Langmuir 1988,4, 1261-1265 to be gradual from the results of the fluorescence anisotropy studies.
Acknowledgment. We thank v. Chen for assistance with the fluorescence depolarization measurements and
1261
Dr. F. G. Prendergast of the Mayo Foundation for the use of his fluorescence spectrophotometer. Registry No. DPH, 1720-32-7; TMA-DPH, 115534-33-3; C8C16DAB, 107004-19-3;n-C6H14,110-54-3;n-Cl0HzZ,124-18-5; n-C16&, 544-76-3.
Changes in the Magnetic Properties of Ni/Ti02 Composites with Reduction Temperature: Evidence for the Formation of Ni,TiO, Hans-Conrad zur Loye and Angelica M. Stacy* Department of Chemistry, University of California, Berkeley, California 94720 Received October 23, 1987. I n Final Form: June 16, 1988 The chemisorptive and magnetic properties of nickel supported on titania were investigated for samples with weight loadings between 0.1% and 20%. The composites were prepared by incipient wetness and reduced at 200 and 500 "C. The amount of hydrogen which chemisorbed depended on both the reduction temperature and the nickel loading. For samples with low loadings of nickel, a suppression of hydrogen chemisorption was observed after reduction at 500 O C . In contrast, the hydrogen uptake for 20% Ni/Ti02 was independent of the reduction temperature. The particle sizes of the nickel were determined by measuring the magnetizations of the composites at 200 K in a field of 100 G. After the samples were reduced at 500 "C, the nickel particles sintered to a small degree, but not enough to account for the large decrease in hydrogen uptake. In order to determine the amount of nickel present as the pure metal, the saturation magnetizations of the composites were measured at 5 K in a field of 40 kG. As the reduction temperature was increased, the saturation magnetization decreased for samples with nickel loadings of less than 1%. This suppression of the saturation magnetization, which correlates with the drop in hydrogen chemisorption, indicates a decrease in the amount of ferromagnetic nickel. The change in the bonding of the nickel, which is associated with the loss of ferromagnetism, suggests that the nickel reacts with the Ti02to form a reduced ternary oxide, Ni,TiO,.
Introduction Transition metals dispersed on oxide supports are among the most important heterogeneous catalysts. While the primary function of the oxide support is to increase the surface area of the metal, some supports can cause pronounced changes in the catalytic and chemisorption properties of the metal. In particular, metals supported on reducible transition-metal oxides, such as TiO,, have attracted much due to the observation that the catalytic activity and selectivity can be altered, depending on the temperature at which the metal-Ti02 composite is r e d ~ c e d . ~In, ~this paper, we will show that the change in surface properties of Ni/Ti02 composites is associated with a change in electronic properties. Typically, transition-metal cations are dispersed on oxide supports by incipient wetness or by ion-exchange techniques and subsequently reduced in flowing hydrogen. As the reduction temperature of the metal-Ti02 composite is increased from 200 to 500 OC, the amounts of Hzand CO which chemisorb on the metal a t 25 O C decrease sharply.'p2 This suppression of chemisorption and the associated change in catalytic activity have been attributed to the following effects: (1)the migration of TiO, (x C 1) species on top of the metal particles;6-20(2) an electronic effect due to either cooperative or localized charge trans(3) the reduction of the TiOz ~ ~ p p o r t fer;12J5J6*21-27 due to hydrogen spillover;6~22~29~30 and (4) alloy formation between the metal and the ~ u p p o r t . ' J ~ JIt~is~important ~~ to point out that these four issues are not really inde-
* Author to whom correspondence should be addressed.
