Nanophase composite catalysts for electrosynthesis

L.Petrik1, V.M.Linkov 1 1Department of Chemistry, University of the Western Cape, Private Bag X17, Bellville 7535, South Africa.

Introduction Nano particles of catalytically active metals may be deposited on inorganic supports, such as zeolites, in order to prepare nanocomposite materials suitable for electrocatalysis. Nanophase electroactive electrodes may be prepared by encapsulation and subsequent support of known electroactive metals, Pt and Ni, in zeolites [1,2]. High loadings or complete void filling of zeolite pores with metals may result in stable nanophases in zeolites, as these materials are known to offer a stable matrix and have angstrom sized pores. Thus, it may be possible to create electroactive nanoclusters upon the external surface of the zeolite or angstrom thick “wires” [3,4], throughout the void space of the zeolite, with the zeolite acting as a stabilizing matrix for the electroactive nanophase – to prevent nanophase aggregation. Furthermore, it is commonly believed that an electrocatalyst must be in contact with the electrode, but in slurry phase electrolysis of water [5] the momentary contact with the feeder electrode of a zeolite containing a loading of electroactive supported nanophase as ultramicroelectrode may allow charge transfer to occur [6]. Moreover, the zeolite may provide a solid-state ionic component to the dispersion medium, which is consistent with a surface conductance [7]. These materials have a strong ionic nature and could act as solid state electrolytes in aqueous or other solvent systems. Enhancement of catalytic reactivity can be expected by interfacial interactions of the support.

The development of novel electrodes, in which the catalytic components (Ni and Pt) are deposited in the form of nanostructures upon various zeolite and membrane supports in preparation of suitable electrode materials for potential application in electrosynthesis, is described for hydrogen generation by water electrolysis. Three methods were investigated for preparation of nanocomposite catalysts. These were firstly, hydrothermal synthesis techniques, secondly, sequential deposition and finally, development of ultramicroelectrodes. The activity of developed composite zeolite modified electrodes was tested for hydrogen production under standardized electrochemical conditions. Estimated hydrogen production costs were calculated based upon the current density obtained at the applied voltage in the different cases tested. The design of small bench scale membrane reactors was optimized for application of nanostructured electrodes in hydrogen production by low or high conductivity water electrolysis for renewable energy systems.

Results and Discussion Rapid hydrothermal synthesis techniques were successfully used to prepare composite zeolite BEA membrane electrodes within 2-4hr on various conductive supports that could subsequently be bifunctionalized by incorporation of electroactive metals such as Pt for application in electrocatalytic processes. Although potentially applicable in gas phase catalysis as well as electrocatalysis, such materials were found costly and difficult to prepare on larger scale.

A less complex process of manufacture to prepare zeolite modified electrodes (ZME) was investigated in which nanoclusters of Pt were synthesized within the pore structure of different zeolites where after the modified zeolite was sequentially deposited in composite configurations by spray coating admixtures of metal-zeolite, conductive phase and binder upon various supports. ZME samples suitable for application in high electrolyte solutions were compared to flat Ni foil electrodes and to carbon based high surface area electrodes. Moreover, ultramicroelectrodes (UME) were prepared by synthesis of nanoclusters of Pt and Ni within the pore structure of different zeolite types. Effective means of metal deposition and suitable reduction methods for preparing active nanophase electro catalysts were developed. The activity of these zeolite based composite nano phase ultramicroelectrodes (UME) was then determined in low electrolyte hydrogen generation. Thus it was possible to separately evaluate nanophase electroactivity apart from other support-induced variables.

Electroactivity of zeolite supported ultramicroelectrodes was found to be a complex function of metal type and concentration, host matrix, zeolite typology, porosity, and acidity as well as supported metal nanocluster size. The nanophase metal type as well as its configuration, which is imposed by the zeolite host matrix, and other zeolite related contributing factors such as pore size (implying diffusional constraints), and the local electrostatic field within the pore are thus shown to have an effect on electroactivity. The optimum supported nanoscale metal cluster size that resulted in significant electroactivity was found to be between 5-10nm. This study provides proof of concept for application of composite zeolite modified nanophase electrode materials and ultramicroelectrodes in electrochemical hydrogen generation by water electrolysis. Cost effective preparation of enhanced surface area nanophase electrodes shows significant progress towards achieving high activity for hydrogen production by water electrolysis.

References 1. Rolison D.R.; E. A. Hayes; W. E. Rudzinski. J.Phys.Chem. 93: 14 (1989) 5524 -5531 2. Bessel C.A., D.R.Rolison. J.Electroanal. Chem. 439 (1997) 97-105 196. 3. Fukuoka A., Higashimoto N., Sakamoto Y., Sasaki M., Sugimoto N., Inagaki S., Fukushima Y. & Ichikawa M. Catalysis Today 66:1 (2001) 23-31. 4. Liu Z., Sakamoto Y., Ohsuna T., Hiraga K., Terasaki O., Ko C.H., Shin H.J. & Ryoo R. Angewandte Chemie - International Edition 39:17 (2000), 3107-3110. 5. Goldman A, J Goldstein, J. Meitav, I.L Arieh, E. Pecherer. US 5196275, 1993 6. Oman S. Talanta, 51:1 (2000)21 – 31 7. Rolison D.R., J.Z. Stemple, D.J.Curran, “Proc. 9th International Zeolite Conference”, Montreal, Eds. R.von Ballmoos et al., (1992) 699-708, Butterworth-Heinemann (USA)