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At least 307 records · Page 17
All sourcesNASA NTRSDOE OSTI
DOE OSTI2024

2D Metal/Graphene and 2D Metal/Graphene/Metal Systems for Electrocatalytic Conversion of CO 2 to Formic Acid

Abstract Efficiently transforming CO 2 into renewable energy sources is crucial for decarbonization efforts. Formic acid (HCOOH) holds great promise as a hydrogen storage compound due to its high hydrogen density, non‐toxicity, and stability under ambient conditions. However, the electrochemical reduction of CO 2 (CO 2 RR) on conventional carbon black‐supported metal catalysts faces challenges such as low stability through dissolution and agglomeration, as well as suffering from high overpotentials and the necessity to overcome the competitive hydrogen evolution reaction (HER). In this study, we modify the physical/chemical properties of metal surfaces by depositing metal monolayers on graphene (M/G) to create highly active and stable electrocatalysts. Strong covalent bonding between graphene and metal is induced by the hybridization of sp and d orbitals, especially the sharp , , and orbitals of metals near the Fermi level, playing a decisive role. Moreover, charge polarization on graphene in M/G enables the deposition of another thin metallic film, forming metal/graphene/metal (M/G/M) structures. Finally, evaluating overpotentials required for CO 2 reduction to HCOOH, CO, and HER, we find that Pd/G, Pt/G/Ag, and Pt/G/Au exhibit excellent activity and selectivity toward HCOOH production. Our novel 2D hybrid catalyst design methodology may offer insights into enhanced electrochemical reactions through the electronic mixing of metal and other p‐block elements.

Cho, Jinwon↗
DOE OSTI2020

Regioselective Generation of Single‐Site Iridium Atoms and Their Evolution into Stabilized Subnanometric Iridium Clusters in MWW Zeolite

Abstract Preparation of supported metal catalysts with uniform particle size and coordination environment is a challenging and important topic in materials chemistry and catalysis. In this work, we report the regioselective generation of single‐site Ir atoms and their evolution into stabilized subnanometric Ir clusters in MWW zeolite, which are located at the 10MR window connecting the two neighboring 12MR supercages. The size of the subnanometric Ir clusters can be controlled by the post‐synthesis treatments and maintain below 1 nm even after being reduced at 650 °C, which cannot be readily achieved with samples prepared by conventional impregnation methods. The high structure sensitivity, size‐dependence, of catalytic performance in the alkane hydrogenolysis reaction of Ir clusters in the subnanometric regime is evidenced.

Liu, Lichen↗
DOE OSTI2024

2D Metal/Graphene and 2D Metal/Graphene/Metal Systems for Electrocatalytic Conversion of CO 2 to Formic Acid

Efficiently transforming CO 2 into renewable energy sources is crucial for decarbonization efforts. Formic acid (HCOOH) holds great promise as a hydrogen storage compound due to its high hydrogen density, non-toxicity, and stability under ambient conditions. However, the electrochemical reduction of CO 2 (CO 2 RR) on conventional carbon black-supported metal catalysts faces challenges such as low stability through dissolution and agglomeration, as well as suffering from high overpotentials and the necessity to overcome the competitive hydrogen evolution reaction (HER). In this study, we modify the physical/chemical properties of metal surfaces by depositing metal monolayers on graphene (M/G) to create highly active and stable electrocatalysts. Strong covalent bonding between graphene and metal is induced by the hybridization of sp and d orbitals, especially the sharp $d_{z^2}$, $d_{yz}$, $d_{xz}$ orbitals of metals near the Fermi level, playing a decisive role. Moreover, charge polarization on graphene in M/G enables the deposition of another thin metallic film, forming metal/graphene/metal (M/G/M) structures. Finally, evaluating overpotentials required for CO 2 reduction to HCOOH, CO, and HER, we find that Pd/G, Pt/G/Ag, and Pt/G/Au exhibit excellent activity and selectivity toward HCOOH production. Our novel 2D hybrid catalyst design methodology may offer insights into enhanced electrochemical reactions through the electronic mixing of metal and other p-block elements.

