Skills and Techniques

As part of our work on catalytic membrane reactors (for oxygen separation, EPOC or SOFCs) and chemical looping we have developed expertise in different techniques that can be used throughout the spectrum of our research activities. A short description of these techniques can be found below.

Pulsed Isotopic Exchange

 

   The Pulse Isotopic Exchange (PIE) technique, using 18O2(g), allows the determination of the overall oxygen surface exchange rates in electroceramic metal oxides.  This will allow us to develop a better understanding of the surface kinetics of oxygen adsorption and dissociation on these systems and can help us design more stable and more active elecrtoceramics for membrane, SOFC and catalytic applications.
 In the PIE technique the sample is packed into a microreactor and exposed to a pulse of labeled oxygen (18O2) at a chosen oxygen partial pressure and at a known sample temperature.  The pulse travels through the sample bed and the 18O2 may undergo exchange with the unlabelled oxygen (16O) at the surface of the sample.  In this event the pulse may contain exchanged i.e., “scrambled” gas phase oxygen (18O-16O) or completely exchanged oxygen (16O2).  From the fraction of exchanged oxygen within the pulse, the rate of oxygen exchange can be evaluated.  

Micro-fabrication and sample characterisation techniques

 

In order to understand better the processes taking place in membrane reactors and systems under dynamic operation (e.g. chemical looping or electrochemical promotion of catalysis) it is often advisable to work on model systems. Such systems should have well defined structure and according to the application this may lead to catalytic systems of low surface area or model membrane reactor modules of very small volumes.  

In our group we have the expertise and the equipment to perform catalytic and electrochemical studies on such systems. We have been recently awarded an EPSRC grant to investigate the role of spillover in catalysis using micro- and nano-patterned catalysts. 

Coulometric titration

Solid oxide materials find applications as the electrodes/electrolyte in solid oxide fuel cells (SOFCs), catalysts for oxidation processes, component materials in solid state oxygen sensors, and membranes in ceramic oxygen generators. The transport of oxygen ions in these materials is common as a result of oxygen non-stoichiometry and the mobility of oxygen across oxygen vacancies in the lattice. Knowledge of the oxygen nonstoichiometry and thermo-chemical stability of solid oxide materials is important since these properties directly relate to the electrochemical properties, mechanical properties, and material reliability. The oxygen non-stoichiometry could be characterized as a function of temperature and oxygen partial pressure by means of coulometric titration.

The electrochemical cell comprises a section of YSZ tube and two sealed YSZ pellets to obtain gas tight conditions. The powder is introduced in a ceramic crucible and inserted into the electrochemical cell, where symmetrical Pt electrodes were painted in the internal and external faces of the tube and were connected by Pt wires to a dc power source. In this situation, the YSZ tube works as an oxygen pump to extract oxygen thus lowering the oxygen partial pressure inside the cell and reducing the sample. The upper YSZ pellets was adapted to work as an oxygen probe by painting Pt electrodes attached to Pt wires, and using a multichannel potentiostat (AMEL 7050 Instrument) for the voltage measurements. Every coulometric titration experiment is performed by applying a step change in applied potential to the oxygen pump and using the Faraday law to extract the oxygen stoichiometry change.

High temperure ceramic sealants

In our work on high temperature membrane reactors (for SOFCs, separation or catalytic) using the appropriate high temperature sealants and developing novel reactor designs to improve sealing and minimise (or eradicate) cross-chamber leaks is of the outmost importance.  In collaboration with Dr M.J. Pascual from ICV-CSIC Madrid we study the properties of high temperatures glass-ceramic sealants developed to suit the application (i.e. membrane composition, temperature of operation and gas atmosphere). We can test the performance, stability and durability of these sealants under a series of temperature cycles and under different gas atmospheres. Post-operation analysis includes SEM-EDS analysis of the bonded surfaces to study the formation of bubbles (porosity), or reaction between the sealing and membrane materials, or between the sealant the reactive gases. The sealant development done in ICV includes tests such as hot stage microscopy, dilatometry and SEM analysis.

Kinetic Analysis and Mass Balances

Most of our work involves catalytic reactions that take place on (i) a porous noble metal catalyst-electrode (ii) the surface of a membrane or (iii) on catalysts in powder form. In order to assess the performance of the catalytic system a set of kinetic experiments are performed. The reaction kinetics in terms of each of the reactants are determined as a function of temperature. Kinetic experiments are performed under differential conversions (typically less than 10%) so that there is no significant change in the gas phase composition of the reactants at the inlet and outlet providing accurate reaction rate calculations. Using techniques such as surface oxygen titration we can calculate the dispersion of catalysts of very low surface areas.

When working with membrane reactors it is critical to obtain accurate mass balances for the whole system. In our work we routinely analyse the gas composition of the outlet of both chambers of a membrane reactor. This allows us to accurately calculate oxygen, carbon and water mass balances and thus gain a better understanding of the membrane performance and the mechanism of ionic transport across the membrane.