Layered niobates for use in photocatalysis

Abstract

The present invention is based on the formula H a A b Sr2Nb3O 10 [wherein H is the element H + and H3O + A represents a group containing K + , Cs + and Rb + wherein 0.6≦a≦1 and 0≦b≦0.4, a+b=1], characterized in that the layered niobates have different layer spacings, as well as a method for producing the same and its use in photocatalysis.

Description

[Technical Field]

[0001] The present invention is based on the formula H a A b Sr2Nb3O 10 [wherein H is the element H + and H3O + A represents a group containing K + , Cs + and Rb + wherein 0.6≦a≦1 and 0≦b≦0.4, a+b=1], characterized in that the layered niobates have different layer spacings, as well as a method for producing the same and its use in photocatalysis. [Background technology]

[0002] During photosynthesis, sunlight is converted into chemical energy, forming sugars from carbon dioxide and water in the form of cellulose, which are stored in plants, and the original energy can be reused by burning the plants. Storage of solar energy in the form of hydrogen is based on a similar principle. Water can be split into hydrogen and oxygen with the help of sunlight in the presence of a catalyst. Reburning the hydrogen releases energy and water, which can be used, for example, as an alternative energy source. Obtaining energy in this form has the advantage that, unlike the combustion of wood or other fossil materials, no environmentally harmful by-products are produced, and, in contrast to wind or solar energy, it is not dependent on the time of day or weather conditions. Compared to classical water electrolysis, photocatalytic water splitting also has the advantage of mild reaction conditions and relatively low technical requirements.

[0003] The first photocatalysts based on titanium dioxide were proposed as early as the 1970s, but the efficiency of this method has not been successfully increased to the point where hydrogen can be produced in large quantities. A class of compounds considered promising in this regard is of the general formula MA n-1 B n O 3n+1 wherein M represents an alkali metal, A represents an alkaline earth metal or a rare earth metal, and B represents a pentavalent metal, usually tantalum or niobium. In the case of niobium, these compounds are also called layered niobates. These compounds have the general formula [A n-1 (B n O 3n+1 )] - These compounds consist of negatively charged perovskite main layers, with alkali metal ions inserted as positive interlayers. Due to their layered structure and the relatively large spacing between the layers, these compounds can be easily modified by ion exchange. For example, the alkali metal ions in the interlayers can be replaced by protons and water molecules, thereby increasing the photocatalytic activity of such compounds, i.e., the amount of hydrogen produced upon irradiation with light.

[0004] Extensive studies have been published in the literature on layered perovskites and their activity for photocatalytic hydrogen evolution.

[0005] In their paper "Ion exchangeable layered niobates as a noble series of photocatalysts" published in Res. Chem. Intermed., Vol. 20, No. 9, pp. 895-908 in 1994, Domen et al. 10 The dependence of the layer spacing and hydrogen evolution on the interlayer spacing was investigated. It was found that the layer spacing increased stepwise with increasing degree of protonation, and that this stepwise increase in the interlayer spacing was due to different degrees of hydration, i.e., the incorporation of water molecules into the interlayer along with protons. A clear increase in the rate of hydrogen formation was observed from a degree of exchange of 60%, which was explained by the fact that, at larger interlayer spacings, methanol molecules reach the interlayer, where they act as hole scavengers. The authors believe that in this way the recombination rate between electrons and holes can be reduced, and more electrons become available for the reduction of water molecules to hydrogen.

[0006] Huang et al. published a paper entitled "Photocatalytic property of partially substituted Pt-intercalated layered perovskite, ASr2Ta" in Solar Energy Materials & Solar Cells 95, (2011) 1019~1027. x Nb 3-x O 10 (A=K, H; x=0, 1, 1.5, 2 and 3) in particular HSR2(Ta / Nb)3O 10 The XRD spectrum and hydrogen formation rate are shown for this compound. A layer spacing of 15.04 Å is described for this compound, which reacts with acid to form HSR2Nb3O 10 The hydrogen evolution from a 10% methanol solution under irradiation with a mercury lamp was observed in the protonated compound HSR2Nb3O 10 It showed higher values ​​than the standard photocatalyst TiO2P25.

