Sustainable Production of High-Purity Hydrogen by Sorption

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Sustainable production of high-purity hydrogen by sorption enhanced steam reforming of glycerol over CeO2promoted Ca9Al6O18-CaO/NiO bifunctional material Marziehossadat Shokrollahi Yancheshmeh, Hamid Radfarnia, and Maria C. Iliuta ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b01627 • Publication Date (Web): 25 Sep 2017 Downloaded from http://pubs.acs.org on October 4, 2017

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ACS Sustainable Chemistry & Engineering

Sustainable production of high-purity hydrogen by sorption enhanced steam reforming of glycerol over CeO2-promoted Ca9Al6O18-CaO/NiO bifunctional material

Marziehossadat Shokrollahi Yancheshmeh1, Hamid R. Radfarnia2, Maria C. Iliuta1* 1

Department of Chemical Engineering, Université Laval, Québec, QC G1V 0A6, Canada 2

CanmetMATERIALS, 183 Longwood Road South, Hamilton L8P 0A5, Canada

*

Corresponding Author Address: 1065 Avenue de la Médecine, Université Laval, Québec City, Québec, Canada G1V 0A6 Phone: 1-418-656-2204 Fax: 1-418-656-5993 E-mail: [email protected] 1 ACS Paragon Plus Environment


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Abstract The present work investigates the sustainable production of high-purity hydrogen through sorption enhanced steam reforming of glycerol (SESRG) over Ca9Al6O18-CaO/xNiO (x = 15, 20, and 25 wt%) and Ca9Al6O18-CaO/20NiO-yCeO2 (y = 5, 10, and 15 wt%) bifunctional catalystsorbent materials. A wet mixing method involving limestone acidification coupled with two-step calcination was employed to prepare the bifunctional materials. Cyclic carbonation/calcination tests revealed that the bifunctional materials promoted with 10 and 15 wt% of CeO2 possessed an excellent CaO conversion (97% in both cases) and a remarkable cyclic stability (up to 15 cycles). This was mainly attributed to the thin shell-connected structure formed by the addition of CeO2 and the oxygen mobility characteristic of CeO2. The use of Ca9Al6O18-CaO/xNiO materials in 5 consecutive SESRG/regeneration cycles revealed that they suffered from fast deactivation owing mainly to CaO sintering and coke deposition. Despite the high H2 purity obtained (∼ 98%), the pre-breakthrough time and hydrogen yield decreased significantly over 5 cycles. Interestingly, the addition of CeO2 to the most efficient catalyst (Ca9Al6O18-CaO/20NiO) resulted in a significant improvement in material stability during cyclic operation. The performance of CeO2promoted materials was shown to depend strongly on the CeO2 content which controlled the number of adjacent Ni active sites, the amount of coke deposition, and the degree of CaO sintering. The bifunctional material promoted with 10 wt% of CeO2 showed the best performance over 5 consecutive SESRG/regeneration cycles, with a stable H2 purity of ∼ 98%, H2 yield of ∼ 91%, and pre-breakthrough time of 48 min. The long-term cyclic stability test of Ca9Al6O18-CaO/20NiO-10CeO2 over 20 cycles exhibited a very stable performance with H2 yield of 91% and H2 purity of 98% within 20 cycles, confirming the high potential of this material for SESRG process.

Keywords: sorption enhanced steam reforming; glycerol; high-purity hydrogen production; bifunctional catalyst-sorbent material.

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Introduction Hydrogen is not only a crucial raw material in chemical and petroleum industries, but it is also considered as a green energy carrier.1,

2

Currently, 95% of hydrogen is produced from

conventional fossil fuel based processes.3 However, these processes have been questioned about their environmental benefit and future energy safety. It has, therefore, attracted a great deal of attention to produce hydrogen from renewable, carbon-neutral fuels, especially from biomass or biomass-derived chemicals.4,

5

Among various bio-derived feedstocks, glycerol has attracted

increasing attention in recent years due to its wide availability, high hydrogen content and less hazardous nature compared to other sources.2,

5, 6

Glycerol can be efficiently converted into

hydrogen through steam reforming (SR) process. This process is quite complex and involves a wide variety of reactions. The main reaction is expressed by Eq. (1) which is a combination of glycerol decomposition and water gas shift reactions. Other reactions such as methanation, methane reforming, and coke formation reactions may also occur depending, mainly, on the operating conditions and the type of catalyst.

C3H8O3 + 3H 2 O ↔ 3CO2 + 7H 2

o ∆H 298 = + 128kJ mol

(1)

Considering the complexity of steam reforming of glycerol (SRG), the hydrogen purity is lowered by undesirable products, especially CO, CO2, and CH4. The maximum theoretical hydrogen purity in the gaseous product stream is 70%, considering complete conversion of glycerol through the SR reaction (Eq. (1)).7,

8

Therefore, purification and separation steps are

imperative in the downstream process to produce high-purity hydrogen, which imposes a very costly and complicated process.9 Sorption enhanced steam reforming (SESR) is a well-known process that is applied to increase the efficiency of reforming process and produce a hydrogen-



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enriched gas stream in a single step. In this process, reforming, WGS and CO2 removal reactions are integrated in a single reactor using a mixture of reforming catalysts and CO2 sorbents. CO2 is selectively removed from the reaction media, driving the equilibrium limited WGS reaction toward hydrogen production according to Le Chatelier principle.8 In addition, SESR lowers the cost of high-purity hydrogen production by reducing the number of processing steps needed for subsequently CO2 capture and improving the thermal efficiency of the reforming process.10, 11 The key factor in the successful application of SESR technology is finding suitable CO2 sorbents and reforming catalysts, which can work efficiently under the harsh operating conditions of SESR process. A highly efficient sorbent for CO2 capture in SESR process must possess specific properties including good thermal stability, high sorption capacity and stability, fast sorption/desorption kinetics, adequate mechanical strength, and reasonable cost. Among all available sorbents for high temperature CO2 capture, CaO-based sorbents have shown a good potential for application in SESR systems.12, 13 The reversible reaction between CaO and CO2 is represented by Eq. (2).

