Reverse Spillover Research Articles

Adsorption sites, metal-support interactions, and oxygen spillover identified by vibrational spectroscopy of adsorbed CO: A model study on Pt/ceria catalysts

Vibrational spectroscopy of adsorbed CO is commonly used for the characterization of oxide-supported metal catalysts, in spite of persisting ambiguities concerning the band assignment. Here, we present reference data on well-defined Pt/CeO2(111)/Cu(111) model catalysts aiming at clear identification of the related features. Growth and morphology of the Pt particles are investigated by scanning tunneling microscopy (STM), the adsorption properties are probed by temperature programmed desorption (TPD), and the electronic structure has previously been studied by photoelectron spectroscopy (PES) and resonant photoelectron spectroscopy (RPES). Combining molecular beam (MB) methods and infrared reflection absorption spectroscopy (IRAS), we correlate this information to the interaction with CO.Two types of defect adsorption sites are identified on the ceria support (2097cm−1, 2120cm−1), which are assigned to structural defects and reduced Ce3+ centers. The relative abundance of these sites changes upon thermal treatment. On the Pt nanoparticles, CO first adsorbs on-top at step and edge sites (2066cm−1), before adsorption on the (111) facets occurs in bridging (1875cm−1) and on-top (2080–2097cm−1) geometry. For the first time, we identify vibrational CO features, which arise from electronic metal-support interactions and from oxygen reverse spillover. On reduced CeO2−x(111), a red-shifted band at 2053cm−1 appears, which is attributed to increased π-backbonding from Pt to CO, as a result of a change in electronic metal-oxide interaction. Thermal treatment also activates oxygen reverse spillover from the support to the Pt nanoparticle, which gives rise to a blue-shifted feature in the CO spectrum (2090–2105cm−1) due to coadsorbed oxygen next to CO sites.

Density functional investigation of reverse hydrogen spillover on zeolite supported Pd6 and Au6 clusters

The present density functional study investigates the stepwise reverse hydrogen spillover from bridging OH groups of the faujasite support to Pd6 and Au6 clusters. Calculations reveal that the process of proton transfer from the bridging OH groups results in hydrogenated clusters, M6Hn/FAU(3−n); M=Pd, Au and n=1–3, which are energetically preferable over faujasite supported bare clusters, M6/FAU(3H). The calculated reverse hydrogen spillover energy per transferred hydrogen atom for faujasite supported Pd6H3 and Au6H3 are −29.33kJmol−1 and −24.50kJmol−1, respectively. For both the metals, hydrogen migration results in partial oxidation of the metal atoms with concomitant reduction of H+ to H−.

Foreign Direct Investment and Managerial Knowledge Spillovers through the Diffusion of Management Practices

abstractAlthough managerial knowledge spillovers have long been claimed to be a major benefit of foreign direct investment (FDI), such spillovers have not yet been systematically analysed. This paper adds to the literature by analysing the nature and extent of managerial knowledge spillovers from FDI through the diffusion of management practices. Taking into account the tacit and explicit elements of management practices and distinguishing between industry and non‐industry specific practices, the paper identifies different types of spillovers and discusses their transmission mechanisms. Evidence from establishment‐level panel data from the UK attests to the existence and significance of intra‐industry, linkage, and non‐linkage based inter‐industry spillovers of managerial knowledge from foreign to local firms, although the strength varies for different types of practices. The spillovers are geographically localized, especially in channels without supply chain linkages. Local firms are selective in the adoption of individual practices and the spillover effects are more significant at the cluster and management system level. Reverse spillovers from local firms to MNEs from industrialized countries appear to be limited despite significant spillovers of practices amongst local firms.

