Intermediate Species Revealed during Sulfidation of Bimetallic

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Intermediate Species Revealed During Sulfidation of Bimetallic Hydrotreating Catalyst: A Multivariate Analysis of Combined Time-Resolved Spectroscopies Amélie Rochet, Bertrand Baubet, Virginie Moizan, Elodie Devers, Antoine Hugon, Christophe Pichon, Edmond Payen, and Valérie Briois J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b03735 • Publication Date (Web): 04 Aug 2017 Downloaded from http://pubs.acs.org on August 10, 2017

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The Journal of Physical Chemistry

Intermediate Species revealed during Sulfidation of Bimetallic Hydrotreating Catalyst: A Multivariate Analysis of Combined Time-Resolved Spectroscopies Amélie Rocheta,b,c, Bertrand Baubeta, Virginie Moizana, Elodie Deversa, Antoine Hugona, Christophe Pichona, Edmond Payend, Valérie Brioisb* a

IFP Energies nouvelles, Etablissement de Lyon, Rond-point de l'échangeur de Solaize, BP 3,

69360 Solaize, France b

Synchrotron SOLEIL, L’Orme des Merisiers, Saint-Aubin, BP 48, 91192 Gif-sur-Yvette

Cedex, France c

Laboratório Nacional de Luz Síncrotron, CEP 13083-970, Caixa Postal 6292, Campinas, São

Paulo, Brazil d

Univ. Lille, CNRS, Centrale Lille, ENSCL, Univ. Artois, UMR 8181 - UCCS - Unité de

Catalyse et Chimie du Solide, F-59000 Lille, France

ACS Paragon Plus Environment

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KEYWORDS: in situ; Quick-XAS; laser Raman spectroscopy; MCR-ALS; NiMo catalyst; transient oxysulfide species; hydrodesulfurization

ABSTRACT

For the first time, the sulfidation process of a bimetallic NiMo catalyst supported on alumina has been followed by combining time-resolved laser Raman spectroscopy -LRS- and X-ray absorption spectroscopy -XAS- quasi simultaneously at both Ni and Mo K edges. Multivariate data analysis reveals that the thermal activation upon 15% H2S/H2 atmosphere of a dehydratedcalcined NiMo(VI) catalyst involves i) a 5-stepped mechanism with oxysulfided or fully sulfided Mo intermediate species and ii) a direct transformation of oxidic nickel species into NiSx and NiMoS ones. Complementary information extracted from LRS and Quick-XAS data permitted to identify at the early stage of the sulfidation the trimeric Mo(V/VI) oxysulfide species [Mo3(µ2O)4(µ2S)µ2{S2}(Ot)2(St)3] grafted to the support surface which is quickly transformed into the Mo(IV) intermediate species [Mo3(µ3S)(µ2S)2µ2{S2}(Ot)2{S2}t]. Above 190 °C, the Mo(IV) second intermediate is transformed into Mo(IV)S3, itself transformed into the final Mo(IV)S2 at T > 220 °C. Thanks to the unambiguous comparison of sulfidation kinetics for both metals the incorporation of promoter into the extended sulfidic molybdenum-based phase has been unprecedentedly related to the formation of the MoS3 intermediate species.


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1. Introduction Hydrodesulfurization process (HDS) is an essential reaction in the petroleum industry 1. It consists in the catalytic removal of sulfur from diesel and gasoline. With strengthened environmental regulations in oil fractions, refiners need to improve the HDS process by a better knowledge of catalysts and activation process. The latter deals with the sulfidation of oxidic precursors to form the supported active phases consisting of Mo disulfide nanocrystallites decorated with Ni or Co atoms yielding the mixed NiMoS (CoMoS) active phase according to the Topsøe’s model 2. By means of a large variety of spectroscopic techniques such as X-ray absorption spectroscopy (XAS), X-ray photoelectron spectroscopy (XPS), Mössbauer emission spectroscopy, laser Raman spectroscopy (LRS) and more recently Fourier transform infra-red spectroscopy (FTIR), numerous studies have been conducted to characterize the local structure and chemical state of the active sites. Efforts were first devoted to identify the structure of final sulfide catalysts and to put in evidence possible intermediates like MoS3 or oxysulfide species 3–5. With the development of in situ cells allowing the characterization of catalysts in temperature under gaseous H2S/H2 atmosphere

6, 7

, LRS studies of the activation process were progressively reported

8, 9

. Actually

this technique is particularly appropriate for supported oxide catalysts and is really straightforward to perform in situ studies of catalysts at elevated temperatures and pressures 10–13. In the 80’s or early 90’s, most of the characterizations were carried out in stepwise sulfiding operations. After sulfidation at discrete temperatures, the sample was cooled down under H2S/H2 or neutral gas flow and the Raman spectra were recorded at room temperature (RT) 8. Then in successive decades, XAS, which provides powerful structural and chemical information about a selected element, became also a widely used technique for characterizing catalysts during


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sulfidation

14–22

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under in situ conditions. Contrary to the early LRS studies for which long

acquisition time was required due to the spectrometer low sensitivity, XAS took advantage of the brightness increase of synchrotron radiation facilities to open the bottleneck in the time-resolved characterization of heterogeneous catalysts. Thus one has access to a deeper and more accurate temporal description of the chemical species involved in the processes, unraveling short-lived intermediates. Nowadays, oxysulfide molybdenum-based species are the commonly accepted intermediate species formed at the early stages of sulfidation process (T < 200-250 °C). Raman spectroscopic fingerprints have been depicted in the pioneering works of Schrader et al.

8

and Payen et al. 9.

Nevertheless, despite the huge number of studies based on LRS, neither molecular formula nor structural models were proposed for the observed transient species. The formation of MoOxSy oxysulfide species was confirmed later in other XAS studies

15, 23

. This species would be

complex amorphous compounds first formed by an O–S exchange on the initial supported oxidic precursor species. Formally the exchange of O by S atoms does not lead to a change of Mo(VI) oxidation state as confirmed by temperature programmed sulfidation (TPS) studies revealing no H2 uptake and only H2S consumption

24

. Then the O-S exchange is followed by reduction of

MoVIOxSy compound leading progressively to intermediate oxysulfide compounds with Mo atoms coordinated by terminal S22-, bridging entities and terminal S or SH groups. The oxidation state of the oxysulfide molybdenum species after intramolecular redox reaction involving sulfido and disulfido ligands is either proposed as Mo(V) 25 or Mo(IV) 24, 26. The high reactivity of molybdenum supported catalysts towards sulfidation is a consensual observation in the literature. It explains the difficulty to isolate as pure phases the possible oxysulfidic intermediates MoVIOxSy, MoVOxSy or MoIVOxSy. To the best of our knowledge,


4

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excepting the assumption of the formation of the model MoOS2 intermediate phase sometimes proposed in the literature

15, 23

(for which the structure was recently studied by extended X-ray

absorption fine structure spectroscopy (EXAFS)27), no oxysulfide species involved in low sulfidation temperature of HDS catalysts was irrevocably identified. At higher sulfidation temperature, the intermediate oxysulfide species are reduced to the final MoS2. This transformation is accompanied by a large H2S production 14. Several studies agreed for proposing that the transformation of MoOxSy species into MoS2 involves the amorphous MoS3 compound 4, 5, 15, 16, 24, 28, 29

.

