Journal of Chemical Engineering of Japan, Vol. 41, No. 9, pp. 915–922, 2008
Research Paper
Investigation on Deactivation and Regeneration of a Commercial Ni/Al2O3 Catalyst in Coal Volatile Decomposition Liuyun LI1, Jun-ichi OZAKI2, Kayoko MORISHITA2, Chiemi IDA2, Masaaki TAKEI2 and Takayuki TAKARADA2 1
Gunma Industry Support Organization, 1-5-1, Tenjin-cho, Kiryu-shi, Gunma 376-8515, Japan 2 Department of Chemical and Environmental Engineering, Gunma University, 1-5-1, Tenjin-cho, Kiryu-shi, Gunma 376-8515, Japan
Keywords: Ni/Al2O3 Catalyst, Coal Volatile Decomposition, Deactivation, Regeneration, Structure Sensitivity The deactivation mechanism of a commercial Ni/Al2O 3 catalyst used during coal volatile decomposition was investigated by transmission electron microscopy–energy dispersive X-ray spectroscopy (TEMEDS), X-ray diffraction (XRD) and nitrogen adsorption. The existence of carbonous species in the reaction system promoted nickel particle growth during coal volatile decomposition, and subsequent coking from volatile cracking. Throughout the catalyst deactivation tests, coke deposits were observed as encapsulating carbon in the spent catalyst, and nickel particles doubled in size from around 10 to 20 nm. The spent catalyst was regenerated in oxygen at relatively moderate conditions by removing the coke deposits. As a result, the catalyst activity was restored remarkably; 1.7 times the surface area and double pore volume were present in the regenerated catalyst compared to the spent catalyst. Also, the regenerated catalyst showed high activity for coal volatile decomposition. Under catalysis of the regenerated Ni/ Al 2O 3, the tarry material in coal volatile matter could transform much more completely, gaining both high gas yields and high carbon balance. We also found that methanation is structure sensitive to nickel particles. Under the action of the regenerated catalyst, CO formed during coal volatile decomposition could not be further converted into methane, and the product gases provided a higher CO concentration. Noticeably, tar decomposition was confirmed to be less structure sensitive to the nickel particles than CO-methanation.
Introduction Research on efficient fossil fuel utilization has focused on topics such as efficient conversion of fossil fuel and high-calorie gas synthesis. For example, studies using group VIII metal catalysts have been conducted for high-calorie SNG synthesis at low temperatures ranging from 500 to 700 K (Rabo et al., 1978; Ishigaki et al., 1989; Nagase et al., 1998). Others have reported that high yields of light hydrocarbons were obtained via nascent-coal volatile catalytic hydrogenolysis under high hydrogen pressures with high-performance catalysts such as Ni, Co and Ru (Chareonpanich et al., 1994, 1995; Xu et al., 1990; Takarada et al., 1992, 1993, 1997). Concerning coal tar conversion, Miura et al. (2003) studied coke oven tar thermal pyrolysis and steam reforming for synthesis gas production, and reported that more than 80% of the tar could be decomposed at temperatures above 1273 K; Onozaki et al. (2006) obtained a higher con-
Received on March 5, 2008; accepted on May 22, 2008. Correspondence concerning this article should be addressed to L. Li (E-mail address:
[email protected]).
Copyright © 2008 The Society of Chemical Engineers, Japan
version ratio for hot coke oven gas (COG) at high temperatures between 1300 and 1600 K, via partial oxidation and steam reforming reactions. In our previous study, we succeeded in transforming coal volatiles to light fuel gases at relatively moderate temperatures around 900 K under a commercial Ni/Al2O3 catalyst and a natural iron ore (Li et al., 2006, 2007). Disappointingly, catalysts do not retain their activity and selectivity permanently; all catalysts deactivate and become less effective with time. Various processes operate with short-lifetime catalysts, but use regular regeneration to afford long periods between new catalyst charges. In most cases, deactivation will manifest as a general loss in activity, but changes in selectivity can also become apparent. Mechanisms for catalyst deactivation can be mainly grouped as coking, thermal sintering and poisoning (Furimsky and Massoth, 1999; Bartholomew, 2001; Moulijn et al., 2001). Alternative remedies can be undertaken to inhibit catalyst deactivation caused by these mechanisms, and proper methods of regeneration need to be considered in terms of catalyst suitability and effectiveness. Thus, it is important to understand the details of these mechanisms, and to ascertain the dominant factors in catalyst deactivation. 915
In this study, further research on the catalytic role during coal volatile decomposition and the deactivation mechanism of the commercial Ni/Al2O3 catalyst was conducted. The characteristics of the catalyst were analyzed by X-ray diffraction (XRD) and transmission electron microscopy–energy dispersive X-ray spectroscopy (TEM-EDS), and with Brunauer– Emmett–Teller (BET) and Barrett–Joyner–Halenda (BJH) algorithms. Regeneration was performed for the spent Ni/Al2O 3 catalyst, and the changes in both the catalyst structure and the catalytic activity were discussed. 1.
