- Email: [email protected]
Electrochemistry of transition metal complexes encapsulated into zeolites
H.G. Karge and J. Weitkamp (Eds.) Zeolite Science 1994: Recent Progress and Discussions Studies in Surface Science and Catalysis, Vol. 98 9 1995 Elsevier Science B.V. All rights reserved.
114
ELECTROCHEMISTRY OF TRANSITION METAL COMPLEXES ENCAPSULATED INTO ZEOLITES Carol A. Bessel and Debra R. Rolison* Surface Chemistry Branch (Code 6170), Naval Research Laboratory 4555 Overlook Avenue, SW, Washington, DC, 20375-5342, USA SUMMARY. The electrochemistry of transition metal complexes (Co(salen) and [Fe(bpy)3]2+ ) encapsulated into zeolites was studied using zeolite-modified electrodes (ZME). Grinding time, binding material, carbon source, electrolyte, solvent, scan rate, and the charge of the encapsulated complex all affect the cyclic and differential pulse voltammetry of these ZMEs. INTRODUCTION. Modification of electrode surfaces with zeolites is of current interest to both electrochemical and solid-state disciplines as the suggested applications include chemical sensing and electrocatalysis. 1-5
In order to demonstrate the utility of transition metal
complexes encapsulated into zeolites for electrosynthesgs, the reactivity of these materials [Z(ML)] at modified electrodes was investigated.
The use of Z(ML) for electrocatalysis
requires insight into the mechanism by which redox activity occurs in the zeolite lattice. Optimization of the variables associated with the synthesis of the modified zeolite and their preparation and use as zeolite-modified electrodes must first be investigated before such issues as changes in the redox reactivity due to interior versus exterior lattice sites, the steric strain to the encapsulated complex imposed by the zeolite and/or bonding of the complex to the zeolite, and electronic communication within the zeolite lattice itself can be addressed. Several fundamental studies to optimize the voltammetry of ZME were conducted using monograin films of Z(ML) applied to the working electrode surface to provide a uniform method for systematically studying the variables that arise in the production and use of 7MEs. EXPERIMENTAL SECTION. NaY (Strem Chemical) was equilibrated in an aq. 0.1 M NaCI for 24 h, dnsed free of CI', and then brought to constant weight over a saturated NH4CI solution. Co(salen)-Y was synthesized following published procedures.3, 6 [Fe(bpy)3]2+-Y was similarly synthesized.
The transition metal encapsulated zeolites were characterized by FT-IR and
diffuse reflectance UV-Vis spectroscopies and by cyclic and differential pulse voltammetry. Electrochemical measurements were conducted in a three-compartment cell containing a coiled Pt auxiliary electrode, a saturated sodium chlodde calomel reference (SSCE) and a zeolite-modified glassy carbon working electrode. The ZME was prepared using a variant of Li and Calzaferri's layered-monograin approach: 7 a MeOH suspension of ground Ultra Pure Carbon and modified zeolite was floated onto the electrode surface, dried, and then covered with a thin layer of polyacrylic acid (PAA, MWavg 450,000) from a methanolic solution.
115
RESULTS AND DISCUSSION. Figure 1 demonstrates one of the variables affecting the cyclic voltammetry of Z(ML)-modified electrodes.
This figure shows that as grinding time of the
zeolite plus carbon powder is increased, the resolution of the redox couple(s) improves and the concentration of the redox species increases.
Grinding the zeolite crystallites appears to
fracture the crystallites and with increased grinding time allows a greater extracrystalline surface area of the zeolite to be exposed to the electrolyte thereby enhancing electrochemical response.
the
This implies that the electrochemical response of the ZME is
caused by electron transfer to transition metal complex located on the exterior of the zeolite. Changes in scan rate, electrolyte, binding material, carbon source, solvent, and charge of the encapsulated complex will be discussed in terms of the cyclic and differential pulse voltammetry of zeolite-modified electrodes.
C
-210
,
;
Volts vs. SSCE
,
.... +2.0
Figure 1. Grinding Study. Cyclic voltammetry of [Fe(bpy)3]2+-encapsulated zeolite Y in 0.1 M LiCIO4/ CH3CN at 10 mV/s. 60 mg of carbon + 60 mg of modified zeolite were ground in a sapphire mortar and pestle for the specified period of time with 10 mg removed to prepare the ZME. A: unmodified glassy carbon electrode; B: ZME with 10 min grinding; C: ZME with 30 min grinding; and D: ZME with 60 rain grinding. The wave negative of-1.2 V is consistent with ligand-centered reduction and the couple positive of 0 V is consistent with Fell/Ill-centered redox for [Fe(bpy)3] 2+.
REFERENCES 1. Rolison, D. R.; Chem. Rev. 1990, 90, 867. 2. Li, Z.; Mallouk, T. E. J. Phys. Chem. 1987, 91,643. 3. Bedioui, F.; De Boysson, E.; Devynck, J. Balkus, K. J. Jr. J. Chem. Soc., Faraday Trans. 1991, 87(24), 3831. 4. Shaw, B. R.; Creasy, K. E.; Lanczycki, C. J.; Sargeant, J. A.; Tirhado, M. J. Electrochem. Soc. 1988, 135, 869. 5. Baker, M. D.; Senaratne, C.; Zhang, J. J. Phys. Chem. 1994, 98, 1668. 6. Herron, N. Inorg. Chem. 1986, 25, 4714. 7. Li, J.; Calzaferri, G. J. Chem. Soc., Chem. Commun. 1993, 1430.
