Combined EXAFS, XRD, DRIFTS, and DFT Study of Nano Copper-Based Catalysts for CO₂ Hydrogenation

Abstract

Highly monodispersed CuO nanoparticles (NPs) were synthesized via continuous hydrothermal flow synthesis (CHFS) and then tested as catalysts for CO 2 hydrogenation. The catalytic behavior of unsupported 11 nm sized nanoparticles from the same batch was characterized by diffuse reflectance infrared fourier transform spectroscopy (DRIFTS), extended X-ray absorption fine structure (EXAFS), X-ray diffraction (XRD), and catalytic testing, under CO 2 /H 2 in the temperature range 25− 500 °C in consistent experimental conditions. This was done to highlight the relationship among structural evolution, surface products, and reaction yields; the experimental results were compared with modeling predictions based on density functional theory (DFT) simulations of the CuO system. In situ DRIFTS revealed the formation of surface formate species at temperatures as low as 70 °C. DFT calculations of CO 2 hydrogenation on the CuO surface suggested that hydrogenation reduced the CuO surface to Cu 2 O, which facilitated the formation of formate. In situ EXAFS supported a strong correlation between the Cu 2 O phase fraction and the formate peak intensity, with the maxima corresponding to where Cu 2 O was the only detectable phase at 170 °C, before the onset of reduction to Cu at 190 °C. The concurrent phase and crystallite size evolution were monitored by in situ XRD, which suggested that the CuO NPs were stable in size before the onset of reduction, with smaller Cu 2 O crystallites being observed from 130 °C. Further reduction to Cu from 190 °C was followed by a rapid decrease of surface formate and the detection of adsorbed CO from 250 °C; these results are in agreement with heterogeneous catalytic tests where surface CO was observed over the same temperature range. Furthermore, CH 4 was detected in correspondence with the decomposition of formate and formation of the Cu phase, with a maximum conversion rate of 2.8% measured at 470 °C (on completely reduced copper), supporting the indication of independent reaction pathways for the conversion of CO 2 to CH 4 and CO that was suggested by catalytic tests. The resulting Cu NPs had a final crystallite size of ca. 44 nm at 500 °C and retained a significantly active surface.

An overview of Cu-based heterogeneous electrocatalysts for CO2reduction

“…In recent years, CO 2 reduction to fuels and chemical products has been investigated by various methods, such as biochemical approaches, 4 building blocks for organic synthesis, 5 thermal hydrogenation, [6][7][8][9][10][11] photocatalysis, [12][13][14] electrocatalysis [15][16][17][18] and dry reforming with methane. 19,20 Among them, electrochemical (EC) reduction has attracted much attention.…”

Synergistic ultraviolet and visible light photo-activation enables intensified low-temperature methanol synthesis over copper/zinc oxide/alumina

“…In addition, the excitations associated with specific components of CZA, namely the ZnO band gap excitation (3.2 eV [ 22 =~388 nm), and Cu LSPR (~590 nm 24 ), will change the electron-populating state of catalyst, thus regulating the intermediate reactions and product distribution. The effects of band gap excitations associated with Cu 2 O and CuO components (2.2 eV and 1.7 eV, respectively 25 ) are negligible due to the very limited presence of these components after reduction 26 .…”

Estudo dos efeitos de estratégias de manipulação química e estrutural em catalisadores heterogêneos aplicados à hidrogenação de CO>sub<2>/sub< a metanol

“…Dentre o vasto número de metais de transição já avaliados, entre os quais estão paládio, ouro, ródio, níquel, molibdênio e tungstênio, os catalisadores baseados em cobre recebem grande destaque por catalisar a reação de hidrogenação do CO2 a metanol com alta eficiência e seletividade comparativamente a outros elementos testados (BERSANI et al, 2016;POROSOFF;YAN;CHEN, 2015;WANG, WEI et al, 2011).…”

“…As energias envolvidas na adsorção e ativação do CO2 em sistemas contendo cobre favorecem a quebra de apenas uma das ligações entre carbono e oxigênio, o que é importante para a formação de compostos oxigenados como o metanol e desfavorece a produção de compostos como o metano (BERSANI et al, 2016;POROSOFF;YAN;CHEN, 2015;WANG, WEI et al, 2011).…”

“…Além disso, catalisadores baseados em cobre apresentam custo consideravelmente reduzido em relação a outros catalisadores que envolvem metais nobres como componente ativo além de apresentar temperaturas menores de redução em relação à maioria dos outros sistemas que utilizam elementos não nobres, levando a vantagens inerentes de menor custo do processo em virtude da utilização de condições de síntese mais brandas (BERSANI et al, 2016;POROSOFF;YAN;CHEN, 2015).…”