Modifications of the metal and support during the deactivation and regeneration of Au/C catalysts for the hydrochlorination of acetylene

Article abstract of DOI:10.1039/c2cy20478a

The effect of the gold oxidation state and carbon structure on the activity of Au/C catalysts for the hydrochlorination of acetylene was investigated by a combined approach using TPR, XPS and porosimetry determinations. The activity of the catalyst in the synthesis of vinyl chloride monomer was found to be dependent on the presence of Au³⁺ species in the catalyst. However, by preparing catalysts with different Au³⁺ content it was possible to determine the existence of a threshold Au³⁺ amount, beyond which the excess of Au³⁺ was not active for the reaction. This was explained by the existence of active sites at the Au/C interface, and not just by the presence of Au³⁺ species on top of Au nanoparticles, as explained by current models for these catalysts. It was also possible to determine the existence of a subset of Au nanoclusters which do not take part in the reaction, as well as changes in the textural properties of the carbon that can affect its long term reusability. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c2cy20478a

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Modifications of the metal and support during the
deactivation and regeneration of Au/C catalysts for
the hydrochlorination of acetylene†
Cite this: Catal. Sci. Technol.,
2013, 3, 128
Marco Conte,*a Catherine J. Davies,a David J. Morgan,a Thomas E. Davies,a
Albert F. Carley,a Peter Johnstonb and Graham J. Hutchings*a
The effect of the gold oxidation state and carbon structure on the activity of Au/C catalysts for the
hydrochlorination of acetylene was investigated by
a combined approach using TPR, XPS and
porosimetry determinations. The activity of the catalyst in the synthesis of vinyl chloride monomer was
found to be dependent on the presence of Au3+ species in the catalyst. However, by preparing catalysts
with different Au3+ content it was possible to determine the existence of a threshold Au3+ amount,
beyond which the excess of Au3+ was not active for the reaction. This was explained by the existence of
active sites at the Au/C interface, and not just by the presence of Au3+ species on top of Au
nanoparticles, as explained by current models for these catalysts. It was also possible to determine the
existence of a subset of Au nanoclusters which do not take part in the reaction, as well as changes in
the textural properties of the carbon that can affect its long term reusability.
Received 9th July 2012,
Accepted 3rd August 2012
DOI: 10.1039/c2cy20478a
www.rsc.org/catalysis
of identifying which of them are responsible for the catalytic
activity. Previous studies showed that it was possible to regenerate
1. Introduction
the Au catalyst using Cl2 and NO.6 Alternatively, it was possible to
demonstrate that an off-line treatment with aqua regia was also an
effective regeneration method.8 However in all these cases the
attention was on the sole change of the gold oxidation state
without considering possible modifications of the support, as
well as postulating Au3+ species on top of Au nanoparticles as
active sites for the reaction,4 without providing clear insight into
the possible correlation between the presence of an excess
amount of Au3+ and the catalytic activity.
