Introduction

The chemoselective hydrogenation of α,β-unsaturated aldehydes to unsaturated alcohols is an important step for the production of pharmaceutical and fine chemicals [1, 2]. However, this selective hydrogenation process is challenging since the activation and hydrogenation of C=O bond is unfavorable over its conjugated C=C bond in the viewpoint of thermodynamic aspects [3]. Therefore, achieving a high yield of unsaturated alcohol by designing a novel catalyst with high efficiency and durability is highly desired. As one of the most intensively studied system developed, Pt catalysts perform good activity but poor selectivity toward the unsaturated alcohol. The selectivity enhancement toward the production of the unsaturated alcohol usually can be achieved by using an active support [4,5,6,7,8,9,10,11], modulating the size and morphology of active metal [5, 12], or adding a more electropositive metal [13,14,15].

It is known that the chemical composition and structure of a support greatly affects the catalytic performance of its supported catalyst by influencing the size, morphology and electronic states of active metal/interfacial species. The utilization of an active support is considered to favor the formation of strong metal-support interaction (SMSI), thus changing the adsorption and hydrogenation of crotonaldehyde (CRAL) molecules. Vannice et al. reported that Pt–TiO x interfacial sites of Pt/TiO2 favored the polarization and hydrogenation of the C=O bond [4]. On the other hand, the Pt (111) epitaxial layer on CeO2, instead of Pt–CeO x interfacial sites or CePt sites, was suggested to be the catalytic sites for the hydrogenation of C=O bond [5]. Lercher’s group concluded that either large Pt particles with high fraction of Pt (111) surfaces or TiO x decorated Pt sites was responsible for the enhanced selectivity toward unsaturated alcohol [16]. Kennedy et al. compared the sum-frequency generation vibrational spectroscopy for Pt/TiO2 and Pt/SiO2, and proposed that a crotyl-oxy surface intermediate through the aldehyde oxygen atom binding to an oxygen vancancy (Ovac) site on the TiO2 surface, should be responsible for the additional catalytic activity and C=O selectivity of Pt/TiO2 relative to Pt/SiO2 [17].

Besides, doping Pt with a more electropositive metal is proposed to effectively improve the catalytic activity, selectivity or stability. Generally, incorporating a second metal M into a Pt catalyst can modify the electronic and geometric properties of Pt owing to a ligand effect or an ensemble size effect [18,19,20]. The first effect is based on the observation of an increased electron density on Pt due to the electron transfer from M to Pt. The increased electron density on Pt is claimed to decrease the probability of C=C bond adsorption, and at the same time, to increase the interaction of the C=O bond with the polar bimetallic sites [18, 21, 22]. For the second one, the unreduced MO x species situated at the edge and corner sites of Pt particles acts as Lewis acid adsorption sites for the aldehyde groups [14, 15, 20, 23]. The ensemble size effect is also observed with proper geometric type of the bimetallic surfaces by a complex set of factors [18, 24, 25]. Various Pt-based bimetallic catalysts have been developed and found to display prominent performance for α,β-unsaturated aldehyde hydrogenation. However, due to the sensitivity of bimetallic nanostructures to the support materials and synthetic parameters, there is no consistent picture for active sites in the bimetallic catalysts.

Lanthanum oxy-carbonate (La2O2CO3) has drawn much attention for its impressive activity in the oxidative coupling of methane, steam reforming of glycerol and biofuel production [26,27,28,29]. As previously reported by our group, the preferential deposit of Pt on the {110} facets of La2O2CO3 nanorods (LOC-NR) displayed superior catalytic behavior for CRAL hydrogenation to that on the particle-counterpart [30]. However, Pt/LOC is limited by the low selectivity (22.8%) to the desired crotyl alcohol (CROL). Herein, a series of PtFe/LOC catalysts with various Fe amount are developed to improve the hydrogenation selectivity of carbonyl groups in the CRAL molecules. Noticeably, an enhanced selectivity toward CROL is attained over the resultant PtFe/LOC catalysts compared to that on the Pt/LOC. The combined experimental and analytical investigations show that Fe addition induces the electronic and geometric changes of surface metals, which are accounted to be responsible for the preferential adsorption and hydrogenation of C=O bonds. Correlating detailed characterizations with the catalytic reaction study, we gain new information about the structure–activity relationship of the bimetallic PtFe/LOC catalysts.

