Metal-support Interaction Effect Research Articles

Effect of Surface Composition and Structure of the Mesoporous Ni/KIT-6 Catalyst on Catalytic Hydrodeoxygenation Performance

A series of Ni/KIT6 catalyst precursors with 25 wt.% Ni loading amount were reduced in H2 at 400, 450, 500, and 550 °C, respectively. The studied catalysts were investigated by XRD, Quasi in-situ XPS, BET, TEM, and H2-TPD/Ranalysis methods. It was found that reduction temperature is an important factor affecting the hydrodeoxygenation (HDO) performance of the studied catalysts because of the Strong Metal Support Interaction Effect (SMSI). The reduction temperature influences mainly the content of active components, crystal size, and the abilityfor adsorbing and activating H2. The developed pore structure and large specific surface area of the KIT-6 support favored the Ni dispersion. The RT450 catalyst, which was prepared in H2 atmosphere at 450 °C, has the best HDO performance. Ethyl acetate can be completely transformed and maintain 96.8% ethane selectivity and 3.2% methane selectivity at 300 °C. The calculated apparent activation energies of the prepared catalysts increased in the following order: RT550 > RT400 > RT500 > RT450.

Kinetic Modelling of Co3O4- and Pd/Co3O4-Catalyzed Wet Lean Methane Combustion

Reaction kinetics of methane combustion is investigated on Co3O4 and Pd/Co3O4 (0.27 wt% Pd) catalysts for a fuel-lean feed. The temperature ranges between 250 and 550 °C in the presence of 5 and 10 vol% water with CH4 concentrations that varied between 1000 and 5000 ppmv. Significant cobalt oxide contribution to the activity of the bimetallic catalyst is observed, especially at higher water concentration and lower temperature (up to 70%). Co3O4 demonstrates first order to CH4, 0 order to H2O and activation energy of 69 kJ/mol. Pd/Co3O4 catalyst shows first order to CH4, negative 0.37 order to H2O and an observed activation energy of 90.7 kJ/mol, which is corrected for water adsorption to 60.6 kJ/mol. The latter is a typical activation energy for Pd/Al2O3 catalyst at similar conditions, indicating that the Co3O4 contribution is not only in performing the methane combustion itself but also in supplying surface oxygen rather than in affecting the activation energy. The kinetic evidence shows that the observed behaviour of Pd/Co3O4 catalyst is not a summation of individual activities of Co3O4 and Pd, but rather the effect of strong metal-support interactions (SMSI).

A Theory/Experience Description of Support Effects in Carbon-Supported Catalysts.

The support plays an important role for supported metal catalysts by positioning itself as a macromolecular ligand, which conditions the nature of the active site and contributes indirectly but also sometimes directly to the reactivity. Metal species such as nanoparticles, clusters, or single atoms can be deposited on carbon materials for various catalytic reactions. All the carbon materials used as catalyst support constitute a large family of compounds that can vary both at textural and at structural levels. Today, the recent developments of well-controlled synthesis methodologies, advanced characterization techniques, and modeling tools allow one to correlate the relationships between metal/support/reactant at the molecular level. Based on these considerations, in this Review article, we wish to provide some answers to the question "How and why anchoring metal nanoparticles, clusters, or single atoms on carbon materials for catalysis?". To do this, we will rely on both experimental and theoretical studies. We will first analyze what sites are available on the surface of a carbon support for the anchoring of the active phase. Then, we will describe some important effects in catalysis inherent to the presence of a carbon-type support (metal-support interaction, confinement, spillover, and surface functional group effects). These effects will be commented on by putting into perspective catalytic results obtained in numerous reactions of thermal catalysis, electrocatalysis, or photocatalysis.

