Adsorbed intermediates of formaldehyde oxidation and their role in the reaction mechanism

Formaldehyde oxidation was studied on the basal planes of platinum single crystals. Electrochemical and IR spectroscopy data give new information on the mechanism of oxidation. Formaldehyde oxidation at platinum electrodes is a surface-sensitive reaction. From the three basal planes of Pt(hkl), Pt(111) is the most active one. The less active surfaces Pt(100) and Pt(110) are blocked by adsorbed carbon monoxide at the initial stages of the reaction as the formaldehyde is admitted in the solution with the electrode polarized at 0.05 V. Besides CO(ad), other adsorbed species are formed. From these, methylene glycolate, H2COO(ad), is the intermediate of the fast oxidation pathways forming CO2 and HCOOH as soluble products. According to IR data the yields of soluble products at Pt(111) were calculated at 0.6 V, giving 63% for HCOOH and 37% for CO2. At 0.05 V the Pt(111) surface becomes slowly blocked by CO(ad), as observed when the electrode was left in contact with the formaldehyde solution over a period of several minutes. The same blockage occurs during a cyclic voltammogram, which causes a lowering of activity during the second potential scan. A general scheme of the reaction is proposed.     

Methanol oxidation on a Pt(111)-OH/O surface

The methanol oxidation on a hydroxylated Pt (Pt(111)-OH) surface has been investigated by means of infrared reflection absorption spectroscopy (IRAS) in ultra-high vacuum (UHV) and in acidic solution. The Pt(111)-OH surface in UHV was prepared by introducing water molecules on a Pt(111)-(2 x 2)-O surface and annealed at temperature higher than 160 K. Methanol was then, introduced to the Pt(111)-OH surface to show the dependence of the reaction intermediate on the annealing temperature. At an annealing temperature below 160 K, IR bands assignable to methanol overlayer were observed and no detectable intermediates, such as CO, formaldehyde and formate, were formed, suggesting that methanol molecules remain stable on Pt(111) surface without dissociation at this temperature region. At an annealing temperature above 160 K, on the other hand, CO and formate were observed. In addition, the oxidation of CO on Pt(111)-OH showed no sign of formate formation, indicating that formate is not derived from CO, but from a direct oxidation of methanol. Methanol oxidation was carried out in 0.1 mol dm(-3) HClO(4) solution on Pt(111) with a flow cell configuration and showed the formation of formate. These results indicate that the formate is the dominant non-CO intermediate both in UHV and in acidic solution, and the preadsorbed oxygen-containing species, in particular OH adsorbates, on Pt(111) surface plays a very important role in the formate formation process in methanol oxidation reaction.     

Formate,an active intermediate for direct oxidation of methanol on pt electrode

The electro-oxidation of methanol on a Pt thin film electrode in acidic solution has been investigated by in situ surface-enhanced IR absorption spectroscopy. A new IR peak is observed at around 1320 cm-1 when the electrode potential is more positive than 0.5 V, where the bulk oxidation of MeOH occurs. This peak has been assigned to the symmetric stretching of formate species adsorbed on the Pt electrode surface. It is the first observation of formate adsorption during the electro-oxidation of methanol on a Pt surface. A near proportional relationship between the intensity of the IR band of the formate species and MeOH electro-oxidation current is observed. A new reaction scheme via non-CO pathway with formate as the active intermediate is proposed for the methanol electro-oxidation process.     

Estimation of Surface Structure and Carbon Monoxide Oxidation Site of Shape‐Controlled Pt Nanoparticles

Surface structures of shape‐controlled Pt nanoparticles have been estimated using cyclic voltammetry (CV) and infrared reflection absorption spectroscopy (IRAS). Cubic and cuboctahedral Pt nanoparticles are prepared using a capping polymer. These nanoparticles give CVs similar to those of single crystal electrodes of Pt in sulfuric acid solution. The CV of cubic nanoparticles is similar to that of the Pt(510) [=5(100)–(110)] electrode, while the CV of cuboctahedral nanoparticles is reproduced well with the convolution of Pt(766) [=13(111)–(100)] and Pt(17 1 1) [=9(100)–(111)] electrodes. These results suggest that the planes of the cubic and cuboctahedral nanoparticles are composed of step‐terrace and atomically flat terraces, respectively. Adsorbed carbon monoxide (CO) on the shape‐controlled nanoparticles gives the IR bands that are assigned to on‐top and bridged CO. The band of on‐top CO is deconvoluted to two bands: the higher and the lower frequency bands are assigned to the CO on the plane and the edges of the nanoparticles, respectively. On‐top CO adsorbed on the edges is oxidized at more negative potential than that on the planes. Edge sites of the nanoparticles promote CO oxidation.     