Experimental

Catalyst preparation

LOC-NR was obtained by the thermal transformation of lanthanum hydroxide nanorods at 500 °C for 3 h [30]. The specific surface area of LOC-NR was 45 m2 g−1, as calculated on the basis of the Barrett-Joiner-Halenda (BJH) method. Monometallic Pt/LOC and Fe/LOC were synthesized via an incipient wet impregnation process through the impregnation of LOC-NR with metal precursors (H2PtCl6·6H2O and Fe(NO3)3·9H2O). After aging and drying at 120 °C for 10 h, the obtained powders were calcined at 400 °C for 2 h, followed by reduction at 600 °C for 1 h under a flowing gas mixture of 20% H2/N2 (v/v). The Fe/Pt/LOC series catalysts with nominal Fe loadings of 0.25, 0.40, 0.50, and 0.75 wt % were prepared through the impregnation of the dried Pt/LOC with Fe(NO3)3 ethanol solution following an identical procedure, and denoted as xFe/Pt/LOC (x = 0.25, 0.40, 0.50, or 0.75).

Catalyst characterization

The actual loadings of metals were analyzed on inductively coupled plasma optical emission spectrometry (ICP-OES) on an Agilent 725-ES.

The phase identification of the catalysts was recorded on an X’Pert Pro Multipurpose diffractometer (PANalytical, Inc.) using Ni-filtered Cu Kα radiation (0.15406 nm) with a scanning angle (2θ) range of 10–80°, conducted at 40 kV and 20 mA.

Morphological analysis of the catalysts was conducted on the TECNAI G2 F20 high-resolution transmission electron microscopy operated at 200 kV. The size distributions of metal particles over the catalysts were assessed from the observation of more than 300 particles in TEM images. The mean diameter was calculated by the following formula: d s  = ∑ nidi 3/ ∑ nidi 2.

XPS spectra were acquired using a Thermo Scientific ESCALAB250xi spectrometer to determine the chemical states and surface atomic composition of the catalysts. All binding energies were referenced to C 1s hydrocarbon peak at 284.8 eV. The surface atomic concentration of each element was calculated from the area of the corresponding peak. The deconvolution of Pt 4f spectra was processed with a curve-fitting routine using the XPSPEAK41 software. The integral of each peak was determined after the subtraction of the background and fitting with some constrains on the fitting parameters including the half width height maximum (FWHM = 1.43), peak area ratio (Pt 4f 7/2: Pt 4f 5/2 = 4:3) and position (the binding energy gap between Pt 4f 7/2 and Pt 4f 5/2 peak is 3.35 eV) with a combination of Gaussian (80%) and Lorentzian (20%) functions.

Experiments on the temperature-programmed reduction (TPR) and desorption (TPD) of hydrogen were taken on a ChemBET Pulsar TPR/TPD analyzer (Quantachrome Instruments U.S.) equipped with a TCD detector. Typically, 50 mg unreduced sample was loaded in a U-tube quartz reactor and pretreated at 400 °C for 30 min in flowing Ar (40 mL min−1); after cooling the reactor to 40 °C, the TPR experiment was carried out from room temperature to 900 °C with a ramp of 20 °C min−1 in flowing 7% H2/Ar (40 mL min−1). The TPD curves of the reduced catatlysts (50 mg) was obtained from room temperature to 900 °C at a ramp of 20 °C min−1 in flowing Ar (40 mL min−1) after hydrogen-adsorption at 70 °C for 60 min.

