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611ProblemSet132010

611ProblemSet132010 - CHEMISTRY C611 ELECTROANALYTICAL...

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Unformatted text preview: CHEMISTRY C611: ELECTROANALYTICAL CHEMISTRY Fall, 2011} Problem Set 13 In a paper by B. Steiger and F. C. Anson (Inorg. Chem. 1994, 33, 5767—5779), a copy of which accompanies this problem set, the authors described the behavior of “ruthenated” cobalt(lI) porphyrins, confined onto surfaces of py-rolytic graphite {glassy carbon) electrodes, as possible electrocatalysts for the four—electron reduction of dioxygen to water. 1. Shown below is a cyclic voltammogram, obtained at a scan rate of 50 mV 3", illus- trating the electrochemical behavior of porphyrin I (shown below) when it is adsorbed onto a glassy carbon electrode; the dashed line represents the background voltammo— gram recorded in the absence of porphyrin I. 0.6 {3,4 0.2 0.0- -o.2 m 0 E. Vvs see on lCoPtthbe-CHSPyJHPFe) (a) According to Steiger and Anson, the area defined by the cathodic peak in the above cyclic voltammo gram (afier subtraction of the area under the dashed curve) corresponds to 1.2 x 10“) mo] cm—2 of adsorbed porphyrin, whereas the analytical quantity of porphyrin applied (in the form of a tiny volume of solution that was subsequently evaporated to dryness) to the surface of the electrode was equivalent to 1.3 x 10—9 mol cm‘2 ofporphyrin I. What is the significance of these obser- vations? (b) What other information or conclusions can be drawn from the above cyclic voltammogram? . Steiger and Anson demonstrated that an adsorbed coba1t(H) porphyrin can be “ruthen- ated” by being immersed into a solution of Ru(NH3)50H22+. For example, each of the three cyano groups of porphyrin 1 (shown above) can be “ruthenatcd” to give porphyrin V (Shown below). Parenthetically, it should be noted here that Steiger and Anson found that the adsorbed “ruthenated” cobait(H) porphyrins are more tenacionsly held on the surface of a glassy carbon electrode if the porphyrin solution is mixed with an alcoholic solution of Nation before being applied to an electrode. A set of cyclic voltammograms, recorded at 50 mV 3", depicting what happens when adsorbed porphyrin I is “ruthenated” and eventually converted into porphyrin V is shown below; the redox potential for the ruthenium(]]1)—ruthenium([l) couple is +0.28 V under the conditions of these experiments. Cyclic voltammetric behavior of glassy carbon electrode coated with 2.1 x 10'9 mol crn’2 of porphyrin I exposed to a 25 mM solution of Ru(NH3)SOI-l22i for (a) 0, (b) 15, (c) 30, (d) 45, (e) 60, and (i) 75 min. The dotted curve was recorded with an uncoated electrode. 0.6 0.4 02 Oil] -0.2 «0.4 E. V VS SCE V cnsutsugsu CNRu£NH5)5Z+ [CoPtPhCNRulNl-I3)5)3(N.c H.3py)]7+ (a) Along with the single cyclic voltammogram shown earlier for porphyrin I, what quantitative information can or could be obtained from this set of cyclic voltanunograms depicting the “ruthenation” of porphyrin I? (b) In this set of cyclic voltannnograms, the peak width at half-height is approxi- mately 190 mV, whereas the theoretical peak width at half-height for a simple, one-electron Nemstian redox couple should be 90.6 mV for an adsorbed species. What might cause the observed and unexpectedly large width at half—height? 3. Shown below is a set of cyclic voltammograms for the electroreduction of dioxygen at electrodes coated withfour drfiizrem adsorbed cobalt(ID porphyrins. Panel A shows the results obtained with adsorbed porphyrin [V (structure below), panel B shows the results obtained with porphyrin VIE (structure below), panel C shows the results obtained with porphyrin I, and panel B shows the results obtained with porphylin V. IV CN CH3 [COPWthN-CI'BPYHMI’FSB VIII 2+ 0.5 0.4 0.2 0.0 .02 -0.4 0.5 0.4 0.2 0.0 -0.2 -0; CNRUU‘HJk e. VVSSCE E. VvsSCE Figure 7. Cyclic voltammogram for the reduction of 0; at electrodes coated with 2.] x It)“I mol cm—2 of cobalt porphjn‘ins plus 1.8 x 1'3"8 mol cm‘2 of Nation sulfonate groups. The supporting electrolyte. 