như một giải pháp cho vấn đề này, các WGSR được kết hợp với các quá trình khí hóa than để sản xuất một sản phẩm hydro tinh khiết. Vì lý tưởng của nền kinh tế hydrogen tăng phổ biến, tập trung vào hydro như một nguồn nhiên liệu thay thế cho các hydrocacbon ngày càng tăng.

Catalysis Today 210 (2013) 2–9 Contents lists available at SciVerse ScienceDirect Catalysis Today journal homepage: www.elsevier.com/locate/cattod The review of Cr-free Fe-based catalysts for high-temperature water-gas shift reactions Dae-Won Lee a , Myung Suk Lee a , Joon Yeob Lee a , Seongmin Kim a , Hee-Jun Eom a , Dong Ju Moon c , Kwan-Young Lee a,b,∗ a Department of Chemical and Biological Engineering, Korea University, Anam-dong, Seongbuk-gu, Seoul 136-713, Republic of Korea Green School, Korea University, Anam-dong, Seongbuk-gu, Seoul 136-713, Republic of Korea c Clean Energy Research Center, Korea Institute of Science and Technology (KIST) 39-1, Hawolgok-dong, Seongbuk-gu, Seoul 136-791, Republic of Korea b a r t i c l e i n f o Article history: Received 31 August 2012 Received in revised form 20 December 2012 Accepted 21 December 2012 Available online 20 February 2013 Keywords: Water-gas shift reaction High-temperature shift Fe/Cr catalysts: Cr-free catalysts a b s t r a c t Since it was patented by Bosch and Wild at 1914, the Fe/Cr-based mixed oxide catalyst has been used for water-gas shift reactions (WGSRs). Until the present, this catalyst has been used as the primary catalyst for industrial high-temperature shift (HTS) reactions. However, because environmental concerns about chromium elements were raised in the early 1980s, the replacement of chromium in HTS catalysts has been intensely studied by many groups. These studies have contributed notable insights into HTS catalysis using Fe-based oxides, especially about the reaction mechanism and functions of promoter elements. In some cases, the potential of using a substituent metal previously neglected because of properties inferior to those of chromium was rediscovered after noteworthy improvements were produced by combining it with other metals in promoting the Fe-oxide catalyst. This paper reviews the recent studies of Cr-free Fe-based HTS catalysts, especially focusing on the roles and functions of the non-chromium promoters in the catalysts. © 2013 Elsevier B.V. All rights reserved. 1. Water-gas shift reaction: general considerations Water-gas shift reaction (WGSR) is a redox-type reaction to convert carbon monoxide and water vapor into carbon dioxide and hydrogen (Eq. (1)), which was first discovered by an Italian physicist Felice Fontana in 1780 [1]. CO (g) + H2 O (v) ↔ CO2 (g) + H2 (g) [ H 0 = −41.1 kJ/mol] (1) WGSR is now mostly associated with the steam reforming of hydrocarbons (natural gas, petroleum gas, naphtha, gasoline, coals and various types of biomass) to produce hydrogen for use in the synthesis of ammonia and methanol and for the Fischer–Tropsch process [2–4]. WGSR generates additional hydrogen using gases remaining after steam reforming, which is generally used to optimize the H2 /CO molar ratio optimal for the production of (liquid) hydrocarbons in the Fischer–Tropsch process. In polymerelectrolyte membrane fuel cells (PEMFC) systems, it is used to remove carbon monoxide, which poisons the electrode catalysts [5,6]. Through WGSR, the carbon monoxide content is reduced from 10–15% to 0.5–1%, which is then further reduced to trace levels ∗ Corresponding author at: Department of Chemical & Biological Engineering, Korea University, Anam-dong, Seongbuk-gu, Seoul 136-713, Republic of Korea. Tel.: +82 2 3290 3299; fax: +82 2 926 6102. E-mail address: kylee@korea.ac.kr (K.-Y. Lee). 0920-5861/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cattod.2012.12.012 ( 89.2% [21] Fe/Ni (67/33) S/C = 2.6 S/G = 0.3 2.0 [0.025 m3 /gcat /h] 1 400 ◦ C: 64% [65%] Selectivity > 88.8% [21] Fe/Ni/Cs (66/31/3) S/C = 2.6 S/G = 0.3 2.0 [0.075 m3 /gcat /h] 1 400 ◦ C: 61% [65%] Selectivity = 100% [66] Fe/Ni/Zn (64/31/5) S/C = 2.6 S/G = 0.3 2.0 [0.030 m3 /gcat /h] 1 400 ◦ C: 65% [65%] Selectivity > 97% [67] Fe/Ni/Alg (37/22/41) S/C = 3.0 S/G = 0.4 1.4d 14,500 h−1 d (10,000 h−1 , dry gas base) 1 400 ◦ C: 54% [57%] [32] Fe–Ni/Ce–Zr (11–10/53–26)d S/C = 3.7 S/G = 0.6 1.1d 15,600 h−1 d (10,000 h−1 , dry gas base) 1 400 ◦ C: 75% [70%] Slight methanation [68] a b c d e f g Steam/CO ratio. Steam/gas ratio. “Wet gas” base. Estimated from the data given in the paper. Prepared with co-precipitation. Prepared with sol–gel method. Prepared by solution-spray plasma method. the U.S. National Research Council published the general guidelines for chromium compound risk assessments in 1983, the EPA has published many practical guidelines for the identification and assessment of hexavalent chromium [52]. The Occupational Health and