Do Olefin Hydrogenation Reactions Remain Structure Insensitive over

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Do Olefin Hydrogenation Reactions Remain Structure Insensitive over Pt in Nanostructured Pt-on-Au Catalyst? Jia-Wei Yang, Wentao Zheng, Zhun Hu, Min Zhang, and Bo-Qing Xu ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b02471 • Publication Date (Web): 02 Oct 2018 Downloaded from http://pubs.acs.org on October 4, 2018

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Do Olefin Hydrogenation Reactions Remain Structure Insensitive over Pt in Nanostructured Pt-on-Au Catalyst? Jia-Wei Yang1,#, Wen-Tao Zheng1,#, Zhun Hu2, Min Zhang1, and Bo-Qing Xu1,∗ 1

Innovative Catalysis Program, Key Lab of Organic Optoelectronics & Molecular Engineering, Department of Chemistry, Tsinghua University, Beijing 100084, China 2 School of Chemical Engineering and Technology, Xi’an Jiaotong University, Xi’an 710049, China # These two authors contribute equally in this work *Corresponding author. Tel: +86 10 62772592; Fax: +86 10 62771149. E-mail: [email protected]

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ABSTRACT: Understanding the effects of catalyst surface structure on a catalytic reaction is of fundamental importance in catalysis chemistry. Herein, a series of Pt-on-Au (PtmÂu, m refers to the atomic Pt/Au ratio; Au size: 3.2 ± 0.4 nm) nanostructures with Pt dispersion in the range of 38% to 100% are used to study the structure sensitivity of the hydrogenation reactions of 1,3butadiene and ethylene, respectively. The specific catalytic rates for both reactions are observed to increase with decreasing the Pt dispersion or increasing m in PtmÂu, demonstrating the structure sensitive nature of both reactions over the PtmÂu nanostructures. These observations strongly contrast to the structure insensitivity of the same reactions over our deliberately prepared counterpart Pt/SiO2 catalysts with Pt dispersion varying in 6%~100% and also those documented in literature, and thus identify a distinct feature of Pt in the PtmÂu nanostructures. Characterization results from XANES spectroscopy and DRIFTS of adsorbed CO uncover that the variation of Pt dispersion or m in PtmÂu resulted in systematical changes in the electronic property of Pt, which uncover the nature of the structure sensitivity. These findings would have important implications for better understanding the dimension and richness of the structure sensitivity concept and the feature of Pt-on-Au nanostructures in catalysis and nanomaterial chemistry, as well as its related fields. KEYWORDS: Surface catalysis, structure sensitivity, hydrogenation reaction, Pt-on-Au nanostructures, electronic effect


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Structure sensitivity and insensitivity of a catalytic reaction over the surface of solid catalysts has been important concepts in catalysis chemistry since it was proposed by Boudart in the middle of 1960s.1-4 The concepts would be considered as the earliest comprehensive ‘atomic/ molecular’ approaches to the nanosize effects in heterogeneous catalysis and define according to the sensitivity/insensitivity of a catalytic reaction by its kinetically relevant specific reaction rate or turnover frequency (TOF, defined as the number of molecules converted/produced per second over per surface active site) to the dispersion (exposed percentage) or overall surface structure of the (nanosized) catalytically active component, such as supported catalytic metal or oxide nanoparticles (NPs). It has been well accepted in heterogeneous catalysis to refer the dispersion of a transition metal (D) as the reciprocal of the metal particle diameter (d) in nanometer, i.e., D