2D Electrocatalysts↗
DOE OSTI2024

Controlling the Phase Transformation of Alumina for Enhanced Stability and Catalytic Properties

Current transition alumina catalysts require the presence of significant amounts of toxic, environmentally deleterious dopants for their stabilization. Herein, we report a simple and novel strategy to engineer transition aluminas to withstand aging temperatures up to 1200 °C without inducing the transformation to low-surface-area α-Al 2 O 3 and without requiring dopants. By judiciously optimizing the abundance of dominant facets and the interparticle distance, we can control the temperature of the phase transformation from θ-Al 2 O 3 to α-Al 2 O 3 and the specific surface sites on the latter. These specific surface sites provide favorable interactions with supported metal catalysts, leading to improved metal dispersion and greatly enhanced catalytic activity for hydrocarbon oxidation. Further, the results presented herein not only provide molecular-level insights into the critical factors causing deactivation and phase transformation of aluminas but also pave the way for the development of catalysts with improved activity for catalytic hydrocarbon oxidation.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2020

Selectivity Control in Catalytic Reductive Amination of Furfural to Furfurylamine on Supported Catalysts

Abstract Amines are widely used in the manufacture of pharmaceuticals, agricultural chemicals, polymers, and surfactants. However, amines are mostly produced via petrochemical means, which motivates amine production from renewable resources, such as biomass. However, biomass compounds present added challenges involving poor carbon balances. We show that furfural reacts homogeneously with ammonia to produce reactive primary imines, which form large side products and leads to significant carbon losses. The carbon balance is improved by mixing furfural with furfurylamine prior to reaction to form a secondary imine for use as the reaction substrate. While controlling the primary to secondary amine selectivity is a common challenge in reductive amination, supported metal catalysts, including Ni/SiO 2 , Co/SiO 2 , and Ru/SiO 2 optimize the primary amine yield to 90 to 94 % by using the secondary imine as the reaction substrate. A qualitative correlation between the primary to secondary amine selectivity with the nitrogen binding energy of metals is identified.

Gould, Nicholas S.↗
DOE OSTI2025

Structurally Regenerable High Entropy Aluminate Spinel Catalysts for Dry Reforming of Methane

The dry reforming of methane reaction is a promising means to convert two potent greenhouse gases, methane and carbon dioxide, into industrially valuable synthesis gas. However, the presence of reducing gases and high operating temperatures degrade conventional nickel catalysts via excessive coke formation and particle sintering. These catalysts are not readily regenerated because the oxidative heat treatments employed to remove coke further promote active particle sintering. In this work, we designed high entropy aluminate spinel oxides (MAl 2 O 4 where M = Co, Mg, Ni, and divalent site vacancies in nominal equimolar concentration) as selective and regenerable reforming catalysts. Under reaction conditions, reducible nickel and cobalt cations exsolved from the spinel lattice to form highly selective bimetallic particles on the oxide surface. Instead of sintering, these particles uniquely redissolved back into the aluminate lattice upon reoxidation and regained the original spinel structure. This phenomenon is ascribed to entropic stabilization, wherein an increase in configurational entropy creates a thermodynamic driving force for redispersing supported metal particles back into the multi‐cationic oxide structure. During the dry reforming reaction, nickel atoms similarly exsolved from a NiAl 2 O 4 sample and reduced to form metallic nickel particles. However, subsequent oxidation of this sample promoted sintering and oxidation of the nickel particles to an inactive state. High entropy materials thus provide a unique mechanism of regeneration, which is inaccessible in conventional catalysts.

