NickelâCobalt on Carbonaceous Supports for the Selective Catalytic

Article pubs.acs.org/IECR

Nickel−Cobalt on Carbonaceous Supports for the Selective Catalytic Hydrogenation of Cinnamaldehyde Lankitsi J. Malobela, Josef Heveling,* Willem G. Augustyn,† and Leskey M. Cele Department of Chemistry, Tshwane University of Technology, Private Bag X680, Pretoria 0001, South Africa S Supporting Information *

ABSTRACT: The catalytic hydrogenation of α,β-unsaturated carbonyl compounds can lead to several different products, of which the unsaturated alcohol (en-ol) is most difficult to obtain. In this regard, cobalt is known to have a positive influence on platinum catalysts. Little is known about such an effect on more cost-effective nickel catalysts. Nickel and cobalt (5 mass % each) were supported on graphite (GRA), multiwalled carbon nanotubes (MWCNT), and activated carbon (AC). The catalysts were characterized by N2-physisorption, X-ray diffraction (XRD), H2-chemisorption, and high-resolution transmission electron microscopy. XRD indicated the formation of a Ni−Co alloy. For cinnamaldehyde as the substrate, the en-ol selectivity and the turnover frequency (TOF) of the catalysts increased in the order GRA < AC < MWCNT. Ni−Co/MWCNT showed the highest selectivity over the whole conversion range, and at a conversion of 63% (TOF: 14.4 h−1) the product contained 62% en-ol and 38% saturated aldehyde. A positive influence of both cobalt and the support (MWCNT) on the selectivity of nickel catalysts is clearly indicated. sterically hindered.6,7 The isomerization of allyl alcohols to the aldehyde (dotted arrow in Scheme 1) is not an important route in liquid-phase hydrogenations8 but was observed for rhodium catalysts.9 The selectivity (CO vs CC hydrogenation) in the hydrogenation of α,β-unsaturated carbonyl compounds is determined by the nature of the catalytically active metal, the presence of a second metal,10 the metal particle size (dispersion), steric constraints imposed by the environment of the active site (e.g., by the support), and possibly by so-called strong metal−support interaction (SMSI).11 The solvent or the polarity of the solvent must also be taken into account.12,13 Electron-donating (or -withdrawing) “ligand” effects induced by deliberate addition of bases or by the catalyst support itself have also been studied.12−15 Thus, an increased selectivity for the formation of the unsaturated alcohol is often explained in terms of a transfer of π-electrons from the graphitic planes of carbonaceous supports to the metal particles. As a result, the charge density on the metal increases, decreasing the probability of CC bond activation16,17 and favoring the formation of allyl alcohols as hydrogenation products.8,18 In this regard, carbon nanotubes (CNT) are especially promising support materials. In addition, the absence of microporosity in bulk CNTs can be expected to prevent significant mass transfer limitations. This can have a beneficial effect on the control of catalyst activity16 and selectivity.19 Table 1 contains a few representative examples for the influence of the catalyst composition (and some reaction parameters) on the hydrogenation of cinnamaldehyde. The turnover frequencies (TOF) shown were calculated as averages

1. INTRODUCTION The reduction of a single functional group in the presence of others is a general selectivity problem in synthetic organic chemistry, and the development of catalysts that are able to deal with such complexities has been a challenge for many years.1 Recent progress made in the field of nanotechnology provides new routes for the preparation of catalysts and catalyst supports, and this should contribute to a more rational design of catalytically active materials.2 In this paper we describe the selective hydrogenation of trans-cinnamaldehyde (Scheme 1) with catalysts based on Scheme 1. Hydrogenation of Cinnamaldehyde (hydrogenation of the aromatic ring excluded)

nanocrystalline nickel−cobalt particles supported on activated carbon (AC), graphite (GRA), and multiwalled carbon nanotubes (MWCNT). Among all the possible hydrogenation products of cinnamaldehyde, hydrocinnamaldehyde and cinnamyl alcohol are of industrial importance, mainly as components of flavors and fragrances, and as building blocks for organic syntheses.3,4 Clearly, the selective catalytic hydrogenation of a carbonyl group in the presence of a carbon−carbon double bond is most difficult to achieve,5 in particular when the double bond is not © 2014 American Chemical Society

Received: Revised: Accepted: Published: 13910

May 26, 2014 August 15, 2014 August 18, 2014 August 18, 2014 dx.doi.org/10.1021/ie502143a | Ind. Eng. Chem. Res. 2014, 53, 13910−13919

Industrial & Engineering Chemistry Research

Article

Table 1. Representative Literature Results for the Hydrogenation of Cinnamaldehyde over Catalysts Containing Pd, Pt, Co, and/or Ni selectivity (mol %) entry

catalyst

solvent

T (°C)

P (MPa)

t (h)

conv. (%)

an-al

en-ol

an-ol

other

TOFa (h−1)

ref

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

5%Pd/AC Pd/MWCNTb 5%Pd/CNF 0.3%Pt/ZrO2 0.3%Pt−0.1%Co/ZrO2 0.3%Pt−0.1%Ni/ZrO2 0.5%Pt/GRA 0.5%Pt/CNT 0.5%Pt−0.17%Co/CNT 0.5%Pt−0.17%Ni/CNT 37%Co/Al2O3 Co−B Co−B Ni−B 10%Ni/SiO2 10%Ni/γ-Al2O3c 10%Co-10%Ni/SiO2c

dioxane dioxane dioxane EtOH EtOH EtOH EtOH EtOH EtOH EtOH EtOH EtOH EtOH − EtOH − −

80 80 80 70 70 70 70 70 60 60 50 30 30 70 100 300 270

0.1 0.1 0.1 2 2 2 2 2 2 2 0.98 0.1 0.1 0.6 2 0.1 0.1

55 25 31 6 6 6 1.5 1.5 1.5 1.5 2 2.5 ? 1 2 0.04 1

98.3 100 97.5 49.9 90.2 70.6 83.6 48.0 86.9 77.6 55 39 100 − 97.6 74.2 63

40 80 94 25.5 5.3 64.3 39.7 57.0 5.4 74.0 0 15 5.4 95 98.8 80.2 59

0 0 0 42.2 88.0 7.8 28.0 43.0 93.0 20.3 99.3 78 70.3 5 − − 0

60 20 6 21.7 6.7 11.8 7.2 − 1.6 5.7 0 7 21.6 − − 0.8 34

− − − 10.6 0.0 16.1 25.0 − − − 0.7 − − − − 19 −

3.6 8.0 6.3 64 62 40 360 275 235 210 1.0 0.07 − 25 21 71 6.8

20−22 22 20−22 15 15 15 23,24 25 23 24 26 27 27 28 29 30 31

a

Calculated as moles of an-al, en-ol + an-ol formed per total moles of Pd, Pt, Co + Ni per hour. bPd confined within the nanotubes. cVapor phase.

