- Email: [email protected]
Studies on Ni–M (M = Cu, Ag, Au) bimetallic catalysts for selective hydrogenation of cinnamaldehyde
G Model
ARTICLE IN PRESS
CATTOD-9862; No. of Pages 7
Catalysis Today xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Catalysis Today journal homepage: www.elsevier.com/locate/cattod
Studies on Ni–M (M = Cu, Ag, Au) bimetallic catalysts for selective hydrogenation of cinnamaldehyde Murthiyamma Gengatharan Prakash a,b , Rajaram Mahalakshmy b , Konda Ramaswamy Krishnamurthy a , Balasubramanian Viswanathan a,∗ a b
National Centre for Catalysis Research, Indian Institute of Technology, Chennai 600036, India Department of Chemistry, Thiagarajar College, Madurai Kamaraj University, Madurai 625009, India
a r t i c l e
i n f o
Article history: Received 13 April 2015 Received in revised form 30 August 2015 Accepted 16 September 2015 Available online xxx Keywords: Ni based bimetallics Chemical reduction Heterojunctions Charge transfer Titania Hydrogenation of cinnamaldehyde
a b s t r a c t Bimetallic catalysts of the type Ni–M with M = Cu, Ag and Au, and supported on TiO2 -P-25 have been prepared by chemical reduction using glucose as the reducing agent. Hydrogenation of cinnamaldehyde (CAL) to yield hydrocinnamaldehyde (HCAL), cinnamyl alcohol (COL) and hydrocinnamyl alcohol (HCOL) has been studied on the catalysts in the temperature range 60–140 ◦ C and at 20 kg/cm2 pressure, with methanol as solvent. Ni crystallite sizes, measured by X-ray line broadening analysis (XLBA), H2 pulse chemisorption and Transmission Electron Microscope (TEM) techniques, are in the range 8–12 nm. Temperature Programmed Reduction (TPR) and Diffuse Reflectance Spectroscopic (DRS) studies indicate the formation of Ni–Cu alloys, while Ni–Ag and Ni–Au exist as bimetallic nanoparticles. High-resolution HRTEM studies show that the bimetallic nanoparticles are in close contact, forming hetero junctions. Changes in the XPS binding energy values for Ni 2p1/2 and Ni 2p3/2 levels reveal that Cu/Ag/Au, tend to increase electron density around Ni, which retards the adsorption of CAL via olefinic bond and weakens Ni H bond strength. H2 TPD measurements also indicated weakening of Ni H bond. Bimetallic catalysts display higher CAL conversion and selectivity to COL vis-à-vis the corresponding monometallic catalysts at lower reaction temperatures, 60–80 ◦ C. But, selectivity to COL decreases at higher temperatures, 100–120 ◦ C. Mode of adsorption of CAL and nature of adsorbed hydrogen on bimetallic catalysts influence their activity and selectivity. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Recent years have witnessed the application of supported bimetallic catalysts for chemo-selective hydrogenation of ␣,unsaturated aldehydes like, cinnamaldehyde, crotonaldehyde, citral, to the corresponding unsaturated alcohols, in view of their higher activity and selectivity compared to the monometallic catalysts [1–13]. The attributes of bimetallic catalysts, like, electronic/ligand as well as geometric/ensemble effects [2,3], synergistic effects [11], reducibility, the state of the second metal [14], alloy formation [15], and core–shell configuration [16], could profoundly affect the catalytic activity and selectivity. In the case of supported Pt catalysts for hydrogenation of cinnamaldehyde (CAL), addition of relatively less electro-positive elements like Fe and Co increases selectivity towards cinnamyl alcohol (COL). Presence of such electro positive elements with Pt tend to increase the
∗ Corresponding author. E-mail address: [email protected] (B. Viswanathan).
electron density around Pt, thus favouring adsorption of CAL through C O vis-à-vis C C bond and hence higher selectivity to COL [17]. Similarly, bimetallic Pt–Co nanoparticles [18] and a number of Ru based bimetallic catalysts are known to display high activity and selectivity towards COL [19,20]. Bimetallic catalysts based on Co, Ni and Cu have also been studied extensively [21–25] with the primary objective of achieving higher selectivity to COL. While this objective could be realized to a considerable extent with Co and Cu based bimetallic catalysts, with Ni, it has not been so. Addition of Ir [26] or Ru [13] to Ni does lead to increase in CAL conversion, the major product being saturated aldehyde, HCAL, with very little COL. On the other hand, Ni–Cu bimetallic nanoparticles confined within the channels of ordered mesoporous SBA-15 display higher activity for the hydrogenation of CAL, compared to monometallic Ni or Cu on SBA-15 [12]. Addition of Cu to Ni up to 0.2 atomic fraction nearly doubles the CAL conversion along with a marginal increase in selectivity to COL. Presence of Ni2+ in the Cu–Al spinel matrix is known to increase the selectivity towards unsaturated alcohol (COL), since Ni2+ acts as a Lewis acid centre, activating C O group. Ni–Cu bimetallic catalysts selectively yield the
http://dx.doi.org/10.1016/j.cattod.2015.09.053 0920-5861/© 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: M.G. Prakash, et al., Studies on Ni–M (M = Cu, Ag, Au) bimetallic catalysts for selective hydrogenation of cinnamaldehyde, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.09.053
G Model CATTOD-9862; No. of Pages 7
ARTICLE IN PRESS M.G. Prakash et al. / Catalysis Today xxx (2015) xxx–xxx
2
hydrogen gas flow at 300 ◦ C for 2 h prior to characterization and hydrogenation experiments. Characteristic UV spectra for the unreduced metal salt solutions and for the corresponding monometallic as well as bimetallic nanoparticles after reduction by alkaline glucose solution are given Fig. S1a–c. Formation of Cu, Ag and Au nanoparticles in solution are indicated by absorption maxima at 262, 405 and 532 nm, respectively, which are in line with the reported data [28]. 2.3. Characterization of catalysts
Fig. 1. XRD patterns for (a) Ni/TiO2 , (b) Ni–Cu/TiO2 , (c) Ni–Ag/TiO2 , (d) Ni–Au/TiO2 , (e) Cu/TiO2 , (f) Ag/TiO2 , (g) Au/TiO2 . A – Anatase, R – Rutile.
corresponding unsaturated alcohols during hydrogenation of crotonaldehyde [25] and citral [22]. Thus, the role of added Cu in the Ni–Cu bimetallic catalyst for hydrogenation of CAL needs a relook. While the addition of Au to Cu [23] and Ag to Ru [19] lead to higher COL selectivity, the synergistic effects on addition of Ag or Au to Ni have not been explored so far. In this context, a systematic study on a series of Ni based bimetallic catalysts, Ni–M, where M = Cu, Ag and Au on TiO2 (P-25) support has been initiated, with a view to achieve higher selectivity towards COL. 2. Experimental 2.1. Materials TiO2 -P-25 (Evonik), AR grade Ni(CH3 COO)2 ·4H2 O (CDH), dglucose (Merck), methanol, ethanol, liquor ammonia (all from Qualigens) and cinnmaldehyde (Aldrich), were used as such. 2.2. Preparation of catalysts Bimetallic catalysts, Ni–M, where M = Cu, Ag and Au were prepared by green chemistry route, using d-glucose as reducing as well as capping agent [27]. Appropriate quantities of Ni salt (nickel acetate, to get 15% w/w Ni or 3 m moles of Ni/g of catalyst) and second metal salts (nitrates for Cu and Ag and auric chloride for Au, to get 0.3 m moles each of Cu/Ag/Au or 1.8% w/w, 3.1% w/w or 5.6% w/w as metal, respectively) were dissolved in 50 ml of water. To this mixture of metal salts solution, a mixture of 40 ml of aqueous d-glucose solution (0.15 M) and 10 ml of liquor ammonia was added drop-wise. Refluxing the mixture for 5 h at 80 ◦ C turned its colour from green to black indicating reduction of Ni2+ along with Cu2+ /Ag2+ /Au3+ ions to metallic state. Formation of bimetallic nanoparticles in solution was confirmed by UV spectroscopic analysis. 1 g of TiO2 (P25) was added to the solution containing colloidal Ni–M bimetallic nanoparticles and stirred for 2 h. The mixture was cooled to ambient temperature, centrifuged, washed with anhydrous ethanol and dried at 60 ◦ C for 24 h. For comparison, monometallic catalysts containing Ni, Cu, Ag and Au (15, 1.8, 3.1 and 5.6, all in % w/w as metals, respectively) on TiO2 P-25 were prepared by the same method. All catalysts were pre-reduced in
Nickel and the second metal contents in the finished catalysts were checked by XRF, after due calibrations. Powder XRD diffraction patterns for the catalysts were recorded by using Rigaku Miniflex II diffractometer with Cu-K␣ ( = 0.15418 nm) radiation in the 2 range of 10–80◦ and at a scan rate of 3◦ /min. Temperature Programmed Reduction (TPR), H2 pulse chemisorption and H2 TPD were carried out using Micromeritics Autochem II 2920 chemisorption analyser. The catalysts were calcined in air at 300 ◦ C, prior to TPR experiments. 50 mg of catalyst was pre-treated at 300 ◦ C in high purity Ar gas (25 cc/min) for 1 h and then cooled to room temperature in Ar flow. The gas was changed to 10% H2 in Ar (25 cc/min) at room temperature. After the stabilization of the baseline, TPR was started from RT to 700 ◦ C with a heating rate 10 ◦ C/min. Ni metal surface area measurements were carried out by H2 pulse chemisorption. 50 mg of catalysts were pre-treated at 300 ◦ C in high purity Ar gas (25 cc/min) for 1 h and then cooled to room temperature under Ar flow. The catalysts were reduced at 300 ◦ C for 2 h under 10% H2 /Ar flow, cooled down to RT under Ar. H2 pulses were injected with Ar as carrier gas, until the eluted peak area of consecutive pulses was constant. For H2 TPD measurements, 50 mg of catalyst was reduced in hydrogen flow (25 cc/min) at 300 ◦ C for 4 h and cooled to ambient temperature. Ar gas flow (30 cc/min) was then introduced and the catalyst was purged for 30 min. After the stabilization of the baseline, TPD of H2 was recorded up to 700 ◦ C at a temperature ramp of 10 ◦ C/min. Transmission electron micrographs were recorded using JEOL 3010 model microscope. Few milligrams of the samples (1–2 mg) were dispersed in few mL (1–2 mL) of ethanol by ultra-sonication for 15 min and a drop of the dispersion was placed on a carbon coated copper grid and allowed to dry in air at room temperature. Selected area elemental profiles were recorded with EDXA attachment to TEM. X-ray photoelectron spectra of the catalysts were recorded using Omicron Nanotechnology instrument with Mg K␣ radiation. The base pressure of the analysis chamber during the scan was 2 × 10−1 mbar. The pass energies for individual scan and survey scan are 20 and 100 eV, respectively. The spectra were recorded with step width of 0.05 eV. The data were processed with the Casa XPS software. DR spectra for the catalysts in the UV–Visible region were recorded using a Thermo Scientific Evolution 600 spectrophotometer equipped with a Praying Mantis DRS accessory. 2.4. Reaction studies Hydrogenation reactions were performed in liquid phase, in a 100 ml Parr reactor (Model-4848). The autoclave was charged with 150 mg of pre-reduced catalyst, 1.2 g of cinnamaldehyde and 16 ml of methanol. After purging first with nitrogen (three times) and then with hydrogen (three times) the autoclave was pressurized with hydrogen to the desired value of 20 kg/cm2 . The reactions were carried out at different temperatures, in the range, 60–140 ◦ C, for
Please cite this article in press as: M.G. Prakash, et al., Studies on Ni–M (M = Cu, Ag, Au) bimetallic catalysts for selective hydrogenation of cinnamaldehyde, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.09.053
G Model
ARTICLE IN PRESS
CATTOD-9862; No. of Pages 7
M.G. Prakash et al. / Catalysis Today xxx (2015) xxx–xxx
3
Fig. 2. TEM images for (a) Ni/TiO2 , (b) Ni–Cu/TiO2 , (c) Ni–Ag/TiO2 , (d) Ni–Au/TiO2 . Table 1 Ni crystallite size, TPR, H2 TPD and surface area data for the catalysts. Catal.