≈ 1/d, or 0.9/d or 1.1/d, depending on the shape of the metal NPs.4 For the reactions catalyzed by transition metal NPs, the structure sensitive (or ‘demanding’) reactions refer to those whose TOFs change significantly with the variation of the metal dispersion or the metal domain size in nanometers;1,2 the structure insensitive (or ‘facile’) reactions, on the other hand, denote those whose TOFs change little with the metal dispersion or the metal domain size.1,4 Olefin hydrogenation reactions involving no C-C cleavage have been well-known as the structure insensitive reactions over either well defined single-crystal metal surfaces5-7 or supported metal NPs.4,8−17 For instance, the hydrogenation reactions of ethylene and 1,3-butadiene were documented as the structure insensitive reactions over Pt NPs loaded on varying support materials4,8,9,12,13,15-17 though these two reactions were also found occasionally to be structure sensitive.16,18,19 However, these earlier observations on the structure insensitivity of ethylene4,5,8,9,16,17 and 1,3-butadiene13,15,16 hydrogenation reactions were mostly made with catalyst samples, in which Pt was dispersed as particles no smaller than ca. 2 nm or with dispersion no higher than 60−70%. As clear and systematic information on the structure sensitivity of olefin hydrogenation reactions is not yet available over highly dispersed (dispersion of higher than 70%) transition metal catalysts, probably due to the difficulty to prepare such highly dispersed metal catalysts on conventional supporting oxides, a full understanding of the structure sensitivity nature of olefin hydrogenation reactions is still lacking. It is of fundamental significance to have the structure sensitivity performance of olefin hydrogenation reactions studied in the full (or possible widest) range of active metal dispersion and answer whether such


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performance would be kept or not when the dispersion of the catalytic metal is made to approach 100%. Earlier work from this laboratory showed that construction of Pt-on-Au20-23 nanostructures based on nearly monodispersive small (2~14 nm) Au NPs (denoted as PtmÂu, m refers to the Pt loading on Au by atomic Pt/Au ratio) can be viable for systematically varying the dispersion (or utilization efficiency) of Pt from ca. 10% up to 100%, by carefully changing the Pt loading (or m), as schematically shown in Figure 1A. The construction of the PtmÂu nanostructures would also make it possible to manipulate the electronic property of Pt at nano-/sub-nanometer scale according to the number m,23 enabling us to gain an insight into the correlation between the electronic property and catalytic activity of Pt in the PtmÂu nanostructures for many different reactions. The structure sensitivity performance of olefin hydrogenation reactions is herein studied over a series of PtmÂu nanostructures loaded on SiO2, an ‘inert’ supporting material for heterogeneous metal catalysts, with Pt dispersion in the range of 38% to 100%. The hydrogenation reactions of 1,3-butadiene and ethylene are employed as two probe reactions for that purpose. The specific catalytic rates for both reactions, which are carefully measured under kinetically relevant reaction conditions, appear to decrease with increasing the dispersion of Pt in PtmÂu. These data strongly contrast to the structure insensitivity of the two reactions in most literature4,8,9,12,13,15-17 and those observed in this study over a series of deliberately prepared reference Pt/SiO2-SEA catalysts with Pt dispersion varying in the range of 6% to 100%, thus uncovering the structure sensitivity nature of the hydrogenation reactions over the present PtmÂu nanostructures and pointing to a distinction of the Pt entities in the PtmÂu nanostructures for catalysis. Characterizations of the PtmÂu nanostructures with X-ray absorption near edge structure (XANES) spectroscopy and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) of adsorbed CO disclose that the variation in m resulted in a systematic change in the electronic property of Pt in PtmÂu, which provides a physical basis for unraveling the nature of structure sensitivity for the hydrogenation reactions. These findings would have important implications for better understanding the dimension and richness of the structure sensitivity concept and distinction of Pt-on-Au nanostructures in catalysis and nanomaterial chemistry, and their related fields.


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Figure 1. (A) Schematic preparation and nanostructures of PtmÂu/SiO2 samples; the Au particle size remained constant during the preparation. (B), (C) Dependences of 1,3-butadiene (TOFBD, B) and ethylene (TOFE, C) turnover frequencies upon the Pt dispersion for the PtmÂu/SiO2 (■ and ○), PtmÂu/SiO2−UVO (□) and Pt/SiO2−SEA−T (▲, ▲ and △) samples. The reaction temperature was 50 oC for 1,3-butadiene and 30 oC for ethylene hydrogenation over the PtmÂu/SiO2 samples but was lowered to 0 oC for both reactions over the Pt/SiO2−SEA catalysts; other information for the reactions is detailed in Tables S2 and S3 for PtmÂu/SiO2, and Tables S5 and S6 for Pt/SiO2−SEA−T . The PtmÂu/SiO2 samples were prepared according to procedures developed earlier in our laboratory (Figure 1A).20-23 PtmÂu nanostructures with intended compositions (m = 0.005−0.4) were first synthesized as colloidal PtmÂu NPs by reductive deposition of Pt atoms onto nearly mono-dispersed polyvinyl alcohol (PVA) stabilized Au NPs of 3.2 ± 0.4 nm (Figure S1, Supporting Information) in aqueous solution, employing K2PtCl6 as the precursors for Pt atoms. The as-synthesized PtmÂu NPs were then immobilized onto SiO2 (Degussa Aerosil 150), a wellknown ‘inert’ supporting material for metal catalysts in heterogeneous catalysis, to obtain the PtmÂu/SiO2 catalysts (Figure 1A); the metal loading by Au was controlled at ca. 1.0 wt% for all


