Fischer-Tropsch synthesis: study of the promotion of Pt on the reduction property of Co/Al₂O₃catalysts byin situEXAFS of CoKand PtL_IIIedges and XPS

Abstract

The addition of platinum metal to cobalt/alumina-based Fischer-Tropsch synthesis (FTS) catalysts increases both the reduction rate and, consequently, the density of active cobalt sites. Platinum also lowers the temperature of the two-step conversion of cobalt oxide to cobalt metal observed in temperature programmed reduction (TPR) as Co3O4 to CoO and CoO to Co0. The interaction of the alumina support with cobalt oxide ultimately determines the active site density of the catalyst surface. This interaction can be controlled by varying the cobalt loading and dispersion, selecting supports with differing surface areas or pore sizes, or changing the noble metal promoter. However, the active site density is observed to depend primarily on the cluster size and extent of reduction, and there is a direct relationship between site density and FTS rate. In this work, in situ extended X-ray absorption fine structure (EXAFS) at the LIII edge of Pt was used to show that isolated Pt atoms interact with supported cobalt clusters without forming observable Pt--Pt bonds. K-edge EXAFS was also used to verify that the cobalt cluster size increases slightly for those systems with Pt promotion. X-ray absorption near-edge spectroscopy (XANES) was used to examine the remaining cobalt clusters after the first stage of TPR, and it revealed that the species were almost entirely cobalt (II) oxide. After the second stage of TPR to form cobalt metal, a residual oxide persists in the sample, and this oxide has been identified as cobalt (II) aluminate using X-ray photoelectron spectroscopy (XPS). Sequential in situ reduction of promoted and unpromoted systems was also monitored through XPS, and Pt was seen to increase the extent of cobalt reduction by a factor of two.

Fischer–Tropsch Synthesis: Preconditioning Effects Upon Co-Containing Promoted and Unpromoted Catalysts

“…Using a standard 10 h H 2 reduction treatment of 350°C, Jacobs et al [11] showed by hydrogen chemisorption/pulse reoxidation measurements that in all cases (Pt, Ru, and Re), gains in active site densities (i.e., number of cobalt atoms exposed at the surface) were directly due to the higher extents of reduction gained by promoter addition. The three promoters, Pt, Ru and Re, have all been observed to exhibit direct contact with Co at the atomic level by EXAFS spectroscopy [12,[17][18][19][20].…”

Recent developments in the application of nanomaterials to understanding molecular level processes in cobalt catalysed Fischer–Tropsch synthesis

“…77 EXAFS studies on incipient wetness and sol gel type Pt promoted catalysts appear to indicate the existence of intimate contact betwen Pt and Co as there is no evidence for Pt-Pt bonding. 78,79 Re appears not to promote cobalt reduction until the Re itself has been reduced. 80 In all three cases the role of Pt or Re in initially reducing the cobalt is implicated in the mechanism of promotion.…”

Characterizing Hf X Zr1−X O2 by EXAFS: Relationship Between Bulk and Surface Composition, and Impact on Catalytic Selectivity for Alcohol Conversion

“…The beamline parameters and crystal detuning procedure have been previously reported [19]. It was imperative for these binary oxide catalysts to first calculate sample thicknesses prior to visiting the synchrotron.…”