Influence of strong π-acceptor ligands on Cr-K-edge X-ray absorption spectral signatures and consequences for the interpretation of surface sites in the Phillips catalyst.

X-ray absorption spectroscopy (XAS) has been central to the study of the Phillips polymerization catalyst (CrO3/SiO2). As Cr K-edge XAS signatures are sensitive to the oxidation state, geometry and types of ligands on surface (active) sites, the superposition of these effects makes their interpretation challenging. Notably, CO has been particularly used as a reductant to generate low valent Cr sites from CrO3/SiO2 and as a structural IR probe for analysing reduced Cr surface sites. Hence, it is essential to establish a solid understanding of the spectroscopic impact of CO on low-valent Cr sites. We thus built a series of fully characterized low-valent Cr molecular compounds bearing isoelectronic isocyanide ligands in place of CO, with the goal of understanding the effect of the coordination of π-acceptor ligands on the XANES signature of Cr sites. Cr K-edge spectra supplemented with DFT calculations elucidate the effect of the coordination of π-acceptor ligands on XAS signatures, giving a sharp resonance at the white line while modifying the fine structure due to short Cr-C distances and stability of low-spin Cr(ii/iii) species. The isocyanide references allow the deconvolution of the XAS spectra of the reduced CrO3/SiO2 catalyst by evaluating the types of surface species and relative amounts of bound CO at different CO pressures and temperatures.

Origin of Reactivity Trends of an Elusive Metathesis Intermediate from NMR Chemical Shift Analysis of Surrogate Analogues.

Olefin metathesis has become an efficient tool in synthetic organic chemistry to build carbon-carbon bonds, thanks to the development of Grubbs- and Schrock-type catalysts. Olefin coordination, a key and often rate-determining elementary step for d0 Schrock-type catalysts, has been rarely explored due to the lack of accessible relevant molecular analogues. Herein, we present a fully characterized surrogate of this key olefin-coordination intermediate, namely, a cationic d0 tungsten oxo-methylidene complex bearing two N-heterocyclic carbene ligands─[WO(CH2)Cl(IMes)2](OTf) (1) (IMes = 1,3-dimesitylimidazole-2-ylidene, OTf-triflate counteranion), resulting in a trigonal bipyramidal (TBP) geometry, along with its neutral octahedral analogue [WO(CH2)Cl2(IMes)2] (2)─and an isostructural oxo-methylidyne derivative [WO(CH)Cl(IMes)2] (3). The analysis of their solid-state 13C and 183W MAS NMR signatures, along with computed 17O NMR parameters, helps to correlate their electronic structures with NMR patterns and evidences the importance of the competition among the three equatorial ligands in the TBP complexes. Anchored on experimentally obtained NMR parameters for 1, computational analysis of a series of olefin coordination intermediates highlights the interplay between σ- and π-donating ligands in modulating their stability and further paralleling their reactivity. NMR spectroscopy descriptors reveal the origin for the advantage of the dissymmetry in σ-donating abilities of ancillary ligands in Schrock-type catalysts: weak σ-donors avoid the orbital-competition with the oxo ligand upon formation of a TBP olefin-coordination intermediate, while stronger σ-donors compromise M≡O triple bonding and thus render olefin coordination step energy demanding.

Probing Homogeneous Catalysts and Precatalysts in Solution by Exchange-Mediated Overhauser Dynamic Nuclear Polarization NMR.

Triphenylphosphine (PPh3) is a ubiquitous ligand in organometallic chemistry that has been shown to give enhanced 31P NMR signals at high magnetic field via a scalar-dominated Overhauser effect dynamic nuclear polarization (OE DNP). However, PPh3 can only be polarized via DNP in the free form, while the coordinated form is DNP-inactive. Here, we demonstrate the possibility of enhancing the 31P NMR signals of coordinated PPh3 in metal complexes in solution at room temperature by combining Overhauser effect DNP and chemical exchange between the free and coordinated PPh3 forms. With this method, we successfully obtain 31P DNP enhancements of up to 2 orders of magnitude for the PPh3 ligands in Rh(I), Ru(II), Pd(II), and Pt(II) complexes, and we show that the DNP enhancements can be used to determine the activation energy of the ligand exchange reaction.

