Production of Ultra Dense Hydrogen has been described by publications of Em. Prof. Leif Holmlid over the last decade.
The method Holmlid can be divided into 4 different states:
Molecular Hydrogen >> Atomic Hydrogen >> Rydberg Hydrogen matter >> Ultra Dense Hydrogen
The transition of Molecular Hydrogen into Atomic Hydrogen uses Palladium as a first 'catalyst'.
The transition of Atomic Molecular Hydrogen into Rydberg Hydrogen matter uses a styrene catalyst type Shell S-105 (iron oxide catalyst doped with K ) as a second 'catalyst'.
The transition of Rydberg Hydrogen matter into Ultra Dense Hydrogen occurs spontaniously, but there will be some conditions (pressure, temperature) required including e.g. a Nickel surface (third catalyst).
I had a closer reading of LENR-Cars's Patent Application titled "method of producing energy from condensed hydrogen clusters" to better understand what UDH creation processes they are suggesting.
This PA describes an example process that I like to translate into simple language from my understanding point of view (open for discussions).
The Hydrogen stages describes are a little different:
Molecular Hydrogen >> Protons >> Atomic Hydrogen >> Rydberg Hydrogen matter >> Ultra Dense Hydrogen
The transition of Molecular Hydrogen into Protons suggests the use of a membrane containing e.g. a thin layer of Palladium, Platinum of similar catalysts. This membrane material is mentioned as 'primary material' and seems to direct to commercially available PEM Fuel Cell membranes.
The transition of Protons to Hydrogen and then into Rydberg Hydrogen matter uses catalysts such as caesium, potassium, lithium, sodium, and/or rubidium (atoms having a low electronegativity).
The transition of Rydberg Hydrogen matter into Ultra Dense Hydrogen uses a mentioned optional 'secondairy material' to facilitate condensation of the Rydberg Hydrogen matter, e.g. ruthenium, rhodium, iridium, and/or nickel.
note: some corrections have been inserted, thanks to can (see below comments). I left the original text strikedthrough.