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Molecular Dye-Sensitized Photocatalysis with Metal-Organic Framework and Metal Oxide Colloids for Fuel Production

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Molecular Dye-Sensitized Photocatalysis with Metal-Organic Framework and Metal Oxide Colloids for Fuel Production energies Article Molecular Dye-Sensitized Photocatalysis with Metal-Organic Framework and Metal Oxide Colloids for Fuel Production Philip M. Stanley and Julien Warnan * Chair of Inorganic and Metal-Organic Chemistry, Department of Chemistry, Technical University of Munich, 85748 Garching, Germany; [email protected] * Correspondence: [email protected]; Tel.: +49-(0)89-289-13147 Abstract: Colloidal dye-sensitized photocatalysis is a promising route toward efficient solar fuel produc- tion by merging properties of catalysis, support, light absorption, and electron mediation in one. Metal- organic frameworks (MOFs) are host materials with modular building principles allowing scaffold prop- erty tailoring. Herein, we combine these two fields and compare porous Zr-based MOFs UiO-66-NH2(Zr) and UiO-66(Zr) to monoclinic ZrO2 as model colloid hosts with co-immobilized molecular carbon diox- 0 0 ide reduction photocatalyst fac-ReBr(CO)3(4,4 -dcbpy) (dcbpy = dicarboxy-2,2 -bipyridine) and photo- 0 0 sensitizer Ru(bpy)2(5,5 -dcbpy)Cl2 (bpy = 2,2 -bipyridine). These host-guest systems demonstrate selective CO2-to-CO reduction in acetonitrile in presence of an electron donor under visible light irradiation, with turnover numbers (TONs) increasing from ZrO2, to UiO-66, and to UiO-66-NH2 in turn. This is attributed to MOF hosts facilitating electron hopping and enhanced CO2 uptake due to their innate porosity. Both of these phenomena are pronounced for UiO-66-NH2(Zr), yielding TONs of 450 which are 2.5 times higher than under MOF-free homogeneous conditions, highlighting synergistic effects between supramolecular photosystem components in dye-sensitized MOFs. Citation: Stanley, P.M.; Warnan, J. Keywords: dye-sensitized; metal-organic frameworks; metal oxides; host-guest photosystems; Molecular Dye-Sensitized molecular catalysis; fuel production; artificial photosystems Photocatalysis with Metal-Organic Framework and Metal Oxide Colloids for Fuel Production. Energies 2021, 14, 4260. https://doi.org/10.3390/ 1. Introduction en14144260 Solar fuel production has emerged as a pertinent possibility on route to address grow- ing energy challenges and shift fossil fuel dependency toward sustainable sources [1]. It Academic Editor: Umberto Desideri aptly merges solar energy harvesting with subsequent energy conversion to form value- adding products [2]. At this cross-section, molecular coordination complexes can play a key Received: 8 June 2021 role as discrete nature-mimicking photosystems combining light sensitizing and chemical Accepted: 9 July 2021 Published: 14 July 2021 reactivity [3]. Previous advances include molecular catalyst and dye engineering to im- prove catalytic parameters such as activity and selectivity on the one hand, as well as light Publisher’s Note: MDPI stays neutral harvesting and antenna effects on the other [4,5]. Additionally, immobilizing such species with regard to jurisdictional claims in to host materials can yield beneficial sustained performance, with dye-sensitized photo- published maps and institutional affil- catalysis (DSP) emerging as a selected approach with distinct advantages [6,7]. Specifically, iations. this methodology bridges the fields of molecular (photo)catalysis and material chemistry by coupling a catalyst and a dye via co-anchoring onto a semiconductor particle—effectively employing the latter as a solid-state electron mediator and a scaffold [6]. While this work- ing principle has been studied for several host particle compounds in colloidal DSP [8,9], applying metal-organic frameworks (MOFs) as the matrix component can provide similar Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. benefits and is an area with burgeoning interest [10–13]. MOFs combine metal-based nodes This article is an open access article with organic multitopic linkers to form porous coordination polymers, thus unlocking distributed under the terms and adjustable chemical properties, topologies, porosities, and molecular complex hosting conditions of the Creative Commons capabilities [14]. Particularly regarding dye-sensitization possibilities, MOFs can substan- Attribution (CC BY) license (https:// tially influence guest photophysics through approaches such as confinement effects as creativecommons.org/licenses/by/ well as scaffold-directed photochromicity [15]. However, their organic/inorganic building 4.0/). principle can result in limited stability or conductivity [16]. This presents the question how Energies 2021, 14, 4260. https://doi.org/10.3390/en14144260 https://www.mdpi.com/journal/energies Energies 2021, 14, x FOR PEER REVIEW 2 of 13 sensitization possibilities, MOFs can substantially influence guest photophysics through Energies 2021, 14, 4260 approaches such as confinement effects as well as scaffold-directed photochromicity2 of[15]. 