3 research outputs found
Systematic Identification of Promoters for Methane Oxidation Catalysts Using Size- and Composition-Controlled Pd-Based Bimetallic Nanocrystals
Promoters enhance the performance
of catalytic active phases by
increasing rates, stability, and/or selectivity. The process of identifying
promoters is in most cases empirical and relies on testing a broad
range of catalysts prepared with the random deposition of active and
promoter phases, typically with no fine control over their localization.
This issue is particularly relevant in supported bimetallic systems,
where two metals are codeposited onto high-surface area materials.
We here report the use of colloidal bimetallic nanocrystals to produce
catalysts where the active and promoter phases are colocalized to
a fine extent. This strategy enables a systematic approach to study
the promotional effects of several transition metals on palladium
catalysts for methane oxidation. In order to achieve these goals,
we demonstrate a single synthetic protocol to obtain uniform palladium-based
bimetallic nanocrystals (PdM, M = V, Mn, Fe, Co, Ni, Zn, Sn, and potentially
extendable to other metal combinations) with a wide variety of compositions
and sizes based on high-temperature thermal decomposition of readily
available precursors. Once the nanocrystals are supported onto oxide
materials, thermal treatments in air cause segregation of the base
metal oxide phase in close proximity to the Pd phase. We demonstrate
that some metals (Fe, Co, and Sn) inhibit the sintering of the active
Pd metal phase, while others (Ni and Zn) increase its intrinsic activity
compared to a monometallic Pd catalyst. This procedure can be generalized
to systematically investigate the promotional effects of metal and
metal oxide phases for a variety of active metal-promoter combinations
and catalytic reactions
Tuning Precursor Reactivity toward Nanometer-Size Control in Palladium Nanoparticles Studied by in Situ Small Angle X‑ray Scattering
Synthesis of monodisperse
nanoparticles (NPs) with precisely controlled
size is critical for understanding their size-dependent properties.
Although significant synthetic developments have been achieved, it
is still challenging to synthesize well-defined NPs in a predictive
way due to a lack of in-depth mechanistic understanding of reaction
kinetics. Here we use synchrotron-based small-angle X-ray scattering
(SAXS) to monitor in situ the formation of palladium (Pd) NPs through
thermal decomposition of Pd–TOP (TOP: trioctylphosphine) complex
via the “heat-up” method. We systematically study the
effects of different ligands, including oleylamine, TOP, and oleic
acid, on the formation kinetics of Pd NPs. Through quantitative analysis
of the real-time SAXS data, we are able to obtain a detailed picture
of the size, size distribution, and concentration of Pd NPs during
the syntheses, and these results show that different ligands strongly
affect the precursor reactivity. We find that oleylamine does not
change the reactivity of the Pd–TOP complex but promote the
formation of nuclei due to strong ligand–NP binding. On the
other hand, TOP and oleic acid substantially change the precursor
reactivity over more than an order of magnitude, which controls the
nucleation kinetics and determines the final particle size. A theoretical
model is used to demonstrate that the nucleation and growth kinetics
are dependent on both precursor reactivity and ligand–NP binding
affinity, thus providing a framework to explain the synthesis process
and the effect of the reaction conditions. Quantitative understanding
of the impacts of different ligands enables the successful synthesis
of a series of monodisperse Pd NPs in the broad size range from 3
to 11 nm with nanometer-size control, which serve as a model system
to study their size-dependent catalytic properties. The in situ SAXS
probing can be readily extended to other functional NPs to greatly
advance their synthetic design
Systematic Structure–Property Relationship Studies in Palladium-Catalyzed Methane Complete Combustion
To
limit further rising levels in methane emissions from stationary
and mobile sources and to enable promising technologies based on methane,
the development of efficient combustion catalysts that completely
oxidize CH<sub>4</sub> to CO<sub>2</sub> and H<sub>2</sub>O at low
temperatures in the presence of high steam concentrations is required.
Palladium is widely considered as one of the most promising materials
for this reaction, and a better understanding of the factors affecting
its activity and stability is crucial to design even more improved
catalysts that efficiently utilize this precious metal. Here we report
a study of the effect of three important variables (particle size,
support, and reaction conditions including water) on the activity
of supported Pd catalysts. We use uniform palladium nanocrystals as
catalyst precursors to prepare a library of well-defined catalysts
to systematically describe structure–property relationships
with help from theory and in situ X-ray absorption spectroscopy. With
this approach, we confirm that PdO is the most active phase and that
small differences in reaction rates as a function of size are likely
due to variations in the surface crystal structure. We further demonstrate
that the support exerts a limited influence on the PdO activity, with
inert (SiO<sub>2</sub>), acidic (Al<sub>2</sub>O<sub>3</sub>), and
redox-active (Ce<sub>0.8</sub>Zr<sub>0.2</sub>O<sub>2</sub>) supports
providing similar rates, while basic (MgO) supports show remarkably
lower activity. Finally, we show that the introduction of steam leads
to a considerable decrease in rates that is due to coverage effects,
rather than structural and/or phase changes. Altogether, the data
suggest that to further increase the activity and stability of Pd-based
catalysts for methane combustion, increasing the surface area of supported
PdO phases while avoiding strong adsorption of water on the catalytic
surfaces is required. This study clarifies contrasting reports in
the literature about the active phase and stability of Pd-based materials
for methane combustion