Article pubs.acs.org/IECR
Nickel−Silicon Intermetallics with Enhanced Selectivity in Hydrogenation Reactions of Cinnamaldehyde and Phenylacetylene Xiao Chen,† Miao Li,† Jingchao Guan,† Xinkui Wang,† Christopher T. Williams,‡ and Changhai Liang*,† †
Laboratory of Advanced Materials and Catalytic Engineering, Dalian University of Technology, Dalian 116024, China Department of Chemical Engineering, University of South Carolina, Columbia, South Carolina 29208, United States
‡
ABSTRACT: Nickel−silicon intermetallics have been prepared by a direct silicification method using SiH4 as the silicon source. The prepared nickel−silicon intermetallics were characterized by X-ray diffraction, transmission electron microscopy, temperature-programmed reduction, temperature-programmed desorption, X-ray photoelectron spectroscopy, and CO chemisorption measurements. The catalytic hydrogenation of cinnamaldehyde and phenylacetylene over the nickel−silicon intermetallics was investigated. Nickel−silicon intermetallics presented much higher selectivity to the intermediate product (hydrocinnamaldehyde) than monometallic nickel catalyst, which may be attributed to the repulsive force between the electronegative silicon atoms in the nickel−silicon intermetallics and oxygen atoms in the CO bond of cinnamaldehyde. In addition, nickel−silicon intermetallics showed excellent selectivity for the hydrogenation of phenylacetylene to styrene (ca. 93%) due to the strong modification of the electronic structure derived from the interaction of nickel and silicon.
1. INTRODUCTION Control of bond formation in reactions is the key to making chemical production more energy efficient and product specific in catalysis, thereby minimizing the formation of unwanted side products.1−4 It is also crucial for developing sustainable industrial processes for manufacturing fuels. The selective hydrogenation of carbon−carbon double or triple bonds (e.g., hydrogenation of phenylacetylene) and the hydrogenation of carbonyl compounds to the corresponding alcohols (e.g., hydrogenation of cinnamaldehyde) have been fundamental and often practical reactions for the synthesis and manufacture of fine chemicals, agrochemicals, and pharmaceuticals for several decades. Transition metals as catalysts are always used for the hydrogenation reaction. Recently, advances in preparation have produced ultrafine intermetallic particles, which combine elemental carbon, nitrogen, silicon, gallium, and phosphorus with transition metals. These materials exhibit superior catalytic properties and improved stability over their metallic counterparts.5−9 However, while metal carbides and nitrides typically exhibit initially high activity, they are easily poisoned by a small amount of sulfur-containing compounds.10 Thermochemical calculations have indicated that transition metal silicides can tolerate much higher H2S concentrations than the corresponding carbides and nitrides.11 This suggests that transition metal silicides have potential as catalysts for selective hydrogenation reactions in a range of applications. Transition metal silicides are formed from the dissolution of silicon atoms into metal lattices, and are thermally, mechanically, and chemically stable due to the lower electronegativity of silicon compared to carbon and the strong modification of electronic structure around the Fermi level of transition metals.12 Due to their unique physical and chemical properties, transition metal silicides are experiencing widespread interest from researchers in fields such as nanoelectronics,13,14 field emitters,15 spintronics,16 thermoelectricity,17,18 and solar energy conversion.19 However, there have only been a few © 2012 American Chemical Society
studies of their catalytic performance in the hydrogenation reaction up to the present. Wallace et al. indicated that both untreated and oxidized alloys Ni2Si, Ni5Si2, and Co2Si prepared by induction melting of the constituent metals exhibited high activity for the formation of methane from carbon monoxide and hydrogen.20−22 In our previous research, supported cobalt silicide catalysts synthesized by metal organic chemical vapor deposition of Co(SiCl3)(CO)4 showed high catalytic activity and selectivity in naphthalene hydrogenation.23,24 Unsupported nickel silicide modified nickel catalyst showed good activity in the hydrogenation of phenylacetylene (PA).25 However, the specific relationship between the structure and catalytic properties of metal silicides in hydrogenation reactions requires further examination. In this work, the applicability of direct silicification to obtain nickel silicide catalysts with different phases at relatively low temperature and atmospheric pressure is discussed. The catalytic properties of these nickel silicide catalysts for hydrogenation of cinnamaldehyde (CMA) and PA are studied. In addition, X-ray diffraction (XRD), transmission electron microscopy (TEM), temperature-programmed reduction (H2TPR), temperature-programmed desorption of hydrogen (H2TPD), CO chemisorption, and X-ray photoelectron spectroscopy (XPS) were used to determine the phase transformation, distribution, reducibility, and active sites of the as-prepared nickel−silicon intermetallics. Nickel silicides exhibit much higher selectivities to the intermediate products (hydrocinnamaldehyde and styrene) toward hydrogenation of CMA and PA compared to metallic nickel, respectively, which is attributed to the electronic modification and isolation of active sites. Received: Revised: Accepted: Published: 3604
