Catalyst During Reaction

Bruckner, A. and Kondratenko, E. (2006) Simultaneous operando EPR/UV-vis/laser-Raman spectroscopy - A powerful tool for monitoring transition metal oxide catalysts during reaction, Catal. Today, 113, 16. 

Characterization is a central aspect of catalyst development [1,2], The elucidation of the structures, compositions, and chemical properties of both the solids used in heterogeneous catalysis and the adsorbates and intermediates present on the surfaces of the catalysts during reaction is vital for a better understanding of the relationship between catalyst properties and catalytic performance. This knowledge is essential to develop more active, selective, and durable catalysts, and also to optimize reaction conditions. 

This polymer is red in its oxidized form and yellow in its reduced form, and lends itself to spectroscopic study. Its activity was compared with that of molybdate catalysts, which are well known to be active for oxidative dehydrogenation and many parallels between the organic and inorganic materials were found. The color changes of this catalyst during reaction made it clear that we are not concerned with a surface reaction, but that at least a part of the bulk of the material participates in the oxidative dehydrogenation reaction, a phenomenon we have mentioned several times in these pages. 

COl" > N03 > eland that the stabilities of the catalysts during reaction in a CO + 3H2 mixture at 500 °C fell in the same order. It was concluded that the main effects of the anions were manifested during the calcination of the coprecipitates. It was also observed that the hydrothermally aged materials gave reduced samples with very similar methanation activities to those derived from the freshly prepared materials. 

Several other successful applications of SIMS in catalyst characterization have been described in the literature [14, 15]. Of particular note are the extensive studies of Vickerman et al. [21] on automotive exhaust catalysts, a SIMS analysis of catalyst coatings in microreactors by Gnaser et al. [22], imaging studies which reveal the lateral distributions of elements [23, 24], and an excellent study on the reactive species present on a supported catalyst during reaction. These will be discussed in a little more detail in the following paragraphs. 

The average oxidation state of a metal in a catalyst during reaction was found to be related to the presence of carbonaceous deposits on the surface. As the feed for propane ODH was depleted in O2, the catalyst was readily reduced (Mul et al., 2003) and amorphous carbon-containing deposits formed. This behavior was corroborated by UV-vis DRS (Mul et al., 2003 Puurunen and Weckhuysen, 2002) and by combination of UV-vis DRS and Raman spectroscopy (Kuba and Knozinger, 2002 Nijhuis et al., 2003). 

Since iron carbonyls are not only involved in chemical synthesis (of e.g., iron nanoparticles) but also can occur in heterogenous iron catalysts during reaction conditions (e g., Fischer Tropsch reactions), we wiU briefly discuss the Mossbauer parameters of this class of compounds. 

Borodzinski A, Bond GC (2006) Selective hydrogenation of ethyne in ethene-rich streams on palladium catalysts. Part 1. Effect of changes to the catalyst during reaction. Catal Rev - Sci Eng 48 91... 

This study focused on the deactivation of the Mn/Ce catalysts during reaction. The catalytic oxidation of phenol in aqueous solution to carbon dioxide, water and other side-products was selected as the test reaction. Catalysts were prepared from amorphous precursors using the citrate method and controlling the calcination temperature. Activity performance as a function of the time on stream was studied by simultaneously analyzing the conversion of phenol, the total organic carbon content of the catiyst, the cations eluted and the elemental composition of both cerium and manganese. Experimental conditions were widely varied. Fresh and used catalysts were also analyzed by BET surface area, X-Ray Diffraction and X-Ray Photoelectron Spectroscopy. 

The relationship between carbon deposition and the decrease in activity was determined through the ratio of XPS C Is /A1 2p peak area. In Table 1, the C/Al ratios of the LTN and HTN catalysts were greater than the others and that of the MTN catalysts was the smallest. This indicates that the drop in activity of the catalysts during reaction was not related to carbon deposition on the surface of the catalyst. To determine if the activity of the nitrided... 

Figure 5 LRS of the catalysts during reaction with butane at 400°C. (a) Bulk catalyst, (b) Catalyst A, (c) Catalyst B.

The primary cause for deactivation in current reforming catalysts, such as supported nickel, is the formation of carbon on the catalyst during reaction [20]. However, when postreaction samples of P-M02C and a-WC were examined by HRTEM, no observable carbon deposition had occurred on the catalyst surface during the reaction. In addition, activity studies demonstrated that M02C had a methane dry reforming activity similar to an active supported noble metal catalyst, namely 5% Ir/Al203 [27]. 

The trend of the variation of TOF with FE can be reversed by secondary effects such as carbon deposition or restructurization by oxygen (284). Clearly, the surface composition of the catalyst during reaction must be measured in order to evaluate these important effects. 

Since SSITKA can decouple the apparent rate of reaction into the contribution from the intrinsic activity ( the reciprocal of surface residence time of intermediates) and the nrnnber of active sites ( surface concentration of intermediates), the cause of deactivation of a catalyst during reaction can often be revealed. SSITKA has been used in a number of studies for this purpose. Catalyst deactivation during n-butane isomerization and selective CO oxidation are good examples. Deactivation studies are conducted by collecting isotopic transient data at particular times-on-stream as deactivation occurs. 

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