- Email: [email protected]
Electrochemical synthesis of urea at gas-diffusion electrodes
Journal of Electroanalytical Chemistry 442 Ž1998. 67–72
Electrochemical synthesis of urea at gas-diffusion electrodes Part II. Simultaneous reduction of carbon dioxide and nitrite ions at Cu, Ag and Au catalysts Masami Shibata 1, Kohji Yoshida, Nagakazu Furuya
)
Department of Applied Chemistry, Faculty of Engineering, Yamanashi UniÕersity, 4-3-11 Takeda, Kofu 400, Japan Received 21 April 1997; received in revised form 19 September 1997
Abstract Simultaneous reduction of CO 2 and nitrite ions was examined by using the gas-diffusion electrodes with Cu, Ag and Au catalysts. The formation of urea, CO, formic acid and ammonia was found in the simultaneous reduction. The maximum current efficiencies of urea formation at Cu, Ag and Au catalysts are 37%, 38% and 26% in the range of y0.75 to y3.0 V ŽSHE., respectively. Urea is supposed to be formed from both an ammonia precursor and CO precursor on the catalysts. q 1998 Elsevier Science S.A. Keywords: CO 2 reduction with nitrite; Urea formation; Gas-diffusion electrode; Cu catalysts; Ag catalysts; Au catalysts
1. Introduction In recent years, evolution of CO 2 and nitrogen oxides attributed to the burning of an enormous volume of fossil fuels has become a global environmental issue, because it causes serious problems such as the ‘greenhouse effect’ and acid rain. Many studies for CO 2 removal have been carried out on the field of physics, chemistry and biology. The electrochemical approach toward the problem becomes advantageous when superfluous electric power and electric power from solar batteries are available. Electrochemical reduction of CO 2 has been investigated at various metal electrodes w1–10x and gas-diffusion electrodes w11–13x with various metal catalysts. We found that the current efficiency of CO formation at gas-diffusion electrodes is larger than that at massive metal electrodes w11x. In most cases, e.g., the combustion of fossil fuels and in the production of Portland cement etc., nitrogen oxides or their ions are formed simultaneously with CO 2 evolution. Therefore, it is interesting that nitrogen oxides and CO 2 can be removed. The electrochemical conversion of CO 2 with nitrite ions into other useful materials with C–N bonds has attracted much attention. ) 1
Corresponding author. E-mail: [email protected]. Also corresponding author.
0022-0728r98r$19.00 q 1998 Elsevier Science S.A. All rights reserved. PII S 0 0 2 2 - 0 7 2 8 Ž 9 7 . 0 0 5 0 4 - 4
The urea molecule, which is one of the simple compounds with C–N bonds, is constructed by a carbonyl group and two amino groups. In order to synthesize urea under mild conditions, the coexistence of activated CO species and activated ammonia species at electrocatalysts is likely to be a necessary condition. These active species are expected to be formed by the simultaneous reduction of carbon dioxide and nitrite ions. We have examined whether urea is formed at a gas-diffusion electrode with Cu catalysts by the simultaneous reduction of carbon dioxide with nitrite ions or not, and reported preliminary results concerning urea formation in this journal w14x. Current efficiencies for CO formation at gas-diffusion electrodes with Ag and Au catalysts on CO 2 reduction are larger than those with Cu catalysts w11x. In this work, we investigated whether urea can be synthesized electrochemically at gas-diffusion electrodes with Cu, Ag and Au catalysts by the simultaneous reduction of CO 2 and nitrite ions.
2. Experimental 2.1. Electrodes Gas-diffusion electrodes w15x were used as working electrodes. The gas-diffusion electrode consists of gas-dif-
68
M. Shibata et al.r Journal of Electroanalytical Chemistry 442 (1998) 67–72
fusion and reaction layers composed of a mixture of the hydrophilic portion and the hydrophobic portion. The hydrophilic portion was made of powder P1 which blends a hydrophilic carbon black and polytetrafluoroethylene. The hydrophobic portion was made of the powder P2 which blends a hydrophobic carbon black and polytetrafluoroethylene. The gas-diffusion layer made of powder P2 has many hydrophobic micropores and has the dual function of supplying CO 2 gas to the reaction layer and of preventing infiltration of the electrolyte. The reaction layer, the gasdiffusion layer and copper mesh for electrical contact were pressed under conditions of 653 K and 6000 N cmy2 . Copper chloride, silver nitrate and chloroauric acid solutions were infiltrated into the reaction layer, dried and oxidized for 1 h at 473 K. The metal oxides in the reaction layer were reduced by hydrogen gas for 2 h at 473 K. The metal catalysts of 0.036 mmol per electrode were loaded in the reaction layer. The metal-loaded gas-diffusion electrode was 40 mm in diameter and had an apparent working area of 12.6 cm2 . A Pt-loaded gas-diffusion electrode provided with hydrogen gas and an SCE were used as the auxiliary electrode and the reference electrode, respectively. All the electrode potentials in this paper refer to the SHE. 2.2. Cell and electrolyte The distance between anode and cathode was a 5 mm gap. 0.2 mol dmy3 KHCO 3 aqueous solution was used as an electrolyte, to which an arbitrary concentration of KNO 2 was added. CO 2 was provided in the cathode gas-chamber during the experiment at a flow rate of 14 ml miny1 , which was measured at outlet by a film flow meter ŽSTEC, SF-1100..
