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DOE OSTI2020

Excited Carrier Dynamics in a Dye-Sensitized Niobate Nanosheet Photocatalyst for Visible-Light Hydrogen Evolution

Dye-sensitized photocatalysts that consist of a light-absorbing dye and a wide gap oxide semiconductor have been studied extensively as components of solar energy conversion systems. Although surface modification by a metal and/or metal oxide has a significant impact on the photocatalytic efficiency, the mechanism by which these modifications increase the activity has not been fully understood. Here a dye-sensitized H 2 evolution system was constructed by using Pt-intercalated HCa 2 Nb 3 O 10 nanosheets, Ru(II) complex photosensitizers ([Ru(4,4’-(CH 3 ) 2- bpy) 2 (4,4’-(PO 3 H 2 ) 2- bpy)] 2+ and [Ru(4,4’-(CH 3 ) 2- bpy) 2 (4,4’-(CH 2 PO 3 H 2 ) 2- bpy)] 2+ , abbreviated as RuP 2+ and RuCP 2+ ; bpy = 2,2'-bipyridine), and amorphous Al 2 O 3 as building blocks. In the presence of iodide as the electron donor, the H 2 evolution rate from Pt/HCa 2 Nb 3 O 10 nanosheets sensitized by RuP 2+ was increased by modification of the nanosheets with Al 2 O 3 . On the other hand, Al 2 O 3 had a negative impact on the H 2 evolution rate when RuCP 2+ was employed. These hybrid materials were studied by transient diffuse reflectance spectroscopy and steady-state emission spectroscopy. A detailed analysis of the transient absorption profiles of the adsorbed Ru(II) complexes revealed that there are at least three states of the complexes on the nanosheet surface. The transient bleaching of the ground state absorbance had different lifetime components ranging from a few μs to several hundred μs, which mainly reflect back electron transfer rates from HCa 2 Nb 3 O 10 to the oxidized Ru(II) complexes. Here, the Al 2 O 3 modifier could inhibit not only the back electron transfer events but also electron injection from the excited state photosensitizer. Interestingly, the negative effect of Al 2 O 3 on the electron injection rate was negligible in the case of RuP 2+ , which also had a higher H 2 evolution rate. This work highlights that suppressing fast back electron transfer from Pt/HCa 2 Nb 3 O 10 to the oxidized Ru(II) complex, which occurs on a time scale of a few μs, and maximizing the electron injection efficiency are both necessary for improving dye-sensitized H 2 evolution.

14 SOLAR ENERGY↗
DOE OSTI2021

Role of Hydrogen-bonded Bimolecular Formic Acid–Formate Complexes for Formic Acid Decomposition on Copper: A Combined First-Principles and Microkinetic Modeling Study

Hydrogen bonding interactions alter the nanoscale reaction mechanisms of many chemistries. Yet, it remains unclear how they affect heterogeneously catalyzed decomposition of formic acid (FA), a reaction of intense interest since FA is a promising hydrogen carrier. In this work, we elucidate how hydrogen bonding affects the reaction mechanisms for FA decomposition on Cu(111) by combining first-principles density functional theory calculations to calculate reaction energetics, Latin-hypercube sampling to elucidate stable high-coverage adsorbate configurations, and coverage self-consistent mean-field microkinetic models to predict reaction kinetics. We demonstrate that hydrogen-bonded complexes of FA with formate (bimolecular FA–HCOO complexes) can play a dominant role in FA decomposition. Specifically, our first-principles calculations show that hydrogen bonding of FA with HCOO may stabilize the crucial monodentate HCOO intermediate and the transition states for HCOO decomposition, especially at low coverages. We predict that, depending on the reaction conditions, 40–80% of the reaction flux goes through pathways involving the bimolecular FA–HCOO complexes. Additionally, the active site for FA decomposition on Cu(111) involves a high coverage (~0.4 monolayers (ML)) of these complexes, which unexpectedly stabilize intermediates and transition states via van der Waals interactions. Our work provides molecular insights consistent with previous experimental observations on supported Cu/Al 2 O 3 catalysts. This paves the way toward the development of novel catalysts for FA decomposition as well as for other industrially important chemistries with intermediates capable of hydrogen bonding, such as ammonia electrooxidation and CO 2 hydrogenation.

