Tuning the Surface Chemistry of Pd by Atomic C and H: A Microscopic Picture

Role of Interfacial Hydrogen in Ethylene Hydrogenation on Graphite-Supported Ag, Au, and Cu Catalysts

A combined surface science/microreactor approach was applied to examine interface effects in ethylene hydrogenation on carbon-supported Ag, Au, and Cu nanoparticle catalysts. Turnover frequencies (TOFs) were substantially higher for supported catalysts than for (unsupported) polycrystalline metal foils, especially for Ag. Spark ablation of the corresponding metals on highly oriented pyrolytic graphite (HOPG) and carbon-coated grids yielded nanoparticles of around 3 nm size that were well-suited for characterization by X-ray photoelectron spectroscopy (XPS), high-resolution (scanning) transmission electron microscopy (HRTEM/STEM), and energy dispersive X-ray spectroscopy (EDX). Polycrystalline metal foils characterized by scanning electron microscopy (SEM), EDX, electron backscatter diffraction (EBSD), XPS, and low-energy ion scattering (LEIS) served as unsupported references. Employing a UHV-compatible flow microreactor and gas chromatography (GC) allowed us to determine the catalytic performance of the model catalysts in ethylene hydrogenation up to 200 °C under atmospheric pressure. Compared to the pure metal foils, the HOPG-supported metal nanoparticles exhibited not only strongly increased activity but also higher stability (slower deactivation) and differing reaction orders. For the most active Ag catalysts, DFT calculations were carried out to determine the adsorption energies of the reacting species on single-crystal surfaces as well as on carbon-supported and unsupported Ag nanoparticles. Adsorption of molecular hydrogen was very weak on all unsupported Ag surfaces, resulting in hydrocarbon-"poisoned" surfaces. However, when a carbon support was present, the adsorption strength of H2 on Ag nanoparticles increased on average by -0.5 eV, driven by changes in Ag-Ag distances near the metal-carbon three-phase boundary (whereas subsurface carbon lowers hydrogen bonding). On Cu particles, the interface effect was calculated to be somewhat weaker than for Ag particles. H2/D2 scrambling experiments on Ag catalysts then corroborated a facilitated hydrogen activation for carbon-supported metals. Thus, the carbon support effect is attributed to an improved hydrogen availability at the metal-carbon interface, controlling performance.

Catalysis of Alloys: Classification, Principles, and Design for a Variety of Materials and Reactions

Alloying has long been used as a promising methodology to improve the catalytic performance of metallic materials. In recent years, the field of alloy catalysis has made remarkable progress with the emergence of a variety of novel alloy materials and their functions. Therefore, a comprehensive disciplinary framework for catalytic chemistry of alloys that provides a cross-sectional understanding of the broad research field is in high demand. In this review, we provide a comprehensive classification of various alloy materials based on metallurgy, thermodynamics, and inorganic chemistry and summarize the roles of alloying in catalysis and its principles with a brief introduction of the historical background of this research field. Furthermore, we explain how each type of alloy can be used as a catalyst material and how to design a functional catalyst for the target reaction by introducing representative case studies. This review includes two approaches, namely, from materials and reactions, to provide a better understanding of the catalytic chemistry of alloys. Our review offers a perspective on this research field and can be used encyclopedically according to the readers' individual interests.

Model Catalysis with HOPG-Supported Pd Nanoparticles and Pd Foil: XPS, STM and C2H4 Hydrogenation

A surface science based approach was applied to model carbon supported Pd nanoparticle catalysts. Employing physical vapour deposition of Pd on sputtered surfaces of highly oriented pyrolytic graphite (HOPG), model catalysts were prepared that are well-suited for characterization by X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). Analysis of the HOPG substrate before and after ion-bombardment, and of Pd/HOPG before and after annealing, revealed the number of “nominal” HOPG defects (~ 10¹⁴ cm⁻²) as well as the nucleation density (~ 10¹² cm⁻²) and structural characteristics of the Pd nanoparticles (mean size/height/distribution). Two model systems were stabilized by UHV annealing to 300 °C, with mean Pd particles sizes of 4.3 and 6.8 nm and size/height aspect ratio up to ~ 10. A UHV-compatible flow microreactor and gas chromatography were used to determine the catalytic performance of Pd/HOPG in ethylene (C₂H₄) hydrogenation up to 150 °C under atmospheric pressure, yielding temperature-dependent conversion values, turnover frequencies (TOFs) and activation energies. The performance of Pd nanocatalysts is compared to that of polycrystalline Pd foil and contrasted to Pt/HOPG and Pt foil, pointing to a beneficial effect of the metal/carbon phase boundary, reflected by up to 10 kJ mol⁻¹ lower activation energies for supported nanoparticles.

Hydrogenation on Palladium Nanoparticles Supported by Graphene Nanoplatelets

Pd nanoparticles (1 wt %; mean size ∼4 nm) were supported on ∼2 μm sized, but few nanometers thick, graphene nanoplatelets (GNPs) and compared to 1 wt % Pd on activated carbon or γ-alumina. Catalyst morphology, specific surface area, and Pd particle size were characterized by SEM, BET, and TEM, respectively. H2-TPD indicated that GNPs intercalated hydrogen, which may provide additional H2 supply to the Pd nanoparticles during C2H4 hydrogenation. Whereas the two types of Pd/GNPs (NaOH vs calcinated) catalysts were less active than Pd/C and Pd/Al2O3 below 40 °C, at 55 °C they were about 3-4 times more active. As for example Pd/GNPs (NaOH) and Pd/Al2O3 exhibited not too different mean Pd particle size (3.7 vs 2.5 nm, respectively), the higher activity is attributed to the additional hydrogen supply likely by the metal/support interface, as suggested by the varying C2H4 and H2 orders on the different supports. Operando XANES measurements during C2H4 hydrogenation revealed the presence of Pd hydride. The Pd hydride was more stable for Pd/GNPs (NaOH) than for Pd/C, once more pointing to a better hydrogen supply by graphene nanoplatelets.

