Selective methanation of CO over supported noble metal catalysts: Effects of the nature of the metallic phase on catalytic performance
Graphical abstract
Methanation of CO, CO2 and CO-CO2 mixtures has been investigated over Al2O3-supported noble metal catalysts. Ru and Rh are more active for the selective methanation of CO, compared to Pt and Pd catalysts, which promote the undesired WGS reaction. Addition of water vapor in the feed does not affect CO hydrogenation but retards CO2 hydrogenation, thereby expanding the temperature window of operation.
Introduction
The methanation reaction has been widely used as a method of removal of carbon oxides from gas mixtures in hydrogen or ammonia plants, and for the purification of hydrogen streams in refineries and ethylene plants [1]. The selective methanation of CO may become attractive as a potentially effective means of reduction of CO content of hydrogen-rich reformate gases to extremely low levels, as required in fuel cell applications [2], [3]. Fuel processors designed for use with polymer electrolyte membrane (PEM) fuel cells typically consist of a reformer, which converts the fuel into a hydrogen-rich gas stream, and a water-gas shift (WGS) unit, which reduces byproduct carbon monoxide to 0.5–1.0 %, thus improving H2 yield. An additional cleanup step is required to reduce CO levels to less than 50 ppm, as dictated by the poisoning limit of PEM fuel cell electrodes [4], [5], [6], [7]. The preferential oxidation of CO (PROX) has been proposed for this purpose and studied by many investigators [8], [9], [10]. However, this approach requires the addition of oxygen (air) in the hydrogen-rich gas stream, which may give rise to various problems related to reduced hydrogen yield, dilution, safety and restrictions in the operating parameters. Thus, CO methanation (Eq. (1)) is investigated as an alternative purification step [2], [3], [8], [11]:CO + 3H2 ↔ CH4 + H2O, ΔH° = −206 kJ/mol
This approach has the advantage that no oxidizing and/or inert gases are mixed with the reformate stream and that the methane produced, which is inert to the PEM fuel cell, can be utilized in the afterburner. However, in order for this method to be effective, a suitable catalyst must be applied to promote selectively the CO methanation reaction at the expense of CO2 methanation, which consumes significant quantities of hydrogen:CO2 + 4H2 ↔ CH4 + 2H2O, ΔH° = −165 kJ/mol
Thus, selectivity of the catalyst in promoting CO methanation versus CO2 methanation () is of paramount importance. This parameter can be defined based on hydrogen consumption as follows:It is of interest to note that under methanation reaction conditions of CO/CO2 mixtures, is relatively high at low temperatures because conversion of carbon dioxide is inhibited until the concentration of carbon monoxide has been reduced to about 200–300 ppm [1], [3], [12].
The reverse water-gas shift (RWGS) reaction may act as an obstacle in the complete, selective methanation of CO:CO2 + H2 ↔ CO + H2O, ΔH° = 41.1 kJ/mol
Consequently, it is important to develop selective CO methanation catalysts characterized by high activity at sufficiently low temperatures, able to retard both the CO2 methanation and the RWGS reactions.
The methanation of CO, in the absence of CO2, has been investigated over a variety of supported metal catalysts, including Ni [13], [14], [15], [16], Ru [2], [13], [17], Pt [17] and Ni-Ru [18]. Nickel catalysts were found to exhibit high activity for the reaction [1], which is also affected by the nature of the support. Fujita et al. [13] reported that the methanation of CO proceeds more rapidly with the use of Ni/Al2O3 catalysts, compared to Ni/SiO2 and Ru/SiO2, whereas Görke et al. [2] reported that Ru/SiO2 catalyst exhibits higher CO conversion and selectivity, compared to Ru/Al2O3. Concerning CO2 methanation, a number of studies have been published dealing with the performance of supported Ru [18], [19], [20], [21], Pt [19], Pd [19], [22], Ir [19], Rh [19], [23], [24], Ni [25], [26], Co [21] and Fe [21] catalysts. It is generally accepted that the hydrogenation of CO2 proceeds through the formation and hydrogenation of CO, with higher selectivity toward methane, compared to CO hydrogenation reaction [13], [22]. For example, Inui et al. [18] reported that the reaction rate for CO2 methanation is higher than that of CO over Ni-La2O3 and Ni-La2O3-Ru catalysts. However, under conditions of simultaneous CO and CO2 methanation, the coexistence of CO and CO2 in the gas mixture, results in complete retardation of CO2 methanation until CO has been totally converted to methane.
A detailed investigation is being carried out in this laboratory in an attempt to identify the key parameters which determine the performance of supported noble metal catalysts for the title reaction and to explore reaction kinetics and mechanism. The objective is to develop active, selective and stable methanation catalysts, able to efficiently and selectively remove CO from reformate gases to levels lower than those dictated by the poisoning limits of fuel cells. In the present study, the effects of the nature of the dispersed metallic phase on the catalytic performance of Al2O3-supported noble metal catalysts is investigated for the methanation of CO, CO2 and their mixtures.
Section snippets
Catalyst preparation and characterization
Supported noble metal catalysts were prepared by impregnation of γ-Al2O3 (Alfa Products) powder with an aqueous solution of the corresponding metal precursor salt (Rh(NO3)3, Ru(NO)(NO3)3, (NH3)2Pt(NO2)2, (NH3)2Pd(NO2)2) (Alfa Products). The resulting slurry was heated slowly to 70 °C under continuous stirring and maintained at that temperature until nearly all the water evaporated. The solid residue was dried at 110 °C for 24 h and then reduced at 300 °C (400 °C for Ru catalysts) in H2 flow for 2 h.
Catalyst characterization
The specific surface area of commercial Al2O3 powder used in the present study, determined with the BET method, was 83 m2/g. The physicochemical characteristics of the synthesized Al2O3-supported Pt, Ru, Rh and Pd catalysts are summarized in Table 1, where the dispersion and mean crystallite size (dM) are listed for all samples investigated. It is observed that metal dispersion is typically higher than 70% for all catalysts studied.
Effect of the nature of the metallic phase
The effect of the nature of the dispersed metallic phase on
Conclusions
Results of the present study show that catalytic performance, apparent activation energy and selectivity to reaction products for the solo- or co-methanation of CO/CO2 depend strongly on the nature of the metallic phase. Generally, methanation activity is much higher for Ru and Rh catalysts, compared to Pd or Pt, which tend to enhance the WGS reaction. Under solo-CO methanation conditions, selectivity to CH4 increases progressively with increasing temperature. Higher hydrocarbons are also
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