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3 Catalysts for Water Oxidation (Production of Solar Fuels)

3 Catalysts for Water Oxidation (Production of Solar Fuels)

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W. Guo et al.

Fig. 13.2 X-ray crystal structure of [CuPW11 O39 ]5 -MOF-199 in combined ball-and-stick and

polyhedral representations. One of the large pores containing the yellow and grey Keggin-type

POM is shown. This POM is orientationally disordered so that the Cu atom is statistically

positioned over the 12 metal positions in the polyanion. The counterions in the smaller pores aren’t

shown for clarity. Color code: oxygen atoms are red; carbon atoms are gray; copper atoms are bluegray; the PO4 tetrahedron inside the POM is gray; and the WO6 octahedra are yellow. (Color figure


have been many reports on both homogenous [61–77] and heterogeneous WOCs

[78–89], with the number of reports of both types rapidly increasing as of mid-2012.

Homogeneous catalysts, including WOCs, are generally much faster than heterogeneous catalysts and their geometrical and electronic structures as well as the

mechanisms of their reactions can be studied both experimentally and computationally more readily and with far more precision than for their heterogeneous

counterparts. The principal advantage of heterogeneous catalysts is that they are

generally much more robust (typically metal oxides for WOCs) and more easily

prepared in quantity and at low cost. Our team’s design concept and thrust was to

use d0 metal oxide cluster anions, and in particular polytungstates, as ligands to

bind and stabilize multiple redox-active transition metals so that the latter could

be sequentially oxidized by two or hopefully the four electrons that are needed for

a viable WOC [90]. This concept was realized with the first POM-based WOC,

13 Multi-electron Transfer Catalysts for Air-Based Organic Oxidations . . .


Fig. 13.3 X-ray crystal structures of polyanions [fRu4 O4 (OH)2 (H2 O)4 g(”-SiW10 O36 )2 ]10 (Ru4 )

(top) and [Co4 (H2 O)2 (PW9 O34 )2 ]10 (Co4 ) (bottom) in combined ball-and-stick and polyhedral

notation. Color code for Ru4 : oxygen is red; ruthenium is magenta; SiO4 tetrahedra are blue; and

WO6 octahedra are gray. Color code for Co4 : oxygen is red; cobalt is blue; PO4 tetrahedra are

yellow; and WO6 octahedra are gray (Color figure online)

[fRu4 O4 (OH)2 (H2 O)4 g(”-SiW10 O36 )2 ]10 (Ru4 ), a complex that was also prepared

via a different synthetic route by the group of Marcella Bonchio in Padova, Italy

[91]. The X-ray crystal structure of Ru4 is shown in Fig. 13.3.

Since the initial papers on Ru4 , this complex has been extensively characterized and shown to function when immobilized on carbon nanotubes as a water

electrooxidation catalyst [92], in solution as a homogeneous catalyst for visiblelight-driven water oxidation [93], and when interfaced with [Ru(bpy)3]2C -sensitized

TiO2 surfaces [94]. Ru4 has shown no evidence of hydrolytic decomposition to

the metal oxides (RuO2 , WO3 ) in any of these studies. The mechanism of water

oxidation by [Ru(bpy)]3C has also been probed in some depth and the principal

catalytic cycle for water oxidation involves sequential oxidation of the resting

oxidation state of Ru4 , which is Ru(IV)4 , to the oxidation state Ru(V)4 [95],

followed by O2 evolution.


W. Guo et al.

The availability and cost of ruthenium in Ru4 compelled us to prepare and

evaluate several of the earth-abundant and inexpensive 3d metals in similar POM

structural motifs, and specifically those bearing a core of multiple 3d metals bridged

by oxygens that is sandwiched between multivalent polytungstate ligands. In 2010

we identified [Co4 (H2 O)2 (PW9 O34 )2 ]10 (Co4 ; X-ray structure in Fig. 13.3) as an

effective water oxidation catalyst that was the fastest WOC known per active site

metal at that time [96]. Subsequently, Co4 was demonstrated to catalyze efficient

H2 O oxidation by persulfate using visible light and the standard photosensitizer

[Ru(bpy)3]2C [97].

An intellectually but not necessarily practically important question pursued for

decades is whether a soluble complex is the actual catalyst or an insoluble material

(particles or film) arising from decomposition of this complex during turnover. This

quandary first surfaced on the reducing side: were soluble organometallic complexes

the catalysts or metal nanoparticles derived from breakdown of the complexes?

A range of studies and techniques surfaced to address this issue [98, 99]. Most

soluble complexes for reductive reactions (hydrogenations, metatheses, reductive

couplings, etc.) remain homogeneous catalysts but many systems do, in fact, form

metal particles that can’t often be readily detected at the outset because the particle

sizes are in the small nanometer size regime. Later this dilemma arose in context

with oxidation catalysts. Specifically, do metal oxide cluster compounds, such as

POMs, function as homogeneous oxidation catalysts, or do they simply serve as

precursors to metal oxide particles or films, which are the true catalysts?

