Self‐Regenerative Property of Nanocrystalline Ce_0.89M_0.11O_2−y (M=Pd, Rh) Mixed Oxides

Abstract

Self‐regenerative properties of nanocrystalline Ce0.89M0.11O2−y (M=noble metal, e.g., Pd and Rh) mixed oxides were investigated by applying the reduction–oxidation treatment. These oxides were synthesised as chemically and structurally homogeneous powders (4–5 nm) with a large surface area (≈100 m2 g−1) by using the water‐in‐oil microemulsion method. The analysis of the reduced oxides revealed the presence of the crystalline M particles (size: 2 nm for Pd and 1 nm for Rh) that demonstrate a preferred orientation on the substrate—(1 1 1)M||(1 1 1)CeO2. The size and arrangement of the metal crystallites were the same after each reduction cycle. After oxidation treatment, the M particles disappeared but no distinct particles of an oxide phase (e.g., PdO or Rh2O3) could be detected. However, a thin amorphous layer was visible on the surface of Ce0.89Pd0.11O2−y crystallites. Temperature‐programmed reduction of hydrogen and temperature‐programmed oxidation confirmed the complete reversibility of the redox properties of Ce0.89Rh0.11O2−y after the reduction–oxidation treatment but only partial reversibility for Ce0.89Pd0.11O2−y .

Redox and Catalytic Properties of RhxCe1–xO2−δ Solid Solution

“…Reduction at 500 °C should lead to a big particle size in comparison with treatment at 300 °C. 22 The capability of the Rh x Ce 1−x O 2−δ samples to undergo reversible transitions between solid solution and segregated Rh n δ+ clusters allows us to consider these samples to some extent as an intelligent catalyst following the works of Tanaka et al 80,81 and Kurnatowska et al 44 Other catalytic behavior is demonstrated by the inhomogeneous Rh/CeO 2 sample calcined at 1000 °C. In this sample, the major part of rhodium is either in the form of Rh 2 O 3 crystallites or nanosized RhO x species localized in the subsurface region of CeO 2 .…”

“…43 These samples demonstrated reversible redox properties after the reduction−oxidation treatments at 500 °C. 44 This might have a great influence on the catalytic characteristics of the systems. In this regard, it is of interest to study the catalytic properties of Rh x Ce 1−x O 2−δ solid solutions with high rhodium content corresponding to the maximum oxygen mobility.…”

“…Solid solutions provide maximum dispersion of the active component as ionic species over a reducible oxide support, giving the highest degree of metal–support interaction. A number of works have been focused on the analysis of solid solutions of Rh with ceria. ,− The incorporation of rhodium into the subsurface ceria layers, , as well as into the CeO 2 bulk, ,, has been shown. The ionic dispersion of rhodium in the CeO 2 matrix enhances the oxygen storage capacity (OSC) of the system and hinders the sintering of CeO 2 ,, improving the catalytic properties of the system …”

Real‐space observation of strong metal‐support interaction: state‐of‐the‐art and what's the next

“…It is noted that the metals doped CeO 2 systems enable self‐regenerate metals by applying the reduction–oxidation treatment or electron irradiation (Zhang & Theil Kuhn, ; Djinović et al ., ; Kurnatowska et al ., ). Rh can be migrated to the doped CeO 2 surface under reduction while it annihilates into the support with a further oxidation (Kurnatowska et al ., ). This reversibility is quite phenomenologically comparable with the SMSI state.…”

Ceria-Based Catalysts Studied by Near Ambient Pressure X-ray Photoelectron Spectroscopy: A Review

“…For instance, it has been demonstrated that Ce 1−x M x O 2−y mixed oxides (where M = Cr [103,104], Mn [104][105][106], Fe [104,106], Co [104][105][106], Ni [105,106], Cu [107,108], Zr [109][110][111], La [112][113][114]) exhibit lower reduction temperatures than pure ceria. The incorporation of noble metals to ceria to generate Ce 1−x M' x O 2−y mixed oxides (M' = Ru [115], Rh [116,117], Pd [117,118]) has also demonstrated to efficiently promote reducibility of cerium ions and greatly decrease the reduction temperatures too [119]. Ikemoto et al investigated the reversible redox activity of Cr 0.19 Rh 0.06 CeO x by means of NAP-XPS, while its catalytic properties were tested with CO and 1-octanol oxidation reactions [120].…”