Lt. Weng

Heat-treated iron and cobalt tetraphenylporphyrins adsorbed on carbon black: Physical characterization and catalytic properties of these materials for the reduction of oxygen in polymer electrolyte fuel cells

Iron and cobalt tetraphenylporphyrins (FeTPP and CoTPP, respectively) have been adsorbed on carbon black (C). The resulting FeTPP/C and CoTPP/C were heat-treated in Ar at various temperatures ranging from 100-1100 degrees C in order to produce catalysts for the electroreduction of oxygen in polymer electrolyte fuel cells. The catalysts have been characterized by XRD, XPS, ToF-SIMS, and bulk analyses. Their electrocatalytic properties have been evaluated by rotating disk electrode (rde) and gas diffusion electrode (gde) measurements. The highest rde activities at 0.70 V vs nhe were recorded for CoTPP/C and FeTPP/C heat-treated in the 500-700 degrees C range. In that temperature range, the increased catalytic activity originates from the well dispersed N-4-metal moiety or from fragments of the original molecule still containing the metal bound to nitrogen. Short term stability tests on the initially most active catalysts revealed the instability of these catalysts in comparison to those obtained at higher pyrolysis temperatures (900-1000 degrees C). At these pyrolysis temperatures the active site of the catalysts is inorganic in nature. The presence of iron and cobalt in their metallic states in these catalysts has been confirmed using XRD. TEM of the catalysts pyrolyzed at these higher temperatures revealed that most of the iron and cobalt are encapsulated in a graphite-like coating. The better stability of these catalysts in the acidic environment of a working fuel cell may be ascribed to the presence of this protective coating. The activity of FeTPP/C pyrolyzed at 1000 degrees C was observed to increase with time in a manner similar to that of platinum-based catalysts. The activity of the former catalyst was determined to be approximately one half that of a commercial platinum catalyst containing the same amount of metal (2 wt%). Copyright (C) 1996 Elsevier Science Ltd.

Is nitrogen important in the formulation of Fe-based catalysts for oxygen reduction in solid polymer fuel cells?

The role of nitrogen and iron in the generation of catalysts for oxygen reduction in acidic media has been investigated by using two independent organic precursors. The Fe and N precursors were polyvinylferrocene adsorbed on carbon black and acetonitrile vapor, respectively. These precursors were pyrolyzed at 1000 degrees C. A catalyst is obtained only if Fe and N are present together in the reactor during pyrolysis. Inactive Fe clusters surrounded by a protective graphitic envelope are produced when adsorbed polyvinylferrocene is pyrolyzed alone at 1000 degrees C. The latter material may, however, be activated by a second pyrolysis in acetonitrile vapor. The characterization of the catalyst indicates that the iron is oxidized (Fe-II and Fe-III), but no strong Fe-N-x bonds were unequivocally detected by ToF SIMS. Lifetime testing of the catalyst in a polymer electrolyte fuel cell demonstrated stable currents for at least 300 h. The current density measured with a catalyst containing 1 wt% Fe was about 1/3 of that measured with a commercial Pt-based catalyst containing 2 wt% metal loading

Physical, Chemical and Electrochemical Characterization of Heat-treated Tetracarboxylic Cobalt Phthalocyanine Adsorbed On Carbon-black As Electrocatalyst for Oxygen Reduction in Polymer Electrolyte Fuel-cells

Tetracarboxylic cobalt phthalocyanine (CoPcTc) has been adsorbed on carbon black (C). The resulting CoPcTc/C has been heat-treated in Ar at various temperatures ranging from 100 to 1100 degrees C in order to produce catalysts for the electroreduction of oxygen in polymer electrolyte fuel cells. Heat-treated CoPcTc/C materials have been characterized by TGA, DSC, bulk elemental analyses, XRD, XPS and ToF SIMS. Their electrocatalytic properties have been evaluated by rde and gde measurements. The highest activity is found for CoPcTc/C heat-treated between 500 and 700 degrees C. In this temperature range, the catalytic site can be traced back either to the intact polymer (< 600 degrees C) or to phthalocyanine fragments still containing Co, even as CoN4 chelates. However, short term life tests on the initially most active catalysts indicate that these catalysts are not stable compared to those obtained after pyrolysis of CoPcTc/C at 900 degrees C. The active site of the latter catalysts is related to inorganic cobalt present as metal and oxides. TEM reveals that inorganic cobalt is surrounded by a protecting graphite shell rendering it chemically stable in acidic media.

