SSITKA experiments

In order to understand the oxygen pathways during the reaction, SSITKA experiments involving 2 136) have also proven useful. Some results... 

Many studies have been done on the CO/H2 reaction over rhodium. In particular, SSITKA experiments have been performed at 220"C over 5% Rh/AFOs for a steady conversion of 1.5% 194). After the change from C0/H2 to - C0/H2, the shape of the CO response curve does not agree with that predicted 17) for a fast, continuously equilibrated exchange with the surface CO. For instance, over Ni the reverse exchange rate divided by the net forward rate b) can be assumed to be infinite 17). Using the data over Rh, the data arc best fit with j3 = 2.2 194). From these data and the response of the CH4 curve, it is determined that the coverage of the active carbon, Q, is only about 0.03. From a series of experiments, it has been possible to estimate the surface coverages of C , Q, CO, and a formate species, COOH, the latter present on the support. 

There is interest in the synthesis of methanol from CO/H2 over Pd, and the reaction has been studied over Pd/Si02 and Li+/Pd/Si02 (245). Only SSITKA experiments have been performed, and it was found that the coverage of surface intermediates is increased for the promoted catalyst. Similar studies have focused on other aspects of the system (246-248). 

The ammonia synthesis reaction is one of the most studied and best understood reactions in heterogeneous catalysis, but it has been the subject of few papers involving transient methods. SSITKA experiments have been performed at 350-500°C and 204-513 kPa using a commercial Haldor-Tops0e KMIR catalyst, with iron triply promoted by AI2O3, K2O, and CaO (262). Similar studies using K-promoted Ru/Si02 have also been reported 263). The promoted Ru catalyst is much more active than Ru alone, and new, very active sites are detected on the promoted catalyst. It seems that the analysis of this type of experiment would benefit from the elementary-step approach, as exemphfied by Kao et al. 107) two kinds of sites can be included in such a model. 

As for the abovementioned steady-state experiments, SSITKA experiments were carried out on catalysts pretreated with water vapor. After reduction, the reactor was cooled down to 453 K in flowing He. The temperature was increased to 523 K before introducing a flow of 1.2 g/h of water vapor, mixed with 2.5 ml/min and 35 ml/min He. After 16 h of water treatment, the catalyst was cooled down to 453 K in flowing He. The further procedure was the same as for the dry catalyst. 

Table 3 shows the experimentally obtained values for the CO conversion, the methane selectivity, the number of active surface species reacting to methane and the corresponding surface residence time for all of the SSITKA experiments. 

CH4/CD4 SSITKA experiments were carried out at 1013 K after 40 min on stream in the DRIFT cell. Fig. 3A shows the spectra obtained a) initially under CH4 flow and b) under CD4 flow, 5 min after the switch when the spectrum was not changing anymore significantly. A very large part of the OH bands at 3715 cm and 3583 cm" was shifted to OD bands at 2737 and 2641 cm" when the deuterated methane (C-D bands at 2257 cm" ) was substituted to light methane (C-H bands at 3061 cm ). Note that C-H and C-D dbrations of adsorbed carbonaceous species could be hidden under the relatively large bands of the methane gas phase. 

Thus, TOFchem will always be smaller (often significantly so) than TOFsite, except where the concentration of metal surface atoms equals exactly the concentration of sites. TOFitk, on the other hand, will always be larger unless the concentration of intermediates (carbon-containing intermediates in the case of methanation) is equal to the concentration of sites. The only possibility of TOFitk ever being smaller than TOFsite is when there is significant readsorption of the product. However, usually in SSITKA experiments, care is taken to minimize readsorption by experiment and/or extrapolation [14]. 

Surface site heterogeneity of Ru/Si02 for ammonia synthesis, as well as the effect of K promotion, has been studied using SSITKA.K is a well known activity promoter for ammonia synthesis on Ru and Fe catalysts. The Ru/Si02 catalyst was studied at 673 K, 204 kPa, H2/N2 ratio of 3 and GHSV between 5000-23000. SSITKA experiments were carried out by switching between N2 and N2. 

SSITKA experiments are typically performed in a fixed-bed reactor system schematically presented in Figure 51.1. The system of mass flow controllers (A1-A3) forms a gas mixture with specified chemical composition and flow rate. The second branch of mass flow controllers (B1-B3) also forms a gas mixture with identical chemical composition and flow rate the only distinction is that one of the components of the gas mixture is... 

SSITKA experiments can be performed in plug flow or mixed flow reactors. This approach was proposed by Happel et al. [18] and further developed by Beimett [19], Biloen [20], and Shaimon and Goodwin [21]. In these experiments a step change or pulsed input is induced in the isotopic label of one reactant in the reactant flow. The total concentration of labeled plus non-labeled reactants, adsorbates, and products is maintained at steady state under isothermal and isobaric conditions. The reactor effluent species are then monitored versus time. The mean surface residence time and abundance of adsorbed surfece... 

To gather information for the role of O2 in the H2-SCR mechanism, similar SSITKA experiments with the use of 02 were conducted at 140 °C. Figure 26.8 presents the transient response curves of N2 0, N2 0, and Ar obtained on Pt/Lao.sCeo.sMnOs (Figure 26.8a) and Pt/Si02 (Figure 26.8b) catalysts after the switch N0/H2/ 02/Ar/He N0/H2/ 02/He was made at 140 °C. As seen in Figure 26.8, the concentration of N2 0 produced by both catalysts is reduced after the isotopic switch, whereas the continuous evolution of N2 0 is noticed. The sum of the steady-state concentrations of N2 0 and N2 0 formed under H2-SCR in the isotopic gas mixture is the same as the steady-state concentration... 

Figure 19.22. Schematic representation of a set-up developed in-house for simultaneous SSITKA experiments and catalyst characterization by in situ time- and spatially-resolved UV/Vis spectroscopy.

The method was developed and actively applied by Happel, Biloen, and Goodwin. A common setup for SSITKA experiments is shown in Fig. 9.14. 

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