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. 2020 Oct 20;124(43):23626–23636. doi: 10.1021/acs.jpcc.0c05735

Dynamics of Pd Dopant Atoms inside Au Nanoclusters during Catalytic CO Oxidation

Clara Garcia †, Vera Truttmann †, Irene Lopez ‡, Thomas Haunold †, Carlo Marini §, Christoph Rameshan †, Ernst Pittenauer ∥, Peter Kregsamer ⊥, Klaus Dobrezberger †, Michael Stöger-Pollach #, Noelia Barrabés †,*, Günther Rupprechter †
PMCID: PMC7604939  PMID: 33154783

Abstract

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Doping gold nanoclusters with palladium has been reported to increase their catalytic activity and stability. PdAu24 nanoclusters, with the Pd dopant atom located at the center of the Au cluster core, were supported on titania and applied in catalytic CO oxidation, showing significantly higher activity than supported monometallic Au25 nanoclusters. After pretreatment, operando DRIFTS spectroscopy detected CO adsorbed on Pd during CO oxidation, indicating migration of the Pd dopant atom from the Au cluster core to the cluster surface. Increasing the number of Pd dopant atoms in the Au structure led to incorporation of Pd mostly in the S–(M–S)n protecting staples, as evidenced by in situ XAFS. A combination of oxidative and reductive thermal pretreatment resulted in the formation of isolated Pd surface sites within the Au surface. The combined analysis of in situ XAFS, operando DRIFTS, and ex situ XPS thus revealed the structural evolution of bimetallic PdAu nanoclusters, yielding a Pd single-site catalyst of 2.7 nm average particle size with improved CO oxidation activity.

1. Introduction

Heterogeneous Au nanoparticle catalysis has been increasingly studied in the past decades because of gold’s versatile catalytic activity, for example in oxidation,1−3 hydrogenation1−4 or C–C coupling reactions.2,4 To understand and control catalytic performance of nanoparticles at the molecular level remains a major challenge. Developing novel nanostructures with atomically controlled key structure parameters is required, such as the number of atoms (size), elemental composition, and surface modification by functional groups. This can be achieved with monolayer-protected gold nanoclusters (Aun(L)m), which have led to advances in nanoscience.5 Heterogeneous catalytic research of atomically precise gold nanoclusters is an emerging field opening new opportunities for accurate studies of size-dependent properties, atomic structure effects, and reaction mechanisms in catalysis.3,6,7 Moreover, because of these properties combined with small particle sizes below 2 nm, nanoclusters exhibit high activity for several catalytic reactions.7

The physical–chemical properties of gold nanoclusters can be fine-tuned by heteroatom doping, which has a strong influence on their stability (toward e.g. thermal and/or chemical treatments) and catalysis.8 Knowledge of the number of incorporated dopant atoms, their exact location in the cluster, and structure–property relationships is required for a thorough understanding. Depending on the nature of the dopant atom, different positions within the Au cluster have been identified, for example in the center (Pd, Pt, Cd), in the outer core–shell (Ag, Cd), or in the protecting Au(I) thiolate staple motifs (Cu, Hg) surrounding the core.6,9−12

Previous studies of AgxAuy nanoclusters revealed the dynamic nature of bimetallic nanocluster structures. Initially reported by Pradeep and co-workers, mobility of atoms between Au25(SR)18 and Ag44(SR)30 in solution was observed, forming Au25–xAgx(SR)18 species.13 This initiated a series of studies on intercluster reactions.13−23 Recently, Bürgi and co-workers reported that metal exchange reactions could also be observed between Au nanoclusters and metal foils (Ag, Cd, Cu), leading to bimetallic nanoclusters.24 Metal migration in Ag2Au25 clusters was found to occur also intramolecularly when exposed to a thiol solution.25 Altogether, this demonstrated that in solution, the structure of doped nanoclusters is not static but evolves under different conditions.

In the case of Au25 clusters, heteroatom doping with Pd, Pt, or Cd induced a drastic change of their redox properties26,27 and increased their stability.8,26,28,29 In addition, the reactivity in catalytic reactions was also altered,6 and in several cases, doped M1Au24 (M = Pd, Pt, Cd) clusters performed superior in comparison to their homogold analogues.26,28−32

Pd-doped Au nanostructures have been applied in catalysis due to their favorable catalytic properties.33,34 Their high activity was often ascribed to electronic effects, i.e., the Pd site(s) being slightly electron deficient compared to the Au ones.29,31,35,36 Pd dopant atoms are typically in center positions, but other locations have also been reported: Monopalladium doping into Au:PVP nanoclusters lead to preferential formation of PdAu33 and PdAu43 particles, with the Pd dopant on the particle surface, leading to a drastic increase in benzyl alcohol oxidation activity.37 Scott and co-workers showed that multiple Pd doping can be achieved by mixing Au25(SR)18 clusters with a Pd(II) compound, leading to the replacement of Au atoms in the staples by Pd, in addition to the usual Pd center position.38−40 After ligand removal treatment, isolated Pd atoms were obtained at the surface of the Au nanoparticles, which led to significantly enhanced allyl alcohol hydrogenation reactivity.38 The presence of monomer (pair) Pd surface sites instead of larger ensembles was also found to increase the activity of Pd/Au(100) and Pd/Au(111) surfaces for acetoxylation of ethylene.34