Catalytic activity and stability of heat-treated iron phthalocyanines for the electroreduction of oxygen in polymer electrolyte fuel cells

Iron phthalocyanine (FePc) and tetracarboxylic iron phthalocyanine (FePcTc) have been adsorbed on carbon black (C). The resulting FePc/C and FePcTc/C have been heat-treated in Ar at various temperatures ranging from 100 to 1100 degrees C to obtain catalysts for the electroreduction of oxygen. The electrochemical properties of these materials have been measured by rotating disk electrode and in polymer electrolyte fuel cells. These properties have been correlated with the bulk and surface characterizations of the catalysts. The most active catalyst is unpyrolyzed FePcTc/C but it is also the least stable one. The only catalysts which are active and stable are those obtained at high pyrolysis temperatures (greater than or equal to 900 degrees C). At those temperatures there is no Fe-N bond anymore, and Fe is-mainly observed as a metal surrounded by a graphitic envelope. After 10 h in a fuel cell at 50 degrees C, 0.5 V Versus reversible hydrogen electrode (RHE), FePcTc/C and FePc/C pyrolyzed at 1000 degrees C yielded currents 37 and 40% that of a commercial Pt catalyst containing the same metalloading (2 wt.%), respectively.

Oxygen reduction in acid media catalysed by heat treated cobalt tetraazaannulene supported on an active charcoal: Correlations between the performances after longevity tests and the active site configuration as seen by XPS and ToF-SIMS

The influence of heat treatment (HT) on CoTAA supported on SX Ultra samples before and after electrochemical tests has been investigated using the rotating disk electrode (RDE) technique and cyclic voltammetry, The heat treated sample configuration was also scrutinized by two different surface spectroscopic techniques: X-ray photoelectron spectroscopy (XPS) and time of flight secondary ion mass spectrometry (ToF-SIMS). After a 100 h polarization period at 500 mV (RHE) (diffusion activation domain), optimal performances were found to occur with the 600 degrees C heat treated samples (HT 600). However, the most salient feature was the observation of a fair activity and stability for the 800 degrees C heat treated samples (HT 800). Similar behavior was exhibited both with H2SO4 and Nafion(R) electrolytes. Collected data could be interpreted on the basis of RDE and cyclic voltammetry theories, suggesting a fast dioxygen mass transfer within the catalytic material in comparison with interfacial process kinetics. After these electrochemical tests a complete demetallation was taking place, as seen by XPS within the HT 800 samples. A protonation process was also clearly identified on the nitrogen ions. An attempt has been made to propose a catalytic cycle in which variations of the different nitrogen oxidation states are occuring. The major obstacle encountered lay in the lack of XPS evidence of the high nitrogen oxidation states. From ToF-SIMS data the existence of bonding between C, N and O can be established with NO3- formation. Under the same treatment conditions this process occurs differently with H(2)TAA precursor. From the collected data it turned out that catalytic cycles involving nitrogen ions possess a lower turnover than those with cobalt ion participation.

Sizing Removal and Functionalization of the Carbon-fiber Surface Studied By Combined Tof Sims and Xps

Time-of-flight secondary ion mass spectrometry (TOF SIMS) and X-ray photoelectron spectroscopy (XPS) have been jointly used to study a two-step surface processing of AS4 carbon fiber: extraction of sizing in CH2Cl2 and functionalization with trimellitic anhydride. The combined information on molecular specificity obtained with TOF SIMS and quantification obtained with XPS allows us to follow qualitatively and quantitatively the changes in functional groups on the carbon surface. The results show that the sizing on AS4 contains at least four different compounds. These compounds can be extracted in CH2Cl2 and the elimination is almost complete for silicone. The functionalization of AS4 with trimellitic anhydride has been realized. The reaction takes place between the amine groups on the carbon fiber and the two types of functional groups in trimellitic anhydride.

Evidence of the Migration of Oxygen Species From Sb2o4 To Moo3 in Moo3-sb2o4 Selective Oxidation Catalysts

Abstract Mechanical mixtures of Mo 16 O 3 and 18 O-labelled (≅25%) α-Sb 2 O 4 and the pure oxides are contacted with propene in the absence of molecular oxygen. α-Sb 2 O 4 alone is totally inactive. Acrolein, CO and CO 2 are formed on MoO 3 and the mixtures. The oxidation products are 18 O labelled when α-Sb 2 O 4 is present, proving that a migration of oxygen from α-Sb 2 O 4 to MoO 3 takes place. The mobile (spillover) oxygen species brings about two effects: (i) direct reaction with propene and (ii) increased rate of incorporation of MoO 3 lattice oxygen. This second effect corresponds to a remote control mechanism, namely the creation and/or regeneration of reaction sites by spillover oxygen.

