Sulfur Moiety as a Double-edged Sword for Realizing Ultrafine

Subscriber access provided by Washington University | Libraries

Energy, Environmental, and Catalysis Applications

Sulfur Moiety as a Double-edged Sword for Realizing Ultrafine Supported Metal Nanoclusters with a Cationic Nature Xinping Duan, Lichao Ning, Yan Yin, Yanting Huang, Jian Gao, Haiqiang Lin, Kai Tan, Huihuang Fang, Linmin Ye, Xin Lu, and Youzhu Yuan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b18952 • Publication Date (Web): 05 Mar 2019 Downloaded from http://pubs.acs.org on March 5, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Sulfur Moiety as a Double-edged Sword for Realizing Ultrafine Supported Metal Nanoclusters with a Cationic Nature

Xinping Duan,#,† Lichao Ning,#,† Yan Yin,† Yanting Huang,† Jian Gao,§ Haiqiang Lin,† Kai Tan,† Huihuang Fang,† Linmin Ye,† Xin Lu,*,† Youzhu Yuan*,† †

State Key Laboratory of Physical Chemistry of Solid Surfaces, National Engineering

Laboratory for Green Chemical Productions of Alcohols-Ethers-Esters, iChEM, College of Chemistry and Chemical Engineering, Xiamen University, No. 422, Siming South Road, Siming District, Xiamen 361005, China § Research

Center of Heterogeneous Catalysis and Engineering Sciences, School of Chemical,

Engineering and Energy, Zhengzhou University, Zhengzhou 450001, China.

* Corresponding Authors: Youzhu Yuan, E-mail: [email protected] Tel: +86-592-2181659. Fax: +86 592 2183047. Xin Lu, E-mail: [email protected]. Tel: +86-592-2181600. Fax: +86 592 2183047.

ORCID ID: Xinping Duan: 0000-0002-9191-1437 Xin Lu: 0000-0003-4968-9462 Youzhu Yuan: 0000-0003-1668-9984

1 ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: Heterogeneously and uniformly dispersed metal nanoclusters with high thermal stability and a stable nonmetallic nature show outstanding catalytic performance. In this work, we report on the role of sulfur moieties in hydrochlorination catalysis over carbon-supported gold (Au/C). A combination of experimental and theoretical analyses shows that the –SO3H and derived –SO2H sulfur species in high oxidation states at the interface between Au and – SO3H at ≥ 180 C give rise to high thermal stability and catalytic activity. By contrast, the grafted thiol group (–SH) and the derived low-valence sulfur species on carbon markedly destabilize the Au nanoclusters, promoting their rapid sintering into large Au nanoparticles and leading to the loss of their cationic nature. Theoretical calculations suggest that –SO3H favorably adsorbs and stabilizes cationic Au species. Compared to Au/C and Au-SH/C with the Auα+/Au0 atomic ratios of 1.02 and 0.24, respectively (α = 1 or 3), the activity and durability of acetylene hydrochlorination are remarkably enhanced by the interaction between the –SO3H moieties and cationic Au species that enables the high oxidation state of Au to be effectively retained (Auα+/Au0 = 3.82). These results clearly demonstrate the double-edged sword effect of sulfur moieties on the catalytic Au component in acetylene hydrochlorination. The double-edged sword effect of sulfur species in the stabilization/destabilization of metal nanoclusters is also applicable to other metals such as Ru, Pd, Pt, and Cu. Overall, this study enriches the general understanding of the stabilization of metal clusters and provides insight into a wet chemistry strategy for stabilizing supported ligand-free nanoclusters. KEYWORDS: Sulfur species, Gold nanocluster, Thermal stability, Hydrochlorination catalysis, Soft chemistry.


Page 2 of 44

Page 3 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60




INTRODUCTION Highly dispersed metal nanoparticles (NPs)/nanoclusters with stable cationic properties are

commonly applied in heterogeneous catalysis such as in H2O2 production, water-gas shift (WGS) reaction, hydrochlorination, and epoxidation.1–6 It has been generally accepted that metallic particles are the active components of supported metal catalysts. However, metal nanoclusters with nonmetallic nature were found to be highly active in various catalysts, with their high activity related to their dispersity, particle size, and interactions with supports.7 Even more interesting is that in many catalytic processes, nonmetallic species rather than metal NPs serve as the catalytically reactive sites.8,9 For example, Xu et al.10 revealed the single Au3+ ions with nonmetallic properties on Au/ZrO2 catalysts were highly active in selective hydrogenation. For the acetylene hydrochlorination reaction, in a typical nonmetallic-species-dominated catalysis, the pioneering studies of Hutchings et al. demonstrated that high-valent Au(III)/Au(I) can be a promising alternative due to its intrinsically high activity.11–16 Strategies of “hard” and “soft” chemistry have been used to address this issue. “Hard” chemical interaction between the metal nanoclusters and oxides (ligand-free) to form MO bond, also referred to as strong metal-support interaction (SMSI), is generally used for highly dispersed nanoclusters with high oxidation states.9,10,17,18 The SMSI effect suggests that metal nanoclusters and/or single atoms of noble metals survive under harsh reaction conditions on certain metal oxides supports such as spinel and ceria.19,20 Fu et al.1 and Valden et al.2 observed active cationic Au species for CO oxidation and WGS reaction. For the SMSI



effect, density functional theory (DFT) calculations revealed that the interaction between the recycles of Ce3+/Ce4+ and Au+/Au3+ supports the high catalytic performance of AuxCe1-xO2 systems, where the active sites are present mostly as cationic Au+ species.20–23 However, the “hard” MO bond on the surface of metal oxide restrains the loading and diversity of the charge-varied active species associated with the requisite properties in the desired reactions.24 As a route of “soft” chemistry, the doping/dedoping mechanism of supports with nonmetal ingredients that interact with metal nanoclusters has been advanced with regard to the role of cations and anions.6 The catalytic activity of gold/titania catalysts for CO oxidation can be dramatically enhanced with the addition of nitrate or sulfate ions to the support.25,26 The common anchoring effect and electronic tunability of metal (noble and other transition metals) nanoclusters by nonmetallic ingredients (groups) with high oxidation state such as sulfate, phosphate, and nitrate have been extensively investigated and revealed.27–29 For instance, Liu et al.30 proposed the concept of “soft” nitriding, where nitrogen-containing species are raised to the carbon surface via mild reaction with NH3 and HCNO. They demonstrated that sub-2-nm, ligand-free, evenly dispersed ultrasmall noble metal (Pt, Pd, Au, Cu, and Ag) nanocatalysts grow in situ on nitrided carbons and are exceptionally active for catalytic methanol oxidation.30 It was revealed that the NH2 and NH functional groups serve as a stabilizing agent, preventing the reduction of Au NPs.27 In this study, we identify the sulfur moieties for stabilizing/destabilizing ultrafine supported Au catalysts in acetylene hydrochlorination reaction. Specifically, we compare the high- and low-valence sulfur species (S4+, S6+ vs. S0, S2) interacting with Au species in order


Page 4 of 44

Page 5 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


to address the following questions. (i) What are the active sites for acetylene hydrochlorination? (ii) What is the principal mechanism for the stabilization of active Au components by a highly charged sulfur moiety? (iii) Can the low-valence sulfur species that are highly poisonous to gold catalysts transform into beneficial groups? Our experiments and DFT calculations clearly reveal that the sulfur species serves as a double-edged sword for the Au nanoclusters in acetylene hydrochlorination: high-valence sulfur species such as –SO3H effectively stabilize the Au sites with highly catalytic performance, whereas low-valence S2 or S0 sulfur species remarkably destabilize the Au sites and inhibit the catalytic activity. Nevertheless, low-valence sulfur species can be oxidized to obtain a positive effect in stabilizing the gold cations. We show that the strong interaction between –SO3H and Auα+ (α = 1 or 3) activates the Au species for acetylene hydrochlorination, whereas –SH species significantly promotes the reduction of cationic Au species and the subsequent agglomeration of metallic Au species.



