Rhodium, active automotive exhaust

As an introductory example we take one of the key reactions in cleaning automotive exhaust, the catalytic oxidation of CO on the surface of noble metals such as platinum, palladium and rhodium. To describe the process, we will assume that the metal surface consists of active sites, denoted as We define them properly later on. The catalytic reaction cycle begins with the adsorption of CO and O2 on the surface of platinum, whereby the O2 molecule dissociates into two O atoms (X indicates that the atom or molecule is adsorbed on the surface, i.e. bound to the site ) ... 

In the past few years the tudy of morphology changes undergone by supported Rh particles during oxidation / reduction thermal treatments has attracted considerable attention (refs 1-4). These phenomena are of particular importance in the deactivation of automotive exhaust catalysts for which rhodium is the main active Ingredient for the reduction of NO. 

The experiments are complicated by the fact that the activity for a certain experimental condition depends on the conditions applied prior to the experiment. Chemieal changes of the catalyst, such as oxidation and reduction of ceria or rhodium and the appearance and disappearance of crystal surfaces, are thought responsible for the latter observation. The importance of the dependence of the catalyst structure and composition on the experimental conditions was reported previously for automotive exhaust catalysts [16] and is in the present paper referred to as extrinsic relaxation. In order to obtain reproducible experimental data, a so-called standard condition was applied in between two subsequent experiments. The conditions corresponding to a given experiment were applied for only 10 min. Next, the standard condition was applied for at least 30 min. In such a way, the catalyst changes due to... 

A ceramic (cordierite) monolith-type three-way automotive exhaust catalyst was used for laboratory-simulated sintering and activity tests and was characterized at Aliieo-Signa1 Materials Research Center. The catalyst contained platinum and rhodium at a weight ratio of 5 to and a total noble metal loading of approximately 1.1 weight percent based on the weight of the... 

Rhodium interacts with y-Al203 at temperatures above 873 K [1,2], sharply decreasing the amount of rhodium oxide that can be reduced at temperatures below 1073 K and lowering the activity for automotive exhaust reactions. A recent study from this laboratory [3] examined the effectiveness of La for preventing interactions between Rli and Y-AI2O3. The La largely blocks the... 

Platinum and palladium have high activities for total oxidation. This property is exploited in automotive exhaust catalysis. Automobile exhaust contains toxic gases such as CO, NO, and hydrocarbons which contribute to formation of photochemical smog and acid rain. Since 1978, catalysts based on platinum, rhodium, and sometimes palladium, supported on a monolithic carrier, are applied to convert exhaust gases to less harmful products. The so-called threeway catalyst enables the following three overall reactions... 

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