PSI - Issue 68

D.M. Tshwane et al. / Procedia Structural Integrity 68 (2025) 39–46 D.M. Tshwane et al. / Structural Integrity Procedia 00 (2025) 000–000

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3.3. Charge density difference To gain insight on the characteristics of chemical bonding, the charge density distribution (CDD) plots were computed and are presented in Fig. 4. The molecules adsorption and contact with the surface, along with the electronic hybridization between its orbital and surface, resulted in the redistribution of charge density. Electron density difference plots are used to give exact details regarding the chemical reactivity and bonding that occur throughout the adsorption process. Eq. 3 defined below was used to determine the charge density difference: #/#012 = $/#&'( − $ − #&'( (3) where $/#&'( , $ and #&'( refer to the relative charge density for hydrogen adsorbed on the Ti 2 AlV surface, the non-interacting hydrogen atom, and the pure surface, respectively. Different colored portions illustrate the size of the charges; yellow denotes depletion while blue indicates build-up charges, as presented in Fig. 4. The charges were found to be more depleted from the surface atoms that interact with the H atoms. However, it was observed that for the Al site, no charge was depleted, suggesting weak interactions, while larger charge accumulation is observed mainly on Ti and V atoms, indicating a stronger interaction. The findings are consistent with the adsorption energy results discussed in Section 3.1. Furthermore, the observation of a directed bond with a spherical shape provides information on the behaviour of ionic interaction. Moreover, the observation of depletion is consistent with the role of the Ti 2 AlV surface as a reservoir for charges. This implies that the Ti 2 AlV surface material will suffer less corrosion through the Al interaction and more through the Ti-V sites. The Mulliken charges (e - ) and the bond population for the surface of the hydrogen adsorption of Ti 2 AlV (110) are listed in Table 1, which can be used to gauge charge accumulation and depletion in the atoms. The Ti and V atoms were found to be positively charged prior to adsorption, while the Al atom was negatively charged. After adsorption it was seen that both Ti and V become slightly positively charged while Al negatively charged. Furthermore, the magnitude of charge difference shows that Ti site charged more than V atoms, and this is a similar trend observed on adsorption energy strength.

Fig. 4. Charge density distribution (CDD) for hydrogen adsorption on Ti 2 AlV (110) surface: (a) Ti-site, (b) V-site, (c) hollow site, (d) bridge Al-V and (e) Al-site, with an iso-surface value of 0.03 eV/ Å ! .