EXPERIMENTAL SECTION 2.1. Reagents. HAuCl44H2O, HPtCl66H2O and similar reagents were analytical grade and

were obtained from the Alfa Aesar Co. (99.9% purity). Activated carbon was purchased from Sinopharm Chemical Reagent Co. Ltd. More details regarding the reagents can be found in Electronic Supporting Information (ESI). 2.2. Synthesis. Carbon was purified prior to application.16 To obtain the function of sulfur moieties, the following modified carbons were obtained: (i) xSO3H/C (x refers to the



normalized S loading).24 (ii) ySH/C (y refers to the normalized S loading). The SH grafted C was obtained by heat treatment of C under H2S/H2 (10 vol.% H2S) at 550 °C. (iii) Oxidized SH/C.5 The obtained SH/C solid samples were oxidized with hydrogen peroxide (30%) and concentrated sulfuric acid (98%) at 80 °C with magnetic stirring for 2 h.24 The oxidized samples were denoted as SH/C-H2O2 and SH/C-H2SO4, respectively. Gold supported on carbon, and sulfur-functionalized carbon samples were prepared by the incipient wetness impregnation method as previously reported in detail elsewhere,6 and are referred to as AuxSO3H/C, AuySH/C, AuySH/C-H2O2, and AuySH/C-H2SO4, respectively. 2.3. Characterization. Various characterization methods such as powder X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning transmission electron microscopy (TEM), hydrogen-temperature-programmed reduction (H2-TPR) experiments, and X-ray photoelectron spectroscopy (XPS) were employed to reveal the activity-structure relationship of the catalysts, indicating the interplay between the sulfur moieties and gold species. Further detailed information regarding the characterization is provided in ESI. 2.4. Catalytic Test. Catalytic activity was tested in a fixed bed microreactor (i.d. of 8 mm, produced by BetterWork Intelligent Technology Company, Xiamen, China) operated at a pressure slightly below the atmospheric pressure (0.11 MPa). Further information regarding the reactants/products analysis associated with the catalyst assessment is provided in ESI.24 2.5. Density Functional Theory Calculations. All of the DFT calculations were carried out with the Gaussian 09 package. All of the structures were fully optimized using the hybrid B3LYP31 functional in combination with the valence double zeta SDD32 basis set with an


Page 6 of 44

Page 7 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


effective core potential used for Au and the standard 6-31G(d) basis set used for the other atoms.33,34 More detailed information regarding DFT calculations is provided in ESI.



RESULTS AND DISCUSSION 3.1. Synthesis and Characterization. For simplicity, sulfuric acid (H2SO4) and hydrogen

sulfide (H2S) were chosen as the typical sulfur species for doping the carbon support. The high-valence sulfur species modified carbon (referred to SO3H/C) supports were synthesized by the impregnation method, followed by drying and thermally decomposing under a nitrogen atmosphere at 110 °C and 550 °C. Low-valence sulfur grafted carbon supports (denoted as SH/C) were prepared by the heat treatment of an H2S/H2 mixture at 550 °C. Sulfur contents of the carbon and Au loadings introduced on the sulfur-containing carbon support by the incipient wetness method to obtain an Au weight loading of 1% were determined by X-ray fluorescence (XRF).3537 The experimental methods are described in detail in the Experimental Section (see ESI). As indicated in Figure S1 and Table S1 (see ESI), carbons modified with different sulfur moieties, namely, –SO3H/C and –SH/C, were investigated using TEM with energy dispersive spectroscopy (STEM-EDS). The sulfur content in –SO3H/C and –SH/C was controlled and measured by elemental analysis to be approximately 1.5 wt% for both carbons. Energy dispersive X-ray (EDX) spectroscopy showed that sulfur species are evenly distributed in the –SO3H/C and –SH/C supports (Figures 1a and 1b). Meanwhile, XPS was used to study the chemical state of the sulfur moieties as indicated in Figures 1c and 1d. For the –SO3H/C



Page 8 of 44

sample, it was found that the most dominant peaks of the featured S–O bonds (167.6 eV and 168.8 eV for 2p3/2) are due to the presence of S4+ and S6+ oxidation states.38 By contrast, only the peak corresponding to the C–S bond (164.1 eV for 2 for 2p3/2) was found for the carbon treated by H2S39. For the –SO3H/C and –SH/C samples, the combination of TEM results and XPS results confirmed the successful introduction of sulfur moieties with high and low oxidation states. Next, we investigated the variation in the surface sulfur functional groups using FT-IR spectroscopy (Figure S1a) and Raman spectroscopy (Figure 3a). An examination of the FT-IR spectra presented in Figure S1a reveals the presence of sulfur-containing function groups after “soft” sulfating or sulfiding. The characteristics 3429 cm–1 and 1385 cm–1 broad peaks are present in the spectra of carbon, –SO3H/C, and –SH/C and are ascribed to the vibration of the =C–H and C–O bonds, respectively. These two peaks reveal the universal existence of oxygen-containing groups (Figure S1a).30 The broad peak at 1122 cm–1 is assigned to the stretching of S=O or S–O from the SOx groups on carbon, whereas no such peak

is

observed

in

the

spectrum

of

the

–SH/C

sample.

Instead,

for

the

hydrogen-sulfide-treated sample, SH/C displays a peak feature at 668 cm–1 corresponding to the stretching of S=C=S groups.40 Furthermore, the Raman spectra provide information on the degree of crystallinity of various samples.6 Typical Raman spectra of carbonaceous materials show two peaks at 1350 cm–1 and 1580 cm–1 that are attributed to the disorder-induced D band and graphitic G band, respectively.4 The Raman spectra obtained in the present work show similar G and D bands for various carbon materials after soft sulfating


Page 9 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(–SO3H/C) and sulfiding (–SH/C) (Figure 3a). Based on the intensity ratio of the two bands, –SO3H/C has a higher crystallinity than its –SH/C counterpart, because the ratios for – SO3H/C and –SH/C are 0.78 and 0.72, respectively (Table S2). Importantly, both –SO3H/C and –SH/C have higher crystallinity values than that of pristine carbon (0.61), once again confirming the effect of soft sulfating and sulfiding processes on the production of turbostratic carbon configuration.27 Figure S2 shows the TEM images of the three as-synthesized catalysts, namely Au/C, Au– SO3H/C, and Au–SH/C. In line with the XRD results, no Au particles or clusters are present on the three catalysts, indicating the high dispersion of Au species (Figure S3a). These results suggest that Au species (chloroauric phase) interact with functional groups consisting of oxygen–, SO3H–, and SH– moieties prior to hydrochlorination catalysis during which harsh conditions, elevated temperature and strong reduction effect by C2H2 were employed. Figures 2a–c show the TEM images of the used catalyst after the acetylene hydrochlorination reaction (at 180 °C, 0.11 MPa, 24 h) combined with the corresponding high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image (Figures 2d–f) and Au particle distribution diagram. For the discernable Au NPs present in the three used catalysts, Au/C shows a medium Au particle size distribution with an average size of 3.42 nm (Figures 2a–c). The used Au–SO3H/C presents the minimum average particle size of 0.81 nm, suggesting the presence of Au nanoclusters (Aun, n = 40200) composed of dozens to approximately two hundred Au atoms.35,36 It is important to note that Au nanoclusters predominantly display the Au(111) facet with a lattice distance of



Page 10 of 44

0.235 nm (insets of Figures 2a–c). By contrast, the presence of –SH groups on carbon markedly increases the mobility of Au atoms and promotes the formation of larger Au crystals with an average particle size of 8.24 nm during the hydrochlorination reaction. The related catalytic performance of these three catalysts will discuss below. To further image the subnanometer Au species, atomic resolution HAADF-STEM was performed combined with size distribution analysis (insets of Figures 2d–f). In agreement with the TEM results, the used Au-SO3H/C shows the Au clusters with subnanometer diameter (Figure 2d). In Au/C (used, Figure 2e), obvious nanocrystals coexist with small Au nanoparticles (2–5 nm), whereas mainly larger Au NPs (7–9 nm) are present exist on the used Au-SH/C catalyst (Figure 2f). Sulfur, nitrogen, and even phosphorus species are commonly applied to modify carbon, metal

oxide,

and

semiconductor

support

matrices

that

support

precious

metal

nanoclusters.4,35,38,43 However, the fundamental role of sulfur in various sulfur species has not been recognized, particularly for stabilizing uniformly dispersed cationic metal nanoclusters in heterogeneous catalysis.7,28 The catalytic performance of cationic Au(III) and Au(I) compounds has been widely investigated.46 For the Au phase on carbon and carbon materials modified by soft sulfating and sulfiding processes, the Au 4f spectra of the catalyst were deconvoluted to reveal the various Au species, as indicated in Figures 2g, S4, and S5. Table S3 summarizes the binding energies (BEs) and relative contents of different Au species in various catalysts. For the Au/C sample, three Au species namely, Au0, Au+, and Au3+ are present with different concentrations. The Au3+ species can be easily reduced to generate the


