Redox properties of CeO2 and Pt-Rh/CeO2 studied by TAP method

Abstract

Redox properties of CeO 2 and Pt-Rh/CeO 2 were studied by temporal analysis of products (TAP) method using alternative pulses of CO and O 2 . A portion of pulsed CO was oxidized to CO 2 and a portion of CO was adsorbed on the surface. Pulsing 18 O 2 onto the catalyst which has surface species derived from CO, evolved CO 2 contained no 18 O suggesting that the surface species will be carbonate ions.KEY WORDS: redox property; ceria; temporal-analysis-of-products; noble metal; promotion effect.Ceria has been widely used as a catalyst support for three-way catalysts in order to enlarge the efficient operating air/fuel (A/F) window. Fast redox character of CeO 2 is expected to work as an oxygen reservoir [1,2] usually; TPR and TPD are used for evaluation of oxidation activity and oxygen storage capacity of catalysts. However, it takes several 10 min to obtain a spectrum and data contains surface and bulk information. The data is sometimes far from the actual situation of catalysis. Temporal analysis of products (TAP) reactor system, an important tool for investigating 'gas-solid' reactions, adopts shots of different gases with a very short interval. Therefore, TAP analysis would be helpful to analyze catalytic reaction on metal oxides. CeO 2 was provided by (Anan Chem. Co., Japan). The loading of Pt and Rh for Pt-Rh/CeO 2 are 0.19 wt% and 0.56 wt%, respectively. The Pt-Rh/SiO 2 was also prepared with same loading amount of Pt and Rh (SiO 2 , Degussa, Aerosil, OX-50). CeO 2 and Pt-Rh/CeO 2 are confirmed to be cubic fluorite-type structure. The BET surface specific areas (SSA) of CeO 2 , Pt-Rh/CeO 2 and Pt-Rh/SiO 2 are 118, 116 and 50 m 2 /g, respectively.TAP reactor system was used for CO-O 2 pulse reaction. Four solenoid-operating gas-valves are arranged to supply a small amount of gas pulse. A quartz tube reactor with an internal diameter of 4 mm is used. Samples were pressed, crushed, and sieved with the size of 32-60 mesh. About 50 mg of a sample was packed in the reactor located in the center of an electrical furnace. The reactor was evacuated to 10 )2 $ 10 )3 Pa by a rotary pump. A capillary tube, of which one end is located just after the catalyst bed, is used for introducing gaseous products into a mass spectrometer. The chamber of mass spectrometer was kept high vacuum ($10 )6 Pa) by a turbo-molecular pump. Prior to measurements, the catalysts are evacuated at 500°C for 3 h and then fully oxidized by O 2 pulses. The accurate amount of each pulse is determined by measuring the pressure of gases in a vessel with known volume after injecting 50 $ 100 pulses with a high accuracy pressure gauge. And the accurate amount of CO (and O 2 ) consumed and CO 2 formed upon each pulse are determined by mass spectrometer. Peaks of CO (m/e = 28), O 2 (m/e = 32), and CO 2 (m/e = 44) are monitored. The relative sensitivity of CO, O 2 and CO 2 are calibrated using each gas. Since CO 2 gave m/e = 28 peak as a fragment peak, which is 11% of the parent peak area of m/e = 44, the amount of CO was corrected by this amount.Approaching...

Oxidation State of Ce in CeO2-Promoted Rh/Al2O3Catalysts during Methane Steam Reforming: H2O Activation and Alumina Stabilization

“…After cooling, a higher fraction of Ce 3+ reoxidizes, which indicates that a higher fraction of cerium is able to do a redox than actually occurs under the influence of water at high temperatures. The oxidation of CeO 2– x by steam is endothermic and was reported to occur at mild temperatures (≈ 500 K). , It is facilitated in the presence of noble metals. ,, Kundakovic et al found that CeO 2– x activates H 2 O and gets oxidized upon H 2 desorption. H 2 desorption starts at 500 K, and it is facilitated by rhodium, but the water activation only depends on the cerium oxidation state.…”

Reactive removal of surface oxygen by H2, CO and CO/H2 on a Au/CeO2 catalyst and its relevance to the preferential CO oxidation (PROX) and reverse water gas shift (RWGS) reaction

“…This O 2 -induced release of CO 2 has been suggested to be a ceria-specific process. 58 The extent was closely related to the reducibility of CeO 2 -based materials, 8,19,[59][60][61] where the latter was shown to depend on the surface area of the materials used as well as on the chemical composition when using mixed oxides, e.g., Ce x Zr 1−x O 2 or doped oxides. 8 The surface oxygen vacancies on ceria-based catalysts were demonstrated to play an important role in stabilizing the CO 2 -containing reaction intermediate (carbonate) species formed during CO oxidation or CO 2 adsorption, 62 and similar findings were reported also in our previous TAP study.…”