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.     

Nanoparticle size effects on methanol electrochemical oxidation on carbon supported platinum catalysts

The particle size effect observed on the performance of Pt/C electrocatalysts toward the methanol oxidation reaction (MOR) has been investigated with differential electrochemical mass spectrometry (DEMS). The investigation has been conducted under both potentiodynamic and potentiostatic conditions as research on methanol electrochemical oxidation is closely related to interest in direct methanol fuel cells. The particle size effect observed on the MOR is commonly regarded as a reflection of different Pt-CO and Pt-OH bond strengths for different particle sizes. This work focuses mainly on the mechanism of methanol dehydrogenation on platinum which is central to the problem of the optimization of the efficiency of methanol electro-oxidation by favoring the CO(2) formation pathway. It was found that the partitioning of the methanol precursor among the end products on supported platinum nanoparticles is strongly dependent on particle size distribution. Also, it is postulated that the coupling among particles of different sizes via soluble products must be considered in order to understand the particle size effects on the observed trends of product formation. An optimum particle size range for efficiently electro-oxidizing methanol to CO(2) was found between 3 and 10 nm, and loss in efficiency is mostly related to the partial oxidation of methanol to formaldehyde on either too small or too large particles. The possible reasons for these observations are also discussed.     

甲醇电氧化催化剂Pt/CeO2-CNTs与PtRu/C的比较研究

为认识合成催化剂Pt/CeO2-CNTs与商用催化剂PtRu/C(E-TEK)的催化性能和结构特点, 用CO溶出法和恒电位氧化法比较了这两种催化剂对CO的电氧化活性, 运用循环伏安法和恒电位氧化法比较了这两种催化剂对甲醇的电氧化活性. CO电氧化实验结果表明, PtRu/C上CO的电氧化活性明显优于Pt/CeO2-CNTs; 甲醇电氧化实验结果却表明, Pt/CeO2-CNTs与PtRu/C上甲醇电氧化表观活性相当. 为从结构特点上解释PtRu/C上CO电氧化和甲醇电氧化活性的不一致, 对PtRu/C进行了循环伏安扫描和CO溶出实验. 结果表明, PtRu/C的甲醇电氧化电流之所以没有预期高, 一是由于Pt比表面积不够大, 同时Pt-Ru之间协同作用有待提高. 本研究结果表明, 尽管Ru对Pt上CO电氧化有显著助催化作用, 但要充分发挥其对Pt上甲醇电氧化的助催化作用, 需同时提高Pt表面积和Pt-Ru接触界面. 该结论对设计甲醇电氧化催化剂具有普适意义.     

In situ FTIR studies on the effect of temperature on the electro-oxidation of small organic molecules at the Ru(0001) electrode

The adsorption and electro-oxidation of formaldehyde, formic acid and methanol at the Ru(0001) electrode in perchloric acid solution have been studied as a function of temperature, potential and time using in situ FTIR spectroscopy, and the results interpreted in terms of the surface chemistry of the Ru(000 1) electrode and compared to those obtained during our previous studies on the adsorption of CO under the same conditions. It was found that no dissociative adsorption or electro-oxidation of methanol takes place at Ru(0001) at potentials < 900 mV vs. Ag/AgCl, and at all three temperatures employed, 10, 25 and 50 degrees C. However, both formaldehyde and formic acid did undergo dissociative adsorption, even at -200 mV, to form linear (CO(L)) and 3-fold-hollow (COH) binding CO adsorbates. In contrast to the adsorption of CO, it was found that increasing the temperature to 50 degrees C markedly increased the amount of CO adsorbates formed on the Ru(0001) surface from the adsorption of both formaldehyde and formic acid. On increasing the potential, the electro-oxidation of the CO adsorbates to CO2 took place via reaction with the active (1 x 1)-O oxide. Formic acid was detected as a partial oxidation product during formaldehyde electro-oxidation. At all three temperatures employed, it was found that adsorbed CO species were formed from the adsorption of both formic acid and formaldehyde, and were oxidised to CO2 faster than was observed in the experiments involving CO adsorbed from CO(g), suggesting a higher mobility of the CO adsorbates formed from the adsorption of the HCOOH and HCHO. At potentials > 1000 mV, both the oxidation of formic acid to CO2 and the oxidation of formaldehyde to both CO2 and formic acid were significantly increased, and the oxidation of methanol to CO2 and methyl formate was observed, all of which were attributed to the formation of an active RuO2 phase on the Ru(0001) surface.     

