Controlling Hydrogen Activation, Spillover, and Desorption with Pd-Au Single-Atom Alloys

Key descriptors in hydrogenation catalysis are the nature of the active sites for H2 activation and the adsorption strength of H atoms to the surface. Using atomically resolved model systems of dilute Pd-Au surface alloys and density functional theory calculations, we determine key aspects of H2 activation, diffusion, and desorption. Pd monomers in a Au(111) surface catalyze the dissociative adsorption of H2 at temperatures as low as 85 K, a process previously expected to require contiguous Pd sites. H atoms preside at the Pd sites and desorb at temperatures significantly lower than those from pure Pd (175 versus 310 K). This facile H2 activation and weak adsorption of H atom intermediates are key requirements for active and selective hydrogenations. We also demonstrate weak adsorption of CO, a common catalyst poison, which is sufficient to force H atoms to spill over from Pd to Au sites, as evidenced by low-temperature H2 desorption

Enhancement of low-energy electron emission in 2D radioactive films

High-energy radiation has been used for decades; however, the role of low-energy electrons created during irradiation has only recently begun to be appreciated. Low-energy electrons are the most important component of radiation damage in biological environments because they have subcellular ranges, interact destructively with chemical bonds, and are the most abundant product of ionizing particles in tissue. However, methods for generating them locally without external stimulation do not exist. Here, we synthesize one-atom-thick films of the radioactive isotope (125)I on gold that are stable under ambient conditions. Scanning tunnelling microscopy, supported by electronic structure simulations, allows us to directly observe nuclear transmutation of individual (125)I atoms into (125)Te, and explain the surprising stability of the 2D film as it underwent radioactive decay. The metal interface geometry induces a 600% amplification of low-energy electron emission (<10 eV; ref. ) compared with atomic (125)I. This enhancement of biologically active low-energy electrons might offer a new direction for highly targeted nanoparticle therapies

Atomic-Scale Picture of the Composition, Decay, and Oxidation of Two-Dimensional Radioactive Films

Two-dimensional radioactive (125)I monolayers are a recent development that combines the fields of radiochemistry and nanoscience. These Au-supported monolayers show great promise for understanding the local interaction of radiation with 2D molecular layers, offer different directions for surface patterning, and enhance the emission of chemically and biologically relevant low-energy electrons. However, the elemental composition of these monolayers is in constant flux due to the nuclear transmutation of (125)I to (125)Te, and their precise composition and stability under ambient conditions has yet to be elucidated. Unlike I, which is stable and unreactive when bound to Au, the newly formed Te atoms would be expected to be more reactive. We have used electron emission and X-ray photoelectron spectroscopy (XPS) to quantify the emitted electron energies and to track the film composition in vacuum and the effect of exposure to ambient conditions. Our results reveal that the Auger electrons emitted during the ultrafast radioactive decay process have a kinetic energy corresponding to neutral Te. By combining XPS and scanning tunneling microscopy experiments with density functional theory, we are able to identify the reaction of newly formed Te to TeO2 and its subsequent dimerization. The fact that the Te2O4 units stay intact during major lateral rearrangement of the monolayer illustrates their stability. These results provide an atomic-scale picture of the composition and mobility of surface species in a radioactive monolayer as well as an understanding of the stability of the films under ambient conditions, which is a critical aspect in their future applications

Pt/Cu single-atom alloys as coke-resistant catalysts for efficient C–H activation

The recent availability of shale gas has led to a renewed interest in C–H bond activation as the first step towards the synthesis of fuels and fine chemicals. Heterogeneous catalysts based on Ni and Pt can perform this chemistry, but deactivate easily due to coke formation. Cu-based catalysts are not practical due to high C–H activation barriers, but their weaker binding to adsorbates offers resilience to coking. Using Pt/Cu single-atom alloys (SAAs), we examine C–H activation in a number of systems including methyl groups, methane and butane using a combination of simulations, surface science and catalysis studies. We find that Pt/Cu SAAs activate C–H bonds more efficiently than Cu, are stable for days under realistic operating conditions, and avoid the problem of coking typically encountered with Pt. Pt/Cu SAAs therefore offer a new approach to coke-resistant C–H activation chemistry, with the added economic benefit that the precious metal is diluted at the atomic limit

Novel catalytically active pd/Ru bimetallic nanoparticles synthesized by Bacillus benzeovorans

