Electrooxidation of methanol and formic acid on PtCu nanoparticles

Improving the catalytic activity of the anode catalyst is an important task in direct methanol and formic acid fuel cell development. In the present work, catalytic activity of shape-controlled PtCu nanoparticles toward methanol and formic acid oxidation was investigated. The results show that the addition of Cu to Pt increases the catalytic activity of both reactions. In addition, the shape of PtCu nanoparticles plays an important role on improving the reactivity of both reactions. Cubic PtCu nanoparticles are more active for methanol oxidation while spheres are better for formic acid oxidation. The present study demonstrates controlling shape of Pt alloy catalysts is an effective way of improving catalytic activity. Likely mechanisms of the activity enhancement are briefly discussed.     

Improved kinetics of methanol oxidation on Pt/hollow carbon sphere catalysts

Hollow carbon spheres (HCSs) have been prepared by combining the hydrothermal method and intermittent microwave heating (IMH) technique. The preparation factors affecting the performance of the HCSs are studied. The results show that Pt nanoparticles supported on HCSs (Pt/HCS), which were heated for 3 min in a microwave oven, give the best performance for methanol oxidation. The higher electrochemical active surface area of the Pt/HCS catalysts results in higher catalytic activity for methanol oxidation compared to that of the commercial Pt/C catalyst at the same Pt loadings. Higher exchange current density and lower reaction activation free energy are observed on Pt/HCS catalysts, indicating improved kinetics. It is recognized that the hollow structure of the Pt/HCS with open microspores and nanochannels is responsible for this higher catalytic activity for methanol oxidation.     

One-Step Flame Synthesis of an Active Pt/TiO2 Catalyst for SO2 Oxidation—A Possible Alternative to Traditional Methods for Parallel Screening

Flame synthesis as a route for production of composite metal oxides has been employed for the one-step synthesis of a supported noble metal catalyst, i.e., a Pt/TiO₂ catalyst, by simultaneous combustion of Ti-isopropoxide and platinum acetylacetonate in a quench-cooled flame reactor. The average size of the platinum particles supported on aggregated nanoparticles of TiO₂ is approximately 2 nm. The high SO₂ oxidation activity of the catalyst proves that platinum is not “hidden” in the titania matrix. The flame-produced catalyst showed catalytic activity similar to that of samples prepared by wet platinum impregnation of pure titania.     

Electrochemical effect of gold nanoparticles on Pt/α-Fe2O3/C for use in methanol oxidation in alkaline solution

A catalyst containing gold nanoparticles with Pt/α-Fe₂O₃/C was prepared by a co-precipitation method and its catalytic activity for the oxidation of methanol, formaldehyde, and formic acid in alkaline solutions was evaluated by an electrochemical method and high-performance liquid chromatography (HPLC). The addition of gold nanoparticles improved catalytic activity only for the oxidation of methanol and formaldehyde, and not for the oxidation of formic acid. HPLC analysis was performed for methanol oxidation to detect the oxidative products. In HPLC analysis, only formate anion could be detected in the electrolyte solution and the ratio of formate anion obtained to the total passed charge in Pt/nano-Au/α-Fe₂O₃/C was less than that in Pt/C, indicating that formic acid is not the final product of methanol oxidation. These results show that gold nanoparticles promoted methanol oxidation up to CO₂.     

Size-selected Pt Nanoparticles Synthesized via Micelle Encapsulation: Effect of Pretreatment and Oxidation State on the Activity for Methanol Decomposition and Oxidation

The effect of pretreatment conditions on the oxidation state and activity of micelle-synthesized Pt nanoparticles supported on ZrO₂ was studied for methanol decomposition and oxidation reactions. An O₂-pretreatment is observed to be effective for producing clean, stable, and active nanoparticles. Pt-oxide species formed during such pretreatments were found to have little influence in methanol decomposition reactions due to their tendency to reduce. However, these same species are stable during methanol oxidation and appear to take part in a Mars-van Krevelen-type of process, in which bound-oxygen (nanoparticle shell) may be replenished with oxygen from the gas phase.     