Experimental
1.1 Samples A bituminous coal (Witbank coal, particle diameter: 0.5–1.4 mm) was used. Sample properties were as follows: proximate analysis (wt%, as received): moisture (3.0), volatile matter (32.0), fixed carbon (57.4) and ash (7.8); ultimate analysis (wt%, daf): C (82.7), H (4.5), N (2.2), S (0.6) and O (10.0, by difference). An alumina-supported nickel catalyst (Ni/Al2O3, No. C13-4, Süd-Chemie Catalysts Japan, Inc.) with 20 ± 2 wt% Ni-loading was used. 1.2 Coal volatile decomposition Coal volatile decomposition was performed in a batch, two-stage fixed-bed quartz reactor. The schematic diagram of the reactor and the experimental details has been described in previous reports (Li et al., 2006, 2007). Product gases were analyzed by an offline gas chromatograph (GC-14B, using a flame ionization detector; Shimadzu Corp.), and the catalyst activity was estimated from the gas yields of coal volatile decomposition. 1.3 Catalyst deactivation and aging treatment One Ni/Al2O3 sample was repeatedly used for coal volatile decomposition. Before the activity tests, the Ni/Al2O3 catalyst was treated under a hydrogen flow (1 ml s–1 per gram nickel catalyst) for 40 min at 923 K, and this reduction treatment was omitted during subsequent use of the catalyst. In each run, a 1-g coal sample was fed and heated from room temperature to 1173 K at 10 K min–1. The volatile matter emitted during coal pyrolysis was streamed into the catalyst stage for decomposition, where the temperature was preset at 923 K. After coal pyrolysis, the reactor was cooled down to room temperature in order to remove char, and another coal sample was then fed for pyrolysis. The Ni/Al2O3 sample was repeatedly used until its activity was essentially reduced to zero. As reference, thermal treatments for Ni/Al 2O 3 catalyst were conducted. Two catalyst samples were reduced at 923 K for 40 min first, and were then kept at this temperature in a 50 vol% hydrogen and 50 vol% nitrogen atmosphere for 80 and 1380 min, respectively.
916
1.4 Catalyst characterisation and reactivation TEM (JEM-2010, JEOL Ltd.) was employed to investigate the distributions of nickel on Al2O3 support and coke in the catalyst. An Oxford Instruments INCAX-sight EDS detector, operating at an accelerating voltage of 200 kV, was fitted to the microscope to provide chemical information about the samples. Samples were prepared for analysis by spreading them on a holey carbon film supported on a copper grid. Nitrogen adsorption characterization of the samples was performed on equipment for automatic gas and vapor adsorption measurement (BELSORP-max, BEL Japan Co., Ltd.) at liquid nitrogen temperature (77 K). Prior to the adsorption measurement, the sample was degassed at 573 K for 3 h under a dynamic vacuum. The BET theory was used to estimate catalyst pore properties and total surface area. Pore distribution curves were calculated from the adsorption branches of isotherms using the BJH algorithm (Nakai, 2001). The identification of the catalyst phases was performed using a powder X-ray diffractometer with a CuK α X-ray source (M03XHF22, Mac Science Co., Ltd.). The spent catalyst was regenerated by removing coke deposits at 873 K in a 20 vol% O2 atmosphere diluted by N2, and the O2 flow rate was set at 0.1 ml s–1 per gram nickel of the catalyst. The amount of coke deposit was determined by analyzing CO2 and CO from coke combustion. The regenerated Ni/Al2O3 catalyst was tested for coal volatile decomposition in order to estimate its activity recovery. In the experiments, 1 g of catalyst and 0.4-g coal samples were employed. The temperature of the catalyst bed was set at 923 K, and 60 ml min–1 of H2 and 60 ml min–1 of N2 were fed into the reactor. The gas yields, gas components and carbon balance from coal volatile decomposition were discussed, but the tar products were not analyzed. The carbon balance in each experiment was estimated by summing up the carbon in coal char, in the gas products and in the coke deposits, and it showed up as a percentage of the carbon in the coal sample. A temperature-programmed reaction (TPR) for CO-methanation under Ni/Al 2 O 3 catalyst was performed in a fixed bed apparatus; 20 mg catalyst sample was used. After 30 min of pre-treatment in hydrogen at 923 K, the catalyst was cooled to 373 K. For TPR characterizations, the catalyst was heated at a rate of 10 K min –1 in He (18 ml min–1), H2 (24 ml min–1) and CO (8 ml min–1) from 373 to 1173 K. Immediately after passing through the catalyst bed, the gases were continuously analysed using a mass spectrometer. 2.
