Surface composition changes of CuNi-ZrO2 during methane decomposition: An operando NAP-XPS and density functional study

AbstractBimetallic CuNi nanoparticles of various nominal compositions (1:3, 1:1, 3:1) supported on ZrO2 were employed for operando spectroscopy and theoretical studies of stable surface compositions under reaction conditions of catalytic methane decomposition up to 500°C. The addition of Cu was intended to increase the coke resistance of the catalyst. After synthesis and (in situ) reduction the CuNi nanoparticles were characterized by HR-TEM/EDX, XRD, FTIR (using CO as probe molecule) and NAP-XPS, all indicating a Cu rich surface, even when the overall nanoparticle composition was rich in Ni. Density functional (DF) theory modelling, applying a recently developed computational protocol based on the construction of topological energy expressions, confirmed that in any studied composition Cu segregation on surface positions is an energetically favourable process, with Cu preferentially occupying corner and edge sites. Ni is present on terraces only when not enough Cu atoms are available to occupy all surface sites.When the catalysts were applied for methane decomposition they were inactive at low temperature but became active above 425°C. Synchrotron-based operando NAP-XPS indicated segregation of Ni on the nanoparticle surface when reactivity set in for CuNi-ZrO2. Under these conditions C 1s core level spectra revealed the presence of various carbonaceous species at the surface. DF calculations indicated that both the increase in temperature and especially the adsorption of CHx groups (x=0-3) induce the segregation of Ni atoms on the surface, with CH3 providing the lowest and C the highest driving force.Combined operando and theoretical studies clearly indicate that, independent of the initial surface composition after synthesis and reduction, the CuNi-ZrO2 catalyst adopts a specific Ni rich surface under reaction conditions. Based on these findings we provide an explanation why Cu rich bimetallic systems show improved coke resistance

In Situ spectroscopy on Cu and ceria modified Ni-zirconia catalysts for methane reforming

