Sintering, noble metals

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. 

Additives such as rare earth or noble metals are generally introduced into industrial cobalt FTS catalysts as structural or reduction promoters.92 The addition of various promoters to cobalt catalysts has also been shown to decrease the amount of carbon produced during the FTS.84 87 93 94 Also, the addition of promoter elements may decrease the temperature of regeneration, preventing the possible sintering of supported cobalt particles during such treatments.92... 

Non-noble metal molybdenum containing carbonyls were studied for ORR activity.189-191 Mox(CO) and Mo,Sr(C 0) were studied side by side.189 190 These high-nuclearity carbonyl compounds were prepared by sintering. It was found that the Mox(CO) was amorphous by XRD, however, as S was added, a polycrystalline/ amorphous combination was formed in the catalyst. While neither Mox(CO) or Mo SyCO), compounds were found to be very active for ORR, the addition of S to Mox(CO) appeared to nullify any activity towards ORR.189 190 MoxSeJ,(CO) was produced by both screen printing and sintering191 as well as chemical synthesis.192 The screen printed catalysts were more active than the sintered catalysts, but as the amount of Se increased, the ORR activity decreased.191 192... 

SMSI is also thought to affect methanation catalysts (normally transition metal or noble metals supported on alumina), which are used in the producton of substitute natural gas (SNG). In general, heating in H2 causes sintering on alumina and silica supports and heating in O2 or steam can cause dispersion and particle coalescence at 200 °C (Rukenstein and Lee 1984,1987, Nakayama et al 1984). The data have been based on ex situ EM studies. Here EM methods, especially under dynamic reaction conditions, can provide a wealth of new insights into metal-support interactions under reaction conditions. 

Effect of Noble Metal Addition to Ni-Based Catalysts. Unmodified Ni-based catalysts tend to deactivate rapidly due to both carbon deposition and sintering. As suggested by the thermodynamic analysis above, carbon formation can be significantly limited by the addition of steam, which can react with carbon by the reverse of reaction (8). Figure 12 shows typical results for such a catalyst, which contained 5.4 mg carbon per gram of catalyst after 54 h on stream. 

Continuous exposure of catalysts to high temperatures may cause an alteration in its components and gradually lead to its deactivation. Thermal degradation may have an undesirable impact on both the catalyst substrate and noble metal load in various ways. Thermal degradation covers two phenomena sintering and solid-state transformation. 

Recent evaluations of S02 oxidation over noble metal catalysts (Pt, Pd, and Rh) have given some information on one particular secondary reaction. It was observed in car tests that S03 formation under the conditions of automobile exhaust is highly vulnerable to catalyst deactivation either by thermal sintering or by poisoning (78, 79). At the same time, the data indicated a lesser sensitivity of CO and hydrocarbon oxidation to catalyst aging. The results were confirmed in laboratory experiments (80). This is one example of preferential suppression of an undesirable side reaction. Obviously, the importance of a given poison on the different secondary reactions will vary widely with catalyst formulation and operating conditions. 

Miyoshi, K., Kamegaya, Y. and Matsumura, M. (2004a) Electrochemical reduction of organohalo-gen compound by noble metal sintered electrode. Chemosphere 56, 187-193. 

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