Well-Defined Nanoparticles for Model Studies in Sustainable Industrial Chemistry

C H A P T E R

19 Well-Defined Nanoparticles for Model Studies in Sustainable Industrial Chemistry Iurii Suleimanov, Chlo
e Thieuleux University of Lyon, Institute of Chemistry of Lyon, UMR 5265-CNRS-UCBL-ESCPE Lyon, Laboratory of Chemistry, Catalysis, Polymers and Processes (C2P2), Villeurbanne, France

O U T L I N E 1 Introduction 2 General Methodology for the Preparation of Silane-Stabilized Nanoparticles 3 Silane-Stabilized Nanoparticle Catalysts for NOx Assisted Soot Oxidation 4 Silane-Stabilized Pt Nanoparticles for Alkene Hydrosilylation

399 400 402 407

1 INTRODUCTION The preparation of highly active and selective heterogeneous catalysts is of interest for the scientific community because 80% of industrial processes involve catalytic reactions. Heterogeneous catalysts are mainly composed of supported metal- or metal-oxide nanoparticles

Studies in Surface Science and Catalysis, Volume 178 https://doi.org/10.1016/B978-0-444-64127-4.00019-7

5 Silane-Stabilized Nickel Nanoparticles for the Low Temperature Dry Reforming of Methane 409 6 Silane-Stabilized Au NPs for Alkenes Epoxidation 412 7 Conclusion 413 References 413

(NPs) which are catalytically active sites. Such NPs are highly desirable if compared to bulk materials as they exhibit a large surface (where the catalysis takes place) to volume ratio. For such reasons, a huge effort has been directed toward the development of synthetic procedures that secure the dispersion, the size, and the shape of NPs onto supports. Of the classical techniques used in industry, one may mention (i) metal ion exchange reactions followed by

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# 2019 Elsevier B.V. All rights reserved.

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in situ reduction of the metal [1], (ii) decomposition of metal clusters [2], or (iii) chemical vapor deposition of metal compounds [3]. Such techniques that allow the successful incorporation of metallic NPs onto supports are not universal (they need fine tuning of the reaction conditions for each metal NPs and each support) and sometimes do not provide a sufficient control over the metal particle size or the particle’s distribution on the whole sample. This lack of control could be responsible for the premature sintering of the NPs in the course of catalysis, leading to the reduced productivity of the catalyst. As an alternative to these techniques, the development of methodologies allowing the low temperature preparation of well-defined NPs (composition, size, and shape) in solution and their further impregnation onto supports has been investigated [4]. In this field, many reports have shown the successful preparation of small NPs in solution by the reaction between an organometallic complex or salt (source of metal), stabilizing molecules that interact with the NPs and prevent their growth to bulk metals, or their further agglomeration in solution and a reducing agent (chemical reagents such as NaBH4 or H2, for instance). Standard approaches for attaining NPs stability in solution are based on creating electrostatic or/and steric repulsion barriers between particles using stabilizing molecules. Various organic capping agents have been used over several past decades for protecting metal NPs from agglomeration and subsequent precipitation. These molecules are generally polymers (for example, polyvinylalcohol, or polyvinylpyrrolidinone) or functional ligands containing a polar “head” often formed by dSH, dNH2, dCOOH, dPR2, or dOH functional groups and an apolar “tail” formed by a long aliphatic chain. For example, long chain n-alkylthiols have been widely used for capping gold NPs in solution. However, when polymers are used, their incomplete removal may occur at low temperature, leading

to less productive catalysts. At the reverse, when high temperature treatments are applied, metal NPs sintering can be observed. Concerning the use of smaller functional ligands, which are less strongly adsorbed to the metal surface, agglomeration issues can be encountered and the presence of functional groups (as those mentioned above) may lead to metal surface poisoning (for instance when using sulfur-containing ligands for the stabilization of gold NPs). To overcome these issues, organosilanes were used as stabilizing ligands and co-reductant because: (i) they strongly interact with a wide range of metals via Si-M bonds allowing the formation of very small metal NPs, which were found extremely stable in solution, (ii) they do not lead to metal poisoning as compared to other functional ligands [5], and (iii) they can be easily removed after the impregnation of NPs onto supports by thermal treatments leading to the co-deposition of silica patches that were inert or beneficial for catalysis by changing the metal-support interactions [6]. This chapter will therefore focus mainly on the development of well-defined NPs in solution using silanes as stabilizing ligands and their use to generate heterogeneous catalysts for industrially relevant reactions.

