M.J. Rossiter

Bio: M.J. Rossiter is an academic researcher. The author has contributed to research in topics: Absorption spectroscopy & Mössbauer spectroscopy. The author has an hindex of 1, co-authored 1 publications receiving 69 citations.

Operando Analysis of NiFe and Fe Oxyhydroxide Electrocatalysts for Water Oxidation: Detection of Fe4+ by Mössbauer Spectroscopy

Abstract: Nickel-iron oxides/hydroxides are among the most active electrocatalysts for the oxygen evolution reaction. In an effort to gain insight into the role of Fe in these catalysts, we have performed operando Mossbauer spectroscopic studies of a 3:1 Ni:Fe layered hydroxide and a hydrous Fe oxide electrocatalyst. The catalysts were prepared by a hydrothermal precipitation method that enabled catalyst growth directly on carbon paper electrodes. Fe(4+) species were detected in the NiFe hydroxide catalyst during steady-state water oxidation, accounting for up to 21% of the total Fe. In contrast, no Fe(4+) was detected in the Fe oxide catalyst. The observed Fe(4+) species are not kinetically competent to serve as the active site in water oxidation; however, their presence has important implications for the role of Fe in NiFe oxide electrocatalysts.

Mössbauer Effect in α-Fe2O3

Abstract: The antiferromagnetic compound α-FeOOH is studied between 90 and 440°K by the Mossbauer technique. Analysis of the data gives a Neel temperature TN of 393.3°K and an effective magnetic field, extrapolated to 0°K, of 510 kOe. From the quadrupole splitting above and below TN and from the field gradient, calculated from the lattice positions, it is concluded that the direction of the spins of the iron ions lies along the crystallographic z-axis. This is in agreement with susceptibility measurements. The shape of the peaks in the Mossbauer spectra is discussed and possible explanations of the shapes in the vicinity of TN are given. The temperature dependence of the sublattice magnetization is compared with the predictions of the two-particle cluster theory for a Heisenberg magnet. If the reduced sublattice magnetization is represented as m(T) = D(1 − T/TN)f it is found that f = 0.32. Finally, a comparison is made with other experimental Mossbauer data on goethite and the discrepancies are explained. Die antiferromagnetische Verbindung α-FeOOH (Goethit) wird zwischen 90 und 440°K mittels der Mossbauermethode untersucht. Aus den gesammelten Daten last sich eine Neeltemperatur TN = 393,3°K und ein effektives Magnetfeld - extrapoliert auf 0°K - von 510 kOe ableiten. Aus der Quadrupolaufspaltung ober- und unterhalb TN sowie aus dem Feldgradienten - berechnet aus den Gitterpositionen - wird geschlossen, das die Richtung der Spins der Eisenionen im Einklang mit Suszeptibilitatsmessungen parallel zur kristallographischen z-Achse liegt. Die Linienform in den Mossbauerspektren wird diskutiert, und mogliche Deutungen fur die Linienform in der Umgebung der Neeltemperatur werden angegeben. Die Temperaturabhangigkeit der Untergittermagnetisierung wird mit Voraussagungen nach der Zweiteilchen-„cluster”-theorie fur einen Heisenbergmagnet verglichen. Wenn die reduzierte Untergittermagnetisierung als m(T) = D(1 − T/TN)f geschrieben wird, bekommt man fur f = 0,32. Abschliesend werden unsere Ergebnisse mit anderen veroffentlichten experimentellen Mossbauerdaten von Goethit verglichen und die Diskrepanzen gedeutet.

Neutron Diffraction Studies of α‐FeOOH

Abstract: Neutron spectrometric investigations are made of goethite (α-FeOOH) of mineralogical and synthetic origin (α-FeOOH of natural isotopic composition) and also of α-FeOOD using a monochromatic neutron beam of λ = 1.12 A. Some powder diffraction patterns in the temperature range from −190 to +120°C are measured. The obtained coordinates for all atoms (including those for hydrogen) in the unit cell are given. The existence of antiferromagnetic spin ordering at temperatures below +80°C is observed. A model is proposed in which the spins are aligned parallel to the b-axis.

Unexpectedly efficient CO2 hydrogenation to higher hydrocarbons over non-doped Fe2O3

Abstract: Since CO 2 hydrogenation into higher hydrocarbons is an attractive approach to mitigate greenhouse effect and to reduce reliance on fossil feedstock, this reaction has been intensively studied over Fe-based catalysts. They had, however, to be promoted with oxides of e.g. K, Cu, Mn and/or Ce to reduce undesired CH 4 formation and to increase olefin selectivity. Here, we demonstrate, for the first time, that similar improvements can be achieved without using dopants but by preparing bare Fe 2 O 3 according to a template-assisted synthesis method. Under optimized reaction conditions (total pressure of 15 bar, 623 K and H 2 /CO 2 = 3/1), the selectivity to C 2 –C 4 hydrocarbons with the olefin to paraffin ratio of 2.7 was about 37% at CO 2 conversion of 40%, while the selectivity to C 5+ hydrocarbons, CH 4 and CO was around 36, 12 and 15% respectively. Our thorough characterization study with complementary techniques (H 2 -TPR, XRD, and Mossbauer spectroscopy) enabled us to conclude that this preparation method affects reducibility of Fe 2 O 3 and, thus, its ability for in situ transformation into catalytically active iron carbide(s) under CO 2 -hydrogenation conditions.

Mössbauer Effect in Antiferromagnetic Fine Particles

Abstract: Mossbauer measurements were made on fine particles of antiferromagnetic α-FeOOH with various particle sizes to examine the Mossbauer spectra in fluctuating fields produced by the relaxation of electron spins. In ultra fine samples, the collapse of the internal field due to superparamagnetism was observed. The critical volume at 294°K was found to be 3×10 -17 cm 3 and the critical temperature for the sample of 1×10 -17 cm 3 to be 210°K. The effective anisotropy constant was estimated to be of the order of 10 4 erg/cm 3 . The transitional patterns from the ordinary magnetic splitting (6-line) to the superparamagnetic doublet are discussed and the assignment of the doublet is attempted in comparison with Blume's prediction.