Influence of Supports on Structure and Performance of Nickel

Mar 10, 2009 - (24, 33, 35) However, when Al2O3 is used as the support, the formation of ... Their supported nickel phosphides were prepared by means ...
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Ind. Eng. Chem. Res. 2009, 48, 3812–3819

Influence of Supports on Structure and Performance of Nickel Phosphide Catalysts for Hydrodechlorination of Chlorobenzene Jixiang Chen,*,† Shaojun Zhou,† Donghui Ci,† Jianxiang Zhang,‡ Rijie Wang,† and Jiyan Zhang† Tianjin Key Laboratory of Applied Catalysis Science and Technology, Department of Catalysis Science and Engineering, School of Chemical Engineering and Technology, Tianjin UniVersity, Tianjin 300072, China, and Xinao New-energy (Beijing) Science and Technology Corporation, Beijing 100176, China

SiO2, TiO2, γ-Al2O3, and HY zeolite supported phosphide catalysts were prepared by the hydrogen temperatureprogrammed reduction method from phosphate precursors. The physicochemical properties of the catalysts were characterized by means of N2 adsorption-desorption, hydrogen temperature-programmed reduction, X-ray diffraction, X-ray photoelectron spectroscopy, hydrogen temperature-programmed desorption, inductively coupled plasma atomic emission spectroscopy, energy-dispersion X-ray spectroscopy, and thermal gravimetric analysis. The catalyst performance in the hydrodechlorination of chlorobenzene was evaluated in a fixed-bed reactor at atmospheric pressure. It has been found that the support property remarkably affects the formation of nickel phosphides. With the same Ni/P molar ratio (about 0.7) in the precursors, Ni2P is prepared on SiO2 and TiO2; however, Ni and Ni3P form on γ-Al2O3 and Ni and Ni12P5 form on HY. This phenomenon is attributed to some phosphorus reacting with γ-Al2O3 and HY to form AlPO4, and the phosphorus reacting with nickel is scarce. Under identical reaction conditions, the hydrodechlorination performance of the catalysts decrease in the order of SiO2-supported N2P, γ-Al2O3-supported Ni-Ni3P, TiO2-supported N2P, and HYsupported Ni-Ni12P5. The catalyst performance is closely related to the properties of active phases and hydrogen species. Nickel phosphides have better performance than metallic nickel due to the electron deficiency of nickel, and the spilt-over hydrogen species also contribute to the hydrogenolysis of C-Cl bond. The chlorobenzene conversion exceeds 99% over SiO2-supported Ni2P during 130 h at 573 K. The excellent performance is ascribed to the strong poison resistance of Ni2P to chlorine and the abundant hydrogen species. TiO2-supported N2P and HY-supported Ni-Ni12P5 have good initial activities; however, their deactivation is remarkable, especially HY-supported Ni-Ni12P5. Their deactivation is mainly owing to the carbonous deposition. 1. Introduction Catalytic hydrodechlorination (HDC) is a promising method for disposing of chlorinated compound pollutants. Compared to physical separation (e.g., adsorption, air or steam stripping) and chemical degradation or destruction (e.g., thermal incineration, catalytic or wet air oxidation), catalytic HDC has many advantages. These include mild reaction conditions, the transformation of chlorinated compound pollutants to valuable raw materials, and no emission of CO2 and NOx.1 The catalysts used for HDC have mainly been metallic, either monometallic (Pt,2,3 Pd,4-8 Rh,9,10 Ni11-16) or bimetallic (Ni-Au,17 Pd-Ni,18 Pd-Au,19 Pd-Ln (Ln ) La, Ce, Sm, Eu, Gd, and Yb),20 Pd-Rh21). Recently, SiO2-supported nickel phosphides catalysts have been used in the HDC of chlorobenzene and exhibit excellent performance.22-25 HDC has been shown to be structure sensitive over Pd7,8 and Ni11,16 catalysts. There is also an evidence that spilt-over hydrogen species are contributed to the HDC.22,24,26-28 The bimetallic synergism can markedly promote the catalyst performance.17,20 The excellent performance of SiO2-supported nickel phosphides is attributed to the special physicochemical properties of nickel phosphides and abundant spilt-over hydrogen species.22,24 A consensus has emerged from the literature that catalytic hydrodechlorination is markedly affected by the support properties, and this is mainly due to different metal-support and reactant/product-surface interactions.2,4,11,12,17,29,30 * To whom correspondence should be addressed. Tel.: +86-2227890865. Fax: +86-22-87894301. E-mail: [email protected]. † Tianjin University. ‡ Xinao New-energy (Beijing) Science and Technology Corporation.

