A growing awareness of issues
related to anthropogenic climate change and an increase in global
energy demand have made the search for viable carbon-neutral
sources of renewable energy one of the most important challenges
in science today. The chemical community is therefore seeking
efficient and inexpensive catalysts that can produce large
quantities of hydrogen gas from water. Here we identify a
molybdenum-oxo complex that can catalytically generate gaseous
hydrogen either from water at neutral pH or from sea water. This
work shows that high-valency metal-oxo species can be used to
create reduction catalysts that are robust and functional in
water, a concept that has broad implications for the design of
‘green’ and sustainable chemistry cycles.
Figure 1: Reaction of [(PY5Me2)Mo(CF3SO3)]1+ with water to form
[(PY5Me2)MoO]2+ and release H2.
* Figure 2: Cyclic voltammograms of compounds 2 and 7.
reduction step is immediately followed by or even coupled to a
proton transfer, is depicted in Supplementary Fig. 7. Author
information
Molecular Molybdenum Persulfide and Related
Catalysts for Generating Hydrogen from Water
US2012217169
Inventor:
LONG JEFFREY R [US]
CHANG CHRISTOPHER
New metal persulfido compositions of matter are described. :In one
embodiment the metal is molybdenum and the metal persulfido
complex mimics the structure and function of the triangular active
edge site fragments of MoS2, a material that is the current
industry standard for petroleum hydro desulfurization, as well as
a promising low-cost alternative to platinum for electrocatalytic
hydrogen production. This molecular [(PY5W2)MoS2]x+ containing
catalyst is capable of generating hydrogen from acidic-buffered
water or even seawater at very low overpotentials at a turnover
frequency rate in excess of 500 moles H2 per mole catalyst per
second, with a turnover number (over a 20 hour period) of at least
19,000,000 moles H2 per mole of catalyst.
BACKGROUND OF THE INVENTION
[0003] 1. Field of the Invention
[0004] This invention relates generally to a new composition of
matter and, more specifically, to a new high oxidation state
metal-oxo catalyst which can be used for generating hydrogen from
water, in one embodiment the high oxidation state metal being
molybdenum.
[0005] 2. Brief Description of the Related Art
[0006] Owing to issues of climate change and accelerating global
energy demands, the search for viable carbon-neutral sources of
renewable energy is amongst the foremost challenges in science
today. One such alternative is hydrogen, which can potentially be
used as a clean replacement for fossil fuels in many applications,
including transportation in cars, buses, trucks, trains, and
airplanes. It can further be used in fuel cells for powering
mobile devices such as lap-top computers and cell phones, as well
as for meeting power requirements in buildings and industry. Many
industries also use hydrogen as a reactant. One example is the
Haber-Bosch process that produces ammonia, which currently relies
on steam reforming of natural gas or liquefied petroleum for the
production of hydrogen. This is expensive, environmentally
unsustainable (based on finite resources of fossil fuel and
produces carbon dioxide and hydrogen sulfide, two major
atmospheric pollutants) and necessitates removal of sulfur which
deactivates the catalyst used for ammonia production. Hydrogen is
also used as a reducing agent for metal ores, for the production
of hydrochloric acid and as a hydrogenating agent for unsaturated
fats and oils.
[0007] In this context, where hydrogen has emerged as an
attractive candidate for a clean, sustainable fuel as well as a
precursor to many essential compounds, an intense interest in
creating artificial systems that utilize earth-abundant catalysts
for efficient hydrogen production from water has developed. A
major quest of this renewable energy research is the search for
efficient catalysts for the production of hydrogen from water,
which rely on cheap, earth-abundant elements.
[0008] Hydrogenase enzymes possessing earth-abundant iron and/or
nickel cofactors have been found to catalytically evolve H2 from
neutral aqueous solution at its thermodynamic potential, with
turnover frequencies of 100-10,000 mol H2/mol catalyst per second.
