Hydrodeoxygenation of phenol as a bio-oil model compound over intimate contact noble metal–Ni₂P/SiO₂ catalysts

Abstract

This study investigates phenol hydrodeoxygenation on supported Ni₂P, prepared via the sol–gel and TPR methods, and noble metal (Pd, Pt and Ru)–Ni₂P catalysts, prepared from Ni₂P by the partial in situ reduction of a noble metal precursor. 10% Ni₂P/SiO₂ had a relatively uniform distribution of Ni₂P nanoparticles and a highly active activity for phenol hydrodeoxygenation. Phenol conversion increased with increasing reaction temperature and the main products on noble metal–Ni₂P/SiO₂ also changed from cyclohexanol at 453 K to cyclohexane at 493 K. In comparison with supported Ni₂P or noble metal catalysts and their physical mixture, Pd–Ni₂P/SiO₂ presented the highest conversion activity and cyclohexane selectivity. Physicochemical characterization showed that the number of active sites on the catalysts increased and electron transfer occurred from Ni₂P to the noble metal due to the intimate contact between Ni₂P and the noble metal. A synergistic effect of the deoxygenation and carbonyl hydrogenation from Ni₂P and the hydrodeoxygenation from Pd resulted in an improvement of the catalytic activity and differences in the selectivity of the catalysts.

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1. Introduction

Many efforts have been focused on the production of renewable and clear biofuels and chemicals due to energy consumption and environmental deterioration. Lignin, typically composed of methoxy-substituted phenyl propanoid units, is the second most abundant component in biomass and can be regarded as a potential resource for the production of biofuels and special chemicals.¹ Bio-oil made from lignin, however, cannot be used without prior refining due to its high viscosity, low heating value and corrosiveness. Thus, hydrodeoxygenation (HDO) has been developed to convert this biofuel with these undesirable properties into feasible vehicle fuel.

Up to now, hydrodeoxygenation catalysts mainly include the traditional MoS₂, noble metal or transition metal phosphide based catalysts.^2–4 Among them, nickel phosphide has been prepared using the traditional temperature programmed reduction (TPR) method. However, there are still problems such as high preparation temperatures and the large and uncontrollable particle sizes of nickel phosphide with this method. For this reason, several other processes have been developed, such as the solution-phase method,⁵ the decomposition of metal hypophosphites⁶ and converting metal nanoparticles into metal phosphides.⁷

At the same time, a series of composite catalysts for the hydrodeoxygenation of guaiacol or phenol, the common model chemicals in biofuel, have also been studied.^8,9 For example, Yan et al.¹⁰ catalyzed the hydrodeoxygenation of phenols using Ru nanoparticle catalysts combined with Brønsted acidic ionic liquids. Hong et al.¹¹ prepared Pd/WOx/γ–Al₂O₃ for guaiacol hydrodeoxygenation. With respect to bimetallic catalysts containing a transition metal phosphide, Pd–Ni₂P/C has presented a 1.5 times higher power density than commercial Pd for direct formic acid fuel cells.¹² Ni₂P could also enhance the activity and durability of the Pt anode catalyst in direct methanol fuel cells.¹³ In addition, some researchers also noticed that the programmed reduction temperature for phosphate decreased and the hydrodesulfurization activity increased with the addition of Pd or Pt into a Ni₂P supported catalyst.¹⁴ However, these catalysts have usually been obtained by impregnating a passivated phosphide or phosphate with a noble metal precursor solution. It is difficult to ensure the directional loading of the noble metal and the formation of an intimate contact between the phosphide and noble metal due to the nonselective deposition of the noble metal on the support or phosphate. It is noteworthy that a transition metal phosphide requires further passivation to avoid the drastic oxidation of nickel phosphide when it is exposed to air. This means that phosphide has a certain reducibility.

