Catalyst for thermochemical water splitting

Abstract

The present invention relates to a catalyst for the thermochemical generation of hydrogen from water and/or the thermochemical generation of carbon monoxide from carbon dioxide comprising a solid solution of cerium dioxide and uranium dioxide.

Claims

1. A method for the generation of hydrogen from water and/or the generation of carbon monoxide from carbon dioxide comprising: reducing at least part of a catalyst at a first elevated temperature to produce diatomic oxygen; contacting the catalyst with at least one of water and carbon dioxide at a second elevated temperature; and oxidizing the at least partially reduced catalyst to generate the hydrogen and/or the carbon monoxide; wherein the second elevated temperature may be the same or different from the first elevated temperature; and wherein the catalyst comprises a solid solution of cerium dioxide and uranium dioxide.

2. The method of claim 1, wherein completion of oxidation of the catalyst is accomplished in less than 20 minutes.

3. The method of claim 1, wherein the catalyst further comprises zirconium dioxide.

4. The method of claim 1, wherein the catalyst consists essentially of cerium dioxide and uranium dioxide.

5. The method of claim 1, wherein the catalyst is free of zirconium dioxide.

6. The method of claim 1, wherein the catalyst was produced by dissolving a cerium salt, a uranium salt and optionally a zirconium salt in water; and then precipitating at a pH of at least 8 by addition of a base to form a precipitate, and calcining the precipitate.

7. The method of claim 1, wherein the second elevated temperature is lower than the first elevated temperature.

8. The method of claim 1, wherein the first elevated temperature is from 600 C. to 1200 C.

9. The method of claim 1, wherein the first elevated temperature is from 1000 C. to 1200 C.

10. The method of claim 1, wherein the second elevated temperature is from 600 C. to 1000 C.

11. The method of claim 1, wherein the second elevated temperature is from 600 C. to 900 C.

12. The method of claim 1, wherein the first and or second elevated temperature is obtained by heating using solar energy.

13. The method of claim 1, wherein a molar ratio of cerium and uranium is from 50 to 0.05.

14. The method of claim 13, wherein the molar ratio of cerium and uranium is from 10 to 0.1.

15. The method of claim 1, wherein the solid solution has a fluorite crystal structure.

16. The method of claim 1, wherein a total amount of cerium dioxide, uranium dioxide and optionally zirconium dioxide in the solid solution is at least 95.0 wt % based on the weight of the solid solution.

17. The method of claim 16, wherein the total amount of cerium dioxide, uranium dioxide and optionally zirconium dioxide in the solid solution is at least 99.0 wt % based on the weight of the solid solution.

18. The method of claim 1, wherein a molar ratio of cerium and uranium is from 10 to 1; the solid solution has a fluorite crystal structure; a total amount of cerium dioxide and uranium dioxide in the solid solution is at least 99.0 wt %; and wherein the catalyst is free of zirconium dioxide.

19. The method of claim 17, wherein the catalyst consists essentially of cerium dioxide and uranium dioxide.

Description

(1) The present invention will now be further explained on the basis of the following non-limiting figures and examples.

(2) FIG. 1 shows the thermochemical generation of hydrogen for several catalysts not according to the present invention.

(3) FIG. 2 shows the results of temperature programmed reduction (TPR) experiments for several catalysts according to the present invention.

(4) FIG. 3 shows the thermochemical generation of hydrogen for a catalyst not according to the present invention.

(5) FIG. 4 shows the thermochemical generation of hydrogen for a catalyst according to the present invention.

(6) The present inventors have applied Density Functional Theory calculations using the Quantum Espresso code with the Generalised Gradient Approximation, in order to determine the energy of formation of an oxygen vacancy. The calculations were conducted upon creation of one oxygen vacancy assuming the oxygen vacancy and the metal cation (M.sup.4+) are nearest neighbours and wherein in a CeO.sub.2 fluorite structure the ratio M.sup.4+ to Ce.sup.4+0.03.

(7) The present inventors found that zirconium, when added to a fluorite CeO.sub.2 crystal structure reduces the energy of formation of an oxygen vacancy, E.sub.vo, from which the present inventors conclude that oxygen may be removed from the CeO.sub.2 fluorite crystal structure more easily and hence that the material requires less energy for the reduction. In addition, the present inventors noted that the ionic radius of zirconium (Zr.sup.4+) differs substantially from the ionic radius of cerium (Ce.sup.4+) which has the effect that it is more difficult to maintain the desirable fluorite crystal structure upon higher amounts of zirconium.

