1,720,993 research outputs found
Nature of Active Sites and Their Quantitative Measurement in Two-Dimensional Pt Metal Catalysts
No full textAbstract: Quantitative measurement of the number of active surface sites on two-dimensional (2D) catalysts is one of the most crucial points in heterogeneous catalysis because it is used to determine the turnover frequency (TOF), which refers to the catalytic activity of model catalysts. However, because of the difficulty in identifying the effective active surface area on 2D heterogeneous catalysts, there is still the assumption that each metal atom is an active site. To shed light on these issues and to bridge the activity gaps between 2D and three-dimensional (3D) heterogeneous catalysts, we present an experimental approach that uses Pt nanoparticle (NP) arrays on a thin silicon wafer probed with CO pulse chemisorption, a widely used surface-sensitive technique, to determine the number of active sites and the area of the effective active surface. A Pt thin film and Pt NP arrays with two different NP sizes (i.e., 2.1 and 4.5 nm) were prepared as model systems for 2D catalysts. The effective active metal surface area determined using CO pulse chemisorption for these 2D catalysts is 53–79% of the apparent metal surface area that was obtained by measuring the surface area based on scanning electron microscopy images. This discrepancy between the active and apparent surface area is attributed to the presence of hydrocarbon contamination and organic capping layers on the catalysts. The results indicate that estimating the active sites of 2D catalysts by apparent surface area is reasonably in agreement with the number measured by chemisorption that is used to characterize 3D nanocatalysts. This experimental technique on 2D catalysts can be expected to provide information for extracting the true TOF of product molecules on 2D catalysts in gas-phase catalytic reactions. Graphical Abstract: [Figure not available: see fulltext.] © 2016, Springer Science+Business Media New York3
High methane selective Pt cluster catalyst supported on Ga2O3 for CO2 hydrogenation
No full textIn heterogeneous catalysis, the CO2 hydrogenation reaction is an important class of reactions that has been widely studied for decades. Particularly, the particle size of the metal plays a crucial role in controlling the selectivity of the CO2 hydrogenation reaction. However, there have only been a few studies investigating the selectivity for sub-nanometer sized particles. Here, we report the effect of Pt particle size on the catalytic activity and selectivity of CO2 hydrogenation. The size of the Pt particles was tuned by changing the amount of loading. With low Pt loading on gallium oxide, Pt clusters were formed; however, Pt nanoparticles were synthesized with high Pt loading. The Pt clusters were mainly edge and step sites where CO2 adsorbs more strongly, while the Pt nanoparticles were mainly composed of terrace sites. The catalytic performance of the Pt catalysts was examined using CO2 hydrogenation. The Pt clusters showed a higher methane selectivity than that of the Pt nanoparticles. The reaction mechanism was analyzed by diffuse reflectance infrared fourier transform (DRIFT) spectroscopy at reaction conditions. The Pt clusters mainly showed the formate peak, while showing fewer carbonate peaks than the support. These results suggest that the formation of CH4 follows the formate route11Nsciescopu
Hot electron flux at solid–liquid interfaces probed with Pt/Si catalytic nanodiodes: Effects of pH during decomposition of hydrogen peroxide
No full textHydrogen peroxide (H2O2) is an effective oxidizing agent that is commonly used in industry. In the presence of metal catalysts, H2O2 can decompose into water and oxygen. Understanding this process at a fundamental level is extremely important for a number of industrial applications, e.g. the direct synthesis of H2O2 from H2 and O2. Here, we studied the rates of H2O2 decomposition on Pt/n-Si catalysts using a chemicurrent approach that is based on the detection of hot electrons created during dissociative adsorption of H2O2 molecules on platinum. We showed that both the rate of H2O2 decomposition and the corresponding chemicurrent are sensitive to the pH of the reactive solution. This phenomenon is explained by variation of the potential barrier for electron transfer at the Pt/solution interface caused by adsorption of H+ and OH− species from the solution on the catalytic surface. © 2017 Elsevier B.V11sciescopu