Results and Discussion
2.1 Structural features of Ni/Al2O 3 catalyst 2.2.1 TEM characterisation Figure 1 shows the TEM images of a freshly reduced catalyst (H40) and a spent Ni/Al2O 3 catalyst (H40U17). The spent catalyst JOURNAL OF CHEMICAL ENGINEERING OF JAPAN
Fig. 1
TEM images of H40 and H40U17, and the EDS spectra of platelet and granular regions in H40U17 encircled by solid and dashed lines, respectively
was obtained from deactivation experiments. It was used for 17 cycles of coal volatile decomposition at 923 K over a period of 23 h. Platelet and granular regions can be observed in the TEM images of both samples. Metallic nickels, visible as dark spherical grains, disperse on alumina support with particle sizes around 10 nm in H40. Larger nickel particles, with sizes in the 10–20 nm range, are observed in H40U17. Furthermore, some particles located around the border of the granular assemblage and the edge of the alumina support grew to as large as 30–40 nm. From the EDS measurement, the existence of coke was identified in both the granular region and the alumina-rich platelet regions. In a highly magnified TEM image, carbon orVOL. 41 NO. 9 2008
dering in the layer is observed around the nickel cores. The carbon shell layer shows a graphite annulus, and the interlayer spacing is about 0.34 nm, which is the same as that for the graphite (002) planes. There exists a clear interface between the shell and core, indicating tight encapsulation. 2.2.2 Structure changes of Ni/Al2O 3 catalyst The Ni/Al2O 3 catalyst samples were characterized by XRD and gas (nitrogen) adsorption measurement. Aging treatments were performed over fresh Ni/Al2O3 catalyst as a reference; the treatment consisted of reduction for 40 min and aging for 80 or 1340 min at 923 K in a H2/N2 atmosphere. The aged samples were labelled as H40A80 and H40A1340, respectively. 917
Table 1
Changes in the Ni crystallite size, total BET surface area and total BET volume of the Ni/Al2O3 samples after various treatments at 923 K
Time on stream [min]
Ni/Al 2 O3 catalyst
d Ni [nm, XRD]
Total BET area [m2 g – 1 ]
Total BET volume [mm3 g – 1 ]
0
Raw
11.6*
87.6
213
40
H40
10.9
88.4
294
H40A80 H40U1
12.1 16.5
90.2 78.6
266 214
H40A1340 H40U17
13.4 20.5
88.2 32.8
293 123
120 1380
*Average crystallite size of NiO in the raw catalyst
The structural properties of the Ni/Al2O3 catalysts throughout the various treatments are compared in Table 1. The average crystallite sizes of Ni and NiO were calculated using the Scherrer formula [L = K·λ/ B2θ cos θ ] from the peaks around 43° and 44°, assuming the value of the constant K as 0.9. Nickel dispersed on the Al 2O 3 support as NiO in the raw Ni/Al2O 3 catalyst, showing a fine average crystallite size of 11.6 nm. Throughout the reduction and aging treatments, the Ni crystallite sizes were found to be 10.9, 12.1 and 13.4 nm for samples H40, H40A80 and H40A1340, respectively. These results indicate that the growth of nickel crystallites was slow in a system without the existence of carbonous species, even after a long aging treatment at the high temperature of 923 K. Furthermore, high catalytic activities of the H40A80 and H40A1340 samples in coal volatile decomposition were confirmed, and high gas yields comparable to those obtained with a fresh Ni/Al2O3 catalyst (H40) were achieved. In contrast, after using the catalyst for coal volatile decomposition, the nickel particles were observed to grow, with average crystallite sizes of 16.5 nm in H40U1 and 20.5 nm in H40U17. These results suggest that the causes of catalyst deactivation are often not independent. During coal volatile decomposition, coking around the nickel metal simultaneously caused the nickel particles of higher mobility to agglomerate. Christensen et al. (2006) also reported that there was a strong correlation between nickel crystal size and resistance to carbon formation. Meerten et al. (1983) investigated the changes in the nickel structure of a supported nickel catalyst during methanation, and reported that nickelcarbon formation was an important intermediate in the transport of nickel from smaller to larger crystallites. Regarding the coke deposits, as in the preceding discussion, coke deposits in the spent Ni/Al2O3 catalyst were confirmed to be encapsulating carbon. Their formation was reported extensively, and the deposits are likely to coat all the catalyst active sites and lessen catalyst pore volume and activity (Chen et al., 2001, 2005; Bonura et al., 2006; Alberton et al., 2007). In
