electrochemical hydrogenation of chlorodifluoromethane (hcfc-22) at metal- and...

8/6/2019 Electrochemical hydrogenation of chlorodifluoromethane (HCFC-22) at metal- and metal–phthalocyanine-supported gas diffusion electrodes

http://slidepdf.com/reader/full/electrochemical-hydrogenation-of-chlorodifluoromethane-hcfc-22-at-metal- 1/5

Advances in Environmental Research 8 (2004) 287–291

1093-0191/04/$ - see front matter 2002 Elsevier Science Ltd. All rights reserved.PII: S109 3-0 191 Ž0 2 .0 0 1 0 3 - X

Electrochemical hydrogenation of chlorodifluoromethane(HCFC-22) at metal- and metal–phthalocyanine-supported gas

diffusion electrodes

Noriyuki Sonoyama*, Tadayoshi Sakata

Department of Electronic Chemistry, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology,

4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan

Received 8 August 2002; received in revised form 9 August 2002; accepted 17 August 2002

Abstract

Electrochemical hydrogenation of chlorodifluoromethane (HCFC-22) was attempted using metal- and metal–phthalocyanine (PC)-supported gas diffusion electrodes (GDEs). Among these modified GDEs, only the Co–PC-supported GDE showed electrocatalytic activity for HCFC-22 hydrogenation, with decomposition to methane anddifluoromethane (HFC-32). The efficiency for hydrogenation and HFC-32 selectivity increased with negative increasein the potential during electrolysis. This potential dependence of hydrogenation efficiency suggests that electrons aretransferred to HCFC-22 not from the Co(I) metal center, but from the reduced PC ring, and that the Co(I) metalcenter acts as the adsorption site for HCFC-22.

2002 Elsevier Science Ltd. All rights reserved.

Keywords: Chlorodifluoromethane (HCFC-22); Electrochemical hydrogenation; Metal–phthalocyanine; Gas diffusion electrode

1. Introduction

Chlorofluorocarbons (CFCs) have been widely usedas refrigerants in refrigerators and air conditioners, andas aerosols, foaming agents, etc. Since the 1970s, theirhigh potential for depletion of the ozone layer in thestratosphere has been pointed out (Molina and Rowland,

1974) and the production of specified CFCs has beendiscontinued since 1995. Recently, hydrochlorofluoro-carbons (HCFCs) have mainly been used as substitutesfor CFCs. In particular, a large amount of HCFC-22(CHClF ) is used as a substitute for CFC-12 (CCl F )2 2 2

as the refrigerant in refrigerators and air conditioners.However, HCFCs also have high ozone-depletion poten-tial, although not as high as CFCs, and it is planned toreplace HCFCs with HFCs, which have no ozone-

*Corresponding author. Tel.: q81-45-924-5400; fax: q81-45-924-5489.

E-mail address:[email protected](N. Sonoyama).

depletion potential. Therefore, HCFC-22 is one of theHCFCs for which appropriate methods of treatmentshould be developed in a short time.

The conventional method for the treatment of HCFCsis an incineration method using a high-temperatureincinerator. This method decomposes HCFCs to carbondioxide, hydrogen chloride and hydrogen fluoride,

which are considered not favorable for release into theenvironment. Among these, hydrogen fluoride is verytoxic to human health and needs secondary treatment asa dangerous industrial waste. The most desirable treat-ment of HCFCs would be to replace the chlorine inHCFCs, which gives HCFCs the potential for ozonelayer depletion, with other atoms without removing thefluorine atoms, which give unique chemical and physicalcharacteristics to HCFCs. We have carried out electro-chemical reduction of CFCs and reported that CFC-12and CFC-13 (CClF3) can be converted to HFC-32(CH2F2) and HFC-32 (CHF3) with a current efficiencyof over 70% (Sonoyama and Sakata, 1998a,b). TheseHFCs are recyclable as substitutes for CFCs and HFCFs.