Zusammenfassung in deutscher SpracheNi/ZrO2 Katalysatoren werden für die Reformierung von Methan, einem wichtigen Prozess für die industrielle Wasserstoffproduktion und in Festoxidbrennstoffzellen, eingesetzt. Obwohl Nickelkatalysatoren generell eine hohe Aktivität für diverse Methanreformierungsreaktionen aufweisen, führen Kohlenstoffablagerungen, die den Katalysator rasch deaktivieren, zu großen Problemen. Das Ziel dieser Arbeit war, Ni/ZrO2 dahingehend zu modifizieren, dass die Bildung von Kohlenstoffablagerungen unterdrückt wird. Dazu wurde zwei Ansätze verfolgt und untersucht: Der Einfluss der Zumischung von Kupfer zur metallischen Nickel-Komponente des Katalysators und der Modifizierung des Trägermaterials mit Ceroxid. Durch die Zugabe von Kupfer wird eine erhöhte Beständigkeit gegenüber Kohlenstoffablagerungen auf Nickel aufgrund elektronischer Wechsel-wirkung und durch die Veränderung der Oberflächenzusammensetzung erwartet. Aufgrund der besseren Verfügbarkeit von Oberflächensauerstoff eines CeO2/ZrO2-Mischoxides gegenüber ZrO2 kann bereits abgelagerter Kohlenstoff leichter von der Katalysatoroberfläche weg-oxidiert werden. Kupfer und Nickel wurde mittels Ko-Imprägnierung auf das ZrO2-Trägermaterial aufgebracht. CeO2/ZrO2-Mischoxide wurden auf verschiedenen Wegen, darunter Imprägnierung, Verbrennung der Ausgangsstoffe, Ko-Fällung und Tensid-unterstützer Ko-Fällung, hergestellt. Nickel wurde mittels Imprägnierung und einer nominellen Beladung von 5 % w/w. auf die Trägermaterialien aufgebracht. Die Katalysatoren wurden mittels Transmissionselektronenmikroskopie (TEM), Wasserstoffchemisorption, Infrarotspektroskopie mit CO als Sondenmolekül, Röntgendiffraktion (XRD) und Röntgenabsorptions-spektroskopie (XAS) charakterisiert. Um die Aktivität für partielle Methan-oxidation, Methanzersetzung, -dampfreformierung und -kohlendioxid-reformierung zu untersuchen, wurden die Katalysatoren in einem Durchflussreaktor getestet. In situ Spektroskopie liefert Informationen über den Katalysator unter relevanten Reaktionsbedingungen wie hohe Temperaturen und eine reaktive Gasatmosphäre. Die Katalysatoren in dieser Arbeit wurden mittels in situ Infrarotspekroskopie, in situ XAS und Röntgenphotoelektronen-spektroskopie (XPS) unter Drücken nahe Atmosphärendruck (NAP-XPS) untersucht. Nach der Reaktion wurden mittels temperaturprogrammierter Oxidation (TPO) Informationen über die Menge der Kohlenstoff-ablagerungen sowie der Temperatur, die für deren Entfernung durch Oxidation erforderlich ist, gewonnen. Mittels TEM konnten Kohlenstoff-ablagerungen auf den Proben direkt beobachtet werden. Auf den ZrO2-geträgerten bi-metallischen Katalysatoren konnte die Bildung einer CuNi-Legierung während der Reduktion mehrfach bestätigt werden. Auf den TEM Bildern von zuvor reduzierten Proben konnten etwa 20 nm große Teilchen, die sowohl Kupfer als auch Nickel enthalten, beobachtet werden. Diese bimetallischen Partikel enthielten mehr Nickel als Kupfer, während zusätzlich fein verteilt reine Kupfer-Partikel am Trägermaterial gefunden wurden. Informationen über die Cu:Ni Zusammensetzung der Oberfläche wurden mittels Wasserstoff-chemisorption gewonnen. In Einklang mit den Ergebnissen der mit Infrarotspektroskopie unter-suchten CO-Adsorption wurde eine Anreicherung der Oberfläche mit Kupfer beobachtet.[1] In einem temperaturprogrammierten Experiment konnte gezeigt werden, dass Nickel deutlich aktiver als Kupfer für Methanzersetzung in Abwesenheit von Sauerstoff hinsichtlich der Wasserstoffproduktion war. Die Aktivität des bi-metallischen Cu/Ni Katalysators war zunächst so gering wie von Cu-ZrO2, bis die H2-Bildung schlagartig bei einer spezifischen Temperatur zwischen 650 und 735 K einsetzte. Im gleichen Temperaturbereich konnte mittels in situ NAP-XPS ein Konzentrations-anstieg reduzierter Nickel-Atome an der Katalysatoroberfläche beobachtet werden. Die Zugabe von Kupfer führte zur Verringerung der Kohlen-stoffablagerung, jedoch ist die Stabilität der CuNi-Legierung unter relevanten Reaktionsbedinungen stark limitiert.[2] Die strukturellen Eigenschaften der verschieden hergestellten CeO2/ZrO2 Trägermaterialien zeigten große Unterschiede. Tetragonale und kubische Mischoxidphasen mit unterschiedlichen Kristallitgrößen und spezifischen Oberflächen wurden gefunden. Die größten spezifischen Oberflächen wurden für die Trägermaterialien, welche mittels Ko-Fällung und Tensid-unterstützter Ko-Fällung hergestellt wurden, gemessen. Die Nickel-Katalysatoren wurden für Dampfreformierung und Kohlendioxidreformie-rung von Methan getestet. Außer dem mittels Tensid-unterstützter Ko-Fällung hergestellten Katalysator waren alle für diese Reformierungs-reaktionen aktiv. Die größte Aktivität unter den Mischoxid-geträgerten Proben zeigte der mittels Ko-Fällung hergestellte Katalysator. Obwohl