However, the large size and relative instability of these enzymes
under aerobic, ambient conditions has led to the search for
well-defined molecular complexes outside the biological milieu
that can produce H2 from water. Although many examples of air- and
moisture-sensitive synthetic iron-sulfur clusters have provided
insight into hydrogenase structure and reactivity, they catalyze
proton reduction from acids in organic solvents at fairly negative
potentials of -0.9 to -1.8 V vs. SHE (the Standard Hydrogen
Electrode). Metal complexes that evolve H2 at more positive
potentials still require organic acids, additives, and/or
solvents. As such, the creation of earth-abundant molecular
systems that produce H2 from water with high catalytic activity
and stability remains a significant basic scientific challenge.
BRIEF SUMMARY OF THE INVENTION
[0009] According to one aspect of this invention a new chemical
has been synthesized which achieves the goal of H2 generation
through the discovery of a well-defined organo metal-oxo complex
that catalytically generates hydrogen from water at neutral pH. In
one embodiment, the organo metal-oxo complex is an organo
molybdenum-oxo complex, which has been successfully used to
generate hydrogen for at least 3 days, with a turnover frequency
of 1.47 million mol H2/mol catalyst per hour and a turnover number
of 105 million mol H2/mol catalyst. Moreover, this same molecular
system was used to evolve H2 from seawater, the earth's most
abundant source of protons. Thus, demonstrated herein is that a
high-valent metal-oxo unit can be exploited to create reduction
catalysts that are robust and functional in water, an approach
that has broad implications for the design of green and
sustainable chemistry cycles.
[0010] The rates of hydrogen production using the organo metal-oxo
catalysts are at least one to two orders of magnitude higher than
other known molecular electro-catalysts that operate in
organic/acidic media. In addition the catalysts of this invention
are significantly cheaper than other solid state catalysts
currently in use. In the case of molybdenum, for example, its cost
is about 74 times lower than the cost of platinum, the current
preferred catalyst for hydrogen production. Unlike solid-state
catalysts such as platinum metal, the molecular catalyst of the
type described has the further advantage that it is also amenable
to structural tuning through ligand modification and metal
substitution, which may further improve production efficiencies.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] The foregoing aspects and others will be readily
appreciated by the skilled artisan from the following
description of illustrative embodiments when read in conjunction
with the accompanying drawings.
[0012] FIG. 1 is a generic, structural formula for the
organo metal-oxo complex according to an embodiment of this
invention.
[0013] FIG. 2 is a representative depiction of a possible
reaction sequence whereby hydrogen gas is evolved by the
reaction of water and a molybdenum-oxo complex according to an
embodiment of the invention.
[0014] FIG. 3 is a series of graphs depicting
electrochemical data obtained for a 7.7 [mu]M solution of
[(PY5Me2)MoO](PF6)2 in a 0.6 M phosphate buffer at pH 7.
[0015] FIG. 4 is a plot of extended electrolysis data for a
2 [mu]M solution of [(PY5Me2)MoO](PF6)2 in a 3 M pH 7 phosphate
buffer.
[0016] FIG. 5 is a plot of electrochemical data obtained
for a 4.2 [mu]M solution of [(PY5Me2)MoO](PF6)2 in 1 M KCl.
[0017] FIG. 6 is a series of graphs similar to those
illustrated in FIG. 3 for a 7.7 [mu]M solution of
[(PY5Me2)MoO](PF6)2 in sea water.
DETAILED DESCRIPTION
[0018] The preferred embodiments are illustrated in the context of
the use of a molybdenum-oxo catalyst for the generation of
hydrogen. The skilled artisan will readily appreciate, however,
that the materials and methods disclosed herein will have
application to a number of variants of this composition.
[0019] It has been discovered that a certain class of molecules
can be particularly useful as catalysts for the generation of
hydrogen gas from water. More particularly, these molecules are
salts wherein the positive moiety comprises a PY5 metal-oxo ion,
and even more particularly where the metal of the PY5 metal-oxo
ion is molybdenum. (As used herein, PY stands for pyridine and PY5
indicates the presence of five pyridyl rings). The positively
charged cations of the compositions of matter of this invention
are described by the general formula [(PY5W2)MO]<2+>,
wherein PY5W2 is (NC5XYZ)(NC5H4)4C2W2, M is a high oxidation state
metal, and W, X, Y, and Z are selected from the group comprising
H, R, a halide, CF3, or SiR3, where R is an alkyl or aryl group.
