In Situ EXAFS of Nickel Species during Pyrolysis of Brown Coals

1012

Energy & Fuels 1997, 11, 1012-1018

In Situ EXAFS of Nickel Species during Pyrolysis of Brown Coals Masayuki Shirai,* Kenji Murakami,† and Yoshiyuki Nishiyama Institute for Chemical Reaction Science, Tohoku University, Katahira, Sendai, 980-77 Japan Received March 18, 1997. Revised Manuscript Received June 30, 1997X

An extended X-ray absorption fine structure (EXAFS) spectroscopy technique has been used to examine the behavior and properties of highly dispersed ion-exchanged nickel species in Loy Yang brown coal samples during pyrolysis. At low loadings (1.4-1.6 wt %) the nickel species aggregate to metal particles at 750 K. At high loadings (6.4-7.5 wt %), however, the nickel species aggregate to metal particles at 650 K. The 100 K higher transition temperature at low nickel loadings is explained by stable nickel containing functional groups and by the low amount of nickel species in the coals.

Introduction Catalysts are utilized in coal conversion during gasification, liquefaction, and pyrolysis. Low-rank coals, including brown coals, generally contain more oxygencontaining groups (e.g., carboxyl groups and phenolic hydroxyl groups) than bituminous coals. These surface functional groups play an important role for cation exchange. By exchanging metal cations with surface functional groups on brown coals, high catalyst dispersions can be achieved.1 Because the activity of catalysts prepared by ion exchange for coal utilizaion is higher than that of catalysts prepared by other methods,2-10 it is important to understand the properties and behavior of the catalysts. Schafer investigated the relation between the decomposition of functional groups and evolved gas in the pyrolysis of acid-washed coal and alkali or alkaline earth metal exchanged coals.11-13 According to Schafer,13 CO2 and CO, emitted from the acid-washed coal during heat treatment, originate from carboxyl groups and phenolic groups, respectively. For cation-exchanged coals, other oxygen-containing groups associated with carboxyl groups yield CO2 before the carboxylate groups decompose into CO2. Murakami et al. reported that the C1 gas evolution * Corresponding author. Tel. and Fax: +81-22-217-5631. E-mail: [email protected]. † Present address: Department of Materials Engineering and Applied Chemistry, Mining College, Akita University, Akita, 010 Japan. X Abstract published in Advance ACS Abstracts, August 15, 1997. (1) Schafer, H. N. S.; Durie, R. A. The Science of Victorian Brown Coals; Butterworth: London, 1991; pp 323-358. (2) Hengel, T. D.; Walker, P. L., Jr. Fuel 1984, 63, 1214-1220. (3) Salinas-Martinez de Lecea, C.; Almela-Alarcon, M., LinaresSolano, A. Fuel 1990, 69, 21-27. (4) Takarada, T.; Nabatame, T.; Ohtsuka, Y.; Tomita, A. Energy Fuels 1987, 1, 308-309. (5) Ohtsuka, Y.; Asami, K. Ind. Eng. Chem. Res. 1991, 30, 19211926. (6) Agnew, J. B.; Jackson, W. R.; Larkins, F. P.; Rash, D.; Rogers, D. E.; Thewlis, P.; White, R. Fuel 1984, 63, 147-152. (7) Joseph, J. T.; Forrai, T. R. Fuel 1992, 71, 75-80. (8) Taghiei, M. M.; Huggins, F. E.; Mahajan, V.; Huffman, G. P. Energy Fuels 1994, 8, 31-37. (9) Taghiei, M. M.; Huggins, F. E.; Ganguly, B.; Huffman, G. P. Energy Fuels 1993, 7, 399-405. (10) Otake, Y.; Walker, P. L., Jr. Fuel 1993, 72, 139-149. (11) Schafer, H. N. S. Fuel 1979, 58, 667-672. (12) Schafer, H. N. S. Fuel 1979, 58, 673-679. (13) Schafer, H. N. S. Fuel 1980, 59, 295-301.

