Global energy consumption is projected to increase, even in the face of substantial declines in energy intensity, at least 2-fold by midcentury relative to the present because of population and economic growth. This demand could be met, in principle, from fossil energy resources, particularly coal. However, the cumulative nature of CO2 emissions in the atmosphere demands that holding atmospheric CO2 levels to even twice their preanthropogenic values by midcentury will require invention, development, and deployment of schemes for carbon-neutral energy production on a scale commensurate with, or larger than, the entire present-day energy supply from all sources combined. Among renewable energy resources, solar energy is by far the largest exploitable resource, providing more energy in 1 hour to the earth than all of the energy consumed by humans in an entire year. In view of the intermittency of insolation, if solar energy is to be a major primary energy source, it must be stored and dispatched on demand to the end user. An especially attractive approach is to store solar-converted energy in the form of chemical bonds, i.e., in a photosynthetic process at a year-round average efficiency significantly higher than current plants or algae, to reduce land-area requirements. Scientific challenges involved with this process include schemes to capture and convert solar energy and then store the energy in the form of chemical bonds, producing oxygen from water and a reduced fuel such as hydrogen, methane, methanol, or other hydrocarbon species.

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Powering the planet: Chemical challenges in solar energy utilization

The interest in nanoscale materials stems from the fact that new properties are acquired at this length scale and, equally important, that these properties * To whom correspondence should be addressed. Phone, 404-8940292; fax, 404-894-0294; e-mail, mostafa.el-sayed@ chemistry.gatech.edu. † Case Western Reserve UniversitysMillis 2258. ‡ Phone, 216-368-5918; fax, 216-368-3006; e-mail, burda@case.edu. § Georgia Institute of Technology. 1025 Chem. Rev. 2005, 105, 1025−1102

Chemistry and properties of nanocrystals of different shapes.

The mass production of proton exchange membrane (PEM) fuel-cell-powered light-duty vehicles requires a reduction in the amount of Pt presently used in fuel cells. This paper quantifies the activities and voltage loss modes for state-of-the-art MEAs (membrane electrode assemblies), specifies performance goals needed for automotive application, and provides benchmark oxygen reduction activities for state-of-the-art platinum electrocatalysts using two different testing procedures to clearly establish the relative merit of candidate catalysts. A pathway to meet the automotive goals is charted, involving the further development of durable, high-activity Pt-alloy catalysts. The history, status in recent experiments, and prospects for Pt-alloy cathode catalysts are reviewed. The performance that would be needed for a cost-free non-Pt catalyst is defined quantitatively, and the behaviors of several published non-Pt catalyst systems (and logical extensions thereof), are compared to these requirements. Critical research topics are listed for the Pt-alloy catalysts, which appear to represent the most likely route to automotive fuel cells.

Activity benchmarks and requirements for Pt, Pt-alloy, and non-Pt oxygen reduction catalysts for PEMFCs

5.1. Nanoalloys of Group 11 (Cu, Ag, Au) 865 5.1.1. Cu−Ag 866 5.1.2. Cu−Au 867 5.1.3. Ag−Au 870 5.1.4. Cu−Ag−Au 872 5.2. Nanoalloys of Group 10 (Ni, Pd, Pt) 872 5.2.1. Ni−Pd 872 * To whom correspondence should be addressed. Phone: +39010 3536214. Fax:+39010 311066. E-mail: ferrando@fisica.unige.it. † Universita di Genova. ‡ Argonne National Laboratory. § University of Birmingham. | As of October 1, 2007, Chemical Sciences and Engineering Division. Volume 108, Number 3

Nanoalloys: From Theory to Applications of Alloy Clusters and Nanoparticles

The recent advances in electrocatalysis for oxygen reduction reaction (ORR) for proton exchange membrane fuel cells (PEMFCs) are thoroughly reviewed. This comprehensive Review focuses on the low- and non-platinum electrocatalysts including advanced platinum alloys, core–shell structures, palladium-based catalysts, metal oxides and chalcogenides, carbon-based non-noble metal catalysts, and metal-free catalysts. The recent development of ORR electrocatalysts with novel structures and compositions is highlighted. The understandings of the correlation between the activity and the shape, size, composition, and synthesis method are summarized. For the carbon-based materials, their performance and stability in fuel cells and comparisons with those of platinum are documented. The research directions as well as perspectives on the further development of more active and less expensive electrocatalysts are provided.

