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TiO2 Fischer–Tropsch catalysts
Applied Catalysis A: General 181 (1999) 201±208
The effect of boron on the catalyst reducibility and activity of Co/TiO2 Fischer±Tropsch catalysts Jinlin Li, Neil J. Coville* Applied Chemistry and Chemical Technology Centre, Department of Chemistry, University of the Witwatersrand, PO Wits 2050, Johannesburg, South Africa Received 10 September 1998; received in revised form 16 December 1998; accepted 17 December 1998
Abstract The effect of boron (as H3BO3) on the CO hydrogenation ability of Co/TiO2 catalysts was investigated using XRD, LRS, TGA, DRIFTS and a ®xed bed ¯ow reactor. The introduction of boron (0.02±1.5%) into a 10 wt% Co/TiO2 catalyst decreased the Co3O4 crystallite size (26±16 nm) in the oxidic catalysts (calcined at 3008C) and decreased the hydrogen uptake (0.35± 0.9 ml/g cat) in the reduced catalyst. Reduction of the Co/TiO2 catalyst was made more dif®cult by the presence of boron. The CO conversion and overall hydrogenation rate decreased with decreasing ease of reducibility and decreasing cobalt dispersion caused by boron. Turnover frequency (ca. 2010ÿ3 sÿ1), however, remained constant throughout and was independent of the extent of reduction and dispersion of the catalysts. This provides further evidence of the structure-insensitivity of supported Co Fischer±Tropsch catalysts. Addition of small amounts of B (<0.1%) do, however, result in an increase in , less CH4 production and an increase in the ole®n/paraf®n ratio. This suggests an increase in the monomer propagation to termination ratio. At higher B loadings, product selectivity shifted to the lower molecular weight hydrocarbons and CO2 selectivity increased (0±2.5%). # 1999 Elsevier Science B.V. All rights reserved. Keywords: Cobalt; Titania; Boron; Fischer±Tropsch synthesis
1. Introduction It is well known that small amounts of chemical additives can both increase or decrease the activity and selectivity of the cobalt catalysed Fischer±Tropsch (F±T) reaction [1]. For example, the activity of cobalt supported catalysts is increased by the presence of ruthenium, which enhances the cobalt reducibility [2,3].
*Corresponding author. Tel.: +27-11-716-2219; fax: +27-11339-7967; e-mail: [email protected]
Decreases in activity (poisoning) are also known. For example, many F±T catalysts suffer rapid and substantial loss of activity in the presence of sulphur poisons at ppm levels. There is thus a considerable interest in the development of more sulphur tolerant catalysts. One approach to remove small amounts of catalyst poisons is by the use of catalysts containing small amounts of complexes or elements that preferentially react with the poison. Iron, nickel and cobalt borides, which contain small amounts of boron, have been reported to be active and selective catalysts for the Fischer±Tropsch reaction [4±6] and importantly these
0926-860X/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S0926-860X(98)00434-7
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metal borides have also been shown to be signi®cantly more resistant to sulphur poisoning in F±T synthesis [6]. The in¯uence of a boron additive on supported cobalt catalysts has been studied by Houalla and Delmon [7] who showed that an increase in both cobalt dispersion and cobalt surface phase formation occurs as a result of boron addition to an alumina support. Similarly, Vorobev et al. [8] have reported an increase in surface phase formation for nickel oxide supported on boron modi®ed alumina. The effect of small amounts of boron on the active phase/support interaction, the reduction capability and the activity of Co/TiO2 catalysts in the F±T reaction has been investigated in this work. This has been achieved by the use of XRD, LRS (laser Raman spectroscopy), TGA, DRIFTS and H2 chemisorption studies and the results are reported below. 2. Experimental 2.1. Catalyst preparation A series of boron-modi®ed titania supports (0.02± 1.5 wt% boron) were prepared by pore volume impregnation of titania (Degussa P25, BET surface area: 50 m2/g; pore volume: 0.51 ml/g) with solutions containing various amounts of boric acid. Samples were dried at 1208C for 16 h and then calcined at 4008C in air for 6 h. Cobalt (1±10 wt%) was deposited on the boron-modi®ed titania by pore volume impregnation from cobalt nitrate solutions. Samples were redried at 1208C for 16 h. All catalysts are represented as Co(y)/B(x)/TiO2, in which y is the percentage loading of cobalt and x is the percentage loading of boron. Atomic absorption spectroscopy (AAS) was employed to determine the cobalt and boron loadings. The results were very close to the expected values. 2.2. Characterisation techniques 2.2.1. X-ray diffraction (XRD) A Phillips spectrometer (PW 1830 generator) equipped with a Cu radiation source was used to analyse powdered disc samples (particle size: 100 micron) in the 0±708 2 range at a generator voltage of 40 kV and a generator current of 20 mA. A scan rate of 2 s per step (step size: 0.028 2) was
used during a continuous scan in the above-mentioned range. The Scherrer equation was used [9] to calculate the Co3O4 crystallite size in an oxidic catalyst from the diffractogram. 2.2.2. Laser Raman spectroscopy (LRS) Laser Raman spectra were obtained with a JobinYvon T 6400 Raman spectrometer that utilises holographic gratings. The excitation source was the 514.5 nm line from a coherent argon ion laser. Samples were pressed into pellets with a KBr support (90 wt% KBr) and rotated off-axis to prevent excessive heating by the laser beam. 2.2.3. Thermogravimetric analysis (TGA) Thermogravimetric analyses of the catalysts were conducted in a Du Pont 951 thermogravimetric analyser. The reduction of the catalysts was measured in a ¯owing stream of pure hydrogen at atmospheric pressure. The gas ¯ow rate (3.0 cm3/s), catalyst charge (40 mg) and heating rate (0.1 K/s) were kept constant in all experiments. All samples were reduced in pure hydrogen at a temperature ranging from room temperature to 7008C. 2.2.4. Infrared spectroscopy (DRIFTS) The IR spectra of CO adsorbed on the reduced catalysts were obtained using a Nicolet Impact 420 IR Fourier Transform spectrometer. The diffuse re¯ectance Fourier Transform IR (DRIFTS) spectra were recorded using a standard re¯ection accessory (Harrick Scienti®c) equipped with a ¯ow cell into which the ground catalyst (100 mg) was loaded. The catalysts were treated with pure carbon monoxide at 2 bar and 1008C after being reduced for 16 h at 3008C under hydrogen. In order to record a spectrum without gaseous carbon monoxide, an earlier spectrum consisting of 1024 scans (0±30 min) was subtracted from all subsequent spectra. 2.2.5. H2 chemisorption and O2 titration H2 chemisorption was carried out in a Micromeritics ASAP 2010 instrument. The calcined catalysts were ®rst reduced with ¯owing hydrogen (5 ml H2/ g cat s) for 16 h at various temperatures, and then evacuated to less than 10 mmm Hg to remove all chemisorbed hydrogen. The catalysts were then cooled to 1008C and isotherms measured at nine to
