Lean NOx reduction over Rh/ZrP2O7 catalyst under steady-state and perturbation conditions

Redox Dynamics of Rh Supported on ZrP2O7 and ZrO2 Analyzed by Time-Resolved In Situ Optical Spectroscopy

“…Two catalysts, Rh/ZrP 2 O 7 and Rh/ZrO 2 , showed optical absorption at 650 nm that decreased upon lean-to-rich switching of the gas feed and reversibly increased upon rich-to-lean switching, in accordance with the redox behavior between Rh 3+ and Rh 0 on the surface. Rh/ZrP 2 O 7 exhibited faster reduction of Rh 3+ , but slower reoxidation of Rh 0 , compared with Rh/ZrO 2 , as previously demonstrated by XAFS/XPS studies. , These different behaviors can be rationalized by the activation energies for the reduction and oxidation of Rh in each of the supported catalysts. In addition, the present in situ technique provided information on the Rh oxidation state under fast lean–rich perturbation conditions at different intervals.…”

“…Under real automotive TWC conditions, the A/F ratio fluctuates between high (lean) and low (rich) values at frequencies of ≥1 Hz. , We previously reported that the NO conversion efficiency can be enhanced by applying fast perturbations, which can be simulated by switching two simulated gas feeds with different A/F values at shorter intervals (1 s) . Although the perturbation effect can be rationalized by the stabilization of metallic Rh, this hypothesis needs to be demonstrated via in situ characterization.…”

“…Our previous studies revealed that metallic Rh formed on metal phosphate supports was only slowly oxidized upon exposure to excess O 2 , whereas Rh on oxide supports was readily oxidized to Rh 2 O 3 . − The unusual redox feature of Rh supported on metal phosphates contributed to enhancing the lean NO conversion efficiency (A/F = 15.0). On the contrary, Rh/ZrP 2 O 7 exhibited lower conversions of CO and C 3 H 6 than Rh/ZrO 2 under a rich condition (A/F = 14.1).…”

“…The metallic Rh (Rh 0 ) formed during A/F < 14.6 exhibits high NO conversion, whereas the oxide (Rh 3+ ) formed during A/F > 14.6 is less active. We recently reported that Rh supported on metal phosphates showed a unique characteristic: the Rh oxide was more easily reduced to the active metallic Rh, which was only slowly oxidized compared with Rh supported on metal oxides. − One practical example of importance was that a ZrP 2 O 7 support significantly enhanced the NO conversion efficiency during A/F > 14.6 compared with the ZrO 2 support. , The efficiency was further enhanced by switching the two simulated gas feeds (stoichiometric (A/F = 14.6) and lean (A/F = 14.9)) at short intervals (∼1 s) . Considering the faster reduction of Rh oxide in contrast to the slower reoxidation of Rh on ZrP 2 O 7 , the perturbation effect can be rationalized by assuming the stabilization of metallic Rh species, which is more active than Rh oxide for catalytic NO reduction.…”

Change in the Pd Oxidation State during Light-Off of Three-Way Catalysis Analyzed by in situ Diffuse Reflectance Spectroscopy

“…Unlike many other industrial catalytic processes, however, the TWC is characterized by unsteady-state operation, where the reaction atmosphere and/or temperature fluctuate rapidly. An example of this is the perturbation between fuel-rich and fuel-lean conditions, which is expressed in terms of the air-to-fuel ratio ( A / F ) on a weight basis below and above 14.6, respectively. ,, Precious metals are in their metallic state under rich conditions ( A / F < 14.6), while their oxides tend to form under lean conditions ( A / F > 14.6). ,,− The resulting redox behavior is closely related to the catalytic activity. Another significant feature is the so-called light-off characteristic, which can be assessed by increasing/decreasing the feed temperature, while monitoring the reactor outlet concentration or the fractional conversion efficiency of each gas.…”