PdO‑CeO₂ Rod‑Like Nanoporous Catalysts with Superior CO Oxidation and Methane Combustion Performance

Abstract

We report a facile, green synthesis of rod‑like nanoporous PdO/CeO₂ composites via dealloying Al–Ce–Pd alloy ribbons followed by calcination. The resulting catalysts display outstanding activity for CO oxidation (T₉₉ = 80 °C) and CH₄ combustion (T₉₉ = 380 °C). Remarkably, the material retains >95 % activity after 100 h of continuous operation in 2 × 10⁵ ppm H₂O, demonstrating excellent water resistance and long‑term stability. The superior performance arises from highly dispersed PdO nanoparticles, a large BET surface area (~102 m² g⁻¹), strong PdO–CeO₂ interaction, and abundant surface‑active oxygen species.

Background

Reducing CO and CH₄ emissions is critical for environmental protection and climate change mitigation. While noble‑metal oxides, particularly PdO, are known for high catalytic efficiency, their cost and scarcity necessitate support‑enhanced designs. CeO₂ is an ideal support owing to its oxygen‑storage capacity and thermal stability. Conventional wet‑chemistry routes often suffer from surfactant contamination and low yield, hindering scalable production. Here we present a dealloying‑calcination approach that eliminates organic reagents, preserves a clean, highly dispersed PdO phase, and yields a reproducible, scalable catalyst.

Methods

Materials

High‑purity Al, Ce, and Pd (≥99.90 wt %) were used as received. 20 wt % NaOH solution was prepared with analytical‑grade NaOH.

Synthesis

Al–Ce–Pd alloy ribbons (Al_92–XCe₈Pd_X, X = 0.1–1.1 at %) were fabricated by arc melting under Ar, followed by rapid solidification on a rotating copper roll. The ribbons were immersed in 20 wt % NaOH at 25 °C for 2 h, then at 80 °C for 10 h to remove Al. After thorough rinsing, the samples were calcined in O₂ (18 mL min⁻¹) from 200–600 °C for 2 h.

Characterization

XRD (Cu Kα), SEM/EDS, TEM/HRTEM, BET (N₂ adsorption), XPS, and H₂‑TPR were employed to probe phase composition, morphology, surface area, oxidation state, and reducibility.

Catalytic Testing

CO oxidation (1 vol % CO/10 vol % O₂/89 vol % N₂) and CH₄ combustion (1 vol % CH₄/10 vol % O₂/89 vol % N₂) were evaluated in a stainless‑steel tubular reactor (30000 h⁻¹ space velocity). Reaction rates were calculated per Eq. (2). Stability tests ran for 100 h in 20 vol % H₂O. CO₂ tolerance and water‑resistance were assessed by adding 15–30 vol % CO₂ or H₂O.

Results and Discussion

Structure and Composition

XRD confirms pure CeO₂; Pd signals are absent due to high dispersion. SEM/TEM reveal a 10 nm nanorod skeleton with interconnected pores (12–14 nm). EDS shows Ce:Pd ~11.5:1, matching the precursor. XPS indicates 91 % Pd²⁺ (PdO) after calcination, while untreated samples are metallic Pd. Ce³⁺ content rises from 21 % to 23 % upon Pd loading, evidencing strong PdO–CeO₂ interaction and increased oxygen vacancies. H₂‑TPR shows low‑temperature PdO reduction peaks (P₁), confirming high reactivity.

Catalytic Performance

CO oxidation: T₅₀ = 15 °C, T₉₉ = 80 °C, light‑off below –20 °C. CH₄ combustion: T₅₀ = 305 °C, T₉₉ = 380 °C. Calcination at 400 °C yields optimal activity; higher temperatures reduce surface area and PdO loading. The catalyst exhibits minimal deactivation after 100 h, even with high H₂O. CO₂ addition shifts CO conversion from 99 % to ~12 % under anoxic conditions, but 0.5 % O₂ recovers activity, illustrating the role of lattice oxygen. CH₄ combustion shows only modest sensitivity to CO₂, owing to higher operating temperatures.

Mechanistic Insights

CO and CH₄ adsorb onto PdO surfaces, reacting with activated surface oxygen to form CO₂/H₂O (or CO₂/O₂ for CH₄). PdO–CeO₂ synergy enhances oxygen mobility, enabling rapid turnover even under low O₂. The rod‑like architecture provides abundant active interfaces and preserves high surface area during operation.

Conclusions

The dealloying‑calcination route yields a clean, highly dispersed PdO/CeO₂ catalyst with rod‑like nanoporous morphology, delivering low‑temperature CO oxidation (T₉₉ = 80 °C) and methane combustion (T₉₉ = 380 °C). Its robust water resistance, CO₂ tolerance, and long‑term stability make it a promising candidate for practical emissions control. The method is scalable and free from surfactant contamination, offering a versatile platform for other noble‑metal oxide catalysts.