Inverse Catalysts Research Articles

Efficient Ceria-Platinum Inverse Catalyst for Partial Oxidation of Methanol.

Ceria-platinum-based bilayered thin films deposited by magnetron sputtering were developed and tested in regard to their catalytic activity for methanol oxidation by employing a temperature-programmed reaction (TPR) technique. The composition and structure of the samples were characterized by X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). Both conventional (oxide-supported metal nanoparticles [NPs]) and inverse configurations (metal with oxide overlayer) were analyzed to uncover the structural dependence of activity and selectivity of these catalysts with respect to different pathways of methanol oxidation. We clearly demonstrate that the amount of cerium oxide (ceria) loading has a profound influence on methanol oxidation reaction characteristics. Adding a noncontinuous adlayer of ceria greatly enhances the catalytic performance of platinum (Pt) in favor of partial oxidation of methanol (POM), gaining an order of magnitude in the absolute yield of hydrogen. Moreover, the undesired by-production of carbon monoxide (CO) is strongly suppressed, making the ceria-platinum inverse catalyst a great candidate for clean hydrogen production. It is suggested that the methanol oxidation process is facilitated by the synergistic effect between both components of the inverse catalyst (involving oxygen from ceria and providing a reaction site on the adjacent Pt surface) as well as by the fact that the ability of ceria to exchange oxygen (i.e., to alter the oxidation state of Ce between 3+ and 4+) during the reaction is inversely proportional to its thickness. The increased redox capability of the discontinuous ceria adlayer shifts the preferred reaction pathway from dehydrogenation of hydroxymethyl intermediate to CO in favor of its oxidation to formate.

Spillover Reoxidation of Ceria Nanoparticles

Interest in resolving the mechanisms behind ceria’s activity has been intense due to the numerous industrial applications including those in heterogeneous catalysis. In this work, we study the reduction and reoxidation of ultrathin CeO2(111) nanoislands on Rh(111) and Pt(111) substrates, so-called inverse model catalysts, with a combination of real and reciprocal space techniques based on X-ray photoemission electron microscopy (XPEEM) and low energy electron microscopy. Soft X-ray microfocused illumination was employed to reduce the ceria islands, which we are able to control by varying the oxygen partial pressure within the measurement chamber. Low energy electron diffraction measurements of the irradiated ceria films demonstrate the formation of an ordered array of oxygen vacancies leading to a (√7 × √7)R19.1° superstructure attributed to the ι-phase (Ce7O12)(111). Resonant photoelectron spectroscopy provides the required high sensitivity to detect small changes in Ce3+ concentration. The high spatial ...

Water-Activated Lattice Oxygen in FeO(111) Islands for Low-Temperature Oxidation of CO at Pt–FeO Interface

The role of water in low-temperature oxidation of CO catalyzed at metal–oxide interfaces is of great interest. In this paper, we prepared Fe16O(111)/Pt(111) and Fe18O(111)/on Pt(111) inverse model catalysts with the same coverage of FeO(111) monolayer islands and comparatively studied C16O oxidation with 18O2 or D218O on Fe16O(111)/Pt(111) inverse model catalyst and C16O oxidation with 16O2 or D216O on Fe18O(111)/Pt(111) inverse model catalyst. Lattice oxygen in FeO does not participate into CO oxidation with O2 but participates into CO oxidation with water via the mechanism of CO oxidation with hydroxyl groups formed by water dissociation at the FeO(111)–Pt interface. These results provide unambiguous experimental evidence for the unique role of water in activating surface lattice oxygen of oxides to oxidize CO at low temperatures, advancing the fundamental understandings of low-temperature CO oxidation involving hydroxyl groups catalyzed by metal–oxide interfaces.

β-cyclodextrin grafted on lignin as inverse phase transfer catalyst for the oxidation of benzyl alcohol in H2O

β-cyclodextrin grafted on lignin (l-β-CD) was prepared by the reaction of β-cyclodextrin (β-CD) and lignin with epichlorohydrin (EPI) as a cross-linking agent. The polymer was characterized by Fourier-transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), Scanning electron microscopy (SEM), and solid-state nuclear magnetic resonance spectroscopy (CP-MAS-NMR). It was used as an inverse phase transfer catalyst for selective oxidation of benzyl alcohol (BzOH) to benzaldehyde (BzH) with hydrogen peroxide as the oxidant. The effects of various reaction variables, e.g., catalyst, the amount of catalyst, stirring speed, reaction temperature, oxidant, and cocatalyst on the catalytic performances were investigated. Under optimal conditions, high selectivity (>99%) was observed for the oxidation of BzOH under rather mild conditions. A plausible mechanism for the liquid-solid phase oxidation of BzOH has been proposed. Synergistic effects based on weak interactions between catalyst and support played an important role for the effective oxidation of BzOH under solution condition. The catalyst can be easily recycled and reused without a significant loss in reaction activity and selectivity, which would lead to its potential application foreground for environmental friendly synthesis of BzH.