2 GENERAL METHODOLOGY FOR THE PREPARATION OF SILANESTABILIZED NANOPARTICLES As introduced previously, the use of silanes— and more particularly long chain n-alkylsilanes— seems to be a promising alternative to already well-established capping agents. The first studies concerning the utilization of silyl hydrides for metal surface functionalization were reported in the late 1990s to early 2000s. All these studies refer to clean gold surfaces on which silyl hydrides were chemisorbed [7–11]. It has also been shown that primary n-alkylsilanes could

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2 GENERAL METHODOLOGY FOR THE PREPARATION OF SILANE-STABILIZED NANOPARTICLES

be covalently bonded to some transition metal surfaces containing native oxide coverings through the formation of MdOdSi bonds and dihydrogen release [12]. The reaction takes place in heptane at 65–70°C for 48 h. Octadecylsilane was used as a digestive ripening agent to convert a highly polydispersed gold colloid into a nearly monodispersed one [13]. Silyl hydrides have pronounced reducing properties and are widely used in organic synthesis for the reduction of different functional groups [14]. In this way, primary long chain n-alkylsilanes can be promising candidates for the preparation of metallic NPs acting at the same time as reducing and stabilizing agents. The general route for the preparation of silanestabilized metal NPs is illustrated in Fig. 19.1. The first example concerning the use of primary silanes for the synthesis of noble metal NPs was demonstrated in 2004 with the preparation of small crystalline Ru NPs [15]. The authors have reported the decomposition of a ruthenium organometallic precursor, namely [Ru (COD) (COT)], where COD is 1,3-cyclooctadiene and COT is 1,3,5-cyclooctatetriene. One equiv. of n-octylsilane in pentane was added to a solution of the precursor in pentane at 193 K, pressurized with 3 bars of H2 and left to slowly reach room temperature. This procedure led to the formation of Ru NPs of
2.3 nm in diameter with a narrow-size distribution. In addition to this example, other noble metal particles were prepared using silanes but the experimental conditions were not optimized and led to bigger NPs with broader size distributions. For instance, palladium NPs were prepared in toluene using

FIG. 19.1

General scheme for the synthesis of silanestabilized NPs.

401

Pd(OAc)2 (OAc ¼ acetate) and n-octadecylsilane in a 1:4 M ratio. [16] TEM analysis revealed the presence of small Pd NPs (<10 nm) aggregates of worm-like morphology. The authors mentioned that the Pd colloid was only stable for 6 h, while after 24 h a significant amount of precipitate was observed. In order to further stabilize the colloid, the addition of an external stabilizing agent, such as triphenylphosphine or trioctylamine, was needed. With a similar method, four equivalents of n-octadecylamine were used as reductant and capping agents in toluene to yield smaller NPs of ca. 3 nm in diameter and a narrow size distribution. Polymeric silyl hydrides were also used to synthesize metal NPs: Pd NPs were obtained by mixing Pd(OAc)2 with five equivalents of polymethylhydrosiloxane (PMHS) in benzene at room temperature [17]. The addition of this PMHS compound led to the complete disappearance of Pd(OAc)2 within 5 min with gas evolution, presumably H2. The average particle size was found to be ca. 6 nm. A catalyst poisoning test showed that these NPs were active in silylesterification reactions. Moreover, after separation and washing, the Pd NPs exhibited a similar catalytic activity to that of those generated in situ. In the same vein, rationally designed siloxane polymers bearing hydrido, silanol, and cyanopropyl groups were also employed to synthesize and stabilize Pd NPs [18]. In this case, SidH groups acted as reducers and cyanopropyl groups coordinated to the NPs surface to prevent their agglomeration. Pd(OAc)2 was used to prepare samples with different metal loadings and different CN/Pd ratios. Small NPs of
1.3 nm were obtained for metal loadings going up to 5%wt keeping CN/Pd ratio around 10. The particle sizes for a given loading could be tuned by varying the quantity of CN groups in the polymer—the lower CN/Pd ratio, the bigger particles are formed. The authors also suggested that, beside their role of reducing agents, the SidH groups may