This prompted us to investigate the Au/C catalysts by a series
of sequential treatments using H2 as a reducing agent and aqua
regia as an oxidizing agent, in order to identify the role of Au3+
and Au0 in the nanoparticles deposited over the carbon support
with respect to catalytic activity. The catalysts were therefore
investigated by means of high resolution X-ray photoelectron
spectroscopy complemented with temperature programmed
reduction analysis, the latter being a bulk technique capable
of quantifying oxidized gold species and changes in the carbon
functional groups that could also affect the final reactivity of
the catalyst,9 in the presence of active gold centers at the Au/C
interface. In addition, porosimetry determinations were also
carried out, as this is proven to be a useful tool for the
determination of carbon textural properties.10 In fact, an aspect
so far neglected in previous studies is the influence of oxidizing
The hydrochlorination of acetylene using HCl is an important
industrial route to the manufacture of vinyl chloride monomer
(VCM), which is the building block for polyvinyl chloride. It was
previously shown that Au/C catalysts can be the best catalysts to
carry out this reaction using the direct acetylene hydrochlorination
route.1–3 However, the catalyst was found to deactivate, possibly by
reduction of Au3+ species to Au0 4,5 when the reaction is carried out
in the 120–180 1C temperature range, and by oligomer formation
over its surface when the reaction is carried out in the 60–100 1C
temperature range.6 At present, most VCM is obtained by an
oxychlorination reaction using ethylene, O2 and Cl2 as reactants.7
Due to the current increase in energy cost, coal-based derived
feedstocks, like acetylene, are now becoming economically advanta-
geous. Current industrial catalysts for the direct hydrochlorination
reaction make use of mercuric chloride, and the efficiency of this
VCM manufacture route is severely affected by the volatility of this
metal chlorinated salt. These factors prompted us to explore the
nature of the active species present over Au/C catalysts with the aim
a Cardiff Catalysis Institute, School of Chemistry, Cardiff University, Cardiff,
CF10 3AT, UK. E-mail: ConteM1@cardiff.ac.uk, Hutch@cardiff.ac.uk
b Johnson Matthey Catalysts, Orchard Road, Royston, Herts SG8 5HE, UK
† Electronic supplementary information (ESI) available: Catalyst testing, and
characterization by XRD, SEM, XPS and porosimetry. See DOI: 10.1039/
c2cy20478a
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acids in the impregnating mixture that was used to prepare the The sample (100 mg) was heated up to 800 1C at a ramp rate
catalyst. This could affect the formation of oxygenated species of 5 1C minꢁ1, under a flow of hydrogen (10% in Ar, 20 mL minꢁ1
)
over the carbon surface11 as well as the pore structure of the for the reduction and the oxidation step respectively. Calibration
carbon,12 with possible effects on the reusability of this material. of the detector, and hydrogen consumption, was carried out
As Au/C catalysts are widely used for reactions other than from the integrated TPR (TCD) signal using CuO as a standard19
hydrochlorination e.g. partial oxidation of alcohols13,14 and (Sigma-Aldrich, 10 mg), subject to the same temperature
hydrocarbons,15,16 we foresee that this work may have a broader program used for the Au/C catalysts. The Au3+ amount is
application to catalyst design for such reactions.
reported as a ratio of the Au3+ to the total Au amount and the
absolute amount in mmol (see ESI†).
Peak integration was carried out using SpecView Software.
The thermogram was subjected to baseline correction and the area
integrated using a cumulative counts algorithm (Fig. S1, ESI†).
CO2 from decarboxylation reactions was qualitatively identified
2. Experimental
2.1 Catalyst preparation
All the catalysts were prepared by a wet impregnation method.
The gold precursor, HAuCl4ꢀxH2O (Alfa Aesar, 40 mg, assay 49%), by heating the carbon in a He/H2 atmosphere and collecting the
was dissolved in aqua regia (3 : 1 HCl (Fisher, 32%) : HNO3 effluent gases in a saturated solution of BaCl2ꢀ2H2O, the formation
(Fisher, 70%) by volume, 5.4 ml) and the solution added dropwise of a BaCO3 precipitate was detected.
with stirring to the activated carbon support (Norit ROX 0.8)
2.3.2 SCANNING ELECTRON MICROSCOPY. Scanning electron micro-
(1.98 g) in order to obtain a catalyst with a final metal loading scopy (SEM) and energy dispersive X-ray (EDX) analyses were
of 1 wt%. Stirring was continued at ambient temperature until performed using a Carl Zeiss EVO-40 microscope and an Oxford
NOx production subsided, approximately 10 minutes. The product instrument SiLi detector respectively. Samples were mounted
was dried for 16 h at 140 1C and used as a catalyst.
on aluminium stubs using adhesive carbon discs. EDX data
Catalyst oxidation was carried out by stirring the material in were averaged from multiple overscans and spot analysis is
a minimum amount of aqua regia to obtain a slurry at room considered representative of the sample. SEM was used to analyse
temperature for 10 min (i.e. until NOx production subsided) the morphology of the samples, while SEM-EDX to determine the
followed by drying overnight at 140 1C.
chemical composition of the samples.
