Selective Hydrogenation of Crotonaldehyde over Ir ... - ACS Publications

Article pubs.acs.org/JPCC

Selective Hydrogenation of Crotonaldehyde over Ir−FeOx/SiO2 Catalysts: Enhancement of Reactivity and Stability by Ir−FeOx Interaction Qin Yu,† Kyoko K. Bando,‡ Ju-Fang Yuan,† Ce-Qi Luo,† Ai-Ping Jia,† Geng-Shen Hu,† Ji-Qing Lu,*,† and Meng-Fei Luo*,† †

Key Laboratory of the Ministry of Education for Advanced Catalysis Materials, Institute of Physical Chemistry, Zhejiang Normal University, Jinhua 321004, China ‡ Research Institute for Chemical Process Technology, National Institute of Advanced Industrial Science and Technology, Ibaraki 305-8565, Japan S Supporting Information *

ABSTRACT: A series of FeOx-promoted Ir/SiO2 catalysts were prepared and tested for gas phase selective hydrogenation of crotonaldehyde. It was found that a catalyst containing Ir nanoparticles contacting with highly dispersed FeOx clusters (3Ir/0.1Fe/SiO2) showed excellent activity and stability, with a 5-fold enhanced steady state crotyl alcohol yield (59.6%) compared to that of the bare Ir/SiO2 (12.4%), while a catalyst promoted with high content of Fe (3Ir/3.5Fe/SiO2) had high initial activity but deactivated rapidly. A catalyst with similar highly dispersed FeOx clusters but prepared by an inverse impregnation sequence (0.1Fe/3Ir/SiO2) also suffered severe deactivation. Various characterizations revealed that the observed behaviors were closely related to the Ir−FeOx interactions induced by different morphologies of the catalysts. The active sites generated at Ir−FeOx interface were responsible for the better performance. The catalyst deactivation was attributed to the deposit of heavy compound and strong adsorption of CO on the surface, which was induced by the strong Ir− FeOx interaction due to heavy decoration of FeOx on the Ir surface.

1. INTRODUCTION The selective hydrogenation of α, β-unsaturated aldehyde with H2 is the most atomically efficient route to produce α, βunsaturated alcohols, which is very important in the syntheses of fragrances and pharmaceuticals.1 Yet, it remains a difficult task due to the much greater susceptibility to hydrogenation of the CC bond compared with the CO bond for both thermodynamic and kinetic reasons.2 As a model reaction, the selective hydrogenation of crotonaldehyde to crotyl alcohol has been extensively investigated and noble metals are employed as the catalysts, including Pt,3−11 Au,12−19 and Ir.20−24 High selectivity to the unsaturated alcohol is an essential issue for selective hydrogenation of α, β-unsaturated aldehyde. In the case of selective hydrogenation of crotonaldehyde, the selectivity to the crotyl alcohol is usually low on the unsupported metal catalysts or metal catalysts supported on inert oxides. On Pt/SiO2 catalysts, the selectivity to crotyl alcohol was less than 30%.25 The improvement of the selectivity as well as activity could be achieved via different approaches. One approach is the promotion of metal catalysts with other metal/metal salts such as Fe,3,26 Sn,27,28 NaCl,29 In,30 Ti,31 and Zn.32 Such promotion could remarkably improve the selectivity © XXXX American Chemical Society

to crotyl alcohol due to the electronic effects and/or alloy formation, which influences the CO bond polarization and/ or the inhibition of the α, β-unsaturated aldehyde adsorption through the CC bond.2 For example, in our previous work,29 a NaCl-promoted Pt/ZrO2 catalyst showed a crotyl alcohol selectivity of 60% after 5 h reaction, which was much higher ́ et al.31 than the unpromoted Pt/ZrO2 (31%). Ruiz-Martinez compared the catalytic behaviors of Ru/SiO2 and RuTi/SiO2 catalysts and found that the RuTi/SiO2 catalyst exhibited almost 5-fold activity (TOF = 0.087 s−1) compared the Ru/ SiO2 (TOF = 0.016 s−1), along with the much improved selectivity to the crotyl alcohol (35% for the RuTi/SiO2 vs 3% for the Ru/SiO2). A very recent work by Lin et al.33 reported AuAg/SBA-15 catalysts for selective hydrogenation of crotonaldehyde, in which Ag promotion significantly enhanced the catalytic performance compared to the Au/SBA-15 or Ag/ SBA-15 counterpart. Another approach is the selection of the support. The employment of partially reducible oxides such as Received: January 15, 2016 Revised: April 6, 2016

A

DOI: 10.1021/acs.jpcc.6b00456 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C CeO2,6,18,34 TiO2,4,20,25 ZnO,7,35 SnO2,36 and ZrO237 is beneficial to the improvement of both activity and selectivity as the generation of strong metal−support interactions (SMSI) which is proposed to be responsible for the improved performance.2 Compared to the relatively low selectivity, catalyst stability is a more important issue in practical applications. Unfortunately, almost all the currently employed catalysts suffer severe deactivation, particularly for gas phase reaction. Pt catalysts, for example, lost about 50% of initial activity after 1 h reaction.4,25,38 Similar deactivation was also observed on Au 12,13,15 or Ir20−24 catalysts. The causes of catalyst deactivation in selective hydrogenation of crotonaldehyde include the strong adsorption of CO on the metal surface via decarbonylation reaction (CH3CHCH−CHO → C3H6 + CO)25 and the blockage of the active sites due to deposition of asymmetric carboxylate and heavy products with conjugated CO and CC bonds on the catalyst surface, especially for the metal supported on a reducible oxide such as TiO2.20 It can be seen that although progress has been made, the selective hydrogenation of α, β-unsaturated aldehyde remains very challenging with respect to high selectivity and catalyst stability.39 Therefore, the design of highly efficient catalyst systems is desirable. Note that the catalytic performance of a catalyst for this reaction is usually governed by several factors, such as types of metal/promoter and support, catalyst preparation methods, and reaction conditions; delicately designed catalyst systems are necessary in order to achieve high performance. Herein, we reported gas phase selective hydrogenation of crotonaldehyde over a series of FeOx-doped Ir/SiO2 catalysts. The aim of this work is to investigate the effect of Ir−FeOx interaction on the behaviors of the catalysts, which was approached from two aspects. By either changing the impregnation sequences (first Ir, then Fe versus first Fe, then Ir) or changing the Fe contents in the catalyst, the structures/ morphologies of the catalysts could be tuned, which consequently altered the interaction between Ir and FeOx species and thus the catalytic performance.

An Ir supported on FeOx catalyst (Ir/FeOx) was also prepared for comparison. The synthesis of FeOx was as follows: 5 mmol of FeCl3·6H2O (1.35 g) was dissolved in 75 mL of glycol under stirring, and then 3.6 g of sodium citrate was added. The mixture was transferred to a 200 mL autoclave and kept at 200 °C for 16 h. Then the solid was separated and washed with deionized water and ethanol for three times, followed by vacuum drying at 60 °C and calcination at 400 °C for 4 h. The impregnation of Ir on the FeOx support followed the same procedure as that of the 3Ir/xFe/SiO2 catalysts. 2.2. Catalyst Characterizations. Elemental compositions of the catalysts were determined by X-ray fluorescence (XRF) analysis on an ARLADVANT’X Intelli Power 4200 scanning Xray fluorescence spectrometer. Specific surface areas of the catalysts were determined by the modified BET method from the N2 sorption isotherms at liquid nitrogen temperature (77 K) on a NOVA 40000e surface area and pore size analyzer. Power X-ray diffraction (XRD) patterns were performed on a PANalytic X’Pert PW3040 diffractiometer with Cu Kα radiation (40 kV, 40 mA). The average particle size of Ir in the catalyst was estimated using the Scherrer equation. The Ir dispersion was determined by CO chemisorption operated on a Quantachrame CHEMBET-3000 instrument. The catalyst was placed in a U-shape quartz tube reactor and pretreated in a 5% H2−95% N2 flow (30 mL min−1) at 300 °C for 1 h and then was cooled down to 30 °C in a pure He flow. Then pulses of CO were fed into the stream of carrier gas with a precision analytical syringe. The dispersion was calculated based on the assumption that CO/surface Ir atom = 1, and the Ir particle size was calculated based on the equation dIr (nm) = 1.1/D (D = dispersion).20 High resolution transmission electron microscopy (HRTEM) analysis was performed on a JEM-2100F microscope with a field emissive gun, operated at 200 kV and with a point resolution of 0.24 nm. Prior to the measurement, the sample was reduced in high purity hydrogen (>99.999%, 30 mL min−1) at 300 °C for 1 h. X-ray photoelectron spectra (XPS) were recorded using a ESCALAB 250Xi spectrometer with an Al anode Kα radiation (1486.6 eV), and the charging effects were corrected by adjusting the binding energy of Si 2p peak of silicon dioxide to 103.5 eV. Prior to the measurement, the sample was in situ reduced in a pretreatment chamber in high purity hydrogen (>99.999%, 30 mL min−1) at 300 °C for 1 h, and then the sample was cooled down to room temperature. Then the sample was directly moved to analysis chamber without being exposed to the environment. To improve the quality of the spectra (particularly for those with very low metal loadings), accumulative 200 scans were performed. The XAFS measurements were conducted at Photon Factory (PF) and Photon Factory Advanced Ring (PF-AR), Institute of Materials Structure Science (IMSS), High Energy Accelerator Research Organization (KEK) in Japan. The catalyst was ground to fine power and put in a glass reactor which is equipped with a XAFS cell. The catalyst was treated at 300 °C for 1 h under a flow of hydrogen (99.999%, 50 mL min−1). After the treatment, the sample was moved to the XAFS cell without exposure to air. XAFS measurements were conducted for these samples kept in H2. Ir LIII-edge spectra were obtained in a transmittance mode. Fe K-edge spectrum for 3Ir−3.5Fe/ SiO2 was measured under the same conditions. For 0.1Fe/SiO2 and 3Ir−0.1Fe/SiO2, Fe K-edge XAFS spectra were obtained in