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PtmÂu/SiO2 samples (Table S1). The Pt dispersion in PtmÂu, as measured by electrochemical method,20,21,23 decreased with increasing m. Specifically, the Pt dispersion was higher than 99% at m ≤ 0.05 and declined to 38% on increasing m up to 0.4 (Tables S1). The catalytic performance of the PtmÂu/SiO2 samples for the hydrogenation reactions of 1,3-butadiene and ethylene were investigated at 50 and 30 °C, respectively, under atmospheric pressure by proper change of the reaction feed velocity to keep the conversion levels of the reactant olefin no higher than 10% (Figure S2, Tables S2 and S3). The hydrogenation reaction of 1,3-butadiene mainly produced its semi-hydrogenated products (i.e., butenes including 2-butene, cis- and trans-2-butene) but its fully hydrogenated product (i.e., butane) increased with decreasing the Pt dispersion or the increase of m in PtmÂu/SiO2 (Figure S3 and Table S2). The reference gold catalyst (Au/SiO2) without Pt showed no measureable activity for both hydrogenation reactions at temperatures up to 80 °C, which points to that exposed Pt atoms in PtmÂu/SiO2 were serving as the catalytic active sites for both reactions. The reaction rates measured at TOS (time-on-stream) = 3-5 min are used to compare the specific activity of Pt by TOF of 1,3-butadiene (TOFBD) and ethylene (TOFE) over the exposed Pt atoms in the PtmÂu/SiO2 samples, which are correlated with the dispersion of Pt in Figures 1B and 1C, respectively; apparently, both TOFBD and TOFE decreased with increasing the dispersion of Pt. These results demonstrate that the hydrogenation reactions of 1,3-butadiene and ethylene are both structure sensitive over the PtmÂu/SiO2 catalysts, which are contrasting to many earlier observations on the structure insensitivity of the same reactions on Pt catalysts.4,8,9,12,13,15-17 The hydrogenation reactions of 1.3-butadiene and ethylene were also studied over a series of Pt/SiO2 catalysts (Pt/SiO2-SEA) with Pt dispersion in the range of 6% to 100% (Table S4), to further check their structure insensitivity on conventional Pt catalyst (Figures S4 and S5). These Pt/SiO2-SEA catalysts were deliberately prepared by making use of strong electrostatic adsorption (SEA) of Pt(NH3)42+ ions on SiO2 (also Degussa Aerosil 150) from a strongly basic solution, following the procedure of Miller et al.24 The temperature was lowered to 0 °C for measuring the specific activity of these Pt/SiO2-SEA catalysts due to their much higher activity for both reactions (Figure S4, Tables S5 and S6). The catalytic reaction rates by TOFBD and TOFE were also added to Figures 1B and 1C, which clearly show that variation of the dispersion of Pt (from 6% to 100%) in these Pt/SiO2-SEA samples essentially produced no effect on the


