Catalysis of the Electrooxidation of Biomass-Derived Alcohol Fuels

Chapter 19

Downloaded by UNIV OF CALIFORNIA SANTA BARBARA on September 19, 2013 | http://pubs.acs.org Publication Date: April 16, 2007 | doi: 10.1021/bk-2007-0954.ch019

Catalysis of the Electrooxidation of Biomass-Derived Alcohol Fuels Corey R. Anthony, Daniel Serra, and Lisa McElwee-White Department of Chemistry and Center for Catalysis, University of Florida, Gainesville, F L 32611-7200

Oxidation of biomass-derived fuels such as methanol can serve as an energy source in applications such as the direct methanol fuel cell (DMFC). Electrocatalysis of methanol oxidation by heterobimetallic complexes provides a possible alternative to catalysis on the surface of bulk Pt/Ru anodes in DMFCs. Electrochemical oxidation of methanol has been demonstrated to be catalyzed by a series of RuPt, RuPd and RuAu complexes.

Introduction Currently 85% of humanity's energy needs are supplied by fossil fuels. The use of fossil fuels has made possible the high standard of living in an industrialized nation. However, these benefits come at a cost, especially to the United States, which annually imports 50% of its fossil fuel needs. The adverse effects of an energy economy based on fossil fuels have been reviewed. " These shortcomings have combined to make renewable energy sources appealing. One such energy source is biomass. With a few exceptions the cost of energy from biomass is currently on par if not cheaper than the energy derived from fossil fuels. When properly used, biomass is an environmentally friendly energy source that can be converted into high energy biofuels such as hydrogen, 1

4

5

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© 2007 American Chemical Society In Materials, Chemicals, and Energy from Forest Biomass; Argyropoulos, D.; ACS Symposium Series; American Chemical Society: Washington, DC, 2007.

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methanol and ethanol. Due to its ease of transportation and high fuel efficiency, the electrochemical oxidation of methanol has received a significant amount of attention. Research in this area has been focused on the development of new catalytic systems and can be roughly grouped into two sets: heterogeneous (fuel cell) studies and homogeneous studies. The challenge is to link information obtained in these two sets of studies to improve catalysis in fuel cells that directly utilize methanol (DMFC) and other complex fuels.

Direct Methanol Fuel Cells and Heterogeneous Electrooxidation of Methanol Fuel cells have been suggested as the power generation system of the immediate future, poised to replace not only internal combustion engines but also advanced alkali batteries. Fuel cells typically operate at 40-60% efficiency, making them much more efficient than conventional fossil fuel engines that operate at less than 20% efficiency. Due to their simplicity and high energy efficiency, direct methanol fuel cells (DMFCs) are especially suited for use in portable electronic devices. In DMFCs, aqueous methanol is electrochemically oxidized at the anode (Eq. 1) to C 0 while oxygen is reduced at the cathode (Eq. 2) to form water. When combined, the two half reactions result in the overall cell reaction (Eq. 3). 6,7

8

2

Anodic Reaction: Cathodic Reaction: Overall Reaction:

CH OH + H 0 1/2 0 + 6H + 6e" 3/2 0 + CH OH 3

2

+

2

2

3

+

C 0 + 6H + 6e" 3H 0 C0 + 2 H 0 2

2

2

2

(1) (2) (3)

9

The mechanism for this oxidation was reviewed by Parsons in 1988 and as a result two key reactions were identified: i) ii)

Electrosorption of methanol to the electrode surface Formation of C 0 from adsorbed carbonaceous intermediates 2

Very few electrode materials are capable of performing both reactions, and of these only platinum and platinum-based electrodes display any significant activity and stability in an acidic medium. When the electrooxidation of methanol is performed with platinum as the anode, the reaction is thought to occur at the surface and involve several adsorbed intermediates. After adsorption of methanol, the reaction mechanism is thought to proceed through a series of dehydrogenation products (Eq. 4-7). Adsorbed C O is a key intermediate of methanol oxidation reaction, and its presence has been observed with the aid of various in situ spectroscopic

In Materials, Chemicals, and Energy from Forest Biomass; Argyropoulos, D.; ACS Symposium Series; American Chemical Society: Washington, DC, 2007.

