Au-based catalyst design for selective CO oxidation in hddrogen-rich streams

Abstract

The objective of this study was to investigate the selective CO oxidation over Au/ -Al2O3 catalyst modified with various metal oxide promoters (such as Mg, Mn, Fe, Ni, Ce, and Co) in hydrogen-rich environment. All catalysts contained 1 wt.% Au, 1.25 wt.% promoter over Al2O3 as a support. The catalysts were prepared by the impregnation of the metal oxides to -Al2O3 support followed by the homogeneous deposition precipitation of the gold over composite MOx/ -Al2O3 support and tested in a microflow reaction system both in the absence and the presence of H2O and CO2. The temperature programmed oxidation (TPO) technique was performed in order to investigate the activity of catalyst in the temperature range of 80-150 oC. The Au loading of the catalysts were verified by Atomic Absorption Spectrometry using an ATI Unicam 929. In order to understand the surface structure and distribution of nanosized gold particles, catalysts were characterized by using high resolution transmission electron microscopy (HRTEM). Compare to Au/ -Al2O3 catalyst, the metal oxide promoted (especially MnOx and MgO) Au/MOx/ -Al2O3 exhibited higher catalytic activity towards CO oxidation at the gradient temperatures. The effect of CO2 in the reaction stream was negative as expected but it was balanced even improved by the addition of H2O for Au/MgO/ -Al2O3 catalyst, which elucidated the best performance under realistic reaction conditions (in presence of H2O and CO2). The catalysts containing 1.25 weight per cent and 2.5 weight per cent Mg exhibited comparable CO conversions while the conversion decreased drastically by the further increase of Mg content to 5 per cent. At the temperatures above 130 oC, the H2O loading of 0 vol.%, 5 vol.%, 10 vol.% demonstrated no sign of enhancement in CO conversion over Au/MgO/ -Al2O3 while the addition of H2O increased the conversion significantly at lower temperatures and this increase become more apparent at the higher H2O per cent. The CO conversion decreased as the CO2 content of the feed increased (mostly appeared in the temperatures of 120-150 oC). This effect was significant when the conversion in the absence and presence of 5 vol.% CO2 were compared, while there was no remarkable difference between 5 vol.% to 25 vol.% CO2 content at operational temperature of fuel cell.