Fig. 1. Current efficiency for NH 3 formation on the reduction of NOy 2 at the gas-diffusion electrode with Cu, Ag and Au catalysts as a function of electrode potential. I Cu catalysts, ` Ag catalysts, ^ Au catalysts.
for formation of ammonia at gas-diffusion electrodes with Cu, Ag and Au catalysts are approximately 37%, 37% and 32% at y0.75 V, respectively, and decrease with increasing overpotential. The formation of hydrazine and hydroxylamine were negligible. The formation of nitrogen gas was not observed. 3.2. Reduction of carbon dioxide Current efficiencies of products on CO 2 reduction at the gas-diffusion electrode with Cu catalysts are plotted against the electrode potential in Fig. 2. The current efficiencies for the formation of CO and formic acid increase with increasing overpotential and reach maxima of about 35%
2.3. Electrolysis and analysis of products After an electrolysis for 4 min at 283 K, the products were determined by a gas chromatograph ŽHitachi, 263-30., a liquid chromatograph ŽShimadzu, LC-6A. and a steam gas chromatograph ŽOhkura Riken, SSC-1.. The urea formed was decomposed by urease into ammonia, which was measured by using a spectrophotometer ŽHitachi, U2000..
3. Results and discussion 3.1. Reduction of nitrite ions The reduction of nitrite ions was examined at a gas-diffusion electrode with Cu, Ag and Au catalysts. The nitrite ions are reduced to ammonia with hydrogen evolution. Fig. 1 shows the correlation between the electrode potential and the current efficiency for ammonia formation on the reduction of 0.02 mol dmy3 nitrite ions. The current efficiencies
Fig. 2. Current efficiency for the major reduction products from CO 2 at a Cu loaded gas-diffusion electrode as a function of electrode potential. I CO, ^ HCOOH, \ CH 4 .
M. Shibata et al.r Journal of Electroanalytical Chemistry 442 (1998) 67–72
69
Fig. 3. Current efficiency for the major reduction products from CO 2 at a Ag loaded gas-diffusion electrode as a function of electrode potential. I CO, ^ HCOOH.
Fig. 5. Current efficiency for the major reduction products from CO 2 and NOy 2 at a Cu loaded gas-diffusion electrode as a function of electrode potential. ` NH 3 , I CO, ^ HCOOH, l ŽNH 2 . 2 CO.
and 20%, respectively. Methane formation was observed at potentials lower than y1.75 V. These results are similar to the previous ones w2x. Current efficiencies of products on CO 2 reduction at the gas-diffusion electrode with Ag catalysts are plotted against the electrode potential in Fig. 3. The current efficiencies for the formation of CO and formic acid increase with increasing overpotential and reach maxima of about 85% and 15%, respectively. These results are similar to the previous ones w2x. Current efficiencies of products on CO 2 reduction at the gas-diffusion electrode with Au catalysts are plotted against the electrode potential in Fig. 4. The current efficiency for the formation of CO increases with increasing overpoten-
tial and reaches a maximum at about 47%. The current efficiency for the formation of formic acid is about 10% at potentials lower than y0.75 V. These results are similar to previous ones w2x.
Fig. 4. Current efficiency for the major reduction products from CO 2 at a Au loaded gas-diffusion electrode as a function of electrode potential. I CO, ^ HCOOH.
Fig. 6. Current efficiency for the major reduction products from CO 2 and NOy 2 at a Ag loaded gas-diffusion electrode as a function of electrode potential. ` NH 3 , I CO, ^ HCOOH, l ŽNH 2 . 2 CO.
3.3. Simultaneous reduction of CO2 and nitrite ions Fig. 5 shows the correlation between the electrode potential and the current efficiency of products at a Culoaded gas diffusion electrode on the simultaneous reduction of CO 2 and 0.02 mol dmy3 nitrite ions. The current efficiency for the formation of CO increases with increasing overpotential and becomes constant at about 32%, which is lower than that on the reduction of CO 2 without
70
M. Shibata et al.r Journal of Electroanalytical Chemistry 442 (1998) 67–72
Fig. 7. Current efficiency for the major reduction products from CO 2 and NOy 2 at a Au loaded gas-diffusion electrode as a function of electrode potential. ` NH 3 , I CO, ^ HCOOH, l ŽNH 2 . 2 CO.