pendent.17J9s20*32 However, the majority of recent reports suggest that the migration of TiO, species on top of the ~~
(1) Tauster, S. J.; Fung, S. C.; Garten, R. L. J. Am. Chem. SOC.1978, 100, 170. (2) Tauster, S. J.; Fung, S. C. J. Catal. 1978, 55, 29. (3) Strong Metal Support Interactions; Baker, R. T. K., Tauster, S. J., Dumesic, J. A., Eds.; ACS Symposium Series 298; American Chemical Society: Washington, DC, 1986. (4) Vannice, M. A.; Garten, R. L. J. Catal. 1979, 56, 236. (5) Bartholomew, C. H.; Pannell, R. B.; Butler, J. L. J. Catal. 1980, 65, 335. (6) Jiang, X. Z.; Hayden, T. F.; Dumesic, J. A. J. Catal. 1983,83, 168. (7) Santos, J.; Phillips, J.; Dumesic, J. A. J. Catal. 1983, 81, 147. (8) Raupp, G. B.; Dumesic, J. A. J. Phys. Chem. 1984,88, 660. (9) Jiang, X. Z.; Stevenson, S. A,; Dumesic, J. A. J. Catal. 1985,91,11. (10)Baker, R. T. K.; Chludzinski, J. J.; Dumesic, J. A. J.Catal. 1985, 93, 312. (11) Marcelin, G.; Lester, J. E. J. Catal. 1985, 93, 270. (12) Resasco, D. E.; Haller, G. L. J. Catal. 1983,82, 279. (13) Spencer, M. S. J. Phys. Chem. 1984,88, 1046. (14) Spencer, M. S. J. Catal. 1985, 93. 216.
(15) Kao, C. C.; Tsai, S. C.; Bahl, M. K.'; Chung, Y. W.; Lo, W. J. Surf. Sci. 1980, 95, 1. (16) Chung, Y. W.; Xiong, G.; Kao, C. C. J. Catal. 1984, 85, 237. (17) Takatani, S.; Chung, Y. W. J. Catal. 1984, 90, 75. (18) KO,C. S.; Gorte, R. J. J. Catal. 1984, 90, 59. (19) Tau, L. M.; Bennett, C. 0. J. Catal. 1984, 89, 285. (20) Simoens, A. J.; Baker, R. T. K.; Dwyer, D. J.; Lund, C. R. F.; ~Madon, ~ ~R. JJ. J. ~ Catal. ~ ~1984,86, ~ ~ 359. ~ (21) Raupp, G. B.; Dumesic, J. A. J. Catal. 1986, 97, 85. (22) Conesa, J. C.; Malet, P.; Munuera, G.; Sanz, J.; Soria, J. J. Phys. Chem. 1984,88, 2986. (23) Sexton, B. A.; Hughes, A. E.; Foger, K. J. Catal. 1982, 77, 85. (24) Herrmann, J. M. J. Catal. 1984, 89, 404. (25) Horsley, J. A. J. Am. Chem. SOC. 1979, 101, 2870.
0743-7463/88/2404-1261$01.50/0 0 1988 American Chemical Society
1262 Langmuir, Vol. 4 , No. 6, 1988
zur Loye and Stacy
metal particles is the main cause for the dramatic change exciting. A t 500 "C, nickel does not react with TiOz in surface chemistry. powder to form a bulk ternary phase. In fact, reactions Although the four effects listed above often are discussed between two solids to form thermodynamically stable as separate models, they may all play a role in producing phases usually do not occur a t these low temperatures the observed changes in the chemistry of metal-Ti02 because of the large energy of activation for diffusion. Our catalysts. It has been shown that the TiO, species are results suggest that if the area of contact between the formed due to hydrogen spillover from the metal to the reactants is large, it is possible to form thin films of new support followed by reduction of the support.6,22~29*30 Once metastable ternary oxides. Studies of such reactions are formed, the reduced titanium oxide is thought to migrate not only important for the design of new catalysts but also on top of the metal particles, thereby blocking the metal for producing coatings and electronic devices, as well as from interacting with adsorbate^.^-'^ However, it is posunderstanding adhesion. sible that these TiO, species also cause a chemical efThe results of our study of how the surface and bulk f e ~ t by~mixing ~ J with ~ the ~ metal ~ ~ particles;18 ~ ~ ~ the reproperties of Ni/Ti02 catalysts vary as a function of action of the metal and TiO, may form an alloy1 with very loading and reduction temperature are presented below. different electronic properties from the pure metal. BeComposites with nickel loadings of 0.1-20.0% were precause it is difficult to obtain clear experimental evidence, pared by incipient wetness techniques. The surface and the importance of electronic effects and the possibility of magnetic properties of the catalysts were measured after the formation of an alloy are debated. reduction a t 