硫化镍本身在加氢精制反应中的加氢脱硫（HDS）活性并不理想, 主要作为硫化钼或钨体系的助催化剂, 组成γ-Al₂O₃或SiO₂担载的Ni-Mo-S和Ni-W-S催化剂. 磷化镍具有较高的HDS、加氢脱氮（HDN）活性和较高的耐硫性能, 有可能替代传统的过渡金属硫化物催化剂用于加氢精制反应. 文献中关于硫化镍和磷化镍的比较研究很少. 由于原料和制备方法对催化剂的组成和性能影响很大, 比较硫化镍和磷化镍并非易事.

我们采用共沉淀法制备了高担载量和高分散度的80%Ni/Al₂O₃催化剂, 经350 ℃焙烧后获得了高分散的NiO/Al₂O₃催化剂前驱体, 其中NiO的粒径约为3 nm. 用CS₂在310 ℃将NiO/Al₂O₃硫化4 h, 获得了高分散的硫化镍（Ni₃S₂和Ni₃S₄）, 粒径约为5.8 nm. 用三苯基膦（PPh₃）将上述硫化镍磷化, 如果磷化温度较低（170~220 ℃）, 硫化镍就无法被磷化, 但其粒径会逐渐增大至8.4 nm. 在270 ℃时部分硫化镍已被磷化成Ni₂P, 粒径增大至11.7 nm. 磷化温度升至320 ℃, 硫化镍全部转化为Ni₂P, 得到NiS_x（P）/Al₂O₃催化剂, Ni₂P粒径也进一步增大到13.3 nm.

如果将所得的Ni/Al₂O₃用PPh₃在320 ℃磷化36 h, NiO就转化为纯Ni₂P物相（粒径约为7.0 nm）, 得到Ni₂P/Al₂O₃催化剂. 用CS₂硫化Ni₂P/Al₂O₃时, 即使温度提高到360 ℃, 催化剂中仍只检测到Ni₂P物相, 表明Ni₂P物相比硫化镍（Ni₃S₂和Ni₃S₄）物相更加稳定, 难以被CS₂硫化, 这是Ni₂P具有较高耐硫性能的重要原因. 硫化后的Ni₂P（S）/Al₂O₃催化剂中Ni₂P粒径增大到9.5 nm, 但仍比320 ℃磷化硫化镍得到的Ni₂P的粒径（13.3 nm）小得多, 证明Ni₂P物相和粒径都比较稳定.

分析催化剂的化学组成发现, 硫化镍经PPh₃磷化后仅剩余了3%的S, 即绝大部分S已被P取代, NiS_x转化成了Ni₂P, 而磷化镍经CS₂硫化后只有1.1%的S进入催化剂中, 且未检测到硫化物物相, 表明Ni₂P确实具有很强的抗硫性能. 此外, 无论是将硫化镍磷化, 还是将磷化镍硫化, 所得样品中P/Ni比值相似（0.74）, 均高于Ni₂P的化学计量比0.5, 说明除了Ni₂P外, 催化剂中还可能存在富磷磷化镍或其他含P物种.

将上述直接磷化或硫化得到的Ni₂P/Al₂O₃和NiS_x/Al₂O₃催化剂, 以及硫化镍再磷化（NiS_x（P）/Al₂O₃）和磷化镍再硫化（Ni₂P（S）/Al₂O₃）后的催化剂分别用于模型柴油（含1.72%（质量分数）的二苯并噻吩（DBT）、0.185%（质量分数）的喹啉、5%（质量分数）的四氢萘）的加氢精制反应. 反应后, Ni₂P/Al₂O₃和NiS_x/Al₂O₃催化剂的物相都没有发生变化, 表明Ni₂P和NiS_x物相在加氢精制反应过程中均是稳定的. NiS_x（P）/Al₂O₃经过HDS反应, Ni₂P的XRD衍射峰依然存在, 且较强, 表明该催化剂在反应条件下没有被DBT硫化, 即由硫化镍磷化而转化来的Ni₂P在加氢精制反应条件下也是稳定的. Ni₂P（S）/Al₂O₃经过HDS反应, 仍以Ni₂P物相为主, 但出现了NiP₂, 可能是因为催化剂中残留了过量P, 它在温度较高时与Ni₂P反应, 生成了过磷化物NiP₂.

