**Abstract**

In this work, catalysts based on cobalt supported on ZrO2 and CeO2 and CoCeMnOx were studied for the CO preferential oxidation (COPrOx) in hydrogenrich stream able to feed fuel cells. Among them, the CoCeMnOx formulation showed the highest CO conversion at low temperatures, while the cobalt oxide supported on ceria presented the best selectivity toward CO2. The Co3O4 spinel was the active phase for the CO preferential oxidation detected in all catalysts. However, the CoOx-CeO2 and CoCeMnOx catalysts resulted more active than cobalt oxide supported on zirconia. The presence of ceria close to cobalt species promotes the redox properties and enhances the catalytic activity. In the CoCeMnOx catalyst prepared by coprecipitation, the incorporation of Mn represented an additional positive effect. The presence of Mn promoted the reoxidation of Co2+ to Co3+ and, consequently, the activity increased at low temperature. By X-ray diffraction (XRD) of CoOx-ZrO2 and the CoOx-CeO2 catalysts, the Co3O4 spinel and ZrO2 or CeO2 were identified in agreement with laser-Raman spectra. At the same time, the CoCeMnOx catalyst, prepared by coprecipitation of precursor salts, showed an incipient development of a new phase (Mn,Co)3O4 mixed spinel, due to the intimate contact between elements.

**Keywords:** COPrOx, (Mn,Co)3O4 mixed spinel, redox couple, CoCeMnOx, CeO2 support, XPS, laser-Raman spectroscopy

## **1. Introduction**

The global demand for energy has been inexorably growing in the last decades. The increasing use of fossil fuels in order to generate energy causes serious problems to the environment due to the gaseous emissions. This fact has produced a global movement toward trying to remove the contaminants from combustion effluents. Besides, the exploration for alternatives to fossil fuels, biofuels or hydrogen as an energy vector, has gained an immediate and future significance because this could contribute to the depletion of greenhouse gases [1–3].

Fuel cells are devices that are being actively developed, because they are power generation systems that can produce energy with significantly less impact on the environment. Among several types of fuel cells, proton exchange membrane fuel cells (H2-PEMC) are considered to be the most technically advanced for such application [4]. Hydrogen produced by means of the steam reforming or autothermal process of hydrocarbons or alcohols should contain less than 10 ppm of carbon monoxide before entering the cell since CO poisons the Pt anode of the fuel cells (**Figure 1**) [5, 6]. After the reforming step, the hydrogen production process

**Figure 1.**

*Schematic flow diagram of a typical fuel cell processor with COPrOx.*

continues with the water gas shift reaction (WGSR), where the stream is enriched in hydrogen and the CO content is diminished to 1%. As the CO concentration in the hydrogen stream is so high to enter to the cell, it is necessary to reduce it to the desired levels. Among various methods such as catalytic methanation, Pd-based membrane, and catalytic CO preferential oxidation (COPrOx), the latter is considered as the most adequate due to its simplicity and effectiveness. Thus, given the importance of the hydrogen purification process, the last few years have witnessed the surge of a renewed interest in the CO oxidation reaction, and several contributions dealing with this issue have recently been published [5–7].

In our research group, catalysts for the removal of contaminants from vehicles, industrial facilities, and power-generating sources have been studied [8, 9]. In addition, catalysts and catalytic reactors have also been investigated for the production and purification of H2 to be used in fuel cells [10–13].

On the other hand, due to their redox properties, cobalt-containing catalysts have been the object of numerous publications in the environmental catalysis field and in the purification of H2 stream, among other applications. For example, catalysts based on cobalt oxides have been studied for soot combustion [14], NOx selective reduction in oxygen excess [15], and CO preferential oxidation reaction [16].

The challenges involved in the design of successful COPrOx catalysts are the following: (i) high CO oxidation activity at low temperatures, (ii) high selectivity for CO oxidation against the undesired H2 oxidation, and (iii) good resistance to deactivation caused by the H2O and CO2 in the feed [5, 6, 17].

