Steric and Orbital Effects Induced by Isovalent Dopants on the Surface Chemistry of ZrO₂

Abstract

Sometimes, dopants in oxide surfaces are referred to as single-atom catalysts, at least when these species are incorporated in the supporting lattice. Usually, single atom catalysts are transition metal atoms stabilized on an oxide surface, and the activity is due to the valence electrons of these species. However, the surface chemistry can be modified also by the presence of isovalent heteroatoms, where the total number of valence electrons of the active site is the same as for the regular surface. The effect of isovalent dopants on the chemical reactivity of tetragonal ZrO 2 has been studied with first principles calculations. Zr ions in the bulk, subsurface, and surface sites have been replaced with Si, Ge, Sn, Pb, Ti, Hf, and Ce ions. Surface or subsurface sites are clearly preferred. The dopants modify the local structure of the surface and introduce new empty states in the band gap, thus affecting the Lewis acid properties of the surface. We studied the effect of the dopants on the decomposition of HCOOH. This can follow four paths with desorption of (a) H 2, (b) CO, (c) H 2 O, or (d) CO 2 . On pure ZrO 2 reaction (a) dehydrogenation is preferred followed by decarbonylation (b). Ti, Hf, and Ce have some effect on the decomposition but do not change the order of reactivity. On the contrary, in the presence of Si, decarbonylation becomes the preferred path. If Ge occupies surface sites, reaction (d) loss of CO 2 is by far more favorable. With Sn, dehydrogenation remains energetically preferred but the ordering of the other reactions changes, while Pb makes CO 2 desorption slightly preferred over release of H 2 . These effects virtually disappear when the dopants occupy subsurface sites. The study shows that "steric" and/or "orbital" effects of isovalent dopants on a catalyst surface are sufficient to change the reaction products compared to the undoped system.

Microwave-Induced One-Pot Preparation of Bifunctional N-Fe/BC Catalysts and Oriented Production of Phenol-Enriched Bio-Oil from Biomass Pyrolysis: Catalyst Synthesis, Performance Evaluation, and Mechanism Insight via Theoretical Calculations

“…Moreover, the reduction of N and Fe and the transformation of species containing N and Fe reduced the intensity and number of weak acid sites and medium acid sites, resulting in the reduction in catalytic activity (Figure f). The number of strong acid sites in the recycled catalyst might be promoted by the empty orbitals in ferrite (Fe 2 O 3 , Fe 3 O 4 ) and the formation of acid sites and oxygen vacancies during microwave–catalytic pyrolysis. ,, According to an SEM image of the catalysts after application (Figure g), carbon deposition at active sites and a partial pore structure collapse might have been responsible for the decreased selectivity and activity of N-Fe/BC. As a result, the S BET value of N-Fe/BC decreased from 265.21 to 52.27 m 2 g –1 (Figure S3), and the average pore volume decreased from 0.14 to 0.09 cm 3 g –1 Figure h).…”

Cation-doping strategies for tuning of zirconia acid–base properties

“…The most stable system identified here was that in which both Ce ions and O vacancy are segregated to the surface, with these species at the NNN position from each other ( figure 2 c ). Maleki & Pacchioni [ 38 ] have recently explored isovalent dopants on t-ZrO 2 (101) surface, and their results when replacing one Zr by one Ce atom have also shown preferential segregation of this dopant to the surface [ 38 ].…”

Unveiling the Selectivity of CO2 Reduction on Cu2ZnSnS4: The Effect of Exposed Termination

“…53 It is speculated to be an effective active site like a monatomic catalyst. 54 On the contrary, the structure of sulfur termination alters slightly after optimization. In light of the spin charge density, all atoms of metal termination are in the spin nonpolarized state.…”

“…In the relaxed metal termination (Figure a), the Sn–S bond is weakened and stretched from 2.45 to 2.57 Å, inducing the large radius Sn atoms to protrude from the surface, which is akin to Cu 3 SnS 4 . It is speculated to be an effective active site like a monatomic catalyst . On the contrary, the structure of sulfur termination alters slightly after optimization.…”