Tuneable electronic control of noble metal catalysis by polymeric supports

Abstract

The electronic structure of catalyst supports strongly influences the activity and selectivity of supported metal nanoparticles. Unlike rigid inorganic supports, organic polymers enable precise tuning of the electronic environment through functional design. Here, this tunability regulates Pt oxidation states, producing distinct catalytic behaviour, particularly in low-temperature liquid-phase reactions.

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Reducing CO₂ emissions associated with petroleum use requires new transformation strategies based on renewable carbon sources.¹ Lignocellulosic biomass is among the most promising alternatives to fossil feedstocks, yet its structural complexity hinders the selective production of defined platform molecules.¹ This challenge has stimulated the development of tuneable catalytic systems combining carbon-rich supports with metal nanoparticles or single-atom sites.²

Catalyst supports are now recognised as active components rather than passive carriers.^3–6 Modern catalyst design therefore relies on supports capable of modulating the geometric and electronic structure of active sites under dynamic reaction conditions. Functionalisation with ligand-like groups such as phosphines, pyridines, imidazoles, triazines and amines is especially important.^7–11 These moieties stabilise metal nanoparticles against aggregation, poisoning and leaching while tuning electron density at the metal centres, thereby controlling catalytic behaviour.

A representative example is the oxidation of biomass-derived phenolics into value-added benzofurans and quinones.^12,13 Supported Pd(II) catalysts selectively produce benzofurans when reduction of the active sites is prevented, although this generally requires strong oxidants and non-green solvents.¹³ Quinone formation is typically achieved over platinum catalysts combined with Lewis acid co-catalysts.¹² Importantly, platinum-group metals are not interchangeable because their electronic structures govern donor–acceptor interactions and ultimately product distribution.

Conjugated polymers such as polypyrrole (PP) and polyaniline (PA) are attractive tuneable supports for platinum-group metals. Their π-conjugated frameworks and nitrogen donor sites promote strong M–N interactions (M = Pd, Pt, Rh, etc.).^14–16 Considerable effort has been devoted to tailoring these interactions through polymerisation conditions, doping and heteroaggregation.^17–19 However, the influence of polymer oxidation state and conductivity on metal–support interactions and thermocatalytic performance remains insufficiently understood. Although Pt–PA composites perform well in electrocatalysis and photocatalysis, their thermocatalytic applications are less explored. It is notable that these structures demonstrated superior catalytic performance in (electrocatalytic) oxidation reactions with high selectivity, for example in methanol oxidation. In addition, the PANI carriers have been shown to provide a useful platform to shield the platinum active sites from leaching out of the composite structures.^20–22

Previously, we developed a synthesis strategy involving immobilisation of capped Pt nanoparticles on organic supports followed by plasma treatment to remove the capping agent (CA).²³ This method preserved the small particle size (∼2.1 nm), prevented aggregation and enhanced metal–support interactions without degrading the support. Building on this approach, we developed a two-step wet impregnation–plasma treatment route to prepare Pt–PA composites. We hypothesised that (i) these materials would be active in both cross-coupling and oxidative transformations of biomass-derived phenols, and (ii) modulation of the PA oxidation state would tune metal–support interactions, enabling direct modulation of the catalytic performance as determined by Raman, TEM, XPS and ICP-MS analyses.

Using established synthetic procedures, four PANI forms with different oxidation states were prepared: emeraldine salt (ES), emeraldine base (EB), leucoemeraldine salt (LS) and leucoemeraldine base (LB) (Scheme S1).²⁴ Raman spectra agreed with literature reports (Fig. 1A and Fig. S1A). Emeraldine samples exhibited characteristic bands at 1168, 1218, 1496, 1560 and 1598 cm⁻¹ corresponding to C–H bending, C–N stretching, C=N stretching, N–H bending and C=C stretching vibrations, confirming the partially oxidised emeraldine structure.^24,25

Fig. 1 Raman spectra (A) of the pure leucoemeraldine supports and their platinum modified counterparts. TEM images of (B) LS, (C) LS + Pt and (D) LS + Pt after plasma treatment.