Oxygen activation on the interface between Pt nanoparticles and mesoporous defective TiO2 during CO oxidation
Get PDF© 2019 Author(s).Platinum-based heterogeneous catalysts are mostly used in various commercial chemical processes because of their high catalytic activity, influenced by the metal/oxide interaction. To design rational catalysts with high performance, it is crucial to understand the relationship between the metal-oxide interface and the reaction pathway. Here, we investigate the role of oxygen defect sites in the reaction mechanism for CO oxidation using Pt nanoparticles supported on mesoporous TiO2 catalysts with oxygen defects. We show an intrinsic correlation between the catalytic reactivity and the local properties of titania with oxygen defects (i.e., Ti3+ sites). In situ infrared spectroscopy observations of the Pt/mesoporous TiO2-x catalyst indicate that an oxygen molecule bond can be activated at the perimeter between the Pt and an oxygen vacancy in TiO2 by neighboring CO molecules on the Pt surface before CO oxidation begins. The proposed reaction pathways for O2 activation at the Pt/TiO2-x interface based on density functional theory confirm our experimental findings. We suggest that this provides valuable insight into the intrinsic origin of the metal/support interaction influenced by the presence of oxygen vacancies, which clarifies the pivotal role played by the support11sciescopu
Probing surface oxide formations on SiO2-supported platinum nanocatalysts under CO oxidation
No full textFormations of an ultrathin oxide layer on noble metal catalysts affect the characteristics of fundamental molecular behaviours such as adsorption, diffusion, and desorption on their surfaces. That is directly correlated to enhancement of catalytic activity under operating conditions because the kinetics of catalytic reactions are also simultaneously influenced. Especially, a sub-monolayered surface oxide is known as having a key role for improving catalytic activity, but revealing its existence in catalysis is challenging due to their fast chemical conversion. Herein, we report the first evidence of surface oxide formations on platinum (Pt) nanocatalysts under CO oxidation probed with a diffuse reflectance infrared Fourier transform (DRIFT) technique. Spectroscopic information demonstrates that the abrupt blue shift of adsorbed CO molecules vibrational frequencies of CO stretching mode on the reduced Pt nanocatalyst surface is initiated prior to aggressive CO conversion to CO2 gas molecules. Site-specific replacements of the adsorbed CO molecule with dissociative oxygen occur just before the ignition temperature that is supposed to be an important reaction step for CO oxidation over a Pt nanocatalyst. Density functional theory (DFT) calculation results support this phenomenon as a function of relative atomic fractions between CO and O on a Pt model surface and consistently show a similar trend with experimental evidence. © 2017 The Royal Society of Chemistry
Effect of the metal-support interaction on the activity and selectivity of methanol oxidation over Au supported on mesoporous oxides
No full textTo elucidate the factors affecting the catalytic properties of supported Au catalysts on the metal oxide support we investigated Au NPs deposited on crystallized mesoporous transition-metal oxides (m-oxides: Co3O4, NiO, and α-Fe2O3) prepared using the nanocasting method. The metal-oxide interaction in Au/mesoporous oxides resulted in higher catalytic activity for converting methanol to CO2 as a full oxidation product than pure m-oxides. Au/m-Fe2O3 exhibited high activity and low selectivity for methyl formate as a partial oxidative coupling product. We correlate the change in activity and selectivity with the interface between the Au and m-oxides. © 2018 The Royal Society of Chemistr
Titania-Encapsulated Hybrid Nanocatalysts as Active and Thermally Stable Model Catalysts
No full textMetal–oxide hybrid nanocatalysts with ultrathin
oxide encapsulation can be a new platform to test the
metal–support interaction. Metal nanoparticles (Ru, Rh, or
Pt) capped with polymer/citrate were deposited on functionalized