CO oxidation is one of most extensively studied reactions, also in Au nanoparticle catalysis, reflected in recent reviews.1,2 The exact reaction mechanism, especially with regard to O2 activation, is still intensively debated. Among many support materials, reducible oxides such as TiO2 have been found particularly suitable for CO oxidation.1,2,41,42 An active Au/TiO2 (among other 3d transition metal oxides) catalyst was already described by Haruta and co-workers, who noticed very high activity of 3.5 nm Au nanoparticles prepared by deposition–precipitation even at 0 °C.43−45 The active sites in this low-temperature CO oxidation processes are supposed to be the perimeter sites between the Au nanoparticles and the support.46 Overbury and co-workers employed supported Au22(L8)6 nanoclusters (L = 1,8-bis(diphenylphosphino)octane) in CO oxidation and found an activity order of CeO2 > TiO2 > Al2O3 for the support material used. Because of the presence of eight uncoordinated metal sites in the cluster structure, even the unpretreated Au22(L8)6/TiO2 exhibits CO oxidation activity above room temperature.47

This is in striking contrast to thiolate-protected Au nanoclusters, where CO oxidation has so far been focused on CeO2 as support material,48−54 because Au25(SR)18/TiO2 was found to be almost inactive.49 Jin and co-workers found the highest activity for Au25(SR)18/MxOy (M = Ce, Fe, Ti) catalysts in CO oxidation for CeO2 as support material (∼50% conversion at 150 °C with 0.1 g of catalyst with 2 wt % cluster), while the TiO2 catalysts did not exhibit any significant activity.49 This was attributed to the crucial role of the Ce4+ perimeter sites in the allocation of lattice oxygen atoms, with the CO oxidation in that case mainly following a Mars–Van Krevelen mechanism.50 Also, Au38SR24/CeO2 catalysts activated by a mild O2 pretreatment (<175 °C) exhibited conversions of around 90% at 100 °C (50 mg of catalyst, 1 wt % cluster).48 Li et al. also reported striking differences in the CO oxidation activity in series of Au38(SR)24/CeO2, Au36(SR)24/CeO2, and Au25(SR)18/CeO2 catalysts depending on how much the specific protecting thiol ligand influenced the accessibility of the Au–thiol–CeO2 interfacial sites. They further found that different geometric cluster core structures (icosahedral vs FCC) can determine whether thermal activation is required.52

Furthermore, heteroatom doping of Au clusters for CO oxidation was so far limited to replacing multiple Au atoms by Cu or Ag. This was found to alter the CO oxidation activity, with CuxAu25–x(SR)18/CeO2 performing slightly better but AgxAu25–x(SR)18/CeO2 worse than Au25(SR)18/CeO2. This was attributed to the different CO adsorption energies on the metal surfaces.54 Similar effects had already been reported for CO oxidation on bimetallic AuPd(100) surfaces, with their low temperature efficiency ascribed to a decrease in CO adsorption energy (and thus weaker CO poisoning).55

Multiple studies have also indicated that for heterogeneous nanocluster catalysis the (partial) removal of the thiolate protecting groups is necessary.36,38,51,56−58 Several strategies are known to expose the active metal surfaces of the clusters,57,59 among which thermal pretreatments36,38,51,53,56,60 are probably most often applied. For CO oxidation on CeO2-supported Au38 nanoclusters, Jin and co-workers reported that the reaction can only take place if the ligands at the cluster–support border were removed by O2 pretreatments.51 Au38(SR)24/CeO2 catalysts were found to be most active for CO oxidation after an oxidative pretreatment at 250 °C, which resulted in exposing the bare Au metal surface. The same procedure at 150 °C only led to a collapse of the clusters’ staple structure, with S still poisoning the Au core.60 A combination of O2 and a reductive pretreatment (with CO or H2) at 80 °C led to 100% CO conversion (80 °C, 50 mg of catalyst, 1 wt % cluster) for Au144(SR)60/CeO2 catalysts in CO oxidation.53 An oxidative pretreatment in air followed by reducing in H2 atmosphere (both at 250 °C) also led to the best results in activating AuPd particles by formation of segregated Pd sites by Scott and co-workers.38 The benefit of a reductive pretreatment step before CO oxidation was also observed for PdAu/TiO2 nanoparticle systems.61

Kurashige et al.29 recently investigated water splitting with Au24M–BaLa4Ti4O15 (M = Pd, Pt) catalysts. Both Pd and Pt heteroatom doping were found to strongly influence the catalytic activity. The exact location of the dopant atom played a critical role. As revealed by extended X-ray absorption fine structure (EXAFS) measurements at the Pd K-edge and Pt L3-edge, the heteroatom dopants migrated upon ligand removal at 300 °C, finally occupying a position at the Au cluster/support interface (Pt) or a Au cluster surface position (Pd). In the case of Pd, Pd–S interaction was still observed after pretreatment but disappeared during the water splitting reaction, indicating the presence of exposed Pd metal. Luneau et al. observed a redistribution of Pd in silica supported Pd0.04Au0.96 from internal to surface sites, stabilized by formation of Pd–O interactions. Exposure to H2 atmospheres at elevated temperatures led to reoccupation of subsurface positions.62 Pd surface segregation was also found in several PdAu nanoparticle systems when exposed to CO atmospheres.55,63−65 Accordingly, to be able to correlate and understand the catalytic behavior of bimetal nanocluster systems, studies of their structural evolution during pretreatment and reaction are of tremendous importance.

