The Consequences of Surface Heterogeneity of Cobalt Nanoparticles on the Kinetics of CO Methanation
CO hydrogenation reaction was studied at methanation conditions (H2/CO > 3, 250-300ºC) on Co/SiO2 catalysts with different mean Co nanoparticle size (dp = 4 nm, 13 nm y 33 nm). Catalysts were prepared by incipient wetness impregnation; the cobalt loading and reduction conditions were suitably selected in order to produce catalysts with different dp. Operando-FTIR and kinetic measurements provided consistent data for the heat of CO adsorption at low and full coverage. The results confirm the structural sensitivity of CO hydrogenation reaction on supported cobalt catalysts and demonstrate the energetic heterogeneity of cobalt surface. A model derived from the Temkin formalism accurately fits the kinetic data and renders the heterogeneity of cobalt surface by capturing the change in the heat of CO adsorption with the CO coverage. This change is normalized by the heat of CO adsorption at high coverage (QCO/Q1) in order to quantify and compare the heterogeneity of cobalt surfaces. The CO hydrogenation turnover rates are lower on the catalyst with smaller nanoparticles. This surface shows higher heterogeneity and lower apparent activation energy, which is attributed to a larger amount of the more active B5-B sites in this catalyst.