Transition-metal phosphides are novel catalytic materials with excellent performance in hydrodenitrogenation (HDN), hydrodesulfuration (HDS),31-38 and HDC.22-25 The supported or unsupported nickel phosphides are usually prepared by the temperature-programmed reduction method,31,32 and the prepared phases (Ni3P, Ni12P5, and Ni2P) are related to the Ni/P molar ratio in the precursor24,33,35,36 and the kind of support.36,37 Silica24,33,35,36 and mesoporous silica-based materials (such as SBA-1538,39 and MCM-4140,41), Al2O3,36,37,40 KUSY,42 CMK5,38 and carbon43 have been used as the supports, especially silica and Al2O3. Usually, silica-supported Ni and Ni3P or Ni12P5 can be prepared when the Ni/P ratio is g2 in the precursor, and silica-supported Ni2P can be prepared as the Ni/P ratio is e1 in the precursor.24,33,35 However, when Al2O3 is used as the support, the formation of Ni2P needs a Ni/P ratio e0.5 because P can react with Al2O3 to form AlPO4, which leads a lack of P reacting with nickel.36 In the present work, in order to investigate the effect of the supports on the structure and performance of nickel phosphides in the hydrodechlorination of chlorobenzene, SiO2, γ-Al2O3, TiO2, and HY zeolite have been adopted since they have different physicochemical properties. From the phosphate precursors with similar Ni/P molar ratios, the supported catalysts were prepared by the temperature-programmed reduction method and their physicochemical properties were investigated. 2. Experiments 2.1. Catalyst Preparation. Commercial spherical SiO2 (Qingdao Haiyang Chemicals Co. Ltd., China), TiO2 (Nanjing

10.1021/ie8018643 CCC: $40.75  2009 American Chemical Society Published on Web 03/10/2009

Ind. Eng. Chem. Res., Vol. 48, No. 8, 2009 3813 Table 1. Properties of Catalysts BET surface area (m2/g)

catalyst Ni2P/SiO2 spent Ni2P/SiO2 Ni-Ni3P/γ-Al2O3 spent Ni-Ni3P/γ-Al2O3 Ni2P/TiO2 spent Ni2P/TiO2 Ni-Ni12P5/HY spent Ni-Ni12P5/HY a

f

pore diameter (nm)

pore volume (cm3/g)

Ni content (wt.%)

bulk composition

378 316 113

4.3c 5.0 8.8c

0.526 0.475 0.331

7.2

Ni1.7P1.00

6.8

Ni0.7P1.00

14.8

31.4c

0.102

7.7

Ni1.8P1.00

394 (312a; 82b) 324 (258a; 67b)

4.7c; 0.55d 5.0c; 0.54d

0.088e; 0.149f 0.072e; 0.120f

7.1

Ni0.7P1.00

carbon depositiong (wt %)

Cl/Ni atomic ratioh

0.8

0.14 0.60

4.5

b

c

t-Plot micropore area. t-Plot external surface area. BJH desorption average pore diameter. Micropore volume. g Measured by TGA. h Measured by EDS.

