The study of the OH adsorption process on Pt single crystals is of paramount importance since this adsorbed species is considered the main intermediate in many electrochemical reactions of interest, ...in particular, those oxidation reactions that require a source of oxygen. So far, it is frequently assumed that the OH adsorption on Pt only takes place at potentials higher than 0.55 V (versus the reversible hydrogen electrode), regardless of the Pt surface structure. However, by CO displacement experiments, alternating current voltammetry, and Raman spectroscopy, we demonstrate here that OH is adsorbed at more negative potentials on the low coordinated Pt atoms, the Pt steps. This finding opens a new door in the mechanistic study of many relevant electrochemical reactions, leading to a better understanding that, ultimately, can be essential to reach the final goal of obtaining improved catalysts for electrochemical applications of technological interest.
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.
Oxidation is an important method for the synthesis of chemical intermediates in the manufacture of high-tonnage commodities, high-value fine chemicals, agrochemicals and pharmaceuticals: but ...oxidations are often inefficient. The introduction of catalytic systems using oxygen from air is preferred for 'green' processing. Gold catalysis is now showing potential in selective redox processes, particularly for alcohol oxidation and the direct synthesis of hydrogen peroxide. However, a major challenge that persists is the synthesis of an epoxide by the direct electrophilic addition of oxygen to an alkene. Although ethene is epoxidized efficiently using molecular oxygen with silver catalysts in a large-scale industrial process, this is unique because higher alkenes can only be effectively epoxidized using hydrogen peroxide, hydroperoxides or stoichiometric oxygen donors. Here we show that nanocrystalline gold catalysts can provide tunable active catalysts for the oxidation of alkenes using air, with exceptionally high selectivity to partial oxidation products (∼98%) and significant conversions. Our finding significantly extends the discovery by Haruta that nanocrystalline gold can epoxidize alkenes when hydrogen is used to activate the molecular oxygen; in our case, no sacrificial reductant is needed. We anticipate that our finding will initiate attempts to understand more fully the mechanism of oxygen activation at gold surfaces, which might lead to commercial exploitation of the high redox activity of gold nanocrystals.
The voltammetry of Pt{111}, Pt{100}, Pt{110} and Pt{311} single crystal electrodes as a function of perchloric acid concentration (0.05-2.00 M) has been studied in order to test the assertion made in ...recent reports by Watanabe et al. that perchlorate anions specifically adsorb on polycrystalline platinum. Such an assertion would have significant ramifications for our understanding of electrocatalytic processes at platinum surfaces since perchlorate anions at low pH have classically been assumed not to specifically adsorb. For Pt{111}, it is found that OHad and electrochemical oxide states are both perturbed significantly as perchloric acid concentration is increased. We suggest that this is due to specific adsorption of perchlorate anions competing with OHad for adsorption sites. The hydrogen underpotential deposition (H UPD) region of Pt{111} however remains unchanged although evidence for perchlorate anion decomposition to chloride on Pt{111} is reported. In contrast, for Pt{100} no variation in the onset of electrochemical oxide formation is found nor any shift in the potential of the OHad state which normally results from the action of specifically adsorbing anions. This suggests that perchlorate anions are non-specifically adsorbed on this plane although strong changes in all H UPD states are observed as perchloric acid concentration is increased. This manifests itself as a redistribution of charge from the H UPD state situated at more positive potential to the one at more negative potential. For Pt{110} and Pt{311}, marginal changes in the onset of electrochemical oxide formation are recorded, associated with specific adsorption of perchlorate. Specific adsorption of perchlorate anions on Pt{111} is deleterious to electrocatalytic activity in relation to the oxygen reduction reaction (ORR) as measured using a rotating disc electrode (RDE) in a hanging meniscus configuration. This study supports previous work suggesting that a large component of the ORR activity on platinum is governed by simple site blocking by specifically adsorbed anions and/or electrosorbed oxide.
