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