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also favor the stabilization of small particles along with the CN groups. Concerning noble metal NPs, Pt NPs were also yielded in situ during the reaction of alkene hydrosilylation with low valent Pt-olefin complexes. It was demonstrated that a stable Pt colloid was formed from Pt(COD)Cl2 (COD ¼ 1,5-cyclooctadiene) and (EtO)3SiH (EtO ¼ ethoxy) in CH2Cl2 with a mean size of 2.3 nm [19]. Interestingly, Pt(COD)2 and (EtO)3SiH in benzene yielded bigger NPs. Both colloids showed higher turnover frequency (TOF) than Pt(COD)Cl2 and Pt(COD)2 and thus the colloid formation was proposed as an important step in the hydrosilylation mechanism (vide infra). More recently, small and well-defined Pt NPs were obtained from Pt(dba)2 (dba¼ dibenzylideneacetone) under mild conditions (3 bars of H2, 20°C) [20,21]. Different amounts of n-octylsilane—a stabilizing agent—were used, resulting in 2 nm-sized NPs. Spherical and crystalline NPs were obtained when 0.5 equivalent of n-octylsilane was used. Complete hydrogenation of styrene was achieved using these NPs with TOF of 36 s1, whereas particles stabilized with 4-hexadecylaniline showed TOF of about 10 s1. Concerning nonnoble metal NPs, silver heptafluorobutyrate in toluene was used as a precursor for silver NPs [22]. A fourfold amount of n-octadecylsilane was added at room temperature and the NPs formation was complete within 1.5 h. TEM analysis carried out at different reaction times revealed particles of 4–8 nm size at the beginning of the reaction and two major populations with an average size of 5–8 nm and 9–15 nm at the end of the reaction. It should be mentioned that even with four equivalents of n-octadecylsilane, the Ag NPs were prone to precipitate after 24 h of storage, even though they could be easily re-dispersed with vigorous stirring. Shorter chain silanes have also been tested and were found to be less efficient stabilizers for Ag NPs. Copper NPs were also synthesized from Cu(OAc)2 using n-octadecylsilane as a reductant in different

conditions [23]. Here, Cu NPs with a broad size distribution were embedded in a polymerized octadecylsilane micelles network using 10 M equivalents of silane at room temperature. The resulting suspension was not stable toward precipitation and a significant amount of precipitate was observed after 48 h. Interestingly, an increase in temperature to 110°C, using the same copper precursor-to-silane ratio, led more rapidly to smaller Cu NPs (1 h reaction time), but still with a broad size distribution from 2 nm to 12 nm. Beside these examples, the preparation of highly stable (more than 6 months) and small NPs of Pt, Au, Cu, and Ni (from 1.5 to 3 nm, depending on the metal) could be obtained by finely tuning the experimental conditions and the metallic precursors [5,6,24–26]. Their small size and their important stability prompted the authors to use them as catalysts (in solution or supported onto supports) for relevant catalytic reactions. Their achievements will be described in the next paragraphs.

3 SILANE-STABILIZED NANOPARTICLE CATALYSTS FOR NOx ASSISTED SOOT OXIDATION The amount of particulate matter (PM) and nitrogen oxides (NOx) contained in car emissions is being progressively restricted by European Union directives. This incited current car manufacturers to search for new efficient pollutant-abatement systems. One of the possible solutions to fight against PM and NOx emission is a catalytic oxidation process. PM hereinafter referred to as “soot,” while being removed via oxidation reaction, needs active oxygen participation. Even mediated with catalysts, this reaction occurs effectively at temperatures above 550°C [27,28]. In order to lower the soot combustion temperature, an approach involving NOx as a third reagent was proposed [28,29]. In these conditions, an efficient oxidation

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3 SILANE-STABILIZED NANOPARTICLE CATALYSTS FOR NOx ASSISTED SOOT OXIDATION

catalyst was required. Here, NOx assisted soot oxidation catalysts based on Pt, CuO, and Pt3Sn NPs were chosen and supported on CeO2. The Pt and CuO NPs were prepared using silanes as stabilizing ligands whereas SnH(n-Bu)3 was used to generate the Pt3Sn alloyed NPs (vide infra). Two cerium dioxide supports—a spongy and microstructured ceria prepared by solution combustion synthesis (referred to as CeO2-SCS), and CeO2 nanocubes (referred to as CeO2-NC)— were prepared. Pt NPs were prepared by reaction of Pt(dba)2 with n-octylsilane in Tetrahydrofuran (THF) under 3 bars of H2 [30]. Pt3Sn NPs were first obtained by contacting Pt(dba)2 with SnH(n-Bu)3 in THF [31]. TEM images of the as-obtained Pt and Pt3Sn NPs are shown in Fig. 19.2. One may notice that the stabilization of Pt NPs by organosilanes resulted in a narrower particle size distribution compared to Pt3Sn NPs. The resulting NPs were deposited on ceria supports by wetness impregnation (WI), dried in

