Transient and isotopic infrared study of adsorbed species on heterogeneous catalysts.

摘要

Heterogeneous catalysis involves adsorption, surface reaction, and desorption. Understanding the reaction mechanism can lead to design of more effective catalyst systems. Elucidation of the reaction mechanism requires determination of the structure of the surface intermediates and their behavior under different conditions (temperature, feed composition, etc.). We have utilized a technique that couples in situ infrared (IR) spectroscopy with mass spectrometry (MS) in the study of redox and acid-base type reactions. This technique allows the surface of the catalyst to be studied under reaction conditions, which generally leads to a unique perspective. We have chosen CO₂ reforming of CH₄ as a model reaction of the redox type due to its importance in production of synthesis gas. Pyridine adsorption onto sulfated zirconia was chosen as a model acid-base reaction due to its unique acid properties.; The CO₂-CH₄ reaction over Rh/Al₂O ₃ and Rh/CeO₂ was studied with pulse and step transient techniques. Linear CO was found to be the major species on Rh/Al₂O₃ during the reaction by IR; its accumulation on Rh0 sites revealed that the surface of Rh crystallites on Al₂O₃ remained in a reduced state throughout the study. Steady-state isotopic transient studies at 773 K and 0.1 MPa showed that the formation of gaseous 13CO closely followed that of adsorbed 13CO, indicating that linear CO is an active adsorbate. Pulsing CH₄ into CO₂ flow showed that the formation of H₂ led that of CO, revealing that the first step of the reaction sequence is the decomposition of CH₄ into *CH_x species and hydrogen. Hydrogen activated CO₂ to produce linear CO. O₂ pulsing into CO₂/CH₄ over Rh/Al₂O₃ resulted in: (i) total oxidation of CH₄ to CO₂ and H₂O and then (ii) a net increase in the formation of the desired products, CO/H₂, at a ratio of 1:1, revealing promotion of the CO₂-CH₄ reaction. O₂ pulsing into CO₂/CH₄ flow over Rh/CeO ₂ led to only total oxidation, revealing that the catalyst is inactive for partial oxidation. A reaction mechanism is proposed and discussed.; The dynamic behavior of adsorbed pyridine and its interaction with the surface of sulfated zirconia and Pt-sulfated zirconia was elucidated by IR/MS. IR analysis confirmed that pretreating SZ in flowing He led to the formation of S=O species. Pyridine adsorption caused desorption of sulfur in the form of SO₃. IR and MS analyses coupled with a temperature-programmed desorption (TPD) study confirmed that pyridine adsorbed on the Lewis acid sites was oxidized to CO₂ while the pyridine-Brønsted acid site complexes showed little desorption or oxidation. Addition of Pt onto sulfated zirconia led to enhanced Brønsted acidity when treated with H₂; higher loading of Pt led to decreased thermal stability of the sulfate group, promoting desorption of SO₂ during the TPD.