Reactivity Switch of Platinum with Gallium: From Reverse Water Gas Shift to Methanol Synthesis.

The development of efficient catalysts for the hydrogenation of CO2 to methanol using "green" H2 is foreseen to be a key step to close the carbon cycle. In this study, we show that small and narrowly distributed alloyed PtGa nanoparticles supported on silica, prepared via a surface organometallic chemistry (SOMC) approach, display notable activity for the hydrogenation of CO2 to methanol, reaching a 7.2 molCH3OH h-1 molPt-1 methanol formation rate with a 54% intrinsic CH3OH selectivity. This reactivity sharply contrasts with what is expected for Pt, which favors the reverse water gas shift reaction, albeit with poor activity (2.6 molCO2 h-1 molPt-1). In situ XAS studies indicate that ca. 50% of Ga is reduced to Ga0 yielding alloyed PtGa nanoparticles, while the remaining 50% persist as isolated GaIII sites. The PtGa catalyst slightly dealloys under CO2 hydrogenation conditions and displays redox dynamics with PtGa-GaOx interfaces responsible for promoting both the CO2 hydrogenation activity and methanol selectivity. Further tailoring the catalyst interface by using a carbon support in place of silica enables to improve the methanol formation rate by a factor of ∼5.

Metadynamics simulations reveal alloying-dealloying processes for bimetallic PdGa nanoparticles under CO2 hydrogenation.

Supported bimetallic nanoparticles (NPs) often display improved catalytic performances (activity and/or selectivity). Yet, structure-activity relationships are difficult to derive due to the multitude of possible compositions, interfaces and alloys. This is notably true for bimetallic NPs used in the selective hydrogenation of CO2 to methanol, where the NPs respond dynamically to the chemical potential of the reactants and products. Herein, we use a combined computational and experimental approach that leverages ab initio Molecular Dynamics (AIMD) and Metadynamics (MTD) in conjunction with in situ X-ray absorption spectroscopy, chemisorption and CO-IR, to explore the dynamic structures and interactions with adsorbates under various CO2 hydrogenation conditions in highly active and selective silica-supported PdGa NPs. We find that PdGa alloying generates isolated Pd sites at the NP surface, changing the dominant binding modes of relevant adsorbates compared to pure Pd NPs: CO molecules mainly occupy atop sites and hydrides switch from mainly internal to atop and bridge sites. Under more oxidizing conditions, akin to CO2 hydrogenation, Ga is partially oxidized, forming a GaOX layer on the NP surface, with a partially dealloyed PdGa core and some remaining isolated Pd surface sites. Overall, these bimetallic NPs show high structural dynamics and a variable extent of alloying depending on the adsorbates relevant to CO2 hydrogenation. This work highlights that AIMD/MTD is a powerful approach to elucidate structural dynamics at a single particle level in complex catalytic systems.

Tracking Coordination Environment and Reaction Intermediates in Homogeneous and Heterogeneous Epoxidation Catalysts via Ti L2,3-Edge Near-Edge X-ray Absorption Fine Structures.

