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Photocatalytic CO2 reduction by Re(I) polypyridyl complexes immobilized on niobates nanoscrolls Leandro A Faustino, Breno L Souza, Barbara N Nunes, Anh-Thu Duong, Fabian Sieland, Detlef W. Bahnemann, and Antonio Otavio Toledo Patrocinio ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04713 • Publication Date (Web): 22 Mar 2018 Downloaded from http://pubs.acs.org on March 23, 2018

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ACS Sustainable Chemistry & Engineering

Photocatalytic CO2 reduction by Re(I) polypyridyl complexes immobilized on niobates nanoscrolls

Leandro A. Faustino1, Breno L. Souza1, Barbara N. Nunes1, Anh-Thu Duong2, Fabian Sieland2, Detlef W. Bahnemann1,3, Antonio Otavio T. Patrocinio1,2*

1

Laboratory of Photochemistry and Materials Science, Insititute of Chemistry, Universidade Federal de Uberlandia,Uberlandia, Brazil

2

3

Institute of Technical Chemistry, Leibniz University Hannover, Hannover, Germany

Laboratory “Photoactive Nanocomposite Materials”, Saint-Petersburg State University, Saint-Petersburg, Russia

* to whom the correspondence should be addressed: [email protected]

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Abstract Immobilization of Re(I) CO2 reduction photocatalysts on metal oxide surfaces is an interesting approach to improve their stability and recyclability. In this work, we describe the photocatalytic activity of two Re(I) complexes (fac[Re(NN)(CO)3(Cl)], NN = 4 4-dicarboxylic acid-2 2-bipyridine, 1, or 5,6-dione1,10-phenantroline, 2) on the surface of hexaniobate nanoscrolls. After adsorption the turnover number for CO production (TONCO) in DMF/TEOA of 1 was increased from 9 to 58, which is 20% higher than that observed on TiO2, being among the highest reported values for a Re(I)-based photocatalyst under visible light irradiation without any sensitizer. The complex 2 is inactive in solution under visible light irradiation, but has a TONCO of 35 when immobilized on hexaniobate nanoscrolls. Transient absorption spectroscopy studies reveal that the slow back electron transfer and the higher reducing power of the hexaniobate conduction band electrons play a major role for the photocatalytic process. The results provide new insights concerning the role of the metal oxide substrate on Re(I)-based molecular systems for CO2 reduction.

Keywords: photocatalytic CO2 reduction; Re(I)-based molecular catalysts; layered materials; transient absorption spectroscopy


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Introduction Artificial photosynthetic systems able to reduce CO2 into useful chemicals, such as CO, and fuels, e.g. CH4, have been intensively investigated due to the growing concerns about the consequences of CO2 accumulation in the atmosphere. The possibility to sustainably convert CO2 into CO using solar radiation is a very attractive pathway to mitigate the concentration of this greenhouse gas in the atmosphere and also provides a green source for CO, an important chemical feedstock to form syngas

1

and to feed industrial processes

based on the Fischer-Tropsch chemistry. Different photo- or photoelectrocatalysts for CO2 reduction have been reported based on metals and semiconductors 2-4, immobilized enzymes 5, 6 and coordination compounds

7-9

with large variability in terms of selectivity and

stability. While metals and semiconductors exhibit great stability but poor selectivity, enzymes and metal complexes can selectively convert CO2 into CO, but they lack from long term stability. One simple and relatively robust catalyst for CO2 reduction into CO is based on Re(I) complexes of general formula fac[Re(CO)3(NN)(L)], NN = polypyridyl ligand, L = leaving group (typically Cl- or Br-) 10, 11

. These species are one of the few examples of mononuclear complexes

able to work simultaneously as light harvester and catalyst. As a drawback, in these conditions, they exhibit low stability and poor absorption of solar radiation. For example, the highest reported turnover number (TONCO) for fac[Re(CO)3(bpy)(Cl)] is 30 in a DMF/triethanolamine (TEOA) mixture (λ>400 nm) 12

. Since the first reports concerning the photo and electrochemical activity

of these compounds for CO2 conversion

12-14

, a great number of studies have



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appeared in the literature with improved TONCO and the turnover frequencies (TOF). Successful strategies include variations of the polypyridyl ligand combination with sensitizers 28

22-25

15-21

and optimization of the reaction conditions

,

26-

. One interesting approach involves the immobilization of the homogeneous

Re(I) catalyst in suitable substrates, enabling the heterogenised system could be easier recycled and used in different solvents or reaction conditions

29-37

.