Page 11 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


corresponding Au+ and Au0 generated by exothermic adsorption in the as-synthesized Au/C catalyst.46 It is well-accepted for acetylene hydrochlorination that the Auα+ species is more reactive than the Au0 phase.11,47 Therefore, the strategy for enhancing Au dispersion and maintaining the high oxidation state of Au phase is the focus of this process. Thus, the variation of Auα+/Au0 atomic ratio can serve as a descriptor and reveal the activity and stability of the targeted catalyst in the hydrochlorination reaction.14 These Au+ and Au0 are generated during the preparation procedure, and the BE of Au0 is approximately 1 eV higher than that of the present Au3+ species.15 Figures S5a and S5b show the change in the Au species on the Au/C catalyst before and after the hydrochlorination reaction. The Auα+/Au0 atomic ratios for the fresh and used Au/C catalyst are 1.02 and 0.45, respectively. Importantly, the as-synthesized Au–SO3H/C catalyst shows a higher Auα+/Au0 atomic ratio of 3.85. Even though the Au–SO3H/C catalyst was subjected to harsh reaction conditions, the used Au–SO3H/C catalyst still exhibits the Auα+/Au0 atomic ratio of 3.42 (Figures 2g and S4, and Table S3). Based on DFT analysis, we inferred that Auα+ species attaches to the oxygen atom bound to the sulfur that stabilizes the Auα+ species, as schematically illustrated in Figure 2h. By contrast, it is interesting to note that Au on –SH/C sample presents the lowest Auα+/Au0 atomic ratio of 0.32, indicating that Au species interact with –SH group, resulting in the major formation of Au0 species (Figures S5c and S5d). This demonstrates that the abundant Auα+ transforms into the Au0 species because all Au species act as the precursors of the Au3+ phase during the preparation for which the reduction of Au3+ by S2– occurs. Sulfur species such as S2–, S0, and Sm2– are recognized as poisons for transition



metal particles due to their electron-rich nature.44 For our system, the Au atoms interacting with S2– largely segregate into Au crystalline NPs after the hydrochlorination reaction, as verified by the TEM and XRD results (Figures 2 and 3b). Subsequently, the used Au–SH/C catalyst presents the Auα+/Au0 ratio of 0.24, matching its catalytic activity. XRD patterns presented in Figure 3b confirm the above observation, showing the formation of Au NPs over the used Au/C and Au–SH/C with the estimated average particle sizes of 5.9 and 9.3 nm, respectively.8 The thermal stabilities of different catalysts including Au/C, Au–SH/C, and Au–SO3H/C were also determined using XRD patterns. The above three catalysts were heated under N2 atmosphere at 350 °C for 3 h. The obtained XRD results presented in Figure 3c show that Au catalysts with high-valence sulfur species (–SO3H) exhibit a weakly visible Au characteristic peak (111), whereas heat-treated Au/C and Au–SH/C reveal the apparent diffraction peaks of the (111), (200), (220), and (311) lattice planes, thus indicating the presence of the featured monocrystal. Thermal stability measurement confirmed the stabilization role of –SO3H moieties for the Au species related to the mobility of Au atoms to form monocrystal Au NPs driven by the –SH moiety on the carbon surface. Figure 3c shows that TPR results can provide valuable information regarding the relationship between the amount of Au3+ and the effect of the sulfur species introduction on the carbon. All of the samples show a broad reduction peak ranging from 550 °C to 650 °C due to the reduction of the oxygen group on the surface of carbon and the decarboxylation reactions.48 The fresh Au/C catalyst exhibit one reduction characteristic peak between 200 °C and 300 °C, with the peak center at


Page 12 of 44

Page 13 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


approximately 265 °C due to the reduction of Au3+ to Au0.7 Similar to the Au/C, after adding –SO3H to the Au catalysts (Au–SO3H/C), reduction behavior of Au3+ was observed. Notably, unlike for carbon blank, the reduction of SO3H/C provides no reduction signal for temperatures lower than 500 °C, indicating the thermal stability of –SO3H moiety on the carbon surface. We carried out the XPS measurements for the sulfur species present on the Au–SO3H/C and Au–SH/C catalysts in order to determine the form of the sulfur phase. Figures 2g and S5 show that the Au–SO3H/C catalyst demonstrates the characteristic sulfur peaks of S6+ (168.7 eV) and S4+ (167.3 eV).44 The Au–SH/C catalyst displays a strong peaks at 164.0 eV and a very week peak at 167.7 eV, relative to S2– and Sm2– species. Compared with the XPS result of –SH/C (Figure 1d, Table S4), the weak response of S4+ on the as-synthesized Au–SH/C validates the interaction and redox reaction between the initial Au3+ and S2– species in the preparation procedure.6 Furthermore, we used the concentrated H2SO4 and H2O2 oxidant agents to modulate the oxidation state of the sulfur species following the deposition of Au species on the generated –SH/C, respectively. Figures S6c and S6d demonstrate the S 2p XPS results, where Au–SH/C-H2SO4 (oxidized by H2SO4) shows the characteristic peaks of S6+ (168.6 eV) and S4+ (167.4 eV), while Au–SH/C-H2O2 (oxidized by H2O2) only reveals the features of the Sm2– and S2– species at 163.7 eV and 164.1 eV (Table S4), which is indicative of sulfur agglomeration on the surface of the carbon matrix.41 Meanwhile, the sulfur species oxidized from S2– to S4+ and S6+ also stabilize the high oxidation state of the Au species (Au– SH/C-H2SO4), as indicated in Figure S5e. Nonetheless, the obtained main Sm2– species



suggests the destabilizing effect on the Auα+ species (Figure S5f) similar to that of its S2– counterpart. 3.2. Catalytic Performance. First, we measured the catalytic activity of Au/C, Au– SO3H/C (sulfur content, 1.5 wt%), and Au–SH/C (sulfur content, 1.5 wt%) catalysts for acetylene hydrochlorination under 180 C, V(HCl)/V(C2H2) = 1.2, and varied GHSV(C2H2). It should be noted that selectivity for VCM is steadily maintained at above 99% for all of catalysts evaluated in our work. Figure 4a shows the acetylene hydrochlorination catalytic activity where the Au–SO3H/C catalyst shows the maximum activity with acetylene conversion of up to 90% and excellent stability under high GHSV(C2H2) = 1150 h1. The Au/C catalyst displays the common initial activity of 78% acetylene conversion and a rapid deactivation from 78% to 58% acetylene conversion under identical reaction conditions. Meanwhile, the Au–SH/C catalyst shows inferior initial and unstable activity in acetylene conversion ranging from 41% to 14%, indicating the significant deactivation of Au active phase induced by the interaction between Au and –SH moieties (Figure 4a). Based on the combination of the XPS, XRD, TPR, TEM characterization results, and the catalytic performance results, we speculated that sulfur species with a high oxidation state such as S4+ and S6+ stabilize ultrafine supported cationic Au nanoclusters (Auα+) that have optimal acetylene hydrochlorination activity. On the other hand, the low-valence sulfur species (S2–) are the driving force for the sintering of Au0 into Au nanocrystals that leads to the loss of the catalytic activity. We performed further controlled experiments to understand the effect of the sulfur valence change on the stabilization of Auα+ and on the activity of the corresponding


