Wojciech Juszczyk

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.

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.

Reforming reactions onPd/Al2O3 active sites in variously pretreated catalysts

Abstract Chlorine-freePd/γ-Al 2 O 3 catalysts were studied in conversions of n -hexane, 2,2-dimethylbutane and methylcyclopentane in excess hydrogen. All these reactions appeared as useful probes for diagnosing changes inPd/Al 2 O 3 catalysts caused by their different pretreatments. The effect of high temperature reduction on catalytic properties ofPd/Al 2 O 3 is twofold: it generates Lewis acidity in γ-Al 2 O 3 and, at the same time, induces Pd-alumina interactions leading to the formation of Pd Al species very active in isomerization. The use of two, similarly dispersed,Pd/Al 2 O 3 catalysts of different metal loading allows to distinguish contributions of two types of active centers mentioned above. The reaction of 2,2-dimethylbutane conversion appeared the most convenient catalytic probe because it did not virtually suffer with time-on-stream, and, simultaneously, displayed largest variations with the catalyst pretreatment. Among various reaction pathways, alkane isomerization experiences most pronounced changes and, in effect, becomes the most sensitive reaction route in diagnosing transformations in supported palladium catalysts. Methylcyclopentane conversion at 260°C and below proceeds primarily on palladium surface. Acidic sites in γ-Al 2 O 3 and Pd Al species contribute only slightly to the overall activity, however, these centers generate a noticeable ring enlargement selectivity.

Hydrodechlorination of CCl2F2 (CFC-12) by carbon- and MgF2-supported palladium and palladium-gold catalysts

Carbon- and MgF 2 -supported palladium catalysts exhibited comparable activity, selectivity pattern and stability in hydrodechlorination of CCl 2 F 2 . Selective hydrodehalogenation to CH 2 F 2 was a prevailing reaction. Introduction of gold to supported palladium catalysts produced different results, depending on whether Pd/C or Pd/MgF 2 was doped. In the case of Au addition to 2 wt% Pd/MgF 2 , the selectivity to CH 2 F 2 was considerably enhanced, from ∼72 to 86%, whereas an analogous modification of 1 wt% Pd/C caused only insignificant changes. TPR and XRD studies of the catalysts indicated considerable differences in the extent of the homogeneity of the Pd-Au bimetal: the degree of alloying was substantially higher for the former catalyst. This suggests that an intimate contact between Pd and Au is essential for improving the selectivity to CH 2 F 2 . XRD of used catalysts showed carbon incorporation into a palladium lattice. A majority of this carbon can be removed by a short H 2 purge at 200°C.

Hydrocarbon reactions on Pd-Re/Al2O3 catalysts

Abstract The reactions of n -hexane, 2,2-dimethylbutane and methylcyclopentane in excess hydrogen were studied on alumina-supported Pd–Re catalysts characterized by medium metal dispersion. Palladium is much less active than rhenium in catalyzing alkane conversions. Even very small amounts of added rhenium to palladium cause big catalytic consequences. All Re-containing samples behave similarly, i.e. exhibit comparable activation energies (much lower than in the case of Pd/Al 2 O 3 ) and a radical loss of isomerization capability. Accordingly, examination of these variables cannot effectively serve in probing Pd–Re surfaces. However, other useful ‘fingerprints’ have been recognized, among which the fragmentation factor in alkane conversions and the 2MP/3MP product ratio in methylcyclopentane reaction are the most valuable. Their profound specific variations with Pd–Re composition clearly suggest that rhenium interacts with surface palladium atoms.

Transformation of Pd/SiO2 Catalysts During High Temperature Reduction

During reduction in dihydrogen at 873 K, a silica-supported palladium catalyst is initially converted to Pd4Si which further reacts with silicon-containing species to a Pd3Si phase. Such a stepwise phase change suggests that the mechanism of Pd silicide formation involves incorporation of silicon into a palladium phase, not vice versa, as was suggested in our previous work. This transformation has a great effect on catalytic properties of Pd–silica-containing systems in 2,2-dimethylpropane hydroconversion.