Safety Administration (OSHA) under the U.S. Department of Labor enforced strict regulations regarding worker exposure to hexavalent chromium in several industries [53]. In Europe, the recently published European Restriction of Hazardous Substances (RoHS) banned the use of six hazardous materials, including hexavalent chromium, in all electronic–electrical equipment [54]. It is only a matter of time before these regulations are expanded to cover entire industries. Returning to the Fe/Cr HTS catalyst, the chromium species in a fresh Fe/Cr catalyst is usually Cr+3 (Cr2 O3 ), which is much less toxic than Cr+6 . The Cr+6 content is low, but workers must take precautions when handling the catalyst throughout the span of the operation. Moreover, Cr+6 is water-soluble and is leached from the catalyst by condensed steam or cold water, which could be a threat to the environment, even with minimal disposal [51]. There are several possibilities for producing hexavalent chromium during the manufacturing of the catalyst. For instance, some of the Cr+3 ions that were not precipitated can be oxidized into Cr+6 when the catalyst is calcined at high temperature with the mineral base (Na+ ) present in the precipitates [51]. The motivation for replacing chromium with other elements mostly lies in these environmental and health concerns [55]. In addition to WGSR, the issue of replacing chromium is being similarly discussed in the development of catalysts for fatty alcohol production (the hydrogenation of fatty esters) [56,57]. 3. Studies on Cr-free Fe-based HTS catalysts Note: The HTS activities of Cr-free and Cr-containing catalysts are listed in Tables 2 and 3. It was not straightforward to compare a catalyst with another in activity, because each catalyst had been tested in a different reaction condition (S/C ratio, R factor, w/f, temperature, etc.). So the activities were listed in a table format with full reaction conditions provided. 3.1. Early studies: 1980–1990 Chinchen first tried using a Cr-free Fe-based HTS catalyst to replace chromium with an element capable of forming a spinel structure in iron oxide without unduly diluting the catalytic activity [58]. The chrome replacements were chosen according to ionic size and oxidation number [28,58]. Among the candidates, Ca, Ce and Zr were capable of forming spinel structures with Fe. Fe/Ce and Fe/Zr showed higher surface areas than the commercial Fe/Cr catalysts, but their specific activities (activity per total catalyst weight) were lower than those of commercial catalysts. A similar attempt by Rethwisch and Dumesic was also unsuccessful. They tried using Zn(II) and Mg to replace Cr, but the activities of Fe/Zn and Fe/Mg were 30 times lower than that of magnetite [59]. It was argued that the element (Zn or Mg) displaced all of the Fe2+ ions from the octahedral site of inverse spinel lattice, preventing Fe2+ /Fe3+ redox transfers, which are the driving force of the regenerative mechanism for magnetite catalysis. Rethwisch et al. dispersed un-promoted magnetite over a graphite support (Fe3 O4 /C) to enhance the catalytic turnover rate 6 D.-W. Lee et al. / Catalysis Today 210 (2013) 2–9 and decelerate the thermal agglomeration of magnetite [60]. The catalyst initially showed high activity, which then decreased after a few hours. The magnetite agglomerated during the reaction, but the authors argued that the decline in activity was due to the constriction of pores within magnetite clusters, which was driven by the surface hydroxylation of magnetite under wet atmospheres. The activity recovered to some degree after the catalyst was dehydroxylated under a dry carbon monoxide atmosphere. 3.2. Fe/Al-based catalysts: Al/Ce or Al/Cu as replacements for Cr: 1995–2010 The first promising results for developing Cr-free Fe-based catalysts were arguably those of Ladebeck and Kochloefl [61], who replaced Cr in a Fe/Cu/Cr catalyst with Al/Ce. The resulting Fe/Cu/Al/Ce catalyst showed activity superior to that of a commercial catalyst. Since then, Al has been studied more intensively than any other element as a replacement for chromium in Fe-based HTS catalysts. From now, it will be reviewed in its proper chronological order. Araújo and Rangel proved that the activity promoted by Al becomes more prominent when Cu is included in the magnetite texture [33]. Under low steam-to-gas ratios (S/G = 0.4; estimated R factor = 0.9), the Fe/Al/Cu catalyst was similar in HTS activity but better in selectivity (i.e., methanation suppression) compared to a commercial Cr-containing catalyst. The authors argued that Al/Cu promotes the formation of the magnetite phase during prereduction and stabilizes the phase against further reduction. They deemed that Cu acted as a textural promoter rather than a functional promoter (creating or