Catalytic reforming of methane with carbon dioxide over nickel catalysts I. Catalyst characterization and activity

~ ELSEVIER

APPLIED CATALYSIS A:GENERAL

Applied Catalysis A: General 142 (1996) 73-96

Catalytic reforming of methane with carbon dioxide over nickel catalysts I. Catalyst characterization and activity Michael C.J. Bradford, M. Albert V a n n i c e * The Pennsylvania State University, University Park, PA 16802-4400, USA

Received 16 October 1995; revised 22 January 1996; accepted 24 January 1996

Abstract The reforming of methane with carbon dioxide was studied over nickel supported on MgO, TiO 2, SiO 2, and activated carbon. The influence of the support on catalyst activity and carbon deposition resistivity was markedly different in each case. Although considerable formation of filamentous carbon was observed over N i / S i O 2 (confirmed by TEM and TPO), there was negligible initial loss of catalytic activity. The catalytic activity of N i / C was very similar to that of N i / S i O 2, but no filamentous carbon appeared to be formed. In contrast to N i / S i O 2, substantially less coking was observed over either the N i / T i O 2 or the N i / M g O catalysts. Evidence of strong metal-support interaction (SMSI) in the N i / T i O 2 catalyst indicated that large ensembles of nickel atoms, active for carbon deposition, are deactivated or removed by the presence of mobile TiO, species. The identification of two TiO x phases and a NisTiO 7 phase was made possible by direct measurement of crystalline d spacings with TEM. The N i / M g O catalyst was both active and very stable for up to 44 h time on stream. Chemisorption, XRD and TEM results indicate the formation of a partially reducible N i O - M g O solid solution, which appears to stabilize the reduced nickel surfaces and provide resistance to carbon deposition. Keywords: Methane; Carbon dioxide; Reforming; Nickel; Titania; Silica; Magnesium oxide; Carbon

1. Introduction

During the last several years, there has been renewed interest in the catalytic reforming of methane with carbon dioxide, rather than steam, for the production of synthesis gas. This process is attractive industrially for it yields lower * Corresponding author. Tel. (+ 1-814) 8634803, Fax. (+ 1-814) 8657846. 0926-860X/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved. PH S0926- 860X(96)00065- 8

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M.C.J. Bradford, M.A. Vannice / Applied Catalysis A: General 142 (1996) 73-96

H 2 / C O product ratios, which are preferable feeds for Fischer-Tropsch plants because high H 2/CO ratios favor methanation and suppress chain growth [1,2]. Furthermore, many remote natural gas fields contain significant amounts of carbon dioxide. Environmentally, this reaction is appealing due to the reduction of carbon dioxide and methane emissions as both gases are contributors to the greenhouse effect [3]. Also, methane reforming with carbon dioxide has been studied for viability in chemical energy transmission systems [4,5]. Specifically, the Solchem Process [6,7] and the CLEA Project [8] utilize the forward CH4-CO 2 reforming reaction and the reverse H 2-CO methanation reaction as a means of converting solar energy to chemical energy for storage and transportation, while the Eva-Adams Process [9] utilizes steam reforming of methane in an analogous reaction network. Some industrial processes which utilize CH4-CO 2 reforming, such as the Caleor Process [10,11] and the SPARG Process [12], are already in operation. All of the group VIIIA metals, with the exception of osmium, have been studied on a variety of supports for C H 4 - C O 2 reforming [13-18]. Industrially, the metal of choice for the catalyst is nickel due to its inherent availability and lower cost in comparison to other more noble metals. However, because the reforming reaction is strongly endothermic, equilibrium yields of synthesis gas require high temperatures and, under such reaction conditions, several studies with alumina-supported metals [5,13-15] have shown that nickel is more susceptible to coking than noble metals; regardless, economic and availability constraints typically prohibit use of noble metal catalysts on an industrial scale, and it is still worthwhile to pursue development of supported nickel catalysts. The aforementioned SPARG process [12] ameliorates the coking problem via sulfur passivation of the nickel catalyst, which provides adsorbed sulfur atoms that block catalyst sites believed to be active for carbon nucleation. Whether a similar role can be played by a support, rather than sulfur, is of interest. The primary purpose of this study was to elucidate the role of the support on catalyst activity and coking resistivity of supported nickel catalysts, and to compare these results with previous work from the open literature.

2. Experimental Four supported nickel catalysts were studied in this investigation. The N i / S i O 2 catalyst was originally prepared by Smith et al. [19] by impregnating high surface area SiO 2 (Davison Grade 57 220 m2/g) with Ni(NO3) 2 • 6 H 2 0 (Aldrich) using an incipient wetness technique. This catalyst was studied primarily as a control, as previous results have shown that it is both active and exhibits carbon deposition [4,16,20]. Ni(NO3) 2 • 6 H 2 0 dissolved in doubly distilled, deionized water was also used to prepare a N i / C catalyst by impregnation of an activated carbon (Norit, 800 m2/g) after HNO 3 treatment [21], and a

