Activity For Ethylene Hydrogenation Research Articles

Dynamic structural evolution of MgO-supported palladium catalysts: from metal to metal oxide nanoparticles to surface then subsurface atomically dispersed cations.

Supported noble metal catalysts, ubiquitous in chemical technology, often undergo dynamic transformations between reduced and oxidized states-which influence the metal nuclearities, oxidation states, and catalytic properties. In this investigation, we report the results of in situ X-ray absorption spectroscopy, scanning transmission electron microscopy, and other physical characterization techniques, bolstered by density functional theory, to elucidate the structural transformations of a set of MgO-supported palladium catalysts under oxidative treatment conditions. As the calcination temperature increased, the as-synthesized supported metallic palladium nanoparticles underwent oxidation to form palladium oxides (at approximately 400 °C), which, at approximately 500 °C, were oxidatively fragmented to form mixtures of atomically dispersed palladium cations. The data indicate two distinct types of atomically dispersed species: palladium cations located at MgO steps and those embedded in the first subsurface layer of MgO. The former exhibit significantly higher (>500 times) catalytic activity for ethylene hydrogenation than the latter. The results pave the way for designing highly active and stable supported palladium hydrogenation catalysts with optimized metal utilization.

Activation by O2 of AgxPd1-x Alloy Catalysts for Ethylene Hydrogenation.

A composition spread alloy film (CSAF) spanning all of AgxPd1-x composition space, xPd = 0 → 1, was used to study catalytic ethylene hydrogenation with and without the presence of O2 in the feed gas. High-throughput measurements of the ethylene hydrogenation activity of AgxPd1-x alloys were performed at 100 Pd compositions spanning xPd = 0 → 1. The extent of ethylene hydrogenation was measured versus xPd at reaction temperatures spanning T = 300 → 405 K and inlet hydrogen partial pressures spanning PH2in = 70 → 690 Torr. The inlet ethylene partial pressure was constant at PC2H4in = 25 Torr, and the O2 inlet partial pressure was either PO2in = 0 or 15 Torr. When PO2in = 0 Torr, only those alloys with xPd ≥ 0.90 displayed observable ethylene hydrogenation activity. As expected, the most active catalyst was pure Pd, which yielded a maximum conversion of ∼0.4 at T = 405 K and PH2in = 690 Torr. Adding a constant O2 partial pressure of PO2in = 15 Torr to the feed stream dramatically increased the catalytic activity across the CSAF at all experimental conditions and catalyst compositions without inducing catalytic ethylene combustion and without measurable O2 consumption. The presence of PO2in = 15 Torr more than doubled the maximum achievable conversion on Pd to ∼0.9 and activated alloys with as little as xPd = 0.6 for ethylene hydrogenation. Measurement of the reaction order with respect to hydrogen, nH2, showed that nH2 ≈ 0 when PO2in = 15 Torr on high xPd alloys but that nH2 increases to values between 0.5 and 1 as xPd decreases or when PO2in = 0 Torr. We attribute this PO2in-induced change in nH2 to a change in the reaction mechanism resulting from different functional catalyst surfaces: one that is O2-activated and Pd-rich and one that is Ag-capped with low activity. Both are extremely sensitive to the bulk alloy composition, xPd, and the reaction temperature, T. These results show that the activity of AgPd catalysts for ethylene hydrogenation depends strongly on the operational conditions. Furthermore, we demonstrate that the exposure of AgPd catalysts to 15 Torr of O2 at moderate temperatures leads to enhanced catalyst performance, presumably by stimulating both Pd segregation to the topmost surface and Pd activation for ethylene hydrogenation.

