SiO2 Catalyst: Overall

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Dry reforming of model biogas on a Ni/SiO catalyst: overall performance and mechanisms of sulfur poisoning and regeneration Xuejing Chen, Jianguo Jiang, Feng Yan, Kaimin Li, Sicong Tian, Yuchen Gao, and Hui Zhou ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02251 • Publication Date (Web): 25 Sep 2017 Downloaded from http://pubs.acs.org on October 2, 2017

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Dry reforming of model biogas on a Ni/SiO2 catalyst: overall performance and mechanisms of sulfur poisoning and regeneration Xuejing Chen a, b, Jianguo Jiang a, c, d*, Feng Yana, Kaimin Li a, Sicong Tian a,Yuchen Gaoa, and Hui Zhou b a. School of Environment, Tsinghua University, Beijing 100084, China b. Department of Earth and Environmental Engineering, Columbia University, New York, NY 10027, USA c. Key Laboratory for Solid Waste Management and Environment Safety, Ministry of Education of China, Beijing 100084, China d. Collaborative Innovation Center for Regional Environmental Quality, Beijing 100084, China

Abstract: Carbon-neutral application of renewable biogas to valuable chemical raw materials has received much attention in sustainable area, while the sulfur poisoning remains a big problem in biogas dry reforming process. This paper the sulfur deactivation and regeneration performance of a Ni/SiO2 catalyst in model biogas dry reforming and related mechanisms were studied. The effect of H2S content (50 and 100 ppm) and reaction temperature (700 to 800°C) on biogas dry reforming were investigated. Three regeneration methods (H2S feeding cessation, temperature programmed calcination (TPC), and O2 activation) were applied. The results showed that the presence of H2S caused server deactivation in catalytic activity, and higher H2S content led to faster deactivation. The deactivation was not reversed simply by stopping H2S feeding and TPC, but O2 activation could totally recover deactivated catalysts. The formation of Ni7S6, detected for the first time in a biogas conditioning catalytic processes, which was confirmed by X-ray diffraction and X-ray photoelectron spectroscopy, led to sulfur poisoning, as well as catalyst sintering and carbon deposition. This paper revealed that sulfur poisoning and regeneration mechanism is the formation of elimination of Ni7S6, and concluded that oxygen activation was the most effective method for reviving the catalytic activity, preventing sintering, and reducing carbon deposition. These findings will contribute to the industrial application of syngas production from biogas dry reforming.

Corresponding Author: Prof. Jianguo Jiang, E-mail: [email protected] ACS Paragon Plus Environment


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Key words: Biogas, dry reforming, sulfur poisoning, regeneration, nickel sulfide (Ni–S)

Introduction Biogas, an important renewable source of green energy, is produced from the anaerobic digestion of biomass, including landfill, food waste, and municipal sludge. With increasing concerns over climate change caused by anthropogenic CO2 emissions, the carbon-neutral application of biogas to valuable chemical raw materials has received much attention. Biogas dry reforming can convert the two main greenhouse gases (GHGs) to syngas (H2 and CO), which are valuable energy-related chemicals (Eq. 1). This process have both environmental and commercial benefits1-4. A typical biogas from municipal solid waste contains 50–75% CH4, 25–50% CO2, 0– 10% N2, and 0–3% H2S5. The H2S impurities in biogas may poison catalysts, especially when non-precious metals (mainly Ni) are used, which is a major concern in biogas dry reforming6-7. Catalyst deactivation can also be caused by carbon deposition, which is mainly caused by CH4 decomposition (Eq. 2) and CO disproportionation (Eq. 3)3, 8-9. In addition, an important side reaction, the reverse water gas shift (RWGS, Eq. 4), usually occurs simultaneously with dry reforming, leading to additional consumption of H2 and production of CO, thus decreasing the H2/CO ratio10-11. CH4 + CO2 2CO + 2H2; ∆H298K = 247 kJ mol–1 (1) CH4 CS + 2H2; ∆H298K = 75 kJ mol–1 (2) 2CO CS + CO2; ∆H298K = –172 kJ mol–1 (3) CO2 + H2 CO + H2O; ∆H298K = 41 kJ mol–1 (4) The sulfur poisoning effect of nickel-based catalysts was first studied in the catalytic process of biomass/coal gasification12-13 and steam reforming5, 14-16. Recently, sulfur poisoning in the dry reforming process has also been investigated1, 6 because sulfur impurities are inevitably produced during the process of biogas production, and the presence of even low sulfur levels after desulfurization may lead to the deactivation of nickel catalysts 17-18. It has been accepted for decades that catalysts can be deactivated by sulfur via the chemisorption of sulfur with active metal to form metal sulfides, which occupy the surface active sites, deactivating the catalyst17, 19-

