Intrinsic relation between catalytic activity of CO oxidation on Ru nanoparticles and Ru oxides uncovered with ambient pressure XPS.

Recent progress in colloidal synthesis of nanoparticles with well-controlled size, shape, and composition, together with development of in situ surface science characterization tools, such as ambient pressure X-ray photoelectron spectroscopy (APXPS), has generated new opportunities to unravel the surface structure of working catalysts. We report an APXPS study of Ru nanoparticles to investigate catalytically active species on Ru nanoparticles under oxidizing, reducing, and CO oxidation reaction conditions. The 2.8 and 6 nm Ru nanoparticle model catalysts were synthesized in the presence of poly(vinyl pyrrolidone) polymer capping agent and deposited onto a flat Si support as two-dimensional arrays using the Langmuir-Blodgett deposition technique. Mild oxidative and reductive characteristics indicate the formation of surface oxide on the Ru nanoparticles, the thickness of which is found to be dependent on nanoparticle size. The larger 6 nm Ru nanoparticles were oxidized to a smaller extent than the smaller Ru 2.8 nm nanoparticles within the temperature range of 50-200 °C under reaction conditions, which appears to be correlated with the higher catalytic activity of the bigger nanoparticles. We found that the smaller Ru nanoparticles form bulk RuO(2) on their surfaces, causing the lower catalytic activity. As the size of the nanoparticle increases, the core-shell type RuO(2) becomes stable. Such in situ observations of Ru nanoparticles are useful in identifying the active state of the catalysts during use and, hence, may allow for rational catalyst designs for practical applications.

In situ oxidation study of Pt(110) and its interaction with CO.

Many interesting structures have been observed for O(2)-exposed Pt(110). These structures, along with their stability and reactivity toward CO, provide insights into catalytic processes on open Pt surfaces, which have similarities to Pt nanoparticle catalysts. In this study, we present results from ambient-pressure X-ray photoelectron spectroscopy, high-pressure scanning tunneling microscopy, and density functional theory calculations. At low oxygen pressure, only chemisorbed oxygen is observed on the Pt(110) surface. At higher pressure (0.5 Torr of O(2)), nanometer-sized islands of multilayered α-PtO(2)-like surface oxide form along with chemisorbed oxygen. Both chemisorbed oxygen and the surface oxide are removed in the presence of CO, and the rate of disappearance of the surface oxide is close to that of the chemisorbed oxygen at 270 K. The spectroscopic features of the surface oxide are similar to the oxide observed on Pt nanoparticles of a similar size, which provides us an extra incentive to revisit some single-crystal model catalyst surfaces under elevated pressure using in situ tools.

Rh(1-x)Pd(x) nanoparticle composition dependence in CO oxidation by oxygen: catalytic activity enhancement in bimetallic systems.

Bimetallic 15 nm Rh(1-x)Pd(x) nanoparticle catalysts of five different compositions and supported on Si wafers have been synthesized, characterized using TEM, SEM, and XPS, and studied in CO oxidation by O(2) in two pressure regimes: atmospheric pressure and 100-200 mTorr. The RhPd bimetallic nanocrystals exhibited similar synergetic effect of increased reaction activity at both atmospheric (760 Torr) and moderate (100-200 mTorr) pressures compared with pure Pd or Rh. The magnitude of the effect depends on the relative pressures of the CO and O(2) reactant gases and the reaction temperature. The catalytic activity of the nanocrystals measured at moderate pressure is directly correlated to the APXPS studies, which were carried out in the same pressure. The APXPS studies suggest that the Pd-Rh interfaces are important for the enhanced activity of the bimetallic nanoparticles.

Size effect of ruthenium nanoparticles in catalytic carbon monoxide oxidation.

Carbon monoxide oxidation over ruthenium catalysts has shown an unusual catalytic behavior. Here we report a particle size effect on CO oxidation over Ru nanoparticle (NP) catalysts. Uniform Ru NPs with a tunable particle size from 2 to 6 nm were synthesized by a polyol reduction of Ru(acac)(3) precursor in the presence of poly(vinylpyrrolidone) stabilizer. The measurement of catalytic activity of CO oxidation over two-dimensional Ru NPs arrays under oxidizing reaction conditions (40 Torr CO and 100 Torr O(2)) showed an activity dependence on the Ru NP size. The CO oxidation activity increases with NP size, and the 6 nm Ru NP catalyst shows 8-fold higher activity than the 2 nm catalysts. The results gained from this study will provide the scientific basis for future design of Ru-based oxidation catalysts.

Evolution of structure and chemistry of bimetallic nanoparticle catalysts under reaction conditions.

Three series of bimetallic nanoparticle catalysts (Rh(x)Pd(1-x), Rh(x)Pt(1-x), and Pd(x)Pt(1-x), x = 0.2, 0.5, 0.8) were synthesized using one-step colloidal chemistry. X-ray photoelectron spectroscopy (XPS) depth profiles using different X-ray energies and scanning transmission electron microscopy showed that the as-synthesized Rh(x)Pd(1-x) and Pd(x)Pt(1-x) nanoparticles have a core-shell structure whereas the Rh(x)Pt(1-x) alloys are more homogeneous in structure. The evolution of their structures and chemistry under oxidizing and reducing conditions was studied with ambient-pressure XPS (AP-XPS) in the Torr pressure range. The Rh(x)Pd(1-x) and Rh(x)Pt(1-x) nanoparticles undergo reversible changes of surface composition and chemical state when the reactant gases change from oxidizing (NO or O(2) at 300 degrees C) to reducing (H(2) or CO at 300 degrees C) or catalytic (mixture of NO and CO at 300 degrees C). In contrast, no significant change in the distribution of the Pd and Pt atoms in the Pd(x)Pt(1-x) nanoparticles was observed. The difference in restructuring behavior under these reaction conditions in the three series of bimetallic nanoparticle catalysts is correlated with the surface free energy of the metals and the heat of formation of the metallic oxides. The observation of structural evolution of bimetallic nanoparticles under different reaction conditions suggests the importance of in situ studies of surface structures of nanoparticle catalysts.

