Can the state of platinum species be unambiguously determined by the stretching frequency of adsorbed CO probe molecule?

The paper addresses possible ambiguities in the determination of the state of platinum species by the stretching frequency of a CO probe, which is a common technique for characterization of platinum-containing catalytic systems. We present a comprehensive comparison of the available experimental data with our theoretical modeling (density functional) results of pertinent systems - platinum surfaces, nanoparticles and clusters as well as reduced or oxidized platinum moieties on a ceria support. Our results for CO adsorbed on-top on metallic Pt0, with C-O vibrational frequencies in the region 2018-2077 cm−1, suggest that a decrease of the coordination number of the platinum atom, to which CO is bound, by one lowers the CO frequency by about 7 cm−1. This trend corroborates the Kappers-van der Maas correlation derived from the analysis of the experimental stretching frequency of CO adsorbed on platinum-containing samples on different supports. We also analyzed the effect of the charge of platinum species on the CO frequency. Based on the calculated vibrational frequencies of CO in various model systems, we concluded that the actual state of the platinum species may be mistaken based only on the measured value of the C-O vibrational frequency due to overlapping regions of frequencies corresponding to different types of species. In order to identify the actual state of platinum species one has to combine this powerful technique with other approaches

Subsurface carbon: a general feature of noble metals

Carbon moieties on late transition metals are regarded as poisoning agents in heterogeneous catalysis. Recent studies show the promoting catalytic role of subsurface C atoms in Pd surfaces and their existence in Ni and Pt surfaces. Here energetic and kinetic evidence obtained by accurate simulations on surface and nanoparticle models shows that such subsurface C species are a general issue to consider even in coinage noble-metal systems. Subsurface C is the most stable situation in densely packed (111) surfaces of Cu and Ag, with sinking barriers low enough to be overcome at catalytic working temperatures. Low-coordinated sites at nanoparticle edges and corners further stabilize them, even in Au, with negligible subsurface sinking barriers. The malleability of low-coordinated sites is key in the subsurface C accommodation. The incorporation of C species decreases the electron density of the surrounding metal atoms, thus affecting their chemical and catalytic activity

Charting the Atomic C Interaction with Transition Metal Surfaces

Carbon interaction with transition metal (TM) surfaces is a relevant topic in heterogeneous catalysis, either for its poisoning capability, for the recently attributed promoter role when incorporated in the subsurface, or for the formation of early TM carbides, which are increasingly used in catalysis. Herein, we present a high-throughput systematic study, adjoining thermodynamic plus kinetic evidence obtained by extensive density functional calculations on surface models (324 diffusion barriers located on 81 TM surfaces in total), which provides a navigation map of these interactions in a holistic fashion. Correlation between previously proposed electronic descriptors and ad/absorption energies has been tested, with the d-band center being found the most suitable one, although machine learning protocols also underscore the importance of the surface energy and the site coordination number. Descriptors have also been tested for diffusion barriers, with ad/absorption energies and the difference in energy between minima being the most appropriate ones. Furthermore, multivariable, polynomial, and random forest regressions show that both thermodynamic and kinetic data are better described when using a combination of different descriptors. Therefore, looking for a single perfect descriptor may not be the best quest, while combining different ones may be a better path to follow

Framework Stability of Heteroatom-Substituted Forms of Extra-Large-Pore Ge-Silicate Molecular Sieves: The Case of ITQ-44

International audienceThe paper reports computational modeling with periodic density functional method of the relative stabilities of pure silicate, germanosilicate, aluminosilicate, titanosilicate, and zincosilicate forms of zeolite with IRR-type topology (ITQ-44). The calculated relative substitution energies of Ge in different crystallographic positions correspond very well with the experimentally observed occupation of Ge in ITQ-44 zeolite. The results also suggest that the Si form of ITQ-44 zeolite is stable without the presence of Ge in the framework. The incorporation of Al in structural positions in the silicate and germanosilicate form stabilizes the framework and aluminosilicate form of zeolite is found more stable than the pristine germanosilicate. Taking into account that, so far, ITQ-44 cannot be synthesized without Ge, one can suggest that a potential role of Ge is the shift of the equilibrium in the initial system to species that favor the formation of the IRR structure. The preference for replacement of Ge or Si in the IRR structure, which is related with possible post-synthesis isomorphic substitution in the framework, has also been studied

Framework Stability of Heteroatom-Substituted Forms of Extra-Large-Pore Ge-Silicate Molecular Sieves: The Case of ITQ-44

International audienceThe paper reports computational modeling with periodic density functional method of the relative stabilities of pure silicate, germanosilicate, aluminosilicate, titanosilicate, and zincosilicate forms of zeolite with IRR-type topology (ITQ-44). The calculated relative substitution energies of Ge in different crystallographic positions correspond very well with the experimentally observed occupation of Ge in ITQ-44 zeolite. The results also suggest that the Si form of ITQ-44 zeolite is stable without the presence of Ge in the framework. The incorporation of Al in structural positions in the silicate and germanosilicate form stabilizes the framework and aluminosilicate form of zeolite is found more stable than the pristine germanosilicate. Taking into account that, so far, ITQ-44 cannot be synthesized without Ge, one can suggest that a potential role of Ge is the shift of the equilibrium in the initial system to species that favor the formation of the IRR structure. The preference for replacement of Ge or Si in the IRR structure, which is related with possible post-synthesis isomorphic substitution in the framework, has also been studied