ELECTROCHEMICAL PROCESS FOR THE REDUCTION OF MOLECULAR OXYGEN

Abstract

The present invention relates to an electrochemical process for the reduction of molecular oxygen in alkaline solutions in the presence of nitrogen-doped carbon nanotubes, in which no hydrogen peroxide forms as a by-product of the reduction.

Claims

1-8. (canceled)

9. Process for the electrochemical reduction of molecular oxygen to oxygen ions having a double negative charge in solutions having a pH greater than or equal to 8, which comprises contacting said molecular oxygen in said solutions with nitrogen-doped carbon nanotubes containing pyridinic and quaternary nitrogen, and which are free of metal and semimetal constituents, under the influence of an applied electrical voltage.

10. Process according to claim 9, wherein the nitrogen-doped carbon nanotubes have a diameter of 3 to 150 nm.

11. Process according to claim 9, wherein the nitrogen-doped carbon nanotubes have an aspect ratio of at least 2.

12. Process of claim 9, wherein the nitrogen-doped carbon nanotubes have ratios of pyridinic to quaternary nitrogen of greater than or equal to 1.

13. Process according to claim 9, wherein the nitrogen content of the nitrogen-doped carbon nanotubes is greater than or equal to 1 atom %.

14. Process according to claim 9, wherein said voltage is +0.2 V to 0.8 V, measured against an Ag/AgCl reference electrode.

15. Electrolysis apparatus for the electrochemical reduction of molecular oxygen to oxygen ions having a double negative charge, comprising a first electrode space (1) filled with a solution having a pH greater than or equal to 8, in which an electrode (1a) comprising nitrogen-doped carbon nanotubes having pyridinic and quaternary nitrogen and being free of metal and semimetal constituents is present, which electrode has an electrically conductive connection via a voltage source (3) to a further electrode (2a) in a further electrode space (2), a membrane (4) being present between the first and the further electrode spaces.

16. The process of claim 10, wherein said diameter is 4 to 100 nm.

17. The process of claim 16, wherein said diameter is 5 to 50 nm.

18. The process of claim 11, wherein said aspect ratio is at least 5.

19. The process of claim 18, wherein said aspect ratio is at least 10.

20. The process of claim 12, wherein said ratio of pyridinic nitrogen to quaternary nitrogen is greater than or equal to 1.5.

21. The process of claim 21, wherein said ratio of pyridinic nitrogen to quaternary nitrogen is greater than or equal to 2.

22. Process according to claim 9, wherein the nitrogen-doped carbon nanotubes have a diameter of 5 to 50 nm, wherein said aspect ratio is at least 10, wherein the nitrogen content of the nitrogen-doped carbon nanotubes is greater than or equal to 1 atom %, and wherein said ratio of pyridinic nitrogen to quaternary nitrogen is greater than or equal to 2.

Description

EXAMPLES

Example 1

Oxygen Reduction According to the Invention

[0073] 40 mg of nitrogen-doped carbon nanotubes, prepared by catalytic decomposition of pyridine at 650 C. in a fixed-bed reactor, over a cobalt-molybdenum-magnesium oxide catalyst (consisting of 19% by weight of Co, 4% by weight of Mo and 77% by weight of MgO), were first dispersed in 50 ml of acetone after they been freed from catalyst residues by means of washing in concentrated hydrochloric acid solution, so that a first dispersion A was obtained.

[0074] The nitrogen-doped carbon nanotubes were investigated beforehand by means of electron spectroscopy for chemical analysis (ESCA; from ThermoFisher, ESCALab 220iXL; method according to the manufacturer's instructions) and by means of transmission electron microscopy (TEM; from FEI, apparatus type: Tecnai20, Megaview III; method according to the manufacturer's instructions).

[0075] It was found here that the nitrogen-doped carbon nanotubes had a proportion of 6.5 atom % of nitrogen, that they had a ration of pyridinic to quaternary nitrogen of 2.88, and that they had a median diameter d.sub.50 of about 10 nm and a minimum length of about 150 nm, so that they had a aspect ratio of greater than 10.

[0076] 120 l of the dispersion A obtained were introduced dropwise onto a polished electrode surface of a rotating annular disc electrode (from Jaissle Elektronik GmbH).

[0077] After the evaporation of the acetone, 10 l of a dissolved sulphonated tetrafluoroethylene polymer (Nafion solution; from DuPont) were introduced dropwise thereon in a concentration of 26 mg/ml in isopropanol for fixing the solid present in dispersion A.

