Ionization processes accompaning interaction of excited atoms with CH3CN, CH3COOH, and HCOOH molecules

19. V. Balzani and F. Scandola, in: M. Gratzel (editor), Energy Resources through Photochemis- try and Catalysis, Academic Press, New York (1983) ~Russian translation: Mir, Moscow (1986), pp. 9-59~.

20. L. I. Kristalik, "Reorganization energy of the medium in homogeneous and electrode reactions," J. Electromnal. Chem., 136, No i, 7-17 (1982).

21. S. Salem, Electrons in Chemical Reactions, Wiley, New York (1982): [Russian translation: Mir, Moscow (1985)].

22. S. Nagase and K. Morokuma, "An ab initio molecular orbital study of organic reactions," J. Am. Chem. Soc., 100, No. 6, 1666-1672 (1978).

23. J.-Y. Liang and W. N. Kipscomb, "Theoretical study of the uncatalyzed hydration of carbon dioxide in the gas phase," ibid., 108, No. 17, 5051-5058 (1986).

24. R. P. Bell, The Tunnel Effect in Chemistry, Chapman and Hall, London (1980). 25. H. Eyring, S. H. Lin, and S. M. Lin, Basic Chemical Kinetics, Wiley, New York (1980)

[Russian translation: Mir, Moscow (1983)]. 26. E. D. German and A. M. Kuznetsov, "A quantum-mechanical model for nucleophilic substitu-

tion reactions in methyl halides," J. Chem. Soc., Faraday Trans. II, 82, 1885-1912 (1986).

27. L. V. Vilkov and Yu. A. Pentin, Physical Methods of Investigation in Chemistry [in Rus- sian], Vysshkaya Shkola, Moscow (1987).

28. W. P. Jencks, Catalysis in Chemistry and Enzymology, McGraw-Hill, New York (1969) [Rus- sian translation: Mir, Moscow (1973)].

IONIZATION PROCESSES ACCOMPANING INTERACTION OF EXCITED ATOMS

WII~H CH3CN, CH~COOH, AND HCOOH MOLECULES

A.N. Stepanov, A. A. Perov, V. A. Peregontsev, and A. P. Simonov

UDC 543.51+621.384.8

The fragmentation processes of the molecular ions formed as a result of single collisions of metastable and highly excited Rydberg atoms Gf noble gases with molecules of acetonitrile, formic acid, and acetic acid have been investigated by a mass- spectrometric method. The correlation between the observed Penning-dissociative- ionization mass spectra and the degree of overlap of the moelcular orbitals with vacant orbitals of the metastable atoms determined from the available energy spectra of the electrons formed during Penning ionization has been examined. Complex ions appearing during associative ionization have been discovered. The mechanisms for the formation of the observed ions have been discussed.

In the present work we investigated the fragmentation processes of the molecular ions formed as a result of single collisions of slow (thermal) metastable and highly excited atoms of noble gases with unexcited CH3CN, CH3COOH, and HCOOH molecules. The ~-nvestigation was carried out by a mass-spectrometric method [i, 2] with the use of a two-chamber ion source [3]; the total pressure of the binary mixtures did not sxceed 6 mPa. Data on the energy spectra of the electrons formed during the Penning ionization of the molecules just indicated by metastable helium atoms are found in [4-8]; the ionic component of the reaction products (the m~ss spectra) was not previously investigated.

It was shown in studies of Penning ionization with the aid of electron spectroscopy that in this process the impinging metastabl~-atom behaves as an electrophilic particle, which captures an electron from a molecular orbital in an inner vacant orbital of its own, while an electron passes from an outer orbital of the metastable atom into the continuum, carrying off the excess energy. In this case, the probability of an ionization transition should depend on the degree of overlap of the orbitals protruding beyond van der Waals repulsion surface with the unfilled orbital of the impinging metastable atom [5, 8]. Therefore, it may be postulated that a difference should be observed between the ionization processes

L. Ya. Karpov Scientific-Research Institute of Physical Chemistry, Moscow. Translated from Teoreticheskaya i ~ksperimental'naya Khimiya, Vol. 26, No. i, pp. 39-46, January-February, 1990. Original article submitted May 16, 1988.

