Ceria additives/catalysts

Ronning, Holmen, and coworkers—Ce doping of Cu/Zn/Al catalysts improves stability. Ronning et al,339 explored the impact of ceria addition to Cu/ZnO catalysts. Catalysts were prepared by co-precipitation of Cu, Zn, and Al from their corresponding nitrates. Ceria was incorporated into the catalyst by impregnation of cerium nitrate either before or after calcination (6 hours at 350 °C or 400 °C). The chemical compositions of the resulting catalysts are reported in Table 62. 

The authors339 found that the best catalysts were those whereby the ceria was impregnated prior to calcination of the hydrotalcite-based Cu-Zn precursors. The addition of ceria led to an important improvement in catalyst stability. For example, during stability tests (0.7 nl/min N2, 0.3 nl/min CO, 0.3 nl/min H20, T = 300 °C, P = 3 atm), the undoped catalyst calcined at 400 °C dropped to 25% of its initial activity within the first couple of hours on-stream. In contrast, the ceria-doped catalyst slowly decreased to 25% of its initial activity in about 50 hours. The activity was linked to the metallic Cu surface. In either case, the deactivation is considered to be very rapid. 

A marked effect of the Ce02/Zr02 composition (in samples containing 40 wt.% NiO) on the catalytic activity was noticed. The catalysts with Ce Zr =1 1 (6A) were not only more active (than 7A and 8A) but were also stable during the reaction. Sample 8A containing no zirconia in the support showed a low activity. The NiO crystallite size (Table 11.2) in these compositions varied in the order 7A < 6A < 8A. It may be recalled that on ceria-based catalysts the crystallite size of nickel metal was similar to that of NiO. The higher activity for 6A than 7A indicates that in addition to accessibility of... 

Ceria-based catalysts are intensively used because of their high chemical and physical stability, high oxygen mobility and high oxygen vacancy concentrations, which are characteristic of fluorite-type oxides. The possibility of cycling easily between reduced and oxidized states (Ce Ce" ) permits the reversible addition... 

A wealth of experimental chemisorption data are presently available for NM/Ce(M)02-, catalysts. As reported in section 4.3.2.2, the increase of generally induces significant modifications on their chemical behaviour. In most of cases, partial rather than complete inhibition of their chemisorption capability is reported. In many cases, however, the techniques and/or experimental routines do not allow an unequivocal interpretation of the reported H(CO)/NM data. As noted in section 4.3.2.2, quite often, the role played by a number of very important side effects, like the metal or support sintering, the adsorption of the probe molecules (H2 and CO) onto the supports, the presence of chlorine in them, or the reversibility of the deactivation phenomena, has not been established. By contrast, there are a number of recent studies (97,117,163,235) from which meaningful conclusions may be drawn. Moreover, some of them (97,163) have provided some additional fine details about the nature of the metal/support interaction effects occurring in ceria-based catalysts. 

These results evidence that OSC is larger for Ir-based catalysts than for Rh catalysts in the whole temperature range from 200 to SOO C. Additionally, OSC for ceria-supported catalysts varies slightly with temperature and reaches a maximum after full reduction of the surface. In this case> OSC appears to be limited by surface diffusion. On the opposite, for ceiia-zirconia supported catalysts, OSC is multiplied by a factor of 3 to 4 - depending on the metal - between 200 and 500°C. Bulk reduction is then responsible for such a large increase of the OSC. In that case, oxygen storage would be limited by bulk diffusion. 

Under catalytic reaction conditions, one should not necessarily expect species to proceed to the thermodynamic final state. An additional complication comes from the fact that the redox properties of catalytically active ceria and of ceria-zirconia mixed oxides appear to be quite different from the bulk thermodynamic values for ceria [37,38]. For example, ceria films calcined above 1270 K no longer promote the WGS [22] or steam-reforming reactions [20] and are much more difficult to reduce upon heating in vacuum [39]. These observations appear to be explained by calorimetric studies, which have shown that the heat of reoxidation for reduced Pd/ceria and Pd/ceria-zirconia catalysts is much lower than bulk thermodynamics would suggest [38]. Therefore, bulk thermodynamic information may not be entirely relevant for describing the nature of sulfur-containing species on catalytically active materials. 