20

air at 120°C for 2 h and further calcined at 320°C for 10 h yielding catalysts with Pt loading ca. 0.9%wt for Pt/CeO2. 0.22 and 0.74%wt of Pt were found for Pt3Sn/CeO2-SCS, and Pt3Sn/ CeO2-NC, respectively. The supported NPs were found to be well dispersed on the ceria and their sizes were slightly bigger than those found for the colloidal solutions. The catalysts were fully characterized by X-ray photoelectron spectroscopy to identify the oxidation states of Ce and Pt. These four catalysts were examined in oxidation reactions of CO, NO, soot, and NOx-assisted soot oxidation. Pt/CeO2-SCS catalyst demonstrated the highest activity among the series in CO oxidation reaction reaching 100% conversion at about 80°C. In the NO oxidation reaction both Pt/CeO2 catalysts demonstrated similar activity with NO conversion of ca. 53% at 400°C however, the specific rate was found three times higher for the NC sample than for the SCS one. The two Pt3Sn samples showed moderate NO conversion that

20

C

12

12

Count

16

Count

16

8

0

8 4

4

10 nm

D

0

1

2

3

4

10 nm

Particle diameter (nm)

FIG. 19.2

0

0

1

2

3

4

Particle diameter (nm)

TEM images of Pt (A) and Pt3Sn (B) NPs with their size distributions (C and D respectively). Data from T. Andana, M. Piumetti, S. Bensaid, L. Veyre, C. Thieuleux, N. Russo, D. Fino, E.A. Quadrelli, R. Pirone, Ceria-supported small Pt and Pt3Sn nanoparticles for NOx-assisted soot oxidation, Appl. Catal. B Environ. 209 (2017) 295–310.

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could be explained by reduced NO adsorption due to the presence of Sn along with the lower Pt loading. Soot oxidation experiments were conducted in the presence and in the absence of NOx. The NOxfree experiment showed that Pt/CeO2-SCS had the same activity as the neat CeO2-SCS support, whereas Pt/CeO2-NC was slightly more active compared to CeO2-NC. These surprising results might be explained by a mechanism proposed in [32] where Pt NPs capture and transfer oxygen to the ceria surface. Therefore, the low activity may come either from the low O2 adsorption rate or the low spillover rate. Pt3Sn alloyed NPs deposited on CeO2-NC phase were found to be the most active catalysts in the series with the

highest rate constant. It was proposed that Sn might prevent Pt from CO poisoning, thus favoring free active sites for the O2 adsorption. Also, PtdSn bimetallic alloys demonstrated higher oxygen chemisorption with increasing Sn content [33]. Finally, in the NOx-assisted soot oxidation Pt/ CeO2-NC catalyst was found to be the most active in the case of the copper-oxide based catalysts. Cu NPs were produced by the reaction of mesitylcopper (I) with n-octylsilane under 3 bar of H2 (Eq. 19.1). The obtained Cu NPs were characterized by TEM analysis (see Fig. 19.3). Using this route, small Cu NPs with a mean diameter of 3 nm and a narrow size distribution ranging from 2 nm to 4 nm were obtained [26].

FIG. 19.3 TEM image and size distribution of Cu NPs. Data extracted from T. Andana, M. Piumetti, S. Bensaid, L. Veyre, C. Thieuleux, N. Russo, D. Fino, E.A. Quadrelli, R. Pirone, CuO nanoparticles supported by ceria for NOx-assisted soot oxidation: insight into catalytic activity and sintering, Appl. Catal. B Environ. 216 (2017) 41–58.

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405

3 SILANE-STABILIZED NANOPARTICLE CATALYSTS FOR NOx ASSISTED SOOT OXIDATION

SYNTHESIS OF SILANE-STABILIZED Cu NPs

ð19:1Þ

The Cu NPs in solution were further impregnated onto the two aforementioned ceria supports using the WI method. The resulting heterogeneous catalysts contained ca. 1 and 3%wt of Cu. Before catalysis, the solids were dried under air and calcined at 320°C for 10 h resulting into CuO-NPs/CeO2 catalysts. The catalysts were characterized by TEM coupled with Energy Dispersive X-ray (EDX) analysis. It should be noted that ceria-supported CuO NPs did not show a high contrast in TEM images because Cu is lighter than Ce. Nevertheless, it was found that the CuO NPs size supported on CeO2-SCS remained in the range of 2 nm to 5 nm. At the reverse, the CeO2-NC-supported CuO NPs appeared much bigger (up to 10 nm) than the initial Cu colloid. The sintering of the Cu NPs was correlated to the small surface area of CeO2-NC phase with respect to CeO2-SCS. The EDX mapping confirmed the homogeneous distribution of Cu over the CeO2-SCS phase as well as the presence of nonsupported CuO NPs in the case of CeO2NC. The oxidation state of copper and cerium was studied through X-ray Photoelectron Spectroscopy (XPS). Reducibility of the catalysts was studied by temperature-programmed reduction (TPR) technique and H2 uptake was measured. After full characterization of the as-obtained catalysts, their catalytic performances in CO oxidation reaction were studied (see Fig. 19.4). The catalyst using CeO2-SCS as support appeared more active than that using CeO2-NC. At the same time, it was shown that the presence