34 0040-5760/90/2601-0034512.50 �9 1990 Plenum Publishing Corporation

TABLE i. Mass Spectra of Acetonitrile Obtained with Different Ionization Methods

Reaction products

>7 i

Mass spectra

...&

.g

CHsCN + 12,19 100 100 100 100 100

CH._,CN+ + H I3.99 51.9 78,7 74,0 27,0 31,9 14,01

CHCN++H~ 15,25 19,2 23,0 23,0 8,0 6,9 15,1

C2N~-+H2+H 19.69 1,2 1,4 - - CH2N++CH 15,56 3,2 5,2 2,5 CH3++CN 15,34 4,4 2,0 2,0 - -

�9 %

25,0 100 100

100 2,8 --

15,3

caused by metastable He(21,3S) and Ne(sP0,2) atoms, since the centrosymmetric atomic orbitals of the s type overlap a molecular orbital less strongly than do the extended p orbitals of the neon atom.

RESULTS AND DISCUSSION

Acetonitri!e. Tab!e 1 presents the mass spectra of the products of Penning dissociative ionization formed in binary mixtures of acetonitrile molecules with atoms of noble gases, as well as the mass spectra obs~rTed in the cases of electron impact [9] and photoionization [i0, ii]. The ionization potential of the valence 2e orbital of CH3CN (12.194 eV) exceeds the excitation energies of metastable argon, krypton, and xenon atoms. As a result, the process of the ionization and dissociative ionization of acetonitrile in mixtures with these atoms can take place only as-a result of inelastic collisions with atoms in long-lived high Ryd ~ berg states, as is confirmed by the appearance thresholds and the form of the ionization functions of the ions observed. In addition, in the case of the interaction of CH3CN with Xe** atoms, formation of the molecular ion is possible only in the ground state CHsCN+(X2E). In the case of collisions with highly excited krypton and argon atoms, the molecular ion can also form in the excited A2AI state. In the mass spectrum of Ar** + CHsCN, the CH2CN+ion has the greatest intensity, and the yield of the CHCN + ion is far smaller. Its formation (the appearance potential is equal to 15.25 eV) requires emcitation of the molecular ion to the second excited state<B2E, whose populating apparently takes place with a low efficiency.

When the partners in a collision are helium and neon atoms, processes with the participation ofmetastable atoms, which form in fairly large numbers under the conditions of our experiment, make the main contribution to the formation of all the ions observed in the mass spectrum. The appearance thresholds of the ions observed are consistent with the excitation energies of the metastable states o~ helium and neon. The electron spectrum of He(23S) + CH3CN [5] shows four bands, which are assigned to the 2e(~CN), 7at(n), le(o), and 6at(o) orbitals. In this case, the o bands have much lower intensities due to the small electronic population of these orbitals beyond the molecular repulsion surface. In the electron spectrum of Ne(3P2) + CH3CN [6], the bands assigned to o orbitals are not observed, and the relative intensity of the 7al(N) band is far greater than its intensity in the spectrum of He(21'~S) + CHaCN.. This may be attributed to the different degrees of overlap of this molecular orbital with the vacant orbitals of the metastable helium and neon atoms. The large relative population of the second band in the spectrum of the Penning electrons (the A2AI state of CH3CN +) in the case of neon apparently causes some difference between the Penning-ionization mass spectra for helium and neon (see Table i). For example, in the case of neon, the spectrum reveals a large relative yield for the CH2CN + ion, which forms as a result of the fragmentation of the molecular ion in the A2AI state:

Ne(SPo.~) + C H 3 C N - + N e + CH3CN+(A2A~) + e - , , -e + CH=CN + + H + e. (i)

The formation of other fragment ions may be attributed to the fragmentation of the molecular ion in the excited B=E and C2AI states. In the case of metastable neon, the excitation of the B2E state of CHsCN + to lower vibrational levels, in which fragmentation results in th formation of a CHCN* ion is feasible. In the case of collisions with metastable helium, C2N +, CH2N +, and CH~3 ions can appear only as a result of fragmentation of the molecular ion in the excited C2AI state. The low intensity of these ions is in good agreement with the low population of the o bands in the electron spectrum [5].

35

TABLE 2. Mass Spectra of Acetic Acid Obtained with Different Ionization Methods

Reaction products

>: L~

Mass spectra

7.

