Catalyst platinum/ceria

In AI2O3 supported catalysts, both ceria and platinum limit the deactivation of rhodium at high temperatures in an oxidizing medium. Nevertheless, while the rhodium in PtRh/CeA can be regenerated by a reducing treatment, Rh/CeA catalyst cannot be regenerated. 

Figure 14.5 (a) Jellyroll monolith prepared from platinum/ceria catalyst, (b) comparison of propane conversion, hydrogen yield and H2/CO ratio for fixed bed, jelly roll, and monolithic reactors as determined for different GHSV [64]. 

When fixed beds of industrial WGS catalysts are applied in small fuel processors, the shift stages dominate the overall system volume and weight by up to 50% owing to the low catalyst utilization [128]. As an alternative to catalysts developed for the industrial scale, precious metal catalysts coatings show at least an order of magnitude higher activity. Certainly the most prominent noble metal catalyst formulation is platinum/ceria [4]. 

Pino et al. reported significant activity and favourable selectivity patterns for their platinum/ceria catalyst. However, the experiments were performed at a very high S/C ratio of 3.6 and a high O/C ratio of 1.3, which shifts the conditions into the field of steam supported partial oxidation [233]. 

Cheekatamarla and Lane [268] observed 50% loss of specific surface area for their 1 wt.% platinum/ceria catalyst after 56 h of autothermal reforming of synthetic low sulfur (10 ppm) diesel fuel. In parallel, the dispersion of the platinum decreased from 51 to 41%. These workers did not attribute a sintering process of the platinum to the latter effect. However, platinum in particular is known to suffer from dispersion losses when exposed to an oxidising atmosphere even at temperatures exceeding not more than400 °C [269]. Thus, both the carrier material and the active species suffered from the elevated temperature of the reforming process. 

A 5% weight increase of the platinum/ceria catalyst was observed when exposed to sulfur dioxide and an excess of air at a 400 °C reaction temperature. Almost identical results were gained for a pure ceria carrier, which supports the assumption that ceria is extremely sensitive to sulfur poisoning. 

Precious metals such as platinum, palladium, rhodium and ruthenium on ceria supports are reported in the literature as formulations for a water-gas shift [164]. Certainly the most prominent formulation is platinum/ceria, which has the drawback of a particular activity towards methane formation [164] at temperatures exceeding 375-425 °C, depending on the catalyst formulation. Its long-term stability is usually limited to a maximum temperature of between 425 and 450 °C due to the sintering effects of platinum. 

NexTech Materials Company have developed a platinum/ceria catalyst, which is highly active and long-term stable in the temperature range between 300 and 400 °C [ 164]. Johnson Matthey have developed platinum-containing precious metal catalysts for water-gas shift, which are long-term stable, show high activity even at 250 °C and no selectivity towards methanation up to an operating temperature of 450 °C [164]. 

Phatak et al. performed kinetic measurements over platinum/alumina and plati-num/ceria catalysts for a water-gas shift [306]. They observed similar activation energy in the range between 70 and 80 kj mol , but a 30 times higher turnover rate for 1 wt.% platinum/ceria at 200 °C than for platinum/alumina at 285 °C. A power-law expression was chosen for the kinetics ... 

The ceria/titania based catalyst was also more stable. The catalyst showed 60-h durability under the conditions provided in Figure 4.24. However, the weight hourly space velocity of21.2 L (hgcat) was relatively low. About ten-times higher yields can be achieved with platinum/ceria systems. The activity of the platinum/ceria catalyst is approximately 15-times higher compared with a platinum/alumina catalyst for the... 

Germani et al. prepared platinum/ceria water-gas shift catalysts in microchannels containing between 0.8 and 1.4wt.% platinum and between 8 and 20vrt.% ceria [308]. The sample containing 1.4wt.% platinum and 8 wt.% ceria showed the highest conversion, which was decreased when the catalyst was reduced prior to the activity test. Kolb et al. varied the platinum content of their platinum/ceria wash-coated catalysts between 1 and 5 wt.%, while the ceria content ranged between 6 and 40 wt.% [269]. The optimum platinum content was determined in the range between 3 and 5 wt.%, while the optimum ceria content was between 12 and 24 wt.%. 

Choung et al. [313] reported significant activity and stability gains for their platinum/ceria/zirconia catalysts, which were supported on a mixture of 46 atom-ic% of ceria and 54 atomic% of zirconia when adding 1-2 wt.% rhenium to the catalyst formulation. 

Liu et al. investigated the performance of a platinum/ceria catalyst exposed to reformate at a low temperature of 60 °C [310]. They found significant aging of the catalyst after just 5-min exposure to a mixture containing 3% carbon monoxide and 15% carbon dioxide as shown in Figure 4.26, which was attributed to the formation of... 

Figure 4.25 1000-h stability test of a platinum/ceria catalyst coating in microchannels for water- as shift weight hourly space velocity 72 L (h gca,) first 500 h, 144 L (h g ) second 500h temperature 400°C feed composition 50 vol.% hydrogen, 9.4vol.% carbon dioxide, 8.0 vol.% carbon monoxide, 32.7 vol.% steam (source I MM). 

Figure 4.26 Effect of aging by exposure to reformate at 60°C for 5 min on the performance of a platinum/ceria catalyst reformate composition2.2vol.%carbon monoxide, 11 vol.% carbon dioxide, 36 vol.% hydrogen, 29 vol.% nitrogen, 26 vol.% steam WHSV 20000 (h ) , fresh catalyst ...

