Observation of substituent effects in the electrochemical adsorption and hydrogenation of alkynes on Pt{hkl} using SHINERS

By combining cyclic voltammetry (CV) and shell-isolated nanoparticle-enhanced Raman spectroscopy (SHINERS), the adsorption behavior of two alkynes, propargyl alcohol (PA) and 2-methyl-3-butyn-2-ol (MeByOH), undergoing hydrogenation on Pt basal plane single-crystal electrodes is investigated. It is found that PA and MeByOH give rise to strong surface sensitivities in relation to both hydrogenation activity and molecular fragmentation into adsorbed species such as CO. For PA, irreversible adsorption is strongly favored for Pt{100} and Pt{110} but is weak in the case of Pt{111}. It is suggested that the presence of the primary alcohol substituent is key to this behavior, with the order of surface reactivity being Pt{100} > Pt{110} > Pt{111}. In contrast, for MeByOH, strong irreversible adsorption is observed on all three basal plane Pt surfaces and we propose that this reflects the enhanced activity of the alkyne moiety arising from the inductive effect of the two methyl groups, coupled with the decreased activity of the tertiary alcohol substituent toward fragmentation. Pt{111} also exhibits singular behavior in relation to MeByOH hydrogenation in that a sharp Raman band at 1590 cm–1 is observed corresponding to the formation of a di-σ/π-bonded surface complex as the alkyne adsorbs. This band frequency is some 20 cm–1 higher than the analogous broadband observed for PA and MeByOH adsorbed on all other basal plane Pt surfaces and may be viewed as a fingerprint of Pt{111} terraces being present at a catalyst surface undergoing hydrogenation. Insights into the hydrogenation activity of different Pt{hkl} surfaces are obtained using quantitative comparisons between Raman bands at hydrogenation potentials and at 0.4 V vs Pd/H, the beginning of the double-layer potential region, and it is asserted (with support from CV) that Pt{110} is the most active plane for hydrogenation due to the presence of surface defects generated via the lifting of the (1 × 2) to (1 × 1) clean surface reconstruction following flame annealing and hydrogen cooling. Our findings are also consistent with the hypothesis that Pt{111} planes are most likely to provide semihydrogenation selectivity of alkynes to alkenes, as reported previously

Active site manipulation in MoS2 cluster electrocatalysts by transition metal doping

The development of non-platinum group metal catalysts for the hydrogen evolution reaction (HER) in water electrolyser devices is essential for their widespread and sustainable deployment. In recent years, molybdenum disulfide (MoS2) catalysts have received significant attention as they not only exhibit good electrocatalytic HER activity but also, crucially, acid-stability. However, further performance enhancement is required for these materials to be competitive with Pt and to that end transition metal doping of MoS2 has been explored as a route to further increasing its catalytic activity. In this work, cluster beam deposition was employed to produce controlled cobalt-doped MoS2 clusters (MoS2–Co). We demonstrate that, in contrast to previous observations of performance enhancement in MoS2 resulting from nickel doping (MoS2–Ni), the introduction of Co has a detrimental effect on HER activity. The contrasting behaviours of Ni and Co doping are rationalized by density functional theory (DFT) calculations, which suggest that HER-active surface vacancies are deactivated by combination with Co dopant atoms, whilst their activity is retained, or even partially enhanced, by combination with Ni dopant atoms. Furthermore, the adatom dopant–vacancy combination kinetics appear to be more than three orders of magnitude faster in MoS2–Co than for MoS2–Ni. These findings highlight a fundamental difference in the influence of transition metal dopants on the HER performance of MoS2 electrocatalysts and stress the importance of considering surface atomic defects when predicting their behaviour

Insights into Self-Poisoning during Catalytic Hydrogenation on Platinum Surfaces Using ATR-IR Spectroelectrochemistry

Attenuated total reflection infrared (ATR-IR) spectroscopy has been combined with electrochemical methods to investigate molecular decomposition and self-poisoning processes on platinum surfaces under the conditions of catalytic hydrogenation. In aqueous 0.1 M H₂SO₄ the α-keto ester ethyl pyruvate (EP) is found to decompose on polycrystalline platinum electrodes to yield surface-adsorbed CO, but the observed behavior is highly dependent on the electrode potential, a parameter intimately linked to the surface-adsorbed hydrogen coverage. In the potential range −0.2 to −0.4 V (vs mercury/mercurous sulfate electrode) where the hydrogen coverage is negligible, CO is readily produced at the platinum surface along with other molecular fragments but the decomposition process becomes inhibited at high EP solution concentrations. At −0.5 V only very low coverages of CO are observed due to competing hydrogen adsorption at Pt(100) step sites which most favor EP decomposition. At more negative potentials, during the onset of catalytic EP hydrogenation, CO is generated rapidly but other intermediates or products are not observed in the ATR-IR spectra. Together these observations suggest two different mechanisms of EP decomposition, the first occurring directly upon EP adsorption and the second occurring after a single hydrogen atom transfer under hydrogen rich conditions. This ability to control substrate decomposition by tuning the surface hydrogen coverage may be used as a potential route to mitigating catalyst poisoning and deactivation during hydrogenation reactions

