Cu/Al2O3와 Cu/Ce/Al2O3 촉매의 표면구조가 CO와 CH4

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박 포 원

Center for Catalysis and Surface Science

Northwestern University, Evanston, IL 60208

The Influence of Surface Structure on the Catalytic Activity of

Alumina Supported Copper/Cerium Oxides Catalysts:

Oxidation of Carbon Monoxide and Methane

Paul Worn Park

Center for Catalysis and Surface Science

Northwestern University, Evanston, IL 60208

Introduction

Copper oxide is a well known component of catalysts for CO [1,2], hydrocarbon [3], chlorinated hydrocarbon [4] and alcohol oxidation [5] as well as for NOx [6] and SO2 reduction [7].  These air purification reactions using heterogeneous catalysts have been important topics due to their application to automobile exhaust converters and industrial waste incinerators.  Copper oxide based catalysts have been considered as potential candidates for substituting noble metal catalysts in emission control applications due to their high catalytic activity for the purification reactions, tolerance to SO2 [8] and refractory nature [9]. Several researchers reported that copper oxide based catalysts show similar activity to noble metal catalysts for CO oxidation [10], butanal and mercaptan oxidation [11], and reduction of NO by CO [12].

Rare earth oxide additives have been widely applied as promoters for supported noble metal catalysts.  Cerium addition prevents □-alumina from sintering at high temperature and improves the dispersion and thermal stability of noble metals [13].  In addition, cerium oxide has been added to noble metal catalysts for treatment of automobile exhaust to promote the water-gas shift reaction [14], to suppress the CO inhibition effect [15], and to remove CO, hydrocarbons, and NOx simultaneously using its oxygen storage ability [16].

Several previous studies of Cu/Al2O3 catalysts have focused on the structure or activity of the catalyst.  Little effort has been devoted to investigating systematically the relationship between the surface structure and CO and CH4 oxidation activity of Cu/Al2O3 catalysts.  The present work is a study to investigate structure-reactivity correlations for cerium oxide promoted copper oxide based emission control catalysts [17,18].

Experimental

X-ray photoelectron spectroscopy(XPS) and X-ray diffraction(XRD), and electron spin resonance(ESR) have been used to determine the effect of Cu loading on the chemical state and dispersion of copper species supported on □-alumina. These techniques also determined the effect of catalyst precursors and cerium promotion on the chemical state and dispersion of copper oxide phases in a series of Cu/Ce/Al2O3 catalysts.  The information derived from these techniques is correlated with CO and CH4 oxidation activity to develop a more complete understanding of Cu/Al2O3 and Cu/Ce/Al2O3 catalysts.

Catalysts were prepared by pore volume impregnation of □-alumina (200 m2/g, <230 mesh) using solutions of metal nitrate or alkoxide.  The impregnated samples were dried in air at 120 oC for 24 h and calcined in air at 500 oC for 16 h.  The copper content of the Cu/Al2O3 series was varied from 0 to 20 wt.% CuO (Cu/Al atomic ratio 0-0.13). Cu/Ce/Al2O3 catalysts(12 wt.% CuO and 5.1 wt.% CeO2) were prepared by impregnation of cerium first, followed by copper. The impregnated samples derived from the aqueous nitrate solution were designated "N" and catalysts prepared with alkoxide precursors were designated "A", such as CuN, CuA etc. Catalytic activity measurements were performed in a flow microreactor.  Prior to the first activity measurement, catalysts were pretreated with a mixture of 5% O2/He stream (143 cc/min) at 350 oC for 1 hr.  CO oxidation reactions were performed with a constant flow rate (80 cm3/min) of 4.8% CO/9.8% O2/85.4% He gas mixture in the temperature range of 100-220 oC. CH4 oxidation reactions were performed with a constant flow (15 cm3/min) of 0.98% CH4/5.25% O2/93.77% He gas mixture in the temperature range of 360-420 oC.

Results and Discussions

Copper/Alumina Catalysts.  For catalysts with Cu/Al atomic ratios < 0.051, XRD and XPS   data indicated that most of the Cu was present as a dispersed surface phase. This has been attributed to the formation of a surface spinel phase (CuAl2O4). ESR results showed that the ratio of isolated/interacting copper surface phase decreased with increasing Cu content.  For catalysts with Cu/Al atomic ratio > 0.077, the increase in XPS determined copper particle size with increasing Cu loading is consistent with the appearance of X-ray diffraction lines characteristic of CuO.  These results indicated that at a critical metal loading, the support becomes saturated with copper surface phase and further addition of the metal results in formation of CuO crystallites.

CO oxidation activity data (Figure) indicate that the activity (Turn Over Frequency) increased with increasing Cu content. The TOF increases with increasing Cu content by nearly a factor of 300 after normalization with Cu dispersion. The significant increase in CO oxidation TOF and decrease in activation energy as a function of Cu content observed for the Cu/Al2O3 catalysts suggests that CuO is the active phase for CO oxidation.  In addition, when one considers that the crystalline CuO is poorly dispersed compared to the copper surface phase, it is clear that CuO is significantly more reactive than the copper surface phase.  The high CO oxidation activity of the CuO phase can be attributed to more facile generation of oxygen vacancies during the CO oxidation reaction compared to the copper surface phase. TOF for CH4 oxidation decreased with increasing Cu content up to Cu/Al = 0.051.  This was paralleled by a significant decrease in the relative Cu ESR signal measured for the catalysts.  This indicates that the isolated Cu surface phase is more active for CH4 oxidation than the interacting copper surface phase or crystalline CuO.  For the high loading catalysts (Cu/Al > 0.077), the CH4 oxidation activity is independent of Cu content, which can be ascribed to the formation of poorly dispersed CuO crystallites.

Copper/Cerium/Alumina Catalysts.  Table shows the TOFs for CO oxidation over Cu/Al2O3 and Cu/Ce/Al2O3 catalysts.  The CuN catalyst shows higher TOF compare to the CuA catalyst.  Comparison of the CO oxidation activities measured for the CuN and CuA catalysts indicates that the catalyst containing more crystalline CuO (i.e. CuN) is more active than catalysts containing less CuO phase (CuA).  This is consistent with the result of Cu/Al2O3, which showed that crystalline CuO was more active for CO oxidation than a dispersed copper surface phase.

Table. Specific Turn Over Numbers of CO and CH4 Oxidation Measured for Cu/Ce/Al2O3 Catalysts.

Catalysts	CO Oxidation (s-1 x 102) at 160 oC	CH4 Oxidation (s-1 x 104) at 390 oC
CuN	1.1	3.7
CuA	0.3	1.2
CuNCeN	8.7	3.7
CuACeN	3.0	1.3
CuNCeA	13.7	4.5
CuACeA	1.5	1.4

Ce addition increases the TOFs measured for the Cu/Al2O3 catalysts by a factor of 5 to 12.  The higher CO oxidation TOF for cerium promoted copper catalysts indicates that cerium oxide is very effective promoter when one considers that only a small amount of cerium oxide (5.1 wt.%) was used in this study.  Changing crystallinity and dispersion of cerium oxide species after Cu addition indicates that copper oxide interacts strongly with cerium oxide during catalyst preparation.  The Cu/Ce/Al2O3 catalysts derived from CuN show higher CO oxidation activity than the catalyst prepared with CuA.  This is consistent with the result of CuN and CuA catalysts, which shows higher CO oxidation activity for the catalyst containing more crystalline CuO.

Table also shows the TOFs for CH4 oxidation calculated at 390 oC.  The TOFs of the catalysts derived from CuN are approximately 3 times higher than the catalysts prepared with CuA.  Ce addition has little effect on the CH4 oxidation TOF for copper catalysts.  In our study, CuA catalyst may contain less isolated copper surface phase than CuN catalyst, although Cu dispersion of CuA is higher than that of CuN.  The catalyst preparation step (sol-gel) may lead to the formation of an interacting copper surface phase (i.e. clustered copper species) and/or amorphous copper oxide which may be a less active phase than isolated copper species for CH4 oxidation.  The absence of a cerium promoter effect on CH4 oxidation reaction for Cu/Ce/Al2O3 catalysts can be explained by limited interaction between poorly dispersed cerium oxide and isolated copper surface phase.

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