CH3COOH + 10,35 63,6 85,0 54,1 46.2 100 - - 4,2 19,0 10,644

C O O H + + C H 3 12,27 93,8 100 100 100 - - 10O 9,3 19,6 C2H40++O 12,9 C H3C O++OH 5,3 -- 19,1 I6,1 . . . . CH2CO++ (H20?) 11,4 100 95,0 82,9 91,4 52,8 - - 12,4 100 CH30+ + (HCO?) 20,2 - - 15,2 11,2 33,0 - - I00 - - HCO + + CH3CO 4,5 -- 5,0 3,9 . . . . CO++CH3OH 12,1 - - I0,2 5,5 . . . . CH4++ (CO2?) 15,3 7,2 - - 17,2 10,9 . . . . CH3~+COOH 5,6 5,0 4,5 4,7 . . . . C H F + (?) 14,0 34,5 40,0 56,3 4,5 . . . . ArCH3COOH+ 7,4 - - 5,7 4,1 . . . .

0,02

Since differential evacuation of the excitation and collision chambers was no~-carried out in our experiment with separate admission of the noble gases and the target molecules, molecules of the target gas could penetrate into the excitation chamber and could be sub- jected to the action~oI the electron beam. In addition, the highly excited atoms formed as a result of the dissociative excitation of the target molecules by electron impact could enter the collision chamber. Having sufficient excitation energy, such atoms can cause not only dissociation, but also ionization of the original molecules in the collision chamber, as was established, for example, in the case of ethyl halide molecules in [i]. In the case of the acetonitrile molecule, a contribution of the highly excited H** atoms to the formation of CH3CN + and CH2CN + ions is also observed, the relative portion of this contribution being different for different noble gases. For example, in the case of a binary Ar + CH3CN mix-

ture, a contribution of H** was noted for the molecular ion. In the case of a Kr + CH~CN mixture, the contribution of H** became appreciable already for the CH3CN + and CH2CN + ions. In the case of a mixture of acetonitrile with Xe, a contribution of H** to the formation of the molecular ion was also observed, and the CH2CN + fragment ion formed only as a result of the interaction of the original molecule with a highly excited H** atom. Figure i shows plots of the dependence of the probability of the formation of CH3CH + and CH2CN + ions on the energy of the exciting electrons in a mixture of xenon with acetonitrile. We note that Table 1 presents the mass spectra without consideration of the contribution of H** to the ionization process (the mass spectrum of acetonitrile obtained without the admission of noble gases was ~ubtracted).

Acetic Acid. Table 2 presents the mass spectra of the products of Penning dissociative ionization formed in binary mixtures of vapors of acetic acid with atoms of noble gases, as well as the electron-impact and photoionization mass spectra. The ionization potential of the CH3COOH molecule corresponding to the removal of an electron from the outer 13a'orbita! is equal to 10.644 eV [12], and, for this reason, the ionization processes occurring when CH3COOH molecules collide with excited:krypton and xenon atoms can be evoked only by high-lying Rydberg states of these atoms. As in the case of acetonitrile, when molecules of acetic acid interact with highly ex~Lted krypton and xenon atoms, a contributionto the ionization of these molecules due to the interaction of the latter with the Rydberg fragments formed as a result of their dissociative excitation by electron impact is observed. In the case of acetic acid molecules, contributions to the ionization are also made by the high-lying 0"* Rydberg molecules, whose excitation energy is sufficient for the formation of CH3COOH +, COOH +, and CH3CO + ions. The appearance threshold and the ionization function of the COO}[ +. ion in a mixture with argon attests to the fact that this ion forms as a result of collisions of CH3C00H molecules with highly excited argon molecules. In the case of interactions with metastable Ar(3P2,0) atoms, the molecular ion can be excited only to the first excited state A 2A'' and in this case, according to [13], fragmentation of the molecular ion takes place along the channel

Ar (~P2.0) + CH~COOH-+ Ar + CH3COOH + (A2A ") + e -+ Ar + CHsCO + + OH + e. ( 2 )

In addition, the mass spectrum displays the CH2CO + ion, which forms in the process

Ar(~P2.0) + CHsCOOH~Ar ~ CH=CO + + H20+e. (3)

36

4 rel .units

12 16 20 24 re J2 3[ 40 fe eV Fig. !

