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.     

Influence of structural defects on the electrocatalytic activity of platinum

Structural defects play major role in catalysis and electrocatalysis. Nanocrystalline (or nanostructured) materials composed of nanometer-sized crystallites joined via grain boundaries have been recognized for their specific structure and properties, differentiating them from single crystals, coarsely grained materials or nanometer-sized supported single-grained particles (Gleiter, Nanostruct Mater 1:1–19, 1992). In this paper, we use Pt electrodes, prepared by electrodeposition on glassy carbon and gold supports, as model nanocrystalline materials to explore the influence of grain boundaries and other structural defects on electrocatalysis of CO and methanol oxidation. We build on the recently established correlations between the nanostructure (lattice parameter, grain size, and microstrains) of electrodeposited Pt and the deposition potential (Plyasova et al., Electrochim. Acta 51:4447–4488, 2006) and use the latter to obtain materials with variable density of grain boundary regions. The activity of electrodeposited Pt in the oxidation of methanol and adsorbed CO exceeds greatly that for Pt(111), polycrystalline Pt, or single-grained Pt particles. It is proposed that active sites in nanostructured Pt are located at the emergence of grain boundaries at the surface. For methanol electrooxidation, the electrodes with optimal nanostructure exhibit relatively high rates of the “direct” oxidation pathway and of the oxidation of strongly adsorbed poisoning intermediate (CO_ads), but not-too-high methanol dehydrogenation rate constant. These electrodes exhibit an initial current increase during potentiostatic methanol oxidation explained by the CO_ads oxidation rate constant exceeding the methanol decomposition rate constant.

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.     

Modification of a Pt surface by spontaneous Sn deposition for electrocatalytic applications. 2. Oxidation of CO,formaldehyde, formic acid,and methanol

The electrocatalytic activity of a spontaneously tin-modified Pt catalyst, fabricated through a simple dip-coating method under open-circuit conditions and characterized using surface analysis methods, was studied in electrooxidation reactions of a preadsorbed CO monolayer and continuous oxidation of methanol, formic acid, and formaldehyde in the potentiodynamic and potentiostatic modes. The catalytic activity of the tin-modified Pt surface is compared with that of a polycrystalline Pt electrode. Spontaneously Sn-modified Pt catalyst shows a superior activity toward adsorbed CO oxidation and thus can be promising for PEFC applications. The methanol oxidation rate is not enhanced on the Sn-modified Pt surface, compared to the Pt electrode. Formic acid oxidation is enhanced in the low potential region on the Sn-modified surface, compared to the Pt electrode. The formaldehyde oxidation rate is dramatically increased by modifying tin species at the most negative potentials, where anodic formaldehyde oxidation is completely suppressed on the pure Pt electrode. The results are discussed in terms of poisoning CO intermediate formation resulting from dehydrogenation of organic molecules on Pt sites, and oxidation of poisoning adsorbed CO species via the surface reaction with OH adsorbed on neighboring Sn sites.     

Mechanism of CO oxidation on Pt(111) in alkaline media

Electrochemical techniques, coupled with in situ scanning tunneling microscopy, have been used to examine the mechanism of CO oxidation and the role of surface structure in promoting CO oxidation on well-ordered and disordered Pt(111) in aqueous NaOH solutions. Oxidation of CO occurs in two distinct potential regions: the prepeak (0.25-0.70 V) and the main peak (0.70 V and higher). The mechanism of reaction is Langmuir-Hinshelwood in both regions, but the OH adsorption site is different. In the prepeak, CO oxidation occurs through reaction with OH that is strongly adsorbed at defect sites. Adsorption of OH on defects at low potentials has been verified using charge displacement measurements. Not all CO can be oxidized in the prepeak, since the Pt-CO bond strength increases as the CO coverage decreases. Below theta(CO) = 0.2 monolayer, CO is too strongly bound to react with defect-bound OH. Oxidation of CO at low coverage occurs in the main peak through reaction with OH adsorbed on (111) terraces, where the Pt-OH bond is weaker than on defects. The enhanced oxidation of CO in alkaline media is attributed to the higher affinity of the Pt(111) surface for adsorption of OH at low potentials in alkaline media as compared with acidic media.     

