The Nature of Hydrogen Adsorption on Platinum in the Aqueous Phase

Article abstract of DOI:10.1002/anie.201813958

The thermodynamic state of H₂ adsorbed on Pt in the aqueous phase was determined by kinetic analysis of H₂ reacting with D₂O to HDO, HD, and D₂, and by DFT-based ab initio molecular dynamics simulations of H₂ adsorption on Pt(111), Pt(110), and Pt nanoparticles. Dissociative adsorption of H₂ on Pt is significantly weakened in the aqueous phase compared to adsorption at gas–solid interfaces. Water destabilizes the adsorbed H atoms, decreasing the heat of adsorption by 19–22 kJ (Formula presented.) while inducing an additional entropy loss of 50–70 J (Formula presented.) K^?1. Upon dissociative adsorption of H₂, the average distance of water from the Pt surface increases and the liquid adopts a structure that is more ordered than before close to the Pt surface, which limits the translation mobility of the adsorbed H atoms. The presence of hydrated hydronium ions next to the Pt surface further lowers the H?Pt bond strength.

Full text of DOI:10.1002/anie.201813958

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Accepted Article
Title: Nature of Hydrogen Adsorption on Platinum in the Aqueous
Phase
Authors: Guoju Yang, Sneha A. Akhade, Xi Chen, Yue Liu, Mal-Soon
Lee, Vassiliki-Alexandra Glezakou, Roger Rousseau, and
Johannes Lercher
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10.1002/anie.201813958
Angewandte Chemie International Edition
COMMUNICATION
Nature of Hydrogen Adsorption on Platinum in the Aqueous
Phase
Guoju Yang,[a] [b] Sneha A. Akhade,[c] Xi Chen,[a] Yue Liu,*[a] Mal-Soon Lee,*[c] Vassiliki-Alexandra
Glezakou,[c] Roger Rousseau,[c] and Johannes A. Lercher*[a] [c]
Abstract: The thermodynamic state of H2 adsorbed on Pt in the
aqueous phase was determined by kinetic analysis of H2 reacting with
D2O to HDO, HD and D2 andbyDFT based ab inito molecular dynamic
simulation of H2 adsorption on Pt (111), Pt (110), and Pt nanoparticles.
Dissociative adsorption of H2 on Pt is significantly weakened in the
aqueous phase compared to adsorption at gas-solid interface. Water
destabilizes adsorbed H, decreasing the heat of adsorption by 19 −
solution, displacement, and adsorption processes. Conceptually,
the question arises whether the dissociative adsorption on metal
surfaces in the condensed phase can be described solely in terms
of the solvation of H2 in water, the desorption of water from the
metal surface, and the adsorption of the dissociated H2 or whether
additional ordering effects take place (non-ideal behavior).
Experimentally, we have chosen a kinetic approach to
determine the thermodynamic properties of adsorbed H using the
adsorption of H2 (g) on supported Pt nanoparticles immersed in
D2O (l), monitoring the formation rate of HD and D2. Theoretical
simulations of the free energy of adsorption of H2 on the Pt (111)
surface complement the experiments. In comparison with H2
adsorption in the gas phase, we will show, how the presence of
liquid water enthalpically and entropically affects the H2
adsorption, and how the presence of hydrated hydronium ions
influences the dissociative adsorption of H2.
22 kJ·molH −1, while inducing an additional entropy loss of 50 − 70
2
J·molH −1∙K−1. Upon dissociative H2 adsorption, the averaged distance
2
of water from the Pt surface increases and it assumes a more ordered
structure. In turn, the more ordered water layer limits the translation
mobility of adsorbed H. The presence of hydrated hydronium ions next
to Pt further reduces the H-Pt bond strength.
Dissociative adsorption of H2 is the first elementary step in most
catalyzed reactions involving hydrogen,[1] including hydrogen fuel
cell anode reactions.[2] It involves physisorption of H2, cleavage of
the H-H bond and formation of a H-metal bond.[3] As adsorbed
surface H reacts with other molecules in catalyzed reactions, it is
critical to understand its thermodynamic state in order to
rationalize and design catalytic processes.
