Ind. Eng. Chem. Res. 1988,27, 790-795
790
TOA= feed temperature for reactor A, K T , = coolant temperature, K U = global heat-transfer coefficient, cal/ (m2.K.s) u = reaction mixture velocity, m/s Xn= number-average degree of polymerization 8,= weight-averagedegree of polymerization z = axial distance, m 2, = relative axial distance, z, = z / L A Greek Symbols
6 = reaction order for oxygen initiation
AH
= reaction enthalpy, cal/mol
p = reaction mixture density, pc = coolant density, kg/m3
kg/m3
X
= weight-average branch point number (per molecular weight) A, = bivariate moment of orders m and n of the radical chain length distribution R i ( x ) , mol/L pmn = bivariate moment of orders m and n of the dead polymer chain length distribution Pi(x),mol/L Registry No. Ethylene, 74-85-1; polyethylene, 9002-88-4.
Literature Cited Agrawal, S.; Han, C. D. AZChE J . 1975, 21, 449. Benedict, M.; Webb, G. B.; Rubin, L. C. J . Chem. Phys. 1940,4334. Brown, K. M.; Dennis, J. E. Numerische Mathematik 1972, 18, 289. Buback, M. Macromol. Chem. 1980, 181, 373. Chen, C. H.; Vermeychuk, J. G.; Howell, S. A.; Ehrlich, P. AZChE J . 1976, 22, 463. Donati, L.; Marini, M.; Marziano, G.; Mazzaferri, C.; Spampinato, M.; Langiani, E. In Chemical Reaction Engineering; Wei, J., Georgakis, C., Eds.; ACS Symposium Series 196; American Chemical Society: Washington, D.C., 1982; pp 579-590. Ehrlich, P.; Mortimer, G. A. Adu. Polym. Sci. 1970, 1 , 386.
Foster, G. N.; Hamielec, A. E.; MacRury, T. B. In Size Exclusion Chromatography (GPC);Provder, T., Ed.; ACS Symposium Series 138; American Chemical Society: Washington, D.C., 1980; pp 131-148. Gear, C. W. Numerical Initial Value Problems in Ordinary Differential Equations; Prentice Hall: Englewood Cliffs, NJ, 1971. Goldman, K. In Ethylene and its Industrial Deriuatiues; Miller, S . A., Ed.; Ernest Benn Limited: London, 1969; pp 150-167. Goto, S.; Yamamoto, K.; Furui, S.; Sugimoto, M. J . Appl. Polym. Sci., Appl. Polym. Symp. 1981, 36, 21. Hulburt, H. M.; Katz, S. Chem. Eng. Sci. 1964, 19, 55. Katz, S.; Saidel, G. M. AIChE J . 1967, 13, 319. Laird, R. K.; Morrell, A. G.; Seed, L. Discuss. Faraday SOC.1956,22, 126. Lee, K. H.; Marano, J. P., Jr. In Polymerization Reactors and Processes; Henderson, J. N., Bouton, T. C., Eds.; ACS Symposium Series 104; American Chemical Society: Washington, D.C., 1979; pp 221-252. Luft, G.; Lim, P.; Yokawa, M. Makromol. Chem. 1983, 184, 207. Michels, A.; Geldermans, M. Physica 1942, 9, 967. Michels, A.; Geldermans, M.; De Groot, S. R. Physica 1946,12, 105. Ogo, Y. J . Macromol. Sci.-Rev. Macromol. Chem. 1984, C24, 1. Powell, M. J. D. Math. Programming 1977, 12, 241. Ray, W. H.; Laurence, R. L. In Chemical Reactor Theory; Lapidus, L., Amundson, N., Eds.; Prentice Hall: Englewood Cliffs, NJ, 1977; p 532. Saidel, G. M.; Katz, S. J . Polym. Sei. 1968, 6 , 1149. Shirodkar, P. P.; Tsien, G. 0. Chem. Eng. Sci. 1986, 41, 1031. Takahashi, T.; Ehrlich, P. Macromolecules 1982, 15, 714. Tatsukami, Y.; Takahashi, T.; Yoshioka, H. Makromol. Chem. 1980, 181, 1107. Woodbrey, J. C.; Ehrlich, P. J . Am. Chem. SOC.1963, 85, 1580. Yamamoto, K.; Sugimoto, M. J . Macromol. Sci. (Chem.) 1979, A13, 1067. Receiued for review March 5, 1987 Accepted November 3, 1987