Windle and co-workers have shown that the immobilization of the phosphonated derivative fac-[Re(CO)3(P-bpy)(Br)], P-bpy = 2,2’-bipyridine-4,4’-bisphosphonic acid, on TiO2 leads to an increase of the TONCO from 2 to 52 in DMF/TEOA (λ>420 nm) 37. This enhancement was attributed to an increase in lifetime of the reaction intermediates due to the electron injection into TiO2. Later, Abdellah and co-workers performed a detailed investigation of the system based on timeresolved infrared spectroscopy and proposed a reaction mechanism for the TiO2 immobilized Re(I) catalyst in DMF/TEOA

38

. The authors concluded that

the improved photocatalytic efficiency of the Re(I) catalyst upon immobilization onto TiO2 is due to the slower charge recombination and the high oxidative power of the ReII species after electron injection as compared to the metal-toligand charge transfer (MLCT) state of the unbound Re(I) species. The authors proposed that during the catalytic cycle, a fraction of the injected electrons could catalytically reduce the reaction intermediate [ReI(P-bpy)(CO)3(CO)]+ in order to promote the CO release. In this work, we report the substitution of TiO2 nanoparticles by hexaniobate nanoscrolls (KxH(4-x)Nb6O17) as suitable substrates for the immobilization of Re(I)-based molecular catalysts for CO2 reduction. The hexaniobates exhibit a more negative conduction band energy (E0CB ≈ -0.75 V


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vs NHE

39, 40

) in relation to TiO2 (E0CB ≈ -0.3 V vs NHE

40

), increasing the

reducing ability of the injected electrons. Moreover, its scrolled lamellar morphology can favor the electron mobility in relation to TiO2 nanoparticles by promoting a vetorial charge transfer through the individual layers, which avoid ohmic losses due to grain boundaries

41

. Two different Re(I) complexes were

evaluated as photocatalysts and the results provide new insights regarding the role of the semiconductor and the anchor group on the photocatalytic efficiency of immobilized Re(I) complexes.

Experimental Section [ClRe(CO)5], 1,10-phenanthroline, 4,4’-dicarboxylic acid-2,2’-bipyridine (dcbH2), tetrabutylammonium hydroxide (TBAOH), DMF, TEOA and K2CO3 from Aldrich, Nb2O5 from “Companhia Brasileira de Metalurgia e Mineração” (CBMM) and TiO2 (Hombikat UV 100) from Sachtleben GMBH were used without any further purification. All solvents employed were HPLC grade. Syntheses. fac-[ClRe(CO)3(dcbH2)] (1) was modifications to literature procedures

prepared with some

42, 43

. 0.19 g (0.5 mmol) of [ClRe(CO)5]

and 0.11 g (0.6 mmol) of dcbH2 were suspended in 10 mL of ethanol and the mixture was heated to reflux for 7 h. The crude product precipitated as an orange solid, which was collected by filtration. Recrystallization was performed by suspending the solid in 90 ml of acetone. The unreacted polypyridyl ligand was removed by centrifugation (5000 rpm, 20 min) and then, hexane was employed to precipitate the product from the bright orange supernatant. Yields 70%. Anal. Calc. for ReC15H8N2O7Cl: C, 32.76%; H, 1.47%; N, 5.09%; Found:



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C, 32.40%; H, 1.98%; N, 5.27%. 1H NMR ((CD3)2CO δ / ppm) 10.04 (dd, 2H); 9.78 (s, 2H); 8.92 (dd, 2H). The ligand phdo (5,6-dione-1,10-phenantroline) was synthesized as previously reported

