SHEET NIOBATES FOR USE IN PHOTOCATALYSTS

Abstract

A layered niobate which is used as a photocatalyst. The layered niobate has the formula [H.sub.aA.sub.b].sup.+[Sr.sub.2Nb.sub.3O.sub.10].sup.. [Sr.sub.2Nb.sub.3O.sub.10].sup. forms main layers. [H.sub.aA.sub.b].sup.+ forms interlayers, wherein H includes H.sup.+ and H.sub.3O.sup.+, A is K.sup.+, Cs.sup.+ and Rb.sup.+, 0.6a1, 0b0.4, and a+b=1. The layered niobate has different spacings between the main layers.

Claims

1-15. (canceled)

16. A layered niobate having a formula [H.sub.aA.sub.b].sup.+[Sr.sub.2Nb.sub.3O.sub.10].sup., wherein, [Sr.sub.2Nb.sub.3O.sub.10].sup. forms main layers, and [H.sub.aA.sub.b].sup.+ forms interlayers, wherein, H comprises H.sup.+ and H.sub.3O.sup.+, A is an element of the group consisting of K.sup.+, Cs.sup.+ and Rb.sup.+, 0.6a1, 0b0.4, and a+b=1, and the layered niobate has different spacings between the main layers.

17. The layered niobate as recited in claim 16, wherein the layered niobate is a layered perovskite of a Dion-Jacobson type.

18. The layered niobate as recited in claim 16, wherein the layered niobate has a hydrated phase and a dehydrated phase.

19. The layered niobate as claimed as recited in claim 16, wherein, the layered niobate has a degree of protonation of at least 60%, and the degree of protonation is determined by via an energy-dispersive x-ray spectroscopy (EDX).

20. The layered niobate as recited in claim 16, wherein the layered niobate comprises at least one of water molecules and hydrated hydronium ions (H.sub.3O.sup.+*H.sub.2O) in the interlayers.

21. The layered niobate as recited in claim 16, wherein a 002 reflection and a 004 reflection each appear as a double peak in an XRD diagram of the layered niobate.

22. The layered niobate as recited in claim 16, wherein A is a potassium ion.

23. The layered niobate as recited in claim 16, wherein the layered niobate is produced by a method comprising: treating a compound having a formula ASr.sub.2Nb.sub.3O.sub.10 with an aqueous nitric acid (HNO.sub.3), where A is an element of the group consisting of K.sup.+, Cs.sup.+ and Rb.sup.+.

24. The layered niobate as recited in claim 23, wherein the treating with the aqueous nitric acid (HNO.sub.3) is performed at a temperature of 40 C. to 70 C.

25. The layered niobate as recited in claim 23, wherein the aqueous nitric acid (HNO.sub.3) has a concentration of 0.5 to 2.5 M.

26. A process for producing the layered niobate as recited in claim 16, the method comprising: treating a compound having a general formula ASr.sub.2Nb.sub.3O.sub.10 with an aqueous nitric acid (HNO.sub.3) at a temperature of 40 C. to 70 C., where A is an element of the group consisting of K.sup.+, Cs.sup.+ and Rb.sup.+.

27. A layered niobate obtainable by the process as recited in claim 26.

28. A method of using the layered niobate as rectied in claim 16 as a photocatalyst, the method comprising: providing the layered niobate; and using the layered niobate as a photocatalyst.

29. The method of using as rectied in claim 28, wherein the photocatalyst provides for a photoinduced splitting of water.

30. A photocatalyst comprising the layered niobate as recited in claim 16.

31. The photocatalyst as recited in claim 30, wherein the photocatalyst further comprises a rhodium cocatalyst.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The present invention is described in greater detail below on the basis of embodiments and of the drawings in which:

[0013] FIG. 1 shows an XRD diagram of an inventive layered perovskite of the formula [H.sub.aK.sub.b]Sr.sub.2Nb.sub.3O.sub.10 (A) with different degrees of protonation, where K.sup.+ was exchanged for H.sup.+, with (a) reference diagram for the 100% proton-exchanged, completely dry compound and (b) reference diagram for the 100% proton-exchanged, completely hydrated compound;

[0014] FIG. 2 shows an enlarged section of an XRD diagram of a layered niobate according to the present invention, where the 001 peaks of the double-structured layered niobate HSr.sub.2Nb.sub.3O.sub.10*x H.sub.2O can clearly be seen, and where the spacing between the layers is 15.3 or 16.9 ;

[0015] FIG. 3 shows the length of the c axis or the layer spacing in a layered niobate according to the present invention depending on the degree of exchange of K.sup.+ for H.sup.+;

[0016] FIG. 4 shows the dependency of the hydrogen formation rate on the degree of exchange of K.sup.+ for H.sup.+ in a layered niobate according to the present invention; and

[0017] FIG. 5 shows a comparison of the XRD diagrams of the layered niobates in Table 1, which were used in the Examples, where the division of the layer spacings (double peak) can clearly be seen (Example 1 and Example 2).