In ES sample, additional bands at 1323 and 890 cm⁻¹ corresponded to C–N⁺ stretching and C–N⁺–C deformation vibrations, confirming positively charged nitrogen centres associated with conductive bipolaron structures. In contrast, leucoemeraldine forms lacked C=N and C=C functionalities, explaining the absence of the 1496 and 1598 cm⁻¹ bands. Instead, broad features centred at 1605 and 1570 cm⁻¹ appeared, originating from C–H-related vibrations. Similar spectral trends were observed for both oxidation states.^24,25

The benzenoid-to-semiquinone ratio was quantified using the oxidation-state parameter (R), calculated from the integrated intensities of the 1168 and 1218 cm⁻¹ bands. The resulting values—0.51 (ES), 0.54 (LS), 0.62 (LB) and 0.64 (EB)—matched literature data and confirmed successful synthesis of the targeted PANI forms.²⁶ These values remained essentially unchanged after Pt immobilisation and plasma treatment (Fig. 1A and Fig. S1A), indicating preservation of the bulk polymer oxidation state.

Spectral changes were nevertheless observed after Pt deposition on salt forms. The C–N⁺ stretching band shifted from 1323–1330 to 1335–1340 cm⁻¹, indicating coordinative interactions between Pt nanoparticles and positively charged nitrogen sites. TEM analysis showed uniform nanoparticle dispersion without aggregation (Fig. 1B–D and Fig. S1B–D). Particle sizes remained between 1.95 and 2.15 nm even after plasma treatment, confirming effective suppression of nanoparticle sintering (Table 1 and Fig. S2). ICP-MS measurements further confirmed Pt loadings close to the intended 2 wt% value for all supports.

Table 1 Physico-chemical characteristics of the PA supported platinum catalysts

PANI	Specific surface area (m^2 g^−1)^a	Pt content (wt%)^b	Average Pt particle size (nm)^c
a Determined by BET method.b Determined by ICP-MS.c Determined by TEM.d Synthesized with the method to create supported sub-nano particles (Pt^US–LS).
Leucoemeraldine salt	24.5	1.94 ± 0.05	1.95 ± 0.26
Leucoemeraldine base	30.3	2.01 ± 0.03	2.05 ± 0.15
Emeraldine salt	26.4	1.96 ± 0.03	2.06 ± 0.30
Emeraldine base	32.5	1.99 ± 0.05	1.96 ± 0.21
Leucoemeraldine salt^d	30.7	1.45 ± 0.03	—

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The catalytic properties were first evaluated in Suzuki–Miyaura coupling of chlorobenzene and phenylboronic acid (Fig. 2E1(A and B)).²³ Catalytic activity strongly depended on the PA support, with biphenyl yields ranging from 15 to 99 mol%. Base-type supports (EB, LB) exhibited low activity, whereas salt-type supports (ES, LS) enabled nearly complete conversion and high biphenyl selectivity without detectable side products. These differences correlate not with the oxidation-state parameter (R), but with the presence of positively charged nitrogen centres that likely strengthen ionic Pt interactions.

Fig. 2 Kinetic curves (A) and substrate scope (B) of Pt–PA composites catalysed Suzuki–Miyaura coupling of phenylboronic acid and chloro-benzene. Substrate scope was completed in the presence of Pt–ES catalyst. Blue numbers are for product yields, green numbers are for selectivities Reaction conditions: 1.0 × 10⁻³ mol of chlorobenzene or its derivatives, 1.2 × 10⁻³ mol of phenylboronic acid, 1.2 × 10⁻³ mol of Bu₄NCl, 1.4 × 10⁻³ mol of Cs₂CO₃, 0.3 mol% catalyst, 2.0 mL of MEK : EtOH = 3 : 1. T = 75 °C, and t = 3 h. Catalytic performance of different Pt-containing catalyst for oxidation of 2-allylphenol (C). Substrate scope reactions for selective oxidation of 2-allylphenol derivatives over a Pt–LS composite (D). Initial reaction conditions: 2.0 × 10⁻³ mol of 2-allylphenol, 6 × 10⁻³ mol of H₂O₂, 3 mL of DMF, T = 75 °C, t = 180 minutes, under a nitrogen atmosphere. Optimized reaction conditions for catalyst with salt-type carriers: 2.0 × 10⁻³ mol of 2-allylphenol, 2.2 × 10⁻³ mol of H₂O₂, 3 mL of DMSO/water (1 : 1), T = 50 °C, t = 120 minutes, under a nitrogen atmosphere (red numbers: 2-methylbenzofuran yields, green numbers: 2-methylbenzofuran selectivities).