The Activity and Stability of Ni/Si02 Catalysts in Water and Methane Reaction Abdurahman S. Al-Ubaid Chemical Engineering Department, T h e College of Engineering, King Saud University, P.O. Box 800, Riyadh 11421, Saudi Arabia
The activity and stability of Ni/Si02 catalysts were investigated for steam reforming of methane at 565 "C. Catalysts were prepared by the homogeneous-precipitation-deposition method, and the influence of the following parameters was investigated nickel loading, steaming, pretreatment, and steam-methane ratio during the reaction. The activity and characterization results show t h e formation of two types of nickel hydrosilicate on the surface of prepared Ni/SiOz catalysts, and these catalysts had permanent deactivation when used at high steam-methane ratio, which occurs via surface transformation, forming a nickel hydrosilicate layer. Nickel-based catalysts are widely used in many industrial processes, including hydrogenation, methanation of coal synthesis gas, and steam reforming of hydrocarbons. In these processes, Ni is commonly supported on the surface of an oxide support. The purpose of the support is to facilitate the formation of finely divided metallic particles, thus providing high surface catalytic area. Different supports have been used to achieve these goals such as SiOz,A1,0,, MgO, and TiOz (Martin et al., 1981; Bartholomew, 1976). Several parameters, such as the preparation method, pretreatment, nickel loading, etc., influence the activity of the catalysts. In the preparation of Ni/SiOz catalyst, for example, using a basic ammonia solution (Martin et al., 1981) favors nickel dispersion. The support is not necessarily a passive inert carrier, as was the concept; it may play a more significant role in
affecting the catalysts' activity. Taylor et al. (1964, 1965) found that the activity of Ni/silica-alumina is much lower than the activity of Ni/SiOz for the hydrogenolysis of ethane. Furthermore, this activity varies significantly with nickel loading (1-570 Ni). In recent years, much attention has been devoted to investigate the role of the support (Burch and Flambard, 1984; Cairns et al., 1983; Ozdagan et al., 1983; Vance and Bartholomew, 1983; Duprez et al., 1982) in Ni catalysts. It has been found that the activity for COz hydrogenation (Ozdagan et al., 1983) and CO hydrogenation (Vance and Bartholomew, 1983) increases with increasing metal support interaction in the order Ni/SiOz, Ni/Al2O3, and Ni/Ti02. It has been suggested (Burch and Flambard, 1984) that this behavior is part of an electronic or a structural effect, but it has not been possible to differen-
088S-5SS5/88/2627-0790~0~.50/0 0 1988 American Chemical Society
Ind. Eng. Chem. Res., Vol. 27, No. 5, 1988 791 Table I. Conditions Used in the Preparation of Ni/Si02 Catalysts Ni(N03).6H20,mol/L Ni loading," % sample designation 0.07 7.5 wt % Ni/SiO, 7.5 15 wt % NijSiO; 15.0 0.14 0.14 30 wt % Ni/SiOz 30.0 50 wt % Ni/SiOz 50.0 0.14 a
urea, mol/L 0.21 0.42 0.62 0.42
silica,* kg/m3 15.2 15.2 15.2 7.6
X
lo3
mixing time, h 2 3 5 3
Determined from AA analysis. *Mass concentration in the solution.
