J . Chern. Soc., Fa-raduy Trans. I , 1987, 83 (lo), 3189-3197 Control of Ni Metal Particle Size in Ni/SiO, Catalysts by Calcination and Reduction Temperatures Hiroyuki Tamagawa, Kyuichi Oyama, Tsuyoshi Yamaguchi, Hiroshige Tanaka, Hideyasu Tsuiki and Akifumi Ueno" Department of Materials Science, Toyohashi University of Technology, Tempaku, Toy ohash i, A ich i 440, Japan It has been found that the particle size of Ni in Ni/SiO, catalysts prepared by the alkoxide technique can be controlled by the calcination and reduction temperatures as well as by the extent of Ni loading in the catalyst. The mechanisms of sintering of the Ni metal and of redispersion of NiO particles during reduction and calcination respectively, are discussed. The sintering occurs by a particle-migration mechanism and the redispersion may occur by direct splitting to small particles. The catalytic property of Ni particles whose sizes were controlled either by their Ni loading or by their calcination and reduction temperatures was examined by using the catalyst for the Boudouard reaction.Since a catalytic reaction over a supported metal catalyst takes place on the surfaces of ' metal particles, the physical and chemical properties of these surfaces will be reflected in their catalytic activity and selectivity. The properties of the surfaces of metal particles are considered to change with additives dissolved in the metal particles and with the supports employed; alloying effects upon the catalysis' are attributed to the former and the so-called SMS12 (strong metal-support interaction) effects are ascribed to the latter.The control of metal particle size is also one of the candidates for modifying the properties of a metal s~rface,~ hence there has been much research effort in this area.* In a previous paper5 we reported that the Ni particle size in silica-supported Ni catalysts can be controlled by adjusting the Ni concentration in the catalyst. The catalyst was prepared by hydrolysis of a mixed solution of ethyl silicate and nickel nitrate dissolved in ethylene glycol (the alkoxide technique). The alkoxide technique has the added advantage of preparing catalysts free from any impurities which will affect catalytic reactions. A few disadvantages have been claimed for catalysts prepared using this technique, one of them being that the metal particle size might not be controlled in the catalyst with a certain concentration of metal.In most of our work the catalysts have been prepared by the alkoxide technique using calcination and reduction temperatures around 500°C. The purpose of the present work is to study the effects of calcination and reduction temperatures upon Ni particle size in Ni/SiO, catalysts prepared by the alkoxide technique. The change in the catalytic activity for CO disproportionation (Boudouard reaction) with the change in Ni particle size is also discussed. Experimental Catalyst Preparation The catalysts employed is Ni/SiO, prepared by the alkoxide technique using ethyl silicate and nickel nitrate dissolved in ethylene glycol. The preparation procedures are summarized in scheme 1.Ni loadings in all the catalysts used are 2.5 wt YO, as revealed by X-ray fluorescence measurements. 3189 105-23190 ethylene glycol- Si( OE t)4 .D H2O h Control of Ni Particle Size v drying at 1 10 "C for 24 h i calcination in air reduction by H, Ni/SiO, In order to study the effects of calcination and reduction temperatures on Ni particle size, the catalysts were prepared by two different methods: (A) the catalysts were calcined at 500 "C in air for 4 h, followed by reduction in flowing hydrogen at 500, 700 and 900 "C for 4 h, respectively; and (B) the catalysts were calcined in air at 500,700 and 900 "C for 4 h, respectively, followed by reduction in flowing hydrogen at 500 "C for 4 h. Hydrogen was supplied from a cylinder and was passed through a cooled trap packed with molecular sieves for removal of impurities.Measurements of Ni Particle Size by T.E.M. The particle sizes of NiO in the calcined catalyst and of Ni metal in the reduced catalyst