(603a) Impact of Water On the Mechanism of Fischer-Tropsch Synthesis On Ru Catalysts

Impact of Water on the Mechanism of Fischer-Tropsch
Synthesis on Ru Catalysts

David D. Hibbitts¹, Brett T. Loveless¹,
Matthew Neurock^2* and Enrique Iglesia^1*

¹Department
of Chemical and Biomolecular Engineering, University of California at Berkeley,
Berkeley, California 94720, United States

²Departments
of Chemical Engineering and Chemistry, University of Virginia, Charlottesville,
Virginia 22904, United States

*iglesia@berkeley.edu, mn4n@virginia.edu

Fischer-Tropsch
synthesis (FTS) forms large hydrocarbons from synthesis gas (H₂-CO)
derived from biomass, coal, or natural gas. The specific mechanism of C-O
cleavage and C-C bond formation steps in FTS remains the subject of active
debate, even after many experimental and theoretical inquiries. Rigorous
kinetic and isotopic data and theoretical calculations have shown that at the
saturation CO* coverages prevalent during FTS [1-5], C-O activation occurs predominantly
with assistance by chemisorbed H-atoms (H*) on Ru and Co surfaces, instead of via
direct CO dissociation on vicinal vacant sites [2,3]. Previous studies have
also shown that H₂O increases FTS turnover rates and product
molecular weight on Co catalysts, [4,5] but without definitive mechanistic
interpretations. The effects of water on turnover rates and selectivities on
Ru-based catalysts are reported here and interpreted mechanistically using density
functional theory (DFT) calculations.

CO
turnover rates are proportional to H₂ pressure on Ru/SiO₂
and obey a rate equation (Eq. 1) similar to that reported on Co catalysts [2,3,4]
at low CO conversions and H₂O pressures.

                                                         (1)

DFT
calculations indicate that a sequence of elementary steps involving H-addition to
CO* to form formyl (HCO*) species followed by a kinetically-relevant H-addition
to form hydroxymethylene (*HCOH*), a species that decomposes irreversibly to
form methylidyne and hydroxyl (CH* + OH*) is consistent with Eq. 1. The
kinetically-relevant transition state has an energy of 193 kJ mol^-1
compared to a CO*-covered surface and H₂ in the gas phase, the
resting state of the catalyst.

CO
turnover rates on Ru/SiO₂ increase monotonically with H₂O
pressure (up to 0.3 MPa) before reaching nearly constant values (Figure 1).
This suggests that H₂O may increase the rate of kinetically-relevant
CO activation steps and that a H₂O-derived intermediate may block
active sites on the catalyst surface at higher H₂O pressures. DFT
calculations were employed to determine how H₂O could alter the
kinetically-relevant step or increase its rate constant. Results show that H₂O
can mediate the formation of COH* via a H-shuttling mechanism in which H* transfers
to a nearby H₂O molecule to form a short-lived H₃O⁺
intermediate which can then protonate the O of CO*. Following this, COH*
undergoes H‑addition at the C to form *HCOH* which then dissociates with
H₂O nearby as a solvent in the kinetically-relevant step. The
transition state for this step has an energy of 122 kJ mol^‑1
compared to a CO*-covered surface with H₂ and H₂O in the
gas phase.

Figure 1. CO consumption rate (r or p), selectivity to CH₄ (™ or ˜), and selectivity to C₅₊
products (£ or ¢) as a function of the average H₂O
partial pressure on a 5 wt% Ru/SiO₂ catalyst (463 K, 2.9 MPa, H₂/CO
= 4.5). (Open symbols) Space velocity runs; (Closed symbols) H₂O-addition
runs.

The
presence of a H₂O-mediated path requires the addition of a H₂O-dependent
term to the rate equation (Eq. 1). Furthermore, the weaker effects of H₂O
observed above 0.3 MPa indicate the presence of one or more H₂O-derived
intermediates blocking active sites on the catalyst surface, leading to a
revised rate law in Eq. 2.

                      (2)

References

1.   Mims, C., McCandlish, L., J. Phys. Chem. 91,
929 (1987).

2.   Ojeda, M., Nabar, R., Nilekar, A.,
Ishikawa, A., Mavrikakis, M., Iglesia, E., J. Catal. 272, 287
(2010).

3.   Loveless, B. T., Buda, C., Neurock, M., Iglesia,
E., J. Am. Chem. Soc. 135, 6107 (2013).

4.   Iglesia, E., Reyes, S., Madon, R., Soled, S., Adv.
Catal. 39, 221 (1993).

5.   Krishnamoorthy, S., Tu, M., Ojeda, M.P., Pinna,
D., and Iglesia, E., J. Catal. 211, 422 (2002).