Ayastuy et al. reported higher activity at a much lower temperature for their platinum/ceria catalysts [330]. While the platinum/alumina catalysts showed fiill conversion at 175 °C, the ceria supported samples showed full conversion by 80 °C. However, it is questionable whether platinum/ceria is a suitable catalyst for preferential oxidation, because it has excellent activity for the water-gas shift. This will impair the performance of the catalyst under partial load in a fuel processor environment due to the reverse water-gas shift taking place, as discussed in Sections 3.10.2 and 5.2.2. 

However, precious metal based catalysts without an oxygen carrier or additive, such as the rhenium/alumina catalyst as used here for the calculations, are known to have much lower activity compared with catalytic systems, such as platinum/ceria (see Section 4.5.1). 

An early stage of plate a heat-exchanger for water-gas shift in the kW size range was described by Kolb et al. [543]. The reactor still had a three stage cross-flow design for the sake of easier fabrication. Platinum/ceria catalyst was wash-coated onto the metal plates, which were sealed by laser welding. The reactor was tested separately and showed equilibrium conversion under the experimental conditions. It was subsequently incorporated into a breadboard fuel processor (see Section 9.5). 

Germani, G., Alphonse, P., Courty, M., Schuurman, Y. and Mirodatos, C. (2005) Platinum/ceria/alumina catalysts on microstructures for carbon monoxide conversion. Catal. Today, 110,1-2.114—120. 

Catalyst systems for the WGS reaction that have recently received significant attention are the cerium oxides, mostly loaded with noble metals, especially platinum 42—46]. Jacobs et al. [44] even claim that it is probable that promoted ceria catalysts with the right development should realize higher CO conversions than the commercial Cu0-Zn0-Al203 catalysts. Ceria doped with transition metals such as Ni, Cu, Fe, and Co are also very interesting catalysts 37,43—471, especially the copper-ceria catalysts that have been found to perform excellently in the WGS reaction, as reported by Li et al. [37], They have found that the copper-ceria catalysts are more stable than other Cu-based LT WGS catalysts and at least as active as the precious metal-ceria catalysts. 

C. Bozo, N. Guilhaume, and J.-M. Herrmann, The role of the ceria-zirconia support in the reactivity of platinum and palladium catalysts for methane total oxidation under lean conditions, J. Catal. 393, 393 06 (2001). 

A. Martinez-Arias, J. M. Coronado, R. Cataluna, J. C. Conesa, and J. C. Soria, Influence of mutual platinum-dispersed ceria interactions on the promoting effect of ceria for the CO oxidation reaction in a R/Ce02/Al203 catalyst, J. Phys. Chem. B 102,4357 365 (1998). 

Platinum and Palladium Based Catalysts. Researchers at ANL have also developed an ATR catalyst formulation comprised of a transition metal element supported on an oxide ion-conducting substrate, such as ceria. 

Gasolines contain a small amount of sulfur which is emitted with the exhaust gas mainly as sulfur dioxide. On passing through the catalyst, the sulfur dioxide in exhaust gas is partially converted to sulfur trioxide which may react with the water vapor to form sulfuric acid (1,2) or with the support oxide to form aluminum sulfate and cerium sulfate (3-6). However, sulfur storage can also occur by the direct interaction of SO2 with both alumina and ceria (4,7). Studies of the oxidation of SO2 over supported noble metal catalysts indicate that Pt catalytically oxidizes more SO2 to SO3 than Rh (8,9) and that this reaction diminishes with increasing Rh content for Pt-Rh catalysts (10). Moreover, it was shown that heating platinum and rhodium catalysts in a SO2 and O2 mixture produces sulfate on the metals (11). 

Table 4.9 summarises our findings for the growth of metal particles on the two major type of ceria surfaces, (111) and (001). Results are identical for Rh and Pt catalysts. Moreover, the orientation relationships described in this table do hold for reduction temperatures in the range 473 K - 1173 K, whenever the supported particles remain metallic type and monocrystalline. As we will describe further, in the case of platinum catalysts, a transformation of the metallic particles into an intermetallic phase takes place at 1173 K. Though in this case specific orientation relationships have also been observed with respect to the support, their characteristics differ from those related in Table 4.9. 

Figures 4.30(c) and 4.31(c) show HREM images representative of the catalysts reduced at 1173 K and further oxidised in pure O2 at 1173 K. The structure of both catalysts is clearly different from that observed after re-oxidation at 773 K. Notice that in this case both materials seem to be formed by small, crystalline, metal particles dispersed over the ceria surface. Fringe analysis confirms that these crystallites consist of metallic rhodium and platinum, respectively. Thus, the DDPs of the larger particles observed in the image of the Pt catalyst show 0.8 nm Moire-type fringes aligned with the (111 )-Ce02 reflections. These spots arise from double diffraction in the (lll)-Pt and (Ill)-Ce02 planes under a parallel orientation relationship. Therefore this result, in addition to confirm the presence of metallic Pt particles in the sample oxidised at 1173 K, suggest that these particles are epitaxially grown on the support. A detailed inspection also reveals that the exposed surfaces of these particles are clean, i.e. free from support overlayers.

A recent study of Bozo et al concerning platinum deposited onto ceria-zirconia solid solution has been published [45]. Pt/CeOo 7ZrOo33 was a most attractive catalysts which showed an activity much higher than for platinum deposited onto alumina T50 for methane combustion was lowered from 470 to 300 C according to their experimental conditions. This activity was attributed to enhanced oxygen species mobility onto ceria containing solids. Unfortunately a continuous... 

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