Page 14 of 44

Page 15 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


catalysts. Figure 4b shows the hydrochlorination activity data for the Au–SH/C-H2SO4 and Au–SH/C-H2O2 catalysts. Interestingly, the acetylene hydrochlorination activity of Au–SH/C was almost recovered through reoxidation of SH/C by concentrated H2SO4 and the subsequently supported Au species. The Au–SH/C-H2SO4 catalyst shows 77% acetylene conversion and good stability. The S2– species in SH/C is efficiently oxidized to the S4+ and S6+ species, as proven by the XPS results (Figures S6c and S6d), and thus they stabilize reactive Auα+ (Figures S5e and f). However, the H2O2 oxidized Au–SH/C catalyst exhibits much lower activity than the Au–SH/C sample. As verified by the XPS results, H2O2 mainly oxidizes S2– to Sm2–, leading to an Auα+/Au atomic ratio of 0.24, which is the lowest value among the catalysts investigated in this study, corresponding to the most inferior catalytic performance.7 This once again confirms the positive effect of the high-valence sulfur species (S4+ and S6+) in the stabilization of reactive Auα+ species and the negative role of the low-valence sulfur (S2– and Sm2–).9 In other words, these results indicate the double-edged sword role played by the sulfur moieties in stabilizing/destabilizing the Auα+ species for acetylene hydrochlorination. Interestingly, we directly observed the change of the Au nanoclusters to Au NPs over Au–SO3H/C catalyst. Figure 5 shows the HRTEM images of the Au nanoclusters and small NPs ( 1 nm), together with the HAADF-STEM results, where the Au nanoclusters and small Au NPs featuring the Au (111) facet coexist, as marked by yellow circles and red circles, respectively. When we changed the sulfur content in the mode of –SO3H and –SH on the carbon matrix, the obtained corresponding Au-based catalysts showed a strong dependence of the catalytic



activity on the different types of sulfur species (Figure 4c). In short, the Au-based catalysts with high-valence sulfur species (–SO3H) exhibit higher activity than the sulfur-free Au/C. Although the optimal content loading was found to be 1.5 wt%, we believe that the sulfur content is correlated to the given Au loading. The plots of acetylene conversion against sulfur content reveal a positive correlation (R2 = 0.858, KSO3H = 20.8). On the other hand, the presence of low-valence sulfur species (–SH) inhibits the catalytic activity for acetylene hydrochlorination, regardless of the sulfur content. The corresponding plot for –SH/C demonstrates a negative correlation (R2 = 0.929, KSH = –10.1). The deactivation rate is a crucial parameter for the acetylene hydrochlorination reaction.49 We estimated the deactivation rate over various sulfur-containing Au-based catalysts based on the catalytic performance during a reaction period of 24 h (Figure 4d). Interestingly, the deactivation rates of the –SO3H-containing catalysts display a volcano type behavior, confirming our conjecture that the optimal sulfur content (in the mode of –SO3H) is related to Au loading. This result means that excessive –SO3H moieties can influence the hydrochlorination reaction, likely affecting the total adsorption of C2H2 or/and HCl. Additionally, it is not surprising that – SH-containing Au catalysts with varying sulfur content give high deactivation rates, which is consistent with the their catalytic performance results (Figures 4a, 4b, and 4d). Catalyst stability is a crucial issue for obtaining highly promising alternative catalysts for future industrial use. We conducted a long-term evaluation of the Au/C, Au–SO3H/C, and Au–SH/C catalysts. Figure 4e shows the durability of the three catalysts for acetylene hydrochlorination. The Au–SO3H/C catalyst shows excellent stability for this reaction even


Page 16 of 44

Page 17 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


under high GHSV(C2H2) of 1150 h–1 at 180 °C, while only a slight decrease in activity from 92.1% to 84% for acetylene conversion after reaction for 120 h is observed. Then, when we adjusted the GHSV(C2H2) from 1150 h–1 to 360 h–1, the conversion of acetylene increased from 84 to more than 99%, suggesting the outstanding durability of the Au–SO3H/C catalyst. The Au/C and Au–SH/C catalysts exhibit rapid deactivation for acetylene conversion in the reaction carried out for 260 h, thereby suggesting their inferior stability. We note that different sulfur-containing moieties grafted on the carbon matrix affect both the nature and the dispersion of the Au species present on the fresh catalysts. As displayed in thermal stability (Figures 3c and 2) and the initial Auα+/Au0 atomic ratio displayed in the as-prepared catalysts, Au–SO3H/C, Au/C, and Au–SH/C, the stabilizing efficiency of cationic Au species decreases according to the interaction and redox properties between sulfur/oxygen moieties following the order –SO3H  –O–  –SH. Based on the catalytic performance characteristics of induction period and durability, we can conclude that the sulfur in high oxidation state hosts the cationic Au component. Associated with the various characterizations (Figures 2, S5, and Table S3) the high ratio of cationic Au species on Au nanoclusters/NPs were confirmed to be high catalytic activity for hydrochlorination and short-term induction on Au– SO3H/C relative to the Au/C and Au–SH/C catalysts, while the latter both catalysts develop active Au species with low efficiency showed rapid sintering of gold and longer induction period.24 Furthermore, we examine whether the above double-edged sword role is applicable for the other precious metals used as catalysts for this reaction. As reported by Hutchings et al.49, the



catalytic activity for hydrochlorination is correlated with the standard electrode potentials of the various metal/metal cations species. We tested different metal/metal cations including Pt, Pd, Ru, and Cu for acetylene hydrochlorination. Notably, these metals are known as reactive metal species, but their initial activity is much lower than that of the Au species.11 Therefore, we lowered the GHSV(C2H2) to 240 h–1 to obtain the distinct difference induced by the highor low-valence sulfur species. Figure S7 shows that for these four metal species, compared to the sulfur-free counterparts, the presence of sulfur in high oxidation state (–SO3H) promotes their activities, whereas the low-valence sulfur (–SH) inhibits their activities for the acetylene hydrochlorination reaction. XPS analysis was conducted to explore the decisive effect of the sulfur species on the catalytic activity. Figure S8 shows the results for different metal species over the various catalysts containing –SO3H before and after the hydrochlorination reaction. Similar results to Au–SO3H/C were observed for the Pt, Pd, Ru, and Cu metal species supported on –SO3H/C, with the nonmetallic properties retained after the hydrochlorination reaction (Table S5). 3.3. DFT Calculations and the Activity Improvement Mechanism. It is still unclear how the sulfur-containing moieties stabilize the catalytically active Au components or drive the mobility of Au species, and a detailed understanding of the reaction network and interaction between Au active sites and sulfur moieties is currently unavailable. Therefore, DFT calculations were performed to elucidate the mechanism of the Au components stabilization by the –SO3H/–SH groups and of the hydrochlorination reaction at the Au active sites. Presumably, AC treated by concentrated sulfuric acid (H2SO4) can be abundantly


Page 18 of 44

Page 19 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


functionalized by –SO3H and –OSO3H groups with the –OSO3H groups being labile and readily decomposed.44,50 Our DFT calculations on the chemical stability of the C14H9SO3H and C14H9OSO3H model compounds confirmed that the C–S bond between the –SO3H group and AC carbon site is thermally stable under the working conditions whereas cleavage of the C–O bond between the –OSO3H and AC carbon site occurs readily (see ESI). Furthermore, the –SO3H group is predicted to anchor and disperse AuCl3 by effectively preventing its dimerization (Eq. 1), affording AC–SO3HAuCl3 species 1 on the fresh Au–SO3H/C catalyst.

C14H9SO3H + 1/2Au2Cl6 → C14H9SO3HAuCl3 (ΔE= –8.3 kcal mol–1; ΔG298K = –2.2 kcal mol–1)

(1)