Stripping voltammetry of carbon monoxide oxidation on stepped platinum single-crystal electrodes in alkaline solution

The electrochemical oxidation of a CO adlayer on Pt[n(111)x(111)] electrodes, with n = 30, 10, and 5, Pt(111), Pt(110) as well as a Pt(553) electrode (with steps of (100) orientation) in alkaline solution (0.1 M NaOH) has been studied using stripping voltammetry. On these electrodes, it is possible to distinguish CO oxidation at four different active oxidation sites on the surface, i.e. sites with (111), (110) and (100) orientation, and kink sites. The least active site for CO oxidation is the (111) terrace site. Steps sites are more active than the (111) terrace sites, the (110) site oxidizing CO at lower potential than the (100) site. The CO oxidation feature with the lowest overpotential (oxidation potential as low as 0.35 V vs. RHE) was ascribed to oxidation of CO at kink sites. The amount of CO oxidized at the active step or kink sites vs. the amount of CO oxidized at the (111) terrace sites depends on the concentration of the active sites and the time given for the terrace-bound CO to reach the active site. By performing CO stripping on the stepped surfaces at different scan rates, the role of CO surface diffusion is probed. The possible role of electronic effects in explaining the unusual activity and dynamics of CO adlayer oxidation in alkaline solution is discussed.     

Electrochemical oxidation of ethylene glycol on Pt single crystal electrodes with basal orientations in acidic medium

Ethylene glycol electrooxidation on platinum using voltammetry is shown to be a structure sensitive reaction. Among the three basal planes, Pt(111) shows the lowest activity for the oxidation of the organic, although the reaction takes place at lower potential values. The poisoning intermediate can be isolated on each orientation and identified as a CO-like species, its formation taking place in a wide potential range. The stability of the voltammetric profiles during continuous cycling suggests that these CO-like species are the main stable adsorbed residues on Pt(100) and Pt(111). The loss of activity observed with repeated cycling on Pt(110) in a potential range excluding oxygen adsorption is explained by a structural modification of the surface.     

SFG study of the ethanol in an acidic medium--Pt(110) interface: effects of the alcohol concentration

Ethanol in an acidic solution-Pt(110) interface was studied by SFG spectroscopy (between 1820 and 2325 cm(-1)) to explore primarily the effects of the alcohol concentration. Stretching bands of H-Pt (ca. 1970 or 2050 cm(-1)) and CO (ca. 1980 and 2040 cm(-1)) species, produced by the ethanol oxidation, were detected during the adsorption and oxidation of 0-1 mol L(-1) ethanol in a 0.1 mol L(-1) HClO(4) solution on the electrode surface. Hydrogen and CO coadsorb stably on Pt(110) between 0.05 and 0.15 V in ethanol-containing solutions. In this potential range, the blue shift of the hydrogen resonance (ca. 80 cm(-1)) reveals a weakening of the hydrogen bonding between adsorbed hydrogen and water molecules in the double layer. After the hydrogen desorption (0.15 V), the formation of compact CO islands, depending on the ethanol concentration, lifts the Pt(110) surface reconstruction. In ethanol-free solution, the surface remains reconstructed. The lower-frequency CO band is assigned to the CO species adsorbed on (1 x 2) reconstructed Pt(110) domains, having smaller local coverages, while the higher-frequency CO band is attributed to the close-packed CO species adsorbed on (1 x 1) patches. The reaction pathway forming CO(2) is less favored with increasing ethanol concentration.     

IR chemiluminescence probe of the vibrational energy distribution of CO2 formed during steady-state CO oxidation on Pt(111) and Pt(110) surfaces

The infrared (IR) chemiluminescence spectra of CO2 were measured during the steady-state CO + O2 reaction over Pt(110) and Pt(111) surfaces. Analysis of the IR emission spectra indicates that the bending vibrational temperature (TVB), as well as the antisymmetric vibrational temperature (TVAS), was higher on Pt(110) than on Pt(111). On the Pt(110) surface, the highly excited bending vibrational mode compared to the antisymmetric vibrational mode was observed under reaction conditions at low CO coverage (theta(CO) < 0.2) or at high surface temperatures (TS > or = 700 K). This can be related to the activated complex of CO2 formation in a more bent form on the inclined (111) terraces of the Pt(110)(1 x 2) structure. On the other hand, at high CO coverage (theta(CO) > 0.2) or at low surface temperatures (TS < 650 K), TVAS was higher than TVB, which can be caused by the reconstruction of the Pt(110)(1 x 2) surface to the (1 x 1) form with high CO coverage.     