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eleven hydrogen pressures between 100 and 800 mm Hg. Adsorption isotherms were extrapolated to zero pressure to obtain chemisorption uptake. The equation given below [10] was used to calculate the dispersion values:
Table 1 Co3O4 crystallite size in Co(10)/B(x)/TiO2 catalysts calcined at various temperatures Boron loading (wt%)
Crystallite size (nm) 2008C
3008C
4008C
D% Vm =Vmol = W%=Wa Fs 100; 100;
0.00 0.02 0.05 0.10 0.50 1.0
27.5 26.0 23.1 18.6 19.0 19.7
25.9 24.0 21.4 19.0 19.4 16.5
21.5 19.3 19.0 14.6 14.9 13.1
(1)
where Vm is the total volume of hydrogen chemisorbed, Vmol the hydrogen molar volume, W% the percent of cobalt by weight, Wa the cobalt atomic weight, and Fs is the stoichiometry factor (Fs2 for H2). The number of accessible active sites on a surface, Ns, was used to calculate the turnover frequency. Ns Vm NA Fs =Vmol ;
(2)
where NA is Avogadro's number. The extent of reduction of the catalysts was measured by oxygen uptake measurements at 573 K after reduction, assuming the formation of Co3O4 during oxidation [25]. 2.3. Reactor study Fischer±Tropsch reactions were performed in three stainless steel reactors [11]. Samples (2 g) were loaded into the three reactors and calcined at 3008C for 24 h at 2000 hÿ1 in ¯owing air, followed by reduction in pure H2 at 3008C for 24 h at 500 hÿ1 before Fischer±Tropsch synthesis. Fischer±Tropsch reaction was carried out at: a pressure of 8 bar, 2508C, GHSV of 350 hÿ1 and a CO/H2 1:2 ratio for more than 200 h. Mass balances were commenced after the catalysts were on line for >80 h. On-line GC and off-line GC were used to analyse the composition of the product spectrum.
(1 1 1) diffraction line of Co3O4 was used to estimate the Co3O4 crystallite size. The results, shown in Table 1, revealed that the Co3O4 crystallite size decreased with increasing boron loading. It is also clear that the higher the temperature of calcination, the smaller the Co3O4 crystallite size [12]. To further investigate the in¯uence of boron on the Co3O4 formation, a series of catalysts containing different amounts of cobalt with or without boron were prepared and X-ray diffraction was used to check for the presence or absence of crystalline Co3O4 in these catalysts. It was found (data not shown) that the samples containing cobalt with wt% greater than 3.0 exhibited lines characteristic of Co3O4 for the boronfree catalysts. However, for the 2 wt% boron loaded system, only the catalysts containing cobalt with wt% greater than 7.0 exhibited lines characteristic of Co3O4. Boron thus suppresses large crystallite Co3O4 formation. Fig. 1 shows the Raman spectra of calcined catalysts containing 7 wt% cobalt supported on boron-free
3. Results and discussion 3.1. Bulk characterisation of oxidic catalysts The composition of the boron impregnated TiO2 was found, by XRD, to have an anatase to rutile ratio of 70.7:29.3, close to that of pure TiO2 (70.0% anatase, 30.0% rutile), indicating that the impregnation and calcination steps did not have any signi®cant in¯uence on the composition of the support. For all Co(y)/B(x)/TiO2 catalysts, line broadening of the
Fig. 1. The Raman spectra of Co(7)/B(x)/TiO2 catalysts (a: 0.0% boron; b: 2.0% boron).
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Table 2 Reduction peak position of Co(10)/B(x)/TiO2 catalystsa measured by TGA Boron loading (wt%) 0.00 0.02 0.05 0.10 0.50 1.00 a
Peak position (8C) Second peakb
Third peakb
290 290 335 360 410 450
425 430 475 490 520 570
Calcined at 3008C. See Fig. 2.
b
TiO2 and on 2 wt% boron/TiO2. Four Raman bands can be observed. The 400, 524 and 640 cmÿ1 bands are due to the titania support, while the 695 cmÿ1 band indicates the presence of Co3O4 [12]. It is apparent that the Co3O4 peak of Co(7)/B(2)/TiO2 is smaller than that of Co(7)/TiO2; again indicating the suppression of large crystallite Co3O4 formation by boron. Attempts to quantify the size of the Co3O4 crystallites from the size of the 695 cmÿ1 peak were not successful.
3.2. Catalyst reduction study The reduction of the catalysts was studied using TGA (under H2) and DRIFTS and the percentage reduction was measured by oxygen titration. TGA and DTA spectra were recorded of the Co(10)/ B(x)/TiO2 catalysts under H2 after calcination at 3008C (Table 2 and Fig. 2). Three reduction peaks were observed. The ®rst peak at about 2008C was attributed to the reduction of residual NOx groups from Co(NO3)26H2O. This peak became smaller and disappeared after calcination at 4008C due to the complete decomposition of the NOx at the higher temperature. The reduction of Co3O4 supported on titania proceeded in two stages [13], namely a primary reduction of Co3O4 to CoO (2508C; second peak) and a subsequent reduction of CoO to Co metal (4508C; third peak). A typical TGA/DTA spectrum showing the reduction behaviour of Co(10)/B(0.02)/ TiO2 is to be seen in Fig. 2. It is clear from Table 2 that the addition of boron caused the reduction peaks of Co3O4 to shift to higher temperatures, i.e. the boron has made the Co3O4 more dif®cult to reduce. To further understand the effect of boron on the reduction of catalysts, DRIFTS was used to measure
Fig. 2. TG-DTA analysis of Co(10)/B(0.02)/TiO2 catalyst in pure hydrogen.
