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Photochemical and electrochemical hydrogenevolution reactivity of lanthanide-functionalizedpolyoxotungstates†

Marzieh Arab Fashapoyeh, ‡ab Masoud Mirzaei, *a Hossein Eshtiagh-Hosseini,a

Ashwene Rajagopal,‡b Manuel Lechner,b Rongji Liubc and Carsten Streb *b

The first example of hydrogen evolution reactivity (HER) of

lanthanide-functionalized Keggin-based polyoxotungstates under

photochemical and electrochemical conditions is reported. The

HER activity under homogeneous, visible light-driven conditions

and under heterogeneous, electrochemical conditions depends on

the type of lanthanide functionalization, so that a new class of model

HER catalysts for systematic reactivity studies is now accessible.

The production of ‘‘solar hydrogen’’, either by direct visiblelight-driven water splitting or by photovoltaic-coupled waterelectrolysis is a major research field for sustainable energyschemes. To-date, molecular catalysts capable of performingthe hydrogen evolution reaction (HER) are attracting significantinterest from the research community.1–4 While initially, noblemetals were prime catalysts due to their high reactivity, to-date,the field is focused on earth-abundant catalysts, owing to theirtechnological importance for large-scale deployment. Recently,molecular metal oxides – so-called polyoxometalates,5 havereceived significant interest as potential HER catalysts. POMsare based on high-valent, early transition metals (e.g. V, Mo, W)and their catalytic reactivity can be tuned by incorporation of awide range of metal cations. POMs have found widespread usein complex proton-coupled multi-electron transfer reactionsincluding water oxidation,6–10 carbon dioxide activation11,12

and more recently, HER catalysis:13–15 most studies on homo-geneous, light-driven POM-HER catalysis have been focused ontransition metal-containing lacunary Keggin polyoxotungstatesfunctionalized e.g. with Ti,16 Ni,17 Mn18 or Co.19 Under

homogeneous light-driven conditions, typical turnover numbers(TONs) in the range of 30–100 were reported for these systems,see ESI† for a summary table.

In addition, the deposition of HER-active POMs on electrodeshas recently received significant interest as a facile means fortechnological deployment.20–22 Only a few reports are currentlyavailable on polyoxometalates in heterogeneous, electrocatalyticHER. In one exemplary study, Zn-functionalized Keggin-typepolyoxomolybdates were integrated into POM-based frameworks(POM-OFs) and promising electrocatalytic HER was observed.23

In an extension of this work, the authors demonstrated that theHER reactivity of the frameworks can be controlled by modifica-tion of the counter-cations present, opening new paths forelectrocatalytic reactivity control.24

Here, for the first time, we explore the HER performanceof lanthanide-functionalized polyoxotungstates under homo-geneous, light-driven conditions and when immobilized onconductive substrates for heterogeneous HER electrocatalysis.In the continuation of previous works reported by some ofthe authors, we use the lanthanide-functionalized species[LnSiW11O39]5� (Ln = La, Pr, Dy) as model compounds.25–28

The compounds were synthesized under hydrothermal condi-tions (130 1C, 5 days) by reaction of the lacunary tungstateK8[a-SiW11O39] (= {W11}) with the respective Ln(NO3)3 hydrateand 3,4-pyridinedicarboxylate (pdc, to stabilize the lanthanideions on the POM and prevent lanthanide-precipitation), givingthe crystalline compounds K6Ln[(pdc)2Ln(H2O)2SiW11O39]�xH2O(= K6Ln{LnW11}�xH2O, Ln = La, Pr, Dy), see Fig. 1. The com-pounds were fully characterized by single-crystal X-ray diffraction,elemental analyses, FT-IR spectroscopy and thermogravimetry,see ESI.† Briefly, the {LnW11} anions feature a Ln3+ cationcoordinated to a [a-SiW11O39]8� cluster via four Ln–O coordina-tive bonds; the Ln3+ is further coordinated by two pdc and twooxo-based ligands, giving a distorted square prismatic coordina-tion environment see Fig. 1 and ESI.†