在DBT的HDS反应中, Ni₂P/Al₂O₃的HDS活性远高于NiS_x/Al₂O₃. 在360 ℃时, 以Ni₂P为主要物相的Ni₂P/Al₂O₃、NiS_x（P）/Al₂O₃和Ni₂P（S）/Al₂O₃的DBT加氢脱硫活性均达到了100%, 而NiS_x/Al₂O₃的DBT转化率只有75%. 同时, Ni₂P（S）/Al₂O₃的HDS活性高于Ni₂P/Al₂O₃, 例如在260 ℃时, 它们的HDS转化率分别为66%和45%. Ni₂P在CS₂硫化时物相没有发生变化, 但有1.1%的S进入了催化剂, 表明Ni₂P经CS₂硫化后有Ni-P-S表面物种生成, 其HDS加氢活性高于纯Ni₂P. NiS_x（P）/Al₂O₃中也含有3%的S, 但其HDS活性却与Ni₂P/Al₂O₃差别不大, 可能是由于NiS_x（P）/Al₂O₃中Ni₂P的粒径（13.0 nm）远大于Ni₂P/Al₂O₃的粒径（7.0 nm）. 可见, 由于S的存在, NiS_x（P）/Al₂O₃中Ni₂P的活性还是显著提高了. 因此, 在360 ℃, DBT在这些催化剂上的转化率顺序为: Ni₂P（S）/Al₂O₃ > NiS_x（P）/Al₂O₃ > Ni₂P/Al₂O₃ > > NiS_x/Al₂O₃.

在产物选择性方面, 随着反应温度提高, 所有催化剂上生成环己基苯（CHB）的选择性增大, 表明在这些催化剂上, DBT的HDS反应以先加氢再脱硫路线为主, 而非生成联苯（BP）产物的直接脱硫路线. 4个催化剂上HDS反应生成CHB的选择性顺序为Ni₂P/Al₂O₃ > NiS_x（P）/Al₂O₃ > Ni₂P（S）/Al₂O₃ > NiS_x/Al₂O₃, 表明催化剂中的S不利于加氢脱硫反应路线.

在260~360 ℃下, 4种催化剂都表现出很高的加氢脱氮活性, 喹啉转化率均达到100%. 虽然各催化剂上丙基环己烷（PCH）的选择性都随反应温度提高而提高, 但不同催化剂对PCH的选择性也存在差异（较低温度下尤为明显）. 260 ℃时, NiS_x/Al₂O₃、Ni₂P/Al₂O₃、Ni₂P（S）/Al₂O₃和NiS_x（P）/Al₂O₃上的PCH选择性依次为30%、90%、80%和88%. 可见, Ni₂P对喹啉的苯环加氢活性远高于硫化镍.

不同方法得到的磷化镍对四氢萘加氢精制反应的催化活性均远高于硫化镍. 例如, 360 ℃时四氢萘在NiS_x/Al₂O₃上的转化率仅有0.6%, 但在NiS_x（P）/Al₂O₃、Ni₂P/Al₂O₃和Ni₂P（S）/Al₂O₃上四氢萘的转化率分别达到了5.7%、9.2%和13.8%, 与催化剂的DBT加氢脱硫活性顺序一致. 四氢萘在加氢精制反应中既可发生加氢反应生成反式和顺式十氢萘, 也可发生脱氢反应生成萘. Ni₂P的十氢萘选择性远高于NiS_x, 而含硫Ni₂P催化剂的十氢萘选择性则降低了, 再次表明Ni₂P中含硫不利于芳环加氢反应.

1 Introduction

Nickel sulfides have various applications in the fields such as semiconductors, optoelectronics, energy and catalysis^[1-5]. However, they are not highly active for the hydrotreating reactions^[6-10], and were mainly used as promoters for the supported Ni-Mo-S and Ni-W-S catalysts^[11-13]. On the other hand, the transition metal phosphides（especially Ni₂P）were found to be highly active for the hydrotreating reactions and expected to become a new generation of catalysts to play the same functions as the traditional catalysts of metal sulfides. In fact, different techniques have been developed for the preparation of metal phosphides, for example, the technique of temperature-programmed reduction（TPR）, the gas phase phosphidation with PH₃ and liquid phase phosphidation with PPh₃^[14-16].