Therefore, within this study, we have employed titania-supported Pd-doped Au nanoclusters to investigate structural changes upon pretreatment and reaction (Scheme 1). The influence of pretreatment was studied for PdAu24/TiO2, and its CO oxidation activity was compared to that of Au25/TiO2. The bimetallic cluster was found to strongly increase the CO conversion, which seems to be related to Pd atom migration from the cluster center to the outer surface. To test this hypothesis, we have also prepared multiply doped PdxAuy clusters (Pd/Au: 0.25–0.33) and conducted in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) as well as EXAFS (Pd-K and Au-L3 edges) and X-ray photoelectron spectroscopy (XPS) after pretreatment and reaction. The observation of Pd–S interactions clearly indicated that Pd atoms were initially located in the staples but then migrated to the cluster surface, forming a PdAu alloy exhibiting isolated Pd sites. Altogether, the presented studies should aid an improved understanding of the specific catalytic activity of Pd-doped Au nanoclusters.

Scheme 1. Illustration of the Structure of PdAu24 and PdxAuy (with Pd5Au20 as an Example), Displaying the Different Possible Locations of the Pd Doping Atom(s).

Scheme 1

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PdAu24 features only core center doping, whereas doping both at the core center and in the staples is possible for PdxAuy. The hydrocarbon backbone of the ligands is not shown. Au atoms in the staple region are depicted smaller than the ones in the kernel for better visibility of the overall cluster structure.

2. Experimental Section

2.1. Nanocluster Synthesis

[Au25(SR)18]− (SR = SC2H4Ph; 2-phenylethanethiolate; 2-PET) clusters were prepared by following Shivare’s method.66 PdAu24(SC2H4Ph)18 was synthesized following protocols for synthesis67 and postsynthetic separation68 reported by Negishi and co-workers. Both types of clusters were characterized by ultraviolet–visible (UV–vis) spectroscopy and matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS). Multiply doped PdxAuy nanoclusters (PdxAuy(SR)z) were obtained following the previous synthesis procedures67,68 with some adjustments and were characterized by UV–vis. Details and UV–vis and MALDI-MS spectra can be found in the Supporting Information.

2.2. Nanoparticle Synthesis

Metal nanoparticles (Au, Pd) were synthesized by the citrate method.69,70 The detailed protocol, UV–vis spectra, and particle size estimation can be found in the Supporting Information.

2.3. Catalyst Preparation

To produce catalysts with an anticipated 2 wt % metal loading, the corresponding amounts of clusters were dissolved in toluene and stirred together with the TiO2 support material (Degussa P25 TiO2, rutile:anatase 85:15, 99.9% purity, 20 nm average particle size) for 24 h. Then, catalysts were separated from the solvent by centrifugation and decantation. Remains of toluene were removed at 60 °C under reduced pressure (1 h). Supporting of the Au and Pd nanoparticles followed the same protocol, except the use of H2O instead of toluene.

2.4. Characterization

The actual Au loading (wt %) of the catalysts was quantified by total reflection X-ray fluorescence (TXRF).

X-ray photoelectron spectroscopy (XPS) was measured in a UHV system (base pressure: 5 × 10–10 mbar) equipped with a Phoibos 100 hemispherical analyzer and a XR 50 X-ray source (all SPECS GmbH). Pure clusters were dropcast as a dichloromethane solution on highly oriented pyrolytic graphite (HOPG). The catalyst powders PdAu24(SR)18/TiO2 and PdxAuy(SR)z/TiO2) were placed on UHV-compatible conductive carbon tape on transferrable sample holders. Spectra were acquired at room temperature by using Al Kα radiation (1486.61 eV) and a photoelectron emission angle of 0°, with the analyzer operated in “large area” transmission mode. By use of CasaXPS, all spectra were referenced to the C 1s signal (C–C, 284.6 eV). Subsequently, peaks were fitted after Shirley background subtraction utilizing Gauss–Lorentz sum functions and consistent values of full width at half-maxima (FWHM). Peak positions (Au 4d5/2: 333–335 eV; Pd 3d5/2: 335–339 eV), doublet separation (Au 4d: 18.1 eV; Au 4f: 3.7 eV; Pd 3d: 5.3 eV), and peak area ratios (d5/2:d3/2 = 3:2 and f7/2:f5/2 = 4:3) were constrained according to the NIST XPS database.

