XPS study of a bulk WP hydrodesulfurization catalyst

Journal of Catalysis 352 (2017) 557–561

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Research Note

XPS study of a bulk WP hydrodesulfurization catalyst Xiang Li a,b,c, Song Tian a,b, Anjie Wang a,b,c,⇑, Roel Prins d, Congcong Li a,b, Yongying Chen a,b a

State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, No. 2 Linggong Road, Dalian 116024, PR China Liaoning Key Laboratory of Petrochemical Technology and Equipments, Dalian University of Technology, No. 2 Linggong Road, Dalian 116024, PR China c Penn State and Dalian University of Technology Joint Center for Energy Research (JCER), Dalian University of Technology, No. 2 Linggong Road, Dalian 116024, PR China d Institute of Chemical and Bioengineering, ETH Zurich, 8093 Zurich, Switzerland b

a r t i c l e

i n f o

Article history: Received 11 April 2017 Revised 15 June 2017 Accepted 17 June 2017

Keywords: WP Hydrodesulfurization XPS Sulfur-containing phase NH3

a b s t r a c t The X-ray photoelectron spectroscopy characterization of a bulk WP hydrodesulfurization (HDS) catalyst demonstrated that the actual active surface phase of WP in the HDS of dibenzothiophene (DBT) is a sulfur-containing phase. This tungsten phosphosulfide phase formed faster than the molybdenum phosphosulfide phase, but possessed a lower sulfur content than the corresponding Ni2P and MoP phases. NH3 was the dominant nitrogen-containing species detected in the WP catalyst after simultaneous HDS and hydrodenitrogenation reactions. NH3 decreased the DBT conversion strongly but less than piperidine, indicating that not only N-containing compounds but also NH3 inhibits the HDS activity of WP. Ó 2017 Published by Elsevier Inc.

1. Introduction The metal-rich transition-metal phosphides (e.g. Ni2P, MoP, and WP) have attracted increasing attention as a novel class of hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) catalysts, due to their high activity and stability under hydrotreating conditions [1,2]. They are covalent compounds and have metallic character [3,4], and thus are also promising for a variety of important reactions catalyzed by metals, such as hydrodeoxygenation, hydrogen evolution, and hydrogenation [5–8]. Unlike their metallic counterparts and noble metal catalysts, sulfur plays a positive role in some reactions over these transition-metal phosphides. For the working Ni2P catalysts, a nickel phosphosulfide has been established to be the actual active phase during HDS reactions. Duan et al. [9] reported that the HDS activity of a Ni2P/MCM-41 correlated well with the sulfur content of the spent catalyst. A density functional theory (DFT) calculation indicated a surface with 50% phosphorus replaced by sulfur and some atomic sulfur species deposited on the threefold hollow sites was an accurate representation for the actual active phase, or the so-called ‘‘phosphosulfide” surface, of Ni2P at typical hydrotreating conditions [10]. The formation of a Ni–S bond on the surface of the working Ni2P catalysts

⇑ Corresponding author at: School of Chemical Engineering, Dalian University of Technology, Dalian 116024, PR China. E-mail address: [email protected] (A. Wang). http://dx.doi.org/10.1016/j.jcat.2017.06.023 0021-9517/Ó 2017 Published by Elsevier Inc.

was observed directly by Kawai et al. [11] by means of in situ X-ray absorption spectroscopy. Through a combination of in situ quick X-ray absorption fine structure (QXAFS) and FT-IR, Bando et al. [12] concluded that a surface nickel phosphosulfide was the active phase of a Ni2P/MCM-41 catalyst. Recently, we obtained this nickel phosphosulfide phase by low-temperature reduction of a Ni2P2S6 precursor in H2, and found that the HDS activity and the selective hydrogenation performance of this sulfur-containing phase were superior to the bulk Ni2P catalyst prepared by using the conventional phosphate precursor [13]. The same holds for MoP. Both Phillips et al. [14] and Wu et al. [15] reported that the initial HDS activities of MoP/SiO2 catalysts increased gradually with reaction time. Wu et al. [15] proved that the surface of MoP/SiO2 was gradually sulfided during HDS, based on the shift of the IR band of CO adsorbed on the Mo sites of MoP at 2045 to ca. 2100 cm 1. Our previous results showed that new active sites, which possessed a higher direct desulfurization activity than the sites on the surface of fresh MoP catalyst, were formed during the HDS of dibenzothiophene (DBT) over bulk MoP [16,17]. The X-ray photoelectron spectroscopy (XPS) analysis of the spent bulk MoP catalyst suggested that the formation of this new active phase is due to the incorporation of sulfur in the surface of MoP [16]. Compared with Ni2P and MoP, the actual active surface of WP in HDS reactions has not been adequately studied yet. The catalytic properties of transition-metal phosphides depend on the metal and on the reactant. Oyama and co-workers reported that the overall activity of the transition-metal phosphides in the