CaO(s) + CO 2 ↔ CaCO 3

o ∆H 298 = − 178 kJ mol

(2)

However, although CaO-based sorbents have been widely applied for CO2 capture in different processes, their cyclic stability still needs to be improved to make them optimal candidates for SESR process. Among different possible strategies,12 the incorporation of inert high-temperature resistant metal oxides into the sorbent structure is one of the most effective ways for enhancing the cyclic stability of CaO-based sorbents.14-20 These inert oxides generally act as physical barriers preventing the agglomeration of CaO particles. Moreover, there are some studies reporting the enhancement effect of oxygen vacancy possessing oxides on the cyclic


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stability of CaO-based sorbents.21, 22 In this regard, although CeO2 was shown to be an efficient candidate, there was no consensus on the mechanism by which this oxide acted as stabilizer.22-24 While Lu et al. 23 and Wang et al. 24 reported that CeO2 behaved as a suitable physical barrier to prevent the sintering of CaO particles because of its high Tamman temperature (1064 ºC), Phromprasit et al.

22

found that CeO2-promoted CaO sorbent presented a stable CO2 capture

capacity due to its oxygen storage capacity which facilitated the CO2 diffusion from the surface to the bulk and vice versa. The most well-known metal catalysts utilized in SRG are Pt, Ru, Rh, Pd, Ir, Co, and Ni. Although noble metals are more active and less susceptible to coke formation in comparison to Ni, they are not commonly used in industrial applications because of their high price and limited availability. On the other hand, Ni-based catalysts with different supports and promoters have been well commercialized for steam reforming.1,

25, 26

Since Ni-based catalysts are prone to

sintering and coke formation, different strategies have been proposed to eliminate these obstacles such as development of highly dispersed Ni catalysts with strong metal-support interaction and promotion in the surface oxygen mobility.27 In this regard, CeO2 has been widely used to enhance the metal-support interaction and promote the removal of carbon species.1, 5, 26, 28 Another important factor affecting the success of SESR process is the mixing pattern of catalyst and sorbent. Combining catalyst and sorbent in a single particle (bifunctional catalystsorbent) rather than physical mixing of catalyst and sorbent can eliminate the mass transfer limitations, reduce the reactor volume, and make the operation simpler.12, 13 To date, there have been some experimental studies dealing with high-purity hydrogen production via SESRG process. Most of them have considered the physical mixing of sorbent and catalyst

4, 7, 9, 10, 29-32

and only a few have addressed the utilization of bifunctional catalyst-sorbent materials.8, 33-37 A



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summary of these studies is presented in Table S1 (Supporting Information). Although there have been a few studies on the high-purity H2 production via SESRG, this process is still facing substantial challenges, such as the increase of pre-breakthrough time and the enhancement of cyclic stability during multi-cyclic operations, which are main parameters in practical applications. Moreover, while the materials developed for SESRG process can be evaluated in a better way if both H2 purity and yield are considered, this has been ignored in most available studies. This work investigates the development of Ca9Al6O18-CaO/xNiO (x = 15, 20, and 25 wt%) and Ca9Al6O18-CaO/20NiO-yCeO2 (y = 5, 10, and 15 wt%) bifunctional materials and their application

in the SESRG process. A detailed study was also performed to highlight the influence of CeO2 on the material stability in cyclic carbonation/calcination operation. To rationalize the role of CeO2 on CO2 capture and catalytic performance of the developed materials, the fresh and spent materials were completely characterized by XRD, SEM, BET, TEM, and TGA. Results and Discussion Characterization of Fresh Samples The XRD patterns of the calcined bifunctional materials are represented in Figure 1(a). The diffractograms of all samples show reflections attributed to NiO (ICDD # 01-073-1523), CaO (ICDD # 00-037-1497), and Ca9Al6O18 (ICDD # 01-070-0839). For Ni20Cey materials, the diffractions associated with the fluorite structure of CeO2 are present (ICDD # 00-034-0394) and become more intense as the CeO2 content increases. As can be seen in Figure 1(a), the diffraction peaks of CaO and NiO slightly shifted toward lower angles with the addition of CeO2 into the structure of Ni20. A magnified view of the shift of diffraction peaks at 2θ of 37.35° and 43.25° is illustrated in Figure 1(b). These shifts can be explained by the distortion of CaO and NiO crystal 6 ACS Paragon Plus Environment

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lattices resulted from the insertion of Ce4+ into their structures. It is known that various combinations of solid solutions including CaO-CeO2, NiO-CeO2, NiO-CaO, and NiO-CaO-CeO2 can be formed from CaO, NiO, and CeO2.38 Interestingly, the distortion of NiO and CaO crystal lattices decreased as the content of CeO2 increased from 5 to 15 wt%. This trend can be more clearly seen by examining the d-spacing values of three main reflections of CaO and NiO provided in Table S2. The XRD data were used to determine the average crystallite sizes of CaO, NiO, and CeO2 using the Scherrer equation (Table 1). Obviously, the crystallite sizes of CaO and NiO for Ni20Cey samples are smaller than those of Ni20. Moreover, the crystallite size of CeO2 remains almost constant for all CeO2 concentrations, which is in contrast with the tendency of CeO2 to form aggregates at high concentrations.26,

39

These results can be attributed to the

incorporation of Ce4+ into CaO and NiO crystal structures, which controlled the growth of crystal structures. The morphology of the freshly calcined bifunctional materials was analyzed using scanning electron microscopy (SEM), and the micrographs are displayed in Figure 2. It can be seen that there are obviously some significant differences in the morphological features of developed materials. Two different mechanisms were involved in the development of porous structures. In the first mechanism, calcium citrate decomposed into CaO, carbon and some gases (CO, CH4, and H2O) during the primary calcination in argon atmosphere. Produced carbon was expected to limit the particle growth and so, control the final morphology of particles. When argon was replaced with air in the second step of calcination, all the carbon formed during the first step burnt off and left behind a porous structure.40 The second mechanism involved the nitrate-citrate redox reaction, in which a large amount of gases (including CO2, N2, and H2O) was produced within a few second at around 260 °C, forming a well-dispersed powder.41 For Ni15 and Ni20,



