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Applied Surface Science 280 (2013) 836– 844

Contents lists available at SciVerse ScienceDirect

Applied Surface Science

jou rn al h omepa g e: www.elsev ier .com/ locate /apsusc

d and S binding energies and Auger parameters on a modelilica-supported Suzuki–Miyaura catalyst: Insights intoatalyst activation

ohammad A. Hanif, Iraklii I. Ebralidze, J. Hugh Horton ∗

epartment of Chemistry, Queen’s University, Kingston, Ontario K7L 3N6, Canada

a r t i c l e i n f o

rticle history:eceived 7 March 2013eceived in revised form 14 May 2013ccepted 15 May 2013vailable online 23 May 2013

eywords:

a b s t r a c t

Model Suzuki–Miyaura reaction catalysts have been developed by immobilizing palladium on a mercap-topropyltrimethoxysilane (MPTMS) functionalized Si substrate. Two types of Pd species were found onthe fresh catalysts that may be attributed to a S-bound Pd (II) species and Pd nanoparticles. The bindingenergy of the nanoparticles is strongly size dependent, and is higher than that of metallic Pd. A sulfurspecies that has not been previously reported on this class of catalysts has also been observed. A sys-tematic investigation of various palladium/sulfur complexes using XPS was carried out to identify this

esoporous siliconupported catalystagner plot

hotoelectron spectroscopyd nanoparticlescheffé simplex-lattice

species, which may be assigned to high oxidation state sulfur formed by oxidation of thiol during thereduction of the Pd(OAc)2 used to load the catalyst with Pd. Shifts in binding energy observed for bothPd and S spectra of the used catalysts were examined in order to probe the change of electronic envi-ronment of reactive palladium center and the thiol ligand during the reaction. Electron and atomic forcemicroscopic imaging of the surfaces demonstrates the formation of Pd nanoparticles on fresh catalystsand subsequent size reduction of the Pd nano-particles following reaction.

© 2013 Elsevier B.V. All rights reserved.

. Introduction

Palladium has been widely used as a catalyst in organic syn-hesis, especially in C C couplings such as the Suzuki–Miyauraeaction [1]. This class of reaction has gained particular attention forts role in the synthesis of fine chemicals and pharmaceutical ingre-ients [2,3]. As a consequence of the increased complexity of targetolecules in industry and academic research, the development of

C bond formation methodology and especially better under-tanding the processes that take place during Suzuki–Miyauraeaction remains an important aspect of organic chemistry. Usingalladium catalyzed coupling reactions in the synthesis of activeharmaceutical ingredients offers advantages, such as the mildeaction conditions and relatively non-toxic nature of the catalysts4]. However, while homogeneous Pd complexes are commonlysed as catalysts, their efficient separation after reaction for sub-equent recycling remains a challenge with both economic andnvironmental implications [3,5].

Heterogeneous catalysts, by contrast, can be often filtered outf the product mixture and reused without significant loss ofctivity. A number of immobilization protocols have been used

∗ Corresponding author. Tel.: +1 613 533 2379; fax: +1 613 533 6669.E-mail addresses: jhh@queensu.ca, hortonj@chem.queensu.ca (J.H. Horton).

169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.apsusc.2013.05.070

to heterogenize the homogeneous palladium catalysts. The moststraightforward of these is the physisorption of Pd onto a solid sup-port [6]. However, the weakly bound Pd may leach significantlyduring the reaction. Another strategy is to covalently tether the Pdto a suitable ligand that is itself covalently linked to a solid support[4]. Alumina, silica and carbon are commonly used in such a catalystdue to their thermal stability and suitable physical properties [6].Among these, mesoporous silicates have been used widely by manygroups as the potential supports for palladium catalysts [7–9]. Oneof the most frequent methods of creating palladium metal com-plexes, as well as stabilizing palladium nanoparticles is the additionof an organic ligand that typically contains a heteroelement, whichmay act as a Lewis base. While phosphines [10], amines [11], andcarbenes [12] may serve as the Lewis base, the strong interactionbetween the platinum group metals and soft, sulfur-containing lig-ands make these very promising and highly efficient stabilizers[13]. Mercaptopropyltrimethoxysilane (MPTMS), a compound thatcontains both the siloxane group suitable for hydroxylated surfacefunctionalization and a thiol group to interact with palladium, hasbeen intensively used by ourselves [14] and others [8,9] to cre-ate effective catalyst for Suzuki–Miyaura reactions. Although bulk

palladium metal efficiently catalyzes the Suzuki–Miyaura reaction[15], a higher activity of a supported catalyst is achieved due tolarge active surface area and small particles size, i.e. a high dis-persion of the active phase [6]. Thus, mesoporous silica supporting

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M.A. Hanif et al. / Applied Su

oth a mercaptopropylsiloxane Pd(II) complex together with smalld clusters, was found to be more active in Suzuki coupling thand metal particles alone [7].