44, 45

. The fac-[ClRe(CO)3(phdo)] complex (2) was prepared

also following a methodology reported in the literature

46-49

. Briefly, 0.45 g (1.24

mmol) of [ClRe(CO)5] and 0.42 g (2.00 mmol) of phdo were dissolved in 20 mL of toluene and the mixture was heated to reflux for 5 h under argon atmosphere. The

product

was

collected

by

filtration

and

recrystallized

from

dichloromethane/n-hexane. Yields 60%. Anal. Calc. for ReC15H6N2O5Cl: C, 34.92%; H, 1.17%; N, 5.43%. Found: C, 34.75%; H, 1.24%; N, 5.44%. 1H NMR (DMSO-d6 δ / ppm) 9.26 (d, 2H); 8.71 (d, 2H); 7.88 (t, 2H). Bulk K4Nb6O17⋅3H2O was prepared by a solid state method

50, 51

. Nb2O5

and K2CO3 in a molar proportion of 2:3 were grinded and heated at 1,100 ºC for 10 h. The final product was washed with deionized water and then dried at 80ºC. The bulk material was then submitted to an acid treatment to partially exchange K+ cations by protons. 3.0 g of K4Nb6O17⋅3H2O were stirred in 150 mL of 0.2 mol L-1 H2SO4 during 3 days. The solid was subsequently separated by filtration, washed with water and dried at 80ºC. The hexaniobate nanoscrolls were obtained by exfoliation of bulk K4-xHxNb6O17 (0.5 g) with 100 mL of 8x10-3 mol L-1 tert-butylammonium hydroxide during seven days

52, 53

. After the stirring

is stopped the non-exfoliated materials tend to precipitate, while the exfoliated layers remain as a stable suspension. The suspension was collected and few drops of HNO3 were added to promote the precipitation of the hexaniobate


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layers. The final powder were collected by filtration, washed with water and ethanol and dried at 80 ºC overnight. The immobilization of the Re(I) complexes on the surface of the niobates was carried out by suspending 300 mg of the nanoscrolls in plain water (pH = 6.8). Under constant stirring, a given amount of 1 or 2 previously dissolved in ethanol was added to the niobate suspension, which was kept under stirring for 3 hours. Two different concentrations were employed for each complex: 90 µmol g-1 and 35 µmol g-1. After adsorption, the powder was recovered by centrifugation, washed several times with ethanol and acetone, and, finally, dried under vacuum overnight. For comparison, the same procedure was adopted to immobilize the complexes on Hombikat UV100 anatase TiO2 particles (SBET = 220 m2 g-1). The amount of complex adsorbed at each condition was determined spectroscopically by analyzing the remaining concentration in the supernatant after the solid removal. Methods. Electronic absorption spectra were recorded on a double beam Shimadzu UV-1650 spectrophotometer equipped with an integrating sphere. Barium sulfate was used as reference. 1H NMR spectra were recorded on a DRX-400 MHz Bruker Ascend 400 spectrometer and the residual solvent signals were used as internal standard. Attenuated total reflectance infrared (ATR-FTIR) spectra were recorded using a Perkin-Elmer Frontier spectrometer. The measurements were performed in a diamond crystal plate, using 32 scans at a resolution of 2 cm-1. X ray diffractograms were obtained using a Shimadzu XRD-6000 diffractometer, with a CuKα (λ = 1.54148 nm) monochromatic source. Specific surface area measurements were determined recording N2 adsorption/desorption isotherms using the BET methodology

54

and a 7

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Quantachrome NOVAtouch LX1 surface analyzer. X ray photoelectron spectroscopy was carried out in a Leybold Heraeus analyser equiped with an Al non monochromatic X ray source and a hemispherical electron energy analyzer. The beam energy was 1484 eV and the residual C1s peak at 284.6 eV was used as internal standard. Transmission electron microscopy images were obtained in a Tecnai G2 F20 TMP (FEI Company) at 200 kV acceleration voltage. Transient absorption spectroscopy (TAS) was carried out employing a LKS80 nanosecond laser flash photolysis spectrometer (Applied Photophysics) equipped with the proper diffuse reflectance accessory as described previously 55