200 and 500 "C. Hydrogen chemisorption There have been various studies of metals supported on measurements were used to follow changes in the surface Ti02that have used techniques sensitive to bulk electronic chemistry of the nickel. We determined the particle size properties, such as magnetic s ~ s c e p t i b i l i t y , l ~ferro~ ~ ~ - ~ ~ of the metal and the amount of nickel present as pure magnetic resonance,20and Mossbauer s p e c t r o s ~ o p y . ~ J ~ nickel ~ ~ ~ metal using magnetism. By correlating the results In most of these investigation^,'*"^^^^^^^^^*^^ little or no deof these measurements, the effects related to reaction of viation in the properties of the supported metal compared the metal with the support could be distinguished from with the properties of the bulk metal were observed. changes in particle size. Therefore, it is tempting to conclude that only the surface Experimental Section of the metal particles is affected by the Ti02 support. However, in all of these studies, only samples with relaMaterials Preparation. Samples of nickel dispersed on TiOz tively large metal loadings (15%) were examined. We and SiOz supports were prepared by incipient wetness techniques. suggest that in such samples the amount of metal affected Approximately 2-4 g of the metal oxides was slurried with 1mL of a solution of reagent grade Ni(NO&6H20 (Mallincroft, 99.95% by the support relative to the total amount of metal pure) dissolved in distilled, deionized water. The concentration present is small, and, therefore, it is difficult to detect of the nickel solution was adjusted t o prepare samples with electronic effects. Furthermore, a strong interaction beloadings from 0.1% to 20.0%. After adding 3-4 mL of acetone, tween the metal and the support is more pronounced on the slurry was ground until dry and then further dried under well-dispersed catalyst^;'^^^^ the best dispersions are obvacuum for 12 h. The TiOz (Cerac, 99.9% pure) used had a BET tained for catalysts with low metal loadings. surface area of 5.1 m2/g and consisted of 10-15% rutile and We chose to study the magnetism of Ni/Ti02 composites 85-90% anatase as estimated from the intensities of the powder as a function of nickel loading to determine the importance X-ray diffraction lines. Several samples employing SiOz (AESAR, of electronic effects. The ferromagnetism of the nickel 99.5% pure), with a surface area of 162 m2/g, were prepared for comparison. provides an excellent probe of changes in the electronic Two reaction temperatures t o reduce the Ni2+ t o Nio were properties of the composites. The results presented below studied: a low-temperature reduction a t 200 "C and a highshow that an electronic change does occur for Ni/Ti02 as temperature reduction at 500 "C.During the low-temperature a function of reduction temperature and is particularly reduction, the samples were treated in flowing hydrogen at 200 pronounced for nickel loadings below 2%. After reduction "C for 20 h and then evacuated at 200 "C for 30 min and finally at 500 "C, we find that a large fraction of the nickel is not cooled slowly to room temperature under dynamic vacuum. For present as pure nickel metal. This result confirms the the high-temperature reduction, the samples which had been report by Simoens et al. that the ferromagnetic resonance reduced previously a t 200 "C were reduced for 1 h in flowing intensity of Ni/Ti02 is only half of its previous value after hydrogen at 500 OC and then evacuated at 500 "C for 15 min before reduction a t 550 0C.20 We propose that the nickel and cooling to room temperature under dynamic vacuum. Typically, 2-4-g samples were reduced in a flow-through quartz cell with TiO, react to form a reduced ternary oxide, Ni,TiO,. a flow rate of 100 cm3 of Hz/min, in the vacuum system described From the standpoint of the synthesis of new materials, below. the possibility of producing new ternary oxides