Al2O3 Catalyst

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The role of citric acid in preparing highly active CoMo/Al2O3 catalyst: From aqueous impregnation solution to active site formation Jianjun Chen, Jinxing Mi, Kezhi Li, Xiqin Wang, Elizabeth Dominguez Garcia, Yanning Cao, Lilong Jiang, Laetitia Oliviero, and Francoise Mauge Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b02877 • Publication Date (Web): 14 Nov 2017 Downloaded from http://pubs.acs.org on November 14, 2017

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Industrial & Engineering Chemistry Research

The role of citric acid in preparing highly active CoMo/Al2O3 catalyst: From aqueous impregnation solution to active site formation Jianjun Chen1,2, Jinxing Mi1, Kezhi Li2, Xiqin Wang1, Elizabeth Dominguez Garcia3, Yanning Cao1, Lilong Jiang1*, Laetitia Oliviero3*, Françoise Maugé3 1. National Engineering Research Center of Chemical Fertilizer Catalyst, School of Chemical Engineering, Fuzhou University, Fuzhou 350002, China 2. School of Environment, Tsinghua University, Beijing 100084, China 3. Laboratoire Catalyse et Spectrochimie, ENSICAEN, Université de Caen, CNRS, 6 bd du Maréchal Juin, 14050 Caen, France Corresponding: [email protected]; [email protected];

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ACS Paragon Plus Environment


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Abstract: In this work, the role of citric acid (CA) in preparing CoMo/Al2O3 catalyst was investigated from aqueous impregnation solution to catalytic active site formation by laser Raman, UV-vis, in-situ IR and mass spectrometry characterizations. COS and CO were employed as probe molecules to illustrate the formation of active site during and after catalyst sulfidation, respectively. Citric acid mainly coordinates with Mo, resulting in the presence of Mo-CA complex,

which consequently enlarge

the temperature window of MoS2 formation. Parallel between in-situ IR and activity test suggests that citric acid addition increases the concentration of CoMoS active site without significantly changing its intrinsic activity. It is concluded that the essential role of CA in preparing CoMo/Al2O3 catalyst is mainly ascribed to a tuning effect on the 2D morphology of MoS2 slabs, resulting in MoS2 exposes more S-edge to accommodate cobalt atom to form more CoMoS site. Keywords: Citric acid; Hydrodesulfurization; MoS2; CoMoS; Complex.

2


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1

Introduction The sulfur in fuels transforms into SOx after engine combustion and results in a

heavy pollution around the world. The worldwide demand for clean fuels leads to increasing efforts to enhance the catalytic performance of hydrodesulfurization (HDS) catalysts, which are widely used in modern industrial refineries to remove sulfur from the oil feedstock 1. Mixed CoMo sulfides supported by Al2O3 are among the most important HDS catalysts used in oil refineries. The surface structure of CoMo catalysts can be depicted by the so-called Co-Mo-S model developed by Topsøe and co-workers

2, 3

, in which the catalytic active sites are proposed to be the CoMoS sites

formed by the decoration of Co atoms on the edge of 2D MoS2 slabs. Besides MoS2 and CoMoS phase, several other phases, such as Mo oxysulfide species (MoOxSy), CoSx clusters, and CoAl2O4-like species, are also present on the catalysts surface, which are generally believed to be not active in HDS reaction 4-7. According to the Co-Mo-S model, intensive research efforts have been made towards increasing the concentration and/or intrinsic activity of CoMoS site to improve the catalytic performance of CoMo catalyst. The use of citric acid (CA) has been well documented to be an effective method of preparing highly active CoMo catalyst. An improvement of HDS activity after CA addition was widely reported in the literature 8-12, and an increase of MoS2 dispersion by CA addition was generally proposed to explain the improvement of catalytic activity

10, 12-14

. Recently, Castillo-Villalón and co-workers

15

reported with low

temperature CO adsorption followed by IR spectroscopy that CA addition increases 3



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the number of CoMoS sites on sulfide CoMo/Al2O3, but detailed relationship between CA addition and CoMoS formation was not discussed. To explain the origin of the beneficial effect of CA, Kubota et al.16 investigated the sulfidation process by X-ray absorption near edge structure (XANES) and ascribed the selective formation of CoMoS sites to a prior or parallel sulfidation of Mo comparing with the sulfidation of Co. In another manner, Klimov and co-workers

17

proposed that citrate coordinates

with both Co and Mo to form a bimetallic Co-Mo complex, which will increase the proximity of Co and Mo and thus favors the formation of CoMoS sites. A careful analysis of the above-mentioned studies

16, 17

indicates that both propositions were

made based on the prerequisite that Co2+ cation coordinate with CA in the aqueous solutions. However, several studies

18-20

found that Co2+ only coordinates with citrate

ligands to form Co-CA complex in neutral or basic solutions, whereas dissolving CA in aqueous solutions generally results in an acidic solution (the pKa1 of CA is 3.15). Therefore, the role of CA in the formation of catalytic CoMoS sites has not been clearly illustrated. Previously we demonstrated

21

that an important role of CA in preparing

Mo/Al2O3 catalyst is that CA can progressively change the 2D morphology of MoS2 slabs from a triangle-like shape with mainly Mo-edge to a hexagon exposing both Mo-edge and S-edge. In this work, we extend our study to go deeper insight into the role of CA in preparing CoMo/Al2O3 catalyst. For this purpose, a series of CoMo/Al2O3 catalysts were prepared with variable amounts of CA by impregnation method. The Mo and Co complexes in aqueous impregnation solutions as well as on 4


