Carbon Number Dependence of Reaction Mechanism and Kinetics in CO Hydrogenation on a Co-Based Catalyst

Article abstract of DOI:10.1021/acscatal.6b01376

A detailed mapping of the kinetic parameters involved in the reaction network of CO hydrogenation on a Co-Re/CNT catalyst has been performed. Multicomponent steady-state isotopic transient kinetic analysis (SSITKA) has been used to deconvolute the rates of chain growth and termination to olefins and paraffins with various carbon numbers into the concentration of the surface intermediates and their reactivity, expressed as their rate constants k_g, k_o, and k_p, respectively, at a molecular level. The site coverage of the different products (θ_Cn) measured by the multicomponent analysis of their isotopic distribution allows the study of their effect on chain growth, as well as on olefin and paraffin formation. The insights into the reaction mechanism were gained from the kinetic dependence of different reaction steps on the hydrogen pressure. The results revealed a significant carbon number dependence of the mechanism for the chain growth and termination to paraffin and olefin reactions: an enol-like intermediate is dominating for the formation of C₃ hydrocarbons, while the alkenyl intermediate is dominating for the formation of C₄ hydrocarbons. Moreover, the SSITKA-assisted kinetic study provided the carbon number dependence of the rate constants of various reaction steps for the paraffin formation and chain growth, both decrease concurrently with increasing carbon number. However, the rate constant for the olefin formation is constant, regardless of the carbon number, suggesting the formation rate of olefins with different carbon number depends mainly on the concentration of the corresponding surface intermediates.

Full text of DOI:10.1021/acscatal.6b01376

Subscriber access provided by Northern Illinois University
Article
Carbon Number Dependency of Reaction Mechanism and
Kinetics in CO hydrogenation on a Co-based Catalyst
Cristian Ledesma, Jia Yang, Edd A. Blekkan, Anders Holmen, and De Chen
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b01376 • Publication Date (Web): 29 Aug 2016
Downloaded from http://pubs.acs.org on September 2, 2016
Just Accepted
“Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted
online prior to technical editing, formatting for publication and author proofing. The American Chemical
Society provides “Just Accepted” as a free service to the research community to expedite the
dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts
appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been
fully peer reviewed, but should not be considered the official version of record. They are accessible to all
readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered
to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published
in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just
Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor
changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers
and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors
or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
ACS Catalysis is published by the American Chemical Society. 1155 Sixteenth Street
N.W., Washington, DC 20036
Published by American Chemical Society. Copyright © American Chemical Society.
However, no copyright claim is made to original U.S. Government works, or works
produced by employees of any Commonwealth realm Crown government in the course
of their duties.

Page 1 of 44
ACS Catalysis
1
2
3
4
5
6
7
8
9
Carbon Number Dependency of Reaction Mechanism and Kinetics in
CO Hydrogenation on a Coꢀbased Catalyst
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Cristian Ledesma, † Jia Yang, ‡ Edd A. Blekkan, † Anders Holmen, † and De Chen*, †
†Department of Chemical Engineering; Norwegian University of Science and Technology
(NTNU); Trondheim, Nꢀ7491 (Norway)
‡SINTEF Materials and Chemistry; Trondheim, Nꢀ7465 (Norway)
ACS Paragon Plus Environment
1

ACS Catalysis
Page 2 of 44
1
2
3
4
Abstract
5
6
7
8
9
A detailed mapping of the kinetic parameters involved in the reaction network of CO
hydrogenation on a CoꢀRe/CNT catalyst has been performed. Multicomponent SteadyꢀState
Isotopic Transient Kinetic Analysis (SSITKA) has been used to deconvolute the rates of chain
growth and termination to olefins and paraffins with various carbon numbers into the
concentration of the surface intermediates and their reactivity, expressed as their rate constants
kg, ko and kp, respectively at a molecular level. The site coverage of the different products (θCn)
measured by the multicomponent analysis of their isotopic distribution allows the study of their
effect on chain growth, as well as on olefin and paraffin formation. The insights into the reaction
mechanism were gained from the kinetic dependency of different reaction steps on the hydrogen
pressure. The results revealed a significant carbon number dependency of the mechanism for the
chain growth and termination to paraffin and olefin reactions: an enolꢀlike intermediate is
dominating for the formation of C3 hydrocarbons, while the alkenyl intermediate is dominating
for the formation of C4 hydrocarbons. Moreover, the SSITKA assisted kinetic study provided the
carbon number dependency of the rate constants of various reaction steps for the paraffin
formation and chain growth, both decrease concurrently with increasing carbon number.
However, the rate constant for the olefin formation is constant, regardless of the carbon number,
suggesting the formation rate of olefins with different carbon number depends mainly on the
concentration of the corresponding surface intermediates.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Keywords: CO hydrogenation, multicomponent SSITKA, isotopic labelling, kinetic study,
mechanism
ACS Paragon Plus Environment
2

Page 3 of 44
ACS Catalysis
1
2
3
4
List of Symbols
5
6
7
8
9
Cn*
precursors for chain termination and chain growth
Cobalt dispersion
[ꢀ]
ꢀ1
DCo
[gCo,s gCo ]
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
FCO,in
FCO,out
FCn
molar flow rate of CO at the inlet of the reactor
molar flow rate of CO at the outlet of the reactor
molar flow of the compound with carbon number n
normalized curve of the compound with carbon number n
[µmol sꢀ1]
[µmol sꢀ1]
[mol sꢀ1]
[ꢀ]
FCn(t)
intensity of the fragmentation pattern of a given isotopic
combination at its m/e value
ref
fij
[ꢀ]
intensity of the fragmentation pattern observed in the GCꢀMS
for each m/e value of a given isotopic combination
fiobs
[ꢀ]
FT
total flow in the reactor
[NmL minꢀ1]
ꢀ1
GHSV
i.d.
Gas Hourly Space Velocity
internal diameter
[mL hꢀ1 gcat
]
[ꢀ]
kg
rate constant of the chain growth reaction
[sꢀ1]
rate constant of the chain growth reaction of the compound
with carbon number n
kg,n
ko
[sꢀ1]
[sꢀ1]
[sꢀ1]
[sꢀ1]
[sꢀ1]
rate constant of the olefin formation reaction
rate constant of the olefin formation reaction of the compound
with carbon number n
ko,n
kp
rate constant of the paraffin formation reaction
rate constant of the paraffin formation reaction of the
compound with carbon number n
kp,n
rate constant of the chain growth reaction for the simple
kinetic model of the compound with carbon number n
ks,g,n
ks,o,n
ks,p,n
[sꢀ1]
[sꢀ1]
[sꢀ1]
rate constant of the olefin formation reaction for the simple
kinetic model of the compound with carbon number n
rate constant of the paraffin formation reaction for the simple
kinetic model of the compound with carbon number n
ACS Paragon Plus Environment
3