HT Nano Material Co. Ltd., China), Al2O3, and HY zeolite (Wenzhou Jingjing Aluminum Oxide Co. Ltd., China) were used as the supports. Their supported nickel phosphides were prepared by means of H2 temperature-programmed reduction (H2 TPR) from the supported nickel phosphates. First, the support was incipiently impregnated with an aqueous solution of Ni(NO3)2 and NH4H2PO4 and left at room temperature for 12 h. The sample was then dried in air at 393 K for 12 h and calcined in air at 773 K for 4 h, and the supported nickel phosphate precursor was prepared, in which the Ni/P molar ratio was about 0.7 measured by an inductively coupled plasma atomic emission spectrometer. In the next step, the nickel phosphate precursor was reduced in a fixed-bed quartz reactor placed in a furnace controlled by a temperature programmer according to the following conditions: the precursor was heated from room temperature to 473 K at 8 K/min and then from 473 to 923 K at 1 K/min and was maintained at 923 K for 6 h. During the reduction, the H2 (99.999%) flow was set at 250 mL/min per gram of precursor. According to the phases obtained from X-ray diffraction (XRD), the SiO2, TiO2, Al2O3, and HY zeolite supported catalysts are designated as Ni2P/SiO2, Ni2P/TiO2, Ni-Ni3P/ γ-Al2O3, and Ni-Ni12P5/HY, respectively. The nickel contents and the Ni/P molar ratios in the catalysts are shown in Table 1. Before N2 adsorption, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), inductively coupled plasma (ICP), and hydrogen temperature-programmed desorption (H2 TPD) characterizations, the fresh catalyst was passivated in a 0.5 vol % O2/N2 flow for 6.0 h at room temperature. 2.2. Catalyst Characterization. The reducibility of the catalyst precursors was characterized by hydrogen temperatureprogrammed reduction (H2 TPR) in a quartz U-tube reactor, into which a 50 mg sample was loaded. The reduction was conducted in a 10 vol % H2/N2 flow of 40 mL/min at a heating rate of 4 K/min, and the hydrogen consumption signal was determined using a thermal conductivity detector (TCD). X-ray diffraction (XRD) patterns of the catalysts were obtained on a PANalytical X’pert Pro diffractometer with Co KR (λ ) 0.17902 nm) radiation (40 kV, 40 mA). The average size of the crystallites was calculated using Scherrer equation, d ) (0.9λ)/(β cosθ), where d is the crystallite size, λ is the wavelength of the radiation, β is the width of the peak at halfmaximum intensity, and θ is the Bragg angle. N2 adsorption-desorption isotherms of Ni2P/SiO2, Ni2P/TiO2, and Ni-Ni3P/γ-Al2O3 were obtained on a Micromeritics TriStar 3000 apparatus, and those of Ni-Ni12P5/HY were obtained on a Micromeritics ASAP 2020 apparatus. The Brunauer-EmmettTeller (BET) equation was used to calculate the specific surface area, SBET. The t-plot method was used to determine the micropore area and the external surface area of HY-supported catalyst. Total pore volumes, Vp, were estimated at a relative

0.16

3.2 d

HK median pore diameter.

0.28 e

Mesopore volume.

pressure of 0.99. The Barrett-Joyner-Halenda (BJH) method applied to the desorption isotherm was used to determine the mesopore size distribution, and the Horvath-Kawazoe (HK) method was used to determine the micropore size distribution. X-ray photoelectron spectroscopy (XPS) was performed on a PHI-1600 ESCA instrument with Mg KR radiation (1253.6 eV). Binding energies were determined with C1s (284.6 eV) as the reference. Hydrogen temperature-programmed desorption (H2 TPD) was determined using a TP-D/R/O 1100 Series unit (Finnigan). For these measurements, 400 mg of passivated sample was loaded in a quartz reactor and reduced in an H2 flow at 723 K for 1 h. The sample was then cooled to 303 K and left for 0.5 h in an H2 atmosphere. H2 TPD was performed in a 50 mL/min N2 flow at a heating rate of 10 K/min. The water derived from the dehydroxylation of the supports was removed by soda-lime, and the desorbed H2 was detected by a thermal conductivity detector (TCD). Thermogravimetric analysis (TGA) was carried out on a Mettler TGA/SDTA851 instrument. During the measurement, an air flow with 150 mL/min was used and a heating rate was 10 K/min. Elemental analysis of the catalysts was carried out on an inductively coupled plasma atomic emission spectrometer [ICP9(N + M), Thermo Jarrell-Ash Corp.]. Cl/Ni atomic ratios in the spent catalysts were measured by an Oxford ISIS-300 energy-dispersion X-ray spectroscopy (EDS). 2.3. Catalyst Activity Evaluation. The catalytic HDC reaction was performed in an atmospheric fixed-bed quartz reactor (i.d. ) 12 mm). For the evaluation, 1.0 g of catalyst precursor was supported on quartz cotton and a layer of ceramic beads was placed on the catalyst bed; the sample was then prepared in situ as described in section 2.1. Following the preparation procedure, the catalyst bed was adjusted to the reaction temperature and the flow rate of H2 was then designated as 4 × 103 mL/h. A 3 mL/h portion of chlorobenzene was delivered by a pump and vaporized before passing the catalyst bed. To avoid the loss due to volatilization, the products were absorbed with anhydrous ethanol, which was subsequently analyzed by a gas chromatograph equipped with a hydrogen flame ionization detector and an OV-101 capillary column. 3. Results and Discussion 3.1. Catalyst Characterization. 3.1.1. H2 TPR Results. Figure 1 displays the H2 TPR profiles of the catalyst precursors. In the trace of the Ni2P/SiO2 precursor, two reduction peaks centered at about 954 and 1120 K are attributed to the nickel phosphates.23,24 In the precursor, the nickel species exist in the form of Ni-O-P, while the phosphorus species can exist as

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Figure 1. H2 TPR profiles of (a) Ni2P/SiO2 precursor; (b) Ni-Ni3P/γ-Al2O3 precursor; (c) Ni-Ni12P5/HY precursor; (d) Ni2P/TiO2 precursor.