by relaChemisorption Measurements. A glass vacuum system tively low temperature reactions at the interface between equipped with grease-free valves was used for surface area a transition metal and a reducible metal oxide is quite measurements and hydrogen chemisorption studies. The system connected to an oil diffusion pump, backed by a rough pump, and could achieve a dynamic vacuum of about l0-B Torr. Pressures were measured to 0.5% accuracy by using a Vacuum General capacitance manometer. The BET surface areas of the supports were determined by using argon at liquid nitrogen temperature. In the calculation, a cross section of 0.146 nm2/Ar and a value of 210 Torr for Po were assumed. Prior to use, the argon gas (Matheson, 99.999% pure) was passed through an Oxysorb cartridge (tradename, Messer Griesheim Industries) to remove traces of oxygen. Changes in the surface chemistry of the nickel as a function of the reduction temperature were monitored by the chemisorption of hydrogen. The chemisorption measurements were carried out at 25 O C in a 0-250-Torr pressure regime. In order to allow for slow uptake of hydrogen, the samples were equilibrated with each new pressure of hydrogen for 60 min. The H / N i ratios, moles WBS
(26) Horsley, J. A. J . Catal. 1984, 88, 549. (27) Henrich, V. E. J . Catal. 1984,88, 519. (28) Baker, R. T. K.; Prestridge, E. B.; Garten, R. L.J. Catal. 1979, 56. 390. ~(29) DeCanio, S. J.; Miller, J. B.; Michel, J. B.; Dybowski, C. J . Phys. Chem. 1983,87, 4619. (30) Huizinga, T.; Prins, R. J. Phys. Chem. 1981, 85, 2156. (31) Mustard, D. G.; Bartholomew, C. H. J. Catal. 1981, 67, 186. (32) Dumesic, J. A.; Stevenson, S. A.; Sherwood, R. D.; Baker, R. T. K. J . CataE. 1986, 99, 79. (33) Von Engels, S.; Morke, W.; Wilde, M.; Roschke, W.; Freitag, B.; Siegel, H. Z.Anorg. Allg. Chem. 1981, 472, 162. (34) Jiang, X. Z.; Stevenson, S. A.; Dumesic, J. A,; Kelly, T. F.; Casper, R. J. J. Phys. Chem. 1984,88, 6191. 135) zur Loye, H. C.; Stacy, A. M. J. Am. Chem. SOC.1985,107,4567. (36) KO,E. I.; Winston, S.; Woo, C. J . Chem. SOC.,Chem. Commun. 1982, 740. ~
Langmuir, Vol. 4, No. 6,1988 1263
Magnetic Properties of NilTi02 us Reduction Temperature
o"21A O
.
l Ni/Ti02
~
o reduced at 200% reduced a t 500T
.z _ 0. I 0
OLtt 0
4
I
I
I
I
I
1
2
3
4
5
A
v
W t % Loading of NI
I
18
I
19
20
0
on T i 0 2
0
e
Figure 1. Dependence of Hzchemisorption at 25 "C on the wt % loading of Ni supported on TiOzand on the reduction tem-
perature of the composite. Open circles are for Ni/TiOz samples reduced at 200 "C and closed circles for reduction at 500 "C. The arrows indicate that for 0.1% Ni/TiOZreduced at 200 and 500 "C and for 0.3% Ni/TiOz reduced at 500 "C the hydrogen chemisorption was below the detection limit of the apparatus. of H atoms absorbed per mole of nickel added to the support, were calculated by extrapolationof the high-pressure linear portion of the isotherm back to zero pressure; this extrapolation to zero pressure gives a means of comparingthe saturation values for the various samples. Prior to use in reduction or chemisorption, the hydrogen gas (Matheson, 99.999% pure) was passed through a 4A molecular sieve drying trap. Magnetic Measurements. The magnetizations of the samples were measured with a SHE SQUID magnetometer at temperatures ranging from 1.7 to 280 K and in magnetic fields from 10 G to 40 kG. In order to avoid contamination with oxygen and water, the samples were transferred under vacuum from the chemisorption line directly to a Vacuum Atmospheres drybox. The samples were loaded inside the drybox into a container made of Kel-F (tradename,3M Co.) and sealed under argon. We note that we have used this method on highly air-sensitive, flammable organometallic samples with good success. The magnetizations reported have been corrected for the diamagnetism of the container, which is typically 2-3 orders of magnitude smaller than the sample magnetization.
