Operando neutron characterisation for chemical looping

Abstract

The central concept of chemical looping (CL) is to split a target reaction into two or more separate reactions which are facilitated by an intermediate material. This ensures that reactants are never directly mixed and so products do not require separation, therefore improving the efficiency of the overall process. CL can improve hydrogen production from the water-gas-shift (WGS) reaction by using the novel concept of the memory reactor. By employing a non-stoichiometric mixed metal oxide perovskite (ABO3−δ) as an oxygen carrier material (OCM), and operating with reverse flow operation, an oxygen chemical potential gradient is developed through the bed that allows the thermodynamic equilibrium of the WGS reaction to be overcome and a higher conversion to hydrogen achieved. To develop this process further, it is paramount to understand how the OCM’s crystal structure changes during reduction and oxidation and relate these changes to the degree of oxygen non-stoichiometry (δ). Neutron diffraction can offer new insight into these structures due to its high sensitivity to oxygen atomic position. Ex situ neutron total scattering data and pair distribution function (PDF) analysis has provided new insight into the local, short-range structure of these materials in their working states. This project, in conjunction with the ISIS neutron and muon source, has developed a novel methodology allowing a working CL memory reactor to be monitored operando by neutron diffraction for the first time. A large (100 g) reactor bed has been analysed in discrete sections, directly demonstrating the oxygen gradient, and allowing the process to be proven on an intermediate scale. This represents a significant advancement of operando neutron capability, which has traditionally been limited to small scale testing (< 10 g) under simulated reaction conditions. It is hoped that the methodology developed can be applied to further investigations of CL processes by future users of ISIS and other neutron facilities and extended to packed bed reactors more generally.