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

Mechanistic Understanding of Methane Combustion over Ni/CeO 2 : A Combined Experimental and Theoretical Approach

Catalytic oxidation of methane (CH 4 ) over nonprecious Ni/CeO 2 catalysts has received a lot of attention due to the large natural gas reserves found in North America and the prohibitive cost of palladium-based catalysts, commonly used for CH 4 oxidation. However, the catalytic mechanism of CH 4 oxidation over Ni/CeO 2 still remains unclear. Moreover, the parameters affecting the reaction rates, the interaction between nickel and CeO 2 , and the reaction intermediates are still not well understood. In this study, kinetic model fitting, CH 4 temperature-programmed reduction-mass spectroscopy (CH 4 TPR-MS), in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and density functional theory (DFT) calculations were combined to elucidate the mechanism of complete oxidation of CH 4 over Ni/CeO 2 . CH 4 TPR-MS showed that the complete oxidation of CH 4 over Ni/CeO 2 requires 55–120 °C lower compared to bare CeO 2 or Ni/quartz sand; complete oxidation of CH 4 took place when the surface oxygen species were abundant, while partial oxidation products (CO, H 2 ) were formed when the oxygen species were depleted. In situ DRIFTS showed that CH 3 , CH 2 , CO, and CO 2 were formed after CH 4 activation over Ni/CeO 2 , while CH 3 O species were not observed. Combining those findings with kinetic model fitting, a redox Mars–van Krevelen (MvK) mechanism showed the best description of the experimental observations. The MvK mechanism involves the reaction of dissociated oxygen species with gas-phase CH 4 while water inhibits the reaction rate by adsorbing on the oxidized sites. Moreover, CH 4 activation leads to the reduction of the active sites and oxygen vacancy formation followed by reoxidation of the active sites by gas-phase O 2 . A CH 4 oxidation reaction pathway over Ni/CeO 2 is proposed by DFT calculations. In summary, the findings shown here suggest that CH 4 oxidation over Ni/CeO 2 follows a redox MvK mechanism and provides guidance for the rational design of non-precious-metal catalysts for CH 4 oxidation reactions.

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

Entropic Control of HD Exchange Rates over Dilute Pd-in-Au Alloy Nanoparticle Catalysts

Dilute Pd-in-Au alloy catalysts are promising materials for selective hydrogenation catalysis. Extensive surface science studies have contributed mechanistic insight on the energetic aspect of hydrogen dissociation, migration and recombination on dilute alloy systems. Yet, translating these fundamental concepts to the kinetics and free energy of hydrogen dissociation on nanoparticle catalysts operating at ambient pressures and temperatures remains challenging. Here, the effect of the Pd concentration and Pd ensemble size on the catalytic activity, apparent activation energy and rate limiting process is addressed by combining experiment and theory. Experiments in a flow reactor show that a compositional change from 4 to 8 atm% Pd of the Pd-in-Au alloy catalyst leads to strong increase in activity, even exceeding the activity per Pd atom of monometallic Pd under the same conditions, albeit with an increase in apparent activation energy. First-principles calculations show that the rate and apparent activation enthalpy for HD exchange increase when increasing the Pd ensemble size from single Pd atoms to Pd trimers in a Au surface, suggesting that the ensemble size distribution shifts from mainly single Pd atoms on the 4 atm% Pd alloy to larger Pd ensembles of at least three atoms for the 8 atm% Pd/Au catalyst. The DFT studies also indicated that the rate-controlling process is different: H 2 (D 2 ) dissociation determines the rate for single atoms whereas recombination of adsorbed H and D determines the rate on Pd trimers. Both experiment and theory suggest that the increased reaction rate with increasing Pd content and ensemble size stems from an entropic driving force. Lastly, our results support hydrogen migration between Pd sites via Au and indicate that the dilute alloy design prevents the formation of subsurface hydrogen, which is crucial in achieving high selectivity in hydrogenation catalysis.