tiate between the relative importance of these two potential contributing factors. Determining the preparation method which will produce the optimal combination of high dispersion and high metal loading is of great importance. Small crystallite size ensures high specific metal area, but sintering results when the nickel loading increases. The maximum nickel surface per unit volume of the catalyst (Dixon and Singh, 1969) usually appears in the range 30-50 w t % Ni. A preparation method via impregnation produces inhomogeneous crystallite size, except for low metal loadings. In addition, it was found (Carter et al., 1966) that the catalytic activity of Ni/Si02-A1,03 catalyst in the hydrogenolysis of ethane decreased as the crystallite size increased, which emphasizes the need to have better preparation methods, ensuring the formation of small crystallite size for certain reactions. Van Dillen et al. (1976) reported a method which uses a slow homogeneous-precipitation-deposition of nickel hydroxide on silica in an aqueous solution of urea. The slow urea decomposition in water at 90 OC, producing hydroxyl ions, is the controlling step. These hydroxyl ions combine with nickel ions to slowly precipitate nickel hydroxide homogeneously on the silica. Moreover, the precipitated nickel hydroxide combines with silica (upon precipitation) to form an immobile nickel hydrosilicate complex. These authors assumed that the nickel hydrosilicate formed was Ni,Si,Olo(OH)s (antigomite)-a conclusion based on the catalyst dehydration in a thermobalance from which nickel-water ratio was subsequently calculated. This indirect technique leaves uncertainty as to the actual structure of the nickel silicate formed. The catalyst prepared by this method (Van Dillen et al., 1976) has very small crystallites (1-2-nm particle size) and a narrow crystallite size distribution (Richardson and Dubus, 1978) and is stable (Richardson and Crump, 1979) against sintering at moderate reduction temperature (S500 "C). As part of our study to investigate the role of the support with Ni catalyzing steam reforming of methane using different types of supports, the homogeneous-precipitation-disposition method was selected to prepare the Ni/Si02 catalysts, since it was shown (Van Dillen et al., 1976; Richardson and Dubus, 1978) to produce good and stable Ni/Si02 catalyst via the formation of a nickel hydrosilicate layer which could provide stability and have more resistance for deactivation of catalyst.
Experimental Section Catalyst Preparation. Four different nickel loadings of Ni/Si02 catalysts were prepared by using the method described by Van Dillen et al. (1976). The materials used in the catalyst preparation were 0.8 L of distilled water, Ni(Ni03),.6H20 (AR grade), CO(NH2)2 (AR grade), and silica (Cab-0-Sil) Cobot EH-5. The preparation was carried out using a constant-temperature water bath, fitted with an immersion heater circulator. The solution mixture of the distilled water, nickel nitrate, and silica was well stirred by a magnetic bar. At a constant temperature of 90 OC, urea was added to the agitated
r -- - - *- -
---
. UANlFOLO .RE4CTOR . STEAM GENERLTOR F . FURNACE c .C O N D E N S E R
M
R SG
P
G GC Y
-
PUMP
P P E S ~ U R E GAUGE
.GAS C H R O M A T O G R A P H
. SIX
PORT VALVE
w .W A T E R S U P P L Y
.._._____ HEAIEO LINES
Iu(
Figure 1. Schematic diagram of the reaction apparatus.