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simultaneous HDS of DBT and HDN of quinoline increased in the order: Fe2P < CoP < MoP < WP < Ni2P [18,19]. Tungsten phosphide has the manganese phosphide structure [3]. It was found to be more active in HDS and HDN than tungsten carbide, tungsten nitride, tungsten sulfide, and more active in HDN than a commercial Ni-Mo-S/Al2O3 catalyst [3]. Previously, we studied the kinetics of the HDS of DBT, 4,6-dimethyldibenzothiophene (4,6-DMDBT), and their hydrogenated intermediates over bulk Ni2P [20,21], MoP [16], and WP [22] catalysts. The results demonstrated that WP possessed a higher hydrogenation/dehydrogenation activity than Ni2P and MoP. It is particularly interesting that the total HDS reactivity of 4,6-DMDBT over WP was higher than that of DBT, which makes WP attractive for deep HDS [22]. Nevertheless, WP is highly sensitive to nitrogen-containing compounds [22]. This explains that although WP was superior to Ni2P in the HDS of 4,6DMDBT in the absence of piperidine, its activity in the simultaneous HDS and HDN was lower than that of Ni2P [22]. In the present work, two bulk WP catalysts were characterized by XPS after the HDS of DBT, both in the presence and absence of piperidine, and compared with the fresh WP catalyst. Our aim is to study the working surface of WP and to better understand the inhibition effect of nitrogen-containing compounds on the HDS performance of transition-metal phosphides.

2. Experimental methods Tungsten phosphate was prepared by dropwise adding an aqueous solution of 1.58 g (NH4)2HPO4 (12.0 mmol) in 10 mL deionized water to a solution of 3.0 g (NH4)6W12O39
xH2O (1.0 mmol) in 15 mL deionized water under stirring. The resulting clear solution was stirred and water was evaporated, then dried at 120 °C for 12 h and calcined at 500 °C for 3 h to obtain the precursor, which had a molar composition of WO3
0.5P2O5. The HDS reactions were carried out in a stainless-steel tubular reactor (8.0 mm i.d.). The obtained precursor was pelleted, crushed, and sieved to 20–40 mesh. Prior to the HDS reaction, the precursor (0.10 g) was transformed into bulk WP by temperature-programmed reduction under 1.0 MPa in a 150 mL/ min H2 flow by heating from room temperature to 120 °C at a heating rate of 10 °C/min and keeping this temperature for 1 h, then heating to 400 °C at 5 °C/min, and finally heating at 1 °C/min to 650 °C and holding at 650 °C for 2 h. After the precursor had been converted to the active WP phase, the reactor was cooled to the reaction temperature of 340 °C and the total pressure was increased to 4.0 MPa. The feed consisted of 1 kPa DBT (J&K Scientific Ltd.), 0 or 0.2 kPa piperidine (Sinopharm Chemical Reagent Co., Ltd.), 165 kPa decalin (as solvent, Sinopharm Chemical Reagent Co., Ltd.), and about 3.8 MPa H2. Weight time [23] was defined as s = wcat/nfeed, where wcat is the catalyst weight and nfeed is the total molar flow to the reactor. The weight time was changed by varying the flow rates of the liquid and gas, while keeping their ratio constant. The XRD patterns of the catalysts were measured on a Rigaku D/ Max 2400 diffractometer with nickel-filtered Cu Ka radiation at 40 kV and 100 mA. The CO uptake of the fresh WP catalyst (determined as described elsewhere [22]) was 2.6 lmol/g. The XPS spectra were recorded on a Multilab 2000 X-ray photoelectron spectrometer, using an Al Ka source. For the individual energy regions, a pass energy of 20 eV was used. All binding energies were referenced to the C 1s peak at 284.6 eV. Before the characterization, the fresh WP sample was prepared according to the same reduction conditions as used in the in situ reduction, followed by a passivation with 0.5% (volume) O2 in Ar. The spent WP catalysts obtained in the absence and presence of piperidine were denoted as WP-S and WP-N, respectively. They were used directly for

XRD and XPS measurements after being flushed by n-heptane and dried with an Ar flow.

