Solid-solid palladium-catalysed water reduction with zinc: Mechanisms of hydrogen generation and direct hydrogen transfer reactions

Article abstract of DOI:10.1039/B001655L

Facile generation of hydrogen gas from water takes place under moderate conditions in the presence of zinc powder and catalytic palladium on carbon; 82% conversion of zinc is obtained. An unusually large kinetic isotope effect is observed using D₂O (k(H)/k(D) = 14), which may reflect the cleavage of both O-H bonds in the rate-determining step. Experiments using D₂O-H₂O mixtures evidence that water molecules adsorbed on the catalyst surface undergo H-D exchange reactions (with molecules from the solvent bulk) that are approximately 100 times faster than the hydrogen generation reaction. The primary factors in this system appear to be palladium-hydrogen and zinc- oxygen interactions. Conversely, in the presence of an organic hydrogen acceptor, such as benzaldehyde, a different course is realised, consisting of direct hydrogen transfer from 'zinc-activated' water to the substrate, without the participation of Pd-H intermediates. Quantitative hydrogenation of benzaldehyde to benzyl alcohol, and of aromatic nitro compounds to the corresponding amines, is obtained. Another application of the above system is the specific deutero-dehalogenation of aromatic halides. Possible mechanisms and the implications of a chemical reaction involving two macroscopic solid particles are discussed.

Full text of DOI:10.1039/B001655L

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Solid–solid palladium-catalysed water reduction with zinc:
mechanisms of hydrogen generation and direct hydrogen transfer
reactions¤
Sudip Mukhopadhyay, Gadi Rothenberg,” Harold Wiener and Yoel Sasson*
Casali Institute of Applied Chemistry, Hebrew University of Jerusalem, 91904, Israel.
E-mail: ysasson=vms.huji.ac.il
Received (in Montpellier, France) 23rd February 2000, Accepted 1st March 2000
Published on the Web 12th April 2000
Facile generation of hydrogen gas from water takes place under moderate conditions in the presence of zinc powder
and catalytic palladium on carbon; 82% conversion of zinc is obtained. An unusually large kinetic isotope e†ect is
observed using D O (k /k \ 14), which may reÑect the cleavage of both OÈH bonds in the rate-determining step.
2
H D
Experiments using D OÈH O mixtures evidence that water molecules adsorbed on the catalyst surface undergo
2
2
HÈD exchange reactions (with molecules from the solvent bulk) that are approximately 100 times faster than the
hydrogen generation reaction. The primary factors in this system appear to be palladiumÈhydrogen and
zincÈoxygen interactions. Conversely, in the presence of an organic hydrogen acceptor, such as benzaldehyde, a
di†erent course is realised, consisting of direct hydrogen transfer from ““zinc-activatedÏÏ water to the substrate,
without the participation of PdÈH intermediates. Quantitative hydrogenation of benzaldehyde to benzyl alcohol,
and of aromatic nitro compounds to the corresponding amines, is obtained. Another application of the above
system is the speciÐc deutero-dehalogenation of aromatic halides. Possible mechanisms and the implications of a
chemical reaction involving two macroscopic solid particles are discussed.
The efficient catalytic reduction of water for generation of
hydrogen is one of the most challenging transformations in
chemistry.1 Water is the ultimate hydrogen source, being both
safe and plentiful. Hydrogen gas, in addition to its potential as
an eco-friendly energy resource, is one of the fundamental feed