Results Hydrogen Chemisorption. The amount of hydrogen which chemisorbed at 25 "C on Ni/Ti02 composites showed that the surface chemistry of these materials depends on both the reduction temperature and the nickel loading as shown in Figure 1. First consider the effect of the reduction temperature. We observed a suppression of hydrogen chemisorption as the reduction temperature of the composite was increased from 200 to 500 "C. For example, for a nickel loading of 0.5%, the H/Ni value of 0.096 mol of H/1 mol of Ni for the 200 "C reduction decreased to 0.012 for the sample reduced at 500 "C. In contrast, the amount of hydrogen which chemisorbed at 25 "C on 0.5% Ni/Si02 showed the opposite trend; the H/Ni ratio for the sample reduced a t 200 "C was 0.129 while H/Ni a t 500 "C increased to 0.210. Next, consider the effect of the nickel loading on H/Ni. For the samples reduced at 200 "C, H/Ni increased sharply with increasing nickel loading. As the nickel loading is increased above 1.070, a decrease in H/Ni is observed. Samples reduced at 500 "C showed a gradual increase in chemisorption from a value below the detection limit of the apparatus for 0.1% Ni/Ti02 to H/Ni = 0.030 for 20.0% Ni/TiOz. For nickel loadings of 20.0%, the chemisorption values observed at both reduction temperatures were equivalent.
u
.
o
.
0
o .
.
e
E
8 ) 1.0% N i / T i O g
0
'\reduced
10 -
.
.
at 500°C
C ) 5.0% Ni/TiOp
Figure 2. Magnetization,G/g of Ni, versus the applied magnetic field measured at 5 K for (A) 0.5% Ni/TiOz, (B)1.0% Ni/TiOz, and (C) 5.0% Ni/TiOz composites. Open circles indicate that the sample was reduced at 200 "C, and closed circles are for 500 "C reduction. The magnetization of a sample of pure nickel powder is included in A for comparison.
Magnetization vs Field: Determination of the Amount of Elemental Nickel. Since nickel is ferromagnetic, the saturation magnetization M , is a measure of the amount of nickel present as elemental nickel. By measuring M , for the Ni/TiOz composites and comparing the value observed with that expected if all the nickel were present as elemental nickel, 57.5 G/g of Ni,37changes in the electronic properties of the nickel as a function of loading and reduction temperature can be monitored. Figure 2 shows the magnetization at 5 K versus applied field for nickel loadings of 0.5%, 1.0%, and 5.0% Ni on TiOz. Data for a sample of nickel powder (Puratronic, 99.999% pure, 100 mesh) are shown for comparison. M , is calculated from these curves by extrapolation of the magnetization at fields greater than 25 kG to zero field. In order to determine M, accurately, it is important to use low temperatures (near 4.2 K) and high fields (in excess of 25 kG); only under these conditions are all the moments aligned with the applied field such that M , is a good measure of the number of spins, which is related to the number of nickel atoms.38 Often M , is calculated by plotting M , versus 1/H and extrapolating to infinite field.38 This method was not (37) Kittel, C. Introduction to Solid State Physics; Wiley: New York, 1976. (38) Selwood, P. W. Chemisorption and Magnetism; Academic: New York, 1975; Chapter 4.