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. The FeOx promoted Ir/SiO2 catalysts were prepared using an impregnation method. In detail, SiO2 (80−120 mesh, SBET = 320 m2 g−1) was impregnated with an aqueous solution of Fe(NO3)3·9H2O for 10 h, followed by drying at 120 °C overnight and calcination at 500 °C for 4 h to obtain the FeOx/SiO2 support. Then, Ir was impregnated on the FeOx/SiO2 support with an solution of iridium(III) acetylacetonate (Ir(acac)3, >98.1%) in benzene for 3 h, followed by drying at 80 °C for 8 h and calcination at 400 °C for 4 h to obtain the final catalyst. The nominal contents of Ir in all the catalysts were 3 wt %. The catalysts were designated as 3Ir/xFe/SiO2, with x referring to the content of Fe in the catalyst. For comparison, a reference catalyst was prepared in similar manner as the 3Ir/xFe/SiO2 catalyst, but with an inverse impregnation method. In this process, the Ir(acac)3 precursor was impregnated on the SiO2, followed by drying at 120 °C for 8 h and calcination at 400 °C for 4 h. Then the Ir/SiO2 solid was impregnated with an aqueous solution of Fe(NO3)3·9H2O, followed by drying at 80 °C for 8 h and calcination at 500 °C for 4 h to obtain the final catalyst. The catalyst was denoted as 0.1Fe/3Ir/SiO2, and the nominal contents of Fe and Ir were 0.1 and 3 wt %, respectively. B


Article

The Journal of Physical Chemistry C a fluorescence mode. Analysis of the spectra was conducted with a data processing software (Rex2000, Rigaku Co.). Diffuse reflectance infrared Fourier transform (DRIFT) spectra of CO chemisorption on the samples were recorded using a Nicolet 6700 spectrometer equipped with a MCT detector and a DRIFTS (Harrick, CHC-CHA-3). Prior to the measurement, the sample was in situ reduced in a H2−Ar mixture (20 mL min−1, 5 vol % H2) at 300 °C for 1 h and then purged by a flow of Ar (30 mL min−1) at 300 °C for 40 min. After that, the sample was cooled down to 50 °C and exposed to a CO−Ar mixture (30 mL min−1, 1 vol % CO) for 30 min. Finally, the sample was purged by Ar for another 30 min, and the spectra were recorded. In all cases the spectra were taken at 50 °C, with a resolution of 4 cm−1 and cumulative 32 scans. The hydrogen temperature-programmed reduction (H2TPR) technique was employed to analyze the reducibility of the catalysts. 25 mg of the as-prepared (calcined) catalyst was placed in a quartz reactor and pretreated in a N2 flow (20 mL min−1) at 300 °C for 60 min in order to remove the adsorbed water and carbonates, and then the sample was cooled down to 40 °C in a N2 flow (20 mL min−1). After that, the sample was heated from 40 to 700 °C with a heating rate of 10 °C min−1 under a mixture of 5% H2−95% N2 (20 mL min−1). The amount of H2 consumption was determined by a gas chromatograph with a thermal conductivity detector (TCD). Temperature-programmed oxidation (TPO) measurement was carried out in a quartz microreactor with an inner diameter of 6 mm. 50 mg of the spent catalyst was placed in the middle of reactor and heated in an O2 flow (20 mL min−1) from rt to 600 °C at a rate of 10 °C min−1. The outlet gas was analyzed online by mass spectrometry (Qic-20 Benchtop, Hiden Analytical). The mass numbers of 44 and 18 were selected to monitor desorption of CO2 and H2O, respectively. Raman spectra were collected on a Renishaw RM1000 confocal microprobe under ambient conditions. The excitation wavelength of laser was 514 nm. The power of each laser was kept at about 3 mW to prevent local heating effect, and the resolution of the spectrometer was 1 cm−1 with the diameter of the analyzed spot being ca. 1 μm. 2.3. Catalytic Testing. The gas phase crotonaldehyde hydrogenation was performed in a fixed bed reaction system at atmospheric pressure, using a quartz tube (i.d. = 8 mm) reactor. 200 mg of catalyst was loaded in the reactor with a thermal couple placed in the middle of the catalyst bed to monitor the reaction temperature. Before running the catalytic test, the catalyst was reduced at 300 °C for 1 h in ultrapure H2 atmosphere (99.999%, 20 mL min−1), and then it was cooled down to 80 °C. The crotonaldehyde was introduced in a trap set before the reactor tube and maintained at 0 °C to achieve a constant crotonaldehyde partial pressure (1.06 kPa), which was carried to the catalyst by a hydrogen flow (99.999%, 26 mL min−1). The space velocity was about 8100 mL gcat−1 h−1. The gas line was kept at about 50 °C to avoid any condensation. The reaction products and reactant were analyzed on line using a gas chromatograph (Shimadzu GC-2014) equipped with a flame ionization detector (FID) and a DB-Wax capillary column (30 m × 0.25 mm × 0.25 μm).

Table 1. Physical Properties of 0.1Fe/SiO2, 3Ir/SiO2, 3Ir/ 0.1Fe/SiO2, 0.1Fe/3Ir/SiO2, and 3Ir/3.5Fe/SiO2 Catalysts contenta (wt %)

sample

Ir

0.1Fe/SiO2 3Ir/SiO2 3Ir/0.1Fe/SiO2 0.1Fe/3Ir/SiO2 3Ir/3.5Fe/SiO2

2.74 2.78 2.87 2.89

Fe 0.13 0.12 0.12 3.34

surface area (m2 g−1)

Ir dispersionb (%)

Ir particle sizec (nm)

320 325 304 304 302

23.1 23.5 21.0 21.0

4.7 4.6 5.3 5.3

a

Measured by XRF. bMeasured by CO chemisorption. cdIr (nm) = 1.1/D. D: Ir dispersion.

