(583fs) Correlations Between Catalytic and Magnetic Response in Titania-Supported Gold

Atomically-dispersed noble metals on metal oxide supports are considered as important systems for catalytic applications such as water-gas shift (WGS) reactions for hydrogen production and ambient-temperature carbon monoxide oxidation.‎[1],‎[2] The improved catalytic behavior in such systems is commonly related to the facilitation of strong interfacial metal-support interactions.‎[1] Optimizing the processing conditions to attain enhanced catalytic activity in titania-supported gold species requires a fundamental understanding of the gold activity and its effect on the physical characteristics and functional properties of titania. Magnetic characterization of titania nanostructures can provide unique information that connects magnetism, optical activity and catalysis ‎[3],‎[4]to improve our understanding of the modulation of the properties of titania by the presence of atomically-dispersed gold on its surface.

In this work, gold clusters were supported on commercial titania nanoparticles (Millennium Inorganic Chemicals, G5) via a deposition/precipitation (DP) method, followed by a short time exposure to UV irradiation in ethanol under anaerobic conditions to bind the gold atoms on the titania surface. [2] Chemical leaching using sodium cyanide solutions removes the excess gold with the remaining gold (approximately 1 wt.%) dispersed atomically on the titania support. The new material shows significantly better activity for the WGS reaction than the conventional DP sample, and maintains good stability in long-time cool-down and start-up operation up to 473 K. [2] The morphology and crystal structure, magnetic behavior and electronic structure of the processed gold-attached titania nanoparticles were characterized. The saturation magnetization of the commercial titania nanoparticles increases with the atomically dispersed cationic gold incorporation, and is accompanied by an increased metallic character of titanium oxide, as quantified by an increased Pauli paramagnetic contribution to the magnetic susceptibility. In analogy, atomically dispersed gold attached to well-defined defect- rich/ poor crystalline TiO_xnanotube arrays have been characterized to confirm the mechanism. These results reveal a positive correlation between magnetic character and catalytic activity and clarify one main aspect of the interaction of metal ions with the metal oxide support surfaces. Other characterizations of the gold/titania materials, including electron paramagnetic resonance (EPR) and x-ray photoelectron spectroscopy (XPS), corroborate the above findings, and also help to elucidate the chemical state of gold in the as prepared materials.

Research supported by thethe US Department of Energy under Grant No. DE-FG02-05ER15730,  and by the National Science Foundation under Grant No. DMR-0906608.

References:

[1]    M. Flytzani-Stephanopoulos, B. Gates, Annu. Rev. Chem. Biomol. 3 (2012) 545.

[2]    M. Yang, L.F. Allard, M. Flytzani-Stephanopoulos, J. Amer. Chem. Soc.135 (2013) 3768.

[3]    J. T. Richardson, J. Appl. Phys., 49 (1978) 1781.

[4]    D. J. Siegel, M. van Schilfgaarde, J. C. Hamilton, Phys. Rev. Lett., 92 (2004) 086101.