Reverse hydrogen spillover on and hydrogenation of supported metal clusters: insights from computational model studies

"Reverse" spillover of hydrogen from hydroxyl groups of the support onto supported transition metal clusters, forming multiply hydrogenated metal species, is an essential aspect of various catalytic systems which comprise small, highly active transition metal particles on a support with a high surface area. We review and analyze the results of our computational model studies related to reverse hydrogen spillover, interpreting available structural and spectral data for the supported species and examining the relationship between metal-support and metal-hydrogen interactions. On the examples of small clusters of late transition metals, adsorbed in zeolite cavities, we showed with computational model studies that reverse spillover of hydrogen is energetically favorable for late transition metals, except for Au. This preference is crucial for the chemical reactivity of such bifunctional catalytic systems because both functions, of metal species and of acidic sites, are strongly modified, in some cases even suppressed - due to partial oxidation of the metal cluster and the conversion of protons from acidic hydroxyl groups to hydride ligands of the metal moiety. Modeling multiple hydrogen adsorption on metal clusters allowed us to quantify how (i) the support affects the adsorption capacity of the clusters and (ii) structure and oxidation state of the metal moiety changes upon adsorption. In all models of neutral systems we found that the metal atoms are partially positively charged, compensated by a negative charge of the adsorbed hydrogen ligands and of the support. In a case study we demonstrated with calculated thermodynamic parameters how to predict the average hydrogen coverage of the transition metal cluster at a given temperature and hydrogen pressure.

Nature of Ag Islands and Nanoparticles on the CeO2(111) Surface

Ag nanoparticles have been deposited on stoichiometric and reduced thin CeO2 films grown on Pt(111). The nucleation and growth of the Ag nanoparticles has been characterized by STM and XPS (X-ray photoemission spectroscopy) measurements complemented with DFT calculations on Ag atoms, clusters, and extended layers deposited on slab models of the CeO2(111) surface. The XPS spectra clearly show a reduction of the ceria support by Ag deposition (formation of Ce3+ ions). This is accompanied by a positive shift of the Ag 3d core levels, in which final state effects related to the finite size of the Ag deposits come into play. The DFT calculations support the view of a direct electron transfer from the Ag clusters and nanoparticles to the ceria support. Other possible origins of the reduction of the ceria substrate, like the occurrence of oxygen reverse spillover on the Ag nanoparticles, are ruled out based on energy considerations.

Hydrogen spillover monitored by resonant photoemission spectroscopy

We have employed resonant photoemission spectroscopy to investigate the phenomenon of hydrogen spillover on well-defined model Pt/ceria catalysts. On Pt/CeO2(111)/Cu(111), hydrogen spillover and reverse spillover give rise to reversible changes of the oxidation state of the surface cerium ions. These changes are monitored with highest sensitivity via resonant enhancement of Ce3+- and Ce4+-related features in the valence band photoemission spectra. The temperature regions are precisely determined in which specific processes such hydrogen spillover, Ce3+ transport, and reverse spillover are activated.

Growth, Structure, and Stability of Ag on CeO2(111): Synchrotron Radiation Photoemission Studies

The growth and interfacial electronic properties of Ag on CeO2(111) thin films have been studied by synchrotron radiation photoemission spectroscopy (SRPES), low energy electron diffraction (LEED), and X-ray photoelectron spectroscopy (XPS). Stoichiometric CeO2(111) thin films were grown on a Ru(0001) substrate. Ag grows as three-dimensional (3D) particles on the well-ordered CeO2(111) surface at 300 K with a number density of ∼1 × 1012 particles/cm2. When the CeO2(111) surface has a high density of defects, Ag initially populates these defect sites, leading to a two-dimensional (2D) island growth at low coverages followed by 3D islanding at high coverages. The binding energy of Ag 3d increases when the Ag particle size decreases, which is mainly attributed to the final-state screening. No strong interaction between Ag and CeO2(111) is found. The CeO2(111) surface is slightly reduced upon Ag deposition, which can be ascribed to the reverse spillover of oxygen atoms from the Ag−CeO2 boundary to the Ag nanoparticles. The Ag particles on CeO2(111) experience significant sintering when the temperature increases before they desorb from the surface.