0.5 M Nth Pris-0.5 M HClOt, was saturated with Ar [dashed lines) or air (solid lines). (A) porphyrin IV {Figure 2); (B) porphyrin VIII (Figure 3}: (C) porphyrin I (Figure 2): (D) porphyrin V (Figure 3). Scan rate = 50 mV 5“. cuJ [CoPtPhCNRutNH3J5JIN-CH3pylsls+ (a) In their paper, Steiger and Anson state that the cyclic voltannnogram shown in panel A for an air-saturated solution (solid line) corresponds to the two-electron reduction of 02 to H202 catalyzed by porphyrin IV. With this information in mind, how would yen interpret the results shown in panels B, C, and D? 4. Steiger and Anson sought to verify their conclusions about the stoichiometry of the catalytic reduction of dioxygen by porphyrins 1 and V by employing hydrodynamic voltammetry with a rotating ring-disk electrode; the disk was a glassy carbon electrode (coated with the appropriate coba1t(11)porphyrin) and the ring was of platinum, and for all experiments the potential of the platinum ring was held at a value (+1.00 V) where H202 undergoes a two-electron oxidation to 02. In addition, it was established experimentally that the collection efficiency (N) for the ring—disk electrode was 0.39. (a) What is meant by the term “collection efficiency” (N), and how can the col- lection efficiency for a ring—disk electrode be changed? (b) Two sets of voltammograms obtained with the ring-disk electrode (rotated at 100 rpm) for the catalytic reduction of dioxygen by porphyrin l and porphyrin V are shown below. What does panel A reveal about the stoichiometry of the catalytic reduction of dioxygen by porphyrin I? What does panel B reveal about the stoichiometry of the catalytic reduction of dioxygen by porphyrin V? Support your conclusions with calculations. 0.5 I14 0.2 0.0 -0_2 43.4 0.6 0.4 0.2 0.0 -0.2 414 E. V vs 505 E. V vs SCE Figure 9. Reduction of O; at a rotating platinum ring-pytolytic graphite disk electrode. The disk was coated with 2.] X It!" moi ctri'2 of EA] porphyrin [ (Figure 2) or (B) porphyrin V (Figure 3) plus 1.8 x [0“ mol em‘2 of Nafton sulfonate groups. The potential of the platinum ring electrode was set at [.0 V. Supporting electrolyte: 0.5 M NHaPFs—Dj M l-lClOt saturated with air. Rotation rate = 100 rpm. Scan rate = 5 m‘V s". 5. One electroanalytical technique that Steiger and Anson did not employ to verify the stoichiometry of the catalytic reduction of dioxygen is called thin-foyer eiectrochem- fstry. In essence, thin—layer electrochemistry is a microcoulometric method in which one measures the quantity of electricity that is passed to electrolyze a species confined (trapped) within a very thin layer (typically, a few tens of micrometers in thickness) of the solution at the surface of an electrode; in fact, the thickness of the thin layer of solution is small compared with the usual thickness of a diffusion layer, so that of! of the electroactive species in the thin layer of solution can be reduced or oxidized. How could you apply the technique of thin-layer electrochemistry to determine the stoichiometry of the catalytic reduction of dioxygcn by, for example, porphyrin V? Define carefully the parameters of the experiment, and show how you would calculate how many electrons are transferred to each dioxygen molecule in the catalytic reduction. Inorg. Chem. 1994, 33. 5767—5779 5T6? New Electrocatalysts for the Four-Electron Reduction of Dioxygen Based on (5,10,!5-Tris(pentaamminernthenium(II}-4~cyanophenyli-ZO- (t-methylpyridiniurn-4—yt)porphyrinato)cobalt(II) Immobilized on Graphite Electrodes“ Beat Steiger and Fred C. Anson' Arthur Amos Noyes Laboratories. Division of Chemistry and Chemical Engineering, Califomia Institute of Technology. Pasadena, California 91125 Received July l. £9946 Coordination of Ru(N'l-Ig}5“‘ centers to the nitrile sites in (5.10.l5-tris(4—cyanophertyl)—20-(1-methylpyridinium— 4~yl}porphyrinato)oobalt(fl) immobilized on pyrolytic graphite electrodes produces the triruthenated complex which acts as an electrocatalyst for the four—electron