Zbigniew Karpiński

Hydrodechlorination of CCl2F2 (CFC-12) over γ-alumina supported palladium catalysts

Abstract Alumina displays low catalytic activity at the initial stage of the reaction of CCl 2 F 2 with hydrogen, giving mainly halogen exchange products, and this activity quickly decays with time-on-stream. In the case of Pd/Al 2 O 3 catalysts, the contribution of the support is negligible at 180°C. Catalytic activity of Pd/γ-Al 2 O 3 in CCl 2 F 2 hydrodechlorination strongly depends on metal dispersion: poorly dispersed Pd samples exhibit the highest turnover frequencies. The same samples also show the highest selectivities towards the formation of CH 2 F 2 . Time-on-stream behaviour and considerable amounts of carbon found in used catalysts suggest that the catalytic properties of Pd/Al 2 O 3 are regulated by incorporation of carbon into Pd lattice, or the formation of Pd carbide. Poorly dispersed Pd catalysts contain a higher proportion of plane atoms and, therefore, are subjected to a more severe carbiding. Such a transformation generates surfaces which bind freon molecules less strongly, resulting in higher activity and selectivity to partial dehalogenation, i.e formation of CH 2 F 2 . High-temperature reduction at 600°C does not much change the overall activity of Pd/Al 2 O 3 . However, the selectivity to CH 2 F 2 is somewhat increased. It is believed that the Pd Al 2 O 3 interface changes upon high-temperature reduction, leading to a Pd Al compound. At corrosive conditions of hydrodehalogenation of CCl 2 F 2 , the Pd Al would be converted to AlF x species much more easily than the Al 2 O 3 species at the Pd Al 2 O 3 interface of mildly reduced Pd/Al 2 O 3 catalysts.

Characterization of supported palladium catalysts: III. PdAl2O3

In Part I of this series (W. Juszczyk et al., in Proceedings, 9th International Congress on Catalysis, Calgary, 1988 (M.J. Phillips and M. Ternan, Eds.), Vol. 3, p. 1238. The Chemical Institute of Canada, Ottawa, 1988) using the catalytic conversion of neopentane as a virtually noncoking probe, the authors found that after reduction at moderate temperatures (573-773 K), Pd/Al{sub 2}O{sub 3} is roughly two orders of magnitude more active than Pd/SiO{sub 2}. An even higher, though transient, catalytic activity is now reported after reducing Pd/Al{sub 2}O{sub 3} at 873 K and extensively purging it in He at this temperature. The IR spectra of adsorbed CO reveal the presence of Pd{sup n+} ions in superactive Pd/Al{sub 2}O{sub 3}; their concentration positively correlates with catalytic activity. Since other potential causes such as strong acid sites are excluded, it is proposed that Pd{sup n+} ions are sites of high catalytic activity. A possible mechanism of their formation and a model of neopentane hydrogenolysis are briefly discussed. Completely reduced Pd appears necessary for neopentane isomerization.

Hydrodechlorination of CCl2F2 (CFC-12) over silica-supported palladium–gold catalysts

Abstract Two series of well-mixed Pd–Au/SiO 2 catalysts prepared by direct redox method were tested in the reaction of CCl 2 F 2 (CFC-12) hydrodechlorination. A moderate selectivity for CH 2 F 2 exhibited by monometallic Pd/SiO 2 (∼40%) is significantly increased, up to ∼95%, with Au addition. Proper Pd–Au alloying is essential to obtain such a selectivity enhancement. Poorly mixed Pd–Au/SiO 2 catalysts show the catalytic behaviour not very much different from that of Pd/SiO 2 , producing a lot of methane. Changes in the apparent activation energy are also associated with the degree of Pd–Au alloying. After reaction, the Pd/SiO 2 and Pd–Au/SiO 2 catalysts contain dissolved carbon. Hydrogen treatment of carbonized Pd–Au/SiO 2 catalysts removes this carbon. XRD study of the regenerated samples shows no change in the structure and composition of Pd–Au phase, compared to freshly prereduced bimetallic catalysts.

Catalysis by supported, unsupported, and electron-deficient palladium

Publisher Summary This chapter focuses on the catalysis by supported, unsupported, and electron-deficient palladium. At lower temperatures, in the presence of hydrogen, the formation of p-palladium hydride has a great effect on the reaction course and/or on the reconstruction of the Pd surface. Another important aspect is the presence of electron-deficient palladium in highly dispersed Pd catalysts. In particular, an interpretation of the positive shift of electron core-levels (XPS) of metals highly dispersed over insulating carriers should be given an unequivocal explanation (initial-state versus final-state effect). Chemical probing of Pd surfaces in supported catalysts is a useful alternative to physical studies. Several reactions are very sensitive to the kind of Pd surface, making them genuine diagnostic tools for surface characterization. But, unlike in the case of platinum, comprehensive information for well-defined Pd surfaces is not plentiful. This fact makes all comparisons with supported catalysts, which are complex by nature, more difficult. Hence, studies involving various model forms of Pd catalysts, such as flat, stepped, and kinked surfaces of palladium, are needed. But palladium remain an important objective of fundamental and technological studies, certainly in the field of heterogeneous catalysis.

