Microkinetic modeling of the decarboxylation and decarbonylation of propanoic acid over Pd(111) model surfaces based on parameters obtained from first principles

Comparative DFT study of methanol decomposition on Mo2C(001) and Mo2C(101) surfaces

CONTEXT

In this study, the complete reaction mechanism of methanol decomposition on metallic Mo₂C(001) and Mo/C-mixed Mo₂C(101) hexagonal Mo₂C crystalline phases was systematically investigated using plane-wave-based periodic density functional theory (DFT). The main reaction route for Mo₂C(001) is as follows: CH₃OH → CH₃O + H → CH₂O + 2H → CHO + 3H → CO + 4H → C + O + 4H. Hence, C, O, and H are the main products. It was found that the energy barrier for CO dissociation was low. Therefore, it was concluded that the Mo₂C(001) surface was too active to be easily oxidized or carburized. The optimal reaction pathway for Mo₂C(101) is as follows: CH₃OH → CH₃O + H → CH₂O + 2H → CH₂ + O + 2H → CH₃ + O + H → CH₄ + O. Therefore, CH₄ is the major product. The hydrogenation of CH₃ leading to CH₄ showed the highest energy barrier and the lowest rate constant and should be the rate-determining step. In addition, the formation of CO + 2H₂ was competitive on Mo₂C(101), and the optimal path was CH₃OH → CH₃O + H → CH₂O + 2H → CH₂ + O + 2H → CH + O + 3H → C + O + 4H → CO + 2H₂. The computed energy barrier and rate constant indicate that the rate-determining step is the last step in CO formation. In agreement with the experimental observations, the results provide insights into the Mo₂C-catalyzed decomposition of methanol and other side reactions.

METHODS

All calculations were performed by using the plane-wave based periodic method implemented in Vienna ab initio simulation package (VASP, version 5.3.5), where the ionic cores are described by the projector augmented wave (PAW) method. The exchange and correlation energies were computed using the Perdew, Burke and Ernzerhof functional with the latest dispersion correction (PBE-D3).

The surface phase structure evolution of the fcc MoC (001) surface in a steam reforming atmosphere: systematic kinetic and thermodynamic investigations

Systematic ab initio-based calculations were performed to clarify the surface structure evolution of the fcc MoC (001) surface at different H2O/H2 pressures.

Surface structure sensitivity of hydrodeoxygenation of biomass-derived organic acids over palladium catalysts: a microkinetic modeling approach

A microkinetic DFT model for HDO of CH3CH2COOH on Pd(100) describes experimental observations while Pd(111) is orders of magnitude less active.

Surface phase structures responsible for the activity and deactivation of the fcc MoC (111)-Mo surface in steam reforming: a systematic kinetic and thermodynamic investigation

Multiscale investigation on MoC surface phase evolution to clarify surface structures responsible for reactivity and deactivation in steam reforming.

Exploring the Reaction Mechanisms of Furfural Hydrodeoxygenation on a CuNiCu(111) Bimetallic Catalyst Surface from Computation

In this study, the selectively catalytic hydrodeoxygenation of furfural (F-CHO) to 2-methylfuran (F-CH₃) on the CuNiCu(111) bimetallic catalyst surface was systematically investigated based on the periodic density functional theory, including dispersion correction. The formation of furfuryl alcohol (F-CH₂OH) involved two steps: the preferred first step was the hydrogenation of the branched C atom, forming the alkoxyl intermediate (F-CHO + H = F-CH₂O), and the second step was H addition to the alkoxyl group, resulting in furfuryl alcohol (F-CH₂O + H = F-CH₂OH), which was the rate-controlling step. In contrast, in the formation of 2-methylfuran, the first step was the dehydroxylation of furfuryl alcohol, resulting in alkyl (F-CH₂) and OH (F-CH₂OH = F-CH₂ + OH) groups, the second step was the hydrogenation of F-CH₂ (F-CH₂ + OH + H = F-CH₃ + OH), and the rate-controlling step was the hydrogenation of OH to H₂O (OH + H = H₂O). Based on the comparison results of the NiCuCu(111), Cu(111), and CuNiCu(111) surfaces, it was concluded that the catalytic performance of the catalyst was closely related to the adsorption structure of furfural. These results provide a basis for studying the intrinsic activity of NiCu catalysts during the hydrodeoxygenation of refined oxygenated compounds involving biomass-derived oils.