coke deposition↗
DOE OSTI2021

Single Atom Catalysts: A Review of Characterization Methods

Abstract Single atom catalysts (SACs) harbor a potential to exceed nanoparticle catalysts in terms of activity, stability and selectivity in a growing number of chemical reactions. Although their investigation is attracting significant attention, important fundamental questions focusing on key physicochemical properties of SACs (e. g., structure – property relationships, structural dynamics, reaction‐driven restructuring) remain unanswered. A main challenge for research in the field is how to reliably characterize the environments of single atoms in the presence of complicating factors such as low weight loadings, strong metal‐support interactions, and atomic and multiscale heterogeneity of bonding in the single atom sites. This review addresses this challenge – identifying catalytically relevant features of physicochemical properties of single atoms (charge state, electronic structure, atomic configuration, bonding interactions with a support) and surveying advanced tools/methods for characterizing them. The review places a strong emphasis on multimodal methods exploiting X‐ray absorption, emission and photoelectron spectroscopies, and provides several examples from the authors’ research that demonstrate their use as powerful tools for SAC characterization.

Kottwitz, Matthew↗
DOE OSTI2021

Toward efficient single-atom catalysts for renewable fuels and chemicals production from biomass and CO 2

Transformation of biomass and CO 2 into renewable value-added chemicals and fuels has been identified as a promising strategy to fulfill high energy demands, lower greenhouse gas emissions, and exploit under-utilized resources. Cost-effective and performance-efficient catalysts are of great importance to lowering the conversion cost of biomass and CO 2 . Significant progress has been made to advance the catalyst design for these processes, with metal catalysts playing a critical role in many involved catalytic reactions. Traditional nanoparticle-based metal catalysts still require improvement in metal utilization rates, stability, and selectivity tunability. Single-atom catalysts, which have maximum atomic efficiency and a uniform and tunable metal center, as well as an adjustable metal-support interaction, provide potential opportunities to boost catalyst efficiency and thermal stability. Their well-defined and uniform structure also provides advantages to fundamental studies for understanding of the intrinsic reaction mechanism and site requirement in biomass and CO 2 conversion. In this article, we summarize and highlight the recent advances in converting biomass and CO 2 to renewable fuels and chemicals using single-atom catalysts. We discuss the design principles of single-atom catalysts and their potential applications to biomass and CO 2 upgrading as well as the origins of catalytic activity. Moreover, we compare the catalytic efficiency of various catalysts reported thus to provide a fair assessment of these catalysts. Finally, perspectives are given on the interesting fields that may guide future studies.

09 BIOMASS FUELS↗
DOE OSTI2020

Enhanced activity and stability of MgO-promoted Ni/Al 2 O 3 catalyst for dry reforming of methane: Role of MgO

Highly dispersed nickel nanoparticles (NPs) were deposited on Al 2 O 3 NPs by atomic layer deposition (ALD). Various amounts of MgO were loaded on Ni/Al 2 O 3 catalysts by the incipient wetness (IW) method for dry reforming of methane. Fresh and used catalysts were systematically characterized to reveal the effects of MgO on the catalytic performance. MgO was found to increase the basic amount and basic strength of catalyst surfaces, which provided additional surface oxygen species and assisted the adsorption and activation of CO 2 . Although the formation of NiO-MgO solid solution, during the calcination of incipient wetness, did not improve the overall reducibility, the Ni NPs from NiO-MgO solid solution after reduction formed an intimate interaction with MgO, which could inhibit Ni NPs from sintering and provide sufficient metal-support interface for CO 2 activation. The MgO-promoted Ni/Al 2 O 3 reached a methane reforming rate of 1780 L CH4 g Ni -1 h -1 at 850 °C, which is 26% more than that of the pristine Ni/Al 2 O 3 . Finally, the higher CO 2 activity enhanced the oxidation rate of the surface carbon generated from side-reactions, thereby resulting in a higher reforming rate and inhibiting coke formation, especially the detrimental graphitic encapsulating carbon on the active nickel surface.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