over the given reaction time (t) and are based on the total amount of catalytically active metals (Pd, Pt, Co, and/or Ni) present on the catalysts. As Table 1 reveals, the active metal(s) used have an overriding influence on the selectivity. High yields of the saturated aldehyde (an-al) can be obtained over palladium at low hydrogen pressure; as a rule, the unsaturated alcohol (en-ol) is not detected. However, the reaction rates (TOFs) over Pd are rather low. Entries 2 and 3 reveal the beneficial effect of MWCNT and carbon nanofiber (CNF) supports on the selectivity in comparison to activated carbon (entry 1). Notably, for the catalyst of entry 2 the metal particles were confined within the tubes. Platinum is the most frequently studied metal. Pt is very active, but the hydrogenation of cinnamaldehyde leads usually to a mixture of the an-al, en-ol, and an-ol (entries 4, 7, and 8). However, addition of cobalt to platinum increases the selectivity to the en-ol (entries 5, 9), while addition of nickel increases the selectivity to the an-al (entries 6, 10). Monometallic cobalt catalysts have a high preference for the hydrogenation of the carbonyl group. Nitta et al. reported the first notable results (entries 12, 13). Over a cobalt boride catalyst they obtained 70% of the en-ol at 100% cinnamaldehyde conversion (entry 13). Ando et al. obtained 99.3% en-ol over a 37% Co/Al2O3 catalyst at a conversion of 55% (entry 11). However, the activity of pure cobalt catalysts is extremely low (compare the TOFs given in Table 1). As is obvious from the examples discussed above for platinum, the addition of platinum to cobalt does not change the principle selectivity pattern (entries 5, 9), but greatly increases the turnover frequency. As for Pt−Ni catalysts, the principal reaction over monometallic nickel is the hydrogenation of the CC bond, and the primary product obtained is the an-al (entries 14−16). Although the selectivity patterns of cobalt and nickel and their positive effects on the performance of platinum catalysts are known, the mutual effects of these two metals on each other have not been studied in any detail. However, in a continuous vapor phase operation at a conversion of 63%, Reddy et al. found the following selectivities for a 10%Co−10%Ni/SiO2

catalyst: 59% an-al and 34% an-ol (Table 1, entry 17). That is, both the carbonyl group and the carbon−carbon double bond were hydrogenated, but none of the desirable en-ol was detected. Luo et al. employed an amorphous Ni−Co−B catalyst for the hydrogenation of furfural in the liquid phase.32 In this case, the catalyst showed higher activity and selectivity for the formation of furfuryl alcohol than either Ni−B, Co−B or a physical mixture of the two. In summary, nickel is much more active than cobalt, but cobalt is highly selective for the hydrogenation of α,βunsaturated carbonyl compounds to the en-ol. (For a rationalization of these observations see Nitta et al.)27 The addition of cobalt to platinum significantly increases the en-ol selectivity of platinum catalysts, while high activities are maintained. If cobalt would have a similar effect on nickel, a promising route to selective, but less expensive catalysts for the formation of sought-after α,β-unsaturated alcohols could be developed. This paper deals therefore with the hydrogenation activity of supported nickel−cobalt particles. Activated carbon, graphite, and multiwalled carbon nanotubes were chosen as supports in order to profit from any positive carbonaceous support effects of the nature discussed above.

2. MATERIALS AND METHODS 2.1. Materials. Activated carbon (Darco KB-G) and multiwalled carbon nanotubes (>95%) were obtained from Sigma-Aldrich (product numbers 694185 and 675326, respectively); graphite from the National Carbon Company (grade SP-2); Ni(NO3)2·6H2O (>96%): Saarchem (4460700); Co(NO3)2·6H2O (>99%): BDH (10083); trans-cinnamaldehyde (99%): Sigma-Aldrich (C80687); trans-cinnamyl alcohol (98%), hydrocinnamaldehyde (95%), and 3-phenyl-1-propanol (99%) from Alfa Aesar (A13025, A10367, and A13022, respectively); Triton X-100 [4-(1,1,3,3-tetramethylbutyl)phenyl-polyethylene glycol] (>98%) from BDH-Merck; and hydrazine monohydrate (64−65% hydrazine in water) from Sigma-Aldrich. 2.2. Synthesis of the Catalysts. An acid/oxidative pretreatment was performed on all carbonaceous supports by 13911

dx.doi.org/10.1021/ie502143a | Ind. Eng. Chem. Res. 2014, 53, 13910−13919


Article

After flushing with hydrogen, the pressure was adjusted to 500 kPa H2 pressure. Under stirring, the temperature was slowly increased from room temperature to 200 °C and maintained at 200 °C for 10 min. This procedure was followed in order to make certain that the catalysts were in a reduced state before exposure to the substrate (cinnamaldehyde). Pressure and temperature were respectively reduced to 100 kPa and 30 °C, and the test substrate (20 mL) was injected. The stirring speed was set to 1000 rpm. After pressurizing to 500 kPa with hydrogen, the reactor was heated to 150 °C. Continuous feeding with hydrogen started when the reactor reached a temperature of about 100 °C, and hydrogenation continued for up to 10 h. Cinnamaldehyde conversions and product distributions were followed by gas chromatographic analysis of samples periodically withdrawn from the reactor. After each run, the pressure was released, and the reactor was allowed to cool down under nitrogen. The remaining reaction mixture was filtered (glovebox) the filter cake washed with ethanol and the wet catalyst stored under ethanol in airtight containers for further characterization and/or reuse. 2.5. Analysis of the Reaction Mixtures. The samples were analyzed on a Varian Chrompack CP-3800 gas chromatograph (GC), equipped with a Hewlett-Packard column (Zebron ZB-5, 30 m × 0.25 mm i.d. × 0.25 μm stationary phase thickness). The conditions used for analysis are given as follows. Solvent: ethanol; sample concentration: 5%; injection volume: 1 μL; split ratio: 40:1; injector temperature: 260 °C; mobile phase: He; column flow: constant at 5 mL/min; detector: FID (300 °C); temperature program: 1 min at 80 °C, 25 °C/min to 150 °C, 1 min at 150 °C, 25 °C/min to 180 °C, 1 min at 180 °C, 25 °C/min to 240 °C, 1 min at 240 °C. The peaks were identified and calibrated using pure compounds as standards. Conversions and product distributions were calculated from the GC analyses. Individual product selectivities are defined as mole percentages of the total product mixture formed (starting material excluded).

