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Water-gas shift reaction on metal-oxide nanocatalysts for hydrogen production

Final Report Summary - NANOWGS (Water-gas shift reaction on metal-oxide nanocatalysts for hydrogen production)

This project aims at elucidating the molecular-level structure and functioning of metals supported on reducible metal oxides as catalytic systems for the water-gas shift (WGS: H2O + CO --> H2 + CO2). To this end, we propose to create suitable atomistic models for these catalysts and apply state-of-the-art computational chemistry methods. The results obtained from these theoretical studies will expectedly provide relevant insight into the main parameters involved in the performance and mechanism of the WGS reaction, aiding the improvement and design of more efficient catalysts.

The work performed since the beginning of the project has involved the development of computational models of Ni/ceria catalysts for the WGS reaction by means of first principles density functional theory (DFT). We have explored the electronic, magnetic, vibrational, and chemical properties changes occurring on the metal due to size effects and the presence of the metal-oxide interface. In addition, we have investigated the stand-alone ability of the bare ceria surface as catalyst. From a physical viewpoint, we have also assessed the role of van der Waals (vdW) dispersion forces in the adsorption and reactivity of key chemicals on extended metal surfaces. Additionally, the change of host in September 2013 from the Institute of Catalysis and Petrochemistry (ICP) to CIC Energigune (CICE) introduced the possibility of applying some of the theoretical methods of this project to energy storage systems, which has resulted in a number of side studies that complemented the main research lines and impact of the project.

The main results achieved during the 4-year integration period are:

1. Atomic understanding of the interaction between isolated water molecules and Ni/ceria surfaces. We have clarified the nature of the adsorption state, with water molecularly adsorbed with one O−H hydrogen bonds or as a hydroxyl pair.

2. Evidence of a strong metal-support interaction for the adsorption of CO and C on Ni/ceria catalysts. We have rationalized the effect of Ni coverage on the CO methanation and WGS reactions: the strong electronic perturbations in the Ni atoms produce a drastic change in their chemical properties, leading to stronger C-O bonds.

3. We found for the first time the applicability of ceria as a catalyst for olefin production and potentially broaden its use for the hydrogenation of polyunsaturated and polyfunctionalized substrates containing triple bonds.

4. Elucidation of the interaction mechanism for benzene derivatives on metal surfaces. With the aid of state-of-the-art vdW-inclusive methods, we were able to account accurately enough for the description of physisorption states and, therefore, rationalize the conventional notion of the metastable and short-lived precursor state.

5. We unveiled the underlying mechanisms for the dissociation of molecular hydrogen and the diffusion and clustering of the resulting atomic species on the ceria (111) surface.

6. Elucidation of the effect of the support on O-H bond cleavage activity for Ni/ceria systems. In addition we found a fast dissociation of water on Ni/ceria, which has a dramatic effect on the activity and stability of this system as a catalyst for the WGS reaction.

7. Exploration of the performance of the Ni/ceria catalytic system towards the dry reforming of methane. Our results revealed a strong metal-support interaction which enhance the reactivity of Ni towards methane dissociation. We put forward that Ni/ceria is a highly efficient, stable and non-expensive catalyst for methane dry reforming at relative low temperatures. This extends the catalytic applications of the Ni/ceria sytem beyond the WGS reaction, increasing the impact of this project.

8. Exploration of the effect of atomic structure and morphology on the electronic properties of sodium superoxide for Na-air rechargable batteries. We theoretically confirmed the insulating nature of this material ranging from its bulk form down to the nanoscale. This insight helped to understand the discharge and charge mechanisms during the operation of these high-density energy storage materials.

The results obtained during these 4 years have provide valuable insight into the fundamental understanding and functioning of hydrogen generation systems based on oxide-supported metal nanoparticles for the WGS reaction. Metal-oxide catalysts are particularly difficult to study and our knowledge of the elementary reaction steps occurring on these systems remains in its infancy. Here we have clarified important questions in this quest. A key result has been the elucidation of the water dissociation process in an economically viable catalyst, Ni/ceria. Controlling the ability of a catalyst to dissociate water is highly desirable and helped us to propose a high-performance material with low degradation and low-cost targets for the WGS reaction. In addition, we found that the Ni/ceria and stand-alone ceria systems also show prominent activity towards other catalytic processes: dry reforming of methane and olefin production, respectively. Therefore, the outcomes of this project will certainly contribute to enhance EU competitiveness.