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Titania - Supported transition metals sulfides as photocatalysts for hydrogen production from
propan-2-ol and methanol
C. Maheu, Eric Puzenat, Christophe Geantet, Luis Cardenas, Pavel Afanasiev
To cite this version:
C. Maheu, Eric Puzenat, Christophe Geantet, Luis Cardenas, Pavel Afanasiev. Titania - Supported transition metals sulfides as photocatalysts for hydrogen production from propan-2-ol and methanol.
International Journal of Hydrogen Energy, Elsevier, 2019, �10.1016/j.ijhydene.2019.05.080�. �hal- 02935057�
1 Titania - supported transition metals sulfides as photocatalysts for hydrogen production 1
from propan-2-ol and methanol 2
Clement Maheu*, Eric Puzenat, Christophe Geantet, Luis Cardenas, Pavel Afanasiev*
3
Institut de Recherches sur la Catalyse et l’Environnement de Lyon IRCELYON, UMR 5256, 4
CNRS – Université Lyon 1, 2 av A. Einstein 69626 Villeurbanne Cedex (France); Fax: 33 04 5
7244 5399; Tel: 33 04 72 44 5466;
6 7 8
E-mail: [email protected] 9
11 12
Abstract 13
A series of transition metals sulfides deposited on anatase titania (MSx/TiO2) were prepared 14
by precipitation of transition metals salts with thioacetamide in aqueous medium under reflux.
15
The solids were characterized by XRD, XPS, temperature programmed reduction and 16
transmission electron microscopy. The properties of as obtained catalysts were compared for 17
the photocatalytic hydrogen evolution reaction (PHER) in pure methanol and water- 18
isopropanol mixture. The sequences of PHER activity were compared with electrochemical 19
HER and thiophene hydrodesulfurization (HDS) activity of the corresponding sulfides 20
prepared by the same technique. For PHER, in both alcohols the most active photocatalysts 21
contain hydrogenating sulfides of Co and Ru. However the PHER activity does not follow the 22
same trend as electrocatalytic HER and thiophene HDS. Some sulfides, such as HgS or CuS, 23
show poor activity in HDS and electrocatalytic HER, but have the PHER activity comparable 24
with that of the best samples. This difference suggests that the PHER rate is not merely 25
related to the hydrogen activating properties of the co-catalyst, but is enhanced by the transfer 26
of photogenerated electrons from TiO2 towards the sulfide. The ranking of the co-catalysts 27
and the PHER activity depend also on the nature of the alcohol molecule, the overall PHER 28
rates in water-isopropanol mixture being lower than in methanol.
29 30
Keywords:
31
Hydrogen evolution reaction; transition metal sulfides; photocatalysts; titanium oxide 32
33
© 2019 published by Elsevier. This manuscript is made available under the Elsevier user license https://www.elsevier.com/open-access/userlicense/1.0/
Version of Record: https://www.sciencedirect.com/science/article/pii/S0360319919319366 Manuscript_e2d3289b89e0d9fb215783198a4bf081
2 1. Introduction
1
Direct conversion of solar energy to chemical energy of hydrogen is considered to be a very 2
promising strategy to mitigate the global warming and to address the problem of alternative 3
energy sources [1,2]. Titanium dioxide TiO2 is by far the most studied semiconductor for the 4
artificial photosynthesis design [3,4]. Versatile aspects of surface chemistry and bulk 5
properties are important for photocatalytic hydrogen evolution reaction (PHER) using titania, 6
such as surface hydroxylation, type of exposed crystallographic facets [5], or carrier dynamics 7
[6,7]. Intense current research efforts are directed to the design of efficient co-catalysts, 8
present on the surface of TiO2 in order to enhance the utilization of photogenerated electrons 9
and holes in redox processes involved in the targeted reactions. PHER catalysts are widely 10
studied that include proton reduction by photogenerated electrons and oxidation of organic 11
molecules by photogenerated holes. Association of titania with electron accepting and/or 12
hydrogenating materials, such as noble metals, proves favorable for PHER, one of the best 13
benchmark reference PHER catalyst being Pt/TiO2. The relationships between the PHER 14
performance and the properties of co-catalysts are widely studied but not fully understood.
15
The improvement of PHER performance in composite catalysts vs. bare titania was attributed 16
to increased charge separation [8], plasmonic effects and increased rate of hydrogen atoms 17
recombination [9]. The impact of a co-catalyst is complex as it might simultaneously 18
influence several thermodynamic or kinetic parameters in the system, including the lifetimes 19
of charge carriers or adsorbed intermediates, which are usually unknown. Beside the efforts of 20
fundamental understanding, great number of research works report on the empirical studies of 21
novel catalysts combining titania and nanoparticles of various co-catalysts, aiming to enhance 22
PHER performance. Much attention has been recently paid to the studies of layered sulfides 23
on titania, particularly to the MoS2/TiO2 composites [10], which has been recently reviewed 24
[11]. MoS2 is considered as a cheap alternative to platinum, both for electrochemical HER 25
and PHER. However the specific PHER rates even for the best MoS2/TiO2 systems are still 26
lover than for Pt/TiO2. 27
Other sulfides were also investigated as co-catalysts with TiO2, though less extensively 28
studied than MoS2. Tungsten sulfide WS2 demonstrated PHER performance comparable to 29
MoS2 [12,13]. Copper sulfide on Aeroxide® P25 titania (0.5-15% wt. CuS) has been prepared 30
hydrothermally using thiourea and showed good activity for methanol reforming, the best 31
catalyst containing 1wt%. CuS [14]. Core-shell composite of NiS and anatase TiO2 was 32
3 applied for PHER using methanol as an organic substrate and demonstrate promising activity 1
[15]. Cobalt sulfide quantum dots on TiO2 nanoparticles prepared with a precipitation- 2
deposition method were tested using ethanol as a sacrificial reagent and showed PHER rate 3
exceeding that of the pure TiO2 by more than 35 times [16]. Cadmium sulfide CdS had been 4
widely studied for PHER, but rather as a semiconductor than as a co-catalyst [17]. Moreover 5
CdS as a semiconductor could be promoted by other TM sulfide and non-sulfide co-catalysts 6
[18,19]. However CdS value for photocatalytic applications is limited because of its 7
solubilisation by photocorrosion, releasing toxic cadmium ions. Composite materials 8
containing bismuth [20], zinc [21] and indium [22] sulfides in combination with TiO2 showed 9
promising HER activities, as well as bimetallic and trimetallic sulfide materials as CuS/ZnS 10
[23] or ZnS–In2S3–CuS [24]. Multi-metal titania–based composite materials have been 11
applied as photocatalysts, containing up to four metals chalcogenides such as, for instance, 12
ZnS/CdS-Mn/MoS2/CdTe/TiO2 [25]. Comparison of PHER efficiency measured in different 13
laboratories is always difficult because the conditions of the experiments (preparation 14
methods, reactor geometry, light source, oxidized molecule, support...) are strongly varied.
