Enhanced Catalytic Hydrogenation Activity and Selectivity of Pt-MxOy

Mar 20, 2013 - Ningbo Institute of Material Technology and Engineering, Chinese ... Faculty of Science, Ningbo University, Ningbo 315211, P. R. China...
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Enhanced Catalytic Hydrogenation Activity and Selectivity of Pt‑MxOy/Al2O3 (M = Ni, Fe, Co) Heteroaggregate Catalysts by in Situ Transformation of PtM Alloy Nanoparticles Xiangdong Wang,‡ Hongbo Yu,† Dayin Hua,*,‡ and Shenghu Zhou*,† †

Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, 519 Zhuangshi Avenue, Ningbo, Zhejiang 315201, P. R. China ‡ Department of Physics, Faculty of Science, Ningbo University, Ningbo 315211, P. R. China S Supporting Information *

ABSTRACT: PtM (M = Ni, Fe, Co) alloy nanoparticles were synthesized by a liquid phase reduction method employing butyllithium as a reducing agent. The alumina-supported PtM materials were then used as precursors to obtain the Pt-MxOy/Al2O3 catalysts through calcination. The influence of synthesis conditions of PtM alloy nanoparticles and the catalytic performance of the Pt-MxOy/Al2O3 catalysts for p-chloronitrobenzene hydrogenation reaction were investigated. The relevant characterizations such as XRD, XPS, and TEM were conducted for PtM alloy nanoparticles and Pt-MxOy/Al2O3 catalysts, and the result showed that the PtM nanoparticles are uniform alloy. Moreover, compared to PtM alloy nanoparticles, the Pt particle size of Pt-MxOy/Al2O3 using PtM alloy nanoparticle precursors did not increase by calcination, indicating good thermal stability. The catalytic activities of Pt-MxOy/Al2O3 for p-chloronitrobenzene hydrogenation reaction were significantly higher than that of control Pt/Al2O3 catalysts due to its strong Pt-MxOy interaction.

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INTRODUCTION

interface with high activity and selectivity for certain reactions.22−33 Reducible oxides such as TiO2, CeO2, and Fe2O3 are usually employed as supports to prepare the strong metal−support interaction catalysts.20,21,34−43 Au/TiO2 for CO oxidation and high-temperature hydrogen reduced Pt/TiO2 catalysts for hydrogenation reaction are typical examples.20,21 Reducible oxides such as TiO2,23 CeO2,44 and Fe2O345 have the valence change of Ti4+/Ti3+, Ce4+/Ce3+, and Fe3+/Fe2+, respectively. Under suitable conditions such as high-temperature hydrogen reduction, the partial reduced TixOy, CexOy, and FexOy will anchor metal particles to supports and produce catalytic active interfaces for some reaction.23 Reducible oxide supported Au catalysts for CO oxidation were intensely studied, and hightemperature hydrogen reduction pretreatment is not required for supported Au catalysts. Most active catalysts for CO oxidation were Au/TiO2,46 Au/CeO2,44 and Au/Fe2O347 catalysts. However, nonreducible oxides such as Al2O3 and SiO2 supported metal catalysts are rarely found to have a strong metal−support interaction.20,32,44 Pt/SiO223 catalysts for methanation reaction of CO/H2, and Au/Al2O332 and Au/ SiO248 catalysts for CO oxidation showed poor catalytic activity unless some special techniques32,44,48 were employed to provide strong metal−support interaction. Developing highperformance hydrogenation catalysts using Al2O3 or SiO2 as

Catalytic hydrogenation has been widely used to produce fine chemicals, and enhancing catalytic activity, selectivity, and stability is an important issue in this field.1−7 Achieving high activity and selectivity together in most cases is difficult when supported noble metals are used as hydrogenation catalysts. For example, supported noble metal catalysts are usually alloyed or poisoned by another element to enhance selectivity for hydrogenation of substituted nitrobenzenes.8 However, the enhancement of selectivity is usually obtained at expense of activity by alloying or poisoning methods.9−13 Supported Pd, Pt, or Ni bimetallic catalysts were sucessfilly used for hydrodechlorination of chlorinated hydrocarbons.14−19 However, in most cases of hydrogenation of chlorinated nitroarenes, dechlorination reaction was not desired and the selectivity of traditional Pd, Pt, or Ni bimetallic catalysts was not satisfied. Recently, TiO2-supported Pt20 and Au21 catalysts have exhibited high performance for catalytic hydrogenation of substituted nitrobenzenes. Corma and co-workers have reported that TiO2 or Fe2O3 supported Au nanoparticles (NPs) have good selectivity for catalytic hydrogenation of functionalized nitroarenes with H2.21 The same group also developed a high-temperature hydrogen reduced Pt/TiO2 catalyst with high performance for hydrogenation of substituted nitrobenzenes.20 High temperature hydrogen reduced Pt/TiO2 was found to have an interface between Pt and TixOy due to the partial reduction of TiO2.20,22,23 The strong interaction between Pt and TixOy not only anchors Pt into support to prevent the Pt NPs from sintering but also provides a new © 2013 American Chemical Society