Effects of Pt Particle Size on Hydrogen Storage on Pt-Doped Metal−Organic Framework IRMOF-8

A series of Pt-doped IRMOF-8 samples with different Pt catalyst sizes have been prepared via organometallic chemical-vapor deposition (CVD). The Pt catalyst size effects on the spillover storage on IRMOF-8 (or any MOF) were studied for the first time. Our studies showed that the Pt catalyst size is a crucial factor determining whether a significant enhancement in storage can be achieved in the hydrogen spillover system at ambient temperature. Compared to undoped IRMOF-8, the spillover storage capacities on Pt-doped IRMOF-8 samples were enhanced by factors ranging from 1.1 to 1.9 because of different Pt catalyst sizes. The doped Pt size was controlled by varying the CVD conditions to yield mean sizes of 2.2, 3.9, and 9.1 nm, and the results showed that smaller sizes were needed for spillover. Temperature-programmed desorption of dosed H2/D2 further shed light on the mechanism of spillover and reverse spillover, confirming undercoordinated sites on the particle-enhanced spillover.

Effect of aging on the NOx storage and regeneration characteristics of fully formulated lean NOx trap catalysts

In order to elucidate the effect of washcoat composition on lean NOx trap (LNT) aging characteristics, fully formulated monolithic LNT catalysts containing varying amounts of Pt, Rh and BaO were subjected to accelerated aging on a bench reactor. Subsequent catalyst evaluation revealed that in all cases aging resulted in deterioration of the NOx conversion as a consequence of impaired NOx storage and NOx reduction functions, while increased selectivity to NH3 was observed in the temperature range 250–450°C. Elemental analysis, H2 chemisorption and TEM data revealed two main changes which account for the degradation in LNT performance. First, residual sulfur in the catalysts, associated with the Ba phase, decreased catalyst NOx storage capacity. Second, sintering of the precious metals in the washcoat occurred, resulting in decreased contact between the Pt and Ba, and hence in less efficient NOx spillover from Pt to Ba during NOx adsorption, as well as decreased rates of reductant spillover from Pt to Ba and reverse NOx spillover during catalyst regeneration. For the aged catalysts, halving the Pt loading from 100 to 50g/ft3 was found to result in a significant decrease in overall NOx conversion, while for catalysts with the same 100g/ft3 Pt loading, increasing the relative amount of Pt on the NOx storage components (BaO and La-stabilized CeO2), as opposed to an Al2O3 support material (where it was co-located with Rh), was found to be beneficial. The effect of Rh loading on aged catalyst performance was found to be marginal within the range studied (10–20g/ft3), as was the effect of BaO loading in the range 30–45g/L.

Enhanced reactivity of Pt nanoparticles supported on ceria thin films during ethylenedehydrogenation

The adsorption and reaction of ethylene on Pt/CeO(2-x)/Cu(111) model catalysts were studied by means of high resolution photoelectron spectroscopy (HR-PES) in conjunction with resonant photoemission spectroscopy (RPES). The dehydrogenation mechanism is compared to the HR-PES data obtained on a Pt(111) single crystal under identical conditions. It was found that the Pt nanoparticle system shows a substantially enhanced reactivity and several additional reaction pathways. In sharp contrast to Pt(111), partial dehydrogenation of ethylene on the supported Pt nanoparticles already starts at temperatures as low as 100 K. Similar to the single crystal surface, dehydrogenation occurs via the isomer ethylidene (CHCH(3)) and then mainly via ethylidyne (CCH(3)). In the temperature region between 100 and 250 K there is strong evidence for spillover of hydrocarbon fragments to the ceria support. In addition, splitting of ethylene to C(1) fragments is more facile than on Pt(111), giving rise to the formation of CH species and CO in the temperature region between 250 and 400 K. Upon further annealing, carbonaceous deposits are formed at 450 K. By heating to 700 K, these carbon deposits are completely removed from the surface by reaction with oxygen, provided by reverse spillover of oxygen from the ceria support.

