Unravelling structure sensitivity in CO 2 hydrogenation over nickel

TLDR: In this paper, a unique set of well-defined silica-supported Ni nanoclusters (1-7 nm) and advanced characterization methods were used to prove how structure sensitivity influences the mechanism of catalytic CO2 reduction.

Abstract: Continuous efforts in the field of materials science have allowed us to generate smaller and smaller metal nanoparticles, creating new opportunities to understand catalytic properties that depend on the metal particle size. Structure sensitivity is the phenomenon where not all surface atoms in a supported metal catalyst have the same activity. Understanding structure sensitivity can assist in the rational design of catalysts, allowing control over mechanisms, activity and selectivity, and thus even the viability of a catalytic reaction. Here, using a unique set of well-defined silica-supported Ni nanoclusters (1–7 nm) and advanced characterization methods, we prove how structure sensitivity influences the mechanism of catalytic CO2 reduction, the nature of which has been long debated. These findings bring fundamental new understanding of CO2 hydrogenation over Ni and allow us to control both activity and selectivity, which can be a means for CO2 emission abatement through its valorization as a low- or even negative-cost feedstock on a low-cost transition-metal catalyst.

Figures

Figure 2| a, Methane activity profiles normalized to Ni surface area in m 2 (assuming hemispherical particles) for catalyst samples A-H (see table 1) the dotted grey line denotes the temperature profile, H2/CO2 = 4, 1 bar. b, The influence of Ni particle size on the TOF in grey (400 °C, H2/CO2 = 4, 1 bar). c, The influence of Ni particle size on activity normalized to the Ni loading (400 °C, H2/CO2 = 4, 1 bar).

Figure 3| Combined operando FT-IR and catalyst activity measurements a, b, Consecutive FT-IR spectra of the CO stretching absorption region plotted against time-on-stream for catalyst A and catalyst H, respectively. At 2060 cm-1, C=O stretching vibration are observed, ascribed to linearly/terminally adsorbed CO atop a single Ni atom. At 2030 cm-1 C=O stretching vibrations can be seen ascribed to linearly/terminally adsorbed CO atop a single Ni(II) or Ni(III) atom. The peak at 1918 cm-1 is attributed to bridged or 3-fold carbonyl C=O stretching vibrations, existing only for the larger particle sizes but convoluted with SiO2 framework stretching. The band at 1847 cm -1 shows SiO2 framework stretching vibration peaks, and the one at 1591 cm-1 shows conjugated C-O/C=O stretching vibration from formate species.34–37,47–50. c-f, FT-IR spectra with highlighted peak areas for methane, gaseous CO, adsorbed CO, and formate species. g, Integrated FT-IR peak areas as a function of time-on-stream. h, Consecutive operando FT-IR spectra recorded during post-reaction flushing with N2 of the catalyst with the highest (Catalyst A), and the lowest dispersion (Catalyst H). i, Operando FT-IR spectra recorded during CO2 hydrogenation experiments pulsed with labeled feedstocks. Solid lines denote non-pulsed feedstock, while a dotted line denotes a subsequent pulse of either 13CO2, or D2.

Table 1. Characteristics of the set of well-defined Ni/SiO2 catalysts (A-H), listing their Ni loadings, and Ni particle sizes after reduction, and of spent catalysts as determined by TEM, HAADF-STEM, XRD, and XAS.;

Figure 1| Mechanisms of catalytic CO2 hydrogenation. Schematic overview of the mechanisms behind CO2 hydrogenation as currently proposed in literature, with simplification of certain non-rate determining steps (RDS) following the purple panels. Pathway 1 and 2, preceding green and red boxes, indicate reverse-water-gas-shift (RWGS) mechanisms, in which a darker colored atom in the cluster represents the higher oxidation state of Ni resulting from each step. The depiction is simplified to merely top-adsorption but it is important to ascertain the coordination of Ni sites in each reaction step.