were monitored by t.e.m. (transmission electron microscopy, Hitachi H-800), operated at an accelerating voltage of 200 kV with a magnification of lo5. The size distribution curves were obtained by measuring the size of more than lo3 paorticles in each catalyst. The lower limit for the measurement of particle sizes is ca. 20 A. Measurements of Ni Particle Size by H, Chemisorption The mean particle size of Ni in the reduced catalyst was also measured by hydrogen chemisorption measurements at room temperature following the procedures proposed by Wilson and The specific surface area of Ni metal particles on theTedwed catalyst was estimated assuming the cross-section of an Ni atom to be 6.8 A2.' The degree of reduction of Ni ions to Ni metal in the reduced catalyst is required for the estimation of the specific surface area and the mean size of Ni metal particles.The saturation magnetization of the reduced catalyst was measured by a vibrating sampleJ. Chem. SOC., Faraday Trans. I , Vol. 83, part 10 ( A ) Plate 1 Plate 1. Micrographs of Ni metal particles in catalysts: (A) for catalysts first calcined at 500 "C and then reduced at (a) 500, (b) 700 and (c) 900 "C; and (B) for catalysts first calcined at (a) 500, (b) 700 and (c) 900 "C and then reduced at 500 "C. H. Tamagawa et al. (Facing p . 3191)H.Tamagawa et al. 3191 magnetometer (VSM-2, Toeikogyo Co.) with a magnetic field at 2 T and a vibrating frequency of 80Hz. The degree of reduction of Ni ions was estimated from the saturation magnetization thus obtained, assuming that the saturation magnetization of Ni metal does not depend upon the particle size, 54.6 emu g-1.8 Ni metal is, of course, a ferromagnetic substance, while NiO is anti-ferromagnetic. Decrease in Ni Metal Surface with Time of Reduction Because of sintering at high temperatures the specific surface area of Ni metal particles decreased with the time of reduction. The change in the Ni surface area with reduction time was monitored by hydrogen chemisorption measurements. The catalyst calcined at 500 "C for 4 h was reduced at 500,700 and 900 "C, respectively, in flowing hydrogen and was then evacuated for 1 h at each reduction temperature, followed by cooling to room temperature for H, chemisorption.Dependence of Ni Metal Catalytic Activity on Particle Size The change in the specific activity of the reduced catalyst for the Boudouard reaction with change in Ni particle size was studied using a closed circulation system made of glass, the total volume of the system being 272 cm3. The dried catalyst (0.5 g) was placed in a reactor made of quartz and was then calcined and reduced at the desired temperatures, followed by introduction of CO gas with an initial pressure of 220 mmHgt. The reaction was carried out at 300°C and the amounts of CO, formed and CO consumed were analysed by gas chromatography using a column packed with active carbon.The activity of the catalyst was expressed in terms of turnover frequency by measuring the amount of CO, formed during the initial 5 or 10 min of the reaction. Results Ni Particle Size measured by T.E.M. Typical micrographs of Ni metal particles in the catalysts calcined and reduced at various temperatures are shown in plate 1. The particles shown in plate 1 (a) are Ni metal particles in the catalysts calcined at 500 "C, followed by reduction at 500, 700 and 900 "C, respectively. In plate 1 (b) are shown the catalysts calcined at 500, 700 and 900 "C, respectively, followed by reduction at 500 "C in flowing hydrogen. The particles in each catalyst have a small spread of size distribution, as shown in fig. 1. The mean particle sizes were calculated using the equation d = C ni di/ni i where di is the particle size measured and ni is the number of particles with the size di.The mean particle sizes thus calculated are shown in fig. 2 and are summarized in table 1. The particle sizes of NiO in the catalysts calcined at 500, 700 and 900 "C, respectively, were also monitored by t.e.m. and the mean particle sizes are shown in fig. 3 with those of Ni metal in the catalysts