Vibrational spectroscopy on platinum single-crystal electrodes: Part I. In-situ infrared spectroscopic studies of the adsorption and oxidation of CO on Pt (111) in sulphuric acid

Platinum single-crystal electrodes of 5 mm diameter were prepared for in situ infrared spectroscopic measurements by melting platinum wires. The linear potential sweep voltammograms of hydrogen adsorption/desorption on Pt (111), (110) and (100) in 0.5 M sulphuric acid are in excellent agreement with those observed on smaller platinum single-crystal surfaces.The adsorption and oxidation of CO on Pt (111) in 0.5 M sulphuric acid was studied by in situ polarization modulated infrared reflection absorption spectroscopy. The effects of the initial adsorption potential and surface reconstruction on the nature and oxidation mechanism of the adsorbed CO layer are reported.     

Electrocatalytic oxidation of ammonia on Pt(111) and Pt(100) surfaces

The electrocatalytic oxidation of ammonia on Pt(111) and Pt(100) has been studied using voltammetry, chronoamperometry, and in situ infrared spectroscopy. The oxidative adsorption of ammonia results in the formation of NH(x) (x = 0-2) adsorbates. On Pt(111), ammonia oxidation occurs in the double-layer region and results in the formation of NH and, possibly, N adsorbates. The experimental current transients show a hyperbolic decay (t(-1)), which indicates strong lateral (repulsive) interactions between the (reacting) species. On Pt(100), the NH(2) adsorbed species is the stable intermediate of ammonia oxidation. Stabilization of the NH and NH(2) fragments on Pt(111) and Pt(100), respectively, is in an interesting agreement with recent theoretical predictions. The Pt(111) surface shows extremely low activity in ammonia oxidation to dinitrogen, thus indicating that neither NH nor N (strongly) adsorbed species are active in dinitrogen production. Neither nitrous oxide nor nitric oxide is the product of ammonia oxidation on Pt(111) at potentials up to 0.9 V, as deduced from the in situ infrared spectroscopy measurements. The Pt(100) surface is highly active in dinitrogen production. This process is characterized by a Tafel slope of 30 mV decade(-1), which is explained by a rate-determining dimerization of NH(2) fragments followed by a fast decay of the resulting surface-bound hydrazine to dinitrogen. Therefore, the high activity of the Pt(100) surface for ammonia oxidation to dinitrogen is likely to be related to its ability to stabilize the NH(2) adsorbate.     

Structural effects of electrochemical oxidation of formic acid on single crystal electrodes of palladium

Structural effects on the rates of formic acid oxidation have been studied on Pd(111), Pd(100), Pd(110), and Pd(S)-[n(100) x (111)] (n = 2-9) electrodes in 0.1 M HClO4 containing 0.1 M formic acid with use of voltammetry. On the low index planes of Pd, the maximum current density of formic acid oxidation (jP) increases in the positive scan as follows: Pd(110) < Pd(111) < Pd(100). This order differs from that on the low index planes of Pt: Pt(111) < Pt(100) < Pt(110). Pd(S)-[n(100) x (111)] electrodes with terrace atomic rows n > or = 3 have almost the same jP as Pd(100), except Pd(911) n = 5. The value of jP on Pd(911) n = 5 is 20% higher than those of the other surfaces. Pd(311) n = 2, of which the first layer is composed of only step atoms, has the lowest jP in the Pd(S)-[n(100) x (111)] series. The adsorption geometry of the reaction intermediate (formate ion) is optimized by using density functional theory.     

Effects of the surface mobility on the oxidation of adsorbed CO on platinum electrodes in alkaline media. The role of the adlayer and surface defects

The oxidation of adsorbed CO on Pt single crystal electrodes has been studied in alkaline media. The surfaces used in this study were the Pt(111) electrode and vicinal stepped and kinked surfaces with (111) terraces. The kinked surfaces have either (110) steps broken by (100) kinks or (100) steps broken by (110) kinks and different kink densities. The voltammetric profiles for the CO stripping on those electrodes show peaks corresponding to the oxidation of CO on the (111) terraces, on the (100) steps/kinks and on the (110) steps/kinks at very distinctive potentials. Additionally, the stripping voltammograms always present a prewave. The analysis of the results with the different stepped and kinked surfaces indicates that the presence of the prewave is not associated with defects or kinks in the electrode surface. Also, the clear separation of the CO stripping process in different peak contributions indicates that the mobility of CO on the surface is very low. Using partial CO stripping experiments and studies at different pH, it has been proposed that the low mobility is a consequence of the negative absolute potential at which the adlayers are formed in alkaline media. Also, the surface diffusion coefficient for CO in these media has been estimated from the dependence of the stripping charge of the peaks with the scan rate of the voltammetry.     