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. However, the detailed sulfur poisoning performance and mechanism in a biogas dry reforming

process with regard to the formation and elimination of Ni–S species still remains unclear. H2S + Ni Ni–S + H2 (5) According to Mojdeh Ashraf et al.14, a high H2S/H2 ratio is responsible for the formation of bulk sulfide, and only surface nickel sulfide (Ni–S) is formed at a low H2S/H2 ratio. The Ni–S species that are formed have been mostly represented as Ni3S2 in the literature20, 22; however, the formation of Ni–S species is influenced by reaction temperature, gas phase composition, and reactor parameters. Furthermore, the Ni–S species formed may undergo phase transportation and may be oxidized during handling, which makes the identification of Ni–S species more difficult20. It is therefore important to determine the formation and elimination of Ni–S species during the deactivation and regeneration of a Ni/SiO2 catalyst in biogas dry reforming. Because the chemisorption process is theoretically reversible and exothermal, surface Ni–S can be reduced by stopping H2S feeding or by temperature enhancement. Srinivas Appari et al. 5 applied H2S removal, temperature enhancement, and a vapor treatment method to regenerate deactivated Ni/γ–Al2O3 in a methane steam reforming process. Mojdeh Ashraf et al.14 studied three regeneration methods and found that a temperature increase was more effective than removing sulfur from the feed gas or an oxidative treatment in Ni/Al2O3 in a biogas steam. Choudhary et al.23 conducted sulfur regeneration of a NiCoMgCeOx catalyst in an oxy–methane reforming process by air treatment, and reported that it was very effective. To the certain of our knowledge, the compared study on reversibility of sulfur poisoning in a Ni/SiO2 catalyst in a biogas dry reforming process has not been undertaken. The effect of sulfur poisoning on a nickel catalyst in a typical biogas reforming process and the poisoning mechanism remain unknown; an effective regeneration method needs to be developed. The aim of this study was to evaluate the deactivation and regeneration performance of a Ni/SiO2 catalyst under various operating conditions and for different regeneration methods during a biogas dry reforming process. Catalytic activity and selectivity under different temperatures and H2S concentrations were tested, and three regeneration methods (stopping H2S feeding, temperature-programmed calcination (TPC), and O2 activation) were applied to compare the effects on reversibility. Furthermore, physical adsorption, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and thermal gravimetric analysis (TGA) were conducted to characterize the sulfur poisoning and carbon deposition of freshly reduced, original deactivated,



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and regenerated catalysts by different regeneration methods to further determine the sulfur deactivation mechanism.

Experimental Methods Preparation of support and catalyst A SiO2 support was prepared by vapor-phase hydrolysis of SiCl4 at 150°C, as previously described by the authors24. A degas procedure in a vacuum oven at 70°C for 4 h and a calcination process in a muffle furnace at 800 for 4 h were applied for support pretreatment. A SiO2 supported catalyst with a Ni loading of 10% wt, was synthesized by wet impregnation. Five grams of calcined SiO2 was dispersed into 50 mL ethanol under ultrasonic oscillation, and Ni(NO3)2·6H2O (99%, Sigma Aldrich, St Louis, MO, USA) was dissolved into 50 mL ethanol to obtain the active metal precursor. Then, the two solutions were mixed together using a magnetic stirrer at 70°C at a rotation rate of 650 rpm for ~12 h to evaporate the ethanol. The supported catalyst powders were further dried at 60°C in a vacuum oven for 2 h, followed by a TPC procedure to 800°C in an air flow of 30 mL min–1. Finally, catalyst powders were tableted and screened to obtain particles of 0.42–0.84 mm for catalytic testing. Dry reforming experiments The biogas dry reforming experiments were conducted in a fixed-bed quartz reactor (inner diameter: 6 mm, outer diameter: 8 mm, length: 520 mm) heated by an electric furnace. Catalyst particles (0.42-0.84 mm, 100 mg) were loaded into the center of the reactor beside a K-type thermocouple to detect temperatures. Four flows of gas feeding, including reducing gas (10% vol H2 in N2 balance), model biogas mixture (CH4/CO2/N2 = 0.4:0.4:0.2), model H2S mixture (200 ppmv H2S in N2 balance), and standard air, were controlled by four separate mass flow controllers (MFC). The outlet gas mixture underwent a cooling water circulation process before being analyzed by an online micro GC apparatus (Micro 490, Agilent), which is equip with two


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independent channels installing a Molecular Sieve 5A PLOT column and a PoraPLOT U PLOT column, respectively, coupling with two thermal conductivity detectors (TCD). N2 was used as the internal standard gas for calibration.

