Layered niobates for use in photocatalysts

Abstract

Layered niobate of formula [H a A b ] + [Sr2Nb3O 10 ] - , where [Sr2Nb3O 10 ] - the main layers and [H a A b ] + forming the intermediate layers, where H represents a group consisting of H + and H3O + consists and A for an element of the group K + , Cs + and Rb + stands, with 0.6 ≤ a ≤ 1 and 0 ≤ b ≤ 0.4, with a + b = 1, characterized in that the layered niobate has different distances between the main layers.

Description

[0001] The present invention relates to layered niobates of formula H a A b Sr2Nb3O 10 , where H represents a group containing the elements H + and H3O + includes and A for an element of the group K + , Cs + and Rb + stands, with 0.6 ≤ a ≤ 1 and 0 ≤ b ≤ 0.4, with a+b = 1, which are characterized by having different layer distances, a method for their production and their use in photocatalysts.

[0002] Photosynthesis converts sunlight into chemical energy, producing sugar from carbon dioxide and water. This sugar is stored in plants as cellulose, and the original energy is later released through combustion of the plant material. A similar principle underlies the storage of solar energy in the form of hydrogen. Water can be split into hydrogen and oxygen using sunlight and a catalyst. When this hydrogen is burned, it releases energy and water, thus serving as an alternative energy source. This form of energy generation has the advantage that, unlike the combustion of wood or other fossil fuels, it produces no environmentally harmful byproducts and, unlike wind or solar energy, is independent of the time of day and weather conditions.Compared to classical water electrolysis, photocatalytic water splitting continues to offer the advantages of moderate reaction conditions and relatively low technical requirements.

[0003] The first photocatalysts based on titanium dioxide were introduced as early as the 1970s; however, it proved impossible to increase the efficiency of this process sufficiently to produce hydrogen in large quantities. One class of compounds considered promising in this regard are layered perovskites of the Dion-Jacobson type, with the general formula [formula missing in original text]. MA n-1 B n O 3n+1 , where M represents an alkali metal, A an alkaline earth metal or rare earth metal, and B a pentavalent metal, usually tantalum or niobium, with the case of niobium these compounds also being called layered niobates. These compounds consist of negatively charged perovskite main layers of the general formula [A n-1 (B n O 3n+1 )] - These compounds are structured in layers between which alkali metal ions are embedded as positive interlayers. Due to their layered structure and the relatively large distance 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—that is, the amount of hydrogen produced when irradiated with light—of such compounds.

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

[0005] Domen et al. investigated the dependence of the layer spacing and hydrogen evolution for KCa2Nb3O in their article “Ion exchangeable layered niobates as a noble series of photocatalysts”, published in 1994 in Res. Chem. Intermed., Vol. 20, No. 9, pages 895 to 908. 10It was found that the layer spacing increased stepwise with increasing protonation level, with this stepwise increase attributed to the different degrees of hydration, i.e., the incorporation of water molecules along with the protons into the interlayers. From an exchange level of 60%, a significant increase in the hydrogen production rate was observed. This was explained by the fact that at large layer spacings, methanol molecules enter the interlayers and act as electron hole scavengers. The authors assume that this reduces the recombination rate between electrons and electron holes, thus making more electrons available for the reduction of water molecules to hydrogen.

[0006] Huang et al show in the article “Photocatalytic property of partially substituted Ptintercalated layered perovskite, ASr2Ta x Note 3-x O 10(A = K, H; x = 0, 1, 1.5, 2 and 3)“, published in Solar Energy Materials & Solar Cells 95, (2011) 1019-1027, XRD spectra and hydrogen formation rates including for HSr2(Ta / Nb)3O 10 For this compound, a layer spacing of 15.04 Å is given, which, after reaction with acid, yields HSr2Nb3O. 10 at 16.53 Å. Hydrogen evolution from a 10% methanol solution under irradiation with a mercury vapor lamp showed the following for the protonated compound HSr₂Nb₃O 10 higher values ​​than for the standard photocatalyst TiO2 P25.