S0887-0624(97)00042-X CCC: $14.00

profile (rate and yield) of the nickel-exchanged coal depends on the loadings in the decomposition of the brown coal.14 These results suggest that the catalytic properties depend on the condition of active species during pyrolysis. To understand how catalysts affect coal pyrolysis, it is necessary to characterize the catalytic species. Also, it is important to have techniques capable of determining the composition, structural characteristics, and particle size distribution of the catalysts at the reaction conditions. However, the structural forms of the catalysts are difficult to determine in general because of their usually high dispersion and relatively dilute concentrations. X-ray absorption fine structure (XAFS) is a powerful technique that provides nondestructive structural information on highly dispersed metal or metal oxide phase on supports under reaction conditions.15 XAFS has been used to examine the chemical structure of catalysts in coals for liquefaction,16-21 gasification,20-24 and pyrolysis19,20,25 The XAFS technique should be useful in determining local strucure of highly dispersed catalytic species in coal. Our objective is to examine the influence of nickel loadings on brown coals with an in situ XAFS technique. (14) Murakami, K.; Shirato, H.; Ozaki, J.; Nishiyama, Y. Fuel Process. Technol. 1996, 46, 183-194. (15) Iwasawa, Y. X-ray Absorption Fine Structure for Catalysts and Surfaces; World Scientific: Singapore, 1995. (16) Sandstorm, D. R.; Filby, R. H.; Lytle, F. W.; Greegor, R. B. Fuel 1982, 61, 195-197. (17) Zhao, J.; Huggins, F. E.; Zhen, F.; Lu, F.; Shah, N.; Huffman, G. P. J. Catal. 1993, 143, 499-509. (18) Huffman, G. P.; Ganguly, B. G.; Rao, K. R. P. M.; Shah, N.; Feng, Z.; Huggins, F. E.; Taghiei, M. M.; Lu, F.; Wender, I.; Pradham, V. R.; Tierney, J. W.; Seehra, M. S.; Ibrahim, M. M.; Shabtai, J.; Eyring, E. M. Energy Fuels 1993, 7, 285-296. (19) Rao, V. U. S. Energy Fuels 1994, 8, 44-47. (20) Wasserman, S. R.; Winans, R. E.; McBeth, R. Energy Fuels 1996, 10, 392-400. (21) Shah, N.; Zhao, J.; Huggins, F. E.; Huffman, G. P. Energy Fuels 1996, 10, 417-420. (22) Yamashita, H.; Yoshida, S.; Tomita, A. Ind. Eng. Chem. Res. 1991, 30, 1651-1655. (23) Yamashita, H.; Nomura, M.; Tomita, A. Energy Fuels 1992, 6, 656-661. (24) Cazorla-Amoros, D.; Linares-Solano, A.; Salinas-Martines, C.; Yamashita, H.; Kyotani, T.; Tomita, A.; Nomura, M. Energy Fuels 1993, 7, 139-145. (25) Huggins, F. E.; Shah, N.; Huffman, G. P.; Shoenberger, R. W.; Walker, J. S.; Lytle, F. W.; Greegor, R. B. Fuels 1986, 65, 621-632.