. The samples were placed in a closed quartz cuvette, purged with N2 and

were excited with a 420 nm laser beam (2.4 mJ cm-2 per pulse). The pump beam was produced by an optical parametric oscillator (OPOTEK) pumped by a Nd:YAG laser (Quantel; Brilliant B; third harmonic, 355 nm). The transient signals were collected by a PMT (Hammatsu R928) connected to a DSO9064A oscilloscope (Agilent). The transient decay is presented as the variation in the reflectance (∆J), which is calculated according to Equation 1

56

, where I0 is

incident intensity of the analyzing light; J0 is the diffuse reflected light without laser pulse (ground light level); Jx is the diffuse reflected light with the laser pulse. It has been reported that the ∆J value can be correlated to the transient absorption provided that ∆J is less than 0.3

57, 58

, thus to describe the results

obtained by the detection of the diffuse reflected light, the term transient absorption will be used here.

∆ =

/ / /

=

(1)


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Photocatalytic studies. CO2 photoreduction experiments were carried out in a closed double-jacket glass reactor with total volume of 75 mL. The reactor was loaded with 28 mg of the photocatalysts suspended in 50 mL of 5:1 DMF/TEOA. The mixture was then purged with CO2 until saturation and exposed to a 300W Xe lamp equipped with a water filter and a 420 nm cutoff filter. The reactor was kept at 25º C degrees by using a water circulator system. At given time intervals, 500 µL of headspace atmosphere were sampled using a gas-tight syringe and analyzed using a quadrupole mass spectrometer for gas analysis (Hiden HPR-20)

59, 60

. The system was calibrated by injection of

different amounts of CO (m/z = 28; sensitivity 1.05) into the reactor filled with the same amount of solvent employed in the photocatalytic assays and saturated with CO2. The headspace was then sampled and analyzed by mass spectroscopy. During the assays, the O2 levels were also monitored in order to possibly detect air contamination inside the reactor, which can directly influence the CO quantification since N2 and CO are isobaric. The signals coming from eventual N2 contamination were subtracted from the raw signals obtained at m/z=28 using the 4:1 ratio in relation to the main O2 signal at m/z=32 (sensitivity 1.0). The initial signal at m/z = 28 related to CO2 fragmentation or eventual CO present in the CO2 gas bottle was also subtracted from the signals collected during the photoreaction.

Results and discussion The as-synthesized K4Nb6O17 exhibits typical lamellar structure as confirmed by XRD, Figure 1. The diffraction peaks can be indexed according to the JCPDS 21-1297 crystallographic sheet corresponding to K4Nb6O17.3H2O. 9 ACS Paragon Plus Environment


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The strong peaks at low angle (020, 040, 060) are characteristic for the lamellar structure, in which asymmetrical [Nb6O17]4- layers composed by edge- and corner-shared NbO6 octhaedra are stacked 61. As expected, after exfoliation the intensity of these peaks is strongly reduced as the stacked structure is lost and the layers are randomly organized. Nevertheless, the (040) diffraction peak can still be observed, which is an indication that some re-ordering along the stacking axis of K4Nb6O17 occurs when the unilamellar colloid is precipitated as nanoscrolls

62

. The strongest diffraction peaks at 2θ = 23.5 and 27.5º can be

ascribed respectively to the (220) and (002) reflections thus confirming that the in-plane crystalline order within the individual nanoscrolls persisted after the exfoliation 62-64. SEM images of the as-synthesized powder, Figure 2a, allow the visualization of the initial stacked layers. After treatment with TBA(OH) solution, the layers are separated from each other and tend to curl due to electrostatic forces, leading to the formation of nanoscrolls, as shown in Figure 2b-c. As a result of the exfoliation, the specific surface area determined by N2-sorption isotherms increases by 2 orders of magnitude (from 2 m2 g-1 to 110 m2 g-1). Elemental analysis of the exfoliated layer shows nitrogen and carbon contents below 0.5%, indicating that the hexaniobate layers precipitated preferentially as KxH(4-x)Nb6O17. Additionally, the absence of the characteristic C-H stretching peaks at 2800–3000 cm–1 in the FTIR spectra

65

(Figure S1, supporting

information) corroborates for the conclusion that no TBA+ cations are present in the powder. X ray photoelectron spectroscopy of the as prepared material (Figure S2, supporting information) also does not show any nitrogen content in the sample surface and confirms the presence of K+ as counter ion.