subsequently reduced at 500 "C in flowing hydrogen. Ni Particle Size estimated by H, Chemisorption By measuring the amount of hydrogen chemisorbed the specific surface area of Ni metal particles was obtained since the degree of reduction of Ni ions in the catalysts have been measured previously (table 1). The degree of reduction was found to increase with t 1 mmHg x 133.3 Pa.3192 Control of Ni Particle Size 50 100 150 50 100 150 50 100 150 I I l 7 5 - ( b 1 50 - 7 - -.25 - - 0 1 1 I 50 100 150 particle size/A 75 50 25 0 75 50 25 0 50 100 150 50 100 150 Fig. 1. Particle size distributions resulting from the micrographs in plate 1 ; the notations of (A) and (B) are the same as given in plate 1. 80 - 1 Z; 60 - 0 .-I 5 LO - 20 - 0 500 700 900 T/"C Fig. 2. Mean particle sizes of Ni in the catalysts measured by t.e.m. : the abscissa gives temperature of reduction (0) and calcination (0) and the notations of (A) and (B) are the same as given in plate 1 . increasing reduction temperature but did not depend on the calcination temperature. The mean size of Ni metal particles in the reduced catalyst was calculated by the equation d = 6/pS assuming that Ni particles are spheres, where p is the density of Ni metal (8.85 g cmP3)' and S is the specific surface area of Ni metal particles in the catalyst.The mean particleH . Tamagawa et al. 3193 Table 1. Characterization of Ni particles in the catalysts (see text for catalyst treatments) calcination temperature/OC : 500 500 500 500 700 900 reduction temperature/OC : 500 700 900 500 500 500 Ni surface area/m2 g-' 88 82 81 88 83 164 d (from H, chemisorption)/A 45 55 71 45 47 24 d (from t.e.m.)/A 34 58 74 34 32 22 reduction (%) 70 80 100 70 69 69 ~~ ~ ~ ~~ 100 80 2 60 .r( a d 40 a 20 0 I I I I I I 500 700 900 T/"C Fig. 3. Mean particle sizes of NiO in the catalysts calcined at 500, 700 and 900 "C and those of Ni produced by reduction of these calcined catalysts at 500 "C. 0, NiO; 0, Ni.sizes of Ni thus obtained are summarised in table 1 and are in good agreement with those monitored by t.e.m. Sintering Mechanism of Ni Metal Particles As shown in fig. 4, the specific surface area of Ni in the silica-supported catalyst decreased with the reduction time, the reducing temperatures being 500,700 and 900 "C, respectively. In order to discuss the sintering mechanism the specific surface area of Ni was converted to the mean particle size using the equation mentioned above. In fig. 5 are shown the plots of log d us. log t at various reduction temperatures. Effect of Ni Metal Particle Size on Boudouard Reaction In order to avoid the change in the surface properties of Ni metal particles during the reaction, the amounts of CO, formed were measured during the initial 5 or 10 min of the reaction and were used to calculate the turnover frequency over the catalysts.The effects of size on the turnover frequency are shown in fig. 6. The open circles in fig. 6 represent the specific rates over the catalysts in which the sizes of Ni metal particles were controlled by Ni loading and solid circles denote the specific rates over the catalyst whose Ni particle sizes were controlled by the calcination and reduction temperatures.3194 Control of Ni Particle Size 100 F 0 7 0 5 10 15 20 reduction time/h Fig. 4. Decrease in the specific surface area of Ni with reduction time. The catalysts were first calcined at 500 "C and then reduced at 500 (O), 700 (0) and 900 "C (a).1.0 I I I 1 I 0.3 0.6 0.9 1.2 1.5 log t Fig. 5. Determination of sintering exponent n. 0, 500; 0, 700; a, 900 "C. Discussion As previously menti~ned,~ one of the features of the alkoxide technique for catalyst preparation lies in the dispersion of metal ions in the dried catalyst. The metal ions may coagulate during the calcination procedure to form the metal oxide clusters with an even size. Thus, the size of the oxide clusters is a function of the calcination temperature as well as the metal concentration in the catalyst. Although the metal oxides are generally considered to increase in size with increase in calcination temperature, a few