Metal disp. (%)a
Cryst. sizeb (nm)
Cryst. sizec (nm)
Cryst. sized (nm)
TPR max (◦ C)
H2 TPD max (◦ C)
Surf. area (m2 /g)
Ni/TiO2 Ni–Cu/TiO2 Ni–Ag/TiO2 Ni–Au/TiO2
10.7 7.9 8.7 8.9
9.5 9 9.3 8.5
12 10 11 10.3
10 8.8 9.4 8.2
352, 690 383, 431, 633 376, 435, 650 366, 419, 621
123 110 108 118
32.1 15.4 17.4 21.4
a,b
H2 chemisorption. TEM. d XLBA. c
Fig. 3. HRTEM images for (a) Ni–Cu/TiO2 , (b) Ni–Ag/TiO2 , (c) Ni–Au/TiO2 .
1 h, with stirring rate of 600 rpm. No increase in conversion was observed at stirring rates higher than 600 rpm indicating that at this rate mass transfer limitations could be ruled out. The reaction products were separated by filtration and analyzed on Perkin Elmer Clarus-500 GC with ZB-1 capillary column and FID. 3. Results and discussions 3.1. Characterization of mono and bimetallic catalysts XRD patterns for the mono and bimetallic catalysts in the reduced state are given in Fig. 1. Besides the characteristic d-lines
due to titania, two prominent d-lines at 2 = 44.49◦ and 51.8◦ , attributed to Ni metal, are observed in the pattern for Ni/TiO2 -P25 catalyst. In the case of Ni–Cu bimetallic catalyst, the two high intensity d-lines, in terms of 2, for Ni (JCPDS 8-70712) and Cu (JCPDS 8-51326) at 44.49◦ & 51.85◦ and 43.31◦ & 50.4◦ , respectively are observed. As the line widths are broad, overlapping of the peaks is observed and hence, no conclusive evidence for alloy formation could be obtained from the diffractograms. Since Cu loading is low (1.8 w/w %), concentration of Ni–Cu alloy would be very low. Two maximum intensity lines for Ag (JCPDS-8-70720) and Au (JCPDS-6-52870), at 2 values of 38.2◦ & 38.1◦ and 44.4◦ &
Please cite this article in press as: M.G. Prakash, et al., Studies on Ni–M (M = Cu, Ag, Au) bimetallic catalysts for selective hydrogenation of cinnamaldehyde, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.09.053
G Model CATTOD-9862; No. of Pages 7 4
ARTICLE IN PRESS M.G. Prakash et al. / Catalysis Today xxx (2015) xxx–xxx
Fig. 4. TPR profiles for (a) Ni/TiO2 , (b) Ni–Cu/TiO2 , (c) Ni–Ag/TiO2 , (d) Ni–Au/TiO2 , (e) Cu/TiO2 , (f) Ag/TiO2 , (g) Au/TiO2 .
44.39◦ , respectively, are observed. In the diffractograms for Ni–Ag and Ni–Au, the former pair of the d-lines merges with that of titania (37.8◦ ) and the later pair is close to that for Ni (44.49◦ ). Again, no conclusive evidence for alloy formation could be obtained. Monometallic Cu, Ag and Au on titania catalysts reveal the characteristics d-lines for the respective metals. TE micrographs along with the histograms for monometallic Ni/TiO2 , along with those for bimetallic catalysts are presented in Fig. 2. While monometallic Ni shows mean crystallite size of 12.0 ± 1.96 nm, bimetallic catalysts indicate a slight decrease in size, 10–11 ± 1.4 nm. Ni crystallite sizes for the three bimetallic and monometallic Ni catalysts, measured by X-ray line broadening analysis (XLBA), TEM and H2 pulse chemisorption techniques, as given in Table 1, are in the range 8–12 nm, indicating good agreement between the values obtained by the three techniques. High-resolution TE micrographs for the three bimetallic catalysts are presented in Fig. 3a–c. Ni, Cu, Ag and Au nanoparticles are identified with their respective lattice spacing values, −0.20 nm for Ni (8-70712), 0.21 nm for Cu (8-51326), 0.23 nm for Ag (8-93722) and 0.23 nm for Au (6-62870) corresponding to the respective (1 1 1) planes. It is observed that the respective constituent nanoparticles are in close contact, forming perhaps hetero junctions. Since nanoparticles are generated in solution under continuous stirring, their distribution is expected to be homogeneous. This is supported by the selected area elemental mapping profiles for bimetallic catalysts presented in Fig. S2 which show that the constituent elements (Ni–M with M = Cu, Ni and Au) are fairly well-distributed. Crystallite sizes of monometallic Ag and Au, as measured by X-ray line broadening analysis (XLBA) are 12.3 nm and 11.5 nm, respectively. The intensity of the characteristic d-line for Cu was very low due to low loading and hence crystallite size measurement was not possible. Based on the crystallite size values measured by XRD, it is observed that Ni crystallite sizes of bimetallic catalysts are smaller than those for monometallic Ni, Cu, Ag and Au. TPR patterns for the catalysts are presented in Fig. 4. Monometallic catalysts display major TPR maxima at 352 ◦ C (Ni), 415 ◦ C (Cu), 485 ◦ C (Ag) and 420 ◦ C (Au). Cu2+ is expected to get reduced at a lower temperature than Ni2+ since the free energy of reduction of CuO is lower (−100.65 kJ/mol at 25 ◦ C) than that for NiO (−12.31 kJ/mol at 25 ◦ C) [29]. However, in present case, CuO gets reduced at higher temperature, possibly due to the interaction with the support. In the case of bimetallic catalysts, Ni2+ , which is reduced at lower temperature, catalyses the reduction of the second metal ion(s). This is indicated
Fig. 5. DR spectra for (a) Ni/TiO2 , (b) Ni–Cu/TiO2 , (c) Ni–Ag/TiO2 , (d) Ni–Au/TiO2 , (e) Cu/TiO2 , (f) Ag/TiO2 , (g) Au/TiO2 .