13 However, their organic/inorganic building principle can result in limited stability or conductivity [16]. This presents the question how dye-sensitized MOFs (DSMs) directly dye-sensitizedcompare to their MOFs commonly (DSMs) directlyused metal compare oxide-based to their commonly particle counterparts used metal oxide-basedin colloidal particlesolar fuel counterparts production. in colloidal solar fuel production. WeWe deviseddevised aa conceptualconceptual side-by-sideside-by-side modelmodel comparisoncomparison ofof COCO22 reducingreducing DSPDSP andand DSMDSM systemssystems basedbased onon ZrOZrO22 nanoparticlesnanoparticles alongsidealongside thethe UiO-66(Zr)UiO-66(Zr) MOFMOF composedcomposed ofof ZrZr66O4(OH)(OH)44 nodes,nodes, simulating simulating the the oxide oxide support support (Figure (Figure 11a,b)a,b) [[17].17]. FigureFigure 1.1. SchematicSchematic overviewoverview ofof thethe studiedstudied hostshosts andand molecularmolecular complexes.complexes. ((aa)) RepresentationRepresentation ofof thethe porousporous MOFMOF hostshosts UiO-66-NHUiO-66-NH22(Zr) ((66-NH22) and UiO-66(Zr)UiO-66(Zr) ((6666),), asas wellwell asas monoclinicmonoclinic ZrOZrO22 nanoparticles.nanoparticles. ChemicalChemical structuresstructures ofof thethe molecular photocatalyst fac-ReBr(CO)3(4,40 ′-dcbpy) (1) and the [Ru(bpy)2(5,50′-dcbpy)]Cl2 photosensitizer (2). (b) Anchoring molecular photocatalyst fac-ReBr(CO)3(4,4 -dcbpy) (1) and the [Ru(bpy)2(5,5 -dcbpy)]Cl2 photosensitizer (2). (b) Anchoring interactions between the molecular species 1 and 2 to 66, 66-NH2 and ZrO2, respectively. interactions between the molecular species 1 and 2 to 66, 66-NH2 and ZrO2, respectively. UiO-66UiO-66 isis well-studiedwell-studied duedue toto its versatility and chemical stability [[18].18]. Additionally, aa fewfew reportsreports showedshowed UiO-66(Zr)’sUiO-66(Zr)’s applicabilityapplicability inin DSMDSM usingusing PtPt nanoparticlesnanoparticles forfor HH22 evolutionevolution [19[19,20].,20]. Functional Functional group group implementation implementation through through linker linker variation variation offers further offers advantagesfurther advantages [18]. For example,[18]. For UiO-66-NH example,2(Zr), UiO-66-NH constructed2(Zr), from 2-aminoterephthalicconstructed from acid2-aminoterephthalic linkers, allows improvedacid linkers, gas allows and molecular improved guest gas and adsorption molecular capabilities, guest adsorption as well ascapabilities, shifts the as optical well absorptionas shifts the through optical ab aminesorption group through incorporation amine group [10,21 incorporation]. Due to its small[10,21]. pore Due apertures to its small and diameters,pore apertures molecular and diameters, species are typicallymolecular loaded species onto are the typically MOFs’ surfaceloaded onto [10,22 the]. MOFs’ ZrO2 has surface also [10,22]. been previously ZrO2 has also applied been as previous a supportly applied for DSP as primarilya support enablingfor DSP primarily so-called “onenabling particle” so-called electronic “on particle” communication electronic between communication PS and catalyst between due PS to theand high catalyst energy due level to the of high the conduction energy level band of the [3, 23conduction]. band [3,23]. 0 WeWe selectedselected aa molecularmolecular COCO22 reductionreduction catalystcatalyst (CRC)(CRC)fac fac-ReBr(CO)-ReBr(CO)33(4,4(4,4′-dcbpy)-dcbpy) 0 0 (dcbpy(dcbpy == dicarboxy-2,2dicarboxy-2,2′-bipyridine)-bipyridine) ((11)) andand thethe photosensitizer photosensitizer [Ru(bpy) [Ru(bpy)2(5,52(5,5-dcbpy)]Cl′-dcbpy)]Cl22 (bpy = 2,20-bipyridine) (2) as a benchmark photosystem (Figure1a). These have been studied together under homogeneous conditions in the presence of a sacrificial electron donor (SED) [10,24,25], as well as in DSP upon anchoring to semiconductor supports such Energies 2021, 14, 4260 3 of 13 as TiO2, SiO2, and SrTiO3, as well as to MOFs, often delivering moderate photoactivity with turnover numbers (TONs) in the range of 10–600 [10,11,26–29]. Ligands with carboxylic acid groups were chosen to enable outer surface anchoring to MOF nodes, free amines for UiO-66-NH2(Zr), as well as to ZrO2 surface hydroxyl groups (Figure1b) [ 11,12,30]. This approach has previously yielded stable host-guest colloidal assemblies, allowing for photoinduced electron transfers from the SED to the sensitizing units and subsequently to vicinal catalysts for CO2 reduction [10–13,30]. Herein, we report the synthesis, thorough characterization, and photocatalytic CO2 reduction performance of DSM UiO-66-NH2(Zr) and UiO-66(Zr), in direct comparison to DSP with their metal oxide counterpart ZrO2. This revealed key working principles and considerations for
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