[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 are produced by exfoliation of the corresponding Dion-Jacobson layered perovskites (HCa2Nb3O). 10 , HSr2Nb3O 10and HLaNb2O7) with tetra(n-butyl)ammonium and subsequent treatment with hydrochloric acid, and their photocatalytic properties were measured and compared with those of conventional compounds.

[0008] Fang et al describe in the Journal of Wuhan University of Technology - Mater. Sci. Ed., 2002, Vol. 7, No. 2, under the title “Synthesis and characterization of a new triple-layered Perovskite KSr2Nb3O 10 and its protonated compounds“ the production of KSR2Nb3O 10 by solid-state synthesis, followed by treatment with acid to synthesize the compound HSr2Nb3O via proton exchange. 10 *1.5 H2O. The layer spacing for the two compounds is 15.0 Å (KSr2Nb3O). 10 ) or 16.4 Å (HSr2Nb3O 10 *1.5 H2O).

[0009] Further state of the art is known from the following articles: DOMEN, Kazunari; EBINA, Yasuo; KONDO, Junko: Ion exchangeable layered niobates as a noble series of photocatalysts. In: Research on Chemical Intermediates, Vol. 20, 1994, No. 9, pp. 895–908. KULISCHOW, Natalia; LADASIU, Calin; MARSCHALL, Roland: Layered Dion-Jacobson type niobium oxides for photocatalytic hydrogen production prepared via molten salt synthesis. In: Catalysis Today, Vol. 287, 2017, pp. 65–69.

[0010] Although a number of compounds are already known to exhibit good photocatalytic activity, there remains a need for photocatalysts with improved efficiency that can be used for the solar chemical splitting of water.

[0011] It was surprisingly found that this task is solved by novel layered niobates, which are characterized by having different layer spacings in the protonated state.

[0012] Therefore, a first object of the present invention is a layered niobate of formula [H a A b ] + [Sr2Nb3O 10 ] - , where [Sr2Nb3O 10 ] - the main layers and [H a A b ] + forming the intermediate layers, where H represents a group consisting of the elements H + and H3O + consists and A for an element of the group K + , Cs + and Rb + stands, with 0.6 ≤ a ≤ 1 and 0 ≤ b ≤ 0.4, with a + b = 1, characterized in that the layered niobate has different distances between the main layers.

[0013] Preferably, the layered niobate according to the invention is one having the composition [H a A b ] + [Sr2Nb3O 10 ] - with 0.6 < a ≤ 1 and 0 ≤ b ≤ 0.4, with a + b = 1, preferably 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 preferably of the type of layered perovskites of the Dion-Jacobson type M[Sr2Nb3O 10 ], where M represents [H a A b ] stands as defined above. Thus, the layered niobate according to the invention is characterized by the fact that between the negatively charged main layers [Sr2Nb3O 10 ] - the positively charged elements M + are embedded. The distances between the individual layers, which can be determined using XRD measurements, correlate with the size of the embedded elements M. +In the case according to the invention, it was surprisingly found that different layer spacings form in the layered niobate, which is expressed in the XRD diagram by the doubling of the corresponding reflections (“double peak”). Without being bound to a specific theory, it is assumed that an inhomogeneous incorporation of water and / or hydrated hydronium ions into the interlayers leads to the layer spacing between some layers being larger than between others, meaning that the layered niobate according to the 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 water incorporation into the interlayers. Surprisingly, it was found that the photocatalytic activity of the layered niobate according to the invention increases significantly with the presence of the two phases, hydrated and dehydrated.Therefore, an embodiment of the present invention is preferred in which the layered niobate comprises a hydrated phase and a dehydrated phase. The hydrated phase comprises water molecules and / or hydrated hydronium ions (H3O). + *H2O) in the interlayers, while the dehydrated phase does not have corresponding molecules in the interlayers.

[0015] It is known from the prior art that the photocatalytic activity of layered perovskites can be increased if at least some of the alkali metal ions typically incorporated in the interlayers are exchanged for protons. Therefore, an embodiment of the present invention is preferred in which the layered niobate has a degree of protonation of at least 60%, preferably more than 70%, and particularly preferably 80 to 100%. Within the scope of the present invention, the degree of protonation is defined as the proportion of alkali metal ions in the interlayers that have been exchanged for protons, so that the degree of protonation can be determined by measuring the content of the exchanged alkali metal ion, for example, by EDX. A degree of protonation of 60% is therefore to be understood as meaning that 60% of the alkali metal ions typically incorporated in the interlayers have been replaced by protons.The degree of protonation can be measured by comparing the alkali metal ion content to that of the unprotonated compound, as described above.

