An Investigation into the Effects of Mn Promotion on the Activity and Selectivity of Co/SiO₂ for Fischer–Tropsch Synthesis: Evidence for Enhanced CO Adsorption and Dissociation

Abstract

Mn is an effective promoter for improving the activity and selectivity of Co-based Fischer-Tropsch synthesis (FTS) catalysts, but the mechanism by which this promoter functions is poorly understood. The work reported here was aimed at defining the manner in which Mn interacts with Co and how these interactions affect the activity and selectivity of Co. Detailed measurements are reported for the kinetics of FTS as a function of Mn/Co ratio, temperature, and reactant partial pressures. These data are described by a single, two-parameter rate expression. Mn-promotion was found to increase both the apparent rate constant for CO consumption and the CO adsorption constant. Further evidence for enhanced CO adsorption and dissociation was obtained from measurements of temperature-programmed desorption of CO and CO disproportionation rates, respectively. Quantitative analysis of elemental maps obtained by STEM-EDS revealed that the promoter accumulates preferentially on the surface of Co nanoparticles at low Mn loadings, resulting in a rapid onset of improvements in the product selectivity as the Mn loading increases. For catalysts prepared with loadings higher than Mn/Co = 0.1, the additional Mn accumulates in the form of nanometer-scale particles of MnO on the support. In situ IR spectra of adsorbed CO show that Mn promotion increases the abundance of adsorbed CO with weakened C-O bonds. It is proposed that the cleavage of the C-O bond is promoted through Lewis acid-base interactions between the Mn 2+ cations located at the edges of MnO islands covering the Co nanoparticles and the O atom of CO adsorbates adjacent to the MnO islands. The observed decrease in selectivity to CH4 and the increased selectivity to C5+ products with increasing Mn/Co ratio are attributed to a decrease in the ratio of adsorbed H to CO on the surface of the supported Co nanoparticles.

On the superior activity and selectivity of PtCo/Nb2O5 Fischer Tropsch catalysts

“…3, right). Pressure-dependent influence of niobia on the catalyst selectivity was previously observed for niobia-promoted catalysts [8] in line with reports on MnOpromoted catalysts [13,17]. At 1 bar, the cobalt weight-normalized activity of Co/c-Al 2 O 3 and Co/Nb 2 O 5 was found to increase by a factor of 1.7 and 2.8 upon Pt-promotion (Fig.…”

“…The selectivity of cobalt catalysts toward liquid hydrocarbons (C 5+ ) is affected by process conditions such as temperature [6,7], pressure [8] and CO conversion [9] and catalyst properties such as support acidity [10,11] and pore diameter [5,10] and can be promoted by partially reducible transition metal oxides such as MnO and ZrO 2 [12][13][14][15][16][17]. These promoters can be present as small particles in the close proximity to cobalt or as support material in the case of Co/TiO 2 [16,18,19] or Co/Nb 2 O 5 [6,[20][21][22][23][24] catalysts.…”

Role of ZrO2 in Promoting the Activity and Selectivity of Co-Based Fischer–Tropsch Synthesis Catalysts

“…Both the carbon number selectivities and the olefin to paraffin ratio became insensitive to the promoter loading at a Zr/Co ratio close to 1. This diminishment in the effects of the promoter with increasing promoter loading is quite similar to that reported for Mn-promoted catalysts (23). However in the case of Zr promotion, ten times as much promoter on an atomic basis is required to reach the selectivity plateau.…”

“…In this study, it was determined that the Zr-promoted catalyst kinetics follow a rate law that is consistent with H-assisted CO dissociation as the rate-limiting step. As reported previously, the same rate law holds for the unpromoted catalyst (23). Differences in the activities of the Zr-promoted and unpromoted catalysts originate from differences in the apparent rate coefficients and CO adsorption constants.…”

“…Ali et al speculated that Co active sites at the interface with ZrO2 may exhibit enhanced activity toward CO hydrogenation (13), a hypothesis that originates from studies of metal oxide-promoted Rh catalysts (20) (21). This hypothesis has also been used to rationalize the catalyst structure-performance relationships resulting from Mn promotion of Co-based FTS catalysts (22) (23). In this study, we demonstrate that Zr promotes the catalyst activity and selectivity in a manner analogous to promotion by Mn.…”