ALD Functionalized Nanoporous Gold: Thermal Stability, Mechanical Properties, and Catalytic Activity

Nanoporous metals have many technologically promising applications but their tendency to coarsen limits their long-term stability and excludes high temperature applications. Here, we demonstrate that atomic layer deposition (ALD) can be used to stabilize and functionalize nanoporous metals. Specifically, we studied the effect of nanometer-thick alumina and titania ALD films on thermal stability, mechanical properties, and catalytic activity of nanoporous gold (np-Au). Our results demonstrate that even only one-nm-thick oxide films can stabilize the nanoscale morphology of np-Au up to 1000 C, while simultaneously making the material stronger and stiffer. The catalytic activity of np-Au can be drastically increased by TiO{sub 2} ALD coatings. Our results open the door to high temperature sensor, actuator, and catalysis applications and functionalized electrodes for energy storage and harvesting applications

Thermal Activation of Nanoporous Gold for Carbon Monoxide Oxidation

Heterogeneous catalysts based on gold have attracted considerable attention over the last 30 years. In spite of its chemical inertness, the research on gold catalysis has proven that in oxidation reactions, for instance, high activities and, in particular, selectivities can be achieved. While mostly Au nanoparticles supported on suitable oxides have been focused on in the literature, more recently, nanoporous gold (npAu) was found to be an alternative that shows similarly good catalytic performance, even though the characteristic structural length (diameter of ligaments vs particle diameter) is an order of magnitude larger. To date, however, no recipe has been reported to reliably activate npAu for total oxidations, such as CO oxidation. Here, we present such an activation procedure that allows obtaining high conversion levels on the time scale of an hour, independently of the prehistory of the samples. It consists of one or several steps where the catalyst is subjected to a thermal treatment in the reaction mixture (CO + O2) at 300 °C for 1 min only. Previously, it was assumed that such a high-temperature treatment would result in coarsening of the nanoporous structure and thus in an intolerable loss of surface area. According to our results, however, short annealing steps hardly alter the ligament size but effectively remove residuals from the preparation and/or modify the surface chemistry so that a catalytically active state is obtained quickly. Reproducibly, rates can be achieved, which are in good agreement with values previously reported in the literature and are stable for at least several days on stream. Our results open the way for a practical use of npAu as well as for further studies on the mechanistic origin of its catalytic activity

Influence of Organic Amino and Thiol Ligands on the Geometric and Electronic Surface Properties of Colloidally Prepared Platinum Nanoparticles

The influence of organic ligands on the geometric and electronic surface properties of colloidally prepared Pt nanoparticles (Pt NPs) was investigated by means of diffuse reflectance infrared spectroscopy (DRIFTS) and using CO as probe molecule. Dodecylamine (DDA) and dodecylthiol (DDT) were used as surface functionalizing ligands that exhibit the same hydrocarbon tail but different functional groups. While for DDA an electronic donor effect as well as a geometric effect was found, the effect of DDT was mainly geometric in nature. For high DDA and DDT coverages, on-top sites were blocked for CO adsorption to a higher extent than bridge and threefold hollow sites. Whereas DDA is only weakly adsorbed and displaced by strongly binding adsorbates (e.g., CO), DDT is strongly attached to the particle surface forming an evenly distributed capping layer. With decreasing thiol coverage, blocking of bridge and threefold hollow sites became more pronounced than on-top site blocking. The influence of both ligands on the selectivity of the hydrogenation of crotonaldehyde catalyzed by Pt NPs was investigated. For amino-functionalized NPs the catalytic properties did not differ from that of “unprotected” Pt NPs. In contrast, an increased selectivity can be found for thiol-functionalized particles, which, according to the IR-spectroscopic investigations, was attributed to a geometric modification of the surface by the ligand

Transient Au–CO Complexes Promote the Activity of an Inverse Ceria/Gold Catalyst: An Insight from <i>Ab Initio</i> Molecular Dynamics

To probe particle–support interactions and their mechanistic role for catalytic CO oxidation on nanoporous gold (np-Au) coated with ceria nanoparticles, we carried out ab initio molecular dynamics (AIMD) simulations and standard density functional theory (static DFT) computations. To this end, we studied ceria clusters (Ce10O20/19) supported on a Au(321) surface exhibiting a high density of steps and kinks. Our theoretical model represents the structurally inverse situation compared to more commonly studied ceria-supported Au nanoparticle systems. In agreement with previous results for Au(111), we find that reduced (Ce10O19) as well as stoichiometric (Ce10O20) ceria nanoparticles transfer electrons to the Au(321) support. This charge transfer (particularly strong in the case of Ce10O19) reflecting a strong chemical interaction between ceria and Au is probably responsible for the stabilization of np-Au against thermal coarsening experimentally observed upon deposition of oxide nanoparticles. The adsorption energies of the ceria cluster on Au(321) are more negative than on the Au(111) surface by around ∼0.5 eV. AIMD simulations were employed to study the mechanism of catalytic CO oxidation with O2 for the ceria/Au(321) system. We found that a CO molecule adsorbed near the ceria/gold perimeter interface can extract a Au atom from the surface in the form of a mobile linear Au–CO complex, which results in a very low activation energy when this species reacts with lattice O to CO2. The released bare Au adatom subsequently attaches to a step edge of the gold surface, leading to a dynamic restructuring of the Au support. Next, an activated O2 molecule adsorbed at a perimeter site between ceria and Au reacts with a second CO molecule to CO2 and an adsorbed O atom, which eventually fills the vacancy site created in the first half of the cycle. As compared to ceria particles supported on Au(111), the reactivity is enhanced as a new low-energy mechanism is enabled, revealing the positive impact of the stepped structure of Au(321)