3. Results and discussion The XRD patterns of WP, WP-S, and WP-N (Fig. S1 in Supplementary Material) indicate that WP had formed by the reduction of the phosphate precursor, and was preserved after the HDS reactions. The XPS spectra of the three catalysts in the W 4f, P 2p, S 2p, and N 1s binding energy regions are illustrated in Fig. 1. The W 4f spectrum consists of two W 4f7/2 and W 4f5/2 peaks, due to spinorbit interaction, with a separation of 2.18 eV, an intensity ratio of 0.78, and equal width [24]. For the bulk WP sample (Fig. 1A), the major doublet located at 31.0 eV (W 4f7/2) and 33.1 eV (W 4f5/2) with a separation (2.1 eV) close to the theoretical one can be attributed to Wd+ species (0 < d  4), which is related to the tungsten species in the WP phase [25]. A doublet with a low intensity was detected at 35.5 and 37.5 eV (Fig. 1A). It is due to W6+, formed by exposure of the sample to air before XPS analysis [25]. The doublet in the P 2p region at 128.9 and 129.8 eV (Fig. 1B) shows that the P species in the surface of WP can be assigned to P bonded to W in the form of a phosphide [26]. Transition-metal phosphides are covalent compounds, in which the electron density is shared between the metal and phosphorus atoms [27]. Thus, the binding energy locations of W and P in WP are near the elemental values [25]. Both the XRD and XPS results demonstrate that the tungsten phosphate precursor was completely reduced to WP. The complete reduction of tungsten phosphate under similar preparation conditions has been proved by Oyama et al. by means of NMR and XAS [3]. The XPS element ratios of transition-metal phosphides may deviate from the real values if the difference in kinetic energies of the collected electrons for metals and P is large. One example is Ni2P, with XPS binding energies for Ni 2p and P 2p of 860 and 130 eV, respectively. The measured XPS Ni/P ratio of a stoichiometric Ni2P phase was near 1.0 rather than 2.0 [28]. However, in our case, this deviation may not be as significant as for Ni2P, because the XPS binding energies of W 4f, P 2p, and S 2p are all close to 100 eV (Fig. 1). Therefore, the relative atomic ratios of the surface elements (Table 1) are used for the qualitative and comparative study of the surface compositions of all the WP catalysts. The XPS P/W atomic ratio of WP was 0.85 (Table 1), which agreed well with our previous observations [22]. Sulfur was detected in the surface of the bulk WP catalyst after the HDS of DBT in the absence of piperidine (Fig. 1C). The S 2p peaks can be deconvoluted into two doublets (2p3/2 and 2p1/2): one doublet (161.8 and 163.0 eV) that arises from a sulfide species (S2 ), and the other doublet (162.6 and 163.7 eV) indicative of sulfur in a thiolate-type environment [26]. These sulfur species are similar to the ones found in the surface of a bulk MoP catalyst after the HDS of DBT [16] or a molybdenum phosphosulfide [26]. Hence, the actual active phase of the bulk WP catalyst in the HDS of DBT is also a sulfur-containing phase or a tungsten phosphosulfide phase, and the possibility that more than one kind of sulfur species might be present in this phase cannot be excluded based on the present data. Accompanied by the presence of sulfur, a significant increase in the binding energies (ca. 1.0 eV) of the W and P species in WP-S was observed (Figs. 1A and 1B), indicating that the W and P species are in an electron-deficient state. This is probably because sulfur is more electronegative than phosphorus. Previously, we found that the HDS activity of a bulk MoP catalyst varied during the HDS of DBT due to the formation of a new surface phosphosulfide phase [16,17]. When the HDS of DBT over the MoP catalyst was carried out in a cyclic manner (high weight time ? low weight time ? high weight time), substantial increases in the DBT conversion and the selectivity of biphenyl

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A

32.9

128.9

B

129.8

35.1

37.2

WP-N

WP-N 31.9

34.0 38.2

Intensity (a.u.)

Intensity (a.u.)

30.7

36.1

WP-S 31.0

33.1

129.9 130.8

WP-S 128.9 129.8

35.5

37.5

WP

WP

40

38

36

34

32

30

134

28

132

130

128

126

Binding energy (eV)

Binding energy (eV)

400.8

C WP-N

Intensity (a.u.)

Intensity (a.u.)

D

WP-N

WP-S

168

166

164

162

160

158

Binding energy (eV)

408

406

404

402

400

398

396

Binding energy (eV)

Fig. 1. The XPS spectra of the fresh WP, WP-S, and WP-N samples in the W 4f (A), P 2p (B), S 2p (C), and N 1s (D) regions.