this study, the spent catalyst, H40U17, showed only 35–40% of the total surface area and total pore volume of the fresh catalyst. Removing the coke deposits should, therefore, be a prior consideration for catalyst reactivation. 2.3 Catalyst regeneration First, catalyst regeneration was studied by burning off the coke deposits. The spent catalyst was treated at 873 K in an O2/N 2 atmosphere, and the O 2 feed was set at a very low flow rate of 0.1 ml s –1 g–1-nickel to avoid overheating the catalyst sample. At the present conditions, the temperature increase, owing to the exothermic reactions of coke combustion and nickel oxidation, was less than 10 K in the catalyst bed. The amount of coke deposits in H40U17 was 56.4 mg of coke in 1 g of catalyst. After removing coke deposits, the catalyst sample was treated in flowing hydrogen at 923 K for 40 min, and the regenerated catalyst was labelled H40U17RH. The TEM image and TEM-EDS spectra of H40U17RH are shown in Figure 2 and the textural properties are compared in Figure 3. In the TEM image of the regenerated catalyst, nickel particles are seen to have a spherical shape, with particle sizes around 10–20 nm. From the EDS spectra of the granular region, it was evident that nickel was a dominant component, and there was no carbon peak around the nickel particles, indicating that the coke was burnt off completely by the present method. In Figure 3, the pore distributions reflect a more intuitive change in the regenerated catalyst, with their porosity restored as a whole compared with the spent catalyst sample. In particular, pores larger than 16.5 nm increased significantly, reaching the level of the fresh catalyst. Compared with the spent catalyst, the regenerated catalyst had 1.7 times the total surface area and double the total pore volume. In addition, the average nickel crystallite size in the regenerated catalyst was 19.1 nm, which was not larger than that of the spent catalyst, i.e. regeneration did not cause further growth of the nickel particles. Therefore, the regeneration method considered in this
918
JOURNAL OF CHEMICAL ENGINEERING OF JAPAN
Fig. 2
TEM images of H40U17RH and EDS spectra of platelet and granular regions encircled by solid and dashed lines, respectively
study is not only efficient for catalyst regeneration, but also appropriate for catalyst structure protection. 2.4 Performance of the regenerated catalyst The regenerated Ni/Al2O3 catalyst was tested during coal volatile decomposition to estimate its activity recovery. Gas yields, gas components and carbon balance are summarized in Table 2. For coal volatile decomposition, the catalyst activity showed remarkable recovery. Tarry materials in coal volatiles were almost completely converted over the regenerated catalyst, the total gas yields reached the level of a fresh catalyst, and the carbon balance was obtained to be as high as 99.5%. On the other hand, in the gas products shown in Table 2, more CO and less CH 4 gases were obtained with the regenerated catalyst compared with the case of a fresh catalyst. This indicated that structural VOL. 41 NO. 9 2008
changes in the nickel catalyst had an influence on gas reforming selectivity; the regenerated catalyst with larger nickel particles was not active enough for efficient CO-methanation. Thus, the CO-methanation reaction is inferred as having a strong structure sensitivity to nickel particles. In order to gain additional insight into the structure sensitivity for CO-methanation, a temperature-programmed reaction experiment was conducted. The signals for m/z = 2 (H 2 ), 16 (CH 4 ), 18 (H2O), 28 (CO) and 44 (CO2) obtained in this experiment are presented in Figure 4. Using a fresh Ni/Al 2O3 catalyst, H2 and CO intensities went down from 500 K, and additional temperature increases led to H2 and CO conversion with peaks at 700 K. In the region between 500 and 1000 K the decreases in H2 and CO intensities were attributed to the methanation reaction (Figure 4(a-1) due to the sharp increase in the 919
