How to properly treat potassium-iron oxide catalysts

  • Over the past few months I tried to collect as much information as I could find on the iron-potassium oxide catalysts routinely used in the work by Holmlid et al. After much research and note taking, it seems apparent to me that the probable main reason why experimenters have been having several difficulties with replicating the results is the preparation involved for activating the catalysts, or in other words to render them catalytically active.


    This information is widely published in the mainstream scientific literature, but this is not immediately obvious by just reading Holmlid's papers. In fact, very little is directly available about them just by reading his papers. One has to follow the references and read the related papers cited there. These are the two main works he usually cites when referring to these catalysts:

    To cut a long explanation short, it seems that upon specific conditions, in particular those similar to the industrial processes they're normally involved with, these catalysts undergo various structural transformations. The main defining characteristic is the formation of an unstable potassium ferrite (KFeO2) compound on their surface, which is generally considered to be the main catalytically active phase (this compound in isolation is as active as the industrial catalyst).


    My understanding is that this compound is formed during these conditions:

    • During catalysts synthesis from precursor materials at about 800-850 °C in air
    • During actual process conditions in the presence of hydrocarbons and oxygen at about 600-650°C (typical operating temperature)
    • Simply by heating the catalyst in a non-reducing residual atmosphere under vacuum conditions above 600°C

    The reaction is as follows:


    Fe2O3(s) + K2CO3(s) => 2 KFeO2(s) + CO2(g)


    Interestingly, the compound is characterized by a dark olive green color, which can be used to detect its formation. Under ambient air conditions the compound is reported to quickly decompose back into red hematite (Fe2O3) due to CO2 absorption, with water catalyzing the reaction. "As received" catalysts from long-term storage under uncontrolled conditions therefore are mostly composed (at least on the surface) of hematite and potassium carbonates, and are not immediately catalytically active. Any activation procedure would have to be preferably done in-situ.




    A few weeks ago to test the hypothesis that just calcinating (heating in air) for a relatively short period of time pre-formed, commercially available industrial iron oxide catalysts similar to those used by Holmlid could restore at least part of the potassium ferrite surface layer (by the above reaction) with a possibly noticeable change of color, I asked Alan Smith (our LENR-Forum moderator), who had a certain amount of these catalysts at disposal, to calcinate some of them for 90 minutes at 900°C and take a couple photos before and after the treatment. The entire procedure overall required minimal effort on his part.


     


    It did appear that the "after" catalysts changed slightly in color, at least in part confirming existing information in the literature and that at the right conditions potassium ferrites (KFeO2) might have been indeed formed. Note that the white patina visible in the "before" photos is likely potassium carbonate. Unfortunately I haven't been provided photos showing their state several hours later after storage in air. From various reports it seems that this occurs visually within hours. The so-treated catalysts have not been tested in an experimental apparatus under a hydrogen atmosphere either.




    I'm wondering if other experimenters with access to basic materials and equipment would be interested in performing some tests to confirm some of the characteristics of these catalysts. I can provide all references and information needed, but in absence of public of interest on the subject putting too much effort early on this thread would probably not be very useful.


    Potentially interesting activity diagram from http://dx.doi.org/10.1006/jcat.2002.3725



  • References suggesting that the KFeO2 compound formed has a dark olive green color. Plausibly, this might be mistaken for a black color or reduction to Fe3O4 which is black and non-active. At low concentrations it can give a light ochre or beige coloration (observation from earlier tests at lower—insufficient—temperature with the same catalysts used above) to the rest of the catalytic material, but prolonged heating to elevated temperature also in a vacuum should increase its concentration on the surface due to diffusion (and desorption) processes taking place, eventually rendering the change more apparent.

    Among other things, the references above may contain useful information regarding the procedures used to synthesize KFeO2. It can be useful to know for example that temperatures significantly greater than 800 °C have to be avoided in order to not synthesize K2Fe22O34 instead (if the aim is only obtaining KFeO2), which unlike the other is very easily reduced in a pure hydrogen atmosphere. In actual catalysts however the active state is apparently a metastable balance between the two compounds.



    Below are some excerpts in image form suggesting that potassium ferrite can also be formed from "as received" catalysts in the process of heating in a vacuum (due to potassium diffusing from the bulk to the surface of the catalyst and reacting with free Fe2O3 phases), and/or that it's unstable.


