Inert Can Be Advantageous: Advisable Reconstruction and Application of Palladium Chloride for the Preferential Oxidation of the Hydrogen Impurity in Carbon Monoxide Streams

Article abstract of DOI:10.1002/cctc.201600301

Preferential oxidation of the H₂ impurity in CO streams is crucial for the catalytic conversion of CO into ethylene glycol. It is uncertain as to whether H₂ can overthrow the dominance of CO and finally lead the reaction. Not only is this a typical research issue, but it is also extremely critical for practical applications. So far, no catalyst has shown selectivity higher than 50 % owing to competitive adsorption of CO. In this work, we report a PdCl_x-based catalyst that can readily overcome the challenge mentioned above. Over this catalyst, the selectivity of H₂ oxidation is promoted by more than 40 % relative to that over the conventional Pd/Al₂O₃ catalyst and reaches a value of 87 %. The turnover frequency of undesired CO oxidation is inhibited by at least one order of magnitude. We found that the reconstructed palladium chloride was highly selective to this reaction by facile inhibition of the adsorption and oxidation of CO.

Full text of DOI:10.1002/cctc.201600301

DOI: 10.1002/cctc.201600301
Communications
Inert Can Be Advantageous: Advisable Reconstruction and
Application of Palladium Chloride for the Preferential
Oxidation of the Hydrogen Impurity in Carbon Monoxide
Streams
[a]
[a]
[a]
[b]
[a]
Luyang Qiao, Qiaohong Li, Zhangfeng Zhou, Rui Si, and Yuangen Yao*
Preferential oxidation of the H impurity in CO streams is cru-
world’s first plant of coal to ethylene glycol with a capacity of
200000 tons (see Figure S1, Supporting Information).
2
cial for the catalytic conversion of CO into ethylene glycol. It is
uncertain as to whether H can overthrow the dominance of
H oxidation is a common reaction that has been studied ex-
2
2
CO and finally lead the reaction. Not only is this a typical re-
search issue, but it is also extremely critical for practical appli-
cations. So far, no catalyst has shown selectivity higher than
tensively as a model reaction and as an available method for
[
3]
H O production. Nevertheless, early studies on this reaction
2
2
were generally performed under mild conditions, other than
a competitive reactant-rich atmosphere, such as CO. As far as
5
0% owing to competitive adsorption of CO. In this work, we
report a PdCl -based catalyst that can readily overcome the
we know, research on the PrOx of H in CO-rich streams is
x
2
challenge mentioned above. Over this catalyst, the selectivity
a largely unexplored area. The best catalyst ever manufactured
for the PrOx of H is Pd/Al O , which offers benchmarked per-
of H oxidation is promoted by more than 40% relative to that
2
2
2
3
over the conventional Pd/Al O catalyst and reaches a value of
formance and is highly active for H conversion. Nevertheless,
2
3
2
8
7%. The turnover frequency of undesired CO oxidation is in-
this catalyst is generally better able to oxidize CO than H2
owing to the adsorption of CO on the metal surface, which sig-
hibited by at least one order of magnitude. We found that the
reconstructed palladium chloride was highly selective to this
reaction by facile inhibition of the adsorption and oxidation of
CO.
[
4,5]
nificantly inhibits the adsorption and activation of H2.
De-
pending on the feedback from a large-scale plant of coal to
ethylene glycol, the selectivity for H oxidation over the Pd/
2
Al O catalyst is generally lower than 50% despite the fact that
2
3
the conversion of H is high. Low selectivity inevitably induces
2
Through the coupling of CO to dimethyl oxalate and following
the consumption of the CO feed and the accumulation of CO2,
both of which are not benign to subsequent dimethyl oxalate
and ethylene glycol synthesis.