Catalytic performance evaluation

Catalytic reaction was carried out in a stainless high pressure reactor of 100 mL capacity, in which 1 mL of CRAL was hydrogenated using 150 mg of catalyst and 24 mL of ethanol. After purging four times with hydrogen, the catalytic test was conducted at 70 °C with a hydrogen pressure of 2.0 MPa. The products were analyzed using an Agilent GC-7890A equipped with an FID detector and a HP-5 capillary column. The CRAL conversion (Conv.) and products selectivity (Sel.) were calculated as follows.

$$Conv.{\text{C}}_{4} {\text{H}}_{6} {\text{O}} = \frac{{^{n} {\text{C}}_{4} {\text{H}}_{6} {\text{O}}_{(a)} - {\,}{^{n}} {\text{C}}_{4} {\text{H}}_{6} {\text{O}}_{(b)} }}{{^{n} {\text{C}}_{4} {\text{H}}_{6} {\text{O}}_{(a)} }}\, \times \,100\%$$
(1)
$$Sel._{x} = \frac{{n_{x} }}{{^{n} {\text{C}}_{4} {\text{H}}_{6} {\text{O}}_{(a)} -\,{^{n}}{\text{C}}_{4} {\text{H}}_{6} {\text{O}}_{(b)} }} \times 100\%$$
(2)

Here \(^{n} {\text{C}}_{4} {\text{H}}_{6} {\text{O}}_{(a)}\) and \(^{n} {\text{C}}_{4} {\text{H}}_{6} {\text{O}}_{(b)}\) is the amount of substance of the initial and residual CRAL, respectively. n x is the mole of the product.

The cycle experiment was carried out under the same conditions except with the recovered catalyst. The spent catalyst was recovered from the mixture of hydrogenation system by centrifugation, desiccation at 120 °C and calcination at 300 °C for 1 h.

Results and discussion

Catalyst characterization

As revealed by the ICP-OES data in Table 1, the Pt loading of the Pt-containing catalysts is in the range of 1.25–1.32%; the Fe loading of Fe/Pt/LOC and xFe/Pt/LOC catalysts is 0.48, 0.21, 0.39, 0.44, and 0.78%, which are basically consistent with the nominal values.

Table 1 The metal loadings of the catalysts determined by ICP-OES analysis
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As shown in Fig. 1, the diffraction patterns of all catalysts can be mainly assigned to a mixture of hexagonal La2O2CO3 (JCPDS No. 25-0424) and hexagonal La2O3 (JCPDS No. 02-0688). Noticeably, Pt/LOC performs a higher phase composition of La2O3 as indicated by the higher intensity ratio of I (2θ = 29.2º) to I (2θ = 26.1º). It is known that La2O2CO3 holds in an arrangement of (La2O2)n+ layers separated by CO3 2− ions, and incorporation of a noble metal onto the lanthanum support can induce the phase transformation depending on the gas atmosphere during the thermal treatment [31,32,33]. Presumably, Pt adatoms onto the catalyst surface promote the phase transformation of La2O2CO3 into La2O3, suggesting a strong metal-support interaction of Pt/LOC. The Fe addition inhibits this process to some extent, as indicated by the decreased diffraction signals of La2O3 in the Fe-promoted catalysts. No reflections associated with metals and their oxides or PtFe alloys can be detected for the supported catalysts, implying a high dispersion of metals on the catalyst surface [34, 35].

Fig. 1
figure 1

Standard diffraction patterns of hexagonal La2O2CO3 and hexagonal La2O3, and XRD patterns of the samples

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As shown in Fig. 2 and Fig. S1, the rod-like morphology of the support with diameters of 15–20 nm and lengths of 200–400 nm is well maintained for all catalysts. In the case of the Pt/LOC (in Fig. S1a and Fig. S2), Pt particles centered at 1.5 nm with a narrow size distribution in the range of 0.8–2.6 nm are uniformly dispersed on the support surface. It can be clearly observed from Fig. S1b that ellipsoidal or spheroidal particles with diameters of about 10–20 nm are distributed on the Fe/LOC catalyst surface, suggesting a weak interaction between iron oxides and the support. The HRTEM investigation identifies the lattice fringe spacing of 3.40 Å, which can be assignable to the facets of orthorhombic Fe3O4. TEM images in Figs. 2a–2d and the statistical results in Fig. S2 demonstrate that incorporating Fe into Pt/LOC does not practically affect the size distribution of Pt particles over the xFe/Pt/LOC catalysts. Noticeably, morphological analysis confirms the successful loading of Fe on Pt/LOC and the generation of new particles related to FeO x or Pt–FeO x entities, whose size is determined as about 5–20 nm for 0.25Fe/Pt/LOC. As Fe loading increases, more particles with decreased size (2–10 nm for 0.75Fe/Pt/LOC) are observed. Combined with the XRD results, it can be speculated that the Fe addition may induce the migration of Pt atoms from catalyst surface to interact with iron and fracture large FeO x particles into small ones upon H2 reduction [36, 37].

