Fabrication of supported Pd–Ir/Al2O3 bimetallic catalysts for 2‐ethylanthraquinone hydrogenation

A series of χ wt % Pd‐(1‐χ) wt % Ir (χ = 0.75, 0.50, and 0.25) catalysts supported on γ‐Al₂O₃ have been prepared by co‐impregnation and calcination‐reduction, and subsequently employed in the hydrogenation of 2‐ethylanthraquinone—a key step in the manufacture of hydrogen peroxide. Detailed studies showed that the size and structure of the bimetallic Pd–Ir particles vary as a function of Pd/Ir ratio. By virtue of its small metal particle size and the strong interaction between Pd and Ir, the 0.75 wt % Pd–0.25 wt % Ir/Al₂O₃ catalyst afforded the highest yield of H₂O₂, some 25.4% higher than that obtained with the monometallic 1 wt % Pd catalyst. Moreover, the concentration of the undesired byproduct 2‐ethyl‐5,6,7,8‐tetrahydroanthraquinone (H₄eAQ) formed using the Pd–Ir bimetallic catalysts was much lower than that observed with the pure Pd catalyst, which can be assigned to the geometric and electronic effects caused by the introduction of Ir. © 2017 American Institute of Chemical Engineers AIChE J, 63: 3955–3965, 2017     

Mono-atomically dispersed Pd on TiO2 as a catalyst for epoxidation of light olefins at low temperatures in the presence of H2 and O2

Palladium supported at extremely low loading on TiO₂ catalyzed epoxidation of light olefins (except ethylene) in the presence of O₂ and H₂ at ambient temperature. Propylene oxide (PO) was obtained from propylene (1–2% yield; 30–60% selectivity) together with propane. Simple oxidation of H₂ to water was slow, and the ratio of H₂ used for PO formation in the reacted H₂ was 10–25%. A Pd concentration of 0.005–0.01 atom-Pd/nm²–TiO₂ was optimum for epoxidation, and the major product at 0.02 atom-Pd/nm² was acetone. That the distributions for a Pd/TiO₂ catalysts prepared from Pd nitrate and from a Pd-tetraphenylporphyrin complex at the same Pd loading were similar suggests that Pd was supported mono-atomically on TiO₂. We propose that PO formed via reaction between propylene adsorbed on Pd and an active species derived from reaction of H₂ and O₂ on TiO₂ (perhaps ·OOH).     

FeOx supported single‐atom Pd bifunctional catalyst for water gas shift reaction

Water gas shift reaction on supported noble metal catalysts is an essential process for upgrading hydrogen source industrially. Here a series of Pd/FeO_x catalysts were detected for this reaction with Pd/Al₂O₃ as reference. It was found that Pd/FeO_x exhibited higher CO conversion than Pd/Al₂O₃ with a good stability even in the presence of CO₂ and H₂. Along the loading decreasing, the turnover frequency of exposed Pd atoms increased with the dispersion from subnanometer (～1 nm) to single atoms. Various characterizations suggested that Pd single atoms greatly enhanced the reducibility of FeO_x and facilitated the formation of oxygen vacancies, which served as sites to promote the dissociation of H₂O to form H₂ and atomic O. The atomic O was ready to react with the linear adsorbed CO species on Pd single‐atom sites through a redox mechanism, which resulted in low activation energy of ～30 kJ mol^?1. © 2017 American Institute of Chemical Engineers AIChE J, 63: 4022–4031, 2017     

Novel Au/TiO2/Al2O3 · xH2O catalysts for CO oxidation

Au/Al₂O₃ · xH₂O and Au/TiO₂/Al₂O₃ · xH₂O (x = 0–3) catalysts were prepared by assembling gold nanoparticles on neat and TiO₂-modified Al₂O₃, AlOOH, and Al(OH)₃ supports, and their catalytic activity in CO oxidation was tested either as synthesized or after on-line pretreatment in O₂–He at 500 °C. A promotional effect of TiO₂ on the activity of gold catalysts was observed upon 500 °C-pretreatment. The catalyst stability as a function of time on stream was tested in the absence or presence of H₂, and physiochemical characterization applying BET, ICP-OES, XRD, TEM, and ²⁷Al MAS NMR was conducted.     

