Visibility of Al Surface Sites of γ‑Alumina: A Combined Computational and Experimental Point of View

The nature of γ-alumina (γ-Al₂O₃) surface sites leaves many open questions today, and solid-state NMR spectroscopy has been proposed and used as a tool for assessing their structure. Here, we calculated ²⁷Al NMR parameters from first principles in periodic boundary conditions for a large number of Al sites with different coordination, potentially present on the alumina surface. The nature and accordingly the NMR parameters of these sites change with the level of hydroxylation and thereby the pretreatment temperature of γ-Al₂O₃. While the Al chemical shift is little affected by hydroxylation, the magnitude of the quadrupolar interaction at the Al nucleus is strongly correlated to hydroxylation, with high to very high quadrupolar coupling constant (<i>C</i>_Q) values (20–34 MHz) on the weakly hydrated major (110) termination, which contains highly reactive Lewis acidic “defect sites”, and gradual lowering to bulklike <i>C</i>_Q values of around 5 MHz on highly hydrated surfaces. In addition, we studied the effects of local symmetry on the EFG tensor of Al atoms and the effects of hydrating neighboring Al sites. These calculation were combined with {¹H}²⁷Al cross-polarization NMR experiments, carried out at high magnetic field (20 T) and high magic angle spinning (MAS) frequency (30 kHz) on γ-Al₂O₃ pretreated at different temperatures. We clearly show that this method is mainly sensitive to strongly hydrated surface sites besides the much more abundant bulk Al atoms, and that catalytically important high-<i>C</i>_Q Al centers (Lewis acid sites) and even some types of hydroxylated Al sites are not visible in the spectrum. Therefore, even current high-field NMR experiments are not (yet) able to provide a complete picture of the structure of γ-Al₂O₃ and much caution should be exercised when interpreting ²⁷Al NMR spectra

Unveiling Structural Defects by ¹³⁹La NMR and Raman Spectroscopies at the Origin of Surface Stability for the Design of Cerium-Based Catalysts

Cerium-based complex oxides are essential materials well-suited for numerous applications related to heterogeneous catalysis and electrocatalysis. However, the impact of structural defects on the thermal stability of the surface has still to be elucidated. Probing lanthanum environments by 139La NMR and analyzing, for the first time in parallel, the associated defects by Raman spectroscopy allow for a better view of the structural defects at the surface and in the bulk for samples annealed at T = 600 and 1200 °C, respectively. Moreover, we propose another indexing of the Raman spectra of cerium-based compounds with a fluorite structure by considering two tetrahedral environments around the cationic defects. We evidence the existence of an unusual isolated La pseudo-cubic site, in a series of cerium-, zirconium-, and lanthanum-based oxides. It is found to be stabilized at the subsurface farther from surface’s Zr atoms and oxygen vacancies. La3+ ions in the bulk are preferentially associated with bulk Zr4+ cations as in the La2Zr2O7 ternary compound. When this peculiar La3+ environment surrounded exclusively by Ce4+ cubic sites is thermally stable, the specific surface area remains interesting for catalytic application at high temperatures. However, enhancing the La and Zr contents tends to increase the association of La3+ and Zr4+ ions in clusters and induce a loss of surface area

Visibility of Al Surface Sites of γ‑Alumina: A Combined Computational and Experimental Point of View

The nature of γ-alumina (γ-Al₂O₃) surface sites leaves many open questions today, and solid-state NMR spectroscopy has been proposed and used as a tool for assessing their structure. Here, we calculated ²⁷Al NMR parameters from first principles in periodic boundary conditions for a large number of Al sites with different coordination, potentially present on the alumina surface. The nature and accordingly the NMR parameters of these sites change with the level of hydroxylation and thereby the pretreatment temperature of γ-Al₂O₃. While the Al chemical shift is little affected by hydroxylation, the magnitude of the quadrupolar interaction at the Al nucleus is strongly correlated to hydroxylation, with high to very high quadrupolar coupling constant (<i>C</i>_Q) values (20–34 MHz) on the weakly hydrated major (110) termination, which contains highly reactive Lewis acidic “defect sites”, and gradual lowering to bulklike <i>C</i>_Q values of around 5 MHz on highly hydrated surfaces. In addition, we studied the effects of local symmetry on the EFG tensor of Al atoms and the effects of hydrating neighboring Al sites. These calculation were combined with {¹H}²⁷Al cross-polarization NMR experiments, carried out at high magnetic field (20 T) and high magic angle spinning (MAS) frequency (30 kHz) on γ-Al₂O₃ pretreated at different temperatures. We clearly show that this method is mainly sensitive to strongly hydrated surface sites besides the much more abundant bulk Al atoms, and that catalytically important high-<i>C</i>_Q Al centers (Lewis acid sites) and even some types of hydroxylated Al sites are not visible in the spectrum. Therefore, even current high-field NMR experiments are not (yet) able to provide a complete picture of the structure of γ-Al₂O₃ and much caution should be exercised when interpreting ²⁷Al NMR spectra