In order to reduce the CO concentration without consuming H2, several catalysts have been studied with promising results. Catalysts of Pt supported on bimetallic systems achieved excellent results [17–19], but the high cost of noble metals led the investigations toward the use of other active phases. For instance, CuO/CeOx catalysts have shown a catalytic performance in the COPrOx comparable to those based on noble catalysts [20–22].

In this vein, it has been shown that the combined effect of cobalt oxide and ceria strongly influences the morphological and redox properties of the composite oxides, by dispersing the Co3O4 phase and promoting the efficiency of the Co3+/Co2+ redox couple [23, 24]. Numerous publications have shown that the addition of other metals to a cerium-based mixed oxide (Zr or Mn) increases the oxygen storage capability of the ceria. This effect is originated by the increase in the surface oxygen mobility due to chemical interactions between cobalt, cerium, and other metals [25–27].

On the other hand, the use of structured catalysts as those supported onto honeycomb monoliths has long been considered in the chemical industry, and it has increased with the significance of environmental catalysis in pollution abatement applications due to catalysts with a high attrition resistance and a low pressure drop that are required. In addition the thin catalytic coating allows high efficiency and

**55**

*Cobalt-Based Catalysts for CO Preferential Oxidation DOI: http://dx.doi.org/10.5772/intechopen.88976*

for the monolithic catalysts [12, 13, 15, 16].

**2. Materials and methods**

volume ca. 0.013 cm3

overnight at 120°C.

500°C at 10°C/min.

**2.2 Characterization**

*2.2.1 Chemical composition quantification*

**2.1 Preparation of cobalt-based catalysts**

selectivity. Cordierite offers high mechanical strength, high resistance to elevated temperatures, and temperature shocks due to its low thermal expansion coefficient. Moreover, it presents great adhesion stability, which is a very important property

concentration, and chemical nature of species present on catalysts.

their relation to the centers active in the CO preferential oxidation.

impregnation and labeled CoZ. A commercial zirconia support (6.8 m2

In our previous work with structured catalysts, we showed that cobalt supported on zirconia and ceria and CoCeMnOx mixed oxides resulted in efficient catalysts for the COPrOx. However, it is of great interest both to identify the cobalt species present in an active catalyst and to analyze the interactions with the support (CeO2 or ZrO2) and their influence on the catalytic behavior. It is also important to understand the role of cobalt in the CoCeMnOx mixed oxides. In the present work, a very detailed analysis of the species present in these complex systems was carried out. The catalysts were analyzed by X-ray diffraction (XRD) and temperatureprogrammed reduction (TPR). The X-ray photoelectron spectroscopy (XPS) and laser-Raman spectroscopy (LRS) were used to characterize the oxidation state,

Three main groups of Co-based catalysts with 10 wt.% of cobalt were prepared— CoOx/ZrO2, CoOx-CeO2, and CoCeMnOx—in order to study the cobalt species and

/g) was impregnated with an aqueous solution of Co(NO3)2.

/g, pore

The CoOx-ZrO2 catalyst with 10 wt.% cobalt on ZrO2 was prepared by wet

The mixture was evaporated until achieving a paste, which was placed in the oven

CoOx-CeO2 catalysts were prepared by two different methods. The CoCe-I sample was produced by the wet impregnation method with 10 wt.% of cobalt, using an aqueous solution of Co(NO3)2 and CeO2 powder as support which was obtained by precipitation of the Ce(NO3)3 solution with NH4(OH). The mixture was evaporated under continuous agitation until achieving a paste, which was placed in the oven overnight at 110°C. By means of the coprecipitation method, CoCe-P catalyst was obtained. The aqueous solution of Co(NO3)2 and Ce(NO3)3 was precipitated with NH4(OH) added drop by drop under vigorous stirring. The resulting precipitate was filtered and washed several times with distilled water and dried overnight at 110°C. The CoCeMnOx oxide mixture catalyst with 10 wt.% of Co and Mn/Co molar ratio of 1/4 was prepared by the coprecipitation method, adding NH4(OH) to an aqueous solution of Co(NO3)2, Ce(NO3)3, and Mn(NO3)2. The mixture was kept 2 h under continuous stirring at room temperature. The precipitate obtained was washed several times with deionized water and then dried overnight at 110°C. After drying, all obtained solids were calcined for 5 h at 500°C under flowing air (25 mL/min), and the temperature was ramped from room temperature up to

Elemental analyzes were performed by inductively coupled plasma atomic emission spectroscopy (ICP-AES) on an ICP Optima 2100 DV PerkinElmer instrument.

