L. P. Teha,
S. Triwahyono*ab,
A. A. Jalilcd,
C. R. Mamata,
S. M. Sidikd,
N. A. A. Fatahd,
R. R. Muktie and
T. Shishidof
aDepartment of Chemistry, Faculty of Science, Universiti Teknologi Malaysia, 81310 UTM Johor Bahru, Johor, Malaysia. E-mail: sugeng@utm.my
bIbnu Sina Institute for Fundamental Science Studies, Universiti Teknologi Malaysia, 81310 UTM Johor Bahru, Johor, Malaysia
cInstitute of Hydrogen Economy, Universiti Teknologi Malaysia, 81310 UTM Johor Bahru, Johor, Malaysia
dDepartment of Chemical Engineering, Faculty of Chemical Engineering, Universiti Teknologi Malaysia, 81310 UTM Johor Bahru, Johor, Malaysia
eDivision of Inorganic and Physical Chemistry, Faculty of Mathematics and Natural Sciences, Institut Teknologi Bandung, Jl Ganesha No 10, Bandung 40132, Indonesia
fDepartment of Applied Chemistry, Graduate School of Urban Environmental Sciences, Tokyo Metropolitan University, 1-1 Minami-Osawa, Hachioji, Tokyo 192-0397, Japan
First published on 22nd July 2015
Nickel-promoted mesoporous ZSM5 (Ni/mZSM5) was prepared for CO methanation. XRD, NMR and SEM analysis confirmed the structural stability of Ni/mZSM5 with coffin type morphology. The nitrogen physisorption and pyrrole adsorbed FTIR analyses indicated the presence of micro–mesoporosity and a moderate amount of basic sites on both mZSM5 and Ni/mZSM5. At 623 K, Ni/mZSM5 showed a high rate of CO conversion (141.6 μmol CO g-cat−1 s−1) and 92% CH4 yield. Ni/mZSM5 showed better catalytic performance than Ni/MSN (82.4 μmol CO g-cat−1 s−1, 82% CH4 yield), Ni/HZSM5 (29.0 μmol CO g-cat−1 s−1, 54.5% CH4 yield), and Ni/γ-Al2O3 (14.5 μmol CO g-cat−1 s−1, 38.6% CH4 yield). It is noteworthy that the superior catalytic performance of Ni/mZSM5 could be attributed to the presence of both micro–mesoporosity and basicity, which led to a synergistic effect of Ni metal active sites and the mZSM5 support. In situ FTIR spectroscopy showed that CO and H2 may be adsorbed on Ni metal followed by spillover to form adsorbed CO and adsorbed H on the mZSM5 surface. Then, two possible mechanisms for CO methanation were proposed. In the first mechanism, the adsorbed CO may be reacted with H2 to form CH4 and H2O. In the second mechanism, the adsorbed H may be reacted with CO to form CH4 and CO2. However, in this case, the former is the predominant pathway as the methanation reaction is favored by inhibition of the water–gas shift reaction.
In previous reports, catalytic performances for CO methanation have been mostly investigated on various supports, such as silica, alumina, and mesoporous material.4–10 Yan et al. reported the use of plasma prepared Ni/SiO2 on CO methanation.5 It gave about 82% CO conversion at 673 K. Guo et al. studied the effect of ZrO2 in Ni/Al2O3 for CO methanation.6 100% CO conversion was obtained at 623 K. On the other hand, Liu et al. studied the influence of V2O3 in the catalytic performance of Ni/Al2O3 for CO methanation.7 At 673 K, it showed nearly 100% CO conversion and 89% CH4 yield. Moreover, Zhang et al. reported that 10 wt% Ni-MCM-41 exhibited excellent activity and stability in the CO methanation with 95.7% CH4 yield at 623 K.8 Besides, Gao et al. prepared the high surface area Ni supported on barium hexaaluminate (Ni/BHA) for improved CO methanation compared with the conventional Ni/BHA.9 It gave 100% CO conversion and 95.7% CH4 yield at 673 K. In addition, Jia et al. reported the improved CO methanation with the use of nickel supported on the perovskite oxide CaTiO3 (Ni/CTO).10 At 673 K, it showed 100% CO conversion and 84% CH4 yield. Nevertheless, they are seldom supported on zeolites. For many catalytic reactions, structure and activity were greatly influenced by the nature of the support material.11–14
Zeolites have proven to be suitable for a variety of applications in industrial heterogeneous catalysis, separation, and adsorption processes. Zeolite ZSM5 is a crystalline aluminosilicate with an MFI structure. It possesses both acidic and basic sites. The bridging OH groups, the trigonally coordinated and extra framework aluminum contributed to the acidity.15 While, the basicity is due to the basic framework oxygen atoms bearing the negative charge. The negative charge on the oxygen atoms is enhanced as the electropositive character of the nonframework compensating cations increases.16 The extraordinary catalytic performance of zeolite catalysts is due to their crystalline frameworks and topological channel structures.17 However, the relatively small individual micropores in zeolites cause diffusion limitations and significantly influence the transportation to and from the active site, severely limiting their application in industry. Moreover, deactivation caused by coke formation is also a severe problem that routinely arises in catalytic applications catalyzed by zeolites.18 Therefore, mesoporous zeolites possessing micro–mesoporosity are urgently needed as an effective solution to overcome these drawbacks.
