Crystallographic Dependence of CO Activation on Cobalt Catalysts: HCP versus FCC

Abstract

Identifying the structure sensitivity of catalysts in reactions, such as Fischer−Tropsch synthesis from CO and H 2 over cobalt catalysts, is an important yet challenging issue in heterogeneous catalysis. Based on a first-principles kinetic study, we find for the first time that CO activation on hexagonal close-packed (HCP) Co not only has much higher intrinsic activity than that of face centered-cubic (FCC) Co but also prefers a different reaction route, i.e., direct dissociation with HCP Co but Hassisted dissociation on the FCC Co. The origin is identified from the formation of various denser yet favorable active sites on HCP Co not available for FCC Co, due to their distinct crystallographic structure and morphology. The great dependence of the activity on the crystallographic structure and morphology of the catalysts revealed here may open a new avenue for better, stable catalysts with maximum mass-specific reactivity.I dentifying the structure sensitivity of catalysts in chemical reactions to achieve the maximum mass-specific yet stable reactivity is one of the most challenging goals in heterogeneous catalysis. 1 A growing number of studies are conducted to understand the nature of the structure sensitivity, supported by well-defined preparation methods, in situ characterizations, surface science studies, and ab initio calculations. A prime example of this complex structure sensitivity is the Fischer− Tropsch synthesis (FTS) converting CO and H 2 from coal, natural gas, and biomass to hydrocarbon over cobalt catalysts. 2 Over the years, two notable structure sensitivities have been observed for FTS over Co catalysts, i.e., crystallographic structure and particle size. First, it is noted that Co can exist in two crystallographic structures, namely, the hexagonal closepacked (HCP) phase and the face-centered cubic (FCC) phase, and both phases are found in FTS. It has been reported by many that HCP Co has higher FTS activity than FCC Co. 3 This structure sensitivity is, however, complicated by a phase transition from HCP to FCC upon decreasing the catalyst size, 4 varying the supports and promoters, and pretreating the catalysts. 5 To our best knowledge, it remains open as to whether and why HCP Co catalysts have higher FTS activity than FCC ones, which prevents the full exploration of this structure sensitivity. Second, there are a number of reports on the effect of particle size of Co catalysts on FTS activity; namely, the turnover frequency (TOF) was constant for crystallites above a certain diameter, but when the diameter became smaller, the TOF decreased. 6 It is unclear whether the decrease of FTS activity at the smaller particle size is related to the phase transition of the Co catalyst from HCP to FCC, because of the complexity of the FTS and the absence of sufficient crystallographic structure data. Nevertheless, it is clear that one cannot increase the mass-specific activity of Co catalysts in FTS simply by decreasing the particle size.We report here a density functional theory (DFT)-base...

Recent Progresses in Understanding of Co-Based Fischer–Tropsch Catalysis by Means of Transient Kinetic Studies and Theoretical Analysis

“…-1.74 ± 0.088 eV) than that for FCC Co (i.e. -1.66 ± 0.058 eV) [113]. However, the small difference on CO and H adsorption energies between HCP and FCC Co, 0.08 and 0.07 eV, respectively [113], reveals the weak structure sensitivity for the adsorption of the reactant.…”

“…-1.66 ± 0.058 eV) [113]. However, the small difference on CO and H adsorption energies between HCP and FCC Co, 0.08 and 0.07 eV, respectively [113], reveals the weak structure sensitivity for the adsorption of the reactant. The effect of CO coverage on the chemisorption of some surface species, H, CO, CH, CH 2 , CH 3 , HCO, HCOH, CH 2 O and so on, has been investigated on Co (0001) with different CO coverages, 1/4, 1/3 and 1/2 ML, as listed in Table 1 [32,40,117].…”

“…Large efforts have been devoted to the studies of adsorption of intermediates on hcp Co (0001), which is the thermodynamically most stable surface of the industrial Co catalysts at FTS condition [110][111][112][113]. The chemisorption energies of the most stable configurations for the related surface species on Co (0001) are summarized in Table 1.…”

“…High binding energies may largely decrease the activation barrier of the bond-breaking steps, indicating that the defect sites on the step surface might be active for CO activation, which will be discussed later. The structure sensitivity of the adsorption process has been further investigated in the refs [113,115,118]. It can be seen that the average binding energy of CO is slightly larger for HCP Co (i.e.…”

Controllable Cobalt Oxide/Au Hierarchically Nanostructured Electrode for Nonenzymatic Glucose Sensing

“…Traditionally drugs in photothermal therapies are delivered via polymeric structures or liposomes [99,[227][228][229][230][231][232]. However, these delivery agents are hindered by several factors.…”