nitrite ions. The current efficiency for the formation of HCOOH increases with increasing overpotential and becomes constant at about 15% at lower than y1.25 V. The current efficiency for the formation of ammonia is 35% at y0.75 V and this decreases with increasing overpotential, which is lower than that on the reduction of nitrite ions without CO 2 . The formation of urea was found during the simultaneous reduction of CO 2 and nitrite ions. The current efficiency of urea is approximately 37% at y0.75 V and then decreases with increasing overpotential. Fig. 6 shows the correlation between electrode potential and the current efficiency of products at a Ag-loaded gas-diffusion electrode during the simultaneous reduction of CO 2 and nitrite ions. The current efficiency for the
Fig. 8. Partial current density for urea formation on the simultaneous reduction of CO 2 and NOy 2 at the gas-diffusion electrode with Cu, Ag and Au catalysts as a function of electrode potential. I Cu catalysts, ` Ag catalysts, ^ Au catalysts.
formation of CO increases with increasing overpotential, reaches a maximum of 52% at y2.6 V and then decreases. The current efficiency for the formation of HCOOH increases slightly with increasing overpotential and becomes constant at about 15% at potentials lower than y2.0 V. The current efficiency for the formation of ammonia is 61% at y0.75 V and this decreases with increasing overpotential. The formation of urea was found at the simultaneous reduction of CO 2 and nitrite ions. The current efficiency of urea is approximately 37% at y0.75 V and this decreases with increasing overpotential. Fig. 7 shows the correlation between electrode potential and the current efficiency of products at a Au-loaded gas-diffusion electrode on the simultaneous reduction of CO 2 and nitrite ions. The current efficiency for the formation of CO increases with increasing overpotential, reaches a maximum of 41% at y1.6 V and this decreases. The current efficiency for the formation of HCOOH decreases with increasing overpotential. The current efficiency for the formation of ammonia is 46% at y0.75 V and this decreases with increasing overpotential, which is lower than that on the reduction of nitrite ions without CO 2 . The formation of urea was found during the simultaneous reduction of CO 2 and nitrite ions. The current efficiency of urea increases with increasing overpotential and becomes constant at about 25%. The current efficiency for urea formation around y1.0 V on Cu catalysts is similar to that on Ag catalysts and is larger than that on Au catalysts. But the current efficiency for urea formation at values lower than y2.0 V on Au catalysts is larger than that on the other catalysts. Rates of urea formation on the simultaneous reduction of CO 2 and NOy 2 were examined at gas-diffusion electrodes with Cu, Ag and Au catalysts. Fig. 8 shows the
Fig. 9. Current efficiency for the major reduction products from CO 2 and NOy at a Cu loaded gas-diffusion electrode as a function of the 2 ŽM s mol dmy3 .. ` NH 3 , I CO, ^ HCOOH, concentration of NOy 2 l ŽNH 2 . 2 CO.
M. Shibata et al.r Journal of Electroanalytical Chemistry 442 (1998) 67–72
71
Fig. 10. Current efficiency for the major reduction products from CO 2 and NOy 2 at a Ag loaded gas-diffusion electrode as a function of the ŽM s mol dmy3 .. ` NH 3 , I CO, ^ HCOOH, concentration of NOy 2 Ž . l NH 2 2 CO.
Fig. 11. Current efficiency for the major reduction products from CO 2 and NOy 2 at a Au loaded gas-diffusion electrode as a function of the ŽM s mol dmy3 .. ` NH 3 , I CO, ^ HCOOH, concentration of NOy 2 l ŽNH 2 . 2 CO.
correlation between the partial currents for urea formation and the electrode potentials. The rates for urea formation around y1.0 V on Cu catalysts are similar to those on Ag catalysts and are larger than those on Au catalysts. But the rate for urea formation on Au catalysts at values lower than y2.0 V is larger than on the other catalysts.
ing nitrite concentration in the electrolyte. When the current efficiency of ammonia formation increases at high concentration of nitrite, that of CO formation decreases, i.e., when the current efficiency of urea formation increases, the CO formed is consumed by the formation of urea. Fig. 11 shows the current efficiencies of the products at the simultaneous reduction of CO 2 and various concentration of nitrite at y1.5 V. The current efficiency of ammonia formation increases with increasing nitrite concentration in the electrolyte and then becomes constant at concentrations larger than 0.5 mol dmy3 . The current efficiency of urea formation increases with increasing nitrite
3.4. Influence of concentration of nitrite ions on urea formation Fig. 9 shows the current efficiencies of the products for the simultaneous reduction of CO 2 and various concentration of nitrite ions at y1.5 V. The current efficiency of ammonia formation increases with increasing nitrite concentration in the electrolyte. The current efficiency of urea formation increases with increasing nitrite concentration in the electrolyte, i.e., the tendency is similar to that of ammonia formation. In contrast, the current efficiency of CO formation decreases with increasing nitrite concentration in the electrolyte. When the current efficiency of ammonia formation increases at high concentration of nitrite, that of CO formation decreases, i.e., when the current efficiency of urea formation increases, the CO formed is consumed by the formation of urea. Fig. 10 shows the current efficiencies of the products for the simultaneous reduction of CO 2 and various concentrations of nitrite at y1.5 V. The current efficiency of ammonia formation increases with increasing nitrite concentration in the electrolyte and then becomes constant at concentrations larger than 0.02 mol dmy3 . The current efficiency of urea formation increases with increasing nitrite concentration in the electrolyte, which tendency is similar to that of ammonia formation. In contrast, the current efficiency of CO formation decreases with increas-
Fig. 12. Current efficiency for the major reduction products from CO 2 and NH 3 at a Cu loaded gas-diffusion electrode as a function of the concentration of NH 3 . I CO, ^ HCOOH, l ŽNH 2 . 2 CO.
