Microkinetic simulation and mechanistic insights into CO2 hydrogenation to methanol catalyzed by Pd/In2O3

2,2′‐Biphenol‐derived phosphoric acid is an effective catalyst for the dehydrative esterification of an equimolar mixture of carboxylic acids and alcohols in toluene at 100°C without the need to remove water from the reaction. The acidic P–OH and basic P ═ O moieties can work together effectively as a cooperative acid–base catalyst system. In this study, mechanistic insights into this catalytic reaction are obtained through experimental kinetics studies, density functional theory (DFT) calculations, and microkinetic simulations of 2,2′‐biphenol‐derived phosphoric acid and related catalysts. The kinetics and DFT analysis support a reaction mechanism that involves the formation of a phosphate–carboxylate complex and activation of the alcohol by the catalyst. The catalyst structure affects the reaction barrier of the dehydration step, thereby determining the overall activity. This step is further modulated by noncovalent interactions, including steric and π–π effects, which depend on the steric demand and flexibility of the catalyst backbone. Microkinetic simulations based on DFT‐derived energy profiles successfully reproduce the experimentally observed conversion trends. Overall, the 2,2′‐biphenol‐derived phosphoric acid is found to have ideal properties for the catalysis of the present dehydrative esterification.

The dominating hydrocarbon pool species (HCPs) in zeolites for methanol-to-olefin (MTO) conversion have been the subject of intense debate for decades due to the diversity of structures and the complexity of reaction networks. We performed microkinetic simulations in a three-site model to study the MTO conversion in industrially relevant H-SAPO-34 zeolite under a wide range of operating conditions. The energetics of 229 and 342 elementary reaction steps were employed, respectively, in the aromatic-based and olefin-based cycles. The dynamic distribution and evolution of the retained aromatic or olefinic HCPs and the origin of olefin products were revealed with respect to reaction conditions. We corroborate that the olefin-based cycle dominates the MTO conversion in H-SAPO-34 under most reaction conditions, and the contribution of the aromatic-based cycle increase with increasing temperature, decreasing pressure, and/or decreasing water partial pressure. The paring route, dedicated highly to propene formation, prevails in the aromatic-based cycle; the side chain route, favoring exclusively ethene formation, only prevails at extremely lower temperatures, higher pressures, and higher water contents. The inherent activity of each aromatic HCP via the paring cycle increases, while its population retained in H-SAPO-34 usually decreases, with the methylation degree remarkably from tetramethylbenzene to hexamethylbenzene. The contents of higher methylbenzenes increase with decreasing water partial pressure, leading to the enhanced contribution of the aromatic-based cycle. The olefin-based cycle contributes to the formation of both ethene and other olefins, and the product distribution is drastically sensitive to reaction conditions. Ethene predominantly comes from the cracking of C5 and C6 species, and propene comes from C5 to C7 species. The olefin-based cycle shifts from the interconversion of C2–C7 olefins toward C2–C5 ones with increasing temperature and water partial pressure to enhance ethene formation. Under industrially relevant conditions, the conversion rate of methanol via the olefin-based cycle is 40-fold greater than that via the aromatic-based cycle (6.6 vs 0.15 s–1), and the former agrees unexpectedly with the experimental value in low-silica AlPO-34 (7.0 s–1). This work thus solves the puzzle of dominating HCPs as a function of reaction conditions in H-SAPO-34 and provides mechanistic insights into the kinetic behaviors, which are the basis for the optimization of the MTO catalyst and process.

The development of high-efficiency dehydrogenation catalysts for methylcyclohexane, a key medium for liquid organic hydrogen storage, is crucial for advancing the hydrogen economy. This study systematically investigates the dehydrogenation performance of methylcyclohexane (MCH) by DFT calculations, including eight types of NixRhy surface and bulk alloys along with six Ni3M1 (M = Ru, Rh, Pd, Os, Ir, or Pt) alloy catalysts. The study reveals that the optimal catalytic activity is achieved at a Ni:M ratio of 3:1, with the top-performing catalysts Ni3Rh1, Ni3Ir1, Ni3Os1, and Ni3Ru1 exhibiting remarkably low reaction energy barriers of 0.70, 0.73, 0.75, and 0.78 eV, respectively. It is further found that the valence electron counts of doped metals and the d-bandwidth of modified Ni-based catalysts collectively affect dehydrogenation energy barriers. Notably, a distinct Brønsted-Evans-Polanyi (BEP) relationship exists between the Gibbs free energy change (ΔG) of the rate-determining step (RDS) in MCH dehydrogenation and the activation energy barrier for the first dehydrogenation step (ΔGa). Based on these correlations, a two-parameter descriptor incorporating valence electron counts and d-bandwidth was established, which can enable the rapid screening of high-performance catalysts. Meanwhile, microkinetic simulations further confirmed that the four optimal catalysts maintain excellent reactivity and high turnover frequencies even under low hydrogen coverage. These findings elucidate the electronic structure modulation mechanism of Ni-based alloys for catalytic MCH dehydrogenation, providing fundamental theoretical guidance for the rational design of cost-effective and high-performance dehydrogenation catalysts.