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the amount of nitrate ions present in the precursors was less than the content of citrate ions. Therefore, decomposition of citrate ions through two-step calcination method was the dominant mechanism for the porous structure formation, creating a structure containing twisted peanutshaped particles separated by small pores. With the increase of nitrate anion content for Ni25 and Ni20Ce5, the nitrate-citrate redox reaction played a more prominent role in the development of morphological structure. The large amount of gases evolved in a short period of time during the redox reaction caused forces on the particles, which was expected to form a morphological structure containing larger pores and agglomerates in some areas compared to Ni15 and Ni20. While both of these features were clearly observed in the case of Ni25, a completely different morphological structure was detected for Ni20Ce5. As illustrated in Figure 2, the morphology of Ni20Cey samples differs remarkably from Nix samples. While Nix materials include a nubby structure (hard skeleton), the morphology of Ni20Cey materials looks like a thin shell-connected structure including many pores. Similar morphological changes in the structure of CaO-based sorbent upon the addition of CeO2 were reported by Wang et al.24 Hence, while Ni20Ce5 shows larger pores caused by both the presence of CeO2 and the redox reaction, there are no agglomerations like the ones found for Ni25. With a further increase in the amount of nitrate ions for Ni20Ce10, the forces on the particles induced by the redox reaction were maximized. Consequently, a thin shell-connected structure with the largest pores was created. In the case of Ni20Ce15, much more gases were released during the redox reaction, forming a structure containing a large number of smaller pores compared to that observed for Ni20Ce10. The SEM results were also confirmed by the N2 physisorption analysis. As shown in Table 1, Ni15 and Ni20 possess similar BET surface area, which is consistent with the fact that the dominant mechanism for the porous structure formation is the same in both cases. For Ni25,


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BET surface area presents a decrease compared with Ni15 and Ni20. This phenomenon could be caused by the particle agglomeration during the nitrate-citrate redox reaction. Introducing CeO2 into the Ni20 material resulted in an increase in the BET surface area, which may be related to the formation of a thin shell-connected structure in the presence of CeO2. Figure 3 shows representative TEM images and Ni particle size distributions of reduced Ni20 and Ni20Ce10 bifunctional materials. Ni particles can be observed as the darkest spots dispersed on the surface of CaO. It is clearly seen that the addition of CeO2 into the Ni20 bifunctional material yielded a smaller Ni particle size and a more homogeneous distribution of Ni on the CaO surface. The average Ni particle size was decreased from 35.1 to 20.8 nm by the incorporation of CeO2 into Ni20. These results indicate that CeO2 could effectively prevent Ni particles from sintering, enhancing the Ni dispersion. It seems that the Ni particles have been embedded in the surrounding CaO/CeO2 matrix (inset, Figure 3), which could produce synergetic interaction with Ni particles and prevent them from sintering at high temperatures. These results are in complete agreement with the XRD data. CO2 Capture Characteristics The CO2 capture performance of the synthesized bifunctional materials was evaluated over 15 carbonation/calcination cycles (Figure 4). It is worth noting that the operating conditions of CO2 capture experiments were carefully chosen to simulate the conditions of SESRG tests. For Nix samples, the initial CaO conversion was 73.5%, 78.8%, and 74.4% for the NiO content of 15, 20 and 25 wt%, respectively. By the addition of CeO2 to the Ni20 sample, the initial CaO conversion was significantly improved; the CaO conversion reached as high as 84.1%, 96.5%, and 96.8% for Ni20Ce5, Ni20Ce10, and Ni20Ce15, respectively. In order to discuss these results in greater detail, the conversion profiles of these materials at the 1st cycle are shown in Figure 5. 9 ACS Paragon Plus Environment


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It is known that the carbonation reaction takes place in two stages, reaction controlled and diffusion controlled. As can be seen in Figure 5, the higher initial CaO conversion of Ni20 compared with Ni15 can be ascribed to its higher carbonation rate in the reaction controlled stage. The carbonation rate in the reaction controlled stage depends on various parameters, including the carbonation temperature, CO2 concentration, reaction surface area per unit volume, porosity of the particles, and initial pore length in the porous system per unit volume.42 Since the carbonation temperature and CO2 concentration were kept constant in all experiments, the difference in the carbonation rates of Ni15 and Ni20 can be attributed to their differences in morphological characteristics. In the case of Ni25, although the carbonation rate of the reaction controlled stage was identical to that of Ni20, the transition between two stages happened sooner. This can be ascribed to the agglomerated particles formed during the preparation step of Ni25 due to the nitrate-citrate redox reaction. The sorbents with small particle size allow the carbonation to occur mainly in the reaction controlled stage, while the diffusion controlled stage is dominant for the sorbents with larger particle size.43 For Ni20Cey materials, the great enhancement of initial CaO conversion can be attributed to several parameters, including the thin shell-connected structure created in the presence of CeO2, the large pores formed by the nitratecitrate redox reaction during the synthesis procedure and the oxygen mobility characteristics of CeO2. While the thin shell-connected structure exposed more active CaO to CO2, the large pores and the oxygen mobility characteristics of CeO2 facilitated the diffusion of CO2 from the surface to the bulk.22, 24 Therefore, both rate and duration of the reaction controlled stage were increased for the CeO2-promoted samples (Figure 5). The Ni20Ce10 and Ni20Ce15 samples showed the best performance in terms of CaO conversion and carbonation rate. The carbonation reaction


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mainly took place in the reaction controlled stage, such that the CaO conversion reached as high as 90% within only 6 min. The cyclic stability of CaO-based sorbents is another important parameter that should be considered for practical applications. According to Figure 4, the cyclic stability of the developed bifunctional materials over 15 cycles was enhanced in the following order: Ni15 < Ni20 ≈ Ni20Ce5 < Ni25 ≈ Ni20Ce10 < Ni20Ce15. The XRD results (Figure 1) confirmed the formation of temperature-resistance Ca9Al6O18 for all samples. The uniform distribution of Ca9Al6O18, which is considered as a physical barrier, among CaO particles could to some extent suppress the sintering of CaO particles. However, it cannot explain the difference in the cyclic stability of the synthesized materials because the weight ratio of CaO to Ca9Al6O18 was identical for all materials. For Nix samples, the enhancement of cyclic stability by increasing the NiO content can be attributed to the formation of larger pores during the nitrate-citrate redox reaction, which prevented the sintering of CaO particles by creating larger distance between adjacent particles. It may also be related to the increase in the content of NiO distributed as an inert material between CaO particles. In the case of Ni20Cey, the cyclic stability was clearly improved by increasing the content of CeO2. Considering the same textural and physical properties of Ni20Ce5 and Ni20Ce15, this enhancement can be assigned to the increase in the amount of CeO2 which can act as a stabilizer. As mentioned earlier, there is no general agreement in the literature about how CeO2 plays a role in enhancing the cycle stability of CaO-based sorbents. To clarify the mechanism by which CeO2 improved the cyclic stability of the Ni20Cey samples, the cyclic CO2 capture performance of Ni20 and Ni20Ce10 was studied in the absence and presence of water. As shown in Figure 6, the CO2 capture capacity of Ni20 and Ni20Ce10 decreased significantly by 58 and 56 wt.%,



