Hydrogenation on palladium nanoparticles supported by graphene nanoplatelets

Pd nanoparticles (1 wt %; mean size ∼4 nm) were supported on ∼2 μm sized, but few nanometers thick, graphene nanoplatelets (GNPs) and compared to 1 wt % Pd on activated carbon or γ-alumina. Catalyst morphology, specific surface area, and Pd particle size were characterized by SEM, BET, and TEM, respectively. H2-TPD indicated that GNPs intercalated hydrogen, which may provide additional H2 supply to the Pd nanoparticles during C2H4 hydrogenation. Whereas the two types of Pd/GNPs (NaOH vs calcinated) catalysts were less active than Pd/C and Pd/Al2O3 below 40 °C, at 55 °C they were about 3–4 times more active. As for example Pd/GNPs (NaOH) and Pd/Al2O3 exhibited not too different mean Pd particle size (3.7 vs 2.5 nm, respectively), the higher activity is attributed to the additional hydrogen supply likely by the metal/support interface, as suggested by the varying C2H4 and H2 orders on the different supports. Operando XANES measurements during C2H4 hydrogenation revealed the presence of Pd hydride. The Pd hydride was more stable for Pd/GNPs (NaOH) than for Pd/C, once more pointing to a better hydrogen supply by graphene nanoplatelets

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

Adsorption and reaction of CO on (Pd–)Al2O3 and (Pd–)ZrO2: vibrational spectroscopy of carbonate formation

γ-Alumina is widely used as an oxide support in catalysis, and palladium nanoparticles supported by alumina represent one of the most frequently used dispersed metals. The surface sites of the catalysts are often probed via FTIR spectroscopy upon CO adsorption, which may result in the formation of surface carbonate species. We have examined this process in detail utilizing FTIR to monitor carbonate formation on γ-alumina and zirconia upon exposure to isotopically labelled and unlabelled CO and CO2. The same was carried out for well-defined Pd nanoparticles supported on Al2O3 or ZrO2. A water gas shift reaction of CO with surface hydroxyls was detected, which requires surface defect sites and adjacent OH groups. Furthermore, we have studied the effect of Cl synthesis residues, leading to strongly reduced carbonate formation and changes in the OH region (isolated OH groups were partly replaced or were even absent). To corroborate this finding, samples were deliberately poisoned with Cl to an extent comparable to that of synthesis residues, as confirmed by Auger electron spectroscopy. For catalysts prepared from Cl-containing precursors a new CO band at 2164 cm−1 was observed in the carbonyl region, which was ascribed to Pd interacting with Cl. Finally, the FTIR measurements were complemented by quantification of the amount of carbonates formed via chemisorption, which provides a tool to determine the concentration of reactive defect sites on the alumina surface