A combined in situ electrochemical attenuated total reflection-surface enhanced IR absorption spectroscopy, microkinetic simulation, and density functional theory calculation study shows that not only can the adsorbed sulfide disproportionally affect the surface binding of OOH* (EOOH* ) vs OH* (EOH* ), i.e., breaking the original scaling relationship of pure metals (Ir, Pd, Pt, Au), to enhance oxygen reduction reaction (ORR) activity but can also be used as a reaction pathway alternating species to help deepen our mechanistic understanding of ORR.

Dry reforming of methane (DRM) holds significant potential for converting greenhouse gases (CH4 and CO2) into synthesis gas. However, its application is severely limited by severe deactivation of Ni-based catalysts due to coking. This study systematically investigates the micromechanisms of the DRM reaction on Ni9/γ-Al2O3(110) catalysts using density functional theory (DFT) calculations and microkinetic simulations. Results indicate that CH4 dissociation preferentially occurs on the surface of Ni nanoparticles, while CO2 activation and coke removal are dominated by the metal-support interface. The energy barrier for CO2 hydrogenation at the interface is significantly lower than that for direct dissociation, and the key decarbonation step (C* + O* → CO) exhibits a markedly reduced barrier of 0.64 eV, effectively suppressing coke formation. Microkinetic simulations further reveal that CO is primarily generated via the CHO decomposition pathway, with the rate-determining step dynamically shifting from CO2 hydrogenation to surface decarbonization as temperature increases. Additionally, negative external electric fields universally reduce energy barriers across key steps. This study elucidates the anticoking mechanism of Ni/γ-Al2O3 catalysts at the atomic scale, providing crucial theoretical foundations for rationally designing highly efficient and stable DRM catalysts.

Understanding the mechanism of CO2 hydrogenation to methanol is important in the context of renewable energy storage from societal and technological point of view. We use density functional theory calculations to study systematically the effect of the size of Cu clusters on the binding strengths of reactants and reaction intermediates as well as the activation barriers for the elementary reaction steps underlying CO2 hydrogenation. All the elementary reaction barriers exhibit linear scaling relationships with CO and O adsorption energies. Used in microkinetics simulations, we predict that medium-sized Cu19 clusters exhibit the highest CO2 hydrogenation activity which can be ascribed to a moderate CO2 coverage and a low CO2 dissociation barrier. The nanoscale effect is evident from the strong variation of CO and O adsorption energies for clusters with 55 or less Cu atoms. The reactivity of larger clusters and nanoparticles is predicted to depend on surface atoms with low coordination number. Optimum activity is correlated with the bond strength of reaction intermediates determined by the d-band center location of the Cu clusters and the extended surfaces. The presented size-activity relations provide useful insight for the design of better Cu catalysts with maximum mass-specific reactivity for CO2 hydrogenation performance.

Coverage effect of surface oxygen vacancy on In2O3-catalyzed CO2 hydrogenation revealed by first principles-based microkinetic simulations