72
M. Shibata et al.r Journal of Electroanalytical Chemistry 442 (1998) 67–72
concentration in the electrolyte, which tendency is similar to that of ammonia formation. In contrast, the current efficiency of CO formation decreases with increasing nitrite concentration in the electrolyte. When the current efficiency of ammonia formation increases at high concentration of nitrite, that of CO formation decreases, i.e., when the current efficiency of urea formation increases, the CO formed is consumed by the formation of urea. 3.5. Urea formation at gas-diffusion electrodes To elucidate whether CO formed on CO 2 reduction combines with ammonia in the electrolyte to form urea, the simultaneous reduction of CO 2 with various concentrations of ammonia was carried out. Fig. 12 shows correlation between ammonia concentrations and the current efficiencies of the products at a gas-diffusion electrode with Cu catalysts. The formation of formic acid and CO was observed, but urea formation could not be observed. Furthermore, to elucidate whether ammonia formed on the reduction of nitrite ions combines with CO provided to urea or not, the reduction of nitrite ions with CO gas instead of CO 2 gas was carried out. As a result, ammonia formation was observed, but no formation of urea could be found. In addition, urea formation cannot be found on the simultaneous reduction of CO with ammonia, formic acid with nitrite ions and formic acid with ammonia. Namely, urea is formed on the simultaneous reduction of CO 2 and nitrite ions. CO 2 and nitrite ions are supposed to be reduced to a CO precursor and ammonia precursor on the catalysts, respectively. Urea is expected to be formed by the reaction of both the ammonia precursor and the CO precursor on the catalysts. Although these precursors were not detected, the urea formation could be described as follows: CO 2 Ž g . q 2Hqq 2ey™ CO Ž pre . q H 2 O q y NOy 2 q 6H q 5e ™ NH 2 Ž pre . q 2H 2 O
CO Ž pre . q 2NH 2 Ž pre . ™ Ž NH 2 . 2 CO
where COŽpre. and NH 2 Žpre. represent CO precursor and ammonia precursor, respectively, which would be adsorbed on the catalysts. 4. Conclusions The formation of urea, CO, formic acid and ammonia in the simultaneous reduction of CO 2 and nitrite ions was found at the gas-diffusion electrodes with Cu, Ag and Au catalysts. The maximum current efficiencies of urea formation at Cu, Ag and Au catalysts are 37%, 38% and 26%, respectively. Urea is supposed to be formed from the ammonia precursor formed from nitrite ions and CO precursor formed from CO 2 on the catalysts. References w1x I. Taniguchi, in: J.O’M. Bockris, R.E. White, B.E. Conway ŽEds.., Modern Aspects of Electrochemistry, Vol. 20, Plenum, New York, 1989, p. 327. w2x Y. Hori, K. Kikuchi, S. Suzuki, Chem. Lett. 1695 Ž1985. . w3x S. Ikeda, T. Takagi, K. Ito, Bull. Chem. Soc. Jpn. 60 Ž1987. 2517. w4x R.L. Cook, R.C. MacDuff, A. Sammells, J. Electrochem. Soc. 134 Ž1987. 2375. w5x J.K.W. Frese, S. Leach, J. Electrochem. Soc. 132 Ž1985. 259. w6x D.P. Summers, S. Leach, J.K.W. Frese, J. Electroanal. Chem. 205 Ž1986. 219. w7x D.P. Summers, J.K.W. Frese, Langmuir 4 Ž1988. 51. w8x J.J. Kim, D.P. Summers, J.K.W. Frese, J. Electroanal. Chem. 245 Ž1988. 223. w9x M. Azuma, K. Hashimoto, M. Hiramoto, M. Watanabe, T. Sakata, J. Electrochem. Soc. 137 Ž1990. 1772. w10x M. Watanabe, M. Shibata, A. Kato, M. Azuma, T. Sakata, J. Electrochem. Soc. 138 Ž1991. 3382. w11x N. Furuya, K. Matsui, S. Motoo, Denki Kagaku 56 Ž1988. 980. w12x R.L. Cook, R.C. MacDuff, A.F. Sammells, J. Electrochem. Soc. 137 Ž1990. 607. w13x M. Schwartz, M.E. Vercauteren, A.F. Sammells, J. Electrochem. Soc. 141 Ž1994. 3119. w14x M. Shibata, K. Yoshida, N. Furuya, J. Electroanal. Chem. 387 Ž1995. 143. w15x S. Motoo, M. Watanabe, N. Furuya, J. Electroanal. Chem. 160 Ž1984. 351.