Scope studies further supported this interpretation (Fig. 2B and Fig. S3). Electron-donating substituents were converted efficiently over ES- and LS-supported catalysts, whereas electron-withdrawing groups significantly reduced yields. This behaviour suggests preferential interaction of more ionic platinum centres with electron-rich substrates. Since surface area, particle size and metal loading were similar for all catalysts (Table 1), the observed differences primarily originate from electronic effects.

The Pt–PA systems were further examined in selective oxidation of 2-allylphenol (Fig. 2 E2(C and D)). Under conditions optimised for Pd catalysts,²⁷ Pt catalysts supported on base-type PA and commercial Pt catalysts showed low activity (≤19 mol% conversion) and almost exclusive 2-allyl-p-quinone formation. In contrast, salt-supported catalysts achieved significantly higher conversions (40–52 mol%) and 2-methylbenzofuran selectivities approaching 70 mol%.

Reaction optimisation (Tables S1 and S2) led to increased conversions across all systems (Fig. 2C), but selectivity remained largely unchanged, indicating that product distribution is primarily governed by the nature of the support. Catalysts prepared without plasma treatment or via conventional one-step methods showed no detectable activity (Fig. S4), underscoring the importance of the tailored synthesis approach and the resulting metal–support interactions. The probable changes in the oxidation state cannot alone result in changes in the catalytic behaviour as can be seen for the reactions over Pt(II) and Pt(IV)-containing PA systems (Fig. 2C). As observed in cross-coupling reactions, reduced conversion and yield are exhibited for 2-allylphenol derivatives with electron-withdrawing groups over ES or LS supported platinum catalysts (Fig. 2D and Fig. S5), confirming the predominance of electronic effects over steric factors.

The heterogeneous nature of the catalysts, particularly for salt-supported systems, was confirmed by hot filtration and recycling experiments. As can be seen in Fig. S6A, the Pt–ES and Pt–LS systems are recyclable for up to five cycles presenting similar catalytic markers to those in the first cycle. Thereafter, a rapid decline in the markers takes place. Fortunately, both hot filtration tests (Fig. S6B) and ICP-MS studies (Table S3) confirmed that no leaching of metal sites occurred. Raman spectra of the spent catalysts, however, demonstrate that their utilisation has a remarkable impact on the supported catalysts, leading to changes in the oxidation state and loss of bipolaron structure of the PANI (Fig. S6C and D). As a result, on the rearranged surfaces, the noticed metal to support interaction (MSI) cannot prevail. In contrast, for composites with base-type carriers, leaching of the active sites began immediately in the first recycling run with its consequences becoming notable in the second run and thereafter (Table S3 and Fig. S6A). This phenomenon again suggests a difference in the interactions formed between the salt forms and base forms of PANI and the metal sites. The catalytic performance of composites based on the salt forms of PANI was lower than that of the benchmark materials with palladium active centres, but higher than those with platinum active sites (Table S4). Notably, the exceptional activity of the palladium-containing system is observed only in reactions with aryl halides other than aryl chlorides. Our system can catalyse reactions involving aryl chlorides, as demonstrated. In addition, our catalysts operate at relatively low temperatures in a green solvent mixture.

XPS was used to elucidate the origin of these effects (Fig. 3A). Since Pt particle sizes are similar in all samples, differences in binding energy can be assigned to electronic changes. Pt on base-type supports resembles metallic nanoparticles, especially with CA, whereas salt-type supports show altered binding energies and increased peak separation, indicating a more ionic Pt character. Accordingly, the apparent oxidation state of Pt lies between 0 and +1, closer to +1.^28–30

Fig. 3 Pt 4f energy region of the XP spectra of the PA supported platinum catalysts and platinum nanoparticle with a CA (A). Nitrogen 1s energy region of the XP spectra of ES supported platinum catalyst (B) and EB supported platinum catalyst (C). CO–DRIFTS spectra of emeraldine salt (black line) and base (red line) supported platinum catalysts (D).

