Following the thermal and chemical activation of supported Au clusters using X-ray absorption spectroscopy

Abstract

Al₂O₃-supported Au₂₅(SC₈H₉)₁₈ clusters with various Au loadings were thermally and chemically treated in order to determine the most efficient method towards the removal of thiolate stabilizers while avoiding unwanted increases in cluster size due to agglomeration and sintering. X-ray absorption spectroscopy (XAS) and transmission electron microscopy (TEM) were used to investigate samples before and after thermal and chemical treatments. Results show that while 250 °C thermal treatment leads to nearly complete removal of thiolate stabilizers, it comes with a concomitant increase in cluster size as sintering becomes problematic. In contrast, chemical reduction treatments using borohydride reducing agents does not lead to significant growth in cluster size, but only allows for partial thiolate removal. These results are important as many researchers look to determine optimal activation conditions for ultra-monodisperse Au and bimetallic clusters for use as model catalysts without destroying their original structures.

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Introduction

Au clusters, owing to their stability and well-defined structures, have recently emerged as an important class of materials in order to study the size and structure related properties of Au nanocatalysts.^1–3 Among all existing Au clusters, Au₂₅(SR)₁₈ clusters have gained special attention due to their high yield synthesis and exceptional stability.^4–6 Recently, several research groups have shown that Au₂₅(SR)₁₈ clusters in solution and on various support materials can act as catalysts for a variety of catalytic reactions such as CO oxidation, hydrogenation of nitrobenzene and its derivatives, styrene oxidation, semi-hydrogenation of terminal alkynes, and alcohol oxidations.^7–13 In many of these studies, the catalytic activity of Au clusters has been found to be affected by the steric bulk of the thiolate stabilizers, and a positive effect on the catalytic activity was found upon removal of the stabilizers. Removal of thiolate stabilizers has been mostly carried out by calcining these clusters at moderate temperatures, which leads to the partial or complete removal of thiolate stabilizers. For example; Tsukuda and coworkers found that thermal activation of porous carbon supported Au₂₅(SR)₁₈ clusters at 450 °C led to an enhancement in the catalytic activity and selectivity for benzyl alcohol oxidation.⁷ Wu et al. found that CeO₂-supported Au₂₅ clusters can be activated for CO oxidation catalysis by partially removing thiolate stabilizers at 150 °C.¹¹ Our group reported that mesoporous carbon-supported Au₂₅ clusters can be thermally activated for p-nitrophenol reduction catalysis by removing thiolate stabilizers at temperatures as low as 125 °C.¹⁰

While the effect of temperature on the removal of thiolate stabilizers from supported Au₂₅ clusters for catalysis has been studied by a number of groups, the effect of chemical reducing agents on the removal of thiolate stabilizers from supported Au clusters has not been studied much. This is despite the presence of literature reports suggesting that borohydride species can reductively desorb thiolate stabilizers from the surface of thiolate-protected Au nanoparticles in solution.^14,15 Recently Asefa and coworkers showed that BH₄⁻ treatment of supported thiolate-protected Au₂₅ and Au₁₄₄ clusters led to an enhancement in the catalytic activity for styrene oxidation and this enhancement was attributed to the partial removal of thiolate stabilizers after BH₄⁻ treatment.¹²

Herein, we document a detailed investigation of the removal of thiolate stabilizers from supported Au₂₅(SC₈H₉)₁₈ clusters with various Au loadings using thermal and chemical strategies using X-ray Absorption Spectroscopy (XAS) and TEM particle size analyses. Thermal treatment of supported Au₂₅(SC₈H₉)₁₈ clusters at 250 °C leads to nearly complete removal of thiolate stabilizers, albeit with a concomitant growth in cluster size. Treatment using various hydride reducing agents; however, did not lead to significant growth in cluster size, but only leads to partial thiolate removal. Our data show that very distinct features in the X-ray absorption near edge (XANES) and extended X-ray absorption fine structure (EXAFS) spectra can be used to follow the thiolate removal process from supported Au₂₅(SC₈H₉)₁₈ clusters and associated changes in cluster size during thermal and chemical treatments. To the best of our knowledge, this is the first study showing EXAFS evidence of the removal of thiolate stabilizers from supported Au₂₅(SC₈H₉)₁₈ clusters using borohydride reducing agents.