September 28, 2011 February 10, 2012 February 13, 2012 February 13, 2012 dx.doi.org/10.1021/ie202227j | Ind. Eng. Chem. Res. 2012, 51, 3604−3611
Industrial & Engineering Chemistry Research
Article
2. EXPERIMENTAL SECTION 2.1. Preparation of Nickel−Silicon Intermetallics. Nickel−silicon intermetallic catalysts were prepared by the reaction of NiO nanoparticles with gas mixture containing silane and hydrogen. NiO was obtained by thermal calcination of the as-prepared Ni(OH)2 in air at 300 °C for 2 h. The Ni(OH)2 was prepared by precipitation of nickel acetylacetonate dissolved in ethylene glycol with sodium carbonate aqueous solution at 120 °C.26−28 The NiO sample was further reduced with hydrogen at 350 °C for 3 h and then silicidized by 10 vol % SiH4 in H2 at relatively low temperature and atmospheric pressure.25 Then the product was passivated in 1% O2/Ar overnight. The as-prepared catalyst was labeled as TNiSix (where T represents temperature). 2.2. Characterization. XRD patterns were recorded on a Rigaku D/Max-RB diffractometer with a Cu Kα monochromatized radiation source, operated at 40 KV and 100 mA. Diffraction data were collected between 5 and 90° (2θ) with a resolution of 0.02°. TEM was performed using a Philips CM200 FEG transmission electron microscope (accelerating voltage 200 kV) using high-resolution imaging, and EDX (energy-dispersive Xray spectroscopy). Powder samples were ultrasonicated in ethanol and dispersed on copper grids covered with a holey carbon film. Elemental mapping was conducted under STEM mode with the EDX detector as recorder. H2-TPR and H2-TPD analysis was carried out using the Micromeritics AutoChem 2720 equipment, which incorporates a thermal conductivity detector. Before starting TPR runs, about 30 mg of catalyst was reduced in a 10% H2/Ar gas mixture flow at a rate of 50 mL min−1 from room temperature to 850 at 10 °C min−1. For an H2-TPD experiment, 50 mg of the sample was reduced in a 10% H2/Ar gas mixture at 350 °C (heating rate of 10 °C min−1) for 3 h under a flow rate of 50 mL min−1. Once the catalysts were cooled to room temperature, the sample was purged in an Ar flow for 1 h. Subsequently, the temperature was linearly increased from room temperature to 850 °C at 10 °C min−1. The chemisorption of CO was carried out on a Micromeritics AutoChem II 2920 automated catalyst characterization system. About 0.10 g of a passivated sample was in situ reduced in H2 at 400 °C for 2 h and then cooled to room temperature in a He stream. The sample was pulsed of 5% CO/He, which was not stopped until the CO peak area remained constant. The CO uptakes were obtained by the accumulated CO adsorbed. It was assumed that the adsorption stoichiometriy is CO:Ni = 1 for all samples. Surface compositions were investigated by X-ray photoelectron spectroscopy (XPS) employing an ESCALAB250 (Thermo VG, USA) spectrometer with Al KR (1486.6 eV) radiation with a power of 150 W. Survey and individual highresolution spectra were recorded with a pass energy of 50 eV. Ni 2p and Si 2p lines were monitored. All core-level spectra were referenced as the C 1s neutral carbon peak at 284.6 eV and were deconvoluted into Gaussian component peaks. 2.3. Catalytic Hydrogenation. CMA hydrogenation (Scheme 1) and PA hydrogenation (Scheme 2) reactions were performed on the nickel and nickel silicide catalysts at a certain H2 pressure in stainless steel autoclave reactors. Temperature was controlled by a water bath with a stirrer. Before the reaction, catalysts were activated in 102 mL min−1 H2 for 1 h at 350 °C. For CMA hydrogenation, the reaction was
Scheme 1. Reaction Network of Cinnamaldehyde Hydrogenation
Scheme 2. Reaction Network of Phenylacetylene Hydrogenation
carried out at 80 °C and 3.0 MPa H2 with 0.20 g of catalyst in 12.5 mL of ethanol, and 0.01 mol of CMA. For PA hydrogenation, the reaction was carried out at 50 °C and 0.41 MPa H2 with 0.20 g of catalyst in 10 mL of 1 M ethanolic substrate solution. For both reactions, the reaction product composition was analyzed using a gas chromatograph (GC 7890F) with a flame ionization detector and an SE-54/52 capillary column. Turnover frequencies (TOFs) of the hydrogenation reaction over the Ni−Si intermetallic compounds were calculated from the formula
TOF =
FC tWM
(I)
where F (mol) is the moles of reactant, C is the conversion of reactant, t (s) is the reaction time, W (g) is the catalyst weight, and M (mol g−1) is the moles of the site calculated from the CO chemisorption.