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Lu, Li, Kiely, C h ri s to p h e r J., Be r b e n , Pe t e r H., M ei r er, Flo ri a n a n d
Weck h uys e n , B e r t M. 2 0 1 8. U n r av ellin g s t r u c t u r e s e n sitivity in CO 2
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1
Unraveling Structure Sensitivity in CO
2
Hydrogenation over Ni 1
Charlotte Vogt
†
, Esther Groeneveld
‡
, Gerda Kamsma
‡
, Maarten Nachtegaal
§
,
Li Lu
¥
, Christopher J. 2
Kiely
¥
, Peter H. Berben
‡
, Florian Meirer
†
, Bert M. Weckhuysen
†,
* 3
†Inorganic Chemistry and Catalysis group, Debye Institute for Nanomaterials Science, Utrecht University, Universiteitsweg 4
99, 3584 CG Utrecht, the Netherlands 5
‡ BASF Nederland B.V., Strijkviertel 61, 3454 PK De Meern, the Netherlands 6
§
Paul Scherrer Institute (PSI), 5232 Villigen PSI, Switzerland 7
¥Department of Materials Science and Engineering, Lehigh University, 5 East Packer Avenue, Bethlehem, PA 18015, USA 8
*B.M.Weckhuysen@uu.nl 9
10
11
12
13
14
ABSTRACT (100-150 words): Continuous efforts in the field of materials science have allowed us to generate smaller and 15
smaller metal nanoparticles, creating new opportunities to understand catalytic properties that depend on the metal particle 16
size. Structure sensitivity is the phenomenon where not all surface atoms in a supported metal catalyst have the same activity. 17
Understanding of it can assist in the rational design of catalysts allowing control over mechanisms, activity and selectivity, 18
and thus even the viability of a catalytic reaction. Using a unique set of well-defined silica-supported Ni nanoclusters (1-7 19
nm) and advanced characterization methods, we prove how structure sensitivity influences the mechanism of catalytic CO
2
20
reduction, the nature of which has been long debated. These findings bring fundamental new understanding of CO
2
21
hydrogenation over Ni and allow us to control both activity and selectivity, which can be a means for CO
2
emission 22
abatement through its valorization as a low, or even negative cost feedstock, on a low-cost transition metal catalyst. 23

2
The reduction of CO
2
emissions into the earth’s atmosphere is gaining legislative importance in view of its impact on the 1
climate
1–5
. Reduction of the harmful effect of these emissions through reclamation of CO
2
is made attractive because CO
2
2
can be a zero- or even negative-cost carbon feedstock
6,7
. The conversion of renewably produced hydrogen and CO
2
into 3
methane, or synthetic natural gas (SNG), over Ni is a solution which combines the potential to reduce CO
2
emissions, with a 4
direct answer to the temporal mismatch in renewable electricity production capacity and demand
8–17
. Chemical energy 5
storage in the form of hydrogen production by electrolysis is a relatively mature technology, however the required costly 6
infrastructure, and inefficiencies in distribution and storage deem it inconvenient for large-scale application in the near 7
future. Point source CO
2
hydrogenation to methane yields an alternative with higher energy density. Furthermore, methane is 8
more easily liquefied and can be stored safely in large quantities through infrastructures that already exist
18,19
. 9
The search for fossil fuel alternatives, and application of a process such as that described above can arguably only be 10
achieved with the help of advances in catalysis and the closely related field of nanomaterials. Continuous efforts in both 11
fields have allowed us to make increasingly smaller and catalytically more active (metal) particles. However, it is already 12
known that making infinitesimally smaller supported catalyst particles doesn’t necessarily linearly correspond to higher 13
catalytic activity
20–22
. This phenomenon, where not all atoms in a supported metal catalysts have the same activity, is called 14
structure sensitivity and is often attributed to the distinctly different chemistries on different lattice planes for π-bond 15
activation in CO
2
, or σ-bond activation in H
2
dissociation and C-H propagation
20,23
. The availability of stepped (less 16
coordinated) versus terrace (more coordinated) sites on the surface of supported catalyst nanoparticles obviously changes 17
with particle size, and atomic geometries become particularly interesting below 2 nm where for example π-bond activation is 18
believed to not be able to occur
20
. While particle size effects have extensively been studied for CO hydrogenation over 19
Co
22,24
, understanding of structure sensitivity effects of these critical particle sizes are lacking as sub 2 nm particles prove 20
difficult to synthesize for first row transition metals (Co, Fe and Ni). In this work we used a unique set of SiO
2
-supported Ni 21
nanoparticles with diameters ranging from 1-7 nm in size, and show not only the existence of a distinct particle size effect, 22
but also evidence that allows us tounderstand the structure-sensitivity of CO
2
hydrogenation over Ni as a model structure 23
sensitive reaction. 24
Classically, CO
2
hydrogenation over nickel is considered to follow a 2-step, Langmuir-Hinshelwood type mechanism 25
whereby first CO
2
dissociatively adsorbs with H
2
to form CO
and H
2
O in the reverse water gas shift (RWGS) reaction. The 26
CO is then subsequently directly hydrogenated or dissociates to atomic C
ads
and is then hydrogenated as schematically 27
depicted in Figure 1
6,7
. However, recent experimental and theoretical studies show that this reaction mechanism, particularly 28
on surfaces of non-model catalysts, is not fully understood
25–30
. The reverse water gas shift reaction is believed to follow 29
either of two mechanisms: firstly, a surface carbonate to formate reaction pathway (pathway 1 in Figure 1), and secondly, the 30