zur Loye and Stacy
1264 Langmuir,Vol. 4, No. 6, 1988 Table 1. Magnetic Data for Ni/Ti02 Samples M,(5 K)/ M(200 K)/ (a2/u)X 1018cm3 d, A % Ni g o f Ni g of Ni Samples Reduced at 200 "C 28.4 0.517 1.49 36 0.1 45 26.2 0.962 3.01 0.3 46 31.7 1.23 3.18 0.5 71 27.1 3.97 12.0 1.0 103 39.0 17.2 36.2 2.0 29.0 5.0 29.0 20.0 0.1 0.3 0.5 1.0 2.0 5.0 20.0
Samples Reduced at 500 "C 10.1 0.384 3.14 19.0 2.03 8.76 14.8 2.33 12.9 28.2 9.77 28.4
46 64 73 95
49.0 51.5
chosen because it exaggerates the magnetization due to paramagnetic impurities and may give a value for MEwhich is too high. In our samples, unreduced Ni2+and any Ti3+ species might be paramagnetic. On the other hand, if the 25-kG field is not high enough to saturate the magnetic moments, then the extrapolation to zero field will give a value for M Ewhich is too low. We feel that the zero field value, in most cases, will be more accurate since 25 kG is usually a large enough field to reach saturation. The values obtained for MEindicate that the amount of nickel present as pure nickel metal depends on the reduction temperature and the nickel loading. Figure 2B shows, for 1.0% Ni/Ti02, that M Eis roughly the same for the sample reduced at 200 "C,ME(200)= 26 G/g of Ni, as that for the sample reduced at 500 OC, ME(500)= 27 G/g of Ni. For 5.0% Ni/TiOz, shown in Figure 2C, ME(200) = 29 G/g of Ni is less than ME(500)= 49 G/g of Ni. The opposite trend is observed for the 0.5% sample as shown in Figure 2 4 M,(200) = 32 G/g of Ni is larger than M,(500) = 15 G/g of Ni. The saturation magnetization data for all the samples measured are summarized in Table I. The general trend is that for high loadings of nickel ME(500) is larger than ME(200),while for low loadings the opposite holds true, with ME(200)larger than ME(500).While M,(200) is approximately equal to 30 G/g of Ni, independent of the nickel loading, ME(500)increases with increasing nickel loading. Magnetization vs Temperature: Determination of Particle Size. Small ferromagnetic particles show a phenomenon known as superparamagnetism,under certain conditions of temperature and field.38 From the superparamagnetic behavior it is possible to determine the particle sizes of the nickel for the various composites prepared by using the Langevin low-field method described by Selwood.38 For uniform particles at low fields and high temperatures, the average particle volume U2/u is obtained from the initial slope of the curve of the magnetization M plotted with respect to the applied field H as follows: u2 3kTM - =u ISpHMS where IEpis the spontaneous magnetization and M Eis the saturation magnetization, both determined at the temperature T used for the measurement. In our studies, the magnetizationM was determined at 200 K in a field of 100 G. This low field was chosen to measure M because the criterion for superparamagnetism is that magnetization vs field/temperature curves superimpose. Another way to state this criterion is that the magnetization must be linear with field in order for all the particles to be in the su-
nl 5.01
,
I
,
I
I
I
4.0
0
0.5% N i / T i 0 2 o r e d u c e d at 200°C b reduced at 500OC
0
01
02
03
04
05
06
07
Field (kG)
Figure 3. Magnetization, G/g of Ni, versus the applied magnetic field at low fields for 0.5% Ni/Ti02. Open circles indicate that the sample was reduced at 200 "C, and closed circles are for reduction at 500 O C . Only below -200 G are the curves linear, indicative of superparamagnetism.
