Use of galvanic displacement in the synthesis of a Pd(Cu) hydrodechlorination catalyst

Article abstract of DOI:10.1016/j.mencom.2010.01.004

The high prospects of galvanic displacement for the preparation of high-reactivity mixed catalysts with a small content of platinum metal have been shown for a Pd(Cu)/support system.

Full text of DOI:10.1016/j.mencom.2010.01.004

Mendeleev
Communications
Mendeleev Commun., 2010, 20, 10–11
Use of galvanic displacement in the synthesis
of a Pd(Cu) hydrodechlorination catalyst
Evgeniya A. Tveritinova, Yurii M. Maksimov, Yurii N. Zhitnev,*
Boris I. Podlovchenko and Valery V. Lunin
Department of Chemistry, M. V. Lomonosov Moscow State University, 119991 Moscow, Russian Federation.
Fax: +7 495 939 4575; e-mail: zhitnevyun@mail.ru
DOI: 10.1016/j.mencom.2010.01.004
The high prospects of galvanic displacement for the preparation of high-reactivity mixed catalysts with a small content of
platinum metal have been shown for a Pd(Cu)/support system.
Due to the high cost and deficiency of platinum metals, decreas-
2
ing the content of such metals in a catalyst remains one of the
most important problems in catalysis. In order to solve this
problem, it was suggested to use the galvanic displacement
method in the creation of nanostructured catalysts (electro-
catalysts) for fuel elements.1–3 Displacement of a non-noble
metal with a noble metal may result in the formation of both a
‘core-shell’ catalyst1,2 and 3D nanoparticles of the noble (catalyti-
cally active) metal on an inert support.1,3 In this work, the galvanic
displacement method has been used to dope the Cu/support
system with micro-quantities of palladium, since the latter is an
active hydrodechlorination (HDC) catalyst.4–6
100
80
60
40
20
0
1
200
240
280
320
360
t/°C
Nanoparticles of copper deposited on quartz or Aerosil particles
0.5–1.0 mm in size served as the precursor for the palladium-
copper catalyst with a very small content of palladium. This
copper catalyst was prepared by thermal decomposition of copper
oxalate pre-deposited onto particles of the support. Decomposi-
tion was carried out under conditions of dynamic vacuum (~2 Pa)
at 330 °C. Samples with 10 wt% copper were used.
X-ray diffraction analysis of the oxalate decomposition product
under these conditions showed that small amounts of copper(I)
oxide and copper(II) oxide were present in addition to metallic
copper, and that the elementary sizes of crystallites in copper
particles were 20–40 nm.
Figure 1 Plot of the reaction mixture composition vs. temperature in
the hydrodechlorination of chlorobenzene on a Pd(Cu)/quartz catalyst:
(1) benzene, (2) chlorobenzene.
tion as a function of temperature for the HDC of chlorobenzene
on the Pd(Cu)/quartz catalyst. It is evident that a 100% conver-
sion of chlorobenzene in a single pulse, where the substrate
contacts the surface for less than 1 s, is observed at 350 °C;
furthermore, the reaction selectivity with respect to benzene is
100% in the entire temperature range. The yield of benzene at
350 °C was 1.37 μmol mg–1 Pd (Figure 2, curve 1). The amount
of Pd in the catalyst portion did not exceed 0.8 mg.
In order to dope micro-quantities of palladium by galvanic
displacement, the copper catalyst was contacted for 1 h at 19 1 °C
with a deaerated aqueous 2×10–3 M solution of PdCl2. The volume
of the PdCl2 solution was varied depending on the amount of
the copper catalyst to be treated, in such a way that the Pd:Cu
ratio was 1:30, assuming the complete deposition of Pd2+.
Pd(Cu)/quartz catalysts were studied on a JSM-6390LA JEOL
scanning electron microscope allowing the elementary composi-
tion to be derived from characteristic X-ray spectra.
An even higher reactivity of the catalyst was observed for the
Pd(Cu) catalyst on Aerosil support (Figure 2, curve 2), where the
yield of benzene (W) at 350 °C amounted to 2.14 μmol mg–1
Pd, at a Pd content in the sample equal to 0.2 mg. The differ-
ence in the reactivities of the Pd(Cu)/quartz and Pd(Cu)/Aerosil
catalysts is likely due to the high difference in the porosity of
2
2.0
The reactivity of the resulting palladium catalyst was studied
in the hydrodechlorination of chlorobenzene by the pulse micro-
catalytic method,7 which allowed us to observe the substrate
(chlorobenzene) conversion during a single pulse containing
1.1 μmol of chlorobenzene. Hydrogen simultaneously plays the
roles of the carrier gas and the hydrogenating agent. The catalytic
conversion products were determined on a Chrom-5 chromato-
graph with a Porapak-N column.
The original copper catalyst that served as the precursor for
synthesising the palladium catalyst showed the complete absence
of reactivity in HDC in the temperature range of 200–350 °C.
Figure 1 demonstrates a plot of the reaction mixture composi-
1.6
1
1.2
0.8
0.4
0.0
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200
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t/°C
320
360
Figure 2 Effect of temperature on the yield of benzene (W): (1) Pd(Cu)/
quartz, (2) Pd(Cu)/Aerosil.
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© 2010 Mendeleev Communications. All rights reserved.