which suggests that the Ir particle sizes in these catalysts are 4.6−5.3 nm. For the 3Ir/11Fe/SiO2, the Ir dispersion is quite low (8.6%) probably due to the partial decoration of Ir particles by FeOx species at the Ir−FeOx interface during the thermal treatment (in either calcination or prereduction process), which consequently results in a larger Ir particle size (Table S1). Similar decorations were also observed in Pt/TiO225 and Pt/ CeO2−SiO2 systems.5 Also, the XRD patterns of the catalysts (Figure S1) reveal the presence of metallic Ir in all the catalysts. No distinct diffractions of FeOx species could be observed in the 3Ir/0.1Fe/SiO2, indicating the presence of highly dispersed FeOx species in this catalyst due to its very low loading. In contrast, a weak diffraction peak at 2θ of 44.4° is detected in the 3Ir/3.5Fe/SiO 2 , implying the formation of Fe 3 O 4 crystallites, while the diffraction peaks of Fe3O4 are more pronounced in the 3Ir/11Fe/SiO2. Average Ir particle sizes in the catalysts are 4.2−6.2 nm (Table S1) as estimated by the Scherrer equation, which are consistent with the values obtained by the CO chemisorption. Figure 1 shows the catalytic behaviors of the catalysts. The 0.1Fe/SiO2 is inactive, while the 3Ir/SiO2 shows a gradual increase in crotonaldehyde conversion and reaches a steady state after about 10 h (15.8%, Figure 1a). Interestingly, the addition of very small amount of FeOx (3Ir/0.1Fe/SiO2) leads to a remarkable enhancement in crontonaldehyde conversion (65.6% after 10 h). In addition, an induction period (about 6 h) is observed on the 3Ir/SiO2 and 3Ir/0.1Fe/SiO2 catalysts. The possible changes of the catalyst structures during the reaction could be safely ruled out because the catalysts were pretreated at high temperatures (calcined at 400 °C and prereduced at 300 °C) while the reaction temperature is low (80 °C). Therefore, such induction period is probably due to the competitive adsorption of crotonaldehyde and hydrogen molecules on the catalyst surface. Before the reaction, the exposed Ir surface could be covered by H adatoms because the catalyst was prereduced in a hydrogen flow. As the crotonaldehyde was introduced to the system, these molecules may gradually occupy the surface Ir sites and replace the hydrogen adatoms, which could result in an increasing coverage of crotonaldehyde on the surface and consequently an enhancement of the conversion. This hypothesis is confirmed by changing the reaction temperatures (Figure S2). The induction period in the conversion level at 80 °C (Figure S2a) disappears at reaction temperature of 100 °C (Figure S2b). This comparison implies that high reaction temperature favors the chemisorption of crotonaldehyde molecules on the catalyst surface, which leads to a higher conversion level (Figure S2b) although gradual deactivation is observed. The addition of relatively large

3. RESULTS AND DISCUSSION Table 1 lists the physical properties of the catalysts. The catalysts have similar surface areas (302−325 m2 g−1) and Ir contents (2.74−2.89 wt %, except for the 0.1Fe/SiO2). The Ir dispersions measured by CO chemisorption are 21.0−23.5%, C


Article

The Journal of Physical Chemistry C

Figure 1. (a) Crotonaldehyde conversion. (b) Selectivity to crotyl alcohol as a function of reaction time over various catalysts. (c) Stability of 3Ir/ 0.1Fe/SiO2.

Table 2. Summary of Reaction Results on 3Ir/SiO2, 3Ir/0.1Fe/SiO2, 0.1Fe/3Ir/SiO2, and 3Ir/3.5Fe/SiO2 Catalysts selectivity (%) sample

conva (%)

react rateb (μmol gIr−1 s−1)

yield (%)

3Ir/SiO2 3Ir/0.1Fe/SiO2 0.1Fe/3Ir/SiO2 3Ir/3.5Fe/SiO2

15.8 65.6 39.2 37.9

5.8 23.8 13.6 13.2

12.4 59.6 32.7 33.7

TOFc (s−1) 3.7 1.8 1.1 1.1

× × × ×

10−3 10−2 10−2 10−2

crotyl alcohol

butanal

othersd

78.3 90.8 83.4 88.9

15.2 3.7 8.7 5.2

6.5 5.5 7.9 5.9

a Conversions taken at reaction time of 10 h. bμmol of crotonaldehyde converted/(per g of Ir)−1 s−1 (taken at reaction time of 10 h in order to reach the quasi-steady state). cTOF = mol of crotyl alcohol formed/(per mol of exposed metal sites based on Ir dispersion)−1 s−1. dOther products are butanol (saturated alcohol), C8 compounds (2,4,6-octatrienal) formed by polymerization, and C3 compounds (propane and propylene) formed by decarbonylation.

the coverage of crotonaldehyde molecule gradually increases by replacing the H adatoms on the surface, the attack of CC might be sufficiently suppressed and thus enhance the selectivity to crotyl alcohol. When the reaction is conducted at 100 °C (Figure S2b), the selectivity to crotyl alcohol is suppressed, accompanied by the significant formation of butanol. This observation suggests that high reaction temperature may accelerate further hydrogenation of crotyl alcohol to saturated butanol. It also should be noticed that the generation of new active sites under reaction environment (even though the reaction temperature is low) could not be ruled out because no concrete evidence could be provided at present, and this issue is still open for further investigation. Nevertheless, it is obvious that the FeOx promotion is beneficial to the enhancement of both activity and selectivity. The most exciting finding is the remarkable stability of the 3Ir/ 0.1Fe/SiO2, which possesses constant conversion and selectivity in 100 h reaction (Figure 1d). Detailed catalytic results of these catalysts are summarized in Table 2. The yield of crotyl alcohol on the 3Ir/0.1Fe/SiO2 is 59.6%, which is almost 5-fold as high as that on the 3Ir/SiO2 (12.4%). The specific reaction rate on the 3Ir/0.1Fe/SiO2 (23.8 μmol gIr−1 s−1) is comparable to the reported results on Pt, Au, and Ir catalysts while the turnover frequency (0.018 s−1) is higher than most of the reported values (Table S2). However, Englisch et al.25 reported a Pt/TiO2 catalyst with extremely high activity (598.6 μmol gPt−1 s−1 and a TOF of 0.29 s−1) and a reasonable selectivity of

amount of FeOx (3Ir/3.5Fe/SiO2) results in a dramatic increase in the initial conversion (91.2%), but it declines rapidly (37.9% after 10 h). Another finding is that although the 0.1Fe/3Ir/SiO2 has almost identical Ir and Fe compositions as the 3Ir/0.1Fe/ SiO2, it has very similar behavior as the 3Ir/3.5Fe/SiO2, namely, high initial conversion (81.6%) and rapid deactivation (39.2% after 10 h). Concerning the selectivity to crotyl alcohol (Figure 1b), high selectivity (80−90%) is obtained over the FeOx-doped samples after a few hour reaction, while the 3Ir/SiO2 has relatively low selectivity (less than 80%). The selectivity on the 0.1Fe/SiO2 catalyst was not presented because of the extremely low conversion. Also, it seems that the selectivity to crotyl alchol also has an induction period on all the catalysts; namely, it rapidly increases in the first 3−4 h and gradually reaches a relatively stable level. Detailed analyses of the trends of conversion and selectivity were conducted on the 3Ir/0.1Fe/ SiO2 (Figure S2a). It is found that the growth of the selectivity to crotyl alcohol is accompanied by the rapid loss in the selectivity to butanal and slight decline in the selectivity to butanol. This finding implies the competitive hydrogenation of CC and CO bonds in the crotonaldehyde molecule in the initial state, and such a switch is also probably related to the coverage of H adatoms on the surface. The H coverage on the catalyst surface must be high in the initial state because of the prereduction treatment, and the H adatom could randomly attack either CC or CO bond in the crotonaldehyde, which would thus lower the selectivity to crotyl alcohol. When D


Article


HRTEM images of three representative samples (3Ir/SiO2, 3Ir/0.1Fe/SiO2, 0.1Fe/3Ir/SiO2, and 3Ir/3.5Fe/SiO2) are shown in Figure 3. The presence of metallic Ir particles is

64%. Nevertheless, the 3Ir/0.1Fe/SiO2 shows the highest selectivity compared to those in the literature (Table S2). For example, a crotyl alcohol selectivity of 82% was reported over a Pt/ZnO catalyst,7 but the TOF was quite low (2.2 × 10−3 s−1). The detected byproducts in the current work include butanal, 2,4,6-octatrienal, butanol, propylene, and propane, indicating the occurrence of side reactions such as hydrogenation of C C bond, polymerization, full hydrogenation of both CC and CO bonds, and decarbonylation. More importantly, such a stable catalyst (3Ir/0.1Fe/SiO2) has not been reported as most of the catalysts in the literature deactivated rapidly (Table S2). Figure 2 demonstrates the effect of Fe content on the catalytic performance, and the detailed results are summarized

Figure 3. HRTEM images of (a) 3Ir/SiO2, (b) 3Ir/0.1Fe/SiO2, (c) 0.1Fe/3Ir/SiO2, and (d) 19 3Ir/3.5Fe/SiO2 catalysts.

