Co-decorated MWCNTs As Novel Promoter Of Cu-ZrO₂ Oxide-based Catalyst For Methanol Synthesis From CO₂ Hydrogenation

CO 2 exists widely in atmosphere and water in which the amount of carbon is 10 times more than that of petroleum, coal and natural gas. Conversion of CO₂ to useful chemicals is widely investigated by many workers from the view of point of finding technology for suppressing the green house effect caused by CO₂ emission.The methanol synthesis from CO2 and H₂ is considered as a promising process. Catalysts containing Cu are known to be efficient for production of methanol from CO₂ and H₂. Because of thermodynamic limitation, the yield of methanol is not high enough for single-pass operation. The utilization of industrial Cu-Zn-Al catalysts which exhibited a high activity for methanol synthesis from syngas, was not successful in CO₂ hydrogenation. It is interesting to develop new catalysts with high activity.Multi-walled carbon-nanotubes (MWCNTs) have been drawing increasing attention over the last decade. This new form of carbon is structurally close to hollow graphite fiber, except that it has a much higher degree of structural perfection. The MWCNTs possesses highly graphitized tube-wall, nanosized channel and sp²-C-constructed surface. They display exceptionally high mechanical strength, high thermal/electrical conductivity, medium to high specific surface areas, and excellent performance for adsorption and spillover of hydrogen, all of which render this kind of nanostructured carbon materials full of promise to be a novel catalyst carrier and/or promoter; while modification of some transition-metals to MWCNTs might be expected to further develop their applicable field and/or improve their some performance.In the present work, a series of Co-decorated MWCNTs, noted as y%Co/ MWCNTs (where y% represented mass percentage), were prepared by an intermittent microwave irradiation-assisted polyol reduction-deposition method, and characterized through TEM, SEM, EDX, XRD and H₂-TPD (temperature programmed desorption) measurements. Using the y%Co/MWCNTs as promoter, y%Co/MWCNTs-promoted Cu-ZrO₂ oxide-based catalysts were prepared. Their catalytic performance was evaluated for methanol synthesis from CO₂ hydrogenation and compared with the related reference systems. The results shed some light on the properties of the Co-decorated MWCNTs-based composite material and the prospect of its applications in catalysis. The progress obtained in the present work was briefly described as fellows:1. Preparation and characterization of Co-decorated MWCNTsWith the homemade MWCNTs as substrate material, a type of metallic cobalt-decorated MWCNTs, y%Co/MWCNTs, was prepared using an intermittent microwave irradiation-assisted polyol reduction-deposition method. TEM and SEM measurements of the produced materials showed that Co nanoparticles were quite uniform in shape and size and well dispersed on the MWCNTs surface. Most of the Co-particle diameters were below 10 nm. EDX analysis demonstrated that carbon, oxygen and cobalt were the only three elements at the surface of 4.3%Co/MWCNTs, with atomic percentage of 82.08%, 14.51% and 3.42%, respectively. The surface oxygen most probably originated from the pre-oxidation/carboxylation treatment of the MWCNTs by the concentrated nitric add. The measurement of H₂-TPD showed that the modification of the metallic cobalt to the MWCNTs led to a marked increase in its hydrogen-adsorbing capacity.2. Metallic Co-decorated MWCNTe-promoted Cu-ZrO₂ catalyst for methanol synthesis from CO₂ hydrogenationUsing the y%Co/MWCNTs as promoter, a type of y%Co/MWCNTs-promoted Cu-ZrO₂ catalysts, noted as Co_iZr_j-x%(y%Co/MWCNTs), for methanol synthesis from CO₂ hydrogenation was prepared by co-precipitation method. It was experimentally found that the Co-decorated MWCNTs-promoted catalyst