SiO2 and then an ultrathin layer of TiO2 was
selectively coated on the SiO2 surface to prevent sintering
and to provide high thermal stability while maximizing the
metal–oxide interface for higher catalytic activity. Transmission
electron microscopy studies confirmed that
2.1–2.3 nm metal nanoparticles were well dispersed and
distributed throughout the surface of the 25 nm SiO2
nanoparticles, and that a 2 nm ultrathin TiO2 layer existed
on the surface of the particles. The metal nanoparticles
were still well exposed to the outer surface, thus allowing
for surface characterization and catalytic activity. Even
after calcination at 600 C, the structure and morphology
of the hybrid nanocatalysts remained intact, confirming
high thermal stability. The catalytic activities of the hybrid
nanocatalysts with ultrathin oxide encapsulation (SiO2/M/
TiO2, M = Pt, Rh, or Ru) were evaluated using the CO
oxidation reaction. Hybrid nanocatalysts encapsulated by
the ultrathin oxide layer allowed us to obtain high thermal
stability and better exposure of the metal active sites for a
strong metal–support interaction between the metals and
the ultrathin TiO2.1221sciescopu
Influence of Support Acidity of Pt/Nb 2 O 5 Catalysts on Selectivity of CO 2 Hydrogenation
Get PDF© 2019, Springer Science+Business Media, LLC, part of Springer Nature. Abstract: In solid acid catalysis, understanding the impact of support acidity on catalytic performance has remained a controversial issue. The selected catalytic systems often rely on mixing different substances to control the degree of acidity, which in turn, also modifies other parameters in the system, making it challenging to perform a definitive study. To specifically investigate the role of support acidity, we performed a systematic study employing Nb 2 O 5 as the catalyst support, which acidity can be controlled by calcination. The catalytic behavior of the fabricated Pt/Nb 2 O 5 catalysts was evaluated using CO 2 hydrogenation to methanol (MeOH) and dimethyl ether (DME). An increase in the acidity of the support resulted in an improvement in the CO 2 conversion owing to the strong interaction between the Pt and the catalyst support, but it was detrimental for the production of MeOH because of the unfavorable adsorption of CO 2 molecules and the formation of carbon-containing species on the surface of the support with high acidity. DME selectivity was enhanced with an increase in catalyst acidity, confirming the role of solid acids for the production of DME from CO 2 reduction. Graphical Abstract: By controlling the calcination temperature of Nb 2 O 5 , tunable support acidity was obtained. CO 2 conversion increased while the selectivity of methanol and dimethyl ether decreased with increasing support acidity.11sciescopu
A tailored oxide interface creates dense Pt single-atom catalysts with high catalytic activity
No full textHighly reactive dense Pt single-atoms stabilized on an oxide support can resolve a grand challenge in the economic use of Pt in catalysis. The maximized number density of reaction sites provided by dense Pt single-atoms guarantees the improved catalytic performance of Pt combined with high efficiency. By manipulating the chemical nature of multi-component interfaces, we synthesized CO-tolerant dense Pt single-atoms highly reactive for the CO oxidation reaction, which governs the key steps for chemical energy conversion and emission control. The addition of 1 wt% of Ce to TiO2 support particles creates a CeOx–TiO2 interface that stabilizes Pt single-atoms by strong electronic interactions. Dense Pt singleatoms formed on CeOx/TiO2 oxides exhibit 15.1 times greater specific mass activity toward CO oxidation at 140 1C compared with a bare Pt/TiO2 catalyst. We elaborate how the CeOx–TiO2 interfaces activate the interface-mediated Mars–van Krevelen mechanism of CO oxidation and protect Pt single-atoms from CO-poisoning. Through a comprehensive interpretation of the formation and activation of dense Pt single-atoms using operando X-ray absorption spectroscopy, density functional theory calculations, and experimental catalyst performance tests, we provide a key that enables the catalytic performance of noble metal single-atom catalysts to be optimized by atomic-scale tuning of the metal–support interface.11Nsciescopu
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