Benzene desorption

Benzene adsorption has advantages over nitrogen adsorption in that nitrogen is limited to a maximum relative pressure of around 0.99 due to pressure fluctuations, whereas with benzene the adsorption temperature is close to room temperature so that fluctuations in (PIPq) are small at pressures near Pq. Further (t/Vi/RT) for benzene is 2.2 times the value for nitrogen so that a lower relative pressure is required for a given pore radius, so that benzene isotherms give information down to a smaller size (1.58 cf 25) nm. 

Benzene was used by Naono etal. [106] for sizing mesopores and macropores of silica of BET surface area 1.57 nfi g-1. They used the 

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In the case of SiC12T sample and benzene as a wetting liquid the desorption curve is smooth without characteristic step. It may be explained by not satisfactory wetting of this silica by benzene and restricted penetration of narrow pores. Observed effect is confirmed by small pore volume for the same sample, derived from benzene desorption data. The localization of desorption steps on temperature axis corresponds to emptying of pores with dominant share in total pore volume. Converting the temperature into pore radius, by using the Kelvin equation, the dimensions of pores and pore size distributions PSD, AV Affp vs. R, may be calculated in the manner described earher [9]. 

The chemisorption of acetylene on the Sn/Pt(100) surface alloys revealed similar chemistry and provided additional information on the structure sensitivity of these reactions [53, 54]. While 15% of the adsorbed acetylene monolayer was converted to gaseous benzene during TPD on the (3V2xV2)R45°-Sn/Pt(100) alloy, no such benzene desorption occurred from related surfaces, as shown in Fig. 2.6. 

Fig. 2.6 Comparison of TPD spectra obtained from a (i -acetylene monolayer on various Sn/Pt(100) surfaces adsorbed at 100 K. Several benzene desorption peaks in the top curve arise from multiple kinetic pathways. Adapted with permission from [54]. Copyright 2001 American Chemical Society...

Alloying Sn in the Pt(lll) surface to form the (2x2) and Vs alloys eliminates the thermal decomposition of benzene on these surfaces under UHV conditions [39]. In a subsequent reinvestigation of this chemistry, Wandelt and co-workers using TPD, UPS and HREELS confirmed this result and found benzene physisorbed on both alloys [45]. They gave evidence for the adsorption of benzene exclusively on atop sites on Pt(lll) and proposed that mixed domains complicated the earlier work [39], which enable benzene desorption at temperatures above 200 K. 

Thus, the reaction on Pd/Fsc is rate limited in the last step, the conversion of (C4H4)(C2H2) into CeHe- This is different from the Pd(lll) surface where the rate determining step for the reaction is benzene desorption. The calculations are consistent with the experimental data. In fact, on Pdi/Fsc, the computed barrier of 0.98 eV corresponds to a desorption temperature of about 300K, as experimentally observed (Fig. 1.100). On Pd (111) surfaces, the bonding of benzene is estimated to be 1.9eV. This binding is consistent with a desorption temperature of 500 K as observed for a low coverage of CeHe on Pd(lll) [489],... 

The rate constants for bond breakage and bond formation, fcs and ki, are each assumed to be much smaller than, the rate constant for benzene desorption. Since fcs is much smaller than fcs, the rate constant for cumene desorption (4), it is reasonable to expect that fcs is also much less than fco, the rate constant for the desorption of benzene, a weak adsorbent. In addition, we know from scheme (I) that... 

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