displayed high activity for CO₂ hydrogenation to methanol. Over the catalyst of Co₁Zr₁-10%(4.3%Co/MWCNTs) under reaction condition of 5.0 MPa, 513 K, V(H₂):V(CO₂): V(N₂) = 69:23:8, GHSV = 8000 mL/(h·g), the observed STY of methanol reached 176 mg/(h·g). This value was 1.16 and 1.13 times that of the MWCNTs-free substrate Cu₁Zr₁ and the simple MWCNTs-promoted counterpart Cu₁Zr₁-10%MWCNTs, respectively, under the same reaction conditions.3. Characterization of Co-decorated MWCNTs-promoted Cu-ZrO₂ based catalystsThe XRD post-analysis of the tested catalysts showed that in the position and shape of XRD features, there was little marked difference between the XRD patterns taken on the three tested catalysts (i.e., Cu₁Zr₁-10%(4.3%Co/MWCNTs), Cu-1Zr₁-10% MWCNTs and Cu₁Z₁i). The Cu-Zr components existed mainly in the forms of Cu (Cu_x⁰) (2θ= 43.4°, 50.4°and 74.1°), CuO (2θ= 35.5°and 38.3°) and ZrO₂ (2θ= 30.5°and 31.7°).H₂-TPD measurements showed that the MWCNTs (especially Co-decorated MWCNTs)-containing catalysts could adsorb considerably greater amount of hydrogen than the MWCNTs-free counterpart. The relative area-intensities of the H₂-TPD profiles taken on the three H2-prereduced catalysts in the temperature region of 473-773 K was: S₍Cu1Zr1-10%(4.3%Co/MWCNTs) / S₍Cu1Zr1-10%MWCNTs/ S_Cu1Zr1 = 100/81/61. This sequence was in line with the sequence of reactivity of CO₂ hydrogenation to methanol over these catalysts.XPS measurements of the tested catalysts demonstrated that little marked difference in the Zr(3d)-XPS spectra but a certain difference in the Cu(2p)-XPS spectra existed among the three tested catalysts in the position and shape as well as relative intensity of the XPS peaks associated with the Zr or Cu species. The analysis and fitting of those XPS spectra have been done. The results showed that, at the surface of functioning catalysts, the observed Zr-species all were +4 valence-state, with the corresponding binding energy (B. E.) of Zr⁴⁺(3d_5/2,3/2) at 182.2/184.5 eV; while the dominant Cu-species were Cu⁰ and Cu⁺, with the corresponding B. E. of Cu⁰(2p_3/2,1/2 and Cu⁺(2p_3/2,1/2 at 932.7/952.7 and 931.6/951.6 eV. The sequence of relative content of Cu⁰-species at the surface of the three tested catalysts was Gu₁Zr₁-10%(4.3%Co/CNT) > Cu₁Zr₁-10%CNT > Cu₁Zr₁, in line with the sequence of turn-over frequency (TOF) of CO₂ hydrogenation over the three catalysts. This result provided a strong support to the suggestion that the surface Cu⁰ was the catalytically active Cu-species responsible for CO₂ hydrogenation.4. Nature of the Promoter action by MWCNTs-based materialsIt was experimentally found that proper incorporation of a minor amount of the MWCNTs or Co-decoreted MWCNTs into the Cu-ZrO₂ host catalyst did not cause a marked change in the apparent activation energy (E_α) for the methanol synthesis from CO₂ hydrogenation reaction. On the above three catalysts, the observed E_αof CO₂ hydrogenation was 30.2, 29.7 and 29.9 kJ/mol, respectively. This most likely implied that the addition of a minor amount of the MWCNTs or the Co-decorated MWCNTs to the Cu-ZrO₂ did not alter the major reaction pathway of methanol synthesis from CO₂ hydrogenation.It is quite evident that the considerably better performance of the MWCNT-containing catalysts for CO₂ hydrogenation to methanol is closely associated with the unique structure and properties of the MWCNTs or Co-decorated MWCNTs as promoter. The TPD tests revealed that the MWCNTs, especially Co-decorated MWCNTs, promoted Cu-ZrO₂ catalyst could reversibly adsorb a greater amount of hydrogen at temperatures ranging from room temperature to 623 K. This unique feature would help to generate a micro-environment with higher concentration of active H-adspecies at the surface of the functioning catalyst, thus increasing the rate of surface hydrogenation reactions.