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.     

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.     

Metal Oxide Promoted Electrocatalysts for Methanol Oxidation

An important research target in DMFCs is to find better catalyst materials that are cheaper, less-prone to poisoning and more catalytically active. In this context, metal oxides with good catalytic properties and stronger interaction with Pt nanoparticles can generate active interfacial regions for electrocatalysis. Pt catalysts promoted by certain metal oxides show enhanced methanol electro-oxidation activity and CO tolerance behavior. In this paper we summarize the recent progress from our laboratory which explored the possibility of developing Pt–MoO₃/C and Pt–Nb₂O₅/C electrocatalysts in acidic media, and Pt–V₂O₅/C electrocatalyst in alkaline media for direct electro-oxidation of methanol. The oxide electrocatalysts have been prepared by a fast and efficient method of loading the metal oxide on carbon black (Vulcan XC-72) employing an intermittent microwave heating (IMH) method. These materials are found to achieve higher activity and stability towards methanol electro-oxidation.     

A theoretical study of alcohol oxidation by ferrate

The conversion of methanol to formaldehyde mediated by ferrate (FeO(4)2-), monoprotonated ferrate (HFeO4-), and diprotonated ferrate (H2FeO4) is discussed with the hybrid B3LYP density functional theory (DFT) method. Diprotonated ferrate is the best mediator for the activation of the O-H and C-H bonds of methanol via two entrance reaction channels: (1) an addition-elimination mechanism that involves coordination of methanol to diprotonated ferrate; (2) a direct abstraction mechanism that involves H atom abstraction from the O-H or C-H bond of methanol. Within the framework of the polarizable continuum model (PCM), the energetic profiles of these reaction mechanisms in aqueous solution are calculated and investigated. In the addition-elimination mechanism, the O-H and C-H bonds of ligating methanol are cleaved by an oxo or hydroxo ligand, and therefore the way to the formation of formaldehyde is branched into four reaction pathways. The most favorable reaction pathway in the addition-elimination mechanism is initiated by an O-H cleavage via a four-centered transition state that leads to intermediate containing an Fe-O bond, followed by a C-H cleavage via a five-centered transition state to lead to formaldehyde complex. In the direct abstraction mechanism, the oxidation reaction can be initiated by a direct H atom abstraction from either the O-H or C-H bond, and it is branched into three pathways for the formation of formaldehyde. The most favorable reaction pathway in the direct abstraction mechanism is initiated by C-H activation that leads to organometallic intermediate containing an Fe-C bond, followed by a concerted H atom transfer from the OH group of methanol to an oxo ligand of ferrate. The first steps in both mechanisms are all competitive in energy, but due to the significant energetical stability of the organometallic intermediate, the most likely initial reaction in methanol oxidation by ferrate is the direct C-H bond cleavage.     

Kinetic isotope effects in complex reaction networks: formic acid electro-oxidation.

The determination of kinetic isotope effects (KIEs) for different reaction pathways and steps in a complex reaction network, where KIEs may affect the overall reaction in various different ways including dominant and minority pathways or the buildup of a reaction-inhibiting adlayer, is demonstrated for formic acid electrooxidation on a Pt film electrode by quantitative electrochemical in situ IR spectroscopic measurements under controlled mass-transport conditions. The ability to separate effects resulting from different contributions--which is not possible using purely electrochemical kinetic measurements--allows conclusions on the nature of the rate-limiting steps and their transition state in the individual reaction pathways. The potential-independent values of approximately 1.9 for the KIE of formic acid dehydration (CO(ad) formation) in the indirect pathway and approximately 3 for the CO(ad) coverage-normalized KIE of formic acid oxidation to CO2 (direct pathway) indicate that 1) C-H bond breaking is rate-limiting in both reaction steps, 2) the transition states for these reactions are different, and 3) the configurations of the transition states involve rather strong bonds to the transferred D/H species, either in the initial or in the final state, for the direct pathway and--even more pronounced--for formic acid dehydration (CO(ad) formation).     

