abstracts - Институт катализа им. Г.К. Борескова

OP-III-11Experimentally, the enthalpy of CO-OC reorientation ΔН° could be found from the slopeof van’t Hoff plot – dependence of logarithm of equilibrium constant, expressed via theobserved intensity ratio A HF / А LF of the high-frequency (HF) and the low-frequency (LF)bands, on the reciprocal temperature:ln К = ln[(A HF /А LF ) (ε LF /ε HF )] = ln(A HF / А LF )- ln(ε HF /ε LF ) = -ΔН°/RТ + ΔS°/RExtrapolation of ln(A HF / А LF ) dependence to 1/T=0 gives the ln(ε HF /ε LF ) + ΔS°/R, and ifthe value of entropy of CO-OC isomerization ΔS° can be evaluated, we can estimate the ratioof extinction coefficients ε HF /ε LF .The thermodynamic parameters can be obtained for certain spectroscopically identifiedsurface species from the lnp plot against 1/T for the set of pressure and temperature valuescorresponding to certain integral intensity of IR band. The slope of the plot gives us theisosteric heat of adsorption, while extrapolation to 1/T=0 gives the entropy of adsorption ΔS a .The results of such analysis of the variable temperature spectroscopy data for COadsorbed on several adsorbents are shown in the table.Adsorbent ν, cm -1 ΔS a , J/mol . K ΔH a ,kJ/mol ΔS a /ΔH aSiO 2 2138 -47 -11 4,3Si-OH … CO 2154 -64 -12 5,1CsX 2124 -71 -25 2,8H- OH … CO 2173 -105 -22 4,7Y OH … OC 2124 -58 -13 4,5In the spectra of aerosil pretreated at 870K the band of H-bonded CO at 2154 cm -1predominates and can be easily separated from that of “physisorbed” molecules at 2138 cm -1 ,whose temperature dependence was studied for methylated aerosil sample with low OHconcentration. This band has rotational wings and is supposed to be due to side-on COcomplexes with O atoms of siloxane bridges. Framework oxygen atoms of X zeolites havemuch greater negative charge and we assign CO band at 2124 cm -1 in the spectrum of CsXzeolite to complexes with Si-O-Al or Al-O-Al bridges. Rotational freedom of CO moleculesin side-on complexes with anions should manifest itself in lower values of entropy change onadsorption. In fact, the related values of adsorption entropy ΔS a /ΔH a , given in the rightcolumn, are the lowest for CO on CsX and physisorbed on silica. For the bands of C-bondedand O-bonded CO complexes with the acidic OH groups of HY zeolites (bands at 2173 and2124 cm -1 , respectively), the ΔS a /ΔH a values are almost equal, despite the great difference inadsorption enthalpy. This regularity was used to predict the entropy change on reorientation.Thus, for NaY the van’t Hoff plot gives the reorientation entropy ΔН°= 2.4 kJ mol -1 [3],333