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Unpromoted and manganese-promoted zeolite Y supported rhodium catalysts were tested for CO hydrogenation at 1 MPa and 250°C. The conditions of catalyst preparation by ion exchange, programmed calcination, programmed reduction and neutralization of acid sites were varied systematically in order to compare the promoter effects of different species of manganese. It is found that only MnO particles, and not Mn 2+ ions, promote the Rh-catalyzed formation of the abundant oxygenates ethanol and ethyl acetate. No reduction in Mn 0 or Rh-Mn alloy formation takes place. Temperature-programmed reduction profiles of Mn/NaY and RhMn/NaY show that reduction of MnO 2 to MnO is dramatically enhanced by Rh, indicating that in the reduced catalyst Rh clusters and MnO clusters are in physical contact. Direct interaction between Rh and MnO also follows from EXAFS data which show that the Rh-Rh coordination number is much lower in NaY when MnO particles are present. TPD results indicate that MnO affects the propensity of Rh to chemisorb hydrogen. Tentative reaction mechanisms for CO hydrogenation are considered, consistent with the experimental findings. |
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