Herein, supported nickel phosphide was firstly prepared from the sol–gel and the traditional TPR method. Thus, Ni₂P was uniformly dispersed over SiO₂. The as-prepared supported nickel phosphide was then impregnated with a noble metal precursor solution to obtain the noble metal–Ni₂P catalyst. This preparation process not only favors the distribution of the noble metal and minimizes its usage due to the controllable distribution of reductant Ni₂P, but also facilitates the formation of an intimate contact structure and intensifies the interaction between the noble metal and Ni₂P. Catalytic HDO evaluation and catalyst characterizations were conducted on the supported Ni₂P, noble metal and noble metal–Ni₂P catalysts. The results showed that noble metal–Ni₂P/SiO₂ had higher catalytic performances for phenol hydrodeoxygenation, due to more active sites and electron transfer from Ni₂P to Pd, than the supported Ni₂P or noble metal catalysts. Based on these outcomes, the relationship between the structure and catalytic performance were also discussed. The results may provide valuable information for developing Ni₂P or noble metal based catalysts to upgrade bio-crude-oil to feasible fuel.

2. Experimental

2.1. General

Chemical reagents (GC grade) used for analysis, such as benzene, phenol, cyclohexanol, cyclohexane, cyclohexene, cyclohexanone, isooctane, dodecane and the noble metal precursors, were from Aladdin Chemistry Co., Ltd. All other chemicals were of analytical grade from Sinopharm Chemical Reagent Co., Ltd, and were used as received without any further purification.

2.2. Preparation of the Ni₂P/SiO₂ catalysts and noble metal–Ni₂P/SiO₂

Ni₂P/SiO₂ was prepared via a sol–gel process according to ref. 7. In the typical preparation, a homogeneous solution of 2.62 g Ni(NO₃)₂·6H₂O, 4.2 g urea and 225 mL H₂O was kept in a conical flask at 303 K, and its pH value was adjusted to 2.5 using a diluted HNO₃ solution. A solution of 22.4 mL tetraethyl orthosilicate (TEOS) and 20 mL ethanol was dropwise added to the above solution under stirring. The as-prepared sample was then heated to 363 K, filtered at pH 8.0, dried at 393 K for 12 h and calcined in U-tube quartz reactor at 823 K for 5 h to obtain Ni/SiO₂. Ni₂P/SiO₂ was further prepared via the traditional TPR method using a NH₄H₂PO₄ solution to impregnate the as-prepared Ni/SiO₂ sample. The nominal Ni/P ratio of the catalyst was 1.0. After impregnation for 12 h, the sample was calcined at 823 K in an air atmosphere for 4 h. After being cooled to room temperature, the sample was reduced at 923 K in a H₂ flow for 3 h and passivated under 1.5% O₂/N₂ for 6 h at room temperature.

The noble metal–Ni₂P/SiO₂ was prepared by an in situ reduction of the noble metal precursors. The detailed procedure is as follows. After the preparation of the fresh Ni₂P/SiO₂ in the U-tube quartz reactor was completed, the H₂ flow was switched to a N₂ atmosphere. This reactor filled with N₂ was placed in an ultrasonic cleaner (KQ-250DE). A certain concentration of a noble metal precursor solution, such as PdCl₂, H₂PtCl₆, or RuCl₃, was injected using a syringe into the freshly prepared Ni₂P/SiO₂ in the U-tube quartz reactor under ultrasonic conditions. The concentration of the noble metal precursor was 0.01 g mL⁻¹ and the corresponding liquid volume used was based on the nominal loading of the noble metal, 1%. After 40 min, the sample was washed, dried under H₂ at 393 K for 2 h and passivated under 1.5% O₂/N₂ for 3 h.

For comparison, the supported pure noble metal catalysts were prepared using the impregnation method. The preparation of the support SiO₂ was similar to that of Ni/SiO₂ except for the addition of the nickel precursor. After impregnating with the noble metal precursor solution for 12 h, the samples were calcined under H₂ at 723 K for 3 h.

2.3. Physicochemical characterization

The X-ray powder diffraction (XRD) measurements were carried out on a PAN-alytical X-ray diffractometer with Cu Kα (λ = 0.15406 nm) radiation. A continuous scan mode was used to collect the 2θ data from 10° to 90° with a step width of 0.0167°. Transmission electron microscopy (TEM) images were collected using a JEOL JEM-1400 instrument. The samples were prepared by dispersing the powders in ethanol and then dropping a small volume onto a carbon-coated copper grid.