evidenced by the measurement of lattice fringe (Ir(111) = 0.221 nm), with mean particle sizes of 4.0−5.0 nm. For the 3Ir/ 0.1Fe/SiO2, no distinct FeOx species could be observed because of its very low content (Figure 3b). Metallic Ir particles but no FeOx species are observed in the 0.1Fe/3Ir/ SiO2 (Figure 3c). However, for the 3Ir/3.5Fe/SiO2 catalyst, the presence of Fe2O3 is observable (Fe2O3(104) = 0.270 nm, Figure 3d). In addition to the isolated Fe2O3 entities, some Fe2O3 particles are contacting with Ir particles (indicated by the white line), suggesting interactions between Ir and Fe2O3. The mean Ir particle size distributions were also analyzed, as shown Figure S4. It is found that the Ir particle sizes in the 3Ir/SiO2, 3Ir/0.1Fe/SiO2, and 0.1Fe/3Ir/SiO2 catalysts are generally centered at about 4−5 nm while that of the 3Ir/3.5Fe/SiO2 is centered about 5−7 nm, which are consistent with the results obtained by CO chemisorption and XRD analyses (Table S1). Oxidation states of the catalysts are measured by XPS (Figure 4). The Ir 4f spectra can be deconvoluted to four peaks at 60.4, 62.0. 63.4, and 64.8 eV. The peaks at BEs of 60.4 and 63.4 eV are assigned to metallic state species (Ir0), and those at 62.0 and 64.8 eV are assigned to electron-deficient iridium species (Irδ+).20,40 Also, the surface concentrations of Ir0 are about 63.5−66.6% in the 3Ir/SiO2 and 3Ir/0.1Fe/SiO2, which are slightly lower than those in the 0.1Fe/3Ir/SiO2 (70.5%) and the 3Ir/3.5Fe/SiO2 (69.9%, Table 3). This also suggests an interaction between Ir and Fe species and the Ir species in the FeOx-promoted samples is more negatively charged compared to those in the 3Ir/SiO2. For the Fe 2p spectra, BE at 710.4 suggests the presence of Fe2+ species (FeO) in the catalysts.41 These results suggest the dominant species in the catalysts are Ir0 and Fe2+, which is understandable because the catalysts were subjected to a reduction pretreatment. Table 3 also lists surface Ir/Fe ratios in the catalysts. The surface Ir/Fe molar ratio in the

Figure 2. (a) Crotonaldehyde conversion and (b) selectivity to crotyl alcohol as a function of reaction time over Ir−Fe/SiO2 catalysts with different FeOx contents.

in Table S3. The Ir supported on bulk FeOx is inactive. For the FeOx-promoted Ir/SiO2 catalysts, low content of Fe in the catalyst (0.01−1.1 wt %) favors the catalytic performance, with the 3Ir/0.03Fe/SiO2, 3Ir/0.1Fe/SiO2, and 3Ir/1.1Fe/SiO2 catalysts having similar performance (Figure 2a, Figure 2b, and Table S3), and these three catalysts all show excellent stability in long-term reaction (60−100 h, Figure S3). But high content of Fe (>1.1 wt %) leads to drastic catalyst deactivation although higher initial conversions were obtained on these catalysts (Figure 2a). Also, it seems that high content of Fe in the catalyst suppresses the selectivity to crotyl alcohol, with relatively low selectivity (about 80%) obtained on the 3Ir/ 11Fe/SiO2 (Figure 2b). The catalytic results (Figure 1, Figure 2, Table 2, and Table S3) clearly imply that the enhanced performance on the FeOxpromoted Ir/SiO2 catalysts lies in the generation of new active sites, which is likely due to the interaction between Ir and FeOx species. Therefore, it is certainly worthwhile to investigate the structural and electronic features of these catalysts, which might be helpful to understand the observed catalytic behaviors. Since the 3Ir/SiO2 is an unpromoted sample, the 3Ir/0.1Fe/SiO2 is the best performed sample, and the 3Ir/3.5Fe/SiO2 and 0.1Fe/ 3Ir/SiO2 deactivate rapidly, these four catalysts were chosen as the representatives, and the following characterizations and discussion are focused on these samples. Nevertheless, the characterizations of other catalysts could be found in the Supporting Information. E


Article


Figure 4. (a) Ir 4f and (b) Fe 2p XPS spectra of 3Ir/SiO2, 3Ir/0.1Fe/SiO2, 0.1Fe/3Ir/SiO2, and 3Ir−3.5Fe/SiO2 catalysts.

Table 3. Atomic Ratios of Ir0/Irδ+ and Ir/Fe over 3Ir/SiO2, 3Ir/0.1Fe/SiO2, 0.1Fe/3Ir/SiO2, and 3Ir/3.5Fe/SiO2 Catalysts Ir contentsa (%)

catalysts, the similar FT spectra of the Ir species (Figure 5a) indicate the same structures of Ir species in these samples. The

Ir/Fe atomic ratio

catalyst

Ir0

Irδ+

bulkb

surfacec

3Ir/SiO2 3Ir/0.1Fe/SiO2 0.1Fe/3Ir/SiO2 3Ir/3.5Fe/SiO2

63.5 66.6 70.5 69.9

36.5 33.4 29.5 30.1

6.77 6.99 0.25

0.71 0.22 0.52

a

Calculated based on XPS results in Figure 4. bCalculated based on XRF results in Table 1. cCalculated based on XPS results in Figure 4.

3Ir/0.1Fe/SiO2 (0.71) is much lower than the bulk ratio (6.77), implying that the Ir particles are quite large or the FeOx entities are highly dispersed. In contrast, the 0.1Fe/3Ir/SiO2 has a much lower surface Ir/Fe ratio (0.22) compared to that of the 3Ir/0.1Fe/SiO2 although they have similar bulk values (6.77 and 6.99), suggesting that Fe atoms are enriched on the surface of the 0.1Fe/3Ir/SiO2. The surface Ir/Fe molar ratio in the 3Ir/ 3.5Fe/SiO2 (0.52) is higher than the bulk ratio (0.25), indicating the FeOx entities in this sample are large. These results are in good agreement with the XRD (Figure S1) and HRTEM (Figure 3) findings. In addition, the XPS analyses (Figure S5 and Table S4) on the catalysts with different Fe contents reveal the presence of Ir0, Irδ+, and Fe2+. In general, the concentration of surface Ir0 increases with increasing Fe content, but the surface Ir/Fe molar ratio follows an opposite trend. However, one must keep in mind that the XPS results, particularly the surface contents of the Ir0 and Irδ+, strongly depend on the deconvolution process, and considerable error might exist. Therefore, quantitative analyses of the exact concentrations of Ir0 and Irδ+ species cannot be done at the current stage; nevertheless, a qualitative remark still could be reached, that is, the Ir species in the high FeOx content samples are more negatively charged than those in the low FeOx content samples. The similar XANES spectra of the Ir species (Figure S6a) in the catalysts suggest the same electronic states of Ir species, while the XANES spectra of the Fe species (Figure S6b) indicate the presence of Fe2+ species, which are well consistent with the XPS results (Figure 4). For the EXAFS spectra of the

Figure 5. (a) Ir LIII-edge and (b) Fe K-edge Fourier transform of EXAFS spectra of 3Ir/SiO2, 3Ir/0.1Fe/SiO2, 0.1Fe/3Ir/SiO2, and 3Ir− 3.5Fe/SiO2 catalysts.

calculated Ir−Ir coordination numbers for the 3Ir/SiO2, 3Ir/ 0.1Fe/SiO2, 0.1Fe/3Ir/SiO2, and 3Ir−3.5Fe/SiO2 are 12.7, 11.6, 12.5, and 12.3, respectively, which agree well with spherical particle models as large as 4 nm and the HRTEM (Figure 3) and CO chemisorption (Table 1) results. The FT spectra of the Fe species (Figure 5b) indicate large oxide particles in the 3Ir/3.5Fe/SiO2 because the contribution of the second-nearest neighbor clearly appears, while very highly dispersed FeO entities are formed in the 3Ir−0.1Fe/SiO2 and the 0.1Fe/3Ir/SiO2 as only Fe−O is observed. It should be noted that the detection of Fe2+ species in the 3Ir/3.5Fe/SiO2 by XPS (Figure 4) and XANES (Figure S6a) results are not consistent with the finding of Fe3+ (Fe2O3) by the HRTEM results (Figure 3d). This is because that the sample was prereduced before the XPS and XANES measurements and without exposure to air in order to investigate the relevant properties, while the sample for HRTEM measurement was exposed to air so that Fe2+ could be reoxidized to Fe3+. F


Article


Nevertheless, the Ir−FeOx interaction was further confirmed by H2-TPR results, as shown in Figure 7. The 0.1Fe/SiO2

In situ DRIFTS measurements of CO chemisorption on the catalysts were conducted in order to probe the electronic properties of the catalyst surfaces (Figure 6). For the 3Ir/SiO2

Figure 6. In situ DRIFT spectra of CO chemisorption on (a) 3Ir/ SiO2, (b) 3Ir/0.1Fe/SiO2, (c) 0.1Fe/3Ir/SiO2, and (d) 3Ir/3.5Fe/SiO2 catalysts. Figure 7. H2-TPR profiles of as-prepared (calcined) catalysts.