Cobalt‐Based Catalyst Supported on Different Morphologies of Alumina for Non‐oxidative Propane Dehydrogenation: Effect of Metal Support Interaction and Lewis Acidic Sites

AbstractDeveloping an economically viable and eco‐friendly catalyst is essential for the dehydrogenation reaction. In this context, cobalt‐based catalysts supported on different morphologies of γ‐Al2O3, nano‐sheet (Al2O3−NS), nano‐fiber (Al2O3−NF) and nano‐plate (Al2O3−NP) were synthesized using wetness impregnation method and tested for non‐oxidative propane dehydrogenation reaction. Metal support interaction and Lewis acidic center play a crucial role in propane dehydrogenation reaction; therefore this study focuses on investigating the role of these parameters on the catalytic activity for cobalt‐based catalysts. A higher metal support interaction was observed in Co/Al2O3−NS as compared to Co/Al2O3−NF and Co/Al2O3−NP. The presence of tetrahedral coordinated Co+2 in Co/Al2O3−NS leads to higher selectivity towards propylene. The formation of metallic cobalt from Co3O4 precursor present in Co/Al2O3−NP promoted side reactions to form lower hydrocarbons and carbon, which caused catalyst deactivation. In‐situ DRIFTS analysis revealed the interaction of propane and propylene gases with the catalyst, which was an important revelation to find the source of carbon deposition.

Ruthenium/cobalt binary oxides supported on hollow alumina microspheres as highly efficient catalyst for vinyl chloride oxidation

Alumina hollow microspheres (Al2O3-hms) were synthesized and used to support metal oxide and binary metal oxide particles to prepare three catalysts, designated as RuOx/Al2O3-hms, CoOx/Al2O3-hms and RuCoOx/Al2O3-hms. Their catalytic properties in vinyl chloride oxidation were investigated and found to follow this order: RuCoOx/Al2O3-hms > RuOx/Al2O3-hms > CoOx/Al2O3-hms. In comparison with a commercial Al2O3 support, the Al2O3-hms provide a higher specific surface area and unique hollow spherical morphology, benefiting for uniform distribution of catalyst particles. It was also found that the strong metal-support interaction between metal nanoparticles and Al2O3-hms has significant effects on the low-temperature reducibility, the surface oxygen abundance and mobility, giving rise to different oxidation activities. The Al2O3-hms supported metal catalysts remained relatively high catalytic activity after consecutive catalytic runs and high-temperature annealing. The anti-sintering ability enhancement of metal particles was attributed to the stabilization and metal-support interaction effects with Al2O3-hms.

Tailoring performance of Co–Pt/MgO–Al2O3 bimetallic aerogel catalyst for methane oxidative carbon dioxide reforming: Effect of Pt/Co ratio

Co–Pt/MgO–Al2O3 bimetallic aerogel catalysts were synthesized via a sol-gel combined with supercritical drying method. The catalysts were characterized by XRD, BET, HRTEM, STEM-HAADF, XPS, H2-TPR, H2-TPD, TG/DSC, FESEM and their catalytic performances in CH4 oxidative CO2 reforming were evaluated. The H2 spillover effect between Pt and Co enhanced the reducibility of the catalyst, while the strong metal-support interaction (SMSI) effect in the bimetallic aerogel catalysts confined the agglomeration of metal particles. Pt/Co ratio played a key role on the existence of surface metal species, leading to different catalytic performances. The optimal Pt/Co ratio was Pt/Co = 0.02 w/w, on which a 50% higher activity in terms of CH4 conversion than monometallic Co or Pt aerogel catalysts was obtained. Whereas the impregnated catalyst with an identical composition showed a much lower activity. The Co–Pt aerogel catalysts also showed high resistance to inactive carbon formation. The oxidation temperature of the carbon species deposited on the spent Co–Pt aerogel catalyst was only 275 °C and no filamentous or graphitic carbon was identified, disclosing that the formation of inactive carbon was inhibited due to the synergy between Co and Pt and the SMSI effect.

Active and stable Pt-Ceria nanowires@silica shell catalyst: Design, formation mechanism and total oxidation of CO and toluene

Cerium oxide is one of the most important rare earth metal oxides in catalysis, however, the sintering problem of noble metals and CeO2 at higher temperature (e.g., >700 °C) is still unresolved. Herein, Pt nanoparticles self-assembled on ultra-thin CeO2 nanowires (NWs) and then confined inside a thermally robust porous silica shell (Pt-CeO2NW@SiO2) were introduced. The thickness of CeO2 NWs was just ˜2.0 nm. Moreover, Pt-CeO2 NW@SiO2 showed significantly enhanced catalytic performances for total oxidation of CO and toluene. The increased catalytic properties are attributed to the strong metal-support interaction effect between Pt and CeO2 NWs at sub-nanoscale. Most importantly, the special core-shell structure also affords excellent sintering resistance retention up to 700 °C for 100 h, due to the guarding effect of porous silica shell. Finally, the formation mechanism of Pt-CeO2 NW@SiO2 was investigated in detail. Current strategy should inspire many rational designs of rare-earth metal-based nanocatalysts for real-world catalysts.