A large number of supported metal catalysts have been reported to be active for CO methanation. Various transition metals like Ni, Co, Rh, Ru, Pd, Pt, and so on have been investigated over different supports.19,20 However, some noble metals such as Rh and Ru are not economical for large-scale production of methane due to their high cost. Therefore, the use of nickel-based catalysts is preferred from the commercial standpoint because of their low cost and wide availability. It should be noted that the catalytic performance of the nickel-based catalysts depends not only on the active nickel metal sites but also on the chemical and physical properties of the supporting materials.
In our previous work, we prepared mesoporous ZSM5 (mZSM5) by the dual templating method and tailored the zeolite properties by varying the aging time.21 In the present work, we prepared nickel-promoted mesoporous ZSM5 (Ni/mZSM5) for CO methanation. The correlation of their physicochemical properties with the catalytic performances is presented and discussed. For comparison purposes, we also studied different types of supports such as commercial HZSM5, γ-Al2O3, and mesostructured silica nanoparticles (MSN). Moreover, in situ FTIR spectroscopy of CO methanation using mZSM5 and Ni/mZSM5 catalyst was also performed in order to provide deeper insight into the reaction mechanism. The high activity of structurally stable Ni/mZSM5 for CO methanation was strongly determined by the presence of both micro–mesoporosity and basicity, which led to a synergistic effect between Ni metal active sites and the mZSM5 support.
A commercial HZSM5 (Zeolyst International) with Si/Al atomic ratio of 23 was used as a catalyst support. A commercial γ-Al2O3 (Sigma-Aldrich) was used as a catalyst support. Prior to modification, HZSM5 and γ-Al2O3 was treated at 823 K. MSN was prepared by the sol–gel method according to a report by Aziz et al.22 In brief, cetyltrimethylammonium bromide (CTAB), ethylene glycol (EG), and NH4OH solution were dissolved in water with the following molar composition of CTAB:
EG
:
NH4OH
:
H2O = 0.0032
:
0.2
:
0.2
:
0.1. After vigorous stirring for about 30 min at 353 K, 1.2 mmol of tetraethyl orthosilicate and 1 mmol of 3-aminopropyl triethoxysilane were added to the clear mixture to give a white suspension solution. This solution was then stirred for another 2 h, and the sample was collected by centrifugation at 3000 rpm. The synthesized MSN was dried at 333 K and calcined at 823 K for 3 h.
The 5 wt% Ni-promoted supports were prepared by the wet impregnation method over mZSM5, HZSM5, γ-Al2O3, and MSN supports. The aqueous nickel nitrate (Ni(NO3)2·6H2O) was impregnated on the support at 353 K, and was then dried in an oven at 383 K overnight before calcination in air at 823 K for 3 h.