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Fig. 3. IR spectra of Co(10)/B(x)/TiO2 catalysts. The spectra were recorded after 30 min of CO adsorption at 1008C and 2 bar on reduced catalysts (a: 0.0% B; b: 0.1% B; c: 0.5% B; d: 1.0% B).
the CO adsorption on the surface of the reduced catalysts. The catalysts were reduced for 16 h at 3008C under ¯owing hydrogen (¯ow rate: 2 ml/min) before introduction of CO. Carbon monoxide, adsorbed onto the reduced Co/TiO2 (no boron) catalyst at 1008C, produced an IR spectrum with peaks at 2173, 2114, 2060, 2032, 2011, 1945 and 1857 cmÿ1 (Fig. 3(A)). The peaks at 2167 and 2113 cmÿ1 are due to gaseous carbon monoxide [14]; 2060 and 2032 cmÿ1 are due to (CO) in Co±CO [14±17]; 2012 cmÿ1 is due to (CO) in Co0±CO; 1945 and 1857 cmÿ1 are due to (CO) in a bridging mode [16]. When boron was added onto the Co/TiO2 catalyst, a number of changes occurred in the IR spectrum (Fig. 3(b)±(d)). These include: (i) a shift to lower wave numbers of the linear Co±CO (from 2055 cmÿ1) and bridging CO (1934 cmÿ1) peaks, possibly due to a decreased CO dipole±dipole interaction [16,17]; (ii) a decrease in the intensity of the linear Co±CO, linear Co0±CO and bridging CO (1857 cmÿ1) peaks, indicating a decrease of the number of reduced cobalt atoms; and (iii) the appear-
ance of a new peak (1980 cmÿ1) with increasing boron loading. Oxygen titration data, shown in Table 3, revealed that the percentage catalyst reduction strongly depends on the boron loading. When the boron loading increased, the percentage reduction decreased. The overall effect as revealed by DSC, DRIFTS and O2 titration studies is that of a decrease in the reducibility of the cobalt with an increase in boron content. This could be due to two effects: ®rstly, boron could Table 3 The extent of reduction of Co(10)/B(x)/TiO2 catalystsa Boron loading (wt%)
Degree of reduction (%)
0.00 0.02 0.05 0.10 0.50 1.00 1.50
59.8 58.6 51.3 42.5 31.6 34.6 29
a
Calcined at 3008C, reduced at 3008C for 16 h.
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1. for low loaded catalysts (B <0.05 wt%), the CO conversion and the reaction rate are almost invariant; however, when the boron loading is greater than 0.1 wt%, CO conversion and the reaction rate decrease with increasing boron loading; 2. products shift to higher hydrocarbons, CH4 content decreases and the value increases for low B content catalysts (<1%), while the reverse occurs when the boron loading increases from 0.1 to 1.5 wt%; 3. the olefin/paraffin ratio increased at the lower boron loading and decreased at the higher boron loading; 4. CO2 was not detectable for low loaded boron catalysts. CO2 selectivity, however, generally increased with increasing boron loading (0±2.5%). There are thus competing effects at work after addition of the boron to Co/TiO2. For low B loaded catalysts, decreased chain termination and/or enhanced chain propagation, as re¯ected by the results shown in Table 4, are suggestive of decreased H2 activation at the catalyst surface. At a higher B loading (0.1±1.5%), decreases, and the methane content increases, CO2 increases while the rate of the reaction decreases. This latter behaviour is typical of unreduced catalysts [22], e.g. unreduced surface cobalt compounds (CoTiO3-like phase), which are inactive for F±T synthesis but active for the water-gas shift (WGS) reaction [23].
increase the interaction between Co3O4 and TiO2; secondly, the addition of boron may result in the formation of a surface cobalt±boron compound which is more dif®cult to reduce. The formation of a surface cobalt±titania compound in a Co/TiO2 catalyst has been reported in the literature. Ho et al. [18] found that after calcination of Co/TiO2 at 4008C a surface CoTiO3-like phase (involving Co2/3) was present in addition to the Co3O4 phase. They found that the amount of CoTiO3 in Co/TiO2 was dependent on the cobalt loading and calcination temperature. It is possible that B could enhance this effect. Indeed, Stranick et al. [12] have reported that boron facilitated the formation of surface cobalt compounds and suppressed the formation of Co3O4. 3.3. Fischer±Tropsch synthesis Percentage CO conversion, reaction rate (mmole CO converted per gram catalyst per second) and selectivity data are listed in Table 4 for the Fischer±Tropsch synthesis on reduced Co(10)/TiO2 catalysts containing different amounts of boron. The pretreatment conditions (calcination temperature: 3008C; reduction temperature: 3008C) and synthesis conditions (temperature: 2508C; pressure: 8 bar; 2H2:CO) were kept constant for all the catalysts in order to compare their catalytic properties. While it is recognised that the product selectivity will vary with the % conversion, the following trends are apparent from Table 4:
Table 4 Catalytic properties of Co(10)/B(x)/TiO2 catalysts for Fischer±Tropsch synthesisa Boron loading (wt%) CO conversion (%) Reaction rate (mmol/g cat s)
0.00 48.6 0.76
0.02 48.1 0.74
0.05 49.0 0.78
0.10 43.2 0.69
0.50 37.6 0.62
1.00 31.2 0.52
1.50 23.3 0.33
Selectivity (% by mass) C1 C2±C4 C5±C11 C12±C18 C18
21.5 13.1 47.0 11.1 7.3
17.8 10.2 54.0 10.1 7.9
14.2 10.5 57.1 12.0 6.0
11.5 9.4 52.3 18.5 8.8
11.3 11.6 49.0 18.1 10.0
19.6 11.9 44.0 17.5 6.9
29.2 18.9 32.1 14.6 5.0
Olefin/paraffin WGS extent (%)b ASF chain grow value ( ) a
0.07 0 0.63
0.08 0 0.69
0.11 0 0.76
0.11 0 0.79
0.06 1.1 0.77
0.05 1.9 0.65
0.04 2.5 0.56
Reaction conditions: calcination: 3008C, 1 atm, 2000 hÿ1, 16 h, flowing air; reduction: 3008C, 1 atm, 2000 hÿ1, 16 h, 100% H2; synthesis: 2508C, 8 bar, 350 hÿ1, 200 h, 2H2:1CO. b Calculated from CO2/(CO2H2O), by mass.