Novel Spillover Interrelating Reversible Electrocatalysts for Oxygen and Hydrogen Electrode Reactions

New striking prospects both in low and medium temperature polymer electrolyte membrane fuel cell (PEMFC) and in water electrolysis (WE) have been opened by the interactive supported individual (Pt) or prevailing hyper-d-electronic nanostructured metal clusters (WPt3, NbPt3, HfPd3, ZrNi3), grafted upon and within high altervalent capacity hypo-d-oxides (WO3, NbO2, TaO2, TiO2) and their proper mixed valence compounds, to create a novel type of alternating polarity (alterpolar) interchangeable composite electrocatalysts for hydrogen and oxygen electrode reactions. Whereas in aqueous media Pt (Pt/C) features either chemisorbed catalytic surface properties of Pt−H or Pt═O, missing any effusion of other interacting species, a new generation of composite SMSI (strong metal−support interaction) electrocatalysts in condensed wet state primarily characterizes interchangeable extremely fast reversible spillover of either H-adatoms or the primary oxides (Pt−OH, Au−OH) or the invertible bronze type behavior of these s...

Microscopic Insights into Methane Activation and Related Processes on Pt/Ceria Model Catalysts

Ceria-based supported noble-metal catalysts release oxygen, which may help to reduce the formation of carbonaceous residues, for example during hydrocarbon reforming. To gain insight into the microscopic origins of these effects, a model study is performed under ultrahigh-vacuum conditions using single-crystal-based supported model catalysts. The model systems are based on ordered CeO(2)(111) films on Cu(111), on which Pt nanoparticles are grown by physical vapor deposition. The growth and structure of the surfaces are characterized by means of scanning tunneling microscopy, and the electronic structure and reactivity are probed by X-ray photoelectron spectroscopy. Specifically, it is shown that the fully oxidized CeO(2) thin films undergo slight reduction upon Pt deposition (CeO(1.99)). This effect is enhanced upon annealing (CeO(1.96)), thus indicating facile oxygen release and reverse spillover. The model system is structurally stable up to temperatures exceeding 700 K. The activation of methane is investigated using high-kinetic-energy CH(4) (0.83 eV), generated by a supersonic molecular beam. It is shown that dehydrogenation occurs under rapid formation of CH or C species without detectable amounts of CH(3) being formed, even at low temperatures (100 K). The released hydrogen spills over to the CeO(2) support, which leads to the formation of OH groups. At 200 K and above, the OH groups start to decompose leaving additional Ce(3+) centers behind (CeO(1.97-1.94)). At up to 700 K, carbon deposits are quantitatively removed by reaction with oxygen, which is supplied by reverse spillover from the CeO(2) film, thus leading to substantial reduction of the support (approximately CeO(1.90-1.85)).

TAP study on the active oxygen species in the total oxidation of propane over a CuO–CeO 2/γ-Al 2O 3 catalyst

The activation of propane both in the absence and presence of gas-phase O 2 or CO 2 over a CuO–CeO 2/γ-Al 2O 3 catalyst is investigated in a TAP reactor between 623 and 873 K. Two different types of oxygen species are involved dependent on the mixture introduced: (1) lattice oxygen from the surface and bulk of the catalyst and (2) weakly adsorbed oxygen species produced either from gas-phase O 2 or CO 2. Both types of oxygen species participate in the total oxidation of propane. If pulses of pure propane are admitted, lattice oxygen from CuO and CeO 2 is consumed, reducing these oxides to a certain extent. Apart from surface lattice oxygen from CuO and CeO 2, also bulk lattice oxygen is involved in the activation of propane, demonstrating the high O mobility within the lattice of these metal oxides. Lattice oxygen of alumina is not available for reaction with propane. Introduction of O 2 reoxidizes the reduced sites on CuO and CeO 2, as expected for a Mars–van Krevelen mechanism. Also CO 2 can replenish O vacancies in the active phase by dissociative adsorption on alumina and ceria. The adsorption on alumina is followed by reverse spillover of reactive O species to the active phase. The second type of oxygen consists of weakly adsorbed oxygen species, produced either from O 2 on both CuO and CeO 2, or from CO 2 on CeO 2, and enhances strongly the catalytic activity. In the presence of both O 2 and CO 2 in the propane feed, the predominant reaction pathways will only involve the two types of active species produced from O 2, and not from CO 2.