refluxing the material for 6 h in 55% nitric acid. After dilution with deionized water and filtration, the filter cake was washed with deionized water until the filtrate gave a neutral pH measurement. The samples were left to dry overnight at 40 °C. For the preparation of the catalyst precursors, the support materials (0.5 g) were sonicated for 10 min in 100 mL of an aqueous 0.1% Triton X-100 solution. To this dispersion were added 0.14 g of Ni(NO3)2·6H2O and 0.14 g of Co(NO3)2· 6H2O. The mixtures were warmed to 80 °C and stirred with a magnetic stirrer for 24 h. Hereafter the materials were filtered, washed, and dried at 100 °C for 1 h. The catalyst precursors obtained were prereduced over a period of 1 h by stirring with an excess of hydrazine monohydrate (40 mL) at 100 °C in a nitrogen glovebox. The samples were kept at 85 °C until excess hydrazine had evaporated. The reduced catalysts are highly pyrophoric and must be enclosed in airtight containers. 2.3. Characterization Techniques. The mass % of cobalt and nickel deposited on the supports were determined by inductively coupled plasma optical emission spectrometry, using a Spectro Cirosccd ICP-OES spectrometer. Nitrogen physisorption measurements (BET) were carried out using a Micromeritics ASAP 2020 surface area and porosity analyzer. This instrument was also equipped to perform hydrogen chemisorption measurements. For this purpose the samples were evacuated and prereduced with H2 at 450 °C, and H2 chemisorption was carried out at 30 °C. Metal dispersions and particle sizes were calculated collectively for cobalt and nickel, assuming a stoichiometric factor between the surface metal atoms and the chemisorbed gas atoms of 2:1.33 XRD diffractograms were recorded on a Bruker D8 Avance X-ray diffractometer, using nickel-filtered Cu Kα radiation (40 kV, 40 mA). The scans were performed at room temperature in 2θ steps of 0.02°, using enclosed sample holders to avoid air exposure. The phases were identified with the help of the Bruker DIFFRACplus evaluation software (EVA) in combination with the ICDD powder diffraction database (International Centre for Diffraction Data). The same software was used to calculate crystallite sizes. A Horiba Jobin Yvon HR 800 Raman spectrometer, equipped with Olympus BX 41 lenses, a 50× magnification lens, and a 514.5 nm (2.41 eV) argon laser beam with a power of 3.3 mW, was employed for Raman spectroscopy. A lower density filter (D1) was used, and the spectral range was scanned from 1000 to 3000 cm−1. The acquisition time was kept at 10 scans per region with a sampleto-laser exposure time of 3 s. Thermogravimetric analyses (TA Instruments, SDT Q6OO) were conducted in air between 30 and 1000 °C at a heating rate of 10 °C/min. High-resolution transmission electron microscopy (HRTEM) analyses of the catalysts were conducted at CSIR (Pretoria) using a JEOL2010F microscope, operating at 200 kV accelerating voltage with a beam current of 30 A. The samples were dispersed in ethanol under ultrasonic vibration and placed on carbon-coated copper grids. Approximately 100 particles from three different HRTEM images per sample (i.e., in total 300 particles per sample) were measured in relation to the scale given on the respective images to determine the particle-size distribution. The ImageJ software was used for this purpose. 2.4. Hydrogenation Reactions. Hydrogenation reactions were carried out in 50 mL stirred minireactor units described earlier.19 The catalyst (0.5 g) and ethanol (30 mL) were transferred into the mini-autoclaves in a nitrogen glovebox. The autoclaves were subsequently connected to the gas supply of the test unit, and purged several times with nitrogen (500 kPa).

3. RESULTS AND DISCUSSION 3.1. Characterization of the Support Materials and the Catalysts. Before and after acid/oxidative treatment the support materials were subjected to thermogravimetric analysis (TGA) in air. The thermograms are presented in Figure 1. AC shows a continuous mass loss from close to ambient starting temperature to 100 °C, associated with loss of moisture. Another significant decline in mass starts at 450 °C and is

Figure 1. Thermogravimetric analysis of the supports before and after acid/oxidative treatment with HNO3. 13912



Article

polyaromatic basal plane of graphite.39 This peak is sharper and narrower for GRA than for AC and MWCNT. The D-band results from structural disorder and a loss in translational symmetry.40,41 The 2D- or G′-band is attributed to the overtone of the disorder-induced D-band and is an intrinsic property of the two-dimensional (2D) graphene lattice.41,42 It is even present in high purity graphite crystals, where the D-band is completely absent. The relative intensity of the D-band divided by that of the G-band (ID/IG) is an indicator that increases as the sp2 symmetry of a sample decreases.43,44 The ID/IG values obtained for GRA, MWCNT, and AC before and after treatment with HNO3 are shown in Table 2. The value of

attributed to the loss of carbon by combustion (as CO2/CO). In contrast to the two-step mass decline of AC, MWCNT and GRA show only single-step mass losses at elevated temperatures. MWCNT keeps an almost constant weight up to 600 °C, and oxidation is complete at ∼750 °C, indicating that the sample is thermally stable and structurally well organized.34 GRA appears to be most inert and exhibits no weight loss up to 800 °C. For all three samples a reduction in oxidation temperature is observed after HNO3 treatment. This reduced thermal stability is most likely due to partial disintegration caused by the introduction of functional groups and defect sites during the acid/oxidative cure. According to Jiang et al.35 and Mathur et al.,36 less ordered structures tend to oxidize at lower temperatures, while highly graphitized carbon requires relatively higher temperatures. Overall, the results obtained are in good agreement with those found in literature reviews on the thermogravimetric analysis of AC, MWCNT, and GRA.37,38 It is also noteworthy that the residue (ash content) found for HNO3-treated AC is considerably less than that of the untreated sample. This indicates that much of the noncombustibles are removed from AC during acid treatment. The purity of as-received MWCNT (and GRA) is reflected by a low ash content already before acid treatment. The carbonaceous support materials were also analyzed by Raman spectroscopy, again before and after acid/oxidative treatment with HNO3. Figure 2 shows the corresponding spectra for MWCNT. The spectra for AC and GRA can be found in the Supporting Information (SI) as Figures S1 and S2. The G-band around 1580 cm−1, the D-band around 1340 cm−1, and the 2D- or G′-band around 2600 cm−1 are present on all samples. The G-band is associated with the Raman-allowed phonon mode E2g, which is the vibrational mode of the

Table 2. ID/IG Values Obtained for the Supports before and after HNO3 Treatment support

ID/IG (before)

ID/IG (after)

GRA MWCNT AC

0.17 0.61 0.80

0.24 0.87 1.17

0.61 obtained for MWCNT (before pretreatment) is in agreement with those reported by other authors.44 After acid/oxidative treatment changes in the graphitic structure of all samples are indicated, and the relative intensity of the Drelative to that of the G-band increases noticeably, although the overall concentration of structural defects remains lowest for GRA and highest for AC. It is well-known that concentrated oxidizing acids not only generate oxygen-containing functional groups attached to carbon, but also cause an increase in defective sites.45 These have an influence on the reactivity and the stability of carbonaceous materials.45 The specific surface areas and some pore properties of the supports, as determined by nitrogen physisorption, are summarized in Table 3 together with the results obtained for the corresponding Ni−Co loaded catalysts. Of the three supports, activated carbon has the highest surface area (860 m2/g), followed by MWCNT (170 m2/g) and graphite (6.1 m2/g). With a micropore volume of 0.1 cm3/g a large fraction of the surface area of AC is due to micropores (260 m2/g). The results obtained are in good agreement with the findings made by other researchers.46−48 Adsorption and desorption isotherms are shown in Figure 3. In spite of considerable microporosity, the isotherm of AC (Figure 3a) is dominated by Type IV features. The Type IV model isotherm is found for mesoporous materials.49 The isotherms of MWCNT (Figure 3b) and GRA (Figure 3c) can be classified as Type II with H3 and H4 type hysteresis loops, respectively. H3 is associated with slit-shaped pores, and H4, with plate-like particles.49 All three Ni−Co catalysts show a decrease in BET surface area, pore area (>micropores) and average pore width compared to the unloaded supports (see Table 3). This could be attributed to some pore blockage caused by the metal particles. The shapes of the isotherms of the fresh catalysts (not shown) are very similar to those of the respective support materials seen in Figure 3. This indicates that the inherent pore structure of the supports did not change by incorporation of the metals. X-ray diffraction patterns of the supports (before acid/ oxidative treatment) are shown in Figure 4. GRA displays characteristic peaks at 2θ values of 26.6°, 42.4°, 44.5°, 54.6°, and 77.5°, corresponding to the (002), (100), (101), (004), and (006) planes, respectively.48,50 MWCNT exhibits peaks at