Towards Mixed Fuels: The Electrochemistry of Hydrazine in the Presence of Methanol and Formic Acid

The electrochemistry of formic acid, carbon monoxide and methanol have been investigated and evaluated in combination with hydrazine. Hydrazine was observed to display the anticipated steady‐state oxidation waves at platinum (Pt) microelectrodes by cyclic voltammetry, and upon introduction of carbon monoxide (CO) gas, the Pt surface was fully passivated (prior to CO oxidation). However, the two individual responses of hydrazine and formic acid (HCOOH) are to be additive when combined in solution. No detrimental effects were observed upon the hydrazine voltammetry, even in the presence of excess formic acid, despite formic acid clearly displaying characteristic self‐poisoning tendencies (primarily due to the formation of CO) in its own voltammetry. Effects intermediate to those of CO and formic acid were observed when methanol was present. Currents were essentially additive at low methanol content, but hydrazine oxidation current decreased by about 40 % when an 100‐fold excess of methanol was present, corresponding to poisoning by methanol dehydrogenation intermediates. These results are discussed with relevance to mixed fuels for more flexible or powerful fuel cells, and the possible formation of a random microelectrode array (templated by strongly adsorbed poison) on the microelectrode surface.     

An NMR investigation of CO tolerance in a Pt/Ru fuel cell catalyst.

We report the first combined application of solid-state electrochemical NMR (EC NMR), cyclic voltammetry (CV), and potentiostatic current generation to investigate the topic of the ruthenium promotion of MeOH electro-oxidation over nanoscale platinum catalysts. The CV and EC NMR results give evidence for two types of CO: CO on essentially pure Pt and CO on Pt/Ru islands. There is no NMR evidence for rapid exchange between the two CO populations. CO molecules on the primarily Pt domains behave much like CO on pure Pt, with there being little effect of Ru on the Knight shift or on Korringa relaxation. In sharp contrast, COs on Pt/Ru have highly shifted (13)C NMR resonances, much weaker Korringa relaxation, and, at higher temperatures, they undergo thermally activated surface diffusion. For CO on Pt, the correlation observed between the 2pi* Fermi level local density of states and the steady-state current suggests a role for Ru in weakening the Pt-CO bond, thereby increasing the CO oxidation rate (current). The combined EC NMR/electrochemistry approach thus provides new insights into the promotion of CO tolerance in Pt/Ru fuel cell catalysts, in addition to providing a novel route to investigating promotion in heterogeneous catalysis in general.     

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.     

The electrooxidation of small organic molecules on platinum nanoparticles supported on gold: influence of platinum deposition procedure

The electrocatalytic properties of small platinum nanoparticles were investigated for the oxidation of CO, methanol, and formic acid using voltammetry, chronoamperometry, and surface-enhanced Raman spectroscopy. The particles were generated by galvanostatic deposition of platinum on a polished gold surface from an H₂PtCl₆ containing electrolyte and ranged between 10 and 20 nm in diameter for low platinum surface concentrations, 10 and 120 nm for medium concentrations, and full Pt monolayers for high concentrations. CO stripping and bulk CO oxidation experiments on the particles up to 120 nm in diameter displayed pronounced structural effects. The CO oxidation current-time transients show a current decay for low platinum coverages and a current maximum for medium and high coverages. These results were also observed in the literature for particles of 2- to 5-nm size and agglomerates of these particles. The similarities between the literature and our results, despite large differences in particle size and morphology, suggest that particle structure and morphology are also very important catalytic parameters. Surface-enhanced Raman spectroscopy data obtained for the oxidation of CO on the Pt-modified Au electrodes corroborate this conclusion. A difference in the ratio between CO adsorbed in linear- and bridge-bonded positions on the Pt nanoparticles of different sizes demonstrates the influence of the surface morphology. The oxidation activity of methanol was found to decrease with the particle size, while the formic acid oxidation rate increases. Again, a structural effect is observed for particles of up to ca. 120 nm in diameter, which is much larger than the particles for which a particle size effect was reported in the literature. The particle shape effect for the methanol oxidation reaction can be explained by a reduction in available “ensemble sites” and a reduction in the mobility of CO formed by decomposition of methanol. As formic acid does not require Pt ensemble sites, decreasing the particle size, and thus, the relative number of defects, increases the reaction rate. Dedicated to Prof. Dr. Teresa Iwasita on the occasion of her 65th birthday in recognition of her numerous contributions to interfacial electrochemistry.     