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

Structure-Activity Relationships of Hydrothermally Aged Titania-Supported Vanadium-Tungsten Oxide Catalysts for SCR of NOx Emissions with NH3

Supported V2O5-WO3/TiO2 materials employed as selective catalytic reduction (SCR) catalysts for NOx emission control from power plants. Fresh SCR catalysts usually receive exposure to harsh treatments in industry to accelerate catalyst activation (calcination in air at 650oC) and catalyst aging (hydrothermal aging at 650oC) in a way that represents various points in the catalyst/product lifetime. The present study investigates the catalyst structural and chemical changes occurring during such harsh treatments. Three series of supported V2O5-WO3/TiO2 catalysts were prepared by incipient-wetness impregnation of aqueous ammonium metavanadate and metatungstate precursors. The catalysts were subsequently dried and calcined at 550oC in O2, 650oC in O2, and hydrothermal conditions (10% O2, 8% H2O, 7% CO2 and 75% N2) at 650oC. The resulting catalysts were physically characterized by numerous techniques (in situ Raman, in situ IR, in situ High Field-High Spinning solid-state 51V MAS NMR, in situ EPR, XRD, BET surface area and ICP) and chemically probed with adsorbed ammonia, SCR-TPSR, and the SCR reaction. The surface WOx sites on the TiO2 support behave as a textural promoter that stabilizes the TiO2 (anatase) phase from sintering and transforming to the undesirable crystalline TiO2 (rutile) phase that can lead to formation of a Ti1-xVxO2 (rutile) solid solution with reduced V4+ cations (~7-15%). The surface VOx sites are mostly oligomerized as surface V+5Ox sites (~50-85% oligomers) and the extent of oligomerization tends to increase with surface WOx coverage and calcination temperature. A major difference between the calcined and hydrothermally treated catalysts was the low concentration of surface NH3* species on Lewis acid sites for the hydrothermally treated catalysts, yet the SCR activity was almost comparable for both catalysts. This finding suggests that surface NH4+*, primarily associated with the surface VOx sites, are able to efficiently perform the SCR reaction. Given that multiple catalyst parameters were simultaneously varying during these treatments, it was difficult to correlate the SCR activity with any single catalyst parameter. A correlation, however, was found between the SCR TOF/activity and the sum of the surface NH3* and NH4+* species, which is dominated by the surface NH4+* species.

SCR, NO, NH3, catalyst, V2O5, WO3, TiO2, promotion↗
DOE OSTI2021

Temperature-Dependent Communication between Pt/Al 2 O 3 Catalysts and Anatase TiO 2 Dilutant: the Effects of Metal Migration and Carbon Transfer on the Reverse Water–Gas Shift Reaction

In heterogeneous catalysis, unexpected effects from the supposedly inert reactor dilutant are not rare, but related understanding is lacking and inconsistent. Here we report investigations of the impacts of the anatase TiO 2 dilutant on the reverse water-gas shift (rWGS) reaction over Pt/Al 2 O 3 catalysts. Combining detailed kinetic data with microscopic and spectroscopic results, we demonstrate that the catalyst-dilutant communication depends on temperature. At high temperature (400 °C), Pt migrate from Al 2 O 3 to TiO 2 , resulting in higher dispersion and activity, without changing reaction mechanisms. The Pt migration is general to catalysts of different Pt nuclearity and various oxide dilutants. In contrast, at low temperature (250 °C), carboxylic acids (in particular acetic acid) present in the ambient air and adsorbed on TiO 2 are transferred onto Al 2 O 3 in close contact, effectively blocking the formate rWGS pathway. As a result, the rWGS can only proceed through the carboxyl pathway, and hence is significantly slower. The acetate transfer affects catalysts of different Pt nuclearity and support, but is unique to anatase TiO 2 dilutant. As the acetate layer is slowly removed under H 2 or the rWGS stream, the activity recovers. This work elucidates the complicated communication between catalysts and dilutants, which has general implications in heterogeneous catalysis, and resolves inconsistency in related reports in the literature. Finally, the impacts that anatase TiO 2 dilution has on the rWGS also unveil mechanistic understanding that further confirms the two co-existing rWGS pathways.

