Supporting Information: Mechanistic Insight into the Interaction Between a Titanium Dioxide Photocatalyst and Pd Co-catalyst for Improved Photocatalytic Performance Ren Su †,‡, Nikolaos Dimitratos╞,╫, Jinjia Liu ‡,╪, Emma Carter ╡, Sultan Althahban ║, Xueqin Wang †, Yanbin Shen †,‡, Stefan Wendt †, Xiaodong Wen ‡,╪, J.W. (Hans) Niemantsverdriet ‡,╬, Bo B. Iversen †,╧, Christopher J. Kiely ║, Graham J. Hutchings╞,╫* and Flemming Besenbacher†* †
Interdisciplinary Nanoscience Centre (iNANO), Aarhus University, DK-8000 Aarhus C, Denmark.
‡
SynCat@Beijing, SynfuelsChina Co. Ltd., Leyuan South Street II, No.1, Yanqi Economic Development
Zone C#, Huairou District, Beijing, 101407, China. ╞
Cardiff Catalysis Institute, School of Chemistry, Cardiff University, Cardiff, CF10 3AT, UK.
╫
The UK Catalysis Hub, Research Complex at Harwell, Rutherford Appleton Laboratory, Oxfordshire,
OX11 0FA, UK. ╡
School of Chemistry, Cardiff University, Cardiff, CF10 3AT, UK.
║
Department of Materials Science and Engineering, Lehigh University, 5 East Packer Avenue, 18015-
3195, Bethlehem, Pennsylvania, USA. ╧
Department of Chemistry, Aarhus University, Langelandsgade 140, DK-8000 Aarhus C, Denmark.
╪
State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, CAS, Taiyuan, China
╬SynCat@DIFFER, Syngaschem BV, Eindhoven, The Netherlands Corresponding Author *
[email protected] (Graham Hutchings),
[email protected] (Flemming Besenbacher)
Supercritical synthesis of anatase TiO2
3.
Anatase TiO2 samples were prepared by supercritical synthesis using a continuous flow reactor.1, 2 By rapid mixing of the cold reactant solution with the super-heated solvent stream in a continuous process, a high degree of super-saturation and a rapid nucleation can be achieved, facilitating the formation of primary crystallites in large quantities. The supercritical solvent was isopropanol mixed with deionized (DI) water and the reactant was titanium isopropoxide (TTIP) (ACROS, 98 %). Pure TiO2 nanoparticles with a precise control of crystallinity and crystallite size were synthesised by varying the concentration of TTIP, the composition of the solvent, flow rate, temperature, and pressure, as shown in Table S1. The asprepared TiO2 samples in suspension were centrifuged, washed in DI water, and subsequently dried overnight at 120 oC. The fixed crystallinity series (size varied) and fixed crystallite size series (degree of crystallinity varied) are labelled as S1 – S6 and C1 – C5, respectively. Table S1. Parameters for synthesis of anatase nanoparticles with various crystallite size and crystallinity ID C1 C2 C3 C4 C5 S1 S2 S3 S4 S5 S6
[TTIP] /M 1.0 0.5 1.0 1.0 1.0 0.5 0.5 0.5 0.5 0.5 0.5
H2O a /% 11.0 10.0 12.0 14.0 18.0 100.0 100.0 10.0 5.4 3.5 2.6
Flow / g.min-1 10.0 5.0 10.0 10.0 10.0 2.5 5.0 5.0 5.0 5.0 2.5
Temp. / oC 255 225 265 275 350 400 350 350 350 350 300
Pressure / MPa 25 30 25 25 25 30 30 30 30 30 30
d b/ nm 9.3 7.8 9.2 9.2 9.3 6.6 8.9 11.3 15.9 20.4 26.6
Cry. b /% 12.6 53.8 60.3 75.5 82.0 74.2 82.8 82.3 82.6 89.2 86.3
a: The volume concentration of water in the isopropanol-water solvent. b: Derived from XRD patterns.