Fig. 2
figure 2

TEM images of 0.25Fe/Pt/LOC (a), 0.40Fe/Pt/LOC (b), 0.50Fe/Pt/LOC (c) and 0.75Fe/Pt/LOC (d)

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To investigate the interaction of the surface composites over the catalyst, H2-TPR measurement was conducted. As displayed in Fig. 3, Pt/LOC presents two reduction peaks: the first one, in the temperature range of 400–600 °C, is associated with the reductive decomposition of platinum oxychlorine and the reduction of the support surface in the vicinity of Pt centers; the second one above 700 °C can be related to the reductive decomposition of the lanthanum support [38]. It can be seen that higher temperature for Pt-containing catalysts than that of the bare LOC and Fe/LOC is needed for the reductive decomposition of the lanthanum support, suggesting a higher phase composition of La2O3 for the Pt-doped catalysts, which is in consistence with the XRD results. In addition, incorporating iron into the Pt/LOC decreases the peak area in the temperature region of 400–600 °C, suggesting that the bonding strength of the Pt-lanthanum interfaces is much weakened upon Fe addition. A new peak centered at 311 °C is observed at a small amount of Fe (0.25%), while two peaks centered at 305 and 280 °C appear when Fe loading increases to 0.40%. Further increasing the Fe loading, the low-temperature reduction peak starts to grow in intensity and peak area at the expense of the middle-temperature one. The emergence of new peaks at lower temperatures can be interpreted as the reductive decomposition of platinum and iron precursors, which is much lower than that of Pt/LOC (500 °C) and Fe/LOC (408 °C). It is clearly shown that the Fe addition greatly improves the reducibility of the catalysts. As TEM analyses display no difference in the size distribution of Pt particles with the Fe addition, we attribute the enhanced reducibility of the Fe-doped catalysts to the promotional effect of iron on the hydrogen spillover phenomenon. It is known that Pt can act as active adsorption sites for molecular hydrogen [5, 24]. Once dissociated, highly reactive hydrogen species (i.e. H+, H−, H3+) spills over to reduce the support surface, and it can also be used to reduce Fe ionic species [39]. It has been documented that the reduction of a support could play an important role in the metal-support interaction, producing either reduced cations or oxygen vacancy Ovac sites [5, 38, 40]. The spillover of hydrogen species from platinum to the transition metal M usually induces the formation of low-coordinated MO x species or the alloying structure [21, 39]. These two effects compete and compromise in bimetallic catalysts and influence the electronic and geometric characteristics of Pt. In conjunction with the XRD and TEM results, it is reasonable to accept that the incorporation of Fe induces the migration of Pt atoms from the Pt-lanthanum interfaces to interact with the Fe species. Then, a close contact between Pt sites and the Fe species would improve the increasing of activated hydrogen number due to the enhanced hydrogen spillover phenomenon.

Fig. 3
figure 3

H2-TPR profiles of the samples obtained from room temperature to 900 °C with a ramp of 20 °C min−1 in flowing 7% H2/Ar (40 mL min−1)

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Fig. 4 displays the profiles of the desorbed H2 versus temperature for the catalysts in the temperature range of 100–900 °C. The wide desorption peaks in H2-TPD curves appear as a result of different activation energies for desorption, indicating the surface heterogeneity of the adsorption sites. It can be seen that there is an analogous peak shape of hydrogen desorption on the La2O2CO3 and La2O3, which initiates from 250 °C and the peak maximum concentrated at 400–600 °C. The desorption temperature of hydrogen on the supported metal catalysts can be divided into three regions: the low-temperature range of 200–500 °C initiates from the hydrogen desorption from metal sites, the mid-temperature one in the region of 500–700 °C stems from the hydrogen desorption from the support surface, and the high-temperature peak above 700 °C originates from the thermal decomposition of La2O2CO3 [41]. In the case of Pt/LOC, the low-temperature peak can be deconvoluted into three peaks centered at 318, 396 and 454 °C, which can be assigned to hydrogen desorbed from the different kinds of active metal sites. Quantification of total H2 desorption below 500 °C for Pt/LOC is 3.15 mmol g −1Pt (listed in Table 2). When the Fe loading increases from 0.25 to 0.75%, the amount of hydrogen desorption from metal sites decreases continuously from 2.95 to 2.42 mmol g −1Pt . The variation in the composition and structure of active sites usually contributes to the desorption behavior of hydrogen. It is reported that the MO x deposits preferentially onto the low-coordinated Pt sites at small amount of iron, which in consequence reduces the exposure degree of Pt sites [42, 43]. In this sense, it is suggested that the Fe-promoted catalyst has a lower Pt accessibility than Pt/LOC because of the site blocking effect of Pt by the Fe species.

