From two-component enzyme complex to nanobiohybrid for energy-efficient water-gas shift reaction

Abstract

The water-gas shift reaction (WGSR, CO + H₂O ⇔ CO₂ + H₂) is widely used for the upgrading of syngas, a key substrate for various chemical processes. However, the industrial WGSR requires high pressure and temperature, and has low selectivity. Here, we have designed a biohybrid catalyst by combining CODH from Rhodospirillum rubrum, which catalyzes CO-to-CO₂ conversion and a bioinspired nickel bisdiphosphine complex, which catalyzes the hydrogen evolution reaction, immobilized on carbon nanotubes. Carbon nanotubes enable the dual functioning of both catalysts providing efficient electrical conductivity and allowing electroless CO-to-CO₂ conversion and H₂ evolution. Owing to CO tolerance of the Ni complex, this bioinspired nanohybrid catalyst shows high performance by reaching 100% conversion yield and maximum TOF of 30 s^-1 towards WGSR at ambient temperature and pressure in the presence of either pure CO or syngas.

Fig. 1. Bioinspired WGSR principle.

Fig. 2. (A) Cyclic voltammetry (CV) of NiPNP^Arg-modified ^NAMWCNT electrode (red) under Ar, and Rec-RrCODH modified ^ADAMWCNT electrode (blue) under CO in 50 mM Bis–Tris propane buffer pH 7.0 (v = 5 mV s⁻¹): (B) half-wave potential and I_maxversus pH for NiPNP^Arg-modified MWCNT electrode (red) under Ar, and Rec-RrCODH-modified MWCNT electrode (blue) under CO in 50 mM Bis–Tris propane buffer pH 7.0.

Fig. 3. (A) CV of the bifunctionalized MWCNT electrode under Ar and CO in 50 mM Bis–Tris propane buffer pH 7.0 (v = 5 mV s⁻¹); (B) CV of the Rec-RrCODH-modified MWCNT electrode (blue) and NiPNP^Arg-modified MWCNT electrode (red) under CO in 50 mM Bis–Tris propane buffer pH 7.0 (v = 5 mV s⁻¹) and the corresponding sum of both CV (black, capacitive contribution of one CV was removed).

Fig. 4. (A) Quantity of gas in headspace during WGSR experiment determined by GC over time for H₂, CO and CO₂ (200 μmol CO injected at t₀). (B) Quantity of H₂ measured after 24 h at various CO concentration in 50 mM Bis–Tris propane buffer, pH 7.0 at 25 °C (pristine MWCNT film, 0.9 nmol per cm²Rec-RrCODH and 110 nmol per cm²NiPNP^Arg) (C) WGSR conversion yield for a nonmodified film, film modified with NiPNP^Arg, film modified with Rec-RrCODH and film modified with both NiPNP^Arg and Rec-RrCODH; (D) quantity of H₂ in headspace of WGSR experiment determined by GC over time for pristine MWCNT, ^NAMWCNT and ^ADAMWCNT in 50 mM Bis–Tris propane buffer pH 7.0 at 25 °C (2 nmol Rec-RrCODH and 240 nmol NiPNP^Arg, 280 μmol injected CO).

Fig. 5. (A) Quantity of H₂ in headspace of WGSR experiment determined by GC after 24 h versus pH (1 mL CO injected at t₀), 0.7 nmol per cm²Rec-RrCODH and 14 nmol per cm²NiPNP^Arg, (buffer used: 50 mM citric acid pH 3, 50 mM sodium acetate pH 4 to 5.5, 50 mM Bis–Tris propane buffer pH 6 to 9.5); (B) turnover frequency (TOF) measured after 1 h WGSR, according to the total amount of Rec-RrCODH, and turnover number (TON) measured after 24 h versus starting amount of injected CO (50 mM Bis–Tris propane buffer pH 7, 0.9 nmol per cm²Rec-RrCODH and 110 nmol per cm²NiPNP^Arg).

Fig. 6. (A) SEM micrograph and (B) XPS spectra of Ni 2p core energy levels for MWCNT film modified with both NiPNP^Arg- and Rec-RrCODH before (a) and after (b) 24 h WGSR accompanied with XPS simulation (red line).

Fig. 7. Evolution of the proportions of H₂ and CO detected by GC in the headspace of a WGSR experiment over time and its corresponding H₂/CO ratio (vial flushed with syngas mix (30% CO, 30% H₂, 20% CO₂, 10% CH₄, 10% N₂ at t₀, corresponding to 7 mL gas mix at ambient pressure and 25 °C, 0.9 nmol per cm²Rec-RrCODH and 110 nmol per cm²NiPNP^Arg)).