Beiträge zur Untersuchung anorganischer nichtstöchiometrischer Verbindungen. XIV. Oxydationsprodukte von orthorhombischem Nb12O29 Elektronenoptische Untersuchung

Contributions to the Investigation of Inorganic Non-stoichiometric Compounds. XIV. Oxidation Products of Orthorhombic Nb₁₂O₂₉, Electron Optical Investigation An electron optical investigation shows that the orthorhombic starting material Nb₁₂O₂₉(BII) is well ordered. The oxidation products Nb₂O₅(Ox1BII) and Nb₂O₅(Ox2BII) are different from each other in structures as well as in their reactions. Nb₂O₅(Ox1BII) is unstable in the electron beam and differs from BII by characteristic point-defects. The radiation load can lead to the reduction to BII or to a transition into a defect structure with R-type-tunnels. The not well ordered structure of Nb₂O₅(Ox2BII) is stable in the electron beam. Characteristic is the sequence of [2×5] and [3×4] blocks, the latter in two different orientations. The observed composition O/Nb = 2.500 can be described by the present structural modell assuming vacant niobium tetrahedral sites. The large structural differences between the oxidation products of the orthorhombic and the monoclinic Nb₁₂O₂₉ are remarkable.     

Beiträge zur Untersuchung anorganischer nichtstöchiometrischer Verbindungen. XLIII. Darstellung und elektronenmikroskopische Untersuchung von (Cr,Nb)12O29

Contributions to the Investigation of Inorganic Non-Stoichiometric Compounds. XLIII. Preparation and Electron Microscopic Investigation of (Cr, Nb)₁₂O₂₉ Transport experiments in the system Cr/Nb/O yielded the compound (Cr, Nb)₁₂O₂₉ as crystalline green needles. The lattice parameters of the monoclinic cell are a = 31.162(8) Å, b = 3.8324(6) Å, c = 20.579(6) Å, β = 113.22(3)°. (Cr, Nb)₁₂O₂₉ is isotypic with monoclinic Nb₁₂O₂₉ as shown by Guinier diagrams and high resolution transmission electron microscopy. Characteristic structural building elements are [3 × 4]-blocks which are columnar sections from the ReO₃ structure. In the real structure one finds numerous defects. Under the influence of the electron beam we observed reversible changes of contrast which indicates that the positions of the M-particles (M = Cr, Nb) have shifted.     

Beiträge zur Untersuchung anorganischer nichtstöchiometrischer verbindungen. XV. Weitere metastabile Nb2O5-Modifikationen,Produkte der langsamen Oxydation von NbOx-Phasen

Contributions to the Investigation of Inorganic Non-stoichiometric Compounds. XV. Some Further Metastable Nb₂O₅-Modifications, Products of the Slow Oxidation of the NbO_x Phases If in contrast to previous investigations the blue-black NbO_x phases (2.4 < x < 2.5) are not oxidized in air, but in a rarefied oxygen atmosphere or with steam, five further metastable Nb₂O₅ forms can be obtained in addition to the numerous already known Nb₂O₅ modifications. Nb₁₂O₂₉ (mon. and orh.) and Nb₄₇O₁₁₆ lead to one new Nb₂O₅ form each. Nb₂₂O₅₄ yields two new Nb₂O₅ modifications. It can be deduced from the thermal behaviour that in addition to unexspected transitions between the known and the new metastable modifications in all cases H-Nb₂O₅ appears as final product at high temperature. The electron optical investigation of the new oxidation products of Nb₁₂O₂₉ (mon. and orh.) shows that their structures are determined by R-type tunnels as the characteristic structure element. By this they clearly differ from the already known oxidation products. With this for the first time sodium free niobium oxide structures occur whose structural concept is substantially shaped by R-type tunnels which are known from NaNb₁₃O₃₃.     