15
Despite a great number of publications on the sulfide-containing photocatalysts, systematic 16
comparison of PHER performance for a series of sulfides on titania prepared by a uniform 17
technique have never been reported. The present work aims to study an extended series of 18
sulfides prepared by means of thioacetamide precipitation technique, in an attempt to provide 19
a qualitative insight into the key properties defining their relative PHER performance.
20 21
2. Experimental 22
2.1 Preparation of catalysts 23
The series of MSx/TiO2 supported catalysts (M= Ag, Co, Cu, Hg, Mo, Ni, Ru) were prepared 24
with the aqueous deposition method under the conditions similar to those reported by Girel et 25
al. [26] and with the same molar amount of co-catalyst metals (1.3 at % of metal). Typically, 26
2.0 g of commercial TiO2 (CristalACTiVTM PC500) and 900 mg of thioacetamide were 27
suspended in 100 mL of deionized water. For each synthesis, transition metal precursor salt 28
was added, in an amount containing 350 µmol equivalent of metal. Then, aqueous suspension 29
was refluxed under stirring for 1 h. In the synthesis using ammonium heptamolybdate, 2.0 ml 30
of 2.5 M HCl was added to accelerate precipitation. The solid materials were isolated by 31
centrifugation, washed and dried under N2. 32
4 Bulk transition metal sulfides were prepared in the same conditions as titania-deposited 1
catalysts, but with a tenfold increase of the precursor amount and without addition of TiO2. 2
They were used as reference samples for characterization and phase identification of sulfide 3
phases as well as for comparison of the electrochemical HER activity. A reference catalyst 4
containing 1 wt.% Pt on P25 TiO2 was prepared by impregnation, drying and reduction with 5
H2, as described earlier [27].
6 7
2.2. Characterizations 8
Temperature-programmed reduction (TPR) was carried out in a quartz reactor. The samples of 9
sulfides (0.05–0.1 g) were heated under H2 flow (50 ml min−1) from room temperature to 10
1050 °C at a rate 5° min−1. The H2S produced in the reduction reaction was detected by a 11
Thermo Prolab quadrupole mass-spectrometer. The amount of H2S released from the solid 12
was quantified after calibration of the quadrupole detector with a known H2S content gas 13
mixture. Transmission electron microscopy (TEM) was carried out on a JEOL 2010 device 14
with an accelerating voltage 200 keV. The samples were dispersed in ethanol by ultrasound, 15
and then put onto a lacey carbon on a copper grid. In order to protect them from oxidation by 16
air, the samples still covered with liquid hexane were immediately introduced into the TEM 17
vacuum chamber. The X-ray diffraction (XRD) patterns were obtained on a Bruker D8 18
Advance diffractometer with Cu-Kα emission and identified using standard JCPDS files. The 19
metal content in the solids was determined by plasma-coupled atomic emission spectroscopy 20
(ICP-OES Activa Jobin Yvon) after dissolution in a HNO3/H2SO4 mixture. Elemental analysis 21
of light elements (CHNS) was performed on an analyzer Thermo Fisher Flash 2000. X-ray 22
photoelectron spectroscopy (XPS) measurements were performed using a commercial Axis 23
Ultra DLD spectrometer (Kratos Analytical). A monochromatic Al-Kα X-ray source at 24
1486.6 eV was used and a flood electron gun to minimize surface charging. Energy 25
calibration was made fixing both the binding energy of the C 1s intense peak assigned to C-C 26
or C-H bonds at 284.5 eV and the binding energy of the Ti 2p3/2 state at 458.5 eV. Data 27
analysis was performed using CasaXPS, after a Shirley background subtraction. The C 1s and 28
S 2p signals were decomposed into a combination of Voigt functions. For Ru 3d an 29
asymmetric lineshape was used: product of a Doniach Sunjic with a Gaussian/Lorentzian 30
function. UV-Vis diffuse reflectance was measured at room temperature with an optical fiber 31
set-up. Six optical fibers illuminate the powder sample with a Deuterium-Halogen light source 32
(AvaLight-DHS, Avantes). A seventh fiber collects the diffused light which is then analyzed 33
5 by a CCD dectector. Pure BaSO4 was used as a reference and to dilute TiO2-based powders.
1
Kubleka-Munk (KM) function was applied to interpret spectra. Note that KM theory is valid 2
only for thick samples (here 2 mm) with weak absorption ( > 0.6). To do so we diluted the 3
samples in BaSO4 (0.1 wt.% MSx/TiO2 in BaSO4) [28].