Figure 2. Raman spectra of MWCNT before and after refluxing in HNO3. (A Gaussian−Lorentzian fit is used to allocate the exact positions of the D- and G-bands.). 13913



Article

Table 3. Nitrogen Physisorption of the Carbon Support Materials (after acid/oxidative treatment) and of the Corresponding Supported 5%Ni−5%Co Catalysts measurement

GRA

Ni−Co/GRA

MWCNT

Ni−Co/MWCNT

AC

Ni−Co/AC

BET surface area (m2/g) area of pores >micropores by t-plot (m2/g) micropore area by t-plot (m2/g) micropore volume by t-plot (cm3/g) mesopore volume by BJH (cm3/g) average pore width (nm)

6.1 5.7 0.50 0.0002 0.42 12

3.0 3.2 0.50 0.0003 0.40 8.6

170 81 31 0.01 0.33 8.2

150 80 10 0.0055 0.30 7.7

860 410 260 0.10 0.43 3.3

850 380 210 0.096 0.70 2.7

Figure 4. XRD patterns and the Miller indices found for graphite (a), MWCNT (b), and activated carbon (c).

delocalized π-electrons.54 The exact position of this peak is influenced by the spacing (d002) between the graphitic layers.48 Compared to AC (0.35−0.36 nm),55 MWCNTs have an interlayer spacing of 0.34 nm,56 close to that of the ideal graphite spacing of 0.335 nm.51,55,56 These values correspond to the 2θ shifts observed in Figure 4. The intensities of the carbon (002) reflections in the XRD spectra correlate with the Raman ID/IG ratios (Table 2). The Raman G-band and the (002) signal are related, since both result from the basal graphitic plane.57 A low ID/IG ratio predicts that the C(002) diffraction peak should be narrow and intense. XRD diffractograms of the three catalysts are shown in Figure 5. Compared to the corresponding diagrams of the

Figure 3. BET isotherms of activated carbon (a), MWCNT (b), and graphite (c) showing adsorption and desorption branches.

25.7°, 42.7°, and 61.5°, matching the (002), (100), and (110) reflections.51,52 AC shows its lack of crystallinity or long-range order by giving two broad peaks at 2θ angles of 24.0° (002) and 42.7° (101).50,53 It is clearly seen that GRA exhibits higher crystallinity and larger domains of ordered structure in comparison to MWCNT and AC. The results obtained correspond well with data found for similar materials in the literature.48,50−53 The most prominent peak for all carbonaceous materials is found at ∼25° and corresponds to the (002) basal plane. The basal plane is a flat, honeycomb-like structure of graphitic sp2-hydridized C atom sheets, associated with

Figure 5. XRD patterns obtained for 5%Ni−5%Co deposited on graphite (a), MWCNT (b), and activated carbon (c). 13914



Article

The exposed metal surface areas were determined by hydrogen chemisorption. From these, dispersions and average particle sizes were calculated. However, for bimetallic systems it is often not possible to determine the surface area of the two metals separately. This is particularly true for nickel and cobalt, as these two metals are very similar in their physical and chemical properties. Only the collective Ni and Co surface area is therefore taken as an estimate for the active catalyst surface. This, as suggested by XRD, corresponds to a Ni−Co alloy. The chemisorption results are included in Table 4. The MWCNTsupported catalyst gave the smallest average particle size (7 nm), followed by AC and GRA. HRTEM images of the supports taken before acid/oxidative treatment reveal that the samples are homogeneous and no impurities in the form of metal particles are visible. AC is amorphous and has no defined structure, while GRA is characterized by a plate-like morphology (as expected from the H4-type hysteresis loop of the physisorption isotherms shown in Figure 3c). For MWCNT low- and highmagnification micrographs are shown in Figure 6. The MWCNTs are well graphitized and typically consist of 15 concentric tubes with a tube length of several micrometers. The inner and outer diameters are approximately 5 and 25 nm, respectively. The tubes appear to be open-ended, curved, and twisted together in bundles. Notably, no amorphous carbon or catalyst residues (from the preparation procedure) are observed. The latter is in agreement with the low ash content found by TGA. HRTEM images of the three freshly prepared catalysts are seen in Figure 7. The metal particles appear as dark circular dots before the lighter background of the support. The images indicate that the metals are uniformly dispersed on the surfaces of the MWCNT and AC supports (b and c of Figure 7), but not on graphite (Figure 7a). The particle sizes on MWCNT and AC follow roughly a normal distribution, which is cut off at the lower end (particle size = 0). This is evident from the insets

supports, three new peaks (at 2θ = 44°, 52°, and 76°) are observed, marked with their corresponding indices (111), (200), and (220). The new (111) peak and the (101) peak of carbon are superimposed. The XRD peak allocations (ex ICDD) for a Ni50Co50 alloy are shown at the bottom of Figure 5. According to several publications, the three observed peaks indicate that the phase of the alloy is purely face-centered cubic (fcc).58−62 Unalloyed cobalt should primarily be present in its most stable hexagonal close packed (hcp) phase,58−61 which is absent. The absence of Co(hcp) suggests therefore that cobalt has alloyed with the nickel. Li et al. proposed the following mechanism for alloy formation during the reductive precipitation of nickel and cobalt salts: as the reduction of Ni2+ is more facile, nickel nucleates first as Ni(fcc); then cobalt is reduced and precipitates together with nickel on the preformed nickel nuclei. The nickel dopant stabilizes the fcc phase of cobalt, and the face-centered cubic cobalt−nickel alloy is produced.60 For each sample, the full-width at half-height of the strongest peak (111) was used to calculate the average crystallite size using the Scherrer equation.63 The results are listed in Table 4. The metal crystallite sizes determined by XRD vary with the support material and increase in the order MWCNT < AC < GRA. Table 4. Characterization of 5%Ni−5%Co Supported on Carbonaceous Materials by XRD and H2 Chemisorption XRD

H2 chemisorption

catalyst

metal crystallite size (nm)

metal surface area (m2/g)

metal dispersion (%)

metal particle size (nm)

