Characterization of Pt–Au and Ni–Au Clusters on TiO2(110)

The interaction of size-selected Ru3 clusters with TiO2: depth-profiling of encapsulated clusters

Ru is a metal of interest in catalysis. Monodisperse Ru3 clusters as catalytic sites are relevant for the development of catalysts because clusters use significantly lower amounts of precious materials for forming active sites due to the small size of the cluster. However, retaining the mono-dispersity of the cluster size after deposition is a challenge because surface energy could drive both agglomeration and encapsulation of the clusters. In the present work Ru3 clusters are deposited by chemical vapor deposition (CVD) of Ru3(CO)12 and cluster source depositions of bare Ru3 onto radio frequency sputter-deposited TiO2 (RF-TiO2) substrates, TiO2(100), and SiO2. When supported on RF-TiO2, bare Ru3 is encapsulated by a layer of titania substrate material during deposition with a cluster source. Ligated Ru3(CO)12 is also encapsulated by a layer of titania when deposited onto sputter-treated RF-TiO2, but only through heat treatment which is required to remove most of the ligands. The titania overlayer thickness was determined to be 1-2 monolayers for Ru3(CO)12 clusters on RF-TiO2, which is thin enough for catalytic or photocatalytic reactions to potentially occur even without clusters being part of the very outermost layer. The implication for catalysis of the encapsulation of Ru3 into the RF-TiO2 is discussed. Temperature-dependent X-ray photoelectron spectroscopy (XPS), angle-resolved XPS, and temperature-dependent low energy ion scattering (TD-LEIS) are used to probe how the cluster-surface interaction changes due to heat treatment and scanning transmission electron microscopy (STEM) was used to image the depth of the surface from side-on.

Optimizing the performance of Auy/Nix/TiO2NTs photoanodes for photoelectrochemical water splitting

Water splitting using photoelectrochemical (PEC) techniques is thought to be a potential method for creating green hydrogen as a sustainable energy source. How to create extremely effective electrode materials is a pressing concern in this area. In this work, a series of Nix/TiO2 anodized nanotubes (NTs) and Auy/Nix/TiO2NTs photoanodes were prepared by electrodeposition via cyclic voltammetry and UV-photoreduction, respectively. The photoanodes were characterized by several structural, morphological, and optical techniques and their performance in PEC water-splitting for oxygen evolution reaction (OER) under simulated solar light was investigated. The obtained results revealed the nanotubular structure of TiO2NTs was preserved after deposition of NiO and Au nanoparticles while the band gap energy was reduced allowing for effective utilization of solar light with lower charge recombination rate. The PEC performance was monitored and it was found that the photocurrent densities of Ni20/TiO2NTs and Au30/Ni20/TiO2NTs were 1.75-fold and 3.25-fold that of pristine TiO2NTs, respectively. It was confirmed that the performance of the photoanodes depends on the number of electrodeposition cycles and duration of photoreduction of gold salt solution. The observed enhanced OER activity of Au30/Ni20/TiO2NTs could be attributed to the synergism between the local surface plasmon resonance (LSPR) effect of nanometric gold which increased solar light harvesting and the p-n heterojunction formed at the NiO/TiO2 interface which led to better charge separation and transportation suggesting its potential application as an efficient and stable photoanode in PEC water splitting for H2 production.

Formation of Catalytically Active Nanoparticles under Thermolysis of Silver Chloroplatinate(II) and Chloroplatinate(IV)

The thermal behaviour of Ag₂[PtCl₄] and Ag₂[PtCl₆] complex salts in inert and reducing atmospheres has been studied. The thermolysis of compounds in a helium atmosphere is shown to occur in two stages. At the first stage, the complexes decompose in the temperature range of 350–500 °C with the formation of platinum and silver chloride and the release of chlorine gas. At the second stage, silver chloride is sublimated in the temperature range of 700–900 °C, while metallic platinum remains in the solid phase. In contrast to the thermolysis of Ag₂[PtCl₆], the thermal decomposition of Ag₂[PtCl₄] at 350 °C is accompanied by significant heat release, which is associated with disproportionation of the initial salt to Ag₂[PtCl₆], silver chloride, and platinum metal. It is confirmed by DSC measurements, DFT calculations of a suggested reaction, and XRD. The thermolysis of Ag₂[PtCl₄] and Ag₂[PtCl₆] compounds is shown to occur in a hydrogen atmosphere in two poorly separable steps. The compounds are decomposed within 170–350 °C, and silver and platinum are reduced to a metallic state, while a metastable single-phase solid solution of Ag_0.67Pt_0.33 is formed. The catalytic activity of the resulting nanoalloy Ag_0.67Pt_0.33 is studied in the reaction of CO total (TOX) and preferential (PROX) oxidation. Ag_0.67Pt_0.33 enhanced Pt nano-powder activity in CO TOX, but was not selective in CO PROX.

Au nanoparticles on Fe-modified rutile TiO2(110): Dispersion, thermal stability, and CO adsorption

Gold clusters on an iron-modified rutile TiO₂(110) surface have been characterized via scanning tunneling microscopy and x-ray photoelectron spectroscopy. This study is focused on the impact of submonolayer preadsorbed Fe on the morphologies, surface compositions, and thermal stabilities of bimetallic Au-Fe systems by comparing them to elemental Au and Fe adsorbates. We found that a submonolayer gold adsorbate followed the nucleation mode of the iron precursor, which considerably enhanced the dispersion of nano-gold while improving its thermal stability. Finally, the temperature-programmed CO desorption spectra of Au and Au-Fe nanoparticles on TiO₂(110) were compared.

Electronic metal–support interactions enhance the ammonia synthesis activity over ruthenium supported on Zr-modified CeO2 catalysts

Doping Zr⁴⁺ alters the electronic properties of the support. The electronic metal–support interaction produces the upshift of d-band center of Ru nanoparticles, which further influences the catalytic activity of ammonia synthesis.

Symmetry breaking and morphological instabilities in core-shell metallic nanoparticles

Nanoalloys are bi- or multi-component metallic particles in the size range between 1 and 100 nm. Nanoalloys present a wide variety of structures and properties, which make them suitable for many applications in catalysis, optics, magnetism and biomedicine. This topical review is devoted to the structural properties of nanoalloys of weakly miscible metals, which are expected to present phase-separated arrangements of their components, such as core-shell and Janus arrangements. The focus is on singling out size- and composition-dependent transitions between these arrangements, showing that several transitions can be rationalized by a unifying concept, that is symmetry breaking, caused by the accumulation of strain at the atomic level and its subsequent release. The driving forces that rule the interplay between core-shell and other structures and determine the actual shapes of core and shell, and the placement of the core inside the shell are analyzed. Several systems, such as Ag-Cu, Ag-Co, Ag-Ni, Au-Co, Au-Pt, and Ir-Pt are treated, comparing computational results to experimental observations and simple analytical models. After treating the lowest-energy structures, which are representative of the equilibrium configurations at sufficiently low temperatures, high-temperature and growth kinetics effects are considered.

Recent advancements in Pt and Pt-free catalysts for oxygen reduction reaction

Based on the understanding of the ORR catalytic mechanism, advanced Pt-based and Pt-free catalysts have been explored.