Catalytic Effects of TiF3 on Hydrogen Spillover on Pt/Carbon for Hydrogen Storage

Recent studies have shown that using the hydrogen spillover phenomena is a promising approach for developing new materials for hydrogen storage at ambient temperature. However, the rates need to be improved. Significant catalytic effects on both spillover (i.e., adsorption) and reverse spillover (i.e., desorption) on Pt-doped carbon by TiF(3) were found. By doping 2 wt % TiF(3) on the Pt-doped Maxsorb (a superactivated carbon), both adsorption and desorption rates were significantly increased while the storage capacity decreased only slightly due to decreased surface areas. The effect of the heat treatment temperature (473 K vs 673 K) of the doped TiF(3) on its catalytic effects was also studied. XPS analyses showed that C-F bonds were formed upon heat treatment and that the amount of C-F bonds increased with the heat treatment temperature. The catalytic effects also increased with the heat treatment temperature, indicating that the catalytic mechanism possibly involved the formation of C-F bonds on the carbon edge sites. In addition, the issue of proper sample preparation of Pt/carbon was briefly addressed; missteps in metal doping and consequently poor metal dispersion will result in significantly diminished spillover enhancements (Stadie et al.).

Hydrogen spillover phenomenon: Enhanced reversible hydrogen adsorption/desorption at Ta 2O 5-coated Pt electrode in acidic media

The current study is concerned with the preparation and characterization of tantalum oxide-loaded Pt (TaO x /Pt) electrodes for hydrogen spillover application. XPS, SEM, EDX and XRD techniques are used to characterize the TaO x /Pt surfaces. TaO x /Pt electrodes were prepared by galvanostatic electrodeposition of Ta on Pt from LiF–NaF (60:40 mol%) molten salts containing K 2TaF 7 (20 wt%) at 800 °C and then by annealing in air at various temperatures (200, 400 and 600 °C). The thus-fabricated TaO x /Pt electrodes were compared with the non-annealed Ta/Pt and the unmodified Pt electrodes for the hydrogen adsorption/desorption (H ads/H des) reaction. The oxidation of Ta to the stoichiometric oxide (Ta 2O 5) increases with increasing the annealing temperature as revealed from XPS and X-ray diffraction (XRD) measurements. The higher the annealing temperature the larger is the enhancement in the H ads/H des reaction at TaO x /Pt electrode. The extraordinary increase in the hydrogen adsorption/desorption at the electrode annealed at 600 °C is explained on the basis of a hydrogen spillover–reverse spillover mechanism. The hydrogen adsorption at the TaO x /Pt electrode is a diffusion-controlled process.

Investigation of Reverse-Hydrogen Spillover on Zeolite-Supported Palladium Tetramer by ONIOM Method

The present study explores favorable chemical state of zeolite-supported Pd4 clusters using the ONIOM (our-own-N-layered integrated molecular orbital + molecular mechanics) approach, as implemented in the Gaussian03 code. Lowest-energy structures of Pd4 in the gas phase as well as on a zeolite support are found to be in the triplet state. The calculations reveal that reverse hydrogen spillover from bridging OH groups of zeolite support onto Pd4 results in a hydrogenated Pd4H/Zeo((m − 1)H), m = 1−3 species, which are energetically preferable over the bare zeolite-supported form of Pd4/Zeo(mH). The process of single hydrogen transfer from the zeolite support to the Pd4 cluster is found to be exothermic and more favorable than two- and three-proton transfer processes.