288 N. Sonoyama, T. Sakata / Advances in Environmental Research 8 (2004) 287–291

Table 1Current efficiency for hydrogenation of HCFC-22 (CHClF ) and product formation at the metal-supported gas diffusion electrodes2

(GDEs) and metal–PC-supported GDEs

Electrocatalyst Potential Current efficiency (%)

(V)

HGa

Methane Hydrogen CH F2 2 Total

Agb y1.8 7.8 0.9 72.9 6.9 80.7Cub y1.8 7.0 4.0 74.3 3.0 81.3Pbb y1.8 6.3 2.4 81.3 3.9 87.6Inb y1.8 5.8 0.2 76.8 5.6 82.6Cob y1.5 2.9 0.2 83.7 2.7 86.6Nib y1.5 1.2 0.2 84.6 1.0 85.8Znb y1.8 1.3 0.2 80.7 1.1 82.0Pbb y1.8 1.2 0.1 84.3 1.1 85.5Feb y1.5 1.2 0.2 81.6 1.0 82.8Crb y1.5 0.9 0.1 84.5 0.8 85.4Snb y1.5 1.1 0.2 82.7 0.9 83.8Ptb y1.8 1.0 NDd 85.6 1.0 86.6

Rub y1.8 0.2 0.2 84.3 ND 84.5

Cu–PCc y2.0 4.5 2.9 80.7 1.6 85.2Zn–PCc y1.7 1.8 ND 84.2 1.8 86.0Co–PCc y1.7 55.7 30.2 35.0 25.5 90.7Fe–PCc y1.8 2.4 0.6 86.5 1.8 88.9Ni–PCc y1.5 1.3 ND 87.2 1.3 88.5Pd–PCc y1.5 1.5 ND 85.1 1.5 86.6H –PCc

2 y1.7 15.2 1.3 65.3 13.9 80.5

Hydrogenation of HCFC-22; the total current efficiency for product formation except for hydrogen.a

Electrolysis conditions: current density 63.7 mA cm under 10 atm of HCFC-22 at room temperature.b y2

Electrolysis conditions: current density 31.9 mA cm under 10 atm of HCFC-22 at room temperature.c y2

ND, not detected.d

Catalytic decomposition of HCFC-22 has been reportedby several groups (Li et al., 1996; Morato et al., 1999,2001; Romelaer et al., 2001). Recently, the reduction of HCFC-22 at a catalyst under hydrogen flow was activelycarried out (Morato et al., 2001, 1999; Romelaer et al.,2001). This method indicated high activity for hydro-genation of HCFC-22; however, the use of hydrogen isdangerous and limits the facility for HCFC treatment.The electrochemical method does not need a specialfacility, because the electrochemical reaction proceedsat room temperature and needs no reactive reagents.

In this paper, we have attempted to hydrogenateHCFC-22 electrochemically using metal-supported gasdiffusion electrodes (GDEs) and metal–phthalocyanine(PC)-supported GDEs.

2. Experimental

All electrolyses were carried out in a stainless auto-clave at room temperature. The structure of the auto-clave has already been described in a previous paper(Sonoyama and Sakata, 1998a). Unsupported and Pt-and Ru-supported GDEs were purchased from TanakaNoble Metal Ltd. Other metals (Ni, Zn, Ag, Cu, Pd,Pb, Co, Fe, Sn, Cr and In) were supported on the GDEs

by the impregnation method. Co(II)–, Fe(II)–,Zn(II)–, Cu(II)–, Ni(II)–, Pd(II)– and H –PCs and2

Fe(III)–PCCl (Kanto Chemicals Co) were used withoutfurther purification. Modification of metal–PCs on theGDEs was carried out according to the method of Furuya and Matsui (1989). These metal–PC-supportedGDEs were used as the working electrodes (cathode).The diameter of the working electrodes was 1 cm, witha 1 M NaOH aqueous solution used as the electrolytesolution. As the counter and reference electrodes, weused a Pt wire and AgyAgCl, respectively. Purified Argas was bubbled into the solution for at least 20 min toremove dissolved oxygen. HCFC-22 (Asahi Glass CoLtd) was directly introduced into the autoclave until thepressure reached 10 atm. Electrolyses were carried out(passage of 125 C) using a potentiostat–galvanostat(Hokuto model HA-305) connected to a Coulomb–ampere–hour meter (Hokuto model HF-201). The gassampled from the autoclave was analyzed by gas chro-matography. The products were determined by GC-MS(Shimazu QP-5050A).

3. Results and discussion

The current efficiency for the hydrogenation of HCFC-22 and product formation at metal- and metal–




Fig. 1. Current–voltage curves under Ar and 10-atm HCFC-22atmosphere at: (a) the Cu-; (b) the Zn–PC-; and (c) the Co–PC-supported GDE.

