Chemical Composition Of Catalyst Research Articles

A Photoelectrochemical and Spectroscopic Studies of Photoelectrodes with Hematite∣Catalyst∣Electrolyte Interface Toward Oxygen Evolution Reaction

One of the recent issues in the world is climate change caused by global warming. Many countries have set an ultimate goal for carbon neutrality to prevent from further increase of the global temperatures. In this regard, research on development of renewable energy sources without any carbon emissions has been intensively conducted. Among many promising chemical fuels with sustainability, Hydrogen (H2) can be produced by water (H2O) splitting.Especially, water splitting driven by photoelectrochemical reactions has great advantages in H2 generation: photovoltage generated by light absorption in semiconductors may significantly reduce the applied overpotential which should be provided by external circuits. This photoelectrochemical system, called photoelectrochemical cell (PEC) for water splitting is basically composed of photoelectrodes, catalysts, and membranes for generation of photogenerated charge carriers, reducing overpotential by improving charge transfer kinetics, and prevention of H2 and Oxygen (O2) crossover, respectively. When photons from solar light with energies greater than band gap are incident on the photoelectrodes, an electron-hole pair (EHP) is formed in the semiconductor. These EHPs are separated and transported to the surface, driven by band bending formed at the semiconductors. In photoanodes, the most of the charge carriers arrived at the electrode surface are holes (h+s), which oxidizes H2O at the photoelectrode-electrolyte interface. The transfer of holes accompanies the proton transfers, thereby producing O2. The separated electrons (e−s) should travel to the (photo)cathode along the external circuit and generate H2. However, the overall efficiency of water splitting using PEC still confronts many challenges for wide commercialization, including the low solar-to-hydrogen (STH) efficiencies. In order to improve the performance, the semiconductor efficiently utilizes the photons in the visible light region, which takes 45% of solar irradiation. In this perspective, hematite (α-Fe2O3) is one of the most promising candidates for the semiconductor as photoanodes owing to is narrow band gap of 1.9 ~ 2.2 eV, which is in the range between 550 ~ 650 nm. Meticulous cares should be also taken in catalysts localization to prevent from the parasitic photon loss, as well as the selection of catalytic materials. Because catalysts attribute to the improved charge transfer kinetics but also simultaneously become the sites that obstruct the light penetration to the semiconductor, resulting in the lower light absorption. Furthermore, physical and chemical composition of catalysts on the semiconductor surface significantly influence the charge transfer rates for the photoelectrochemical reactions by varying the electrical field gradient formed at the semiconductor∣catalyst∣electrolyte interface. As several procedures are additionally involved in the PEC, the electrochemical reaction pathways become more complicated rather than the one in electrolyzers utilizing conductive electrodes. Therefore, underlying understanding in semiconductor∣catalyst∣electrolyte interface is crucial to achieve the high performance in PECs.Herein, we demonstrate the hematite∣catalyst∣electrolyte interface as the photoanodes to investigate the photoelectrochemical reaction pathways in water oxidation. Hematite is the perfect material as the semiconductor model, not only due to the narrow band gap in the region of visible light but also the excellent stability even when exposed to the electrolytes and electrochemical environments. The hematite is fabricated by simple hydrothermal reactions, followed by step-wise heat treatment. The structure of hematite is optimized by varying hydrothermal temperatures, affecting the crystallinity and exposed area of the conductive substrate, fluorine-doped tin oxide (FTO). The recombination by back reduction is occurred at the exposed FTO surface by pinholes in hematite, which directly contact the electrolyte. Moreover, the localization of Co-based catalysts is also easily controlled by light-guided electrodeposition. In this hematite∣catalyst∣electrolyte interface, the overall water oxidation pathways are investigated by photoelectrochemical and Raman spectroscopic studies. It is expected that our study in here contributes to the in-depth understanding on the complicated photoelectrochemical reaction mechanism, might leading to the improved performance for wide commercialization of PEC.

Ti3C2OH MXene supported NiPtP nanoparticles with low noble metal content as hydrazine dehydrogenation catalysts

