N Dynamic Nuclear Polarization Surface Enhanced NMR Spectrosco

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C: Surfaces, Interfaces, Porous Materials, and Catalysis 15

Discerning #-Alumina Surface Sites with N Dynamic Nuclear Polarization Surface Enhanced NMR Spectroscopy of Adsorbed Pyridine Ilia Borisovitch Moroz, Kim Larmier, Wei-Chih Liao, and Christophe Copéret J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b01823 • Publication Date (Web): 23 Apr 2018 Downloaded from http://pubs.acs.org on April 24, 2018

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The Journal of Physical Chemistry

Discerning γ-Alumina Surface Sites with 15N Dynamic Nuclear Polarization Surface Enhanced NMR Spectroscopy of Adsorbed Pyridine Ilia B. Moroz, Kim Larmier, Wei-Chih Liao, Christophe Copéret* Department of Chemistry and Applied Biosciences, ETH Zürich, Vladimir-Prelog-Weg 1-5, 8093 Zürich, Switzerland.

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ABSTRACT: Low-temperature 15N dynamic nuclear polarization surface enhanced NMR spectros-

copy (DNP SENS) of adsorbed pyridine in combination with FTIR measurements and DFT calcula-

tions was applied to investigate the surface sites of γ-alumina, which can be divided in four groups:

1) groups 1 and 2, associated with less shielded 15N chemical shifts and the lowest ν(8a) frequen-

cies of adsorbed pyridine, correspond to weakly adsorbed H-bonding pyridine to hydroxyl group or chemisorbed water on alumina, 2) group 3, with intermediate 15N chemical shifts and ν(8a) fre-

quencies, corresponds to pyridine coordinated to specific Lewis acid sites, namely five-coordinated (AlV) aluminum atoms of the (100) facet, as well as some H-bonded pyridine, 3) group 4, associated

with the most shielded 15N chemical shift and the highest ν(8a) frequency, is selectively assigned to Lewis acid Al sites located on the (110) facet and corresponds to both four- and five-coordinated

aluminum atoms (AlIV and AlV). Noteworthy, a correlation between the 15N chemical shift and the adsorption energy of pyridine, that is H-bonded or coordinated to Al Lewis acid sites, was identi-

fied: the stronger the adsorption, the lower the 15N chemical shift. According to Natural Chemical

Shielding (NCS) analysis, this behavior is traced back to the bonding interaction of the lone pair of

pyridine and the Lewis acid or OH sites, that controls a specific principal component of the chemical shift tensor of pyridine on the one hand, and the adsorption energy on the other hand. This correla-

tion between 15N chemical shift and adsorption energy is likely general since it has a well-defined, molecular origin and can be thus extended to other oxides.


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Introduction Transition aluminas, in particular γ-Al2O3, are of great interest due to their broad range of appli-

cations. For instance, aluminas are used as catalysts by themselves (for instance for the Claus process or alcohol dehydration reactions)1-2 or as catalyst supports, to stabilize both isolated metal

sites in olefin metathesis, ethylene polymerization or propane dehydrogenation catalysts3 or nano-

particles for reforming or hydrodesulphurization.2, 4 In all cases, the acido-basic properties of the

alumina surface are key to the overall performance of the material,2-4 and has therefore been the subject of numerous and detailed studies for more than 40 years.5-16

Today, the most advanced studies have focused on understanding the role of specific surface sites

and facets in the reactivity of γ-alumina. For instance, an acid-base pair consisting of acidic penta-

coordinated AlV and basic O sites on the (100) facet is required to efficiently catalyze the dehydra-

tion of alcohols (ethanol and isopropanol),17-19 while more demanding reactions like the activation of the C-H bond of methane or the binding of N2 require the high reactivity of the AlIII sites present

on the (110) facet.15, 20-22 In other cases, pairs of aluminum sites (AlIII and AlIVb sites on the (110) facets) have been shown to be essential for the olefin metathesis activity of Re2O7 or CH3ReO3 sup-

ported on alumina.23-24 Thus, distinguishing different types of surface sites and/or facets is an es-

sential step to better understand the surface reactivity of alumina and its role as a catalyst or catalyst support.

The adsorption of probe molecules combined with spectroscopy – typically infrared – and com-

putational studies have become a classical toolset to assess the structure of surface sites.6-11 CO2,

CO, NH3 and pyridine are among the most typically used probe molecules.25 CO2 is usually consid-

ered as an acidic probe to the surface basic sites, while the others probe acidic sites. Adsorption of

NH3 is associated with a very complex surface chemistry, that can include coordination, protonation

and dissociative adsorption, making its use as a probe molecule to understand surface sites on ox-

ide supports not recommended.26 CO is a more commonly used probe. It cannot be protonated, and ACS Paragon Plus Environment

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the observed change in its stretching frequency is considered to correlate with the acidity of the site on which CO is adsorbed. However, this correlation is often ambiguous, since the CO coverage level

also influences the vibration frequency, and the extrapolation to zero coverage must be performed. Additionally, it has been shown that the CO stretching frequency actually probes the local electric

field, which does not directly correlate to the acidity strength of the surface site.27 Pyridine remains

the mostly-used probe molecule to study solid acids. It is a rather strong base that can be proto-

nated by Brønsted acid sites, H-bonded to weaker Brønsted acid sites, and coordinated to Lewis

acid sites. All three types of species can be identified by the IR stretching frequencies of the pyridine ring.28 The adsorption of pyridine is most easily performed by saturation of the sample and

evacuation at increasing temperature. 15N solid-state NMR can also be a powerful tool to probe the

acidity of solids by the adsorption of pyridine.13, 29-32 For instance, the 15N isotropic chemical shift (δiso) of pyridine and pyridinium (protonated pyridine) differs by more than 100 ppm.33-35 This

large chemical shift range can be used to assess the acidic character of the surface site interacting with pyridine, but the NMR studies has to be carried out at low temperature to avoid rapid ex-

change between surface sites, that averages chemical shift values and can thereby lead to misassignment.30, 36

While NMR studies of surface species concentrate on distinguishing surface sites from their spe-

cific isotropic chemical shift values, the large chemical shift anisotropy (CSA) of 15N is particularly

interesting yet not fully exploited. Indeed, CSA contains key information regarding the electronic

structure of molecules, and more specifically it provides information about frontier molecular or-

bitals (FMOs) and hence a link to reactivity.37-39 We thus reason that extracting the CSA parameters

can provide a molecular understanding of the 15N chemical shift of the adsorbed pyridine on acid sites.