Despite significant success in the development of effective het-rogeneous Pd catalysts, there is much evidence suggesting thatven traditional heterogeneous catalysts such as bulk palladium15] or palladium on carbon [16,17] act by releasing small amountsf soluble Pd, which then may redeposit at the completion of theeaction or remain in the reaction mixture. For example, oxida-ive addition of aryl iodide to Pd(0) results in the dissolution ofd nanoparticles and formation of ArPdI or [ArPdI3]2− species [18].atalysis using Pd foil [15] demonstrated that the coupling reactionccurs by dissolution of Pd species followed by re-deposition of Pd,referentially at the cooler edges of the heated reaction site, ratherhan uniformly across the surface. Thiol functionalized mesoporousilicates are known to be efficient scavengers for Pd and displayxcellent ability to remove palladium from organic and aqueousolutions [9,19]. Therefore, they appear to be good candidates asupports for Pd nanoparticles in a heterogeneous catalyst.

On the other hand, the processes taking place on the surfacef silica-supported Pd catalysts, particularly the oxidation state ofhe reactive Pd species, and the role that the thiol tether plays inctivation of the catalyst, remain poorly understood. Our previousork using X-ray photoelectron spectroscopy (XPS) and the Augerarameter [14] to characterize Pd catalysts supported on the meso-orous silicate SBA-15 found that Pd forms a new species of lowerinding energy during the coupling reaction; this was attributed tohe formation of a metallic Pd nanoparticle. Importantly, the bind-ng energies observed still lay in the region normally attributedo Pd(II) species, as opposed to Pd(0) as would be expected for Pdanoparticles: this was attributed to a reduction in polarizability

n the Pd environment, due to the small size of the nanoparticlesormed. Binding energy shifts between two species, �EB, dependot only on the difference in ground state energy between twopecies, �ε, but also on the extra-atomic relaxation, �Rea [20,21]hich itself is a measure of the polarizability of the atomic envi-

onment:

EB = �ε − �Rea (1)

Chemical shifts of the Auger parameter, ˛′, the sum of the XPSinding energy and X-ray induced Auger electron spectroscopicransition (XAES):

′ = KE(Auger) + BE(photoelectron) (2)

re proportional to the extra-atomic relaxation term:

˛′ = 2�Rea (3)

herefore, by plotting both XPS binding energy and XAES kineticnergy on a single plot (Wagner plot), we may obtain a more com-lete picture of the electronic environment for a particular chemicalpecies. A Wagner plot is also known as a chemical state plot, aseveloped by C.D. Wagner in 1970s and provides chemical infor-ation regarding the species associated with a particular X-ray

hotoelectron transition [20]. Here, we have chosen to explore aeries of model catalysts in which Pd is deposited on an oxidizedingle crystal Si(1 0 0) surface terminated with mercaptopropyl-iloxane (MPTMS). While these catalysts are not as effective ashose on mesoporous silicate supports, they have the advantagehat a wider range of Pd particle sizes can be grown, as nanoparticlerowth is no longer constrained by pore size. This is important as weill explore here the effect nanoparticle size has on the Pd binding

nergy. These flat catalysts are also better suited for imaging with

tomic force microscopy (AFM) and scanning electron microscopySEM). In addition, the cavity structure of the mesoporous silicatesesults in a catalyst with a range of active sites, which contribute ton average XPS signal; some sites deep within the pore structure

cience 280 (2013) 836– 844 837

may not be observed due to electron attenuation effects. The useof a single layer, flat model catalyst minimizes electron attenua-tion effects, and should lead to a more homogeneous surface layer.Finally, we will explore the changes in binding energy associatedwith the thiol ligand, which has often been neglected in previousXPS studies of these catalytic systems.

2. Experimental

2.1. Materials

Palladium acetate, mercaptopropyltrimethoxysilane (MPTMS),octadecyltrichlorosilane, phenylboronic acid, 1-iodo-4-methylbenzene, and pinacol were purchased from Sigma–Aldrich;ammonium hydroxide 30% solution in water was obtained fromCaledon Labs; 30% hydrogen peroxide and 98% and fuming sul-furic acid were bought from Fisher Scientific. Phenylboronic acidpinacol ester was synthesized according to the published [22]procedure: phenylboronic acid (3.4 mmol) and pinacol (3.4 mmol)were dissolved in anhydrous Et2O; the mixture was stirred for 1 hat r.t. and concentrated under reduced pressure. The crude productwas purified by chromatography (3:7 EtOAc–EtPet), yielding aftersolvent evaporation the ester in the form of white crystallinesolid. Pd(DMSO)2Cl2 was prepared according to the published [23]procedure: 50 mg of PdCl2 was dissolved in 1 mL of DMSO at 50 ◦C.The crystals were formed via slow vapor diffusion of diethyl etherinto the DMSO solution at room temperature. Pd4(OAc)4(SEt)4and Pd6(SEt)12 were prepared according to published procedures[24,25] via recrystallization from solutions of the relevant com-pound in 1,2-dichloroethane. Pd nanoparticles were synthesizedaccording to the procedure by Shen et al. [26].

