First Principles Calculations on Oxide-Based Heterogeneous Catalysts and Photocatalysts: Problems and Advances

Abstract

Density functional theory (DFT) has become an essential complement of experiments to interpret, rationalize, and understand structures, spectroscopic data, microscopy analyses, etc. of relevance in heterogeneous catalysis and photocatalysis. However, one of the major goals of theory in catalysis remains the prediction of reaction enthalpies and entropies, of transition state structures, and the identification of reaction mechanisms. While accurate theoretical methods are currently available to study the thermochemistry of molecular systems, this is much less so when reactions involve solid surfaces and in particular oxide materials. The problem stems from the approximate nature of the exchange-correlation functionals used in all DFT approaches. Attempts to improve the accuracy of reaction energies is at the core of the efforts made in the past 30 years to generate new functionals. In this review we will discuss some recent advances in the theoretical description of oxides and their surfaces in heterogeneous catalysis and photocatalysis. In particular, we will focus on two problems: (1) the determination of an oxide band gap and the proper alignment of the occupied and unoccupied levels with respect to the vacuum level, an aspect relevant for the red-ox properties of the material, and (2) the calculation of reaction energies at oxide surfaces. To this end, we will discuss and comment on the performance of current implementations of exchange-correlation functionals (standard GGA, GGA?U, hybrid functionals, meta-GGA, etc.) or alternative approaches (like the quasiparticle GW method), in predicting band alignment and chemical reactivity at oxide surfaces. The role of dispersion forces will be also briefly discussed.

Defect calculations in semiconductors through a dielectric-dependent hybrid DFT functional: The case of oxygen vacancies in metal oxides

“…24 We considered bulk 72-atom 2 × 2 × 3 and 96-atom 2 √ 2 × 2 √ 2 × 2 supercells for rutile and anatase TiO 2 , respectively. For RT γ-monoclinic WO 3 , we adopted a model supercell comprising 64 atoms, obtained by doubling the primitive cell along the a crystallographic axis. For tetragonal ZrO 2 , a 108-atom 2 × 2 × 3 supercell was employed.…”

“…Moreover, defects control and engineering play a fundamental role in a wide range of applications, such as in catalysis and photocatalysis, 3,4 information technology, 5 gas sensors, 6 and smart windows. 7 The standard approach to point defects computational modeling in materials is rooted within density-functional theory (DFT) and the supercell approximation.…”

Electronic structure and phase stability of oxide semiconductors: Performance of dielectric-dependent hybrid functional DFT, benchmarked againstGWband structure calculations and experiments

“…In a typical catalytic reaction, in which the semiconductor plays the role of the active catalyst, low-coordinated oxygen atoms at the semiconductor surface move into the reactant, leaving behind oxygen vacancies in the original material, which are subsequently filled by molecular oxygen present in the atmosphere. Thus, the semiconductor undergoes a change in stoichiometry for which an accurate description of the energetics is determinant in order to properly predict its performance as a catalyst [33]. The formation of a vacancy also possibly strongly affects its electronic properties.…”

The behavior of active diffusiophoretic suspensions: An accelerated Laplacian dynamics study

“…The importance of the search for a universal functional and validating functionals against a wide range of problems can hardly be overemphasized. For example, in reviewing the current state of density functional methods for catalysis and photocatalysts, Pacchioni said 34 "It should be stressed, however, that it is very important to be able to identify which method is the most appropriate for a given problem, and that the quality of the results critically depends on this specific capability. At the moment, there is no single universal solution for every chemical or physical problem."…”

“…[291][292][293][294] Since surfaces may be considered (from one point of view) as large molecules, medium-range correlation can also be important in heterogeneous catalysis. 34,295 Therefore, in the quest for a universal functional, our goal is to obtain useful accuracy for attractive noncovalent interactions as well as for bonding interactions.…”