Towards this goal, we have thus explored the use of low-temperature 15N dynamic nuclear polari-

zation surface enhanced NMR spectroscopy (DNP SENS)37, 40-43 and 15N labeling in order to obtain ACS Paragon Plus Environment

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the CSA parameters of specific sites within acceptable experimental time thanks to the increase of

the NMR signal through DNP44-46 using exogenous dinitroxyl radicals.47 This study is complemented

by FTIR measurements and DFT calculations on periodic models of the (100) and (110) termina-

tions of Al2O3 at different water coverage. We have also investigated the origin of the chemical shift through Natural Chemical Shielding (NCS) analysis on selected models in order to identify at the

molecular level the origin of the chemical shift difference. Based on the results, we propose an assignment for the NMR and IR signals and rationalize the relationship between 15N chemical shift and adsorption energy of pyridine.

Experimental and computational details Preparation of alumina. γ-Al2O3 (purchased from SASOL TH 100/150) was dehydroxylated at a

given temperature (300, 500 or 700 °C) in a flow of dry synthetic air (SA) for 15 h. The sample was

then cooled down under high vacuum (ca. 10-5 mbar). Dehydroxylated alumina was stored in argon

atmosphere prior to pyridine adsorption.

Determination of OH-group density. Ca. 50 mg of dehydroxylated alumina were loaded into an

airtight J-young NMR tube inside the glovebox together with a large excess of [Mg(CH2Ph)2(THF)2]

(denoted further as MgBn2) with respect to OH-groups dissolved in C6D6. Ferrocene (ca. 15 mg) was

added as an internal standard. The amount of toluene evolved, detected by 1H NMR (2.1 ppm), is

related to the amount of accessible hydroxyl groups (Al–OH) on the surface of alumina according to the titration equation (equation 1):

Al–OH + MgBn2 = Al–O–MgBn + Toluene

Pyridine adsorption on alumina. About 100 mg of dehydroxylated alumina were exposed to

(1)

pyridine (non-labeled or 15N-labeled) saturation vapor at room temperature for 10 min. The excess of pyridine was removed in high vacuum (about 10-5 mbar) at room temperature for 3 h. The sam-

ples were stored under inert atmosphere (argon) prior to 15N DNP NMR and FTIR measurements.


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Pyridine desorption experiments. Samples containing either 15N-labeled pyridine or non-la-

beled 14N-pyridine adsorbed on Al2O3 were evacuated under high vacuum at a given temperature

(100, 150 or 400 °C) for 1 h prior to 15N DNP NMR and FTIR measurements.

NMR experiments. All 15N DNP SENS experiments were conducted on a Bruker 600 MHz (14.1

T) spectrometer equipped with gyrotron microwaves emitting at 395 GHz and output power 6-

10W. A Bruker low-temperature 3.2 mm double-resonance probe was used. The static magnetic

field was externally referenced by setting the 13C higher frequency peak of adamantane to 38.5

ppm.

All DNP samples were prepared in an argon-filled glove box. The 15N-pyridine treated solids were

incipient wetness impregnated with minimum amount of 16 mM TEKPol47 solution in 1,1,2,2-tetra-

chloroethane (TCE).48 The mixture was packed into a 3.2 mm (O.D.) sapphire rotor, and a Teflon insert was placed into the rotor to prevent solvent spill. The rotor was finally closed with a corresponding zirconia cap. After taking the sample out of the glove box, it was quickly (within ca. 5

minutes) inserted into the cold probe at 100 K cooled by a cryogenic heat exchanger system. DNP

build-up time (TDNP) was measured by 1H saturation-recovery experiment with microwaves turned

on. 15N{1H} cross-polarization magic-angle spinning (CP-MAS) experiments were used. Recycle delay for CP-MAS experiments was set as 1.3*TDNP. The 1H excitation and decoupling radiofrequency

(rf) fields were set as 100 kHz. The CP condition was optimized to fulfill Hartmann-Hahn condition

under MAS with minor adjustment to reach the best CP efficiency experimentally.

Computational details. Periodic DFT were performed with VASP49-50 using the PBE exchange-

correlation functional,51 and dispersion corrections at the D3 level.52 The calculations were carried

out using pseudo-potentials obtained by the Projected-Augmented Wave (PAW) method53 as imple-

mented in VASP. The cutoff energy was set to be 400 eV. The energy criterion for the convergence of the self-consistent field (SCF) cycles was fixed to 10-5 eV. Geometry optimizations were carried

out using a conjugate-gradient algorithm and a convergence criterion on the forces exerted on all ACS Paragon Plus Environment

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relaxed atoms of 0.01 eV Å-1. During geometry optimizations, the two upper atomic layers of the

slab (see Models) were allowed relaxing together with the adsorbates. The adsorption energy of pyridine was calculated according to equation 2:

∆𝐸𝐸𝑎𝑎𝑎𝑎𝑎𝑎 = 𝐸𝐸(𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠 + 𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝) − 𝐸𝐸(𝑠𝑠𝑠𝑠𝑠𝑠𝑠𝑠) − 𝐸𝐸(𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝)

(2)

where E(surf) stands for the ab initio total energy of the surface, E(pyridine) of the gas-phase pyr-

idine molecule, and E (surf + pyridine) of the adsorbed molecule on the surface. Vibration analysis

was performed using the finite difference method by displacement of each pyridine atom by 0.01 Å

in each direction. Chemical shift calculations were performed using the linear response method implemented in VASP; for these calculations, the convergence criterion on the SCF cycles was set to 10-8 eV.

For selected models, the density of states (DOS) and site-projected density of states (pDOS) were

calculated as implemented in VASP. The band-decomposed charge density could then be calculated

and plotted for orbitals of interest using VESTA as a visualization software.54 Energy intervals of 0.5 to 0.8 eV centered on the maximum of the pDOS-N were used to this end; note that the band-de-

composed charge density is not site-projected and thus can also contain density of charge located on the acidic moiety of similar energy. Isosurface values of 2.5·10-2 e Å-3 were used.