(8) The present inventors further noted that the ionic radius of uranium (U.sup.4+) is close to the ionic radius of cerium (Ce.sup.4+) whereas the E.sub.vo is significantly lower than the E.sub.vo for a cerium dioxide fluorite crystal structure. The E.sub.vo is further reduced compared to a crystal structure based on CeO.sub.2 and ZrO.sub.2. Given the similar ionic radius U.sup.4+ may be added to the CeO.sub.2 fluorite structure in substantially any amount without losing the fluorite crystal structure.

Examples 1 to 6

(9) CeO.sub.2, ZrO.sub.2, Ce(Zr)O.sub.2, Ce(Zr,U)O.sub.2, and UO.sub.2 catalysts were prepared by the precipitation or co-precipitation method respectively using NH.sub.4OH at a pH of 8-9. The metal precursors were cerium nitrate, uranium nitrate and zirconium chloride. The single or mixed precipitated hydroxides were washed with de-ionized water until neutral pH, dried overnight at 100 C. and then calcined to make the oxides at 500 C. for 5 hours or more. X-ray diffraction, temperature programmed reduction (TPR), BET surface area, and X-ray photoelectron spectroscopy were conducted to further identify and study the catalysts.

(10) The FIGS. 1 and 2 show the results of temperature programmed reduction experiments that were performed on catalysts 1 to 5 of Table 1 below. Temperature programmed reduction is a technique known to the skilled person for the characterization of solid materials and is often used in the field of heterogeneous catalysis to find the most efficient reduction conditions. An oxidized catalyst is submitted to a programmed temperature rise while a reducing gas, in the present case a mixture of hydrogen (H.sub.2) and nitrogen is flowed over it. The change in composition of this reducing gas is measured as a function of temperature by means of a gas chromatograph equipped with a thermal conductivity detector. This change is reflected in FIGS. 1 and 2 on the vertical axis, which plots the signal of said thermal conductivity detector, referred to as TCD signal.

(11) The following catalysts were subjected to the temperature programmed reduction measurement:

(12) TABLE-US-00001 TABLE 1 Hydrogen consumption Example Composition [ml/g catalyst] 1* Ce.sub.0.66Zr.sub.0.34O.sub.2 22 2* CeO.sub.2 24 3 Ce.sub.0.5U.sub.0.5O.sub.2 34 4* UO.sub.2 28 5 Ce.sub.0.65Zr.sub.0.25U.sub.0.1O.sub.2 27 6* ZrO.sub.2 0 *= Not according to the invention

(13) The hydrogen consumption as shown in Table 1 is determined by measuring the area under the curves of FIGS. 1 and 2.

(14) From Table 1 it is clear that the catalyst of Example 3 shows the highest hydrogen consumption per gram of catalyst. In addition to that it is clear from FIG. 2 that this Example 3 catalyst is reduced at relatively low temperature compared to the catalysts of Examples 1, 2 and 4 for which the TPR curves are shown in FIG. 1. These results show that a cerium dioxide based catalyst according to the present invention not only allows a higher level of reduction, but also that reduction may be carried out at lower temperature, i.e. requiring less energy. The present inventors observe that the higher level of reduction also results in a higher level of hydrogen and/or carbon monoxide generation when such reduced catalysts are oxidised again using water and/or carbon dioxide.

(15) Table 1 further confirms that ZrO.sub.2 cannot be used as a suitable catalyst for the thermochemical generation of hydrogen from water and/or the thermochemical generation of carbon monoxide from carbon dioxide.

(16) This is reflected in FIG. 3 and FIG. 4 which show the hydrogen generation upon oxidising with water for a CeO.sub.2/ZrO.sub.2 (Example 1) and a CeO.sub.2/UO.sub.2 (Example 3) catalyst respectively.

(17) On the horizontal axis the time during the oxidation step is shown, whereas on the vertical axis the moles of hydrogen that are generated are plotted. Both catalysts were produced using the same method and both were reduced at 1100 C. Oxidation was carried out by contacting the catalysts with steam using nitrogen as a carrier gas at a temperature of 1000 C.

(18) The total amount of hydrogen that was formed upon oxidation of the CeO.sub.2/ZrO.sub.2 catalyst was 410.sup.6 mol/gram catalyst whereas the CeO.sub.2/UO.sub.2 catalyst according to the invention produced 710.sup.6 mol/gram catalyst. The present inventors further found that completion of the oxidation of the CeO.sub.2/ZrO.sub.2 catalyst may take considerable time, i.e. well over 20 minutes, whereas the completion of oxidation for the catalyst according to the present invention was completed in less than 20 minutes, namely approximately 15 minutes.

