cat c the sprayed powder is granulated dried and sintered as in example 1 the activity of the catalysts a b c according to the invention have been tested in an experimental reactor with the results shown in the attached diagram fig2 as compared to catalyst cat d known from u s pat no 4 073 749 the activity curves in the diagram show that the cerium activated catalyst compared with the known catalyst at the same experimental conditions i e space velocity s v h 1 20 000 h 1 pressures 315 and 150 kg cm 2 and respectively with the same average sphere diameter 1 5 2 5 mm presents a very high activity more particularly it has been found that the efficiency of a cerium activated catalyst gets higher as the temperature gets lower 350 400 c the catalysts of examples 1 3 have been submitted to thermal resistance tests consisting in determining the loss of activity at the same test conditions before and after the same charge has been maintained for 10 hours at 600 u00b0 c and 315 abs atm and a spacial velocity of 20 000 from the measures carried out it appeared that the catalyst a b c maintained their initial activity furthermore as pointed out pelletisation of the known catalyst of u s pat no 4 073 749 is effected by adding water and bentonite to the catalyst powder while pelletisation of the catalyst according to this invention is effected by adding only an aqueous solution of cerium nitrate the behaviour of the nitrate is unexpected indeed.