Mendeleev Commun., 2010, 20, 10–11
(a)
(b)
100
Cu
CuO
Spectrum 1
80
60
40
Cu
1 μm
1 μm
Cu2O
Spectrum 2
Cu
CuO
20
Figure 3 Electron micrographs: (a) original Cu/quartz catalyst; (b) Pd(Cu)/
quartz (‘spectrum 1’ and ‘spectrum 2’ are areas where probing for elementary
composition was performed).
0
20
30
40
50
60
70
2q/°
the supports. The relatively small change in the yield of benzene
with an increase in the temperature from ~230 °C can be explained
by diffusion limitations.
Figure 4 X-ray diffraction pattern of the original copper catalyst.
Apparently, the formation of copper oxides occurs both during
the synthesis of the copper catalyst and during its treatment
with an aqueous PdCl2 solution.
Thus, it is believed that the high catalytic activity of the
Pd(Cu)/support catalysts (with very low Pd content) synthesised
in this study by galvanic displacement is mainly due to the high
extent of Pd dispersion. From the practical point of view, an
advantage of the method for synthesising a Pd(Cu) catalyst
suggested here is that it is technologically simple to introduce
the second component.
Thus, galvanic displacement makes it possible to obtain a mixed
Pd(Cu) catalyst that manifests high activity in HDC reactions at
very small Pd concentrations. It was important to determine the
structure of the catalysts obtained in order to reveal the nature
of their activity. Figure 3(a) shows an electron micrograph of
the original copper catalyst. One can see the presence of Cu
and/or mixed Cu + Cu2O, CuO particles, 100 to 400 nm in size.
It can be concluded from Figure 3(b) that treatment of the copper
catalyst with a PdCl2 solution produces a surface containing both
areas virtually free of Pd [the upper part of Figure 3(b)] and
areas [the lower part of Figure 3(b)] coated with a ‘mesh’ of finely
dispersed palladium (Pd nanoparticles contacting each other).
The non-uniform distribution of Pd is additionally supported
by the results of microprobe elementary analysis. Palladium was
not found in metal particles shown in the upper part of the mixed
catalyst micrograph [Figure 3(b), ‘spectrum 1’ area], whereas
the Pd:Cu atomic ratio in the particles shown in the lower part
[Figure 3(b), ‘spectrum 2’ area] amounted to ~1:10. Hence,
palladium is almost not deposited on some of the copper
particles, whereas on other particles, obviously, replacement
of copper atoms (Cus) on certain ‘active’ sites of the surface
by palladium atoms initially occurs: Cus + Pd2+ ® Cu2+ + Pds.
Further, nano-sized Pd particles are formed due to electro-
deposition of palladium on palladium, i.e., the structure of the
mixed Pd(Cu) catalyst is mostly formed due to ‘operation’ of
the galvanic couple:
This work was supported by the Russian Foundation for
Basic Research (project 09-03-00498a).
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Pd2+ + 2e ® Pd0,
Cu0 – 2e ® Cu2+
under conditions where copper ionisation and discharge of
palladium ions are spatially separated. This suggests that dif-
ferent areas of copper particles differ in activity. Apparently, this
is mostly due to the presence of copper oxides (Cu2O and CuO)
in the original catalyst Cu/support (Figure 4), as noted above.
The oxidized areas on the surface of copper particles are not
involved in the exchange with Pd2+ ions (E0CuO/Cu O = 0.67 V).8
Received: 15th July 2009; Com. 09/3366
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