reduction of dioxygcn to water. For comparison. RutINl-Igls“ centers were also coordinated to the nitrile sites in (S.lmbist4eymophenyI)—l5.20-bistl-methylpyfidinium-4- yllporphyrinatokobalttfl), (5.15~bis(4-eya.nophenyl)—l(LID-bidl-methylpyridiniurn-tt—y])porphyrinato)eobalt(m and (S-(d—cyanophenyl}10.l5.20—tt'istl-methylpyridiniurn~4~yl}porphyrinato)cobalt(l'l) to produce the corresponding di- and monoruthenated complexes. The dinithenaled complexes exhibit some eloctrocatalytic activity for the four-electron reduction of dioxygen. whereas the monoruthenated complex calalyres only the two-electron reduction to hydrogen peroxide. None of the ruthenated cobalt porphyrins are catalysts for the reduction of hydrogen peroxide. The synthesis and the electrocatalytle behavior of the set of structurally related porphyrins are described. It is argued that the ruthenated porphyrins achieve their catalytic activity for the reduction of dioxygen to water by means of back-bonding interactions between the Ru(N‘ll;)s“ groups and the cobalt center of the porphyrin ring. Introduction In recent reports it has been demonsuated‘that the coordina- tion of four Roam-13);.2+ groups to the pyridine sites of t5.10.15mteuakist4-pvfidvllmrphyfinatélcobaltfln (Comm) on electrode surfaces converts the complex from a two- to a four-electron catalyst for the electroneduetion of 0;.” Related cobalt porphyrins containing combinations of 4-pyridyl or phenyl groups. Col’fl’hldpy), (Pit = phenyl;1 + y m 4; y = l. 2. 3. 4) behaved as catalysts for the two—electron reduction of 02 following coordination of RUMJJ§1+ groups to the available pyridine sites when y = l or 2 but were convened to four— electron catalysts when y = 3 or 4.3 Four-electron reduction catalysts were also fashioned from combinations of CoP(py)4 and coordinated Ru(NHy)g(Ol-Ifl.“ groups (x = l. 2)‘ but coordination of four Ru(edta)1‘ {edra = etltylettcdiarnine- tetraacetate) groups to CoPtpyL; resulted in no change in catalytic activity: only the two-electron reduction of 02 was observed? In continuing efforts to identify the factors that are important in the conversion of cobalt porphy'rins from two-electron to four- electron catalysts for the reduction of 02 by rulhertalion of peripheral ligands on the porphyrin ring. we prepared a set of cabalt porphyri ns containing various combinations of N—rneth- ylpyridinium-It-yl {hLCHpr} and 4-cyanophenyl groups as Peripheral ligands. The 4—cyanopheriyl ligands were used to coordinate Rquflglg.“ groups to the porphyrin ring and the resulting complexes. confined to the surface of pyrolytic graphite electrodes. were tested as electrocalalysts during the reduction or 02. Certain of the ruthenated cobalt porphyrirts exhibited notable catalytic activity and accomplished the reduction of Oz ' Contribution No. 8963. ‘9 Abstract published in Advance AC5 Abstracts. November |. I994. (I) Sill. Ct; Anson. F. C. J. Am. Chum 39;. 1991, ”3. 9564. (2] 3h]. C.; Anson. F. C. l'norg. Chem, 1992. 31'. find. (3] Steiger. 13.; Slii. Cs. Anson. F. C. foarg. Chem. 1993. 32. 2101' {4} Shi. Cs Anson. F. C. triers. Grim. Acre 1994. 225. BIS. 0020-1669r‘Q4H3 33-5'l6750450l'fl to-Hgo. The preparation of diesc new cobalt porphyrins. their behavior as electrocatalysls and some suggestions about the mechanisms by which they Operate are described in this report. Results and Discussion Syntheses. For reasans that are explained in what follows. it proved desirable to utilize cobalt porphyrins containing mixtures of 4—cyahophcnyl and N—methylpyridinium—d-yl (N- Cngy) groups as peripheral ligands. To obtain the desired porphytirls. the molecules shown in Figure l were prepared by a variation cfa procedure utilized in our recent report1 (see the Experimental Semen}. The porphyrins containing pyridyl groups were then converted to the corresponding MCI-{my derivates and isolated as PFf.