Selectivity For Hydrogenation Of Acetylene Research Articles

Same size, same support, same spectator? Selective acetylene hydrogenation on supported Pd nanoparticles.

The selective hydrogenation of acetylene catalyzed by Pd nanoparticles is industrially used to increase the purity of ethylene. Despite the implementation of Pd based catalysts on an industrial scale, little is known about metal-support interactions on a fundamental level due to the complexity of these systems. In this study, the influence of metal-support interactions between Pd nanoparticles and two electronically modified a-SiO2 thin films on acetylene hydrogenation is investigated under ultra-high vacuum (UHV) conditions. The hydrogenation is performed under isothermal reaction conditions using a pulsed molecular beam reactive scattering (pMBRS) technique. Besides the activity and selectivity of clean Pd particles also the impact of dehydrogenated species intentionally introduced a priori is elucidated, whereas the active phase of the catalyst is additionally characterized by CO infrared reflection-absorption spectroscopy (IRRAS) and post-mortem temperature-programmed reaction (TPR). Metal-support interactions are found to influence the catalytic properties of Pd particles by charge-transfer, where positive charging leads to increased activity for acetylene hydrogenation. However, the increased activity is accompanied by formation of undesired byproducts. The active sites for acetylene and ethylene hydrogenation are shown to be different as previously proposed by the A and E model. The availability of the two different active sites on the Pd nanoparticles is determined by dehydrogenated species, whose nature and stability can be tuned by metal-support interactions. Based on these findings an electronic model is proposed how selectivity for acetylene hydrogenation can be steered solely by metal-support interactions leading to blocking of unselective sites in situ.

Microkinetic simulations of acetylene(acetylene-d2) hydrogenation(deuteration) on Ag nanoparticles

The semi-hydrogenation(deuteration) of alkynes plays an essential role in high-value chemical production, and the reaction mechanism between H and D remains elusive. In this work, advanced microkinetic modeling is combined with the energetics from the first principles calculation to investigate the activity and selectivity of acetylene hydrogenation (acetylene-d2 deuteration) on Ag nanoparticles. By constructing a polyhedral model, it can be found that the olefin production on Ag nanoparticles converged when the size was at the diameter of 6 nm. Furthermore, it is found that the effective production rates are contributed by Ag(100) and Ag55 (corner site). The temperature considerably affects the free reaction energy (ΔG) between hydrogenation and deuteration. Consistent with the experiment, our kinetic simulations indicate that hydrogenation gives rise to a reaction rate approximately 3 times larger than the TOF of deuteration.

Study on the reaction mechanism of acetylene selective hydrogenation catalysts Pd-Ag/Al2O3

Taking active component Pd-Ag alloy and catalyst carrier Al2O3 as the research object, the catalytic mechanism of acetylene hydrogenation reaction was explored by quantum mechanics theory and Monte Carlo method. In this paper, the activation energy on the catalyst surface, H2 dissociation process, acetylene and ethylene hydrogenation reaction were calculated. From the thermodynamic point of view, the increase of the co-catalyst Ag content enhances the desorption of ethylene on the catalyst surface, reduces the probability of ethane formation, and thus enhances the selectivity of acetylene hydrogenation. Through further process simulation studies, the quantum chemistry method and Monte Carlo method can be used to obtain the mechanism and optimal conditions for the selective catalytic hydrogenation of acetylene, with saving a lot of manpower and material resources.

Effect of IB-metal on Ni/SiO2 catalyst for selective hydrogenation of acetylene

The IB metal (Au, Ag and Cu) alloyed Pd single atom catalysts had been proved to be efficient in promoting the selectivity for hydrogenation of acetylene to ethylene. As a base metal in the same group as Pd, the Ni-based catalysts are also active for hydrogenation reactions. Herein, the effects of the IB metals on the Ni/SiO2 catalyst for the selective hydrogenation of acetylene were systematically studied. Different from the Pd/SiO2 catalyst, the monometallic Ni/SiO2 catalyst is not active at low temperatures. The addition of the IB metals to the Ni/SiO2 catalysts can greatly enhance the activity. Besides, the catalytic activity of the AuNix/SiO2 and CuNix/SiO2 catalysts increase with the reduction temperature, while the AgNix/SiO2 catalysts are not sensitive to the pretreatment temperature. The origin of the effect of the different IB metals on the Ni-based catalysts for selective hydrogenation of acetylene is discussed based on the characterizations by XRD, TPR and microcalorimetric measurements.

Pd Single-Atom Catalysts on Nitrogen-Doped Graphene for the Highly Selective Photothermal Hydrogenation of Acetylene to Ethylene.

