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

An electrochemical investigation of oxygen adsorption on Pt single crystal electrodes in a non-aqueous Li+ electrolyte

Cyclic voltammetry has been used to probe the initial stages of oxygen reduction and oxidation in lithium-containing dimethyl sulfoxide at well-defined Pt single crystal electrodes in order to elucidate any catalytic effects ascribable to surface structure. In contrast to previous work involving sodium-oxygen, lithium-oxygen studies did not yield any significant differences for reaction on the three basal planes of platinum. Rather, all three planes generated a similar voltammetric response. However, by judicious use of various potential sweep limits, the formation of superoxide together with both a “conformal” or surface adlayer of lithium peroxide (Li2O2) together with a “microcrystallite” surface Li2O2 phase was resolved. Voltammetric peak intensity versus sweep rate measurements confirmed that superoxide electrooxidation was diffusion limited whereas electrooxidation of the two Li2O2 phases displayed behaviour typical of a surface-confined process. Under steady-state conditions for the formation of superoxide, it was found that for both the conformal and microcrystallite Li2O2 phases, electrooxidation followed zero-order kinetics, pointing to the importance of free surface sites in facilitating these reactions. A marked change in the rate of Li2O2 formation was found to coincide with a coverage of 0.25 monolayers, as measured by the charge density of the conformal Li2O2 electrooxidation peak. We postulate that electron tunnelling through both the conformal Li2O2 layer and microcrystallites deposited on this surface layer coincides with this coverage and accounts for such behaviour. This phenomenon of electron tunnelling through single conformal and mixed conformal/microcrystallite structures should prove vitally important in governing the overall electrooxidation rate

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

Oxygen reactions on Pt{hkl} in a non-aqueous Na+ electrolyte: site selective stabilisation of a sodium peroxy species

Sodium–oxygen battery cathodes utilise the reversible redox species of oxygen in the presence of sodium ions. However, the oxygen reduction and evolution reaction mechanism is yet to be conclusively determined. In order to examine the part played by surface structure in sodium–oxygen electrochemistry for the development of catalytic materials and structures, a method of preparing clean, well-defined Pt electrode surfaces for adsorption studies in aprotic solvents is described. Using cyclic voltammetry (CV) and in situ electrochemical shell-isolated nanoparticle enhanced Raman spectroscopy (SHINERS), the various stages of oxygen reduction as a function of potential have been determined. It is found that on Pt{111} and Pt{110}-(1 × 1) terraces, a long lived surface sodium peroxide species is formed reversibly, whereas on Pt{100} and polycrystalline electrodes, this species is not detected

Hydrochlorination of acetylene using supported bimetallic Au-based catalysts

A detailed study of the hydrochlorination of acetylene and higher alkynes using a supported gold catalyst is described and discussed. A series of reactions using sequential exposure of the catalysts to C2H2 and HCl demonstrate that exposure to HCl prior to reaction of C2H2/HCl leads to enhanced activity whereas exposure to C2H2 leads to deactivation. The reaction of higher alkynes is affected by steric factors with the trend in activity being: acetylene (ca. 40 % conversion)>> hex-1-yne (10%)>phenylacetylene (7 %) > hex-2-yne (2 %) under standard reaction conditions. Using 1H-NMR spectroscopy we have found that for hex-1-yne and phenyl acetylene the anti-Markovnikov product is formed by anti addition of HCl. However, the Markovnikov products are equivalent for syn- and antiaddition of HCl, and hence we investigated the reaction using deuterated substrates and confirmed the products are formed by the anti addition of HCl. The reaction mechanism is discussed in detail

A supercritical-fluid method for growing carbon nanotubes

Large‐scale generation of multiwalled carbon nanotubes (MCNTs) is efficiently achieved through a supercritical fluid technique employing carbon dioxide as the carbon source. Nanotubes with diameters ranging from 10 to 20 nm and lengths of several tens of micrometers are synthesized (see figure). The supercritical‐fluid‐grown nanotubes also exhibit field‐emission characteristics similar to MCNTs grown by chemical‐vapor deposition

Microbial synthesis of core/shell gold/palladium nanoparticles for applications in green chemistry

We report a novel biochemical method based on the sacrificial hydrogen strategy to synthesize bimetallic gold (Au)–palladium (Pd) nanoparticles (NPs) with a core/shell configuration. The ability of Escherichia coli cells supplied with H2 as electron donor to rapidly precipitate Pd(II) ions from solution is used to promote the reduction of soluble Au(III). Pre-coating cells with Pd(0) (bioPd) dramatically accelerated Au(III) reduction, with the Au(III) reduction rate being dependent upon the initial Pd loading by mass on the cells. Following Au(III) addition, the bioPd–Au(III) mixture rapidly turned purple, indicating the formation of colloidal gold. Mapping of bio-NPs by energy dispersive X-ray microanalysis suggested Au-dense core regions and peripheral Pd but only Au was detected by X-ray diffraction (XRD) analysis. However, surface analysis of cleaned NPs by cyclic voltammetry revealed large Pd surface sites, suggesting, since XRD shows no crystalline Pd component, that layers of Pd atoms surround Au NPs. Characterization of the bimetallic particles using X-ray absorption spectroscopy confirmed the existence of Au-rich core and Pd-rich shell type bimetallic biogenic NPs. These showed comparable catalytic activity to chemical counterparts with respect to the oxidation of benzyl alcohol, in air, and at a low temperature (90°C)

Copper/molybdenum nanocomposite particles as catalysts for the growth of bamboo-structured carbon nanotubes

Bamboo-structured carbon nanotubes (BCNTs), with mean diameters of 20 nm, have been synthesized on MgO-supported Cu and Mo catalysts by the catalytic chemical vapor deposition of methane. BCNTs could only be generated using a combination of Cu and Mo catalysts. No BCNTs were produced from either individual Cu or Mo catalysts. In combination, Mo was found to be essential for cracking the methane precursor, while Cu was required for BCNT formation. Energy dispersive X-ray analysis of the individual particles at the tips of the nanotubes suggest that Cu and Mo are present as a “composite” nanoparticle catalyst after growth. First-principles modeling has been used to describe the interaction of the Cu/Mo catalyst with the nanotubes, suggesting that the catalyst binds with the same energy as traditional catalysts such as Fe, Ni, and Co

Polymers of intrinsic microporosity in electrocatalysis:Novel pore rigidity effects and lamella palladium growth

Two polymers (i) the polymer of intrinsic microporosity (or PIM) ethanoanthracene TB-PIM (P1, PIM-EA-TB, MW 70 kDa, BET surface area 1027 m2 g−1) and (ii) the structurally less rigid polymer based on dimethyldiphenylmethane units (P2, BDMPM-TB, MW 100 kDa, BET surface area 47 m2g−1) are compared to highlight the benefits of the newly emerging PIM membrane materials in electrocatalysis and nanostructure formation. Binding sites and binding ability/capacity in aqueous environments are compared in films deposited onto glassy carbon electrodes for (i) indigo carmine dianion immobilisation (weakly binding from water–ethanol) and (ii) PdCl42− immobilisation (strongly binding from acidic media). Nano-lamella growth for Pd metal during electro-reduction of PdCl42− is observed. Electrocatalytic oxidation of formic acid (at pH 6) is investigated for P1 and P2 as a function of film thickness. The more rigid high BET surface area PIM material P1 exhibits “open-pore” characteristics with much more promising electrocatalytic activity at Pd lamella within polymer pores