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the surface of dried catalysts were analyzed respectively by laser Raman and UV-vis spectroscopies. The formation of CoMoS sites during catalyst sulfidation were in-situ monitored by two probe molecules: COS and CO, while the catalytic activity of the CoMoS sites was evaluated in thiophene HDS reaction. Finally, comprehensive analysis of characterizations from aqueous solution to active CoMoS site formation allows concluding the essential role of CA in preparing CoMo/Al2O3 catalyst and pointing out the design strategy for highly active CoMo-based HDS catalysts. 2 2.1

Experimental Preparations of aqueous solutions and solid catalysts The aqueous impregnation solutions, which are denoted as CoMo(CA/M=x) (x

refers to the molar ratio of CA/M, (M=Co+Mo)), were prepared by dissolving variable amounts of citric acid (C6H8O7.H2O) and fixed amounts of ammonium heptamolybdatetetrahydrate ((NH4)6Mo7O24.4H2O) and cobalt (II) nitrate hexahydrate (Co(NO3)2.6H2O) in deionized water at room temperature. The composition and pH of the solutions are shown in Table 1. Note that the pH of the solutions were tested directly after the preparation without further adjustment. All the reagents was employed without further purification. Table 1: Composition and pH of the CoMo(CA/M=x) aqueous solutions Aqueous Solutions

Mo (mol.L-1)

Co (mol.L-1)

Citric acid (mol.L-1)

pH

CoMo(CA/M=0)

1.60

0.69

0

4.40

CoMo(CA/M=0.5)

1.60

0.69

1.14

0.16

CoMo(CA/M=1.0)

1.60

0.69

2.29

0.12

CoMo(CA/M=2.0)

1.60

0.69

4.57

0.01

The series of CoMo catalysts, which are referred as CoMo(CA/M=x)/Al2O3, were prepared by the classic one-step pore volume impregnation. The Al2O3 support 5



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(SASOL, specific surface area: 252 m2.g-1; pore volume: 0.84 mL.g-1) was added into the as-prepared CoMo(CA/M=x) solutions and matured for 2 hours with strong shake. To maintain the citric acid in its initial form, the catalysts were only dried for 4 hours at 383 K without further calcination. Hereinafter, the x refers to the molar ratio of CA/M (M=Co+Mo) (x=0, 0.5, 1.0, and 2.0). For all the CoMo(CA/M=x)/Al2O3 catalysts, the content of Mo was fixed with 3.2 Mo atoms per nm2 and the Co/(Co+Mo) ratio was equal to 0.3. 2.2

Laser Raman and UV-vis characterizations Laser Raman spectra of the aqueous CoMo(CA/M=x) solutions and

CoMo(CA/M=x)/Al2O3 catalysts were recorded on a Renishaw InVia Reflex spectrometer. The laser wavelength is 532 nm (500 mW) and the intensity is 1%. The spectrum resolution is 0.65 cm-1. The Raman spectroscopy was calibrated before each experiment with Si wafer showing 0 and 520 nm lines. The UV-vis spectra of the aqueous solutions were collected on a UV-Lambda950 spectrophotometer in the range of 200-800 nm. The absorption mode was used for the solutions with deionized water as the reference. 2.3

H2-TPR characterizations The H2-TPR characterizations were fulfilled on a Micromeritics Autochem 2920.

First, around 50 mg of dried catalyst (precisely weighted) was pre-treated in He for 30 min at 300 ℃ (ramping rate of 3 ℃.min-1) . Then the catalyst was heated in a 10 mL.min-1 10% (v/v) H2/Ar fluent from room temperature to 1000 ℃ with 10 ℃.min-1. The consumption of H2 was analyzed by a TCD detector. The carrier 6


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gas flows through flows through a cold trap to wipe off the water before going through the TCD detector. 2.4

Low temperature CO adsorption followed by IR characterization (IR/CO) IR/CO characterizations were fulfilled on a homemade cell called CellEx. A

detailed description of CellEx was reported in our previous work22. (1) Catalyst sulfidation. First, the catalyst was made into a self-supporting pellet about 8 mg.cm-2 (precisely weighted) and put into the reactor. Second, the reactor was evacuated by a mechanical vacuum pump. Then, the catalyst pellet was sulfided at 350 ℃ (3 ℃.min-1) and 0.1 MPa under a 30 mL.min-1 10%H2S/H2 for 2 h. After that, the reactor was flushed by Ar for 30 min. Finally, after cooled down to room temperature under Ar, the sulfided pellet was transferred to the IR cell without contacting with air through a transfer connection. (2) Low temperature CO adsorption and IR characterization. Before CO adsorption, the sulfided pellet was firstly evacuated (10-3 Pa) at 350 ℃ for 1 h. Then, the IR cell was immerged into liquid nitrogen for CO adsorption. After that, small calibrated doses of CO at variable pressures was introduced and finally 133 Pa CO at equilibrium was reached in the IR cell. The IR spectra after CO adsorption were taken by a Nicolet spectrometer equipped with a MCT detector. For comparison, all the spectra in this work were normalized to a sulfided pellet of 5 mg.cm-2. (3) IR spectrum decomposition and sulfide site quantification. To calculate the concentration of sulfide sites on the catalysts, the IR/CO spectra obtained at 133 Pa CO equilibrium were further decomposed using the Peakfit V4.12. Details of 7