ACS Catalysis
Page 4 of 44
1
2
3
4
5
rate constant lumped with the site coverage that cannot be
measured experimentally
ks,n
[sꢀ1]
6
7
8
9
k’’o,n
k’’p,n
KH2
K25a
K25b
k25c
m
k lumped including ko,n; KH2m2/2 and ꢀ*m2
k lumped including kp,n; KH2m1/2 and ꢀ*m1
hydrogen adsorption equilibrium constant
equilibrium constant for the reaction step Eq. 25a
equilibrium constant for the reaction step Eq. 25b
rate constant for the reaction step Eq. 25c
number of different m/e values for each hydrocarbon
[ꢀ]
[ꢀ]
[ꢀ]
[ꢀ]
[ꢀ]
[sꢀ1]
[ꢀ]
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
reaction order with respect to adsorbed atomic hydrogen for
the paraffin formation reaction
m1
m2
[ꢀ]
reaction order with respect to adsorbed atomic hydrogen for
the olefin formation reaction
[ꢀ]
MCo
n
molecular weight of Cobalt
[g molꢀ1]
[ꢀ]
number of isotopic combinations
concentration of adsorbed compounds with carbon number n
concentration of total active sites in the catalyst
total pressure
ꢀ1
NCn
Ns
[mol gcat
[mol gcat
[bar]
]
ꢀ1
]
PT
PCO
PH2
rCn
CO partial pressure
[bar]
H2 partial pressure
[bar]
reaction rate of the compound with carbon number n
molar rate of CO consumption per gram of catalyst
[mol (gcat s)ꢀ1]
[mol (gcat s)ꢀ1]
RCO
reaction rate of the olefin formation for the compound with
carbon number n
ro,n
[s]
reaction rate of the paraffin formation for the compound with
carbon number n
rp,n
[s]
SCn
S(x)
T
carbon based selectivity to compound with carbon number n
objective function to minimize
[mol molꢀ1]
[ꢀ]
K
temperature
ACS Paragon Plus Environment
4

Page 5 of 44
ACS Catalysis
1
2
3
4
5
6
7
8
9
wcat
catalyst mass
[gcat]
[sꢀ1]
[sꢀ1]
[sꢀ1]
[ꢀ]
TOFg,n
TOFo,n
TOFp,n
wt.
turnover frequency for chain growth reaction
turnover frequency for olefin formation reaction
turnover frequency for paraffin formation reaction
weight
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
XCO
CO conversion
[µmol µmolꢀ1]
ꢀ1
xCo
Cobalt loading on the catalyst
[gCo gcat
[ꢀ]
]
contribution of a given isotopic combination to the mixture at
its m/e value
xij
site coverage of species with carbon number n
[ꢀ]
ꢀꢁꢂ
ꢀC*
site coverage of the monomer to be incorporated in the
growing chain
[ꢀ]
site coverage of intermediates with carbon number n
corresponding to the formation of olefins
ꢀꢃ,ꢂ
ꢀꢄ,ꢂ
[ꢀ]
[ꢀ]
site coverage of intermediates with carbon number n
corresponding to the formation of paraffins
ꢀH
site coverage of hydrogen
[ꢀ]
[ꢀ]
[ꢀ]
[ꢀ]
[ꢀ]
[s]
site coverage of the alkyl intermediate
site coverage of the alkenyl intermediate
site coverage of the enolꢀlike intermediates
site vacancy
ꢀRCH2CH2
ꢀRCHCH
ꢀRCH2CHO
ꢀ*
residence time of compounds with carbon number n
ꢅꢁꢂ
ACS Paragon Plus Environment
5

ACS Catalysis
Page 6 of 44
1
2
3
4
Introduction
5
6
7
8
9
FischerꢀTropsch (FT) synthesis is one of the most important industrial processes to convert
synthesis gas into valuable chemicals and clean liquid fuels. Although the process is known since
the 1920s, the reaction mechanism for CO activation, chain growth and termination is still in
debate1–3. The FT product spectrum, which can be generally described by the AndersonꢀSchulzꢀ
Flory (ASF) model for chain polymerization kinetics, contains a complex mixture of linear and
branched hydrocarbons and oxygenated products, as a result of several intermediate steps:
reactant adsorption, chain initiation, chain growth, chain termination, product desorption,
readsorption and further reaction4–6. Several mechanisms have been developed over the years to
explain the different product distribution obtained in this process, and they can be summarized in
three different groups: the carbide mechanism7, originally proposed by FischerꢀTropsch; the CO
insertion mechanism8 and the hydroxycarbene mechanism9. The key surface intermediates (Cn*)
responsible for the chain growth and the termination to hydrocarbons with different carbon
number could depend on the reaction mechanism. The nature of the intermediates corresponding
to the chain growth and termination, together with the reaction mechanism of FT synthesis
remains in debate.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
The identity of metal, particle size, support and promoter, as well as reaction conditions have
been intensively studied, and observed to largely influence the catalyst activity and selectivity10–
16. However, about the nature of such effects very little is known. As an important step towards a
rational design of FT catalysts, a kinetic diagnostic tool is highly desired, which could provide
fundamental understanding of the catalyst properties and reaction conditions on adsorption and
ACS Paragon Plus Environment
6