Surface reaction network of CO oxidation on CeO2/Au(110) inverse model catalysts.

CeO2/Au(110) inverse model catalysts were prepared and their activity toward the adsorption and co-adsorption of O2, CO, CO2 and water was studied by means of X-ray photoelectron spectroscopy, low energy electron diffraction, thermal desorption spectra and temperature-programmed reaction spectra. The Au surface of CeO2/Au(110) inverse model catalysts molecularly adsorbs CO, CO2 and water, and the polycrystalline CeO2 surface of CeO2/Au(110) inverse model catalysts molecularly adsorbs O2, and molecularly and reactively adsorbs CO, CO2 and water. By controllably preparing co-adsorbed surface species on CeO2/Au(110) inverse model catalysts, we successfully identified various surface reaction pathways of CO oxidation to produce CO2 with different barriers both on the CeO2 surface and at the Au-CeO2 interface, including CO oxidation by various oxygen species, and water/hydroxyl group-involved CO oxidation. These results establish a surface reaction network of CO oxidation catalyzed by Au/CeO2 catalysts, greatly advancing the fundamental understandings of catalytic CO oxidation reactions.

CeO2 decorated CuO hierarchical composites as inverse catalyst for enhanced CO oxidation

CeO2 decorated CuO hierarchical composites were prepared and was used as inverse catalyst for enhanced CO oxidation.

ChemInform Abstract: Tuning the Properties of Copper‐Based Catalysts Based on Molecular in situ Studies of Model Systems

AbstractReview: 42 refs.

Growth of Ceria Nano-Islands on a Stepped Au(788) Surface.

The growth morphology and structure of ceria nano-islands on a stepped Au(788) surface has been investigated by scanning tunneling microscopy (STM) and low-energy electron diffraction (LEED). Within the concept of physical vapor deposition, different kinetic routes have been employed to design ceria-Au inverse model catalysts with different ceria nanoparticle shapes and arrangements. A two-dimensional superlattice of ceria nano-islands with a relatively narrow size distribution (5 ± 2 nm2) has been generated on the Au(788) surface by the postoxidation method. This reflects the periodic anisotropy of the template surface and has been ascribed to the pinning of ceria clusters and thus nucleation on the fcc domains of the herringbone reconstruction on the Au terraces. In contrast, the reactive evaporation method yields ceria islands elongated in [01-1] direction, i.e., parallel to the step edges, with high aspect ratios (~6). Diffusion along the Au step edges of ceria clusters and their limited step crossing in conjunction with a growth front perpendicular to the step edges is tentatively proposed to control the ceria growth under reactive evaporation conditions. Both deposition recipes generate two-dimensional islands of CeO2(111)-type O–Ce–O single and double trilayer structures for submonolayer coverages.

Tuning the properties of copper-based catalysts based on molecular in situ studies of model systems.