[1,2]
selective hydrogenation,
the route for the catalytic conver-
sion of CO into ethylene glycol has become a challenging and
attractive subject of C1 chemistry. As a raw material, CO is
mainly derived from the reforming of abundant resources such
as coal and shale gas. Nevertheless, desirable CO usually con-
tains trace amounts of H2 even after cryogenic separation,
which is severely poisonous to the following process of CO
With the aim to elevate the selectivity, the rational design of
novel catalysts based on fundamental understanding of this re-
action is very critical. To drive the competitive reaction in a tar-
geted direction, much effort has been dedicated to compre-
hending the effects of promoters by adjusting the structural or
[
6,7]
coupling; thus, the amount of H needs to be reduced to an
electronic texture of the catalyst. However, another pathway
to upgrade the selectivity by prohibiting the undesired side re-
action has rarely been reported. Inhibitors can also be advanta-
geous. Purposely exploiting a catalytically inert or even poison-
ous material towards one reaction can possibly proffer its
counterpart certain enhanced effects. As a kind of well-known
catalytic poison, chloride usually plays a negative role in the
2
[1b]
acceptable level (below 100 ppm). To address this problem,
the preferential oxidation (PrOx) of H in a CO-rich stream was
2
developed as the most promising approach among the chemi-
cal and physical methods. This technique is realized in the
[
a] Dr. L. Qiao, Dr. Q. Li, Dr. Z. Zhou, Prof. Y. Yao
Key Laboratory of Coal to Ethylene Glycol and Its Related Technology
State Key Laboratory of Structural Chemistry
Fujian Institute of Research on the Structure of Matter
Chinese Academy of Sciences
[
8]
reaction of CO oxidation or PrOx. Owing to the presence of
chlorine, all the active sites on the catalyst will be occupied
and the adsorption of the reactants will be blocked. This phe-
nomenon provides us with some clues on how to promote the
Yangqiao West Road 155
Fuzhou, Fujian 350002 (P.R. China)
selectivity of H oxidation by inhibiting the adsorption and oxi-
2
E-mail: yyg@fjirsm.ac.cn
dation of CO.
[
b] Prof. R. Si
Shanghai Synchrotron Radiation Facility
Herein, we first designed a PdCl -based catalyst that was
x
verified to be highly selective towards the PrOx of H in CO-
2
Shanghai Institute of Applied Physics
Chinese Academy of Sciences
Shanghai 201204 (P.R. China)
rich streams. In contrast with the benchmark Pd/Al O catalyst,
2
3
CO oxidation was inhibited by at least one order of magnitude
and the selectivity was promoted by more than 40% on the
novel catalyst. It is proposed that the coordinatively unsaturat-
Supporting Information for this article can be found under:
http://dx.doi.org/10.1002/cctc.201600301.
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Communications
2
À1
ed (CU) configuration of PdCl is responsible for this break-
bare Al O support (310 m g ) owing to pore blocking by im-
2 3
x
through. By intentionally reconstructing the texture of palladi-
um chloride, the best balance of activity and selectivity was
achieved. The catalyst (denoted as CU-PdCl /Al O ) was pre-
pregnation (see Table S1).
The properties of the catalysts were evaluated under simu-
lated industrial conditions (2 vol% H , 4 vol% O , and 94 vol%
x
2
3
2
2
[
9]
pared by incipient wetness impregnation. For comparison, the
PdCl /Al O and Pd/Al O catalyst were also prepared. The Pd
CO). As the most efficient catalyst ever manufactured for the
PrOx of H , Pd/Al O was initially tested to benchmark the ac-
2
2
3
2
3
2
2
3
loadings of all the discussed catalysts were measured by induc-
tively coupled plasma optical emission spectrometry (ICP-OES)
and were found to be between 0.9 and 1.1 wt%, which corre-
spond well to theoretical values. The BET surface areas of all
the discussed catalysts were slightly lower than that of the
tivity and selectivity of the system. Figure 1a illustrates the
profiles of the H conversions as a function of reaction temper-
2
ature. The conversion of H approximated to 50% at 1008C
2
and reached 100% at 1508C over Pd/Al O . With a similar ten-
2
3
dency, the H conversion over CU-PdCl /Al O reached 99.5%
2
x
2
3
at 1508C, and the complete oxidation of H was realized at
2
1
558C. However, the activity of PdCl /Al O was much lower
2 2 3
than that of CU-PdCl /Al O . The conversion of H was negligi-
x
2
3
2
ble at 1008C and decreased to 50% at 1808C. This can proba-
bly be attributed to the relative intact structure of PdCl , and
2
therefore, the adsorption of the reactants is significantly hin-
dered.