Electrochemical synthesis of urea at gas-diffusion electrodes Part II. Simultaneous reduction of carbon dioxide and nitrite ions at Cu, Ag and Au catalysts Masami Shibata 1, Kohji Yoshida, Nagakazu Furuya
)
Department of Applied Chemistry, Faculty of Engineering, Yamanashi UniÕersity, 4-3-11 Takeda, Kofu 400, Japan Received 21 April 1997; received in revised form 19 September 1997
Abstract Simultaneous reduction of CO 2 and nitrite ions was examined by using the gas-diffusion electrodes with Cu, Ag and Au catalysts. The formation of urea, CO, formic acid and ammonia was found in the simultaneous reduction. The maximum current efficiencies of urea formation at Cu, Ag and Au catalysts are 37%, 38% and 26% in the range of y0.75 to y3.0 V ŽSHE., respectively. Urea is supposed to be formed from both an ammonia precursor and CO precursor on the catalysts. q 1998 Elsevier Science S.A. Keywords: CO 2 reduction with nitrite; Urea formation; Gas-diffusion electrode; Cu catalysts; Ag catalysts; Au catalysts
1. Introduction In recent years, evolution of CO 2 and nitrogen oxides attributed to the burning of an enormous volume of fossil fuels has become a global environmental issue, because it causes serious problems such as the ‘greenhouse effect’ and acid rain. Many studies for CO 2 removal have been carried out on the field of physics, chemistry and biology. The electrochemical approach toward the problem becomes advantageous when superfluous electric power and electric power from solar batteries are available. Electrochemical reduction of CO 2 has been investigated at various metal electrodes w1–10x and gas-diffusion electrodes w11–13x with various metal catalysts. We found that the current efficiency of CO formation at gas-diffusion electrodes is larger than that at massive metal electrodes w11x. In most cases, e.g., the combustion of fossil fuels and in the production of Portland cement etc., nitrogen oxides or their ions are formed simultaneously with CO 2 evolution. Therefore, it is interesting that nitrogen oxides and CO 2 can be removed. The electrochemical conversion of CO 2 with nitrite ions into other useful materials with C–N bonds has attracted much attention. ) 1
Corresponding author. E-mail: [email protected]. Also corresponding author.
0022-0728r98r$19.00 q 1998 Elsevier Science S.A. All rights reserved. PII S 0 0 2 2 - 0 7 2 8 Ž 9 7 . 0 0 5 0 4 - 4
The urea molecule, which is one of the simple compounds with C–N bonds, is constructed by a carbonyl group and two amino groups. In order to synthesize urea under mild conditions, the coexistence of activated CO species and activated ammonia species at electrocatalysts is likely to be a necessary condition. These active species are expected to be formed by the simultaneous reduction of carbon dioxide and nitrite ions. We have examined whether urea is formed at a gas-diffusion electrode with Cu catalysts by the simultaneous reduction of carbon dioxide with nitrite ions or not, and reported preliminary results concerning urea formation in this journal w14x. Current efficiencies for CO formation at gas-diffusion electrodes with Ag and Au catalysts on CO 2 reduction are larger than those with Cu catalysts w11x. In this work, we investigated whether urea can be synthesized electrochemically at gas-diffusion electrodes with Cu, Ag and Au catalysts by the simultaneous reduction of CO 2 and nitrite ions.
2. Experimental 2.1. Electrodes Gas-diffusion electrodes w15x were used as working electrodes. The gas-diffusion electrode consists of gas-dif-
68
M. Shibata et al.r Journal of Electroanalytical Chemistry 442 (1998) 67–72
fusion and reaction layers composed of a mixture of the hydrophilic portion and the hydrophobic portion. The hydrophilic portion was made of powder P1 which blends a hydrophilic carbon black and polytetrafluoroethylene. The hydrophobic portion was made of the powder P2 which blends a hydrophobic carbon black and polytetrafluoroethylene. The gas-diffusion layer made of powder P2 has many hydrophobic micropores and has the dual function of supplying CO 2 gas to the reaction layer and of preventing infiltration of the electrolyte. The reaction layer, the gasdiffusion layer and copper mesh for electrical contact were pressed under conditions of 653 K and 6000 N cmy2 . Copper chloride, silver nitrate and chloroauric acid solutions were infiltrated into the reaction layer, dried and oxidized for 1 h at 473 K. The metal oxides in the reaction layer were reduced by hydrogen gas for 2 h at 473 K. The metal catalysts of 0.036 mmol per electrode were loaded in the reaction layer. The metal-loaded gas-diffusion electrode was 40 mm in diameter and had an apparent working area of 12.6 cm2 . A Pt-loaded gas-diffusion electrode provided with hydrogen gas and an SCE were used as the auxiliary electrode and the reference electrode, respectively. All the electrode potentials in this paper refer to the SHE. 2.2. Cell and electrolyte The distance between anode and cathode was a 5 mm gap. 0.2 mol dmy3 KHCO 3 aqueous solution was used as an electrolyte, to which an arbitrary concentration of KNO 2 was added. CO 2 was provided in the cathode gas-chamber during the experiment at a flow rate of 14 ml miny1 , which was measured at outlet by a film flow meter ŽSTEC, SF-1100..