Investigating the effect of metal nuclearity on activity for ethylene hydrogenation by metal-organic-framework-supported oxy-Ni(II) catalysts

Metal-organic frameworks (MOFs), thanks to their well-defined coordination sites, are promising for studying catalytically active structures. To understand the effect of metal nuclearity in MOF-supported catalysts, we adjusted the nuclearity of oxy-Ni(II) supported on the zirconia-like nodes of MOF NU-1000 by modulating the Ni loading, where the organic linkers served to prevent node-to-node migration of grafted nickel ions. At the single-node level, we grafted auxiliary structural linkers, naphthalene dicarboxylate, to reduce the number of binding/grafting sites. We found higher catalytic rates at higher Ni loading for ethylene hydrogenation on a per-nickel-ion basis, despite the similar chemical environment of Ni(II) at different loadings; catalysts consisting mainly of pairs of nickel ions were more reactive than those of single nickel ions. These observations illustrate the need for at least two proximal nickel ions for effective catalysis – presumably one for hydrogen binding and heterolytic splitting and one for ethylene binding and activation.

The Molecular Path Approaching the Active Site in Catalytic Metal-Organic Frameworks.

How molecules approach, bind at, and release from catalytic sites is key to heterogeneous catalysis, including for emerging metal-organic framework (MOF)-based catalysts. We use in situ synchrotron X-ray scattering analysis to evaluate the dominant binding sites for reagent and product molecules in the vicinity of catalytic Ni-oxo clusters in NU-1000 with different surface functionalization under conditions approaching those used in catalysis. The locations of the reagent and product molecules within the pores can be linked to the activity for ethylene hydrogenation. For the most active catalyst, ethylene reagent molecules bind close to the catalytic clusters, but only at temperatures approaching experimentally observed onset of catalysis. The ethane product molecules favor a different binding location suggesting that the product is readily released from the active site. An unusual guest-dependence of the framework negative thermal expansion is documented. We hypothesize that reagent and product binding sites reflect the pathway through the MOF to the active site and can be used to identify key factors that impact the catalytic activity.

Front Cover: Tuning Ligand‐Coordinated Single Metal Atoms on TiO2 and their Dynamic Response during Hydrogenation Catalysis (ChemSusChem 18/2021)

The Front Cover shows ethylene hydrogenation catalysis by single Pt atoms stabilized by organic ligands on titanium dioxide powder. Ligand-coordinated supported catalysts (LCSCs) are of growing interest for heterogeneous single-atom catalysis because of their high stability and uniform active sites. In this work, the choice of ligand has a significant effect on ethylene hydrogenation activity and on the stability of the catalyst. The supported Pt-ligand catalyst showed excellent in-situ stability against structural and chemical change. Ir LCSCs on TiO2 also showed excellent stability in the hydrogenation of ethylene. More information can be found in the Full Paper by X. Zhou et al.

A strategy for fast and facile embedding platinum nanoparticles in silicalite‑1 crystallites with a stable and catalytic active structure

A strategy for embedding ultrasmall Pt nanoparticles in silicalite-1 (S-1) was proposed. It consists of treating the zeolite with tetrapropylammonium hydroxide (TPAOH) solution, loading Pt, and recrystallizing the resultant composite in a silicon-containing Na+-free TPAOH solution at 170 °C for 6 h. Complementary characterizations confirm that, the first step is facile and effective for creating plenty of mesopores in the zeolite, and the last step is fast and favorable for embedding ultrasmall Pt nanoparticles in S-1 yielding the Pt@S-1 hybrid. This material displayed excellent resistance to sintering and much higher activity for ethylene hydrogenation and CO oxidation than conventional Pt-loading catalyst.

Structure and Activity Transition from Oxidized to Metallic Tungsten for Catalytic Hydrogenation: A Density Functional Theory Study