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Fig. 1 XRD patterns of mZSM5, HZSM5, γ-Al2O3, MSN, and Ni-promoted catalysts; the inset shows NiO (*) peaks. |
Fig. 2 shows the N2 adsorption–desorption isotherms and NLDFT pore size distribution of mZSM5, HZSM5, γ-Al2O3, MSN, and Ni-promoted catalysts. For mZSM5-based catalysts (mZSM5and Ni/mZSM5), all isotherms were type IV adsorption isotherms with type H1 hysteresis loops, which is typically exhibited by uniform mesoporous material according to the IUPAC classification. A sharp uptake at low relative pressure indicated the presence of microporosity. In addition, an increased uptake at relative pressures of P/P0 = 0.2–0.4 was due to the presence of mesoporosity. The first step at a relative pressure of 0.2–0.4 was due to the presence of intraparticle pores, while the second step at P/P0 = 0.9–1.0 was due to the presence of interparticle pores.28 These results confirm the permanence of the mesoporous phase in parallel with the microporous phase in mZSM5. Besides, it is noteworthy that the second step at higher partial pressure was slightly decreased for Ni/mZSM5, which could be attributed to the fact that Ni particles blocked some of the interparticle pores of mZSM5. On the contrary, commercial HZSM5 demonstrated a type I isotherm with type H4 hysteresis loops, which is usually exhibited by microporous solids.29 No obvious changes were observed upon the introduction of Ni. For Al2O3-based catalysts (γ-Al2O3 and Ni/γ-Al2O3), all isotherms were type IV adsorption isotherms (according to the IUPAC classification) with type H1 hysteresis loops, which is characteristic of mesoporous materials, broad pore size distribution, and uniform cylindrical shape.30,31 No significant difference was noticed for Ni/γ-Al2O3 with respect to the bare γ-Al2O3. Moreover, MSN-based catalysts (MSN and Ni/MSN) exhibited a type IV isotherm with a type H1 hysteresis loop, confirming a typical adsorption profile for a mesostructured material. The filling of intraparticle and interparticle pores was observed at P/P0 = 0.2–0.4 and 0.9–1.0, respectively. The decrease of the step at high partial pressure could be attributed to the fact that the Ni particles blocked the interparticle pores of MSNs.
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Fig. 2 N2 adsorption–desorption isotherms and NLDFT pore size distribution of mZSM5, HZSM5, γ-Al2O3, MSN, and Ni-promoted catalysts. |
The pore size distribution of all catalysts was calculated by the non-local density functional theory (NLDFT) method. Significantly, narrow pore size distributions in the range of 3–6 nm were observed for mZSM5. It is noteworthy that mZSM5 has both micropores and mesopores and the amount of mesopores is higher than in commercial HZSM5. With Ni–metal loading, the pore size of the mZSM5 shifted towards a higher occurrence of micropores and slightly higher mesopore size. For HZSM5, an obvious decrease of the pore volume at a pore size of 2.4 nm led to the evolution of smaller observed pore size after the introduction of Ni onto HZSM5. For Al2O3-based catalysts, the pore size distribution was centered at 9.4 nm. Only a slight decrease in the pore volume was observed on Ni/γ-Al2O3. For MSN-based catalysts, a bimodal pore size distribution of 3.7 and 43.0 nm was observed. A marked decrease in pore volume was observed on Ni/MSN.
The summary data on surface areas and total pore volumes of all catalysts are listed in Table 1. In all cases, it can be seen that the surface area and total pore volume decreased considerably after the introduction of Ni, suggesting that a portion of the Ni particles were dispersed in the pores of the supports.
Catalysts | Surface area (m2 g−1) | Total pore volume (cm3 g−1) |
---|---|---|
mZSM5 | 733 | 0.248 |
Ni/mZSM5 | 477 | 0.203 |
HZSM5 | 389 | 0.222 |
Ni/HZSM5 | 367 | 0.199 |
γ-Al2O3 | 198 | 0.531 |
Ni/γ-Al2O3 | 184 | 0.485 |
MSN | 965 | 1.573 |
Ni/MSN | 769 | 0.867 |
