the antimony catalyst and it is much less expensive than the equivalent germanium only catalyst silicon shows no significant catalytic activity on its own however when combined with germanium silicon enhanced the catalytic activity of the germanium various germanium silicon levels are shown each of which shows enhanced activity compared with the same level of germanium on its own at 40 parts per million germanium addition b8 b9 an equivalent activity to germanium only of 50 to 60 parts per million addition was achieved manganese showed some catalytic activity on its own however when combined with germanium manganese enhanced the catalytic activity of the germanium by a greater amount than would be expected based on the manganese activity on its own various germanium manganese levels have been tested each of which demonstrated enhanced activity compared with the same level of germanium on its own at 50 parts per million germanium addition b11 an equivalent activity to germanium only of 60 parts per million addition was achieved the tests demonstrate that germanium content can be further reduced by combining the u201c enhanced u201d effect of two or more of the additives previously described tests a29 and a30 contain the same addition levels of germanium and aluminium as test a15 however the addition of a further promoting element silicon in a29 and lithium in a30 enhanced the activity further from approximately 60 parts per million germanium only equivalent to around 100 parts per million germanium only equivalent the information gained from the tests on the above described reactor have been validated by external tests on both continuous and large scale.