Catalysis plays an important role in optimizing chemical processes in terms of cost efficiency and
energy consumption. Further, the emission of poisonous or greenhouse gases can be reduced, leading
to higher health and living standards. Catalytic processes are know for their complexity, which
hampers catalysis research. New paths need to be taken to simplify reaction systems for investigations
at ambient pressure. In heterogeneous catalysis metal nanoparticles are common active sites. [1]
To get around the variation in particle size and size distribution of nanoparticles thiolate protected
nanoclusters can be utilized.
Thiolate protected Au nanoclusters
Aun(SR)m supported on oxides have
been proven to be active in several catalytic
reactions with high yields. In
contrast to nanoparticles, which usually
have a size distribution, nanoclusters
are well defined with resolved
structures and offer the possibility to
atomically design a truly homogenous
system. This leads to optimal conditions
for reaction and mechanism studies
in catalytic research. Limited stability
under harsher conditions has been overcome by supporting the clusters on solid materials like
CeO2.
[2-5] When deposited on a support, the ligands can be successively thermally removed to expose
the metal atoms and obtain a truly monodisperse heterogenous catalyst. Different activity and
selectivity in catalysis were observed, depending on the atomic composition, degree of ligand removal
and pre-treatment. [4-7] However, surface studies on the state of the cluster structure and ligand effects
after pretreatment or during catalytic reactions have not been performed yet, but are crucial for
a mechanistic understanding. The present work represents the first operando XAFS studies of monolayer
protected gold cluster catalysts under pretreatment and reaction conditions. Investigations were
performed with a Au38(SC2H4Ph)24/CeO2 catalyst in a CO oxidation model reaction. The structure
of this specific cluster is shown in Figure 1.
EXPERIMENTAL
Au38(SC2H4Ph)24 clusters were made by a wet synthesis approach through ligand exchange. To
investigate the cluster´s stability during pre-treatment and under reaction conditions, XAFS measurements
were performed. At CLAESS beamline of ALBA Synchrotron, in-situ XANES studies at Au
L3-edge of the catalyst were performed during thermal treatment in oxidizing atmosphere and during
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CO oxidation. CO conversion was followed simultaneously with MS to analyze the catalyst´s activity
profile. Additional ex-situ EXAFS at Au L3-edge and XANES measurements at S K-edge were performed
on samples pretreated under various conditions to investigate the pretreatment effect from the
point of view of the ligands. Investigation of the oxidation state and particle size of the support and
the deposited clusters before and after reaction were done by XPS and STEM-HAADF, respectively.
RESULTS AND DISCUSSION
The XAFS measurements represent the first evidence for a redistribution of the thiol ligands between
nanoclusters and support. Further conversions of oxidation state in the S species were observed upon
thermal pre-treatment, leading to different activity profiles in the model reaction. The selectivity
changes in several reactions, depending on the level of ligand removal. This can be explained by
the continuous evolution of new oxidized S species. [6, 7] Au L3-edge measurements point out the
importance of a soft pretreatment in order to obtain a stable catalyst. The XANES spectra of the
unpretreated samples during CO oxidation reaction evidence the changes of the clusters, related to
modifications in both the ligand shell (Au-S) and also the remaining Au core (Au-Au). Once the
samples had been pretreated, no relevant changes in the XANES spectra were observed, denoting the
high stability of these catalysts.
CONCLUSION
Thus, these results obtained by XAFS, together with complementary studies (XPS, STEM-HAADF),
confirm highly active and stable catalysts in form of a well defined cluster catalyst system, which are
tunable by atom number, support material, different ligands and through the introduction of dopant
atoms to find optimum conditions for different reactions.
REFERENCES
[1] S. Navalon, H. Garcia, Nanomaterials, 2016, 6(7), 123
[2] T. A. Dreier, et al., Chem. commun. (Camb), 2015, 51, 1240
[3] S. Yamazoe, et al., Acc. Chem. Res., 2014, 47, 816
[4] T. Yoskamtorn, et al., ACS catal. 2014, 4, 3696
[5] X. Nie, et al., Nanoscale, 2013, 5, 5912-5918
[6] B. Zhang, et al., J. Phys. Chem. C, 2015, 119(20), 11193
[7] B. Zhang, et al., ACS Catalysis, submitted (2017)