Only a few comparative studies on the nickel sulfides and phosphides were reported in literature. Phimsen et al.^[17] prepared a 15%Ni/Al₂O₃ catalyst, which was sulfided by H₂S/H₂ to nickel sulfides（Ni₃S₂ and Ni₃S₄）or phosphided by the TPR method to nickel phosphides（Ni₁₂P₅ and Ni₃P）. They found that the nickel phosphides showed the higher hydrogenation and hydrodeoxygenation activities than the nickel sulfides while the nickel sulfides exhibited the lower methanation and cracking activities than the nickel phosphides for the production of bio-fuels from the spent coffee oil. Stinner et al.^[18] found that the presence of H₂S in feed strongly inhibited the hydrodenitridation（HDN）activity of a Ni₂P catalyst, and the activity could not be recovered even after the removal of H₂S from the feed. The authors argued that Ni₂P was more stable than NiS_x and Ni₂P could not be sulfided by H₂S so that the decrease of HDN activity must be due to the irreversible block of active sites of Ni₂P by the strongly adsorbed H₂S. However, others reported that sulfur could be incorporated into the surface of metal phosphides during the HDS reactions. For example, Oyama et al.^[19-20] detected Ni-S bonds on the surface of used nickel phosphides, suggesting the formation of nickel phosphosulfide surface species, which might be the real active species for the HDS reactions. Wang et al.^[21] found that the Ni₂P/MCM-41 passivated by H₂S was more active than that passivated by O₂ for the HDS reaction, and the catalyst with the higher HDS activity contained more sulfur after the reaction. The authors also suggested that the surface sulfides or phosphosulfide species were the active sites for the HDS reaction. Similar results were obtained on the FeP/SiO₂ prepared with FeS as the precursor^[22]. Nelson et al.^[23] indicated that up to 50% of surface P atoms in Ni₂P could be replaced by S according to their DFT calculations.

It is not easy to adequately compare nickel sulfides with phosphides since different precursors and preparation methods could significantly affect the composition and performance of catalysts. To a large extent, the relatively low HDS activities of nickel sulfides might be due to their low surface area, poor dispersion and thus little active sites^[7-9]. It can be expected that more active sites and consequently higher HDS activity could be achieved with nickel sulfides. Recently, highly dispersed Ni₂P^{[16, 24-27]} andFexP^[28-29] catalysts were prepared in our group, with the highly loaded（60%~80% weight percent）and dispersed precursors. The Ni₂P catalysts thus prepared possessed small particles and abundant active sites and exhibited high activities for the HDS reactions of dibenzothiophene（DBT）. In particular, the phosphidation of the 80% Ni/Al₂O₃（with the high H₂ uptake of 1017 μmol/g）generated a Ni₂P/Al₂O₃ catalyst with the CO uptake of 345 µmol/g, which is the highest value reported so far^[26]. In this work, the similar nickel precursor was used to prepare nickel sulfides and compared with Ni₂P. However, since the nickel sulfides were prepared by the direct sulfidation with CS₂, Ni₂P in comparison must be prepared via the direct phosphidation with the same calcined precursor without the pre-reduction. In brief, the highly dispersed 80% Ni/Al₂O₃ catalyst was prepared, calcined and then sulfided by CS₂ or phophided by PPh₃ in liquid phases to obtain the highly dispersed nickel sulfides（NiS_x）or Ni₂P catalysts. Then the NiS_x（or Ni₂P）was further treated by PPh₃（or CS₂）to observe the phase changes. All the catalysts were tested for the HDS of DBT, HDN of quinoline and hydrogenation of tetralin.