The afore described schematic transformation of the supported Mo species into oxysulfide ones and later fully sulfided species does not seem to be affected by the presence of Ni/Co species, at least at the early stages of activation 15. Most of the XAS studies of the sulfidation of the promoter reports progressive sulfidation of oxidic species with no intermediate species 15. Finally, the formation of MoS2 slabs decorated by the Ni or Co ions is difficult to be evidenced by in situ XAS under high temperature. Actually, to the best of our knowledge, the sulfidation stage at which interactions between Mo and promoter occur has never been elucidated in any studies focused on the promoter behavior. It is only proposed that the decoration of edges of MoS2 slabs by Ni/Co atoms is strongly dependent on the order of sulfidation of both metals. The optimal condition to form mixed active phases would be to let formation of MoS2 slabs, after which Ni/Co can decorate the most reactive MoS2 sites situated at the edges or better to have a simultaneous sulfidation of Ni/Co and Mo (e.g.

14–16, 30, 31

). These features could explain that,

except for peculiar precursors presenting at the oxide state structural interactions between Mo and promoter (such as in heteropolyanions based oxidic precursor 32), no interaction between Mo and Ni/Co can be detected before the advanced stages of sulfidation and formation of MoS2.


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Nevertheless, these conclusions were deduced from XAS experiments carried out twice: one experiment probing the local order around Mo and another one around the promoter, with sometimes different heating rates

15

and catalyst mass in the cell. It is then of prime importance

to confirm these assumptions by characterizing both metals in a single experiment with a unique time scale. Taking advantage of the recent development in time-resolved Raman

33

and XAS

34

spectroscopies, the goal of the present study is to simultaneously combine both techniques for the first time to characterize the sulfidation process of Mo and Ni species of a bimetallic supported catalyst. Multivariate curve regression with alternating least square (MCR-ALS) fitting analysis, which recently emerged as a powerful skill for isolating pure spectra of intermediate species involved in time-resolved XAS studies

35–37, 13, 38

, is fully exploited herein.

The sulfidation of a NiMo/Al2O3-supported catalyst loaded in a dedicated cell 39 is followed in one single experiment at both edges thanks to the edge jumping capability offered by the SOLEIL’s Quick-EXAFS beamline 34, 40. This leads to the unambiguous comparison of the order of sulfidation stage for both metals. The complementary information provided by the combination with LRS analyzed with MCR-ALS allows us to propose structural models for the intermediate species isolated from XAS by MCR-ALS. In order to match the lifetime of intermediate species to the time scale of our spectroscopies, we adopted a strategy to decrease the catalyst reactivity towards H2S. Since it has been reported that hydrated catalysts are easier to sulfide than dehydrated ones 41, 42, we collected data upon sulfidation of an in situ dehydrated-calcined NiMo/Al2O3 catalyst (i.e. preserved from any contact to air moisture). The preparation of the oxidic precursor of our model catalyst is described in the experimental section. Nevertheless its structural characterization has been


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reported in-depth in a previous paper

12

and only the main conclusions were gathered herein.

Briefly, the oxidic sample presents highly distorted isolated or partially condensed tetrahedral Mo units with terminal mono-oxo groups. Bulk and/or surface NiAl2O4-type and NiO-type species have been identified for the nickel species. Neither interaction of both metals nor the formation of mixed NiMo heteropolyanions has been observed at the dehydrated oxidic state. The in-depth LRS and Quick-XAS in situ sulfidation study of this model dehydrated-calcined NiMo/Al2O3-supported catalyst is reported herein.

2. Experimental Section 2.1. Sample preparation The bimetallic model NiMo/Al2O3-supported catalyst was synthesized by an incipient impregnation procedure over a δ-alumina support (140 m2/g) with a solution of MoO3 (0.6 M) dissolved into H2O2 (0.2 M) in which nickel nitrate (Fluka) is subsequently added (molar Ni/Mo ratio = 0.5, 8wt%. MoO3; 2wt%. NiO). Subsequently to the impregnation step, the solid was matured in a saturated water atmosphere at room temperature (RT) during 12h followed by a drying step at 120°C overnight in a static air oven in order to ensure a good diffusion of the molybdenum and nickel precursors into the alumina porosity.

2.2. In situ experiment Powdery oxidic precursor was loaded in a dedicated oven allowing the simultaneous characterization of the catalyst under real conditions by X-ray absorption and laser Raman


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spectroscopies. This capability has been fully used herein. Technical details on the oven are presented in Figures 3 and 4 of the reference 39. The oxidic precursor was calcined in situ for 2 hours at 450°C (air) and maintained without any contact with air moisture after cooling down to RT. Then the dehydrated-calcined oxidic precursor was sulfided by raising the temperature up to 400°C under a flow of a mixture of 15% of H2S into H2 (2 mL/min) at atmospheric pressure. After reaching 400°C, the catalyst was maintained under sulfiding atmosphere for 2 h before to be cooled down at RT. The outlet gas products were analyzed by mass spectrometry (MS) using a MKS-Cirrus apparatus (as detailed described in 13). The in situ characterizations carried out on the sample are described in the Supporting Information.

3. Results 3.1. Molybdenum K edge XAS Characterization Figure 1 displays the Mo K edge Quick-XAS evolution vs temperature during the sulfidation of the dehydrated-calcined NiMo catalyst ((A) XANES, (B) EXAFS and (C) corresponding Fourier transforms (FT) moduli). Relevant spectra in the evolution are emphasized by colored lines. The spectral shapes strongly change during the heating under H2/H2S resulting in the transformation of oxidic species into sulfidic ones. The spectral modifications occur very early after the exposure to H2S/H2 atmosphere. Firstly, a shift to lower energy of the absorption ramp is observed as soon as 40 °C, indicating that transformations already occur changing either the metal oxidation state or/and the ligand nature. For this later contribution, it was demonstrated that the pre-edge intensity is particularly


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sensitive to the exchange of O by S

43

. Indeed, this feature is attributable to formally dipole-

forbidden 1s4d transitions gaining intensity from the hybridization of 4d with p(Mo) orbitals along M-O bond vectors. Therefore, the more oxygen belongs to the first coordination shell, the more intense is the pre-edge. At 50 °C, the XANES spectrum (red curve, Figure 1 (A)) presents a defined pre-edge feature and a resonance around 20049 eV suggesting that the oxidic structure described previously as formed by highly distorted isolated or partially condensed tetrahedral Mo units with terminal mono-oxo groups is still present at this temperature. The corresponding EXAFS spectrum (Figure 1 (B)) displays a global intensity reduction of the oscillations with slight shift in position and appearance of a new contribution around 4.5 Å-1.