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Table 5 Percentage dispersion, reaction rates and turnover frequencies for Fischer±Tropsch synthesis on Co(10)/B(x)/TiO2 catalysts Boron loading (wt%)
Total H2 uptake (ml/g cat)
%Da
Reaction rate (mmol/g cat s)
TOFb (103 sÿ1)
0.00 0.02 0.05 0.10 0.50 1.00 1.50
0.35 0.34 0.41 0.32 0.29 0.27 0.17
1.9 1.85 2.2 1.71 1.59 1.46 0.92
0.76 0.74 0.78 0.69 0.62 0.52 0.33
24.3 24.6 21.3 24.2 23.9 21.1 19.5
a
Calculated using Eq. (1). Calculated from Eq. (2) and reaction rate data.
b
Table 5 lists the percentage reduction, percentage dispersion, reaction rate and turnover frequency (TOF) data that were calculated from Table 4. It is to be noted that the `percentage dispersion' calculated re¯ects the number of accessible reduced Co sites relative to the total number of cobalt atoms. It is clear that the reaction rate decreases with decreasing percentage reduction and percentage dispersion. However, the turnover frequency (TOF) remained constant and does not depend on the percentage dispersion or reduction for the catalysts studied. This implies that the reaction rate is only determined by the number of reduced cobalt atoms and that the reaction is a structure insensitive reaction [24]. However, it is clear that the environment of each of the cobalt atoms is in¯uenced by its surroundings in that the product distribution varies with dispersion and reduction. Ho et al. [19] have studied the effect of calcination temperature on the surface characteristics and CO hydrogenation activity of a Co/SiO2 catalyst. They did not ®nd any change in TOF with dispersion. Iglesia et al. [2] have investigated metal dispersion and support effects on reaction activity and selectivity of cobalt and ruthenium catalysts. Their results revealed that the hydrocarbon synthesis rate is proportional to metal dispersion, but site-time yield (TOF) was independent of metal dispersion. They concluded that Fischer±Tropsch synthesis on cobalt catalysts is a structure-insensitive reaction in the investigated dispersion range (0.45±9.5%). Many previous reports prior to this work have, however, suggested strong dispersion effects on the TOF (CO hydrogenation) on cobalt catalysts. Fu and Bartholomew [20] reported an increase in TOF as Co dispersion decreased from 30% to 15%. They suggested that CO hydrogenation on Co/
Al2O3 is structure-sensitive and requires sites where CO is strongly co-ordinated. Lee et al. [21] also reported an increase in TOF with increasing Co crystallite size on Al2O3. Our results are consistent with the Ho and Iglesia data, indicating that in the dispersion range 0.5±10%, F±T synthesis on a Co/TiO2 catalyst is structure-insensitive. 4. Conclusion The addition of small amounts of boron to a 10 wt% Co/TiO2 catalyst was found to decrease the Co3O4 crystallite size in the oxidic catalyst. The reducibility of the catalysts was found to decrease with increasing boron loading. This was characterised by the increase of reduction temperatures of the Co peaks in the TGDTA spectra and by the decreased percentage reduction. The amount of metal atoms exposed on the surface, determined by H2 chemisorption, was found to decrease with boron loading. The decrease in surface exposed metal atoms was complimented by an equivalent decrease in the rate of F±T synthesis, yielding a practically constant turnover frequency. This important result is consistent with the recent reports on Co supported F±T catalysis in which the reaction was shown to be structure-insensitive. The decrease in the rate of F±T synthesis with increasing boron loading was due to a decrease in the number of surface cobalt sites. Product selectivity shifted to lower molecular weight hydrocarbons and the CO2/ H2O ratio increased with increasing boron loading. This effect may be due to the abundance of stable oxide surface sites in the poorly reduced catalysts, which is in¯uenced by the presence of boron.
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Acknowledgements The authors gratefully acknowledge the ®nancial support from the University, Sasol and the FRD, and the technical assistance of Mr. B. Chassoulas and the recording of the laser Raman spectra (Dr. P. Murphy). References [1] R.B. Anderson, The Fischer±Tropsch Synthesis, Academic Press, New York, 1984. [2] E. Iglesia, S.L. Soled, R.A. Fiato, J. Catal. 137 (1992) 212. [3] E. Iglesia, S.L. Soled, R.A. Fiato, G.H. Via, J. Catal. 143 (1993) 345. [4] C.A. Brown, H.C. Brown, J. Am. Chem. Soc. 85 (1963) 1005. [5] R.C. Wade, D.G. Hoah, A.N. Hughes, B.C. Hui, Catal. Rev.Sci. Eng. 14 (1976) 211. [6] C.H. Bartholomew, R.M. Bowman, Appl. Catal. 15 (1985) 59. [7] M. Houalla, B. Delmon, Appl. Catal. 1 (1981) 285. [8] V. Vorobev, A. Agzamkhodzhaeva, V. Mikita, M. Abidova, Kinet. Katal. 25 (1984) 154. [9] H.P. Klug, I.E. Alexander, X-ray Diffraction Procedure, Wiley, New York, 1954, 385 pp.