Experimental

Hydrogen tetrachloroaurate(III) trihydrate (HAuCl₄·3H₂O, 99.9% on metal basis, Alfa Aesar), tetraoctylammonium bromide (TOAB, 98%, Aldrich), phenylethanethiol (C₈H₉SH, 99%, Acros Organics), sodium borohydride (NaBH₄, 98%, EMD), and aluminum oxide (Al₂O₃, 58 Å, ∼150 mesh) were used as received. High purity acetonitrile was purchased from Fischer Scientific. High purity tetrahydrofuran was purchased from EMD and 100% ethanol was purchased from Commercial Alcohols. 18 MΩ cm Milli-Q (Millipore, Bedford, MA) deionized water was used throughout.

Synthesis of Au₂₅(SC₈H₉)₁₈ clusters

Synthesis of Au₂₅(SC₈H₉)₁₈ clusters is reported elsewhere.⁴ Briefly, 500 mg of HAuCl₄·3H₂O in 50 mL of THF was mixed with 1.2 equiv. of TOAB. To this solution, 5 equiv. of phenylethanethiol was added dropwise and the mixture was left for stirring till it became clear. Subsequently, 10 equiv. of NaBH₄ in 10 mL of ice cold water was added all at once. The resulting solution was stirred for four days. After four days, the reaction was stopped and the solvent was evaporated completely. The resulting solid was then washed with a mixture of ethanol/water several times to remove excess leftover thiol and disulfide species, and finally the residue was dried using a rotary evaporator. The resulting Au₂₅(SC₈H₉)₁₈ clusters were extracted with acetonitrile.

Synthesis of Al₂O₃-supported Au₂₅(SC₈H₉)₁₈ clusters

Supported clusters were prepared by immobilizing the required amount on Au₂₅(SC₈H₉)₁₈ clusters dissolved in THF on 200 mg of Al₂O₃ support using a wetness impregnation approach, and the obtained solid was dried by purging the sample using N₂ gas. As prepared supported clusters were stored at 0 °C under inert atmosphere prior to TEM and EXAFS analysis.

Thermal and hydride treatment

200 mg of supported Au₂₅(SC₈H₉)₁₈ clusters with different metal loadings were treated at 250 °C under air atmosphere for 90 minutes. Treatment with LiAlH₄ and LiBH₄ was carried out by suspending 100 mg of supported samples in 5 mL of hexane, followed by the addition of 70 equiv. of LiAlH₄ and LiBH₄ with respect to Au₂₅(SC₈H₉)₁₈ clusters. These solutions were stirred for 10 minutes and subsequently washed with hexane and water. Treatment with NaBH₄ was carried out by first mixing 100 mg of supported samples and 3500 equiv. of NaBH₄ with respect to Au₂₅(SC₈H₉)₁₈ clusters. To this mixture was then added 5 mL of deionized water and resulting mixture was allowed to stir for 20 minutes. After that, the resulting solid was washed with plenty of water and dried under vacuum.