Fig. 1. Current efficiency for NH 3 formation on the reduction of NOy 2 at the gas-diffusion electrode with Cu, Ag and Au catalysts as a function of electrode potential. I Cu catalysts, ` Ag catalysts, ^ Au catalysts.
for formation of ammonia at gas-diffusion electrodes with Cu, Ag and Au catalysts are approximately 37%, 37% and 32% at y0.75 V, respectively, and decrease with increasing overpotential. The formation of hydrazine and hydroxylamine were negligible. The formation of nitrogen gas was not observed. 3.2. Reduction of carbon dioxide Current efficiencies of products on CO 2 reduction at the gas-diffusion electrode with Cu catalysts are plotted against the electrode potential in Fig. 2. The current efficiencies for the formation of CO and formic acid increase with increasing overpotential and reach maxima of about 35%
2.3. Electrolysis and analysis of products After an electrolysis for 4 min at 283 K, the products were determined by a gas chromatograph ŽHitachi, 263-30., a liquid chromatograph ŽShimadzu, LC-6A. and a steam gas chromatograph ŽOhkura Riken, SSC-1.. The urea formed was decomposed by urease into ammonia, which was measured by using a spectrophotometer ŽHitachi, U2000..
3. Results and discussion 3.1. Reduction of nitrite ions The reduction of nitrite ions was examined at a gas-diffusion electrode with Cu, Ag and Au catalysts. The nitrite ions are reduced to ammonia with hydrogen evolution. Fig. 1 shows the correlation between the electrode potential and the current efficiency for ammonia formation on the reduction of 0.02 mol dmy3 nitrite ions. The current efficiencies
Fig. 2. Current efficiency for the major reduction products from CO 2 at a Cu loaded gas-diffusion electrode as a function of electrode potential. I CO, ^ HCOOH, \ CH 4 .
M. Shibata et al.r Journal of Electroanalytical Chemistry 442 (1998) 67–72
69
Fig. 3. Current efficiency for the major reduction products from CO 2 at a Ag loaded gas-diffusion electrode as a function of electrode potential. I CO, ^ HCOOH.
Fig. 5. Current efficiency for the major reduction products from CO 2 and NOy 2 at a Cu loaded gas-diffusion electrode as a function of electrode potential. ` NH 3 , I CO, ^ HCOOH, l ŽNH 2 . 2 CO.
and 20%, respectively. Methane formation was observed at potentials lower than y1.75 V. These results are similar to the previous ones w2x. Current efficiencies of products on CO 2 reduction at the gas-diffusion electrode with Ag catalysts are plotted against the electrode potential in Fig. 3. The current efficiencies for the formation of CO and formic acid increase with increasing overpotential and reach maxima of about 85% and 15%, respectively. These results are similar to the previous ones w2x. Current efficiencies of products on CO 2 reduction at the gas-diffusion electrode with Au catalysts are plotted against the electrode potential in Fig. 4. The current efficiency for the formation of CO increases with increasing overpoten-
tial and reaches a maximum at about 47%. The current efficiency for the formation of formic acid is about 10% at potentials lower than y0.75 V. These results are similar to previous ones w2x.
Fig. 4. Current efficiency for the major reduction products from CO 2 at a Au loaded gas-diffusion electrode as a function of electrode potential. I CO, ^ HCOOH.
Fig. 6. Current efficiency for the major reduction products from CO 2 and NOy 2 at a Ag loaded gas-diffusion electrode as a function of electrode potential. ` NH 3 , I CO, ^ HCOOH, l ŽNH 2 . 2 CO.
3.3. Simultaneous reduction of CO2 and nitrite ions Fig. 5 shows the correlation between the electrode potential and the current efficiency of products at a Culoaded gas diffusion electrode on the simultaneous reduction of CO 2 and 0.02 mol dmy3 nitrite ions. The current efficiency for the formation of CO increases with increasing overpotential and becomes constant at about 32%, which is lower than that on the reduction of CO 2 without
70
M. Shibata et al.r Journal of Electroanalytical Chemistry 442 (1998) 67–72
Fig. 7. Current efficiency for the major reduction products from CO 2 and NOy 2 at a Au loaded gas-diffusion electrode as a function of electrode potential. ` NH 3 , I CO, ^ HCOOH, l ŽNH 2 . 2 CO.
