Investigation of the Pd(1-x)Znxalloy phase diagram usingab initiomodelling approaches

The identification of the stable phases in alloy materials is challenging because composition affects the structural stability of different intermediate phases. Computational simulation, via multiscale modelling approaches, can significantly accelerate the exploration of phase space and help to identify stable phases. Here, we apply such new approaches to understand the complex phase diagram of binary alloys of PdZn, with the relative stability of structural polymorphs considered through application of density functional theory coupled with cluster expansion (CE). The experimental phase diagram has several competing crystal structures, and we focus on three different closed-packed phases that are commonly observed for PdZn, namely the face-centred cubic (FCC), body-centred tetragonal (BCT) and hexagonal close packed (HCP), to identify their respective stability ranges. Our multiscale approach confirms a narrow range of stability for the BCT mixed alloy, within the Zn concentration range from 43.75% to 50%, which aligns with experimental observations. We subsequently use CE to show that the phases are competitive across all concentrations, but with the FCC alloy phase favoured for Zn concentrations below 43.75%, and that the HCP structure favoured for Zn-rich concentrations. Our methodology and results provide a platform for future investigations of PdZn and other close-packed alloy systems with multiscale modelling techniques.

Tracking C-H bond activation for propane dehydrogenation over transition metal catalysts: work function shines

The activation of the C-H bond in heterogeneous catalysis plays a privileged role in converting light alkanes into commodity chemicals with a higher value. In contrast to traditional trial-and-error approaches, developing predictive descriptors via theoretical calculations can accelerate the process of catalyst design. Using density functional theory (DFT) calculations, this work describes tracking C-H bond activation of propane over transition metal catalysts, which is highly dependent on the electronic environment of catalytic sites. Furthermore, we reveal that the occupancy of the antibonding state for metal-adsorbate interaction is the key factor in determining the ability to activate the C-H bond. Among 10 frequently used electronic features, the work function (W) exhibits a strong negative correlation with C-H activation energies. We demonstrate that e^-W can effectively quantify the ability of C-H bond activation, surpassing the predictive capacity of the d-band center. The C-H activation temperatures of the synthesized catalysts also confirm the effectiveness of this descriptor. Apart from propane, e^-W applies to other reactants like methane.

From Lab to Technical CO2 Hydrogenation Catalysts: Understanding PdZn Decomposition

The valorization of CO₂ to produce high-value chemicals, such as methanol and hydrocarbons, represents key technology in the future net-zero society. Herein, we report further investigation of a PdZn/ZrO₂ + SAPO-34 catalyst for conversion of CO₂ and H₂ into propane, already presented in a previous work. The focus of this contribution is on the scale up of this catalyst. In particular, we explored the effect of mixing (1:1 mass ratio) and shaping the two catalyst functions into tablets and extrudates using an alumina binder. Their catalytic performance was correlated with structural and spectroscopic characteristics using methods such as FT-IR and X-ray absorption spectroscopy. The two scaled-up bifunctional catalysts demonstrated worse performance than a 1:1 mass physical mixture of the two individual components. Indeed, we demonstrated that the preparation negatively affects the element distribution. The physical mixture is featured by the presence of a PdZn alloy, as demonstrated by our previous work on this sample and high hydrocarbon selectivity among products. For both tablets and extrudates, the characterization showed Zn migration to produce Zn aluminates from the alumina binder phase upon reduction. Moreover, the extrudates showed a remarkable higher amount of Zn aluminates before the activation rather than the tablets. Comparing tablets and extrudates with the physical mixture, no PdZn alloy was observed after activation and only the extrudates showed the presence of metallic Pd. Due to the Zn migration, SAPO-34 poisoning and subsequent deactivation of the catalyst could not be excluded. These findings corroborated the catalytic results: Zn aluminate formation and Pd⁰ separation could be responsible for the decrease of the catalytic activity of the extrudates, featured by high methane selectivity and unconverted methanol, while tablets displayed reduced methanol conversion to hydrocarbons mainly attributed to the partial deactivation of the SAPO-34.