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75

N i / T i O 2 catalyst by impregnation of TiO 2 (Degussa P-25, 40 m2/g). In addition, a N i / M g O catalyst (20 mZ/g) was prepared using a coprecipitation method, and after precipitation of the catalyst with NazCO 3, it was thoroughly rinsed with deionized water to remove trace sodium [22]. The N i / M g O catalyst was separated into two sieve fractions: < 70 mesh (8.2%Ni/MgO) and > 70 mesh (10. l % N i / M g O ) , and the latter sample was ground to < 70 mesh prior to testing. Extensive study of the 8.2%Ni/MgO catalyst showed that it was inactive due to formation of an irreducible N i O - M g O solid solution [23] Parmaliana et al. [24,25] have similarly shown that Ni loadings of less than about 11 wt.% for N i / M g O catalysts are inactive for CH4-H20 reforming due to formation of an irreducible solid solution. All further results referred to herein as N i / M g O pertain to the 10.1%Ni/MgO catalyst. After preparation, all catalysts were calcined at 373 K in air for roughly 12 h prior to storage in a desiccator. A reduction at 773 K, adapted from that used by Smith et al. [19], was used with the N i / S i O 2, N i / C , and N i / T i O 2 catalysts. The first step of this pretreatment consisted of heating in flowing hydrogen (12 sccm) for 30 min at 423 K, primarily to remove water, then heating to 773 K and reducing in flowing hydrogen for 60 min to remove possible ammonia and carbonate impurities as well as to reduce nickel oxide to a zero-valent state. The results of Sridhar et al. show clearly that a 773 K reduction is adequate to reduce nickel oxide [26]. In the case of the N i / T i O 2 catalyst, this 773 K reduction is also required to obtain an SMSI state [27]. After cooling the catalyst in flowing H 2 to the desired reaction temperature, ca. 673 K, the catalyst was purged with flowing helium (36 sccm) for 30 min to remove adsorbed hydrogen from the surface, because adsorbed hydrogen has been shown to promote dissociative carbon dioxide adsorption [28]. A higher reduction temperature was used only for the N i / M g O catalyst because the 773 K pretreatment did not activate the catalyst [23]. This high temperature pretreatment consisted of a temperature ramp to 1123 K in flowing He, followed by reduction in flowing hydrogen for 30 min at 1123 K. The catalyst was then cooled to 773 K in flowing hydrogen and purged with flowing helium for 30 min. This pretreatment has been shown to provide good catalytic behavior [29]. Flame emission spectrophotometry (FES) was used to determine the concentration of trace sodium in the N i / M g O catalyst [30]. This was necessary because the N i / M g O catalyst was prepared using a sodium precursor [22], and it has been reported that trace alkali content in catalysts has a detrimental affect on the catalyst activity for CH4-H20 reforming [31]. Inductively coupled plasma spectrophotometry (ICPS) was used to determine the metal loading of the N i / C , N i / M g O and N i / T i O 2 catalysts [30], and neutron activation analysis (NAA) was used to determine the metal loading of the N i / S i O 2 catalyst [19]. Hydrogen and carbon monoxide chemisorption were performed at 300 K on all catalysts with a stainless steel volumetric apparatus to estimate irreversible

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gas uptake, metal dispersion and particle size via application of the dual isotherm method [32]. Additional carbon monoxide isotherms were obtained at 473 K on N i / T i O 2 to estimate the heat of adsorption by fitting the carbon monoxide isotherms to a non-dissociative Langmuir adsorption isotherm model. In addition, carbon dioxide chemisorption was also performed on some catalysts. To obtain accurate chemisorption uptakes for the fresh catalysts, large amounts, ca. 1 g of sample, were typically utilized. Thus, for reasons of accuracy, chemisorption was typically not performed on the used catalysts because of the small sample size and the fixed experimental uncertainty of 0.46 Ixmol/sample [33] (see, for example, used N i / S i O 2 in Table 2). X-ray diffraction (XRD), performed with a Scintag VAX 3100 system using filtered Cu K s radiation, was used to identify the bulk phases of the fresh and used catalysts. When possible, the nickel crystallite size was calculated using the N i ( l l l ) reflection and the Scherrer formula with Warren's correction for instrumental line broadening [34], i.e., d v = 0.9~k/[([~ 2 - B2)1/2cos 0], where X = 1.54059 ,~ and the line broadening at 44.5°20 is B = 0.09°20 [23]. Transmission electron microscopy (TEM), performed with a JEOL system with a high tension voltage of 200 kV [21], was used to investigate the surface morphology and crystallinity of the used catalysts. The point-to-point resolution of this instrument is reported at ca. 2-3 A, and in the case of the N i / T i O 2 catalyst, measurement of lattice fringes permitted identification of the possible surface phases. When possible, a particle size distribution was also obtained to calculate the number-weighted ( d n = Enidi/Eni) , volume-weighted ( d v = 2nid4/S, nid 3) and surface-weighted (d~ = S, nid3/~ni d2) particle sizes for comparison with values calculated from XRD and chemisorption measurements. Temperature programmed oxidation (TPO) was utilized to quantify the amount of carbonaceous deposits on the Ni/SiO2, N i / T i O 2 and N i / M g O catalysts after reaction. The temperature was ramped ( _+ standard deviation) at a rate of 10 _ 1 K / r a i n from 298 to 1073 K, while a 50% mixture of oxygen in helium was passed through the catalyst bed at an absolute pressure of 1 atm. Quantification of carbon deposition was possible by monitoring the rate of carbon dioxide evolution, which was measured in situ every two minutes with an on-line gas chromatograph. The assumption that all gasified carbon evolved primarily as carbon dioxide and not carbon monoxide is supported from (a) simple thermodynamic considerations, in which carbon dioxide is the more energetically favored product, and (b) kinetic considerations, in which any carbon monoxide formed is rapidly oxidized to carbon dioxide over the nickel surface. Similar TPO procedures have been reported in the literature [35-37]. TPO was not utilized to characterize the N i / C catalyst because, in the absence of isotopically labeled carbon, it was not possible to differentiate between oxidation of the support and oxidation of carbon deposits formed during reaction; however, temperature programmed desorption (TPD) was utilized to obtain desorption spectra of gases adsorbed on the surface after reaction was