Further DFT calculations were performed to explore the catalytic process of acetylene hydrochlorination by using C14H9SO3HAuCl3 (1) as the molecular model of the fresh Au– SO3H/C catalyst (Figure 6). The as-formed AC–SO3HAuCl3 species 1 can directly react with acetylene to form vinyl chloride, followed by the addition of HCl to recover its original structure. As shown in Figures 6 and 7, the addition of both proton and anionic Cl ligand from species 1 to an incoming acetylene proceeds concertedly through either the cis-addition (transition state TS1) or trans-addition (transition state TS1) pathways, with the cis-addition pathway favored by 7.8 kcal mol–1. Desorption of C2H3Cl from the as-formed intermediate 2 (or 2) generates a chelate-structured species 3, AC–SO3 > AuCl2. Then, dissociative chemisorption of HCl on the chelate species 3 via TS3 ′ regenerates species 1. This catalytic cycle following a concerted addition mechanism has the overall free energy barrier



of 32.7 kcal mol–1, which is 8.5 kcal mol–1 higher than the activation barrier (~24.2 kcal mol–1, see insertion in Figure 7) required for the thermal desorption of HCl from species 1. That is, the chelate species 3 can be more effectively generated by thermal desorption of HCl from species 1 than by the concerted-addition reaction of C2H2 with species 1. Notably, after the chelate species 3 is formed, the adsorption of acetylene, rather than the aforementioned chemisorption of HCl, on the chelate species 3 occurs preferentially to form the [Au]–C2H2 d- complex 4 via transition state TS3, for which the free energy barrier is 3.5 kcal mol–1 lower than that of the dissociative chemisorption of HCl (13.4 kcal mol–1 vs. 16.9 kcal mol–1). Further nucleophilic addition of HCl to d- complex 4 gives rise to Au–vinyl chloride intermediate 5. Protonation of the vinyl chloride species in 5 can occur in an intramolecular manner via TS5 (G‡ ~14.3 Kcal/mol) to afford [Au]–chloroethene d- complex 6 or in a less favored intermolecular manner by the addition of another HCl via TS5 (G‡ ~19.5 kcal/mol) to afford another Au–chloroethene d- complex 6 with a coadsorbed HCl. Finally, the release of chloroethene directly from 6 with regeneration of 3 via TS6 (G‡ ~14.0 kcal/mol) is found to be favored over the ligand substitution pathway (via TS6, G‡ ~16.8 kcal/mol) by 2.8 kcal/mol in the activation free energy. In short, the calculations revealed that the chelate-structured AC–SO3>AuCl2 species 3, rather than the AC– SO3HAuCl3 species 1 available on the fresh AuCl3/AC–SO3H catalyst, acts as the key catalytic species in acetylene hydrochlorination and the working catalytic cycle includes the following steps: the activated adsorption of acetylene to 3 to form [Au]–C2H2 d- complex 4, addition of HCl to form chlorovinyl–[Au] intermediate 5, intramolecular protonation of


Page 20 of 44

Page 21 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


chlorovinyl to form [Au]–chloroethene d- complex 6, and release of chloroethene with regeneration of 3. Further DFT calculations revealed that the –SH group available on Au–SH/C is also capable of dispersing AuCl3, because the computed free energy (ΔG298K) of the AC–SH + 1/2Au2Cl6  AC–SH–AuCl3 reaction is 11.5 kcal mol1. This is consistent with the TEM (Figure S2) and XRD (Figure S3a) observations that no Au nanoclusters/NPs were present on the fresh Au-SH/C. However, our experiments showed that the Au+ species on the fresh Au–SH/C catalyst can be readily reduced, affording Au NPs on the used Au–SH/C catalyst. Therefore, we do not further explore the mechanism of acetylene hydrochlorination over the AC–SHAuCl3 species. 3.4. Mechanism Aspect of the Sulfur Double-edged Sword Function. To demonstrate the common principle of the double-edged role of sulfur moieties, we proposed a possible mechanism for the Au species interacting with sulfur at different valences to control and stabilize the ultrafine cationic Au nanoclusters, as indicated in Figure 8. Generally, the deposition of AuCl3 on a carbon support the surface of which contains abundant oxygen moieties (carbonyl and hydroxyl groups) leads to the direct interaction between Au3+ and oxygen atoms.49 The Au–O–C bond is unstable during the hydrochlorination reaction due to the effect of hard donor ligands and analogs induced by oxygen atoms, wherein C2H2 significantly reduces the Au3+ to Au0, leading to the rapid deactivation of Au/C (Figures 4a and 4f).11 Subsequently, the final used Au/C displayed an average particle size of 3.42 nm under



reaction conditions as verified by the TEM and XRD results (Figures 2, 3, and S3). Figure 8 illustrates that the approaching pathway is found in the middle line. For –SH doping on the carbon surface under 500 °C and protected by N2, the oxygen atoms are likely replaced by the sulfur atoms that serve as another kind of hard donor ligand.30 Following the deposition of AuCl on –SH/C, Au3+ cations will preferentially interact with S2– atoms because of the high electron density on the S2– atoms.41 Although no Au nanoclusters/NPs can be seen on the fresh Au–SH/C, as validated by the TEM and XRD results (Figures S2 and S3a), the reduction of Au3+ by S2– occurred as evidenced by the XPS analysis of the as-synthesized Au–SH/C (Figure S5c). During the hydrochlorination reaction, the highly dispersed but reduced Au0 species on Au-SH/C sharply migrate and form Au nanoclusters and then large Au NPs due to the rapid catalyst deactivation (Figures 2c and 3b). Finally, the used Au– SH/C catalyst exhibited the largest particle size of Au NPs. By contrast, the introduction of – SO3H groups on the carbon surface also induces the interaction between the oxygen atoms on the carbon with the –SO3H groups at high temperature. The Au3+ species arising from AuCl3 then preferentially interact with the oxygen atoms bound to sulfur atoms, as confirmed by DFT calculations (Figures 6 and 7). However, the added –SO3H in the carbon matrix can serve as a kind of ligands such as thiourea, thiosulfate, and thiocyanate that exist as soft donor atoms via the bonded oxygen atoms, leading to the remarkable stability for Au3+ and Au+.13 Nevertheless, under harsh reaction conditions, the used Au–SO3H/C catalyst exhibits a slight variation in the Au particle size, ultimately resulting in the appearance of ultrafine dispersed Au nanoclusters (1 nm), as schematically illustrated for the left pathway in Figure


Page 22 of 44

Page 23 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


8. Furthermore, we intentionally tune the oxidation states of sulfur on SH/C to verify the role of the various sulfur species in the Au-based catalytic performance by oxidizing S2– using H2O2 and concentrated H2SO4.45 Then, we found that the H2SO4-oxidized SH/C (Au3+ subsequently deposited on it) returns to the pathway of Au–SO3H/C, whereas H2O2 oxidized SH/C followed by supported Au3+ produces a rapid sintering of Au species for the formation of Sm2– (the pathway on the right in Figure 8).41 Taken together, the proposed schematic model presents a series of competing factors that influence the stabilization of uniform Au nanoclusters on the carbon.



CONCLUSION Our experiments and DFT calculations jointly show that the activity/stability of Au-based

catalysts in acetylene hydrochlorination can be tuned by changing the oxidation states of the sulfur species on the carbon. The Au–SO3H/C catalyst with the S6+ or S4+ sulfur species displays high activity and stability, whereas Au0 is preferentially formed in the presence of the S2– or Sm2– low-valence sulfur species on the Au-SH/C catalyst. The sulfur-containing groups act as a double-edged sword for the stabilization/destabilization of cationic Au components that are catalytically active for acetylene hydrochlorination. The results in this study provide essential understanding regarding how high-valence sulfur moieties that stabilize Au components with a nonmetallic nature can be used to obtain effective catalysts for acetylene hydrochlorination.





ASSOCIATED CONTENT

Supporting Information Experimental details including catalyst preparation, catalyst assessment, characterization technologies, and DFT calculations; TEM images of pristine carbon and its modified derivate; FT-IR spectra of carbon supports and their counterparts; XRD patterns of fresh and used catalysts; Raman spectra data; XPS date of various metal and sulfur; Au species or sulfur contents calculated by a fitting method derived from XPS; Catalytic performance of different metal catalysts.



AUTHOR INFORMATION

Corresponding Authors: * Youzhu Yuan, E-mail: [email protected] (Y. Y) Tel: +86-592-2181659. Fax: +86 592 2183047. * Xin Lu, E-mail: [email protected]. (X. L) Tel: +86-592-2181600. Fax: +86 592 2183047. Author Contribution: # These authors contributed equally to this work.



ACKNOWLEDGMENT We thankfully acknowledge the National Key Research and Development Program of

China (2017YFA0206801), the Natural Science Foundation of China (Nos. 21403178, 21473145, 21503173, 91545105 and 91545115), the National High-tech R&D Program


Page 24 of 44

Page 25 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(2015AA03A402, 20720150096), the Program for Innovative Research Team in Chinese Universities (No. IRT_14R31), and the Fundamental Research Funds for the Central Universities (20720170024).



