Synthesis of a Highly Stable Pd@CeO₂ Catalyst for Methane Combustion with the Synergistic Effect of Urea and Citric Acid

Abstract

Making use of synergy between urea and citric acid, a core–shell Pd@CeO 2 catalyst with spherical morphology was facilely synthesized by a hydrothermal method. The formation mechanism of the core–shell structure in the presence of citric acid and hydrogen peroxide was studied. Results showed that the Pd@CeO 2 catalyst exhibited high catalytic activity in methane oxidation. Pd nanoparticles were well stabilized by CeO 2 shell encapsulation, resulting in high stability of the catalyst. A high CH 4 conversion of 99% was retained after 50 h on-stream reaction at 500 °C. Additionally, many tiny pores on the CeO 2 shell surface were beneficial for the full contact between reactants and active components. Pd nanoparticles were highly dispersed inside the shell, improving the utilization efficiency of active components. The results also demonstrated that the Pd species in the catalyst existed in the form of oxidation state, mainly in PdO (ca. 66.6%), which played an essential part in methane combustion.

Effect of Support on the Reactive Adsorption of CO from Low CO Concentration Streams on the Surface of Pd Based Catalysts

“…20 The intensity of the LT peak increases significantly in the order Al 2 O 3 < ZrO 2 < CeO 2 accompanied by a shift of its maximum from 78 to 94 °C, indicating that the number and strength of basic sites increases following the above order. 20,21 The total amount of desorbed CO 2 was estimated from the area below the CO 2 response curve and was found equal to 45 μmol/g for Al 2 O 3 , 100 μmol/g for ZrO 2 , and 144 μmol/g for CeO 2 , which further confirmed the higher basicity of CeO 2 compared to ZrO 2 and Al 2 O 3 in accordance to previous studies. 22 It has been reported that metal oxides are generally characterized by two types of basic sites, the basic hydroxyl groups and basic surface oxygen, where CO 2 can be adsorbed resulting in carbonates and bicarbonates production.…”

Structure-activity relationship in Pd/CeO2 methane oxidation catalysts

“…The role of ceria on methane oxidation performance in core-shell nanoparticles has been further confirmed by the comparison of Pd@CeO2 with Pd@TiO2 both supported on SiO2/Al2O3, with the latter showing a much lower catalytic activity (~35% methane conversion at 573 K for Pd-Ce system against ~15% conversion for Pd-Ti; CH4/O2 = 2 and GHSV≈170000 ml/(g•h)) [39]. A similar configuration with Pd@CeO2 core-shell units, in this case without support, has been recently described by Cai et al [92]. Ceria shell helps to avoid the aggregation of Pd particles as well as to maintain them in oxidized state.…”

“…For example, in our group we have observed that when the catalyst formulation involves Pd entities embedded into ceria lattice the overall stability in presence of water is very good and higher with respect to the impregnated analogues containing Pd nanoclusters, both in transient and in steady state conditions, the loss in conversion during time on stream experiments being comprised between 10% and 20% against 75% of impregnated catalysts [17,22]. The same does not hold for core-shell Pd@CeO2 units, supported [42] or unsupported [92], for which the formation of stable OH groups on ceria is proposed to inhibit the oxygen exchange with PdO and its accessibility to the gas phase [42]. For the alumina-supported core-shell units an additive deactivation in presence of water and phosphorus has also been observed, and ascribed to the agglomeration of CeO2 nanoparticles with subsequent incorporation of Pd active phase [138].…”

“…There are several papers, mostly published in the last decade, which report that catalysts showing a strong Pd-CeO2 interaction possess unique properties for methane oxidation and in general outperform conventional samples. Pd-based catalysts in which Pd and ceria are in strong physical contact and/or Pd is embedded into ceria lattice are nowadays regarded as the most active formulations for methane oxidation [20,22,41,75,92,93], also on the basis of repeated reaction cycles and exposure to high temperature which are important factors in light of practical applications. Cargnello et al [41] described the superior activity of a catalyst in which Pd-ceria core-shell nanoparticles were deposited on Si-functionalized hydrophobic alumina.…”

Cooperative Catalysis of Methane Oxidation through Modulating the Stabilization of PdO and Electronic Properties over Ti-Doped Alumina-Supported Palladium Catalysts

“…As shown in Figure 5b, the Pd/0TA catalyst shows that the reduction peak of finely dispersed PdO is located at 130 °C, which is consistent with the results reported in the literature. 38 The peak appearing at 410 °C is attributed to the reduction of subsurface palladium species, 39 and the broad peak located at 800–900 °C is due to the reduction of bulk oxygen. 40 Notably, the Pd/15TA catalyst exhibits the lowest reduction temperature of PdO (ca.…”