2 Experimental 2.1 Preparation of catalysts

The Ni/Al₂O₃ with about 80% Ni in weight was prepared by the co-precipitation method and dried with n-butanol^[26]. In brief, desired amounts of Ni（NO₃）₂and Al（NO₃）₃ were dissolved in 100 mL de-ionized water to obtain a solution, while desired amount of Na₂CO₃ was dissolved in 100 mL de-ionized water to obtain another solution. The two solutions were simultaneously added drop-wise into 200 mL de-ionized water under stirring at 80 ℃. The precipitate formed waswashed thoroughly with de-ionized water, and then dispersed into 200 mL n-butanol which was evaporated at 80 ℃. The sample was then further dried at 120 ℃for 12 h, and calcined in air at 350 ℃ for 3 h. The samplethus obtained was denoted as NiO/Al₂O₃.

The sample was sulfided by CS₂ in liquid ph-ase^[29-30]. Typically, the NiO/Al₂O₃ was loaded into a fix-bed reactor with an inner diameter of 10 mm and sandwiched with quartz sands. Then, a heptanes solution containing 2%（weight percentage）CS₂ was pumped into the reactor along with H₂ flow at a weight hourly space velocity（WHSV）of 2 h^-1 and H₂/oil of 300 v/v. The reactor was heated from room temperature to 230 ℃ with a rate of 1 ℃/min, kept at 230 ℃ for 2 h, then heated to 310 ℃ with the same rate, and kept at 310 ℃ for 4 h. The sample thus sulfided directly was denoted as NiS_x/Al₂O₃.

The NiO/Al₂O₃ was phosphided by PPh₃ in liquidphase^[26]. The procedure was similar to that of sulfi-dation above, but a heptane solution containing 2%（weight percentage）PPh₃ was used here. The temperature was elevated to 320 ℃ with the rate of 1 ℃/min and kept at 320 ℃ for 36 h. The catalyst thus phos-phided directly was denoted as Ni₂P/Al₂O₃.

Then, the NiS_x/Al₂O₃ was treated by PPh₃ with the same phosphidation procedure described above and the sample obtained was denoted as NiS_x（P）/Al₂O₃. Similarly, the Ni₂P/Al₂O₃ was treated by CS₂ with the same sulfidation procedure described above and the sample obtained was denoted as Ni₂P（S）/Al₂O₃.

2.2 Characterization of catalysts

The catalysts for characterizations were passi-vated before unloaded from the fix-bed reactor. X-ray diffraction（XRD）patterns were collected on a Shimadzu XRD-6000 powder diffractometer using a Cu Kα radiation（λ=0.1541 nm）at 40 kV and 30 mA.The 2θ scans covered the range of 10° to 80° with a speed of 6（°）/min and a step of 0.02°. The chem-ical compositions of catalysts were analyzed by an ARL-9800 X-ray fluorescence spectrometer（XRF）.

2.3 Catalytic tests

Model diesel containing 1.72% DBT, 0.185%quinoline, 5% tetralin and 0.5% n-octane（as an internal standard）in balanced n-tetradecane（solvent）was pumped into a tubular stainless steel fix-bed reactor with an inner diameter of 10 mm for the evaluation of catalysts at 260~360 ℃, 3.1 MPa with a WHSV of 2 h^-1and H₂/oil ratio of 1500（v/v）. The products were collected after 24 h when the activity was stabilized, and analyzed on a gas chromatograph equipped with a flame ionization detector（FID）, a flame photometric detector（FPD）and a nitrogen-phosphorus detector（NPD）.

3 Results and discussion 3.1 Phosphidation of NiSx

Fig. 1 shows the XRD patterns for the NiO/Al₂O₃, NiS_x/Al₂O₃ and the samples derived from the phos-phidations of NiS_x/Al₂O₃ at 170~320 ℃ for 36 h, respe-ctively. Only broad diffraction peaks for NiO（PDF 65-2901）were detected for the calcined sample（Fig. 1（a））, indicating that small NiO particles（about 3 nm）were highly dispersed on the support. When the NiO/Al₂O₃ was sulfided by CS₂ at 310 ℃, the diffraction pea-ks for NiO disappeared, and new peaks for Ni₃S₂（PDF 44-1418）appeared at 2θ=21.7°, 31.1°, 37.7°, 44.3°, 49.7° and 55.2° and those for Ni₃S₄（PDF 43-1469）appeared at 2θ=26.5°, 31.3°, 37.9°, 49.9° and 54.8°（Fig. 1（b））, indicating the format-ion of nickel sulfides（denoted as NiS_x）upon the direct sulfidation of NiO/Al₂O₃ by CS₂.