BackgroundHydrazine hydrate is widely used in hydrogen production due to its high hydrogen content and good room temperature stability. However, the decomposition pathway of hydrazine hydrate is related to the type of catalyst, therefore, it is necessary to develop efficient and good selective catalysts. Methodswe have developed a method for preparing NiPtP NPs on MXene using NaH2PO2 as a non-metallic phosphorus source by a one-pot reduction method. The structure and chemical composition of catalysts were studied by Scanning electron microscope (SEM), Energy dispersive spectroscopy (EDS), Transmission electron microscope (TEM), X-ray powder diffractometer (XRD), Inductively coupled plasma-atomic emission spectrometry (ICP-AES), and X-ray photoelectron spectroscopy (XPS). Significant findingsThe results show that the P doping significantly improved the catalytic hydrogen production activity of the Ni0.74Pt0.2P0.06@MXene catalysts in alkaline hydrazine hydrate solutions. The Ni0.74Pt0.2P0.06@MXene catalyst had good catalytic activity for the catalytic production of hydrogen from alkaline hydrazine hydrate solutions under mild conditions, with 100% selectivity for H2 and a TOF value of 794 h−1 at 323 K. This facile synthetic method for the preparation of transition metal phosphides, combined with the high catalytic activity, high hydrogen selectivity and low cost of Ni0.74Pt0.2P0.06@MXene catalysts, may facilitate the practical application of MXene in the field of chemical hydrogen storage, and further fill the gap in the application of transition metal phosphide materials in the field of hydrogen production.

Defective PdRh bimetallic nanocrystals enable enhanced methanol electrooxidation

Although PdRh-based bimetallic nanocrystals have been demonstrated as effective catalysts for methanol electrooxidation, the impact of introduced Rh on CO poisoning resistance and consequently the catalytic activity is still lacking in-depth studies. In this work, defective PdRh bimetallic nanocrystals (PdRh NAs) with different composition ratios are synthesized. The prepared Pd3Rh1 NAs exhibit much higher peak current density and greater durability in comparison with the control sample of Pd1Rh1 NAs, Pd5Rh1 NAs, and the Commercial Pd/C. In addition, we observe that the introduced suitable amount of Rh plays a critical role in promoting the activity and stability of PdRh NAs for methanol electrooxidation reaction (MOR). This work provides an advanced MOR catalyst, and highlights that modulating both surface structure and chemical composition of catalysts is an effective route to realize catalytic property optimization.

The processes of hydrocarbon conversion using catalytic systems

This article describes properties of the catalytic conversion of methane to synthesis gas, makes a review of information on the chemical composition of catalysts and provides assumptions about the mechanism of their action. The facts and generalizations given in the article can be useful in determining ways to improve catalytic systems. The most active and most selective catalytic systems make it possible to optimize existing processes by cutting down energy consumption, cost, emissions and increasing the yield of a valuable product. Increasing the depth of conversion and the integrated use of raw materials, as well as ensuring the environmental cleanliness of the technological processes of processing is achieved by using highly efficient catalysts. With the help of highly efficient catalysts it is probable to increase the depth of conversion, the integrated use of raw materials as well as ensuring the environmental cleanliness of the technological processes of its processing.

Effect of Chemical Composition of Zeolite Catalysts on Their Catalytic Properties in Isobutane Alkylation with Butylenes

The effect of the chemical composition of zeolite catalysts in the CaLaHX form on their catalytic properties in isobutane alkylation with butylenes have been studied. Optimum concentrations of metal oxides providing a butylene conversion of 97–100 wt %, an alkylate yield that is close to the theoretical value (95–100 wt %), and a trimethylpentane (TMP) selectivity at a level of 73 wt % have been determined (wt %: Na2O, 0.5; CaO, 0.6; La2O3, 18.3). It has been found that there is a fair good correlation between the optimum chemical composition of the catalyst samples and their acidity determined by temperature-programmed desorption of ammonia.

Discussion on the reaction mechanism of the photocatalytic degradation of organic contaminants from a viewpoint of semiconductor photo-induced electrocatalysis

Up to now, the well-known mechanism on the photocatalytic degradation of contaminants is mainly based on the reactions derived from the photogenerated electron-hole pairs and their subsequent oxidizing species, and the degradation of organic dyes are especially focused. Whereas, it is rarely concerned the relationship of the oxidation ability of photocatalysts with the oxidative behaviors of contaminants. In this paper, a semiconductor photo-induced electrocatalysis model is proposed to clarify the photocatalytic mechanism by correlating the band structures of BiOI, Bi4O5I2, and Bi4O5Br2 with their visible-light photocatalytic reactivity to phenol, bisphenol A, methylparaben, and propylparaben. It is revealed that the essential condition for degrading a certain contaminant is the valence band potential of photocatalyst is positive than the oxidation potential of contaminant. The valence band position of a photocatalyst can be determined by its optical absorption and electrochemical flat band potential measurement, and the oxidation potential of a contaminant can be measured by cyclic voltammetry technique. In particular, it is indicated that the oxidation potential of contaminants are generally correlated with their molecular orbital energy, which can be predicted by density functional theory (DFT) calculation. And the band positions of photocatalysts evaluated by an empirical equation based on the element electronegativity is in accordance with the actual band potentials. In addition, the bands composition calculated using DFT not only matches the experimental results but also provides the related information on the band structure with the chemical composition of catalysts. Thus, from the viewpoint of photo-induced electrocatalysis and with the help of experimental and theoretical analysis, the most photocatalytic reactions can be predicted and designed, which is universal for the environmental photocatalysis.

