Photocatalytic proton reduction with ruthenium and cobalt complexes immobilized on fumed reversed-phase silica

Abstract

Heterogeneous photocatalytic hydrogen production with a non-covalently immobilized molecular ruthenium based photosensitizer (PS) and a cobalt polypyridyl based water reducing catalyst (WRC) is reported. PS and WRC were derivatized with C₁₈-alkyl chains and immobilized by adsorption on hydrophobic fumed silica. The resulting loaded support was suspended in water with anionic or cationic surfactants and subjected to heterogeneous photocatalytic H₂ production with ascorbate as sacrificial electron donor (SED). No leaching was observed under catalytic conditions, thus catalysis was truly heterogeneous. The catalytic performance of immobilized PS and WRC clearly exceeded that of homogeneous catalysis at low concentrations. At high concentration, diffusion and light limitation lead to lower reaction rates, but the same stability as for homogeneous reactions was still achieved. WRC concentration variations indicated a relatively high stability (up to 1300 H₂/Co) and mobility of amphiphilic catalysts on the hydrophobic silica surface. Comparison of fumed silica with porous and non-porous silica showed, that a high BET surface area along with a good accessibility from the reaction media are crucial for catalytic performance. Mechanistic investigations by transient absorption spectroscopy displayed reductive quenching of excited PS by ascorbate followed by on particle electron transfer to WRC as reaction pathway. Particles with additional cationic surfactants exhibited a significantly higher catalytic performance as compared to anionic surfactants. Non-covalent anchoring of correspondingly derivatized WRCs or PSs to reversed-phase silica offers a rapid and versatile transition from homogeneous to heterogeneous molecular proton reduction.

Scheme 1. Structure of water reducing catalyst [Co^IIBr(appy)]Br 1.,

Scheme 2. Schematic representations of syntheses towards C₁₈/C₁₉-derivatized WRC and PS: (i) NaH, DMF, rt, 45 min; (ii) C₁₈H₃₇-I, rt, 15 h; (iii) Co(ClO₄)₂, MeOH, rt, 3 h; (iv) Ru(bpy)₂Cl₂, EtOH/H₂O, 100 °C, 24 h; (v) NH₄PF₆, H₂O. Detailed synthetic procedures are given in the experimental part.

Scheme 3. Schematic illustration of WRC and PS adsorption on hydrophobic silica. (i) 3, 5, 7 (or 6), MeOH, rt, 30 min (ii) 0.1 M NaOTf electrolyte, MeOH evaporation. Detailed synthetic procedures are given in the experimental part.

Fig. 1. Representative TEM micrograph of hydrophilic fumed silica without (a) and hydrophobic fumed silica with adsorbed PS 5 from a diluted aqueous suspension with 7 as surfactant (b).

Fig. 2. Hydrogen evolution rate courses (solid lines) and total amounts of H₂ (dotted lines) in 1 M ascorbate buffer (pH 4) with 0.1 M NaOTf. Black: 20 μM 3 and 200 μM 5 adsorbed on hydrophobic fumed silica with 300 μM [C₁₆-NMe₃][OAc] (7) as surfactant. Green: same as black, but loaded silica was filtered off and the residual solution irradiated, then 0.5 mM [Ru(bpy)₃]Cl₂ was added and irradiation continued (green arrows). Red: 0.5 mM [Ru(bpy)₃]Cl₂, no WRC. See Table SI3.

Fig. 3. Rates (solid lines) and amounts of H₂ (dashed lines) for homo- and heterogeneous photocatalytic reactions in 1 M ascorbate buffer (pH 4) with 0.1 M NaOTf. Black: 20 μM PS 5 and 1 μM WRC 3 immobilized on f-SiO₂-C₁₈ with 300 μM 7. Red: 20 μM [Ru(bpy)₃]Cl₂ and 1 μM 1. Grey: 10 μM PS 5 and 0.5 μM WRC 3 immobilized on f-SiO₂-C₁₈ with 150 μM 7. Magenta: 10 μM [Ru(bpy)₃]Cl₂ and 0.5 μM 1. Detailed values are shown in Table 1.

Fig. 4. [WRC] dependency study with 200 μM PS 5 and varying amounts of WRC 3 (0, 0.2, 2, 5, 10 and 20 μM) adsorbed on f-SiO₂-C₁₈ in 1 M ascorbate buffer, 0.1 M NaOTf and 300 μM [C₁₆-NMe₃][OAc] (7) as surfactant. The amount of H₂ from the blank experiment (no WRC) was subtracted at each concentration and the corresponding TONs in Co (H₂/Co) calculated (Table SI6†).

Fig. 5. Maximal H₂ evolution rates (black squares, left scale) and total amounts of evolved H₂ (red squares, right scale) in 1 M ascorbate buffer (pH 4) with 0.1 M NaOTf and 200 μM PS 5 and 20 μM WRC 3 immobilized on f-SiO₂-C₁₈ at different surfactant concentrations (mM) multiplied by their charge (negative: Na[C₁₂-PhSO₃], 6; positive: [C₁₆-NMe₃][OAc], 7). Results are summarized in Table SI5 and the structures of 6 and 7 depicted in Scheme SI1.

Fig. 6. Comparison of rates (solid lines, left scale) and H₂ amounts (dotted lines, right scale) with WRC 3 and PS 5 immobilized on different types of hydrophobic silica in 1 M ascorbate buffer (pH = 4, 0.1 M NaOTf). Black: 200 μM PS 5 and 40 μM WRC 3 on f-SiO₂-C₁₈ with 2.5 mM 6. Red: 4.8 μM 5 and 1 μM 3 on non-porous spherical silica with 100 μM 6. Green: 200 μM 5 and 40 μM 3 on porous silica with 2.5 mM 6.

Scheme 4. Proposed reaction mechanism of photocatalytic H₂ production on hydrophobic silica: (1) photoexcitation of 5 → 5*; (2) reductive quenching of 5* → 5^–; (3) electron transfer 5^– → 3; (4) H⁺ reduction by 3^–.