2.2. Model Pd catalyst preparation

n-type Si(1 0 0) wafers were purchased from SEH America Inc.The 1 cm × 1 cm sized silicon wafer surfaces were cleaned andoxidized to form a hydroxylated surface layer first with piranhasolution (mixture of 7:3 (v/v) H2SO4 and 30% H2O2, at 85 ◦C, for1 h) and then using the RCA cleaning protocol (1:5:1 (v/v) ammo-nium hydroxide 30% solution:deionized water:hydrogen peroxide30% solution, at 70 ◦C, for 20 min) [27]. The substrates were subse-quently washed with DI water and dried under N2 stream. Caution!Piranha solution is an extremely dangerous oxidizing agent and shouldbe handled with care using appropriate personal protection. In the sec-ond stage of the catalyst preparation process, the hydroxylated Siwafer was immersed into a 5 mM toluene solution of MPTMS for24 h under inert atmosphere in order to form the thiol-terminatedsurface. All glassware used in this stage of the preparation processwas passivated by immersing in a toluene solution of octadecyl-trichlorosilane for 24 h to avoid any competitive adsorption ofMPTMS onto the walls of the glassware. The thiol-terminated Siwafers were thoroughly washed by toluene followed by sonica-tion in toluene and chloroform for 10 min each to remove anyphysisorbed MPTMS from the surface. The samples were driedunder an N2 stream.

The third stage of the catalyst preparation process involved Pddeposition. A series of catalysts were prepared by Pd immobiliza-tion on the MPTMS functionalized silicon substrate by immersionin a 1 mM solution of Pd(OAc)2. The effects of temperature, thecomposition (ethanol/THF) of the solvent, and the exposure timeon Pd deposition were explored by using Scheffé’s simplex-lattice

experimental design for three components [28,29]. Ten differ-ent catalysts were prepared, under the conditions summarized inTable 1. Ternary plots were then plotted from this data using regres-sion coefficients calculated using Scheffé’s algorithm [28].

838 M.A. Hanif et al. / Applied Surface Science 280 (2013) 836– 844

Table 1Conditions used to deposit Pd on the thiol-terminated Si substrate.

Sample Pd deposition conditions: 1.0 mM Pd(OAc)2

Temperature (◦C) Solvent composition, vol% Exposure time (min)

Ethanol THF

1 50 100 0 602 20 0 100 603 20 100 0 2404 42 72 28 605 28 28 72 606 42 100 0 1087 28 100 0 190

28

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67

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2

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2

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8 20

9 20

10 30

.3. Other surface modification processes

Disulfide functionalized surfaces (R-S-S-R) were obtained by modified procedure of Aida et al. [30] in which the MPTMS-unctionalized surfaces described above were immersed in aolution of 3 mg of iodine in DMSO:DI water (1 mL:0.1 mL) mix-ure for 1 h, followed by surface washing by DMSO:DI waterolution. Alkyl thioacetate functionalized surfaces (R-S-Ac) werebtained by immersing MPTMS-functionalized surfaces in aceticnhydride for 1 h, followed by surface washing by DI water.lkylsulfate functionalized surfaces (R-O-SO3H) were obtained byrst functionalizing the hydroxylated Si(1 0 0) substrate using 2-trimethoxysilyl)ethyl acetate by immersion in a 5 mM solutionf 2-(trimethoxysilyl)ethyl acetate in toluene for 24 h. These werehen placed in a 1 M NaOH solution for 20 min to hydrolyze thecetyl group, washed by deionized (DI) water, and dried under N2tream. The resulting alkyl hydroxy-terminated surface was sub-erged in fuming sulfuric acid (oleum) for 30 min, followed by

urface washing by DI water.

.4. Suzuki–Miyaura coupling reaction

A solution of 1-iodo-4-methylbenzene (0.035 mmol, 7.63 mg)nd phenylboronic acid pinacol ester (0.0525 mmol, 10.7 mg) in.8 mL of anhydrous DMF were added to a solution of cesium car-onate (0.525 mmol, 17.1 mg) in DI water (0.2 mL) and loaded withodel Pd catalyst, sealed and flushed with argon. The reaction mix-

ure was then heated at 70 ◦C without stirring under Ar for 24 h,ooled to room temperature, and the Si wafer supporting the cat-lyst was removed from the reaction mixture, rinsed with DMF:DIater (10:1, v/v) solution, then DI water, and finally dried under

n N2 stream. It was then taken for further surface characterizationnd recycling, as appropriate. GC analysis was performed using angilent 6850 series GC with a FID detector equipped with 19091J-13E nonpolar column and operated under HP GC ChemStationoftware.