resultsH. Tamagawa et al. 3195 0- 50 100 particle size of Ni/A Fig. 6. Change in the turnover frequency for Boudouard reaction with the change in mean particle size of Ni in the catalyst.The reaction was carried out at 300 "C. 0, particle size controlled by Ni concentration ; , particle size controlled by calcination and reduction temperatures. indicating the opposite trend have been reported." This phenomenon has been called redispersion and has been used to explain the strong metal-support interaction of the Pt/TiO,'' and Fe/Ti0,12 systems. The process of redispersion has been studied extensively13 and two probable mechanisms have been proposed: one is the actual splitting of particles into smaller ones during oxidation'* and the other is a spread of oxide particles over the support surfa~e.'~ Depending upon the volatility and stability of metal oxides either mechanism will be favourable for the redispersion of the oxide particles.As is shown in fig. 3, tiny NiO particles were well dispersed in the silica support and were not spread over the support surface. Nevertheless the melting point of NiO is not significantly high (1998 "C). The size of NiO particles as obtained by t.e.m. measurements decreased dramatically when the catalyst was calcined at 900 "C (fig. 3). This indicates that very small, stable NiO particles were produced in silica during calcination at high temperature. Ni metal particles were formed by reducing the oxidised catalyst at 500 "C in flowing hydrogen and the size of Ni particles formed was in proportion to that of NiO in the oxidised catalyst (fig. 4). Thus, the Ni particle size can be controlled by varying the calcination temperature.As shown in plate 1 (b) and fig. 1, the particle size was made almost uniform even when the catalyst was calcined at the temperature as high as 900 "C. On the other hand, the Ni particle size increased with increasing reducing temperature, as shown in plate 1 (a) and fig. 1, probably due to the sintering of Ni particles at high reducing temperature. The sintering has also been studied extensively and several mechanisms have been proposed." The mechanism involving vapour transport or particle migration has received much attention. l7 These two mechanisms could be distinguished by measuring the surface area of metal particles as a function of reduction time and by using the sintering kinetics expressed as17 dS/dt = - KS" where S is the exposed surface area of metal per unit area of support and n is the sintering exponent, which can be predicted to be between 4 and 8 for a particle-migration mechanism and to be < 2 and > 13 for a vapour-transport or atomic-migration mechanism." According to Wynblatt and Gjosteinlg the above equation can be modified to (n - 1) log (d/do) = c+ log ( t )3196 Control of Ni Particle Size where do and d are the mean particle sizes of metal particles with the reduction time of 0 and t, respectively, at a certain temperature and C is a constant. Thus, plots of log(d/do) or log(d) us.log(t) should result in straight lines from which the order of sintering, n, can be obtained.20 From the results given in fig. 5, n was found to be 5 for the present silica-supported Ni catalyst. This seems to indicate that the sintering of Ni metal particles in the present catalyst takes place via a particle-migration mechanism, although Bartholomew and Sorensen20 reported a shift in the sintering mechanism from particle migration to atomic migration with increasing temperature and time of sintering. Although the distribution curve of Ni particles in the catalyst reduced at 900 "C was broader than those reduced at 500 and 700 "C, it is still possible to say that the Ni particle size was controlled within a small range [plate ] ( a ) and fig.13. Consequently, the Ni particle size in catalysts prepared by the alkoxide technique can be controlled within a small range and the range can be varied by the catalyst calcination and reduction temperatilres as well as by Ni loading in the catalyst.These conclusions are summarized in fig. 2. The specific rates for the