In situ K-edge X-ray absorption spectroscopy of the ligand environment of single-site Au/C catalysts during acetylene hydrochlorination

Article abstract of DOI:10.1039/d0sc02152k

The replacement of HgCl2/C with Au/C as a catalyst for acetylene hydrochlorination represents a significant reduction in the environmental impact of this industrial process. Under reaction conditions atomically dispersed cationic Au species are the catalytic active site, representing a large-scale application of heterogeneous single-site catalysts. While the metal nuclearity and oxidation state under operating conditions has been investigated in catalysts prepared from aqua regia and thiosulphate, limited studies have focused on the ligand environment surrounding the metal centre. We now report K-edge soft X-ray absorption spectroscopy of the Cl and S ligand species used to stabilise these isolated cationic Au centres in the harsh reaction conditions. We demonstrate the presence of three distinct Cl species in the materials; inorganic Cl-, Au-Cl, and C-Cl and how these species evolve during reaction. Direct evidence of Au-S interactions is confirmed in catalysts prepared using thiosulfate precursors which show high stability towards reduction to inactive metal nanoparticles. This stability was clear during gas switching experiments, where exposure to C2H2 alone did not dramatically alter the Au electronic structure and consequently did not deactivate the thiosulfate catalyst.

Full text of DOI:10.1039/d0sc02152k

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DOI: 10.1039/D0SC02152K
In situ K-edge X-ray absorption spectroscopy of the ligand
environment of single-site Au/C catalysts during acetylene
hydrochlorination.
Grazia Malta1, Simon A. Kondrat,2 Simon J. Freakley,3 David J. Morgan,1 Emma K. Gibson4,5,
Peter P. Wells,6,7 Matteo Aramini7, Diego Gianolio7, Paul B. J. Thompson8,9, Peter Johnston10
and Graham J. Hutchings1*
1) Cardiff Catalysis Institute, School of Chemistry, Cardiff University, Main Building, Park Place,
Cardiff, CF10 3AT, UK.
2) Department of Chemistry, Loughborough University, Loughborough, Leicestershire, LE113TU, U.K
3) Department of Chemistry, University of Bath, Bath, BA2 7AY, U.K.
4) School of Chemistry, Joseph Black Building, University of Glasgow, Glasgow G12 8QQ, U.K.
5) UK Catalysis Hub, Research Complex at Harwell, RAL, Oxford, OX11 0FA, UK.
6) School of Chemistry, University of Southampton, Southampton, SO17 1BJ, U.K.
7) Diamond Light Source, Harwell Science and Innovation Campus, Chilton, Didcot OX11 0DE, UK.
8) XMaS, UK CRG, ESRF, 71 Avenue des Martyrs, 38043 Grenoble, France
9) Department of Physics, University of Liverpool, Oliver Lodge Laboratory, Liverpool L69 7ZE, UK
10) Process Technologies, Johnson Matthey PLC, Billingham, TS23 1LB, UK.
*Hutch@Cardiff.ac.uk
Key words: Heterogeneous Catalysis; X-ray Absorption Spectroscopy; Single-Site Catalysis
1

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Abstract: The replacement of HgCl2/C with Au/C as a catalyst for acetylene hydrochlorination
represents a significant reduction in the environmental impact of this industrial process. Under reaction
conditions atomically dispersed cationic Au species are the catalytic active site, representing a large-
scale application of heterogeneous single-site catalysts. While the metal nuclearity and oxidation state
under operating conditions has been investigated in catalysts prepared from aqua regia and
thiosulphate, limited studies have focused on the ligand environment surrounding the metal centre. We
now report K-edge soft X-ray absorption spectroscopy of the Cl and S ligand species used to stabilise
these isolated cationic Au centres in the harsh reaction conditions. We demonstrate the presence of
three distinct Cl species in the materials; inorganic Cl-, Au-Cl, C-Cl and how these species evolve
during reaction. Direct evidence of a Au-S interactions is confirmed in catalysts prepared using
thiosulfate precursors which show high stability towards reduction to inactive metal nanoparticles. This
stability was clear during gas switching experiments, where exposure to C2H2 alone did not
dramatically alter the Au electronic structure and consequently did not deactivate the thiosulfate
catalyst.
2

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Introduction
The commercialisation of carbon-supported gold catalysts (Au/C), as a replacement for toxic mercuric
chloride (HgCl2/C), represents a significant reduction in environmental impact of large scale vinyl
chloride monomer (VCM) production via acetylene hydrochlorination.1,2,3,4 Since the prediction that Au
would be an effective catalyst,5 intensive studies to understand and optimise these catalysts have been
on-going.6,7,8,9,10 These catalysts were originally developed using strongly acidic and oxidising solvents,
including impregnation of HAuCl4 from aqua regia (Au/C-AR).11,12 The preparation and use of such
Au/C catalysts at an industrial scale represents an economic and technical challenge, which hindered
the validation of this type of catalyst. Moreover, the high activity of these Au/C materials is often
coupled with poor lifetimes under extended testing. The reduction of the active cationic Au to metallic
nanoparticles and the development of acetylene-oligomers on the catalyst surface at strong acid sites
introduced from the aqua regia are the primary deactivation mechanisms.1,13 In comparison, catalyst
preparation from aqueous HAuCl4 results in the formation of large Au nanoparticles which have limited
activity.14 Alternative solvents to aqua regia, such as “organic aqua regia (OAR)”, have been developed
and utilised for the preparation of mono and bimetallic catalysts;15,16 however, despite the appreciable
catalytic performances this still does not provide a solution to large scale catalyst synthesis.