Table 2 XPS data for bimetallic catalysts. Catalyst
Ni/TiO2 Ni–Cu/TiO2 Ni–Ag/ TiO2 Ni–Au/TiO2
Binding energy values (eV) Ni2p1/2
Ni2p3/2
NiO 2p3/2
870.2 869.4 869.3 869.3
852.7 852.1 852.2
853.8
by the shift of TPR maximum of Ni2+ to higher temperature and those of Cu2+ , Ag2+ and Au2+ to lower temperature. Ni and Cu are known to form bulk alloys, while Ni–Ag and Ni–Au are reported to form surface alloys [30–33]. XRD data, however, do not indicate shift in d-lines of Ni due to alloy formation. High temperature (>600 ◦ C) reduction peaks are due to the reduction of the support. Diffuse reflectance spectra for the monometallic as well as bimetallic catalysts presented in Fig. 5 bring out the possible interactions within bimetallic catalysts. While monometallic Ni and Cu (profiles a and e) display broad bands in the range 250–325 nm, monometallic Ag and Au (profiles f and g) show the characteristic surface plasmon resonance (SPR) bands at 480 and 540 nm, respectively. Bimetallic Ni–Cu (profile-b) presents a combined broad band around 250–300 nm, indicating possibility of alloy formation. The profiles c and d for Ni–Ag and Ni–Au display distinct SPR bands at 470 and 530 nm, respectively, indicating the presence of Ag and Au nanoparticles separately and ruling out the formation of bulk alloys with Ni. Results from the DRS study support the inferences drawn from TPR study. Ni 2p1/2 and Ni2p3/2 XPS lines profiles for the three bimetallic catalysts and monometallic Ni are depicted in Fig. 6a–d and binding energy values are listed in Table 2. With respect to the binding energy (BE) values of 870 eV and 853 eV for metallic Ni, monometallic Ni/TiO2 shows BE values of 870.2 and 852.7 eV attributed to Ni0 . On deconvolution, presence of small amount of NiO corresponding to BE of 853.8 is observed. With respect to the BE values for monometallic Ni, all the three bimetallic catalysts show a decrease in BE, to 869.3–869.4 eV and 852.1–852.3 eV. The shift in BE towards lower value is indicative of charge transfer from Cu/Ag/Au to Ni electronic levels, resulting in an increase in electron density on Ni, wherein adsorption of CAL via C C bond may not be favoured.
Please cite this article in press as: M.G. Prakash, et al., Studies on Ni–M (M = Cu, Ag, Au) bimetallic catalysts for selective hydrogenation of cinnamaldehyde, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.09.053
G Model CATTOD-9862; No. of Pages 7
ARTICLE IN PRESS M.G. Prakash et al. / Catalysis Today xxx (2015) xxx–xxx
5
Fig. 6. XPS profiles for (a) Ni, (b) Ni–Cu, (c) Ni–Ag, (d) Ni–Au, catalysts-all supported on TiO2 -P-25.
and Au adsorb relatively less hydrogen, the adsorption/desorption behaviour observed for bimetallic catalysts is only due to Ni species. Monometallic Cu, Ag and Au also display typical H2 TPD maxima at 132, 116 and 146 ◦ C, respectively, which are again at temperatures higher than the corresponding desorption maxima for the bimetallic catalysts. Ni based bimetallic catalysts are thus characterized by weaker Ni H bonds vis-à-vis monometallic catalysts. As indicated by XPS data for bimetallic catalysts (Table 2), charge transfer from Cu/Ag/Au to Ni increases electron density around the metal, wherein adsorption of CAL via C C bond is not favoured. Further, due to the charge transfer, the quantity of hydrogen adsorbed is reduced and Ni H bond strength weakened. Weakening of Ni H bond in Ni based bimetallic catalysts has been observed by Lazar et al. [29] and Lin et al. [26]. Availability of labile and active hydrogen could facilitate hydrogenation reactions.
4. Studies on hydrogenation of cinnamaldehyde Fig. 7. H2 TPD profiles for (a) Ni/TiO2 , (b) Ni–Cu/TiO2 , (c) Ni–Ag/TiO2 , (d) Ni–Au/TiO2 , (e) Cu/TiO2 , (f) Ag/TiO2 , (g) Au/TiO2 .
H2 TPD maxima (Fig. 7) observed for the bimetallic catalysts, Ni–Cu: 110 ◦ C, Ni–Ag: 108 ◦ C and Ni–Au: 108 ◦ C are lower than that observed for monometallic Ni, at 123 ◦ C, indicating that Ni H bond is weakened in the bimetallic systems. Since monometallic Cu, Ag
4.1. Monometallic catalysts Distribution of all the products during hydrogenation of cinnamaldehyde (CAL) in the first 1 h, for different catalysts at various reaction temperatures are given in Table S1. Of the four monometallic catalysts studied, Ni displays maximum activity and good selectivity for COL, which increases with temperature
Please cite this article in press as: M.G. Prakash, et al., Studies on Ni–M (M = Cu, Ag, Au) bimetallic catalysts for selective hydrogenation of cinnamaldehyde, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.09.053
G Model CATTOD-9862; No. of Pages 7 6
ARTICLE IN PRESS M.G. Prakash et al. / Catalysis Today xxx (2015) xxx–xxx
and CAL conversion, up to 120 ◦ C [34]. This observation is not in line with the literature data. Though CAL hydrogenation on supported monometallic Ni catalysts have been investigated earlier [7,12,26,35–37], comparative evaluation of the results is difficult, since the nature of support, catalysts preparation methods, metal dispersion, metal-support interactions, hydrogenation reaction conditions adopted and the solvent used vary widely. Ni/TiO2 [26] and Ni/SBA-15 [12] are reported to display very high selectivity for HCAL and very little COL is formed. It is to be noted that in the present work with Ni/TiO2 P-25, formation of COL increases with temperature at the cost of HCAL, while HCOL formation picks up slowly (Table S1). At 140 ◦ C, HCOL, thermodynamically the most stable product, predominates while at lower temperatures, HCAL is the major product. Separate experiments on Ni/TiO2 P-25 have shown that hydrogenation of COL to HCOL proceeds at a faster rate, vis-a-vis the rate for conversion of HCAL to COL. Besides, addition of small amount HCAL (0.1 g), along with feed CAL (1.1 g), increases the selectivity to HCOL at 120 ◦ C, while the addition of COL (0.13 g) to CAL (1.07 g) in the same manner, lowers the selectivity towards HCAL, from 31.0% to 19.6% [34]. It is likely that adsorbed COL promotes adsorption of CAL in on-top 1 mode [1,3] via C O bond due to steric hindrance. Breen et al. [35] have observed that on Ir supported on carbon, adsorbed COL causes steric hindrance, thus inhibiting the adsorption of CAL via C C bond [35]. Higher COL selectivity observed for Ni/TiO2 is attributed to the preferential adsorption of CAL through C O bond, due to (a) the steric hindrance caused by adsorption of COL and (b) the presence of small amounts of Ni2+ along with Ni0 which acts as Lewis acid site that activates C O [34]. Methanol as the solvent/medium is preferred due to higher solubility of hydrogen and higher dipole moment. Trends in the formation of products at different temperatures for the other three monometallic catalysts, Cu/TiO2 , Ag/TiO2 and Au/TiO2 , are similar to those obtained with NiO/TiO2 . Though the overall CAL conversion is 25% or less, HCAL is the major product and as the temperature/conversion increases, formation of COL picks up at the cost of HCAL. Au/TiO2 displays better performance in terms of CAL conversion and COL selectivity when compared to Cu/TiO2 and Ag/TiO2 . Yuan et al. [23] reported equal
selectivity (49%) for COL and HCAL with CAL conversion of 16% at 373 K on 6% w/w Cu/SiO2 catalyst. Unsupported Cu nanoparticles of size 3–4 nm display very high selectivity (87%) for COL and activity, but when supported on MCM-41 or CeO2 show very less conversion and selectivity [38]. Though CAL conversion on Cu catalysts with different supports [12,23,24] has been studied, the data on Cu/TiO2 is not available for comparison. In the present work 1.8%w/w Cu/TiO2 P-25 shows 15.3% CAL conversion and very high selectivity of 84.8% for HCAL at 373 K. Similar trends in CAL conversion and selectivity for COL are observed, for Ag/TiO2 P25 and Au/TiO2 P-25. No supporting data from literature for the hydrogenation of CAL on Ag/TiO2 are available, though Ag/SiO2 is reported [39] to be active and selective for C O bond hydrogenation in crotonaldehyde. Ide et al. [40] have used 1.6% w/w Au/TiO2 (Cryst. size-2.6 nm) from World Gold Council and observed 42% conversion of CAL at 333 K and 2 atmospheres pressure, with selectivity of 52% and 39% for COL and HCAL, respectively. Support plays a critical role, as indicated in the investigation by Yuan et al. [23] who observed that 1.9% w/w Au/SiO2 (Au cryst. size-5.4 nm) catalyst showed 10% conversion, with 15% and 84% selectivity for COL and HCAL, respectively at 373 K. In the present work, 5.6% w/w Au/TiO2 P-25 with an average crystallite size of 11.5 nm was used, which displayed CAL conversion of 22.2% with 15.3% and 78.1% selectivity for COL and HCAL, respectively, and is in line with the observations by Yuan et al. [23]. 4.2. Bimetallic catalysts Bimetallic catalysts are more active than the corresponding monometallic catalysts, displaying higher CAL conversion in the temperature range 60–120 ◦ C (Table S1 and Fig. S3). Table 3 gives the product selectivity for the catalysts at iso-conversion level. While bimetallic catalysts display higher activity and selectivity at lower temperatures, bimetallic Ni–Au displays maximum selectivity to COL. Comparison of CAL conversion and COL selectivity for mono vs bimetallic catalysts at 333 K is presented in Fig. S3. Such improvements in Ni based bimetallic catalysts for selective hydrogenation of ␣,-unsaturated aldehydes have been reported for Ni–Cu [12], Ni–Ir [26], Ni–Ru [13]. Synergistic interactions between Ni and Cu involving charge transfer from Cu to Ni [22]
Scheme 1. Hydrogenation of cinnamaldehyde – reaction pathways.
Please cite this article in press as: M.G. Prakash, et al., Studies on Ni–M (M = Cu, Ag, Au) bimetallic catalysts for selective hydrogenation of cinnamaldehyde, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.09.053
G Model
ARTICLE IN PRESS
CATTOD-9862; No. of Pages 7
M.G. Prakash et al. / Catalysis Today xxx (2015) xxx–xxx Table 3 Selectivity to COL on bimetallic catalysts at iso conversion level. Catalysts
Ni (100 ◦ C) Ni–Cu (80 ◦ C) Ni–Ag (80 ◦ C) Ni–Au (80 ◦ C)
Conv %
73.0 76.0 76.0 77.0
7
roles in controlling activity and selectivity for the hydrogenation of cinnamaldehyde.
