Thermal stability of metal-supported catalysts for reduction of cold-startemissions in a wood-fired domestic boiler

The aim of the present work is to develop a catalyst based on a mixture of manganese oxides and platinum supported on a metallic monolith for abatemen t of emissions from wood combustion, particularly during the cold-start pha se. The activity and the thermal stability of the catalysts have been studi ed in the laboratory, before performing tests in a wood-stove. The effect o f the hydrothermal treatment at 900 degrees C on the adherence of the washc oat onto a metallic substrate was studied using scanning electronic microsc ope. It revealed well-adhering washcoat onto the metallic support due to th e growth of the alumina whiskers during the treatment. The influence of the amount of washcoat, as well as the influence of the concentration of manga nese oxides in it (Mn: 5 to 20 mol%/Al2O3) on the activity of fresh and hyd rothermally-treated catalysts were studied. The activity tests were carried out using a mixture of carbon monoxide, naphthalene and methane in the pre sence of air, steam and carbon dioxide to resemble the flue gases from wood combustion. On the fresh catalysts, containing the same total amount of ma nganese, a high concentration of manganese oxides in the washcoat favoured the oxidation of carbon monoxide and naphthalene, whereas a lower concentra tion of manganese oxides in the washcoat gave higher activity for the oxida tion of methane. An increased total amount of manganese oxides in the catal ysts, which had the same amount of washcoat, resulted in an increase in act ivity for the oxidation of the three combustibles. After thermal treatment at 900 degrees C for 270 h in steam, most of the manganese oxide catalysts were activated for the oxidation of carbon monoxide and naphthalene while o nly being slightly deactivated for the oxidation of methane. The addition o f manganese oxides in the washcoat, however, lowers the temperature of the gamma- to alpha-alumina phase transformation. Platinum (0.5 mol%) was added to the manganese oxide (10 mol%) catalyst to improve its activity. A plati num catalyst was also tested for comparison. The platinum and the mixed cat alysts showed similar activity for the oxidation of carbon monoxide and nap hthalene, while the mixed catalysts were more active for the oxidation of m ethane. A similarly mixed MnOx-Pt (10-0.5 mol%) catalyst supported on Al2O3 stabilised with 3% lanthanum, but at larger scale, was tested in a wood-st ove. The possibility of pre-heating the catalyst during the start-up phase was studied. The tests revealed a strong decrease of the carbon monoxide an d unburned hydrocarbons emissions during the start-up phase when the cataly st was pre-heated with hot air compared with no pre-heating or no catalyst. (C)1999 Elsevier Science B.V. All rights reserved.