A catalytic converter unit for pre-filtering dirt particles from a gas discharge containing an ozone-laden gas discharge from electrographic copy machines, in advance of the passage of the gas discharge through a gas-neutralizing catalyst bed, to exclude the passage of the dirt particles which can cause contamination and degradation of the catalyst, and plugging of the catalyst bed resulting in reduced gas-permeability and the creation of a significant pressure drop thereacross which interferes with or substantially reduces the rate of gas flow through the unit. The catalytic converter units incorporate an easily-removable, cleanable and/or replaceable dirt filter screen which is supported for simple removal from the unit for periodic cleaning or replacement.