suspension of silica in a nickel nitrate solution. The concentrations of nickel nitrate and urea, the amount of support, and the mixing time were varied to produce different nickel loadings. To stop the reaction, the immersion heater circulator was turned off and the bath water was drained, while the solution continued to be stirred. The green precipitate produced was filtered and washed with hot distilled water several times and then was dried at 120 OC in air for 16 h. Atomic absorption (AA) analysis was used to determine the total nickel content of each sample. Table I summarizes the variables used in the preparation and the nickel loadings for the four catalysts. Kinetic Apparatus. It consisted of a flow reactor with a gas feed section, steam generator, and gas analysis section (Figure 1). The gas feed section included metering valves, a manifold, and flow meters to measure the flow rate of each gas stream. Steam was generated in a heated stainless steel tube (1-in. 0.d.) filled with Rashig rings in.) and supplied with a constant flow of water by a fluid metering pump. The reactor consisted of a quartz tube (12-in.long, ' I 2in. i.d.) heated by a 1200-W furnace with a feedback temperature control. The catalyst was placed in powder form on a porous frit, and its temperature was measured by a chromel-alumel thermocouple inserted in a thermowell. The reactor inlet and outlet compositions were measured by an on-line gas chromatograph (GC) equipped with an 18-ft-long, 1/8-in.-o.d. column filled with 801100 Porapack S, operated at 115 "C using He as a carrier gas. An ice bath, placed between the reactor exit and the GC sampling valve, condensed the steam and removed the water. Prior to each experiment, 0.5 g of catalyst was pretreated in situ with 80 cm3/min of hydrogen a t 400 "C for 12 h. After reduction, hydrogen was replaced by He, and the reactor temperature was raised to its desired value. Helium was passed through the steam generator heated to 200 "C, and after a constant flow rate was attained, the steam-He flow was mixed with methane, and the composition of the inlet stream was measured. The feed was then introduced into the reactor, and the composition of the gases leaving the reactor was measured by the GC as a function of time on stream. The methane, helium, and steam flow rates were 5.19 x 7.76 X and 3 X low2mol/min, respectively, for high steam-methane ratio activity experiments and 6.54 x 6.54 x and 1.9 X mol/min, respectively,
792 Ind. Eng. Chem. Res., Vol. 27, No. 5, 1988 0.25 1/2gm N i i S i O 2 Catalyst N i c k e l loading
= 50 % o = 30 % = 1 5 '1.
0
-
v
::
0.2 0
I
A
7 5%
%
T = 565'1 .3 CH,= 2 28x10 g m o l e l l H 2 0 z 0 013 gmole 11 R =57
g-
0.15
50 Ni I S i O z
m
T = 5 6 5 'C 1 I 2 gm Catalyst
a W
>
z
0.1 0
0 V W
z
2
0.05
I-
-
W
I
0 10
20
3 0
40
1.0
0
50
R E A C T I O N T I M E (hour)
Figure 2. Deactivation behavior of Ni/SiO, catalysts with varied nickel loadings a t high steam-methane ratio reaction.
for the low steam-methane ratio activity experiments. Hydrogen Chemisorption, Hydrogen chemisorption measurements were carried out in situ by using the pulse method in a flow chemisorption unit described elsewhere (Varghese, 1979). The catalyst was first pretreated as previously described, degassed at 500 "C for l/z h, and then cooled to room temperature in argon flow. Chemisorption was then carried out by sending pulses of hydrogen, via a six-port valve, into the argon carrier gas stream. After saturation, the unadsorbed hydrogen fraction was detected by a thermal conductivity cell. X-ray Diffraction. XRD patterns of the catalyst were obtained in a Diano diffractometer using a Cu K a source to irradiate 0.4 g of the catalysts pressed a t 10OOO psi into a thin wafer of about 0.1-cm thickness and 1-cm diameter. The XRD scan was carried out from 5" to 75" by using the step scanning technique which minimizes the noise in the analysis of highly dispersed supported catalysts. X-ray Photoelectron Spectroscopy. XPS measurements were obtained in a Varian spectrometer (Model IEE-15) using a Mg K a source. The samples, in powder form, were dusted onto double-sided scotch tape mounted in a cylindrical aluminum holder. The binding energies were calibrated against the C 1s 285-eV line, and the integrated intensities were corrected for cross section and sensitivity. The peaks corresponding to Ni 2p, p2/3, Si 2p, and A1 2p transitions yielded Ni/Si and Ni/A1 ratios, assumed to be proportional to the metal's relative surface concentration. Samples of the untreated, reduced, and reactor-used catalyst were characterized by H2 chemisorption, X-ray diffraction, and X-ray photoelectron spectroscopy.
