Improving the activity of gold nanoparticles for the water-gas shift reaction using TiO2-Y2O3: an example of catalyst design.

In the last ten years, there has been an acceleration in the pace at which new catalysts for the water-gas shift reaction are designed and synthesized. Pt-based catalysts remain the best solution when only activity is considered. However, cost, operation temperature, and deactivation phenomena are important variables when these catalysts are scaled in industry. Here, a new catalyst, Au/TiO2-Y2O3, is presented as an alternative to the less selective Pt/oxide systems. Experimental and theoretical techniques are combined to design, synthesize, characterize and analyze the performance of this system. The mixed oxide demonstrates a synergistic effect, improving the activity of the catalyst not only at large-to-medium temperatures but also at low temperatures. This effect is related to the homogeneous dispersion of the vacancies that act both as nucleation centers for smaller and more active gold nanoparticles and as dissociation sites for water molecules. The calculated reaction path points to carboxyl formation as the rate-limiting step with an activation energy of 6.9 kcal mol-1, which is in quantitative agreement with experimental measurements and, to the best of our knowledge, it is the lowest activation energy reported for the water-gas shift reaction. This discovery demonstrates the importance of combining experimental and theoretical techniques to model and understand catalytic processes and opens the door to new improvements to reduce the operating temperature and the deactivation of the catalyst.

Effects of the capping ligands, linkers and oxide surface on the electron injection mechanism of copper sulfide quantum dot-sensitized solar cells.

Quantum dot-sensitized solar cells, QDSCs, are a clean and effective alternative to fossil fuels to reduce CO2 emissions. However, the different components that constitute the QDSCs and the difficulty of isolating experimentally their effects on the performance of the whole system slow down the development of more efficient devices. In this work, DFT calculations are combined with a bottom-up approach to differentiate the effect of each component on the electronic structure and absorption spectra. First, Cu2S QDs were built including a U parameter to effectively describe the localization of electrons. The effect of capping agents is addressed using ligands with different electron-donating/withdrawing groups. The role of linkers and their adsorption on the oxide surface are also examined. Finally, we propose a main indirect electron injection mechanism based on the position of the peaks of the spectra.

Ag2S Quantum Dot-Sensitized Solar Cells by First Principles: The Effect of Capping Ligands and Linkers.

Quantum dots solar cells, QDSCs, are one of the candidates for being a reliable alternative to fossil fuels. However, the well-studied CdSe and CdTe-based QDSCs present a variety of issues for their use in consumer-goods applications. Silver sulfide, Ag2S, is a promising material, but poor efficiency has been reported for QDSCs based on this compound. The potential influence of each component of QDSCs is critical and key for the development of more efficient devices based on Ag2S. In this work, density functional theory calculations were performed to study the nature of the optoelectronic properties for an anatase-TiO2(101) surface sensitized with different silver sulfide nanoclusters. We demonstrated how it is possible to deeply tune of its electronic properties by modifying the capping ligands and linkers to the surface. Finally, an analysis of the electron injection mechanism for this system is presented.

First principles thermodynamical modeling of the binodal and spinodal curves in lead chalcogenides.

High-throughput ab initio calculations, cluster expansion techniques, and thermodynamic modeling have been synergistically combined to characterize the binodal and the spinodal decompositions features in the pseudo-binary lead chalcogenides PbSe-PbTe, PbS-PbTe, and PbS-PbSe. While our results agree with the available experimental data, our consolute temperatures substantially improve with respect to previous computational modeling. The computed phase diagrams corroborate that in ad hoc synthesis conditions the formation of nanostructure may occur justifying the low thermal conductivities in these alloys. The presented approach, making a rational use of online quantum repositories, can be extended to study thermodynamical and kinetic properties of materials of technological interest.

Design Principles Guided by DFT Calculations and High-Throughput Frameworks for the Discovery of New Diamond-like Chalcogenide Thermoelectric Materials.

Rational design principles are one pathway to discovering new materials. However, technological breakthroughs rarely occur in this way because these design principles are usually based on incremental advances that seldom lead to disruptive applications. The emergence of machine-learning (ML) and high-throughput (HT) techniques has changed the paradigm, opening up new possibilities for efficiently screening large chemical spaces and creating on-the-fly design principles for the discovery of novel materials with desired properties. In this work, the approach is used to discover novel thermoelectric (TE) materials based on quaternary diamond-like chalcogenides. A HT framework that integrates density functional theory calculations, ML, and the solution of the Boltzmann transport equation is used to efficiently rationalize the transport properties of these compounds and identify those with potential as TE materials, achieving ZT values above 2.