Fig. 4
figure 4

H2-TPD profiles of the samples. Each curve-fitting is processed after subtraction of the baseline from the TPD data and then deconvolution into six peaks to isolate the contributions of H2 desorption sites from different surface structures

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Table 2 The H2-TPD results of the samples
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The surface chemical composition and valences of as-prepared catalysts are analyzed by XPS measurement. The fresh Pt/LOC for the Pt 4f 7/2 and Pt 4f 5/2 lines exhibit a distinct asymmetric shape inclined toward high binding energies and the peak area ratios deviate from the theoretical value of 1.33, suggesting the different chemical states of Pt. As shown in Fig. S3 and Table 3, the Pt 4f 7/2 spectra for the Pt/LOC catalyst can be deconvoluted into two features as metallic Pt0 (71.8 eV) and oxidized Pt2+ species (73.6 eV), and the Pt2+/Pt0 ratio is 0.44. The higher binding energy value of Pt0 relative to that of pure Pt foil has been observed by decreasing the particle size of Pt clusters due to the quantum size effect [44]. The oxidized Pt2+ is reported to be originated from the strong coordination of Pt with the oxygen atoms of the support or the residual chlorine species on the catalyst surface [34, 45,46,47]. The broad and weak peak at 710.6 eV of Fe 2p spectra for the Fe/LOC and xFe/Pt/LOC catalysts demonstrates the co-existence of Fe(ΙΙ) and Fe(III) ionic species. Additionally, the Fe addition lowers the binding energy of platinum in the 0.25Fe/Pt/LOC at a shift of 0.3 eV, and at the same time the Pt2+/Pt0 ratio is decreased to 0.31. As suggested by the XRD and H2-TPR results, some Pt–O bonds at the Pt–lanthanum interfaces are broken upon Fe addition, which facilitates the reduction of oxidized Pt2+ to metallic Pt0. The negative shift of Pt 4f spectra for the iron-promoted catalysts can be attributed to the improved Pt–Fe interaction upon Fe addition, which improves the increasing of highly active hydrogen number and results into the presence of electronic-rich Pt.

Table 3 The surface atomic composition and chemical states of the catalysts from XPS measurements
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The quantified elemental distributions over catalyst surface are outlined in Table 3. It is noteworthy that the Pt/Fe ratios of bimetallic catalysts analyzed by XPS are much lower than those from ICP analyses, suggesting the surface enrichment of iron. Furthermore, the decreased O/La ratios with increasing Fe composition imply that more Ovac sites are produced owing to the enhanced hydrogen spillover phenomenon upon iron addition [25].

Evaluation of the catalytic properties

CRAL hydrogenation is chosen as a probe reaction to evaluate the catalytic behavior of the resultant catalysts. As shown in Fig. 5, CRAL is mainly hydrogenated to butyraldehyde (BUAL, hydrogenation product of C=C bond), crotyl alcohol (CROL, hydrogenation product of C=O bond), and saturated hydrogenation product butanol (BUOL, hydrogenation product of C=C and C=O bond). It should be mentioned that Fe/LOC shows almost no catalytic activity. It is clear that Fe is not the active center in catalysis. The evolving catalytic hydrogenation results associated with the Fe loading from 0.25 to 0.75% are summarized in Table 4. In a reaction period of 60 min, the Pt/LOC catalyst exhibits 19.7% selectivity to CROL with 61.2% conversion of CRAL. Under the same reaction conditions, the 0.25Fe/Pt/LOC catalyst shows the highest conversion of CRAL (99.8%), mainly from the high rates of CROL and BUOL formation. However, a further increase in Fe loadings drastically reduces the catalytic reactivity. The CRAL conversion of xFe/Pt/LOC catalysts descends to 92.2, 60.8, and 36.1% when the Fe loading increases to 0.40, 0.50, and 0.75%. Impressively, the selectivity to CROL increases strongly with the promotion of iron, and reaches a maximum value (54.5%) at Fe loading of 0.75%. It should be noteworthy that the selectivity to CROL over the 0.50Fe/Pt/LOC is two-fold higher than that of the Pt/LOC in spite of their close catalytic reactivity. The explanation for this behavior may be associated with the presence of new catalytic active sites when Pt and Fe are brought into atomic contact.

