(Invited) A Visible Light-Active Titania Photocatalyst with Rhodium As a Built-in Redox Mediator

Titanium(IV) oxide (titania) constitutes one of the most common base for highly active photocatalysts due to the proper electronic structure and physicochemical properties.  The main drawback of this wide bandgap semiconductor is the range of the effective light which is limited to ultraviolet radiation.  Bulk modification of titania with some of the transition metal ions allows to extend its photocatalytic activity toward visible light, however, the origin of the modification is often not sufficiently explained. Although a lot of studies were devoted to bulk modification of titania, the questions of the essential conditions for true doping and its influence on the photocatalytic activity still remain.

Utilization of rhodium (Rh) species for titania modification brings attention because of the similarity in ionic radius of Rh with that of titanium (Ti) with equivalent valency.  Trivalent Rh may also strongly influence the light absorption properties of titania and several titania-based photocatalysts prepared by impregnation or doping with Rh species have been already reported [1-4]. Present work reports detailed studies on properties and photocatalytic activity of Rh-modified titania with respect to nature of modification depending on Rh content and preparation conditions.

Rh-modified titania (Rh-TiO₂) was obtained by impregnation of titania (Showa Titanium FP-6, anatase) with Rh(III) chloride followed by calcination in air or oxygen at various temperatures.  The preparation process led to the improvement of visible-light (> 420 nm) photocatalytic activity of base titania for oxidative decomposition of acetaldehyde (AcH) in air.  The titania crystalline structure, photoabsorption properties and photocatalytic performance depended strongly on the Rh content.  Figure 1 shows the activity of Rh-TiO₂ as a function of molar ratio of rhodium/titanium (Rh/Ti).  At extremely low Rh concentration, the activity was improved, presumably due to the participation of visible light-induced interfacial electron transition from isolated Rh(III) to conduction band of titania.  In the middle range of Rh concentration, the activity was rather reduced by possible hindrance of photoabsorption of titania in its absorption edge with deposition of rhodium oxide. Relatively higher concentration (1mol%) induced enhancement of the activity, which might be attributed to "true" doping, substitution of Ti with Rh in titania.

The results of detailed studies on influence of rhodium amount and calcination conditions on anatase to rutile transformation and visible light induced photocatalytic activity, including photo-sensitization mechanism, will be discussed [5]. 

[1] R. Niishiro, et al., J. Phys. Chem. C, 111 (2007) 17420.

[2] W. Choi, A. Termin, M. R. Hoffmann, J. Phys. Chem., 98 (1994) 13669.

[3] Z.-M. Dai, G. Burgeth, F. Parrino, H. Kisch, J. Organomet. Chem., 694 (2009) 1049.

[4] S. Kitano, K. Hashimoto, H. Kominami, Catal. Today, 164 (2011) 404.

[5] J. Kuncewicz, B. Ohtani, Chem. Commun., in press.