Theoretical Investigation of the Lattice Thermal Conductivities of II-IV-V2 Pnictide Semiconductors.

Ternary pnictide semiconductors with II-IV-V2 stoichiometry hold potential as cost-effective thermoelectric materials with suitable electronic transport properties, but their lattice thermal conductivities (κ) are typically too high. Insights into their vibrational properties are therefore crucial to finding strategies to reduce κ and achieve improved thermoelectric performance. We present a theoretical exploration of the lattice thermal conductivities for a set of pnictide semiconductors with ABX2 composition (A = Zn, Cd; B = Si, Ge, Sn; and X = P, As) using machine-learning-based regression algorithms to extract force constants from a reduced number of density functional theory simulations and then solving the Boltzmann transport equation for phonons. Our results align well with available experimental data, decreasing the mean absolute error by ∼3 W m-1 K-1 with respect to the best previous set of theoretical predictions. Zn-based ternary pnictides have, on average, more than double the thermal conductivity of the Cd-based compounds. Anisotropic behavior increases with the mass difference between A and B cations, but while the nature of the anion does not affect the structural anisotropy, the thermal conductivity anisotropy is typically higher for arsenides than for phosphides. We identify compounds such as CdGeAs2, for which nanostructuring to an affordable range of particle sizes could lead to κ values low enough for thermoelectric applications.

High-Throughput Prediction of the Thermal and Electronic Transport Properties of Large Physical and Chemical Spaces Accelerated by Machine Learning: Charting the ZT of Binary Skutterudites.

Thermal and electronic transport properties are the keys to many technological applications of materials. Thermoelectric, TE, materials can be considered a singular case in which not only one but three different transport properties are combined to describe their performance through their TE figure of merit, ZT. Despite the availability of high-throughput experimental techniques, synthesizing, characterizing, and measuring the properties of samples with numerous variables affecting ZT are not a cost- or time-efficient approach to lead this strategy. The significance of computational materials science in discovering new TE materials has been running in parallel to the development of new frameworks and methodologies to compute the electron and thermal transport properties linked to ZT. Nevertheless, the trade-off between computational cost and accuracy has hindered the reliable prediction of TE performance for large chemical spaces. In this work, we present for the first time the combination of new ab initio methodologies to predict transport properties with machine learning and a high-throughput framework to establish a solid foundation for the accurate prediction of thermal and electron transport properties. This strategy is applied to a whole family of materials, binary skutterudites, which are well-known as good TE candidates. Following this methodology, it is possible not only to connect ZT with the experimental synthetic (carrier concentration and grain size) and operando (temperature) variables but also to understand the physical and chemical phenomena that act as driving forces in the maximization of ZT for p-type and n-type binary skutterudites.

Communication: improving the density functional theory+U description of CeO2 by including the contribution of the O 2p electrons.

Density functional theory (DFT) based approaches within the local-density approximation or generalized gradient approximation frameworks fail to predict the correct electron localization in strongly correlated systems due to the lack of cancellation of the Coulomb self-interaction. This problem might be circumvented either by using hybrid functionals or by introducing a Hubbard-like term to account for the on site interactions. This latter DFT+U approach is less expensive and therefore more practical for extensive calculations in solid-state computational simulations. By and large, the U term only affects the metal electrons, in our case the Ce 4f ones. In the present work, we report a systematic analysis of the effect of adding such a U term also to the oxygen 2p electrons. We find that using a set of U(f) = 5 eV and U(p) = 5eV effective terms leads to improved description of the lattice parameters, band gaps, and formation and reduction energies of CeO(2).

High-Throughput Screening of the Thermoelastic Properties of Ultrahigh-Temperature Ceramics.

Ultrahigh-temperature ceramics (UHTCs) are a group of materials with high technological interest because of their applications in extreme environments. However, their characterization at high temperatures represents the main obstacle for their fast development. Obstacles are found from an experimental point of view, where only few laboratories around the world have the resources to test these materials under extreme conditions, and also from a theoretical point of view, where actual methods are expensive and difficult to apply to large sets of materials. Here, a new theoretical high-throughput framework for the prediction of the thermoelastic properties of materials is introduced. This approach can be systematically applied to any kind of crystalline material, drastically reducing the computational cost of previous methodologies up to 80% approximately. This new approach combines Taylor expansion and density functional theory calculations to predict the vibrational free energy of any arbitrary strained configuration, which represents the bottleneck in other methods. Using this framework, elastic constants for UHTCs have been calculated in a wide range of temperatures with excellent agreement with experimental values, when available. Using the elastic constants as the starting point, other mechanical properties such a bulk modulus, shear modulus, or Poisson ratio have been also explored, including upper and lower limits for polycrystalline materials. Finally, this work goes beyond the isotropic mechanical properties and represents one of the most comprehensive and exhaustive studies of some of the most important UHTCs, charting their anisotropy and thermal and thermodynamical properties.