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TOFBD and TOFE numbers (see also Tables S5 and S6). In agreement with the conclusion of many earlier studies,4,8,9,12,13,15-17 these results confirm that the hydrogenation reactions of 1.3butadiene and ethylene are both structure insensitive over conventional Pt catalysts. Thus, the structure sensitive performance of the same hydrogenation reactions observed over Pt in the PtmÂu/SiO2 catalysts (Figure 1) uncovers a distinction of the Pt entities in the PtmÂu nanostructures from those in conventional Pt catalysts. It is known that a part of PVA, which was used as the stabilizer for Au and PtmÂu NPs during the colloidal synthesis, would remain as a kind of residue on the surface of PtmÂu NPs after they were immobilized on SiO2 support.25,26 This PVA residue could serve as a modifier to the surface property and catalysis of PtmÂu/SiO2, thus disturbing the structure sensitivity nature of the hydrogenation reactions. In order to address this issue, as-prepared PtmÂu/SiO2 samples were subjected to UV-ozone treatment to remove the PVA residual.26 We showed earlier that this UV-ozone treatment was effective for completely removal of polymer residues from SiO2immobilized Au NPs without changing the Au morphology and size.26 The catalytic activity data (TOFBD) of these UV-ozone treated PtmÂu/SiO2 samples (PtmÂu/SiO2-UVO) for 1,3-butabiene hydrogenation reaction (Figures S6 and S7) are shown as the open squares in Figure 1B (see also Table S7). Though the TOFBD numbers over the UV-ozone treated samples were, irrespective of m, a little higher than those obtained over their as-prepared counterparts, they showed almost the same dependency on Pt dispersion, that is TOFBD over the UV-ozone treated PtmÂu/SiO2 decreased with increasing the Pt dispersion. These results indicate that the residual PVA stabilizer had no significant influence on the structure sensitivity nature of olefin hydrogenation reaction over the Pt in the PtmÂu NPs. This conclusion was further confirmed by conducting 1,3-butadiene hydrogenation reaction on a Pt/SiO2-SEA-400-PVA catalyst, which was obtained by PVA adsorption on Pt/SiO2-SEA-400 from an aqueous solution (5 mg PVA in 100 ml deionized water, PVA/Pt (molar) = 100) for PVA adsorption; the blue triangle (▲) in Figure 1B shows the specific Pt activity of this Pt/SiO2-SEA-400-PVA catalyst. It appears that the turnover rate of 1,3-butadiene over this Pt/SiO2-SEA-400-PVA (TOFBD = 5.7 s-1) was similar to the rate over its counterpart Pt/SiO2-SEA-400 catalyst free of PVA (TOFBD = 6.3 s-1). The time courses of 1,3-butadiene conversion (Figure S8) were found indistinguishable from each other over these two catalysts, demonstrating no significant effect of PVA adsorption on TOFBD over the Pt/SiO2SEA-400 catalyst.


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In-situ DRIFT spectroscopy measurement of chemisorbed CO was conducted to probe the electronic property of Pt27 in the PtmÂu/SiO2 samples, the result was shown in Figure 2. The spectra of chemisorbed CO on PtmÂu/SiO2 (Figure 2A) were measured at 100 °C and showed a clear signal for linearly adsorbed CO on Pt,28-30 whose position depended significantly on the loading level of Pt or m. The absorbance maximum of the IR signal was at 2051 cm-1 for the sample of m = 0.05, then shifted towards higher wavenumbers with the increase of m and reached to 2076 cm-1 at m = 0.4. In contrast, in-situ DRIFT spectroscopy measurement of chemisorbed CO on the Pt/SiO2-SEA samples at 25 oC shows that absorption maximum of adsorbed CO was insensitive to the Pt dispersion and always appeared at 2078 cm-1 (Figure 2B), which agrees surprisingly with the structure insensitive performance of the hydrogenation reactions of both 1,3-butadiene and ethylene (Figures 1B and 1C).

Figure 2. (A) In-situ DRIFT spectra of adsorbed CO at 100 oC on PtmÂu/SiO2 of m = 0 (a), 0.05 (b), 0.1 (c), 0.2 (d), 0.3 (e) and 0.4 (f). (B) In-situ DRIFT spectra of adsorbed CO at 25 oC on Pt/SiO2-SEA−T samples of T = 100 oC (a), 200 oC (b), 400 oC (c), and 600 oC (d). The spectra were corrected for catalyst backgrounds but were not normalized for the Pt dispersion or Pt surface areas. (C) Dependence of the specific activity of Pt in the PtmÂu/SiO2 samples for 1,3butadiene (TOFBD) and ethylene (TOFE) hydrogenation reactions on the IR absorption frequency of adsorbed CO. It deserves to note that the wavenumbers of the IR signals for chemisorbed CO on the PtmÂu/SiO2 samples (Figure 2A) were all lower than that on the Pt/SiO2-SEA catalysts (Figure 2B). However, an artifact is seen in Figure 2 that when the Pt loading on Au m was increased up