© 1997 American Chemical Society

EXAFS of Nickel Species during Pyrolysis of Brown Coals

Energy & Fuels, Vol. 11, No. 5, 1997 1013

Experimental Section Sample Preparation. Loy Yang brown coal from Victoria, Australia, was used in this study. Raw coal was ground between 250 µm particle size, washed with deionized water, dried at 323 K under vacuum and stored in a desiccator. The analyses for this coal are as follows: C 67.6% (daf), H 5.2% (daf), N 0.8% (daf), O 26.4% (diff), and ash 0.2% (dry). The concentration of carboxyl groups was 3.7 mequiv/g.26 The concentration of phenolic group was assumed to be the same as the carboxyl group concentration.11,12 Details of the ion-exchange procedure were described previously.26 Briefly, the coal sample (10 g) in aqueous solution of nickel chloride (850 mL) at the concentration of 10000 ppm was stirred and the pH of the solution was adjusted by adding ammonia or hydrochloric acid. After the pH stopped changing and remained unchanged for 5 h, the exchange reaction was judged to be at equilibrium. After the equilibrium pH was measured, the coal was separated from the solution, gently rinsed a few times with deionized water, and dried in vacuum at 323 K for 2 h. The extent of exchanged nickel cation was determined to extract the cation leached by hydrochloric acid from the sample. The 1.4 and 1.6 wt % nickel loaded coal were obtained from solution at an equilibrium pH of 3.5. The 6.4 and 7.5 wt % nickel-loaded coal were obtained from solution at an equilibrium pH of 6.1 and 6.5. The ion-exchanged coal sample was evacuated at 373 K for 1 h. Pyrolysis of coal was carried out in a glass tube under vacuum placed inside a furnace. The heating rate was 5 K/min from 373 K to desired temperature. Because the reactor tube projected beyond the end of the furnace from the system during pyrolysis, most of the tars condensed in the cooler part of the tube. Gaseous products were removed via the vacuum. After reaching the desired temperature, samples were cooled at room temperature and transferred into EXAFS cells with Kapton windows. The samples were sealed in EXAFS cells under vacuum. The nickel cation exchanged samples are identified by the amount of loading (wt %), and treated temperature, e.g., 1.4 wt %-573 K for 573 K treated 1.4 wt % nickel-loaded coal (daf). EXAFS. Data were collected at beam line BL-7C at the Photon Factory. The storage ring was operated with an electron energy of 2.5 GeV and a current of 250-340 mA. The Si(111) double crystal monochromator was detuned by 30% to minimize higher harmonics in the X-ray beam. Data were recorded in the transmission mode in the region of the Ni K edge (8331.7 eV) at room temperature. The transmission spectra were collected using ion chambers that were filled with nitrogen gas. EXAFS oscillation data were extracted from the EXAFS raw data by using a cubic spline method and normalized with the edge height.15 The k3-weighted and EXAFS spectra were Fourier transformed to R space over the range 2.0-13.2 Å-1. The inversely Fourier filtered data were analyzed by a curvefitting technique based on the single-scattering plane-wave theory.27 The curve-fitting analysis was for k space over the range 3.0-12.5 Å-1. Experimentally determined phase shifts and backscattering amplitudes for Ni-Ni and Ni-O were obtained from EXAFS data for Ni foil and NiO, respectively. In Figure 1 we show Fourier transforms for k3-weighted EXAFS spectra of Ni foil and NiO, respectively. The first neighbor Ni-Ni metal bond of nickel metal was observed at 2.2 Å in the Fourier transform of Ni foil EXAFS spectra. The first neighbor Ni-O bond and the first adjacent Ni-O-Ni bond in nickel oxide were observed (26) Murakami, K.; Ozaki, J.; Nishiyama, Y. Fuel Process. Technol. 1995, 43, 95-110. (27) Yokoyama, T.; Hamamatsu, H.; Ohta, T. Program EXAFSH ver. 2.1; The University of Tokyo, Tokyo, 1994. (28) Wyckoff, R. W. G. Crystal Structures, 2nd ed.; Wiley: New York, 1963; Vol. 1, p 10. (29) Rooksby, H. P. Acta Crystallogr. 1948, 1, 226-227.

Figure 1. Fourier transforms for EXAFS oscillations of model compounds. These distributions were obtained by the Fourier transform of the k3-weighted EXAFS data (∆k ) 2.0-13.2 Å-1). Table 1. Crystallographic Data Characterizing the Reference Compounds and Fourier Transform Ranges Used in the EXAFS Analysisa crystallographic data

Fourier transform

sample

shell

N

R/Å

∆k/Å-1

∆R/Å

n

Ni foilb NiOc

Ni-Ni Ni-O

12 6

2.488 2.098

2.0-13.2 2.0-13.2

1.4-2.7 1.0-2.0

3 3

a Notation: N, coordination number for absorber-backscatterer pair; R, distance; ∆k, limits of forward Fourier transformation (k is the wave vector); ∆R, limits used for shell isolation; n, power of k used for Fourier transform. b XRD data.28 c XRD data.29

at 1.7 and 2.6 Å, respectively, in the NiO EXAFS Fourier transform. The data are presented without correction for phase shift. In Table 1, we list data on reference materials. The Nyquist theorem was used to determine the number of parameters of freedom (p) for fitting the data15

p ) (2∆R∆k/π) + 1 where ∆R is the R range used in the inverse Fourier transform and ∆k is the curve-fitting range in k space. Four parameters were used for fitting the data for single shell (Ni-O or NiNi) and eight parameters were used for double shells (Ni-O and Ni-Ni).