The supported nanoclusters were further analyzed by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Refer to the Supporting Information for details.

2.5. Pretreatment

Different pretreatments were tested to find the optimal conditions for activation of PdAu24/TiO2 catalyst, while all other parameters were kept constant (250 °C maximum temperature, 10 °C/min heating ramp, 40 min holding time at 250 °C, and 50 mL/min total gas flow): (1) argon pretreatment (pretAr); (2) oxidative pretreatment with 5% O2 in argon, cooldown to RT under argon (pretO2); (3) reductive pretreatment with 5% H2 in argon, cooldown to RT under H2 (pretH2); (4) pretO2 followed by pretAr (pretO2–Ar); (5) pret-Ar followed by pretH2 (pretAr–H2); and (6) pretO2 followed by pretH2 (pretO2–H2). The argon pretreatment step in pretO2–Ar and pretAr–H2 was intended to compensate for the different pretreatment durations, which were reported by Li et al.53 to have a significant effect on the catalyst activation.

2.6. Catalytic Studies

Gas phase CO oxidation was performed in a flow reactor coupled to a Micro GC. Gases were regulated with Bronkhorst mass flow controllers; the reaction temperature in the cylindrical oven was adjusted by a PID controller, with a Ni/NiCr thermocouple placed inside the catalyst bed. The catalyst (15 mg) was located inside a quartz glass tube attached between quartz wool plugs, with a total gas flow of 50 mL/min in all experiments. Pretreatments were performed as described in the previous section. For the optimal pretreatment (pretO2–H2), 40 min oxidation (5% O2 in argon) at 250 °C (10 °C/min rate) followed (after cooling in Ar) by 40 min reduction (5% H2 in argon) at the same temperature and heating rate as the previous oxidation was employed. Once the catalyst had been pretreated, CO oxidation was performed with 1:2 CO:O2 in argon (1% CO, 2% O2, total flow 50 mL/min). The reaction was conducted in temperature steps (150, 175, 200, 225, and 250 °C), and each temperature was held for 30 min to reach steady state.

2.7. In Situ/Operando Studies

Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) studies were performed on a Bruker Vertex 70 spectrometer with a liquid-N2-cooled MCT detector and with 4 cm–1 resolution. The stainless-steel flow cell (Pike) has a CaF2 window and an oven. The inlet of the cell was connected to a gas manifold system with calibrated mass flow controllers to adjust the gas mixtures and a mass spectrometer for kinetic measurements. Each sample was placed into a small ceramic cup, and the exact weight was taken for normalization (∼30 mg). The sample was pretreated as described in section 3.1. Then, the gases were changed to reaction conditions without removing the sample in between. The reaction temperature was increased with a heating ramp of 1 °C/min and kept at the maximum temperature of 250 °C (reached after 125 min) for 2 h. Afterward, the sample was cooled to room temperature (under reaction gas mixture or inert gas flow, depending on experiment). To study the surface composition of the used catalysts, 1% CO in He was flown through the cell for 30 min. Afterward, the cell was purged for 30 min with inert gas to remove the CO. DRIFTS spectra were taken over the course of the whole experiment by averaging 256 scans to achieve a good signal-to-noise ratio. For background removal, the spectra of the unpretreated catalysts in He were used for the operando CO oxidation experiments and the spectra of the catalysts after pretreatment for the CO dosing measurements.

X-ray absorption spectroscopy (XAS) measurements were performed at the CLAESS Beamline at Alba Synchrotron in fluorescence mode (Pd K-edge and Au-L3 edge) in the beamline’s solid/gas reactor multipurpose cell. The catalysts were pressed into pellets. The samples were pretreated inside the multipurpose cell at 250 °C for 40 min under oxygen flow (pretO2; 5% O2 in He; total flow: 45 mL/min; cooldown 40 mL/min He) followed by a reductive pretreatment under hydrogen (pretH2; 5% H2 in He; total flow: 45 mL/min) at the same conditions. After cooling (5% H2 in He; total flow: 45 mL/min), the gas mix was changed to reaction conditions (reaction: 1.7% CO, 3.3% O2 in He; total flow: 45 mL/min). The samples were heated to 250 °C with a ramp of 5 °C/min. The maximum temperature was held for 60 min, and then the reaction chamber was cooled to RT (45 mL/min He). Extended X-ray absorption fine structure (EXAFS) spectra were taken at 40 °C in He at the beginning, after pretreatment and after reaction for each sample, without opening the reaction chamber in between. The Artemis package71 that uses the FEFF8 code72 was applied for EXAFS data treatment and is described in the Supporting Information.

3. Results and Discussion

3.1. Pretreatment Effect on PdAu24/TiO2 Nanocluster Catalysts

As discussed in the Introduction, optimal activation of a catalyst is crucial to achieve high reactivity. Our and other groups’ previous studies have indicated that the local removal of the thiolate ligands from Au is best achieved by oxidative pretreatment at 250 °C, exposing clean Au clusters surfaces.56,60,73 However, for bimetallic PdAu catalysts, because of the easier oxidation of Pd, an additional reductive step is required to create Pd0 active sites.61 To find optimal catalyst activation conditions, six different thermal pretreatments were tested for the PdAu24(SR)18 (SR = 2-PET) clusters supported on titanium dioxide, differing in the gas types and sequence (pretAr, pretO2, pretH2, pretAr-H2, pretO2-Ar, pretO2–H2), before carrying out CO oxidation.