For NCS analysis on selected models, NMR calculations were performed using the ADF 2016

code55 with B3LYP functional and a TZP56 basis set with the all-electron relativistic zeroth-order

regular approximation (ZORA).57 No frozen core approximation was applied. Analysis of scalar-relativistic natural localized molecular orbitals (NLMO) was done with the NBO 6.0 program.58 Calcu-

lated NMR shielding tensors were analyzed using these scalar-relativistic NLMO.59-61

Models. We used the models for the γ-Al2O3 surface developed by Digne et al.62-63 The models are

based on a nonspinel bulk structure.64 The (100) surface model used in the present study consists

in a triperiodic cell with the size of 28.0 × 11.1 × 16.8 Å3 wide, occupied by a 6.0 Å thick alumina

slab (normal to the X-axis) surmounted by a 22 Å wide vacuum slab. The Brillouin zone integration ACS Paragon Plus Environment

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is performed on a 1 × 2 × 1 k-points grid mesh. The (110) surface model has a cell size of 16.1 × 16.8

× 28.0 Å3 containing a 6.0 Å thick alumina slab (normal to the Z-axis) surmounted by a 22 Å wide vacuum slab. The Brillouin zone integration is performed on a 1 × 1 × 1 k-points grid mesh. Both

surfaces are considered in a fully and partially dehydrated state, with OH-coverage increasing from

0 to 15 OH nm-2. For constructing the hydrated models, the appropriate number of water molecules

was adsorbed on the surface and the obtained structure was optimized (Figure S1 and Figure

S2).62-63, 65

Chemical shielding tensor and its principal components. The chemical shift is anisotropic by

nature, and is in fact associated to a chemical shift tensor. This chemical shift tensor can be ex-

pressed on the principal axis system of the molecule of interest, in which case it can be represented by a diagonalized matrix containing the principal components of the chemical shift tensor (δ11 > δ22 𝑟𝑟𝑟𝑟𝑟𝑟

> δ33) (equation 3); it is associated with the related shielding tensor (σ11 < σ22 < σ33), where σ𝑖𝑖𝑖𝑖𝑖𝑖 is

an isotropic shielding of the reference compound. Thus, the orientation of chemical shielding component coincides the one of the corresponding chemical shift component. The shielding tensor can

be calculated with reasonable precision by DFT calculations. The three principal components of the chemical shift tensor describe the chemical shift anisotropy (CSA) and individually provide a detailed understanding of the electronic structure of the probe nuclei.66 𝛿𝛿11 � 0 0

0 𝛿𝛿22 0

0 𝜎𝜎11 1 0 0 𝑟𝑟𝑟𝑟𝑟𝑟 0 � = 𝜎𝜎𝑖𝑖𝑖𝑖𝑖𝑖 ∙ �0 1 0� − � 0 0 𝛿𝛿33 0 0 1

0 𝜎𝜎22 0

0 0 � 𝜎𝜎33

(3)

The chemical shift principal components can be determined experimentally. The three main com-

ponents δ11, δ22, δ33 are often associated to three other quantities according to the Herzfeld-Berger

convention: the isotropic chemical shift (δiso) which is an average of the three principal components

of the chemical shift tensor (equation 4); the span (Ω) and the skew (κ) that describe the magnitude and the asymmetry of the anisotropy, respectively (equation 5 and 6). 1 3

𝛿𝛿𝑖𝑖𝑖𝑖𝑖𝑖 = (𝛿𝛿11 + 𝛿𝛿22 + 𝛿𝛿33 )


(4) 8

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𝛺𝛺 = 𝛿𝛿11 − 𝛿𝛿33

Results and discussion

𝜅𝜅 =

3(𝛿𝛿22 − 𝛿𝛿𝑖𝑖𝑖𝑖𝑖𝑖 ) 𝛺𝛺

(5)

(6)

Effect of Al2O3 dehydroxylation temperature on pyridine adsorption. We first investigate the

influence of the pre-treatment conditions on the pyridine adsorption. γ-alumina is calcined in a flow of synthetic air at different temperatures (X = 300, 500 and 700 °C, denoted as Al2O3-X), a process

thereafter referred to as dehydroxylation (Figure 1). The pure γ-phase is preserved up to 700 °C

(Figure S3). The progressive disappearance of the υ-OH stretching bands in IR spectra (between

3800 and 3500 cm-1) with increasing dehydroxylation temperature (Figure 1a) is indicative of the decrease of the surface OH density, from 3.7 OH nm-2 on Al2O3-300 to 1.4 OH nm-2 on Al2O3-700 (see

Table S1) as determined by chemical titration using MgBn2, which is in line with previous find-

ings.21, 65 These partially dehydroxylated Al2O3-X samples are then contacted with pyridine at room

temperature (Ppyr ~ 18 mbar), and the excess is removed by evacuation under dynamic vacuum (ca.

10-5 mbar) for 3 hours. Figure 1b shows the IR spectra of Al2O3-X samples with pyridine adsorbed in the region of 1400 to 1650 cm-1, where the four bands corresponding to ring vibrational modes 8a,

8b, 19a, and 19b (Figure 1d), are perturbed depending on the nature and the acidic strength of the sites.25 The absence of bands in the regions near 1635 cm-1 and 1550 cm-1, where the 8a and 19b

modes of pyridinium are expected,25 excludes the presence of protonated pyridine species for all

alumina samples (Figure 1b). Adsorptions at 1616 cm-1 and ca. 1442 cm-1 are usually assigned to

8a and 19b ring vibrational modes of pyridine bound to Lewis site (Py-L), respectively while that at 1608-1590 cm-1 is characteristic for 8a mode of H-bonded pyridine (H-Py) to weakly acidic surface OH groups. With increasing dehydroxylation temperature, the band at ca. 1616 cm-1 becomes rela-

tively more intense compared to the bands at 1608 and 1590 cm-1 (Figure 1b), indicating an in-

crease fraction of the associated surface species (vide infra for further discussion).


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Al2O3-X are then further characterized by 15N DNP SENS. The use of DNP and 15N labeling signifi-

cantly improve the 15N NMR sensitivity and allow for extracting the CSA parameters of each ob-

served isotropic site (vide infra). Figure 1c shows 1D 15N DNP SENS spectrum of Al2O3-X samples

after saturation with 15N-pyridine and evacuation at ambient temperature under dynamic vacuum

(10-5 mbar) for 3 hours. The 15N CSA parameters measured for these samples are presented in Ta-

ble S2.