Results Activity and Stability. Results of methane conversion versus reaction time at 565 "C, obtained using catalysts containing 7.5, 15, 30, and 50 wt % Ni, and at a high steam-methane ratio (hereafter designated R) of 5.7, are shown in Figure 2. The results indicate that all four Ni/Si02 catalysts experienced fast, permanent deactivation. The initial activity is roughly proportional to the nickel loading, with the exception of the 30 wt % Ni/Si02 catalyst (which displayed activity higher than 50 wt % Ni/SiO,). Figure 3 shows the activity of 50 and 15 wt % Ni/SiOz catalysts, obtained at 565 "C at lower R (-2.3); the activity at high R is also presented for comparison. The results
3.0
2.0
5.0
4.0
REACTION TIME (hours)
(a1
0.20
I
G
1 0
I
I
O
I 0
0 I c
K
z
0.15
0
m m W
;0.10 0
1 / 2 g m Catalyst 0 R-2.3
V
v RES.?
01
0
I
I
I
I
1.0
2.0
3.0
4.0
i I
5.0
R E A C T I O N TIME ( h o u r s ) (b)
Figure 3. Steam-methane ratio effect on Ni/Si02 catalysts.
indicate that these catalysts (as well as those with intermediate loading) were stable and showed no deactivation during a 5-h reaction period a t low R reaction. The influence of calcination on activity was investigated, using 50 wt % Ni/Si02. The catalyst was calcined in air flow at 400 "C for 2 h, followed by the reduction in hydrogen for 12 h at 400 "C. The results (not presented) indicate that calcination did not change the activity. The effect of steaming on the catalyst was also investigated on the 50 wt % Ni/SiOz. After the regular pretreatment (H2/400"C/ 12 h), the temperature was raised to 565 "C, followed by a helium flow of 8.18 X mol/ mol/min for h. min with steam flow of 1.9 X Methane was then introduced into this mixture, but after steaming the activity had been permanently lost. In order to gain some understanding of the causes of catalyst deactivation, a detailed characterization of the catalyst (summarized below) was carried out both prior to and after the reaction for the 15 and 50 wt % Ni/SiOz catalysts. Hydrogen Chemisorption. The H2 chemisorption results for the various fresh catalysts used are shown in Figure 4 along with the ratio NiR/NiT describing the reduced Ni surface atoms to the total Ni atoms. The active
Ind. Eng. Chem. Res., Vol. 27, No. 5, 1988 793
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
I
0
10
20
30
40
50
N I C K E L L O A D I N G (percent)
Figure 4. Percentage of the reduced nickel and the active area as a function of nickel loading. J
SiO,
A
I
I
1
1
1
1
60
50
40 28
30
20
10
Figure 6. XRD patterns of 50 w t % Ni/Si02 catalysts: (a) untreated, (b) reduced, (c) used at low R reaction, and (d) used at high R reaction.
I
J
I
70
I
1
I
1
I
1
I
70
60
50
40
30
20
10
28
Figure 5. X-ray diffraction patterns of untreated Ni/Si02 Catalysts.
nickel area increases as the nickel approaches a maximum, after which it decreases. The highest nickel loading 50 wt % Ni/Si02 catalyst had less active area than 30 wt % Ni/Si02 catalyst. It was also observed that exposing the reduced catalyst to atmospheric air caused it to get hot. XRD Analysis. XRD has not been used in previous studies (Van Dillen et al., 1976; Richard and Dubus, 1978), possibly due to the formation of very small nickel crystallites which would make these measurements difficult. In order to overcome this problem, the step scanning technique was used to collect XRD data for all Ni/Si02 catalyst samples. Similar results for various untreated Ni/Si02 catalysts (containing increasing amounts