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

Structural and Catalytic Properties of Isolated Pt 2+ Sites in Platinum Phosphide (PtP 2 )

This article describes the synthesis and catalytic properties of supported, 2-3 nm platinum phosphide (PtP 2 ) nanoparticles (NPs). Depending on the P loading, two PtP 2 structures are formed, that is, a PtP 2 surface on a (metallic) Pt core (Pt@PtP 2 ) and single-phase PtP 2 NPs. The structures were determined using extended X-ray absorption fine structure , in situ synchrotron X-ray diffraction, and scanning transmission electron microscopy. In PtP 2 NPs, Pt 2+ ions are geometrically isolated by P 2 2- ions, at a Pt-Pt distance of 4.02 Å, which is much longer than 2.78 Å in (metallic) Pt NPs. The oxidation state of Pt in PtP 2 NPs was determined by in situ X-ray absorption near-edge structure and in situ X-ray photoelectron spectroscopy and was found to be consistent with Pt 2+ ions even after treatment in H 2 at 550 °C. Unlike Pt NPs, which are highly active for propylene hydrogenation at room temperature, PtP 2 NPs are not active below about 150 °C, suggesting the absence of metallic surface Pt. In contrast to metallic Pt, which is poorly selective for acetylene hydrogenation, PtP 2 NPs display high selectivity toward ethylene. PtP 2 , also has high olefin selectivity for propane dehydrogenation, although the rate per g Pt is about 7 times lower than that of metallic Pt NPs of the same size. In situ resonant inelastic X-ray scattering spectroscopy shows that the energy of the filled Pt 5d valence orbitals is 1.5 eV lower than that of metallic Pt, which leads to weaker adsorbate binding consistent with its catalytic properties. Here, a H 2 -stable Pt 2+ site suggests different catalytic applications for these catalysts as compared to Pt NPs.

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

Oxidation Dynamics of Supported Catalytic Cu Clusters: Coupling to Fluxionality

Copper oxide nanoclusters have a wide range of catalytic applications, such as the selective oxidation of hydrocarbons. O 2 binding to the catalyst, activation, and release upon reagent oxidation are key events in these catalytic chemistries. Furthermore, these events are expected to be accompanied by significant structural changes of the Cu clusters, because O atoms integrate into the cluster, rather than bind to its surface. Topping the complexity of the problem, partially oxidized Cu clusters are known to exhibit strong fluxionality and feature diverse and interconverting structures and oxygen contents in conditions of oxidative dehydrogenation (ODH). Hence, a significant dynamic coupling between the “hot” O 2 molecule impacting the cluster at reaction temperatures and the cluster fluxionality can be expected. In this work, we focus on the dynamics of dioxygen integration into a partially oxidized Cu cluster supported on hydroxylated amorphous alumina–a system recently reported to be an exceptionally selective catalyst for cyclohexane ODH with very little CO produced, whose mechanistic underpinnings are of utmost interest. The statistics over a swarm of adsorption and scattering trajectories where O 2 hits various sites on the cluster at reaction temperature shows that the O 2 binding does not only follow the minimal energy paths. O 2 also rarely integrates into the cluster in a single step and instead first binds to a single Cu atom via either an η 1 -O 2 or an η 2 -O 2 mode. Surprisingly, this step often has a higher barrier than the subsequent O 2 integration and dissociation, which in turn take multiple steps and complete the oxidation process. Dynamic trajectories starting from the key transition state of integration of the adsorbed O 2 can also lead to different intermediate structures during or right after the dissociation, due to the energy released from the transition state and the thermal intracluster effects. From these activated O 2 chemisorbed structures, O 2 dissociation occurs with moderate barriers (~0.5 eV), producing multiple final oxidized Cu 4 O 4 states. Hence, a diversity of reaction profiles for the attack of supported Cu cluster by O 2 emerges due to the dynamic effects, with implications for mechanisms, kinetic models, and catalyst design principles.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
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 OSTI2022

