Palladium particle size effects

P-10 gas, 45, 219 Pair production, 290 Palladium, determination by x-ray emission spectrography, 328 Particle size, effect of variations of, in mineral analysis, 200 Philips Autrometer, 252-256, 280 Philips Electronics gas analyzer, 135 Philips Electronics improved Coolidge tubes, 248, 252, 253... 

Three factors determine the activity and selectivity in the hydrogenation of alkadienes (a) the particle size through the effect of complexation of reactants to active sites containing low coordination number atoms, (b) the particle size, through self-poisoning by carbonaceous residues, and (c) with palladium, a particle-size effect through the solubility of hydrogen and the formation of the unselective /l-PdH derivatives. 

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. 

Borodzinski A, Bond GC Selective hydrogenation of ethyne in ethene-rich streams on palladium catalysts, part 2 steady-state kinetics and effects of palladium particle size, carbon monoxide, and promoters, Catal Rjeu Sci Eng 50(3) 379—469, 2008. 

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. 

Takasu Y, Zhang XG, Minoura S, Murakami Y. 1997. Size effects of ultrafine palladium particles on the electrocatalytic oxidation of CO. Appl Surf Sci 121 596-600. 

The same group has looked into the conversion of NO on palladium particles. The authors in that case started with a simple model involving only one type of reactive site, and used as many experimental parameters as possible [86], That proved sufficient to obtain qualitative agreement with the set of experiments on Pd/MgO discussed above [72], and with the conclusion that the rate-limiting step is NO decomposition at low temperatures and CO adsorption at high temperatures. Both the temperature and pressure dependences of the C02 production rate and the major features of the transient signals were correctly reproduced. In a more detailed simulation that included the contribution of different facets to the kinetics on Pd particles of different sizes, it was shown that the effects of CO and NO desorption are fundamental to the overall behavior... 

These spectra show even fewer features than those of the palladium catalysts. Absorption takes place almost exclusively in the region 2000-2100 cm-1. There are some weak bands below 2000 cm-1, but our experimental method did not allow us to determine their frequencies with reasonable accuracy. It is clear that also with the iridium catalysts the particle size has an effect on the spectra. The spectrum of Ir-8 shows only one intense band at 2048 cm-1, whereas the other two have additional bands at higher frequencies. There is also a marked dependence of the intensity of the 2048 cm-1 band on the CO pressure, especially in the case of Ir-37 and Ir-100. We shall not try to interpret the CO spectra of the iridium samples, as we consider the data available insufficient for the purpose. 

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. 

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