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Silver addition to a cobalt Fischer–Tropsch catalyst

Chen, Wei, Pestman, Robert, Chiang, Fu-Kuo, Hensen, Emiel J.M.
Journal of catalysis 2018 v.366 pp. 107-114
catalysts, catalytic activity, cobalt, dissociation, energy-dispersive X-ray analysis, graphene, hydrogenation, infrared spectroscopy, methane, methane production, nanoparticles, nanosilver, oxygen, platinum, probability, silver, surface area, terraces, transmission electron microscopy
Silver is known to promote F–T catalysts as a reduction promoter, although it has also been observed to affect catalytic performance of, for instance, cobalt nanoparticles. In this work, a series of silica-supported cobalt catalysts with a constant platinum content (0.04 wt%) and varying silver content were characterized and tested. Characterization shows that mainly coordinatively unsaturated cobalt atoms are substituted at low silver content (probed by N₂ IR). Adding more silver results in replacement of both coordinatively unsaturated sites and planar sites (probed by H₂-chemisorption and TEM-EDS). The presence of silver on the cobalt surface affects both structure-sensitive CO dissociation and structure-insensitive hydrogenation reactions. The Boudouard reaction shows that the CO dissociation rate decreases with increasing silver content. Hampered CO dissociation is also apparent from temperature-programmed CO IR spectroscopy. However, even at high silver content the surface still contains step-edge sites that can dissociate CO and a small amount of carbon is deposited (quantified by TPH). During the CO hydrogenation at 260 °C, CO conversion is reduced by less than 30% when 8.8 wt% silver is added. This does not correlate well with the loss in cobalt surface area measured by chemisorption (decreases by 93%) or CO dissociation rate probed by CO disproportionation (decreases by 90%). This mismatch is mainly because that the CO consumption rate under methanation condition is largely controlled by carbon and oxygen hydrogenation, rather than CO dissociation. On the other hand, transient experiments show that silver accelerates oxygen-removal steps. It turns out that CO conversion is only moderately affected by silver. The coverage of hydrogenation sites – mostly terrace sites – by silver causes slower CH₄ formation. Longer residence of CHₓ intermediates (determined by SSITKA) accordingly leads to an increase in chain-growth probability and therefore higher C₂₊ selectivity. SSITKA also confirms that the decrease in steady-state CO/CHₓ coverage qualitatively correlates to the loss in cobalt surface area. Another consequence of slower hydrogenation is that the olefins-to-paraffins ratios are higher when silver is added to the catalyst. Importantly, covering the cobalt surface with silver affects the F–T product distribution in a similar manner as graphitic carbon deposited on terrace sites as we reported earlier.