4 5
2.3 Photocatalytic tests 6
Photocatalytic reaction was conducted in liquid phase, in a double-wall semi-batch slurry 7
reactor. Most of the catalytic tests involved 50 mg of catalyst dispersed either in pure 8
methanol or in 50 mL of 50 vol. % mixture of propan-2-ol (IPA) and distilled water. Before 9
irradiation, the reactor was purged under an argon flow for 30 min to conduct the test under 10
anaerobic conditions. Gases were analyzed by micro gas chromatography with thermal 11
conductivity detectors (Agilent Technologies, 300 A). It quantified several gases including 12
oxygen, hydrogen, hydrogen sulphide, propene and propane. Photoreactor temperature was 13
regulated at 20°C by thermostated water recirculation in the double-wall.
14
A 125 W high-pressure mercury lamp (Philips HPK 125) was used to illuminate the reactor 15
through a 20 cm2 area optical window. The reactor Pyrex glassware cut light below 290 nm 16
wavelength. The number of incident photons, likely to be absorbed by anatase, was measured 17
between 290 and 390 nm with a radiometer device, a spectrometer (AvaSpec-2018, Avantes) 18
coupled with one fiber optic probe (AvaSpec, FCR-7UV-400-2-ME) equipped with a cosinus 19
corrector. The intensity value was 1370 µE/h for the screening test in water-IPA mixture and 20
1550 µE/h for the methanol PHER test. Irradiance spectrum is provided in the Supplementary 21
Information (SI), Fig. S1. The photonic efficiency (PE) has been calculated as hydrogen 22
production rate divided by this photonic intensity, following IUPAC recommendations [29].
23
Because of the uncertainty on the amount of hydrogen produced and on the incident amount 24
of photons determination, the error on the PE was estimated to be close to 10%. As the PE 25
depends on the slurry concentration, reference curve of normalized PHER rate as a function of 26
catalyst mass (slurry concentration) was measured (Fig. S2). This curve was applied to 27
compare PHER rates measure in this work with literature data.
28 29
2.4. Electrochemical measurements 30
To prepare the electrodes, 5 mg of a solid were dispersed by ultrasound in a solvent 31
containing 800 µl of ethanol and 400 µl of 0.5% Nafion solution, optionally containing 1 mg 32
6 of acetylene black carbon. Then, 10 µl of suspension was dropped onto rotating glassy carbon 1
electrode (0.0706 cm2) and then dried in argon. The catalyst loading was 0.65 mg/cm2. The 2
electrochemical measurements were performed on a Voltalab three-electrode potentiostat 3
using a Pt counter electrode and SCE reference electrode at 25 °C in purged solutions of 4
0.1 M H2SO4, by bubbling argon. Cyclic voltammograms were obtained in the potential range 5
from 0.5 to −0.1 V vs. SCE at scan rates ranging from 20 to 200 mV s−1. Linear Sweep 6
Voltammetry (LSV) scans were carried out at 2 mV/s rate.
7 8
2.5. Thiophene hydrodesulfurization (HDS).
9
Catalytic activity in thiophene HDS (Scheme 1) was measured at atmospheric pressure in a 10
fixed-bed flow microreactor. Prior to catalytic tests, all samples were re-activated under the 11
standard conditions by means of treatment under a flow of 10% H2S/H2 at 350°C, for 2h.
12
13
Scheme 1. Thiophene HDS pathways 14
Reaction was carried out in the temperature range 280–340 °C, under 50 ml/min gas flow, 15
using 100–200 mg of catalyst and partial pressure of thiophene 2.7 kPa. The plug-flow reactor 16
model was used to calculate the specific reaction rate, Vs, according to the equation 17
Vs = − (F/m)ln(1 − x) 18
where F is thiophene molar flow (mol/s), m is the catalyst mass (g), and x is thiophene 19
conversion. Catalytic activity was estimated at steady state conversion, after at least 16 h on- 20
stream.
21
22
3. Results and discussion 23
S
S
HYD
DDS
7 3.1. Synthesis and characterizations of solid materials (TEM, XRD, TPR)
1
The primary goal in this work was to study the PHER activity for a series of titania-supported 2
sulfides obtained with the same preparation technique, as a function of the nature of transition 3
metal (TM) sulfide. First, the composition and the morphology of sulfides deposited with this 4
technique were investigated. To this aim, the samples of supported catalysts and/or the 5
corresponding reference bulk sulfides have been characterized using elemental analysis, 6
TEM, XRD, XPS and TPR.
7
The set of available sulfides is limited by the possibilities of the preparation technique.
8
Stability of TM sulfides vs. the corresponding oxides increases from the left to the right of the 9
periodic table. Sulfides of oxophilic early TM (Ti, Nb or Zr) could not be precipitated using 10
aqueous thioacetamide reaction. We studied the reactions of 3d metals from Mn to Zn.
11
Additionally we tried several 4d and 5d TMs (Mo, W, Ru, Pt, Ag, Hg).
12
Thioacetamide has been widely used as sulfur source for synthetic and analytic purposes. In 13
various conditions, reaction of metals precursors with thioacetamide has been applied to 14
prepare sulfides of Mo [30], Co and Ni [31], Cd [32], Sn [33] and other metals. Here we use 15
uniform aqueous reflux synthesis conditions. Already by visual observation we could identify 16
the cases where TM sulfides were probably not formed. An inspection of the color change 17
after the reaction suggested that for Mn, Fe, W and Pt the corresponding (yellow or dark 18
brown) sulfides were not formed, but the reactants probably remained in the oxide form.
19
Platinum-containing product remained yellow as is the parent hexachloroplatinate. Zinc 20
nitrate if put into the reaction with thioacetamide does not give any precipitate under the 21
conditions applied (though ZnS could be prepared using thioacetamide in different conditions, 22
[34]). Therefore, W, Pt, Mn, Fe and Zn- containing solids were discarded. From twelve metals 23
precursors initially tried, seven sulfides were obtained and retained for further 24
characterizations and catalytic tests. The TM sulfides for which the thioacetamide technique 25
failed could be prepared using different methods, such as hydrothermal or microwave 26
assisted, but in this work we follow a uniform preparation protocol.