Direct Lanthanide−Transition Metal Interactions: Synthesis of (NH₃)₂YbFe(CO)₄ and Crystal Structures of {[(CH₃CN)₃YbFe(CO)₄]₂·CH₃CN}_∞ and [(CH₃CN)₃YbFe(CO)₄]_∞

The heterometallic complex (NH3)2YbFe(CO)4 was prepared from the reduction of Fe3(CO)12 by Yb in liquid ammonia. Ammonia was displaced from (NH3)2YbFe(CO)4 by acetonitrile in acetonitrile solution, and the crystalline compounds {[(CH3CN)3YbFe(CO)4)]2·CH3CN}∞ and [(CH3CN)3YbFe(CO)4]∞ were obtained. An earlier X-ray study of {[(CH3CN)3YbFe(CO)4]2·CH3CN}∞ showed that it is a ladder polymer with direct Yb−Fe bonds. In the present study, an X-ray crystal structure analysis also showed that [(CH3CN)3YbFe(CO)4]∞ is a sheetlike array with direct Yb−Fe bonds. Crystal data for {[(CH3CN)3YbFe(CO)4]2·CH3CN}∞:  monoclinic space group P21/c, a = 21.515(8) Å, b = 7.838(2) Å, c = 19.866(6) Å, β = 105.47(2)°, Z = 4. Crystal data for [(CH3CN)3YbFe(CO)4]∞:  monoclinic space group P21/n, a = 8.364(3) Å, b = 9.605(5) Å, c = 17.240(6) Å, β = 92.22(3)°, Z = 4. Electrical conductivity measurements in acetonitrile show that these acetonitrile complexes are partially dissociated into ionic species. IR and NMR spectra of the solutions reveal the presence of [HFe(CO)4]-. However, upon recrystallization, the acetonitrile complexes show no evidence for the presence of [HFe(CO)4]- on the basis of their IR spectra. The solid state MAS 2H NMR spectra of deuterated acetonitrile complexes give no evidence for [2HFe(CO)4]-. It appears that rupture of the Yb−Fe bond could occur in solution to generate the ion pair [LnYb]2+[Fe(CO)4]2-, but then the highly basic [Fe(CO)4]2- anion could abstract a proton from a coordinated acetonitrile ligand to form [HFe(CO)4]-. However, upon crystallization, the proton could be transferred back to the ligand, which results in the neutral polymeric species

Elucidation of the Al/Si Ordering in Gehlenite Ca₂Al₂SiO₇ by Combined ²⁹Si and ²⁷Al NMR Spectroscopy/Quantum Chemical Calculations

We have investigated the Al/Si ordering in the pseudoisolated pairs of tetrahedral sites of the structure of crystalline gehlenite Ca2Al2SiO7 by means of 29Si and 27Al NMR and first-principles quantum mechanical calculations. 29Si NMR spectra of isotopically enriched samples enables the precise determination of the population of the two silicon sites Si–(OAl)3‑n(OSi)n (n = 0, 1) and hence the amount of Al–O–Al linkages. This leads to a reliable and model-free quantification of the departure from the Loewenstein rule and to an experimental Al/Si ordering enthalpy of 50.4 ± 1.6 kJ/mol fully reproduced by the quantum mechanical calculations. The seven aluminum sites arising from the Al/Si substitutions Al–(OAl)4‑p(OSi)p (0 ≤ p ≤ 4) and Al–(OAl)3‑p(OSi)p (p = 0, 1) are identified by 27Al MAS, MQMAS, and {29Si}27Al HMQC experiments, with their quantification being consistent with a fully disordered arrangement of the tetrahedral pairs in the a–b plane of the structure. Assignments of those strongly overlapping lines are further confirmed by density functional theory (DFT) calculations performed on a series of 2 × 25 supercells. An experimental and computational variation of −3 ppm of the 27Al isotropic chemical shift is obtained for the substitution of one Al by one Si in the second coordination sphere of a central Al atom. 29Si and 27Al isotropic chemical shifts are seen to be sensitive primarily to short-range structural variations whereas a more complex behavior related to the nearby presence of Loewenstein-violating pairs is observed for the 27Al quadrupolar coupling constant. Decomposition of the calculated EFG tensors into a sum of local, nonlocal, and ionic components demonstrates that it is almost entirely determined by the local electronic structure near the T1 nucleus. The width of the distribution of NMR parameters is seen to strongly correlate to the degree of ordering present in the material. Scalar coupling constants 2J(T–O–T) (with T = Al, Si) are found to be linearly related to the ∠TOT bond angle

¹⁴N and ⁸¹Br Quadrupolar Nuclei as Sensitive NMR Probes of <i>n</i>-Alkyltrimethylammonium Bromide Crystal Structures. An Experimental and Theoretical Study