Ti-based molecules and materials are ubiquitous and play a major role in both homogeneous and heterogeneous catalytic processes. Understanding the electronic structures of their active sites (oxidation state, local symmetry, and ligand environment) is key to developing molecular-level structure-property relationships. In that context, X-ray absorption spectroscopy (XAS) offers a unique combination of elemental selectivity and sensitivity to local symmetry. Commonly, for early transition metals such as Ti, K-edge XAS is applied for in situ characterization and subsequent structural analysis with high sensitivity toward tetrahedral species. Ti L2,3-edge spectroscopy is in principle complementary and offers specific opportunities to interrogate the electronic structure of five-and six-coordinated species. It is, however, much more rarely implemented because the use of soft X-rays implies ultrahigh vacuum conditions. Furthermore, the interpretation of the data can be challenging. Here, we show how Ti L2,3-edge spectroscopy can help to obtain unique information about both homogeneous and heterogeneous epoxidation catalysts and develop a molecular-level relationship between spectroscopic signatures and electronic structures. Toward this goal, we first establish a spectral library of molecular Ti reference compounds, comprising various coordination environments with mono- and dimeric Ti species having O, N, and Cl ligands. We next implemented a computational methodology based on multiplet ligand field theory and maximally localized Wannier orbitals benchmarked on our library to understand Ti L2,3-edge spectroscopic signatures. We finally used this approach to track and predict the spectra of catalytically relevant intermediates, focusing on Ti-based olefin epoxidation catalysts.

Well-Defined Ti Surface Sites in Ziegler-Natta Pre-Catalysts from 47/49Ti Solid-State Nuclear Magnetic Resonance Spectroscopy.

Treatment of Ziegler-Natta (ZN) catalysts with BCl3 improves their activity by increasing the number of active sites. Here we show how 47/49Ti solid-state nuclear magnetic resonance (NMR) spectroscopy enables us to understand the electronic structure of the Ti surface sites present in such treated ZN pre-catalysts, prior to activation with alkyl aluminum. High-field (21.1 T) and low-temperature (∼100 K) NMR augmented by DFT modeling on the pre-catalyst and corresponding molecular analogues enables the detection of 47/49Ti NMR signatures and a molecular level understanding of the electronic structure of Ti surface sites. The associated Ti surface sites exhibit 49Ti NMR signatures (δiso, exp = -170 ppm; CQ, exp = 9.3 MHz; κ = 0.05) corresponding to well-defined fully chlorinated hexacoordinated Ti sites adsorbed on a distorted surface of the MgCl2 support, formed upon post-treatment with BCl3 and removal of the alkoxo ligands, paralleling the increased polymerization activity.

NMR Spectroscopic Signatures of Cationic Surface Sites from Supported Coinage Metals Interacting with N-Heterocyclic Carbenes.

N-heterocyclic carbenes (NHCs) have been extensively studied to modulate the reactivity of molecular catalysts, colloids, and their supported analogues, being isolated sites, clusters, or nanoparticles. While the interaction of NHCs on metal surfaces has been discussed in great detail, showing specific coordination chemistry depending on the type of NHC ligands, much less is known when the metal is dispersed on oxide supports, as in heterogeneous catalysts. Herein, we study the interaction of NHC ligands with Au surface sites dispersed on silica, a nonreducible oxide support. We identify the easy formation of bis-NHC ligated Au(I) surface sites parallel to what is found on metallic Au surfaces. These species display a specific 13C NMR spectroscopic signature that clearly distinguishes them from the mono-NHC Au(I) surface sites or supported imidazoliums. We find that bis-ligated surface species are not unique to supported Au(I) species and are found for the corresponding Ag(I) and Cu(I) species, as well as for the isolobal surface silanols. Furthermore, the interaction of NHC ligand with silica-supported Au nanoparticles also yields bis-NHC ligated Au(I) surface sites, indicating that metal atoms can also be easily extracted from nanoparticles, further illustrating the dynamics of these systems and the overall favorable formation of such bis-ligated species across a range of systems, besides what has been found on crystalline metal facets.

Bonding and Reactivity of d0 Transition Metal Imido Complexes Encoded in Their 15N NMR Signatures.

