- Email: [email protected]
Electroreduction of nitrogen to ammonia on gas-diffusion electrodes loaded with inorganic catalyst
269
J. Electroanal. Chem., 291 (1990) 269-212 Elsevier Sequoia S.A., Lausanne
Preliminary note
Electroreduction of nitrogen to ammonia on gas-diffusion electrodes loaded with inorganic catalyst Nagakazu Furuya and Hiroshi Yoshiba Department
of Applied Chemistry,
Faculty of Engineering,
Yamanashi
Universi@, Takeda-4, Kofu (Japan)
(Received 6 August 1990) INTRODUCTION
Nitrogen can be reduced to ammonia under ambient temperature and pressure by electroreduction using a gas-diffusion electrode modified by metal-phthalocyanines [1,2] in various electrolytes [3]. The current efficiency and the stability of a gas diffusion electrode depends strongly on the nature of the central metal and a series of supporting electrolyte media. The Sn-phthalocyanine is the best catalyst in terms of current efficiency and the stability of the gas-diffusion electrode in 1 M KOH. The current efficiency of ammonia production at -0.4 V vs. RHE was ca. 1.85% at the beginning of electrolysis, and ca. 1.2% after 25 min. However, in the Haber process ammonia is manufactured in the presence of an iron catalyst. We thought that there might be catalysts other than metal phthalocyanines for ammonia synthesis. Inorganic compounds such as metals and metal oxides were made the object of research. A gas diffusion electrode loaded with a fine powder of these inorganic compounds was produced. The electrode was used for experiments on the electroreduction of nitrogen to ammonia. The purpose of the present study was to investigate the activity of these inorganic catalysts. EXPERIMENTAL
The electrolysis cell is the same as that which has been described in the previous paper [3]. The cell consists of the two gas-diffusion electrodes, separated by 1 mm, loaded with ca. lo-20 mg/cm2 inorganic catalysts (cathode) and 0.56 mg/cm2 Pt (anode), respectively. Inorganic catalyst powder (metal, metal sulfides, metal oxides, metal boride, metal selenide and metal carbide) was prepared. The electrode active area was 12.56 cm2 in each case. A gas-diffusion electrode loaded with Pt catalyst was used as the anode; it was depolarized by hydrogen in order to prevent the
270
oxidation of ammonia produced at the cathode. Electrolysis was conducted under potentiostatic conditions at -1.0 V vs. RHE for 1 min and -0.5 V vs. RHE for 30 min, in 1 M KOH under ambient pressure and temperature, 25 o C, while N, and H, were supplied at a constant flow rate from the back sides of the cathode and anode, respectively. The amount of ammonia in the solution produced by the electroreduction was determined by absorptiometry using Nessler’s reagent and an ammonia ion meter.
RESULTS AND DISCUSSION
The current efficiency for ammonia formation is shown in Table 1.
TABLE 1 The current efficiency of ammonia production of Fe fine powder and Carbon black, metal oxides, metal sulfides, and ZnSe, TiB, and Sic, at - 1.0 V vs. RHE catalyst
current I/A
10’ X amount of NH, produced/m01 mm’
current efficiency/%
Fe C
4.10 0.73
10.1 0.72
0.119 0.048
PbO-TiOr Sb-SnO, ZnO Sn-In r03 SnOz
1.25 2.58 4.15 3.83 2.55 4.22 2.85
7.06 11.3 11.9 8.26 4.77 6.50 2.50
0.273 0.211 0.138 0.104 0.090 0.074 0.042
ASS cos
2.13 1.66 2.03 2.02 3.29 1LO.50 4.62 2.51 5.17 3.97 3.69 8.86 6.84 4.37
42.6 29.2 31.2 7.62 11.8 22.8 9.34 5.00 9.11 3.28 2.37 5.75 3.44 1.32
0.964 0.849 0.741 0.182 0.172 0.105 0.098 0.096 0.085 0.040 0.031 0.031 0.024 0.015
ZnSe TiB, Sic
1.81 2.00 1.72
48.5 46.1 3.25
1.293 1.111 0.091
Fe203 TiO ZnS NiS CdS cus Bi,S, FeS MoS Sb,S, SnS MnS PbS PdS
271
Iron and carbon The current efficiency of the iron and carbon powder was 0.119% and 0.048% respectively. It is clear that nitrogen can be reduced to ammonia on iron catalysts. Metal oxides For metal oxides the current efficiency of PbO-TiO, is the highest, (ca. 0.273% at the beginning of electrolysis), and it decreases in the order PbO-TiO, > Sb-SnO, > ZnO > Sn-In,O, > SnO, > Fe,,O, > TiO. It may also be noted that the current efficiency depends on the metal and its oxidation state. Metal sulfides Here too, the efficiency depends strongly on the metal, and is highest for ZnS, ca. 0.964% at - 1.0 V vs. RHE. The efficiency decreases in the order NiS > CdS > CuS > Bi,S, > FeS > MoS > Sb,S, > SnS > MnS > PdS > PbS > AgS > CoS. Metal sulfides are better catalysts than the corresponding oxides. Sic, TiB, and ZnSe The current efficiency of the electrodes loaded with ZnSe; TiB, 1.298, 1.11% and 0.09%, respectively.