In situ infrared reflection absorption spectroscopy of carbon monoxide adsorbed on Pt(S)-[n(100)x(110)] electrodes

Structural effects on the adsorption of CO have been studied using infrared reflection absorption spectroscopy (IRAS) on Pt(S)-[n(100)x(110)] surfaces (n = 2, 5, 9) that have densely packed kink atoms in the step. Coverage and potential dependence of the IRAS spectra are scrutinized. On-top and bridge-bonded CO are found on all of the surfaces examined. CO is adsorbed on only kink at low coverage (thetaCO < or = 0.2). Adsorbed CO on kink gives an IR band at lower frequency than that on step. CO is adsorbed on both kink and terrace at 0.3 < or = thetaCO. Water is adsorbed on the terrace of Pt(510) n = 5 and Pt(910) n = 9 at low CO coverage, but water is not found on Pt(210) n = 2 of which the first layer is composed of only kink atoms. It is suggested that coadsorbed water on the terrace enhances the activity for the oxidation of adsorbed CO on the kink remarkably.     

Surface structural and chemical characterization of Pt/Ru composite electrodes: a combined study by XPS, STM and IR spectroscopy

Electrochemical Ru deposits on Pt(111) surfaces are investigated by STM; the images of the Ru-modified surfaces show islands of monoatomic height and between 2–5 nm in diameter. The density of islands on the surface depends on the Ru deposition potential (observed by STM and XRSD) and the cyclic voltammograms indicate an increasing Ru coverage for lower deposition potentials. The Ru surface coverage is determined by ex-situ XPS measurements and a linear dependence of the Ru coverage on the deposition potential is demonstrated. IR spectra of a monolayer of adsorbed CO on the Ru-modified Pt(111) surfaces show distinct bands for CO adsorbed on Pt and on Ru. For the integrated band intensity of the CO/Ru vibration a linear dependence on deposition potential is found indicating that lateral dipole interactions between CO adsorbed on Pt and Ru are unimportant and that the CO coverage on the Ru islands is constant for the Ru coverages investigated. The possibility of using adsorbate vibrational bands for the determination of the coverage of deposits is discussed. Received: 24 June 1996 / Revised: 6 December 1996 / Accepted: 12 December 1996     

甲醇在铂微粒修饰的聚硫堇电极上的电催化氧化

利用电化学循环伏安和现场ＦＴＩＲ反射光谱等技术研究了甲醇在铂微粒修饰的聚硫堇电极上的电催化氧化。结果表明，循环伏安法制备的铂微粒均匀分散于聚合物膜上，其粒径大小约为３０－１３０ｎｍ；复合修饰电极对甲醇电化学氧化呈现了较高的催化活性，其催化活性的大小依赖于Ｐｔ载量。现场ＦＴＩＲ光谱实验揭示了线性吸附的ＣＯ物种是甲醇在复合电极上氧化的唯一中间体，这种吸附的ＣＯ物种在复合修饰电极上更容易被氧化为最终产物     

Structure sensitivity of methanol electrooxidation pathways on platinum: an on-line electrochemical mass spectrometry study

By monitoring the mass fractions of CO(2) (m/z 44) and methylformate (m/z 60, formed from CH(3)OH + HCOOH) with on-line electrochemical mass spectrometry (OLEMS), the selectivity and structure sensitivity of the methanol oxidation pathways were investigated on the basal planes--Pt(111), Pt(110), and Pt(100)--and the stepped Pt electrodes--Pt(554) and Pt(553)--in sulfuric and perchloric acid electrolytes. The maximum reactivity of the MeOH oxidation reaction on Pt(111), Pt(110), and Pt(100) increases in the order Pt(111) < Pt(110) < Pt(100). Mass spectrometry results indicate that the direct oxidation pathway through soluble intermediates plays a pronounced role on Pt(110) and Pt(111), while, on Pt(100), the indirect pathway through adsorbed carbon monoxide is predominant. In 0.5 M H(2)SO(4), introducing steps in the (111) plane increases the total reaction rate, while the relative importance of the direct pathway decreases considerably. In 0.5 M HClO(4), however, introducing steps increases both the total reaction rate and the selectivity toward the direct oxidation pathway. Anion (sulfate) adsorption on (111) leads to a more prominent role of the direct pathway, but, on all the other surfaces, (bi)sulfate seems to block the formation of soluble intermediates. For both electrolytes, increasing the step density results in more methylformate being formed relative to the amount of CO(2) detected, indicating that the [110] steps themselves catalyze the direct oxidation pathway. A detailed reaction scheme for the methanol oxidation mechanism is suggested based on the literature and the results obtained here.     