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respectively, over 15 cycles under dry condition. These results imply that CeO2 did not act as an effective physical barrier since it was unable to suppress the capacity decay of Ni20Ce10 under dry condition. By the injection of steam during carbonation, both Ni20 and Ni20Ce10 experienced less capacity loss over 15 cycles. This may be attributed to the enhanced solid-state diffusion in the CaCO3 product layer under wet condition, which can improve the cyclic stability of CaO-based sorbents.44-46 On the other hand, the addition of steam improved more the cyclic stability of Ni20Ce10 as compared with Ni20. Under wet condition, the capacity loss of Ni20 and Ni20Ce10 reached 25 and 16 wt%, respectively. The better stability of Ni20Ce10 compared with Ni20 might be a result of the oxygen mobility characteristics of CeO2. Some studies have shown that the presence of oxygen vacancy sites in the structure of CaO-based sorbents could improve their cyclic stability under specific conditions by promoting the diffusion of CO2 through CaCO3 product layer and agglomerated or even sintered CaO particles.21, 22 Yi et al.

21

suggested that an oxygen vacancy site provided O2- ion to CO2 producing CO32-; the latter was then converted to CO2 returning O2- back to the previous site while obtaining O2- from the next site. In this way, CO2 can be transferred through oxygen vacancy sites until it reaches active CaO sites and forms CaCO3 product. Accordingly, it is expected that CeO2, which is well known for its oxygen mobility characteristics, enhances the cyclic stability of CaO-based sorbents by facilitating the CO2 diffusion through oxygen vacancy sites. Nonetheless, while Ce is easily oxidized, its reduction is usually sluggish, which hampers its oxygen mobility during the redox cycle.47 It is known that the redox properties of CeO2 are strongly dependent on the surface structure.48-50 It has already been reported that the steam treatment of CeO2 enhanced the oxygen mobility at its (111) surface.50 This can explain why CeO2 could effectively enhance the cyclic stability of Ni20Cey samples only in the presence of steam in the reaction medium. The role of


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CeO2 in enhancing the cyclic stability of Ni20Cey samples can therefore be related to its oxygen mobility characteristics in the presence of steam. Sorption Enhanced Steam Reforming of Glycerol Ni15, Ni20, and Ni25 bifunctional materials were tested for their activities in SESRG process to determine the optimized NiO loading. To have a better understanding of the effect of in-situ CO2 capture during SRG on gaseous product distribution, the concentration profiles of H2, CO2, CO, and CH4 (Ar- and H2O-free basis) during the 1st cycle of SESRG over Ni20 are displayed in Figure 7. The large difference between H2 concentration in the pre-breakthrough (98.3%) and post-breakthrough (65.5%) stages clearly confirmed the great potential of developed bifunctional material, as well as the suitability of selected operating conditions for high-purity H2 production via SESRG. Figure 8 shows a comparison between the breakthrough curves of Nix materials over 5 SESRG/regeneration cycles. The pre-breakthrough time of Ni15, Ni20, and Ni25 samples reduces significantly with the increase of the cycle number, reaching from 57, 51, and 48 min, respectively, at the 1st cycle of operation to around 15, 30, and 21 min, respectively, at the end of the 5th cycle (pre-breakthrough time corresponds to the H2 purity of more than 95%). Although none of these materials represent a good stability, Ni20 sample possesses the best cyclic stability when compared with Ni15 and Ni25. This observation, however, is not in agreement with the cyclic CO2 capture behavior of Nix bifunctional materials. As shown in Figure 4, Ni25 shows the best cyclic stability with only 2.9% capacity loss within 5 cycles, compared to Ni20 and Ni15 with 6.7% and 13.3% capacity loss, respectively. Obviously, the CO2 capture performance of Nix materials deteriorates much faster during SESRG/regeneration cycles than carbonation/calcination cycles. Accordingly, the rapid decay of CO2 capture capacity in the cyclic SESRG process may not only be ascribed to the sintering of CaO particles. Some 13 ACS Paragon Plus Environment


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studies have reported that CaO-based sorbents experience a much higher rate of deactivation when they come into contact with coke.51-53 Coke formation can reduce the access of CO2 to active CaO sites by blocking the pores and/or adhering to the surface of sorbent at high temperatures. Hence, the decrease of pre-breakthrough time through the cyclic SESRG operation can be attributed to the decay in CO2 capture capacity of CaO due to both sintering of CaO particles and coke deposition on the surface of the developed materials. Figure 9 presents the hydrogen yield versus the number of cycles for the Nix bifunctional materials. It can clearly be seen that the hydrogen yield decreases continuously with the number of cycles for all three samples although its initial value and extent of reduction are a function of NiO loading. As the activity of a steam reforming catalyst strongly depends on the number of Ni active sites adjacent to each other, the increase of Ni active sites is expected to promote the catalyst activity. However, since coke formation requires more adjacent sites than steam reforming, increasing the Ni active sites can also promote coke deposition at the expense of steam reforming.54, 55 It is also worth reminding that when the number of Ni active sites is not sufficient, the oxygenated hydrocarbons produced by thermal decomposition of glycerol cannot efficiently be gasified, leading to low catalyst activity and stability. These oxygenated hydrocarbons are considered as coke precursors and so, promote the coke formation.4,

56

Therefore, there is an optimum number of Ni active sites in terms of both catalyst activity and coke formation. This can explain why the H2 yield in the 1st cycle rose from 67.1% to 82.1% when the NiO content increased from 15 wt% to 20 wt%, whereas it decreased to 74.2% with the further increase in NiO loading to 25 wt%. For Ni20, it appeared that the number of adjacent Ni active sites was high enough to favor the steam reforming of glycerol and intermediate oxygenated hydrocarbons, but small enough to restrict coke formation. In terms of catalyst