Received: September 26, 2012 Revised: March 11, 2013 Published: March 20, 2013 7294

dx.doi.org/10.1021/jp309548v | J. Phys. Chem. C 2013, 117, 7294−7302

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Scheme 1. Schematic Diagram for the Syntheses of PtM/Al2O3 and Pt-MxOy/Al2O3 (M = Ni, Fe, Co)a

a

The small black particles represents MxOy particles.

Chemicals. Deionized water used in the synthesis was from local sources. All the reagents were used as received without further purification. Synthesis. The synthesis procedure of the Pt-based PtM (M = Ni, Fe, Co) alloy NPs was similar to those of AuNi and AuPt NPs by Zhou et al.50,53 In a typical synthesis, 0.13 mmol of H2PtCl6·H2O, 0.13 mmol of Ni(acac)2, and 2 mL of oleylamine were codissolved in 8 mL of 1-octadecene at 40−50 °C, and then the warm solution was injected via a syringe into a 75 °C solution containing 15 mL of 1-octadecene and 1.2 mL of 2.2 M n-butyllithium−cyclohexane solution in a reaction flask with magnetic stirring under a N2 atmosphere. The reaction mixture was stirred for 20 min at 75 °C and then heated to 240 °C and maintained at 240 °C for 2 h. The sample was cooled down to room temperatures, and then 1.5 mL of trioctylphosphine was injected into the reaction mixture to protect the NPs. The product was isolated by centrifuging with ethanol. The resultant black powder product was redispersed in toluene or hexane with a minimum amount of oleylamine for further application. To synthesize PtNi2 and Pt2Ni, 0.065 mmol of H2PtCl6·H2O/0.13 mmol of Ni(acac)2 and 0.13 mmol of H2PtCl6·H2O/0.065 mmol of Ni(acac)2 were used, respectively, and the other synthetic conditions are the same as those for PtNi NPs. To synthesize PtFe, 0.065 mmol of H2PtCl6·H2O and 0.065 mmol of Fe(acac)3 were codissolved in 4 mL of oleylamine at 60−70 °C, and then the warm solution was injected via a syringe into a 75 °C solution containing 20 mL of 1-octadecene and 2.0 mL of 2.2 M n-butyllithium−cyclohexane solution in a reaction flask with magnetic stirring under a N2 atmosphere. The reaction mixture was stirred at 75 °C for 20 min and then heated to 260 °C and maintained at 260 °C for 2 h. The product collection procedure was the same as that of PtNi NPs. To synthesize PtCo, 0.065 mmol of H2PtCl6·H2O and 0.065 mmol of Co(CH3COO)2·4H2O were codissolved in 4 mL of oleylamine at 45−55 °C, and the other procedures are the same as those of PtFe synthesis. The pure Pt particles were synthesized by the same butyllithium reduction method. 0.13 mmol of H2PtCl6·H2O and 2 mL of oleylamine were codissolved in 8 mL of 1-octadecene at 40−50 °C, and then the warm solution was injected via a syringe into a 75 °C solution containing 15 mL of 1-octadecene and 1.0 mL of 2.2 M n-butyllithium−cyclohexane solution in a reaction flask with magnetic stirring under a N2 atmosphere. The reaction mixture

supports has fundamental and industrial importance since these oxides are readily available and cheap. Bimetallic alloy materials have been used to obtain oxides supported metal catalysts. The Rousset group used bulk alloy Au0.5Zr0.5 at room temperature to obtain active Au/ZrO2 catalysts for preferential CO oxidation in H2.49 Because of oxidation of Zr metal at room temperatures, the bulk alloy AuZr turned to be active Au/ZrO2 with close contact of Au and ZrO2. Zhou and co-workers employed AuNi NPs as precursors to obtain highly active Au-NiO/SiO2 catalysts for CO oxidation.50,51 AuNi NPs were in situ transformed into close contact Au-NiO heteraggregates on SiO2 support through proper pretreatment. The origin of high activity for CO oxidation was ascribed to the strong interaction between Au and NiO. Using AuNi alloy NPs as precursors is the key to obtaining the strong interaction of Au-NiO since the in situ transformation of AuNi alloy NPs resulted in the formation of close contact Au-NiO heteroaggregates. Here, we extended the above strategy to obtain high performance Pt-MxOy/Al2O3 (M = Ni, Fe, and Co) catalysts for hydrogenation of p-chloronitrobenzene (p-CNB) with H2. PtM alloy and control Pt NPs were prepared by a liquid phase reduction method employing butyllithium as reducing agents. Butyllithium is a fast reducing agent so that the precursors were coreduced to form alloy.50,52 Pt-MxOy/Al2O3 catalysts were obtained by in situ transformation of PtM/Al2O3 through calcination. The PtM alloy formation was confirmed by XRD and TEM study. The experiments of catalytic hydrogenation of p-CNB with H2 showed that Pt-MxOy/Al2O3 catalysts have significantly better performance than that of control Pt/Al2O3 catalysts, suggesting that the origin of the excellent performance of Pt-MxOy/Al2O3 is due to the strong interaction between Pt and MxOy. The synthesis process is demonstrated in Scheme 1.