I, rel. unit~

, I , , , . , , , . r L , , , . t

/0 l~ /8 ff 28 Jo J4 58 42 s eV Fig. 2

Fig. I. Dependence of the currents of the Xe + (Xe**) (i), CH2CN + (2), and CH3CN + (3) ions on the energy of the electrons in the mass spectrum of a binary mixture of xenon with acetonitrile. 4) Excitation function of an H** atom formed as a result of the dissociative excitation of acetonitrile by electro~ impact.

Fig. 2. Dependence of the current of the C0~ (i) and 0 + (3) ions on the energy of the electrons in the mass spectrum of a binary mixture of helium with formic acid. 2) Excita- tion function of the 0"* atom formed as a result of the dissociative excitation of formic acid by electron impact.

Hence we can evaluate the upper limit of the appearance potential of the CH2CO + ion, which should not exceed the excitation energy of the 3P 0 state of metastable argon (11.72 eV). The validity of the hypotheses regarding the formation of ions in reactions (2) and (3) is confirmed by the agreement between the appearance thresholds of these ions and the excitation energy of metastable argon, as well as by the similarity between the ionization functions of these ions and the excitation function of Ar(3P2,0) [2].

The large excitation energy of the metastable He(21,3S) and Ne(3P2,0 ~) atoms colliding with CHsCOOH produces an extensive set of observable ions. The energy spectrum of the electrons liberated in the He(21'3S) + CH3COOH Penning process [7] shows that the highest excited states of the molecular ion (B, C, and D) have far larger populations than does the first excited state (A2A") and the ground state (X2A ' ) of the CH3COOH + ion. As a whole, the mass spectra of mixtures of acetic acid with helium and neon have a very-similar character. An especially significant difference is observed for the CH~ ion, for which a large yield is also observed in the case of the photoionization of CH3COOH molecules [13] by photons with an energy equal to 21.2 eV and in the electron-impact mass spectrum [9]. The Penning-ionization electron spectra of He(23S) + CH3COOH and the photoelectron spectra of He I were studied in [8]. It was shown that the greatest activity in the Penning process with He(2~S) belongs to the 9a' orbital, which displays an sp-hybrid character on the oxygen atom of the carbonyl group and is extended along the C=O axis beyond the molecular repulsion surface. The peak corresponding

to ionization of the 9a' orbital in the photoelectron spectrum has a low relative intensity. The foregoing statements make it possible to understand the observed increase in the relative yield of the CH~ ion in the mass spectrum of He(21,SS) + CH3COOH in comparison to the photoionization [13]

i + and electron-impact mass spectra [9]. The increase "n the yield of the C2H40 ion in comparison to dissociative ionization by electron impact is apparently caused in a similar manner. In the case of the photoionization (21.2 eV) of CH3COOH, the relative yield of the molecular ion is greater than in the process of Penning ionization by metastable helium. This is consistent with the fact that the 13a' and 3a" orbitals of CH3COOH have little electron density beyond the van der Waals repulsion surface and should, therefore, interact weakly with an impinging metastable helium atom [8].

The main contribution to the formation of all the ions observed in binary mixtures of acetic acid and helium (neon) is made by the collisions with metastable atoms of the noble gases, as is confirmed by the observed dependence of the yields of the ions on the energy of the exciting electrons and by the appearance thresholds of the ions, which coincide with the energies of excitation by metastable helium and neon atoms.

37

In addition to the processes of dissociative ionization, we discovered the existence of a process which results in the formation of a complex ion in a mixture of molecules of acetic acid with argon atoms:

Ar (3P2,o) -5 CH~COOH--+ ArCH~COOH** -+ ArCH3COOH + + e.

Formic Acid. Table 3 presents the Penning-dissociative-ionization mass spectra formed in binary mixtures of molecules of formic acid with atoms of noble gases, as well as the electron-impact and photoionization mass spectra 49, 13, 14].: The ionization potential of HCOOH(XIAI) is equal to 11.05 eV. This is the reason why the metastable krypton and xenon