    Source: https://doi.org/10.1080/01614947408071864


    Source: http://lib.dr.iastate.edu/cbe_pubs/222 (TGA/Thermogravimetric analysis device had a pressure of 0.003 atm)


    Source: https://doi.org/10.1021/ie00023a010 (Holmlid, 1993. Note that I cut much of the original text in the excerpt)

  • Some might be asking what is there so special about this 600 °C threshold mentioned in the excerpts of the previous comment.


    It appears that there's a temperature range (550–650°C) from which the emission of potassium atoms and ions increases to significant levels compared to lower temperatures. Not only this includes the emission of excited (Rydberg) states from the surface, but also the diffusion of ions from the bulk to the upper layers of the catalyst. This means that under active conditions the surface of the catalyst will slowly develop an increased concentration of the active KFeO2 compound.


    A typical desorption signal graph looks like this:



    Source: https://doi.org/10.1023/A:1019013504729 (Available on Researchgate)


    In a more user-friendly linear axis form it becomes like this, from which it becomes clearer that at lower temperatures there is little hope that structural changes in the catalyst will take place in a reasonable amount of time (since as previously mentioned they are driven by diffusion of potassium to the surface):



    Other sources showing similar graphs and behavior from these compounds and industrial iron-potassium oxide catalysts:

  • It might be interesting to know that at least under a low partial H2 pressure (in the order of a few millibar) this active KFeO2 compound will not get easily reduced, not even at 630 °C—the typical operating temperature of the actual industrial catalyst in the styrene process (although apparently reduction will start to take place above 700°C). It seems that once formed it increases the reduction resistance of the industrial iron oxide catalysts, even when present in small amounts, at least under less reducing conditions than a pure H2 environment.


    In the styrene process, which generally operates at higher sub-atmospheric pressures, steam is admitted with ethylbenzene to keep the catalyst in an active oxide form. When using hydrogen only, either a low pressure or low temperatures would have to be used to prevent irreversible reduction processes, taking place with the following reaction as indicated in https://doi.org/10.1021/ie00023a010:




    But perhaps if the catalyst was mostly composed of the catalytically active phase(s) this would be less of a concern.




    Relevant excerpts from http://dx.doi.org/10.1016/j.apcata.2008.11.007









    Other related sources:

  • Catalyst deactivation phenomena in styrene production by Meima and Menon (2001)


    This paper, which has been always referenced by Holmlid over the past years in his ultradense hydrogen studies (it's the newer of the first two cited in the opening post in this thread), offers a general overview of K-Fe oxide catalysts for the production of styrene in the context of deactivation processes. Indirectly and upon careful reading it helps understanding what makes them work properly. A few cited patents (in refs 36 and 37) might be of interest.


    Some time ago I took notes of what I considered the most interesting bits while reading it. Here they are for everybody's benefit, in case there's anybody interested (not that so far I have seen much signs of interest, though). I can't guarantee that they're accurate, so of course the best thing would be reading the actual paper (e.g. using sci-hub).