Characterization methods

Transmission electron micrographs (TEM) were taken with a HT7700 TEM operating at 100 kV. TEM grids were prepared by placing a drop of sample suspended in hexane on graphene-enhanced lacey carbon TEM grids (Electron Microscopy Sciences). 20–30 particles were analyzed in order to calculate the average size and standard deviation. All samples used for EXAFS analysis were prepared by supporting phenylethanethiol-protected Au₂₅ clusters with different loadings, 2.5%, 1.5%, 0.75%, and 0.2% by metal weight on an Al₂O₃ support (58 Å, ∼150 mesh) using a wetness impregnation approach. EXAFS measurements were conducted at the Hard X-ray MicroAnalysis (HXMA) beamline 061D-1 (energy range 5–30 keV; resolution, 1 × 10⁻⁴ ΔE/E) of the Canadian Light Source (CLS, 2.9 GeV storage ring, 250 mA current). Samples was pressed into pellets and measured in transmission (2.5% and 1.5% loadings by metal weight) and fluorescence (0.75 and 0.2% loadings by metal weight) modes by measuring the Au L₃-edge. A double-crystal Si (111) monochromator was employed for energy selection. Higher harmonics were eliminated by detuning double-crystal using Rh coated KB mirrors for the Au L₃-edge. The incident and transmission X-ray intensities were detected by ion chambers filled with helium–nitrogen mixtures for transmission measurements, and a 32 element detector was used for fluorescence measurements. The IFEFFIT software package was used for data processing.¹⁶ For the data analysis, the amplitude reduction factor, S₀², was found from fitting Au foil and subsequently fixed at 0.90. For EXAFS data fitting, a standard Au₂₅L₁₈ crystal structure was using in order to fit the as-synthesized, LiAlH₄ and LiBH₄ treated samples.¹⁰ Au face centered cubic (fcc) and AuS models were used to fit thermally-treated and NaBH₄-treated samples.

Results and discussion

Fig. 1 shows the EXAFS fitting in R-space of Al₂O₃-supported Au₂₅(SC₈H₉)₁₈ clusters with 2.5% loading by Au weight in R-space. The data shows one major peak at ca. 2 Å due to the scattering of photoelectrons from neighboring sulphur atoms along with three much smaller Au–Au peaks (∼2.2–3.2 Å), which have been shown by Zhang and coworkers to be due to different Au–Au bonding pairs present within Au₂₅ clusters.^10,17 The first Au–Au (core) peak is due to the bonding between the central Au atom of the icosahedron core of the clusters and 12 neighboring Au atoms along with six short Au–Au pairs present on Au₁₃ surface with an average Au–Au bond distance of 2.73 Å (designated Au–Au core contribution). The second Au–Au peak is due to the presence of larger Au–Au pairs on the surface of Au₁₃ core with the Au–Au bond distance of 2.94 Å and is given the designation Au–Au surface contribution. The last Au–Au (staple) peak is due to the bonding between Au atoms from staple motifs and Au atoms from the surface of the Au₁₃ core with the Au–Au distance of 3.3 Å.¹⁷ Due to the phase shift of the exited photoelectron, Au–Au bonding pairs in R-space data (Fig. 1) are at lower R values compared to the bond distance values obtained from single crystal data.¹⁸ In order to fit the EXAFS data, different Au–Au coordination numbers were fixed to that obtained from the literature crystal structures and the rest of the parameters were allowed to vary (Table 1). Here, interestingly, we found that the Au–S coordination number value of 1.4(1) was lower than the value reported for intact Au₂₅(SR)₁₈ clusters by both ourselves and others (Au–S, CN = 2.0).^10,17 This is a very interesting finding and we attribute it to the partial removal of thiolate groups after the immobilization of Au₂₅(SC₈H₉)₁₈ clusters on the oxide support.

Fig. 1 EXAFS fitting in R-space of Al₂O₃-supported Au₂₅(SC₈H₉)₁₈ clusters with 2.5% loading by metal weight.