Insights into Hydrogen Gas Environment-Promoted Nanostructural Changes in Stressed and Relaxed Palladium by Environmental Transmission Electron Microscopy and Variable-Energy Positron Annihilation Spectroscopy

Environmental transmission electron microscopy (ETEM) and variable-energy positron annihilation spectroscopy (VEPAS) are used to observe hydrogen-induced microstructural changes in stress-free palladium (Pd) foils and stressed Pd thin films grown on rutile TiO₂ substrates. The microstructural changes in Pd strongly depend on the hydrogen pressure and on the stress state. At room temperature, enhanced Pd surface atom mobility and surface reconstruction is seen by ETEM already at low hydrogen pressures ( p_H < 10 Pa). The observations are consistent with molecular dynamics simulations. A strong increase of the vacancy density was found, and so-called superabundant vacancies were identified by VEPAS. At higher pressures, migration and vanishing of intrinsic defects is observed in Pd free-standing foils. The Pd thin films demonstrate an increased density of dislocations with increase of the H₂ pressure. The comparison of the two studied systems demonstrates the influence of the mechanical stress on structural evolution of Pd catalysts.

Defect generation in Pd layers by 'smart' films with high H-affinity

In this paper, we demonstrate that the microstructure and the surface of a thin palladium (Pd) film can be intentionally altered by the presence of a subjacent niobium (Nb) film. Depending on the thickness of the Nb film and on the hydrogen gas pressure, defects in the Pd film can be healed or created. To demonstrate this effect, Pd/Nb/sapphire (Al₂O₃) stacks are studied during hydrogen gas exposure at room temperature by using scanning tunneling microscopy (STM), X-ray diffraction (XRD) and environmental transmission electron microscopy (ETEM). STM shows that hydrogen-induced topography changes in the Nb films depend on the film thickness which affects the height of the Nb surface corrugations, their lateral size and distribution. XRD measurements show that these changes in the Nb hydride film influence the microstructure of the overlaying Pd film. ETEM reveals that the modifications of the Pd film occur due to the precipitation and growth of the Nb hydride phase. The appearance of new defects, interface and surface roughening is observed in the Pd film above locally grown Nb hydride grains. These results can open a new route to design 'smart' catalysts or membranes, which may accommodate their microstructure depending on the gaseous environment.

Approaching complexity of alkyl hydrogenation on Pd via density-functional modelling

Pd is widely used to catalyse hydrogenation and dehydrogenation reactions. One of them is the hydrogenation of ethylene, which includes the transformation of ethyl species to ethane. Herein, by means of density-functional calculations we address several still insufficiently understood factors affecting the latter process. In particular, we shed light on the following aspects of hydrogenation of alkyls on Pd: (i) the mechanistic details of how subsurface H accelerates the reaction on a (111) surface; (ii) the role of nanoparticle edges; and (iii) the influence of a common spectator ethylidyne, [triple bond, length as m-dash]C-CH₃. These factors are identified as significant for the height of the ethyl hydrogenation barrier on Pd. Moreover, we show that butyl hydrogenation on Pd is also governed by very similar interactions, which suggests a broader applicability of our conclusions. This study highlights the complexity of alkyl hydrogenation and analyses the factors that need to be taken into account for a more realistic description of the hydrogenation processes on metal surfaces.

Elucidation of the higher coking resistance of small versus large nickel nanoparticles in methane dry reforming via computational modeling

Monoatomic C species remain separated in the subsurface regions of small Ni nanoparticles, while in larger particles, carbon chains are formed, which can be considered as precursors for coke or graphene formation.

Carbon dissolution and segregation in platinum

Density functional studies at show the feasibility of C subsurface incorporation in Platinum occupying tetrahedral sites. A comparative with Ni and Pd highlights that surface relaxation is critical in C dissolution, specially at low-coordinated sites of Pt nanoparticles. Results explain phenomena such as C dissolution and segregation to form graphene from below, and may serve to tune the Pt surface chemical reactivity.

How absorbed hydrogen affects the catalytic activity of transition metals

Heterogeneous catalysis is commonly governed by surface active sites. Yet, areas just below the surface can also influence catalytic activity, for instance, when fragmentation products of catalytic feeds penetrate into catalysts. In particular, H absorbed below the surface is required for certain hydrogenation reactions on metals. Herein, we show that a sufficient concentration of subsurface hydrogen, H(sub) , may either significantly increase or decrease the bond energy and the reactivity of the adsorbed hydrogen, H(ad) , depending on the metal. We predict a representative reaction, ethyl hydrogenation, to speed up on Pd and Pt, but to slow down on Ni and Rh in the presence of H(sub) , especially on metal nanoparticles. The identified effects of subsurface H on surface reactivity are indispensable for an atomistic understanding of hydrogenation processes on transition metals and interactions of hydrogen with metals in general.

Understanding the reactivity of metallic nanoparticles: beyond the extended surface model for catalysis

Metallic nanoparticles (NPs) constitute a new class of chemical objects which are used in different fields as diverse as plasmonics, optics, catalysis, or biochemistry.

DFT studies of oxygen dissociation on the 116-atom platinum truncated octahedron particle

Oxygen dissociation studies performed on Pt₁₁₆ nanoparticles highlight the importance of surface flexibility for fast reaction kinetics.