The specific surface area was determined via the N₂ adsorption–desorption isotherm method at 77 K using a Micromeritics ASAP 2020 system and calculated using the BET (Brunauer–Emmett–Teller) method. The pore structure parameters were calculated using the BJH (Barrett–Joiner–Halenda) method. Prior to the analyses, the samples were outgassed at 523 K for 3 h to eliminate volatile adsorbates on the surface.

CO chemisorption was performed (Micromeritics ASAP 2020) to provide an estimate of the number of active sites on the samples. Usually, 0.1 g of a passivated sample was loaded into a quartz reactor and re-reduced in a H₂ flow (40 mL min⁻¹) at 723 K for 2 h. After evacuating for 1 h at this temperature, the samples were cooled down to RT in a He flow (60 mL min⁻¹). Subsequently, the first isotherm was taken. After the first isotherm was measured, the sample was evacuated for 2 h. The second isotherm was performed. The CO chemisorption obtained was calculated by subtracting the amount of adsorbed CO in the second isotherm.

To characterize the structure and composition of the catalysts, high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and energy-dispersive X-ray spectroscopy (EDS) were performed using a FEI TECNAI F30. X-ray photoelectron spectroscopy (XPS) measurements were performed using a Quantum 2000 Scanning ESCA Microprobe system with Al Kα radiation under ultrahigh vacuum. The binding energies were internally referenced to the C1s peak at 284.6 eV.

Transmission infrared spectra of the catalysts were collected in situ in a reactor cell placed in a Bruker Vertex 70 Fourier transform infrared (FTIR) spectrometer. The IR cell was equipped with CaF₂ windows, connections for inlet and outlet flows, and thermocouples to monitor and control the temperature. Before introducing CO, the samples were reduced in H₂ at 723 K for 3 h, then cooled to room temperature in a He flow and exposed to CO for 3 h to achieve saturation. The samples were then purged in an Ar flow for 30 min to remove the gaseous and weakly adsorbed CO species. The spectra were obtained in the absorbance mode and were presented after the subtraction of a background spectrum obtained on the freshly reduced samples.

2.4. Phenol hydrodeoxygenation evaluation

For the hydrodeoxygenation reaction, phenol (0.27 g), dodecane as a solvent, (20 mL) and the catalyst (0.1 g) were firstly loaded into an autoclave (100 mL). The reactor was flushed with N₂ to remove air and then stably pressurized with H₂ to 2.0 MPa at room temperature. After the autoclave was heated to 453 or 493 K, the reaction lasted for a certain amount of time. At last, the liquid product was analyzed with a gas chromatograph (SHIMADZU GC2010) equipped with an auto-sampler and a flame ionization detector, using N₂ as carrier gas. Isooctane was used as the internal standard for the quantification of products.

3. Results and discussion

3.1. Phenol hydrodeoxygenation and physicochemical characterization over Ni₂P/SiO₂

Fig. 1 shows the phenol conversion and product selectivity at 453 and 493 K. With increasing Ni₂P loading, the catalytic activity increases and then decreases at 453 and 493 K. A similar change tendency is also found over Ni₂P/SiO₂ for benzofuran hydrodeoxygenation.¹⁵ Fig. S1† demonstrates the XRD patterns of Ni₂P/SiO₂ with different loadings. There is no prominent Ni₂P diffraction peak at 5% Ni₂P/SiO₂ compared with the other samples due to the low loading and high distribution of Ni₂P. However, excess loading leads to larger particle sizes of Ni₂P and correspondingly decreases the hydrodeoxygenation activity. The appropriate catalyst is 10% Ni₂P/SiO₂ for phenol hydrodeoxygenation. The TEM micrographs in Fig. 2 present the distribution of the Ni₂P particle size in the Ni₂P/SiO₂ catalysts. With the increase in the loading from 5 to 15%, Ni₂P particle size increases from about 2 nm to 6 nm. 10% Ni₂P/SiO₂ has a uniform particle size of 3.2 nm. This is similar to the XRD result using the Scherrer formula.

Fig. 1 Phenol conversion and product selectivities of the different Ni₂P/SiO₂ catalysts: (a and b) 453 K; (c and d) 493 K.

Fig. 2 TEM images and particle size distribution for the different Ni₂P/SiO₂ catalysts: (a) 5%, (b) 10% and (c) 15%.