The two accompanying negative ions (i.e. the counter anion) of the
metal-oxo salt composition may be selected from any number of
anions, including a halide such as Cl<->, I<->, or
PF6<->, CF3SO3<->, and so forth. The exact composition
of the negative moiety is not significant as the anion does not
play a significant role in the water to hydrogen reaction.
[0020] A three dimensional model of the high oxidation state
metal-oxo ion of the composition of this invention is shown in
FIG. 1. Therein, central to the molecule is a metal atom, which in
the illustrated embodiment is molybdenum (Mo). The metal can also
be one of the following transition metals of the periodic table,
including Ti, V, Cr, Mn, Fe, Co, Ni, Cu, and Tungsten. Directly
bound to the metal atom is a single oxygen atom, as well as the
five pyridyl rings. For the base (or axial) pyridyl ring, the
hydrogen atoms at the, X, Y, and Z positions may be substituted
with a halogen such as F, Cl, Br, and I or a group such as R, CF3
or SiR3 where R=alkyl or aryl group. Furthermore, the
substitutions at the X, Y and Z position may be the same or
different. Finally, the group attached to the quaternary carbon at
the position may be either hydrogen, methyl, a higher alkyl, or
aryl group or any one of the halogen, CF3 or SiR3 groups listed
above. These pentapyridine ligand complexes are semi rigid, and in
their salt form easily dissolve in water.
[0021] It has been found that when placed in water, the
molybdenum-oxo salt goes into solution to form the moiety
[(PY5Me2)MoO]<2+>. The hydrogen forming reaction in one
embodiment is electrolysis driven. Herein, to initiate the
hydrogen forming reaction, a negative potential is applied to one
of two electrodes positioned within the water bath containing an
electrolyte such as sodium phosphate or potassium chloride. In an
embodiment, the negative voltage is in the order of 1.0 V to 1.4 V
versus the standard hydrogen electrode which at pH 7 corresponds
to an overpotential of between 0.6 V to 1.0 V. [As used herein,
Overpotential=(applied potential-E(pH 7)), where at pH 7E=-0.4 V.]
Upon application of the voltage, the positive metal-oxo moiety
migrates to the negative electrode where it picks up electrons.
While not intending to be bound by any particular theory regarding
the reaction sequence, it is speculated that the catalytic cycle
depicted in FIG. 2 is representative of the sequence of steps that
results in the conversion of water to hydrogen.
[0022] More particularly, the catalytic cycle of FIG. 2 depicts a
possible pathway for the generation of hydrogen from water
mediated by [(PY5Me2)MoO](PF6)2. One-electron reduction of
[(PY5Me2MoO]<2+> gives [(PY5Me2)MoO]<1+>, with the
addition of a second electron providing a putative [(PY5Me2)MoO]
species. This reduction weakens the Mo-O bond and enhances its
nucleophilicity, enabling it to deprotonate nearby water molecules
to afford the reactive intermediate [(PY5Me2)Mo(H2O)]<2+>
and release two OH<-> anions. The reduced aquo complex in
the present scheme then eliminates H2 to regenerate
[(PY5Me2)MoO]<2+>.