die katalytische Aktivität im Vergleich zu Ni-ZrO2 nicht erhöht wurde, wurde eine stark verringerte Kohlenstoffablagerung, insbesondere von faser-artigem Kohlenstoff, im Vergleich zu Ni-ZrO2 auf dem aktiven Ko-fällungsgeträgerten Katalysator beobachtet.Ni/ZrO2 is used as a catalyst for methane reforming reactions, which are key processes for hydrogen production in industry and in solid oxide fuel cells. Nickel shows high activity but is rapidly deactivated by coke formation, which is a major problem. Aim of this thesis was to modify Ni/ZrO2 in order to prevent coke formation. Two approaches of catalyst modification were studied: The influence of copper addition to nickel and the modification of the zirconia support material by adding ceria were investigated. The promotion of nickel is expected to stabilize nickel electronically or via an ensemble effect against carbon deposition while the ceria-zirconia mixed oxide support releases surface oxygen more easily which oxidizes surface carbon. Copper and nickel were loaded on the zirconia support via co-impregnation followed by calcination. Ceria-Zirconia mixed oxide support materials were prepared via different synthesis routes including impregnation, combustion of precursors, co-precipitation and surfactant assisted co-precipitation. Ni was deposited on these support materials via impregnation with a nominal loading of 5 % w/w. The catalysts were characterized using techniques such as transmission electron microscopy (TEM), H2-Chemisorption, infrared spectroscopy with CO as a probe molecule, X-ray diffraction (XRD), and X-ray absorption spectroscopy (XAS). To study the catalytic performance, the catalysts were tested in a continuous flow setup for partial oxidation of methane, methane decomposition, steam reforming and dry reforming. In situ spectroscopy provides information about the catalyst under relevant reaction conditions such as high temperature and reactive gas atmosphere. In this thesis, the catalysts were studied via in situ infrared spectroscopy, in situ XAS and near ambient pressure (NAP-)XPS. After reaction, the used catalysts were further studied via temperature programmed oxidation (TPO), in order to get information about the amount of carbon deposition and the temperature needed to remove these carbon species via oxidation, and by TEM, where carbon deposition was visible. Formation of a copper-nickel alloy on the zirconia supported bimetallic catalysts during reduction was confirmed by various techniques. TEM images of catalysts reduced prior to microscopy revealed particles of about 20 nm size containing both copper and nickel . These alloy particles were enriched in nickel. Additionally, much smaller particles containing mainly or only copper were found, finely distributed over the zirconium oxide support. In order to get information about the Cu:Ni ratio on the surface, chemisorption of hydrogen as adsorbing agent was performed. The results of these experiments showed that the surface was enriched in copper, in agreement with FTIR spectroscopy of CO adsorption.[1] Temperature Programmed Reaction with mass spectrometry (MS) detection showed that the methane decompositon rate towards hydrogen in absence of oxygen was much higher on nickel than on the copper catalyst. The bimetallic Cu/Ni catalysts showed low initial activity, similar to Cu-ZrO2, until a sudden strong increase of the H2 formation rate was observed at a certain temperature between 650 and 735 K. In the same temperature range, an increase of the concentration of reduced Ni atoms at the catalyst surface in the active state, likely as a consequence of the interaction with C and CHx, was observed by in situ NAP-XPS. Cu addition to the Ni catalyst caused decreased coke formation but the CuNi alloy shows limited stability under relevant reaction conditions.[2] The textural properties of the CeO2/ZrO2 support materials prepared by different synthesis routes were strongly different. Tetragonal and cubic mixed oxide phases with different crystallite size and surface area were observed. The highest specific surface area was observed for the catalysts prepared by co-precipitation and surfactant co-precipitation. The catalytic performance of the supported nickel catalysts was evaluated for methane dry reforming and methane steam reforming. Except of the catalyst prepared by surfactant assisted co-precipitation, all catalysts were active for these methane reforming reactions. The highest activity among the mixed oxide supported catalysts was observed on the catalyst prepared by co-precipitation. Even though the catalytic activity compared to Ni-ZrO2 was not increased, strongly decreased coke formation after methane dry reforming was observed on the active co-precipitation catalyst compared to Ni-ZrO2.10