The selectivities are compared in Figure 1b. For Pd/Al O ,
2
3
the selectivity approached a maximum at 1008C, and then
abruptly decreased to 44.2% upon increasing the temperature
to 1508C. At this temperature, complete conversion of H was
2
achieved at the expense of the selectivity, which suggested
that CO oxidation began to predominate the system. In con-
trast, the selectivity of CU-PdCl /Al O initially attained 87% at
x
2
3
1
008C, and this level was steadily maintained over the entire
temperature region with a slight decrease of only 5%. This
result is significantly better than that on Pd/Al O and indicates
2
3
that the oxidation of CO is probably inhibited efficiently on
CU-PdCl /Al O . In addition, the selectivity of PdCl /Al O was
x
2
3
2
2
3
the highest, but this is meaningless owing to its low activity
for the desired reaction.
To further demonstrate the extraordinarily high selectivity of
H oxidation over CU-PdCl /Al O , the catalytic behavior of the
2
x
2
3
catalysts were examined from the viewpoint of kinetics. The
turnover frequency (TOF) values of the catalysts for all the re-
actions were measured under different conditions (conversions
below 20%) and are shown in Table 1. For H2 oxidation in
a CO-rich stream, the TOF for H conversion over CU-PdCl /
2
x
Figure 1. a) Conversion of H
of reaction temperature over the CU-PdCl
catalysts.
2
and b) selectivity for H
2
oxidation as a function
À2 À1
Al O was 4.8”10
s
at 908C, which approximates to that
x
/Al
2
O
3
, Pd-Al , and PdCl /Al
2
O
3
2
2
O
3
2
3
over Pd/Al O ; this indicates that both catalysts are able to oxi-
2
3
x 2 3 2 3 2
Table 1. Turnover frequencies and apparent activation energies over the CU-PdCl /Al O and Pd/Al O catalysts for the PrOx of H in a CO-rich stream and
CO oxidation in the presence of H at different temperatures.
2
Sample
H
2
oxidation in CO-rich stream
CO oxidation in the presence of H
2
À2
À1 [a]
À1
À2
À1 [a]
À1
T [8C]
TOF”10 [s
]
E
a
[kJmol
]
T [8C]
TOF”10 [s
]
a
E [kJmol ]
CU-PdCl
x
/Al
2
O
3
60
1.8
2.8
3.7
4.8
29.9
110
130
150
170
0.6
1.1
2.1
3.2
38.6
7
8
9
0
0
0
Pd/Al
2
O
3
60
2.4
3.2
4.1
5.0
24.9
110
130
150
170
18.6
19.8
20.9
21.9
3.8
7
8
9
0
0
0
[
a] The dispersion of Pd was measured by CO chemisorption by using a Micromeritics ASAP 2020 M+C by assuming CO/Pd=1:1.
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dize H . For CO oxidation in the presence of H , the TOF for CO
2
2
À2 À1
conversion was only 0.6”10
s
at 110 8C. Relative to that on
Pd/Al O , the TOF on CU-PdCl /Al O is almost 30-fold lower
2
3
x
2
3
under the same conditions. This inhibition indicates that it is
difficult to oxidize CO over CU-PdCl /Al O . On the basis of the
x
2
3
Arrhenius plots (Figure S5), we calculated the apparent activa-
tion energies (E ) of the CU-PdCl /Al O and Pd/Al O catalysts.
a
x
2
3
2
3
À1
The E value of 38.6 kJmol over CU-PdCl /Al O is one order
a
x
2
À1
3
of magnitude higher than that of 3.8 kJmol over Pd/Al O3
2
for CO oxidation. The high activation barrier of CU-PdCl /Al O
3
x
2
correlates well with its efficient inhibition of CO oxidation and
outstanding selectivity towards H oxidation.