REFERENCES

(1) Fu, Q.; Saltsburg, H.; Flytzani-Stephanopoulos, M., Active Nonmetallic Au and Pt Species on Ceria-Based Water-gas Shift Catalysts. Science 2003, 301, 935938. (2) Valden, M.; Lai, X.; Goodman, D. W., Onset of Catalytic Activity of Gold Clusters on Titania with the Appearance of Nonmetallic Properties. Science 1998, 281, 16471650. (3) Guzman, J.; Gates, B. C., Catalysis by Supported Gold: Correlation between Catalytic Activity for CO Oxidation and Oxidation States of Gold. J. Am. Chem. Soc. 2004, 126, 26722673. (4) Kone, I.; Xie, A.; Tang, Y.; Chen, Y.; Liu, J.; Chen, Y.; Sun, Y.; Yang, X.; Wan, P., Hierarchical Porous Carbon Doped with Iron/Nitrogen/Sulfur for Efficient Oxygen Reduction Reaction. ACS Appl. Mater. Interfaces 2017, 9, 2096320973. (5) Tian, W.; Zhang, H.; Duan, X.; Sun, H.; Tade, M. O.; Ang, H. M.; Wang, S., Nitrogenand Sulfur-Codoped Hierarchically Porous Carbon for Adsorptive and Oxidative Removal of Pharmaceutical Contaminants. ACS Appl. Mater. Interfaces 2016, 8, 71847193. (6) Wang, Y. C.; Lai, Y. J.; Song, L.; Zhou, Z. Y.; Liu, J. G.; Wang, Q.; Yang, X. D.; Chen, C.; Shi, W.; Zheng, Y. P.; Rauf, M.; Sun, S. G., S-Doping of an Fe/N/C ORR Catalyst



for Polymer Electrolyte Membrane Fuel Cells with High Power Density. Angew. Chem. Int. Ed. 2015, 54, 99079910. (7) Fierro-Gonzalez, J. C.; Gates, B. C., Catalysis by Gold Dispersed on Supports: The Importance of Cationic Gold. Chem. Soc. Rev. 2008, 37, 21272134. (8) Woodham, A. P.; Fielicke, A., Superoxide Formation on Isolated Cationic Gold Clusters. Angew. Chem. Int. Ed. 2014, 53, 65546557. (9) Flytzani-Stephanopoulos, M., Gold Atoms Stabilized on Various Supports Catalyze the Water–Gas Shift Reaction. Acc.Chem. Res. 2014, 47, 783792. (10) Zhang, X.; Shi, H.; Xu, B. Q., Catalysis by Gold: Isolated Surface Au3+ Ions are Active Sites for Selective Hydrogenation of 1,3-Butadiene over Au/ZrO2 Catalysts. Angew. Chem. Int. Ed. 2005, 44, 71327135. (11) Johnston, P.; Carthey, N.; Hutchings, G. J., Discovery, Development, and Commercialization of Gold Catalysts for Acetylene Hydrochlorination. J. Am. Chem. Soc. 2015, 137, 1454814557. (12) Conte, M.; Carley, A. F.; Heirene, C.; Willock, D. J.; Johnston, P.; Herzing, A. A.; Kiely, C. J.; Hutchings, G. J., Hydrochlorination of Acetylene Using a Supported Gold Catalyst: A Study of the Reaction Mechanism. J. Catal. 2007, 250, 231239. (13) Malta, G.; Kondrat, S. A.; Freakley, S. J.; Davies, C. J.; Lu, L.; Dawson, S.; Thetford, A.; Gibson, E. K.; Morgan, D. J.; Jones, W.; Wells, P. P.; Johnston, P.; Catlow, C. R. A.; Kiely, C. J.; Hutchings, G. J., Identification of Single-site Gold Catalysis in Acetylene Hydrochlorination. Science 2017, 355, 13991402.


Page 26 of 44

Page 27 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(14) Yin, X.; Huang, C.; Kang, L.; Zhu, M.; Dai, B., Novel AuCl3-Thiourea Catalyst with a Low

Au

Content

and

an

Excellent

Catalytic

Performance

for

Acetylene

Hydrochlorination. Catal. Sci. Technol. 2016, 6, 42544259. (15) Liu, B.; Yao, H.; Song, W.; Jin, L.; Mosa, I. M.; Rusling, J. F.; Suib, S. L.; He, J., Ligand-Free Noble Metal Nanocluster Catalysts on Carbon Supports via "Soft" Nitriding. J. Am. Chem. Soc. 2016, 138, 47184721. (16) Ke, J. H.; Zhao, Y. X.; Yin, Y.; Chen, K.; Duan, X. P.; Ye, L. M.; Yuan, Y. Z., Yttrium Chloride-Modified Au/AC Catalysts for Acetylene Hydrochlorination with Improved Activity and Stability. J. Rare Earths 2017, 35, 10831091. (17) Concepción, P.; Carrettin, S.; Corma, A., Stabilization of Cationic Gold Species on Au/CeO2 Catalysts under Working Conditions. Appl. Catal. A Gen. 2006, 307, 4245. (18) Li, X. N.; Yuan, Z.; He, S. G., CO Oxidation Promoted by Gold Atoms Supported on Titanium Oxide Cluster Anions. J. Am. Chem. Soc. 2014, 136, 36173623. (19) Camellone, M. F.; Fabris, S., Reaction Mechanisms for the CO Oxidation on Au/CeO2 Catalysts: Activity of Substitutional Au3+/Au+ Cations and Deactivation of Supported Au+ Adatoms. J. Am. Chem. Soc. 2009, 131, 1047310483. (20) Qiao, B.; Wang, A.; Yang, X.; Allard, L. F.; Jiang, Z.; Cui, Y.; Liu, J.; Li, J.; Zhang, T., Single-Atom Catalysis of CO Oxidation Using Pt1/FeOx. Nat. Chem. 2011, 3, 634641. (21) Yang, M.; Li, S.; Wang, Y.; Herron, J. A.; Xu, Y.; Allard, L. F.; Lee, S.; Huang, J.; Mavrikakis, M.; Flytzani-Stephanopoulos, M., Catalytically Active Au-O(OH)x-Species Stabilized by Alkali Ions on Zeolites and Mesoporous Oxides. Science 2014, 346,



14981501. (22) Jones, J.; Xiong, H. F.; Delariva, A. T.; Peterson, E. J.; Pham, H.; Challa, S. R.; Qi, G. S.; Oh, S.; Wiebenga, M. H.; Hernandez, X. I. P.; Wang, Y.; Datye, A. K., Thermally Stable Single-Atom Platinum-on-ceria Catalysts via Atom Trapping. Science 2016, 353, 150154. (23) Bürgel, C.; Reilly, N. M.; Johnson, G. E.; Mitrić, R.; Kimble, M. L.; Castleman, A. W.; Bonačić-Koutecký, V., Influence of Charge State on the Mechanism of CO Oxidation on Gold Clusters. J. Am. Chem. Soc. 2008, 130, 16941698. (24) Kim, S. M.; Kim, K. K.; Jo, Y. W.; Park, M. H.; Chae, S. J.; Duong, D. L.; Yang, C. W.; Kong, J.; Lee, Y. H., Role of Anions in the AuCl3-Doping of Carbon Nanotubes. ACS Nano 2011, 5, 12361242. (25) Solsona, B.; Conte, M.; Cong, Y.; Carley, A.; Hutchings, G., Unexpected promotion of Au/TiO2 by nitrate for CO oxidation. Chem. Commun. 2005, 23512353. (26) Mohapatra, P.; Moma, J.; Parida, K. M.; Jordaan, W. A.; Scurrell, M. S., Dramatic Promotion of Gold/titania for CO Oxidation by Sulfate Ions. Chem. Commun. 2007, 10441046. (27) Datta, K. K. R.; Reddy, B. V. S.; Ariga, K.; Vinu, A., Gold Nanoparticles Embedded in a Mesoporous Carbon Nitride Stabilizer for Highly Efficient Three-Component Coupling Reaction. Angew. Chem. Int. Ed. 2010, 49, 59615965. (28) Zhao, J.; Gu, S.; Xu, X.; Zhang, T.; Yu, Y.; Di, X.; Ni, J.; Pan, Z.; Li, X., Supported Ionic-Liquid-Phase-Stabilized Au(III) Catalyst for Acetylene Hydrochlorination. Catal.