Figure S1 (Supplementary materials) shows the

experimental set-up of fixed- bed reactor for biogas reforming process. Before each test, the catalyst samples were reduced in situ using a 10% vol H2 in N2 balance of 30 ml min-1 by heating from room temperature (RT) to 800°C at a rate of 10°C min-1, followed by a steady state at 800°C for 60 min. After the reduction, the temperature was lowered to the initial activity test temperature. For H2S tolerance tests, three temperatures (700, 750, and 800°C) and two H2S concentrations (50 and 100 ppmv) were used. First, model biogas without H2S was introduced for about 30 min to obtain the original activity. Different H2S contents were then introduced, and the deactivation of catalysts was observed. The H2S feeding was stopped when the conversion of CH4 and CO2 dropped to ~5%. The biogas dry reforming process was then continued to determine whether the activity could be regenerated by stopping in situ H2S feeding. Table S1 (Supplementary materials) shows the gas components of biogas with different H2S concentrations. During biogas dry reforming process, the flow rate of CH4 and CO2 was always kept at 40mL min-1, which made the gas hourly space velocity (GHSV) stable at 24,000 mL g-1 h-1. In addition to stopping the in situ H2S feeding, two other methods were also applied to study the regeneration of deactivated catalysts. One was the TPC method, during which the deactivated catalysts were calcined from RT to 800°C with a temperature ramp of 10°C min-1, followed by an isothermal step at 800°C for 30 min. The overall TPC process was conducted under a N2 stream of 30 mL min–1. The activity was then tested under related temperatures. Oxygen activation was also applied, in which the deactivated catalysts were calcined in standard air (50 mL min–1) from RT to 800°C by a temperature ramp of 10°C min-1 and kept in an isothermal state for 30 min; then, the catalysts were reduced using a reducing gas (50 mL min–1) at 800°C for 1 h. After regeneration, the catalytic activity tests were kept on stream for 70 min. The conversion (X) of CH4 and CO2 and the H2/CO ratio were calculated by Eq. 6-8:



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(6)

(7)

(8) where [CH4]、[CO2]、[H2]、[CO] refer to the molar flow rate of gas species, mol min-1. Catalyst characterization Catalysts that were freshly reduced, originally deactivated, and regenerated by the different regeneration methods were characterized by physical adsorption, XRD, TGA, and methods. Physical adsorption was carried out by a surface area and porosity analyzer (ASAP2020 HD88: Micromeritics, GA, USA) to obtain the specific surface area (SSA), pore volume, and pore size of different samples. The SSA was calculated by the Brunauer-Emmett-Teller (BET) model, and the average pore diameter was calculated by the Barrett-Joyner-Halenda (BJH) model. XRD patterns were obtained through high-resolution XRD (D8 Advance: Siemens, Munich, Germany) using Cu–Kα radiation (λ = 0.154 nm) with a scanning rate of 2° min-1 and 2θ range of 10–90°. A thermo-gravimetric analyzer (TGA/DSC 2: Mettler Toledo, Columbus, OH, USA) was applied to detect carbon deposition. Samples (~15 mg) were placed in an alumina crucible of 150 µL, then treated from RT to 900°C at a temperature ramp of 10°C min–1 under an air stream of 60 mL min–1 with a protective argon gas stream of 20 mL min–1. XPS measurements were conducted using an ESCALAB 250XI instrument (Thermo Fisher Scientific, Waltham, MA, USA) with monochromatized Al–Kα X-rays (1486.6 eV). The binding energies were corrected using the C 1s peak observed at 284.8 eV.