298 techniques. C O and other carbonaceous intermediates can then be oxidized to carbon dioxide reacting with adsorbed water (Eq. 8-10). Presently D M F C s are faced with a few challenging problems, foremost of which is that the overall reaction is very slow. The electrochemical oxidation of methanol, although thermodynamically favored, is kinetically sluggish, due to the formation of stable intermediates (Eq 5-7). Adsorbed C O in particular can quickly poison the Pt surface resulting in a high overpotential (approximately 0.5 V vs. N H E ) for a reasonable current density.


10,11

C H O H + Pt( 3

— * »

s)

Pt-CH OH + Pt( 2

P t C H O H + Pt( r

»-

S)

(4)

Pt -CHOH + t T + e"

(5)

2

2

(6)

Pt -COH + I T + β"

S)

3

Pt -COH

Pt-CO + 2 Pt< + H * + e

3

H 0 + Pt(

+

Pt-CH OH + H + e"

-

s)

(7)

Pt-OH + 1^ + 6

(8)

Pt-CO + Pt-OH

Pt-COOH

(9a)

Pt-CO + H 0

Pt-COOH + H+ + e"

2

s)

2

•

Pt-COOH

Pt( + C 0 H s)

2

+

+ e

(9b)

-

(10) 12 14

Although there has been moderate success with Pt anodes, ' the high overpotentials required for these fuel cells have made Pt electrodes unacceptable for use in D M F C s . This conclusion has led to an intensive search for other materials that can improve the performance of the Pt anode during the methanol oxidation process. Several methods have been investigated for promoting the formation of C 0 . One method involves alloying Pt with a metal that readily adsorbs and dehydrogenates water within the potential range for methanol oxidation. Studies of Pt alloyed with such metals shows that Ru has by far the largest catalytic effect. 2

9

Considerable effort has been expended to clarify the role of Ru in Pt/Ru binary electrodes. O f the mechanisms proposed, the afunctional theory has gained general acceptance. According to this mechanism the Pt sites in Pt/Ru alloys are responsible for the chemisorption and dehydrogenation of methanol, while the Ru serves as a source of "activated" oxygen, aiding the formation of C 0 ( E q . 11-12). 15

2

H 0 + Ru 2

+

(s)

Pt-CO + Ru-OH

Ru-OH + H + e" C0

+

2

+ H + e"

(11) (12)


299

Homogeneous Electrooxidation of Alcohols In addition to heterogeneous studies, the homogeneous electrochemical oxidation of alcohols with ruthenium complexes has been extensively examined. This reaction can be made catalytic by applying a fixed potential capable of regenerating the active metal species. As an example, [l,3-bis(4-methyl-2pyridylimino)isoindoline]RuCl serves as a catalyst for both electrooxidation and autoxidation of several alcohols, including methanol. The polypyridyl complexes [Ru(trpy)(dppi)(OH)] and [Ru(4,4'-Me bpy) (PPh ) (H 0)](C10 ) catalyze the electrooxidation of benzyl alcohol to benzaldehyde. Cyclic voltammetry of these solutions exhibits an increase in anodic current when an alcohol is introduced; this increase in current is characteristic of the electrooxidation of alcohols by the catalytic species R u = 0 . Extensive mechanistic studies have been performed on the electrooxidation of aqueous alcoholic solutions with ruthenium complexes. Various mechanisms have been proposed, each containing a Ru-oxo complex as the catalytically active species. The Ru-oxo bond can either be generated in situ by reacting with water or be present in the pre-catalyst. The mechanisms which have been proposed include hydride transfer, hydrogen atom abstraction, and oxygen atom transfer. Oxo-bridged Ru dimers can also serve as catalysts for the electrooxidation of alcohols. Meyer reported rapid oxidation of a variety of alcohols, aldehydes, and carboxylates by [(bpy) (0)Ru ORu (0)(bpy) ]. Related binuclear Ru complexes such as [Ru (napy) (H 0) Cl(OH)][C10 ] (napy = 1,8-naphthyridine) have also been shown to be catalysts for the oxidation of primary and secondary alcohols, although the oxidation chemistry was complicated by the instability of the complexes. The binuclear Ru complex (LoMe)(HO)Ru (μ-0) Ru (OH) (LoMe) where L M e [CpCo{P(0)(OMe) } ]" also serves to oxidize alcohols, with a further electrooxidation step of formaldehyde to formate also accessible. 3


