Study Of Co-decorated MWCNTs-promoted CuO-ZnO-ZrO₂ Catalyst For Methanol Synthesis From CO₂ Hydrogenation

Hydrogenation of carbon dioxide has been considered as one of the most economical and effective ways to chemically fix huge amount of emitted CO₂. In order to improve climate conditions, it is desirable to develop methods to convert CO₂ into valuable chemicals. Among the options considered, catalytic hydrogenation of CO₂ to produce methanol has received much attention.Since carbon dioxide is one of the most stable compounds, most reactions including CO₂ as a reactant are not favorable thermodynamically. The synthesis of methanol from CO₂ (CO₂ + 3H₂ = CH₃OH + H₂O,â–³G_523K = 46.5 kJ/mol) is less favored compared to that from CO (CO + 2H₂ = CH₃OH,â–³G_{523 K} = 26.9 kJ/mol), therefore the equilibrium conversion of CO₂ is extremely low even at a commercially desirable operating temperature around 523 K. Development of highly active catalyst, which is able to operate under the reaction conditions of high space velocity of feed-gas so as to gain high single-pass time-space-yield of methanol, is one of ways to overcome deficiency of low thermodynamic equilibrium conversion of CO₂ hydrogenation to methanol. Nevertheless, the activity of the existing catalysts (mainly CuO-based catalysts) is relatively low in eyes of practicability. Under the typical reaction conditions (of 5 - 8 MPa), the reached single-pass time-space yield (STY) of methanol for most systems is in range of 200-350 mg·h^-1·g^-1.To the other front, multi-walled carbon-nanotubes (MWCNTs, simplified as CNTs in next text), as a novel nano-carbon material, have been drawing increasing attention recently. This new form of nano-carbon possesses several unique features, such as graphitized tube-wall, nanometer-sized channel and sp²-C-constructed surface. They display high thermal/electrical conductivity and excellent performance for adsorption of hydrogen, all of which render this kind of nanostructured carbon materials full of promise as a novel catalyst support and/or promoter.In this dissertation, a type of Co-decorated CNT-promoted CuO-ZnO-ZrO₂ catalysts was developed. The catalyst displays higher activity and selectivity for CO₂ hydrogenation to methanol, compared to the CNT-free counterpart and to the reference catalyst promoted by the simple CNTs. The results shed some light on understanding the nature of promoter action by the Co-decorated CNTs and on the design of practicable catalyst for CO₂ hydrogenation to methanol.The obtained progress in the present study was briefly described as fellows:1. Co-decorated CNT-promoted CuO-ZnO-ZrO₂ based catalyst for CO₂ hydrogenation to CH₃OHWith Co-decorated CNTs as promoter, a type of Co-decorated CNT-promoted CuO-ZnO-ZrO₂ catalysts was developed. The catalyst displays excellent performance for CO₂ hydrogenation to methanol. Over a Cu₈Zn₂Zr₅-10%(4.5%Co/CNTs) catalyst under the reaction conditions of 5.0 MPa, 523 K, V(H₂) : V(CO₂) :V(N₂) = 69 : 23 : 8 and GHSV = 25 000 ml_STP·h^-1·g^-1, the observed turnover frequency (TOF) of CO₂ hydrogenation reached 4.99×10^-3 s^-1. This value was 1.16 and 1.08 times that (4.31×10^-3 s^-1 and 4.64×10^-3 s^-1) of the CNT-free substrate Cu₈Zn₂Zr₅ and the simple CNT-promoted counterpart Cu₈Zn₂Zr₅-10%CNTs, respectively, under the same reaction conditions. The selectivity of CH₃OH in the CO₂ hydrogenation products reached 97.9%, with the corresponding STY reaching 699 mg_-CH3OH·h^-1·g_-catal.