of Ni) are shown in Figure 5. The peak a t 28 = 21.9' (corresponding to SO2)shifts slightly to the left as nickel loading increases. The peaks a t 28 = 24.3' and 60.5' belong to Ni6Si4010(OH)8(antigornite), whereas the peaks at 28 = 33.96O and 35.6' belong to Ni-Si-0.H20 (some type of nickel hydrosilicate) (Joint Committee on Powder Diffraction Standards, 1981). No lines corresponding to nickel, nickel oxide, or nickel hydroxide could be identified. XRD results of catalysts with 15% and 50% nickel loadings (obtained before and after pretreatment, and after reaction at high and low steam-methane ratio) show that, after reduction, diffraction lines corresponding to nickel oxide appear a t 28 = 43.3O and 37.3'. The intensity of the lines increased sharply after the catalyst had been used a t high R reaction (R = 5.7/565 OC/3 h). The XRD results for catalysts used a t low R reaction are similar to those of reduced catalysts. The
Table 11. Standard Sample Binding Energy of Nickel Compounds Ni satellite sample Ni 2p3,, splitting ref NiO 854.9 7 this study 5.9 this study 856.0 Ni(OHI2 6.2 this study 856.8 NiS03 Shalvoy and Reucroft (1979) 852.8 Ni Badrinarayanan et al. (1981) 852.6 Ni 854.8 Badrinarayanan et al. (1981) Ni02 7 Shalvoy and Reucroft (1979) 854.6 Ni02 5.8 855.5 Shalvoy and Reucroft (1979) Ni(OH)2 Shalvoy and Reucroft (1979) 6.0 856.7 NiSi03 Table 111. XPS Results of 15 and 50 wt % Ni/Si02 Catalvsts Ni satellite Ni % Ni 2p3 splitting, Ni-Si BE, e 4 eV ratio loading sample 15 untreated 855.7 5.2 0.41 15 reduced 856.7 5.2 0.34 15 after low R rxn 857.4 6.4 0.28 15 after high R rxn 857.4 5.9 0.24 50 untreated 857.0 6.2 0.86 50 reduced 857.0 5.9 0.75 50 after low R rxn 857.0 6.2 0.66 50 after high R rxn 857.3 5.8 0.47
XRD results of the 50 wt 70Ni/Si02 are shown in Figure 6. XPS Results. Surface characterization was carried out by using X-ray photoelectron spectroscopy (XPS); binding energies (BE)for some standard nickel compounds with some of their published literature (Shalvoy and Reucroft, 1979; Badrinarayanan et al., 1981) are summarized in Table 11. The Ni 2p3/, binding energy of metallic nickel is 852.5 eV, whereas the BE'S of NiO and Ni(OH)2 are 854.8 and 856.0 eV, respectively. The binding energy of supported Ni2+may vary over a 2-eV range with different supports; the stronger the interaction of NiO with the support, the closer the BE is to that of Ni(OH)2(Vedrine et al., 1978). Table 111, summarizing XPS results for the 15 and 50 wt % Ni/Si02, catalysts, shows that the BE'S of untreated, reduced, and used catalyst range from 855.7 to 857.5 eV. The Ni BE increases for the reactor used catalyst. The integrated peak intensities of Ni 2pSj2and Si 2p photoemission spectra (corrected for the cross section and
794 Ind. Eng. Chem. Res., Vol. 27, No. 5, 1988
sensitivity) are related to the surface Ni-Si atom ratio. The Ni-Si ratio, shown in Table 111, decreased with exposure to reaction environment. Furthermore, this decrease becomes more pronounced with exposure to high R reaction. The nickel satellite splitting was measured and is listed in Tables I1 and I11 for both the standard samples and the tested catalysts. The satellite splitting of Ni 2p of nickel oxide is 7.0 eV, while it is about 6.0 eV for the nickel hydroxide and nickel silicate. The satellite splitting of Ni 2p for the Ni/Si02 catalysts is in the range 5.2-6.4 eV, indicating that possibilities for the existence of nickel oxide on the surface are very few.