Positive Effect of Water Vapor on CO Oxidation at Low Temperature over Pd/CeO2–TiO2 Catalyst

CO oxidation at low temperature over Pd/CeO₂–TiO₂ catalyst was carried out in the feed containing different contents of water vapor (H₂O). A positive effect of H₂O was observed on the catalytic performance of Pd/CeO₂–TiO₂ in CO oxidation at low temperature. The extent of this effect depends on the content of H₂O in the feed; with a H₂O content being 2.5 vol%, the catalyst Pd/CeO₂–TiO₂ exhibits the highest stability (longest life time for CO oxidation at 80 °C). The results of in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and temperature-programmed reaction (TPReaction) reaction illustrated that H₂O in the feed supplies sufficient OH groups in the presence of O₂, which can react with adsorbed CO on Pd species to produce CO₂. Moreover, H₂O may also enhance the adsorption of CO and suppress the formation of some carbonate species.     

Novel Pd–Au/TiO2 catalyst for the selective catalytic reduction of NOx by H2

Novel Pd–Au/TiO₂ catalyst exhibited high catalytic activity with a wide temperature window for the selective catalytic reduction of NO_x by H₂ in the presence of oxygen. The synergetic effect between Pd and Au contributes to the formation of Pd⁰ and Pd–Au alloy, thus promoting the NO_x reduction to proceed.     

Efficient Reduction of NO x by H2 Under Oxygen-Rich Conditions over Pd/TiO2 Catalysts: An in situ DRIFTS Study

The H₂/NO/O₂ reaction under lean-burn conditions has been studied by means of in situ DRIFTS, reactor measurements and temperature-programmed desorption with the aim of understanding the very different behavior of Pd/TiO₂ and Pd/Al₂O₃ catalysts. The former deliver very high NO _x conversions (70-80%) with good N₂ selectivity whereas the latter show very low activity. In addition, PdTiO₂ exhibits two distinct NO _x reduction pathways, thus greatly extending the useful temperature range. It is shown that the PdTiO₂ low-temperature channel involves adsorption and subsequent dissociation of NO on reduced (Pd⁰) metal sites. The low activity of PdAl₂O₃ is a consequence of palladium remaining in an oxidized state under reaction conditions. The high-temperature NO reduction channel found with PdTiO₂ is associated with the generation and subsequent reaction of NH _x species.     

Significant enhancement of the oxidation of CO by H2 and/or H2O on a FeO x /Pt/TiO2 catalyst

Oxidation of CO on the FeO _x /Pt/TiO₂ catalyst is markedly enhanced by H₂ and/or H₂O at 60 °C, but no such enhancement is observed on the Pt/TiO₂ catalyst, but shift reaction (CO + H₂O → H₂ + CO₂) does not occur on the FeO _x /Pt/TiO₂ catalyst at 60 °C. DRIFT-IR spectroscopy reveals that the fraction of bridge bonded CO increases while that of linearly bonded CO decreases on the FeO _x loaded Pt/TiO₂ catalyst. The in-situ DRIFT IR spectra proved that the bridged CO is more reactive than the linearly bonded CO with respect to O₂, and the reaction of the bridge-bonded CO with O₂ as well as of the linearly bonded CO is markedly enhanced by adding H₂ to a flow of CO + O₂. From these results, we deduced that the promoting effect of H₂ and/or H₂O is responsible for the preferential oxidation (PROX) reaction of CO on the FeO _x /Pt/TiO₂ catalyst, and a following new mechanism via the hydroxyl carbonyl or bicarbonate intermediate is proposed for the oxidation of CO in the presence of H₂O.      

Low-Temperature Combustion of CH4 over CeO2–MO x Solid Solution (M = Zr4+, La3+, Ca2+, or Mg2+) Promoted Pd/γ–Al2O3 Catalysts

The catalytic behavior of Pd (2 wt%) catalysts supported on γ-Al₂O₃ and promoted with CeO₂ ? MO _x (M = Zr⁴⁺, La³⁺, Ca²⁺, or Mg²⁺) solid solution was investigated for methane combustion. The results demonstrated that Pd/γ-Al₂O₃CeO₂ MO _x catalysts can be effective for the low-temperature catalytic combustion of methane and are comparable in activity to other conventional catalysts for this reaction. The XPS and XRD results indicated that an enhanced mobility of lattice oxygen induced by the perturbation of Ce–O lattice was responsible for an increased catalytic performance during oxidation reaction. The most active sites in the catalyst system involve contacts between Pd and the CeO₂–MO _x mixed oxide component. Meanwhile, pre-treatment conditions have significant effect on the catalytic activity in methane combustion.     