Terminal imido complexes containing metal-nitrogen multiple bonds have been widely used in organometallic chemistry and homogeneous catalysis. The role of terminal imido ligands spans from reactive sites to spectator motifs, largely depending on the nature of the metal center and its specific coordination sphere. Aiming at identifying reactivity descriptors for M-N multiple bonds, we herein explore solid-state 15N NMR spectroscopy (ssNMR) on early transition metal terminal imido complexes augmented by computational studies and show that the asymmetry parameter, κ (skew, 1 ≥ κ ≥ -1), readily available from experiments or calculations, is diagnostic for the reactivity of M-N multiple bonds in imido complexes. While inert imido ligands exhibit skew values (κ) close to 1, highly reactive imido moieties display significantly lower skew values (κ ≪ 1) as found in metallocene or bis-imido complexes. Natural chemical shielding analysis shows that skew values away from 1 are associated with an asymmetric development of π-orbitals around the M-N multiple bond of the imido moiety, with a larger double-bond character for reactive imido. Notably, this descriptor does not directly relate to the M-N-C bond angle, illustrating the shortcoming of evaluating bonding and hybridization from geometrical parameters alone. Overall, this descriptor enables to obtain direct experimental evidence for the π-loading effect seen in bis(imido) and related complexes, thus explaining their bonding/reactivity.

Geometry and electronic structure of Yb(III)[CH(SiMe3)2]3 from EPR and solid-state NMR augmented by computations.

Characterization of paramagnetic compounds, in particular regarding the detailed conformation and electronic structure, remains a challenge, and - still today it often relies solely on the use of X-ray crystallography, thus limiting the access to electronic structure information. This is particularly true for lanthanide elements that are often associated with peculiar structural and electronic features in relation to their partially filled f-shell. Here, we develop a methodology based on the combined use of state-of-the-art magnetic resonance spectroscopies (EPR and solid-state NMR) and computational approaches as well as magnetic susceptibility measurements to determine the electronic structure and geometry of a paramagnetic Yb(III) alkyl complex, Yb(III)[CH(SiMe3)2]3, a prototypical example, which contains notable structural features according to X-ray crystallography. Each of these techniques revealed specific information about the geometry and electronic structure of the complex. Taken together, both EPR and NMR, augmented by quantum chemical calculations, provide a detailed and complementary understanding of such paramagnetic compounds. In particular, the EPR and NMR signatures point to the presence of three-centre-two-electron Yb-γ-Me-ß-Si secondary metal-ligand interactions in this otherwise tri-coordinate metal complex, similarly to its diamagnetic Lu analogues. The electronic structure of Yb(III) can be described as a single 4f13 configuration, while an unusually large crystal-field splitting results in a thermally isolated ground Kramers doublet. Furthermore, the computational data indicate that the Yb-carbon bond contains some π-character, reminiscent of the so-called α-H agostic interaction.

109Ag NMR chemical shift as a descriptor for Brønsted acidity from molecules to materials.

Molecular-level understanding of the acid/base properties of heterogeneous catalysts requires the development of selective spectroscopic probes to establish structure-activity relationships. In this work we show that substituting the surface protons in oxide supports by isolobal N-heterocyclic carbene (NHC) Ag cations and measuring their 109Ag nuclear magnetic resonance (NMR) signatures enables to probe the speciation and to evaluate the corresponding Brønsted acidity of the substituted OH surface sites. Specifically, a series of silver N-heterocyclic carbene (NHC) Ag(i) complexes of general formula [(NHC)AgX] are synthesized and characterized, showing that the 109Ag NMR chemical shift of the series correlates with the Brønsted acidity of the conjugate acid of X- (i.e., HX), thus establishing an acidity scale based on 109Ag NMR chemical shift. The methodology is then used to evaluate the Brønsted acidity of the OH sites of representative oxide materials using Dynamic Nuclear Polarization (DNP-)enhanced solid-state NMR spectroscopy.

Solid-State 183W NMR Spectroscopy as a High-Resolution Probe of Polyoxotungstate Structures and Dynamics.