Beiträge zur Untersuchung anorganischer nichtstöchiometrischer Verbindungen. XLIV. Neue elektronenmikroskopische Untersuchung zur Realstruktur der metastabilen Nb2O5-Modifikation Ox1C

Contributions to the Investigation of Inorganic Non-Stoichiometric Compounds. XLIV. New Electron Microscopic Investigation of the Metastable Nb₂O₅ Modification Ox1C Oxidation of the blue-black NbO_x phases (2.4 < x < 2.5) in air or a diluted oxygen atmosphere leads all together to 21 metastable colourless (partly yellowish) Nb₂O₅ modifications. These can be described as compounds having block structures like the NbO_x phases themselves. At 200°C the oxidation process to the metastable phases proceeds quite slowly; at higher temperatures all polymorphs are finally transformed to H? Nb₂O₅. Electron diffraction patterns of the polymorph, which can be obtained by heating crystals of Nb₂₂O₅₄ in atmospheric environment at about 200°C (Ox1C), show extra spots in the c* direction compared to Nb₂₂O₅₄. This oxidation product is quite sensitive to the focused electron beam, so the reduction of the crystals back to Nb₂₂O₅₄ can only be avoided by reducing its intensity. Finally HRTEM images of thin crystal areas showing a superstructure were obtained. Comparing those characteristic image contrasts to calculated images of different point defect models one finds, that mainly one point defect is responsible for the superstructure. By migration of one Niobium atom two interstitial oxygen sites are created. So a structure model can be achieved leading to the formula Nb₄₄O₁₁₀.     

Beiträge zur Untersuchung anorganischer nichtstöchiometrischer Verbindungen. XXXIV. Fehlgeordnete Strukturen mit Großen Blöcken im System Nb2O5/WO3 — Elektronenmikroskopische Untersuchung

Contributions to the Investigation of Inorganic Non-stoichiometric Compounds. XXXIV. Disordered Structures Consisting of Large Blocks in the System Nb₂O₅/WO₃. An Electronmicroscopical Investigation The real structures of samples which have been prepared by quenching procedures using the composition Nb₂O₅: WO₃ = 10:11 are in most cases heavily disordered; some areas show extremly large blocks. The electron microscopic investigation revealed that the blocks consist of [n × m] M? O octahedra and have edge lengths of n = 5 to 14 and of m = 5 to 13 and in one case we even found [n × m] = [5 × 32]. In comparison, samples that were heated below the melting point for a longer time, showed a less disordered structure. Block sizes of [5 × 6] to [5 × 9] in a matrix of [5 × 5] blocks could be observed.     

Beiträge zur Untersuchung anorganischer nichtstöchiometrischer Verbindungen. XXIII. Neue Bauelemente bei metastabilen Blockstrukturen im System Nb2O5/WO3 Elektronenmikroskopische Untersuchung in Hochauflösung

Contributions to the Investigation of Inorganic Non-stoichiometric Compounds. XXIII. New Building Elements of Metastable Block Structures in the System Nb₂O₅/WO₃, Investigation by High Resolution Electron Microscopy Under equilibrium conditions the region of existence of block structures in the system Nb₂O₅/WO₃ ends with a phase having the composition 9 Nb₂O₅:8 WO₃ corresponding to values of O/ΣM = 2.654 (M = Nb, W) and W/Nb = 0.44. We now investigated to what extent this region of existence of block structures could be expanded towards a higher content of WO₃. Starting out from the known phases Nb₂O₅:WO₃ = 6:1, 7:3, 8:5 and 9:8 in which Nb and W are present in their maximum states of oxidation, we obtained series of solid solutions by substituting W for Nb. Thereby the initial structure (block sizes [3×4], [4×4], [4×5] and [5×5] M–O-octahedra) and also the respective O/ΣM remained unchanged. Upon complete oxidation of these series of solid solutions which led into reduced systems (e. g. NbO₂/Nb₂O₅/WO₃) at ～500°C metastable products were gained, which also have block structures. Corresponding to the extent of substitution of the initial series of solid solution and the resulting ratio O/ΣM of the oxidation products the new structures are to a wide extent built of large blocks that have an extension (at maximum [5X15] M–O-octahedra) we have not observed so far. These blocks could be seen using high resolution transmission electron microscopy. The investigations on the oxidation products of the solid solutions of thc 8:5 and 9:8 series and illso it model concerning the mechanism of oxidation are described.     