114
ELECTROCHEMISTRY OF TRANSITION METAL COMPLEXES ENCAPSULATED INTO ZEOLITES Carol A. Bessel and Debra R. Rolison* Surface Chemistry Branch (Code 6170), Naval Research Laboratory 4555 Overlook Avenue, SW, Washington, DC, 20375-5342, USA SUMMARY. The electrochemistry of transition metal complexes (Co(salen) and [Fe(bpy)3]2+ ) encapsulated into zeolites was studied using zeolite-modified electrodes (ZME). Grinding time, binding material, carbon source, electrolyte, solvent, scan rate, and the charge of the encapsulated complex all affect the cyclic and differential pulse voltammetry of these ZMEs. INTRODUCTION. Modification of electrode surfaces with zeolites is of current interest to both electrochemical and solid-state disciplines as the suggested applications include chemical sensing and electrocatalysis. 1-5
In order to demonstrate the utility of transition metal
complexes encapsulated into zeolites for electrosynthesgs, the reactivity of these materials [Z(ML)] at modified electrodes was investigated.
The use of Z(ML) for electrocatalysis
requires insight into the mechanism by which redox activity occurs in the zeolite lattice. Optimization of the variables associated with the synthesis of the modified zeolite and their preparation and use as zeolite-modified electrodes must first be investigated before such issues as changes in the redox reactivity due to interior versus exterior lattice sites, the steric strain to the encapsulated complex imposed by the zeolite and/or bonding of the complex to the zeolite, and electronic communication within the zeolite lattice itself can be addressed. Several fundamental studies to optimize the voltammetry of ZME were conducted using monograin films of Z(ML) applied to the working electrode surface to provide a uniform method for systematically studying the variables that arise in the production and use of 7MEs. EXPERIMENTAL SECTION. NaY (Strem Chemical) was equilibrated in an aq. 0.1 M NaCI for 24 h, dnsed free of CI', and then brought to constant weight over a saturated NH4CI solution. Co(salen)-Y was synthesized following published procedures.3, 6 [Fe(bpy)3]2+-Y was similarly synthesized.
The transition metal encapsulated zeolites were characterized by FT-IR and
diffuse reflectance UV-Vis spectroscopies and by cyclic and differential pulse voltammetry. Electrochemical measurements were conducted in a three-compartment cell containing a coiled Pt auxiliary electrode, a saturated sodium chlodde calomel reference (SSCE) and a zeolite-modified glassy carbon working electrode. The ZME was prepared using a variant of Li and Calzaferri's layered-monograin approach: 7 a MeOH suspension of ground Ultra Pure Carbon and modified zeolite was floated onto the electrode surface, dried, and then covered with a thin layer of polyacrylic acid (PAA, MWavg 450,000) from a methanolic solution.
115
RESULTS AND DISCUSSION. Figure 1 demonstrates one of the variables affecting the cyclic voltammetry of Z(ML)-modified electrodes.
This figure shows that as grinding time of the
zeolite plus carbon powder is increased, the resolution of the redox couple(s) improves and the concentration of the redox species increases.
Grinding the zeolite crystallites appears to
fracture the crystallites and with increased grinding time allows a greater extracrystalline surface area of the zeolite to be exposed to the electrolyte thereby enhancing electrochemical response.
the
This implies that the electrochemical response of the ZME is
caused by electron transfer to transition metal complex located on the exterior of the zeolite. Changes in scan rate, electrolyte, binding material, carbon source, solvent, and charge of the encapsulated complex will be discussed in terms of the cyclic and differential pulse voltammetry of zeolite-modified electrodes.
C
-210
,
;
Volts vs. SSCE
,
.... +2.0
Figure 1. Grinding Study. Cyclic voltammetry of [Fe(bpy)3]2+-encapsulated zeolite Y in 0.1 M LiCIO4/ CH3CN at 10 mV/s. 60 mg of carbon + 60 mg of modified zeolite were ground in a sapphire mortar and pestle for the specified period of time with 10 mg removed to prepare the ZME. A: unmodified glassy carbon electrode; B: ZME with 10 min grinding; C: ZME with 30 min grinding; and D: ZME with 60 rain grinding. The wave negative of-1.2 V is consistent with ligand-centered reduction and the couple positive of 0 V is consistent with Fell/Ill-centered redox for [Fe(bpy)3] 2+.
REFERENCES 1. Rolison, D. R.; Chem. Rev. 1990, 90, 867. 2. Li, Z.; Mallouk, T. E. J. Phys. Chem. 1987, 91,643. 3. Bedioui, F.; De Boysson, E.; Devynck, J. Balkus, K. J. Jr. J. Chem. Soc., Faraday Trans. 1991, 87(24), 3831. 4. Shaw, B. R.; Creasy, K. E.; Lanczycki, C. J.; Sargeant, J. A.; Tirhado, M. J. Electrochem. Soc. 1988, 135, 869. 5. Baker, M. D.; Senaratne, C.; Zhang, J. J. Phys. Chem. 1994, 98, 1668. 6. Herron, N. Inorg. Chem. 1986, 25, 4714. 7. Li, J.; Calzaferri, G. J. Chem. Soc., Chem. Commun. 1993, 1430.