Crossing the divide between homogeneous and heterogeneous catalysis in water oxidation

Abstract

Enhancing the surface binding stability of chromophores, catalysts, and chromophore-catalyst assemblies attached to metal oxide surfaces is an important element in furthering the development of dye sensitized solar cells, photoelectrosynthesis cells, and interfacial molecular catalysis. Phosphonate-derivatized catalysts and molecular assemblies provide a basis for sustained water oxidation on these surfaces in acidic solution but are unstable toward hydrolysis and loss from surfaces as the pH is increased. Here, we report enhanced surface binding stability of a phosphonate-derivatized water oxidation catalyst over a wide pH range (1-12) by atomic layer deposition of an overlayer of TiO 2 . Increased stability of surface binding, and the reactivity of the bound catalyst, provides a hybrid approach to heterogeneous catalysis combining the advantages of systematic modifications possible by chemical synthesis with heterogeneous reactivity. For the surface-stabilized catalyst, greatly enhanced rates of water oxidation are observed upon addition of buffer bases −H 2 PO − 4 /HPO 2− 4 , B(OH) 3 /B(OH) 2 O − , HPO 2− 4 /PO 3− 4 − and with a pathway identified in which O-atom transfer to OH − occurs with a rate constant increase of 10 6 compared to water oxidation in acid.electrocatalysis | surface stabilization H eterogeneous catalysis plays an important role in industrial chemical processing, fuel reforming, and energy-producing reactions. Examples include the Haber-Bosch process, steam reforming, Ziegler-Natta polymerization, and hydrocarbon cracking (1-8). Research in heterogeneous catalysis continues to flourish (9-15) but iterative design and modification are restricted by limitations in materials preparation and experimental access to surface mechanisms. By contrast, synthetic modification of molecular catalysts is possible by readily available routes; a variety of experimental techniques is available for monitoring rates and mechanism in solution for the investigation of homogeneous catalysis (16-23). Transferring this knowledge and the reactivity of homogeneous molecular catalysts to a surface could open the door to heterogeneous applications in fuel cells, dye sensitized photoelectrochemical cells, and multiphase industrial reactions.Procedures are available for immobilization of organometallic and coordination complexes on the surfaces of solid supports. Common strategies include surface derivatization of metal oxides by carboxylate, phosphonate, and siloxane bindings (24-27), carbongrafted electrodes (28-30), and electropolymerization (31-33). These approaches provide a useful bridge to the interface and a way to translate mechanistic understanding and ease of synthetic modification of solution catalysts to heterogeneous applications with a promise of higher reactivity under milder conditions. A significant barrier to this approach arises from the limited stability of surface binding. Surface-bound carboxylates are typically unstable to hydrolysis in water, whereas phosphonates are unstable in neutral or basic...

Visible photoelectrochemical water splitting into H 2 and O 2 in a dye-sensitized photoelectrosynthesis cell

“…ALD deposition of overlayers of TiO 2 or Al 2 O 3 has been shown to greatly enhance surface stability toward hydrolysis even in strongly basic solutions (25,26). We show here, for assembly 1 surface-bound to SnO 2 /TiO 2 , that ALD overlayers of TiO 2 or Al 2 O 3 provide both long-term stabilization on the oxide surface at pH 7 in a phosphate buffer, and, as a bonus, incrementally enhanced efficiencies for water splitting (23).…”

“…The latter is problematic because the rate of water oxidation is enhanced by added buffer bases, conditions that also enhance the rate of water oxidation (5,(17)(18)(19)(20)(21)(22)(23)(24).…”

Base-enhanced catalytic water oxidation by a carboxylate–bipyridine Ru(II) complex

“…Under our conditions, with electrochemical monitoring at pH 7.0, the observed behavior is similar to that observed earlier for related single-site Ru polypyridyl catalysts. For these catalysts, oxidation to Ru V =O is followed by rate-limiting O-atom transfer to H 2 O (11,16,17,19,20). Recently, Sun and coworkers (32) reported that the bdacarbene catalyst, [Ru III (bda)(mmi)(OH 2 )] (mmi is 1,3-dimethylimidazolium-2-ylidene) undergoes single-site catalytic water oxidation at pH 1.0. .…”

“…APT can promote dramatic rate enhancements. In a recent study on surface-bound [Ru(Mebimpy)(4,4′-((HO) 2 OPCH 2 ) 2 bpy)(OH 2 )] 2+ [4,4′-((HO) 2 OPCH 2 ) 2 bpy is 4,4′-bis-methlylenephosphonato-2,2′-bipyridine] stabilized by atomic layer deposition, a rate enhancement of ∼10 6 was observed with 0.012 M added PO 4 3− at pH 12 compared with oxidation at pH 1 (20).…”

Perylene Diimide Aggregates on Sb-Doped SnO2: Charge Transfer Dynamics Relevant to Solar Fuel Generation

“…16 Furthermore, this approach generally involves less synthetic efforts, and it does not require the use of post-treatments based on advanced techniques (e.g., atomic layer deposition 17 ) or the deposition of polymeric overlayers 18 to further stabilize the linkage. On the other hand, the charge injection by these aggregates likely proceeds through π−π* exciton migration within the molecular assembly until achieving separation at the interface between the innermost molecular surface and the semiconductor.…”