Results and Discussion Low Nickel Loaded Coals (1.4-1.6 wt %). Figure 2 shows the Fourier transforms for EXAFS spectra of the low amount of nickel loaded Loy Yang coals pyrolyzed from 373 to 873 K. Figure 3 and Table 2 show the EXAFS curve-fitting analysis. The number of parameters used in our analyses satisfied the Nyquist criterion. The Fourier transform for the EXAFS spectrum of the 1.4 wt %-373 K sample exhibited only one peak between 1 and 2 Å, which can be ascribed to the Ni-O bond. No peak was found for the Ni-O-Ni bond. Only Ni-O was observed in the 1.4 wt %-373 K sample with

1014 Energy & Fuels, Vol. 11, No. 5, 1997

Shirai et al.

Figure 2. Fourier transforms for EXAFS oscillations of low nickel loaded (1.4-1.6 wt %) brown coals treated at temperatures from 373 to 873 K. These distributions were obtained by the Fourier transforms of the k3-weighted EXAFS data (∆k ) 2.0-13.2 Å-1).

the curve-fitting analysis, which showed that highly dispersed nickel atoms existed in these samples. The coordination number of Ni-O at the 1.4 wt %-373 K sample was almost the same as that of nickel oxide in which a nickel atom is octahedrally surrounded by six oxygen atoms. EXAFS results indicate that nickel atoms in the exchanged coals are also octahedrally surrounded by six oxygen atoms. Murakami et al. have reported that nickel atoms exchanged for protons of carboxyl groups on brown coal produce carboxylates.26 The EXAFS results showed that not only oxygen atoms in carboxylate but also oxygen atoms from H2O and OH ions were found to coordinate with nickel atoms. Shah et al. reported that using a XAFS technique iron atoms in ion-exchanged brown coals are surrounded by six oxygen atoms.21 Linares-Salano et al. used a TPD technique and observed that calcium atoms ion-exchanged on Saran char are surrounded by six oxygen atoms.30 These reports support the proposed structure that six oxygen atoms coordinate a nickel atom in the low nickel loaded brown coals.

In the Fourier transformed EXAFS spectra for samples treated from 373 to 723 K, only peaks associated with the Ni-O bond appeared. Only Ni-O bonds were observed following pyrolysis up to 723 K of the low nickel loaded coals. This implies that carboxylate groups were stable at 723 K or that nickel carboxylates were decomposed and nickel atoms were still highly dispersed in the coals. After treatment at 743 K, peaks assigned to Ni-Ni metal bonds were observed in addition to the appearance of the Ni-O bond peaks in the Fourier transform spectrum which indicated nickel atom aggregation. However, the weakness of the intensity of Ni-Ni peak indicated that nickel species were highly dispersed in the form of small metal clusters. With increasing temperature of pyrolysis, the coordination number of Ni-Ni metal bond increased, indicating that the nickel species agglomerated. The coordination number of the Ni-O bond decreased with temperature, (30) Linares-Solano, A.; Salinas-Marting de Lecea, C.; CazorlaAmoros, D. Energy Fuels 1990, 4, 467-474.



Figure 3. Results of experimental EXAFS data (solid line) and the best fitting calculated data (broken line) for low nickel loaded (1.4-1.6 wt %) brown coals treated at temperatures from 373 to 873 K. Table 2. Curve-Fitting Results for EXAFS Data for Low Nickel Loaded (1.4-1.6 wt %) Loy Yang Brown Coala for N (20% and R (0.02 Å sample

Ni-O shell

Ni-Ni shell

Ni/wt%

T/K

back FT range of R/Å

N

R/Å

E0/eV

∆σ2/Å2

1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.4 1.6

373 673 698 723 748 773 793 813 838 873

1.0-2.0 1.0-2.0 1.0-2.0 1.0-2.0 1.0-2.7 1.0-2.7 1.4-2.7 1.4-2.7 1.4-2.7 1.4-2.7

6.4 6.1 5.8 5.6 5.0 4.7 nd nd nd nd

2.03 2.02 2.00 2.01 2.01 1.99 nd nd nd nd

-2.6 -1.3 -2.0 -1.9 0.4 1.0 nd nd nd nd

0.0024 0.0027 0.0039 0.0025 0.0042 0.0057 nd nd nd nd

N

R/Å

E0/eV

∆σ2/Å2

nd nd nd nd 0.9 0.6 4.8 5.0 7.1 8.1

nd nd nd nd 2.50 2.49 2.47 2.47 2.48 2.48

nd nd nd nd -8.7 -10.3 -14.4 -14.2 -13.6 -13.7

nd nd nd nd -0.0009 -0.0002 0.0001 0.0001 -0.0001 -0.0001

a Notation: N, coordination number for absorber-backscatterer pair; R, distance; ∆σ2, differences of Debye-Waller factor from model compounds; E0, inner potential correction; nd, not detected.