(Figure 6a), one asymmetric broad band is observed, which could be further deconvoluted to three peaks at 2080, 2073, and 2055 cm−1 assigning to CO linearly adsorbed on different Ir0 sites.42,43 For the 3Ir/0.1Fe/SiO2 (Figure 6b), three similar peaks are deconvoluted. However, obvious red-shifts toward lower wavenumbers are observed on this catalyst, indicating an increase in the electron density on Ir, which in turn increases the extent of the back-donation to the CO 2π* orbital.44 Interestingly, the much smaller band observed on the 0.1Fe/ 3Ir/SiO2 (Figure 6c) compared to that of the 3Ir/0.1Fe/SiO2 implies the reduced amount of surface Ir atoms in this sample, which is well consistent with the XPS results (Table 3). Besides, three resolved bands systematically shift to lower wavenumbers. For the 3Ir/3.5Fe/SiO2 (Figure 6d), in addition to the redshifts, the much smaller peak area suggests the partial decoration of the surface Ir species by FeO, which is similar to the finding on a Pt/TiO2 catalyst reduced at high temperature.4 Moreover, since the red-shifts on the 0.1Fe/ 3Ir/SiO2 and 3Ir/3.5Fe/SiO2 are more significant than those on the 3Ir/0.1Fe/SiO2, suggesting stronger interactions between Ir−FeOx in the former catalysts compared to the latter; namely, the charge densities of the Ir species in the 0.1Fe/3Ir/SiO2 and 3Ir/3.5Fe/SiO2 are higher than that in the 3Ir/0.1Fe/SiO2. Similar observation was reported by Siani et al.,45 in which an 11 cm−1 red-shift was observed on the PtFe/ SiO2 catalyst compared to the Pt/SiO2. FTIR spectra of CO chemisorption on the catalysts with different Fe contents further confirm the red-shifts (Figure S7). The IR bands gradually shift to lower wavenumbers with increasing Fe content in the catalyst, implying charge transfer from Fe to Ir. These findings are in agreement with the XPS results (Table 3, Figure 4, Table S4, and Figure S5), as the Ir0 content in the high FeOx content sample is higher than that in the low FeOx ones. The spectrum of the 3Ir/11Fe/SiO2 is very weak probably because of the partial decoration of Ir surface atoms by FeOx species.

catalyst shows a weak but broad reduction peak at 400−650 °C, which could be assigned to the sequential reduction of FeOx (i.e., Fe3O4 → FeO → Fe).46 The 3Ir/SiO2 catalyst shows an intense reduction peak centered at about 236 °C (β), which could be assigned to the reduction of IrOx. With the promotion of FeOx, an overlapped reduction peak (α) at temperature range of 150−180 °C emerges, and its area increases with increasing Fe content in the catalyst. This peak could be assigned to combined reduction of FeOx and IrOx species in close contact through a spillover effect.47 Moreover, the α peak gradually shifts to lower temperature with increasing FeOx content (from 178 °C for the 3Ir/0.01Fe/SiO2 to 157 °C for the 3Ir/3.5Fe/SiO2), suggesting that the IrOx−FeOx interaction becomes stronger in the higher FeOx content catalyst. Concerning the 0.1Fe/3Ir/SiO2, the reduction temperature of the peak β centered at about 250 °C is slightly higher than that of the 3Ir/0.1Fe/SiO2 (240 °C), and the α peak in the 3Ir/ 0.1Fe/SiO2 is also absent. The differences in the profiles of these two catalysts with inverse impregnation sequence also reflect the changes of Ir−FeOx interactions induced by their morphologies. The above results reflect a strong interaction between Ir and Fe species in the FeOx-promoted catalysts. Such an interaction changes the electronic properties of the Ir and Fe species, which could possibly explain the enhanced performance of these catalysts compared to the Ir/SiO2. It is believed that the catalytic performance of the selective hydrogenation of crotonaldehyde is strongly dependent on the adsorption mode of the crotonaldehyde molecule on the catalyst surface.9,20 Therefore, a possible adsorption model of crotonaldehyde on the catalyst is proposed in Figure 8, based on the findings on Pt/TiO24,9,25 and Ir/TiO2 catalysts.20 On the Ir/SiO2 catalyst (left part), the carbonyl oxygen is suggested to strongly interact with Lewis acidic Irδ+ sites (σ2) and the Ir0 G


Article


S1), HRTEM (Figure 3d), and EXAFS (Figure 5b) results. Since the 0.1Fe/3Ir/SiO2 and 3Ir/3.5Fe/SiO2 have much higher initial activity compared to the 3Ir/0.1Fe/SiO2 (Figure 1a), it could be deduced that there are more Ir−FeO interfacial active sites in these catalysts compared to that in the 3Ir/0.1Fe/ SiO2. In other words, the perimeters of the Ir−FeO interface in the former two catalysts are larger than that in the latter one. Although this point could not clarified by a quantitative analysis, it is justified by the comparison of the surface Ir/Fe atomic ratios (Table 3). For example, the 0.1Fe/3Ir/SiO2 has a much lower value (0.22) than that of the 3Ir/0.1Fe/SiO2 (0.71), indicating the pronounced coverage of surface Ir atoms by FeO species. Such decoration or coverage of Ir by FeO species is further confirmed by the FTIR results (Figure 6) as the intensities of CO adsorption bands in the 0.1Fe/3Ir/ SiO2 and 3Ir/3.5Fe/SiO2 are much weaker than that in the 3Ir/ 0.1Fe/SiO2. Therefore, the more quantities of contacting Ir− FeO entities result in more Ir−FeO interfacial sites, which accounts for the higher initial activities observed on the 0.1Fe/ 3Ir/SiO2 and 3Ir/3.5Fe/SiO2. However, one must note that although the 0.1Fe/3Ir/SiO2 and the 3Ir/3.5Fe/SiO2 catalysts have high initial activity, they suffer severe deactivation during the reaction. The change of the catalyst structure during the reaction could be ruled out because of the low reaction temperature (80 °C). In addition, the formed products on this catalyst contain propylene and C8 compounds (Table 1), implying the occurrence of side reactions such as decarbonylation and coupling which may account for the catalyst deactivation.21 In order to investigate the surface species on the catalyst surface, TPO measurements were conducted and the results are shown in Figure 10. For the

Figure 8. Proposed adsorption of crotonaldehyde on Ir/SiO2 (left part) and FeOx-promoted Ir/SiO2 (right part) catalysts.

particle is the adsorption center of CC band (π) and carbonyl carbon (σ1). Thus, the hydrogenations of the CC and CO bonds are competitive, which controls the selectivities to butanal and crotyl alcohol, respectively. On the other hand, in the FeOx-promoted catalyst (right part), additional new active sites are generated at the Ir−FeOx interface via the interaction between these two species. The carbonyl group could chemisorb on the Ir−FeOx sites through the charge-deficient carbonyl carbon with charge-rich Ir0 (σ1) and charge-rich carbonyl oxygen with charge-deficient FeO (σ2) to form a di-σCO adsorption mode, which was believed to be responsible for the polarization and activation of the CO bond.4,9,20,25 Obviously, the activation of the CO bond on Ir−FeO entity is much easier than that on the Ir0−Irδ+ couple, which accounts for the 4-fold enhancement in activity obtained on the 3Ir/0.1Fe/SiO2 (65.6%) compared to the 3Ir/SiO2 catalyst (15.8%). Moreover, the formation of such σ-bonded complex [−CO− surface (interface)] is also responsible for the enhanced selectivity to crotyl alcohol.20 However, another possible adsorption mode(s) over bimetallic catalysts could not be ruled out. For example, Nishiyama et al.48 conducted in situ FTIR experiments of propionaldehyde adsorption over silicasupported Rh−Sn bimetallic catalyst and proposed donatingon-top η1 (O) adsorbed species which was one of the active intermediates of hydrogenation of the aldehyde. It is also worthwhile to discuss the differences in the FeOxpromoted catalysts, particularly the 3Ir/0.1Fe/SiO2, 0.1Fe/3Ir/ SiO2, and 3Ir/3.5Fe/SiO2 because these catalysts possess different behaviors. First of all, morphologies of these catalysts should be considered, which are illustrated in Figure 9. The

Figure 9. Simplified structures of 3Ir/0.1Fe/SiO2, 0.1Fe/3Ir/SiO2, and 3Ir/3.5Fe/SiO2 catalysts.

