Monu Kaushik

César Leroy

Zixuan Chen

David Gajan

Elena Willinger

Christoph Müller

Franck Fayon

Dominique Massiot

Alexey Fedorov

Christophe Copéret

Christoph R Mül- Ler §

Christophe Copéret∥

Anne Lesage

Pierre Florian

Atomic-Scale Structure and Its Impact on Chemical Properties of Aluminum Oxide Layers Prepared by Atomic Layer Deposition on Silica

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

Alumina and amorphous aluminosilicates (ASAs) are important classes of industrial catalysts and catalyst supports, with applications ranging from petroleum refining to automotive emission control or biomass conversion. [START_REF] Busca | Acid Catalysts in Industrial Hydrocarbon Chemistry[END_REF] These materials have been extensively studied in attempts to correlate their surface (Lewis and/or Brønsted) acidity with their catalytic activity. In contrast to alumina, which is known for the presence of strong Lewis sites associated with coordinatively unsaturated Al 3+ ions, ASA materials contain Brønsted acid sites that drive their catalytic properties. [START_REF] Busca | Acid Catalysts in Industrial Hydrocarbon Chemistry[END_REF][START_REF] Olah | My Search for Carbocations and Their Role in Chemistry (Nobel Lecture)[END_REF] Their Brønsted acidity, milder than that of crystalline zeolites, was originally proposed to arise from protons compensating the electronic charge of the surface. It is now more precisely described as zeolite-like acidic sites (i.e. bridging Si-OH-Al groups), [START_REF] Hensen | Quantification of Strong Bronsted Acid Sites in Aluminosilicates[END_REF] or more recently as pseudobridging silanols, [START_REF] Larmier | Isopropanol Dehydration on Amorphous Silica-Alumina: Synergy of Brønsted and Lewis Acidities at Pseudo-Bridging Silanols[END_REF][START_REF] Caillot | The variety of Bronsted acid sites in amorphous aluminosilicates and zeolites[END_REF] consisting of a silanol group in close vicinity to an aluminum atom so that Si-O-Al bridges are formed upon proton transfer. Several forms of Lewis acid sites were also identified in ASA materials, stemming from undercoordinated Al sites located at the interface of ASA and alumina-rich domains or at the silica surface. [START_REF] Hensen | Quantification of Strong Bronsted Acid Sites in Aluminosilicates[END_REF] Understanding and controlling acidity and thereby the property of these materials require the detailed knowledge of the chemical nature of the surface sites (i.e. at the atomic scale), which is a challenging experimental task. Most of the studies performed at the gas-solid interface involve the use of molecular probes, allowing a characterization of the strength and amount of the adsorbed species. For instance, the presence of tri-coordinated Al sites on highly dehydroxylated surfaces of g-alumina has been evidenced by their interaction with N2, their reaction with H2 and CH4 [START_REF] Wischert | Optimal Water Coverage on Alumina: A Key to Generate Lewis Acid-Base Pairs that are Reactive Towards the C-H Bond Activation of Methane[END_REF][START_REF] Wischert | gamma-Alumina: The Essential and Unexpected Role of Water for the Structure, Stability, and Reactivity of "Defect" Sites[END_REF] and pyridine adsorption, which has also been helpful in studying acid sites on g-alumina and aluminosilicate materials. [START_REF] Escribano | gamma-Alumina and Amorphous Silica-Alumina: Structural Features, Acid Sites and the Role of Adsorbed Water[END_REF][START_REF] Moroz | Discerning γ-Alumina Surface Sites with Nitrogen-15 Dynamic Nuclear Polarization Surface Enhanced NMR Spectroscopy of Adsorbed Pyridine[END_REF] Advanced spectroscopic techniques have also been implemented to obtain insight into surface acidity and among those FTIR using probe molecules has traditionally played a central role in the assessment of surface acidity. [START_REF] Jystad | Surface Acidity Characterization of Metal-Doped Amorphous Silicates via Py-FTIR and 15N NMR Simulations[END_REF] In addition, solid-state NMR spectroscopy has also been used to a large extent, providing atomic-scale insights into the surface structure of aluminum and mixed oxides, [START_REF] Blumenfeld | Acid sites topology in aluminas and zeolites from high-resolution solid-state NMR[END_REF][START_REF] Taoufik | Heteronuclear NMR Spectroscopy as a Surface-Selective Technique: A Unique Look at the Hydroxyl Groups of γ-Alumina[END_REF][START_REF] Nagashima | Observation of Low-γ Quadrupolar Nuclei by Surface-Enhanced NMR Spectroscopy[END_REF] while being often limited by its lack of sensitivity towards surface species. [START_REF] Wischert | Visibility of Al Surface Sites of γ-Alumina: A Combined Computational and Experimental Point of View[END_REF] This limitation has been alleviated in part with the emergence of hyperpolarization techniques such as dynamic nuclear polarization (DNP) solid-state NMR [START_REF] Berruyer | DNP in Materials Science: Touching the Surface[END_REF][START_REF] Hooper | Dynamic Nuclear Polarization (DNP) 101: A New Era for Materials[END_REF][START_REF] Corzilius | High-Field Dynamic Nuclear Polarization[END_REF] for g-alumina and alumino-silicates. [START_REF] Lee | Primostrato Solid-State NMR Enhanced by Dynamic Nuclear Polarization: Pentacoordinated Al3+ Ions Are Only Located at the Surface of Hydrated gamma-Alumina[END_REF][START_REF] Valla | Atomic Description of the Interface between Silica and Alumina in Aluminosilicates through Dynamic Nuclear Polarization Surface-Enhanced NMR Spectroscopy and First-Principles Calculations[END_REF][START_REF] Rankin | Determining the Surface Structure of Silicated Alumina Catalysts via Isotopic Enrichment and Dynamic Nuclear Polarization Surface-Enhanced NMR Spectroscopy[END_REF][START_REF] Perras | Shedding light on the atomic-scale structure of amorphous silica-alumina and its Bronsted acid sites[END_REF][START_REF] Mouat | Highly Dispersed SiOx/Al2O3 Catalysts Illuminate the Reactivity of Isolated Silanol Sites[END_REF][START_REF] Mouat | Direct Spectroscopic Evidence for Isolated Silanols in SiOx/Al2O3 and Their Formation Mechanism[END_REF] In parallel, the detailed structure of acid sites or the role of facets has been addressed through DFT methods, where

The surface properties of ASA materials are proposed to be linked to the distribution of the Al surface sites on the silica materials, therefore major research efforts have been directed towards the controlled deposition of Al. [START_REF] Copéret | Surface Organometallic and Coordination Chemistry toward Single-Site Heterogeneous Catalysts: Strategies, Methods, Structures, and Activities[END_REF][START_REF] Caillot | Synthesis of amorphous aluminosilicates by grafting: Tuning the building and final structure of the deposit by selecting the appropriate synthesis conditions[END_REF] One of the most powerful methods to control precisely the growth of oxide layers onto oxide supports is atomic layer deposition (ALD), [START_REF] George | Atomic Layer Deposition: An Overview[END_REF] in which the self-limiting reaction of a volatile reactive molecular precursor with specific sites on a substrate (such as surface hydroxyls) is followed by a post-treatment, with ozone or steam, allowing the controlled growth of oxide layers with atomic resolution. Repeated ALD cycles lead to a step-by-step deposition of layers, whose thickness can be controlled by the number of cycles. [START_REF] Copéret | Surface Organometallic and Coordination Chemistry toward Single-Site Heterogeneous Catalysts: Strategies, Methods, Structures, and Activities[END_REF][START_REF] Caillot | Synthesis of amorphous aluminosilicates by grafting: Tuning the building and final structure of the deposit by selecting the appropriate synthesis conditions[END_REF] Thus, ALD, using trimethylaluminum (TMA) as a precursor and silica as a support, allows the deposition and growth of aluminum oxide layers on silica, from grafted submonolayer species [START_REF] Copéret | Surface Organometallic and Coordination Chemistry toward Single-Site Heterogeneous Catalysts: Strategies, Methods, Structures, and Activities[END_REF] to nanometer-sized ultra-thin layers of Al2O3, with correlated changes in their catalytic activities. [START_REF] O'neill | Catalyst Design with Atomic Layer Deposition[END_REF] Yet, while ALD of alumina on silica is a well-studied process, understanding the atomic-scale structure of the aluminum oxide layer, the relation between local structure and acidity (which requires the use of dehydroxylated surfaces), and the detailed molecular-level mechanisms of the chemical reactions involved is still needed. [START_REF] Zaera | Mechanisms of surface reactions in thin solid film chemical deposition processes[END_REF][START_REF] Puurunen | Surface chemistry of atomic layer deposition: A case study for the trimethylaluminum/water process[END_REF] It has already been shown that the coverage and nature of surface species impact the growth rate and purity of the deposited films, [START_REF] Pallister | Surface chemistry of group 11 atomic layer deposition precursors on silica using solid-state nuclear magnetic resonance spectroscopy[END_REF] and the coating protocols largely influence their structure, which in some studies has been assessed using solid state NMR. [START_REF] Han | Influence of Coating Protocols on Alumina-Coated Cathode Material: Atomic Layer Deposition versus Wet-Chemical Coating[END_REF][START_REF] Lakomaa | Surface reactions in Al2O3 growth from trimethylaluminium and water by atomic layer epitaxy[END_REF] Figure 1. (a) Synthetic approach to obtain Al2O3/SiO2 materials by ALD, as well as transmission FTIR spectra of (b) SiO2-500 (grey), TMA1-SiO2-500 (black), TMA5-SiO2-500 (red) and TMA10-SiO2-500 (blue), (c) Al1-SiO2-500 (black), Al5-SiO2-500 (red), and Al10-SiO2-500 (blue), and HRTEM images of (d) Al1-SiO2-500, (e) Al5-SiO2-500, (f) Al10-SiO2-500. The yellow arrows indicate the ALD-grown film.

Here, we exploited ALD of the precursor TMA onto amorphous silica, dehydroxylated at 500 °C (i.e. SiO2-500), and ozone for oxidation of the deposited TMA (Figure 1a), in order to obtain precise control over the atomic composition of the formed layer and to minimize rehydroxylation of the support during film growth. [START_REF] Copéret | Surface Organometallic and Coordination Chemistry toward Single-Site Heterogeneous Catalysts: Strategies, Methods, Structures, and Activities[END_REF] Thin films with thicknesses between a sub-monolayer up to several nanometers were prepared, providing a consistent series of materials with different ratios of bulk to surface Al atoms, thereby allowing the detection of different NMR signatures as the thickness of the layer grows. Reinforced by complimentary information from diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), elemental analysis, highresolution transmission electron microscopy (HRTEM), energy-dispersive X-ray (EDX) spectroscopy and high-angle annular dark-field imaging (HAADF), we describe below how the structure, surface acidity, and domain interface of the silica-supported aluminum oxide layers is uncovered by high magnetic field and DNP surface-enhanced solid-state (DNP SENS) NMR spectroscopies.

