Deactivation Of Pt Research Articles

Electro-oxidation of d-mannitol on platinum single crystal surfaces

The adsorption and oxidation reactions of d-mannitol at platinum single crystal electrodes with basal orientations have been studied by combining cyclic voltammetry and in situ FTIR spectroscopy. Cyclic voltammetric results have shown that, as far as current densities are concerned, the Pt(110) surface is the least active and more poisonable surface among the three platinum basal planes. The deactivation of Pt(100) and Pt(111) surfaces was found to be similar, although, higher current densities are obtained for Pt(100). FTIR spectroscopic experiments have indicated that CO is formed upon d-mannitol dissociative adsorption on the three basal platinum surfaces. This adsorbed intermediate, which was identified as the main poisoning species, is bonded to the platinum surface both in a linearly and bridge bonding mode and on a ratio which depends on the electrode surface orientation. The electro-oxidation process was found to be strongly dependent on the platinum crystalline surface structure. Moreover, additional voltammetric experiments performed on the three sets of platinum stepped surfaces have shown that the adsorption and oxidation reactions of d-mannitol are sensitive to the symmetry of the bidimensional domains as well as to the distribution of orientated defects (monoatomic steps) on the platinum surfaces.

Influence of the metal and of the support on the activity and stability of bifunctional catalysts for toluene hydrogenation

Gas phase hydrogenation of toluene, diluted in inert n-hexane in order to minimise the thermal effects was investigated in a fixed bed reactor at 110°C over series of bifunctional catalysts in which Pt or Pd was deposited on alumina or on an acidic HFAU zeolite. Practically no deactivation was observed when the metals were deposited on alumina, even in mixture with the HFAU zeolite, whereas there was a rapid initial deactivation when Pd and especially Pt were deposited on the acidic zeolite. However, even in this latter case, the activity of the fresh catalysts can be obtained with a good accuracy thanks to the use of a multiple loop value. For given metal and support, a linear correlation was found between the hydrogenating activity and the number of accessible metal atoms. However, Pt was found to be 20–60 times more active than Pd, both metals being more active on HFAU zeolite than on alumina. This acidity effect could be explained by the hydrogenation of aromatic molecules adsorbed on acidic sites by hydrogen spilled over from the metal surface. The initial deactivation of Pt- and Pd-HFAU zeolite catalysts was due to the rapid formation of ‘coke’ molecules inside the zeolite micropores. ‘Coke’ was mainly constituted by C 14 and C 21 products resulting from the acid alkylation of toluene molecules by the olefinic and dienic intermediates of their hydrogenation. While the composition of ‘coke’ as well as its deactivating effect did not depend on the metal (Pt or Pd), the rate of ‘coke’ formation at isoconversion of toluene was found to be lower on Pd than on Pt zeolite catalysts, which explains their slower deactivation.

Deactivation of PT / Zeolite-L Catalysts Studied by Z-Contrast

Abstract When Pt particles of sub-nanometer dimensions are inserted into the linear channels of the potassium ion form of zeolite L, the resultant Pt/KL catalyst converts linear n-C6 hexane into cyclic C6 benzene with remarkable efficiency [1,2]. However, a drawback to Pt/KL catalysts is that they are exquisitely sensitive to sulfur contamination in the n-hexane feed. The catalyst deactivates rapidly when there are trace amounts (> 50 parts per billion) of sulfur in the feed. Bright field TEM studies show that the Pt particles remain finely dispersed within the channels after sulfur poisoning, with typical particle widths < 1.5 nm, implying that a large percentage of the Pt atoms still reside on the surfaces of the particles. For heterogeneous catalysts such as Pt/γ-alumina, where the Pt particles are always readily accessible on the external surface of the alumina support, such sulfur concentrations are tolerated.

Oxygen storage capacity (OSC) of aged Pt/CeO 2/Al 2O 3 catalysts: roles of Pt and CeO 2 supported on Al 2O 3

In order to investigate the behavior of Pt and CeO 2 on the Pt/CeO 2/Al 2O 3 catalyst after aging at 1273 K, the oxygen storage capacity(OSC) has been measured and physically mixed catalysts such as Pt Al 2O 3 + CeO 2 Al 2O 3 , Pt Al 2O 3 + CeO 2 , Pt CeO 2 + Al 2O 3 and Pt CeO 2 + Pt Al 2O 3 catalysts have been employed for comparison. After the aging, the OSC of the physically mixed catalysts whose CeO 2 was not supported on Al 2O 3 became poor because of the large-sized CeO 2 and the deactivation of Pt, although those catalysts provided large OSC assisted by the PtCeO 2 interaction before the aging. The OSC increment calculated by subtraction of the OSC of Pt Al 2O 3 + CeO 2 Al 2O 3 from that of Pt/CeO 2/Al 2O 3 indicated that both the PtCeO 2 interaction and the dispersed CeO 2 on Al 2O 3 are essential for the generation of OSC, even after the aging.

1,3-Butadiene hydrogenation on single crystals of platinum: II. Sulfur poisoning of Pt(110)

The sulfur deactivation of Pt(110) for the 1,3-butadiene hydrogenation and H 2D 2 equilibration reactions at 323–423 K and ≈400 Torr pressure were investigated. It was observed that each sulfur atom poisons one dissociation site for hydrogen without changing the mechanisms of the reactions occurring on the sites remaining free of sulfur. An isotherm of sulfur adsorption was obtained at 373 K during the hydrogenation reaction. This curve indicates that the butadiene molecules adsorbed under the steady-state conditions of hydrogenation significantly decrease the binding energy of sulfur on platinum (15–20%). As a general conclusion of this study, it appears that the deactivation behavior of sulfur for a catalytic reaction involving unsaturated hydrocarbon depends on the adsorption strength of the hydrocarbon and on its concentration on the surface.