Ni−Co/GRA Ni−Co/MWCNT Ni−Co/AC

23 10 15

18 55 43

4 21 14

27 7 19

Figure 6. HRTEM images of MWCNT. Measurements at low magnification (a) and high magnifications (b and c) are shown. 13915



Article

supported metals indicate that the particles are crystalline in nature and have a random orientation. Some authors elaborated on the disagreement between the average metal sizes obtained by different techniques.64 The most reliable and consistent data can be expected from HRTEM and H2 chemisorption, as these techniques provide either a direct image of the particles (HRTEM) or quantify directly the exposed metal atoms (chemisorption). However, particle overlapping and insufficient image contrast can lead to some uncertainty when using HRTEM. As evident from Figure 8, all three methods (XRD, H2 chemisorption, and HRTEM)

Figure 8. Average metal particle/crystallite sizes determined by different techniques for 5%Ni−5%Co/C catalysts.

confirm that the average particle/crystallite size obtained depends on the catalyst support and increases in the order MWCNT < AC < GRA. Finally, elemental analysis by ICP-OES confirmed that both nickel and cobalt were successfully loaded onto the supports in concentrations close to the targeted values of 5 mass %. For each metal 4.9−5.1% was found on all three supports. The exact values shown in Table 5 were used to calculate the molebased TOFs for the hydrogenation of cinnamaldehyde. 3.2. Hydrogenation of Cinnamaldehyde. The catalysts were tested for the hydrogenation of cinnamaldehyde as described in section 2.4. The key results are shown in Table 5. Simultaneous appearance of hydrocinnamaldehyde (an-al) and cinnamyl alcohol (en-ol) is observed over all three catalysts. At high conversion (∼63%) the AC-supported catalyst produces a small amount of the fully saturated 3-phenyl-1-propanol (anol); it is probably formed by consecutive hydrogenation of the en-ol and/or an-al. As is apparent from the graphical presentation in Figure 9, the MWCNT-supported catalyst has the highest activity (TOF), and also shows the highest selectivity for the formation of the unsaturated alcohol (enol). However, according to the TOFs based on the exposed metal surface areas (last column in Table 5), Ni−Co/GRA would emerge as the most active catalyst. On graphite, nickel and cobalt are purely dispersed (compare Table 4), leading to a relatively small active metal surface. Accordingly, the catalyst productivity expressed per total amount of metal (mol mol−1 h−1) is low. TOFs based on metal surface areas can therefore be somewhat misleading and are not necessarily a good estimation of efficient metal usage. For a recent debate on the usefulness of TOFs see refs 65 and 66. (From an economic point of view it would probably be best to report TOFs based on the commercial value of the metal, that is in mol $−1 s−1.) The mole-based TOFs of Table 5 allow for a direct comparison with the values extracted for other catalysts from the literature and shown in Table 1. (TOFs based on the

Figure 7. HRTEM images of 5%Ni−5%Co deposited on graphite (a), MWCNT (b), and activated carbon (c).

provided for each image. Average particle diameters of 2.8 and 7.8 nm were determined for 5%Ni−5%Co/MWCNT and 5% Ni−5%Co/AC, respectively. In contrast, the HRTEM image of 5%Ni−5%Co/GRA shows a bimodal distribution, from which an average particle size of 14.5 nm was calculated. The picture indicates that GRA, which has a surface area of only 6.1 m2/g, is overloaded with metal. In Figure 7b the MWCNT walls (horizontal) are clearly visible, and the lattice planes of the 13916



Article

Table 5. Hydrogenation of Cinnamaldehyde over 5%Ni−5%Co/C Catalysts (150 °C, 500 kPa, in ethanol) selectivity(mol %) catalyst Ni−Co/GRA Ni−Co/MWCNT Ni−Co/AC

Ni (mass %)

Co (mass %)

Ni + Co (m2 g−1)

4.95 5.13 5.13 5.14 5.14

4.89 5.05 5.05 5.10 5.10

18 55 55 43 43

t (h)

conv. (%)

an-al

en-ol

an-ol

TOFa (mol mol−1 h−1)

TOFb (mmol m−2 h−1)

5.0 3.0 8.0 3.5 9.5

29.8 30.0 62.6 29.2 63.2

44.7 31.5 37.9 43.3 50.5

55.3 68.5 62.1 56.7 46.9

0 0 0 0 2.6

11.3 18.4 14.4 15.2 12.1

1.05 0.58 0.45 0.62 0.49

a Calculated as moles of an-al, en-ol, and an-ol per total moles of Ni + Co per hour. bCalculated as moles of an-al, en-ol, and an-ol per total metal surface area per hour.

Figure 10. Selectivity versus cinnamaldehyde conversion observed for 5%Ni−5%Co catalysts supported on three different carbonaceous supports.

Figure 9. TOFs (per mole of Ni + Co) and en-ol selectivities obtained over 5%Ni−5%Co/C catalysts at a constant conversion of ∼30%.

exposed metal surface area can rarely be used for that purpose, as the required metal surface areas are seldom reported.) The activities of our Ni−Co catalysts are higher than those of priorart monometallic cobalt catalysts (Table 1, entries 11−13), and compare well with known pure nickel catalysts used in the liquid phase (entries 14, 15). In addition, all our catalysts are more active than a 10%Co−10%Ni/SiO2 catalyst (Table 1, entry 17). This catalyst (gas-phase operation) produces a mixture of the an-al and the an-ol, while all our catalysts (liquid phase) produce mixtures of the an-al and the en-ol, and the most difficult to obtain en-ol is the major product. If on our catalysts a mutual influence of the two metals on each other were totally absent, one would rather expect the an-al as the major product, as nickel is more active than cobalt and rapidly hydrogenates the carbon−carbon double bond (compare Table 1, entries 14−16). Cobalt is selective for the formation of the en-ol (entries 11−13), but the low reaction rate would limit its contribution to the formation of the final product mixture. Therefore, the results point to a positive effect of cobalt and nickel on each other, as the outcome differs considerably from that expected for a physical mixture of separate cobalt and nickel catalysts. The full potential of this effect needs to be evaluated by further optimization of the catalyst composition and possibly other reaction parameters. Figure 10 demonstrates that the catalysts under the current conditions lack intrinsic selectivity for the formation of a single product. Extrapolation of the plots to zero percent conversion indicates that the en-ol and the an-al are produced simultaneously as primary products over all three catalysts (about 64% en-ol and 36% an-al). Figure 10 also shows clearly that the MWCNT-supported catalyst produces the en-ol with the highest selectivity over the whole conversion range. Whether the superior performance of Ni− Co/MWCNT is primarily due to the smaller metal particle size (compare Figure 8) or due to a genuine support effect cannot be answered with certainty at this stage. However, as the selectivity to cinnamyl alcohol is known to increase with

increasing particle size,8 a support effect is more likely. Thus, it appears that the positive influence of carbon nanotube supports on the performance of palladium and platinum catalysts (as discussed in the introduction) is also applicable to Ni−Co catalysts.

