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•First, principles based microkinetic model developed for the HDO of guaiacol on Ru.•HDO of guaiacol occurs by dehydrogenation of the methoxy group and decarbonylation.•Phenol is the ...main reaction product, and catechol is the most relevant side product.•Benzene production occurs primarily by partial phenyl ring hydrogenation.
The reaction mechanism of the hydrodeoxygenation of guaiacol to aromatic products has been studied by density functional theory calculations and microkinetic modeling over a Ru(0001) model surface. Our model suggests that the dominant hydrodeoxygenation pathway proceeds via O–H bond cleavage of guaiacol, C6H4(OH)(OCH3), to C6H4(O)(OCH3), followed by dehydrogenation of the methoxy group to C6H4(O)(OC), decarbonylation to C6H4O, and finally hydrogenation to phenol. At the adsorbed C6H4(O)(OCH) intermediate, a competitive deoxygenation pathway is identified, which involves methyne group removal to C6H4O2, followed by hydrogenation to C6H4(OH)(O), dehydroxylation to C6H4O, and finally hydrogenation to phenol. In agreement with experimental results, phenol is predicted to be the major product and catechol is the most relevant minority side product. Further deoxygenation of phenol to benzene is found to be slow. Finally, computations predict the last dehydrogenation step of the methoxy species in guaiacol to be at least partially rate controlling over Ru(0001).
Atomically dispersed supported metal catalysts offer unique opportunities for designing highly selective catalysts and maximizing the utility of precious metals that have potential applications in a ...wide variety of industrial chemical reactions. Although substantial advances in understanding the origin of the activity of such highly dispersed metal catalysts have been made for a few chemical reactions, the reaction mechanisms and the nature of the active sitesmall metal clusters versus single atomsare still highly debated. Using a combination of density functional theory and microkinetic modeling, we confirm that a positively charged single Pt atom on TiO2(110) can exhibit a very high low-temperature activity for the water-gas shift reaction (TOF > 0.1 s–1 at 473 K). A comparison of these results with our work on TiO2-supported Pt cluster models provides clear evidence that different active sites are responsible for the experimentally observed activity at low and high temperatures. Finally, we explain why contradictory experimental conclusions have been reported.
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•WGS reaction on CeO2 (111)-supported Pt cluster follow both redox and associative carboxyl with redox regeneration mechanisms.•High activity of Pt/CeO2 interface sites originates ...from a significantly enhanced water activation and dissociation at interfacial oxygen vacancies.•First principles-based microkinetic modeling analysis provides insights on the unique activity of Pt/CeO2 interface.
The mechanism of water–gas shift reaction at the three-phase boundary of Pt/CeO2 catalysts has been investigated using density functional theory and microkinetic modeling to better understand the importance of metal–oxide interface sites in heterogeneous catalysis. Analysis of a microkinetic model based on parameters obtained from first principles suggests that both the “Redox pathway” and the “Associative carboxyl pathway with redox regeneration” could operate on Pt/CeO2 catalysts. Although (1) only few interfacial Pt atoms are found to be catalytically active at low temperatures due to strong adsorption of CO and (2) interfacial O–H bond breakage is difficult due to the high reducibility of ceria, interface sites are 2–3 orders of magnitude more active than Pt (111) and stepped Pt surface sites and therefore effectively determine the overall activity of Pt/CeO2. The high activity of Pt/CeO2 interface sites originates from a significantly enhanced water activation and dissociation at interfacial oxygen vacancies.
Chemical kinetic modeling in heterogeneous catalysis is advancing in its ability to provide qualitatively or even quantitatively accurate prediction of real-world behavior because of new advances in ...the physical and chemical representations of catalytic systems, estimation of relevant kinetic parameters, and capabilities in kinetic modeling. This Perspective describes current trends and future areas of advancement in chemical kinetic modeling, simulation, and parameter estimation: ranging from elementary step calculations to multiscale modeling to the role of advanced statistical methods for incorporating uncertainties in predictions. Multiple new or growing methodologies are covered, examples are provided, and forward-looking topics for advancement are noted.
The origin of the unique activity of small Pt clusters supported on TiO2(110) was investigated for the low-temperature water–gas shift (WGS) reaction. The reaction follows a redox mechanism and the ...elementary steps occurring on the TiO2 support, such as H2O dissociation and H diffusion, largely control the overall rate. Display omitted
•Low-temperature WGS reaction on TiO2(110)-supported Pt cluster follows a novel CO-promoted redox mechanism.•High-temperature WGS reaction on TiO2(110)-supported Pt cluster follows a classical redox mechanism.•Pt–TiO2 interface sites two orders of magnitude more active than Pt(111) surface sites.•Elementary processes occurring on the TiO2 surface, i.e., H2O dissociation and H diffusion largely control the overall rate.•First-principles-based microkinetic model provides deep insights into the unique activity of Pt/TiO2 catalysts.
Periodic density functional theory calculations and microkinetic modeling are used to illustrate the specific role of the three-phase boundary (TPB) in determining the activity and selectivity of TiO2-supported Pt catalysts for the water–gas shift (WGS) reaction. The Pt8/TiO2(110) catalyst model identified from a systematic ab initio atomistic thermodynamics study is used to investigate the redox mechanism and associative pathway with redox regeneration of the WGS reaction. Analysis of a microkinetic model determined exclusively from first principles suggests that a CO-promoted redox pathway dominates in the low-temperature range of 473–623K and the classical redox pathway becomes dominant at temperatures above 673K. The improved activity of the TPB compared to the Pt(111) surface can be explained by a reduced CO adsorption strength on Pt sites at the TPB, an increased number of oxygen vacancy at the TPB, and a significantly facilitated water activation and dissociation.