perparamagnetic regime. As shown in Figure 3 for a sample of 0.5% Ni/TiOP,a magnetization which is linear with field is observed only in fields below -200 G at a temperature of 200 K. Above -200 G, the larger particles of nickel begin to saturate, and therefore the sample is no longer entirely in the superparamagnetic regime. Since we have determined M in a region where the entire sample is superparamagnetic, the particle sizes reported are an upper limit. Table I lists the particle sizes for samples with various nickel loadings after reduction at 200 and 500 "C. For the calculations, a value of Isp= 505 G/g of Ni at 200 K was used;37the magnetization M a t 200 K in a field of 100 G and the saturation magnetization M Eat 5 K in a field of 40 kG used are listed in Table I. For the samples studied, it was not possible to completely saturate the nickel particles at 200 K in the highest fields available on our magnetometer. Therefore, we chose to use the value for M , which we obtained at 5 K and 40 kG as a good estimate of M , at 200 K. The diameters were calculated from the volumes obtained, assuming that the particles are spheres. The particle size increases in going from the 200 to the 500 "C reduction temperature as well as in going to higher loadings. Particle sizes for loadings larger than 2% are not given since in these samples the nickel magnetism does not meet the criteria for superparamagnetism given above.
Discussion The results presented above show that for Ni/TiOz composites the surface chemistry and the electronic properties of the nickel change as a function of nickel loading and reduction temperature. These two parameters affect the particle size of the nickel and the degree of interaction between the nickel and the TiO> In this study, both the hydrogen chemisorption and the magnetism of the various composites were measured in order to distinguish effects related to particle size, such as sintering and incomplete reduction of Ni2+,from effects due to the interaction between nickel and Ti02. We measured the amount of hydrogen chemisorption to follow changes in the surface chemistry of the nickel due to an interaction with the support. However, since the hydrogen chemisorption is influenced also by the degree of dispersion, we determined the particle size of the nickel using magnetism. By measuring the saturation magnetization, we have determined the amount of nickel present as pure nickel metal in each sample. In the following discussion, we will correlate the changes in hydrogen chemisorption with changes in magnetism, first as a function of loading for a given
Magnetic Properties of Ni/Ti02 us Reduction Temperature
Langmuir, Vol. 4, No. 6, 1988 1265
reduction temperature and then as a function of reduction temperature. Before examining how the properties vary as the nickel loading or the reduction temperature is increased, it is important to consider the following general points about supported nickel catalysts. First of all, studies of nickel on various supports have shown that 200 "C may not be high enough to reduce all the Ni2+to NiO; in particular, the temperature needed to reduce Ni2+may increase as the particle size gets smaller in the samples with lower nickel loading^."*^^*^^ A second point is that in contrast to platinum, nickel does not disperse as well; in addition, there is some evidence that sintering begins near 500 0C.20,21Finally, since the Ti02 used in our study has a surface area of only 5.1 m2/g, a loading of roughly 1.0% Ni is enough to completely cover the surface of the support with nickel atoms. We will show in the following discussion that by monitoring changes in the magnetism of the nickel, effects due to an interaction with the support can be distinguished from changes in particle size due to sintering or incomplete reduction. As is typical for group VI11 metals supported on Ti02, we observed a suppression of hydrogen chemisorption for Ni/Ti02 composites reduced at 500 "C compared with those reduced at 200 "C, but only for nickel loadings below 20.0%. At high nickel loadings, the amount of hydrogen chemisorptionis independent of the reduction temperature as is expected for bulk nickel; very large particles of nickel metal are not affected by the Ti02 support. This result confirms other reports that suggest that it is difficult to produce a strong interaction between the metal and support for large metal particle^.