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stability, the loss of active sites due to Ni sintering was not the only reason of catalyst deactivation. Coke deposition and CaO sintering, which restricted the access of reactants to the Ni active sites, could mainly contribute to the deactivation of Nix materials. As can be seen in Figure 9, Ni20 exceeded the performance of both Ni15 and Ni25 in terms of catalyst stability. The decrease in H2 yield over 5 cycles for Ni20 was ∼ 18%, ∼ 2 and 1.5 times less than that for Ni15 and Ni25, respectively. As mentioned above, CaO sintering, Ni sintering, and coke deposition seem to be the main reasons for the deactivation of Nix bifunctional materials in terms of both pre-breakthrough time and average H2 yield in the pre-breakthrough step. In order to eliminate these obstacles, CeO2 can be used to enhance the metal-support interaction and decrease the coke deposition. Since the best performance was obtained for the Ni20 bifunctional material, it was selected to further investigate the effect of CeO2 on the activity and stability of developed bifunctional materials in the cyclic SESRG process. The performance of Ni20Cey bifunctional materials in SESRG process was evaluated over 5 cycles and the results are displayed in Figures 10 and 11. As can be seen in Figure 10, the pre-breakthrough time decreases from 54 and 51 min to 36 min within 5 cycles for Ni20Ce5 and Ni20Ce15, respectively. In the case of Ni20Ce10, however, the prebreakthrough time is almost constant at ∼ 48 min over 5 cycles. Regarding the hydrogen yield (Figure 11), Ni20Ce5 shows a hydrogen yield of 73.1% at the 1st cycle, which is ∼ 10% lower than that of Ni20 (82.1%). For Ni20Ce10 and Ni20Ce15, the hydrogen yield is, respectively, higher than and almost equal to that of Ni20 at the 1st cycle (91.8% and 85.5%, respectively). While the hydrogen yield slightly decreases over 5 cycles for Ni20Ce5 and Ni20Ce15, it is almost constant for Ni20Ce10. In order to discuss these results properly, one must recall some facts:



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(i) First, it has been well established that if the number of adjacent Ni active sites is well controlled and the migration of carbon atoms across the surface is limited, steam reforming over Ni-based catalysts should be boosted at the expense of coke formation.54 This concept was utilized in several studies to restrict both the number of adjacent Ni active sites and the migration of carbon atoms by covering the surface of some Ni atoms using an additive.57-59 The amount of promoter should be optimized to allow steam reforming but to minimize coke deposition.54 (ii) Second, the catalytic reforming is intrinsically a partial oxidation reaction. In the reforming process, the oxidation of carbonaceous species adsorbed on the catalyst surface, including intermediate oxygenated hydrocarbons, CO, and coke deposits, is an important transition reaction which contributes significantly to the selectivity and stability of catalysts. Since the activation of water, which is considered as the oxidant in steam reforming processes, is almost difficult on the metal active sites, oxides with high surface oxygen mobility can be used to promote the oxidative gasification of carbonaceous species. CeO2 is one of the most important rare earth oxides being widely used to promote the activation of water and the oxidative conversion of carbonaceous species into a gas mixture mainly containing H2, CO2, CO, and CH4.27 To contribute to the removal of carbonaceous species adsorbed on the surface of metallic Ni, CeO2 should be in close contact with Ni.60, 61 Considering these facts, the observed differences in the catalytic performance of Ni20Cey materials can be explained based on the geometric effects caused by the presence of different concentrations of CeO2. As mentioned earlier, XRD analysis of calcined samples confirmed the formation of NiO-CeO2 interaction (insertion of Ce4+ into the NiO structure). After the reduction treatment, CeO2 might preferentially segregate to the surface of metallic Ni,61 decreasing the number of adjacent Ni active sites available for steam reforming. Since the intensity of


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interaction between NiO and CeO2 decreased as the CeO2 content increased, larger number of Ni active sites was covered by CeO2 in the case of Ni20Ce5 compared with Ni20Ce10 and Ni20Ce15. For Ni20Ce5, therefore, neither the number of Ni active sites nor the amount of CeO2 was enough to effectively gasify the oxygenated hydrocarbons. Hence, a lower hydrogen yield was observed for Ni20Ce5 compared to Ni20 and the coke formation could not be suppressed in the presence of 5 wt% of CeO2. As can be seen in Figure 10, the pre-breakthrough time decreases by ∼ 34% over 5 cycles in the case of Ni20Ce5, which can be ascribed to both CaO sintering and coke deposition. For Ni20Ce15, the NiO-CeO2 interaction was at its lowest level. As a result of that, CeO2 had the smallest effect on the number of adjacent Ni active sites after reduction. Moreover, CeO2 could not effectively contribute to the removal of carbonaceous species from Ni surface because it was not in intimate contact with metallic Ni. The hydrogen yield is therefore almost the same as that for Ni20 (Figure 11). In addition, it shows ∼ 30% decay in the pre-breakthrough time over 5 cycles (Figure 10). Ni20Ce10 exhibits the best performance with an almost constant pre-breakthrough time and the highest H2 yield over 5 cycles. In this case, some of Ni active sites are covered by CeO2, resulting in the promotion of steam reforming by decreasing the coke formation. In addition, large amount of CeO2 is in close contact with Ni, contributing to the gasification of carbonaceous species. The Ni20Ce10 bifunctional material was subjected to 20 SESRG/regeneration cycles in order to evaluate its long-term cyclic stability (Figure 12). The operating time of SESRG step was limited to 30 min because (1) only the pre-breakthrough time is of interest for high-purity hydrogen production in practical applications and (2) an effective method to prolong the sorbent activity through cyclic operation is the partial carbonation of CaO-based sorbents.62 Moreover, the spent bifunctional material was treated with air (100 mL/min) at 800 °C for 30 min after 10



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cycles to burn off the coke deposits. After air treatment, the reduction step was performed prior to restart the cyclic SESRG process. As can be seen in Figure 12, Ni20Ce10 bifunctional material showed excellent cyclic activity and stability, with an average H2 purity and yield of 98% and 91%, respectively, over 20 cycles. It is worth noting that the pre-breakthrough time remained almost constant through 20 cycles. These results prove that Ni20Ce10 is a promising candidate for high-purity hydrogen production by SESRG process. Characterization of Spent Materials Among the spent bifunctional materials, the Ni20 and Ni20Ce10 samples were fully characterized in order to explore possible structural changes and deactivation phenomena that occurred during 5 SESRG/regeneration cycles. Figure 13 shows the XRD patterns of the spent Ni20 and Ni20Ce10 materials. It can be seen that the diffraction peaks corresponding to CaO, CeO2, Ca9Al6O18, and metallic Ni are clearly present. No diffractions ascribed to NiO were detected in the XRD analyses; hence the catalyst deactivation of Ni20 cannot be attributed to the oxidation of metallic Ni during the reforming process. The XRD data were used to calculate the crystallite size of Ni using the Scherrer equation. While the Ni crystallite size of Ni20Ce10 remained nearly unchanged after 5 SESRG/regeneration cycles (18.1 nm), the Ni crystallite size of Ni20 increased by ∼ 30% (25.4 nm). These results indicate that Ni sintering is significantly reduced in the presence of CeO2, possibly due to the synergetic interaction of CaO and CeO2 with Ni particles, which induces a stronger metal support interaction and so, prevents Ni particles from sintering. Figure 14 displays the SEM images of the spent Ni20 and Ni20Ce10 materials. It can be clearly seen that Ni20Ce10 still possesses a well-developed porous structure after 5 SESRG/regeneration cycles, while Ni20 has a severely agglomerated structure. These 18 ACS Paragon Plus Environment