validity of these models in Figure 9 is evidenced by various characterizations such as HRTEM (Figure 3), XPS (Table 3), EXAFS (Figure 5), and H2-TPR (Figure 7) results. For the 3Ir/ 0.1Fe/SiO2, since the Ir species were impregnated on the FeOx/SiO2 support and the FeOx species are highly dispersed on the SiO2 due to its very low content (0.1 wt %), most of the Ir species would locate on SiO2 and very limited quantities of the Ir species would cover or contact with FeOx species. While for the 0.1Fe/3Ir/SiO2, since the Fe precursor was impregnated on the Ir/SiO2 and the Ir content is relatively high (about 3 wt %), the highly dispersed FeOx species would isolate on the SiO2, and some would be selectively deposited on the Ir surface. For the 3Ir/3.5Fe/SiO2, it has a similar morphology as that of the 3Ir/0.1Fe/SiO2, except that the particle sizes of the FeO entities are much larger, as revealed by the XRD (Figure

Figure 10. TPO profiles of (a) spent 3Ir/SiO2, (b) spent 3Ir/0.1Fe/ SiO2, (c) spent 0.1Fe/3Ir/SiO2, and (d) spent 3Ir/3.5Fe/SiO2 catalysts after 10 h reaction.

spent 3Ir/SiO2 and 3Ir/0.1Fe/SiO2 catalysts, no signals of CO2 and H2O are detected (Figure 10a,b), while distinct desorption of CO2 at 234 and 290 °C and desorption of H2O at 300 °C appear on the spent 0.1Fe/3Ir/SiO2 (Figure 10c). The spent 3Ir/3.5Fe/SiO2 shows desorption of CO2 and H2O at 166 and about 300 °C, with a shoulder of CO2 at 256 °C (Figure 10d). H


Article

The Journal of Physical Chemistry C The simultaneous emergence of CO2 and H2O is due to the combustion of the surface deposits under oxidation environment, while the CO2 signal at 234 or 256 °C is probably due to the oxidation of the adsorbed CO on Ir surface. Furthermore, it also implies that the natures of the surface deposits on the spent 0.1Fe/3Ir/SiO2 and 3Ir/3.5Fe/SiO2 are quite different, and the spent 3Ir/3.5Fe/SiO2 catalyst may contain more complicated surface species compared to the spent 0.1Fe/3Ir/ SiO2. Nevertheless, the presence of such deposits is also confirmed by Raman spectra of the spent catalysts (Figure 11).

Figure 12. Conversion of crotonaldehyde and selectivity to crotyl alcohol on fresh and regenerated (a) 3Ir/3.5Fe/SiO2 and (b) 0.1Fe/ 3Ir/SiO2 catalysts.

samples. According the chemisorption model of crotonaldehyde on the surface (Figure 8), the σ1 and σ2 bonds on the 3Ir/ 3.5Fe/SiO2 and 0.1Fe/3Ir/SiO2 are relatively stronger than those on the 3Ir/0.1Fe/SiO2 because the different charge densities of the Ir and FeOx species in these two catalysts (Figure 6). Products such as CO and C8 compound formed through such strongly bonded crotonaldehyde molecules may stick on the surface and block the active sites and thus lead to the catalyst deactivation. Finally, it is also worthwhile to discuss our strategy of catalyst preparation in the current work. As reported in the literature, noble metals supported on reducible oxides and bimetallic catalyst systems indeed significantly improve both catalytic activity and the selectivity. Also, the interaction between metal and support oxide (or metal−metal interaction in bimetallic system) could be tuned by the adjustment of treatment conditions such as reduction temperature.4,5,20 However, in these systems, the noble metal NPs tend to interact with either bulk oxides (the support) or NPs of promoters (the alloy), which makes subtle tuning of such interaction rather difficult. As demonstrated in the present work, even though the 3Ir/ 3.5Fe/SiO2 catalyst contains Ir and FeOx NPs, the strong interaction between these species may lead to drastic changes in the electronic properties of the catalysts and consequently the strong adsorption of reaction products (e.g., CO and conjugated compound), which could explain the often observed catalyst deactivation in the literature. On the other hand, interaction between Ir NPs and highly dispersed FeOx nanoclusters (3Ir/0.1Fe/SiO2) results in proper electronic features of the active Ir−FeOx interface, which is beneficial to the enhancement in both reactivity and catalyst stability. Actually, a recent work by Aich et al.50 provides a similar strategy in the preparation of active catalysts for selective hydrogenation of acrolein, in which Pd−Ag alloy catalysts with atomically isolated Pd in Ag NPs showed high activity and selectivity to the allyl alcohol due to facile H2 dissociation and changes in the configuration of adsorbed acrolein. However, one must keep in mind that the structural and electronic properties of the catalyst may drastically change by different preparation method and thus play a vital role in the catalytic performance, as it is demonstrated in the current work that the 0.1Fe/3Ir/SiO2 catalyst prepared by an inverse impregnation

Figure 11. Raman spectra of fresh and spent 3Ir/SiO2, 3Ir/0.1Fe/ SiO2, 0.1Fe/3Ir/SiO2, and 3Ir−3.5Fe/SiO2 catalysts.

No Raman bands are observed on the fresh or spent 3Ir/SiO2 and 3Ir/0.1Fe/SiO2, while a broad band centered at about 1532 cm−1 is detected on the spent 3Ir/3.5Fe/SiO2, and a band at about 1613 cm−1 is detected on the spent 0.1Fe/3Ir/SiO2. These bands are is characteristic of conjugated olefin-like species deposited on the catalyst surface.49 Thus, the deactivation of the 3Ir/3.5Fe/SiO2 and 0.1Fe/3Ir/SiO2 could be attributed to the strong adsorption of CO on the surface Ir atoms via decarbonylation reaction (CH3CHCH−CHO → C3H6 + CO)25 and/or the deposition of heavy products with conjugated CO and CC bonds on the catalyst surface.20 The deactivated 3Ir/3.5Fe/SiO2 and 0.1Fe/3Ir/SiO2 catalysts could be regenerated by treating with O2 (30 mL min−1) at 300 °C for 1 h followed by H2 (30 mL min−1) at 300 °C for 1 h, by which the initial performance could be recovered (Figure 12). However, the activity drastically declines in a few hours, indicating the deactivation of the catalyst. Since propylene and C8 compound are also formed on the 3Ir/SiO2 and 3Ir/0.1Fe/SiO2 catalysts but they are stable, it is reasonable to conclude that the chemisorption of CO and C8 compounds on the 3Ir/SiO2 and 3Ir/0.1Fe/SiO2 catalysts is not strong, and these molecules (CO and C8 compounds) could easily desorb from the catalyst surface because no detectable residues are found in these two catalysts (Figures 10 and 11). On the other hand, chemisorbed CO and organic deposits are detected on the spent 3Ir/3.5Fe/SiO2 and 0.1Fe/3Ir/SiO2 surfaces (Figures 10 and 11), implying that part of these molecules could not be easily removed from the surface under reaction conditions. A possible explanation of such deposits on the deactivated catalysts is the electronic features of these I



■

ACKNOWLEDGMENTS This research was financially supported by the National Natural Science Foundation of China (Grant 21173194).

sequence shows a completely different behavior compared to that of the 3Ir/0.1Fe/SiO2, although the former also contains Ir NPs and highly dispersed FeOx nanoclusters. Nevertheless, such systems with NPs interacting with atomic ion/highly dispersed cluster have been recently applied in various reactions, such as Au−Pd catalyst for selective oxidation of glucose;51 alkali-metal-promoted Pt catalysts for water-gas shift reaction,52 formaldehyde oxidation,53 and dichloromethane oxidation;54 single atom Pt supported on FeOx for CO oxidation;55 atomically dispersed Au(OH)x supported on TiO2;56 and single atomic Ir/FeOx57 for water-gas shift reaction. Therefore, this work demonstrates that high performing catalysts could be achieved by finely adjustment of the interaction between the metal and promoter and provides a useful approach for the design of catalyst systems for selective hydrogenation of α, β-unsaturated aldehyde.