The activation and hydrogenation of CO2 at the Cu/TiO2 interfaces that are formed by depositing subnanometer Cun (n = 1–8) clusters on TiO2(110) surfaces have been systematically investigated using density functional theory calculations. The most stable structures with a bent CO2δ− configuration at the Cun/TiO2 interfaces are determined, which indicate that the binding strength of CO2 on the Cun/TiO2(110) surface can be tuned by controlling the size of the deposited Cu cluster. It is interesting that the copper cluster with a specific size of Cu4 exhibits a distinct preference for CO2 activation, and the strongest binding interaction between CO2 and Cu4/TiO2(110) is mainly ascribed to the formation of the strong Cu–C and Ti–O adsorption bonds. The reaction mechanisms of CO2 conversion to CH3OH at the Cu4/TiO2(110) interface via the formate and the reverse water gas shift (RWGS) + CO-hydrogenation pathways are further investigated by microkinetic simulations. The production of CH3OH over Cu4/TiO2 is mainly via the RWGS pathway to yield CO followed by the formation of H3CO* as the most stable intermediate, while the formate pathway is not efficient enough because of the higher apparent activation energy of CH3OH generation and the overly strong binding of HCOO* species at the interface. Compared with other Cun/TiO2 interfaces, the TiO2(110) surface-supported size-selected Cu4 cluster exhibits the highest CO2 hydrogenation activity. The findings obtained in the present work provide useful insight to design Cu/oxide interfaces with high activity toward methanol synthesis from CO2 hydrogenation by precisely controlling the size of copper clusters.

Evidence from density functional theory calculations that the main reaction pathway for the Fischer−Tropsch process on Co{0001} is not the carbide mechanism but an alternative branch starting with the hydrogenation of CO to an oxymethylidyne species. We show that hydrogenation is the main reaction path at realistic pressure using microkinetic simulations and thereby bridge the pressure gap in heterogeneous catalysis.

Cr2O3 has been recognized as a key oxide component in bifunctional catalysts to produce bridging intermediate, e.g., methanol, from syngas. By combining density functional theory calculations and microkinetic modeling, we computationally studied the surface structures and catalytic activities of bare Cr2O3 (001) and (012) surfaces, and two reduced (012) surfaces covered with dissociative hydrogens or oxygen vacancies. The reduction of (001) surface is much more difficult than that of (012) surface. The stepwise or the concerted reaction pathways were explored for the syngas to methanol conversion, and the hydrogenation of CO or CHO is identified as rate-determining step. Microkinetic modeling reveals that (001) surface is inactive for the reaction, and the rates of both reduced (012) surfaces (25−28 s−1) are about five times higher than bare (012) surface (4.3 s−1) at 673 K. These theoretical results highlight the importance of surface reducibility on the reaction and may provide some implications on the design of individual component in bifunctional catalysis.

The hydrogenation of carbon dioxide (CO2) is one of important processes to effectively convert and utilize CO2, which is also regarded as the key step at the industrial methanol synthesis. Water is likely to play an important role in this process, but it still remains elusive. To systematically understand its influence, here we computationally compare the reaction mechanisms of CO2 hydrogenation over the stepped Cu(211) surface between in the absence and presence of water based on microkinetic simulations upon density functional theory (DFT) calculations. The effects of water on each hydrogenation step and the whole activity and selectivity are checked and its physical origin is discussed. It is found that the water could kinetically accelerate the hydrogenation on CO2 to COOH, promoting the reverse water gas shift reaction to produce carbon monoxide (CO). It hardly influences the CO2 hydrogenation to methanol kinetically. In addition, the too high initial partial pressure of water will thermodynamically inhibit the CO2 conversion.

In this work, we performed density functional theory (DFT)-based microkinetic simulations to elucidate the reaction mechanism of methanol synthesis on two of the most stable facets of the cubic In2O3 (c-In2O3) catalyst, namely the (111) and (110) surfaces. Our DFT calculations show that for both surfaces, it is difficult for the H atom adsorbed at the remaining surface O atom around the O vacancy (Ov) active site to migrate to an O adsorbed at the Ov due to the very high energy barrier involved. In addition, we also find that the C-O bond in the bt-CO2* chemisorption structure can directly break to form CO with a lower energy barrier than that in its hydrogenation to the COOH* intermediate in the COOH route. However, our microkinetic simulations suggest that for both surfaces, CO2 deoxygenation to form CO in both pathways, namely the COOH and CO-O routes, are kinetically slower than methanol formation under typical steady state conditions assuming a CO2 conversion of 10% and a CO selectivity of 1%. Although these results agree with previous experimental observations at relatively low reaction temperature, where methanol formation dominates, they cannot explain the predominant formation of CO at relatively high reaction temperature. We tentatively attribute this to the simplicity of our microkinetic model as well as possible structural changes of the catalyst at relatively high reaction temperature. Furthermore, although the rate-determining step (RDS) from the degree of rate control (DRC) analysis is usually consistent with that judged from the DFT calculated energy barriers, for CO2 hydrogenation to methanol over the (111) surface, our DRC analysis suggests homolytic H2 dissociation to be the rate-controlling step, which is not apparent from the DFT-calculated energy barriers. This indicates that CO2 conversion and methanol selectivity over the (111) surface can be further enhanced if homolytic H2 dissociation can be accelerated for instance by introducing transition metal dopants as already shown by some experimental observations.

