High‑Performance Au/Ce‑La Nanorod Catalysts for Low‑Temperature CO Oxidation: Synthesis, Characterization, and Mechanistic Insights

Abstract

One‑dimensional Ce‑La nanorods with varied La content (Ce:La ratios of 1:0, 3:1, 1:1, 1:3, and 0:1) were synthesized by a hydrothermal route. Au was deposited by a modified deposition‑precipitation method to produce Au/Ce‑La nanorod catalysts. Comprehensive characterization—BET, ICP, XRD, SEM, TEM, EDX, XPS, UV‑vis DRS, and H₂-TPR—confirmed that La exists as LaO_x within the rods and that the morphology remains rod‑like across all compositions. Catalytic tests show that the 3:1 Ce/La ratio (Ce_0.75‑La_0.25) yields the most active catalyst: 1.0 %Au/Ce_0.75‑La_0.25 (calcined at 300 °C) achieves complete CO conversion at only 30 °C and retains stability up to 200 °C.

Background

Carbon monoxide (CO) is a potent respiratory toxin that binds irreversibly to hemoglobin, and its presence in indoor air and vehicle exhaust poses serious health risks. Catalytic CO oxidation remains the most effective remediation strategy, especially for low‑temperature applications in pollution control devices, indoor air purifiers, and CO sensors.

Supported gold catalysts have emerged as promising candidates for low‑temperature CO oxidation, owing to the unique reducibility of ceria (CeO₂) and the ability of gold to exist in multiple oxidation states. The catalytic activity of Au/oxide systems depends critically on particle size, metal–support interaction, and the reducibility of the support.

Ceria’s ability to switch between Ce⁴⁺ and Ce³⁺ endows it with high oxygen storage capacity, making it an ideal support. Enhancing ceria with transition‑metal dopants (e.g., La) can further improve surface defects, oxygen vacancy concentration, and metal‑support interactions, thereby boosting catalytic performance.

Experimental

Support Preparation

Ce‑La nanorods were prepared by hydrothermal synthesis: NaOH (9 M) and Ln(NO₃)₃ (Ln = Ce, La, 0.8 M) were mixed and stirred for 30 min at room temperature, then transferred to a Teflon‑lined autoclave, heated to 110 °C for 14 h, air‑cooled, washed, dried at 80 °C, and calcined at 400 °C (5 °C min⁻¹). Products with La contents of 0–100 at.% were denoted Ce, Ce_0.75‑La_0.25, Ce_0.5‑La_0.5, Ce_0.25‑La_0.75, and La nanorods.

Catalyst Preparation

Au/Ce‑La catalysts were prepared by deposition‑precipitation: the support (100 mL deionized water) was mixed with 0.01 M HAuCl₄ (pH ≈ 7). The mixture was stirred for 12 h, refluxed at 100 °C for 4 h, centrifuged, washed, and dried at 80 °C. Au loadings were expressed as weight % and confirmed by ICP‑AES.

Characterization Techniques

BET surface areas were measured by N₂ adsorption at –196 °C. XRD (Rigaku D/Max‑2500, λ = 0.154 nm) revealed the fluorite structure of CeO₂ and the presence of La₂O₃ or La(OH)₃ at higher La loadings. TEM and HRTEM confirmed rod morphology (diameter 5–10 nm, length 100–300 nm) and sub‑nanometer Au clusters. XPS (Kratos Axis Ultra) identified Ce⁴⁺, La³⁺, and Au^δ+ (dominant at 300 °C calcination). UV‑vis DRS detected a surface plasmon resonance band (500–600 nm) attributable to Au nanoparticles. H₂-TPR (PX200) indicated reduced reduction temperatures for Au‑doped samples, reflecting enhanced oxygen mobility.

Results and Discussion

Structural and Surface Properties

BET analysis showed a surface area of 99.7 m² g⁻¹ for pure CeO₂ nanorods, decreasing to 74.1 m² g⁻¹ at 3:1 Ce/La, and further with higher La content. Mesoporous character (H3 hysteresis) was retained across all samples, with pore volumes around 0.23 cm³ g⁻¹ for Ce_0.75‑La_0.25.

Gold Dispersion and Oxidation State

Au loading remained below the nominal value due to precipitation losses, yet TEM confirmed highly dispersed Au sub‑nanometer clusters (< 1 nm) on all supports. XPS showed predominantly Au^δ+ species for catalysts calcined at 300 °C, whereas calcination at 400 °C produced mainly metallic Au⁰, correlating with reduced activity due to sintering.

Reducibility

Pure CeO₂ exhibited reduction peaks at ~410 °C (surface) and ~620 °C (bulk). La‑doped samples displayed a shift to higher temperatures (~520 °C) for the surface reduction peak, indicating stronger Ce–O–La interactions. Au deposition introduced a low‑temperature reduction feature (~200 °C), attributed to the reduction of Au^δ+ and facilitated oxygen spill‑over.

CO Oxidation Activity

Au/Ce_0.75‑La_0.25 (1.0 %Au) achieved 100 % CO conversion at 30 °C, outperforming all other compositions. Increasing La beyond 25 at.% reduced activity, likely due to decreased reducible oxygen sites. Lower Au loadings (0.5 %) still delivered complete conversion at 50 °C, illustrating the crucial role of Au particle size (< 5 nm) and oxidation state. Calcination at 300 °C produced the most active catalyst; temperatures above 300 °C led to particle growth and diminished performance.

Stability

Continuous operation tests at 70 °C and 40 °C maintained 100 % and 60 % CO conversion, respectively, over 10 h with no deactivation. At 200 °C, the 0.3 %Au/Ce_0.75‑La_0.25 catalyst retained 100 % conversion for 50 h, evidencing excellent thermal stability and resistance to sintering.

Conclusions

Ce‑La nanorods synthesized via a simple hydrothermal route retain their rod morphology across all La contents. LaO_x doping enhances the reducibility of the support and stabilizes highly dispersed Au^δ+ clusters, yielding exceptional low‑temperature CO oxidation activity. A 1.0 %Au/Ce_0.75‑La_0.25 catalyst converts CO to CO₂ completely at 30 °C and remains stable up to 200 °C. These findings underscore the potential of 1D binary Ce‑La nanorods as robust supports for precious metal catalysts in practical CO‑removal applications.