Surface spectroscopy on UHV-grown and technological Ni–ZrO2 reforming catalysts: from UHV to operando conditions

The final publication is available at Springer via https://doi.org/10.1007/s11244-016-0678-8.Ni nanoparticles supported on ZrO2 are a prototypical system for reforming catalysis converting methane to synthesis gas. Herein, we examine this catalyst on a fundamental level using a 2-fold approach employing industrial-grade catalysts as well as surface science based model catalysts. In both cases we examine the atomic (HRTEM/XRD/LEED) and electronic (XPS) structure, as well as the adsorption properties (FTIR/PM-IRAS), with emphasis on in situ/operando studies under atmospheric pressure conditions. For technological Ni–ZrO2 the rather large Ni nanoparticles (about 20 nm diameter) were evenly distributed over the monoclinic zirconia support. In situ FTIR spectroscopy and ex situ XRD revealed that even upon H2 exposure at 673 K no full reduction of the nickel surface was achieved. CO adsorbed reversibly on metallic and oxidic Ni sites but no CO dissociation was observed at room temperature, most likely because the Ni particle edges/steps comprised Ni oxide. CO desorption temperatures were in line with single crystal data, due to the large size of the nanoparticles. During methane dry reforming at 873 K carbon species were deposited on the Ni surface within the first 3 h but the CH4 and CO2 conversion hardly changed even during 24 h. Post reaction TEM and TPO suggest the formation of graphitic and whisker-type carbon that do not significantly block the Ni surface but rather physically block the tube reactor. Reverse water gas shift decreased the H2/CO ratio. Operando studies of methane steam reforming, simultaneously recording FTIR and MS data, detected activated CH4 (CH3 and CH2), activated water (OH), as well as different bidentate (bi)carbonate species, with the latter being involved in the water gas shift side reaction. Surface science Ni–ZrO2 model catalysts were prepared by first growing an ultrathin “trilayer” (O–Zr–O) ZrO2 support on an Pd3Zr alloy substrate, and subsequently depositing Ni, with the process being monitored by XPS and LEED. Apart from the trilayer oxide, there is a small fraction of ZrO2 clusters with more bulk-like properties. When CO was adsorbed on the (fully metallic) Ni particles at pressures up to 100 mbar, both PM-IRAS and XPS indicated CO dissociation around room temperature and blocking of the Ni surface by carbon (note that on the partially oxidized technological Ni particles, CO dissociation was absent). The Ni nanoparticles were stable up to 550 K but annealing to higher temperatures induced Ni migration through the ultrathin ZrO2 support into the Pd3Zr alloy. Both approaches have their benefits and limitations but enable us to address specific questions on a molecular level.Austrian Science Fund (FWF

Surface composition changes of CuNi-ZrO2 catalysts during methane decomposition: An operando NAP-XPS and density functional study

Bimetallic CuNi nanoparticles of various nominal compositions (1:3, 1:1, 3:1) supported on ZrO2wereemployed for operando spectroscopy and theoretical studies of stable surface compositions under reac-tion conditions of catalytic methane decomposition up to 500◦C. The addition of Cu was intended toincrease the coke resistance of the catalyst. After synthesis and (in situ) reduction the CuNi nanoparticleswere characterized by HR-TEM/EDX, XRD, FTIR (using CO as probe molecule) and NAP-XPS, all indicatinga Cu rich surface, even when the overall nanoparticle composition was rich in Ni. Density functional (DF)theory modelling, applying a recently developed computational protocol based on the construction oftopological energy expressions, confirmed that in any studied composition Cu segregation on surfacepositions is an energetically favourable process, with Cu preferentially occupying corner and edge sites.Ni is present on terraces only when not enough Cu atoms are available to occupy all surface sites.When the catalysts were applied for methane decomposition they were inactive at low temperaturebut became active above 425◦C. Synchrotron-based operando NAP-XPS indicated segregation of Ni on thenanoparticle surface when reactivity set in for CuNi-ZrO2. Under these conditions C 1s core level spectrarevealed the presence of various carbonaceous species at the surface. DF calculations indicated that boththe increase in temperature and especially the adsorption of CHxgroups (x = 0-3) induce the segregationof Ni atoms on the surface, with CH3providing the lowest and C the highest driving force.Combined operando and theoretical studies clearly indicate that, independent of the initial surfacecomposition after synthesis and reduction, the CuNi-ZrO2catalyst adopts a specific Ni rich surface underreaction conditions. Based on these findings we provide an explanation why Cu rich bimetallic systemsshow improved coke resistance