[0078] The rotating annular disc electrode, now comprising the nitrogen-doped carbon nanotubes, was then used as a working electrode in a laboratory cell containing 3 electrodes (working electrode, opposite electrode and reference electrode).

[0079] The setup used is known to the person skilled in the art in general as a three-electrode arrangement. A 1 molar NaOH solution in water, which was saturated with oxygen beforehand by means of passing through a gas stream of pure oxygen, was used as an electrolyte surrounding the working electrode.

[0080] The reference electrode used was a commercially available Ag/AgCl electrode (from Mettler-Toledo).

[0081] The electrolyte was heated to 60 C. The reduction of the oxygen dissolved in molecular form in the electrolyte was likewise carried out at this temperature, which was controlled.

[0082] Subsequently, the variation of the limiting current was measured in the range from +0.2 V to 0.8 V, applied between the working electrode and the reference electrode. The abovementioned range of +0.2 V to 0.8 V was checked at a speed of 10 mV/s.

[0083] The measurement of the abovementioned range was carried out analogously several times, the rotational speed of the annular disc electrode being varied in each new experiment.

[0084] Altogether, three such measurements were carried out at 400, 900 and 1600 revolutions of the annular disc electrode per minute, for plotting in the Koutecky-Levich diagram of FIG. 1.

[0085] The results of the measurement are shown in the form of a Koutecky-Levich diagram in FIG. 1. A value of about 4.2 is obtained from the slope of the linear approximation, using the formulae (V) and (VI) shown above for the number of electrons n transferred in the process. It follows from this, that in the course of the reduction of the oxygen, no hydrogen peroxide was formed in the reaction according to formula (I), which is the result of the abovementioned advantages of the process.

[0086] By way of example, a single measurement from the abovementioned Koutecky-Levich diagram is shown in FIG. 2 for a measurement at 3600 revolutions of the annular disc electrode per minute (A) in comparison with the corresponding measurement from Comparative Example 1 (B).

[0087] It is evident from this firstly that, on carrying out the process according to the invention in the course of the measurement, a current flow occurs at an applied voltage of only about 0.1 V relative to an Ag/AgCl electrode, whereas, on carrying out the process not according to the invention, this current flow occurs to a significant extent when the applied voltage is about 0.2 V. Thus, the reduction of oxygen in the process according to the invention advantageously occurs earlier than in processes according to the prior art, which leads to a saving of energy for the electrochemical reduction of oxygen. FIG. 2 further shows that the limiting current for the process according to the invention is about twice as high as that of the process not according to the invention. This is due to the abovementioned transfer of four electrons according to the formula (III) in the process according to the invention, whereas only two electrons are transferred according to the formula (I), with formation of hydrogen peroxide, in the process not according to the invention. Accordingly, in the case of the process not according to the invention, an application of an even higher voltage, the further reduction of hydrogen peroxide according to the formula (II) may follow, which is indicated in FIG. 2 in the form of the further bending of the curve at a voltage of about 0.75 V. However, this also implies that the process not according to the invention is always associated with the necessity of applying a higher voltage if a similar yield of oxygen ions having a double negative charge is to be obtained from this process as in the process according to the invention. Thus, such processes are considerably disadvantageous, at least in teems of energy, and as a direct consequence also economically disadvantageous.

Example 2

Further Oxygen Reduction According to the Invention

[0088] An experiment equivalent to that in Example 1 was carried out, with the only difference that, instead of the nitrogen-doped carbon nanotubes used there, nitrogen-doped carbon nanotubes prepared by catalytic decomposition of pyridine at 650 C. in a fixed-bed reactor over a catalyst corresponding to Example 1 of WO 2007 093 337 were now used. Moreover, measurements were carried out at a rotational speed of the annular disc electrode of 2500 revolutions per minute.

[0089] The nitrogen-doped carbon nanotubes were investigated beforehand by means of ESCA. It was found thereby that the nitrogen-doped carbon nanotubes had a proportion of 3.8 atom % of nitrogen and that they had a ratio of pyridinic to quaternary nitrogen of 2.79.

[0090] The results of the measurement are shown in the form of a Koutecky-Levich diagram in FIG. 3. A value of about 3.6 is obtained for the number of electrons n transferred in the process from the slope of the linear approximation, using the formulae (V) and (VI) shown above. It follows from this that, in the course of the reduction of the oxygen, no hydrogen peroxide was formed in the reaction according to formula (I) which has the result of the abovementioned advantages of the process.