M.C.J. Bradford, M.A. Vannice / Applied Catalysis A: General 142 (1996) 73-96

77

terminated. After cooling the catalyst overnight to room temperature in flowing helium, the temperature was ramped at a rate of 12 ___7 K / m i n from 298 to 1173 K in flowing helium, and gas evolution, only hydrogen and carbon monoxide in this case, was measured in situ every two minutes with an on-line gas chromatograph. Limitations of the apparatus did not permit for either rapid cooling of the catalyst or analysis of water desorption. In regards to the former comment, it must be acknowledged that cooling in helium may have resulted in desorption of weakly adsorbed species prior to the TPD experiment. A high temperature reactor system was constructed to test catalyst activity. Tylan FC-260 mass flow controllers were utilized for gas delivery to obtain accurate flow rate measurements and to allow in situ reduction and calcination procedures. Although gases of initially high purity were used - hydrogen (99.999%, Matheson), helium (99.999%, Matheson), methane (99.99%, Matheson), and carbon dioxide (99.99%, Matheson) - all gases were further purified by their passage through an indicating oxygen trap (Alltech, Model D4004) prior to reaction. This was to ensure that trace amounts of oxygen did not influence the kinetics through the oxidative coupling mechanism [38]. A high precision pressure transducer (Cooper Instruments, Model PTA 145) was placed immediately upstream of the catalyst bed to monitor the gas feed pressure to the reactor. The catalyst reactor was constructed of quartz, rather than pyrex or stainless steel, for stability and to eliminate catalysis of carbon monoxide disproportionation [15]. A stainless-steel sheathed thermocouple, inserted into the center of the catalyst bed to monitor and control the reaction temperature (Omega, Model CN2011) was electroplated with a 6 Ixm layer of gold to ensure catalytic inactivity of the thermocouple sheath. The quartz reactor cell was placed within an insulated high temperature furnace (National Element, Inc., Type FD303), which also served as a gas pre-heater, to maintain reaction isothermality. Blank reaction tests without catalyst were performed to show that the reactor and thermocouple itself were not catalytically active. On-line reaction gas analysis was performed with a Hewlett Packard 5890 Series II Gas Chromatograph using a 10' Porapak Q column, as Mindrup has shown that Porapak Q provides for adequate separation of H 2, CO, C H 4, C O 2, and H20 [39]. In addition, thermal response values for quantification of reaction rates and conversions were determined and compared with known literature values to ensure accuracy of measurement [40]. The gas flow rates, reactant partial pressures, and amount of catalyst (ca. 5 - 5 0 mg) used during these experiments were adjusted to maintain conversions away from thermodynamic equilibrium, and typically at less than 15%, so that operation was maintained in a differential, plug-flow, kinetically controlled regime. Calculation of the Weisz criterion (0.01-0.06) for each of the catalysts indicated that the kinetic results were free from mass transfer limitations. Catalyst particle sizes were 70-120 mesh sieve fraction. All activity tests were carried out under an absolute pressure of ca. 740 Torr, with a feed composition

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of C O 2 / C H 4 / H e = 1 / 1 / 1 . 8 and a total feed flow rate of 20 sccm (space velocity ca. 10000-20000 h - l ) over a temperature range of 723 to 823 K. Because most work reported in the literature has been performed at temperatures less than 873 K, operation was maintained beneath this ceiling to facilitate comparison with other investigations. 3. Results A summary of the weight loadings of catalysts determined by ICPS and NAA are compared with nominal values in Table 1. In general, the agreement between nominal and analyzed metal loadings is good. The sodium content of the N i / M g O catalyst determined by FES (Table 1) was fairly small, and thus should have had a negligible influence on catalyst activity. A summary of the chemisorption uptakes, metal dispersions, and particle sizes determined for each catalyst is provided in Table 2. Dispersions, i.e., Nisurf/Nitotal, calculated from either carbon monoxide uptake or hydrogen uptake indicated a low degree of nickel dispersion for all of the catalysts. The surface-weighted particle sizes were estimated from the metal dispersion using d s (nm) = 101/D(%) from Smith et al. [19]. Particle sizes estimated from TEM particle size distributions and XRD line broadening measurements are also provided in Table 2 for comparison. In general, the surface-weighted d S values from TEM are in reasonable agreement with the values determined from carbon monoxide uptake, although the former are larger than the latter for N i / S i O 2. This may be a result of the use of bright-field (BF) imaging techniques. Yacamfin and Dom~nguez have shown that in BF imaging small particles can be out of contrast, thereby skewing particle size distributions to higher values [41]. At 300 K, hydrogen chemisorption was suppressed relative to that of carbon monoxide on the N i / T i O 2 catalyst. This type of SMSI behavior is expected on TiOz-supported catalysts after reduction at 773 K due to migration of TiO~ species onto the nickel surface [19,27]. The carbon monoxide adsorption isotherms used to determine the uptakes in Table 2 were fit to a non-dissociative Langmuir adsorption model, and from the equilibrium adsorption constants obtained it was possible to calculate the heat of adsorption for carbon monoxide.

Table 1 Nickel loadings and sodium content of studied nickel catalysts Catalyst

Ni content (wt.%) Nominal

Ni/TiO 2

1.3

Ni/SiO 2

8.0

Ni/MgO

-

Ni/C

8.2

Na content ICPS 1.22 _+ 0 . 0 6

NAA 6.8 _+ 0.2

(wppm) -

10.1 _+ 0.5

-

30

6.3_+0.3

-

-

M. C.J. Bradford, M.A. Vannice / Applied Catalysis A: General 142 (1996) 73-96

,.2 II

0

II

? N

E E e-

II

l.

© 9 .o

.o .£ 'd'd

.~_, ~ ~

~

~.~ ~ e-

<2

~

e'~

~+1

,,~, . ~

79

80

M.C.J. Bradford, M.A. Vannice /Applied Catalysis A: General 142 (1996) 73-96

The calculated value, roughly 2 kcal/mol, is indicative of weak chemisorption and is well below the values of 10 kcal/mol for carbon monoxide on TiO 2 [42] and 30 kcal/mol for carbon monoxide on nickel reported in the literature [43]. Carbon dioxide chemisorption was also undertaken on the N i / T i O 2 catalyst, and from these adsorption isotherms, the heat of adsorption of carbon dioxide was estimated to be on the order of 1 kcal/mol, which was also indicative of weak adsorption. The entropies of adsorption obtained from these adsorption constants are acceptable within suggested thermodynamic guidelines [44,45]. For both the fresh and used N i / S i O 2 catalysts, the ratio of carbon monoxide uptake to hydrogen uptake at 300 K was slightly less than two, which is expected for non-dissociative carbon monoxide adsorption versus dissociative hydrogen adsorption. The possible reason for the obtained ratios of less than two may be due to carbon monoxide adsorption on both linear and bridged sites. HREELS investigations of carbon monoxide adsorbed on various single crystal surfaces of nickel have shown clearly that carbon monoxide adsorbs on both types of sites, even near saturation coverage [46-48]. In addition, both types of adsorption sites were detected by DRIFTS during in situ reaction studies [49]. The decrease in gas uptake observed on the used, relative to the fresh, catalyst suggested that there was a large degree of site removal induced during the reaction, and comparison of nickel particle sizes (Table 2) indicates that this was due to particle sintering. This result is consistent with particle size distributions obtained by TEM. Suppression of hydrogen chemisorption, relative to that of carbon monoxide, was also observed for the N i / M g O catalyst, but the reasons for this are not entirely clear at this time. This may possibly be related to a geometric or electronic influence imposed on the nickel by the MgO support, such as solid solution formation. Suppression of hydrogen chemisorption, relative to that of carbon monoxide, was also observed for the N i / C catalyst. This type of adsorption behavior has also been reported for P d / C [50], and was attributed to the formation of interstitial carbon during reduction. In general, irreversible hydrogen uptakes on all catalysts indicated that reduced nickel surfaces were present. XRD spectra were obtained for both pure Degussa P-25 TiO 2 and a number of N i / T i O 2 catalyst samples (Fig. 1A). The bulk weight percent composition ( _+ standard deviation) of the TiO2, 32 _+ 3% rutile and 68 _+ 3% anatase, which was calculated using integral intensities of the rutile(ll0), I r, and the anatase(101), Ia, reflections using the formula [51]:

