Solid-state molybdenum sulfide clusters with an octahedral metal framework as hydrogenation, dehydrogenation, and hydrogenolysis catalysts similar to the platinum group metals

Article abstract of DOI:10.1016/j.apcata.2015.04.036

Solid-state molybdenum sulfide clusters with a nonstoichiometric sulfur-deficient site, Cu_xMo₆S_8-δ (x = 2.94 and δ ? 0.3) and Mo₆S_8-δ (δ ? 0.4), catalyze the dehydrogenation of cyclohexene, the hydrogenation of alkyne, alkene, aldehyde, ketone, and nitrobenzene, and the hydrogenolysis of halogenobenzenes in a hydrogen stream above 300 °C. This catalytic behavior of the clusters is similar to that of the platinum group metals, which is attributable to the isoelectronicity of sulfur-coordinating molybdenum atom to the platinum group metals.

Full text of DOI:10.1016/j.apcata.2015.04.036

Accepted Manuscript
Title: Solid-State molybdenum sulfide clusters with an
octahedral metal framework as hydrogenation,
dehydrogenation, and hydrogenolysis catalysts similar to the
platinum group metals
Author: Satoshi Kamiguchi Kanako Arai Kazu Okumura
Hajime Iida Sayoko Nagashima Teiji Chihara
PII:
S0926-860X(15)00302-6
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2015.04.036
APCATA 15382
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
5-3-2015
20-4-2015
21-4-2015
Please cite this article as: S. Kamiguchi, K. Arai, K. Okumura, H. Iida, S.
Nagashima, T. Chihara, Solid-State molybdenum sulfide clusters with an octahedral
metal framework as hydrogenation, dehydrogenation, and hydrogenolysis catalysts
similar to the platinum group metals, Applied Catalysis A, General (2015),
http://dx.doi.org/10.1016/j.apcata.2015.04.036
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Highlights:
• Sulfur-deficient solid-state molybdenum sulfide clusters are applied to catalysis.
• Hydrogenation, dehydrogenation, and hydrogenolysis proceed over the clusters.
• This catalytic behavior is similar to that of the platinum metals.
• The molybdenum atom is isoelecronic with the platinum metals, causing the similarity.
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Solid-State molybdenum sulfide clusters with an octahedral metal framework as hydrogenation,
dehydrogenation, and hydrogenolysis catalysts similar to the platinum group metals
Satoshi Kamiguchi a,b,*, Kanako Arai c, Kazu Okumura d, Hajime Iida d, Sayoko Nagashima c, and Teiji
Chihara c
____________________
a Advanced Catalysis Research Group, RIKEN Center for Sustainable Resource Science, 2-1 Hirosawa,
Wako City, Saitama 351-0198, Japan
b Organometallic Chemistry Laboratory, RIKEN, 2-1 Hirosawa, Wako City, Saitama 351-0198, Japan
c Graduate School of Science and Engineering, Saitama University, Shimo-Okubo, Sakura-ku, Saitama
City, Saitama 338-8570, Japan
d Faculty of Engineering, Kogakuin University, Nakano-machi, Hachioji City, Tokyo 192-0015, Japan
* Corresponding author: Satoshi Kamiguchi
* Phone: (+81)48-467-1349
* Fax: (+81)48-467-9391
* E-mail: kamigu@riken.jp
* Address: RIKEN, Wako, Saitama 351-0198, Japan
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Abstract
Solid-state molybdenum sulfide clusters with a nonstoichiometric sulfur-deficient site, CuxMo6S8–
δ (x = 2.94 and δ ≈ 0.3) and Mo6S8–δ (δ ≈ 0.4), catalyze the dehydrogenation of cyclohexene, the
hydrogenation of alkyne, alkene, aldehyde, ketone, and nitrobenzene, and the hydrogenolysis of
halogenobenzenes in a hydrogen stream above 300 ºC. This catalytic behavior of the clusters is similar
to that of the platinum group metals, which is attributable to the isoelectronicity of sulfur-coordinating
molybdenum atom to the platinum group metals.
Keywords: Molybdenum sulfide cluster; Chevrel phase; Coordinatively unsaturated metal atom;
Platinum-like catalysis
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1. Introduction
The platinum group metals are widely used as catalysts for bulk and fine chemical synthesis.