Crystallinity and crystallite size analysis X-ray diffraction (XRD) was employed to examine the crystallite size and the degree of crystallinity of the nanoparticles using an X-ray diffractometer (SmartLab Rigaku) with Cu-Kα1 radiation. The crystallinity of the sample was measured using the following procedure: 1. Mix a sample with the 100% crystalline standard (CaF2) thoroughly with a mass ratio of 1:1. 2. Collect the XRD pattern of the TiO 2/CaF2 mixture.
The integrated areas of diffraction peaks in a sample are given relative to corrundum Al 2O3 (113) peak in the database. 4. A ratio between the areas of distinct TiO 2 and CaF2 peaks can be obtained from their relative intensities. 5. The absolute crystallinity of the TiO 2 sample is then determined from the area of the peak. According to the database, A anatase(101) / AAl2O3(113) = 5.04, ACaF2(111) / AAl2O3(113) = 3.84, thus Aanatase(101) / ACaF2(111) = 1.31. The crystallinity therefore can be calculated using equation S1:
𝐴𝑏𝑠𝑜𝑙𝑢𝑡𝑒 𝑐𝑟𝑦𝑠𝑡𝑎𝑙𝑙𝑖𝑛𝑖𝑡𝑦 = 1/1.31 × 𝐴𝑎𝑛𝑎𝑡𝑎𝑠𝑒(101) / 𝐴𝐶𝑎𝐹2 (111) × 𝑚𝐶𝑎𝐹2 /𝑚𝑎𝑛𝑎𝑡𝑎𝑠𝑒 × 100 % Eq. (S1) The measured XRD patterns (Fig. S1a, S1b, S1c, and S1d) were refined by Rietveld method using the Fullprof program. 3 Figures S2(e) and S2(f) show examples of the measured data and the refined data for crystallite size and crystallinity determination, respectively. The full-width-at-half-maximum and the peak area can be therefore derived from the refinement.
Deposition of Pd NPs on TiO2 A standard sol immobilisation method was utilised to deposit 1 wt% of Pd NPs on TiO2 supports.4 Firstly, a 1 wt% poly vinyl alcohol solution (PVA, MW=10,000, 80% hydrolysed, Aldrich) was freshly prepared prior to the synthesis of Pd colloid. The required amount of PVA solution was added (w(PVA)/w(Pd) = 1.2) to the PdCl2 solution, followed by addition of a freshly prepared NaBH 4 solution (0.1 M, n(NaBH 4)/n(Pd) = 5). After 30 min of sol generation, the colloid was immobilised by adding TiO2 (acidified to pH 1-2 by H2SO4) under vigorous stirring conditions. After 2 h, the slurry was filtered and washed thoroughly with DI water and dried at 120 C overnight. Due to the scarcity of Pd/TiO 2 samples, we did not perform the crystallinity test that required mixing the samples with CaF2. However, XRD analysis of all Pd/TiO 2 samples revealed that the mean crystallite size remained unchanged after Pd immobilisation. As the Pd deposition was performed under mild conditions compared to that of the TiO 2 synthesis conditions, changes in crystallinity should be negligible.
Microstructure and particle size distributions Figures S2-S5 characterise the microstructure and particle size distributions of TiO 2 and Pd NPs of samples C1, S2, S4, and S6 by STEM.
Fig. S1. XRD patterns of TiO2 with different (a) crystallite size and (b) crystallinity; (c) and (d) XRD patterns of selected samples for crystallinity analysis. The marked peaks belong to CaF2. (e) Refinement result of sample S1. Y obs, Ycal, and Yobs – Ycal are measured pattern, calculated result, and the difference between measurement and calculation, respectively. (f) Refinement result of sample S4.
