The size distribution of supported gold nanoparticles was evaluated by a statistical measurement of 300 randomly selected particles, which can be found in Figure 2d. These particles are in the range 2 to 8 nm and the average size centers at 4.1 nm. Figure 1 XRD patterns of HNTs and Au/HNTs. Black circle, metallic Au. Figure 2 TEM images of the HNTs and Au/HNTs and size distribution . (a) Pure HNTs. (b) Gold nanoparticles in the HNTs. (c)
High-resolution TEM image of gold nanoparticles. (d) Size distribution of supported gold nanoparticles. Figure 3 shows the representative Au 4f core level XPS spectrum of the Au/HNTs catalyst. Broad peaks of Au 4f7/2 and Au 4f5/2 states were observed in the Au/HNTs sample, indicating the presence of both metallic Proteasome inhibitor and ionic gold species [12, 13]. In addition to the main peak characteristic of metallic Au0, the XPS spectra also contain the 4f7/2 signals from Au1+ ions [12, 13]. The deconvolution analysis results of the Au 4f spectra of the Au/HNTs catalysts showed that about 60% of the gold species are oxidized Au1+ species. Similar to our findings, Abad et al. have recently shown by XPS and IR spectroscopy the presence of positive gold ions in Au/CeO2 catalyst . Such species has been suggested to be of vital importance in the rate-controlling step during the oxidation of alcohols involving the hydride shift from alcohol to gold . Figure 3 Representative Au 4f core level XPS
spectrum of Au/HNTs. For the Au/HNTs catalyst, solvent-free aerobic oxidation of Selleck Trichostatin A benzyl alcohol which is often employed as a model reaction for alcohol oxidation was chosen to test its catalytic activity [16–18]. The control experiments using the
pure HNTs reveal that less than 2% of the benzyl alcohol can be selectively these converted to benzaldehyde within 8 h at 110°C. Figure 4 shows a typical set of results for benzyl alcohol conversion over the Au/HNTs catalyst, illustrating the dependence of conversion and selectivity on the reaction time. As the reaction proceeded, the conversion of benzyl alcohol and the selectivity to benzyl benzoate increased, while the selectivity to benzaldehyde decreased. Enache et al.  and Abad et al.  have recently reported very high turnover frequency (TOF) values in the solvent-free oxidation of benzyl alcohol at about 100°C for Au-Pd/TiO2 (TOF = 607 h−1) and Au/CeO2 (TOF = 150 h−1) catalysts, respectively. To compare with other reported catalysts, the catalytic performance of the Au/HNTs catalyst in the solvent-free aerobic oxidation of benzyl alcohol at 110°C under atmospheric pressure was also investigated. The results showed that the Au/HNTs catalyst exhibited a specific rate of 307 h−1 under similar reaction conditions. This value compares favorably with the results reported on Au/CeO2 catalysts , demonstrating that our catalytic system can serve as a promising catalyst for the selective oxidation of alcohols.