It is interesting to consider which factors are important in the formation of cerium sulfates. The presence or absence of Pt had little effect, showing that ceria is able to oxidize SO2 without an additional catalyst [42]. Furthermore, the oxidation of SO2 to S04 occurs on ceria to some extent without the addition of gas-phase O. obviously with the simultaneous reduction of ceria [40]. Finally, bulk sulfates were only formed when the ceria samples were exposed to SO2 at temperatures above 250"C. 

A nice example of cooperation between metal particles and a support with redox properties concerns the three-way catalysts for automotive exhaust gases treatment composed of a noble metal (Pt, Pd, Rh) on a support with ceria additive. The cooperation was put in evidence in the CO/NO/O2 reactions [135-137] and... 

CO oxidation experiments were performed on Pt/alumina and Pt/ceria model catalysts, prepared by colloidal lithography. The samples were prepared without any of the additional plasma or UV-ozone pretreatments steps described in Preparation Procedures. Figure 4.38 shows T50 (the temperature at which 50% of reactant conversion is reached) and E (the apparent activation energy) as a function of CO oxidation cycle (ramping up and down in temperature). It is seen that both T50 and E initially shifts up during... 

The addition of Cu or Ni into ceria has different effect on the sulfur selectivity of the catalyst under fuel-rich conditions. Cu promotes the complete oxidation and it is selective for Sj. Conversely, on Ni/ceria catalysts, side reactions favor HjS production over elemental sulfur. It is worth to note that both the catalysts suppress carbon formation. The high carbon resistance shown by the metal/ceria based catalysts may be attributed to a higher dispersion of metals into this kind of matrices. Moreover, the high mobility of oxygen ions in ceria allows a rapid supply of oxygen to the metal interfaces speeding up the surface oxidation of carbon species and thus inhibiting deposition of carbon on the catalyst surfaces [25,26]. 

Note the overall good performance of the rhodium free catalyst Pd-Ce, which showed almost the same characteristics as the fully promoted palladium catalyst Pd-Rh-Ce. Ceria addition also increased CgHg conversion of the platinum catalyst, but resulted in a lower N2-yield. Generally, the palladium catalysts Pd-Ce and Pd-Rh-Ce showed similar or even superior catalytic performance compared to Pt-Rh-Ce. 

A complete range of metastable cerium-zirconium mixed metal oxide powders (CexZr(i.x)Oy, 0 < X < 1) were prepared through a similar hydroxide precipitation technique reported by Hori, et al. [11]. Cerium (IV) ammonium nitrate and zirconium oxynitrate precursors are completely dissolved in de-ionized water with mild heat and precipitated through the addition of excess ammonium hydroxide (-100 vol%). The ceria-zirconia is thoroughly washed with excess distilled water and allowed to evaporate to dryness overnight. The ceria-zirconia system is calcined in atmosphere for 1 hour at 773 K and subsequently milled into a fine powder. The model ceria-zirconia catalysts are prepared from the ground cerium-zirconium oxide powders using a 13 mm diameter pellet die and hydraulic press. 

The primary function of the ceria-based fuel-tome catalyst (FBC) [20] is to lower the combustion temperature of the soot accumulated in the DPF (currently around 450°C) compared to that of non-catalytically assisted combustion (usually around 600°C), thus allowing the DPF to regenerate more readily (Figure 9-11, combustion temperature of model soot decreases by the addition of a ceria-based catalyst). 

In addition, FBC [21] technology provides constant and continuously fresh ceria-based catalyst to the soot layer. This explains why, unlike many other catalyst-based DPF-regeneration technologies, the ceria-based fuel-borne catalyst is relatively insensitive to fuel-sulfur levels and is able to function with fuel containing over 2,500ppm of sulfur, as demonstrated in marine and stationary applications. However, in the presently described automotive application, the permissible level of sulfur is limited by the sulfur-sensitivity of other components of the complete DPF system. 

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