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observations are in agreement with the N2 physisorption results where 30% reduction in the BET surface area of Ni20 was recorded over 5 SESRG cycles, while the BET surface area of Ni20Ce10 remained almost constant. These findings explain why the pre-breakthrough time decreased significantly over 5 SESRG/regeneration cycles in the case of Ni20, whereas it remained constant for Ni20Ce10. However, the CO2 capture results (Figure 4) shows that both Ni20 and Ni20Ce10 exhibit only 2.9% loss in CO2 capture capacity through 5 carbonation/calcination cycles. Therefore, the sintering of CaO was intensified during the cyclic SESRG/regeneration process in the case of Ni20, which can be attributed to the much higher amount of coke deposited on Ni20 rather than Ni20Ce10. As mentioned previously, the coke deposits may cause particle agglomeration along the surrounding sorbent and coke due to their adhesive properties, thus accelerating the deactivation of CaO-based sorbents.53 The formation, structure, and amount of coke deposits were further studied by TEM and TGA analyses. The spent Ni20 and Ni20Ce10 materials were examined by TEM and representative micrographs and Ni particle size distributions are illustrated in Figure 15. Concerning the Ni sintering, it should be noted that in comparison with the fresh samples (Figure 3), the Ni particle size distribution of the spent Ni20 changed significantly, while that of the spent Ni20Ce10 showed a small change. The average Ni particle size is 49.23 and 25.94 nm for the spent Ni20 and Ni20Ce10 materials, respectively. Comparing these results with those obtained for the fresh samples, it can be suggested that Ni sintering is significantly decreased in the presence of CeO2. Regarding the carbon formation, while almost no carbon deposits were observed on the surface of Ni20Ce10, two forms of deposited carbon were found on the surface of Ni20; filamentous carbon and encapsulating carbon. The formation of filamentous carbon in steam reforming process over Ni-based catalysts is a well-known phenomenon. Ni particles are closely involved



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in the onset of filamentous carbon formation through a nucleation-growth mechanism. The carbon atoms formed on the surface of Ni particles via the dissociation of CO and/or carbonaceous species diffuse through the Ni crystallites, nucleate at the rear side of Ni crystallites and grow to form carbon filaments. The filamentous carbon may lead to loss of catalytic activity by either plugging the pores or entrapping the Ni particles.5, 63 The formation of encapsulating carbon takes place through the polymerization of CnHm groups on Ni surface into a nonreactive film, which can include linear polymers of -CH2 groups and/or aromatic polymers. Such a nonreactive film encapsulates and so deactivates Ni sites for adsorption and/or reaction.55 Both the amount and the type of carbonaceous species deposited on the spent Ni20 and Ni20Ce10 samples were approximately determined by TGA. A summary of the obtained results is provided in Table 2. It can be clearly seen that the amount of coke deposits in the case of Ni20 (∼ 23%) is much higher than that for Ni20Ce10 (∼ 4%). This is in agreement with the results of the cyclic SESRG process, where Ni20 showed a fast deactivation over 5 cycles compared with the stable performance of Ni20Ce10. As mentioned previously, CeO2 can remarkably decrease the coke deposition by controlling the number of adjacent Ni active sites, restricting the migration of carbon atoms across the surface, and promoting coke gasification owing to its oxygen mobility characteristics. Table 2 also reveals that the coke deposits were oxidized at a much lower temperature in the case of Ni20Ce10 (∼ 350 °C) compared with Ni20 (∼ 650 °C). This can be attributed to the cooperative effect of CeO2 in the gasification of coke deposits and/or the less graphitic nature of the carbonaceous species formed in the presence of CeO2.1 For Ni20Ce10, the significantly developed Ni-CeO2 interaction as well as the oxygen mobility characteristics of CeO2 affected the nature of carbonaceous species, producing a soft coke deposition (carbon-containing hydroxylated groups), which could be gasified at lower


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temperatures.28, 60 On the contrary, the carbonaceous species deposited on the Ni20 sample can be probably specified as graphitic-type species which are oxidized at higher temperatures.28 The XRD analysis of the spent Ni20 sample also confirmed the formation of graphite carbon (Figure 13). A reflection assigned to the graphite (2θ = 26.6°)64 was observed in the diffractogram of the spent Ni20 sample. Unlike our observations regarding the coke formation, the thermodynamic studies have shown that coke formation can be suppressed to a large extent in the SESRG process. However, these types of studies do not consider any kinetic constraints, such as temperature gradient happening in the real process. According to our previous investigation on the effect of main operating parameters on the performance of SESRG in a fixed-bed reactor,11 the temperature rapidly dropped in the entrance region of the reactor because the endothermic SRG reaction was much faster than the exothermic sorption reaction. After this region, the CO2 capture reaction became dominant and the heat generated in the CO2 capture reaction led to an increase in the local temperature. A maximum fluctuation of 130 °C was observed around the set point. The temperature fluctuation moved toward the reactor exit with the increase of reaction time. In the cyclic processes, the fixed-bed zones of maximum and minimum temperatures moved along the reactor length with increasing the cycle number. It is well-known that glycerol is not a thermally stable compound and hence, it can to some extent decompose before or while passing through the catalyst bed in SRG process. The thermal decomposition of glycerol results in the formation of various oxygenated hydrocarbons, which are considered as coke precursors. These oxygenated hydrocarbons cannot easily be converted to syngas at low temperatures (lower than 550°C). In other words, at low temperatures, the thermal decomposition of glycerol is favored and the steam reforming reaction efficiency is lowered.4, 56 Therefore, the thermal decomposition of glycerol is