Page 7 of 44
ACS Catalysis
1
2
3
4
5
6
surface reactions in the steps involved in CO activation, methane formation, chain growth and
termination by olefin and paraffin formation.
7
8
9
SSITKA is a powerful technique known since the 1970s that can give valuable kinetic
information about the adsorption and surface reactions at the molecular level by providing the
site coverage and rate constants simultaneously at steadyꢀstate conditions. This technique has
been applied to many reaction systems, including FT17,18. Most of the work deals with the
conventional SSITKA, limited to analysis of C1 compounds only, such as CO conversion and
methane formation19–21. Developments in SSITKA have recently been reviewed18, and one of the
progresses has been the implementation of the multicomponent SSITKA where the responses of
heavier hydrocarbons can be evaluated. Schouten and coꢀworkers developed a mathematical
method based on different assumptions to identify and discriminate efficiently between different
mechanistic models for methane formation as well as for higher hydrocarbon (C2+) formation in
FT synthesis over cobalt22–24 and ironꢀbased catalysts25–27. This type of transient kinetic model of
the SSITKA response curves in combination with Density Functional Theory (DFT) calculations
was found to effectively elucidate the reaction mechanism of CO activation and methane
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
formation over cobaltꢀbased catalysts under methanation conditions19–21
.
In the current work, a new method is developed to extend our previous kinetic studies on
methane formation by using multicomponent SSITKA. This method makes it possible to
determine the site coverage of C2+ hydrocarbons and provide a mapping of the kinetic parameters
of the CO hydrogenation reaction network over a CoꢀRe catalyst supported on carbon nanotubes.
It deconvolutes the rates of chain growth and termination by olefin and paraffin formation with
various carbon numbers into the concentration of the surface intermediates and their reactivity.
Combining the kinetic analysis in terms of the concentration of the intermediates on Co surfaces,
ACS Paragon Plus Environment
7

ACS Catalysis
Page 8 of 44
1
2
3
4
5
6
the reaction mechanism and the chemical identity of intermediates leading to chain growth and
termination are proposed.
7
8
9
Experimental section
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
The CO hydrogenation reaction has been performed using a fixedꢀbed quartz reactor (4 mm i.d.)
over a 20% wt. Co and 0.5% wt. Re supported on carbon nanotubes. Previous investigations
have pointed out that Re has little effect in the intrinsic site activity and apparent activation
energy for the catalysts21. The CoꢀRe/CNT behaves almost identical as the Co/CNT catalyst.
CNT as a support has showed a high surface area and low interaction with cobalt, where the
cobalt particles can be completely reduced28–30. The dispersion of cobalt is 5.1% and the cobalt
particle size is 18.9 nm based on H2 chemisorption. The crystallite size of cobalt is 11.7 nm
found by Xꢀray diffraction. The detailed preparation method, full characterization of the catalyst
and the procedures for H2 chemisorption and Xꢀray diffraction have been described previously21.
The CNT supported Co catalyst with relatively large Co particle size was selected as a model
catalyst in the present work to eliminate possible effects of cobalt size and support on the
reaction mechanism31. Therefore, the mechanistic insight obtained in this study reflects the CO
hydrogenation mechanism on industrial relevant Co catalysts with relatively large Co particles.
Prior to reaction 25 mg of catalyst were reduced in 10 NmL/min H2 at 623 K for 16 h using a
ramping rate of 1 K/min. The catalyst was then cooled down to 483 K, H2/CO/Ar (15/1.5/33.5
NmL/min) mixture was introduced and the pressure was adjusted to 1.85 bar (absolute pressure),
which corresponds to a Gas Hourly Space Velocity (GHSV) of 3600 mLCO/h/gcat. Once steady
state was achieved after 6 hours on stream, the catalytic performance was evaluated. Comparing
ACS Paragon Plus Environment
8

Page 9 of 44
ACS Catalysis
1
2
3
4
5
6
the amount of the different components in the reactor outlet to the composition of the feed gas,
the conversion and selectivity were calculated as shown in Eqs. 1 and 2, respectively.
7
8
9
ꢈ
ꢍꢈ
ꢉꢊ,ꢎꢏꢐ ∙ 100 [%]
(Eq. 1)
(Eq. 2)
ꢉꢊ,ꢋꢌ
ꢆꢁꢇ
=
=
ꢈ
ꢉꢊ,ꢋꢌ
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
ꢈ
ꢉꢌ
ꢑꢁꢂ
∙ 100 [%]
ꢌ
∑
ꢈ
ꢉꢌ
ꢋ
where the terms Fco,in and Fco,out correspond to µmol/s CO flow rate of the feed gas and at the
reactor outlet, respectively; FCn corresponds to the carbon based mol/s of the hydrocarbon n
produced in the reaction and SCn corresponds to the carbon based selectivity of the n product in
%.
SSITKA experiments were carried out at 483 K and a total pressure (PT) of 1.85 bar by
switching between H2/12CO/Ar and H2/13CO/Kr. The reaction order of the reactants has been
12
13
determined by varying their partial pressure. The transient responses of Ar, Kr, CO, CO,
12CH4 and 13CH4 were monitored with a Balzers QMG 422 quadrupole mass spectrometer (MS).
The concentrations of H2, CO, Ar and C1ꢀC6 hydrocarbons were analyzed with a GCꢀMS
(Agilent GC7890B – MSD5977A) equipped with TCD, FID and MSD detectors. The isotopic
distribution of C2ꢀC5 hydrocarbons were calculated as a linear combination of the fragmentation
patterns of the corresponding isotopic products26. At the conditions tested mass or heat transfer
limitations are not observed experimentally32, which were also verified via adequate
correlations33. A repeated set of experiments indicates that the experimental relative error is
typically less than 5%.
Results and discussion
ACS Paragon Plus Environment
9

ACS Catalysis
Page 10 of 44
1
2
3
4
5
1. Effect of CO and H2 pressures on chain growth and termination to olefin and
paraffin
6
7
8
9
The kinetic study of CO hydrogenation reaction has been performed on CoꢀRe/CNT catalyst at
methanation conditions using a mixture of H2/CO/Ar at 483 K and 1.85 bar. The present work
focuses on gaining a better mechanistic understanding of CO hydrogenation, and the experiments
were performed at constant temperature, but at various CO and H2 pressures. The results are
summarized in Table 1.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
ACS Paragon Plus Environment
10