3
direct dissociation of CO
2
to CO via a CO
2
-
ion (pathway 2 in Figure 1). Much of the debate in the literature arises from the 1
direct comparison between model and non-model surface studies. We hypothesize that mechanistic understanding of this 2
reaction is closely related to its structures sensitivity. 3
Enhanced understanding of structure sensitivity and mechanistic aspects behind this reaction will not only be a step towards 4
a feasible method for the valorization of CO
2
, with the potential to reduce its impact on the environment, but it will also aid 5
in understanding similar structure sensitive reactions. Evidence for the impact of different atomic coordinations in metal 6
nanoclusters on the activation of different bond-types, however, can have far greater, multidisciplinary impact as it will allow 7
the rational design of catalysts enabling us to control, at the atomic level, the activity and selectivity of catalytic reactions
31–
8
33
. It may even facilitate the discovery of new, previously unattainable catalytic reactions. 9
10

4
1
Figure 1| Mechanisms of catalytic CO
2
hydrogenation. Schematic
overview of the mechanisms behind CO
2
hydrogenation as currently
proposed in literature, with simplification of certain non-rate determining
steps (RDS) following the purple panels. Pathway 1 and 2, preceding
green and red boxes, indicate reverse-water-gas-shift (RWGS)
mechanisms, in which a darker colored atom in the cluster represents the
higher oxidation state of Ni resulting from each step. The depiction is
simplified to merely top-adsorption but it is important to ascertain the
coordination of Ni sites in each reaction step.

Frequently asked questions

Q1. What are the contributions in this paper?

Vogt, Ch a rlo t e, Groe n eveld, Es t h er, Kam s m a, Ge r d a, N a c h t e g a al, M a a r t e n, Lu, Li, Kiely, Ch ris top h e r J., Be r b e n, Pe t e r H., M ei r er, Flo ri a n a n d Weckh uys e N, Be r t M. this paper 

Q2. What was the purpose of the FT-IR measurements?

Operando Fourier transform infrared (FT-IR) 22spectroscopy measurements were performed to study reactants, intermediates and products in CO2 hydrogenation over Ni. 23 Product formation was followed by on-line gas chromatography. 

Q3. what is the chemistry of a bacterial carbon dioxide reductase?

Reversible hydrogen storage using CO2 and a proton-switchable iridium catalyst in aqueous media under mild temperatures and 5 pressures. 

Q4. What was the frequency of the ionization chambers and the angular encoder?

The Si(111) crystal was rotated at a frequency of 10 Hz across the Ni 17K-edge, and the signals of the ionization chambers and the angular encoder were sampled at a frequency of 2 MHz. 

Q5. How many spectral data were recorded for each catalyst?

For each catalyst sample with differing metal dispersion, 10on-line activity data was collected with a time resolution of 30 s. Simultaneously, successive operando FT-IR spectra were 11 also recorded with a time resolution of 30 s (see section ‘FR-IR Studies of Catalyst Samples’ in the Supplementary 12information). 

Q6. How long did the catalysts remain at these temperatures?

Per these results, all reduction steps in this work were ramped at 5 °C min -1 to 600 °C (catalyst A), 550 °C (catalyst 7B-G), and 500 °C (catalyst H), and held at these respective temperatures for 30 min. 

Q7. How many particles were determined by TEM?

Particle size distributions were 18 determined by TEM for fresh, reduced and spent samples (see section ‘Characterization Results’ in the Supporting 19information).