Dendrimer templates for heterogeneous catalysts: Bimetallic Pt–Au nanoparticles on oxide supports

Polyamidoamine (PAMAM) dendrimers were used to template Pt, Au, and bimetallic Pt–Au dendrimer encapsulated nanoparticles (DENs) in solution. Adjusting the solution pH allowed for slow, spontaneous adsorption of the nanoparticles onto silica, alumina, and titania. After dendrimer removal, the catalysts were characterized with infrared spectroscopy of adsorbed CO and tested with CO oxidation catalysis. Infrared spectroscopy of the monometallic Pt catalysts showed a slight shift in the CO stretching frequency for the different supports. For the bimetallic catalysts, infrared spectra showed CO adsorbed on both Pt and on Au sites. Spectra collected during CO desorption showed substantial interactions between the two bands, confirming the presence of bimetallic particles on all the supports. The bimetallic catalysts were found to be more active than the monometallic catalysts and had lower apparent activation energies. The titania supported Pt–Au catalyst was resistant to deactivation during an extended treatment at 300 °C. Correlations between IR spectra and catalytic activity showed differences between the mono- and bimetallic materials and implicated a bimetallic Pt–Au ensemble at the catalytic active site. This is the first study to show that DENs are appropriate precursors for studying support effects on catalysis by metal nanoparticles, although the magnitude of the effects were small.     

Hydrothermal synthesis of size-dependent Pt in Pt/MWCNTs nanocomposites for methanol electro-oxidation

A hydrothermal method has been developed to prepare size-controlled Pt nanoparticles dispersed highly on multiwalled carbon nanotubes (Pt/MWCNTs). It was found that the size of Pt nanoparticles was strongly dependent on the solution pH in synthesis. The Pt nanoparticles with mean size of 3.0, 4.2 and 9.1 nm were obtained at pHs 13, 12 and 10 separately. After Pt/MWCNTs composites were fabricated, the different properties of cyclic voltammetry and chronoamperometry in electro-oxidation of methanol were found. The results showed that the smaller diameter Pt deposited Pt/MWCNTs nanocomposites exhibited higher electrocatalytic activity for methanol oxidation. By characterization of X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS), size-dependent activities were identified.     

The Nanoscience Revolution: Merging of Colloid Science, Catalysis and Nanoelectronics

The incorporation of nanosciences into catalysis studies has become the most powerful approach to understanding reaction mechanisms of industrial catalysts and designing new-generation catalysts with high selectivity. Nanoparticle catalysts were synthesized via controlled colloid chemistry routes. Nanostructured catalysts such as nanodots and nanowires were fabricated with nanolithography techniques. Catalytic selectivity is dominated by several complex factors including the interface between active catalyst phase and oxide support, particle size and surface structure, and selective blocking of surface sites, etc. The advantage of incorporating nanosciences into the studies of catalytic selectivity is the capability of separating these complex factors and studying them one by one in different catalyst systems. The role of oxide–metal interfaces in catalytic reactions was investigated by detection of continuous hot electron flow in catalytic nanodiodes fabricated with shadow mask deposition technique. We found that the generation mechanism of hot electrons detected in Pt/TiO₂ nanodiode is closely correlated with the turnover rate under CO oxidation. The correlation suggests the possibility of promoting catalytic selectivity by precisely controlling hot electron flow at the oxide–metal interface. Catalytic activity of 1.7–7.2 nm monodispersed Pt nanoparticles exhibits particle size dependence, demonstrating the enhancement of catalytic selectivity via controlling the size of catalyst. Pt–Au alloys with different Au coverage grown on Pt(111) single crystal surface have different catalytic selectivity for four conversion channels of n-hexane, showing that selective blocking of catalytic sites is an approach to tuning catalytic selectivity. In addition, presence and absence of excess hydrogen lead to different catalytic selectivity for isomerization and dehydrocyclization of n-hexane on Pt(111) single crystal surface, suggesting that modification of reactive intermediates by the presence of coadsorbed hydrogen is one approach to shaping catalytic selectivity. Several challenges such as imaging the mobility of adsorbed molecules during catalytic reactions by high pressure STM and removing polymeric capping agents from metal nanoparticles remain.     