Fig. 1（c-f） show the XRD patterns for the NiS_x/Al₂O₃ phosphided at different temperatures（170~320 ℃）for 36 h. When the NiS_x/Al₂O₃ was phosphided at 170 and 220 ℃, only Ni₃S₂ and Ni₃S₄ phases were obs-erved（Fig. 1（c） and （d））, indicating that NiS_x could not be phosphided at the low temperatures. However, the intensities of diffraction peaks for NiS_x increased, indicating the increase in the crystalline sizes of NiS_x during the phosphidation. When the NiS_x/Al₂O₃ was phosphided at 270 ℃, the diffraction peaks for NiS_x dis-appeared, and the diffraction peaks for Ni₂P（PDF 65-1989）appeared at 2θ=40.7°, 44.6°, 47.4° and 54.2° and those for Ni₁₂P₅（PDF 22-1190）appeared at 2θ=48.9°（Fig. 1（e））, indicating the replacement ofS in NiS_x by P during the process. When the phos-phidation temperature increased to 320 ℃, the diffract-ion peak for Ni₁₂P₅ disappeared while the intensities of peaks for Ni₂P increased significantly（Fig. 1（f））, indicating the complete conversion of NiS_x to Ni₂P（NiS_x（P）/Al₂O₃）and growing of the Ni₂P particles.

Table 1 summarizes the information of the phases and crystalline sizes for the samples（a-f）in Fig. 1. The crystalline size of NiO obtained by the calcination of Ni/Al₂O₃ at 350 ℃ was estimated to be about 3 nm, while the direct sulfidation led to the formation of NiS_x with the average crystalline size of about 5.8 nm. Although the NiS_x was not phosphided by PPh₃ at the lower temperatures, the crystalline sizes of NiS_x were gradually increased to 8.4 nm. When NiS_x was phosphided at 320 ℃, the crystalline size of formed Ni₂P grew larger to about 13.3 nm.

3.2 Sulfidation of Ni₂P

Fig. 2 shows the XRD patterns for the NiO/Al₂O₃, Ni₂P/Al₂O₃ and those obtained by the sulfidation of Ni₂P/Al₂O₃ at 310~360 ℃, respectively. When the NiO/Al₂O₃ was phosphided by PPh₃ at 320 ℃, pure Ni₂P phase wasobtained（Fig. 2（b））. However, when the Ni₂P/Al₂O₃ was treated by CS₂ at even higher temperature（360 ℃）, the Ni₂P was still the only phase detected by XRD, indicating that the Ni₂P was significantly more stable than the sulfide phases（Ni₃S₂ and Ni₃S₄）, and highly resistant to S poisoning.

Table 2 displays the changes in the phases and crystalline sizes for the samples shown in Fig. 2. The crystalline size of Ni₂P obtained by the direct phosphidation of NiO/Al₂O₃ with PPh₃ was about 7.0 nm. When the Ni₂P/Al₂O₃ was treated by CS₂ at 310 ℃, both the Ni₂P phase and the crystalline size（6.7 nm）did not change. When it was treated at 360 ℃, the Ni₂P phase still remained, but the crystalline size of Ni₂P increased to 9.5 nm, which was still significantly smaller than that formed by the phosphidation of NiS_x at 320 ℃（13.3 nm）, indicating the high stability of phase and crystalline size of Ni₂P.

Table 3 summarizes the information about the phases, crystalline sizes and compositions for the NiS_x/Al₂O₃, Ni₂P/Al₂O₃, NiS_x（P）/Al₂O₃（NiS_x/Al₂O₃ phosphided at 320 ℃）and Ni₂P（S）/Al₂O₃（Ni₂P/Al₂O₃sulfided at 310 ℃）. It is seen that only 3% sulfur remai-ned in the NiS_x（P）/Al₂O₃, i. e., most S in the NiS_x was replaced by P upon the phosphidation of NiS_x at 320 ℃. In contrast, only 1.1% sulfur was incorporated into the Ni₂P（S）/Al₂O₃ when the Ni₂P/Al₂O₃ was treated by CS₂ at 310 ℃, indicating that Ni₂P was highly resistant to sulfur. The P/Ni atom ratios in the NiS_x（P）/Al₂O₃ andNi₂P（S）/Al₂O₃ were similar（0.74）, higher than the stoichiometric ratio of 0.5 for Ni₂P, probably due to the presence of some other phosphorus species in the catalysts^[16].

