21 research outputs found
Dynamic evolution of atomically dispersed platinum over alumina under adsorption and reaction conditions, and related CO oxidation performance
MICROSCOPIE+ECI2D:ING+DEC:FMO:MAO:JRO:LPIInternational audienceIn order to get insight into the interplay between metal ultradispersion and catalytic performance, atomically dispersed Pt/Îł-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 âatom-economicalâ 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 âsingle-atom catalystsâ (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/Îł-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/Îł-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/Îł-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/Ξ-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/Îł-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
Dynamic evolution of atomically dispersed platinum over alumina under adsorption and reaction conditions, and related CO oxidation performance
MICROSCOPIE+ECI2D:ING+DEC:FMO:MAO:JRO:LPIInternational audienceIn order to get insight into the interplay between metal ultradispersion and catalytic performance, atomically dispersed Pt/Îł-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 âatom-economicalâ 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 âsingle-atom catalystsâ (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/Îł-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/Îł-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/Îł-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/Ξ-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/Îł-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
Atmosphere-dependent stability, mobility and CO oxidation performance of Pt single atoms and clusters on Îł-alumina
SSCI-VIDE+ECI2D:ING+DEC:FMO:JRO:PAF:LPINational audienceIn this work, the stability of Îł-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/Îł-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
Comparison between single atoms and clusters of platinum supported on Îł-Al2O3 in CO oxidation
SSCI-VIDE+ECI2D:ING+TLN:FMO:JRO:PAF:LPIInternational audienceNoble 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/Îł-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), 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. [1]S. Mitchell, E. Vorobyeva, J. PĂ©rezâRamĂrez, Angew. Chem. Int. Ed. 57 (2018) 15316â15329.[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) 12634â12645.[3]K. Ding, A. Gulec, A.M. Johnson, N.M. Schweitzer, G.D. Stucky, L.D. Marks, P.C. Stair, Science 350 (2015) 189â192.[4]C. Dessal, A. Sangnier, C. Chizallet, C. Dujardin, F. Morfin, J.L. Rousset, M. Aouine, M. Bugnet, P. Afanasiev, L. Piccolo, Nanoscale 11, 6897-6904 (2019).[5]C. Dessal, T. Len, F. Morfin, J.L. Rousset, M. Aouine, P. Afanasiev, L. Piccolo, ACS Catal. 9, 5752-5759 (2019)
Comparaison entre les atomes isolés et les clusters de Pt supportés sur γ-Al2O3 en oxydation de CO : activité catalytique et hystérésis
SSCI-VIDE+ECI2D:ING+TLN:DEC:FMO:JRO:PAF:LPINational audienceLes mĂ©taux nobles sont utilisĂ©s pour catalyser de nombreuses rĂ©actions chimiques. Cependant, ils sont rares et onĂ©reux, ce qui limite leurs utilisations industrielles. Il est donc nĂ©cessaire dâaugmenter lâefficacitĂ© de ces catalyseurs. Cette rĂ©flexion, jointe au dĂ©veloppement des microscopes Ă©lectroniques en transmission (TEM) de derniĂšre gĂ©nĂ©ration, ont conduit Ă un intĂ©rĂȘt croissant pour les catalyseurs constituĂ©s non plus de nanoparticules mais dâatomes isolĂ©s (single-atom catalysts) stabilisĂ©s sur un support poreux,gĂ©nĂ©ralement un oxyde tel que la cĂ©rine, lâoxyde ferrique ou lâalumine.La charge en atomes de mĂ©tal (ici le Pt) affecte la probabilitĂ© de rencontre entre deux atomes et influe donc sur le phĂ©nomĂšne dâagrĂ©gation. Les traitements thermiques appliquĂ©s aprĂšs lâimprĂ©gnation peuvent Ă©galement accentuer cette agrĂ©gation, notamment sous atmosphĂšre rĂ©ductrice oĂč les atomes isolĂ©s de Pt se dĂ©stabilisent facilement en formant des clusters (1). De plus, des travaux indiquent que lâajout dâun mĂ©tal alcalin comme le sodium permet de favoriser la dispersion du platine et la stabilitĂ© des atomes isolĂ©s sur lâalumine (2). Dâautres additifs, tels que le lanthane, ont Ă©galement Ă©tĂ© rapportĂ©s dans la littĂ©rature (3).Dans cette Ă©tude, des catalyseurs Pt/(Na, La-)Îł-Al2O3 ont Ă©tĂ© prĂ©parĂ©s par une mĂ©thode conventionnelle dâimprĂ©gnation Ă humiditĂ© naissante suivie dâun traitement thermique oxydant et/ou rĂ©ducteur. Dans le but de comparer la rĂ©activitĂ© des atomes isolĂ©s et des clusters (taille = 1 nm) de Pt, les deux types de catalyseurs correspondants ont Ă©tĂ© testĂ©s en oxydation de CO dans un rĂ©acteur sous flux Ă lit fixe. Les matĂ©riaux ont Ă©tĂ© caractĂ©risĂ©s par TEM Ă balayage (STEM) avant et aprĂšs rĂ©action. Les tests catalytiques montrent une activitĂ© et une stabilitĂ© supĂ©rieures des clusters par rapport aux atomes isolĂ©s. De plus, les courbes de production de CO2 en fonction de la tempĂ©rature (chauffage vs refroidissement) montrent une forte hystĂ©rĂ©sis. Lâorigine de ces phĂ©nomĂšnes, ainsi que lâinfluence du support (Al2O3 vs TiO2 et ZnO), seront discutĂ©es lors de cette prĂ©sentation.(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, 3470â3473.(3) Lakshmanan, P.; Park, J. E.; Kim, B.; Park, E. D. Catalysis Today 2016, 265, 19â26
Comparison between single atoms and clusters of platinum supported on Îł-Al2O3 in CO oxidation
SSCI-VIDE+ECI2D:ING+TLN:FMO:JRO:PAF:LPINational audienceNoble 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/Îł-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), 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. PĂ©rezâRamĂrez, Angew. Chem. Int. Ed. 57 (2018) 15316â15329.[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) 12634â12645.[3]K. Ding, A. Gulec, A.M. Johnson, N.M. Schweitzer, G.D. Stucky, L.D. Marks, P.C. Stair, Science 350 (2015) 189â192.[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