N. I. Jaeger

Electron microscopy study of the interaction of Ni, Pd and Pt with carbon: I. Nickel catalyzed graphitization of amorphous carbon☆

Abstract The aggregation of nickel supported on amorphous carbon during heating in vacuum and at how hydrogen pressures in the temperature range 1000 K was investigated by means of high resolution electron microscopy, electron diffraction and microdiffraction. It was established that the interaction of highly dispersed nickel with amorphous carbon substrates is strong enough to break away carbon atoms from the bulk at relatively low temperatures (∼ 700 K) Carbon atoms dissolved in nickel precipitate as graphite. As a result the aggregation of nickel is accompanied by the conversion of the amorphous carbon substrate to graphitic carbon.

Electron microscopy study of the interaction of Ni, Pd and Pt with carbon: II. Interaction of palladium with amorphous carbon

Abstract A change in the lattice parameter of Pd following the evaporation of the metal onto a carbon support is interpreted by the incorporation of carbon atoms into the Pd lattice. The decomposition of this phase above 700 K leads to the deposition of carbon on top of the Pd crystallites or even to their encapsulation. At temperatures above 870 K. larger Pd crystallites were found to catalyze the graphitization of amorphous carbon.

NEGATIVE COUPLING DURING OSCILLATORY PATTERN FORMATION ON A RING ELECTRODE

Pattern formation during the oscillatory electrodissolution of Co was studied using a Co ring electrode with a small reference electrode at a short distance in the center of the ring. Traveling pulses as well as source points (one-dimensional target patterns) were observed. These findings could be reproduced using a reaction-migration equation (RME) the coupling function of which was derived for the particular geometry from basic potential theory and was found to become negative for increasing distance. During pattern formation (i.e., for inhomogeneous potential distribution) the potential drop across the double layer could actually exceed the external applied voltage giving direct evidence for negative coupling.

Charge transport in thin films of molecular semiconductors as investigated by measurements of thermoelectric power and electrical conductivity

Thin films (~ 100 nm) of molecular semiconductors were prepared by physical vapour deposition on quartz substrates equipped with gold electrodes that allowed simultaneous measurement of the electrical conductivity (specific conductivity σ) under an applied electric field and of the thermoelectric power (Seebeck coefficient S) under an applied temperature gradient. The measurements were performed in vacuo and during exposure to molecular oxygen. The thin film samples were chosen as representatives of three characteristic groups of molecular semiconducting materials: an unsubstituted phthalocyanine (phthalo-cyaninatozinc(II) (PcZn)), a modified phthalocyanine with an electron-withdrawing ligand (tetrapyrido(2,3-b;2′3′-g;2″3″-1;2‴3‴-q) tetra-azaporphyrinatozinc(II) (TPyTAPZn)) and a perylene pigment (N,N′-dimethyl-3,4,9,10-perylenetetracar☐ylic acid di-imide (MePTCDI)). The p-type character of PcZn as reported from a number of studies is confirmed whereas the films of TPyTAPZn and MePTCDI showed n-type behaviour. These results are inferred from the sign of S as well as from the influence of oxygen on σ. Both S and σ of the three materials were studied under variation of the film temperature. Activation energies for S and for σ are derived and lead to a discussion of charge carrier generation and charge carrier transport in each film. It is concluded that both a delocalized charge carrier transport as well as a thermally activated hopping mechanism have to be considered to discuss the electrical properties of the samples.

Interaction of CO with platinum species in zeolites. Evidence for platinum carbonyls in partially reduced zeolite PtNaX

Abstract The decomposition of Pt(NH 3 ) 4 2+ in zeolite NaX both in vacuo and in flowing oxygen ( T max =630 K) yields a variety of ionic species, which appear in IR spectra as PtO-, Pt δ+ -, Pt 2+ - CO and [Pt(CO) 2 ] + complexes formed after the adsorption of CO at 300 K. Subsequent reduction with hydrogen results in characteristic particle size distributions, which are reflected in distinct vibration frequencies of adsorbed CO, i.e., ca . 2010 cm −1 for d ≈1.2 nm (supercage size) and ca . 2070 cm −1 for d ≈4 nm. A hydrated precalcined sample treated with CO shows unusual low platinum carbonyl bands at 1930–1950 and 1720–1740 cm −1 , assigned to [Pt 3 (CO) 6 ] 2− clusters, supported on Pt 2+ /PtO. The appearance of these Chini complexes is most probably connected with the occurrence of the low temperature water gas shift reaction.