Table 1 The relative atomic ratios of W, P, S, and N in the surfaces of WP, WP-S, and WP-N determined by XPS. Catalyst

W

P

S

N

WP WP-S WP-N

1 1 1

0.85 0.81 1.20

0.15 0.20

0.35

(BP) were observed [16]. In the present work, the same cyclic HDS experiment was carried out over the bulk WP catalyst, but neither a difference in the DBT conversion nor a difference in the BP selectivity was seen between the two stages, and the BP selectivity was almost independent of weight time (Fig. 2). All these results indicate that the bulk WP catalyst was stable during the HDS of DBT. Taking into account the different HDS behaviors of MoP and WP as well as their sulfur-containing surfaces, we suggest that the surface sulfidation of WP is faster than that of MoP. It is possible that the sulfur-containing surface of WP or the tungsten phosphosulfide phase was already formed before the first sample was taken from the reactor. To further verify that the tungsten phosphosulfide phase is the actual active phase of WP in HDS reactions and that the formation of this phase is fast, we studied the HDS of DBT in the absence of piperidine over a bulk WP catalyst pretreated with 0.7 vol.% H2S/H2 at 4.0 MPa and 340 °C for 4 h (WP-H2S). The XPS S/ W ratio of WP-H2S was 0.17, which was slightly higher than the value 0.15 of WP-S. As expected, the conversion of DBT over WPH2S was slightly higher than that over WP (Fig. S2 in Supplementary Material). Moreover, the small difference in the S/W ratios of WP-S and WP-H2S indicates that the surface of WP was almost completely converted to the tungsten phosphosulfide phase during the HDS of DBT.

Based on the fact that the XPS P/W ratio decreased from 0.85 for WP to 0.81 for WP-S (Table 1), it is likely that part of the phosphorous species was replaced by sulfur to form the tungsten phosphosulfide phase in the HDS of DBT. A similar mechanism has been proposed for the formation of nickel phosphosulfide [10], but the sulfur content in the tungsten phosphosulfide phase was much lower than that in the nickel phosphosulfide phase. The S/Ni ratio of a stable nickel phosphosulfide phase predicted by DFT calculations was 0.33 [10]. Recently, we prepared a Ni2P catalyst with a distinct sulfur-containing surface by low-temperature reduction of Ni2P2S6 [13]. The surface S/Ni ratio measured by XPS was 0.41, which was close to the theoretical value. The S/W ratio of WP-S was as low as 0.15 (Table 1). This value was also lower than the S/Mo ratio (0.35) of a spent bulk MoP catalyst obtained after the HDS of DBT [16]. According to the available data, the sulfur/metal ratios of the phosphosulfide phases increased in the order: WP < MoP  Ni2P. The low sulfur/metal ratio of the tungsten phosphosulfide phase and the high covalent character of WP might be responsible for the fast formation of the tungsten phosphosulfide phase. A previous DFT study [29] suggested that MoP is more ionic (or less covalent) than WP. To clarify this question, further experimental and theoretical studies are needed. Besides sulfur, quite a few nitrogen-containing species were detected in WP-N. The N 1s spectrum of WP-N (Fig. 1D) was dominated by a peak at 400.8 eV due to NH3 adsorbed on tungsten [30]. There is no sign of nitrides (397.6 eV) [30], N2 (399.2 eV) [30], or pyridine-like nitrogen (398.5 eV) [31]. This indicates that the nitrogen-containing species on the surface of the spent WP catalyst was NH3. The content of nitrogen in the surface of WP-N was 12.8% and the N/W ratio (0.35) was even higher than the S/W ratio (0.2) (Table 1). Since the concentration of piperidine in the feed was only one fifth of that of DBT, the high nitrogen content suggests a strong adsorption of NH3 on WP. Our previous study [22] and the present

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100

A DBT conversion (%)

80

60

The first stage The second stage

40

20

0 0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.4

1.6

Weight time (g·min/mol) 100

B

BP selectivity (%)

80

60

The first stage

40

The second stage

20

0 0.0

0.2

0.4

0.6

0.8

1.0

1.2

Weight time (g·min/mol) Fig. 2. DBT conversion (A) and BP selectivity (B) as a function of weight time in the HDS of DBT over bulk WP at 340 °C and total pressure 4.0 MPa proceeded in a cyclic manner. The first stage (▲): from high weight time to low weight time; the second stage (s): from low weight time to high weight time.

