Spectroscopic Fingerprints of MgCl₂/TiCl₄ Nanoclusters Determined by Machine Learning and DFT

Abstract

Understanding the structure and properties of MgCl2/TiCl4 nanoclusters is a key to uncovering the origin of Ziegler–Natta catalysis. In particular, vibrational spectroscopy can sensitively probe the morphology and active species of MgCl2/TiCl4. Here, we determined vibrational spectroscopic fingerprints of 50MgCl2 and 50MgCl2/3TiCl4 which were obtained by nonempirical structure determination based on an evolutionary algorithm and DFT. The adsorption of CO, TiCl4, and Ti2Cl8 dimers was also modeled on each of the coordinatively unsaturated Mg2+ sites available for binding including so-called defect sites, which are likely present at the surface of activated MgCl2 nanocrystals and plausible sites for strong TiCl4 species adsorption. The outcomes of thermodynamical and vibrational analysis were compared to results on ideal surfaces of MgCl2. Vibrational analysis (IR and Raman) on plausible models of TiCl4/ MgCl2 nanoclusters revealed that IR response is useful for distinguishing between the different ways of binding of TiCl4 on different sites of adsorption, whereas Raman response provides a clear fingerprint of supported TiCl4 species.

Effect of Internal Donors on Raman and IR Spectroscopic Fingerprints of MgCl2/TiCl4 Nanoclusters Determined by Machine Learning and DFT

“…The adsorption energy reveals that the binding of EB is only tiny sensitive to the site of adsorption due to the frontier behaviour of sites. In particular, in the case of model B, where the proximity of TiCl 4 increases the polarizing ability of pentacoordinated Mg 2+ (as we already found in probing cluster sites by CO [55]), the adsorption energy almost equals that of model A. In the case of homogeneous-like complexes, the complex with two EB molecules (model E) is by far more strongly bound to the support, in fact in mono-EB case (models D, G), a large distortion occurs with respect to the geometry adopted from molecule of EB in the gas phase with a cost amounting to 34.5 kJ/mol.…”

“…To inspect the molecular vibrational shifts of main modes involving CO group, O-C-O and TiCl 4 due to both the interaction with the support and the reciprocal interaction of organic and inorganic molecules, we run geometry optimizations followed by harmonic vibrational frequency calculations of the molecular adducts together with the corresponding IR and Raman intensities. Details about the computational set up and the calculations of the vibrational frequencies can be found in previuos works by us [55,78,79].…”

“…Concerning the IR response of MgCl 2 /TixCl y nanoplatelets, it covers a region between 200 and 500 cm −1 [55]; altough the 200-400 cm −1 region refers to bulk modes, strongly dependent on the particle shape and size, the 400-500 cm −1 region contains fingerprints that can be easily correlated to surface sites and adorbed species: peaks at 429 and 445 cm −1 related to exposed tetracoordinated Mg 2+ typical of 110 surfaces, a couple of bands at 465 and 485 cm −1 attributed to symmetric and antisymmetric stretching of Ti-Cl bonds in supported TiCl 4 and 458, 476 and 495 cm −1 bands for supported Ti 2 Cl 8 .…”

“…Inelastic Neutron Scattering (INS) spectroscopy [54], able to sample all the first Brillouin zone (FBZ) and thus predicting the dynamical behaviour of materials, proved to be a powerful method to understand nano-sized systems and measure the degree of disorder of δ-MgCl 2 in terms of both translational and rotational contribution. A step forward to determine the morphology of platelets, with dimensions comparable to those of primary particles typically adopted in industrial catalysis (2-3 nm), have been made by us thanks to the adoption of a genethic algorithm; the calculations have been performed both in presence and in absence of TiCl 4 [55]. Differently than in the case of naked MgCl 2 where the evolutionary plots allowed us to identify structures where the majority of sites are pentacoordinated Mg 2+ sites, in presence of TiCl 4 (in a ratio 50/3 MgCl 2 /TiCl 4 ) couples of tetracoordinated sites are present after the reconstrution of nano-particles.…”

“…Starting from model nanoplatelets (50MgCl 2 and 50MgCl 2 /3TiCl 4 ) identified by nonempirical structure prediction based on the genetic algorithm and DFT, as reported in recent papers of us [55,57,58], different ways of binding were proposed and analysed. Vibrational and Raman simulations were carried out to identify whether or not it is possible to identify clear fingerprints of many possible adducts donor/TiCl 4 on nano-shaped MgCl 2 .…”