Metal-support interaction in the Pd/SiO2 system: Influence of the support pretreatment☆

Transmission electron microscopy, electron diffraction, and convergent beam diffraction have been used to study strong metal-support interaction in the Pd/SiO2 system. It has been shown that heating of the palladium/silica system in a hydrogen atmosphere may lead to chemical (strong) interaction between metal and support and growth of palladium silicide (Pd2Si). Microdiffraction analysis shows an oriented growth of the Pd2Si precipitate with respect to the Pd matrix. The magnitude of metal-support interaction, the agglomeration of metal particles, and the formation of an intermetallic compound are strongly influenced by the thermal pretreatment of the SiO2 substrate. The presence of silanol groups (SiOH) on the silica surface seems to facilitate the metal particle migration and growth by coalescence, in contrast to OH-depleted silica where the change in the particle size proceeds by atomic diffusion. The presence of silanol groups may facilitate chemical metal-support interaction and formation of an intermetallic compound.

Texture of deep bed treated Y zeolites

Abstract Transmission electron microscopy has been used for the determination of the texture of modified Y zeolites. It can be shown that the size and density of secondary pores created during hydrothermal treatment depend, at an equal degree of ion-exchange Na + → NH 4 + of the parent sample, on the temperature of the treatment.

Mechanism of the kinetic oscillations in the oxidation of CO on palladium dispersed within a zeolite matrix

Reaction rate oscillations of the oxidation of CO were studied on highly dispersed palladium embedded within a zeolite matrix under nonisothermal conditions. Transients and periodic oscillations are interpreted in terms of an oxidation-reduction cycle involving the Pd surface which moves the system between two kinetic branches of the reaction. The observation of aperiodic oscillations can be explained by the desynchronization of the catalyst surface.

FTIR evidence of the formation of platinum carbonyls from Pt metal clusters encaged in KL zeolite

Reactivity of Pt metal clusters supported on KL zeolite toward CO was studied by FTIR spectroscopy. Investigation of the CO adsorption was performed within a wide CO pressure range (1–500 mbar). IR data on the CO adsorption at high pressure (500 mbar) suggest the transformation of the finest Pt particles into neutral Pt carbonyls ((Zeol-O:)mPtx(CO)y) stabilized by the basic oxygen atoms of the KL framework. The transformation is found to be readily reversible upon CO adsorption-desorption at room temperature (RT).

Photoelectrochemical oxidation of 2-mercaptoethanol at the surface of octacyanophthalocyanine thin film electrodes

Thin films of octacyanophthalocyanine metal complexes (1) on ITO substrates have been studied in photoelectrochemical experiments in comparison with tetrapyridotetraazaporphyrinatozinc (2), tetrapyrazinotetraazaporphyrinatozinc (3) and unsubstituted phthalocyaninatozinc (4) in contact with aqueous redox electrolytes containing 2-mercaptoethanol or hydroquinone. At electrodes of the zinc(II) complex 1a and at electrodes of 2 and 3 the current under illumination exceeds that in the dark by a factor of up to five, whereas no significant photocurrent is observed at electrodes of 4. The relative concentrations of electrons in the LUMO (CB) and defect-electrons in the HOMO (VB) are influenced by a changing electron density within the inner ring π-system, leading to the observed differences in the semiconducting properties. Electrodes of 1a were investigated in further detail. In spite of the presence of thiyl-radicals the electrodes performed at a high stability. The thiolate anion is found to be the electroactive species. The dependence of the photocurrent on the reactant concentration could be attributed to an adsorption step. A model including charging and discharging processes of these surface states is presented for the analysis of photocurrent transients as obtained by a chopped light beam.