FIG. 19.4 CO conversion vs. temperature. Data from T. Andana, M. Piumetti, S. Bensaid, L. Veyre, C. Thieuleux, N. Russo, D. Fino, E.A. Quadrelli, R. Pirone, CuO nanoparticles supported by ceria for NOx-assisted soot oxidation: insight into catalytic activity and sintering, Appl. Catal. B Environ. 216 (2017) 41–58.

of CuO NPs clearly improved the catalytic activity of both cerium phases by lowering the reaction temperature by ca. 200°C (CeO2-SCS samples). CeO2-SCS supported catalysts in general showed better performances than that of their CeO2-NC counterparts. For instance, 3%wt Cu/CeO2-SCS was the most active catalyst from the series. To further confirm the superiority of these copper

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FIG. 19.5

Molar percentage of NO2 vs. temperature for Cu/ceria catalysts in the absence of soot (A, C) and in the presence of soot (B, D). Data from T. Andana, M. Piumetti, S. Bensaid, L. Veyre, C. Thieuleux, N. Russo, D. Fino, E.A. Quadrelli, R. Pirone, CuO nanoparticles supported by ceria for NOx-assisted soot oxidation: insight into catalytic activity and sintering, Appl. Catal. B Environ. 216 (2017) 41–58.

ceria catalysts, they were tested in NO oxidation with and without soot (see Fig. 19.5). The CeO2-SCS-based catalysts demonstrated higher NO conversion compared to the CeO2NC homologues. In particular, Cu/CeO2-SCS (3%wt) showed the broadest reaction temperature window and the highest NO2 percentage of ca. 60%. This catalyst was found more active in terms of NO2 percentage than the previously

described Pt/CeO2-SCS [26,30]. It worth noting that the Cu/CeO2-NC catalyst showed the lowest activity which might be correlated to the sintering of the Cu NPs into large CuO NPs. NOx-assisted soot oxidation tests revealed a high activity for the 3%wt Cu/CeO2-SCS catalyst in a temperature range of 300–500°C while at higher temperatures, 1%wt Cu/CeO2-NC catalysts dominated (Fig. 19.6).

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4 SILANE-STABILIZED Pt NANOPARTICLES FOR ALKENE HYDROSILYLATION

407

FIG. 19.6 Soot conversion vs. temperature for the ceria supported catalysts in the absence of NO (A, C) and in the presence of NO (B, D). Data extracted from T. Andana, M. Piumetti, S. Bensaid, L. Veyre, C. Thieuleux, N. Russo, D. Fino, E.A. Quadrelli, R. Pirone, CuO nanoparticles supported by ceria for NOx-assisted soot oxidation: insight into catalytic activity and sintering, Appl. Catal. B Environ. 216 (2017) 41–58.

4 SILANE-STABILIZED Pt NANOPARTICLES FOR ALKENE HYDROSILYLATION Hydrosilylation of alkene is a key industrial reaction to prepare functional silanes and fluids [34]. In industry, Pt complexes, of which the well-known Speier’s catalyst (H2PtCl6 in isopropanol) and Karstedt’s complex (Pt2[(Me2SiCH] CH2)2O]3), are used to catalyze this reaction in batch reactors. However, due to the high price of such noble metal, huge efforts have been

directed toward the replacement or the decrease of platinum lost in such reactions [35]. To decrease platinum lost, and to develop even more efficient catalysts, understanding the structure of the catalytically active species and the reaction mechanism is crucial. However, the nature of the active site (isolated Pt atom in solution or NPs) involved in the reaction is still under debate and has been object of controversy for more than 30 years [36]. Some early reports suggested that platinum colloids were the active species and not a