A recent publication reported that Co4 is simply a precatalyst for a cobalt oxide

film which is the actual water oxidation catalyst under electrochemical conditions

[100]. This publication didn’t explicitly state that Co4 in our homogeneous catalytic

studies [96] was not the actual catalyst and that insoluble cobalt oxide was but

this was strongly implied. However, this implication is incorrect: our original

paper reporting homogeneous water oxidation catalyzed by Co4 was under much

different conditions than those in the subsequent study [100]. The original investigation used traditional (micromolar) concentrations of the POM WOC; whereas,

the subsequent electrochemical study used catalyst concentrations two orders of

magnitude higher. The solubility of polyoxometalates near their pH limits of

thermodynamic hydrolytic stability is extremely sensitive to metal concentration.

The original study used the soluble complex, [Ru(bpy)3]3C , as the oxidant; whereas,

the subsequent study used a highly oxidizing glassy carbon electrode as the

oxidant. Also the two studies focused on different time regimes. It should be

noted that the original study of water oxidation catalyzed by Co4 in solution

provided seven different experiments that thoroughly established the stability of

the Co4 under these conditions, including three techniques that directly ruled out

the presence of cobalt oxide particles from decomposition of Co4 during catalysis

[96]. All this data was effectively ignored in the subsequent electrochemical study.

The original study (homogeneous water oxidation catalyzed by Co4 ) has been

reproduced by other research groups (work that has yet to be published). Finally,

several other groups have used quite definitive techniques, including dynamic light

scattering (DLS), to assess the presence of particles in homogeneous water oxidation

13 Multi-electron Transfer Catalysts for Air-Based Organic Oxidations . . .


catalyzed by several different polyoxometalate complexes and none of these studies

have shown POM decomposition to form metal oxide particles or films during

catalysis [77, 101].

There is a key distinction between a soluble metal complex versus an insoluble

product (particles or films) for the reductive versus oxidative domains. For reductive

systems, generally, if not always, the metal is far more stable thermodynamically

than the soluble complex. Thus once the metal forms, that’s it; there is no going back

to homogeneous catalysis. This is NOT the case for POMs (soluble metal-oxygencluster polyanions) and their corresponding metal oxide(s). POMs and metal oxide

represent equilibrium systems in water over a wide range of pH values [102]. In

other words, there are pH ranges where the POM is thermodynamically more stable

than the metal oxide or hydroxide. Indeed, POMs are frequently made by heating

up the metal oxide or hydroxide at a suitable pH in water: the metal oxide dissolves

to form the POM [102]. This fact underlies our group’s approach to develop

multi-electron-transfer catalysts for solar fuel production because these catalysts

must be extremely stable. (Projected turnover numbers, TONs, for the catalysts

in viable solar fuel production devices range from 108 to more than 109 : : : and there

are no known catalysts at present, synthetic or biological, that persist that long.)

The dynamic behavior of metal oxides under catalytic water oxidation conditions is

reflected not only in POM-metal oxide equilibration reactions but also in the evident

equilibration chemistry of the Nocera catalyst (cobalt oxide phosphate film) under

catalytic conditions [80, 103].

Since the development of Co4 , another molecular WOC by Sun, Llobet, Privalov

and co-workers is substantially faster but it contains organic ligands that are rapidly

oxidized [104]. Most recently, a new carbon-free and thus oxidatively stable POMbased WOC has been developed in our group at Emory University. This one is more

thermodynamically stable to hydrolysis than Co4 in basic water, the desired medium

for water oxidation. This new tetra-cobalt-containing polytungstate turns over at a

rate of >1,000 O2 molecules per second at pH 9, making it the fastest WOC, at least

thus far.

But this brings one to a final point regarding solar fuel generation devices. What

matters to the research community and to society is that such devices be efficient,

fast and stable. It doesn’t matter what form the catalysts, light absorber-charge

separators or interfaces are as long as they exhibit these three attributes. If they

do, they could well be viable.

We close by returning to a point noted above and related to the theme of

this book: the oxidation of water and thus WOCs are very complicated, but

in generating viable sunlight-driven solar fuel production systems, the WOC is

only one component; interfacing the WOC with the light absorbing and charge

separating components and these in turn with multi-electron-reduction catalysts

can significantly perturb both the thermodynamics and kinetics of component steps

in the overall process. Complete solar fuel generating entities are examples of

complexity at a leading each of scientific endeavor. At present it’s not possible

to predict the overall efficiency of solar fuel generating nanostructures or devices

because the several component substructures affect each other’s properties and in

turn consequently partially control multiple charge transfer events. Complicating


W. Guo et al.

matters further, these charge transfer events themselves may not necessarily depend

linearly on the usual reaction parameters: local electronic structures, interface

properties, external reaction conditions, and others.

Acknowledgements We thank the U.S. Department of Defense (the Army Research Office and

the Defense Threat Reduction Agency (DTRA)) for funding complex catalysts for air-based

oxidations and the Department of Energy, Office of Basic Energy Sciences for funding our research

on catalytic water oxidation.


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