Activation of molecular hydrogen plays an essential role in many industrial processes such as the synthesis of ammonia, the hydrogenation of organic compounds in petroleum refining. As current processes typically rely on the use of expensive noble metal catalysts or aluminum which is susceptible to oxidation, there is significant interest in identifying cheaper and more efficient ways to activate molecular hydrogen under relatively mild conditions.
As reported in the November issue of Nature Materials (DOI: 10.1038/nmat3123; p. 884), Y.J. Chabal of the University of Texas–Dallas, S. Chaudhuri of Washington State University, and their colleagues have achieved this goal and have demonstrated that very small amounts of titanium incorporated in aluminum surfaces can activate molecular hydrogen at temperatures as low as 90 K. The team combined infrared reflection–absorption spectroscopy and first-principle calculations to identify the atomistic arrangement of the Ti-containing catalytically active sites present on Ti-doped single-crystal Al(111) surfaces. They found that hydrogen can spill over from the catalytic sites onto bare aluminum. It then combines with CO molecules adsorbed on the catalytically active sites to form a complex with activated hydrogen, which can be removed at remarkably low temperatures (115 K; possibly as hydrogenated CO molecules).
The fundamental understanding de rived in this work provides a guide to identifying the active sites needed for the formation of complex metal hydrides for hydrogen storage applications. In addition, these studies show that, in place of expensive and less available noble metals, an inexpensive and abundant metal such as aluminum can be turned into an active catalyst by selectively placing Ti atoms on the surface, thus enabling activation of molecular hydrogen and facilitating CO and hydrogen removal at low tempera tures. Furthermore, even though high concentrations of CO can block the Ti sites, thereby inhibiting catalytic activity toward hydrogen activation, the active sites show a promising tolerance to low contamination levels of CO in H2. Lower desorption temperatures occur under these conditions, which frees up the active sites. This work provides the first direct evidence that Al doped with Ti can carry out the essential first step of molecular hydrogen activation.