and SIC was
Stability of ammonia production Figure 1 shows the time dependence of the current efficiency on the ZnSe, Ti& and PbS at - 0.5 V vs. RHE. The stability of current efficiency depends strongly on the catalyst, and is highest on the ZnSe, ca. 2.24% after 10 min. The highest stability of the gas-diffusion electrode is also observed on the ZnSe. Even after 20 min, the current efficiency is 2.03%. The current efficiency on the NiS, CdS and Fe was 0.0436, 0.04% and O.OO%,respectively, after 10 min at -0.5 V vs. RHE.
Time Of Electrolysis
/ min
Fig. 1. Time dependence of the current efficiency of the NH, production in 1 M KOH at -0.50 RHE. (0) ZnSe, (A) Ti&, (0) ZnS.
V vs.
212
Further studies are now in progress in our laboratory to search for better catalysts for electrochemical nitrogen fixation, such as metal boride. CONCLUSIONS
Electroreduction of nitrogen to ammonia can be conducted under ambient pressure and temperature using a gas-diffusion electrode modified by 26 kinds of inorganic catalyst. The order of current efficiency is ZnSe > TiB, > ZnS > NiS > CdS > . . . . The efficiency of ammonia production depends strongly on the catalysts, and is highest on the ZnSe, ca. 1.29% at - 1.0 V vs. RHE and 2.24% at - 0.5 V RHE, and 1.11% on the TiB, at - 1.0 V vs. RHE. We expect to obtain better catalysts for electroreduction of nitrogen to ammonia. REFERENCES 1 N. Furuya and H. Yoshiba, Denki Kagaku, 57 (1989) 261. 2 N. Furuya and H. Yoshiba, J. Electroanal. Chem., 272 (1989) 263. 3 N. Furuya and H. Yoshiba, J. Electroanal. Chem., 263 (1989) 171.
J. Electroanal. Chem., 291 (1990) 269-212 Elsevier Sequoia S.A., Lausanne
Preliminary note
Electroreduction of nitrogen to ammonia on gas-diffusion electrodes loaded with inorganic catalyst Nagakazu Furuya and Hiroshi Yoshiba Department
of Applied Chemistry,
Faculty of Engineering,
Yamanashi
Universi@, Takeda-4, Kofu (Japan)
(Received 6 August 1990) INTRODUCTION
Nitrogen can be reduced to ammonia under ambient temperature and pressure by electroreduction using a gas-diffusion electrode modified by metal-phthalocyanines [1,2] in various electrolytes [3]. The current efficiency and the stability of a gas diffusion electrode depends strongly on the nature of the central metal and a series of supporting electrolyte media. The Sn-phthalocyanine is the best catalyst in terms of current efficiency and the stability of the gas-diffusion electrode in 1 M KOH. The current efficiency of ammonia production at -0.4 V vs. RHE was ca. 1.85% at the beginning of electrolysis, and ca. 1.2% after 25 min. However, in the Haber process ammonia is manufactured in the presence of an iron catalyst. We thought that there might be catalysts other than metal phthalocyanines for ammonia synthesis. Inorganic compounds such as metals and metal oxides were made the object of research. A gas diffusion electrode loaded with a fine powder of these inorganic compounds was produced. The electrode was used for experiments on the electroreduction of nitrogen to ammonia. The purpose of the present study was to investigate the activity of these inorganic catalysts. EXPERIMENTAL
The electrolysis cell is the same as that which has been described in the previous paper [3]. The cell consists of the two gas-diffusion electrodes, separated by 1 mm, loaded with ca. lo-20 mg/cm2 inorganic catalysts (cathode) and 0.56 mg/cm2 Pt (anode), respectively. Inorganic catalyst powder (metal, metal sulfides, metal oxides, metal boride, metal selenide and metal carbide) was prepared. The electrode active area was 12.56 cm2 in each case. A gas-diffusion electrode loaded with Pt catalyst was used as the anode; it was depolarized by hydrogen in order to prevent the
270
oxidation of ammonia produced at the cathode. Electrolysis was conducted under potentiostatic conditions at -1.0 V vs. RHE for 1 min and -0.5 V vs. RHE for 30 min, in 1 M KOH under ambient pressure and temperature, 25 o C, while N, and H, were supplied at a constant flow rate from the back sides of the cathode and anode, respectively. The amount of ammonia in the solution produced by the electroreduction was determined by absorptiometry using Nessler’s reagent and an ammonia ion meter.