Disordered, Sub-Nanometer Ru Structures on CeO 2 are Highly Efficient and Selective Catalysts in Polymer Upcycling by Hydrogenolysis

We report non-degradable polyolefin plastics pose severe environmental threats, and thus demand efficient upcycling technologies. In this work, we discovered that low-loading (= 0.25 wt%) Ru/CeO 2 exhibits remarkable catalytic performance in the hydrogenolysis of polypropylene (PP), polyethylene (PE), and n-C 16 H 34 that is superior to high-loading (= 0.5 wt%) Ru/CeO 2 . They possess high PP conversion efficiency (7-fold increase over current literature reports), low selectivity towards undesired CH 4 , and good isomerization ability. In the low-loading range, the intrinsic activity of Ru in PP hydrogenolysis increases as the particle size decreases, opposite of the trend in the high-loading range. Detailed characterization revealed that the abrupt changes in catalytic behaviors coincide with Ru species transitioning from well-defined to highly disordered structures in the low-loading domain. The disordered Ru species were shown to be sub-nanometer in size and cationic. Mechanistically, the regioselectivity and the rate dependence on hydrogen pressure of C-C bond cleavage are different on low- and high-loading Ru/CeO 2 , both explained by the higher coverage of adsorbed hydrogen (*H) on low-loading Ru/CeO 2 . This work uncovers the remarkable catalytic performance of highly disordered, sub-nanometer, cationic Ru species in polyolefin hydrogenolysis, opening immense opportunities to develop effective, selective, and versatile catalysts for plastic upcycling.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Kinetic Model for the Reduction of Cu II Sites by NO + NH 3 and Reoxidation of NH 3 -Solvated Cu I Sites by O 2 and NO in Cu-SSZ-13

In this work, a kinetic model is developed for the reduction of Cu II sites by NO + NH 3 and the reoxidation of NH 3 -solvated Cu I sites by O 2 and NO in Cu-SSZ-13. Fourier transform infrared (FTIR) spectroscopy and spatially resolved capillary inlet mass spectrometry (SpaciMS) measurements during transient reactor experiments are utilized to identify the rate parameters associated with NO + NH 3 RHC (reduction half-cycle), proposed to occur via two distinct pathways involving adsorbed NH 3 and gas-phase NH 3 . The resulting NO + NH 3 RHC model is validated using spatiotemporal N 2 measurements covering a wide range of temperatures (200–450 °C) and space velocities (53,000–640,000 h –1 ). N 2 O formation is observed and modeled during NO + NH 3 RHC, with quantitative validation under standard selective catalytic reduction (SCR) conditions. Experimentally measured enthalpic and entropic changes associated with O 2 adsorption on NH 3 -solvated Cu I (ZCu(NH 3 ) 2 ) complexes [ Kamasamudram, K. Catal. Today 2010 , 151 (3–4), 212-222], along with activation energies estimated computationally for the intercage diffusion of ZCu(NH 3 ) 2 complexes [ Paolucci, C. Science 2017 , 357 (6 354), 898-903], are incorporated into a mean field kinetic model for the low-temperature oxidation half-cycle (OHC). Significant NH 3 release is observed during the isothermal oxidation of Cu I sites, attributed to desorption of NH 3 ligands from NH 3 -solvated Cu II dimers (Z 2 Cu 2 (NH 3 ) 4 O 2 ). Reduction of these dimeric complexes leads to the consumption of one NO/Cu II , contradicting the expected reduction stoichiometry. Inclusion of a global Arrhenius rate for the NO titration of Z 2 Cu 2 (NH 3 ) 4 O 2 complexes provides accurate representations of standard SCR on reduced and oxidized catalysts, predicting transient NO and NH 3 consumption between 150 and 250 °C as a function of hydrothermal aging. Deactivation of low-temperature standard SCR by NH 3 is observed at high NH 3 pressures, modeled via the formation of superoxo amino (ZCu(NH 3 ) 3 OO*) complexes during NH 3 titration of Z 2 Cu 2 (NH 3 ) 4 O 2 complexes [ Negri, C. J. Am. Chem. Soc. 2020 , 142 (37), 15884-15896]. The redox kinetic model presented here provides a foundational description of active site redox during low-temperature standard SCR, combining the recent kinetic, spectroscopic, and computational findings on the mechanism of standard SCR over Cu-SSZ-13.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2022

