Title: Oscillatory heat evolution in the Pd/H system

Erwin Lalik

Polish Academy of Sciences, Poland


E. Lalik graduated at the Technical University of Krakow in 1984 in chemical engineering.  He obtained his PhD at the Birkbeck College, University of London, in crystallography in 2003.  He is currently an assistant professor at Jerzy Haber Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, in Krakow (Poland) and is running the Laboratory of Microcalorimetry.


Metallic palladium has a unique property of being able to absorb large amounts of gaseous hydrogen into its crystal lattice, and it is well established that the hydrogen is accommodated within the Pd lattice in the atomic form.  Hence, the dissociation of molecular hydrogen and the formation of atomic hydrogen species must occur on the Pd surface prior to their penetration into the Pd bulk.  However, the actual chemical mechanism of the process is not sufficiently understood.  It has been discovered, that the sorption of hydrogen in Pd may proceed in oscillatory manner.  The oscillations of the rate of heat evolution accompanying the sorption of hydrogen in Pd demonstrating all kinds of dynamics (periodic, quasiperiodic and chaotic) have been recorded microcalorimetrically.  The occurrence of chaos has been confirmed mathematically by detecting deterministic character of microcalorimetric time series.  The oscillation frequency turned out to be a linear function of atomic parameters (more precisely, a linear function of a product of the first ionization potential times the square root of atomic mass) of the inert gases admixed in small proportion to the stream of hydrogen prior to its contact with palladium.  Combination of the gas flow-through microcalorimetry with potentiostatic measurements in situ revealed a correlation between chaotic variations of electric conductivity of Pd powder and the frequency of periodic thermokinetic oscillations in the sorption of H2 and D2 in the metal.  Basing on these findings, a mechanism has been proposed that accounts for the occurrence of oscillatory kinetics in sorption of hydrogen in Pd.  The microcalorimetric measurements have also been used to study the thermal effects of the recombination reaction of H2 and O2 under conditions resembling those affecting performance of the supported Pd-based catalyst used in nuclear reactors to mitigate the hazard of hydrogen explosions.  Deterministic chaos has been revealed to prevail at several occasions in the heat evolution accompanying this very exothermic reaction of H2O formation from elements and, moreover, fortuitously the actual values of heat productions exceeded the thermodynamic value of the heat water formation (242 kJ/mol H2) by a factor of three [5].  The unexpectedly high heat evolution in the H2 + O2 recombination may be a result of interaction of oxygen with atomic hydrogen species gathered on the surface of metallic palladium in a periodic manner during the sorption process.

Audience Take Away:

•    Oscillatory chemical reactions are still considered a curiosity in spite of long history of research in this area.
•    The so called passive autocatalytic recombiners (PAR) are the safety devices installed within the nuclear reactor's confinements in order to prevent a build up of hazardous concentration of gaseous hydrogen by its recombination with oxygen.  The latter is a highly exothermic reaction.  The standard heat of water formation is 242 kJ/mol H2 and this value is supposed to be used in designing the PARs parameters.  It has been shown, however, that accidental heat evolution may reach as high 700 kJ/mol H2, due to the oscillations in the H2+O2 reaction.  Without considering this possibility, the underdisigned PAR may be prone to overheating, and thus in spite of being intended for safety, it may be pose a risk of hydrogen explosion itself.
•    A precise mechanism of the anomalously high heat evolution in the recombination H2+O2 reaction is not fully understood.  It seems possible, however, that it may be related to the phenomenon of the so called low energy nuclear reactions (LENR).  The latter involve atomic hydrogen formation on catalysts such like metallic Pd or Ni.