The role of chromium in iron-based high-temperature water-gas shift catalysts under industrial conditions

M.I Ariëns,V Chlan,P Novák,L.G.A Van De Water,A.I Dugulan,E Brück,E.J.M Hensen

doi:10.1016/j.apcatb.2021.120465

Abstract

Chromium promotion of iron oxide based water-gas shift (WGS) catalysts prepared via co-precipitation/calcination was investigated. Mössbauer spectroscopy and XRD evidence that chromium is incorporated in the calcined hematite (α-Fe2O3) precursor irrespective of the doping level (0−12 wt.%). CO-TPR shows chromium delays the reduction of hematite and the active magnetite (Fe3O4) phase. WGS activity was evaluated under realistic conditions for 4 days. Enhanced CO conversion was observed with increased chromium doping. Mössbauer spectra indicate that chromium incorporates into octahedral sites of magnetite and prevents reduction of Fe3+ to Fe2+ during formation of the active phase, leading to an increased Fe3+/Fe2+ ratio in octahedral sites. The higher Fe3+/Fe2+ ratio did not affect the high CO conversion associated with the structural stabilization mechanism of Cr-doping. Interpretation of the Mössbauer spectra was supported by computational modelling of various chromium and vacancy-doped magnetite structures. The bulk structure of an in situ prepared chromium-doped high-temperature WGS catalyst is best described as a partially oxidized chromium-doped magnetite phase. No surface effects of Cr-doping were found.

Highlights

IntroductionHydrogen is produced by the steam methane reforming (SMR) process, which involves the water-gas shift (WGS) reaction to maximise hydrogen production (1) [1,3]
Hydrogen is an important reagent used mainly in industrial ammonia synthesis [1,2]
The shift of the he matite diffraction lines to higher 2θ values points to contraction of the unit cell, which can be explained by the incorporation of chromium cations (Cr3+) with a smaller radius (62 pm) than iron cations (Fe3+, 65 pm) in the host structure

Summary

Introduction

Hydrogen is produced by the steam methane reforming (SMR) process, which involves the water-gas shift (WGS) reaction to maximise hydrogen production (1) [1,3]. As the WGS reaction is mildly exothermic (ΔH =− 40.6 kJ/mol), high CO conversion is favoured at low temperature [1]. The WGS section is divided into two steps: (i) a high-temperature shift (HTS) step, which is typically performed at 350− 450 ◦C over an iron-chromium-copper-oxide catalyst and removes the bulk of CO from the synthesis gas product stream from the SMR step, and (ii) a low-temperature shift (LTS) step, performed at temperatures in the range of 190− 250 ◦C on a more active copper-zinc-alumina catalyst [3,4,5,6]. 10–15% CO to 2–4 % CO [4].

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