"Honeycomb structure 20 shown in Fig. 2 is disposed in tube 10. Structure 20 has upper- and lower-course end faces which extend at right angles to the axial direction. It also includes a number of cells 21 which allow the combustion gas to flow from the upper-course end face to the lower-course end face, and cause the gas to come into satisfactory contact with a catalyst. The honeycomb structure is formed of a ceramic material, e.g., cordierite, in order to be able to be fully impregnated with the catalyst. Cells 21 are arranged at a density of 16 to 32 per centimetre square (100 to 200 to one inch square).
The pitch of cells 21 ranges from 1.5 to 1.8 mm."
http://ip.com/patent/EP0382335B1

"Catalyst Pitch
Catalyst pitch is a term used in association with honeycomb and metal plate catalyst
and affects the flue gas velocity in interstitial spaces [1]. As shown in Figure 2.5 pitch,
represented as p, is the width of the catalyst cell plus the cell wall thickness, a. For a given flow rate, wider pitch will result in lower interstitial gas velocities. Appropriate catalyst pitch is important to assure that ash will not deposit and bridge over catalyst cells and pores. Plugging of the catalyst reduces the effective surface area by decreasing the number of active sites available for the NOx reduction reaction."
www.epa.gov/ttn/catc/dir1/cs4-2ch2.pdf

"Honeycomb catalytic apparatus - EP 0382335 B1 - IP.com
http://ip.com/patent/EP0382335B1Cached
Dispositif de catalyseur en nid d'abeilles comprenant : ... direction axiale et une pluralité de cellules (21) permettant au fluide de s'écouler ... de la structure en nid d'abeilles puisse d'écouler vers l'aval et le pas des passages ..."