This observation suggests that salt forms of PA withdraw electron density from the metal centres, generating partially cationic platinum species capable of participating in redox processes analogous to those of Pd catalysts. In contrast, more metallic platinum centres act primarily as Lewis acids with electron surplus, resulting in reduced catalytic activity for 2-methylbenzofuran synthesis, consistent with literature reports.

Nitrogen core-level spectra further clarify the metal–polymer interaction (Fig. 3B, C and Fig. S7A, B).³¹ Although all samples were fitted with the three nitrogen environments commonly reported for PANI, the expected differences between salt and base forms were less pronounced than anticipated. Protonated systems should show a dominant C=(NH⁺)–C contribution and a reduced C=N–C signal. The similar fitted components suggest that coordinated Pt species partially mask these intrinsic differences by redistributing or shielding the positive charge on nitrogen centres. Consequently, the nitrogen environments become electronically more similar, confirming that Pt nanoparticles modify the local electronic structure of the polymer.

Further evidence for this interaction was obtained by CO–DRIFTS, which is highly sensitive to the electronic state of metal centres through the balance of σ-donation and π-backdonation (Fig. 3D).²⁸ Emeraldine-base-supported Pt showed a single band at ca. 2070 cm⁻¹, characteristic of linearly adsorbed CO on metallic Pt⁰. In contrast, salt-supported catalysts exhibited a clear splitting of this band, indicating Pt sites in different electronic environments. This behaviour reflects the coexistence of metallic and partially oxidised surface Pt species, consistent with electron-density modulation by the protonated PANI support.

The small separation of the DRIFTS doublet indicates that the oxidised Pt species do not reach a fully Pt²⁺ state, but instead possess apparent oxidation states below +2 due to partial electron transfer from the metal to the polymer. This agrees with the XPS results and, together with the nitrogen spectra, confirms strong metal–support interactions involving Pt–N coordination and mutual electronic modification of the metal and PANI support. To further support this conclusion, Pt–LS containing ultrasmall platinum sites (Pt^US–LS) was prepared following a reported procedure.¹⁶ ICP-MS and Raman analysis confirmed Pt anchoring and metal–support interactions similar to those in Pt–LS (Table 1, Fig. S8A). Catalytic comparison showed that the beneficial effect of LS was even more pronounced for smaller Pt sites (Fig. S8B), in agreement with literature expectations.³² However, these experiments serve only as proof of concept, and further characterisation and catalytic studies are required to fully assess Pt^US–LS.

In conclusion, polyaniline represents a highly effective and tuneable support for controlling the catalytic performance of platinum nanoparticles. The presented two-step impregnation–plasma treatment method enables precise control over metal–support interactions, allowing activation of platinum in reactions where it is typically less active, such as selective oxidation of allyl phenols to 2-methylbenzofurans. Importantly, the key governing factor is not the formal oxidation state of the polymer, but the presence of partially or fully polarised nitrogen centres, characteristic of the salt forms. These centres modulate the electronic structure of the supported metal, thereby determining catalytic activity and selectivity.

Author contributions

Á. Sz.: writing – original draft, visualization, investigation, formal analysis, data curation. A. E.: writing – original draft, visualization, investigation, formal analysis, data curation. V. Cs.: investigation, formal analysis, data curation. L. B.: investigation, formal analysis, data curation. A. S.: writing – review & editing, visualization, methodology, formal analysis, data curation, conceptualization. Á. K.: resources, funding acquisition. Z. K.: resources, funding acquisition. G. V.: writing – review & editing, writing – original draft, visualization, supervision, project administration, methodology, investigation, funding acquisition, formal analysis, data curation, conceptualization.

Conflicts of interest

There are no conflicts to declare.

Data availability

We stated that the data supporting the findings of this study are available within the article and its supplementary information (SI). Supplementary information: experimental part and supporting data. See DOI: https://doi.org/10.1039/d6cc02813f.

Acknowledgements

GV is grateful for STARTING 150872 project, AS gratefully acknowledges the support of FK 143583 and ZK is grateful for K_21 138714 project all from the source of the National Research, Development and Innovation Fund (NRDIF). The authors are grateful for the financial support of the 2019-2.1.11-TÉT-2020-00233, 2021-1.2.6-TÉT-IPARI-MA-2022-00009 from the source of NRDIF.

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Footnote

† These authors contributed equally.

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