Results and Discussion Catalytic activity under different H2S concentrations Catalytic activity was observed under the different H2S concentrations. Figure 1 shows the effect of H2S concentration on biogas dry reforming at 700°C. The conversion profiles can be divided


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into three regimes: before H2S introduction, with H2S exposure, and after stopping H2S feeding. In the first 30 min, biogas reformed in the absence of H2S, and it was clear that both CH4 and CO2 conversion decreased slightly at 700°C, even when no H2S impurities were present; CH4 conversion dropped from 53.4% to 50.5%, and CO2 conversion dropped from 63.8% to 61.5%. When H2S was introduced at a concentration of 100 ppm, the conversion of CH4 and CO2 decreased rapidly, dropping to ~2.5% after 140 min. The use of a lower H2S concentration of 50 ppm also caused an obvious decrease in CH4 and CO2 conversion; however, the deactivation rate was slower than at the higher H2S concentration. By 200 min after the H2S was introduced, CH4 conversion dropped to 7.9%, and CO2 conversion fall to 8.5%. The deactivation rate of conversions were calculated by the conversion drop over time on H2S stream, and results were listed in Table S2. The deactivation rate of CH4 conversion was 22.9% with 50 ppm H2S， and increased to 36% when H2S content was 100 ppm, which indicating that a higher H2S concentration will lead to a faster sulfur deactivation. This situation also went well with CO2 conversion. Conversion profiles were also recorded after H2S feeding was stopped. At the very moment that H2S feeding was stopped, both CH4 and CO2 conversion were revived slightly, but they then resumed the decline. During the tests with the two different H2S concentrations, CH4 conversion was always lower than CO2 conversion at 700°C, which can be explained by the RWGS side reaction, which consumed more CO2 and H2 to produce CO and water vapor, resulting in a H2/CO ratio lower than 125. Figure 1(b) shows the related variation in the H2/CO ratio. Before the introduction of H2S, the H2/CO ratio was ~ 0.8, but the value dropped with the introduction of H2S, displaying a similar trend to that of the CH4 and CO2 conversion profiles, and indicating that H2S in biogas inhibited the production of H2 more than that of CO. A similar conclusion has also been reported for H2 selectivity, which was observed to decrease to a greater degree than that of CO during a deactivation process26.



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Figure 1 Influence of the H2S concentration on biogas dry reforming at 700°C (a) CH4 and CO2 conversion (b) H2/CO ratio; gas hourly space velocity (GHSV) = 24,000 mL g-1 h-1, P = 1 atm. When the biogas dry reforming temperature was increased to 750°C, the influence of the H2S content on CH4 and CO2 conversion, and the H2/CO ratio are shown in Figure 2. The conversion and selectivity profiles were divided into the three same regimes. There was a notable difference, with the conversion of CH4 and CO2 remaining stable before the introduction of H2S, and the difference between CH4 and CO2 conversion being small at 83% for CH4 conversion and 85% for CO2 conversion. The related H2/CO ratio was ~0.92, which was much closer to 1 than the value at 700 °C. A higher temperature can achieve a higher conversion of reactants and help retain the catalytic stability

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. After exposure to a H2S impurity of 100 pm, CH4 conversion


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dropped to 5.5%, and CO2 conversion fell to 7.9% in 180 min, whereas at a H2S concentration of 50 ppm, CH4 conversion fell to 6.5% and CO2 conversion fell to 7.5% in 200 min. When H2S feeding was stopped, the conversions continued to drop slowly. This was the same trend as for the H2/CO ratio.

Figure 2 Influence of the H2S concentration on biogas dry reforming at 750°C (a) CH4 and CO2 conversion (b) H2/CO ratio; gas hourly space velocity (GHSV) = 24,000 mL g-1 h-1, P = 1 atm. Figure 3 shows the influence of the H2S concentration on biogas dry reforming at 800°C. It is clear that both CH4 and CO2 conversion increased as the temperature increased, with the conversion reaching ~88% at 800°C, and the H2/CO ratio was 0.94 before H2S exposure. When the H2S concentration was 100 ppm, CH4 conversion and CO2 conversion fell to 5.1% and 7.9% in 130 min, respectively. It took 150 min for the conversions to drop to a similar level when the