Experimental section

Sample preparation. Materials studied in this work were prepared by ALD using alternating pulses of trimethylaluminum and ozone, at 300 °C, onto silica dehydroxylated at 500 °C (See Supporting Information for additional details). SiO2-500 contained 1.3 OH nm -2 (0.74 mmol g -1 ) of reactive OH sites according to dibenzylmagnesium titration. Further details of the ALD deposition protocol are provided in the Supporting Information. The as-prepared ALD-treated materials, depending on the number of the used ALD cycles denoted as TMA1-, TMA5-, and TMA10-SiO2-500, were calcined in synthetic air (500°C, 4

h). The resulting materials denoted Al1-, Al5-, and Al10-SiO2-500 contained 3.4, 9.2 and 15.4 wt% Al, respectively, according to elemental analysis. For pyridine adsorption, the calcined materials were exposed to [START_REF] Berruyer | DNP in Materials Science: Touching the Surface[END_REF] N pyridine vapor at room temperature for about 1 min, followed by the removal of 15 N pyridine at 150°C (at ca. 10

Results and discussion

Growth of the thin film. The preparation route to ALD-derived materials, which are characterized in detail below, is presented in Figure 1a. TMA was deposited onto SiO2-500 at 300 °C. In these conditions, TMA is expected to graft onto surface silanols and open siloxane (i.e. Si-O-Si) bridges by the interaction with Al-C bonds. [START_REF] Bartram | A reexamination of the chemisorption of trimethylaluminum on silica[END_REF][START_REF] Anwander | Formation of Lewis Acidic Support Materials via Chemisorption of Trimethylaluminum on Mesoporous Silicate MCM-41[END_REF] TMA pulses were followed by ozonolysis at 300 °C. Transmission FTIR data shows that, relative to SiO2-500, the intensity of the silanol band at 3741 cm -1 decreases in all three TMA-SiO2-500 materials (Figure 1b). This is accompanied by the appearance of a new band at 3783 cm -1 , very weak in TMA1-SiO2-500, but notably stronger in TMA5-SiO2-500 and TMA10-SiO2-500. The position of this band is close to that of an aluminol group in γ-Al2O3 (3785 cm -1 ). [START_REF] Copéret | Surface Organometallic and Coordination Chemistry toward Single-Site Heterogeneous Catalysts: Strategies, Methods, Structures, and Activities[END_REF] The broad band of vicinal and geminal silanols, centered in SiO2-500 at ca. 3680 cm -1 , is also present in TMA-SiO2-500 materials, albeit it likely contains a contribution from OH sites interacting with the grafted TMA species. Alkyl groups are identified in TMA-SiO2-500 materials by νCH stretching modes at 2890-3010 cm -1 and δCH bending modes at 1510-1370 cm -1 (Figure 1b), which indicates that ozone pulses at 300 °C oxidize the grafted TMA only partially. This result is consistent with a previous study of ALD of alumina on ZrO2, which gave several CH-containing species after the oxidation of the as-deposited TMA with ozone at 363 to 650 K. [START_REF] Goldstein | Al2O3 Atomic Layer Deposition with Trimethylaluminum and Ozone Studied by in Situ Transmission FTIR Spectroscopy and Quadrupole Mass Spectrometry[END_REF] The band at 1585 cm -1 in the TMA-SiO2-500 materials is assigned to surface carbonates, formed owing to the interaction of the surface with CO2 (released during the oxidation of the grafted TMA by ozone). [START_REF] Rai | Surface Reaction Mechanisms during Ozone and Oxygen Plasma Assisted Atomic Layer Deposition of Aluminum Oxide[END_REF][START_REF] Rose | In Situ Reaction Mechanism Studies on Ozone-Based Atomic Layer Deposition of Al2O3 and HfO2[END_REF] The complete oxidation of the alkyl groups and removal of carbonates is achieved by calcining the grafted materials at 500 °C in synthetic air (Figure 1c). Thus, the resulting Al-SiO2-500 materials do not feature C-H and carbonate bands, but reveal the band at 3741 cm -1 , corresponding to regenerated isolated silanols, intense in Al1-SiO2-500 and notably less intense in Al5 and Al10-SiO2-500. High-angle annular dark-field imaging (HAADF) as well as energy-dispersive X-ray spectroscopy (EDX) were performed to investigate the distribution of the ALD coating on SiO2-500 (Figure S1). Materials were exposed to ambient air shortly before the TEM analysis. A homogeneous distribution of deposited layers on the silica support was observed on Al10-SiO2-500, which was chosen as a representative material for EDX imaging (Figure S2). Next, HRTEM was performed to characterize all three Al-SiO2-500 materials. Al-containing coatings can be clearly visualized in Al1-SiO2-500, Al5-SiO2-500 and Al10-SiO2-500, as indicated by yellow arrows in Figure 1d-f. With an increasing number of ALD cycles, the Al-containing coating layer becomes thicker, i.e., it increases from ca. 0.44 to 0.56 and 0.81 nm for Al1-, Al5-and Al10-SiO2-500 materials, respectively (Figure S3). This result is consistent with the lack of crystalline phases in prepared materials by XRD (Figure S4) and with the Al loadings determined by elemental analysis as discussed above. We observe a few crystalline planes forming a zigzag coating on a silica surface (Figure 1f). These planes can also be observed in images of Al5-SiO2-500 obtained using an air-tight TEM transfer holder, thus excluding air-induced transformations (Figure S2). Nevertheless, those areas do not appear on every silica grain, and a smooth coating (i.e. a coating following the curvature of silica grains) is the most abundant morphology. Evolution (for instance, crystallization) was not observed for any studied specimen under the beam doze rates used. Taking into account the very few occurrences of those nanocrystals, we will not consider them further (additional TEM details can be found in the Supporting Information). Lastly, no porosity can be identified by TEM in the coatings grown, also consistent with the BJH analysis of N2 physisorption data that shows only intergranular porosity of the support used (Figure S5-S6, Table S1).. While instructive, TEM results do not allow to distinguish unambiguously between alumina and aluminosilicate phases of the surface coating. In what follows, we focus on high-magnetic field solid-state NMR spectroscopy to derive at an atomic-scale description of the layer, the interface and the surface. Aluminum environments. Figure 2 shows the [START_REF] Caillot | Synthesis of amorphous aluminosilicates by grafting: Tuning the building and final structure of the deposit by selecting the appropriate synthesis conditions[END_REF] Al NMR Hahn echo spectra of the three Al-SiO2-500 materials and the simulations of these spectra. At least three components can be resolved, corresponding to n-coordinated aluminum species, i.e.

[n] Al where n = 4, 5 or 6 in highly disordered environments, as seen in aluminosilicate glasses. [START_REF] Massiot | Topological, Geometric, and Chemical Order in Materials: Insights from Solid-State NMR[END_REF] There is a clear shift toward higher chemical shifts upon increasing the number of cycles (from (a) to (c) in Figure 2). This shift indicates a progressive substitution of silicon by aluminum in [n] Al(OSi)p(OAl)n-p environments, consistent with a +3 ppm increase of the [START_REF] Larmier | Isopropanol Dehydration on Amorphous Silica-Alumina: Synergy of Brønsted and Lewis Acidities at Pseudo-Bridging Silanols[END_REF] Al isotropic chemical shift upon substitution of one Si by one Al in the aluminum second coordination sphere as determined in Ca2Al2SiO7 and SrAl2SiO8. [START_REF] Florian | Elucidation of the Al/Si Ordering in Gehlenite Ca2Al2SiO7 by Combined 29Si and 27Al NMR Spectroscopy/Quantum Chemical Calculations[END_REF][START_REF] Al Saghir | Transparency through Structural Disorder: A New Concept for Innovative Transparent Ceramics[END_REF] The Gaussian Isotropic Model (GIM) or Czjzek model, [START_REF] Caer | General models for the distributions of electric field gradients in disordered solids[END_REF] based on a statistical distribution of the local environments and implemented in DMFit, [START_REF] Massiot | Modelling one-and two-dimensional solid-state NMR spectra[END_REF] is suitable for simulating this type of spectra. It allows to retrieve the mean isotropic chemical shift 𝛿 ̅

!"# and the width of its Gaussian distribution ∆𝛿 !"# , the mean quadrupolar coupling constant 𝐶 ̅ $ , and the population of each component. As seen in Figure 2a1 and a2, the model indeed provides a very satisfactory simultaneous fit of the Al1-SiO2-500 spectra obtained in fields of 20.0 and 23.5 T. However, this fitting procedure was less successful for Al5-SiO2-500 and Al10-SiO2-500 (see Figure S7), pointing to an additional complexity in those spectra. As seen in Figure 2b and 2c an excellent simulation is obtained when the experimental spectrum of Al1-SiO2-500 is also used as one of the components of the fit, indicative that Al5-SiO2-500 and Al10-SiO2-500 contain almost unaltered Al1-SiO2-500 system. The resulting NMR parameters, reported in Table S2, may nevertheless contain errors due to the use of a non-quantitative pulse sequence. [START_REF] Caillot | Synthesis of amorphous aluminosilicates by grafting: Tuning the building and final structure of the deposit by selecting the appropriate synthesis conditions[END_REF] Al MAS NMR experimental (dark blue) and simulated (red) spectra of (a) Al1-SiO2-500, (b) Al5-SiO2-500 and (c) Al10-SiO2-500. All spectra were recorded at 20.0 T except for (a2) which was obtained at 23.5 T. The various components are shown in different colors: [START_REF] Larmier | Isopropanol Dehydration on Amorphous Silica-Alumina: Synergy of Brønsted and Lewis Acidities at Pseudo-Bridging Silanols[END_REF] Al (green) [START_REF] Caillot | The variety of Bronsted acid sites in amorphous aluminosilicates and zeolites[END_REF] Al (light blue) [START_REF] Wischert | Optimal Water Coverage on Alumina: A Key to Generate Lewis Acid-Base Pairs that are Reactive Towards the C-H Bond Activation of Methane[END_REF] Al (purple). The additional grey lines are the experimental spectra of Al1-SiO2-500 used as a fourth component in the simulation. Dotted lines show the 𝛿 ̅ !"# of each Al1-SiO2-500 components.

A final set of simulations was then performed on spectra obtained with a single quantitative pulse, after baseline subtraction, [START_REF] Yon | Iterative baseline correction algorithm for dead time truncated onedimensional solid-state MAS NMR spectra[END_REF] and taking into account the spinning sidebands of the external transitions (Figure S8). The resulting parameters are reported in Tables 1 andS3, revealing a good agreement between the two approaches (calculated errors for all fits are given in Table S4). We note that the MQMAS performed either at 20.0 T or at 23.5 T (Figure S9) does not clearly resolve the Al1-SiO2-500 component in Al10-SiO2-500 and that simulations could be performed with only three components (Table S5), which could be related to filtering effects in the MQMAS experiments.

The high and very similar 𝛿 ̅

!"# obtained for Al5-SiO2-500 and Al10-SiO2-500 suggests that an aluminum oxide film is formed already after five ALD cycles and the nature of the Al sites does not change significantly with additional cycles. The percentage of the "Al1-like" component decreases as expected with an increasing number of cycles, which is consistent with a description in which Al1-SiO2-500 is, from an aluminum point of view, a good approximation of the alumina/silica interface in thicker films. The fitted Al1 content of 51% after 5 cycles nevertheless suggests that one cycle deposits only a sub-monolayer, leaving the silica surface available for the subsequent interaction with TMA.