Table 1 EXAFS fitting parameters of Al₂O₃-supported Au₂₅(SC₈H₉)₁₈ clusters with 2.5% loading by metal weight

	CN	R (Å)	σ^2 (Å^2)	Eo (eV)
a Au–Au coordination numbers were fixed as discussed in ref. 10 and 17.
Au–S	1.4(1)	2.313(5)	0.001(4)	2.2(7)
Au–Au (core, 1)	1.44^a	2.732(4)	0.007(1)	6.2(1.2)
Au–Au (surface, 2)	1.92^a	2.94(5)	0.016(5)	6.2(1.2)
Au–Au (staple, 3)	2.88^a	3.30(8)	0.04(2)	6.2(1.2)

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Recently, various groups have reported that oxide supported Au₂₅(SR)₁₈ clusters show catalytic activity at moderate temperature without any high temperature thermal activation.^19–21 This finding provides a possible explanation to this by attributing the catalytic activity of these supported clusters to a decrease in Au–S coordination number, which opens up the active sites for catalysis. Fig. 2 shows the EXAFS data in R-space of Al₂O₃ supported Au₂₅(SC₈H₉)₁₈ clusters with 1.5% loading by Au weight after thermal (250 °C) and chemical treatments. BH₄⁻ and thermal treatments led to a decrease in the Au–S contribution and increase in the Au–Au contribution. The decrease in the Au–S contribution shows removal of thiolate stabilizers from the surface of Au clusters, and the increase in the Au–Au contribution shows growth in cluster size. EXAFS fittings in R-space after thermal and chemical treatment is shown in Fig. 3. Full fitting parameters for each of the samples are found in Table 2.

Fig. 2 EXAFS data in R-space of thermally and chemically treated Al₂O₃-supported Au₂₅(SC₈H₉)₁₈ clusters with 1.5% loading by metal weight.

Fig. 3 EXAFS fitting in R-space of thermally and chemically treated Al₂O₃-supported Au₂₅(SC₈H₉)₁₈ clusters with 1.5% loading by metal weight.

Table 2 EXAFS fitting parameters of thermally and chemically treated Al₂O₃-supported Au₂₅(SC₈H₉)₁₈ clusters with 1.5% loading by metal weight

		CN	R (Å)	σ^2 (Å^2)	Eo (eV)
a Au–Au coordination numbers were fixed as discussed in ref. 10 and 17.
LiAlH4	Au–S	1.2(1)	2.30(2)	0.0007(8)	−1.1(1.1)
	Au–Au(1)	1.44^a	2.73(2)	0.007(3)	2.2(3.2)
	Au–Au(2)	1.92^a	2.89(3)	0.012(5)	2.2(3.2)
	Au–Au(3)	2.88^a	3.2(1)	0.05(4)	2.2(3.2)
LiBH4	Au–S	0.8(1)	2.319(8)	0.0004(9)	1.0(1.5)
	Au–Au(1)	1.44^a	2.7(3)	0.007(2)	0.8(1.4)
	Au–Au(2)	1.92^a	2.85(4)	0.019(8)	0.8(1.4)
NaBH4	Au–S	0.6(1)	2.301(7)	0.001	4.2(7)
	Au–Au	6.1(7)	2.83(1)	0.003(1)	4.2(7)
250 °C thermal	Au–Au	9.7(5)	2.846(4)	0.0095(5)	4.9(3)

