An interesting proposition for an advanced transportation fuel would be presented to us - IF (big if) hydrogen can be routinely converted into a denser form on a catalyst, and then expanded in a piston engine configuration. This concept would relate to using argon as a "pseudo oxidizer." Argon is not exactly "inert" to the same extent as helium and other Column eight atoms (Vlll on the periodic table).
AFAIK this exact concept, when transposed into a piston engine configuration, has never been explored... or has it? There is the Papp engine, which used argon and other inert gases but did not use hydrogen; and there is the Laumann engine which included oxygen with argon and no surface catalyst -- but neither of those is precisely the same. According to Wiki, "argonium" is the name for argon hydride which is a (1+) ion species formed by combining a proton with argon into a short-lived molecule (2+ millisecond) life - which has a surprising strong binding energy. Argonium is actually found to be relatively common in interstellar space, despite this short lifetime. In a piston engine a short lifetime could actually be put to good use if an asymmetry exists due to the Mills effect. It would act as a thermal sink. Imagine a closed cycle piston engine which recirculates the two gases H2 ans Ar in such a way that under compression (at TDC) the two are combined on a catalyst surface (such as nickel, palladium, iridium etc) allowing for net energy to be freed as UV photons, which gain would be the result of some combination of the ion binding energy along with a redundant orbital photon emission less the ionization loss - as described by Mills, Holmlid etc. In Mills theory this emission would be related minimally to multiples of 27.2 eV so even if the reaction goes no further that a single redundant hydrogen orbital reduction, an attractive scenario for net gain would exist - even if the protons are lost after a single pass and must be continually replace by electrolysis of water.