Characterization of supported palladium catalysts: II. PdSiO2

The isomerization of neopentane has been investigated over the 0.76 wt% Pd/SiO{sub 2} catalyst. It is found that after high temperature reduction (HTR, at 873 K) the selectivity for isomerization is much higher than that after low temperature reduction (LTR, at 573 K). A variety of experiments, including kinetic, chemisorption (O{sub 2}, H{sub 2}, and CO), temperature-programmed desorption of H{sub 2}, and X-ray diffraction, showed that this selectivity enhancement cannot be interpreted in terms of H{sub 2} retention by catalyst. Instead, the formation of Pd-Si compound(s) (most probably Pd{sub 3}Si) during HTR seems immediately responsible for the catalytic behavior of HTR Pd/SiO{sub 2} catalysts. A mechanism is proposed for the Pd-SiO{sub 2} interaction in which Pd atoms (or ions) are incorporated into the silica support (via oxygen vacancies) and a new phase of palladium silicide is formed. Regeneration by an oxygen treatment of the HTR sample does not fully restore the low isomerization selectivity typical of LTR samples. The additional selectivity is attributed to an overlayer of oxidized silicon species (after oxidation of the HTR sample) which partially cover the metal.

Hydrodechlorination of CCl2F2 (CFC-12) over Pd-Au/C catalysts

Abstract A series of carbon-supported palladium-gold (Pd-Au) catalysts prepared by direct redox reaction method and characterized by various techniques were investigated in the reaction of dichlorodifluoromethane (CFC-12) with dihydrogen. The selectivity towards difluoromethane (desired reaction product) was increased upon introducing gold to palladium, from ∼72 to ∼86%, at 180°C. Such a selectivity enhancement was not observed in our previous studies when Pd-Au/C catalysts prepared by incipient wetness impregnation showed inadequate extent of Pd-Au alloying. Conditions of preparation of Pd-Au/C catalysts by the direct redox reaction method are found to affect the amount of deposited metals and the degree of Pd-Au mixing. The latter factor is essential in determining hydrodehalogenation behavior of the catalysts.

Neopentane conversion over Pd/γ-Al2O3

X-ray diffraction showed that during high temperature reduction at 600°C, chlorine-free Pd/γ-Al2O3 undergoes partial transformation to a Pd-Al alloy, which confirms results of other studies [9]. This evolution appears to have a large effect on the catalytic behaviour in the reaction of neopentane with hydrogen: the selectivity towards isomerization increases from <20 up to ∼80%. At the same time, the activation energy drops from ∼ 60 to ∼ 22 kcal/mol. These changes can be reversed by oxidation at 500°C followed by reduction at 300°C. The presence of residual chlorine (ex-PdCl2 precursor) appears to inhibit the Pd induced reduction of Al2O3 leading to Pd-Al alloy formation.

Catalytic reactions of hydrocarbons over PtPd alloys: III. Skeletal reactions of n-pentane and n-hexane over PtPdSiO2 catalysts

The isomerization, cyclization, and hydrogenolysis kinetics of n-pentane and n-hexane were studied on dispersed silica-supported 1% alloy consisting of 0-100% palladium/platinum. The catalysts were calcined for 25 hr at 600/sup 0/C prior to reduction or only reduced in hydrogen at 400/sup 0/C. The surface composition was probably similar to bulk composition. The catalytic activity vs. composition curves were similar for precalcined and uncalcined catalysts and for various reaction temperatures (300/sup 0/-360/sup 0/C). For n-pentane, hydrogenolysis was higher on palladium than on platinum or low-palladium alloys but more selective towards butane on platinum; isomerization decreased with increasing palladium content; and cyclization went through a maximum on alloy catalysts. The reaction patterns were similar with n-hexane, except that precalcination of the catalyst suppressed cyclization to methylcyclopentane and benzene. The catalysts underwent a poisoning process which was generally highest at the higher temperatures and higher palladium contents. The results were compared with results obtained on Pt-Pd foil and mechanisms were discussed.

Catalytic reactions of hydrocarbons over Pt-Pd alloys: II. Deuterium exchange of methane and ethane over Pt-Pd alloy films. Surface composition of Pt-Pd alloy system

The exchange of deuterium with methane is proposed as a diagnostic tool to estimate the surface composition of Pt--Pd alloy films. Only stepwise exchange has been found on Pt, Pd, and Pt--Pd alloys. From the kinetic parameters (E/sub A/ and log A) obtained on thick and thin films the surface composition of thick films is predicted. The occurrence of a well-defined compensation effect suggests that the total activity may be calculated from the contributions from platinum and palladium individual active centers, supporting the idea of using deuterium-methane exchange for solving the problem of surface composition. It seems that although the surface of Pt--Pd alloys is in fact enriched with Pd, this occurs to a lesser extent than is foreseen by the theory of alloy segregation. This conclusion finds support in the analysis of one Pt--Pd alloy by Auger electron spectroscopy. The results of a study of C/sub 2/H/sub 6/--D/sub 2/ exchange are also reported. 3 figures, 2 tables.

Catalytic reactions of hydrocarbons over Pt-Pd alloys: I. Skeletal reactions of C5 and C6 alkanes over Pt-Pd alloy films

Abstract The reactions of neopentane, n-pentane, and n-hexane in the presence of an excess of hydrogen have been studied on evaporated films of Pt-Pd alloys as catalysts. The selectivity in neopentane isomerization on a pure Pd film has been found to be nonnegligible, in contrast with previous work in the literature. A possible explanation of this involving the carbonization of the surface during the experiments and the effect of texture on the process is proposed. The neopentane isomerization vs alloy composition pattern is interpreted in terms of an “inferred” surface composition. The results may be easily explained assuming a one-Pt-site mechanism for isomerization and a two-Pd-site mechanism for hydrogenolysis. The 1,5-cyclization selectivity pattern in the n-pentane reaction is interpreted in terms of the change in active site number. In the n-hexane conversion the ratio of selectivity towards 1,6-versus 1,5-cyclization is higher for alloys than for pure metals.