Formally, the reactions of H2 (g) with D2O (l) into D2 (g), HD
(g), and HDO (l) on a supported Pt catalyst can be written as:
(Rxn 1)
(Rxn 2)
The adsorption geometry and the enthalpy and entropy of H2
on transition metals are experimentally and theoretically well
studied.[4] However, the majority of these studies explore the
adsorption of H2 from the gas phase. Understanding hydrogen
adsorption in the condensed phase and especially in the presence
of water has been mostly explored in relation to electrochemistry
[2b], but understanding the state of adsorbed hydrogen is equally
important for synthetic chemistry.[1a, 5]
The heat of adsorption of H2 on electrode materials in the
aqueous phase has been determined by electrochemical
methods.[6] However, this approach requires that the catalyst is
electrically conductive. The majority of supported catalysts do not
fulfill this condition. The direct use of calorimetric methods suffers,
on the other hand, from the convolution of the heat resulting from
For both reaction pathways, H2 dissociates on Pt into H atoms,
which partially exchange to D atoms via interaction with adsorbed
D2O. The H and D atoms recombine into H2, HD and D2 desorbing
from the surface. Under a given pressure of H2, the desorption
rates of H2 (rH2) HD (rHD) and D2 (rD2) follow equations 1 and 2
(Derivation details are in the Supporting Information S1).
0.5
0.5
r
 rHD  rD
pH0.5  ka0.5  Ka0.5 r0.5  rD0.5 KIE
(Eq 1)






H2
H2
2
2
2
D2
rH2D KIE


HD
D2
(Eq 2)
rH

2
2
4rD KIE

2
The terms ka and Ka are the adsorption rate constant and
equilibrium constant of H2 adsorption on Pt. KIE refers to the
kinetic isotope effect. [KIE]D is defined as the desorption rate
2
constant ratio of D2 to H2 and [KIE]HD is defined as the desorption
rate constant ratio of HD to H2 (KIE values are compiled in
[a]
Dr. G. J. Yang, X. Chen, Dr. Y. Liu, Prof. Dr. J. A. Lercher
Department of Chemistry and Catalysis Research Center
Technische Universität München
Lichtenbergstrasse 4, 85747 Garching (Germany)
E-mail: yue.liu@tum.de (YL), johannes.lercher@ch.tum.de (JAL)
Supporting Information S2). The rHD and rD are measurable as
2
the formation rates of HD and D2 in the reaction, whereas rH2 is
difficult to measure directly because of the excess of H2, but can
be calculated using rHD and rD2 based on Equation 2. Equation 1
shows that the term (rH + rHD + rD )0.5·pH -0.5 has a linear correlation
2
2
2
[b]
[c]
Dr. G. J. Yang
with the term (rH 0.5 + rD 0.5 [KIE]D -0.5). The slope is the square root
2
2
2
Department of Chemistry and International Center of Future Science
Jilin University, Qianjin Str.2699, 130012, Changchun (P. R. China)
of Ka, and the intercept is the square root of ka.
The reaction of gaseous H2 with liquid D2O was carried out on
silicalite-1 supported Pt (Pt/silicalite-1). The Pt/silicalite-1 consists
of Pt particles (average diameter of 1.0 nm) supported on a non-
acidic MFI-type silicalite-1. A typical reaction result of 30 bar H2
with 30 mL D2O at 323 K is shown in Figure 1 inset, as the yields
of HD and D2 with reaction time. Both HD and D2 increased
linearly from the beginning of the reaction until at least 50 min,
Prof. Dr. J. A. Lercher, Dr. S. A. Akhade, Dr. M. S. Lee, Dr. V. A.
Glezakou, Dr. R. Rousseau
Institute for Integrated Catalysis
Pacific Northwest National Laboratory
P.O. Box 999, Richland, WA 99352 (USA)
E-mail : malsoon.lee@pnnl.gov (MSL)
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201813958
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and van ’t Hoff plot (Supporting Information S4) and compiled in
Table 1. The desorption rate constant k-a increased with
increasing temperature, leading to an activation energy for
desorption of (51 ± 3) kJ∙mol−1. The adsorption equilibrium
constant Kaօ decreased with temperature, showing an exothermic
process with heat of adsorption of 50 ± 3 kJ∙mol−1. The activation
energy for desorption being identical to the heat of adsorption is
because the activation energy for adsorption is close to zero. The
H2 adsorption rate constant ka hardly changed with temperature.
