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Mao, Zhongtian

Publications and source records attributed to Mao, Zhongtian.

12 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2022

Trends and descriptors of heterogeneous hydroformylation activity and selectivity of RhM 3 (M = Fe, Co, Ni, Cu and Zn) catalysts

Here monometallic Rh/MCM-41 and bimetallic RhM 3 /MCM-41 (M = Fe, Co, Ni, Cu, or Zn) catalysts are synthesized and tested for vapor-phase ethylene hydroformylation reaction. Co is found to exhibit a significant promotion effect on the selectivity of hydroformylation to C 3 oxygenates, followed by Fe and Cu, with Ni and Zn showing a negative effect. DFT calculations reveal that the addition of a secondary metal can provide a new type of site, being able to selectively tune the binding energies of reaction intermediates, and thus lead to different catalytic performances.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2022

Catalytic Tandem CO 2 –Ethane Reactions and Hydroformylation for C3 Oxygenate Production

The strategy of tandem hydroformylation reaction for C3 oxygenate production from CO 2 and ethane represents an opportunity of simultaneously upgrading greenhouse gas CO 2 and the large-reserved shale gas into value-added liquid products. One of the challenges is how to tune and achieve the appropriate ethylene/CO/H 2 ratios for the downstream hydroformylation. Herein, the desired ethylene/CO/H 2 ratios were analyzed and identified by considering different combinations of main and side-reactions of CO 2 and ethane, based on which the PtSn 3 /γ-Al 2 O 3 catalyst was identified as promising to enable the catalytic tandem hydroformylation reaction. Further, the combined studies of reactor evaluation, in situ and ex situ characterization and theoretical calculations revealed that the Pt cluster/SnO x interfacial structures dominated the simultaneous dehydrogenation and dry reforming of ethane, thereby allowing the co-formation of ethylene, CO and H 2 that were subsequently converted to C3 oxygenates in the tandem hydroformylation reactor. The current work not only demonstrates the design principles of suitable catalysts for the tandem-reactor strategy, but also highlights the utilization of CO 2 and shale gas to produce value-added oxygenate products.

03 NATURAL GAS↗
DOE OSTI2022

Size-Dependent Adsorption and Adhesion Energetics of Ag Nanoparticles on Graphene Films on Ni(111) by Calorimetry

Interest in the use of carbon supports for late transition metal nanoparticle catalysts has expanded rapidly due to the increasing importance of electrocatalysts for clean energy and environmental technologies and the use and storage of renewable electricity. Compared to oxide supports, almost nothing is known about the effect of metal nanoparticle size on the energies of the metal atoms within carbon-supported nanoparticles, yet these energies are crucial for understanding their surface reactivity and sintering kinetics. Here, the growth morphology and adsorption energetics of vapor-deposited Ag onto clean graphene/Ni(111) surfaces have been studied using a combination of single-crystal adsorption calorimetry (SCAC) and He + low-energy ion scattering (LEIS). The differential heat of Ag adsorption is 207 kJ/mol for making ~30 atom Ag particles on graphene terraces at 100 K and 16 kJ/mol higher for making ~9 atom Ag clusters at defect sites at the same temperature. The heat of adsorption increases rapidly with Ag coverage as 3D Ag nanoparticles nucleate and grow in size, asymptotically reaching within 5 kJ/mol of the bulk Ag sublimation enthalpy (285 kJ/mol) by 2 ML. The heats of adsorption and Ag nanoparticle densities from LEIS (~10 16 /m 2 ) were combined to provide the Ag/graphene adhesion energy (E adh = 1.8 J/m 2 in the large-particle limit) and the Ag chemical potential (μ) versus effective particle diameter (D). The Ag chemical potential was well-fitted by μ(D) = (3γ v /M – E adh )(1 + (1.5 nm)/D)(2V m /D), where γ v /M is the surface energy of bulk Ag and V m is its molar volume. The same equation is known to fit similar data for late transition metals on clean surfaces of metal oxide single crystals. The adhesion energy of Ag measured here on graphene falls within the wide range measured for Ag on those oxide surfaces and is almost as large as on the oxide that binds Ag particles most strongly, namely CeO 2 (111), which is well-known to be very effective at resisting catalyst deactivation by metal sintering. Furthermore, these results imply that carbon supports will be effective at resisting sintering and that Ag particles smaller than 6 nm on graphene will bind small adsorbed reaction intermediates more weakly than supports with weaker adhesion to Ag, like MgO(100).