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EXPERIMENTAL SECTION Chemicals. Chloroplatinic acid hexahydrate (AR), nickel(II) acetylacetonate (95%), iron(III) acetylacetonate (98%), oleylamine (80−90%), 1-octadecene (90%), and methanol (for HPLC) were purchased from Aladdin. Cobalt(II) acetate tetrahydrate (AR), toluene (AR), acetone (AR), absolute ethyl alcohol (AR), and p-chloronitrobenzene (AR) were purchased from Shanghai Chemical Reagent Company. n-Butyllithium (2.2 M in cyclohexane) were purchased from Amethyst 7295

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Figure 1. (a) TEM images of Pt2Ni NPs; (b) HRTEM image of Pt2Ni NPs showing a lattice spacing of (200) plane; (c) TEM image of PtNi NPs; (d) HRTEM image of PtNi NPs showing a lattice spacing of (200) plane; (e) TEM image of PtNi2 NPs; (f) HRTEM image of PtNi2 NPs showing a lattice spacing of (200) plane. Scale bars of (a, c, e) = 20 nm; scale bars of (b, d, f) = 5 nm.

was stirred at 75 °C for 20 min and then heated to 220 °C and maintained at 220 °C for 1 h. The product collection procedure was the same as that of PtNi NPs. The PtM or control Pt NP powders were redispersed in hexane, and a calculated amount of Al2O3 powder (surface area: 185 m2 g−1) was then added, followed by stirring for 1 h under N2 flow to remove hexane. The black products were then centrifuged with ethanol/acetone and followed by drying at 80 °C in oven. To be consistent, the Pt loading of all the supported catalysts for hydrogenation reaction was fixed at 0.2 wt %. The Pt-MxOy/Al2O3 catalyst was obtained by in situ transformation of PtM/Al2O3 through calcination at 500 °C for 2 h. Catalysts with 0.2−2 wt % Pt were prepared for characterization. The syntheses of PtM NPs and supported Pt-MxOy/Al2O3 were carried out more than three times, and the reproducibility was good. Characterization. The structure and morphology of NPs and corresponding Al2O3 supported catalysts were characterized by X-ray diffraction (XRD) on Bruker D8 Advance with Cu Kα radiation source at λ = 1.540 56 nm. Diffraction patterns were measured in the 2θ range from 10° to 90°. XRD samples were prepared by pressuring powders onto a glass plate. Prior to the measurement, the samples were thoroughly washed by ethanol or acetone to remove excess organic chemicals. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images were recorded on a TEI Tecnai F20 microscope operated at an accelerating voltage of 200 kV. For TEM measurements, NPs and catalyst samples were dispersed in toluene solution and were dropped onto a carbon-coated copper grid followed by solvent evaporation in air at room temperatures. X-ray photoelectron spectroscopy (XPS) studies of the supported catalysts were carried out on AXIS ULTRADLD multifunctional X-ray photoelectron spectroscope with an Al source. The data processing was performed using CasaXPS software. The spectra were fit after linear background subtraction. Experimental uncertainties in binding energies were ±0.35 eV. The infrared (IR) spectra of the samples were obtained in the transmission mode in a Bruker Tensor 27 spectrophotometer. The thermal degradation property of the as-synthesized PtM NPs was

measured by a Pyris Diamond thermogravimetric analyzer (TGA). The experimental run was carried out from 50 to 500 °C at a heating rate of 10 °C min−1 in an air atmosphere with a gas flow rate of 50 mL min−1. Catalytic Performance Test. The obtained Pt/Al2O3 and Pt-MxOy/Al2O3 catalysts calcined at 500 °C for 2 h were used for the hydrogenation of p-chloronitrobenzene (p-CNB) with H2 to produce p-chloroaniline (p-CAN) using a high-pressure agitated autoclave with 500 mL capacity. To begin with, the autoclave was filled with 0.2−0.5 g of supported catalysts and 200 mL of absolute ethanol containing 10 g of p-CNB and was flushed with hydrogen more than three times at room temperatures. The reactions were carried out at 30−100 °C and 1−4 MPa hydrogen partial pressures for 2 h. The solution was cooled down to room temperatures and centrifuged to remove the solid catalysts. The liquid product was collected and diluted to a certain concentration for Agilent 1200 HPLC analysis. An SB-C18 column (150 mm × 4.6 mm, particle size of 5 μm) was used with a mixture of methanol/water = 80/20 (v/v) as the mobile phase at a flow rate of 1.0 mL/min and with a UV−vis detector at 278 nm.