^-1, markedly higher than the level of the similar or related catalysts reported in the literature.2. Characterization of Co-decorated CNT-promoted CuO-ZnO-ZrO₂ based catalystThe XRD measurements disclosed that, in the functioning catalysts, the metal Cu component existed mainly in the forms of Cu_x⁰ (2θ= 43.4°, 50.4°, 74.1°), and the presence of crystallite phase of ZrO₂ could not be excluded, while the content of ZnO-phase was under the XRD-detection limit. It is worth noting that, being different from the ZnO-free binary system CuO-ZrO₂, a XRD-observable amount of Cu₂O crystallite phase (2θ= 36.4°) was detected in the ZnO-containing ternary system CuO-ZnO-ZrO₂, which most probably resulted from the so-called "effect of ion-doping-valence-compensation". It is interesting that appropriate addition of an amount of Co-decorated CNTs to the ternary CuO-ZnO-ZrO₂ system led to a marked increase of the content of Cu₂O crystallite phase.XPS post-analysis of the tested catalysts showed that little marked difference in the Zn(2p)-XPS and 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 Zn, Zr and Cu species, respectively. The results of analysis-fitting of those XPS spectra showed that, at the surface of functioning catalysts, the observed Zn-species all were +2 valence-state, with the corresponding binding energy (B.E.) of Zn²⁺(2p_3/2) at 1022.5 eV, and Zr-species all were +4 valence-state, with the corresponding B. E. of Zr⁴⁺(3d_5/2,3/2) at 182.8/185.2 eV, while the dominant Cu-species were Cu⁰, secondary Cu⁺, with the corresponding B.E. of Cu⁰(2p_3/2,1/2) and Cu⁺(2p_3/2,1/2 at 932.5/952.5 and 931.8/951.8 eV, respectively. The ratio of the molar percentage of Cu⁰-species in the total Cu-amount at the surface of the three tested catalysts was Cu₈Zn₂Zr₅-10%(4.5%Co/CNTs): Cu₈Zn₂Zr₅-10%CNTs : Cu₈Zn₂Zr₅ =72 : 69 : 66, this sequence being in line with the sequence of turn-over frequency (TOF) of CO₂ hydrogenation over the three catalysts. This result also provided a strong support to the suggestion that the surface Cu⁰ was the catalytically active Cu-species responsible for CO₂ hydrogenation.H2-TPD tests of the pre-reduced catalysts revealed that the CNTs(especially Co-decorated CNTs)-containing catalysts could adsorb considerably greater amount of hydrogen than the CNTs-free substrate. The ratio of relative area-intensities of the H₂-TPD profiles taken on the three H₂-prereduced catalysts in the temperature region of463 543 K Was:S₍Cu₈Zn₂Zr₅-10%(4.5%Co/CNTs) :S_{Cu8Zn2Zr5-10%CNTs} :S_Cu8Zn2Zr5 = 100 : 87 : 60.This sequence was in line with the sequence of reactivity of CO₂ hydrogenation to methanol over these catalysts.3. Nature of the promoter action by CNTs-based additivesIt was experimentally found that appropriate incorporation of a minor amount of the CNTs or the Co-decoreted CNTs into the CuO-ZnO-ZrO₂ host catalyst did not cause a marked change in the apparent activation energy (E_a) for CO₂ hydrogenation to CH₃OH, implying that the addition of a minor amount of the CNTs or the Co-decorated CNTs to the CuO-ZnO-ZrO₂ did not alter the major reaction pathway of CO₂ hydrogenation to CH₃OH. It is quite evident that the considerably better performance of the CNT-containing catalysts for CO₂ hydrogenation to CH₃OH is closely related to the properties of the CNTs or the Co-decorated CNTs as promoter. The results of the present study demonstrated that the action by the CNT-based promoters (CNTs or Co-decorated CNTs) was mainly in increasing the concentration of surface-species (Cu⁰) active catalytically via affecting the chemical states of catalyst and in improving the capability of catalyst for adsorbing and activating H₂ (one of the reactants). Both the factors are in favor of increasing the reaction rate of CO₂ hydrogenation to CH₃OH.