Paper SS-WeA12
Role of Dissociated Hydrogen in Stabilizing Catalytic Sites and Regulating Surface Reactions for Hydrogen Storage in Metal Hydrides
Wednesday, October 31, 2012, 5:40 pm, Room 21
| Session: |
Catalysis on Metals and Alloys |
| Presenter: |
I. Chopra, University of Texas at Dallas |
| Authors: |
I. Chopra, University of Texas at Dallas
S. Chaudhuri, Washington State University
J.-F. Veyan, University of Texas at Dallas
Y.J. Chabal, University of Texas at Dallas
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| Correspondent: |
Click to Email |
There has been an ongoing quest to find cheaper hydrogen activation routes based on aluminum. Recently it was shown1 that aluminum doped with very small amounts of Ti can activate molecular hydrogen at temperatures as low as 90K. The method is based on the ability to introduce a high flux of molecular hydrogen seeded with a guest molecule (CO) to probe the catalytic activity and H2dissociation . Once dissociated, hydrogen forms a complex with adsorbed CO (CO-H), characterized by a substantially and uniquely blue-shifted CO internal frequency.
We use this new method to determine that Titanium when present on the surface is catalytically more active (~30%) than subsurface Titanium. Dissociated hydrogen stabilizes Ti on the surface and as the surface becomes saturated with hydrogen it is possible for Ti atoms sitting in originally sub surface configurations to be pulled out of their original positions by the dissociated hydrogen into surface configurations. These “pulled” Ti atoms then contribute towards additional hydrogen activation when Ti is present in specific (nearest neighbor and next nearest neighbor) configurations.
Finally we show that this dissociated hydrogen protects the CO in the CO-H complex from additional reactions as is demonstrated by no isotopic exchange between CO16 and CO18. Dissociated hydrogen is present in the hollow sites around the Ti and protects the Ti-Al back-bond from further chemical activation. Similar experiments performed on surfaces with no hydrogen (Al/Ti, AlO/TiOx) show isotopic exchange reactions between CO16 (of the complex) and adsorbed CO18. The barrier for this isotopic exchange is the lowest for adsorbed CO on TiOx (desorption temperature ~ 140K) and highest for CO on Ti (desorption temperature ~ >300K).
These results provide critical information about the role of dissociated hydrogen in stabilizing Ti catalytic sites and the CO-H complex. A complete understanding of such mechanisms will help enhance material performance for hydrogen activation using cheap catalyst materials.
References:
[1] Chopra, I. S., Chaudhuri, S., Veyan, J.-F., and Chabal, Y. J., Nature Materials, 10, 884–889 (2011)