Because these metals are very expensive and very scarce, alternative materials are urgently required.
Studies on solid-state molybdenum sulfide clusters having an octahedral metal framework with eight
face-capping sulfur ligands, the Chevrel phases, have focused mainly on superconductivity. There have
been only a few reports on the application of these clusters for catalysis: the decomposition of nitric
oxide to nitrogen and oxygen [1], the hydrodesulfurization of thiophene [2], and methanethiol synthesis
from methanol and hydrogen sulfide [3]. The clusters have several characteristic features: metal–metal
bonds that are similar to those in bulk metals, multi-metal multi-electron systems, an intermediate
oxidation state of the metal atom around 2+, and high thermal stability expected from high synthesis
temperatures (700–1000 °C). Recently, we have reported the ring-opening of tetrahydrofuran to
butyraldehyde [4] and the decomposition of methyl tert-butyl ether (MTBE) to isobutene [5] over
clusters with a sulfur-deficient molybdenum atom, to expand the scope for the application of cluster
catalysis. In this work, we applied these clusters to hydrogenation, dehydrogenation, and hydrogenolysis,
which are catalyzed by the platinum group metals.
2. Experimental
Solid-state molybdenum sulfide clusters CuxMo6S8–δ (x = 2.94, δ ≈ 0.3) (1) [6], Mo6S8–δ (δ ≈ 0.4)
(2) [7], Li4Mo6S8 [8], and Ni2Mo6S8 [9] were synthesized according to published procedures. Crystals of
these clusters were crushed and screened to 200–325 mesh, then stored in air for more than 90 days
before use as catalysts [4]. Commercial palladium (0.5 wt% on carbon, N.E. Chemcat Co.) and platinum
(0.5 wt% on carbon, N.E. Chemcat Co.) were used as references. The other chemicals (> 98%, Sigma–
Aldrich Chemical Co.) were used as received. The reactions were performed using a conventional
continuous-flow micro-reactor operated at atmospheric pressure [10]. Typically, a weighed crystalline
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cluster sample (30 mg) of CuxMo6S8–δ (x = 2.94, δ ≈ 0.3) (1) stored in air was packed in a borosilicate
glass tube (3 mm i.d.), and placed in the center of an electric furnace. The catalyst sample was initially
heated from room temperature to a fixed temperature between 250 and 500 °C over 15 min in a
hydrogen stream (2.4 L/h), and then held at this temperature for 45 min. Without changing the
temperature, the reaction was initiated by feeding reactant (0.5 mmol/h) into the hydrogen stream using
a syringe pump. The reaction was monitored every 30 min by sampling the reaction gas (1 mL) via a six-
way valve kept at 150 °C, followed by analysis using an on-line gas-liquid chromatograph (GLC). The
reactor effluent was frozen in an ice-water trap for determination of material balance and subsequent
analysis of the products. Catalytic reactions using the other catalysts were performed in the same way
[4].
Powder X-ray diffraction (XRD) data were recorded using a Rigaku RINT-2000 X-ray
diffractometer employing Cu K radiation and a scan rate of 2.5° min–1. Scanning electron microscopy
(SEM) was carried out using a HITACHI S-4200 microscope (acceleration voltage of 5 kV). Elemental
analysis was carried out by the Materials Characterization Team at RIKEN. X-ray photoelectron
spectrum (XPS) was obtained using an ULVAC-PHI Model 558UP spectrometer with Mg K X-ray
source. Before measurement, cluster samples (ca. 20 mg) were cleaned by Ar+ sputtering inside the
spectrometer. The binding energy was corrected using the C 1s peak at 284.6 eV for adventitious carbon
as a reference.
3. Results
The pulverized crystals of 1 stored in air were thermally activated in a hydrogen stream, followed
by the reaction of cyclehexene at the activation temperature. A typical reaction profile at 450 ºC is
plotted in Fig. 1. The catalytic activity decreased with time. No appreciable changes in the XRD pattern
were observed after 7 h reaction under the reaction conditions (see supplementary data, Fig. S1a),
indicating the retention of the cluster integrity [4]. The XPS of 1 before and after the reaction showed
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that the sulfur ligand of the surface was retained as S2–, unlike the case with NiMoS2 which releases the
sulfur species on the surface during a catalytic reaction [11]. Elemental analysis showed that 1 subjected
to the 7 h reaction contained 0.30 wt% carbon, which suggests that coke formed on the catalyst retarded
the reaction. Deactivation of palladium metal in the gas–solid phase dehydrogenation of cyclohexene
has been reported to be attributed to coke deposition [12]. Catalyst deactivation by coke formation is
inevitable in gas–solid-phase reactions at higher temperatures [13]. The SEM images of 1 were shown in
Fig. 2. The primary particle size was ca. 50 nm after the activation (Fig. 2a). After the 7 h reaction, the
particles were sintered (Fig. 2b), suggesting that the sintering is also a cause of the deactivation.