which indicates that the percentage of surface atoms in metal particles decreased with aggregation. High Nickel Loaded Coals (6.4-7.5 wt %). Figure 4 shows the Fourier transforms for EXAFS spectra of the high nickel loaded brown coals pyrolyzed at various temperatures. Figure 5 and Table 3 show the results

of curve-fitting analysis for the EXAFS of high nickel loaded coals at from 373 to 873 K. The Fourier transform EXAFS spectrum of the 6.4 wt %-373 K sample showed one peak between 1 and 2 Å, which is ascribed to the Ni-O bond. No Ni-O-Ni peak was observed in the Fourier transform. The curve-


Shirai et al.

Figure 4. Fourier transforms for EXAFS oscillations of high nickel loaded (6.4-7.5 wt %) brown coals treated at temperatures from 373 to 873 K. These distributions were obtained by the Fourier transforms of the k3-weighted to EXAFS data (∆k ) 2.0-13.2 Å-1).

Figure 5. Results of experimental EXAFS data (solid line) and the best fitting calculated data (broken line) for high nickel loaded (6.4-7.5 wt %) brown coals treated at temperatures from 373 to 873 K.



Table 3. Curve-Fitting Results for EXAFS Data for High Nickel Loaded (6.4-7.5 wt %) Loy Yang Brown Coala for N (20% and R (0.02 Å sample

Ni-O shell

Ni-Ni shell

Ni/wt %

T/K

back FT range of R/Å

N

R/Å

E0/eV

∆σ2/Å2

N

R/Å

E0/eV

∆σ2/Å2

6.4 6.4 6.4 6.4 6.4 6.4 6.4 7.5 7.5

373 573 623 673 698 723 748 773 873

1.0-2.0 1.0-2.0 1.0-2.0 1.0-2.0 1.0-2.0 1.0-2.7 1.0-2.7 1.4-2.7 1.4-2.7

6.2 5.5 5.3 3.5 4.0 nd nd nd nd

2.03 2.01 2.01 2.02 1.99 nd nd nd nd

-2.3 -2.1 -2.2 -0.5 -3.0 nd nd nd nd

0.0024 0.0023 0.0021 0.0045 0.0045 nd nd nd nd

nd nd nd 2.9 1.0 5.0 6.1 7.2 8.3

nd nd nd 2.48 2.47 2.47 2.48 2.48 2.48

nd nd nd -13.7 -15.9 -14.6 -13.7 -13.6 -12.9

nd nd nd -0.0002 0.0004 0.0004 0.0002 0.0003 0.0002

a Notation: N, coordination number for absorber-backscatterer pair; R, distance; ∆σ2, differences of Debye-Waller factor from model compounds; E0, inner potential correction; nd, not detected.

Figure 6. Coordination numbers of Ni-Ni bond as a function of the treatment temperature: (a) low amount of nickel-loaded brown coal; (b) high amount of nickel-loaded brown coal.

fitting analysis also showed that there is no Ni-O-Ni bond in the 6.4 wt %-373 K sample, indicating that nickel atoms were highly dispersed following the 373 K treatment. There are no differences in the Ni-O peaks given by the EXAFS spectra between the lowloaded sample and the high-loaded sample after the 373 K treatment. This suggests that nickel species are octahedrally surrounded by six oxygen atoms derived from either oxygen atoms in carboxylate, H2O, and OH ions. After treatment of 673 K a new peak, ascribed to NiNi metal bond, was observed besides the Ni-O bond in the Fourier transform (Figure 4). The weakness of the intensity of the Ni-Ni peak indicated that nickel species are highly dispersed forming small metal clusters. The intensities of the Ni-Ni bond in the Fourier transforms increased with increasing the pyrolysis temperature, which shows that nickel species agglomerated. The peak intensity associated with the Ni-O bond decreased as the peak associated with the Ni-Ni bond increased. Agglomeration of Nickel Metal Particles. From this study, the aggregation behavior of nickel atoms as a function of nickel loading can be discussed. Figure 6 shows the dependence of Ni-Ni coordination numbers on the pyrolysis temperatures. Nickel species in the low-loaded sample aggregated to metal particles at 750