    Quick notes

    • The styrene process with these catalysts usually runs at 600–650 °C in a partial vacuum.
    • It has been observed that removing coke (carbon deposit) formation with steam quickly increases activity. Coke formation however is not always harmful. Menon classifies it into several types [6].
    • KFeO2 is generally considered to be the active phase of these catalysts. First observed by Hirano [4, 14] then other groups.
    • The active state of the catalyst is an equilibrium between KFeO2 and K2Fe22O34. Reduction by hydrogen causes formation of inactive Fe3O4 [19].
    • Stobbe [16], Goodman [20], Lundin et al [21], Menon and Holmlid [22] also support the notion of the active state being KFeO2.
    • Holmlid et al [24, 25] observed excited states and clusters (Rydberg matter) from used and fresh catalysts at 600–800 °C in an UHV chamber. An energy diagram has been constructed taking into account the mechanisms proposed earlier by Ertl et al [19].
    • In the actual styrene production process the fresh catalyst is partially reduced to Fe3O4, but becomes more stable afterwards.
    • Muhler et al [19,28] note that the precursor materials contain excess Fe3+ ions which form the unstable ternary compound K2Fe22O34, which forms in turn KFeO2 which is considered the active phase.
    • As the above occurs, Fe2O3 is slowly reduced to Fe3O4 which is considered inactive.
    • K2Fe22O34 acts as a K storage medium to prevent reduction of Fe3+; once depleted, the catalyst becomes irreversibly deactivated.
    • In general, it is acknowledged that transformations during actual usage occur.
    • Reduction (to Fe3O4) of Fe3+ -containing phases must be prevented and KFeO2 possibly works beneficially towards this goal.
    • Promoters—including especially Cr—also stabilize the Fe3+ oxidation state, i.e. prevent or slow down reduction to Fe3O4.
    • Reduction to Fe3O4 also physically weakens the catalyst, causing a transition from hexagonal lattice to cubic lattice structure.
    • The loss of the K promoter increases coke formation, which clogs catalyst pores, decreasing the number of active sites.
    • Alkaline earth oxides as promoters tend to improve structural stability. A number of studies and patents exist showing their effect.
    • In conclusion, catalyst deactivation processes are complex. They can be of both permanent and temporary nature. Much research work has gone towards decreasing the former.
    • Stabilizing the Fe3+ oxidation state seems to be the main trend (as of 2001). This also has the effect of increasing the K desorption threshold temperature as proposed by Holmlid et al [33].
    • Patents and studies exist on this aspect too. For example, MgO or ZrO2 appear to be good support materials that can stabilize these catalysts.
    • The pore structure of the catalyst is also important on this regard. Notably, treatment in hydrogen under specific conditions can alter it to prevent further deactivation [37]. This might also promote KFeO2 formation.

    Relevant references

  • I provided some catalyst pellets to Alan Smith last year, which may be what he used in the test for you. I still have an ample quantity available for further testing if you have something in mind. I also had the Chinese-sourced pellets analyzed using PIXE (Particle induced x-ray emission) and RBS by a volunteer. Here are the comments from this very experienced analyst:


    "I used PIXE to get accurate metal ratios. My PIXE set-up does not detect anything lighter than Si, so no Mg or O can be detected with PIXE.

    RBS is used to get depth distributions and measure the light stuff, predominantly O.

    I am not detecting any chemical information, so I cannot determine exactly the composition of the oxides. While I see many metal peaks, I am only detecting one O peak which is a sum of all the oxides, so I presumed that the metal oxides are present in their dominant form. This is easy for K, Ca and Ce which usually only form one oxide type. This is why I can't determine the order of the Fe oxide - could be any of them in any combination.

    The metal ratios determined from PIXE were added into an RBS simulation, and the total [O] was calculated from that.

    The beam from both techniques, 2.3MeV He+ for RBS and 1 MeV H+ for PIXE, penetrate several microns into these type of oxides"

    Here are the combined PIXE/RBS results for the catalyst pellets :

    K2O 63.2 %

    FeOx 32.8 %

    CaO 2.8 %

    CeO2 1.2 %

    Mg trace

  • magicsound

    I can confirm that the test was performed with catalyst pellets provided by you. Thanks to you too for making that test possible.




    After extensive reading it's now clearer that the potassium on the surface from the analyses was probably mostly present in the form of carbonates, as its oxide readily absorbs CO2 from the atmosphere at room temperature. The strongly red appearance of the "as-received" pellets should suggest that the Fe oxide was mostly hematite.


    I think a good test requiring the least amount of efforts and resource expenditure while still being useful would be documenting the stability over time of this active KFeO2 compound, which should be formed in limited amounts on the surface of the catalyst upon prolonged calcination in air at atmospheric pressure at about 800°C by the reaction of Fe2O3 with K2CO3. Higher temperatures promote the formation of other phases. Shell 105-type catalysts are usually calcined during their manufacturing for about 2-3 hours at temperatures in the 850-950 °C range.



    Source: http://dx.doi.org/10.1039/b104263g


    How exactly unstable this compound is in a standard atmosphere? It's been reported in the literature to revert to red hematite within a few hours, but I suspect the bulk of the change occurs within the first couple dozen minutes of exposure to ambient air. A timelapse video could be interesting and be something unique as this sort of visual documentation doesn't appear to be available.


    Source: http://lib.dr.iastate.edu/cbe_pubs/222 (open access)


    More good photos of the freshly calcined pellets showing a more apparent olive green coloration than the one I uploaded would also be good to have. Photographing them together with uncalcined catalyst samples would make any difference more visible.