Thus, we conclude that almost no activation energy, i.e. (1 ± 2)
kJ∙mol−1, is required for the adsorption of H2 on Pt/silicalite-1. This
agrees with the previous adsorption study in the gas phase, which
reported that the dissociative chemisorption process of H2 is
spontaneous on Pt.[4i, 4m, 7]
Based on the adsorption constant, the adsorption isotherm of
H2 on Pt at 343 K was derived (Figure 2, blue curve). It shows a
partial coverage (0.1 ~ 0.5) within the H2 pressure range of 1 − 50
bar. For comparison, the adsorption isotherm of H2 on Pt/silicalite-
1 in the gas phase was determined at 343 K (Supporting
Information S5). The dissociatively adsorbed H is considered as
quasi-irreversibly adsorbed H on Pt, i.e., the adsorption constant
is sufficiently high, that very low H2 pressures have to be used to
lower the coverage.[8] The adsorption isotherm and heat were
determined via subtraction of the isotherm and heat of total
adsorbed H by that of the reversibly adsorbed H. The obtained
isotherm was plotted in Figure 2 (black point). In contrast to the
adsorption in water, the saturated coverage was readily reached
at a H2 pressure below 0.02 bar. The stronger H2 adsorption in
the gas phase is also manifested by the higher heat of adsorption
of 72 ± 5 kJ∙mol−1 compared to 50 ± 3 kJ∙mol−1 in water.
Figure 1. Reaction kinetic results over Pt/silicalite-1 under different pressures
and temperatures plotted using Equation 1. Inset is the yield of HD and D2 with
increasing reaction time at 323 K, 30 bar H2 and 5 mg Pt/silicalite-1.
indicating that the period required to reach steady-state
coverages of H and D on Pt surface was very short. Re-adsorption
of HD and D2 was also negligible within the reaction time, because
the large excess of H2 caused the concentration of D in the gas
phase to be still below 2 atom % after 50 min reaction time. The
desorption rates of HD and D2 were obtained from the slopes as
5.54 ± 0.28 molHD∙molPt∙s−1 and 1.71 ± 0.08 molD ∙molPt∙s−1,
2
respectively. Using these two values and Equation 2, the
desorption rate of H2 was calculated to be 5.30 ± 0.60
molH ∙molPt∙s−1. In the same way, the H2, HD and D2 desorption
2
rates at different H2 pressures and different temperatures were
determined (Supporting Information S3) and plotted in the manner
of Equation 1, as shown in Figure 1. The values of Kaօ and ka were
obtained by linear regression of the plots in this graph. For
example, linear regression of the red plot in Figure 1, which is the
reaction result with Pt/silicalite-1 at 323 K, led to a slope of 0.53
օ
and an intercept of 1.52, corresponding to Ka of 0.28 and ka of
2.3 s−1∙bar−1; then the desorption rate constant (k− ) was
a
օ
calculated to be 8.2 s−1 by division of ka by Ka . All adsorption rate
constants and equilibrium constants on Pt/silicalite-1 are
summarized in Table 1.
Table 1. Adsorption and desorption rate constants and adsorption
equilibrium constants over Pt/silicalite-1 at temperatures from 303 to
343 K. (Relative deviations ~ 8%).
Pt/silicalite-1
Temp. (K)
օ
Ka
k−a (s−1
8.2
19
)
ka (s−1∙bar−1
)
Figure 2. Adsorption isotherms of H2 on Pt at 343 K. Isotherms from
experiments correspond to H2 on Pt/silicalite-1, and isotherms from theory
correspond to H2 adsorption on Pt (111) surface. Shaded regions for isotherms
in water represent the 95% confidence interval.