To obtain effective catalysts using aqueous metal-precursor solutions, strongly coordinating ligands are
required to prevent nanoparticle formation.17,18 Johnston and collaborators,1,19 reported the use of soft
donor ligands such as thiosulphate could produce a class of active and stable catalysts due to the
increased stability constants of the Au-S species compared to the Au-Cl, avoiding the use of aggressive
impregnation conditions. This catalyst (Au/C-S2O3) has been industrially validated as a replacement for
the HgCl2/C. The synthesis is based on the in situ formation of a Au-thiosulfate complex before
immobilisation onto carbon and the active form of the catalyst has been shown to consist of atomically
dispersed cationic Au species under reaction conditions. We have previously demonstrated by
operando X-ray Absorption Spectroscopy (XAS) that catalysts containing chloride or sulphur ligands
had the same structure/activity correlation, with a Au(I) spectroscopic feature being proportional to
VCM productivity and catalysts were shown to be comprise of atomically dispersed Au centres.13 The
two catalysts (Au/C-AR and Au/C-S2O3) displayed differing induction periods; the Au/C-AR required
3 h to reach steady state activity, due to initial oxidation of Au(I) to Au (III) and subsequent re-
equilibration of metal oxidation states, while Au/C-S2O3 achieved steady state immediately due to
higher stability of the Au(I) species with a similar Au(I) to Au(III) ratio. In analogy with homogenous
Au complexes,20 the choice of ligand and solvent, used during the catalyst preparation plays a major
role in determining performance and stability.21,22
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We now report a study using in situ K-edge S and Cl X-ray absorption spectroscopy, under relevant
DOI: 10.1039/D0SC02152K
reaction conditions, to examine Cl and S speciation in the Au/C-AR and Au/C-S2O3 catalysts and
correlate this to the observed catalytic activities. We show how the nature of the Cl and S evolve during
the induction periods and at steady state to give information about the metal ligand environment and
provide further information towards designing effective catalysts. The stability of the Au metal centre
in the Au/C-S2O3 was studied in a operando Au L-edge XAS experiment, where reaction gases were
switched to expose the catalyst to conditions that have already shown to deactivate the Au/C-AR
catalyst.
Results and discussion
Initially, ex situ normalised Cl K-edge XANES spectra of fresh Au/C-AR and Au/C-S2O3 were
compared (Figure 1a). For comparative purposes a Au/C-H2O catalyst containing Au nanoparticles was
also analysed (Figure S1). Three characteristic features, labelled as A, B and C, were observed in the
XANES spectra of the catalysts with varying relative intensity. The absolute amount of Cl, determined
from raw XANES spectra before data reduction23 differs in each sample (Figure S2); Au/C-AR has
significantly more Cl (0.32) than Au/C-S2O3 (0.08) as expected from a preparation method using aqua
regia compared to aqueous thiosulphate solution; the Au/C-H2O sample has lower amounts of Cl than
either of the previous catalysts (0.03). These results are in accordance with Cl(2p) core-level XPS
spectra (Figure 1b, Figure S3), which verifies this difference in Cl concentration with Au/C-AR having
1.98 at% surface Cl and the Au/C-S2O3 0.25 at%. (Table S1).
The XANES feature at 2,828 eV, labelled (A), is assigned to the Cl 1s→4p dipole allowed
transition of an inorganic Cl- species and it is comparable to the ex situ recorded reference material
NaCl (Figure S4). The second feature at 2,824-2,826 eV, labelled (B), which is the predominant feature
in Au/C-AR and a shoulder of the main edge for the Au/C-S2O3, can be assigned to a 1s→3p* transition
and associated with C-Cl functionality.24 This observation is in accordance with the assignments of XPS
binding energies; Au-Cl/NaCl (197.7 eV), C-Cl groups (200 and 202 eV) and loss structure associated
with high levels of carbon chlorination (203.4 and 205.2 eV)25 (Figure 1b, Figure S3). Variation in the
position of XANES C-Cl features is associated with the aliphatic/aromatic nature of the carbon species
and the degree of chlorination;26 higher C-Cl energies, as seen in Au/C-AR, suggest a high degree of
chlorination of the carbon surface. The high relative C-Cl content in Au/C-AR is rationalised as a result
of chlorination from the highly acidic and oxidising aqua regia preparation. The XANES spectra of the
Au/C-H2O catalyst is similar to Au/C-S2O3 however feature B (C-Cl) is more pronounced; aqueous
impregnation of HAuCl4 on to the carbon support results is the deposition of metallic Au(0)
nanoparticles and presumably consequent formation of a greater amount of C-Cl species. 10,13
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Bare activated carbon as received was also shown to contain both Cl and C-Cl species, but in
negligible amounts compared to the catalysts; both XANES and XPS results suggest that after washing
the carbon support with water (C-H2O) the inorganic Cl- functionality was removed while treatment
aqua regia (C-AR) was shown to be able to introduce C-Cl functionality (Figure S5, S6 and Table S2).
The influence of water and aqua regia on the modification of the carbon surface, with and without the
HAuCl4 metal precursor, has been also analysed via O1s XPS. As expected, the aqua regia
impregnation led to the highest oxygen content (Table S3). However, the oxygen content does not seem
to have direct influence on the catalysts’ steady state activity when comparing the Au/C-AR and Au/C-
S2O3 in our previous studies (catalytic data shown in Figure 1c); this evidence also agrees with previous
studies performed also on metal-free catalysts.27
The third feature in the Cl XANES at 2,821 eV, labelled (C), is attributed to the Cl 1s→ mixed
Cl 3p and Au d orbital transition and is a pre-edge feature.28 The intensity of feature is dictated by the
degree of covalency in the Au-Cl bond and is therefore sensitive to the electronic structure of the Au
and its oxidation state. The hybridisation of Au 5d-6s orbitals, due to relativistic affects, allows the
formally d10 electronic configuration of Au(I) to have molecular orbitals with Cl of sufficient character
for electonic dipole allowed transitions. The feature is pronounced in Au/C-AR, while less intense in
Au/C-S2O3 (Figure 1a), showing that less Cl is associated with the Au and/or that the nature of the Au-
Cl bonding is different, due to a change in bond covalency or lower oxidation state in the fresh Au/C-
S2O3. Feature C is also negligible in the Au/C-H2O catalyst, in which the Au-Cl bond is almost absent
(Figure S1).13 Given Au/C-S2O3 is prepared from a gold-thiosulfate complex made from HAuCl4, Au-
Cl species could persist from the precursor. It is not possible to determine if the Au-Cl species is a
discrete AuClx species or a partially chlorinated Au(S2O3)x species, raising the question if multiple Au
speciation exists. However, the clear difference in catalytic stability and induction behaviour of this
catalyst shows that most Au centres in Au/C-S2O3 are significantly different from the Au-Clx in Au/C-
AR (Figure 1c). Similar considerations can be made in the comparison between the Au/C-AR and Au/C-
H2O. From analysis of the Au-Cl pre-edge for the catalysts it is possible to determine the if dimeric
Au2Cl6 species are present. The dimeric Au2Cl6 structure has two bonding Cl environments and Au-Cl
bond lengths, i.e. terminal and bridged. This is reflected in the XANES spectrum of the dimeric standard
(Au2Cl6) as a splitting of the pre-edge (Figure 1d). On inspection and consistent with monomeric
standard (KAuCl4), the XANES of both catalysts showed a single feature (Figure 1d), indicating no
significant population of Au dimers where present, supporting the observations made previously that
the Au is present as mono-dispersed cationic species.Error! Bookmark not defined.