Cyclohexane, cyclohexanol and cyclohexanone are the main products over Ni₂P/SiO₂ at 453 K (Fig. 1). Cyclohexanol and cyclohexanone are intermediates for the phenol hydrodeoxygenation to cyclohexane.¹⁶ The higher reaction temperature favors the formation of more cyclohexane. Thus, the cyclohexane selectivity prominently rises and those of cyclohexanol and cyclohexanone dramatically drop, accompanied by an increase in phenol conversion when the reaction temperature increases from 453 K to 493 K.

3.2. Physicochemical characterization and phenol hydrodeoxygenation of noble metal–Ni₂P/SiO₂

Fig. 3 demonstrates the catalytic results over the noble metal–Ni₂P/SiO₂ catalysts at 453 and 493 K. With the addition of a noble metal, the catalytic activities of Ni₂P/SiO₂ increase dramatically. At the same time, the selectivity toward cyclohexanol increases while for cyclohexanone it decreases at 453 K. In other words, the presence of a noble metal leads to the generation of a great deal of cyclohexanol with the increase in the phenol conversion over the noble metal–Ni₂P/SiO₂ catalyst. In comparison with the supported noble metal catalysts (Fig. S2†), the catalytic activity over noble metal–Ni₂P/SiO₂ changes slightly. However, cyclohexane and cyclohexanol selectivities increase and the cyclohexanone selectivity decreases obviously. This is mainly because the existence of Ni₂P intensifies the deoxygenation and carbonyl hydrogenation over noble metal–Ni₂P/SiO₂. When the reaction temperature further increases up to 493 K, the effects of the addition of a noble metal on the catalytic activities are not prominent, since 10% Ni₂P/SiO₂ has a high phenol conversion, 92.6%, and the rise of the catalytic activities is limited. However, the main products also change from cyclohexanol at 453 K to cyclohexane at 493 K on noble metal–Ni₂P/SiO₂. The effect of reaction time on the product selectivities (Fig. S3†) also confirms that cyclohexanol can gradually convert into cyclohexane when prolonging the reaction time over 1% Pd–10% Ni₂P/SiO₂. Thus, in comparison with the pure noble metal or Ni₂P catalysts, the synergistic effect of the hydrodeoxygenation from Pd and the deoxygenation and carbonyl hydrogenation from Ni₂P gives rise to the increment in the hydrodeoxygenation activity and the differences in the selectivity of the noble metal–Ni₂P catalysts as well as higher reaction temperatures and longer times. In addition, the catalytic activities over the supported catalysts were compared with that of a mixture of 1% Pd/SiO₂ and 10% Ni₂P/SiO₂ ¹⁷ (Fig. 4 and Table S1†). The result showed that 1% Pd–10% Ni₂P/SiO₂ has the higher phenol conversion and cyclohexane selectivity than the pure and physically mixed samples. This implies again the occurrence of a synergistic effect between the noble metal and Ni₂P on 1% Pd–10% Ni₂P/SiO₂.

Fig. 3 Phenol conversion and product selectivities of the different noble metal–Ni₂P/SiO₂ catalysts: (a and b) 453 K; (c and d) 493 K.

Fig. 4 Phenol conversion and product selectivities of 1% Pd–10% Ni₂P/SiO₂ and the physical mixture of Pd/SiO₂ and Ni₂P/SiO₂.

