Catalytic fart flaps for cows

A flap would hang from the tail over the anus of a cow for the purpose of converting the methane into less harmful gases (carbon dioxide, water vapor, oxygen). The flap would need to be a flap so that it can be pushed out of the way when more solid stuff comes out. It would not work perfectly, but it would eliminate some greenhouse gases. The flap would need to pass gases through, and through a catalytic process, convert as much of the methane as possible. Some of the problems would be: What would be the catalyst? Platinum group metals, which are used as catalysts, are expensive. Motivating farmers to put them on their cows.