27
Chemical analysis of the solids after reaction shows quantitative deposition of the transition 28
metals species onto titania (Table S1). For four metals, the chemical form of supported 29
species might be inferred from the XRD of the reaction products of individual precursors.
30
Indeed, Ag2S, CuS, NiS2 and HgS crystalline sulfides are formed upon reflux (Fig. 1-4). Other 31
three sulfides (Mo, Ru, Co) form sulfur-rich amorphous precipitates with no crystalline 32
8 phases detectable by XRD. Secondary reaction of slow oxidation of thioacetamide upon 1
reflux leads to the formation of crystalline sulfur impurities in CoSx, MoSx and RuSx (Fig. 5).
2
As discussed in [26], molybdenum precipitates as amorphous MoSx phase with composition 3
close to MoS3. Ruthenium and cobalt give dark XRD-amorphous precipitates but due to 4
considerable sulfur impurity the exact composition of amorphous metal sulfides is unclear.
5
6
Figure 1. XRD pattern of bulk AgSx (black) and the matching pattern of β-Ag2S (PDF 01- 7
080-5476) (blue).
8
9
Figure 2: XRD pattern of bulk CuSx (black) and the matching pattern of CuS (PDF 00-006- 10
0464) (blue).
11
10 20 30 40 50 60 70 80
(-104)
(-210) (-104)
(210) (133)
(102) (013) (-122) (-113)
(111)
(-102) & (110)(-111)
angle (2θ)
(211) & (220)
(-114) & (122)
(-202)(031)
(022)
(120)(-121)
(100) (-112)(012)
AgSx
β−Ag2S: PDF 01-080-5476
10 20 30 40 50 60 70 80
(006)
CuSx
CuS: PDF 00-006-0464
angle (2θ)
(108) (116) (213)
(105) (208)
(103) (1011)
(100) (110)
(002) (101) (102)
9 1
Figure3: XRD pattern of bulk NiSx (black) and the matching pattern of NiS2 (PDF 04-003- 2
1992) (blue); elemental orthorhombic sulfur (04-012-7311) is also observed.
3 4
5
Figure 4. XRD pattern of bulk HgSx (black) and the matching pattern of HgS (PDF 04-002- 6
6787) (blue).
7
8
10 20 30 40 50 60 70 80
(313)
(026)(222)
(113)
NiSx
NiS2: PDF 04-003-1992 S: PDF 04-012-7311
(311)
(220)
angle (2θ)
(331)
(210) (211)
(111) (200) (222) (023) (321)
10 20 30 40 50 60 70 80
(111) (113) (202) & (015)(021)(104)(110) (114) & (203)
angle (2θ) HgSx
HgS: PDF 04-002-6787
(116)(122) & (205)
(006)
(103)
(012)
(101)(100) (211)(024)
(003)
0 10 20 30 40 50 60 70 80
CoSx RuSx MoSx
angle (2θ)
10 Figure 5. XRD patterns of CoSx, RuSx and MoSx amorphous phases. Narrow peaks 1
correspond to the common impurity of elemental rhombohedral sulfur (00-024-1206) and 2
elemental monoclinic sulfure (04-007-3019) only observed for CoSx. 3
4
Fig. 6 depicts TEM images of the supported sulfide catalysts under study. TEM reveals that 5
sulfides dispersion varies depending on its nature. For Ag and Ni, 100 nm - range particles of 6
Ag2S and NiS2 were observed, so the dispersion is rather poor. Hg and Mo give relatively 7
well-dispersed 10 nm - range particles. MoSx amorphous phase is fragile and rapidly 8
transforms under the electron beam into the MoS2 fringes. For the supported Co and Cu 9
sulfides, we could not identify the sulfide species by means of HREM and STEM. The TM 10
species are ultradispersed, of subnanometer size and are homogeneously smeared over the 11
titania, as attested by the EDS scans. For both cobalt and copper (Cu sample is shown in Fig.
12
S3) only titania grains were observed. Similarly, ruthenium sulfide while detected by EDS 13
forms highly dispersed sulfide species not detectable by HREM (Fig. S4). However, due to 14
the relatively high mass of Ru, STEM contrast was sufficient to detect 1-1.5 nm size clusters 15
of RuSx (Fig. 6).
16
Additional characterization has been carried out using temperature programmed reduction 17
(TPR), in which we detected hydrogen sulfide released upon linear heating of the samples up 18
to 1050°C (Fig. S5-S11). TPR study allows correlating the amount of sulfur released and the 19
temperature of TPR peaks with the chemical nature of sulfide species. In particular the TPR 20
characterization is important to give evidence of the sulfides formation in the case of 21
amorphous species, not detectable by TEM.
22
For the metals that form large sulfide particles (Ag, Ni), TPR of supported catalysts gives the 23
main reduction peaks in the same temperature range as for the corresponding bulk sulfides 24
(Fig. S5, S6). The shape of the ascending part in the high temperature TPR curve region is 25
related to the sulfide formation enthalpy, and therefore is specific for the corresponding metal 26
sulfide phase, as discussed earlier [35]. Beside the sulfide core reduction, additional peaks are 27
observed at low temperatures due to the weakly bonded sulfur species located at the surface of 28
TiO2 and/or at the surface of sulfide particles. The higher is sulfide dispersion, the higher is 29
the area of low-temperature peak. For highly dispersed supported species TPR proves the 30
presence of sulfides not detectable by TEM (Cu, Co, Fig. S7, S8). Beside the low-temperature 31
peaks, TPR attests formation of transition metal sulfides due to systematic presence of 32
11 relatively high temperature feature, related to the core reduction (Fig. S8, Fig. S9). Thus, 1
supported ruthenium sulfide TPR shows a large pic of weakly bonded sulfur at low 2
temperature but also the high temperature core reduction peak, which attests the presence of 3
RuS2 particles, having size of the order of 1 nm (Fig. S9).