Selectivity % HCAL
COL
HCOL
Others
53.9 13.9 12.7 13.1
40.1 46.0 44.2 47.4
5.0 38.5 40.2 36.3
1.0 0.9 3.7 3.2
Catalyst wt. – 150 mg, CAL – 1.2 g, methanol – 16 ml, pressure – 20 kg/cm2 , reaction time – 1 h, Agitation – 600 rpm.
Acknowledgements The authors would like to express their gratefulness to the Dept. of Science & Technology, Govt. of India for establishing research facilities at NCCR. M.G.P. is thankful to CSIR for the award of SRF scholarship. Appendix A. Supplementary data
and formation of Ni–Cu hetero structures with specific geometric and/or electronic properties [12] are considered as the basis for the improvements. XPS data presented in Table S1 lend credence to this view. In the present case, TPR (Fig. 4) studies reveal simultaneous reduction of Ni2+ along with Cu2+ , Ag2+ , Au3+ leading to the formation of bimetallic nanoparticles. HRTEM studies reveal that the bimetallic nanoparticles form hetero junctions. While formation of Ni–Cu alloy is possible, DRS (Fig. 5) studies rule out such possibilities in the cases of Ni–Ag and Ni–Au. However, the influence of the second metals, Cu, Ag and Au, on the electronic character of Ni is revealed by the XPS studies. Such modifications, in turn, influence the mode of adsorption of CAL and the nature of binding of H2 on bimetallic catalyst surfaces, as is clearly observed in the H2 -TPD profiles. These aspects have been discussed earlier in Section 3. H2 -TPD maxima for bimetallic catalysts are observed at temperatures lower than those for the monometallic Ni, indicating the presence weaker Ni H bonds in bimetallic catalysts. Labile and active adsorbed hydrogen species could facilitate CAL hydrogenation and is reflected in relatively higher CAL conversion with bimetallic vs monometallic catalysts. However, unlike the monometallic catalysts, the selectivity towards COL decreases with the increase in temperature in the case of bimetallic catalysts (Table S1, Figs. S4A and B). When the reaction temperatures are in 60–80 ◦ C range, COL selectivity is higher than those displayed by monometallic catalysts and at higher temperatures, in the range 100–120 ◦ C, COL selectivity decreases. Data presented in Table S1 show that as the temperature increases, the selectivity for HCOL formation increases at the cost of COL, with the selectivity for HCAL remaining nearly constant. As can be seen from Scheme 1 for hydrogenation of CAL, conversion of COL to HCOL proceeds at very high rate and is further accelerated by the presence of weakly adsorbed hydrogen in the case of bimetallic catalysts. Hence selectivity to COL decreases with temperature. 5. Conclusions Bimetallic catalysts of the type Ni–M with M = Cu, Ag and Au, and supported by TiO2 (P-25) have been prepared by chemical reduction using glucose as the reducing agent. TPR, DRS and TEM studies indicate the formation of Ni–Cu alloys and Ni–Ag & Ni–Au bimetallic nanoparticles. Ni crystallite sizes measured by XRD, H2 pulse chemisorption and TEM techniques are in the range 8–12 nm. XPS studies reveal charge transfer from Cu/Ag/Au to Ni. Increase in electron density on Ni does not favour CAL adsorption via C C bond and Ni H bond turns weaker compared to monometallic Ni catalyst. H2 -TPD measurements also reveal weakening of Ni H bond. Bimetallic catalysts display higher CAL conversion and selectivity to COL when compared to the corresponding monometallic catalysts at lower temperatures, 60–80 ◦ C. But, selectivity to COL decreases at higher temperatures, 100–120 ◦ C. Mode of adsorption of CAL and the nature of adsorbed hydrogen on bimetallic catalysts play crucial
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cattod.2015.09. 053. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
[23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39]
[40]
P. Gallezot, D. Richard, Catal. Rev. Sci. Eng. 40 (1998) 81. V. Ponec, Appl. Catal. A: Gen. 149 (1997) 27. B. Coq, F. Figueras, J. Mol. Catal. A: Chem. 173 (2001) 117. S.R. Wang, W. Lin, Y.X. Zhu, Y.C. Xie, J.R. McCormick, W. Huang, J.G. Chen, Catal. Lett. 114 (2007) 169. S.K. Singh, Q.A. Xu, Chem. Commun. 46 (2010) 6545. J. Li, W.P. Tian, X. Wang, L. Shi, Chem. Eng. J. 175 (2011) 417. Z.F. Zhao, Z.G. Wu, L.X. Zhou, M.H. Zhang, W. Li, K.Y. Tao, Catal. Commun. 9 (2008) 2191. X. Yang, D. Chen, S. Liao, H. Song, Y. Li, Z. Fu, Y. Su, J. Catal. 291 (2012) 36. D. Xu, W. Li, H. Duan, Q. Ge, H. Xu, Catal. Lett. 102 (2005) 229. J.M. Campelo, D. Luna, R. Luque, J.M. Marinas, A.A. Romero, ChemSusChem 2 (2009) 18. H.-L. Jiang, Q. Xu, J. Mater. Chem. 21 (2011) 13705. A. Ungureanu, B. Dragoi, A. Chirieac, C. Ciotonea, S. Royer, D. Duprez, A.S. Mamede, E. Dumitriu, ACS Appl. Mater. Interfaces 5 (2013) 5301. Z.F. Zhao, Z.J. Wu, L.X. Zhou, M.H. Zhang, W. Li, K.Y. Tao, Catal. Commun. 9 (2008) 2191. A.J. Marchi, A.D.A. Gordo, A.F. Trasarti, C.R. Apesteguía, Appl. Catal. A: Gen. 249 (2003) 53. W. Yu, Y. Wang, H. Liu, W Zheng, J. Mol. Catal. A 112 (1996) 105. N. Oshima, Pure Appl. Chem. 72 (2000) 317. D. Goupil, P. Fouilloux, R. Maurel, React. Kinet. Catal. Lett. 35 (1987) 185. W. Yu, Y. Wang, H. Liu, W. Zheng, J. Mol. Catal. A 112 (1996) 105. B. Coq, P.S. Kumbhar, C. Moreau, P. Moreau, M.G. Warawdekar, J. Mol. Catal. 85 (1993) 215. B. Coq, P.S. Kumbhar, C. Moreau, P. Moreau, F. Figueras, J. Phys. Chem. 98 (1994) 10180. S.A. Ananthan, V. Narayanan, Int. J. Chem. 3 (2011) 55. P. Mäki-Arvela, L.P. Tiainen, M. Lindblad, K. Demirkan, N. Kumar, R. Sjöholm, T. Ollonqvist, J. VäyrynenSalmi, D.Yu. Murzin, Appl. Catal. A: Gen. 241 (2003) 271. X. Yuan, J. Zheng, Q. Zhang, S. Li, Y. Yang, J. Gong, AIChE J. 60 (2012) 1036. E. Asedegbega-Nieto, B. Bachiller-Baeza, A. Guerrero-Ruíz, I. Rodríguez-Ramos, Appl. Catal. A: Gen. 300 (2006) 120. H. Noller, W.M. Lint, J. Catal. 85 (1984) 25. W. Lin, H. Cheng, L. He, Y. Yu, F. Zhao, J. Catal. 303 (2013) 110. M. Vasseem, N. Tripathy, G. Khang, Y. Halm, RSC Adv. 3 (2013) 9698. P. Christian, M. Bromfield, J. Mater. Chem. 20 (2010) 1135. M. Lazar, M. Mihet, M. Dan, V.D. Almasan, P. Marginean, J. Phys. Conf. Ser. 182 (2009) 012049. A.A. Smirnov, S.A. Khromova, O.A. Bulavchenko, V.V. Kaichev, A.A. Saraeva, S.I. Reshetnikov, M.V. Bykova, L.I. Trusov, V.A. Yakovlev, Kinet. Catal. 55 (2014) 69. H.K. Chiu, I.C. Chiang, D.H. Chen, J. Nanopart. Res. 11 (2009) 1137. A.K. Santra, C.N.R. Rao, Appl. Surf. Sci. 84 (1995) 347. J. Xu, U. Herr, T. Klassen, R.S. Averback, J. Appl. Phys. 79 (1996) 5493. M.G. Prakash, R. Mahalakshmy, K.R. Krishnamurthy, B. Viswanathan, Catal. Sci. Technol. 5 (2015) 3313. J.P. Breen, R. Burcha, J. Gomez-Lopez, K. Griffin, M. Hayes, Appl. Catal. A: Gen. 268 (2004) 267. S. Gryglewicz, A.S. liwak, J.S. wikła, G. Gryglewicz, Catal. Lett. 144 (2014) 623. H. Wang, Y. Shu, M. Zheng, T. Zhang, Catal. Lett. 124 (2008) 219. V. Gutiérrez, F. Nadob, G. Radivoy, M.A. Volpe, Appl. Catal. A: Gen. 464 (2013) 109. P. Claus, P. Kraak, R. Schödel, Studies in surface science and catalysis, in: H.U. Blaser, A. Baiker, R. Prins (Eds.), 108: Heterogeneous Catalysis and Fine Chemicals IV, Elsevier, Amsterdam, 1997, p. 281. M.S. Ide, H. Bing, M. Neurock, R.J. Davis, ACS Catal. 2 (2012) 671.