3. RESULTS AND DISCUSSION 3.1. Textural and Structural Properties of Nickel Silicides. The bulk structure of nickel silicides prepared by direct silicification was analyzed by XRD. Figure 1 shows the
Figure 1. XRD patterns for nanoparticles of (a) metallic Ni that have been partially converted to NiO before being exposed to air and nanoparticles of nickel silicides prepared by direct silicification at (b) 250, (c) 350, and (d) 450 °C , respectively. 3605
dx.doi.org/10.1021/ie202227j | Ind. Eng. Chem. Res. 2012, 51, 3604−3611
Industrial & Engineering Chemistry Research
Article
XRD patterns of as-prepared metallic Ni (Figure 1a) and nickel silicides prepared by direct silicification at 250 (Figure 1b), 350 (Figure 1c), and 450 °C (Figure 1d). The diffraction pattern in Figure 1a shows that metallic Ni displays face-centered-cubic lattice structure (JCPDS Card No. 04-0850), which was easily oxidized to NiO because of exposure to air.29 The structure of Ni particles was transformed during silicification at 250 °C from the metallic Ni phase to a mixture of Ni2Si phase (JCPDS Card No. 48-1339) and NiSi phase (JCPDS Card No. 380844). This indicates that the silicon atoms are entering the Ni lattice during the silicification. The Ni2Si phase is a transient phase that forms on route to the NiSi phase,30,31 and thus the phase transition was susceptible to the silicification temperature for silicide formation. When the silicification temperature was increased to 350 °C (Figure 1c), the peak of Ni2Si was almost negligible. However, the proportion of NiSi increases slightly in addition to the appearance of peaks associated with NiSi2 (JCPDS Card No. 43-0989). Further increase of the silicification temperature to 450 °C (Figure 1d) results in NiSi2 being dominant, with the NiSi phase almost completely removed. The relative contents of nickel silicides calculated from XRD patterns of T-NiSix are shown in Table 1. These Table 1. Phase Composition of T-NiSix Calculated from XRD Patterns catalysts Ni 250-NiSix 350-NiSix
450-NiSix a
crystalline phases
rel contenta (%)
Ni NiO Ni2Si NiSi Ni2Si NiSi NiSi2 NiSi NiSi2
57 43 52 48 33 57 10 16 84
Figure 2. (a) TEM and (b) HRTEM images of nickel silicides by direct silicification at 350 °C and (c−e) corresponding fast Fourier transforms of the lattice-resolved images that are indicated by boxes in the HRTEM image.
The relative contents are calculated by the intensity of XRD patterns.
compositions indicate that the formation of nickel silicides involves the sequence Ni 2 Si (orthorhombic) → NiSi (orthorhombic) → NiSi2 (cubic), with increasing silicification temperature. The structural properties and the composition distribution were investigated by TEM and scanning tunneling electron microscopy (STEM). Figure 2 shows the typical TEM and high-resolution TEM images of the 350-NiSix catalyst at different magnifications. From the low magnification image (Figure 2a), it is observed that the as-obtained nickel silicide nanoparticles are weakly aggregated. In order to further reveal the composition distribution of the 350-NiSix catalyst, the HRTEM image of one part of the image is shown in Figure 2b, which clearly shows that the structure of Ni particles was transformed to a mixture of nickel silicide phases due to silicification. The measured lattice spacings observed in HRTEM are 0.203, 0.210, and 0.197 nm, corresponding well to the (220), (310), and (121) lattice spacings of the orthorhombic Ni2Si structure, respectively.32,33 In addition, the measured interplanar spacing for the lattice fringes are 0.237 and 0.246 nm, which correspond well to the (201) and (111) lattice spacings of the orthorhombic NiSi structure, respectively.34 The fast Fourier transforms of the latticeresolved images (Figure 2c−e) show the reciprocal lattice peaks (Ni2Si(220), Ni2Si(201), NiSi(111)), which further indicates
that the 350-NiSix sample is a mixture phase including Ni2Si and NiSi, which is in harmony with the XRD pattern. In addition, the passivated 350-NiSix sample is confirmed by the STEM dark field image and the corresponding elemental maps shown in Figure 3. The elemental distributions of O, Si, and Ni clearly highlight that the NiSix nanoparticles are homogeneously decorated by SiOx overlayer since the sample was handled in air.25,35 H2-TPR and H2-TPD are powerful tools for studying the nature and reducibility of nickel silicides in the catalysts. Figure 4a shows the H2-TPR profiles for passivated nickel and the various nickel silicide catalysts. For the passivated nickel sample, the oxygen-deficient NiO overlayer on the surface was easily reduced to metallic nickel at the low temperature (ca. 160 °C), while a much higher temperature of around 425 °C was required for reduction of stable NiO.36 For the passivated nickel silicide samples, a low temperature peak at 100 °C is attributed to silicon oxide (SiOx) coated on the surface. This oxygen can be removed from the sample by a reduction in hydrogen.25 In addition, there is a peak at 300 °C that is tentatively assigned to the reduction of oxygen species adsorbed on nickel silicide. These results are similar to those of the TPR of CoSi/SBA-15.24 In addition, the peak shifts to the higher temperature with the increased silicification 3606
dx.doi.org/10.1021/ie202227j | Ind. Eng. Chem. Res. 2012, 51, 3604−3611
Industrial & Engineering Chemistry Research
Article
interactions lead to a change in the strength of chemisorbed H2 due to a significantly altered electronic structure.12,39 These results thus further confirm the formation of nickel silicides with different phases that have significantly different surface structures as well as electronic and crystallographic properties, compared to metallic Ni. In addition, the irreversible extent of CO chemisorption decreased from 83.2 to 2.9 μmol g−1 (Table 2), which can be explained as that silicon withdraws electrons Table 2. TOFs of Cinnamaldehyde and Product Selectivity over Ni and T-NiSix Catalysts selectivity (%) sample Ni 250NiSix 350NiSix 450NiSix
Figure 3. Representative STEM dark-field image and corresponding X-ray maps of O, Ni, and Si for passivated nickel silicide by direct silification at 350 °C.