The visible light driven HER by {LnW11} was explored underhomogeneous conditions in degassed H2O : MeOH (9 : 1, v : v)using the respective catalyst {LnW11} (0.5 mM), the photosensitizer

a Department of Chemistry, Faculty of Science, Ferdowsi University of Mashhad,

Mashhad, Iran. E-mail: mirzaeesh@um.ac.ir, mirzaei487@yahoo.comb Institute of Inorganic Chemistry I, Ulm University, Albert-Einstein-Allee 11, 89081

Ulm, Germany. E-mail: carsten.streb@uni-ulm.de; Web: www.strebgroup.netc Key Laboratory of Green Process and Engineering, Institute of Process Engineering,

Chinese Academy of Sciences, 100190, Beijing, China

† Electronic supplementary information (ESI) available: Analytical and catalyticdetails are provided. CCDC 1857777–1857779. For ESI and crystallographic datain CIF or other electronic format see DOI: 10.1039/c8cc06334f‡ These authors contributed equally.

Received 3rd August 2018,Accepted 23rd August 2018

DOI: 10.1039/c8cc06334f

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[Ru(bpy)3]2+ (20 mM, bpy = 2,20-bipyridine) and the sacrificialelectron/proton donor ascorbic acid (0.1 M).29 The solution wasirradiated at room temperature with a monochromatic LEDlight source (lmax = 470 nm, power density B40 mW cm�2), H2

evolution was measured using headspace gas chromatography(each data point was recorded in triplicate). An establishedprotocol based on microfiltration and UV-vis spectroscopy wasused to exclude colloid formation in the catalytic solution.30

No colloids were detected in the experiments reported (seeESI†). Reference experiments were performed where catalyst,photosensitizer or electron/proton donor were absent or thesolution was not irradiated; none of these experiments showedhydrogen evolution.

In the presence of all components given above and underirradiation, hydrogen evolution was observed for the lanthanide-functionalized clusters {LnW11} as well as for the non-functionalized [a-SiW11O39]8�. All compounds examined showedsimilar general HER reactivity. Turnover numbers (TONs) indicatea high activity during the first two to three hours of irradiation,after which catalysis reaches a plateau (Fig. 2). Initial UV-visspectrometric observations suggest that a well-known decay ofthe photosensitizer could be one cause for the observed reactivityloss (see ESI†).31 When comparing the catalysts studied, we noticethat functionalization with La3+ and Dy3+ leads to an increasedhydrogen evolution, while functionalization with Pr3+ results in anotable loss of reactivity: after an irradiation period of 7 h, thenon-functionalized reference {W11} gives TONs of B300, while theLa3+/Dy3+ functionalized species {LnW11} show TON increase ofB40–60%, see Table 1. Under the given reaction conditions, thelanthanide ions are redox-inactive, so their role in modulatingHER activity of {LnW11} could be the variation of the electronicstructure (and hence the proton-coupled electron transferreactivity of the catalysts). Theoretical computations are currentlyunderway to explore this possibility; alternatively, the Lewis-acidity of the lanthanides could affect proton binding and protonreduction, resulting in the reactivity changes observed. We arecurrently undertaking mechanistic studies to investigate thispotential mode of HER tuning further.

To gain further insight into the causes of the reactivitydifferences observed, and to establish whether the {LnW11} clusterscan also be used to develop heterogeneous, electrocatalytic HER

catalysts, we investigated the deposition of the clusters onreduced graphene oxide (rGO) as conductive substrate.32 Briefly,a literature-known method was used32 where graphene oxide(GO) is treated with an electrochemically reduced solution of therespective {LnW11} catalyst, resulting in reduction of GO to rGO,the re-oxidation of the catalyst and the formation of a colloidalcomposite (see ESI† for synthetic and analytical details). Thecomposites were dried and deposited on a glassy carbon rotatingdisk electrode (RDE) for electrochemical analyses.

RDE linear sweep voltammetry was performed in 0.5 M H2SO4

(pH = 0.3) to assess the performance of the composites. Asshown in Fig. 3, electrocatalytic HER activity is observed forall composites studied. The lowest onset potentials and over-potentials are observed for {LaW11} and {DyW11}, while for{PrW11} and {W11}, significantly lower HER reactivity para-meters are observed, see Fig. 3 and Table 1.