Beiträge zur Untersuchung anorganischer nichtstöchiometrischer Verbindungen. XXII. Neue metastabile Blockstrukturen im System Nb2O5/WO3 Elektronenoptische Untersuchung

Contributions to the Investigation of Inorganic Non-stoichiometric Compounds. XXII. New Metastable Block Structures in the System Nb₂O₅/WO₃, Electron Optical Investigation We succeeded in considerably expanding the region of existence of block structures. By substituting W for Nb, while the ratio O/∑M (M ? Nb, W) is kept constant, and starting from the known phases Nb₂O₅: WO₃ = 6:1, 7:3, 8:5 and 9:8 one obtains series of solid solutions whose metastable products of oxidation have block structures too. In contrast to the solid solutions which have the structures of the starting phases i. e. with socalled blocks of [3 × 4], [4 × 4], [4 × 5] and [5 × 5] M? O-octahedra, the products of the oxidation have structures in which some edge sharing octahedra changed their connections to become octahedra sharing corners thus opening up the possibility for the formation of blocks with twice the number of octahedra as before. Rows of these large blocks with e. g. [4 × 6] or [4 × 8] octahedra alternate nonperiodically with rows of smaller blocks of the initial size. Details of these heavily disordered structures could only be discerned with the help of high resolution electron microscopy.     

Beiträge zur Untersuchung anorganischer nichtstöchiometrischer Verbindungen. XXX. Elektronenmikroskopische Untersuchung in Hochauflösung an metastabilen Niob-Wolfram-Oxiden Vergleich mit der rechnerischen Simultation

Contributions to the Investigation of Inorganic Non-stoichiometric Compounds. XXX. High Resolution Electron Microscopy Investigation of Metastable Niobium Tungsten Oxides. Comparison with Computer Simulated Images . Starting with solid solution series (Nb, W)O_x, where WO₃ + NbO₂ was substituted for Nb₂O₅, a gentle oxidation leads from the original block sizes of [3 × 4], [4 × 4], [4 × 5] or [5 × 5] octahedra to a doubling of blocks in the new ternary oxides, e. g. 2 × [5 × 5] → [5 × 10]. As HRTEM of the oxidation products revealed, these new larger blocks are connected in a special way including so-called “L-type-tunnels”. Using a LaB₆-cathode with our electron microscope, we now for the first time can directly see the positions of these tunnels.     

Beiträge zur Untersuchung anorganischer nichtstöchiometrischer Verbindungen. XXXV [1, 2]. Reduktion von Blockstrukturen im Elektronenmikroskop - in situ - Beobachtung einer topotaktischen Reaktion im System Nb2O5/WO3

Contributions to the investigation of inorganic non-stoichiometric compounds. XXXV. Reduction of block structures by heating with the electron beam -- in situ investigation of a topotactic reaction in the system Nb₂O₅/WO₃ In quenched samples with the starting composition Nb₂O₅: WO₃ ≈? 1:1 we observed by using high resolution transmission electron microscopy (HRTEM) areas with large blocks of [5 × n] M–O octahedra (n = 6, 7, 8, 9; M = Nb, W). Upon heating with the electron beam a structural change occurs as an in-situ reaction. The original [5 × n] blocks are splitting up mostly in two [5 × 1/2n] blocks (n even) or when n is odd into one block with [5 × 1/2(n + 1)] and one with [5 × 1/2(n--1)] M–O octahedra. The splitting up of blocks forces some corner sharing octahedra to become edge sharing octahedra. This change of connections constitutes a partial reduction of the sample.     

Beiträge zur Untersuchung anorganischer nichtstöchiometrischer Verbindungen. XII [1, 2]. Rückreduktion metastabiler Nb2O5-Modifikationen Oxydationsprodukte von NbOx-Phasen (2,4 < x < 2,5)

Die Reduktion der metastabilen Oxydationsprodukte der NbO_x-Phasen (2,4 < x < 2,5) mit H₂O/H₂-Mischungen wurde untersucht. Bei niedrigerer Temperatur (≈300°C) ist die Reduktionswirkung noch gering, sie nimmt mit steigender Temperatur beträchtlich zu. Dabei zeigen die Oxydationsprodukte der Zusammensetzung Nb₂O₅ ein gewisses ?Erinnerungsvermögen”?; in allen Fällen läft die Reduktion bis zu den ursprünglichen NbO_x-Phasen zurück. Man gelangt bei um so niedrigerer Reduktionstemperatur zur Zusammensetzung x des jeweiligen Ausgangsprodukts NbO_x zurück, je kleiner die Differenz 2,50?x ist. Bemerkenswert ist, daß die Rückreduktion zum monoklinen Nb₁₂O₂₉ über eine Zwischenstufe der Zusammensetzung 2,439?2,447 O/Nb verläuft. An dieser Stelle liegt eine neue, metastabile NbO_x-Phase mit einem charakteristischen Guinierdiagramm vor. Contributions on the Investigation of Inorganic Non-stoichiometric Compounds. XII. Reduction of Metastable Nb₂O₅ Modifications, Oxidation Products of NbO_x Phases (2.4 < x < 2.5) The reduction of metastable oxidation products of the NbO_x phases (2.4 < x < 2.5) by H₂O/H₂ mixtures was investigated. At lower temperatures (ca 300°C) the effect of the reduction was slight. It increased considerably with rising temperature. Samples oxidized to the composition Nb₂O₅ showed a certain “memory of structure”. The original NbO_x phases can be obtained in all cases from the oxidized products. The smaller the difference 2.50?x the lower the reaction temperature at which the composition x of the corresponding starting material NbO_x is regained. It is remarkable that the re-reduction to the monoclinic Nb₁₂O₂₉ proceeds by way of the intermediate composition 2.439?2.447 O/Nb. At this ratio O/Nb a new metastable NbO_x phase with a characteristic x-ray diagram exists.     

