Herein we investigate the effect of irreversibly adsorbed bismuth on polycrystalline platinum (Pt p) on the electrooxidation of glycerol in alkaline media by combining electrochemical, spectroscopic (in situ FTIR) and analytical (HPLC on line) techniques. We found that the activity of Pt p increases by about fivefold when the optimal quantity of Bi ions is added to the solution. Besides, the adatom strongly impacts the reaction products by suppressing the pathways with CC bond breaking, hindering the formation of CO (and other unknown intermediates) and enhancing the production of Glycerate. Different to the results in acid media for Pt p-Bi systems where Bi block the oxidation pathway through the primary carbon, glycerate is the main product in alkaline media and dihydroxyacetone is either produced in extremely low quantities or not produced. Besides, comparing our results with those in acid media, the peak current recorded at 1 mV.s-1 in this work was one order of magnitude higher. These results show the strong impact of the pH in the reaction rate and selectivity.

In heterogeneous (electro)catalysis, the overall catalytic output results from responses of surface sites with different catalytic activities, and their discrimination in terms of what specific site is responsible for a given activity is not an easy task. Here, we use the electrooxidation of CO as a probe reaction to access the catalytic activity of different sites on high Miller index stepped Pt surfaces with their {110} steps selectively modified by Ru at different coverage. Data from in situ FTIR spectroscopy and cyclic voltammetry evidence that Ru deposited on {110} steps modifies the surface in a non-trivial way, only favoring the electrocatalytic oxidation of CO over {111} terraces. Moreover, these {111} terraces become catalytically active throughout a large potential window. On the other hand, after the deposition of Ru on {110} steps, the partial oxidation of a CO adlayer (by stripping voltammetry and in situ FTIR potential steps) show that those {110} steps that remain free of Ru seem to be not influenced by the presence of this metal. As a result, the remaining CO adlayer is oxidized on these Ru-free {110} steps at potentials identical to those observed in steps of pure stepped Pt surfaces (in absence of Ru). Firstly, these This is a previous version of the article published in ACS Catalysis. 2016, 6(5): 2997-3007. doi:10.1021/acscatal.6b00439 2 findings suggest that COads behaves as a motionless species during its oxidation. Secondly, they evidence that the impact caused by the presence of Ru in the catalytic activity of Pt(s)-[(n-1)(111)×(110)] stepped surfaces depends on the crystallographic orientation of Pt sites. These results help us to shed new light about the role of Ru in the mechanism of oxidation of CO and allow a deeper understanding regarding the CO tolerance in Pt-Ru catalysts.

The adsorption of thiourea (TU) at Au(111) and Au(100) single crystal and evaporated gold thin-film electrodes with preferential (111) orientation was studied in perchloric acid solutions with TU concentrations below 0.1 mM. For this purpose, cyclic voltammetry with gold single crystals was combined with external reflection infrared spectroscopy and surface-enhanced infrared reflection−absorption spectroscopy under attenuated total reflection conditions (ATR-SEIRAS) with gold thin film electrodes. In situ surface enhanced Raman spectroscopy (SERS) experiments were also carried out with these latter samples. In addition, optimized geometries and theoretical harmonic vibrational frequencies, obtained from B3LYP/LANL2DZ, 6-31+G(d) calculations for TU and thioureate species adsorbed on Au clusters with (111) orientation, were used for the interpretation of the experimental spectra. ATR-SEIRAS experiments show irreversible adsorption of TU at 0.10 V, whereas the SERS experiments have confirmed the bonding of the TU molecule to the metal surface through the S atom. The optimized geometry obtained from density functional theory (DFT) calculations for adsorbed TU corresponds to unidentate bonding through the sulfur atom, with the Au−S bond slightly tilted (13°) from the surface normal, whereas the C−S bond appears to be tilted by 45°. In the case of adsorbed thioureate, under the application of an external electric field of 0.01 a.u., a bidentate asymmetrical bridge adsorption configuration is obtained with one N(H) and the S atoms bonded to Au in positions close to "top" adsorption sites and with the molecular plane perpendicular to the metal surface. The observation of an adsorbate band for the asymmetric NCN stretching in the experimental infrared spectra confirms the tilting of the S−C bond of the adsorbed TU at low potentials. Changes of the adsorbate bands in the ATR-SEIRA spectra at potentials around 0.60 vs reversible hydrogen electrode (RHE) can be interpreted on the basis of DFT results as due to the deprotonation of adsorbed TU to form adsorbed thioureate.

The adsorption of cyanate anions at Au(111) and Au(100) single crystal electrodes has been studied spectroelectrochemically in neutral solutions. Potential-dependent in situ InfraRed Reflection Absorption spectra obtained below the onset of cyanate oxidation were compared with previously published data and analyzed on the basis of periodical Density Functional Theory (DFT) calculations. The calculated adsorption energies for cyanate and related species suggest that cyanic and isocyanic acid adsorb weakly at the studied gold surfaces and, thus, seems not to be at the origin of any of the adsorbate bands in the experimental infrared spectra collected in the cyanatecontaining solutions. The latter features can be clearly ascribed to the asymmetric OCN stretching of N-bonded cyanate species. The observation of absorption bands in a wide spectral region, including features above 2200 cm -1 , agrees with the coexistence of N-bonded cyanate species with different adsorption sites and tilting angles. DFT calculations have revealed that although these adspecies can have significantly different frequencies, their adsorption energies are rather close. In addition, the existence of collective in-phase vibrations at relatively high cyanate coverages also contributes to the widening of the absorption bands.

