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Techniques to characterize surfaces
One of many grand challenges of surface chemistry is the difficulty of experimentally determining surface structure. This stems from the significantly small proportion of atoms at the very surface, and the difficulty of detecting them and distinguishing them from those located in the bulk region of the material.1,22 In recent years, many surface science techniques have been developed and adapted to study interfacial phenomena at the molecular level.1
Adsorption-desorption methods5 and microscopies, can provide information about the topology of the surface and its surface area. The microscopies encompass scanning probe microscopies, such as atomic force microscopy (AFM) and scanning tunnelling microscopy (STM),5,22,23 and electron microscopies, such as transmission electron microscope (TEM) and the scanning electron microscope (SEM).24,25 Information on the chemical composition and atomic-level structures of the surfaces can be obtained using surface-sensitive spectroscopies, including X-ray photoelectron and Auger electron spectroscopies, as well as secondary ion mass spectrometry (SIMS). Furthermore, compared to techniques mentioned in the previous paragraph, they can be employed in practical working environments (operando or in-situ).14,26,27 They are based on stimulating the surface of the sample with incoming (incident) particles (photons, electrons and ions) and monitoring the signals produced by the ejected particles (photons, electrons and ions) that are only emitted by the surface, as a result of said stimulation.6,28–30 The analysis can reveal key information about the elemental composition as well as the atomic-level structure of the surface of the studied material.6,14 The main characteristics and type of information that can be extracted from most commonly used surface methods are summarized in Table 1.
Importance of quadrupolar nuclei near surfaces
Many inorganic materials contain quadrupolar nuclei with spin I ≥ 1, which represent about 75% of NMR-active isotopes.88. Nuclei, such as 27Al (I = 5/2), 17O (I = 5/2), 6Li (I = 1), 7Li (I = 3/2), 11B (I = 3/2), 14N (I = 1), 23Na (I = 3/2), 25Mg (I = 5/2), 35Cl (I = 3/2), and 39K (I = 3/2), are present in catalysts, inorganic oxides, polymers, clays, glasses, energy materials, pharmaceutical and biomaterials. Compared to spin-1/2 nuclei, an additional difficulty for the detection of quadrupolar nuclei near surfaces is that their NMR signals are broadened by the quadrupolar interaction, which results from the interaction between the electric quadrupolar moment of the quadrupolar nuclei and the electric field gradient. This interaction is generally stronger than the dipolar and CSA interactions and gives rise to broadening often over several megahertz, which cannot be removed by MAS.88,92 Second, some of the commonly studied nuclei have low gyromagnetic ratios (γ), or high quadrupole moments and or low natural abundances, which represent additional challenges for the NMR detection.93,94 Furthermore, the lower coordination number of surface atoms results in higher electric field gradient and hence, larger quadrupolar interaction. However, structural information can be obtained by evaluating the quadrupolar interaction, which depends on the quadrupolar coupling constant (CQ) and the asymmetry parameter of the electric field gradient (ηQ).88 Both parameters are sensitive to the coordination environment, and combined with isotropic chemical shifts iso, they can help determining the local structure of surface sites.60 The NMR spectra of half-integer spin quadrupolar nuclei (e.g. 27Al, 11B, 17O) are often dominated by the central transition (CT) between energy levels mI = +1/2 and −1/2 since this transition is not broadened by the first-order quadrupolar interaction but only by the second-order quadrupolar interaction.95 Nevertheless, this second-order quadrupolar interaction is not completely averaged out by MAS.61 Conversely, high magnetic fields reduce the broadening due to the second-order quadrupolar interaction and thus, improve the resolution of the NMR spectra of quadrupolar nuclei.96,97 Development of high-field (DNP)-NMR methods to detect quadrupolar nuclei on surfaces 15 Furthermore, multiple-quantum MAS (MQMAS) experiment is a 2D NMR method, which allows the acquisition of high-resolution NMR spectra of quadrupolar nuclei by removing broadening due to second-order quadrupolar interaction.98 This technique correlates different transitions within the spin system and yields an isotropic spectrum from the projection onto the indirect dimension. Other approaches have been proposed to record high-resolution spectra of quadrupolar nuclei. Some of these techniques involve sample rotation around two different angles: simultaneously for double rotation (DOR), and sequentially for dynamic-angle spinning (DAS).99 The MQMAS, DOR and DAS experiments will be presented in more details in Section 1.4. An example of quadrupolar nucleus detected near surface is 27Al, which is present in alumina, a widely used catalyst support.100 Its 100% natural abundance and its moderate gyromagnetic