http://repository.ust.hk/ir/bitstream/1783.1-77094/1/acs.chemrev.5b00462_withlink.pdf

Recent Advances in Electrocatalysts for Oxygen Reduction Reaction

This tutorial review is aimed at chemical scientists interested in understanding and exploiting the remarkable catalytic behavior of the hydrogenases. The key structural features are analyzed for the active sites of the two most important hydrogenases. Reactivity is emphasized, focusing on mechanism and catalysis. Through this analysis, gaps are identified in the synthesis of functional replicas of these fascinating and potentially useful enzymes.

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Small molecule mimics of hydrogenases: hydrides and redox

Homogeneous catalysts for proton reduction 1 are of interest because they are amenable to systematic manipulation, and they represent viable precursors to tailored heterogeneous catalysts, including those using more economically attractive base metals such as Fe. Hydrogenase enzymes represent a structurally unusual but highly efficient hydrogen-processing catalysts that rely on base metals (Ni, Fe). 2 The structures of both major families of hydrogenase enzymes, the Fe-only and the NiFe hydrogenases, are known at high resolution. 3 The active site of the Fe-only hydrogenases consists of an Fe 2(μ-SR)2(CN)2(CO)3Ln core (L) H2O/H2 and a thiolate-linked Fe 4S4(SR)4 cluster, Scheme 1). 4 This core shares key structural features with organometallic complexes Fe2(μ-SR)2(CO)6 that have been known since the 1920s. 5 So stable are the Fe 2(μ-SR)2(CO)6 derivatives that such compounds form under harsh conditions (e.g., 50 -200 MPa at 250°C) from primitive reagents (FeS, RSH, HCO 2H). We have reported that model complex {Fe2[μ-S2(CH2)3](CN)2(CO)4} (1, Scheme 1) reacts with protons to give substoichiometric amounts of dihydrogen. 7 Unfortunately acid also converts 1 (and related dianions) into insoluble and catalytically inactive polymeric derivatives of unknown structure. The unsuitability of 1 as a catalyst is attributable to its highly reducing character, supported by the aforementioned ability to reduce protons directly as well as by electrochemical measurements. 8 This logic led us to investigate the complex {Fe2[μ-S2(CH2)3](CN)(CO)4(PMe3)} (2) which is less reducing than 1. As described below, 2 is an active catalyst for proton reduction, and as such provides the first functional link between organometallic models and the Fe-only hydrogenases. In evaluating the catalysis, we first examined the protonation of 2. Dark red HFe2[μ-S2(CH2)3](CN)(CO)4(PMe4) (3) precipitates in analytical purity from MeCN solutions of 2 upon addition of excess aqueous H 2SO4 (see Scheme 2). The 1H NMR spectrum of this species shows a 31P-coupled doublet signal at δ ) -17 (JH-P ) 23 Hz), consistent with protonation of the Fe -Fe bond. Amine bases do not convert 3 into 2, probably reflecting the kinetic inertness of the hydride. 9 Complexes of the type HFe 2(μ-SR)2(CO)6-xLx were first described by Poilblanc without examination of their redox properties. 10 It is significant that protonation of2 occurs at Fe, not the N of cyanide, as both are basic sites. 11 Further treatment of 3 with toluenesulfonic acid (HOTs) gave a new and relatively air-stable species that exhibits νCO bands ca. 10 cm-1 higher in energy than3 (Figure 1). In contrast, the conversion of 2 to 3 results in a shift inνCO of ca. 60 cm-1. The new species generated by protonation of 3 is assigned as{HFe2[μ-S2(CH2)3](CNH)(CO)4(PMe3)} (3H+). Proton reduction catalysis was demonstrated by cyclic voltammetry (CV) (Figure 2A). The voltammogram of a solution of 2 with 1 equiv of HOTs shows, in addition to the one-electron reduction of2 (Ep ) -2.14 V vs Ag|AgCl), two new reduction peaks at a potential ca . 