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H2S concentration was 50 ppm. After stopping H2S feeding, both the CH4 and CO2 conversions were slowly revived. After 200 min, the conversions were still blow 20%. The H2/CO ratio followed the same trend as that of the CH4 and CO2 conversions. From the results of the tests with H2S, it was apparent that before H2S feeding, the catalytic activity of dry reforming was stable at high temperatures (750 and 800°C); at 700°C, there was a slight deactivation even in the absence of H2S feeding. After the introduction of H2S, catalysts displayed obvious deactivation at all of the temperatures investigated, and higher H2S concentration led to faster deactivation, which was considered reasonable because a higher H2S/H2 ratio has been shown to benefit the chemisorption of H2S on active metal and the formation of Ni–S species20. This shows that the deactivation of H2S poisoning is not reversible simply by stopping the H2S feeding. The activity continued to drop at 700 and 750°C; a high temperature of 800°C led to a slight recovery in catalytic activity, but the original activity could not be attained. Although in theory, it is possible to regenerate the deactivated catalyst by stopping H2S feeding (i.e., by reversing the chemisorption of H2S on Ni), the relatively low driving force and diffusion limit in real experiments leads to a low degree of regeneration of deactivated catalysts14. Both reaction temperature and H2S concentration can influence the degree of catalyst recovery, and full regeneration could not be achieved in these tests, which agrees with previous findings5, 14.


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Figure 3 Influence of H2S concentration on biogas dry reforming at 800°C (a) CH4 and CO2 conversion (b) H2/CO ratio; gas hourly space velocity (GHSV) = 24,000 mL g-1 h-1, P = 1 atm.

Regeneration of Catalysts Because a higher temperature is favorable for catalyst recovery, a TPC method to 800°C was also applied to compare the reversibility of the regeneration method. Figure 4 shows the conversion and selectivity results of deactivated catalysts regenerated by the TPC method. After calcination, the deactivated catalysts showed some recovery of activity, and then the conversions declined during the initial 30 min and remained stable at a lower level. The H2/CO ratio had a similar trend to that of the conversions. Table 1 shows the detailed changes in conversions. Compared to the original activity without the introduction of H2S at 800°C, the CH4 conversion initially



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recovered to 66%, and then it slowly dropped to 44%, followed by a period of stability. The CO2 conversion initially recovered to 75%, and then it slowly dropped to 60%, followed by a period of stability. This phenomenon was similar to the situation at 700 and 750°C, where the conversion of both CH4 and CO2 was revived slightly at first, and then fell to a lower level, where it remained stable. It seems that after regeneration by the TPC method, the activity could be partly revived, but there was still an obvious deactivation process, indicating that the absorbed sulfur was not fully removed by calcination.

Figure 4 Conversions and selectivity of deactivated catalysts regenerated by temperature programmed calcination (TPC): (a) CH4 and CO2 conversion, (b) H2/CO ratio. Table 1 Summary of the conversion of fresh catalysts and regenerated catalysts under different


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temperatures. Temperature

Original

Stopping H2S feeding

°C

XCH4

XCO2

XCH4

XCO2

700

52

63

Drop to ~0

Drop to ~0

750

82

84

Drop to ~0

800

88

88

Rise to 9

TPC XCH4

O2 activation XCO2

XCH4

XCO2

37→9

51→20

49

59

Drop to ~0

47→17

65→29

68

76

Rise to 16

66→44

75→60

88

88

Figure 5 shows the conversion and H2/CO ratios of deactivated catalysts regenerated by the O2 activation method. Both the CH4 and CO2 conversions and the H2/CO ratio were stable after O2 activation at all the temperatures tested. At a high temperature of 800°C, the activities revived to the original level, i.e., ~88% conversion of CH4 and CO2 and a H2/CO ratio of 0.94. When the temperature was 750°C, the conversion of CH4 and CO2 was 68% and 76%, respectively, which is a little lower than the original conversion at the same temperature. For the results at 700°C, the conversion of CH4 and CO2 was 49% and 59% respectively, also slightly lower than the fresh catalysts. The exact conversion data obtained from the O2 activation method, as well as the conversions of fresh and regenerated catalysts achieved by stopping H2S feeding, are listed in Table 1 for comparison. After O2 activation, the deactivated catalyst displayed stable activity, which was recovered to exactly the level of fresh catalysts at a high temperature of 800°C. Previously, Jasmin Blanchard et al. 28 concluded similarly that catalyst activity can be completely recovered by calcining in air at 900°C.