A DFT Insight into the Tuning Effect of Potassium Promoter on the Formation of Carbon Atoms via Carburization Gases Dissociation on Iron-Based Catalysts

The research of the formation mechanism of iron carbides is significant to design the high-performance catalysts for the Fischer–Tropsch synthesis (FTS) process. In this paper, the effect of potassium promoter on the formation of atomic carbon via carburization gases dissociation on the iron-based catalyst, the C2H4, C2H2 and CO/H2 adsorption energies and dissociation paths as well as the rate constants of the corresponding elementary steps are investigated by DFT on the Fe(110), Fe(110)-K2O, Fe(211) and Fe(211)-K2O surfaces. The calculation results demonstrated that the K2O promoter can modify the capabilities of surface C formation via the thermodynamic method as well as the kinetical method. The K2O promoter can increase the CO adsorption energy while decreasing the C2H4 adsorption energy both on Fe(110) and Fe(211) surfaces. Kinetically, via tuning the catalyst surfaces from Fe(110) to Fe(211), the K2O promoter can inhibit the ability of C2H4/C2H2 dissociation to atomic carbon, while enhancing the ability of CO/H2 decomposition to atomic carbon. The C2H4/C2H2 dissociation rate constants on Fe(211) and Fe(211)-K2O are about 107 times slower than that on Fe(110) and Fe(110)-K2O, whereas the dissociation rate constants of CO/H2 on Fe(211) are about 106 times faster than that on Fe(110), and about 107 times faster on Fe(211)-K2O than on Fe(110)-K2O.

Reaction pathway of poly(ethylene) terephthalate carbonization: Decomposition behavior based on carbonized product

Char, a solid product obtained from carbonization of waste Poly(Ethylene) Terephthalate (PET), has high potential to solve the current plastic waste problem through the synthesis of new carbon-based adsorbents. However, thermal degradation reaction of polymer involves multiple series of complex reaction pathways and the formation of char is not clarified. In this study, the phase behavior of PET carbonization and the mechanism of char formation was studied in detail. Based on the van Krevelen diagram, it is evident that rapid thermal decomposition of PET occurs through decarbonylation to form char and decarboxylation to form wax. Based on the analysis of cross-linking behavior, a correlation between the degree of cross-linking as a function of CO and CO₂ and dependent parameters based on the experimental operation was obtained. The findings validified the assumption that scission of CO bond in the ester group through decarbonylation and decarboxylation to release CO and CO₂ leads to the formation of char. The cross-linking behavior was further clarified by studying the distribution of cross-linking structure in char and wax. It was confirmed that decarbonylation reaction to release CO is highly associated with the formation of cross-linking to form char in the solid residue, whereas decarboxylation reaction to release CO₂ is highly associated with the formation of cross-linking to form aromatic compounds in the wax residue.

A Multiple Filter Based Neural Network Approach to the Extrapolation of Adsorption Energies on Metal Surfaces for Catalysis Applications

Computational catalyst discovery involves the development of microkinetic reactor models based on estimated parameters determined from density functional theory (DFT). For complex surface chemistries, the number of reaction intermediates can be very large, and the cost of calculating the adsorption energies by DFT for all surface intermediates even for one active site model can become prohibitive. In this paper, we have identified appropriate descriptors and machine learning models that can be used to predict a significant part of these adsorption energies given data on the rest of them. Moreover, our investigations also included the case when the species data used to train the predictive model are of different size relative to the species the model tries to predict-this is an extrapolation in the data space which is typically difficult with regular machine learning models. Due to the relative size of the available data sets, we have attempted to extrapolate from the larger species to the smaller ones in the current work. Here, we have developed a neural network based predictive model that combines an established additive atomic contribution based model with the concepts of a convolutional neural network that, when extrapolating, achieves a statistically significant improvement over the previous models.

