>Dear subscribers,
>Where can I find background information on platinum as a catalyst for the
>oxygen-CO2 conversion. Most of the new papers dealing with fluorination
>of oxides and subsequent O2-CO2 conversion refer to this step but without
>details. Are there any other catalysts to promote the high-temperature
>formation of CO2 (res. to avoid the CO-formation)?
>best wishes
>Bernd

Dave Mattey give the right answer to the conversion of O2 to CO2. Simply,
don't do it. However, there are still a lot of labs which do not have the
opportunities to analysis O2 directly.

So if you MUST convert O2 to CO2, diamonds are a good compromise down to 1
micromol O2 sample size. We use diamonds for more then two years in our
fluorination line. Diamonds react very fast with O2, don´t degas like hot
graphite rods and are cheap. We have measured the amount of gas before and
after the conversion step and found that at least 99.9% of the oxygen is
converted to CO2 starting with 1 micromol O2. Only 0.1% residual gas might
be CO but I don´t believe that any CO will be produced by O2-CO2
conversion. For the simple reason that conversion of O2 and CO continuous
until all oxygen is frozen as CO2 in liquid nitrogen traps (see Boudouard
equilibrium). A second argument against CO is that an offset on mass 28 by
CO never has been observed in our mass spectrometer.

Anyway precise d18O values for sample sizes below 3 micromol are difficult
to obtain with conventional lines due to other fractionation effects
especially in liquid nitrogen traps (see Wiechert and Hoefs 1995, GCA
59,19,4093-4101). Fractionation do not occur down to 0.5 micromol (minimum
sample size which can be analyzed with our inlet-system) if O2 is
transported in a continuous He gas flow, even if O2 is converted to CO2
afterwards. I presume that observed fractionation in conventional lines is
not caused by O2-CO2 conversion but by adsorbtion of O2 at solid-surfaces
which is strongest in cold traps and at low pressures. This results have
obtained using red hot diamonds for O2-CO2 conversion. I agree with Dave
Mattey that hot graphite causes a lot of problems and is not suitable for
small samples.

Good catalyst to react CO to CO2 in gas mixtures are Ni or Ni/Fe alloys,
whereas Pt has no effect up to several hundred degrees celsius.

Regards

Uwe Wiechert

____________________________________________________________________________

Uwe Wiechert
Geochemisches Institut, Georg-August Universitaet, Goldschmidtstr. 1,
D-37077 Goettingen, Germany
FAX: 0551-393982
Phone: 0551-393985
email: [log in to unmask]