[0023] Observations are consistent with the foregoing
Mo<II>/Mo<IV >cycle. Under reductive catalytic
conditions, controlled potential electrolysis (CPE) of solutions
of green catalyst [(PY5Me2)MoO](PF6)2 initially turn dark yellow
and within about 10 minutes change to a purple-brown color that is
maintained for the remainder of the electrolysis. Once the
potential is switched off, the solution quickly changes back to
the dark yellow color. Chemical reduction of
[(PY5Me2)MoO]<2+> using one equivalent of sodium
naphthalenide results in a similar dark yellow solution displaying
absorption bands at 226, 264, 452, 473, and 737 nm. These data,
together with the reversibility of the first reduction wave in the
cyclic voltammogram of [(PY5Me2)MoO](PF6)2 in water, suggest that
the dark yellow complex is [(PY5Me2)MoO]<1+>. Reaction of
[(PY5Me2)MoO](PF6)2 with two equivalent of sodium naphthalenide
further affords a solution with a purple-brown color matching that
observed after extended electrolysis. Moreover, upon exposure to
air, electrolyzed solutions regenerate green
[(PY5Me2)MoO]<2+>, as verified by electronic (UV-visible
range radiation) and vibrational (infrared radiation)
spectroscopy. It is therefore believed that a PY5Me2 complex of
Mo<II >is responsible for the reductive cleavage of water to
release H2 and OH<-> ions.
[0024] A series of experiments were carried out for one member of
the catalysts of this invention, the species of the embodiment
being [(PY5Me2)MoO](PF6)2. The particular reaction was carried out
in deionized water. The results are illustrated in FIG. 3. More
specifically, the data reported in FIG. 3 are electrochemical data
for a 7.7 [mu]M solution of [(PY5Me2)MoO](PF6)2 in a 0.6 M
phosphate buffer at pH 7. The cyclic voltammograms are for a
solution with (the dark line), and without (the light line) the
catalyst at a scan rate of 50 mV/s. Plot B graphically depicts the
data for charge buildup versus time at various overpotentials.
Plot C shows turnover frequency (TOF) versus overpotential. The
background solvent activity was subtracted from plots 3B and 3C
[Overpotential=applied potential-E(pH7)].
[0025] Controlled potential electrolysis (CPE) experiments were
carried out in a double-compartment cell to assess the efficacy of
the [(PY5Me2)MoO](PF6)2 catalyst. As shown in FIG. 3B, the amount
of charge utilized in 2 min increases with increasing
overpotential until a saturation value of 0.43 C is reached at
0.64 V. This saturation behavior is due to the potential drop
between the working and auxiliary electrodes exceeding the
compliance voltage of the potentiostat at high current densities,
and is not an inherent property of the catalyst. Assuming that
every electron is used for the reduction of protons, and that all
the catalyst molecules in solution were producing hydrogen, the
TOF for the catalyst was also calculated. The TOP increases with
overpotential, reaching a maximum of 1600 mol H2/mol catalyst per
hour (FIG. 3C). Control experiments performed using Na2MoO4 or
PY5Me2 showed no catalytic activity, and no catalytic activity was
observed when fresh electrolyte was added to a used mercury
electrode. Moreover, no solid deposits were observed on the
mercury electrode, which remained shiny even after extended and
repeated electrolysis experiments.
[0026] To optimize catalytic TOP and assess the tong-term
stability of [(PY5Me2)MoO](PF6)2 as a catalyst, extended CPE
experiments were performed using a frit of greater diameter and a
higher concentration of electrolyte (3 M phosphate, pH 7) to
minimize internal resistance. Remarkably, the catalyst maintained
activity under these conditions for at least 71 h, when the
measurement was stopped because the concentration of hydroxide
ions in the working electrode compartment overcame the capacity of
the buffer. Thus, the [(PY5Me2)MoO](PF6)2 catalyst is effective
for long durations at close to neutral pH, with its durability
apparently limited only by the strength of the buffer. The data
obtained in the experiments are reported at FIG. 4, which depicts
extended electrolysis data for a 2 [mu]M solution of
[(PY5Me2)MoO](PF6)2 in a 3 M pH 7 phosphate buffer, showing charge
build-up and turnover number (TON) versus time (open circles), as
well as data for the buffer solution alone (the solid line) with
the cell operating at a potential of -1.40 V vs. SHE.