Page 11 of 44
ACS Catalysis
1
2
3
4
Table 1. Performance of the CoꢀRe/CNT catalyst in CO hydrogenation at T = 483 K and PT = 1.85 bar under different partial
5
6
pressures of the reactants, CO and H2; FT = 50 NmL/min
7
8
9
RCO
(µmol/s/gcat)
SC2H6 SC2H4 SC3H8 SC3H6 SC4H10 SC4H8 SC5H12 SC5H10 SC6H14 SC7+
(%)
PCO
(bar)
PH2
GHSV
SCH4
(%)
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
(bar) (mLCO/h/gcat)
(%)
(%)
0.5
0.5
0.6
0.8
1.3
1.1
0.8
(%)
5.7
7.3
6.5
5.4
4.0
4.4
5.2
(%)
3.8
4.6
7.3
(%)
2.2
3.9
4.1
(%)
3.9
5.9
7.3
8.1
5.5
5.6
4.4
(%)
1.5
2.9
3.3
3.4
1.3
2.4
1.4
(%)
1.2
2.6
3.5
4.2
1.8
1.2
1.6
(%)
0.8
1.7
2.0
2.1
0.4
0.0
0.9
0.056 0.560
0.111 0.560
0.137 0.560
0.166 0.560
0.056 0.296
0.056 0.370
0.056 0.444
3600
1800
2220
2700
3600
3600
3600
4.8
3.6
3.1
2.4
2.4
2.7
3.3
72.3 8.2
62.6 8.1
58.0 7.5
53.3 6.9
69.9 7.1
66.6 7.6
70.8 7.9
0.0
0.0
0.0
1.8
0.0
0.0
0.0
10.1 3.9
7.1
7.1
4.9
1.6
2.0
2.1
ACS Paragon Plus Environment
11

ACS Catalysis
Page 12 of 44
1
2
3
4
5
6
Based on the product distribution, a simplified reaction network including monomer formation,
C1ꢀC6 paraffin and olefin formation is depicted in Scheme 1, where the intermediates of Cn* are
7
8
9
generally defined as the precursors for the chain growth and termination5,35–45
.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Scheme 1. Reaction network of CO hydrogenation, where the rates for chain growth and
termination to paraffin and olefin are marked for each intermediate.
The turnover frequency for olefin formation (TOFo,n), paraffin formation (TOFp,n) and chain
growth (TOFg,n) are obtained from the formation rates of all the products and the mass balance.
The formation rates of olefin and paraffin, expressed as turnover frequency (TOF) based on the
Co active sites measured by hydrogen chemisorption, are calculated by Eq. 3.
ꢗ
∙ꢘ ∙ꢙ
ꢉꢌ ꢉꢊ ꢉꢎ
ꢍꢝ
[
]
ꢒꢓꢔꢄꢕꢃꢖ,ꢂ
=
ꢜ
(Eq. 3)
ꢚ
∙ꢛ
ꢉꢎ ꢉꢎ
where SCn is the carbon based selectivity of the product n, RCO corresponds to the molar rate of
CO consumption per gram of catalyst (mol/gcat/s), which was estimated from the conversion by
the differential reactor design equation, MCo corresponds to the molecular weight of cobalt
(g/mol), DCo is the Co dispersion obtained from H2 chemisorption and xCo corresponds to the
cobalt loading on the catalyst (gCo/gcat). SCn*RCO is the formation rate of the hydrocarbon with
ACS Paragon Plus Environment
12

Page 13 of 44
ACS Catalysis
1
2
3
4
5
6
7
8
9
carbon number n (mol/gcat,s). DCoxCo/MCo represents mole of surface Co per gram of catalyst
(molCo,s/gcat). The turnover frequencies for paraffin and olefin formation are plotted as a function
of CO and H2 partial pressures in Figure 1.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
CH4
C2H6
C3H8
C4H10
C5H12
C6H14
C3H6
C4H8
C5H10
C2H4
Figure 1. Turnover frequencies of paraffin formation (TOFp,n) (a, b) and olefin formation
(TOFo,n) (c, d). (a,c) were measured at different CO pressures and constant H2 pressure of 0.56
bar and (b,d) at different H2 pressures and constant CO pressure of 0.06 bar. T = 483 K and PT =
1.85 bar.
The effect of CO pressure on the paraffin formation is presented in Figure 1 a). The effect
depends obviously on the carbon number. Results indicate that the rate of paraffin formation
ACS Paragon Plus Environment
13

ACS Catalysis
Page 14 of 44
1
2
3
4
5
6
7
8
9
generally decreases with the carbon number. The methane formation rate shows a monotonic
decrease with increasing CO pressure. For C2ꢀC5 paraffin, the formation rates increases with
increasing CO pressure following a decrease at high CO pressures. A maximum rate exists for
each paraffin at a CO pressure of about 0.11 bar. The decrease in the TOFp,n with increasing CO
pressure is less pronounced when the carbon number increases. The effect of H2 pressure on the
paraffin formation is presented in Figure 1 b). The forming rates of all the paraffins increase
almost monotonically with increasing H2 pressure.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
The effects of CO and H2 pressure on the olefin formation are presented in Figure 1 c) and d),
respectively. The amount of ethylene formed at the different conditions tested is very small
(Table 1, Figure 1), possibly due to strong adsorption of ethylene. The ethylene formation rate
did not vary significantly with CO pressure, while the formation rate decreased slightly with
increasing H2 pressure. There are also maxima in the curves for C4ꢀC5 olefin formation with CO
pressure, but the optimum rates appear at a slightly higher CO pressure of about 0.14 bar
compared to the paraffin formation (Figure 1a).
The effective chain growth turnover frequency (TOFg,n) for Cn is calculated based on Eqs. 4
and 5, where the chain growth is generally assumed to be irreversible. Since no significant C6+
hydrocarbon were detected, TOFg,5, the chain growth from C5 is equal to the formation TOF of
C6 paraffin (TOFp,6), with no C6 olefins observed above the GC detection limit. The TOFg,n for
Cn (n=1ꢀ5) is calculated based on the mass balance as described by Eq. 5, comprising the
formation rate to Cn+1 paraffin and olefins and chain growth rate of Cn+1 towards Cn+2, where the
insertion of C1 unit as the chain growth monomer in the chain growth of Cn+1 is subtracted from
the chain growth rate by applying a factor of n+1/n+2.
ACS Paragon Plus Environment
14