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deactivated form of the catalyst [37–39] whereas other publications reported that complexes [40–43] in solution were catalyzing the reaction without the formation of NPs showing that colloid formation was not necessary. In this context, Lewis’s group showed that colloids were formed when using the Karstedt complex and were at the origin of the observed induction period during catalysis [19,44,45]. However, a few years after, this group showed by in situ Extended X-Ray Absorption Fine Structure (EXAFS) that Pt species in the course of catalysis presented only PtdSi and PtdC bonds, thus ruling out the formation of colloids in their experimental conditions (low substrate/catalyst ratios with maximum Turnover number (TON) of 1000) [41]. It is worth noting that such experimental conditions are very different from those used in industrial processes where TONs are about 105. In this context, recent publications investigated the use of Pt colloids stabilized by silane ligands as catalysts in industrially relevant conditions. The authors prepared three colloids at room temperature in toluene using different Pt complexes (Pt(dba)2 (dba ¼ dibenzylideneacetone) or Karstedt’s complex) and n-octylsilane or a polymethylhydrosiloxane (PMHS) containing approximately 50 SiH units as stabilizers under 4 bars of dihydrogen. When testing these colloids that exhibit similar Pt NPs (crystalline and 1.6 nm sized) in the hydrosilylation of 1-octene with PMHS,

TONs higher than 105 were rapidly obtained with similar selectivities than that observed when using the Karstedt’s complex in solution [24] (Table 19.1). These results clearly show that silanestabilized NPs are as efficient as Karstedt’s complex and not a deactivation form of the catalyst, and prompted the author to develop a heterogeneous catalyst containing Pt NPs. Obtaining highly active heterogeneous catalysts would present the advantage of obtaining platinum-free and colorless hydrosilylation products along with the possibility to develop continuous flow processes with reusable catalysts. The challenge here was to generate a highly active solid catalyst (at least as active as complexes in solution) that would be stable toward platinum leaching, which is a recurrent problem in hydrosilylation reactions. This goal was reached by embedding silane-stabilized platinum NPs in the walls of a mesostructured silica framework (see Fig. 19.7). The catalyst was prepared by the controlled growth of a silica framework by sol-gel process around chloropropylsilane-stabilized hydrophilic Pt NPs using a Pluronic 123 surfactant as the silica structure-directing agent, followed by calcination under dry air at 320°C [46]. This methodology allowed to secure the location of the Pt NPs in the silica pores [21] or walls [46] depending on the hydrophilic/hydrophobic character of these NPs. The resulting solid containing 0.3%wt of Pt was characterized by several techniques

TABLE 19.1

Hydrosilylation of 1-Octene and PMHS Using Pt Colloids and Pt Complexes in Batch Reactor

Catalyst

Pt Precursor

Ligand

Mean Size (nm)

SiH Conv. (TON) 0.5 ppm of Pt

1-Octene Isomerization (%)

Pt(dba)2

None

None

–

86% (1.5  106)

12

None

–

87% (1.5  10 )

12

n-Octylsilane

77% (1.3  10 )

11

PMHS

84% (1.5  10 )

11

n-Octylsilane

81% (1.4  10 )

11

Karstedt Colloid 1 Colloid 2 Colloid 3

None Pt(dba)2 Karstedt Karstedt

6 6 6 6

Data from T. Galeandro-Diamant, M.-L. Zanota, R. Sayah, L. Veyre, C. Nikitine, C. de Bellefon, S. Marrot, V. Meille, C. Thieuleux, Platinum nanoparticles in suspension are as efficient as Karstedt’s complex for alkene hydrosilylation, Chem. Commun. 51 (2015) 16194–16196.

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5 SILANE-STABILIZED NICKEL NANOPARTICLES FOR THE LOW TEMPERATURE DRY REFORMING OF METHANE

409

FIG. 19.7

STEM-HAADF micrograph of the catalyst containing Pt NPs in the silica walls. Data from T. Galeandro-Diamant, R. Sayah, M.-L. Zanota, S. Marrot, L. Veyre, C. Thieuleux, V. Meille, Pt nanoparticles immobilized in mesostructured silica: a nonleaching catalyst for 1-octene hydrosilylation, Chem. Commun. 53 (2017) 2962–2965.

such as N2 adsorption at 77 K, H2 chemisorption, and TEM (bright field and High-Angle Annular Dark-Field Scanning Transmission Electron Microscopy (STEM-HAADF)). The material was found to be mesoporous with 6 nm-sized pores and a specific surface area of about 800m2/g. The supported platinum NPs exhibited a mean diameter of 2.8 nm with a narrow size distribution. This solid, along with Pt/SiO2 standards (Pt NPs on fumed silica or in the pores of a mesoporous silica framework), were tested in hydrosilylation of 1-octene with PHMS. The results showed that this specific material was able to catalyze the reaction with TON of ca. 105 without platinum leaching as shown in Fig. 19.8 [25]. This result is in contrast with literature precedents that reported either active solids containing Pt NPs which: (i) needed harsh conditions and exhibited lower selectivity toward the desired product [47], (ii) reached lower TON

[48], or (iii) led to significant Pt leaching [49,50]. This solid also outperformed reported catalysts containing supported Pt(0) complexes in term of productivity (TON) [51–53].