Figure 1. a) Transmission infrared spectra of Al2O3-300 (green), Al2O3-500 (blue) and Al2O3-700 (red).

b) Transmission infrared spectra (pyridine range) of 15N-labeled pyridine adsorbed on Al2O3-300

(green), Al2O3-500 (blue), Al2O3-700 (red) and desorbed at ambient temperature under high vacuum

for 3h. c) 15N DNP SENS spectra of 15N-labeled pyridine on Al2O3-300 (green), Al2O3-500 (blue), Al2O3-700 (red). d) Graphical representation of pyridine ring vibrational modes.


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For the three investigated dehydroxylation temperatures, all spectra contain one peak around

240 ppm and a broad feature between 300 and 250 ppm that can be deconvolved into three main

contributions. The most shielded contribution lies around 265 ppm and hardly depends on the de-

hydroxylation temperature, while the chemical shifts of the two others slightly shifts from 296 and 284 ppm for Al2O3-300 to 288 and 278 ppm for Al2O3-500. No major change is observed between the

spectra of Al2O3-500 and Al2O3-700.

Overall, the data presented show that we can distinguish four signals that can be assigned to four

groups of different types of surface pyridine species (pyridine interacting with surface sites) for all the dehydroxylation temperatures investigated.

Pyridine desorption studies. In order to assess the thermal stability of the different pyridine

surface species, two sets of experiments are carried out. On the one hand, the Al2O3-500 sample ex-

posed to 15N-labeled pyridine is evacuated at ambient temperature, 100, 150 or 400 °C, and for each temperature, a 1D 15N DNP SENS and an IR spectra are collected (Figure 2a and Figure S6, respec-

tively). On the other hand, the Al2O3-500 sample is exposed to non-labeled pyridine and treated in

similar conditions in order to measure IR frequencies comparable with the literature data (usually collected using 14N pyridine). The corresponding FTIR spectra are shown in Figure 2b. According

to the literature,7 the vibration frequency of 8a vibrational mode of pyridine is the most sensitive to

the strength of the acid site, while analysis of other modes (8b, 19a and 19b, see Figure S7) is more

complicated due to overlap of the bands that belong to different types of pyridine interaction with acid sites.25, 63 We thus focus on 8a mode range. Note that the experiments with labeled pyridine

(Figure S6) qualitatively match what is observed with the non-labeled molecule reported below. Heating up the Al2O3-500 sample at 100 °C in vacuum leads to disappearance of the most

deshielded peak (288 ppm) (Figure 2a). The peak at 272 ppm has lower relative intensity than at

ambient temperature. This is accompanied by disappearance of the FTIR band at 1596 cm-1. For the sample treated at 150 °C, only two 15N NMR isotropic peaks are observed, and the peak at 272 ppm ACS Paragon Plus Environment

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disappears. At this temperature, the band at 1608 cm-1 in the IR spectrum also disappears. When

the sample is heated up to 400 °C, only the most shielded 15N NMR peak at 234 ppm remains. This is

accompanied by a significant decrease in intensity of the band at 1612 cm-1 in IR, the band at 1624 cm-1 being essentially unaffected.

Figure 2. a) 15N DNP SENS spectra of 15N-labeled pyridine adsorbed on Al2O3-500 and desorbed un-

der dynamic vacuum at ambient temperature for 3h (blue), 100 °C for 1h (red), 150 °C for 1h

(green), 400 °C for 1h (violet). Relative chemical shift with respect to free pyridine is given in parentheses. b) Transmission infrared spectra of 14N-pyridine adsorbed on Al2O3-500 (8a vibration


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mode range) and then desorbed at room temperature (RT) for 3h (blue), 100 °C for 1h (red), 150 °C for 1h (green), 400 °C for 1h (violet).

The desorption experiments demonstrate that four groups of different surface pyridine species

can be distinguished, each characterized by different desorption temperature (in other words, thermal stability, which is associated to the adsorption energy), the isotropic chemical shift, and the 8a mode vibration frequency. The data are summarized in Table 1.

Finally, the DNP SENS data recorded allows us to obtain the principal components of the chemical

shift tensor (see Experimental section) for each of the 15N NMR peak observed in this study. The

data reported in Table 1 shows that the variation of the isotropic chemical shift observed in the

four groups of peaks (1 to 4) is essentially caused by the variation of the most deshielded compo-

nent δ11 that varies from 495 ppm for Group 1 to 383 ppm for Group 4. The other components are

affected to a much lesser extent: δ22 changes from 385 ppm to 345 ppm, while δ33 does not change

significantly. This information already shows that the adsorption of pyridine mostly affects specific frontier molecular orbitals of pyridine (vide infra for further discussion).66

Table 1. Different types of pyridine-surface interaction with the corresponding desorption temper-

ature (Tdes), characteristic vibration frequency (υvib(8a)) and isotropic chemical shift (δiso) with its principal components (δ11, δ22, and δ33) obtained from experiments on Al2O3-500. group

Tdes, °C

υvib(8a), cm-1

δiso (δiso- δiso,pyridine)a, ppmb

δ11, ppmb

δ22, ppmb

δ33, ppmb

2

150

1605

278 (–29)±9

466±9

386±9

–19±9

1 3 4

aThe

100 400

> 400

1596 1612 1624

288 (–19)±4 266 (–41)±7 237 (–70)±4

495±4 437±7 383±4

385±4 370±7 345±4

difference between chemical shift of the surface species and free pyridine.

bErrors

–16±4 –12±7 –17±4

are determined by peak width.

Computational studies.


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In order to better understand the nature of the possible adsorption sites, we investigate the inter-

action of pyridine with acidic centers existing on periodic models of alumina surface by DFT calcu-

lations (see Figure 3 for representative examples) and calculate the associated spectroscopic signa-

tures (IR frequencies and NMR chemical shifts). The most abundant γ-Al2O3 terminations (100) and (110) are studied (see Experimental Part for details).63 Both terminations are studied in a fully or a

partially dehydroxylated state, with OH-coverage increasing from 0 to 15 OH nm-2.

The calculated chemical shifts are referenced with respect to the calculated isotropic chemical

shift of free pyridine, and we report the relative chemical shifts Δδiso = δiso(surface spe-

cies) – δiso(free pyridine). For comparison, the corresponding experimental value is given in Table

1.