Page 15 of 44
ACS Catalysis
1
2
3
4
5
6
7
8
ꢒꢓꢔ = ꢒꢓꢔꢄ,ꢠ
(Eq. 4)
(Eq. 5)
ꢞ,ꢟ
ꢒꢓꢔ = ꢒꢓꢔꢄ,ꢂꢡꢝ ꢢ ꢒꢓꢔꢃ,ꢂꢡꢝ ꢢ ꢂꢂꢡꢡꢣꢝ ꢒꢓꢔꢞ,ꢂꢡꢝ ; ꢤ = 1 ꢥ 4
ꢞ,ꢂ
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
The dependence of the chain growth with CO and H2 partial pressures is depicted in Figure 2.
n=1
n=2
n=3
n=4
n=5
Figure 2. Turnover frequency of chain growth (TOFg,n; n=1ꢀ5) at a) different CO pressures and
constant H2 pressure of 0.56 bar and b) different H2 pressures and constant CO pressure of 0.06
bar. T = 483 K and PT = 1.85 bar.
Figure 2 indicates that the chain growth rate depends significantly on the carbon number and
the rate decreases with increasing carbon number. Furthermore, a maximum in the curve was
also observed for the chain growth rate with the CO pressure, and the maximum chain growth
rate was located at a CO pressure between 0.11ꢀ0.14 bar (Figure 2a). The position of this is
identical to the peak position of the CO activation rate. The chain growth rate increases
concurrently with increasing hydrogen pressure (Figure 2b), and the effect of hydrogen is more
significant for hydrocarbons with lower carbon number.
ACS Paragon Plus Environment
15

ACS Catalysis
Page 16 of 44
1
2
3
4
5
6
7
8
9
Figure 1 and 2 illustrate the complex dependencies of the formation rates of olefins and
paraffins as well as the chain growth rates on the CO and H2 pressures. In order to get a better
understanding of such effects and thus provide new insights to the reaction mechanism,
multicomponent SSITKA was performed in order to deconvolute these effects to the site
coverage and the reactivity of the intermediates.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
2. Mapping of the kinetic parameters involved in the reaction network
Based on the transient curves of different isotopic labeled products, the site coverages of the
intermediates leading to the main products on the catalyst surface were directly obtained. The
site coverage of each intermediate produced in FT synthesis is defined by Eq. 6, namely the ratio
of the concentration of adsorbed species (NCn) in mol/gcat and the concentration of total active
sites (Ns) in mol/gcat, measured by H2 chemisorption.
ꢦ
ꢉꢌ
ꢀꢁꢂ
=
(Eq. 6)
ꢦ
ꢧ
The number of adsorbed species (NCn) is defined by the product of the reaction rate (rCn),
which can be calculated by Eq. 7 (mol/gcats), and the mean residence time of each intermediate
(s) over the catalytic surface (Eq. 8).
ꢈ
ꢩ
ꢉꢊ
ꢮꢯꢰ
ꢨꢁꢂ
=
ꢉꢊ ∙ ꢑꢁꢂ
ꢭ
ꢵ
ꢶ
ꢜ
(Eq. 7)
(Eq. 8)
ꢱꢲꢳꢴ
ꢪ
ꢫꢬꢐ
ꢮꢯꢰ
ꢷꢁꢂ = ꢨꢁꢂ ∙ ꢅꢁꢂ
ꢭ
ꢵ
ꢶ
ꢱꢲꢳꢴ
where FCO is the CO flow rate in the feed stream (mol/s) and XCO is the CO conversion level in
%. SCn is the selectivity of the product n and wcat corresponds to the weight of catalyst in grams.
ACS Paragon Plus Environment
16

Page 17 of 44
ACS Catalysis
1
2
3
4
5
6
The residence time of CO, Ar, Kr, and CH4 are determined from the area under the normalized
curve obtained by SSITKA experiments using the MS (Eq. 9).
7
8
9
ꢅꢁꢂ
=
ꢹ ꢔꢁꢂ ꢕꢻꢖꢼꢻ
(Eq. 9)
ꢸ
ꢺ
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
The residence time of higher hydrocarbons is determined by the analysis of the fragmentation
pattern of the nonꢀ, partially and fully labelled products after the isotopic switch using a GCꢀMS,
following the procedure developed by Van Dijk and coꢀworkers23–27. Firstly, the fragmentation
pattern of each hydrocarbon produced in the reaction is calculated as a linear combination of the
fragmentation patterns of the nonꢀ, partially and fully labeled compounds and its contribution to
the product. The contribution can be calculated by minimizing the objective function shown in
Eq. 10.
ꢛ
ꢣ
ꣂꣃ꣄
ꢿꣀ
꣈
ꢑ ꢽ = ꢿ꣉ꢝꢾ ꢂꢾꢽꢿꣀꣁ ꣅ ꢥ ꣁꢃ꣆꣇ꣅ → ꢮ꣊ꢤ꣊ꢮ꣋ꢮ
(Eq. 10)
ꢕ ꢖ
∑
∑
ꣀ
ꢿ
where S(x) is the objective function, m is the number of different m/e values for each
hydrocarbon produced, n is the number of isotopic combinations depending on the number of
atoms that can be labelled, xij is the contribution of a given isotopic combination to the mixture
at its m/e value, fijref is the intensity of the fragmentation pattern of a given isotopic combination
at its m/e value, and fiobs is the intensity of the fragmentation pattern observed in the GCꢀMS for
each m/e value of a given isotopic combination. Once the contribution of each isotopic
combination is calculated, the normalized responses can by depicted over time on stream as
shown in Figure 3, which was used to calculate the residence time of the nonꢀ, partially and
fullyꢀlabeled intermediates. The isotopic distribution in ethylene was not shown in Figure 3, due
to very low concentration of ethylene at the conditions studied. The overall residence time of a
ACS Paragon Plus Environment
17

