Pd-doped perovskites

Maitlis filtration test. To investigate whether the Pd-doped perovskite is the actual catalyst, or a reservoir of soluble Pd, a Maitlis filtration test (10) was performed. The reaction of 4-bromoanisole with 4-phenylboronic acid, catalyzed by BaCeo 95Pdo os02 95, was intemipted at 20 s and 1 min, corresponding to conversions of 16 and 45%, respectively, by filtering the hot reaction mixture to remove the solid perovskite. The filtrates were allowed to cool to room temperature without stirring. After 3 h, the biatyl yields in both samples were estimated to be 100% by H NMR. 

Catalyst recycling. The solid Pd-doped perovskite catalysts are easily filtered from the reaction mixtnre for reuse. The activity of the recycled BaCco 95Pdo.o502 95 catalyst was investigated in the coupling of 4-bromoanisole with 4-phenylboronic acid. The results in Table 27.3 show that high activity was retained even after seven cycles of catalyst use. 

Scheme 27.1. Proposed mechanism for Suzuki coupling by Pd-doped perovskite catalysts.

Soot/catalvst ratio A series of experiments with different [soot/catalyst] ratios is carried out with stoicliiometric substituted Pd doped perovskite as catalyst. Results show that the combustion temperature increases when the [soot/catalyst] ratio is higher than 15 wt%. This is probably due to the lack of contact between catalyst surface and all the soot particulates beyond this point. 

The Pd-supported perovskites appeared to be more active than Pd-substituted perovskites, especially in relation to NO reduction. The higher activity of Pd/ LaFeo,8Coo.203 catalysts was associated with the easier reduction of Pd species, compared to Pd in the lattice of Pd-substituted perovskite, which is difficult to be reduced due to the strong Pd-0 interactions [85]. Moreover, the TWC activity of two versions of Pd-doped perovskites was investigated under lean, stoichiometric and rich simulated exhaust conditions [89]. Pd-doped perovskites were found to exhibit similar conversion performance with a commercial Pd-rich TWC catalyst, demonstrating, however, enhanced N2-selectivity. 

In contrast, the nondoped perovskite sample was readily poisoned by sulfur (NO conversion dropped from 69.5 to 30.5% after SO2 was directly added to the feed gas) as well as the Pt/Ba0/Al203 (NO conversion dropped from 99.2 to 54.9% and N2 selectivity dropped from 96.1 to 85.3%) without the ability to recover by H2 reduction at 325 °C. The authors successfully showed that Pd-doped perovskite provides a new possibility for overcoming the problems caused by sulfur poisoning for the NSR systems. 

Li, X, Chen, C, liu, C, Xian, H, Guo, L, Lv, J., Jiang, Z., and Vemoux, P. (2013) Pd-doped perovskite an effective catalyst for removal of NO from lean-bum exhausts with high sulfur resistance. ACS Catal, 3,1071-1075. 

The greater rearrangement of the perovskite stmeture in the catalyst associated with the higher level of Pd-doping may be responsible for the longer indnetion period. After the onset of catalytie activity, the slopes of the two conversion vs. time eurves for X = 0.05 and x = 0.10 in Figure 27. la are very similar, demonstrating that the two catalysts produce the same soluble aetive site in similar amounts. 

The catalytic activity of cation-doped hexaaluminate was not as high as the Pd catalyst or some perovskite-type oxides. But the thermal stability is superior for hexaaluminate to the Pd or perovskite-based catalysts. Pd/cordierite is the popular combustion catalyst due to its very high thermal shock resistance resulting from the low thermal expansion coefficient of the support material. However, the thermal stabilities are not high enough due to its low melting point. 

Ghasdi, M. and Alamdari, H. (2012) Highly sensitive pure and Pd-doped LaFeOa nanocrystalline perovskite-based sensor prepared by high energy ball milling. Adv. Mater. Res., 409,... 