Oxide-based hydrogenation catalysts have attracted intensive interest, but the relationships between their composition, structure, and reaction mechanism are still ambiguous. Here, we conducted density functional theory (DFT) calculation on ethylene hydrogenation over WO3, WO2.72, WO2, and W catalysts to explore how the structure and catalytic activity change with the composition and explain why neither WO3 nor metallic W is a good hydrogenation catalyst but WO3–x is. Calculations on the geometric and electronic structures show a transition from semiconductor to metal-like with more W–W metal bonds appearing from the corresponding oxidized to metallic tungsten. Correspondingly, the H2 dissociation mechanism changes from heterotypic on the semiconductor WO3 and WO2.72 to homotypic on the metal-like WO2 and metallic W, and the adsorption strength of the dissociation product (2H) enhances from oxidized to metallic W. Calculations on the stepwise hydrogenation indicate that H2 dissociation is the rate-limiting step (RLS) for WO3 and WO2.72, whereas the following hydrogenation step (C2H5 + H → C2H6) is the RLS for WO2 and W. By linear fitting the binding energies of H2, 2H, H + C2H5, and C2H6 adsorptions, a perfect correlation between (H + C2H5) and (2H) adsorptions is observed; thus, the catalytic activity and mechanism can be evaluated using 2H adsorption behavior as the descriptor. On the basis of the calculation results, we predict that the best ethylene hydrogenation activity will show at 2 < x < 2.72 for WOx. This work suggests that controlling the composition between the stoichiometric oxide and the metal may be an effective way to develop an active hydrogenation catalyst.

Sinter‐Resistant Platinum Catalyst Supported by Metal–Organic Framework

AbstractSingle atoms and few‐atom clusters of platinum are uniformly installed on the zirconia nodes of a metal‐organic framework (MOF) NU‐1000 via targeted vapor‐phase synthesis. The catalytic Pt clusters, site‐isolated by organic linkers, are shown to exhibit high catalytic activity for ethylene hydrogenation while exhibiting resistance to sintering up to 200 °C. In situ IR spectroscopy reveals the presence of both single atoms and few‐atom clusters that depend upon synthesis conditions. Operando X‐ray absorption spectroscopy and X‐ray pair distribution analyses reveal unique changes in chemical bonding environment and cluster size stability while on stream. Density functional theory calculations elucidate a favorable reaction pathway for ethylene hydrogenation with the novel catalyst. These results provide evidence that atomic layer deposition (ALD) in MOFs is a versatile approach to the rational synthesis of size‐selected clusters, including noble metals, on a high surface area support.

Sinter-Resistant Platinum Catalyst Supported by Metal-Organic Framework.

Single atoms and few-atom clusters of platinum are uniformly installed on the zirconia nodes of a metal-organic framework (MOF) NU-1000 via targeted vapor-phase synthesis. The catalytic Pt clusters, site-isolated by organic linkers, are shown to exhibit high catalytic activity for ethylene hydrogenation while exhibiting resistance to sintering up to 200 °C. In situ IR spectroscopy reveals the presence of both single atoms and few-atom clusters that depend upon synthesis conditions. Operando X-ray absorption spectroscopy and X-ray pair distribution analyses reveal unique changes in chemical bonding environment and cluster size stability while on stream. Density functional theory calculations elucidate a favorable reaction pathway for ethylene hydrogenation with the novel catalyst. These results provide evidence that atomic layer deposition (ALD) in MOFs is a versatile approach to the rational synthesis of size-selected clusters, including noble metals, on a high surface area support.

Ethylene Hydrogenation on a Platinum Nanocoating at Various Electric Potentials

The possibility of controlling the rate of ethylene hydrogenation on a platinum nanocoating is established by applying to it electric potentials of different polarities and magnitudes from an external voltage source. At a negative potential of −10 V, the hydrogenation rate increases by 4%, whereas at a positive potential of +10 V, the hydrogenation rate increases by 42% under the conditions of the experiment at room temperature, atmospheric pressure, and an initial mixture composition of 0.09C2H4 + 0.91H2. Quantum-chemical calculations of the energy of the reaction of platinum hydride with hydrogen, Pt2H2 + H2 → Pt2H3 + H, and the energy characteristics of similar reactions involving negatively and positively charged Pt2H2 are performed. It has been demonstrated that the presence of a negative or positive charge on Pt2H2 lowers the endothermicity of formation of H radicals by 18.4 or 22.5 kcal/mol, respectively. Based on the calculation results, a mechanism is proposed to explain the effect of the charge of a platinum coating on its catalytic activity in ethylene hydrogenation.