CO and methanol adsorption on (2 × 1)Pt(110) and ion‐eroded Pt(111) model catalysts

Methanol adsorption on ion‐sputtered Pt(111) surface exhibiting high concentration of vacancy islands and on (2 × 1)Pt(110) single crystal were investigated by means of photoelectron spectroscopy (PES) and thermal desorption spectroscopy. The measurements showed that methanol adsorbed at low temperature on sputtered Pt(111) and on (2 × 1)Pt(110) surfaces decomposed upon heating. The PES data of methanol adsorption were compared to the data of CO adsorbed on the same Pt single crystal surfaces. In the case of the sputtered Pt(111) surface, the dehydrogenation of H_xCO intermediates is followed by the CO bond breakage. On the (2 × 1)Pt(110) surface, carbon monoxide, as product of methanol decomposition, desorbed molecularly without appearance of any traces of atomic carbon. By comparing both platinum surfaces we conclude that methanol decomposition occurs at higher temperature on sputtered Pt(111) than on (2 × 1)Pt(110). Copyright © 2010 John Wiley & Sons, Ltd.     

Mechanisms of methanol decomposition on platinum: A combined experimental and ab initio approach

The dual path mechanism for methanol decomposition on well-defined low Miller index platinum single crystal planes, Pt(111), Pt(110), and Pt(100), was studied using a combination of chronoamperometry, fast scan cyclic voltammetry, and theoretical methods. The main focus was on the electrode potential range when the adsorbed intermediate, CO(ad), is stable. At such "CO stability" potentials, the decomposition proceeds through a pure dehydrogenation reaction, and the dual path mechanism is then independent of the electrode-substrate surface structure. However, the threshold potential where the decomposition of methanol proceeds via parallel pathways, forming other than CO(ad) products, depends on the surface structure. This is rationalized theoretically. To gain insights into the controlling surface chemistry, density functional theory calculations for the energy of dehydrogenation were used to approximate the potential-dependent methanol dehydrogenation pathways over aqueous-solvated platinum interfaces.     

Adsorption of formaldehyde on Pt(111) and Pt(100) electrodes: cyclic voltammetry and scanning tunneling microscopy

The adsorption of formaldehyde (HCHO) on Pt(111) and Pt(100) electrodes was examined by cyclic voltammetry (CV) and in situ scanning tunneling microscopy (STM) in 0.1 M HClO(4). The extent of HCHO adsorption at both Pt electrodes was evaluated by comparing the CVs, particularly for the hydrogen adsorption and desorption between 0.05 and 0.4 V, obtained in 0.1 M HClO(4) with and without HCHO. The adsorption of HCHO on these Pt electrodes was significant only when [HCHO] >/= 10 mM. Adsorbed organic intermediate species acted as poisons, blocking Pt surfaces and causing delays in the oxidation of HCHO. Compared to Pt(111), Pt(100) was more prone to poisoning, as indicated by a 200 mV positive shift of the onset of HCHO oxidation. However, Pt(100) exhibited an activity 3 times higher than that of Pt(111), as indicated by the difference in peak current density of HCHO oxidation. Molecular resolution STM revealed highly ordered structures of Pt(111)-( radical7 x radical7)R19.1 degrees and Pt(100)-( radical2 x radical2) in the potential region between 0.1 and 0.3 V. Voltammetric measurements further showed that the organic poisons produced by HCHO adsorption behaved differently from the intentionally dosed CO admolecules, which supports the assumption for the formation of HCO or COH adspecies, rather than CO, as the poison. On both Pt electrodes, HCHO oxidation commenced preferentially at step sites at the onset potential of this reaction, but it occurred uniformly at the peak potentials.     

CO2在金属表面活化的能学方法研究

应用UBI-QEP方法, 估算了CO2-在金属表面的吸附热, 并计算了CO2在Cu(111)、Pd(111)、Fe(111)、Ni(111)表面的各种反应途径的活化能垒. 结果表明, CO2-在4种过渡金属表面相对的稳定性和CO2解离吸附的活性顺序一致,均为Fe>Ni>Cu>Pd. 说明CO2-可能是CO2解离吸附的关键中间体. 在Cu、Pd、Ni表面上, CO2解离吸附的最终产物是CO,而在Fe表面其最终会解离成C和O. 在Cu、Fe、Ni表面, CO2加氢活化是一种有效模式, 而在Pd上则不容易进行. 在Cu和Pd表面,碳酸盐物种也可能是CO2活化的重要中间体.