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RESULTS AND DISCUSSION

Bimetallic alloy NPs were usually prepared by coreduction of corresponding precursors, and selection of the reducing agent is important for successful synthesis of alloy NPs, especially for those bimetallic alloys containing two metals with huge difference of reduction potentials. Pt is easily reduced by a number of common reducing agents such as glycol and sodium borohydride while Ni, Co, and Fe are difficult to be reduced by the above-mentioned reducing agents. Therefore, strong reducing agents are necessary to reduce Pt, Ni, Co, or Fe precursors for the PtM alloy NP formation. In this work, butyllithium was used to coreduce the precursors of PtM so that atoms of Pt and M randomly mixed in NPs, and perfect PtM alloy formation was observed through aging at an elevated temperature. In PtNi synthesis, a warm solution of a desired amount of H2PtCl6·6H2O and Ni(acac)2 was injected into a solution containing butyllithium maintained at 75 °C, and the mixture of 7296

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solution instantly changed to a dark color, indicating the reduction of precursors. The resultant colloid was maintained at 75 °C for 30 min and further heated and maintained at an elevated temperature to obtained uniform PtNi alloy formation. Figure 1 showed TEM and HRTEM images of the PtNi alloy NPs synthesized with different atomic ratios of Pt to Ni. Figure 1c,d illustrates TEM and HRTEM images of PtNi alloy NPs. The PtNi NPs basically have a spherical shape with an average particle size of 5.8 nm (Figure 1c). Energy dispersive X-ray spectroscopy (EDS) study revealed an atomic ratio of 57/43 of Pt/Ni, which is close to the starting 50/50 ratio of the starting materials. XRD pattern of PtNi NPs (Figure 2b) suggested a

Figure 3. (a) TEM image of PtCo NPs; (b) HRTEM image of PtCo NPs showing a lattice spacing of (200) plane; (c) TEM image of PtFe NPs; (d) HRTEM image of PtFe NPs showing a lattice spacing of (200) plane. Scale bars of (a, c) = 50 nm; scale bars of (b, d) = 5 nm.

PtCo. Moreover, XRD pattern of PtCo (Figure 2e) is consistent with XRD database, indicating perfect PtCo alloy formation. The synthesized PtFe NPs had an average particle size of 5.2 nm, and the HRTEM image in Figure 3d reveals a lattice spacing of 0.192 nm, suggesting a lattice spacing of (200) plane of the cubic phase PtFe. Again, EDS showed an atomic ratio of 55/45 of Pt/Fe, close to the starting 50/50 ratio of the starting materials. Furthermore, the XRD pattern (Figure 2d) is consistent with database, confirming the successful PtFe alloy formation. The thermal analysis study in an air atmosphere was carried out for as-synthesized PtM NPs. As shown in Figure 4, organic residues such as capping agents could be completely removed under calcination at 500 °C since TGA study showed a flat curve around 500 °C. The thermal behaviors of PtNi and PtFe NPs were quite similar while PtCo NPs showed a different behavior. Removal of capping agents of PtCo NPs requires

Figure 2. XRD patterns of platinum-based NPs: (a) PtNi2, (b) PtNi, (c) Pt2Ni, (d) PtFe, and (e) PtCo.

uniform 1:1 PtNi solid solution alloy formation since the diffractions of PtNi NPs are just between those of pure fcc Pt and pure fcc Ni. A lattice spacing of 0.187 nm (Figure 1d) is clearly visible and consistent with that of (200) plane of 1:1 PtNi alloy. Figure 2c showed XRD patterns of Pt2Ni NPs. Compared to XRD pattern of PtNi NPs, the diffractions of Pt2Ni NPs shifted to low 2θ angles and were consistent with a higher ratio of Pt to Ni in Pt2Ni NPs. Figures 1a and 1b demonstrate TEM and HRTEM images of Pt2Ni NPs, respectively. Pt2Ni NPs have an average particle size of 6.5 nm, and a lattice spacing of 0.191 nm was visible in Figure 1b, which is consistent with that of (200) plane of the solid solution Pt2Ni alloy. Figures 1e and 1f show TEM and HRTEM images of PtNi2 NPs, respectively. PtNi2 NPs basically have a spherical shape with an average particle size of 7.0 nm. When the atomic ratio of Pt to Ni approached 1/2, the X-ray diffractions of the obtained NPs (Figure 2a) shifted to high 2θ angles and were consistent with the lower atomic ratio of Pt to Ni in Pt2Ni NPs. HRTEM images of the PtNi2 NPs (Figure 1f) revealed visible lattice spacing with 0.184 nm, which was consistent with that of the (200) plane of PtNi2 solid solution alloy phase. The developed solution method was also used to synthesize PtCo and PtFe alloy NPs. However, Co and Fe are more difficult to be reduced than Ni. Therefore, the excessive reducing agent n-butyllithium was employed and higher aging temperatures were used to obtain perfect alloy formation. Figures 3a and 3b show TEM and HRTEM images of PtCo NPs, respectively. Basically spherical shape with an average particle size of 5.8 nm was observed for PtCo NPs from Figure 3a. An atomic ratio of 54/46 of Pt/Co was obtained by EDS study. Figure 3b reveals a lattice spacing of 0.187 nm, which was consistent with that of the (200) plane of the fcc phase