stocks in the contemporary chemical industry.2
the organic and in the inorganic systems are discussed. The
application of this process to in situ site-speciÐc deuteration of
aromatic molecules is also described.
Results
The reduction reaction of water with zinc is known to occur
at extreme temperatures and pressures. Thus, ZnO and H
have been detected under supercritical conditions3 at 450 ¡C,
Hydrogen generation from water reduction
2
In a typical experiment [eqn. (1)], 1 equiv. of untreated zinc
powder (40 mesh size) was mixed in an autoclave with an
excessÒ of water and 0.005 equiv. of 5% Pd/C. At 120 ¡C, the
reaction ceased when 82% of the zinc had been consumed (34
or at 250È325 ¡C at 221 bar pressure.4 It has also been report-
ed that electrochemical coating of zinc particles with noble
metals (e.g., Rh, Pt, Pd)5a or with Ni5b can facilitate the reac-
tion between zinc and water at the particle interface. A com-
parable concept has also been patented for maintaining a dry
atmosphere within electronic devices.6 Although the above
techniques require special apparatus, and were only performed
on a small scale, they demonstrate the thermodynamic feasi-
bility of the reaction of water and zinc, provided that the acti-
vation energy barrier can be negotiated.°
atm H pressure), probably due to coating of the zinc particles
2
with ZnO (vide infra). A 100% mass balance was evidenced by
gravimetric analysis of zinc oxide. Analysis of the reaction
proÐle showed that the reaction follows a Ðrst-order rate
equation up to 60% conversion of zinc. Hydrogen generation
was found to be equally facile under both neutral and basic
conditions (6.6 \ pH \ 12.8). Control reactions (25È120 ¡C)
conÐrmed that no hydrogen was generated in the absence of
the catalyst, nor was hydrogen detected when palladium on
carbon was stirred with water in the absence of zinc. Further-
more, blank experiments using glass vessels and TeÑon pro-
pellers gave similar results to those using stainless steel
equipment, indicating that the steel apparatus plays no part in
the reaction.
We have now observed that palladium on carbon catalyses,
under remarkably mild conditions, the heterogeneous reaction
of water with plain zinc powder to zinc oxide and hydrogen
gas. In this paper, this chemical process is studied through the
examination of rate proÐles and kinetic isotope e†ects. A reac-
tion mechanism is proposed and the implications of a chemi-
cal reaction at the microscopic interface between two
macroscopic metallic particles are discussed. Water reduction
experiments in the presence of organic hydrogen acceptors are
also reported, wherein a di†erent reaction pathway, involving
direct hydrogen transfer from water to the organic substrate,
is evidenced. The roles of PdÈH species as intermediates in
(1)
As mentioned above, hydrogen generation stopped after ca.
8
2% zinc conversion. This does not reÑect catalyst deactiva-
¤
Non-SI units employed: 1 bar \ 105 Pa; 1 atm \ 101.3 kPa; 1
psi \ 6.9 ] 103 Pa; 1 kcal B 4.2 kJ.
Current address: Clean Technology Centre, Department of Chem-
tion, as the reaction did not resume when fresh portions of
Pd/C were added. Conversely, when fresh zinc powder was
added, hydrogen generation resumed. The deactivation
became more pronounced when larger zinc particles were
”
istry, University of York, Heslington, York, UK YO10 5DD. E-mail:
gr8=york.ac.uk
DOI: 10.1039/b001655l
New J. Chem., 2000, 24, 305È308
305
This journal is ( The Royal Society of Chemistry and the Centre National de la Recherche ScientiÐque 2000