[0007] In the publication "Comparison of two- and three-layer restacked Dion-Jacobson phase niobate nanosheets as catalysts for photochemical hydrogen evolution" by Maeda et al. in J. Mater. Chem., 2009, 19, 4813-4818, layered niobate nanosheets were compared with the corresponding Dion-Jacobson layered perovskite (HCaNbO 10 , HSR2Nb3O 10 and HLaNb2O7) were prepared by exfoliation with tetra(n-butyl)ammonium followed by treatment with hydrochloric acid, and their photocatalytic properties were measured and compared with those of conventional compounds.

[0008] Fang et al. published an article titled "Synthesis and characterization of a new triple-layered perovskite KSr2Nb3O" in the Journal of Wuhan University of Technology Mater.Sci. Ed., 2002, Vol.7, No.2. 10 and its protonated compounds” and KSr2Nb3O by solid-phase synthesis. 10 was subsequently treated with acid and converted by proton exchange to the compound HSR2Nb3O 10 The layer spacing for the two compounds is 15.0 Å (KSr2Nb3O 10 ) or 16.4Å (HSr2Nb3O 10 ·1.5H2O) is described. [Prior art documents] [Non-patent literature]

[0009] [Non-Patent Document 1] Domen et al, “Ion exchangeable layered niobates as a noble series of photocatalysts”, Res.Chem.Intermed., Vol.20, No.9, 1994, 895~908 [Non-patent document 2] Huang et al, “Photocatalytic property of partially substituted Pt-intercalated layered perovskite, ASr2TaxNb3-xO10(A=K,H; x=0,1,1.5,2 and 3)”, Solar Energy Materials&Solar Cells 95, 2011, 1019~1027 [Non-patent document 3] Maeda et al, “Comparison of two- and three-layer restacked Dion-Jacobson phase niobate nanosheets as catalysts for photochemical hydrogen evolution”, J.Mater.Chem, 2009,19,4813~4818 [Non-patent document 4] Fang et al, “Synthesis and characterization of a new triple-layered Perovskite KSr2Nb3O10 and its protonated compounds”, Journal of Wuhan University of Technology Mater.Sci. Ed., 2002, Vol.7, No.2 Summary of the Invention [Problem to be solved by the invention]

[0010] Even though a series of compounds with good photocatalytic activity are already known, there is still a need for photocatalysts with improved efficiency that can be used for the solar photochemical splitting of water. [Means for solving the problem]

[0011] Surprisingly, it has now been found that this problem is solved by novel layered niobates which are characterized by having different layer spacings in the protonated state.

[0012] A first subject of the present invention is therefore a compound of formula [H a A b ] + [Sr2Nb3O 10 ] - [In the formula, [Sr2Nb3O 10 ] - is the main layer, and [H a A b ] + forms the intermediate layer, and H is the element H +and H3O + represents a group consisting of, and A is K + , Cs + and Rb + of the group, 0.6 ≦ a ≦ 1 and 0 ≦ b ≦ 0.4, a + b = 1, and is a layered niobate characterized by having different intervals between the main layers.

[0013] Advantageously, the layered niobate according to the invention has the composition [H a A b + [Sr2Nb3O 10 - where 0.6 < a ≦ 1 and 0 ≦ b ≦ 0.4, a + b = 1, advantageously 0.7 < a ≦ 1 and 0 ≦ b ≦ 0.3, particularly preferably 0.8 < a ≦ 1 and 0 ≦ b ≦ 0.2, each with a + b = 1.

[0014] The layered niobate according to the invention is advantageously of the Dion-Jacobson type M[Sr2Nb3O 10 [where M represents [H a A b and is of the layered perovskite type. Thus, the layered niobate according to the invention is characterized in that a positively charged element M 10 - is inserted between the negatively charged main layers [Sr2Nb3O + . The interval between the individual layers that can be determined by XRD measurement is the inserted element M + ​​​The layer spacing correlates with the size of the interlayer. In the present invention, it was unexpectedly found that different interlayer spacings are formed in the layered niobate, which is manifested by the corresponding double reflections ("double peaks") in the XRD pattern. Without being bound by any particular theory, it is believed that the non-uniform incorporation of water and / or hydrolyzed hydronium ions into the interlayer leads to the interlayer 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 interlayer spacing is considered to be the hydrated phase, while the phase with the smaller interlayer spacing is interpreted as a phase without additional water incorporation into the interlayer. Surprisingly, it was found that the photocatalytic activity of the layered niobate according to the present invention significantly increases with the appearance of both the hydrated and dehydrated phases. In this respect, an embodiment of the present invention in which the layered niobate has a hydrated phase and a dehydrated phase is preferred. Here, the hydrated phase is composed of water molecules and / or hydrated hydronium ions (HO). + ·H2O) in the interlayer, while the dehydrated phase does not have the corresponding molecules in the interlayer.