As evident from Figure 1a, the type of pretreatment plays a significant role. Thermal pretreatment in argon was not able to remove the thiol ligands, which then blocked the active sites even at 250 °C reaction temperature. Pure oxidation or reduction improved activity at 250 °C, but the maximum and lower temperature activity was obtained by a combination of oxidation and reduction (pretO2–H2). Oxidation allows for removal of ligands49,74 and surface impurities (synthesis residues), while the subsequent H2 treatment reduces oxidized Pd (and Au) atoms.61 Activity set in at 225 °C, which agrees with CO oxidation on (impregnated) Pd/Al2O3 catalysts (below 225 °C, the actives sites are poisoned by CO).75,76Figure 1a also shows the effect of longer heating times (e.g., pretO2 vs pretO2–Ar).

Figure 1.

Figure 1

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(a) Effect of pretreatment on the catalytic CO oxidation activity of PdAu24/TiO2. (b) Catalytic CO oxidation activity of PdAu24/TiO2, Au25/TiO2 and the pure support (all after pretO2–H2).

3.2. Comparison of PdAu24/TiO2 to Au25/TiO2 and TiO2

To evaluate the effect of Pd doping on the nanoclusters’ reactivity, PdAu24/TiO2 was contrasted to undoped Au25/TiO2 and the pure titania support, with pretO2–H2 applied to all samples. As evident from Figure 1b, the catalytic activity was very low below 200 °C, for both PdAu24/TiO2 and Au25/TiO2. For the palladium-doped catalyst, the onset of activity was at 225 °C, reaching more than 90% conversion at 250 °C. Only around 10% conversion was detected for Au25/TiO2 at 250 °C, slightly higher than that of the bare TiO2 support. As oxidation at 250 °C should remove the ligands from Au, the inactivity of Au25/TiO2 can likely be attributed to continued S poisoning of the support. PretO2 works well for Au25/CeO249 or Au38/CeO2,60 but it is insufficient for Au25/TiO2, which was also reported previously. The inactivity of ligands on Au25/TiO2 catalysts in CO oxidation is in striking contrast to ligand-free Au nanoparticle/TiO2 catalysts active even below room temperature43 and has been reported previously.49 Nevertheless, the strong effect of adding only one single Pd dopant atom to a Au nanocluster is evident.

3.3. Operando DRIFTS of CO Oxidation on PdAu24/TiO2

To further examine the effect of doping, operando DRIFTS during CO oxidation was performed on PdAu24/TiO2 after a one-step pretreatment with oxygen (pretO2) and after a pretreatment with both an oxidative and a reductive step (pretO2–H2). Those represent catalysts with a relatively low (pretO2) vs the maximum level of activity (pretO2–H2) obtained in the catalytic tests (Figure 1a). Figure 2 shows temperature-dependent (stepwise heating) infrared spectra of PdAu24/TiO2 after pretO2–H2 (a) and after pretO2 (b).

Figure 2.

Figure 2

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Operando DRIFTS spectra of PdAu24/TiO2 catalysts during CO oxidation: (a) after pretO2–H2 and (b) after pretO2. Spectra taken upon cooling are indicated by dotted lines. Spectra of the reference operando DRIFTS measurements of TiO2 can be found in the Supporting Information (Figure S10). Both “afterpretO2” and “afterpretH2” were acquired in He at RT. Note that absolute intensities of different samples cannot be compared.

Focusing first on catalytic activity, indicated by the CO2 gas phase bands (2400–2300 cm–1), for PdAu24/TiO2 (pretO2–H2), catalytic activity set in above 200 °C (Figure 2a). The PdAu24/TiO2 catalyst after pure oxygen pretreatment became active at 250 °C (cf. the CO2 bands in Figure 2b). The activity trends indicated by the CO2 gas phase bands were corroborated by mass spectroscopic (MS) analysis of the DRIFTS cell exhaust gas (Figure S12), both being in line with the GC measurements in Figure 1b.

Turning to adsorbed species, there were significant differences between pretO2–H2 and pretO2. After pretO2–H2, PdAu24/TiO2 showed a band at 2067 cm–1, clearly indicating CO adsorbed on Pd. No CO adsorption on the TiO2 support could be detected (see Figure S10).