Non-PGM membrane electrode assemblies: Optimization for performance

The performance of non-platinum group metals (non-PGM) iron–nitrogen–carbon (Fe–N–C) based cathode in fuel cell operation conditions was studied. The effect of Nafion to catalyst ratio as well as carbon additives on the activity was studied. It was found that ionomer plays a key role in proton and water transport within the MEA. To compensate intrinsic low electronic conductivity of Fe–N–C materials additional carbons such as Vulcan XC72R, Ketjen Black 600 EC, and carbon nanotubes (CNTs) were added to MEA inks. X-ray Phototelectron Spectroscopy studied the effect of these carbon additives on chemical composition of catalysts. It was determined that inks with 35 wt% of Nafion ionomer resulted in the optimal performance of the Fe–N–C cathode catalyst in the air.

Cobalt-modified nickel–zinc catalyst for electrooxidation of methanol in alkaline medium

Cobalt-modified nickel-zinc catalyst CuNi(Zn)Co is prepared on a copper substrate by using electrodeposition. Its catalytic efficiency for methanol oxidation is studied with cyclic voltammetry, chronoamperometry, and chronopotentiometry techniques. The surface morphology and chemical composition of catalyst are characterized by scanning electron microscopy and energy dispersive X-ray spectroscopy. The oxidation kinetic parameters activation energy (Ea), active species on the surface (Γ), and rate constant (k) are determined from cyclic voltammograms which are performed at different methanol concentrations and temperatures. The results show that Ni(Zn)Co catalyst has higher catalytic activity than Ni, Co, and NiZn coatings as a composite catalyst for a promising choice of methanol electrooxidation in the alkaline medium.

Electrocatalysis of Ni-promoted Cd coated graphite toward methanol oxidation in alkaline medium

The nickel–cadmium (Ni-promoted Cd) catalyst for using direct methanol fuel cell (DMFC) applications is electrochemically deposited on a graphite substrate. The activity of methanol oxidation is assessed by recording cyclic voltammetry, electrochemical impedance spectroscopy (EIS) and chronoamperometry (CA) techniques. The surface morphologies and chemical compositions of catalysts are determined by scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and X-ray florescence spectroscopy (XRF). Mean roughness of catalysts is measured by atomic force microscopy (AFM). Kinetic parameters of oxidation such as the anodic electron transfer coefficient (αa), cathodic electron transfer coefficient (αc) and charge transfer rate constant (ks) are calculated. The effect of methanol concentration and temperature on methanol oxidation is also investigated. The electrochemical measurements show that the addition of Cd to single coating enhances the electrocatalytic properties for methanol oxidation and Ni-promoted Cd/C has the best catalytic activity and stability.

Selection of the optimum composition of an alumina support of Pd/Al2O3 catalysts for pyrolysis gasoline hydrogenation

To optimize the chemical composition of catalysts for the selective hydrogenation of diene and vinyl aromatic hydrocarbons in pyrolysis gasoline, we study the effect of the acid-base and textural characteristics of a support modified with additives of sodium compounds on the activity and stability of the catalysts in reactions of hydrogenation and oligomerization of unsaturated compounds. It is shown that the rate of formation of oligomeric compounds depends on the number of Lewis (QCO > 34 kJ/mol) and Bronsted sites (νOH = 3688 cm−1). An increase in their total amount on the catalyst surface leads to an increase in the rate of formation of oligomeric hydrocarbons. The amount of surface condensation products is determined by the concentration of strong aprotic sites with QCO > 35 kJ/mol. Alumina support samples with a high surface concentration of medium-strong Lewis sites, wedge-shaped or conical pores, and the preferential distribution of porometric volume in pores with a diameter of 5–15 nm are characterized by a significant ability to oligomerize unsaturated compounds. Catalysts that contain 0.5 wt % Na exhibit the lowest oligomerization ability and a high stabile activity in reactions of hydrogenation of diene and vinyl aromatic hydrocarbons in pyrolysis gasoline. For the selective hydrogenation of diene and vinyl aromatic hydrocarbons in pyrolysis gasoline, we recommend a catalyst with 0.5 wt % Pd supported from palladium acetylacetonate on δ-Al2O3 modified with 0.5 wt % sodium; it is characterized by the absence of wedge-shaped or conical pores, the preferential (60.7%) distribution of porometric volume in a range of dp > 15 nm, and a low aprotic acidity (L = 3.1 μmol/g), which contributes to the decrease in the amount of resulting condensation products (V = 3.6 μg/(gcat h)) and a high stable activity (DN = 0.68 g J2/100 g) in reactions of hydrogenation of unsaturated compounds.