[0015] It is known from the prior art that the photocatalytic activity of layered perovskites can be enhanced when at least a portion of the alkali metal ions typically inserted into the interlayer are exchanged with protons. Therefore, preferred embodiments of the present invention are those in which the layered niobates have a protonation degree of at least 60%, advantageously greater than 70%, and particularly preferably 80-100%. In the context of the present invention, the protonation degree indicates the proportion of alkali metal ions in the interlayer that have been replaced by protons, and can be determined by determining the content of exchanged alkali ions, for example, using EDX. Therefore, a protonation degree of 60% should generally be understood to mean that 60% of the alkali metal ions embedded in the interlayer have been replaced by protons. Here, as mentioned above, the protonation degree can be measured by comparing the alkali metal ion content with that of a non-protonated compound.

[0016] The layered niobate according to the present invention is particularly characterized by having two phases with different interlayer spacings. These phases can be identified by XRD measurement. In a preferred embodiment, the X-ray diffraction reflections of 002 and 004 of the layered niobate according to the present invention appear as two reflections (double peaks). Therefore, in the XRD spectrum of the layered niobate according to the present invention, the reflections appear as double reflections, rather than as single reflections as in the spectrum of conventional layered niobates.

[0017] The photocatalytic activity of the layered niobates according to the invention can be enhanced by replacing at least a portion of the alkali metal ions intercalated in the mesophase with protons. This exchange has been found to be particularly efficient when the alkali metal ions are potassium ions. Therefore, the embodiment of the invention in which A is potassium ions is particularly preferred.

[0018] Without being bound by any particular theory, it is believed that it is particularly the protonation step that contributes to the formation of the special structure of the layered niobates according to the present invention. Thus, in a preferred embodiment, the layered niobates have the formula ASrNbO 10 [Wherein A is K + , Cs + and Rb + with aqueous nitric acid (HNO3). Advantageously, the treatment with aqueous nitric acid is carried out at a temperature of 40-70°C, advantageously 50-65°C. The duration of the treatment depends on the desired degree of protonation and can be, in a preferred embodiment, 3-24 hours, advantageously 5-20 hours, and particularly preferably 12-18 hours. Furthermore, it has been found to be advantageous to refresh the aqueous nitric acid solution during the treatment. Particularly preferred is therefore an embodiment in which the aqueous nitric acid solution is replaced with fresh solution every 4-10 hours, advantageously every 5-8 hours. In a further preferred embodiment, the concentration of the aqueous nitric acid solution is 0.5-2.5 M, advantageously 0.5-1.5 M.

[0019] A further subject of the present invention is a method for preparing the layered niobates according to the invention, which have the general formula ASrNbO10 [Wherein A is the element K + , Cs + and Rb + The process comprises treating a compound of formula (III) with aqueous nitric acid (HNO3) at a temperature of 40-70°C, advantageously 50-65°C. The duration of the treatment depends on the desired degree of protonation and can be, in a preferred embodiment, 3-24 hours, advantageously 5-20 hours, and particularly preferably 12-18 hours. Furthermore, it has been found to be advantageous to refresh the aqueous nitric acid solution during the treatment. Particularly preferred is therefore an embodiment in which the aqueous nitric acid solution is replaced with fresh solution every 4-10 hours, advantageously every 5-8 hours. In a further preferred embodiment, the concentration of the aqueous nitric acid solution is 0.5-2.5 M, advantageously 0.5-1.5 M.