Polyoxometalates such as ammonium paratungstate (APT) constitute an important class of metal oxides with applications for catalysis, (opto)electronics, and functional materials. Structural analyses of solid polyoxometalates mostly rely on X-ray or neutron diffraction techniques, which are largely limited to compounds that can be isolated with long-range crystallographic order. While 183W NMR has been shown to probe polyoxotungstate structures and dynamics in solution, its application to solids has been extremely limited. Here, state-of-the-art methods for the detection of solid-state 183W NMR spectra are tested and compared for APT in different hydration states. The highly resolved solid-state spectra distinguish each crystallographically distinct site in the tungstate structure. Furthermore, the 183W chemical shifts are shown to be highly sensitive to the local structure, dynamics, and symmetry of APT, establishing solid-state 183W NMR spectroscopy as a potent probe for analysis of polyoxotungstates and other tungsten-derived materials to complement solution NMR and diffraction-based techniques.

Structure and Role of a Ga-Promoter in Ni-Based Catalysts for the Selective Hydrogenation of CO2 to Methanol.

Supported, bimetallic catalysts have shown great promise for the selective hydrogenation of CO2 to methanol. In this study, we decipher the catalytically active structure of Ni-Ga-based catalysts. To this end, model Ni-Ga-based catalysts, with varying Ni:Ga ratios, were prepared by a surface organometallic chemistry approach. In situ differential pair distribution function (d-PDF) analysis revealed that catalyst activation in H2 leads to the formation of nanoparticles based on a Ni-Ga face-centered cubic (fcc) alloy along with a small quantity of GaOx. Structure refinements of the d-PDF data enabled us to determine the amount of both alloyed Ga and GaOx species. In situ X-ray absorption spectroscopy experiments confirmed the presence of alloyed Ga and GaOx and indicated that alloying with Ga affects the electronic structure of metallic Ni (viz., Niδ-). Both the Ni:Ga ratio in the alloy and the quantity of GaOx are found to minimize methanation and to determine the methanol formation rate and the resulting methanol selectivity. The highest formation rate and methanol selectivity are found for a Ni-Ga alloy having a Ni:Ga ratio of ∼75:25 along with a small quantity of oxidized Ga species (0.14 molNi-1). Furthermore, operando infrared spectroscopy experiments indicate that GaOx species play a role in the stabilization of formate surface intermediates, which are subsequently further hydrogenated to methoxy species and ultimately to methanol. Notably, operando XAS shows that alloying between Ni and Ga is maintained under reaction conditions and is key to attaining a high methanol selectivity (by minimizing CO and CH4 formation), while oxidized Ga species enhance the methanol formation rate.

Active Sites in Cr(III)-Based Ethylene Polymerization Catalysts from Machine-Learning-Supported XAS and EPR Spectroscopy.

The ethylene polymerization Phillips catalyst has been employed for decades and is central to the polymer industry. While Cr(III) alkyl species are proposed to be the propagating sites, there is so far no direct experimental evidence for such proposal. In this work, by coupling Surface organometallic chemistry, EPR spectroscopy, and machine learning-supported XAS studies, we have studied the electronic structure of well-defined silica-supported Cr(III) alkyls and identified the presence of several surface species in high and low-spin states, associated with different coordination environments. Notably, low-spin Cr(III) sites are shown to participate in ethylene polymerization, indicating that similar Cr(III) alkyl species could be involved in the related Phillips catalyst.

Non-Oxidative Coupling of Methane: Interplay of Catalyst Interface and Gas Phase Mechanisms.

Non-oxidative coupling of methane (NOCM) is a sought-after reaction that has been studied for decades. Harsh reaction conditions (T >800°C) in the face of limited catalyst stability lead to rapid catalyst deactivation and strong coking, preventing application thus far. Recent reports have shown the significance of an interplay of catalyst nature and reaction conditions, whereas metal carbides have prevailed to play a crucial role which involves incorporation of carbidic carbon in C2Hx and aromatic products. This perspective gives an overview of proposed mechanistic pathways and considerations about experiment conditions in order to foster a rational catalyst design platform for NOCM.

