Light-off temperatures

The emission control system for LPG is the same as is used for gasoline fueled engines with the exception of the fuel metering system. No evaporative emission system is required. Both Pt—Rh and Pd—Rh catalysts are good for emission control of LPG fuel exhaust. Pt provides the lowest light off temperature for C Hg. The sulfur content of LPG is also very low so that Pd catalysts perform very well. 

Can one further enhance the performance of this classically promoted Rh catalyst by using electrochemical promotion The promoted Rh catalyst, is, after all, already deposited on YSZ and one can directly examine what additional effect may have the application of an external voltage UWR ( 1 V) and the concomitant supply (+1 V) or removal (-1 V) of O2 to or from the promoted Rh surface. The result is shown in Fig. 2.3 with the curves labeled electrochemical promotion of a promoted catalyst . It is clear that positive potentials, i.e. supply of O2 to the catalyst surface, further enhances its performance. The light-off temperature is further decreased and the selectivity is further enhanced. Why This we will see in subsequent chapters when we examine the effect of catalyst potential UWR on the chemisorptive bond strength of various adsorbates, such as NO, N, CO and O. But the fact is that positive potentials (+1V) can further significantly enhance the performance of an already promoted catalyst. So one can electrochemically promote an already classically promoted catalyst. 

Figure 8.62 shows the effect of temperature and of positive potential application on the reaction rates and on the nitrogen selectivity for the C3H6/N0/02 reaction.67,68 Electrochemical promotion significantly enhances both activity and N2 selectivity (e.g. from 58% to 92% at 350°C) and causes a pronounced (60°C) decrease in the light-off temperature of NO reduction in presence of 02. Positive potentials weaken the Rh=0 bond, decrease the O coverage and thus liberate surface sites for NO adsorption and dissociation. 

In the absence ofH hih light-ofF temperature, T50, for NO is strongly improved on the two o imiz catalysts (Table 2), The light-off temperature becomes close to that of the reference Pt-Rh/Al20j solid. Near T50 the conversions obey the sequence ... 

Let us also notice that i) the changes in the activity sequences with the temperature show the necessity to compare the activities at various temperatures, not only near the light-off temperature ii) since the activity sequences depend also on the mixture, for example on the presence or on the absence of H2O, it is necessary to study complex realistic mixtures. 

Light-off temperatures of CH4 combustion in the presence of different oxidizing compounds... 

In contrast to NaZSM-5 zeolite, introduction of CoZSM-5 or HZSM-5 zeolite in the reaction system shifts the "light-off" temperature and modifies the chemistry now not only NO but Nj is formed. Hence, some intermediate species required for Nj formation must be stabilized on the catalyst surface. The "light-off"temperature shifts observed with CoZSM-5 and HZSM-5 catalysts may result from the enhanced redox capacity provided by these catalysts or from the NOj/NO equilibrium achieved more readily than with NaZSM-5. Moreover, equilibrium is approached at a somewhat lower temperature over CoZSM-5 than HZSM-5, and much lower than with the empty reactor (see Fig. 1 of Ref. lOl.The decomposition reaction of NOj into NO -t- occurs readily on these catalysts and the "light-off" temperature of both combustion and SCR is lower in comparison with that of the homogeneous reaction. 

Cant et al [21] focused their attention on the concentration of N20 in the automotive exhaust gas, which are rather low (14 ppm) but quite dependent on the air-to-fuel ratio. Typically 60-80% of NO is converted into N20 below the light-off temperature on Rh and then the selectivity drops at relatively high temperature 370°C [21,22] when the partial pressures of NO tends below lOTorr [22-25]. 

Previous surface science studies showed that Rh is essential for NOx removal over TWC because NO dissociates more readily on metallic Rh than on Pt and Pd sites [26-28]. Nevertheless, the efficiency of Rh to selectively transform NO into N2 is restricted below the light-off temperature with a predominant formation of N20. Future practical developments are closely related to a better understanding of the formation and on the transformation of N20 over noble metals during the cold start engine. Such an aspect is still challenging over TWC and particularly under lean conditions since the extent in NO conversion is usually significantly lowered. 

In industry, the POX of heavy liquid petroleum feedstocks is carried out without the use of a catalyst at temperatures between 1,250 and 1,500°C and pressures between 25 and 80 atm. In smaller systems, the introduction of a catalyst is necessary for operational stability, and to reduce the operating and light-off temperatures. 

Light-off Temperature of Aged Catalysts Catalyst A Contained 1.5g/dm3 Pt, 0.35g/dm3 Rh with CZ and Catalyst Contained the Same Amount of Precious Metals with ACZ... 

These catalysts were aged under the exhaust gas stream at 900°C for 100 h. Catalyst A contained 1.5 g/dm3 Pt, 0.35 g/dm3 Rh with CZ and Catalyst contained the same amount of precious metals with ACZ. As shown in the table, both the sintering of the precious metals and the CZ particles is inhibited in Catalyst compared with Catalyst A. Table III also shows the light-off temperature of those catalysts. The light-off temperature of Catalyst is about 15°C lower than that of Catalyst A. In addition, the NOx emission of a vehicle equipped with Catalyst was about 20% less than that with Catalyst A. 

The catalyst operation not only above, but also around the light-off temperature has been shown to be significantly affected by washcoat diffusion (cfi, e.g. Hayes et al., 2004, 2005 Koci et al., 2004a, b Kryl et al., 2005 Ramanathan et al., 2003 Tronconi and Beretta, 1999 Zygourakis and Aris,... 

Particular care should be given to the inhibition parameters if the DOC is operated also under temporarily rich conditions (i.e., with NSRC or DPF). The light-off temperature varies significantly for lean and rich exhaust gas and depends also on NOx concentration (cf. Fig. 21 in Section VI on the NSRC kinetics). 

A recent paper published by van der Molen (2) presents several sets of experimental data relating initiator consumption and polymerization conditions. It clearly shows that, in a given reactor, the initiator consumption per unit polymer produced is not a monotonic function of the reaction temperature, but it shows a minimum. These data also show that this minimum depends on the initiator type and on process conditions such as mixing, pressure and input flow-rate. Another phenomenon of interest which also depends on initiator characteristics is the light off temperature, below which the polymerization reaction extinguishes. The experimental data (2) clearly demonstrate that this temperature also depends on initiator type, and the operating conditions. 

Figure 2. Effects of 02 in the feed stream on the light-off temperatures for propane oxidation on the thcnnaUy aged catalysts.(2a reduced samples 2b oxidized samples)...

Sulfated catalyst activity was determined with the S02 free feedstream in the absence of water The light-off temperatures reported in Fig 3a for propene oxidation show that sulfation by SO2 induces the same effects on catalyst activity than SO2 in the feedstream in the course of the oxidation reaction (Fig 1b). Thus Pt-Rh catalyst activity is not affected by sulfation while monometallic platinum catalysts are far less active after sulfur storage with 20 ppm SC>2 We must note also that a small inhibiting effect appears after sulfur storage with 4 ppm SO2. 

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