[0020] A further subject of the present invention is a compound of the general formula ASr2Nb3O 10 [Wherein A is the element K + , Cs + and Rb + The layered niobate obtained by treating a compound of the formula [H a A b ] + [Sr2Nb3O 10 ] - and has a composition of [Sr2Nb3O 10 ] - is the main layer, and [H a A b ] + forms the intermediate layer, and H is the element H + and H3O + and A represents the group consisting of K + , Cs + and Rb + where 0.6≦a≦1 and 0≦b≦0.4, a+b=1, and the spacing between the main layers is different.

[0021] The compounds of the general formula ASr2Nb3O serve as starting compounds for the preparation of the layered niobates according to the invention. 10The compounds of formula (I) are advantageously prepared by molten salt synthesis or solid phase synthesis.

[0022] The layered niobates according to the invention are characterized by a high photocatalytic activity.A further subject of the invention is therefore the use of the layered niobates according to the invention as a photocatalyst, preferably in light-induced water splitting.

[0023] Another object of the present invention is a photocatalyst comprising the layered niobate salt according to the present invention. Surprisingly, it has been shown that the amount of hydrogen produced by the photocatalyst according to the present invention is greater than that achieved by conventional photocatalysts under the same conditions. Advantageously, the photocatalyst according to the present invention further comprises a rhodium co-catalyst.

[0024] The present invention will be described in more detail below using examples, but these should not be understood as limiting the scope of the present invention in any way. [Brief explanation of the drawings]

[0025] [Figure 1] 1 shows XRD patterns of layered perovskites according to the invention of formula [HaKb]Sr2Nb3O10(A) with different degrees of protonation. [Figure 2] 1 shows a magnified crop of the XRD pattern of a layered niobate according to the present invention. [Figure 3] 1 shows the dependence of the c-axis length or layer spacing on the degree of exchange of K + for H + in the layered niobate according to the present invention. [Figure 4] 1 shows the dependence of the rate of hydrogen formation on the degree of exchange of K+ for H+ in layered niobates according to the present invention. [Figure 5] Table 1 shows a comparison of XRD patterns of layered niobates. [Example]

[0026] KSr2Nb3O 10was prepared by molten salt synthesis, for example as described in Kulischow et al., Catal. Today 2017, 287, 65-69.

[0027] KSr2Nb3O 10 was stirred in 1 M HNO3 solution at 60 °C for various time intervals. The degree of protonation was followed by energy dispersive X-ray spectroscopy (EDX).

[0028] X-ray diffraction analysis was performed on a PANalytical MPD diffractometer. α The measurements were carried out in the 2θ range of 5° to 30° using a 1000 kJ / s ray (λ = 0.1541 nm).

[0029] EDX elemental analysis was carried out using a Philips LEO Gemini 928 field emission SEM at an accelerating voltage of 20 kV.

[0030] Photocatalytic studies were carried out in a double-walled quartz reactor as described in Kulischow et al., Catal. Today 2017, 287, 65-69. To eliminate thermal effects, the reactor was cooled to 10°C. A 350W Hg lamp was used as the light source. A Shimadzu GC-2014 gas chromatograph equipped with a thermal conductivity detector (TCD) and a RESTEK ShinCarbon ST 100 / 120 column was used to detect the evolved hydrogen. The column temperature was maintained at 35°C during the measurements, and the elution time for H was 1 min.

[0031] In a typical experiment, 0.3 g of the layered niobate according to the invention was suspended in 600 ml of aqueous methanol (10% v / v) with 0.3% by weight of Rh(NH3)5Cl)Cl2 as a cocatalyst under ultrasonication and then irradiated with a 350 W Hg lamp. The starting pH value of the solution was adjusted to 3 using perchloric acid. Before irradiation, the system was purged with argon to ensure complete removal of air. The results of this photocatalytic measurement using a 350 W Hg lamp on materials with different degrees of protonation are shown in Figure 4.

[0032] Additional layered niobates were prepared by varying the temperature and duration of the acid treatment. Treatment with 1 M HNO3 was carried out at 20 °C, 55 °C, 60 °C, and 80 °C. The duration of the treatment was adjusted to achieve a similar degree of protonation at the end of the acid treatment for all experiments, with the longest treatment (172 h) required at 20 °C. The chemical analysis of the resulting layered niobates is summarized in Table 1, where the starting compound KSr2Nb3O is used for comparison. 10 The authors cite the paper. Although the samples used each had a degree of exchange or protonation of 83%, only the samples of Examples 1 and 2, which were subjected to acid treatment at 55°C or 60°C, showed the different layer spacing according to the present invention. The corresponding XRD patterns are shown in Figure 5. Acid treatment at 20°C or 80°C (Comparative Examples 1 and 2) showed only one 002 peak and one 004 peak in the XRD pattern, respectively.

