Temperature-programmed desorption of methanol and oxidation of methanol on Pt–Sn/Al 2O 3 catalysts

Abstract

The temperature-programmed desorption (TPD) of methanol, a widely used solvent contributing to industrial VOC emissions, was studied on two different series of Pt–Sn/Al 2O 3 catalysts that were prepared by coimpregnation and sequential impregnation with Sn first. TPD results showed that methanol decomposed primarily to hydrogen and carbon monoxide. Hydrogen desorbed first, followed by carbon monoxide at higher temperatures. Adding Sn to Pt shifted the desorption peaks of both H 2 and CO to higher temperatures. For the coimpregnated Pt–Sn catalysts, the temperature difference between the H 2 and CO desorption peak maxima increased systematically with increasing Sn content; for the sequentially impregnated catalysts, the temperature difference remained nearly constant. An exception was the catalyst containing 5 wt.% Sn, where the peaks shifted upward by 150 °C. In both catalyst series, the temperature difference between the hydrogen and the carbon monoxide desorption peak maxima increased with increasing Sn content. This study has demonstrated that the adsorption/desorption behavior of methanol on platinum is strongly modified by the presence of tin. The oxidation of methanol over the monometallic Pt and bimetallic Pt–Sn on alumina support was studied in the temperature range 35–300 °C and initial concentrations of methanol in the range 500–1200 ppm and excess oxygen (21% O 2). The experimental results showed that the monometallic Pt catalyst were much more active than the bimetallic catalysts. The coimpregnated catalysts were more active than the sequentially impregnated catalysts. CO 2 and methyl formate (CH 3OCHO) were the only carbon-containing products of methanol oxidation. Methyl formate was the principle product at low temperatures, while CO 2 was the principle product at high temperatures. The reaction order of methanol oxidation was 1.15±0.05. The apparent activation energy of the monometallic platinum catalyst was 14.4 kJ/mol. For the coimpregnated catalysts, addition of tin increased the apparent activation energy while in the sequentially impregnated Pt–Sn the apparent activation energy remained essentially constant over the range 0.6–1.5 wt.% Sn, then shifted to 66.8 kJ/mol for 5 wt.% Sn.