An In Situ Infrared Study of CO₂ Hydrogenation to Formic Acid by Using Rhodium Supported on Titanate Nanotubes as Catalysts

Abstract

Titanate nanotubes (TiNT) were synthesized by hydrothermal method and used as support of rhodium nanoparticles. Results of X-Ray diffraction (XRD) and Raman spectroscopy of TiNT revealed its structure of Na 2 Ti 3 O 7 , while the results of Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM) and physisorption of N 2 confirmed the multilayer nanotubular morphology with external diameter of ∼ 12 nm, length > 100 nm and a BET surface area of 195 m 2 g À 1 . The TEM analysis of the rhodium supported sample (Rh/TiNT) showed evidence of small (∼ 1 nm) and highly dispersed rhodium particles. Results of X-Ray Photoelectron Spectroscopy (XPS) revealed a strong electronic interaction between TiNT and Rh sites. The catalytic activity of Rh/TiNT for the hydrogenation of CO 2 to formic acid at moderate temperature (∼ 40°C) and atmospheric pressure was demonstrated as evidenced by results of Mass Spectrometry (MS) and in-situ Diffuse Reflectance Infrared with Fourier Transform Spectroscopy (DRIFTS). The in-situ studies showed active surface species bonded to support sites and to rhodium sites. It is proposed that under H 2 atmosphere, Na + cations near to Rh particles promote the conversion of CO 2 via dissociated H, allowing the formation of formate species at low temperature. The formate species and the hydride rhodium complexes are considered reaction intermediates.[a] Prof.

Hydrogenation of Carbon Dioxide on Supported Rh Catalysts

“…It was proposed that the Na + content in TiNT plays an important role in the various carbonate formation processes. The reduced Rh/TiNT has the ability to form rhodium hydride complexes which can interact with bidentate carbonates to yield formate species and facilitates the further hydrogenation to formic acid [44]. This proposal was supported by the infrared spectroscopic results on Rh/MgO.…”

“…The CO 2 dissociation occurs on Rh/TiNT (titanate nanotube) and Rh/TiNW (titanate nanowire) at 493 K but at smaller extent than on Rh/TiO 2 under the same experimental conditions [43]. Other authors found that the CO 2 dissociation at room temperature does not occur; CO band was not detected on Rh/TiNT [44].…”

“…On Rh/TiNT at 363 K at atmospheric pressure, besides methane and CO, formic acid formation was detected in the CO 2 + H 2 reaction. The calculated turnover frequency for HCOOH formation was 7.210 −2 h −1 [44]. It was proposed that the Na + content in TiNT plays an important role in the various carbonate formation processes.…”

Rh-induced Support Transformation and Rh Incorporation in Titanate Structures and Their Influence on Catalytic Activity

“…However, the absorption band of the C=O group of formaldehyde adsorbed on Rh/TiO2 appears at lower wavenumbers, at about 1727 cm −1 , and it forms at higher temperatures [109]. Although the vibration frequency of C=O groups in The bands detected between 1550-1570 cm −1 and 1379 cm −1 could be assigned to the asymmetric and symmetric vibration of the OCO group of formate species, respectively [94,[102][103][104][105][106][107]. The absorption found at about 1620 cm −1 could be attributed to water formed in the reaction.…”

“…The hydrogenation of CO 2 was studied extensively on titania (TiO 2 ) supported Rh [20][21][22][23][24][91][92][93] and the reaction was also investigated on titanate (TiONW and TiONT)-supported Rh recently [62,90,94]. In all cases, the supported Rh showed an excellent catalytic activity.…”