    It's likely that crushing the pellets into sub-millimeter granules (but not a too fine powder) could yield better results. A comparative test with the cylindrical pellets after the same treatment might be of interest. In that case it would be also be interesting to know if such crushed pellets from long-term storage also show the same deep red coloration on the inside (probably they do).


    After calcination, would the cylindrical (uncrushed) pellets, once they show a decomposed red appearance, also appear decomposed on the inside? It would be interesting to crush some to find out.


    I think these tests could be done in a single run before and after calcination in a furnace at 800°C for several hours. Anything between 4-6 hours at this temperature would be fine; more than this is probably not needed but it will not harm either. In the following study the authors synthesize KFeO2 directly from powdered precursors with 3.5h under flowing air.


    Source: http://lib.dr.iastate.edu/cbe_pubs/232 (open access)


    I believe this more or less sums up what can be done with a standard furnace to find out more about the temporary nature of the active phase of these catalysts. It would be great if you could perform some of these tests. Thanks for donating some of your lab time!





    On a partially related note, in the industry the actual styrene catalysts are activated simply by exposure to reaction conditions (about 600-650 °C with steam and ethylbenzene at pressures < 1 atm). Steady state conditions are achieved generally within a few days of activity.


    Source: https://doi.org/10.1002/9783527610044.hetcat0163

  • OK,I can easily do the test you suggest. If time permits, I will also seal some of the calcined material in Argon as soon as it cools from the furnace. That might provide some additional insight into the stability of the proposed KFeO2 product in the absence of atmospheric CO2.


    Looking at my schedule, I think Monday is possible.


    AlanG

  • Seiber Troutman

    If you want to contribute constructively to this thread you could try synthesizing potassium ferrite (KFeO2) on your own and note any short-term interesting change in color of the starting hematite.


    The resulting material should appear olive green upon synthesis and it's the one that actually gives the slight green tint to the industrial catalysts upon calcination—as well as making them apparently "turn black" upon heating in a vacuum due to emission of excited states, as linked in one of the firsts posts in this thread. It should also be rather unstable under ambient conditions, more than the treated catalyst pellets themselves.


    Apparently this compound alone is as catalytically active as the industrial catalysts. So, potentially (there are some caveats) there wouldn't be necessarily need to hunt for hard to obtain catalysts for replications of experiments employing them.


    Materials (in equal proportions by weight to simplify things out—in practice usually slightly more potassium carbonate than hematite is used):

    • Fe2O3 (hematite) powder
    • K2CO3 (potassium carbonate)
    • H2O

    Proposed procedure:

    1. Place the solid materials in a crucible and add just enough water to obtain a saturated solution of K2CO3. About the same amount of water by weight should be sufficient according to solubility data provided on Wikipedia.
    2. Stir well as to obtain a more or less homogeneous dense slurry.
    3. Place the crucible in the furnace, increase temperatures to 200°C and hold for 30 minutes to let the solution dry (should be enough).
    4. Remove the crucible from the furnace and crush the resulting powder with a pestle to homogenize it.
    5. Place again the powder in the furnace, increase temperatures to 800-825°C and let it calcinate for 4–6 hours.
    6. Remove the crucible from the furnace and quickly take photos of the calcinated sample.



    Source: https://doi.org/10.1016/0021-9517(92)90295-S



    Source: http://lib.dr.iastate.edu/cbe_pubs/232

  • Seiber Troutman

    Not quite. Styrene is the precursor compound (a liquid hydrocarbon) used to produce its polymerized form (a solid) called polystyrene, which is what many plastic materials are made of. However, in the context of this thread this is not really important, as using these catalysts for styrene production isn't the end goal here.

  • OK, its best I quit , everything I relate is wrong...

    The process your all working with sounds more intersing then mine but I know when to walk away.

    for the record my first design was much more simple.

    I built a chain stone grinder to get the hematite from the ore,

    I removed the steel bits of chain with a magnet

    I used a smaller stainless steel screen to remove the smaller dust and try to keep out the damged stainless screen from the grinder.

    pulled the deep purple wet hematite "water float method" brackish, and mixed it with a bit of clay and high temp mortar into a ball and pushed it down to the desired cathode height.

    with a plastic straw into the now pancake

    with about 3/8" each filled area in the straw let them dry.

    when they fell out i drop them into hot coat powder " I dont know what temp it was. and then the oven 450 or just see the glossy coat.-insulator'

    sand down the ends and installed them in the body you see in my pics. " short lived"

    At the time I was only concerned with spark continuity.