4
The presence of RuSx clusters (x≈2) was also confirmed by XPS measurement, as shown in 5
Fig. S12. Ru 3d, Ru 3p and S 2p signals were analyzed for supported and bulk RuSx. The Ru 6
3p and Ti 2p signals overlap, which makes the interpretation difficult. The state at 279.9 eV 7
(Fig. S12 C) can be either metallic Ru or its sulfide [36,37]. A second state, probably an 8
oxide, is also observed in low surface concentration. The contribution located at 161.9 eV 9
(Fig. S12 D) is attributable to pyrite type sulfur (S-S)2-. It confirms that RuSx is deposited at 10
the surface of TiO2. The molar ratio / Ru was found close to 1.4 for the supported 11
sulfides. We wanted to study sulfides-based material in realistic conditions, as it is present in 12
the catalytic tests. Therefore, no particular precaution was taken to load the photoreactor 13
without air exposure. It explains why some sulfate is observed by XPS (167 eV, Fig. S12 B 14
and D) [38].
15
Finally, BET surface areas of the supported catalysts are high and remain similar to that of 16
bare titania (350 m2/g) and are all greater than 300 m2/g. For example specific surface area is 17
of 321 m2/g for MoSx/TiO2 and 328 m2/g for CoSx/TiO2. The morphology of supported 18
catalysts is dominated by TiO2 support because the TM sulfides loadings are relatively small, 19
whereas no thermal treatments were involved during preparation, which could lead to TiO2 20
sintering.
21
Overall, for seven metals under study, sulfide species deposited on the PC500 anatase TiO2 22
were prepared by thioacetamide method. However the crystallinity and the dispersion of the 23
sulfides as obtained are different.
24 25
3.2. Photocatalytic HER activity 26
The two studied reactions are:
27
- CH3OH HCHO + H2 (1)
28
- CH3CHOHCH3 CH3COCH3 + H2 (2) 29
12 The initial values of PHER activity were compared after several hours of test, in order to 1
characterize the intrinsic ability of TM sulfides to co-catalyze the reactions (1, 2). In all cases, 2
a short induction period (15-20 min) was observed. Then, for the duration of the experiment 3
(3-6 hours) the PHER rate was stable.
4
The PHER rate evolution was also checked on a longer time scale. We studied stability for 5
several days on-stream for the semi-continuous flow IPA test, or for several repetitive runs for 6
the batch methanol test (in the last case - with refreshing the methanol and carrying out purge 7
with argon between the runs). In the water-IPA test we measured PHER rate at the 8
temperatures stepwisely changed between 10 and 45 °C, in order to determine the apparent 9
activation energy (which is beyond the scope of the present work and will be discussed 10
elsewhere).
11
After the initial stabilization period, during several hours on-stream in isothermal conditions 12
the relative variations of PHER rate were less than 5%; an apparent steady state was achieved.
13
However the PHER rates slowly evolved on the several-days scale for the continuous flow 14
test (Fig. S13, S14). In most cases (Co, Cu, Hg, Mo, Ni, Ru) slow activation occurred, 15
probably due to the cleaning of TiO2 surface from the residuals of thioacetamide used in the 16
preparation. In the case of Ag2S slow deactivation occurs, which might be attributed to photo 17
corrosion. The steady state rate measured at 20 °C in the first or in the second temperature 18
cycle was taken to compare the PHER activity (Fig. S13). Batch PHER test in the methanol 19
medium provides good stability for several (4-5) repetitive 3h runs (Fig. S15). In bothcases, 20
the variations of activity with time are lesser than the differences between various TM 21
sulfides and do not influence the conclusions of this work.
22
Post-mortem XPS analysis of the solids after water-IPA tests showed that both metal species 23
and sulfur species are partially transformed to the higher oxidation states (Fig. S16). In 24
agreement with the PHER test results, the most important degree of corrosion was observed 25
for Ag2S/TiO2 sample. After 70 hours of test, surface layers were tatally transformed to 26
sulfate. Note that the stability study was not in the focus of this work, because we are 27
interested in the understanding of titania combined with fresh sulfides.
28
The PHER photon efficiency measured in pure methanol and water-IPA 50/50 vol. mixture is 29
presented in Fig. 7. The absolute and mass - specific PHER rates are given in Table S2. For 30
the water–IPA mixture, the performance changes in the sequence Ru > Ni > Co > Hg > Ag>
31
Mo > Cu. In methanol the sequence is Co > Ni > Ru = Mo = Cu > Ag > Hg. In methanol even 32
13 the least active HgS/TiO2 produced 5 times more hydrogen than bare TiO2. In IPA the PHER 1
activity of the least active CuS/TiO2 was 12 times higher than of bare titania which undergone 2
the thioacetamide reflux procedure.
3
The photocatalytic properties of HgS, Ag2S and RuSx supported on TiO2 are reported for the 4
first time. Moreover, no transition metal sulfides-based photocatalysts were studied 5
previously for IPA dehydrogenation. Therefore no literature references exist for these cases.
6
In methanol, the PHER performance of our most active catalysts (Co and Ru sulfides) is at the 7
level of the best sulfide catalysts reported in the literature (Table S2). As far as comparison is 8
possible, our absolute PHER values are lower than the literature values for copper and nickel 9
sulfides [14,39]. Our values for molybdenum sulfide are lower than for amorphous MoSx
10
reported in [10] but higher than observed earlier for the MoS2 nanosheets [40]. However if 11
approximately re-calculated for the same conditions (lamp power and slurry concentration, 12
Table S2, Fig. S2), the best values reported in the literature for Co, Cu and Mo sulfides are in 13
a remarkable agreement with our data (Table S2). Finally, our values measured in methanol 14
for cobalt and nickel co-catalysts are higher than observed in ethanol for CoSx and in lactic 15
acid for NiS, but since the organic molecules are not the same, such comparison makes lesser 16
sense [39,40,16,41].