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spectrum decomposition were reported in Refs.13, 21. The concentration of MoS2 edge sites and CoMoS site were calculated after spectrum decomposition with the following equation using the CO molar extinction coefficients on unprompted MoS2 and CoMoS site measured in our previously studies 13, 23, which are 20±3 cm.µmol-1 and 43±12 cm.µmol-1, respectively. n=

஺×ௌ ఌ×௠

where: n: the concentration of sulfide site on which CO adsorbs (µmol.g-1); A: the obtained CO adsorption band area (cm-1); S: the surface area of the catalyst pellet (cm2); ε: the CO molar extinction coefficient on the sulfide site (cm.µmol-1); m: the mass of the catalyst pellet after sulfidation (g). 2.5

COS (Carbon oxide sulfide) conversion monitored by mass spectrometry Our previous study

24

has demonstrated that the conversion of COS, which was

presented as an impurity in the H2S/H2 mixture, allows determining the temperature windows of the sulfide phase formation. In this work, when the catalyst pellets was sulfided in the reactor of CellEx, the outlet gases were measured an OmniSTAR (Pfeiffer) mass spectrometer. The catalyst sulfidation procedure is the same as that in IR/CO characterization. The evolutions of COS (m/z = 60), CO (m/z = 28) and CO2 (m/z = 44) were recorded. Note that the species of H2O, H2, H2S and citric acid were also recorded during sulfidation but their evolution will not be reported further in this word because they do not provide relevant information. More details of COS 8


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conversion characterization were reported in Ref. 24. 2.6

TEM characterization of the sulfided catalyst The TEM characterization was executed on a JEOL 2010 FEG operated at 200

kV. The sulfided catalysts were dispersed into ethanol under the protection of Ar after sulfidation without exposure to the air. All the images were taken and digitized with a 2k x 2k CCD camera. The analysis of TEM images was done with the commercial software of GATAN (DIGITALMICROGRAPH). At least 500 stacked MoS2 units each catalyst was manually measured to determine the length and stacking of MoS2 slabs. 2.7

Thiophene hydrodesulfurization (HDS) test The thiophene HDS tests were fulfilled in a plug flow reactor. About 50 mg

catalysts (20 - 40 meshes, precisely weighted) were firstly sulfided with 30 ml.min-1 10% (v/v) H2S/H2 at 350 ℃ (3 ℃.min-1) for 2 hours. Then, the catalytic activity of thiophene HDS was test at 350 ℃ using a 90 ml.min-1 gas mixture consisting of 7.9% thiophene, 90.0% H2 and 2.1% H2S. In all tests, the concentrations of 1-butene, butane, cis-2-butene, trans-2-butene, tetrahydrothiophene (THT) and thiophene in the outlet gas was measured by a Varian 3900 chromatograph using a FID detector. In order to get a differential reactor, the conversion of thiophene is controlled below 10%. The reaction rate (mol. h-1. kg-1) was calculated using the formula as follows: r=(x×F)/m, in which x, F, and m are the thiophene conversion (%), molar flow rate of thiophene (mol.h-1) and mass of the catalyst after sulfidation, respectively.

9



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3

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Results

3.1

The Co and Mo complexes in CoMo(CA/M=x) aqueous solutions

Figure 1: (A) The Raman spectra of the CoMo(CA/M=x) aqueous impregnation solutions; (B) The Raman shift of the main Mo=O band vibration of Mo species in the solution.

Laser Raman Spectroscopy (LRS) characterizations were carried out to identify the Mo complex in the CoMo(CA/M=x) aqueous solutions. As shown in Figure 1(A) all the sample show a Raman peak at 1045 cm-1, being due to the NO3- species from Co(NO3)2 precursor 25. The CoMo(CA/M=0) solution shows a strong peak at 941 cm-1, a shoulder peak at 896 cm-1, and weak peaks at 358 and 212 cm-1. The former two peaks can be assigned to the vibrations of Mo=O bands, while the latter two is attributed respectively to the vibrations of Mo-O-Mo and O-Mo-O bands peaks are characteristics of Mo7O246- species

17

. These

25

, which is in full accordance with

previous report that Mo7O246- is the main species in the aqueous solution of ammonium heptamolybdatetetrahydrate without adjusting the pH of solution 25. With citric acid addition, the Raman spectrum of CoMo(CA/M=0.5) shows the main peak at around 944 cm-1, accompanied by two shoulders at 900 and 861 cm-1, as well as three weak peaks at 547, 384 and 212 cm-1. Accordingly to the literature 17, 25,

10


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these Raman peaks correspond to the characteristic vibrations of [Mo4(Hcitrate)2O11]4complex. Comparing CoMo(CA/M=0.5) with CoMo(CA/M=0), the an upward shift of the main Raman peak was observed, which is due to the larger aggregate of [Mo4(Hcitrate)2O11]4- complex than Mo7O246- species

26

.

