Catalyst pre-treatment

Henriques, C., Ribiero, M., Abreu, C. el al. (1998) An FT-IR study of NO adsorption over Cu-exchanged MFI catalysts Effect of Si/Al ratio, copper loading and catalyst pre-treatment, Appl. Catal. B Environ., 16, 79. 

Furthermore, no significant differences are observed on the methane conversion or on the ignition temperatures of the Pd2HZSHe and the Pd2HZS02 catalysts. This result presumes that the catalyst pre-treatment with oxidant or with inert gas has the same effect on the activity of the Pd-HZSM-5 catalysts prepared by the solid-exchange method. Moreover, a noticeable methane conversion increase is observed when the NaZSM-5 support is used instead of the HZSM-5 zeolite. The ignition temperatures... 

Catalysts pre-treatment (calcination and reduction) was performed in the same testing system or in a parallel automatic activation system prior to reaction test Calcination is carried out at 600 °C under airflow for 8 h and reduction at 250 °C for 2 h under hydrogen flow. Catalytic tests were carried out at 30 bar total pressure, temperature range 200-240°C, and 2.26h-1 WHSV, H2/hydrocarbons molar ratio of 2.93. Each fixed bed microreactor contained 500 mg of catalyst (particle size 0.4—0.6 mm, for which there are no internal diffusion limitations). Reaction products distribution are analysed using a gas chromatograph (Varian 3380GC) equipped with a Plot Alumina capillary column. 

Metal Dispersion by Chemisorption and Titration Selective Chemisorption. - This is the most frequently used technique for determining the metal area in a supported catalyst and depends on finding conditions under which the gas will chemisorb to monolayer coverage on the metal but to a negligible extent on the support. Various experimental methods, conditions, and adsorbates have been tried and studies made of catalyst pre-treatment and adsorption stoicheiometry, viz, the (surface metal atom)/(gas adsorbate) ratio, written here as Pts/H, Bh jQO,etc., and reviews to about 1975 are available. A summary is given in Table IV of ref. 2 of methods used to confirm the various adsorption stoicheiometries proposed, sometimes from infrared studies. These include chemisorption on metal powders of known BET area or, more satisfactorily, one of the instrumental methods reviewed in Section 3 for the determination of crystallite size distributions. For many purposes, a relative measurement of metal dispersion is sufficient, conveniently expressed as the ratio (number of atoms or molecules adsorbed)/(totfl/ number of metal atoms in the catalyst), e.g., H/Ptt. 

Figure 10.20. CO conversion as a flmction of temperature over a dyno-aged (100,000 mile equivalent) Pt/Rh catalyst formulation after various combinations of catalyst pre-treatment (Qxidizing or Reducing) and reaction gas mixtures (Lean or Stoichiometric),...

In Figure 5, however, it is seen that the pre-treatment atmosphere has a significant effect on the low temperature activity of Pd/Co/AbOa. The effect of catalyst pre-treatment is most pronounced for the cobalt oxide catalyst promoted with Pd or Pt (see Table 2). In lean reactant gas, pre-reduced Pd/Co/AbOs has light-off temperatures at 169°C and 177°C for CO and HC, respectively, whereas the light-off temperatures over the same catalyst, but pre-oxidised, are 246°C for both CO and HC. A clear effect of pre-reduction is also seen for Pt/Ce/Ab03, whereas no obvious effect of the pre-treatment atmosphere on the oxidation activities for CO or HC is seen for Pd/Ce/AbOs. 

In a previous investigation, we showed that feeding NO at low temperature (80°C) to Cu over-exchanged (640%) ZSM5 after He pre-treatment at elevated temperatures, a transient production of N2O is observed as result of catalyst re-oxidation [14]. Similar results have been obtained in the present study. Fig.3 shows that, after catalyst pre-treatment of 2 h in He flow at 550°C, causing copper ions reduction from Cu to Cu", the response to a step change of NO gaseous concentration (0-600 ppm) at 80°C, results in the transient formation of N2O due to the re-oxidation of copper sites. 

Despite this limitation, a number of relevant conclusions may be drawn. Thus, in Collins et al hydrogen adsorption by a 3% Au/Ceo.eaZro.sgOg (Au/CZ) catalyst, with a BET surface area, 62.8 m .g" was investigated at room temperature using volumetric adsorption and FTIR techniques. This approach has an unusual, and certainly very fruitful, combination of techniques. As discussed below, it has rendered interesting data on the amount and nature of the hydrogen chemisorbed on Au/CZ and closely related catalysts. Likewise, these techniques have also provided useful information about the influence of different catalyst pre-treatments and of CO co-adsorption, on the chemistry of the Hj-(Au/CZ) system. 

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