atoms can no longer make a contribution to the formation of the HCOOH + molecular ion and the Penning ionization process takes place as a result of interactions with the highly exited Kr** and Xe** atoms. In this case, the excitation energies of the Rydberg states of the xenon atom, which converge at the ionization limit of Xe+(2Pl/2),are sufficient for the formation of the molecular ion in the ground state (X2A ') and in the first excited state (A2A"). No fragment ions were observed in an Xe + HCOOH mixture, although, according to [13], the appearance potentials of the COOH + and HCO + ions from formic acid are equal to 12.26 and 12.65 eV, respectively. A broader spectrum of fragment ions is observed in a Kr + HCOOH mixture. The excitation energy of the high Rydberg states of krypton, which converge at the ionization limit of Kr+(2PI/2) corresponds to the beginning of the third band in the photoelectron spectrum [15], as a result of which new channels for fragmentation of the molecular ion from the second excited state (B2A ' ) that result in the formation of COOH +, C02 +, HCO+, and CO + ions, which were not observed in the case of collisions with xenon, may be opened up.

The situation is altered in a mixture with argon, since metastable Ar(3P2,0) atoms, which have an excitation energy sufficient for the formation of the molecular ion in the ground state, already participate in the reaction with HCOOH molecules along with the highly excited argon atoms. Under the conditions of our experiment, we were not able to isolate the contribution of the highly excited argon atoms to the formation of HCOOH + on a background of the large ~ignal- from the metastable argon atoms. The ionization function of HCOOH + prac ~ tically coincides with the excitation function of the metastable argon atoms. The At** Ryd- berg atoms can form the HCOOH + molecular ion in the excited A2A '', B2A ' , and C2A" states. COOH + and HCO + ions can be formed by means of fragmentation from the states indicated. CO~ ions (their thermochemical appearance potential is equal to 13.7 eV) can appear as a result of fragmentation only from the B2A ' and C2A '' states.

Besides the peaks for the ions formed as a result of dissociative ionization, an intense peak is observed for the ArH + ion, which appear in the process

Ar** + HCOOH-+ArHCOOH**-~ArH + + COOH + + e, (4) as is evidenced by the appearance threshold of the ion and the form of the dependence of the probability of its formation on the energy of the electrons. Similar processes with the formation of the complex iQns ArCO~, ARCH20 +, and ARCH20 ~ were observed in the case of interactions with metastable argon atoms.

In binary mixtures with neon and helium, the main contribution to the formation of all the ions observed is made by metastable Ne(2p2,0) and He(2i,3S) atoms. The Ne(2P2,0) + HCOOH process results in the populating of three excited states of the molecular ion, and the HCOOH + ion appears in the D2A ' state as an additional product in the He(21,~S) + HCOOH process. The latter opens up new channels for fragmentation of the molecular ion, as is seen from Table 3. In particular, the formation of an 0 + ion is apparently observed in the process

He (2"~) + HCOOH--,- He + HCOOH+ (D~A ') + e--,- He + 0 + + (CH~O ?) + e. (5)

~he main process resulting in the formation of an 0 + ion that competes with the reaction just presented is ionization near the metallic surface and in collisions with particles of the highly excited 0"* atom formed as a result of the dissociative excitation of a molecule of HCOOH by electron impa=t::: Figure 2 shows plots of the dependence of the yield of 0 + ions in an He + HCOOH mixture and without the admission of helium on the energy of the exciting electrons. According to [8], the 8a I orbital of HCOOH, which is responsible for the populating of the D=A ' level of the HCOOH + ion, has the highest density of the outer electrons in the C=O group. The existence of ~ channel for fragmentation of the molecular ion from the D2A ' state results in an increase in the yield of CH20 + and CO + ions in comparison to the processes with the participation of metastable neon atoms. The 10a' and 2a" orbitals display weaker activity in the Penning ionization electron spectrum [8] than in the photoionization electron