[0033] [Table 1]

[0034] The goal of photocatalyst development is solar water splitting. Hg lamps produce a high proportion of high-energy UV light, which certainly enhances photocatalytic hydrogen production but is not contained in the solar spectrum. To test the application to solar water splitting, a quartz glass cuvette containing a photocatalyst suspension of the layered niobate salts listed in Table 1 and Rh(NH3)5Cl)Cl2 as a cocatalyst, as described above, was irradiated with a xenon arc lamp (Perkin Elmer Cermax E300BF) instead of a Hg lamp through a solar simulator filter. A water filter was used to avoid temperature rise. The irradiance on the cuvette was 1283.9 mW / cm2. 2 The amount of hydrogen measured after 5 hours is summarized in Table 2: [Table 2]

[0035] As can be seen from Table 2, significantly higher hydrogen production could be achieved by using the layered niobate salts according to the present invention (Examples 1 and 2).

[0036] FIG. 1 shows the formula [H a K b ]Sr2Nb3O 10 (A) shows the XRD pattern of the layered perovskite according to the present invention, where K + H + (a) is a reference diagram of the 100% proton-exchanged, fully dried compound, and (b) is a reference diagram of the 100% proton-exchanged, fully hydrated compound.

[0037] FIG. 2 shows a magnified cutout of the XRD pattern of the layered niobate according to the present invention, where the double-structure layered niobate HSR2Nb3O 10 The 00l peak of xH2O is clearly distinguished. The interlayer spacing is 15.3 Å or 16.9 Å.

[0038] FIG. 3 shows the relationship between the c-axis length or layer spacing in the layered niobate of the present invention and K + H+ The dependence of the exchange rate on

[0039] FIG. 4 shows the hydrogen formation rate versus K in the layered niobate of the present invention. + H + The dependence of the exchange rate on

[0040] FIG. 5 shows a comparison of the XRD patterns of the layered niobates of Table 1 used in the described examples, in which the separation of the layer spacing ("double peak") is clearly discernible (Example 1 and Example 2).

Claims

1. Formula [H a A b ] + [Sr 2 Nb 3 O 10 ] - [In the formula, [Sr 2 Nb 3 O 10 ] - is the main layer, and [H a A b ] + forms the intermediate layer, and H is H + and H 3 O + and A represents the group consisting of K + , Cs + and Rb + wherein 0.6≦a≦1 and 0≦b≦0.4, a+b=1], characterized in that the layered niobate has different spacings between the main layers.

2. 2. The layered niobate according to claim 1, wherein the layered niobate is a Dion-Jacobson type layered perovskite.

3. 3. The layered niobate according to claim 1, wherein the layered niobate has a hydrated phase and a dehydrated phase.

4. 4. The layered niobate according to claim 1, wherein the layered niobate has a degree of protonation of at least 60%, the degree of protonation being determined using EDX.

5. The layered niobate contains water molecules and / or hydrated hydronium ions (H 3 O + ・H 2 5. The layered niobate according to claim 1, wherein the layered niobate comprises at least one of:

6. 6. The layered niobate according to claim 1, wherein the 002 reflection and the 004 reflection each appear as a double peak in an XRD pattern of the layered niobate.

7. 7. The layered niobate according to claim 1, wherein A is a potassium ion.

8. A method for producing the layered niobate according to any one of claims 1 to 7, comprising: 2 Nb 3 O 10 [Wherein A is K + , Cs + and Rb + [representing an element of the group] with nitric acid (HNO 3 ) aqueous solution, said treatment being carried out at a temperature of 40 to 70°C.

9. The method according to claim 8, wherein the concentration of the aqueous nitric acid solution is 0.5 to 2.5M.

10. Use of the layered niobate according to any one of claims 1 to 7 as a photocatalyst.

11. A photocatalyst comprising the layered niobate according to any one of claims 1 to 7.

12. 12. The photocatalyst of claim 11, further comprising a rhodium co-catalyst.

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