DETAILED DESCRIPTION

[0018] The present invention firstly provides a layered niobate of the formula [H.sub.aA.sub.b].sup.+[Sr.sub.2Nb.sub.3O.sub.10].sup., where [Sr.sub.2Nb.sub.3O.sub.10].sup. forms the main layers and [H.sub.aA.sub.b].sup.+ forms the interlayers, where H is a group consisting of the elements H.sup.+ and H.sub.3O.sup.+ and A is an element of the group of K.sup.+, Cs.sup.+ and Rb.sup.+, where 0.6a1 and 0b0.4, where a+b=1, characterized in that the layered niobate has different spacings between the main layers.

[0019] The layered niobate according to the present invention can, for example, be one having the composition [H.sub.aA.sub.b].sup.+[Sr.sub.2Nb.sub.3O.sub.10].sup. where 0.6<a1 and 0b0.4, where a+b=1, for example, 0.7<a1 and 0b0.3, for example, 0.8<a1 and 0b0.2, in each case where a+b=1.

[0020] The layered niobate according to the present invention can, for example, be one of the type of the layered perovskites of the Dion-Jacobson type M[Sr.sub.2Nb.sub.3O.sub.10], where M is [H.sub.aA.sub.b], as defined above. The layered niobate according to the present invention is thus notable in that the positively charged elements M.sup.+ are intercalated between the negatively charged main layers [Sr.sub.2Nb.sub.3O.sub.10].sup.. The spacings between the individual layers, which can be determined by XRD measurements, correlate with the size of the intercalated elements M.sup.+. In the case according to the present invention, it has surprisingly been found that different layer spacings are formed in the layered niobate, this being expressed in the XRD diagram by the doubling of the corresponding reflections (double peak). Without being bound to any particular theory, it is assumed that inhomogeneous incorporation of water and/or hydrated hydronium ions into the interlayers results in the layer spacing between some layers being larger than between others, i.e., the layered niobate according to the present invention has two phases. The phase with the larger layer spacing is assumed to be a hydrated phase, while the phase with the smaller layer spacing is interpreted as a phase without additional incorporation of water in the interlayers. It has surprisingly been found that the photocatalytic activity of the layered niobate according to the present invention increases significantly with the occurrence of the two phases, hydrated and dehydrated. The present invention therefore provides the layered niobate having, for example, a hydrated phase and a dehydrated phase. The hydrated phase here comprises water molecules and/or hydrated hydronium ions (H.sub.3O.sup.+*H.sub.2O) in the interlayers, while the dehydrated phase does not comprise any corresponding molecules in the interlayers.

[0021] It has previously been described that the photocatalytic activity of layered perovskites can be increased if at least some of the alkali metal ions usually intercalated in the interlayers are exchanged with protons. In an embodiment of the present invention, the layered niobate can, for example, have a degree of protonation of at least 60%, for example, more than 70%, for example, 80% to 100%. In the context of the present invention, degree of protonation refers to the proportion of alkali metal ions in the interlayers that has been replaced by protons. The degree of protonation can therefore be determined by determining the content of the replaced alkali metal ion, for example, via EDX. A degree of protonation of 60% is therefore to be understood to mean that 60% of the alkali metal ions usually intercalated in the interlayers have been replaced by protons. The degree of protonation can be measured by comparing the content of alkali metal ions to the unprotonated compound, as described above.

[0022] The layered niobate according to the present invention is particularly notable in that it has two phases with different layer spacings. These phases can be identified via XRD measurements. In an embodiment, the 002 and 004 x-ray diffraction reflections of the layered niobate according to the present invention appear as two reflections (double peak). In the XRD spectrum of the layered niobate according to the present invention, said reflections therefore appear as double reflections, instead of individual reflections as in the spectra of conventional layered niobates.