303
313
323
333
343
2.3
2.3
2.2
2.4
2.4
0.28
0.12
31
0.072
0.048
0.025
50
Theoretical simulations of H2 adsorption were also carried out
on Pt (111). The interfacial system consisted of 192 Pt atoms in 4
layers (48 Pt/layer) with dimension of 19.18 × 16.61 × 6.78 Å; 25
H2 molecules within 20 Å space above the Pt surface for gas
simulation or 25 H2 + 145 H2O molecules in the same space for water
phase simulation. (a detailed model is depicted in Supporting
Information S6, Fig. S5). Figure 3 reports the Helmholtz free energy
associated with dissociative adsorption of H2 on Pt (111) in gas
and water at 343 K. The profile was created using intensive Blue-
96
Activation energy
of desorption
Activation energy
of adsorption
Heat of
adsorption
Energy
(kJ∙mol−1
(Eades
)
(Eaads
)
(Qads
)
)
51 ± 3
1 ± 2
50 ± 3
The activation energy of adsorption and desorption, as well as
the heat of adsorption, were obtained by using the Arrhenius plot
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Moon ensemble AIMD simulations (Detailed procedure reported
in Supporting Information S6). For aqueous phase simulations,
the free energy landscape is essentially flat far away from the Pt
(111) surface (z > 3 Å). At z < 3 Å, the metal-hydrogen interactions
begin to influence the H2 bonding strength and leads to dissociation
for z ≤ 2.6 Å. For gas-phase simulations, H2 dissociation is always
observed in the range we studied, indicating that in the absence of
water, the metal-H interactions can span over a longer range than that
in the presence of water. This suggests that water plays a crucial role
indicating the extent of influence the metal surface can exert on the
bonding behavior of H2 molecules. Data points in Figure 3
correspond to simulations conducted at H2 pressure of 12 bar in
gas and partial H2 pressure of 9000 bar in water. For the
energetics of H2 dissociation in water, a solvation energy
correction, ΔGsolv = −RT ln(KH) was applied for transfer of H2 from
gas to water under standard conditions (KH = 0.00078 mol/kg
bar[9]).
at high temperature and pressure, where water is present as a
liquid phase, the larger loss of entropy contributes equally or even
dominantly (Supporting Information S9).
Table 2. Adsorption thermodynamic properties of H2 on Pt.
Adsorption
enthalpy
Adsorption
entropy
Adsorption
Gibbs free
o
o
energy, 343 K.
Ha
Sa
Gao (kJ·mol−1
)
(kJ·mol−1
)
(J·mol−1∙K−1
)
Gas phase
Water phase
Difference
Gas phase
Water phase
Difference
−72 ± 5
−125 ± 18
−29 ± 3
−50 ± 3
22 ± 6
−59
−174 ± 10
−50 ± 20
−40
10.5 ± 1.1
40 ± 3
−45 ± 7
−1 ± 4
−40
−113
19
−73
44 ± 8
The H2 adsorption heat (negative value of adsorption enthalpy)
on Pt/silicalite-1 in water was measured to be 50 ± 3 kJ∙mol−1
(Table 2). The value is substantially lower than that on Pt/silicalite-
1 gas phase by 22 ± 6 kJ∙mol−1. Theoretical calculations on Pt
(111) also showed a 19 kJ∙mol−1 lower heat of adsorption in water
compared to the gas phase. On Pt (110), one of strongly H
adsorbing surfaces,[11] and on Pt50 nanoparticle supported by
graphene, the difference in the heat of adsorption was calculated
to be ~27 kJ/mol, i.e., also within the experimental range. (see
details in the Supporting Information S10). In water, the Pt surface
was covered by multilayers of (partly) oriented and structured
water molecules.[12] The adsorption of H2 on a Pt surface
competes with that of water, displacing water and disrupting or
restructuring the water layers on Pt. In a thermodynamic cycle
(Supporting Information S7), H2 adsorption on Pt in water (Rxn 3,
Figure 3. Computed Helmholtz free energy (ΔA) profile as a function of z-
distance (surface-normal) from the metal surface for dissociative adsorption of
H2 on Pt (111) in (a) gas and (b) water at 343 K. z = 0 Å denotes the surface of
the metal and z = 4.0/3.0 Å represents the gas-/aqueous-phase bulk region.
Data points in (a) correspond to a H2 pressure of 12 bar and in (b) correspond
to a partial H2 pressure of 9000 bar. H2 solvation correction is also applied in
(b).