This work was supported by a UK Commonwealth scholarship to JBO. BK was supported by the Petroleum Technology Development Funds (PTDF) of Nigeria. The project was funded by NERC grant NE/L014076/1 to LEM. The Science City Photoemission Facility used in this research was funded through the Science Cities Advanced Materials Project 1: Creating and Characterizing Next Generation of Advanced Materials with support from AWM and ERDF funds. The microscopy work was conducted in the “Laboratorio de Microscopias Avanzadas” at “Instituto de Nanociencia de Aragon - Universidad de Zaragoza” Spain. The authors acknowledge the LMA-INA for offering access to their instruments and expertise.Bacillus benzeovorans assisted and supported growth of ruthenium (bio-Ru) and palladium/ruthenium (bio-Pd@Ru) core@shell nanoparticles (NPs) as bio-derived catalysts. Characterization of the bio-NPs using various electron microscopy techniques and high-angle annular dark field (HAADF) analysis confirmed two NP populations (1–2 nm and 5–8 nm), with core@shells in the latter. The Pd/Ru NP lattice fringes, 0.231 nm, corresponded to the (110) plane of RuO2. While surface characterization using X-ray photoelectron spectroscopy (XPS) showed the presence of Pd(0), Pd(II), Ru(III) and Ru(VI), X-ray absorption (XAS) studies of the bulk material confirmed the Pd speciation (Pd(0) and Pd(II)- corresponding to PdO), and identified Ru as Ru(III) and Ru(IV). The absence of Ru–Ru or Ru–Pd peaks indicated Ru only exists in oxide forms (RuO2 and RuOH), which are surface-localized. X ray diffraction (XRD) patterns did not identify Pd-Ru alloying. Preliminary catalytic studies explored the conversion of 5-hydroxymethyl furfural (5-HMF) to the fuel precursor 2,5-dimethyl furan (2,5-DMF). Both high-loading (9.7 wt.% Pd, 6 wt.% Ru) and low-loading (2.4 wt.% Pd, 2 wt.% Ru) bio-derived catalysts demonstrated high conversion efficiencies (~95%) and selectivity of ~63% (~20% better than bio-Ru NPs) and 58%, respectively. These materials show promising future scope as efficient low-cost biofuel catalysts.Funded by NERC grant NE/L014076/

Thermally stable single atom Pt/m-Al2O3 for selective hydrogenation and CO oxidation

Single-atom metal catalysts offer a promising way to utilize precious noble metal elements more effectively, provided that they are catalytically active and sufficiently stable. Herein, we report a synthetic strategy for Pt single-atom catalysts with outstanding stability in several reactions under demanding conditions. The Pt atoms are firmly anchored in the internal surface of mesoporous Al2O3, likely stabilized by coordinatively unsaturated pentahedral Al3+ centres. The catalyst keeps its structural integrity and excellent performance for the selective hydrogenation of 1,3-butadiene after exposure to a reductive atmosphere at 200 °C for 24 h. Compared to commercial Pt nanoparticle catalyst on Al2O3 and control samples, this system exhibits significantly enhanced stability and performance for n-hexane hydro-reforming at 550 °C for 48 h, although agglomeration of Pt single-atoms into clusters is observed after reaction. In CO oxidation, the Pt single-atom identity was fully maintained after 60 cycles between 100 and 400 °C over a one-month period

Carbon Monoxide Mediated Hydrogen Release from PtCu Single-Atom Alloys: The Punctured Molecular Cork Effect

Pt based materials are used extensively in heterogeneous catalytic processes, but are notoriously susceptible to poisoning by CO. In contrast, highly dilute binary alloys formed of isolated Pt atoms in a Cu metal host, known as PtCu single-atom alloys (SAAs), are more resilient to CO poisoning during catalytic hydrogenation reactions. In this article, we describe how CO affects the adsorption and desorption of H2 from a model PtCu(111) SAA surface and gain a microscopic understanding of their interaction at the Pt atom active sites. By combining temperature programmed desorption and scanning tunneling microscopy with first principles kinetic Monte Carlo we identify CO as a Pt site blocker that prevents the low temperature adsorption and desorption of H2, the so-called molecular cork effect, first realized when examining PdCu SAAs. Intriguingly, for the case of PtCu, H2 desorption occurs before CO release is detected. Furthermore, desorption experiments show a non-linear relationship between CO coverage of the Pt sites and H2 desorption peak temperature. When all the Pt atoms are saturated by CO a very sharp H2 desorption feature is observed 55 K above the regular desorption temperature of H2. Our simulations reveal that the origin of these effects is the fact that desorption of just one CO molecule from a Pt site facilitates the fast release of many molecules of H2. In fact, just 0.7 % of the CO adsorbed at Pt sites has desorbed when the H2 desorption peak maximum is reached. The release of H2 from CO corked PtCu SAA surfaces analogous to the escape of gas from a pressurized container with a small puncture. Given that small changes in CO surface coverage lead to large changes in H2 evolution energetics the punctured molecular cork effect must be considered when modeling reaction mechanisms on similar alloy systems