Beiträge zur Untersuchung anorganischer nichtstöchiometrischer Verbindungen. XX. Metastabile Oxydation von Mischkristallreihen — ein Zugang zu W-reichen Blockstrukturen im System Nb2O5/WO3

Contributions to the Investigation of Inorganic Non-stoichiometric Compounds. XX. Metastable Oxidation of a Series of Solid Solutions — an Access to W-rich Block Structures in the System Nb₂O₅/WO₃ The characteristical region of existence of block structures with building elements that are limited in size in two directions ends in the system Nb₂O₅/WO₃, as was shown by previous investigations, under conditions of equilibrium at a maximum value of 2.654 O/ΣM. For the occurring phases with the ratios Nb₂O₅: WO₃ = 6:1, = 7:3, = 8:5 and = 9:8 as well we now were successful in substituting W for Nb. The original block structure and the corresponding ratio O/ΣM were preserved. The “9:8”-phase W_4/4[Nb₁₈W₇O₆₉], for example, forms solid solutions W_4/4[Nb₁₁W₁₄O₆₉] leaving the size of the building elements ([5 times; 5] blocks) unchanged. Hereby the ratio W/Nb is drastically enhanced from 0.444 to 1.364. By metastable oxidation of these solid solutions at temperatures of about 500°C, for instance in air, one comes back to the system Nb₂O₅/WO₃. In this way the region of existence of block structures could be expanded far beyond the limit at 2.654 O/ΣM to higher W/Nb values.     

Weitere neue Nb2O5-Modifikationen,metastabile Oxidationsprodukte von NbOx-Phasen (2,4 <x< 2,5)

Some New Nb₂O₅ Modifications, Metastable Oxidation Products of NbO_x Phases (2.4 <x< 2.5) The blue-black NbO_x Phases (2.4 <x< 2.5) are oxidized metastable in air to colourless (partly yellowish) Nb₂O₅ modifications already at temperatures about 200°C. At further heating stepwise and monotropic phase transformations are noticed; as final product the well known stable H modification appears. Some of the 15 new metastable Nb₂O₅ modifications obtained in this way appear in series. Therewith Nb₁₂O₂₉ (orh. and mon.) changes into two each, Nb₂₂O₅₄ and Nb₄₇O₁₁₆ into four each, Nb₂₅O₆₂ into two and Nb₅₃O₁₃₂ into one Nb₂O₅ phase. During oxidation and the following phase transformations at higher temperatures both the character of single crystal and the principle of building (block structure) of the starting material are maintained. This is especially proven from the electronoptical and X-ray investigations of Nb₁₂O₂₉ (orh. and mon.) and its products obtained by oxidation and further heating.     

Beiträge zur Untersuchung anorganischer nichtstöchiometrischer Verbindungen. XXXVIII. Quaternäre Nioboxidfluoride des Cadmiums und Mangans mit LiNb6O15F-Struktur

Contributions to the Investigation of Inorganic Non-stoichiometric Compounds. XXXVIII. Quaternary Niobium Oxide Fluorides of Cadmium and Manganese with LiNb₆O₁₅F structure For the first time we observed an occupation of the canals in the LiNb₆O₁₅F structure by bivalent metal ions. We obtained wellshaped crystals of M_xNb₆(O, F)₁₆ (M ? Cd, Mn) by heating mixtures of MF₂ and Nb₂O₅. Cd_xNb₆(O, F)₁₆ (Mn_xNb₆(O, F)₁₆) crystallizes in the orthorhombic space group Pmma with a = 16.686 (16.677), b = 3.951 (3.952) and c = 8.907 (8.902) Å. The structure of the cadmium compound was refined to give R = 3.33% and R_w = 2.90%. The resulting formula for the investigated crystal was Cd_0.25Nb₆O_14.5F_1.5.     

