Influence of supported gold particles on the surface reactions of ethanol on TiO2
Graphical abstract
Introduction
The chemistry of gold has gained increasing interest since it was discovered that gold particles have excellent properties as chemical sensors [1], [2] and catalysts [3], [4], [5], [6]. The catalytic activity of metal oxide-supported gold has been tested for numerous reactions, including CO oxidation [7], [8], alkene hydrogenation [9], [10], [11], the reduction of nitro compounds [12], [13], [14], and it was recently reported that supported gold particles also exhibit unique catalytic properties for the selective oxidation of n-alkanes to give alcohols and ketones [15], [16], [17]. Particularly, supported gold catalysts are active and selective at relatively low temperatures for numerous reactions of alcohols, including their selective oxidations [18], [19], [20], [21], dehydrogenations [22], [23] and carbonylations [24], [25] to give aldehydes, ketones, esters and carboxylic acids. Due to the importance of the potential applications of most of these reactions, attempts have been made to elucidate the way in which supported gold catalysts function for the transformations of alcohols. However, there is a debate regarding the exact sites for the activation of alcohols and the identity of the intermediate species that are formed during their reactions on supported gold catalysts [23], [26], [27], [28], [29].
Some authors [26], [27] have proposed that alcohols are activated directly on surface gold atoms of the catalysts during their oxidation or dehydrogenation, while others [28], [29] have suggested that they are activated on sites of the supports. Recently, our group reported the oxidation [28] and the dehydrogenation [23] of 2-propanol catalyzed by γ-Al2O3-supported gold. Infrared (IR) spectra measured under reaction conditions indicate that 2-propanol was activated on the support in the form of 2-propoxide species. Our data suggest that neighboring gold particles provide sites to abstract hydrogen atoms from CH bonds of 2-propoxide species to give acetone bonded to Al3+ sites. Those results indicate that γ-Al2O3-supported gold catalysts are bifunctional, with the alcohol being activated on the support and the role of the gold particles consisting of providing sites for hydrogen abstraction and recombination.
We now extend our investigation to the reactions of ethanol on the surface of TiO2-supported gold catalysts. Ethanol is a simple molecule that lends itself as a good probe to investigate its surface reactions [30], [31], [32] and it has technologically important potential applications, including its use as a raw material for the clean production of hydrogen [33], [34], [35]. Here we report the combined use of IR spectroscopy and mass spectrometry to monitor the reactions of ethanol on TiO2 and TiO2-supported gold samples at increasing temperature. Our results indicate that the presence of gold favors the dehydrogenation of ethanol to give acetaldehyde and H2, while butene and water are preferentially formed on samples of the bare TiO2 support. We propose reaction schemes that attempt to explain the influence of gold particles on the surface reactions of ethanol on TiO2. These results provide insight into the way in which supported gold catalysts function for the transformations of alcohols.
Section snippets
Synthesis of TiO2-supported gold samples
Samples of TiO2-supported gold were prepared by a deposition-precipitation method [23], [28], [36], [37]. In the synthesis, a 1 M solution of NaOH (Sigma–Aldrich) was added dropwise to a solution of AuHCl4 (Sigma–Aldrich) at 60 °C under vigorous stirring until the pH of the resulting mixture reached a value of approximately 5.0. The concentration of the AuHCl4 solution was calculated to give TiO2-supported gold samples with 5.0% wt Au. Then, TiO2 powder (Evonik, P25; 30% rutile and 70% anatase)
Evidence of gold nanoparticles in the fresh TiO2-supported gold samples
TEM images characterizing a TiO2-supported gold sample after it had been treated in air at 110 °C for 24 h (Fig. 1a) indicate the presence of gold particles with an average diameter of 7.5 nm. Consistent with these results, UV–vis spectra characterizing the sample include a peak centered at 555 nm (Fig. 1b). Peaks in the range between 500 nm and 570 nm are attributed to the surface plasmon resonance of gold nanoparticles [38], [39], [40], [41], [42], [43], [44], [45], [46], [47].
X-ray absorption near
Conclusions
We investigated the adsorption and reactions of ethanol on the surfaces of TiO2 and TiO2-supported gold samples prepared by deposition–precipitation. Our results indicate that ethanol is adsorbed both molecularly and dissociatively on the surface of both samples, with ethoxy species being bonded to Ti4+ sites. When gold particles are present, adsorbed ethanol is preferentially dehydrogenated to give acetaldehyde and H2, but it is converted mainly to butene and water on samples of the bare TiO2
Acknowledgments
This research was supported by PROMEP. Contract number: IT-CEL-11. We acknowledge the Brazilian Synchrotron Light National Laboratory (LNLS) for financial support and the staff of beamline D06A-DXAS for assisting with the XANES measurements. We also acknowledge the staff at the Central Laboratory of Electron Microscopy at UAMI for their support during the TEM measurements.
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