Thus, a series of physicochemical characterizations were conducted to further unravel the root of the improvement of the catalytic performances of the noble metal–Ni₂P/SiO₂. Table 1 lists the BET surface area, pore volume, pore size and CO chemisorption for the different catalysts. As shown in Table 1, compared with the Si₂O support, the BET surface area and pore volume of the 10% Ni₂P/SiO₂ decrease with the addition of the phosphorus precursor. This indicates that parts of the catalyst pores were blocked by the phosphorus source during the impregnation process. On the other hand, with the addition of a noble metal, the pore size and surface area increase for noble metal–Ni₂P/SiO₂. The increments of the pore sizes result from the consumption of part of the Ni₂P. At the same time, only noble metal precursors in contact with dispersed Ni₂P can convert into noble metal nanoparticles because of the Ni₂P reducing the noble metal precursors during the preparation process. As a result, the increasing surface area is a compromise between the consumption of Ni₂P and the high dispersion of the generated noble metal nanoparticles. Moreover, with the addition of the noble metal, the number of active sites also increases, except for 1% Ru–10% Ni₂P/SiO₂. If it is assumed that CO is linearly adsorbed on the nickel atoms of Ni₂P, as was theoretically and experimentally shown by Nelson et al.¹⁸ and Layman and Bussell,¹⁹ respectively, and the same type of adsorption exists for the noble metal atoms, except for partially bridged CO over Pd and Ru, then increased CO chemisorption uptake implies that more active sites for hydrodeoxygenation are generated with the addition of a noble metal. As shown in Table 1, the trend for CO chemisorption uptake is 1% Pd–10% Ni₂P/SiO₂ > 1% Pt–10% Ni₂P/SiO₂ > 10% Ni₂P/SiO₂ ≈ 1% Ru–10% Ni₂P/SiO₂. This is roughly consistent with the activity result of the catalysts. Thus, the high hydrodeoxygenation capacity of Pd or Pt–Ni₂P/SiO₂ is partly attributed to the increase in the number of active sites.

Table 1 Textural and structural properties of different catalysts

Catalyst	Pore size (nm)	Pore volume (cm^3 g^−1)	BET surface area (m^2 g^−1)	CO uptake (μmol g^−1)
SiO2–sol–gel	9.30	0.98	421.00	—
10% Ni2P/SiO2	10.03	0.90	238.55	92.25
1% Pt–10% Ni2P/SiO2	11.60	0.85	293.63	109.51
1% Ru–10% Ni2P/SiO2	11.49	0.88	307.39	90.77
1% Pd–10% Ni2P/SiO2	15.62	1.00	257.12	121.32

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Fig. 5 shows the XRD patterns of noble metal–Ni₂P/SiO₂. The XRD peaks of Ni₂P(111), Pd(111), Pt(111) and Ru(111) are 40.714 (PDF#74-1385), 40.365 (PDF#87-0645), 39.796 (PDF#87-0646) and 40.772° (PDF#88-2333), respectively. The intensity of the Ni₂P(111) diffraction peak is relatively weak, whereas the noble metal peak becomes prominent with the addition of noble metal. This implies that the generation of a noble metal nanoparticle is at the expense of Ni₂P. The XRD patterns show that the Ni₂P(111) peak shifts toward lower angles for Pd(111) and Pt(111) with the addition of the noble metal, but not for Ru–Ni₂P/SiO₂ with its weak diffraction. This result indicates that Pd (0.179 nm) or Pt (0.183 nm) with their larger atomic radii is doped into the Ni₂P (Ni (0.124 nm)), or that the addition of Pd or Pt leads to lattice distortion of Ni₂P.²⁰ Furthermore, we believe that an interaction between the noble metals and Ni₂P is formed.

Fig. 5 XRD diffraction patterns of the different noble metal–Ni₂P/SiO₂ catalysts.

HAADF-STEM mapping demonstrates the distributions of palladium, nickel and phosphorus in Fig. 6. The loading site of the phosphorous precursor is uncontrollable while the formation of Pd nanoparticles depends on Ni₂P reducing the Pd precursor. As shown in Fig. 6, the distribution density of Pd is mainly consistent with that of Ni₂P. This result further verifies that a controllable distribution of noble metal nanoparticles is achieved and the intimate contact structure between the noble metal and Ni₂P is formed by Ni₂P reducing the noble metal precursor. Combined with the CO chemisorption and XRD results, we speculate that the controllable distribution of the noble metal nanoparticle results in the intimate contact between noble metal and Ni₂P, further leading to interactions between both of them and the increment in the number of active sites, compared with 10% Ni₂P/SiO₂.

Fig. 6 HAADF-STEM of 1.0% Pd–10% Ni₂P/SiO₂.