The effect of the cooling atmosphere in the preparation of flame-annealed Pt(111) electrodes on CO adlayer oxidation

The effect of the cooling atmosphere on the rate of CO adlayer oxidation on flame-annealed Pt(111) has been studied. Cooling of a flame-annealed Pt(111) electrode in air results in a higher amount of crystalline defects compared to Pt(111) cooled in a hydrogen–argon stream. Although the blank profiles in 0.5 M H₂SO₄ of Pt(111), cooled in air and under oxygen exclusion, are virtually identical, CO adlayer oxidation occurs at significantly lower overpotentials on the former electrode. Three voltammetric peaks are observed for subsaturated CO adlayer oxidation on Pt(111), cooled in Ar+H₂ mixture, while only two peaks develop in the case of a Pt(111) surface cooled in air. Random crystalline defects, introduced via cooling of a flame-annealed Pt(111) in air, enhance CO adlayer oxidation, and apparently also suppress the third high-potential peak observed on a quasi-perfect (111) surface. The high sensitivity of the saturated CO adlayer oxidation to the presence of crystalline defects on Pt(111) can hence be used as a straightforward, sensitive, though qualitative method to assess the degree of crystalline order of the electrode.     

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.     

The origin of the high performance of tungsten carbides/carbon nanotubes supported Pt catalysts for methanol electrooxidation

The Pt supported on WC modified MWCNT catalysts (PtWC/MWCNT) were synthesized by the combination of organic colloidal and intermittent microwave heating (IMH) methods for the first. The results proved the better performance of the PtWC/MWCNT catalyst than that of Pt/C for methanol oxidation in terms of the onset potential and peak current density. The synergistic effect between Pt nanoparticles and WC and the structure effect of the MWCNTs could be the reasons to result in the high activity. The CO stripping test provided the evidence that the onset potential shift for methanol oxidation is consistent with the reduction in the overpotential for the CO oxidation on PtWC/MWCNT catalyst. Therefore, the mechanism of the high performance for methanol oxidation on PtWC/MWCNT catalyst is probably the easier oxidation of CO-like species which cause high overpotential for further oxidation of methanol.     

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.     

CO oxidation on Pt-modified Rh(111) electrodes.

The CO electro-oxidation reaction was studied on platinum-modified Rh(111) electrodes in 0.5 M H2SO4 using cyclic voltammetry and chronoamperometry. The Pt-Rh(111) electrodes were generated during voltammetric cycles at 50 mV s(-1) in a 30 microM H2PtCl6 and 0.5 M H2SO4 solution. Surfaces generated by n deposition cycles were investigated (Ptn-Rh(111) with n=2, 4, 6, 8, 10, and 16). The blank cyclic voltammograms of these surfaces are characterized by a pronounced sharpening of the hydrogen/(bi)sulfate adsorption/desorption peaks, typical for Rh(111), and the appearance of contributions between 0.1 and 0.4 V, which were ascribed to hydrogen/(bi)sulfate adsorption/desorption on the deposited platinum. At higher potentials, the surface oxidation of Rh(111) is enhanced by the presence of platinum. The structure of the Pt-modified electrodes was investigated by STM imaging. At low Pt coverages (Pt2-Rh(111)), monoatomically high islands are formed, which grow three dimensionally as the number of deposition cycles increases. After eight cycles, the monolayer islands have grown in diameter and range from mono- to multiatomic height. At even higher Pt coverage (Pt16-Rh(111)), the islands grow to particles of approx. 10 nm in diameter, which are 5-6 atoms high. The CO stripping voltammetry on these surfaces is characterized by two peaks: A low-potential, structure-insensitive peak, ascribed to CO reacting at the platinum monolayer islands, whose onset is shifted 150, 250, and 100 mV negatively with respect to pure Rh(111), Pt(111), and polycrystalline Pt, respectively, indicating the enhanced CO electro-oxidation properties of the Pt overlayer system. A peak at higher potentials displays strong structure sensitivity (particle-size effect) and was ascribed to CO reacting on the islands of multiatomic height. Current-time transients recorded on the surface with the highest amount of monolayer islands (Pt4-Rh(111)) also indicate enhanced CO-oxidation kinetics. Comparison of the Pt4-Rh(111) current-time transients recorded at 0.635, 0.675, and 0.750 V versus RHE (reversible hydrogen electrode) with those of pure Rh(111) and Pt(111) shows greatly reduced reaction times. A Cottrellian decay at long times indicates surface-diffusion-limited CO oxidation on the bare Rh(111) surface, while the peak visible at short times is indicative of CO reacting at the monolayer platinum islands. The results presented here show that, as indicated by density functional theory (DFT) calculations, the CO-adlayer oxidation for this system is enhanced compared to both pure Rh and Pt.     