The in situ combination of electrochemistry and shell-isolated nanoparticle enhanced Raman spectroscopy (SHINERS) has been used for the first time to investigate the surface structure sensitivity of ...asymmetric catalytic hydrogenation at single-crystal Pt electrodes. The adsorption and hydrogenation behavior of aqueous ethyl pyruvate (EP) at a range of modified and unmodified Pt{hkl} electrodes was measured both by cyclic voltammetry and by recording Raman spectra at hydrogen evolution potentials. Two primary surface intermediates were observed, including the previously reported half-hydrogenation state (HHS), formed by addition of a hydrogen atom to the keto carbonyl group, as well as a new species identified as intact chemisorbed EP bound in a μ2(C,O) configuration. The relative populations of these two species were sensitive to the Pt surface structure; whereas the μ2(C,O) EP adsorbate was dominant at pristine Pt{111} and Pt{100}, the HHS was only observed at these electrodes after the introduction of defects by electrochemical roughening. Intrinsically defective Pt{110} and kinked Pt{321} and Pt{721} surfaces exhibited behavior similar to that of electrochemically roughened basal surfaces, indicating the requirement for low coordination sites for observation of the HHS. Rationalization of the differing behaviors is given on the basis of density functional theory (DFT) calculations, which indicate that the μ2(C,O) EP adsorbate is considerably more stable on basal {111} than on {221} stepped surfaces. A mechanism is proposed in which the μ2(C,O)-bound species is a precursor to the HHS but the rate of the first hydrogen atom addition is slow, leading to a low steady-state population of the HHS at terrace sites. The implications of this in the context of enantioselective hydrogenation at chirally modified Pt are discussed.
A highly rigid amine‐based polymer of intrinsic microporosity (PIM), prepared by a polymerization reaction involving the formation of Tröger’s base, is demonstrated to act as an ionic diode with ...electrolyte‐dependent bistable switchable states.
Ionic flip‐flop: Polymers with intrinsic microporosity introduce ionic diode and metastable switching effects as a result of their rigid backbone and molecular‐structure‐dependent anion versus cation conductivity.
The combination of electrochemical methods with the use of well-defined kinked metal surfaces allows the experimentalist to examine many fundamental aspects of asymmetric reactions at solid surfaces ...in a new and detailed manner. For example, by systematically changing both the stereogenic center on a particular chemisorbing molecule and also the molecular architecture of the metal surface, relationships between adsorption geometry and reactive sites on the metal surface may be deduced. In addition, by measuring the differential rate of chemisorption of chiral auxiliaries on R and S metal surfaces insights in to the mechanism of the so-called “Orito” reaction (whereby enantioselective hydrogenation of α-ketoesters is observed) may be deduced. In the present article, a brief resumé of the relevant electrochemical and heterogeneous catalysis literature is expounded together with an analysis of the problem of chirality in two dimensions in order to show that, by utilizing an electrochemical surface science approach, the nature of asymmetric surface reactivity my be tackled in a systematic and accessible manner. Important questions regarding the surface stability or otherwise of chiral modifiers under reaction conditions may be addressed together with the relevance of surface structure in the chemisorption of such species. Some conclusions regarding the mechanism of the Orito reaction on the basis of new experimental data are discussed.
The enzymeless glucose oxidation process readily occurs on nano‐gold electrocatalyst at pH 7, but it is highly susceptible to poisoning (competitive binding), for example from protein or chloride. Is ...it shown here that gold nanoparticle catalyst can be protected against poisoning by a polymer of intrinsic microporosity (PIM‐EA‐TB with BET surface area 1027 m2 g−1). This PIM material when protonated, achieves a triple catalyst protection effect by (i) size selective repulsion of larger protein molecules (albumins) and (ii) membrane ion selection effects, and (iii) membrane ion activity effects. PIM materials allow “environmental control” to be introduced in electrocatalytic processes.
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