Pd-doped catalysts have been produced by USS [82]. The fingerprint of Pd adopting the octahedral coordination of Fe in LaFeo,95Pdo,o503 has been observed in the XANES spectra of the material prepared by spray synthesis (27m /g) similarly to the preparation by the amorphous citrate method (14m /g) [17,82]. In contrast, the flame-made material of the same composition (22m /g) exposed metallic Pd particles on LaFeOs similarly to preparation by solution combustion. The different nature of the Pd species obtained by changing the synthesis method dramatically influences their catalytic performance, since PdO nanoparticles exposed at the surface of the mixed oxide exhibit catalytic activity, whereas Pd—O species in the bulk of the mixed oxide are inactive, at least in the case of methane oxidation [27]. In contrast to LaFeOs, LaMnOs did not allow Pd to adopt the octahedral coordination irrespective of synthesis method. Therefore, the coordination of Pd strongly depends on both the composition of the perovskite-type oxide and the synthesis method. 

Clearly, the Mn and Cu-based catalysts are very active for CO and HC oxidation reactions, a behaviour which is well known for these two metals, their activity being higher than that of the reference Pt-Rh catalyst. The assistance of small amounts of noble metals (1 wt.% Rh or 2.4 wt.% Pd respectively) improves the activity in NO reduction, which is total around 400°C. Monceaux et al. [14] also observed a pronounced promoting effect of Pt and / or Rh dopes on the three-way catalytic activity of LaogSrojMnOs s perovskites. Rh was shown to be particularly important to obtain a nearly total conversion of NO at 500°C, with a high space velocity (100 000 h ) under stationary conditions. 

Data for samples calcined at lower temperature (800°C) are usually more scattered, even if they confirm the decrease of a with increasing content of the doping metals. These results suggest that the noble metals are incorporated at different oxidation states resulting in some defects of the perovskite crystal structure. In tha case of the Pd-containing sample, this hypothesis is confirmed by a small shift of XRD peaks and decreasing peak sharpness observed at high palladixun content [4]. 

As far as precious metal dopes are concerned, Pd is the most active for the carbon oxidation. Inside the perovskite matrix its oxidation state is Pd + as it was reported that the platinum in the same kind of structure is in the fonn of dissolved tetravalent ion [23]. 

Afterburning processes enable the removal of pollutants such as hydrocarbons and volatile organic compounds (VOCs) by treatment under thermal or catalytical conditions. Combinations of both techniques are also known. VOCs are emissions from various sources (e.g. solvents, reaction products etc. from the paint industry, enaml-ing operations, plywood manufacture, printing industry). They are mostly oxidized catalytically in the presence of Pt, Pd, Fe, Mn, Cu or Cr catalysts. The temperatures in catalytic afterburning processes are much lower than for thermal processes, so avoiding higher NOx levels. The catalysts involved are ceramic or metal honeycombs with washcoats based on cordierite, mullite or perovskites such as LaCoOs or Sr-doped LaCoOs. Conventional catalysts contain Ba-stabilized alumina plus Pt or Pd. 

In a modified version, a perovskite electrolyte (SrCeOs doped with 5 mol% Yb) in the form of a tube was coated with a layer of impermeable Au-Pd alloy to allow access to oxygen gas on the outer surface, while impeding H2O transfer. The inner surface coated with platinum is exposed to both H2O and oxygen (Kumar et al. 1996, Cobb et al. 1996, Fray et al. 1995, Kumar 1997). 

Nanoscale Fe-based perovskites (LaFei ,(Cu,Pd)03) have been found to address considerable activity towards NO reduction by CO [61]. Their superior catalytic performance was ascribed to the facilitation of anion vacancies generation after Cu incorporation, which in turn determines the NO adsorp-tion/dissociation. Furthermore, the enhanced reducibility of Cu-doped LaFe03 perovskites results in oxygen vacancies regeneration and CO oxidation promotion [61]. Advanced preparation methods, such as reverse microemulsion, can also result to physicochemical characteristics (nanosized perovskites with higher surface areas) and catalytic performance (more active crystal phases) modifications [57]. 

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