Discovery of homogeneously dispersed pentacoordinated Al species on the surface of amorphous silica-alumina

The dispersion and coordination of aluminium species on the surface of silica-alumina based materials are essential for controlling their catalytic activity and selectivity. Al and Al are two common coordinations of Al species in the silica network and alumina phase, respectively. Al is rare in nature and was found hitherto only in the alumina phase or interfaces containing alumina, a behavior which negatively affects the dispersion, population, and accessibility of Al species on the silica-alumina surface. This constraint has limited the development of silicaalumina based catalysts, particularly because Al had been confirmed to act as a highly active center for acid reactions and single-atom catalysts. Here, we report the direct observation of high population of homogenously dispersed Al species in amorphous silica-alumina in the absence of any bulk alumina phase, by high resolution TEM/EDX and high magnetic-field MAS NMR. Solid-state Al multi-quantum MAS NMR experiments prove unambiguously that most of the Al species formed independently from the alumina phase and are accessible on the surface for guest molecules. These species are mainly transferred to Al species with partial formation of Al species after adsorption of water. The NMR chemical shifts and their coordination transformation with and without water adsorption are matching that obtained in DFT calculations of the predicted clusters. The discovery presented in this study not only provides fundamental knowledge of the nature of aluminum coordination, but also paves the way for developing highly efficient catalysts. Alumina and its mixed oxides are important catalytic materials both as active catalysts and as functional supports for active metal particles. The catalytic functions of these materials in chemical reactions are mainly dependent on the surface coordination of Al species due to their structure-activity relationship. Most research efforts have been focused on the tetrahedral and octahedral coordination (Al and Al), which are the most popular coordinations of Al species. Pentahedral coordination (Al) were rarely on alumina and silica-alumina and reported to be a transition state to octacoordinated aluminum species and generated during calcination of alumina. Recently, it has been reported that Al species on -alumina are surface active sites for stabilizing metal centers or nanoparticles to suppress sintering. The Al–metal (e.g. Ba, Cu, Ru, Au, Ag, Pt and Pd) interaction was proposed to improve the catalytic activity of metal centers in various reactions, such as CO2 reduction and deNO(x) reactions. Therefore, Al V species have recently attracted great attention, particularly, they were proposed to be coordinatively unsaturated surface centers of supports for anchoring noble metal atoms for emerging single-atom catalysts. However, the previous reports showed that Al species are not highly populating the surface. Although a certain amount of Al species has been found to be generated during phase transformation from -Al2O3 to -Al2O3 (up to 17 %), 1,8,9 only a very small amount of Al species (< 2 at.% ) was stabilized on the surface of -Al2O3, when suitable hydroxides or oxides such as La2O3 and BaO were added to inhibit the phase transformation and stabilize the unsaturated Al ions. It was also reported that Al species only exist nearby Al species on the surface of crystalline Al2O3 and might represent Al VI in the vicinity of an oxygen vacancy, such as the defect spinel structures of the transition aluminas. For silica-aluminas, Al species were proposed to be located on the interface between alumina and silica or alumina and aluminosilicates. Therefore, Al species on Al2O3 or corresponding interfaces were assumed to be poorly distributed on the surface and hardly available for the reactants, which reinforced the doubt that Al species are promising active centers for building highperformance catalysts such as emerging solid acids, single-atom catalysts and spatially confined catalysts due to their poor accessibility. In this study, we discovered a new type of Al species in amorphous silica-alumina (ASA). It is homogeneously distributed and highly populates the surface, thus providing high accessibility for guest molecules. The ASA and Al2O3 nano-particles were prepared by flame spray pyrolysis as described previously, which could offer strong acidity. All prepared particles had a size around 5-10 nm and their crystalline or amorphous structure has been verified by XRD. As shown in Fig. S1, both ASA samples, containing 10 and 30 at.% of aluminum (SA/10 and SA/30), only showed a broad reflection at 22-23 due to amorphous silica. As a reference, a pure Al2O3 sample SA/100 obtained without adding silica during synthesis has been investigated by XRD, as also shown in Fig. 1. Broad and weak signals of crystalline Al2O3 were observed for SA/100 due to its small particle size. The very fine nanoparticles with well-ordered lattice structure did not show significant diffraction and could therefore not be identified by XRD. High-resolution transmission electron microscopy (HRTEM) has also been applied to examine the existence of alumina phase domains in ASAs. 16 For SA/100, the image clearly revealed the well-ordered alumina lattice in Fig. 2a. For ASA samples, the alumina lattice disappeared, and the amorphous nature of both samples was corroborated, as shown in the HRTEM images in Fig. 2b and c. No small alumina clusters were detected on the ASA surface. As revealed by EDX atom mapping images, Al species were homogeneously distributed in the silica network of SA/10 (Fig. 2d-e) and SA/30 (Fig. 2f and g), again, no aggregated aluminum species or small alumina nanoparticles were observed on the surface. Figure 1. XRD pattern of SA/10, SA/30 and Al2O3. 10 20 30 40 50 60 70 SA/10 In te n s it y / a .u . 2 Theta / degree Al2O3