Development of composite anode electrocatalyst for direct methanol fuel cells

Different effects of support hydrophilicity and metal-oxide on the performance of Pt-based catalysts were investigated with the aim of improving the mass activities toward methanol electrooxidation. Both potentiodynamic and potentiostatic measurements revealed that improved surface hydrophilicity of multi-wall carbon nanotubes (MWCNTs) could promote the dispersion of Pt nanoparticles and, consequently, promote the Pt utilization and reduce the polarization in methanol electrooxidation. In addition, WO₃ was shown to play a supportive role in enhancing catalytic activity. The interaction between Pt and WO₃ was examined by CO-stripping and CO oxidation transient experiments. The results suggested that the activity and the kinetics of monolayer CO_ads electrooxidation of Pt nanoparticles are enhanced by the adjacent WO₃ via a bifunctional mechanism, which accounts for improved activity in methanol electrooxidation.     

Hollow nitrogen-containing core/shell fibrous carbon nanomaterials as support to platinum nanocatalysts and their TEM tomography study

Core/shell nanostructured carbon materials with carbon nanofiber (CNF) as the core and a nitrogen (N)-doped graphitic layer as the shell were synthesized by pyrolysis of CNF/polyaniline (CNF/PANI) composites prepared by in situ polymerization of aniline on CNFs. High-resolution transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared and Raman analyses indicated that the PANI shell was carbonized at 900°C. Platinum (Pt) nanoparticles were reduced by formic acid with catalyst supports. Compared to the untreated CNF/PANI composites, the carbonized composites were proven to be better supporting materials for the Pt nanocatalysts and showed superior performance as catalyst supports for methanol electrochemical oxidation. The current density of methanol oxidation on the catalyst with the core/shell nanostructured carbon materials is approximately seven times of that on the catalyst with CNF/PANI support. TEM tomography revealed that some Pt nanoparticles were embedded in the PANI shells of the CNF/PANI composites, which might decrease the electrocatalyst activity. TEM-energy dispersive spectroscopy mapping confirmed that the Pt nanoparticles in the inner tube of N-doped hollow CNFs could be accessed by the Nafion ionomer electrolyte, contributing to the catalytic oxidation of methanol.     

Nanomechanical characterization of chemical interaction between gold nanoparticles and chemical functional groups

Core/shell nanostructured carbon materials with carbon nanofiber (CNF) as the core and a nitrogen (N)-doped graphitic layer as the shell were synthesized by pyrolysis of CNF/polyaniline (CNF/PANI) composites prepared by in situ polymerization of aniline on CNFs. High-resolution transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared and Raman analyses indicated that the PANI shell was carbonized at 900°C. Platinum (Pt) nanoparticles were reduced by formic acid with catalyst supports. Compared to the untreated CNF/PANI composites, the carbonized composites were proven to be better supporting materials for the Pt nanocatalysts and showed superior performance as catalyst supports for methanol electrochemical oxidation. The current density of methanol oxidation on the catalyst with the core/shell nanostructured carbon materials is approximately seven times of that on the catalyst with CNF/PANI support. TEM tomography revealed that some Pt nanoparticles were embedded in the PANI shells of the CNF/PANI composites, which might decrease the electrocatalyst activity. TEM-energy dispersive spectroscopy mapping confirmed that the Pt nanoparticles in the inner tube of N-doped hollow CNFs could be accessed by the Nafion ionomer electrolyte, contributing to the catalytic oxidation of methanol.     

CO Oxidation and Toluene Hydrogenation by Pt/TiO2 Catalysts Prepared from Dendrimer Encapsulated Nanoparticle Precursors

Generation 4 hydroxyl terminated polyamidoamine (PAMAM) dendrimer encapsulated nanoparticles (DENs) were examined as precursors for Pt/TiO₂ catalysts. In this preparation method, the dendrimers were initially used to template and stabilize Pt nanoparticles in solution. DENs were then deposited onto titania, and activation conditions for dendrimer thermolysis were examined. The interactions between PAMAM dendrimers and the titania were found to differ from previous reports of dendrimer-support interactions with silica, alumina, and zirconia. In the case of titania, the amide bonds were found to shift 100 cm^?1, indicating adsorption occurs primarily through amide–titania interactions. Infrared spectroscopy, CO oxidation catalysis, and toluene hydrogenation catalysis were used to evaluate protocols for removing the dendrimer. Thermal decomposition of the DENs in O₂ or CO/O₂ atmospheres led to the formation of surface isocyanates that were preferentially bound to the metal nanoparticles. CO oxidation catalysis was insensitive to the activation protocol used, and infrared spectroscopy of adsorbed CO showed only small differences in the basic surface properties of the resulting Pt catalysts. Toluene hydrogenation catalysis was more sensitive to different activation pretreatments. The most active hydrogenation catalysts resulted from short, low temperature (150 °C) hydrogen treatments while longer treatments at higher temperature (300 °C) resulted in slightly less active catalysts.     