Table 1. [START_REF] Caillot | Synthesis of amorphous aluminosilicates by grafting: Tuning the building and final structure of the deposit by selecting the appropriate synthesis conditions[END_REF] Al NMR parameters derived from the simulation of the one-pulse quantitative spectra. Al(2Si) 35 The NMR parameters derived for the film components (Al5and Al10-SiO2-500), and particularly the large relative amount of [START_REF] Caillot | The variety of Bronsted acid sites in amorphous aluminosilicates and zeolites[END_REF] Al sites, resemble those of amorphous alumina (am-Al2O3).

The average aluminum coordination numbers, [av] Al, are 4.40, 4.43 and 4.50 for Al1-, Al5-and Al10-SiO2-500 materials, respectively. According to X-ray diffraction and pair distribution function analysis, a similar trend but with lower [av] Al has been reported for thicker AlOx films prepared from TMA and H2O. [START_REF] Young | Probing the Atomic-Scale Structure of Amorphous Aluminum Oxide Grown by Atomic Layer Deposition[END_REF] Note that the latter report and a previous electron energy loss spectroscopy study of ALD-grown amorphous alumina films have not been able to show the presence of [START_REF] Caillot | The variety of Bronsted acid sites in amorphous aluminosilicates and zeolites[END_REF] Al species, in contrast to what is reported in this work. [START_REF] Kimoto | Coordination and interface analysis of atomic-layer-deposition Al2O3 on Si(001) using energy-loss near-edge structures[END_REF] However, our proposed model of an alumina layer built on top of the Al1-SiO2-500 interface (with lowest [av] Al in this interface) is consistent with an interface dominated by [START_REF] Larmier | Isopropanol Dehydration on Amorphous Silica-Alumina: Synergy of Brønsted and Lewis Acidities at Pseudo-Bridging Silanols[END_REF] Al species. [START_REF] Kimoto | Coordination and interface analysis of atomic-layer-deposition Al2O3 on Si(001) using energy-loss near-edge structures[END_REF] Some of such am-Al2O3 materials, obtained by various chemical routes, have been probed by [START_REF] Caillot | Synthesis of amorphous aluminosilicates by grafting: Tuning the building and final structure of the deposit by selecting the appropriate synthesis conditions[END_REF] Al NMR, viz. am-Al2O3 obtained by spin coating from aluminum nitrate aqueous solution, 50 magnetron sputtering [START_REF] Lee | Probing of 2 dimensional confinement-induced structural transitions in amorphous oxide thin film[END_REF] or metal organic chemical vapor deposition (MOCVD) of aluminum isopropoxide. [START_REF] Sarou-Kanian | Temperature-Dependent 4-, 5-and 6-Fold Coordination of Aluminum in MOCVD-Grown Amorphous Alumina Films: A Very High Field 27Al-NMR study[END_REF] Using the NMR parameters reported by those three studies (also using the GIM model for simulations) for the thinner films, the following average 𝛿 ̅ !"# and 𝐶 ̅ $ values and associated standard deviations are obtained: 70.0 ± 2.2 ppm and 8.8 ± 2.1 MHz for [START_REF] Larmier | Isopropanol Dehydration on Amorphous Silica-Alumina: Synergy of Brønsted and Lewis Acidities at Pseudo-Bridging Silanols[END_REF] Al; 41.4 ± 1.3 ppm and 8.0 ± 1.0 MHz for [START_REF] Caillot | The variety of Bronsted acid sites in amorphous aluminosilicates and zeolites[END_REF] Al; 11.0 ± 1.4 ppm and 6.0 ± 0.8 MHz for [START_REF] Wischert | Optimal Water Coverage on Alumina: A Key to Generate Lewis Acid-Base Pairs that are Reactive Towards the C-H Bond Activation of Methane[END_REF] Al. The span is small for 𝛿 ̅ !"# and the average values are very similar to those found in this work. Thus the local structure of ALDgrown films features NMR parameters closely mimicking those of am-Al2O3. -500 (open symbols), Al10-SiO2-500 (filled symbols) and gibbsite (crosses), simulating the spectra with components for [START_REF] Wischert | Optimal Water Coverage on Alumina: A Key to Generate Lewis Acid-Base Pairs that are Reactive Towards the C-H Bond Activation of Methane[END_REF] Al (triangles), [START_REF] Caillot | The variety of Bronsted acid sites in amorphous aluminosilicates and zeolites[END_REF] Al (squares) and [START_REF] Larmier | Isopropanol Dehydration on Amorphous Silica-Alumina: Synergy of Brønsted and Lewis Acidities at Pseudo-Bridging Silanols[END_REF] Al (circles).

Based on the above values for "pure" am-Al2O3 and considering that a shift of approx. -3 ppm of diso( [START_REF] Larmier | Isopropanol Dehydration on Amorphous Silica-Alumina: Synergy of Brønsted and Lewis Acidities at Pseudo-Bridging Silanols[END_REF] Al) upon Al to Si substitution in the second coordination sphere of aluminum [START_REF] Florian | Elucidation of the Al/Si Ordering in Gehlenite Ca2Al2SiO7 by Combined 29Si and 27Al NMR Spectroscopy/Quantum Chemical Calculations[END_REF] applies also for [START_REF] Caillot | The variety of Bronsted acid sites in amorphous aluminosilicates and zeolites[END_REF] Al and [START_REF] Wischert | Optimal Water Coverage on Alumina: A Key to Generate Lewis Acid-Base Pairs that are Reactive Towards the C-H Bond Activation of Methane[END_REF] Al sites, one can tentatively assign the observed Al environments in Al1-SiO2-500 to [START_REF] Larmier | Isopropanol Dehydration on Amorphous Silica-Alumina: Synergy of Brønsted and Lewis Acidities at Pseudo-Bridging Silanols[END_REF] Al(3-4Si), [START_REF] Caillot | The variety of Bronsted acid sites in amorphous aluminosilicates and zeolites[END_REF] Al(2Si) and [START_REF] Wischert | Optimal Water Coverage on Alumina: A Key to Generate Lewis Acid-Base Pairs that are Reactive Towards the C-H Bond Activation of Methane[END_REF] Al(1Si) sites. In other words, during the first cycle (followed by calcination), ca. 60% of the Al species form [START_REF] Larmier | Isopropanol Dehydration on Amorphous Silica-Alumina: Synergy of Brønsted and Lewis Acidities at Pseudo-Bridging Silanols[END_REF] Al(3Si,4Si) environments interacting strongly with the silica interface, including possibly a migration into the silica support. This is reminiscent of the behavior observed during grafting of trialkylaluminum species (R3Al, with R = Me, [START_REF] Li | Chemistry of the Silica Surface: Liquid-Solid Reactions of Silica Gel with Trimethylaluminum[END_REF] Et 54 or iBu [START_REF] Kermagoret | Triisobutylaluminum: bulkier and yet more reactive towards silica surfaces than triethyl or trimethylaluminum[END_REF] ) with Al diffusion into the subsurface layers of silica. On the other hand, the remaining 35% [START_REF] Caillot | The variety of Bronsted acid sites in amorphous aluminosilicates and zeolites[END_REF] Al and minor 5% [START_REF] Wischert | Optimal Water Coverage on Alumina: A Key to Generate Lewis Acid-Base Pairs that are Reactive Towards the C-H Bond Activation of Methane[END_REF] Al species are only monoor bis-bonded to silica, and are therefore mostly surface species. A word of caution is needed with respect to this, and further, assignments: note that they refer to the "most probable" species within the framework of a statistical distribution of environments and not "unique" environments. To summarize, the 1D [START_REF] Caillot | Synthesis of amorphous aluminosilicates by grafting: Tuning the building and final structure of the deposit by selecting the appropriate synthesis conditions[END_REF] Al experiments and their quantitative analysis unambiguously establish that the first ALD cycle deposits a submonolayer of aluminum atoms of various coordination numbers, all connected to Si atoms. This interface forms the basis onto which layers containing aluminum sites in amorphous alumina-like environments grow when increasing further the number of ALD cycles. Location of OH groups. The presence of hydroxyl groups in the investigated materials is confirmed by the appearance of characteristic OH bands in the FTIR spectra as discussed above (Figure 1c). While the position of OH bands is indicative of their nature (i.e. silanol, aluminol or bridging Si-OH-Al groups), the assignment can be ambiguous in the presence of SiOHAl species. One approach to access this information by NMR is the use of double resonance 1 H/ [START_REF] Caillot | Synthesis of amorphous aluminosilicates by grafting: Tuning the building and final structure of the deposit by selecting the appropriate synthesis conditions[END_REF] Al methods, recalling that a very strong 1 H probe background prevents a reliable direct observation of diluted OH species. 1 MAS NMR correlation experiments for (a) Al1-SiO2-500 and (b) Al10-SiO2-500 along with two-dimensional simulations and their respective projections: [START_REF] Larmier | Isopropanol Dehydration on Amorphous Silica-Alumina: Synergy of Brønsted and Lewis Acidities at Pseudo-Bridging Silanols[END_REF] Al (green), [START_REF] Caillot | The variety of Bronsted acid sites in amorphous aluminosilicates and zeolites[END_REF] Al (light blue), [START_REF] Wischert | Optimal Water Coverage on Alumina: A Key to Generate Lewis Acid-Base Pairs that are Reactive Towards the C-H Bond Activation of Methane[END_REF] Al (purple). Yellow spectra and projection line correspond to the experimental spectra of Al1-SiO2-500 used as an additional component in the simulation for Al10-SiO2-500. [START_REF] Wischert | Optimal Water Coverage on Alumina: A Key to Generate Lewis Acid-Base Pairs that are Reactive Towards the C-H Bond Activation of Methane[END_REF] Al [START_REF] Caillot | The variety of Bronsted acid sites in amorphous aluminosilicates and zeolites[END_REF] Al [START_REF] Larmier | Isopropanol Dehydration on Amorphous Silica-Alumina: Synergy of Brønsted and Lewis Acidities at Pseudo-Bridging Silanols[END_REF] Al [START_REF] Wischert | Optimal Water Coverage on Alumina: A Key to Generate Lewis Acid-Base Pairs that are Reactive Towards the C-H Bond Activation of Methane[END_REF] Al [START_REF] Caillot | The variety of Bronsted acid sites in amorphous aluminosilicates and zeolites[END_REF] Al [START_REF] Larmier | Isopropanol Dehydration on Amorphous Silica-Alumina: Synergy of Brønsted and Lewis Acidities at Pseudo-Bridging Silanols[END_REF] Al