To understand the evolution of Cl species during the acetylene hydrochlorination reaction, in
situ Cl K-edge XANES characterisation of Au/C-AR and Au/C-S2O3 was performed. On heating to
reaction temperature (200 °C) under He (Figure 2a-c), the position of feature A, associated with
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inorganic Cl species, remains unchanged for both catalysts, with a slight decrease in normalised
intensity in Au/C-S2O3. On the other hand, changes in feature C, associated with Au-Cl, were evident
for both catalysts. In particular, the intensity of the pre-edge increased between 100 – 120 °C for the 2
catalysts, before reducing on further heating to 200 °C (Figures 2c and S7). This behaviour can be
explained as following: on heating the catalysts the Au is oxidised to a higher oxidation state AuClx,
through migration of Cl from the carbon to the Au. This causes an increase in intensity of feature C as
there is an increased Cl coordination number (CN) and a lower occupancy of the 5d-3p hybridized
orbital. . The small decrease in the C-Cl band in Au/C-S2O3 concurrent with the increase in feature C
supports this hypothesis; the absence of any notable C-Cl population change in Au/C-AR is attributed
to the significantly higher initial signal intensity, masking any small change in signal from Cl migration.
The reduced intensity of feature C above 110-130 °C is due to the decomposition of Au(III)Cl to Au(I)
Cl, where fully occupied d orbitals limit the pre-edge feature,29 in agreement with Au L3-edge
experiments.13 This observation suggests that the C-Cl species can be mobile and interact with the Au
species at elevated temperatures by acting as a reservoir of Cl functionality. It is important to
acknowledge that while changes between Cl environments can be inferred, the intensities of allowed
1s→ 4d transitions cannot be proportionately compared with feature C, which is dependent on Cl 3p-
Au 5d/6s hybridisation.
The catalysts were then characterised during the acetylene hydrochlorination reaction (Figure 3a-c).
Over a 100 minutes of reaction, the overall Cl concentration modestly increased for Au/C-S2O3 and was
unchanged for Au/C-AR, which remained higher than the Au/C-S2O3 throughout (Figure S8, S9 and
Table S1) which suggests that gaseous HCl cannot significantly chlorinate the carbon surface. Given
the comparable VCM productivities between the catalysts (Figure 1c),13 there seems to be no correlation
between the total Cl concentration within the catalysts and catalytic activity. Moreover, it suggests that
the Au complex and carbon surface within Au/C-S2O3 catalyst which is not highly chlorinated during
the preparation is relatively resistant to possible chlorination phenomena directly from HCl and it
remains significantly different to the highly chlorinated surface of the Au/C-AR. Cl(2p) XPS of the
used catalysts (Figure S10 and Table S1) show a slight increase in Cl content in the Au/C-S2O3 catalyst
after reaction, especially of the inorganic Cl species, which is attributable to a certain level of
chlorination of the metal centre under reaction condition.
Comparison of the normalised Cl-edge XANES spectra (Figures 3 and S9), show variation in feature C
(Au-Cl) during reaction in both catalysts; shown as a function of reaction time-on-line in Figure 3c.
Feature C intensifies, broadens and shifts to higher energy during the first 20 min and then decreases,
suggesting changes in the Au-Cl bonding, Au oxidation state and Au-Clx geometry during reaction.18
Broadening possibly indicates multiple Au-Cl bond lengths or Au-Cl species. These observations are
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in line with those observed from operando Au L3-edge XAFS. The comparable changes in feature C
with respect to reaction time (Figure 3c) suggest that the Au-Cl species are similar for both catalysts.
However, the significantly lower Cl concentration in Au/C-S2O3 compared to Au/C-AR throughout the
reaction suggests fewer Au-Cl species in Au/C-S2O3. Given all the Au within this catalyst is cationic
and atomically dispersed, this lower Cl signal shows that most of the Au is coordinated to other non-
chloride ligands, most probably S ligands, throughout the reaction and that the Au in this catalyst does
not become excessively chlorinated or convert to a purely AuClx species while operating. Under
operation Au/C-S2O3 contains multiple Au speciation, a robust Au-S complex and a minority Au-Cl
species analogous to Au/C-AR.
