Monoxide carbon frequency shift as a tool for the characterization of TiO2 surfaces: Insights from first principles spectroscopy

The adsorption and vibrational frequency of CO on defective and undefective titanium dioxide surfaces is examined applying first-principles molecular dynamics simulations. In particular, the vibrational frequencies are obtained beyond the harmonic approximation, through the time correlation functions of the atomic trajectories. In agreement with experiments, at low CO coverages we find an upshift in the vibration frequency with respect to the free CO molecule, of 45 and 35 cm-1 on the stoichiometric rutile (110) and anatase (101) faces, respectively. A band falling 8 cm-1 below the frequency corresponding to the perfect face is observed for the reduced rutile (110) surface in the low vacancy concentration limit, where the adsorption is favored on Ti4+ sites. At a higher density of defects, adsorption on Ti3+ sites becomes more stable, accompanied by a downshift in the stretching band. In the case of anatase (101), we analyze the effect of subsurface oxygen vacancies, which have been shown to be predominant in this material. Interestingly, we find that the adsorption of CO on five coordinate Ti atoms placed over subsurface vacancies is favored with respect to other Ti4+ sites (7.25 against 6.95 kcal/mol), exhibiting a vibrational redshift of 20 cm-1 . These results provide the basis to quantitatively assess the degree of reduction of rutile and anatase surfaces via IR spectroscopy, and at the same time allow for the assignment of characteristic bands in the CO spectra on TiO2 whose origin has remained ambiguous.Fil: Lustemberg, Pablo German. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Rosario. Instituto de Física de Rosario (i); Argentina. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de los Materiales, Medioambiente y Energía; Argentina. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Química Inorgánica, Analítica y Quimica Fisica; ArgentinaFil: Scherlis Perel, Damian Ariel. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Química, Física de Los Materiales, Medioambiente y Energía; Argentina. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales. Departamento de Química Inorgánica, Analítica y Quimica Fisica; Argentin

Vibrational frequencies of CO bound to all three low-index cerium oxide surfaces: A consistent theoretical description of vacancy-induced changes using density functional theory

The facet-dependent adsorption of CO on oxidized and reduced CeO$_2$ single crystal surfaces is reviewed, with emphasis on the effect of CO coverage and the ability of state-of-the-art quantum-mechanical methods to provide reliable energies and an accurate description of the IR vibrational frequency of CO. Comparison with detailed, high-resolution experimental infrared reflection absorption spectroscopy data obtained for single crystal samples allows the assignment of the different CO vibrational bands observed on all three low-index ceria surfaces. Good agreement is achieved with the hybrid density functional theory approach with the HSE06 functional and with saturation coverage. It is shown that CO is very sensitive to the structure of cerium oxide surfaces and to the presence of oxygen vacancies. The combined theoretical-experimental approach offers new opportunities for a better characterization of ceria nanoparticles and for unraveling changes occurring during reactions involving CO at higher pressures

Scar functions in the Bunimovich Stadium billiard

In the context of the semiclassical theory of short periodic orbits, scar functions play a crucial role. These wavefunctions live in the neighbourhood of the trajectories, resembling the hyperbolic structure of the phase space in their immediate vicinity. This property makes them extremely suitable for investigating chaotic eigenfunctions. On the other hand, for all practical purposes reductions to Poincare sections become essential. Here we give a detailed explanation of resonances and scar functions construction in the Bunimovich stadium billiard and the corresponding reduction to the boundary. Moreover, we develop a method that takes into account the departure of the unstable and stable manifolds from the linear regime. This new feature extends the validity of the expressions.Comment: 21 pages, 10 figure

Metal-Support Interactions and C1 Chemistry: Transforming Pt-CeO2into a Highly Active and Stable Catalyst for the Conversion of Carbon Dioxide and Methane

[EN] There is an ongoing search for materials which can accomplish the activation of two dangerous greenhouse gases like carbon dioxide and methane. In the area of C1 chemistry, the reaction between CO2 and CH4 to produce syngas (CO/H2), known as methane dry reforming (MDR), is attracting a lot of interest due to its green nature. On Pt(111), high temperatures must be used to activate the reactants, leading to a substantial deposition of carbon which makes this metal surface useless for the MDR process. In this study, we show that strong metal-support interactions present in Pt/CeO2(111) and Pt/CeO2 powders lead to systems which can bind CO2 and CH4 well at room temperature and are excellent and stable catalysts for the MDR process at moderate temperature (500 °C). The behavior of these systems was studied using a combination of in situ/operando methods (AP-XPS, XRD, and XAFS) which pointed to an active Pt-CeO2-x interface. In this interface, the oxide is far from being a passive spectator. It modifies the chemical properties of Pt, facilitating improved methane dissociation, and is directly involved in the adsorption and dissociation of CO2 making the MDR catalytic cycle possible. A comparison of the benefits gained by the use of an effective metal-oxide interface and those obtained by plain bimetallic bonding indicates that the former is much more important when optimizing the C1 chemistry associated with CO2 and CH4 conversion. The presence of elements with a different chemical nature at the metal-oxide interface opens the possibility for truly cooperative interactions in the activation of C-O and C-H bonds.The XRD and XAFS experiments carried out at the Advanced Photon SourceBeamline 17BM (XRD) and 9BM (XAFS) at Argonne National Laboratory was supported by the U.S. DOE under Contract No. DE-AC02-06CH11357. This project has also received funding from the European Union’s Horizon 2020 research and innovation programme under the Marie Skl̷odowska-Curie grant agreement No 832121. M.V.G.P. thanks the support from the MINECO and MICINN-Spain(CTQ2015-71823-R and RTI2018-101604-B-I00, respectively)Peer reviewe