promoting activity by affecting the electronic properties of the major active species [23]). However, the thermal stability of the Fe/Al/Cu catalyst was not within the scope of the study. Liu et al. studied an Al/Ce-promoted Fe catalyst (Fe/Al/Ce), which was given a proprietary name, NBC-1 [62,63]. The basic idea was to adopt ␥-Fe2 O3 (maghemite) as the backbone of the catalyst, which was thought to be more effective than ␣-Fe2 O3 in incorporating promoter elements, by utilizing the vacant sites of an imperfect spinel structure [64]. From the context, it is inferred that the authors regarded Al and Ce both as textural promoters for the magnetite phase. The authors claimed that the catalyst was active and thermo-resistant, by showing that it was comparable to a commercial Fe/Cr catalyst in both HTS activity and specific surface area measured after high-temperature aging (530 ◦ C, 15 h). The achievement would have been more promising if the catalyst had been tested under more reducible conditions. (The R factor of the reaction gas is estimated to be 0.6, which is much lower than the conventional value, 1.0.) Regarding the Fe/Al/Cu HTS catalyst, the Ozkan group have published several noteworthy studies in the last decade [28–31], providing a systematic understanding of the catalyst. Using in situ XRD and TPR studies, the group proved that Al played a role similar to that of Cr, inhibiting the thermal growth of the magnetite phase and stabilizing the phase against further reduction to FeO and Fe [28]. However, the XPS studies indicated that Al did not promote the redox rate of the iron oxide because there was no change in its oxidation state (Al+3 ) during WGS catalysis. Unlike Al, Cr changed its oxidation state (+3 ↔ +6) during catalysis, which was thought to allow it to act as a functional promoter in promoting the WGS activity of the Fe catalyst. Similarly to the results of Araújo and Rangel’s study [33], the HTS activity was greatly enhanced when Al and Cu were included in the catalyst together as Fe promoters (Table 3). Cu was considered a functional promoter for the Fe/Al catalyst because TPR analysis indicated that Cu greatly enhanced the reducibility of the iron oxide [28]. The authors claimed that there are two ways for Cu to participate in the catalysis: (1) the Cu Fig. 2. BET surface areas of Fe/Ni catalysts. FNxxyy denotes a Fe/Ni catalyst with Fe and Ni in xx and yy wt.%, respectively; “Aged” implies that the catalyst was aged with reaction for 3 h under 400 ◦ C, H2 (56.7%), CO (10%), CO2 (6.7%), H2 O (26.7%) and WGSV = 0.025 m3 /gcat /h. species serves as an electronic (functional) promoter and promotes the redox rate of the catalysis and (2) the excluded Cu species is present on the catalyst surface, which is reduced to metallic Cu during the reaction and provides additional active sites, similar to Cu in the Cu/Zn/Al catalyst in LTS reactions. However, because metallic Cu is very prone to thermal sintering, it is desirable to incorporate all Cu species into an iron oxide structure and form a perfect solid solution [29]. The sol–gel preparation of the catalyst fulfilled this objective. In the author’s following papers [29,30], it was proven that Cu was uniformly distributed over the iron oxide matrix when the catalyst was prepared using the sol–gel method at pH 9 with iron acetylacetonate as the Fe precursor (others in nitrates), and C2 H5 OH/NaOH was used as the solvent/precipitant [29]. Such uniformity was not obtained using the conventional co-precipitation method. The sol–gel method is more advantageous in that it allows the formation of ␥-Fe2 O3 to be induced by adjusting the Fe2+ /Fe3+ ratio in the precursor solution and the aging time. The TPR and XPS measurements confirmed that ␥-Fe2 O3 helps incorporate the promoter elements (Al and Cu) into the iron oxide structure to form a uniform solid solution [29]. The authors further improved the preparation method using propylene oxide as the gelation agent, which improves the HTS activity and stability for the Fe/Al/Cu catalyst [31]. 3.3. Fe/Ni-based catalysts: HTS catalysis of Fe/Ni/Zn and Fe/Ni/Cs under high-R-factor conditions: 2009–2011 Ni has been perceived as unsuitable for use as a component of WGS catalysts because it is easily reduced under WGS conditions and manifests high methanation activity [65]. However, our research group has found that Ni is capable of forming a solid solution with iron oxide, producing a Fe/Ni catalyst that exhibits reasonable HTS activity even under highly reducible conditions (R factor = 2) if promoted by another appropriate element [20,21,66,67]. Ni/Fe catalysts were prepared by conventional co-precipitation, which produced inverse spinel NiFeO4 after calcination in air at 500 ◦ C. The inclusion of Ni increased the surface area of the fresh catalyst, but the effect was drastically diminished when the catalyst is aged in HTS reaction (Fig. 2), so it is technically improper to refer to Ni as a textural promoter. Ni can instead be referred to as a functional promoter: under a high R factor of 2, the Ni/Fe (67/33 in mol% (Table 3) or 66/34 in wt.