PO43-, P2O74-, and (PO3-)n.35 The phosphorus species can be reduced to elemental P4 (P2) and/or phosphines (PxHy), which then react with nickel species to form nickel phosphide. In the H2 TPR trace of Ni-Ni3P/γ-Al2O3 precursor, the broad peak centered at about 890 K is related to the reduction of nickel phosphate and/or nickel species interacting with the support.44 Another reduction peak at about 1194 K is perhaps owing to Ni2AlO444 and/or AlPO4.45 The formation of Ni2AlO4 and AlPO4 is related to the defects on the surface of γ-Al2O3. AlPO4, an amorphous material, can easily form on the surface of γ-Al2O3.46,47 However, it can be reduced at above 1023 K, and phosphorus species are released.45 A peak at 760 K in the reduction trace of Ni-Ni12P5/HY is due to Ni2+ species strongly interacting with the support,48 while that at 890 K is attributed to nickel phosphate. A peak starting at 1200 K is also related to the reduction of AlPO type compounds, such as AlPO4, derived from the reaction between phosphate and framework or extra-framework alumina in HY zeolite.49 In the reduction trace of Ni2P/TiO2 precursor, there is only a peak at 874 K attributed to nickel phosphate. This indicates that the reduced species exist in a similar environment. The above results show that the supports remarkably affect the existence states of nickel and phosphorus species in the precursors. 3.1.2. XRD and Textural Property Results. XRD patterns of the catalysts are shown in Figure 2. Apart from the diffraction peaks due to the supports, there are only the diffraction peaks of the Ni2P phase (2θ ) 47.7°, 52.4°, 55.7°, 63.9°) for Ni2P/ SiO2 and Ni2P/TiO2. However, the Ni2P diffraction peaks of Ni2P/TiO2 are sharper than those of Ni2P/SiO2. On the basis of the reflection of Ni2P (111) and calculated with Scherrer equation, the average sizes of Ni2P crystallites in Ni2P/SiO2 and Ni2P/TiO2 are about 10 and 26 nm, respectively. It is clear that the Ni2P crystallites are larger in Ni2P/TiO2 than Ni2P/SiO2. This is mainly due to low specific surface area of Ni2P/TiO2 (see Table 1), which is not beneficial to the dispersion of Ni2P.

