Effect of Rh addition on activity and stability over Ni/γ-Al2O3 catalysts during methane reforming with CO2

Marco Ocsachoquea, Claudia E. Quincocesa, M. Gloria Gonzáleza

a Centro de Investigación y Desarrollo en Ciencias Aplicadas Dr. J.J.Ronco (CINDECA) (CONICET, UNLP), 47 Nro 257, 1900 La Plata, Argentina.

ABSTRACT

The effect of low Rh contents was studied in Ni/γ-Al2O3 catalysts. Activity, stability and carbon deposition on these catalysts were analyzed for methane reforming with CO2. Catalysts were characterized by XRD, TGA, TPR and flow-reaction. Results indicated that the catalyst promoted by Rh showed a higher activity and a similar carbon deposition than the Rh and Ni monometallic catalysts. Rh addition favors an interaction compound in the Rh-Ni/γ-Al2O3 that would improve the methane reforming activity of the supported Ni-based catalysts.

1. INTRODUCTION

In last decades, methane reforming with CO2 to produce synthesis gas has acquired special attention due to appraisement of natural gas and CO2, both of them with impact on the environment and energy resources [1,3]. This reaction is attractive because it can be employed in areas where the water is not available and it produces syngas with lower H2/CO ratios convenient for Fisher-Tropsch synthesis [4] and can contribute to the use of natural gas fields containing considerable CO2 amount.

Reaction conditions (high temperatures, CH4/CO2 ratio) used in methane reforming are the principal deactivation causes of supported Ni catalysts employed at industrial scale. Deactivation produced by carbon deposition,

Natural Gas Conversion VIII F.B. Noronha, M. Schmal, E.F. Sousa-Aguiar (Editors)© 2007 Published by Elsevier B.V.

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metallic sintering and poisoning by sulfur affects their catalytic properties. At the same time, supported catalysts of noble metals presented high catalytic activity for dry reforming and resistance to deactivation by carbon deposition. However, it is important to develop Ni catalysts resistant to deactivation considering the low availability and high cost of noble metals. One of the most important options for enhancing activity and stability in methane reforming reactions is the use of bimetallic catalyst. Several studies [5-11] report that addition of noble metals to Ni- based catalyst increases the thioresistance and reduces carbon deposition. Rostrup-Nielsen reported that Rh and Ru decrease the formation of filamentous carbon due to low carbon solubility in the metal. Ruckennstein and Wang [12] stated that supports as γ-Al2O3, SiO2, La2O3 and MgO have important effect on the metallic Rh formation; this metallic Rh is proposed as active site for reforming with CO2. These results suggest that the addition of noble metals in the formulation of Ni/γ-Al2O3 catalysts can improve stability and resistance to catalyst deactivation, and in this way they are more convenient at industrial scale.

The aim of this work was analized the effect of the addition of low Rh contents in Ni/γ-Al2O3 catalysts. Activity, stability and carbon deposition on these catalysts were studied for methane reforming with CO2.

2. EXPERIMENTAL

Ni and Rh monometallic catalysts were prepared by impregnation at incipient wetness of γ-alumina with solutions of Ni(NO3)2.4H2O and RhCl3.4H2O, respectively. The corresponding bimetallic samples were prepared by impregnating first Rh and then Ni, in nominal contents of 0.5% Rh and 5% Ni onto the support. Then, Ni and Rh-Ni catalysts were dried and calcined at 500°C for 1 h. The Rh monometallic sample was calcined at 380°C. Before the catalytic test, catalysts were reduced in situ with a pure hydrogen flow at 650°C for 1h.

Fresh and used catalysts were characterized by X Ray Diffraction, Temperature Programmed Reduction and Thermal Gravimetric Analysis in order to analyze physical and physicochemical characteristics and their effect on catalytic properties, with special emphasis in deactivation problems.

Identification of the crystalline phase by XRD was determined in a Philips PW 1740 equipment, with CuKα radiation in the range 2θ = 20-80° and with scan speed of 0.02°min-1.

TPR analyses were carried out in a conventional equipment (Quantasorb QS JR-2) with samples of 0.1g heated from RT to 1000°C at a rate of 10°C min-1 using a 10% (v/v) H2/N2 gas flow rate of 20 cm3 min-1.

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The amount of carbonaceous deposits was determined by thermogravimetric analysis (TGA) in a Shimadzu TGA-50H under air stream (20 cm3 min-1). The temperature was increased from 25 to 800°C at a heating rate of 10°C min-1. Experiments were performed over samples extracted from the reactor after 20 reaction hours.

