Understanding the CO Preoxidation and the Intrinsic Catalytic Activity of Step Sites in Stepped Pt Surfaces in Acidic Medium

In order to deepen the knowledge about the origin of the CO preoxidation process and the intrinsic catalytic activity of Pt superficial steps toward CO oxidation, a series of CO stripping experiments were performed on stepped Pt electrodes in acidic medium. For the occurrence of CO preoxidation, it was found that it arises (reproducibly) whenever four interconnected conditions are simultaneously fulfilled: (1) CO adsorption at potentials lower than about 0.2 V; (2) on surfaces saturated with COads; (3) in the presence of traces of CO in solution; (4) in the presence of surface steps. If any of these four conditions is not satisfied, the CO preoxidation pathway does not appear, even though the steps on the electrode surface are completely covered by CO. By controlling the removal of the CO adlayer (voltammetrically), we show that once the CO adlayer has been partially oxidized, the (111) terrace sites of stepped surfaces are released earlier than the (110) step sites. Moreover, if (110) steps are selectively decorated with CO, its oxidation occurs only at potentials ∼150 mV higher than the CO preoxidation peak. Our results systematically demonstrate that step sites are less active to oxidize CO than those ones responsible for the CO preoxidation process. Once the sites responsible for the CO preoxidation are made free, there is no apparent motion of the remaining adsorbed CO layer, suggesting that the activation of the surface controls the whole process, rather than the diffusion of COads toward hypothetically “most active sites”. Voltammetric and chronoamperometric experiments performed on partially covered CO adlayers suggest that adsorbed CO behave as a motionless species during its oxidation, in which the CO adlayer is removed piece by piece. By means of in situ FTIR experiments, the stretching frequency of CO selectively adsorbed on (110) step sites was examined. Band frequency results confirm that those molecules adsorbed on steps are fully coupled with the adsorbed CO on (111) terraces when the surface reaches full coverage.M.J.S. Farias is grateful to the CNPq (Brazil) (grants 200390/2011-2 and 313402/2013-2) for the financial support. G. A. Camara acknowledges financial assistance from CNPq (grants 305494/2012-0 and 405695/2013-6) and FUNDECT (grant 23/200.583/2012). J. M. Feliu thanks the MINECO (Spain) project CTQ2013-44083-P

In situ FTIRRAS study of the electro-oxidation reactions of thiourea and gold in aqueous acid solutions

The electro-oxidation of gold and thiourea (TU) in aqueous acid solutions is investigated by means of in situ FTIRRAS measurements complemented by voltammetry. The formation of soluble species related to the gold(I)-TU complex and TU electrooxidation products is monitored by following the changes in the IR spectra with the applied potential in the range -0.1 -1.5 V. Experimental data show that gold electrodissolution produces soluble [Au(CS(NH2)2)2]2SO4 species. The IR spectrum of solid samples of this complex is reported. The electrodissolution reaction begins at approximately 0.15 V (SHE), concomitantly with the early stages of TU electro-oxidation. The electro-oxidation of TU on gold in acid solutions produces formamidine disulphide (FDS) at potentials below 1.2 V (SHE). The formation of the gold complex and that of FDS are quasi-reversible processes, as revealed by FTIRRAS data. Once the formation of the oxygen-containing layer on gold sets in, the electro-oxidation of TU and FDS yields carbon dioxide, sulphate ions and CN-containing products. Adsorbed sulphate species, which are also detected from 1.2 V upwards, exhibit a band shift of 27 cm-1 V-1.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicada

Combined voltammetry and in situ infrared spectroscopy of tetramethylthiourea on gold in aqueous acid solutions

The voltammetric behaviour of tetramethylthiourea (TMTU) dissolved in aqueous perchloric and sulphuric acid solutions on polycrystalline gold combined with in situ Fourier transform infrared reflection absorption spectroscopy (FTIRRAS) was investigated. Conventional and triangular modulated voltammetry and rotating disc electrode data demonstrate the occurrence of two redox surface processes in the potential ranges −0.1 ≤ E ≤ −0.3 V and 0.7 ≤ E ≤ 0.8 V (versus SHE), respectively. These processes would involve the participation of TMTU adsorbates. The redox surface process observed in the range −0.1 to −0.3 V can be related to the electrosorption of TMTU. The absence of infrared absorption bands from these adsorbates agrees with a flat, almost parallel adsorption of TMTU molecules on gold that has been already reported. In the range 0.4 V ≤ E ≤ 0.8 V both electrochemical and spectroscopic data indicate the anodic formation of soluble gold–TMTU complex and tetramethylformamidine disulphide (TMFDS2+) ions presumably via Au(TMTU)+ adsorbates. The kinetics of soluble gold– TMTU complex ion formation is under diffusion control, whereas the formation of TMFDS2+ ions behaves as a rather irreversible process with the anodic slope, derived from current–potential curves, ∂E/∂ log I = 0.120 V decade−1 at 298 K. At E > 1.2 V, the global electro-oxidation of both TMTU and TMFDS2+ species yields carbon dioxide, sulphate ions and carbonyl-containing compounds, as indicated by IR spectra. These reactions involve the participation of oxygen-containing adsorbates produced from water electro-oxidation on gold.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicada

Electrochemical behaviour of gamma hydroxybutyric acid at a platinum electrode in acidic medium