Theoretical Study of the Mechanism of Furfural Conversion on the NiCuCu(111) Surface

The full potential energy surface for the hydrodeoxygenation of furfural to furan and other ring-opening products has been systematically investigated using periodic density functional theory including dispersion corrections (PBE-D3) on the bimetallic NiCuCu(111) surface. For furan formation, the most favorable first step is the dehydrogenation of furfural into furoyl (F-CHO + H = F-CO + 2H), the successive step is decarbonylation of furoyl into furanyl (F-CO + H = F + CO + 2H), and the third step of furan formation from the hydrogenation of furanyl (F + CO + 2H = FA + CO + H) is the rate-determining step. In addition, on the basis of the most stably adsorbed furan and H, the ring opening of furan was found to be more favorable for producing many chemicals such as propane, butanal, butanol, and butene. In summary, furan is the main product of furfural conversion on the NiCuCu(111) surface. Since results have been obtained only for the NiCuCu(111) surface constructed by replacing the topmost Cu atoms by Ni atoms, the entire experimentally observed reactivity and selectivity of bimetallic CuNi catalysts for different construction methods cannot be fully rationalized. Nevertheless, the results provide the basis for investigating the intrinsic activity of CuNi catalysts in the hydrodeoxygenation of oxygenates involved in the refining of biomass-derived oils.

Adsorption and reactions of propenoic acid and 2-fluoropropanoic acid on Cu(100) and O/Cu(100)

X-ray photoelectron spectroscopy, reflection-absorption infrared spectroscopy, and temperature-programmed reaction/desorption have been employed to investigate the adsorption and reaction pathways of CH₂=CHCOOH and CH₃CHFCOOH on Cu(100) and oxygen-precovered Cu(100) [O/Cu(100)]. In the case of CH₂=CHCOOH on O/Cu(100), CH₂=CHCOO is the surface intermediate detected between 110 K and 400 K. CH₂=CHCOO is adsorbed vertically and can change adsorption sites at a higher temperature. The propenoate (acrylate) decomposes at higher temperatures (>500 K), with formation of >C=C=O (ketenylidene) surface species and gaseous products. On Cu(100), CH₂=CHCOOH is adsorbed in dimer form and can dissociate to generate CH₂=CHCOO and CH₃CHCOO intermediates on the surface. The CH₃CHCOO continuously recombines with the H from deprotonation of CH₂=CHCOOH, resulting in the formation CH₃CH₂COO. The co-existing CH₂=CHCOO and CH₃CH₂COO further decompose at ∼550 K to evolve reaction products, but without >C=C=O being detected. On O/Cu(100), CH₃CHFCOOH readily deprotonates to form CH₃CHFCOO at 120 K. This intermediate reacts on the surface at ∼460 K to evolve gaseous products, also producing CH₂=CHCOO. In the case of Cu(100), deprotonation of CH₃CHFCOOH occurs at ∼250 K, forming CH₃CHFCOO. Without oxygen on the surface, this intermediate decomposes into HF and CH₂=CHCOO at ∼455 K.

Uncovering reaction sequences on surfaces through graphical methods

ZStruct is a graph-based model that generates an ensemble of plausible reaction pathways starting from a given initial state, without requiring prior knowledge of reaction intermediates.

Dense CO Adlayers as Enablers of CO Hydrogenation Turnovers on Ru Surfaces

High CO* coverages lead to rates much higher than Langmuirian treatments predict because co-adsorbate interactions destabilize relevant transition states less than their bound precursors. This is shown here by kinetic and spectroscopic data-interpreted by rate equations modified for thermodynamically nonideal surfaces-and by DFT treatments of CO-covered Ru clusters and lattice models that mimic adlayer densification. At conditions (0.01-1 kPa CO; 500-600 K) which create low CO* coverages (0.3-0.8 ML from in situ infrared spectra), turnover rates are accurately described by Langmuirian models. Infrared bands indicate that adlayers nearly saturate and then gradually densify as pressure increases above 1 kPa CO, and rates become increasingly larger than those predicted from Langmuir treatments (15-fold at 25 kPa and 70-fold at 1 MPa CO). These strong rate enhancements are described here by adapting formalisms for reactions in nonideal and nearly incompressible media (liquids, ultrahigh-pressure gases) to handle the strong co-adsorbate interactions within the nearly incompressible CO* adlayer. These approaches show that rates are enhanced by densifying CO* adlayers because CO hydrogenation has a negative activation area (calculated by DFT), analogous to how increasing pressure enhances rates for liquid-phase reactions with negative activation volumes. Without these co-adsorbate effects and the negative activation area of CO activation, Fischer-Tropsch synthesis would not occur at practical rates. These findings and conceptual frameworks accurately treat dense surface adlayers and are relevant in the general treatment of surface catalysis as it is typically practiced at conditions leading to saturation coverages of reactants or products.

