A catalytic reactor apparatus produces high purity water vapor by reacting hydrogen and oxygen together within a catalytic reaction chamber. The reactants and an inert gas, such as argon gas or nitrogen, are supplied to the catalytic reaction chamber in a controlled fashion by a gas panel. The cylindrical catalytic reaction chamber is preferably constructed of titanium or stainless steel. The catalytic reaction chamber is filled with high purity ceramic pellets of a nonreactive material coated with a catalyst, such a noble metal catalyst. Screens at each end of the reaction chamber prevent the catalyst pellets from being transported outside reaction chamber. The interior of the reaction chamber has two perpendicular baffle plates traversing the length to increase contact area with the catalytic pellets for electrical charge and thermal transport during the reaction. The temperature of the reaction chamber is maintained below 350 C during operation.