ratio, γ(27Al)/γ(1H) facilitate the detection of this isotope near surface. Valuable information on local structure and chemistry of aluminium in its various environments can be obtained from NMR parameters, such as isotropic chemical shifts as well as CQ and ɳQ quadrupolar parameters.88 In particular, the coordination of the aluminium can be deduced from its isotropic chemical shift. For example, Xu et al. used 27Al MAS and MQMAS NMR experiments combined with high magnetic fields to study a series of transition aluminium oxides where the γ-, δ-, or θ-Al2O3 dominates.101 Specifically, MQMAS was used to resolve and obtain the quadrupolar parameters of the different aluminium sites, thus, allowing to distinguish the aluminium environments of each of the -, – and -phases. It was found that -Al2O3 contains three distinct octahedral sites and two aluminium tetrahedral sites. These results allowed to determine also that the -Al2O3 consists of local structures similar to those found in θ-Al2O3 as well as δ-Al2O3 phases but with less ordering, which before could not be observed at low field-NMR. More recently, Kaushik et al. reported the use of 1D and 2D NMR experiments (including 27Al MAS, 27Al{1H} and 29Si{27Al} D-HMQC and 27Al DQ/SQ) to get detailed insight into the structure of the surface species on alumina oxide/silica thin-films prepared by atomic layer deposition (ALD).102 The results showed that in films with 3 wt% of Al, a sub-monolayer is formed and possibly contains aluminium species ([n]Al where n = 4, 5 or. 6) which have been substituted by Si in the second coordination sphere such as [4]Al(3Si), [4]Al(4Si) and [5]Al(2Si) on the silica surface, with most of these sites attached to OH groups, whereas films with 9 or 15 wt% of Al exhibit characteristics of an amorphous alumina phase with a high concentration of [5]Al species and OH groups. In addition, the results gave further evidence that the most likely species at the interface between silica and alumina are [4]Al(2Si), [4]Al(3Si) and [5]Al(2Si). Another isotope of interest for the characterization of surfaces is 17O since it is present at the surface of numerous oxide-based materials. Solid-state NMR spectroscopy of oxygen-17 can provide detailed chemical and structural information of the various oxygen atoms in oxide-based solids.103,104 Nevertheless, 17O NMR is (0.037%) and usually requires isotropic enrichment.58 Merle et al. performed 17O MAS, MQMAS, and D-HMQC NMR experiments to probe the 17O local environment on 17O-surface labelled silica and M-SiO2 (M= W, Ta, Zr).105 MQMAS allowed to extract the quadrupolar parameters of the Si−OH, Si−O−Si, and Si−O−M surface sites for all the materials, while D-HMQC helped to probe the proximities between 17O and 1H sites and gain further information on the spatial configuration of the surface species. These results show that 17O NMR can provide unprecedented information on the interactions between the metallic complexes and their support in supported catalysts. 17O NMR spectroscopy has been also applied to study CeO2 nanoparticles.49,106,107 They made use of hyperpolarization techniques, in particular dynamic nuclear polarisation (DNP), to selectively enhance the NMR signals of the surface species on isotopically-labelled samples. This technique so-called DNP-SENS108 will be discussed in more detail in section 1.5.4.
HIGH-RESOLUTION NMR SPECTRA OF QUADRUPOLAR NUCLEI
The power of solid-state NMR to probe the structure of materials relies strongly on the availability of recording high-resolution spectra.123 As mentioned earlier, the second-order quadrupolar interaction poses a limit to obtaining high resolution spectra of quadrupolar spins, since it cannot be completely removed by MAS due to its complex angular dependence.119 There are several sophisticated methods to remove the broadening caused by this interaction.120 One experimental approach to overcome this is to find an angle such that 2 (cos ) = 4 (cos ) = 0 (where P2 and P4 denote the second- and fourth-order Legendre polynomials) is satisfied.117 Two techniques, DOR (Double Rotation)124 and DAS (Dynamic Angle Spinning)125, were initially developed to remove the second-order broadening of the CT. They involved space averaging of the second-order quadrupolar interaction through mechanical rotation of the sample holder.117 In DOR, the sample is placed within an inner rotor, which is spun inside an outer rotor, and the assembly is spun simultaneously around two different axes (30.56° and 54.74°) relative to B0.126 The DOR technique enables the elimination of the first- and second-order terms of the quadrupolar interaction in a 1D experiment.127 On the other hand, DAS is a 2D experiment that works by rapidly switching the rotor between two axes.128,129 Therefore, it requires specialized instrumentation for the rapid switching and stabilization of the rotor during the experiment.130,131 Since this is a 2D experiment, experimental times are typically longer than those for DOR.116,131 Even if the development of DOR and DAS allowed an elegant solution to the high resolution problem, their use is limited nowadays since they both require dedicated NMR probes.110.