4. CONCLUSION The literature reveals little about combinations of nickel and cobalt used for the catalytic hydrogenation of cinnamaldehyde. For economic and fundamental reasons it is important to know whether cobalt can improve the selectivity of nickel catalysts for the formation of the desired cinnamyl alcohol (en-ol), as is observed for platinum−cobalt catalysts. Graphite (GRA), multiwalled carbon nanotubes (MWCNT), and activated carbon (AC) were used as catalyst support materials. TGA and Raman spectroscopy, before and after acid/ oxidative treatment of the supports with nitric acid, confirmed the envisaged structural changes caused by exposure to HNO3. Nitrogen physisorption and XRD measurements displayed the typical features expected for GRA, MWCNT, and AC. The nickel−cobalt catalysts were prepared by reduction of predeposited metal salts, and elemental analysis (ICP-OES) confirmed that both nickel and cobalt were loaded onto the supports in concentrations close to the targeted values of 5 mass %. XRD pointed to the formation of a Ni50Co50 alloy. The average metal sizes determined by XRD, H2 chemisorption, and HRTEM correspond reasonably well. The metal particle size of the catalysts increases, depending on the support, in the order MWCNT (2.8 nm) < AC (7.8 nm) < GRA (14.5 nm). (The numbers in brackets are the values found by HRTEM.) For the graphite-supported catalyst a bimodal particle-size distribution is observed, and the support appears to be overloaded with metal. When tested for the hydrogenation of cinnamaldehyde, the en-ol selectivities and the TOFs (based on the total amount of 13917



Article

(9) Campelo, J. M.; Garcia, A.; Luna, D.; Marinas, J. M. Rh/AlPO4 Catalysts IX. Liquid-Phase Hydrogenation and Isomerization of α,βUnsaturated Alcohols. J. Catal. 1988, 113, 172−184. (10) Yu, W. W.; Liu, H. Singular Modification Effects of Metal Cations and Metal Complex Ions on the Catalytic Properties of Metal Colloidal Nanocatalysts. J. Mol. Catal. A: Chem. 2006, 243, 120−141. (11) Steffan, M.; Klasovsky, F.; Arras, J.; Roth, C.; Radnik, J.; Hofmeister, H.; Claus, P. Carbon-Carbon Double Bond versus Carbonyl Group Hydrogenation: Controlling the Intramolecular Selectivity with Polyaniline-Supported Platinum Catalysts. Adv. Synth. Catal. 2008, 350, 1337−1348. (12) Yu, W.; Wang, Y.; Liu, H.; Zheng, W. Preparation and Characterization of Polymer-Protected Pt/Co Bimetallic Colloids and Their Catalytic Properties in the Selective Hydrogenation of Cinnamaldehyde. J. Mol. Catal. A: Chem. 1996, 112, 105−113. (13) Yu, W.; Liu, H.; Liu, M.; Tao, Q. Selective Hydrogenation of a,b-Unsaturated Aldehyde to a,b-Unsaturated Alcohol over PolymerStabilised Platinum Colloid and the Promotion Effect of Metal Cations. J. Mol. Catal. A: Chem. 1999, 138, 273−286. (14) Lai, G. H.; Zhou, R. X.; Han, X. X.; Zheng, X. M. Effect of Transition Metals on Selective Hydrogenation of Cinnamaldehyde over Pt/ZrO2 Catalysts. Chin. Chem. Lett. 2004, 15, 1494−1496. (15) Han, X.; Zhou, R.; Yue, B.; Zheng, X. Selective Hydrogenation of Cinnamaldehyde over Pt/ZrO2 Catalyst Modified by Cr, Mn, Fe, Co and Ni. Catal. Lett. 2006, 109, 157−161. (16) Vu, H.; Gonçalves, F.; Philippe, R.; Lamouroux, E.; Corrias, M.; Kihn, Y.; Plee, D.; Kalck, P.; Serp, P. Bimetallic Catalysis on Carbon Nanotubes for the Selective Hydrogenation of Cinnamaldehyde. J. Catal. 2006, 240, 18−22. (17) Toebes, M. L.; Prinsloo, F. F.; Bitter, J. H.; van Dillen, A. J.; de Jong, K. P. Influence of Oxygen-Containing Surface Groups on the Activity and Selectivity of Carbon Nanofiber-Supported Ruthenium Catalysts in the Hydrogenation of Cinnamaldehyde. J. Catal. 2003, 214, 78−87. (18) Giroir-Fendler, A.; Richard, D.; Gallezot, P. Selectivity in Cinnamaldehyde Hydrogenation of Group-VIII Metals Supported on Graphite and Carbon. Stud. Surf. Sci. Catal. 1988, 41, 171−178. (19) Lekgoathi, M. D. S.; Augustyn, W. G.; Heveling, J. NickelCobalt Nanoparticles Supported on Single-Walled Carbon Nanotubes and Their Catalytic Hydrogenation Activity. J. Nanosci. Nanotechnol. 2011, 11, 7001−7005. (20) Pham-Huu, C.; Keller, N.; Ehret, G.; Charbonniere, L. J.; Ziessel, R.; Ledoux, M. J. Carbon Nanofiber Supported Palladium Catalyst for Liquid-Phase reactions. An Active and Selective Catalyst for Hydrogenation of Cinnamaldehyde to Hydrocinnamaldehyde. J. Mol. Catal. A: Chem. 2001, 170, 155−163. (21) Pham-Huu, C.; Keller, N.; Ledoux, M. J.; Charbonniere, L. J.; Ziessel, R. Carbon Nanofiber Supported Palladium Catalyst for LiquidPhase Reactions. An Active and Selective Catalyst for Hydrogenation of CC Bonds. Chem. Commun. 2000, 1871−1872. (22) Ledoux, M. J.; Vieira, R.; Pham-Huu, C.; Keller, N. New Catalytic Phenomena on Nanostructured (Fibers and Tubes) Catalysts. J. Catal. 2003, 216, 333−342. (23) Li, Y.; Zhu, P. F.; Zhou, R.-X. Selective Hydrogenation of Cinnamaldehyde to Cinnamyl Alcohol with Carbon Nanotubes Supported Pt-Co Catalysts. Appl. Surf. Sci. 2008, 254, 2609−2614. (24) Li, Y.; Lai, G. H.; Zhou, R.-X. Carbon Nanotubes Supported Pt−Ni Catalysts and Their Properties for the Liquid Phase Hydrogenation of Cinnamaldehyde to Hydrocinnamaldehyde. Appl. Surf. Sci. 2007, 253, 4978−4984. (25) Li, Y.; Ge, C.-H.; Zhao, J.; Zhou, R.-X. Influence of Preparation Modes on Pt-Ni/CNT Catalysts Used in the Selective Hydrogenation of Cinnamaldehyde to Hydrocinnamaldehyde. Catal. Lett. 2008, 126, 280−285. (26) Ando, C.; Kurokawa, H.; Miura, H. Selective Hydrogenation of Aldehyde Groups in Various a,b-Unsaturated Aldehydes over AluminaSupported Cobalt(0) Catalyst. Appl. Catal., A 1999, 185, L181−L183.