.5. Surface characterization

XPS and AES measurements were performed using a Thermonstrument Microlab 310F surface analysis system (Hastings, UK)sing a Mg K� X-ray source (1253.6 eV) at 15 kV anode potentialnd 20 mA emission current. Ultra high vacuum (UHV) conditions10−9 to 10−10 Torr) were maintained during the experiment. The

1s line at 285.0 eV was used as an internal reference peak for

alibrating the binding energy. A Shirley fit algorithm was usedor background subtraction and a Powell peak-fitting algorithm forata analysis. AFM images were obtained using a Veeco Multimodeicroscope equipped with a IIIa controller. All AFM images were

72 10828 19033 120

acquired under ambient conditions. SEM images were acquiredusing a LEO 1530 field emission SEM.

3. Results

3.1. Characterization of the thiol-terminated substrate

Prior to deposition of palladium, the thiol-terminated surfaceswere fully characterized using XPS, AFM, and water contact anglemeasurements. Fig. 1 summarizes the various preparation stages,and incorporates some typical XP spectra, AFM images and watercontact angle data. Piranha and RCA solutions were used to removecontaminants from the Si and subsequently form silanol sites (Si-OH). Water contact angle measurements demonstrated that thesurface became highly hydrophilic, with a contact angle of 5 ± 2◦

following hydroxylation, compared to 45 ± 3◦ for the unmodifiedSi(1 0 0) wafer. AFM images also showed a homogeneous surface oflow root mean square (rms) roughness of 0.16 nm. Thiol function-alization of the surface was then carried out using hydrolysis ofMPTMS to form a thiol-terminated overlayer. Following 24 h expo-sure to a solution of 5 mM solution of MPTMS in toluene, the watercontact angle increased to 70 ± 2◦, consistent with that previouslyreported by Balachander and Sukenik [31]. The surface rms rough-ness increased slightly to 0.35 nm, with no evidence of polymerizedagglomerate deposition. The presence of a S 2s peak confirmed thedeposition of thiol species. The C 1s to S 2s XPS area ratio (nor-malized with respect to sensitivity factors [32]) was 4.8 ± 0.5:1. Aratio greater than 3:1 (i.e. the C:S ratio of the mercaptopropyl sidechain) would suggest that the MPTMS molecules have not beencompletely hydrolyzed, leaving residual methoxy groups at the sur-face. A 5:1 ratio would indicate that on average, two of the threemethoxy groups remain unreacted. The real fraction is likely lessthan this, due to residual C contamination from the underlying Sisubstrate and exposure of the sample during transfer to the XPSanalysis chamber.

3.2. Pd catalyst reactivity

Having characterized the thiol-terminated surface, an activecatalyst was synthesized by immersing the thiolated silicon sub-strate in a solution of palladium acetate (Fig. 1). A range of Pddeposition conditions, as summarized in Table 1, were exploredin order to determine their influence on both the physical char-acteristics of the catalyst, and its activity. A typical single-factorexperiment for a comprehensive study of a chemical process oftenrequires a large number of experimental runs. In addition, many

single-factor dependencies often cannot be combined into oneoverall. Thus, in order to obtain a better understanding of howseveral factors simultaneously affect the characteristics of the cat-alyst, we used Scheffé’s experimental design methodology [28,29]

M.A. Hanif et al. / Applied Surface Science 280 (2013) 836– 844 839

F ious md

ttr

cctrsdstwicftTaprm

o

ig. 1. Preparation of the model heterogeneous Pd catalysts. A schematic of the varata for each stage.

o explore the influence of three independent variables: tempera-ure, solvent composition and exposure time. In the following, theseesults are presented as ternary diagrams.

The model Pd catalysts were then used to perform the Suzukiross-coupling of p-iodotoluene with phenylboronic acid pina-ol ester (Scheme 1) and the catalysts characterized in order torack the changes that occurred following the catalytic cycle. Theseesults are plotted as ternary diagrams in Fig. 2. It should be empha-ized that the differing conditions noted in Fig. 2 and in later ternaryiagrams refer to the Pd deposition onto the MPTMS-terminatedubstrate, i.e. the catalyst preparation conditions; they do not refero the conditions under which the Suzuki–Miyaura reaction itselfas carried out. The latter was the same in all cases, as detailed

n Section 2. Fig. 2 is a plot of the % yield of the Suzuki cross-oupling reaction (Fig. 2A) and turnover numbers (Fig. 2B) as aunction of three catalyst preparation conditions (Pd depositionemperature; solvent composition; and exposure time to solution).urnover numbers were determined using a classical approach [33]s the amount of product per amount of loaded Pd. The quantity ofalladium present was estimated from the XPS-determined S:Pd

atio, assuming that a complete SAM on silica contains 5 silaneolecules per nm2 [34,35].It is clear from Fig. 2B that the highest turnover numbers were

bserved on those catalysts prepared under conditions of lower

Scheme 1. The Suzuki–Miyaura cross-coupling reaction.

odification stages discussed in the text, together with contact angle, AFM and XPS

deposition temperature and higher THF concentrations (bottomright corner of the ternary diagram). The dependence of the % yieldon catalyst is less pronounced, but with somewhat higher yieldsassociated with the same conditions that led to high turnover num-ber. The one exception is the catalyst Sample 1, deposited at 50 ◦Cfrom ethanol solution, which showed the highest %yield of anycatalyst examined here.