Al(OHAl) 6 gibbsite Al1-SiO 2-500 Al10-SiO 2-500
Figure 3 plots 27 Al{ 1 H} REDOR dephasing curves for [n] Al in Al1-SiO2-500 and Al10-SiO2-500. The dephasing curve obtained for natural gibbsite Al(OH)3 is also shown as a reference of Al(OHAl)6 species in a fully hydroxylated phase. Both ALDmade materials show a quick increase of the DS/S0 dephased fraction, evidencing the presence of protons in the vicinity of aluminum sites. The increase is nevertheless slower than in natural gibbsite, indicating a lower hydroxylation degree of the aluminum sites in the ALD-made thin films. The [START_REF] Wischert | Optimal Water Coverage on Alumina: A Key to Generate Lewis Acid-Base Pairs that are Reactive Towards the C-H Bond Activation of Methane[END_REF] Al environments systematically show a faster dephasing (steeper initial slope) associated with a higher degree of hydroxylation than in [START_REF] Larmier | Isopropanol Dehydration on Amorphous Silica-Alumina: Synergy of Brønsted and Lewis Acidities at Pseudo-Bridging Silanols[END_REF] Al and [START_REF] Caillot | The variety of Bronsted acid sites in amorphous aluminosilicates and zeolites[END_REF] Al, which dephase almost at the same rate. A very similar behavior has been observed for am-Al2O3 films deposited onto a silicon wafer by MOCVD of aluminum isopropoxide, [START_REF] Sarou-Kanian | Temperature-Dependent 4-, 5-and 6-Fold Coordination of Aluminum in MOCVD-Grown Amorphous Alumina Films: A Very High Field 27Al-NMR study[END_REF] indicating a general behavior of aluminates: hexacoordinated [START_REF] Wischert | Optimal Water Coverage on Alumina: A Key to Generate Lewis Acid-Base Pairs that are Reactive Towards the C-H Bond Activation of Methane[END_REF] Al species are more likely to be protonated than tetraor pentacoordinated [START_REF] Larmier | Isopropanol Dehydration on Amorphous Silica-Alumina: Synergy of Brønsted and Lewis Acidities at Pseudo-Bridging Silanols[END_REF][START_REF] Caillot | The variety of Bronsted acid sites in amorphous aluminosilicates and zeolites[END_REF] Al species. The faster REDOR dephasing of the Al1-SiO2-500 components points to a higher OH density close to aluminum centers compared to Al10-SiO2-500. For long dephasing time, all DS/S0 fractions seem to level off at ~ 0.80 which means that in both materials only 20% of the Al sites are at remote distances form protons, i.e. approximately 80% of the aluminum atoms carry OH groups in the form of aluminols and (pseudo)bridging silanols. This high concentration of OH groups allows the surface-proton magnetization to spin-diffuse through the alumina film up to the [START_REF] O'neill | Catalyst Design with Atomic Layer Deposition[END_REF] Si nuclei of the silica support, as demonstrated by DNP SENS 29 Si{ 1 H} cross-polarization transfers in Al10-SiO2-500 (Figure S10). Table 2. [START_REF] Caillot | Synthesis of amorphous aluminosilicates by grafting: Tuning the building and final structure of the deposit by selecting the appropriate synthesis conditions[END_REF] Al NMR parameters derived from the simulation of the 27 Al{ 1 H} D-HMQC (Figure 4). ( * ) Lorentzian broadening.

site % 𝜹 $ 𝒊𝒔𝒐 (ppm) 𝚫𝜹 𝒊𝒔𝒐 (ppm) 𝑪 $ 𝑸 (MHz)
Al1-SiO2-500 The nature of the protonated aluminum sites was also probed using 1 H/ 27 Al D-HMQC experiment that converts the dipolardephased [START_REF] Caillot | Synthesis of amorphous aluminosilicates by grafting: Tuning the building and final structure of the deposit by selecting the appropriate synthesis conditions[END_REF] Al signal into heteronuclear MQ coherences to obtain 1 H/ 27 Al correlation maps, as recently shown for ASA. [START_REF] Wang | Brønsted acid sites based on penta-coordinated aluminum species[END_REF] Figure 4 shows, for Al1-SiO2-500 and Al10-SiO2-500, the 27 Al{ 1 H} D-HMQC spectra obtained with a short excitation time of 400 µs to selectively probe short-range Al-H correlations within Al(OH)X species where X = Al, Si (bridging or pseudobridging silanols) or aluminols. For Al1-SiO2-500, the simulation of the two-dimensional data set is accounted for by three aluminum environments and two proton environments. The NMR parameters extracted from this data are listed in Table 2. Although we used two lines for 1 H, it must be understood as a continuous distribution of 1 H environments arising from a broad range of Si(OH)Al hydrogenbonded Brønsted acid sites [START_REF] Hunger | Multinuclear solid-state NMR studies of acidic and non-acidic hydroxyl protons in zeolites[END_REF][START_REF] Jiang | Solid-state nuclear magnetic resonance investigations of the nature, property, and activity of acid sites on solid catalysts[END_REF][START_REF] Schroeder | Hydrogen Bond Formation of Brønsted Acid Sites in Zeolites[END_REF] and possibly Al(OH)Al sites. [START_REF] Batista | Beyond gamma-Al2O3 crystallite surfaces: The hidden features of edges revealed by solid-state H-1 NMR and DFT calculations[END_REF] This asymmetric distribution, ranging from 0 ppm to 10 ppm with a maximum at 2.6 ppm, is similar for all three coordination states of aluminum. In other words, the proton chemical shift of the [n] Al(OH)Si species is not significantly correlated to the coordination number n of aluminum. [START_REF] Larmier | Isopropanol Dehydration on Amorphous Silica-Alumina: Synergy of Brønsted and Lewis Acidities at Pseudo-Bridging Silanols[END_REF] Al(OH)X and [START_REF] Caillot | The variety of Bronsted acid sites in amorphous aluminosilicates and zeolites[END_REF] Al(OH)X (respectively in green and light blue in Figure 4) display [START_REF] Caillot | Synthesis of amorphous aluminosilicates by grafting: Tuning the building and final structure of the deposit by selecting the appropriate synthesis conditions[END_REF] Al resonances significantly narrower than those observed by direct excitation (Figure 2 and Table 1), corresponding to a decrease in 𝐶 ̅ $ by almost a factor of two. This structural ordering of the Al environments near OH groups mimics the one observed in g-alumina. [START_REF] Wischert | Visibility of Al Surface Sites of γ-Alumina: A Combined Computational and Experimental Point of View[END_REF] On the other hand, their average chemical shifts are identical to the non-protonated environments (obtained from the quantitative one-pulse experiments), showing that the above assignment of [START_REF] Larmier | Isopropanol Dehydration on Amorphous Silica-Alumina: Synergy of Brønsted and Lewis Acidities at Pseudo-Bridging Silanols[END_REF] Al(3-4Si) and [START_REF] Caillot | The variety of Bronsted acid sites in amorphous aluminosilicates and zeolites[END_REF] Al(2Si) environments holds and that both species are either bonded or close to OH groups. In line with its higher REDOR dephasing at short recoupling time, the [START_REF] Wischert | Optimal Water Coverage on Alumina: A Key to Generate Lewis Acid-Base Pairs that are Reactive Towards the C-H Bond Activation of Methane[END_REF] Al component is increased in the 27 Al{ 1 H} HMQC experiment (~20% of this [START_REF] Wischert | Optimal Water Coverage on Alumina: A Key to Generate Lewis Acid-Base Pairs that are Reactive Towards the C-H Bond Activation of Methane[END_REF] Al component is found in the D-HMQC spectrum). It appears that a Lorentzian broadening is needed to account for the [START_REF] Wischert | Optimal Water Coverage on Alumina: A Key to Generate Lewis Acid-Base Pairs that are Reactive Towards the C-H Bond Activation of Methane[END_REF] Al line shape (purple component in Figure 4), rendering a comparison with the direct excitation spectra difficult. For Al10-SiO2-500 (Figure 4b), we performed the simulation of the 2D dataset with the same hypothesis used above for simulating the 1D direct excitation experiments in Figure 2, i.e. the presence of an Al1-SiO2-500 component is accounted for by including its spectra (its simulation was used to avoid additional noise). Along the 1 H dimension, we found a broad and asymmetric line shape, again approximated by a pair of Gaussian lines (H(1), H(2)). The peak maxima of this (H(1), H(2)) distribution for Al1-SiO2-500 was uncorrelated with the coordination number n of the [n] Al environments but for Al10-SiO2-500, a clear shift of the maxima is observed in Figure 4b. This suggests separate (H(1), H(2)) components for each [n] Al environment. We therefore obtain three ( [n] Al, (H(1), H(2))) 2D components on top of an Al1-SiO2-500 contribution; all retrieved parameters are shown in Table 2 (Table S6 contains respected calculated errors).

Similar to Al1-SiO2-500, the recovered 𝛿 ̅ !"# are identical to the "bulk" one derived from the quantitative experiment and 𝐶 ̅ $ are greatly reduced. Considering the high 𝛿 ̅