Page 28 of 44

Page 29 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Sci. Technol. 2016, 6, 32633270. (29) Duan, X. P.; Yin, Y.; Tian, X. L.; Ke, J. H.; Wen, Z. J.; Zheng, J. W.; Hu, M. L.; Ye, L. M.; Yuan, Y. Z., C-X (X = Cl, Br, I) Bond Dissociation Energy as a Descriptor for the Redispersion of Sintered Au/AC Catalysts. Chin. J. Catal. 2016, 37, 17941803. (30) Liu, B.; Yao, H.; Song, W.; Jin, L.; Mosa, I. M.; Rusling, J. F.; Suib, S. L.; He, J., Ligand-Free Noble Metal Nanocluster Catalysts on Carbon Supports via “Soft” Nitriding. J. Am. Chem. Soc. 2016, 138, 47184721. (31) Becke, A. D., Density‐functional Thermochemistry. III. The Role of Exact Exchange. J. Chem. Phy. 1993, 98, 5648-5652. (32) Dolg, M.; Wedig, U.; Stoll, H.; Preuss, H., Energy‐Adjusted ab Initio Pseudopotentials for the First Tow Transition Elements. J. Chem. Phy. 1987, 86, 866-872. (33) Krishnan, R.; Binkley, J. S.; Seeger, R.; Pople, J. A., Self‐Consistent Molecular Orbital Methods. XX. A Basis Set for Correlated Wave Functions. J. Chem. Phy.1980, 72, 650-654. (34) Zhu, Z. H.; Lu, G. Q., Comparative Study of Li, Na, and K Adsorptions on Graphite by Using ab Initio Method. Langmuir 2004, 20, 10751-10755. (35) Lessard, J. D.; Valsamakis, I.; Flytzani-Stephanopoulos, M., Novel Au/La2O3 and Au/La2O2SO4 Catalysts for the Water-Gas Shift Reaction Prepared via an Anion Adsorption Method. Chem. Commun. 2012, 48, 48574859. (36) Manzoli, M.; Boccuzzi, F.; Trevisan, V.; Menegazzo, F.; Signoretto, M.; Pinna, F., Au/ZrO2 Catalysts for LT-WGSR: Active Role of Sulfates During Gold Deposition.



Appl. Catal. B Environ. 2010, 96, 2833. (37) Choi, C. H.; Kim, M.; Kwon, H. C.; Cho, S. J.; Yun, S.; Kim, H. T.; Mayrhofer, K. J.; Kim, H.; Choi, M., Tuning Selectivity of Electrochemical Reactions by Atomically Dispersed Platinum Catalyst. Nat. Commun. 2016, 7, 1092210933. (38) Wu, Z.; Jiang, D.-e.; Mann, A. K. P.; Mullins, D. R.; Qiao, Z.-A.; Allard, L. F.; Zeng, C.; Jin, R.; Overbury, S. H., Thiolate Ligands as a Double-Edged Sword for CO Oxidation on CeO2 Supported Au25(SCH2CH2Ph)18 Nanoclusters. J. Am. Chem. Soc. 2014, 136, 61116122. (39) Duan, X. P.; Tian, X. L.; Ke, J. H.; Yin, Y.; Zheng, J. W.; Chen, J.; Cao, Z. M.; Xie, Z. X.; Yuan, Y. Z., Size Controllable Redispersion of Sintered Au Nanoparticles by Using Iodohydrocarbon and Its Implications. Chem. Sci. 2016, 7, 31813187. (40) Rodriguez, J. A.; Dvorak, J.; Jirsak, T.; Liu, G.; Hrbek, J.; Aray, Y.; González, C., Coverage Effects and the Nature of the Metal−Sulfur Bond in S/Au(111): High-Resolution Photoemission and Density-Functional Studies. J. Am. Chem. Soc. 2003, 125, 276285. (41) Duan, X.; Teng, Y.; Wang, A.; Kogan, V. M.; Li, X.; Wang, Y., Role of Sulfur in Hydrotreating Catalysis over Nickel Phosphide. J. Catal. 2009, 261, 232240. (42) Weng, B.; Lu, K.-Q.; Tang, Z.; Chen, H. M.; Xu, Y.-J., Stabilizing Ultrasmall Au Clusters for Enhanced Photoredox Catalysis. Nat. Commun. 2018, 9, 15431553. (43) Zhu, F. X.; Wang, W.; Li, H. X., Water-Medium and Solvent-free Organic Reactions over a Bifunctional Catalyst with Au Nanoparticles Covalently Bonded to HS/SO3H


Page 30 of 44

Page 31 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Functionalized Periodic Mesoporous Organosilica. J. Am. Chem. Soc. 2011, 133, 1163211640. (44) Yang, Z.; Yao, Z.; Li, G.; Fang, G.; Nie, H.; Liu, Z.; Zhou, X.; Chen, X. a.; Huang, S., Sulfur-Doped Graphene as an Efficient Metal-free Cathode Catalyst for Oxygen Reduction. ACS Nano 2012, 6, 205211. (45) Flytzani-Stephanopoulos, M.; Gates, B. C., Atomically Dispersed Supported Metal Catalysts. Annu. Rev. Chem. Biomol. Eng. 2012, 3, 545574. (46) Zhao, J.; Wang, B.; Xu, X.; Yu, Y.; Di, S.; Xu, H.; Zhai, Y.; He, H.; Guo, L.; Pan, Z.; Li, X., Alternative Solvent to Aqua Regia to Activate Au/AC Catalysts for the Hydrochlorination of Acetylene. J. Catal. 2017, 350, 149158. (47) Yan, W.; Mahurin, S. M.; Pan, Z.; Overbury, S. H.; Dai, S., Ultrastable Au Nanocatalyst Supported on Surface-Modified TiO2 Nanocrystals. J. Am. Chem. Soc. 2005, 127, 1048010481. (48) Hong, G.; Tian, X.; Jiang, B.; Liao, Z.; Wang, J.; Yang, Y.; Zheng, J., Improvement of Performance of a Au-Cu/AC Catalyst using Thiol for Acetylene Hydrochlorination Reaction. RSC Adv. 2016, 6, 38063814. (49) Malta, G.; Freakley, S. J.; Kondrat, S. A.; Hutchings, G. J., Acetylene Hydrochlorination Using Au/carbon: A Journey towards Single Site Catalysis. Chem. Commun. 2017, 53, 1173311746. (50) Garg, B.; Bisht, T.; Ling, Y. C., Graphene-Based Nanomaterials as Heterogeneous Acid Catalysts: A Comprehensive Perspective. Molecules 2014, 19, 1458214616.




Page 32 of 44

Page 33 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure captions

Figure 1. Representative TEM images of typical samples coupled with energy dispersive X-ray mapping images of O (orange) and S (yellow): (a) –SO3H/C, (b) –SH/C; and their corresponding S 2p spectra of (c) –SO3H/C and (d) –SH/C, respectively.

Figure 2. Structures of the Au species on the spent catalysts. (a–c) TEM images indicating the presence of (a) ca. 0.8 nm Au clusters in used Au–SO3H/C, (b) ca. 3.4 nm Au clusters in used Au/C, and (c) Au NPs with the average particle size of ca. 8.2 nm in used Au–SH/C. (d– f) atomic resolution HAADF-STEM images provided for (d) used Au–SO3H/C, (e) Au/C, and (f) Au–SO3H/C. Meanwhile, the resulting histograms of the particle size are displayed in the insets of (d–f). (g) Representative XPS spectrum of Au 4f was deconvoluted with Au3+, Au+, and Au0 phases. (h) Proposed atomistic structure of the Au–SO3H/C catalyst based on the DFT analysis.

Figure 3. (a) Raman spectra of different sulfur-phase-modified carbon supports. (b) XRD patterns of used catalysts containing different sulfur moieties. (c) XRD patterns of heat-treated catalysts containing different sulfur phases under nitrogen flow at 350 °C for 4 h. (d) TPR profiles of different samples involving carbon supports and its supported Au catalysts.