3.3. Pd photoelectron spectroscopy

Each catalyst was characterized using XPS and XAES both priorto and following its use in the cross-coupling reaction. Fig. 3Ashows the Pd 3d XPS spectrum of Sample 1, before and afteruse in the Suzuki cross-coupling reaction. These are comparedto spectra from Pd(OAc)2 powder and to the spectrum of a sam-ple catalyst exposed to a solution of 1-iodo-4-methylbenzene inDMF solvent at reaction temperature (i.e. reaction conditions butwith only one of the two reactants present). Both the Pd 3d3/2and 3d5/2 peaks are shown, but further results and discussion willfocus on the 3d5/2 spectra. The Pd 3d5/2 spectrum of Pd(OAc)2exhibits one peak at 338.5 eV, consistent with previous reportsfor this compound [36]. The catalyst itself exhibits two chemicalstates: one at approximately 338.0 ± 0.2 eV (State 1) and anotherat 335.8 ± 0.2 eV (State 2), before use in the reaction. Followingreaction, both states undergo a small chemical shift, but in oppo-site directions, with State 1 shifting to 337.6 ± 0.2 eV and State 2to 336.2 ± 0.2 eV. Similar spectra were obtained from the remain-ing catalyst samples, and the reported errors indicate the rangeof binding energy values observed for these two states on all

10 samples. As these errors are close to the limits of the reso-lution of our instrument, no further analysis was carried out toreview effects of catalyst preparation conditions on the bindingenergy.

840 M.A. Hanif et al. / Applied Surface Science 280 (2013) 836– 844

ation c

sdseuisltoFattptc

Fp

Fig. 2. Ternary diagrams as a function of catalyst prepar

While the positions of the two Pd peaks were similar in allamples, the relative intensity of these two chemical states of Pdiffered significantly between samples. Fig. 3B is a ternary diagramhowing the relative intensity of the State 1 peak (higher bindingnergy) with respect to State 2 (lower binding energy), on thenused catalyst. State 1 dominates in the unused catalyst when

t is prepared under low temperature deposition conditions inolutions of high THF concentrations, i.e. the same conditions ased to high turnover numbers and relatively higher % yields inhe cross-coupling reaction. Upon reaction, the total Pd loadingn the samples decreases under all conditions, as indicated inig. 3C, which shows the ratio of the total Pd 3d5/2 signal beforend after reaction. In Fig. 3C, the smallest values therefore indicatehe smallest overall loss of Pd, which is again associated with low

emperature and high THF concentrations. State 1 Pd is thereforereferentially retained on the surface relative to State 2, followinghe cross-coupling reaction. This can also be seen qualitatively byomparing the XP spectra in Fig. 3A.

ig. 3. (A) Pd 3d X-ray photoelectron spectra of the Suzuki–Miyaura coupling catalysts. Streparation conditions showing (B) the ratio of the Pd 3d5/2 peak area of State 1 and State

onditions showing (A) %yield and (B) turnover number.

3.4. S photoelectron spectroscopy

Both S 2p and S 2s photoelectron spectra were collected for eachmodel catalyst. Generally, S 2p, being the higher intensity peak, isthe spectrum recorded and presented in the literature. Unfortu-nately, in this case the S 2p signal is obscured by a Si 2p satellite peakassociated with the silicon oxide layer at the surface. This satellitepeak has been previously observed to make proper analysis of any S2p spectrum difficult to carry out [37]. Thus, we report here on thelower intensity S 2s peak. Fig. 4A shows the S 2s spectra of Sample1, before and after use in the Suzuki cross-coupling reaction. Theseare compared to spectra from the bare MPTMS-modified surface(i.e. prior to Pd deposition) and to the spectrum of a sample catalystexposed to a solution of 1-iodo-4-methylbenzene in DMF solvent

at reaction temperature. Fig. 4B shows the total S:Pd atomic ratioof the catalyst (as determined from the relative intensities of the S2s and Pd 3d5/2 XPS peak areas) prior to reaction. The highest S:Pdratios (about 2:1) were generally associated with the same sample

ate 1 and State 2 are discussed in the text; ternary diagrams as a function of catalyst 2 prior to reaction and (C) the total Pd 3d5/2 peak area before and after reaction.

