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In heterogeneous catalysis, structure sensitivity describes a dependence of a reaction's surface-normalized rate on the size or surface structure of the catalyst particles. A reaction is termed structure-sensitive when this rate (often expressed as a turnover frequency) changes with particle size, and structure-insensitive when it shows little or no such dependence over the size range examined.[1][2] The effect arises because nanoparticles of different sizes and shapes expose different proportions of terraces, steps, kinks, edges, corners and other atomic ensembles. Some bond-cleavage and bond-formation steps have a strong preference for particular groupings of surface atoms, whose abundance can therefore affect the measured rate.[2] Structural parameters that can affect the abundance of exposed surface sites include the exposed crystal facets and the metal-support interface can affect these .
The classification of a reaction being structure sensitive or not does not imply that catalyst has a single, uniquely identifiable active site, or that a reaction is inherently structure-sensitive under every condition. The experimental classification depends greatly on the structural range investigated, catalyst composition, how the number of accessible sites is measured, activation parameters, reaction conditions, and so forth.[1][3]
Origin of the effect
[edit]Atoms at terraces, steps, kinks, edges and corners differ in coordination and local electronic environment. They can therefore bind reactants and intermediates differently and can stabilize different transition states. Some elementary steps have a particularly strong requirement for a specific arrangement of the atom(s) the reactants bind to; changing the relative abundance of such ensembles changes the surface-normalized rate.[2][4]
For supported nanoparticles, varying particle size commonly changes the fraction of low-coordination atoms and the proportions of exposed facets. Particle-size trends by themselves are not necessarily proof of structure sensitivity because size can simultaneously alter the degree and dominant type of metal-support interaction, oxidation state, strain, transport properties, adsorption coverage, and so forth.[3][5]
Coordination number has also been used to introduce surface geometry into adsorption-energy scaling relations. Calculations across different transition-metal facets have shown that the slopes of such relations are governed primarily by adsorbate valence, whereas their offsets can vary with the coordination number of the adsorption site. When a scaling relation has a slope close to unity, the coordination-number dependence of the offset can vanish, producing an energetic relationship that is approximately independent of surface structure.[6]
Classes of structure sensitivity
[edit]One proposed interpretation distinguishes two broad particle-size trends according to the type of bond involved in the kinetically important step. In σ-type structure sensitivity, the surface-normalized activity commonly increases as particle size decreases; the greater abundance of low-coordination sites on smaller particles can favour the cleavage of σ bonds. In π-type structure sensitivity, activity would instead decrease with decreasing particle size because activation of a π bond can favour a more highly coordinated or otherwise suitably sized surface ensemble.[7] These categories describe empirical trends, and are not necessarily universal trends because a multistep reaction can contain elementary steps with opposing structural requirements, while adsorbate coverage, support interactions and finite-size effects can alter or obscure the resulting particle-size dependence.[7][6]
Structure-insensitive reactions
[edit]A reaction is conventionally termed structure-insensitive when its site-normalized rate varies little with the particle sizes under examination.[1] This observation does not necessarily demonstrate that every exposed atom has identical reactivity, but can instead result from similar rates on the predominant sites, adsorbate-induced blocking of the sites that would otherwise differ, or dynamic restructuring that changes the surface under reaction conditions.[3][8]
Alkene hydrogenation has historically been used as a representative structure-insensitive reaction, whereas reactions involving bond cleavage or formation over particular multi-atom ensembles often show stronger structure dependence.[4] At the sub-nanometre scale, even this classification can change because clusters have discrete geometries and size-dependent electronic structures.[3]
The apparent absence of a particle-size dependence is counterintuitive because particles of different sizes expose different distributions of surface sites. Proposed explanations include carbonaceous or other adsorbate overlayers that mask structural differences, dominance by a particular type of surface site, and adsorbate-induced restructuring that makes initially different surfaces more similar under reaction conditions.[4][8] Much of the early evidence for these mechanisms came from studies of well-defined single-crystal surfaces under vacuum or low-pressure conditions. Applying those explanations to practical supported catalysts requires caution because supported nanoparticles are structurally heterogeneous and can restructure dynamically during a reaction.[3][9]
Measurement and interpretation
[edit]Structure sensitivity has been studied using single crystal surfaces, shape- and size-controlled nanoparticles, and supported catalysts characterized before, during and after reaction. The comparison should use rates normalized to a defensible count of accessible sites rather than total catalyst mass or geometric area alone.[1] Because the active surface can evolve under reaction conditions, operando or time-resolved measurements are increasingly used to relate rate changes to surface reconstruction, adsorbate coverage and changes at the support interface.[10]
Care is required in assigning a cause to an observed size or shape effect. A reliable interpretation separates geometric effects from changes in composition, support interaction, oxidation state, surface coverage and catalyst dynamics; otherwise, a measured particle-size dependence can be incorrectly attributed to a change in active-site geometry.[2][3]
Examples and current interpretation
[edit]Carbon-dioxide hydrogenation over supported nickel nanoparticles has been reported to show particle-size-dependent rates and reaction pathways, consistent with structure sensitivity.[11] In contrast, experiments on supported nickel have shown that surface restructuring under ethene hydrogenation can make a reaction that is intrinsically structure-sensitive appear insensitive to particle size under particular conditions.[9] Accordingly, contemporary accounts treat structure sensitivity as a useful but conditional structure-activity relationship, rather than a permanent label attached to a reaction independent of catalyst state and reaction environment.[3][12]
See also
[edit]References
[edit]- 1 2 3 4 Boudart, Michel (1995). "Turnover Rates in Heterogeneous Catalysis". Chemical Reviews. 95 (3): 661–666. doi:10.1021/cr00035a008.
