Some considerations related to the use of the Scherrer equation in powder X-ray diffraction as applied to heterogeneous catalysts

This short overview summarises some of the basic considerations which should be undertaken when the Scherrer equation is applied to reflection widths in X-ray diffraction patterns of heterogeneous catalysts in order to extract meaningful information. Frequently, little account has been taken of the apparent complications arising from the presence of microstructural strain and disorder such as that which can be introduced upon doping or of anisotropic effects and such considerations are highlighted. Scanning electron micrograph showing the highly anisotropic nature of biogenic iron oxide found in a natural iron ochre source

Synthesis and characterization of iron and iron nitride microtubes obtained from biogenic iron oxide

With the aim to obtain iron tubular microstructures attractive for various applications, we have used a natural biogenic iron ochre as a raw material and explore various procedures and experimental conditions to achieve our goal. Our experiments included reduction, nitridation and characterization of microtubes derived from biogenic iron ochre. Various temperatures of reduction under streams of H2/N2 or NH3 were tested. Our results show that the tubular structure is maintained after reduction of the natural material under H2/N2. In addition, ammonia was not produced under our experimental conditions, and as expected, hydrogen reduced the material. However, the treatment under NH3 reduces the material and allows the incorporation of nitrogen into the structure of the solid yielding iron nitride microtubes. Reduced and nitrided microstructures were successfully obtained from natural biogenic iron ochre. A temperature of 500 °C seems to be suitable to expose the calcined biogenic iron ochre under H2/N2 for 4 h or under ammonia for 8 h to obtain reduced or nitrided microtubes, respectively. The stability of the tubular structure during reduction/reduction–nitridation is maintained under both treatments. Further interesting applications of this natural biomaterial could be envisaged

The denitridation of nitrides of iron, cobalt and rhenium under hydrogen

The denitridation behaviour of binary iron, cobalt and rehnium nitrides under H2 /Ar has been investigated. The iron nitride was found to lose over 70 % of its as prepared nitrogen content at 400 °C. The cobalt nitride was completely denitrided at 250 °C. Rhenium nitride lost close to 90 % of its nitrogen at 350 °C. In addition, Co-Re4 prepared by ammonolyis was investigated, whilst only traces of NH<sub>3</sub> were lost from this material under H<sub>2</sub>/Ar at 400 °C, with H<sub>2</sub>/N2 it proved to be an active ambient pressure ammonia synthesis catalyst in accordance with previous literature

Towards anti-perovskite nitrides as potential nitrogen storage materials for chemical looping ammonia production: reduction of Co₃ZnN, Ni₃ZnN, Co₃InN, Ni₃InN under hydrogen

The ammonia production properties upon reduction in hydrogen of the anti-perovskite nitrides Co₃ZnN, Ni₃ZnN, Co₃InN, and Ni₃InN have been investigated. Single phases with ideal anti-perovskite structures (Space group: Pm-3m) were prepared for all the nitrides by the ammonolysis of the corresponding precursor oxides and all the nitrides were observed to produce ammonia in high yields when reacted with H2/Ar. The cumulative ammonia production values at 400 °C were 3069, 2925, 289, and 1029 μmol-NH3 g⁻¹ for Co₃ZnN, Ni₃ZnN, Co₃InN, and Ni₃InN, respectively and the order of the release rates was Ni₃ZnN > Co₃ZnN > Ni₃InN > Co₃InN. X-ray diffraction studies revealed that Co₃ZnN and Co₃InN were decomposed upon the loss of lattice N, whereas Ni₃ZnN and Ni₃InN were transformed into Ni₃Zn and Ni₃In via the intermediate phases Ni₃ZnNₓ and Ni₃InNᵧ. The crystal structures of these intermediate phases are related to their initial structures, indicating that the loss of lattice N in Ni₃ZnN and Ni3InN was topotactic

Metal nitrides, the Mars-van Krevelen mechanism and heterogeneously catalysed ammonia Synthesis

Whilst the Mars-van Krevelen mechanism is well established for metal oxide catalysed oxidation reactions, it has been much less studied in the case of nitrogen based reactions catalysed by metal nitrides. In this overview, some of the recent literature in relation to the possible operation of the Mars- van Krevelen mechanism for ammonia synthesis catalysed by metal nitrides is presented, along with some of the literature relating to the synthesis of ammonia via chemical looping approaches

A comparison of the activities of various supported catalysts for ammonia synthesis

The present study presents an empirical screening study of the catalytic performance of a variety of supported materials for ammonia synthesis at 400 and 500 °C. Amongst the materials tested, those derived from Ru/Al2O3 exhibited the best performance. Supported Os and CoRe catalysts also demonstrated comparatively high activities indicating them to be potentially worthy of further investigation

Powder X-ray diffraction and heterogeneous catalysis

In this short review, the application of powder X-ray diffraction to the study of heterogeneous catalysis is described. Within the literature, most applications of powder diffraction in heterogeneous catalysis have centred on its use as a fingerprint technique for phase identification/confirmation. However, there is wealth of additional information potentially available, and in this review some studies, which have gone beyond fingerprint applications, are described. Brief descriptions of selected papers detailing the application of in-situ measurements, the determination of catalyst stoichiometry in-situ, the determination of coke location in ZSM-5 catalysts, line profile analysis, Debye Functional Analysis and combined X-ray absorption and diffraction are given. Many, but not all, of these studies have been performed with standard instrumentation, illustrating some of the range of information available in laboratory studies

Preface

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