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MICROSCOPIE+CATREN:ING+TLN:PAF:FMO:JRO:MAO:LPI; International audience; Stimulated by recent advances in scanning transmission electron microscopy (STEM), single-atom catalysts (SACs) have become prominent materials in heterogeneous catalysis [1]. In spite of the numerous reports on promising catalytic performance and noble-metal saving, single-atom stabilization remains a challenge – in particular for noble metals on oxides [2,3]. In addition, single atoms are not necessarily more efficient than their cluster or nanoparticle counterparts [3,4]. In this communication, the previous issues will be illustrated for two prototypical catalytic systems: CO oxidation on Pt/Al2O3 and photocatalytic hydrogen evolution reaction over Pt/TiO2. The combined use of catalytic testing, STEM, operando X-ray and infrared absorption spectroscopies, and DFT calculations, reveals the influence of the reactive environment on Pt nuclearity and oxidation state (generally clustering and reduction), which in turn correlates to changes in catalytic activity [5–8]. From this knowledge, strategies can be suggested to stabilize the metals in an ultradispersed state. [1]R. Lang, X. Du, Y. Huang, X. Jiang, Q. Zhang, Y. Guo, K. Liu, B. Qiao, A. Wang, T. Zhang, Chem. Rev. 120 (2020) 11986[2]L. Piccolo, Catal. Today 373 (2021) 80[3] L. Liu, D.M. Meira, R. Arenal, P. Concepcion, A.V. Puga, A. Corma, ACS Catal. 9 (2019) 10626[4]A. Beniya, S. Higashi, Nat. Catal. 2 (2019) 590[5]C. Dessal, A. Sangnier, C. Chizallet, C. Dujardin, F. Morfin, J.-L. Rousset, M. Aouine, M. Bugnet, P. Afanasiev, L. Piccolo, Nanoscale 11 (2019) 6897[6]C. Dessal, T. Len, F. Morfin, J.-L. Rousset, M. Aouine, P. Afanasiev, L. Piccolo, ACS Catal. 9 (2019) 5752[7]C. Dessal, L. Martínez, C. Maheu, T. Len, F. Morfin, J.-L. Rousset, E. Puzenat, P. Afanasiev, M. Aouine, L. Soler, J. Llorca, L. Piccolo, J. Catal. 375 (2019) 155[8]L. Piccolo, P. Afanasiev, F. Morfin, T. Len, C. Dessal, J.L. Rousset, M. Aouine, F. Bourgain, A. Aguilar-Tapia, O. Proux, Y. Chen, L. Soler, J. Llorca, ACS Catal. 10 (2020) 12696

SSCI-VIDE+ECI2D:ING+TLN:FMO:JRO:PAF:LPI; National audience; Noble metals are used as catalysts for numerous chemical reactions. However, due to their rarity and high price, their decreased usage or substitution by inexpensive metals is strongly desired. A way to save metals while possibly improving the catalytic performance is to increase metal dispersion up to the single-atom limit. The recent development of aberration-corrected scanning transmission electronic microscopy facilitated the development of so-called single-atom catalysts (SACs) [1]. On the respective performances of SACs and nanoparticulate catalysts, the literature is controversial. In particular, for CO oxidation on Pt/Al2O3, single Pt atoms have been found intrinsically active [2] or, in contrast, inactive [3].. In this work, Pt/γ-Al2O3 catalysts were prepared by incipient wetness impregnation followed by an oxidative and/or a reductive thermal treatment. It was shown that the treatment atmosphere and the Pt loading are decisive for controlling the Pt dispersion [4]. At intermediate Pt loading, operando experiments using diffuse reflectance infrared spectroscopy and X-ray absorption spectroscopy showed that Pt atoms gradually aggregate into subnanometric clusters throughout CO oxidation cycles, making the catalyst more active [5]. Here, from the comparison of the initial CO oxidation activities of catalysts composed of Pt single atoms, clusters (size < 2 nm), or nanoparticles (> 2 nm), it is consistently demonstrated that single atoms are less efficient than clusters and nanoparticles. The kinetics and mechanism of CO adsorption and oxidation on the different types of catalysts will be discussed in this contribution. References[1]S. Mitchell, E. Vorobyeva, J. Pérez‐Ramírez, Angew. Chem. Int. Ed. 57 (2018) 15316–15329.[2]M. Moses-DeBusk, M. Yoon, L.F. Allard, D.R. Mullins, Z. Wu, X. Yang, G. Veith, G.M. Stocks, C.K. Narula, J. Am. Chem. Soc. 135 (2013) 12634–12645.[3]K. Ding, A. Gulec, A.M. Johnson, N.M. Schweitzer, G.D. Stucky, L.D. Marks, P.C. Stair, Science 350 (2015) 189–192.[4]C. Dessal, A. Sangnier, C. Chizallet, C. Dujardin, F. Morfin, J.L. Rousset, M. Aouine, M. Bugnet, P. Afanasiev, L. Piccolo, Nanoscale 11 (2019) 6897-6904.[5]C. Dessal, T. Len, F. Morfin, J.L. Rousset, M. Aouine, P. Afanasiev, L. Piccolo, ACS Catal. 9 (2019) 5752-5759.

