Yunxi Yao

Interface-Confined Ferrous Centers for Catalytic Oxidation

Catalysis at the Edge Many catalysts in solution, such as metalloenzymes and homogeneous metal complexes, create active sites where the metal ion is available to bind and activate reactants. Such coordinately unsaturated ferrous sites, or CUFs, have been created in a supported heterogeneous catalyst by Fu et al. (p. 1141). Ferrous oxide islands grown on platinum single-crystal surfaces were much more reactive for CO oxidation at low temperatures than more oxidized ferric islands. This difference arose from sites at the interface between the islands and the Pt surface that activated oxygen. Silica-supported Pt-Fe catalysts were active for CO removal from hydrogen streams, a reaction critical for maintaining the activity of fuel cells. The interface between ferrous oxide islands and a platinum support contains sites that activate dioxygen for catalytic reactions. Coordinatively unsaturated ferrous (CUF) sites confined in nanosized matrices are active centers in a wide range of enzyme and homogeneous catalytic reactions. Preparation of the analogous active sites at supported catalysts is of great importance in heterogeneous catalysis but remains a challenge. On the basis of surface science measurements and density functional calculations, we show that the interface confinement effect can be used to stabilize the CUF sites by taking advantage of strong adhesion between ferrous oxides and metal substrates. The interface-confined CUF sites together with the metal supports are active for dioxygen activation, producing reactive dissociated oxygen atoms. We show that the structural ensemble was highly efficient for carbon monoxide oxidation at low temperature under typical operating conditions of a proton-exchange membrane fuel cell.

Graphene cover-promoted metal-catalyzed reactions

Significance Carbon deposits have been widely observed on metal surfaces in a variety of catalytic reactions, and the graphitic carbon species are often considered as inhibitors for surface reactions. We demonstrate here that CO adsorption and oxidation can occur on Pt surface covered by monolayer graphene, showing that the space between graphene overlayer and metal surface can act as a two-dimensional (2D) nanoreactor. Inside, CO oxidation happens with lower activation barrier due to the confinement effect of the graphene cover. This finding reminds us to reconsider the role of graphitic carbon in metal-catalyzed surface reactions and further provides a way to design novel catalysts. Graphitic overlayers on metals have commonly been considered as inhibitors for surface reactions due to their chemical inertness and physical blockage of surface active sites. In this work, however, we find that surface reactions, for instance, CO adsorption/desorption and CO oxidation, can take place on Pt(111) surface covered by monolayer graphene sheets. Surface science measurements combined with density functional calculations show that the graphene overlayer weakens the strong interaction between CO and Pt and, consequently, facilitates the CO oxidation with lower apparent activation energy. These results suggest that interfaces between graphitic overlayers and metal surfaces act as 2D confined nanoreactors, in which catalytic reactions are promoted. The finding contrasts with the conventional knowledge that graphitic carbon poisons a catalyst surface but opens up an avenue to enhance catalytic performance through coating of metal catalysts with controlled graphitic covers.

Highly active Pt–Fe bicomponent catalysts for CO oxidation in the presence and absence of H2

Surface Fe ensembles, surface alloyed Fe atoms, and subsurface Fe species have been identified at Pt surfaces on the basis of studies in Fe–Pt(111) model systems and supported Pt–Fe nanoparticles (NPs). The surface Fe ensemble changes to ferrous oxide and forms a highly active and stable “FeO-on-Pt” structure in preferential oxidation of CO in the presence of H2 (PROX), which, however, gets fully oxidized in CO oxidation in the absence of H2 (COOX) and becomes inactive in the reaction. The surface alloyed Fe remains stable under the H2-rich and O2-rich reaction conditions, which are active for both PROX and COOX reactions. Accordingly, highly efficient Pt–Fe catalysts for the PROX and COOX reactions can be prepared via mild reduction and/or acid leaching.

Size-dependent surface reactions of Ag nanoparticles supported on highly oriented pyrolytic graphite.

Various sizes of Ag particles were grown on highly oriented pyrolytic graphite (HOPG) surfaces, which had previously been modified with nanopits to act as anchoring sites. Surface reactions of O2, CHCl3, and CCl4 on the Ag particles and bulk Ag(111) surfaces were studied by X-ray photoelectron spectroscopy (XPS), and it has been shown that size dependence of O2 and CHCl3 reactions on Ag differs from that of CCl4. Weak reactions of O2 and CHCl3 were observed on the bulk Ag(111) surfaces, while strong reactions occur on Ag particles with medium Ag coverage, suggesting that the reactions are controlled by the number of surface defect sites. On the contrary, the dissociation of CCl4 is mainly determined by the exposed Ag facet area, mainly Ag(111) facet, and strong dissociation reaction happens on the bulk Ag(111) surface. The results suggest that the size effects, which are often discussed in heterogeneous catalysis, are strongly dependent on the reaction mechanism.

