Elemental composition of catalyst

The objective of this study was to determine the elemental composition of a Pt5K10Ce85Ox catalyst coated onto Saffil CG and Saffil HX alumina fibers, and to interpret the catalytic activity behavior of the supported catalyst, the catalyst preparation is described in Chapter 4.3.2. The concentrations of the Pt, K and Ce stock solutions were determined separately for verification, and the catalyst composition was analyzed by inductively coupled plasma optical emission spectrometry (ICP-OES), see Chapter 4.4.2.

Table 3.5 Elemental analysis of 0.2 molar stock solutions Nominal element

concentration Measured element concentration (mg ml -1₎ (mg ml-1) Pt K Ce Pt 67.57 66.26 n.d.* n.d.* K 7.82 0.82 7.56 0.31 Ce 28.02 n.d.* n.d.* 27.89 * not detected

The elemental composition of 0.2 molar stock solutions is summarized in Table 3.5. The measured Pt, K and Ce concentrations were within 97 and 100% of the nominal values. K was detected in the Pt and Ce precursors at 0.8 and 0.3 mg ml-1, respectively, although no K should be contained and contributed to its higher final concentration in Pt5K10Ce85Ox catalyst.

60 3 Results and discussion

The target and current elemental values of the metals in the coated Saffil CG and Saffil HX are presented in Table 3.6. From the table it is obvious that the measured concentrations of Pt, K and Ce on the Saffil CG fibers were higher than on the Saffil HX fibers. The amount of K on Saffil CG was found to be approx. 20% higher than the nominal amount, which could be explained by the presence of K, previously measured in the Pt and Ce precursors. The Pt concentration of 9.1 mg ml-1 agrees with the nominal concentration of 9.2 mg ml-1. The Ce is the main component in the ternary composition, its measured concentration was 80% of this. The investigated concentrations of all three components coated onto Saffil HX were less than expected. K and Ce were found at only about 70% of the nominal concentration, and Pt content at 80% with the target concentration of 7.6 mg ml-1.

Table 3.6 Elemental analysis of 0.55 molar Pt5K10Ce85Ox catalyst composition coated onto alumina fibers

Carrier material Pt (mg ml-1) K (mg ml-1) Ce (mg ml-1)

Target 9.23 2.15 65.5

Saffil CG 9.1 2.6 57.7

Saffil HX 7.6 1.4 44.2

A correlation was observed between the specific surface area and cumulative pore volume, and the detected elemental analysis. While Saffil CG shows high BET surface area and high porosity of 150 m2 g-1 and 0.3 cm3 g-1 respectively, Saffil HX exhibits low BET surface area of only <5 m2 g-1 and is non-porous. The experiment using Saffil CG showed that the catalyst has penetrated predominantly into the pores and is not present on the surface. This is shown by the analysis results, where the measured concentrations of the various elements agree well with the nominal values. The amount of Pt, K and Ce found on the Saffil HX fibers was lower in comparison to Saffil CG fibers. Since the Saffil HX carrier exhibits a low BET surface area and is non-porous, the catalyst is forced to accumulate on its surface forming weakly adhesive surface agglomerates. During the transfer of the loaded fibers it is most likely that some catalyst agglomerates remained in the wells, so concentrations of only 65 to 80% of the nominal single component were detected.

3.4 Characterization of alumina support 61

3.4.6 Discussion

Two types of ceramic fibers, Saffil CG and Saffil HX, with different properties were impregnated with catalysts and characterized in order to study the low relative activity measured by ecIRT. The elemental compositions of the catalyst stock solutions and the catalysts coated onto the carriers were analyzed.

The investigation showed that the stock solutions of Pt and of Ce contained additional 15% more K than expected and the detected concentrations of the three metals in their respective stock solutions were in agreement with the nominal values. The amount of catalyst elements supported on the Saffil CG fibers was found to be close to the expected amount, except for K, which was about 20% higher, due to its elevated concentration in the Pt and Ce stock solutions.

The structure characterization of Saffil CG showed high BET surface area of 145 m2 g-1 and high cumulative pore volume of 0.3 cm3 g-1, and its predominant crystalline form is θ-Al2O3 (boehmit). In order to obtain a loose contact mode the Saffil CG fibers loaded with catalysts were mixed by spatula. The increased catalyst concentration did not affect the temperature T50 which remained nearly constant, with a drop of only 10 °C observed. The temperature T50 at a concentration of 0.05 mol l-1 was 591 °C but only 582 °C was measured at a concentration of 0.55 mol l-1 although the total catalyst concentration was detected. This relates to the catalysts penetration and deposition respectively into the pores resulting in poor contact between soot and catalyst. The soot oxidation rate is dependant on the catalyst surface area associated with the active sites which are in contact with soot particles. The contact between soot and catalyst was poor. These observations are in agreement with the results of investigation of ceramic fibers impregnated with catalysts in the literature [176, 177]. It was found that contact between soot and catalyst is of critical importance for the ignition and oxidation of soot, as discussed by several authors [177].

In contrast to Saffil CG the measured concentrations of the single elements loaded onto the Saffil HX fibers was between 20 and 35% lower than the expected catalyst concentration. Saffil HX is non-porous and the crystalline structure is α-Al2O3 (corundum) which has a characteristic low specific surface area (< 5 m2 g-1). The study

62 3 Results and discussion

showed that the temperature T50 correlates with the catalyst concentration, thus generally it can be concluded that the activity rose with increased concentration. The T50-values were nearly equal to these of the unsupported catalysts. Because of the low BET surface area and non-porosity of Saffil HX, the catalysts agglomerates on the fibers surface, as confirmed by the high catalytic activity at concentration of 0.55 mol l-1, which increased by 90 °C for loaded Saffil HX fibers, and by 110 °C for the unsupported catalyst. The difference of these 20 °C is associated with lower measured concentration of the elements coated onto the carrier caused by losses during the transportation of the loaded carrier from the slot where the catalysts remained.