Ultrathin metal films and particles on oxide surfaces: structural, electronic and chemisorptive properties

Modern trends in studying the influence of the support on the electronic state, morphology, and catalytic properties of supported metal particles are reviewed. The analysis of new developments in techniques for catalyst characterization is given. Metal–support interaction effects are discussed in terms of electronic modification of the supported clusters and their morphological transformations induced by the support. Special attention is paid to the recent concepts of the structure of the metal–support interface. The support effects on the catalytic properties of metal particles are revealed as (1) changes due to metal particle charging, (2) effects related to variations in metal particle shape and crystallographic structure and, (3) appearance of the specific active sites at the metal–support boundary.

Effects of the support on the morphology and electronic properties of supported metal clusters : modern concepts and progress in 1990s

Molecular beam experiments on model catalysts

The hydrogenation of carbon dioxide (CO2) to formic acid (FA; HCOOH), a renewable hydrogen storage material, is a promising means of realizing an economical CO2-mediated hydrogen energy cycle. The development of reliable heterogeneous catalysts is an urgent yet challenging task associated with such systems, although precise catalytic site design protocols are still lacking. In the present study, we demonstrate that PdAg alloy nanoparticles (NPs) supported on TiO2 promote the efficient selective hydrogenation of CO2 to give FA even under mild reaction conditions (2.0 MPa, 100 °C). Specimens made using surface engineering with atomic precision reveal a strong correlation between increased catalytic activity and decreased electron density of active Pd atoms resulting from a synergistic effect of alloying with Ag atoms. The isolated and electronically promoted surface-exposed Pd atoms in Pd@Ag alloy NPs exhibit a maximum turnover number of 14 839 based on the quantity of surface Pd atoms, which represents a m...

Surface Engineering of a Supported PdAg Catalyst for Hydrogenation of CO2 to Formic Acid: Elucidating the Active Pd Atoms in Alloy Nanoparticles

Thin oxide films (from one to tens of monolayers) of SiO2, MgO, NiO, Al2O3, FexOy, and TiO2 supported on refractory metal substrates have been prepared by depositing the oxide metal precursor in a background of oxygen (ca 1 x 10(-5) Torr). The thinness of these oxide samples facilitates investigation by an array of surface techniques, many of which are precluded when applied to the corresponding bulk oxide. Layered and mixed binary oxides have been prepared by sequential synthesis of dissimilar oxide layers or co-deposition of two different oxides. Recent work has shown that the underlying oxide substrate can markedly influence the electronic and chemical properties of the overlayer oxide. The structural, electronic, and chemical properties of these ultrathin oxide films have been probed using Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), electron energy loss spectroscopy (ELS), ion-scattering spectroscopy (ISS), high-resolution electron energy loss spectroscopy (HREELS), infrared reflectance absorption spectroscopy (IRAS), temperature-programmed desorption (TPD), scanning tunneling microscopy (STM), and scanning tunneling spectroscopy (STS).

The physical and chemical properties of ultrathin oxide films.

The adsorption of CO on Pd/Al2O3/Ta(110) model catalysts was studied using infrared reflection adsorption spectroscopy (IRAS) for two different Pd coverages, corresponding to 25–30 and 60–70 A average particle sizes. Striking similarities between the IRAS data for the larger particles and Pd single crystals indicate that the particles are comprised predominantly of 〈111〉 and 〈100〉 facets. The isosteric heat of adsorption of CO extrapolated to zero coverage was determined to be 38.9 kcal/mol on the 60–70 A particles. IRAS and Auger results provide evidence of CO dissociation on the smaller particles.

Adsorption of CO on Pd/Al2O3/Ta(110) model catalysts

A multiscale model for proton exchange membrane fuel cells with order-structured catalyst layers

/pdf/modeling-proton-exchange-membrane-fuel-cells-with-fiber-3q99pp0u.pdf

Modeling proton exchange membrane fuel cells with fiber-based microporous layers

Platinum-group-metal-free (PGM-free) catalysts hold great potential to replace their expensive Platinum-based counterparts in proton exchange membrane fuel cells because of their exceptional catalytic activity, outstanding abundance, and low cost. However, inferior cell performance resulting from the ill catalyst layer design remains a critical barrier for their practical applications in fuel cells. Here, we present a refined two-phase model to reveal the effects of the design and operating parameters on the performance of the fuel cells. To accurately capture the cathodic process, key descriptors (site density, turnover frequency, and active site availability) of PGM-free catalysts are incorporated into the agglomerate model. Numerical results show that catalyst loading, porosity, and site density significantly affect cell performance. In contrast to the high catalyst loadings usually employed in the reported literature, we find that a moderate catalyst loading is beneficial for achieving the best cell performance due to the balanced activity, ohmic and concentration polarizations. Moreover, the optimal catalyst loading can be further reduced with higher site densities. Optimal porosity of the cathode catalyst layer also exists due to the contradiction between mass transport and effective electrochemical reaction rate in agglomerates. Additionally, our results show that PGM-free catalyst layer should be optimized accordingly when the fuel cells operate under H2-O2 and H2-air conditions. This work provides insightful understanding into the electrochemical reaction and mass transport behavior in the fuel cells with PGM-free catalysts, paving the way for the development of cost-effective and high-performance fuel cells. 

Modeling proton exchange membrane fuel cells with platinum-group-metal-free catalysts

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Adsorption of CO on Pd/Al2O3/Ta(110) model catalysts

A multiscale model for proton exchange membrane fuel cells with order-structured catalyst layers

Modeling proton exchange membrane fuel cells with fiber-based microporous layers

Modeling proton exchange membrane fuel cells with platinum-group-metal-free catalysts