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

Facet-dependent stability of near-surface oxygen vacancies and excess charge localization at CeO2surfaces

To study the dependence of the relative stability of surface (V A) and subsurface (VB) oxygen vacancies with the crystal facet of CeO2, the reduced (100), (110) and (111) surfaces, with two different concentrations of vacancies, were investigated by means of density functional theory (DFT + U) calculations. The results show that the trend in the near-surface vacancy formation energies for comparable vacancy spacings, i.e. (110) < (100) < (111), does not follow the one in the surface stability of the facets, i.e. (111) < (110) < (100). The results also reveal that the preference of vacancies for surface or subsurface sites, as well as the preferred location of the associated Ce3+ polarons, are facet- and concentration-dependent. At the higher vacancy concentration, the V A is more stable than the V B at the (110) facet whereas at the (111), it is the other way around, and at the (100) facet, both the V A and the VB have similar stability. The stability of the V A vacancies, compared to that of the V B, is accentuated as the concentration decreases. Nearest neighbor polarons to the vacant sites are only observed for the less densely packed (110) and (100) facets. These findings are rationalized in terms of the packing density of the facets, the lattice relaxation effects induced by vacancy formation and the localization of the excess charge, as well as the repulsive Ce3+-Ce3+ interactions.Fil: Pérez Bailac, Patricia. Universidad Autónoma de Madrid; España. Consejo Superior de Investigaciones Científicas; EspañaFil: Lustemberg, Pablo German. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Rosario. Instituto de Física de Rosario. Universidad Nacional de Rosario. Instituto de Física de Rosario; Argentina. Consejo Superior de Investigaciones Científicas; EspañaFil: Ganduglia Pirovano, M. Verónica. Consejo Superior de Investigaciones Científicas; Españ

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

Nature of the Active Sites on Ni/CeO2Catalysts for Methane Conversions

Effective catalysts for the direct conversion of methane to methanol and for methane's dry reforming to syngas are Holy Grails of catalysis research toward clean energy technologies. It has recently been discovered that Ni at low loadings on CeO2(111) is very active for both of these reactions. Revealing the nature of the active sites in such systems is paramount to a rational design of improved catalysts. Here, we correlate experimental measurements on the CeO2(111) surface to show that the most active sites are cationic Ni atoms in clusters at step edges, with a small size wherein they have the highest Ni chemical potential. We clarify the reasons for this observation using density functional theory calculations. Focusing on the activation barrier for C-H bond cleavage during the dissociative adsorption of CH4 as an example, we show that the size and morphology of the supported Ni nanoparticles together with strong Ni-support bonding and charge transfer at the step edge are key to the high catalytic activity. We anticipate that this knowledge will inspire the development of more efficient catalysts for these reactions.Fil: Lustemberg, Pablo German. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Rosario. Instituto de Física de Rosario. Universidad Nacional de Rosario. Instituto de Física de Rosario; ArgentinaFil: Mao, Zhongtian. University of Washington; Estados UnidosFil: Salcedo, Agustín. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Tecnologías del Hidrogeno y Energias Sostenibles. Universidad de Buenos Aires. Facultad de Ingeniería. Instituto de Tecnologías del Hidrogeno y Energias Sostenibles; ArgentinaFil: Irigoyen, Beatriz del Luján. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Ciudad Universitaria. Instituto de Tecnologías del Hidrogeno y Energias Sostenibles. Universidad de Buenos Aires. Facultad de Ingeniería. Instituto de Tecnologías del Hidrogeno y Energias Sostenibles; ArgentinaFil: Ganduglia Pirovano, M. Verónica. Universidad de Buenos Aires; ArgentinaFil: Campbell, Charles T.. University of Washington; Estados Unido

Vibrational Frequencies of Cerium-Oxide-Bound CO: A Challenge for Conventional DFT Methods