2
Previous studies illustrated that the presence of CO usually
[
10,11]
has a negative effect on reactions in which H is involved.
2
To investigate the influence of a high concentration of CO on
the PrOx of H , we tested the adsorption behavior of the cata-
2
lysts by static chemisorption. In the individual tests, Table S2
shows that the adsorption of both H and CO on Pd/Al O is
2
2
3
more facile than on CU-PdCl /Al O . Relative to the amount of
x
2
3
CO adsorbed without competition of H , the amount of CO ad-
2
sorbed slightly decreased on both CU-PdCl /Al O (from 24.5 to
x
2
3
À1
À1
1
8.8 mmolg ) and Pd/Al O (from 43 to 34.6 mmolg ) after H
2 3 2
preadsorption. With a reverse order of CO/H adsorption, how-
2
ever, the behavior of these two catalysts was very different.
After CO preadsorption, a significant decrease (from 24.6 to
À1
5
.9 mmolg ) in the amount of H adsorbed was observed on
2
2
Figure 2. a) XANES spectra and b) k -weighted Fourier transform Pd K-edge
EXAFS spectra of the standard PdCl power (A), PdCl /Al (B), CU-PdCl
Al (C), Pd/Al (D), and standard Pd foil (E).
Pd/Al O . This indicates that a strong interaction between CO
2
3
2
2
2
O
3
x
/
and the Pd sites is established on Pd/Al O , and then the ad-
2
3
2
O
3
2 3
O
sorption of H is blocked. On the contrary, only a small de-
2
À1
crease (from 14.3 to13 mmolg ) in the amount of H adsorbed
2
was found on CU-PdCl /Al O . The coordination of Cl around
suggests that the PdÀCl bond increases in length as the
amount of excess Cl decreases and the configuration of PdClx
becomes coordinatively unsaturated. Besides that, no PdÀPd
bond can be detected in CU-PdCl /Al O , and consequently, no
x
2
3
the Pd sites should be responsible for this result, as this tends
to deplete backdonation from the d bands of Pd to the anti-
[12]
bonding 2p* orbitals of CO. Thus, the interaction between
x
2
3
CO and Pd is weakened. Such fragile adsorption of CO on the
metallic Pd particles are formed.
catalyst makes CO labile and commutable by H . Finally, the
Notably, the E values acquired from the Arrhenius plots are
2
a
adsorption gap between CO and H can be minimized over
apparent in that the diffusional effect was not eliminated;
therefore, an intrinsic investigation about the behavior of the
catalysts is essential. DFT calculations were performed to ex-
plore the inhibitory mechanisms of CO oxidation over the
PdCl structure. The chosen Pd (111) surface of Pd/Al O and
2
CU-PdCl /Al O by deliberately restricting access to CO, and
x
2
3
then the opportunity for H is maximized.
2
Both the electronic properties and the short-range local
structures of the discussed catalysts were determined from the
extended X-ray absorption fine structure (EXAFS) spectra and
X-ray absorption near-edge structure (XANES) spectra of the
Pd K-edge. As shown in Figure 2a, the XANES curve of PdCl2/
Al O fits well with the standard PdCl sample, but a slight de-
x
2
3
the defective PdCl (140) surface of CU-PdCl /Al O were iden-
x
x
2
3
tified through characterization by high-resolution transmission
electron microscopy, X-ray absorption fine structure, and X-ray
photoelectron spectroscopy (see Section S6 in the Supporting
Information). The comparable energetic landscapes of all the
pathways for CO oxidation on the surfaces are depicted in
Schemes S2 and S3. We propose two mechanisms for CO oxi-
2
3
2
viation is found in CU-PdCl /Al O , which suggests that the
x
2
3
local electron density of Pd increases as the coordination of
electron-withdrawing Cl becomes unsaturated. This verifies
that the intrinsic structure of PdCl is not intact. In contrast,
dation depending on whether H is involved or not.