Effect of Mo promotion on the activity and selectivity of Ru/Graphite catalysts for Fischer-Tropsch synthesis

In this paper molybdenum oxide promoted graphite supported ruthenium catalysts highly active and selective for Fischer-Tropsch synthesis (FTS) are presented. The effect of Mo loading (0–5 wt%) of promoted graphite supported Ru (2 wt%) catalysts on the syngas conversion was studied at 523 K, H2/CO = 2 and 3.5 bar. Mean diameters of Ru nanoparticles were all close to 3 nm independently of the molybdenum loading used. Microcalorimetric measurements during CO adsorption at 330 K reveal a clear interaction between Ru and Mo observing an important increase of CO adsorption heats for Mo/Ru ratios of ≤0.26. XPS analysis performed to 2Ru0.5Mo/G catalyst after in-situ reduction showed the presence of MoVI (MoO3) and MoIV (MoO2) species on the catalyst surface. It is argued that these Mo species could be located around the ruthenium nanoparticles acting as Lewis acids and therefore facilitating the CO dissociation. Mo promotion increased up to four times the activity with respect to unpromoted Ru catalyst, increased the selectivity to C5+ products and improved the olefin to paraffin ratio in the C2-C3 hydrocarbons range. These conclusions contribute to understand the Mo promotion effect in the FTS, because realistic deductions have been obtained using a proper inert support for studying the molybdenum promotion effect in ruthenium based catalysts, avoiding strong metal-support interaction effects.

Light-Triggered Boost of Activity of Catalytic Bola-Type Surfactants by a Plasmonic Metal-Support Interaction Effect.

The maximization of activity is a general aim in catalysis research. The possibility for light-triggered enhancement of a catalytic process, even if the process is not photochemical in nature, represents an intriguing concept. Here, we present a novel system for the exploration of the latter idea. A surfactant with a catalytically active head group, a protonated polyoxometalate (POM) cluster, is attached to the surface of a gold nanoparticle (Au NP) using thiol coupling chemistry. The distance of the catalytically active center to the gold surface could be adjusted precisely using surfactants containing hydrocarbon chains (Cn) of different lengths (n = 4–10). Radiation with VIS-light has no effect on the catalytic activity of micellar aggregates of the surfactant. The situation changes, as soon as the surfactants have been attached to the Au NPs. The catalytic activity could almost be doubled. It was proven that the effect is caused by coupling the surface plasmon resonance of the Au NPs with the properties of the POM head group. The improvement of activity could only be observed if the excitation wavelength matches the absorption band of the used Au NPs. Furthermore, the shorter the distance between the POM group and the surface of the NP, the stronger is the effect. This phenomenon was explained by lowering the activation energy of the transition state relevant to the catalytic process by the strong electric fields in the vicinity of the surfaces of plasmonic nanoparticles. Because the catalytic enhancement is wavelength-selective, one can imagine the creation of complex systems in the future, a system of differently sized NPs, each responsible for a different catalytic step and activated by light of different colors.

Metal-support interaction controlled migration and coalescence of supported particles

The particle migration and coalescence (PMC) kinetics of a supported metal are the main deactivation mechanisms restricting the successful industrialization of nanoparticles, but the theoretical insights regarding these kinetics are lacking. One key issue is the lack of a physical model to predict the effects of metal-support interaction (MSI) on PMC kinetics. In this paper, we report a theoretical study of PMC kinetics and their dependence on MSI. A new particle diffusion model is proposed based on the surface premelting hypothesis that considers the contact angle of a hemispherical particle on the support. Enhanced MSI suppresses PMC by increasing the radius of curvature and the interfacial adhesion energy, even though the accompanying reduction in the geometry factor partially promotes PMC kinetics. The increased surface energy increases the chemical potential of the atoms in the particle, which is conducive to PMC; an increased surface energy also results in enhanced MSI, which suppresses PMC. The competition between these two contradictory effects leads to a critical contact angle where the surface energy has no influence on the diffusion and resulting PMC kinetics. The proposed diffusion theory mode lincluding the effects of the support and the corresponding kinetic simulations, shed light onto the support-dependence of PMC kinetics and provide a foundation for further optimization and design of supported particles with better stability.