3.3 Hydrotreating activities of nickel sulfides and phosphides

The catalysts NiS_x/Al₂O₃, Ni₂P/Al₂O₃, NiS_x（P）/Al₂O₃ and Ni₂P（S）/Al₂O₃ were compared for the hydr-otreating reactions. Fig. 3 shows the XRD patterns of the used catalysts after the hydrotreating reactions. It is seen that NiS_x and Ni₂P phases in NiS_x/Al₂O₃（7.7 nm）and Ni₂P/Al₂O₃（8.0 nm）remained after the reactions（Fig. 3（a） and （c））, indicating that both NiS_x and Ni₂P phases were stable during the HDS reactions.

The diffraction peaks for Ni₂P in the NiS_x（P）/Al₂O₃also remained with similar intensities（~13.0 nm）after the HDS reactions（Fig. 3（b））. However, some diffraction peaks for NiP₂（PDF 21-0590）were observed in the used catalyst after the HDS reaction. The reaction of Ni₂P with extra P in the catalyst during the HDS reaction might account for the formation of NiP₂.

The diffraction peaks for Ni₂P in the Ni₂P（S）/Al₂O₃ did not seem to change（7.0 nm）after the HDS rea-ctions（Fig. 3（d））, indicating the high stability of Ni₂P in this catalyst.

Fig. 4（a） shows the activities of the NiS_x（P）/Al₂O₃, Ni₂P/Al₂O₃, NiS_x（P）/Al₂O₃ and Ni₂P（S）/Al₂O₃ for theHDS of DBT. Apparently, the Ni₂P/Al₂O₃ catalysts were significantly more active than the NiS_x/Al₂O₃. It is known that the Ni₂P was more active than sulfide catalysts for the hydrotreating reactions^[14]. However, the comparisons were usually made between Ni₂P and the commercial Ni-Mo-S catalysts^{[14, 20, 31-33]}. In the current work, the Ni₂P and NiS_x were prepared from the same precursor. In addition, the NiS_x/Al₂O₃（without Mo）prepared in this work showed the high activity for the HDS reaction. The conversion of DBT reached 75% at 360 ℃. This must due to that the NiS_x/Al₂O₃ catalysts were prepared from the highly dispersed NiO/Al₂O₃ with small NiO particles（~ 3 nm）.

It is worth noting that the Ni₂P（S）/Al₂O₃ was moreactive than the Ni₂P/Al₂O₃ for the HDS of DBT. For example, at 260 ℃, the conversion of DBT over the Ni₂P（S）/Al₂O₃ and Ni₂P/Al₂O₃ was about 66% and 45%, respectively. Some authors^{[19-22, 34]} proposed that the M-P-S（M=metal）species formed on the surfaces of transition metal phosphides during the HDS reactions might be the real active species. When the Ni₂P/Al₂O₃ was treated with CS₂, the phase of Ni₂P did not change, but some S（1.1%）was incorporated into the Ni₂P（S）/Al₂O₃ according to the XRF analysis. Thus, our current result agreed well with reported that surface Ni-P-S species must be more active than pure Ni₂P for the HDS reactions.

The NiS_x（P）/Al₂O₃ contained 3% sulfur exhibi-ted the similar activity to the Ni₂P/Al₂O₃ for the HDS reaction. However, the crystalline size of Ni₂P in the NiS_x（P）/Al₂O₃ was significantly larger（13.3 nm）thanthat in the Ni₂P/Al₂O₃（7.0 nm）. Thus, the HDS activity of Ni₂P in the NiS_x（P）/Al₂O₃ was significantly higher than that in the Ni₂P/Al₂O₃, due to the presence of S.

The above results showed that the conversion of DBT at 360 ℃ followed the order of Ni₂P（S）/Al₂O₃ > NiS_x（P）/Al₂O₃ ~ Ni₂P/Al₂O₃ > > NiS_x/Al₂O₃.

