Investigation of Co-based DeNOx catalysts for the treatment of natural gas lean-burn engine exhaust

The catalytic reduction of nitric oxide with methane in lean conditions has been investigated on cobalt-based powder catalysts such as Co-exchanged ferrierite, Co-exchanged beta zeolite and Co-ZrO2. Co-exchanged ferrierite was also prepared in monolith shape. From catalyst laboratory testing the following activity scale was obtained: Co-ferrierite > Co-beta >>>Co-ZrO2, suggesting a significant role of the support catalytic matrix. At 30,000 h-1, in the absence of water, a maximum value of NOx 50 % conversion was obtained with Co-ferrierite. Water addition to the feed resulted in marked reduction of NOx conversion and shifting to higher temperatures of CH4 conversion curve. Co exchanged monolith samples showed something lower NOx reduction activity with respect to powder catalysts. In the tests at the engine exhaust, the best results, after water separation from the exhaust, were obtained with Co-ferrierite (65 % NO conversion and simultaneous total conversion of both CH4 and CO at 480 °C, 20,000 h-1). However, in the presence of the very high concentration of water, typical of the engine exhaust, the performances were dramatically reduced.