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Figure 1. X ray diffractograms of the hexaniobate before (a) and after exfoliation (b), along with the standard diffraction data (JCPDS 21-1297) for K4Nb6O17.3H2O (red).

Figure 2. SEM images of the K4Nb6O17 as synthesized (a) and after exfoliation (b). (c) TEM image of the hexaniobate nanoscrolls along with selected area diffraction pattern.

The nanoscrolls were then exposed to ethanolic solutions of the Re(I) complexes. Two different concentrations of the Re(I) complexes were employed in order to obtain different surface coverages. The same procedure was adopted for the sensitization of the TiO2 nanoparticles. The sensitized metal oxides were characterized by UV-Vis absorption spectroscopy, Figure 3. After adsorption of the complexes no changes were observed in the morphology of the nanoscrolls. Thus, despite the free energy related to the curling/uncurling of the niobate nanosheets are relatively small

64

, under our experimental

conditions, , the curled morphology is favored. 11 ACS Paragon Plus Environment


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Figure 3. UV-Vis spectra of bare (black) and sensitized metal oxide particles with 1 (blue) or 2 (red). (a) hexaniobate nanoscrolls (b) TiO2 nanoparticles. The dashed and solid lines indicate different surface concentrations as detailed in Table 1. Insets: molecular structure and electronic absorption spectra of 1 (blue) and 2 (red) in acetonitrile.

After adsorption of the Re(I) complexes on the surfaces of the oxides, a new absorption feature appears in the visible region that resembles the MLCT absorption bands of the respective complexes in acetonitrile solution. While for the complex 1 only a slight red-shift in the absorption maximum is observed after adsorption, for 2 the MLCT maximum shifts from 372 nm in acetonitrile to about 400 nm on the surface of the oxides. This indicates a strong interaction between the carbonyl groups in the phdo ligand and the oxide surfaces that reduces the electron density on the bipyridine rings and consequently favors the metal-to-ligand charge transfer upon excitation. The FTIR spectrum of the sensitized hexaniobate, Figure 4, confirms this strong interaction with the ketone groups in the phdo ligand. As shown in Figure 4(a) the spectrum of 2 in the 1500-2200 cm-1 region is characterized by ν(C≡O) stretching modes at 1887, 1940 and 2037 cm-1, corresponding to the A1 and the two E modes expected for the facial geometry of the complex 66-68. Moreover, a strong peak is observed at 1700 cm-1, ascribed to the ν(C=O) stretching mode of the ketone


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groups

69

. After adsorption, this peak can no longer be observed indicating that

the complex is anchored at the oxide surface through the carbonyl groups of the diketone. Additionally, the peaks associated with the E modes of the ν(C≡O) are slightly shifted to higher energies and become unresolved. A broad peak at 1628 cm-1 is present on the bare niobate and it is attributed to the symmetric deformation of adsorbed water molecules δs (H2O)

70

. The same trends were

also observed after adsorption on TiO2 (Figure S3 - supplementary information) suggesting that the adsorption on both oxides proceeds in a similar fashion. Similar behavior is also observed for 1, in which the intensities of the ν(C=O) peaks at 1737 and 1700 cm-1, attributed to the carbonyl groups of the carboxylates in the dcbH2 ligand, are strongly decreased after adsorption. However, these peaks can still be observed for the complex adsorbed on the niobate nanoscrolls, which may indicate that for some molecules only one of the two carboxylate groups are bonded to the niobate surface. Upon adsorption on the TiO2 nanospheres (Figure S3), these peaks disappeared completely, which suggests that both carboxylate groups are attached to the TiO2 surface. This result corroborates with those reported by Anfuso et al. for adsorption of 1 on TiO2 rutile (001) surface 31.



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Figure 4. FTIR spectra of 1 (blue) and 2 (red) before (a) and after (b) immobilization on the niobate nanoscrolls. The spectrum of the bare niobate (black) is shown for comparison.

