Carbon oxidation to enhance hydrogen storage by
hydrogen spillover
Qixiu Li, Angela D. Lueking
Department of Energy and Mineral Engineering, The Pennsylvania
State University, PA-16801, USA
Introduction
Several recent studies have established that small amounts of
transition metals may activate carbon nanomaterials for hydrogen
adsorption.1-6 The activation has been explained by Lueking and
Yang with the concept of hydrogen spillover,1 and by Dillon, Heben,
et al. by the formation of ligands for well-dispersed metals on
carbon.7 Ab initio molecular simulation studies of Fe-doped C60
suggest that application of hydrogen to doped nanocarbons may
induce defects that lead to additional carbon adsorption sites with a
hydrogen binding energy between that of classical physisorption and
chemical adsorption.8 The binding energy for spilled over hydrogen
for certain sites on a graphene sheet has been theoretically predicted
to be in the range of 24 kJ/mole,9 which is in the DOE desired range
for mobile applications of 10-50 kJ/mol. The binding energy
translates to a moderate operating temperature for hydrogen
adsorption.
Experimentally, Lueking and Yang have shown hydrogen
spillover to increase the uptake of a multi-wall carbon nanotube
(MWNT) by up to 40%;1 high-pressure studies with this material
showed adsorption and desorption to be 3.7% and 3.6% hydrogen by
weight at 69 bar and 300 K.2 Temperature-programmed desorption
showed a secondary low temperature desorption peak that was
attributed to hydrogen spilled over to the carbon surface. Subsequent
studies by Yang et al. have shown carbon bridges formed between a
primary transition metal catalyst (i.e. either Pd or Pt on activated
carbon) and a secondary high surface area support will further
increase uptake.10-12 Formation of carbon bridges via carbonization
increased hydrogen adsorption by a factor of 2.9 for AX-21 activated
carbon and 1.6 for SWNT at 298 K and 100 kPa, and an
enhancement was also observed at pressures up to 10 MPa. Yang et
al. then showed the overall uptake of a carbonized Pt/AC and MOF-5
mixture was 4% at 298 K and 100 bar.11 The formation of carbon
bridges enhanced the hydrogen uptake of the MOF-5 by a factor of
3.3 at the given conditions. Work by other groups has also shown
increased uptake in carbon materials after metal-doping.13-15
Many of these experimental hydrogen spillover studies rely on a
transition metal supported on activated carbon (i.e. Pt/AC) or a
nanocarbon (Pt/NC) as the primary source of atomic hydrogen. This
primary catalytic ‘hydrogen source’ is then mixed with a high surface
area material that acts as a hydrogen receptor. The catalytic source
increases the hydrogen uptake of the secondary receptor and
increases the operative adsorption temperature. These “secondary
spillover” studies have the advantage that they provide a constant
hydrogen source, allowing modification of the receptor sites without
compromising or altering the hydrogen supply. Here we use
secondary spillover studies to explore the role of oxidation of a high
surface area carbon support in enhancing hydrogen spillover.
Oxidation is expected to provide surface sites that may act as specific
chemical receptors for spilled over hydrogen, while also altering the
porosity of the carbon material.
Experimental
Synthesis. The KOH modified Activated Carbon (neutral) is
prepared according to [16]. In brief, a high surface area activated
carbon (AC, The Kansai Coke & Chemicals Co.) is impregnated with
a 1 M KOH (EMD Chemicals, Inc.) solution by magnetically stirring
for 12 or 24 h (denoted as AC_KOH_12hrs and AC_KOH_24hrs) at
room temperature at a KOH:AC ratio of 4:1. The KOH-modified AC
was washed to neutral, filtered, and dried at 373 K. AC(pH9) was
prepared under similar conditions but washed to pH=9 (tested by pH
paper), then dried at 373 K.
A H2O2 oxidized activated carbon was prepared according to the
method in [17]. In brief, AC was stirred with 10 mL of 30% H2O2
(VWR) for 4 or 6 days (denoted as AC_H2O2_4d and AC_ H2O2_6d
) at room temperature. To keep sufficient concentration of H2O2, 5
ML of H2O2 was added to the mixture every day. The oxidized AC
was washed, filtered to neutral, then dried overnight in the oven.