% [21]) catalyst showed high initial CO conversion (64%), close to the equilibrium value (65%), whereas the commercial Fe/Cu/Cr catalyst showed only 50% conversion [21]. TPR measurements confirmed the D.-W. Lee et al. / Catalysis Today 210 (2013) 2–9 Fig. 3. HTS activities of Cs-promoted Fe/Ni catalysts; FN: Fe/Ni (66/34 in wt.%), xCsFN: x wt.%-Cs impregnated Fe/Ni (66/34 in wt.%); H2 (56.7%), CO (10%), CO2 (6.7%) and H2 O (26.6%), R factor = 2; 400 ◦ C; WHSV = 0.075 m3 /gcat /h [66]. Ni-enhanced redox rate of iron oxides [67]. Even with a high R factor (R = 2) and at high temperature (400 ◦ C), the catalyst managed to maintain its initial activity for over 11 h. However, part of the catalyst was reduced to FeNi3 (awaruite) during the reaction, which is attributed at least in part to the methanation side reaction [21]. Methane was produced from both CO and CO2 [21], from which the selectivity of the HTS reaction over the Fe/Ni catalyst is estimated as 85–90% (Table 3). The problem of low selectivity, that is, the occurrence of methanation, was overcome by promoting the Fe/Ni catalyst with cesium [66] or zinc [67]. By impregnating Cs on the Fe/Ni catalyst, the HTS activity was greatly enhanced and the methanation was effectively restrained (Fig. 3) [66]. The catalysts were tested under a weight hour space velocity (WHSV = 0.075 m3 /gcat /h) three times higher than that used in the previous study (Table 3). Because of the increase in WHSV, the CO conversion of un-promoted Fe/Ni (NF, in Fig. 3) was almost halved to 32%. Under such adverse conditions, the Fe/Ni/Cs catalysts (3.9CsNF and 6.0CsNF, in Fig. 3) showed near-equilibrium CO conversion (63% and 61%) with almost 100% selectivity. Based on CO2 -TPD analysis, the improvement of the catalytic performance was attributed to the increase in the number of weakly basic sites by Cs promotion, on which the formate-intermediated associative mechanism was thought to progress. Zn promotion also enhanced the HTS activity of the Fe/Ni catalyst [67]. Zn was co-precipitated with Fe and Ni to form a solid solution of (Zn,Ni)Fe2 O4 inverse spinel species. The Zn-promoted Fe/Ni (Fe/Ni/Zn) showed near-equilibrium CO conversion with excellent methanation restraint (selectivity over 98%, Table 3), which was similar to the previous Cs-promoted Ni/Fe catalyst. The catalyst showed very stable performance, maintaining its activity over 15 h. However, Zn promotion is thought to be inferior to Cs promotion in terms of activity enhancement because such a level of activity was obtained under a WHSV of 0.035 m3 /gcat /h, whereas Fe/Ni/Cs achieved a similar level of activity under a nearly doubled WHSV of 0.075 m3 /gcat /h (Table 3). Zn performs the role of functional promoter for the Fe/Ni catalyst very well: first, Zn prevents the reduction or disintegration of the inverse spinel phase during reaction. Through time-dependent XRD analysis, it was found that FeNi3 was formed from the disintegration of an unstable, incomplete layer of zinc–nickel ferrite near the catalyst surface. When the unstable layer was used up, the core crystal of (Zn,Ni)Fe2 O4 was intact, and the reduced phase (FeNi3 ) did not grow further throughout the rest of reaction [67]. Second, Zn enhanced the reducibility of the catalyst, promoting CO oxidation with lattice oxygen, which 7 leads to an increase in the WGS rate and selectivity. The improved reducibility of Fe/Ni/Zn was confirmed by H2 -TPR and CO-TGA measurements [67]. Watanabe et al. showed that combining Fe/Ni with Al resulted in excellent HTS activity without significant methanation [32] (Table 3). The authors prepared the Fe/Ni/Al catalyst with the solution-spray plasma technique to produce Fe/Ni species welldispersed on the hollow Al2 O3 sphere. During the reaction, the Fe/Ni species were partially reduced to Ni–Fe alloy (FeNi3 ), which the authors noted was a crucial species in suppressing hydrogen adsorption and CO methanation. The catalyst showed the best performance in terms of HTS activity and methanation suppression when the Fe/(Fe + Ni) atomic ratio was between 0.5 and 0.8. The authors developed this idea into dispersing Fe/Ni species on the mesoporous CeO2 –ZrO2 support prepared by the “hardtemplate method” using KIT-6 as a template material [68]. The purpose of this study was also to minimize methanation over the Fe/Ni species. The basic ideas were, first, to improve Ni (or Fe–Ni) dispersion using a support with a large specific surface area, and second, to use a reducible oxide support that improves the transfer rate of lattice oxygen (in order to suppress methanation and increase the selectivity). Both requirements were simultaneously satisfied by impregnating Fe/Ni species on the mesoporous CeO2 –ZrO2 prepared by the hard template method. The catalyst showed improved thermal stability, HTS activity and methanation suppression compared to the catalyst prepared using conventional, co-precipitated CeO2 –ZrO2 support. In particular, the formation of FeNi3 in a highly dispersed state over Fe–Ni/CeO2 –ZrO2 (hard template) led to a more effective suppression of methanation. 