H-Spillover through the Catalyst Saturation: An Ab Initio Thermodynamics Study

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Hydrogen is a carrier of clean
energy1,2 and can be easily generated
from renewable sources. A
cost-effective, safe, and efficient storage
medium is the key to utilize its full potential.
Among the various possibilities, the
carbon-based adsorbents3
8 are recognized
as strong candidates, where large surface
area and lighter weight make the substantial
volumetric and gravimetric content possible.
Furthermore, the storage capacity of
graphitic materials (e.g., nanotubes and
fullerenes) can be significantly enhanced
by decorating them with metal atoms,9
14
which absorb multiple H2 molecules via
Kubas interaction.15 Although promising,
the experimental efforts in synthesizing the
metal-decorated nanotubes and fullerenes
have not been successful so far. Additionally,
the tendency of metal atoms to
cluster16,17 leads to considerable reduction
in potential storage capacity. In contrast,
the metal cluster supported on graphitic
materials acts as a catalyst and enhances
the hydrogen uptake of substrates via
spillover.18
21
The spillover process involves the transport
of an active species (e.g., H) formed on
a catalyst onto a receptor that does not sorb
the species22 under the same conditions.
Current growing interest in efficient storage
of hydrogen brought this long-known
phenomenon into the spotlight. The most
widely used catalysts for spillover of H atoms
on graphitic materials are Ni, Pd, Pt,
and other transition metal atoms. Recently,
several experiments have shown the enhancement
of H2 adsorption via spillover
on activated carbons and MOF.19,23
25 Up
to 4 wt % of adsorption has been reported
for IRMOF-8 at 298 K and 10 MPa.26 Furthermore,
it is empirically established that the
spillover can be enhanced by adding socalled
bridges24 between the catalyst and
receptor. Although exact distribution and
the binding sites of the H remain experimentally
unspecified, it is reasonable to
suggest that the best coverage of the H on
graphitic substrates can be achieved when
they are hydrogenated on both sides,27,28
and spillover is considered as a possible
path to achieving it. Thus, fully hydrogenated
graphene would have stoichiometry
CH, with 7.7 wt%of hydrogen,27
30 meeting
the DOE goals. Even though the spillover
of the H on graphitic surfaces was observed
decades ago,22 it is still not well-understood
how a H binds to graphene when it seems
energetically more favorable to stay on the
catalyst or even to remain in a molecular H2
form in the gas phase. To better understand
the spillover mechanism, with the
goal to optimize its kinetics, it is important
to compare the relative energy states of the
hydrogen in its (i) dihydrogen gas form, (ii)
at the metal catalyst, and (iii) on the receptor
substrate (Figure 1). The energy states
available for H will depend on the degree of
saturation or substrate coverage.
*Address correspondence to
biy@rice.edu.
Received for review November 6, 2008
and accepted June 10, 2009.
10.1021/nn9004044 CCC: $40.75
© XXXX American Chemical Society
ABSTRACT The spillover phenomenon, which essentially involves transfer of H from a metal catalyst to a
graphitic receptor, has been considered promising for efficient hydrogen storage. An open question about the
spillover mechanism is how a H atom binds to graphene instead of forming the thermodynamically preferred H2.
Using ab initio calculations, we show that the catalyst saturation provides a way to the adsorption of hydrogen on
the receptor by increasing the H chemical potential to a spillover favorable range. Although it is energetically
unfavorable for the spillover to occur on a pristine graphene surface, presence of a phase of hydrogenated
graphene facilitates the spillover by significantly improving the C
H binding. We show that thermodynamic
spillover can occur, both from the free-standing and from the receptor-supported clusters. Further, the computed
energy barrier of the motion of a H from the catalyst to the hydrogenated graphene is small (0.7 eV) and can be
overcome at operational temperatures.
KEYWORDS: hydrogen storage · spillover · catalysis · graphene · ab initio
thermodynamics
ARTICLE
www.acsnano.org VOL. XXX ▪ NO. XX ▪ 000–000 ▪ XXXX A
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Published on June 17, 2009 on http://pubs.acs.org | doi: 10.1021/nn9004044
Indeed, Cheng et al. have studied31,32 the dissociative
chemisorption of molecular hydrogen and desorption
of atomic hydrogen on Pt clusters and concluded
that the number of adsorbed H2 increases with the increasing
size of the cluster. Furthermore, they show that
the binding strengths decrease with the increasing coverage,
that is, energy states available for H raise, approximately
representing the increase of the chemical
potential H since the entropy contribution is less significant.
They find that in low coverage the adsorption
strengths are very large and at the saturation level are
closer to the energies on a fully H-covered Pt(111)
surface.31,32 In contrast, the strength of a H binding to
sp2-carbon receptor is shown to increase with the
greater coverage, due to its clustering and CH phase
formation.30 This analysis reconciles the fact of spillover
with too weak binding of the H to the bare substrate,
by stressing the role of nucleation of condensed CH
phase, which must be forming in the process of spillover
on a graphene receptor, as it is more favorable
than a H2 molecule.30 In our previous study, the catalyst
per se was not considered, and the focus was on the
variation of the hydrogen binding to the receptor and
its thermodynamic comparison with gaseous H2.
Here we report the details of hydrogen binding to
the catalyst particle, which serves as a gateway to the
entire process and whose saturation is also an important
aspect of spillover. Combining molecular dihydrogen
gas phase, H dissolved on the catalyst, and H in the
“storage phase” on the receptor, a conceptual quantitative
diagram of spillover is drawn in Figure 1. On the
left, the blue line marks the energy of H in its molecular
form and the additional broad (also blue) range is
the chemical potential of H including the entropic contribution
at different gas conditions. On the right
side, a family of thin dark-blue lines corresponds to
the energies of H bound to graphene, which vary
with the size and the configuration of the cluster island30
and converge to the CH phase energy. The
midsection pink block shows the range of energies
of H at the catalyst as computed and analyzed below.
The first H2 molecule dissociates and binds to
the catalyst rather strongly; and therefore, H lies
deep in this picture. However, the energies of the
subsequent H2 binding gradually decrease, raising
the H. For the spillover of a H to occur from the
metal, the H must exceed the CH state energy level
shown by the gray line, before the metal cluster
saturates (i.e., becomes unable to further accept
new H2 molecules). The catalyst plays an important
role in bringing the H into this range. Possible
metal-hydride phase formation imposes an additional
constraint on the H. The former must lie
above the H of the receptor to avoid formation of
metal-hydride before the spillover (assuming that
the hydride phase would inhibit the catalytic activity).
In this work, we validate this model by exploring
through ab initio computations the gradual energy
change of H on the catalyst to reveal how it fits between
the energy on the receptor and as free gas. Comparing
these H values identifies the range of chemical potential
favorable for the spillover. The role of catalyst saturation
and binding strength of H with the receptor in
bringing the H in this desirable range is explored. Furthermore,
to understand the first kinetic step, the barrier
involved in the motion of a H atom from catalyst to
receptor is computed and compared with the experimental
observation