[0027] In this experiment, the current leveled out at 179 mA,
whereas a control experiment run under identical conditions, hut
without the catalyst, showed a current of just 1.1 mA. The charge
accumulated over this period, after subtracting the contribution
from the blank solution, resulted in a TON of 606,000 mol H2/mol
catalyst with a TOF of 8510 mol H2/mol catalyst per hour (FIG. 4).
It is believed these values are significantly higher than those
for other reported molecular catalysts for electrochemical
hydrogen production from neutral water, including di-nickel and
mono cobalt complexes with TONs of just 100 and 5 mol H2/mol
catalyst, respectively. Moreover, the activity of
[(PY5Me2)MoO](PF6)2 is comparable to hydrogenase enzymes on a per
volume basis, (packing together the number of catalyst molecules
needed to fill the volume of a single hydrogenase protein yields
hydrogen production rates of 1000-3400 H2 molecules/s) with far
greater stability for the former.
[0028] To test the stability of [(PY5Me2)MoO](PF6)2 in the absence
of a buffer, CPE experiments were performed in a 1 M aq. KCl
solution. Here, accumulation of hydroxide anions as H2 is
generated, leads to an increase in pH. The accumulated charge
within a given time period can be used to calculate the amount of
H2 produced, and, therefore, the concentration of OH<-> ions
in solution. FIG. 5A plots charge build-up over time at an applied
potential of -1.40 V vs. SHE. The linear fit of the data (where
the charge vs. time slope {y=0.42x}) evidences current is constant
during the measurement (i.e. the catalytic activity is constant,
with no decomposition occurring). FIG. 5B, plots the measured
change in solution pH with time during electrolysis (squares) and
the calculated change in pH assuming the catalyst performs at
Faradaic efficiency (continuous line) is plotted. Notably, the
agreement between calculated and observed pH changes during a
60-min electrolysis, establishes that the catalyst indeed operates
at Faradaic efficiency. Mass spectrometry studies indicate a
reduced stability for [(PY5Me2)MoO]<2+> at high pH, with a
significant dissociation of the molybdenum center from the PY5Me2
ligand occurring above pH 12.
[0029] With data showing that the catalyst can tolerate impurities
and still show activity in water, performance was evaluated in
seawater, the earth's most abundant proton source (FIG. 6). Upon
adding [(PY5Me2)MoO](PF6)2 to a sample of California seawater with
no added electrolyte, the onset of catalytic current was observed
at ea, -0.81 V vs. SHE. In the absence of [(PY5Me2)MoO](PF6)2, a
catalytic current was not apparent until a potential of -1.60 V
was attained. To obtain an accurate blank subtraction for
controlled potential electrolysis (CPE), the charge generated from
seawater atone was subtracted from the charge generated from the
catalyst solution at the same overpotential, as determined from
the solution pH at the end of the electrolysis. CPE experiments
performed for short durations in seawater were remarkably similar
to the results obtained in pH 7 buffered water. The current
saturated at 0.32 C at an applied potential of -1.40 V vs. SHE,
corresponding to a turnover frequency of 1225 mol H2/mol catalyst
per hour at an overpotential of 0.78 V. The background solvent
activity was subtracted from plots 6B and 6C
[Overpotential=applied potential-E(pH at the end of the
electrolysis)].
[0030] An exemplary synthesis route for the obtaining of
[(PY5Me2)MoO](PF6)2 is set forth in the following paragraphs. All
chemical synthese were conducted under strictly air and
moisture-free conditions using standard glove-box and Schlenk-line
techniques, unless otherwise noted. The compound MoI2(CO)3(MeCN)2
was synthesized as described in Baker, P. K., Fraser, S. G. Keys,
E. M., "The synthesis and spectral properties of some highly
reactive new seven-coordinate molybdenum(II) and tungsten(II)
bisacetonitrile dihalogenotricarbonyl complexes", J. Organomet,
Chem. 309, 319-321 (1986). The compound PY5Me2, was synthesized as
described in Canty, A. J., Minchin, N. J., Skelton, B. &
White, A. H., "Interaction of Palladium (II) Acetate with
Substituted Pyridines, Including a Cyclometalation Reaction and
the Structure of [Pd{meso-[(py)PhMeC]2-O5H3N}(O2CMe)][O2CMe]3H2O",
J. Chem. Soc., Dalton Trans. 10, 2205-2210 (1986). All other
reagents were purchased from commercial vendors and used without
further purification. Electronic grade Hg (99.9998%), and platinum
gauze were purchased from Alfa Aesar for the electrochemical
studies. Toluene, acetonitrile and diethylether were dried and
degassed using the VAC 103991 solvent system and stored over 3-A
molecular sieves under a nitrogen atmosphere. Water was deionized
with the Millipore Milli-Q UF Plus system.