t°r = 1 + ( k r l a / k a l r )

(1)

where k r = 3.40 and k, = 4.30 [51], remained fairly unchanged for the fresh, reduced, and used catalysts. The calculated TiO 2 composition is in good

M.C.J. Bradford, M.A. Vannice / Applied Catalysis A: General 142 (1996) 73-96 6-

Si(l | 1)

Si(220)

Si(31I)

Ni(|l 1)

..~4-,. 3-

a

1~

20

30

Si(~31)

MgO(3ll) Mg0(222)

.

Ni(~lI 1)

[ A(101) -~,~..A R(Jl0)

.

Mg~220,

Mg]200)

1--,:(ooa),

|

~

Mg~lll)

•~ 2 ~ -

81

~

~ . (C)

Ni(200)

. . . . . . . . . . . . . . . . . . . . . . . . . . . A(200) R(2~l)A(211) 40

50

60

70

80

°20 Fig. 1. XRD spectra of (A) the Ni/TiO 2 catalyst after reduction at 773 K, (B) the Ni/SiO 2 catalyst after 5.5 h on stream, (D) the Ni/C catalyst after 6 h on stream, and (C) the Ni/MgO catalyst after reduction at 1123 K.

agreement with the composition reported by Singh et al. [52], and slightly different than that reported by Busca et al. [53]. The absence of nickel diffraction peaks in the XRD spectra of the N i / T i O 2 catalyst is probably due to poorer XRD resolution at relatively low nickel loadings (Table 1). XRD spectra of the used N i / S i O 2 samples (Fig. 1B) showed distinct Ni(111) and Ni(200) reflections, and a fairly broad peak at 26°20, due to either the graphite(002) or the SiO2(101) reflection. The low signal to noise ratio in these spectra, possibly due to a combination of the low degree of crystallinity of the SiO 2 and the low nickel loading of the catalyst, may be the reason for the poor agreement between the volume-weighted particle sizes calculated from the particle size distribution and the Ni(ll 1) reflection (Table 2). o The bulk lattice constant for the N i / M g O catalyst, a = 4.2085 + 0.0005 A, was calculated from XRD spectra (Fig. 1C) by extrapolating to zero a linear plot of the lattice constant for each reflection versus cos20/sin 0. This procedure corrects for displacement of the specimen from the diffractometer axis, typically the largest source of error in XRD measurements [34]. The data reported by Kale [54] permit an estimation of the weight percent of nickel in the form of a N i O - M g O solid solution from measurement of this lattice poarameter; however, the lattice parameter for pure MgO reported by Kale, 4.210 A, differs from that reported in the JCPDS, 4.213 A. Thus, some uncertainty is associated with conclusions drawn from calculations with his data. Nonetheless, the calculation result of 10.0 ___1.8 wt.% nickel is in good agreement with the total nickel loading of the catalyst (Table 1), indicating that most of the nickel is in the form of a N i O - M g O solid solution. This should not be surprising considering that nickel and magnesium almost perfectly fit the Hume-Rothery criteria [55] for formation of an extensive solid solution, i.e., both cations have similar ionic

82

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Fig. 2. TEM micrograph of a used Ni/TiO 2 catalyst illustrating lattice fringes.

radii, ca. 0.78 A [55], the same common oxidation state (2 + ), and the same bulk oxide structure, NaCl-type [56]. If a majority of the nickel in the N i / M g O catalyst is in the bulk and not dispersed on the MgO surface, then the surface-weighted particle size as determined by carbon monoxide chemisorption (Table 2) should be considered as an overestimate of the true particle size. XRD spectra of the N i / C catalyst (Fig. 1D) diluted with ground silicon ( < 325 mesh) clearly show the Ni(111), Ni(200) and graphite (002) reflections. The latter carbon reflection is consistent with results that the activated carbon support is partially graphitic [21]. TEM results with a used N i / T i O 2 catalyst point towards evidence of a strong metal-support interaction. The catalyst morphology consisted of very thin platelets, typically with well defined facets (Fig. 2). Due to the exceptional point-to-point resolution of the instrument [21], it was also possible in many micrographs to measure some crystal d spacings (Table 3), some of which were in very good agreement with previously reported values for T i 4 0 7 determined by electron diffraction [57]. A thorough search of the JCPDS database resulted in direct identification of three possible phases - T i 4 0 7 , T i 3 0 5 , and N i s T i O 7 -

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83

Table 3 TEM measured lattice spacings for the used Ni/TiO 2 catalyst compared with possible literature identifications Measured d (A) c

dhkl a and Dhd, d,,h2kzt2 b (~) Ti407 (18-1402)

3.04 _+0.06 3.11 _+0.07 3.30 -+ 0.07 4.87 _+0.10 6.38_+0.13 7.09 _+0.15 8.38_+0.18 9.48 _+0.20 17.69 _+0.37

3.020132 3.110o23 3.350o20

Ti305 (11-217)

NisTiO 7 (31-917)

TiO 2 (21-1276)