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to m = 0.4 the IR absorption frequency for chemisorbed CO on PtmÂu/SiO2 seemingly get close to the IR absorption signal on the Pt/SiO2-SEA samples. When the DRIFT measurement for chemisorbed CO on Pt/SiO2-SEA was conducted at 100 oC (i.e., the same temperature at which the spectra for PtmÂu/SiO2 in Figure 2A were obtained), the IR absorption maximum appeared at 2089 cm-1 (not shown), which was significantly higher than that for the Pt0.4Âu/SiO2 sample (2076 cm-1). It has been known that the IR absorption mavenumber of linearly adsorbed CO on a transition metal adsorption site (e.g., a surface Pt atom) is sensitive to the coordination state of the metal site. The higher the coordination number of the metal site the higher the wavenumber of the IR absorbance maximum.29,30 The Pt entities of higher dispersion in PtmÂu/SiO2 would mean lower coordination number and their adsorbed CO molecules should produce IR signals at lower wavenumbers, which is clearly supported by the present observations (Figure 2A). Also, the continued shift towards higher wavenumbers of the IR absorbance maximum on increasing the loading level of Pt (or number m) in the PtmÂu/SiO2 samples would indicate that the weakening of the triple C≡O bond by CO chemisorption on the surface Pt atoms was enhanced by increasing the Pt dispersion (or lowering the number m) in the PtmÂu/SiO2 samples. A referee of this work pointed out that these explanations take no consideration of the Pt ensemble size and dipole-dipole coupling between chemisorbed CO on adjacent surface Pt atoms. It is understood, however, that the coordination state of the surface Pt atoms would be an averaged comprehensive index involving the ensemble (electronic) effect. The dipole-dipole coupling between the adjacent chemisorbed CO, which shifts the IR signal for the adsorbed CO towards higher wavenumbers, is associated with the geometric effect30,31 of the Pt entities. Our results in Figure 2A are also consistent with an increased dipole-dipole coupling between chemisorbed CO on adjacent surface Pt atoms in the PtmÂu/SiO2 samples as the percentage of such adjacent Pt atoms would increase with increasing the number m (or lowering the Pt dispersion). On the other hand, the insensitivity to Pt dispersion change of the IR signal for chemisorbed CO on the Pt/SiO2-SEA samples (Figure 2B) would suggest very similar dipole-dipole coupling between chemisorbed CO on Pt surfaces of the different Pt/SiO2-SEA samples. These discussions all point to that Pt entities in the PtmÂu/SiO2 samples are electronically and geometrically different from those in the Pt only Pt/SiO2-SEA samples. An attempt is then made in Figure 2C to correlated the specific activity of Pt for the two hydrogenation reactions (TOFBD and TOFE) with the IR absorbance maximum of chemisorbed


