Elsevier

Applied Catalysis A: General

Volumes 464–465, 15 August 2013, Pages 374-383
Applied Catalysis A: General

Influence of supported gold particles on the surface reactions of ethanol on TiO2

https://doi.org/10.1016/j.apcata.2013.06.019Get rights and content

Highlights

  • TiO2-supported gold favors the dehydrogenation of ethanol to acetaldehyde.

  • Ethanol is converted mainly to butene and water on the bare TiO2 support.

  • Au atoms tune the reactions of ethanol on TiO2, providing sites for H recombination.

Abstract

The adsorption and reactions of ethanol on the surfaces of TiO2 and TiO2-supported gold samples were investigated by infrared (IR) spectroscopy and mass spectrometry. IR spectra measured as the samples were treated in flowing ethanol at room temperature indicate that the alcohol was adsorbed both molecularly and dissociatively in the form of ethoxy species bonded to Ti4+ sites. Our results suggest that the presence of neighboring gold particles facilitates cleavage of the β-Csingle bondH bond of the ethoxy species at increasing temperature, leading to the formation of acetaldehyde linearly bonded to Ti4+ sites, which is readily desorbed. In the process, hydrogen adatoms are recombined on gold particles to give H2. In absence of gold, Ti4+ sites abstract hydrogen atoms from the ethoxy species to give titanium hydride species, which react with hydroxyl groups to produce water and leave oxygen vacancies on the TiO2 surface. Acetaldehyde is formed in the process and it is adsorbed in a bidentate configuration on the oxygen vacancies, where it undergoes reductive coupling to give butene. Our data show that the ability of gold particles to abstract and recombine hydrogen atoms prevents the formation of oxygen vacancies on the support and, in consequence, hinders the subsequent reactions of the produced acetaldehyde.

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 Csingle bondH 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.

References (107)

  • N. Funazaki et al.

    Sens. Actuators B

    (1993)
  • G. Neri et al.

    Sens. Actuators B

    (2003)
  • G.J. Hutchings et al.

    J. Catal.

    (2006)
  • M. Haruta et al.

    J. Catal.

    (1993)
  • D.A.H. Cunningham et al.

    J. Catal.

    (1998)
  • M. Okumura et al.

    Catal. Today

    (2002)
  • A. Ueda et al.

    Appl. Catal. B: Environ.

    (1998)
  • B.E. Solsona et al.

    Appl. Catal. A: Gen.

    (2006)
  • A.V. Biradar et al.

    Appl. Catal. A: Gen.

    (2012)
  • L. Prati et al.

    J. Catal.

    (1998)
  • O.A. Simakova et al.

    J. Catal.

    (2012)
  • Y. Guan et al.

    Appl. Catal. A: Gen.

    (2009)
  • Z. Martinez-Ramirez et al.

    Surf. Sci.

    (2012)
  • Z. Martinez-Ramirez et al.

    J. Mol. Catal. A: Chem.

    (2011)
  • J.I. Di Cosimo et al.

    J. Catal.

    (2000)
  • J. Llorca et al.

    J. Catal.

    (2002)
  • H.H. Kung et al.

    J. Catal.

    (2003)
  • A.J. Maira et al.

    J. Catal.

    (2001)
  • S. Zhu et al.

    Appl. Catal. B: Environ.

    (2012)
  • A.M. Nadeem et al.

    Catal. Today

    (2012)
  • X. Pan et al.

    Appl. Catal. A: Gen.

    (2013)
  • R. Zanella et al.

    J. Catal.

    (2004)
  • Á. Veres et al.

    Catal. Today

    (2012)
  • A. Naldoni et al.

    Appl. Catal. B: Environ.

    (2013)
  • J. Huang et al.

    J. Catal.

    (2007)
  • G. Martra

    Appl. Catal. A: Gen.

    (2000)
  • J. Raskó et al.

    Appl. Catal. A: Gen.

    (2006)
  • J. Raskó et al.

    Appl. Catal. A: Gen.

    (2004)
  • M. El-Maazawi et al.

    J. Catal.

    (2000)
  • M. Dömök et al.

    Appl. Catal. B: Environ.

    (2007)
  • B.A. Sexton et al.

    Surf. Sci.

    (1982)
  • G.A. Flores-Escamilla et al.

    J. Mol. Catal. A: Chem.

    (2012)
  • E. Finocchio et al.

    Catal. Today

    (1999)
  • L. Huang et al.

    J. Mol. Catal. A: Chem.

    (2008)
  • J. Raskó et al.

    Appl. Catal. A: Gen.

    (2005)
  • H. Hollenstein et al.

    Spectrochim. Acta A

    (1971)
  • A. Yee et al.

    J. Catal.

    (1999)
  • A. Yee et al.

    Catal. Today

    (2000)
  • P.Y. Sheng et al.

    J. Catal.

    (2002)
  • J. Llorca et al.

    J. Catal.

    (2004)
  • S.M. de Lima et al.

    Appl. Catal. A: Gen.

    (2009)
  • S.M. de Lima et al.

    J. Catal.

    (2009)
  • H. Madhavaram et al.

    J. Catal.

    (2004)
  • C.K.S. Choong et al.

    Appl. Catal. A: Gen.

    (2011)
  • A. Erdőhelyi et al.

    Catal. Today

    (2006)
  • E. Iglesia et al.

    Catal. Today

    (1997)
  • K. Fujimoto et al.
  • M. Manzoli et al.

    Catal. Today

    (2012)
  • I. Nakamura et al.

    Surf. Sci.

    (2012)
  • P. Serna et al.

    J. Catal.

    (2009)
  • Cited by (19)

    • Reverse hydrogen spillover during ethanol dehydrogenation on TiO<inf>2</inf>-supported gold catalysts

      2017, Molecular Catalysis
      Citation Excerpt :

      Accordingly, bands at 1735, 1704 and 1420 cm−1 appeared in the spectra (Fig. 5A). The former two bands are assigned to the υCO vibration mode acetaldehyde in the gas-phase and linearly bonded to Ti4+ sites, respectively [10,46,47], whereas the latter might be assigned to the δCH3 vibration mode of adsorbed acetaldehyde [46,47,49]. The appearance of those bands is consistent with the dehydrogenation of ethoxy species on the sample, in agreement with mass spectra of the effluent gases from the flow reactor/DRIFT cell showing formation of acetaldehyde (Fig. 4A).

    • Promoting effect of alcohols and formic acid on Au-catalyzed one-pot myrtenol amination

      2017, Molecular Catalysis
      Citation Excerpt :

      Latter can allow selectivity control in the case of substrates possessing several functional groups which can provoke competitive hydrogenation. Herein the study was focused on the improvement of the hydrogen transfer to produce the desired amine, since the imine accumulation was detected in the case of myrtenol amination due to competitive CC bond hydrogenation and/or insufficient hydrogen formation on the surface of gold particles as a result of non-specific myrtenol dehydrogenation involving both support active sites and gold nanoparticles [24,27,28]. In our previous work it was demonstrated that introduction of additional external molecular hydrogen in the presence of Au/ZrO2 catalyst permitted a more effective hydrogen transfer with enhanced yield of the target amines [29].

    View all citing articles on Scopus
    View full text