Figure 4. TGA curves of as-synthesized PtM NPs in an air atmosphere. 7297

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higher temperatures than those of PtNi and PtFe NPs, possibly due to the stronger coordination. The as-synthesized PtM alloy NPs were then used as precursors to prepared Pt-MxOy/Al2O3 catalysts. The PtM NPs were first isolated by centrifugation, redispersed in hexane, and then impregnated with Al2O3 powders. The obtained materials were then slightly heated to remove hexane under N2 flow and calcined at 500 °C for 2 h to obtain Pt-MxOy/Al2O3 catalysts. Different Pt loading catalysts were prepared. One is 0.2 wt % Pt loading catalyst for catalytic study of hydrogenation, and a catalyst with a higher Pt loading is just for characterization if the catalyst with 0.2 wt % Pt loading has weak signal for some characterization techniques. The FT-IR studies of supported PtM/Al2O3 and Pt-MxOy/ Al2O3 are shown in Figure 5. The PtM/Al2O3 with 0.2 wt % Pt

Figure 6. TEM images of supported Pt-based catalysts with a Pt loading of 0.2 wt % by calcination at 500 °C showing (a) Pt/Al2O3 catalysts and insert is individual Pt particles (3.7 nm); (b) Pt-NiO/ Al2O3 catalysts; (c) Pt-Fe2O3/Al2O3 catalysts; (d) Pt-CoxOy/Al2O3 catalysts. Scale bars of (a−d) and inset of (a) = 50 nm.

Figure 5. FTIR spectra of (a) PtNi/Al2O3 catalysts and (b) Pt-NiO/ Al2O3 catalysts after 500 °C calcination.

loading showed only information from Al2O3, and no signal from organic residues was found due to very low Pt loading. Increasing Pt loading to 0.5 wt % resulted in the absorption at wavenumbers of 2924 and 2853 cm−1, which can be ascribed to the characteristic frequencies of antisymmetric and symmetric stretching vibration of methylene, respectively.54 However, after 500 °C calcination, the absorption peak of methylene disappeared, indicating the complete removal of organic residues. TEM images of supported Pt-based catalysts with a Pt loading of 0.2 wt % after 500 °C calcination are shown in Figure 6. The TEM image of control Pt/Al2O3 catalyst is shown in Figure 6a. The Pt NPs were prepared with the same method and had an average particle size of 3.7 nm (Figure 6a inset). While the presynthesized Pt NPs were very small, the supported Pt/Al2O3 catalyst after calcination at 500 °C for 2 h had an average Pt particle size of 6.0 nm (Figure 7a), indicating Pt particle growth due to thermal treatment. However, for those Pt-MxOy/Al2O3 catalysts, the Pt particle size decreased. The individual particle sizes of PtNi, PtFe, and PtCo NPs were 5.8, 5.2, and 5.8 nm, respectively. The Pt particle sizes of Pt-NiO/Al2O3, Pt-Fe2O3/Al2O3, and Pt-CoxOy/ Al2O3 (Figures 7b−d) were 4.3, 4.2, and 4.5 nm, respectively, suggesting the higher thermal stability of Pt-MxOy/Al2O3 catalysts. The decrease of particle size in Pt-MxOy/Al2O3 is due to the phase separation of MxOy from the original PtM NPs by calcination and MxOy particles cannot be differentiated from Al2O3 by TEM.

Figure 7. Pt particle size distribution of the alumina supported Ptbased catalysts with a Pt loading of 0.2 wt % by calcination at 500 °C showing (a) Pt/Al2O3 catalysts, (b) Pt-NiO/Al2O3 catalysts, (c) PtFe2O3/Al2O3 catalysts, and (d) Pt-CoxOy/Al2O3 catalysts.

