Nature of Ni–Al developed phases during thermal activation in relation to the preparation techniques. Part II: reduction and catalytic properties
Introduction
Due to the great demand of hydrogen, both for petroleum refining processes and petrochemical industry, its production by reforming of methane has increased in importance. While most syngas is produced by steam reforming of methane, some other routes as partial oxidation or CO2 reforming of methane may be more attractive, depending on factors such as H2/CO ratio, environmental restrictions, etc.
Catalysts based on Ni are widely used for steam reforming, partial oxidation or CO2 reforming 1, 2, 3, 4.
In Part I it was studied the evolution of different systems (bulk and supported catalysts) during calcination step by using differential thermal analysis (DTA) and thermogravimetry analysis (TGA–DTG), complemented with X-ray diffraction (XRD).
In this work the H2 reducibility of calcined phases in function of preparation method is studied by TGA–DTG. The relation between nature and reducibility of active phase (Ni) with catalytic properties (stability) is analysed, taking as reaction test the methane steam reforming.
Section snippets
Experimental methods
Catalytic systems were prepared following the methodology presented in Part I. Table 1 summarizes prepared catalytic systems and their nomenclature.
The Ni reducibility was studied following the weight change observed (TGA–DTG) when previously calcined samples (calcination temperature, 1173 K) were exposed to hydrogen flow. Samples were placed in alumina crucibles and α-Al2O3 was used as a reference. The equipment was evacuated and then filled with N2 to eliminate the oxygen present. The
Reduction step
The reduction of supported metallic catalysts is considered of great interest because of: (i) the reaction temperature must be known to obtain the adequate active phase and (ii) the reduction temperature may be related with the nature of this phase and with the interactions generated with the support and/or promoters.
Fig. 1 shows the TGA–DTG diagrams of the reduction step for the NiEV solid achieved by calcination of nickel nitrate (Ni(NO3)2·6H2O). In these conditions the solid phase represents
Conclusions
The use of TGA–DTG permits to study the H2 reducibility of different catalysts based on Ni (bulk and supported), as a function of the different preparation techniques used.
Those preparations that lead to a strong interaction between Ni and Al (co-precipitation, co-impregnation, deposition–precipitation), generate Ni phases more difficult to be reduced. This is produced by the formation of mixed oxides (spinel type non-stoichiometric) when a Al/Ni = 0.5 atomic ratio is used.
The catalytic stability
Acknowledgements
This work was sponsoned by the Consejo Nacional de Investigaciones Cientı́ficas y Técnicas (CONICET), Argentina. We thank Lic. S. Conconi for the performing of the thermal analysis.
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