Surface, catalytic, and magnetic properties of small iron particles: The effect of chemisorption of hydrogen on magnetic anisotropy

Abstract

The superparamagnetic behavior of very small particles of metallic iron (ca 1.5 nm), with about half of their atoms at the surface, is changed reversibly by adsorption and desorption of hydrogen below the superparamagnetic transition EXPERIMENTALThe Mossbauer spectrometer, the cells, and the gas handling systems have been described elsewhere (1, 4). The essential feature is the capability of collecting Mossbauer spectra with the sample in flowing palladium-diffused hydrogen or purified helium between 300 and 800 K within ±1 K. Helium was used as a convenient, noncontaminating alternative to high vacuum.The magnetic susceptibility data were obtained by the Faraday method, with an apparatus described elsewhere (5). In brief, a water-cooled electromagnet provided variable fields up to 300 kA-m-(1 G = 103 A-m-') and field gradients up to 3 MA-m-2 (1 Oe = 79.6 A-m-2). The multipurpose gas-handling vacuum system that allowed gassing with H2 or He and evacuation to O-4 Pa between 77 and 720 K. For purification, the H2 was passed through a Deoxo unit, and the He was passed through copper turnings at 500 K. Both gases were subsequently passed through separate molecular sieve (Linde 13X) traps at 77 K. The samples, prepared and characterized as described elsewhere (1), were 1%, 3%, and 8% iron (wt/wt) supported on magnesium oxide with metallic iron dispersions of 0.5, 0.2, and 0.1, respectively (dispersion, D, of the metal is defined as the fraction of the iron atoms that are surface atoms). The corresponding surface average particle sizes were 1.5, 4.0, and 8.0 nm, respectively. The fraction of the iron present as 57Fe for these three samples was 0.26, 0.32, and 0.022, respectively. For convenience these three samples will be referred to as Fe(D = 0.5), Fe(D = 0.2), and Fe(D = 0.1), respectively. RESULTSMossbauer Spectroscopy. A known quantity of the sample Fe(D = 0.5) was reduced in flowing H2 according to the previously described reduction schedule (1). M6ssbauer spectra were then collected in flowing H2 at 298 and 683 K (Fig. 1) and in flowing He at 683 K (Fig. 2). The qualitative features of these spectra reveal two different states of iron in the reduced sample: metallic iron giving rise to the magnetically split spectral component, and Fe2+ in MgO which is responsible for the central spectral doublet (1). In the present study, it is the metallic iron species that is of interest. It represents about half of the total iron in all the reduced samples.The particle size dependence on the above effect of the gaseous atmosphere over the sample was studied by obtaining spectra of sample Fe(D = 0.2). This sample was reduced, and a spectrum was taken at 390 K in flowing H2 with the velocity offset method, which allows the scanning of the two metallic iron peaks that are located the furthest from the zero of velocity (6). This method provides greater sensitivity to change in the metallic iron spectrum than that possible with the constant acceleration mode used to collect the data of Figs. 1 and 2. The sample was t...

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