Synergetic effect between Pd2+ and Ir4+ species promoting direct ethane dehydrogenation into ethylene over bimetallic PdIr/AC catalysts

This work reported the efficient Pd–Ir pairs on the Pd7Ir2/AC-B catalyst achieved a TOF (C2H4) of 756.6 h−1 at 500 °C, and the direct ethane dehydrogenation (EDH) rationale and deactivation mechanism were proposed.

Multifunctional Catalyst Combination for the Direct Conversion of CO2 to Propane

The production of carbon-rich hydrocarbons via CO₂ valorization is essential for the transition to renewable, non-fossil-fuel-based energy sources. However, most of the recent works in the state of the art are devoted to the formation of olefins and aromatics, ignoring the rest of the hydrocarbon commodities that, like propane, are essential to our economy. Hence, in this work, we have developed a highly active and selective PdZn/ZrO₂+SAPO-34 multifunctional catalyst for the direct conversion of CO₂ to propane. Our multifunctional system displays a total selectivity to propane higher than 50% (with 20% CO, 6% C₁, 13% C₂, 10% C₄, and 1% C₅) and a CO₂ conversion close to 40% at 350 °C, 50 bar, and 1500 mL g^-1 h^-1. We attribute these results to the synergy between the intimately mixed PdZn/ZrO₂ and SAPO-34 components that shifts the overall reaction equilibrium, boosting CO₂ conversion and minimizing CO selectivity. Comparison to a PdZn/ZrO₂+ZSM-5 system showed that propane selectivity is further boosted by the topology of SAPO-34. The presence of Pd in the catalyst drives paraffin production via hydrogenation, with more than 99.9% of the products being saturated hydrocarbons, offering very important advantages for the purification of the products.

C-H bond activation in light alkanes: a theoretical perspective

Alkanes are the major constituents of natural gas and crude oil, the feedstocks for the chemical industry. The efficient and selective activation of C-H bonds can convert abundant and low-cost hydrocarbon feedstocks into value-added products. Due to the increasing global demand for light alkenes and their corresponding polymers as well as synthesis gas and hydrogen production, C-H bond activation of light alkanes has attracted widespread attention. A theoretical understanding of C-H bond activation in light hydrocarbons via density functional theory (DFT) and microkinetic modeling provides a feasible approach to gain insight into the process and guidelines for designing more efficient catalysts to promote light alkane transformation. This review describes the recent progress in computational catalysis that has addressed the C-H bond activation of light alkanes. We start with direct and oxidative C-H bond activation of methane, with emphasis placed on kinetic and mechanistic insights obtained from DFT assisted microkinetic analysis into steam and dry reforming, and the partial oxidation dependence on metal/oxide surfaces and nanoparticle size. Direct and oxidative activation of the C-H bond of ethane and propane on various metal and oxide surfaces are subsequently reviewed, including the elucidation of active sites, intriguing mechanisms, microkinetic modeling, and electronic features of the ethane and propane conversion processes with a focus on suppressing the side reaction and coke formation. The main target of this review is to give fundamental insight into C-H bond activation of light alkanes, which can provide useful guidance for the optimization of catalysts in future research.

Dehydrogenation of light alkanes to mono-olefins

In the past several decades, light alkane dehydrogenation to mono-olefins, especially propane dehydrogenation to propylene has gained widespread attention and much development in the field of research and commercial application. Under suitable conditions, the supported Pt-Sn and CrO_x catalysts widely used in industry exhibit satisfactory dehydrogenation activity and selectivity. However, the high cost of Pt and the potential environmental problems of CrO_x have driven researchers to improve the coking and sintering resistance of Pt catalysts, and to find new non-noble metal and environment-friendly catalysts. As for the development of the reactor, it should be noted that low operation pressure is beneficial for improving the single-pass conversion, decreasing the amount of unconverted alkane recycled back to the reactor, and reducing the energy consumption of the whole process. Therefore, the research direction of reactor improvement is towards reducing the pressure drop. This review is aimed at introducing the characteristics of the dehydrogenation reaction, the progress made in the development of catalysts and reactors, and a new understanding of reaction mechanism as well as its guiding role in the development of catalyst and reactor.