20035

(C)

(B)

20017

400°C

-4

FT moduli, k (Å )

(A)

Normalized XANES

218°C

3

-3 3

183°C

k χ(k) (Å )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


50°C RT

20049

19980

20020 20060 Energy (eV)

20100

2

3

4

5

6

7

8

9

10 11

0

−1

k (Å )

1

2

3

4

5

R (Å)

Figure 1. Evolution of the Mo K edge XANES (A), EXAFS (B) spectra and corresponding Fourier transform moduli (C) collected during sulfidation.


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Around 183 °C, the XANES resonances are also shifted at lower energies together with the rising edge (green curve). Furthermore the pre-edge structure totally vanishes. Besides, at this temperature the EXAFS oscillations present stronger intensity in the 4-8 Å-1 k-range (Figure 1 (B)), in agreement with the scattering amplitude in this k-range for sulfur backscatters compared to oxygen ones. These features are characteristic of the transformation of the oxidic local order environment into a sulfidic one. It is worth to note that the first contribution of neighbors in the FT is located at the same position as that reported for MoS2 only around 218 °C (blue curve, Figure 1 (C)). Thus at this temperature the Mo is mainly in a sulfur environment with a Mo-S distance characteristic of the final phase (~ 2.41 Å). This later presents a spectrum characteristic of MoS2 (orange curve) with a well-resolved shoulder around 20017 eV. The second contribution, around 2.93 Å on the FTs, characteristic of the Mo-Mo distance in MoS2 (3.16 Å), is well evidenced above 300 °C indicating that this phase is yet formed at this temperature. Finally, as far as it can be observed by Quick-XAS, the nature of the Mo-based phase is unchanged during the 2h-plateau at 400 °C. In order to unravel the complex transformations occurring during sulfidation, a multivariate analysis was carried out over the set of time-resolved XAS data. Herein, PCA-SVD analysis (principal component analysis - singular value decomposition) indicates that five components are necessary to describe the Mo K edge data set (Supporting Information). Those components, labeled hereafter Comp.1, Comp.2, Comp.3, Comp.4 and Comp.5, were determined by further MCR-ALS fitting analysis as fully explained in Supporting Information.


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1.2

100

(A)

Percentage of components

1.0 Normalized XANES

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.8 0.6 0.4

Comp. 1 Comp. 2 Comp. 3 Comp. 4 Comp. 5

0.2 0.0 19980

20020

20060

(B)

80

60

40

20

0

20100

50

Energy (eV)

100

150

200

250

300

350

Temperature (°C)

Figure 2. (A) Mo K edge XANES spectra of the MCR-ALS components describing the sulfidation. (B) Concentration profiles of each component.

Figure 2 (A) displays the XANES spectra of the MCR-ALS components. XANES spectra of Comp.1 and Comp.5 are superimposable to the experimental spectra of the initial dehydratedcalcined NiMo catalyst (RT) and to the final MoS2 phase (400 °C), respectively (Figure S3). The edge position, defined at half height of the normalized absorption, shifts from 20018.3 eV for the dehydrated-calcined Mo(VI) precursor (Comp.1) to 20010.9 eV for the final Mo(IV)S2 phase (Comp.5). Between these extreme phases, the edge position moves to 20013.9 eV for Comp.2, 20011.4 eV for Comp.3 and 20011.2 eV for Comp.4. The Mo edge position is often used for the determination of the Mo oxidation state

44

. Using the literature, overlapped energy

regions (Figure S4) are defined as regions in which Mo(VI), Mo(V) and Mo(IV) oxidation states are present for Mo compounds with different proportions of Mo-O/Mo-S bonds and several bond


11

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lengths 43, 45–48. However it is noteworthy that the rising edge position of the last three MCR-ALS components, i.e. Comp.3 to Comp.5, falls in a position where no overlap exists (orange domain, Figure S4). It allows us to conclude that for these species, Mo presents a tetravalent oxidation state. For the first intermediate, Comp.2, the position of the rising edge is consistent with both Mo(VI) and Mo(V) oxidation states (green and blue domains, Figure S4). Thus, further information is necessary to be more conclusive about the Mo oxidation state of Comp.2. Besides, comparing the pre-edge regions of XANES spectra reported Figure 2 (A), first conclusions for the ligand nature in the first coordination shell can be obtained. Comp.2 still displays a pre-edge structure. Less resolved than the one of the dehydrated-calcined tetrahedral polyoxomolybdate catalyst, this suggests a possible exchange of oxygen by sulfur ligands. The pre-edge peak is almost absent for the other intermediate species revealing a further strong substitution of oxygen by sulfur ligands around molybdenum compared to the initial precursor and Comp.2. Finally in Figure 2 (A), the shift to lower energy of the first EXAFS oscillation of the intermediates species compared to the one of the dehydrated-calcined precursor suggests a lengthening of Mo-ligand distances for these species. Furthermore, we clearly evidence from the MCR-ALS intermediates concentration profiles (Figure 2 (B)) that the transformations occur gradually with a successive interconversion of the intermediates. We can find some temperature ranges in which each intermediate species is the dominant ones. Starting at T > 40 °C with a mixture of two entities (Comp.1 – the starting oxidic precursor phase and Comp.2), the third one rapidly appears at T > 50 °C to reach a maximum around 175 °C. Then this third entity is transformed into the fourth one. It is only when this fourth compound reaches 35% around 200 °C that the transformation into MoS2 starts.


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3.2. LRS Characterization Figure 3 (A) and Figure S5 display the LRS data collected simultaneously to the XAS data during the sulfidation (upon heating). The assignment of the lines characterizing the LRS spectrum of the oxidic dehydrated-calcined NiMo precursor can be found in 12. The lines marked by a star are due to the mica window of the cell 12. In agreement with the modifications observed by Quick-XAS, strong modifications of the LRS data also occur very early after the exposure of the catalyst to H2S/H2. Despite the weakness of the scattering signal characterizing the Raman data recorded at temperatures lower than 230 °C, several lines are clearly observed below 600 cm-1. In particular, a double line at 451 and 476 cm1

and a broad signal centered around 535 cm-1 are observed for the spectra recorded at the early

stages of sulfidation (T ~ 40 °C). Moreover, it is worth to note that the line at 588 cm-1, presents when the catalyst is in contact with the H2S/H2 atmosphere, is the more intense line characteristic of gaseous H2 in the 200-1000 cm-1 spectral range (as shown in Figure S7 with the spectrum of pure H2). This intense line at 588 cm-1 is always accompanied to a line at 355 cm-1. They correspond to the rotational modes of both ortho and para hydrogen molecules. This identification of the line at 588 cm-1 to gaseous H2 rotational mode allows to definitely refute its previous assignation, proposed by one of us 9, 42, as a fingerprint of a metal-S-H bond. Around 40 °C the broad line at 800-1020 cm-1, related to Mo-O stretching vibrations of the dehydrated-calcined precursor, strongly decreases in intensity (Figure S5).