Swiss CAT+, a Data-driven Infrastructure for Accelerated Catalysts Discovery and Optimization.

The Catalysis Hub - Swiss CAT+ is a new infrastructure project funded by ETH-domain, co-headed by EPFL and ETHZ. It offers the scientific community a unique integrated technology platform combining automated and high-throughput experimentation with advanced computational data analysis to accelerate the discoveries in the field of sustainable catalytic technologies. Divided into two hubs of expertise, homogeneous catalysis at EPFL and heterogeneous catalysis at ETHZ, the platform is open to academic and private research groups. Following a multi-year investment plan, both hubs have acquired and developed several high-end robotic platforms devoted to the synthesis, characterization, and testing of large numbers of molecular and solid catalysts. The hardware is associated with a fully digitalized experimental workflow and a specific data management strategy to support closed-loop experimentation and advanced computational data analysis.

Geometry and Local Environment of Surface Sites in Vanadium-Based Ziegler-Natta Catalysts from 51V Solid-State NMR Spectroscopy.

Since its emergence over 50 years ago, the structure of surface sites in Ziegler-Natta catalysts, which are responsible for a major fraction of the world's supply of polyethylene (PE) and polypropylene (PP), has remained elusive. This is in part due to the complexity of these systems that involve multiple synthetic steps and components, namely, the MgCl2 support, a transition-metal chloride, and several organic modifiers, known as donors, that are used prior and in some instances during the activation step with alkyl aluminum. Due to the favorable nuclear magnetic resonance (NMR) properties of V and its use in Ziegler-Natta catalysts, we utilize 51V solid-state NMR spectroscopy to investigate the structure of VOCl3 on MgCl2(thf)1.5. The resulting catalyst shows ethylene polymerization activity similar to that of its Ti analogues. Using carefully benchmarked density functional theory (DFT) calculations, the experimental 51V NMR signature was analyzed to elucidate the structure of the surface sites. Using this approach, we demonstrate that the 51V NMR signature contains information about the coordination environment, i.e., the type of ancillary ligand, and the morphology of the MgCl2 support. Analysis of the NMR signature shows that the adsorption of VOCl3 on MgCl2(thf)1.5 generates a well-defined hexacoordinated V-oxo species containing one alkoxy and four chloride ligands, whose local geometry results from the interaction with an amorphous MgCl2 surface. This study illustrates how NMR spectroscopy, which is highly sensitive to the local environment of the investigated nuclei, here V, enables us to identify the exact coordination sphere and to address the effect of the support morphology on surface site structures.

Small Cobalt Nanoparticles Favor Reverse Water-Gas Shift Reaction Over Methanation Under CO2 Hydrogenation Conditions.

Cobalt-based catalysts are well-known to convert syngas into a variety of Fischer-Tropsch (FTS) products depending on the various reaction parameters, in particular particle size. In contrast, the reactivity of these particles has been much less investigated in the context of CO2 hydrogenation. In that context, Surface organometallic chemistry (SOMC) was employed to synthesize highly dispersed cobalt nanoparticles (Co-NPs) with particle sizes ranging from 1.6 to 3.0 nm. These SOMC-derived Co-NPs display significantly different catalytic performances under CO2 hydrogenation conditions: while the smallest cobalt nanoparticles (1.6 nm) catalyze mainly the reverse water-gas shift (rWGS) reaction, the larger nanoparticles (2.1-3.0 nm) favor the expected methanation activity. Operando X-ray absorption spectroscopy shows that the smaller cobalt particles are fully oxidized under CO2 hydrogenation conditions, while the larger ones remain mostly metallic, paralleling the observed difference of catalytic performances. This fundamental shift of selectivity, away from methanation to reverse water-gas shift for the smaller nanoparticles is noteworthy and correlates with the formation of CoO under CO2 hydrogenation conditions.