Surface Pd loadings and oxidation states of Pd Table S2 shows the surface Pd loadings and Pd 0:Pd2+ ratios of all samples determined from XPS survey spectra and Pd 3d high resolution spectra, respectively. The surface Pd loading of individual samples varied from 0.6 to 0.9 at%, which probably originated from analytic errors, synthesis errors, and slightly inhomogeneous distribution of the Pd NPs. The Pd 0:Pd2+ ratios increased following the increase of the crystallinity of the TiO 2, whereas increasing crystallite size showed a negligible effect on the Pd0:Pd2+ ratios (sample S1 is exceptional). Table S2. Surface Pd loadings and Pd0:Pd2+ ratios of all Pd/TiO2 samples as determined by XPS. ID Pd loading / at% Pd0:Pd2+
C1 0.9 2.4
C2 0.6 3.0
C3 0.6 3.3
C4 0.8 5.3
C5 0.9 6.7
ID Pd loading / at% Pd0:Pd2+
S1 0.6 13.7
S2 0.6 6.0
S3 0.8 8.9
S4 0.6 7.0
S5 0.7 8.8
S6 0.9 8.9
Fig. S2. (a, b) BF- and HAADF-STEM images from Pd on TiO 2 (sample C1). (c, d) HAADF image and particle size distribution of the TiO2 particles; (e, f) HAADF image and particle size distribution of the Pd NPs.
Fig. S3. (a, b) BF- and HAADF-STEM images of Pd on TiO2 (sample S2). (c, d) HAADF image and particle size distribution of the TiO2 particles; (e, f) HAADF image and particle size distribution of the Pd NPs.
Fig. S4. (a, b) BF- and HAADF-STEM images of Pd on TiO2 (sample S4). (c, d) HAADF image and particle size distribution of the TiO2 particles; (e, f) HAADF image and particle size distribution of the Pd NPs.
Fe(II)-1,10-phenanthroline complex, which presents a characteristic absorption maximum at 510 nm (Fig. S6B). The absorbance at 510 nm shows a linear correlation to the irradiation time (Fig. S6C), thus the photon flux (q) can be calculated using equation (S3):
q = ∆A/t·(V1·V3/V2)·(NA/ФFe2+·ε510nm·l)
(S3)
where, ∆A/t is the slope of the absorbance change per time (shown in Fig. S3C); V1 is the total volume illuminated (130 mL); V2 is the aliquot volume (2 mL); V3 is the remaining volume of the liquid; NA is Avogadro’s number; ФFe2+ is the quantum yield of ferrioxalate at 365 nm (1.25 ± 0.02);5 ε510nm is the extinction coefficient of the Fe(II)-1,10phenanthroline at 510 nm (11835.1 ± 28.9 mol -1L-1cm-1); l is the path length of light in the cuvette (1 cm). Figure S6(D) depicts the calculated photon flux as a function of the irradiation time. The average photon flux is ~ 4 x 1017 photons /sec.
Fig. S5. (a, b) BF- and HAADF-STEM images of Pd on TiO 2 (sample S6). (c, d) HAADF image and particle size distribution of the TiO2 particles; (e, f) HAADF image and particle size distribution of the Pd NPs.
Photocatalytic testing A full characterisation of the light source (e.g., emission spectrum and photon flux) was performed to estimate the quantum efficiency (QE) of the photocatalytic H 2 production, as shown in Fig. S6. The emission spectrum (Fig. S6A) of the UV LED (365 nm, Optimax 365) light source indicates that the light source has an emission peak at 365 nm with a full-widthat-half-maximum (FWHM) of 10 nm. The photon flux of the light source was measured using a standard ferrioxalate actinometry method,5 as described in Eq. S2.
2[Fe(C2O4)3]3- → 2Fe2+ + 5C2O42- + 2CO2
(S2)
The light intensity measurement was executed under dark conditions. 2 mL of the 130 mL actinometer solution (6 mM) subjected to various irradiation times, 2 mL of 0.1 wt% ophenanthroline solution, 1 mL of buffer solution (0.6 M CH3COOH and 0.18 M H 2SO4), and 15 mL DI water were added into a 20 mL volumetric flask and mixed immediately. The mixture was developed in the dark for 1h to form the
Fig S6. (A) Light source emission spectra; (B) Ferrioxalate solution absorption spectrum under irradiation; (C) The absorbance of the ferrioxalate solution at 510 nm as a function of irradiation time; (D) Photon flux of the light source calculated from ferrioxalate actinometry analysis.