On the Relative Stability of Near-Surface Oxygen Vacancies at the CeO2(111) Surface upon Zr Doping

Ceria (CeO2)-based materials are of great importance in numerous technological applications such as three-way catalysts (TWCs) [Catal. Today 62, 35-50 (2000) and Chem. Rev. 116, 5987-6041 (2016)], hydrocarbon reforming [Chem. Rev. 116, 5987-6041 (2016) and Catalysis by Ceria and Related Materials; Trovarelli, A.; Fornasiero, P., Eds.; 2nd Edition; Imperial College Press: London, 2013] and solid oxide fuel cells (SOFC) [Chem. Rev. 116, 5987-6041 (2016) and Catalysis by Ceria and Related Materials; Trovarelli, A.; Fornasiero, P., Eds.; 2nd Edition; Imperial College Press: London, 2013]. These materials possess a property that is key to most of such applications, namely, their capability for easy conversion between the Ce4+ and Ce3+ oxidation states, which is achieved by releasing oxygen atoms from the crystal lattice and forming oxygen vacancies. In particular, the replacement of Ce by Zr to form CeO2-ZrO2 solid solutions was found to facilitate the reducibility of the oxide as well as to increase the oxygen storage capacity and the system thermal stability, compared to pure CeO2. This theoretical work employing DFT+U calculations, is a systematic study of the effects of Zr doping on the stoichiometric and reduced CeO2(111) surfaces to determine the preferred location of the Zr dopants at various concentrations, as well as to pinpoint how Zr doping affects the stability of near-surface oxygen vacancies -including the position of the Ce3+ ions. We found that for a given Zr content, the more stable structures do not correspond to those configurations with Zr located in the topmost O-Ce-O trilayer (TL1), but in inner layers, and the stability decreases with increasing Zr concentration. Regarding the formation of oxygen vacancies, it was found that the most stable configuration corresponds to the Zr atom located in the surface layer (TL1) neighboring a subsurface oxygen vacancy with next-nearest neighbor Ce3+, being the formation energy equal to 1.16 eV. The corresponding surface oxygen vacancy is 0.16 eV less stable. These values are by 0.73 and 0.92 eV, respectively, smaller than the corresponding ones for the pure CeO2(111) surface. The results are explained in terms of Zr- and vacancy-induced lattice relaxation effects. This study provides microscopic insight into the interplay between Zr-doping, vacancy formation, lattice relaxations, and the localization of the excess charge that will be key to understanding surface chemistry and catalysis on Zr-doped ceria surface, as well as conductive ceria-based materials for advanced applications.<br /

Tuning Selectivity in the Direct Conversion of Methane to Methanol: Bimetallic Synergistic Effects on the Cleavage of C-H and O-H Bonds over NiCu/CeO2 Catalysts

The efficient activation of methane and simultaneous water dissociation are crucial in many catalytic reactions on oxide-supported transition metal catalysts. On very low-loaded Ni/CeO2 surfaces, methane easily fully decomposes, CH4 -> C + 4H, and water dissociates, H2O-> OH + H. However, in important reactions such as the direct oxidation of methane to methanol (MTM), where complex interplay exists between reactants (CH4, O2), it is desirable to avoid the complete dehydrogenation of methane to carbon. Remarkably, the barrier for the activation of CH bonds in CHx (x= 1-3) species on Ni/CeO2 surfaces can be manipulated by adding Cu, forming bimetallic NiCu clusters, whereas the ease for cleavage of OH bonds in water, is not affected by ensemble effects, as obtained from density functional theory-based calculations. CH4 activation occurs only on Ni sites and H2O activation on both Ni and Cu sites. The MTM reaction pathway for the example of the Ni3Cu1/CeO2 model catalyst predict higher selectivity and a lower activation barrier for methanol production, compared with that for Ni4-CeO2. These findings point toward a possible strategy to design active and stable catalysts which can be employed for methane activation and conversions