Studying catalytic processes at the molecular level is extremely challenging, due to the structural and chemical complexity of the materials used as catalysts and the presence of reactants and products in the reactor's environment. The most common materials used on catalysts are transition metals and their oxides. The importance of multifunctional active sites at metal/oxide interfaces has been long recognized, but a molecular picture of them based on experimental observations is only recently emerging. The initial approach to interrogate the surface chemistry of catalysts at the molecular level consisted of studying metal single crystals as models for reactive metal centers, moving later to single crystal or well-defined thin film oxides. The natural next iteration consisted in the deposition of metal nanoparticles on well-defined oxide substrates. Metal nanoparticles contain undercoordinated sites, which are more reactive. It is also possible to create architectures where oxide nanoparticles are deposited on top of metal single crystals, denominated inverse catalysts, leading in this case to a high concentration of reactive cationic sites in direct contact with the underlying fully coordinated metal atoms. Using a second oxide as a support (host), a multifunctional configuration can be built in which both metal and oxide nanoparticles are located in close proximity. Our recent studies on copper-based catalysts are presented here as an example of the application of these complementary model systems, starting from the creation of undercoordinated sites on Cu(111) and Cu2O(111) surfaces, continuing with the formation of mixed-metal copper oxides, the synthesis of ceria nanoparticles on Cu(111) and the codeposition of Cu and ceria nanoparticles on TiO2(110). Catalysts have traditionally been characterized before or after reactions and analyzed based on static representations of surface structures. It is shown here how dynamic changes on a catalyst's chemical state and morphology can be followed during a reaction by a combination of in situ microscopy and spectroscopy. In addition to determining the active phase of a catalyst by in situ methods, the presence of weakly adsorbed surface species or intermediates generated only in the presence of reactants can be detected, allowing in turn the comparison of experimental results with first principle modeling of specific reaction mechanisms. Three reactions are used to exemplify the approach: CO oxidation (CO + 1/2O2 → CO2), water gas shift reaction (WGSR) (CO + H2O → CO2 + H2), and methanol synthesis (CO2 + 3H2 → CH3OH + H2O). During CO oxidation, the full conversion of Cu(0) to Cu(2+) deactivates an initially outstanding catalyst. This can be remedied by the formation of a TiCuOx mixed-oxide that protects the presence of active partially oxidized Cu(+) cations. It is also shown that for the WGSR a switch occurs in the reaction mechanism, going from a redox process on Cu(111) to a more efficient associative pathway at the interface of ceria nanoparticles deposited on Cu(111). Similarly, the activation of CO2 at the ceria/Cu(111) interface allows its facile hydrogenation to methanol. Our combined studies emphasize the need of searching for optimal metal/oxide interfaces, where multifunctional sites can lead to new efficient catalytic reaction pathways.

Direct Visualization of Catalytically Active Sites at the FeO-Pt(111) Interface.

Within the area of surface science, one of the "holy grails" is to directly visualize a chemical reaction at the atomic scale. Whereas this goal has been reached by high-resolution scanning tunneling microscopy (STM) in a number of cases for reactions occurring at flat surfaces, such a direct view is often inhibited for reaction occurring at steps and interfaces. Here we have studied the CO oxidation reaction at the interface between ultrathin FeO islands and a Pt(111) support by in situ STM and density functional theory (DFT) calculations. Time-lapsed STM imaging on this inverse model catalyst in O2 and CO environments revealed catalytic activity occurring at the FeO-Pt(111) interface and directly showed that the Fe-edges host the catalytically most active sites for the CO oxidation reaction. This is an important result since previous evidence for the catalytic activity of the FeO-Pt(111) interface is essentially based on averaging techniques in conjunction with DFT calculations. The presented STM results are in accord with DFT+U calculations, in which we compare possible CO oxidation pathways on oxidized Fe-edges and O-edges. We found that the CO oxidation reaction is more favorable on the oxidized Fe-edges, both thermodynamically and kinetically.

Hydrogen spillover enhanced hydroxyl formation and catalytic activity toward CO oxidation at the metal/oxide interface.

H2-promoted catalytic activity of oxide-supported metal catalysts in low-temperature CO oxidation is of great interest but its origin remains unknown. Employing an FeO(111)/Pt(111) inverse model catalyst, we herewith report direct experimental evidence for the spillover of H(a) adatoms on the Pt surface formed by H2 dissociation to the Pt-FeO interface to form hydroxyl groups that facilely oxidize CO(a) on the neighboring Pt surface to produce CO2. Hydroxyl groups and coadsorbed water play a crucial role in the occurrence of hydrogen spillover. These results unambiguously identify the occurrence of hydrogen spillover from the metal surface to the noble metal/metal oxide interface and the resultant enhanced catalytic activity of the metal/oxide interface in low-temperature CO oxidation, which provides a molecular-level understanding of both H2-promoted catalytic activity of metal/oxide ensembles in low-temperature CO oxidation and hydrogen spillover.