A theoretical insight into the catalytic effect of a mixed-metal oxide at the nanometer level: the case of the highly active metal/CeOx/TiO2(110) catalysts.

The structural and electronic properties of CeO(x) species supported on the rutile TiO(2)(110) surface have been examined by means of periodic density-functional calculations that use a generalized gradient approximation functional including a Hubbard-like type correction. Deposition of Ce atoms leads in a first step to Ce(3+) ions bound to the surface through bridge and in-plane oxygen atoms, the released electrons occupying the Ti 3d empty orbitals. Further addition of Ce and molecular oxygen gives place to Ce(2)O(3) dimers diagonally arranged on the surface, in agreement with the spots observed in the scanning tunnel microscope images. The formation process of CeO(x) nanoparticles (NPs) on the TiO(2) surface is highly exothermic and our calculations show that the redox properties of the Ce(III)-Ce(IV) couple are significantly altered when it is supported on TiO(2). In particular the reactivity against CO/O(2) indicates that on the surface the presence of Ce(III) is favored over Ce(IV) species. Our results also indicate that the CeO(x)/TiO(2) interface should be seen like a real mixed-metal oxide rather than a supported NP of ceria. Finally, in the context of the high catalytic activity of the M/CeO(x)/TiO(2) (M=Au,Cu,Pt) systems in the water-gas shift reaction, we have examined the dissociation of water on the CeO(x)/TiO(2) surface and estimated a barrier as small as 0.04 eV, i.e. approximately 8 times smaller than that computed for a TiO(2) oxygen vacancy. This result agrees with the experimental superior catalytic activity of the M/CeO(x)/TiO(2) systems over M/TiO(2).

Vinyl acetate synthesis on homogeneous and heterogeneous Pd-based catalysts: a theoretical analysis on the reaction mechanisms.

Vinyl acetate can be synthetized by both homogeneous and heterogeneous processes involving Pd atoms as reaction centers. We have determined the reaction mechanisms by means of density functional theory applied to molecular models for the homogeneous catalyst and to slabs that model the most active heterogeneous ensemble to unravel the similarities and differences in the reaction networks under these different conditions. We find that although the reaction network is similar, the rate determining step is different. Thus, direct extrapolations from organometallic chemistry to gas-phase heterogeneous catalysis should be handled with care.

Charting the complete elastic properties of inorganic crystalline compounds.

The elastic constant tensor of an inorganic compound provides a complete description of the response of the material to external stresses in the elastic limit. It thus provides fundamental insight into the nature of the bonding in the material, and it is known to correlate with many mechanical properties. Despite the importance of the elastic constant tensor, it has been measured for a very small fraction of all known inorganic compounds, a situation that limits the ability of materials scientists to develop new materials with targeted mechanical responses. To address this deficiency, we present here the largest database of calculated elastic properties for inorganic compounds to date. The database currently contains full elastic information for 1,181 inorganic compounds, and this number is growing steadily. The methods used to develop the database are described, as are results of tests that establish the accuracy of the data. In addition, we document the database format and describe the different ways it can be accessed and analyzed in efforts related to materials discovery and design.

Comparative Study on the Performance of Hybrid DFT Functionals in Highly Correlated Oxides: The Case of CeO2 and Ce2O3.

The outstanding catalytic properties of cerium oxides rely on the easy Ce(3+) ↔ Ce(4+) redox conversion, which however constitutes a challenge in density functional based theoretical chemistry due to the strongly correlated nature of the 4f electrons present in the reduced materials. In this work, we report an analysis of the performance of five exchange-correlation functionals (HH, HHLYP, PBE0, B3LYP, and B1-WC) implemented in the CRYSTAL06 code to describe three properties of ceria: crystal structure, band gaps, and reaction energies of the CeO2 → Ce2O3 process. All five functionals give values for cell parameters that are in fairly good agreement with experiment, although the PBE0 hybrid functional is found to be the most accurate. Band gaps, 2p-4f-5d in the case of CeO2 and 4f-5d in the case of Ce2O3, are found to be, in general, overestimated and drop off when the amount of Hartree-Fock exchange in the exchange-correlation functional decreases. In contrast, the reaction energies are found to be underestimated, and increase when the amount of HF exchange lowers. Overall, at its standard formulation, the B1-WC functional seems to be the best choice as it provides good band gaps and reaction energies, and very reasonable crystal parameters.