13


(A)


(B)

* 400°C

*

(C)

*

464 320

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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505 403* 411 447 480 490

218°C 181°C

588

297 476 540 451

49°C RT 200

300 400 500 600 -1 Raman shift (cm )

250

350

450

550

650

-1

Raman shift (cm )

650

750 850 950 -1 Raman shift (cm )

Figure 3. (A) In situ LRS monitoring of the sulfidation. (B and C) Raman spectra determined from MCR-ALS of the data recorded upon the sulfidation process. The peaks marked by a star symbol are due to the mica window.

The spectral shape characteristic of the final MoS2 phase is evidenced for temperature higher than 230 °C showing that if the sulfidation is done after a dehydration (without transfer in the air moisture), the MoS2 formation proceeds at higher temperature than the one observed with nondehydrated samples

9, 41

. The intensities of both lines continuously increase up to 400 °C and

during the 2h-plateau (Figure S6) whereas their full width at half maximum decreases. Furthermore a shift from 402 to 400 cm-1 and from 372 to 370 cm-1 occurs by increasing the temperature. These lines correspond to the out-of-plane υ(A1g) mode (409 cm-1) and the in-plane υ(E2g) mode (384 cm-1) in crystalline 2H-MoS2 49. The downshift of the catalyst lines compared


14

1050

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to the crystalline phase is commonly observed in the literature stacking degree in the catalyst

50

and of a temperature effect

9

7

and could result of the low

combined with the presence of

sulfiding atmosphere in the cell 7. Finally the changes observed above 300 °C and during the prolonged aging at 400 °C are correlated to an increase of the MoS2 crystallinity. To complete this qualitative analysis, in the same way to the use of MCR-ALS on XAS data, the chemometric approach was applied to Raman data. The PCA-SVD analysis revealed that the variance of the Raman data set can also be reproduced by five components (Figure S1). Figure 3 (B and C) displays the Raman spectra of those 5 species and the existence domains of those components are presented Figure S8. Even if slight differences can be observed, the existence domains of each of the species, except for Comp.4, are in line with the concentration profiles determined by MCR-ALS of the Quick-XAS data presented in Figure 2 (B). The differences existing between both techniques are discussed in Supporting Information. In the next section we will only discuss the main vibrational characteristics of the MCR-ALS Raman spectra of the components Comp.2 to Comp.4 since the Raman spectra of Comp.1 and Comp.5 are unambiguously related to the one of the oxidic dehydrated-calcined NiMo catalyst and to the supported MoS2 slabs, respectively. The Raman lines observed Figure 3 (B) have been reported in many works and fall in the typical ranges characterizing sulfur-based species. Hexavalent Mo-oxysulfide compounds of general formula (MoVIO4-xSx)2- 51 and reduced sulfidic species like (n-Bu4N)2(MoV2O2S2(S2)2) 52 or (NH4)2(MoIV3S13].H2O

53

are known to present lines in three characteristic regions 300-

400 cm-1, 420-480 cm-1 and 500-550 cm-1 (Table S1). The attributions in each region have been clearly reported by Schrader et al. 8.


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The spectral range 420-480 cm-1 allows the unambiguous identification of vibrational modes involving sulfur since no overlap with Mo-O vibrations is expected. The lines can be assigned to the stretching vibration mode of bridging MoV-µ2S-MoV bonds as for (n-Bu4N)2 (MoV2O2S2(S2)2) 52 and MoIV-µ3S-MoIV bonds as for (NH4)2(MoIV3S13].H2O

53

or to the one of

the Mo-St bonds (here St stands for a terminal sulfido ligand) like encountered for the Raman spectrum of the (MoVIO4-xSx)2- anion 54, 55. In the 500-550 cm-1 region, the broad lines are generally associated to the stretching mode of bridged disulfide (S22-) 52 or terminal one anion 53. However, this region is also a fingerprint of the Mo-O-Mo symmetric stretching mode of polyoxomolybdates, like in (NH4)2Mo7O24.4H2O

8

(Table S1). Finally, in the 300-400 cm-1 region, deformation modes of Mo-O-Mo bridge and stretching modes of Mo-S-Mo bridging bonds are also overlapping, making the identification less straightforward (Table S1). Hopefully, the observation of lines in the 800-1000 cm-1, characteristic of the Mo-O stretching mode (Mo=Ot or Mo-O(X) bonds) can be used for unscrambling the complex information contained in the Raman spectra. Comp.2 is characterized by well-defined lines centered at ~297, 451, 476, 540 cm-1 and a broad line between 740 and 1000 cm-1 (red spectrum). As already discussed for the oxidic precursor in 12, this later line is generally assigned to the asymmetric stretching mode of the MoO as terminal or bridging Mo-O(X) stretching modes where X can be Mo or Al. The defined bands at 451 and 476 cm-1 could be assigned to stretching vibrational modes of the Mo-S(2-) bond in bridging and terminal positions, respectively. Indeed hexavalent molybdenum oxysulfidic species, (MoOxS4-x)2- displays single line at ~ 473 cm-1 for x=2 or 3

54, 55

whereas stretching

vibrations of bridging sulfido ligands are encountered between 420-460 cm-1 as reported in Table


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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


S1. Finally taking into account the complete lack of lines in the 500-600 cm-1 range for the spectrum of Comp.1 characteristic of the oxidic precursors, we propose to attribute the line at 540 cm-1 to the stretching mode of S-S disulfido bridge as previously ascribed by Payen et al. 9. Comp.3 (green spectrum) displays a more featureless spectrum with lines at ~ 411, 447, 480, 490 cm-1 and a broad line between 500 and 570 cm-1.This later can be still attributed to the S-S disulfido stretching mode in terminal and/or bridging position as ascribed in 9. Furthermore, as aforementioned, the lines in the 440-480 cm-1 range are assigned to the stretching mode of bridging Mo-S-Mo bonds, Mo being either Mo(V) or Mo(IV). It is noteworthy that the Mo(IV) oxidation state is the only one consistent with reported XAS results. Finally, Comp.4 (blue spectrum) exhibits a broad and intense line at 403 cm-1 due to the mica window, which is particularly higher than the sample signal. Underlying lines at ~ 320, 464 and 505 cm-1 are tentatively ascribed by reference to the literature to the formation of MoS3 species. Indeed works reporting the Raman spectrum of thermal decomposition of (NH4)2MoS4 in inert gas atmosphere associate broad lines around 430-450 cm-1 and 525-528 cm-1 to the formation of amorphous MoS3

4, 28

. This assumption is also supported by the fact that the signal in the 800-

1000 cm-1 region, corresponding to Mo=O, Mo-O(X) lines, is no more observed. In this context, the line at 320 cm-1 is ascribed to vibrational modes of the Mo-S-Mo or Mo-(S2) bridging bond. In summary, Raman spectroscopy suggests that: - Comp.2 is an oxysulfidic species grafted at the support surface with characteristic lines of oxo/sulfido and disulfido bridging ligands and terminal sulfido and oxo ones. - Comp.3 presents characteristic lines of Mo(IV) oxysulfidic species with Mo-S-Mo, Mo-(S2)Mo and terminal disulfido characteristic vibrations. - Comp.4 is a fully sulfided species tentatively identified as MoS3.


