The catalyst carrier can also affect the catalytic selectivity?

chemical reactionto a large extentCatalyst-dependentAt present, the commonly used heterogeneous catalysts are based onInexpensive solid metal oxide supports supporting transition metal nanoparticles. The high surface to volume ratio of the support increases the utilization of the metal, such as a monatomic catalyst.Reducing the size of the noble metal affects the reactivity of the catalyst,Carrier and metal-The chemical nature of the support interactions (MSIs) strongly regulates the activity of the catalyst.CeO₂It is a key component of automotive catalytic converters, especially with the strength of precious metals.MSIsIt is known for keeping them stable in highly dispersed form. To limit precious metalsThe agglomeration of the active phase in SACs, usually using low metal loading, high surface area support, and nanostructures can significantly change the CeO₂The chemical nature.but currentlyCeO₂The effect of particle size on the catalytic performance of atomically dispersed noble metals has not been investigated.

Solution ideas

Based on this,Eindhoven University of Technology in the NetherlandsEmiel J. M. Hensen团队Useflame spray pyrolysis(FSP) Pd-CeO with variable carrier size (4~18 nm) were prepared in one step₂SACs. UseCO oxidation was used as a probe reaction to demonstrate the preparation of nanocompositesThe reaction activity is strongly dependent onCeO₂The size of the particles. By combining advanced in-situ spectroscopy tools and steady-state kinetic studies,Pd-CeO₂The size-dependent redox characteristics of the interface have a strong influence on the catalytic performance.The relevant results are based on 《Size of cerium dioxide support nanocrystals dictates reactivity of highly dispersed palladium catalysts》为题发表在ScienceOn.

different sizesCeO₂Supported palladium catalyst

Using a solution containing two metal precursors, in one step1wt % Pd/CeO prepared by FSP process₂Catalyst(PdFSP)。HAADF-STEM图像(Figure1A) shows the synthesis of Pd/CeO₂is octahedronPdFSP catalyst:(I) small ~ 4 nm NPs, (ii) medium ~ 8 nm NPs, (iii) large ~ 13 nm NPs.XRD only shows CeO₂reflection, indicating that there is noPd/PdO相(Figure1B)。H₂-TPR determinationPd-CeO in PdFSP nanocomposites₂The interactionCeO₂Strong influence of size(Figure1C). The first reduction peak (α) of small PdFSP NPs at ~ 200°C indicates that CeO₂andThere is a strong interaction between Pd.WithThe increase in the size of PdFSP NPs, the feature shifts to low temperature, and finally splits into two peaks, indicating that the MSIs effect is weak.XPS results show that Pd in medium and small PdFSP samples (Figure2A) in highly dispersed Pd² morphological existence, andCeO₂strong interaction. For largerPdFSP NPs (>10 nm), an additional weak component was observed at ~ 336 eV, attributed to PdO_xClusters.EXAFS shows that the Pd-oxo of all samples are mainly isolated, with about 4 O atoms in the first coordination shell and 2 to 3 Ce atoms in the second coordination shell (Figure2B). In the EXAFS of large PdFSP NPs, the weak contribution of Pd-Pd scattering at ~ 2.7 Å and coordination number ~ 1 indicates the presence of small PdOx clusters.In order to determineMorphology of Pd on the surface of NPs, DRIFTS adsorbed at -20°C using CO(Figure2C). In-situ drift spectra of small and medium-sized PdFSP NPs show that at ~ 2140 cm^{− 1}There is a major infrared band, is the oxidation of single atomsLinear adsorption of CO by Pd species. For larger PdFSP NPs, at lower frequencies (2096, 2050 and 1900 cm^{− 1}The additional CO band observed under) may be related to the presence of semi-reduced and aggregated Pd species.Palladium in largeCeO₂NPs surface enrichment, thereby reducing the dispersion of palladium.

Figure 1: Preparation of different sizes of CeO by FSP method₂carrier1wt% Pd-CeO₂Structure and Reduction Properties of Nanocomposites

Figure 2: small, medium and large CeO₂NanoparticlesSpectral characterization of PdFSP samples