Water Adsorption and Dissociation at Metal-Supported Ceria Thin Films: Thickness and Interface-Proximity Effects Studied with DFT+U Calculations

The chemistry of several catalytic processes can be controlled by tuning metal–oxide interfaces, as demonstrated by fundamental studies on inverse model catalysts. We investigate the effects of the metal–oxide interface on the surface reactivity of ceria (CeO2) thin films supported by a copper metal surface. Our density functional theory (DFT+U) calculations reveal that the interface has impact on the surface water adsorption and dissociation when the thickness of the ceria film is below ≈9 Å. On thinner films, the energetics of adsorption and dissociation display a significant variation, which arises from a combination of thickness and interface-proximity effects, and which we rationalize in terms of charge-density response at the adsorbate-oxide and oxide-metal interfaces. The adsorption energy is maximized for film thicknesses of 5.5 Å (corresponding to two O–Ce–O trilayers), while thinner films affect primarily the relative stability between molecular, semidissociated, and dissociated water adsorption. These results provide useful insights into the effect of low-dimensional ceria species in Cu/CeO2 catalysts.

Tailoring metal-oxide interfaces of inverse catalysts of TiO2/nanoporous-Au under hydrogen oxidation.

Engineering metal-oxide interfaces in TiO2/nanoporous (np) Au inverse catalysts results in enhancement of H2 oxidation activity. While the intrinsic activity of the novel np-Au prepared from a Au-Si alloy is low, the activity increased as the weight fraction of the TTIP (amount of TiO2) was increased to 0.5 weight%. We correlate the change in activity with the active sites at the perimeter interface between the TiO2 and np-Au.

Unraveling the Nature of the Oxide–Metal Interaction in Ceria-Based Noble Metal Inverse Catalysts

Recently, it has been shown that model catalysts consisting of cerium oxide nanoparticles supported on noble metal surfaces are very convenient to clarify the reaction mechanism of important industrial reactions such as water–gas shift, CO oxidation, and methanol synthesis from CO2. Moreover, these special inverse catalysts are highly active, even more than the regular metal/oxide catalyst in some cases. However, there is a lack of fundamental knowledge about the electronic nature of the supported CeOx nanoparticles (NPs) and their interaction with the noble metal support. In this work we have carried out DFT calculations on CeOx/Cu(111), CeOx/Ag(111), and CeOx/Au(111) interfaces for both supported particles and periodic 2D film. The most stable and most reduced CeOx NPs are found when supported on Cu(111), followed by Ag(111) and Au(111). A similar behavior is observed for periodic 2D CeO2(111) films supported on the M(111) surfaces, although the reducibility is lower and the stability higher in extended...

Ab Initio Study of Thin Oxide–Metal Overlayers as an Inverse Catalytic System for Dioxygen Reduction and Enhanced CO Tolerance

Using first-principles density functional theory calculations, we used a thin oxide overlayer, such as MgO, on a metal surface as an inverse catalyst for dioxygen reduction. Surface distortions in the oxide layer, combined with the tunneling of electron from the underneath metal, activated the adsorbed O2 in the form of a superoxo or peroxo. On the other hand, the thin MgO overlayer readily prevents the π-back-bonding between CO and the metal surface, thereby efficiently mitigating the affinity of the metal surface for CO. The operating potential and overpotential for the oxygen reduction reaction (ORR) process have been estimated for various combinations of thin insulators and metals. The strongest binding intermediate in the overall reaction pathway influenced the overpotential. We show that for a Ag(100)-supported MgO surface, the ORR commences with a low overpotential, which is comparable to that of the Pt(111) surface. This suggests that an optimally chosen insulator–metal overlayer structure can yield a sharply tuned free energy profile for ORR.

Mineralizer-Assisted Shape-Control of Rare Earth Oxide Nanoplates

Rare earth oxides are important emerging materials because of their unique properties. Nanosized, two-dimentional (2D) materials have received increasing attention due to their high surface-to-volume ratios and ultrathin layered structures. Here, we synthesize 2D rare earth (RE) oxide nanoplates in the presence of a mineralizer. The use of a mineralizer not only facilitates the synthesis of RE oxide nanoplates (i.e., increases the yield and allows mild reaction parameters), but also allows for shape-control. To emphasize the importance of RE oxide nanoplates in materials science and engineering, we demonstrate that (1) ceria nanoplates can be used in a ceria/Cu inverse catalyst to enable an enhanced CO oxidation activity, and (2) Y2O3 nanoplates doped with Eu3+ enable photon energy down-conversion.