3.04203 3.146003 3.30711 l, 3.329202 4.72oo 2

6.170oo 2 7.09ff2,Joo

5.06 Jo2 6.06002 7.25]0t

6.36200.21 ] 8.3722o.3ol

9.46 ffo.Joo 17.7602 L ~02

9.42OOl 17.473o2,2o i

a The number in parentheses refers to the compound location in the JCPDS database. b Dh,k,i,.h2k2t2 =dl d 2 / ( d l _ d2 ) is the moir6 fringe spacing resulting from parallel planes with different d spacings, d~ and d 2, where hikil i are the Miller indices for a plane with spacing dg. Measurement uncertainty ( + 2.1%) determined by calibration with graphite whiskers.

which imply the formation of TiO x species during reduction a n d / o r reaction and implicate the miscibility of nickel and TiO x phases. Chung et al. have shown previously that the interdiffusion of nickel and TiO X can occur extensively above 573 K [58-60]. Although none of the measured lattice fringes is a d spacings of either metallic nickel or rutile or anatase, some of the larger fringe spacings are consistent with calculated moir6 fringes for rutile. There were also some indications of encapsulation of nickel particles, but it was not clear whether it was due to TiOx, carbon deposits formed during reaction, or an oxide layer formed during air exposure. TEM micrographs of the N i / S i O 2 catalyst (Fig. 3) provided clear evidence of whisker carbon formation during the methane reforming reaction. From the TEM micrographs it was possible to determine a particle size distribution (Fig. 4), from which surface- and volume-weighted particle sizes were calculated (Table 2). Although the number-averaged particle size is in reasonable agreement with the XRD line broadening measurement (Table 2), the calculated volume-weighted particle size does not agree well. Comparison with the particle size distribution of the fresh catalyst indicated that in addition to coking, substantial nickel particle sintering had also occurred during reaction. The concomitant occurrence of these two phenomena is consistent with the comments of Bhattacharyya and Chang [61]. TEM analysis of the used N i / M g O catalyst confirmed the presence of small nickel particles supported on the surface of the MgO (Fig. 5), and allowed for an estimate of the nickel particle size distribution (Fig. 6), from which the surfaceand volume-weighted particle sizes were calculated (Table 2). Although the d s value for the used catalyst is slightly less than that calculated from the carbon monoxide chemisorption uptake, this result is still consistent. As mentioned

84

M.CJ. Bradford, M.A. Vannice / Applied Catalysis A: General 142 (1996) 73-96

Fig. 3. TEM micrograph of a used N i / S i O 2 catalyst illustrating well-faceted nickel particles and filamentous carbon whiskers.

previously, d~ obtained from chemisorption is an upper limit to the particle size due to extensive formation of a N i O - M g O solid solution. The TPO results obtained for the N i / S i O 2, N i / T i O 2, and N i / M g O catalysts are presented in Fig. 7. The low temperature peaks observed near 300 K in each spectrum may be due to the presence of either an extremely reactive carbonaceous deposit or to the oxidation of chemisorbed carbon monoxide present on the catalyst surface after termination of reaction. The TPO spectra for N i / S i O 2 showed one extremely sharp peak at 806 K, which is attributed to the combustion of graphitic carbon. This conclusion is based upon TEM results which revealed that the carbon was deposited in the form of graphitic whisker fibers. Swaan et al. performed TPO on a N i / S i O 2 catalyst after 6 h on stream and reported a strong carbon dioxide peak at approximately 940 K [35]. The difference in peak temperatures may be due to a combination of the faster heating rate of 20 K / m i n and the lower oxygen partial pressure ( O 2 / H e = 3 / 7 ) used by Swaan et al. [35]. Two carbon dioxide peaks at 643 and 770 K were apparent in the N i / T i O 2 TPO spectra. Comparison with the N i / S i O 2 TPO

M.C.J. Bradford, M.A. Vannice /Applied Catalysis A: General 142 (1996) 73-96 150

85

A

100 # of Particles 50

Particle Size (nm)

oo]

B

804 | # of Particles

I

60 ¸ 40 20

0 ~ e q t ~ eq 1~ e q t ~ e q t ~ eq t ~ eq t ~ e q I,~ eq

o o o o ~ o o

~

eqeq

Particle Size (nm) Fig. 4. Particle size ( + 2.5 nm) distributions obtained for the Ni/SiO 2 catalyst (A) after reduction at 773 K (298 particles) and (B) after reaction at 723 K (428 particles).

spectra suggests that the latter peak at 770 K for N i / T i O 2 may be due to deposit of a graphitic carbon, the formation of which may have been the cause of the observed deactivation of the N i / T i O 2 catalyst (Fig. 9). Two weak peaks were also observed on the N i / M g O catalyst, although at lower temperatures than either the N i / S i O 2 or the N i / T i O 2 catalyst, which suggests that the surface carbon was more reactive on the N i / M g O catalyst. The amount of carbon deposited during reaction was quantified via integration of the TPO spectra in Fig. 7, and the mean rate of carbon deposition during reaction was then estimated from the total amount of carbon deposited normalized to the catalyst time on stream (Table 4). Clearly, the amount of carbon deposited on the N i / S i O 2 catalyst was orders of magnitude greater than on the other catalysts, indicating that TiO 2 and MgO interact with nickel particles to suppress coke formation.

86

M.C.J. Bradford, M.A. Vannice /Applied Catalysis A: General 142 (1996) 73-96

Fig. 5. TEM micrograph of a used N i / M g O catalyst illustrating supported nickel particles.

During the TPD with the N i / C catalyst, only hydrogen and carbon monoxide desorption were detected. Hydrogen evolution, measured from ca. 850-1150 K, was erratic and showed no signs of typical desorption behavior. This suggested 50-

40-

30# of Particles 201

100 ~

1

3

5

7

9

II

13

15

17

19

21

23

25

d (nm) Fig. 6. The estimated particle size ( + 1 nm) distribution obtained for the N i / M g O catalyst from TEM measurement of 155 particles.

M. C.J. Bradford, M.A. Vannice / Applied Catalysis A: General 142 (1996) 73-96

87

0.07" N i / S i O 2 ( x l 0 -2 )

Xa

0.06" 0.05' rco2

0.04

(I-tmol/s)

0.03

Ni / TiO2

0.02 Ni / MgO

/

~

0.01 0.00 200

400

600

800

1000

T (K) Fig. 7. TPO spectra for Ni/TiO 2, N i / S i O 2 and Ni/MgO. Reaction conditions: 0 2 / H e = 1/ 1, P = 740 Ton., Heating rate = 10_+ 1 K/min.