The effects of SMSI on m-Cresol Hydrodeoxygenation over Pt/Nb 2 O 5 and Pt/TiO 2

The hydrodeoxygenation (HDO) of m-cresol was studied on Pt/Nb 2 O 5 /MgAl 2 O 4 and Pt/TiO 2 /MgAl 2 O 4 catalysts to understand the effects of Strong Metal Support Interactions (SMSI). The Nb 2 O 5 and TiO 2 supports were prepared as 0.7-nm films on MgAl 2 O 4 by Atomic Layer Deposition (ALD) to ensure that the structures of the catalyst were the same. When reduced at 773 K to place Pt in the encapsulated state, Pt/Nb 2 O 5 /MgAl 2 O 4 was much less active than Pt/MgAl 2 O 4 at 573 K but much more active at 623 K. While Pt/MgAl 2 O 4 deactivated rapidly due to coking, Pt/Nb 2 O 5 /MgAl 2 O 4 showed significantly better coke tolerance and was almost 100% selective towards toluene production. Pt/Nb 2 O 5 /MgAl 2 O 4 reduced at lower temperatures exhibited intermediate catalytic properties. The effect of reduction temperature on Pt/TiO 2 /MgAl 2 O 4 was much less and this catalyst was more similar to Pt/MgAl 2 O 4 than its Nb 2 O 5 counterpart. Lastly, the implications of these results for understanding the nature of oxide promoters on HDO of m-cresol are discussed.

09 BIOMASS FUELS↗
DOE OSTI2022

Evidence for redispersion of Ni on LaMnO 3 films following high-temperature oxidation

LaMnO 3 films, 0.5-nm thick, were deposited by atomic layer deposition (ALD) onto γ-Al 2 O 3 that had been modified with 15-wt% CaO. The CaO was shown to be effective in preventing formation of LaAlO 3 that formed when La 2 O 3 was deposited directly onto γ-Al 2 O 3 . Lattice fringes on the resulting CaAl 2 O 4 /γ-Al 2 O 3 substrate were weakly resolved, allowing a detailed characterization of the LaMnO 3 films. High-resolution transmission electron microscopy (HR-TEM) images showed that the LaMnO 3 formed two-dimensional crystallites, ~10 to 15 nm wide, that covered most of the surface. Crystallites with (001) and (111) orientation were clearly identified. High-temperature oxidation caused Ni to spread over the LaMnO 3 film, suggesting there is a reaction of the Ni 2+ cations with the perovskite lattice. Ni formed by high-temperature reduction on these films remained well dispersed and significantly more active for CO 2 reforming of CH 4 compared to Ni on MgAl 2 O 4 , even after repeated oxidation and reduction cycles at 1073 K. Finally, the implications of these results for understanding metal-support interactions between Ni and LaMnO 3 are discussed.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

Conversion of ethanol to 1,3–butadiene over Ag–ZrO 2 /SiO 2 catalysts: The role of surface interfaces

A series of Ag–ZrO 2 /SiO 2 catalysts with different metal–support interfaces were synthesized in an effort to elucidate the roles of specific interfaces in controlling the ethanol to 1,3–butadiene conversion and selectivity. According to the results of detailed characterizations (e.g. CO/pyridine–DRIFTS, XPS, TEM, NH 3 –TPD, and 1 H MAS NMR), it was found that the Ag–O–Si interfaces significantly enhanced the dehydrogenation of ethanol while the presence of ZrO 2 improved the interaction between Ag and ZrO 2 /SiO 2 , creating more Ag δ+ active sites. In this research, the high dispersion of ZrO 2 on SiO 2 generated abundant Zr–O–Si interfaces with medium and weak Lewis acidity, promoting the condensation of acetaldehyde to crotonaldehyde. These Zr–O–Si interfaces in close interaction with Ag δ+ species played a critical role in the enhanced H transfer during the MPV reduction of crotonaldehyde to crotyl alcohol. The synergies among the interfaces resulted in retarded ethanol dehydration reactivity, balanced ethanol dehydrogenation and condensation reactions, and a subsequent high 1,3–butadiene yield.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2021