纳米结构Pt膜方波电位法制备及特殊红外性能

运用方波电位法处理Pt微电极,制备一系列具有较高表面粗糙度的纳米结构薄膜.扫描隧道显微镜（STM）观察到薄膜由Pt金属岛组成,并测得所研制的薄膜虽然其平均厚度从58 nm 增加到139 nm,但Pt金属岛的平均尺度仅从102 nm增加到114 nm,而岛的平均高度也只在15～18 nm之间变化.以CO为探针分子,结合原位FTIR反射光谱研究,发现所制备的纳米结构薄膜上吸附态CO的红外吸收都给出类Fano光谱,初步归结为Pt金属岛的尺度效应.     

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.     

Pt-Pb六边形纳米板电催化甲醇氧化反应中的表面元素分布效应

由于副产物CO可降低纯Pt催化剂的活性,因此双金属Pt基催化剂已经广泛用于提高直接甲醇燃料电池的性能.本文合成了Pt-Pb六边形纳米板,作为模型催化剂用于甲醇氧化反应(MOR),并通过乙酸(HAc)处理进一步控制纳米板表面Pt和Pb的分布,从而得到Pt-Pb合金表面均相分布的Pt-Pb纳米板以及非均匀分布的HAc处理的Pt-Pb纳米板.结果表明,与HAc处理的Pt-Pb纳米板相比, Pt-Pb纳米板的MOR催化活性和稳定性提高,这主要是由于亲氧性Pb的加入提高了CO容忍度并修饰了Pt的电子结构.     

Property and structure of various platinum catalysts for low-temperature carbon monoxide oxidations

The oxidation of carbon monoxide (CO) has received more attention in the last two to three decades owing to its importance in different fields. To control this CO pollution, catalytic converters have been investigated. Different types of catalysts have been used in a catalytic converter for CO emission control purposes. Platinum (Pt)-based noble metal catalysts show great potential for CO oxidation in catalytic converters with high thermal stability and tailoring flexibility. Pt metal catalysts modified with promoters such as alkali metals and reducible metal oxides have received great attention for their superior catalytic activities in CO oxidation. Temperature, close environment of the catalyst, and chemical composition in the surface layer of the catalyst have a huge effect on the active phase dispersion and O₂ adsorption capacity of the Pt metal catalysts. The main difference in activities of Pt metal catalyst for CO oxidation in O₂ or H₂ atmosphere has found. The addition of supports in Pt metal catalysts has improved their performances and reduced their cost. These improvement strongly depends on the surface structure, morphology, number of active sites, and various Pt-O interactions. Many research articles have already been published in CO oxidation over Pt metal catalysts, but no review article dedicated to CO oxidation is available in the literature.     

铂催化甲醇氧化开始步骤的研究

用ab initio和密度泛函(DFT)方法研究甲醇在铂低指数晶面的脱氢步骤. 在经典的Bagotzky模型计算的基础上提出了三种新的吸附脱氢模型, 并通过计算证明了三种模型的可行性, 计算证实了原位波谱法检测到的甲醇在铂上脱氢的三种中间体, CHOH, CH₂OH和CH₃O. 得出了甲醇脱氢的根本原因在于形成的强Pt—H键, 且在Pt不同的晶面上, 催化性能及反应历程、中间产物都不同.     

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.