An apparatus for treating gaseous pollutants in an air stream such as a regenerative thermal oxidizer or a selective catalytic reduction system, including a reaction chamber wherein the reaction media bed within the chamber comprises a plurality of nonprismatic ceramic blocks each having end faces, side faces and a chamfered face extending at an angle relative to the side faces of the blocks. The ceramic blocks are stacked in end-to-end and side-to-side relation within the reaction chamber and the ceramic blocks each include spaced parallel passages extending through the end and chamfered faces. The chamfered faces of adjacent blocks form voids between the blocks promoting turbulent gas flow through the voids and equalizing the pressure within the reaction chamber. The ceramic blocks may be symmetrical, wherein only a portion of the gas flows into the voids between the blocks and the remainder flows through the channels through the end faces or the ceramic blocks may be nonsymmetrical, wherein the chamfered faces are located vertically opposite an end face, such that all of the gas flowing through the reaction chamber flows through the gas passages into the voids between the blocks at alternative levels.