NANOWGS · Optimization of Water-Gas-Shift catalysts: a fundamental and mechanistic approach

Periodic Report Summary 2 - NANOWGS (Optimisation of water-gas-shift catalysts: a fundamental and mechanistic approach)

Hydrogen and fuel cells are considered a key solution for the 21st century clean energy demand. At present, hydrogen is mainly produced from the reforming of crude oil, coal, natural gas, wood, organic waste and biomass. The high content of carbon monoxide (CO) (1 - 10 %) in the reformed fuel degrades the performance of the Pt electrode used in fuel-cell systems. Water gas shift (WGS) reaction is a critical process for providing clean hydrogen. For each mole of CO removed a mole of hydrogen is produced. WGS allows not only for the removal of CO but also for an increase in fuel-cell efficiency by improving the hydrogen concentration. For mobile fuel cell applications conventional WGS catalysts are not suitable and advanced systems are required. Commercial Cu-Zn-based catalysts are pyrophoric, require special activation procedures and are intolerant to oxidation. Due to the expected exposure to many start-up / shutdown cycles of fuel process WGS catalysts, desired catalysts characteristics include tolerance to redox cycles and condensation steam. Ceria and titania-based nanocatalysts have been investigated extensively in recent years and are expected to be part of the next generation of WGS catalysts. To obtain low temperature WGS activity cerium oxide is usually loaded with reduction promoter metals such as Pt, Cu or Au. The design and optimisation of new catalysts for the WGS reaction is hindered by the intricate reaction mechanism and the difficulty of identifying active species. The ability of characterising the catalysts and reaction intermediates at the pressure and temperature for WGS operation is therefore a challenge. The main objectives of the projects are the identification of the active phase for the subsequent rational design of an improved generation of catalysts. As described in the attached scheme, this project involves a multidisciplinary approach and includes novel in situ characterisation of powder samples, surface science studies on model systems and theoretical density functional theory (DFT) calculations. During the outgoing phase of the project execution the work focused on the characterisation of the active site under reaction conditions. The oxidation state of the active phase was determined by in situ X-ray absorption spectroscopy while the long range interactions (crystalline structure) have been analysed by X-ray diffraction. In situ capabilities available at the National Synchrotron Light Source (Brookhaven National Laboratory) are a powerful tool for identifying the active phase. Complementary studies of model systems were also performed by X-ray photoelectron spectroscopy of single crystals. Overall, the project has advanced into gaining knowledge of the active phase. The study of an inverse ceria supported over copper catalysts outlined the role of CeOx species in reaction mechanism. This is a bi-functional system in which CeOx particles help in water bond breaking while reduced Cu participate in the CO oxidation step. When analysing Pt supported over CeOx / TiO2 matrix, the correlation between the catalytic performance and the reducibility of the CeOx species highlighted the strong interaction between noble metal particles and support. Ni-CeO2 solid solutions have also been studied and again the strong metal-support interaction provides the system with promising catalytic properties for WGS and ethanol steam reforming reactions. During the returning phase the work has focused on the optimisation of support properties by carful control of the CeOx incorporation over TiO2. CeOx dispersion and interaction with TiO2 depends on the kind of precursor employed, the electrostatic interaction between ionic salts and support surface during impregnation and also on the calcination conditions. The information achieved so far shows the importance of both metal-phase and metal-oxide support in reaction mechanism. To optimise catalytic performance one has not only to work on the structure of the noble metal particles but also on the support properties.

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