Rhodium sintering

In contrast, when Claus salt ( [Rh(NH ) Cl ]C1 ) is used as the precursor for rhodium, the initial decomposition product upon calcination is rhodium metal which retains a relatively low particle size (Fig. 12(B)). As the temperature is increased rhodium is converted to rhodium (III) oxide and particle growth increases markedly. Thus, rhodium sinters as the oxide and a parallel, although not entirely coincident, increase occurs in CO oxidation light-off temperature. 

The rhodium dispersion becomes progressively worse on the higher temperature and, therefore, lower surface area alumina phases, NO uptake also falls sharply as the ageing temperature of each Rh/A1 0 phase is increased. The lower NO uptake can be explained partially by rhodium sintering (as the oxide) and also by a metal support interaction (Ref. 36). The interaction is less for the high temperature, less reactive alumina phases but even here NO absorption is not measurable after ageing at 850 C. The rhodium/alumina interaction is also observed when temperature programmed reduction (TPR) is performed (Fig. 14(A) and (B). 

A TWC catalyst must be able to partition enough CO present in the exhaust for each of these reactions and provide a surface that has preference for NO adsorption. Rhodium has a slight preference for NO adsorption rather than O2 adsorption Pt prefers O2. Rh also does not cataly2e the unwanted NH reaction as does Pt, and Rh is more sinter-resistant than Pt (6). However, the concentrations of O2 and NO have to be balanced for the preferred maximum reduction of NO and oxidation of CO. This occurs at approximately the stoichiometric point with just enough oxidants (O2 and NO ) and reductants (CO, HC, and H2). If the mixture is too rich there is not enough O2 and no matter how active the catalyst, some CO and HC is not converted. If the mixture is too lean, there is too much O2 and the NO caimot effectively compete for the catalyst sites (53—58). 

Nonselective catalytic reduction systems are often referred to as three-way conversions. These systems reduce NO, unbumed hydrocarbon, and CO simultaneously. In the presence of the catalyst, the NO are reduced by the CO resulting in N2 and CO2 (37). A mixture of platinum and rhodium has been generally used to promote this reaction (37). It has also been reported that a catalyst using palladium has been used in this appHcation (1). The catalyst operation temperature limits are 350 to 800°C, and 425 to 650°C are the most desirable. Temperatures above 800°C result in catalyst sintering (37). Automotive exhaust control systems are generally NSCR systems, often shortened to NCR. 

The ideal operating temperatures for the three-way catalyst lie between 350 and 650 °C. After a cold start it takes at least a minute to reach this temperature, implying that most CO and hydrocarbons emission takes place directly after the start. Temperatures above 800 °C should be avoided to prevent sintering of the noble metals and dissolution of rhodium in the support. 

Rhodium(III) chloride trihydrate (2 g.) is dissolved in 70 ml. of ethanol (95%) in a 500-ml. round-bottomed flask fitted with gas inlet tube, reflux condenser, and gas exit bubbler. A solution of 12 g. of triphenylphosphine (freshly crystallized from ethanol to remove triphenylphosphine oxide) in 350 ml. of hot ethanol is added and the flask purged with nitrogen. The solution is refluxed for about 2 hours, and the crystalline product which forms is collected from the hot solution on a Buchner funnel or sintered-glass filter. The product is washed with small portions of 50 ml. of anhydrous ether yield 6.25 g. (88% based on Rh). This crystalline product is deep red in color. 

In addition to palladium, the catalysts used commercially always contain alkali salts, preferably potassium acetate. Additional activators include gold, cadmium, platinum, rhodium, barium, while supports such as silica, alumina, aluminosilicates or carbon are used. The catalysts remain in operation for several years but undergo deactivation. The drop in activity is due to a gradual sintering of the palladium particles which causes the catalytically active area to decrease progressively. Under reaction conditions potassium acetate is slowly lost from the catalyst and must continuously be replaced. 

While the use of a support or carrier stabilizes small metal crystallites against growth and surface area loss, reactions of support and inetal may occur, especially if the catalyst is subjected to elevated temperatures The main factors affecting these phenomena are temperature, time of exposure and atmosphere. Most previously reported sintering studies were for supported platinum and particularly for Pt/Al20 catalysts (1,2). Considerably less information is available on the sintering of other supported metal catalysts (3,4), and thus there is very little information on the behavior of biiDetallic catalyst systems such as supported platinum-rhodium. 

Table 20. Sintering behavior of platinum, palladium and rhodium as a function of the aging atmosphere (washcoat La203-doped AI2O3, precious metal content 0.14wt.%). Reprinted from refs. [50, 51] with kind permission of Elsevier Science.

---