Deactivation of MoS2 in the hydrodesulfurization has been related to sintering [14]. The selectivity
remained steady after a period of 3 h. Dehydrogenation yielding benzene proceeded with 77%
selectivity, along with hydrogenation yielding cyclohexane with 19% selectivity. We have previously
reported that the Brunauer-Emmett-Teller surface area of 1 after the activation was small (5.8 m2/g) [4].
Nevertheless, the turnover frequency (TOF) per Mo6 cluster core during a period of 3–4 h was 6.1 h–1,
assuming that all the cluster cores were catalytically active. The material balance was 96% at 3–5 h after
the start of the reaction.
Table 1 lists the catalytic activity of some molybdenum sulfide clusters and related compounds at
3 h after the start of the reaction at 450 ºC. Neutral cluster 2 (entry 2) as well as 1 (entry 1)
predominantly catalyzed the dehydrogenation of cyclohexene to yield benzene. No appreciable changes
were observed in the XRD pattern of 2 after the reaction (see supplementary data, Fig. S1b), indicating
that 2 served as a catalyst with the retention of the cluster integrity. Molybdenum disulfide (entry 5) and
molybdenum metal (entry 6) exhibited low catalytic activity and different selectivity from those of 1 and
2: the main reaction was hydrogenolysis to yield gaseous products such as methane, ethane, and
ethylene. Table 1 also shows that palladium and platinum metals selectively catalyzed the
dehydrogenation (entries 8 and 9). Thus, the molybdenum clusters 1 and 2 exhibited catalytic behavior
similar to that of these platinum group metals, though the TOF values of the molybdenum clusters are
much lower than those of the platinum group metals. Nickel and lithium salts of the clusters, which have
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no sulfur-deficient sites [4], had low catalytic activity and selectivity for the dehydrogenation (entries 3
and 4).
The effect of the reaction temperature on activity and selectivity over 1 at 3 h after the start of the
reaction is shown in Fig. 3, in which the activation and the subsequent reaction were performed at the
same temperature. The catalytic activity developed above 300 °C and increased with increasing
temperature. Hydrogenation yielding cyclohexane was the main reaction below 350 °C and
dehydrogenation selectively yielding benzene proceeded above 400 °C. According to Le Chatelier’s
principle, a reaction that increases the number of product molecules is thermodynamically favorable at
higher temperatures under constant pressure. The temperature-dependent selectivity is consistent with
this principle. The same temperature dependence has been reported for supported palladium, iridium,
and platinum metals [15].
Hydrogenation of various compounds over 1 and 2 was examined at 300 °C. Table 2 lists the
reactivities of alkynes and alkenes. Phenyl acetylene was partially hydrogenated to styrene with little
formation of a complete hydrogenation product, ethylbenzene (entries 1 and 2). When styrene was
allowed to react under identical conditions, ethylbenzene was selectively obtained with higher
conversions (entries 3 and 4). These reactivities indicate that phenyl acetylene was strongly adsorbed on
these cluster catalysts, leading to the prevention of the further hydrogenation of styrene. When 1-pentyne
or 1-hexyne was allowed to react over 1, partial hydrogenation proceeded to yield the corresponding 1-
alkene along with a small amount of the isomerized products, while a complete hydrogenation product,
an alkane, was not detected (entries 5 and 6).
The reactivities of oxygen- and nitrogen-containing compounds are shown in Table 3. Cluster 1
selectively hydrogenated aldehydes such as propanol and butanal to yield the corresponding primary
alcohols with 70–93% selectivity (entries 1 and 3). When ketones such as acetone and 3-pentanone were
allowed to react over 1, hydrogenation proceeded to yield the corresponding secondary alcohols,
followed by dehydration to yield alkenes (entries 4 and 6) [16]. The reactivities of propanal and acetone
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over 2 were similar to those over 1 (entries 2 and 5). Nitrobenzene was almost exclusively hydrogenated
to aniline over 1 and 2 (entries 7 and 8).