TEM images and Pt particle size distribution of supported Pt-based catalysts with a Pt loading of 2 wt % are shown in Figures S1 and S2 (Supporting Information). Comparison of particle size distribution of Pt-based catalyst with 0.2 and 2 wt % Pt loadings suggested that increasing Pt loading to 2 wt % did not cause particle agglomeration on Al2O3 supports. XRD patterns of supported Pt-based catalysts with a Pt loading of 0.2 wt % are shown in Figure S3, and there is no obvious difference between supported Pt-based catalysts and individual Al2O3 due to very low Pt loading. XRD patterns of Pt-MxOy/Al2O3 catalysts with a Pt loading of 2 wt % are shown in Figure 8. Although supported Pt-based catalysts with a Pt loading of 2 wt % are not real catalysts used in hydrogenation 7298

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Figure 9. XPS spectra of Pt-NiO/Al2O3 with different Pt loadings showing (a) Pt XPS spectrum and (b) Ni XPS spectrum.

Figure 8. XRD patterns of Al2O3 and supported Pt-based catalysts with a Pt loading of 2 wt % by calcination at 500 °C showing (a) Al2O3, (b) Pt-NiO/Al2O3 catalysts, (c) Pt-Fe2O3/Al2O3 catalysts, (d) Pt-CoxOy/Al2O3 catalysts, and (e) Pt/Al2O3 catalysts.

Table 1. Atomic Compositions of Pt and M with Different Oxidation States from XPS Analysis for Supported Pt-Based Catalysts with Pt Loading of 2 wt %

reaction, XRD study of Pt-MxOy/Al2O3 catalysts with a Pt loading of 2 wt % can give some information since TEM study of Pt-MxOy/Al2O3 catalysts with 2 wt % Pt loading did not show particle agglomeration on Al2O3 supports. XRD pattern of Al2O3 supports used in this work are illustrated in Figure 8a. Although the Pt diffractions of Pt-MxOy/Al2O3 catalysts overlapped with Al2O3 diffractions, the XRD diffraction around the 39.7° 2θ angle were all intensified for all Pt-MxOy/Al2O3, indicating the presence of Pt. Moreover, for XRD patterns of Pt/Al2O3 (Figure 8e), the (111) Pt diffraction was more intense than that of Pt-MxOy/Al2O3 and high angle Pt diffraction around 81.3° 2θ angle was clearly visible, confirming a relative bigger Pt particle size of Pt/Al2O3 catalysts from TEM image (Figures 6a and 7a). It is well-known that Pt will not be oxidized, and Ni, Fe, or Co is easily oxidized under 500 °C calcination used in this work. The presence of Pt was confirmed by XRD patterns, but there are no visible XRD diffractions from MxOy. The absence of XRD diffractions from MxOy for those Pt-MxOy/Al2O3 catalysts is probably due to the formation of amorphous MxOy phases. EDS studies of a large area of TEM images confirmed the presence of Ni, Fe, and Co elements. The atomic ratios of Pt/Ni, Pt/Fe, and Pt/Co in Pt-MxOy catalysts by EDS were 51/49, 48/52, and 56/44, respectively, and were close to the 1/1 ratio of Pt/M in starting materials. XPS spectra confirmed that the Pt was metallic and M was in the state of MxOy. XPS study of Pt-NiO/Al2O3 after 500 °C calcination is shown in Figure 9. As well as XRD study, signals from Pt-NiO/Al2O3 with a Pt loading of 0.2 wt % were very weak. Therefore, catalysts with a Pt loading of 2 wt % were used to obtain information about valence of Pt and M elements. The binding energies of Pt 4d5/2 were 314.7 and 317.3 eV, which corresponded to metallic Pt and oxidized Pt2+, respectively. Moreover, the atomic ratio of metallic Pt and oxidized Pt was 46.43/8.57 (Table 1), indicating that Pt was mainly in zero valence state. It is well-known that surface atoms of metals except Au will be oxidized at room temperatures due to their low coordination number. In contrast, Ni was completely in the oxidation state. The binding energies of Ni 2p3/2 and 2p1/2 were 855.1 and 872.3 eV, respectively, indicating the presence of NiO. XPS spectra of Pt-Fe2O3/Al2O3, Pt-CoxOy/Al2O3, and control Pt/Al2O3 are shown in Figures S4, S5, and S6, and

materials Pt-NiO/ Al2O3 Pt-Fe2O3/ Al2O3 Pt-CoxOy/ Al2O3

metallic Pta (%)

oxidized Ptb (%)

metallic Mc (%)

oxidized Md (%)

