|Publication type:||Conference paper|
|Type of review:||Peer review (abstract)|
|Title:||Operando spectroscopic monitoring of active species in catalytic hydrogenation of CO2 to methanol at elevated pressure and temperature : steady-state versus transient analysis|
Meier, Daniel Matthias
|Conference details:||Materials Research Meeting, Yokohama, Japan, 13-16 December 2021|
|Subject (DDC):||660: Chemical engineering|
|Abstract:||The global concerns and interests in CO2 capture and utilization as carbon feedstock are triggering both academic and industrial research on catalytic hydrogenation of CO2. Deep understanding of such hydrogenation reaction requires transient spectroscopic analysis because active species involved in catalytic cycles change on a short time scale1). However, operando monitoring of catalyst surface and gas-phase components at high pressure and temperature under transient condition has never been realised because of technical difficulties. We herein report the first example of operando analysis of hydrogenation of CO2 by modulation excitation spectroscopy (MES) at high pressure and temperature. As a model catalyst, Cu-Zn-Ba-Zr/Al2O3 was prepared by a conventional wet-impregnation method. A newly developed operando set-up enables both steady-state and transient analysis of surface species and gas-phase components by diffuse reflectance IR Fourier transform spectroscopy (DRIFTS) and mass spectroscopy (MS) at elevated pressure (max. 100 bar) and temperature (max. 1000 oC). Figure 1 shows (a, b) concentration profile and (c-d) DRIFTS spectra monitored during CO2 hydrogenation at 250 o C and 30 bar with CO2-concentration modulation. As shown in Figure 1a, the first 4 cycles were unstable, and therefore the next 20 cycles were used for MES analysis. Figure 1b clearly indicates time-delay in methanol formation in the gas-phase. Operando DRIFTS spectra plotted in time-domain, phase-domain and contour map demonstrated that there were three surface species; carbonate, formate and methoxy species. Upon CO2 adsorption, carbonate was immediately formed, followed by rapid hydrogenation to formate. The formate underwent further hydrogenation to methoxy. Interestingly, the formation of methoxy species on the surface (in-phase angle of 77 o) was in line with the delay of gas-phase methanol. Therefore, the reaction of methoxy(a)+ H(a) is considered to be the rate-limiting step. Such information on surface species and their kinetics can only be obtained by operando transient spectroscopic analysis, which is advantageous over conventional steady-state spectroscopic analysis. Based on the rate-limiting step unveiled in this study, we tried to accelerate this step by enhanced dissociation of H2 molecules with 0.1 wt % Pd-added catalyst. Only a small amount of the Pd additive dramatically enhanced the selectivity to methanol. We believe that operando MES-DRIFTS operated at elevated pressure and temperature allows such a rational design of catalytically active materials in an elegant way. References: 1) N. Maeda, F. Meemken, K. Hungerbühler, A. Baiker, Chimia 66, 664-667 (2012).|
|Fulltext version:||Accepted version|
|License (according to publishing contract):||Licence according to publishing contract|
|Departement:||School of Engineering|
|Organisational Unit:||Institute of Materials and Process Engineering (IMPE)|
|Appears in collections:||Publikationen School of Engineering|
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