PC-supported GDEs are summarized in Table 1, where

the current efficiency is defined as the ratio of thecharge used for the formation of each product to thetotal charge passed during electrolysis, expressed as

follows:

mFn E s (1)C

C

where E , m, F , n and C are current efficiency, molarC

number of the product, Faraday constant, the number of

electrons needed for the formation of the product andthe total charge passed during electrolysis, respectively.The current efficiency for the hydrogenation of HCFC-

22 is defined as the total of the current efficiency valuesfor all products formed on electrochemical reaction of HCFC-22. The electrolyses were carried out at least

twice with an error of at most 10% in current efficiency.Metal-supported GDEs have hardly any activity forhydrogenation of HCFC-22, whereas some of the PC-

supported GDEs showed activity. Only the Co–PC-supported GDE showed high activity for hydrogenationof HCFC-22 and the H –PC-supported GDE showed2

low activity. This result is very different from that forhydrogenation of CFC-12 (CCl F ) (Sonoyama and2 2

Sakata, 1998a). In the case of CFC-12, many metal-

supported GDEs (Cu, Ag, Pb, In, Zn and Sn) showedactivity for CFC-12 hydrogenation, and Cu-, Pb- andIn-supported GDEs showed very high activity ()50%

current efficiency)

. This activity of the Co–PC-support-ed GDE is inherent in the Co–PC complex, because theCo-supported GDE showed no activity for hydrogena-

tion of HCFC-22, as listed in Table 1. These activityvalues for metal–complex hydrogenation of HCFC-22do not agree with those for CFC-12. In the electrochem-

ical hydrogenation of CFC-12 on metal–PC-supportedGDEs, Cu–, Zn–, Ni– and Pd–PC showed electrocatal-ytic activity, as well as Co–PC (Sonoyama et al., in

press). In the electrochemical hydrogenation of CFC-12on metal–tetra-phenylporphyrin (TPP)-supportedGDEs, with a similar structure to metal–PCs, Cu– and

Zn–TPP-supported GDEs showed high activity and theCo–TPP-supported GDE showed low activity (Sonoy-ama and Sakata, in press). Concerning the selectivity of

the products, methane and difluoromethane (HFC-32)

were produced with almost the same selectivity on theCo–PC-supported GDE, whereas difluoromethane was

selectively produced from HCFC-22 at the H –PC-2

supported GDE. In all the metal- and metal–PC-sup-

ported GDE systems, a slight amount of trifluoromethane was detected. It is presumed that thiscompound formed from the disproportionation of HCFC-22 in aqueous solution was not due to theelectrolysis, because trifluoromethane was also detectedfrom the system without current flow.

Current–voltage (C –V ) curves for the Cu- and Zn–

and Co–PC-supported GDEs are shown in Fig. 1. Atthe Cu- and Zn–PC-supported GDEs, a slight negativeshift was observed in the C –V curve under HCFC-22compared with that under Ar, whereas the current at theCo–PC-supported GDE under HCFC-22 indicated asteep increase from y1.5 V with a negative increase inthe potential, and a positive shift was found at the Co–PC-supported GDE. This positive shift at the Co–PC-supported GDE suggests a strong interaction betweenHCFC-22 and Co–PC at potential values more negativethan y1.5 V.

The relationship between the potential during elec-trolysis and the current efficiency for hydrogenation of HCFC-22 and product formation at the Co–PC-support-




Fig. 2. Relationship between the potential during electrolysisand the current efficiency for hydrogenation of HCFC-22 andfor product formation at the Co–PC-supported GDE.

Table 2The redox potential of Co–PC in DMF and redox state at themetal center and the PC ring

Potentiala Metal PC ring(V) center

y0.37 II™Iy1.40 y2™y3

y1.80 y3™y4y2.08 y4™y5

From Lever et al. (1993).a

ed GDE is shown in Fig. 2. The Co–PC-supported GDE

showed hydrogenation activity at y1.5 V, and thecurrent efficiency for HCFC-22 hydrogenation increasedwith negative increase in the potential. The selectivityof the products was also dependent on the potential.The current efficiency for difluoromethane formationincreased with negative increase in the potential, anddifluoromethane became the main product at y3.0 V(45% current efficiency), whereas that of methaneformation was not largely dependent on the potential.

