How Gold Deposition Affects Anatase Performance in the Photo-catalytic Oxidation of Cyclohexane
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How Gold Deposition Affects Anatase Performance in the Photo-catalytic Oxidation of Cyclohexane Joana T. Carneiro Æ Chieh-Chao Yang Æ John A. Moma Æ Jacob A. Moulijn Æ Guido Mul
Received: 5 September 2008 / Accepted: 25 November 2008 / Published online: 9 January 2009 Ó The Author(s) 2009. This article is published with open access at Springerlink.com
Abstract Gold deposition on Hombikat UV100 was found to negatively affect the activity of this Anatase catalyst in selective photo-oxidation of cyclohexane. By ammonia TPD and DRIFT spectroscopy it was determined that the Au deposition procedure leads to a significant decrease in OH-group density (mol m-2BET) on Hombikat, suggesting that the amount of surface OH-groups, rather than the presence or absence of Au, is determining the catalytic performance. The importance of surface OH-groups was demonstrated by comparing the performance of Hombikat (with and without Au deposition) to surface propoxylated TiO2, synthesized by a sol–gel method from titanium (IV) isopropoxide. The effect of the deposition recipe of noble metals on the surface composition of TiO2 should thus be taken into account in evaluating and explaining photocatalytic performance of TiO2 modified by noble metals (Au), in particular in non-aqueous phase reactions. Keywords TiO2 Titanium (IV) isopropoxide Au OH-group density Cyclohexane Oxidation Photocatalysis Sol–gel DRIFT NH3
J. T. Carneiro C.-C. Yang J. A. Moulijn G. Mul (&) Catalysis Engineering, DelftChemTech, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands e-mail: [email protected] J. A. Moma Project Au-TEK Catalysis, Advanced Materials Division, MINTEK, Private Bag X3015, Randburg, South Africa
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1 Introduction Liquid phase oxidation of cyclohexane is an important reaction for the conversion of cyclohexane via cyclohexanone in caprolactam, which is the monomer for nylon-6 production. The current commercial process is very energetically intensive, and several studies have focused on the direct use of photon energy to selectively oxidize cyclohexane at room temperature using a photocatalyst [1–11]. Although a high ketone over alcohol selectivity is obtained in photocatalytic cyclohexane oxidation, a low conversion (quantum efficiency) and rapid catalyst deactivation prevent application in practice. In order to overcome these problems several improvements can be made in catalyst, reactor [12], and process design [13]. Of the many options to enhance the efficiency of TiO2 photocatalyst, noble metal deposition (Pt, Pd) has been shown to be effective in many occasions. Various studies show that metal deposition enhances the photocatalytic activity of TiO2 in photo degradation reactions in the aqueous phase [14–16], as well as in gas phase applications [17, 18]. Besides modification by Pt or Pd, modification of Titania by Au has recently been reported to enhance performance in photocatalytic applications involving waste water treatment [19–22]. Most of these studies attribute the enhancement in
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