Ethylene Hydrogenation over Pt/TiO2: A Charge-Sensitive Reaction

Controlled charge transfer between a support and small metal particles provides unique opportunities to tune the activity of supported metal catalysts, as first proposed by Schwab [G. M. Schwab et al., Angew. Chem. 1959, 71, 101–104]. By controlling the thickness of polycrystalline anatase TiO2 films, the TiO2 carrier concentration can be manipulated by an order of magnitude. When 1 nm Pt particles are deposited on these TiO2 films, the variation in the charge transfer between the TiO2 support and the Pt particles is found to dramatically increase the ethylene hydrogenation activity. The sensitivity of ethylene hydrogenation to charge transfer was anticipated from the large effect of the Pt charge on the ethylene and ethylidyne adsorption energy, e.g., compared to CO and H. Our results demonstrate that the controllable Schwab effect provides a powerful tool to tune catalytic activity. An even larger effect can be expected for supported sub-nanometer clusters, and for the selectivity of hydrogenation reactions.

Use of Hydrogen Chemisorption and Ethylene Hydrogenation as Predictors for Aqueous Phase Reforming of Lactose over Ni@Pt and Co@Pt Bimetallic Overlayer Catalysts

Overlayer Pt on Ni (Ni@Pt) or Co (Co@Pt) were synthesized and tested for H2 generation from APR of lactose. H2 chemisorption descriptor showed that Ni@Pt and Co@Pt overlayer catalysts had reduced H2 adsorption strength compared to a Pt only catalyst, which agree with computational predictions. The overlayer catalysts also demonstrated lower activity for ethylene hydrogenation than the Pt only catalyst, which likely resulted from decreased H2 binding strength decreasing the surface coverage of H2. XAS results showed that overlayer catalysts exhibited higher white line intensity than the Pt catalyst, which indicates a negative d-band shift for the Pt overlayer, further providing evidence for overlayer formation. Lactose APR studies showed that lactose can be used as feedstock to produce H2 and CO under desirable reaction conditions. The Pt active sites of Ni@Pt and Co@Pt overlayer catalysts showed significantly enhanced H2 production selectivity and activity when compared with that of a Pt only catalyst. Th...

Hydrodeoxygenation of guaiacol over Ni@Pd and Ni@Pt bimetallic overlayer catalysts

First principle computational studies have predicted that formation of Pd or Pt overlayer on Ni base metal would cause a negative shift of the d-band center of surface Pd or Pt, resulting in reduced binding strength for adsorbed species. In this paper, silica alumina supported Ni@Pd and Ni@Pt overlayer catalysts were synthesized via directed deposition technique and tested for hydrodeoxygenation of guaiacol at atmospheric pressure. Several characterization techniques such as hydrogen chemisorption, ethylene hydrogenation descriptor reaction, XRD and TEM studies were employed to characterize the catalysts. H2 chemisorption results showed that Ni@Pd and Ni@Pt overlayer catalysts had reduced H2 adsorption strength compared to Pd or Pt only catalysts, which agreed with the computational prediction. As predicted, the overlayer catalysts also showed lower activity for ethylene hydrogenation than Pd or Pt only catalysts. XRD and TEM studies indicated that overlayers have been successfully deposited atop supported nickel. Guaiacol hydrodeoxygenation results showed that, especially at low reaction temperature, Pd and Pt active sites of overlayer catalysts showed significantly enhanced deoxygenation activity compared with that of Pd or Pt only catalysts. Further studies showed that guaiacol could be completely deoxygenated using silica alumina supported metal catalysts, yielding benzene, toluene, and xylenes as major products.