The various oxygenated ACs are doped with either 1% Pt-C or 5
% Pt-C (STEM chemicals) by grinding with an agate mortar and
pestle for 30 min to produce physical mixtures. The ratio of AC to
Pt-C was fixed at 9:1. After grinding, the mixtures were calcined in
flowing Ar at 673 K for 2hrs in a quartz tube furnace, after the
method of [10]. The samples are denoted as 1% Pt-C+AC (pH9), 5%
Pt-C+AC (pH9), 5% Pt-C+AC (H2O2_4d), and 5% Pt-C+AC
(H2O2_6d).
Characterization. BET specific surface area and pore size
distribution
(PSD)
were
calculated
from
nitrogen
adsorption/desorption isotherms at 77K using Micromeritics ASAP
2010. Before each test, the samples were vacuumed for 12 hours at
393 K. The analysis model used for PSD is DFT slit pore model.
On-going tests (to be presented at the conference) include: (1)
XRD tests to show the changes in structure of carbon samples before
and after treatment by KOH and H2O2; (2) XPS technique to obtain
the chemical bonding information on the carbon surface after the
modification; and (3) TPD to see the thermal desorption of oxygen
functional groups in carbons (CO or CO2).
Hydrogen Adsorption. Hydrogen adsorption isotherms were
conducted on an Intelligent gravimetric analyzer (IGA)-003 (Hiden
Isochema) in static mode at pressures from 0-20 bar and a
temperature of 294 K. Sample buoyancy corrections were made by
using He density measurements of the sample after the hydrogen
isotherms were complete. Prior to measurement the samples were
pretreated in flow mode using methods similar to those of Srinivas
and Rao18: the samples were reduced in hydrogen (50 ml/min) at 523
K for 6 h and degassing in ultrahigh vacuum (10 -6 mbar) at 673 K for
a minimum of 8 h. Ultra-high-purity hydrogen (99.999%) and
helium (99.999%) were used for all pretreatments and measurements.
Molecular sieve 3A purifiers were used on each gas stream to ensure
purity was maintained in all experiments. On-going tests (to be
presented at the conference) include: temperature-programmed
desorption to assess activation energy of hydrogen desorption from
the surface and temperature desorption spectroscopy to ensure the
high hydrogen adsorption is not due to artifacts.
Results and Discussion
Effect of Oxidation on Carbon Structure. The BET surface
area of original AC, KOH activated AC (12 h) and KOH activated
AC (24 h) are 3145 m2/g, 2720 m2/g and 2270 m2/g, respectively.
The surface areas measured from the DFT model were 1872; 1888;
and 1485 (m2/g), respectively (Fig. 1b). (The DFT model may
provide a more accurate estimate of surface area for materials with
significant microporosity, however DFT tends to underestimate
compared to NLDFT.) The pore size distribution of original AC and
KOH modified AC are shown in Fig.1. KOH treatment shifts the
PSD to lower pore widths of around 5 Å (ultramicropores). The
contribution of pores located in the 10-15 Å range (micropores) to
the total surface area is decreased after KOH treatment at different
activation time. KOH treatment increased ultramicroporosity and
decreased the surface area of AC by shifting the PSD from
Prepr. Pap.-Am. Chem. Soc., Div. Fuel Chem. 2008, 53 (1), xxxx
micropores to ultramicropores, the former had higher total surface
area as shown by Fig 1b.
Figure 1. Incremental (a) and cumulative (b) pore size distribution
of KOH modified AC. The cumulative (b) PSD shows how
ultramicroporosity may be introduced without a significant alteration
(or even a decrease) of surface area.
