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

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 Na2Ti3O7, 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 approximate to 12nm, length > 100nm and a BET surface area of 195m(2)g(-1). The TEM analysis of the rhodium supported sample (Rh/TiNT) showed evidence of small (approximate to 1nm) 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 CO2 to formic acid at moderate temperature (approximate to 40 degrees 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 CO2 via dissociated H, allowing the formation of formate species at low temperature. The formate species and the hydride rhodium complexes are considered reaction intermediates.