Another difference between the two catalysts Cl speciation, under reaction conditions, is the change in
C-Cl species, feature B. During heating to 200 °C and during reaction this gradually reduces in intensity
for Au/C-S2O3 (Figure 3a). This behaviour can be attributed to the migration of Cl from the carbon to
the Au centre, confirming the Cl(2p) XPS results on the used sample (S10). Although the corresponding
feature appears to remain stable in Au/C-AR, this is again due to the high C-Cl concentration making
small differences difficult to observe. The relatively high amount and possible mobility or evolution of
C-Cl species in the Au/C-AR catalyst could also explain the induction period typically observed in this
type of catalyst.13 On heating of the catalyst, we first observe an increase in the amount of Au(III),
13
confirmed by both XAS of the Au L3-edge and Cl K-edge, before conversion to Au(I) beyond the
thermal decomposition temperatures of AuCl3, which is approximately 130 - 160 ºC. This observation
of Au (III) is likely the result of oxidation of Au(I) rather than the disproportionation reaction (3 AuCl
 2Au + AuCl3) as we do not observe metallic Au in these or our previous experiments.13 We have
previously shown that exposure to HCl alone does not result in the oxidation of Au(I) to Au(III) species
and that the induction period observed is related to the formation of Au(III) on exposure to both
reactants with activity increasing as the ratio of Au(I) to Au(III) equilibrates. The presence of large
amounts of surface chlorination could facilitate evolution of Cl2 from the surface at reaction temperature
which is a strong enough oxidant to convert AuCl into AuCl3 in addition to residual NOx species
remaining from the preparation. The evolution of these species could be accelerated by the large initial
exotherm passing through the bed on introduction of the reactant gases and only when the evolution of
oxidant has subsided can the catalyst equilibrate to a Au(I)/Au(III) ratio determined by catalytic
turnover and steady state activity.
This behaviour is observed to a much lesser extent for Au/C-S2O3, due to the C-Cl reservoir being
quickly depleted. The increased C-Cl in Au/C-AR results in a more extensive oxidation of the Au(I)
chloride like species to a Au(III) type chloride species and consequently the process of Au(I)
reformation, which is correlated to activity, is considerably slower, resulting in a notable induction
period as the species equilibrate.
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Simple impregnation of the metal precursor, as in the aqua regia preparation, is however
preferable to the in situ formation of the Au-thiosulphate complex followed by its impregnation onto
the carbon support required in the preparation of Au/C-S2O3. Clearly the use of an alternative non-
chlorinated/organic solvent to aqua regia for the dissolution of the HAuCl4 metal precursor could be
an efficient solution for the preparation of active, stable and scalable Au/C catalysts for this reaction.
Recently we have shown that catalysts can be prepared using organic solvents, such as acetone.30 It is
important to note that these catalysts which are prepared using HAuCl4 and without aqua regia (and
therefore without a high Cl content) did not have an induction period and behaved analogously to Au/C-
S2O3 catalysts. The Cl K-edge XANES of a catalyst prepared using HAuCl4 and an acetone solvent is
shown in Figure 4. Critically, the C-Cl feature is notably less intense than that seen in Au/C-AR,
confirming the relationship of this species to the catalytic induction period. Although the Au/C-acetone
catalyst does not have a pronounced induction period and is similar in initial activity to the Au/C-S2O3
catalyst, a proportion of the Au within the catalyst was found to reduce to form Au(0) nanoparticles.
This reduction was not observed for Au/C-S2O3 catalysts, suggesting that in addition to limiting C-Cl
surface species the presence of sulphur species increases the stability of the Au(I) active site.
To understand the role of sulphur-based ligands in Au/C-S2O3 in situ S K-edge XANES experiments
were also performed. The S1s→S3p electronic transitions are extremely sensitive to the sulphur oxidation
state.31 Moreover, in the case of oxyanions, where the sulphur can be present in various oxidation states,
the spectrum originated will show one feature (or edge) for each oxidation states of the element in the
sample. Sulphur compounds have unique patterns of transitions on the absorption edge, making the
sulphur K-edge XANES suitable for qualitative determination of sulphur species in samples with
complex composition by using appropriate reference materials. 32,33
Figure 5a shows the S K-edge XANES of Au/C-S2O3 and Na3S2O3 (XANES spectra of other standards
materials recorded are reported in Figure S11), where the sulphide can be differentiated from the more
oxidised sulphate like species. While the formal oxidation state of sulphur in thiosulfate ions is debated,
34,35,36 it is clear that species of comparable, but not identical, oxidation state are present in Na3S2O3 and
Au/C-S2O3. Features in Na2S2O3 at 2472 and 2482 eV, associated with 1s→3p transitions of the
different sulphur environments, are shifted to higher energy in Au/C-S2O3 (2473 and 2483 eV) (Figure
5a). However, features observed in Na2S2O3, between at 2477-2479 eV which are not satisfactorily
explained in the literature, are absent in Au/C-S2O3. Ab-Initio DFT simulations of the XANES spectra
of [S2O3]2- show that these undefined features can be assigned to transitions to excited states where
atomic orbitals of the 2 S atoms and O 2p from thiosulfate are strongly hybridized (Figure 6a). [S2O3]2-
interaction with the carbon support (via C-π orbitals) only weakly affect these excited states (Figure
6b). A relevant decrease of the intensity of the corresponding peaks could only be reproduced by
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including a strong chemical interaction in the models for the simulations (such as the addition Voiefw HArticle Online
DOI: 10.1039/D0SC02152K
covalently bonded to O of SO3, or the chemical binding of S2O3 to the C-based support, whose spectra
are shown in Figure 6b). The absence of these features in Au/C-S2O3 therefore suggests that the
thiosulphate ligand has not remained intact on deposition onto the carbon and has potentially undergone
2-
a disproportionation reaction to S2- and SO3 . Furthermore, the XANES simulations of the model
structures where a strong chemical interaction is consistently included, also successfully predict an
increase in the energy of the first electronic transition to 2473 eV consistent with the shift of the first
peak observed in experimental spectra shown in Figure 5 between Na2S2O3 and Au/C-S2O3.