[0016] The layered niobate according to the invention is characterized in particular by having two phases with different layer spacings. These phases can be identified by means of XRD measurements. In a preferred embodiment, the 002 and 004 X-ray diffraction reflections of the layered niobate according to the invention appear as two reflections (“double peaks”). In the XRD spectrum of the layered niobate according to the invention, the aforementioned reflections therefore appear as double reflections instead of as single reflections as in the spectra of conventional layered niobates.

[0017] The photocatalytic activity of the layered niobates according to the invention can be increased by replacing at least some of the alkali metal ions embedded in the interlayers with protons. This exchange has proven to be particularly efficient when the alkali metal ions are potassium ions. Therefore, an embodiment of the present invention is particularly preferred in which A is a potassium ion.

[0018] Without being bound to a specific theory, it is assumed that it is primarily the protonation step that contributes to the formation of the special structure of the layered niobates according to the invention. Therefore, in a preferred embodiment, the layered niobate is produced by reacting a compound of the formula ASr₂Nb₃O₄. 10 , where A represents an element of the group K + , Cs + and Rb +The surface is treated with aqueous nitric acid (HNO3). Preferably, the treatment with the aqueous nitric acid is carried out at a temperature of 40 to 70 °C, more preferably 50 to 65 °C. The duration of the treatment depends on the desired degree of protonation and can, in a preferred embodiment, be 3 to 24 hours, more preferably 5 to 20 hours, and particularly preferably 12 to 18 hours. Furthermore, it has proven advantageous to renew the aqueous nitric acid during the treatment. Therefore, an embodiment in which the aqueous nitric acid solution is replaced with a fresh solution every 4 to 10 hours, more preferably every 5 to 8 hours, is particularly preferred. In a further preferred embodiment, the concentration of the aqueous nitric acid solution is 0.5 to 2.5 M, more preferably 0.5 to 1.5 M.

[0019] Another object of the present invention is a method for producing the layered niobate according to the invention, wherein the method involves treating a compound of the general formula ASr2Nb3O 10 , where A represents an element of the group Elements K + , Cs + and Rb +The treatment process involves the preparation of a solution containing aqueous nitric acid (HNO3) at a temperature of 40 to 70 °C, preferably 50 to 65 °C. The duration of the treatment depends on the desired degree of protonation and, in a preferred embodiment, can be 3 to 24 hours, more preferably 5 to 20 hours, and particularly preferably 12 to 18 hours. Furthermore, it has proven advantageous to renew the aqueous nitric acid during the treatment. Therefore, an embodiment in which the aqueous nitric acid solution is replaced with a fresh solution every 4 to 10 hours, preferably every 5 to 8 hours, is particularly preferred. In a further preferred embodiment, the concentration of the aqueous nitric acid solution is 0.5 to 2.5 M, more preferably 0.5 to 1.5 M.

[0020] Another object of the present invention is a layered niobate obtainable by treatment of a compound of the general formula ASr2Nb3O 10, where A represents an element of the group Elements K + , Cs + and Rb + The niobate is treated with aqueous nitric acid (HNO3) at a temperature of 40 to 70 °C, preferably 50 to 65 °C. The layered niobate obtained in this way has a composition of the formula [H a A b ] + [Sr2Nb3O 10 ] - , where [Sr2Nb3O 10 ] - the main layers and [H a A b ] + forming the intermediate layers, where H represents a group consisting of the elements H + and H3O + consists and A for an element of the group K + , Cs + and Rb + stands, with 0.6 ≤ a ≤ 1 and 0 ≤ b ≤ 0.4, with a + b = 1, and has different distances between the main layers.

[0021] The compound of the general formula ASr2Nb3O 10, which serves as a starting compound in the production of the layered niobates according to the invention, is preferably produced by means of molten salt synthesis or solid-phase synthesis.

