Pt single sites are highly attractive due to their high atom economy and can be generated on CeO2 by an oxidative high-temperature treatment. However, their location and activity are strongly debated. Furthermore, reaction-driven structural dynamics have not been addressed so far. Here, we were able to evidence Ptinduced CeO2 surface restructuring, locate Pt single sites on CeO2 and track the variation of the active state under reaction conditions using a complementary approach of density functional theory calculations, in situ infrared spectroscopy, operando high-energy-resolution fluorescence detected X-ray absorption spectroscopy and catalytic CO (as well as C3H6 and CH4) oxidation. We find that the onset of CO oxidation is linked to the migration of Pt single sites from four-fold hollow sites to form small clusters containing few Pt atoms. This demonstrates that operando studies on single sites are essential to assess their fate and the resulting catalytic properties. a promise as they lower the noble metal content significantly as all atoms are potentially active species. [5][6][7][8][9] Exploiting the strong noble metal support interaction between Pt and CeO2, metallic Pt particles can be formed orin contrast to weakly interacting supports like Al2O3redispersed, with tremendous impact on the catalyst activity. [10][11][12][13] The preparation of SAC has been demonstrated for Pt, which can be atomically dispersed when using CeO2 as a support through an oxidizing treatment at 800 °C. 14 However, the exact structure of the single sites, their reactivity and, particularly, their state and dynamics during reaction are still unknown and heavily debated. 4,15,16 The location of Pt single sites is claimed to range from surface adsorbates on {111} ceria steps, 17,18 {111}, 19 {110} 20,21 or {100} 6,22,23 ceria facets to surface 21,24,25 or bulk Ce substitutes [26][27][28] forming Ce1-XPt 2+ XO2-Y-composites. During change of the gas atmosphere and of the temperature, the structure of the single sites may strongly change resulting in a new and more active state. For example, after a high temperature treatment strong Pt-O-Ce bonds are reported to over-stabilize the single sites which are thus less active. 29 During the catalytic oxidation, oxygen is suggested to be provided by the support, while the reactant e.g. CO is adsorbed directly on Pt, 22,25 similar to Pt nanoparticles on CeO2. 30 Bera et al. correlated the intensity of the Pt-O-Ce bond observed by Extended X-ray Absorption Fine Structure (EXAFS) measurements with the catalytic activity for Ce1-XPt 2+ XO2-Y, 31 and Nie et al. 24demonstrated that the catalytic activation of a Pt single atom catalyst can be increased by steam treatment. It is suggested that this treatment leads to the formation of Ce1-XPt 2+ XO2-YH-OH species that are catalytically more active than Ce1-XPt 2+ XO2-Y. 24 In contrast, other studies report an increase in catalytic activity after a reductive treatment at temperatures below 300 °C. 9,32-34 Importantly, such
Extending the toolbox from mono-to bimetallic catalysts is key in realizing efficient chemical processes 1 . Traditionally, the performance of bimetallic catalysts featuring one active and one selective metal is optimized by varying the metal composition 1-3 , often resulting in a compromise between the catalytic properties of the two metals 4-6 . Here we show that by designing the atomic distribution of bimetallic Au-Pd nanocatalysts, we obtain a synergistic catalytic performance in the industrially relevant selective hydrogenation of butadiene. Our single-crystalline Au-core Pd-shell nanorods were up to 50 times more active than their alloyed and monometallic counterparts, while retaining high selectivity. We find a shell-thickness-dependent catalytic activity, indicating that not only the nature of the surface but also several subsurface layers play a crucial role in the catalytic performance, and rationalize this finding using density functional theory calculations. Our results open up an alternative avenue for the structural design of bimetallic catalysts.Synergy arises when two catalytically active metals are combined such that the catalytic performance exceeds that of the monometallic counterparts 1 . This makes bimetallics an interesting class of materials for catalysing a variety of chemical processes ranging from selective hydrogenation 1,5,7 to oxidation 2,4,8 and electrochemical reactions [9][10][11] . The main focus has been on alloyed nanoparticles, as they are easily accessible with standard catalyst preparation methods and allow facile tuning of their catalytic properties via the average metal composition. However, the arrangement in which the atoms are assembled is also crucial; gas induced and thermally induced metal redistribution can have a large impact on the catalytic performance 12,13 . Thanks to recent advances in material science, it is now possible to synthesize bimetallic nanoparticles with precisely defined atomic arrangements, such as single-atom alloys 14,15 , intermetallic structures 16 and core-shell materials 17,18 . Yet, only a limited number of studies systematically link the metal distribution to the performance of bimetallic catalysts 12,13 . In particular, the catalytic behaviour of core-shell nanoparticles is largely unexplored, despite successful demonstrations of core-shell catalysts in electrocatalysis, where changes in the electronic properties of the shell atoms induced by the underlying core resulted in enhanced catalytic performances [9][10][11]19 .Here, by employing colloid synthesis 18,20 , we prepared a welldefined Au-Pd model system with a precisely tunable atomic structure allowing a direct correlation between the metal distribution, to six atomic Pd-shell layers. Our study highlights the importance of tuning the atomic distribution in bimetallic catalysts, and lays a foundation for the rational design of bimetallic catalysts with optimal synergistic performances.
