The structure of the catalyst used in a passive autocatalytic recombiner (PAR) is crucial for making the PAR reliably functional in environments of high humidity and for concentrations of hydrogen above 8 to 10 vol %. The temperature of the catalyst has to be kept below 500 degrees C to avoid the autoignition of hydrogen. A new type of catalyst for the PAR, a hydrophobic catalyst on a low porous metal carrier with a screen [HCm(screen)], was designed by Russian Energy Technologies. It consists of a porous Ti plate with the adsorbtion metal Pt. The surface of the catalyst was completely covered by a metal grid. In a series of tests with different small-scale PARs, the HCm(screen) catalyst was found to function under concentrations of hydrogen up to at least 20 vol %. The effects of mass and heat transfer processes (Fick diffusion, Knudsen diffusion, and Stefan flow) on the thermal regime and characteristics of the working catalyst are discussed. Metal grids of dense weaving appear to be the most suitable for a screen because they have a double function: removing the heat and acting as a gas separation membrane enriching with hydrogen the gas mix in the zone of the catalytic reaction.