Please cite this article in press as: M.G. Prakash, et al., Studies on Ni–M (M = Cu, Ag, Au) bimetallic catalysts for selective hydrogenation of cinnamaldehyde, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.09.053
ARTICLE IN PRESS
CATTOD-9862; No. of Pages 7
Catalysis Today xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Catalysis Today journal homepage: www.elsevier.com/locate/cattod
Studies on Ni–M (M = Cu, Ag, Au) bimetallic catalysts for selective hydrogenation of cinnamaldehyde Murthiyamma Gengatharan Prakash a,b , Rajaram Mahalakshmy b , Konda Ramaswamy Krishnamurthy a , Balasubramanian Viswanathan a,∗ a b
National Centre for Catalysis Research, Indian Institute of Technology, Chennai 600036, India Department of Chemistry, Thiagarajar College, Madurai Kamaraj University, Madurai 625009, India
a r t i c l e
i n f o
Article history: Received 13 April 2015 Received in revised form 30 August 2015 Accepted 16 September 2015 Available online xxx Keywords: Ni based bimetallics Chemical reduction Heterojunctions Charge transfer Titania Hydrogenation of cinnamaldehyde
a b s t r a c t Bimetallic catalysts of the type Ni–M with M = Cu, Ag and Au, and supported on TiO2 -P-25 have been prepared by chemical reduction using glucose as the reducing agent. Hydrogenation of cinnamaldehyde (CAL) to yield hydrocinnamaldehyde (HCAL), cinnamyl alcohol (COL) and hydrocinnamyl alcohol (HCOL) has been studied on the catalysts in the temperature range 60–140 ◦ C and at 20 kg/cm2 pressure, with methanol as solvent. Ni crystallite sizes, measured by X-ray line broadening analysis (XLBA), H2 pulse chemisorption and Transmission Electron Microscope (TEM) techniques, are in the range 8–12 nm. Temperature Programmed Reduction (TPR) and Diffuse Reflectance Spectroscopic (DRS) studies indicate the formation of Ni–Cu alloys, while Ni–Ag and Ni–Au exist as bimetallic nanoparticles. High-resolution HRTEM studies show that the bimetallic nanoparticles are in close contact, forming hetero junctions. Changes in the XPS binding energy values for Ni 2p1/2 and Ni 2p3/2 levels reveal that Cu/Ag/Au, tend to increase electron density around Ni, which retards the adsorption of CAL via olefinic bond and weakens Ni H bond strength. H2 TPD measurements also indicated weakening of Ni H bond. Bimetallic catalysts display higher CAL conversion and selectivity to COL vis-à-vis the corresponding monometallic catalysts at lower reaction temperatures, 60–80 ◦ C. But, selectivity to COL decreases at higher temperatures, 100–120 ◦ C. Mode of adsorption of CAL and nature of adsorbed hydrogen on bimetallic catalysts influence their activity and selectivity. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Recent years have witnessed the application of supported bimetallic catalysts for chemo-selective hydrogenation of ␣,unsaturated aldehydes like, cinnamaldehyde, crotonaldehyde, citral, to the corresponding unsaturated alcohols, in view of their higher activity and selectivity compared to the monometallic catalysts [1–13]. The attributes of bimetallic catalysts, like, electronic/ligand as well as geometric/ensemble effects [2,3], synergistic effects [11], reducibility, the state of the second metal [14], alloy formation [15], and core–shell configuration [16], could profoundly affect the catalytic activity and selectivity. In the case of supported Pt catalysts for hydrogenation of cinnamaldehyde (CAL), addition of relatively less electro-positive elements like Fe and Co increases selectivity towards cinnamyl alcohol (COL). Presence of such electro positive elements with Pt tend to increase the
∗ Corresponding author. E-mail address: [email protected] (B. Viswanathan).
electron density around Pt, thus favouring adsorption of CAL through C O vis-à-vis C C bond and hence higher selectivity to COL [17]. Similarly, bimetallic Pt–Co nanoparticles [18] and a number of Ru based bimetallic catalysts are known to display high activity and selectivity towards COL [19,20]. Bimetallic catalysts based on Co, Ni and Cu have also been studied extensively [21–25] with the primary objective of achieving higher selectivity to COL. While this objective could be realized to a considerable extent with Co and Cu based bimetallic catalysts, with Ni, it has not been so. Addition of Ir [26] or Ru [13] to Ni does lead to increase in CAL conversion, the major product being saturated aldehyde, HCAL, with very little COL. On the other hand, Ni–Cu bimetallic nanoparticles confined within the channels of ordered mesoporous SBA-15 display higher activity for the hydrogenation of CAL, compared to monometallic Ni or Cu on SBA-15 [12]. Addition of Cu to Ni up to 0.2 atomic fraction nearly doubles the CAL conversion along with a marginal increase in selectivity to COL. Presence of Ni2+ in the Cu–Al spinel matrix is known to increase the selectivity towards unsaturated alcohol (COL), since Ni2+ acts as a Lewis acid centre, activating C O group. Ni–Cu bimetallic catalysts selectively yield the
http://dx.doi.org/10.1016/j.cattod.2015.09.053 0920-5861/© 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: M.G. Prakash, et al., Studies on Ni–M (M = Cu, Ag, Au) bimetallic catalysts for selective hydrogenation of cinnamaldehyde, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.09.053
G Model CATTOD-9862; No. of Pages 7
ARTICLE IN PRESS M.G. Prakash et al. / Catalysis Today xxx (2015) xxx–xxx
2
hydrogen gas flow at 300 ◦ C for 2 h prior to characterization and hydrogenation experiments. Characteristic UV spectra for the unreduced metal salt solutions and for the corresponding monometallic as well as bimetallic nanoparticles after reduction by alkaline glucose solution are given Fig. S1a–c. Formation of Cu, Ag and Au nanoparticles in solution are indicated by absorption maxima at 262, 405 and 532 nm, respectively, which are in line with the reported data [28]. 2.3. Characterization of catalysts
Fig. 1. XRD patterns for (a) Ni/TiO2 , (b) Ni–Cu/TiO2 , (c) Ni–Ag/TiO2 , (d) Ni–Au/TiO2 , (e) Cu/TiO2 , (f) Ag/TiO2 , (g) Au/TiO2 . A – Anatase, R – Rutile.
corresponding unsaturated alcohols during hydrogenation of crotonaldehyde [25] and citral [22]. Thus, the role of added Cu in the Ni–Cu bimetallic catalyst for hydrogenation of CAL needs a relook. While the addition of Au to Cu [23] and Ag to Ru [19] lead to higher COL selectivity, the synergistic effects on addition of Ag or Au to Ni have not been explored so far. In this context, a systematic study on a series of Ni based bimetallic catalysts, Ni–M, where M = Cu, Ag and Au on TiO2 (P-25) support has been initiated, with a view to achieve higher selectivity towards COL. 2. Experimental 2.1. Materials TiO2 -P-25 (Evonik), AR grade Ni(CH3 COO)2 ·4H2 O (CDH), dglucose (Merck), methanol, ethanol, liquor ammonia (all from Qualigens) and cinnmaldehyde (Aldrich), were used as such. 2.2. Preparation of catalysts Bimetallic catalysts, Ni–M, where M = Cu, Ag and Au were prepared by green chemistry route, using d-glucose as reducing as well as capping agent [27]. Appropriate quantities of Ni salt (nickel acetate, to get 15% w/w Ni or 3 m moles of Ni/g of catalyst) and second metal salts (nitrates for Cu and Ag and auric chloride for Au, to get 0.3 m moles each of Cu/Ag/Au or 1.8% w/w, 3.1% w/w or 5.6% w/w as metal, respectively) were dissolved in 50 ml of water. To this mixture of metal salts solution, a mixture of 40 ml of aqueous d-glucose solution (0.15 M) and 10 ml of liquor ammonia was added drop-wise. Refluxing the mixture for 5 h at 80 ◦ C turned its colour from green to black indicating reduction of Ni2+ along with Cu2+ /Ag2+ /Au3+ ions to metallic state. Formation of bimetallic nanoparticles in solution was confirmed by UV spectroscopic analysis. 1 g of TiO2 (P25) was added to the solution containing colloidal Ni–M bimetallic nanoparticles and stirred for 2 h. The mixture was cooled to ambient temperature, centrifuged, washed with anhydrous ethanol and dried at 60 ◦ C for 24 h. For comparison, monometallic catalysts containing Ni, Cu, Ag and Au (15, 1.8, 3.1 and 5.6, all in % w/w as metals, respectively) on TiO2 P-25 were prepared by the same method. All catalysts were pre-reduced in
Nickel and the second metal contents in the finished catalysts were checked by XRF, after due calibrations. Powder XRD diffraction patterns for the catalysts were recorded by using Rigaku Miniflex II diffractometer with Cu-K␣ ( = 0.15418 nm) radiation in the 2 range of 10–80◦ and at a scan rate of 3◦ /min. Temperature Programmed Reduction (TPR), H2 pulse chemisorption and H2 TPD were carried out using Micromeritics Autochem II 2920 chemisorption analyser. The catalysts were calcined in air at 300 ◦ C, prior to TPR experiments. 50 mg of catalyst was pre-treated at 300 ◦ C in high purity Ar gas (25 cc/min) for 1 h and then cooled to room temperature in Ar flow. The gas was changed to 10% H2 in Ar (25 cc/min) at room temperature. After the stabilization of the baseline, TPR was started from RT to 700 ◦ C with a heating rate 10 ◦ C/min. Ni metal surface area measurements were carried out by H2 pulse chemisorption. 50 mg of catalysts were pre-treated at 300 ◦ C in high purity Ar gas (25 cc/min) for 1 h and then cooled to room temperature under Ar flow. The catalysts were reduced at 300 ◦ C for 2 h under 10% H2 /Ar flow, cooled down to RT under Ar. H2 pulses were injected with Ar as carrier gas, until the eluted peak area of consecutive pulses was constant. For H2 TPD measurements, 50 mg of catalyst was reduced in hydrogen flow (25 cc/min) at 300 ◦ C for 4 h and cooled to ambient temperature. Ar gas flow (30 cc/min) was then introduced and the catalyst was purged for 30 min. After the stabilization of the baseline, TPD of H2 was recorded up to 700 ◦ C at a temperature ramp of 10 ◦ C/min. Transmission electron micrographs were recorded using JEOL 3010 model microscope. Few milligrams of the samples (1–2 mg) were dispersed in few mL (1–2 mL) of ethanol by ultra-sonication for 15 min and a drop of the dispersion was placed on a carbon coated copper grid and allowed to dry in air at room temperature. Selected area elemental profiles were recorded with EDXA attachment to TEM. X-ray photoelectron spectra of the catalysts were recorded using Omicron Nanotechnology instrument with Mg K␣ radiation. The base pressure of the analysis chamber during the scan was 2 × 10−1 mbar. The pass energies for individual scan and survey scan are 20 and 100 eV, respectively. The spectra were recorded with step width of 0.05 eV. The data were processed with the Casa XPS software. DR spectra for the catalysts in the UV–Visible region were recorded using a Thermo Scientific Evolution 600 spectrophotometer equipped with a Praying Mantis DRS accessory. 2.4. Reaction studies Hydrogenation reactions were performed in liquid phase, in a 100 ml Parr reactor (Model-4848). The autoclave was charged with 150 mg of pre-reduced catalyst, 1.2 g of cinnamaldehyde and 16 ml of methanol. After purging first with nitrogen (three times) and then with hydrogen (three times) the autoclave was pressurized with hydrogen to the desired value of 20 kg/cm2 . The reactions were carried out at different temperatures, in the range, 60–140 ◦ C, for
Please cite this article in press as: M.G. Prakash, et al., Studies on Ni–M (M = Cu, Ag, Au) bimetallic catalysts for selective hydrogenation of cinnamaldehyde, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.09.053
G Model
ARTICLE IN PRESS
CATTOD-9862; No. of Pages 7
M.G. Prakash et al. / Catalysis Today xxx (2015) xxx–xxx
3
Fig. 2. TEM images for (a) Ni/TiO2 , (b) Ni–Cu/TiO2 , (c) Ni–Ag/TiO2 , (d) Ni–Au/TiO2 . Table 1 Ni crystallite size, TPR, H2 TPD and surface area data for the catalysts. Catal.