With increasing the

addition of citric acid, the Raman spectra of CoMo (CA/M=0.5, 1.0 and 2.0) are rather similar, indicating the same Mo-CA complex in these solutions. Nevertheless, a deliberately analysis of the spectra reveals that the position of the main peak corresponding to symmetric Mo=O stretching is gradually downward shifted with the increase of citric acid amount (Figure 1(B)). This is due to a lower carboxyl group deprotonation of the citrate with the decrease of pH values of the solutions (Ref.

25

and Table 1).

Figure 2: The UV-vis spectra of the CoMo(CA/M=x) aqueous impregnation solutions

UV-vis spectroscopy was applied to study the Co complexes in the aqueous solutions. As shown in Figure 2, the UV-vis spectrum recorded on CoMo(CA/M=0) solution presents an asymmetric absorption band centered at 511 nm with a shoulder at 462 nm, which can be assigned to the 4T1g→4T1g (P) transition of [Co(H2O)6]2+ 11



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complex

Page 12 of 31

20

. The UV-vis spectra of CoMo(CA/M=0.5, 1.0, 2.0) solutions are rather

similar to that of CoMo(CA/M=0) solution, except that the band intensity gradually decreases. The complexation between Co2+ and citric acid in aqueous solutions was widely studied in the literature 20, 27-29. In brief, under acidic conditions protonation of the citrate carboxylate groups is almost complete, and no complexation takes place between Co2+ and citrate, resulting in [Co(H2O)6]2+ complex mainly present. At a pH higher than 5, the citrate carboxylate groups are totally deprotonated and [Co(Hcitrate)(H2O)3]- or [Co(Hcitrate)2]4- complexes will be formed depending on the Co:CA

ratios.

The

UV-vis

band

positions

of

[Co(Hcitrate)(H2O)3]-

and

[Co(Hcitrate)2]4- were reported respectively at 513 and 509 nm 29, which are close to that of the [Co(H2O)6]2+ complex. However, the extinction coefficients of [Co(citrH)(H2O)3]- and [Co(citrH)2]4- are respectively 9.2 and 13.4 M-1 cm-1, both of which are much bigger than that of [Co(H2O)6]2+ (4.4 M-1 cm-1) 29. In the absorption mode of UV-vis measurements presented in Figure 2, the band intensities of Co-complexes should be directly proportional to the extinction coefficients because the Co concentrations are the same in all the solutions. A remarkable increase of band intensity should be observed if Co-CA complex was formed in the aqueous solution, which was not observed in our experiment. Such result suggests that the Co-CA complex should not formed in the acidic solutions used in this work.

12


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3.2

The characterizations on dried CoMo(CA/M=x)/Al2O3 catalysts

Figure 3: (A) The Raman spectra of dried CoMo(CA/M=x)/Al2O3 catalysts; (B) The Raman shift of the main Mo=O band vibration of Mo species.

Laser Raman spectroscopy was further used to characterized the Mo species on the

dried

CoMo(CA/M=x)/Al2O3

catalysts.

As shown

in

Figure

3,

the

CoMo(CA/M=0)/Al2O3 catalyst shows a main peak at 943 cm-1, a shoulder peak at 900 cm-1, and weak peaks at 550, 345 and 216 cm-1. From previous studies

10, 28, 30

,

these peaks can be ascribed to the Anderson-type [Al(OH)6Mo6O18]3- species. The formation of these species was proposed to involve the Mo-assisted dissolution of Al2O3 support in molybdate solution, which was discussed in detail by Carrier et al. 31. After citric acid addition, the main Raman peak of CoMo(CA/M=0.5)/Al2O3 catalyst is upward shifted to 945 cm-1, which was also observed in the solutions (Figure 1) and can be attributed to the larger aggregate of Mo-citrate than [Al(OH)6Mo6O18]3- species 26

. Meanwhile, a shoulder at 859 cm-1 and a weak peak at 380 cm-1 were detected.

These Raman vibrations are similar as these of CoMo(CA/M=0.5) aqueous solution (Figure 1(A)), suggesting the similar [Mo4(Hcitrate)2O11]4- complex is present on the catalyst surface. With increasing the amount of citric acid, the position of the main Raman peak is gradually downward shifted (Figure 1 (B)), indicating the lower 13



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deprotonation of free carboxyl group in citric acid, which is consistent with the observation on aqueous solutions. The Raman result in Figure 3 indicates that the impregnation of Al2O3 support with CoMo(CA/M=0.5/1.0/2.0)/Al2O3 solutions did not destroy the main structure of Mo-CA complex formed in the aqueous solution, and the Mo-CA complex was well-conserved on the dried catalyst surface.

Figure 4: H2-TPR profiles of the dried CoMo(CA/M=x)/Al2O3 catalysts.

H2-TPR characterizations were performed to explore the effect of citric acid on the reducibility of Mo species on dried CoMo(CA/M=x)/Al2O3 catalysts. As shown in Figure 4, the reduction of CoMo(CA/M=0)/Al2O3 catalyst requires hydrogen consumption in a broad temperature range from ~350 to 950 ℃ with three main reduction peaks at 436, 577 and 812℃ (denoted as peak LT, MT and HT according to the low, middle and high temperature ranges). According to the literature

11, 32

, the

peaks on CoMo(CA/M=0)/Al2O3 catalyst below 700 ℃ can be attributed to the reduction of Mo from Mo(VI) to Mo(IV) in the polymeric Mo species, while the peak higher than 700 ℃ is associated to the complete reduction of polymeric Mo from Mo(IV) to metallic Mo and the reduction of tetrahedral Mo species. As the amount of citric acid increases, the peak-LT shifts toward higher temperature from 436 to 459 ℃ along with a gradual decrease of the peak area. The peak-MT also shifts toward higher temperature from 577 to 641 ℃, but the peak area stepwise increases. The peak-HT also shifts to higher temperature with the peak area gradually decreases with 14