(19) Set forth below are some embodiments of a catalyst for the thermochemical generation of hydrogen from water and/or the thermochemical generation of carbon monoxide.

Embodiment 1

(20) A catalyst for the thermochemical generation of hydrogen from water and/or the thermochemical generation of carbon monoxide from carbon dioxide comprising a solid solution of cerium dioxide and uranium dioxide.

Embodiment 2

(21) The catalyst according to Embodiment 1, wherein a molar ratio of cerium and uranium is from 50 to 0.05, preferably from 10 to 0.1, more preferably from 10 to 1.

Embodiment 3

(22) The catalyst according to Embodiment 1 or 2, wherein the solid solution further contains zirconium dioxide in an amount of at most 40.0 mol % based on the total amount of cerium dioxide, uranium dioxide, and zirconium dioxide.

Embodiment 4

(23) The catalyst according to one or more of the preceding Embodiments 1-3, wherein the solid solution has a fluorite crystal structure.

Embodiment 5

(24) The catalyst according to one or more of the preceding Embodiments 1-4, wherein a total amount of cerium dioxide, uranium dioxide, and optionally zirconium dioxide in the solid solution is at least 95.0 wt % based on the weight of the solid solution.

Embodiment 6

(25) The catalyst according to one or more of the preceding Embodiments 1-4, wherein a total amount of cerium dioxide, uranium dioxide, and optionally zirconium dioxide in the solid solution is at least 99.0 wt % based on the weight of the solid solution.

Embodiment 7

(26) The catalyst according to one or more of the preceding Embodiments 1-4, wherein a total amount of cerium dioxide, uranium dioxide, and optionally zirconium dioxide in the solid solution is at least 99.9 wt % based on the weight of the solid solution.

Embodiment 8

(27) A solid solution comprising cerium dioxide, uranium dioxide and zirconium dioxide, wherein the amount of zirconium dioxide is at most 40 mol % based on the total amount of cerium dioxide, uranium dioxide, and zirconium dioxide.

Embodiment 9

(28) A method for producing the solid solution according to any one or more of the preceding Embodiments 1-8 comprising: dissolving a cerium salt, a uranium salt, and optionally a zirconium salt in water followed by precipitation at a pH of at least 8 by addition of a base.

Embodiment 10

(29) A method for producing the catalyst according to any one or more of preceding Embodiments 1-7 comprising the method for producing the solid solution according to Embodiment 9 followed by drying the obtained precipitate and calcining the precipitate at elevated temperature.

Embodiment 11

(30) A method for the generation of hydrogen from water and/or the generation of carbon monoxide from carbon dioxide comprising: a) providing a catalyst according to any one or more of preceding Embodiments 1-7; b) reducing at least part of said catalyst at a first elevated temperature; c) oxidising the at least partially reduced catalyst of step b by contacting said at least partially reduced catalyst with water and/or carbon dioxide at a second elevated temperature which may be the same or different from the first elevated temperature.

Embodiment 12

(31) The method of Embodiment 11, wherein the second elevated temperature is lower than the first elevated temperature.

Embodiment 13

(32) The method of any one or more of the previous Embodiments 11-12 wherein the first elevated temperature is 600 C. to 1,200 C.

Embodiment 14

(33) The method of any one or more of the previous Embodiments 11-13 wherein the first elevated temperature is 1,000 C. to 1,200 C.

Embodiment 15

(34) The method of any one or more of the preceding Embodiments 11-14 wherein the second elevated temperature is 600 C. to 1,000 C.

Embodiment 16

(35) The method of any one or more of the preceding Embodiments 11-15 wherein the second elevated temperature is 600 C. to 900 C.

Embodiment 17

(36) The method of one or more of the preceding Embodiments 11-14, wherein the first and or second elevated temperature is obtained by heating using solar energy.

Embodiment 18

(37) Reactor for generating hydrogen and/or carbon monoxide comprising a reaction zone comprising the catalyst according to any one or more of preceding Embodiments 1-7, means for heating said reaction zone, means for introducing gasses into the reaction zone and means for extracting gasses from the reaction zone.

Embodiment 19

(38) The use of a solid solution of cerium dioxide and uranium dioxide as a catalyst for the thermochemical generation of hydrogen from water and/or the thermochemical generation of carbon monoxide from carbon dioxide.