The electrooxidation of Gamma Hydroxybutyric Acid (GHB) on a polycrystalline platinum electrode is studied by cyclic voltammetry in acidic medium. Two oxidation peaks, A and B, are obtained in the positive scan within the potential range of the double layer region and of the platinum oxide region, respectively. In the negative going potential sweep an inverted oxidation peak with an onset partially overlapping with the tail of the cathodic peak for the reduction of the platinum oxide formed during the anodic scan is obtained (peak C). This inverted peak can be observed at a potential close to +0.2 V (vs Ag/AgCl at pH 2) and separated 0.4 and 0.8 V from the two other oxidation peaks obtained during the anodic scan and in such conditions that the surface is particularly activated to favour this electrochemical process. The response obtained in the electronic current for the different peaks when GHB concentration and scan rate were changed to allows inferring that these are the result of a potential dependent mechanism. The behaviour observed is according with the oxidation of the alcohol group to the corresponding aldehyde and carboxylic acid (succinic acid) as main products

In-situ infrared spectroscopy applied to the study of the electrocatalytic reduction of CO2 : Theory, practice and challenges

Acknowledgements RK and WAS thank the European Research Council (ERC) for the financial support under the European Union's Horizon 2020 research and innovation programme (grant agreement no. 759743 – WUTANG). AC acknowledges the support of the Leverhulme Trust (RPG-2015-040) and OA thanks the University of Aberdeen for an Elphinstone Fellowship.Peer reviewedPostprin

Comparative voltammetric and FTIRRAS study on the electro-oxidation of thiourea and methyl-thioureas on platinum in aqueous acid solutions

A comparative voltammetric and spectroscopic study on thiourea, methylthiourea, 1,3 dimethylthiourea and tetramethylthiourea electro-oxidation is presented. The investigation was performed at platinum electrodes in aqueous acid solutions in the potential range between 0.05 and 1.6 V (versus SHE). The electro-oxidation of thioureas starts at ca. 0.55 V and it involves at least two reaction stages irrespective of their nature. The first electro-oxidation stage occurs in the range 0.55–0.9 V and involves the formation of a soluble disulphide derivative influenced by the blockage of the electrode surface. The second electro-oxidation stage occurs in the range 0.9–1.6 V and it involves a complex electrochemical process yielding products such as carbon dioxide, sulphate ions, CN- and CO-containing species. The second electro-oxidation stage is likely mediated by the oxygen-containing layer that is produced electrochemically on platinum in that range of potential. The gradual depletion of thioureas and the progressive formation of soluble products in the thin solution layer in contact with the electrode are followed through changes in the band intensities of IR spectra. From the correlation between voltammetric and spectroscopic data a description of global reactions involving the electro-oxidation of thioureas to soluble products in both potential ranges is presented. The proposed reaction pathways are consistent with the participation of different adsorbates as was reported earlier for thioureas on platinum.Instituto de Investigaciones Fisicoquímicas Teóricas y Aplicada

Study of the Nature of Formic Acid Adsorbates on Rough Pt and its Interaction with CO

应用现场红外反射光谱（ＦＴＩＲ）和微分电化学质谱（ｄｉｆｆｅｒｅｎｔｉａｌｅｌｅｃｔｒｏｃｈｅｍｉｃａｌｍａｓｓｓｐｅｃｔｒｏｍｅｔｅｒ，ＤＥＭＳ）研究了甲酸在多孔Ｐｔ上吸附质的性质．伏安结果显示了甲酸在多孔Ｐｔ上吸附质的氧化与吸附电位和吸附时间有关．在线质谱结果表明从甲酸吸附质氧化成二氧化碳所需要的电子数（ｎｅ）与吸附电位无关．显然地ｎｅ＝２．２说明了甲酸吸附质是由ＣＯ和ＣＯＨ组成，而不是只有单一的ＣＯ或者ＣＯＨ．另一方面，在１２６２ｃｍ－１和２０４８ｃｍ－１出现的现场红外光谱带证实了甲酸在多孔Ｐｔ上的吸附质中存在ＣＯ和ＣＯＨ表面物质．此外，用ＤＥＭＳ技术和碳－１３同位素标的甲酸作探针研究了甲酸吸附质和ＣＯ的相互作用．结果表明溶液中的甲酸不能取代先吸附的ＣＯ，而溶液中的ＣＯ可以与７５％先吸附的甲酸吸附质发生交换．The nature of formic acid adsorbates formed on porous platinum electrode has been studied by in situ infrared reflectance spectroscopic (FTIR) and differential electrochemical mass spectroscopic (DEMS) techniques. By carefully avoiding the contamination of the environment( i.e ., air) voltammetric experiments show that the oxidation of formic acid adsorbates depends on both adsorption potential and adsorption time. The online DEMS results show furthermore that the number of electrons necessary for the formation of one molecule of CO 2 from adsorbed formic acid is independent of the adsorption potential. Its value of 2.2 demonstrates that the composition of formic acid adsorbates is a mixture of CO and COH, but not CO or COH alone. The existence of these two species has been clearly proved by in situ FTIR experiments on electrodeposited platinum with a roughness factor of 11. The bands at 1 262 cm -1 , 1 840 cm -1 , and in the frequency range of 2 048 to 2 060 cm -1 are due to the surface adsorbed COH, bridge and linearly bound CO ad , respectively. In addition, the interaction between formic acid adsorbates and CO has been investigated by using DEMS via isotopic labelled 13 C formic acid as a probe as well. These experiments show that the surface species previously fromed from CO and formic acid cannot be exchanged by formic acid in solution. whereas about 75 percent of the firstly formed formic acid adsorbates can be replaced by bulk CO.作者联系地址：波恩大学物理化学所Author's Address: Institute of Physical Chemistry, Bonn University, Wegelerstrabe 12, D 53115 Bonn 1, German