Discussion Van Dillen et al. (1976) concluded that Ni&3i40,0(OH)8 (antigornite) would form on the surface of Ni/Si02 catalysts prepared with the homogeneous-precipitation-deposition method. As mentioned earlier, however, their conclusion was based on an indirect method, leaving uncertainty about the actual compounds formed. The XRD results (Figure 5) reveal the presence of two types of nickel hydrosilicates, Ni6Si4010(OH)8and NiSi-0.H20, and the presence of silica. Neither nickel nor nickel hydroxide nor nickel oxide was present in the untreated catalysts. The intensity of the two nickel hydrosilicate lines increases with nickel loading. A t the same time, as more nickel hydrosilicate forms, the intensity of the silica peak (20 = 21.9’) decreases and shifts toward the left side, where one of the Ni6Si40,0(OH)8peaks at 20 = 24.3’ is located. These XRD results provide the determination of two types of nickel hydrosilicate on the prepared Ni/Si02 catalysts. The hydrogen chemisorption results indicate the difficulty of reducing Ni/Si02 catalysts (measured by the reduced percentage of the total used nickel, NiR/NiT), since NiR/NiTin the best case did not exceed 3%. It was reported (Schuit and Van Reijen, 1958) that the formation of nickel hydrosilicate structure is invariably accompanied by a decrease in reducibility. It has also been noted (Swift, 1977) that the nickel reduction from combined nickel materials (such as montmorillonite, a type of nickel hydrosilicate) is more difficult to accomplish than the reduction of noncombined nickel (such as NiO/Si02). However, XRD results of the Ni/Si02 catalysts (Figure 5) show that only nickel hydrosilicate was present after preparation on the support @io2). Therefore, our results (H2chemisorption and XRD) support the fact that combined nickel/silica catalysts (such as the ones prepared by homogenous-precipitation-deposition)are the most difficult to reduce. This explains the low degree of reduction of Ni/Si02 catalysts. As Figure 4 shows, the optimum nickel loading (i.e., the one which yields the highest active area and the highest NiR/NiT)will be in the 15-30% range, since the nickel active area reached its maximum a t 30% and the maximum NiR/NiToccurs with 15% Ni loading. Initial activity increased with nickel loading from 7.5% to 30% Ni and then decreased as Ni loading increased further (toward 50% Ni/Si02 catalyst). This behavior can be explained simply on the basis of the decrease in the amount of exposed nickel area of the 50% Ni/Si02 compared to that of 30% Ni/Si02 catalysts. The activity results of the Ni/Si02 catalysts showed a strong dependence on the steam-methane ratio in the steam reforming of methane reaction at 565 OC. At low R (2.31, the catalysts were stable, and no deactivation occurred during a 5-h period. Once the reaction was stopped, however, the catalyst activity could not be re-
gained. Furthermore, when the Ni/Si02 catalyst was used at high R (5.7), all the catalysts experienced a fast, irreversible deactivation over a 3-h period. Steaming the catalyst prior to the reaction also caused a permanent catalyst deactivation. The effect of R, from low to high (i.e., to steaming), shows that steam is one of the main causes of the Ni/Si02 catalyst’s deactivation. XRD results show that the untreated Ni/Si02 catalysts contain a bulk nickel hydrosilicate only, and that no detectable bulk nickel hydroxide or oxide is present (Figure 5). After reduction (H2/400 ‘C/12 h), some NiO is formed. Since the reduction would produce metallic nickel, it seems that exposing the reduced Ni/Si02 catalyst to the atmosphere (during handling and transferring the sample to the X-ray machine) oxidizes the reduced nickel, producing nickel oxide. This conclusion is supported by the observation that the reduced sample (Ni/Si02) got very hot when exposed to the atmosphere. After catalysts were used a t high R, the NiO XRD intensities sharply increased and the peaks became narrower. Conversely XRD patterns obtained after reaction a t low R are similar to those obtained with the reduced catalyst. These results strongly suggest that, since the formation of bulk NiO does change significantly after reaction, sintering might be involved in the deactivation at high R reaction. These XPS results cannot be used unambiguously to detect a buildup of carbon on the surface since the use of diffusion pumps in the XPS spectrometer always necessitates at least some carbon contamination in the spectrometer chamber. Signal intensities, however, are similar for both freshly reduced and for deactivated catalysts for the carbon 1s level. If the deactivation were the result of carbon overlayer, an increase in the carbon