Fig. 5
figure 5

Reaction pathways for CRAL hydrogenation

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Table 4 The hydrogenation results of CRAL over the catalysts
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The time-dependent hydrogenation results over 0.50Fe/Pt/LOC in Fig. 6 show that the desired CROL (60.1%) is the main product while the byproducts BUAL and BUOL are limited to a relatively low level when the reaction time is 20 min. As the reaction time increases, the conversion of CRAL is boosted from 31.6 to 79.8%, while the selectivity to CROL decreases continuously from 60.2 to 40.1%, resulting in the further hydrogenation of CROL to BUOL.

Fig. 6
figure 6

Time-dependent results of CRAL hydrogenation over 0.50Fe/Pt/LOC. Reaction condition: 150 mg of catalyst, 1 mL of CRAL, 24 mL of ethanol as solvent, T = 70 °C, H2 pressure = 2.0 MPa

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The stability is one of the parameters to evaluate the catalytic performance of heterogeneous catalysts. The reusability of Pt/LOC and 0.50Fe/Pt/LOC is investigated, and the results are presented in Fig. 7. The Pt/LOC presents a good reproducibility, while an obvious drop in the catalytic activity and CROL selectivity of 0.50Fe/Pt/LOC is detected during the cycles. To investigate the deactivation related to the active center, the spent Pt/LOC-C and 0.50Fe/Pt/LOC-C after five cycles are collected for characterization after calcination at 300 °C for 1 h. TEM analyses of the cycled catalyst in Fig. S4 demonstrate a high dispersion of metal particles over the support surface. Pt particles on the Pt/LOC-C are confined at 1.3 nm with a narrow size distribution. By contrast, 0.50Fe/Pt/LOC-C shows the homogeneous metal particles with an average particle size of 2.1 nm, larger than the highly dispersed Pt clusters while smaller than the FeO x or Pt–FeO x ensembles on the fresh one. This further affirms the prevailing influence of the enhanced metal-support interaction on the stabilization of Pt particles over the Pt/LOC. As indicated by XPS in Fig. S5, the new peak at 292.2 eV in C 1s spectra of 0.50Fe/Pt/LOC-C clearly manifests the existence of residual organic molecules (by-products from hydrogenation reaction) on the catalyst surface, which can be readily removed from the lanthanum surfaces in the vicinity of Pt centers for Pt/LOC-C [26, 27, 48]. The carbon deposit and an increase of Pt particles is proposed to be the dominant factor for the activity drop of 0.50Fe/Pt/LOC-C [5, 24]. Platinum in the cycled catalysts composed of Pt0 and Pt2+ is detected, and the Pt2+/Pt0 ratio of 0.50Fe/Pt/LOC-C is 0.61, much higher than that of the fresh one. In conjunction with the decreased Pt/Fe ratio and the uniform metal particles over the 0.50Fe/Pt/LOC-C, it is suggested the structure evolution of bimetallic ensembles after five cycles of hydrogenation-oxidation treatment is accounted to be the primary reason responsible for the decreased selectivity of aldehyde hydrogenation.