[0031] The precursor [(PY5Me2)MoI]I2 was first synthesized. Solid
PY5Me2 (200 mg, 0.45 mmol) was added to a 20-mL toluene solution
of MoI2(CO)3(MeCN)2 (350 mg, 0.67 mmol) and the mixture was heated
at reflux for 3 days. The solution was then cooled to room
temperature and filtered to afford an orange solid, which was
washed repeatedly with toluene until the filtrate was colorless.
The solid was then extracted into 50 mL of acetonitrile, layered
with 30 mL of diethylether, and allowed to stand for 2 days to
yield orange rod-shaped crystals. The crystals were washed with 20
mL of diethylether to give a combined yield of 360 mg (87% with
respect to PY5Me2).
[0032] [(PY5Me2)MoO]I2: Solid (PY5Me2)MoI]I2 (620 mg, 0.67 mmol)
was added to 40 mL of deionized water and stirred in air for one
day to give a green suspension. The water was then removed under
reduced pressure and the green solid was washed with cold (0[deg.]
C.) acetonitrile until the color of the filtrate changed from
brown to green. The solid was then washed with 20 mL of
diethylether to yield 470 mg (85%) of product.
[0033] [(PY5Me2)MoO](PF6)2: A 20-mL acetonitrile solution of
TI(PF6) (401 mg, 1.15 mmol) was added drop wise to a stirred 20-mL
acetonitrile solution of [(PY5Me2)MoO]I2 (465 mg, 0.575 mmol), and
the mixture was stirred in air for 12 h. The solution was cooled
to 0[deg.] C. and filtered to remove the yellow thallium iodide,
and the bright green filtrate was concentrated to a volume of 20
mL under reduced pressure. Diffusion of diethylether vapor into
this solution over the course of 3 days afforded 484 mg (95.0%) of
product as green rod-shaped crystals.
[0034] Electrochemical studies employed a mercury pool working
electrode with a surface area of 19.6 cm<2>, which was
stirred constantly during the CPE experiments. Electrical contact
to the mercury pool was achieved through a platinum wire that
remained immersed below the surface of the mercury, thereby
avoiding contact with the solution, A 20.5 cm<2 >platinum
gauze (52 mesh, woven from 0.1 mm diameter wire) was utilized as
the auxiliary electrode and was separated from the solution in the
working electrode compartment by a medium-porosity sintered-glass
frit. The reference electrode was a commercially available aqueous
Ag/AgCl electrode, which was positioned within 5 mm of the working
electrode, and the potentials are reported with respect to SHE by
adding 0.195 V to the experimentally obtained values. The working
electrode compartment contained 5-100 mL of electrolyte solution
which was thoroughly sparged and kept under a blanket of
water-saturated nitrogen during the experiments. A 0.6 M pH 7
phosphate buffer was used as electrolyte. Extended electrolyses of
greater than 1 h were conducted in a larger cell containing 170 mL
of 3 M pH 7 phosphate buffer in each compartment. The solutions in
both compartments were vigorously stirred during the electrolysis.
Sea water was obtained from Ocean Beach, San Francisco, and was
passed through a course paper fitter prior to use in order to
remove any particulate matter, iR compensation was employed for
all experiments to account for the voltage drop between the
reference and working electrodes using the BAS CV-50W software.