The advent of 2D experiments such as MQMAS121,132 and later satellite transition MAS (STMAS)133,134, have revolutionized the observation of half-integer quadrupolar nuclei.116 They use conventional MAS hardware and provide high-resolution NMR spectra by cancelling out the broadening due to both first- and second order quadrupolar interactions.110,117.
Table of contents :
Abstract
Acknowledgements
Table of Contents
List of Figures
List of Tables
General introduction
Chapter 1: Observation of quadrupolar nuclei near surfaces using solid-state NMR
1.1 IMPORTANCE OF SURFACES
1.1.1 Applications to materials science
1.1.2 Techniques to characterize surfaces
1.1.3 NMR of surfaces
1.1.4 Importance of quadrupolar nuclei near surfaces
1.2 QUADRUPOLAR INTERACTION
1.3 EFFECTS OF RF FIELD FOR QUADRUPOLAR NUCLEI
1.4 HIGH-RESOLUTION NMR SPECTRA OF QUADRUPOLAR NUCLEI
1.4.1 MQMAS
1.4.2 STMAS
1.5 METHODS TO ENHANCE THE SENSITIVITY FOR QUADRUPOLAR NUCLEI
1.5.1 High-fields
1.5.2 Population transfer
1.5.3 QCPMG
1.5.4 Dynamic Nuclear Polarization (DNP)
1.6 OUTLINE OF THE THESIS
Chapter 2: Improved NMR transfer of magnetization from 1H to half-integer quadrupolar nuclei at 20 and 62.5 kHz MAS
2.1 STATEMENT OF CONTRIBUTION
2.2 INTRODUCTION
2.3 THEORETICAL BACKGROUND
2.3.1 Symmetry-based dipolar recouplings
2.3.2 Scaling factor κ
2.3.3 Adiabatic pulses
2.3.4 PRESTO-III sequence
2.3.5 Recoupling schemes for PRESTO
2.3.6 D-RINEPT sequence
2.3.7 Recoupling schemes for D-RINEPT
Development of high-field (DNP)-NMR methods to detect quadrupolar nuclei on surfaces x
2.4 1H→ 27Al PRESTO AND D-RINEPT NMR EXPERIMENTS
2.4.1 Samples and experimental conditions
2.4.2 Performances of PRESTO and D-RINEPT at R = 20 kHz
2.4.3 Performances of PRESTO and D-RINEPT at R = 62.5 kHz
2.5 2D 1H→ 27Al HETCOR EXPERIMENTS
2.7 CONCLUSIONS
References
Chapter 3: Comparison of double and single quantum dipolar NMR correlations of quadrupolar nuclei
3.1 STATEMENT OF CONTRIBUTION
3.2 INTRODUCTION
3.3 PULSE SEQUENCES
3.4 SYMMETRY-BASED MATHEMATICAL DESCRIPTION
3.4.1 SQ-SQ
3.4.2 DQ-SQ
3.5 MATERIALS AND METHODS
3.5.1 Samples
3.5.2 Synthesis of lithium diborate
3.5.3 Phase identification
3.5.4 NMR experiments
3.5.4.1 Experiments at 9.4 T
3.5.4.2 Experiments at 18.8 T
3.6 RESULTS AND DISCUSSION
3.6.1 11B-11B D-HOMCOR experiments on Li2B4O7
3.6.1.1 SQ-SQ
3.6.1.2 DQ-SQ
3.6.1.3 SQ-SQ vs DQ-SQ
3.6.2 27Al-27Al experiments on AlPO4-14
3.6.2.1 SQ-SQ
3.6.2.2 DQ-SQ
3.6.2.3 SQ-SQ vs DQ-SQ
3.7 CONCLUSIONS
References
Chapter 4: Boron nitride and oxide coated dendritic fibrous nanosilica for oxidative dehydrogenation: Insights into the catalytic sites from solid-state NMRh
4.1 STATEMENT OF CONTRIBUTION
4.2 INTRODUCTION
4.3 STATE OF THE ART
4.3.1 Catalysts for ODH of light alkanes
4.3.2 Catalytic activity of DFNS/BN and DFNS/B2O3
Development of high-field (DNP)-NMR methods to detect quadrupolar nuclei on surfaces xi
4.4 SOLID-STATE NMR EXPERIMENTS
4.4.1 Samples
4.4.2 Experiments at 9.4 T
4.4.3 Experiments at 18.8 T
4.5 RESULTS AND DISCUSSION
4.5.1 Probing 1H and 11B environments and their proximities
4.5.2 Probing 29Si environments and 29Si-11B proximities
4.5.3 Insights into the boron species
4.6 CONCLUSIONS
References
