Synthsis And Structure-activity Relations Of Gold Nanoparticle Supported On Iron-based Oxides Or Hydroxides

CO oxidation is normally used as probe reaction to investigate the catalytic mechanism of gold catalysis. According to previous findings, catalytic performance of supported gold catalysts can be influenced by various factors, including the size and the valence state of gold nanoparticles, the properties of oxide support and the gold-support interaction. Currently, the surface hydroxyl group effect to enhance the activity of gold catalysts has attracted great attention. However, different factors mentioned above are always mixing and intertwining, those are very difficult to be distinguished from each other in gold catalysis. Therefore, debate over the origin of high activity of gold catalysts as well as the reaction mechanism of low-temperature CO oxidation has continued for more than two decades due to the ultra complexity and sensitivity of the oxide-supported gold system.Gold on iron-oxide (Au/FeOx) is a typical system for the study of gold catalysis. For the widely studied co-precipitation Au/FeOx catalysts, it has been discovered that the wide size distribution of gold nanoparticles make the precise identification of active gold species extremely difficult. Besides, it has been reported that the calcined samples were less active than as-dried samples. In fact, the calcination on gold-iron oxide catalyst usually results in simultaneous structural changes of both gold and the oxide support, i.e., small gold clusters are transformed into nanoparticles; in addition, the phase of the support changes from hydroxylated ferrihydrite to dehydrated hematite. For such a complex catalyst system, it is difficult to find out the origin of high activity. On the other hands, research work related to hydroxyl group effect is rarely reported, and many issues need to be explored in depth.Therefore, in this work, we aims to synthesize a series iron-based oxide and hydroxide supported gold catalyst with various quantities of hydroxyl groups. We futher explored the catalytic behavior of CO oxidation over these catalysts and their structure-activity relationship, which will provide important guidance on the synthesis of high-performance gold catalysts. The work is shown as follows:(一) Hydroxylated (Fe_OH) and dehydrated iron oxide (Fe_O) support have been prepared by chemical precipitation. Two kinds of iron-based gold catalysts then synthesized by either deposition-precipitation or colloidal-deposition methods. A combination of high-resolution transmission electron microscopy (HRTEM), ex-situ and in-situ X-ray absorption fine structure (XAFS), and in-situ X-ray diffraction (XRD) techniques were used to study the structure and texture of all catalysts. Two kinds of catalysts exhibit an inverse order of catalytic activity between DP (Au/Fe_OH< Au/Fe_O) and CD (Au/Fe_OH> Au/Fe_O). High level of effectiveness of gold-support interaction favours the high activity for gold catalysts by DP method, in which gold nanoparticles are locally generated. On the other hand, hydroxyl surface benefits the reactivity of gold catalysts by CD method, for which gold nanoparticles are externally introduced.(3) Fe_OH_pH support with different amounts of hydroxyl groups are synthesized by controlling the pH of precipitation. And Fe_O_T support with a small quantity of hydroxyl groups are prepared by calcinating Fe_OH support at different temperatures. Two series of gold catalysts were synthesized by deposition-precipitation method. A combination of transmission electron microscopy (TEM) and temperature-programmed reduction by hydrogen (H2-TPR) techniques were used to study the structure and texture of all catalysts. Two series of catalysts exhibit respective order of catalytic activity. For DP_Au/Fe_OH_pH catalysts, higher pH value results in more hydroxyl groups and the higher catalytic activity. On the other hand, for DP_Au/Fe_O_T catalysts, higher calcination temperature results in less hydroxyl groups and lower catalytic activity.