Metal disp. (%)a
Cryst. sizeb (nm)
Cryst. sizec (nm)
Cryst. sized (nm)
TPR max (◦ C)
H2 TPD max (◦ C)
Surf. area (m2 /g)
Ni/TiO2 Ni–Cu/TiO2 Ni–Ag/TiO2 Ni–Au/TiO2
10.7 7.9 8.7 8.9
9.5 9 9.3 8.5
12 10 11 10.3
10 8.8 9.4 8.2
352, 690 383, 431, 633 376, 435, 650 366, 419, 621
123 110 108 118
32.1 15.4 17.4 21.4
a,b
H2 chemisorption. TEM. d XLBA. c
Fig. 3. HRTEM images for (a) Ni–Cu/TiO2 , (b) Ni–Ag/TiO2 , (c) Ni–Au/TiO2 .
1 h, with stirring rate of 600 rpm. No increase in conversion was observed at stirring rates higher than 600 rpm indicating that at this rate mass transfer limitations could be ruled out. The reaction products were separated by filtration and analyzed on Perkin Elmer Clarus-500 GC with ZB-1 capillary column and FID. 3. Results and discussions 3.1. Characterization of mono and bimetallic catalysts XRD patterns for the mono and bimetallic catalysts in the reduced state are given in Fig. 1. Besides the characteristic d-lines
due to titania, two prominent d-lines at 2 = 44.49◦ and 51.8◦ , attributed to Ni metal, are observed in the pattern for Ni/TiO2 -P25 catalyst. In the case of Ni–Cu bimetallic catalyst, the two high intensity d-lines, in terms of 2, for Ni (JCPDS 8-70712) and Cu (JCPDS 8-51326) at 44.49◦ & 51.85◦ and 43.31◦ & 50.4◦ , respectively are observed. As the line widths are broad, overlapping of the peaks is observed and hence, no conclusive evidence for alloy formation could be obtained from the diffractograms. Since Cu loading is low (1.8 w/w %), concentration of Ni–Cu alloy would be very low. Two maximum intensity lines for Ag (JCPDS-8-70720) and Au (JCPDS-6-52870), at 2 values of 38.2◦ & 38.1◦ and 44.4◦ &
Please cite this article in press as: M.G. Prakash, et al., Studies on Ni–M (M = Cu, Ag, Au) bimetallic catalysts for selective hydrogenation of cinnamaldehyde, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.09.053
G Model CATTOD-9862; No. of Pages 7 4
ARTICLE IN PRESS M.G. Prakash et al. / Catalysis Today xxx (2015) xxx–xxx
Fig. 4. TPR profiles for (a) Ni/TiO2 , (b) Ni–Cu/TiO2 , (c) Ni–Ag/TiO2 , (d) Ni–Au/TiO2 , (e) Cu/TiO2 , (f) Ag/TiO2 , (g) Au/TiO2 .
44.39◦ , respectively, are observed. In the diffractograms for Ni–Ag and Ni–Au, the former pair of the d-lines merges with that of titania (37.8◦ ) and the later pair is close to that for Ni (44.49◦ ). Again, no conclusive evidence for alloy formation could be obtained. Monometallic Cu, Ag and Au on titania catalysts reveal the characteristics d-lines for the respective metals. TE micrographs along with the histograms for monometallic Ni/TiO2 , along with those for bimetallic catalysts are presented in Fig. 2. While monometallic Ni shows mean crystallite size of 12.0 ± 1.96 nm, bimetallic catalysts indicate a slight decrease in size, 10–11 ± 1.4 nm. Ni crystallite sizes for the three bimetallic and monometallic Ni catalysts, measured by X-ray line broadening analysis (XLBA), TEM and H2 pulse chemisorption techniques, as given in Table 1, are in the range 8–12 nm, indicating good agreement between the values obtained by the three techniques. High-resolution TE micrographs for the three bimetallic catalysts are presented in Fig. 3a–c. Ni, Cu, Ag and Au nanoparticles are identified with their respective lattice spacing values, −0.20 nm for Ni (8-70712), 0.21 nm for Cu (8-51326), 0.23 nm for Ag (8-93722) and 0.23 nm for Au (6-62870) corresponding to the respective (1 1 1) planes. It is observed that the respective constituent nanoparticles are in close contact, forming perhaps hetero junctions. Since nanoparticles are generated in solution under continuous stirring, their distribution is expected to be homogeneous. This is supported by the selected area elemental mapping profiles for bimetallic catalysts presented in Fig. S2 which show that the constituent elements (Ni–M with M = Cu, Ni and Au) are fairly well-distributed. Crystallite sizes of monometallic Ag and Au, as measured by X-ray line broadening analysis (XLBA) are 12.3 nm and 11.5 nm, respectively. The intensity of the characteristic d-line for Cu was very low due to low loading and hence crystallite size measurement was not possible. Based on the crystallite size values measured by XRD, it is observed that Ni crystallite sizes of bimetallic catalysts are smaller than those for monometallic Ni, Cu, Ag and Au. TPR patterns for the catalysts are presented in Fig. 4. Monometallic catalysts display major TPR maxima at 352 ◦ C (Ni), 415 ◦ C (Cu), 485 ◦ C (Ag) and 420 ◦ C (Au). Cu2+ is expected to get reduced at a lower temperature than Ni2+ since the free energy of reduction of CuO is lower (−100.65 kJ/mol at 25 ◦ C) than that for NiO (−12.31 kJ/mol at 25 ◦ C) [29]. However, in present case, CuO gets reduced at higher temperature, possibly due to the interaction with the support. In the case of bimetallic catalysts, Ni2+ , which is reduced at lower temperature, catalyses the reduction of the second metal ion(s). This is indicated
Fig. 5. DR spectra for (a) Ni/TiO2 , (b) Ni–Cu/TiO2 , (c) Ni–Ag/TiO2 , (d) Ni–Au/TiO2 , (e) Cu/TiO2 , (f) Ag/TiO2 , (g) Au/TiO2 .
Table 2 XPS data for bimetallic catalysts. Catalyst
Ni/TiO2 Ni–Cu/TiO2 Ni–Ag/ TiO2 Ni–Au/TiO2
Binding energy values (eV) Ni2p1/2
Ni2p3/2
NiO 2p3/2
870.2 869.4 869.3 869.3
852.7 852.1 852.2
853.8
by the shift of TPR maximum of Ni2+ to higher temperature and those of Cu2+ , Ag2+ and Au2+ to lower temperature. Ni and Cu are known to form bulk alloys, while Ni–Ag and Ni–Au are reported to form surface alloys [30–33]. XRD data, however, do not indicate shift in d-lines of Ni due to alloy formation. High temperature (>600 ◦ C) reduction peaks are due to the reduction of the support. Diffuse reflectance spectra for the monometallic as well as bimetallic catalysts presented in Fig. 5 bring out the possible interactions within bimetallic catalysts. While monometallic Ni and Cu (profiles a and e) display broad bands in the range 250–325 nm, monometallic Ag and Au (profiles f and g) show the characteristic surface plasmon resonance (SPR) bands at 480 and 540 nm, respectively. Bimetallic Ni–Cu (profile-b) presents a combined broad band around 250–300 nm, indicating possibility of alloy formation. The profiles c and d for Ni–Ag and Ni–Au display distinct SPR bands at 470 and 530 nm, respectively, indicating the presence of Ag and Au nanoparticles separately and ruling out the formation of bulk alloys with Ni. Results from the DRS study support the inferences drawn from TPR study. Ni 2p1/2 and Ni2p3/2 XPS lines profiles for the three bimetallic catalysts and monometallic Ni are depicted in Fig. 6a–d and binding energy values are listed in Table 2. With respect to the binding energy (BE) values of 870 eV and 853 eV for metallic Ni, monometallic Ni/TiO2 shows BE values of 870.2 and 852.7 eV attributed to Ni0 . On deconvolution, presence of small amount of NiO corresponding to BE of 853.8 is observed. With respect to the BE values for monometallic Ni, all the three bimetallic catalysts show a decrease in BE, to 869.3–869.4 eV and 852.1–852.3 eV. The shift in BE towards lower value is indicative of charge transfer from Cu/Ag/Au to Ni electronic levels, resulting in an increase in electron density on Ni, wherein adsorption of CAL via C C bond may not be favoured.
