Vol.:(0123456789) 1 3
Topics in Catalysis
https://doi.org/10.1007/s11244-020-01306-y
ORIGINAL PAPER
Reactivity of C
3
H
x
Adsorbates in Presence of Co‑adsorbed CO
and Hydrogen: Testing Fischer–Tropsch Chain Growth Mechanisms
C. J. Weststrate
1
· Devyani Sharma
1,2
· Daniel Garcia Rodriguez
1,2
· Michael A. Gleeson
2
· Hans O. A. Fredriksson
1
·
J. W. Niemantsverdriet
1,3
© Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract
The identity of the surface intermediates involved in chain growth during Fischer–Tropsch synthesis remains a topic of ongo-
ing debate. In the present work we use a combination of temperature programmed reaction spectroscopy and high resolution
X-ray photoemission spectroscopy to study the reactivity of C
3
H
x
adsorbates on a Co(0001) single crystal surface in order
to explore the stabilities of the diferent C
3
H
x
surface intermediates and to study elementary reaction steps relevant to chain
growth and chain termination. Propene (H
3
C–CH=CH
2
) and propyl (H
3
C–CH
2
–CH
2
–) adsorbates react below 200 K already,
either by desorption of propene or by dehydrogenation to adsorbed propyne (H
3
C–C≡CH). Co-adsorbed H
ad
and CO
ad
do
not afect the temperature at which propyl and propene react, but they do suppress the dehydrogenation pathway in favour of
propene desorption. Their high reactivity under simulated FTS conditions disqualifes them as feasible intermediates for FTS,
which requires long-lived intermediates to match the low monomer formation rate. Propyne, the most stable C
3
H
x
adsorb-
ate in the absence of CO
ad
, is hydrogenated to propylidyne (H
3
C–CH
2
–C≡) > 230 K when both CO
ad
and H
ad
are present.
Propylidyne dimerization occurs around 313 K and produces a 3-hexyne (H
5
C
2
–C≡C–C
2
H
5
) surface intermediate which is
hydrogenated to 3-hexene (H
5
C
2
–CH=CH–C
2
H
5
) above 350 K. These fndings are of direct relevance to FTS: they show
that the high coverage of CO
ad
and H
ad
present during the reaction infuence the reactivity of C
x
H
y
adsorbates involved in
chain growth and ultimately steer product selectivity. The fndings provide further experimental support for the previously
proposed alkylidyne chain growth mechanism on close-packed cobalt terraces: CO stabilizes C
x
H
y
growth intermediates in
the alkylidyne form and growth proceeds via coupling of a long chain alkylidyne and methylidyne (CH).
Keywords Fischer–Tropsch synthesis · Chain growth mechanism · Cobalt catalysts · Synchrotron XPS · Near-ambient
pressure XPS · Elementary surface reactions
1 Introduction
Fischer–Tropsch synthesis (FTS) is an industrially applied
process in which iron- or cobalt-based catalysts are used to
convert synthesis gas, a mixture of CO and H
2
, into long
chain hydrocarbons. Synthesis gas is most often manufac-
tured using fossil sources such as natural gas or coal, but
it can also be produced in a renewable manner, by using
biomass or by partial reduction of CO
2
using hydrogen pro-
duced via electrochemical water splitting. Fischer–Tropsch
synthesis can then be employed to convert renewable synthe-
sis gas into liquid hydrocarbon fuels for applications that are
difcult to electrify, such as airplanes, long haul road trans-
port and shipping. Cobalt-based catalysts are particularly
suited for this application [1] due to their high activity, high
stability and high selectivity towards straight, long chain
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s11244-020-01306-y) contains
supplementary material, which is available to authorized users.
* C. J. Weststrate
c.j.weststrate@syngaschem.com
1
SynCat@DIFFER, Syngaschem BV, Eindhoven,
The Netherlands
2
Dutch Institute for Fundamental Energy Research (DIFFER),
Eindhoven, The Netherlands
3
SynCat@Beijing, Huairou, Beijing,
People’s Republic of China