XPS analysis of the Au/C-S2O3 catalyst supports this supposition. Figure 6c shows the S(2p) region for
Na3Au(S2O3)2 and Au/C-S2O3; with two sulphur states in the former (S6+ and S2- at 166.7 and 162.1
eV) and several additional sulphur states in the catalyst, attributed to Au-S (162.6 eV), S-S (163.8 eV),
S-H (164.8 eV), SO3 (168.1 eV) and SO4 (169.6 eV), consistent with disproportionation. The presence
of Au-S bonds in Au/C-S2O3 proposed based on XPS binding energy and splitting of the XANES S2-
feature, as seen in Na3Au(S2O3)2 (Figure S11), confirms that the Au is predominantly bound to the
sulphide species. To understand the dynamic nature of S within Au/C-S2O3, in situ S K-edge XANES
studies were performed. During heating to reaction temperature under an inert atmosphere, the two
main absorption edges did not change in intensity or position (Figure S12), in contrast with Cl
speciation. Demonstrating that S speciation in Au/C-S2O3 was more thermally stable than the Cl species.
Introduction of reaction gases resulted in a shift in the absorption-edge of the S2- bound to Au, during
2-
the first 20 min of reaction, while the SO3 group remains stable (Figure 5b and S13). The shift to
higher absorption-edge for the sulphide species shows a decrease in electron density and oxidation of
this S species bound to Au. We suggest that Au(I) oxidation, caused by C-Cl surface species on the
carbon is suppressed by the polarisable Au-S bond. Therefore, in addition to the significantly lower
concentration of C-Cl species, the active Au(I) species in Au/C-S2O3 do not over oxidise and the catalyst
is immediately active. This supports the proposed detrimental role of the Cl excess on the carbon
surface. The S 2p XPS of the Au/C-S2O3 catalysts before and after reaction (Figure S14 and Table S4)
show that multiple sulphur states can be observed as in Figure 5c with identical speciation assignments.
It should be noted however, binding energies for the species differ by ca. 0.2 - 0.6 eV, the smaller
difference attributable to experimental errors and uncertainty in peak fits whilst the larger difference
may again be attributed to agglomeration of sulphur species and/or higher uncertainty in the peak fits
due to the signal to noise in the data.
The stability of the S containing Au/C-S2O3 catalyst was investigated at the L3-edge using an
“accelerated deactivation test”, in which the catalyst is sequentially exposed to; (i) HCl + C2H2, (ii) HCl
only, (iii) HCl + C2H2 (iv) C2H2 only and finally (v) HCl + C2H2 during one experiment. We have
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already reported that exposure to HCl only (i.e. in the absence of C2H2) resulted in Au/C-AR catalytic
DOI: 10.1039/D0SC02152K
performance being perturbed on the re-introduction of C2H2, resulting in a second induction period to
regain steady state acticity.1, 37 Also, Au/C-AR rapidly deactivates after treatment with only C2H2.37
This deactivation was attributed to an interaction with AuClx and C2H2 that on the re-introduction of
HCl causes the formation of Au(0) nanoparticles. To clarify the influence of the individual reactants on
the Au/C-S2O3, the VCM productivity (Figure S15) and the Au L3-edge XANES (Figure 7) and EXAFS
(Figure 8 and Table 1) were correlated during an operando experiment in a fixed bed reactor. XANES
and EXAFS spectra of the Au/C-AR catalyst, reported previously,37 are also shown for comparison.
These previous studies show that under steady state reaction conditions a combination of Au(I) and
Au(III) chloride/sulphur compounds are present, with EXAFS determined coordination numbers
matching those determined from XANES analysis, by white line height or linear combination fitting.13,37
After a small drop in productivity over the first 20 min time-on-line Au/C-S2O3 remained stable for the
complete test duration with minimal deactivation despite periods of being exposed to only HCl or C2H2.
The XANES spectra of Au/C-S2O3 showed a slight decrease in white-line height, as seen for Au/C-AR,
under the HCl environment. It is clear that for both catalysts the Au(III/I) centres are not strongly
influenced by the dilute HCl stream and that neither catalyst is oxidised to a higher degree of
chlorination under these conditions. This is corroborated by the EXAFS which shows no change in the
Au-Cl/S (indistinguishable by EXAFS) coordination number of 2.5 or bond lengths between reaction
(i) and under HCl (ii).
The difference in catalyst performance on re-introduction of both reactants following HCl treatment
(step iii) between Au/C-S2O3 and Au/C-AR is therefore due to the C-Cl content on the two respective
support structures. Namely, the absence of significant C-Cl content in Au/C-S2O3 resulting in an
immediate return to reactivity, without an induction period. Under a C2H2 flow, the energy of the
absorption edge increased for Au/C-S2O3, with the observed white-line shifting from 11921.6 eV to
11922.7 eV. Comparison with the change in XANES seen for Au/C-AR under identical C2H2 conditions
showed similar trends to that observed for Au/C-S2O3. The change in white-line for both catalysts is
attributed to an uncharacterised Au-acetylene interaction.37 However, the extent of the edge shift was
significantly less for Au/C-S2O3 than that seen for Au/C-AR. The Fourier Transformed Au L3-edge
EXAFS (Figure 9) showed a decrease in the magnitude of the signal at 1.8 Å for both catalysts. Again,
the dampening of the signal was notably less for Au/C-S2O3 under C2H2 relative to Au/C-AR.
The damping of oscillations seen for Au/C-AR was previously attributed to out of phase multiple Au-
Cl path lengths.37 The catalyst was considered to be Au(I) based on the edge position but with a
significant change in geometry. The rational for this interpretation being that χ space data showed a
damping of oscillations across the k range under C2H2, with no change in the phasing between a catalyst
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under C2H2 (step iv) or under steady state reaction conditions (step i-iii) (Figure S16), sugDgOeIs:t1i0v.1e03Vo9ief/wDnA0orStiCcl0e2O1n5li2nKe
new scatter being present and an increase in disorder. However, an unresolved question of the cause of
this affect, in addition to the significant change in the XANES of both the Au/C-AR and Au/C-S2O3
catalysts under C2H2, prompts consideration of an alternative interpretation of the data. A compelling
hypothesis being that in addition to Au-Cl paths there is a Au-C path attributable to Au- C2H2 bonding.