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.6b00456. Tables S1−S4: physical properties; comparison of catalytic results of various catalysts in the literature; summary of reaction results and XPS results on various catalysts in the current work; Figures S1−S7: XRD patterns of various catalysts; conversions and selectivities on the 3Ir/0.1Fe/SiO2 catalyst at reaction temperatures of 80 and 100 °C; stability of 3Ir−0.03Fe/SiO2, 3Ir/ 0.1Fe/SiO2, and 3Ir/1.1Fe/SiO2 catalysts; Ir particle size distributions of 3Ir/SiO2, 3Ir/0.1Fe/SiO2, 0.1Fe/3Ir/ SiO2, and 3Ir/3.5Fe/SiO2 catalysts; XPS spectra of various catalysts; XANES spectra of Ir and Fe species in 3Ir/SiO2, 3Ir/0.1Fe/SiO2, 0.1Fe/3Ir/SiO2, and 3Ir/ 3.5Fe/SiO2 catalysts; in situ DRIFT spectra of CO chemisorption on various catalysts (PDF)

■

REFERENCES

(1) Claus, P. Selective hydrogenation of α, β-unsaturated aldehydes and other CO and CC bonds containing compounds. Top. Catal. 1998, 5, 51−62. (2) Mäki-Arvela, P.; Hájek, J.; Salmi, T.; Murzin, D. Y. Chemoselective hydrogenation of carbonyl compounds over heterogeneous catalysts. Appl. Catal., A 2005, 292, 1−49. (3) Englisch, M.; Ranade, V. S.; Lercher, J. A. Hydrogenation of crotonaldehyde over Pt based bimetallic catalysts. J. Mol. Catal. A: Chem. 1997, 121, 69−80. (4) Dandekar, A.; Vannice, M. A. Crotonaldehyde hydrogenation on Pt/TiO2 and Ni/TiO2 SMSI catalysts. J. Catal. 1999, 183, 344−354. (5) Silvestre-Albero, J.; Rodríguez-Reinoso, F.; Sepúlveda-Escribano, A. Improved metal-support interaction in Pt/CeO2−SiO2 catalysts after zinc addition. J. Catal. 2002, 210, 127−136. (6) Concepción, P.; Corma, A.; Silvestre-Albero, J.; Franco, V.; Chane-Ching, J. Y. Chemoselective hydrogenation catalysts: Pt on mesostructured CeO2 nanoparticles embedded within ultrathin layers of SiO2 binder. J. Am. Chem. Soc. 2004, 126, 5523−5532. (7) Ammari, F.; Lamotte, J.; Touroude, R. An emergent catalytic material: Pt/ZnO catalyst for selective hydrogenation of crotonaldehyde. J. Catal. 2004, 221, 32−42. (8) Ammari, F.; Milone, C.; Touroude, R. Selective hydrogenation of crotonaldehyde on Pt/ZnCl2/SiO2 catalysts. J. Catal. 2005, 235, 1−9. (9) Kennedy, G.; Baker, L. R.; Somorjai, G. A. Selective amplification of CO bond hydrogenation on Pt/TiO2: Catalytic reaction and sum-frequency generation vibrational spectroscopy studies of crotonaldehyde hydrogenation. Angew. Chem., Int. Ed. 2014, 53, 3405−3408. (10) Ramos-Fernández, E. V.; Samaranch, B.; Ramirez de la Piscina, P.; Homs, N.; Fierro, J. L. G.; Rodríguez-Reinoso, F.; SepúlvedaEscribano, A. Pt/Ta2O5−ZrO2 catalysts for vapour phase selective hydrogenation of crotonaldehyde. Appl. Catal., A 2008, 349, 165−169. (11) Hidalgo-Carrillo, J.; Aramendía, M. A.; Marinas, A.; Marinas, J. M.; Urbano, F. J. Support and solvent effects on the liquid-phase chemoselective hydrogenation of crotonaldehyde over Pt catalysts. Appl. Catal., A 2010, 385, 190−200. (12) Bailie, J. E.; Hutchings, G. J. Promotion by sulfur of gold catalysts for crotyl alcohol formation from crotonaldehyde hydrogenation. Chem. Commun. 1999, 21, 2151−2152. (13) Zanella, R.; Louis, C.; Giorgio, S.; Touroude, R. Crotonaldehyde hydrogenation by gold supported on TiO2: structure sensitivity and mechanism. J. Catal. 2004, 223, 328−339. (14) Claus, P. Heterogeneously catalysed hydrogenation using gold catalysts. Appl. Catal., A 2005, 291, 222−229. (15) Campo, B.; Volpe, M.; Ivanova, S.; Touroude, R. Selective hydrogenation of crotonaldehyde on Au/HSA-CeO2 catalysts. J. Catal. 2006, 242, 162−171. (16) Lin, H.; Zheng, J.; Zheng, X.; Gu, Z.; Yuan, Y.; Yang, Y. Improved chemoselective hydrogenation of crotonaldehyde over bimetallic AuAg/SBA-15 catalyst. J. Catal. 2015, 330, 135−144. (17) Chen, H. Y.; Chang, C. T.; Chiang, S. J.; Liaw, B. J.; Chen, Y. Z. Selective hydrogenation of crotonaldehyde in liquid-phase over Au/ Mg2AlO hydrotalcite catalysts. Appl. Catal., A 2010, 381, 209−215. (18) Campo, B. C.; Ivanova, S.; Gigola, C.; Petit, C.; Volpe, M. A. Crotonaldehyde hydrogenation on supported gold catalysts. Catal. Today 2008, 133−135, 661−666. (19) Guzmán, C.; Del Angel, G.; Fierro, J. L. G.; Bertin, V. Role of Pt oxidation state on the activity and selectivity for crotonaldehyde hydrogenation over Pt−Sn/Al2O3−La and Pt−Pb/Al2O3−La catalysts. Top. Catal. 2010, 53, 1142−1144. (20) Reyes, P.; Aguirre, M. C.; Melián-Cabrera, I.; López Granados, M.; Fierro, J. L.G. Interfacial properties of an Ir/TiO2 system and their

4. CONCLUSIONS In summary, highly active and stable FeOx-promoted Ir/SiO2 catalysts were synthesized for gas phase selective hydrogenation of crotonaldehyde. On one hand, the addition of of FeOx in the catalyst significantly enhances the catalytic performance due to the formation of new active sites at the Ir−FeO interface. On the other hand, the stability of the catalyst is governed by the electronic property of such Ir−FeO assembly which is closely related to the morphologies of the catalysts. The suitable Ir− FeO interaction is beneficial for the maintenance of catalyst stability due to the proper adsorption strength of crotonaldehyde molecules on the catalyst surface (as in the 3Ir/0.1Fe/ SiO2), while the strong interaction leads to the difficult desorption of the reaction products and thus the catalyst deactivation (as in the 3Ir/3.5Fe/SiO2 and 0.1Fe/3Ir/SiO2).

■

Article

AUTHOR INFORMATION

Corresponding Authors

*Tel +86 579 82287325; Fax +86 579 82282595; e-mail [email protected] (J.Q.L.). *Tel +86 579 82287325; Fax +86 579 82282595; e-mail [email protected] (M.F.L.). Notes