Adsorption on (100) termination. The fully dehydrated (100) termination exposes 4 types of

aluminum Lewis acid sites, all of them being five-coordinated (labeled as AlVa, AlVb, AlVb’ and AlVc, see

ESI). The adsorption energy of pyridine varies from –155 kJ mol-1 for AlVa to –112 kJ mol-1 for AlVc

(see Table S3). The calculated 15N chemical shifts slightly depend on the type of AlV and vary from Δδiso = –63 ppm for AlVa to –51 ppm for AlVb (see Table S3).

The adsorption of water on the (100) termination proceeds either via dissociation on adjacent

AlV,O sites with the formation of a terminal (μ1) OH group (AlV–OH) and OH group on adjacent oxy-

gen or via molecular adsorption on the AlV sites (Table S3).21, 62 As a result of water adsorption, the

aluminum atom becomes inaccessible for pyridine, while new sites for pyridine adsorption appear,

namely hydroxyl groups of various possible coordination (terminal μ1-OH or bridging μ2-OH and μ3OH), which can act as potential Brønsted acid sites.


14

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Figure 3. Top view (left) and side view (right) for optimized structures of pyridine surface species:

a) pyridine coordinated to AlVa on (100) facet; b) pyridine H-bonded to μ1–OH group on (100) facet; c) pyridine H-bonded to μ3–OH group on (100) facet; d) pyridine coordinated to AlIII on (110) facet;

e) pyridine coordinated to AlIVb on (110) facet; f) protonated pyridine on (110) facet. Only the two

top layers of the periodical slab of the alumina are shown as balls and sticks for clarity. Colour code is as follows: Al (pink), O (red), N (blue), C (black), and H (white). A black dashed line indicates the surface unit cell.


15


Page 16 of 39

The calculations show that none of the Brønsted acid sites on (100) termination are able to proto-

nate pyridine which is in line with previous calculations63 and in agreement with experimental re-

sults reported here and elsewhere.25 The pyridine interaction with the Brönsted acid sites forms

hydrogen-bonded species. They are less stable (–70 to –90 kJ mol-1) compared to pyridine com-

plexes with Lewis acid sites (Table S3). The Δδiso of the hydrogen-bonded species varies from –21

to –45 ppm. (Table S3) However, no clear trend is observed that would allow for distinguishing different types of OH groups (e.g. terminal vs. bridging), neither in terms of chemical shifts nor in adsorption energies.

Adsorption on (110) termination. The fully dehydrated (110) termination exposes three types

of aluminum Lewis acid sites: one tri-coordinated (AlIII) and two types of tetra-coordinated (AlIVa

and AlIVb). While pyridine binds very strongly on AlIII (–208 kJ mol-1), the adsorption energy on AlIVa (–128 kJ mol-1) and AlIVb (–161 kJ mol-1) is close to that on AlVa (100) (see Table S3). Pyridine on

AlIII is characterized by higher Δδiso (–92 ppm), if compared to AlIVb (–80 ppm) and AlIVa (–68 ppm). The partially dehydrated (110) termination is obtained by adsorption of water molecules on

Lewis acid sites. While the most acidic aluminum sites are covered with water, others are still able

to adsorb pyridine with Eads. of –129 kJ mol-1 to –187 kJ mol-1. At high water coverage there are for-

mally five-coordinated AlIV(110)–OH sites (AlV on (110)) that are also able to bind pyridine with Eads of –125 to –131 kJ mol-1 and Δδiso of –63 to –73 ppm. Pyridine adsorption on terminal and bridging

OH-groups, or adsorbed water molecules, leads to various surface species with Eads ranging from – 48 to –95 kJ mol-1 and Δδiso ranging from –32 to –50 ppm. In few cases surface OH groups are able

to protonate pyridine yielding pyridinium species that are shielded with respect to other types of

pyridine binding (Δδiso of –96 to –100 ppm). However, the existence of such species is unlikely due

to the presence of neighboring Lewis sites on which the adsorption of pyridine is energetically

much more favorable (e.g. Eads = –187 for Py on AlIVb vs. –92 kJ mol-1 for PyH+), again in line with the experimental results (absence of pyridinium) and previous computational findings.63 ACS Paragon Plus Environment

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The isotropic chemical shift, computed by DFT and referenced to free pyridine, can be decom-

posed into the three main components of the chemical shift tensor (see Table S3). The data set

shows that δiso essentially depends on the variation of the most deshielded δ11 component, which

varies from 313 ppm for pyridine to 53 ppm for Py_AlIII. The other two components depend much

less on the type of coordination: δ22 changes from 83 ppm to 32 ppm, while δ33 – from –395 ppm to

–354 ppm. This result follows the same trend as observed experimentally (vide supra), i.e. the change of δiso mostly depends on its most deshielded component δ11.

The results of the calculations are summarized on Figure 4a (additional data are reported in Ta-

ble S4) using three different parameters: adsorption energy, 15N NMR isotropic chemical shift and

υ(8a) frequency (calculated for selected representative models). The experimental spectroscopic data are also included with the label 1 to 4 by order of increasing thermal stability (see Table 1).

On Figure 4a, we sorted the data obtained for the various models of adsorbed pyridine on the alu-

mina surface (hydrated and dehydrated) according to the type of pyridine interaction with the surface. The first type (red) includes pyridine in interaction with various Brønsted acid sites without

protonation, the so-called hydrogen-bonded pyridine (H-Py). This type of interaction is character-

ized by the lowest stability and lowest absolute value of Δδiso (–21 to –50 ppm). The second type

(blue) includes pyridine adsorbed on five-coordinated aluminum atoms of (100) termination (–47 to –63 ppm). The third type 3 (green) contains the models in which pyridine is interacting with

four-coordinated aluminum atom on the (110) termination (–67 to –83 ppm). AlV sites on (110),

resulting from the hydroxylation of AlIV(110) sites, are also included in this type due to similar cal-

culated values. The last two types represent pyridine coordinated to AlIII sites (violet) on (110) ter-

mination (–92 to –93 ppm), or protonated (PyH+) (orange) (–96 to –100 ppm). Overall, the results

of chemical shift calculation show that the NMR signature of adsorbed pyridine can be in principle used to distinguish Al sites on (100) and (110) terminations, but not four- and five-coordinated Al sites on the (110) termination.


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Page 18 of 39

Figure 4a shows that a stronger adsorption of pyridine is associated at the same time to a higher

shielding of the 15N NMR isotropic chemical shift and to a higher υ(8a) frequency. This matches well

the experimental results that also associate a higher thermal stability (qualitatively associated to a stronger adsorption of the pyridine) to a higher IR frequency and a lower isotropic chemical shift.

