Pathways for Carboxylic Acid Decomposition on Transition Metal Oxides

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Abstract
The concept of structure sensitivity is well established for reactions catalyzed by metals as it has been generally demonstrated by the use of supported metal catalysts exhibiting different particle size [l-71. The con-cept of structure sensitivity in catalysis by metal oxides is considerably less well developed than in catalysis by metals, in spite of the growing number of examples of such reactions. Characterization of oxide catalyst is generally more problematical than that of metal; it is difficult, for example, to titrate the active surface areas of supported oxides by chemisorption techniques. Carboxylic acid decomposition could be used as a probe to establish struc-tural dependence and selectivity on metal oxides. For example, in the case of formic acid decomposition, bimolecular decomposition of two adsorbed formates occurs on a surface with Ti4+ cation of fourfold oxygen coordi-znation while unimolecular decomposition occurs in the case of formates adsorbed on Ti4+ fivefold coordinated cations [8]. Similarly, acetic acid adsorbed on Ti02 with Ti0,(011) leads to the production of ketene by unimolecular dehydration whereas on the Ti02(l 14) faceted surface, bi-molecular reaction of surface acetate takes place to form acetone [9, 101. Thus understanding the pathway of decomposition of carboxylic acid is likely not only to throw light on the new catalyst search for ketene formation but also to establish the use of this molecular decomposition model as a probe for the study of the structural property of metal oxide catalysts.