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

Comparison of Heterogeneous Hydroformylation of Ethylene and Propylene over RhCo 3 /MCM-41 Catalysts

The reaction mechanisms of heterogenous hydroformylation of ethylene and propylene were compared at 413-453 K using RhCo 3 /MCM-41 as catalysts. The reaction rates of propylene for both hydroformylation and the undesired side reaction of hydrogenation were found to be about one order of magnitude lower than those for ethylene in flow reactor studies. The difference in the kinetic behavior between ethylene and propylene was investigated by measuring the reaction orders and apparent activation energies, and these macro-kinetic observables were analyzed using the degree of rate control (DRC) method. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) experiments were performed to characterize the surface intermediates formed during the reactions. When the reactant was changed from ethylene to propylene, the IR peak corresponding to adsorbed CO exhibited a significant increase, while the IR peaks of alkyl group decreased in magnitude. Combined with the DRIFTS results, DRC analysis indicates that the first step of olefin hydroformylation, the formation of an alkyl group on the catalyst surface, plays a key role in the difference between ethylene and propylene. This step is kinetically non-relevant when ethylene is the reactant, but it is one of the rate-controlling steps for propylene. Furthermore, the low concentration of the adsorbed propyl group, which is a common intermediate shared by both hydroformylation and hydrogenation of propylene, decreases the rates of both reaction pathways as compared to ethylene.

10 SYNTHETIC FUELS↗
DOE OSTI2021

Analysis and prediction of reaction kinetics using the degree of rate control

“Degree of rate control” (DRC) analysis provides a quantitative approach for analysing the kinetics of multi-step reaction mechanisms that has been widely applied to both heterogeneous and homogeneous catalysis research, as well as electrocatalysis. The DRC of any given transition state or intermediate is defined as a partial derivative such that it approximately equals the fractional increase in net rate to the product of interest per differential decrease in its standard-state free energy for that species (÷RT), holding constant the standard-state free energies of all other transition states and intermediates. Even very complex mechanisms usually have only a few species with non-zero DRCs and are thus the species whose interactions with the catalyst most strongly affect the net rate. These key DRC values thus offer a simple and intuitive route to optimize catalyst materials, especially with the assistance of computational methods like density functional theory (DFT). These high-DRC species are also the species whose energetics must be most accurately measured or calculated to achieve an accurate kinetic model for any reaction mechanism. In simple cases with a single “rate-determining step”, the DRC for its transition state (TS) is + 1. Catalyst-bound intermediates, on the other hand, often have negative DRCs equal to a small integer times their fractional occupancy of catalyst sites. The apparent activation energy equals a weighted average of the standard-state enthalpies (relative to reactants) of all the species (intermediates, transition states and products) in the reaction mechanism, each weighted by its DRC (+RT). It has been shown that the apparent transfer coefficient in electrocatalysis, an inverted form of the Tafel slope, is a weighted average of the number of electrons transferred to generate each intermediate or product species in the mechanism, weighted again by the DRC. Quantitative analysis of kinetic isotope effects (KIEs, or the ratio of net rates for different reactant isotopes) in complex mechanisms has shown that the logarithm of the KIE equals the weighted average over all species in the mechanism of the difference between the two isotopes in their standard-state free energies (÷RT), again weighted by the DRC. The reaction orders with respect to fluid-phase concentrations of reactants, products and intermediates have also been proven to be directly related to DRCs. Thus, there are numerous experimental observables which equate to short linear combinations of DRCs, so that combinations of experimental measurements might provide access to DRC values. Since its invention for transition states in 1994 and its generalization to include intermediates in 2009, the DRC has thus far mainly been calculated for microkinetic models based either entirely on DFT or on DFT with the key energies (i.e., those for high-DRC species) being fine-tuned to match experiments. The relationships summarized in this work provide new opportunities for using experiments earlier in the development and optimization of microkinetic models that require input from computational catalysis.

