Capturing metal-support interactions in situ during the reduction of a Re promoted Co/γ-Al₂O₃ catalyst

Abstract

The reduction of a Re promoted Co/γ-Al2O3 catalyst was monitored in situ by synchrotron X-ray powder diffraction (XRPD) under H2 environment. Whole powder pattern analysis showed a non-linear expansion of the unit cell of γ-Al2O3 during the reduction process suggesting the diffusion of cobalt cations into the structure of the γ-Al2O3 support material. The cell expansion coincided with the formation of CoO phase. Evidence for the negative effect of the partial pressure of indigenous H2O on the reduction process was obtained by alternating XRPD probing of the inlet and outlet ends of the reactor.Catalyst activation is a critical step of the start-up procedure for most industrial catalytic processes. 1 Commonly a solid precursor is been subjected to specific conditions that allow transformation to the catalytically active component, ex situ or in situ. In many catalytic applications, the metallic surface of nanoparticles is the active component and therefore reduction of supported metal oxide precursors precedes. The reduction process is affected by various parameters such as the nature of the precursor, the size of the nanoparticles, the reactivity of the support, the reducing atmosphere. The execution of the activation step has an impact on catalyst structure and performance, avoiding risks of sintering or a lower final degree of reduction. 2 In the last decades, in situ investigations have boosted catalysis research and allowed the deconvolution of such complex phenomena. 3 Cobalt nanoparticles (NPs) supported on high surface area porous materials such as γ-Al2O3 are used in various processes. One of the most important industrial applications is the FischerTropsch synthesis (FTS). 4 FTS is the heart of Gas-to-Liquid (GTL) technologies and a tool for the utilization of synthesis gas derived from different feedstocks i.e. natural gas, coal and biomass. In cobalt based FTS carbon monoxide and hydrogen are converted into a mixture of linear long-chain hydrocarbons while significant amounts of steam are co-produced. The active metallic Co is commonly formed by reduction in H2 of the Co3O4 spinel precursor produced after drying and calcination of the impregnating source. The activation procedure is important for the optimization of catalysts selectivity 5 and stability 6 .The reduction of promoted and un-promoted γ-alumina supported Co3O4 NPs has been studied in detail either by conventional temperature programed reduction (TPR) 7-9 or by advanced in situ techniques including X-ray powder diffraction (XRPD) 10 , X-ray absorption spectroscopy (XAS) 11-14 and transmission electron microscopy (TEM) 15 . From the majority of the reports, it is evident that the reduction is a two-step process that reaches the polycrystalline metallic Co through a CoO intermediate. The use of dopants can either promote or impede this reduction process. Co3O4 + H2 → 3CoO + H2O(1)CoO + H2 → Co 0 + H2O (2) Although most of the reports agree on the steps of the reduction procedure the formation of mixed compounds of Co with the γ-Al2O3 support ...

α-alumina as catalyst support in Co Fischer-Tropsch synthesis and the effect of added water; encompassing transient effects

“…This temperature range can be correlated with observed intrusion of Co II into the lattice of γ-alumina during the reduction process. 33 A small sharp peak appears only for catalysts on α-alumina, and we consequently ascribe this feature to reduction of rhenium oxide. It has previously been shown that Re as promoter only shifts the second reduction step of cobalt to lower temperatures.…”

“…32 Intrusion of Co II into the alumna lattice has been observed to occur during reduction. 33 Although γ-Al2O3 is the standard alumina-based catalyst carrier, several transition phases occur during calcination before the thermodynamically stable α-alumina phase is reached. The crystal morphology of alumina is rather complex, and a large number of polymorphs exist and are formed upon dehydration.…”

Evaluation of Reoxidation Thresholds for γ-Al2O3-Supported Cobalt Catalysts under Fischer–Tropsch Synthesis Conditions

“…As already mentioned, highly dispersed Co-support phases are invisible by electron microscopic techniques meaning that they are not part of the Co PSD obtained by TEM. In situ X-ray diffraction has been used to probe them indirectly through the detection of their lattice expansion 32,62 . Here it is worth mentioning that an in situ XRD approach in the current case would give valuable input.…”

Structure–Performance Relationships on Co-Based Fischer–Tropsch Synthesis Catalysts: The More Defect-Free, the Better

“…Exposure of bulk hcp-Co to temperatures above ~420°C has been shown to initiate/trigger transformation to fcc-Co 47,48 . The topic is still a matter of scientific debate and various deviations from this transformation temperature have been reported and related to various factors such as NP size, metal-support interactions and pre-existing concentration of stacking faults [49][50][51] . Here, with in situ TPR-XRD, the transformation from hcp-Co to fcc-Co is followed.…”