M.A. Hanif et al. / Applied Surface S

Fig. 4. (A) S 2s X-ray photoelectron spectra of the Suzuki–Miyaura coupling catalystsaS

pd

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4

Soles2smto

nd (B) ternary diagram as a function of catalyst preparation conditions showing the:Pd atom ratio on the catalyst prior to reaction, as determined using XPS.

reparation conditions that led to higher turnover numbers andominance of Pd State 1 on the catalyst surface.

Prior to deposition of Pd, the S 2s spectrum of the MPTMS-unctionalized surface shows the presence of a single S state, at

binding energy of 227.7 eV. This peak remains present on the sur-ace following deposition of Pd, but is reduced in intensity and aecond peak at binding energy of 231.7 eV is observed. While thisigher binding energy state generally appeared to become stronger

n intensity following the cross-coupling reaction, the relativelyigh signal-to-noise of the S 2s spectra precluded any full analysissing the ternary diagrams as was carried out for Pd.

. Discussion

Previous reports of Pd XP spectra on catalysts supportinguzuki–Miyaura cross-coupling reactions have generally focusedn thiol-terminated silica and mesoporous silica-supported cata-ysts such as SBA-15-SH or MCM-21-SH [7–9,14,19,38]. Generally,ither one or two chemical states of Pd have been reported, corre-ponding roughly in binding energy to those of State 1 and State

observed here. Typically State 1 has been assigned as a Pd(II)

pecies, and State 2 as a Pd(0) species; shifts in binding energyay also take place after the catalyst has undergone reaction. In

hese cases, THF was mainly used as a solvent for Pd(OAc)2. On thether hand, Yang [39] reports the use of alcohols as a deposition

cience 280 (2013) 836– 844 841

solvent. The presence of ethanol has been reported to lead to Pd(II)reduction and formation of Pd(0) species in the form of nanoparti-cles [40]. Thus, the influence of THF/ethanol solvent mixtures wasexplored here. Peaks similar in binding energy to State 2 have alsobeen observed when reducing PdCl2 or Pd(acac)2 using H2 on car-bon black or MgO supports, and again have been assigned to Pd(0)species [41].

The nature of the various forms of Pd present on our model cat-alyst, and some insight into these previous reports, may be madeby plotting our current XPS and XAES results on a Wagner plot,together with data from a range of well-characterized compounds.The Wagner plot in Fig. 5 is a plot of the XAES kinetic energy dataversus XPS binding energy for a given species. The Auger param-eter (˛′), as defined by Eq. (2), is shown as diagonal lines in thefigure. While the lack of XAES data for many of the systems notedabove precludes their inclusion in the Wagner diagram, data for avariety of Pd compounds is available. In Fig. 5, colored data pointsrepresent results obtained here and the black points represent datapreviously reported in the literature, including data from SBA-15-SH-supported Pd catalysts previously described by us [14].

The State 1 Pd observed here appears (within experimentalerror) to correspond to the Pd species we have previously observedon SBA-15-SH. Previous workers have attributed peaks in thisregion as a “Pd(II)” species: either unreacted Pd(OAc)2 or someform of thiol/acetate complex. Notably, Shimizu et al. [7] observed,using X-ray absorption near edge structure (EXAFS), that on FSM-15-SH supports Pd(OAc)2 forms a Pd species with (on average) 2.3Pd S bonds and 0.3 Pd O bonds; the former to thiol ligands on thesupport and the latter, presumably, from residual acetate ligands.While our C 1s spectra show minimal evidence for the carbonylspecies of the acetate in the as-prepared catalyst, our results areotherwise supportive of this: the S:Pd atomic ratio is 2:1 on thosecatalysts in which State 1 dominates (Fig. 4B) and, before use in theSuzuki–Miyaura reaction, the State 1 XPS binding energy and Augerkinetic energy are close to those we have observed for the com-pound Pd4(OAc)4(SEt)4 (Fig. 5). In Pd4(OAc)4(SEt)4, the Pd centreis surrounded by two (monodentate) acetate and two thiol ligands[24].

Following use of the catalyst in the Suzuki–Miyaura couplingreaction, State 1 shifts to lower binding energies. In our previ-ous work on SBA-15-SH-supported catalysts, we attributed thisspecies to the formation of small (<10 nm diameter) Pd nanopar-ticles [14]: SEM imaging of SBA-15-SH catalysts and of similarcatalysts [19,42] indicated the presence of such nanoparticles, andwe attributed the relatively high binding energy, compared to thatof metallic Pd, to the low polarizability of Pd environment withinthe nanoparticle, resulting in reduced stabilization of the core holeand hence a reduced value of Rea (Eq. (1)). However, State 1 can-not correspond to a Pd nanoparticle on our model catalyst here:as will be discussed below, there is strong evidence from both XPSand SEM data that it is State 2 that corresponds to Pd nanoparti-cles on the surface. Post-reaction, the binding energy and Augerkinetic energy of State 1 correspond closely to those of PdS (Fig. 5);presumably Pd undergoes further loss of acetate ligands upon par-ticipating in the coupling reaction. As catalysts in which State 1 isthe main Pd species present exhibit high turnover numbers andgenerally higher yields, and State 1 is preferentially retained on thecatalyst surface following the coupling reaction, State 1 must cor-respond to the active Pd species in the Suzuki–Miyaura couplingreaction. A species similar in binding energy to State 1 was previ-ously observed using Pd foil as the catalyst [15]. This was ascribedto insertion complex of the aryl iodide to Pd. In this case, exposure