16

+

2

2

IV

3

2

4

2

17

18,19

20

21

IV

v

22

2

2

2

2

2

4

4

23

IV

IV

2

=

0

2

3

24

Cooperative Effect in Bimetallic Catalysts The introduction of binuclear complexes has contributed to the development of important applications due to the fact that the two metals can cooperate with each other and show different reactivities from the monometallic compounds. This effect can result in a significant modification of the individual metal properties where the catalytic activity of one metal is mediated by the presence of the other one. It is possible to observe effects of the chemical, electrochemical or photochemical modifications of one metal center on the properties of another. The intramolecular electronic interaction largely depends 25

26


300 on the structure of the complex including the ancillary ligands and bridging ligands but also on the nature of the metals and their oxidation state. Information on the metal-metal interactions has been obtained from spectroscopy, crystallography and theoretical investigations of such model complexes. The cooperative bimetallic effect has been also demonstrated in many examples of heterobimetallic complexes with unsaturated hydrocarbon bridges, complexes based on bridging ligands containing bipyridine, phenanthroline, and terpyridine chelating units, as well as complexes containing μ-p-(ΟΗ )-Υ (Y = P , S ) , ' ' μ-thiolate, μ-οχο or μ-halide bridged type ligands. Recently, Severin and co-workers have used the heterobimetallic complex ( η C Ph CO)Rh^-Cl) Ru(PPh ) (acetone) as catalyst in the Oppenauer-type oxidation of primary and secondary alcohols. The beneficial effect of two metal centers could be demonstrated since the homobimetallic Ru or Rh analogues are not active under the same conditions. One way to demonstrate metal-metal interaction in heterobimetallic complexes is to study their cyclic voltammetry (CV) processes, since the results can be compared with mononuclear model systems. Introduction of a second metal usually shows shifts in redox potentials which are strong evidence of metal-metal communication. There has been previous work which has examined several Mo/Pt and Ru/Pt heterobimetallic complexes in electrochemical processes. The shifts in the oxidation potentials reflect the ability of the metal centers to communicate with each other through the bridging ligands. For example, the cationic [Mo(CO) ^-dppm) Pt(H)]PF complex shows a 400 mV positive shift for both Mo(II/III) and P t ( M V ) waves compared to the neutral Mo(CO) ^-dppm) Pt(H)Cl complex. Since the only change in the compounds occurs at the Pt center, this result demonstrates the electronic interaction between the metal centers through the bridging ligand. 27


28

26

2

27

29

30

31

32

33

4

4

4

3

3

2

34

35

3

3

2

6

2

Product Detection and Analysis The electrooxidation of methanol has been investigated since the 1960's and several reviews on methanol oxidation have been published. As previously described by Parson and Iwasita for oxidation on an anode surface, the sixelectron oxidation process involves a complicated multistep mechanism (Eq. Π Ι 5), where methanol is converted to C 0 via formation of formaldehyde and C O . However, when the process is performed in solution in presence of homogeneous catalysts, formic acid is formed as the 4e" oxidation product instead of carbon monoxide (Eq. 16, 17). When methanol is used in excess during the homogeneous electrooxidation reaction, both formaldehyde and formic acid undergo condensation reactions with methanol to form dimethoxymethane and methyl formate as the 2e" and 4e" 11,36