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employed, such that with 1 mm diameter particles the reaction
stopped after less than 15% of the zinc had been consumed.
(k
B k ), and then levels out with a rate corresponding
1
, Mix2
H2O
tothat of pure D O reduction (k
rate constant for the HÈD exchange reaction at about two
B k ). This sets the
2
2, Mix2
D2O
orders of magnitude higher than the catalytic reduction of
Deuterium isotope e†ect studies
D O on the palladium surface.
2
Water, of course, could not be used as diluent in the above
When the reaction was performed with D O instead of H O,
2
2
isotope e†ect studies. Dry PhMe was therefore used, after
blank reactions showed that its presence does not a†ect the
reaction.
an uncommonly large kinetic isotope e†ect8a was measured
(
(
Fig. 1, k /k \ 14.2), indicating the participation of OÈH
H2O D2O
or OÈD) bond scission in the rate-determining step. This
unusual magnitude8b of the isotope e†ect may be attributed to
two OÈH bonds being broken simultaneously in the rate-
determining step.
Using water as a direct hydrogen donor
Remarkably, an equimolar mixturep (Mix1, Fig. 2) of D O
The zincÈwater system was also found to a†ect direct hydro-
gen transfer reactions from water to organic substrates. For
example, benzaldehyde was quantitatively reduced to benzyl
alcohol [eqn. (2)]. Excellent ([99%) yields were similarly
obtained in the reductions of nitrobenzene to aniline, and of
4-nitroanisole to 4-aminoanisole.
2
and H O, for which a value of k /k
itively expected, actually displayed the same activity as pure
B 7 could be intu-
2
H2O Mix1
H O (i.e., k
\ k
). This observation may be explained by
2
H2O
Mix1
taking into account fast OÈH and OÈD exchange reactions,
which can occur not only in solution but also (and apparently
much faster) when the molecule is activated on the catalyst
surface (cf. aqueous formate salts and Pd/C10). In the case of
Mix1, when a D O molecule is adsorbed on the palladium
catalyst, it is activated and may freely exchange its D atoms
(2)
2
for H atoms with an H O molecule from the bulk of the
2
solvent. Of course, the e†ects of this fast reaction would not be
We believe that this reaction is a true hydrogen-transfer
process from ““zinc-activatedÏÏ water to the substrate, rather
than a dehydrogenation-hydrogenation sequence involving
molecular hydrogen. This is supported by the following Ðnd-
ings: (i) experiments in open and closed vessels evidenced
similar hydrogenation rates; (ii) no free hydrogen was released
observable in pure H O or D O. However, in a mixture such
as Mix1, D O molecules would be apt to exchange their D
2
2
2
atoms for H atoms and H gas would be released more often
2
than HD or D . As these kinetic studies are based on the
2
measurements of initial rates, Mix1, which contains about 25
mol% of H O, would display the same behaviour as pure
2
from the system; (iii) the reduction of benzaldehyde with D O
H O.
2
2
to benzyl-d-alcohol-d showed only a low isotope e†ect
In order to try and assess the relative rates of the HÈD
(
k /k \ 1.4);** and (iv) initial H production rates (without
H D
an organic substrate) were slower than the initial rates (up to
exchange and the water reduction reactions, we performed
another experiment using a 98 : 2 molar ratio mixture of
D O : H O (Mix2). As shown in Fig. 2, the pressure initially
2
20% conversion) of reduction in the presence of an organic
2
2
substrate, the ratio k
2 : 1.
: k
being approximately
rises with a rate corresponding to that of pure H O reduction
reduction H2 generation
2
Another interesting application for the method outlined
above is the deuteration of aryls at speciÐc sites on the aro-
matic ring. This can be done simply by reductive deutero-
dehalogenation of haloaromatics. As shown in eqn. (3), for
example, 1-deuterobenzene, C H D, was isolated in 56% yield
6
5
from bromobenzene (72% conversion), the only other product
being biphenyl.
(3)
Discussion
Fig. 1 Pressure generated in the presence of (L) H O and (…) D O.
2
2
Since Pd0 does not react with water in the absence of zinc, it
is unlikely that palladium oxide is an intermediate in our
system.¤¤ Rather, it appears that zinc acts as an oxygen
acceptor, while palladium interacts only with the hydrogen
atoms. The sensitivity of the reaction to the zinc surface area,
plus the fact that the deactivation is dependent on zinc, rather
than on palladium, all point to zincÈoxygen and palladiumÈ
hydrogen interactions.
Reaction conditions: 150 mmol Zn, 300 mmol H O or D O, 1 mol%
2
2
Pd (5% Pd/C), 75 ¡C, 100 ml dry PhMe.
Based on the experimental results, we may envisage two
possible mechanistic options for this remarkable transform-
ation. The Ðrst, and more probable, is that the reaction occurs
at the interface between two macroscopic particles (see Fig. 3).
In this model, water Ðrst adsorbs on the palladium surface,
whereby the OÈH bonds are activated (see Fig. 4). Following
this, zinc donates two electrons to the oxygen atom and both
OÈH bonds are cleaved simultaneously in the rate-
determining step. The two H~ radicals interact with Pd0 to
form palladium hydrides, and then, by reductive elimination,
Fig. 2 Pressure generated in the presence of (L) Mix1 and (…)
Mix2. Reaction conditions: 150 mmol Zn, 300 mmol Mix1 or Mix2, 1
mol% Pd (5% Pd/C), 75 ¡C, 100 ml dry PhMe.
H leaves the palladium surface.
2
306
New J. Chem., 2000, 24, 305È308

