Pentacoordinated Al3+‐Stabilized Active Pd Structures on Al2O3‐Coated Palladium Catalysts for Methane Combustion

Duan, Huimei; You, Rui; Xu, Shutao; Li, Zhaorui; Qian, Kun; Cao, Tian; Huang, Weixin; Bao, Xinhe

doi:10.1002/ange.201904883

Cited by 12 publications

(3 citation statements)

References 62 publications

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“…Such palladium active sites were introduced into the reaction sequence by postulating the presence of oxygen vacancies; oxygen vacancies are regenerated at the end of a catalytic cycle by recombination of surface hydroxyl groups formed in C-H bond activation steps, which allows explaining the negative reaction order of water (equation 1). Recently, PdO x and Pd-PdO x structures were also found on Al 2 O 3decorated Pd/SiO 2 catalysts synthesized by means of atomic layer deposition 22 .…”

Section: Introductionmentioning

confidence: 94%

Role of Water on the Structure of Palladium for Complete Oxidation of Methane

Wang

Roy

et al. 2020

ACS Catal.

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Palladium-based catalysts are attractive for methane combustion on natural gas vehicles at low temperature. By means of ambient pressure x-ray photoelectron spectroscopy, we investigated the reaction on a palladium foil exposed to different mixtures at increasing temperature. Water affects the long-term catalyst stability and blocks the active sites, ascribed to the hydroxyl inhibition effect. We investigated such an effect both under steady state and under transient reaction conditions, to understand the mechanism of inhibition.The hydroxyl formation on the surface of palladium blocks the sites for methane activation, postponing the formation of the active palladium oxide phase in the bulk.

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Section: Introductionmentioning

confidence: 94%

Role of Water on the Structure of Palladium for Complete Oxidation of Methane

Wang

Roy

et al. 2020

ACS Catal.

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“…Ag, Pt, and Pd) to improve their catalytic activity and stability. [11][12][13] Besides, theoretical calculations have suggested that Al V species could promote BAS formation in ASAs as typically proposed for Al IV . 14 Therefore, increasing the Al V concentration in alumina and ASAs is promising for both enhancing surface Lewis acidity and Brønsted acidity, thereby overcoming the limitations of current efforts in tuning the concentration of Al IV and Al VI species.…”

Section: Introductionmentioning

confidence: 77%

Pentacoordinated Aluminum Species: New Frontier for Tailoring Acidity-Enhanced Silica–Alumina Catalysts

et al. 2020

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CONSPECTUS: Silica-alumina catalysts, including zeolites and amorphous silica-aluminas (ASAs), are amongst the most widely used solid acid catalysts and supports to produce petrochemicals, fine chemicals, and renewable energy. The coordination, distribution, and interactions of aluminum in ASAs have an enormous impact on their acidic properties and catalytic performance. Unsaturated tetracoordinated aluminum (Al IV ) species are commonly accepted as the key sites in generating catalytically active Brønsted acid sites (BASs) in silica-alumina catalysts.Extensive efforts focus on increasing the concentration of Al IV as the main route to enhance their Brønsted acidity for efficient catalysis. However, increasing the Al IV concentration either weakens the acid strength in zeolites or lowers Brønsted acidity in ASAs at high Al/Si ratios, impeding acidity enhancement of these popular catalysts."Penta-coordinated aluminum (Al V ) species" are potential unsaturated Al species like Al IV but rarely observed in silica-aluminas, and thus, are widely considered unavailable for BAS formation or surface reactions. In this Account, we will describe novel strategies for the controlled synthesis of Al V -enriched ASAs and corresponding supported metal nanocatalysts using flame spray pyrolysis (FSP) techniques and highlight the contribution of Al V species in acidity enhancement, together with their structure-activity relationship in the conversion of biomass-derived compounds into valuable chemicals. Using various in situ and advanced 2D solid-state NMR (SSNMR) experiments, the studies of the acidic properties and local structure of Al V -enriched ASAs reveal that Al V species can highly populate on ASA surfaces, promote BASs formation and facilitate adaptable tuning of BASs from moderate to zeolitic strength by synergy with neighboring Al species. Moreover, the BASs with enhanced acidity can work jointly with surface Lewis acid sites or metal active species for bifunctional catalysis on Al V -enriched ASAs. Compared to zeolites, these Al V -enriched ASAs are highly active in acid-catalyzed biomass conversion, including 3 alcohol dehydration and sugar conversion reactions, as well as in promoting the performance of supported metal catalysts in chemoselective hydrogenation of aromatic ketones. These new insights provide a state-of-the-art strategy for strongly enhancing the acidity of these popular silica-alumina catalysts, which offers an interesting potential for a wide range of acid and multifunctional catalysis.

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“…This is consistent with DFT calculations which show that a surface oxide is more active than bulk PdO nanoparticles 17,18 . However, when Pd based catalysts are used for methane combustion, the metal Pd nanoparticle transforms to PdO, leading to the decrease in catalytic activity 19 . A well-known approach for improving the stability and reactivity of Pd based catalysts is to add Pt, which stabilizes the Pd phase in a metallic state by forming bimetallic PtPd even under oxidizing conditions 20 , leading to enhanced resistance to adsorption of water molecules.…”

Section: Introductionmentioning

confidence: 99%

Engineering catalyst supports to stabilize Pd/PdO 2D rafts for water-tolerant methane oxidation

Xiong¹,

Kunwar²,

García‐Vargas³

et al. 2020

Preprint

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The size and morphology of the active phase (metal or metal oxide) are critical for the performance of heterogeneous catalysts. Conventional approaches for catalyst synthesis involve the modification of pore size and structure, the use of ligands to anchor the metal during preparation or the use of nanostructured oxides with well-defined facets to provide suitable sites for metal nucleation and growth. However, these approaches may not yield durable catalysts for high temperature applications, such as the treatment of unburnt methane from natural gas fueled engines. Here we demonstrate an approach that relies on the trapping of metal single atoms on the support surface, in thermally stable form, to modify the nature of deposited metal/metal oxide clusters. By anchoring Pt ions on the catalyst support we can tailor the morphology of the deposited phase. In particular, two-dimensional (2-D) rafts of Pt/PtOx on the engineered catalyst support are formed by this approach, as opposed to three-dimensional (3-D) metal oxide nanoparticles on conventional supports. Adopting this approach for the synthesis of bimetallic catalysts via addition of Pd to the atom-trapped catalyst support (Pt@CeO2) we found that the resulting Pd/Pt@CeO2 catalyst provides improved thermal stability and water tolerance during methane oxidation. We attribute the improved performance to the 2-D morphology of the Pd/PdO phase present on the atom-trapped catalyst support. The results show that modifying the support by trapping single atoms could provide an important addition to the toolkit of catalyst designers to engineer catalyst supports for controlling the nucleation and growth of metal and metal oxide clusters in heterogeneous catalysts.

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Pentacoordinated Al³⁺‐Stabilized Active Pd Structures on Al₂O₃‐Coated Palladium Catalysts for Methane Combustion

Cited by 12 publications

References 62 publications

Role of Water on the Structure of Palladium for Complete Oxidation of Methane

Role of Water on the Structure of Palladium for Complete Oxidation of Methane

Pentacoordinated Aluminum Species: New Frontier for Tailoring Acidity-Enhanced Silica–Alumina Catalysts

Engineering catalyst supports to stabilize Pd/PdO 2D rafts for water-tolerant methane oxidation

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