17
Comparison with the existing literature is only approximate as the conditions applied from 18
one study to another are never the same and recommendations for reporting the activities 19
[42,43] are not always followed. Hence a more reliable activity assessment might be achieved 20
by comparison with a well-known Pt/TiO2 benchmark reference (Table S2). The overall 21
activity in methanol is much higher than in water-IPA, showing the importance of organic 22
substrate for this type of studies. In methanol, the best cobalt sulfide promoted catalyst shows 23
approximately 30 % PHER rate of the 1% Pt/TiO2 reference (Table S2). In water-IPA the 24
relative activity of TM sulfides attains, in the best case of RuSx, only 16 % of activity of the 25
Pt/TiO2 reference. The observed PHER activity changes depending on the alcohol, in 26
agreement with previous works [44]. With metal supported on TiO2 the activity is often 27
higher in methanol than in IPA [45,46].
28
The observed PHER trends show both expected and unexplained features. The best catalysts 29
in methanol and water-IPA media are, respectively, Co and Ru sulfides, which are known as 30
active hydrogen production catalysts [47,48]. On the other hand, MoSx, which is by far the 31
most studied sulfide for PHER, is not among the best co-catalysts in both media. We closely 32
14 reproduce the PHER activity in methanol of MoSx/TiO2, observed in the previous work, which 1
used similar thioacetamide preparation [26]. However MoSx/TiO2 activity in methanol is 2
almost the same as for CuS/TiO2, which is not among the front-running PHER catalysts.
3
Moreover, MoSx/TiO2 shows poor performance in water-IPA, where it appears less active 4
than HgS/TiO2. Surprisingly, HgS is a reasonably good co-catalyst for PHER, 5
To understand the observed trends of PHER, various properties of the systems under study 6
were compared such as the morphology, the electrocatalytic HER activity and their thiophene 7
Hydrodesulfurization (HDS) activity. Deposition of sulfides at the surface of TiO2 does not 8
alter the light absorption properties, important for PHER. The observed color change is due to 9
light absorption by sulfides in visible region, which does not interfere with electron-hole 10
generation in titania. Indeed, Kubelka-Munk transforms of UV-vis spectra (Fig. S17) are 11
similar between 5.5 and 3.2 eV. The band gap (Eg) was determined as the intersection 12
between the x-axis in the Tauc plots. The Eg for seven samples under study is included within 13
the region between 3.2 and 3.35 eV. The Eg of seven catalysts and pure PC500 titania is the 14
same within the accuracy of the experimental method and corresponds to the literature value 15
for PC500 anatase [49]. Therefore the amount of (useful) photons absorbed by the samples is 16
similar, so other properties are responsible for the differences of observed PHER activity.
17
The dispersion degree of the co-catalyst species seems to be less important than the nature of 18
the TM sulfide. Indeed, two most active co-catalysts in the methanol medium are CoSx and 19
NiS2. However, CoSx species are too finely dispersed to be observed by conventional TEM, 20
whereas the NiS2 particles are large (near 100 nm). On the other hand, poorly dispersed Ag2S 21
shows better activity in water-IPA than well - dispersed MoSx. We already noted in the 22
previous works that for PHER in methanol, the dispersion of sulfide co-catalyst on titania 23
does not seem to be a crucial parameter [26].
24
We are compelled to conclude that the differences of PHER rates are related to the intrinsic 25
properties of sulfides deposited onto titania. The property of a co-catalyst obviously relevant 26
to the PHER performance should be its ability to reversibly activate hydrogen. This ability is 27
revealed in the heterogeneous catalysis or in the electrochemical HER. Seven TM sulfides 28
considered here are known to possess very different ability to activate hydrogen and to 29
catalyze hydrogenation or hydrogenolysis of organic substrates. Ru and Mo sulfides are well- 30
known highly active hydrogenating catalysts [36,38,50]. Co and Ni are used as promoters in 31
sulfide heterogeneous catalysts but the activity of Co and Ni binary sulfides is low, though 32
15 non-negligible. Finally, CuS, HgS and Ag2S have never been shown to act as hydrogen 1
activators or hydrogenating heterogeneous catalysts. To check in what extent the hydrogen 2
activation ability of TM sulfides is important, we compare the trends in HDS and in the 3
electrochemical HER with the observed PHER trend. The corresponding literature data are 4
dispersed; the preparation techniques and the reaction conditions are very different from one 5
work to another. To provide a reliable series of activity, we measured the electrochemical 6
HER performances and HDS activity for the catalysts studied above in PHER, as follows.
7
8
Figure 6 TEM and STEM HAADF images of selected solids. The transition metals forming co- 9
catalyst sulfides are indicated on the images. White spots in the STEM images correspond to the 10
TM sulfides particles.
11
16 0
1 2 3 4
CuS MoSx Ag2S HgS CoSx NiS2 RuS2
Photon Efficiency (%) isopropanol - water methanol
1
Figure 7. PHER activity of titania-deposited sulfides in methanol and in water-IPA mixture.
2 3
3.3. Electrocatalytic HER and thiophene HDS activity 4
PHER via alcohols dehydrogenation certainly follows a complex mechanism. To get a better 5
insight, we used model HER and HDS reactions, in order to disentangle the intrinsic 6
hydrogen-activating properties of TM sulfides from their influence on the titania 7
semiconductor. HER and thiophene HDS reactions include respectively the H-H bond 8
formation and breaking on the sulfide surface. These properties are correlated, because of the 9
microreversibility principle. So HDS and HER both estimate the ability of a catalyst to 10
activate H2 and as such they mimic the last PHER step, where molecular H2 is formed.
