Fischer–Tropsch synthesis: effect of ammonia impurities in syngas feed over a cobalt/alumina catalyst

•Addition of ammonia resulted in significant irreversible catalyst deactivation.•The deactivation rate was almost constant at all the concentrations of ammonia exposure.•Ammonia addition appears to poison cobalt metallic sites to some extent.•Prolonged exposure at similar concentrations does not further change the activity.•A mild in situ hydrogen treatment partly recovered the activity of the catalyst.

The effect of co-fed ammonia in syngas on the performance of a traditional cobalt catalyst (platinum promoted cobalt/alumina) was investigated during Fischer–Tropsch synthesis (FTS) using a continuously stirred tank reactor (CSTR). The addition of 1, 10, 100, and 1200 ppmw (concentration of ammonia with respect to the syngas feed) of ammonia resulted in significant irreversible deactivation of the catalyst under identical reaction conditions. The percentage of deactivation was almost constant (±5%) at all the concentrations (1–1200 ppmw) of ammonia exposure. The addition of ammonia (either NH3 gas or aqueous NH4OH) particularly at higher concentrations exhibits lower methane selectivity compared to ammonia-free synthesis conditions. This enhancement in C5+ hydrocarbon formation may be attributed to the selective poisoning of metallic cobalt sites by addition of ammonia.

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