Improving electrochemical activity of PtRu/SnO_2/C catalyst by reduction treatment and alkaline etching

PtRu/SnO_2/C catalyst was prepared in a polyol process, followed by reduction treatment and alkaline etching. X-ray diffraction, transmission electron microscope with energy dispersive spectrometer and Xray photoelectron spectroscopy were used to characterize the morphology, structure and composition of the catalysts. CO and methanol electro-oxidation activities of the catalysts were evaluated by CO stripping voltammetry, cyclic voltammetry and chronoamperometry measurements. Reduction treatment of the prepared PtRuSnO_2/C catalyst in a polyol process induced the enrichment of Sn on the surface, inhibiting methanol dissolution and CO adsorption on Pt. Alkaline etching removed Sn or SnO_x and thus exposed PtRu on the surface, resulting in enhanced activities for CO and methanol electro-oxidation due to the synergy effects of PtRu on the surface and Sn species beneath.     

Effect of the iridium oxide thin film on the electrochemical activity of platinum nanoparticles

The influence of the iridium oxide thin film on the electrocatalytic properties of platinum nanoparticles was investigated using the electro-oxidation of methanol and CO as a probe. The presence of the IrO(2) thin film leads to the homogeneous dispersion of Pt nanoparticles. For comparison, polycrystalline platinum and Pt nanoparticles dispersed on a Ti substrate in the absence of an IrO(2) layer (Ti/Pt) were also investigated in this study. Inverted and enhanced CO bipolar peaks were observed using an in situ electrochemical Fourier transform infrared technique during the methanol oxidation on the Pt nanoparticles dispersed on a Ti substrate. Electrochemical impedance studies showed that the charge transfer resistance was significantly lower for the Ti/IrO(2)/Pt electrode compared with that of the massive Pt and Ti/Pt nanoparticles. The presence of the IrO(2) thin film not only greatly increases the active surface area but also promotes CO oxidation at a much lower electrode potential, thus, significantly enhancing the electrocatalytic activity of Pt nanoparticles toward methanol electro-oxidation.     

Electrocatalytic mechanism and kinetics of SOMs oxidation on ordered PtPb and PtBi intermetallic compounds: DEMS and FTIRS study

The electrocatalytic activities and mechanisms of PtPb and PtBi ordered intermetallic phases towards formic acid, formaldehyde and methanol oxidation have been studied by DEMS and FTIRS, and the results compared to those for a pure polycrystalline platinum electrode. While PtPb exhibits an enhanced electrocatalytic activity for the oxidation of all three organic molecules when compared to a Pt electrode, PtBi exhibits an enhanced catalytic activity towards formic acid and formaldehyde oxidation, but not methanol. FTIRS data indicate that adsorbed CO does not form on PtPb or PtBi intermetallic compounds during the oxidation of formic acid, formaldehyde and methanol, and therefore their oxidation on both PtPb and PtBi intermetallic compounds proceeds via a non-CO(ads) pathway. Quantitative DEMS measurements indicate that only CO(2) was detected as a final product during formic acid oxidation on Pt, PtPb and PtBi electrodes. At a smooth polycrystalline platinum electrode, the oxidation of formaldehyde and methanol produces mainly intermediates (formaldehyde and formic acid), while CO(2) is a minor product. In contrast, CO(2) is the major product for formaldehyde and methanol oxidation at a PtPb electrode. The high current efficiency of CO(2) formation for methanol and formaldehyde oxidation at a PtPb electrode can be ascribed to the complete dehydrogenation of formaldehyde and formic acid due to electronic effects. The low onset potential, high current density and high CO(2) yield make PtPb one of the most promising electrocatalysts for fuel cell applications using small organic molecules as fuels.     