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presence of Pd/C. Possibly, this reaction occurs at the inter-
face between two macroscopic solids. In the presence of
organic hydrogen acceptors, direct hydrogen transfer from
water to the organic substrate is realised.
Experimental
General
Fig. 3 Cartoon of the solidÈsolid water reduction reaction.
GC and GC-MS analyses were performed using an HP-5890
gas chromatograph with a 50% diphenylÈ50% dimethyl-
polysiloxane packed column (25 m/0.53 mm). Chemicals were
purchased from commercial Ðrms ([98% pure) and used
without further puriÐcation. Satisfactory spectral analyses
were obtained for all products. Reactions were performed in a
300 ml stainless steel Parr autoclave equipped with a six-
bladed propeller, an external heating mantle and a gas cooling
system.
Procedures
Water reduction reaction. Zn powder (40 mesh size, 15.0 g,
30 mmol) was mixed in an autoclave with 2.0 g 5% Pd/C (0.9
2
Fig. 4 Suggested mechanism for hydrogen formation.
mmol Pd) and 125 ml H O at 75 ¡C. An 82% conversion of
2
zinc was achieved after 72 h, with a pressure of 34 atm H .
2
Direct hydrogen transfer reactions. As an example, using the
hydrogen transfer from water to benzaldehyde reaction: 5.0 g
of benzaldehyde (47 mmol), 3.3 g (52 mmol) Zn powder, 0.2 g
5
% Pd/C (0.09 mmol Pd), 1 g PEG-400, and 45 ml H O were
2
mixed at 100 ¡C for 8 h, after which quantitative conversion to
benzyl alcohol was measured. Hydrogenations of nitrobenzene
(
4
44 mmol, 12 h, 1.2 mol% Pd, [99% yield to aniline) and of
-methoxynitrobenzene (44 mmol, 14 h, 1.2 mol% Pd, 100%
yield to 4-methoxyaniline) were similarly performed.
Deuteration of halobenzenes. For C H D from C H Br: 5.0
6
5
6 5
g C H Br (32 mmol), 4.2 g (64 mmol) Zn powder, 0.35 g 5%
6
5
Pd/C (0.16 mmol Pd), and 25 ml D O were mixed at 100 ¡C
2
for 18 h (72% conversion of bromobenzene). Filtration, fol-
Fig. 5 Suggested mechanism for hydrogen transfer from ““zinc-
activatedÏÏ water to an organic substrate S.
lowed by extraction with 25 ml CH Cl and distillation
a†orded 1.0 g (56% isolated yield based on starting material)
2
2
of 1-deuterobenzene, C H D. GC-MS m/z 79 (cf. for C H ,
6
5
6 6
7
8).
A second possible option is that a classical electrochemical
mechanism occurs via direct contact between the solid par-
ticles. However, inasmuch as any chemical reaction is an
Kinetic studies. For the transfer hydrogenation of benz-
aldehyde: 76 mmol Zn, 75 mmol benzaldehyde, 15 ml H O
or D O) and 0.3 mol% of Pd/C were stirred in a Ñask at
00 ¡C for 10 h. Periodic GC analysis gave the following k
““electrochemical reactionÏÏ, it should be noted that zinc, zinc
2
(
1
oxide and Pd/C are all insoluble in water under the reaction
conditions. The isotopic e†ects observed, plus the fact that the
reaction contains no electrolyte, make a classical electro-
chemical mechanism unlikely in this case.
2
obs
11
values:
k \ 7.65 ] 10~2
h~1
(r2 \ 0.999
for
H
observations); k \ 4.75 ] 10~2 h~1 (r2 \ 0.9985 for 11
D
observations). For the hydrogen generation reactions: 150
In the presence of an organic hydrogen acceptor (S), the
experimental results point to a di†erent reaction route. This
pathway (Fig. 5) consists of direct hydrogen transfer from
water to the adsorbed substrate, with no PdÈH species. T he
latter cannot participate as intermediates, since their formation
is rate-determining in the slower hydrogen generation process.
Accordingly, the rate-determining step may now be the
adsorption of water to a free active site. Indeed, it has been
shown in similar cases that the adsorption of the H-donor (in
this case, water) is rate determining.11 Furthermore, the pres-
mmol zinc, 300 mmol H O (or D O or Mix1 or Mix2), 1
mol% Pd/C and 100 ml dry PhMe were stirred in an auto-
clave at 75 ¡C for 6 h. Periodic GC analysis gave the following
2
2
k
values: k
\ 6.2 ] 10~4 min~1 (r2 \ 0.989 for 12
obs
H2O
observations); k
observations); k
observations); k
observations); k
observations).
\ 4.3 ] 10~5 min~1 (r2 \ 0.99 for 11
D2O
\ 6.1 ] 10~4 min~1 (r2 \ 0.988 for 11
\ 6.0 ] 10~4 min~1 (r2 \ 0.99 for 5
Mix1
1, Mix2
\ 4.2 ] 10~5 min~1 (r2 \ 0.999 for 8
2, Mix2
ence of the organic substrate S and/or of its product SH
2
Notes and references
alters the system, so that the deactivation of the zinc particles
described above is evidently prevented (quantitative hydro-
genation of S to SH
lents of zinc and S).
°
Indeed, this is an exothermic reaction, *H0 \ [14.9 kcal mol~1,
r
2 takes place when using molar equiva-
and *G0 \ [19.4 kcal mol~1.7
r
Ò 1 : 1 molar ratios can be used, but the reaction is much slower.
Moreover, for e†ective stirring in the 300 ml autoclave a minimal
volume is required. For these reasons, an excess (50È100 ml in this
case) of water was used.
Conclusion
p The equilibrium constant for the reaction H O ] D O H HDO at
2
2
Water molecules that are activated by zinc particles can
decompose catalytically to hydrogen and zinc oxide in the
75 ¡C was reported9 as K \ 3.12. Therefore, Mix1 contains a 2 : 1 : 1
eq
equilibrium ratio of HDO : D O : H O.
2
2
New J. Chem., 2000, 24, 305È308
307

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*
* This low isotope e†ect may be attributed to OÈH (OÈD) bond
5
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catalyst.
¤
¤ The reaction Pd ] H O H PdO ] H is thermodynamically unfa-
6
7
8
2
2
vourable, with *G0 [ ]30 kcal mol~1.7
r
1
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2
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New J. Chem., 2000, 24, 305È308