ΔH4
=
−50 kJ∙mol−1) consists of three elementary steps: H2
dissociative adsorption on Pt (Rxn 4, ΔH4 = −72 kJ∙mol−1), D2O
desorption from Pt to liquid phase (Rxn 5, ΔH5 = 6 kJ∙mol−1) [12][13]
and solvation of adsorbed H with D2O (Rxn 6, ΔH6).
The H2 adsorption isotherms on Pt (111) in the gas phase and
in water at 343 K determined by theoretical simulations were
compared in Figure 2. Within the whole pressure range of 0.001
to 100 bar, Pt (111) is fully covered by H in the adsorption in the
gas phase. In contrast, in water Pt (111) coverage evolves from
0.1 to 0.9 in this pressure range. The partial coverage of Pt (111)
by H is in line with the experimental results by Marković et al. who
showed a 0.66 monolayer coverage of Hupd (underpotential
deposited H) at Pt (111) at a potential equivalent to an H2 pressure
of ≤ 1 bar.[10]
(Rxn 3)
(Rxn 4)
(Rxn 5)
(Rxn 6)
It should be noted here that adsorbed H is solvated by liquid
D2O and the solvation enthalpy is calculated by
Both experimental and theoretical adsorption isotherms show
that H2 adsorption on Pt in water is much weaker than that in the
gas phase. This is also reflected by the lower adsorption Gibbs
H  H  H / 2  H  5 kJmol-1
(Eq 3)

4 
6
3
5
Such an enthalpic increment is hypothesized to be contributed
by both solvating H atom by D2O, and equilibrating the redox
reaction at the established hydrogen electrode at the D2O-Pt
interface (Rxn 7).
o
free energy change (Ga ). Taking 343 K for example, H2
o
adsorption on Pt/silicalite-1 in gas phase had a Ga of −29 ± 3
kJ∙mol−1, while in water the value of Gao was 10.5 ± 1.1 kJ∙mol−1;
on Pt (111) H2 adsorption had a Gao of −45 ± 3 kJ∙mol−1, while in
water it was −1 ± 4 kJ∙mol−1 (Table 2). These results show a
destabilization of H adsorbed on Pt by the presence of liquid water.
The destabilization effect by water is attributed to both a lower
heat of adsorption and a larger entropy loss in water. At low
temperatures where most surface adsorption study of Pt was
carried out, the entropy has only a minor contribution compared
to the heat of adsorption. However, for most catalytic applications
(Rxn 7)
A brief estimation of the charge (electron) transfer in Rxn 7 by
Nernst equation shows a reduction potential of ~ -0.4 V (relative
to standard hydrogen electrode) on such a Pt electrode at 1 bar
H2 and pH neutral conditions at 343 K (Eq 4). Since such an
equilibrium in Rxn 7 depends on the HD2O+ concentration and the
state of electron in Pt, the pH in water and electric potential on Pt
would change the measured heat of adsorption of H2.
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RT [HD2O+ ]
Maybe the most surprising outcome of the simulations is the fact
that the presence of water layers reduces the mobility of H on Pt
surface. Using the mean squared displacements of H atoms at
the gas-solid and at the liquid-solid interface (Supporting
Information S6, Table S8), we show that the diffusion coefficient
of adsorbed H on Pt is significantly lower in the presence of water.
In particular, the surface diffusion coefficient of adsorbed H on Pt
(111) decreased by ~80%, from 33 × 10−5 to 7 × 10−5 cm2 s−1 in
the presence of interfacial water. The results reflect the mutual
influence between adsorbed H and water on Pt. Water at the
interface re-orders in the presence of adsorbed H, while reducing
the diffusion of adsorbed H, both effects contributing to the low
entropy of adsorbed H on Pt in water. We speculate at present
that this low mobility is associated with polar interactions and the
more positive charge atomic hydrogen adsorbed on Pt assumes
in water.
As dissociative adsorption of H2 on Pt is affected by water, the
number density of hydronium ions is hypothesized to influence
hydrogen adsorption. Indeed, H2 adsorption on Pt depended on
the concentration of hydronium ions. Comparing H-ZSM-5,
silicalite-1 and activated carbon (detailed measurement data, e.g.
adsorption constants are compiled in Supporting Information S8),
Pt/HZSM-5 shows the lowest heat of adsorption of 41 ± 3 kJ·mol−1.