Isomerisierung von n-Alkanen über Pt hältigem sulfatisierten Zirkonoxid

Gesättigte Kohlenwasserstoffe, Hauptbestandteile von Rohöl, sind bis heute die wichtigste Quelle für die Herstellung von Treibstoffen. Da geradkettige n-Alkane eine sehr niedrige Oktanzahl aufweisen, müssen sie zu verzweigten Alkanen isomerisiert werden. Als Katalysatoren für die Isomerisierung werden bifunktionelle Katalysatoren mit sauren und metallischen Zentren eingesetzt. Sulfatisiertes Zirkonoxid (SZ) hat großes Interesse hervorgerufen, da es eine ausgezeichnete Aktivität für die Isomerisierung von kurzkettigen Alkanen bei tiefen Temperaturen aufweist. Allerdings ist die Art der aktiven Zentren von SZ noch ungeklärt.Ziel dieser Arbeit war, die Art der aktiven Zentren und den Einfluß des Sulfatgehalts mittels verschiedener Charakterisierungsmethoden, in situ IR Spektroskopie sowie der Isomerisierung von n-Hexan und n-Heptan als kinetische Testreaktionen zu untersuchen.Während der Reaktion von n-Alkanen in inertem Trägergas deaktiviert SZ sehr rasch aufgrund von Kohlenstoffablagerungen am Katalysator. Erst in Gegenwart von Wasserstoff und Pt findet man eine sehr stabile Aktivität.Für die Isomerisierung von n-Hexan wurde eine ausgezeichnete Isomerisierungsselektivität beobachtet. Hingegen bei der Reaktion von n-Heptan fand hauptsächlich Cracken zu Propan und iso-Butan statt, was auf einen säurekatalysierten Crackmechanismus hinweist.Es wurden zwei verschiedene Arten von Sulfatgruppen an der Oberfläche von SZ identifiziert. Eine Spezies, die nach Reduktion schwächer an der Oberfläche gebunden ist bzw. erst bei einer Bedeckung mit mehr als einer halben Monolage Sulfaten gebildet wird, ist verantwortlich für die katalytische Aktivität, während der Großteil der Sulfatgruppen inaktiv für die Umsetzung von n-Alkanen ist. Als aktive Spezies werden Pyrosulfatgruppen diskutiert. Pyridinadsorption ergab, dass ein inaktives Material nur Lewis saure Zentren (LS) besitzt, während ein aktiver Katalysator sowohl Brønsted saure Zentren (BS) als auch LS aufweist. Eine möglichst große Nähe der sauren und metallischen Zentren zueinander führt zu einer signifikant besseren Aktivität und Isomerisierungsselektivität. Ensembles, die Brønsted saure Zentren an Pyrosulfatgruppen und benachbarte Metallzentren besitzen, werden als aktive Zentren für die n-Alkanisomerisierung angenommen.Saturated hydrocarbons as the main components of raw oil are one of the most important sources of fuels. Since n-alkanes possess a low octane number isomerization to branched alkanes is necessary.Bifunctional catalysts containing metal and acid sites are used for isomerization. Among the solid acid catalysts sulfated zirconia (SZ) has reached a lot of interest due to its high activity for the isomerization of light alkanes at low temperatures. Until now a discussion exists about the nature of the active sites on SZ. The aim of this thesis is to investigate the nature of active sites and the influence of the sulfate content by means of in situ infrared spectroscopy, various characterization techniques and kinetic measurements using n-hexane and n-heptane conversion as test reaction.During the conversion of n-alkanes in inert carrier gas SZ deactivates very fast due to carbon deposition on the catalyst. A stable and high activity was only observed in the presence of hydrogen and Pt. The selectivity to isomerization was excellent for hexane, whereas for n-heptane the main reaction products were propane and iso-butane indicating an acid catalyzed cracking mechanism.It was found that there are at least two different sulfate species present on the surface of sulfated zirconia. Sulfate species, which are more weakly bonded to the surface after reduction and which are only formed at sulfate loadings of more than half a monolayer, are essential for catalytic activity, whereas most of the sulfate groups are inactive for n-alkane conversion.As active species pyrosulfate groups are proposed. Pyridine adsorption on an inactive sample showed only the presence of Lewis acid sites (LS), whereas active samples possess both - Brønsted (BS) and Lewis acid sites. A close vicinity of the metal and acidic sites leads to a significantly better performance of the catalyst. Thus, ensembles containing Brønsted acid sites connected to pyrosulfate species and adjacent metal sites are suggested as active sites for n-alkane conversion.11

The mechanism of carbonate formation on Pd–Al2O3 catalysts

Vibrational spectroscopic investigations of the adsorption of isotopically labelled and unlabelled CO and CO2 reveal that carbonate formation on Pd–alumina catalysts occurs via an oxygen down reaction of CO with hydroxyl groups on the support, whereas CO dissociation on Pd can be excluded

Infrared Studies on Bimetallic Copper/Nickel Catalysts Supported on Zirconia and Ceria/Zirconia

Infrared spectroscopy has been employed for a detailed characterization of ZrO2 and CeO2/ZrO2 supported nickel and copper/nickel catalysts to be utilized for methane decomposition. Adsorption of CO at 303 K was performed in order to determine the surface composition and accessible adsorption sites. Alloy formation occurred during reduction, as indicated by a red-shift of the vibrational band of CO on Ni: by 27 cm−1 on nickel-rich CuNi alloy, by 34 cm−1 on 1:1 Cu:Ni and by 36 cm−1 on copper-rich CuNi alloy. CuNi alloy formation was confirmed by X-ray absorption spectroscopy during reduction revealing a considerably lower reduction temperature of NiO in the bimetallic catalyst compared to the monometallic one. However, hydrogen chemisorption indicated that after reduction at 673 K copper was enriched at the surface of the all bimetallic catalysts, in agreement with IR spectra of adsorbed CO. In situ IR studies of methane decomposition at 773 K demonstrated that the addition of Cu to Ni strongly reduced coking occurring preferentially on nickel, while maintaining methane activation. Modification of the zirconia by ceria did not have much effect on the adsorption and reaction properties. Ceria-zirconia and zirconia supported samples exhibited very similar properties and surface chemistry. The main difference was an additional IR band of CO adsorbed on metallic copper pointing to an interaction of part of the Cu with the ceria.ISSN:1011-372XISSN:1572-879