In fact, Figure 4b shows that, excluding the pyridinium models, relation between adsorption en-

ergy and 15N chemical shift qualitatively follows a linear trend on the ranges investigated. Pyri-

dinium does not follow this trend; it is very likely due to the protonation of pyridine associated with the cleavage and formation of new bonds that altogether contributes to the adsorption energy. It is

thus not surprising that the coordination of pyridine to OH or Al sites and the protonation does not follow the same trend in terms of spectroscopic properties and adsorptions energies.

From the comparison between experimental and computed 15N NMR Δδiso, the following assign-

ment of the experimentally observed groups can be proposed: -

Groups 1 and 2 correspond both to H-bonded pyridine on weak Brønsted acid sites. However,

as mentioned above, our computational data do not allow for distinguishing these two groups in

-

terms of chemical structure or termination;

Group 3 corresponds to Py_AlV on (100) termination, having more shielded δiso and higher ad-

sorption energy than species of group 1 and 2. However, some H-bonded pyridine can also be

included in this group due to overlap between these two types of species (Py_AlV and H-Py) in

-

terms of chemical shift and adsorption energy (Figure 4a);

Group 4 is selectively associated with pyridine adsorbed on the Lewis acid sites of the (110) termination, namely the Py_AlIV and Py_AlV. These species have more shielded δiso and higher ad-

sorption energies than on the other termination.

As a general trend, this assignment is also consistent with the changes observed upon dehydrox-

ylation of alumina, which is expected to be associated with an increased fraction of exposed Lewis


18

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acid sites.20, 62, 65 This is, in fact, observed by IR spectroscopy: the intensity of the band at higher fre-

quency (1616 cm-1) relatively increases with respect to the lower-frequency bands at 1608 cm-1 and 1590 cm-1 when the dehydroxylation temperature increases (Figure 1b).

Worthy of note, the calculated IR frequencies of pyridine interacting with AlIII and AlIV sites are in

the same range, presuming that these sites cannot be readily distinguished by IR spectroscopy.

However, the calculated 15N chemical shifts are significantly distinct (see Figure 4a). Therefore,

NMR spectroscopy of adsorbed pyridine would in principle be able to distinguish AlIII from AlIV. In

this respect, according to the NMR data collected, no signal corresponding to pyridine adsorbed on AlIII sites could be detected under our experimental conditions, possibly due to their very low

amount (ca. 0.05 per nm-2) by comparison to other sites (1-5 per nm2).21 Another possible reason is

that pyridine adsorption perturbs the surface through the competitive adsorption of pyridine and

water, which can lead to a change the location of hydroxyl group and chemisorbed water at the surface of alumina, leading to disappearance of the small amount of AlIII sites. This would also explain

why only minor changes in the NMR spectra of alumina dehydroxylated at different temperatures are observed (Figure 1c).


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Page 20 of 39

Figure 4. a) Calculated chemical shift (Δδiso), adsorption energy (Eads) and 8a-mode vibration fre-

quency – υvib(8a) – for different types of pyridine surface species: hydrogen-bonded pyridine (red),

pyridine on AlV (blue), pyridine on AlIV (green), pyridine on AlIII (violet) and protonated pyridine

PyH+ (orange). Colored lines represent the calculated values and colored areas represent the range of obtained results for a same type of species. The black bars represent the experimental Δδiso and

υvib(8a) values for group 1-4 (Table 1). b) Calculated 15N chemical shift (Δδiso) vs. pyridine adsorption energy (Eads). The black line corresponds to linear fit (y = 0.43x – 6, R2 = 0.73) of the data ex-

cluding PyH+ species.


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Nevertheless, the fundamental origin of the correlation between 15N isotropic chemical shift of

pyridine and its adsorption energy observed on Figure 4b remains puzzling. Considering that

chemical shift and in particular the principal components of the chemical shift contain direct infor-

mation about specific frontier molecular orbitals (equation 8), we further carry out a detailed analysis on the principal components of chemical shielding tensor (see equation 3 for relation between

shielding σ and chemical shift δ) accompanied by a localized orbital analysis, named Natural Chemi-

cal Shielding (NCS) analysis, to extract the contribution of individual orbitals to the particular principal component of the chemical shielding tensor. Since this analysis is not currently available for periodic models, we investigated molecular analogues of surface pyridine species. Principal components of 15N chemical shielding tensor.

We chose to carry out the study on selected models that can be considered as analogs of the dif-

ferent types of adsorbates that we distinguished on the γ-alumina surface models:

- H-bonded pyridine (II), where (CH3O)3Si–OH moiety is used to model a weak surface Brønsted site;

- pyridine on Al Lewis acid site (III), where AlCl3 models a prototypical Al Lewis acid site; - protonated pyridine (as called, pyridinium ion PyH+) (IV).

The binding energy of pyridine with the (CH3O)3Si–OH and AlCl3 moieties was calculated to be –

31 and –141 kJ mol-1, respectively, which follow a similar trend to what is obtained on the γ-alu-

mina surface sites (see above). Free pyridine (I) was also considered as a reference. For each model, the shielding tensor was calculated, and projected on the three main natural axes of the pyridine

molecule (X, Y and Z) as defined on Figure 5a. The axes direction is kept identical for all models, IIV.


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Page 22 of 39

Figure 5. a) The principal axis system (X, Y, Z) of the pyridine molecule. This orientation is pre-

served for all structures. b) Orientations and values of principal components of shielding tensor for pyridine (I), H-bonded pyridine (II), pyridine on Lewis site (III), protonated pyridine (IV) and 3D

representation of the calculated shielding tensors (orange for deshielding, blue for shielding). Note

that the orientation and the value of σ11 tensor component are shown in red, σ22 – in green and σ33 –

in blue. Positive value indicates shielding with respect to free N atom, negative ones – deshielding.

For models II-IV, the isotropic chemical shift Δδiso = δiso – δiso, pyridine is calculated based on computed σ11, σ22 and σ33. Colour code is as follows: Al (pink), O (red), N (blue), C (black), Si (pale yellow), Cl (lime) and H (white).

Figure 5b shows the optimized structures I-IV with the calculated principal components of the

chemical shielding tensor and their orientation with respect to pyridine ring. While the principal

components oriented along Y-axis and Z-axis (σyy and σzz, respectively) do not change significantly

from I to IV, the shielding tensor component oriented along the X-axis (σxx) is significantly affected by the coordination to acid sites changing from –407 ppm for I to 12 ppm for IV (see Figure 5b).