Therefore, the fitting of the EXAFS data of Au/C-AR and Au/C-S2O3 catalysts under C2H2 was
performed using two models; (1) a single Au-Cl path with a floated 2σ2 parameter to account for an
increased structural disorder due to multiple Au-Cl paths or (2) fitting Au-Cl and Au-C paths.
Both models provided reasonable fits of the data. Model 1 showed a significant increase in 2σ2 from
0.0037 Å2 under steady state (step i) to values under C2H2 of 0.006(2) Å2 and 0.011(2) for Au/C-AR
and Au/C-S2O3 respectively. Model 2 provides a viable Au-C path length of 2.04(7) Å which, despite
the significant error, fits with distances seen for Au(I) σ-bonded alkyne complexes at 1.98 Å .38 Freeing
of the Au-Cl/S coordination number showed that this path was more significant in Au/C-S2O3 and
suggest that more free Au(I) is present in this catalyst than Au/C-AR. Comparison of the fits shows,
however, no statistical improvement on the addition of a Au-C path, further this model significantly
misrepresented the first oscillation of the χ space data where a soft scatterer would be best represented
(low k distances, Figure S17). It is concluded, that whist compelling to evoke Au-alkyne complex
formation it could not be definitively proven with the current data. Regardless of the fitting model used
it is clear that C2H2 has a similar influence on Au/C-S2O3, although it is far less dramatic than that seen
for Au/C-AR.
The effect of reintroduction of both reactant gases in step v on Au speciation was notably different for
the two catalysts. While Au/C-AR was found to deactivate after the re-introduction of HCl, Au/C-S2O3
remained relatively stable with only minimal deactivation. As stated previously, the growth of Au(0)
responsible for deactivation in Au/C-AR, occurred only after re-introduction of HCl (step iv) and not
under C2H2.37 Au-Au paths could be fitted for both catalysts at the end of reaction step iv, at all other
points during the reaction no acceptable fit of this path was found. The proportion of Au(0) could not
accurately be determined by EXAFS due to the large error in Au-Au coordination numbers.
Conclusion
S and Cl K-edge XANES have been used to gain further insight into the structure-activity relationship
of cationic Au catalysts in acetylene hydrochlorination. Three Cl species; unreactive inorganic Cl,
covalently bound C-Cl and Au-Cl species are found on Au/C-AR and Au/C-S2O3. The concentration of
these species was significantly higher in Au/C-AR. Au-Cl species are dynamic during heating and under
reaction conditions, with changes in Au-Cl bond hybridisation and bond length. In addition, surface
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bound C-Cl is found to be titrated from the carbon surface during reaction suggesting DthOeI: 1p0.o10te39n/tDia0SlC02152K
importance of these surface groups in influencing Au-Cl speciation. High concentrations of C-Cl, as
seen in Au/C-AR, therefore result in a catalytic induction period. The presence of Au-Cl in Au/C-S2O3
was concluded to be from a minority phase that coexists with Au-S species. The presence of intact
[S2O3]2- ligands could not be confirmed on deposition of the Au complex onto the carbon support.
However, the S species present were found to be stable under reaction conditions and stabilises Au(I)
to produce a robust and commercially viable acetylene hydrochlorination catalyst.
Experimental
Catalysts preparation
1 wt. % gold supported on activated carbon catalysts were prepared by wet impregnation of the HAuCl4
precursor dissolved in aqua regia or water (denoted as Au/C-AR and Au/C-H2O respectively).
Activated carbon was initially ground to obtain a 100 - 140 mesh powder. The gold precursor,
HAuCl4·xH2O (Alfa Aesar, 99.9% (metals basis), Au 49%) was dissolved in aqua regia (3 parts by
volume HCl [(Fisher, 32 wt.%)]: 1 part by volume HNO3 [(Fisher, 70 wt. %)]) or water. The gold
precursor solution was then added drop-wise with stirring to the acid washed, steam activated wood
carbon. Stirring was continued at ambient temperature for 1 hour or until NOx production subsided. The
product was then dried for 16 h at 140 ºC under an inert flow of nitrogen. A different procedure was
employed for the preparation of the 1wt% Au/C-S2O3 catalyst: an aqueous HAuCl4·3H2O solution was
mixed with an aqueous solution of sodium thiosulfate, Na2S2O3, in order to obtain a NaAuS2O3
complex. The mixture obtained was added slowly in aliquots to the support while stirring. The product
was then dried at 110 °C overnight under a nitrogen flow.
Catalysts testing
In situ K-edge XANES experiment:
A microreactor was located inside an environmental chamber or vacuum vessel that allow a vacuum or
a helium atmosphere. Thus, specifics microreactor (flow cell) and reaction conditions have been used.
The set-up (vacuum vessel and cell) used have been developed by Thomson and Newton and it is widely
described elsewhere in the literature.39 The flow cell is placed in the core of the environmental chamber.
Within this setup the dedicated space for the catalyst bed is very limited and the amount of catalyst used
was ca. 10 mg. For this reason, the total flow has been reduced to 5 ml min-1: C2H2/He (2,5 mL min-1)
HCl/He (2,5 mL min-1). Dilute gas mixtures were used C2H2/He (4.97% balanced in He, Air Liquide)
and HCl/He (5.00% balanced in He, Air Liquide). The gases were dried, using moisture traps, prior to
introduction to the reactor setup. In all cases, the reactor was purged with He (99.99 %, Air Liquide),
heated to 200 °C at a ramp rate of 2.5 °C min-1 and held at temperature for 30 min, all under a flow of
He (5 mL min-1), prior to admitting the hydrochlorination reaction mixture. The outlet gas line was
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connected to a mass spectrometer (Hiden QGA), to detect VCM during reactions and showed that the
DOI: 10.1039/D0SC02152K
catalysts were functioning.
Operando Au L3-edge XAS and laboratory activity test:
The catalysts were tested using a completely automated reactor system with the same setup as
previously described.13 All of the predilute gases 5% C2H2/Ar (BOC), 5% HCl/Ar BOC), and Ar
(99.99% BIP, Air Products) were dried using moisture traps before being introduced into the reactor.