Theoretical insights into non-oxidative propane dehydrogenation over Fe3C

Identifying catalysts for non-oxidative propane dehydrogenation has become increasingly important due to the increasing demand for propylene coupled to decreasing propylene production from steam cracking as we shift to lighter hydrocarbon feedstocks. Commercialized propane dehydrogenation (PDH) catalysts are based on Pt or Cr, which are expensive or toxic, respectively. Recent experimental work has demonstrated that earth-abundant and environmentally-benign metals, such as iron, form in situ carbide phases that exhibit good activity and high selectivity for PDH. In this work, we used density functional theory (DFT) to better understand why the PDH reaction is highly selective on Fe3C surfaces. We use ab initio thermodynamics to identify stable Fe3C surface terminations as a function of reaction conditions, which then serve as our models for investigating rate-determining and selectivity-determining kinetic barriers during PDH. We find that carbon-rich surfaces show much higher selectivity for propylene production over competing cracking reactions compared to iron-rich surfaces, which is determined by comparing the propylene desorption barrier to the C-H scission barrier for dehydrogenation steps beyond propylene. Electronic structure analyses of the d-band center and the crystal orbital Hamilton population (COHP) of the carbides demonstrate that the high selectivity of carbon-rich surfaces originates from the disruption of surface Fe ensembles via carbon. Finally, we investigated the role of phosphate in suppressing coke formation and found that the electron-withdrawing character of phosphate destabilizes surface carbon.

Metal-based catalysts for the non-oxidative dehydrogenation of light alkanes to light olefins

This review provides an overview of metal-based catalysts, including Pt-, Pd-, Rh- and Ni-based bimetallic catalysts for non-oxidative dehydrogenation of light alkanes to olefins.

CO2 hydrogenation to methanol and hydrocarbons over bifunctional Zn-doped ZrO2/zeolite catalysts

The tandem process of carbon dioxide hydrogenation to methanol and its conversion to hydrocarbons over mixed metal/metal oxide-zeotype catalysts is a promising path to CO₂ valorization.

Surface Hexagonal Pt1Sn1 Intermetallic on Pt Nanoparticles for Selective Propane Dehydrogenation

A series of 2-3 nm Pt-Sn bimetallic nanoparticles with different Pt-Sn coordination numbers were synthesized by a stepwise approach including electrostatic adsorption and temperature-programmed reduction of metal precursors on the SiO₂ support. In situ synchrotron X-ray diffraction (XRD) and X-ray absorption fine structure (XAFS) demonstrated a highly ordered hexagonal Pt₁Sn₁ intermetallic shell on Pt nanoparticles. The turnover rates (TORs), propylene selectivity, and stability of these bimetallic catalysts significantly surpass those of the monometallic Pt catalyst for propane dehydrogenation. At the same time, TORs increase with increasing the Pt-Sn coordination number, whereas propylene selectivity is not significantly influenced by the Pt-Sn coordination number. Combined with experiments and theoretical calculations, the high propylene selectivity of Pt-Sn bimetallic nanoparticles is attributed to the geometric effects of Sn that reduce the Pt ensembles, and the high TORs are due to the electronic effects that weaken Pt-hydrocarbon chemisorption energies.

Structural trends in the dehydrogenation selectivity of palladium alloys

Alloying is well-known to improve the dehydrogenation selectivity of pure metals, but there remains considerable debate about the structural and electronic features of alloy surfaces that give rise to this behavior. To provide molecular-level insights into these effects, a series of Pd intermetallic alloy catalysts with Zn, Ga, In, Fe and Mn promoter elements was synthesized, and the structures were determined using in situ X-ray absorption spectroscopy (XAS) and synchrotron X-ray diffraction (XRD). The alloys all showed propane dehydrogenation turnover rates 5–8 times higher than monometallic Pd and selectivity to propylene of over 90%. Moreover, among the synthesized alloys, Pd₃M alloy structures were less olefin selective than PdM alloys which were, in turn, almost 100% selective to propylene. This selectivity improvement was interpreted by changes in the DFT-calculated binding energies and activation energies for C–C and C–H bond activation, which are ultimately influenced by perturbation of the most stable adsorption site and changes to the d-band density of states. Furthermore, transition state analysis showed that the C–C bond breaking reactions require 4-fold ensemble sites, which are suggested to be required for non-selective, alkane hydrogenolysis reactions. These sites, which are not present on alloys with PdM structures, could be formed in the Pd₃M alloy through substitution of one M atom with Pd, and this effect is suggested to be partially responsible for their slightly lower selectivity.