Figure 4. Catalytic performance of various catalysts. (a) Catalytic activity of the sulfur-containing Au catalysts together with the Au/C catalyst. (b) Catalytic performance of the Au–SH/C catalysts reverse oxidized by concentrated H2SO4 and H2O2. (c) Acetylene conversion for various Au catalysts containing different sulfur phases (Au–SO3H/C and Au– SH/C) plotted versus sulfur content. (d) Deactivation rates of various sulfur-contained catalysts plotted versus sulfur content. (e) Catalyst durability of the three catalysts: Au– SO3H/C, Au/C, and Au–SH/C. Reaction conditions: 180 °C, 0.11 MPa, V(HCl)/V(C2H2) = 1.2, and GHSV(C2H2) = 360~1150 h1.

Figure 5. Typical HRTEM image (a) and corresponding HAADF–STM image of the used Au–SO3H/C catalyst, indicating the coexistence of noncrystalline (yellow circles) and nanocrystalline (red circles) clusters. Reaction conditions: 180 °C, 0.11 MPa, V(HCl)/V(C2H2) = 1.2, and GHSV(C2H2) = 1150 h1.

Figure 6. Optimized key structures for acetylene hydrochlorination reaction catalyzed by AuCl3 anchored by the sulfonic acid group (–SO3H) on the activated carbon surface. (Bond lengths are given in Å. Atom color: white, hydrogen; gray, carbon; orange, sulfur; red, oxygen; yellow, gold; green, chlorine.)

Figure 7. Energy profile of main (black line) and side (red line) reaction pathways for the acetylene hydrochlorination catalyzed by the AuCl3 anchored by the sulfonic acid group (–


Page 34 of 44

Page 35 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


SO3H) on the activated carbon surface. The inset depicts the energy profile of dissociative adsorption of HCl onto C14H9-SO3>AuCl2.

Figure 8. Schematic of the stabilization/destabilization induced by the sulfur double-edged sword effect. As indicated, the sulfur in the mode of –SO3H phase effectively ensures the high dispersion of Au nanoclusters, whereas –SH moiety rapidly destabilizes the Au clusters into nanocrystals, leading to the loss of their activity for the hydrochlorination reaction (light blue approach). Upon the reoxidation treatment of Au–SH/C by concentrated H2SO4 or 30 wt% H2O2, the catalytic activity was reversed to become nearly equal to Au–SO3H/C, even though H2O2-treated Au–SH/C is significantly deactivated compared with the Au–SH/C catalyst (dark red route). Red spheres: –SO3H moieties; purple spheres: oxygen-containing groups; green spheres: –SH moieties; gold spheres: gold species (nanoclusters or nanoparticles).



Figure 1. Representative TEM images of typical samples coupled with energy dispersive X-ray mapping images of O (orange) and S (yellow): (a) –SO3H/C, (b) –SH/C; and their corresponding S 2p spectra of (c) –SO3H/C and (d) –SH/C, respectively.


Page 36 of 44

Page 37 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. Structures of the Au species on the spent catalysts. (a–c) TEM images indicating the presence of (a) ca. 0.8 nm Au clusters in used Au–SO3H/C, (b) ca. 3.4 nm Au clusters in used Au/C, and (c) Au NPs with the average particle size of ca. 8.2 nm in used Au–SH/C. (d– f) atomic resolution HAADF-STEM images provided for (d) used Au–SO3H/C, (e) Au/C (e), and (f) Au–SO3H/C (f). Meanwhile, the resulting histograms of the particle size are displayed in insets of (d–f). (g) Representative XPS spectrum of Au 4f was deconvoluted with Au3+, Au+, and Au0 phases. (h) Proposed atomistic structure of the Au–SO3H/C catalyst based on the DFT analysis.



b

G

Intensity (a.u.)

D

SO3H/C SH/C C

c

1000

1500

2000

111

Au-SH/C 40

50

60

d

200

Au-SH/AC

50

60

2 Theta ()

70

80

70

638 Au-SO3H/C 265

606

SO3H/C Au/C 257 C

Au/C 40

Au/C

2 Theta ()

Au-SO3H/C

30

Au-SO3H/C

30

2500

1

Raman shift (cm )

Intensity (a.u.)

500

111 200

Intensity (a.u.)

a

Intensity (a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

80

150

300

450

600

Temperature (C)

750

Figure 3. (a) Raman spectra of different sulfur phase modified carbon supports. (b) XRD patterns of used catalysts containing different sulfur moieties. (c) XRD patterns of heat-treated catalysts containing different sulfur phases under nitrogen flow at 350 °C for 4 h. (d) TPR profiles of different samples involving carbon supports and its supported Au catalysts.


Page 38 of 44

Page 39 of 44

a Au/C

60 40 20

Au-SH/C

60 40 20

10 12 14 16 18 20 22 24

H2SO4-treated

80

H2O2-treated 10 12 14 16 18 20 22 24

Time on stream (h)

Time on stream (h) 2

4

6

8

d 10

Conversion (%)

2

RSO H = 0.858 kSO H = 20.8

80

3

3

60 40 20

2

RSH = 0.929 kSH = 10.1

0

e

0

1

2

SH content (%)

SO3H content (%)

1

12

3

4

2

3

4

0.7

1

100

SO3H content (%)

1

0

Deactivation rate (% h )

c

1.4

0.6

1.2 1.0

Au-SH/C

0.8

0.5

0.6

0.4

0.4

0.3

0.2 Au-SO3H/C 0.0

Deactivation rate (% h )

Au-SO3H/C

80

Conversion (%)

Conversion (%)

b

0

0.2

1

2

3

SH content (%)

1

1150 h

100

1

Conversion (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


360 h

80

Au-SO3H/C

60 40

Au/C 1

360 h

20

Au-SH/C

0 0

50

100

150

200

250

Time on stream (h)

300

Figure 4. Catalytic performance of various catalysts. (a) Catalytic activity of the sulfur-containing Au catalysts together with the Au/C catalyst. (b) Catalytic performance of the Au–SH/C catalysts reverse oxidized by concentrated H2SO4 and H2O2. (c) Acetylene conversion for various Au catalysts containing different sulfur phases (Au–SO3H/C and Au– SH/C) plotted versus sulfur content. (d) Deactivation rates of various sulfur-contained catalysts plotted versus sulfur content. (e) Catalyst durability of the three catalysts: Au– SO3H/C, Au/C, and Au–SH/C. Reaction conditions: 180 °C, 0.11 MPa, V(HCl)/V(C2H2) = 1.2, and GHSV(C2H2) = 360~1150 h1. 39 ACS Paragon Plus Environment


Figure 5. Typical HRTEM image (a) and corresponding HAADF–STM image (b) of the used Au–SO3H/C catalyst, indicating the coexistence of noncrystalline (yellow circles) and nanocrystalline (red circles) clusters. Reaction conditions: 180 °C, 0.11 MPa, VHCl/VC2H2 = 1.1, and GHSV(C2H2) = 1150 h1.


Page 40 of 44

Page 41 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 6. Optimized key structures for acetylene hydrochlorination reaction catalyzed by AuCl3 anchored by the sulfonic acid group (–SO3H) on the activated carbon surface. (Bond lengths are given in Å. Atom color: white, hydrogen; gray, carbon; orange, sulfur; red, oxygen; yellow, gold; green, chlorine.)



Figure 7. Energy profile of main (black line) and side (red line) reaction pathways for the acetylene hydrochlorination catalyzed by the AuCl3 anchored by the sulfonic acid group (– SO3H) on the activated carbon surface. The insert depicts the energy profile of dissociative adsorption of HCl onto C14H9-SO3 > AuCl2.


Page 42 of 44

Page 43 of 44 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 8. Schematic of the stabilization/destabilization induced by the sulfur double-edged sword effect. As indicated, the sulfur in the mode of –SO3H phase effectively ensures the high dispersion of Au nanoclusters, whereas –SH moiety rapidly destabilizes the Au clusters into nanocrystals, leading to the loss of activity for the hydrochlorination reaction (light blue approach). Upon the reoxidation treatment of Au–SH/C by concentrated H2SO4 or 30 wt% H2O2, the catalytic activity was reversed to become nearly equal to Au–SO3H/C, even though H2O2-treated Au–SH/C is significantly deactivates compared with the Au–SH/C catalyst (dark red route). Red spheres: –SO3H moieties; purple spheres: oxygen-containing groups; green spheres: –SH moieties; gold spheres: gold species (nanoclusters or nanoparticles).



Graphical abstract


Page 44 of 44

Sulfur Moiety as a Double-edged Sword for Realizing Ultrafine

Recommend Documents