ACS Catalysis
Page 18 of 44
1
2
given hydrocarbon can be calculated by the overall 12C content of a hydrocarbon with n Cꢀatoms
where i atoms are labelled, as shown in Eq. 11.
3
4
5
6
7
8
9
ꢝ
ꢂ
∑
ꢅꢁꢂ
=
ꢿ꣉ꢝ ꣊ ∙ ꢅꢝꢣ
(Eq. 11)
ꢝ꣌
ꢉꢋ ꢉꢌ꣍ꢋ
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
ꢂ
ACS Paragon Plus Environment
18

Page 19 of 44
ACS Catalysis
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
b)
1.2
1.0
0.8
0.6
0.4
0.2
0.0
-0.2
12C3H8
13C3H8
12C213CH8
12C13C2H8
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
time (min)
ACS Paragon Plus Environment
19

ACS Catalysis
Page 20 of 44
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 3. Isotopic distribution of the main intermediates C2H6 (a), C3H8 (b), C3H6 (c), C4H10 (d),
C4H8 (e), C5H12 (f), C5H10 (g) over time on stream at 483 K, PT = 1.85 bar, PCO = 0.17 bar and
PH2 = 0.56 bar.
CO
CH4
C3H8
C2H6
C4H10
C4H8
C3H6
C5H10
C2H4
ACS Paragon Plus Environment
20

Page 21 of 44
ACS Catalysis
1
2
3
4
5
6
7
8
9
Figure 4. Site coverage of CO, and the intermediates leading to the formation of CH4 and C2+
(C2H6, C3H8, C4H8) paraffins (a, b) and to the formation of C2+ olefins (C2H4, C3H6, C4H8, C5H10)
(c, d). (a,c) were measured at different CO pressures and constant H2 pressure of 0.56 bar and (b,
d) were measured at different H2 pressures and constant CO pressure of 0.06 bar. T = 483 K and
PT = 1.85 bar.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 4 presents the changes of the site coverage of CO and the main surface intermediates
leading to the formation of paraffin (Figure 4 a and b) and olefin (Figure 4, c and d) of various
carbon numbers with CO and H2 pressure. Figure 4a shows an increase in CO site coverage with
increasing CO pressure, while Figure 4b shows no significant changes in CO site coverage with
increasing H2 partial pressure. The results clearly revealed the adsorbed CO species as the most
abundant surface species on Co surfaces at the studied conditions, suggesting a stronger
adsorption of CO compared to other species. However, the monotonic increase in CO site
coverage is inconsistent with the volcano type of relationship between the CO conversion rate
and CO pressure (Table 1). Based on the previously reported CO hydrogenation mechanism, the
HCO*+H* step is the rate determining step for CO activation34, and the CO conversion rate is
2
then proportional to θCOθH . Unfortunately, it is not possible to accurately measure the H surface
coverage by H2/D2 switch in SSITKA experiments due to exchange reactions with surface and
gas phase OH groups. However, the adsorption of CO and H is competitive on the Co surfaces,
and CO adsorption is much stronger than H adsorption. It is then expected that increasing CO
pressure increases the CO site coverage and consequently decreases the H site coverage. The
opposite changes in CO and H site coverages resulted in a volcano type change of the CO
conversion rate with CO pressure. The increase of reaction rate with the increase in CO pressure
is a result of more significant increase in CO site coverage compared to the decrease in H site
coverage. At high CO pressures, increasing CO pressure might result in a more significant
decrease in the H site coverage, thus the reaction rate decreases.
ACS Paragon Plus Environment
21

ACS Catalysis
Page 22 of 44
1
2
3
4
5
6
7
8
In general, the dependency of site coverage of intermediates leading to olefin and paraffin
formation with CO and H2 pressures (Figure 4) corresponds well with the dependencies of olefin
and paraffin formation rates (Figure 1). This is expected from a kinetic point of view, the CO and
hydrogen pressures should mainly influence their site coverage. Similar to the CO pressure
dependency of the paraffin formation rate (Figure 1a), a volcanoꢀshaped curve was also observed
for the CO pressure dependency of the site coverage of the intermediates leading to paraffin
formation. Although the detailed kinetic expression for the formation of these intermediates
could be complicated, the results suggest that the intermediate formation rate is a function of the
site coverage of CO and H. The maximum in the site coverage of intermediates at a moderate CO
pressure is possibly a result of the positive effect on the CO site coverage and a negative effect
on the hydrogen site coverage with increasing CO pressure. Moreover, a more significant
decrease in the site coverage of intermediates leading to paraffin formation with increasing CO
pressure at relatively high CO pressures was observed, compared to changes in intermediates
leading to olefin formation. It is a result of more hydrogen involved in the elementary reactions
leading to paraffin formation compared to olefin formation, which will be discussed later in more
detail.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figures 1ꢀ4 show that the CO and H2 pressure dependency of the formation rates of olefin and
paraffin and chain growth, as well as the concentration of intermediates, and such dependency is
clearly a function of the carbon number. A clearer carbon number dependency of the formation
rates of olefin and paraffin and chain growth is presented in Figure 5a. Both paraffin formation
and chain growth rates decrease with increasing carbon number, while the olefin formation rate
increases with carbon number up to 4, followed by a decrease for higher carbon numbers. The
ACS Paragon Plus Environment
22

Page 23 of 44
ACS Catalysis
1
2
3
4
5
6
7
8
9
reaction rates follow exactly same trend as the selectivity as a function of carbon number.
SSITKA data could provide a better understanding of the carbon number dependency of TOFs in
terms of site coverage and the rate constant.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Similar to previous modeling of the SSITKA data17,18, the TOF of the formation of olefin,
paraffin and chain growth can be expressed by the site coverage of corresponding intermediates
and their reactivity, namely the inverse of the corresponding residence time (Eqs. 12ꢀ14):
ꢒꢓꢔ = ꣎꣇,ꢄ,ꢂ ꢄ,ꢂ
ꢀ
(Eq. 12)
(Eq. 13)
(Eq. 14)
ꢄ,ꢂ
ꢒꢓꢔ = ꣎꣇,ꢃ,ꢂ ꢃ,ꢂ
ꢀ
ꢃ,ꢂ
ꢒꢓꢔ = ꣎꣇,ꢞ,ꢂ ꢃ,ꢂ
ꢀ
ꢞ,ꢂ
where n represents the intermediate with carbon number of n; TOFp,n, TOFo,n, are the turnover
frequencies for the formation of paraffin, olefin, respectively, TOFg,n is the propagation, and
ks,p,n, ks,o,n, ks,g,n are the rate constants for the formation of paraffin and olefin and propagation,
respectively for the simple kinetic model; θp,n, θo,n are the site coverage of the intermediate
corresponding to the formation of paraffin and olefin, respectively. Table 2 and Figure 5 provide
a detailed mapping of the surface coverage and kinetic rate constants of the steps involved in the
reaction network of the CO hydrogenation on CoꢀRe/CNT.
ACS Paragon Plus Environment
23