Conversion of Undaria pinnatifida residue to glycolic acid with recyclable methylamine in low temperature hydrothermal liquefaction

The conversion of Undaria pinnatifida residue to glycolic acid was carried out using methylamine as catalyst by hydrothermal method at relatively low temperature. GC-MS and HPLC were used to identify the composition of bio-oil and liquid products which provide the knowledge of the chemical reaction pathways of the hydrothermal liquefaction. The main liquid product was organic acid which contained glycolic acid, lactic acid, formic acid and acetic acid. And the major organic acid was glycolic acid with the highest yield of 46.52% or 33.98% of dry biomass. Methylamine promoted the dissolution of cellulose from Undaria pinnatifida residue, and significantly improved the yield of glycolic acid. The mechanism of HTL was investigated and the results show that the carbocation C3 was attacked by methylamine molecule which led to the high yield of glycolic acid. In addition, the recovery of methylamine was studied and the highest recovery rate reached 99.28%.

Carboxylic acid formation by hydroxyl insertion into acyl moieties on late transition metals

With a DFT approach, we systematically examined the barriers for OH insertion into acyl moieties on late transition metals, a reaction pertinent to the catalytic decarboxylation of biomass.

Selective Deoxygenation of Biomass-Derived Bio-oils within Hydrogen-Modest Environments: A Review and New Insights

Research development of processes for refining bio-oils is becoming increasingly popular. One issue that these processes possess is their high requirement for H2 gas. In response, researchers must develop catalysts that perform deoxygenation while minimizing H2 consumption-selective deoxygenation. Unlike traditional deoxygenation processes, selective deoxygenation reactions and catalysts represent an information gap that, prior to this publication, has yet to be reviewed. This review addresses the gap by providing both a summary of recent research developments and insight into future developments of new catalytic materials. Bifunctional catalysts containing a combination of oxophilicity and an active metal phase appear to be the most beneficial for selective deoxygenation processes in a H2 -modest environment. It is important that catalysts have a supply of disassociated hydrogen, because without such, activity and stability will suffer. The authors recommend to maximize the use of internally available hydrogen in bio-fuel, which may be the only viable approach for deoxygenation if external H2 gas is limited. This would be possible through the development of catalysts that promote both the water-gas-shift and deoxygenation reactions.

Theoretical study about Mo2C(101)-catalyzed hydrodeoxygenation of butyric acid to butane for biomass conversion

To explore the conversion mechanism of fatty acids to long-chain alkanes using molybdenum carbide catalysts, the full potential energy surface of the hydrogenation of butyric acid to butane on the H-pre-covered hexagonal Mo₂C(101) surface has been systematically computed.

Mechanisms of H- and OH-assisted CO activation as well as C–C coupling on the flat Co(0001) surface – revisited

The mechanisms of H- and OH-assisted CO activation and the consecutive C–C coupling on the flat Co(0001) surface have been computed at the level of periodic RPBE density functional theory.

Theoretical investigation of the hydrodeoxygenation of methyl propionate over Pd (111) model surfaces

Theoretical study of the hydrodeoxygenation of methyl propionate over Pd (111) model surfaces.

Hydrodeoxygenation of propanoic acid over silica-supported palladium: effect of metal particle size

A detailed discussion on the effect of Pd/SiO₂ nanoparticle size on the gas-phase hydrodeoxygenation (HDO) of propanoic acid (PAc) is presented.