Applications of isotopic tracers in Fischer–Tropsch synthesis

Abstract

In this review, five different applications of isotopic tracers to the Fischer-Tropsch synthesis reaction on cobalt, iron, and ruthenium catalysts are reviewed. By co-feeding molecules containing a radioactive 14 C-label and monitoring the fate of the species in the product distribution, researchers have identified molecules that are candidates for chain initiation or reincorporation into the growing chain. Kinetic isotope effects resulting from H 2 /D 2 switching have provided insights into the rate limiting steps involved in chain initiation and propagation. H 2 /D 2 switching has also been employed in order to account for the holdup of heavier products in the reactor, thus allowing one to correct for the product accumulation effect on the ASF plot so that the true impact of olefin reincorporation on the product distribution can be assessed. The competitive adsorption of H 2 and D 2 on the catalyst surface has also been explored in order to identify if H-D partitioning effects could influence the probability for one isotope to be favored for reaction during FTS and thus influence KIE results. Finally, steady state isotopic transient kinetic analysis (SSITKA) has been utilized to obtain a number of important kinetically relevant parameters (e.g., number of active sites, rate constant, TOF, etc.), to quantify the pool of reactive intermediates on the catalyst surface, and explore how co-adsorbed molecules may influence the size or reactivity of the reactive pool.

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

“…4 5.a). The spectrum should a large band located at 2041 cm -1 due to linear (on-top) carbonyls 5 adsorbed on metallic cobalt sites and ill-resolved bands below 2000 cm -1 associated with 6 bridged and multibonded CO [4]. The bands due to gas-phase CO were clearly visible at 2176 7 and 2114 cm -1 .…”

“…2 The optical path within the powdered sample bed is complex, since it derives from a 3 succession of transmissions through the sample particles and reflection at the surface of those. 4 The pathlength through the sample bed filled with the cobalt-based catalyst was estimated 5 using the same method as before, i.e. by comparing the area of the R-branch of the CO signal 6 collected with the DRIFTS cell loaded with the sample (Fig.…”

“…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. The renewed interest in 7 Fischer-Tropsch synthesis require high pressure analyses, for which the signal of gas-phase 8 CO becomes overwhelming and must be subtracted to unravel the signal of adsorbed CO. 9 The effect of temperature on the signal of gas-phase CO is, to our knowledge, largely 10 overlooked, while it can lead to significant signal distortion when comparing spectra collected 11 at different temperatures.…”

“…12 DRIFTS exhibits a higher sensitivity with respect to surface species and strongly 13 absorbing samples such as carbon-based materials can be studied [22]. It was demonstrated 14 that DRIFTS could be used for quantitative analysis to determine the surface concentration of 15 adsorbates [5,23,24,25] and the heat of adsorption of CO on oxides and supported metals [26]. 16 In particular, DRIFTS analyses were able to show that the heat of CO adsorption on a Pt-Sn 17 alloy was about half the value measured on plain Pt, and that Pt-Sn segregation took place 18 reversibly in the presence of CO when the temperature was lower than 175°C [27].…”

“…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.…”