27Al MAS NMR and 29Si MAS NMR offer a strong and effective tool for characterizing the structure of zeolite. In general, species with different structures or different chemical environments of the aluminum and silicon atoms will have different chemical shifts in their 27Al MAS NMR and 29Si MAS NMR spectra.32 Fig. 3A and B show the 27Al MAS NMR and 29Si MAS NMR spectra of all catalysts, respectively. The 27Al MAS NMR was carried out to detect the presence of tetrahedral coordinated atoms (in the framework sites) and octahedral coordinated aluminum atoms (possibly as extra-framework aluminum, EFAL). As shown in Fig. 3A, three signals were observed for mZSM5: one signal at 61 ppm and two signals at around 0 ppm. A sharp resonance at 61 ppm corresponds to the tetrahedrally coordinated aluminum in the framework structure. This demonstrated that most of the aluminum atoms are incorporated into the zeolite framework. Additionally, two resonance signals were observed around 0 ppm, corresponding to the octahedral aluminum species in a highly symmetric environment and distorted octahedral aluminum species. For Ni/mZSM5, three signals were observed. A sharp signal at 59.5 ppm can be assigned to tetrahedral framework aluminum species. In addition, two octahedral aluminum species can be detected, both with an isotropic shift around 0 ppm, one type in a highly symmetric environment and one more distorted.33 As compared with mZSM5, the intensity of the signal at around 0 ppm increased obviously may be due to the occurrence of dealumination during the calcination at 823 K, which then increased the extra-framework aluminum species.34 On the other hand, only two signals were observed for HZSM5, at 56.5 and 0 ppm, which are attributed to tetrahedral and octahedral aluminum species, respectively. For Ni/HZSM5, two signals were observed at 55.6 and 0 ppm, corresponding to tetrahedral and octahedral aluminum species, respectively. For Al2O3-based catalysts (γ-Al2O3 and Ni/γ-Al2O3), two signals centered at 71.5 and 11 ppm were observed and can be assigned to tetrahedrally coordinated Al and octahedrally coordinated Al, respectively.35 In Fig. 3B, only a dominant signal was observed at −106 ppm, which is assigned to the crystallographically equivalent site of (SiO)4Si for both mZSM5 and Ni/mZSM5.32 No significance difference was observed upon the introduction of Ni. For HZSM5, a dominant signal was observed at −104.4 ppm. Additionally, two shoulder peaks appeared at −98 and −93.5 ppm, indicating the formation of (
SiO)3Si and (
SiO)2Si, respectively. For MSN and Ni/MSN, three signals at −102, −93.5, and −84.5 ppm were observed, which can be assigned to (
SiO)4Si, (
SiO)3Si, and (
SiO)2Si, respectively.32
Fig. 4 shows the SEM images of mZSM5 and Ni/mZSM5. As illustrated in the images, mZSM5 possessed a smooth surface with a typical coffin-type morphology. Similarly, Ni/mZSM5 also had a coffin-type morphology but the surface was covered with some Ni metal. Xin et al. observed that the parent ZSM-5 had a smooth surface with typical coffin shape and uniform crystallite size of 1.5–2.5 μm.36 In addition, Zhou et al. reported the synthesis of mesoporous ZSM-5 zeolite crystals by conventional hydrothermal treatment under stirring. Without stirring, conventional MFI morphology was quite smooth and no mesopores or growth steps on crystal surfaces were observed.37 On the contrary, rough and moustache-like surfaces were observed with stirring. The authors proposed that this mesoporous ZSM-5 crystal contains a microporous coffin-shaped core crystal wrapped by a mesoporous shell composed of uniformly aligned zeolite nanocrystals.
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Fig. 4 SEM images of (A) mZSM5, (B) Ni/mZSM5, (C) closed up single particle of mZSM5, and (D) closed up single particle of Ni/mZSM5. |
Fig. 6A shows the rate of CO conversion for all catalysts as a function of the reaction temperatures. The activity of all Ni-promoted catalysts showed an obvious increase with increasing temperature. It is noteworthy that the catalytic performance of Ni/mZSM5 was superior compared to that of other Ni-promoted catalysts (Ni/MSN, Ni/HZSM5, and Ni/γ-Al2O3); it presents a significant catalytic activity (rate of CO conversion = 141.6 μmol CO g-cat−1 s−1) at 623 K. Additionally, Ni/mZSM5 exhibited the highest yield of CH4 of 92.0% at 623 K, which increased notably to 96.1% at 673 K, as demonstrated in Fig. 6B. Meanwhile, Ni/MSN only gave 82.4 μmol CO g-cat−1 s−1 and 82% CH4 yield, Ni/HZSM5 gave 29.0 μmol CO g-cat−1 s−1 and 54.5% CH4 yield, and Ni/γ-Al2O3 gave 14.5 μmol CO g-cat−1 s−1 and 38.6% CH4 yield at 623 K. Besides, Ni-promoted ZSM5 (Ni/ZSM5) gave 37.5 μmol CO g-cat−1 s−1 and 60.3% CH4 yield (inset figure in Fig. 6). As shown in Fig. 6C, only a small amount of CO2 (<10%) was produced for all catalysts under the reaction temperatures studied. These results showed that the existence of Ni metal sites inhibited the water–gas shift reaction of CO to CO2 and favored the methanation reaction. The presence of low CO2 yield for all Ni-promoted catalysts corroborates this suggestion.