x
2
the curve of Pd/Al O shows two metallic peaks at energies of
On Pd (111), pathway I is illustrated by a redox mechanism
2
3
[
13]
approximately 24367 and 24391 eV. These features are identi-
cal to those of a standard Pd foil sample.
in which H is only a spectator. As shown in Figure 3, ad-
2
sorbed CO* is directly attacked by O to yield a carbonate in-
2
2
The curves of the Fourier-transformed k -weighted Pd K-
termediate (CO *) with a barrier of 1.66 eV via transition state 1
3
edge EXAFS are shown in Figure 2b. The PdÀCl coordination
number (CN) of CU-PdCl /Al O (3.3Æ0.4) is markedly lower
(TS1), and it then decomposes to CO and O* via TS2. CO is
2
2
readily desorbed from the Pd surface and O* approaches an-
other adsorbed CO* species to produce another molecule of
x
2
3
than that of the standard PdCl sample and that of PdCl /Al O
3
2
2
2
(
5.3Æ0.9), and the PdÀCl distance (R) of PdCl is smaller, which
CO via TS3. Differing in the adsorption modes, other similar
x
2
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On PdCl (140), on the basis of our calculations all the pro-
x
posed pathways need to overcome a higher energy barrier. As
shown in Figure 5, preferred pathway II for CO oxidation was
comparatively investigated on this surface. Initially, two dissoci-
ated H* grasp gaseous O to form a HOOH* species (other
2
Figure 3. Pathway I for CO oxidation on the Pd (111) surface with TS1–TS3.
pathways were also investigated (see Section S5.2). Although
the barriers for these other pathways are slightly lower than
that for pathway I, the odds that they will occur are low, be-
cause their initial states are contradicted by the real circum-
x
Figure 5. Pathway II for CO oxidation on the PdCl (140) surface with TS8
and TS9.
stances of our system. With the involvement of H , an associate
2
mechanism is proposed in pathway II. As shown in Figure 4,
than OH*) on the defective sites (V ) via TS8 owing to the lack
Cl
of exposed Pd. Then, adsorbed CO* readily substitutes a hy-
droxy group in HOOH* to yield a OCOH* intermediate. If this
replaced OH* intends to associate with OCOH* again to ab-
stract a hydrogen atom, a higher barrier of 1.86 eV must be
surmounted via TS9. Besides that, the barrier of pathway I,
which abides by a redox mechanism, also increases from 1.66
to 2.71 eV on the PdCl (140) surface. This indicates that the
x
oxidation of CO is undoubtedly inhibited on the PdCl (140)
x
surface, regardless of whether H is involved or not.
2
We then performed in situ diffuse reflectance infrared Fouri-
er transform spectroscopy (DRIFTS) experiments to further
verify the inhibitory mechanisms proposed by DFT calculations.
The spectra of CO oxidation in the presence of H were record-
2
ed in the range of 40 to 2008C over the discussed catalysts.
À1
For clarity, only the region of 3000 to 1150 cm is extracted
Figure 4. Pathway II for CO oxidation on the Pd (111) surface with TS4–TS7.