[0023] The photocatalytic activity of the layered niobates according to the present invention can be increased by replacing at least some of the alkali metal ions intercalated in the interlayers with protons. This exchange has proven to be particularly efficient if the alkali metal ions are potassium ions. In an embodiment of the present invention, A can, for example, be a potassium ion.

[0024] Without being bound to any particular theory, it is assumed that it is in particular the step of protonation that contributes to the formation of the special structure of the layered niobates according to the present invention. In an embodiment, the layered niobate can, for example, be produced by treating a compound of the formula ASr.sub.2Nb.sub.3O.sub.10, where A is an element of the group of K.sup.+, Cs.sup.+ and Rb.sup.+, with aqueous nitric acid (HNO.sub.3). The treatment with the aqueous nitric acid can, for example, be performed at a temperature of 40 C. to 70 C., for example, 50 C. to 65 C. The duration of the treatment is guided by the desired degree of protonation and, in an embodiment, may be 3 to 24 hours, for example, 5 to 20 hours, for example, 12 to 18 hours. It has also proven advantageous to renew the aqueous nitric acid during the treatment. In an embodiment, the aqueous nitric acid solution can, for example, be replaced by a fresh solution every 4 to 10 hours, for example, every 5 to 8 hours. In an embodiment, the concentration of the aqueous nitric acid solution can, for example, be 0.5 to 2.5 M, for example, 0.5 to 1.5 M.

[0025] The present invention further provides a process for producing the layered niobate according to the present invention, the process comprising the treatment of a compound of the general formula ASr.sub.2Nb.sub.3O.sub.10, where A is an element of the group of elements K.sup.+, Cs.sup.+ and Rb.sup.+, with aqueous nitric acid (HNO.sub.3) at a temperature of 40 C. to 70 C., for example, 50 C. to 65 C. The duration of the treatment is guided by the desired degree of protonation and can, for example, be 3 to 24 hours, for example, 5 to 20 hours, for example, 12 to 18 hours. It has also proven to be advantageous to renew the aqueous nitric acid during the treatment. In an embodiment, the aqueous nitric acid solution can, for example, be replaced by a fresh solution every 4 to 10 hours, for example, every 5 to 8 hours. In an embodiment, the concentration of the aqueous nitric acid solution can, for example, be 0.5 to 2.5 M, for example, 0.5 to 1.5 M.

[0026] The present invention further provides a layered niobate obtainable by treating a compound of the general formula ASr.sub.2Nb.sub.3O.sub.10, where A is an element of the group of elements K.sup.+, Cs.sup.+ and Rb.sup.+, with aqueous nitric acid (HNO.sub.3) at a temperature of 40 C. to 70 C., for example, 50 C. to 65 C. The layered niobate obtained in this way has a composition of the formula [H.sub.aA.sub.b].sup.+[Sr.sub.2Nb.sub.3O.sub.10].sup., where [Sr.sub.2Nb.sub.3O.sub.10].sup. forms the main layers and [H.sub.aA.sub.b].sup.+ forms the interlayers, where H is a group consisting of the elements H.sup.+ and H.sub.3O.sup.+ and A is an element of the group of K.sup.+, Cs.sup.+ and Rb.sup.+, where 0.6a1 and 0b0.4, where a+b=1, and has different spacings between the main layers.

[0027] The compound of the general formula ASr.sub.2Nb.sub.3O.sub.10, which is used as a starting compound in the production of the layered niobates according to the present invention can, for example, be produced via molten salt synthesis or solid phase synthesis.

[0028] The layered niobates according to the present invention feature high photocatalytic activity. The present invention therefore further provides for the use of the layered niobate according to the present invention as a photocatalyst, for example, as a photocatalyst in the photoinduced splitting of water.

[0029] The present invention further provides a photocatalyst comprising a layered niobate according to the present invention. It has surprisingly been found that the amount of hydrogen generated by the photocatalyst according to the present invention is higher than that achieved by conventional photocatalysts under the same conditions. The photocatalyst according to the present invention can, for example, further comprise a rhodium cocatalyst.

[0030] The present invention is illustrated in greater detail below on the basis of examples which should, however, in no way be considered as a limitation of the concept of the present invention.

Examples

[0031] KSr.sub.2Nb.sub.3O.sub.10 was produced by molten salt synthesis, as described, for example, by Kulischow et al. in Catal. Today 2017, 287, 65-69.