‘ salts as described in the Experimental Section. The identities of the products resulting From the syntheses Were established from their '1! NMR spectra. The N—CHpr substlnlents exhibited tlte expected N-melhyl-H resonance? and produced typical downfield shifts in the pyridyl-H resonancess (see the Experimental Section). The more complex spectra of the porpbyrins with the two types of substitucnts were assignable on the basis of previously published spectra of analogous asymmetrically substituted porphyrinsit-a No evidence for mthylation of lhe pyrrolic nitrogen atoms of the porphyr'irts was present in the spectra of the porpbyrins containing the NvCHpr groups“ in agreement with the report of Pasternack and etc-vrortoers.9 Coballfl'l] Was incorporated into each of the isolated porphy— tins by standard trictl'tcids“:I to produce the cobaltfll} porpltyrins shown in Figure 2 and the resulting cobalt porphyrins were adsorbed on the surface of pyrolytic graphite electrodes to be ruthenalcd in place by reaction with RMNHfisOHg” as in our (5) Czuchajowslu'. 1...}. Habdas. l.; Niedbola. 1-1.: Wandreltar. V. I. Hererocyci. Chem. 1992. 29. 479. (5} Walker: F. A: Bathe. V. L.‘. McDem‘tolt. G. A. foam. Chen-r. 1982. 2!. 3342. Q 1994 American Chemical Society 5763 Inorganic Chemistry. Vol. 33, Na. 25. 1994 HzPEI’hG'JM cisdH2P(PhCN}2(p}'32 Sleiger and Anson H2P(PhCN)3(pyl CH tmrrs~H2P(PhCN11(py)2 HzPWhCmtpm Figure 1. Structures of Ihe metal-free porphyrins exmined in this stud},I with the abbreviations used for them in the text. previous study with pyridyl por'phyrins.J However, the present cobalt porphyrins did not remain on the graphite surfaces for sufficiently long times to complete the ruthenation reactions. It was. therefore. necessary to mix the p'orphyn'ns with a dilute alcoholic solution of Nation before applying them to the electrode surfaces to obtain coatings which were retained on {T} LavalIcc, D. K; Gcbala. A. E. lnorg. Chem. 1914. :3. 2004. [3) AI-Hazimi. H. M. 0.: Jackson. A. 11.; Johnson. A. W; Winter. M. J. Chem Soc. Perkin Trans. 1' 197?. 9‘3. (9} Pasternack, R. F.'. Huber. l‘. its, Boyd. 9.: Engasser. 13.: Francescuni. L: Gibbs. En, Fisciltr, P.; Venture, G. C; Hinds. L. deCJ. Him. Cimn. .Sac. 1912. 94. 45“. (10] Adler. A. 1).: Loose. F. R.'. Kanpur. F; Kim. J. Inorg. Nari. Chem. 1970, 32, 24.13. the graphite surface (see the Experimental section}. Exposure of the resulting electrodes to solutions of Rufl‘fl-IflsOHIH produced stable electrode coatings consisting of the ruthenaled cobalt porpltyrirts shown in Figure 3. Voltammetry of the Adsorbed Cobalt Pot-phyrins heron: Ruthenatiou. The voltanunctric responses exhibited by the Cofll'lHCom} couple of most cobalt porphyrins in aqueous media are illvtlefined and difficult to identify. Such was the case with the Campy): and the Ctzll5‘lil’h),i(pyr)Jr (x + y = 4', x = 1, 2. 3) porphyfins that were adsorbed on graphite and examined in our previous sl:l.1dic.s.z-J Adsorbed ConPhCNh behaves similarly. A cyclic vollarnmogram for the latter complex adsorbed on graphite is shown in Figure 4A. Surprisingly. a Electrocatalysts for Reduction of 02 I CH CH [CoPlPhCNhIN-Cflapyllfi’l‘fil III CN CH lmns-[CoPCPhCN'lzm-CflapylszFglz Figure 2. Structures of the cobaltlll] porphyrins examined in this study. much better-defined response resulted from coatings of per- phyrin l of Figure 2 as shown in Figure 4B. The area lying between the dashed and solid lines of the cathodic peaks in the vollarnmograrn in Figure 48 corresponds to 1.2 x 10—9 mol our—1 of adsorbed porphyriu compared with the l .3 x ill—9 mol crrt’l that were dissolved in the aliquot of the standard solution of porphyrin I that was transferred to the electrode surface to prepare the coating. This reasonable agreement demonstrates that essentially all oftitc cobalt centers in the adsorbed porphyrin contribute to the voltammetric re5ponse. (The correSponding measurement with Figure 4A indicated that only tilt: of the total cobalt porphyrin deposited on the electrode surface contributed to the broad cathodic peak.) When the coating containing the same amount oil was prepared from the cobalt porphyrin—Nafion mixture, the cathodic response became broader