Promotion and inhibition effects of TiOx species on Rh inverse model catalysts

Abstract The effect of TiO x on the bonding of CO test molecules to different Rh surfaces has been investigated in model systems under UHV conditions. The TiO x overlayers had a twofold effect both on Rh nanoparticles and Ar + -ion sputtered Rh films prepared on TiO 2 (1 1 0). They suppressed the uptake of molecular CO by a site blocking mechanism. In the other mechanism the recombinative CO desorption states went through a maximum at intermediate TiO x coverages on both kinds of rhodium surfaces, indicating the promotion of CO decomposition. These findings are in harmony with earlier high pressure studies which showed that the methanation rate of CO on rhodium is maximal at intermediate TiO x coverages. The effect of a TiO x film underneath a Rh deposit (0.4 monolayer) on its surface chemistry has also been addressed. A Rh–TiO x –Rh structure was formed at 230 K by the deposition of Rh on a continuous TiO x film supported by a 20 monolayer thick Rh multilayer. CO desorption curves indicated a highly reactive Rh overlayer at 230 K and also a dramatic structural change in the rather narrow temperature range of 230–265 K. The rearrangement is accompanied by the migration of TiO x species onto the post-deposited Rh as it can be deduced from the appearance of a CO desorption feature with T p = 360 K. The driving force of the process is the formation of strong Rh–Rh bonds and the low surface free energy of TiO x overlayer with respect to the bare Rh surface.

Isolation and characterization of formates on CeOx–CuyO/Cu(1 1 1)

Formate species have been proposed to be either critical intermediates or spectators in the water–gas shift (WGS) and methanol synthesis processes. CeOx–CuyO/Cu(111) has been shown to be a very active inverse catalyst for the WGS reaction. We present here the study of formate species obtained from the deprotonation of formic acid (HCOOH) on the inverse catalysts. Exposure of CeOx–CuyO/Cu(111) to HCOOH at 300K leads to the formation of formates on both ceria and Cu sites. The formates isolated on CeOx–CuyO/Cu(111) systems cannot be hydrogenated even at a pressure of 200Torr H2 at 300–350K. The formate species localized on ceria sites are thermally more stable than those on Cu sites, and the thermal decomposition of all of the formates occurs by dehydrogenation releasing CO2 and H2. Evidence of reverse spillover of formates from the oxide to the metal was observed on CeO2−x/Cu(111) inverse catalysts.

Oxidation catalysis by large trisoctahedral gold nanoparticles: Mind the step!

To understand the origin of catalysis by gold and to probe the role of defects and interfaces, benchmark reactions namely CO oxidation and benzyl alcohol oxidation were carried out on supported trisoctahedral gold nanoparticles and on the inverse catalyst system. The sizes of these particles which are between 45nm and 110nm, are well beyond the quantum size regime and well above the optimum size range where gold is considered to be catalytically active. The periodic steps sites in contact with oxide support interface were able to promote the reaction.

In Situ Characterizations of Nanostructured SnOx/Pt(111) Surfaces Using Ambient-Pressure XPS (APXPS) and High-Pressure Scanning Tunneling Microscopy (HPSTM)

We have conducted in situ measurement of “inverse catalysts” of SnOx nanostructures supported on Pt(111) using ambient-pressure X-ray photoelectron spectroscopy (APXPS) and high-pressure scanning tunneling microscopy (HPSTM) techniques under CO exposure at room temperature and 450 K. Nanostructures of SnOx were prepared by depositing Sn on Pt(111) precovered by O2 layers at liquid nitrogen temperature. APXPS data show that the prepared SnOx nanoparticles are highly reduced, with Sn2+ being the dominant oxide species. The relative Sn2+concentration, compared to Sn4+ and Sn0, in the SnOx nanoparticles decreases slightly with increasing Sn coverage. In situ study of SnOx/Pt(111) inverse catalyst shows that for lower coverage of SnOx (0.25 monolayers (ML)), the amount of Sn2+ decreased steadily, while Sn0 amount steadily increased with negligible Sn4+ amount, as the surface was heated under CO exposure at 450 K. Meanwhile, for the higher coverage (1.0 ML), the decrease of Sn2+ is followed by sharp increase in the amount of Sn4+ and Sn0. HPSTM images show that small islands of SnOx are randomly formed on the substrate, with the size and density increasing with SnOx coverage. HPSTM images show morphology differences between low and high coverages of SnOx on Pt(111) under both UHV and CO exposure conditions.