Mechanism of the catalytic synthesis of water on silver

Article abstract of DOI:10.1016/j.mencom.2015.03.028

A mechanism of the macroscopic step of water synthesis in the oxidation of hydrogen on silver has been revealed.

Full text of DOI:10.1016/j.mencom.2015.03.028

Mendeleev
Communications
Mendeleev Commun., 2015, 25, 155–156
Mechanism of the catalytic synthesis of water on silver
Eugene V. Dokuchits,*a,b Alexander V. Khasinb and Alexander A. Khassina,b
a Novosibirsk State University, 630090 Novosibirsk, Russian Federation
b G. K. Boreskov Institute of Catalysis, Siberian Branch of the Russian Academy of Sciences,
630090 Novosibirsk, Russian Federation. Fax: +7 383 330 8056; e-mail: oschtan@catalysis.ru
DOI: 10.1016/j.mencom.2015.03.028
A mechanism of the macroscopic step of water synthesis in the oxidation of hydrogen on silver has been revealed.
The oxidation of hydrogen on silver is of interest for the theory
of chemical kinetics and catalysis, and it can be used as a model
redox catalytic reaction. This reaction makes it possible to study
surface intermediates in order to understand the mechanisms of
many important catalytic hydrogenation and oxidation reactions.
This reaction is also of practical importance for the development
of fuel cells.
The study was performed using a volumetric static method in
a glass vacuum setup equipped with McLeod gauges for pressure
measurements. Hydrogen was purified by diffusion through a
heated palladium capillary. Oxygen was produced by the vacuum
decomposition of potassium permanganate (chemically pure).
The setup made it possible to prepare hydrogen–oxygen mixtures
with specified pressure and composition.
Early studies1–3 have demonstrated that the test reaction
proceeds by the mechanism consisting of two consecutive steps:
the chemisorption of oxygen and the interaction of gaseous
hydrogen with adsorbed oxygen. However, further studies have
revealed that the reaction has a more complicated mechanism
and occurs via the intermediate formation of surface hydroxyl
groups4,5 by the three-step mechanism:
A stationary adsorbed layer was obtained by consecutive
transformations of three portions of a stoichiometric hydrogen–
oxygen mixture (a 30% conversion in each portion). The unreacted
mixture was promptly removed by evacuation, and the reaction
was carried out with a new portion of the mixture. The initial
pressure of the reaction mixture was 22.8 Pa, and the reaction
temperature was 298 K. The temperature was controlled by water
circulating through a reactor jacket and measured by a thermo-
couple at the center of the catalyst layer. The resulting stationary
adsorbed layer interacted with hydrogen at the same temperature
and an initial pressure of 15.7 Pa, which was close to the hydrogen
pressure in the stoichiometric mixture portions (15.2 Pa con-
stitutes 2/3 of 22.8 Pa). Although the reaction is highly exo-
thermal, the probable adiabatic rise in temperature is less than
0.2 K since polycrystalline silver had low dispersion and only
1 mmol of the reaction mixture interacted over 5 g of the catalyst
during the experiment. Therefore, we considered that the collected
data correspond to a constant temperature of 298 K.
O2 = 2Oads
H2 + 2Oads = 2OHads
H2 + 2OHads = 2H2O.
,
(1)
(2)
(3)
,
The kinetics and mechanism of steps (2) and (3) are unknown.
The adsorption of oxygen (1) and step (2) proceed more rapidly,
as compared to step (3).6 In the steady state, the preponderant
fraction of adsorbed oxygen will reside most likely on the silver
surface as the OHads groups. In this case, the rate of hydrogen
interaction with the silver surface, which has a stationary com-
position, will be the rate of step (3) in the test reaction mechanism.
The aim of this work was to elucidate the regularities of this step.
Step (3) is highly unlikely to be considered elementary, as
it was emphasized by Hillary and Stoukides.7 Involving two
adjacent hydroxyls in the collision with the hydrogen molecule
should require the reaction being second-order with respect to
surface hydroxyl groups. If step (3) is a macroscopic one and
consists of elementary steps, it may contain the elementary rate-
determining step whose activated complex has only one hydroxyl
group; in this case, step (3) will be a first-order reaction with
respect to surface hydroxyl groups. The reaction scheme by Hillary
and Stoukides considered the dissociation of dihydrogen and
the consecutive reactions of adsorbed oxygen and the resulting
OH group with adsorbed hydrogen atoms; the latter stage was
concluded being rate-determining. The weak point of the proposed
scheme is that molecular hydrogen is not dissociatively adsorbed
on a metallic silver surface.8–10
Kinetic curves of the stoichiometric mixture flow rate are
shown in Figure 1. Equal initial rates of the reaction during the
transformation of second and third portions indicate the steady-
state occurrence of the process. It can be concluded that the
stationary adsorbed layer formed at 298 K and 22.8 Pa; the steady-
state reaction rate was 1.18×10–9 mol m–2 s–1 at an average
pressure of 21.4 Pa.
3.0
1
2.5
2.0
2
1.5
3
1.0
0.5
0.0
This study was carried out with polycrystalline silver obtained
via the reduction of silver oxide by hydrogen. Silver oxide was
