Light Induced Carbon Dioxide Reduction by Water at Binuclear ZrOCo^II Unit Coupled to Ir Oxide Nanocluster Catalyst

Abstract

An all-inorganic polynuclear unit consisting of an oxo-bridged binuclear ZrOCo(II) group coupled to an iridium oxide nanocluster (IrO(x)) was assembled on an SBA-15 silica mesopore surface. A photodeposition method was developed that affords coupling of the IrO(x) water oxidation catalyst with the Co donor center. The approach consists of excitation of the ZrOCo(II) metal-to-metal charge-transfer (MMCT) chromophore with visible light in the presence of [Ir(acac)3] (acac: acetylacetonate) precursor followed by calcination under mild conditions, with each step monitored by optical and infrared spectroscopy. Illumination of the MMCT chromophore of the resulting ZrOCo(II)-IrO(x) units in the SBA-15 pores loaded with a mixture of (13)CO2 and H2O vapor resulted in the formation of (13)CO and O2 monitored by FT-IR and mass spectroscopy, respectively. Use of (18)O labeled water resulted in the formation of (18)O2 product. This is the first example of a closed photosynthetic cycle of carbon dioxide reduction by water using an all-inorganic polynuclear cluster featuring a molecularly defined light absorber. The observed activity implies successful competition of electron transfer between the IrO(x) catalyst cluster and the transient oxidized Co donor center with back electron transfer of the ZrOCo light absorber, and is further aided by the instant desorption of the CO and O2 product from the silica pores.

Towards a Molecular Level Understanding of the Multi-Electron Catalysis of Water Oxidation on Metal Oxide Surfaces

“…Hence, photodepositon is a required step for establishing charge transfer coupling between the Co donor of the light absorber and the Ir oxide catalyst. The observed conversion of CO 2 to CO and O 2 (DG°= 62 kcal mol -1 ) with H 2 O as electron source under illumination with 355 nm photons is consistent with the energetics of the reaction and in agreement with our previous results on the corresponding oxidation and reduction half reactions [66]. The quantum efficiency is estimated at 17 % assuming that there is no energy transfer between individual photocatalytic sites.…”

“…The oxide is used in commercial electrolyzers [62] and has been explored as a catalytic component in a various electrochemical cells [1,63], photoanode assemblies [64] and integrated artificial photosystem designs [65,66]. Large scale use of this noble metal oxide as water oxidation catalyst is prohibited by its scarcity, but mechanistic understanding will be valuable given the observed high efficiency.…”

“…74, with permission Water oxidation to molecular oxygen is a mandatory reaction of any photosynthetic process because it provides the electrons and protons that enable the formation of a fuel upon reaction with carbon dioxide. As an example, we discuss here the coupling O 2 evolution from water at a metal oxide catalyst with carbon dioxide reduction at an all-inorganic, oxo-bridged heterobinuclear light absorber [66].…”

“…The detailed structure of several units on the surface of mesoporous silica supports, including the ZrOCo II system shown in Fig. 17a has been determined by EXAFS spectroscopy [66,96,97]. Typical metal-to-metal distances are around 3.4 A.…”

“…Development of a photodeposition method for proper positioning of an Ir oxide nanocluster catalyst adjacent to the Co donor center of a ZrOCo unit in the channels of mesoporous silica SBA-15 allowed us to synthesize polynuclear units consisting of a ZrOCo charge-transfer chromophore coupled to an IrO x nanocluster catalyst. The spatially directed assembly of the Ir oxide clusters was monitored by optical and FT-IR spectroscopy, and their size of the 2 nm established by HAADF-STEM imaging and EDX measurements [66].…”