Due to the nature of the catalysts preparation procedure
2.3.3 X-RAY PHOTOELECTRON SPECTROSCOPY. X-ray photoelectron
used, wet impregnation,17 no filtration of the carbon or catalyst spectroscopy (XPS) was performed with a Kratos Axis Ultra DLD
washing was carried out, and the metal loading should be spectrometer using a monochromatised AlKa X-ray source
considered as equal to the nominal amount of metal impregnated (120 W) with an analyzer pass energy of 160 eV for survey scans
into the support.18
and 40 eV for detailed elemental scans. Binding energies are
referenced to the C(1s) binding energy of carbon, taken to be
284.7 eV. As it is well known that cationic Au species can be
2.2 Catalytic tests and characterization of the products
Catalysts were tested for acetylene hydrochlorination in a fixed- reduced to the zero-valent state by secondary electron emission
bed glass microreactor. Acetylene (5 mL minꢁ1, 0.5 bar) and during XPS analysis, the Au(4f) region was recorded at the
HCl (6 mL minꢁ1, 1 bar) were fed through a mixing vessel and a beginning and end of the analysis.20 The Au3+ and Au0 amounts
preheater (70 1C), and further mixed in a N2 flow (10 mL minꢁ1
,
are reported as the percentage of the total Au amount, and the
1 bar) via calibrated mass flow controllers in a heated glass absolute amount in mmol (see ESI†).
reactor containing catalyst (200 mg), with a total GHSV of
2.3.4 X-RAY POWDER DIFFRACTION. X-ray powder diffraction
740 hꢁ1. A reaction temperature of 180 1C was chosen, and (XRPD) spectra were acquired using a X’Pert PANalytical
blank tests using an empty reactor filled with quartz wool did diffractometer operating at 40 kV and 40 mA selecting the CuKa
not reveal any catalytic activity, even at 250 1C with the radiation. Analysis of the spectra was carried out using X’Pert
reactants under these flow conditions, SiC (2 ꢂ 2.5 g) was used HighScore Plus software. Particle size was determined using the
to extend the bed length, above and below the catalyst itself, Scherrer equation assuming spherical particle shapes and a K
separated by quartz wool. The pressure of the reactants, HCl, factor of 0.89. The line broadening was determined using a
C2H2 and N2, was chosen both for safety reasons and to test the Voigt profile function convoluting the Gaussian and Lorentzian
catalyst under mild conditions. The gas phase products were profile part of the reflection peak and the instrumental broadening
analyzed on-line by GC using a Varian 450GC equipped with a for the Bragg–Brentano geometry used was estimated to be 0.061 2y.
flame ionisation detector (FID). Chromatographic separation
2.3.5 SURFACE AREA AND POROSIMETRY DETERMINATION. Surface area
and identification of the products were carried out using a and pore size analysis was determined by N2 adsorption at 77 K
Porapak N packed column (6 ft ꢂ 1/800 stainless steel).
using a Quantachrome Autosorb AS1. Samples were degassed
for a minimum of 24 h at 120 1C before the analysis. Surface
areas were evaluated using the BET method and the pore size
2.3 Characterization of the catalyst
2.3.1 TEMPERATURE PROGRAMMED REDUCTION. Temperature pro- distribution was determined by the Barrett–Joyner–Halenda
grammed reduction (TPR) analysis was carried out on a Thermo (BJH) method.21 The microporous surface area and the external
TPD/R/O 1100 Series instrument equipped with a thermal surface area (the area of those pores which are not micropores)
conductivity detector (TCD), recording the signal in mV. were determined by the t-plot.
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3. Results and discussion
3.1 Correlation between the catalytic activity and gold
oxidation state by TPR determinations
In order to verify the effect of oxidized gold species on Au/C
catalysts, a catalytic test was carried out with a fresh catalyst
(1 wt% Au) and with this material reduced in a H2 atmosphere
prior to the hydrochlorination reaction (Fig. 1). The fresh
catalyst displayed a conversion to vinyl chloride monomer of
ca. 60% with a selectivity virtually of 100% to VCM, with trace
amounts (o0.1%) of 1,2-dichloroethane and chlorinated oligomers
only.4,6 In contrast, for the catalyst pre-treated in H2 the activity was
limited to only ca. 10% conversion.
