| AVS 55th International Symposium & Exhibition | |
| Energy Science and Technology Focus Topic | Tuesday Sessions |
| Session EN+BI+SS+SE-TuM |
| Session: | Catalysis for Energy Sustainability |
| Presenter: | P. Liu, Brookhaven National Laboratory |
| Authors: | P. Liu, Brookhaven National Laboratory S. Ma, Brookhaven National Laboratory J.A. Rodriguez, Brookhaven National Laboratory J. Hrbek, Brookhaven National Laboratory |
| Correspondent: | Click to Email |
The water-gas shift (WGS) reaction (CO + H2O → H2 + CO2) is a critical process in providing pure hydrogen for fuel cells and other applications. However, current industrial catalysts (Fe-Cr or Zn-Al-Cu oxides) are pyrophoric and require complex activation steps before usage. A fascinating puzzle has recently emerged: Au/CeO2 and Au/TiO2 nanomaterials show high activity for WGS catalysis. This is remarkable since in bulk form Au, ceria and titania are not known as WGS catalysts. The nature of the active phase(s) in these metal-oxide nanocatalysts is unclear at the present time, which impedes the design and optimization of WGS catalysts. We have carried out coordinated experimental and theoretical studies to address this problem. The experiments show that the model catalysts, Au/CeO2(111) or TiO2(110) and inverse CeO2 or TiO2/Au(111), display activities comparable to good WGS catalysts ( i.e., Cu(100) and Cu(111)). Theoretical calculations based on density functional theory (DFT) are also carried out to understand the active sites in the oxide-gold catalysts, by probing reaction scenarios on Au, titania, and Au-TiO2 (Au/TiO2 and TiO2/Au(111)) catalyst model structure. In accordance with experiments, our calculations show a very high barrier for the dissociation of water on Au and the formation of very stable formate species on titania that prevents the production of H2 and CO2. The model Au-TiO2 catalyst overcomes these bottlenecks: the moderate chemical activity of gold is coupled to the more reactive oxide. The dissociation of water takes place on the oxide easily, a reaction that extended surfaces and nanoparticles of Au cannot perform. CO adsorbs on gold sites located nearby (bifunctional catalyst). Then all the subsequent steps occur at the oxide-metal interface at a reasonable speed. Our results imply that the high activities of Au/CeO2 and Au/TiO2 nanocatalysts in the WGS reaction depend heavily on the direct participation of oxide-metal interface. The diversity of nanoparticle structures and the interplay with the support highlight the importance of identifying critical structural motifs to model catalyst function. This research was carried out at Brookhaven National Laboratory and supported by the US Department of Energy (Chemical Sciences Division, DE-AC02-98CH10886).