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CA addition, which totally disappears on CoMo(CA/M=2.0)/Al2O3 catalyst. The gaseous molecules produced in the decomposition of Mo-CA complex were probably responsible for the negative peaks in the region of 500-600 ℃ for the catalysts prepared with citric acid. This is supported by the fact that the negative peaks disappears in the H2-TPR profiles of the CoMo(CA/M=0)/Al2O3 catalyst. The shifts of peak positions and the variation of peak area in Figure 4 suggests that citric acid addition gives a convincing effect on the reduction of Mo species. The upward shifts of H2 consumption peaks indicate the starting reduction of Mo species becomes difficult after the Mo-CA complex formation, whereas the increase of peak area below 700 ℃ suggests that more Mo species are reduced at relative lower temperature. 3.3

COS conversion monitored by mass spectrometry in catalyst sulfidation

Figure 5: Evolution of mass spectrometry signals of COS, CO and CO2 during sulfidation of (A) CoMo(CA/M=0)/Al2O3 and (B) CoMo(CA/M=2)/Al2O3 catalysts with 10% (v/v) H2S/H2 at atmospheric pressure.

The influence of citric acid addition on the formation of sulfide phase was investigated by the probe reaction of carbon oxide sulfide (COS) conversion. Our previous study 24 has illustrated that the conversion of COS, present as an impurity in the H2S/H2 flow, allows determining the temperature window of the sulfide phase formation. As shown in Figure 5(A), three stages can be identified according to the evolution of COS during the sulfidation of CoMo(CA/M=0)/Al2O3 catalyst. Stage (1): 15



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when T < T1 (T1 = ca. 195 ℃), the concentration of COS stays almost constant, indicating no MoS2 or CoMoS sulfide phase are formed. Stage (2): in the temperature range of [T1, T2] (T2 = ca. 220 ℃), the COS concentration decreases sharply with the increase of temperature. Meanwhile, CO as well as small amount of CO2 is generated. The production of CO2 decrease quickly after a short time. According to our previous study 24, this stage corresponds to the formation of sulfide sites. The decrease of COS concentration is mainly ascribed to the desulfurization of COS on the sulfide phase (COS + H2 = CO + H2S). Meanwhile, small amount of COS converts through hydrolysis reaction (COS + H2O = CO2 + H2S) which produces small amount of CO2 in a small temperature range. Stage (3): When T > T2, the conversion of COS becomes constant with CO the sole product. In this stage, the formation of sulfide active phase is finished. During the sulfidation of CoMo(CA/M=2)/Al2O3 catalyst, as shown in Figure 5(B), three stages can also be identified according to the evolution of COS molecules. However, the T1 temperature at which COS concentration starts to decrease is slightly shifted to higher temperature (T1 = ca. 200 ℃), indicating the formation temperature of sulfide sites becomes higher after citric acid addition. This is consistent with the increase of activation energy of Mo species reduction as shown by H2-TPR results. More importantly, the T2 temperature at which full conversion of COS is obtained is remarkably shifted towards higher temperature (T2 = ca. 280 ℃). Therefore, the temperature window of active site formation on CoMo(CA/M=2)/Al2O3 catalyst is from 200 to 280 ℃, which is remarkably wider than that on CoMo(CA/M=0)/Al2O3 16


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catalyst (from 195 to 220 ℃). It is also noticeable that the MS signals of CO and CO2 are much higher than those on CoMo(CA/M=0)/Al2O3 catalyst, which is resulted from the decomposition of citric acid under H2-rich condition. 3.4

IR/CO characterizations of sulfided CoMo(CA/M=x)/Al2O3 catalysts

Figure 6: (A) IR spectra of CO adsorption (-173 ℃ and 133 Pa CO at equilibrium) on CoMo(CA/M=x)/Al2O3 catalysts sulfided with 10% (v/v) H2S/H2 at atmospheric pressure and 350 ℃ for 2 hours ; (B) Site Concentrations of sulfide sites detected by CO on sulfided CoMo(CA/M=x)/Al2O3 catalysts

Low temperature (liquid nitrogen temperature) CO adsorption followed by in-situ IR spectroscopy (IR/CO) is a well-established technique to probe the MoS2 and CoMoS sites on the sulfided CoMo catalyst 13, 21-23, 33-36. IR/CO technique was used in this work to inquire the effect of citric acid on the concentration of sulfide sites on CoMo(CA/M=x)/Al2O3 catalysts after sulfidation. Figure 6 (A) reports the IR/CO spectra obtained on the series of CoMo(CA/M=x)/Al2O3 catalysts. The IR/CO spectra are in accordance with our previous observations on the CoMo catalyst prepared with other chelating agents such as ethylene diaminotetraacetic acid (EDTA) triacetic acid (NTA)

37

and nitrilo

38

. The two bands at around 2189 and 2156 cm-1 are ascribed

respectively to CO interaction with Lewis acid sites and hydroxyl groups of the Al2O3

17



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support

Page 18 of 31

35, 36

. The strong band at around 2072 cm-1 is attributed to the CO adsorption

on Co-promoted MoS2 edges (the so-called CoMoS sites), while the small shoulder at 2108 cm-1 is attributed to CO adsorption on unprompted MoS2 sites 35, 36. It was clearly recorded in Figure 6 (A) that the CO adsorption band at 2072 cm-1 steadily increases with CA/M ratio, demonstrating that citric acid addition favors the formation

of

CoMoS

sites.