Decomposition and oxidation of methanol on platinum: A study by in situ X-ray photoelectron spectroscopy and mass spectrometry

The reactions of the catalytic oxidation and decomposition of methanol on the atomically smooth and high-defect Pt(111) single-crystal surfaces were studied using in situ temperature-programmed reaction and X-ray photoelectron spectroscopy. It was found that the decomposition of methanol on both of the surfaces occurred via two reaction pathways: complete dehydrogenation to CO and decomposition with the C-O bond cleavage. Although the rate of reaction via the latter pathway was lower than the rate of dehydrogenation by three orders of magnitude, the carbon formed as a result of the C-O bond cleavage can be accumulated on the surface of platinum to prevent the further course of the reaction. It was shown that oxygen exhibits high activity toward the formed carbon deposits. As a result, the rate of methanol conversion in the presence of oxygen in a gas phase increased by one or two orders of magnitude; in this case, CO₂ and water appeared in the composition of the reaction products as a result of the oxidation of CO and hydrogen, respectively. The high-defect surface of platinum was more active in the reactions of methanol decomposition and oxidation than the atomically smooth Pt(111) single-crystal surface. On the former, selectivity for the formation of methanol dehydrogenation products in oxygen deficiency was higher than on the latter. The main reaction pathways of the decomposition and oxidation of methanol on platinum were considered.     

Electro-oxidation of carbon monoxide on well-ordered Pt(111)/Sn surface alloys

The electro-oxidation of CO on model platinum-tin alloy catalysts has been studied by ex-situ electrochemical measurements following the preparation of the Pt(111)/Sn(2x2) and Pt(111)/Sn(radical3 x radical3)R30 degrees surfaces. A surface redox couple, which is associated with the adsorption/desorption of hydroxide on the Sn sites, is observed at 0.28 V(RHE)/0.15 V(RHE) in H(2)SO(4) electrolyte on both surfaces. Evidence that it is associated with the adsorption of OH comes from ex-situ photoemission measurements, which indicate that the Sn atoms are in a metallic state at potentials below 0.15 V(RHE) and an oxidized state at potentials above 0.28 V(RHE). Specific adsorption of sulfate anions is not associated with the surface process since there is no evidence from photoemission of sulfate adsorption, and the same surface couple is observed in the HClO(4) electrolyte. CO is adsorbed from solution at 300 K, with saturation coverages of 0.37 +/- 0.05 and 0.2 +/- 0.05 ML, respectively. The adsorbed CO is oxidatively stripped at the potential coincident with the adsorption of hydroxide on the tin sites, viz., 0.28 V(RHE). This strong promotional effect is unambiguously associated with the bifunctional mechanism. The Sn-induced activation of water, and promotion of CO electro-oxidation, is sustained as long as the alloy structure remains intact, in the potential range below 0.5 V(RHE). The results are discussed in the light of the requirements for CO-tolerant platinum-based electrodes in hydrogen fuel cell anode catalysts and catalysts for direct methanol electro-oxidation.     

Multi-dimensional Pt–Mo/Co@NC nanocomposites with low platinum contents for methanol oxidation

Journal of Solid State Electrochemistry - The high cost, CO poisoning, and slow electro-oxidation kinetics of Pt-based noble metal catalysts limit the merchandizing of direct methanol fuel cell....     

甲酸在Aucore@Ptshell/Pt电极上电催化氧化的原位SERS

用化学还原法合成了Aucore@Ptshell纳米粒子, 并用扫描电子显微镜(SEM)及X射线衍射(XRD)等技术对纳米粒子进行表征; 采用电化学原位表面增强拉曼光谱(SERS)技术对甲酸的电催化氧化过程进行了研究, 成功地获得了甲酸在Aucore@Ptshell/Pt电极上解离吸附的原位SERS. 结果显示, 在开路电位时, 甲酸能在Aucore@Ptshell/Pt电极表面自发氧化, 解离生成强吸附中间体COad和弱吸附中间体HCOOad, 在电位为+0.10 V时检测到氧化产物CO2的谱峰. 研究结果表明, Aucore@Ptshell/Pt电极对甲酸的氧化具有较高的催化活性和较强的SERS效应, 甲酸在Aucore@Ptshell/Pt电极上的电催化氧化过程遵循双途径机理.     