The evolved H2 can be quantified by the following equations:6
n(H2)gas = p(H2)rea x Vrea / RT
(S4)
p(H2)rea = RSF(H2) x p(H2)det x p(Air)rea / p(Air)det
(S5)
where, p(H2)rea is the partial pressure of H 2 in the reaction chamber; p(H2)det is the partial pressure of H 2 detected by the MS; p(Air)rea is the pressure of air in the reactor (100 kPa); p(Air)det is the pressure of air detected by the MS; Vrea is the gas-phase volume of the reactor (190 mL); RSF(H2) is the relative sensitivity factor of H 2 (0.284).
Full absorption minimum path length estimation: The absorption coefficient (α) of bulk TiO 2 at 365 nm is 4.89 x104 cm-1, therefore α is 23.2 cm-1 for the 2 g·L-1 suspension. Therefore, a full absorption (> 99 %) of the light requires a depth of 0.20 cm of the suspension. Since the TiO 2 suspension scatters the light dramatically, the attenuation coefficients of the suspensions should be larger than the ideal absorption coefficient. By assuming that the number of scattered photons is equal to that of the absorbed photons (attenuation coefficients = 2α), the minimum full absorption length will be 0.1 cm for the 2 g·L-1 suspension, which is much smaller than the depth of suspension (2.6 cm). Therefore the loss of photons should be negligible.
UV-vis spectrometer (UV-1800, Shimadzu, JP). The measured UV-vis spectra of selected samples are shown in Fig. S7. The extinction coefficients of phenol, benzoquinone, catechol, and hydroquinone were measured independently to quantify the concentration of phenolic intermediate products, as shown in Table S3.7 Table S3. Extinction coefficients of phenol, hydroquinone, catechol, and benzoquinone at specific wavelengths. Products WL Phenol Benzoquinone Catechol Hydroquinone
246 nm 89 16649 255 231
Coefficient / M-1cm-1 270 nm 275 nm 1571 1282 689 618 1964 2283 874 1305
289 nm 9 708 258 2055
The concentration of phenol (P), hydroquinone (H), catechol (C), and benzoquinone (B) were calculated by solving the spectrum via the Beer – Lambert Law using the following equations.7
𝐴246 ≈ 𝜀𝐵246 × 𝐶𝐵
Eq. (S6)
𝐴289 ≈ 𝜀𝐵289 × 𝐶𝐵 + 𝜀𝐻289 × 𝐶𝐻 + 𝜀𝐶289 × 𝐶𝐶 Eq. (S7) 270 270 270 270 𝐴270 = 𝜀𝐵 × 𝐶𝐵 + 𝜀𝐻 × 𝐶𝐻 + 𝜀P × 𝐶P + 𝜀𝐶 × 𝐶𝐶 Eq. (S8) 𝐴275 = 𝜀𝐵275 × 𝐶𝐵 + 𝜀𝐻275 × 𝐶𝐻 + 𝜀P275 × 𝐶P + 𝜀𝐶275 × 𝐶𝐶 Eq. (S9) where, AWL is the apparent absorbance at these specific wavelengths (WL), which corresponds to the sum of the absorbance from each of the compounds; WLn is the extinction coefficient of n at specific wavelength; Cn is the concentration of n. The concentrations of B, C, H, and P can be therefore calculated from the following equations:
𝐶𝐵 = 𝐴246 ⁄16649 Fig. S7. UV-vis spectra of the evolved intermediates during the photodegradation phenol process collected at given time intervals using Pd on (a) C1, (b) C2, (c) C4, (d) C5, (e) S1, (f) S2, (g) S5, and (h) S6, respectively.