Swinging Hydrogen Evolution to Nitrate Reduction Activity in Molybdenum Carbide by Ruthenium Doping

A common challenge for electrochemical ammonia synthesis in an aqueous phase is the consumption of Faradaic charge by the competing hydrogen evolution reaction (HER), which reduces the Faradaic efficiency for the desired conversion, i.e., the nitrate reduction reaction (NO 3 RR) to ammonium. This problem is particularly severe when a single-phase catalyst is operated at high current limits, thus a cocatalyst system that works synergistically for hydrogen acquisition and deoxygenation is needed to promote NO 3 RR over HER. For this study, we select a well-known HER catalyst Mo 2 C and investigate how metal doping can switch its kinetics from HER-dominated to NO 3 RR-dominated pathways. At 3.8 wt % Ru doping of Mo 2 C, a 75% single pass conversion of nitrate (0.1 M) to ammonium in a 16 cm 2 flow electrolyzer was achieved, corresponding to an ammonium yield rate of 9.07 mmol h –1 at a full cell voltage of 2 V. As confirmed by DFT calculations and kinetic isotope experiments, ruthenium dopants in the matrix serve as the sink point for adsorbed hydrogen during NO 3 RR to promote the cooperative deoxygenation of *NO 3 and *NO 2 on the Ru–Mo cocatalytic site. Our study suggests that optimizing hydrogen acquisition and deoxygenation reactions in cocatalytic systems is an effective strategy for electrochemical synthesis.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

A Coverage Self-Consistent Microkinetic Model for Vapor-Phase Formic Acid Decomposition over Pd/C Catalysts

An iterative approach utilizing density functional theory (DFT, PW91-GGA)-informed mean-field microkinetic models and reaction kinetics experiments is used to determine the reaction mechanism and the active site for formic acid (HCOOH, FA) decomposition over a Pd/C catalyst. Models parametrized using DFT energetics on clean Pd(100) and Pd(111) required large corrections to the DFT energetics for capturing our experimental data. Further, both Pd(111) and Pd(100) models predicted a high coverage of adsorbed CO (CO*), inconsistent with the assumption of a clean surface at which the rate parameters for these models were calculated. To better represent the active site under reaction conditions and explicitly account for the presence of CO*, subsequent microkinetic models were formulated using DFT energetics that were calculated on partially (5/9 ML) CO*-covered Pd (111) and (100) facets. Upon parameter adjustment, the resultant 5/9 ML CO*-covered Pd(100) model, although consistent in terms of CO* coverage, was unable to capture the dehydration path measured in the experiments and was, therefore, deemed not to offer an accurate representation of the active site for FA decomposition over Pd/C. In contrast, a partially CO*-covered Pd(111) model was better at representing the catalytic active site, as in addition to being consistent in terms of CO* coverages, it required small adjustments of the DFT parameters to accurately capture the experimental data set (both dehydrogenation and dehydration). Our results suggest that the reaction occurs via the spectroscopically elusive carboxyl (COOH*) intermediate and that spectator CO*-assisted decomposition pathways play an important role under typical experimental conditions. In addition, our study highlights the importance of striving for coverage self-consistent microkinetic models and for including spectator-assisted mechanisms in order to develop an improved picture of the active site under reaction conditions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Insights into the Oxygen Evolution Reaction on Graphene-Based Single-Atom Catalysts from First-Principles-Informed Microkinetic Modeling