[10] P.A. Webb, C. Orr, Analytical Methods in Fine Particle Technology, Micromeritics, 1997, 229 pp. [11] J. Price, Ph.D. Thesis, University of the Witwatersrand, Johannesburg, South Africa, 1994. [12] M.A. Stranick, M. Houalla, D.M. Hercules, J. Catal. 104 (1987) 396. [13] J.H. Martens, H.F.J. Van't Blik, J. Catal. 97 (1986) 200. [14] A.A. Davydov, N.J. Coville, Russ. Chem. Bull. 44 (1995) 1866. [15] H.M. Heal, E.C. Leisegang, R.G. Torrington, J. Catal. 51 (1978) 314. [16] A.A. Davydov, Infrared Spectroscopy of Adsorbed Species on the Surface of Transition Metal Oxides, Wiley, Chichester, 1990. [17] B. Mothebe, D.J. Duvenhage, V.D. Sokolovskii, N.J. Coville, Stud. Surf. Sci. Catal. 107 (1997) 187. [18] S.W. Ho, J.M. Cruz, M. Houalla, D.M. Hercules, J. Catal. 135 (1992) 173. [19] S.W. Ho, M. Hovalla, D.M. Hercules, J. Phys. Chem. 94 (1990) 6396. [20] L. Fu, C.H. Bartholomew, J. Catal. 92 (1985) 376. [21] J.H. Lee, D.K. Lee, S.K. Ihm, J. Catal. 113 (1988) 544. [22] H.H. Stoch, N. Golumbic, R.B. Anderson, The Fischer± Tropsch and Related Synthesis, Wiley, New York, 1951. [23] D.S. Newsome, Catal. Rev.-Sci. Eng. 21 (1980) 27. [24] R.L. Toomes, D.A. King, Surf. Sci. 349 (1996) 1. [25] C.H. Bartholomew, R.J. Farrauto, J. Catal. 41 (1976) 45.
The effect of boron on the catalyst reducibility and activity of Co/TiO2 Fischer±Tropsch catalysts Jinlin Li, Neil J. Coville* Applied Chemistry and Chemical Technology Centre, Department of Chemistry, University of the Witwatersrand, PO Wits 2050, Johannesburg, South Africa Received 10 September 1998; received in revised form 16 December 1998; accepted 17 December 1998
Abstract The effect of boron (as H3BO3) on the CO hydrogenation ability of Co/TiO2 catalysts was investigated using XRD, LRS, TGA, DRIFTS and a ®xed bed ¯ow reactor. The introduction of boron (0.02±1.5%) into a 10 wt% Co/TiO2 catalyst decreased the Co3O4 crystallite size (26±16 nm) in the oxidic catalysts (calcined at 3008C) and decreased the hydrogen uptake (0.35± 0.9 ml/g cat) in the reduced catalyst. Reduction of the Co/TiO2 catalyst was made more dif®cult by the presence of boron. The CO conversion and overall hydrogenation rate decreased with decreasing ease of reducibility and decreasing cobalt dispersion caused by boron. Turnover frequency (ca. 2010ÿ3 sÿ1), however, remained constant throughout and was independent of the extent of reduction and dispersion of the catalysts. This provides further evidence of the structure-insensitivity of supported Co Fischer±Tropsch catalysts. Addition of small amounts of B (<0.1%) do, however, result in an increase in , less CH4 production and an increase in the ole®n/paraf®n ratio. This suggests an increase in the monomer propagation to termination ratio. At higher B loadings, product selectivity shifted to the lower molecular weight hydrocarbons and CO2 selectivity increased (0±2.5%). # 1999 Elsevier Science B.V. All rights reserved. Keywords: Cobalt; Titania; Boron; Fischer±Tropsch synthesis
1. Introduction It is well known that small amounts of chemical additives can both increase or decrease the activity and selectivity of the cobalt catalysed Fischer±Tropsch (F±T) reaction [1]. For example, the activity of cobalt supported catalysts is increased by the presence of ruthenium, which enhances the cobalt reducibility [2,3].
*Corresponding author. Tel.: +27-11-716-2219; fax: +27-11339-7967; e-mail: [email protected]
Decreases in activity (poisoning) are also known. For example, many F±T catalysts suffer rapid and substantial loss of activity in the presence of sulphur poisons at ppm levels. There is thus a considerable interest in the development of more sulphur tolerant catalysts. One approach to remove small amounts of catalyst poisons is by the use of catalysts containing small amounts of complexes or elements that preferentially react with the poison. Iron, nickel and cobalt borides, which contain small amounts of boron, have been reported to be active and selective catalysts for the Fischer±Tropsch reaction [4±6] and importantly these
0926-860X/99/$ ± see front matter # 1999 Elsevier Science B.V. All rights reserved. PII: S0926-860X(98)00434-7
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metal borides have also been shown to be signi®cantly more resistant to sulphur poisoning in F±T synthesis [6]. The in¯uence of a boron additive on supported cobalt catalysts has been studied by Houalla and Delmon [7] who showed that an increase in both cobalt dispersion and cobalt surface phase formation occurs as a result of boron addition to an alumina support. Similarly, Vorobev et al. [8] have reported an increase in surface phase formation for nickel oxide supported on boron modi®ed alumina. The effect of small amounts of boron on the active phase/support interaction, the reduction capability and the activity of Co/TiO2 catalysts in the F±T reaction has been investigated in this work. This has been achieved by the use of XRD, LRS (laser Raman spectroscopy), TGA, DRIFTS and H2 chemisorption studies and the results are reported below. 2. Experimental 2.1. Catalyst preparation A series of boron-modi®ed titania supports (0.02± 1.5 wt% boron) were prepared by pore volume impregnation of titania (Degussa P25, BET surface area: 50 m2/g; pore volume: 0.51 ml/g) with solutions containing various amounts of boric acid. Samples were dried at 1208C for 16 h and then calcined at 4008C in air for 6 h. Cobalt (1±10 wt%) was deposited on the boron-modi®ed titania by pore volume impregnation from cobalt nitrate solutions. Samples were redried at 1208C for 16 h. All catalysts are represented as Co(y)/B(x)/TiO2, in which y is the percentage loading of cobalt and x is the percentage loading of boron. Atomic absorption spectroscopy (AAS) was employed to determine the cobalt and boron loadings. The results were very close to the expected values. 2.2. Characterisation techniques 2.2.1. X-ray diffraction (XRD) A Phillips spectrometer (PW 1830 generator) equipped with a Cu radiation source was used to analyse powdered disc samples (particle size: 100 micron) in the 0±708 2 range at a generator voltage of 40 kV and a generator current of 20 mA. A scan rate of 2 s per step (step size: 0.028 2) was