Photodecomposition of phenol was carried out under ambient conditions using the aforementioned catalysts. All samples were UV irradiated for two hours to remove the surface adsorbed organic molecules prior to the measurements. The loading of the catalyst was 1 g/L, and the concentration of phenol was 400 µM. The suspension was kept in the dark for 1h to establish adsorption equilibrium. The light source was the same LED diode (Optima 365). An aliquot of the suspension was collected at given time intervals (0, 1, 2, 5, 10, 20, 30, 40, 60, 80, 100, 120, 150, 180, 210, 240, 270, 300, 330, and 360 min), and was immediately centrifuged and analysed using an
Eq. (S10)
𝐶𝐶 = (𝐴275 − 0.816𝐴270 − 0.288𝐴289 + 0.008𝐴246 )/ 606.02 Eq. (S11) 𝐶𝐻 = (𝐴289 − 0.043𝐴246 − 258𝐶𝐶 )⁄2055
Eq. (S12)
𝐶𝑃 = (𝐴270 − 0.023𝐴246 − 0.425𝐴289 − 1854.35𝐶𝐶 )⁄1571 Eq. (S13)
Fig. S8. (a)-(d): Evolution of phenol and phenolic intermediates during photocatalytic phenol decomposition using the 9 nm pristine anatase (left column) and 1 wt% Pd NPs modified anatase (right column) with different crystallinity. (e)-(h): Evolution of phenol and phenolic intermediates during photocatalytic phenol decomposition using the 82% pristine anatase (left column) and 1 wt% Pd NPs modified anatase (right column) with different mean crystallite size.
Fig. S10. Comparison of the ESR spectra for pure P25, S3, S4, and S6 samples that were irradiated under (a) high vacuum and (b) low vacuum for 30 min at 77 K. Comparison of the ESR spectra for Pd deposited on P25, S3, S4, and S6 that were irradiated under (c) high vacuum and (d) low vacuum.
All spectra were integrated in the g value range of 1.85 to 2.10, therefore the integrated intensity in Fig. 6(e) and (f) corresponding to all active paramagnetic species.
Additional ESR spectra and integration method
Additional calculation details
The ESR spectra of each sample were measured twice and confirmed to be essentially identical. Figure S9 shows the ESR spectra of S4 both before and after Pd deposition.
Density functional theory (DFT) was used to calculate the formation energy of the photo-excited electrons that been trapped at surface and subsurface sites by creating a variety of possible surface and subsurface oxygen vacancies (Vo). Subsequently, the impact of surface and subsurface V o defects on the surface density of states (DOS) and the distribution of the lowest unoccupied molecular orbitals (LUMO) were also evaluated. A three O-Ti-O layer anatase model (144 atoms) was used to simulate the (101) surface (surface area 2 8 10.44×15.57 Å ). The bottom layer was fixed and the other two layers were fully relaxed. A vacuum layer of 15 Å was used to exclude the influence of vertical periodic images. The starting anatase unit cell had a lattice parameter of a=b= 3.78 Å and c= 9.49 Å, and became a=b= 3.89 Å and c= 9.69 Å after full relaxation. Vienna Ab-Initio Simulation Package (VASP) with the frozen-core projected-augmented wave (PAW) method was used with application of the generalised gradient 9-12 approximation (GGA) of Pedew-Burke-Ernzerhof (PBE). The Ti3d states were described by GGA + U with a U value of 3.5 eV 13, 14 to correct for the on-site Coulomb interactions. The energy and the electronic properties of the anatase (101) surface were computed using a plane wave cutoff of 500 eV and a 15 Monkhorst–Pack grid of (2×2×1) k-points. The computational
Fig. S9. Repetition of the ESR spectra for (a) sample S4 and (b) sample Pd/S4 that have both been UV irradiated for 30 min at 77 K under low vacuum.
The ESR spectra of pure Degussa P25 and Pd deposited P25 (Pd/P25) after UV irradiation under different conditions are shown in Fig. S10 for comparison with samples S3, S4, and S6.
resources for the project were supplied by the Tianhe-2 in Lvliang, Shanxi province.
Fig. S11. (a) The three layer anatase (101) surface model with five possible oxygen vacancy (VO).defects variants. (b-f) The computed formation energies of the five different VO defects at 0 K under vacuum.
Fig. S11 shows the five possible V O sites and their formation energies at 0 K in vacuum. It is clear that VO1 and VO4 are thermodynamically most stable defects in the anatase (101) surface. Fig. S12 shows the influence of V O1 and VO4 on the surface DOS of the O 2p.
Fig. S12. The surface DOS of O2p with VO1 and VO4. In the three layer anatase (101) surface model at 0 K in vacuum.
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