In all cases the reactor was heated to 200 °C at a ramp rate of 5 °C/min and held at this temperature for
30 min under a flow of argon prior to admitting the hydrochlorination reaction mixture. The tests were
performed using a fixed-bed polyimide (Kapton) microreactor containing the catalysts, keeping the total
flow of 50 mL min−1 and a total gas hourly space velocity (GHSV) of ∼14 000 h−1. When both reactants
were present, the C2H2:HCl ratio was kept at a constant value of 1:1.02.
The reaction mixture was analysed on-line by mass spectrometry (Hiden QGA), and Professional
Edition software was used for both qualitative and quantitative analyses. The catalyst activity presented
is shown in terms of productivity toward vinyl chloride monomer (VCM). The response factor of the
mass spectrometer toward VCM was correlated with the productivity (mol kgcat−1h−1) obtained by using
a Varian 450 gas chromatograph equipped with a flame ionization detector (FID). Chromatographic
separation and identification of the products was carried out using a Porapak N packed column (6 ft
×1/8″ stainless steel).
The sequential flow experiment was performed simultaneously monitoring the Au L3-edge XAS and
catalytic activity. Reaction sequence employed the following gas compositions: step i = HCl/C2H2/Ar,
step ii = HCl/Ar, step iii = HCl/ C2H2/Ar, step iv = C2H2/Ar, and step v = HCl/C2H2/Ar. The duration
of each step in the sequence was not the same. The gas composition during the experiment was altered
only when no further change in the XAS spectra was observed.
Catalysts characterisation:
-
X-ray Absorption Spectroscopy (XAS)
In situ and ex situ XANES spectra have been acquired at the BM28 (XMaS) beamline at the European
Synchrotron Radiation Facility (ESRF), situated on the soft end of an ESRF dipole magnet, which
provides a wide range of X-ray techniques making use of white beam and monochromatic energies in
the range of 2.4 to 15 keV. At the BM28 it is possible to run a so-called XESCAN.MAC – Extended
Escan: Variable point density. In particular, it is possible to perform energy scan for multiple
consecutive energy regions, with equal or different step (variable point density). The sample chamber
was purged with helium for at least an hour to remove air introduced during sample loading. The beam
upstream of the sample chamber is contained within a helium-filled tube to minimise X-ray absorption
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by the air. The fluorescent signal was detected using a silicon drift diodes detector. AllDsOpIe: 1c0t.r1a03Vh9ie/awDvA0erStiCcl0e2O1n5li2nKe
been acquired in fluorescence mode.
In situ and ex situ XAFS at the Au L3-edge and Cl K-edge have been also acquired at the B18. The Cl
K-edge X-ray absorption spectra (XAS) have been measured to probe chloride-gold bonding. Spectra
for the Au/C-AR samples at different time-on-line were recorded ex situ at the Cl K absorption edge in
fluorescence mode, using beamline B18 of the Diamond Light Source, Harwell, UK. The measurements
were performed using a QEXAFS setup with a fast-scanning Si (111) double crystal monochromator
and a 36 element Ge fluorescence detector. The K-edge absorption spectrum of Cl when bound to a
transition metal shows a pre-edge feature due to the forbidden 1s3d transition. This transition
becomes partially allowed and therefore observed when the Cl p-orbitals mix with the metal d-orbitals.
The position of the pre-edge feature is dependent on several factors, namely (i) the Cl 1s energy, related
to the charge on the chloride, and (ii) the metal d-orbital energy which is itself determined by both the
oxidation state of the metal and the coordination number. The intensity of the pre-edge feature is
dependent on the mixing of the Cl orbitals and metal d-orbitals and so the bonding characteristic of the
Cl to the metal.
-
X-ray Photoelectron Spectroscopy (XPS)
XPS was carried out using a Thermo Scientific K-alpha photoelectron spectrometer with microfocused
monochromatic Al Kα radiation operating at 72W (6 mA x 12 kV), the value of the C(1s) peak under
the operating conditions was found to be 284.5 eV, typical of activated carbons with graphic character
as per carbon supports used here.. The resulting spectra were processed in CasaXPS (v2.3.21) using a
Shriley type background removal, Scofield cross-sections and an electron energy dependence of -0.6.
-
DFT simulations
Within the framework of DFT simulations, we used the software package ORCA v4.140 and
compared two levels of DFT theory, including generalised-gradient functionals (BP86 and PBE)41 and
hybrid functionals (B3LYP) 42, obtaining comparable spectral simulations for the S K edge.
We tested the consistency of the results obtained with 6-311G*, triple-ζ split-valence plus polarization
and double-ζ split-valence basis set for both S and O atoms. Here we show the results obtained with
def2-TZVP basis set. Geometries of the molecules were relaxed within an energy tolerance of 5.0e-6
a.u. and a maximum displacement of 4.0e-3 a.u. The electronic properties of the ground state were
converged to an energy threshold of 1.0e-07 a.u. Sulphur K edge spectra were then calculated within
the TD-DFT formalism for the excited states.43 A rigid shift has been applied consistently among all
the spectra shown in Figure SIM1 to align them at the energy of the experimental S K absorption spectra.
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Acknowledgment
We acknowledge Diamond Light Source for time on Beamline B18 under Proposal SP15214-1.
The K-edge experiments were performed on beamline BM28 (XMaS - UK CRG Beamline) at the
European Synchrotron Radiation Facility (ESRF), Grenoble, France. We are grateful to Dr Paul
Thompson, Dr Laurence Bouchenoire and Dr Didier Wermeille at the ESRF for providing assistance in
using beamline BM28 (XMaS - UK CRG Beamline). We thank S. Morris for technical support.