3.4. Other noteworthy studies: 1998–2011 Costa et al. studied the use of Th as a replacement for Cr in ˚ Fe/Cr/Cu catalysts [26]. Because the ionic radius of Th4+ (0.94 A) ˚ Th4+ was not incoris considerably larger than that of Fe3+ (0.69 A), porated into the iron oxide matrix; instead, it went to the surface, forming a segregated phase. However, the presence of Th resulted in the formation of smaller iron oxide particles and hindered the thermal sintering of the particles. In addition, although Th was present at the surface, it stabilized the magnetite phase against deeper reduction. Hence, Th can be categorized as a textural promoter for Fe-based HTS catalysts. Except for its presence on the surface, Th is almost identical to Al in its characteristics and role as a promoter. Like Al, its activity is also largely enhanced by the use of Cu as a co-promoter. The author claimed that the Fe/Th/Cu catalyst is more active than a commercial Fe/Cr/Cu catalyst at 370 ◦ C, S/G = 0.6 (S/C = 6) and R factor = 0.8. Júnior et al. tried using V(IV) (vanadium) as a chrome replacement [19]. In this study, vanadium-doped magnetite was prepared by heating sol–gel-prepared, iron (III)–vanadium (IV) hydroxoacetate under nitrogen. Because magnetite was produced directly with this method, pre-reduction was not needed when using this catalyst in the HTS reaction. Vanadium was located mainly on the surface as V2+ and V5+ species. There is some doubt about the claim that vanadium acted as a textural promoter for the magnetite phase because the specific surface area of V-doped magnetite was already small (25–28 m2 /g) in the fresh state and the difference from that of un-doped magnetite was marginal. However, the vanadium stabilized Fe3+ and increased the activity and selectivity of the magnetite phase, implying that it acts as a functional promoter. Martos et al. studied Fe/Mo(VI)/Cu as a Cr-free HTS catalyst, using the oxidation-reduction method to prepare the catalyst [23]. Mo6+ was incorporated perfectly into the magnetite structure due to its ˚ compared to Fe3+ (0.69 A). ˚ Although smaller ionic radius (0.62 A) the catalyst was prepared without thermal calcination, the specific surface area of Fe/Mo was quite small (32 m2 /g). However, a 8 D.-W. Lee et al. / Catalysis Today 210 (2013) 2–9 linear relationship was found between the Mo content and BET area, which implies that Mo is a textural promoter for the magnetite phase. However, the activity enhancement and stabilization of the magnetite phase were obtained when Mo was paired with Cu, which was very similar to the cases of Fe/Al/Cu and Fe/Th/Cu described previously. Boudjemaa et al. [69] examined the influence of acid-base properties on Cr-free Fe-based catalysts in HTS reaction using in situ DRIFT measurements as a major analytical tool. Hematite(later in reaction, magnetite-) supported SiO2 , TiO2 and MgO were used as the catalysts, and the order of activity was related to the basicity of the materials (measured by the activity in isopropanol dehydrogenation): Fe2 O3 /MgO Fe2 O3 /TiO2 > Fe2 O3 (not supported) Fe2 O3 /SiO2 . The high activity of Fe2 O3 /MgO was explained in terms of a formate-intermediated associate mechanism, in which the carbonyl species adsorbed on Fe reacts with hydroxyl groups on the Fe–MgO interface to produce formate intermediates. It was proposed that the decomposition rate of formate species governs the overall reaction rate, which is facilitated by the weaker metal-oxygen bond in the basic oxides. Mahadevaiah et al. incorporated Fe into the CeO2 crystalline network, and the resultant catalyst exhibited impressive performances under LTS and HTS conditions [70]. It is generally accepted that CeO2 is a good redox material for the regenerative mechanism in WGS catalysis. In CeO2 , the lattice oxygen easily interacts with adsorbed CO (turning into CO2 ), and the depleted oxygen site is restored with the release of H2 from H2 O. The WGS activity is further augmented if CO adsorption is promoted by another element. The noble metals (Pt, Pd, Rh) are usually chosen for such purpose [71], but the authors used Fe instead, partially substituting the CeO2 with Fe to form a solid solution of