46.43

8.57

0

45

51.23

4.77

0

44

44.77

7.23

0

48

a Pt 4d5/2 binding energy (eV): 314.7. bPt2+ 4d5/2 binding energy (eV): 317.3. cNo metallic M. dNi2+ 2p1/2 and 2p3/2 binding energies (eV): 872.3 and 855.1, respectively. Fe3+ 2p1/2 and 2p3/2 binding energies (eV): 723.9 and 710.3; Co was assigned as CoxOy.

the results indicated that Fe was complete in the state of Fe2O3. Co in the Pt-CoxOy/Al2O3 was difficult to clearly assign the valence state from XPS spectrum, but no binding energy of metallic Co was found, which is consistent with the literature.55 Table 1 summarizes atomic compositions of Pt and M with different oxidation states of supported Pt-based catalysts. The atomic ratios of Pt/Ni, Pt/Fe, and Pt/Co from XPS spectra were 55/45, 56/46, and 52/48, respectively, and were consistent with EDS results and the ratios of the starting materials. To study the catalytic activity of the nanocatalysts, the reaction of selective hydrogenation of p-chloronitrobenzene (pCNB) was used to test the catalytic performance of Pt-MxOy/ Al2O3 and control Pt/Al2O3 catalysts. The main byproduct of hydrogenation of p-CNB with H2 is aniline,56 which is produced by dechlorination reaction of p-chloroaniline (pCAN). The effects of reaction conditions on catalytic performance were studied for the hydrogenation using 0.2 wt % Pt-MxOy/Al2O3 and Pt/Al2O3 catalysts with anhydrous ethanol as medium. Products were analyzed by HPLC to calculate the conversion of p-CNB and selectivity of p-CAN. The effect of temperature on the conversion and selectivity for Pt-NiO/Al2O3 catalysts was studied in the temperature range 30−100 °C, and the results are presented in Figure 10. The conversion to p-CNB was found to increase from 24% at 30 °C to 100% at 80 °C, and conversion remained constant with further temperature increasing from 80 to 100 °C. In all conditions, aniline formed by dechlorination reaction was not observed, and some intermediates with 0.45−2.42% selectivity were found. 7299

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Figure 10. Effect of temperature on p-CNB conversion and p-CAN selectivity for hydrogenation of p-CNB with H2. Reaction conditions: p-CNB, 10 g; Pt-NiO/Al2O3 catalysts, 0.2 g; 200 mL of EtOH; 4.0 MPa of H2; reaction time 2 h; speed of agitation 500 rpm.

Figure 12. Effect of reaction time on p-CNB conversion and p-CAN selectivity for hydrogenation of p-CNB with H2. Reaction conditions: p-CNB, 10 g; Pt-NiO/Al2O3 catalysts, 0.2 g; 200 mL of EtOH; 3.0 MPa of H2; 80 °C; speed of agitation 500 rpm.

The effect of hydrogen partial pressure on catalytic performance for Pt-NiO/Al2O3 catalysts was studied in the range 1.0−4.0 MPa at 80 °C As shown in Figure 11, the

S12. All the Pt-MxOy/Al2O3 catalysts reached 100% p-CNB conversion and above 96% selectivity of p-CAN under reaction conditions of hydrogen partial pressure 3.0 MPa, reaction temperature 80 °C, and reaction time 2 h. Among Pt-MxOy/ Al2O3 catalysts, Pt-Fe2O3/Al2O3 catalysts have the highest catalytic activity at low temperatures and obtained 75.52% conversion of p-CNB (Figure S7). The catalytic performance of Pt-MxOy/Al2O3 and control Pt/ Al2O3 catalysts is summarized in Table 2. The control Pt/Al2O3 Table 2. Product Distribution for Hydrogenation of p-CNB with H2 Using Pt/Al2O3 and Pt-MxOy/Al2O3 Catalysts selectivity (%) catalytic systemsa 0.2 0.5 0.2 0.2 0.2

Figure 11. Effect of hydrogen partial pressure on p-CNB conversion and p-CAN selectivity for hydrogenation of p-CNB with H2. Reaction conditions: p-CNB, 10 g; Pt-NiO/Al2O3 catalysts, 0.2 g; 200 mL of EtOH; 80 °C; reaction time 2 h; speed of agitation 500 rpm.

g, g, g, g, g,

0.2 0.2 0.2 0.2 0.2

wt wt wt wt wt

% % % % %

Pt/Al2O3 Pt/Al2O3 Pt-NiO/Al2O3 Pt-Fe2O3/Al2O3 Pt-CoxOy/Al2O3

conv (%)

p-CAN

AN

others

0 8 100 100 100

0 97.62 96.40 97.38 96.95

0 0 0 0 0

0 2.38 3.60 2.62 3.05

Reaction conditions: p-CNB, 10 g; 200 mL of EtOH; 80 °C; 3.0 MPa of H2 partial pressure; reaction time 2 h; speed of agitation 500 rpm.