This is the first time a comprehensive study has been carried out on n-alkyltrimethylammonium bromide salts using 14N and 81Br solid state NMR, X-ray diffraction, and theoretical calculations. The investigation represents a necessary step toward further 14N and 81Br NMR characterization of the environment of cationic and anionic groups in materials, accounting for the amphiphilic properties of cationic surfactants. The NMR spectra of five CxH2x+1(CH3)3N+Br− polycrystalline samples with different n-alkyl chain lengths (x = 1, 12, 14, 16, 18) were recorded and modeled. The 14N and 81Br quadrupolar coupling interaction parameters (CQ, ηQ) were also estimated from spectrum modeling and from computer simulation. The obtained results were discussed in depth making use of the experimental and reoptimized crystal structures. In the study, both 14N and 81Br nuclei were found to be sensitive probes for small structural variations. The parameters which influence the NMR properties the most are mobility, deviation of C−N−C bond angles from Td angles, and variations in r(N−Br) distances

Temperature-Dependent 4‑, 5- and 6‑Fold Coordination of Aluminum in MOCVD-Grown Amorphous Alumina Films: A Very High Field ²⁷Al-NMR study

The only easy way to prepare amorphous alumina is via thin film deposition. For this reason, the disorder in amorphous alumina has not yet been fully investigated. We have used very high-field (20 T) solid state ²⁷Al NMR spectroscopy to analyze the structural modifications of amorphous alumina thin films with deposition temperature (<i>T</i>_d). The films were deposited by metalorganic chemical vapor deposition in the <i>T</i>_d range of 360–720 °C. Depending on <i>T</i>_d, film composition is either AlO_1+<i>x</i>(OH)_{1–2<i>x</i>} (0 ≤ <i>x</i> ≤ 0.5) or Al₂O₃. From ²⁷Al 1D magic angle spinning (MAS) and 2D multiple-quantum magic angle spinning (MQMAS) NMR analyses, the films grown between 360 and 600 °C contain between 38 and 43 atom % of 5-fold coordinated aluminum sites (^[5]Al). The percentages of ^[6]Al and ^[4]Al sites vary spectacularly, reaching their respective minimum (5 atom %) and maximum (54 atom %) around 515 °C. The analysis of a very thin film (85 nm) of Al₂O₃ reveals the presence of metallic aluminum at the interface with the substrate and suggests that the respective percentages of ^[<i>n</i>]Al sites slightly differ from those in thicker films. The observed <i>T</i>_d dependence of amorphous alumina structure can be correlated with that of film properties previously reported, namely, Young’s modulus, hardness, and corrosion protection

High-Field ¹⁷O MAS NMR Investigation of Phosphonic Acid Monolayers on Titania

High-field 17O MAS NMR was used to investigate the binding of self-assembled monolayers of 17O-enriched phosphonic acids deposited on a titania anatase support. The spectra were recorded at two different magnetic fields (9.4 and 17.6 T), to improve the reliability of the simulations of the different resonances. The spectra recorded at 17.6 T offer an excellent resolution between the different oxygen sites, PO, P−O−H, and P−O−Ti, thus greatly facilitating their quantification. The data reported here give direct evidence of the extensive formation of Ti−O−P bonds in the surface modification of titania by phosphonic acids. The presence of residual PO and P−O−H sites indicates the presence of several different binding modes in phosphonic acid monolayers. The chemical shift of P−O−Ti sites is consistent with bridging (as opposed to chelating) modes

Structure and Sodium Ion Dynamics in Sodium Strontium Silicate Investigated by Multinuclear Solid-State NMR

The high oxide ion conductivity of the proposed sodium strontium silicate ion conductors Sr0.55Na0.45SiO2.775 (>10–2 S·cm–1 at 525 °C) and its unusual alkali metal substitution strategy have been extensively questioned in the literature. Here, we present a comprehensive understanding of the structure of this material using a combination of XRD and multinuclear 17O, 23Na, and 29Si solid-state NMR spectroscopy data and a detailed investigation of the Na ion dynamics by high temperature 23Na NMR line shape analysis and relaxation rates measurements. Both 23Na and 29Si NMR spectra demonstrate the absence of Na doping in strontium silicate SrSiO3 and the presence of an amorphous phase identified as Na2O·2SiO2 glass as the Na-containing product. Devitrification at 800 °C yields crystallization of the Na2O·2SiO2 glass into the known crystalline α-Na2Si2O5 phase which was positively identified by its XRD pattern and the extensive and clear 17O, 23Na, and 29Si NMR fingerprints. High temperature 23Na NMR reveals that the Na ions are mobile in the Na2O·2SiO2 amorphous component below its glass transition temperature (∼450 °C). In contrast, 23Na NMR data obtained on the crystalline α-Na2Si2O5 shows limited Na dynamics below ∼650 °C, and this result explains the large discrepancy in the conductivity observed in the literature which strongly depends on the thermal history of the Sr0.55Na0.45SiO2.775 material. These insights demonstrate that the high conductivity observed in Sr0.55Na0.45SiO2.775 is due to Na conduction in the Na2O·2SiO2 glass, and this motivates the quest for the discovery of low temperature fast ion conductors in noncrystalline solids