New concept of radiolytic synthesis of gold nanoparticles in continuous flow

Radiolytic synthesis has been used to produce a plethora of unsupported and supported metallic nanoparticles in batch. As proof of a new concept, we evaluated the formation of gold nanoparticles by radiolytic synthesis in continuous flow using a millifluidic reactor. A mixture of gold ions solution, branched polyethyleneimine, and ethylene glycol yielded gold nanoparticles with a mean size of 3.4 ± 1.0 nm after irradiation with an X-ray irradiator (127 Gy/min) at an absorbed dose of 5.1 kGy. The morphology and size distribution of gold nanoparticles was influenced by AgNO 3 , where its absence resulted in a mean size of 20.9 ± 15.8 nm. The radiolytic synthesis of gold nanoparticles in continuous flow was achieved in short reaction times (<60 min), at room temperature, and with low concentrations of both stabilizing and reducing agents. Furthermore, these results demonstrate the potential of radiolytic synthesis in continuous flow for high-throughput formation of metallic nanoparticles with controlled specifications in size and distribution.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2023

Surface Lattice-Embedded Pt Single-Atom Catalyst on Ceria-Zirconia with Superior Catalytic Performance for Propane Oxidation

Tuning the metal–support interaction and coordination environment of single-atom catalysts can help achieve satisfactory catalytic performance for targeted reactions. Herein, via the facile control of calcination temperatures for Pt catalysts on pre-stabilized Ce 0.9 Zr 0.1 O 2 (CZO) support, Pt single atoms (Pt 1 ) with different strengths of Pt–CeO 2 interaction and coordination environment were successfully constructed. With the increase in calcination temperature from 350 to 750 °C, a stronger Pt–CeO 2 interaction and higher Pt-O-Ce coordination number were achieved due to the reaction between PtO x and surface Ce 3+ species as well as the migration of Pt 1 into the surface lattice of CZO. The Pt/CZO catalyst calcined at 750 °C (Pt/CZO-750) exhibited a surprisingly higher C 3 H 8 oxidation activity than that calcined at 550 °C (Pt/CZO-550). Here, through systematic characterizations and reaction mechanism study, it was revealed that the higher concentration of surface Ce 3+ species/oxygen vacancies and the stronger Pt–CeO 2 interaction on Pt/CZO-750 could better facilitate the activation of oxygen to oxidize C 3 H 8 into reactive carbonate/carboxyl species and further promote the transformation of these intermediates into gaseous CO 2 . The Pt/CZO-750 catalyst can be a potential candidate for the catalytic removal of hydrocarbons from vehicle exhaust.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Deactivation by Potassium Accumulation on a Pt/TiO 2 Bifunctional Catalyst for Biomass Catalytic Fast Pyrolysis

Biomass-derived feedstocks bring significant challenges to the longevity of the catalysts used for their conversion, and alkali metals, for example, K, in the feedstock have been widely ascribed as one of the important factors causing catalyst deactivation. To address this challenge, it is critical to understand the mechanism of catalyst deactivation caused by K accumulation to guide the improvement of catalysts and processes and the development of catalyst regeneration strategies. In this work, we report a deep understanding of the impact of K on a bifunctional Pt/TiO 2 catalyst, which is an efficient catalyst for the ex situ catalytic fast pyrolysis of biomass. We simulated the K-poisoning of Pt/TiO 2 catalysts by purposely loading different amounts of K (100-6000 ppm by weight) on the catalysts. A series of characterization approaches, including scanning transmission electron microscopy, Fourier transform infrared spectroscopy, and chemical titration, were combined with a kinetic assessment of multiple probe reactions to elucidate the mechanism of Pt/TiO 2 deactivation by K accumulation. At low K loadings (<800 ppm), K preferentially poisons the strong Lewis acid sites, leading to significantly reduced activity for acid-catalyzed alcohol dehydration. However, acetone condensation is less sensitive to the poisoning of strong Lewis acid sites. Reactions that occur on Pt sites or at the metal-support interface, including alkene hydrogenation, m-cresol hydrodeoxygenation (HDO), and CO oxidation, were not impacted. At high K loadings (>800 ppm), K starts to accumulate on the Pt-TiO 2 interfacial area, poisoning the interfacial active sites for HDO and CO oxidation reactions. We further found that the poisoning of the Pt/TiO 2 catalyst by K is reversible, and water washing can successfully remove the accumulated K and recover the activities for both alcohol dehydration and m-cresol HDO reactions.