Alloying is well-known to improve the dehydrogenation selectivity of pure metals, but there remains considerable debate about the structural and electronic features of alloy surfaces that give rise to this behavior.

On the conversion of CO2 to value added products over composite PdZn and H-ZSM-5 catalysts: excess Zn over Pd, a compromise or a penalty?

Zn was found to possess a dual role in composite PdZn–H-ZSM-5 catalysts for CO₂ hydrogenation reactions: it promotes methanol formation when alloyed with Pd, but inhibits hydrocarbon formation by ion exchange with Brønsted acid sites in H-ZSM-5.

Effects of phosphorus addition on selectivity and stability of Pd model catalysts during cyclohexene dehydrogenation

P addition to Pd increases dehydrogenation selectivity and increases catalyst stability.

Effects of metal cation doping in CeO2 support on catalytic methane steam reforming at low temperature in an electric field

Catalytic methane steam reforming was conducted at low temperature using a Pd catalyst supported on Ce_1−xM_xO₂ (x = 0 or 0.1, M = Ca, Ba, La, Y or Al) oxides with or without an electric field (EF). The effects of the catalyst support on catalytic activity and surface proton hopping were investigated. Results show that Pd/Al-CeO₂ (Pd/Ce_0.9Al_0.1O₂) showed higher activity than Pd/CeO₂ with EF, although their activity was identical without EF. Thermogravimetry revealed a larger amount of H₂O adsorbed onto Pd/Al-CeO₂ than onto Pd/CeO₂, so Al doping to CeO₂ contributes to greater H₂O adsorption. Furthermore, electrochemical conduction measurements of Pd/Al-CeO₂ revealed a larger contribution of surface proton hopping than that for Pd/CeO₂. This promotes the surface proton conductivity and catalytic activity during EF application.

Temperature dependence of electron/ion conductivity of Pd/CeO₂ and Pd/Al-CeO₂ under wet conditions with application of an electric field.

Diffusion-Limited Formation of Nonequilibrium Intermetallic Nanophase for Selective Dehydrogenation

Nonequilibrium intermetallic phases in the nanoscale were realized by diffusion-controlled solid-state transformation, forming SiO₂ supported NPs with Pd core and a CsCl type Pd₁M₁ shell, where M is Sn or Sb. The core-shell geometry is identified from scanning transmission electron microscopy and infrared spectroscopy and the crystal structure is confirmed from in situ synchrotron X-ray diffraction and X-ray absorption spectroscopy. The highly symmetric Pd₁M₁ intermetallic phase has not been reported previously and contains catalytic ensembles with high selectivity toward dehydrogenation of propane. The kinetically limited solid-state reaction is generally applicable to nanoparticle synthesis and could produce materials with desired structures and properties beyond conventional structural limits.

Visualizing Formation of Intermetallic PdZn in a Palladium/Zinc Oxide Catalyst: Interfacial Fertilization by PdHx

Controllable synthesis of well-defined supported intermetallic catalysts is desirable because of their unique properties in physical chemistry. To accurately pinpoint the evolution of such materials at an atomic-scale, especially clarification of the initial state under a particular chemical environment, will facilitate rational design and optimal synthesis of such catalysts. The dynamic formation of a ZnO-supported PdZn catalyst is presented, whereby detailed analyses of in situ transmission electron microscopy, electron energy-loss spectroscopy, and in situ X-ray diffraction are combined to form a nanoscale understanding of PdZn phase transitions under realistic catalytic conditions. Remarkably, introduction of atoms (H and Zn in sequence) into the Pd matrix was initially observed. The resultant PdH_x is an intermediate phase in the intermetallic formation process. The evolution of PdH_x in the PdZn catalyst initializes at the PdH_x /ZnO interfaces, and proceeds along the PdH_x ⟨111⟩ direction.