Table 4 Activity of supported nickel catalysts for CH 4-CO 2 reforming at 723 K (reaction conditions: CO 2 :CH 4 :He =1:1:1.8, P = 7 4 0 T o r r ) Catalyst

1.2%Ni/TiO 2 6.3%Ni/C 6.8%Ni/SiO 2 10.1%Ni/MgO d a u c d

CH 4 conv.

Initial activity for CO at 723 K a

Carbon deposition rate

(%)

Ixmol/s gcat

~mol/s gNi b

C/Nisurf h

3.2 2.8 2.9 1.6

30.2 34.2 66.9 3.6

2475 543 984 36

TOF (s- I) c 1.73 0.58 0.36 0.04

1 184 0.04

Initial activity is defined as activity at 30 min time on stream. Calculated using the nickel content determined by ICPS and NAA (Table 1). Calculated using the irreversible carbon monoxide uptake at 300 K (Table 2). Calculated using the activation energy for carbon monoxide formation, Eco = 21 kcal/mol [49].

'21 1.0 0.8 0.6" CO 0.4 0.2 0.0 3OO

500

700

900

1 loo

T(K) Fig. 8. Carbon monoxide TPD spectra for the N i / C catalyst. Reaction conditions: PHe ~ 740 Ton-, heating rate = 12_+7 K/min.

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TOFco

(s-1)

0

10 20 30 40 Time on Stream (h)

50

Fig. 9. Catalyst activity of ( • ) N i / M g O at 823 K and (C)) Ni/TiO2, ( 0 ) N i / S i O 2, and (A) N i / C at 723 K. Reaction conditions: CO 2 / C H 4 / H e = 1 / 1 / 1 . 8 , P = 740 Torr.

that the hydrogen was not adsorbed on the nickel surfaces, but was being removed from the carbon. In contrast, the carbon monoxide desorption spectra showed a strong desorption peak at 999 K (Fig. 8), suggestive of dissociative adsorption on the nickel surface. The small shoulder at roughly 1070 K may be indicative of a second adsorption state or just a slight experimental artifact. Analysis using the methodology put forth by Gorte [62] for desorption from porous catalysts revealed that readsorption and concentration gradients were significant, and thus the carbon monoxide binding energy on N i / C could not be determined from the TPD spectrum. Activity maintenance was examined with the N i / M g O catalyst at 823 K and with the N i / T i O 2, N i / S i O 2 and N i / C catalysts at 723 K (Fig. 9). The N i / T i O 2 catalyst, which was initially more active than all other tested catalysts on both a per gram of nickel and turnover frequency (TOF) basis (Table 4), exhibited gradual deactivation. The cause of this deactivation process, though not established, may be either site blockage by deposited surface carbon, additional site blockage induced by migrating TiO x, or particle sintering. Initial deactivation of the N i / S i O 2 and N i / C catalysts is presumably due to carbon deposition a n d / o r nickel sintering. The N i / C catalyst had slightly higher initial activity than the N i / S i O 2 catalyst on a TOF basis. Comparatively, though least active of all tested catalysts, the N i / M g O catalyst remained stable without detectable deactivation for up to 44 h on stream.

4. Discussion of results

All of the catalysts in this investigation except N i / M g O exhibited some deactivation. Correspondingly, Takayasu et al. have already shown that at 1073

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K, N i / S i O 2 deactivates quickly due to carbon formation [20]. In addition, Swaan et al. have measured a relative linear deactivation rate of 0 . 7 % / h at 973 K for a N i / S i O 2 catalyst [35]. In contrast to our result for N i / M g O , Swaan et al. reported very low activity at 823 K for a 5 % N i / M g O catalyst and a linear deactivation rate of 1 . 3 % / h [35]. Takayasu also reported severe deactivation for a N i / M g O catalyst at 1073 K [20]. However, Fujimoto and coworkers have also obtained excellent activity maintenance for a N i / M g O catalyst [63]. Many authors have presented calculations which predict the thermodynamic potential of graphitic carbon deposition as a function of operating conditions, such as Reitmeier et al. [64], White et al. [65] and Gadalla and Bower [1]. Conclusions drawn from these calculations typically suggest operation at high C O 2 / C H 4 ratios above 1 and high temperatures, ca. 1000 K to avoid regions where there is a thermodynamic potential for carbon formation; however, from an industrial standpoint, it may be desirable to operate at lower temperatures with C O 2 / C H 4 ratios near unity. This requires a catalyst which kinetically inhibits carbon formation under conditions which are thermodynamically favorable for deposition. The origin of inactive carbon during methane reforming may be from either carbon monoxide disproportionation (Eq. (2)) or methane decomposition (Eq. (3)). 2CO + *

~

C O 2 -4- C *

(2)

CH 4 + * ~

C* + 2H 2

(3)

Thermodynamically, carbon monoxide disproportionation is exothermic and the equilibrium constant decreases with increasing temperature; conversely, methane decomposition is endothermic and the equilibrium constant increases with increasing temperature. The calculations of Reitmeier et al. [64] illustrate, for any reaction mixture of H 2, CO, H 2 0 , C O 2 and C H 4 at thermodynamic equilibrium, that the extent of graphitic carbon deposition during reforming decreases at higher reaction temperatures, in agreement with experimental observations reported in the literature [5,66]. This result suggests that the main contributor to carbon deposition during reaction is carbon monoxide disproportionation. There is other evidence that carbon monoxide disproportionation is primarily responsible for the formation of inactive carbon deposits during methane reforming. The carbon formed during reaction is often in the form of filamentous whiskers [67], and Rodriguez, in an extensive review of the literature about carbon nanofiber growth, reports that the rate determining step for the formation of filamentous whisker carbon is the diffusion of carbon through a metal particle [68]. The driving force for this diffusion process is considered to be heat generated by an exothermic surface process, such as carbon monoxide disproportionation. In addition, Tsipouriari et al. [36] and Swaan et al. [35] have independently used isotope labeling and TPO to reveal that carbon deposits formed during C H 4 - C O 2 reforming originate from both

