Dipl. Ing. Matthias Mayerhofer Technische Universität München ...

42 Precious Metal Catalysts
The effectiveness of the catalysts for the decomposition of different compounds was ethylene>methane>ammonia
(Rönkkönen,2011b). Hence, they are more effective in decomposing
higher hydrocarbons. It was observed though, that this decomposition is evident at high temperatures
mainly 800-900⁰C. The amount of benzene though, after the catalytic bed increased, for Pd,
Ru and Pt use of catalysts (Rönkkönen,2010), but at the temperatures from 600 to 800⁰C, but at
900⁰C it decreased. The gas composition had the less amount of benzene with the use of Rh catalyst
at 900⁰C (300 mg/m 3 N) (Rönkkönen,2011b).
Deactivation:
The main reasons for deactivation of the precious metal catalysts are sulfur poisoning, coke formation
and particle size growth. However, the decrease of their performance is evident after long
hours on stream. After 6 hours on stream the catalysts started to show a decrease in their activity
due to sulfur poisoning. So they were flushed overnight with N2, to restore their performance and
after that the tar conversion was even higher than before (Rönkkönen,2010).
As for the coke formation, after the 10 hours stability test Pd was the catalyst with very clear carbon
deposition on its surface. The catalysts showed deactivation due to coke in the following order
Pd>Ru>Rh>Pt= none. It is evident that the Rh shows quite good tolerance to carbon deposition
(Rönkkönen,2011b).
Results:
Generally, the precious metal catalysts are more tolerant to sulfur poisoning compared to nickel
based catalysts which rapidly deactivate at temperatures below 900 ⁰C (Rönkkönen,2011b)The
most promising catalyst for adequate tar conversion was Rh/m- ZrO2, above 800⁰C
(Rönkkönen,2010), (Rönkkönen,2011b), (Rönkkönen,2011a). However, the only catalyst that had
the ability to decompose ammonia in sulfur containing environment was Ru/m- ZrO2. The Pt and Pd
catalysts showed similar activity over 95% for very high temperatures, but below 900 ⁰C the Pt
catalyst was less effective than Pd in decomposition of naphthalene, toluene and aromatic hydrocarbons.
The minimum conversion occurred at 800 ⁰C and it was 50% for the Pd and 25% for the
Pt catalyst (Rönkkönen,2011b). In addition, Rh, Ru,Ni had very low conversion below 800⁰C, but
the total hydrocarbon conversion for 900⁰C was 98%, 70% and 91% respectively.
The metal loading may slightly influence the activity of the catalyst (Rönkkönen,2011b). Increasing
the amount of Rh from 0.5 to 5wt% did not improve its performance (Rönkkönen,2011a). The most
promising amount proved to be 0.5wt% metal loading. However, the particle size of the metal may
have a more severe influence on the activity of the catalyst particularly at low time on stream. It
may influence also the formation of carbon.
Although the sulfur content poisoned the active site of the catalysts, a promoting effect of its presence
was observed regarding naphthalene oxidation. At low temperatures (600-700 ⁰C) the addition
of sulfur selectively enhances the conversion of naphthalene for the Pt catalyst but it poisons
the Rh one.
The conversion increases with the increase of temperature in all catalysts. Metal additions of modified
zirconium for temperatures