Comparison between homogeneous photochemical and hetero-geneous electrochemical HER reactivity shows that in both cases,the lanthanum and dysprosium-functionalized species show higherreactivity (i.e. higher TON/TOF and lower overpotential) than thepraseodymium species and the non-functionalized tungstateprecursor. Notably, the non-functionalized {W11} does howevershow both photochemical and electrochemical HER activity

Fig. 1 Lanthanide-functionalized lacunary polyoxotungstates [LnSiW11O39]5�

(Ln = La, Pr, Dy) used as HER catalysts.Fig. 2 HER activity of {LnW11} catalysts under homogeneous light-drivenconditions. Conditions: [[Ru(bpy)3]Cl2] = 20 mM, [catalyst] = 0.5 mM,[ascorbic acid] = 0.1 M, H2O : MeOH (9 : 1, v : v), irradiation at 470 nm. H2

detection via headspace gas chromatography.

Table 1 Photochemical and electrochemical HER activity of the{LnW11} compounds

Photochemical TONa TOFinitialc/h�1

{LaW11} 525 324{DyW11} 448 258{PrW11} 162 65{W11} 325 166

Electrochemical Overpotentialb/V vs. RHE Tafel slope/mV dec�1

{LaW11}@rGO 0.14 71{DyW11}@rGO 0.17 86{PrW11}@rGO 0.41 185{W11}@rGO 0.54 87

a After tirradiation = 7 h. b At J = �3 mA cm�2. c After tirradiation = 1 h.

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which has – to the best of our knowledge – not been reported inthe literature and requires further study to explore whetherpure tungsten oxide clusters could be relevant for HERcatalysis. Notably, we observe that {PrW11}@rGO shows higherelectrocatalytic HER activity than {W11}@rGO (based on over-potential and Tafel slope, Table 1) while the opposite trend isfound under homogeneous light-driven conditions. Reasons forthis divergent reactivity could be the strong interaction between{PrW11} and rGO (see X-ray photoelectron spectroscopy (XPS)analysis in ESI,† Fig. S7 and Table S5) or the divergent solutioninteractions between the catalysts and the photosensitizer underhomogeneous conditions. Detailed photophysical, electrochemicaland theoretical studies are currently underway to rationalizethis observation. Notably, while this is the first observation oflanthanide-controlled HER activity in POM chemistry, similarreactivity modulation has been reported for lanthanide-dopedsolid-state33,34 and molecular titanium oxides.35 In sum, thesystem presented could be a unique model system to system-atically rationalize the effect of lanthanide substitution on HERreactivity of polyoxometalates, thus enabling correlationsbetween photochemical and electrochemical HER catalysis, afield which is currently not well established.

In conclusion, we present the first example of lanthanide-functionalized polyoxometalate HER catalysts suitable forhomogeneous, light-driven hydrogen evolution and for depositionon conductive substrates for heterogeneous electrocatalytic HER.Tuning of the photochemical and electrochemical reactivity arepossible by variation of the lanthanide ion employed (here: La3+,Pr3+, Dy3+). For both photochemical and electrochemical HER, wenote that under the given conditions, the lanthanide-functionalizedspecies {LaW11} shows the most promising HER activity. While thelow earth-abundance of lanthanides precludes these materials fromimmediate technological relevance, the materials class neverthelessis well suited to explore reactivity tuning of POM-HER catalysts byintroduction of redox-inactive f-block elements. Future work willuse combined experimental and theoretical methods to rationalizethe reactivity differences observed for this catalyst class and explorewhether the knowledge gained can be transferred to technologicallymore relevant heterometal types, e.g. 3d elements.

This research was supported by the Ferdowsi University ofMashhad (grant no. 37981/3-2015/06/14). Ulm University and theHelmholtz Gemeinschaft (HGF) are gratefully acknowledgedfor financial support. R. L. acknowledges the Alexander-von-Humboldt-Foundation for a postdoctoral fellowship. M. M.acknowledges the Iran Science Elites Federation for financialsupport. C. S. acknowledges the Deutsche ForschungsgemeinschaftDFG is for generous financial support (TRR234’’CataLight’’).

Conflicts of interest

There are no conflicts to declare.

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