In the XRD patterns of Ni-Ni12P5/HY and Ni-Ni3P/γ-Al2O3, no Ni2P phase is detected. Ni (2θ ) 52.2°, 61.0°, 91.8°), Ni12P5 (2θ ) 38.2°, 44.9°, 48.9°, 55.1°, 57.5°), and HY phases are detected in Ni-Ni12P5/HY, while Ni and Ni3P (2θ ) 42.5°, 48.9°, 51.1°, 53.1°, 54.7°, 61.2°, 62.1°, 65.3°, 90.5°) are detected in Ni-Ni3P/γ-Al2O3 although there is an overlap between the diffraction peaks of Ni, Ni3P, and γ-Al2O3 (2θ ) 37.7°, 44.3°, 54.1°, 72.0°, 80.0°). In order to prepare Ni2P phase, surplus P element in the precursor is necessary due to the loss of P element during the TPR process.35 However, Ni2P does not form from the γ-Al2O3 and HY-supported precursors with Ni/P molar ratio of 0.7. As shown in Table 1, the bulk Ni/P molar ratios in Ni-Ni12P5/HY and Ni-Ni3P/γ-Al2O3 are about 0.7. This indicates that some phosphorus are not reduced and remained in the catalyst, that is, the phosphorus species reacting with nickel species is scarce. This is due to the formation of AlPO4,35 which has been shown in the H2 TPR results and can be further confirmed by the XPS results (see section 3.1.3). Table 1 lists the textural properties of the catalysts, and there is a marked difference in specific surface area, pore size, and pore volume for the different catalysts. This can mainly be ascribed to the properties of the supports. Figure 3 shows the pore size distribution of Ni2P/SiO2, Ni-Ni3P/γ-Al2O3, and Ni2P/ TiO2, all of which are characteristic of mesopores. Ni-Ni12P5/ HY has both mesopores and micropores as shown in Figure 4, and its micropore volume is about 63% in total pore volume (see Table 1). The micropore of about 0.55 nm is attributed to HY zeolite. 3.1.3. XPS Results. XPS results of the passivated catalysts in the Ni 2p3/2 and P 2p3/2 regions are shown in Table 2. Oxidized Ni and P species exist in every catalyst due to the passivation of the catalysts. There are two electronic binding energies for Ni 2p3/2 and P 2p3/2. This result is similar to that reported by Bussell and co-workers,35 Smith and co-workers,50 and us.22,24 For Ni2P/SiO2, Ni-Ni12P5/HY, and Ni-Ni3P/ γ-Al2O3, the electronic binding energies of about 856.6 and 134.4 eV are assigned to Ni2+ and P5+ species in the oxide layer, while those of about 853.1 eV for Ni 2p3/2 and 129.5 eV for P 2p3/2 indicate an electron transfer from Ni to P.22,24 As a result, the Ni species has a small positive charge (Niδ+) and the P species has a small negative charge (Pδ-). This is characteristic of nickel phosphides.51-53 The DFT studies32,52 indicate that the Ni-P bonds in Ni2P are covalent; that is, there is a “ligand effect” for Ni-P bonds. However, this effect is weak. Ni3P, Ni12P5, and Ni2P show evidence of exhibiting metallic character.35,54 It is worth noting that the electronic binding energies of Ni 2p3/2 and P 2p3/2 in Ni2P/TiO2 are lower that those in the other catalysts, indicating that there is relatively greater electron density of Ni and P species. This is perhaps related to the strong interaction between Ni2P and reduced TiO2. It is well-known that TiO2 can be reduced to TiOx (x < 2) species at above 673 K. Similar to the interaction with Ni,55 TiOx (x < 2) species may create strong interactions with Ni2P and decorate Ni2P crystallites. It has been demonstrated that the electron density of Ni crystallites supported on TiO2 is greater than that associated with Ni supported on SiO256 due to the interaction between Ni and TiOx. But then, the electron transfer from Ni to P should still take place due to existence of Ni2P in Ni2P/ TiO2. Ni-Ni3P/γ-Al2O3 and Ni-Ni12P5/HY catalysts have the higher binding energy of P5+ species than Ni2P/SiO2 and Ni2P/ TiO2 (see Table 2). The value of 134.6 eV is close to the binding energy of P in AlPO4 molecular sieves (134.6-134.8 eV).57

Ind. Eng. Chem. Res., Vol. 48, No. 8, 2009 3815

Figure 2. XRD patterns of catalysts (a) Ni2P/SiO2; (b) Ni2P/TiO2; (c) Ni-Ni12P5/HY; (d) Ni-Ni3P/γ-Al2O3. Time on stream for spent catalysts: Ni2P/SiO2 130 h; Ni2P/TiO2 33 h; Ni-Ni12P5/HY 10 h; Ni-Ni3P/γ-Al2O3 32 h.

which has been suggested by Li and co-workers.58 For Ni-Ni3P/ γ-Al2O3 and Ni-Ni12P5/HY, phosphorus enrichment is more remarkable. One reason for this is the surface oxidation, and the other important reason is due to the formation of AlPO4 on the catalyst surface, which leads to less P element loss during TPR. 3.1.4. H2 TPD Results. The H2 TPD profiles of the different catalysts are shown in Figure 5. All of the H2 TPD traces can be divided to two parts: one below 673 K and another above 673 K. Generally, the hydrogen species desorbed below 673 K are ascribed to those adsorbed on the metal Ni and/or nickel phosphides, and those desorbed above 670 K are ascribed to spilt-over hydrogen species.20,22,59-62

Figure 3. Pore size distribution of Ni2P/SiO2, Ni-Ni3P/γ-Al2O3, and Ni2P/ TiO2 according to the BJH model.