38

TABLE 3. Mass Spectra of Formic Acid Obtained with Different Ionization Methods

Reaction products

Hco6~+ COOH++H CO++(H~?) CH20 e + 0 HCO++OH

11,05 60,9 89,0 12,26 47,6 I00

10,0

1,6 12,65 100 65.0

C O + + ( H 2 0 ?) 14 ,26 17,2 O + + ( C H 2 0 ?) 5.2 RCO +

RCH20 + RH + RCH20 ~

50,0 62,5

I00

Mass spectra

9,2 12,I 100 - - 22,9 100 13,8 17,9 - - 5,9 26,0 --

100 i00 -- 13,8 40,2 -- 6,6 O.G 2,6 -- -- --

15,8 19,8 - - 14,9 100 -- 19,2 9,8 -- -- 10.2 -- 15,I . . . . . - - 0,5 0,05 ~ - - - -

- - - - 0 , 5 - - - - - -

- - - - - - 1 0 0 - - - -

- - 0,06 0.75 -- -- --

spectrum, in agreement with the weak localization of the electrons in these orbitals beyond the molecular repulsion surface. This results in a decrease in the relative yield of the HCOOH +, COOH +, and HCO + ions in the mass spectrum of He(21,aS) + HCOOH. The increased intensity of the COOH + and HCO + ions in the Ne(3P2,0) + HCOOH process in comparison to the ioniza ~ tion process of formic acid in collisions with metastable helium may be caused by the different degree of overlap of the molecular orbitals with an unfilled orbital of the impinging metastable atom, as was observed in the case of the collisions of the metastable helium and neon atoms with an acetonitrile molecule.

The formation of the complex ions NeCO~ and NeCH20 ~ along with the fragment ions was observed in a process with collisions between molecules of formic acid and metastable neon atoms; however, the efficiency of their formation was very low.

LITERATURE CITED

i. A. N. Stepanov, A. A0 Perov, S. P. Kabanov, and A. P. Simonov, "Ionization processes in binary mixtures of atoms of noble gases with molecules of C2D5CI, C2DsBr, and CD3CH2I molecules," Khim. Vys. Energ., 19, No. i, 308 (1985).

2. A. N. Stepanov, A. A. Perov, S. P. Kabanov, and V. A. Peregontsev, "Experimental investigation of ionization processes with the participation of metastable atoms and molecules," in: Metastable States of Atoms and Molecules and Methods for Their Investigation [in RUST~ ~ sian], Izd. Chuvash. Univ., Cherboksary (1981), pp. 11-28.

3. S. E. Kupriyanov, A. N. Stepanov, A. A. Perov, and S. P. Kabanov, "Mass-spectrometric investigation of the excitation functions of long-lived highly excited states of atoms of noble gases," Khim. Vys. ~nerg., 16, No. i, 10-14 (1982).

4. D. S. C. Yee and C. E. Brion, "Electron spectroscopy using excited atoms and photons. 6. Penning ionization of HCN and some related compounds," J. Electron Spectrosc. Related Phenom., 8, No. 4, 313-323 (1976).

5. K. Ohno, S. Matsumoto, K. Imai, and Y. Harada, "Penning ionization spectroscopy of nitriles," J. Phys. Chem., 8_88, No. 2, 206-209 (1984).

6. V. Cermak and A. J. Yencha, "Penning ionization electron spectroscopy of molecules containing the C~N group," J. Electron SpectrQsc.; Related Phenom., 8, No. 2, 109-1.21 (1976).

7. D. S. C. Yee and C. E. Brion, "Electron spectroscopy using excited atoms and photons. I. Penning ionization of some carbonyl-containing compounds," ibid., 8, No. 5, 377-387 (1976).

8. K. Ohno, Sn Takano, and K. Mase, "Penning ionization electron spectroscopy of molecules containing the~C=O group. Aldehydes and carboxylic acids," J. Phys. Chem., 9_O0, No. 10, 2015-2019 (1986).

9. A. Cornu and R. Massot, Compilation of Mass Spectral Data, Heyden, London (1966). 10. D. M. Rider, G. W. Ray,~ E. J. Darland, and G. E. Leroi, "A photoionization mass spectro-

metric investigation of CH3CN and CDaCN," J. Chem. Phys., 7__~&, No. 3, 1672-1660 (1981). II. A. V. Golovin, M. E. Akopyan, and Yu. L. Sergeev, "Investigation of fragmentation pro~

cesses of acetonitrile and pyridine ions by the ion-electron correspondence method," Khim. Vys. Energ., 17, No. 4, 378~380 (1983).

39

12. D. J. Knowles and A. J. C. Nicholson, "Ionization energies of formic and acetic acid monomers," J. Chem. Phys., 60, No. 3, 1810-1811 (1974).

13. A. V. Golovin, M. E. Akopyan, F. I. Vilesov, and Yu. L. Sergeev, "Investigation of the photoionization processses of formic acid and acetic acid by the ion-electron correspondence method," Khim. Vys. Energ., 13, No. 3, 200-204 (1979).