Based on previous studies of Pd77 and PdAu alloy64 nanoparticles, 2067 cm–1 points to atop CO on isolated Pd atoms. After synthesis (and before pretreatment), the Pd dopant atom is located in the center of the Au nanocluster core (Scheme 1).9,12 This would indicate that the Pd atom should not be accessible to CO bonding. This seems to hold true for the pretO2 sample (Figure 2b), for which no Pd-related bands could be detected. Therefore, the CO-Pd band in Figure 2a indicates that pretO2–H2 induces migration of the Pd atom from the cluster center position to the surface of the Au core. Mobility of metal atoms within a nanocluster structure13−23,29 and Pd segregation to the surface of alloy PdAu particles were indeed reported previously.62−64,78 The band around 1962 cm–1 at 250 °C became more pronounced after cooling (dotted traces in Figure 2a) and was assigned to bridging CO on Pd–Au alloy sites based on experimental and theoretical studies.63,75,79,80 Zhu et al. corroborated DFT calculations by several groups by experimental DRIFTS measurements, assigning the bands between 1950 and 1969 cm–1 to bridged CO on PdAu sites.63,79,81

The formation of PdAu observed by operando DRIFTS is also consistent with the higher activity of PdAu24/TiO2 below 250 °C, when CO oxidation is continued (see Figure S8). Not only pretO2–H2 but also the reaction conditions led to further activation of the PdAu24(SR)18/TiO2 catalyst. This is in agreement with the observations by Luneau et al., who reported changes in the palladium surface content of PdAu nanomaterials, induced by two consecutive treatments (O2 and H2) or under CO exposure.62 Upon H2 treatment, Pd moved subsurface but migrated back to the Au surface in the presence of CO.

The region of the CO gas phase vibration rotations (yellow area in Figure 2) overlaps with possible bands associated with stretching vibrations of CO adsorbed on Au60 (marked as CO–Au in Figure 2). Slight shifts to higher wavenumbers were observed, from 2169 and 2115 cm–1 (gas phase CO, see Figures S10 and S11) to 2175 and 2120 cm–1, which may be related to the contribution of CO–Au bands. Therefore, CO dosing experiments were performed with the samples after different pretreatments (Figure S11), detecting low-intensity CO–Au bands around 2108 and 2118 cm–1. For PdAu24/TiO2 after pretO2–H2, an intense CO–Pd band at 2046 cm–1 was detected, confirming the observations of the operando DRIFTS measurements. No additional bands in the region around 1900 cm–1 appeared, neither in the CO-dosing experiments (Figure S11b,c), which rules out the presence of bridge/hollow CO–Pd vibrations, characteristic of larger Pd ensembles.

3.4. Structural Evolution of PdxAuy/TiO2 Catalysts

In the following, catalysts with more than one Pd dopant atom per cluster were prepared to investigate how the structure of supported bimetallic PdAu nanoclusters evolved during CO oxidation. The Pd:Au ratio of supported and pretreated (pretO2–H2) clusters was about 1:4 or 1:3, as determined by TXRF and XPS (see the Supporting Information). According to STEM, the particle size of PdxAuy on TiO2 was around 1.3 nm before pretreatment, indicating that the clusters contained ∼25–30 metals atoms (see Figure S6 and Table S2). Changes in size were noted after pretreatment, with particle sizes of around 2.7 nm observed by HAADF-STEM (Figure S7). The catalytic activity of PdxAuy/TiO2 is compared to that of monopalladium PdAu24/TiO2 in Figure 3 (both after pretO2–H2). The higher number of (∼6) Pd dopant atoms resulted in a small increase of catalytic activity.

Figure 3.

Figure 3

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Comparison of the catalytic activity of PdAu24/TiO2 and PdxAuy/TiO2 (x = ∼5–8; y = ∼25–30; values estimated for unpretreated catalyst from STEM and TXRF) catalysts (both pretO2–H2).

The activity of the PdxAuy/TiO2 catalyst was also compared to those of (impregnated) Au and Pd nanoparticles (NPs) supported on the same TiO2 (see Figure S9). The activity of the nanocluster catalyst was significantly lower than that of supported Au NPs, which seems to be related to the clusters’ protecting ligands blocking adsorption sites even after the pretreatment. CO conversion of supported Pd NPs was found to be slightly higher than those of PdxAuy/TiO2, both showing a similar temperature trend, which points to CO poisoning of Pd below ∼200 °C.

3.5. In Situ DRIFTS of CO Oxidation on PdxAuy/TiO2

The higher catalytic activity obtained by multiple Pd doping was better visible from operando DRIFTS via stronger CO2 bands at corresponding temperatures (Figure 4a). The CO gas phase bands at 2168 and 2122 cm–1 are overlapping with potential CO–Au bands (≈2130 cm–1), which is why their presence was confirmed by postreaction CO dosing experiments (Figure 4c). The region related to CO–Pd vibrations displayed a strong component at 2060 cm–1 and a shoulder at 2075 cm–1, due to different on-top CO species on Pd atoms, with the vibrational frequency depending on the coordination numbers of Pd in the AuxPdy alloy clusters.63,78

Figure 4.

Figure 4

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(a) Operando DRIFTS spectra during CO oxidation on PdxAuy/TiO2 (pretO2–H2). (b) Enlarged view. (c) Postreaction DRIFTS spectra upon 1% CO dosing and evacuation.

