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J. Zhao et al. / Journal of Catalysis 365 (2018) 153–162
Recently, Hutchings et al. proposed that deposition of HAuCl4
A series of carbon-supported Au catalysts were prepared via a
wet impregnation method. In a typical synthesis of the 1.0 wt%
Au/ROX08(AP) catalyst, 0.6 mL of HAuCl4 (0.05 gAu/mL) aqueous
solution was added into 5.4 mL of H2O2/HCl mixture (1:3
H2O2(30 wt%): HCl (36 wt%)) in a 20 mL flask under magnetic stir-
ring. The solution was stirred for 1 h after complete addition of the
HAuCl4 solution. Then, the solution of the gold precursor was
added dropwise to 2.97 g of pretreated ROX08 carbon support.
The mixture was stirred vigorously for 4 h and then dried at
110 °C under vacuum for 12 h. The synthetic procedures for
preparing Au/ROX08(AP) with different Au loadings ranging from
0.1 wt% to 0.5 wt% were similar to that for 1.0 wt% Au/ROX08
(AP), except for the concentration and amount of HAuCl4 solution.
HAuCl4ꢀ4H2O solutions in 36 wt% hydrochloric acid or 30 wt%
hydrogen peroxide solutions were also prepared to obtain Au
catalysts with 1.0 wt% loading. The samples were designated
Au/ROX08(A) and Au/ROX08(P), respectively. For comparison, the
Au/ROX08(AQ) catalyst was also prepared using aqua regia (1:3
HNO3 (>65 wt%): HCl (>36 wt%)) as solvent in an identical proce-
dure. This catalyst was used as a reference.
as a precursor onto a carbon support in the presence of aqua
regia as an impregnation solvent led to considerably active
hydrochlorination catalysts because of the formation of single-
site cationic Au entities (AuClx), which act as the active sites
in the final material [13,14]. The study also demonstrated that
the active species consist of a single-site Au+ and Au3+ cationic
species [14]. However, metallic Au0 species are inactive and
are not involving in the reaction [15]. Despite the demonstrated
high activity of the Au/C catalyst prepared using aqua regia,
there are several drawbacks to this technique that make it
incompatible with common industrial manufacturing processes.
First, aqua regia is the origin of toxic NOCl, Cl2 and NOx, which
would cause serious environmental impacts and limit process
safety. Second, the Au/C catalysts made with aqua regia are
unstable during long reactions due to the dynamic nature of
the surface Au species [16]. Finally, the use of the aqua regia
for the preparation of Au/AC catalysts lead to considerable
agglomeration of the gold on the carbon support, which makes
it impossible to decreasing the metal loading [17–19]. To cir-
cumvent those issues, alternative catalyst preparation methods
that are greener and more effective are necessary.
Thus, as a continuation of Hutchings’ efforts, we describe an
environmentally benign approach using a mixture of hydrogen
peroxide and hydrochloric acid (H2O2/HCl), i.e., a ‘‘green aqua
regia”, which provides facile entry to the production of active Au/
AC catalysts and is suitable for practical use. This H2O2/HCl mixture
was also found to efficiently produce single-site active AuClx enti-
ties on the carbon support through a combination of the oxidizing
effect of the H2O2/HCl mixture on the metallic Au0 species which
was related with the reduction property of carbon towards Au3+
and the carbon support, which facilitates high-valent and atomi-
cally dispersed cationic Au sites. These observations were sup-
ported by substantial catalyst characterization studies. Then, the
catalytic performances of the prepared Au-based catalysts were
investigated in the hydrochlorination of ethyne. In contrast to
the catalyst prepared using aqua regia (Au/ROX08(AQ)), we
demonstrated that the new preparation method using the H2O2/
HCl mixture leads to catalysts (Au/ROX08(AP)) with improved
activity and stability. Mechanistic studies revealed that the surface
oxygen-containing functional groups (SOGs) created by the H2O2/
HCl mixture are better than aqua regia at stabilizing single-site
cationic Au sites. More importantly, density functional theory
(DFT) calculations showed that isolated cationic AuCl entities
directly bonded to SOGs (e.g., carbonyl groups) on the carbon sup-
port could catalyze the reaction more effectively than AuCl on a
graphite surface. In addition, the used catalyst could be efficiently
reactivated using H2O2/HCl mixture treatment. This strategy
allows the green preparation of highly dispersed and active gold/-
carbon catalysts.