With 658 µmol BAS confined in 0.16 cm3 micropores, the local
number density corresponds to ~ 4.1 mol∙L−1. The absence of
hydronium ions in silicalite-1 (although strictly it is slightly acidic
showing a point of zero charge (PZC) of 4.0 at 298 K) resulted in
an H2 adsorption heat of 50 ± 3 kJ·mol−1. In Pt/C, the activated
carbon support provides a slightly basic environment with a PZC
of 8.5 at 298 K. In line with the changes from a Brønsted acidic to
a neutral environment, in this slightly basic environment the H2
adsorption heat, 54 ± 3 kJ·mol–1 was slightly higher than for the
other two supports. The heat of adsorption of H2 (Hydrogen
Binding Energy, HBE) on Pt is sensitive to the acid-base
properties of the surrounding environments.[6, 18] For example, the
HBE of both Pt (110) and (100) surface decreased about 15
kJ·molH−1 with the pH changing from 13 to 0.[18a] The interpretation
for such a dependence of HBE on pH is still under debate in
electrochemistry: whether it is due to a pH-dependent behavior of
water at Pt-water interface[19] or due to a change in the near-
interface ions, e.g. hydroxyl species.[20] This work measured the
enthalpy of adsorbed H on Pt in the absence of electrolyte and
under a high pressure of H2 (10-50 bar). The high H2 pressure
makes the Pt surface free of hydroxyl and the absence of
electrolyte excludes the impact of cations/anions at the Pt-water
interface. Therefore, the observed dependence of the enthalpy of
adsorbed H on the hydronium ion concentration is likely the result
of restructuring/reorientation of water at the Pt-water interface.
It is hypothesized that the presence of adjacent hydronium
ions impacts the dipole orientation or dipole moment of adsorbed
water on the Pt surface,[6, 18b, 21] strengthening the adsorption of
water and consequently weakening the competitive adsorption of
H2. Other factors that may influence the water at the water-Pt
interface, i.e., metal cations, hydroxide ions, etc., may additionally
change the dissociative adsorption of H2 on Pt.[22]
(Eq 4)
E  E(SHE)

ln
F
P0.5
H2
The presence of liquid water induces a higher loss of entropy
for the H2 adsorption on Pt. The H2 adsorption entropy on
Pt/silicalite-1 and Pt (111) in water were −174 ± 10 (experiments)
and −113 J∙mol−1∙K−1 (theory), respectively, which are 50 ± 20 and
73 J∙mol −1∙K−1 lower than their counterparts at the gas-solid
interface. The larger loss of entropy is the second contribution
weakening the adsorption in water. This extra entropy loss by
water is hypothesized to be due to (i) the restructuring of water at
the water-Pt interface mediated by the surface layer of adsorbed
H, and (ii) a limitation of H mobility on Pt surface by the
surrounded water. An ice-like structure has been observed
experimentally and predicted theoretically on Pt (111) at ultrathin
layers of water at low temperature (e.g., 140 K) and high
vacuum.[14] However, theoretical calculations show that when the
Pt (111) surface was covered by hydrogen, the water layer
located at a larger distance from the Pt surface and exhibited a
greater order than the water layer on clean Pt (111).[15]
Experiments at low temperature and high vacuum showed that
co-adsorption of D2 destabilizes water on Pt (211),[16] Pt (533), Pt
(755) and Pt (977).[17] It is also seen in simulations of sorption on
Pt (111) that water layers shifted ~ 0.5 Å to a larger distance from
Pt surface in the presence of adsorbed H (Supporting Information
S6, Fig. S7).
⃗
Figure 4. Angle distribution between water dipole moment (푑) and the normal
to the surface (푛ꢀ). FWHM is estimated from Gaussian fitting.