The selective hydrogenation of acetylene to ethylene in an ethylene-rich gas stream is an important process in the chemical industry. Pd-based catalysts are widely used in this reaction due to their excellent hydrogenation activity, though their selectivity for acetylene hydrogenation and durability need improvement. Herein, the successful synthesis of atomically dispersed Pd single-atom catalysts on nitrogen-doped graphene (Pd1 /N-graphene) by a freeze-drying-assisted method is reported. The Pd1 /N-graphene catalyst exhibits outstanding activity and selectivity for the hydrogenation of C2 H2 with H2 in the presence of excess C2 H4 under photothermal heating (UV and visible-light irradiation from a Xe lamp), achieving 99% conversion of acetylene and 93.5% selectivity to ethylene at 125 °C. This remarkable catalytic performance is attributed to the high concentration of Pd active sites on the catalyst surface and the weak adsorption energy of ethylene on isolated Pd atoms, which prevents C2 H4 hydrogenation. Importantly, the Pd1 /N-graphene catalyst exhibits excellent durability at the optimal reaction temperature of 125 °C, which is explained by the strong local coordination of Pd atoms by nitrogen atoms, which suppresses the Pd aggregation. The results presented here encourage the wider pursuit of solar-driven photothermal catalyst systems based on single-atom active sites for selective hydrogenation reactions.

The DFT Study of Single-Atom Pd1/g-C3N4 Catalyst for Selective Acetylene Hydrogenation Reaction

Inspired by the recently experimental discovered that the single-atom Pd1/g-C3N4 catalyst exhibited higher ethylene selectivity for hydrogenation of acetylene, we systematically investigate the mechanism of such reactions over Pd1/g-C3N4 by using the B3LYP method of density functional theory. We found that Pd1/g-C3N4 can catalytic acetylene hydrogenation with high of selectivity but low of activity energy, which is consistent with the experimental results. The diagram of the whole reaction path over single-atom Pd1/C3N4 catalyst and the corresponding energy barrier at each step.

The reaction mechanism and selectivity of acetylene hydrogenation over Ni-Ga intermetallic compound catalysts: a density functional theory study.

Intermetallic compounds (IMCs) have shown excellent catalytic performance toward the selective hydrogenation of acetylene, but the theoretical understanding on this reaction over Ni-based IMCs is rather limited. In this work, the adsorptions of the C2 species, Bader charge, projected density of states (PDOS) and the reaction pathways were calculated by the density functional theory (DFT) method to investigate the mechanism and selectivity for the acetylene hydrogenation on the (111) surface of NinGa (n = 1, 3) IMCs, with a comparative study on the pristine Ni(111) surface. The results indicate that the adsorption energy of acetylene increased along with the Ni/Ga ratio, therefore a feasible acetylene adsorption on the Ga-rich surface guaranteed a low effective barrier, leading to the best activity for the NiGa(111) surface among three surfaces. Bader charge analysis shows that electrons transferred from Ga atoms to Ni atoms and further delivered to C2 species, decreasing the adsorption capacity of C2 species on NiGa(111) in comparison with those on Ni(111) and Ni3Ga(111). The reaction pathway of acetylene hydrogenation to ethylene via vinyl or even over-hydrogenation to ethane via ethyl is more favorable than the pathway involving the ethylidene intermediate on all surfaces. Moreover, the ethylene selectivity has a positive correlation with the gallium content by comparing the desorption barrier with the hydrogenation barrier of ethylene, thus the NiGa(111) surface also exhibits the best selectivity. Therefore, the NiGa(111) surface demonstrates to be an excellent reaction facet for the semihydrogenation of acetylene, which agreed with the experimental findings, and would provide helpful instructions for designing and preparing highly-selective and noble-substitute catalysts of alkyne semihydrogenation.

Influence of surface structures, subsurface carbon and hydrogen, and surface alloying on the activity and selectivity of acetylene hydrogenation on Pd surfaces: A density functional theory study

The selective hydrogenation of acetylene to ethylene on several Pd surfaces (Pd(111), Pd(100), Pd(211), and Pd(211)-defect) and Pd surfaces with subsurface species (carbon and hydrogen) as well as a number of Pd-based alloys (Pd–M/Pd(111) and Pd–M/Pd(211) (M=Cu, Ag and Au)) are investigated using density functional theory calculations to understand both the acetylene hydrogenation activity and the selectivity of ethylene formation. All the hydrogenation barriers are calculated, and the reaction rates on these surfaces are obtained using a two-step model. Pd(211) is found to have the highest activity for acetylene hydrogenation while Pd(100) gives rise to the lowest activity. In addition, more open surfaces result in over-hydrogenation to form ethane, while the close-packed surface (Pd(111)) is the most selective. However, we also find that the presence of subsurface carbon and hydrogen significantly changes the reactivity and selectivity of acetylene toward hydrogenation on Pd surfaces. On forming surface alloys of Pd with Cu, Ag and Au, the selectivity for ethylene is also found to be changed. A new energy decomposition method is used to quantitatively analyze the factors in determining the changes in selectivity. These surface modifiers are found to block low coordination unselective sites, leading to a decreased ethane production.