XPS and reactivity study of bimetallic nanoparticles containing Ru and Pt supported on a gold disk

We report a new method of immobilization of catalytic metal/alloy nanoparticles on a gold disk for transfer from an electrochemical cell to UHV (without sample exposure to air) for XPS analyses. Using this immobilization approach, several samples were examined: a core-shell Pt-on-Ru catalyst prepared from Ru black onto which Pt was spontaneously deposited, commercial Pt/Ru 50:50 nanoparticle alloy, as well as single metal Ru and Pt nanoparticle samples. The catalysts were characterized for the Ru oxidation state and for the methanol electrooxidation activity (as Pt was always metallic). For all bimetallic samples, we found that the reduced nanoparticles were more active towards methanol oxidation than the fully or partially oxidized samples. Regardless the Ru oxidation state however, the activity was lower than that previously reported for Ru decorated Pt nanoparticle catalysts (Ru-on-Pt). Possible reasons for the reactivity differences are discussed.     

Synthesis and electro-catalytic activity of methanol oxidation on nitrogen containing carbon nanotubes supported Pt electrodes

Template synthesis of various nitrogen containing carbon nanotubes using different nitrogen containing polymers and the variation of nitrogen content in carbon nanotube (CNT) on the behaviour of supported Pt electrodes in the anodic oxidation of methanol in direct methanol fuel cells was investigated. Characterizations of the as-prepared catalysts are investigated by electron microscopy and electrochemical analysis. The catalyst with N-containing CNT as a support exhibits a higher catalytic activity than that carbon supported platinum electrode and CNT supported electrodes. The N-containing CNT supported electrodes with 10.5% nitrogen content show a higher catalytic activity compared to other N-CNT supported electrodes. This could be due to the existence of additional active sites on the surface of the N-containing CNT supported electrodes, which favours better dispersion of Pt particles. Also, the strong metal-support interaction plays a major role in enhancing the catalytic activity for methanol oxidation.     

Electrocatalytic activity of Pt nanoparticles deposited on porous TiO2 supports toward methanol oxidation

Porous TiO₂ thin films were prepared on the Si substrate by hydrothermal method, and used as the Pt electrocatalyst support for methanol oxidation study. Well-dispersed Pt nanoparticles with a particle size of 5–7 nm were pulse-electrodeposited on the porous TiO₂ support, which was mainly composed of the anatase phase after an annealing at 600 °C in vacuum. Cyclic voltammetry (CV) and CO stripping measurements showed that the Pt/TiO₂ electrode had a high electrocatalytic activity toward methanol oxidation and an excellent CO tolerance. The excellent electrocatalytic performance of the electrode is ascribed to the synergistic effect of Pt nanoparticles and the porous TiO₂ support on CO oxidation. The strong electronic interaction between Pt and the TiO₂ support may modify CO chemisorption properties on Pt nanoparticles, thereby facilitating CO oxidation on Pt nanoparticles via the bifunctional mechanism and thus improving the electrocatalytic activity of the Pt catalyst toward methanol oxidation.     

甲醇氧化电催化剂Pt、Ru和PtRu的性能研究

应用恒电位沉积法制得Pt、Ru和PtRu直接甲醇燃料电池阳极催化剂,并对三种催化剂的甲醇氧化活性和稳定性进行了考察。动电位和恒电位实验结果均表明,Ru的加入使PtRu的甲醇起始氧化电位相对于Pt催化剂负移,催化活性和稳定性得到明显的改善。     

The use of nitrogen-doped graphene supporting Pt nanoparticles as a catalyst for methanol electrocatalytic oxidation

Nitrogen-doped graphene (N-G) was prepared by thermal annealing of graphene oxide in ammonia at different temperatures. The resultant N-G was used as a conductive support for Pt nanoparticles (Pt/N-G) and the electrocatalytic activity of the Pt/N-G catalysts towards methanol oxidation was examined. To investigate the microstructure and morphology of the synthesized catalysts, X-ray diffraction, scanning and transmission electron microscopy and X-ray photoelectron spectroscopy were used. The catalytic activity of the catalysts towards the oxidation of methanol was evaluated by cyclic voltammetry. Compared to a control catalyst of Pt loaded on undoped graphene, the Pt/N-G materials show higher electrochemical activity towards methanol oxidation. The excellent electrochemical performance of Pt/N-G is mainly attributed to the nitrogen doping and the uniform distribution of Pt particles on the doped graphene support. These results indicate that N-doped graphene has great potential as a high-performance catalyst support for fuel cell electrocatalysis.     