In ceria-based catalysis, the shape of the catalyst particle, which determines the exposed crystal facets, profoundly affects its reactivity. The vibrational frequency of adsorbed carbon monoxide (CO) can be used as a sensitive probe to identify the exposed surface facets, provided reference data on well-defined single crystal surfaces together with a definitive theoretical assignment exist. We investigate the adsorption of CO on the CeO2(110) and (111) surfaces and show that the commonly applied DFT(PBE)+U method does not provide reliable CO vibrational frequencies by comparing with state-of-the-art infrared spectroscopy experiments for monocrystalline CeO2 surfaces. Good agreement requires the hybrid DFT approach with the HSE06 functional. The failure of conventional density-functional theory (DFT) is explained in terms of its inability to accurately describe the facet- A nd configuration-specific donation and backdonation effects that control the changes in the Câ"O bond length upon CO adsorption and the CO force constant. Our findings thus provide a theoretical basis for the detailed interpretation of experiments and open up the path to characterize more complex scenarios, including oxygen vacancies and metal adatoms.Fil: Lustemberg, Pablo German. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Rosario. Instituto de Física de Rosario. Universidad Nacional de Rosario. Instituto de Física de Rosario; Argentina. Consejo Superior de Investigaciones Científicas; EspañaFil: Plessow, Philipp N.. Karlsruher Institut fur Technologie; AlemaniaFil: Wang, Yuemin. Karlsruher Institut fur Technologie; AlemaniaFil: Yang, Chengwu. Karlsruher Institut fur Technologie; AlemaniaFil: Nefedov, Alexei. Karlsruher Institut fur Technologie; AlemaniaFil: Studt, Felix. Karlsruher Institut fur Technologie; AlemaniaFil: Wöll, Christof. Karlsruher Institut fur Technologie; AlemaniaFil: Ganduglia Pirovano, Maria Veronica. Karlsruher Institut fur Technologie; Alemani

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

Direct Conversion of Methane to Methanol on Ni-Ceria Surfaces: Metal-Support Interactions and Water-Enabled Catalytic Conversion by Site Blocking

[EN] The transformation of methane into methanol or higher alcohols at moderate temperature and pressure conditions is of great environmental interest and remains a challenge despite many efforts. Extended surfaces of metallic nickel are inactive for a direct CH → CHOH conversion. This experimental and computational study provides clear evidence that low Ni loadings on a CeO(111) support can perform a direct catalytic cycle for the generation of methanol at low temperature using oxygen and water as reactants, with a higher selectivity than ever reported for ceria-based catalysts. On the basis of ambient pressure X-ray photoemission spectroscopy and density functional theory calculations, we demonstrate that water plays a crucial role in blocking catalyst sites where methyl species could fully decompose, an essential factor for diminishing the production of CO and CO, and in generating sites on which methoxy species and ultimately methanol can form. In addition to water-site blocking, one needs the effects of metal-support interactions to bind and activate methane and water. These findings should be considered when designing metal/oxide catalysts for converting methane to value-added chemicals and fuels.The work carried out at Brookhaven National Laboratory was supported by the U.S. Department of Energy (Chemical Sciences Division, DE-SC0012704). S.D.S. is supported by a U.S. Department of Energy Early Career Award. This research used resources of the Advanced Light Source (Beamline 9.3.2),which is a DOE Office of Science User Facility under contract no. DE-AC02-05CH11231. Authors acknowledge contribution of Dr. Ethan Crumlin for assistance with AP-XPS measurements. M.V.G.-P. acknowledges the financial support of the /Ministry of Economy and Competitiveness MINECO-Spain (Grant No. CTQ2015-78823-R) and P.G.L. that of the Agencia Nacional de Promocion Científiica y Tecnologica-Argentina (Grant No. PICT-2016-2750). Computer time provided by the BIFI-ZCAM, RES at the Marenostrum and La Palma nodes, SNCAD (Sistema Nacional de Computación de Alto Desempeño, Argentina), and the DECI resources BEM based in Poland at WCSS and Archer at EPCC with support from the PRACE aislb, is acknowledged. M.V. thanks the Ministry of Education, Youth and Sports of the Czech Republic for financial support under Project LH15277. R.M.P. was partially funded by the AGEP-T (Alliance for Graduate Education and the Professoriate−Transformation) which is funded by the National Science Foundation, award #131131

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

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.Fil: Zhang, Feng. State University of New York. Stony Brook University; Estados UnidosFil: Gutiérrez, Ramón A.. Universidad Central de Venezuela; VenezuelaFil: Lustemberg, Pablo German. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Rosario. Instituto de Física de Rosario. Universidad Nacional de Rosario. Instituto de Física de Rosario; Argentina. Consejo Superior de Investigaciones Científicas; EspañaFil: Liu, Zongyuan. Brookhaven National Laboratory; Estados UnidosFil: Rui, Ning. Brookhaven National Laboratory; Estados UnidosFil: Wu, Tianpin. Argonne National Laboratory; Estados UnidosFil: Ramírez, Pedro J.. Zoneca-cenex; México. Universidad Central de Venezuela; VenezuelaFil: Xu, Wenqian. Argonne National Laboratory; Estados UnidosFil: Idriss, Hicham. King Abdullah University of Science and Technology; Arabia SauditaFil: Ganduglia Pirovano, M. Verónica. Consejo Superior de Investigaciones Científicas; EspañaFil: Senanayake, Sanjaya D.. Brookhaven National Laboratory; Estados UnidosFil: Rodriguez, José A.. Brookhaven National Laboratory; Estados Unidos. State University of New York. Stony Brook University; Estados Unido

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