The

spectra

obtained

on

the

sulfide

CoMo(CA/M=x)/Al2O3 catalysts were further decomposed with Peakfit V4.12, and the concentrations of sulfide sites was calculated (more details in experimental section). As shown in Figure 6 (B), the CoMoS site concentration steadily increases with the addition of citric acid from 40 µmol.g-1 on CoMo(CA/M=x)/Al2O3 catalyst to 120 µmol.g-1 on CoMo(CA/M=2)/Al2O3 catalyst. Meanwhile, the amount of unprompted MoS2 is around 20 µmol.g-1, which slightly decreases with citric acid addition. 3.5

TEM analysis of the sulfided CoMo(CA/M=x)/Al2O3 catalysts

Figure 7: TEM results of the slab length (A) and slab stacking (B) on CoMo(CA/M=0)/Al2O3 and CoMo(CA/M=2)/Al2O3 catalysts sulfided by 10% H2S/H2 at 350 ℃ and 0.1 MPa

HRTEM was used to assess the effect of citric acid addition on the MoS2 slab

18


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length and stacking on sulfided CoMo(CA/M=x)/Al2O3 catalysts. As shown in Figure 7 (A), the citric acid addition slightly increases the MoS2 slab length. The average slab length on CoMo(CA/M=0)/Al2O3 was calculated to be 2.84 nm, while it increases to 3.09 nm on CoMo(CA/M=2)/Al2O3. The increase of slab length may be stemmed from a weaker MoS2-support interaction after citric acid. However, such effect on the slab-support interaction could also be modulated by others effects such as isolating effect as reported by van Dillen et al.

28

. Meanwhile, the slab stacking is

also modified by citric acid addition as displayed in Figure 7 (B). Compared with CoMo(CA/M=0)/Al2O3 catalyst,

CoMo(CA/M=2)/Al2O3 sample

presents less

monolayer and double-layer MoS2 slabs with an increase frequency of triple-layered ones.

The

average

slab

stacking

on

CoMo(CA/M=0)/Al2O3 catalyst

and

CoMo(CA/M=2)/Al2O3 were estimated to be 1.17 and 1.19, respectively. 3.6

Thiophene HDS activity of sulfided CoMo(CA/M=x)/Al2O3 catalysts

Figure 8: Catalytic activity of sulfided CoMo(CA/M=x)/Al2O3 catalysts in thiophene HDS reaction. Test condition: T = 350 ℃, P = 0.1 MPa, differential condition.

The catalytic performance of sulfided CoMo(CA/M=x)/Al2O3 catalysts was 19



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evaluated by thiophene HDS reaction. As shown in Figure 8, citric acid addition shows a promoting effect on the catalytic activity of CoMo catalysts in thiophene HDS reaction. The thiophene HDS rate gradually increases from 17 mol.h-1. kg-1 on CoMo(CA/M=0)/Al2O3 to 42 mol.h-1. kg-1 on CoMo(CA/M=2.0)/Al2O3 catalyst. Dugulan et al.

39

reported that the thiophene HDS rate was around 39 to 86 mol.h-1.

kg-1 on carbon-supported CoMo catalysts. Considering that the Mo loading is around 1.2 mol Mo per kilogram catalyst in this work, the measured kinetic data is comparable with the results reported in the literature

39

. Moreover, the improvement

of catalytic activity in thiophene HDS reaction consists with previous observations on HDS of dibenzothiophene and 4,6-dimethyldibenzotiophene

11, 15

, indicating that the

addition of citric acid has a beneficial effect on the catalytic performance of CoMo catalyst in HDS of various sulfur-containing molecules.

Figure 9: Relationship between CoMoS site concentration and thiophene HDS rate measured under differential condition at 350 ℃ and 0.1 MPa.

In order to demonstrate the effect of citric acid addition on the intrinsic activity of CoMoS sites, the thiophene HDS rate was plotted against the CoMoS site concentration probed by low tempertature CO adsorption. As shown in Figure 9, a 20


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linear relationship is obtained between the HDS rate and CoMoS site concentration. The slope of the line is 246 h-1, corresponding to the turnover frequency (TOF) of thiophene molecules on CoMoS sites. The TOF of CoMoS sites measured in this work is comparable with the result of Lelias et al. 38, which is 435 h-1 for the CoMo catalyst prepared with another chelating agent of nitrilo triaceticacid (NTA) and around ten times higher than the ones obtained for unprompted MoS2 sites

21

in

accordance with the well-known promoting effect of Co. The correlation analysis in Figure 9 suggests that citric acid addition on CoMo/Al2O3 catalyst increases the CoMoS site concentration without changing significantly its TOF value. It should be noted that the interception of the linear relationship between HDS activity and CoMoS site concentration is 7.2 mol.h-1. kg-1. Considering the low concentration of unprompted MoS2 sites in Figure 6 (B), the contribution of this activity may origin not only from the unprompted MoS2 sites but also from other species on sulfided CoMo/Al2O3 catalysts such as small sulfide Co clusters in close contact with MoS2 as proposed in the literature 38, 40. 4 4.1

Discussion The complexation between citric acid and Co, Mo cations The Raman results in this work (Figure 1 and Figure 3) clearly demonstrated that

the use of citric acid as chelating agent results in the presence of Mo-CA complex on the surface of dried CoMo(CA/M=x)/Al2O3 catalysts. It is well established that the Mo-CA complex formation can avoid the re-agglomeration of Mo during catalyst sulfidation and thus increase the dispersion of Mo on the sulfided catalyst 21


10, 12-14, 21

.