International audience

SSCI-VIDE+ECI2D:ING+DEC:FMO:JRO:PAF:LPI; National audience; In this work, the stability of γ-alumina-supported single Pt atoms formed by oxidative treatment of an impregnated Pt precursor has been monitored by operando X-ray absorption spectroscopy (XAS). Their destabilization into subnanometric clusters under reductive treatment has been studied by XAS and environmental scanning transmission electron microscopy (E-STEM). DFT calculations allow us to fully rationalize these behaviors in terms of nuclearity and adsorbate coverage (O or H), which governs the cluster size, shape and interaction with the support.Pt/γ-Al2O3 SACs were also submitted to CO oxidation heating/cooling cycles separated by a reduction treatment and the catalysts were analyzed by operando DRIFTS, operando XAS and STEM.

SSCI-VIDE+ECI2D:ING+TLN:DEC:FMO:JRO:PAF:LPI; National audience; Les métaux nobles sont utilisés pour catalyser de nombreuses réactions chimiques. Cependant, ils sont rares et onéreux, ce qui limite leurs utilisations industrielles. Il est donc nécessaire d’augmenter l’efficacité de ces catalyseurs. Cette réflexion, jointe au développement des microscopes électroniques en transmission (TEM) de dernière génération, ont conduit à un intérêt croissant pour les catalyseurs constitués non plus de nanoparticules mais d’atomes isolés (single-atom catalysts) stabilisés sur un support poreux,généralement un oxyde tel que la cérine, l’oxyde ferrique ou l’alumine.La charge en atomes de métal (ici le Pt) affecte la probabilité de rencontre entre deux atomes et influe donc sur le phénomène d’agrégation. Les traitements thermiques appliqués après l’imprégnation peuvent également accentuer cette agrégation, notamment sous atmosphère réductrice où les atomes isolés de Pt se déstabilisent facilement en formant des clusters (1). De plus, des travaux indiquent que l’ajout d’un métal alcalin comme le sodium permet de favoriser la dispersion du platine et la stabilité des atomes isolés sur l’alumine (2). D’autres additifs, tels que le lanthane, ont également été rapportés dans la littérature (3).Dans cette étude, des catalyseurs Pt/(Na, La-)γ-Al2O3 ont été préparés par une méthode conventionnelle d’imprégnation à humidité naissante suivie d’un traitement thermique oxydant et/ou réducteur. Dans le but de comparer la réactivité des atomes isolés et des clusters (taille = 1 nm) de Pt, les deux types de catalyseurs correspondants ont été testés en oxydation de CO dans un réacteur sous flux à lit fixe. Les matériaux ont été caractérisés par TEM à balayage (STEM) avant et après réaction. Les tests catalytiques montrent une activité et une stabilité supérieures des clusters par rapport aux atomes isolés. De plus, les courbes de production de CO2 en fonction de la température (chauffage vs refroidissement) montrent une forte hystérésis. L’origine de ces phénomènes, ainsi que l’influence du support (Al2O3 vs TiO2 et ZnO), seront discutées lors de cette présentation.(1) Duan, S.; Wang, R.; Liu, J. Nanotechnology 2018.(2) Yang, M.; Liu, J.; Lee, S.; Zugic, B.; Huang, J.; Allard, L. F.; Flytzani-Stephanopoulos, M. Journal of the American ChemicalSociety 2015, 137, 3470–3473.(3) Lakshmanan, P.; Park, J. E.; Kim, B.; Park, E. D. Catalysis Today 2016, 265, 19–26.