!"# ( 1 H) obtained here, we observe mainly a wide variety of hydrogen-bonded [n] AlOH [p] Al sites such as those found in g-alumina, [START_REF] Batista | Beyond gamma-Al2O3 crystallite surfaces: The hidden features of edges revealed by solid-state H-1 NMR and DFT calculations[END_REF] on top of the minor Al(OH)Si ones described above and accounted for by the "Al1-SiO2-500" component. An additional resonance which, upon simulation, corresponds to a 1 H chemical shift of 0.4 ppm and a 27 Al line with 𝛿 ̅ !"# = 72.7 ppm and 𝐶 ̅ $ = 6.38 MHz is also clearly seen (orange component in Figure 4b). Based on its absence in Al1-SiO2-500, its low 1 H chemical shift and the presence of an IR band at 3783 cm -1 (Figure 1c) this latter component is assigned to terminal [START_REF] Larmier | Isopropanol Dehydration on Amorphous Silica-Alumina: Synergy of Brønsted and Lewis Acidities at Pseudo-Bridging Silanols[END_REF] AlOH aluminols [START_REF] Taoufik | Heteronuclear NMR Spectroscopy as a Surface-Selective Technique: A Unique Look at the Hydroxyl Groups of γ-Alumina[END_REF][START_REF] Jiang | Solid-state nuclear magnetic resonance investigations of the nature, property, and activity of acid sites on solid catalysts[END_REF] possibly located on the surface of the am-Al2O3 film. We observe a -0.5 ppm change in the 1 H chemical shift for higher aluminum coordination numbers, suggesting that the proton becomes slightly less acidic with increasing aluminum coordination, a behavior not observed for Al(OH)Si environments (vide supra) but previously inferred from 27 Al{ 1 H} NMR experiments on g-alumina. [START_REF] Taoufik | Heteronuclear NMR Spectroscopy as a Surface-Selective Technique: A Unique Look at the Hydroxyl Groups of γ-Alumina[END_REF] As seen from the line width of each of 1 H components (Table 2), the range of proton environments increases with decreasing aluminum coordination number and is on average broader than that for Al(OH)Si sites. To summarize, a combination of 1D 27 Al{ 1 H} REDOR and 2D 27 Al{ 1 H} D-HMQC experiments demonstrates clearly that hydroxyl groups are located in the ALD-deposited aluminum oxide. The density of the OH groups decreases with increasing thickness of the am-Al2O3 layer, with a significant fraction of hydroxyls involved in hydrogen bonding. Silica-alumina interface. Accessing the signal from the SiO2/Al2O3 interface is challenging owing to the paucity of nuclei present at the interface and the low natural abundance of [START_REF] O'neill | Catalyst Design with Atomic Layer Deposition[END_REF] Si, yet we were nevertheless successful in performing [START_REF] Caillot | Synthesis of amorphous aluminosilicates by grafting: Tuning the building and final structure of the deposit by selecting the appropriate synthesis conditions[END_REF] Al{ [START_REF] O'neill | Catalyst Design with Atomic Layer Deposition[END_REF] Si} correlation experiments on both Al1-SiO2-500 and Al10-SiO2-500 (Figure 5a and 5b respectively). Although both through-bond and through-space experiments are feasible at silica/alumina interfaces under DNP conditions, [START_REF] Valla | Atomic Description of the Interface between Silica and Alumina in Aluminosilicates through Dynamic Nuclear Polarization Surface-Enhanced NMR Spectroscopy and First-Principles Calculations[END_REF] we focused here on through-space dipolar-based HMQC experiments, using a rather long (6.4 ms) excitation time to obtain an optimum signal-to-noise ratio and hence possibly obtain correlations between distant Al/Si nuclei pairs. As seen in Figure 5, both materials yield similar projections along the [START_REF] Caillot | Synthesis of amorphous aluminosilicates by grafting: Tuning the building and final structure of the deposit by selecting the appropriate synthesis conditions[END_REF] Al dimension meaning that both experiments are retrieving the same type of aluminum environments. The close similarity with the direct spectra of Al1-SiO2-500 (Figure 2a1) suggests that the 27 Al{ 29 Si} D-HMQC spectra indeed probe the silica-alumina interface. This feature is highlighted in Figure 6a, which compares the [START_REF] O'neill | Catalyst Design with Atomic Layer Deposition[END_REF] Si-filtered and the 1 H-filtered [START_REF] Caillot | Synthesis of amorphous aluminosilicates by grafting: Tuning the building and final structure of the deposit by selecting the appropriate synthesis conditions[END_REF] Al spectra of Al1-SiO2-500 and Al10-SiO2-500 with the direct acquisition data. This similarity is not seen for the silicon projections in Figure 5. In particular, the silicon-29 projection of the Al10-SiO2-500 sample shows a much broader peak tailing towards higher chemical shifts than the direct [START_REF] O'neill | Catalyst Design with Atomic Layer Deposition[END_REF] Si spectra, suggesting the presence of either less polymerized or more Al-connected species than in Al1-SiO2-500 as discussed below. This feature is again illustrated in Figure 6b that compares the 27 Al-filtered (orange) and the direct acquisition (brown) [START_REF] O'neill | Catalyst Design with Atomic Layer Deposition[END_REF] Si spectra recorded on Al10-SiO2-500. To obtain further insights into the spectra of Figure 5, we performed a simulation of the 2D line shapes, assuming that two Gaussian lines are needed to render the [START_REF] O'neill | Catalyst Design with Atomic Layer Deposition[END_REF] Si dimension. It appeared that two [START_REF] Larmier | Isopropanol Dehydration on Amorphous Silica-Alumina: Synergy of Brønsted and Lewis Acidities at Pseudo-Bridging Silanols[END_REF] Al environments (green) are required to obtain a satisfactory simulation result whereas only one [START_REF] Caillot | The variety of Bronsted acid sites in amorphous aluminosilicates and zeolites[END_REF] Al en-vironment (blue) is sufficient to account for the observed correlations. The two [START_REF] Wischert | Optimal Water Coverage on Alumina: A Key to Generate Lewis Acid-Base Pairs that are Reactive Towards the C-H Bond Activation of Methane[END_REF] Al lines (purple) are very likely part of the same component, but are needed to account for the appearance of a sharp ridge on the 2D line shape. Nevertheless, the low abundance of this component (less than 5%) does not allow its 29 Si frequency (ppm) [START_REF] Caillot | Synthesis of amorphous aluminosilicates by grafting: Tuning the building and final structure of the deposit by selecting the appropriate synthesis conditions[END_REF] Al frequency (ppm) reliable simulation and it will not be discussed further. The results of the simulations are given in Table 3 and the calculated errors in Table S7.

Based on the chemical shifts reported in Table 3 and literature data, [START_REF] Lippmaa | Investigation of the Structure of Zeolites by Solid-State High-resolution Si-29 NMR-Spectroscopy[END_REF] the silicon environments are assigned to 𝑄 )*+ [-] sites at -104.0 ppm (in Al1-SiO2-500) and -104.6 ppm (in Al10-SiO2-500), as well as to 𝑄 )*+ [/] sites at -94.7 ppm in Al1-SiO2-500 and to 𝑄 0*+ [/] sites at -91.9 ppm in Al10-SiO2-500. These shifts differ from the bulk signal of the silica core (mostly 𝑄 [START_REF] Larmier | Isopropanol Dehydration on Amorphous Silica-Alumina: Synergy of Brønsted and Lewis Acidities at Pseudo-Bridging Silanols[END_REF] ) and its hydrated species (mostly 𝑄 [START_REF] Hensen | Quantification of Strong Bronsted Acid Sites in Aluminosilicates[END_REF] ), as shown in the direct (brown) and CP (green and blue) [START_REF] O'neill | Catalyst Design with Atomic Layer Deposition[END_REF] Si spectra in Figure 6b. Details of the simulation are given in Figure S11 and Table S8. Clearly, increasing the thickness of the ALD-deposited alumina layer promotes the connectivity between silicon and aluminum through the Q [START_REF] Hensen | Quantification of Strong Bronsted Acid Sites in Aluminosilicates[END_REF] sites, but, within error bars, the Q [START_REF] Larmier | Isopropanol Dehydration on Amorphous Silica-Alumina: Synergy of Brønsted and Lewis Acidities at Pseudo-Bridging Silanols[END_REF] (Al)/Q [3] (Al) ratio stays constant at approx. 60/40. We also observe a shift in 𝛿 ̅ !"# ( [START_REF] Larmier | Isopropanol Dehydration on Amorphous Silica-Alumina: Synergy of Brønsted and Lewis Acidities at Pseudo-Bridging Silanols[END_REF] Al) with increased layer thickness, and the [START_REF] Larmier | Isopropanol Dehydration on Amorphous Silica-Alumina: Synergy of Brønsted and Lewis Acidities at Pseudo-Bridging Silanols[END_REF] Al sites are assigned to [START_REF] Larmier | Isopropanol Dehydration on Amorphous Silica-Alumina: Synergy of Brønsted and Lewis Acidities at Pseudo-Bridging Silanols[END_REF] Al(3-4Si) in Al1-SiO2-500 and to [START_REF] Larmier | Isopropanol Dehydration on Amorphous Silica-Alumina: Synergy of Brønsted and Lewis Acidities at Pseudo-Bridging Silanols[END_REF] Al(2-3Si) in Al10-SiO2-500 whereas the chemical shifts of the [START_REF] Caillot | The variety of Bronsted acid sites in amorphous aluminosilicates and zeolites[END_REF] Al sites, assigned in both cases to [START_REF] Caillot | The variety of Bronsted acid sites in amorphous aluminosilicates and zeolites[END_REF] Al(2Si), do not appear to vary with the thickness of the ALD layer. Table 3. [START_REF] Caillot | Synthesis of amorphous aluminosilicates by grafting: Tuning the building and final structure of the deposit by selecting the appropriate synthesis conditions[END_REF] Al and [START_REF] O'neill | Catalyst Design with Atomic Layer Deposition[END_REF] Si NMR parameters derived from the simulation of the 2D 27 Al{ 29 Si} D-HMQC experiments (Figure 5).

site % 𝜹 $ 𝒊𝒔𝒐 (ppm) 𝚫𝜹 𝒊𝒔𝒐 (ppm) 𝑪 $ 𝑸 (MHz)
Al1-SiO2-500 [START_REF] Larmier | Isopropanol Dehydration on Amorphous Silica-Alumina: Synergy of Brønsted and Lewis Acidities at Pseudo-Bridging Silanols[END_REF] Al(3Si) 18.6 61.1 18.0 9.31 [START_REF] Larmier | Isopropanol Dehydration on Amorphous Silica-Alumina: Synergy of Brønsted and Lewis Acidities at Pseudo-Bridging Silanols[END_REF] Al(4Si) 32.1 56. 5 14.0 8.01 [START_REF] Caillot | The variety of Bronsted acid sites in amorphous aluminosilicates and zeolites[END_REF] Al(2Si) 44.9 35.9

14.4 10.0 [START_REF] Wischert | Optimal Water Coverage on Alumina: A Key to Generate Lewis Acid-Base Pairs that are Reactive Towards the C-H Bond Activation of Methane[END_REF] Al(1)

1.8 9.1 2.4 8.37 [START_REF] Wischert | Optimal Water Coverage on Alumina: A Key to Generate Lewis Acid-Base Pairs that are Reactive Towards the C-H Bond Activation of Methane[END_REF] Al(2)

2.6 6.3 0.9 5.57

Q [3] (1Al)
37.8 -94.7 17.0 -

Q [4] (1Al)
Q [3] (2Al)
44.2 -91.9 17.3 -

Q [4] (1Al)
55.8 -104.6 13.6 -

The connectivity pattern derived from the 2D 27 Al{ [START_REF] O'neill | Catalyst Design with Atomic Layer Deposition[END_REF] Si} spectra (see Table S9 for details) leads to the most probable connectivity of [START_REF] Larmier | Isopropanol Dehydration on Amorphous Silica-Alumina: Synergy of Brønsted and Lewis Acidities at Pseudo-Bridging Silanols[END_REF] Al(4Si) -𝑄 )*+ [-] and [START_REF] Larmier | Isopropanol Dehydration on Amorphous Silica-Alumina: Synergy of Brønsted and Lewis Acidities at Pseudo-Bridging Silanols[END_REF] Al(3Si) -𝑄 )*+ [/] for Al1-SiO2-500 and [START_REF] Larmier | Isopropanol Dehydration on Amorphous Silica-Alumina: Synergy of Brønsted and Lewis Acidities at Pseudo-Bridging Silanols[END_REF] Al(3Si) -𝑄 )*+ [-] and [START_REF] Larmier | Isopropanol Dehydration on Amorphous Silica-Alumina: Synergy of Brønsted and Lewis Acidities at Pseudo-Bridging Silanols[END_REF] Al(2Si) -𝑄 0*+ [/] for Al10-SiO2-500. In other words, the proton-free fully polymerized 𝑄 )*+ [-] species are preferably linked to the [START_REF] Larmier | Isopropanol Dehydration on Amorphous Silica-Alumina: Synergy of Brønsted and Lewis Acidities at Pseudo-Bridging Silanols[END_REF] Al environments with few AlOAl linkages. Apart from a slight decrease of the amount of [START_REF] Caillot | The variety of Bronsted acid sites in amorphous aluminosilicates and zeolites[END_REF] Al(2Si) -𝑄 )*+ [-] , the [n] Al -𝑄 [START_REF] Busca | Acid Catalysts in Industrial Hydrocarbon Chemistry[END_REF] connectivity pattern does not seem to be strongly dependent on the number of ALD cycles, showing again that most of the alumina/silica interface is built within 1 ALD cycle. We note that the ~14 ppm and ~17 ppm widths of the, respectively, Q [START_REF] Larmier | Isopropanol Dehydration on Amorphous Silica-Alumina: Synergy of Brønsted and Lewis Acidities at Pseudo-Bridging Silanols[END_REF] (1Al) and Q [START_REF] Hensen | Quantification of Strong Bronsted Acid Sites in Aluminosilicates[END_REF] (pAl), resonances (Table 3) are significantly broader than the 10.8 ppm width obtained in a direct or CP acquisitions (Table S8 and Figure 6b). This suggests that those broad lines contain more environments than the single Q [n] (pAl) unit that we assigned from their maxima. Again, we emphasize that all components evidenced in this study must be seen as the "most probable" environments within the framework of a purely statistical (i.e. Gaussian) distribution of environments. [START_REF] Caillot | Synthesis of amorphous aluminosilicates by grafting: Tuning the building and final structure of the deposit by selecting the appropriate synthesis conditions[END_REF] Al-(orange) and 1 H-(blue and green) filtered [START_REF] O'neill | Catalyst Design with Atomic Layer Deposition[END_REF] Si spectra compared with direct CPMG signal (brown), (c) [START_REF] Caillot | Synthesis of amorphous aluminosilicates by grafting: Tuning the building and final structure of the deposit by selecting the appropriate synthesis conditions[END_REF] Al-(top) and [START_REF] O'neill | Catalyst Design with Atomic Layer Deposition[END_REF] Si-(bottom) filtered 1 H spectra.