of the catalyst to the aryl iodide alone under reactions condi-tions resulted in a relatively unchanged Pd 3d spectrum (Fig. 3A)and only a trace quantity of iodine was observed in the I 3d XPSspectrum.

842 M.A. Hanif et al. / Applied Surface Science 280 (2013) 836– 844

322

323

324

325

326

327

328

329

330

334335336337338339

As-prep ared Pd catalysts Used Pd catalyst

SBA-15-SH supported Catalysts (as prepared) SBA-15-SH supported Catalysts (used)

Pd

M4N

45N

45

Kin

etic E

ne

rgy (

eV

)

Pd 3d5/2 Binding Energy (eV)

Auge

r Pa

ram

ete

r, α΄

(eV

)

Metallic Pd

PdO

PdS

State 1 State 2

Pd6(SE t)12

Pd(OAc)2

PdCl2(DMSO)2

Pd nanop articles

Pd4(OAc)4(SE t)4

Fig. 5. Wagner chemical state plot for Pd 3d5/2 XPS and Pd MNN Auger electron spectroscopy data. Colored symbols represent data reported here; black symbols are datar cataly( rried ot

vstpTt

Fns

eported previously elsewhere. Two chemical states were observed for the model

triangles) and the model catalyst developed here (circles), measurements were cahe range of binding energy and Auger kinetic energy values observed.

The binding energy of State 2 is similar to those values pre-iously reported in the literature as corresponding to a “Pd(0)”pecies and generally assigned to metallic Pd, or to Pd nanopar-icles. SEM images of the catalyst and size distributions of the

articles present on the surface were acquired, as shown in Fig. 6.hese show that before the catalyst is used in the coupling reac-ion (Fig. 6A), Pd particles of average diameter of 14 nm are present

ig. 6. SEM images of the model catalyst surfaces (Sample 1 in Table 1) (A) prior and (B)anoparticles prepared according to the procedures published by Shen et al. [26], and suhow the particle size distribution.

sts used here, denoted State 1 and State 2. In the case of the SBA-15-SH catalystsut on multiple samples prepared under differing conditions. The error bars reflect

on the surface. Following reaction, the particles are still present atroughly the same density, but the average diameter is decreased to10 nm (Fig. 6B). This is consistent with the XPS data: following reac-tion, the particles shrink in size, leading to the observed decrease

in the overall Pd signal, as seen in the ternary diagram in Fig. 3C.The XPS peak also shifts to higher binding energy, as the smallerparticles provide a less polarizable environment for the ionized Pd

subsequent to use in the Suzuki–Miyaura coupling reaction. (C) SEM image of Pdbsequently deposited on a Si substrate for imaging. The accompanying histograms

M.A. Hanif et al. / Applied Surface S

Fig. 7. Comparative S 2s X-ray photoelectron spectra of the model catalyst withthat of other compounds exhibiting Pd S bonding (Pd6(SEt)12, Pd4(OAc)4(SEt)4, andPR

actFtm

fHuwcariprihmt

t(wibbpedscsasi

cI

least prone to the undesirable leaching of Pd, are those that min-

dCl2(DMSO)2), surface-bound S species (alkylthioacetate, R-S-Ac, disulfide, R-S-S-, sulfonate R-O-SO3H, and thiol, R-SH) of varying oxidation state.

tom, leading to reduced stabilization of the core hole. In order toonfirm this dependence of binding energy on particle size, we syn-hesized Pd nanoparticles of average diameter 20 nm, as shown inig. 6C. These larger particles have a binding energy smaller thanhat of the smaller State 2 particles, but greater than that of the

ore polarizable metallic Pd (Fig. 5).There are relatively few data available on sulfur XP spectra

or thiol-supported Pd catalysts used in Suzuki–Miyaura reactions.owever, the S 2s peak at 227.7 eV is evidently associated withnreacted S sites on the surface as it undergoes no chemical shifthen Pd is present, but does undergo a reduction in intensity. Its

ontinued presence indicates that not all the S sites on the surfacere available to react with Pd. This is inconsistent with previouseports on mesoporous silica catalysts, in which only one S chem-cal state was observed under any given conditions [14,42]. Theresence of unreacted S sites may be due to the substrate prepa-ation methods: as noted earlier, the C:S area ratio at the surfacendicated that the surface MPTMS molecules were not completelyydrolyzed; nonetheless, any degree of cross-linking on the surfaceight lead to SH groups being overlaid by further MPTMS, making