The reaction mechanism of the mild hydrogenation of guaiacol over Pt(111) has been investigated by density functional theory calculations and microkinetic modeling. Our model suggests that at 573 K, ...catechol is the preferred reaction product and that any deoxygenation to, for example, phenol or benzene is at least 4 orders of magnitude slower than the production of catechol. Slow deoxygenation of guaiacol can occur by decarbonylation and possibly by hydrogenation of the phenyl ring followed by C–OH bond cleavage. Direct −OH removal without activation of the phenyl ring is found to be at least 5 orders of magnitude slower. Overall, this study suggests that Pt(111) sites are not active deoxygenation sites and that the experimentally observed deoxygenation activity of Pt catalysts originates likely from the involvement of the catalyst support or Pt step and corner sites.
Computational catalyst screening has the potential to significantly accelerate heterogeneous catalyst discovery. Typically, this involves developing microkinetic reactor models that are based on ...parameters obtained from density functional theory and transition-state theory. To reduce the large computational cost involved in computing various adsorption and transition-state energies of all possible surface states on a large number of catalyst models, linear scaling relations for surface intermediates and transition states have been developed that only depend on a few, typically one or two descriptors, such as the carbon atom adsorption energy. As a result, only the descriptor values have to be computed for various active site models to generate volcano curves in activity or selectivity. Unfortunately, for more complex chemistries the predictability of linear scaling relations is unknown. Also, the selection of descriptors is essentially a trial and error process. Here, using a database of adsorption energies of the surface species involved in the decarboxylation and decarbonylation of propionic acid over eight monometalic transition-metal catalyst surfaces (Ni, Pt, Pd, Ru, Rh, Re, Cu, Ag), we tested if nonlinear machine learning (ML) models can outperform the linear scaling relations in prediction accuracy when predicting the adsorption energy for various species on a metal surface based on data from the rest of the metal surfaces. We found linear scaling relations to hold well for predictions across metals with a mean-absolute error of 0.12 eV, and ML methods being unable to outperform linear scaling relations when the training dataset contains a complete set of energies for all of the species on various metal surfaces. Only when the training dataset is incomplete, namely, contains a random subset of species’ energies for each metal, a currently unlikely scenario for catalyst screening, do kernel-based ML models significantly outperform linear scaling relations. We also found that simple coordinate-free species descriptors, such as bond counts, achieve as good results as sophisticated coordinate-based descriptors. Finally, we propose an approach for automatic discovery of appropriate metal descriptors using principal component analysis.
The oxidation mechanism of H2 fuel at the three-phase boundary of Ni/YSZ has been investigated under experimental solid oxide fuel cell conditions by a combination of density functional theory and ...microkinetic modeling. It is shown that the O migration pathway is 2–4 orders of magnitude faster than the H spillover and OH migration pathways. Bulk oxygen diffusion in YSZ is rate-limiting at low temperatures, and H transfer from Ni to YSZ to form water becomes rate-limiting at high temperatures.
Conversion of biomass into fuels or chemicals often requires a processing step limited by hydrodeoxygenation of organic acids. Various pathways have been proposed for the deoxygenation of these acids ...into hydrocarbons, with the decarboxylation and decarbonylation requiring less hydrogen than the reductive deoxygenation without C–C bond cleavage. In this paper, we present the reaction mechanism for the decarboxylation and decarbonylation of propanoic acid over Pd(111) model surfaces determined by first-principles electronic structure calculations based on density functional theory. Our calculations suggest that the most significant decarbonylation pathways proceed via a dehydroxylation of the acid to produce propanoyl (CH3CH2CO) followed by either full α-carbon dehydrogenation and CH3C–CO bond scission to produce CH3C and CO, or first α-carbon dehydrogenation followed by β-carbon dehydrogenation and CH2CH–CO bond scission to produce CH2CH and CO. The decarboxylation mechanism starts with O–H bond cleavage followed by direct C–CO2 bond scission or possibly α-carbon dehydrogenation prior to C–CO2 bond cleavage. As a result, in both mechanisms the most favorable pathways likely involve some level of α- and/or β-carbon dehydrogenation steps prior to C–C scission, which distinguishes these deoxygenation pathways from the reductive deoxygenation without C–C bond cleavage that has previously been shown to not involve dehydrogenation steps.
The development of porous well-defined hybrid materials (e.g., metal–organic frameworks or MOFs) will add a new dimension to a wide number of applications ranging from supercapacitors and electrodes ...to “smart” membranes and thermoelectrics. From this perspective, the understanding and tailoring of the electronic properties of MOFs are key fundamental challenges that could unlock the full potential of these materials. In this work, we focused on the fundamental insights responsible for the electronic properties of three distinct classes of bimetallic systems, M x–y M′ y -MOFs, M x M′ y -MOFs, and M x (ligand-M′ y )-MOFs, in which the second metal (M′) incorporation occurs through (i) metal (M) replacement in the framework nodes (type I), (ii) metal node extension (type II), and (iii) metal coordination to the organic ligand (type III), respectively. We employed microwave conductivity, X-ray photoelectron spectroscopy, diffuse reflectance spectroscopy, powder X-ray diffraction, inductively coupled plasma atomic emission spectroscopy, pressed-pellet conductivity, and theoretical modeling to shed light on the key factors responsible for the tunability of MOF electronic structures. Experimental prescreening of MOFs was performed based on changes in the density of electronic states near the Fermi edge, which was used as a starting point for further selection of suitable MOFs. As a result, we demonstrated that the tailoring of MOF electronic properties could be performed as a function of metal node engineering, framework topology, and/or the presence of unsaturated metal sites while preserving framework porosity and structural integrity. These studies unveil the possible pathways for transforming the electronic properties of MOFs from insulating to semiconducting, as well as provide a blueprint for the development of hybrid porous materials with desirable electronic structures.
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