Chlorobenzene total oxidation over palladium supported on ZrO2, TiO2 nanostructured supports

Two series of supported Pd catalysts were synthesized on new mesoporous–macroporous supports (ZrO₂, TiO₂) labelled M (Zr and Ti). The deposition of palladium was carried out by wet impregnation on the calcined TiO₂ and ZrO₂ supports at 400 °C (Pd/Zr₄, Pd/Ti₄) and 600 °C (Pd/Zr₆, Pd/Ti₆) and followed by a calcination at 400 °C for 4 h. The pre-reduced Pd/MX catalysts were investigated for the chlorobenzene total oxidation and their catalytic properties where compared to those of a reference catalyst Pd/Ti-Ref (TiO₂ from Huntsman Tioxide recalcined at 500 °C) and of a palladium supported on the fresh mesoporous–macroporous TiO₂ (Pd/Ti). Based on the activity determined by T₅₀, the Pd/Ti and Pd/Ti₄ catalysts have been found to be more active than the reference one. Moreover activity decreased owing to the sequence: Pd/TiX  Pd/ZrX and in each series when the temperature of calcination of the support was raised. The overall results clearly showed that the activity was dependant on the nature of the support. The better activity of Pd/TiX compared to Pd/ZrX was likely due to a better reducibility of the TiO₂ support (Ti⁴⁺ into Ti³⁺) leading to an enhancement of the oxygen mobility. Production of polychlorinated benzenes PhCl_x (x = 2–6) and of Cl₂ was also observed. Nevertheless at 500 °C the selectivity in HCl was higher than 90% for the best catalysts.     

Mechanism in chlorine-enhanced Pd catalyst for H2O2 in situ synthesis in electro-Fenton system

Palladium-based catalysts have been widely employed in the electro-Fenton process for in situ generation of H₂O₂. However, the process is still far from being practical on a large scale. In this work, a series of Cl_xFePd/γ-Al₂O₃/Al catalysts were prepared by a three-step-impregnation method. They exhibited excellent activity in H₂O₂ in situ synthesis and high efficiency in phenol degradation. The characterization results showed that Cl could assist in increasing the content of Pd⁰ and reducing the isoelectric point of catalysts, which led to the drastic promotion in the synthesis of H₂O₂. Theoretical calculations further demonstrated that Cl doping could facilitate the main reaction in H₂O₂ synthesis, as well as inhibit side reactions such as dissociation of the O O bond. Furthermore, kinetic models were proposed and fitted. A plausible reaction mechanism as well as degradation pathways were elaborated based on electron spin resonance and gas chromatography–mass spectrometry results. These findings illustrate the value of palladium-based Cl_xFePd/γ-Al₂O₃/Al catalysts for their application in the electro-Fenton process.     

From single crystals to supported nanoparticles in experimental and theoretical studies of H2 oxidation over platinum metals (Pt, Pd): Intermediates, surface waves and spillover

The present paper reviews our investigations concerning the mechanism of H₂ + O₂ reaction on the metal surfaces (Pt, Pd) at different structures: single crystals (Pt(1 1 1), Pt(1 0 0), Pd(1 1 0)); microcrystals (Pt tips); and nanoparticles (Pd–Ti³⁺/TiO₂). Field electron microscopy (FEM), field ion microscopy (FIM), high-resolution electron energy loss spectroscopy (HREELS), XPS, UPS, work function (WF), TDS and temperature-programmed reaction (TPR) methods have been applied to study the kinetics of H₂ oxidation on a nanolevel. The adsorption of both O₂ and H₂ and several dissociative products (H_ads, O_ads, OH_ads) was studied by HREELS. Using the DFT technique the equilibrium states and stretching vibrations of H, O, OH, H₂O, adsorbed on the Pt(1 1 1) surface, have been calculated depending on the surrounding of the metal atoms. Sharp tips of Pt, several hundreds angstroms in radius, were used to perform in situ investigations of the dynamic surface processes. The FEM and FIM studies on the Pt-tip surface demonstrate that the self-oscillations and waves propagations are connected with periodic changes in the surface structure of nanoplane (1 0 0)-(hex) ↔ (1 × 1), varying the catalytic property of metal. The role of defects (Ti³⁺-□_O) in the adsorption centers formation, their stabilization by the palladium nanoparticles, and then the defects participation in H₂ + O₂ steady-state reaction over Pd–Ti³⁺/TiO₂ surface have been studied by XPS, UPS and photodesorption techniques (PhDS). This reaction seems to involve the “protonate” hydrogen atoms (H⁺/TiO_x) as a result of spillover effect: diffusion of H_ads atoms from Pd particles on a TiO_x surface. The comprehensive study of H₂, O₂ adsorption and H₂ + O₂ reaction in a row: single crystals → tips → nanoparticles has shown the same nature of active centers over these metal surfaces.     

