| 1. |
- Grönbeck, Henrik, 1966, et al.
(författare)
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Oxygen Adsorption on Graphene-Encapsulated Palladium Nanoparticles Imaged by Kelvin Probe Force Microscopy
- 2019
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Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 123:40, s. 24615-24625
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Tidskriftsartikel (refereegranskat)abstract
- Graphene-encapsulated metal nanoparticles (G@NPs) offer a possibility to observe confined reactions in the nanocontainer formed by the NP's facets and graphene. However, direct experimental detection of adsorbed atomic and molecular species under the graphene cover is still challenging, and the mechanisms of intercalation and adsorption are not well understood. Here, we show that Kelvin probe force microscopy can largely contribute to the understanding of adsorption and desorption at the single NP level, which we exemplify by comparing oxygen adsorption experiments obtained at as-prepared PdNPs and G@PdNPs, both supported on highly oriented pyrolytic graphite and studied under ultrahigh vacuum (UHV) conditions. We show that oxygen adsorption at room temperature occurs at a much higher partial oxygen pressure on G@PdNPs compared to as-prepared PdNPs. Similarly, the removal of oxygen via a reaction with the residual gas of the UHV is slower on the G@PdNPs compared to as-prepared PdNPs. The differences can be explained by a limited facility for reactant and product molecules to enter and desorb from the nanocontainer via the defects of the graphene. Experimental observations are supported by assisting density functional theory calculations.
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| 2. |
- Grönbeck, Henrik, 1966, et al.
(författare)
-
Revealing Carbon Phenomena at Palladium Nanoparticles by Analyzing the Work Function
- 2019
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Ingår i: Journal of Physical Chemistry C. - : American Chemical Society (ACS). - 1932-7447 .- 1932-7455. ; 123:7, s. 4360-4370
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Tidskriftsartikel (refereegranskat)abstract
- Carbon at metal nanoparticles (NPs) plays a fundamental role in heterogeneous catalysis. However, as experimental detection of small amounts of carbon is difficult, in particular when occupying subsurface sites, reaction mechanisms involving absorbed carbon are highly debated. Here, we show that the work function (WF) of metal NPs can be used as a measure of carbon adsorption and absorption, which we demonstrate by Kelvin probe force microscopy and density functional theory calculations for (111)-faceted palladium NPs (PdNPs) on graphite. Growth of PdNPs between 150 and 480 °C leads to carbon etching of the graphite steps and carbon absorption into the first subsurface layer below the NP's facets. This strongly reduces the WF of Pd(111) by up to -1 eV. During a 1 h long postannealing at 650 °C, more carbon is etched from the graphite steps, leading to a carbon precursor structure adsorbed on the NP's facets, as verified by scanning tunneling microscopy. The carbonaceous structures are replaced by graphene upon further annealing (1 to 2 h), followed by a decrease in the WF by ∼-1.4 eV. Similar phenomena are observed after short-time ethylene decomposition at PdNPs at 650 °C. Apart from subsurface carbon, we suggest that the large WF shifts observed experimentally could be attributed to structural defects on NP's facets.
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