In order to assess the differences in the Au species for the
fresh and the pre-reduced catalyst, temperature programmed
reduction (TPR) was used (Fig. 2). It is known that Au3+ can
reduce in the range of 120–180 1C, and that the catalyst can
form oligomers over its surface in the temperature range of
60–100 1C.6 However, the analyses carried out to date were all
focused on pure changes of the oxidation state of the metal. In
contrast, by means of TPR we can also probe changes in the
carbon support matrix, and therefore acquire a more accurate
picture of the catalyst requirements to be active in the case of
metal–support interaction. For our catalysts, the fresh Au/C
material presents a characteristic reduction band between 230
and 300 1C (with center at 267 1C, Fig. 2 and Fig. S2, ESI†),
Fig. 2 TPR profiles of Au/C catalysts subject to reduction and oxidation cycles,
(a) fresh catalyst, (b) reduced catalyst, (c) first catalyst oxidation, (d) second
catalyst reduction, (e) second catalyst oxidation, (f) third catalyst reduction.
acetylene.23 These impurities were identified using SEM-EDX
(see ESI†, Fig. S3 and S4) and the following catalyst atomic
composition was determined: C 86.44%, O 9.69%, Na 0.11%, Al
0.05%, Si 0.13%, S 0.22%, Cl 2.34%, K 0.04% and Au 1.04%.
In view of this, and to evaluate systematically the effect of
Au3+ centres on the catalytic activity, the catalyst was treated
off-line with aqua regia.8 We have previously demonstrated that
a short aqua regia treatment of the catalyst can regenerate the
catalytic activity without loss of gold metal (as determined by
atomic absorption spectroscopy). This was the case also for the
Norit carbon support used in the present study, and the
catalytic activity was recovered (Fig. 2). More importantly, if
the catalyst was subjected to a reduction treatment the catalytic
activity was again lost, but if the catalyst was re-oxidized it was
able to recover its original activity without any apparent con-
version loss when compared to the fresh catalyst.
which is diagnostic of Au3+ 22
Comparison of the TCD signal
.
with a standard permits an estimate of the Au3+ amount to be
ca. 26% of the total Au loading. In contrast, TPR of the pre-
reduced catalyst showed that no Au3+ species remained as this
reduction band was absent. Control tests with the carbon
support without Au showed that the activity of the pre-reduced
Au/C catalyst was identical to that of the support. This residual
activity of the carbon can arise from the presence of trace
amounts of K+ and Al3+ in the carbon matrix, as these metals
can display some activity to the hydrochlorination reaction of
Using TPR we were able to monitor the amount of Au3+
present at each step of the reduction–oxidation cycles, and it is
clearly possible to observe that at every oxidation Au3+ is
restored, and catalytic activity is present, while at each reduction
Au3+ is absent, and negligible catalytic activity is detected
accordingly (Fig. 2). This is an important result, because it shows
a high degree of reversibility of the Au3+–Au0 system, as well as it
implies that the catalyst can support a high number of regenera-
tion by sequential oxidation–reduction, and this could be extre-
mely useful in an industrial context.
An important aspect, in the use of TPR tools for the analysis
of the catalysts reported in this study, is that they allow the
determination of not just the gold oxidation state, and quanti-
fication of the bulk Au3+ amount, but also changes in structural
groups present in the carbon matrix. In fact, it should be
stressed that activated carbon is not a matrix constituted of
carbon only as an element, but it has a complex structure
characterized by the presence of oxygen in the form of carboxylic,
ester, ether and lactone groups12,24 as well as heteroatoms e.g.
phosphorus nitrogen or sulphur, present as phosphates, amines
and thiols.25
Fig. 1 Acetylene conversion over Au/C (1 wt%) catalysts subject to reduction and
oxidation cycles, (’) fresh catalyst, (J) reduced catalyst, (K) first catalyst
oxidation, (D) second catalyst reduction, (m) second catalyst oxidation, (ꢂ) third
catalyst reduction.