This further proves that AlPO4 forms in Ni-Ni3P/γ-Al2O3 and Ni-Ni12P5/HY catalysts. Table 2 also shows the surface Ni/P ratios of the different catalysts. Compared with the bulk Ni/P ratio (see Table 1), the surface Ni/P ratio of Ni2P/TiO2 and Ni2P/SiO2 is lower, i.e., phosphorus is enriched on the catalysts surface. This phenomenon is related to the surface oxidation during the passivation,

Comparing with the other catalysts, there are more hydrogen species desorbed below 673 K over Ni2P/SiO2, that is, more hydrogen species adsorb on Ni2P. Perhaps due to the blocking of nickel or nickel phosphides with phosphorus, there are less hydrogen species adsorbed on nickel and nickel phosphides in Ni-Ni12P5/HY and Ni-Ni3P/γ-Al2O3. However, Ni2P/SiO2, Ni-Ni12P5/HY, and Ni-Ni3P/γ-Al2O3 all have a great amount of spilt-over hydrogen species. This phenomenon is related to the properties of nickel phosphides and the supports. As suggested in the previous reports,22,24 hydrogen spillover occurs easily over the supported nickel phosphides; moreover, there

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Figure 4. Pore size distribution of Ni-Ni12P5/HY according to the HK (a) and BJH models (b). Table 2. XPS Results of Catalysts electronic bindings energies (eV) catalyst

Ni 2p3/2

P 2p3/2

Ni/P atomic ratioa

Ni2P/SiO2 Ni-Ni3P/γ-Al2O3 Ni2P/TiO2 Ni-Ni12P5/HY

856.4; 853.3 856.9; 853.2 855.4; 852.9 856.3; 853.1

134.2; 129.4 134.5; 129.7 132.7; 129.0 134.5; 129.6

1.3 0.3 1.5 0.2

a

Atomic content on surface of catalyst.

Figure 5. H2 TPD profiles of (a) Ni-Ni12P5/HY; (b) Ni2P/TiO2; (c) Ni-Ni3P/γ-Al2O3; (d) Ni2P/SiO2.

are abundant hydroxyl groups on the surface of HY, γ-Al2O3, and SiO2, which can facilitate the hydrogen spillover.62 In comparison with other catalysts, there is a lesser amount of hydrogen species on the surface of Ni2P/TiO2, both adsorbed on Ni2P and spilt-over. This should be ascribed to the decoration of TiOx (x < 2) species on the Ni2P crystallites, which not only decreases the available surface of Ni2P crystallites but also inhibits hydrogen spillover.

3.2. Catalyst Performance. Figure 6 shows the activities of the catalysts in the gas-phase hydrodechlorination of chlorobenzene. When the reaction occurs at 573 K, the chlorobenzene conversion maintains at about 99% over Ni2P/SiO2 and Ni-Ni3P/γ-Al2O3 catalysts during 33 h on stream. When the reaction temperature is 543 K, the chlorobenzene conversion over Ni2P/SiO2 is about 98%, whereas that over Ni-Ni3P/γAl2O3 catalyst is about 98% at the first hour, and then, it decreases and eventually is steady at about 90%. Although the chlorobenzene conversion initially exceeds 99% over Ni2P/TiO2 at 573 K, it decreases slowly during the first 10 h and sharply from the eleventh hour, and it decreases to about 37% after reaction for 33 h. At 573 K, the chlorobenzene conversion is about 96% over Ni-Ni12P5/HY at the first hour; however, it directly decreases and is about 35% at the tenth hour. This indicates that the deactivation of Ni-Ni12P5/HY is serious. Considering both the activity and the stability, the catalyst performance decreases in the following sequence: Ni2P/SiO2, Ni-Ni3P/γ-Al2O3, Ni2P/TiO2, and Ni-Ni12P5/HY. As discussed previously,23,24 the excellent performance of Ni2P/SiO2 is ascribed to the structural properties of Ni2P and a great amount of hydrogen species (both that on Ni2P crystallites and spilt-over). It is known that chlorine acts as an electron acceptor with respect to transition metals.10,63 The adsorption strength of chlorine is enhanced as the electronic density of the metal atom increases, which is not beneficial to the reduction of the metal chloride. However, lowing the electronic density of the metal atom is propitious to HDC, because surface poisoning due to chlorine is one of main reasons for catalyst deactivation.64,65 Indicated in our previous reports,22-25 the superior activity of SiO2-supported nickel phosphides to SiO2supported nickel is closely related to the electron deficiency of nickel in nickel phosphides; however, the electron-enriched nickel species due to the existence of the boron element are unfavorable to hydrodechlorination. Similarly, Gopinath and coworkers6 attribute the good HDC activity of Pd/ZrO2 catalyst prepared by the deposition-precipitation to the electron-deficient

Ind. Eng. Chem. Res., Vol. 48, No. 8, 2009 3817

Figure 7. Stability of Ni2P/SiO2 catalyst in HDC. Reaction conditions: 0.1 MPa, 573 K, chlorobenzene 3 mL/h, H2 4 × 103 mL/h.

