Ultradispersed Mo/TiO2 catalysts for CO2 hydrogenation to methanol

Mo/TiO2 catalysts with atomic dispersion of molybdenum appear active and stable in the gas-phase hydrogenation of CO2. A comparison between various titania materials shows a crucial effect of the support surface structure on the methanol yield. Molybdenum supported at low coverage on rutile titania nanorods is the most active and methanol-selective system. From catalyst characterization by aberration-corrected scanning transmission electron microscopy, near-ambient pressure X-ray photoelectron spectroscopy, diffuse reflectance UV-vis spectroscopy, and temperature-programmed techniques, we suggest that the most active catalysts for methanol production involve ultradispersed molybdate species with high reducibility and strong interaction with the rutile support.Peer ReviewedPostprint (author's final draft

Water adsorption, dissociation and oxidation on SrTiO3 and ferroelectric surfaces revealed by ambient pressure X-ray photoelectron spectroscopy

Water dissociation on oxides is of great interest because its fundamental aspects are still not well understood and it has implications in many processes, from ferroelectric polarization screening phenomena to surface catalysis and surface chemistry on oxides. In situ water dissociation and redox processes on metal oxide perovskites which easily expose TiO-terminated surfaces, such as SrTiO, BaTiO or Pb(Zr,Ti)O, are studied by ambient pressure XPS, as a function of water vapour pressure. From the analysis of the O1s spectrum, we determine the presence of different types of oxygen based species, from hydroxyl groups, either bound to Ti and metal sites or lattice oxygen, to different peroxide compounds, and propose a model for the adsorbate layer composition, valid for environmental conditions. From the XPS analysis, we describe the existing surface redox reactions for metal oxide perovskites, occurring at different water vapour pressures. Among them, peroxide species resulting from surface oxidative reactions are correlated with the presence of Ti ions, which are observed to specifically promote surface oxidation and water dissociation as compared to other metals. Finally, surface peroxidation is enhanced by X-ray beam irradiation, leading to a higher coverage of peroxide species after beam overexposure and by ferroelectric polarization, demonstrating the enhancement of the reactivity of the surfaces of ferroelectric materials due to the effect of internal electric fields

Dynamic Structure and Subsurface Oxygen Formation of a Working Copper Catalyst under Methanol Steam Reforming Conditions: An in Situ Time-Resolved Spectroscopic Study

"This document is the Accepted Manuscript version of a Published Work that appeared in final form in ACS Catalysis, copyright © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see https://doi.org/10.1021/acscatal.8b05042."[EN] The dynamic behavior of a CuO/ZnO/Ga2O3 catalyst under methanol steam reforming (MSR) reaction conditions promoted by a high dispersion of the copper nanoparticles and defect sites of a nonstoichiometric ZnGa2O4 spinel phase has been observed, where structural changes taking place in the initial state of the reaction determine the final state of the catalyst in stationary reaction conditions. Mass spectrometry (MS) studies under transient conditions coupled to X-ray photoelectron spectroscopy (XPS) have shown copper oxidation to Cu+ in the initial state of the reaction (TOS = 4 min), followed by a fast reduction of the outer shell to Cu-0, while keeping dissolved oxygen species in the inner layers of the nanoparticle. The presence of this subsurface oxygen impairs a positive charge to the uppermost surface copper species, that is, Cu delta+, which undoubtedly plays an important role on the MSR catalytic activity. The detection of these features, unperceived by conventional spectroscopic and catalytic studies, has only been possible by combining synchrotron NAP-XPS studies with transient studies performed in a low volume catalytic reactor connected to MS and linked with Raman and laboratory scale XPS studies.The research leading to these results has received funding from the European Union's Seventh Framework Programme (FP/2007-2013) for the Fuel Cells and Hydrogen Joint Technology Initiative under Grant Agreement No. [303476]. Part of this work was financially supported by the following projects: (i) Project POCI-01-0145-FEDER-006939 (Laboratory for Process Engineering, Environment, Biotechnology and Energy UID/EQU/00511/2013) funded by the European Regional Development Fund (ERDF), through COMPETE2020 - Programa Operacional Competitividade e Internacionalizacao (POCI) and by national funds, through FCT - Fundacao para a Ciencia e a Tecnologia; (ii) NORTE-01-0145-FEDER-000005 - LEPABE-2-ECO-INNOVATION, supported by North Portugal Regional Operational Programme (NORTE 2020), under the Portugal 2020 Partnership Agreement, through the European Regional Development Fund (ERDF); and (iii) the Spanish Government-MINECO through "Severo Ochoa" Excellence Programme (SEV-2016-0683). D.R. thanks European Research Council project SYNCATMATCH (671093). J.C. thanks the Spanish Government (MINECO) for a "Severo Ochoa" grant (BES-2015-075748). The NAP-XPS experiments were performed at the NAPP branch of the CIRCE beamline at the ALBA Synchrotron with the collaboration of ALBA staff.Ruano-Sánchez, D.; Cored-Bandrés, J.; Azenha, C.; Pérez-Dieste, V.; Mendes, A.; Mateos-Pedrero, C.; Concepción Heydorn, P. (2019). Dynamic Structure and Subsurface Oxygen Formation of a Working Copper Catalyst under Methanol Steam Reforming Conditions: An in Situ Time-Resolved Spectroscopic Study. ACS Catalysis. 9(4):2922-2930. https://doi.org/10.1021/acscatal.8b05042S292229309