Dual-atom catalysts (DACs) have emerged as a promising platform for converting CO2 into valuable chemicals, addressing critical energy and environmental challenges. Here, we theoretically designed M1-P1/VBN catalysts by embedding single transition metal (M = Ir, Rh, and Co) and phosphorus atoms into defective h-BN. Extensive first-principles calculations were employed to investigate the mechanisms of CO2 thermal hydrogenation to HCOOH and CO2 cycloaddition with propylene oxide (PO) to produce propylene carbonate (PC). The metal-phosphorus dual-active sites were predicted to facilitate simultaneous activation and adsorption of CO2, H2, and PO, enabling detailed exploration of the reaction pathways. By combining static electronic structure calculations and microkinetic simulations, this work demonstrates that M1-P1/VBN sheets show excellent catalytic performance for both reactions under relatively mild conditions, particularly for CO2 hydrogenation on Co1-P1/VBN and cycloaddition on Rh1-P1/VBN. Stability analysis confirms the robustness of transition-metal-doped M1-P1/VBN systems. Notably, the binding strength of small molecules strongly correlates with metal type, and a strong linear correlation was observed between the adsorption free energies of reactive intermediates. This study offers valuable theoretical insights into the thermocatalytic mechanisms of CO2 hydrogenation and cycloaddition mediated by M1-P1/VBN catalysts, laying a foundation for designing multifunctional DACs to advance CO2 utilization technologies.

Density functional theory (DFT) calculations and microkinetic simulations were performed to study the structure-performance relationship of In2O3 and Zr-doped In2O3 catalysts for methanol synthesis, focusing on the In2O3(110) and Zr-doped In2O3(110) surfaces. These surfaces are expected to follow the oxygen vacancy-based mechanism via the HCOO route for CO2 hydronation to methanol. Our DFT calcualtions show that the Zr-In2O3(110) surface is more favorable for CO2 adsorption than the In2O3(110) surface, and although the energy barriers are not lowered, most intermediates in the HCOO route are stablized with the introduction of the Zr dopant. Microkinetic simulations suggest that the CH3OH formation rate is improved by ∼10 times and CH3OH selectivity increased significantly from 10% on In2O3(110) to 100% on the Zr1-In2O3(110) catalyst model at 550 K. We find that the higher CH3OH formation rate and CH3OH selectivity on the Zr1-In2O3(110) surface than those on the In2O3(110) surface can be attributed to the slightly increased OV formation energy and the stablization of the reaction intermediates, whereas the much lower CH3OH formation rate on the Zr3-In2O3(110) surface is due to the much higher OV formation energy and the over binding of the H2O at the OV site.

Massive ethanol production has long been a dream of human society. Despite extensive research in past decades, only a few systems have the potential of industrialization: specifically, Mn-promoted Rh (MnRh) binary heterogeneous catalysts were shown to achieve up to 60% C2 oxygenates selectivity in converting syngas (CO/H2) to ethanol. However, the active site of the binary system has remained poorly characterized. Here, large-scale machine-learning global optimization is utilized to identify the most stable Mn phases on Rh metal surfaces under reaction conditions by exploring millions of likely structures. We demonstrate that Mn prefers the subsurface sites of Rh metal surfaces and is able to emerge onto the surface forming MnRh surface alloy once the oxidative O/OH adsorbates are present. Our machine-learning-based transition state exploration further helps to resolve automatedly the whole reaction network, including 74 elementary reactions on various MnRh surface sites, and reveals that the Mn-Mn dimeric site at the monatomic step edge is the true active site for C2 oxygenate formation. The turnover frequency of the C2 product on the Mn-Mn dimeric site at MnRh steps is at least 107 higher than that on pure Rh steps from our microkinetic simulations, with the selectivity to the C2 product being 52% at 523 K. Our results demonstrate the key catalytic role of Mn-Mn dimeric sites in allowing C-O bond cleavage and facilitating the hydrogenation of O-terminating C2 intermediates, and rule out Rh metal by itself as the active site for CO hydrogenation to C2 oxygenates.