It is noteworthy that our results are up to par with previous reported literature reviews (Table 2). Derekaya and Yasar reported CO methanation over NaY–zeolite in which Ni/ZrO2/NaY appeared to be the most active catalyst with 100% conversion at 548 K.44 In addition, Ding et al. reported the high activity of Ni/Al2O3–CeO2 with 91.6% CO conversion, 92% CH4 selectivity, and 84% CH4 yield at 623 K.45 Moreover, Variava et al. studied carbon-nanotube supported catalysts for CH4 production.46 Based on their results, 13 wt% Ni/MWNT achieved the highest activity with ∼95% CO conversion, ∼85% CH4 selectivity, and ∼81% CH4 yield at 623 K. Shinde et al. reported the implementation of 23 wt% Ni/TiO2 for CH4 production.47 They studied the sonication and conventional impregnation methods, and the former showed higher activity for CH4 formation, with ∼99% CO conversion, 88% CH4 selectivity, and 87% CH4 yield at 593 K.
Catalyst | Catalytic performance [%] | Reaction conditions | Reference | |||
---|---|---|---|---|---|---|
CO conversion | CH4 selectivity | CH4 yield | Temperature [K] | Pressure [MPa] | ||
Ni/mZSM5 | 100 | 92 | 92 | 623 | 0.1 | This study |
Ni/HZSM5 | 59 | 93 | 54 | 623 | 0.1 | This study |
Ni/γ-Al2O3 | 40 | 96 | 39 | 623 | 0.1 | This study |
Ni/MSN | 87 | 95 | 82 | 623 | 0.1 | This study |
Ni/ZrO2/NaY | 100 | — | — | 548 | — | 44 |
Ni/Al2O3–CeO2 | 91.6 | 92 | 84 | 623 | 0.1 | 45 |
Ni/MWNT | ∼95 | ∼85 | ∼81 | 623 | 0.1 | 46 |
Ni/TiO2 | ∼99 | 88 | 87 | 613 | 0.1 | 47 |
Recently, we reported a study of mesoporous ZSM5 having both intrinsic acidic and basic sites for cracking and methanation and we concluded that the co-existence of micro–mesoporosity with the presence of inter- and intra-particle pores and dual intrinsic acidic–basic sites is vital for acid-catalyzed and base-catalyzed reactions. In the present work, we focus on base-catalyzed CO methanation reaction for methane production. Fig. 7 shows the relationship of the basic sites with the catalytic activity at 623 K. Conversion of carbon monoxide to methane is essentially catalyzed by the support over the basic sites and therefore the presence of these basic sites is a key point in CO methanation to produce methane. With bare support, the presence of basic sites did not show any significance effect on the catalytic performance (rate of CO conversion and yield of CH4 and CO2). However, the catalytic activity is enhanced in the presence of Ni metal active sites and thus a synergistic effect of Ni metal active sites and mZSM5 support could be claimed to occur. Results from pyrrole adsorbed FTIR (Fig. 5) showed that the concentration of basic sites in Ni/mZSM5 is higher than in Ni/HZSM5 and Ni/γ-Al2O3 catalysts but lower than in Ni/MSN. Notably, an optimum amount of basic sites is needed to obtain a high yield of methane. These results are in accordance with other studies reported in the literature.31,48,49
The FTIR adsorption spectra of CO + H2 adsorption on mZSM5 and Ni/mZSM5, the interaction of H2 with pre-adsorbed CO, and the interaction of CO with pre-adsorbed H2 on Ni/mZSM5 are presented in Fig. 8. The blank reaction (without catalyst) of CO + H2 showed no significant peak, which showed that adsorbed species is needed for the methanation reaction. Furthermore, the adsorption of CO + H2 on Ni showed no IR adsorption peak as the experiment could not proceed because the Ni pellet became black after being reduced by the hydrogen flow. As mentioned earlier, CO methanation on Ni was negligible, indicating that a methanation reaction was probably not taking place on the Ni surface. For in situ FTIR spectroscopy of CO + H2 (Fig. 8A and B), the adsorption bands at 2170 and 2110 cm−1 were observed for both mZSM5 and Ni/mZSM5 catalysts, which can be assigned to the gaseous CO. A band at 1625 cm−1 was observed on mZSM5, which was assigned to atomic hydrogen. It can be suggested that bare mZSM5 has a low ability to adsorb and dissociate molecular hydrogen to atomic hydrogen. From our previous results, it is known that high methanation activity over mZSM5 only happens at high temperature (at 723 K). Therefore, in the present study, Ni metal was introduced to mZSM5 support to convert gaseous CO and H2 to adsorbed species on mZSM5 support, which allowed high interaction between the two reactants and lowered the reaction temperature. A band was observed at 1850 cm−1 and shifted to 1810 cm−1 at higher temperature, indicating the presence of adsorbed carbonyls on Ni0 sites (Ni0–CO) on the Ni/mZSM5.51,52 The band at 1850 cm−1 shifted to 1810 cm−1 upon temperature increase, which is likely caused by the destabilization of Ni0–CO species. This may also be due to the CO desorption from more labile adsorption on Ni sites. Moreover, the evolution of the adsorption band at 1625 cm−1 is attributed to the presence of atomic hydrogen. Furthermore, the formation of adsorbed carbonate species was only observed on Ni/mZSM5, as evidenced by the adsorption bands at 1510 and 1360 cm−1.53 At 623 K, Ni/mZSM5 showed a fully diminished in gaseous CO bands, a notable depletion in Ni0–CO bands, and progressive formation of carbonate species. At this temperature, the system's energy starts to be high enough for it to dissociate and then hydrogenate or to hydrogenate CO directly until methane formation.