17


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Page 18 of 47

3.3. Ni K edge Characterization Ni K edge Quick-XAS spectra recorded simultaneously with the Mo K edge data are presented in Figure 4 (A). The starting oxidic species has been described as a mixture of bulk/surface octahedral Ni oxo species and bulk/surface NiAl2O4-like entities

12

. At first, upon sulfidation, a

significant decrease of the white line intensity is observed, whereas the first EXAFS oscillation is progressively shifted to higher energy. Around 200 °C, the EXAFS oscillation is in phase with that of the final sulfidic Ni species. A reminiscence of the white line is still present at the end of the sulfidation. This feature is ascribed to the presence of NiAl2O4-like species after calcination, as discussed in

12

. The spectra of pure species determined by MCR-ALS of the Ni K edge data

are reported in Figure 4 (B). The bulk/surface octahedral Ni oxo species (Comp.Ni-1) and bulk/surface NiAl2O4-like entities (Comp.Ni-2), identified in our previous study 12, are directly transformed into a sulfided species (Comp.Ni-3). This latter one presents on its Fourier transform a contribution of first neighbors at higher distances than the Ni-O contribution of the oxide catalyst (Figure S9). It corresponds to a shell of 3.1 ± 0.8 sulfur atoms at 2.21 Å (Table S2), which could be consistent with the decoration of Ni at the edges of MoS2 slabs 56–58. Although no direct evidence of the formation of the mixed NiMoS species with a significant identification of a Ni-Mo contribution arises from the EXAFS fitting of the spectrum recorded at 400°C, the comparison of its XANES spectrum is clearly different from the spectrum measured for monometallic nickel sulfidic NiSx species supported on δ-alumina 38 prepared and sulfided in the same conditions that the sample studied herein (Figure S10). This strongly suggests that in addition to the expected formation of NiSx species

59, 60

, even if no quantification can be done

from the presented data, the active NiMoS phase is also formed upon sulfidation of the studied catalyst.


18

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(B)

(A)

Comp. Ni-1 Comp. Ni-2 Comp. Ni-3

Normalized XANES

RT

Normalized XANES

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


400°C

8310

8340

8370 Energy (eV)

8400

8430

8310

8340

8370 Energy (eV)

8400

8430

Figure 4. (A) Evolution of the Ni K edge XANES spectra collected during sulfidation and comparison of the Ni K edge XANES spectra of the components determined by MCR-ALS (B).

4. Discussion Our study reveals that the sulfidation of the dehydrated-calcined NiMo(VI) oxide precursor involves a five-stepped mechanism. The use of MCR-ALS method was of prime importance for isolating the Quick-XAS and Raman spectra of short-lived Mo-based intermediates formed during sulfidation. In this section, we will first consider the structure of those Mo-based intermediates, resulting from the EXAFS fitting of MCR-ALS components, driven by the identification of characteristic vibrations of the MCR-ALS LRS spectra. Then the mechanisms underlying the sulfidation will be discussed. Finally, from the unambiguous comparison of the concentration profiles obtained for Mo-based intermediates and Ni-based ones, the synergy between Mo and Ni species will be discussed.


19


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Page 20 of 47

4.1. Structure of Mo-based MCR-ALS species Scheme 1 displays the models for Comp.2 and Comp.3 for which the best agreements were achieved for the EXAFS fittings of the species presented Figure 5. Those models are based on inputs from the Raman analysis with the identification of characteristic bonds and the oxidation states identified by XANES analysis. Therefore Comp.2, with an oxidation state Mo(V)/Mo(VI), presents Mo-O-Mo, Mo-S-Mo (sulfido and disulfido) bonds, terminal sulfido and oxo ones in interaction with the support. Comp.3, with a tetravalent oxidation state, presents Mo-S-Mo, Mo(S2)-Mo bonds. It is noteworthy that the structural models shown in Scheme 1 are not unique and small variations of the coordination numbers could satisfactorily reproduce the data. Nevertheless, the bonds in these structures are unambiguously identified by LRS and have always been present in the different tested models. In order to limit the number of parameters for the fitting procedures, the coordination number associated to the different ligands was fixed during the fit. Table 1 reports the structural parameters determined by fitting whereas fits with these parameters are shown in Figure S11. Scheme 1. Proposed structural models for the molybdenum intermediate species observed at the early stages of sulfidation (T < 200 °C). Red spheres: O atom, green spheres: Mo atom, yellow spheres: S atom, grey surface: Al2O3 support.


20

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12

(A)

(B)


8

3

-4

FT moduli, k (Å )

10

3

-3

k χ(k) (Å )

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


6

4

2

0 2

3

4

5

6

7

8

9

10

11

0

1

-1

2

3

4

5

R (Å)

k (Å )

Figure 5. Mo K edge EXAFS (A) and Fourier transform moduli (B) of the components determined by MCR-ALS.

The Fourier transform of the EXAFS spectrum of the first intermediate species (Comp.2, red curve, Figure 5) can be satisfactorily reproduced considering the trimeric model grafted at the support surface, with formula [Mo3(µ2O)4(µ2S)µ2{S2}(Ot)2(St)3] (Scheme 1) (hereafter simplified as (Mo3S6O6(Al)3 model). The refined parameters for the oxidic network are fully consistent with Mo-ligand distances found in known hexavalent Mo-based compounds like (NH4)2Mo7O24.4H2O 61

with Mo=Ot bond at 1.66 Å and oxo bridging ligand at 1.74 Å giving rise to Mo-Al and Mo-

Mo distance at 3.06 and 3.42 Å. The distance for the sulfidic network found at 2.24 Å, is consistent with a mixture of short distances characteristic of terminal sulfido ligands (typically between 2.09-2.19 Å depending on the Mo oxidation state

62

) and long distances around 2.28-

2.35 Å associated to bridging sulfido ligands as encountered in (Mo2O2S6)2- moiety

63

. In the


21


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Page 22 of 47

case of sulfido bridging ligands, the mean Mo-Mo distance is found at 2.82 Å 63 which is slightly longer than our refined Comp.2 distance (2.78 Å). We ascribe that the additional bridging by disulfido ligand of both Mo could be responsible for the Mo-Mo distance shortening. Finally, the distance for disulfido bridging ligands found at 2.47 Å is well in line with the one reported in the literature for Mo(V) complexes

64

. These structural features, together with the position of the

rising edge reported at 20013.9 eV for the XANES spectrum of Comp.2, allow us to propose the formation of Mo(V) oxysulfide or mixed valence Mo(V)/Mo(VI) oxysulfide phase. The Fourier transform of the EXAFS signal of the second intermediate species (Comp.3, green curve,