[0032] KSr.sub.2Nb.sub.3O.sub.10 was stirred in 1M HNO.sub.3 solution at 60 C. for various time intervals. The degree of protonation was monitored via energy-dispersive x-ray spectroscopy (EDX).

[0033] X-ray diffraction analyses were carried out via a PANalytical MPD diffractometer with CuK.sub.a radiation (=0.1541 nm) in the 2 range from 5 to 30.

[0034] EDX elemental analysis was carried out using a Philips LEO Gemini 928 field emission SEM at a 20 kV acceleration voltage.

[0035] The photocatalytic investigations were carried out as described in Kulischow et al. in Catal. Today 2017, 287, 65-69 in a double-walled quartz reactor. The reactor was cooled to 10 C. in order to rule out thermal influences. The light source used was a 350 W Hg lamp. For the detection of the hydrogen formed, use was made of a Shimadzu GC-2014 gas chromatograph, equipped with a detector for thermal conductivity (TCD) and a RESTEK ShinCarbon ST 100/120 column. The column was maintained at a temperature of 35 C. during the measurement and the elution time for H.sub.2 was 1 minute.

[0036] In a typical experiment, 0.3 g of the layered niobate according to the present invention with 0.3% by weight of Rh(NH.sub.3).sub.5Cl)Cl.sub.2 as cocatalyst was suspended in 600 ml of aqueous methanol solution (10% v/v) with ultrasound treatment and then irradiated with the 350 W Hg lamp. The starting pH of the solution was adjusted to 3 with perchloric acid. Prior to irradiation, the system was purged with argon in order to ensure that air was completely removed. The results of these photocatalytic measurements with the 350 W Hg lamp on the materials with different degrees of protonation are illustrated in FIG. 4.

[0037] Further layered niobates were produced by varying the temperature and the duration of the acid treatment. The treatment with 1M HNO.sub.3 was carried out at 20 C., 55 C., and 80 C. The duration of the treatments was adjusted so that a similar degree of protonation was achieved in all experiments at the end of the acid treatment, with the longest treatment (172 h) being necessary at 20 C. The chemical analysis of the layered niobates obtained is summarized in Table 1, with the starting compound KSr.sub.2Nb.sub.3O.sub.10 being listed for comparison purposes. The samples used each had a degree of exchange or protonation of 83%, but only the samples of Examples 1 and 2, in the case of which the acid treatment was carried out at 55 C. and 60 C., respectively, showed the different layer spacings according to the present invention. The corresponding XRD diagrams are shown in FIG. 5. The acid treatments at 20 C. and 80 C. (Comparative Examples 1 and 2, respectively) each showed only one 002 and 004 peak in the XRD diagrams.

TABLE-US-00001 TABLE 1 Analyses Molar ratios based K Sr Nb on Nb = 3 Temperature [% [% [% H [ C.] by weight] by weight] by weight] (1-K) K Sr Nb KSr.sub.2Nb.sub.3O.sub.10 5.86 26.75 42.82 0.98 1.99 3 Comparative 20 1.06 27.84 44.35 0.83 0.17 2.00 3 Example 1 Example 1 55 1.05 27.83 44.26 0.83 0.17 2.00 3 Example 2 60 1.05 27.81 44.14 0.83 0.17 2.00 3 Comparative 80 1.07 27.84 44.22 0.83 0.17 2.00 3 Example 2

[0038] The aim of the photocatalyst development is the solar splitting of water. Hg lamps generate a high proportion of high-energy UV radiation, which increases the photocatalytic evolution of hydrogen but is not included in the solar spectrum. In order to test the application for the solar splitting of water, a quartz glass cuvette having the photocatalyst suspension of the layered niobates from Table 1 with Rh(NH.sub.3).sub.5Cl)Cl.sub.2 as cocatalyst as described above was irradiated with a xenon arc lamp (Perkin Elmer Cermax E300BF), instead of the Hg lamp, through a solar simulator filter. A water filter was used to avoid an increase in temperature. The illuminance was 1283.9 mW/cm 2 on the cuvette. The measured amount of hydrogen after 5 hours is summarized in Table 2:

TABLE-US-00002 TABLE 2 Comparatve Comparative Sample Example 1 Example 1 Example 2 Example 2 H.sub.2 [mol/h] 400 493 545 418

[0039] As can be seen from Table 2, the use of the layered niobates according to the present invention (Example 1 and Example 2) made it possible to achieve significantly higher hydrogen production.

[0040] The present invention is not limited to embodiments described herein; reference should be had to the appended claims.