5 SILANE-STABILIZED NICKEL NANOPARTICLES FOR THE LOW TEMPERATURE DRY REFORMING OF METHANE Steam reforming of natural gas is a key reaction to yield syngas because of its high H2/CO ratio (syngas ratio, cf. Eq. 19.1). However, for the needs of off-stream processes, such as Fischer-Tropsch [54–56], CO2 reforming of methane (also called dry reforming of methane) is needed to tune this H2/CO ratio and provide lower syngas composition (Eqs. 19.2 and 19.3) [57]. Moreover, due to the uprising worldwide concern toward global warming and the

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FIG. 19.8 (left) TON and Pt leaching comparison for various heterogeneous Pt catalysts after 5 h of reaction; (right) Split test (hot filtration) during a semibatch test of the catalyst containing Pt NPs in the silica walls. The period in gray, from 0 to 52 min, is the addition of PMHS in the reactor. Data from T. Galeandro-Diamant, R. Sayah, M.-L. Zanota, S. Marrot, L. Veyre, C. Thieuleux, V. Meille, Pt nanoparticles immobilized in mesostructured silica: a non-leaching catalyst for 1-octene hydrosilylation, Chem. Commun., 53 (2017) 2962–2965.

installation of carbon tax, the dry reforming of methane has become more and more attractive as it consumes CH4 and CO2 (two major greenhouse gases) to generate syngas. CH4 + H2 O>CO + 3 H2

(19.2)

CH4 + CO2 >2 CO + 2 H2

(19.3)

Of the potential candidate catalysts to perform this reaction, nickel-based catalytic systems were preferred to noble metals for economic reasons even if they exhibited a lower stability [58–62]. However, despite a tremendous research effort, increasing nickel dispersion above 35%–50% (i.e., a diameter of 3–2 nm) and particle stability are still very challenging [63–66]. In this context, the preparation of very small nickel colloids using silane ligands was investigated. The nickel colloids were obtained by contacting a solution of Ni(COD)2 in THF with n-octylsilane under 3 bars of H2 at 55–65°C. The resulting black suspension showed a remarkable stability toward NPs agglomeration (more than 2 months without sedimentation). Using several analytical techniques (monitoring of the reaction by IR, liquid state 1H

NMR, diffusion-ordered NMR spectroscopy (DOSY) and XANES/EXAFS experiments), it was shown that the resulting NPs were in fact very small aggregates (1.3 nm) of amorphous nickel silicides clusters stabilized by surface silicon-alkyl groups. When these nickel silicides NPs were exposed to dry air, a fast color change from black to slightly yellow was observed. This was correlated to the oxidation of nickel silicide NPs into nickel oxide (NiO) as confirmed by in situ XAS experiments. Analyses of the resulting NPs by TEM showed the presence of small isolated particles of ca. 2.4 nm (see Fig. 19.9). The resulting NiO NPs were further supported onto silica by the Incipient Wetness Impregnation (IWI) method and subjected to dihydrogen at 500°C. The resulting solid containing ca. 0.8%wt of Ni was fully characterized by EXAFS, TEM (see Fig. 19.10) and H2 chemisorption. The results showed the presence of very small metallic nickel NPs of 1.3 nm size on the silica support. The solid containing these very small nickel NPs was further tested in dry reforming of methane at 500°C. This catalyst presented a very

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5 SILANE-STABILIZED NICKEL NANOPARTICLES FOR THE LOW TEMPERATURE DRY REFORMING OF METHANE

411

FIG. 19.9 TEM image and the corresponding particle size distribution of the Ni based colloids after exposure to dry air at room temperature. Data from D. Baudouin, K.C. Szeto, P. Laurent, A. De Mallmann, B. Fenet, L. Veyre, U. Rodemerck, C. Cop
eret, C. Thieuleux, Nickel-silicide colloid prepared under mild conditions as a versatile Ni precursor for more efficient CO2 reforming of CH4 catalysts, J. Am. Chem. Soc. 134 (2012) 20624–20627.