The reactivities of halogenated benzenes are summarized in Table 4. Cluster 1 catalyzed the
hydrogenolysis of chloro-, bromo-, and iodo-benzenes to yield benzene exclusively (entries 2, 4, and 5).
In contrast, fluorobenzene did not react at all (entry 1). The hydrogenolysis of chlorobenzene was also
catalyzed by 2 (entry 2).
4. Discussion
The selective partial hydrogenation of alkynes to alkenes is catalyzed by a palladium-based
Lindlar catalyst [17–19]. Aliphatic aldehydes are reported to be hydrogenated to primary alcohols over
ruthenium metals [20, 21]. The hydrogenation of aliphatic ketones to secondary alcohols [21, 22] and
subsequent dehydration to alkenes [16] are catalyzed by platinum metal. The hydrogenation of
nitrobenzene to aniline is catalyzed by palladium and platinum metals [23, 24]. Chloro-, bromo-, and
iodo-benzenes are reported to be effectively hydrogenolyzed to benzene over palladium; in contrast, the
hydrogenolysis of fluorobenzene does not proceed over palladium [25, 26]. The hydrogenation activity
of molybdenum metal is quite low: an evaporated molybdenum film hydrogenates ethylene but not
propylene [27]. Nevertheless, the catalytic activity of the molybdenum atom in the clusters 1 and 2 is
high. The selectivity of these clusters is similar to that of the platinum group metals, though the TOF
values of the clusters at 300 ºC (Tables 2–4) are much lower than the reported values of the platinum
group metals at lower temperatures: 4500 h–1 for hydrogenation of 1-pentyne over supported Pd at 85 ºC
[28], 90–190 h–1 for hydrogenation of nitrobenzene over supported Pd at 223 ºC [29], and 60 h–1 for
hydrogenolysis of chlorobenzene over supported Pd at 80 ºC [30].
We have investigated the activation and deactivation processes of 1 under hydrogen and air by
XRD patterns, thermogravimetric analyses, Fourier transform infrared spectra, temperature programmed
desorption of ammonia, elemental analyses, and gravimetric measurements [4, 5]. A proposed
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mechanism for the activation can be summarized as follows. Upon storage in air, 1 reacts with moisture
in air. The gravimetric measurements under air showed that the weight of 1 rapidly increases by 1.0%
within 24 h, followed by a slow weight increase by 4.5% over 90 days. This observation suggests that
the surface of 1 is rapidly passivated, resulting in the retarded oxygenation of the inside of 1. After the
90 days, a hydroxo ligand is formed on the copper cation, and an oxo ligand is incorporated at the
sulfur-deficient site of the cluster anion (Eq. (1)). When this oxygenated cluster is heated in a hydrogen
stream, it releases the hydroxo ligand of the copper cation at around 200 C (Eq. (2)) and the oxo ligand
of the cluster up to 300 C, leading to the re-formation of sulfur-deficient 1 (Eq. (3)) [4, 5]. As Fig. 3
illustrates, the catalytic activity for cyclohexene developed above 300 ºC, indicating that the sulfur-
deficient site or, in other words, the coordinatively unsaturated molybdenum atom, is catalytically active.
The XPS of 1 activated under hydrogen at 400 ºC exhibited the binding energy of Cu 2p3/2 at 932.5 eV,
which is attributable to Cu0 [31]. As Table 1 (entry 7) shows, copper metal had little activity and its
selectivity differed from that of 1 under identical conditions: hydrogenolysis was the main reaction.
Thus, copper atom in 1 did not participate in the reaction. The catalytic active site of the neutral cluster 2
is also attributable to the coordinatively unsaturated molybdenum atom [4].