Figure 6. HDC activities of different catalysts: (a) 573; (b) 543 K. Reaction conditions: 0.1 MPa, chlorobenzene 3 mL/h, H2 4 × 103 mL/h.

Pd species. It is also reported by Coq and co-workers10 that the PdSn catalyst shows a lower activity than the Pd catalyst due to its higher electronic density of Pd. The spilt-over hydrogen species also plays an important role in HDC.22,24,26-28 Whether chlorobenzene adsorbs on Ni2P, the support, or the interface of Ni2P and the support, the abundant spilt-over hydrogen species can promote the hydrogenolysis of C-Cl. In addition, chloride ions can be converted to HCl with spilt-over H+ species during the reaction,66 that is, the spilt-over hydrogen species have the role of “cleaning” the catalyst surface. Figure 7 shows the activity of Ni2P/SiO2 catalyst during 130 h. Although there is a decrease in the surface area and pore volume of the spent catalyst (see Table 1), the chlorobenzene conversion is still above 99%. Figure 2a shows that the phase composition of the spent catalyst is same as that of the fresh one; moreover, the average Ni2P crystallites size (about 10 nm, based on the reflection of Ni2P (111)) is similar to that in the fresh catalyst. It is clear that the Ni2P crystallites have strong antisintering during the HDC process. This is very different from the Ni catalyst, in which chlorine induces the sintering of nickel crystallites through the formation of NiCl2 and leads to the irreversible catalyst deactivation.64,65 As shown in Table 1, among the spent catalysts, Cl/Ni atomic ratio on the spent Ni2P/ SiO2 is the least although Ni2P/SiO2 was used for the longest time. This indicates that Ni2P/SiO2 showed a good poison

resistance to chlorine. In addition, TG results show that there is only 0.8 wt % carbonous deposition on the spent Ni2P/SiO2. Summarizing the above results, it is suggested that the good stability of Ni2P/SiO2 can be ascribed to the stability of Ni2P crystallites and low carbonous deposition. Ni-Ni3P/γ-Al2O3 has similar activity to Ni2P/SiO2 at 573 K. This can be ascribed to the existence of Ni3P and abundant spiltover hydrogen species. The previous study has shown that the supported Ni3P has better activity than the supported Ni, and Ni3P has a similar role to Ni2P in HDC.24 In addition, there are weak Lewis acidic sites on the surface of γ-Al2O3. Because the chlorobenzene molecule has a dipole moment, that is, a polar Clδ--Cδ+ bond, the chlorobenzene molecule seems to be coordinately adsorbed on the Lewis acid site (L) of γ-Al2O3 in the form of L r Cl-C6H5. The adsorbed chlorobenzene molecule is considered to react with hydrogen species easily.66 Although the Cl/Ni ratio in the spent Ni-Ni3P/γ-Al2O3 is bigger (see Table 1), Ni-Ni3P/γ-Al2O3 has better HDC stability than Ni2P/TiO2 and Ni-Ni12P5/HY. In addition, comparing with the fresh Ni-Ni3P/γ-Al2O3, the diffraction peaks of Ni in the XRD pattern of the spent sample are not remarkable. The above results are perhaps related to the amphoteric property of γ-Al2O3. The produced HCl during the reaction may induce the structural change of Ni-Ni3P/γ-Al2O3. The further reasons will be investigated. When the reaction occurs at 543 K, the activity of Ni-Ni3P/γ-Al2O3 is lower than that of Ni2P/SiO2. This may be owing to lower activity of metallic Ni. The initial activity of Ni2P/TiO2 is comparable to Ni2P/SiO2 and Ni-Ni3P/γ-Al2O3 (Figure 6a), which is related to the following reasons. Although the Ni2P surface area of Ni2P/TiO2 is small due to decorating Ni2P with TiOx, the role of Ni2P in the hydrodechlorination is also important just as that in Ni2P/ SiO2. Moreover, the oxygen vacancies in titania (especially reduced titania, i.e., TiOx) are known to capture chloride species.30,67,68 As a result, the C-Cl band is perhaps polarized and attacked easily by hydrogen species, and the hydrogenolysis is promoted. However, the deactivation of Ni2P/TiO2 is remarkable from the eleventh hour. The XRD result shows that the spent Ni2P/TiO2 (Figure 2b) has same phases and similar Ni2P crystallites size to the fresh one. Moreover, Cl/Ni ratio in the spent Ni2P/TiO2 is close to that in the spent Ni2P/SiO2. It is clear that Ni2P crystallites are stable during the HDC. The main reason for the Ni2P/TiO2 deactivation is not the sintering of Ni2P crystallites and chlorine poison. TG test shows that the carbonous deposition on the spent Ni2P/TiO2 is about 4.5%. Considering the low surface area of Ni2P/TiO2 (see Table 1), it is