used during a continuous scan in the above-mentioned range. The Scherrer equation was used [9] to calculate the Co3O4 crystallite size in an oxidic catalyst from the diffractogram. 2.2.2. Laser Raman spectroscopy (LRS) Laser Raman spectra were obtained with a JobinYvon T 6400 Raman spectrometer that utilises holographic gratings. The excitation source was the 514.5 nm line from a coherent argon ion laser. Samples were pressed into pellets with a KBr support (90 wt% KBr) and rotated off-axis to prevent excessive heating by the laser beam. 2.2.3. Thermogravimetric analysis (TGA) Thermogravimetric analyses of the catalysts were conducted in a Du Pont 951 thermogravimetric analyser. The reduction of the catalysts was measured in a ¯owing stream of pure hydrogen at atmospheric pressure. The gas ¯ow rate (3.0 cm3/s), catalyst charge (40 mg) and heating rate (0.1 K/s) were kept constant in all experiments. All samples were reduced in pure hydrogen at a temperature ranging from room temperature to 7008C. 2.2.4. Infrared spectroscopy (DRIFTS) The IR spectra of CO adsorbed on the reduced catalysts were obtained using a Nicolet Impact 420 IR Fourier Transform spectrometer. The diffuse re¯ectance Fourier Transform IR (DRIFTS) spectra were recorded using a standard re¯ection accessory (Harrick Scienti®c) equipped with a ¯ow cell into which the ground catalyst (100 mg) was loaded. The catalysts were treated with pure carbon monoxide at 2 bar and 1008C after being reduced for 16 h at 3008C under hydrogen. In order to record a spectrum without gaseous carbon monoxide, an earlier spectrum consisting of 1024 scans (0±30 min) was subtracted from all subsequent spectra. 2.2.5. H2 chemisorption and O2 titration H2 chemisorption was carried out in a Micromeritics ASAP 2010 instrument. The calcined catalysts were ®rst reduced with ¯owing hydrogen (5 ml H2/ g cat s) for 16 h at various temperatures, and then evacuated to less than 10 mmm Hg to remove all chemisorbed hydrogen. The catalysts were then cooled to 1008C and isotherms measured at nine to
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eleven hydrogen pressures between 100 and 800 mm Hg. Adsorption isotherms were extrapolated to zero pressure to obtain chemisorption uptake. The equation given below [10] was used to calculate the dispersion values:
Table 1 Co3O4 crystallite size in Co(10)/B(x)/TiO2 catalysts calcined at various temperatures Boron loading (wt%)
Crystallite size (nm) 2008C
3008C
4008C
D% Vm =Vmol = W%=Wa Fs 100; 100;
0.00 0.02 0.05 0.10 0.50 1.0
27.5 26.0 23.1 18.6 19.0 19.7
25.9 24.0 21.4 19.0 19.4 16.5
21.5 19.3 19.0 14.6 14.9 13.1
(1)
where Vm is the total volume of hydrogen chemisorbed, Vmol the hydrogen molar volume, W% the percent of cobalt by weight, Wa the cobalt atomic weight, and Fs is the stoichiometry factor (Fs2 for H2). The number of accessible active sites on a surface, Ns, was used to calculate the turnover frequency. Ns Vm NA Fs =Vmol ;
(2)
where NA is Avogadro's number. The extent of reduction of the catalysts was measured by oxygen uptake measurements at 573 K after reduction, assuming the formation of Co3O4 during oxidation [25]. 2.3. Reactor study Fischer±Tropsch reactions were performed in three stainless steel reactors [11]. Samples (2 g) were loaded into the three reactors and calcined at 3008C for 24 h at 2000 hÿ1 in ¯owing air, followed by reduction in pure H2 at 3008C for 24 h at 500 hÿ1 before Fischer±Tropsch synthesis. Fischer±Tropsch reaction was carried out at: a pressure of 8 bar, 2508C, GHSV of 350 hÿ1 and a CO/H2 1:2 ratio for more than 200 h. Mass balances were commenced after the catalysts were on line for >80 h. On-line GC and off-line GC were used to analyse the composition of the product spectrum.
(1 1 1) diffraction line of Co3O4 was used to estimate the Co3O4 crystallite size. The results, shown in Table 1, revealed that the Co3O4 crystallite size decreased with increasing boron loading. It is also clear that the higher the temperature of calcination, the smaller the Co3O4 crystallite size [12]. To further investigate the in¯uence of boron on the Co3O4 formation, a series of catalysts containing different amounts of cobalt with or without boron were prepared and X-ray diffraction was used to check for the presence or absence of crystalline Co3O4 in these catalysts. It was found (data not shown) that the samples containing cobalt with wt% greater than 3.0 exhibited lines characteristic of Co3O4 for the boronfree catalysts. However, for the 2 wt% boron loaded system, only the catalysts containing cobalt with wt% greater than 7.0 exhibited lines characteristic of Co3O4. Boron thus suppresses large crystallite Co3O4 formation. Fig. 1 shows the Raman spectra of calcined catalysts containing 7 wt% cobalt supported on boron-free
3. Results and discussion 3.1. Bulk characterisation of oxidic catalysts The composition of the boron impregnated TiO2 was found, by XRD, to have an anatase to rutile ratio of 70.7:29.3, close to that of pure TiO2 (70.0% anatase, 30.0% rutile), indicating that the impregnation and calcination steps did not have any signi®cant in¯uence on the composition of the support. For all Co(y)/B(x)/TiO2 catalysts, line broadening of the
Fig. 1. The Raman spectra of Co(7)/B(x)/TiO2 catalysts (a: 0.0% boron; b: 2.0% boron).
