High Catalytic Efficiency of a Nanosized Copper-Based Catalyst for Automotives: A Physicochemical Characterization

Abstract

The global trend in restrictions on pollutant emissions requires the use of catalytic converters in the automotive industry. Noble metals belonging to the platinum group metals (PGMs, platinum, palladium, and rhodium) are currently used for autocatalysts. However, recent efforts focus on the development of new catalytic converters that combine high activity and reduced cost, attracting the interest of the automotive industry. Among them, the partial substitution of PGMs by abundant non-PGMs (transition metals such as copper) seems to be a promising alternative. The PROMETHEUS catalyst (PROM100) is a polymetallic nanosized copper-based catalyst for automotives prepared by a wet impregnation method, using as a carrier an inorganic mixed oxide (CeO₂-ZrO₂) exhibiting elevated oxygen storage capacity. On the other hand, catalyst deactivation or ageing is defined as the process in which the structure and state of the catalyst change, leading to the loss of the catalyst's active sites with a subsequent decrease in the catalyst's performance, significantly affecting the emissions of the catalyst. The main scope of this research is to investigate in detail the effect of ageing on this low-cost, effective catalyst. To that end, a detailed characterization has been performed with a train of methods, such as SEM, Raman, XRD, XRF, BET and XPS, to both ceria-zirconia mixed inorganic oxide support (CZ-fresh and -aged) and to the copper-based catalyst (PROM100-fresh and -aged), revealing the impact of ageing on catalytic efficiency. It was found that ageing affects the Ce-Zr mixed oxide structure by initiating the formation of distinct ZrO₂ and CeO₂ structures monitored by Raman and XRD. In addition, it crucially affects the morphology of the sample by reducing the surface area by a factor of nearly two orders of magnitude and increasing particle size as indicated by BET and SEM due to sintering. Finally, the Pd concentration was found to be considerably reduced from the material's surface as suggested by XPS data. The above-mentioned alterations observed after ageing increased the light-off temperatures by more than 175 °C, compared to the fresh sample, without affecting the overall efficiency of the catalyst for CO and CH₄ oxidation reactions. Metal particle and CeZr carrier sintering, washcoat loss as well as partial metal encapsulation by Cu and/or CeZrO₄ are identified as the main causes for the deactivation after hydrothermal ageing.

Keywords: BET; Ce–Zr mixed oxides; CuPdRh/CeZr washcoat; PGMs substitution; Raman; SEM; XDR; XPS; ageing; copper catalyst; three-way catalysts.

Figure 1

(a) Raman spectra of the CZ support material fresh and aged (b) same spectra normalized with respect to main peak.

Figure 2

Raman spectrum of Cu/Pd/Rh doped on the CZ support, PROM100-fresh.

Figure 3

(a) Raman spectrum of PROM100-aged; (b) focus on the low-intensity bands observed.

Figure 4

XRD patterns of the CZ- and PROM100-fresh samples. Refractive peaks indicated with asterisk correspond to Ni which was added to samples for calibration purposes.

Figure 5

(a) XRD diffractograms for PROM100-fresh and PROM100-aged samples; (b) focus on the low-intensity peaks observed. The refractive peak indicated with asterisk correspond to Ni, which was added to samples for calibration purposes.

Figure 6

Scanning electron microscopy images of CZ-fresh (a) and -aged (b) and PROM100-fresh (c) and -aged (d).

Figure 7

(a) Pd-Zr3p XP spectra window and (b) Cu2p XP spectra of PROM100-fresh and -aged.

Figure 8

Deconvoluted O1s XP spectra of CZ carriers and PROM100-fresh and -aged.

Figure 9

Light-off curves of the conversion of CO, CH₄ and NO of PROM100-fresh catalytic washcoat: (A) under rich-burn conditions (λ = 0.99) and (B) under lean-burn conditions (λ = 1.03).

Figure 10

Light-off curves of the conversion of CO, CH₄ and NO of PROM100-aged catalytic washcoat: (A) under rich-burn conditions (λ = 0.99) and (B) under lean-burn conditions (λ = 1.03).