Catalytic properties were determined in a flow system with fixed bed reactor connected in series to a Perkin Elmer chromatograph containing a 4 m Porapack Q column, a thermal conductivity detector and He gas (40 cm3 min-1) as carrier. The reactor was fed with 100 cm3 min-1 of a CH4/CO2=1.2 mixture and He as balance. A mass of 0.05 g catalyst was loaded into the quartz reactor. Catalytic reaction was performed at 650°C and atmospheric pressure in conditions of chemical control. The sulfur resistance was measured by adding 0.3 ppm of H2S to the reactor feed.

3. RESULTS AND DISCUSSION

Catalytic test

Table 1 shows the effect of Rh addition on the initial activity and stability after 15 reaction hours. The deactivation degree (RD) is defined as the ratio between initial conversion to conversion at 15 h in reaction.

Results indicate higher activity for the bimetallic catalyst and a similar deactivation degree for tested samples. The investigation will be continued under more severe temperature conditions to analyze the behavior of Rh-Ni catalysts with respect to carbon deposition.

Table 1. Catalytic activity and stability.

Catalyst %XCH4(1h) RD(1)

Rh/γ- Al2O3 36.6 0.95

Ni/γ- Al2O3 37.0 0.90

Rh-Ni/γ- Al2O3 42.7 0.90

Thermogravimetric analysis, in air current, shows that carbon contents in Rh-Ni catalyst (2.5%) and Ni catalyst (2.1%) were similar (Fig.1a). dTGA as a function of temperature for these catalysts is represented in Fig. 1b, where it is observed that the Ni catalyst presents an only broad peak between 370 and 700°C while the bimetallic catalyst show two signals, at 400 and 680 oC.

399 Effect of Rh addition on activity and stability over Ni/g-Al2 O3 catalysts

Fig.1a. Thermogravimetric diagrams of Ni monometallic (black curve) and Rh-Ni bimetallic

(red curve) supported catalysts.

Fig.1b. dTGA for Ni (black curve) and Rh-Ni (red curve) catalysts.

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The signal at 400°C may be related with a superficial carbide and the second species above 600°C is identified as whisker-like filamentous carbon. According to literature [13], species at 400°C may be reaction intermediate and their reactivity can be correlated with catalytic activity and with CO formation. The whisker- like filamentous carbon corresponds to absorbed carbon atoms derived from methane decomposition and CO dissociation.

Concerning the stability, the bimetallic catalyst showed high resistance to carbon formation, instead, this catalyst was sensitive to poisoning by sulfur presenting low thioresistance which leads to a decrease of its activity to 35% of its initial activity, after 23 hours.

Characterization of catalysts Figure 2 shows TPR profiles of mono and bimetallic catalysts. For the

monometallic Ni catalyst, two peaks at 420 and 640°C can be attributed to Ni interaction with the support. The peak around 640°C had a long tail toward higher temperature and a significantly stronger intensity than that at 420°C. These results indicate that at least two types of NiO species were present in the Ni catalyst. One type was NiO, which was weakly interacted with γ- Al2O3. The other NiO species was assignable to a strong interaction with the support surface by a solid-state reaction between NiO and γ-Al2O3. For Rh-Ni catalyst, a broad peak from 300 to 493°C can be observed and this is attributed to interaction of reduced Rh with the support [14]. It is well known that Rh and alumina interact by Rh diffusion into subsurface and bulk

Fig.2. Temperature Programmed Reduction profiles of mono and bimetallic catalysts.

of γ-Al2O3 when the catalyst is calcined in an oxidizing atmosphere. This makes it possible that Rh oxide is less reducible [15,16].

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umed

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.)401 Effect of Rh addition on activity and stability over Ni/g-Al2 O3 catalysts

Spectra of Rh-Ni bimetallic catalyst show a broad peak at 750°C. This signal is attributed to an Rh-Ni interaction compound that implies an intimate contact between Ni and Rh.

X-Ray diffractograms of reduced Ni-Rh/γ-Al2O3 do not show the Ni° signal, which is observed in the Ni/γ-Al2O3 sample. XRD results would indicate the presence of small Ni particles in the bimetallic catalyst and would suggest that the Rh addition favors Ni dispersion on the support surface.

CONCLUSIONS

Conclusions of this work can be summarized as follows: -The addition of 0.5% Rh in Ni/γ-Al2O3 catalysts improves the catalytic

activity by modifications of the active phase. -TPR results for Rh-Ni/γ-Al2O3 catalysts show the existence of a species that

is reduced at high temperature assigned to an interaction compound between the two metallic species. By XRD, the disappearance of the Ni signal in the bimetallic catalyst suggests that the Rh presence favors Ni dispersion.

-The addition of Rh favors the formation of more reactive carbonous species that are correlated with catalytic activity.

-The interaction between Rh and Ni and the Ni dispersion would be the responsible for the activity increase in the bimetallic catalyst.

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