Table 2
Comparison of carbon conversion and gas yields from coal volatile decomposition with the Ni/Al 2 O 3 catalysts following different treatments: Catalytic temperature, 923 K; feed gas rates, 60 ml min –1 H2 and 60 ml min –1 N2
Catalyst
balance [%] None Fresh catalyst Spent catalyst Regenerated catalyst
Gas yields [mmol g – 1 -coal daf]
Carbon
Total carbon in C1 –C2 gas products
CO
CO2
CH4
C2 H4
C2 H6
9.9 15.2 10.1 15.4
0.8 2.5 2.8 4.7
0.4 0.1 0.1 0.1
6.6 12.5 6.7 10.5
0.5 0 0.02 0.01
0.6 0 0.2 0.06
64.0 98.0 65.0 99.5
CH4 and H2O signals; the low CO2 intensity (Figure 4(a-2) was deemed to be a by-product of the CO shiftreaction with H2O. Higher temperatures over 1000 K were not favourable for CO conversion. In contrast, in the case of the regenerated catalyst, consumption of H2 and CO for methanation was hardly observed in Figure 4(b-1), and there were no distinct peaks for the products in any of the temperature ranges, except for two very small H2O peaks between 510 and 530 K and at around 1100 K, as shown in Figure 4(b-2). This indicates that the nickel sintering during volatile decomposition results not only in a decrease in the nickel surface area and the number of active sites left in the larger nickel crystals, but also in a loss of activity for CO-methanation.
The structure sensitivity is in agreement with early data (Schoubye, 1969; Goodman and Kiskinowa, 1981; Rostrup-Nielsen et al., 2007). Furthermore, the new data obtained by theoretical calculations strongly indicated that the methanation reaction was structure sensitive to metal particles (Andersson et al., 2006). Schoubye (1969) reported that the structure sensitivity of the methanation reaction could be improved by the addition of an alkali to the catalysts. In addition, based on an investigation regarding the specific activities of a number of catalysts, Rostrup-Nielsen et al. (2007) concluded that the two other reactions during steam reforming and hydrogenolysis were not as structure sensitive as methanation. Therefore, the nickel structure change due to thermal degradation influenced methanation selectivity; however, it hardly affected coal volatile decomposition. The catalyst activity for coal volatile decomposition could be remarkably recovered by improving the fouling due to coking 2.5 Sulfur evolution from coal pyrolysis and its effect on catalyst activity Sulfur poisoning is another serious problem in many catalytic processes (Poels et al., 1995; Srinakruang et al., 2006). Regarding the sulfur evolution behavior, Liu et al. (2007) conducted a comparative study and reported that organic sulfur evolution mainly occurred between 673 and 973 K, accompanied by coal volatilization, and that the sulfur transformation into gas and tar phases upon reaching 973 K was less than 60% of the total sulfur in the coal sample. Thus, the sulfur content in coal volatile matter in this study is estimated to be less than 0.33 mg g –1-coal, based on our operating conditions. The sulphide (assumed to be H2S) concentration in the reaction system is calculated to be 100 ppm. This is significantly lower than 1000 and 2000 ppm, which are the H2S concentrations used for Ni/Al2O 3 sulfur poisoning capacity tests in our earlier research. We also found that feeding H2S slightly affected Ni/Al2O 3 activity under the present operating conditions (Li et al., 2006). In addition, adsorbed sulfur was not detected in the TEM-EDS spectra of the spent catalyst (in Figure 1). Therefore, the effect of evolved sulphates on catalyst activity during coal carbonization was considered to be irrelevant in this study.