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Figure 5 Conversion and selectivity of deactivated catalysts regenerated by O2 activation: (a) CH4 and CO2 conversion, (b) H2/CO ratio

Analysis of the mechanism of H2S poisoning and regeneration The characterization of catalyst support, fresh reduced catalyst, original deactivated catalyst, and, regenerated catalysts by different methods were conducted to analyze the mechanism of H2S poisoning and regeneration. Table 2 shows the SSA, pore volume, and average pore size of different samples. After wet-impregnation and reduction process, the SSA and pore volume was reduced by 42.0% and 34.3%, respectively, and the average pore size decreased slightly by 2.5%, this was cause by pore blocking during the wet-impregnation process29-31 and the thermal contraction and/or condensation by dehydration and dehydroxylation of catalyst support during


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the high temperature reduction process32-33. After catalyst was deactivated under H2S tolerance for about 150 min, the SSA of original deactivated catalyst was 180.9 m2g–1, there was no obvious change in pore volume and average pore size comparing with fresh reduced catalyst. After regeneration through different methods, all the regenerated catalysts have a decrease in SSA and pore volume, and an increase in average pore size. However, the loss in SSA and pore volume for catalyst regenerated by stopping H2S feeding was most obvious. While catalysts regenerated by TPC and O2 activation have less decrease in SSA and pore volume. The degree of SSA and pore volume loss by different regenerated catalysts decreased by stop H2S feeding > TPC > O2 activation. Considering that catalyst poisoned by H2S was related to the chemical sorption with active metal species, and the change in SSA and pore volume was caused by the change of textural structure of catalyst support5, 34, so the reaction time of regenerated catalysts played an important role in affecting the SSA and pore volume of catalysts. The reaction time of catalyst regenerated by stopping H2S feeding, which was 200 min, was responsible for the most obvious loss in SSA and pore volume. When the reaction time of catalyst regenerated by TPC and O2 activation was the same, the SSA of catalyst regenerated by O2 activation was higher, indicating that O2 activation was more effective. Table 2 Textural properties of different catalysts.

#

Samples

SSA

PV

BJH average pore volume

Average crystal sizea

m2 ·g–1

cm3 ·g–1

nm

Nm

1

Support

337.0

1.28

15.20

-

2

Reduced

195.7

0.84

14.82

5.64

3

Original deactivation

180.9

0.89

12.00

9.54

4

Stop H2S feeding

98.5

0.72

29.25

7.54

5

TPC

117.3

0.78

18.09

6.53

6

O2 activation

147.5

0.82

13.21

6.09

a. Average crystal size was obtained through Scherrer's equation from the diffraction of the Ni (200) plane.

XRD analysis was used to characterize the change in the crystal phase in different catalysts. Figure 6 shows the XRD patterns of freshly reduced, originally deactivated, and regenerated catalysts by stopping H2S feeding, TPC, and O2 activation. All of the catalysts had diffraction peaks of Ni° (PDF 04–0850), while the peak intensity of the original deactivation for Ni° was weak. The broad peak at ~22° in all samples was attributed to the diffraction of the amorphous



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SiO2 support24. It is also important to note that only the original deactivated catalyst had a new crystal phase of Ni7S6 (PDF 14-0364)35, which was caused by the reaction of H2S in biogas with the active nickel of the catalysts to form Ni–S. The existence of Ni7S6 species in the original deactivated catalyst is further proof that the sulfur poisoning mechanism (i.e., the formation of Ni–S blocks the active sites) leads to severe deactivation of catalysts. The Ni7S6 species has been detected in the sulfur poisoning process for the first time. The crystal sizes of nickel was calculated using Scherrer's equation, and the results are listed in Table 2. The freshly reduced catalyst had an average Ni° crystal size of 5.64 nm, whereas the size of the crystal deactivated by H2S poisoning was 9.54 nm, indicating obvious sintering. Although the peak intensity of Ni7S6 is small, we can use it to calculate the crystal size of Ni7S6 phase through Scherrer equation. The diameter result was calculated to be ~42 nm, which is much bigger than the 9.5 nm of Ni. The formation of Ni7S6 in large particle size may change the surface energy of nickel particle and lead to catalyst sintering. After various regeneration methods, the average nickel crystal size changed to differing degrees. The diameter of the Ni° crystal size decreased in the order of original deactivation > stopping H2S feeding > TPC > O2 activation > freshly reduced catalyst, indicating that sulfur poisoning may also enhance catalyst sintering and that O2 activation is the most effective regeneration method for retarding catalyst sintering. This is in accordance with the conclusion of Changkun Yuan et al., who reported that the adsorption of sulfur might block the active sites and accelerate sintering as well 36.