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This was evident from careful analysis of the TPR profiles,
Catalysis Science & Technology
where the oxidation step in the presence of aqua regia not only
regenerated Au3+, but also effected the oxidation of the carbon
support (Fig. 2). From the thermograms (Fig. 2), it is inferred that
bands in the range from 450 to 800 1C are assigned to decarboxyla-
tion reactions of oxygenated carbon functional groups at the carbon
surface, which releases CO and CO2 with concomitant reduction of
oxygenated groups by H2 in the TPR stream. More specifically,
decarboxylation and reduction of oxygenated functional groups in
the range of 400–650 1C are attributed to carboxylic acids and
carboxylic anhydrides,26 while bands above 650 1C are usually
assigned to lactones and phenols.27 Therefore while the pre-treat-
ment of the catalyst in H2 also reduced the surface carboxylic
groups on the carbon, the treatment with aqua regia is capable of
inducing carboxyl functionality of the carbon surface as well as Au3+
recovery. This effect on the carbon matrix should be considered a
consequence of the presence of HNO3 in the regeneration step. In
fact, this acid is known to induce modifications on the carbon
surface inducing oxygenated functional groups,28 as well as
changes in the textural properties of the carbon support24 and this
was confirmed by XPS and porosimetry (this will be discussed
subsequently in Sections 3.2 and 3.3).
Fig. 3 Conversion (K) after 5 h and Au3+ (&) amount for Au/C catalysts
containing different amounts of Au3+. The catalysts were obtained from
impregnation using solutions containing different HNO3 volume fraction
in the HCl/HNO3 mixture.
(which are not active for the reaction), and the oxidized catalyst
with an amount of Au3+ (which are active for the reaction), by
analyzing the amount of Au3+ for the fresh catalyst, the catalyst
after first oxidation, and the catalyst after second oxidation
(Fig. 2), the Au3+ content was ca. 26, 36 and 40% respectively.
This would suggest that despite Au3+ being clearly needed for
the reaction, above a given limit an excess of Au3+ does not
increase the activity further. In view of this, we prepared a set of
catalysts using a different HCl/HNO3 ratio in the starting
impregnation mixture spanning from 0 to 0.5 HNO3 volume
fraction. In fact in HCl/HNO3 mixtures HNO3 is responsible for
changes in the oxidation state of gold, as well as textural properties
of the carbon.24 The Au3+ content was determined for each catalyst
by means of TPR, and compared with the corresponding catalytic
activity after 5 h (Fig. 3 and Fig. S7 and S8, ESI†). Interestingly, a
volcano plot is obtained (Fig. 3 and Fig. S7, ESI†), centred at the
aqua regia composition (0.25 HNO3). This shows that if the amount
of Au3+ increases (in the range of 0 to 0.25 HCl–HNO3) the activity
also increases. However, if the amount of Au3+ increases further,
no actual gain in activity is obtained but rather a decrease in
conversion. It should also be stressed that in this set of experi-
ments at a different HCl/HNO3 ratio, an increased Au3+ amount
was obtained in line with the increased HNO3 amount, rather
than using a repeated aqua regia treatment as it was done
initially. This result would suggest that the most active catalyst
is not necessarily the material with the highest Au3+ content, but
possibly the material that has Au3+ at the appropriate catalyst
sites. Thus suggesting the existence of active Au3+ sites at the
Au/C interface and not just at the surface of Au nanoparticles as
previously postulated.4
It is worth noting that previous studies on Au/C catalysts for
hydrochlorination4,29 showed a monotonic deactivation trend
ascribed to the reduction of Au3+ to Au0 and not an activity
trend with enhanced activity per time on stream, like the one
we are detecting in the current study. On the other hand, for the
catalyst reported in the present study, XRPD patterns (Fig. S5,
ESI†) permit an estimate of the mean particle size to be around