ACS Catalysis
Page 24 of 44
1
2
3
4
5
6
Table 2. Compilation of the site coverage, turnover frequency and rate constants for the different
compound formation on the catalytic surface of CoꢀRe/CNT at 483 K, PT = 1.85 bar, PCO = 0.11
bar and PH2 = 0.56 bar.
7
8
9
Compound TOF (10ꢀ3 sꢀ1)[a] θ[b][e]
ks,n (sꢀ1)[d]
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
CO
0.6423
0.0811 0.1630
CH4
13.2195
5.0849
1.7029
0.0939
4.9321
1.5444
0.9712
3.2220
0.8196
1.2403
1.4527
0.6206
0.5390
0.3517
0.3517
[c]
C1ꢀC2
C2H6
C2H4
C2ꢀC3
C3H8
C3H6
C3ꢀC4
0.0627
0.0365 0.0467
0.0021 0.0449
2.3587
0.0340 0.0454
0.0216 0.0449
0.1490
C4H10
C4H8
0.0183 0.0448
0.0276 0.0449
0.0526
C4ꢀC5
C5H12
C5H10
C5ꢀC6
C6H14
0.0142 0.0437
0.0127 0.0425
0.0278
0.0078
[a] TOF of chain growth, paraffin and olefin formation reactions. [b] Site coverage of the main intermediates
adsorbed on the catalytic surface, which were determined by transient response curves of labeled species as shown
in Figure 3. [c] CnꢀCn+1 represents the chain growth reaction of Cn, [d] ks,n is the rate constant lumped with the site
coverage which cannot be measured directly, for chain growth ks,n=kg,nθC*, paraffin formation (ks,n =kp,n(θH)m1) and
olefin formation (ks,n =ko,n(θH)m2) reactions, respectively. ks,n were estimated from measures TOFs and site
coverages based on Eq. 12ꢀ14; [e] The summary of the site coverages in Table 2 is not 1, and the rest corresponds
the site coverage of H, OH, site vacancy and others.
ACS Paragon Plus Environment
24

Page 25 of 44
ACS Catalysis
1
2
3
4
5
6
7
8
9
Figure 5 provides a detailed mapping of the surface coverage of intermediates and kinetic rate
constants of the steps involved in the reaction network of the CO hydrogenation on Co catalysts,
at the conditions of 483 K, PT = 1.85 bar, PCO = 0.11 bar and PH2 = 0.56 bar, as an example.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
paraffin
chain growth
olefin
Figure 5. Kinetic data from multicomponent SSITKA and its dependence on carbon number. a)
Changes in chain growth rate (●), the formation rate of olefin (■) and paraffin (▲). b) Evolution
of site coverage of intermediates leading directly to the formation of olefin (■) and paraffin (▲).
c) Rate constants obtained by Eq. 12ꢀ14 for the chain growth (●), olefin (■) and paraffin (▲)
formation in FT synthesis at 483 K, PT = 1.85 bar, PCO = 0.11 bar and PH2 = 0.56 bar. Dashed
line corresponds to estimated value.
The changes in site coverage of the intermediates leading to paraffin and olefin formation are
presented in Figure 5b as a function of the carbon number. As reported previously, the site
coverage of intermediates leading to methane is much higher than for intermediates leading to
heavier paraffins19. The site coverage of the intermediates is reported only up to carbon number
5, since there is too large uncertainty in the measurement of the site coverage of the heavier
olefins due to the low concentrations obtained at the conditions tested. The site coverage of the
intermediate for ethylene formation is very low, possibly due to the strong adsorption of ethylene
and therefore leading to a fast chain growth reaction. It explains a very low observed formation
ACS Paragon Plus Environment
25

ACS Catalysis
Page 26 of 44
1
2
3
4
5
6
7
8
9
rate of ethylene. A volcano curve of the intermediates leading to olefin formation is observed
with the carbon number and the largest site coverage is found for carbon number 4.
Figure 5c shows the evolution of the rate constants for chain growth, paraffin and olefin
formation with the carbon number. The rate constant for the olefin formation is constant
regardless of the carbon number, indicating that the carbon number dependency of the olefin
formation rate (Figure 5a) is mainly governed by the carbon number dependency of the site
coverage of the intermediates, more precisely the adsorption strength of the olefin. It is
interesting to note that the rate constants for paraffin formation are very similar to the ones for
olefin formation and constant for C2ꢀC6, except for methane, which is higher than the others.
Regarding the chain growth, there is a significant decrease in the rate constant with increasing
carbon numbers.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
It should be noted that the above simple kinetic model deconvolutes the TOF into the site
coverage and the reactivity (or rate constant) of the intermediates, which provides a useful tool to
study the effect of catalyst properties and operation conditions on the catalytic performance.
However, this type of simple kinetic model alone cannot provide any information on the nature
of the intermediates, which is a key factor for a better understanding of the mechanism of the
formation of paraffin and olefin of each carbon number as well as the chain growth.
Furthermore, the rate constants in the simple model are lumped parameters, and the exact
physical meaning depends on the detailed reaction mechanism, which will be discussed in the
next section.
3. Carbon number dependence of reaction mechanism and kinetics
3.1 Olefin and paraffin formation
ACS Paragon Plus Environment
26

