The Effect of the State of Pd on Methane Combustion in Pd-Doped LaFeO₃

Abstract

Pd-based LaFeO3 perovskite-type catalysts have been prepared by the amorphous citrate method with the aim to expose Pd at the surface of LaFeO3 (Pd/LaFeO3) and to force Pd to form a solid solution with LaFeO3 (LaFe0.95Pd0.05O3). The catalysts have been tested for methane combustion in the temperature range of 200−900 °C. The state of Pd in the various samples has been accurately characterized using XANES, EXAFS, XPS, H2-TPR, and DRIFTS. The shape of the white line of the Pd K edge XANES spectrum of LaFe0.95Pd0.05O3 indicates that palladium most probably exhibits distorted octahedral coordination. Hence, Pd is fully incorporated in the perovskite structure, substituting Fe, which is also confirmed by EXAFS refinement. On the contrary, the XANES spectrum of Pd/LaFeO3 corresponds more to Pd in square planar coordination, suggesting that Pd is mainly distributed on the surface of LaFeO3, most probably in the form of PdO. The different oxidation states and local coordination of Pd in LaFe0.95Pd0.05O3 (most likely Pd3+ in distorted octahedral coordination) and Pd/LaFeO3 (predominantly Pd2+ in square planar coordination) reflect the distinct availability of Pd for reaction and coincide with the different catalytic performances of the catalysts. Pd/LaFeO3 exhibits high performance for the oxidation of methane (T 50% = 460 °C), which correlates with the high concentration of Pd species on the oxide surface. On the contrary, when Pd is incorporated within LaFeO3, the corresponding catalyst shows poor performance similar to that of LaFeO3. Reaction at 900 °C causes serious changes in the structure of the catalysts, which were interpreted in light of the initial structure, the “self-regenerative property” of perovskite-type oxides, and the PdO ↔ Pd equilibrium.

First Principles Modeling of Pd‐doped (La,Sr)(Co,Fe)O3 Complex Perovskites

“…In contrast, the magnetic moment on Pd is small, its effective charge is also smaller than that for a nominally 3+ ion, thus we assign it as the low-spin Pd 2+ (d 8 ) ions. The presence of palladium cations in the perovskite lattice is in agreement with previous results [9] and as well with experimental studies on Pd in other oxides where Pd 2+ (and or Pd 4+ ) species were detected [19][20][21].…”

Preparation of LaFe0.95Pd0.05O3 perovskites and their catalytic performances in methane combustion

“…The comparable activity between LFP-1073 and LaFeO 3 in CH 4 combustion confirms that LaFeO 3 acted as the real catalyst and the contribution of Pd in the LaFe 0.95 Pd 0.05 O 3 sample was almost negligible. This observation is in accord with a previous report that LaFe 0.95 Pd 0.05 O 3 only exhibited comparable activity with LaFeO 3 in CH 4 combustion [25]. Most likely, the Pd species in the LFP-1073 sample strongly interacted with the surrounding perovskite structure, exhibiting negligible effect on the surface adsorption properties and not influencing the status of the adsorbed oxygen, and thereby was almost inert for CH 4 combustion.…”

“…This was mainly because of the larger surface area that usually exposed more active sites. The T 50 of these LaFe 0.95 Pd 0.05 O 3 samples ranged from 842 to 903 K, being generally consistent with the results of Pdcontaining perovskites, such as 768 K for LaFe 0.9 Pd 0.1 O 3 [14] and 848 K for LaFe 0.95 Pd 0.05 O 3 [25].…”

“…This pre-treatment enhanced the activity for CH 4 combustion by decreasing the T 50 to 824 K, as shown in Figure 5. During the pre-treatment, the Pd n+ ions within the LaFe 0.95 Pd 0.05 O 3 perovskite decomposed to metallic Pd and emerged on the perovskite surface [9,25,29]. Upon lowering the temperature under the reaction gas, Pd was partially oxidized to PdO.…”

Dynamic Behavior of Pd Species in an LaFe0.95Pd0.05O3 Perovskite

“…For the reference Pd/ LaFeO 3 catalyst, PdO was readily reduced to metallic Pd at 318 K. The minor peak at 446 K was assigned to the reduction of Pd n? incorporated into the perovskite structure [18]. The peak at 714 K corresponded to reduction of Fe 3? to Fe 2?…”