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DOI: 10.1039/D0SC02152K
Table 1. EXAFS Fitting for Au L3-edge data of Au/C-S2O3 and Au/C-AR catalysts
during sequential gas experiments
Coordination
number
2σ2
R (Å)
Ef (eV)
R factor
(Å2)5
Reaction
step1
Scattering
path2
Au/C-
S2O3
Au/C-
Au/C-
S2O3
Au/C-
AR
Au/C-
S2O3
Au/C-
Au/C-
Au/C-
AR3
1(1)
3(1)
3(1)
Au/C-
S2O3
0.011
0.012
0.022
Au/C-
AR3
0.012
0.07
0.04
0.031
AR3
AR3
S2O3
2(1)
1(1)
2(1)
1(2)
i
ii
iii
Au-Cl
Au-Cl
Au-Cl
2.5(1)
2.5(1)
2.6(2)
2.2(3)
1.6(1)
1
2.5(1)
2.4(1)
2.5(1)
2.0(4)
0.9(2)
1
0.0037
2.27(1)
2.27(1)
2.28(1)
2.28(1)
2.28(2)
2.28(1)
2.27(1)
2.88(2)
2.27 (1)
2.27 (1)
2.27(1)
2.29(2)
2.30(3)
2.04(7)
2.26(1)
2.84(5)
iv (model 1)6
Au-Cl
0.006(2)
0.011(2)
1(2)
Au-Cl (1)
0.0037
iv (model 2)
v
1(1)
1(4)
0.020
0.019
0.037
0.017
Au-C4
0.0090
0.0037
0.007
Au-Cl
2.2(1)
1.0(4)
1.7(3)
0.6(4)
1(1)
2(2)
Au-Au4
Fixed parameters: So2 = 0.75, 2σ2 (Å2) = 0.0037 (Au-Cl) and 0.007(Au-Au)
1EXAFS data taken under steady state conditions
2Fitting of multiple Au-Cl paths was attempted for all spectra. Only is step iv did this provide
realistic fits.
3Au/C-AR data published in ref 37.
4Au-C CN fixed at 1
5 Debye-Waller and amplitude reduction factors were determined from fitting of KAuCl4 with
a fixed CN of 4 or Au(0) with fixed CN of 12.
6 Debye-Waller fitted for model 1(step iv)
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Figures
Figure 1. a) Comparison of the normalised ex situ Cl K-edge XANES of the freshly prepared Au/C-
AR (blue solid line) and Au/C-S2O3 (black solid line) , b) Cl(2p) core-level spectra for Au/C-AR and
Au/C-S2O3 with associated binding energies; 197.7 eV - Au-Cl/NaCl (red), 200 and 202 eV - C-Cl
functional groups (blue) and 203.4 and 205.2 eV loss structure associated with Cl species (orange). c)
Catalytic activity of the Au/C-AR (-●-, black), Au/C-S2O3 (-■-, red) and Au/C-H2O (-♦-, blue) (Test
conditions: 90 mg catalyst, 23.5 mL min-1 C2H2, 23.7 mL min-1 HCl and 2.7 mL min-1 Ar, 200 °C). 1d)
XANES of gold chloride standards. KAuCl4 (black solid line), Au2Cl6 (blue solid line) and AuCl (red
solid line).
a)
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DOI: 10.1039/D0SC02152K
b)
c)
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d)
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Figure 2. Cl K-edge XANES during the heating ramp of Au/C-AR (a) and Au/C-S2O3 (b) at start of
DOI: 10.1039/D0SC02152K
heating ramp (black), maximum pre-edge intensity (red) and end of heating ramp (blue). (c) Pre-edge
intensity against temperature for Au/C-AR (-●-, black) and Au/C-S2O3 (-■-, red).
a)
b)
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c)
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Figure 3. In situ Cl K-edge XANES spectra of (a) Au/C-AR and (b) Au/C-S2O3 catalysts dVuierwinAgrticle Online
DOI: 10.1039/D0SC02152K
acetylene hydrochlorination at: beginning of the reaction (black line) max of pre-edge intensity (red
line) and at steady state (blue line). (c) Change in pre-edge intensity with reaction time for Au/C-AR (-
●-, black) and Au/C-S2O3 (-■-,red).
a)
b)
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c)
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Figure 4. Comparison of the normalised ex situ Cl K-edge XANES of the freshly prepared Au/C-
DOI: 10.1039/D0SC02152K
acetone (dashed orange line) and Au/C-AR (black solid line).
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Figure 5. S K-edge XANES of the Au/C-S2O3. (a) Ex situ XANES normalised absorption spectra of
DOI: 10.1039/D0SC02152K
Au/C-S2O3 (red solid line) with Na2S2O3 standard (blue solid line). (b) In situ XANES normalised
absorption spectra during reaction time-on-line: start of reaction (red solid line) and after 20 minutes
(red dashed line).
a)
b)
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Figure 6. Sulphur selected orbital K-edge XANES obtained by DFT simulations. a) ExcitedVsietwaAterticle Online
DOI: 10.1039/D0SC02152K
2-
responsible for transition at 2477 eV. b) Comparison of simulated S K edge in thiosulfate S2O3 and
model structures. (c) S(2p) core-level spectra for Na2Au(S2O3)2 and the Au/C-S2O3 catalyst with
associated binding energy; two sulphur states in the former (S6+ and S2- at 166.7 and 162.1 eV) and
several sulphur states in the catalyst, attributed to Au-S (162.6 eV), S-S (163.8 eV), S-H (164.8 eV),
SO3 (168.1 eV) and SO4 (169.6 eV), consistent with disproportionation.
a)
b)
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c)
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Figure 7. Au L3-edge XANES of Au/C-S2O3 and Au/C-AR during the sequential gas exposure
DOI: 10.1039/D0SC02152K
experiments. (a) Au/C-S2O3 and (b) Au/C-AR at steady state for steps i (red line), steps ii (blue line)
and steps iii (black line). (c) Au/C-S2O3 and (d) Au/C-AR at steady state for steps iii (black line), steps
iv (green line) and steps v (purple line).
a)
b)
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c)
d)
29