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Moreover, the formation of Mo-CA complex should also result in a beneficial effect on the sulfide phase formation, since the decomposition of COS (carbon oxide sulfide) during catalyst sulfidation followed by mass spectrometry illustrates that the temperature window of sulfide phase formation is significantly enlarged after citric acid addition (Figure 5). To our best knowledge, this is the first report on such a role of citric acid addition on CoMo/Al2O3 catalyst. It is proposed (as further discussed in Section 4.2) that such effect of citric acid will be close related to a morphology change of MoS2 slabs. By contrast, the UV-vis and laser Raman spectroscopy characterizations did not provide clear evidence of Co-CA complex formation in CoMo(CA/M=0.5, 1.0 and 2.0) impregnation solutions. Previous study

27

showed that citrate ligands only

coordinate with Co2+ when pH > 4.5. The result in Figure 2 demonstrated that both the UV-vis band position and intensity are similar for all the CoMo(CA/M=x) aqueous solutions with different amounts of citric acid, indicating that probably Co(H2O)62+ is the main Co species in these solutions. Using Co(CH3COO)2 as cobalt precursor, Klimov and co-workers 17, 41 reported that a Co2Mo4-CA complex was formed both in the solution and on the dried CoMo/Al2O3 catalyst, which was proposed to increase the proximity of Co and Mo and thus favors the formation of CoMoS active sites 17, 41. According to their study13,

40

, two Co cations can coordinate with one

[Mo4O11(C6H5O7)2]4- anion via terminal oxygen and carboxyl groups, thus decreasing the NMR signals of Mo=O and –COO- groups. In our work, the NO3- in the CoMo(CA/M=x) solutions, which origins from the Co(NO3)2 precursor, can be used 22


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as an internal standard to normalized the Raman intensities of ν(Mo=O)

25

. As

displayed in Figure 10, the normalized Raman intensity of ν(Mo=O) is increased after citric acid addition rather than decreased. The increase of Raman intensity of ν(Mo=O) can be tentatively ascribed to the decrease of degree of polymerization of Mo species (Mo7O246- VS [Mo4(Hcitrate)2O11]4-). Nevertheless, such results suggest that in our work Co2Mo4-CA complex were probably not formed in the aqueous solutions.

Figure 10: Comparison of Raman spectra of CoMo(CA/M=x) solutions (the NO3- species from Co(NO3)2 precursor is used as an internal standard and its Raman intensity was normalized to be identical for all the solutions as done in Ref. 25)

4.2

The role of citric acid in the formation of CoMoS active sites Although the Raman and UV-vis spectra in this work suggest that citrate ligand

coordinates with Mo rather than with cobalt as discussed in 4.1, the IR/CO results (Figure 7) clearly display that citric acid addition favors the formation of CoMoS sites. To date, two propositions exist in the literature to explain the positive role of citric acid on the formation of CoMoS sites. At the first place, Klimov and co-workers

17

proposed that citrate coordinates with both Co and Mo to form a bimetallic Co2Mo4-CA complex, which will increase the proximity of Co and Mo and thus

23



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favors the formation of CoMoS sites. By contrast, Kubota et al.

Page 24 of 31

16

investigated the

sulfidation process by XANES and ascribed the selective formation of CoMoS sites to the prior or parallel sulfidation of Mo comparing with the sulfidation of Co. Based on the results in this work, it seems that neither of the propositions can explain satisfyingly the role of citric acid in the formation of CoMoS site observed in this work. According to the Co-Mo-S model2, 3, the CoMoS sites are formed by the decoration of Co on the edges of 2D MoS2 slabs. Further study

42

established that

MoS2 slabs expose generally two kinds of edges: the so-called Mo-edge and S-edge. Density functional theory (DFT) calculations

43-45

suggested that the formation of

CoMoS sites is energetically more favorable on the S-edge than on the Mo-edge. Such result was further confirmed in the study of Lauritsen et al. 3 with scanning tunneling microscopy (STM) on Au-supported MoS2 model catalyst. Therefore, it is expected that the formation of CoMoS sites is closely related to the morphology of MoS2 slabs, i.e., the different edge sites exposed by MoS2. A relationship between citric acid addition and MoS2 morphology was recently established in our study

21

. With IR/CO technique we illustrated that the 2D

morphology of MoS2 can be modified progressively from a triangle-like shape with mainly Mo-edge to a hexagon exposing both Mo-edge and S-edge by increasing citric acid addition on Mo/Al2O3 catalysts

21

. It was quantitatively demonstrated that the

addition of citric acid on Mo/Al2O3 catalysts can increase the amount of S-edge on MoS2 slabs by almost 3 times (from 13 to 37 µmol.g-1 as shown in Ref. 24