The catalysts prepared using deionized water as the solvent on
the untreated ROX08 carbon or ROX08 carbon pretreated by the
H2O2/HCl mixture or aqua regia were labeled Au/ROX08-H2O, Au/
ROX08(AP)-H2O and Au/ROX08(AQ)-H2O, respectively.
2.2. The regeneration of used Au/ROX08(AP) catalyst
Regeneration treatments of the used catalysts were carried out
in a sealed glass vial. In a typical treatment, 2 mL of the H2O2/HCl
mixture (1:3 H2O2: HCl) was prepared in a sealed sample vial. After
that, 1 g of the used Au/ROX08(AP) catalyst was added to the
mixed solution. The samples were vigorously stirred at 70 °C under
ambient pressure for 4 h. After treatment, the samples were dried
at 110 °C for 12 h to give the regenerated catalyst, which was
labeled Au/ROX08(AP)-R.
2.3. Catalyst characterization and computational procedures
X-ray diffraction (XRD) measurements were carried out on a
PANalytical-X’Pert PRO generator with Cu
Ka radiation (k =
0.1541 nm) operating at 60 kV and 55 mA. Diffraction patterns
were recorded at a scanning rate of 2° minꢁ1 and at a step angle
of 0.02°. Transmission electron microscopy (TEM) images were
acquired on a Cs-corrected FEI Titan G2 60–300 Microscope oper-
ating at 300 kV using an HAADF detector. The solid samples were
finely ground and dispersed ultrasonically in ethanol and then
transferred to a carbon/Cu grid. Images that clearly reveal single
Au atoms were typically recorded at 10 M x direct magnification.
Because the nanoparticles and sub-nm species have almost equal
chances to be imaged on the support surfaces, the various gold spe-
cies (>200 total counts) near the edges of the samples in different
regions were counted and analyzed. X-ray photoelectron spec-
troscopy (XPS) was performed with a Kratos AXIS Ultra DLD spec-
trometer to determine the elemental surface composition of the
catalysts. The monochromatized aluminum X-ray source was
1486.6 eV, and the passed energy with an electron analyzer was
40 eV. The pressure in the analysis chamber was lower than
5 ꢂ 10ꢁ10 Torr during data acquisition. The samples were out-
gassed under vacuum for 4 h before loading. Binding energies were
referred to the C1s line at 284.8 eV. Temperature-programmed
reduction (TPR) was performed in a micro-flow reactor fed with
hydrogen (10% in Ar) at a flow rate of 45 mL minꢁ1. The tempera-
2. Experimental
2.1. Catalyst preparation
A commercially extruded activated carbon species, Norit ROX08
(surface area of 1100–1200 m2/g, pore volume of 0.63 cm3/g, den-
sity of 400 g/L, maximum diameter of 0.5 mm, and length of 1–5
mm; ROX08 is a steam-activated carbon that can be thermally
reactivated) was used as the catalyst support. First, the ROX08 car-
bon was ground to obtain a powder (80–100 mesh) and then pre-
treated with HCl (>36 wt%) to remove residual metal ions (such as
trace amounts of K+ and Al3+). The pretreated ROX08 carbon was
filtered, washed with deionized water until the pH of the filtrate
was 7, and then dried for use.
ture was increased from 30 °C to 850 °C at a rate of 10 °C minꢁ1
.
A thermal conductivity detector (TCD) was used to measure the
hydrogen consumption. The integrated TPR signal using CuO as a
standard was used to calibrate the TCD. The AuClx amount is