A Mössbauer investigation of iron-containing catalysts prepared at low temperatures and active for carbon monoxide hydrogenation

Abstract

The changes which are induced in iron-containing precipitates, which are sometimes potassium-doped, by low-temperature pretreatment in controlled gaseous atmospheres and under reaction conditions of carbon monoxide and hydrogen have been examined. The results show that potassium influences the changes which occur in both amorphous and crystalline unsupported iron-containing solids when heated in carbon monoxide and hydrogen. Amorphous materials, formed by calcination of both the non-potassium-doped and potassium-doped precipitates in air at 120 °C contained iron(III) species in oxide or oxyhydroxide coordination and were partially dehydrated by heating in nitrogen at 270 °C. Treatment in carbon monoxide at 270 °C for 2 h induced the formation of small-particle superparamagnetic Fe3O4. Continued exposure of the non-potassium-doped material to carbon monoxide for a total of 12 h results in the formtion of a product containing large-particle Fe3O4 and the iron carbide of composition χ-Fe5C2, whilst under identical conditios the potassium-doped material was virtually completely converted into iron carbide. Subsequent treatment of the non-potassium-doped material in hydrogen at 270 °C for 12 h resulted in partial conversin into metallic iron and complete conversion into the elemental state after 24 h. In contrast, the potassium-doped sample failed to respond to treatment in hydrogen under these conditions and only after 36 h was evidence of metallic iron detected. Treatment of both reduced materials in a 1 : 1 mixture of carbon monoxide and hydrogen at 270 °C resulted in the formation of iron carbides. Crystalline materials, formed by the calcination of the air-dried precipitates at 600 °C in air, were characterises as α-Fe2O3 and underwent little change when heated in nitrogen at 270 °C. Exposure of both the crystalline α-F2O3 phases to carbon monoxide for 2 h resulted in the formation of magnetically ordered Fe3O4. Further treatment of the non-potassium-doped material resulted in its partial conversion into the iron carbide χ-Fe5C2. Similar treatment of the potassium-doped phase resulted in complete conversion into the iron carbide. The non-potassium-doped, carbided material was readily transformed to metallic iron by treatment in hydrogen for 12 h, whilst the potassium-doped phase was changed only by prolonged exposure to hydrogen. Both phases were converted into iron carbide when treated in a 1:1 mixture of carbon monoxide and hydrogen at 270 °C. The enhanced formation of iron carbide phases in potassium-doped materials may be associated with the role of the dopant in increasing the rate of dissociative chemisorption of carbon monoxide. The subsequent resistance of the potassium-doped carbide phases to hydrogenation can be related to the inhibiting influence of the excess carbon which is deposited duirng the initial reaction in carbon monoxide.