In order to clarify the predominant reaction pathway for CO methanation over Ni/mZSM5, the interaction of H2 with pre-adsorbed CO and interaction of CO with pre-adsorbed H2 was examined by in situ FTIR (Fig. 8C and D). In the study of the interaction of H2 with pre-adsorbed CO, four adsorption bands were observed at 1780, 1625, 1510 and 1360 cm−1. The band at 1780 cm−1, which was assigned to Ni0–CO species, was significantly decreased at 623 K. No obvious changes of the other three adsorption bands were observed. On the other hand, in the study of the interaction of CO with pre-adsorbed H2, the results showed one additional adsorption band at 2340 cm−1, which was assigned to gaseous CO2. CO2 species may result from the interaction of the adsorbed CO with the oxide surface of mZSM5 or as a consequence of the water–gas shift reaction and accumulation on the support. With increasing temperature, reduction of the adsorption bands of Ni0–CO and atomic hydrogen at 1810 and 1625 cm−1, respectively, were observed. Moreover, a methanation reaction can occur on the Ni0 sites as well as on the mZSM5 support. However, in this case, the methanation sites on the mZSM5 support are more active as compared to the ones on the Ni0 sites. From these results, we can propose two possible mechanisms for CO methanation. In the first mechanism, the adsorbed CO species may be reacted with H2 to form CH4 and H2O. In the second mechanism, the adsorbed H may be reacted with CO to form CH4 and CO2. However, in this case, the former is the predominant pathway as the methanation reaction is favored by inhibition of the water–gas shift reaction. Therefore, a plausible reaction mechanism of CO methanation over Ni/mZSM5 is shown in Scheme 1.
Under the experimental conditions used in the present work, the presence of metal carbonyl (Ni0–CO) was observed. This may suggest that the route to methane formation was formed via metal carbonyl. Unfortunately, in this experiment, the CHx vibration bands in the 2800–3000 cm−1 region were not detected for these catalysts. Chen et al. studied the reaction mechanism of Si–Ni/SiO2 catalyst by temperature-programmed reaction FTIR (TPR-FTIR) and temperature-programmed desorption FTIR (TPD-FTIR).51 From their results, three possible mechanisms emerged: (1) the gas phase CO was initially absorbed on the surface of nickel silicide as a linear-and bridge-type species; (2) the bridge-type CO was easily dissociated to Cs (adsorbed carbon) and CO2 on the surface of nickel silicide; and (3) the linearly adsorbed CO and the Cs were then quickly reacted with dissociated hydrogen (Hs) to form the CH4. Zarfl et al. reported on the DRIFTS study of commercial Ni/γ-Al2O3 for CO methanation.54 They suggested that atomic C and H produced by CO and H2 dissociation on Ni during methanation and C–H species may recombine to form methane product. However, these experimental results cannot confirm the role of hydrogenation of adsorbed CO species. Zhang et al. proposed a mechanism of carbon monoxide methanation on a Ru(0001) surface based on a density functional theory (DFT) study.2 Their result showed that the reaction pathway for CO methanation proceeds via either a COH or a CHO intermediate from CO dissociation, resulting in active C and CH species, respectively. The active C and CH species subsequently undergo stepwise hydrogenation to CH4. Zhen et al. studied CO2 methanation on Ni–Ru/γ-Al2O3 and proposed that CO2 was dissociated on Ru species surfaces to form carbon species (COads) and oxygen species (Oads) and then reacted with activated H on Ni centers to form methane and water.55 However, the role of Ni and Ru species was not discussed in detail. Besides, the methanation reaction can also proceed through a hydrogen-assisted CO dissociation mechanism (formate route), which has been proposed in the literature.56,57 However, in the present work, the absence of formate species suggested that it is not a possible route for CO methanation over mZSM5-based catalyst.
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