Single-atom transition metals embedded in nitrogen-doped graphene have emerged as promising electrocatalysts due to their high activity and low material cost. These materials have been shown to catalyze a variety of electrochemical reactions, but their active sites under reaction conditions remain poorly understood. Using first-principles density functional theory calculations, we develop a pH-dependent microkinetic model to evaluate the relative performance of transition metal catalysts embedded in fourfold N-substituted double carbon vacancies in graphene for the oxygen evolution reaction. We find that reaction pathways involving intermediates co-adsorbed on the metal site are preferred on all transition metals. These pathways lead to enhancements in catalytic activity and broaden the activity peak when compared with purely thermodynamics-based predictions. Furthermore, these findings demonstrate the importance of investigating reaction pathways on graphene-based catalysts and other two-dimensional (2D) materials that involve metal active centers decorated by spectator intermediate species.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Modeling the Effect of Surface Platinum–Tin Alloys on Propane Dehydrogenation on Platinum–Tin Catalysts

Uncertainty analysis, reported experimental literature data, and density functional theory were synthesized to model the effect of surface tin coverage on platinum-based catalysts for nonoxidative propane dehydrogenation to propylene. Here, this study tests four different platinum–tin skin surface models as potential catalytic sites, Pt 3 Sn/Pt(100), PtSn/Pt(100), Pt 3 Sn/Pt(111), and Pt 2 Sn/Pt(211), and compares them to the corresponding pure Pt surface sites using an uncertainty analysis methodology that uses BEEF-vdW with its ensembles (BMwE) to generate the uncertainty for the energies of the intermediates and transition states. One experimental data set with two experimental observations, selectivity to propylene and turnover frequency of propylene, was used as a calibration data set to evaluate the impact of the experimental data on informing the models. This study finds that the prior model for Pt 3 Sn/Pt(100) is the most active and Pt 2 Sn/Pt(211) is the most selective toward propylene. Active sites on the (100) facet have the highest probability of being responsible for C 1 and C 2 product formations (C–C bond cleavage). Increasing the Sn coverage on the (100) surface facet to a PtSn/Pt(100) active site leads to a significantly reduced rate and might explain the experimentally observed higher selectivity of Sn-doped catalysts relative to pure Pt catalysts. Next, this study finds that for all surfaces, except PtSn/Pt(100), the rate-controlling steps are the initial dehydrogenation steps alongside some partially rate-controlling second dehydrogenation steps. For PtSn/Pt(100), only the initial terminal dehydrogenation step to CH 3 CH 2 CH 2 * and second dehydrogenation steps are rate-controlling. Next, the calibrated models for all surfaces were found to be selective toward propylene production and model the reported turnover frequency successfully. Nevertheless, Pt 2 Sn/Pt(211) emerges as the active site with some (minor) evidence as the main active site based on Jeffreys’ scale interpretation of Bayes factors. This observation agrees with prior studies that also found step sites to be most likely the most relevant active sites for pure Pt catalysts. Overall, the results indicate that tin, in addition to affecting the binding strength of the adsorbed species, prevents deeper dehydrogenation (reducing coking) and cracking reactions through increasing activation barriers for unwanted side reactions.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2023

Pt Nanoparticles on Atomic-Metal-Rich Carbon for Heavy-Duty Fuel Cell Catalysts: Durability Enhancement and Degradation Behavior in Membrane Electrode Assemblies