20 nm while in previous cases catalysts having an average
particle size in the range of 4–5 nm were used, and this could
affect the observed trend. Additional tests on the differences in
drying temperature that could affect the different particle size
and the metal/support interface were also carried out. Two
catalysts were prepared at a drying temperature of 110 and 180 1C
(Fig. S6, ESI†). The latter was chosen because it is the same as the
reaction temperature, in order to evaluate if the conversion trend
could be affected by the temperature itself. The catalyst dried at
180 1C has a similar trend to those dried at 110 1C, in contrast, the
one dried at 140 1C is markedly different with an increase in
activity from ca. 10% at the beginning of the reaction to up to 70%
after 5 h of time on stream. It is possible that modifications of the
Au/C interface, also induced by the particle size, are responsible
for the observed phenomenon. XRPD patterns for the set of
catalysts dried at 110, 140 and 180 1C (Fig. S5, ESI†) showed a
particle size distribution centered at o2 nm, 20 and 3 nm,
respectively, suggesting that particles that are too small could
not be active for the reaction. On the other hand, previous studies
showed that under reaction conditions, the catalyst can be
affected by a small, yet detectable, degree of sintering.29
3.1.1 EXCESS AMOUNT OF AU3+. A further important aspect of
the analysis of the Au3+ content is to assess if there is a
threshold beyond which the amount of Au3+ is uneffective, or
3.2 X-ray photoelectron spectroscopy and particle size
distribution
up to which extent the amount of Au3+ present in the catalyst In order to correlate bulk changes of the catalyst structure with
can influence the catalytic activity. In fact, while there is a clear the surface of gold nanoparticles, X-ray photoelectron spectro-
distinction between the reduced Au/C catalysts with no Au3+ scopy (XPS) was systematically carried out for all these samples
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Fig. 4 XPS profiles of Au/C catalysts subject to reduction and oxidation cycles, (A) fresh catalyst, (B) reduced catalyst, (C) first catalyst oxidation, (D) second catalyst
reduction, (E) second catalyst oxidation, (F) third catalyst reduction and (G) third catalyst oxidation.
(Fig. 4 and Fig. S9 to S15, ESI†). It should be noted that where indicate that the deactivated catalysts contain Au0 only. Similarly,
more than one Au species was evident, curve fitting was for the active catalysts, the Au3+ content was found to follow the
employed to determine the ratio of each species. As shown in same general trend obtained from TPR. Quantification of the Au3+
Table 1, the XPS results confirm the TPR measurements and content reveals a fluctuation between ca. 12 and 25% of the overall
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Table 1 Quantification and identification from XPS of Au species over Au/C
Table 3 Textural properties of the carbon precursor, the Au/C catalyst and a
catalyst oxidized with aqua regia
catalysts subject to reduction and oxidation cycles
Au species (%)
Catalyst treatment Au3+ Au0
Binding energies (eV)
SBET
Smicro
Sext
Vtot
Vmic
D
(nm)
Sample (m2 gꢁ1
)
(m2 gꢁ1
)
(m2 gꢁ1
)
(cmꢁ3 gꢁ1
)
(cmꢁ3 gꢁ1
)
Au0-s Au3+
Au0
Au0-s
Carbon 1153
(Norit
938
215
0.5318
0.4859
2.2
Fresh catalyst
1st reduction
1st oxidation
2nd reduction
2nd oxidation
3rd reduction
3rd oxidation
20.5
0
13.1
0
24.5
0
12.3
65.5 14.0
87.1 12.9
75.2 11.7
89.5 10.6
86.4
—
86.3
—
86.2
—
86.2
83.8
83.9
83.9
84.0
83.9
84.0
83.8
84.9
85.4
85.2
85.1
85.1
85.3
84.8
ROX0.8)
Au/C
fresh
Au/C
1181
1213
838
827
343
385
0.5391
0.5442
0.4275
0.4251
2.3
2.4
67.4
8.1
90.0 10.0
70.2 17.5
oxidised
pore changes in the carbon structure by nitric acid,16,17 and
therefore gold particles are present also inside the carbon pores
and not just at the surface of these. In fact when SEM-EDX was
used in view of the major penetration depth of this method a
composition close to 1% was detected.
