How to properly treat potassium-iron oxide catalysts

  • Rotational spectroscopy has been used. Due to the small size, rotational transitions fall in the of the visible range for the H–H pairs, whereas for ordinary molecules they would be in the GHz (microwave) range.

    Otherwise, laser time-of-flight has been most often used in the "Coulomb Explosion" experiments. When laser photons remove electrons from the H–H pairs (or larger fragments) in the ultra-dense clusters, the ions will repel each other by Coulomb repulsion, and by measuring the kinetic energy of the fragments it's possible to see that the bond energy was much higher than with ordinary molecules.


    A more accessible method would be observing nuclear reactions, since it is argued that they are easily possible only if the hydrogen atoms are at a very close distance with each other.



    I think all of these methods are outside the scope of this thread, which was initially started because information on the catalysts used for UDH production wasn't too well known. With the recent publication of a paper dedicated on the catalysts by Holmlid et al., it has probably run its course.

  • can - In the first post of this thread (~2,5 years ago) you wrote that your understanding is that potassium ferrite (KFeO2) is the compound of interest, and that it is formed during various conditions including heating the catalyst to ~600 degrees.


    Can I ask – is that still your understanding, or has it changed with the last years’ papers on the subject?

  • milton

    It hasn't changed, in that it indeed is the active compound of iron-oxide styrene catalysts and likely also other alkali-promoted iron oxide catalyst types. It could perhaps be added that the decomposition of this compound at higher temperatures (>800–900 °C) and/or vacuum conditions yields a potassium-deficient ferrite phase (K2Fe22O34, also known as potassium β-ferrite) from which potassium loss should be even faster, which could in turn lead to faster potassium alkali Rydberg matter formation. This potassium loss is industrially disadvantageous, so it is usually avoided with stabilizers (oxide additives).


    https://doi.org/10.1016/j.ijhydene.2021.02.221

    Quote

    [...] The use of additives to delay the loss of the promoter may give further options [87,88]. One of the strategies to stabilize alkali in the mixed oxides is to hinder the alkali ion bulk diffusion, as observed for Cr doping of β-ferrite. This strategy may not be so useful for H(0) formation, since the promoter needs to be able to diffuse and desorb for Rydberg states and Rydberg matter to be formed.


    https://doi.org/10.1006/jcat.2002.3725 (2002)

    Quote

    Abstract: Thermal desorption of potassium ions and atoms from K-doped iron oxides (Fe3O4, Fe2O3) and potassium ferrites (KFeO2, K2Fe22O34) that are the principal phases of the iron oxide catalysts for dehydrogenetion of ethylbenzene to styrene was investigated. From the Arrhenius plots the activation energies for desorption of K and K+ were determined in the process temperature range for each of the phases. Based on these results the desorption energies obtained previously for the commercial styrene catalysts were reinterpreted and the K storage and release phases were explicitly identified. The results were discussed in terms of a surface stability diagram. It was shown that in the active state of the catalyst the K2Fe22O34 component is responsible for excessive potassium release. The proposed optimal morphology of the catalyst grain consists of a core K2Fe22O34 surrounded by a compact shell of active KFeO2, while a core and cracked-shell model was adapted to account for the potassium desorption data from the real catalysts.


    Key figures from the above paper:




    Another related more recent publication on the topic: https://doi.org/10.1016/j.jcat.2007.02.009



    In a 2006 paper by Alpermann and Holmlid it was inferred that Rydberg matter formation was more intense from potassium diffusing from graphite patches and this K2Fe22O34 phase: https://doi.org/10.1016/j.saa.2006.09.003

  • Hi,

    I just want to say thanks for these whole post. Thanks for the information!

    I believe there is a paper talking about "chemical transport deactivation": It has been observed that, the concentration of K in the used catalyst is higher in the center than at the surface. This indicated that the aAs-formed KOH under the presence of steam in the styrene process would transport as liquid phase/vapor in the pore, which would be recrystallized/stable at the center of the catalyst pellet.

    This may not be relevant to the deactivation mechanism, though.

    I will post the link once I found that

  • JL123

    Thanks, although as I mentioned earlier, with the recent publication of a paper on the catalysts for ultra-dense hydrogen production by Holmlid, Kotarba, Stelmachowski, this thread has probably run it course.



    Some authors, e.g. Muhler et al. in https://doi.org/10.1016/0021-9517(90)90003-3 , do indeed describe the final inactive state of iron oxide styrene catalysts as a "spatially-disintegrated" core-shell structure, where the core would be enriched with segregated KOH.



    I don't know if this is entirely applicable for the experiments by Holmlid et al. where steam is not introduced and thus KOH would not form, but it has been sometimes pointed out that the loss of promoter (K) from the catalyst (due to diffusion and desorption by heating in a vacuum) eventually leads to deactivation and inability to form ultra-dense hydrogen and Rydberg matter, e.g. in https://doi.org/10.1007/s10876-011-0387-1 :