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CO for the PtmÂu/SiO2 samples, which reveals that those exposed Pt atoms on which chemisorbed CO molecules would absorb IR lights of higher wavenumbers are intrinsically more active for the hydrogenation reactions. XANES spectroscopy was also employed to probe the electronic property of Pt in the PtmÂu/SiO2 samples. Figure 3A and Figure S10 shows the normalized XANES spectra for the Pt L3 and L2 edges, respectively; the spectrum for a Pt foil (a standard reference) is also included for comparison. The Pt L3 (ca. 11564 eV) and L2 (ca. 13273 eV) edges feature the electron transitions from the 2p3/2 and 2p1/2 core levels to the 5d5/2 and 5d3/2 valence levels, respectively. The first intense absorption in the Pt L3-edge measures the whiteline and its intensity, which characterizes the number of holes in the 5d orbits,32-36 increased with the decrease of m in the PtmÂu/SiO2 samples. However, compared with the spectrum for the Pt foil, the whiteline intensity for PtmÂu/SiO2 was always higher regardless of m. Similar changes were also observed in the whitelines for Pt L2 edge (Figure S10). These results indicate that the number of holes in the Pt 5d orbits increased with increasing the Pt dispersion the PtmÂu/SiO2 samples, or Pt entities in PtmÂu/SiO2 were essentially in an electron deficient state in comparison with the Pt foil (the reference) and their electron deficiency decreased with the increase of m or the decline of Pt dispersion. These observations imply that a charge (electron) transfer to Au from Pt occurred more or less in the PtmÂu/SiO2 samples owing to interaction between the Pt entities and their carrying Au NP surface, which is in line with the fact that Au has a higher electronegativity than Pt.36 To quantify the electron deficiency of Pt in the PtmÂu/SiO2 samples, the Pt whiteline intensity was quantitatively analyzed to determine the holes (h3/2 and h5/2) in the Pt 5d states for each of the samples, using the method reported by Sham et al.33-35 As shown in Table 1, the overall holes in the Pt 5d states (h3/2 + h5/2) decreased with increasing m of the PtmÂu/SiO2 samples, compared with that of the Pt foil (reference). One would wonder that the ex-situ XANES results in Figure 3A and Figure S10 were obtained on PtmÂu/SiO2 samples without any pretreatment and might not be used to correlate the catalyst activity. Quasi in-situ XANES measurements were then conducted, in which the PtmÂu/SiO2 catalysts were pretreated, before collecting the room temperature XANES spectra, in-situ in flowing He at 110 oC (i.e., simulating the pretreatment of PtmÂu/SiO2 prior to the olefin hydrogenation reactions) or 5%H2/He at 50 oC (i.e., the temperature for 1,3-butadiene


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hydrogenation reaction). Figures S11 shows the XANES spectra for Pt L3 and L2 edges after the pretreatment in flowing 5%H2/He at 50 oC, and Figure S12 the XANES spectra for Pt L3-edge after the pretreatment in flowing He at 110 oC. The quantitative Pt whiteline parameters for the samples pretreated at 50 oC in flowing 5%H2/He are shown in Table S8. These spectra also demonstrate that the Pt entities in PtmÂu/SiO2 are essentially in electron deficient states and their electron deficiency decreased with the increase of m or the decline of Pt dispersion, being qualitatively consistent with results obtained for the samples without pretreatment (Table 1). Therefore, the pretreatments showed little influence on the dependence of the electron deficiency of Pt on number m or Pt dispersion for the PtmÂu/SiO2 samples. The specific activity of Pt by TOFBD and TOFE in the PtmÂu/SiO2 catalysts are then correlated, respectively, with the overall holes in the Pt 5d states by “h3/2 + h5/2” in Figure 3B, which demonstrates that the TOFBD and TOFE numbers depend sensitively on the electron deficiency level of the Pt entities in the PtmÂu/SiO2 catalysts and the lower the electron deficiency level the higher the specific activity of Pt for the hydrogenation reactions.

Figure 3. (A) Normalized XANES spectra at Pt L3-edge for the PtmÂu/SiO2 samples of m = 0.05 (red), 0.2 (blue), 0.3 (purple) and 0.4 (olive), and Pt foil (reference, black); the inset shows the difference spectra between those for the PtmÂu/SiO2 samples and that for Pt foil. (B) Dependence of the specific activity of Pt for 1,3-butadiene (TOFBD) and ethylene (TOFE) hydrogenation reactions on the overall unoccupied density in the Pt 5d states for the PtmÂu/SiO2 samples.


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Table 1. Quantitative Pt Whiteline Parameters for the PtmÂu/SiO2 Samples Sample Pt0.05Âu/SiO2