Reversible structural transformation of FeOx nanostructures on Pt under cycling redox conditions and its effect on oxidation catalysis

Understanding dynamic changes of catalytically active nanostructures under reaction conditions is a pivotal challenge in catalysis research, which has been extensively addressed in metal nanoparticles but is less explored in supported oxide nanocatalysts. Here, structural changes of iron oxide (FeO(x)) nanostructures supported on Pt in a gaseous environment were examined by scanning tunneling microscopy, ambient pressure X-ray photoelectron spectroscopy, and in situ X-ray absorption spectroscopy using both model systems and real catalysts. O-Fe (FeO) bilayer nanostructures can be stabilized on Pt surfaces in reductive environments such as vacuum conditions and H2-rich reaction gas, which are highly active for low temperature CO oxidation. In contrast, exposure to H2-free oxidative gases produces a less active O-Fe-O (FeO2) trilayer structure. Reversible transformation between the FeO bilayer and FeO2 trilayer structures can be achieved under alternating reduction and oxidation conditions, leading to oscillation in the catalytic oxidation performance.

Unique Reactivity of Confined Metal Atoms on a Silicon Substrate

When small metal assembles such as single atoms or small clusters consisting of a few atoms are supported on the semiconducting or oxide substrates, the sp electrons in the metal can be confined by the metal-substrate interface, which may be critical to surface reactivity. To shed lights on the electron confinement effect, in particular the sp electron confinement, in supported metal atoms and its effect on the catalytic activity, we report here a comparative reactivity study of bulk Ag(111) surface and a Ag monolayer film on Si(111) surface by means of scanning tunneling microscopy (STM), ultraviolet and X-ray photoelectron spectroscopy (UPS and XPS), photoemission electron microscopy (PEEM), and density functional theory (DFT) calculations. Ag deposited on Si(111) can form a stable periodic array of Ag atoms confined in a 2D Ag monolayer film on Si(111) with the (√3×√3) symmetry. The most simple halogen methane, CCl4, was chosen as the probe molecule to study the surface chemistry of the Ag surfaces due to the high activity of halogen towards Ag. Our study shows that these two surfaces present distinct reactivity towards CCl4 dissociation. Monolayer Ag is inert toward dissociation of CCl4 compared to bulk Ag. Specifically, it is found that confinement of 5sp electron of Ag atoms in the √3×√3-Ag-Si surface, which is delocalized in the bulk Ag(111) surface, is decisive to the different reactivity.

Modulation of surface reactivity via electron confinement in metal quantum well films: O2 adsorption on Pb/Si(111)

Pb quantum well films with atomic-scale uniformity in thickness over macroscopic areas were prepared on Si(111)-7x7 surfaces. As a probe molecule, O(2) was used to explore the effect of electron confinement in the metal films on the surface reactivity. X-ray photoelectron spectroscopy results showed clear oscillations of oxygen adsorption and Pb oxidation with the thickness of the Pb films. The higher reactivity to O(2) on the films with 23 and 25 ML Pb has been attributed to their highest occupied quantum well states being close to the Fermi level (E(F)) and the high density of the electron states at E(F) (DOS-E(F)), as evidenced by the corresponding ultraviolet photoelectron spectroscopy. A dominant role of DOS-E(F) was suggested to explain the quantum modulation of surface reactivity in metal quantum well films.

Formation of Periodic Arrays of O Vacancy Clusters on Monolayer FeO Islands Grown on Pt(111)

Abstract The structural evolution of a Pt surface with 0.4 monolayer (ML) subsurface Fe on annealing in 1.1 × 10−7 kPa O2 was studied by scanning tunneling microscopy and X-ray photoelectron spectroscopy. When the annealing temperature was 600 K, only dissociative adsorption of O2 occurred, which induced the local restructuring of surface regions. At 750 K, subsurface Fe atoms segregated onto the surface and were oxidized. When the annealing temperature was increased to 850 K, well defined monolayer FeO islands with periodic arrays of defects were formed. The defects were located at the hexagonal closest packed sites (fcc) of FeO Moire unit cells and were either single oxygen vacancies or multiple oxygen vacancies consisting of six missing O atoms. The formation of periodic defects on monolayer FeO islands may be a way to construct active sites on Pt-Fe model catalysts.

A comparative study of CCl4 reactions on Ag and Si surfaces by in situ ultraviolet photoemission electron microscopy

The reactivity of a bulk Ag surface, an Ag monolayer film on Si(111)- 7 × 7 (denoted as the [Formula: see text]-Ag-Si surface), and Si(111)-7 × 7 to CCl(4) was investigated by x-ray photoelectron spectroscopy (XPS) and ultraviolet photoemission electron microscopy (UV-PEEM). In situ UV-PEEM was used to monitor simultaneously the CCl(4) dissociation on different surface domains, including the bulk Ag, [Formula: see text]-Ag-Si, and Si(111). The PEEM results combined with XPS data show that CCl(4) adsorbs dissociatively on bulk Ag(111) and Si(111) but adsorbs molecularly on the [Formula: see text]-Ag-Si surface, and the surface reactivity follows the order of [Formula: see text]-Ag-Si.