Selective Dispersion of the Ru-RuO x /TiO 2 Catalyst for Methanation of CO 2 at Room Temperature and Atmospheric Pressur
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SELECTIVE DISPERSION OF THE Ru-RuOx/I'iO2 CATALYST FOR METHANATION OF CO 2 AT ROOM TEMPERATURE AND ATMOSPHERIC PRESSURE
P. Ruterana, P-A. Buffat, K.R. Thampi* and M. Graetzel* 12M and ICP* The Federal Institute of Technology Lausanne PhB-Ecublens, Ch-1015 Lausanne, Switzerland
ABSTRACT A structural investigation of the Ru-RuOx/TiO 2 catalyst was carried out in order to determine the role played by the support, the size of the particles, their dispersion, structural relationship with the support and the preparation conditions on the activity. In the ideal catalyst the metallic particles are 10-155A in size. Moreover they are selectively dispersed on the TiO 2 crystallites with the rutile structure. This uneven distribution is not found for other metals ( Rh, Pt, Co, Ni, etc...) on P25 though the preparation procedure was similar. I. INTRODUCTION Efficient techniques to transform carbon oxides to fuels remain an important goal in catalysis. They could eventually lead to the production of useful hydrocarbons while recycling the potentially dangerous levels of CO2 in the atmosphere. The recent finding' that C02 may be the primary source in the synthesis of methanol from CO-CO 2-H 2 mixture is another important step towards the possible use of CO2 as an industrial feedstock. The main route to hydrocarbon product from C0 2 is via the so called Sabatier reaction: C0
2
+ 4H 2 -4 CH4 + 2H 2 0
(1)
In principle, this reaction is thermodynamically downhill (AG 0 2 98 K = -27 kcal/mol). However the reduction of CO2 (8 electrons) to CH4 by H2 is difficult to achieve. The formation of by products is common. Conventionally, a catalyst at elevated temperature and high pressure are required 2 ,3 for methane generation at significant rates and yield. Recently 4 , the authors have shown that the Ru-RuOx(,2friO 2 catalyst is active in methanation of CO 2 at room temperature and atmospheric pressure. Moreover it's activity is substantially enhanced upon illumination during the reaction 5 . Our preliminary results on electron microscopy study 6 have shown that the ideal catalyst is made of a non uniform dispersion of 10-15A RuOx particles on the TiO 2 (Degussa P25 80% anatase and 20% rutile) support. Here we present a detailed structural analysis of this catalyst in comparison with the catalytic activity measurements. The latter will only briefly discussed, for most of them have already been dealt with in detail elsewhere4 ,5 . They are used here to illustrate the role played by the support and the preparation procedure upon the catalytic activity. II. EXPERIMENTAL The catalyst preparation follows a well defined procedure called deposition / precipitation 4 . The TiO 2 powder (55 g.m"2 support is suspended in an acidic solution of RuC13. The mixture is then hydrolized at 70'C by adjusting the pH to 4-4.5 with NaOH. This pH setting was found to be a critical parameter in order to produce an ideal catalyst made of very small RuRuOx particles . The powder is then dried by evaporation. Two calcination steps are then carried Mat. Res. S
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