data (Fig. S3 in Supplementary Material) demonstrated that WP was highly sensitive to nitrogen-containing compounds. Based on the CO uptake of the fresh WP catalyst and DBT conversions, the turnover frequencies (TOF) of DBT over WP in the absence and presence of piperidine at s = 0.37 g min/mol were estimated to be 0.20 and 0.034 s 1, respectively. These values are in good agreement with our previous work [22]. The TOF of DBT over WP in the presence of piperidine was almost one order of magnitude lower than that in the absence of piperidine. Under the identical reaction conditions, the TOF of DBT over a bulk MoP catalyst only decreased from 0.26 s 1 in the absence of piperidine to 0.11 s 1 in the presence of 0.25 kPa piperidine [22], demonstrating that MoP is more nitrogen tolerant than WP. Moreover, the surface nitrogen content of a spent bulk Ni2P determined by XPS was only 1.9%, almost one order of magnitude lower than that of WP-N. This may explain why Ni2P is more nitrogen tolerant than WP [22]. Due to the low surface nitrogen content, only a weak N 1s peak at 400.6 eV assigned to NH3 was observed in the XPS spectrum of the spent Ni2P catalyst (Fig. S4 in Supplementary Material). Although the intensity of the N 1s peak of the spent Ni2P is too weak to allow an analysis, its broad and asymmetric line-shape suggests the coexistence of multiple nitrogen-containing species (Fig. S4 in Supplementary Material). A detailed study of these species is helpful to understand the structure-performance relationships of the transition-metal phosphide hydrotreating catalysts. The inhibitory effect of nitrogen-containing compounds on HDS is usually ascribed to their competitive adsorption with sulfur-

containing compounds [32]. However, our results show that the strong adsorption of NH3 (the HDN product of nitrogencontaining compounds) on the active phase is another important factor that needs to be considered to account for the inhibition effect of nitrogen-containing compounds on the HDS performance of transition-metal phosphides. To study the influence of NH3 on the HDS activity of WP, we first carried out the HDS of DBT at s = 1.5 g min/mol in the presence of 0.2 kPa NH3 (equivalent to the complete conversion of piperidine). NH3 was introduced by using a standard gas of NH3 in H2. After 8 h, the feed gas was switched to pure H2. In the presence of NH3, the DBT conversion was as low as ca. 20% (Fig. S5 in Supplementary Material), much lower than ca. 60% in the absence of NH3 but higher than the 10.5% in the presence of piperidine (Fig. S3 in Supplementary Material). This indicates that NH3 is one of the important factors inhibiting the activity of WP. The adsorption of NH3 is fairly strong, because it took 6 h for the recovery of the HDS activity of WP after the feed gas was switched to pure H2 (Fig. S5 in Supplementary Material). One of the significant consequences of the adsorption of NH3 is the strong negative shifts of the binding energies of the W, P, and S species (Fig. 1). The binding energies of all these species of WP-N were about 1.0 eV lower than those of WP-S, and were very close to the values of the fresh WP catalyst. This can be an indication of strong electronic interactions between adsorbed NH3 and the surface species of WP. It is possible that NH3 may play a role as strong base or electron donor, leading to an increase in the electron density of the surface of WP. Furthermore, the surface P/W and S/ W ratios of WP-N increased to 1.2 and 0.20, respectively (Table 1). The surface S/W ratio of WP-N was even higher than that of WPH2S (0.17). All these results suggest a decrease in the surface metal content of WP-N. The W, P, and S species detected in the surface of WP-N were almost identical to those in the surface of WP-S, and the S/P ratio of WP-N (0.17) was similar to that of WP-S (0.19). Considering the measurement error, the presence of nitrogencontaining compounds does not seem to influence the structure of the tungsten phosphosulfide phase. 4. Conclusion The actual active phase of WP in the HDS of DBT was a sulfurcontaining phase or a tungsten phosphosulfide phase. The formation of this sulfur-containing phase was faster than the case of MoP, but the sulfur content in the tungsten phosphosulfide phase was lower than in the nickel and molybdenum phosphosulfide phases. The presence of sulfur led to a significant increase in the binding energies of the surface W and P species. The nitrogencontaining species detected in the surface of the WP catalyst after the simultaneous HDS of DBT and the HDN of piperidine was NH3. NH3 decreased the DBT conversion strongly but less than piperidine, indicating that not only nitrogen-containing compounds but also NH3 inhibits the HDS activity of WP. The adsorption of NH3 is fairly strong, because it took 6 h for the HDS activity of WP to recover when NH3 was removed from the feed gas. The presence of nitrogen-containing compounds does not seem to influence the structure of the tungsten phosphosulfide phase. The increase in the surface P/W and S/W ratios and the decrease in the binding energies of the surface W, P, and S species suggest that both the composition and the electronic property of the WP surface were affected by nitrogen-containing compounds. Acknowledgement This work was financially supported by the Natural Science Foundation of China (U1162203, 21473017, and 21673029), the

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