- 1 2 3 4 Musselwhite, Nathan (2013). "Investigations of Structure Sensitivity in Heterogeneous Catalysis: From Single Crystals to Monodisperse Nanoparticles". Catalysis Letters. 56 (15–17): 1277–1283. doi:10.1007/s11244-013-0150-y.
- 1 2 3 4 5 6 7 Crampton, Andrew S.; Rötzer, Marian D.; Ridge, Claron J.; Yoon, Bokwon; Schweinberger, Florian F.; Landman, Uzi; Heiz, Ueli (2016). "Assessing the concept of structure sensitivity or insensitivity for sub-nanometer catalyst materials". Surface Science. 652: 7–19. Bibcode:2016SurSc.652....7C. doi:10.1016/j.susc.2016.02.006.
- 1 2 3 Che, Michel; Bennett, Charles O. (1991). "The structure sensitivity and insensitivity of catalytic reactions in light of the adsorbate induced dynamic restructuring of surfaces". Catalysis Letters. 7 (1–4): 169–182. doi:10.1007/BF00764500.
- ↑ Koper, Marc T. M. (2011). "Structure sensitivity and nanoscale effects in electrocatalysis". Nanoscale. 3 (5): 2054–2073. Bibcode:2011Nanos...3.2054K. doi:10.1039/C0NR00857E. PMID 21399781.
- 1 2 Calle-Vallejo, Federico; Loffreda, David; Koper, Marc T. M.; Sautet, Philippe (2015). "Introducing structural sensitivity into adsorption-energy scaling relations by means of coordination numbers". Nature Chemistry. 7 (5): 403–410. Bibcode:2015NatCh...7..403C. doi:10.1038/nchem.2226. PMID 25901818.
- 1 2 Vogt, Charlotte (2023). "Adsorbate Bond Number Dependency for σ- and π-Bonds in Linear Scaling Relationships". The Journal of Physical Chemistry C. 127 (11): 5416–5424. doi:10.1021/acs.jpcc.3c00727.
- 1 2 Crampton, Andrew S.; Rötzer, Marian D.; Ridge, Claron J.; Schweinberger, Florian F.; Heiz, Ueli; Yoon, Bokwon; Landman, Uzi (2016). "Structure sensitivity in the nonscalable regime explored via catalysed ethylene hydrogenation on supported platinum nanoclusters". Nature Communications. 7 10389. Bibcode:2016NatCo...710389C. doi:10.1038/ncomms10389. PMC 4738346. PMID 26817713.
- 1 2 Vogt, Charlotte; Meirer, Florian; Monai, Matteo; Groeneveld, Esther; Ferri, Davide; van Santen, Rutger A.; Nachtegaal, Maarten; Unocic, Raymond R.; Frenkel, Anatoly I.; Weckhuysen, Bert M. (2021). "Dynamic restructuring of supported metal nanoparticles and its implications for structure insensitive catalysis". Nature Communications. 12 (1) 7096. Bibcode:2021NatCo..12.7096V. doi:10.1038/s41467-021-27474-3. PMC 8651646. PMID 34876582.
- ↑ Leybo, Denis; Etim, Ubong J.; Monai, Matteo; Bare, Simon R.; Zhong, Ziyi; Vogt, Charlotte (2024). "Metal-support interactions in metal oxide-supported atomic, cluster, and nanoparticle catalysis". Chemical Society Reviews. 53 (21): 10450–10490. doi:10.1039/D4CS00527A. PMC 11445804. PMID 39356078.
- ↑ Vogt, Charlotte; Monai, Matteo; Sterk, Ellen B.; Palle, Jonas; Zijlstra, Bart; Groeneveld, Esther; Berben, Peter H.; Boereboom, Jelle; Hensen, Emiel J. M.; Meirer, Florian; Filot, Ivo A. W.; Weckhuysen, Bert M. (2018). "Unravelling structure sensitivity in CO2 hydrogenation over nickel". Nature Catalysis. 1 (2): 127–134. doi:10.1038/s41929-017-0016-y.
- ↑ Vogt, Charlotte; Weckhuysen, Bert M. (2022). "The concept of active site in heterogeneous catalysis". Nature Reviews Chemistry. 6 (2): 89–111. doi:10.1038/s41570-021-00340-y. PMID 37117296.