Under reaction conditions, the on-top and bridged CO on Pd (2076 cm–1) became even more pronounced. This points to rearrangements, resulting from Pd and Au mobility, and possible formation of Pd dimers and larger ensembles. The latter are known to effectively dissociate O2.78

Figure 4c shows the postreaction characterization of PdxAuy/TiO2 (pretO2–H2) after cooldown and purging with He. Upon CO dosing, the same bands as observed under reaction conditions appeared, slightly shifted due to the different gas atmosphere. CO–Au and CO–Pd vibrations can be identified at 2125 and 2080 cm–1, respectively. The bands at 1968 and 1950 cm–1 confirm Pd segregation and formation of dimer and larger sites.55,63,64 However, other studies reported that these bands originated from PdAu alloy sites instead.79 This seems to apply to our PdxAuy/TiO2 also, as EXAFS measurements (discussed below) detected no significant contribution of Pd–Pd bonds. After pretreatment and CO oxidation, Pd is therefore expected to be located on the surface of a Au cluster in the form of isolated and/or neighboring Pd atoms/sites.

3.6. Ex Situ XPS

The composition and oxidation state of Pd and Au in the PdxAuy nanoclusters were additionally investigated by ex situ XPS. For the PdxAuy bimetallics, the Pd 3d and Au 4d region is most informative (Figure 5a). When the as-prepared (untreated) clusters were deposited on HOPG, the Pd 3d binding energies suggested oxidized Pd species. This is in accordance with the expected location of the Pd dopants in the staples of the clusters (as only one Pd atom can be at the core center). Upon supporting, a small (∼0.4 eV) shift was observed. In the Au 4f region (Figure 5b), changing from a HOPG to TiO2 support, the peaks also shifted, leading to BEs typical of ligand-protected nanoclusters (Au0 in the core and Au+ in the staples).56

Figure 5.

Figure 5

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Ex situ XPS spectra of the PdxAuy catalyst: (a) Pd 3d and Au 4d region; (b) Au 4f region: clusters supported on HOPG (clusters), clusters supported on TiO2 (fresh catalyst), the latter after pretreatment (after pretO2–H2), and after reaction (after COox).

After pretreatment (pretO2–H2), the Pd 3d signal was shifted to even lower binding energy (336.1 eV), indicative of metallic Pd in a PdAu alloy.65,82,83 Small shifts to even lower binding energies were also found for the Au 4d signal. The Au 4f spectra corroborated these compositional changes and confirmed PdAu alloy formation. Overall, the XPS measurements agreed with the operando DRIFTS spectra (Figure 4a,b), indicating surface Pd species and PdAu alloy formation.65,82−84

3.7. In Situ EXAFS

The structural evolution of the TiO2-supported bimetallic PdxAuy nanoclusters was studied by in situ XAFS at the Pd K-edge and the Au L3-edge. Figure 6a and Table 1 show results of the Pd K-edge EXAFS fitting for each step. Supporting the PdxAuy clusters on TiO2 by impregnation does not lead to significant changes (Figure 6), confirming the stability of the cluster structure. As can be seen from Table 1, Pd K-edge EXAFS of the as-prepared sample showed a strong peak at 2.33 Å, followed by low-intensity peak at 2.81 Å, related to Pd–S and Pd–Au bonds, respectively. Together with the coordination numbers (CNs) obtained (2.2 and 0.9, respectively), this indicates Pd atoms being located in the staples, in agreement with previous reports.39,85,86 The absence of Pd–Pd bonds indicates isolated Pd atoms in the clusters and apparently a lower percentage of Pd than Au, based on TXRF, XPS, and previous studies on PdAu nanoparticles.38,61,64,82 This is also confirmed by the fitting results at Au L3-edge, where no Au–Pd bonds were detected in the fresh sample (Table 1). Au–Au bond contributions from the fresh sample are barely visible, resulting in a low CN value. This is related to the clusters being measured immobilized on the TiO2 support and also to the conditions and has been reported previously.87−89

Figure 6.

Figure 6

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R-space EXAFS of spectra (without phase correction) of the PdxAuy/TiO2 catalyst: (a) Pd K-edge and (b) Au L3-edge. The structural evolution of the cluster structure is monitored starting with the fresh catalyst to after pretreatment and after CO oxidation. Pd and Au foil are included as references.

Table 1. EXAFS Fitting Results for the PdxAuy/TiO2 Catalyst.

  Pd K-edge
 Au L3-edge
  Pd–S
 Pd–Au
 Au–S
 Au–Au
 Au–Pd
  CN R (Å) CN R (Å) CN R (Å) CN R (Å) CN R (Å)
fresh 2.3 (3) 2.33 (1) 0.9 (5) 2.81 (3) 2.1(4) 2.31 (1) 0.6 (1) 2.74 (6)    
after pretO2–H2 0.9 (3) 2.30 (2) 4.1 (6) 2.82 (1) 0.2(3) 2.31 (1) 8.5 (7) 2.84 (1) 0.6 (5) 2.80 (1)
after COox 0.2 (1) 2.30 (3) 6.2 (6) 2.80 (2)     8.7 (8) 2.83 (1) 0.9 (5) 2.80 (1)
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After pretO2–H2, most Pd–S and Au–S bonds disappeared, denoting the removal of S from the clusters. However, some remaining Pd–S may be related to the staples collapsing on the cluster core surface, as already observed previously.60,73 Simultaneously, Pd–Au bonds were established, denoting PdAu alloy formation. The alloying process, initiated by the migration of Pd from the staples to the Au core surface (maybe during the staple-collapse), continues under CO oxidation reaction. No evidence for Pd–Pd bond formation was detected (also corroborated by EXAFS simulations; see the Supporting Information). Therefore, the results indicate that isolated Pd single atom surface sites are present on PdAu nanoalloys.33,40,82 The fitting results at the Au L3-edge after the CO oxidation reaction denoted a higher CN number of Au–Au in comparison to Au–Pd, corroborating the single Pd atoms dispersed across the nanocluster surface.64