Proton exchange membrane fuel cells (PEMFCs) are a promising zero-emission power source for heavy-duty vehicles (HDVs). However, long-term durability of up to 25,000 h is challenging because current carbon support, catalyst, membrane, and ionomer developed for traditional light-duty vehicles cannot meet the stringent requirement. Therefore, understanding catalyst degradation mechanisms under the HDV condition is crucial for rationally designing highly active and durable platinum group metal (PGM) catalysts for high-performance membrane electrode assemblies (MEAs). Herein, we report a PGM catalyst consisting of platinum nanoparticles with a high content (40 wt %) on atomic-metal-site (e.g., MnN 4 )-rich carbon support. MEAs with the Pt (40 wt %)/Mn–N–C cathode catalyst achieved significantly enhanced performance and durability, generating 1.41 A cm –2 at 0.7 V under HDV conditions (0.25 mgPt cm –2 and 250 kPa abs pressure) and retaining 1.20 A cm –2 after an extended and accelerated stress test up to 150,000 voltage cycles. Electron microscopy studies indicate that most fine Pt nanoparticles are retained on or/and in the carbon support covered with the ionomer throughout the catalyst layer at the end of life. During the long-term stability test, the observed electrochemical active surface area reduction and performance loss primarily result from Pt depletion in the catalyst layer due to Pt dissolution and redeposition at the interface of the cathode and membrane. Importantly, the first-principle density functional theory calculations further reveal a support entrapment effect of the Mn–N–C, in which the MnN 4 site can specifically adsorb the Pt atom and further retard the Pt dissolution and migration, therefore enhancing long-term MEA durability.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
DOE OSTI2024

Catalytic Reaction Triggered by Magnetic Induction Heating Mechanistically Distinguishes Itself from the Standard Thermal Reaction

As a recent advancement in chemical engineering, magnetic induction heating (MIH) is utilized to initiate the intended reactions by enabling the self-heating of the ferromagnetic catalyst particles. While MIH can be energy-efficient and industrially scalable, its full potential has been underappreciated in catalysis because of the perception that MIH is merely an alternative heating approach. Unexpectedly, we show that the MIH-triggered reaction could go beyond standard thermal catalysis. Specifically, by probing the representative Pt/Fe 3 O 4 catalysts with CO oxidation in both thermal and MIH modes with consistent temperature profiles and catalyst structures, we found that the MIH mode boosts the reactivity more than 25 times by modifying Pt-FeO x interfacial synergies and promoting facile oxidation of the adsorbed carbonyl species by atomic oxygen. Further, as we preliminarily observed, this beneficial MIH-catalysis can be translational to other thermal reactions, potentially paving the way to launch MIH-catalysis as a distinct reaction category.

CO oxidation↗
DOE OSTI2024

Investigation of Ethane Dehydrogenation and Hydrogenolysis on Pt(111), Pt(211), and Pt(100): Bayesian Quantification and Correction of DFT-Based Enthalpic and Entropic Uncertainties

Computational investigations of heterogeneously catalyzed reactions using density functional theory (DFT) are often inaccurate, largely due to uncertainties in the choice of DFT functional (enthalpic uncertainty) and approximations for modeling adsorbate movement along the catalyst surface (entropic uncertainty). This work illustrates that both uncertainties are significant in the investigation of ethane dehydrogenation (EDH) and hydrogenolysis on Pt catalysts by considering the complete deconstruction of ethane on Pt(111), Pt(211), and Pt(100) using microkinetic modeling (MKM). Hence, this work uses both noncalibrated and Bayesian-calibrated MKMs to quantify and correct inaccuracies in macroscopic properties due to both uncertainties. A Bayesian approach to the correction of entropic errors was introduced using a “Modified Fermi Function (MFF)” to calibrate between the two bounds of entropy represented by the harmonic oscillator (HO) and free translator (FT) approximations. Regardless of enthalpic and entropic uncertainties, all three surfaces are capable of ethane activation; however, Pt(211) was found to be the most active and is largely responsible for methane production. Next, Pt(111) is largely responsible for acetylene production, and Pt(100) has the highest ethylene selectivity but is most susceptible to coking. By comparison of different calibrated models, the FT entropy approximation was found to better describe EDH under typical experimental conditions. Statistical evidence was found to support Pt(111) as the active site for EDH, assuming that one single site is responsible for the chemistry. On the three surfaces, competing second dehydrogenations to CH 2 CH 2 and CH 3 CH were observed as well as isomerization of CH 3 CH back to CH 2 CH 2 and deeper dehydrogenation of CH 3 CH. In conclusion, C–C cleavage was found to largely proceed via the CH 3 C intermediate on Pt(100) and Pt(111), while on Pt(211), it was via both CHC and CH 3 C.

Bayesian model selection↗