Implications of Ga promotion and metal-oxide interface from tailored PtGa propane dehydrogenation catalysts supported on carbon.

Propane Dehydrogenation is a key technology, where Pt-based catalysts have widely been investigated in industry and academia, with development exploring the use of promoters (Sn, Zn, Ga, etc.) and additives (Na, K, Ca, Si, etc.) towards improved catalytic performances. Recent studies have focused on the role of Ga promotion: while computations suggest that Ga plays a key role in enhancing catalytic selectivity and stability of PtGa catalysts through Pt-site isolation as well as morphological changes, experimental evidence are lacking because of the use of oxide supports that prevent more detailed investigation. Here, we develop a methodology to generate Pt and PtGa nanoparticles with tailored interfaces on carbon supports by combining surface organometallic chemistry (SOMC) and specific thermolytic molecular precursors containing or not siloxide ligands. This approach enables the preparation of supported nanoparticles, exhibiting or not an oxide interface, suitable for state-of-the art electron microscopy and XANES characterization. We show that the introduction of Ga enables the formation of homogenously alloyed, amorphous PtGa nanoparticles, in sharp contrast to highly crystalline monometallic Pt nanoparticles. Furthermore, the presence of an oxide interface is shown to stabilize the formation of small particles, at the expense of propene selectivity loss (formation of cracking side-products, methane/ethene), explaining the use of additives such as Na, K and Ca in industrial catalysts.

Nature of GaOx Shells Grown on Silica by Atomic Layer Deposition.

Gallia-based shells with a thickness varying from a submonolayer to ca. 2.5 nm were prepared by atomic layer deposition (ALD) using trimethylgallium, ozone, and partially dehydroxylated silica, followed by calcination at 500 °C. Insight into the atomic-scale structure of these shells was obtained by high-field 71Ga solid-state nuclear magnetic resonance (NMR) experiments and the modeling of X-ray differential pair distribution function data, complemented by Ga K-edge X-ray absorption spectroscopy and 29Si dynamic nuclear polarization surface enhanced NMR spectroscopy (DNP SENS) studies. When applying one ALD cycle, the grown submonolayer contains mostly tetracoordinate Ga sites with Si atoms in the second coordination sphere ([4]Ga(Si)) and, according to 15N DNP SENS using pyridine as the probe molecule, both strong Lewis acid sites (LAS) and strong Brønsted acid sites (BAS), consistent with the formation of gallosilicate Ga-O-Si and Ga-µ2-OH-Si species. The shells obtained using five and ten ALD cycles display characteristics of amorphous gallia (GaOx), i.e., an increased relative fraction of pentacoordinate sites ([5]Ga(Ga)), the presence of mild LAS, and a decreased relative abundance of strong BAS. The prepared Ga1-, Ga5-, and Ga10-SiO2-500 materials catalyze the dehydrogenation of isobutane to isobutene, and their catalytic performance correlates with the relative abundance and strength of LAS and BAS, viz., Ga1-SiO2-500, a material with a higher relative fraction of strong LAS, is more active and stable compared to Ga5- and Ga10-SiO2-500. In contrast, related ALD-derived Al1-, Al5-, and Al10-SiO2-500 materials do not catalyze the dehydrogenation of isobutane and this correlates with the lack of strong LAS in these materials that instead feature abundant strong BAS formed via the atomic-scale mixing of Al sites with silica, leading to Al-µ2-OH-Si sites. Our results suggest that [4]Ga(Si) sites provide strong Lewis acidity and drive the dehydrogenation activity, while the appearance of [5]Ga(Ga) sites with mild Lewis activity is associated with catalyst deactivation through coking. Overall, the atomic-level insights into the structure of the GaOx-based materials prepared in this work provide a guide to design active Ga-based catalysts by a rational tailoring of Lewis and Brønsted acidity (nature, strength, and abundance).