Steam reforming of n-dodecane over mesoporous alumina supported nickel catalysts: Effects of metal-support interaction on nickel catalysts

Developing a highly active and stable Ni-based catalyst is still a challenge for the generation of on-site hydrogen through steam reforming of long-chained hydrocarbons, such as kerosene fuels. Ni nanoparticles (ca. 5 nm) on mesoporous alumina prepared by atomic layer deposition (ALD) were employed in steam reforming of n-dodecane, and exhibited a turnover frequency (TOF) of 477.6 h−1, whereas Ni nanoparticles on commercial alumina support prepared by impregnation method exhibited a TOF of 100 h−1. The high activity of ALD Ni catalysts was ascribed to high reduction degree, as confirmed by X-ray diffraction (XRD), transmission electron microscopy (TEM), and H2-chemisorption. A deactivation was also observed on the ALD prepared catalysts, which was ascribed to the weak metal-support interaction, as confirmed by H2 temperature-programmed reduction (TPR). The ALD Ni/Al2O3 catalysts were further modified with CeO2 and they showed enhanced stability with 8% deactivation degree in steam reforming of n-dodecane. Further characterizations of spent catalysts showed that the presence of CeO2 was favorable for stabilizing Ni nanoparticles by enhancing moderate metal-support interaction, and reducing the formation of coke on the interfaces of NiCeO2.

Core-shell structure Ag@Pd nanoparticles supported on layered MnO2 substrate as toluene oxidation catalyst

The silver palladium bimetallic core-shell structure nanoparticles (Ag@Pd NPs) were synthesized by a thin Pd shell slowly generating on the outmost of Ag nanoparticle according to galvanic replacement mechanism. Then, the 2D layered structure manganese dioxide (MnO2) substrate was used to support the as-synthesize Ag@Pd NPs to prepare Ag@Pd/MnO2 catalyst for toluene purification in oxidation reaction. Physicochemical properties of the samples were characterized by a number of different analytical techniques. It is found that the Ag@Pd NPs are homogeneously spherical shape with an average particle diameter size of 7.1 nm. And the MnO2 substrate can not only uniformly disperse Ag@Pd active component on its surface for its high specific surface area and mesopore volume, but also provide the reactive lattice oxygen by its mixed oxidation states of Mn3+/Mn4+. Moreover, the core-shell configuration of Ag@Pd NPs can improve the state phase transformation from Pd0 to PdO2 with the aid of lattice oxygen of MnO2 substrate, because of the lattice oxygen migration controlled strong metal-support interaction (SMSI) effect between palladium active component and MnO2 substrate. So, there would be more exposed PdO2 active sites on the Ag@Pd/MnO2 catalyst relative to monometallic Pd/MnO2 catalyst. Hence, the as-prepared bimetallic core-shell Ag@Pd/MnO2 catalyst exhibits greatly higher toluene oxidation activity at lower reaction temperature than monometallic Pd/MnO2 catalyst, indicating it is a promising catalyst for use in volatile organic compounds (VOCs) purification.

Strong Metal-Support Interaction (SMSI) Effect between Metal Catalysts and Carbide Supports

Strong Metal-Support Interaction (SMSI) Effect between Metal Catalysts and Carbide Supports

Molecular-level understanding of reaction path optimization as a function of shape concerning the metal–support interaction effect of Co/CeO2 on water-gas shift catalysis

In this work, the active sites generated in hydrogen reduction and the reaction pathways for the water gas shift (WGS) reaction over Co/CeO2 catalysts were studied by in situ XAS and XPS coupled with DFT+U calculations.

Pt Nanoparticles Loaded on W18O49 Nanocables-rGO Nanocomposite as a Highly Active and Durable Catalyst for Methanol Electro-Oxidation.