2


301 oxidation products, respectively (Eq. 18, 19). It is important to point out that the equilibria for these reactions are shifted to the right in the presence of an excess of methanol. As highlighted by Rand and Sermon, these reactions are also favored by the presence of acid catalysts. 37

CH OH

HCHO + 2H+ + 2e

(13)

HCHO

CO + 2H+ + 2e"

(14)

C0

(15)

3


38

CO + H 0 2

+ 2H+ + 2e

2

HCHO + H 0

HCOOH + 2 H + 2e"

(16)

HCOOH

CO2 + 2H " + 2e

(17)

H C(OCH ) + H 0

(18)

HCOOCH3 + H 0

(19)

+

2

HCHO + 2

1

CH3OH

2

HCOOH + CH3OH

3

2

2

2

It is possible to follow the oxidation process by analysis of the products. For example, a direct FTIR investigation of the methanol oxidation in a prototype direct methanol fuel cell (DMFC) demonstrated that dimethoxymethane, methyl formate and C 0 were the products of the electrooxidation process with pure methanol in the anode feed. However, the product distribution is dependent on different factors including the activity of the catalyst, the methanol/water ratio as well as the temperature of operation. 2

39

Heterobimetallic dppm-Bridged Catalysts for the Electrooxidation of Methanol The development of heterobimetallic catalysts for electrooxidation of methanol was initially motivated by literature results on introducing a second metal into bulk metal anodes, » " and also by the possibility that each metal center could exhibit cooperative activity or a unique mechanistic function. The choice of bis(diphenylphosphino)methane (dppm) as a bridging ligand was directed by the fact that metal-phosphorus bonds are often very strong and two metals can be locked together in close proximity by a bidentate phosphine. The first generation of heterobimetallic complexes C p ( P P h ) R u ^ - C l ) ^ dppm)PtCl ( l ) , Cp(PPh )Ru^-Cl)fa-dppm)PdCl (2), and Cp(PPh )RuClfadppm)AuCl(3) was prepared by the reaction of CpRuiPPhsXr^-dppnOCl (4) with Pt(COD)Cl , Pd(COD)Cl and Au(PPh )Cl, respectively. A l l of them possess a dppm linkage between Ru and the second metal center with a threelegged piano stool geometry at Ru (Figure 1). Complexes 1 and 2 possess a bridging chloride that links Ru centers and the quasi square planar Pt or Pd, in a 11

40

42

3

35

2

43

3

2

3

43

2

2

3


In Materials, Chemicals, and Energy from Forest Biomass; Argyropoulos, D.; ACS Symposium Series; American Chemical Society: Washington, DC, 2007. 35

43

Figure 1. Thermal ellipsoid drawings of the molecular structures of complexes I 2 and 3. Thermal ellipsoids are plotted at 50% probability. Phenyl rings and most hydrogen atoms are omitted for clarity.


ο

303 distorted six-membered ring. In contrast, complex 3 is linked only via dppm in a pendant fashion with a linear configuration at the A u center. Cyclic voltammetry of complexes 1-3 in the presence of methanol led to significant enhancement of oxidative current, consistent with a catalytic process. Bulk electrolysis of methanol in the presence of the heterobimetallic complexes resulted in much higher current efficiencies than those obtained from the mononuclear models complexes CpRu(PPh ) Cl (5), and CpRu(r) -dppm)Cl (6) suggesting that the second metal center enhances the catalytic activity. 35,43,44

2


3

2

Table I. Formal Potentials of Complexes 1-3. Complex Ru/Pt(l) Ru/Pd (2) Ru/Au(3) Ru' Ru* Pt(n -dppm)Cl Pd(Ti -dppm)Cl Au(PPh )Cl

Couple Ru(II/III) Ru(MII) Ru(II/III) Ru(II/III) Ru(II/III)

43

35

43

2

1.13* 1.29 0.89 0.61 0.87*

45 2

2

2

46

3

2

Couple Pt(II/IV) Pd(II/IV) Au(I/III)

Em

Catalysis of the Electrooxidation of Biomass-Derived Alcohol Fuels

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