Ce1−x Fex O2−ı . As a result, Fe not only promoted CO adsorption in WGS catalysis but also enhanced the oxygen storage capacity by synergetic redox interaction between Ce4+ /Ce3+ and Fe3+ /Fe2+ [70]. Nearequilibrium CO conversion was achieved using Ce0.67 Fe0.33 O1.835 at temperatures above 450 ◦ C, and the activity was expanded to the LTS region (∼285 ◦ C) when Pt was co-doped inside the catalyst (Ce0.67 Fe0.33 Pt0.02 O1.785 ). Another example of fixing Fe onto a non-magnetite structure was suggested by the work by Sun et al., in which the perovskite structure was utilized as a catalyst matrix for Fe [72]. The basic idea was as follows: in the LaFeO3 perovskite structure, La3+ lowers the binding energy of oxygen in FeO6 octahedra, promoting the transfer rate of lattice oxygen (␣-oxygen) to the adsorbed CO (possibly on Fe) to enhance the WGS rate in a regenerative (redox) mechanism. In addition, the authors adopted a general method to increase the redox property of the perovskite catalyst, substituting another cation (Ce4+ ) for the cations in LaFeO3 . Because of the restriction in ionic radius, Ce4+ is incorporated exclusively into the A-site (i.e., La3+ ), making the perovskite structure non-stoichiometric, which is higher in oxygen storage capacity and thermal stability than stoichiometric LaFeO3 . Hence, the non-stoichiometric La0.9−x Cex FeO3 catalyst is more active in HTS reactions than the LaFeO3 catalyst. In addition, its activity at temperatures above 550 ◦ C was higher than those of commercial Fe/Cr/Cu catalysts operating at 450 ◦ C. The accommodation of Ce4+ in this catalyst was limited to low values (x = 0.2; above 0.2, the perovskite became unstable); however, a small amount of CeO2 was always found in the catalysts. It was claimed that the segregated CeO2 phase also enhances the WGS activity via its intrinsic redox property. 4. Summary and future perspectives We have summarized the results of previous studies of Cr-free Fe-based HTS catalysts, especially those addressing the promoters used to replace Cr in Fe-based HTS catalysts. (1) First, a promoter element should form a solid solution with iron oxides or at least be located in the surface layer in a well-dispersed state. In the Fe-based WGS catalysts, the role of promoter is divided into textural and functional roles. A textural promoter, whether it exists as individual crystallite or fuses into iron oxide lattices, enhances the catalyst microstructure (surface area, porosity, grain size) and behaves as a barrier for thermal growth of iron oxide crystallites (i.e., thermal sintering). A functional promoter enhances the redox rate of the catalyst with its own redox activity or by facilitating the redox cycle of iron oxide. Regardless of role, it is more desirable for a promoter to form a homogeneous solid solution with iron oxides. (2) Chromium in a commercial Fe/Cr or Fe/Cr/Cu HTS catalyst acts as a textural and functional promoter. Among the chrome replacement promoters, Al and Th have functionalities as textural promoters, preventing thermal agglomeration and excessive reduction of the magnetite phase. Ce and Cu function as functional promoters for the magnetite or ‘promoted’ magnetite (e.g., Fe/Cr, Fe/Al, Fe/Th) phases, improving the redox properties of active species, which in turn increases the intrinsic WGS activity of the catalyst. (3) To date, there is no known single elemental promoter that plays a dual role (textural and functional) with a promoting functionality comparable to that of chromium. In developing chrome replacement promoters, the most effective strategy is to combine more than two non-chromium elements (usually one textural and one functional), in which the matching between elements is very crucial. For instance, Zn is not a proper promoter for iron oxide for itself; it increases the specific surface area but does not increase the WGS activity [23]. However, Zn plays a prominent role as a functional promoter if paired with Ni in promoting iron oxide, which not only improves the WGS activity but also hinders methanation [67]. (4) In general, magnetite-based WGS catalysis follows a regenerative mechanism. Hence, it is used to produce a good result when dispersing magnetite (or promoted magnetite species) over reducible oxide support with superior lattice oxygen mobility. Cerium oxides and their derivatives can be used for such a purpose. (5) The use of base promoters, such as alkaline or alkaline-earth metals, promotes the formate-intermediated associative mechanism in WGS catalysis. This approach could exhibit synergy when combined with other functional/textural promoters for promoting iron oxide catalysts. (6) The immobilization of Fe into a crystalline matrix is sometimes effective in stabilizing the active species (FeOx ) against thermal sintering