Highly active and durable electrocatalysts are vital for commercialization of direct methanol fuel cells. In this work, a three-dimensional nanocomposite consisting of platinum nanoparticles, W18O49 nanocables, and reduced graphene oxide composite (Pt/W18O49 NCs–rGO) has been prepared as an electrocatalyst for methanol oxidation reaction (MOR). The catalyst is prepared through a two-step method. The W18O49 nanocables and the reduced graphene oxide composite are prepared by a solvothermal process. Then, Pt nanoparticles are loaded on the W18O49 nanocables and the reduced graphene oxide composite by a hydrogen reduction at ambient condition. The obtained catalyst has a special three-dimensional architecture consisting of two-dimensional nanosheets, assembled one-dimensional nanocables, and the loaded nanoparticles on their surface. The Pt/W18O49 NCs–rGO catalyst shows 1.56 time mass activities than the Pt/C, with the current density of the forward anodic peak reaching 1624 mA/mgPt at 0.854 V versus reversible hydrogen electrode potential in 0.1 M HClO4 and 0.5 M CH3OH mixed electrolyte. It also shows a strong antipoisoning property toward CO. For the durability testing, the current density of Pt/W18O49 NCs–rGO shows a 37% decay, whereas the current of Pt/C catalyst shows a 41% degradation from 600 to 3600 s at 0.7 V. The high activity toward MOR, good antipoisoning for intermediate products, and excellent stability are ascribed to strong metal–support interaction effects between the Pt nanoparticles and the W18O49 NCs.

Carbide-Supported Au Catalysts for Water-Gas Shift Reactions: A New Territory for the Strong Metal-Support Interaction Effect.

Strong metal-support interaction (SMSI) has been regarded as one of the most important concepts in heterogeneous catalysis, which has been almost exclusively discussed in metal/oxide catalysts. Here, we show that gold/molybdenum carbide (Au/MoC x) catalysts feature highly dispersed Au overlayers, strong interfacial charge transfer between metal and support, and excellent activity in the low-temperature water-gas shift reaction (LT-WGSR), demonstrating the active SMSI state. Subsequent oxidation treatment results in strong aggregation of Au nanoparticles, weak interfacial electronic interaction, and poor LT-WGSR activity. The two interface states can be transformed into each other by alternative carbonization and oxidation treatments. This work reveals the active SMSI effect in metal/carbide catalysts induced by carbonization, which opens a new territory for this important concept.

Electrochemical Promotion of Bimetallic Nanoparticles with Low-Noble Metal Content Interfaced with Yttria-Stabilized Zirconia Solid-Electrolyte