RESULTS AND DISCUSSION
The current efficiency for ammonia formation is shown in Table 1.
TABLE 1 The current efficiency of ammonia production of Fe fine powder and Carbon black, metal oxides, metal sulfides, and ZnSe, TiB, and Sic, at - 1.0 V vs. RHE catalyst
current I/A
10’ X amount of NH, produced/m01 mm’
current efficiency/%
Fe C
4.10 0.73
10.1 0.72
0.119 0.048
PbO-TiOr Sb-SnO, ZnO Sn-In r03 SnOz
1.25 2.58 4.15 3.83 2.55 4.22 2.85
7.06 11.3 11.9 8.26 4.77 6.50 2.50
0.273 0.211 0.138 0.104 0.090 0.074 0.042
ASS cos
2.13 1.66 2.03 2.02 3.29 1LO.50 4.62 2.51 5.17 3.97 3.69 8.86 6.84 4.37
42.6 29.2 31.2 7.62 11.8 22.8 9.34 5.00 9.11 3.28 2.37 5.75 3.44 1.32
0.964 0.849 0.741 0.182 0.172 0.105 0.098 0.096 0.085 0.040 0.031 0.031 0.024 0.015
ZnSe TiB, Sic
1.81 2.00 1.72
48.5 46.1 3.25
1.293 1.111 0.091
Fe203 TiO ZnS NiS CdS cus Bi,S, FeS MoS Sb,S, SnS MnS PbS PdS
271
Iron and carbon The current efficiency of the iron and carbon powder was 0.119% and 0.048% respectively. It is clear that nitrogen can be reduced to ammonia on iron catalysts. Metal oxides For metal oxides the current efficiency of PbO-TiO, is the highest, (ca. 0.273% at the beginning of electrolysis), and it decreases in the order PbO-TiO, > Sb-SnO, > ZnO > Sn-In,O, > SnO, > Fe,,O, > TiO. It may also be noted that the current efficiency depends on the metal and its oxidation state. Metal sulfides Here too, the efficiency depends strongly on the metal, and is highest for ZnS, ca. 0.964% at - 1.0 V vs. RHE. The efficiency decreases in the order NiS > CdS > CuS > Bi,S, > FeS > MoS > Sb,S, > SnS > MnS > PdS > PbS > AgS > CoS. Metal sulfides are better catalysts than the corresponding oxides. Sic, TiB, and ZnSe The current efficiency of the electrodes loaded with ZnSe; TiB, 1.298, 1.11% and 0.09%, respectively.
and SIC was
Stability of ammonia production Figure 1 shows the time dependence of the current efficiency on the ZnSe, Ti& and PbS at - 0.5 V vs. RHE. The stability of current efficiency depends strongly on the catalyst, and is highest on the ZnSe, ca. 2.24% after 10 min. The highest stability of the gas-diffusion electrode is also observed on the ZnSe. Even after 20 min, the current efficiency is 2.03%. The current efficiency on the NiS, CdS and Fe was 0.0436, 0.04% and O.OO%,respectively, after 10 min at -0.5 V vs. RHE.
Time Of Electrolysis
/ min
Fig. 1. Time dependence of the current efficiency of the NH, production in 1 M KOH at -0.50 RHE. (0) ZnSe, (A) Ti&, (0) ZnS.
V vs.
212
Further studies are now in progress in our laboratory to search for better catalysts for electrochemical nitrogen fixation, such as metal boride. CONCLUSIONS
Electroreduction of nitrogen to ammonia can be conducted under ambient pressure and temperature using a gas-diffusion electrode modified by 26 kinds of inorganic catalyst. The order of current efficiency is ZnSe > TiB, > ZnS > NiS > CdS > . . . . The efficiency of ammonia production depends strongly on the catalysts, and is highest on the ZnSe, ca. 1.29% at - 1.0 V vs. RHE and 2.24% at - 0.5 V RHE, and 1.11% on the TiB, at - 1.0 V vs. RHE. We expect to obtain better catalysts for electroreduction of nitrogen to ammonia. REFERENCES 1 N. Furuya and H. Yoshiba, Denki Kagaku, 57 (1989) 261. 2 N. Furuya and H. Yoshiba, J. Electroanal. Chem., 272 (1989) 263. 3 N. Furuya and H. Yoshiba, J. Electroanal. Chem., 263 (1989) 171.