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

Abstract

The effects of Zr promotion on the structure and performance of Co-based Fischer-Tropsch synthesis (FTS) catalysts were investigated. Inclusion of Zr in the catalysts was found to increase the FTS turnover frequency and the selectivity to C5+ hydrocarbons and to decrease the selectivity to methane under most operating conditions. These improvements to the catalytic performance are a function of Zr loading up to an atomic ratio of Zr/Co = 1.0, above which the product selectivity is insensitive to higher concentrations of the promoter. Characterization of the Co nanoparticles by different methods demonstrated that the optimal Zr loading corresponds to half monolayer coverage of the Co surface by the promoter. Measurements of the rate of FTS at different pressures and temperatures established that the kinetics data for both the Zr-promoted and unpromoted catalysts are described by a twoparameter Langmuir-Hinshelwood expression. The parameters used to fit this rate law to the experimental data indicate that the apparent rate coefficient and the CO adsorption constant for the Zrpromoted catalysts are higher than those for the unpromoted catalyst. Elemental mapping by means of STEM-EDS provided evidence that Zr is highly dispersed over the catalyst surface and has limited preference for association with the Co nanoparticles. In situ X-ray absorption spectroscopy confirmed the absence of mixing between the Zr and Co in the nanoparticles. These results suggest that Zr exists as a partial layer of ZrO2 on the surface of the Co metal nanoparticles. Accordingly, it is proposed that Zr promotion effects originate from sites of enhanced activity at the interface between Co and ZrO2. The possibility that ZrO2 acts as a Lewis acid to assist in CO dissociation as well as to increase the ratio of CO to H adsorbed on the catalyst surface is discussed.

Morphology Evolution of Hcp Cobalt Nanoparticles Induced by Ru Promoter

“…[18] The higher activity of hcp Co nanoparticles is identified from various favorable active sites on exposed facets that not available for fcc Co nanoparticles. In addition, Ge et al have proposed that the corrugated Co (10)(11)(12) and Co (10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24) surfaces can significantly enhance the reactivity toward CO dissociation by reducing the activation barrier compared to the flat Co(0001) and corrugated Co (11)(12)(13)(14)(15)(16)(17)(18)(19)(20) surfaces. [19] The identification of the first CÀ C bond formation in chain growth by Su et al revealed that the preferential carbide mechanism on Co(10-11) is more selective to chain growth rather than methanation, while the preferential CO insertion mechanism on Co(0001) is opposite.…”

Fischer–Tropsch Synthesis Over Zr-Promoted Co/γ-Al2O3 Catalysts

“…Indeed, Rohr et al [15] found that the effect of Zr on the catalyst activity at high pressure FTS conditions was not as significant as its effect at low pressure conditions. Johnson and Bell [33] also observed that the promoting effect of Zr on Co/SiO 2 was already very small at syngas pressures of 10 bar and a temperature of 210 °C. This decreasing promoting effect of Zr with increasing pressure, together with a faster deactivation, may explain the low performance of the CoZrAl ME catalyst.…”

“…Unfortunately, these results cannot be explained by differences in support acidity, factor that is believed to play a crucial role in the FTS activity [46,47]. According to Johnson and Bell [33], promotion with Zr originates from sites with enhanced activity at the Co-ZrO 2 interface. The results are therefore surprising since a larger contact between ZrO 2 and cobalt and thus, a higher activity, was expected for the CoZrAl ME catalyst.…”

“…Besides some studies [27], Zr has often been reported to increase the TOF [28][29][30][31][32][33]. However, some researchers [28][29][30] found that ZrO 2 enhances the reducibility while others [31] claim that it does not have any influence.…”

Higher Alcohol Synthesis Over Rh Catalysts: Conditioning of Rh/N-CNTs by Co and Mn Entrapped in the Support

“…Model catalysts based on Rh exhibit a unique selectivity to oxygenates due to moderate ability for CO dissociation [6,8,[10][11][12][13]. Here, a variety of transition metals were identified as efficient dopants, e.g., Mn, Fe, Co, Mo, or Zr [8,[14][15][16].…”