CuxMo6S8–δ δ (1) + (x + δ)H2O  Cux(OH)xMo6S8–δOδ + (x/2 + δ)H2
Cux(OH)xMo6S8–δOδ + (x/2)H2  CuxMo6S8–δOδ + x H2O
CuxMo6S8–δOδ + δ H2  CuxMo6S8–δ δ (1) + δ H2O
(1)
(2)
(3)
Isoelectronic metal atoms occasionally exhibit the same catalysis as is exemplified by the nitration
of alkylbenzene catalyzed by Hg(II), Tl(III), or Pb(IV) [32]. A series of [Y(η5-C5H5)2(alkyl)], [Zr(η5-
C5H5)2(alkyl)]+, and [Nb(η5-C5H5)(η4-C4H6)]+ moieties show similar polymerization activity [33]. It is
well known that Ru metals in [RuCl2(PPh3)3] and [RuHCl(PPh3)3] are isoelectronic with Rh metal in
[RhCl(PPh3)3], and that these complexes catalyze the hydrogenation of olefinic bonds [34].
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Molybdenum nitride and carbide have been reported to undergo similar catalyses to the platinum group
metals. This similarity has been explained by the addition of the valence electrons of the carbon and
nitrogen atom to the molybdenum atom, which changes its electron distribution to one similar to that of
the platinum group metals [35, 36]. The unsaturated molybdenum atom of 1 and 2 could be isoelectronic
with the platinum group metals by taking some electrons from the sulfur ligands, resulting in similar
activity and selectivity to that of the platinum group metals. We have previously reported that a
molybdenum halide cluster (H3O)2[(Mo6Cl8)Cl6]·6H2O exhibits catalytic behavior similar to that of the
platinum group metals, and this similarity has been explained in the same way [37–40].
5. Conclusions
Solid-state molybdenum sulfide clusters, the Chevrel phases, catalyzed various reactions that are
catalyzed by the platinum group metals: the dehydrogenation of cyclohexene, the hydrogenation of
alkyne, alkyne, aldehyde, ketone, and nitrobenzene, and the hydrogenolysis of halogenobenzenes. This
similarity can be explained by the isoelectronicity of the molybdenum atom with the platinum group
metals. Thus, the molybdenum clusters could be materials alternative to the platinum group metals.
Acknowledgments
This work was supported by a Grant-in-Aid (25410079) for Scientific Research (C) from the
Ministry of Education, Science, and Culture.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in the online version.
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Figure captions
Fig. 1 A typical reaction profile of cyclohexene over CuxMo6S8–δ (x = 2.94, δ ≈ 0.3) (1) in a hydrogen
stream at 450 °C. Conversion = products/(products + recovered cyclohexene) × 100 (%); selectivity
= product/(total amount of products) × 100(%). Legend: conversion (●); selectivity for benzene
(○); selectivity for cyclohexane (∆), and selectivity for hydrogenolysis products such as methane
and ethane (□).
Fig. 2 SEM image of CuxMo6S8–δ (x = 2.94, δ ≈ 0.3) (1): (a) after 1-h activation with hydrogen at 450
ºC and (b) after subsequent 7-h reaction with cyclohexene in a hydrogen stream at 450 ºC.
Fig. 3 Effect of temperature on activity and selectivity in the reaction of cyclohexene over CuxMo6S8–δ
(x = 2.94, δ ≈ 0.3) (1) in a hydrogen stream at 3 h after the start of the reaction. Legend: conversion
(●); selectivity for benzene (○); selectivity for cyclohexane (∆), and selectivity for hydrogenolysis
products such as methane and ethane (□).
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Table 1. Reactivity of cyclohexene at 450 ºCa
Entry
Catalyst
Conversion (%)
TOFb (h–1)
Selectivity (%)
Cyclohexane
18.9
Benzene
76.6
62.0
15.9
3.2
Othersc
4.5
1
2
3
4
5
6
7
8
9
CuxMo6S8–δ (1)
Mo6S8–δ (2)
Ni2Mo6S8
Li4Mo6S8
MoS2
36.2
32.2
3.9
1.02
0.733
0.103
0.067
0.061
0.053
0.034d
493
22.7
15.3
62.3
73.3
79.7
76.9
77.7
2.1
21.8
2.8
23.5
2.3
1.9
18.4
Mo metal
Cu metal
Pd/Ce
3.3
1.7
21.4
3.2
1.6
20.7
46.3
81.5
94.0
95.0
1.8
3.9
Pt/Ce
1590
-
1.9
3.1
10
None
1.0
27.9
70.3
a
b
c
At 3 h after the start of the reaction. Turnover frequency per molybdenum atom at 3–4 h after the start of the reaction. Hydrogenolyzed
products such as methane, ethane, ethylene, etc. d Per copper atom. e 0.5 wt%, 10 mg. f Per palladium atom. g Per platinum atom.