Please cite this article in press as: M.G. Prakash, et al., Studies on Ni–M (M = Cu, Ag, Au) bimetallic catalysts for selective hydrogenation of cinnamaldehyde, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.09.053
G Model CATTOD-9862; No. of Pages 7
ARTICLE IN PRESS M.G. Prakash et al. / Catalysis Today xxx (2015) xxx–xxx
5
Fig. 6. XPS profiles for (a) Ni, (b) Ni–Cu, (c) Ni–Ag, (d) Ni–Au, catalysts-all supported on TiO2 -P-25.
and Au adsorb relatively less hydrogen, the adsorption/desorption behaviour observed for bimetallic catalysts is only due to Ni species. Monometallic Cu, Ag and Au also display typical H2 TPD maxima at 132, 116 and 146 ◦ C, respectively, which are again at temperatures higher than the corresponding desorption maxima for the bimetallic catalysts. Ni based bimetallic catalysts are thus characterized by weaker Ni H bonds vis-à-vis monometallic catalysts. As indicated by XPS data for bimetallic catalysts (Table 2), charge transfer from Cu/Ag/Au to Ni increases electron density around the metal, wherein adsorption of CAL via C C bond is not favoured. Further, due to the charge transfer, the quantity of hydrogen adsorbed is reduced and Ni H bond strength weakened. Weakening of Ni H bond in Ni based bimetallic catalysts has been observed by Lazar et al. [29] and Lin et al. [26]. Availability of labile and active hydrogen could facilitate hydrogenation reactions.
4. Studies on hydrogenation of cinnamaldehyde Fig. 7. H2 TPD profiles for (a) Ni/TiO2 , (b) Ni–Cu/TiO2 , (c) Ni–Ag/TiO2 , (d) Ni–Au/TiO2 , (e) Cu/TiO2 , (f) Ag/TiO2 , (g) Au/TiO2 .
H2 TPD maxima (Fig. 7) observed for the bimetallic catalysts, Ni–Cu: 110 ◦ C, Ni–Ag: 108 ◦ C and Ni–Au: 108 ◦ C are lower than that observed for monometallic Ni, at 123 ◦ C, indicating that Ni H bond is weakened in the bimetallic systems. Since monometallic Cu, Ag
4.1. Monometallic catalysts Distribution of all the products during hydrogenation of cinnamaldehyde (CAL) in the first 1 h, for different catalysts at various reaction temperatures are given in Table S1. Of the four monometallic catalysts studied, Ni displays maximum activity and good selectivity for COL, which increases with temperature
Please cite this article in press as: M.G. Prakash, et al., Studies on Ni–M (M = Cu, Ag, Au) bimetallic catalysts for selective hydrogenation of cinnamaldehyde, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.09.053
G Model CATTOD-9862; No. of Pages 7 6
ARTICLE IN PRESS M.G. Prakash et al. / Catalysis Today xxx (2015) xxx–xxx
and CAL conversion, up to 120 ◦ C [34]. This observation is not in line with the literature data. Though CAL hydrogenation on supported monometallic Ni catalysts have been investigated earlier [7,12,26,35–37], comparative evaluation of the results is difficult, since the nature of support, catalysts preparation methods, metal dispersion, metal-support interactions, hydrogenation reaction conditions adopted and the solvent used vary widely. Ni/TiO2 [26] and Ni/SBA-15 [12] are reported to display very high selectivity for HCAL and very little COL is formed. It is to be noted that in the present work with Ni/TiO2 P-25, formation of COL increases with temperature at the cost of HCAL, while HCOL formation picks up slowly (Table S1). At 140 ◦ C, HCOL, thermodynamically the most stable product, predominates while at lower temperatures, HCAL is the major product. Separate experiments on Ni/TiO2 P-25 have shown that hydrogenation of COL to HCOL proceeds at a faster rate, vis-a-vis the rate for conversion of HCAL to COL. Besides, addition of small amount HCAL (0.1 g), along with feed CAL (1.1 g), increases the selectivity to HCOL at 120 ◦ C, while the addition of COL (0.13 g) to CAL (1.07 g) in the same manner, lowers the selectivity towards HCAL, from 31.0% to 19.6% [34]. It is likely that adsorbed COL promotes adsorption of CAL in on-top 1 mode [1,3] via C O bond due to steric hindrance. Breen et al. [35] have observed that on Ir supported on carbon, adsorbed COL causes steric hindrance, thus inhibiting the adsorption of CAL via C C bond [35]. Higher COL selectivity observed for Ni/TiO2 is attributed to the preferential adsorption of CAL through C O bond, due to (a) the steric hindrance caused by adsorption of COL and (b) the presence of small amounts of Ni2+ along with Ni0 which acts as Lewis acid site that activates C O [34]. Methanol as the solvent/medium is preferred due to higher solubility of hydrogen and higher dipole moment. Trends in the formation of products at different temperatures for the other three monometallic catalysts, Cu/TiO2 , Ag/TiO2 and Au/TiO2 , are similar to those obtained with NiO/TiO2 . Though the overall CAL conversion is 25% or less, HCAL is the major product and as the temperature/conversion increases, formation of COL picks up at the cost of HCAL. Au/TiO2 displays better performance in terms of CAL conversion and COL selectivity when compared to Cu/TiO2 and Ag/TiO2 . Yuan et al. [23] reported equal
selectivity (49%) for COL and HCAL with CAL conversion of 16% at 373 K on 6% w/w Cu/SiO2 catalyst. Unsupported Cu nanoparticles of size 3–4 nm display very high selectivity (87%) for COL and activity, but when supported on MCM-41 or CeO2 show very less conversion and selectivity [38]. Though CAL conversion on Cu catalysts with different supports [12,23,24] has been studied, the data on Cu/TiO2 is not available for comparison. In the present work 1.8%w/w Cu/TiO2 P-25 shows 15.3% CAL conversion and very high selectivity of 84.8% for HCAL at 373 K. Similar trends in CAL conversion and selectivity for COL are observed, for Ag/TiO2 P25 and Au/TiO2 P-25. No supporting data from literature for the hydrogenation of CAL on Ag/TiO2 are available, though Ag/SiO2 is reported [39] to be active and selective for C O bond hydrogenation in crotonaldehyde. Ide et al. [40] have used 1.6% w/w Au/TiO2 (Cryst. size-2.6 nm) from World Gold Council and observed 42% conversion of CAL at 333 K and 2 atmospheres pressure, with selectivity of 52% and 39% for COL and HCAL, respectively. Support plays a critical role, as indicated in the investigation by Yuan et al. [23] who observed that 1.9% w/w Au/SiO2 (Au cryst. size-5.4 nm) catalyst showed 10% conversion, with 15% and 84% selectivity for COL and HCAL, respectively at 373 K. In the present work, 5.6% w/w Au/TiO2 P-25 with an average crystallite size of 11.5 nm was used, which displayed CAL conversion of 22.2% with 15.3% and 78.1% selectivity for COL and HCAL, respectively, and is in line with the observations by Yuan et al. [23]. 4.2. Bimetallic catalysts Bimetallic catalysts are more active than the corresponding monometallic catalysts, displaying higher CAL conversion in the temperature range 60–120 ◦ C (Table S1 and Fig. S3). Table 3 gives the product selectivity for the catalysts at iso-conversion level. While bimetallic catalysts display higher activity and selectivity at lower temperatures, bimetallic Ni–Au displays maximum selectivity to COL. Comparison of CAL conversion and COL selectivity for mono vs bimetallic catalysts at 333 K is presented in Fig. S3. Such improvements in Ni based bimetallic catalysts for selective hydrogenation of ␣,-unsaturated aldehydes have been reported for Ni–Cu [12], Ni–Ir [26], Ni–Ru [13]. Synergistic interactions between Ni and Cu involving charge transfer from Cu to Ni [22]
Scheme 1. Hydrogenation of cinnamaldehyde – reaction pathways.
Please cite this article in press as: M.G. Prakash, et al., Studies on Ni–M (M = Cu, Ag, Au) bimetallic catalysts for selective hydrogenation of cinnamaldehyde, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.09.053
G Model
ARTICLE IN PRESS
CATTOD-9862; No. of Pages 7
M.G. Prakash et al. / Catalysis Today xxx (2015) xxx–xxx Table 3 Selectivity to COL on bimetallic catalysts at iso conversion level. Catalysts
Ni (100 ◦ C) Ni–Cu (80 ◦ C) Ni–Ag (80 ◦ C) Ni–Au (80 ◦ C)
Conv %
73.0 76.0 76.0 77.0
7
roles in controlling activity and selectivity for the hydrogenation of cinnamaldehyde.