11
However, these reactions occur under different conditions and may involve different species.
12
Thus, HER proceeds with a participation of hydrated protons, whereas HDS obviously cannot 13
involve such species.
14
TM sulfides and in particular MoS2 recently emerged as HER electro catalysts with high 15
activity and good stability in acidic medium [51]. The HER performance of sulfides strongly 16
depends on the nature of TM metal and on the preparation technique. To understand how the 17
nature of TM sulfides obtained by thioacetamide technique impacts its electrochemical HER 18
performance, we compared the initial activities of seven sulfides as prepared. Such initial 19
HER activities are provided by steady–state characteristics which are achieved after several 20
LSV cycles. Since pure titania is inactive, the electrocatalytic properties of diluted sulfides 21
species supported on inert PC500 are difficult to measure. By this reason we proceeded with 22
17 the HER measurements for bulk TM sulfides obtained from the thioacetamide reaction. Here 1
we assume that similar TM sulfide species are formed with and without TiO2 and they have 2
similar electrochemical properties.
3
The Tafel plots and LSV curves are presented in Fig 8a, b. The numeric values of Tafel slopes 4
and η10 are summarized in Table 1. Low Tafel slope and low values of η10 (overpotential 5
necessary to attain the 10 mA/cm2 current) are characteristics of high HER performance. The 6
overall ranking of sulfides HER performance as a function of metal is Mo > Ru ≈ Ag > Cu >
7
Co > Ni > Hg. Worth emphasizing that we do not claim this ranking to be a general trend, 8
transposable to any sulfides of these metals, but merely a particular sequence for the materials 9
prepared using thioacetamide method.
10
The specific rate of thiophene HDS reaction (Scheme 1) could be considered as another 11
descriptor of hydrogen-activation and hydrogenating activity. With the exception of Ag, we 12
see that highly hydrogenating sulfides (Ru, Mo) demonstrate the highest HER activity and 13
overall correlation between the electrocatalytic HER and HDS performances is good (Table 14
1). The activities per mole of sulfided metal of Cu, Co and Ni sulfides are usually found to be 15
low, whereas the activities of Ru and Mo sulfides are high. This is true for both hydrogenation 16
and HDS reactions [52] and for binary [53] or ternary sulfides including these elements [54].
17
Silver sulfide Ag2S was recently studied as HER electrocatalyst and shown moderate but non- 18
negligible performance [55]. However, its HDS activity appeared negligible, probably 19
because relatively harsh HDS conditions modify the initial sulfide. Finally, mercury sulfide 20
HgS was never studied in HDS/HER and here we attest its zero HDS activity as well as poor 21
electrochemical performance.
22
Confronting the results of photocatalytic HER, electrocatalytic HER and thiophene HDS, we 23
see that if the two latter activities are well correlated, there is no clear relationship between 24
any of them and observed PHER. Therefore, photocatalytic performance depends more 25
strongly on other parameters influenced by the co-catalysts (charges separation, lifetimes etc.) 26
than by their hydrogen-activating ability.
27
18
-600 -500 -400 -300 -200 -100
-0,8 -0,3 0,2
Overmpotential vs. RHE (mV)
Log J (mA/cm2) HgS 209
NiS2240 CuS 134 CoSx 170
MoSx87 RuSx124
Ag2S 127
(a)
-20 -16 -12 -8 -4 0
-600 -400 -200 0
J (mA/cm2)
Overmpotential vs. RHE (mV) HgS
CoSx
Ag2S RuSx NiS2
CuS
MoSx
(b)
1
Figure 8. Electrochemical HER activity of TM sulfides: Tafel plots (a) and LSV curves (b).
2
Table 1 Overall performance in the PHER, thiophene HDS and electrochemical HER.
3
property\sulfide Ag2S CoSx CuS HgS MoSx NiS2 RuS2
Thiophene HDSa 0.1 2.1 0.3 0.0 16 0.5 91
Electro HER Tafel b 127 170 134 209 87 240 124
Electro HER η10c 460 670f 580 910f 433 770f 445 Photo HER, methanold 1.50 4.25 2.50 0.50 2.45 4.00 2.70 Photo HER, IPA-H2Oe 0.30 0.40 0.01 0.35 0.10 0.50 1.90
a reaction rate,10-8 mol.g-1.s-1 measured at 300 °C (per gram of metal sulfide); b mV/decade; c 4
-mV; d,e photon efficiency, %; f extrapolation.
5
3.4. Mechanisctic considerations 6
The observed HDS and HER trends have many common points, which could be expected. In 7
the most general case, the overall (HER, HDS) performance of a sulfide material is a product 8
of two terms. The first term is the intrinsic chemical performance and the second is 9
morphology –related term describing the availability of a sulfide surface to the reactants.
10
Beside these factors, the electric conductivity of sulfide plays an important role for the 11
electrochemical HER, though in some extent it is leveled off by applying conductor binders 12
(usually Nafion and carbon). While morphology of sulfides can be extremely versatile 13
depending on the preparation ways, the intrinsic chemical trends are relatively well 14
understood on the basis of generalized Sabatier principle [56–58].The energies of S - vacancy 15
creation and those of hydrogen dissociative adsorption and desorption govern both the HDS 16
and HER trends. Thus, M-S bond energy and hydrogen adsorption energy are recognized to 17
be useful descriptors for HDS activity. So-called volcano plots allow predicting which 18
sulfides could be potentially active HDS (HER) catalysts [59,60].