or excessive reduction. The use of cerium-based perovskite is a good example. Presently, the development of Cr-free HTS catalysts is an important, ongoing topic in the catalyst industry. We have introduced some studies about Cr-free “Fe-based” HTS catalysts, but the major developmental trend involves noble metal catalysts, which exhibit high activity in a compact catalyst bed and maintain this activity even in oxidative atmospheres [55]. These catalysts are quite promising in specific fields, such as automobile applications. However, Fe-based catalysts are still advantageous in terms of material cost, which is highly desirable for reducing the operation costs of hydrogen plants [73]. Thus, the need for reasonably priced HTS catalysts is providing an impetus for continuous studies of Cr-free Fe-based catalysts. Based on this review, these studies should focus on developing well-matched (non-chromium) promoter groups and crystalline matrixes for the active iron species. D.-W. Lee et al. / Catalysis Today 210 (2013) 2–9 Acknowledgements This work was supported by the Human Resources Development of the Korea Institute of Energy Technology Evaluation and Planning (20114010203050) grant funded by the Korea government Ministry of Knowledge Economy. This work was supported from GTL-FPSO project (KIST, 2M29760) funded by the Korean government, Ministry of Knowledge Economy (Project number: MKE2011T100200023) and Industries (DSME, KOGAS, JNK Heaters, Hy-lok Korea). Dr. Dae-Won Lee is supported by a Korea University grant. References [1] D. Mendes, A. Mendes, L.M. Madeira, A. Iulianelli, J.M. Sousa, A. Basile, AsiaPacific Journal of Chemical Engineering 5 (2010) 111. [2] L. Lloyd, D.E. Ridler, M.V. Twigg, in: M.V. Twigg (Ed.), Catalyst Handbook, 2nd ed., Wolfe Publishing Ltd., London, 1989, pp. 283–292. [3] K. Kochloefl, in: G. Etrl, H. Knozinger, J. Weitkamp (Eds.), Handbook of Heterogeneous Catalysis, vol. 4, Wiley-VCH, Weinheim, 1997, pp. 1831–1843. [4] C.N. Satterfield, Heterogeneous Catalysis in Industrial Practice, 2nd ed., McGraw-Hill, New York, 1991, pp. 442–446. [5] J.J. Baschuk, X.G. Li, International Journal of Energy Research 25 (2001) 695. [6] X. Cheng, Z. Shi, N. Glass, L. Zhang, J. Zhang, D. Song, Z-S. Liu, H. Wang, J. Shen, Journal of Power Source 165 (2) (2007) 739. [7] E.D. Park, D. Lee, H.C. Lee, Catalysis Today 139 (4) (2009) 280. [8] J.M. Moe, Chemical Engineering Progress 58 (2) (1962) 33. [9] D.S. Newsome, Catalysis Reviews: Science and Engineering 21 (2) (1980) 275. [10] G.J. Hutchings, R.G. Copperthwaitet, F.M. Gottschalk, R. Hunter, J. Mellor, S.W. Orchard, T. Sangiorgio, Journal of Catalysis 137 (2) (1992) 408. [11] J.R. Mellor, R.G. Copperthwaite, N.J. Coville, Applied Catalysis 164 (1–2) (1997) 69. [12] D. Andreev, V. Idakiev, T. Tabakova, A. Andreev, R. Giovanoli, Applied Catalysis A: General 134 (2) (1996) 275. [13] V. Idakiev, Z.-Y. Yuan, T. Tabakova, B.-L. Su, Applied Catalysis A: General 281 (1–2) (2005) 149. [14] D. Tibiletti, A. Amieiro-Fonseca, R. Burch, Y. Chen, J.M. Fisher, A. Goguet, C. Hardacre, P. Hu, D. Thompsett, Journal of Physical Chemistry B 109 (47) (2005) 22553. [15] G. Jacobs, L. Williams, U. Graham, G.A. Thomas, D.E. Sparksand, B.H. Davis, Applied Catalysis 252 (1) (2005) 66–74. [16] J.W. Lywood, M.V. Twigg, Eur. Patent Appl., 0361648, 1990. [17] C. Rhodes, G.J. Hutchings, A.M. Ward, Catalysis Today (1995) 43. [18] C. Rhodes, B.P. Williams, F. King, G.J. Hutchings, Catalysis Communications 3 (2002) 381. [19] I.L. Júnior, J.M.M. Millet, M. Aouine, M.C. Rangel, Applied Catalysis A: General 283 (2005) 91. [20] J.Y. Lee, D.-W. Lee, K.-Y. Lee, Y. Wang, Catalysis Today 146 (2009) 260. [21] J.Y. Lee, D.-W. Lee, Y.-K. Hong, K.-Y. Lee, International Journal of Hydrogen Energy 36 (2011) 8173. [22] G.K. Reddy, P. Boolch, P.G. Smirniots, Journal of Catalysis 282 (2011) 258. [23] C. Martos, J. Dufour, A. Ruiz, International Journal of Hydrogen Energy 34 (10) (2009) 4475. [24] Y.S. Oh, H.S. Roh, K.W. Jun, Y.S. Baek, International Journal of Hydrogen Energy 28 (2003) 1387. [25] S.H. Kim, S.W. Nam, T.H. Lim, H.I. Lee, Applied Catalysis B: Environmental 81 (2008) 97. [26] J.L.R. Costa, G.S. Marchetti, M.C. Rangel, Catalysis Today 77 (2002) 205. [27] A.L.C. Pereira, G.J.P. Berrocal, S.G. Marchetti, A. Albornoz, A.O. de Souza, M.C. Rangel, Journal of Molecular Catalysis A: Chemical 281 (2008) 66. [28] S. Natesakhawat, X. Wang, L. Zhang, U.S. Ozkan, Journal of Molecular Catalysis A: Chemical 260 (2006) 82. [29] L. Zhang, X. Wang, J.M.M. Millet, P.H. Matter, U.S. Ozkan, Applied Catalysis A: General 351 (2008) 1. [30] L. Zhang, J.M.M Millet, U.S. Ozkan, Applied Catalysis A: General 357 (2009) 66. 