09 BIOMASS FUELS↗
DOE OSTI2023

Ceria Incorporation in Sinter-Resistant Platinum-Based Catalysts

Platinum group metals (PGM) are widely used for exhaust emission abatement. Sintering during the high-temperature emission control conditions decreases noble metal utilization efficiency. Efficient use of scarce noble metals requires sinter-resistant catalysts. Here we extend an approach to synthesize catalysts consisting of platinum nanoparticles encapsulated in a mixture of cerium and aluminum oxides (Pt@Al 2 O 3 -CeO 2 ). We tested the activity of this catalyst toward carbon monoxide, propene, and propane oxidation chosen as model oxidation reactions for emission control catalysts. Pt@Al 2 O 3 -CeO 2 catalysts demonstrated similar activity and stability upon aging as the comparison system without ceria, Pt@Al 2 O 3 , while maintaining small Pt nanoparticles and ceria crystallites. Additionally, we studied the influence of various thermal treatments on CO oxidation activity and determined that a steam treatment can activate low temperature CO oxidation activity of Pt@Al 2 O 3 -CeO 2 . STEM-EDS analysis revealed that thermal treatments led to the co-location of Pt and CeO 2 and temperature programmed-reduction analysis revealed that the steam treatment specifically enhanced CO oxidation activity through surface reduction of the CeO 2 . As a result, we demonstrate the versatility of this encapsulation approach to generate mixed metal-oxide supports with improved metal-support interactions without hindering nanoparticle stability.

77 NANOSCIENCE AND NANOTECHNOLOGY↗
DOE OSTI2024

Hydrogen Spillover Is Regulating Minority Rh 1 Active Sites on TiO 2 in Room-Temperature Ethylene Hydrogenation

The complicated dynamics of active sites on single-atom catalysts under reducing conditions limits their applications in hydrogenation reactions and mechanistic understanding. Herein, we report that on Rh 1 /TiO 2 , *H spillover during room-temperature ethylene hydrogenation hydroxylates and reduces TiO 2 , enhancing the intrinsic activity of Rh 1 by 9-fold. Spectroscopic and kinetic evidence suggests that the spillover of *H is suppressed by their facile reaction with C 2 H 4 , most of the spilled *H are nonreactive spectators, and >99% turnovers occur on a small subset (<20%) of exposed “active Rh 1 ”. Steady-state kinetics indicates competitive adsorption between H and C 2 H 4 , H 2 dissociation is the rate-determining step, and the apparent activation barrier (E a,app ) of the reaction is ~48 kJ/mol. The evolution of Rh 1 under H 2 was further tracked by spectroscopic and microscopic techniques at elevated temperatures. At 200 °C, more Rh 1 are exposed, but these Rh 1 are at least 5-fold less active than that of the “active Rh 1 ”. At 300 °C, Rh clusters derived from Rh 1 become the main active sites, shifting E a,app to 62 kJ/mol, characteristic of Rh nanoparticles. At ≥400 °C, larger and more active Rh particles in the strong metal–support interaction state are created. In conclusion, this work revealed the unexpected regulation effects of *H spillover on M 1 active sites under ambient conditions, differentiated the minority active M 1 sites, and demonstrated how the stability of M 1 under reducing atmospheres affects hydrogenation catalysis.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