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believed to increase the coke formation and lower consequently the H2 yield. Based on previous investigation,11 it is believed that under the conditions of the present work, the temperature might decrease to ∼ 420°C in some regions. At such temperatures, the oxygenated hydrocarbons formed during the thermal decomposition of glycerol are not effectively steam reformed, significantly promoting the coke formation. This discussion implies that the thermodynamics analyses cannot itself provide a comprehensive picture of the SESRG process at real conditions due to its complexity. It is then required to consider both thermodynamic and kinetic analyses at the same time to understand such a complicated process. Conclusion The present study dealt with the development of an efficient bifunctional catalyst-sorbent material for high-purity hydrogen production through sorption enhanced steam reforming of glycerol (SESRG). Two series of bifunctional materials, Ca9Al6O18-CaO/xNiO (x = 15, 20, and 25 wt%) and Ca9Al6O18-CaO/20NiO-yCeO2 (y = 5, 10, and 15 wt%), were synthesized by a wet mixing method, which involved limestone acidification followed by two-step calcination. The developed materials were tested in cyclic carbonation/calcination process as well as cyclic SESRG/regeneration operation. The obtained results can be summarized as follows: (1) The Ca9Al6O18-CaO/20NiO-10CeO2 and Ca9Al6O18-CaO/20NiO-15CeO2 bifunctional materials showed excellent CO2 capture performance in terms of CaO conversion, carbonation rate, and cyclic stability. In both cases, CaO conversion of 90% was achieved in only ∼ 6 min. Moreover, the conversion decreased only very slightly over 15 cycles. These results were attributed to the promotion of carbonation in both reaction and diffusion controlled stages resulted from the formation of a thin shell-connected structure in the presence of CeO2 and the oxygen mobility characteristics of CeO2. 22 ACS Paragon Plus Environment

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(2) Both CO2 capture capacity and catalytic activity of all Ca9Al6O18-CaO/xNiO bifunctional materials were declined over 5 SESRG/regeneration cycles due to CaO sintering, Ni sintering, and coke formation. In this group, Ca9Al6O18-CaO/20NiO showed the best performance. The bifunctional materials containing 15 and 25 wt% NiO were more susceptible to coke deposition in comparison with the sample containing 20 wt% NiO. This behavior was attributed to the low efficiency of the steam reforming of oxygenated hydrocarbon intermediates for Ni15 caused by the insufficient number of Ni active sites and the promotion of coke formation at the expense of steam reforming in the presence of the large number of adjacent Ni active sites for Ni25. In addition, Ca9Al6O18-CaO/15NiO was more prone to CaO sintering than the other two samples according to the results of cyclic carbonation/calcination experiments. (3) The influence of CeO2 on the performance of Ca9Al6O18-CaO/20NiO in cyclic SESRG/regeneration process depended strongly on CeO2 loading. For 5 wt% CeO2, the number of adjacent Ni active sites decreased considerably due to the segregation of CeO2 at the surface of Ni atoms after the reduction step. Therefore, the hydrogen yield was diminished in the case of Ca9Al6O18-CaO/20NiO-5CeO2 compared to Ca9Al6O18-CaO/20NiO. For 15 wt% CeO2, since the insertion of Ce4+ into the NiO structure was at its lowest level, the H2 yield was almost identical to that of Ca9Al6O18-CaO/20NiO. For both Ca9Al6O18-CaO/20NiO-5CeO2 and Ca9Al6O18CaO/20NiO-15CeO2 samples, the pre-breakthrough time was significantly decreased over the last 3 cycles due to CaO sintering and coke formation. Ca9Al6O18-CaO/20NiO-10CeO2 showed the best performance among the studied bifunctional materials because of the suitable number of adjacent Ni active sites and the intimate contact between NiO and CeO2, which decreased the coke deposition and increased the coke gasification.



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(4) Ca9Al6O18-CaO/20NiO-10CeO2 was tested in a long-term cyclic SESRG operation, where the reaction time was limited to 30 min. According to the results (stable H2 purity of 98% and H2 yield of 91% over 20 SESRG/regeneration cycles), this bifunctional material is believed to be a promising candidate for high-purity hydrogen production by SESRG process. Associated Content The Supporting Information is available free of charge on ACS Publications website: summary of literature results for SESRG process (Table S1), experimental section (Section S1), d-spacing values for three main reflections of CaO and NiO (Table S2).


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Acknowledgements

Financial support from the Natural Sciences and Engineering Research Council of Canada (NSERC) is gratefully acknowledged.



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Table caption Table 1: Chemical composition and physical properties of the calcined Nix and Ni20Cey samples. Table 2: Summary of the TGA results for the spent Ni20 and Ni20Ce10 bifunctional materials after 5 SESRG/regeneration cycles.


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Table 1: Chemical composition and physical properties of the calcined Nix and Ni20Cey samples. Bifunctional

Chemical composition (wt%)

BET surface area

Crystallite size (nm)

materials

CaO

Ca9Al6O18

NiO

CeO2

(m2/g)

CaO

NiO

CeO2

Ni15

66.2

18.8

15

-

11.8

31.8

20.1

-

Ni20

62.3

17.7

20

-

12.5

30.1

19.6

-

Ni25

58.4

16.6

25

-

9.6

28.8

21.0

-

Ni20Ce5

58.4

16.6

20

5

19.5

22.4

12.9

15.9

Ni20Ce10

54.5

15.5

20

10

15.4

23.3

16.6

14.6

Ni20Ce15

50.6

14.4

20

15

20.0

23.1

17.9

15.8



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Table 2. Summary of the TGA results for the spent Ni20 and Ni20Ce10 bifunctional materials after 5 SESRG/regeneration cycles. Sample Ni20 Ni20Ce10

Type of coke deposits Graphite Small carboncontaining hydroxylated groups

Oxidation temperature of coke deposits 550-700 °C

Amount of coke deposits ∼ 23%

300-400 °C

∼ 4%


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Figure captions Figure 1: (a) XRD patterns of calcined bifunctional materials. The following compounds were detected: ( ) CaO; (

) CeO2; ( ) NiO; (

)Ca9Al6O18. (b) A magnified view of the shift of

diffraction peaks at 2θ of 37.35° and 43.25°.

Figure 2: SEM images of freshly calcined bifunctional materials.

Figure 3: Representative TEM images and Ni particle size distributions of reduced Ni20 and Ni20Ce10 bifunctional materials.

Figure 4: CO2 capture capacity (a) and molar conversion of CaO (b) for Nix and Ni20Cey. Carbonation conditions: T = 550 °C, flowrate = 150 mL/min containing 15 vol% CO2, 9.5 vol% H2O, and 75.5 vol% Ar; Calcination conditions: T = 800 °C, 100 vol% Ar.