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Data show that the Au–S contribution below 2 Å decreases in intensity after treatment with different reducing agents and completely disappears after thermal treatment at 250 °C. After LiAlH₄ treatment, the Au–Au contributions experienced very little change; however, significant changes were observed after treatment with LiBH₄ and NaBH₄. After LiBH₄ treatment, the Au–Au staple (Au–Au 3) contribution could not be fit, which is likely due to the fact that BH₄⁻ reducing agent reacted with the thiolate stabilizers present in these staple motifs. This is evident by the decrease in Au–S coordination number from 1.4(1) in the case of Al₂O₃-supported Au₂₅(SC₈H₉)₁₈ clusters to 0.8 (1) after LiBH₄ treatment. Treatment of Al₂O₃-supported Au₂₅(SC₈H₉)₁₈ clusters with NaBH₄ led to significant changes in both the Au–S and Au–Au coordination number values, such that only one Au–Au first shell fcc contribution could be fit. The Au–S coordination number decreased to 0.6(1) and the Au–Au coordination number was found to be 6.1(7). The low Au–Au coordination number value after NaBH₄ treatment (6.1 ± 0.7) compared to thermal treatment (9.7 ± 0.5) shows very little growth in cluster size after NaBH₄ treatment. The correlation between coordination numbers obtained from EXAFS analysis and particle size (calculated theoretically using imperial formulae) suggests that cluster size changes from 25 Au atoms to ca. 30–40 Au atoms after NaBH₄ treatment; however, after thermal treatment cluster size changes to ca. 400 Au atoms.¹⁰ It is important to note that the NaBH₄ studies were done at much higher concentrations than LiBH₄/LiAlH₄ studies in order to try to attempt to fully remove thiol stabilizers, and thus differences between LiBH₄ and NaBH₄ are likely mostly due to higher concentrations of NaBH₄ used. We have also analyzed EXAFS data in R-space for Al₂O₃-supported Au₂₅(SC₈H₉)₁₈ clusters with 2.5% loading by metal weight (see ESI†), and data shows similar trends to the 1.5% by weight sample data.

TEM analysis (Fig. 4) of the Al₂O₃-supported Au₂₅(SC₈H₉)₁₈ clusters was also carried out. The initial clusters are hard to discriminate from the Al₂O₃ support, but those measured have an average size of 1.0 ± 0.2 nm, which is consistent with their expected size. After thermal treatment at 250 °C, the clusters grew to 1.8 ± 0.4 nm, while BH₄⁻ treatment led to a final size of 1.3 ± 0.3 nm. TEM data show that while thermal treatment leads to significant growth in cluster size, borohydride treatments leads to almost negligible growth in size, which further corroborates our previous EXAFS findings. Correlation of first-shell coordination number values obtained by EXAFS after thermal and NaBH₄ treatments with number of Au atoms obtained theoretically suggest that after NaBH₄ treatment, the cluster size grew from 25 Au atoms to ∼30–40 Au atoms, and after thermal treatment, the number of Au atoms in the clusters grows significantly to ca. 400 Au atoms.¹⁰

Fig. 4 TEM images of (a) as synthesized Au₂₅(SC₈H₉)₁₈ clusters with 1.5% loading by metal weight, (b) thermally treated Au₂₅(SC₈H₉)₁₈ clusters with 1.5% loading by metal weight, and (c) NaBH₄ treated Au₂₅(SC₈H₉)₁₈ clusters with 1.5% loading by metal weight.

Fig. 5 shows the Au-L₃ edge XANES data of Al₂O₃-supported Au₂₅(SC₈H₉)₁₈ clusters with 1.5% loading by Au weight before and after thermal and chemical treatments. The inset shows the XANES edges for each of the samples. While there are only minimal differences at the white line (at ca. 11 930 eV) suggesting no change in oxidation state, a clear trend can be seen beyond the white-line region (marked peaks), with a new peak growing at ca. 11 950 eV as the clusters show greater thiolate removal. Comparing this data with the Fourier transformed EXAFS data in R-space (Fig. 3) suggests that these new features are related to the decrease in Au–S contribution and increase in Au–Au contribution, and thus increase in cluster size. We believe this 11 950 eV peak is a multi-scattering peak, such that its' intensity is related to the number of shells around absorbing atoms; small Au clusters which do not have many upper-shell Au neighbours thus show a much lowered intensity.

Fig. 5 XANES data of thermally and chemically treated Al₂O₃-supported Au₂₅(SC₈H₉)₁₈ clusters with 1.5% loading by metal weight.