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Table 2 Reduction peak position of Co(10)/B(x)/TiO2 catalystsa measured by TGA Boron loading (wt%) 0.00 0.02 0.05 0.10 0.50 1.00 a
Peak position (8C) Second peakb
Third peakb
290 290 335 360 410 450
425 430 475 490 520 570
Calcined at 3008C. See Fig. 2.
b
TiO2 and on 2 wt% boron/TiO2. Four Raman bands can be observed. The 400, 524 and 640 cmÿ1 bands are due to the titania support, while the 695 cmÿ1 band indicates the presence of Co3O4 [12]. It is apparent that the Co3O4 peak of Co(7)/B(2)/TiO2 is smaller than that of Co(7)/TiO2; again indicating the suppression of large crystallite Co3O4 formation by boron. Attempts to quantify the size of the Co3O4 crystallites from the size of the 695 cmÿ1 peak were not successful.
3.2. Catalyst reduction study The reduction of the catalysts was studied using TGA (under H2) and DRIFTS and the percentage reduction was measured by oxygen titration. TGA and DTA spectra were recorded of the Co(10)/ B(x)/TiO2 catalysts under H2 after calcination at 3008C (Table 2 and Fig. 2). Three reduction peaks were observed. The ®rst peak at about 2008C was attributed to the reduction of residual NOx groups from Co(NO3)26H2O. This peak became smaller and disappeared after calcination at 4008C due to the complete decomposition of the NOx at the higher temperature. The reduction of Co3O4 supported on titania proceeded in two stages [13], namely a primary reduction of Co3O4 to CoO (2508C; second peak) and a subsequent reduction of CoO to Co metal (4508C; third peak). A typical TGA/DTA spectrum showing the reduction behaviour of Co(10)/B(0.02)/ TiO2 is to be seen in Fig. 2. It is clear from Table 2 that the addition of boron caused the reduction peaks of Co3O4 to shift to higher temperatures, i.e. the boron has made the Co3O4 more dif®cult to reduce. To further understand the effect of boron on the reduction of catalysts, DRIFTS was used to measure
Fig. 2. TG-DTA analysis of Co(10)/B(0.02)/TiO2 catalyst in pure hydrogen.
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Fig. 3. IR spectra of Co(10)/B(x)/TiO2 catalysts. The spectra were recorded after 30 min of CO adsorption at 1008C and 2 bar on reduced catalysts (a: 0.0% B; b: 0.1% B; c: 0.5% B; d: 1.0% B).
the CO adsorption on the surface of the reduced catalysts. The catalysts were reduced for 16 h at 3008C under ¯owing hydrogen (¯ow rate: 2 ml/min) before introduction of CO. Carbon monoxide, adsorbed onto the reduced Co/TiO2 (no boron) catalyst at 1008C, produced an IR spectrum with peaks at 2173, 2114, 2060, 2032, 2011, 1945 and 1857 cmÿ1 (Fig. 3(A)). The peaks at 2167 and 2113 cmÿ1 are due to gaseous carbon monoxide [14]; 2060 and 2032 cmÿ1 are due to (CO) in Co±CO [14±17]; 2012 cmÿ1 is due to (CO) in Co0±CO; 1945 and 1857 cmÿ1 are due to (CO) in a bridging mode [16]. When boron was added onto the Co/TiO2 catalyst, a number of changes occurred in the IR spectrum (Fig. 3(b)±(d)). These include: (i) a shift to lower wave numbers of the linear Co±CO (from 2055 cmÿ1) and bridging CO (1934 cmÿ1) peaks, possibly due to a decreased CO dipole±dipole interaction [16,17]; (ii) a decrease in the intensity of the linear Co±CO, linear Co0±CO and bridging CO (1857 cmÿ1) peaks, indicating a decrease of the number of reduced cobalt atoms; and (iii) the appear-
ance of a new peak (1980 cmÿ1) with increasing boron loading. Oxygen titration data, shown in Table 3, revealed that the percentage catalyst reduction strongly depends on the boron loading. When the boron loading increased, the percentage reduction decreased. The overall effect as revealed by DSC, DRIFTS and O2 titration studies is that of a decrease in the reducibility of the cobalt with an increase in boron content. This could be due to two effects: ®rstly, boron could Table 3 The extent of reduction of Co(10)/B(x)/TiO2 catalystsa Boron loading (wt%)
Degree of reduction (%)
0.00 0.02 0.05 0.10 0.50 1.00 1.50
59.8 58.6 51.3 42.5 31.6 34.6 29
a
Calcined at 3008C, reduced at 3008C for 16 h.
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1. for low loaded catalysts (B <0.05 wt%), the CO conversion and the reaction rate are almost invariant; however, when the boron loading is greater than 0.1 wt%, CO conversion and the reaction rate decrease with increasing boron loading; 2. products shift to higher hydrocarbons, CH4 content decreases and the value increases for low B content catalysts (<1%), while the reverse occurs when the boron loading increases from 0.1 to 1.5 wt%; 3. the olefin/paraffin ratio increased at the lower boron loading and decreased at the higher boron loading; 4. CO2 was not detectable for low loaded boron catalysts. CO2 selectivity, however, generally increased with increasing boron loading (0±2.5%). There are thus competing effects at work after addition of the boron to Co/TiO2. For low B loaded catalysts, decreased chain termination and/or enhanced chain propagation, as re¯ected by the results shown in Table 4, are suggestive of decreased H2 activation at the catalyst surface. At a higher B loading (0.1±1.5%), decreases, and the methane content increases, CO2 increases while the rate of the reaction decreases. This latter behaviour is typical of unreduced catalysts [22], e.g. unreduced surface cobalt compounds (CoTiO3-like phase), which are inactive for F±T synthesis but active for the water-gas shift (WGS) reaction [23].