What Changes on the Inverse Catalyst? Insights from CO Oxidation on Au-Supported Ceria Nanoparticles Using Ab Initio Molecular Dynamics

Gold-supported ceria nanoparticles (CeOx/Au), constituting an inverse system with respect to the more commonly studied ceria-supported gold nanoparticles, were previously identified as an excellent catalyst for water–gas shift reaction, CO oxidation, and steam reforming of methanol. However, the electronic structure and reactivity of such inverse catalysts have not been well understood. To probe the inherent nanoparticle–support interactions and their mechanistic role for the catalytic CO oxidation over this composite catalyst, ab initio molecular dynamics simulations and static density functional theory computations have been carried out for Au(111)-supported ceria clusters (Ce10O20/19), as a realistic model system of an inverse CeOx/Au catalyst. We have identified the perimeter of the supported ceria nanoparticle as the most favorable O vacancy formation site; however, the vacancy further migrates to an inner interface site during the thermalization process, simultaneously triggering electron transfer from ceria to Au. Our study shows that the Au(111) surface always withdraws electron density from ceria, irrespective of the chemical environment, namely, in a reducing (Ce10O19) as well as oxidizing (Ce10O20) environment. To mimic a realistic catalytic environment, CO and O2 molecules were preadsorbed on the surface of a composite catalyst. We find a vacancy diffusion-assisted Mars–van Krevelen type of reaction mechanism in which the first CO molecule reacts with a lattice O atom of ceria rather than with an activated O22– species, forming CO2 and leaving one O vacancy behind. This vacancy becomes subsequently refilled by an O atom diffusing from the site of O2 reaction with a second CO molecule, recovering the stoichiometry of the Ce10O19 cluster and closing the catalytic cycle. Finally, we discuss differences and similarities between ceria/Au and Aun/ceria with respect to surface dynamics, charge transfer between the gold and the oxide phases, and the mechanism of CO oxidation

Chemisorbed Oxygen on the Au(321) Surface Alloyed with Silver: A First-Principles Investigation

The adsorption of oxygen on kinked Au(321) slabs is investigated theoretically on the basis of density functional theory. On-surface, subsurface, and surface-oxide forms of O are analyzed and compared on pure gold and on the surfaces containing silver atoms. At low O coverage (0.1 ML) subsurface O species are shown to be unstable both thermodynamically and kinetically due to a low barrier for conversion to stronger bound on-surface chemisorbed oxygen. The presence of Ag in the near-surface region was shown to increase the binding strength of on-surface as well as subsurface O, but the activation barrier for releasing subsurface O to the surface remains essentially unaffected by the presence of Ag. At oxygen coverage 0.2 ML or higher, the most stable surface arrangements of O atoms are chain-like structures consisting of linear −O–Au–O– fragments. Subsurface O atoms being a part of such chains are significantly stabilized. We examine phase transitions between the clean surface and possible stable oxidized surface structures as a function of temperature and O₂ partial pressure. Ag atoms replacing Au on the Au(321) surface are shown to stabilize the O-covered surface with respect to the clean surface. Pre-existent chemisorbed atomic oxygen is predicted to facilitate the dissociation of molecular oxygen on the pure and alloyed gold surfaces

What Changes on the Inverse Catalyst? Insights from CO Oxidation on Au-Supported Ceria Nanoparticles Using Ab Initio Molecular Dynamics

Gold-supported ceria nanoparticles (CeOx/Au), constituting an inverse system with respect to the more commonly studied ceria-supported gold nanoparticles, were previously identified as an excellent catalyst for water–gas shift reaction, CO oxidation, and steam reforming of methanol. However, the electronic structure and reactivity of such inverse catalysts have not been well understood. To probe the inherent nanoparticle–support interactions and their mechanistic role for the catalytic CO oxidation over this composite catalyst, ab initio molecular dynamics simulations and static density functional theory computations have been carried out for Au(111)-supported ceria clusters (Ce10O20/19), as a realistic model system of an inverse CeOx/Au catalyst. We have identified the perimeter of the supported ceria nanoparticle as the most favorable O vacancy formation site; however, the vacancy further migrates to an inner interface site during the thermalization process, simultaneously triggering electron transfer from ceria to Au. Our study shows that the Au(111) surface always withdraws electron density from ceria, irrespective of the chemical environment, namely, in a reducing (Ce10O19) as well as oxidizing (Ce10O20) environment. To mimic a realistic catalytic environment, CO and O2 molecules were preadsorbed on the surface of a composite catalyst. We find a vacancy diffusion-assisted Mars–van Krevelen type of reaction mechanism in which the first CO molecule reacts with a lattice O atom of ceria rather than with an activated O22– species, forming CO2 and leaving one O vacancy behind. This vacancy becomes subsequently refilled by an O atom diffusing from the site of O2 reaction with a second CO molecule, recovering the stoichiometry of the Ce10O19 cluster and closing the catalytic cycle. Finally, we discuss differences and similarities between ceria/Au and Aun/ceria with respect to surface dynamics, charge transfer between the gold and the oxide phases, and the mechanism of CO oxidation