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This is in line with the experimental and computational results discussed above, where only the

most deshielded component (δ11) varies. We can now assign this variation – and thus the change of

isotropic chemical shift – to the variation of the chemical shielding along the X-axis (σxx) of the pyri-

dine molecule caused by the adsorption process.

We thus further investigate which molecular orbitals contribute to the corresponding σxx compo-

nent, which is the main component affected by coordination/protonation. The analysis of the other components σyy and σzz is described in ESI (Figure S8 and Figure S9).

NCS analysis. The chemical shielding (σ) consists of diamagnetic and paramagnetic terms (equa-

tion 7).66

σ = σ𝑑𝑑𝑑𝑑𝑑𝑑 + σ𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝

(7)

The first term (σdia), associated with the electronic ground state of the molecule, leads to shield-

ing and is usually similar for a given nucleus within a broad range of environment. The second term (σpara) is proportional to the coupling of occupied (𝜓𝜓𝑜𝑜𝑜𝑜𝑜𝑜 ) and virtual (𝜓𝜓𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣 ) molecular orbitals via the angular momentum operator (equation 8),66 𝜎𝜎𝑝𝑝𝑝𝑝𝑝𝑝𝑝𝑝,𝑖𝑖𝑖𝑖 ≈ 𝐾𝐾 � �

𝑜𝑜𝑜𝑜𝑜𝑜 𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣

�𝜓𝜓𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣 �𝐿𝐿�𝑖𝑖 �𝜓𝜓𝑜𝑜𝑜𝑜𝑜𝑜 ��𝜓𝜓𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣 � 𝐿𝐿�𝑖𝑖 ⁄𝑟𝑟 3 �𝜓𝜓𝑜𝑜𝑜𝑜𝑜𝑜 � ∆𝐸𝐸𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣−𝑜𝑜𝑜𝑜𝑜𝑜

(8)

where �𝜓𝜓𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣 �𝐿𝐿�𝑖𝑖 �𝜓𝜓𝑜𝑜𝑜𝑜𝑜𝑜 � stands for the overlap between orbitals coupled via operator 𝐿𝐿�𝑖𝑖 (i = X, Y and Z,

the direction of action), ∆𝐸𝐸𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣−𝑜𝑜𝑜𝑜𝑜𝑜 is the energy difference between the two considered wavefunc-

tions, and K is a multiplicative constant. σpara is typically negative; it thus leads to deshielding and is

very sensitive to the local environment of the nuclei. It depends on the overlap between occupied

orbital rotated by 90° (under the action of operator 𝐿𝐿�𝑖𝑖 ) and virtual orbital, and is also inversely proportional to the term ∆𝐸𝐸𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣−𝑜𝑜𝑜𝑜𝑜𝑜 , which is the energy difference between the coupled orbitals. Thus σpara is dominated by the coupling between frontier molecular orbitals that are closer in energy.66

These orbitals may also participate in pyridine coordination to surface acid sites, and thus their energy should depend on the strength of the coordination.


23


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NCS analysis of σxx tensor component. NCS analysis allows the extraction of the contribution of

individual orbitals to individual principal components of σ tensor.66 Figure 6 shows the splitting of

the shielding tensor component along the X-axis (σxx) in its dia- and paramagnetic terms, and the

decomposition of the latter on the main frontier orbitals of the pyridine molecule localized on nitrogen according to equation 8. The NCS analysis shows that the diamagnetic contribution is similar for all systems. The variation of σxx is thus mainly associated with the paramagnetic term, and in

particular with the component associated to the nitrogen lone pair or non-bonding orbital – N(LP)

– coupled with the virtual π*-orbital via the operator 𝐿𝐿�𝑥𝑥 for models I-III. For the pyridinium ion IV,

the main component is replaced by the σ-bonding orbital σ(N-H), which has the same symmetry as

N(LP), as the main contribution to the paramagnetic shielding along the X-axis. All other orbitals in

structures I-IV have a very similar and small contribution.

Upon interaction of pyridine with either a weak Brønsted acid site (II) or a Lewis acid site (III)

and further upon protonation to pyridinium (IV), the component of the paramagnetic shielding

along the X-axis associated to the lone pair (or σN-H bond), decreases in absolute value, and hence

causes the increase of the σxx – from negative values (–407 ppm for free pyridine I) to slightly posi-

tive values (+42 ppm for pyridinium IV) (see Figure 6). As a result, the overall isotropic chemical shift decreases (see Δδiso on Figure 5).


24

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Figure 6. Histograms of orbital contributions to σxx tensor component for models I-IV obtained by

NCS analysis.

The change in chemical shift can be interpreted based on the frontier orbitals of the pyridine mol-

ecule and more specifically the orbital involving the lone pair, and their change in energy upon coordination. For models I to III, we calculate the density of states projected on the pyridine moiety

C5H5N (pDOS-pyridine) and specifically on the nitrogen atom (pDOS-N) in order to identify the orbitals involved in the chemical shift, namely N(LP) and π*(N) (Figure 7a).


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Page 26 of 39

Figure 7. a) Projected density of state on the pyridine moiety (pDOS-pyridine) and on the nitrogen atom (pDOS-N) on models I-III. The band-decomposed charge density associated to the energy

states assigned to the main frontier orbitals with a contribution from the nitrogen atom is also

shown (isosurface values are 2.5·10-2 e Å-3). Colour code is as follows: Al (pink), O (red), N (blue), C

(black), Si (pale yellow), Cl (lime) and H (white). b) Schematic representation of ∆𝑬𝑬𝒗𝒗𝒗𝒗𝒗𝒗𝒗𝒗−𝒐𝒐𝒐𝒐𝒐𝒐 for pyri-

dine adsorption on weak and strong acid site. σ* stands here for the orbital of the acid interacting

with pyridine (i.e. an empty orbital with a σ-type symmetry around the N-acid bond).