a

catalysts did not exhibit any catalytic activity when using 0.2 g of catalysts, and the conversion of p-CNB was just 8% even when 0.5 g of catalysts was used. The traditional Pt/MxOy/ Al2O3 catalysts with the same Pt loading prepared by the common coimpregnation method showed decreased selectivity and conversion, and the dechlorination reaction was observed (Table S1). However, Pt-NiO/Al2O3, Pt-Fe2O3/Al2O3, and PtCoxOy/Al2O3, catalysts all exhibited a 100% conversion using 0.2 g of catalysts, suggesting a unique advantage arising from Pt-MxOy heteroaggregate structure. Moreover, all the Pt-MxOy/ Al2O3 catalysts had a good selectivity for formation of p-CAN, and the aniline formed by the dechlorination reaction was not observed. Table S2 summarizes the catalytic stability test of PtNiO/Al2O3 catalysts. The decreasing catalytic conversion with cycle to cycle was not due to the leaching of catalyst component since the catalyst had been calcined under 500 °C and ICP analysis of product solution did not find any detectable concentrations of catalyst components. Although the

conversion of p-CNB increased from 71% at 1.0 MPa to 100% at 3.0 MPa. At 1.0 MPa, the intermediate selectivity reached 6% and decreased with H2 partial pressure increasing, suggesting that high H2 partial pressure will favor the formation of the final p-CAN and decrease the concentration of hydrogenation intermediates in final products. The effect of reaction time on catalytic performance for PtNiO/Al2O3 catalyst was studied in the range 0.5−3.0 h at 80 °C under hydrogen partial pressure of 3.0 MPa. As shown in Figure 12, the conversion of p-CNB increased from 80% at 0.5 h to 100% at 2.0 h and remained 100% with further increasing reaction time. In all reaction time studied, aniline formed by dechlorination reaction was not observed, and some intermediate with 2.00−2.60% selectivity was found. The effects of reaction temperatures, hydrogen partial pressures, and reaction time on catalytic performance of PtFe2O3/Al2O3 and Pt-CoxOy/Al2O3 are shown in Figures S7− 7300

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Pt-NiO/Al2O3 showed decreased performance with recycling times increasing, the p-CNB conversion (35.8%) of Pt-NiO/ Al2O3 with 0.2 g of catalysts after 4 times catalytic runs is still much better than the conversion (8%) of the control Pt/Al2O3 with 0.5 g of catalysts for the first time catalytic run. Compared with the control Pt/Al2O3 catalyst, the advantage of Pt-MxOy/Al2O3 catalyst is that the platinum particle size did not increase under high-temperature treatment. In contrast to the Pt particle size increasing from 3.7 to 6.0 nm for Pt/Al2O3 catalysts under 500 °C calcinations for 2 h, the Pt particle size of Pt-MxOy/Al2O3 decreased about 1 nm due to the phase separation of MxOy from PtM alloy NPs under the same calcination condition. The formation of amorphous MxOy in the proximity of Pt NPs on Al2O3 supports can protect Pt NPs from sintering but also produce a new interface of Pt-MxOy with high catalytic activity for hydrogenation reaction. This kind of interaction was also observed from Au-NiO/SiO2 catalysts for CO oxidation.50

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CONCLUSIONS To summarize, the Pt-based PtM alloy NPs were prepared by a facile solution-phase method using butyllithium as a reducing agent. The PtM alloy NPs were used as the precursors to synthesize Pt-MxOy/Al2O3 catalysts. Through in situ phase transformation of PtM to Pt-MxOy on supports, the Pt-MxOy/ Al2O3 catalysts illustrated much higher catalytic activities for the hydrogenation of p-CNB with H2 than the control Pt/Al2O3 catalyst. Importantly, this work provides a generic method to synthesize PtM alloy NPs and also extends the strategy using bimetallic alloy NPs as precursors to obtain highly active catalysts for hydrogenation of substituted nitrobenzenes with H2.

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ASSOCIATED CONTENT

S Supporting Information *

TEM images of the supported 2 wt % Pt-based catalysts, XRD patterns of 0.2 wt % Pt-based catalysts, XPS spectra of PtFe2O3/Al2O3, Pt-CoxOy/Al2O3, and Pt/Al2O3 with different Pt loadings, figures of effect of temperature, hydrogen partial pressure, and reaction time on p-CNB conversion and p-CAN selectivity for hydrogenation of p-CNB with H2, tables of catalytic performance of traditional Pt/MxOy/Al2O3 catalysts and stability test of Pt-MxOy/Al2O3. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Fax (+86) 574-86685043; e-mail [email protected]; [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for generous funding from China Zhejiang Provincial Natural Science Foundation under Grant Y4110116 and the Ministry of Science and Technology of the People’s Republic of China under Grant 2012DFA40550.

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