Interesting, from E. Lalik et al. Oscillatory Behavior and Anomalous Heat Evolution in Recombination of H2 and O2 on Pd-based Catalysts,
June 29, 2015
https://pubs.acs.org/doi/abs/10.1021/acs.iecr.5b00686
"In designing nuclear reactors, safety must be kept uppermost in one’s mind and prominent among the safety issues is that of hydrogen safety. Due to various adverse processes, the water that is used as a cooling medium within the reactor confinement can also be a source of gaseous hydrogen. The latter’s concentration, if it is accumulated in an uncontrolled manner, may easily reach an explosive limit in a mixture with air. A hydrogen explosion is therefore a threat which, due to the fact that it may take place in a close vicinity to the nuclear core, can have potentially disastrous consequences, including an uncontrollable release of radioactive material. To mitigate the hazard posed by hydrogen, passive autocatalytic recombiners (PARs) are widely applied in nuclear reactors as a remedy. In spite of the name, the recombination reaction of H2 + O2→ H2O, which proceeds within the PAR, is not truly an autocatalytic one. It requires solid catalyst for the recombination to occur at a moderate temperature, and supported noble metals (Pd, Pt) are mostly used for this purpose. The idea behind the PAR design is that it should work reliably without constant human control, and crucially, without an external energy source such as electricity, since in case of an accident, such factors might well be expected to fail. The catalytic recombination of H2 and O2 should be, therefore, ideally left to its own means in the task of prevention of hydrogen gas accumulation. However, complexity of the reaction mechanism and energetics will have a bearing on effectiveness of PAR technology. The formation of water is a highly exothermic reaction (∆H°(298.15 K) = –242 kJ/mol H2 for H2Og formation from elements, and so a potentially high rate of heat evolution must be carefully considered in effective PAR design. Currently, this is addressed by assuming a twofold role played by natural convection. Apart from the removal of the evolving heat, it is also supposed to ensure an efficient gas circulation within the PAR interior. This assumes that the heat evolution in this reaction is “well-behaved”. But there are hints that the system may behave in a less than predictable manner. In fact, the reaction is known to have rather intricate kinetics. It is capable of attaining multiple steady states6,7, a trait it shares with other Pd-catalyzed reactions. The notoriously evasive hysteretic phenomena, in other words multiple steady states in the metallic Pd catalysis, may not be featured often in literature, but are a frequent subject of conversations among scientists working in the field. The metal-catalyzed reaction is also capable of reaching oscillatory regime(s). The oscillatory kinetics in the hydrogen oxidation have been reported on palladium, platinum, and nickel, i.e., on the metals that are also known for dissociative sorption of H2. Although oscillatory oxidation of H2 on Pd is not studied very often, metallic Pd is by no means a stranger in oscillatory catalysis. In fact, oscillatory oxidations of CO on metallic Pd or Pt are classic systems widely studied for their nonlinear dynamics, also leading to the Nobel Prize in chemistry in 2007. As for the H/Pd system, a handful of results on oscillatory hydrogenations have been published. Thermokinetic oscillations in the Pd/H system, i.e., the oscillatory rate of heat evolution accompanying the sorption of hydrogen and/or deuterium in metallic palladium have been reported recently. Moreover, an incidence of anomalously high thermal effects has been observed in the H2/O2/Pd system, as well as in the H2/O2/Ni, and also with noble metals catalysts such as Au/TiO2, considerably exceeding the thermodynamically expected heat of water formation. Here we use the gas flow-through microcalorimetric method to detect both the thermokinetic oscillations as well as anomalous heat evolutions in the catalytic recombination of H2 and O2 on the Pd supported catalysts, including Pd/Al2O3, Pd/Al2O3 and PdAu/Al2O3, i.e. of the type usually applied in the PARs. We show that instantaneous thermal effects as high as 700 kJ/mol H2 which is nearly three times as much as the “normal” heat of water formation, may unexpectedly appear in this reaction. We believe that those findings are important for the hydrogen safety of nuclear reactors, and that heat evolutions much larger than the thermodynamic values of 242 kJ/mol H2 should be considered in the PAR design proceedings."