It is noteworthy that [START_REF] Larmier | Isopropanol Dehydration on Amorphous Silica-Alumina: Synergy of Brønsted and Lewis Acidities at Pseudo-Bridging Silanols[END_REF] Al(3Si,4Si) and Q [START_REF] Hensen | Quantification of Strong Bronsted Acid Sites in Aluminosilicates[END_REF] (1Al) are replaced by [4] Al(2Si,3Si) and Q [START_REF] Hensen | Quantification of Strong Bronsted Acid Sites in Aluminosilicates[END_REF] (2Al) going from Al1-SiO2-500 to Al10-SiO2-500. This indicates that the first cycle deposits only a sub-monolayer of aluminum on the surface of silica, possibly forming dispersed "islands" of alumina clusters, with the remaining silica surface accessible to subsequent deposition connects the clusters to form a homogeneous film. This is fully consistent with the observation that the growth per cycle in the ALD process is less than a monolayer. [START_REF] Puurunen | Surface chemistry of atomic layer deposition: A case study for the trimethylaluminum/water process[END_REF] Our findings from 2D experiments here are consistent with, and refine our observations from the analysis of Figure 2: the first ALD cycle grafts [START_REF] Larmier | Isopropanol Dehydration on Amorphous Silica-Alumina: Synergy of Brønsted and Lewis Acidities at Pseudo-Bridging Silanols[END_REF] Al(3Si) and [START_REF] Larmier | Isopropanol Dehydration on Amorphous Silica-Alumina: Synergy of Brønsted and Lewis Acidities at Pseudo-Bridging Silanols[END_REF] Al(4Si) species as well as bis-bonded [START_REF] Caillot | The variety of Bronsted acid sites in amorphous aluminosilicates and zeolites[END_REF] Al(2Si) onto the silica surface (Table 1). Only a very small amount of [START_REF] Wischert | Optimal Water Coverage on Alumina: A Key to Generate Lewis Acid-Base Pairs that are Reactive Towards the C-H Bond Activation of Methane[END_REF] Al is actually connected to the silica surface. Subsequent cycles add aluminum onto the "free" silica surface, left after the first cycle, leading to a silica/alumina interface mainly composed of [START_REF] Larmier | Isopropanol Dehydration on Amorphous Silica-Alumina: Synergy of Brønsted and Lewis Acidities at Pseudo-Bridging Silanols[END_REF] Al(3Si), [START_REF] Larmier | Isopropanol Dehydration on Amorphous Silica-Alumina: Synergy of Brønsted and Lewis Acidities at Pseudo-Bridging Silanols[END_REF] Al(2Si) and [START_REF] Caillot | The variety of Bronsted acid sites in amorphous aluminosilicates and zeolites[END_REF] Al(2Si). Increasing the number of ALD cycles further builds an am-Al2O3 film on top of this intermediate layer with additional (hydrated) [START_REF] Wischert | Optimal Water Coverage on Alumina: A Key to Generate Lewis Acid-Base Pairs that are Reactive Towards the C-H Bond Activation of Methane[END_REF] Al environments. For Al1-SiO2-500, the slightly increased 𝐶 ̅ $ (Table 3) suggests that we observe here, on average, less of the "protonated" sites than detected with the [START_REF] Caillot | Synthesis of amorphous aluminosilicates by grafting: Tuning the building and final structure of the deposit by selecting the appropriate synthesis conditions[END_REF] Al/ 1 H experiments (Table 2).

Figure 7. Sketch of the aluminum/silicon most probable connectivity for the Al1-SiO2-500 surface. The aluminum environments are depicted in dark green ( [START_REF] Larmier | Isopropanol Dehydration on Amorphous Silica-Alumina: Synergy of Brønsted and Lewis Acidities at Pseudo-Bridging Silanols[END_REF] Al(4Si)-𝑄 )*+ [-] ), light green ( [START_REF] Larmier | Isopropanol Dehydration on Amorphous Silica-Alumina: Synergy of Brønsted and Lewis Acidities at Pseudo-Bridging Silanols[END_REF] Al(3Si) -𝑄 )*+ [/] ), dark blue ( [START_REF] Caillot | The variety of Bronsted acid sites in amorphous aluminosilicates and zeolites[END_REF] Al(2Si) -𝑄 )*+ [-] ), light blue ( [START_REF] Caillot | The variety of Bronsted acid sites in amorphous aluminosilicates and zeolites[END_REF] Al(2Si) -𝑄 )*+ [/] ), dark purple ( [START_REF] Wischert | Optimal Water Coverage on Alumina: A Key to Generate Lewis Acid-Base Pairs that are Reactive Towards the C-H Bond Activation of Methane[END_REF] Al(1Si) -𝑄 )*+ [-] ), light purple ( [START_REF] Wischert | Optimal Water Coverage on Alumina: A Key to Generate Lewis Acid-Base Pairs that are Reactive Towards the C-H Bond Activation of Methane[END_REF] Al(1Si) -𝑄 )*+ [/] ) and silicon 𝑄 )*+ [/] species in dark red.

To complete the structural analysis and assess the Al-O-Al connectivity, we performed [START_REF] Caillot | Synthesis of amorphous aluminosilicates by grafting: Tuning the building and final structure of the deposit by selecting the appropriate synthesis conditions[END_REF] Al DQ/SQ (double-quantum singlequantum) correlation experiments using a dipolar-based R2 2 1 experiment with short (0.5 ms) excitation times. Sheared and symmetrized spectra for Al1-SiO2-500 and Al10-SiO2-500 are given in Figure S12 along with their simulations and the parameters derived are given in Table S10. The Al10-SiO2-500 spectrum displays strong correlations between the three 27 Al resonances, indicating a full connection between all of the various [n] Al species, in agreement with the am-Al2O3 nature of the film. For Al1-SiO2-500, the 2D map clearly shows that [START_REF] Larmier | Isopropanol Dehydration on Amorphous Silica-Alumina: Synergy of Brønsted and Lewis Acidities at Pseudo-Bridging Silanols[END_REF] Al sites have no linking preferences and share bonds to all [n] Al species (n = 4, 5 and 6) whereas [START_REF] Wischert | Optimal Water Coverage on Alumina: A Key to Generate Lewis Acid-Base Pairs that are Reactive Towards the C-H Bond Activation of Methane[END_REF] Al sites connect neither to themselves nor to [START_REF] Caillot | The variety of Bronsted acid sites in amorphous aluminosilicates and zeolites[END_REF] Al. This reinforces the that [START_REF] Wischert | Optimal Water Coverage on Alumina: A Key to Generate Lewis Acid-Base Pairs that are Reactive Towards the C-H Bond Activation of Methane[END_REF] Al species are rather isolated on the outer side of an [START_REF] Larmier | Isopropanol Dehydration on Amorphous Silica-Alumina: Synergy of Brønsted and Lewis Acidities at Pseudo-Bridging Silanols[END_REF][START_REF] Caillot | The variety of Bronsted acid sites in amorphous aluminosilicates and zeolites[END_REF] Al/Si interface and the possible "synergy" between two [START_REF] Larmier | Isopropanol Dehydration on Amorphous Silica-Alumina: Synergy of Brønsted and Lewis Acidities at Pseudo-Bridging Silanols[END_REF][START_REF] Caillot | The variety of Bronsted acid sites in amorphous aluminosilicates and zeolites[END_REF] Al units forming [START_REF] Larmier | Isopropanol Dehydration on Amorphous Silica-Alumina: Synergy of Brønsted and Lewis Acidities at Pseudo-Bridging Silanols[END_REF] Al(SiOH) [START_REF] Caillot | The variety of Bronsted acid sites in amorphous aluminosilicates and zeolites[END_REF] Al or [5] Al(SiOH) [START_REF] Caillot | The variety of Bronsted acid sites in amorphous aluminosilicates and zeolites[END_REF] Al pairs enhancing acid strength. [START_REF] Wang | Acidity enhancement through synergy of penta-and tetra-coordinated aluminum species in amorphous silica networks[END_REF] Finally, in an attempt to characterize protons located at the silica-alumina interface, two-dimensional 29 Si{ 1 H} CP-CPMG experiments were recorded on Al1-SiO2-500 and Al10-SiO2-500 with a 500 µs contact time at 7.0 T (Figure S13). The projections in the indirect proton dimension are shown in Figure 6c at the bottom. There are no significant differences between Al1-SiO2-500 and Al10-SiO2-500, both displaying a single narrow peak at around 2.5 ppm. However, these projections are very different from those obtained from 2D 27 Al{ 1 H} spectra of Figure 4 (shown on top in Figure 6c). This indicates that the majority of the hydroxyl groups evidenced here are of the SiOH type, consistent with the IR spectra (Figure 1c). [START_REF] Hunger | Multinuclear solid-state NMR studies of acidic and non-acidic hydroxyl protons in zeolites[END_REF][START_REF] Zeng | Proton Environments in Hydrous Aluminosilicate Glasses: A 1H MAS, 1H/27Al, and 1H/23Na TRAPDOR NMR Study[END_REF] To summarize, the extensive NMR characterization of the silica-alumina interface demonstrates that the average connectivity (coordination number) between aluminum and silicon atoms increases with the growth of the layer, confirming the deposition of a submonolayer in the first ALD cycle. The environment of the silicon and aluminum sites at the interface corresponds mainly to Q3 and Q4 silicon sites, and to four-and five-coordinated aluminum sites. A representation of the most probable environments found at the interface (i.e., Al1-SiO2-500) is given in Figures 7 andS14. Acidity of surface sites. The catalytic activity of alumina aluminosilicates is directly linked to their surface acidity, i.e. the type, location and local environment of their acid sites. A selective characterization of those surface sites is out of reach of conventional solid-state NMR, partly due to their low concentration, which is exacerbated by the limited intrinsic sensitivity of NMR spectroscopy. Both of these obstacles can typically be overcome by implementing DNP SENS approaches. [START_REF] Berruyer | DNP in Materials Science: Touching the Surface[END_REF] Figure S15 shows the DNP enhanced [START_REF] Caillot | Synthesis of amorphous aluminosilicates by grafting: Tuning the building and final structure of the deposit by selecting the appropriate synthesis conditions[END_REF] Al NMR spectra of the series of Py-Al-SiO2-500 samples, measured at 9.4 T (proton enhancement >140) and 18.8 T (proton enhancement >40). As was demonstrated above, protons are present in the aluminum oxide thin film and are expected to relay the nuclear hyperpolarization from the surface to the inner part of the oxide layer through spin diffusion. Therefore, the DNP enhanced [START_REF] Caillot | Synthesis of amorphous aluminosilicates by grafting: Tuning the building and final structure of the deposit by selecting the appropriate synthesis conditions[END_REF] Al NMR experiments are not surface-selective. We note indeed that the [START_REF] Caillot | Synthesis of amorphous aluminosilicates by grafting: Tuning the building and final structure of the deposit by selecting the appropriate synthesis conditions[END_REF] Al spectra recorded at 18.8 T (Figure S15) are similar to those recorded at ambient temperature and at a magnetic field of 20 T (Figure 2). In other words, the DNP enhanced [START_REF] Caillot | Synthesis of amorphous aluminosilicates by grafting: Tuning the building and final structure of the deposit by selecting the appropriate synthesis conditions[END_REF] Al spectra cannot be exploited to extract unambiguously structural information of the surface acid sites. Therefore, we probed the acidity of the surface layer by monitoring its interaction with pyridine probe molecule. The 15 N NMR pyridine chemical shifts and resonance intensities arising from various sites of adsorption has been used previously to identify the acidic nature of surface species in zeolites and amorphous aluminosilicates. [START_REF] Escribano | gamma-Alumina and Amorphous Silica-Alumina: Structural Features, Acid Sites and the Role of Adsorbed Water[END_REF][START_REF] Gunther | Interrogating the Lewis Acidity of Metal Sites in Beta Zeolites with 15N Pyridine Adsorption Coupled with MAS NMR Spectroscopy[END_REF] The 15 N chemical shift of free pyridine has been reported at 294-317 ppm, whereas the chemical shift of the pyridinium ion (PyH + ), formed upon protonation of pyridine is at 201-211 ppm. [START_REF] Jiang | Hyperpolarized 15N-pyridine Derivatives as pH-Sensitive MRI Agents[END_REF] H-bonded pyridine on weak Brønsted acid sites (H-Py) or pyridine bonded to Lewis acid sites (Py-L), appear as [START_REF] Berruyer | DNP in Materials Science: Touching the Surface[END_REF] N resonances between these values, with H-Py sites typically found between ca. 290-260 ppm, while Py-L sites are found between ca. 280-230 ppm. The NMR sensitivity was increased here by combining the use of isotopically-labelled [START_REF] Berruyer | DNP in Materials Science: Touching the Surface[END_REF] N pyridine with DNP SENS. In addition, the cryogenic temperature (~100 K) at which the experiments are carried out, reduces molecular motions that partly average magnetic interactions at Al [START_REF] Caillot | The variety of Bronsted acid sites in amorphous aluminosilicates and zeolites[END_REF] Al Al [START_REF] Caillot | The variety of Bronsted acid sites in amorphous aluminosilicates and zeolites[END_REF] Al Corresponding FTIR spectra of each material using the same color code, and highlighting PyH + and Py-L vibration frequencies in light grey and red respectively.