The particle size of the AuxPdy particles after pretreatment and reaction was then extracted based on the CN numbers of the Au L3-edge EXAFS, considering the Au–Au and Au–Pd CNs from the first shell, related to the number of next-neighbor atoms. For bulk fcc gold, the Au–Au CN = 12, but it is significantly lower for nanoclusters (CN ∼ 3–9).39,90,91 For PdxAuy/TiO2, the coordination numbers of the first shell (Au–Au and Au–Pd) increased from about 8 to 9 during pretreatment and reaction, indicating an increase in particle size. Based on previous studies, PdxAuy clusters after reaction may consist of 40–80 atoms (depending on the structure/shape) with a particle size of ∼2 nm.36,38,39,84,90,91 A mean particle size of around 2.7 nm was observed by HAADF-STEM for pretO2–H2 and after CO oxidation (see Figure S7 and Table S2).

Scheme 2. Illustration of the Structure Evolution of PdxAuy Clusters Supported on TiO2 upon Pretreatment and CO Oxidation.

Scheme 2

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4. Conclusions

This work demonstrates the dynamic structure of Pd-doped Au25 nanoclusters upon pretreatment and catalytic reaction. When one Pd atom was doped into the center of a Au25 cluster, a combination of oxidative and reductive pretreatment was required to obtain maximum CO oxidation activity. This was related to the migration of the Pd atom to the surface of the cluster core. A CO–Pd vibrational band, characteristic of single site bonding, was observed by operando DRIFTS. Therefore, the activity enhancement of the doped system is related to a single Pd atom located on the Au cluster surface. In the case of multiply-Pd-doped Au nanoclusters, the Pd atoms were initially mainly located in the staples. Upon pretreatment, the particle size increased to 2.7 nm, and migration of Pd to the core surface was evidenced primarily by in situ XAFS, forming a PdAu alloy. The latter was confirmed by ex situ XPS. Because XAFS ruled out Pd–Pd bonds, isolated Pd single sites were present on the cluster surface, as further supported by operando DRIFTS.

Thus, the evolution of the Pd-doped Au clusters to PdAu nanoalloys with single Pd atom surface sites was revealed by combined XAFS, DRIFTS, and XPS analysis. Overall, this study contributes to a better understanding of the dynamics of supported doped nanoclusters upon pretreatment and reaction, which is a key information for future design and application of bimetallic nanocluster catalysts.

Acknowledgments

N.B. thanks the TUW Innovative Project GIP165CDGC. G.R. acknowledges financial support by the Austrian Science Fund (FWF) via grants SFB FOXSI (F4502), DK+ Solids4Fun (W1243), and Single Atom Catalysis (I 4434-N). The authors thank Stephan Pollitt for his advice in synthesizing Pd-doped Au nanoclusters. ALBA synchrotron is acknowledged for beamtime at CLAESS beamline (Proposal ID: 2019023443) and Dr. Vlad Martin-Diaconesu for assistance during measurements.

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.jpcc.0c05735.

  • Detailed synthesis procedures, UV–vis and MS spectra of unsupported nanoclusters, UV–vis spectra and DRS of the nanoparticle reference catalysts, HAADF-STEM images, additional DRIFTS spectra and kinetic measurements, EXAFS fitting (PDF)

Author Contributions

□ C.G. and V.T. contributed equally. Sample preparations were performed by C.G., V.T., I.L., and N.B. EXAFS measurements were performed by C.G., V.T., I.L., and N.B. EXAFS fitting was performed by C.M. Catalytic tests were performed by C.G., V.T., and I.L. Operando DRIFTS measurements and data analysis were done by C.G. and I.L. XPS measurements and analysis were performed by T.H., C.R., and V.T. HAADF-STEM measurements were performed by M.S.P. and K.D. MALDI analysis was done by E.P., and XRF measurements were performed by P.K. Final interpretation and manuscript preparation were led by N.B., V.T., C.G., and G.R., with contributions from all authors. Funding was acquired by G.R.

The authors declare no competing financial interest.

Special Issue

Published as part of The Journal of Physical Chemistry virtual special issue “Metal Clusters, Nanoparticles, and the Physical Chemistry of Catalysis”.

Supplementary Material

jp0c05735_si_001.pdf (2MB, pdf)

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