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

Predicting a Key Catalyst-Performance Descriptor for Supported Metal Nanoparticles: Metal Chemical Potential

The chemical potential of metal atoms in supported nanoparticles provides a convenient descriptor of their performance as heterogeneous catalysts that captures the effects of particle size, support and alloying. Based on microcalorimetric measurements, the chemical potential is shown in this work to be predictable as a function of monometallic particle diameter and the adhesion energy of the particle to the support, and, for oxide supports, this adhesion energy correlates predictably with metal oxophilicity. These correlations provide predictions of metal chemical potential that can enable catalyst design. They also suggest an improvement in the Gibbs-Thomson relation for free-standing nanoparticles, whether metals or molecular solids.

metal/oxide interface↗
DOE OSTI2021

Energetics of Ag Adsorption on and Adhesion to Rutile TiO 2 (100) Studied by Microcalorimetry

The adsorption and adhesion energies of vapor-deposited Ag on rutile TiO 2 (100) films have been measured using single crystal adsorption calorimetry (SCAC) and He + low energy ion scattering spectroscopy (LEIS). Ag grows as three-dimensional (3D) nanoparticles at 300 and 100 K. The saturation particle density is 8 × 10 16 particles/m 2 at 300 K and 2.5 × 10 17 particles/m 2 at 100 K. The differential heat of adsorption starts low, increases with Ag coverage, and finally approaches the sublimation enthalpy of Ag at both 300 and 100 K. At 300 K, the differential heat of adsorption starts from 208 kJ/mol and rises rapidly to 265 kJ/mol by 1 ML. At 100 K, Ag grows nanoparticles with smaller particle size on the terraces, while at 300 K, the Ag nanoparticles have bigger particle size and grow on step edges with stronger binding strength to the nanoparticles. Thus, the heat of Ag adsorption at 100 K starts at 141 kJ/mol (near the monomer size limit on terrace sites) and remains lower than that at 300 K until 1.5 ML. Here, the adhesion energy of Ag(solid) to rutile TiO 2 (100) is found to be large (~2.44 J/m 2 ), supporting a trend of decreasing adhesion energy with the enthalpy of oxide reduction.

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

Energetics and Structure of Nickel Atoms and Nanoparticles on MgO(100)

The growth morphology and interfacial energetics of vapor deposited Ni on the MgO(100) surface at 300 and 100 K have been studied using single crystal adsorption calorimetry (SCAC), He + low-energy ion scattering spectroscopy (LEIS), X-ray photoelectron spectroscopy (XPS), and low-energy electron diffraction (LEED). At 300 K, the Ni atoms grow as three-dimensional nanoparticles with a saturation number density of 5 × 10 16 particles/m 2 . The differential heat of adsorption at 300 K increases rapidly with coverage, from 276 (initially) to 311 kJ/mol by 0.4 ML. Thereafter, it slowly increases asymptotically to the sublimation enthalpy of bulk Ni (430 kJ/ml) by 9 ML. The Ni 2p 3/2 XPS peak binding energy at 300 K is initially (i.e., at 0.16 ML) 1.4 eV higher than that for bulk Ni(solid), but it decreases to that value at high coverage. The Ni atoms form a metastable phase at 300 K when in nanoparticles with diameter <2.5 nm, and the adhesion energy of such Ni nanoparticles to MgO(100) was found to be 3.05 J/m 2 . At 100 K, the Ni atoms form single adatoms and then 0.17 nm thick 2D islands at low coverage with fewer Ni-Ni bonds compared to the Ni nanoparticles formed at 300 K. Thus, the initial heat (i.e., for the first ~0.03 ML) is 148 kJ/mol at 100 K, 128 kJ/mol lower than at 300 K, and remains lower for the 2D islands. With increasing coverage at 100 K, the tiny 2D Ni islands grow in size to cover nearly the entire surface before thickening. Lastly, the XPS Ni 2p 3/2 peak binding energy for 0.21 ML Ni on MgO(100) at 100 K is 2.2 eV higher than that for bulk Ni(solid), suggesting charge transfer from Ni to MgO(100) and formation of Ni 2+ at very low coverage.

