Pulse reactions

In order to study the mechanism of POM reaction, a series of pulse reactions were performed. The results of CH4 pulse reaction on the fi edi LiNiLa0x/Al203 sample were shown in Fig.9. The products of the CH4 pulse reaction were H2, CO and CO2. With pulse... 

Fig.9 Results of CH4 pulse reaction on the LiNiLa0x/Al203 catalyst at 1123K...

To distinguish the different actions of different oxygen species over the LiNiLa0x/Al203 on the CO selectivity, the sanq)le was pretreated by 5% H2/Ar flow at 1123K for 0.5 hr. After the pretreatment, the NiO was reduced to the reduced nickel and the surface adsorbed oxygen species was conqrletely consumed, and then the CH4 pulse reaction was performed. The results are different from the results of CH4 pulse reaction on the fresh sample. The... 

The question remaining now to be addressed is the role of reactants and catalyst phases in the network of reactions leading to CO, Hj and carbon. For that purpose pulse reaction/titration experiments using the individual reactants mostly in sequence were applied. 

The use of IR pulse technique was reported for the first time around the year 2000 in order to study a catalytic reaction by transient mode [126-131], A little amount of reactant can be quickly added on the continuous flow using an injection loop and then introduce a transient perturbation to the system. Figure 4.10 illustrates the experimental system used for transient pulse reaction. It generally consists in (1) the gas flow system with mass flow controllers, (2) the six-ports valve with the injection loop, (3) the in situ IR reactor cell with self-supporting catalyst wafer, (4) the analysis section with a FTIR spectrometer for recording spectra of adsorbed species and (5) a quadruple MS for the gas analysis of reactants and products. 

Figure 4.10. Scheme of the experimental system used for transient pulse reaction [126],... 

The bands in 1580 - 1983 cm 1, which are not observed in the gaseous PO, could be due to the adsorbed C3 oxygenates produced from adsorbed PO. The spectra of adsorbed PO observed in the 800 - 2000 cm"1 during TPD resembled those of the pulse reaction studies in Fig. 1, suggesting that adsorbed PO from gaseous PO may have the same structure as the adsorbed epoxidation intermediate. 

Pulse reaction studies of gallium modified H-ZSM5 catalysts with propane... 

Keywords Pulse Reactions, Propane, Zeolites, Gallium Modification, Cyclar Process. 

Figure 1. - Pulse reaction studies results. Left Propane pmol converted per gram of catalyst. Right Hydrogen pmol produced per gram of catalyst. Squares - catalyst with 2 wt%. of gallium. Circles - catalyst with 3 wt.% of gallium.

EXAFS analysis for the sample after the fifth pulse reaction of benzene and O2 revealed the formation of Re monomers with Re=0 bonds (CN = 3.7 0.2) at 0.173 O.OOlnm and Re-O bond (CN = 1.3 0.6) at 0.211 0.002nm (Table 10.7). The monomeric structure (vi) in Table 10.7 was similar to that after the steady state reaction (i). The Re monomers (vi) were transformed into the Rejo clusters again by NH3 treatment for 2h. NH3 has two roles, N supplier and reduc-tant, in producing the catalytically active N-interstitial Re cluster, which converts benzene and O2 into phenol with a selectivity of 93.9%, accompanied with oxidation of the cluster to the inactive Re monomer (Scheme 10.4). Thus, the formation of the N-interstitial Rejo clusters and the decomposition of the Rejo clusters to the Re monomers are balanced under the steady-state reaction conditions [73]. 

For the 8.2 V/nm sample, the products observed for the pulse reaction at 400°C consisted of only dehydrogenation products (butenes and butadiene) and carbon oxides. No oxygenates were observed, and the carbon balance for each pulse was satisfied within experimental error. The selectivity for dehydrogenation is shown in Fig. 3a as a function of 0. It shows that the selectivity was very low when the catalyst was in a nearly fully oxidized state, but increased rapidly when the catalyst was reduced beyond 0 = 0.15. It should be noted that the dependence of selectivity for dehydrogenation on 0 shown in the figure was not... 

While preadsorbed oxygen has no effect, the presence of gaseous oxygen drastically changes the product distribution in the thermal desorption and pulse reaction of butene on a-Fe203, as can be seen from results shown in Table VI (6). On this oxide, thermal desorption in an 02 instead of an He carrier results in a much lower yield of hydrocarbons and a much higher yield of C02, The same is observed in pulse reactions. Thus, on a-Fe203, adsorbed butene, adsorbed butadiene, and/or butadiene precursors must be very... 

In 10 7 molecules m 2 for thermal desorption, in 10 7 molecules in pulse reaction. 

Table 2.5 Catalytic performance of pulse reactions on the CVD-Re/ HZSM-5 (Si02/AI203 = 19) catalyst (Re 0.58 wt%) for direct phenol synthesis from benzene and 02 at 553 K in the absence of NH3a .

Re monomer was observed after the reaction. This structure-reactivity gap can be explained by the difference in rates between the reduction and oxidation processes of Re species. Oxygen not only oxidizes benzene to phenol but also oxidizes the active Re10 clusters (4) to the inactive Re04 monomers ((5), Figure 2.7) competitively. Indeed, the amount of phenol molecules produced in the pulse reactions was estimated to be 3% of the amount of Re10 clusters (4) [170], Under steady-state conditions in the presence of NH3, the population of the active Re10 clusters (4) was only 3.7% of the supported Re species, which cannot be captured even by in situ EXAFS under the reaction conditions. 

Finally Miyamoto et al. again confirmed this mechanism in a kinetic study using the pulse reaction technique.114... 

Fig. 2. Reaction of 2-propanol to propylene over Fe 03-1 treated with 0.5 M HiS()4 ( ) or (NH4LSO4 (O) and calcined at various temperatures. Pulse reaction conditions He carrier 30 cm/min, pulse size 0.4 p.1 (liquid), catalyst 30 mg, temperature 170°C.

Both catalysts were active for the skeletal isomerizations of butane and isobutane at room temperature (130). The reaction of butane was carried out under pulse reaction conditions the catalytic activities were shown as a function of calcination temperature of the catalyst (Fig. 4) (129). Butane was converted to isobutane and propane, and the catalytic activity of Ti02-I was higher than that of TiOrlI. The maximum activity was observed with calcination at 525°C. The quantity of S was estimated to be 2.11 and 0.01% for the TiOrI catalyst calcined at 525 and 650°C, respectively (56). The activity enhancement of Ti02 by an addition of ammonium sulfate was also reported by Tanabe et at. (137). 

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