A coverage-dependent study of pt spontaneously deposited onto Au and Ru surfaces: direct experimental evidence of the ensemble effect for methanol electro-oxidation on Pt

Direct experimental evidence that can be unambiguously attributed to the need of an ensemble of a minimum number of neighboring Pt atoms for methanol electro-oxidation has been observed for the first time. This was realized by a Pt coverage-dependent investigation of methanol and CO electro-oxidation on Pt sites generated via spontaneous deposition onto both Au and Ru surfaces. CO stripping voltammograms also show clear evidence of a substantially strengthened CO-Pt bonding for submonolayer Pt deposited on the Au substrate over a range of ca. 0.22 to 0.77, which is in qualitative agreement with the theoretical prediction based on the Hammer-N?rskov d-band center model. However, the degree of the bond strengthening depends on the Pt coverage, being stronger for lower coverage. Additionally, evidence of an Ostwald ripening process for Pt islands formation has also been observed.     

An unexpected enhancement in methanol electro-oxidation on an ensemble of Pt(111) nanofacets: a case of nanoscale single crystal ensemble electrocatalysis

Pt nanoparticles having the same size ( approximately 10 nm) but different shapes (cubic or octahedral/tetrahedral), as determined by transmission electron microscopy, were synthesized via a polyol-based synthetic procedure. Their respective electrocatalytic activities for methanol oxidation were characterized by cyclic voltammetry and chronoamperometry in both sulfuric and perchloric acid electrolytes, which showed clear shape (surface orientation) dependences. Furthermore, the octahedral/tetrahedral Pt nanoparticles displayed an unexpectedly large enhancement in methanol electro-oxidation activity; about 3-fold increase in transient intrinsic activity and 10-fold increase in CO tolerance steady-state activity when compared to commercial Pt black. Gaseous and methanolic CO adsorption on the synthesized nanoparticles were also investigated by surface-enhanced IR absorption spectroscopy in perchloric acid electrolyte, which suggested that the different trends observed might be related to the electronic effects specific to a given ensemble of the nanofacets.     

Reaction mechanisms of methanol oxidation by FeIVO biomimetic complex

Density functional theory calculations were carried out to investigate the reaction mechanism of methanol oxidation mediated by [(bpg)Fe^IVO]⁺ ( A ). Two models (CH₃CN‐bound ferryl model B and CH₃OH‐bound ferryl model C ) were also studied in this work to probe ligand effect. Mechanistically, both direct and concerted hydrogen transfer (DHT and CHT) pathways were explored. It is found that the initial step of methanol oxidation by A is C? H bond activation via a DHT pathway. Addition of different equatorial ligands has considerable influence on the reaction mechanisms. Methanol oxidation mediated by B commences via O? H bond activation; in sharp contrast, the oxidation mediated by C stems from C? H bond activation. Frontier molecular orbital analysis showed that the initial C? H bond activation by all these model complexes follows a hydrogen atom transfer (HAT) mechanism, whereas O? H bond activation proceeds via an HAT or proton transfer. © 2016 Wiley Periodicals, Inc.     

炭载Pt-Ag双金属催化剂对甲醇氧化反应的电催化

以NaBH4为还原剂,将K2PtCl6和AgNO3前体进行共还原制备了一系列具有不同组成的碳载PtmAg/C合金催化剂（m为Pt/Ag原子比,m为0.05～1.0）,在酸性介质中考察了该系列催化剂对甲醇氧化反应的电催化性能。 与单组分Pt/C催化剂相比,系列PtmAg/C催化剂呈现出较高的催化氧化甲醇的活性与抗CO毒化能力,而且该催化剂的性能与其组成密切相关。 随m值增加,PtmAg/C催化剂对甲醇氧化反应的质量比催化活性(MSA)、本征催化活性(IA)与稳定性均逐步增加,当m=0.5时催化活性达到最高,其MSA和IA分别是Pt/C催化剂的5.1和4.8倍。