    I will wait and see what you all come up with.

  • OK,I can easily do the test you suggest. If time permits, I will also seal some of the calcined material in Argon as soon as it cools from the furnace. That might provide some additional insight into the stability of the proposed KFeO2 product in the absence of atmospheric CO2.


    The test is now under way. I'll post photos at the conclusion.

  • Thanks again. Looking forward to reading and seeing the result



    There was a bit of color change, but no hint of green there. These are in sequence: first the raw material before baking, then fresh out of the furnace after 6 hours at 800°C. Finally, the same material after cooling for 10 minutes in air. All photos are from an iphone6 with flash on. The color accuracy is pretty good.



  • magicsound

    Good job. Some pellets do appear on my screen like they've acquired what has been described in the literature as an olive green color, but unexpectedly this happened mostly on the surface of some of the larger ones, while the rest of the powder except for a few of the larger chunks has now an overall slightly darker red (crimson) appearance than before the treatment.


    This makes me wonder if at this temperature and calcination time another phase generally associated with KFeO2 has formed instead (K2Fe22O34), except for the spots where the local concentration of Fe2O3 and K2CO3 was higher. Either way, if anything this shows that the composition can vary greatly from pellet to pellet also from the same batch.


    Actually I have a photo from Alan Smith showing such a color on the pellets calcinated at 900 °C for 90 minutes immediately after being put out of the furnace, but as the lighting conditions weren't controlled and the pellets were still hot I thought this was for the most part a camera effect (artifact), not an actual change. Most of this crimson tint appeared to have been gone a few minutes later. So apparently this darker compound was unstable too:





    Your "10-minute after" photo—if it has good color accuracy—appears to show too that some of this crimson coloration disappeared, while the larger pellets still retained part of their yellow-green (ochre?) color.


    It will be interesting to check out again under the same lighting conditions in one hour (although at the time of posting such time probably already passed) and then after several hours (overnight).



    EDIT: added composite image of the catalysts calcinated at 800°C for 6 hours.



  • can


    Exactly how potassium iron oxide assists in the formation of Rydberg matter of H is not clear to me. Fom the publications I read on the subject (I have no direct expertise in surface catalysis), I can see two main hypotheses:

    - breaking the H2 molecule in atomic H at the surface of the iron oxide with a simultaneous desorption of K in an excited Rydberg state K* and a rapid transfer of the excitation energy of the K* to the atomic H to produce an excited Rydberg H*, or

    - absorption of atomic H within the bulk of the porous iron oxide followed by its desorption in an excited Rydberg H*, this without any apparent role of K.


    Holmlid seems to favor the 1st hypothesis, going even further in suggesting that Rydberg states K* are key in the process of ethylbenzene dehydrogenation (somewhat in contradiction with the attributed role of KFeO2), but he also alluded that a direct desorption of H in H* is possible. In any case, for what it's worth, this doesn't say why KFeO2 would be the active form.

  • JulianBianchi

    It's my understanding too that at least in the context of the K-Fe2O3 catalysts he's most often suggested the first explanation, i.e. excitation transfer of the K* energy to the atomic H. The so-called K promoter reportedly makes this transition easier than normally possible. A short explantion of the process has been described in his lapsed patent in the section Catalytic conversion: https://patents.google.com/patent/EP2680271A1/


    However, this transition can also be achieved in other ways, not just from these catalysts. See for example a very general explanation in section 2.2 Formation of RM in this open access paper here: http://aa.springer.de/papers/0358001/2300276.pdf


    Speaking again of the K-Fe2O3 catalysts of this thread, it could be very simplistically summarized that according to Holmlid et al. K atoms can desorb thermally directly in an excited state from the KFeO2 phase of their activated form. This probably doesn't occur only on the surface, but also in the internal macropores inherently composing their structure, but for ethylbenzene dehydrogenation the latter might not be a very significant factor.


    Compounds like KAlO2 do not seem to show the same behavior. In this paper available on Researchgate the authors conclude:



    These catalysts have been used for decades and other authors have studied the role of K promotion on catalytic activity in practical terms, without necessarily focusing on Rydberg emission like Holmlid and colleagues did. In this old review (1974) by Lee, the relative activity between different metal oxides, K-promoted and unpromoted, was analyzed. Al + K was not found to be catalytically active.