9 [31] P. Gawade, B. Mirkelamoglu, B. Tan, U.S. Ozkan, Journal of Molecular Catalysis A: Chemical 321 (2010) 61. [32] K. Watanabe, T. Miyao, K. Higashiyama, H. Yamashita, M. Watanabe, Catalysis Communications 10 (2009) 1952. [33] G.C. Araújo, M.C. Rangel, Catalysis Today 62 (2000) 201. [34] C. Bosch, W. Wild, Canada Patent 153,379, 1914. [35] W. Ruettinger, O. Ilinich, in: S. Lee (Ed.), in: Encyclopedia of Chemical Processing, Taylor & Francis, 2006. [36] L. Lloyd, D.E. Ridler, M.V. Twigg, in: M.V. Twigg (Ed.), Catalyst Handbook, 2nd ed., Wolfe Publishing Ltd., London, 1989, pp. 293–295. [37] C. Rhodes, G. Hutchings, Physical Chemistry Chemical Physics 5 (2003) 2719. [38] G.K. Reddy, P.G. Smirniotis, Catalysis Letters 141 (1) (2011) 27. [39] A. Khan, P. Chen, P. Boolchand, P.G. Smirniotis, Journal of Catalysis 253 (2008) 91. [40] V. Idakiev, D. Mihajlova, B. Kunev, A. Andreev, Reaction Kinetics and Catalysis Letters 33 (1) (1987) 119. [41] C. Martos, J. Dufour, A. Ruiz, International Journal of Hydrogen Energy 34 (2009) 4475. [42] G.K. Reddy, K. Gunasekera, P. Boolchand, J. Dong, P.G. Smirniotis, Journal of Physical Chemistry C 115 (2011) 7586. [43] T. Popa, G. Xu, T.F. Barton, M.D. Argyle, Applied Catalysis A: General 379 (2010) 15. [44] L. Lloyd, D.E. Ridler, M.V. Twigg, in: M.V. Twigg (Ed.), Catalyst Handbook, 2nd ed., Wolfe Publishing Ltd., London, 1989, pp. 302–304. [45] L. Lloyd, D.E. Ridler, M.V. Twigg, in: M.V. Twigg (Ed.), Catalyst Handbook, 2nd ed., Wolfe Publishing Ltd., London, 1989, pp. 304–306. [46] F. Domka, A. Basinska, R. Fieldcrow, Surface Technology 18 (1983) 275. [47] G.C. Chinchen, R.H. Logan, M.S. Spencer, Applied Catalysis 12 (1984) 89. [48] G.C. Chinchen, R.H. Logan, M.S. Spencer, Applied Catalysis 12 (1984) 93. [49] M. Robbins, G.K. Wertheim, R.C. Sherwood, D.N.E. Buchanan, Journal of Physics and Chemistry of Solids 32 (1971) 717. [50] M.A. Edwards, D.M. Whittle, C. Rhodes, A.M. Ward, D. Rohan, M.D. Shannon, G.J. Hutchings, C.J. Kiely, Physical Chemistry Chemical Physics 4 (2002) 3902. [51] C. Pellerin, S.M. Booker, Environmental Health Perspectives 108 (2000) a402. [52] P.C. Grevatt, Toxicological Review of Hexavalent Chromium, U.S. Environmental Protection Agency, 1998. [53] U.S. Department of Labor, Hexavalent Chromium, OSHA 3373-10, 2009. [54] Official Journal of the European Union 46 (L37) (2003) 19–37. [55] C. Ratnasamy, J.P. Wagner, Catalysis Reviews 51 (2009) 325. [56] Y. Hattori, K. Yamamoto, J. Kaita, M. Matsuda, S. Yamada, Journal of the American Oil Chemists Society 77 (12) (2000) 1283. [57] J. Ladebeck, T. Regula, Studies in Surface Science and Catalysis 121 (1999) 215. [58] G.C. Chinchen, Eur. Patent Appl. 0062410, 1982. [59] D.G. Rethwisch, J.A. Dumesic, Applied Catalysis 21 (1986) 97. [60] D.G. Rethwisch, J. Phillips, Y. Chen, T.F. Hayden, J.A. Dumesic, Journal of Catalysis 91 (1985) 167. [61] J. Ladebeck, K. Kochloefl, Studies in Surface Science and Catalysis 91 (1995) 1079. [62] Q. Liu, W. Ma, R. He, Z. Mu, Catalysis Today 106 (2005) 52. [63] H. Jin, Q. Liu, Z. Mu, L. Diao, X. Zhang, W. Ma, Chin. Patent ZL95 121834.4, 1999. [64] M.L. Kundu, A.C. Sengupta, G.C. Maiti, B. Sen, S.K. Ghoshi, V.I. Kuznetsov, G.N. Kustova, E.N. Yurchenko, Journal of Catalysis 112 (1988) 375. [65] A. Andreev, V. Idakiev, D. Mihajlova, D. Shopov, Applied Catalysis 22 (1986) 385. [66] J.Y. Lee, D.-W. Lee, M.S. Lee, K.-W. Lee, Catalysis Communications 14 (2011) 37. [67] M.S. Lee, J.Y. Lee, D.-W. Lee, D.J. Moon, K.-W. Lee, International Journal of Hydrogen Energy 37 (2011) 11218. [68] K. Watanabe, T. Miyao, K. Higashiyama, H. Yamashita, M. Watanabe, Catalysis Communications 12 (2011) 976. [69] A. Boudjemaa, C. Daniel, C. Mirodatos, M. Trari, A. Auroux, R. Bouarab, Comptes Rendus Chimie 14 (2011) 532. [70] N. Mahadevaiah, P. Singh, B.D. Mukri, S.K. Parida, M.S. Hedge, Applied Catalysis B: Environmental 108–109 (2011) 117. [71] T. Blunesin, R.J. Gorte, G.W. Graham, Applied Catalysis B: Environmental 15 (1998) 107. [72] Y. Sun, S.S. Hla, G.J. Duffy, A.J. Cousins, D. French, L.D. Morpeth, J.H. Edwards, D.G. Roberts, International Journal of Hydrogen Energy 36 (2011) 79. [73] FreedomCAR and Fuel Partnership, Hydrogen Production Roadmap: Technology Pathways to the Future (2009): www1.eere.energy.gov/ hydrogenandfuelcells/pdfs/h2 production roadmap.pdf  ... development of catalysts for fatty alcohol production (the hydrogenation of fatty esters) [56,57] Studies on Cr-free Fe-based HTS catalysts Note: The HTS activities of Cr-free and Cr-containing catalysts. .. monoxide atmosphere 3.2 Fe/Al-based catalysts: Al/Ce or Al/Cu as replacements for Cr: 1995–2010 The first promising results for developing Cr-free Fe-based catalysts were arguably those of Ladebeck... previous studies of Cr-free Fe-based HTS catalysts, especially those addressing the promoters used to replace Cr in Fe-based HTS catalysts (1) First, a promoter element should form a solid solution