Selectivity % HCAL
COL
HCOL
Others
53.9 13.9 12.7 13.1
40.1 46.0 44.2 47.4
5.0 38.5 40.2 36.3
1.0 0.9 3.7 3.2
Catalyst wt. – 150 mg, CAL – 1.2 g, methanol – 16 ml, pressure – 20 kg/cm2 , reaction time – 1 h, Agitation – 600 rpm.
Acknowledgements The authors would like to express their gratefulness to the Dept. of Science & Technology, Govt. of India for establishing research facilities at NCCR. M.G.P. is thankful to CSIR for the award of SRF scholarship. Appendix A. Supplementary data
and formation of Ni–Cu hetero structures with specific geometric and/or electronic properties [12] are considered as the basis for the improvements. XPS data presented in Table S1 lend credence to this view. In the present case, TPR (Fig. 4) studies reveal simultaneous reduction of Ni2+ along with Cu2+ , Ag2+ , Au3+ leading to the formation of bimetallic nanoparticles. HRTEM studies reveal that the bimetallic nanoparticles form hetero junctions. While formation of Ni–Cu alloy is possible, DRS (Fig. 5) studies rule out such possibilities in the cases of Ni–Ag and Ni–Au. However, the influence of the second metals, Cu, Ag and Au, on the electronic character of Ni is revealed by the XPS studies. Such modifications, in turn, influence the mode of adsorption of CAL and the nature of binding of H2 on bimetallic catalyst surfaces, as is clearly observed in the H2 -TPD profiles. These aspects have been discussed earlier in Section 3. H2 -TPD maxima for bimetallic catalysts are observed at temperatures lower than those for the monometallic Ni, indicating the presence weaker Ni H bonds in bimetallic catalysts. Labile and active adsorbed hydrogen species could facilitate CAL hydrogenation and is reflected in relatively higher CAL conversion with bimetallic vs monometallic catalysts. However, unlike the monometallic catalysts, the selectivity towards COL decreases with the increase in temperature in the case of bimetallic catalysts (Table S1, Figs. S4A and B). When the reaction temperatures are in 60–80 ◦ C range, COL selectivity is higher than those displayed by monometallic catalysts and at higher temperatures, in the range 100–120 ◦ C, COL selectivity decreases. Data presented in Table S1 show that as the temperature increases, the selectivity for HCOL formation increases at the cost of COL, with the selectivity for HCAL remaining nearly constant. As can be seen from Scheme 1 for hydrogenation of CAL, conversion of COL to HCOL proceeds at very high rate and is further accelerated by the presence of weakly adsorbed hydrogen in the case of bimetallic catalysts. Hence selectivity to COL decreases with temperature. 5. Conclusions Bimetallic catalysts of the type Ni–M with M = Cu, Ag and Au, and supported by TiO2 (P-25) have been prepared by chemical reduction using glucose as the reducing agent. TPR, DRS and TEM studies indicate the formation of Ni–Cu alloys and Ni–Ag & Ni–Au bimetallic nanoparticles. Ni crystallite sizes measured by XRD, H2 pulse chemisorption and TEM techniques are in the range 8–12 nm. XPS studies reveal charge transfer from Cu/Ag/Au to Ni. Increase in electron density on Ni does not favour CAL adsorption via C C bond and Ni H bond turns weaker compared to monometallic Ni catalyst. H2 -TPD measurements also reveal weakening of Ni H bond. Bimetallic catalysts display higher CAL conversion and selectivity to COL when compared to the corresponding monometallic catalysts at lower temperatures, 60–80 ◦ C. But, selectivity to COL decreases at higher temperatures, 100–120 ◦ C. Mode of adsorption of CAL and the nature of adsorbed hydrogen on bimetallic catalysts play crucial
Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cattod.2015.09. 053. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22]
[23] [24] [25] [26] [27] [28] [29] [30] [31] [32] [33] [34] [35] [36] [37] [38] [39]
[40]
P. Gallezot, D. Richard, Catal. Rev. Sci. Eng. 40 (1998) 81. V. Ponec, Appl. Catal. A: Gen. 149 (1997) 27. B. Coq, F. Figueras, J. Mol. Catal. A: Chem. 173 (2001) 117. S.R. Wang, W. Lin, Y.X. Zhu, Y.C. Xie, J.R. McCormick, W. Huang, J.G. Chen, Catal. Lett. 114 (2007) 169. S.K. Singh, Q.A. Xu, Chem. Commun. 46 (2010) 6545. J. Li, W.P. Tian, X. Wang, L. Shi, Chem. Eng. J. 175 (2011) 417. Z.F. Zhao, Z.G. Wu, L.X. Zhou, M.H. Zhang, W. Li, K.Y. Tao, Catal. Commun. 9 (2008) 2191. X. Yang, D. Chen, S. Liao, H. Song, Y. Li, Z. Fu, Y. Su, J. Catal. 291 (2012) 36. D. Xu, W. Li, H. Duan, Q. Ge, H. Xu, Catal. Lett. 102 (2005) 229. J.M. Campelo, D. Luna, R. Luque, J.M. Marinas, A.A. Romero, ChemSusChem 2 (2009) 18. H.-L. Jiang, Q. Xu, J. Mater. Chem. 21 (2011) 13705. A. Ungureanu, B. Dragoi, A. Chirieac, C. Ciotonea, S. Royer, D. Duprez, A.S. Mamede, E. Dumitriu, ACS Appl. Mater. Interfaces 5 (2013) 5301. Z.F. Zhao, Z.J. Wu, L.X. Zhou, M.H. Zhang, W. Li, K.Y. Tao, Catal. Commun. 9 (2008) 2191. A.J. Marchi, A.D.A. Gordo, A.F. Trasarti, C.R. Apesteguía, Appl. Catal. A: Gen. 249 (2003) 53. W. Yu, Y. Wang, H. Liu, W Zheng, J. Mol. Catal. A 112 (1996) 105. N. Oshima, Pure Appl. Chem. 72 (2000) 317. D. Goupil, P. Fouilloux, R. Maurel, React. Kinet. Catal. Lett. 35 (1987) 185. W. Yu, Y. Wang, H. Liu, W. Zheng, J. Mol. Catal. A 112 (1996) 105. B. Coq, P.S. Kumbhar, C. Moreau, P. Moreau, M.G. Warawdekar, J. Mol. Catal. 85 (1993) 215. B. Coq, P.S. Kumbhar, C. Moreau, P. Moreau, F. Figueras, J. Phys. Chem. 98 (1994) 10180. S.A. Ananthan, V. Narayanan, Int. J. Chem. 3 (2011) 55. P. Mäki-Arvela, L.P. Tiainen, M. Lindblad, K. Demirkan, N. Kumar, R. Sjöholm, T. Ollonqvist, J. VäyrynenSalmi, D.Yu. Murzin, Appl. Catal. A: Gen. 241 (2003) 271. X. Yuan, J. Zheng, Q. Zhang, S. Li, Y. Yang, J. Gong, AIChE J. 60 (2012) 1036. E. Asedegbega-Nieto, B. Bachiller-Baeza, A. Guerrero-Ruíz, I. Rodríguez-Ramos, Appl. Catal. A: Gen. 300 (2006) 120. H. Noller, W.M. Lint, J. Catal. 85 (1984) 25. W. Lin, H. Cheng, L. He, Y. Yu, F. Zhao, J. Catal. 303 (2013) 110. M. Vasseem, N. Tripathy, G. Khang, Y. Halm, RSC Adv. 3 (2013) 9698. P. Christian, M. Bromfield, J. Mater. Chem. 20 (2010) 1135. M. Lazar, M. Mihet, M. Dan, V.D. Almasan, P. Marginean, J. Phys. Conf. Ser. 182 (2009) 012049. A.A. Smirnov, S.A. Khromova, O.A. Bulavchenko, V.V. Kaichev, A.A. Saraeva, S.I. Reshetnikov, M.V. Bykova, L.I. Trusov, V.A. Yakovlev, Kinet. Catal. 55 (2014) 69. H.K. Chiu, I.C. Chiang, D.H. Chen, J. Nanopart. Res. 11 (2009) 1137. A.K. Santra, C.N.R. Rao, Appl. Surf. Sci. 84 (1995) 347. J. Xu, U. Herr, T. Klassen, R.S. Averback, J. Appl. Phys. 79 (1996) 5493. M.G. Prakash, R. Mahalakshmy, K.R. Krishnamurthy, B. Viswanathan, Catal. Sci. Technol. 5 (2015) 3313. J.P. Breen, R. Burcha, J. Gomez-Lopez, K. Griffin, M. Hayes, Appl. Catal. A: Gen. 268 (2004) 267. S. Gryglewicz, A.S. liwak, J.S. wikła, G. Gryglewicz, Catal. Lett. 144 (2014) 623. H. Wang, Y. Shu, M. Zheng, T. Zhang, Catal. Lett. 124 (2008) 219. V. Gutiérrez, F. Nadob, G. Radivoy, M.A. Volpe, Appl. Catal. A: Gen. 464 (2013) 109. P. Claus, P. Kraak, R. Schödel, Studies in surface science and catalysis, in: H.U. Blaser, A. Baiker, R. Prins (Eds.), 108: Heterogeneous Catalysis and Fine Chemicals IV, Elsevier, Amsterdam, 1997, p. 281. M.S. Ide, H. Bing, M. Neurock, R.J. Davis, ACS Catal. 2 (2012) 671.
Please cite this article in press as: M.G. Prakash, et al., Studies on Ni–M (M = Cu, Ag, Au) bimetallic catalysts for selective hydrogenation of cinnamaldehyde, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.09.053