nitrite ions. The current efficiency for the formation of HCOOH increases with increasing overpotential and becomes constant at about 15% at lower than y1.25 V. The current efficiency for the formation of ammonia is 35% at y0.75 V and this decreases with increasing overpotential, which is lower than that on the reduction of nitrite ions without CO 2 . The formation of urea was found during the simultaneous reduction of CO 2 and nitrite ions. The current efficiency of urea is approximately 37% at y0.75 V and then decreases with increasing overpotential. Fig. 6 shows the correlation between electrode potential and the current efficiency of products at a Ag-loaded gas-diffusion electrode during the simultaneous reduction of CO 2 and nitrite ions. The current efficiency for the
Fig. 8. Partial current density for urea formation on the simultaneous reduction of CO 2 and NOy 2 at the gas-diffusion electrode with Cu, Ag and Au catalysts as a function of electrode potential. I Cu catalysts, ` Ag catalysts, ^ Au catalysts.
formation of CO increases with increasing overpotential, reaches a maximum of 52% at y2.6 V and then decreases. The current efficiency for the formation of HCOOH increases slightly with increasing overpotential and becomes constant at about 15% at potentials lower than y2.0 V. The current efficiency for the formation of ammonia is 61% at y0.75 V and this decreases with increasing overpotential. The formation of urea was found at the simultaneous reduction of CO 2 and nitrite ions. The current efficiency of urea is approximately 37% at y0.75 V and this decreases with increasing overpotential. Fig. 7 shows the correlation between electrode potential and the current efficiency of products at a Au-loaded gas-diffusion electrode on the simultaneous reduction of CO 2 and nitrite ions. The current efficiency for the formation of CO increases with increasing overpotential, reaches a maximum of 41% at y1.6 V and this decreases. The current efficiency for the formation of HCOOH decreases with increasing overpotential. The current efficiency for the formation of ammonia is 46% at y0.75 V and this decreases with increasing overpotential, which is lower than that on the reduction of nitrite ions without CO 2 . The formation of urea was found during the simultaneous reduction of CO 2 and nitrite ions. The current efficiency of urea increases with increasing overpotential and becomes constant at about 25%. The current efficiency for urea formation around y1.0 V on Cu catalysts is similar to that on Ag catalysts and is larger than that on Au catalysts. But the current efficiency for urea formation at values lower than y2.0 V on Au catalysts is larger than that on the other catalysts. Rates of urea formation on the simultaneous reduction of CO 2 and NOy 2 were examined at gas-diffusion electrodes with Cu, Ag and Au catalysts. Fig. 8 shows the
Fig. 9. Current efficiency for the major reduction products from CO 2 and NOy at a Cu loaded gas-diffusion electrode as a function of the 2 ŽM s mol dmy3 .. ` NH 3 , I CO, ^ HCOOH, concentration of NOy 2 l ŽNH 2 . 2 CO.
M. Shibata et al.r Journal of Electroanalytical Chemistry 442 (1998) 67–72
71
Fig. 10. Current efficiency for the major reduction products from CO 2 and NOy 2 at a Ag loaded gas-diffusion electrode as a function of the ŽM s mol dmy3 .. ` NH 3 , I CO, ^ HCOOH, concentration of NOy 2 Ž . l NH 2 2 CO.
Fig. 11. Current efficiency for the major reduction products from CO 2 and NOy 2 at a Au loaded gas-diffusion electrode as a function of the ŽM s mol dmy3 .. ` NH 3 , I CO, ^ HCOOH, concentration of NOy 2 l ŽNH 2 . 2 CO.
correlation between the partial currents for urea formation and the electrode potentials. The rates for urea formation around y1.0 V on Cu catalysts are similar to those on Ag catalysts and are larger than those on Au catalysts. But the rate for urea formation on Au catalysts at values lower than y2.0 V is larger than on the other catalysts.
ing nitrite concentration in the electrolyte. When the current efficiency of ammonia formation increases at high concentration of nitrite, that of CO formation decreases, i.e., when the current efficiency of urea formation increases, the CO formed is consumed by the formation of urea. Fig. 11 shows the current efficiencies of the products at the simultaneous reduction of CO 2 and various concentration of nitrite at y1.5 V. The current efficiency of ammonia formation increases with increasing nitrite concentration in the electrolyte and then becomes constant at concentrations larger than 0.5 mol dmy3 . The current efficiency of urea formation increases with increasing nitrite
3.4. Influence of concentration of nitrite ions on urea formation Fig. 9 shows the current efficiencies of the products for the simultaneous reduction of CO 2 and various concentration of nitrite ions at y1.5 V. The current efficiency of ammonia formation increases with increasing nitrite concentration in the electrolyte. The current efficiency of urea formation increases with increasing nitrite concentration in the electrolyte, i.e., the tendency is similar to that of ammonia formation. In contrast, the current efficiency of CO formation decreases with increasing nitrite concentration in the electrolyte. When the current efficiency of ammonia formation increases at high concentration of nitrite, that of CO formation decreases, i.e., when the current efficiency of urea formation increases, the CO formed is consumed by the formation of urea. Fig. 10 shows the current efficiencies of the products for the simultaneous reduction of CO 2 and various concentrations of nitrite at y1.5 V. The current efficiency of ammonia formation increases with increasing nitrite concentration in the electrolyte and then becomes constant at concentrations larger than 0.02 mol dmy3 . The current efficiency of urea formation increases with increasing nitrite concentration in the electrolyte, which tendency is similar to that of ammonia formation. In contrast, the current efficiency of CO formation decreases with increas-
Fig. 12. Current efficiency for the major reduction products from CO 2 and NH 3 at a Cu loaded gas-diffusion electrode as a function of the concentration of NH 3 . I CO, ^ HCOOH, l ŽNH 2 . 2 CO.
