Synthesis of complex transition metal nitrides and their ammonia synthesis activity

Abstract

In this study, some nanocrystalline quaternary interstitial nitrides adopting the filled b-Mn and the η-6 carbide type structures have been synthesised using a modified Pechini method. This method, which involves dissolving metal salts with citric acid followed by the evaporation and ammonolysis of the resulting oxide precursors, has resulted in the nanoparticle materials. It has been shown to be a useful tool for producing homogenous quaternary nitrides in a fast and simple way. The structural properties of the samples were examined by powder X-ray diffraction and the data was further analysed and quantitatively refined using Rietveld refinement methods. Other characterisation techniques such as SEM, TEM, TGA and elemental analysis of carbon, hydrogen and nitrogen content were also undertaken. The surface areas of the materials were measured by the Brunauer- EmmettTeller (BET) method, using nitrogen adsorption isotherms. The refinement of Ni2-xCuxMo3N and Ni2WxMo3-xN powder X-ray diffraction data confirms a linear dependence of lattice parameters versus Cu or W content where ! = 0.0, 0.1, 0.2, 0.3, and then the lattice parameters no longer change as the doping level is increased, indicating that copper or tungsten can be substituted at the nickel or molybdenum sites in Ni2Mo3N structure up to a solubility limit of ! = 0.3. Ni2-xFexMo3N was observed to have a linear increase in the lattice parameters up to ! = 1. These metal nitrides show good ammonia synthesis activity at 500 °C under ambient pressure. The activity fell with copper content and increased with iron and tungsten content, although these changes appear to be related to changing surface areas of the catalysts. The η-6 Co3No3M structure is a promising material for ammonia synthesis. The substitution of molybdenum with tungsten into this structure in the formula Co3WxMo3N resulted in some single phases where ! = 0.1, 0.2, 0.4, 0.7 and 0.8. However, the compositions with ! = 0.3, 0.5 and 0.6 were found to have a trace impurity of Co3Mo. These materials showed good activity for ammonia synthesis, but less than that of the parent compound Co3Mo3N. The oxide intermediates from the citrate gel process cleanly converted to the metal nitride under the ammonia synthesis gas stream (75% H2, 25% N2) providing more active catalysts than those pre-nitrided by ammonolysis.

More information

Identifiers

ORCID for Andrew Hector: orcid.org/0000-0002-9964-2163

Catalogue record

Export record