(773b) Hydrogen Production by Catalytic Partial Oxidation of Methane On Reticulated Rhodium and Platinum Foam Catalysts

Hydrogen Production by Catalytic Partial Oxidation of Methane on Reticulated
Rhodium and Platinum Foam Catalysts

        Catalytic partial oxidation (CPO) of
methane is an alternative to steam reforming for industrial production of
synthesis gas or hydrogen. Since Hickman and Schmidt showed in their pioneering
work [1] that equilibrium yields of synthesis gas can be obtained in
millisecond contact times by methane CPO on autothermally
operated noble metal coated foam catalysts this reaction has received a lot of attention
in academia and industry.

By
measuring species and temperature profiles through Rh
and Pt coated a-Al₂O₃
foam catalysts during methane CPO [2] it was shown that H₂ and CO
are being produced partly in presence of gas phase O₂ by partial
oxidation (Eq. 1) and partly by steam reforming after gas phase O₂
is fully consumed (Eq. 2).

CH₄
+ 1/2O₂®
CO + 2H₂   D_rH° = -36 kJ×mol^-1  
(1)

CH₄
+ H₂O ®
CO + 3H₂   D_rH° = +206 kJ×mol^-1  
(2)

Whereas
the steam reforming pathway to synthesis gas can be rationalized because CH₄
and H₂O are present at reaction temperatures around 1000°C, the
co-existence of CO and in particular H₂ with O₂ over a
noble metal surface at these high temperatures comes as a surprise raising the
question why they are not burned to CO₂ and H₂O. One
interpretation that is pushed forward in the literature is that H₂
and O₂ do only co-exist because methane CPO is fully film transport
limited leading to an effective O₂ concentration close to zero at
the catalyst surface [3]. This argumentation is based on numerical and
experimental studies which show without doubt that methane CPO on reticulated Rhodium
foam catalysts is indeed fully film transport limited [4].

In
the present paper it will be shown that in contrast to Rh
foam catalysts methane CPO on Pt foam catalysts is much slower and largely
kinetically controlled probably due to formation of oxidation resistant carbon
deposits as revealed by in situ Raman spectroscopy. To verify kinetic control
on Pt foam catalysts spatial reactor profiles were measured and analyzed in
terms of the achieved O₂ conversion rate which is clearly below of
what can be expected in the film transport limit. Furthermore flow rate and
pressure variations were conducted whose results are also in line with kinetic
control.

If
catalytic data on Rh and Pt foam catalysts are now
compared it can be shown that up to 50% H₂ selectivity is obtained
in the oxidation zone on a Rh foam catalyst under
full film transport control but that already 30% selectivity is achieved on a Pt
foam catalyst under kinetic control. Therefore film transport has indeed a
strong beneficial effect on the achievable H₂ selectivity but it is
not the sole explanation of the co-existence of H₂ and O₂
as argued in the literature. In fact Pt and most certainly also Rh produce a non-vanishing selectivity to H₂ even
under high temperature and high pressure conditions making them interesting
catalysts for industrial application.

References

[1]   D.
A. Hickman, L. D. Schmidt, Science 1993, 259, 343-346

[2]   R. Horn, K. A. Williams, N. J. Degenstein et
al., J. Catal.
2007, 249, 380-393

[3]   A. Donazzi, M. Maestri, B. C. Michel et al., J. Catal. 2010, 275, 270-279

[4]   D. Dalle Nogare, N. J. Degenstein, R. Horn et
al., J. Catal.
2008, 258, 131-142