[0022] The layered niobates according to the invention are characterized by high photocatalytic activity. Therefore, a further object of the present invention is the use of the layered niobate according to the invention as a photocatalyst, preferably as a photocatalyst in photoinduced water splitting.

[0023] Another object of the present invention is a photocatalyst comprising a layered niobate according to the present invention. It has surprisingly been found that the amount of hydrogen produced by the photocatalyst according to the invention is higher than that obtained by conventional photocatalysts under the same conditions. Preferably, the photocatalyst according to the invention further comprises a rhodium cocatalyst.

[0024] The invention will be explained in more detail below using examples, which should in no way be understood as a limitation of the inventive concept. Examples:

[0025] KSR2Nb3O 10 was produced by molten salt synthesis, as described, for example, by Kulischow et al in Catal. Today 2017, 287, 65-69.

[0026] KSR2Nb3O 10The sample was stirred in 1M HNO3 solution at 60 °C for various time intervals. The degree of protonation was monitored using energy-dispersive X-ray spectroscopy (EDX).

[0027] X-ray diffraction analyses were performed using a PANalytical MPD diffractometer with Cu-K α - Radiation (λ = 0.1541 nm) in the 2Θ range from 5° to 30°.

[0028] EDX elemental analysis was performed using a Philips LEO Gemini 928 field emission SEM at 20 kV acceleration voltage.

[0029] The photocatalytic investigations were carried out in a double-walled quartz reactor as described in Kulischow et al. in Catal. Today 2017, 287, 65-69. To eliminate thermal influences, the reactor was cooled to 10 °C. A 350 W mercury 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 hydrogen produced. The column was maintained at a temperature of 35 °C during the measurement, and the elution time for H₂ was 1 minute.

[0030] In a typical experiment, 0.3 g of the layered niobate according to the invention was suspended with 0.3 wt% Rh(NH3)5Cl)Cl2 as a cocatalyst in 600 ml of aqueous methanol solution (10% v / v) under ultrasonic treatment and then irradiated with a 350 W mercury lamp. The initial pH of the solution was adjusted to 3 with perchloric acid. Before irradiation, the system was purged with argon to ensure complete removal of air. The results of these photocatalytic measurements with the 350 W mercury lamp on materials with different degrees of protonation are presented in Fig. 4 shown.

[0031] Further 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 treatment durations were adjusted to achieve a similar degree of protonation at the end of the acid treatment in all experiments, with the longest treatment (172 h) required at 20 °C. The chemical analysis of the obtained layered niobates is summarized in Table 1, with the starting compound KSr2Nb3O included for comparison. 10 The samples used each had an exchange or protonation degree of 83%, but only the samples in Examples 1 and 2, where the acid treatment was carried out at 55 °C and 60 °C respectively, showed the different layer spacings according to the invention. The corresponding XRD diagrams are shown in Fig.5. The acid treatments at 20 °C and 80 °C (comparative examples 1 and 2) showed only one 002 and one 004 peak in the XRD diagrams. Table 1: Analyses Molar ratios based on Nb = 3 Temperature [°C] K[Wt. %] Sr[Wt. %] Nb[Wt. %] H(1-K) K Sr Note KSr2Nb3O 10 - 5,86 26,75 42,82 - 0,98 1,99 3 See 1 20 1,06 27,84 44,35 0,83 0,17 2,00 3 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 See 2 80 1,07 27,84 44,22 0,83 0,17 2,00 3

[0032] The aim of photocatalyst development is solar water splitting. Mercury lamps produce a high proportion of high-energy UV radiation, which, while increasing photocatalytic hydrogen evolution, is not present in the sunlight spectrum. To test its application for solar water splitting, a quartz glass cuvette containing the photocatalyst suspension from the layered niobates listed in Table 1, with Rh(NH3)5Cl)Cl2 as a cocatalyst as described above, was irradiated with a xenon arc lamp (Perkin Elmer Cermax E300BF) through a solar simulator filter instead of a mercury lamp. A water filter was used to prevent a temperature increase. The illuminance was 1283.9 mW / cm². 2on the cuvette. The measured amount of hydrogen after 5 hours is summarized in Table 2: sample See 1 Example 1 Example 2 See 2 H2 [µmol / h] 400 493 545 418

[0033] As can be seen from Table 2, a significantly higher hydrogen production could be achieved by using the layered niobates according to the invention (Ex. 1 and Ex. 2).