△A3a

△A2a

h5/2b

h3/2b

7.75

3.29

0.72

0.15

Pt0.1Âu/SiO2 Pt0.2Âu/SiO2

7.49

3.18

0.70

0.14

7.38

2.90

0.69

0.13

Pt0.3Âu/SiO2

7.27

2.82

0.67

0.13

Pt0.4Âu/SiO2

6.91 6.07

2.60 1.79

0.64 0.57

0.12 0.08

Pt foil a

△A3 and △A2 refer to the intensity by areas of the normalized Pt L3- and L2-edge whitelines, respectively. The areas were obtained, respectively, by subtracting those for the Au L3- and L2-edge whitelines from the Pt L3- and L2-edge ones in each of the XANES spectra for the PtmÂu/SiO2 samples. b h5/2 and h3/2 refer to the holes in Pt 5d5/2 and 5d3/2 orbits, respectively. It was demonstrated that the unoccupied d-orbits or d-holes play a vital role to the activity of transition metal catalysts.36,37 For olefin hydrogenation reactions, the reaction order in olefin was nearly zero, and the reaction order in hydrogen was close to one, implying that adsorbed olefin was the most abundant species on the catalyst surface during the reactions.16,38-42 Thus, the catalytic activity would depend strongly on the olefin-metal interaction at the metallic catalyst surface.13,42-45 Owing to that the Pt atoms in PtmÂu/SiO2 became more electron-deficient when the number m was lowered or the dispersion of Pt become higher, the adsorption of olefin molecules (1,3-butadiene or ethylene), which are molecular electron donors in essence, would become stronger on the Pt entities of the PtmÂu/SiO2 sample having a smaller m or higher Pt dispersion. Thus, the structure sensitive behavior of the 1,3-butadiene and ethylene hydrogenation reactions (Figure 1) would point to a correlation between the specific activity of Pt (TOFBD or TOFE) and the adsorption strength of 1,3-butadiene or ethylene molecules.13,37,38,39 It is likely that the present PtmÂu/SiO2 samples are all located on the right side of the activity‘olefin adsorption strength’ correlation (assuming a volcano-shaped curve). This inference seems to be supported by the facts that even the most active Pt0.4Âu/SiO2, among the PtmÂu/SiO2 samples, was much less active than the Pt/SiO2-SEA ones for the hydrogenation reactions (Figure 1). In summary, this work addresses for the first time the structure sensitivity issue of olefin hydrogenation reactions on Pt entities with dispersion up to 100% in Pt-on-Au (PtmÂu)


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nanostructures. The Pt entities deposited on the surface of small Au NPs (ca. 3.2 nm) in the nanostructures were found to be electron-deficient; their electron deficiency decreased with the increase of m or the decrease of Pt dispersion. The specific (or intrinsic) activity of Pt in the PtmÂu/SiO2 samples for the hydrogenation of both 1,3-butadiene and ethylene varied sensitively according to the Pt dispersion (and thus electron deficiency in Pt) such that the entities with a lower Pt dispersion always offered a higher activity, which clearly demonstrates that the hydrogenation reactions were essentially structure sensitive over the PtmÂu nanostructures. These results strongly contrast to those obtained in this study on reference Pt/SiO2-SEA catalysts with the possible widest Pt dispersions (5%~100%), as well as those in earlier documentations, that the olefin hydrogenation reactions over Pt catalysts are structure insensitive. The catalytic data thus identify a distinct feature of the Pt entities in PtmÂu for catalysis, over which the olefin hydrogenation reactions could no longer remain as structure insensitive but become instead structure sensitive. The spectroscopic data have enabled us to correlate the structure sensitivity of the hydrogenation reactions with the electronic property of Pt in the PtmÂu nanostructures. The present findings would have important implications for better understanding the dimension and richness of the structure sensitivity concept and the feature of transition metal (TM) domains in TM-on-Au nanostructures in catalysis and nanomaterial chemistry, and their related fields. Associated content Supporting Information. Details on the materials, sample preparations, characterizations and catalytic hydrogenation reaction tests (S1). Information on sample compositions and detailed results for catalytic reactions (Tables S1− −S7 & Figures S1− −S8). Table S8 and Figures S9− −S12 present the additional XANES data. This information is available free of charge on the ACS Publications website. Notes The authors declare no competing financial interests. ACKNOWLEDGMENTS The authors are indebted to Drs. Zheng Jiang and Jingyuan Ma at Shanghai Synchrotron Radiation Facility (SSRF) and Drs. Jing Zhang and Zhan Shi of Beijing Synchrotron Radiation Facility (BSRF) for their professional help in situ XANES measurements. This work was


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TOC Graphic:


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Do Olefin Hydrogenation Reactions Remain Structure Insensitive over

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