Heterogeneous catalysts play a central role in production of a variety of useful chemicals, energy generation and greenhouse gas emissions reduction. A heterogeneous catalyst usually consists of fine metal or metal oxide particles in the nanometer range dispersed on a high surface area supports. The catalytic activity and/or selectivity of the catalyst could be tuned in by using chemical promoters that are added to the catalyst during its fabrication or by choosing a suitable catalyst supports via metal-support interaction (MSI) effect. Another way to efficiently promote catalytic activity and in some instances catalyst selectivity is the use of electrochemistry via Electrochemical Promotion of Catalysis (EPOC), also called Non-faradaic Electrochemical Modification of Catalytic Activity (NEMCA) phenomenon [1, 2]. EPOC or NEMCA is ascribed to significant changes in the catalytic properties of metal or metal oxide catalysts deposited on solid-electrolytes caused by the application of small current (in the range of a few μA) or potential (up to 2V). The application of electric stimuli results in the supply of ionic species from the solid-electrolyte to the active surface, where the catalytic reaction takes place. These ionic species act as promotes and their addition results in significant catalytic rate increase due to changes in the catalyst work function [2-4]. This phenomenon has been applied to over 100 catalytic systems and reactions and continues to make an impact to surface science, catalysis and solid-state electrochemistry. The application of EPOC to nanostructured, highly dispersed catalysts has attracted significant interest in the last decade and several studies have demonstrated dramatic EPOC effect with noble metal (Pt, RuO2, Pd, etc.) nanoparticles (NPs) as low as 1.5 nm in size and metal loading down to 0.1 mg metal/cm2 [5-9]. Despite this tremendous progress, further improvement in metal utilization for practical advancement of EPOC is essential. In the present work, we investigated EPOC of bi-metallic, highly dispersed Ni90Pd10 and Ru45Fe55 (at. %) nanoparticles for two environmentally important reactions: CH4 combustion and CO2 hydrogenation. Both CH4 and CO2 are greenhouse gases that have negative impact on the environment and contribute to climate change, therefore methane conversion to CO2 and CO2 transformed to useful chemicals have several advantages. Ni90Pd10 and Ru45Fe55 NPs were synthesized using polyol reduction method by using ethylene glycol as reducing and stabilizing agent. TEM/SEM and XRD analysis were carried out to determent the particle and crystallite size, as well as alloy formation in bi-metallic NPs. The resulting colloidal solution containing NPs were deposited on yttria-stabilized zirconia (8 mol % Y2O3-ZrO2, (YSZ)) solid-electrolyte disk (D = 19 mm, thickness = 1 mm) with loading of 0.2 - 0.3 mg metal/cm2 and served as a catalyst-working electrode. Inert gold counter and pseudo-reference electrode were deposited on the opposite side of YSZ disk. Gold mesh served a current collector for NPs. The solid-electrolyte cell was placed in the CSTR-type quartz reactor (atm. pressure) [9] for CH4 combustion (T = 400 – 500 oC) and CO2 hydrogenation (T = 275 – 400 oC) reactions. Galvanostatic and potentiostatic EPOC transient experiments were carried out under various gas compositions and temperatures. The EPOC results on bi-metallic nanoparticles will be presented and the role of Ni and Fe on the catalytic activity and EPOC efficiency will be discussed. References Stoukides, C.G. Vayenas, J. Catal., 70 (1981), 137-146G. Vayenas, S. Bebelis, S. Ladas, Nature 343 (1990) 625–627.G. Vayenas, S. Bebelis, C. Pliangos, S. Brosda, D. TsiplakidesElectrochemical Activation of Catalysis: Promotion, Electrochemical Promotion and Metal-support Interactions, Kluwer Academic/Plenum, New York (2001)Vernoux, L. Lizarraga, M.N. Tsampas, F.M. Sapountzi, A. De Lucas-Consuegra, J.L. Valverde, S. Souentie, C.G. Vayenas, D. Tsiplakides, S. Balomenou, E.A. Baranova, Chem. Rev. 113 (2013) 8192–8260.Kambolis, L. Lizarraga, M.N. Tsampas, L. Burel, M. Rieu, J.P. Viricelle, Vernoux Electrochem. Commun., 19 (2012), pp. 5-8.A.E. Dole, E.A. Baranova, Implementation of Nanostructured Catalysts in the Electrochemical Promotion of Catalysis in Handb. Nanoelectrochemistry M. Aliofkhazraei, H.A.S. Makhlouf (Eds.), Springer International Publishing, Cham (2015), 1-27A.E. Dole, A. Costa, M. Couillard, E.A. Baranova, J. Catal. 333 (2016) 40–50.M. Hajar, K.D. Patel, U. Tariq, E.A. Baranova, J. Catal. 352 (2017) 42–51.

Hydrogen reduction and metal-support interaction in a metastable metal-oxide system: Pd on rhombohedral In2O3

Structural and chemical consequences of reactive metal-support interaction (RMSI) effects occurring under hydrogen reduction in a metastable metal-oxide system have been exemplified for small PdO particles loaded onto rhombohedral In2O3 (PdO/rh-In2O3) using synchrotron-based in situ X-ray diffraction experiments at temperatures up to 500 °C. rh-In2O3 is a meta-stable In2O3 polymorph that is prone to gas-phase dependent phase transformation to cubic In2O3 (c-In2O3). Cross-influence of metal-support interaction and phase transformation can therefore be expected in similar temperature regimes. To separate both effects, comparable experiments have also been conducted on pure rh-In2O3. Phase transformation of pure rh-In2O3 to cubic In2O3 in hydrogen occurs between 415 °C and 450 °C. On Pd/rh-In2O3, a sequence of PdO reduction to Pd metal, followed by PdH0.706 hydride formation and subsequently, InPd and In3Pd2 intermetallic compound formation have been observed between 30 °C and 500 °C. After the intermetallic compound formation is finished at around 400 °C, the phase transformation to c-In2O3 sets in at exactly the same temperature as on pure rh-In2O3 and extends over the same temperature range. This proves that the phase transformation of rh-In2O3 to c-In2O3 is not influenced by the reduction and the intermetallic compound formation. In contrast to Pd on c-In2O3, In-Pd compound formation from rh-In2O3 occurs at much lower temperatures (230 °C vs. 300 °C), despite finally approaching the same compound stoichiometries (InPd and In3Pd2). This points to a high structural stability of reduced rh-In2O3/stability of oxygen vacancies (compared to c-In2O3) as well as to facilitated diffusion of reduced In(-O) species at low temperatures.