Effect of the Amount of H2O on the Crystallization of TiO2 Films Prepared from Alkoxide Solutions Containing Acetylacetone

Sol–gel‐derived TiO₂ films were prepared by dip‐coating from Ti(OC₃H₇ⁱ)₄–CH₃COCH₂COCH₃–HNO₃–H₂O–C₂H₅OH solutions, and the effect of the H₂O content in solutions on the crystallization of TiO₂ films during the heat treatment at 800°C was discussed. The crystalline phase, crystallite size, grain size, and refractive index of the TiO₂ films depended on the H₂O/Ti(OC₃H₇ⁱ)₄ mole ratios (x) in the coating solutions. Highly dense and crystalline rutile films were obtained at x = 0.5–10, where the crystallites and grains became larger with increasing x. The further increase in x from 10 to 50 reduced the crystallite size and refractive index of the films, where anatase phase appeared at x = 30–50.     

Activation of a Au/TiO2 Catalyst by Loading a Large Amount of Fe–Oxide: Oxidation of CO Enhanced by H2 and H2O

Catalytic activity of a 1 wt% Au/TiO₂ catalyst is markedly improved by loading a large amount of FeO_x, on which the oxidation of CO in excess H₂ is selectively promoted at temperature lower than 60 °C. Oxidation of CO with O₂ on the FeO_x/Au/TiO₂ catalyst is markedly enhanced by H₂, and H₂O moisture also enhances the oxidation of CO but its effect is not so large as the promotion by H₂. We deduced that activation of Au/TiO₂ catalyst by loading FeO_x is not caused by the size effect of Au particles but a new reaction path via hydroxyl carbonyl intermediate is responsible for the superior activity of the FeO_x/Au/TiO₂ catalyst.     

Mechanism of sulfur poisoning on supported noble metal catalyst — The adsorption and transformation of sulfur on palladium catalysts with different supports

A series of supported palladium catalysts (Pd/Al₂O₃, Pd/MgO and Pd/TiO₂) were prepared by the impregnating method and treated with H₂S, H₂ +O₂ or O₂, among which H₂S is used as a poison and H₂ +O₂ or O₂ are as purging atmospheres. The S^2– species in the supports was introduced by means of mechanically mixing Na₂S with the supports or catalysts. X-ray photoelectron spectroscopy (XPS) was employed to determine the changes in the chemical states of oxygen, palladium and sulfur in the catalysts before and after the treatment, while infrared (IR) spectroscopy was used to measure the SO^2– ₄ group produced in the catalysts and supports. The results show that on MgO and TiO₂ carriers whose acidities are weak, there exist two kinds of oxygen species, one is the lattice oxygen, the other one is the active species of oxygen. The latter can oxidize the S^2– into SO^2– ₄ even at room temperature in air. Because of the weak acidities and smaller specific surface area of MgO and TiO₂, the S^2– is liable to adsorb on the catalysts and to transform into SO^2– ₄. But for the case of Al₂O₃ support its acidity is rather strong, and its surface oxygen species under the experimental conditions is not so active as that in MgO and TiO₂ carries. The poison H₂S on the Al₂O₃ support only experiences a process of physical adsorption-desorption. In Pd/Al₂O₃ catalyst, the negatively charged sulfur ions are not so easily adsorbed and transformed as those in Pd/MgO and Pd/TiO₂. It is also implied that the properties of the carriers are related to the ability of self-regeneration of the corresponding catalysts. Pd/Al₂O₃ catalyst is more able to self-regenerate than Pd/MgO and Pd/TiO₂ catalyst.     

MnOx-WO3/TiO2NH3选择性还原NOx的催化性能与动力学

研究了Mn-W/TiO2用于NH3选择性催化还原NOx体系的催化反应性能,在很宽的温度范围和各种气体条件下,该催化剂显示了较高的催化活性.在GHSV 18900 h-1、100～350℃条件下,NOx转化率高达80.3%～99.6%,Nz选择性达98.7%～100%;当反应气体中有0.01%SO2(分压比,下同)和6%...     