It is well known that Co complexes are good electro-catalysts for carbon dioxide reduction. Lieber and Lewis(1984) reported that carbon dioxide was efficiently

reduced to carbon monoxide at a Co–PC-modifiedcarbon electrode at potential values from y0.95 to y

1.2 V vs. SCE. Some investigators also reported that a

reaction current was observed at approximately y1 Vat Co complex-modified electrodes. (Atoguchi et al.,

1991; Sonoyama, et al., 1999) Atoguchi et al. concludedthat a reduced Co(I) metal center has high activity for

CO reduction, because the first reduction potential,2

where the accepted electron reduces the metal center of the complex wCo(II)™Co(I)x, is almost consistent with

the potential value at which CO reduction began to2

proceed (Atoguchi et al., 1991). The reduction potential

and site for Co–PC in DMF (Lever et al., 1993) arelisted in Table 2. In the case of HCFC-22 reduction atthe Co–PC-supported GDE, the current in the C –V

curve begins to rise at approximately y1.5 V, withhardly an reaction current flow at potential values morepositive than y1.5 V. These results suggest that further

reduced Co–PC, rather than wCo(I)–PCx , is involvedy

in the reduction of HCFC-22. From the results men-tioned above, it is presumed that the reduction processoccurs via HCFC-22 adsorption on the reduced metal

Co(I) center, and that electrons are transferred not from

the Co(I) metal center, but from the reduced PC ring.

4. Conclusion

Electrochemical hydrogenation of HCFC-22 only pro-

ceeded at the Co–PC-supported GDE, whereas Cu- and

Zn–PC-supported GDEs that have high activity for

electrochemical hydrogenation of CFC-12 showed noelectrocatalytic activity. The Co metal center seems to

act as the adsorption site for HCFC-22 and electrons

are transferred via the reduced PC ring. Further study

of electrochemical reaction mechanism would support

the efficient electrochemical conversion of HCFCs.

References

Atoguchi, T., Aramata, A., Kazusaka, A., Enyo, M., 1991.

Cobalt (II)–tetraphenylporphyrin–pyridine complex fixed

on a glassy carbon electrode and its prominent catalyticactivity for reduction of carbon dioxide. J. Chem. Soc.

Chem. Commun., 156–157.

Furuya, N., Matsui, K., 1989. Electroreduction of carbon

dioxide on gas-diffusion electrodes modified by metal

phthalocyanines. J. Electroanal. Chem. 271, 181–191.

Lever, A.B.P., Milaeva, E.R., Speier, G. (Eds.), 1993. Phthal-

ocyanines, Vol. 1, VCH Publishers, New York, Chapter 1.

Li, G.-L., Ishihara, T., Moro-oka, Y., Takita, Y., 1996. Catalytic

decomposition of HCFC-22(CHClF ). Appl. Catal. B: Envi-2

ron. 9, 239–249.

Lieber, C.M., Lewis, N.S., 1984. Catalytic reduction of CO 2

at carbon electrodes modified with cobalt phthalocyanine. J.Am. Chem. Soc. 106, 5033–5034.




Molina, M.J., Rowland, F.S., 1974. Stratospheric sink for

chlorofluoromethanes. chlorine atom-catalyzed destruction

of ozone. Nature 249, 810–812.

Morato, A., Alonso, C., Medina, F., et al., 1999. Conversion

under hydrogen of dichlorodifluoromethane and chlorodi-

fluoromethane over nickel catalysts. Appl. Catal. B: Environ.23, 175–185.

Morato, A., Alonso, C., Medina, F., et al., 2001. Palladium

hydrotalcites as precursors for the catalytic hydroconversion

of CCl F (CFC-12) and CHClF (HCFC-22). Appl. Catal.2 2 2

B: Environ. 32, 167–197.

Romelaer, R., Kruger, V., Baker, J.M., Dolbier Jr, W.R., 2001.

Pyrolyses of chlorodifluoromethane and trifluoromethane in

the presence of hydrogen. Mechanism and optimization of reaction conditions. J. Am. Chem. Soc. 123, 6767–6772.

Sonoyama, N., Sakata, T., 1998. Electrochemical decomposi-tion of CFC-12 using gas diffusion electrodes. Environ. Sci.Technol. 32, 375–378.

Sonoyama, N., Sakata, T., 1998. Electrochemical hydrogena-tion of CFC-13 using metal-supported gas diffusion elec-trodes. Environ. Sci. Technol. 32, 4005–4009.

Sonoyama, N., Sakata, T. Electrochemical decomposition of CFC-12 at metal–tetra-phenylporphyrin-supported gas dif-fusion electrodes. Adv. Environ. Res., in press.

Sonoyama, N., Kirii, M., Sakata, T., 1999. Electrochemicalreduction of CO at metal–porphyrin–supported gas diffu-2

sion electrodes under high-pressure CO . Electrochem. Com-2

mun. 1, 213–216.Sonoyama, N., Fujii, H., Sakata, T. Electrochemical hydrogen-

ation of CFC-12 at the metal–phthalocyanine-supported gasdiffusion electrodes. J. Electrochem. Soc., in press.

electrochemical hydrogenation of chlorodifluoromethane (hcfc-22) at metal- and...

Documents