Dear powerful XAFS community,
Thanks for your sincere reply, Jason.
I am thinking to try SEXAFS because its advantage to probe surface atoms, but since SEXAFS is usually used on light adsorbate atoms(in my case C),there is still a problelm of how to ensure the mononlayer adsorption of CO, is there any method to improve this skill? And I think the SEXAFS spectra of light atoms may not be very complete due to the narrow range of energy, so can I still get the extended x-ray data in the spectra? And even if I can get the complete spectra, is the analysis of light atom's SEXAFS spectra the same with EXAFS data analysis?or it uses a different data analysis theory and method?
And I also found one paper mentioned that fluorescence detection in grazing incidence geometry can also help improve surface sensitivity, is there any body familiar with this technique and can give me some suggestions?

Thanks so much~


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Today's Topics:

1. Re: How to write feff.inp file for CO adsorbed Cu? (Jason Gaudet)


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Message: 1
Date: Thu, 6 Sep 2012 11:30:13 -0400
From: Jason Gaudet
To: XAFS Analysis using Ifeffit
Subject: Re: [Ifeffit] How to write feff.inp file for CO adsorbed Cu?
Message-ID:

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Tian,

Thanks for these articles, although I think they (kind of) confirm what I
was saying before.

As you say, the Phys Rev article was taken at the O K edge, not the Cu K
edge, so of course it's going to probe O-C-Cu structure much more
effectively than using a Cu absorber. (Although that's not to say SEXAFS
is easy!) I have no experience in this method so I can't say if O K edge
would be an appropriate technique for you or not.

The J Am Chem Soc article concerns a Ru catalyst that was atomically
disperse or nearly so under at least one condition, allowing C,O scatterers
to be more clearly observed. I'm extremely suspicious about the peak
assignments in Fig 5a; they look to me like Ru-O and Ru-Ru paths, and
without good curve fits to that plot I find the assignments speculative.
If I were a reviewer I would ask for such fits, but since this is a 22 year
old paper I would simply suggest caution about that one specific aspect, as
by itself, it may not be as convincing today as it was when it was
published. Perhaps one of their citations makes a solid case.

Interestingly, I'm running an experiment right now on that same model of
Nicolet FTIR. Those things last forever!

IME, every time I'm at a beamline, I set aside at least a few hours for
exploratory experiments. These exploratory experiments let me get an idea
of the type of data I can get from new samples, and allow me to try out a
handful of completely new techniques. If something works, I'll plan a big
experiment around it for the next trip. If nothing works, I don't feel too
bad about those few hours of beamtime not leading to anything of immediate.

In suggesting you exercise caution, I don't mean to say don't run these
experiments, I mean to say be prepared for difficulty, and don't let this
be your one and only plan.

My personal experience is this: I've run some in situ experiments with Pt,
Pd, Cu catalysts, under NO and CO and although I can see secondary effects
of adsorption - changes in oxidation state, metal-oxide and metal-metal
bonding - I've yet to actually observe the adsorbed species in EXAFS, even
when the catalysts are very disperse. Ribeiro does a lot of operando EXAFS
and his recent paper on Cu/SSZ-13, where the Cu is in fact atomically
disperse within the lattice, does not directly observe Cu-N paths (
dx.doi.org/10.1016/j.cattod.2011.11.037).

In situations similar to yours, I have used XAS simultaneous with FTIR,
because that's the best way to observe the metal oxidation state and the
adsorbates at the same time. This is not an easy task but it might be the
appropriate one for you. And if you're lucky, and can see Cu-C paths, you
have that same data confirmed with IR. This of course means compromises;
an experiment ideal for both techniques is going to be less than ideal for
either one separately.

In summary: metal edge XAS is really useful in catalysis because we can
observe all sorts of changes in oxidation state and morphology. But it is
not the best tool for observing adsorbates, because it averages the
contribution of adsorbing sites with the very large number of non-adsorbing
sites. You cannot see Cu-C in CO adsorbed to a small particle or flat
surface of Cu as easily as you can see Cu-C in copper cyanide, because in
the adsorbed case the Cu-Cu contributions are many and strong while the
Cu-C contributions are few and weak. Even if you can see Cu-C paths in
your samples (and I hope you do!) that still does not mean that you're
going to get a quantification of Cu-CO adsorption sites anywhere near as
good as you could by IR and TPD.

-Jason

On Wed, Sep 5, 2012 at 11:12 PM, Zhaomo Tian wrote:

> Hi,Jason!
> I think you are really professional and pointed out the most important
> point at the beginning !
>
> Actually I am really worried about my experiment too, I also have a doubt
> about whether I can see C or O contributions to the EXAFS in my sample.But
> I think maybe no one can give the answer for sure. So at least I need to
> try first and see. Actually the originality of my work is to study the
> electrochemical reduction of CO2 on metal electrodes. Since CO is noticed
> as an important adsorbed intermediate in the multistep reaction and only
> copper has been found to be unique among the metals in its ability to
> produce a high quantities of hydrocarbon fuels, i start to first
> investigate the interaction between adsorbed CO and copper.Thats why I
> believe that using EXAFS to study the chemisorbed CO gas-copper system will
> contribute to some reasonable explanation.
>
> I sent you two references, in the first one there is evidence for CO
> adsorbed Ru taken at Ru k-edge,where CO is observed in EXAFS, and the
> second one SEXAFS study on CO adsorbed on Cu, but it is taken at oxygen K
> edge.I think even if there is not exact the same evidence with my system
> which is EXAFS on Cu k-edge of CO adsorbed Cu, but it may still work. Why
> do you think my experiment is not promising? Could you please explain some
> reasons to me? Is there any conditions for metal and adsorbed gas that
> their EXAFS signal can be detected? If there are,would you please to give
> me some modification of my experiment?
>
> Thanks a million.
>
> Best Regards,
>
>
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