CatalysisFormation of palladium during CO oxidation

used for researchDetermination of Pd/CeO Prepared by FSP by CO Oxidation₂activity of nanocomposites.AllPdFSP catalysts can oxidize CO (Figure3A)。low temperatureCO oxidation activity (<100 ℃) and CeO₂The size of the carrier is closely related. at moderate temperature(~ 150°C), the activity of small PdFSP NPs was lower than that of medium and large PdFSP NPs. According to the ignition catalytic test,~8 nm的CeO₂NPs are best suited for low-temperature CO oxidation on PdFSP NPs(Figure3B)。Using in situ drift to reveal low temperatureThe morphology of Pd under CO oxidation conditions (75°C). Figure 3Cdisplay: even inAfter the reaction at 300°C, the Pd single atom is still dominant in the small PdFSP NPs.the dispersion of atoms in a medium-sized samplePd concentration is higher. On large PdFSP NPs, the proportion of metal and cluster palladium is significantly higher than that of atomically dispersed palladium, which is the reason for its limited low-temperature activity.In order to determinePd-oxo the stability at high temperature (300°C), a near-ambient pressure XPS(NAP-XPS) based on a surface-sensitive synchrotron is used. at high temperatureDuring CO oxidation, palladium is stable in the form of highly dispersed Pd-ox-Ce groups (Figure3D). The drift of medium and large PdFSP NPs shows that,Reduced formation at low temperaturesPd clusters, the formation of reduced Pd clusters after heating to 300 ℃ is more obvious.SmallSintering Resistance of Monoatomic Palladium in PdFSP NPs Derived from Strong MSI.WithCeO₂As the size increases, these interactions become weaker, resulting in atomic dispersion under reaction conditions.The stability of Pd decreases. The observed CeO₂The carrier size effect may be derived from the redox properties ando differences in mobility.

Figure 3: Catalyst performance and Pd morphology under reaction conditions

Pd-CeO₂oxygen mobility at the interface

In situ resonance photoelectron spectroscopy(RPES) for the preparation of nanocomposites in Pd-CeO₂The redox characteristics of the interface were studied.SmallValence band photoelectron spectra of PdFSP NPs (Figure4A) shows Ce³ Resonant peak relativeCe⁴ peak and the relative intensity of the non-resonance spectrum is the highestWhat, what?³ with the contentCeO₂The reduction of NPs size decreases, indicating that the reducibility of the surface is related to size.size-inducedChanges in the redox properties of PdFSP NPs strongly alter the kinetics of CO oxidation. For~ 4 NM admin₂The catalyst,The reaction order in CO is particularly high (1.3), while for larger NPs (>7 nm), the reaction order is greatly reduced, and the PdFSP NPs at 13 nm are 0.2 (Figure4B). this shows that,CO Oxidation on Large PdFSP NPs May Follow Mars-van Krevelen Mechanism. in the smallThe unusual sequence of reactions observed in PdFSP NPs suggests that O₂Poisoned with smallCeO₂The Pd site in the catalyst of NPs requires high pressure CO to obtain high catalytic activity. Under CO-rich reaction conditions, small PdFSP NPs showed higher activity than large PdFSP NPs (Figure4C)。

In order to directly determineEvolution of CO coverage on Pd sites, in situ DRIFTS experiments were performed under conditions similar to those used in the reaction sequence studies (Figure 4D). For largerPdFSP NPs, higher CO partial pressure caused only Pd speciation and CO₂small changes in the intensity of the infrared band, reflectingOxidation activity of CO. For medium PdFSP NPs, the addition of more CO increased the activity, but also caused the decrease and aggregation of Pd-oxo species.Through in situ Raman spectroscopy and isotope labeling experiments, it is further proved that it has a small.CeO₂The PdFSP catalyst of NPs has high O mobility. Pd induced O during low-temperature CO oxidation_vformed and promotedO₂of activation.

Figure 4：Pd-CeO₂InterfaceCO Oxidation Kinetics and O Transfer

Summary

CeO₂The size of the carrier can strongly affect the highly dispersedPd-CeO₂Catalyst PairReactivity of CO oxidation.Pd和CeO₂between the strongHigh O Mobility of MSI and Small PdFSP NPs (4nm) Lead to Pd Monoatomic O₂Poisoning, limiting activity. MediumPdFSP NPs (8 nm)中CeO₂The moderate reducibility of NPs and the mobility of O at the metal-support interface lead to a lower reaction sequence of CO and an earlier ignition time of CO oxidation. Due to weak MSI and weak O transfer, single Pd atoms are easily reduced and sintered in large PdFSP NPs (13 nm), which leads to poor low-temperature activity of the catalyst.Metal-CeO₂The concept that the redox properties of the interface are dependent on the carrier size can be extended to otherCe³ /Ce⁴ Kinetically important catalytic reactions, suchCO₂Hydrogenation, water-gas shift reaction and propane dehydrogenation.

References:

https://www.science.org/doi/10.1126/science.adf9082

Valery Muravev et al.Size of cerium dioxide support nanocrystals dictates reactivity of highly dispersed palladium catalysts.Science (2023).

DOI：10.1126/science.adf9082