Gold-based catalysts are promising for efficient oxidation of CO and selective oxidation of propene. This is a fascinating set of catalytic processes because only nanoscopic gold is effective as a catalyst. These nanoscopic gold catalysts are active at much lower temperature compared to platinum-group catalysts for CO oxidation. Likewise, Au-based catalysts are effective for selective oxidation of propene, despite the very labile allylic hydrogen in this molecule. Because of the complexity of the Au-based catalysts, there is significant debate regarding the underlying basis for the activity of these nanoscale materials. One important factor is that large Au particles and Au single crystals are not active for dissociation of O2, which is likely to be a key step. Dissociation of O2 is clearly related to the size and shape of gold particles and possibly to interactions with the water moiety and oxide support. Even the bonding of molecular oxygen (peroxo-type species), which has been proposed as a possible intermediate, is affected by the characteristics of Au. Model studies on oxidized Au(111) demonstrate, however, that atomic oxygen is very active for low-temperature oxidation of CO. These model studies further demonstrate that the interaction of Au with the metal oxide support is not necessary to impart activity to the Au and that metastable oxygen phases are particularly active at low temperature. At the same time, there is still a need to determine which features of the catalyst determine the long-term stability of the Au nanoparticles so that practical improvements can be made for improvement in, e.g., automotive catalysts. Propene oxidation by gold catalyst is less well understood, in part because of the complexity of the reaction system. Metastable oxygen phases on Au(111) are active for partial oxidation of propene; however, acrolein, not the epoxide, is the primary oxidation product. While these model studies indicate that disordered phases of atomic oxygen are active for partial oxidation, there are still many unanswered questions as to how to efficiently promote epoxidation using Au-based nanocatalysts. Nevertheless, the efficient oxidation of propene - an important industrial process - might be achieved with Au-based nanocatalysts using O2 as an oxidant. Enhancing the stability of gold catalysts during propene oxidation is also a critical issue for future commercial application. Even though Ti-modified silica support is known to enhance the stability of gold catalyst, there is little understanding of the underlying basis of this phenomenon, demanding more systematic study with the assistance of a model system. This may enable further understanding the mechanism of propene oxidation by some special gold catalysts where gold particles are supported on TiO2 or Ti-modified silica, which may help to design a more efficient gold catalyst for propene oxidation.
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