72
M. Shibata et al.r Journal of Electroanalytical Chemistry 442 (1998) 67–72
concentration in the electrolyte, which tendency is similar to that of ammonia formation. In contrast, the current efficiency of CO formation decreases with increasing nitrite concentration in the electrolyte. When the current efficiency of ammonia formation increases at high concentration of nitrite, that of CO formation decreases, i.e., when the current efficiency of urea formation increases, the CO formed is consumed by the formation of urea. 3.5. Urea formation at gas-diffusion electrodes To elucidate whether CO formed on CO 2 reduction combines with ammonia in the electrolyte to form urea, the simultaneous reduction of CO 2 with various concentrations of ammonia was carried out. Fig. 12 shows correlation between ammonia concentrations and the current efficiencies of the products at a gas-diffusion electrode with Cu catalysts. The formation of formic acid and CO was observed, but urea formation could not be observed. Furthermore, to elucidate whether ammonia formed on the reduction of nitrite ions combines with CO provided to urea or not, the reduction of nitrite ions with CO gas instead of CO 2 gas was carried out. As a result, ammonia formation was observed, but no formation of urea could be found. In addition, urea formation cannot be found on the simultaneous reduction of CO with ammonia, formic acid with nitrite ions and formic acid with ammonia. Namely, urea is formed on the simultaneous reduction of CO 2 and nitrite ions. CO 2 and nitrite ions are supposed to be reduced to a CO precursor and ammonia precursor on the catalysts, respectively. Urea is expected to be formed by the reaction of both the ammonia precursor and the CO precursor on the catalysts. Although these precursors were not detected, the urea formation could be described as follows: CO 2 Ž g . q 2Hqq 2ey™ CO Ž pre . q H 2 O q y NOy 2 q 6H q 5e ™ NH 2 Ž pre . q 2H 2 O
CO Ž pre . q 2NH 2 Ž pre . ™ Ž NH 2 . 2 CO
where COŽpre. and NH 2 Žpre. represent CO precursor and ammonia precursor, respectively, which would be adsorbed on the catalysts. 4. Conclusions The formation of urea, CO, formic acid and ammonia in the simultaneous reduction of CO 2 and nitrite ions was found at the gas-diffusion electrodes with Cu, Ag and Au catalysts. The maximum current efficiencies of urea formation at Cu, Ag and Au catalysts are 37%, 38% and 26%, respectively. Urea is supposed to be formed from the ammonia precursor formed from nitrite ions and CO precursor formed from CO 2 on the catalysts. References w1x I. Taniguchi, in: J.O’M. Bockris, R.E. White, B.E. Conway ŽEds.., Modern Aspects of Electrochemistry, Vol. 20, Plenum, New York, 1989, p. 327. w2x Y. Hori, K. Kikuchi, S. Suzuki, Chem. Lett. 1695 Ž1985. . w3x S. Ikeda, T. Takagi, K. Ito, Bull. Chem. Soc. Jpn. 60 Ž1987. 2517. w4x R.L. Cook, R.C. MacDuff, A. Sammells, J. Electrochem. Soc. 134 Ž1987. 2375. w5x J.K.W. Frese, S. Leach, J. Electrochem. Soc. 132 Ž1985. 259. w6x D.P. Summers, S. Leach, J.K.W. Frese, J. Electroanal. Chem. 205 Ž1986. 219. w7x D.P. Summers, J.K.W. Frese, Langmuir 4 Ž1988. 51. w8x J.J. Kim, D.P. Summers, J.K.W. Frese, J. Electroanal. Chem. 245 Ž1988. 223. w9x M. Azuma, K. Hashimoto, M. Hiramoto, M. Watanabe, T. Sakata, J. Electrochem. Soc. 137 Ž1990. 1772. w10x M. Watanabe, M. Shibata, A. Kato, M. Azuma, T. Sakata, J. Electrochem. Soc. 138 Ž1991. 3382. w11x N. Furuya, K. Matsui, S. Motoo, Denki Kagaku 56 Ž1988. 980. w12x R.L. Cook, R.C. MacDuff, A.F. Sammells, J. Electrochem. Soc. 137 Ž1990. 607. w13x M. Schwartz, M.E. Vercauteren, A.F. Sammells, J. Electrochem. Soc. 141 Ž1994. 3119. w14x M. Shibata, K. Yoshida, N. Furuya, J. Electroanal. Chem. 387 Ž1995. 143. w15x S. Motoo, M. Watanabe, N. Furuya, J. Electroanal. Chem. 160 Ž1984. 351.