Surface characterization by time-of-flight SIMS of a catalyst for oxygen electroreduction: pyrolyzed cobalt phthalocyanine-on-carbon black

Cobalt phthalocyanine (CoPc) loaded on carbon black (Vulcan XC-72) has been heat-treated at various temperatures ranging from 400 to 1100-degrees-C in order to obtain catalysts (CoPc/XC-72) for the electroreduction of O2 in solid polymer electrolyte fuel cells. The CoPc precursor and catalysts have been analyzed by time-of-flight secondary ion mass spectrometry (ToF SIMS) in positive and negative ion modes. The electroactivity of the catalysts has also been measured and its evolution with the heat-treatment temperature has been compared with the chemical analysis of the extreme surface of the catalysts. SIMS results show that CoPc loaded on carbon black pyrolyzes at temperatures above 400-degrees-C. All heat-treated CoPc/XC-72 spectra display peaks that are either present in the spectrum of CoPc powder or in the spectrum of XC-72 carbon black support. There is no additional peak corresponding to other molecular fragments. By combining SIMS results with XPS and TEM results already obtained on the same catalysts, one concludes that: (i) Co metal surrounded by graphite layers is the only species responsible for the catalytic activity of CoPc/XC-72 pyrolyzed above 700-degrees-C; (ii) either Co metal and/or CoPc fragments containing Co are responsible for the catalytic activity of CoPc/XC-72 pyrolyzed at 600 or 700-degrees-C where the maximum of the electrocatalytic activity occurs. SIMS results indicate that a modified carbon surface - Co ion complex, where the carbon surface would be modified either by N(x) or by other functional groups, cannot be a model for the active site of CoPc/XC-72 pyrolyzed at 600 or 700-degrees-C.

Phase Cooperation Between Tin and Antimony Oxides in Selective Oxidation of Isobutene To Methacrolein .1. Mechanical Mixtures of Sno2 and Alpha-sb2o4

Results are reported concerning the cooperation between SnO2 and α-Sb2O4 particles in the selective oxidation of isobutene to methacrolein. The catalysts were prepared by mechanically mixing the corresponding powders. A conspicuous catalytic synergy was observed when methacrolein production and selectivity to methacrolein formation were considered. The catalysts, both fresh and used, were characterized by XRD, 119Sn Mossbauer spectroscopy, XPS, analytical electron microscopy (AEM), and ESR in order to investigate the origin of the synergy observed. The joint use of these techniques yielded no indication that a new phase (or solid solution) formed or that mutual surface contamination during either the preparation of the mixture or the catalytic test took place. Within the sensitivity limits of the techniques used, the mechanical mixtures can be considered as composed of two pure separate phases in good contact. The origin of the observed synergy and the other experimental observations is explained in a satisfactory manner by the existence of a “remote control” mechanism, i.e., that α-Sb2O4 produces a mobile oxygen species, namely spillover oxygen, which, by flowing onto the surface of SnO2, creates on the surface of the latter new selective sites and/or regenerates those which have become deactivated. Spillover oxygen produced by α-Sb2O4 seems to control the selective catalytic sites on SnO2 by inhibiting their transformation to reduced, nonselective sites. Spillover oxygen also inhibits the formation of carbonaceous deposits.

Classification of the roles of oxides as catalysts for selective oxidation of olefins

Abstract The communication focusses on the different roles that oxides can play in multiphase oxidation catalysts. The explanation rests on the remote control effect, namely the creation or preservation of active and selective sites on Acceptor oxides thanks to the flow on their surface of spill-over oxygen emitted by separate Donor phases. The bond properties of Donors and Acceptors are discussed. Certain oxides may, according to the other oxides they are mixed with, play the role of Donors or Acceptors. It is speculated that optimal activity might necessitate a cooperation between Donor and Acceptor phase, the Donor/Acceptor mixtures being inherently better than mixed oxides with both abilities. Approximate scales of Donors and Acceptors are proposed.