Fig. 7
figure 7

Recyclability of Pt/LOC and 0.50Fe/Pt/LOC for the liquid-phase CRAL hydrogenation during five cycles. Reaction conditions: 150 mg of catalyst, 1 mL of CRAL, 24 mL of ethanol as solvent, T = 70 °C, H2 pressure = 2.0 MPa, t = 60 min

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Discussion

According to the reactive performance of the resultant catalysts, the Fe addition exerts great influence on the catalytic activity and selectivity of PtFe catalysts. As TEM analyses show that incorporation of Fe do not cause obvious change in the size distribution of Pt particles over the PtFe/LOC catalysts, the size effect of Pt particles is excluded from the promoting effect of Fe. Therefore, the distinct catalytic behavior of Fe-promoted catalysts most likely results from the dilution of Pt sites by FeO x entities, which determines the adsorption mode of CRAL molecule and the hydrogenation routes.

According to the XRD, H2-TPR and TEM results, the strong metal-support interaction is attained on the Pt/LOC. The strong metal-support interaction makes the rearrangements at the metal-support interfaces kinetically and thermodynamically limited, thus producing fine Pt particles and preventing the migration and aggregation of Pt particles. Considering the fact that the Fe/LOC and bare LOC show almost no reactivity for hydrogenation, it is reasonable to accept that platinum in the catalyst is the main active sites for adsorbing and cleaving H2 molecule into active hydrogen species. The Ovac site owing to the spillover effect of Pt to lanthanum support is proposed to be the active center for the polarization of O atom in the aldehyde group [17]. The adsorbed C=O bond on the Ovac site is hydrogenated by the dissociative hydrogen on the neighboring Pt. As suggested by XPS measurements, Pt with oxychlorine precursors are coordinated by the O atom of the lanthanum support and remain unreduced as Pt2+ even after 600 °C reduction [45].

Upon Fe addition, some Pt–O bonds are broken depending on the H2-treatment conditions and chemical surroundings of Pt cluster [36]. Concerning the H2-TPR measurement, it can be seen that the promotional effect of iron on the increasing number of active hydrogen species due to the enhanced hydrogen spillover phenomenon [39]. The decrease in the Pt2+/Pt0 ratio provides further evidence of the promoted reducibility in the Fe-doped catalysts. As suggested by the XPS results, an increased electron density on metallic Pt is detected, which is reported to enhance the repulsive four-electron interactions with the C=C bond and favor a d-electron donation to the π* orbital of the polar C=O bond [49]. In addition, the positively charged Feδ+ species as Lewis acid adsorption sites, would prefer the adsorption and activation of the C=O bond towards the improved production of unsaturated alcohol [42, 50]. Evidently, the hydrogenation rate of C=O bond on the Ptδ−–Feδ+ couple outperforms the Pt–Ovac couple on the Pt/LOC, greatly improving the alcohol selectivity at low Fe loading of 0.25%. With the increment in Fe loading, more FeO x or Pt–FeO x particles with decreased size are constituted. As expected, the carbonyl hydrogenation is substantially improved accompanied by the evident decrease of BUAL and BUOL formation. Especially, the increase of carbonyl selectivity is predominant at Fe loadings of 0.25–0.50%. As a function of activity, at low Fe loading (0.25%), the prevailing effect of Fe is that of activating the substrate through a polarization of the carbonyl groups, which can react more easily with hydrogen chemisorbed on Pt. However, a further increase in Fe loading reduces the accessibility of Pt centers for hydrogen chemisorption because of the site blocking effect of Pt by metal oxide [51]. Accordingly, the total reactivity of CRAL hydrogenation is greatly decreased. Analogous composition-dependency of hydrogenation results has been reported on the selective hydrogenation of unsaturated aldehyde over active carbon-supported Pt–Sn catalysts and Rh–Sn/SiO2 [43, 52]. Therefore, it seems that there is an optimum of surface Fe concentration allowing for the best catalytic reactivity and selectivity on the Fe-promoted Pt catalysts.

Conclusions

In the chemoselective hydrogenation of CRAL, incorporating Fe into the Pt/LOC catalyst is an effective way to improve the carbonyl reactivity. Upon hydrogen reduction, by varying Fe loading, the geometric and electronic characteristics of metals are modified owing to the composition-dependent interaction between Pt and FeO x , which dominates the hydrogenation reactivity and selectivity. The suitable Pt–FeO x interaction is likely to change the adsorption mode of CRAL molecules and contributes to the selective hydrogenation routes. This structure–activity study of PtFe/LOC catalysts provides new evidence and insight into the design and fabrication of bimetallic surfaces for specific reaction.