We also examined ordering of water near the surface by
calculating the dipole moment of water molecules within the first
water layer with and without adsorbed H on Pt (111). Figure 4
shows angle distribution between water dipole and the normal to
the surface, where a peak with adsorbed H is sharper than that
without adsorbed H. The estimated full width at half maximum
(FWHM) with H* from Gaussian fitting is 27o, smaller than that
without H* (31o). These observations imply a more ordered water
structure with adsorbed H, resulting in a larger loss of entropy
compared to the situation in the absence of H*. The hydrogen
electrode like situation established by the presence of water and
adsorbed H (Rxn 7) may also help to reorient the water dipole by
the charges in HD2O+ and electron in Pt. In analogy, the presence
of an external electric field, e.g., in an electrochemical cell, would
also change the orientation of water molecules close to the Pt-
water interface, thus altering the adsorption entropy of H2.
In conclusion, the adsorption isotherm and thermodynamic
properties of H2 on Pt in water were probed by two approaches:
(i) reaction kinetics of H2 with D2O into D2 and HD over supported
Pt catalysts; (ii) theoretical simulations of the free energy change
during H2 adsorption onto Pt (111) surface in water. In contrast to
the adsorption at the gas-solid interface, which reaches saturation
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at H2 pressures below 0.02 bar at 40 – 70 oC, the H2 adsorption
in the aqueous phase is much weaker, causing that only partial
coverage is reached at H2 pressure of 1 bar in the same
temperature range. The weak adsorption of H2 in water is caused
both by a lower adsorption enthalpy and a larger loss of entropy
compared to adsorption at the gas-solid interface. Experimental
results of H2 on Pt/Silicalite-1 shows 22 ± 6 kJ·mol−1 lower heat of
adsorption and 50 ± 20 J∙mol–1∙K–1 larger entropy loss in water
compared to the gas phase; and these differences are
approximately 19 kJ·mol-1 and 73 J∙mol–1∙K–1, respectively in
theoretical calculations.
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J. Zheng, W. Sheng, Z. Zhuang, B. Xu, Y. Yan, Sci. Adv. 2016, 2,
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The adsorbed H is solvated by the liquid water and
equilibrated with hydronium ion via a redox reaction at the
established hydrogen electrode at the D2O-Pt interface, inducing
an enthalpic compensation and partially compensate the
adsorption heat. The formed surface Pt-H induces a re-orientation
of water at the interface to Pt. In turn, the presence of water layers
limits the translation mobility of the adsorbed H on Pt surface.
Both effects contribute to the significant loss of entropy in the
adsorbed state of H atoms. The acid-base environment at the Pt-
liquid interface influences the adsorption of H2. This is shown as
the decreasing of the heat of adsorption and HBE in the sequence
of Pt/C > Pt/silicalite-1 > Pt/HZSM-5 with the increase of local
acidity provided by the support near Pt in the sequence of
Pt/HZSM-5 > Pt/silicalite-1 > Pt/C.
Acknowledgements
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GY is grateful for the support by the 111Project (B17020) of China
and the postdoctoral international exchange program of China.
SA, MSL, VAG, RR, and JAL were supported by the Laboratory
Directed Research and Development program at Pacific
Northwest National Laboratory. Computational resources were
provided by PNNL’s Platform for Institutional Computing (PIC)
and the National Energy Research Scientific Computing Center
(NERSC) at Lawrence Berkeley National Laboratory.
Keywords: hydrogen • adsorption • water phase • platinum •
hydrogen binding energy
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10.1002/anie.201813958
Angewandte Chemie International Edition
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COMMUNICATION
The thermodynamic state of H2
Guoju Yang, Sneha A. Akhade, Xi Chen,
Yue Liu,* Mal-Soon Lee,* Vassiliki-
Alexandra Glezakou, Roger Rousseau,
Johannes A. Lercher*
adsorbed on Pt in the aqueous phase
was determined via kinetic analysis of
H2 reacting with D2O to HDO, HD and
D2 and by DFT based ab inito
Page No. – Page No.
molecular dynamic simulation of H2
adsorption on Pt (111). Compared to
gas-solid interface, the presence of
liquid water weakens the dissociative
adsorption of H2 on Pt by decreasing
the heat of adsorption and inducing a
larger loss of entropy.
Nature of Hydrogen Adsorption on
Platinum in the Aqueous Phase
This article is protected by copyright. All rights reserved.