DRIFTS/MS Studies during Chemical Transients and SSITKA of the CO/H₂ Reaction over Co-MgO Catalysts

Abstract

Diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS) has been employed along with chemical and isotope transients to study the catalytic CO hydrogenation over Co/MgO catalysts in a single fixed-bed reactor at T = 523 K and ambient pressure conditions (H2/CO = 3). According to the operando DRIFTS measurements, the catalyst surface contains hydroxyl groups, adsorbed CO, formate, and methylene groups in the steady-state of the reaction. Transient experiments following fast changes in the feed (chemical transient kinetics, CTK) or isotope composition (steady-state isotopic transient kinetic analysis, SSITKA) have been carried out during DRIFTS and demonstrate that the formate/methylene “seen by DRIFTS” plays no role as imminent intermediates of the ambient pressure Fischer−Tropsch (FT) reaction. The SSITKA experiments (replacing 12CO by 13CO) show that the exchange rate of formate/methylene is significantly lower than that of ethane, which is one of the main reaction products of CO hydrogenation (followed by mass spectrometry). Formate is most probably bound as bidentate μ2-species to MgO or at the Co/MgO interface, while methylene stands for skeleton CH2 in either hydrocarbon or carboxylate.

Recent Progresses in Understanding of Co-Based Fischer–Tropsch Catalysis by Means of Transient Kinetic Studies and Theoretical Analysis

“…However, the surface species detected by IR do not necessarily participate in the reaction, may just be a spectator. A powerful tool for identifying active surface intermediates is SSITKA coupled with diffuse reflectance Fourier Transform Infrared Spectroscopy (DRIFS) [103][104][105]. This coupled technique has been used to investigate formate species in the water-gas-shift reactions [106][107][108].…”

“…This coupled technique has been used to investigate formate species in the water-gas-shift reactions [106][107][108]. However, fewer studies have been carried out to study the mechanism of CO hydrogenation [109].…”

Spectrum baseline artefacts and correction of gas-phase species signal during diffuse reflectance FT-IR analyses of catalysts at variable temperatures

“…In situ and operando diffuse reflectance FT-IR spectroscopy (DRIFTS) have brought about 3 many valuable insights into the mechanism of heterogeneous catalytic reactions, such as CO + 4 NO reaction [1], syngas conversion [2,3,4], water-gas shift reaction [5,6,7,8], CO oxidation 5 [9,10,11,12], hydrocarbon or ammonia selective reduction of NOx [13,14,15] or deactivation 6 processes by surface poisoning [16,17,18]. A DRIFTS reactor is particularly suited to this 7 type of investigation since the catalyst powder can be used directly as such and therefore 8 DRIFTS is becoming a major technique for catalytic applications at the gas-solid interface.…”

“…3 The correction of gas-phase signal is particularly important in the case of CO-based 4 analyses since the signals of gas-phase CO and adsorbed CO may overlap extensively. This 5 problem is more acute in the case of DRIFTS than in the case of transmission FT-IR because 6 of the significant IR beam pathlength in the volume of DRIFTS cell.…”

Density functional theory simulations of complex catalytic materials in reactive environments: beyond the ideal surface at low coverage

“…22,37,38 Of course, both approaches can be combined, and have led to significant achievements in the understanding of metallic and oxide surfaces in particular. 39 Computational chemistry is a precious and complementary approach that can provide a better description and understanding of heterogeneous catalytic active sites at the atomic scale.…”