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C H 4 and CO 2, suggesting that C H 4 decomposition is not the dominant carbon

formation mechanism. Tavares et al. [69] reported interesting microscopy results of carbon deposits formed from C O / C O 2 and CH4//H2 gas mixtures, although they did not provide the data. In the former case, the metal particles were rough and the carbon deposit was of an encapsulating type; in the latter case, metal particles were well faceted and carbon deposits were ordered and filamentous. Although their study suggests that carbon monoxide disproportionation may not significantly contribute to filamentous carbon deposition during CO2-CH 4 reforming, the analogy may not be valid for reaction mixtures of all four gases. The results of Tavares et al. [69] are consistent with reports of carbon morphology on nickel-containing catalysts formed via methane decomposition [70]. It has been suggested that carbon atoms, formed by methane decomposition on the (100) and (110) nickel surfaces, diffuse across the nickel particles and deposit on the (111) surfaces, forming ordered graphite layers aligned parallel to the metalcarbon interface [70]. The driving force for carbon diffusion in this process is assumed to be a carbon concentration gradient. There is strong evidence for a support influence on resistivity to catalyst coking on nickel via stabilization of different CH x surface intermediates. Matsumoto observed different CH x species during the adsorption of hydrocarbons on different nickel catalysts, i.e., CH0.08 on nickel foil, CH0. 5 on N i / S i O 2, and CH 2 on N i / M g O , and he verified that CH x intermediates with lower values of x were more susceptible to form carbonaceous deposits [71-73]. In agreement with this, Erkelens and Wosten used low-field magnetic and infrared measurements to show that on a N i / S i O 2 catalyst at 298 K between 4 and 5 surface bonds are formed per molecule of methane chemisorbed [74]. More recently, Osaki et al. used pulse surface reaction rate analysis during a study of C H 4 - C O 2 reforming and found that the support has a strong influence on the stabilization of different CHx intermediates, giving values of CH 2.7 for N i / M g O , CH2. 5 for N i / Z n O , CH2. 4 for Ni/A1203, CHI. 9 for Ni/TiO2, and CH1. o for N i / S i O 2 [75]. These results are consistent with the order of carbon formation resistivity for the catalysts used in this investigation, i.e., N i / M g O > N i / T i O 2 > N i / S i O 2 (Table 4). The stabilization of different CH x intermediates may be directly related to hydrogen spillover because it is possible that increased surface concentrations of hydrogen, spilled over onto the oxide support, can inhibit carbon deposition. Takayasu et al. showed that the stability of nickel catalysts at 1073 K could be greatly increased by physical mixing with either MgO or A1203 [20], and Inui has also provided evidence of coking suppression due to hydrogen spillover [76]. Nakamura et al. showed that the activity of R h / S i O 2 could be greatly increased by physical mixing with either MgO, TiO 2, or A1203, although they did not attribute their results to hydrogen spillover [9]. In addition, many investigations have shown that the extent of coking is minimal on TiO zand MgO-supported metals [28,36,77,78]. The preceding analysis is consistent

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with the report that coking occurs less readily on a catalyst with a basic support such as MgO [29,76]. It has thus been suggested that on a basic support, carbon dioxide surface concentrations are larger and carbon dioxide affinity for surface carbon through Eq. (2) minimizes carbon accumulation [17]. In summary, there is substantial evidence in the literature to suggest that both carbon monoxide disproportionation and methane decomposition contribute to the formation of inactive carbon deposits during C H 4 - C O 2 reforming. The TPO results provide evidence that the choice of support greatly influences catalyst coking susceptibility. Although the relative catalyst resistivity to coking, N i / M g O > N i / T i O 2 >> N i / S i O 2 may be directly correlated with the support ability to stabilize CH x decomposition, carbon monoxide disproportionation is also a likely mechanism for carbon deposition during C H 4 - C O 2 reforming. Thus, an additional question to address is how each support influences the mechanism of carbon monoxide disproportionation. From a compilation of surface science studies, it is possible to speculate on carbon monoxide disproportionation and subsequent carbon fiber growth on nickel surfaces. Numerous studies suggest that carbon monoxide dissociation on transition metal surfaces is initiated via adsorption at multiply coordinated sites [79-81]. The carbon monoxide heat of adsorption on these sites should be greater than that for an atop site. Adsorbed carbon monoxide then proceeds through a bent transition state essentially parallel to the surface prior to dissociation [82]. The XPS results of Zdansky et al. show that subsequent carbon monoxide adsorption on a C / N i surface induces migration of carbon to subsurface nickel layers [83]. LEED work reported by Somorjai has shown that these adsorbed carbon atoms induce a local reconstructing of the nickel surface, lengthening nearby Ni-Ni bonds and permitting a deeper carbon penetration into the nickel lattice [84]. At this point, carbon diffusion through the nickel lattice may occur until the carbon atoms deposit in eventual graphitic layers on the back-side of the nickel particle. Energetically, this process is thus dependent upon the carbon monoxide heat of adsorption, Qad, the activation barrier for carbon monoxide dissociation, Edit, and the activation barrier for carbon diffusion, EdifWith this mechanism in mind, one can elucidate the particular mechanistic influence which each support plays during carbon deposition and whisker growth. It is suggested that TiO x migrating onto the nickel surface geometrically blocks primarily high coordination adsorption sites, thus inhibiting the mechanism at its initial step. This is supported by the TEM identification of TiO x phases, suppressed hydrogen chemisorption on the N i / T i O 2 catalyst, and the extremely low Qad value of 2 k c a l / m o l obtained from carbon monoxide adsorption isotherms, which suggests that carbon monoxide is weakly bound. This site blockage scenario is analogous to the effects of sulfur passivation of nickel catalysts during C H a - H 2 0 reforming [85], and the influence of c(2 × 2)S overlayers on Ni(100) during carbon monoxide adsorption [81].