In contrast to catalytically active metal single atoms deposited on oxide nanoparticles, the crystalline nature of metal‐organic frameworks (MOFs) allows for a thorough characterization of reaction mechanisms. Using defect‐free HKUST‐1 MOF thin films, we demonstrate that Cu+/Cu2+ dimer defects, created in a controlled fashion by reducing the pristine Cu2+/Cu2+ pairs of the intact framework, account for the high catalytic activity in low‐temperature CO oxidation. Combining advanced IR spectroscopy and density functional theory we propose a new reaction mechanism where the key intermediate is an uncharged O2 species, weakly bound to Cu+/Cu2+. Our results reveal a complex interplay between electronic and steric effects at defect sites in MOFs and provide important guidelines for tailoring and exploiting the catalytic activity of single metal atom sites.
The potential of oxide-supported rhodium single atom catalysts (SACs) for heterogeneous hydroformylation was investigated both theoretically and experimentally. Using high-level DLPNO-CCSD(T) calculations, both stability and catalytic activity were investigated for Rh single atoms on different oxide surfaces. Atomically dispersed, supported Rh-catalysts were synthesized on MgO and CeO2. While the CeO2-supported rhodium catalyst is found to be highly active, this is not the case for MgO, most likely due to increased confinement, as determined by EXAFS, that diminishes the reactivity of Rh complexes on MgO. This agrees well with our computational investigation, where we find that rhodium carbonyl hydride complexes on flat oxide surfaces such as CeO2( 111) have catalytic activities comparable to those of molecular complexes. For a step edge on a MgO(301) surface, however, calculations show a significantly reduced catalytic activity. At the same time, calculations predict that stronger adsorption at the higher coordinated adsorption site leads to a more stable catalyst. Keeping the balance between stability and activity appears to be the main challenge for oxide supported Rh hydroformylation catalysts. In addition to the chemical bonding between rhodium complex and support, the confinement experienced by the active site plays an important role for the catalytic activity.
Understanding and tuning the catalytic properties of metals atomically dispersed on oxides are major stepping-stones towards a rational development of single-atom catalysts (SACs). Beyond individual showcase studies, the design and synthesis of structurally regular series of SACs opens the door to systematic experimental investigations of performance as a function of metal identity. Herein, a series of single-atom catalysts based on various 4d (Ru, Rh, Pd) and 5d (Ir, Pt) transition metals has been synthesized on a common MgO carrier. Complementary experimental (X-ray absorption spectroscopy) and theoretical (Density Functional Theory) studies reveal that, regardless of the metal identity, metal cations occupy preferably octahedral coordination MgO lattice positions under step-edges, hence highly confined by the oxide support. Upon exposure to O2-lean CO oxidation conditions, FTIR spectroscopy indicates the partial de-confinement of the monoatomic metal centers driven by CO at pre-catalysis temperatures, followed by the development of surface carbonate species under steady-state conditions. These findings are supported by DFT calculations, which show the driving force and final structure for the surface metal protrusion to be metal-dependent, but point to an equivalent octahedral-coordinated M 4+ carbonate species as the resting state in all cases. Experimentally, apparent reaction activation energies in the range of 96±19 kJ/mol are determined, with Pt leading to the lowest energy barrier. The results indicate that, for monoatomic sites in SACs, differences in CO oxidation reactivity enforceable via metal selection are of lower magnitude than those evidenced previously through the mechanistic involvement of adjacent redox centers on the oxide carrier, suggesting that tuning of the oxide surface chemistry is as relevant as the selection of the supported metal.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.