Surface composition of AgPd single-atom alloy catalyst in an oxidative environment

Single-atom alloys (SAAs) have recently gained considerable attention in the field of heterogeneous catalysis research due to their potential for novel catalytic properties. While SAAs are often examined in reactions of reductive atmospheres, such as hydrogenation reactions, in the present work, we change the focus to AgPd SAAs in oxidative environments since Pd has the highest catalytic activity of all metals for oxidative reactions. Here, we examine how the chemical reactivity of AgPd SAAs differs from its constituent Pd in an oxidative atmosphere. For this purpose, electronic structure changes in an Ag0.98Pd0.02 SAA foil in 1 mbar of O2 were studied by in situ x-ray photoemission spectroscopy and compared with the electronic structure of a Pd foil under the same conditions. When heated in an oxidative atmosphere, Pd in Ag0.98Pd0.02 partly oxidizes and forms a metastable PdOx surface oxide. By using a peak area modeling procedure, we conclude that PdOx on Ag0.98Pd0.02 is present as thin, possibly monolayer thick, PdOx islands on the surface. In comparison to the PdO formed on the Pd foil, the PdOx formed on AgPd is substantially less thermodynamically stable, decomposing at temperatures about 270 °C lower than the native oxide on Pd. Such behavior is an interesting property of oxides formed on dilute alloys, which could be potentially utilized in catalytic oxidative reactions such as methane oxidation

Inelastic electron scattering by the gas phase in near ambient pressure XPS measurements

X‐ray photoemission spectroscopy (XPS) measurements in near‐ambient pressure (NAP) conditions result in a signal loss of the primary spectrum as a result of inelastic scattering of photoelectrons in the gas phase. The inelastic scattering of the primary electrons gives rise to a secondary signal that can result in additional and often unwanted features in the measured spectrum. In the present work, we derive equations that can be used to model the resulting signal and provide equations that can be used to simulate or remove the inelastic scattering signal from measured spectra. We demonstrate this process for photoemission spectra of a wide range of kinetic energies, measured from Au, Ag, and Cu, in a variety of gases (N2, He, H2, and O2). The work is supplemented with an open‐source software in which the algorithms described here have been implemented and can be used to remove the gas phase inelastic scattering signal

Isolated Pd atoms in a silver matrix: Spectroscopic and chemical properties

Over the past decade, single-atom alloys (SAAs) have been a lively topic of research due to their potential for achieving novel catalytic properties and circumventing some known limitations of heterogeneous catalysts, such as scaling relationships. In researching SAAs, it is important to recognize experimental evidence of peculiarities in their electronic structure. When an isolated atom is embedded in a matrix of foreign atoms, it exhibits spectroscopic signatures that reflect its surrounding chemical environment. In the present work, using photoemission spectroscopy and computational chemistry, we discuss the experimental evidence from Ag0.98Pd0.02 SAAs that show free-atom-like characteristics in their electronic structure. In particular, the broad Pd4d valence band states of the bulk Pd metal become a narrow band in the alloy. The measured photoemission spectra were compared with the calculated photoemission signal of a free Pd atom in the gas phase with very good agreement, suggesting that the Pd4d states in the alloy exhibit very weak hybridization with their surroundings and are therefore electronically isolated. Since AgPd alloys are known for their superior performance in the industrially relevant semi-hydrogenation of acetylene, we considered whether it is worthwhile to drive the dilution of Pd in the inert Ag host to the single-atom level. We conclude that although site-isolation provides beneficial electronic structure changes to the Pd centers due to the difficulty in activating H2 on Ag, utilizing such SAAs in acetylene semi-hydrogenation would require either a higher Pd concentration to bring isolated sites sufficiently close together or an H2-activating support

Pocket Monte Carlo algorithm for classical doped dimer models

We study the correlations of classical hardcore dimer models doped with monomers by Monte Carlo simulation. We introduce an efficient cluster algorithm, which is applicable in any dimension, for different lattices and arbitrary doping. We use this algorithm for the dimer model on the square lattice, where a finite density of monomers destroys the critical confinement of the two-monomer problem. The monomers form a two-component plasma located in its high-temperature phase, with the Coulomb interaction screened at finite densities. On the triangular lattice, a single pair of monomers is not confined. The monomer correlations are extremely short-ranged and hardly change with doping.Comment: 6 pages, REVTeX

Sub therapeutic drug levels among HIV/TB co-infected patients receiving Rifampicin in northwestern Tanzania: A cross sectional clinic based study

Background: Tuberculosis/Human Immunodeficiency Virus (TB/HIV) is a very common co-infection which carries a high mortality rate. Though World Health Organization recommends co-treatment of TB/HIV to improve its outcome, Rifampicin potentially induces metabolism and sub-therapeutic antiretroviral plasma levels of non nucleoside reverse transcriptase inhibitors and protease inhibitors which may cause inadequate virological suppression if corrections are not timely done. In Tanzania Therapeutic drug monitoring is not done; so the proportion of sub-therapeutic ARV plasma levels among TB/HIV patients co-treated with anti-tuberculous drugs is not known. The aim of this study was therefore to determine the magnitude and risk factors of sub-therapeutic ARV plasma levels among adult HIV patients co-treated with anti tuberculous Medications.Materials and methods: A cross sectional hospital based study was conducted among adult HIV patients on ARV and TB co-treatment for at least one month. Patients were serially enrolled through routine HIV care and treatment services until the sample size was reached. The information about demographic, clinical and adherence level, Anti-TB duration, viral load, baseline and enrollment CD4 counts, Hepatitis B co-infection and ARV plasma levels was collected and analyzed using STATA 12 software.Results: In total 118 patients were included in this study; of whom 26 (22%) had sub-therapeutic ARV plasma levels. The sub-therapeutic ARV levels were independently associated with adherence <95% (OR =6.8, p= 0.001), female gender (OR = 3.4, p= 0.028) and virological failure (OR= 3.8, p= 0.016). NVP based regimen was associated with sub-therapeutic drug levels on univariate model (OR = 2.1, p= 0.010).Conclusion: The magnitude of sub-therapeutic ARV plasma levels is high among adult HIV/TB coinfected patients on anti-TB co-treatment in Tanzania. These patients stand a high risk of inadequate virological suppression with a potential resistance development and a long term poor clinical outcome. Identifying at risk patients and adherence enhancement could potentially improve the overall outcome of this subgroup of patients in resource restricted setting like ours where TDM is not available.Keywords: HIV; HIV/TB co-infection; HIV/TB co-treatment Rifampicin; Antiretroviral therapy; Plasma ARV drug levels; Therapeutic drug monitorin