TOFs × 102 (s−1)
HCMA
CMO
HCMO
99.9 99.9
1.7 8.7
4.1 44.1
0.2 0.3
95.7 55.6
3.0
99.9
45.6
54.8
0.2
45.0
2.9
99.9
48.5
39.8
0.5
59.7
CO uptake (μmol g−1)
conversion (%)
83.2 15.9
a
b
a The reaction was carried out at 80 °C and 3.0 MPa H2 for 120 min, with 0.20 g of catalyst, 12.5 mL of ethanol, and 0.01 mol of CMA. bIt was assumed that the adsorption stoichiometriy is CO:Ni = 1 for all samples.
from the less electronegative nickel substrate, decreasing πback-bonding between Ni and CO. On the other hand, the TCD signal of CO chemisorption on pure silicon (silicon powder from Aladdin Chem. Co. Ltd.) is almost zero, which indicates that CO molecules are not chemisorbed on silicon. This indicates that Ni atoms diluted by the formation of Ni−Si phases also decreased the quantity of CO chemisorption from the geometric effect. To investigate the electronic effect, XPS was used to characterize the Ni−Si intermetallic compounds (Figure 5). The Ni 2p core level spectra have been deconvoluted into Gaussian component peaks by XPSPeak software. The Ni 2p peaks of metallic Ni sample at 825.5, 855.4, and 860.9 eV were attributed to metallic Ni, NiO, and the satellite peak, respectively. The NiO peak was also observed because the sample was oxidized when it was exposed to air. After the silicification, the peak due to the Ni 2p in metallic Ni (852.5 eV) shifted to higher binding energy, which is attributed to the covalent interactions between Ni and Si, further demonstrating the formation of nickel silicide. The Ni 2p peak at about 852.6 eV can be attributed to the Ni2Si phase for the 250-NiSix sample. With increase of the silicification temperature to 350 °C, two peaks at 852.2 and 852.9 eV were observed and can be attributed to the Ni2Si and NiSi phase, respectively. With further increase of the temperature to 450 °C, the peak at 853.4 eV due to the NiSi2 phase can be observed in the 450-NiSix sample. This is similar to the XRD results. In addition, the peaks in the Si 2p region at 99 eV corresponding to silicon in the intermetallic Ni−Si compounds shifted to the lower binding energy due to electron shift from nickel to silicon atoms with the increased content of Si, which further indicated the interaction between Ni and Si in the Ni−Si intermetallic compounds. Meanwhile, there is an extra broad peak at 101− 104 eV in the Si 2p spectra due to Si−Ox, which is attributed to the formation of passivated layers on the Ni−Si intermetallic compounds. The Si−Ox layers protected the nickel silicides from violent oxidation when the sample was exposed to air.25
Figure 4. (a) H2-TPR profiles of nickel and nickel silicide samples with different silicification temperatures and (b) H2-TPD profiles of nickel and nickel silicide samples with different silicification temperatures.
temperature, probably due to the stronger interaction of Ni and Si as more silicon atoms enter the metallic Ni lattice. This result is similar to that obtained during TPR of Ni−Sn intermetallic compound particles.37 Interestingly, a reduction temperature at 170 °C in the 450-NiSix sample is observed, which may due to the reduction of NiO containing some oxygen vacancies. This suggests that the main composition (NiSi2) in the passivated 450-NiSix sample may be oxidized upon exposure to air as the following reaction indicates: NiSi2 + O2 → SiOx