Ni and Co present on each support) increase in the order GRA < AC < MWCNT. For Ni−Co/MWCNT the product contains 62% en-ol and 38% hydrocinnamaldehyde (an-al) at a conversion of 63%. The corresponding TOF is 14.4 h−1. In particular at low conversions, high en-ol selectivities are also observed for Ni−Co/AC and Ni−Co/GRA, but over the whole conversion range the MWCNT-supported catalyst is significantly more selective than the other two catalysts. The en-ol selectivities are high. In particular the Ni−Co/ MWCNT catalyst combines the high activity of monometallic nickel catalysts with the high selectivity of cobalt catalysts known from the literature. The results are very promising and can probably be further improved by fine-tuning the catalyst composition (metal loading and cobalt-to-nickel ratio) and possibly other reaction parameters. A positive effect of both cobalt and the support (MWCNT) on the selectivity of nickel catalysts is clearly indicated.

■

ASSOCIATED CONTENT

S Supporting Information *

Figure S1. Raman spectra of AC before and after refluxing in HNO3. Figure S2. Raman spectra of GRA before and after refluxing in HNO3. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*Tel.: +27-012-3826125. Fax: +27-012-3826286. E-mail: [email protected]. Present Address †

The South African Nuclear Energy Corporation, R&D, Applied Chemistry, Section Delta-F, PO Box 582, Pretoria 0001, South Africa.

Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported financially by the National Research Foundation of South Africa under Grant Unique Number 63266. The authors thank CSIR (Pretoria) for help with catalyst characterization.

■

REFERENCES

(1) Heveling, J. Catalysis at Lonza: From Metallic Glasses to Fine Chemicals. Chimia 1996, 50, 114−118. (2) Chng, L. L.; Erathodiyil, N.; Ying, J. Y. Nanostructured Catalysts for Organic Transformations. Acc. Chem. Res. 2013, 46, 1825−1837. (3) Kohlpaintner, C.; Schulte, M.; Falbe, J.; Lappe, P.; Weber, J. Aldehydes, Aliphatic and Araliphatic. Ullmann’s Encyclopedia of Industrial Chemistry; Wiley-VCH: Weinheim, 2003; Vol. 2, pp 65−103. (4) Bauer, K.; Garbe, D.; Surburg, H. Flavors and Fragances. Ullmann’s Encyclopedia of Industrial Chemistry; Wiley-VCH: Weinheim, 2003; Vol. 14, pp 73−199. (5) Salman, F.; Park, C.; Baker, R. T. K. Hydrogenation of Crotonaldehyde over Graphite Nanofiber Supported Nickel. Catal. Today 1999, 53, 385−394. (6) Rylander, P. N. Hydrogenation Methods; Academic Press: London, 1985; pp 70−71. (7) Nishimura, S. Handbook of Heterogeneous Catalytic Hydrogenation for Organic Synthesis; Wiley: New York, 2001; pp 178−185. (8) Gallezot, P.; Richard, D. Selective Hydrogenation of α,βUnsaturated Aldehydes. Catal. Rev.Sci. Eng. 1998, 40, 81−126. 13918



Article

(27) Nitta, Y.; Imanaka, T.; Teranishi, S. Hydrogenation Activity and Selectivity of Cobalt Boride and Cobalt Nickel Binary Boride Catalysts. Bull. Chem. Soc. Jpn. 1980, 53, 3154−3158. (28) Schreifels, J. A.; Maybury, P. C.; Swartz, W. E., Jr. Comparison of the Activity and Lifetime of Raney Nickel and Nickel Boride in the Hydrogenation of Various Functional Groups. J. Org. Chem. 1981, 46, 1263−1269. (29) Fan, Q.; Liu, Y.; Zheng, Y.; Yan, W. Preparation of Ni/SiO2 Catalyst in Ionic Liquids for Hydrogenation. Front. Chem. Eng. China 2008, 2, 63−68. (30) Chin, S.-Y.; Lin, F.-J.; Ko, A.-N. Vapour Phase Hydrogenation of Cinnamaldehyde over Ni/γ-Al2O3 Catalysts: Interesting Reaction Network. Catal. Lett. 2009, 132, 389−394. (31) Reddy, B. M.; Kumar, G. M.; Ganesh, I.; Khan, A. Vapour Phase Hydrogenation of Cinnamaldehyde over Silica Supported Transition Metal-Based Bimetallic Catalysts. J. Mol. Catal. A: Chem. 2006, 247, 80−87. (32) Luo, H.; Li, H.; Zhuang, L. Furfural Hydrogenation to Furfuryl Alcohol over a Novel Ni-Co-B Amorphous Alloy Catalyst. Chem. Lett. 2001, 5, 404−405. (33) Aguinaga, A.; Montes, M.; Asua, J. M. Effect of the Preparation on the Initial Toxicity of Supported Nickel Catalysts Deactivated by Thiophene. J. Chem. Technol. Biotechnol. 1991, 52, 369−381. (34) Vander Wal, R. L.; Tomasek, A. J.; Street, K.; Hull, D. R.; Thompson, W. K. Carbon Nanostructure Examined by Lattice Fringe Analysis of High-Resolution Transmission Electron Microscopy Images. Appl. Spectrosc. 2004, 58, 230−237. (35) Jiang, W.; Nadeau, G.; Zaghib, K.; Kinoshita, K. Thermal Analysis of the Oxidation of Natural Graphite: Effect of Particle Size. Thermochim. Acta 2000, 351, 85−93. (36) Mathur, R. B.; Seth, S.; Lal, C.; Rao, R.; Singh, B. P.; Dhami, T. L.; Rao, A. M. Co-Synthesis, Purification and Characterisation of Single- and Multi-Walled Carbon Nanotubes Using the Electric Arc Method. Carbon 2007, 45, 132−140. (37) Raffaelle, R. P.; Landi, B. J.; Harris, J. D.; Bailey, S. G.; Hepp, A. F. Carbon Nanotubes for Power Applications. Mater. Sci. Eng., B 2005, 116, 233−243. (38) Lee, J.; Kim, M.; Hong, C. K.; Shim, S. E. Measurement of the Dispersion Stability of Pristine and Surface-Modified Multiwalled Carbon Nanotubes in Various Nonpolar and Polar Solvents. Meas. Sci. Technol. 2007, 18, 3707−3712. (39) Krüger, A. Neue Kohlenstoffmaterialien; Teubner Verlag: Wiesbaden, 2007; pp 211−220. (40) Ando, Y.; Zhao, X.; Shimoyama, H.; Sakai, G.; Kaneto, K. Physical Properties of Multiwalled Carbon Nanotubes. Int. J. Inorg. Mater. 1999, 1, 77−82. (41) Souza Filho, A. G.; Jorio, A.; Swan, A. K.; Ü nlü, M. S.; Goldberg, B. B.; Saito, R.; Hafner, J. H.; Lieber, C. M.; Pimenta, M. A.; Dresselhaus, G.; Dresselhaus, M. S. Anomalous Two-Peak G′-Band Raman Effect in One Isolated Single-Wall Carbon Nanotube. Phys. Rev. B 2002, 65, 1−8. (42) Tessonnier, J.-P.; Rosenthal, D.; Hansen, T. W.; Hess, C.; Schuster, M. E.; Blume, R.; Girgsdies, F.; Pfänder, N.; Timpe, O.; Su, D. S.; Schlögl, R. Analysis of the Structure and Chemical Properties of Some Commercial Carbon Nanostructures. Carbon 2009, 47, 1779− 1798. (43) Kumar, R.; Cronin, S. B. Raman Scattering of Carbon Nanotube Bundles under Axial Strain and Strain-Induced Debundling. Phys. Rev. B 2007, 75, 1−4. (44) Ouyang, Y.; Cong, L. M.; Chen, L.; Liu, Q. X.; Fang, Y. Raman Study on Single-Walled Carbon Nanotubes and Multi-Walled Carbon Nanotubes with Different Laser Excitation Energies. Physica E 2008, 40, 2386−2389. (45) Rosca, I. D.; Watari, F.; Uo, M.; Akasaka, T. Oxidation of Multiwalled Carbon Nanotubes by Nitric Acid. Carbon 2005, 43, 3124−3131. (46) Chen, M.; Yu, H.-W.; Chen, J.-H.; Koo, H.-S. Effect of Purification Treatment on Adsorption Characteristics of Carbon Nanotubes. Diamond Relat. Mater. 2007, 16, 1110−1115.