but a clear anodie peak remained {Figure 4C) with an area corresponding to that expected for the total quantity of cobalt porphyrin in the aliquot of the standard solution which was evaporated on the electrode surfECe‘ The results shown in Figure 4B and 4C showed that essentially all of the porphyrin deposited on the electrode surface remained on this surface when the electrode was immersed in the supporting electrolyte solution. The quantities of porphyrin present on the electrode surfaces were therefore delenrlfuted either by measuring lhe areas of cyclic voltamrnograms or from the volumes of the standard porphyrin solutions transferred to the electrode surfaces. Inorganic Chemistry, Vol. 33. No. '25. 1994 5’36? cis-{CoPCPl‘tmnW-ClbpylfllPE-‘filz IV on FF;‘ PF;- menu"F \ 1' \N—cn, f l l+ PFs' 'IIH:f lCoI’tthlm-Cflapylalu’FsB With the more highly charged cobalt porphyrins. I]. ill and IV in figure 2. no significant yoltammetric peaks were obtained front porphyrin —Na.fion coatings. We believe this behavior is a reflection of the strong nsmiation of the more highly charged complexes with the sull'onatc groups of the Nafion present in lhe coatings. The stronger binding greatly diminishes the rates with which the porphyrins can move from the interior of the Nation poiyeleetrolyte to the electrode surface to be oxidized or reduced. For these porphyrins. the quantities contained in the coatings were estimated from the volumes of the standard solutions transferred to the electrode surfaces. As noted in the Experimental Section. to obtain coatings in which all of the cobalt centers were eiectroactive it was necessary to transfer the porphyrin solutions to the electrode surface in a series of small aliquots each of which was allowed to evaporate before the next was applied When the same quantity of porphyriu was transferred in a single aliquot the porphyrin deposits appeared to be less solvated which dimin- ished the voltarrunetric response from the ColflIN'CoflI) couple. The reason that the presence of the N-Cllgpy group on the porphyrin ring converts a negligible voltammetric response to the reasonably well formed peaks in Figures 413 and 4C may also involve the degree of solvalion of the porphyrins in the coatings. The oxidation of the Coal] centers to C0(I1I) requires that suitable axial ligands have access to the cobalt centers and 5770 Inorganic Chemistry. Vol. 33. N0. 25. i994 V CNRu(HH,)52+ cnnutnttgé’r [CePfl’hmRuiNfialslgiN-Cfl3pyll7+ VII CNRuIENI-lflszl' iram-[Cortrhcneutmt3)5}2t1~t-(313930216+ Stciger and Anson VI cumfltuflé’r HLC—H f_\ fi 0 NHUINHJlSZI t CH}, cis-lCoPiPhCNRuINHslslziN-Cflapylzlé+ VIII CNRIKNHJ)§1+ [coemmnuontatst-Ctopytafir Figure 3. Structures ofthe ru'thettated cobaitul) porphy'rins examined in this study. such access is likely to be facilitated by the salvation of the porphyrin complexes which is induced by the charged N-Ctlgpy groups. Veltetnmetry of the Adsorbcd Cobalt Perphyrins after Ruthenation. To ooordinate Ru(NI-i;);3+ complexes to the cobalt porphyrins in the coatings on the electrode surface. the electrodes were immersed in a 25 mM solution of Ru(NHj}5— 0H2". After various reaction times the electrodes were transferred toe ruthenium-free supporting electrolyte where cyclic voltammograms were recorded. The electrodes were then returned to the solution of RutNH})st‘ZZtH;2+ to allow the coordination of additional Ru(NH1}51+ groups to proceed. This process was continued until the voltarnmetric peak current corresponding to the Ru(NH;)5(NCPh)3m+ couple reached its maximum value. A typical example of the cyclic volumino— grams obtained during a ruthenation experiment is shown in Figure SA. The dotted line shows the voltamrnetric response obtained at the uncoaled graphite electrode. The dashed curve was obtained after the cobalt porphyn'n—Nztfion mixture was deposited on the electrode surface and the alcoholic solvent was allowed to evaporate at room temperature. The small peak currents from the Co(l...
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