Factors affecting the photoelectrochemical ®xation of carbon dioxide with semiconductor colloids Brian R. Eggins a,* , Peter K.J. Robertson b , Eileen P. Murphy a , Elaine Woods a , John T.S. Irvine c a School of Biological and Chemical Sciences, University of Ulster at Jordanstown, Shore Road, Newtownabbey, Co. Antrim, BT37 0QB, Northern Ireland, UK b School of Applied Sciences, The Robert Gordon University, St Andrew Street, Aberdeen, AB25 1HG, Scotland, UK c Department of Chemistry, University of St. Andrews, College Gate, North Street, St. Andrews, KY16 9AJ, Scotland, UK Received 17 July 1998; accepted 29 July 1998 Abstract Carbon dioxide was reduced photocatalytically using aqueous CdS or ZnS colloids containing tetramethylammonium chloride to give the dimeric and tetrameric products namely, oxalate, glyoxylate, glycolate and tartrate. A model is presented to explain the role of the tetramethylammonium ions. Studies were also performed using ZnO, SiC, BaTiO 3 and Sr TiO 3 , which in the absence of tetramethylammonium ions produced formate and formaldehyde. The relative quantum ef®ciencies of the six semiconductors were related to their band gaps and conduction band potentials. The role and effectiveness of several `hole acceptor' (electron donor) compounds in this process is shown to be related to their redox potentials. # 1998 Elsevier Science S.A. All rights reserved. Keywords: Photochemical ®xation; Semiconductor colloids; Tetramethylammonium ions 1. Introduction The electrochemical and photoelectrochemical reduction of carbon dioxide has been a topic of interest and sometimes controversy for many years [1]. Usually the formation of single carbon products such as formate or formaldehyde have been reported [1]. We have con®rmed and developed the observations of Bewick [2,3] that the electrochemical reduction of carbon dioxide in aqueous solutions containing tetraalkylammonium salts results in the generation of dimeric and tetrameric products. These observations have been rationalised by ourselves [4] and Gressin [5] with the mechanisms shown in Figs. 1 and 2. The addition of one electron to a carbon dioxide molecule produces a carbon dioxide radical anion, which has been detected by Aylmer-Kelly [6] and Lamy [7]. This radical may be protonated leading to formate; it may be dispropor- tionate to carbon monoxide and carbonate or it may dimerise giving oxalate (Fig. 1). It is with this latter route that our work is concerned. Fig. 2 shows the range of products possible from further reduction of oxalate. Electrochemical reduction of carbon dioxide under these conditions has been shown to lead to oxalate [8], glyoxylate [4,9], glycolate [2,10], tartrate [9] and malate [3]. Our analysis of the cyclic voltammetry of carbon dioxide in aqueous tetramethylammonium perchlorate has further developed the mechanism at the electrode surface [11]. Semiconductors are powerful photo-redox catalysts. A semiconductor particle can act as a microelectrochemical cell [12], which is activated by photolysis. The use of simple semiconductor particles to reduce carbon dioxide was ®rst reported by Halmann et al. in 1978 [13±17]. The products they obtained were mainly formate, and occasionally for- maldehyde and methanol. Henglein [17] continued these studies using mainly cadmium sulphide (CdS) or zinc sulphide (ZnS) particles. With the latter he claimed to obtain quantum yields for formate of up to 0.8 using sulphite or propan-2-ol as hole acceptors. In 1988 and 1990 we [18,19] showed that in the presence of tetramethylammonium chloride the formation of glyoxylic, acetic and oxalic acids occurred as well as the expected formate, formaldehyde and methanol when carbon dioxide was photolysed over cadmium sulphide. Similar results were then obtained with zinc sulphide with the additional formation of glycolic and tartaric acids [20]. Journal of Photochemistry and Photobiology A: Chemistry 118 (1998) 31±40 *Corresponding author. Tel.: +44-1232-366113; fax: +44-1232-366812; e-mail: br.eggins@ulst.ac.uk 1010-6030/98/$ ± see front matter # 1998 Elsevier Science S.A. All rights reserved. PII: S1010-6030(98)00356-6