Steen Mørup

In situ Mössbauer emission spectroscopy studies of unsupported and supported sulfided CoMo hydrodesulfurization catalysts: Evidence for and nature of a CoMoS phase

Information regarding the structure and type of phases present in sulfided alumina-supported, as well as unsupported, CoMo catalysts is obtained from in situ Mossbauer emission spectroscopy (MES) studies. The results give the first direct evidence of the presence of a CoMoS phase in alumina-supported and unsupported catalysts with similar CoMo ratios. Information regarding the nature of the CoMoS phase is obtained from a detailed study of the Mossbauer parameters, their temperature dependence and their sensitivity to changes in the gaseous environment. Previously proposed structural models cannot explain all the observed features of the CoMoS phase. It is proposed that in alumina-supported catalysts the CoMoS phase is present as single SMoS slabs (i.e., one layer of the MoS2 structure) with cobalt most likely present at molybdenum sites. For unsupported catalysts the CoMoS phase consists of several slabs with bulk MoS2-like structure. The present observations suggest that the previously observed similarities between supported and unsupported catalysts are associated with the presence of the CoMoS phase in both catalyst systems. A possible reaction mechanism for hydrodesulfurization involving cobalt in the CoMoS phase is proposed. Information regarding other phases in the catalysts is obtained by MES studies of CoAl2O3, CoMo2S4, Co9S8, CoS2, Co3S4, and CoS1 + x samples. It is observed that for a sulfided CoMoAl2O3 catalyst with a composition typical of those used industrially, part of the cobalt is located in the alumina. For unsupported catalysts the effect of changing the cobalt concentration and preparation method is investigated and it is observed that under certain conditions also the thermodynamically stable cobalt sulfide, Co9S8, may be formed.

On the catalytic significance of a CoMoS phase in CoMoAl2O3 hydrodesulfurization catalysts: Combined in situ Mössbauer emission spectroscopy and activity studies

Abstract A series of sulfided CoMo Al 2 O 3 catalysts with different Co Mo ratios but with constant molybdenum content is investigated. The catalysts are characterized by in situ Mossbauer emission spectroscopy (MES) and investigated for their thiophene hydrodesulfurization activity. The catalytic activity shows a pronounced maximum at a Co Mo ratio of about 1.0. The MES spectra reveal that cobalt may be present in three distinctly different phases: cobalt located in the alumina lattice (Co:Al 2 O 3 ), cobalt in Co 9 S 8 , and cobalt located in the CoMoS surface phase discussed in the preceding paper. It is found that the relative amounts of the three phases depend strongly on the Co Mo ratio. The Co:Al 2 O 3 phase and the CoMoS phase are observed in all catalysts studied, whereas Co 9 S 8 is observed only in catalysts with Co Mo ≳ 0.4 . It is shown that the presence of Co 9 S 8 cannot explain the promoting role of cobalt in the CoMo Al 2 O 3 catalysts. However, a linear relation between the catalytic activity and the amount of Co in the CoMoS phase leads to the conclusion that the promoting effect of cobalt is associated with the presence of this phase.

Magnetic hyperfine splitting in mössbauer spectra of microcrystals

Abstract Below the superparamagnetic blocking temperature of a microcrystal the magnetization direction is in general not fixed, but fluctuates in directions close to an easy direction of magnetization. Such fluctuations (collective magnetic excitations) result in a reduction in the magnetic splitting of the Mossbauer spectrum. A low-temperature approximation for this reduction is derived for microcrystals with arbitrary magnetic energy. Moreover, explicit expressions are presented for particles with special types of magnetic anisotropy and for particles exposed to external magnetic fields. The reduction in the magnetic hyperfine field has its maximum value just below the blocking temperature but does not exceed 5–15% in isolated particles with uniaxial anisotropy in zero applied magnetic field. However, ferro- and ferrimagnetic particles, exposed to external magnetic fields, and particles for which exchange anisotropy is predominant, may exhibit any magnetic hyperfine splitting between zero and the saturation value. It is shown that in special cases an assembly of microcrystals in close contact with each other may behave like a spin-glass. We discuss how studies of the magnetic hyperfine splitting ofMossbauer spectra of microcrystals give information of the particle size and the prevailing magnetic anisotropies.

A new interpretation of Mössbauer spectra of microcrystalline goethite: “Super-ferromagnetism” or “super-spin-glass” behaviour?