^^^^^ We will use the properties of the 20.0% Ni/Ti02 sample as a reference point to which the properties of the samples with lower loadings can be compared. First consider the samples reduced at 200 OC for 20 h. In all of these samples, the saturation magnetization is about half of the value of 57.5 G/g of Ni expected if all the nickel were present as pure nickel metal. We attribute this suppressed magnetization to incomplete reduction of Ni2+ at 200 "C, as has been found by other investigat o r ~ . It~is~interesting * ~ ~ ~to~note ~ that about half of the Ni2+ is reduced, independent of the particle size of the nickel, which increases as the loading increases. Therefore, within the range of particle sizes studied and for a relatively long reduction time, the amount of nickel reduced does not depend on dispersion. A maximum in the number of moles of hydrogen chemisorbed per mole of nickel is observed for samples reduced at 200 "C with nickel loadings near 1.0%. As the nickel loading is increased above 1.070, the chemisorption per mole of nickel decreases. This decrease in chemisorption can be attributed to a change in dispersion since the particle size of the nickel is found to increase with loading. Curiously, for nickel loadings below LO%, the chemisorption is suppressed as the loading decreases even though the number of surface atoms increases. We suggest that for these samples with small nickel particles, an interaction with the Ti02 support occurs already at reduction temperatures as low as 200 "C. This is consistent with other reports that suggest that the metal-support interaction is more pronounced for well-dispersed catalyst^.'^^^^
Next, consider the samples reduced at 500 "C for 1 h. An increase in the particle size of the nickel is observed for these samples compared with those reduced at 200 OC for the same nickel loading. The increase in particle size is due to the reduction of the remaining Ni2+to NiO at 500 "C and perhaps to a small amount of sintering.20.21The hydrogen chemisorption per mole of nickel after reduction at 500 "C increases with increasing loading, despite the fact that the particle size also increases. We suggest that smaller particles chemisorb less hydrogen because there is a stronger interaction of the metal with the support compared with the samples containing larger nickel particles. Since 500 "C is high enough to reduce all the Ni2+,39the saturation magnetization should increase to 57.5 G/g of Ni, the value for bulk nickel. A value close to this is obtained for the samples with 5.0% and 20.0% Ni, in agreement with other studies of the magnetization of Ni/Ti02 samples with loadings greater than 5 % However, as the loading is decreased below 5.0%, the saturation magnetization decreases; for samples with loadings below 1.0%,M,(500) is less than M,(200).35 Since it is not possible that the samples reacted in hydrogen at 500 "C are reduced less than those reacted at 200 "C, the suppressed magnetization indicates that much of the nickel (up to 80% for 0.1% Ni/Ti02) is not present as pure nickel metal. We also note here that for samples reduced for 20 h at 500 "C, a further decrease in M,was observed. Since this loss of bulk nickel after the high-temperature reduction correlates well with the suppression of hydrogen chemisorption, we conclude that the change in surface properties of the nickel is associated with a change in the form of the nickel present in the composite. To account for a change in the form of the nickel and explain the breaking of nickel-nickel bonds associated with the loss of ferromagnetism, we propose that the nickel diffuses a few layers into the TiO, moieties. It is possible to argue that the loss of ferromagnetismis due to a wetting of the Ti02 surface by a monolayer of nickel atoms. However, the surface area of the support used is only 5 m2/g, just large enough to accommodate one monolayer of nickel for a 1% loading. Since a nickel film which is only a few layers thick shows no deviation in the saturation magnetization from that of bulk nickel,4l it is difficult to believe that the suppressed magnetization is due simply to the nickel wetting the Ti02 surface. We suggest that a more plausible explanation of the decrease in saturation magnetization is that the nickel reacts with the support to form a reduced ternary oxide, NiyTiO,, which is kinetically stable and not ferromagnetic. The formation of a NiyTiO, alloy would explain also the observed change in surface chemistry of the composite. This model of a reaction between the TiO, moieties and the nickel is similar to the recent suggestion that the TiO, species cause a chemical effect in addition to simply blocking surface sites by migrating onto the nickel particle^.^^^^^
(39)Robertson, S. D.; McNicol, B. D.; deBaas, J. H.; K l o e t m Jenkins, J. W. J. Catal. 1975, 37, 424. (40) Bartholomew, C. H.; Pannell, R. P.; Butler, J. L. J. Catal. 1980, 65, 335.
.11933p34
Acknowledgment. This research is supported by a Presidential Young Investigator Award to A.M.S. from the National Science Foundation (Grant no. CHE83-51881) and matching funds from E. I. du Pont de Nemours and Company. Registry No. Ni, 7440-02-0;TiOz,13463-67-7;H2, 1333-74-0. (41) Tersoff, J.; Falicov, L. M. Phys. Rev B: Condem. Matter 1982, B62, 6186.