Page 27 of 44
ACS Catalysis
1
2
3
4
5
6
7
8
9
In order to better understand the carbon number dependency of the kinetics, knowledge of the
reaction mechanism and identification of the nature of the intermediates involved in the
elementary reactions for each carbon number are essential. Here we will employ a kinetic study
at the elementary step level by varying the CO and H2 pressure and combining a more detailed
kinetic modeling, as a tool to elucidate the possible nature of the intermediates in Scheme 1.
Based on the reaction network proposed in Scheme 1 and analysis of all the data, intermediates
leading to olefin formation was selected as Cn* in the kinetic analysis. The elementary reaction
steps from the intermediate Cn* to the corresponding olefin and paraffin involve several
hydrogenation or dehydrogenation reaction steps. The exact number of elementary steps or how
many H atoms are involved in the reaction steps depend on the detailed mechanism. Based on
the rate determining step of CnH2n+1*+H* suggested by DFT studies19,46 and the assumption of
reaction orders of m1 and m2 H involved in the elementary reaction steps towards the formation
of paraffin and olefin with the carbon number of n, the rate expressions can be described as in
Eqs. 15ꢀ16, respectively:
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
ꢕ
ꢖ꣈ꢝ
ꢒꢓꢔ = ꣎ꢄ,ꢂꢀꢁꢂ ꢀ꣏
(Eq. 15)
(Eq. 16)
ꢄ,ꢂ
ꢕ
ꢖ꣈ꢣ
ꢒꢓꢔ = ꣎ꢃ,ꢂꢀꢁꢂ ꢀ꣏
ꢃ,ꢂ
where kp,n, ko,n are the rate constant for the formation of paraffin and olefin, respectively; θCn and
θH, are the site coverage of the intermediate and hydrogen, respectively, measured by SSITKA;
and m1 and m2 are the reaction order with respect to adsorbed atomic hydrogen for the paraffin
and olefin formation from the intermediates, respectively. Unfortunately, the site coverage of
hydrogen is not directly measureable by D2/H2 switches in SSITKA, due to exchange reactions
with OH groups on the support as well as in gas phase. By assuming an equilibrium of hydrogen
ACS Paragon Plus Environment
27

ACS Catalysis
Page 28 of 44
1
2
3
4
5
6
adsorption and desorption, the site coverage of hydrogen and vacant sites can be expressed as
shown in Eqs. 17 and 18, respectively.
7
8
9
ꢀ = ꣑ ꣒ ꢀ∗
(Eq. 17)
(Eq. 18)
꣐
꣏
꣏ꢣ ꣏ꢣ
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
ꢌ
∑
ꢝꢍꢾ
꣓ ꣅ
ꢋ
ꢋ꣔꣕
ꢀ∗ =
ꢝꢡ ꣖
꣙
꣐
꣗꣘ ꣗꣘
where KH2 is the H adsorption equilibrium constant, estimated to be 1.68 at 483 K in our
previous studies combining transient and steadyꢀstate modeling19, PH2 is the hydrogen partial
pressure and θi is the site coverage of all the compounds adsorbed on the catalytic surface except
hydrogen, this includes CO, CH4 and higher hydrocarbons. It was found that site vacancy is
almost constant at the conditions studied in this work. It is partially due to opposite changes in
site coverage of H and CO when CO or hydrogen pressure changes which compensates each
other to a certain degree. Therefore, we could lump the site vacancy (θ*) together with the rate
constant. By rearranging Eqs. 15ꢀ16 and combining with Eqs. 17ꢀ18, Eqs. 19 and 20 are
obtained in order to evaluate the reaction order with respect hydrogen:
lnꢕꢒꢓꢔ /ꢀꢁꢂꢖ = ꢰꢤ꣎꣚꣚ ꢢ ꢮꢝ/2ꢰꢤ ꣒꣏ꢣ
(Eq. 19)
(Eq. 20)
ꢕ
ꢖ
ꢖ
ꢄ,ꢂ
ꢄ,ꢂ
lnꢕꢒꢓꢔ /ꢀꢁꢂꢖ = ꢰꢤ꣎꣚꣚ ꢢ ꢮꢣ/2ꢰꢤ ꣒꣏ꢣ
ꢕ
ꢃ,ꢂ
ꢃ,ꢂ
where k’’p,n =kp,nKH2m1/2θ*m1 and k’’o,n =ko,nKH2m2/2θ*m2, where KH2 is the equilibrium constant of
hydrogen adsorption and θ* is the site vacancy. ln(TOFp,n/θCn) and ln(TOFo,n/θCn) were plotted
as a function of ln(PH2), as depicted in Figure 6.
ACS Paragon Plus Environment
28

Page 29 of 44
ACS Catalysis
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Figure 6. Plot of ln(TOFi,n/θCn) vs. ln(PH2) for (a) paraffin formation (i=p) and (b) olefin
formation (i=o) reactions at different carbon numbers: ■: C3, ▲: C4, at: 483 K, PCO = 0.06 bar
and PT = 1.85 bar.
A linear relationship was found both for C3 and C4 and the slope of the line gives the reaction
order with respect to hydrogen of 2.2 for the paraffin formation and 0.7 for the olefin formation
from the C3 intermediate. Similarly, the reaction order with respect to hydrogen is found to be
1.4 for the paraffin formation and 0.3 for the olefin formation for the C4 intermediate. The
carbon number dependency of the hydrogen reaction order suggests therefore a possible carbon
number dependency of the reaction mechanism for the termination reactions of Cn* to paraffin
and olefin. As discussed above, the reaction order with respect to hydrogen provides the number
of H* involved in the kinetic relevant elementary steps19,46 from the intermediates to the
products. The last hydrogenation step of CnH2n+1*+H* has been suggested as the rate
determining step for paraffin formation. Using that as the starting point, the chemical identity of
possible intermediate can be suggested based on the reaction order of hydrogen for the paraffin
and olefin formation.
The most probable chemical identity of the intermediates could be related to alkyl5,37,47
,
alkenyl5,23,47,48 or hydroxycarbene49–51 like species depending on the reaction mechanism. In the
carbide mechanism, alkyl and alkenyl intermediates could be formed by the dissociation of
adsorbed CO followed by hydrogenation of the carbide species to generate alkylene species
(CH2ꢀ), which finally polymerize to generate alkyl like intermediates. Alternatively they can be
ACS Paragon Plus Environment
29