Boudouard reaction were compared to demonstrate the identical catalytic property of Ni particles whose sizes were controlled either by Ni concentration in the catalyst or by the calcination and reduction temperatures. The specific rate, expressed in terms of turnover frequency, increased with increasing Ni particle size (fig. 6). This indicates that the reaction between adsorbed oxygen (originating from dissociatively adsorbed CO) and adsorbed or gaseous carbon monoxide might be the rate-determining step of the reaction,21 since oxygen ions adsorbed on small Ni particles are more strongly bound to the metal atoms than those on large particles and resulted in less reactivity with gaseous or adsorbed CO to form carbon dioxide.As can be seen in fig. 6, the specific rates over Ni particles with sizes controlled either by Ni loading or by the calcination and reduction temperatures fit well on a curve. Thus, the catalytic performances of Ni particles with sizes controlled by Ni loading are identical to those controlled by calcination and reduction temperatures. References 1 J. H. Sinfelt and G. H. Via, J. Catal., 1979,56, 1 ; H. C. de Jongste, V. Ponec and F. G. Gault, J. Catal., 1980, 64, 389; X-Z. Jiang, S. A. Stevenson and J. A. Dumesic, J . Catal., 1985, 91, 11. 2 S. J. Tauster, S. C. Fung and R. L. Carten, J. Am. Chem. SOC., 1980, 100, 180; S. J. Tauster and S.C. Fung, J. Catal., 1978, 55, 29; K. Kunimori, S. Matsui and T. Uchijima, J . Catal., 1984, 85, 253. 3 M. Boudart, J. Catal., 1965, 4, 704; P. H. Otero-Scipper, W. A. Wachter, J. B. Butt, R. L. Burwell Jr and J. B. Cohen, J. Catal., 1978, 53, 414; J. R. Anderson, Structure of Metallic Catalysts (Academic Press, New York, 1975). 4 J. L. Carter, J. A. Cusumano and J. H. Sinfelt, J. Phys. Chem., 1966, 70, 2257; M. Boudart, Adv. Catal., 1969, 20, 153; M. Primet, J. M. Basset, E. Garbowski and M. V. Mathieu, J. Am, Chem. Soc., 1975,97,3655; Y. Takasu, R. Unwin, B. Tesche, A. M. Bradshaw and M. Grunze, Surf. Sci., 1978,77, 219; Y . Takai, A. Ueno and Y. Kotera, Bull. Chem. SOC. Jpn, 1983, 56, 2941. 5 A. Ueno, H. Suzuki and Y. Kotera, J . Chem. SOC., Faraday Trans. I , 1983, 79, 127; K.Tohju, Y. Udagawa, S. Tanabe and A. Ueno, J. Am. Chem. Soc., 1984, 106, 612. 6 G. R. Wilson and W. K. Hall, J. Catal., 1970, 17, 190. 7 D. G. Mustard and C. H. Bartholomew, J. Catal., 1981, 67, 186. 8 Kagaku Binran (Kiso-hen, Zr) (Chem. SOC. of Jpn, Maruzen, Tokyo, 1975), p. 1233. 9 J. R. Anderson, Structure of Metallic Catalysts (Academic Press, New York, 1975), p. 447. 10 M. F. L. Johnson and C. D. Keith, J. Phys. Chem., 1963,67,200; R. M . J. Fiedorow, B. S. Chahar and S. E. Wanke, J . Catal., 1978, 51, 193; S. Takasaki, H. Suzuki, K. Takahashi, S. Tanabe, A. Ueno and Y. Kotera, J. Chem. SOC., Faraday Trans. I , 1984, 80, 803. 11 R. T. K. Baker, E. B. Prestridge and R. L. Carten, J. Catal., 1979, 59, 293. 12 B. J. Tatarchulk and J. A. Dumesic, J. Catal., 1981, 70, 308. 13 K. Forger and H. Jaeger, J . Catal., 1981, 70, 53; 1985, 92, 64; K. Forger, D. Hay and J. Jaeger, 14 E. Ruckenstein and J. L. Malhotra, J . Catal., 1976, 41, 303, D. B. Dadyburjor, J. Catal., 1979, 57, J . Catal., 1985, 96, 154; T. Wang and L. D. Schmidt, J. Catal., 1981, 70, 187; 1980, 66, 301. 504.H. Tamagawa et al. 3197 15 E. Ruckenstein and Y. F. Chu, J. Catal., 1979, 59, 109; J. E. Stulga, P. Wynblatt and J. K. Tien, 16 Sintering and Catalysis, ed. G. C . Kuchynski (Plenum Press, New York, 1975). 17 A. Williams, G. A. Butler and J. Hammonds, J. Catal., 1972, 24, 352; J. T. Richardson and J. G. Crump, J. Catal., 1979, 57, 417; S. E. Wanke and P. C. Fynn, Catal. Rev. Sci. Eng., 1975, 12, 93; P. C. Fynn and S. E. Wanke, J. Catal., 1974, 34, 400. J . Catal., 1980, 62, 59. 18 K-T. Kim and S-K. Ihm, J. Catal., 1985, 96, 12. 19 P. Wynblatt and N. A. Gjostein, Progr. Solid State Chem., 1975, 9, 21. 20 C. H. Bartholomew and W. L. Sorensen, J. Catal., 1983, 81, 131. 21 E. G. M. Kuupers, A. J. H. M. Kock, M. W. C. M. A. Nieuwesteeg and J. W. Geus, J. Catal., 1985, 95, 13. Paper 612486; Received 30th December, 1986