Particle growth behavior of carbon-supported Pt,Ru, PtRu catalysts prepared by an impregnation reductive-pyrolysis method for direct methanol fuel cell anodes

Carbon-supported Pt, Ru, and binary PtRu catalysts were prepared by an impregnation-reductive pyrolysis method at various temperatures, with Pt(NH₃)₂(NO₂)₂ and Ru(NO₃)₃ as precursors. The effect of the reductive pyrolysis temperature on the structure of the metal particles and its relationship to the electrocatalytic activity toward methanol and preadsorbed carbon monoxide (CO_ad) oxidation was examined. The decomposition temperature of the Pt₅₀Ru₅₀ mixed precursor shifted to a temperature lower than that of the Ru single-source precursor. High-resolution scanning electron microscopy, X-ray diffraction, and CO_ad stripping voltammetry of Pt/C and Ru/C indicated that Ru nanoparticles tend to grow drastically when the pyrolysis temperature is increased, whereas Pt nanoparticles are more resistant to particle growth. Scanning transmission electron microscopy coupled with energy-dispersive X-ray spectroscopy analysis showed that there is a slight compositional variation between individual nanoparticles, depending on the particle size. The Pt₅₀Ru₅₀/C catalyst prepared at 200 °C exhibited the maximum electrocatalytic activity toward methanol oxidation per mass of PtRu, which is discussed based on the appropriate balance of precursor decomposition and particle growth.     

碳纳米管表面Pd纳米颗粒对甲醇的电催化氧化研究

采用电化学循环伏安法分别在碳纳米管(CNT)和玻碳(GC)电极表面沉积Pd纳米颗粒。扫描电镜(SEM)和XRD分析显示了Pd纳米颗粒均匀分散于碳纳米管表面,而在GC表面则趋向于堆积形成Pd金属薄膜。比较研究了Pd/CNT和Pd/GC电极在碱液中对甲醇的电催化氧化性能,循环伏安结果发现,Pd/CNT对甲醇的催化活性要高于Pd/GC电极;而交流阻抗谱研究发现,Pd/CNT电极对甲醇具有更快的催化氧化速率。另外,不同Pd载量,不同环境温度以及不同甲醇浓度的研究表明,相对于Pd/GC电极,Pd/CNT电极对甲醇的催化氧化具有更高的灵敏度和电化学稳定性。     

Effects of textural parameters and noble metal loading on the catalytic activity of cryptomelane-type manganese oxides for formaldehyde oxidation

Nanoporous manganese oxides were synthesized by a sol–gel method with maleic acid as an organic reducing agent. Noble metals (Pt and Ag) were then loaded by an impregnation method. The as-prepared materials are tunneled manganese oxides, cryptomelane (K-OMS-2). Further characterization was performed using X-ray diffraction (XRD), transmission electron microscopy (TEM) and N₂ adsorption/desorption techniques. The structure and textural properties are heavily dependent on reaction conditions. Reaction solutions using low temperature produced well-crystallized nanorods, whereas solutions at high temperature yielded poorly-crystallized nanoparticles. Formaldehyde oxidation was carried out to evaluate the catalytic activity of these materials. The cryptomelane nanoparticles that possess higher specific surface area and more open pores showed better catalytic performance than the cryptomelane nanorods. Effects of Pt and Ag on the cryptomelane catalysts indicate that not only noble metal types, but also the existence form of metals on the catalyst surface greatly affects the catalytic activity. Pt/K-OMS-2 consisting of ca. 2 nm Pt nanoparticles presents higher activity than Ag/K-OMS-2, but lower activity than the corresponding cryptomelane materials without metal loading, due to the cover of Pt on the active sites of manganese oxides.