These orbitals can be distinguished in the density of state based on the contribution of the nitrogen

to the DOS as well as to the charge density projected on the associated energy bands (see ESI – the

frontier orbitals having no contribution from nitrogen are labeled π(C) and π*(C)). The diagrams in

Figure 7a show that the energy of the N(LP) is strongly affected by the interaction with an acid site, either HO–Si(OCH3)3 or AlCl3. In the latter case, the energy of N(LP) is even shifted lower than the π(C) orbital due to the strong binding. The energy of the π*(N) orbital is comparatively less af-

fected. As a result, the energy difference ∆𝐸𝐸𝑣𝑣𝑣𝑣𝑣𝑣𝑣𝑣−𝑜𝑜𝑜𝑜𝑜𝑜 increases from 3.99 eV in pyridine (I) to 4.29 eV

in II and 5.45 eV in III. Hence the corresponding term in equation 8 decreases in absolute value,


26

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which causes the change in shielding and thus in chemical shift described above. The situation can be schematically described as in Figure 7b.

For the pyridinium ion IV, the trend is still conserved when considering the σ(N-H) orbital in

place of the lone pair. In this respect, in the pyridinium ion, the pyridine-proton bond is stronger

than H-Py or Al-Py bonds in hydrogen-bonded pyridine and pyridine coordinated to Lewis acid site, respectively. As a consequence, the paramagnetic contribution is very weak and the σxx value even

becomes positive.

To summarize, the observed correlation between adsorption energy and 15N chemical shift δiso

has a molecular origin. In general, we show here that it mainly probes the strength of the A-N bond

(where A is the acid, Lewis or Brønsted). Thus, it is correlated to the acidic strength of the sites.

However, in the case of OH sites strong enough to protonate pyridine (as pyridinium), the acidic

strength also depends on the conjugated base stability, which has little influence on the 15N chemical shift. Thus pyridinium does not follow the same correlation. Conclusions

The adsorption of pyridine on γ-alumina was investigated by a combination of low-temperature

DNP surface enhanced NMR spectroscopy (DNP SENS), FTIR measurements and periodic DFT cal-

culations including the simulation of the spectroscopic features of the adsorbed pyridine and Natural Chemical Shielding (NCS) analysis. Four distinct groups of sites can be observed from the NMR and FTIR signatures of the adsorbed pyridine. These adsorbed species are also characterized by

specific desorption temperature, that is related to the adsorption energy of pyridine. However, the

dehydroxylation temperature has only a slight effect on the IR and NMR signatures of adsorbed pyridine, suggesting that in all cases the surface species result from the competitive adsorption of pyri-

dine and preadsorbed water on alumina so that pyridine is not an ideal for probing sites that are specifically generated upon dehydroxylation.


27


Page 28 of 39

However, the four types of sites, revealed by pyridine adsorption, can be assigned as follows: i)

the two types of sites – associated with low temperatures of pyridine desorption and the most

deshielded 15N chemical shifts and lower ν(8a) frequencies of adsorbed pyridine – are assigned to

chemisorbed hydroxyl group or water on alumina, which weakly interact with pyridine via H-bond-

ing (groups 1 and 2), ii) the sites, with the most strongly adsorbed pyridine – associated with the most shielded 15N chemical shift and the highest ν(8a) frequency – are assigned to AlIV and AlV

Lewis acid sites on the (110) facet, which strongly interact with pyridine via a Lewis acid–base in-

teraction (group 4), iii) the sites with intermediate pyridine adsorption strength – associated with

15N

chemical shift and a ν(8a) frequency in between those of the weakly and strongly adsorbed pyr-

idine – can be attributed to AlV sites of the (100) facet, but also to some hydroxyl group or chemi-

sorbed water on alumina (group 3). Thus, pyridine adsorption allows distinguishing Al Lewis acid sites from (110) and (100) facets and even selectively addressing Al sites on the (110) facet. This

study also shows that, while AlIII sites are not expected to display specific IR signature, one would expect to distinguish them from AlIV and AlV sites by the 15N NMR chemical shift of the adsorbed

pyridine. However, a specific – more shielded – signal for pyridine adsorbed on AlIII sites is not ob-

served experimentally by NMR under the conditions explored. This is in sharp contrast with previ-

ous studies, where N2, H2 and CH4 could selectively probe AlIII sites.21-22, 67 The difference may be rationalized by a very low concentration of these sites, that cannot be detected by NMR. However, as mentioned above the competitive adsorption of pyridine and preadsorbed water at the surface of

alumina may affect the location of hydroxyl group and chemisorbed water, and thereby lead to the disappearance of AlIII sites. This illustrates the difficulty to understand the surface of oxide sup-

ports, using probe molecules that might perturb the surface upon adsorption.

Noteworthy, the 15N chemical shift of H-bonded and coordinated pyridine almost linearly corre-

lates with the pyridine adsorption energy on these sites, while the one of protonated pyridine does not. Chemical shift analysis shows that the isotropic chemical shift of adsorbed pyridine is mostly ACS Paragon Plus Environment

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directed by one of the principal components of the chemical shift tensor, δxx, that is in plane of and

tangential to pyridine ring. The paramagnetic term of the δxx component is mostly associated with

the coupling of the lone pair on nitrogen and the π*-orbital of the pyridine ring located on nitrogen via the angular momentum operator. Its value thus decreases (shielding) with an increased energy difference between these two molecular orbitals (equation 8); since this energy difference in-

creases with the strength of pyridine adsorption, the sites with higher adsorption energy lead to more shielded 15N chemical shift for the adsorbed pyridine, hence the observed correlation be-

tween 15N chemical shift and adsorption energy. The exception is associated with the OH sites

where pyridine is protonated because the adsorption energy is now a combination of both N-H

bonding strength and the stability of the conjugated base. Nevertheless, the observed correlation

between adsorption energy and 15N chemical shift is likely general since it has a well-defined, mo-

lecular origin and can be thus extended to other oxides. This is currently under investigation. ASSOCIATED CONTENT

Supporting Information Materials and general procedures, surface models considered in the DFT studies, XRD patterns of

alumina samples, additional NMR and FTIR spectra, NCS analysis of σyy and σzz tensor components,

PDOS for molecular models I-III, additional experimental and computational data in Tables S1-S4. AUTHOR INFORMATION Corresponding Author *[email protected]

Notes

The authors declare no competing financial interest. ACKNOWLEDGMENT


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Page 30 of 39

I.B.M. is grateful to the Swiss Government Excellence Scholarship for financial support. K.L. is fi-

nanced in part from the SCCER Energy Storage. The work of W.C.L. is financially supported by SNF (200020_149704). We also acknowledge Dr. Paula Macarena Abdala (ETH Zürich) for XRD measurements and Christopher P. Gordon for helpful discussions.

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