[0035] A mercury electrode was used in this study to reduce
background activity of direct water reduction at the electrode at
the high overpotentials needed to evaluate the catalyst. For
catalysts which require lower overpotentials, other electrodes
such as graphite and steel may be used.
[0036] It is to be noted that the turnover numbers and turnover
frequencies reported in the text and figures of this application
were generated prior to the filing of the referenced Provisional
application, and these initial results were published thereafter
(H. I. Karunadasa, C. J. Chang, J. R. Long, Nature 464, 1329
(2010)). At the time, it was assumed that all the metal-oxo
catalyst molecules in solution contributed toward hydrogen
generation. It was subsequently determined that only the metal-oxo
catalyst molecules adsorbed on the surface of the mercury
electrode of the experiments were catalytically active. Using
cyclic voltammetry, the surface coverage of the catalyst molecules
on the electrode was calculated to be ca. 10<-10
>mols/cm<2 >(according to methods detailed in A. J. Bard,
L. R. Faulkner, Electrochemical Methods, Wiley, New York, 1980).
By following this approach, we were able to derive a more accurate
estimate of the rates of hydrogen production, which are about 2
orders of magnitude greater than that earlier reported, and
published. For example, in the extended electrolysis the turnover
frequency was first reported as 8510 mols H2/mol catalyst per h,
when in fact (applying the new calculus) it was actually about
1,500,000 mol H2/mol catalyst per h. Similarly, the reported
turnover number of 606,000 mol H2/mol catalyst is more accurately
about 105,000,000 mol H2/mol catalyst. These higher rates are
comparable to those of hydrogenase enzymes on a per molecule
basis, while packing together the number of catalyst molecules
needed to fill the volume of a single hydrogenase protein yields
hydrogen production rates of 200,000-600,000 H2 molecules.
[0037] The discovery of a molecular molybdenum-oxo catalyst for
generating hydrogen from water without use of additional acids
and/or organic co-solvents establishes a new chemical paradigm for
creating reduction catalysts that are highly active and robust in
aqueous media. Importantly, the system employs an inexpensive,
earth-abundant metal to achieve catalytic H2 evolution from
neutral buffered water or even seawater, maintaining long-term
activity with TOF and TON values of 1.5 million mol H2/mol
catalyst per hour and 105 million mol H2/mol catalyst,
respectively. An overpotential of between 0.6 V to 1.0 V at the
cathode leads to an efficiency of 67%-55% respectively for the
cell, assuming that the rest of the cell operates at ideal
efficiencies. The total voltage necessary for the cell depends on
the reaction at the anode and well as the internal resistance of
the cell which depends heavily on cell design, which does not
constitute a part of this invention.
[0038] This invention has been described herein in considerable
detail to provide those skilled in the art with information
relevant to apply the novel principles and to construct and use
such specialized components as are required. However, it is to be
understood that the invention can be carried out by different
equipment, materials and devices, and that various modifications,
both as to the equipment and operating procedures, can be
accomplished without departing from the scope of the invention
itself.
MOLECULAR METAL-OXO CATALYSTS FOR
GENERATING HYDROGEN FROM WATER
WO2011043893 / US2012228152
Abstract
A composition of matter suitable for the generation of hydrogen
from water is described, the positively charged cation of the
composition having the general formula [(PY5 W2)MO]2+, wherein
PY5W2 is (NC5XYZ)(NC5H4)C2W2, M is a transition metal, and W, X,
Y, and Z can be H, R, a halide, CF3, or SiR3, where R can be an
alkyl or aryl group. The two accompanying counter anions, in one
embodiment, can be selected from the following Cl', I', PF6", and
CF3SO3". In embodiments of the invention, water, such as tap water
containing electrolyte or straight sea water can be subject to an
electric potential of between 1.0 V and 1.4 V relative to the
standard hydrogen electrode, which at pH 7 corresponds to an
overpotential of 0.6 to 1.0 V, with the result being, among other
things, the generation of hydrogen with an optimal turnover
frequency of ca. 1.5 million mol H2/mol catalyst per h.
