Supported palladium particle size effects

The poisoning effect of chloride observed for the catalyst prepared by impregnation with the PdCl2 precursor and calcined at 500°C, decreases when the solid is calcined at 800°C. The decrease of the inhibition effect of chloride with the growth of the palladium particle size and/or the migration of chloride to the alumina support seems to be the cause. 

CH3) as alternatives to symmetrically-bonded species, either in the predominant route to ethene" and ethane, or just in conditions of low selectivity " (ii) the probable operation of two or three separate types of site during ethyne hydrogenation with excess ethene (Table 9.6) (iii) the likely importance of carbonaceous deposits in determining selectivity or in creating sites at which selective reaction can occur " and (iv) face sensitivity. Other imponderables already noted include the possible formation of carbide and hydride phases in palladium. To add to the misery, we have seen that even the sense of the particle size effect on TOF and selectivity cannot be agreed (Table 9.6), and supports appear to exert an important but poorly understood influence. 

Ramos ALD, Alves PS, Aranda DAG, Schmal M. Characterization of carbon supported palladium catalysts inference of electronic and particle size effects using reaction probes. AppI Catal A General. 2004 277 71. 

Takasu Y, Matsuda Y, Toyoshima I. 1984. A photoelectron spectroscopic study of the effect of particle-size on the adsorbed state of carbon-monoxide over supported palladium catalysts. Chem Phys Lett 108 384-387. 

The Effect of Particle Size on the Reactivity of Supported Palladium... 

Besson M, Gallezot P, Lahmer E, Eleche G, Euertes P (1993) Oxidation of glucose on palladium catalysts particle size and support effects. In Kosak JR, Johnson TA (eds) Catalysis of organic reactions. Marcel Dekker, New York... 

The details of the sample preparation and studies of the nature of the supported-metal samples have been described in a paper dealing with the effect of surface coverage on the spectra of carbon monoxide chemisorbed on platinum, nickel, and palladium (1). The samples consist of small particles of metal dispersed on a nonporous silica which is produced commercially under the names Cabosil or Aerosil.f This type of silica is suitable as a support because it is relatively inert and has a small particle size (150-200 A.). The small particle size is important because it reduces the amount of radiation which is lost by scattering. A nonporous small particle form of gamma-alumina, known as Alon-C, is also available. This material is not so inert as the silica and will react with gases such as CO and CO2 at elevated temperatures. 

A detrimental effect of excess nitric acid on the ZnO support was observed, resulting in a reduced ZnO particle size and losses of surface area. This excess was present in the palladium nitrate solution that was applied for catalyst impregnation. Additionally it was found that the PdZn alloy was not only formed during the initial reduction step but also in situ in the hydrogen-rich reaction mixture of methanol steam reforming. 

Vannice (232) measured turnover numbers for methanation on a variety of well-characterized palladium catalysts. The supported catalysts were all more active than unsupported palladium (Pd black) and PdHY was intermediate between Pd/Si02 and Pd/Al203 in specific activity (see Table VII). As is evident from the data there is no obvious correlation betwen particle size and turnover number. It was therefore suggested that the enhanced activity of various supported catalysts was due to a metal-support interaction. Figureas et al. (105) also found good evidence for a support effect during benzene hydrogenation studies. In this case the palladium zeolite... 

The influence of the nature of the support on the 2,4-DNT hydrogenation is reported in Table 3. Catalysts with similar dispersion have been compared in order to neglect the effect of metal particle size. The specific activity of palladium on the different supports investigated is similar. A different products distribution is instead observed. 

The effect of 25 ppm SO2 with respect to the complete oxidation of methane over supported palladium catalysts has been examined. Pd nitrate was impregnated onto alumina, Ba-modified alumina and La-modified alumina by incipient wetness impregnation. The calculated metal loading was 2.5 wt%. Two different sets of catalysts have been prepared one calcined at 500 C and the other at 1000 C. The main purpose of these two calcinations temperatures was to obtain different metal particle sizes. The results show that the combustion activity is strongly affected by the nature of the support when large particles are present. On the other hand, the small Pd particles exhibit similar behavior regardless of the nature of the support. When SO2 is added to the gas stream, a strong deactivation is observed and the presence of additive to stabilize the support increases the loss of activity of the supported Pd particles. 

The outstanding quality of palladium in both these respects is well known but not well understood. Foremost among the complexities that have to be addressed are (i) the tendency to form hydride phases, and (ii) the frequent formation, especially with ethyne, of a carbonaceous overlayer, derived perhaps from oligomers, some of which escape into the fluid phase. Both these factors are invoked to account for palladium s remarkable properties, but both are responsive to reaction conditions and especially to particle size support effects may also operate. So many factors have to be kept in play when discussing mechanisms that one may safely conclude that none so far suggested is wholly satisfactory. The rich literature on alkyne hydrogenation deserves careful attention, and should be a fine source of inspiration for further research. 

---