6004 J. Phys. Chem. 1988, 92, 6004-6009 Magnetic Study of Catalytic Particles: Application to NiKeO, A. Mauger, Groupe de Physique des Solides, ENS, UniversitP Paris VII, zyxwvu 2 Place Jussieu, zyxwv 75007 Paris CPdex 05, France M. Escorne, V. Paul-Boncour, A. Percheron-Guegan,* J. C. Achard, Laboratoire de Chimie MPtallurgique des Terres Rares, CNRS, 1 Place A . Briand, 921 90 Meudon, France and J. Barrault Laboratoire de Catalyse Organique. UniversitP de Poitiers, 40 Avenue du Recteur Pineau, 86022 Poitiers CPdex, France (Received: August 28, 1987) In this work, we present a new method to determine the particle size distribution of nickel-supported catalysts, based on the analysis of magnetic properties beyond the superparamagnetic approximation. Field-cooled magnetization curves of Ni supported on CeO, reduced under hydrogen at two different temperatures (250 and 400 "C) have been measured for T zy < 620 K. Below 500 K, a progressive spin freezing of nickel particles is evidenced by the observation of a remanent magnetization and a deviation of the magnetic susceptibility from the Curie-Weiss law. We show that the blocking of the nickel particles is due to a strong strain anisotropy (3 X lo6 erg ~ m-~) and that the two samples differ by the particle size distribution, although the mean size d is about the same (7.8 nm). We conclude that the difference in the catalytic activity between the two samples cannot be explained by a difference of surface area. 1. Introduction Catalysts of transition metal deposited on rare-earth oxide have been recently investigated in order to provide new catalytic systems for isomerization,' water gas shift reaction,2 hydrogenation re- a~tion,~,~ and automotive exhaust cataly~is.~ Our studies on cobalt and rare-earth oxide system in CO hydrogenation3 showed the influence of the nature of the rare earth on the catalytic properties. We have also tested nickel supported on cerium oxide in this reaction and observed a significant effect of the reduction tem- perature during the preparation of the catalyst on the catalytic properties. When reduced at 250 *C under hydrogen, Ni/Ce02 behaves like a classical Ni/SiOz or Ni/Al2O3 catalyst. However, when the reduction temperature is raised up to 400 "C, the catalysts become inactive and it is necessary to perform the re- action at 400 "C instead of 250 "C to obtain some catalytic properties6 To explain such a decrease of activity, we can invoke either a strong decrease of surface area or a strong interaction between the nickei and the support. The question is then, what is the surface area? H2 and CO chemisorption experiments failed to answer this question, since when the reduction temperature increases, the H2 chemisorption vanishes and the CO chemisorption increases by a factor of 2. Such a result only shows that an interaction of the reactant with CeO, should be taken into account and it becomes impossible to deduce a number of active sites. We therefore tried to determine the particles size distribution by physical method as X-ray dif- fraction (XRD), transmission electron microscopy (TEM), and magnetization measurements. In fact magnetization measure- ments prove to be the only way of investigation of the particles size distribution since XRD spectra revealed that Ni particles are too large to be sizeable and the contrast between nickel and cerium was too weak in TEM images. Since the nickel particles are large in both Ni/Ce02 compounds, the sample is not in the super- paramagnetic phase, even at room temperature, so that previous (1) Le Normand, F.; Girard, P.; Hilaire, L.; Ravet, M. F.; Krill, G.; Maire, G. J. Catal. 1984, zyxwvutsrqpo 89, 1. (2) Mendelovici, L.; Steinberg, M. J. Catal. 1985, 96, 285. (3) Barrault, J.; Guilleminot, A.; Achard, J. C.; Paul-Boncour, V.: Per- cheron-Guegan, A.; Hilaire, L.; Coulon, M. Appl. Catal. 1986, 22, 273. (4) Sudhakar, zyxwvutsrqponm C.; Vannice, M. A. J. Catal. 1985, zyxwvutsrq 95, 227. (5) Yao, H. C.; Yu Yao, Y. F. J. Catal. 1984, 86, 254. (6) Barrault, J.; Guilleminot, A.; Paul-Boncour, V.; Percheron-Guegan, A,; Achard, J. C.; Hilaire, L., to be published. models of superparamagnetism used by other authors to determine the particle sizes' cannot be used in our case. Therefore we determine the particle-size distribution (f( V)) from the analysis of magnetic properties, following the pioneering work by Weil in Cu/Co This paper is organized as follows: in the section 2 we present the theoretical model which describes the spin freezing of the magnetic particles. In section 3, the experimental magnetic properties of our samples are reported. In section 4, the magnetic anisotropy and granulometry of the samples are derived from the fit of the experimental data by the theoretical model. 2. Theory 2.1. Theoretical Background. We first assume that the Ni particles are single magnetic domains; Le., all the Ni atoms that belong to the same particle are spin-polarized along the same axis. For a spherical particle, it is straightforward to show that the single domain is stable provided the diameter d is smaller than a critical value9 d < 9y/2irMs2 where M, is the saturation magnetization per unit volume and y the superficial energy density of a Block wall. Second, we assume the magnetic interaction between Ni par- ticles is negligible. This hypothesis is necessary for the following reason. Magnetic measurements are only sensitive to magnetic clusters. If the magnetic interaction between the Ni particles is negligible, the magnetic clusters are identical with the Ni particles; Le., the magnetic moments of Ni atoms belonging to the same particle are aligned ferromagnetically along a common local quantization axis; however, there is no correlation between the quantization axes of two particles. To the contrary, if the magnetic interaction between Ni particles is significant, the system un- dergoes a transition to a ferromagnetic ordered phase at a finite Curie temperature T,, which depends on the concentration x of Ni atoms in the matrix. The ferromagnetic phase below T,(x) corresponds to a spontaneous alignment of the magnetic moments (7) Bonneviot, L.; Che, M.; Olivier, D.; Martin, G. A,; Freund, E. J. Phys. (8) Weil, L. Colloque National de MagnCrisme: Centre National de la (9) See, for example: Herpin, A. ThCorie du MngnCtisme; Presses Univ- Chem. 1986, 90, 2112. Recherche Scientifique: Paris, 1957; p 147. ersitaires de France: Paris, 1968; p 753. 0022-3654/88/2092-6004$01.50/0 0 1988 American Chemical Society