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On the N i / S i O 2 catalyst there appeared to be no evidence of an interaction between nickel and SiO2, and extensive coking was observed. Geometrically, the presence of high coordination carbon monoxide adsorption sites on these nickel surfaces permit extensive carbon monoxide adsorption. The heat of adsorption of carbon monoxide on N i / S i O 2 has been suggested by Tavares et al. to be roughly 26 k c a l / m o l [69]. In addition, Edis can be estimated from the EHMO calculation results of Yin-Sheng and Xiao-Le to be about 6.6 kcal/mol on N i / S i O 2, which is significantly lower than that of 24.7 kcal/mol on N i / T i O 2 [82]. Energetically, a combination of higher carbon monoxide binding energy and a lower barrier for carbon monoxide dissociation makes N i / S i O z more susceptible to extensive carbon formation than N i / T i O 2. Thus, the large differences between the coking resistivity of TiO 2- and SiO2-supported nickel catalysts can be explained in terms of a geometric effect in the N i / T i O 2 catalyst. The possible reasons for the coking resistance of the N i / M g O catalyst are more speculative. The low reducibility of the N i / M g O catalyst, as determined through XRD and chemisorption measurements, is evidence of increased N i - O bond strength as a result of solid solution formation. It is suggested that formation of a partially reducible N i O - M g O solid solution also directly increases the stability of Ni-Ni bonds on the reduced surfaces. This greater surface stability prohibits nickel surface reconstruction via bond relaxation, inhibits carbon diffusion into the nickel lattice, and thus prevents the formation of carbon whiskers. This proposal is analogous to results reported in the literature for Ni/A120 3 catalysts. The difference in activation energies for the reduction of NiO (4.3 kcal/mol) and NiA1204 (32 kcal/mol) is indicative of a relative strengthening of the N i - O bond in NiAI204 [26]. Chen and Ren studied C H 4 - C O 2 reforming on Ni/AI203 and have convincingly shown that carbon deposition is markedly suppressed if NiAlzO 4 is formed during the pretreatment procedure [86]. In addition, Blom et al. improved the stability of a Ni/A1203 catalyst by modification with La203 [87]. X-ray diffraction results provided evidence of bulk LaA103 and LaAl12019 after pretreatment, and temperature programmed reduction provided evidence that a partially reducible LaNiA111019 phase also had formed. Alternatively, the coking resistivity of MgO-supported catalysts has also been described in terms of high catalyst basicity, evidenced by the extremely high heat of adsorption of 58 kcal/mol for CO 2 on MgO [88]. However, the observation of significant carbon deposition on a 1% N i / M g O catalyst at temperatures near 850 K by Rostrup-Nielsen and Bak-Hansen [67] suggests that MgO basicity did not play a major role in carbon inhibition during their study. Osaki et al. have reported that the activity of a N i / T i O 2 catalyst at 723 K is higher than that of either N i / M g O or N i / S i O 2 [75], in agreement with the results obtained here. Although, in contrast, Swaan et al. reported that a N i / T i O 2 catalyst completely deactivated at 673 K within one hour on stream

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[35], this is most likely due to the harsh 10 h reduction at 1023 K pretreatment used by Swaan et al. to activate the catalyst. Takatani and Chung have shown with N i / T i O 2 catalysts that TiO x coverage increases and carbon monoxide uptake decreases with increasing reduction time [59]. This suggests that excessive reduction time may block entirely any reduced nickel sites responsible for catalyst activity. Although the activity of N i / S i O 2 (per gram of catalyst) is similar to that reported by Osaki et al. [89], the specific turnover number reported by Osaki at 723 K is higher than that reported here, possibly an artifact due to the extremely low dispersion, ca. 1.2%, of their catalyst. In addition, the activity of 1 7 % N i / L a 2 0 3 at 723 K, 1000 _+ 220 txmol C O / s gNi, as estimated from the data of Zhang and Verykios [90] using an activation energy of 14 _+ 3 kcal/mol, is comparable to the activity of the nickel catalysts in this investigation. The observed order of initial activity for reforming on a TOF basis is N i / T i O 2 > N i / S i O 2 > N i / M g O (Table 4). Although this may just be related to the available nickel surface area in each catalyst, another possible explanation for these differences may be obtained from a brief discussion about methane activation. Dissociative adsorption of methane primarily requires substantial deformation away from its tetrahedral geometry [91], followed by C - H bond cleavage and surface bond formation. Activation of the C - H bond cleavage requires electron donation from the surface to any of the lowest unoccupied antibonding molecular orbitals in methane. Experimentally, Imelik and Vedrine have reported that the order of binding energy of Nizp,/2 electrons is 854.5 + 0.1 eV for Ni/TiO2, 856.3 _+ 0.1 eV for Ni/SiO2, and 856.7 +_ 0.1 eV for N i / M g O [92]. They also report that introduction of SMSI into TiOz-supported metal catalysts can be correlated with an increase in electron density in the metal crystallites [92]. These two findings indicate that, from an electronic point of view, the ability to activate C - H bond cleavage in methane via electron donation to anti-bonding orbitals should be in the order N i / T i O 2 > N i / S i O 2 > N i / M g O , which is the observed order of measured activities of the three catalysts (Table 4). Although support-induced electronic effects should be negligible for large (ca. 10-40 nm) metallic particles, electronic effects may become significant when the nickel is spread in thin rafts on the support surface, as is the case for N i / T i O 2, or when the nickel interacts with the support to form a solid solution, as is the case for N i / M g O .

5. Conclusions

The role of the support regarding carbon deposition and activity for C H 4 - C O 2 reforming has been investigated for nickel catalysts. A lack of metal-support interaction in N i / S i O 2 permitted substantial formation of filamentous whisker carbon. However, SMSI produced a drastic reduction of carbon formation with

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N i / T i O 2, which was attributed to site blockage by TiO x and a large activation barrier for carbon monoxide dissociation. Formation of a partially reducible NiO-MgO solid solution appeared to stabilize surface Ni-Ni bonds and prevent carbon diffusion into the nickel particles. In addition, it is suggested that the support may also influence the relative catalyst activity by altering the electrondonating ability of the reduced nickel surfaces.

Acknowledgements The authors would like to thank a Japanese international joint research program, NEDO, for sponsoring this study, the US Department of Education for providing the GAANN fellowship, Professor K. Fujimoto and his research group for providing the N i / M g O catalyst used in this investigation, and Professor R.T.K. Baker and Professor N.M. Rodriguez for TEM analyses and helpful discussions.

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