for detailed illustration. As shown in Figure S6, injecting the re-
actants into the cell initially yielded CO bands of the gaseous
À1
À1
type ( n˜ =2173 and 2120 cm ), linear type ( n˜ =2090 cm ), and
À1
[17]
adsorbed O * initially dissociates into two O* (TS4), own of
bridged type ( n˜ =1968 cm ) at 408C. Furthermore, bands at
2
À1
which abstracts H* to form hydroxy groups (OH*) on the Pd
n˜ =1435 and 1230 cm were also observed, and they were
(
111) surface via TS5. Then, one OH* associates with CO* to
generally attributed to the OÀCÀO stretching vibration of the
[
18]
yield a carboxylate intermediate (OCOH*) via TS6 by overcom-
ing a barrier of 1.4 eV. Finally, this OCOH* species transfers a hy-
drogen atom to another neighboring OH* spontaneously to
yield CO and H O via TS7, in which a long OCOHOH* inter-
CO * species. With an increase in the temperature, a series
3
À1
of new bands ( n˜ =1596, 1396, 1374 cm ) corresponding to
[
19]
the OÀCÀO stretching vibration of the OCOH* species
emerged and became clearer above 1308C. Simultaneously,
2
2
mediate geometry is formed, as previously speculated by Oji-
the intensities of the bands associated with the CO * species
3
[
14]
finni et al. The lower barrier of pathway II indicates that CO
gradually decreased as the new bands emerged. This suggest-
ed that the associate mechanism began to dominate the reac-
oxidation is probably enhanced by participation of H . This
2
effect is assisted by the initial formation of OH*, which is sup-
ported by studies on hydroxy-enhanced CO oxidation and the
tion. Coupled with maximization of the OCOH* species, the
À1 [15a]
bands for CO ( n˜ =2362 and 2342 cm )
abruptly increased
2
[15,16]
water–gas shift reaction.
in intensity and remained almost steady in the range of 130 to
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2
008C. The dramatic increase in the intensity of the CO bands
2
Experimental Section
indicates that the oxidation of CO is enhanced by the partici-
All catalysts were prepared by incipient wetness impregnation.
pation of H through pathway II.
2
Typically, commercial g-Al O (2 g) was impregnated into an aque-
2
3
For CU-PdCl /Al O , the spectra depicted in Figure 6 exhibit
x
2
3
ous solution of PdCl2 (10 mL, Aldrich, 99.9%) at an appropriate
concentration at 258C for 6 h. The pH value of the solution was
controlled at pHꢀ2 by adding hydrochloric acid. After impregna-
tion, the sample was dried at 1008C under vacuum for 12 h and
was then treated in a microwave reactor for 10 min. The patterned
PdCl /Al O catalyst was thus obtained. We treated this catalyst
certain differences upon comparison to the spectra of Pd/
Al O . The bands of the OCOH* or CO * intermediate are not
2
3
3
observed over the entire range of temperatures, which indi-
cates that all the pathways for CO oxidation abiding by an as-
sociate or redox mechanism do not occur. Furthermore, the in-
2
2
3
(
1 g) with 0.5 vol% aqueous vapor and 10 vol% H balanced with
2
tensity of the CO signal is approximately only 1/20 of that on
2
Ar under 1508C for 30 min. The resulting catalyst was denoted CU-
Pd/Al O . This wide margin unequivocally demonstrates that
2
3
PdCl /Al O . With the same procedures, the patterned Pd/Al O cat-
x
2
3
2
3
CO oxidation is inhibited by at least one order of magnitude
on CU-PdCl /Al O , which correlates well with our DFT calcula-
alyst was also synthesized by replacing PdCl2 with Pd(NO3)2 (Al-
drich, 99.9%), and the sample was reduced further under 2008C
x
2
3
tions.
with 10 vol% H /Ar mixture. Details of the catalytic measurements,
2
characterization data, and DFT calculations are given in the Sup-
porting Information.
Acknowledgements
The authors are thankful for financial support from the 973 Pro-
gram of China (2011CBA00505), the Strategic Priority Research
Program of the Chinese Academy of Sciences (XDA07070200 and
XDA09030102), and the National Key Technology R&D Program
(
2012BAE06B08). The work was also supported by the BL14W1
beam line of the Shanghai Synchrotron Radiation Facility.
Keywords: C1 building blocks · hydrogen · oxidation ·
palladium · selectivity
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2 3
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[
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Received: March 15, 2016
Published online on May 23, 2016
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ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim