This paper investigates the effects of CuO contents in the CuO-CeO2 catalysts to the variation in physical properties of CuO/CeO2 catalysts and correlates them to their catalytic activities on selective CO oxidation. The characteristic of crystallites were revealed by X-ray diffraction, and their morphological developments were examined with TEM, SEM, and BET methods. Catalytic performance of catalysts was investigated in the temperature range of 90–240°C. The results showed that the catalyst was optimized at CuO loading of 20 wt.%. This was due to the high dispersion of CuO, high specific surface area, small crystallite sizes, and low degree of CuO agglomeration. Complete CO conversion with near 100% selectivity was achieved at a temperature below 120°C. The optimal performance was seen as a balance between CuO content and dispersion observed with growth, morphology, and agglomeration of nanostructures.
The generation of clean electrochemical hydrogen energy via proton exchange membrane fuel cells (PEMFCs) has attracted wide interests for stationary and mobile applications with high efficiency, high power density, and rapid startup. A multistep process requires for producing hydrogen on board usually accomplished by reform of hydrocarbons or methanol, followed by high and low temperature water gas shift reaction (WGSR) [
Binary CuO-ZnO and ternary CuO-ZnO-Al2O3 mixed oxide catalysts have been widely employed commercially for the WGSR. Unfortunately, the problems for applying CuO-ZnO catalysts under these conditions are pertain to poor thermal stability and the pyrophoricity of the material [
The characteristic properties that affect the catalytic performance are usually observed in terms of surface area, particle size, dispersion of the active metals, and structural defects such as oxygen vacancies [
In this work, the variation in physical properties and CO oxidation performances of coprecipitated CuO/CeO2 catalysts were investigated over whole range of CuO loadings. The activity and selectivity of the catalysts were discussed and correlated with their characterizations results obtained from XRD, TEM, SEM, and BET measurements. The changes on morphology of nanostructures and agglomeration of structures observed with CuO loadings could assist us in identifying influential factors in term of synergistic effect on performance of the catalyst.
Catalysts of various CuO/CeO2 weight ratios were prepared by coprecipitation method. For each weight ratio, the proper amount of Cu(NO3)2·3H2O (BDH) and Ce(NO3)3·6H2O (Aldrich) was dissolved separately in 400 mL of deionized water. Then, 100 mL each of these two salt solutions were mixed well using a magnetic stirrer. NH4OH solution (Richer Chemicals) of 1 M was slowly added into the previous solution under continuous stirring. The precipitate was observed at pH 9. The solution was continuously stirred for 30 min. After stirring, the precipitate was washed with deionized water several times to remove excess ions. The cleaned precipitate was dried at 110°C for 24 h and then calcined at 500°C for 10 h. The obtained powder was ground and sieved to mesh sizes of 80–100. The weight ratios of CuO to CeO2 investigated in this work were 10/90, 20/80, 40/60, 60/40, 80/20, and 90/10. The catalyst will thereafter be referred to by the CuO weight %; that is, 20% CuO represents the catalyst containing 20% of CuO and 80% of CeO2 by weight.
The crystalline structure of the catalysts was characterized by X-ray diffraction (XRD) technique. Powder XRD patterns were recorded at room temperature using a Bruker D8 advance powder diffractometer equipped with a CuK
A TEM (JEOL JSM-2100) equipped with an EDX system operated at 200 kV was used to determine the characteristics of nanostructures as well as observing the dispersion of the 20% CuO catalyst. The SEM images (JEOL JSM-6510) were taken at 5 kV for all CuO loadings for observing agglomeration.
BET measurement was used to determine the surface area and pore size of CuO/CeO2 catalysts. The measurement was carried out with adsorption-desorption isotherms of liquid N2 using Autosorption-1 C from Quantachrome. Approximately 100 mg of calcined catalyst was placed in a quartz reactor. Before measuring, the catalyst was heated at 200°C for 0.5 h under N2 gas purging as a pretreatment. The BET surface area and pore size of all CuO/CeO2 catalysts with different weight ratios were calculated at N2 adsorption-desorption isotherms with P/Po between 0.05 and 0.35.
The activity tests were carried out in a fixed-bed reactor (4 mm ID) at atmospheric pressure. For each test, 80 mg of catalyst was loaded inside the reactor on top of quartz wool. The reaction temperature inside the reactor was measured by a
The CO conversion was obtained by comparing the CO concentration at the bypass line and the outlet stream from the reactor. Selectivity to CO oxidation was defined as the ratio of oxygen consumed by CO oxidation to the total oxygen consumption (obtained by subtracting the O2 concentration at the reactor outlet from the O2 concentration in the feed). The amount of O2 not used in CO oxidation reaction was assumed to oxidize H2. Importantly, there was no methane formation observed under reaction conditions performed in this study. The selectivity can be expressed as the follows:
Figure
XRD patterns of CuO/CeO2 catalysts for various CuO loadings.
The presence of the CuO phases for catalysts containing less than 20 wt.% CuO was confirmed with TEM results in Figure
TEM results of 20% CuO/CeO2: (a) typical TEM image, (b) SAED, and (c) HRTEM images.
The average crystallite sizes of CeO2 and CuO, calculated from the CeO2(111) peak and CuO(111) peak using Sherrer’s equation, are reported in Table
Physical properties of the catalysts.
Sample |
|
Average pore |
Average crystallite size | |
---|---|---|---|---|
(wt.%) | (m2/g) | size (nm) | CuO (111) | CeO2 (111) |
CuO/CeO2 = 0/100 | 40.6 | — | — | 17.1 |
CuO/CeO2 = 10/90 | 105.7 | 4.2 | — | 11.02 |
CuO/CeO2 = 20/80 | 109.2 | 4.4 | 5.9 | 10.4 |
CuO/CeO2 = 40/60 | 106.7 | 5.8 | 8.84 | 10.4 |
CuO/CeO2 = 60/40 | 73.6 | 7.8 | 10.1 | 9.5 |
CuO/CeO2 = 80/20 | 30.5 | 12.4 | 15.7 | 8.7 |
CuO/CeO2 = 90/10 | 12.5 | — | 17.7 | — |
CuO/CeO2 = 100/0 | 64.9 | — | 20.2 | — |
The BET surface area and average pore size of these catalysts are listed in Table
The morphology development of the catalyst is demonstrated by a sequence of SEM micrographs in Figure
SEM micrographs of the catalysts with the percentage numbers below each picture indicated the level of CuO loading by wt.%.
For the catalysts of CuO content between 10 and 20 wt.%, a uniform morphology with narrow particle size distribution was observed. This indicates the effectiveness of CeO2 support in anchoring and dispersing of CuO, and consequently the catalyst exhibits a low degree of CuO agglomeration. The aggregation of mixed nanoparticles generates voids between particles. The low degree of CuO agglomeration provides a larger interface area and together with a small CuO crystallite is responsible for high surface area (~109.2 m2/g) of the catalyst.
For higher contents of CuO (>40 wt.%), crystallites were agglomerated into larger particles with increasing CuO content. Some researchers had revealed the composite distribution of the catalysts over several ranges of CuO content. Such a result that is the EFTEM elementary maps of Ce, Cu, and O particles had been shown with indiscernible of CeO2 and CuO composites [
At very high copper contents (>80 wt.%), the XRD results present with strong crystallinity of CuO. Due to the low amount of CeO2 to adequately disperse and oxidize the active metal, CuO was agglomerated into larger bulk CuO. This bulk CuO is the third form of copper in the composite oxides which can be examined by XRD; however, it makes little contribution to catalytic activity.
The catalytic performance of the CuO/CeO2 catalysts was firstly investigated on CO oxidation with the feeding gases containing 1% CO, 1% O2, and He balance. The CO conversion results for the catalysts of different CuO contents (wt.%) are presented along with the performance of the pure oxides in Figure
CO oxidation performances of CuO/CeO2 catalysts for various weight ratios with their pure oxides are also shown. Feed composition (v/v): 1% CO, 1% O2, and 98% He.
Pure CeO2 shows constantly low activity over the temperature range, while the CO conversion for the pure CuO rises sharply after 130°C and reaches 70% conversion at around 190°C. The catalyst of 10% CuO loading lowers the 50% CO conversion (T50) temperature by almost 80°C. The remarkable increase of catalytic activity at low temperature of the catalyst is well known and extensively reported as the synergetic effects. The low activity group of catalysts is seen with 10, 80, and 90 wt.% of CuO content. The sluggish activities of the catalysts are result from different effects and can clearly be observed in Table
The 10% CuO catalyst possesses with high specific surface area and small CuO crystallite size, but its adverse characteristic is the larger CeO2 crystallite size of (11.02 nm) and as a result lowers surface oxygen storage capability. The catalysts with 80% and 90% CuO content suffered directly from the low specific surface areas in addition to the large CuO crystallite sizes [
The high activity group of catalysts (CuO loading of 20, 40, and 60 wt%) can reach the full CO oxidation at temperature below 120°C. Considering the CuO loading for this group (20–60 wt.%), the development of cluster structures and agglomeration into larger particles was previously discussed with SEM and TEM results with increasing CuO content. An optimal number of CuO crystallites agglomerated into a nanostructured composite particle, referred to by Ayastuy et al. [
Following the results from the catalytic oxidation of the mixed oxides, the group with high activity (CuO loading of 20, 40, and 60 wt.%) was selected for further investigation on selective CO oxidation in H2 rich stream. Plots of the CO conversion, O2 consumption, and CO selectivity as functions of the reaction temperature are shown in Figures
Selective CO oxidation performances of CuO/CeO2 catalysts of various weight ratios: (a) CO conversion, (b) O2 consumption, and (c) selectivity to CO. Feed composition (v/v): 1% CO, 1% O2, 60% H2, and balance with He. SV = 60,000 cm−3 g−1 h−1.
The O2 consumption in Figure
In addition, the catalytic activities of CuO-CeO2 catalysts were related to their physical properties: crystallite sizes, morphology development of structures such as cluster, and agglomeration of particles and BET surface area. The catalysts had been identified to exist in a form of mixed crystallites of monoclinic CuO and cubic CeO2 structures. At low CuO content (10–20 wt.%), CeO2 support can effectively anchors and disperse CuO, and this result in a uniform morphology and narrow particle size distribution of copper species. The aggregation of nanoparticles generates voids and together with small CuO crystallite size of ~5.9 nm is responsible for the high specific surface area (~109.2 m2/g) of the mixed oxides. As the content of CuO was increased, CuO began to form loosely packed CuO cluster structures (CuO content between 20 and 40 wt.%) and eventually the excess CuO aggregated and form bulk-like crystalline CuO (>40 wt.% CuO). Drastic reduction of BET surface area was seen in catalysts with high CuO content due to the formation of large bulk-like crystalline CuO. The agglomeration of crystallites into larger copper particles reduces the interface area and results in the weakening of the metal-support interaction. A reverse morphology, described as finely dispersed of ceria crystallites over a copper oxide support, occurred at the CuO content greater than 80 wt.%. The optimal CuO loading (between 10 and 20 wt.%) provides the catalysts with small crystallite sizes, high specific surface area, and the formation of small CuO cluster, identified by researchers [
The CO poisoning of PEMFCs is a major problem to deplete the efficiency and energy conversion of PEMFCs. To remove a trace amount of CO in the reformed gas is essential. In this work, the catalytic performance of CuO-CeO2 in selective CO oxidation in the presence of excess hydrogen has been studied. The content of CuO in the catalysts has strong affects to their physical properties and their catalytic activities to the reaction. High CO conversion at low temperature was obtained when the catalysts contained the small crystallite sizes, high specific surface area, and the formation of small CuO cluster. Under our catalyst preparation condition, we found that the optimal CuO content of CuO-CeO2 catalysts using in CO-PROX was 20% with high CO conversion and high selectivity to CO oxidation over the temperature range of 110–130°C.
The authors gratefully acknowledge the financial support from Faculty of Engineering, Burapha University under the Contract no. 16/2550.