1 V less negative. Shifts of reduction potential parallel the changes in the νCO. These two peaks are ascribed to the reduction of 3H+ and3 at Ep ) -1.03 andE1/2 ) -1.13 V, respectively. The value of ∆Ep (difference between the peak potentials for reduction and reverse oxidation) indicates that the reduction of3 is a one-electron process (Figure 2B). With increasing acid concentration (HOTs, H 2SO4, and HCl give similar results) the height of the first reduction peak increases, and its potential is shifted toward more negative potentials, as expected for catalytic proton reduction. 13 At more negative potentials the only significant electrochemical event observed is the reduction of 2. In the presence of excess protons ( g3 equiv), solutions derived from2 are completely stable under catalytic conditions for hours at room temperature. In a preparative-scale reaction, a solution of 10-3 M of 2 with 50 equiv H2SO4 was electrolyzed at -1.2 V (ca.-1 V vs NHE). Over the course of 15 min, 12 F per mol of 2 were passed. This corresponds to six turnovers for the bulk solution, close to the theoretical maximum because of the (1) Koelle, U.New J. Chem. 1992, 16, 157-169. (2) (a) Cammack, R. Nature1999, 397, 214-215. (b) Collman, J. P. Nat. Struct. Biol.1996, 3, 213-217. (c) Adams, M. W. W.; Stiefel, E. I. Curr. Opin. Chem. Biol.2000, 4, 214-220. (3) (a) Volbeda, A.; Garcin, E.; Piras, C.; de Lacey, A. L.; Fernandez, V. M.; Hatchikian, E. C.; Frey, M.; Fontecilla-Camps, J. C. J. Am. Chem. Soc. 1996, 118, 12989-12996. (b) Fontecilla-Camps, J. C.; Ragsdale, S. W. AdV. Inorg. Chem.1999, 47, 283-333. (c) Peters, J. W.; Lanzilotta, W. N.; Lemon, B. J.; Seefeldt, L. C. Science1998, 282, 1853-1858. (d) Nicolet, Y.; Piras, C.; Legrand, P.; Hatchikian, C. E.; Fontecilla-Camps, J. C. Structure1999, 7, 13-23. (e) Nicolet, Y.; de Lacey, A. L.; Verne `d , X.; Fernandez, V. M.; Hatchikian, E. C.; Fontecilla-Camps, J. C. J. Am. Chem. Soc. 2001, 123, 15961601. (4) Nicolet, Y.; Lemon, B. J.; Fontecilla-Camps, J. C.; Peters, J. W. Trends Biochem. Sci. 2000, 25, 138-143. (5) Reihlen, H.; v. Friedolsheim, A.; Ostwald, W. Justus Liebigs Ann. Chem. 1928, 465, 72-96. (6) Cody, G. D.; Boctor, N. Z.; Filley, T. R.; Hazan, R. M.; Scott, J. H.; Sharma, A.; Yoder, H. S., Jr. Science2000, 289, 1337-1340. (7) Schmidt, M.; Contakes, S. M.; Rauchfuss, T. B. J. Am. Chem. Soc. 1999, 121, 9736-9737. (8) (a) Gloaguen, F.; Lawrence, J. D.; Schmidt, M.; Wilson, S. R.; Rauchfuss, T. B. Submitted for publication. (b) Le Cloirec, A.; Best, S. P.; Borg, S.; Davies, S. C.; Evans, D. J.; Hughes, D. L.; Pickett, C. J. Chem. Commun.1999, 2285-2286. (9) Abdur-Rashid, K.; Gusev, D. G.; Landau, S. E.; Lough, A. J.; Morris, R. H. J. Am. Chem. Soc. 1998, 120, 11826-11827. (10) (a) Fauvel, K.; Mathieu, R.; Poilblanc, R. Inorg. Chem.1976, 15, 976978. (b) Arabi, M. S.; Mathieu, R.; Poilblanc, R. J. Organomet. Chem . 1979, 177, 199-209. (11) Amrhein, P. I.; Drouin, S. D.; Forde, C. E.; Lough, A. J.; Morris, R. H. J. Chem. Soc., Chem. Commun. 1996, 1665-1666. (12) Mathieu, R.; Poilblanc, R.; Lemoine, P.; Gross, M. J. Organomet. Chem.1979, 165, 243-252. (13) Bhugun, I.; Lexa, D.; Saveant, J.-M. J. Am. Chem. Soc. 1996, 118, 3982-3983. Scheme 1