Reverse Spillover of Hydrogen on Carbon-Based Nanomaterials: Evidence of Recombination Using Isotopic Exchange

The hydrogen spillover phenomenon at ambient temperature is of interest in the development of adsorbents for storage applications. The mechanism of hydrogen spillover is studied using equilibrium dosing of the deuterium hydride molecule. Temperature programmed desorption spectra for both primary and secondary spillover materials, with the deuterium peak appearing first, have supported partitioning of the desorption behavior into reverse spillover and recombination directly from the atomic receptor. Desorption kinetics are faster relative to adsorption for hydrogen and deuterium on all carbon-based adsorbents in the study. Diffusion time constants are computed using a surface diffusion model based on Fick’s second law. Kinetic data are studied for pressures from atmospheric to 100 atm and equilibrium adsorbed amounts to 5.5 mmol/g. Values follow an inverse relationship with amount and are 2 orders of magnitude higher (10−4 s−1) than those reported for nitrogen physisorption on carbon adsorbents. Adsorption...

Zeolite-Supported Organorhodium Fragments: Essentially Molecular Surface Chemistry Elucidated with Spectroscopy and Theory

Structures of zeolite-anchored organorhodium complexes undergoing conversions with gas-phase reactants were characterized by infrared spectra bolstered by calculations with density functional theory and analysis of the gas-phase products. Structurally well-defined zeolite-supported rhodium diethylene complexes were synthesized by chemisorption of Rh(C(2)H(4))(2)(acac) (acac = CH(3)COCHCOCH(3)) on dealuminated Y zeolite, being anchored by two Rh-O bonds, as shown by extended X-ray absorption fine structure (EXAFS) spectroscopy. In contrast to the nonuniformity of metal complexes anchored to metal oxides, the near uniformity of the zeolite-supported species allowed precise determination of their chemistry, including the role of the support as a ligand. The anchored rhodium diethylene complex underwent facile, reversible ligand exchange with deuterated ethylene at 298 K, and ethylene ligands were hydrogenated by reverse spillover of hydrogen from support hydroxyl groups. The supported complexes reacted with CO to form rhodium gem-dicarbonyls, which, in the presence of ethylene, gave rhodium monocarbonyls. The facile removal of ethylene ligands from the complex in H(2)-N(2) mixtures created coordinatively unsaturated rhodium complexes; the coordinative unsaturation was stabilized by the site isolation of the complexes, allowing reaction with N(2) to form rhodium complexes with one and with two N(2) ligands. The results also provide evidence of a new rhodium monohydride species incorporating a C(2)H(4) ligand.

Density Functional Studies on the Structure and Reverse Hydrogen Spillover in Au6 Cluster Supported on Zeolite

We have studied the structure of Au6 cluster supported on acidic form of faujasite zeolite using density functional theory. In the gas phase Au6 minimizes to a triangular structure while on zeolite support it has a three-dimensional structure with three apical centers bending towards the support. The hydrogenated clusters Au6H/(2H)-FAU, Au6H2/H-FAU and Au6H3/FAU generated by stepwise reverse hydrogen spillover from bridging OH groups of zeolite are energetically preferred over the Au6/(3H)-FAU structure. The calculated reverse hydrogen spillover energy per hydrogen atom for zeolite supported Au6H, Au6H2 and Au6H3 clusters are −67.6, −65.6 and −59.1 kJ/mol, respectively.

Absorption of D(g) atoms in ultrathin Mg films and Pd-catalyzed decomposition of MgD 2

We have investigated the interaction of the gas-phase D atoms with ultrathin Mg films thermally evaporated on SiO 2 by temperature programmed desorption. The gas-phase D atoms impinging upon a Mg (100 Å) film directly penetrate into the bulk with a constant probability of 0.22 until the top 16 Å is deuterated, whereafter the MgD 2(s) growth quickly slows down. Upon heating the partially deuterated Mg film, Mg atoms diffuse out to the surface to desorb at the same temperature as a pure Mg film. On the other hand, MgD 2(s) decomposes to result in simultaneous desorption of D 2 and Mg at 590 K with an activation energy of 133 ± 5 kJ/mol. A Pd layer of 20 Å thickness deposited on top of MgD 2(s) acts as a catalyst to greatly reduce the decomposition temperature from 590 to 350 K. We interpret the catalytic effect of Pd in terms of reverse spill-over, in which the surface of Pd clusters in contact with MgD 2(s) provides an intermediate adsorption sites with a low activation energy for the recombinative desorption of D atoms.