Characterization of Ni@Pt and Co@Pt overlayer catalysts using adsorption, reactivity descriptors, and X-ray absorption spectroscopy

Alumina supported bimetallic overlayer catalysts of platinum on nickel (Ni@Pt) and platinum on cobalt (Co@Pt) were synthesized using the directed deposition technique. First principle computational and single crystal literature studies in overlayer systems have indicated that overlayer formation can have an electronic effect on the surface Pt metal that ultimately will result in decreased binding strength for adsorbed species such as H2 and CO. In order to determine electronic modifications to the platinum metal surface from overlayer synthesis, several characterization techniques were examined. These techniques include: hydrogen and carbon monoxide chemisorption, an ethylene hydrogenation descriptor reaction, and X-ray absorption spectroscopy studies. Non-structured bimetallic alloy catalysts were also examined (Ni–Pt and Co–Pt) and when compared to overlayer catalysts exhibited notably different behavior. Hydrogen and carbon monoxide chemisorption results demonstrated that overlayer catalysts showed decreased adsorption of both species when compared to both their Pt and non-structured bimetallic counterparts indicating the formation of an overlayer system rather than alloyed or monometallic clusters on the support surface. Ni@Pt and Co@Pt catalysts also demonstrated reduced activity for ethylene hydrogenation when compared to both Pt only and non-structured bimetallic alloy catalysts, further indicating overlayer formation. An observed increase in XAS white line intensity for the overlayer catalysts when compared to pure Pt also indicates a change in the electronic structure of the Pt overlayer consistent with chemisorption, reactivity, and computational predictions. These findings all suggest that the directed deposition technique effectively modified the behavior of the surface Pt in a manner consistent with literature predictions.

Overlayer Bimetallic Catalysts or Particle Size Effects? A Comparison of Re@Pd Catalysts with Different Particle Sizes

Alumina supported Re@Pd bimetallic overlayer catalysts of different loading, dispersion, and particle size were synthesized using the directed deposition technique. Past computational and empirical studies have shown that through depositing a monoatomic pseudomorphic overlayer of one metal atop a different base metal it is possible to alter the adsorption and catalytic properties of the system when compared to both monometallic counterparts. It has been shown that a 2 wt% Re@Pd overlayer catalyst was capable of having lower hydrogen adsorption strength and lower activity for ethylene hydrogenation than its pure Pd counterpart. Particle size effects have long been known to play a role in the behavior of various catalysts as smaller particles tend to have more unsaturated, highly reactive, metal sites (e.g., kinks, corners, edges, etc.). In order to demonstrate that the observed changes in activity were not merely particle size effects, 7 wt% Re@Pd catalysts have been synthesized through similar methods and consist of particles much larger than those in the 2 wt% sample (viz., 6.3 and 2.7 nm). Similar reductions in hydrogen adsorption quantity and strength were seen for both Re@Pd samples and ethylene hydrogenation activity proved consistent. Both results indicate that the formation of a Pd overlayer was reason for this observed change in adsorption and activity and the effect cannot be merely attributed to particle size effects.

Surface‐Mediated Synthesis of Dimeric Rhodium Catalysts on MgO: Tracking Changes in the Nuclearity and Ligand Environment of the Catalytically Active Sites by X‐ray Absorption and Infrared Spectroscopies

The preparation of dinuclear rhodium clusters and their use as catalysts is challenging because these clusters are unstable, evolving readily into species with higher nuclearities. We now present a novel synthetic route to generate rhodium dimers on the surface of MgO by a stoichiometrically simple surface-mediated reaction involving [Rh(C(2)H(4))(2)] species and H(2). X-ray absorption and IR spectra were used to characterize the changes in the nuclearity of the essentially molecular surface species as they formed, including the ligands on the rhodium and the metal-support interactions. The support plays a key role in stabilizing the dinuclear rhodium species, allowing the incorporation of small ligands (ethyl, hydride, and/or CO) and enabling a characterization of the catalytic performance of the supported species for the hydrogenation of ethylene as a function of the metal nuclearity and ligand environment. A change in the nuclearity from one to two Rh atoms leads to a 58-fold increase in the catalytic activity for ethylene hydrogenation, a reaction involving unsaturated, but stable, dimeric rhodium species.