920
JOURNAL OF CHEMICAL ENGINEERING OF JAPAN
Fig. 3
Pore distribution curves of the fresh (H40), spent (H40U17) and regenerated (H40U17RH) Ni/Al2O3 catalysts
Fig. 4
Methanation of CO over fresh and regenerated Ni/Al2O3 catalysts: P(H 2), 48 kPa; P(CO), 16 kPa; P(He), 36 kPa; flow rate, 50 ml min –1
Conclusions The catalyst deactivation mechanisms were mainly confirmed as coking during coal volatile decomposition and catalyst structural changes due to thermal degradation. Compared with a mere aging treatment on catalyst samples, coal volatile decomposition and subsequent coking was found to have a more distinct effect on the catalytic properties. The coke deposits were confirmed to be a kind of encapsulating carbon. By removing the coke in oxygen at a relatively moderate temperature of 873 K, the catalyst structure and activity were restored remarkably. Indeed, compared with the spent catalyst, 1.7 times the surface area and double the pore volume were obtained in the regenerated catalyst. The regenerated catalyst showed high activity for coal volatile decomposition. With the regenerated catalyst, tarry material in the coal volatile matter was converted almost completely, and high gas yields and a high carbon balance were obtained. In the product gases, however, more CO gas was obtained. COmethanation was found to be structure sensitive to nickel particles, and the nickel structural changes due to thermal degradation weakened the methanation reaction. Nevertheless, coal volatile decomposition was hardly affected, and tar decomposition was not as structure sensitive as methanation.
VOL. 41 NO. 9 2008
Acknowledgments The authors thank the Gunma Prefecture Collaboration of Regional Entities for the Advancement of Technological Excellence, the Japan Science and Technology Agency for financial support and Mr. K. Usami of BEL Japan, Inc. for technical assistance with the gas adsorption analysis. Literature Cited Alberton, A. L., M. M. V. M. Souza and M. Schmal; “Carbon Formation and Its Influence on Ethanol Steam Reforming over Ni/Al 2O 3 Catalysts,” Catal. Today, 123, 257–264 (2007) Andersson, M. P., T. Bligaard, A. Kustov, K. E. Larsen, J. Greeley, T. Johannessen, C. H. Christensen and J. K. Nørskov; “Toward Computational Screening in Heterogeneous Catalysis: ParetoOptimal Methanation Catalysts,” J. Catal., 239, 501–506 (2006) Bartholomew, C. H.; “Mechanisms of Catalyst Deactivation,” Appl. Catal., 212, 17–60 (2001) Bonura, G., O. D. Blasi, L. Spadaro, F. Arena and F. Frusteri; “A Basic Assessment of the Reactivity of Ni Catalysts in the Decomposition of Methane for the Production of ‘COx–Free’ Hydrogen for Fuel Cells Application,” Catal. Today, 116, 298– 303 (2006) Chareonpanich, M., T. Takeda, H. Yamashita and A. Tomita; “Catalytic Hydrocracking Reaction of Nascent Coal Volatile Matter under High Pressure,” Fuel, 73, 666–670 (1994) Chareonpanich, M., Z. G. Zhang, A. Nishijima and A. Tomita; “Effect of Catalysts on Yields of Monocyclic Aromatic Hydrocarbons in Hydrocracking of Coal Volatile Matter,” Fuel, 74, 1636– 1640 (1995) Chen, D., R. Lødeng, A. Anundskås, O. Olsvik and A. Holmen; “Deactivation during Carbon Dioxide Reforming of Methane over Ni Catalyst: Microkinetic Analysis,” Chem. Eng. Sci., 56, 1371–1379 (2001)
921