K. On the other hand, nickel species aggregated at 650 K in the high-loaded sample. The transition temperature for the low-loaded sample was 100 K higher than that of the high-loaded sample. Two main steps are imagined during formation of metal particles from highly dispersed nickel species. The first one is the decomposition step of the functional groups associated with nickel. The second step involves diffusion and coalescence of the nickel atoms in the coal. The first step (decomposition) seems to be sensitive to the structure of functional groups. From XAFS analysis, both low- and high-loaded samples have six oxygen atoms surrounding each nickel atom. Murakami et al. investigated the local structures of nickel carboxylate groups with IR. They showed that nickel carboxylates in low nickel loaded samples (below 4 wt %) have bidentate structure, that is, one nickel atom bound to the two oxygen atoms of a carboxylate group. The researchers also showed that nickel carboxylates in high nickel loaded sample (above 4 wt %) have bridge structure, that is, two separate nickel atoms bound to each oxygen atom of a carboxylate.31 These results mean that for low-loaded samples six oxygen atoms coordinate each nickel atom and two of the six oxygen atoms are derived from a carboxylate group and that for high-loaded samples six oxygen atoms coordinate each nickel atom and one of the six oxygen atoms is derived from a carboxylate group. The different carboxylates between bidentate structure and bridge structure would affect their stability and their decomposition temperature. Shah et al. reported that the transformation temperature from iron oxyhydroxide to pyrrhotite in coal liquefaction system depends on the catalyst dispersion of the structure.21 Shah’s observation supports the conclusion that the decomposition temperature depends on the structure of the nickel species. However, our EXAFS measurement shows that the local structures of nickel species (nickel bound to six oxygen atoms) are independent of nickel loadings. The second step (coalescence) relates to the amount of nickel atoms in the coal. After the decomposition of the carboxylates, nickel atoms diffuse and coalesce in the coals during pyrolysis. The probability that a nickel atom meets with another nickel atom in coal after the decomposition increases with nickel loading. A sufficient number of nickel atoms have diffused and coalesced in high-loaded samples at lower temperature. It is possible that the aggregation behavior of nickel metal particles could affect their catalytic behavior. The (31) Unpublished work.


yield of C1 gases in coal pyrolysis increased with coexisting nickel species. Murakami et al. reported that CO gas evolution profiles depend on the nickel loadings.14 The evolution temperature of CO is around 873 K for the acid-washed brown coal. However, the peak shifts to 923 K in the nickel-loaded coal. Moreover, another CO peak appears at 673 K with above 4 wt % nickel-loaded coal, and the new peak shifts to 773 K with below 4 wt % nickel-loaded coal. The peak shift at about 100 K suggests that the state and the effect of nickel species in coals are different below and above 4 wt %. We expect that the transition temperature for nickel particle formation, shown in Figure 6, is related to the CO evolution. Small nickel metal clusters after decomposition of functional groups probably catalyze CO production during pyrolysis. Conclusions The in situ XAFS technique can be used to determine local structure in ion exchanged coal. On the basis of

Shirai et al.

in situ XAFS, nickel that has undergone ion exchange with brown coal is initially presented in a highly dispersed state and is coordinated by six oxygen atoms. During the coal pyrolysis, ion-exchanged nickel species were reduced to their base metal state. In 1.4-1.6 wt % nickel-loaded coals, the aggregation to metal particles from highly dispersed nickel species occurred at 750 K. In 6.4-7.5 wt % nickel-loaded coals, the aggregation occurred at 650 K. Acknowledgment. This work was carried out under the approval of PF advisory committee (No.94G008). The authors thank the former Coal Corp. of Victoria (now joined to HRL) for providing specimens. This work was financially supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture of Japan. EF970042H

In Situ EXAFS of Nickel Species during Pyrolysis of Brown Coals

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