A Langmuir-Hinshelwood model for a hydrogen transfer mechanism in the selective hydrogenation of acetylene over a Pd/gamma-Al2O3 catalyst prepared bythe sol-gel method

This study represents an effort to fit a Langmuir-Hinshelwood-type mechanis m which involves a proposed hydrogen transfer step from a carbonaceous depo sit with observed rate data, spectroscopic data, and reported literature re sults on a supported Pd/gamma -Al2O3 catalyst prepared by the sol-gel metho d. Our data was taken in a differential reactor as the temperature was vari ed from 100 to 225 degreesC in six increments. A change in the apparent rea ctant orders and activation energies support a multiple mechanism rate expr ession. Our data correlates best with an expression composed of two mechani sms: a competitive, low-temperature mechanism (mechanism I) with an activat ion energy of 29 kJ/mol, and a non-competitive, high-temperature mechanism (mechanism II) with an activation energy of 168 kJ/mol. Both mechanisms wer e fit using an enthalpy of adsorption of -124 kJ/mol for acetylene. The ent halpy of the hydrogen transfer was found to be 133 kJ/mol, indicating an en dothermic process. Mechanism II is predicted to emerge and dominate at high temperatures. (C) 2001 Elsevier Science BN. All rights reserved.