Figure 8a shows the 15 N DNP SENS spectra of pyridine that remained adsorbed on the surface of the materials after a desorption step at 150 °C (see details in the SI). Three resonances are observed whose intensity varies as a function of the thickness of the aluminum oxide layer, i.e. on the number of ALD cycles. The line width of each of the three resonances reflect broad chemical shift distributions for the 15 N nuclei, likely due to slight differences in local structure and environment. Notably the broad resonance at around 200-210 ppm could not be fitted by a single component but can still be assigned to pyridinium ions. Since pyridine adsorbed through hydrogen bonds on weak Brønsted acid sites is expected to be largely desorbed at 150 °C and therefore not be detected under our experimental conditions, we assign the peaks at around 240 and 260 ppm to Py interacting with Lewis acid sites in preference to Brønsted acid sites. This assignment is confirmed by 15 N{ 27 Al} J-HMQC experiments that detect [START_REF] Berruyer | DNP in Materials Science: Touching the Surface[END_REF] N spins having a through-bond coupling to [START_REF] Caillot | Synthesis of amorphous aluminosilicates by grafting: Tuning the building and final structure of the deposit by selecting the appropriate synthesis conditions[END_REF] Al sites (shown in Figure S16). The proposed also agrees with previous report on g-alumina. [START_REF] Moroz | Discerning γ-Alumina Surface Sites with Nitrogen-15 Dynamic Nuclear Polarization Surface Enhanced NMR Spectroscopy of Adsorbed Pyridine[END_REF] The presence of each type of acidic site is additionally corroborated by FTIR spectroscopy. The IR absorption peaks at 1620 cm -1 and 1450 cm -1 are characteristic of the ring vibrational modes of pyridine bound to a Lewis site, 8 and are observed for all samples (Figure 8b). PyH + vibration modes appear at 1635 cm -1 and 1545 cm -1 and are clearly visible in the IR spectra of Py-Al1-SiO2-500, Py-Al5-SiO2-500 and to a much lesser extent in the spectrum of Py-Al10-SiO2-500. While the NMR signal of the pyridinium ion is becoming less intense for thicker layers, the intensity of the peaks corresponding to the Lewis acid sites shows a reverse trend. Previous work on ASA materials showed that the formation of Brønsted acid sites is directly related to the presence of [START_REF] Larmier | Isopropanol Dehydration on Amorphous Silica-Alumina: Synergy of Brønsted and Lewis Acidities at Pseudo-Bridging Silanols[END_REF] Al connected to silicon atoms and that silicon connected to aluminum having a higher coordination number would not form Brønsted acid sites. [START_REF] Valla | Atomic Description of the Interface between Silica and Alumina in Aluminosilicates through Dynamic Nuclear Polarization Surface-Enhanced NMR Spectroscopy and First-Principles Calculations[END_REF] As elaborated in the previous sections, Py-Al1-SiO2-500 contains a large fraction of [START_REF] Larmier | Isopropanol Dehydration on Amorphous Silica-Alumina: Synergy of Brønsted and Lewis Acidities at Pseudo-Bridging Silanols[END_REF] Al(O)Si. Most of these sites are expected to be protonated (from REDOR data) and therefore accessible for pyridine adsorption and protonation. Strong Brønsted acid sites (providing PyH + ) are thus observed in the 15 N DNP SENS spectrum of Py-Al1-SiO2-500 as an intense resonance at 204 ppm (in black, Figure 8a). This surface Brønsted acidity is more prevalent than Lewis acidity as little Al(O)Al sites are formed at this stage. The nature of surface acidity clearly changes with the growth of the aluminum oxide layer concomitantly with the number of ALD cycles. With the growth of the aluminum oxide layer, pyridine adsorption sites become further away from the alumina/silica interface where protonated [START_REF] Larmier | Isopropanol Dehydration on Amorphous Silica-Alumina: Synergy of Brønsted and Lewis Acidities at Pseudo-Bridging Silanols[END_REF] Al(OH)Si sites are found. As a result, the relative fraction of Brønsted acid sites decreases in Py-Al5-SiO2-500 and Py-Al10-SiO2-500 with respect to that of Lewis acid sites, as observed in the 15 N DNP SENS and FTIR spectra. The 15 N DNP SENS spectrum of Py-Al10-SiO2-500 (shown in blue) is thus dominated by the resonances owing to pyridine on Lewis acid sites while the peak corresponding to the strong Brønsted acid sites, PyH + , becomes minor. This spectrum closely resembles that of pyridine adsorbed on g-alumina (recorded after similar pyridine adsorption and desorption at 150°C). 9

Conclusions

We have applied a broad range of one-and two-dimensional solid-state NMR characterization approaches to understand the growth processes at an atom level in the deposition of alumina onto a silica surface via ALD. The first ALD cycle produces an aluminosilicate layer with aluminum environments consisting of 60% [START_REF] Larmier | Isopropanol Dehydration on Amorphous Silica-Alumina: Synergy of Brønsted and Lewis Acidities at Pseudo-Bridging Silanols[END_REF] Al, 35% [START_REF] Caillot | The variety of Bronsted acid sites in amorphous aluminosilicates and zeolites[END_REF] Al and 5% [START_REF] Wischert | Optimal Water Coverage on Alumina: A Key to Generate Lewis Acid-Base Pairs that are Reactive Towards the C-H Bond Activation of Methane[END_REF] Al. Subsequent cycles build on top of this amorphous alumina film characterized by approx. 55% [START_REF] Larmier | Isopropanol Dehydration on Amorphous Silica-Alumina: Synergy of Brønsted and Lewis Acidities at Pseudo-Bridging Silanols[END_REF] Al, 40% [START_REF] Caillot | The variety of Bronsted acid sites in amorphous aluminosilicates and zeolites[END_REF] Al and 5% [START_REF] Wischert | Optimal Water Coverage on Alumina: A Key to Generate Lewis Acid-Base Pairs that are Reactive Towards the C-H Bond Activation of Methane[END_REF] Al. Based on the [START_REF] Caillot | Synthesis of amorphous aluminosilicates by grafting: Tuning the building and final structure of the deposit by selecting the appropriate synthesis conditions[END_REF] Al average isotropic chemical shifts obtained both from one-dimensional [START_REF] Caillot | Synthesis of amorphous aluminosilicates by grafting: Tuning the building and final structure of the deposit by selecting the appropriate synthesis conditions[END_REF] Al quantitative spectra and twodimensional 27 Al{ 29 Si} correlation experiments, we could decipher that the first aluminosilicate layer is made of [START_REF] Larmier | Isopropanol Dehydration on Amorphous Silica-Alumina: Synergy of Brønsted and Lewis Acidities at Pseudo-Bridging Silanols[END_REF] Al(3Si), [4] Al(4Si) and [START_REF] Caillot | The variety of Bronsted acid sites in amorphous aluminosilicates and zeolites[END_REF] Al(2Si) species grafted onto the silica surface (Figure 7). The silica surface is not fully covered after the first ALD cycle. During subsequent cycles, a film of amorphous am-Al2O3 (42% of [START_REF] Caillot | The variety of Bronsted acid sites in amorphous aluminosilicates and zeolites[END_REF] Al and less than 5 % of [START_REF] Wischert | Optimal Water Coverage on Alumina: A Key to Generate Lewis Acid-Base Pairs that are Reactive Towards the C-H Bond Activation of Methane[END_REF] Al) is formed and the interface between the deposited alumina and the surface of the silica support is mainly built up of [START_REF] Larmier | Isopropanol Dehydration on Amorphous Silica-Alumina: Synergy of Brønsted and Lewis Acidities at Pseudo-Bridging Silanols[END_REF] Al(3Si), [START_REF] Larmier | Isopropanol Dehydration on Amorphous Silica-Alumina: Synergy of Brønsted and Lewis Acidities at Pseudo-Bridging Silanols[END_REF] Al(2Si) and [START_REF] Caillot | The variety of Bronsted acid sites in amorphous aluminosilicates and zeolites[END_REF] Al(2Si) species. Two-dimensional 27 Al{ 1 H} correlation and REDOR experiments show that approximately 80% of the aluminum environments are hydroxylated. The hydroxylation level of the various Al species vary with n = 4, 5, 6 as well as with the number of ALD cycles. Six-fold coordinated species show a high level of hydroxylation with more than one OH group attached, whereas [4] Al and [START_REF] Caillot | The variety of Bronsted acid sites in amorphous aluminosilicates and zeolites[END_REF] Al show a lower hydroxylation level. Overall, hydroxylation is higher for [n] AlOSi environments (one ALD cycle) than for [n] AlOAl ones (> 5 ALD cycles). Clear evidence of the presence of aluminols [START_REF] Larmier | Isopropanol Dehydration on Amorphous Silica-Alumina: Synergy of Brønsted and Lewis Acidities at Pseudo-Bridging Silanols[END_REF] AlOH is seen in the alumina film but not at the silica/alumina interface, consistent also with FTIR data. It follows from the above description that as the number of cycles increases, the surface evolves from an ASA-like surface towards a pure alumina one, i.e. from [START_REF] Larmier | Isopropanol Dehydration on Amorphous Silica-Alumina: Synergy of Brønsted and Lewis Acidities at Pseudo-Bridging Silanols[END_REF][START_REF] Caillot | The variety of Bronsted acid sites in amorphous aluminosilicates and zeolites[END_REF] AlOSi environments towards [START_REF] Larmier | Isopropanol Dehydration on Amorphous Silica-Alumina: Synergy of Brønsted and Lewis Acidities at Pseudo-Bridging Silanols[END_REF][START_REF] Caillot | The variety of Bronsted acid sites in amorphous aluminosilicates and zeolites[END_REF] AlO [START_REF] Larmier | Isopropanol Dehydration on Amorphous Silica-Alumina: Synergy of Brønsted and Lewis Acidities at Pseudo-Bridging Silanols[END_REF][START_REF] Caillot | The variety of Bronsted acid sites in amorphous aluminosilicates and zeolites[END_REF] Al environments. Both types of species can carry OH groups, forming thereby weak Brønsted acid sites. Strong Brønsted acid sites, formed by an [n] Al species nearby a silanol SiOH group, are found only for small numbers of ALD deposition cycles. On the other hand, Lewis acid sites [n] Al are found after several ALD cycles. To obtain a direct insight into the presence, type and strength of those surface acid sites we used [START_REF] Berruyer | DNP in Materials Science: Touching the Surface[END_REF] N DNP SENS after 15 N-labeled pyridine adsorption.