This work investigates the mechanisms by which Sn and Ru can improve or inhibit the site-specific catalytic activity of Pt (neighboring or distant from foreign atoms) at bimetallic surfaces. For this purpose, we decorated Pt stepped surfaces (non-equivalent sites) by site-selective electrodeposition of different coverages of either Sn or Ru on (110) Pt steps, forming Snsteps/Pt(hkl) and Rusteps/Pt(hkl) bimetallic surfaces, and we used CO adlayer electro-oxidation as a surface probe reaction, monitored by in situ FTIR and cyclic voltammetry techniques. The results showed that both Sn and Ru selectively accelerated the reaction pathway of CO electro-oxidation only at the (111) Pt terrace sites, but importantly played different underlying roles in favoring activity at these active sites.In case of Snsteps/Pt(hkl) catalysts, the CO adlayer oxidation started at lower potentials than on Rusteps/Pt(hkl), but Sn only improved the activity at sites on atoms of the first rows of (111) Pt terraces, while the catalytic benefit of Ru seemed to extend further along the (111) Pt terraces. Compared to unmodified Pt surfaces, Ru did not influence the activity at the line of the (110) Pt steps, while Sn slightly inhibited the activity there, which characterized a slight contrasting effect in catalytic activity at the (111) terraces compared to the (110) step sites. In this regard, the chemical modification by irreversible deposition of either Sn or Ru at lines of Pt steps on a stepped Pt surface interestingly resulted in a non-uniform synergistic effect or balancing of energies involving different site-specific catalytic activities at non-equivalent Pt surface sites. Since the electro-oxidation of CO takes place at the (111) Pt terrace sites away from Sn or Ru, and because COads behaves as an immobile species during its oxidation, it is reasonable to assume that the classical bifunctional mechanism completely fails as a model to interpret the enhancement of catalytic activity towards CO electro-oxidation at Snsteps/Pt(hkl) or Rusteps/Pt(hkl) This is a previous version of the article published in ACS Catalysis. 2017Catalysis. , 7(5): 3434-3445. doi:10.1021 catalysts. The selective alteration in site-specific catalytic activity of Pt was related to: (i) the type and coverage of foreign atoms (Sn or Ru) at the lines of Pt steps; (ii) the crystallographic orientation of Pt sites, whether (111) terraces or steps; and (iii) the width of the (111) Pt terraces.

One of the main current goals of humanity is the CO 2 conversion into high energy compounds for facilitating both a diminution of the CO 2 atmospheric levels and the development of energy storage strategies. In many studies, TiO 2 has been successfully used as a photocatalyst for CO 2 reduction, but there is still a lack of understanding of its catalytic behavior. In this context, CO 2 reduction has been studied on nanoporous TiO 2 electrodes in acetonitrile media by means of (spectro) electrochemical methods (ATR-IR and UV−vis). Importantly, the onset of the cathodic Faradaic processes related with CO 2 reduction on TiO 2 electrodes is located at −0.81 V versus SHE, which is less negative than that observed for metal electrodes under similar conditions. UV−vis spectroelectrochemical results indicate that the electrocatalytic behavior of TiO 2 is related to the generation of oxygen vacancies and Ti 3+ sites at its surface and promoted by electrolytes with nonintercalating cations in agreement with recent results on WO 3 electrodes. ATR-IR spectroelectrochemical measurements allow for monitoring of the TiO 2 /solution interfacial state as reduction proceeds. Specifically, IR bands for carbon monoxide and carbonyl groups related with carbonate and oxalate are observed. Additionally, a chromatographic analysis shows CO and oxalate as main products. With controlled water addition (0.5 M), methanol and CO were found to be the main products. Based on these results, a mechanism for CO 2 reduction on TiO 2 electrodes is presented in which the regeneration of the TiO 2 surface by oxide electrodissolution/deposition is a critical step.

Spectroelectrochemical detection of specifically adsorbed cyanurate anions at gold electrodes with (111) orientation in contact with cyanate and cyanuric acid neutral solutions.

The adsorption and oxidation of squaric acid (3,4-dihydroxycyclobut-3-ene-1,2-dione, H 2 C 4 O 4 , SQA) at platinum and gold electrodes were studied spectroelectrochemically in perchloric acid solutions. Voltammetric experiments demonstrate that reversible adsorption takes place at gold electrodes in the double-layer region. As a difference with platinum electrodes, no dissociative adsorption processes leading to the blocking of the electrode surface are detected. ATR-SEIRAS experiments show potential-dependent adsorbate bands at potentials below 1.20 V RHE that, according to DFT calculations, can be assigned to adsorbed squarate. For platinum electrodes, the potential-dependent adsorption of squarate anions is coupled with the oxidative stripping of adsorbed carbon monoxide, which is formed upon dissociative SQA adsorption. Bonding of squarate species to the platinum and gold surfaces involves two oxygen atoms in a bidentate configuration, with the molecular plane perpendicular to the metal surface. The ATR-SEIRA spectra obtained for gold electrodes in the SQA oxidation region show bands for adsorbed bicarbonate anions formed from dissolved carbon dioxide molecules. In the case of platinum, distinct bands are observed for adsorbed oxidation products which probably are formed upon opening of the SQA ring.