Fig. 7 shows the XPS spectra of the different pure noble metal and composite catalysts. It has been noted that the signal to noise ratio is similar for the three couples of samples containing Pd, Pt and Ru, respectively, and that the intensity differences between the SiO₂ and Ni₂P/SiO₂ supported samples are in the same direction. This implies that noble metal–Ni₂P/SiO₂ might have a stronger signal in the XPS spectra than the pure noble metal catalysts. In addition, Table S2† shows the surface atomic ratios with respect to Si, O and the noble metal of these catalysts. A comparison also showed that the composite catalysts have higher loading ratios of the noble metal on the surface than the SiO₂ supported catalyst. This result might be related to Ni₂P reducing the noble metal precursor into nanoparticles. The existence of reductant Ni₂P contributes to the generation of more noble metal particles. On the other hand, the spectrum peak of Pd in 1% Pd–10% Ni₂P/SiO₂ shifts toward a lower binding energy compared to the pure noble metal catalysts. The XPS signal of the supported metal particles shifts to a higher binding energy with decreasing particle size.²¹ Meanwhile, the electrons transferring from Ni₂P to the noble metal render the latter electron rich and move the corresponding signal toward a lower binding energy. Therefore, the Pd spectrum peak shifting toward the lower binding energy in Fig. 7 is a compromise, resulting from the smaller noble metal particles and electron transferring. However, this electron transferring is likely the reason for the high catalytic performance of Pd–Ni₂P/SiO₂. As proposed by Ueckert et al.,²² a metal site with a high electron density favors the formation of a π back bond between the aromatic ring and metal sites, which promotes the hydrogenation of the aromatic ring. Moreover, since the lowest unoccupied molecular orbital (LUMO) of C–O is antibonding and surfaces that are able to transfer electron density to this orbital facilitate the dissociation of the C–O bond,²³ the increased electron density on Pd accounts for the higher hydrodeoxygenation catalytic activity of Pd–Ni₂P/SiO₂. For product selectivity, cyclohexanol increases while cyclohexanone decreases over the noble metal–Ni₂P/SiO₂ catalysts due to the improvement in the hydrogenation capacity in comparison with 10% Ni₂P/SiO₂ at 453 K. As mentioned above, the synergistic effect of the hydrodeoxygenation from Pd and the deoxygenation and carbonyl hydrogenation from Ni₂P results in the promotion of the catalytic performance of the noble metal–Ni₂P catalysts. Herein, this effect is mainly due to the interaction between Ni₂P and the noble metal and the electron transfer from Ni₂P to Pd on Pd–Ni₂P/SiO₂. As the reaction temperature rises from 453 K to 493 K, the main product changes from cyclohexanol to cyclohexane because of the intensifying deoxygenation at higher temperatures. In contrast, although electrons also transfer from Ni₂P to Pt on the noble metal–Ni₂P catalysts,¹¹ 1% Pt–10% Ni₂P/SiO₂ exhibits a lower catalytic selectivity as well as a higher conversion than Ni₂P/SiO₂. That is, the synergistic effect of Pt and Ni₂P on the catalytic performance is less prominent than Pd–Ni₂P/SiO₂. Ru exists in the form of Ru⁴⁺ in 1% Ru–10% Ni₂P/SiO₂ because Ru is prone to oxidation again. Ru that has been oxidized and fewer active sites on 1% Ru–10% Ni₂P/SiO₂ account for a lower catalytic performance than of the other composite catalysts.

Fig. 7 XPS spectra of the different noble metal–Ni₂P/SiO₂ catalysts.

The electronegativity of an element represents the capability of an atom to draw an electron to itself. A larger electronegativity means that this atom has a greater ability to draw electrons. For Ni₂P, Ni^δ+ (0 < δ < 2) covalently bonds with P^δ− (0 < δ < 1). Ni has the lower electronegativity (1.91) while the value for phosphorus is 2.19. There is a small electron transfer from Ni to P in nickel phosphide. In contrast, the electronegativities of Pd, Pt and Ru are 2.20, 2.28 and 2.20, respectively. A noble metal in contact with Ni₂P probably accepts electrons from nickel due to the larger electronegativity of the noble metal.