Beiträge zur Untersuchung anorganischer nichtstöchiometrischer Verbindungen. XXVI. Darstellung und elektronenmikroskopische Untersuchung von V16W9O65

Contributions to the Investigation of Inorganic Non-Stoichiometric Compounds. XXVI. Preparation and Electron Optical Investigation of V₁₆W₉O₆₅ The mixed vanadium-tungsten oxide with the composition V₁₆W₉O₆₅ (V_0.64W_0.36O_2.60) was prepared by chemical transport reactions. The dimensions of the tetragonal unit cell are a = 25.074 Å; c = 3,714 Å, determined by the X-ray-Guinier method. The probable space group is 14/mmm. The electron microscopical investigation directly reveals the block structure of the new compound. The blocks are made up of [5 × 5 × ∞] corner sharing M-O-octahedra. Other unusual block sizes are observed as defects. The blocks are connected by common corners like in the related compound (V_0.65W_0.35)₂O₅ with [4 × 4 × ∞] blocks.     

Beiträge zur Untersuchung anorganischer nichtstöchiometrischer Verbindungen. XXXIII. Darstellung und elektronenmikroskopische Untersuchung von Hf2Nb20O54, M3Nb44O116 und MNb24O62 (M = Zr,Hf)

Contributions to the Investigation of Inorganic Non-stoichiometric Compounds. XXXIII. Preparation and Electron Microscopical Investigation of Hf₂Nb₂₀O₅₄, M₃Nb₄₄O₁₁₆, and MNb₂₄O₆₂ (M = Zr, Hf) The phases Hf₂Nb₂₀O₅₄, Zr₃Nb₄₄O₁₁₆, Hf₃Nb₄₄O₁₁₆, and β-HfNb₂₄O₆₂ have been prepared for the first time. The oxides MO₂ (M = Zr, Hf) and Nb₂O₅ were fused with (NH₄)₂SO₄ and their mixed precipitations were heated at 1350°C. In the same way we also obtained pure β-ZrNb₂₄O₆₂. The new compounds have block structures, as the structure investigation with HRTEM shows. Hf₂Nb₂₀O₅₄ is isostructural with Nb₂₂O₅₄, the same is valid for Zr₃Nb₄₄O₁₁₆ and Hf₃Nb₄₄O₁₁₆ with respect to Nb₄₇O₁₁₆. β-HfNb₂₄O₆₂ has the same structure as β-ZrNb₂₄O₆₂ and β-Nb₂₅O₆₂.     

Giant Hollow Heterometallic Polyoxoniobates with Sodalite‐Type Lanthanide–Tungsten–Oxide Cages: Discrete Nanoclusters and Extended Frameworks

The first series of niobium–tungsten–lanthanide (Nb‐W‐Ln) heterometallic polyoxometalates {Ln₁₂W₁₂O₃₆(H₂O)₂₄(Nb₆O₁₉)₁₂} (Ln=Y, La, Sm, Eu, Yb) have been obtained, which are comprised of giant cluster‐in‐cluster‐like ({Ln₁₂W₁₂}‐in‐{Nb₇₂}) structures built from 12 hexaniobate {Nb₆O₁₉} clusters gathered together by a rare 24‐nuclearity sodalite‐type heterometal–oxide cage {Ln₁₂W₁₂O₃₆(H₂O)₂₄}. The Nb‐W‐Ln clusters present the largest multi‐metal polyoxoniobates and a series of rare high‐nuclearity 4d‐5d‐4f multicomponent clusters. Furthermore, the giant Nb‐W‐Ln clusters may be isolated as discrete inorganic alkali salts and can be used as building blocks to form high‐dimensional inorganic–organic hybrid frameworks.     

