1,720,967 research outputs found
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
Operando Surface Studies on Metal-Oxide Interfaces of Bimetal and Mixed Catalysts
No full text© The formation of metal-oxide interfaces in catalytic systems exhibits a synergistic phenomenon between metal and reducible oxides, often referred to as the strong metal-support interaction (SMSI). This unusual characteristic has been connected to the origin of highly enhanced catalytic performance for decades, but the mechanistic explanation of SMSI remains a long-standing issue in heterogeneous catalysis. To understand this matter at the molecular level, geometric and electronic functions of metal-oxide interfaces during catalytic reactions should be clearly interpreted using advanced microscopic and spectroscopic analysis techniques. In this Review, we highlight recently performed investigations at metal-oxide interfaces by operando characterization tools to identify active sites in working conditions. We introduce two kinds of catalysts, platinum-based bimetallic alloys and mixed metal-oxide catalysts. Selected operando techniques reveal their atomic-scale morphology, surface electronic structure, and charge transfer/transport at surfaces under oxidation, reduction, and gas mixture environments. With bimetallic model catalysts, topographic morphology observations present critical evidence for the structural modulation between the topmost layer and the subsurface lattice in oxygen conditions. For the mixed metal-oxide catalysts, we note that metal nanoparticles on reducible oxides demonstrate the catalytic activity enhancement, which is obviously influenced by the change of oxidation states at the metal-oxide interface. Environmental transmission electron microscopy images unveil the atomic-scale redox behaviors at the nanoparticle interfaces with evolutions of the reducible oxide under the catalytic reaction. The important role of reactive interfaces between the transition metal atom and oxide-support explains the surface chemistry and heterogeneous catalysis over active sites on well-defined single crystal model surfaces, as well as nanoparticle catalysts. Overall, operando studies for metal-oxide interfaces can shed light on mechanistic insights into the tuning of catalytic activity at the molecular level and on improving catalytic performance by the SMSI effect.11Nsciescopu
Size-controlled model Ni catalysts on Ga2O3 for CO2 hydrogenation to methanol
No full text© 2019 Elsevier Inc.The effect of particle size for Ni nanoparticles supported on β-Ga2O3 was investigated for CO2 hydrogenation to methanol at 0.5 MPa. Model Ni nanoparticles ranging from 3.3 to 10.2 nm were synthesized using the hot injection method by controlling the reaction temperature and time. The smallest Ni nanoparticles (3.3 nm) showed the highest catalytic activity across the entire temperature range and the largest Ni nanoparticles (10.2 nm) showed the highest methanol selectivity. The apparent activation energies for methanol with Ni nanoparticles increased from 6.0 to 18.4 kcal mol−1 as the nanoparticle size increased. Furthermore, it was found that the smallest Ni nanoparticles favor the reverse water gas shift reaction. In situ DRIFT analysis revealed that the gallium oxide itself could produce an intermediate species and the addition of Ni on the oxide support increases the hydrogenation rate. The Ni supported catalysts showed a CO peak, but the smallest Ni nanoparticles showed a larger CO peak than that for the largest Ni nanoparticles, which clearly supports that the smaller nanoparticles favor the reverse water gas shift reactio
Defective Nb2O5-supported Pt catalysts for CO oxidation: Promoting catalytic activity via oxygen vacancy engineering
No full text© 2019 Elsevier Inc.We introduced oxygen vacancies into Nb2O5 via thermal treatment at 700 °C under oxygen-deficient conditions for different lengths of time. Niobia-supported Pt catalysts were subsequently synthesized using the wet impregnation method with the fabricated Nb2O5. The catalytic activity of the synthesized catalysts for CO oxidation exhibited a significant increase from that obtained for a counterpart having the Nb2O5 support treated under saturated oxygen conditions (i.e., in air). Moreover, increasing the number of oxygen vacancies was found to increase the catalytic activity. At 150 °C, the TOF calculated for the catalyst with the most oxygen vacancies was 0.36 s−1, which was much higher than that obtained from the catalyst possessing the fewest oxygen vacancies (0.05 s−1). Systematic characterization of the synthesized catalysts revealed the crucial impact of oxygen vacancies and active lattice oxygen on the enhancement of catalytic activit
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
Synthesis of High Surface Area TiO2 Aerogel Support with Pt Nanoparticle Catalyst and CO Oxidation Study
No full textWe demonstrate the preparation and catalytic testing of nanocatalysts composed of TiO2 synthesized in aerogel using a supercritical drying process with Pt nanoparticles deposited on the mesopore oxide surface. By controlling the aging time, we found conditions that led to TiO2 with a surface area among the highest ever reported for a TiO2 material in aerogel form (similar to 685 m(2) g(- 1)). The chosen synthesis route also features good control over the pore volume and pore size. Even though our aerogel TiO2 support showed an amorphous structure before calcination, 90.2% anatase and 9.8% rutile phases co-existed after calcination at 500 A degrees C for 2 h with an average particle size of about 17 nm based on XRD results. The as-synthesized TiO2 and Pt/TiO2 were both used as catalysts for the CO oxidation reaction. Uncalcined Pt/TiO2 showed 100% conversion at 300 A degrees C. After calcination, Pt/TiO2 exhibited a significantly higher catalytic activity, showing 100% conversion at 125 A degrees C and an activation energy of only 13.4 kcal mol(- 1). We show that TiO2 synthesized by the aerogel method can be an excellent support material for nanocatalysis because of its high surface area and facile and scalable synthesis. © Springer Science+Business Media, LLC, part of Springer Nature 201
The facet effect of ceria nanoparticles on platinum dispersion and catalytic activity of methanol partial oxidation
No full textThe effect of platinum-supported nano-shaped ceria catalysts on methanol partial oxidation and methyl formate product selectivity has been investigated. A Pt-supported CeO2 nanocube catalyst had a higher turnover frequency than nanosphere catalysts; however, nanosphere catalysts showed higher selectivity towards methyl formate. The observed ceria shape effect in catalysis was associated with the shape-dependent Pt dispersion and its oxidation states. Furthermore, in situ studies revealed that the reduced platinum and mono-dentate methoxy group were responsible for the higher turnover frequency.11Nsciescopu
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
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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