prepared by precipitation from an aqueous solution of silver
nitrate [chemically pure, GOST (State Standard) 1277-75] with
potassium hydroxide (high purity). After the precipitation, silver
oxide was carefully washed with distilled water and dried. The
sample had a weight of 5.00 g, a silver grain size of 0.25–0.50 mm,
and a specific surface area of 0.75 m2 g–1.
0
500
1000 1500 2000 2500
t/s
Figure 1 Kinetic curves of the stoichiometric mixture flow rate in the
oxidation of hydrogen on silver at 298 K and 18.0–22.8 Pa. The conversion
reached 30% in each portion. (1), (2) and (3) are the first, second and third
portions of the mixture, respectively.
© 2015 Mendeleev Communications. Published by ELSEVIER B.V.
on behalf of the N. D. Zelinsky Institute of Organic Chemistry of the
Russian Academy of Sciences.
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Mendeleev Commun., 2015, 25, 155–156
which differs from the value obtained earlier11 for the hydro-
9
6
3
0
genation of surface hydroxyl groups, 4.25×10–5 s–1 Pa–1, only
by a factor of 1.14. This reliably confirms the accuracy of the
obtained result.
In the mechanistic scheme (1)–(3), step (3) is really a macro-
scopic one, which can be presented as the sum of two elementary
steps, (4) and
Hads + OHads = H2O.
(5)
0
1000
2000
3000
Each elementary act of step (4) is immediately accompanied
by the interaction of the adsorbed hydrogen atom with the second
hydroxyl group, which results in the formation of a second water
molecule by reaction (5). Thus, step (4) is the key step of
hydrogen oxidation on silver, and the surface hydroxyl group is
a key intermediate directly involved in the formation of water.
A hydrogen molecule and a surface hydroxyl group interact with
each other to produce directly a water molecule without any
intermediate species. Thus, it has been shown for the first time
that the hydrogenation of surface hydroxyl groups during the
oxidation of hydrogen on silver follows a first-order kinetic
law with respect to hydroxyl groups. Chemical equation (4) was
earlier unknown in catalysis. Some processes resembling those
examined in this work were proposed when a model of hydrogen
interaction with oxygen adsorbed on silver was analyzed using
the density functional theory.12,13 The results obtained in this study
are novel and essential; in particular, a mechanistic similarity
between the catalytic oxidation of hydrogen on silver and the
branched chain reaction of hydrogen combustion including the
similar step:14
t/s
Figure 2 Interaction of hydrogen with the stationary adsorbed layer pro-
duced by the transformation of three successive portions of the stoichiometric
mixture (2H2 + O2). Initial hydrogen pressure, 15.7 Pa; temperature, 298 K.
The following assumptions can be made:
(1) The initial rate of the interaction of hydrogen with the
stationary adsorbed layer equals one-half of the rate of the catalytic
oxidation of hydrogen because equal amounts of hydrogen are
consumed in two steps: the formation of hydroxyl groups and
the hydrogenation of surface hydroxyl groups.
(2) The hydrogenation of surface hydroxyl groups follows
the first-order law with respect to surface hydroxyl groups, and
it can be expressed as
H2 + OHads = H2O + Hads
.
(4)
To verify these assumptions, the interaction of hydrogen with
the stationary adsorbed layer, which was produced during the
transformation of the stoichiometric mixture, was investigated
(Figure 2).
The initial rate of hydrogen interaction with the stationary
adsorbed layer is W = 6.00×10–10 mol m–2 s–1, and the steady-
state rate of hydrogen oxidation in the reaction of stoichiometric
mixture oxidation is 1.18×10–9 mol m–2 s–1. It means that the
initial rate of hydrogen interaction with the stationary adsorbed
layer is two times lower than the rate of the steady-state catalytic
oxidation of hydrogen under identical reaction conditions.
Figure 3 shows that the reaction obeys a first-order equation
with respect to surface hydroxyl groups at a constant hydrogen
pressure: ln[1/(1 – x)] = kt. Here, k is the rate constant (s–1),
and x is the conversion of surface hydroxide. These data were
acquired at relatively small changes in the hydrogen partial pres-
sure (average value of ~15.7 Pa). Nevertheless, these changes
provided the accurate determination of the conversion of the
adsorbed layer during its reaction with hydrogen.
•OH + H2 = H2O + •H,
(6)
where a hydroxyl group is the key species directly involved in
the formation of water, is of interest.
Thus, the rate-determining step of the oxidation of hydrogen
on silver is the interaction of a hydrogen molecule with a surface
hydroxyl group giving a water molecule and an adsorbed hydrogen
atom, which is described by equation (4). The adsorbed hydro-
gen atom rapidly interacts with the second surface hydroxyl group
according to equation (5).
This study was supported by the Ministry of Education and
Science of the Russian Federation (project no. 2014/139/2211).
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t/s
Figure 3 Experimental data on the interaction of hydrogen with the sta-
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Received: 29th July 2014; Com. 14/4434
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