signal after the reaction should have been detected. The XPS results show that the main oxidation state of nickel species on the surface is Ni2+for all tested catalysts. The BE values of Ni and NiO are 852.7 and 854.8 eV, respectively, and the BE values for tested catalysts are in the range 855.7-857.5 eV (which are higher than the BE of nickel oxide). This higher BE could be due to a strong interaction of NiO with the support (Vedrine et al., 1978) or to nickel compounds (where nickel is associated with higher electronegative charges, as in nickel hydrosilicate). XRD results for the untreated Ni/Si02 support the second case, since NiO was not detected. However, the satellite splitting of the tested samples ranges from 5.2 to 6.4 eV, while that of NiO is 7.0 eV. This result supports the argument that nickel oxide was not present on the catalyst surface (with strong support interaction). Instead, it is most probable that some type of nickel hydrosilicate covers the surface. XRD results for the deactivated (high R reaction) Ni/SiQ2 show that NiO is present in appreciable amounts (in the bulk of the catalyst). This is in conflict with the XPS results (of the catalyst’s surface). Considering the fact that XRD indicates bulk properties-while XPS indicates surface properties-it is more likely that nickel oxide is covered with nickel hydrosilicate. Sholvoy and Reucroft (1979) encountered a similar problem. That study suggested either that NiO may be present in the pores of the Si02support or that it is covered by an amorphous nickel silicate layer. The NiO in these locations would be detected by XRD but would not be seen by the XPS. The Ni-Si ratio on the catalyst’s surface measured by XPS (Table 111) decreased by reduction and dropped further after being used in the reaction. The decrease in
Ind. Eng. Chem. Res., Vol. 27, No. 5, 1988 795 N I C K E L H Y D R O S I L I C A T E LAYER
XXXXXX REDUCTION H2
1'c
400
N i M E T A L + SILICA + N I C K E L HYDROSILICATE
REA C T I O N H I G H H2 0 I C or S T E A M I N G
1
565 C '
NICKEL HYDROSILICATE FILM
\
\
\
-NICKEL OXIDE
Figure 7. Proposed mechanism of Ni/SiO catalysts deactivation of high nickel loading at high steam-methane ratio in steam reforming of methane.
the Ni-Si ratio is a t a maximum after being used in high R reaction. This Ni-Si ratio change suggests that the deactivation is related to changes on the surface. It is not clear, however, precisely how the change took place. The presence of steam a t high temperature caused the sintering of NiO (as the XRD line became narrower and the intensity got sharper and stronger). As XPS results indicate, some type of nickel hydrosilicate overlayer formed on the surface as a consequence. It could be that silicon atoms dissolved through nickel oxide to the surface (as reported by Pease et al. (1980)). The availability of water during the reaction increases the likelihood that a nickel hydrosilicate overlayer will form-a formation process in which the water ultimately participates. Phase transformation on the surface, where NiO immigrated through the amorphous nickel hydrosilicate, is another possibility. Schuit and Van Reijen (1958) explain the resistance of the coprecipitated Ni/SiOz catalyst to nickel removal by CO (as Ni(CO),) by assuming the formation of a silicate skin. During reduction, the formation of a skin of nickel silicate begins to take place at 400 "C. The silica skin formation and the reduction are two independent processes.
These characterization results were used to postulate the scheme shown in Figure 7. The dried, green precipitate formed during preparation was a nickel hydrosilicate/silica (untreated) (Van Dillen et al., 19?6). After reduction, some of the nickel hydrosilicate broke down to Ni (which, by exposure to air, formed the detected NiO), SOz,and H 2 0 (Van Dillen et al., 1976), and the Ni and Si02are in mixed composition. By use of the reduced catalysts in high R reaction, several processes appeared to occur: oxidation of the nickel by steam, sintering of the resulting crystallites, and reaction of surface nickel with silica and steam to form an amorphous nickel hydrosilicate layer over the nickel oxide. In summary, the studies presented in this work show the formation of two types of nickel hydrosilicate on the surface of the Ni/SiOz catalyst prepared by the homogeneous-precipitation-deposition method. These Ni/Si02 catalysts were stable a t low R, but it deactivated permanently a t high R values or by steaming. The characterization clearly indicates that deactivation occurs via surface transformation. Registry No. Ni, 7440-02-0;CH,, 74-82-8;Ni6Si4010(OH)8, 75321-23-2.
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