Reaction-induced restructuring

However, we have found that reaction induced restructuring of Bi2FeMo20]2 produces a narked enhancement in catalytic behavior (Table II . X-ray diffraction, in-situ Raman spectroscopy, and photoelectron spectroscopy reveal that the restructuring produces a multiphase system consisting of unreacted Bi FeMo20.2, in combination with Bi2MOj0.2, 0-FeHoO and a small amount of Bi2MoO. The key features of the Raman spectra of the activated catalyst are summarized in Table III. 

Experimental Case Studies with Nanofabricated Model Catalysts Catalytic Reactions and Reaction-Induced Restructuring... 

Here, we describe several approaches to study catalytic surface reactions (primarily CO oxidation) on lithographically fabricated model catalysts at atmospheric pressures, in UHV, and in electrocatalysis. In addition, we discuss reaction-induced restructuring of these lithographically fabricated samples. 

Fig. 4.41. EM images of the before (insets) and after reaction 40-rnn Pt/ceria and Pt/alumina model catalysts, (a) Pt/ceria after 5h in / = Pco/ Pco + P02) = 0.67 (stoichiometric) at 673K (b) Pt/ceria after 5h in / = 0.1 (O-rich) at 673K (c) 40-nm Pt/alumina after 5h in / = 0.66 at 673 K. It is seen that on Pt/ceria the prolonged stoichiometric CO oxidation (/ = 0.67) leads to pronounced restructuring, with a height-to-width ratio much larger than expected from thermodynamics (Wulff shape). On Pt/AbOa, the reaction induces a reshaping that approaches the thermodynamically predicted H/W-iaXio of 0.6 (from [98])...

Ad.sorplion-induced restructuring can occur on the chemisorption time scale (isf 10 see for charge transfer or — lO" see for vibrational times). There is evidence, however, (hat adsorbate-induced restmeturing can occur on the lime scale of catalytic reactions (seconds). CO oxidation to CO or ammonia reacting with NO to produce nnd H O show osciliaiiiry behavior under certain cireumsiance.s of temperature and renctant partial pressures. The rcuction rate alternates periodically bc-... 

Adsorbate induced restructuring can also explain some of the important properties of catalytic reactions. The classification of... 

The effects of water-vapor and ammonia pretreatment on the initial rate of ammonia synthesis over Fe, Al O /Fe, and K/Al O /Fe surfaces can be summarized as follows. The presence of aluminum oxide promotes the restructuring of iron during the water-vapor pretreatment, but it inhibits the ammonia-induced restructuring. The presence of potassium shows no effect in the ammonia pretreatment and it inhibits water-vapor-induced restructuring of iron. These results suggest that to form the most active ammonia synthesis catalyst, the iron should first be restructured in ammonia before aluminum oxide is added. After aluminum oxide is added the surface should be treated in water vapor, and finally potassium should be added to serve as a promoter at high ammonia synthesis reaction conversions. 

---