Abstract The temperature dependence of the magnetic hyperfine field of microcrystalline goethite has been studied in detail and the results have been compared to the behaviour of well-crystallized goethite. It is found that the magnetic properties of the microcrystalline goethite can not be described by existing theories for collective magnetic excitations and superparamagnetic relaxation. However, a model, in which the magnetic interaction among the particles is taken into account, may explain the results. Specifically, a modified Weiss mean field theory for interacting particles gives an excellent fit to the results. This model, which is denominated “super-ferromagnetism”, gives a significantly better fit than a “super-spin-glass” model, in which a Gaussian distribution in the magnetic coupling constants is assumed. These models for interacting crystallites also suggest that an apparent reduction in the magnetic transition temperature may be observed in other fine-grained polycrystalline materials.

Mössbauer studies of thermal excitations in magnetically ordered microcrystals

A simple model is developed to explain the often-observed diminished values of the magnetic hyperfine fields in microcrystals. It is shown that below the superparamagnetic blocking temperature thermally excited oscillations of the magnetization around an energy minimum reduce the average magnetization and the magnetic hyperfine splitting in the Mössbauer spectrum. In a microcrystal of volumeV these quantities are reduced by a factor of about 1−kT/(2κV), wherekT is the thermal energy and κ is related to the anisotropy constant. Mössbauer spectra of 60 Å, 100 Å, and 120 Å Fe3O4 particles and 120 Å α-Fe2O3 particles show excellent agreement with the theory.

Models for the dynamics of interacting magnetic nanoparticles

Abstract A critical review of models for the dynamics of interacting magnetic nanoparticles is given. It is shown that the basic assumptions in the Dormann–Bessais–Fiorani model are unrealistic. The experimental observations on systems of interacting magnetic nanoparticles can, at least qualitatively, be explained by the model derived by Morup and Tronc for weakly interacting particles, in combination with a transition to an ordered state in the case of strong interactions.

Magnetic interactions between nanoparticles

Summary We present a short overview of the influence of inter-particle interactions on the properties of magnetic nanoparticles. Strong magnetic dipole interactions between ferromagnetic or ferrimagnetic particles, that would be superparamagnetic if isolated, can result in a collective state of nanoparticles. This collective state has many similarities to spin-glasses. In samples of aggregated magnetic nanoparticles, exchange interactions are often important and this can also lead to a strong suppression of superparamagnetic relaxation. The temperature dependence of the order parameter in samples of strongly interacting hematite nanoparticles or goethite grains is well described by a simple mean field model. Exchange interactions between nanoparticles with different orientations of the easy axes can also result in a rotation of the sub-lattice magnetization directions.

Estimation of blocking temperatures from ZFC/FC curves

We present a new method to extract the parameters of a log-normal distribution of energy barriers in an assembly of ultrafine magnetic particles from simple features of the zero-field cooled and field cooled magnetisation curves. The method is established using numerical simulations and is tested on two experimental data sets.

Experimental and theoretical studies of nanoparticles of antiferromagnetic materials

The magnetic properties of nanoparticles of antiferromagnetic materials are reviewed. The magnetic structure is often similar to the bulk structure, but there are several examples of size-dependent magnetic structures. Owing to the small magnetic moments of antiferromagnetic nanoparticles, the commonly used analysis of magnetization curves above the superparamagnetic blocking temperature may give erroneous results, because the distribution in magnetic moments and the magnetic anisotropy are not taken into account. We discuss how the magnetic dynamics can be studied by use of magnetization measurements, Mossbauer spectroscopy and neutron scattering. Below the blocking temperature, the magnetic dynamics in nanoparticles is dominated by thermal excitations of the uniform mode. In antiferromagnetic nanoparticles, the frequency of this mode is much higher than in ferromagnetic and ferrimagnetic nanoparticles, but it depends crucially on the size of the uncompensated moment. Excitation of the uniform mode results in a so-called thermoinduced moment, because the two sublattices are not strictly antiparallel when this mode is excited. The magnetic dipole interaction between antiferromagnetic nanoparticles is usually negligible, and therefore such particles present a unique possibility to study exchange interactions between magnetic particles. The interactions can have a significant influence on both the magnetic dynamics and the magnetic structure. Nanoparticles can be attached with a common crystallographic orientation such that both the crystallographic and the magnetic order continue across the interfaces.

Superparamagnetism and Spin Glass Ordering in Magnetic Nanocomposites

Mossbauer spectra of samples containing nano-sized magnetic particles exhibit a transition from a magnetically split spectrum to a doublet or singlet when the temperature is varied, and zero-field-cooled magnetization measurements show a maximum at a certain temperature. When the interactions are weak these features can be explained by superparamagnetic blocking, but in the case of strong interactions they can be explained by ordering of the magnetic moments. A phase diagram, which elucidates these possibilities, is presented.