  • JulianBianchi

    Kotarba et al studied the role and the loss of potassium from dehydrogenation catalysts under a plethora of different conditions; I also suggest checking out other papers from them on the subject. His latest one was in 2015 with a K-Mn oxide material, also available on Researchgate. Sort of off-topic for this thread, but it shows that Rydberg K emission (and possibly Rydberg matter) isn't specific to K-Fe oxide catalysts.


    On a related note, a couple months ago I contacted Kotarba via email and he confirmed that in his past studies "[the industrial styrene catalysts] were always good emitters of K* also without any special activation procedure - just as received". However these studies most often involved heating them in an ultra-high vacuum to process temperature (~600-650 °C) and in earlier comments in this thread I already posted some excerpts from other researchers who noted the apparent formation of KFeO2 on the surface of these catalysts in a vacuum at high temperature.



    EDIT: some actual references of related studies by Kotarba and others (either as an author or coauthor):

  • The Fe2O3 compound is known as a LENR H* catalyst since the early days.


    The problem with the understanding of chemical papers/ their relevance for LENR is that they use H*/K* for ordinary Rydberg matter, where as Holmlid/Mills mostly use H* for the shrunken form. Mills claims that in a disproportion reaction H* (going to H 1/4) releases the exact amount of energy to split the catalyst. Holmlid assumes that H* has a different spin state. The combination of the two models would imply that the proton has a so far unknown quantization of its spin that somehow coincides with Mills quantization rules.


    What could be the role of the catalyst?


    One fact we know is that energy in the "some eV" range can only be added/removed from a molecule in a kinetic transaction (might be the exchange of magnetic flux quanta in the form of a temporary generated charge density). Thus a good LENR catalyst must provide a collision point that allows to efficiently transfer the energy quantum. Whether this is the same effect as in a pure chemical reactions has to be shown. But I believe this is very unlikely as the reaction frequency for catalysts is magnitudes larger than we see for LENR.

    Nevertheless the origin of both effects could be the same, but the fine-tuning will be different.

  • The Fe2O3 compound is known as a LENR H* catalyst since the early days.


    Since the start of this thread I have been showing sources explaining that the K-Fe2O3 catalysts have to undergo certain transformations in order to efficiently work and form excited atoms in desorption. Fe2O3 isn't very active on its own.


    The problem with the understanding of chemical papers/ their relevance for LENR is that they use H*/K* for ordinary Rydberg matter, where as Holmlid/Mills mostly use H* for the shrunken form.


    Read Holmlid's explanation from this patent (section "Catalytic conversion"). He's not using the H*/K* notation for what he calls the ultradense state.



  • For those interested in forming KFeO2 directly by heating in a vacuum instead of calcinating the material ex-situ at 800 °C or above, it might be worth mentioning that Ndlela and Shanks reported in Reduction behavior of potassium-promoted iron oxide under mixed steam/hydrogen atmospheres (2006) that partial reduction to Fe3O4 seems necessary for KFeO2 to form under very mild vacuum conditions (0.057 atm in the experiments described) at temperatures in the range of 600-630 °C (typical styrene process temperature).



    However, excessive reduction to Fe3O4 or metallic Fe (e.g. with a too high hydrogen pressure) will prevent KFeO2 formation.


    It's possible that at the deeper vacuum levels implied in the excerpts posted previously, with potassium ions diffusing from the bulk to the surface of the material (and reacting with the oxides present) more quickly, such partial reduction might not be required.


    On the other hand, under prolonged heating under such conditions, it can occur that vacuum pump oils volatilize in the atmosphere and eventually crack (decompose) on the surface of the catalysts, depositing a thin carbon layer and also partially reducing the catalyst with the hydrogen produced in the process (which is however expected to be usually rather slow unless deliberately or inadvertently promoted).


    Apparently Leif Holmlid—who used to perform prolonged experiments with the catalysts under high vacuum conditions—has observed such phenomenon in his experiments throughout the years and sometimes very briefly reported it in his papers.



    Source: http://doi.org/10.1021/jp046288m


    Source: https://doi.org/10.1007/s11051-011-0543-4



    Source: https://doi.org/10.1016/j.saa.2006.09.003



    Source: https://dx.doi.org/10.1088/0953-8984/19/27/276206