Photo-catalytic oxidation of cyclohexane over TiO₂: a novel interpretation of temperature dependent performance

Abstract

The rate of cyclohexane photo-catalytic oxidation to cyclohexanone over anatase TiO 2 was studied at temperatures between 23 and 60 1C by in situ ATR-FTIR spectroscopy, and the kinetic parameters were estimated using a microkinetic model. At low temperatures, surface cyclohexanone formation is limited by cyclohexane adsorption due to unfavorable desorption of H 2 O, rather than previously proposed slow desorption of the product cyclohexanone. Up to 50 1C, the activation energy for photocatalytic cyclohexanone formation is zero, while carboxylates are formed with an activation energy of 18.4 AE 3.3 kJ mol À1 . Above 50 1C, significant (thermal) oxidation of cyclohexanone contributes to carboxylate formation. The irreversibly adsorbed carboxylates lead to deactivation of the catalyst, and are most likely the predominant cause of the non-Arrhenius behavior at relatively high reaction temperatures, rather than cyclohexane adsorption limitations. The results imply that elevating the reaction temperature of photocatalytic cyclohexane oxidation reduces selectivity, and is not a means to suppress catalyst deactivation.

Cl2 Production by Photocatalytic Oxidation of HCl over TiO2

“…We studied the influence of temperature on the reaction to find suitable conditions for further experiments. For this purpose, we used the commercial anatase TiO 2 Hombikat UV100, which possesses high catalytic activity in various photooxidation reactions …”

“…We studied the influence of temperature on the reactiont o find suitable conditions for furthere xperiments.F or this purpose, we used the commercial anatase TiO 2 Hombikat UV100, which possesses high catalytic activity in various photooxidation reactions. [11,12] Figure 1s hows the result of the temperaturev ariation experiment. The rate gradually increased with temperature to a maximum value of 16 mmol h À1 m À2 at 140 8C.…”

“…Similart emperature dependencies reported in the literature were ascribed to adsorption-desorption equilibria. [9,10] Mul and co-workers [12,13] encountered ar elated issue for the photooxidation of cyclohexaneo ver TiO 2 .T he unfavorable desorption of water limited the adsorption capacity for cyclohexane at low temperature. This explanation maya lso hold for the Figure 1.…”

In Situ Study of the Photodegradation of Carbofuran Deposited on TiO2Film under UV Light, Using ATR-FTIR Coupled to HS-MCR-ALS

“…Mul et al have used ATR-IR spectroscopy to the study the kinetics of the photochemical oxidation of cyclohexane on solid surfaces. [18][19][20] This methodology was also utilized by Bürgi et al to examine the photochemical decomposition of malonic acid. 21,22 Frei et al have used the rapid scan technique to identify surface intermediates in the oxidations of water.…”

The influence of water vapour on the photocatalytic oxidation of cyclohexane in an internally illuminated monolith reactor

“…by addition of water [18,19] or solvents [19][20][21] • regeneration of deactivated photocatalysts [22] • optimisation of the operation temperature [23] • optimisation of (the wavelength of) the irradiance [15,24] • optimisation of the oxygen concentration [21] • optimisation of the reactor design [25] • mechanistic understanding [26,27] to allow rational design of improved catalyst materials…”