Chen, D., K. O. Christensen, E. Ochoa-Fernández, Z. Yu, B. Tøtdal, N. Latorre, A. Monzón and A. Holmen; “Synthesis of Carbon Nanofibers: Effects of Ni Crystal Size during Methane Decomposition,” J. Catal., 229, 82–96 (2005) Christensen, K. O., D. Chen, R. Lødeng and A. Holmen; “Effect of Supports and Ni Crystal Size on Carbon Formation and Sintering during Steam Methane Reforming,” Appl. Catal., 314, 9–22 (2006) Furimsky, E. and F. E. Massoth; “Deactivation of Hydroprocessing Catalysts,” Catal. Today, 52, 381–495 (1999) Goodman, D. W. and M. Kiskinowa; “Chemisorption and Reactivity Studies of H 2 and CO on Sulfided Ni(100),” Surf. Sci. Lett., 105, 265–270 (1981) Ishigaki, Y., M. Uba, S. Nishida and T. Inui; “Application of CoMn 2O3-Ru Catalyst to the Process for Producing High-Calorie Substitute Natural Gas from Coke Oven Gas,” Appl. Catal., 47, 197–208 (1989) Li, L., K. Morishita and T. Takarada; “Conversion of Hot Coke Oven Gas into Light Fuel Gas over Ni/Al 2O3 Catalyst,” J. Chem. Eng. Japan, 39, 461–468 (2006) Li, L., K. Morishita and T. Takarada; “Light Fuel Gas Production from Nascent Coal Volatiles Using a Natural Limonite Ore,” Fuel, 86, 1570–1576 (2007) Liu, F., W. Li, H. Chen and B. Li; “Uneven Distribution of Sulfurs and Their Transformation during Coal Pyrolysis,” Fuel, 86, 360–366 (2007) Meerten, R. Z. C. V., A. H. G. M. Beaumont, P. F. M. T. V. Nisselrooij and J. W. E. Coenen; “Structure Sensitivity and Crystallite Size Change of Nickel during Methanation of CO/H 2 on Nickel– Silica Catalysts,” Surf. Sci., 135, 565–579 (1983) Miura, K., M. Kawase, H. Nakagawa, R. Ashida, T. Nakai and T. Ishikawa; “Conversion of Tar in Hot Coke Oven Gas by Pyrolysis and Steam Reforming,” J. Chem. Eng. Japan, 36, 735– 741 (2003) Moulijn, J. A., A. E. V. Diepen and F. Kapteijn; “Catalyst Deactivation: Is It Predictable?: What to Do?,” Appl. Catal., 212, 3–16 (2001)
922
Nagase, S., S. Takami, A. Hirayama and Y. Hirai; “Development of a High Efficiency Substitute Natural Gas Production Process,” Catal. Today, 45, 393–397 (1998) Nakai, K.; “Characterization of Catalysis by Using Gas Adsorption Technique” (in Japanese), Catal. Soc. Jpn., 43, 575–580 (2001) Onozaki, M., K. Watanabe, T. Hashimoto, H. Saegusa and Y. Katayama; “Hydrogen Production by the Partial Oxidation and Steam Reforming of Tar from Hot Coke Oven Gas,” Fuel, 85, 143–149 (2006) Poels, E. K., W. P. V. Beek, W. D. Hoed and C. Visser; “Deactivation of Fixed-Bed Nickel Hydrogenation Catalysts by Sulfur,” Fuel, 74, 1800–1805 (1995) Rabo, J. A., A. P. Risch and M. L. Poutsma; “Reactions of Carbon Monoxide and Hydrogen on Co, Ni, Ru, and Pd Metals,” J. Catal., 53, 295–311 (1978) Rostrup-Nielsen, J. R, K. Pedersen and J. Sehested; “High Temperature Methanation: Sintering and Structure Sensitivity,” Appl. Catal., 330, 134–138 (2007) Schoubye, P.; “Methanation of CO on Some Ni Catalysts,” J. Catal., 14, 238–246 (1969) Srinakruang, J., K. Sato, T. Vitidsant and K. Fujimoto; “Highly Efficient Sulfur and Coking Resistance Catalysts for Tar Gasification with Steam,” Fuel, 85, 2419–2426 (2006) Takarada, T., T. Tonishi, H. Takezawa and K. Kato; “Pyrolysis of Yallourn Coal in a Powder-Particle Fluidized Bed,” Fuel, 71, 1087–1092 (1992) Takarada, T., T. Tonishi, Y. Fusegawa, K. Morishita, N. Nakagawa and K. Kato; “Hydropyrolysis of Coal in a Powder-Particle Fluidized Bed,” Fuel, 72, 921–926 (1993) Takarada, T., Y. Onoyama, K. Takayama and T. Sakashita; “Hydropyrolysis of Coal in a Pressurized Powder-Particle Fluidized Bed Using Several Catalysts,” Catal. Today, 39, 127– 136 (1997) Xu, W.-C., H. Yamashita and A. Tomita; “Catalytic Hydrocracking Reactions of Nascent Coal Volatiles in a Two-Stage Reactor,” Fuel, 69, 1007–1011 (1990)
