Chrome Oxide Reduction - an Atomic Level modelling and Spectrographic Experimental Approach.

The environmental impact of metal production would be significantly reduced by substituting methane for coal and coke. However, not all metal oxides are as suitable for methane reduction as others, and the reason for the differences is not obvious. Pure chrome oxide is, for example, readily reduced by methane, while naturally occurring chromite ores, are not. It is our hypothesis that atomic scale properties of the surfaces of the oxides govern the interactions with methane, and that any explanation of the difference in reduction behaviour must include a detailed description of the surface properties at the atomic scale. The project has had two parallel activities: one experimental and one based on atomistic modelling. In the experimental part, a lot of effort has been put into synthesising chrome oxides. In nature, chrome is found in minerals together with iron, magnesium and aluminium in various levels. By using synthetic chrome oxides, the various levels of these other metals can be controlled. This way, the effect of these metals on the reduction process with methane can be investigated. Some reduction experiments have been carried out, but focus on the impure oxides will be intensified and finished in 2017. The modelling part of the project has so far dealt with finding an atomic level description of the decomposition of methane on chrome oxide. This involves trying to find the specific sites on the surface with which the methane molecules are likeliest to interact, and how the partially decomposed methane molecules can move on the surface. In particular, the energy barriers for the different steps of the decomposition reaction is of interest. When these are in place, it is possible to do probabilistic simulations of reactions (where a higher energy barrier corresponds to a lower possibility for a given reaction step to occur). Another important investigation has been the study of interactions between methane and chrome oxide surfaces with iron, aluminium and magnesium atoms present. When complete, these can be compared with the reduction experiments from the experimental part of the project with impure oxides. The goal is then to understand the differences in interactions between methane and different chrome oxide surfaces in light of both microscopic and macroscopic considerations. Hopefully, the obtained experience will all ow for generalisation on what surface properties to look for in ores intended for methane reduction, and perhaps give hints on what pre-treatments could alter the surface properties of otherwise unsuitable ores.

The environmental impact of metal production would be significantly reduced by substituting methane for coal and coke. However, not all metal oxides are as suitable for methane reduction as others, and the reason for the differences is not obvious. Pure c hrome oxide is, for example, readily reduced by methane, while naturally occurring chromite ores, are not. Since the interactions between a surface and gas-phase molecules are strongly dependent on the local geometry, chemical composition and electronic structure at the exposed surfaces, any explanation will need to be based on a detailed understanding of the properties at the atomic scale. This can only be obtained through a well interlinked experimental and modelling effort focusing on the atomic leng th and time scales. PES/XPS, TEM and X-ray diffraction will be used to extract surface and bulk structure information. DFT will be used to calculate the energy barriers associated with transitions between different chemical states and adsorption sites. These estimates of the energy barriers will translate to reaction rates of single reaction and diffusion steps, or in terms of Monte Carlo simulations, transition probabilities. These will form the basis for the development of a Kinetic Monte Carlo model which will then be used to describe the overall kinetics of the complex CH4-oxide chemistry, and compared with the experimental kinetics studies. Upon completion of the project, the difference in macroscopic and microscopic interactions observed between methane and pure chrome oxide and synthetically grown chromite ores will be explained based on surface characteristics at the atomic level. Hopefully, the obtained experience will allow for generalisation on what surface properties to look for in ores i ntended for CH4 reduction, and perhaps give hints on what pre-treatments could alter the surface properties of otherwise unsuitable ores.