[0034] Fig. Figure 1 shows an XRD diagram of a layered perovskite of the formula [H] according to the invention. a K b ]Sr2Nb3O 10 (A) with different degrees of protonation, where K + against H + was replaced with (a) reference diagram of the 100% proton-exchanged, fully dry compound and (b) reference diagram of the 100% proton-exchanged, fully hydrated compound.

[0035] Fig. Figure 2 shows an enlarged section of an XRD diagram of a layered niobate according to the invention, clearly showing the 00l peaks of the bistructured layered niobate HSr2Nb3O10 * x H2O can be seen. The distance between the layers is 15.3 Å and 16.9 Å, respectively.

[0036] Fig. Figure 3 shows the length of the c-axis or the layer spacing in a layered niobate according to the invention as a function of the exchange rate K. + against H + .

[0037] Fig. Figure 4 shows the dependence of the hydrogen production rate on the exchange rate K. + against H + in a layered niobate according to the invention.

[0038] Fig. Figure 5 shows a comparison of the XRD diagrams of the layer niobates from Table 1, which were used in the examples described, clearly showing the splitting of the layer intervals (“double peak”) (Ex. 1 and Ex. 2).

Claims

[1] Layered niobate of formula [H a A b ] + [Sr2Nb3O 10 ] - , where [Sr2Nb3O 10 ] - the main layers and [H a A b ] + forming the intermediate layers, where H represents a group consisting of H + and H3O + consists and A for an element of the group K + , Cs + and Rb + stands, with 0.6 ≤ a ≤ 1 and 0 ≤ b ≤ 0.4, with a + b = 1, characterized by that the layered niobate exhibits different distances between the main layers. [2] Layered niobate according to claim 1, characterized by that the layered niobate is a layered perovskite of the Dion-Jacobson type. [3] Layered niobate according to at least one of the preceding claims, characterized by that the layered niobate has a hydrated and a dehydrated phase. [4] Layered niobate according to at least one of the preceding claims, characterized bythat the layered niobate has a degree of protonation of at least 60%, preferably more than 70%, particularly preferably 80 to 100%, wherein the degree of protonation is determined by EDX. [5] Layered niobate according to at least one of the preceding claims, characterized by that the layered niobate water molecules and / or hydrated hydronium ions (H3O) + *H2O) in the intermediate layers. [6] Layered niobate according to at least one of the preceding claims, characterized by , that in the XRD diagram of the layered niobate, the 002 and 004 reflections each appear as a double peak. [7] Layered niobate according to at least one of the preceding claims, characterized by , that A is a potassium ion. [8] Layered niobate according to at least one of the preceding claims, characterized by that the layered niobate is produced by a compound of the formula ASr2Nb3O 10 , where A represents an element of the group K + , Cs+ and Rb + is treated with aqueous nitric acid (HNO3). [9] Layered niobate according to claim 8, characterized by that the treatment with aqueous nitric acid takes place at a temperature of 40 to 70 °C, preferably 50 to 65 °C. [10] Layered niobate according to at least one of claims 8 or 9, characterized by that the concentration of the aqueous nitric acid solution is 0.5 to 2.5 M, preferably 0.5 to 1.5 M. [11] Method for producing a layered niobate according to at least one of claims 1 to 10, comprising treating a compound of the general formula ASr2Nb3O 10 , where A represents an element of the group K + , Cs + and Rb + stands, with aqueous nitric acid (HNO3), characterized by that the treatment takes place at a temperature of 40 to 70 °C, preferably 50 to 65 °C. [12] Layered niobate obtainable by a method according to claim 11. [13] Use of a layered niobate according to at least one of the preceding claims as a photocatalyst, preferably for photoinduced water splitting. [14] Photocatalyst comprising a layered niobate according to at least one of claims 1 to 10 or claim 12. [15] Photocatalyst according to claim 14, characterized by that the photocatalyst still contains a rhodium cocatalyst.

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