Figure 6 X-ray diffraction (XRD) patterns of fresh, deactivated, and regenerated catalysts using


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different regeneration methods

Figure 7 X-ray photoelectron spectroscopy (XPS) spectrum of original deactivated catalysts and regenerated catalysts using different regeneration methods: (a) Ni 2p and (b) S 2p

XPS analysis was used to determine the surface composition and chemical state of the nickel and sulfur species of the deactivated and regenerated catalysts. Figure 7 shows the Ni 2p 3/2 and S 2p XPS spectra of the original deactivated catalyst and regenerated catalysts using different regeneration methods. The strong peaks at 852.6 eV of Ni 2p in three regenerated catalysts indicates the existence of Ni° 37, whereas the peak at 856.1 eV and ~861.5 eV refers to the main and satellite peaks of Ni2O336, indicating that slight oxidation occurred during the biogas dry reforming process. The Ni° peak of the original deactivated catalyst shifted to a higher value (853.4 eV) than the regenerated catalyst, probably due to a different chemical environment of Ni° caused by the existence of Ni7S6. When the original deactivated catalyst was regenerated by different methods, the peak area of Ni° at 852.6 eV increased in the order of the original deactivation < stopping H2S feeding < TPC < O2 activation, and the peak of Ni2O3 at 856.1 eV decreased in the same order, which implied that sulfur poisoning had an oxidative effect on the nickel catalyst36 and the oxidative composition of nickel decreased to a different extent after regeneration by different methods. The S 2p spectrum in Figure 7(b) exhibited an obvious noise signal, as the formation of bulk Ni–S was unstable17; however, a broad peak at 160.5–163.0 eV was still noticeable in the original deactivated catalysts, which could be attributed to a S2- species according to Struis et al.38. The results also confirmed the formation of



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Ni–S in the original deactivated catalyst, which agreed with the XRD results. The Ni–S species were only detected in the original deactivated catalysts, implying that the regeneration methods also helped to eliminate Ni–S, at least to a level making it undetectable by XRD and XPS.

Table 3 Calculation of carbon deposition amount of catalyst regenerated by different methods. Catalyst

Fresh reduced

Original deactivation

Stopping H2S feeding

TPC

O2 activation

Average crystal size (nm)

5.64

9.54

7.54

6.53

6.09

Weight increase (%)

--

2.73

2.10

1.95

2.08

Weight loss (%)

--

3.97

1.10

0.94

0.86

Carbon deposition amount (%)

--

3.28

1.10

0.94

0.86

Carbon deposition rate (mmol h-1)

--

0.86

0.29

0.25

0.22

Figure 8 shows the TGA profiles of the original deactivated and regenerated catalysts using different regeneration methods. At temperatures below 200°C, all four samples experienced a slight weight loss of ~0.5%, which was attributed to the loss of adsorbed gas and surface water. From 200 to 400°C, the three regenerated catalysts exhibited an obvious weight increase of ~2%, which was due to the oxidation of Ni° to NiO39-40. When the temperature increased above 400°C, the three regenerated catalysts started to lose weight in the air stream, which was ascribed to the oxidation of deposited carbon41. The carbon deposition rate is calculated by the amount of carbon deposited over time on stream, and the results were listed in Table 3. The regenerated catalysts experienced a slight carbon deposition amount of ~1%. The original deactivated catalyst had a TGA profile that was distinguishable from the three regenerated ones above 200°C. The weight increase process occurred from 200 to ~530°C, with the increase being 2.73%, which was much larger than that of the regenerated catalysts. Because the same catalyst was applied here, the difference in weight increase was attributed to the oxidation of Ni–S, which was shown to be exist by the XRD and XPS analysis. During O2 activation by an air stream, Ni–S can be oxidized to nickel sulfate and decomposed to nickel oxide under an air stream42-43 (Eq. 9 -10). A reduction process was then applied to reduce NiO to metal Ni, thus reviving the catalytic activity43-44.


