J . Phys. Chem. 1984, 88, 6191-6198
6191
Characterization of NiFe Alloy Particles Supported on Titania and Alumina: Scanning Transmisslon Electron Microscopy, Magnetic Susceptlbillty, Mossbauer Spectroscopy, and Chemisorption Measurements Xuan-zhen Jiang? Scott A. Stevenson, J. A. Dumesic,* Thomas F. Kelly,f and Richard J. Caspert Department of Chemical Engineering, University of Wisconsin, Madison, Wisconsin 53706 (Received: August 16, 1984) Nickel-iron alloy particles were prepared on titania and alumina via coimpregnation, followed by reduction in hydrogen at 713 and 773 K. The existence of alloy particles was confirmed by scanning transmission electron microscopy coupled with energy dispersive X-ray analysis, magnetic susceptibility, and Mossbauer spectroscopy. The sizes of the NiFe particles were measured by using the above techniques, and these agreed well with the metal sizes estimated from hydrogen desorption measurements. The extent of alloy formation was greater on the titania support compared to that on alumina. This was related to the fact that iron was highly reducible to the zero-valent state on titania, whereas significant amounts of Fe2+ were stabilized on alumina. Increasing the reduction temperature from 713 to 773 K led to an appreciable increase in the size of the NiFe particles supported on titania (from 5 to 10 nm), while the NiFe particles on alumina were apparently more resistant to sintering in hydrogen (increasing from 3 to 4 nm).
Introduction It is now well-known that the catalytic and chemisorptive properties of group 8 metals may be altered when these metals are supported on titania and reduced at temperatures near 750 Perhaps the most useful consequence of these so-called "strong metalsupport interactions" is their effect on the properties of metals for methanation and Fischer-Tropsch synthesis.M For example, compared to conventionally supported nickel catalysts, nickel supported on titania has been found to have a higher activity for methanation, a greater stability under methanation reaction conditions, and an increased selectivity toward higher hydroc a r b o n ~ . ~ . ~The - ~ majority -~~ of the previous studies in this area have focused on the properties of pure nickel supported on titania. The present study involves the attempt to prepare titania-supported NiFe alloy particles. The primary reason for the choice of iron as the metal to be alloyed with nickel was that Mossbauer spectroscopy could then be used to study the metal particles after reduction and under methanation reaction conditions, thereby providing additional information about the nature of the strong metal-support interactions. Low loadings of iron were chosen so that the iron could probe the nickel particles without severely altering their catalytic properties. Accordingly, alloys were studied with Ni:Fe ratios near 1O:l. The techniques used to characterize the NiFe/Ti02 samples prepared in this study were scanning transmission electron microscopy, magnetic susceptibility, Mossbauer spectroscopy, and hydrogen chemisorption measurements. For comparison, samples of nickel-iron supported on alumina were also prepared and characterized. In a related study," the catalytic properties of these titania- and alumina-supported NiFe samples for the methanation reaction have been examined and in situ Mossbauer spectroscopy has been used to study the state of iron in these catalysts under methanation reaction conditions. Compared to alumina-supported samples, the titania-supported NiFe samples exhibit (i) higher methanation activity, (ii) slower deactivation for this reaction, and (iii) greater selectivity to higher hydrocarbons.
Experimental Section Samples of nickel and nickel-iron supported on titania (Degussa P-25) and y-alumina (Davison SMR-7) were prepared by incipient wetness impregnation using nitrate salts. Prior to impregnation, the titania was cleaned of organic contaminants in the manner described by Munuera et a1.12 and Santos et al.;13this procedure involved high-temperature treatment in oxygen and under vacuum Visiting scholar from the Peoples' Republic of China. Permanent address:
De artment of Chemistry, Zhejiang University, Hangzhou, China. !Materials Science Center, University of Wisconsin.
0022-3654/84/2088-6191$01.50/0
followed by exposure to water at 380 K and evacuation for 16 h at the same temperature. In order to facilitate collection of Mossbauer spectra for the supported NiFe samples, 57Fewas used in the catalyst preparation. Specifically, 57Fe-enriched Fe203 powder (86% 57Fe,Oak Ridge National Laboratories) was reduced in Hz at 723 K for 24 h and dissolved in 30 wt % H N 0 3 at room temperature without exposure to air. The pH of the ferric nitrate solution was adjusted by evaporation and addition of distilled water. This was done in view of the possible importance of pH in the preparation of titania-supported metals, as suggested earlier.I3 In particular, it is possible that catalyst preparation at low pH favors the presence of titania species on the surface of titania-supported metal particles. Thus, one series of samples was prepared with an impregnation solution having a pH near zero, while a second series was prepared at a pH near one (the highest pH usable without the precipitation of iron hydroxides). These samples will be designated by the symbols (S) and (W), respectively. Nickel nitrate was then added to the 57Fe-containing solution for coimpregnation of the titania and alumina supports. Approximately 0.25 and 0.60 mL of solution was used per gram of titania and alumina, respectively. Following impregnation, the samples were dried overnight in air at 380 K. The samples were then reduced in flowing hydrogen at 713 K for 2-3 h. After catalyst characterization, as discussed below, the samples were treated in hydrogen for an additional 2 h a t 773 K and characterized again. Nickel loadings were approximately 5 wt % for all samples, while the iron loading was about 0.5 wt %. In particular, the following samples were studied (loadings determined by chemical analysis at Galbraith Laboratories): 5.2% Ni-0.55% Fe/Ti02 (S), 6.1% Ni-0.48% Fe/TiOz (W), 5.8% Ni-0.35% Fe/A1,03 (S), 6.1% Ni-0.49% Fe/Al2O3 (W), and 7.4% Ni/TiOz. Chemisorption Measurements. Chemisorption studies were carried out volumetrically in a glass, high-vacuum system capable (1) Tauster, S. J.; Fung, S. C.; Garten, R. L. J. Am. Chem. SOC.1978, ZOO, 170. (2) Tauster, S. J.; Fung, S. C. J . Caral. 1978, 55, 29. (3) Tauster, S. J.; Fung, S. C.; Baker, R. T. K.; Horsley, J. A. Science 1981, 211, 1121. (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, 7-25