Figure 5: Conversion profiles of Nix and Ni20Cey as a function of time at the 1st cycle. Carbonation conditions: T = 550 °C, flowrate = 150 mL/min containing 15 vol% CO2, 9.5 vol% H2O, and 75.5 vol% Ar; Calcination conditions: T = 800 °C, 100 vol% Ar.

Figure 6: CO2 capture capacity (a) and molar conversion of CaO (b) in the absence and presence of water. Carbonation conditions: T = 550 °C, flowrate = 150 mL/min containing 15 vol% CO2, 9.5 vol% H2O, and 75.5 vol% Ar under wet condition and containing 15 vol% CO2 and 85 vol% Ar under dry condition; Calcination conditions: T = 800 °C, 100 vol% Ar.

Figure 7: Typical gaseous product distribution during the first cycle of SESRG over Ni20 bifunctional material. SESRG conditions: P = 1 atm, T = 550 °C, S/C = 9, WHSV = 1.55 h-1. 35 ACS Paragon Plus Environment


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Figure 8: Breakthrough curves of (a) Ni15, (b) Ni20, and (c) Ni25 over 5 cycles. SESRG conditions: P = 1 atm, T = 550 °C, S/C = 9, WHSV = 1.55 h-1.

Figure 9: Average hydrogen yield as a function of cycle number for Nix bifunctional materials. SESRG conditions: P = 1 atm, T = 550 °C, S/C = 9, WHSV = 1.55 h-1. Figure 10: Breakthrough curves of (a) Ni20Ce5, (b) Ni20Ce10, and (c) Ni20Ce15 over 5 cycles. SESRG conditions: P = 1 atm, T = 550 °C, S/C = 9, WHSV = 1.55 h-1.

Figure 11: Average hydrogen yield as a function of cycle number for Ni20Cey bifunctional materials. SESRG conditions: P = 1 atm, T = 550 °C, S/C = 9, WHSV = 1.55 h-1.

Figure 12: Average hydrogen purity and yield as a function of cycle number for Ni20Ce10 bifunctional materials. SESRG conditions: P = 1 atm, T = 550 °C, S/C = 9, WHSV = 1.55 h-1. Figure 13: XRD patterns of spent Ni20 and Ni20Ce10 bifunctional materials upon 5 SESRG/regeneration cycles. The following compounds were detected: ( ) CaO; ( )CeO2; ( Ni; ( ) Ca9Al6O18; ( )Graphite. Figure 14: SEM images of spent Ni20 and Ni20Ce10 after 5 SESRG/regeneration cycles.

Figure 15: Representative TEM images and Ni particle size distributions of spent Ni20 and Ni20Ce10 after 5 SESRG/regeneration cycles.


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Figure 1. XRD patterns of calcined bifunctional materials. The following compounds were detected: ( ) CaO; ( ) CeO2; ( ) NiO; ( )Ca9Al6O18. (b) A magnified view of the shift of diffraction peaks at 2θ of 37.35° and 43.25°.



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Figure 2. SEM images of freshly calcined bifunctional materials.


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Figure 3. Representative TEM images and Ni particle size distributions of reduced Ni20 and Ni20Ce10 bifunctional materials.



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Figure 4. CO2 capture capacity (a) and molar conversion of CaO (b) for Nix and Ni20Cey. Carbonation conditions: T = 550 °C, flowrate = 150 mL/min containing 15 vol% CO2, 9.5 vol% H2O, and 75.5 vol% Ar; Calcination conditions: T = 800 °C, 100 vol% Ar.


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Figure 5. Conversion profiles of Nix and Ni20Cey as a function of time at the 1st cycle. Carbonation conditions: T = 550 °C, flowrate = 150 mL/min containing 15 vol% CO2, 9.5 vol% H2O, and 75.5 vol% Ar; Calcination conditions: T = 800 °C, 100 vol% Ar.



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Figure 6. CO2 capture capacity (a) and molar conversion of CaO (b) in the absence and presence of water. Carbonation conditions: T = 550 °C, flowrate = 150 mL/min containing 15 vol% CO2, 9.5 vol% H2O, and 75.5 vol% Ar under wet condition and containing 15 vol% CO2 and 85 vol% Ar under dry condition; Calcination conditions: T = 800 °C, 100 vol% Ar. 42 ACS Paragon Plus Environment

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Figure 7. Typical gaseous product distribution during the first cycle of SESRG over Ni20 bifunctional material. SESRG conditions: P = 1 atm, T = 550 °C, S/C = 9, WHSV = 1.55 h-1.



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Figure 8. Breakthrough curves of (a) Ni15, (b) Ni20, and (c) Ni25 over 5 cycles. SESRG conditions: P = 1 atm, T = 550 °C, S/C = 9, WHSV = 1.55 h-1.


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Figure 9. Average hydrogen yield as a function of cycle number for Nix bifunctional materials. SESRG conditions: P = 1 atm, T = 550 °C, S/C = 9, WHSV = 1.55 h-1.



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Figure 10. Breakthrough curves of (a) Ni20Ce5, (b) Ni20Ce10, and (c) Ni20Ce15 over 5 cycles. SESRG conditions: P = 1 atm, T = 550 °C, S/C = 9, WHSV = 1.55 h-1.


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Figure 11. Average hydrogen yield as a function of cycle number for Ni20Cey bifunctional materials. SESRG conditions: P = 1 atm, T = 550 °C, S/C = 9, WHSV = 1.55 h-1.



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Figure 12. Average hydrogen purity and yield as a function of cycle number for Ni20Ce10 bifunctional materials. SESRG conditions: P = 1 atm, T = 550 °C, S/C = 9, WHSV = 1.55 h-1.


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Figure 13. XRD patterns of spent Ni20 and Ni20Ce10 bifunctional materials upon 5 SESRG/regeneration cycles. The following compounds were detected: ( ) CaO; ( )CeO2; ( )Ni; ( ) Ca9Al6O18; ( )Graphite.



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Figure 14. SEM images of spent Ni20 and Ni20Ce10 after 5 SESRG/regeneration cycles.


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Figure 15. Representative TEM images and Ni particle size distributions of spent Ni20 and Ni20Ce10 after 5 SESRG/regeneration cycles.


Sustainable Production of High-Purity Hydrogen by Sorption

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