The final surface coverages (mol mg-1) determined for the complexes on the two different oxides employed here are shown in Table 1. For the complex 1, the surface coverage was found to be only dependent on the initial concentration, with similar values being obtained for both, TiO2 nanospheres and niobate nanoscrolls. However, for 2, a different behavior was observed for the niobates in relation to TiO2. The surface coverage on the niobate nanoscrolls was twice as high when the initial concentration of the complex was increased, while for TiO2, a saturation limit around 3-4x10-8 mol mg-1 seems to be reached. This behavior is not related to the respective specific surface area, since the TiO2 nanoparticles employed in this study have a surface area twice as high than that of the niobate nanoscrolls (220 m2 g-1 in contrast to 110 m2 g1

). A possible reason for the better dye uptake by the niobate nanoscrolls is the

fact that the Nb5+ cations exhibit an increased charge/radius ratio as compared to Ti4+, which should result in a stronger Lewis acidity thus favoring the chelation by the dione in the phdo ligand

71

. It is worth noting here that the


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amount of Re(I) complexes being adsorbed through their anchor groups on the hexaniobates nanoscrolls was in the same order than the values reported for cationic metal complexes, such as [Ru(bpy)3]2+ 62 or its phosphonated derivative 72

.

Table 1. Surface coverage of 1 and 2 on hexaniobate nanoscrolls and TiO2 under different adsorption conditions Initial Surface concentration coverage Complex Substrate Sample code (µmol g-1) (µmol mg-1) 1-Nb-HC 90 0.091 KxH4-xNb6O17 1-Nb-LC 35 0.036 1 1-TiO2-HC 90 0.090 TiO2 1-TiO2-LC 35 0.038 90 0.120 2-Nb-HC KxH4-xNb6O17 35 0.051 2-Nb-LC 2 90 0.040 2-TiO2-HC TiO2 2-TiO2-LC 35 0.034 The sensitized oxides were employed as photocatalysts for CO2 reduction under experimental conditions similar to those reported by Windle et al

37

. A fixed amount of each powder was suspended in 5:1 DMF/TEOA

solutions and exposed to visible-light (λ>400 nm) irradiation under CO2 atmosphere. The possible gaseous products were analyzed by mass spectrometry with CO being the only detected photoproduct. Control experiments were carefully performed with the bare oxides or in the absence of TEOA and did not show any CO generation. Long term irradiation of the immobilized complexes in the absence of gaseous CO2 (under argon atmosphere) also did not yield in any CO, which therefore, exclude the possibility of CO photodissociation during the photolysis. These experiments show that the CO detected during the photocatalytic experiment comes from the reduction of the gaseous CO2 by the Re(I)-based catalysts in the presence of



TEOA, that act as sacrificial agent as previously reported

Page 16 of 41

73, 74

. The turnover

numbers (TONCO), defined as the number of molecules of CO formed per molecule of the Re(I) catalyst, were determined for each sample after 20 h irradiation and are listed in the Table 2. As two different surface coverages were tested for each photocatalyst, the powders are referred to LC and HC for low and high surface concentrations, respectively, as specified in Table 1. For clarity, the total number of mols of the respective Re(I) complex in the reactor is shown in Table 2. Figure 5 exemplifies the mass spectra of the reactor atmosphere having 1 immobilized on niobate nanoscrolls in DMF/TEOA before and after irradiation. It is possible to clearly observe the increase in the m/z = 28 peak after irradiation, which is attributed to photorreduction of CO2 into CO by the Re(I) complex adsorbed on the niobate nanoscrolls.

Figure 5. Mass spectra of the photoreactor atmosphere before (black) and after (blue) 20 h of irradiation. The reactor (50 mL) was filled with 25 mL of DMF/TEOA (4:1) solution having 28 mg of niobate nanoscrolls sensitized by 1. Argon is used as carrier gas.


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Table 2. Turnover number for photoreduction of CO2 to CO employing the bare or the immobilized Re(I) complexes. TONCO after 20 h µmols of Re(I) Photocatalyst TOF (h-1) irradiation catalyst 1 2.8 0.7 9±4 2 3.0

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