Síntese de compostos C2+ a partir de CO2 e H2O aplicando materiais de Cu e Cu-Zn: estudos na catálise heterogênea em fase gasosa e eletrocatálise

Abstract

Strategies aimed at recycling of CO2 in producing chemicals and fuels are needed to reduce the environmental impacts caused by this gas and attend the industrial interests. Among the all applied strategies, we highlight those that include gas-phase catalysis and electrocatalysis. Through catalysis and electrocatalysis it is possible to produce chemicals with high industrial applicability, such as liquid fuels and other molecules containing two or more carbons (C2+ compounds) from CO2. In the electrochemical system, the synthesis of C2+ compounds are extensively investigated by applying pure Cu and Cu-based electrodes (eg CuZn, CuAu and CuAg). In this system, applying an electric potential and providing up of an CO2-saturated aqueous solution (containing ion conductors - electrolyte), Cu-based electrode can be produced, among others, ethylene, ethanol, and propanol. On the other hand, in the gas phase catalytic system, applying temperature, pressure and Cu-based catalysts, the hydrogenation of CO2 with H2 results only in C1 compounds (methanol, CO, and CH4). Many questions arise about these differences, such as the why in the electrocatalytic system C2+ compounds can be formed while in the gas phase catalytic system these compounds are not identified. One of the differences between both systems is the use of water as a source of atomic H in the electrochemical system whereas H2 is used in the gaseous system. Since water can play an important role in the synthesis of C2+ compounds, this work aimed investigate the synthesis of C2+ compounds from CO2 with water by applying Cu and Cu-Zn based catalysts in the electrocatalytic and gas phase catalytic systems. Regarding the electrocatalytic results, it was possible to show that the distribution of C2+ products was influenced by the surface metallic area of the electrode, the composition of Cu-Zn and by the geometry of the particles exposed in the electrodes. However, although the increase in surface roughness and changes in chemical composition have led to a higher faradaic efficiency for C2+ compounds, the production of these molecules was more strongly influenced by the nanoparticle geometry exposed in the electrodes. In the gas-phase catalytic system, CO2 hydrogenation with water steam at atmospheric pressure over pure Cu resulted in the formation of ethanol (a result not observed when the CO2 hydrogenation with H2 was performed). For the first time, it is reported the possibility of synthesizing ethanol at atmospheric pressure in the CO2 hydrogenation in gas-phase using water as unique source of hydrogen. The ethanol productivity was investigated under different conditions and it was shown to be impacted by the reaction temperature, Cu surface area, particle geometry, stability, and catalyst composition. Cu/ZnO/Al2O3 and X-Cu/ZnO/Al2O3 (X = Li, Na, K or Cs) were also investigated in the CO2 hydrogenation with water steam and, by chemometric optimization, it was possible to show that the increase in size of the cation added to the catalyst is a more significant variable than the temperature in ethanol productivity. The role of water was evaluated by DRIFTS tests and it was possible to show that the wavenumbers referring to the adsorption of CO at Cu sites, pointed out as a key intermediary of the reaction, was lower in the presence of water than when in the presence of H2. This result suggest that the C-O bond of the adsorbate attached to the metal surface has been weakened in the presence of water while the carbon-metal bond has been strengthened. Therefore, these compounds are more likely to continue reacting rather than desorbing as reaction products. This result may explain why C2+ compounds are observed in the presence of water while only C1 compounds are identified in the presence of H2. Finally, it is believed that the results obtained in this work have an impact not only on studies at a fundamental level with regard to the hydrogenation of CO2 in gas-phase, but also have the potential for direct applications in industry.