hese SH sites unavailable for bonding with Pd.One report is available for a Pd catalyst supported on sulfur-

erminated GaAs(0 0 1) [43]. As with our model catalyst, they foundusing a synchrotron X-ray source) that the S 1s peak associatedith unreacted S sites on the surface remained present follow-

ng deposition of Pd. A second state, shifted some 7 eV to higherinding energies, and trace amounts of S species at intermediateinding energy were also observed. Precise assignments of theseeaks were not made by the authors. As S 2s is not the photo-lectron transition routinely acquired in XPS, there is also littleata available in the literature on S 2s binding energies of otherpecies. Therefore, in order to assign the S 2s peak at 231.7 eV asso-iated with the presence of Pd, we acquired S 2s binding energypectra for the compounds Pd6(SEt)12 and Pd4(OAc)4(SEt)4 as wells surface-bound S species of varying oxidation state. These arehown, together with the S spectrum for Sample 1 for comparisonn Fig. 7.

It is evident from Fig. 7 that the S 2s binding energy does notorrespond too closely to that in Pd6(SEt)12 and Pd4(OAc)4(SEt)4.n these compounds, the oxidation state of sulfur is formally

cience 280 (2013) 836– 844 843

−2. The binding energy observed indicates a S oxidation statehigher than that of surface-bound disulfide (R-S-S-R) or alkylth-ioacetate (formally −1 and −2 respectively), but lower than that of asulfonate-terminated surface (formal oxidation state +6). It appearsclosest to the binding energy for S in the compound PdCl2(DMSO)2in which the S of the DMSO ligand is formally in a +2 oxidationstate and has been shown to be the donor site to the Pd center [23].This is in agreement with our previous inference of a +2 oxidationstate of S in SBA-15-SH-supported Pd catalysts using the S 2p XPspectrum [14].

Together with the Pd XPS data, these results suggest that, uponreaction with Pd(OAc)2, the S species on the thiol-terminated sup-port is formally oxidized, leading to the reduction of Pd and theformation of both Pd nanoparticles and a Pd surface complex boundto the S of MPTMS on the silica support, as shown schematicallyin Fig. 1. Lower temperatures and Pd deposition from solutionsof higher THF concentration lead to preferential formation of thePd surface complex, which appears to be the reactive species inthe Suzuki–Miyaura coupling reaction. When ethanol is used asthe deposition solvent, the amount of deposited Pd is significantlyhigher, but much of this is in the form of the State 2 species, whichhas been assigned to the formation of Pd nanoparticles. This formof Pd is preferentially lost when the catalyst is used in the cou-pling reaction. While Pd leaching from the surface is an undesirablephenomenon, the leached metal is often claimed to be the activespecies in coupling reactions [44]. This likely explains the relativelyhigh %yield associated with the higher-ethanol conditions, but oth-erwise low turnover numbers as the Pd is rapidly lost from thesurface.

5. Conclusions

The Suzuki–Miyaura coupling reaction is one of the most effec-tive available to synthetic chemistry for the formation of C Cbonds. Obtaining a clearer picture of processes that take place onthe heterogeneous Pd catalyst surface during this reaction, and par-ticularly what Pd species are present and their role in the reactionmechanism, remain a challenge. Furthermore, leaching of Pd intosolution must be minimized in order to reduce contamination ofthe products, particularly in pharmaceutical applications. Here, wehave reported on a model palladium catalyst in which Pd(OAc)2 isused to deposit Pd on a thiol-terminated, oxidized single-crystalSi(1 0 0) substrate. While these catalysts do not exhibit catalyticbehavior as efficient as those based on porous silica, presumablydue to their relatively small surface area, our design does makethem readily amenable to correlating catalyst activity with surfacemorphology and composition.

Palladium XP spectra indicated that two chemical states of Pdwere present on the surface. In conjunction with S 2s spectra, oneof these was assigned to a S-bound Pd(II) species in which thesurface-grafted S ligand is formally in a +4 oxidation state. The sec-ond state was associated with formation of Pd nanoparticles onthe surface. Following use in the Suzuki–Miyaura coupling reac-tion, these nanoparticles decreased in size, due to leaching of Pdinto solution. This resulted in an upwards shift in the XPS bind-ing energy that may be correlated to changes in the extra-atomicrelaxation term as smaller particles exhibit a less polarizable envi-ronment and hence reduced stabilization of the core hole of ionizedPd. The catalysts that exhibited the highest turnover numbers andlowest loss of Pd were those in which the Pd(II) species domi-nated. This suggests that that the most effective catalysts, and those

imize the formation of metallic Pd in the form of nanoparticlesand which maximize the number of S-bound Pd species on thesurface.

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cknowledgment

We acknowledge the Natural Sciences and Engineering Researchouncil of Canada for financial support.

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