3818 Ind. Eng. Chem. Res., Vol. 48, No. 8, 2009

suggested that the main reason for Ni2P/TiO2 deactivation is carbonous deposition. Ni-Ni12P5/HY also has a good initial activity, which should be attributed to Ni12P5 and spilt-over hydrogen species; however, its stability is very poor. The XRD patterns (Figure 2c) show that the phases and the nickel crystallites size in spent Ni-Ni12P5/ HY catalyst does not change remarkably in comparison with the fresh one, indicating that the deactivation of Ni-Ni12P5/ HY does not result from the sintering of the active phases. It is well-known that there are a great amount of acidic sites (both Lewis sites and Brøsted sites) on the surface of HY zeolite, which favors the carbonous deposition. TG test shows that the carbonous deposition is about 3.2 wt %. HY zeolite has many micropores with about 0.55 nm in diameter. The narrow pores are not beneficial to mass transfer and are prone to being blocked with carbonous deposition. Carbonous deposition accounts for the decrease in the surface area and pore volume of Ni-Ni12P5/ HY after reaction for 10 h (see Table 1). Furthermore, the Cl/ Ni ratio on in the spent Ni-Ni12P5/HY is higher than that in spent Ni2P/SiO2 and the spent Ni2P/TiO2, which is perhaps related to the existence of metallic nickel. Analyzing the above results, the Ni-Ni12P5/HY deactivation is mainly due to carbonous deposition. 4. Conclusions From the phosphate precursors with an Ni/P ratio of 0.7, Ni2P/ SiO2 and Ni2P/TiO2 catalysts were prepared; however, Ni-Ni3P/ γ-Al2O3 and Ni-Ni12P5/HY were prepared due to some phosphorus species reacting with the supports. There are a great amount of hydrogen species on Ni2P/SiO2, both adsorbed on the Ni2P crystallites and spilt-over on the support. For Ni-Ni3P/γ-Al2O3 and Ni-Ni12P5/HY, less hydrogen species adsorb on the surface of Ni and Ni3P or Ni12P5; however, there are abundant spilt-over hydrogen species. Perhaps due to the decorating of Ni2P crystallites with TiOx (x < 2) species, the amount of hydrogen species on Ni2P/TiO2 is less. In the hydrodechlorination of chlorobenzene, the catalyst performance decreases in the following sequence: Ni2P/SiO2, Ni-Ni3P/γ-Al2O3, Ni2P/TiO2, and Ni-Ni12P5/HY. The poison resistance of nickel phosphides to chlorine due to the Ni electron deficiency and abundant hydrogen species (especially those spiltover) contribute to the catalyst activity. The excellent activity and stability of Ni2P/SiO2 in the HDC of chlorobenzene are attributed to its good structural stability and resistance to carbonous deposition. Although Ni-Ni12P5/ HY and Ni2P/TiO2 had a good initial activity, their activity loss was remarkable, which is mainly due to carbonous deposition. Acknowledgment This work was supported by the Natural Science Foundation of Tianjin (No. 08JCYBJC01600) and the Program of Introducing Talents to the University Disciplines (No. B06006). Literature Cited (1) Keane, M. A. Advances in greener separation processes - case study: recovery of chlorinated aromatic compounds. Green Chem. 2003, 5, 309. (2) Creyghton, E. J.; Burgers, M. H. W.; Jansen, J. C; Bekkum, H. Vapour-phase hydrodehalogenation of chlorobenzene over platinum/H-BEA zeolite. Appl. Catal. A: Gen. 1995, 128, 275–288. (3) Yoneda, T.; Takido, T.; Konuma, K. Hydrodechlorination reactivity of para-substituted chlorobenzenes over platinum/carbon catalyst. J. Mol. Catal. A: Chem. 2007, 265, 80–89.

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ReceiVed for reView December 4, 2008 ReVised manuscript receiVed January 30, 2009 Accepted February 5, 2009 IE8018643