Figure

5)

can

be

satisfactorily

reproduced

considering

the

trimeric

[Mo3(µ3S)(µ2S)2µ2{S2}(Ot)2{S2}t] structure (hereafter simplified as (Mo3S7O2) model) (Scheme 1). This model, directly inspired from the (NH4)2[Mo3S13].H2O structure tetravalent cubane-like structure [Mo3S4(H2O)9]4+

66

65

and the associated

, involves a µ3-sulfido ligand bridging the

three Mo(IV) centers. Despite no conclusive information about the presence of terminal oxo ligand was arisen from LRS, the use of a Mo-O contribution at 1.65 Å ascribed to terminal oxo group is necessary for achieving a satisfactorily fit. The Mo-Mo distance found by the fit at 2.72 Å perfectly matches those reported for [Mo3S4(H2O)9]4+ 66 and (NH4)2[Mo3S13].H2O 65. Thanks to Raman spectroscopy, Comp.4 has been identified as a fully sulfidic phase, different from MoS2. The structure of MoS3 and in particular its oxidation state is still subject of numerous discussions due to its amorphous nature

28, 67

. Nevertheless, the Mo-S and Mo-Mo

distances determined by least-square fitting of the EXAFS spectrum isolated for the Comp.4 is well in line with those consensually reported in the literature for MoS3 65, 68.


22

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Table 1. Best fitted EXAFS parameters of the intermediates species identified during the sulfidation. Italic parameters are fixed values. S0² = 0.96. σ² (Å²) x R-factor 103 Comp.2 O 0.66 1.66 ± 0.04 1.3 ± 2.7 O 1.66 1.74 ± 0.09 34 ± 11 S 1.66 2.24 ±0.03 9.5 ± 3.0 0.0146 S 1.33 2.46 ± 0.03 4.8 ± 3.2 Mo 0.66 2.78 ±0.08 10.5 ± 8.2 Al 1.0 3.06 ±0.11 18.0 ± 22 Mo 0.66 3.42 ±0.07 9.2 ± 9.3 -1 ∆k = 2.3 – 10.8 Å , ∆R = 1.0 – 3.5 Å, E0 + ∆E= 20013.8 eV Comp.3 O 0.66 1.65 ± 0.01 2.0 ± 3.6 S 2.33 2.38 ± 0.03 17 ± 5.5 0.0035 S 2 2.45 ± 0.01 2.2 ± 1.0 Mo 2 2.72 ± 0.03 16 ± 4.3 ∆k = 2.3 – 10.3 Å -1, ∆R = 1.0 – 2.9 Å, E0 + ∆E = 20012.5 eV Comp.4 S 3.3 ± 0.6 2.22 ± 0.02 11.8 ± 2.5 S 2.7 ± 0.6 2.38 70.0 0.0240 Mo 0.9 ± 0.6 2.78 19.2 ± 10.0 ∆k = 2.3 – 10.3 Å-1, ∆R = 1.0 – 3.0 Å, E0 + ∆E = 20011.7 eV Atoms

N

࣑૛ࣇ

R (Å)

3205

5791

23158

4.2. Sulfidation mechanism of the dehydrated-calcined Ni-Mo/Al2O3 catalyst The Mo intermediates involved during the sulfidation of the NiMo/Al2O3 dehydrated-calcined catalyst are well in line with the scheme proposed by Payen et al.

9

with the LRS study of

sulfidation of a 14% Mo/Al2O3 catalyst. As herein, their scheme involved oxysulfide and MoS3 compounds. At the early exposure to H2S/H2 (T < 60 °C), the fully oxidic precursor consisting in distorted tetrahedral polyoxomolybdate Mo(VI) species is transformed into the oxysulfide species (Comp.2). This transformation is a two-step process which firstly involves the exchange of oxygen ligands by sulfur ones without reduction of Mo in agreement with the TPS results of


23


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Page 24 of 47

Arnoldy et al. 24. Afterwards, the partial transformation of terminal sulfido groups into bridging sulfido and disulfido ligands between two Mo centers takes place. Due to the high reactivity of the terminal sulfido ligands and the time resolution for LRS and XAS measurements (considering blind periods for molybdenum characterization, whilst measurements were performed at the Ni K edge), our work does not allow us to isolate the species where only exchange of O by S ligands occurs. The bridging of Mo centers by sulfido and disulfido ligands is accompanied by the formation of Mo(V) centers as discussed in the previous section. The presence of such paramagnetic center is consistent with the electron spin resonance signal reported at the early stage of sulfiding (5 min at 60 °C) and interpreted by Payen et al.

9

and Blanchard et al.

69

as resulting from the

formation of Mo(V) oxysulfide species. A step further was achieved in our present work compared to the aforementioned papers with a more detailed identification of the oxysulfide species, with formula Mo3S6O6(Al)3. At higher temperature (80 < T < 200 °C), the oxysulfide Comp.2 is transformed into the Mo(IV) oxysulfide Comp.3. This reaction involves further O-S exchange of the remaining bridging Mo-O groups: Mo-O-Mo or between Mo center and Al support. In agreement with the proposition made by Weber et al. 23 on the basis of XPS and infrared emission spectroscopy results, this exchange of bridging oxo group definitely breaks the link with the support. Taking into account our models, the transformation of Comp.2 to Comp.3, represented according to the reaction (1): Mo3S6O6 + H2S + 3H2 → Mo3S7O2 + 4 H2O

(1),

involves an intra-redox oxidation of the remaining terminal S2- ligands into disulfido group (2S2- → S22- + 2e)

70

and simultaneously the reduction of the Mo(VI)/Mo(V) centers into

Mo(IV).


24

Page 25 of 47

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The formation of Mo(IV) oligomeric species is consistent with results reported by Leliveld et al. during sulfidation of a 15% Mo/Al2O3 catalyst intermediate

formed

between

RT

and

175

26

°C,

. Indeed, they proposed as possible a

Mo(IV)

dimer

with

formula

[Mo2(µ2O)2µ2S({S2}t)2] in which only a single sulfido ligand bridges both Mo(IV) centers and with terminal disulfido group coordinating each Mo atom. Nevertheless, as recognized by Leliveld et al. 26, the depicted structures are not in full agreement with their EXAFS coordination numbers. On the contrary, using the constrained structural model presented in Scheme 1, our results successfully reproduce the experimental EXAFS data for Comp.3. The discrepancy emphasized by Leliveld et al. is probably related to their analysis of spectra of non-pure species, contrary to the ones isolated herein with chemometric analysis. At the third stage (165 < T < 220 °C), the Mo(IV) trimeric Comp.3 intermediate is transformed into MoS3 (Comp.4) according to reaction (2): Mo3S7O2 + 2 H2S → 3 MoS3 + 2 H2O Weber et al.