MICROSCOPIE+ECI2D:ING+DEC:FMO:MAO:JRO:LPI; International audience; In order to get insight into the interplay between metal ultradispersion and catalytic performance, atomically dispersed Pt/γ-Al2O3 catalysts with 0.1 or 0.5 wt% Pt loading, were prepared by impregnation-based methods and tested for CO oxidation. According to scanning transmission electron microscopy (STEM), only single Pt atoms (SAs) are present on the 0.1 wt% catalyst, while they coexist with a small fraction of subnanometric clusters in the 0.5 wt% one. The Pt SAs are stable during CO oxidation up to 300 °C in oxygen excess, but exhibit relatively low activity. In contrast, after H2 pretreatment, the SAs aggregate into subnanometric clusters, which exhibit high CO oxidation performance. Environmental STEM under 5 mbar of H2 shows that the SA-to-clusters aggregation already occurs at room temperature, and the resulting Pt clusters appear stable in size up to 800 °C in this reducing atmosphere.1. ScopeNoble metals are highly active in many catalytic reactions but they are rare and expensive, limiting their industrial use and requiring a maximization of their efficiency, or their replacement by cheaper metals. An “atom-economical” strategy consists in dispersing small metal amounts, ideally in the form of isolated atoms, on a suitable support. This induces a change in surface electronic structure, which can lead to different/superior performances (activity, selectivity, stability) as compared to conventional catalysts. However, the preparation of so-called “single-atom catalysts” (SACs)1 is challenging because noble metal atoms tend to aggregate in order to minimize their surface energy.2. Results and discussionIn this work, we have prepared Pt/γ-Al2O3 catalysts using conventional impregnation methods. To maximize the metal dispersion, relatively low metal loadings (0.1-0.5 wt%) and an oxidizing thermal treatment were used. Aberration-corrected scanning transmission electron microscopy (STEM) analyses of the as-prepared 0.1 wt% Pt/γ-Al2O3 catalysts exclusively show single Pt atoms (Figure 1a). The increase in the Pt loading (0.5 wt%) leads to the formation of some subnanometric clusters in addition to single atoms (Figure 1b).The 0.1 wt% Pt1/γ-Al2O3 SAC was tested for CO oxidation in a flow-fixed-bed reactor. Three CO oxidation heating/cooling cycles separated by calcination or reduction treatments were performed (Figure 2). The catalyst was analyzed at each step by STEM. As a result, the Pt SAs are retained after the first calcination/reaction step, then partially converted into subnanometric clusters after reduction, and finally replaced by 2 nm-NPs after the following steps (Figure 2a).Considering only the first (heating) reaction runs, the CO oxidation activity follows the order: SAs < NPs < clusters (Figure 2b). Moreover, a strong hysteresis is observed in all cases, the catalyst being always much more active during the cooling stage than during the heating one. The relatively low activity of the SAC is consistent with the results of Moses-DeBusk et al. for Pt/θ-Al2O3.23. ConclusionsMany recent works praise the use of SACs in various catalytic reactions, however the stability and the homogeneity of the metal dispersion during the reactions are not always systematically assessed. In this work, we have combined CO oxidation tests of atomically dispersed Pt/γ-Al2O3 catalysts with STEM analysis performed before, after or during the reaction/adsorption processes. Two kinds of Pt entities were present depending on the synthesis conditions: SAs only (0.1 wt% Pt loading) or a mixture of SAs and subnanometric clusters (0.5 wt%). Despite their low activity, Pt SAs are stable during O2-rich CO oxidation cycles up to 300 °C. In contrast, SAs aggregate into stable clusters upon H2 pretreatment. The latter are much more active than SAs and NPs in CO oxidation. The interplay between the size, the structure, the oxidation state and the catalytic performance of the supported Pt entities, together with the strong hysteresis phenomena, will be discussed in the light of additional microscopic and spectroscopic analyses.References1. J. Liu, ACS. Catal. 2017, 7, 34-59.2. M. Moses-DeBusk, M. Yoon, L. F. Allard, D. R. Mullins, Z. Wu, X. Yang, G. Veith, G. M. Stocks and C. K. Narula, J. Am. Chem. Soc. 2013, 135, 12634-12645.

International audience

Palladium platinum (PdPt) has been intensively studied these last decades due to high conversion rate in hydrogen oxidation at room temperature with significant exothermic effects. These remarkable properties have been studied by measuring the temperature variations of alumina (Al2O3) supported nanosized PdPt nanoparticles exposed to different hydrogen concentrations in dry air. This catalyst is expected to be used as a sensing material for stable and reversible ultrasensitive hydrogen sensors working at room temperature (low power consumption). Structural and gas sensing characterisations and catalytic activity of PdPt / Al2O3 systems synthesised by co-impregnation will be presented. Catalytic characterisations show that the system is already active at room temperature and that this activity sharply increases with rise in temperature. Moreover, the increase of the PdPt proportion in the co-impregnation process improves the activity, and very high conversion can be reached even at room temperature. The thermal response (about 3 °C) of only 1 mg of PdPt / Al2O3 is reversible, and the time response is about 5 s. The integration of PdPt / Al2O3 powder on a flat substrate has been realised by the deposition onto the powder of a thin porous hydrophobic layer of parylene. The possibility of using PdPt in gas sensors will be discussed.

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