Those experiments show the presence of both pyridinium PyH + ions (formed upon the transfer of a proton from strong Brønsted acid sites) and pyridine molecules coordinated to Al Lewis acid sites. As expected, the amount of the former, requiring silanol groups, is decreasing with increasing alumina film thickness, in agreement with the fact that the surface acidity changes from that of an ASA-like to that of an am-Al2O3 surface.

The extensive structural characterization performed here describes with an unprecedented level of details, the mechanisms at play during atomic layer deposition of trimethylaluminum onto partially dehydroxylated silica. It also dwells on the evolution of the surface acid sites as the thickness of the deposited alumina layer grows. Such in-depth characterization and molecular level understanding of the surface structure is crucial for the design of improved heterogeneous catalysts whose catalytic activity, selectivity and stability are often directly influenced by the surface and interfacial properties, in particular the type and the strength of the acidity.

Figure 2 .

 2 Figure 2.[START_REF] Caillot | Synthesis of amorphous aluminosilicates by grafting: Tuning the building and final structure of the deposit by selecting the appropriate synthesis conditions[END_REF] Al MAS NMR experimental (dark blue) and simulated (red) spectra of (a) Al1-SiO2-500, (b) Al5-SiO2-500 and (c) Al10-SiO2-500. All spectra were recorded at 20.0 T except for (a2) which was obtained at 23.5 T. The various components are shown in different colors:[START_REF] Larmier | Isopropanol Dehydration on Amorphous Silica-Alumina: Synergy of Brønsted and Lewis Acidities at Pseudo-Bridging Silanols[END_REF] Al (green)[START_REF] Caillot | The variety of Bronsted acid sites in amorphous aluminosilicates and zeolites[END_REF] Al (light blue)[START_REF] Wischert | Optimal Water Coverage on Alumina: A Key to Generate Lewis Acid-Base Pairs that are Reactive Towards the C-H Bond Activation of Methane[END_REF] Al (purple). The additional grey lines are the experimental spectra of Al1-SiO2-500 used as a fourth component in the simulation. Dotted lines show the 𝛿 ̅

  site% 𝜹 $ 𝒊𝒔𝒐 (ppm) 𝚫𝜹 𝒊𝒔𝒐 (ppm) 𝑪 $ 𝑸 (

Figure 3 .

 3 Figure3.[START_REF] Caillot | Synthesis of amorphous aluminosilicates by grafting: Tuning the building and final structure of the deposit by selecting the appropriate synthesis conditions[END_REF] Al{ 1 H} REDOR MAS NMR experiments obtained on Al1-SiO2-500 (open symbols), Al10-SiO2-500 (filled symbols) and gibbsite (crosses), simulating the spectra with components for[START_REF] Wischert | Optimal Water Coverage on Alumina: A Key to Generate Lewis Acid-Base Pairs that are Reactive Towards the C-H Bond Activation of Methane[END_REF] Al (triangles),[START_REF] Caillot | The variety of Bronsted acid sites in amorphous aluminosilicates and zeolites[END_REF] Al (squares) and[START_REF] Larmier | Isopropanol Dehydration on Amorphous Silica-Alumina: Synergy of Brønsted and Lewis Acidities at Pseudo-Bridging Silanols[END_REF] Al (circles).

Figure 4 .

 4 Figure 4.[START_REF] Caillot | Synthesis of amorphous aluminosilicates by grafting: Tuning the building and final structure of the deposit by selecting the appropriate synthesis conditions[END_REF] Al{ 1 H} D-HMQC SR4[START_REF] Olah | My Search for Carbocations and Their Role in Chemistry (Nobel Lecture)[END_REF] 1 MAS NMR correlation experiments for (a) Al1-SiO2-500 and (b) Al10-SiO2-500 along with two-dimensional simulations and their respective projections:[START_REF] Larmier | Isopropanol Dehydration on Amorphous Silica-Alumina: Synergy of Brønsted and Lewis Acidities at Pseudo-Bridging Silanols[END_REF] Al (green),[START_REF] Caillot | The variety of Bronsted acid sites in amorphous aluminosilicates and zeolites[END_REF] Al (light blue),[START_REF] Wischert | Optimal Water Coverage on Alumina: A Key to Generate Lewis Acid-Base Pairs that are Reactive Towards the C-H Bond Activation of Methane[END_REF] Al (purple). Yellow spectra and projection line correspond to the experimental spectra of Al1-SiO2-500 used as an additional component in the simulation for Al10-SiO2-500.

Figure 5 .

 5 Figure 5. 27 Al{ 29 Si} D-HMQC SR4 2 1 correlation experiments for (a) Al1-SiO2-500 and (b) Al10-SiO2-500 with projections (dark blue) along each dimension. Simulations of the 2D line shapes are given in grey and individual components are given in the projections in color with their sum in red.

Figure 6 .

 6 Figure 6. Various 27 Al{ 1 H} D-HMQC SR4 2 1 and 29 Si{ 1 H} CP-CPMG MAS NMR experiments performed on Al1-SiO2-500 and Al10-SiO2-500 showing: (a) 1 H-(blue and green) and[START_REF] O'neill | Catalyst Design with Atomic Layer Deposition[END_REF] Si-(orange) filtered[START_REF] Caillot | Synthesis of amorphous aluminosilicates by grafting: Tuning the building and final structure of the deposit by selecting the appropriate synthesis conditions[END_REF] Al signals, compared with the direct spectrum (dark blue), (b)[START_REF] Caillot | Synthesis of amorphous aluminosilicates by grafting: Tuning the building and final structure of the deposit by selecting the appropriate synthesis conditions[END_REF] Al-(orange) and 1 H-(blue and green) filtered[START_REF] O'neill | Catalyst Design with Atomic Layer Deposition[END_REF] Si spectra compared with direct CPMG signal (brown), (c)[START_REF] Caillot | Synthesis of amorphous aluminosilicates by grafting: Tuning the building and final structure of the deposit by selecting the appropriate synthesis conditions[END_REF] Al-(top) and[START_REF] O'neill | Catalyst Design with Atomic Layer Deposition[END_REF] Si-(bottom) filtered 1 H spectra.

66 Figure 8 .

 668 Figure 8. (a)[START_REF] Berruyer | DNP in Materials Science: Touching the Surface[END_REF] N DNP enhanced NMR spectra of Py-Al1-SiO2-500 (black), Py-Al5-SiO2-500 (red), and Py-Al10-SiO2-500 (blue). (b) Corresponding FTIR spectra of each material using the same color code, and highlighting PyH + and Py-L vibration frequencies in light grey and red respectively.

  -5 mbar, 2 h). Unless specified otherwise, all ALD-derived materials described in this work were handled without exposure to ambient air. High-field NMR experiments. Single resonance[START_REF] Caillot | Synthesis of amorphous aluminosilicates by grafting: Tuning the building and final structure of the deposit by selecting the appropriate synthesis conditions[END_REF] Al solidstate NMR experiments were performed at 20.0 T and 23.5 T using a 2.5 mm double resonance probe with a MAS rate of 33.3 kHz. Double resonance 27 Al{ 1 H} experiments were performed with the same probe at 20.0 T whereas 29 Si{ 27 Al} experiments were performed with a spinning speed of 15 kHz and a 3.2 mm MAS probe at 20.0 T. Chemical shifts are referenced to a 1 M solution of Al(NO3)3 in HNO3 for[START_REF] Caillot | Synthesis of amorphous aluminosilicates by grafting: Tuning the building and final structure of the deposit by selecting the appropriate synthesis conditions[END_REF] Al and to tetramethylsilane for 1 H and[START_REF] O'neill | Catalyst Design with Atomic Layer Deposition[END_REF] Si. All samples were packed in an argon-filled glove box and NMR experiments performed under pure nitrogen. Additional details are given in the Supporting Information.

DNP NMR experiments. DNP experiments were performed using 1.3 mm triple-resonance low-temperature magic angle spinning (MAS) probes at 9.4 T/263 GHz or 18.8 T/526 GHz on Bruker Avance III spectrometers. DNP samples were packed in a glove box after being impregnated by a 1,1,2,2-tetracholorethane (TCE) solution containing 16 mM TEKPol 35 or HyTEK2 36 for experiments respectively at 9.4 and 18.8 T. Additional experimental details are provided in the SI or in the legends of the figures.

Acknowledgements

ETH Zürich (ETH-40 17-2) and the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program under grant agreement No. 819573 are acknowledge for partial funding. Financial support from Equipex contracts ANR-10-EQPX-47-01, ANR-15-CE29-0022-01, and ANR-17-CE29-0006-01 are gratefully acknowledged. M.K. acknowledges financial support from the Deutsche Forschungsgemeinschaft (KA 5221/1-1).

Supporting Information

Experimental procedures and additional experimental data.

Corresponding Authors * P.F.: pierre.florian@cnrs-orleans.fr * A.L.: anne.lesage@ens-lyon.fr

Author Contributions #M.K. and C.L. contributed equally.

Notes

The authors declare no competing financial interest.