Darstellung,Kristallstruktur und elektronenmikroskopische Untersuchung von UNb6O16 – einem neuen niobreichen Oxid im System U/Nb/O

Preparation, Crystal Structure and Electron Microscopic Investigation of UNb₆O₁₆ – a New Niobium-rich Phase in the System U/Nb/O Powdery UNb₆O₁₆ was produced by heating (1 000°C or 1 100°C; evacuated silica tube) mixed powders of UO₂, NbO₂ and Nb₂O₅ (1:2:2). Single-crystals of UNb₆O₁₆ were obtained by chemical transport in a small temperature gradient (1 000°C → 990°C; transport agent NH₄Cl). The lattice constants are a = 22 339(4) Å; b = 3.7750(6) Å; c = 7.249(3) Å; β = 97.61(3)° and Z = 2. The structure determination (space group C2) let to R = 0.026 (R_w = 0.026). Eight oxygen atoms surround U⁴⁺ like a trans-bis-capped octahedron, Nb⁴⁺ and Nb⁵⁺ are coordinated distorted octahedraly. The structure was checked and the occupation of the positions O8 and O9 was clarified with the program MAPLE4 [3]. A through focus series of high resolution transmission electron microscopic images was obtained which is in acceptable agreement with images calculated on the basis of the multi-slice method.     

Zur Präparation und Struktur gemischtvalenter Niob-Wolframoxide der Zusammensetzung (Nb,W)17O47

Preparation and Structure of Niobium Tungsten Oxides (Nb,W)₁₇O₄₇ with Mixed Valency The formal substitution of 2Nb⁵⁺ by Nb⁴⁺ or W⁴⁺, respectively, and W⁶⁺ leads to tungsten niobium oxides (Nb,W)₁₇O₄₇ with mixed valency. The phases Nb_8-nW_9+nO₄₇ with n = 1 to 5 could be obtained by heating (1 250°) mixtures of NbO₂ or WO₂, respectively, with Nb₂O₅ and WO₃. The products crystallize with the structure of Nb₈W₉O₄₇. This is proved by X-ray powder diffraction and transmission electron microscopy. A further decrease of the Nb-content results in two-phase products.     

{Nb288O768(OH)48(CO3)12}: A Macromolecular Polyoxometalate with Close to 300 Niobium Atoms

A protein‐sized (ca. 4.2×4.2×3.6 nm³) non‐biologically derived molecule {Nb₂₈₈O₇₆₈(OH)₄₈(CO₃)₁₂} ( Nb₂₈₈ ) containing up to 288 niobium atoms has been obtained, which is by far the largest and the highest nuclearity polyoxoniobate (PONb). Particularly, in terms of metal nuclearity number, Nb₂₈₈ is the second largest cluster so far reported in classic polyoxometalate chemistry (V, Mo, W, Nb, and Ta). Nb₂₈₈ can be described as a giant windmill‐like cluster aggregate of six nanoscale high‐nuclearity PONb units {Nb₄₇O₁₂₈(OH)₆(CO₃)₂} ( Nb₄₇ ) joined together by six additional Nb ions. Interestingly, the 47‐nuclearity Nb₄₇ units generated in situ can be isolated and bridged by copper complexes to form an inorganic–organic hybrid three‐dimensional PONb framework, which exhibits effective catalytic activity for hydrolyzing nerve agent simulant of dimethyl methylphosphonate. The unique Nb₄₇ cluster also provides a new type of topology to very limited family of Nb‐O clusters.     

Characterization of Thermally Stable Brønsted Acid Sites on Alumina‐Supported Niobium Oxide after Calcination at High Temperatures

Thermally stable Brønsted acid sites were generated on alumina‐supported niobium oxide (Nb₂O₅/Al₂O₃) by calcination at high temperatures, such as 1123 K. The results of structural characterization by using Fourier‐transform infrared (FTIR) spectroscopy, TEM, scanning transmission electron microscopy (STEM), and energy‐dispersive X‐ray (EDX) analysis indicated that the Nb₂O₅ monolayer domains were highly dispersed over alumina at low Nb₂O₅ loadings, such as 5 wt %, and no Brønsted acid sites were presents. The coverage of Nb₂O₅ monolayer domains over Al₂O₃ increased with increasing Nb₂O₅ loading and almost‐full coverage was obtained at a loading of 16 wt %. A sharp increase in the number of hydroxy groups, which acted as Brønsted acid sites, was observed at this loading level. The relationship between the acidic properties and the structure of the material suggested that the bridging hydroxy groups (Nb? O(H)? Nb), which were formed at the boundaries between the domains of the Nb₂O₅ monolayer, acted as thermally stable Brønsted acid sites.