The authors declare no competing financial interest. J


Article

The Journal of Physical Chemistry C relevance in crotonaldehyde hydrogenation. J. Catal. 2002, 208, 229− 237. (21) Chen, P.; Lu, J.; Xie, G.; Hu, G.; Zhu, L.; Luo, L.; Huang, W.; Luo, M. Effect of reduction temperature on selective hydrogenation of crotonaldehyde over Ir/TiO2 catalysts. Appl. Catal., A 2012, 433−434, 236−242. (22) Hong, X.; Li, B.; Wang, Y.; Lu, J.; Hu, G.; Luo, M. Stable Ir/ SiO2 catalyst for selective hydrogenation of crotonaldehyde. Appl. Surf. Sci. 2013, 270, 388−394. (23) Li, B.; Hong, X.; Lin, J.; Hu, G.; Yu, Q.; Wang, Y.; Luo, M.; Lu, J. Promoting effect of Ir on the catalytic property of Ru/ZnO catalysts for selective hydrogenation of crotonaldehyde. Appl. Surf. Sci. 2013, 280, 179−185. (24) Yu, Q.; Zhang, X.; Li, B.; Lu, J.; Hu, G.; Jia, A.; Luo, C.; Hong, Q.; Song, Y.; Luo, M. Effect of reduction temperature on Ru−Ir/ZnO catalyst for selective hydrogenation of crotonaldehyde. J. Mol. Catal. A: Chem. 2014, 392, 89−96. (25) Englisch, M.; Jentys, A.; Lercher, J. A. Structure sensitivity of the hydrogenation of crotonaldehyde over Pt/SiO2 and Pt/TiO2. J. Catal. 1997, 166, 25−35. (26) Marinelli, T. B. L. W.; Nabuurs, S.; Ponec, V. A study on the selectivity in acrolein hydrogenation on platinum catalysts: A model for hydrogenation of α, β-unsaturated aldehydes. J. Catal. 1995, 151, 431−438. (27) Margitfalvi, J. L.; Vankó, Gy.; Borbáth, I.; Tompos, A.; Vértes, A. Characterization of Sn−Pt/SiO2 catalysts used in selective hydrogenation of crotonaldehyde by Mössbauer spectroscopy. J. Catal. 2000, 190, 474−477. (28) Merlo, A. B.; Machado, B. F.; Vetere, V.; Faria, J. L.; Casella, M. L. PtSn/SiO2 catalysts prepared by surface controlled reactions for the selective hydrogenation of cinnamaldehyde. Appl. Catal., A 2010, 383, 43−49. (29) Wang, X. X.; Zheng, H. Y.; Liu, X. J.; Xie, G. Q.; Lu, J. Q.; Jin, L. Y.; Luo, M. F. Effects of NaCl on Pt/ZrO2 catalysts for selective hydrogenation of crotonaldehyde. Appl. Catal., A 2010, 388, 134−140. (30) Tian, L.; Yang, X.; Jiang, Z.; Zhu, Y.; Pei, Y.; Qiao, M.; Fan, K. Highly chemoselective hydrogenation of crotonaldehyde over Ag-In/ SBA-15 fabricated by a modified “two solvents” strategy. Chem. Commun. 2011, 47, 6168−6170. (31) Ruiz-Martinez, J.; Fukui, Y.; Komatsu, T.; Sepulveda-Escribano, A. Ru-Ti intermetallic catalysts for the selective hydrogenation of crotonaldehyde. J. Catal. 2008, 260, 150−156. (32) Galloway, E.; Armbruster, M.; Kovnir, K.; Tikhov, M. S.; Lambert, R. M. Bromine-promoted PtZn is very effective for the chemoselective hydrogenation of crotonaldehyde. J. Catal. 2009, 261, 60−65. (33) Lin, H.; Zheng, J.; Zheng, X.; Gu, Z.; Yuan, Y.; Yang, Y. Improved chemoselective hydrogenation of crotonaldehyde over bimetallic AuAg/SBA-15 catalyst. J. Catal. 2015, 330, 135−144. (34) Sepúlveda-Escribano, A.; Coloma, F.; Rodríguez-Reinoso, F. Promoting effect of ceria on the gas phase hydrogenation of crotonaldehyde over platinum catalysts. J. Catal. 1998, 178, 649−657. (35) Hidalgo-Carrillo, J.; Marinas, A.; Marinas, J. M.; Delgado, J. J.; Raya-Miranda, R.; Urbano, F. J. Water as solvent in the liquid-phase selective hydrogenation of crotonaldehyde to crotyl alcohol over Pt/ ZnO: A factorial design approach. Appl. Catal., B 2014, 154−155, 369−378. (36) Liberková, K.; Touroude, R. Performance of Pt/SnO2 catalyst in the gas phase hydrogenation of crotonaldehyde. J. Mol. Catal. A: Chem. 2002, 180, 221−230. (37) Zhu, L.; Lu, J. Q.; Chen, P.; Hong, X.; Xie, G. Q.; Hu, G. S.; Luo, M. F. A comparative study on Pt/CeO2 and Pt/ZrO2 catalysts for crotonaldehyde hydrogenation. J. Mol. Catal. A: Chem. 2012, 361-362, 52−57. (38) Gebauer-Henke, E.; Grams, J.; Szubiakiewicz, E.; Farbotko, J.; Touroude, R.; Rynkowski, J. Pt/Ga2O3 catalysts of selective hydrogenation of crotonaldehyde. J. Catal. 2007, 250, 195−208. (39) Boudart, M. Fine-tuning metal clusters. Nature 1994, 372, 320− 320.

(40) Reyes, P.; Aguirre, M. C.; Pecchi, G.; Fierro, J. L. G. Crotonaldehyde hydrogenation on Ir supported catalysts. J. Mol. Catal. A: Chem. 2000, 164, 245−251. (41) Wilson, D.; Langell, M. A. XPS analysis of oleylamine/oleic acid capped Fe3O4 nanoparticles as a function of temperature. Appl. Surf. Sci. 2014, 303, 6−13. (42) Solymosi, F.; Novák, É.; Molnár, A. Infrared spectroscopic study of CO-induced structural changes of iridium on an alumina support. J. Phys. Chem. 1990, 94, 7250−7255. (43) Zhang, W.; Wang, A.; Li, L.; Wang, X.; Zhang, T. Design of a novel bifunctional catalyst IrFe/Al2O3 for preferential CO oxidation. Catal. Today 2008, 131, 457−463. (44) Blyholder, G. Molecular orbital view of chemisorbed carbon monoxide. J. Phys. Chem. 1964, 68, 2772−2777. (45) Siani, A.; Alexeev, O. S.; Lafaye, G.; Amiridis, M. D. The effect of Fe on SiO2-supported Pt catalysts: Structure, chemisorptive, and catalytic properties. J. Catal. 2009, 266, 26−38. (46) Basinska, A.; Józwiak, W. K.; Góralski, J.; Domka, F. The behaviour of Ru/Fe2O3 catalysts and Fe2O3 supports in the TPR and TPO conditions. Appl. Catal., A 2000, 190, 107−115. (47) Zhang, W.; Huang, Y.; Wang, J.; Liu, K.; Wang, X.; Wang, A.; Zhang, T. IrFeOx/SiO2-A highly active catalyst for preferential CO oxidation in H2. Int. J. Hydrogen Energy 2010, 35, 3065−3071. (48) Nishiyama, S.; Hara, T.; Tsuruya, S.; Masai, M. Infrared spectroscopy study of aldehydes adsorbed on Rh-Sn bimetallic systems: Selective activation of aldehydes by tin. J. Phys. Chem. B 1999, 103, 4431−4439. (49) Zerda, T. W.; Xu, W.; Zerda, A.; Zhao, Y.; Von Dreele, R. B. High pressure Raman and neutron scattering study on structure of carbon black particles. Carbon 2000, 38, 355−361. (50) Aich, P.; Wei, H.; Basan, B.; Kropf, A. J.; Schweitzer, N. M.; Marshall, C. L.; Miller, J. T.; Meyer, R. Single-atom alloy Pd−Ag catalyst for selective hydrogenation of acrolein. J. Phys. Chem. C 2015, 119, 18140−18148. (51) Zhang, H.; Watanabe, T.; Okumura, M.; Haruta, M.; Toshima, N. Catalytically highly active top gold atom on palladium nanocluster. Nat. Mater. 2012, 11, 49−52. (52) Zhai, Y.; Pierre, D.; Si, R.; Deng, W.; Ferrin, P.; Nilekar, A. U.; Peng, G.; Herron, J. A.; Bell, D. C.; Saltsburg, H.; et al. Alkali-stabilized Pt-OHx species catalyze low-temperature water-gas shift reactions. Science 2010, 329, 1633−1636. (53) Zhang, C.; Liu, F.; Zhai, Y.; Ariga, H.; Yi, N.; Liu, Y.; Asakura, K.; Flytzani-Stephanopoulos, M.; He, H. Alkali-metal-promoted Pt/ TiO2 opens a more efficient pathway to formaldehyde oxidation at ambient temperatures. Angew. Chem., Int. Ed. 2012, 51, 9628−9632. (54) Wang, Y.; Liu, H. H.; Wang, S. Y.; Luo, M. F.; Lu, J. Q. Remarkable enhancement of dichloromethane oxidation over potassium-promoted Pt/Al2O3 catalysts. J. Catal. 2014, 311, 314−324. (55) Qiao, B.; Wang, A.; Yang, X.; Allard, L. F.; Jiang, Z.; Cui, Y.; Liu, J.; Li, J.; Zhang, T. Single-atom catalysis of CO oxidation using Pt1/ FeOx. Nat. Chem. 2011, 3, 634−641. (56) Yang, M.; Allard, L. F.; Flytzani-Stephanopoulos, M. Atomically dispersed Au−(OH)x species bound on titania catalyze the lowtemperature water-gas shift reaction. J. Am. Chem. Soc. 2013, 135, 3768−3771. (57) Lin, J.; Wang, A.; Qiao, B.; Liu, X.; Yang, X.; Wang, X.; Liang, J.; Li, J.; Liu, J.; Zhang, T. Remarkable performance of Ir1/FeOx singleatom catalyst in water gas shift reaction. J. Am. Chem. Soc. 2013, 135, 15314−15317.

K


Selective Hydrogenation of Crotonaldehyde over Ir ... - ACS Publications

Recommend Documents