14. J. Niwa, T. Nishimura, F. Isogai, and T. Tsuchiya, "Fragmentation of energy selected formic acid ions," Chem. Phys. Lett., 74, No. i, 40-42 (1980).

15. W. Von Niessen, G. Bieri, and L. Asbrink,"30.4-nm He(II) photoelectron spectra of organic molecules.3. Oxo Compounds (C, H, 0)," J. Electron. Spectrosc. Related Phenom., 21, No. 2, 175-191 (1980).

16. V. H. Dibeler and K. S. Liston, "Mass-spectrometric study of photoionization~ 9. Hydro- gen cyanide and acetonitrile," J. Chem. Phys., 48, No. I0, 4765-4768 (1968).

17. H. M. Rosenstock, K. Draxl, B. W. Steiner, and J. T. Herron, "Energetics of gaseous ions," J. Phys. Chem. Ref. Data, ~, Suppl. No. 1 (1977).

ELECTROCATALYTIC REDUCTION OF CARBON DIOXIDE ON ELECTRODES MODIFIED BY

POLYPYRROLE WITH AN IMMOBILIZED COMPLEX OF NICKEL WITH

1,4,8,11-TETRAAZACYCLOTETRADECANE

O. V. Zhalko-Titarenko, O. A. Lazurskii, and V. D. Pokhodenko

UDC 541.138.3+542.973+541.67

Electrodes modified by polypyrrole with the imobilized complex Ni(cyclam)Cl 2 have been obtained by means of the electrochemical polymerization of pyrrole in a solution containing the complex of nickel with 1,4,8,11-tetraazacyclotetradecane (cyclam). It has been shown by means of preparative electrolysis that such electrodes can serve as an efficient electrocatalytic system for the reduction of CO2: the potential and rate of the catalyzed process differ only slightly from the case of homogeneous catalysis, but the number of catalytic cycles is increased significantly.

At the present time there is special interest in investigations of the processes of the electrochemical activation of CO and CO 2 molecules which are aimed at obtaining useful organic products such as formaldehyde, formic acid, oxalic acid, methanol, ethanol, methane, etc. from them. In view of the fact that the electrochemical reduction of CO 2 on metallic electrodes takes place with a high overvoltage (a potential more negative than -2.0 V relative to an s.c.e, is required to attain appreciable current densities) [I], the main direc- tion of the investigations in this area has been the search for efficient electrocatalysts, which lower the reduction potential of CO 2 and are selective with respect to the final products [2].

A number of compounds which display electrocatalytic properties in the reduction of CO 2 have been proposed in the past few years; among them we can single out the phthalocya- nines of nickel and cobalt [3], the clusters of iron with sulfur [4], the tetraaza macrocyclic complexes of nickel and cobalt [5], and the complexes of rhenium [6], rhodium [6], ruthenium [8], etc. It was shown in [9] that the complex of nickel with 1,4,8,11-tetraazacyclotetradecane (cyclam) is an exceptionally efficient and selective homogeneous electro~atalyst of the reduction of C02: the current efficiency with respect to CO in an aqueous solution on a mer- cury cathode is equal to 100%. On the other hand, it was noted in [10] that a 100-fold increase in the concentration of the complex [Ni(cyclem)]Cl2 results in only a threefold increase in the rate of the electrocatalytic reduction of CO 2. This finding attests to the fact that the rate of the electrocatalytic reaction indicated is determined mainly by the selective adsorption of the catalyst on the surface of the electrode and is dependent to a lesser extent on its concentration in the solution [ii]. In this context it would be wise to create electrodes having electrocatalytic properties by modifying their surfaces with electroactive redox particles, particularly with complexes of transition metals which are efficient homogeneous catalysts of the reduction of C02.

L. V. Pisarzhevskii Institute of Physical Chemistry, Academy of Sciences of the Ukrain- ian SSR, Kiev. Translated from Teoreticheskaya i Eksperimental'naya Khimiya, Vol. 26, No. I, pp. 46-51, January-February, 1990. Original article submitted August 18, 1988.

40 0040-5760/90/2601-0040512.50 �9 1990 Plenum Publishing Corporation

Documents

Ionization processes accompaning interaction of excited atoms with CH3CN, CH3COOH, and HCOOH molecules