Active and Regioselective Ru Single-Site Heterogeneous Catalystsfor Alpha-Olefin Hydroformylation

[EN] A heterogeneous ruthenium catalyst consisting ofisolated single atoms and disordered clusters stabilized in a N-doped carbon matrix has been synthesized with very good activityand remarkable regioselectivity in the hydroformylation of 1-hexene. The role of the nitrogen heteroatoms has been probedessential to increase the catalyst stability and activity, enabling thestabilization of Ru(II)-N sites according to X-ray photoelectronspectroscopy (XPS) and XANES. Intrinsic size-dependent activityof Ru species of different atomicity has been extracted, correlatingthe observed reaction rate and the particle size distributiondetermined by means of aberration-corrected high-angle annulardark-field scanning transmission electron microscopy, permittingthe identification of single-atom sites as the most active ones. This catalyst appears as a promising alternative with respect to itsheterogeneous counterparts, paving the way for designing improved Ru heterogeneous catalysts.The research leading to these results has received funding from the Spanish Ministry of Science, Innovation and Universities, through the "Severo Ochoa" Excellence Programme (SEV2016-0683) and RTI2018-099668-B-C21 and PGC2018101247-B-100 "Programa Estatal de Generacion de Conocimiento". P.C acknowledges the financial support from the "Generalitat Valenciana" through the project AICO/2020/205. HR-HAADF-STEM measurements were performed at the DME-UCA node of ELECMI ICTS with financial support from FEDER/MINECO (MAT2017-87579-R and PID2019110018GA-I00); XAS experiments were performed at the BL22-CLAESS beamline at the ALBA Synchrotron with the collaboration of ALBA staff as part of projects 2019093692 and 2020024106. XPS experiments were performed at the BL24CIRCE beamline at the ALBA Synchrotron with the collaboration of ALBA staff. J.E.B acknowledges the Polytechnical University of Valencia for the economic support through the grant of an FPI scholarship associated with the PAID programme "Programa de Ayudas de Investigacion y Desarrollo".Escobar-Bedia, FJ.; Lopez-Haro, M.; Calvino, JJ.; Martin-Diaconescu, V.; Simonelli, L.; Pérez-Dieste, V.; Sabater Picot, MJ.... (2022). Active and Regioselective Ru Single-Site Heterogeneous Catalystsfor Alpha-Olefin Hydroformylation. ACS Catalysis. 12(7):4182-4193. https://doi.org/10.1021/acscatal.1c057374182419312

Bimetallic NiFe nanoparticles supported on CeO2 as catalysts for methane steam reforming

Ni-Fe nanocatalysts supported on CeO2 have been prepared for the catalysis of methane steam reforming (MSR) aiming for coke-resistant noble metal-free catalysts. The catalysts have been synthesized by traditional incipient wetness impregnation as well as dry ball milling, a green and more sustainable preparation method. The impact of the synthesis method on the catalytic performance and the catalysts’ nanostructure has been investigated. The influence of Fe addition has been addressed as well. The reducibility and the electronic and crystalline structure of Ni and Ni-Fe mono- and bimetallic catalysts have been characterized by temperature programmed reduction (H2-TPR), in situ synchrotron X-ray diffraction (SXRD), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy. Their catalytic activity was tested between 700 and 950 °C at 108 L gcat-1 h-1 and with the reactant flow varying between 54 and 415 L gcat-1 h-1 at 700 °C. Hydrogen production rates of 67 mol gmet-1 h-1 have been achieved. The performance of the ball-milled Fe0.1Ni0.9/CeO2 catalyst was similar to that of Ni/CeO2 at high temperatures, but Raman spectroscopy revealed a higher amount of highly defective carbon on the surface of Ni-Fe nanocatalysts. The reorganization of the surface under MSR of the ball-milled NiFe/CeO2 has been monitored by in situ near-ambient pressure XPS experiments, where a strong reorganization of the Ni-Fe nanoparticles with segregation of Fe toward the surface has been observed. Despite the catalytic activity being lower in the low-temperature regime, Fe addition for the milled nanocatalyst increased the coke resistance and could be an efficient alternative to industrial Ni/Al2O3 catalysts.Peer ReviewedPostprint (published version