increase the interaction between Co3O4 and TiO2; secondly, the addition of boron may result in the formation of a surface cobalt±boron compound which is more dif®cult to reduce. The formation of a surface cobalt±titania compound in a Co/TiO2 catalyst has been reported in the literature. Ho et al. [18] found that after calcination of Co/TiO2 at 4008C a surface CoTiO3-like phase (involving Co2/3) was present in addition to the Co3O4 phase. They found that the amount of CoTiO3 in Co/TiO2 was dependent on the cobalt loading and calcination temperature. It is possible that B could enhance this effect. Indeed, Stranick et al. [12] have reported that boron facilitated the formation of surface cobalt compounds and suppressed the formation of Co3O4. 3.3. Fischer±Tropsch synthesis Percentage CO conversion, reaction rate (mmole CO converted per gram catalyst per second) and selectivity data are listed in Table 4 for the Fischer±Tropsch synthesis on reduced Co(10)/TiO2 catalysts containing different amounts of boron. The pretreatment conditions (calcination temperature: 3008C; reduction temperature: 3008C) and synthesis conditions (temperature: 2508C; pressure: 8 bar; 2H2:CO) were kept constant for all the catalysts in order to compare their catalytic properties. While it is recognised that the product selectivity will vary with the % conversion, the following trends are apparent from Table 4:
Table 4 Catalytic properties of Co(10)/B(x)/TiO2 catalysts for Fischer±Tropsch synthesisa Boron loading (wt%) CO conversion (%) Reaction rate (mmol/g cat s)
0.00 48.6 0.76
0.02 48.1 0.74
0.05 49.0 0.78
0.10 43.2 0.69
0.50 37.6 0.62
1.00 31.2 0.52
1.50 23.3 0.33
Selectivity (% by mass) C1 C2±C4 C5±C11 C12±C18 C18
21.5 13.1 47.0 11.1 7.3
17.8 10.2 54.0 10.1 7.9
14.2 10.5 57.1 12.0 6.0
11.5 9.4 52.3 18.5 8.8
11.3 11.6 49.0 18.1 10.0
19.6 11.9 44.0 17.5 6.9
29.2 18.9 32.1 14.6 5.0
Olefin/paraffin WGS extent (%)b ASF chain grow value ( ) a
0.07 0 0.63
0.08 0 0.69
0.11 0 0.76
0.11 0 0.79
0.06 1.1 0.77
0.05 1.9 0.65
0.04 2.5 0.56
Reaction conditions: calcination: 3008C, 1 atm, 2000 hÿ1, 16 h, flowing air; reduction: 3008C, 1 atm, 2000 hÿ1, 16 h, 100% H2; synthesis: 2508C, 8 bar, 350 hÿ1, 200 h, 2H2:1CO. b Calculated from CO2/(CO2H2O), by mass.
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Table 5 Percentage dispersion, reaction rates and turnover frequencies for Fischer±Tropsch synthesis on Co(10)/B(x)/TiO2 catalysts Boron loading (wt%)
Total H2 uptake (ml/g cat)
%Da
Reaction rate (mmol/g cat s)
TOFb (103 sÿ1)
0.00 0.02 0.05 0.10 0.50 1.00 1.50
0.35 0.34 0.41 0.32 0.29 0.27 0.17
1.9 1.85 2.2 1.71 1.59 1.46 0.92
0.76 0.74 0.78 0.69 0.62 0.52 0.33
24.3 24.6 21.3 24.2 23.9 21.1 19.5
a
Calculated using Eq. (1). Calculated from Eq. (2) and reaction rate data.
b
Table 5 lists the percentage reduction, percentage dispersion, reaction rate and turnover frequency (TOF) data that were calculated from Table 4. It is to be noted that the `percentage dispersion' calculated re¯ects the number of accessible reduced Co sites relative to the total number of cobalt atoms. It is clear that the reaction rate decreases with decreasing percentage reduction and percentage dispersion. However, the turnover frequency (TOF) remained constant and does not depend on the percentage dispersion or reduction for the catalysts studied. This implies that the reaction rate is only determined by the number of reduced cobalt atoms and that the reaction is a structure insensitive reaction [24]. However, it is clear that the environment of each of the cobalt atoms is in¯uenced by its surroundings in that the product distribution varies with dispersion and reduction. Ho et al. [19] have studied the effect of calcination temperature on the surface characteristics and CO hydrogenation activity of a Co/SiO2 catalyst. They did not ®nd any change in TOF with dispersion. Iglesia et al. [2] have investigated metal dispersion and support effects on reaction activity and selectivity of cobalt and ruthenium catalysts. Their results revealed that the hydrocarbon synthesis rate is proportional to metal dispersion, but site-time yield (TOF) was independent of metal dispersion. They concluded that Fischer±Tropsch synthesis on cobalt catalysts is a structure-insensitive reaction in the investigated dispersion range (0.45±9.5%). Many previous reports prior to this work have, however, suggested strong dispersion effects on the TOF (CO hydrogenation) on cobalt catalysts. Fu and Bartholomew [20] reported an increase in TOF as Co dispersion decreased from 30% to 15%. They suggested that CO hydrogenation on Co/
Al2O3 is structure-sensitive and requires sites where CO is strongly co-ordinated. Lee et al. [21] also reported an increase in TOF with increasing Co crystallite size on Al2O3. Our results are consistent with the Ho and Iglesia data, indicating that in the dispersion range 0.5±10%, F±T synthesis on a Co/TiO2 catalyst is structure-insensitive. 4. Conclusion The addition of small amounts of boron to a 10 wt% Co/TiO2 catalyst was found to decrease the Co3O4 crystallite size in the oxidic catalyst. The reducibility of the catalysts was found to decrease with increasing boron loading. This was characterised by the increase of reduction temperatures of the Co peaks in the TGDTA spectra and by the decreased percentage reduction. The amount of metal atoms exposed on the surface, determined by H2 chemisorption, was found to decrease with boron loading. The decrease in surface exposed metal atoms was complimented by an equivalent decrease in the rate of F±T synthesis, yielding a practically constant turnover frequency. This important result is consistent with the recent reports on Co supported F±T catalysis in which the reaction was shown to be structure-insensitive. The decrease in the rate of F±T synthesis with increasing boron loading was due to a decrease in the number of surface cobalt sites. Product selectivity shifted to lower molecular weight hydrocarbons and the CO2/ H2O ratio increased with increasing boron loading. This effect may be due to the abundance of stable oxide surface sites in the poorly reduced catalysts, which is in¯uenced by the presence of boron.
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