Pulse chemisorption

This argument is confirmed by the study of CO pulse chemisorption by Biffis at al., mentioned above. In this piece of investigation, the authors prepared a 2% (w/w) palladium catalyst supported by Lewatit UCP 118, a macroreticular resin (nominal cld = 18 %) from Bayer. Its TEM characterization showed a remarkably heterogeneous distribution of the metal nanoclusters, which are apparently located close to the surface of the polymer nodules [62] (Figure 9). 

FIGURE 16 Chemisorption on a metal surface. A, Chemisorption isotherm showing approach to monolayer coverage B, typical data from a pulsed chemisorption technique. 

TPR of the catalyst precursors was used to determine the presence of metal-metal interaction. Pulse chemisorption of hydrogen was employed to calculate the catalyst dispersion. Both techniques were carried out in an Autochem 2910 Micromeritics apparatus. Before TPR experiments, the catalyst precursor was conditioned in N2, rising the temperature from ambient to 323 K. TPR data were obtained from 223 to 773 K in a flow of Hz/Ar = 5/95, through a thermal conductivity detector (TCD). 

Prior to the pulse chemisorption experiments the catalysts were degassed by heating from ambient temperature up to 773 K in Ar. Then the sample was cooled to 273 K and pulses of hydrogen were injected into the stream of argon. The hydrogen amount adsorbed by the sample was measured by TCD. 

Metallic contents of precursor and catalysts, in wt%, results of pulse chemisorption, TOF values and selectivity towards paraffinic fraction at 503 K. 

Results of pulse chemisorption experiments are shown in Table 1. Using the standard evaluation method and a stoichiometric ratio H2/Metal=2 for both ruthenium and platinum, the active metal dispersion was calculated, and from it the mean metal particle size, L. 

The metallic surface area and dispersion are measured by CO pulse chemisorption, performed on a Micromeritics Pulse ChemiSorb 2700. 

Pulse chemisorption involves exposing the reduced catalyst to a number of pulses of adsorbate gas from a calibrated loop which are passed over the catalyst in a flow of inert carrier at atmospheric pressure. Early studies employed only a single pulse with a recommended volume of ca. twice the volume likely to be consumed.36 The quantity of gas remaining (that is, not adsorbed) from the pulse is measured by TCD and in a typical experiment the first two or three pulses would be completely consumed, while only fractions of the subsequent peaks would be taken up by the sample. Eventually, the area of the peaks detected reaches a constant level indicating that the sample has reached monolayer capacity. The volume of gas adsorbed is calculated from the sum of the areas of peaks fully consumed plus the areas of the partially consumed peaks by equating the area of a peak eluted where no gas was taken up, with the volume of gas involved in... 

CO pulse chemisorption measurements were carried out on a Micromeritics AutoChem 2910. Pd metal dispersion of catalyst was calculated from CO pulse chemisorption results. Before introducing CO, the catalyst samples were reduced at 75°C using 4% hydrogen in argon for 45 minutes. Catalyst samples were cooled to room temperature for CO chemisorption. Helium carrier gas flow rate was about 30 ml/min. 

The dispersion of Pt was determined from the amount of chemisorbed CO using pulse method. The experiments were carried out using a (Micromeritics 2910 AutoChem) instrument. The Pt modified catalysts were reduced with hydrogen in a U- shaped quartz tube and then cooled to 313 K under flow of He and CO pulse chemisorption was performed. More detail description of the R dispersion can be found in reference [11], The dispersion of R measured by CO chemisorption was the highest for R-MCM-41 catalyst prepared by ion-exchange method Table 1. 

The quantity of molecules selectively chemisorbed by the metallic component of the catalyst may be determined by what are commonly described as static (volumetric) or dynamic (flow) methods. The former is performed at reduced pressure and involves allowing the system to reach equilibrium between the adsorbed and gaseous states. The later group of methods, which involves pulsed chemisorption, are generally performed at atmospheric pressure and the equilibrium between adsorbed and gas states is not achieved (or maintained). 

F re 2.2 Example of pulse chemisorption where 0.285 cm pulses of CO are introduced to the catalyst at 298 K every 10 min and the volume adsorbed is apparent from differences between the peak area of the initial peaks compared to the... 

The total concentration of active sites Ct is independent from the operating conditions and is typically determined from independent measurements. For acid catalysis, NH3 TPD measurements are at hand to titrate the number of active sites, whereas for metallic catalysts H2 and/or O2 pulse chemisorption techniques may be used. However, in view of the uncertainties as to the nature and, hence, the real concentration of active sites, Ct is often incorporated in the rate coefficients of the rate expression. 

The combination of the described techniques and the integration of the experimental results produce a detailed picture of the investigated catalyst, allowing a better comprehension of the reaction mechanisms in complicated processes and a detailed characterisation of catalyst activity and selectivity. Most of the experimental results shown in the present paper have been obtained in the application lab of CE Instruments (ThermoQuest S.p.A.), Milan - Italy. All the graphs related to static volumetric chemisorption have been obtained by the adsorption apparatus Sorptomatic 1990, while the graphs related to TPD, TPR/0 and pulse chemisorption analyses with the dynamic apparatus TPDRO 1100. 

Pulse chemisorption in flow Active sites surface area Degree of dispersion Determination of strong gas-solid interaction Acid/base surface properties Isostheric heat of adsorption... 

Figure 6. Diagram of a multipurpose apparatus for pulse chemisorption, TPD, TPR and TPO.

Figure 7. Pulse chemisorption analysis performed by the TPDRO 1100 (CE Instruments).

In case of pulse chemisorption technique, the best way to evaluate the monolayer volume is to take into account the total volume of gas adsorbed during the experiment. In fact, in this case it is not necessary any volume correction as the carrier flow during the run removes continuously the physisorbed or weak chemisorbed probe gas. 

Dynamic methods are very fast and relatively easy to handle, even by inexperienced users. The analytical results, especially for pulse chemisorption, can be compared to the static methods ones only taking into consideration the basic differences between the two systems. In fact, in pulse chemisorption, the weak chemisorbed species are removed as they forms and it is not possible to state that there is a real equilibrium between the probe gas phases (gaseous and bound). 

Pulse chemisorption measurements were performed using Micromeritics Autochem II 2920 instrument. The samples were pre-treated for 2h in a flow of hydrogen at 200 C, in order to reduce the Pd in the catalyst, then the samples were cooled to 100°C in a stream of argon. Pulse chemisorption measurements were performed at this temperature. 

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