Enhanced methane activation on diluted metal–metal ensembles under an electric field: breakthrough in alloy catalysis

Synergy between an electric field and Pd–Zn alloy allows improved catalytic activities in the steam reforming of methane.

Identification of the structure of the Bi promoted Pt non-oxidative coupling of methane catalyst: a nanoscale Pt3Bi intermetallic alloy

Identification of a Pt₃Bi nanoscale, surface intermetallic alloy catalyst for non-oxidative coupling of methane (NOCM).

Two-dimensional transition metal carbides as supports for tuning the chemistry of catalytic nanoparticles

Supported nanoparticles are broadly employed in industrial catalytic processes, where the active sites can be tuned by metal-support interactions (MSIs). Although it is well accepted that supports can modify the chemistry of metal nanoparticles, systematic utilization of MSIs for achieving desired catalytic performance is still challenging. The developments of supports with appropriate chemical properties and identification of the resulting active sites are the main barriers. Here, we develop two-dimensional transition metal carbides (MXenes) supported platinum as efficient catalysts for light alkane dehydrogenations. Ordered Pt₃Ti and surface Pt₃Nb intermetallic compound nanoparticles are formed via reactive metal-support interactions on Pt/Ti₃C₂T_x and Pt/Nb₂CT_x catalysts, respectively. MXene supports modulate the nature of the active sites, making them highly selective toward C–H activation. Such exploitation of the MSIs makes MXenes promising platforms with versatile chemical reactivity and tunability for facile design of supported intermetallic nanoparticles over a wide range of compositions and structures.

The performance of supported metal nanoparticle catalysts can be tailored by metal-support interactions, but their use in catalyst design is still challenging. Here, the authors develop two-dimensional transition metal carbides as platforms for designing intermetallic compound catalysts that are efficient for light alkane dehydrogenations.

MOF-Confined Sub-2 nm Atomically Ordered Intermetallic PdZn Nanoparticles as High-Performance Catalysts for Selective Hydrogenation of Acetylene

Controllable synthesis of ultrasmall atomically ordered intermetallic nanoparticles is a challenging task, owing to the high temperature commonly required for the formation of intermetallic phases. Here, a metal-organic framework (MOF)-confined co-reduction strategy is developed for the preparation of sub-2 nm intermetallic PdZn nanoparticles, by employing the well-defined porous structures of calcinated ZIF-8 (ZIF-8C) and an in situ co-reduction therein. HAADF-STEM, HRTEM, and EDS characterizations reveal the homogeneous dispersion of these sub-2 nm intermetallic PdZn nanoparticles within the ZIF-8C frameworks. XRD, XPS, and EXAFS measurements further confirm the atomically ordered intermetallic phase nature of these sub-2 nm PdZn nanoparticles. Selective hydrogenation of acetylene evaluation results show the excellent catalytic properties of the sub-2 nm intermetallic PdZn, which result from the energetically more favorable path for acetylene hydrogenation and ethylene desorption over the ultrasmall particles than over larger-sized intermetallic PdZn as revealed by density functional theory (DFT) calculations. Moreover, this protocol is also extendable for the preparation of sub-2 nm intermetallic PtZn nanoparticles and is expected to provide a novel methodology in synthesizing ultrasmall atomically ordered intermetallic nanomaterials by rationally functionalizing MOFs.

Direct solid state NMR observation of the 105Pd nucleus in inorganic compounds and palladium metal systems

Although ¹⁰⁵Pd is a very challenging nucleus for solid state NMR, these initial observations demonstrate its potential for characterising catalytically relevant Pd metal systems.

Pd–In intermetallic alloy nanoparticles: highly selective ethane dehydrogenation catalysts

2 nm PdIn intermetallic alloy (cubic, CsCl type) nanoparticle catalyst was near 100% selective to ethane dehydrogenation at 600 °C (at 15% conversion) with a dehydrogenation TOR almost 10 times higher than that of monometallic Pd.