Restructuring of hex-Pt(100) under CO gas environments: formation of 2-D nanoclusters.

The atomic-scale restructuring of hex-Pt(100) induced by carbon monoxide with a wide pressure range was studied with a newly designed chamber-in-chamber high-pressure STM and theoretical calculations. Both experimental and DFT calculation results show that CO molecules are bound to Pt nanoclusters through a tilted on-top configuration with a separation of approximately 3.7-4.1 A. The phenomenon of restructuring of metal catalyst surfaces induced by adsorption and, in particular, the formation of small metallic clusters suggests the importance of studying structures of catalyst surfaces under high-pressure conditions for understanding catalytic mechanisms.

Mobility on the reconstructed Pt(100)-hex surface in ethylene and in its mixture with hydrogen and carbon monoxide.

By using high-pressure scanning tunneling microscopy and ambient-pressure X-ray photoelectron spectroscopy, we studied the mobility along with composition, structure and reactivity on the Pt(100)-hex surface. Adsorbates are mobile under 1 Torr of C2H4 and C2H4-H2 mixtures, but adding 3 mTorr of CO quenches the mobility. Ethylene-related adsorbates can also weaken Pt-Pt bonds and thus facilitate displacements in the hexagonal layer.

Break-up of stepped platinum catalyst surfaces by high CO coverage.

Stepped single-crystal surfaces are viewed as models of real catalysts, which consist of small metal particles exposing a large number of low-coordination sites. We found that stepped platinum (Pt) surfaces can undergo extensive and reversible restructuring when exposed to carbon monoxide (CO) at pressures above 0.1 torr. Scanning tunneling microscopy and photoelectron spectroscopy studies under gaseous environments near ambient pressure at room temperature revealed that as the CO surface coverage approaches 100%, the originally flat terraces of (557) and (332) oriented Pt crystals break up into nanometer-sized clusters and revert to the initial morphology after pumping out the CO gas. Density functional theory calculations provide a rationale for the observations whereby the creation of increased concentrations of low-coordination Pt edge sites in the formed nanoclusters relieves the strong CO-CO repulsion in the highly compressed adsorbate film. This restructuring phenomenon has important implications for heterogeneous catalytic reactions.

Reaction-driven restructuring of Rh-Pd and Pt-Pd core-shell nanoparticles.

Heterogeneous catalysts that contain bimetallic nanoparticles may undergo segregation of the metals, driven by oxidizing and reducing environments. The structure and composition of core-shell Rh(0.5)Pd(0.5) and Pt(0.5)Pd(0.5) nanoparticle catalysts were studied in situ, during oxidizing, reducing, and catalytic reactions involving NO, O2, CO, and H2 by x-ray photoelectron spectroscopy at near-ambient pressure. The Rh(0.5)Pd(0.5) nanoparticles underwent dramatic and reversible changes in composition and chemical state in response to oxidizing or reducing conditions. In contrast, no substantial segregation of Pd or Pt atoms was found in Pt(0.5)Pd(0.5) nanoparticles. The different behaviors in restructuring and chemical response of Rh(0.5)Pd(0.5) and Pt(0.5)Pd(0.5) nanoparticle catalysts under the same reaction conditions illustrates the flexibility and tunability of the structure of bimetallic nanoparticle catalysts during catalytic reactions.

The evolution of model catalytic systems; studies of structure, bonding and dynamics from single crystal metal surfaces to nanoparticles, and from low pressure (<10(-3) Torr) to high pressure (>10(-3) Torr) to liquid interfaces.

The material and pressure gap has been a long standing challenge in the field of heterogeneous catalysis and have transformed surface science and biointerfacial research. In heterogeneous catalysis, the material gap refers to the discontinuity between well-characterized model systems and industrially relevant catalysts. Single crystal metal surfaces have been useful model systems to elucidate the role of surface defects and the mobility of reaction intermediates in catalytic reactivity and selectivity. As nanoscience advances, we have developed nanoparticle catalysts with lithographic techniques and colloidal syntheses. Nanoparticle catalysts on oxide supports allow us to investigate several important ingredients of heterogeneous catalysis such as the metal-oxide interface and the influence of noble metal particle size and surface structure on catalytic selectivity. Monodispersed nanoparticle and nanowire arrays were fabricated for use as model catalysts by lithographic techniques. Platinum and rhodium nanoparticles in the 1-10 nm range were synthesized in colloidal solutions in the presence of polymer capping agents. The most catalytically active systems are employed at high pressure or at solid-liquid interfaces. In order to study the high pressure and liquid interfaces on the molecular level, experimental techniques with which we bridged the pressure gap in catalysis have been developed. These techniques include the ultrahigh vacuum system equipped with high pressure reaction cell, high pressure Sum Frequency Generation (SFG) vibration spectroscopy, High Pressure Scanning Tunneling Microscopy (HP-STM), and High Pressure X-ray Photoemission Spectroscopy (HP-XPS), and Quartz Crystal Microbalance (QCM). In this article, we overview the development of experimental techniques and evolution of the model systems for the research of heterogeneous catalysis and biointerfacial studies that can shed light on the long-standing issues of materials and pressure gaps.