Effect of TiO2 support on the structural and electronic properties of PdmAgn clusters: a first-principles study

First-principles density functional theory studies have been carried out to investigate the effects of perfect and defective anatase TiO2 supports on the structural stabilities and electronic properties of PdmAgn(m + n = 2-5) bimetallic clusters. Our results showed that the structures of supported Pd-Ag bimetallic clusters are distorted compared to their structures in the gas phase, which is caused by the balance of the cluster inner-interaction and the metal-support interfacial interaction. In particular, Pd1Ag3 and Pd1Ag4 clusters prefer to form three-dimensional structures on both perfect and defective anatase TiO2 support while their most stable structures in the gas phase are planar. In the most stable structures of supported PdmAgn bimetallic clusters, Pd atoms always occupy the most active sites of TiO2(101) surface, which induced Pd enriched at the interface of TiO2 support and Ag atoms exposed at the surface of the bimetallic cluster. As Ag% increases, the perfect TiO2 support gets more electrons from the Pd-Ag bimetallic cluster, which reduced the stability of the supported Pd-Ag cluster. The Mulliken population and electron density difference analysis demonstrated that the co-deposition of Ag induced the charge of adsorbed Pd on the perfect TiO2 support from positive to negative as a result of charge transfer from the half-filled s-orbital of Ag(5s(1)) to the d-orbit of Pd, and the negative charges of Pd on the defective TiO2 support were also increased by Pd-Ag charge polarization. Therefore, the selectivity of acetylene hydrogenation is enhanced by anatase-TiO2 supported Pd-Ag bimetallic catalyst as it serves as an electron donor.

TiO 2-modified nano-egg-shell Pd catalyst for selective hydrogenation of acetylene

Pd-based egg-shell nano-catalysts were prepared using porous hollow silica nanoparticles (PHSNs) as support, and the as-prepared catalysts were modified with TiO 2 to promote their selectivity for hydrogenation of acetylene. Pd nanoparticles were loaded evenly on PHSNs and TiO 2 was loaded on the active Pd particles. The effects of reduction time and temperature and the amount of TiO 2 added on catalytic performances were investigated by using a fixed-bed micro-reactor. It was found that the catalysts showed better performance when reduced at 300 °C than at 500 °C, and if reduced for 1 h than 3 h. When the amount of Ti added was 6 times that of Pd, the catalyst showed the highest ethylene selectivity.

Effect of nickel concentration profile on selectivity of acetylene hydrogenation.

The effect of the nickel concentration profile on the selectivity of acetylene hydrogenation was investigated by using nickel/alumina catalysts with different concentration profiles: egg shell of thin layer, egg shell of thick layer, uniform, and egg yolk. A model calculation of selectivity was also carried out. The selectivity of ethylene showed the following order: egg shell of thin layer > egg shell of thick layer > uniform > egg yolk. The result of the model calculation showed comparatively good agreement with the experimental results.

Acetylene hydrogenation selectivity control using Pd/Cu catalysts

The hydrogenation of acetylene impurity in ethylene has been studied over 0.04 wt% Pd/Al2O3 catalysts modified by Cu. The undesired hydrogenation of ethylene could be suppressed by Cu.14C labelling proved that the intrinsic selectivity is always 95–98% of Cu or gaseous CO reduces the parallel hydrogenation of ethylene on the support, leaving the intrinsic selectivity unchanged.

Flow reactor for kinetic studies with simultaneous X-ray phase analysis of a catalyst

A flow reactor for kinetic studies of heterogeneous catalytic reactions with simultaneous X-ray phase analysis of the catalyst is described. As an example, the correlation between the selectivity of acetylene hydrogenation and the β-Pd hydride concentration in palladium catalysts is shown.

Radiochemical studies of chemisorption and catalysis: VII. Direct observation of ethylene-C 14 behavior on nickel, rhodium, palladium, iridium, and platinum during chemisorption and hydrogenation

Ethylene-C 14 tracer studies have been made of the adsorption and hydrogenation of ethylene at 20°C using 5% alumina-supported nickel, rhodium, palladium, iridium, and platinum catalysts. The effects of an equimolar mixture of nonradioactive ethylene and hydrogen, nonradioactive ethylene, and hydrogen on preadsorbed ethylene-C 14 have been observed using an end-window G-M counter placed within the catalyst vessel. In all cases only part of the preadsorbed ethylene was removed from the surface and it was independent of the subsequent treatment. Average percentages of initial radioactivity retained in the surface were Pd, 63.5%; Ni, 24.0%; Rh, 22.5%; Ir, 16;% and Pt, 6.5%. Evacuation following ethylene-C 14 adsorption gave similar retention values. With nickel catalysts, pretreatment with hydrogen at 20°C did not change the retention of radioactivity, and at surface saturation an equilibrium between gas-phase and adsorbed ethylene was observed. Ethylene-C 14 containing 8% acetylene-C 14 behaved similarly to ethylene-C 14 with respect to retention of radioactivity. However, acetylene-C 14 could displace ethylene from the metal surfaces. Attempts to elucidate the nature of the adsorbed species have been made, correlations being sought between retention, heat of adsorption, and the ability of the metals to break carbon-hydrogen bonds. A possible correlation between retention and selectivity for acetylene hydrogenation is also discussed.