Direct H2-to-H2O2 Oxidation in Aqueous Acidic Medium Containing Br Promoter Over Pd/Al2O3 and Pd/C Catalysts Thermally Pretreated Under Different Conditions

Influence of thermal pretreatments (under N₂, air or H₂ gas atmosphere at 500 °C or 700 °C) has been investigated for the Pd/Al₂O₃ and Pd/Carbon catalysts in terms of its effect on their Pd particle size and performance in the H₂-to-H₂O₂ oxidation and H₂O₂ destruction (by decomposition and/or hydrogenation) reactions in aqueous acidic medium containing Br promoter. The influence on the net H₂O₂ formation is found to depend strongly upon the catalyst support due to support–Pd cluster interactions. For both the catalysts, the thermal treatments (except in air) caused a large increase in their Pd particle size. The increase in Pd particle size caused an increase in the H₂O₂ formation activity of Pd/Al₂O₃ but a decrease in the H₂O₂ formation activity of Pd/C.     

Oxygen-enhanced water gas shift on ceria-supported Pd–Cu and Pt–Cu bimetallic catalysts

Aiming at enhancing H₂ production in water gas shift (WGS) for fuel cell application, a small amount of oxygen was added to WGS reaction toward oxygen-enhanced water gas shift (OWGS) on ceria-supported bimetallic Pd–Cu and Pt–Cu catalysts. Both CO conversion and H₂ yield were found to increase by the oxygen addition. The remarkable enhancement of H₂ production by O₂ addition in short contact time was attributed to the enhanced shift reaction, rather than the oxidation of CO on catalyst surface. The strong dependence of H₂ production rate on CO concentration in OWGS kinetic study suggested O₂ lowers the CO surface coverage. It was proposed that O₂ breaks down the domain structure of chemisorbed CO into smaller domains to increase the chance for coreactant (H₂O) to participate in the reaction and the heat of exothermic surface reaction helping to enhance WGS kinetics. Pt–Cu and Pd–Cu bimetallic catalysts were found to be superior to monometallic catalysts for both CO conversion and H₂ production for OWGS at 300 °C or lower, while the superiority of bimetallic catalysts was not as pronounced in WGS. These catalytic properties were correlated with the structure of the bimetallic catalysts. EXAFS spectra indicated that Cu forms alloys with Pt and with Pd. TPR demonstrated the strong interaction between the two metals causing the reduction temperature of Cu to decrease upon Pd or Pt addition. The transient pulse desorption rate of CO₂ from Pd–Cu supported on CeO₂ is faster than that of Pd, suggesting the presence of Cu in Pd–Cu facilitate CO₂ desorption from Pd catalyst. The oxygen storage capacity (OSC) of CeO₂ in the bimetallic catalysts indicates that Cu is much less pyrophoric in the bimetallic catalysts due to lower O₂ uptake compared to monometallic Cu. These significant changes in structure and electronic properties of the bimetallic catalysts are the result of highly dispersed Pt or Pd in the Cu nanoparticles.     

Novel cobalt-doped ZrSnO4 catalysts for direct nitrous oxide decomposition

Novel ZrSn_1−xCo_xO_4−δ catalysts for the direct decomposition of nitrous oxide (N₂O) were synthesized using a co-precipitation method. Metastable ZrSnO₄ with an α-PbO₂-type structure was used as the mother solid because of its interstitial open spaces derived from its lattice distortion that may be effective for N₂O adsorption. The doping of Co^2+/3+ into the ZrSnO₄ lattice improved the N₂O decomposition activity, likely owing to the enhancement of the redox properties and the increase in the number of oxygen vacancies. Among the prepared catalysts, Zr_1.17Sn_0.73Co_0.10O_4−δ exhibited the highest activity decomposing N₂O completely decomposed at 550°C. In addition, the Zr_1.17Sn_0.73Co_0.10O_4−δ catalyst showed high durability in the presence of CO₂, O₂, and H₂O vapor.     

Gas‐Phase Hydrogenolysis of Glycerol Catalyzed by Cu/MOx Catalysts

The catalytic activities of Cu/MO_x (MO_x = Al₂O₃, TiO₂, and ZnO) catalysts in the gas‐phase hydrogenolysis of glycerol were studied at 180–300 °C under 0.1 MPa of H₂. Cu/MO_x (MO_x = Al₂O₃, TiO₂, and ZnO) catalysts were prepared by the incipient wetness impregnation method. After reduction, CuO species were converted to metallic copper (Cu⁰). Cu/Al₂O₃ catalysts with high acidity, high specific surface areas and small metallic copper size favored the formation of 1,2‐propanediol with a maximum selectivity of 87.9 % at complete conversion of glycerol and a low reaction temperature of 180 °C, and favored the formation of ethylene glycol and monohydric alcohols at high reaction temperature of 300 °C. Cu/TiO₂ and Cu/ZnO catalysts exhibited high catalytic activity toward the formation of hydroxyacetone with a selectivity of approx. 90 % in a wide range of reaction temperature.