Titania-supported catalysts

Rutile is the thermodynamically most stable Ti02 crystal structure, and the transformation from anatase to rutile takes place at about 1300 K. This temperature can be much lower in the presence of foreign atoms, which can catalyze the transformation. In a series of studies of the properties of titania, Shanon and Pask (45) postulated that the transformation to rutile 

On the other hand, the impregnation process involves a combination of both adsorption of metal complex ions and deposition of the solute as the solvent is evaporated. Thus, the resulting metal dispersion depends on the fraction of metal ions undergoing exchange with support species relative 

Metal dispersion (H/M) as measured by H2 chemisorption after reduction at 473 K of a series of 2-wt% Rh/Ti02 catalysts as a function of pH of the solution used during the impregnation step in the catalyst preparation. (After Ref. 46.) 

As written, only the Rh has been reduced and all the titanium cations remain Ti4+. This is a reasonable assumption for the low-temperature reduction (LTR usually 473 K) case. When the reduction temperature increases, surface oxygen vacancies may be created, causing the reduction of Ti4+ to Ti3+. In that case, the HTR step may be written as 

In both cases, the resulting structure would be one-dimensional rows of Rh with the spacing of the Ti rows, as observed experimentally (52). 

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Whereas the effect of water on deactivation and on the overall activity of the FTS varies with the support, similar effects of water on the selectivity is reported for all catalysts, to a certain degree independent of the support, promoter and conditions. The effect can be summarized as an increase in C5 + selectivity, a decrease in methane selectivity, and in some instances a weak enhancement of the C02 selectivity is observed. Fig. 4 illustrates the effect on the C5 + and methane selectivity of adding water to cobalt catalysts supported on alumina, silica and titania, and both unpromoted and Re-promoted catalysts are shown. At the outset these selectivities are strong functions of the conversion, the C5 + selectivity increasing and the methane decreasing with increasing conversion, as illustrated by the trendlines in the figures. The points for methane are below, and C5 + -selectivity is above the line when water is added. Similar results were reported by many authors for alumina-supported catalysts,16-19 23 30 silica-supported catalysts,30 37 46-48 and titania-supported catalysts.19 30... 

HCOOH decomposition, 29 25-26 schematic apparatus, 29 18-19 small metal particles, 36 108, 109 spectra, 29 7-9, 25-26 titania-supported catalysts, 36 203-205, 208-209... 

Hadjivanov, K.I. and Klissurski, D., Surface chemistry of titania and titania-supported catalysts, Chem. Soc. Rev., 25, 61-69, 1996. 

Figure 3. Performance of a titania-supported catalyst in the oxidation of 1-butene. Conditions 363 K, atmospheric pressure, 4 g of catalyst, flow rate 1.67 cm3/s. Feed 1 kPa 1-butene, 7 kPa water, 93 kPa air. Catalyst 0.84 wt% PdS04, 8.3 wt% V2Os on Ti02.

Fig. 3 shows the activity of a titania-supported catalyst in the oxidation of 1-butene. The application of titania as a support not only results in an improvement of the stability of the catalyst but also enhances its activity. Although the decrease in activity for the titania-supported catalyst is smaller than for the 7-alumina-supported one, the two stages of... 

Figure 4. TPR profiles of fresh (A) and spent (B) titania-supported catalyst. (C) FID signal during reduction of spent catalyst. Heating rate 0.17 K/s. Catalyst 0.84 wt% PdSO, 8.3 wi% V3Os on Ti03 (anatase).

Although the relative deactivation of titania-supported catalysts is smaller than for y-alumina-supported ones, the activity still decreases considerably over the first 150 h of operation (see Fig. 3), resulting in a loss of approximately 40% of the initial activity (neglecting Stage 1). After 150 h the activity and butanone selectivity remains stable for a period of more than 650 h. After 800 h the catalyst was taken from the reactor and investigated to reveal the differences with the fresh catalyst. 

The results of the above characterization studies indicate that also in titania-supported catalysts the vanadium oxide layer slightly sinters. Since the vanadium oxide dispersion strongly effects the activity of the catalyst [16], it is likely that this sintering process is causing the deactivation observed in Fig. 3. The TPR and TPD results show that also some carbonaceous deposits are formed under reaction conditions, but these deposits are only present in low concentrations and, therefore, not likely to cause the deactivation of the catalyst. 

It is interesting to note also that the mixed oxide supported catalysts increase their activity with reduction temperature, although the impregnated sample performs better. In all cases ethylene selectivity was lower for alumina-supported sample and higher for titania-supported catalyst, and after an initial period the selectivity remained constant. 

When considering metal-support interaction effects, the whole set of Electron Microscopy data presented in the previous section point out some important differences between the behaviour of noble metal catalysts supported on ceria and that of titania-supported catalysts. Much higher reduction temperatures are required in the case of ceria-type supports to observe nanostructural features similar to those described for the so called SMS I efTect. 

It is interesting to compare our reaction rates to those reported for actual supported catalysts.(2, ) The rates for our oxide-covered surfaces are almost identical to those reported in the literature for titania-supported catalysts. 

A flow reactor was used to determine the catalytic performance of the different catalysts. The experiments were carried out with typically 0.30 g of catalyst and a gas flow of 50 Nml/min (GHSV 10,000 h ). In the epoxidation experiments, a gas mixture was used similar to that in most research in the literature 10% of oxygen, 10% of hydrogen, and 10% of propene in helium (all gas compositions given in vol.%). The pressure was 1.1 bar. In this study, the activity was determined at 323, 373,423, and 473 K the lowest temperature being most appropriate for the titania-supported catalyst, the higher temperatures being more optimal for the Ti-silica-supported catalysts. 

The highest propene oxide yields were obtained with both the Ti-SBA-15- and the Ti-silica-supported catalysts, although a higher reaction temperature was needed in comparison to the titania-supported catalyst. The deactivation for these catalysts was also considerably less. At lower temperatures (up to 423 K), all catalysts had an inhibition period for both propene oxide and water formation, which is explained by product adsorption on the support. The side products produced by all catalysts were similar. Primarily, carbon dioxide and acetaldehyde were produced as side products and, in smaller quantities, also propanal, acrolein, acetic acid, and formaldehyde. Propanol (both 1- and 2- as well as propanediol), acetone, carbon monoxide, and methanol were only observed in trace amounts. 

Figure 12.6 shows a plot for the titania-supported catalyst of the catalytic activity versus the amount of propene oxide produced. It can be seen that initially (from the maximum activity to half of the maximum activity) the activity decreases linearly with the amount of propene oxide produced and only later on, that the rate of deactivation decreases. This behavior can be explained on the basis of the deactivation model based on spectroscopic data published... 

Hagenmaier and Mittelbach [150] also studied the influence of dedusted flue gas from a waste incinerator on an SCR catalyst. The relative activity of the titania-supported catalyst decreased after 2000 h on stream at 513 K and remained constant during 6000 h at 533 K. The activity drop for 513 K is higher than that for 533 K. Dioxins are converted over SCR catalysts at temperatures above 573 K. High conversions were found for low ammonia concentrations. These low concentrations occur at the exit of the SCR catalyst. 

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