Carbon-supported ruthenium catalysts for NH3 synthesis doped with caesium nitrate: Activation process, working state of Cs–Ru/C

The process of an active Cs–Ru/carbon catalyst formation (reduction) was studied in detail, using graphitised carbons as supports for ruthenium and caesium nitrate as a promoter precursor. In situ XRD and TPR-MS techniques were applied to monitor the changes in the specimens when heating in H2 and H2 + Ar mixtures, respectively. The postactivation state of the catalysts (i.e., the state corresponding to ammonia synthesis conditions) was characterised chemically via interaction of the reduced samples with water vapour at 50 °C (H2 evolution) and also via interaction with oxygen at 0 °C. These experiments were supplemented with those of ammonia synthesis. Ruthenium was shown to facilitate the decomposition of caesium nitrate; whereas the Ru-free CsNO3/C reference materials are stable in a flowing H2 + Ar mixture up to about 400 °C, the CsNO3 decomposition starts at 100–120 °C for the CsNO3–Ru/C catalysts (XRD, TPR-MS) and proceeds via a CsOH⋅H2O-intermediate product that turns into an amorphous species at elevated temperatures, as indicated by XRD. Characterisation studies of the postactivation catalysts showed that caesium is partially reduced during operations and reacts with oxygen (O2 consumption) and water vapour (H2 evolution). The degree of promoter reduction resulting from H2 liberation varies from 0.25 to 0.45, depending on the Cs loading, Ru loading, and kind of carbon. Combining the O2 consumption and H2 evolution data suggests that a substoichiometric oxide (CsxOy; x/y=2.7–3.6x/y=2.7–3.6) exists on the catalyst surface rather than Cs0 + CsOH. High activities (TOFs) of the optimally promoted Cs–Ru/C systems in ammonia synthesis (63 bar, 370 and 400 °C) were ascribed to the strong promotional effect of partly reduced caesium (CsxOy) covering the Ru surface. A peculiar S-like shape of the TOF trace versus Cs loading is suggested to be a consequence of the promoter distribution between the carbon surface and surface of ruthenium. Clearly, the trend in TOF reflects that in Ru coverage by the CsxOy groups, the latter being controlled by the heats of CsxOy adsorption on ruthenium and on carbon, respectively.