15
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Table 2. Hydrogenation of alkyne and alkene at 300 ºC a
Entry
Reactant
Catalyst
Conversion (%)
TOFb (h–1)
Selectivity (%)
Alkene
96.3
93.6
-
Alkane
2.3
Others
1.5
1
2
3
4
5
6
Phenyl acetylene
Phenyl acetylene
Styrene
22.6
33.7
77.6
99.2
48.2
48.4
0.63
0.77
2.18
2.26
1.35
1.36
CuxMo6S8–δ (1)
Mo6S8–δ (2)
5.4
1.0
99.7
>99.9
0.0
0.3
CuxMo6S8–δ (1)
Mo6S8–δ (2)
Styrene
-
<0.1
28.2d
25.4f
1-Pentyne
71.8c
74.8e
CuxMo6S8–δ (1)
1-Hexyne
0.8
CuxMo6S8–δ (1)
a
b
c
At 3 h after the start of the reaction. TOF per molybdenum atom at 3–4 h after the start of the reaction. 1-Pentene (58.6%), cis-2-pentene
d
e
(7.9%), and trans-2-pentene (5.3%). 2,4-Pentadiene (24.7%), 2-pentyne (2.5%). 1-Hexene (64.1%), cis-2-hexene (8.1%), and trans-2-hexene
(2.6%). f Conjugated pentadienes (5.7%).
16
Page 16 of 22

Table 3. Reactivity of oxygen- or nitrogen-containing compounds under hydrogen at 300 ºC a
Entry
Reactant
Catalyst
Conversion (%)
TOFb (h–1)
Hydrogenation
product
Selectivity (%)
1
2
3
4
5
6
7
8
Propanal
52.5
70.9
49.3
14.0
40.2
7.0
1.47
1.61
1.38
0.39
0.92
0.20
2.42
2.68
1-Propanol
1-Propanol
1-Butanol
2-Propanol
2-Propanol
3-Pentanol
Aniline
93.4
68.6
69.9
31.7c
7.7d
20.8e
99.4
98.2
CuxMo6S8–δ (1)
Mo6S8–δ (2)
Propanal
Butanal
CuxMo6S8–δ (1)
CuxMo6S8–δ (1)
Mo6S8–δ (2)
Acetone
Acetone
3-Pentanone
Nitrobenzene
Nitrobenzene
CuxMo6S8–δ (1)
CuxMo6S8–δ (1)
Mo6S8–δ (2)
86.6
95.7
Aniline
a At 3 h after the start of the reaction. b TOF per molybdenum atom at 3–4 h after the start of the reaction. c The main product was propene (64.7%).
d The main product was propene (73.6%). e The main products were pentenes (75.3%).
17
Page 17 of 22

Table 4. Reactivity of halogenated benzenes at 300 ºC a
Entry
Reactant
Catalyst
Conversion (%)
TOFb (h–1)
Hydrogenolysis
product
Selectivity (%)
1
2
3
4
5
Fluorobenzene
Chlorobenzene
Chlorobenzene
Bromobenzene
Iodobenzene
0.0
25.4
91.0
30.4
5.7
0.0
CuxMo6S8–δ (1)
CuxMo6S8–δ (1)
Mo6S8–δ (2)
0.71
2.07
0.85
0.16
Benzene
Benzene
Benzene
Benzene
99.0
99.5
99.5
99.3
CuxMo6S8–δ (1)
CuxMo6S8–δ (1)
a At 3 h after the start of the reaction. b TOF per molybdenum atom at 3–4 h after the start of the reaction.
18
Page 18 of 22

Figure
Fig. 1
100
80
60
40
20
0
0
2
4
6
t / h
Page 19 of 22

Fig. 2
a
b
200 nm
200 nm
Page 20 of 22

Fig. 3
40
30
20
100
80
60
40
20
0
10
0
250
300
350
400
450
T / ºC
Page 21 of 22

Graphical abstract
S
H2
+
Mo
H2
H2
H2
+
+
O
Cu
OH
NH2
H
NO2
+
X
H2
+
(X = Cl, Br, I)
Page 22 of 22