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Since Haruta reported that Au nanoparticles are surprisingly active if prepared and deposited on suitable carriers [1], the activity of Au has become widely recognized for various reactions [2]. Catalytic properties of Au depend on the size and shape of Au particles [3], but also on the nature of the support or additives and their interaction with Au [4, 5].
Highly active (un)promoted Au/Al2O3 catalysts were prepared by deposition-precipitation with urea and characterized by means of various techniques such as: XRD, HRTEM, DR-UV/Vis, TPR and LEIS. The samples were tested in various reactions relevant to air pollution control: CO oxidation in the absence or presence (PROX) of hydrogen, total oxidation of (un)saturated hydrocarbons, NO and N2O reduction. XRD, HRTEM and DR-UV/Vis have shown that Au is present as very small nanoparticles (< 4nm) on the support. Based on TPR and DR-UV/Vis results we have concluded that the active state of Au is most probably Au0. Additives such as alkali (earth) metal oxides significantly influence the stability of Au nanoparticles against sintering (structural promoters). Additives such as ceria act as both structural and chemical promoters. However, the interaction between Au and additives is not fully understood. We applied, for the first time on Au-based catalysts, LEIS as a versatile technique to study the composition/structure of the catalytic surface. The results suggest that surface reconstruction may take place after the addition of Au to the (mixed) support, which, in turn, can influence the catalytic activity.
The results suggest that more reaction channels may be active during the catalytic process. The Au nanoparticles (by means of an increased number of steps, kinks and edges), along with the perimeter Au-support, act probably as the main reaction centers. In addition, new reaction paths may become available as a function of catalyst composition and reaction temperature.
[1] M. Haruta, T. Kobayashi, N. Yamada, Chem. Lett. 2 (1987) 405.
[2] A. C. Gluhoi, M. A. P. Dekkers, B. E. Nieuwenhuys, J. Catal. 219 (2003) 197.
[3] A. Wolf, F. Schuth, Appl. Catal. A 226 (2002) 1.
[4] A. C. Gluhoi, S. D. Lin, B. E. Nieuwenhuys, Catal. Today 90 (2004) 175.
[5] A. C. Gluhoi, N. Bogdanchikova, B. E. Nieuwenhuys, J. Catal. 229 (2005) 154. |