Fig. 4（b） shows the selectivities for the HDS of DBT over the catalysts at different temperatures. With the increase of temperature, the selectivity to cyclohexylbenzene（CHB）increased over all the four catalysts. The high selectivity to CHB indicated that the HDS of DBT underwent mainly through the indirect desulfurization route（HYD）over the catalysts, where the desulfurization occurred after an aromatic ring in DBT was hydrogenated, rather than the direct desulfurization route（DDS）with the formation of bi-phenyl. It is also seen that the selectivity to CHB over the catalysts followed the order of Ni₂P/Al₂O₃ > NiS_x（P）/Al₂O₃ > Ni₂P（S）/Al₂O₃ > NiS_x/Al₂O₃, indicating that the presence of sulfur might be unfavorable to the HYD route over Ni₂P for the HDS of DBT.

Fig. 5 shows the results for the conversion of quinoline and selectivity to propyl-cyclohexane（PCH）over the catalysts at different temperatures. The conversion of quinoline over all the catalysts reached 100% at 260~360 ℃, indicating the high HDN activityof these catalysts. Although the selectivity to PCH increased with temperature over the catalysts, the catalysts exhibited different selectivities to PCH（especially at low temperatures）. For example, the selectivity to PCH at 260 ℃ over the NiS_x/Al₂O₃, Ni₂P/Al₂O₃, Ni₂P（S）/Al₂O₃ and NiS_x（P）/Al₂O₃ were about 30%, 90%, 80% and 88%, respectively, indicating that Ni₂P was much more active than NiS_x for the hydrogenation of aromatic ring in quinoline.

Fig. 6（a） shows the conversions of tetralin over the catalysts. It is seen again that the Ni₂P（S）/Al₂O₃ catalyst exhibited much higher activities than the Ni₂P/Al₂O₃ and NiS_x/Al₂O₃ catalysts. For example, the conversion of tetralin at 360 ℃ was only 0.6% over theNiS_x/Al₂O₃, while that was 5.7%, 9.2% and 13.8% over the NiS_x（P）/Al₂O₃, Ni₂P/Al₂O₃ and Ni₂P（S）/Al₂O₃, respectively, in consistence with the results for the HDS of DBT.

Tetralin might be hydrogenated to decalin or dehydrogenated to naphthalene during the hydrotreating reactions. Fig. 6（b） shows the selectivity to decalin over the catalysts. Apparently, Ni₂P exhibited much higher selectivity to decalin than NiS_x, and the Ni₂P catalysts containing a small amount of S showed the moderate selectivities, indicating again that the presence of S was unfavorable to the hydrogenation of aromatic rings.

4 Conclusions

A highly loaded and dispersed 80% Ni/Al₂O₃ catalyst was prepared and calcined at 350 ℃ to obtain a NiO/Al₂O₃ sample with small NiO particles（~ 3 nm）.Then the NiO/Al₂O₃ was sulfided by CS₂ at 310 ℃ to Ni-S_x/Al₂O₃（Ni₃S₂ and Ni₃S₄）or phosphided by PPh₃ at 320 ℃ to Ni₂P/Al₂O₃.

When the NiS_x/Al₂O₃ was phosphided with PPh₃ at 320 ℃, NiS_x was converted into Ni₂P（13.3 nm）（NiS_x（P）/Al₂O₃）, which was as active as the directly phosphided Ni₂P/Al₂O₃（7.0 nm）, indicating that theNi-P-S species must be more active than the Ni-P species for the HDS of DBT, consistent with the results reported^[20-21].

When the Ni₂P/Al₂O₃ was treated with CS₂ at 310~360 ℃, the Ni₂P phase did not change, indicating that Ni₂P was much more stable than NiS_x（Ni₃S₂ and Ni₃S₄）. Moreover, the Ni₂P（S）/Al₂O₃（6.7 nm）treat-ed with CS₂ at 310 ℃ was more active than the directly phosphided Ni₂P/Al₂O₃（7.0 nm）, indicating again that the Ni-P-S species must be more active than the Ni-P species for the HDS of DBT.

Acknowledgments: Financial supports from the NSFC（21773108）, NSFC-DFG（21761132006）and fundamental research funds for central universities are acknowledged.