Try 'activating' the graphite by heating at 1000oC in air. We found that
'burning-in' the graphite in a furnace for about 5-10 minutes at 1000oC
helped get the conversion up to speed; the 'activated' graphite has a
porous surface which, as Torsten says, greatly improves reaction rate.

Dave Mattey

Geology Department, Royal Holloway College, University of London, Egham
Hill, Egham, Surrey TW20 0EX

Tel:      01784  443587          Office
                        443637          Isotope lab (Prism)
                        443629          Isotope lab (Optima)
                        471780          FAX