19
19 With this respect Mo and Ru sulfides have the M-S bond energies are close to the optimal 1
value [60], whereas Ag2S and HgS have it too weak (∆Hf0 values are −31.8 and −58.2.
2
KJ/mol , respectively [61]. The observed HDS trend is therefore in agreement with the 3
existing volcano plots.
4
The HDS and HER properties are probably related to the electronic band structures of the 5
corresponding sulfides via the M-S bond energy. However there are neither clear rules for 6
such relationship, nor any features of the band structure could be potential descriptors for the 7
HDS and/or HER activity. The HDS and HER rates for the sulfides having similar electronic 8
bands vary by many orders or magnitude. Thus, semiconducting sulfides with Eg between 1 9
and 2 eV can be both highly active (MoS2, RuS2) and very poor (HgS) HDS catalysts.
10
On the other hand the PHER rate sequence vs. the nature of sulfide and its dispersion has 11
much lesser variations. All tested sulfides induce a significant increase of the PHER rate on 12
titania, even if the ranking is not the same for two alcohols. This observation suggests that the 13
action of different sulfides might be explained on a common basis. For PHER, not the 14
hydrogen-activating power but the electronic structure of transition metal sulfide is the most 15
important property.
16
Consideration of relative positions of electronic bands shows that similar alignment between 17
TiO2 and all sulfides should occur. The electron affinity of all sulfides under study is higher 18
than for titania. The Fermi levels are slightly higher than Fermi level of TiO2. Position of VB 19
slightly varies from one sulfide to another, but in all cases the top of VB is due to the states 20
dominated by S 3p electrons. After the band alignment the bottom of CB for semiconductor 21
sulfides (Ef for metallic sulfides) will be always below the bottom of CB for TiO2 (Scheme 2).
22
23
Scheme 2. Relative bands positions of titania and seven sulfides studied in this work; values 24
adopted from [62].
25
20 Therefore the photo generated electrons from the bottom CB of titania (or shallow trapped 1
electrons) wouldtransferred to the sulfides CB. That means, photo generated electrons will be 2
attracted towards the interface between sulfide and TiO2. If a sulfide possess strong hydrogen 3
- activating power then the overall PHER rate might be additionally increased due to the 4
catalytic action of sulfide surface (similarly to the well-known Pt/TiO2 reference case).
5
However, even if the sulfide possesses no hydrogen-activating properties at all, the PHER rate 6
is still higher than for bare titania, due to an enhance charge carrier separation. In that case, 7
TiO2 is the active site for both oxidation and reduction half reactions.
8
The second important finding to be explained is that the dispersion of TM sulfide particles 9
does not play a crucial role. We suppose that due to polarization of TiO2 agglomerate by 10
adjacent sulfide particle, the photo generated electrons which have high mobility and 11
relatively long lifetime, might be collected towards the interface between TiO2 and sulfide via 12
an antenna-like mechanism [63], as depicted in Scheme 3.
13
14
Scheme 3 Harvesting of photo generated electrons from distant regions of a TiO2 agglomerate by 15
a TM sulfide particle.
16
Finally there are two distinct groups of co-catalysts with respect to their relative activity in 17
water-IPA and in methanol media. In the first group, water-IPA and methanol tests give the 18
PHER rates ratio close to 1. This group includes Ru, Hg sulfides and Pt metal co-catalysts 19
(Table S2). For the second group including five of seven tested sulfides (Co, Cu, Mo, Ag and 20
Ni), the PHER rate in methanol is at least an order of magnitude greater than in water-IPA.
21
Both groups include relatively poor and highly active catalysts in PHER as well as in HER 22
and HDS. Both groups include relatively poorly dispersed and highly dispersed sulfides. No 23
21 property descriptor correlated with this activity difference could be identified for the time 1
being. However such a striking difference of behavior suggests that some fundamental 2
mechanistic difference exists between two types of co-catalysts, though it remains yet 3
unexplained.
4
4. Conclusions 5
In this work we prepared a series of transition metals sulfides on PC500 titania using 6
thioacetamide solution reaction. The properties of obtained catalysts were compared in PHER 7
in methanol and water-isopropanol media, as well as electrocatalytic HER in 0.5 M H2SO4 8
and heterogeneous gas-phase thiophene HDS reaction. The following conclusions can be 9
drawn from our work.
10
(i) Several transition metals sulfides of the first and the second raw show high PHER activity 11
in the methanol and water-isopropanol media, superior to that of frequently studied MoS2. 12
(ii) Commonly known hydrogen-activating sulfides (Ru, Co, Ni) show the best photocatalytic 13
HER performance. However, other metals sulfides such as Hg or Cu have a poor activity for 14
HDS but show a PE many times higher than bare titania. Similarly, the trends established for 15
the seven sulfides differ in photocatalytic and electrocatalytic HER. Therefore proton 16
reduction or hydrogen activation by the co-catalyst, albeit favorable, is not a critical parameter 17
in the PHER.
18
(iii) Dispersion of the sulfide species has no determining effect on the PHER performance.
19
Porly dispersed NiS2 and highly dispersed CoSx species show comparable high PHER 20
activity. In agreement with previous observations, sulfide particles, even moderately 21
dispersed, can efficiently harvest electrons from extended areas of adjacent titania.
22
(iv) The observed PHER rates strongly depend on both the nature of sulfide and that of the 23
organic substrate. However, this observation does not allow concluding on which step in the 24
multistep PHER scheme is rate limiting, because the organic molecule might interfere with 25
both electron and hole evolution. In order to understand this issue, integrated models should 26
be developed with rate equation including all the essential steps of the process. A better 27
insight can be obtained further measuring apparent activation energy of the process.
28
Supplementary data 29
22 Supplementary material is available including experimental conditions, samples
1
compositions, photocatalytic results, EDS analyses,TPR patterns, XPS spectra and UV-Vis 2
absorption spectra.
3
23 References
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