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April 14

Mechanism of hydrogen absorption by palladium

Why exactly is palladium so good at absorbing hydrogen? Nickel and platinum are in the same group, and they certainly are not. Double sharp (talk) 16:28, 14 April 2020 (UTC)[reply]

This is a rather old paper and highly technical, but may be a good start. This is a newer and more accessible synopsis article, which may be closer to helping answer the question; in summation there's something specific in the crystal lattice of the palladium metal that allows H2 to efficiently adsorb onto the palladium that metal atoms of different metallic radius or crystal shapes would not work as well. --Jayron 32 19:53, 14 April 2020 (UTC)[reply]

That first ref notes that nickel does take up hydrogen, but that the amount is more limited for the bulk metal because of poor penetration below the surface. Might be some useful info to add to the nickel hydride article? Nickel is at least good enough for use in some hydrogenation catalysts, so please be careful with a blanket statement that nickel is not good at it. Doi:10.1016/j.ijhydene.2006.07.004 discusses use of nickel as a hydrogen-storage material. I found that last ref by google-scholar search for [adsorption of hydrogen onto nickel]. DMacks (talk) 23:45, 14 April 2020 (UTC)[reply]

Well, yes, what I meant was that Ni was certainly not as good as Pd at this. ;)

Hmm. Jayron32's comment about metallic radius and crystal structure prompted me to check other similar metals. Palladium has a metallic radius of 137 pm and has an fcc crystal structure; well, platinum is 138.5 pm and also fcc. But apparently platinum is not good at absorbing hydrogen, even under pressure. It looks like rhodium is the second-best platinum metal at doing this, but RhH_x is thermodynamically unstable. (Well, it is also fcc and 134 pm, so not far away.) So it must really be very sensitive to small changes. Double sharp (talk) 03:43, 15 April 2020 (UTC)[reply]

Anything relevant in a comparison of reduction potentials, thinking about forming an alloy with more M–H bonding than just an inclusion in the M structure? Doi:10.1039/B701402C is an experimental analysis of the structural changes of Pt (and Au) upon absorption of H2. DMacks (talk) 08:33, 15 April 2020 (UTC)[reply]

I just want to clarify that it isn't just the metallic radius and crystal lattice to the exclusion of all other possible factors which can explain palladium's properties here. There are probably dozens of properties relevant to our discussion (coordination number, electron affinity, electron configuration, reduction potential, atomic mass, spin-spin coupling, etc. etc. etc., along with many others I either can't think of or are too bored to list) each of which adds a non-trivial contribution to palladium's particular properties, causing it to hit that perfect sweet spot in terms of hydrogen adsorption. The fact that I found articles that highlighted those particular properties should in no way imply that the answer to the question was a simple as that, and that one could ignore everything else. --Jayron 32 12:20, 15 April 2020 (UTC)[reply]