Facile condensation of indole with benzaldehyde over Keggin-type heteropoly compounds: An initial effort toward catalyst design

A series of catalysts derived from Keggin-type heteropoly compounds (HPCs) were prepared and used as catalysts for the reaction of indole with benzaldehyde to afford bis(3-indolyl)phenylmethane. The results showed that the difference in catalytic activity of HPCs in this reaction depends on the number of introduced vanadium atoms in H 3+ n PMo 12− n V n O 40, kind of support, cesium content in Cs x H 3− x PW 12O 40 and nature of the metal cation in [( n-C 4H 9) 4N] (7− n) PMo 2W 9(M n+ ·H 2O)O 39. The product yield was strongly dependent on the acidic characteristics which, in turn, depend on chemical composition of catalysts. In the presence of H 3+ n PMo 12− n V n O 40 and Cs x H 3− x PW 12O 40, the catalytic activity and acidity decreased with increasing V atom or cesium content. Among supported H 3+ n PMo 12− n V n O 40, H 5PMo 10V 2O 40/γ-Al 2O 3 is a weaker catalyst than bulk one. [( n-C 4H 9) 4N] (7− n) PMo 2W 9(M n+ ·H 2O)O 39 catalysts exhibit a high electron-acceptor character and interesting structural and coordination properties. Catalytic activity of these compounds is related to the difference in their ability to coordinate substrates.

Influence of support recrystallization techniques on catalyst performance in olefin polymerization

This paper is a comparative study of the performance of TiCl 4 catalysts supported on recrystallized MgCl 2 through different techniques for the polymerization of ethylene, propylene and ethylene–propylene copolymers. MgCl 2 was dissolved in 1-hexanol and recrystallized through solvent evaporation, quick cooling and precipitation with SiCl 4. The effect of the recrystallization conditions during the catalyst preparation on the chemical composition of catalysts was discussed with the help of IR spectroscopy. The variations of dealcoholation levels due to the different recrystallization techniques highly influenced the catalytic activity. The catalyst obtained through SiCl 4 recrystallization was not only the most active, but it also showed the highest isotacticity indexes for propylene polymerization.

Dehydroamination of methylcyclohexanols

The catalytic dehydroamination of 2-, 3-, and 4-methylcyclohexanols on Pd- and Ni-containing heterogeneous systems in the presence of ammonia was studied. The main reaction products were ortho-, meta-, and para-toluidines; 2,2"-, 3,3"-, and 4,4"-dimethyldiphenylamines were formed as by-products. The effects of the chemical composition of catalysts and reaction conditions on the yields of desired products were found.

Bulk and surface composition of Raney copper catalysts

1. It was established that considerable differences exist in the chemical composition of catalysts on the surface and in the interior. 2. A direct relation does not exist between the chemical composition of the surface and the activity of Raney copper catalysts.

Catalytic deoxygenation of heavy gas oil

The extent of oxygen removal during catalytic hydrotreatment of heavy gas oils depends on the chemical composition of catalysts. Deoxygenation increases with the concentration of MoO3 up to about 6 wt %. Further increases in the MoO3 concentration do not appreciably increase oxygen removal. Deoxygenation is also improved by the addition of cobalt and nickel oxides to the molybdate catalysts. Independent acidity measurements on the liquid products suggest the presence of phenols.

Linear free energy relationships in heterogeneous catalysis: IV. Regional analysis for solid acid catalysis

A new method, “regional analysis” for heterogeneous catalysis, is presented in order to find quantitative correlations between the reaction rate over catalysts and either acid strength distribution over solid acids for an acid catalysis or excess oxygen distribution over oxides for an oxidation-reduction catalysis. This analysis is based on the assumption that, in the case of acid catalysis, the reaction rate on an acid site is determined solely by the acid strength of the site, without depending upon the chemical composition of catalysts as far as similar catalysts are concerned. Methods of least squares are employed to find the regional rates or regional rate constants. Oligomerization of propylene and depolymerization of paraldehyde, both on nickel sulfate calcined at various temperatures, were analyzed as examples. In the latter reaction, it was suggested that the Brönsted catalysis law may hold even in heterogeneous catalysis.