(47) Liu, Z.-T.; Wang, C.-X.; Liu, Z.-W.; Lu, J. Selective Hydrogenation of Cinnamaldehyde over Pt-Supported Multi-Walled Carbon Nanotubes: Insights into the Tube-Size Effects. Appl. Catal., A 2008, 344, 114−123. (48) Sápi, A.; Rémiás, R.; Kónya, Z.; Kukovecz, Á .; Kordás, K.; Kiricsi, I. Synthesis and Characterisation of Nickel Catalysts Supported on Different Carbon Materials. React. Kinet. Catal. Lett. 2009, 96, 379− 389. (49) Gregg, S. J.; Sing, K. S. W. Adsorption, Surface Area and Porosity; Academic Press: London, 1982. (50) Li, L.; Zhu, Z. H.; Yan, Z. F.; Lu, G. Q.; Rintoul, L. Catalytic Ammonia Decomposition over Ru/Carbon Catalysts: The Importance of the Structure of Carbon Support. Appl. Catal., 2007, 320, 166−172. (51) Belin, T.; Epron, F. Characterisation Methods of Carbon Nanotubes: A Review. Mater. Sci. Eng., B 2005, 119, 105−118. (52) Wang, F.; Arai, S.; Park, K. C.; Takeuchi, K.; Kim, Y. J.; Endo, M. Synthesis of Carbon Nanotube-Supported Nickel-Phosphorus Nanoparticles by an Electroless Process. Carbon 2006, 44, 1307−1310. (53) Wesseling, P.; Ko, B. C.; Lewandowski, J. J. Quantitative Evaluation of a-Al Nano-Particles in Amorphous Al87Ni7Gd6 Comparison of XRD, DSC, and TEM. Scripta Mater. 2003, 48, 1537−1541. (54) Chambers, A.; Nemes, T.; Rodriguez, N. M.; Baker, R. T. K. Catalytic Behavior of Graphite Nanofiber Supported Nickel Particles. 1. Comparison with Other Support Media. J. Phys. Chem. B 1998, 102, 2251−2258. (55) Hadoun, H.; Sadaoui, Z.; Souami, N.; Sahel, D.; Toumert, I. Characterization of Mesoporous Carbon Prepared from Date Stems by H3PO4 Chemical Activation. Appl. Surf. Sci. 2013, 280, 1−7. (56) Hsieh, C.-T.; Lin, Y.-T.; Chen, W.-Y.; Wei, J.-L. Parameter Setting on Growth of Carbon Nanotubes over Transition Metal/ Alumina Catalysts in a Fluidized Bed Reactor. Powder Technol. 2009, 192, 16−22. (57) Cuesta, A.; Dhamelincourt, P.; Laureyns, J.; Martínez-Alonso, A.; Tascón, J. M. D. Comparative Performance of X-Ray Diffraction and Raman Microprobe Techniques for the Study of Carbon Materials. J. Mater. Chem. 1998, 8, 2875−2879. (58) Aymard, L.; Dumont, B.; Viau, G. Production of Co-Ni Alloys by Mechanical-Alloying. J. Alloys Compd. 1996, 242, 108−113. (59) Leslie-Pelecky; Diandra, L.; Bonder, M.; Martin, T.; Kirkpatrick, E. M.; Zhang, X. Q.; Kim, S.-H.; Rieke, R. D. Chemical Synthesis of Nanostructured Cobalt at Elevated Temperature. IEEE Trans. Magn. 1998, 34, 1018−1020. (60) Li, Y. D.; Li, L. Q.; Liao, H. W.; Wang, H. R. Preparation of Pure Nickel, Cobalt, Nickel-Cobalt and Nickel-Copper Alloys by Hydrothermal Reduction. J. Mater. Chem. 1999, 9, 2675−2677. (61) Syukri; Ban, T.; Ohya, Y.; Takahashi, Y. A Simple Synthesis of Metallic Ni and Ni−Co Alloy Fine Powders from a Mixed-Metal Acetate Precursor. Mater. Chem. Phys. 2003, 78, 645−649. (62) Jayakumar, O. D.; Salunke, H. G.; Tyagi, A. K. Synthesis and Characterization of Stoichiometric NiCo Nanoparticles Dispersible in Both Aqueous and Non-Aqueous Media. Solid State Commun. 2009, 149, 1769−1771. (63) Bueno-Ferrer, C.; Parres-Esclapez, S.; Lozano-Castelló, D.; Bueno-López, A. Relationship between Surface Area and Crystal Size of Pure and Doped Cerium Oxides. J. Rare Earths 2010, 28, 647−653. (64) Han, K. S.; Moon, Y.-S.; Han, O. H.; Hwang, K. J.; Kim, I.; Kim, H. Heat Treatment and Potential Cycling Effects on Surface Morphology, Particle Size, and Catalytic Activity of Pt/C Catalysts Studied by 13C NMR, TEM, XRD and CV. Electrochem. Commun. 2007, 9, 317−324. (65) Kozuch, S.; Martin, J. M. L. “Turning Over” Definitions in Catalytic Cycles. ACS Catal. 2012, 2, 2787−2794. (66) Lente, G. Comment on “Turning Over” Definitions in Catalytic Cycles. ACS Catal. 2013, 3, 381−382.

13919


NickelâCobalt on Carbonaceous Supports for the Selective Catalytic

Recommend Documents

NickelâCobalt on Carbonaceous Supports for the Selective Catalytic