Development and evaluation of a multiple-locus variable-number tandem-repeats analysis assay for subtyping Salmonella Typhi strains from sub-Saharan Africa

Purpose: Molecular epidemiological investigations of the highly clonal Salmonella enterica subspecies enterica serovar Typhi (S. Typhi) are important in outbreak detection and in tracking disease transmission. In this study, we developed and evaluated a multiple-locus variable-number tandem-repeats (VNTR) analysis (MLVA) assay for characterization of S. Typhi isolates from sub-Saharan Africa. Methodology: Twelve previously reported VNTR loci were evaluated and an MLVA assay consisting of five polymorphic loci was adopted. The MLVA assay was developed for use on capillary electrophoresis systems by testing a collection of 50 S. Typhi isolates. This S. Typhi strain panel consisted of six outbreak related isolates and 44 epidemiologically unlinked isolates. Amongst these were nine S.Typhi haplotype H58 isolates. Results: The MLVA assay characterized the 50 isolates into 47 MLVA profiles while PFGE analysis of the same isolates revealed 34 pulsotypes. MLVA displayed higher discriminatory power (Simpson’s index of diversity (DI) 0.998 [95% confidence interval (CI) 0.995–1.000)] as compared to pulsed-field gel electrophoresis [Simpson’s DI 0.984 (95% CI 0.974–0.994)]. Conclusion: The MLVA assay presented in this study is a simple, rapid and more accessible tool that serves as a good alternative to other molecular subtyping methods for S. Typhi

The role of tungsten oxide in enhancing the carbon monoxide tolerance of platinum-based hydrogen oxidation catalysts

Significant reductions in total cost of ownership can be realized by engineering PEM fuel cells to run on low-purity hydrogen. One of the main drawbacks of low-purity hydrogen fuels is the carbon monoxide fraction, which poisons platinum electrocatalysts and reduces the power output below useful levels. Platinum-Tungsten oxide catalyst systems have previously shown high levels of CO tolerance during both ex situ and in situ investigations. In this work, we explore the mechanism of enhanced tolerance using in situ electrochemical attenuated total reflection-infrared (ATR-IR) and Raman spectroscopy methods and investigate, using a mixture of Pt/C and WO3 powders, the role of the WV/WVI redox couple in the oxidation of adsorbed CO

A universal polymer shell-isolated nanoparticle (SHIN) design for single particle spectro-electrochemical SERS sensing using different core shapes

Shell-isolated nanoparticles (SHINs) have attracted increasing interest for non-interfering plasmonic enhanced sensing in fields such as materials science, biosensing, and in various electrochemical systems. The metallic core of these nanoparticles is isolated from the surrounding environment preventing direct contact or chemical interaction with the metal surface, while still being close enough to enable localized surface plasmon enhancement of the Raman scattering signal from the analyte. This concept forms the basis of the shell isolated nanoparticle-enhanced Raman spectroscopy (SHINERS) technique. To date, the vast majority of SHIN designs have focused on SiO 2shells around spherical nanoparticle cores and there has been very limited published research considering alternatives. In this article, we introduce a new polymer-based approach which provides excellent control over the layer thickness and can be applied to plasmonic metal nanoparticles of various shapes and sizes without compromising the overall nanoparticle morphology. The SHIN layers are shown to exhibit excellent passivation properties and robustness in the case of gold nanosphere (AuNP) and anisotropic gold nanostar (AuNS) core shapes. In addition,in situSHINERS spectro-electrochemistry measurements performed on both SHIN and bare Au nanoparticles demonstrate the utility of the SHIN coatings. Correlated confocal Raman and SEM mapping was achieved to clearly establish single nanoparticle SERS sensitivity. Finally, confocalin situSERS mapping enabled visualisation of the redox related molecular structure changes occurring on an electrode surface in the vicinity of individual SHIN-coated nanoparticles

Epidemiological evidence that garden birds are a source of human salmonellosis in England and Wales

The importance of wild bird populations as a reservoir of zoonotic pathogens is well established. Salmonellosis is a frequently diagnosed infectious cause of mortality of garden birds in England and Wales, predominantly caused by Salmonella enterica subspecies enterica serovar Typhimurium definitive phage types 40, 56(v) and 160. In Britain, these phage types are considered highly host-adapted with a high degree of genetic similarity amongst isolates, and in some instances are clonal. Pulsed field gel electrophoresis, however, demonstrated minimal variation amongst matched DT40 and DT56(v) isolates derived from passerine and human incidents of salmonellosis across England in 2000-2007. Also, during the period 1993-2012, similar temporal and spatial trends of infection with these S. Typhimurium phage types occurred in both the British garden bird and human populations; 1.6% of all S. Typhimurium (0.2% of all Salmonella) isolates from humans in England and Wales over the period 2000-2010. These findings support the hypothesis that garden birds act as the primary reservoir of infection for these zoonotic bacteria. Most passerine salmonellosis outbreaks identified occurred at and around feeding stations, which are likely sites of public exposure to sick or dead garden birds and their faeces. We, therefore, advise the public to practise routine personal hygiene measures when feeding wild birds and especially when handling sick wild birds