Jithin John Varghese

Using periodic Density Functional Theory calculations, propane oxidative dehydrogenation (ODH) and overoxidation over bare and hydroxylated Ni-doped CeO2 nanorods with predominantly exposed (1 1 0) facets were studied. Ab-initio thermodynamics-based surface phase analysis and computational Raman spectroscopic analysis predicted an 8.3 % surface oxygen vacancy concentration (Ce0.83Ni0.17O1.83) at typical ODH conditions. Only one-third of the surface oxygens, adjacent to the dopant, were selective for propene formation. Moreover, activated oxygen (O22–*) favored the formation of overoxidation products over propene. A monolayer hydroxyl coverage from water dissociation was stable at typical ODH conditions. This reduced the activation energy barrier for propene formation by 0.38 eV, increased the barrier for undesired acetone formation by 0.54 eV, and increased the barrier for propene activation by 0.6 eV. These promotional effects were due to the destabilization and induced hyperconjugation effects in the C3 adsorbates due to surface hydroxylation. Hence, surface hydroxylation (Lewis Base addition) is a potential strategy to improve propene selectivity.

Modern heterogeneous catalysis has benefitted immensely from computational predictions of catalyst structure and its evolution under reaction conditions, first-principles mechanistic investigations, and detailed kinetic modeling, which are rungs on a multiscale workflow. Establishing connections across these rungs and integration with experiments have been challenging. Here, operando catalyst structure prediction techniques using density functional theory simulations and ab initio thermodynamics calculations, molecular dynamics, and machine learning techniques are presented. Surface structure characterization by computational spectroscopic and machine learning techniques is then discussed. Hierarchical approaches in kinetic parameter estimation involving semi-empirical, data-driven, and first-principles calculations and detailed kinetic modeling via mean-field microkinetic modeling and kinetic Monte Carlo simulations are discussed along with methods and the need for uncertainty quantification. With these as the background, this article proposes a bottom-up hierarchical and closed loop modeling framework incorporating consistency checks and iterative refinements at each level and across levels.

Propane oxidative dehydrogenation (ODH) was studied over VOx/CeO2 nanoparticle catalysts for various vanadia nuclearities using Density Functional Theory simulations. On monomers and dimers, propene formation involved V=O, bridged (V–O–Ce, V–OV) and ceria surface oxygens but with low selectivity, while on trimers and possibly higher oligomers it was kinetically favourable over ceria surface oxygens, with higher selectivity than on monomers and dimers. On trimer, overoxidation via acetone formation was inhibited, but at the expense of overall catalyst reactivity. Doping of ceria support with Ni induced strong Ni–VOx interactions, stabilizing the vanadia monomer. The V–O–Ni bridged oxygen was less accessible for overoxidation, resulting in an increase in activation energy barrier for acetone formation by 0.69 eV on VO2/Ce0.89Ni0.11O2(1 1 1), while there was a minimal impact on V3O6/Ce0.89Ni0.11O2 (1 1 1). Hence, transition metal doping of ceria support is a potential strategy to improve propene selectivity over VOx/CeO2 catalysts for a wide range of V loadings.

Using density functional theory calculations, the promotional effects of vanadia deposition over ceria-based nanorods on propane oxidative dehydrogenation (ODH) to propene were studied. Reliable catalyst models were modeled using thermodynamic and computational IR analyses. Electronic transfer between vanadia and the ceria surface kept a check on oxygen vacancy formation and ensured a lower degree of surface site heterogeneity. Ni2+ doping of the ceria support stabilized the vanadia clusters by strong interactions. Propane ODH occurred in two pathways (isopropyl radical and isopropoxide mediated), based on the type of the first C-H activation intermediate formed. The isopropyl radical-mediated mechanism favored propene formation via spontaneous C-H activation of the formed radical over the bridged oxygen of vanadia. The isopropoxide-mediated mechanism, with the second C-H activation occurring among the intra-row surface oxygen combinations, led to favorable propene formation, while those occurring between the inter-row surface oxygens led to the undesirable acetone formation and eventual direct oxidation of propane. Ni doping of the ceria support enhanced the first and second C-H activations kinetically, facilitated more sites for propene formation, and improved catalyst activity and propene selectivity. Increased vanadia coverage was shown to significantly improve propene selectivity via a heuristic prediction based on the mechanistic insights developed.