28

(2)

established from chemical extrusion experiments of amorphous MoS3 a

relationship between the MoS3 structure model and the aggregation of triangular Mo3 cluster fragments such as {Mo3S7}4+ and {Mo3S4}4+. As aforementioned, the structure proposed for Comp.3 in Scheme 1 is actually inspired from those {Mo3S7}4+ and {Mo3S4}4+ fragments described in 65,

66

, with internal structures composed of Mo-S bonds found in each fragment.

Furthermore, as already discussed, the edge position reported for Comp.4 at 20011.2 eV is characteristic of Mo(IV) oxidation states. Consequently the bottom-up description of the MoS3 formation, upon sulfidation of the sample studied herein, and, resulting from the aggregation of those fragments together with the tetravalent oxidation state of molybdenum in Comp.4 significantly contributes to the debate related to the MoS3 structure. For these supported entities,


25


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Page 26 of 47

the chain model, proposed by Hibble et al. and based on the MoV(S2-)2{S22-}0.5 formula

68

, is

refuted. Above 200 °C, MoS3 is finally transformed into MoS2 according to reaction (3): MoS3 + H2 → MoS2 + H2S

(3)

According to the concentration profile (Figure 2 (B)), this transformation is complete at the end of the heating ramp (~ 380°C). For the last stages of sulfidation, the reaction sequence can be discussed in light of the H2 and H2S signals measured by mass spectrometry (MS) and of the raw-XAS absorption level measured at the Mo K edge (Figure 6). The absorption level is informative about the capacity of the catalyst to trap H2S or elemental S0. Indeed, an increasing slope of the absorption level means that the sample is less transparent to X-rays due to the physical adsorption of H2S/S0 whereas a decreasing slope points out their desorption from the catalyst surface. Even if non quantitative analysis could be done with our MS data, it is noteworthy that the observed MS and raw-XAS absorption level evolutions are fully in agreement with the TPS results reported by Arnoldy et al. 24

. H2S surface adsorption is observed up to 125 °C followed by a steady state before a rapid

desorption in the 175 – 220 °C temperature range (Figure 6 (A)). The MS profiles, shown Figure 6 (B), simultaneously display the peaks of H2S release (blue arrow) and H2 consumption (red arrow) which is interpreted as the consequence of the desorption of physically adsorbed elemental sulfur according to the reaction (4): H2 + Sadso → H2S

(4)

The desorption of H2S/S0 in the 175 – 220 °C temperature range is also concomitant with the transformation of Comp.3 into Comp.4 (Figure 2 (B)). At higher temperature, the raw-XAS absorption level is still slowly decreasing during the slow transformation of MoS3 into MoS2. Considering that the decomposition of amorphous MoS3 into crystalline MoS2 in inert gas


26

Page 27 of 47

atmosphere above 310 °C

51

yields also the formation of elemental sulfur S0 28, we propose, in

agreement with the variation of the raw-XAS level, that a better description of reaction (3) should be the one described with the sequences: MoS3 → MoS2 + Sadso

(5a)

H2 + Sadso → H2S

(5b)

The desorption of elemental sulfur from the support (reaction (4) and reaction (5b)), together with the temperature increase, cause a higher mobility of Comp.3 and Comp.4 species that present no more interaction with the support as shown by EXAFS. This higher mobility allows the fast solid-state polymerization of Comp.3 into Comp.4, whereas aggregated fragments of Mo(IV) sulfur cluster-type, which compose MoS3, can be reorganized at higher temperature into MoS2 through the complex possible pathways reviewed by Müller 62.

(B)

(A)

H2S release

MS signal (a.u.)

Raw Mo K edge X-ray absorption level (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


H2S

H2 H2 uptake

100

200

300

400

Temperature (°C)

100

200

300

400

Temperature (°C)

Figure 6. (A) Raw Mo-K edge X-ray absorption level measured before the rising edge and (B) smoothed normalized MS signal obtained during the sulfidation treatment (heating ramp). The signal for H2S was multiplied by 5 and the curves are plotted with an arbitrary offset. Regarding the MS spectra, the signals are disturbed up to ~125 °C due to dilution of the reactive gases with


27


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Page 28 of 47

the neutral gas initially filling the cell that is the reason why only the signal above this temperature is presented. Above 125 °C, the observation of positive MS peak is consistent with gas release, whereas negative MS peak is related to gas consumption.

4.3. Synergy between nickel and molybdenum to form the active phase NiMoS A single sulfided species has been isolated by MCR-ALS analysis of the data set recorded at the Ni K edge upon sulfidation of the initial amorphous nickel-based aluminate and octahedral oxo nickel species. This finding is not in agreement neither with the literature 56, 58 reporting the formation of bulk NiSx species and NiMoS, nor with our comparison with the experimental XANES spectrum recorded at 400°C of the sulfided monometallic supported species (Figure S10). This latter excludes the lonely formation of bulk NiSx species in addition to the EXAFS fitting of the first coordination shell consistent with the NiMoS formation. We ascribe the inability of the MCR-ALS to separate bulk NiSx species and NiMoS one from the XAS data set to a rank deficiency data issue

71

associated to co-evolving profiles for both species at a quite

similar transformation rate. Despite this limitation, it is noteworthy that the sulfidation rate of those nickel sulfided species is strongly related to the sulfidation of Mo species. Indeed, comparing both Mo and Ni speciation profiles presented in Figure 7, we firstly evidenced a quite constant sulfidation rate (2‰ per °C) for nickel in the temperature range in which Comp.3 is mainly formed, i.e. for 60 < T < 190 °C. While Mo3S7O2, i.e. Comp.3, is transformed into MoS3 (Comp.4) (T ≥ 200°C), we observe (Figure 7) a deviation in the nickel sulfidation rate with a significant increase from 2‰ to 6‰ per °C. The slope of the conversion rate remains constant to this 6‰ per °C up to ~ 240 °C.


28

Page 29 of 47

After this temperature, the Ni sulfidation rate slows down and the remaining 30% of oxidic nickel species are sulfided with a sulfidation rate of 1‰ per °C. Finally, above 370 °C, the Mo and Ni sulfidation states are simultaneously stable with a few percents of remaining oxidic Ni species associated to the bulk aforementioned NiAl2O4.

100 Percentage of components

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


80 Comp. Ni-1 Comp. Ni-2 Comp. Ni-3 Comp. 1 Comp. 2 Comp. 3 Comp. 4 Comp. 5

60 40 20 0 50

100

150 200 250 300 Temperature (°C)

350

400

Figure 7. Comparison of concentration profiles for Mo (solid lines) and Ni (symbols) species recorded during the sulfidation treatment (heating ramp).