Role of the Ni:Fe Ratio in Ethylene Hydrogenation Activity for Silica-Supported Ni–Fe Clusters Prepared by Dendrimer-Templating

Ni–Fe nanoparticles of ∼1 nm in size were synthesized by the dendrimer-templating technique using hydroxyl-terminated fourth-generation poly aminoamide (PAMAM) dendrimers. Spectroscopy and electron microscopy results are consistent with 40 atom clusters, in which all atoms are at the surface, as dictated by the synthesis strategy. Following immobilization of the clusters onto silica, the role of Ni:Fe ratio in the bimetallic was addressed for ethylene hydrogenation. A distinctive enhancement (factor of 10) in the catalytic activity was observed for the Ni-rich (Ni:Fe = 3:1) nanoparticles as compared to pure Ni, which was more active than pure Fe and a physical mixture of the pure parent components. Examination of the rate dependencies upon temperature and ethylene partial pressure indicates ethylene and its intermediates’ strong binding as a key kinetic limitation. Using an available semiempirical model, the d-band center of the Ni–Fe systems correlated to turnover frequencies calculated for increasing st...

In-situ carbonization synthesis and ethylene hydrogenation activity of ordered mesoporous tungsten carbide

A new replication- in-situ carbonization route has been developed to synthesize mesoporous tungsten carbide using the block copolymer (poly(ethylene glycol)-block-poly (propyleneglycol)-block-poly(ethylene glycol)) in-situ present in the pore channels of mesoporous silica template as carbon source. The results indicate that the synthesized mesoporous WC has high surface area (143 m 2 g −1) and well-crystallized framework, and possesses high and stable catalytic activity for ethylene hydrogenation reaction. The influence of H 2/C 2H 4 mole ratio and space velocity on the mesoporous WC catalyst performance in the ethylene hydrogenation reaction was studied. This mesoporous WC material with high surface area and porous structure showed much better ethylene hydrogenation activity than the reference WC bought from the company, and the complete ethylene hydrogenation was reached at 250 °C on the prepared mesoporous WC material.

In situ synthesis of catalytic metal nanoparticle-PDMS membranes by thermal decomposition process

Nanoparticles of various transition elements such as palladium, iron, and nickel were synthesized in situ in the polydimethylsiloxane (PDMS) matrix by thermal decomposition of their corresponding acetylacetonate salts. Various complementary techniques such as XRD, TEM and XPS were used to characterize the nanoparticles formed in the polymer matrix. This synthesis route results in relatively monodisperse nanoparticles with a narrow particle size distribution. In addition, the composite films are pore-free and mechanically stable, making them attractive for a range of applications. Palladium-PDMS membranes can be used as catalytic membrane reactors and show enhanced catalytic activity in ethylene hydrogenation.

Effect of organic capping layers over monodisperse platinum nanoparticles upon activity for ethylene hydrogenation and carbon monoxide oxidation

The influence of oleylamine (OA), trimethyl tetradecyl ammonium bromide (TTAB), and polyvinlypyrrolidone (PVP) capping agents upon the catalytic properties of Pt/silica catalysts was evaluated. Pt nanoparticles that were 1.5 nm in size were synthesized by the same procedure (ethylene glycol reduction under basic conditions) with the various capping agents added afterward for stabilization. Before examining catalytic properties for ethylene hydrogenation and CO oxidation, the Pt NPs were deposited onto mesoporous silica (SBA-15) supports and characterized by transmission electron microscopy (TEM), H 2 chemisorption, and elemental analysis (ICP-MS). PVP- and TTAB-capped Pt yielded mass-normalized reaction rates that decreased with increasing pretreatment temperature, and this trend was attributed to the partial coverage of the Pt surface with decomposition products from the organic capping agent. Once normalized to the Pt surface area, similar intrinsic activities were obtained regardless of the pretreatment temperature, which indicated no influence on the nature of the active sites. Consequently, a chemical probe technique using intrinsic activity for ethylene hydrogenation was demonstrated as an acceptable method for estimating the metallic surface areas of Pt. Amine (OA) capping exhibited a detrimental influence on the catalytic properties as severe deactivation and low activity were observed for ethylene hydrogenation and CO oxidation, respectively. These results were consistent with amine groups being strong poisons for Pt surfaces, and revealed the need to consider the effects of capping agents on the catalytic properties.