Biomimetic hydrogen evolution catalyzed by an iron carbonyl thiolate.

Electron and proton transfers at diiron dithiolate sites relevant to the catalysis of proton reduction by the [FeFe]-hydrogenases

A series of models for the active site (H-cluster) of the iron-only hydrogenase enzymes (Fe-only H2-ases) were prepared. Treatment of MeCN solutions of Fe2(SR)2(CO)6 with 2 equiv of Et4NCN gave [Fe2(SR)2(CN)2(CO)4](2-) compounds. IR spectra of the dicyanides feature four nu(CO) bands between 1965 and 1870 cm(-1) and two nu(CN) bands at 2077 and 2033 cm(-1). For alkyl derivatives, both diequatorial and axial-equatorial isomers were observed by NMR analysis. Also prepared were a series of dithiolate derivatives (Et4N)2[Fe2(SR)2(CN)2(CO)4], where (SR)2 = S(CH2)2S, S(CH2)3S. Reaction of Et4NCN with Fe2(S-t-Bu)2(CO)6 gave initially [Fe2(S-t-Bu)2(CN)2(CO)4](2-), which comproportionated to give [Fe2(S-t-Bu)2(CN)(CO)5](-). The mechanism of the CN(-)-for-CO substitution was probed as follows: (i) excess CN(-) with a 1:1 mixture of Fe2(SMe)2(CO)6 and Fe2(SC6H4Me)2(CO)6 gave no mixed thiolates, (ii) treatment of Fe2(S2C3H6)(CO)6 with Me3NO followed by Et4NCN gave (Et4N)[Fe2(S2C3H6)(CN)(CO)5], which is a well-behaved salt, (iii) treatment of Fe2(S2C3H6)(CO)6 with Et4NCN in the presence of excess PMe3 gave (Et4N)[Fe2(S2C3H6)(CN)(CO)4(PMe3)] much more rapidly than the reaction of PMe3 with (Et4N)[Fe2(S2C3H6)(CN)(CO)5], and (iv) a competition experiment showed that Et4NCN reacts with Fe2(S2C3H6)(CO)6 more rapidly than with (Et4N)[Fe2(S2C3H6)(CN)(CO)5]. Salts of [Fe2(SR)2(CN)2(CO)4](2-) (for (SR)2 = (SMe)2 and S2C2H4) and the monocyanides [Fe2(S2C3H6)(CN)(CO)5](-) and [Fe2(S-t-Bu)2(CN)(CO)5](-) were characterized crystallographically; in each case, the Fe-CO distances were approximately 10% shorter than the Fe-CN distances. The oxidation potentials for Fe2(S2C3H6)(CO)4L2 become milder for L = CO, followed by MeNC, PMe3, and CN(-); the range is approximately 1.3 V. In water,oxidation of [Fe2(S2C3H6)(CN)2(CO)4](2-) occurs irreversibly at -0.12 V (Ag/AgCl) and is coupled to a second oxidation.

Frédéric Gloaguen

Papers

Small molecule mimics of hydrogenases: hydrides and redox

Biomimetic hydrogen evolution catalyzed by an iron carbonyl thiolate.

Electron and proton transfers at diiron dithiolate sites relevant to the catalysis of proton reduction by the [FeFe]-hydrogenases

Synthetic and structural studies on [Fe2(SR)2(CN)x(CO)6-x](x-) as active site models for Fe-only hydrogenases.

Catalysis of the electrochemical H2 evolution by di-iron sub-site models