Table 2 Surface atomic composition of Au/C catalysts subject to reduction and
oxidation cycles
Surface atomic composition (%)
Catalyst treatment
Au 4f
C 1s
Cl 2p
Na 1s
O 1s
3.3 Textural properties of the Au/C catalysts
Fresh catalyst
1st reduction
1st oxidation
2nd reduction
2nd oxidation
3rd reduction
3rd oxidation
0.08
0.07
0.06
0.04
0.06
0.04
0.04
93.30
96.21
93.27
97.42
94.53
97.25
92.58
0.80
0.32
1.20
0.13
1.08
0.13
1.26
0.00
0.33
0.00
0.18
0.40
0.05
0.08
5.81
3.06
5.48
2.23
3.93
2.54
6.04
To corroborate these experimental results, porosimetry deter-
minations were carried out on an untreated carbon support, a
fresh catalyst and an catalyst after oxidation treatment with
aqua regia.
Adsorption of N2 resulted in the formation of a type 1
isotherm typical of microporous material (Fig. S16 and S17,
gold content after regeneration (Table 1 and Table S1, ESI†), ESI†). The narrow pore size distribution curves we obtained are
however regardless of the overall amount all catalysts are indicative of uniform pore channels in the small mesopore to
capable of reaching similar conversion values of ca. 60%. This micropore region. Application of the BJH method shows that
the average pore size is in the range of 2–3 nm but given the
high N2 uptake at P/Po o 0.2 the material likely occupies the
result may further indicate that it is not just the presence of
Au3+ which is responsible for the observed reactivity, but also
the location of such species (the Au3+ amount obtained by XPS microporous to mesoporous range (1–2 nm). The untreated
are always lower than those obtained by TPR as the former is a carbon has the lowest surface area of 1153 m2 gꢁ1 and the
surface method, while the latter a bulk technique). The lowest total pore size volume of 0.5318 cm3 gꢁ1 (Table 3).
observed increase in surface oxygen content (Table 2) after Treatment of the catalyst with acid appears to decrease micro-
each regeneration cycle is a consequence of the oxidising effect porosity indicated by the decrease in the Smic and an increase in
of the nitric acid in the aqua regia regeneration medium24,30 Sext, which accounts for the external surface area and meso-
and could be responsible for stabilising the high Au3+ oxidation
porous contribution. The microporosity of the catalyst is further
decreased by oxidation. It has been shown previously10,24 that
state, particularly as there is no decrease in the surface oxygen
content of the reduced samples, together with the absence of oxidation and acid treatment of activated carbon result in a
any Au3+ species.
decrease in the microporous volume due to the formation of
However, the most important feature of the XPS spectra is surface oxides at the micropore entrance. It was also shown that
possibly that in the preparation of these catalysts small metallic strong oxidation can result in a total loss of surface area due to
gold clusters (hereafter labelled Au0-s) are also formed, the destruction of the pore walls and network.12 The blocking of
binding energy of which is ca. 1 eV higher than the Au(4f) micropores as a result of Au addition should also be considered.
binding energy for the majority of the Au0 species31,32 (Table 2
These factors have been so far fully neglected in the acetylene
and Fig. 4). As we as well as other researchers20 have previously
hydrochlorination reaction, and they suggest that they could be
observed, Au+ will also reduce to Au0; which is not observed by important for the long term usability of this catalyst.
prolonged X-ray analysis here. However, we consider the Au0-s
species to be nothing more than a spectator species, since the
4. Conclusions
reduced catalysts also contain these Au0-s nanoclusters and are
inactive, although small gold clusters are active in a range of
reactions such as CO oxidation.33
It has been confirmed that the activity of Au/C catalysts for
the hydrochlorination reaction is invariably associated with the
presence of Au3+ species, although our data suggest that the
final activity can also be a consequence of the Au3+ location, and
not just its concentration, as an excess amount of Au3+ does not
contribute to the reaction, with important implications in the
catalyst design. In addition, the current work provides insight
Therefore, considering the XPS and XRD reported in this
study, as well as previous work, these data could suggest a
particle size range for the particles activity from 529 to 20 nm.
It should also be underlined that the apparent decrease in
Au content detected from XPS analysis is an effect induced by
c
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into the active particle size distribution for this catalyst for this
reaction, showing that Au nanoclusters are not active for this
reaction. This will have important implications in the catalyst
preparation targeted to obtain nanoparticles >1 nm and possibly
centered at the size range of 5–20 nm. The catalysts also showed a
large extent of reusability, although changes in textural properties
of the carbon were detected.
´
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