These findings allow us to propose the following interpretation for the formation of the active NiMoS species. At the early stages of sulfidation (T < 200 °C), we assume that the sulfidation of oxidic species leads to the formation of NiSx species without interaction with the Mo-based sulfide species. Indeed, the successful EXAFS fitting of Comp.2 and Comp.3 based on models presented in Scheme 1 clearly excludes such interaction between both metals. The change of sulfidation rate of nickel and also of molybdenum during the transformation of Comp.3 into MoS3 is a strong indication that a synergetic mechanism involving both metals takes place at this


29


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Page 30 of 47

stage. Thus we assume that the incorporation of nickel into an extended sulfidic molybdenum species occurs and provides the pre-requisite to the further decoration of the MoS2 slabs by the nickel promoter. We may suggest that only the amount of Ni transformed in this range of temperature (~ 23 Ni at.%) will be in decoration position of the active phase. Similar amount was determined by XPS 72. Such low decoration ratio is here in relation with the low Mo sulfidation rate as expected for dehydrated oxidic precursor. This Ni-MoS3 interaction would be at the origin of the control of the MoS3 polymerization and subsequently of the MoS2 nanocrystallite length. This control of length by the promotor has been previously suggested by Chassard

72

.

Unfortunately, due to the extended structure of the MoS3 intermediate now formed, there is no way to prove this assumption by our local range order techniques, at least in the frame of in situ experiments carried out under temperature.

5. Conclusion For the first time, using the edge jumping capability available with the Quick-EXAFS monochromators installed at SOLEIL, Ni and Mo K edges have been investigated quasi simultaneously during the sulfidation of a model dehydrated-calcined NiMo catalyst prepared by the peroxo method. It is only thanks to the combination of Quick-XAS and laser Raman spectroscopy that the complex sulfidation process of the bimetallic catalyst has been followed and results into the identification of Mo intermediates. The initial Mo(VI) polyoxomolybdate consisting in distorted tetrahedral units is transformed into oxysulfide species identified for this specific catalysts as [Mo3(µ2O)4(µ2S)µ2{S2}(Ot)2(St)3]


30

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1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


grafted to the support surface which is then transformed into [Mo3(µ3S)(µ2S)2µ2{S2}(Ot)2{S2}t] before being converted into MoS3 with an oxidation degree of +IV and finally transformed into MoS2. Considering the proposed structure of [Mo3(µ3S)(µ2S)2µ2{S2}(Ot)2{S2}t] based on the {Mo3S7} triangular cluster fragment, the formation of MoS3 is consistent with the polymerization mechanism of those fragments proposed by Weber et al. reported for the dehydrated oxydic calcined catalysts

9

28

. Since, similar Raman spectra are

irrespective to the preparation route

starting from polyoxomolybdate salts, the Mo-based species identified in this work resulting from the sulfidation of distorted isolated or partially condensed tetrahedral Mo units with terminal mono-oxo groups can be considered quite general. Obviously, the intermediate species resulting from the sulfidation of heteropolyanions in which Mo and promoter are associated in the same molecular structure will give rise to different oxysulfide species, at the early stages of sulfidation as shown in 73. Although no isolation of NiMoS was done by MCR-ALS analysis of the Ni K-edge XAS data recorded during the monitoring of the sulfidation, the formation of NiSx and NiMoS species is strongly suggested from the comparison of sulfided species formed for monometallic nickel supported on alumina 38 and from the first coordination shell fitting of the final sulfided species. Both unambiguous comparison of sulfidation kinetics for Ni and Mo and correlation of sulfidation rates points out the stage at which the incorporation of promoter into the extended sulfidic molybdenum-based phase occurs. We assume that the promoter incorporation occurs during the formation of MoS3. The presence of the MoS3 intermediate with an oxidation degree of IV and change in sulfidation rate of the promoter simultaneously to the formation of MoS3 have been observed with other hydrodesulfurization catalytic systems

73

. Moreover the gas

effluent composition determined by mass spectrometry simultaneously to the Mo and Ni


31


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Page 32 of 47

speciation points out a concomitant release of H2S and upload of H2, resulting from the desorption of elemental sulfur, and the transformation of [Mo3(µ3S)(µ2S)2µ2{S2}(Ot)2{S2}t] into MoS3.

ASSOCIATED CONTENT Supporting Information. Experimental section. PCA and MCR-ALS results. Characteristic Raman vibrations. LRS spectra of the catalyst during the sulfidation. Gas LRS data. XAS data of the dehydrated-calcined and sulfide catalyst. EXAFS fittings of intermediate species. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *[email protected] Present Addresses †If an author’s address is different than the one given in the affiliation line, this information may be included here. Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ‡These authors contributed equally. (match statement to author names with a symbol)


32

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ACKNOWLEDGMENT This work was supported by a public grant overseen by the French National Research Agency (ANR) as part of the “Investissements d’Avenir” program (reference: ANR-10-EQPX-45). The authors are grateful to SOLEIL committees for beam time allocated on the SAMBA and ROCK beamlines.

ABBREVIATIONS HDS, hydrodesulfurization; XAS, X-ray absorption spectroscopy; XPS, X-ray photoelectron spectroscopy; LRS, laser Raman spectroscopy; EXAFS: extended X-ray absorption fine structure; XANES, X-ray absorption near edge structure; Quick-XAS, Quick-EXAFS; MS, mass spectrometry; FT, Fourier transform; MCR-ALS, multivariate curve regression with alternative least square; PCA-SVD analysis, principal component analysis - singular value decomposition; RT, room temperature; TPS, temperature programmed sulfidation. REFERENCES [1]

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Insert Table of Contents Graphic and Synopsis Here


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Evolution of the Mo K edge XANES (A), EXAFS (B) spectra and corresponding Fourier transform moduli (C) collected during sulfidation. 594x355mm (300 x 300 DPI)



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(A) Mo K edge XANES spectra of the MCR-ALS components describing the sulfidation. (B) Concentration profiles of each component. 233x119mm (300 x 300 DPI)


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(A) In situ LRS monitoring of the sulfidation. (B and C) Raman spectra determined from MCR-ALS of the data recorded upon the sulfidation process. The peaks marked by a star symbol are due to the mica window. 233x139mm (300 x 300 DPI)



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(A) Evolution of the Ni K edge XANES spectra collected during sulfidation and comparison of the Ni K edge XANES spectra of the components determined by MCR-ALS (B). 594x304mm (300 x 300 DPI)


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Mo K edge EXAFS (A) and Fourier transform moduli (B) of the components determined by MCR-ALS. 594x304mm (300 x 300 DPI)



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(A) Raw Mo-K edge X-ray absorption level measured before the rising edge and (B) smoothed normalized MS signal obtained during the sulfidation treatment (heating ramp). The signal for H2S was multiplied by 5 and the curves are plotted with an arbitrary offset. Regarding the MS spectra, the signals are disturbed up to ~125 °C due to dilution of the reactive gases with the neutral gas initially filling the cell that is the reason why only the signal above this temperature is presented. Above 125 °C, the observation of positive MS peak is consistent with gas release, whereas negative MS peak is related to gas consumption. 297x209mm (300 x 300 DPI)


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Comparison of concentration profiles for Mo (solid lines) and Ni (symbols) species recorded during the sulfidation treatment (heating ramp). 297x209mm (300 x 300 DPI)


Intermediate Species Revealed during Sulfidation of Bimetallic

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