Reduced Oxide Species Research Articles

Promoting Effect of Palladium on ZnAl2O4-Supported Catalysts Based on Cobalt or Copper Oxide on the Activity for the Total Propene Oxidation.

Palladium-modified Co-ZnAland Cu-ZnAl materials were used and found active for the catalytic oxidation of propene and propane. According to the results obtained by XRD, TPR and XPS, the zinc aluminate-supported phases are oxide phases, Co3O4, CuO and PdOx for Co-ZnAl, Cu-ZnAl and Pd-ZnAl catalysts, respectively. These reducible oxide species present good catalytic activity for the oxidation reactions. The addition of palladium to Co-ZnAl or Cu-ZnAl samples promoted the reducibility of the system and, consequently, produced a synergic effect which enhanced the activity for the propene oxidation. The PdCo-ZnAl sample was the most active and exhibited highly dispersed PdOx particles and surface structural defects. In addition, it exhibited good catalytic stability. The H2 pre-treated PdCu-ZnAl, PdCo-ZnAl and Pd-ZnAl samples showed higher activity than the original oxide catalysts, evidencing the important role of the oxidation state of the species, mainly of the palladium species, on the catalytic activity for the propene combustion. The synergic effect between metal transition oxides and PdOx could not be observed for the propane oxidation.

Promotional effect of ZrO2 and WO3 on bimetallic Pt-Pd diesel oxidation catalyst.

Diesel oxidation catalysts Pt-Pd-(y)ZrO2-(z)WO3/CeZrOx-Al2O3 with total Pt & Pd loading of only 0.68 wt.% were prepared and investigated for oxidation activity and stability of CO, C3H6, and NO. Introduction of ZrO2 greatly improved low-temperature activities and retained stability especially for CO and C3H6 oxidation after treated at 800 °C. With the optimal loading amount of 6 wt% ZrO2, 2 wt% WO3 was introduced to the system and showed higher activity. Reaction temperature for 50% CO and C3H6 conversiondeclined to 160 and 181 °C, and the maximal NO conversion increased to 50%. By using XRD, TEM, CO chemisorption, XPS, and H2-TPR analysis, it was found that ZrO2 could inhibit aggregation of Pt and Pd, improve metal dispersion, and increase surface-chemisorbed oxygen after high-temperature treatment, accounting for promoted performance. Also, there were more reducible oxide species in ZrO2-doped catalysts. ZrO2 could induce reduction of noble metal oxides and surface ceria by weakening Pt-O-Ce interaction, which increased the ability to dissociate H2 and spillover effect of dissociated hydrogen to ceria. Doping WO3 increased metal dispersion of fresh samples and brought more Pt0 species that were active sites for oxidation reactions. Thus, ZrO2 and WO3 could be effective additives for oxidation catalysts to synergistically improve their activities and thermal stability.

Rhodium‐Catalyzed Methanation and Methane Steam Reforming Reactions on Rhodium–Perovskite Systems: Metal–Support Interaction

AbstractMetal–support interaction in rhodium–perovskite systems was studied using LSF (La0.6Sr0.4FeO3−δ) and STF (SrTi0.7Fe0.3O3−δ) supports to disentangle different manifestations of strong or reactive metal–support interaction. Electron microscopy and catalytic characterization in methane steam reforming/CO2 methanation reveal that reduction in hydrogen at 673 K and 873 K causes different extents of Fe exsolution. Depending on the perovskite reducibility, Fe–Rh alloy particles are observed. No signs of strong metal–support interaction (i.e., encapsulation of metal particles) by reduced oxide species were observed. As re‐oxidation in oxygen at 873 K did not fully restore the initial structures, the interaction between Rh and the perovskites manifests itself in irreversible alloy formation. Catalytic effects are the suppression of methane reactivity with increasing prereduction temperature. The results show the limits of the strong metal–support interaction concept in complex metal–oxide systems.