FIG. 19.10 Pictures of the Ni based catalyst after reduction at 500°C under H2. Data from D. Baudouin, K.C. Szeto, P. Laurent, A. De Mallmann, B. Fenet, L. Veyre, U. Rodemerck, C. Cop
eret, C. Thieuleux, Nickel-silicide colloid prepared under mild conditions as a versatile Ni precursor for more efficient CO2 reforming of CH4 catalysts, J. Am. Chem. Soc. 134 (2012) 20624–20627.

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high activity when compared to a conventional Ni/SiO2 catalyst as expected due to its very high Ni dispersion [67]. However, the tested catalysts also exhibited a drastic deactivation. The Ni silicide colloid was thus supported on cerium oxide; as such, support is known to provide a better stability to the Ni NPs. The catalyst was prepared using the same IWI technique and exposed to dihydrogen at 500°C. The resulting solid was compared with a conventional Ni/CeO2 catalyst exhibiting similar Ni dispersion (54% for the new solid vs. 47% for conventional Ni/CeO2 sample) in dry reforming of methane at 500°C. Despite an increased stability for both solids, the solid obtained via the impregnation of Ni silicide colloid also displayed an activity 34% higher than that of the traditional catalyst, thus suggesting that the development of catalysts using a colloidal approach and silanes as stabilizing agents is of interest to generate more active and stable catalysts [6].

6 SILANE-STABILIZED Au NPs FOR ALKENES EPOXIDATION Epoxidation of unsaturated hydrocarbons is an important area of academic and industrial research as epoxides give direct access to a variety of products [68]. Promising results were found for the catalytic epoxidation of alkenes with gold NPs [5,69–72]. Silane-stabilized Au NPs were prepared by reduction of HAuCl4 in a biphasic system using tetraoctylammonium bromide as a phase transfer agent and n-octylsilane as a stabilizing agent (see Fig. 19.11). Small gold NPs of ca. 2 nm size were obtained as shown by TEM (see Fig. 19.12). Two more samples used as standards were obtained following

FIG. 19.11

Synthesis of silane-stabilized Au NPs.

FIG. 19.12

TEM image of the silane-stabilized Au NPs. Data from M. Boualleg, K. Guillois, B. Istria, L. Burel, L. Veyre, J.-M. Basset, C. Thieuleux, V. Caps, Highly efficient aerobic oxidation of alkenes over unsupported nanogold, Chem. Commun. 46 (2010) 5361.

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413

FIG. 19.13 Conversion (squares) and yield (circles) for the trans-stilbene epoxidation reaction using Au NPs stabilized with n-octylsilane (red), n-octylthiol (green), and tetraoctylammonium bromide (orange). Data from M. Boualleg, K. Guillois, B. Istria, L. Burel, L. Veyre, J.-M. Basset, C. Thieuleux, V. Caps, Highly efficient aerobic oxidation of alkenes over unsupported nanogold, Chem. Commun. 46 (2010) 5361.

the same procedure with n-octylthiol and tetraoctylammonium bromide as a stabilizing agent respectively (instead of silane). It should be noted that similar size and distribution were obtained for the octylthiol- and ammonium stabilized NPs. Catalytic tests were then conducted in order to estimate the activity of these NPs toward aerobic epoxidation of trans-stilbene in methylcyclohexane solution in the presence of 5%mol of tert-butyl hydroperoxide. The best result was obtained for the Au NPs stabilized with n-octylsilane (see Fig. 19.13). Full trans-stilbene conversion was achieved in 72 h with 90% selectivity whereas the two other colloids exhibited lower conversions and selectivities [5]. This superiority of the silane-stabilized colloids might be attributed to the specific behavior of surface bonded octylsilane fragments that are oxidized into siloxane during catalysis, providing a high stability of the NPs as compared to ammonium stabilized NPs.

7 CONCLUSION This chapter disclosed the use of silane compounds (particularly long chain silanes) as promising capping agents for the synthesis of small

and well defined noble and nonnoble metal NPs. By contacting organometallic complexes and n-alkylsilane derivatives under reductive atmosphere, the preparation of very stable and highly dispersed metal colloids in mild conditions is possible. These colloids, used as obtained in solution or after deposition onto supports, revealed high catalytic activity in industrially relevant reactions such as CO, NO and soot oxidation, dry reforming of methane, alkenes hydrosilylation, and alkene epoxidation. Improved catalytic performances and resistance to sintering were observed for these silane-stabilized NPs, thus highlighting the advantages of using silanes to generate NPs using a colloidal route.

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