The generality of these features was established by analyzing XANES data at various Au loadings (see ESI†); similar trends in the XANES were observed for various Au loadings. This is important as it shows that XANES, as well, can be used to study small changes in the cluster size and structure during thiolate removal, which is sometimes very difficult to study with EXAFS data alone due to the poor quality of data at low Au loadings.¹¹

It has been reported in the literature that desorption of two-dimensional thiolate self-assembled monolayers from Au surfaces takes place at a potential of ca. 1.0 V to −1.3 V (vs. NHE). NaBH₄ has a standard oxidation potential of 1.24 V vs. NHE in basic conditions, and therefore can desorb thiolate monolayers from Au surface.²² For a planar Au surface covered with two-dimensional self-assembled thiolate monolayers, a dissociation mechanism is rather simple and involves the reductive desorption of organic thiolates via one electron transfer.²³ However, in the case of Au₂₅(SR)₁₈ clusters, due to the presence of two spatially different thiolates in staple motifs, the exact determination of the point of cleavage is not clear. Asefa and coworkers have reported that thiolate desorption from Au₂₅(SR)₁₈ clusters occurs at a negative potential of ca. −1.5 V; however, no comment was made on the selectivity of thiolate removal.¹² Jin and coworkers showed using NMR spectroscopy that in the case of glutathione protected Au₂₅ clusters, thiolates bonded with only staple Au atoms are more vulnerable to attack by Ce(SO₄)₂ than thiolates bonded with the Au₁₃ core directly.²⁴ In our study, in the case of LiBH₄ treatment suggests that Au₁₃ core was completely preserved, as we were able to fit EXAFS data after fixing Au–Au (core) and Au–Au (surface) coordination numbers representative of Au₁₃ core. However, the inability to fit Au–Au (staple) contributions suggests that BH₄⁻ was perhaps reducing thiolates bonded with only staple Au atoms. After NaBH₄ treatment, both types of thiolates were found to be mostly desorbed as the increase in Au–Au coordination number value due to the growth in cluster size was observed. In our previous study, we showed that Au₂₅(SR)₁₈ clusters were relatively more stable towards NaBH₄ in solution than their larger counterparts.²⁵ The present study however suggests that oxide supported Au₂₅(SR)₁₈ clusters are less stable towards NaBH₄ than the same clusters in solution, as some thiols are removed upon absorption onto the oxide support. Our data also suggests that by varying the amount and the strength of the reducing agents, one can partially remove the thiolate stabilizers under relatively mild conditions (chemical treatments). This result shows promise towards developing strategies that allow for selectively accessing active sites for catalysis.

Conclusions

In summary, we have shown that thiolate stabilizers can be removed partially from oxide-supported Au₂₅(SR)₁₈ clusters under milder conditions (using borohydride reducing agents) with particle sizes much smaller than that seen under harsher temperature treatments (250 °C thermal treatment). TEM data suggest that thermal treatment leads to significant growth in cluster size; however, borohydride treatment leads to almost negligible growth in size during thiolate removal. EXAFS data suggests that partial desorption of thiolate stabilizers was observed after the immobilization of these clusters on oxide supports. This is an important finding, as it can help explain the catalytic activity of oxide supported Au₂₅(SR)₁₈ clusters before any thermal activation step. Finally we have shown that very distinct features in the XANES region can be used to follow this thiolate removal process and the concomitant growth in cluster size. This is particularly important in the case of low Au loadings, where it becomes very difficult to obtain high-quality EXAFS data.

Acknowledgements

The authors acknowledge financial assistance from the National Sciences and Engineering Research Council of Canada (NSERC) and thank Ning Chen and Weifeng Chen at the Canadian Light Source for assistance with XAS measurements. EXAFS experiments described in this paper were performed at the Canadian Light Source, which is supported by the Natural Sciences and Engineering Research Council of Canada, the National Research Council Canada, the Canadian Institutes of Health Research, the Province of Saskatchewan, Western Economic Diversification Canada, and the University of Saskatchewan.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: XANES and EXAFS fitting data for 2.5% wt samples. See DOI: 10.1039/c6ra10941a

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