CO-FTIR spectroscopy was also performed to further verify the electronic contribution of the supported catalysts (Fig. 8). The FTIR spectra of adsorbed CO shifts from 2077 cm⁻¹ to higher wavenumbers, 2094, 2083 and 2079 cm⁻¹ with the addition of Pd, Pt and Ru, respectively. According to the Blyholder model,²⁴ this shift toward higher frequencies is indicative of the strengthening of the CO bond due to decreased donation of the metal d-electrons into the CO π* antibonding orbital by back-electron transferring.²⁵ Since the presence of Pd gives rise to the electron donation from Ni₂P to Pd, electron enrichment on Pd and electron deficiency on Ni₂P occur, as also reported by the XPS data. Thus, metal d-electron transfer into the CO π* antibonding orbital decreases and the stretching vibration of the adsorbed CO shifts toward higher frequencies.

Fig. 8 Infrared spectra of adsorbed CO on 10% Ni₂P/SiO₂, 1% Pd–10% Ni₂P/SiO₂, 1% Pt–10% Ni₂P/SiO₂ and 1% Ru–10% Ni₂P/SiO₂.

In addition, the vibration frequency of CO adsorbed on the catalysts between 2077 and 2094 cm⁻¹ also indicates that most of the CO is linearly adsorbed on noble metal–Ni₂P/SiO₂, in agreement with the assumption for the determination of active sites in CO chemisorption. However, we also notice that the intensity change of the CO stretching vibration with the addition of the noble metal is not consistent with the active site number by CO chemisorption. Utilizing CO-FTIR for quantification is inaccurate due to the effects of the loading of catalysts and the roughness of the tested surface. In comparison with CO-FTIR, CO chemisorption is a typical method to determine the active sites of catalysts. Therefore, herein CO-FTIR is performed to only confirm the electron transfer from Ni₂P to the noble metal on the catalyst surface.

The evaluation and characterization results suggested that the key factors influencing the catalyst activity and the selectivities toward different products were the increased number of active sites and the electron transfer from Ni₂P to Pd upon addition of the noble metal as well as the appropriate reaction temperature and time. Furthermore, the increment of the active sites and the electron transfer are from the reduction of the noble metal precursor on Ni₂P/SiO₂ and the interaction between Ni₂P and the noble metal. Based on experimental results, a possible deoxygenation scheme over noble metal–Ni₂P/SiO₂ is also suggested as follows (Fig. 9). Pure Ni₂P has a higher selectivity for cyclohexanone and cyclohexanol. In contrast, cyclohexanone can continue to hydrogenate into cyclohexanol over noble metal–Ni₂P/SiO₂ under the same reaction conditions due to the synergistic effect of Ni₂P and Pd. When the reaction temperature further increases or the hydrodeoxygenation time is prolonged, cyclohexanol can continue to convert into cyclohexane over noble metal–Ni₂P/SiO₂. In terms of the Ni₂P catalyst system, benzene, however, is generated rarely.

Fig. 9 A possible deoxygenation scheme for noble metal–Ni₂P/SiO₂.

4. Conclusions

10% Ni₂P/SiO₂, prepared from the sol–gel and TPR method, has a uniform distribution of Ni₂P nanoparticles and a relatively high activity for phenol hydrodeoxygenation. With the addition of a noble metal, especially Pd, noble metal–Ni₂P/SiO₂ catalysts generate more cyclohexanol, compared to the pure Ni₂P or noble metal supported catalysts at 453 K. When the reaction temperature rises, the main products change from cyclohexanol at 453 K to cyclohexane at 493 K on Ni₂P/SiO₂ or noble metal–Ni₂P/SiO₂. In addition, 1% Pd–10% Ni₂P/SiO₂ presents higher catalytic performances than the supported or physically mixed catalysts. As for the noble metal and Ni₂P composite catalyst, the controllable distribution of the noble metal and the intimate contact between Ni₂P and the noble metal result in enhanced interactions between both of them and facilitate the increase in the number of active sites and electrons transferring from Ni₂P to the noble metal. The synergistic effect of deoxygenation and carbonyl hydrogenation from Ni₂P and hydrodeoxygenation from Pd gives rise to the increment of the hydrodeoxygenation activity and the generation of a large amount of cyclohexanol at 453 K and cyclohexane at 493 K.

Acknowledgements

This project was supported financially by the National Natural Science Foundation of China (Grant No. 21476188, 21106118). We would like to appreciate Xing Zhang’s contribution to the preparation of the manuscript.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra11203f

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