Example 3

Even Further Oxygen Reduction According to the Invention

[0091] An experiment equivalent to that in Example 2 was carried out, with the only difference that, instead of the nitrogen-doped carbon nanotubes used there, nitrogen-doped carbon nanotubes prepared by catalytic decomposition of pyridine at 650 C. in a fixed-bed reactor over a catalyst corresponding to Example 2 of WO 2007 093 337 were now used.

[0092] The nitrogen-doped carbon nanotubes were investigated beforehand by means of ESCA. It was found thereby that the nitrogen-doped carbon nanotubes had a proportion of 5.8 atom % of nitrogen and that they had a ratio of pyridinic to quaternary nitrogen of 1.61.

[0093] The results of the measurement are shown in the form of a Koutecky-Levich diagram in FIG. 4. A value of about 4.1 is obtained for the number of electrons n transferred in the process from the slope of the linear approximation, using the formulae (V) and (VI) shown above. It follows from this that, in the course of the reduction of the oxygen, no hydrogen peroxide was formed in the reaction according to formula (I) which has the result of the abovementioned advantages of the process.

Comparative Example 1

Oxygen Reduction Not According to the Invention, Using Carbon Black

[0094] An experiment equivalent to that in Example 1 was carried out, with the only difference that, instead of the nitrogen-doped carbon nanotubes used there, carbon black (Vulcan XC72, from Cabot) was used.

[0095] The comparison between this process not according to the invention and the process according to the invention, according to Example 1, is shown in FIG. 2, the differences having already been explained in the context of Example 1 according to the invention.

Comparative Example 2

Further Oxygen Reduction Not According to the Invention, Using Other Nitrogen-Doped Carbon Nanotubes

[0096] An experiment equivalent to that in Example 1 was carried out, with the only difference that, instead of the nitrogen-doped carbon nanotubes used there, nitrogen-doped carbon nanotubes which, according to ESCA, had a ratio of pyridine to quaternary nitrogen of 0.63 were now used. These nitrogen-doped carbon nanotubes were prepared by catalytic decomposition of pyridine at 750 C. in a fixed-bed reactor over a catalyst corresponding to Example 2 of WO 2007 093 337.

[0097] The results of the measurement are shown in the form of empty squares (V2) in the Koutecky-Levich diagram of FIG. 5. A value of about 2.2 is obtained for the number of electrons n transferred in the process according to this comparative example from the slope of the linear approximation of these measured data, which is likewise shown as a thin dashed line in FIG. 5 and is characterized by V2, using the formulae (V) and (VI) shown above.

[0098] In comparison with the oxygen reduction according to the invention, according to Examples 1 to 3, which are shown in the form of respective solid lines (1, 2, 3) and in the form of the solid circles, squares and triangles (1, 2, 3), likewise in FIG. 5, the slope of the linear approximation which is only half as great is evident.

[0099] It follows from this that, in the course of the reduction of the oxygen according to the comparative example carried out here, a reduction according to the formula (I) with formation of hydrogen peroxide takes place, which is disadvantageous for the abovementioned reasons.

Comparative Example 3

Further Oxygen Reduction Not According to the Invention, Using Carbon Nanotubes Not Doped with Nitrogen

[0100] An experiment equivalent to that in Example 1 was carried out, with the only difference that, instead of the nitrogen-doped carbon nanotubes used there, commercially available carbon nanotubes (BayTubes, from BayTubes) were now used.

[0101] The results of the measurement are shown in the form of empty circles (V3) in the Koutecky-Levich diagram of FIG. 5. A value of about 2.1 is obtained for the number of electrons n transferred in the process according to this comparative example from the slope of the linear approximation of the measured data, which is likewise shown as a thin dashed line in FIG. 5 and is characterized by V3, using the formulae (V) and (VI) shown above.

[0102] In comparison with the oxygen reduction according to the invention, according to Examples 1 to 3, which are shown in the form of respective solid lines (1, 2, 3) and in the form of the solid circles, squares and triangles (1, 2, 3), likewise in FIG. 5, the slope of the linear approximation which is only half as great is evident.

[0103] It follows from this that, in the course of the reduction of the oxygen according to the comparative example carried out here, a reduction according to the formula (I) with formulation of hydrogen peroxide takes place, which is disadvantageous for the abovementioned reasons.