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

Ni Nanoparticles on CeO 2 (111): Energetics, Electron Transfer, and Structure by Ni Adsorption Calorimetry, Spectroscopies, and Density Functional Theory

The morphology, interfacial bonding energetics and charge transfer of Ni clusters and nanoparticles on slightly-reduced CeO 2-x (111) surfaces at 100 to 300 K have been studied using single crystal adsorption calorimetry (SCAC), low-energy ion scattering spectroscopy (LEIS), Xray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED) and density functional theory (DFT). The initial heat of adsorption of Ni vapor decreased with the extent of pre-reduction (x) of the CeO 2-x (111), showing that stoichiometric ceria adsorbs Ni more strongly than oxygen vacancies. On CeO 1.95 (111) at 300 K, the heat dropped quickly with coverage in the first 0.1 ML, attributed to nucleation of Ni clusters on stoichiometric steps, followed by the Ni particles spreading onto less favorable terrace sites. At 100 K, the clusters nucleate on terraces due to slower diffusion. Adsorbed Ni monomers are in the +2 oxidation state, and they bind by ~45 kJ/mol more strongly to step sites than terraces. The measured heat of adsorption versus average particle size on terraces is favorably compared to DFT calculations. The Ce 3d XPS lineshape showed an increase in Ce 3+ /Ce 4+ ratio with Ni coverage, providing the number of electrons donated to the ceria per Ni atom. The charge transferred per Ni is initially large but strongly decreases with increasing cluster size for both experiments and DFT, and shows large differences between clusters at steps versus terraces. This charge is localized on the interfacial Ni and Ce atoms in their atomic layers closest to the interface. This knowledge is crucial to understanding the nature of the active sites on the surface of Ni-CeO 2 catalysts for which metal-oxide interactions play a very important role in the activation of O-H and C-H bonds. In conclusion, the changes in these interactions with Ni particle size (metal loading) and the extent of reduction of the ceria help to explain how previously reported catalytic activity and selectivity change with these same structural details.

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

Kinetic Isotope Effects: Interpretation and Prediction Using Degrees of Rate Control

Kinetic isotope effects (KIEs) have been used for decades in catalysis research as a tool for clarifying reaction mechanisms. Significant primary kinetic isotope effects have usually been interpreted as being a result of isotope substitution at a site of bond breaking (or forming) in the rate-determining step in the reaction mechanism. However, quantitative analysis of the magnitude of the KIE in complex multistep reaction mechanisms is seldom reported. We prove here that the logarithm of the rate ratio for two isotopes is the weighted average over all species in the mechanism of their standard-state free energy difference between the two isotopes, divided by RT. The weighting factor is the degree of rate control (DRC) for that species (e.g., transition state, intermediate, reactant) when the rate is measured separately for each isotope. It is instead the degree of selectivity ratio control (DSRC) when the KIE is measured as the product selectivity in a parallel competition between two isotopes within the same reactant molecule. Since only a few species have non-zero DRCs (or DSRCs) for most reactions, only these few contribute to this weighted average and the KIE. We show that this provides a simple way to interpret and quantitatively predict kinetic isotope effects that is powerful in the insights it provides, allowing one to evaluate directly which species contribute most to the KIE. By applying it to H/D KIEs in several example mechanisms, we further show that the traditional way of interpreting KIEs that focuses only on the rate-determining step can easily lead to misunderstanding of KIE and the reaction mechanism. Finally, this highlights the importance to consider the effect of isotope substitution on the energies of all species with large DRCs (i.e, those whose energies are kinetically relevant). This method also offers opportunities for quantitative validation of mechanism-based microkinetic models.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