*Cobalt-Based Catalysts for CO Preferential Oxidation DOI: http://dx.doi.org/10.5772/intechopen.88976*

*Cobalt Compounds and Applications*

**Figure 1.**

continues with the water gas shift reaction (WGSR), where the stream is enriched in hydrogen and the CO content is diminished to 1%. As the CO concentration in the hydrogen stream is so high to enter to the cell, it is necessary to reduce it to the desired levels. Among various methods such as catalytic methanation, Pd-based membrane, and catalytic CO preferential oxidation (COPrOx), the latter is considered as the most adequate due to its simplicity and effectiveness. Thus, given the importance of the hydrogen purification process, the last few years have witnessed the surge of a renewed interest in the CO oxidation reaction, and several contribu-

In our research group, catalysts for the removal of contaminants from vehicles, industrial facilities, and power-generating sources have been studied [8, 9]. In addition, catalysts and catalytic reactors have also been investigated for the production

On the other hand, due to their redox properties, cobalt-containing catalysts have been the object of numerous publications in the environmental catalysis field and in the purification of H2 stream, among other applications. For example, catalysts based on cobalt oxides have been studied for soot combustion [14], NOx selective reduction in oxygen excess [15], and CO preferential oxidation reaction [16]. The challenges involved in the design of successful COPrOx catalysts are the following: (i) high CO oxidation activity at low temperatures, (ii) high selectivity for CO oxidation against the undesired H2 oxidation, and (iii) good resistance to

In order to reduce the CO concentration without consuming H2, several catalysts have been studied with promising results. Catalysts of Pt supported on bimetallic systems achieved excellent results [17–19], but the high cost of noble metals led the investigations toward the use of other active phases. For instance, CuO/CeOx catalysts have shown a catalytic performance in the COPrOx comparable to those

In this vein, it has been shown that the combined effect of cobalt oxide and ceria strongly influences the morphological and redox properties of the composite oxides, by dispersing the Co3O4 phase and promoting the efficiency of the Co3+/Co2+ redox couple [23, 24]. Numerous publications have shown that the addition of other metals to a cerium-based mixed oxide (Zr or Mn) increases the oxygen storage capability of the ceria. This effect is originated by the increase in the surface oxygen mobility due

to chemical interactions between cobalt, cerium, and other metals [25–27]. On the other hand, the use of structured catalysts as those supported onto honeycomb monoliths has long been considered in the chemical industry, and it has increased with the significance of environmental catalysis in pollution abatement applications due to catalysts with a high attrition resistance and a low pressure drop that are required. In addition the thin catalytic coating allows high efficiency and

tions dealing with this issue have recently been published [5–7].

deactivation caused by the H2O and CO2 in the feed [5, 6, 17].

based on noble catalysts [20–22].

and purification of H2 to be used in fuel cells [10–13].

*Schematic flow diagram of a typical fuel cell processor with COPrOx.*

**54**

selectivity. Cordierite offers high mechanical strength, high resistance to elevated temperatures, and temperature shocks due to its low thermal expansion coefficient. Moreover, it presents great adhesion stability, which is a very important property for the monolithic catalysts [12, 13, 15, 16].

In our previous work with structured catalysts, we showed that cobalt supported on zirconia and ceria and CoCeMnOx mixed oxides resulted in efficient catalysts for the COPrOx. However, it is of great interest both to identify the cobalt species present in an active catalyst and to analyze the interactions with the support (CeO2 or ZrO2) and their influence on the catalytic behavior. It is also important to understand the role of cobalt in the CoCeMnOx mixed oxides. In the present work, a very detailed analysis of the species present in these complex systems was carried out. The catalysts were analyzed by X-ray diffraction (XRD) and temperatureprogrammed reduction (TPR). The X-ray photoelectron spectroscopy (XPS) and laser-Raman spectroscopy (LRS) were used to characterize the oxidation state, concentration, and chemical nature of species present on catalysts.
