Heats of adsorption of the linear and bridged CO species on a Ni/Al_2O_3 catalyst by using the AEIR method

摘要

The heats of adsorption of the adsorbed CO species on Ni supported catalysts are rarely studied by using classical analytical methods (i.e.microcalorimetry and temperature programmed desorption (TPD)) due to experimental difficulties linked to different parallel surface reactions to the adsorption/desorption processes.In the present study,the heats of adsorption of the linear and bridged CO species (denoted L and B CO,respectively) on a reduced 20% Ni/Al_2O_3 catalyst are studied by using the adsorption equilibrium infra red method developed previously.After a stabilization procedure of the catalyst,the evolutions of the IR bands of the L and B CO species:2053 and 1927 cm~(-1),respectively,at T_a=500 K and P_a=1 kPa,with the adsorption temperature T_a are studied at a constant adsorption pressure P_a.These data provide the evolutions of the coverages theta_L=f(T_a) and theta_B=f(T_a) of the two adsorbed CO species with the increase in T_a in isobar conditions.These curves allow us to determine the heats of adsorption EL_(theta L) and EL_(theta B) of the L and B CO species for several values of the coverage theta_L and theta_B according to an adsorption model,i.e.EL_1=100,EL_0=153,EB_1=106 and EB_0=147 kJ/mol at coverages 1 and 0,respectively.These values are consistent with literature data on Ni single crystals.In particular,it is shown that the mathematical formalism of the AEIR method can be applied to literature data on Ni(l 1 1) providing ECO_(theta) values similar to those of the present study showing the limited impact of the Ni particle size on the CO adsorption properties.The presence of hydrogen (2% CO/10% H_2/He) during the measurement decreases slightly the coverages of the two CO species at high temperatures due to either a decrease in the heats of adsorption (i.e.EL_1=97,EL_O=131,EB_1=89 and EB_0=138 kJ/mol) or the perturbation of the adsorption equilibrium by hydrogenation of the adsorbed CO species.