In situ monitoring of the phenomenon of electrochemical promotion of catalysis

In this work we investigate by in-situ near-ambient pressure photoemission (NAPP) spectroscopy the phenomenon of Electrochemical Promotion of Catalysis (EPOC). We studied the reduction and diffusion kinetics of alkaline ions in a solid electrolyte cell formed by a nickel electrode supported on K+-β-alumina electrolyte. Experiments in ultra-high vacuum and in the presence of steam showed that the amount of potassium atoms supplied to the surface is probably affected by nickel electronic modifications induced by adsorbed OH− groups. It was also deduced that part of the segregated potassium would be adsorbed at inner interfaces where it would be inaccessible to the photoelectron analyzer. A migration mechanism of the promoter is proposed consisting in: (i) the electrochemical reduction of the alkali ions (potassium) at the Ni/solid electrolyte/gas interface; (ii) the spillover of potassium atoms onto the Ni gas-exposed surface; and (iii) the diffusion of potassium atoms to Ni inner grain boundary interfaces.En este trabajo investigamos mediante espectroscopía de fotoemisión a presión cercana al ambiente in situ (NAPP) el fenómeno de la Promoción Electroquímica de la Catálisis (EPOC). Estudiamos la cinética de reducción y difusión de iones alcalinos en una celda de electrolito sólido formada por un electrodo de níquel soportado sobre electrolito K + -β-alúmina. Los experimentos en vacío ultraalto y en presencia de vapor mostraron que la cantidad de átomos de potasio suministrados a la superficie probablemente se ve afectada por modificaciones electrónicas de níquel inducidas por OH adsorbido .grupos También se dedujo que parte del potasio segregado sería adsorbido en las interfaces internas donde sería inaccesible para el analizador de fotoelectrones. Se propone un mecanismo de migración del promotor que consiste en: (i) la reducción electroquímica de los iones alcalinos (potasio) en la interfase Ni/electrolito sólido/gas; (ii) el derrame de átomos de potasio sobre la superficie expuesta al gas Ni; y (iii) la difusión de átomos de potasio a las interfaces de límite de grano interno de Ni

Graphene Formation Mechanism by the Electrochemical Promotion of a Ni Catalyst

In this work, we show that multilayer graphene forms by methanol decomposition at 280 °C on an electrochemically promoted nickel catalyst film supported on a K-βAl2O3 solid electrolyte. In operando near ambient pressure photoemission spectroscopy and electrochemical measurements have shown that polarizing negatively the Ni electrode induces the electrochemical reduction and migration of potassium to the nickel surface. This elemental potassium promotes the catalytic decomposition of methanol into graphene and also stabilizes the graphene formed via diffusion and direct K–C interaction. Experiments reveal that adsorbed methoxy radicals are intermediate species in this process and that, once formed, multilayer graphene remains stable after electrochemical oxidation and back migration of potassium to the solid electrolyte upon positive polarization. The reversible diffusion of ca. 100 equivalent monolayers of potassium through the carbon layers and the unprecedented low-temperature formation of graphene and other carbon forms are mechanistic pathways of high potential impact for applications where mild synthesis and operation conditions are required.En este trabajo mostramos que el grafeno multicapa se forma por descomposición del metanol a 280 °C sobre una película de catalizador de níquel promovida electroquímicamente soportada sobre un K-βAl 2 O 3electrolito sólido. Operando cerca de la espectroscopia de fotoemisión a presión ambiental y las mediciones electroquímicas han demostrado que la polarización negativa del electrodo de Ni induce la reducción electroquímica y la migración del potasio a la superficie del níquel. Este potasio elemental promueve la descomposición catalítica del metanol en grafeno y también estabiliza el grafeno formado a través de la difusión y la interacción directa K-C. Los experimentos revelan que los radicales metoxi adsorbidos son especies intermedias en este proceso y que, una vez formado, el grafeno multicapa permanece estable después de la oxidación electroquímica y la migración de regreso del potasio al electrolito sólido tras la polarización positiva. La difusión reversible de ca

Evolución de la estructura electrónica y superficie de Fermi de interfases Ag/Si (111) en función de la cantidad de plata depositada

Tesis doctoral inédita leída en la Universidad Autónoma de Madrid, Facultad de Ciencias Físicas, Departamento de Física de la Materia Condensada. Fecha de lectura: 15-11-200

Characterization of model and real catalysts by APXPS

In this contribution, I first briefly summarize some of the recent advances relevant for the investigation of heterogeneous catalysis with Ambient Pressure X-ray Photoelectron Spectroscopy (APXPS). In the second part, two examples of the research done at the CIRCE beamline of the synchrotron ALBA are described: CO oxidation on a model curved crystal Pd(111) catalyst and methanol steam reforming on powder bimetallic supported catalysts, PdCu/ monoclinic and cubic zirconia