Figure 2. Hydrogen adsorption isotherms of original AC, 1%PtC+AC (pH9) and 5%Pt-C+AC (pH9) at 21oC
Hydrogen Adsorption. The hydrogen adsorption capacity of the
original AC, the physical mixture of 1% Pt-C and AC (pH9), and
mixture of 5% Pt-C and AC (pH9) are shown in Fig.2. Obvious
enhancement in storage on 1%Pt-C+AC (pH9) is achieved by just
using basic AC samples. Even higher storage capacity is obtained
when higher metal content catalyst is used. Using the BensonBoudart extrapolation method to assess ‘zero pressure’ spillover, the
H:Pt(total) ratio for 1% and 5% Pt-C+AC(pH9) are 287 and 220,
respectively. A H:Pt(total) greater than 1 is a clear indication of
hydrogen spillover, particularly since the normalization is based on
total Pt atoms rather than surface Pt atoms which are expected to bind
one hydrogen atom. The preparation method is based on studies that
unequivocally show that oxygen groups are introduced by the
treatments; on-going characterizations are quantifying various
functional groups of the materials. On-going work is exploring the
relative effects of introduced oxygen functional groups versus
alterations in ultramicroporosity in hydrogen spillover.
Discussion. The results suggest a strategy for enhancing
hydrogen storage via hydrogen spillover. However, the effects of
porosity versus the introduction of oxygen groups are not clear, and
are the subject of on-going work. Clearly, the greatest effect for the
treatment is realized at low pressure, consistent with the traditional
concept of hydrogen spillover in the catalysis literature, in which
most experiments are performed at pressures less than 1 bar.
Metal-doped carbon systems remain a key candidate for
moderate temperature hydrogen adsorption, although the mechanism
for hydrogen adsorption is not well understood. The highest
experimental uptakes thus far have been reported by Yang et al.: 4%
at 298 K and 100 bar.11 The authors use the hydrogen spillover
hypothesis to account for this uptake at moderate temperature
conditions, and further imply the material may meet DOE hydrogen
storage targets with moderate increases in pressure above 100 bar. An
implication of this statement is that the uptake of these materials has
not plateaued at 100 bar. This would further imply that hydrogen
spillover is a function of pressure or that physisorption increases
hydrogen storage at high pressures, possibly due to surface
modifications induced by Pt or spilled over hydrogen. The former
pressure functionality of hydrogen spillover is often thought to be
proportional to P0.5, based on a Langmuirian analysis. However, it is
not clear how the surface diffusion of hydrogen from the metal to
support via hydrogen spillover would retain this relationship as the
P0.5 relationship is derived for hydrogen chemisorption to the metal
and not the subsequent surface diffusion process. The similarity in
isotherm slope of AC to the metal-doped materials in Fig. 2 suggest
the pressure functionality of materials exhibiting hydrogen spillover
is dictated by the pressure relationship of the receptor rather than the
pressure relationship of the metal. Our analysis and model of the
pressure-dependence of hydrogen spillover, supporting experimental
data, and the implications of low-pressure hydrogen spillover for
hydrogen storage will be discussed.
The key advantage of metal-carbon materials is an increased
adsorption temperature due to high binding energies relative to high
surface area materials that rely on physisorption. The results
presented here suggest a further advantage if the uptake of the
materials can be shifted to low pressure.
On-going work includes further characterization of the materials
to develop a mechanistic understanding of the structural features that
lead to the enhanced hydrogen spillover and hydrogen adsorption,
including temperature-programmed desorption tests (with mass
spectroscopy) to calculate the binding energy between H2 and the
surface.
Conclusions
Oxidation of carbon materials alters the porosity of the carbon
material, leading to enhancements in ultramicroporosity. The
oxidation process is based on known methods to introduce oxygen
functional groups. The results presented here show a significant
enhancement in low-pressure hydrogen spillover and storage.
Shifting the hydrogen storage to low pressure may have important
implications for DOE hydrogen storage goals. Forthcoming work
will also explore the relative roles of porosity versus surface
chemistry in the observed low-pressure enhancement of hydrogen
storage via spillover.
Acknowledgement. This work was funded by the Department
of Energy’s University Coal Research Program administered through
the National Energy Technology Laboratory and the Pennsylvania
State University.
Prepr. Pap.-Am. Chem. Soc., Div. Fuel Chem. 2008, 53 (1), xxxx
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