Flame spray pyrolysis synthesized CuO-TiO2 nanoparticles for catalytic combustion of lean CO

In this work, CuO-TiO2 nanoparticles with different CuO mass contents of 2%, 8%, 12%, and 20% are synthesized by flame spray pyrolysis (FSP) method and applied to catalytic combustion of lean CO. The nano-catalyst is characterized by N2-physisorption isotherms, X-ray diffraction (XRD), transmission electron microscopy (TEM), H2-TPR (temperature-programmed reduction) and X-ray photoelectron spectroscopy (XPS). All the catalysts possess a high specific surface area, of which the CuO-TiO2 nanoparticles with 2 wt.% Cu (2CT) is as high as 98 m2/g, and exhibits a spherical structure with a diameter of 15–20 nm. Compared with other methods, the FSP method can significantly improve the loading of CuO without producing large crystalline CuO particles on the catalyst surface. Interestingly, the addition of CuO will essentially change the lattice structure of TiO2 for all catalysts, including its crystal spacing and XRD diffraction angle. Copper cations are embedded in TiO2 lattice to promote the transformation from anatase to rutile by producing oxygen defect at high flame temperature. The interaction between CuO and TiO2 has significant influence on its physicochemical properties. A lower onset reduction temperature on the sample with higher CuO loading is obtained due to the hydrogen spillover effect in H2-TPR test. Moreover, the loaded CuO increases the content of more stable rutile phase in the materials, so that it reduces the strong metal-support interaction (SMSI) effect of CuO and anatase phase to improve the properties of CO catalytic combustion. The synthesized CuO-TiO2 nanoparticles can achieve complete combustion conversion of lean CO at lower temperature of 120°C.

In situ NAP-XPS spectroscopy during methane dry reforming on ZrO2/Pt(1 1 1) inverse model catalyst

Due to the need of sustainable energy sources, methane dry reforming is a useful reaction for conversion of the greenhouse gases CH4 and CO2 to synthesis gas (CO + H2). Syngas is the basis for a wide range of commodity chemicals and can be utilized for fuel production via Fischer–Tropsch synthesis. The current study focuses on spectroscopic investigations of the surface and reaction properties of a ZrO2/Pt inverse model catalyst, i.e. ZrO2 particles (islands) grown on a Pt(1 1 1) single crystal, with emphasis on in situ near ambient pressure x-ray photoelectron spectroscopy (NAP-XPS) during MDR reaction. In comparison to technological systems, model catalysts facilitate characterization of the surface (oxidation) state, surface adsorbates, and the role of the metal-support interface. Using XPS and infrared reflection absorption spectroscopy we demonstrated that under reducing conditions (UHV or CH4) the ZrO2 particles transformed to an ultrathin ZrO2 film that started to cover (wet) the Pt surface in an SMSI-like fashion, paralleled by a decrease in surface/interface oxygen. In contrast, (more oxidizing) dry reforming conditions with a 1:1 ratio of CH4 and CO2 were stabilizing the ZrO2 particles on the model catalyst surface (or were even reversing the strong metal support interaction (SMSI) effect), as revealed by in situ XPS. Carbon deposits resulting from CH4 dissociation were easily removed by CO2 or by switching to dry reforming conditions (673–873 K). Thus, at these temperatures the active Pt surface remained free of carbon deposits, also preserving the ZrO2/Pt interface.