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Mojdeh Ashrafi et al. reported that H2S poisoning was not reversible by air treatment14, probably because of the absence of a reduction process after air treatment. Ni-S+ O2 → NiSO4 NiSO4→NiO+SO2

(9) (10)

Assuming that the four samples would have the same weight increase due to nickel oxidation, the actual amount of carbon deposited on the original deactivation catalyst would be the weight loss at 530°C minus the difference during the weight increase process of nickel and nickel sulfide oxidation. The carbon deposition rate on the original deactivated catalyst was calculated to be 0.86 mmol h-1, which was much more than that on the regenerated catalyst. The amount of carbon formed followed the order of the original deactivation > stopping H2S feeding > TPC > O2 activation, implying that sulfur poisoning blocked the active metal and inhibited carbon gasification45-46. However, if sulfur concentration was precisely controlled, the existence of trace sulfur can inhibit carbon formation due to ensemble of Ni atoms where carbon growth is not active, although the reaction rate would decrease47. Since that the catalytic activity was related to the number of active nickel sites, with the increase of time on stream (TOS), the number of active nickel site will decrease, and the coverage of nickel sulfide (Ni-S/Nitotal) was lower than 1. For the original deactivated catalyst, the CH4 and CO2 conversions almost dropped to 0, which means the total loss of active nickel sites. However, the XRD and XPS results proved that metal Ni° dominated in original deactivated catalyst. One probable explanation is that the residual active Ni was blocked by deposited carbon. Usually, carbon deposition is prohibited at high conversions since the main dry reforming reaction dominated this process. While, when conversions are low, side reaction to form coke are more favorable. Thus conversion loss was caused by a combined and sequential poisoning effect of sulfur and carbon deposition2, 48. All three of the regeneration methods could effectively remove Ni–S and reduce carbon formation, however, the O2 activation method was the most effective for reviving catalytic activity, preventing sintering, and reducing carbon deposition.



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Figure 8 Thermal gravimetric analysis (TGA) profiles of the original deactivated catalysts and regenerated catalysts using different regeneration methods (60 mL min−1 air stream).

Conclusions To better understand the deactivation and regeneration performance and mechanism of sulfur poison in a biogas dry reforming process, an evaluation of the deactivation and regeneration of a Ni/SiO2 catalyst under various operating conditions was conducted. Three regeneration methods, stopping H2S feeding, TPC, and O2 activation, were applied to compare their reversibility. The testing of catalytic activity and selectivity under different temperatures and H2S concentrations showed that H2S exposure in biogas feeding could cause severe catalyst deactivation, and a higher H2S concentration led to a faster deactivation. The deactivation of H2S poisoning was not reversible by simply stopping the H2S feeding. Only at a high temperature of 800°C was a slight catalytic activity recovery evident, but the original activity could not be attained due to the diffusion limit. After being regenerated by the TPC method, the activity could be partly revived, but there was still obvious deactivation due to the incomplete removal of Ni–S. Oxygen activation stabilized catalytic activity at a certain level, and was the most effective way to regenerate the deactivated catalyst. Furthermore, XRD, XPS, and TGA were applied to characterize the mechanism of sulfur poisoning and carbon deposition in freshly reduced, originally deactivated, and regenerated catalysts using different regeneration methods. The formation of Ni7S6, detected for the first time through XRD in a sulfur poisoning process. The XPS spectrum also revealed the existence of


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Ni–S species, which blocked the active sites, led to catalyst sintering and carbon deposition. The reversibility of different regeneration methods was dependent on the degree of elimination of Ni– S. This paper provides that the key factor for sulfur deactivation and regeneration on nickel catalyst is the formation and removal of Ni7S6, and O2 activation method is the most effective for reviving catalytic activity, preventing sintering, and reducing carbon deposition. Supporting Information. Scheme of the experimental set-up, table of gas components of biogas with different H2S concentrations, and calculation of H2S deactivation rate under various conditions.

Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENTS This work was financially supported by the Major Science and Technology Program for Water Pollution Control and Treatment (2017ZX07202005), National Natural Science Foundation of China

(21576156)

and

Tsinghua

University

Initiative

Scientific

Research

Program

(2014z22075). The English in this document has been checked by at least two professional editors, both are native English speakers. For a certificate, please see: http://www.textcheck.com/certificate/1H0Ldp

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DOI: 10.1016/j.apcata.2010.12.019



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TOC

The sulfur poisoning and regeneration mechanism of Ni in dry reforming process is the formation and elimination of Ni7S6.


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97x44mm (300 x 300 DPI)


SiO2 Catalyst: Overall

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