21

). In this

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work as shown in Figure 5 by the conversion of COS, the enlarged temperature window of sulfide phase formation after citric acid addition suggests that more MoS2 slabs are formed at higher temperature. In this manner, more S-edge should be formed on MoS2 slabs as we have shown in another work

22

that increasing sulfidation

temperature leads to MoS2 exposing more S-edge. Therefore, the COS conversion in this work also suggests that citric acid addition will result in MoS2 slabs exposing relatively more S-edge than Mo-edge on sulfided CoMo/Al2O3 catalysts. Accordingly it was observed that citric acid addition on CoMo/Al2O3 catalysts can increase the CoMoS concentration by a factor of ~ 3 in this work (38 to 128 µmol.g-1). Therefore, taking into account the DFT calculations previous studies

43-45

, STM study 3, and our

21, 22

, the results in this work allow us to propose the role of citric

acid in CoMoS site formation as that the complexation between citric acid and Mo in the aqueous solution and on dried catalysts results in MoS2 exposing more S-edge so that more CoMoS sites are formed on this edge during catalyst sufidation (Figure 11). Since the CoMoS sites are formed on the same edge of MoS2 slabs, such proposition explains the kinetic observation in Figure 9 that citric acid addition only increases the amount of CoMoS sites without remarkably changing their intrinsic activity. Such proposition also points out the importance of 2D morphology of MoS2 slabs in the formation of CoMoS catalytic active sites. Accordingly, a design strategy for highly active CoMo-based catalysts is to reasonably control the morphology of MoS2 nano-slabs.

25



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Figure 11: The role of citric acid in the formation of catalytic CoMoS active site on CoMo/Al2O3 hydrodesulfurization (HDS) catalyst

5

Conclusions In this work, the role of citric acid in preparing CoMo/Al2O3 catalyst was

thoroughly examined from aqueous solution to the active CoMoS site formation. Laser Raman and UV-vis spectroscopies demonstrated that Mo-CA complex is formed in the aqueous solutions and presented on the surface of dried CoMo/Al2O3 catalysts, whereas Co probably does not coordinate with citric acid during catalyst preparation. The H2-TPR characterization showed that the formation of Mo-CA complex increases the reduction temperature of Mo species, which results in a remarkably wider temperature window of CoMoS site formation as indicated by the mass spectrometry of COS conversion. Parallel between in-situ IR spectroscopy of low temperature CO adsorption and thiophene HDS test suggests that citric acid addition increases the concentration of CoMoS actives site without changing significantly its intrinsic activity. It is concluded that the essential role of citric acid in preparing CoMo/Al2O3 catalysts is mainly related to a tuning effect on MoS2 2D morphology, leading to MoS2 exposes more S-edge to accommodate cobalt atoms to form more CoMoS active site. The findings in this work highlight the importance of 26


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MoS2 morphology in the formation of CoMoS catalytic active site, pointing out that a general design strategy for highly active CoMo-based catalysts is to reasonably control the 2D morphology of MoS2 nano-slabs. Acknowledgement This work was supported by the Natural Science Foundation of China (No. 21606047 and No. 21576051), Natural Science Foundation of Fujian Province (2017J01415), National High-tech R&D Program (No. 2015AA03A402), and the National Key Research and Development Program (No. 2016YFC0203900). References (1) Topsoe, H.; Clausen, B. S.; Massoth, F. E., Hydrotreating catalysis. Springer: Berlin-Heidelberg, 1996. (2) Topsoe, H., The role of Co-Mo-S type structures in hydrotreating catalysts. Appl Catal. A.Gen.

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(40) Ramos, M.; Berhault, G.; Ferrer, D. A.; Torres, B.; Chianelli, R. R., HRTEM and molecular modeling of the MoS2-Co9S8 interface: understanding the promotion effect in bulk HDS catalysts.

Catal. Sci. Technol. 2012, 2, (Copyright (C) 2013 American Chemical Society (ACS). All Rights Reserved.), 164-178. (41) Klimov, O. V.; Pashigreva, A. V.; Bukhtiyarova, G. A.; Budukva, S. V.; Fedotov, M. A.; Kochubey, D. I.; Chesalov, Y. A.; Zaikovskii, V. I.; Noskov, A. S., Bimetallic Co-Mo complexes: A starting material for high active hydrodesulfurization catalysts. Catal. Today. 2010, 150, 196-206. (42) Lauritsen, J. V.; Kibsgaard, J.; Helveg, S.; Topsoe, H.; Clausen, B. S.; Laegsgaard, E.; Besenbacher, F., Size-dependent structure of MoS2 nanocrystals. Nat. Nanotechnol. 2007, 2, 53-58. (43) Byskov, L. S.; Norskov, J. K.; Clausen, B. S.; Topsoe, H., DFT calculations of unpromoted and promoted MoS(2)-based hydrodesulfurization catalysts. J. Catal. 1999, 187, 109-122. (44) Sun, M. Y.; Nelson, A. E.; Adjaye, J., On the incorporation of nickel and cobalt into MoS2-edge structures. J.Catal. 2004, 226, 32-40. (45) Schweiger, H.; Raybaud, P.; Toulhoat, H., Promoter sensitive shapes of Co(Ni)MoS nanocatalysts in sulfo-reductive conditions. J. Catal. 2002, 212, 33-38.

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