Need some more information in your question....?

What is the active metal you have used for the reaction...?

Surface area alone is not the factor for the activity of all heterogeneously catalysed reactions....?

first one should know that which characteristic of material is required for the conversion of CO to CH4 in your case.

I hope your reaction is a metal active reaction so the difference in your results was due to the content of the active metal.....

Good Luck. 

The BET surface area is usually not directly correlated with catalyst activity. You should check the active metal surface area, since this is where the reaction takes place. You could check this by chemisorption of CO or H2. If you do not have equipment for chemisorption measurement, you could try to correlate the measured reaction rate with the content (mass fraction) of active material in your catalyst. Active material for CO methanation is typically Nickel in its metallic form.

Dear Haribabu, Dear Robert

The active metal used in this experiment is nickle

the surface area of this active metal is much less than the samples with CNTs

Furthermore, in contrary to SBET, the crystallite size of both samples (10 and 30wt%) are fairly constant, and also the NiO phase was observed in the sample with higher amount of CNTs ......  The catalytic activity can affect by these factors?

Dear Haribabu, could you give some more details on the catalyst concerning composition. It obviously consists of Nickel, CNT and some further material. What is the approximate mass fraction of all these components? At the end activity should only depend on the nickel content as long as the Nickel cristallite size is the same in all samples.

Dear Robert,

The composition of catalyst materials are Ni-10wt%CNTs and Ni-30wt%CNTs.

In the XRD patterns of the Ni-30wt%CNTs some peaks related to the NiO phase was observed. what is the effect of NiO phase on the catalytic activity? can this phase deteriorates the catalytic performance?

Thanks

So this means your catalyst consists of Nickel and CNT only? This explains you results: In the sample with 10 % CNT is 90 % related to nickel, while it is 30 % CNT and 70 % Nickel in the other sample. CNT itself is not active in CO methanation, while Nickel is the active compound. NiO should be inactive, since the metallic nickel is the active phase.

Dear Robert,

Thanks a lot for your valuable informations

I will be so grateful if you would share some references in this regard.

Sincerely yours