Catalytic stepwise nitrate hydrogenation in batch-recycle fixed-bed reactors

Pd (1.0 wt.%)-Cu (0.3 wt.%) bimetallic and Pd (1.0 wt.%) monometallic catalysts were synthesized by means of incipient-wetness impregnation technique and deposited on alumina spheres (dp=1.7 mm). The prepared catalysts were tested at T=298 K and p(H2)=1.0 bar in the integrated process of catalytic liquid-phase hydrogenation of aqueous nitrate solutions, in which the denitration step was carried out consecutively in separate, single-flow fixed-bed reactor units operating in a batch-recycle mode. In the first reactor packed with a Pd-Cu bimetallic catalyst, nitrate ions were transformed to nitrites at pH 12.5 with a selectivity as high as 93%; the rest was found in the form of ammonium ions. Liquid-phase nitrite hydrogenation to nitrogen in the second reactor unit packed with a Pd monometallic catalyst was conducted at low pH values of 3.7 and 4.5, respectively. Although these values are well below the pHpzc of examined catalyst (6.1), which assured that the nitrite reduction was carried out over a positively charged catalyst surface, up to 15% (23% in the presence of 5.0 g/l NaCl in the solution) of initial nitrite content was converted to undesired ammonium ions. Since a negligible amount of these species (below 0.5mg/l) was produced at identical operating conditions over a powdered Pd/gamma-Al2O3 catalyst, it is believed that the enhanced production of ammonium ions observed in the second fixed-bed reactor is due to the build-up of pH gradients in liquid-filled pores of spherical catalyst particles. Both Pd-Cu bimetallic and Pd monometallic catalysts were chemically resistant in the investigated range of pH values.     

A comparative study of catalytic partial oxidation of methane over CeO2 supported metallic catalysts

In the present study, the catalytic partial oxidation of methane (CPOM) over various active metals supported on CeO2 (M/CeO2, M = Ir, Ni, Pd, Pt, Rh and Ru) has been investigated. The catalysts were characterized by X-ray diffraction (XRD), BET surface area, H2-temperature programmed reduction (H2-TPR), CO chemisorption and transmission electron microscope (TEM) analysis. Ir/CeO2 catalysts showed higher BET surface area, higher metal dispersion, small active metal nano-particles (approximately 3 nm) than compared to other M/CeO2 catalysts. The catalytic tests were carried out in a fixed R(mix) ratio of 2 (CH4/O2) in a fixed-bed reactor, operating isothermally at atmospheric pressure. From time-on-stream analysis at 700 degrees C for 12 h, a high and stable catalytic activity has been observed for Ir/CeO2 catalysts. TEM analysis of the spent catalysts showed that the decrease in the catalytic activity of Ni/CeO2 and Pd/CeO2 catalysts is due to carbon formation whereas no carbon formation has been observed for Ir/CeO2 catalysts.     

Synthesis and characterization of Co–Fe nanoparticles supported on mesoporous silicas

Co–Fe bimetallic samples containing 25 wt% total of metal content were prepared by incipient wetness impregnation of cobalt nitrate and iron nitrate salts over hexagonal mesoporous silica (HMS) and SBA-15 supports. Changes in the textural properties and reduction behavior were compared with monometallic cobalt/iron-based samples. The samples were characterized by N₂ physisorption, X-ray diffraction (XRD), H₂-temperature programmed reduction (TPR), transmission electron microscopy (TEM) and H₂ chemisorption. The amount of incorporated metal was estimated by atomic absorption spectroscopy (AAS). Morphological properties revealed that after introduction of the metal to the SBA-15 support, the specific area, pore volume and pore diameter decreased to a lesser extent for bimetallic samples. XRD measurements detected the formation of Co₃O₄ and CoFe₂O₄ phases for both bimetallic samples. TPR profiles indicated similar behavior for both the bimetallic and monometallic samples. Higher temperatures were observed for the reducibility of Co–Fe/HMS as compared to Co–Fe/SBA-15. Dispersion values of the bimetallic samples were higher than Fe monometallic samples and lower than Co monometallic samples according to hydrogen chemisorption. The particle size distribution of the bimetallic samples estimated by TEM microphotographs showed a smaller fraction of larger size particles for Co–Fe/SBA-15.     

Catalytic activities of sonochemically prepared Au-core/Pd-shell-structured bimetallic nanoparticles immobilised on TiO2 and its dependence on Pd-shell thickness

Catalytic activities of sonochemically prepared Au-core/Pd-shell-structured bimetallic nanoparticles (NPs) immobilised on TiO₂ were evaluated. Comparing with the mixture of monometallic Au and Pd NPs on TiO₂, core/shell-immobilised catalysts exhibited higher activities for the partial reduction of nitrobenzene (NB) to aniline (AN), suggesting that the synergistic effect originating from the core/shell structure enhanced the catalytic activities. In the case of high Au/Pd ratios, where the Pd-shell thickness was calculated to be 0.5 nm or lower, infrared spectroscopic measurements of adsorbed CO showed that the Au cores were successfully covered with Pd shells. It was found that a thin Pd shell of one layer or two layers of Pd atoms effectively catalysed the reduction of NB under ambient temperature, whereas the formation of AN was not confirmed on monometallic Au NP-immobilised catalysts.     

Hydrodechlorination of chlorobenzene over polymer-stabilized palladium-platinum bimetallic colloidal nanocatalysts

Poly(N-vinyl-2-pyrrolidone) (PVP)-stabilized Pd, Pt, Pd-Pt nanocatalysts were prepared and characterized by transmission electron microscopy (TEM). Hydrogenation of chlorobenzene was carried out over these colloidal nanocatalysts under ambient conditions. The catalytic properties for the hydrogenation of chlorobenzene depended on the composition of the bimetallic nanocatalysts. The conversion of chlorobenzene over PVP-Pd (83.64%) was higher than that of PVP-Pt (66.67%), which indicated that the activity of Pd was higher than that of Pt. In 10 hrs. the conversions of all the bimetallic nanocatalysts were higher than that of PVP-Pt (66.67%) monometallic nanocatalysts, and the maximum conversion of chlorobenzene (95.34%) was achieved using PVP-Pd/Pt = 1/1 catalytic system, which was much higher than that of the physical mixture of monometallic nanocatalysts (PVP-Pd and PVP-Pt) at the same Pd/Pt ratio as the PVP-Pd/Pt bimetallic nanocatalysts used. The selectivity to benzene and cyclohexane of the bimetallic nanocatalysts (with < or = 40 mol% Pt) was similar to that of PVP-Pd monometallic nanocatalysts, and nearly approximately 100% selectivity to benzene could be obtained, the selectivity to cyclohexane increased slowly with increasing of platinum content in bimetallic nanocatalysts.     

Catalytic wet air oxidation of phenol over CeO2-TiO2 catalyst in the batch reactor and the packed-bed reactor

CeO2-TiO2 catalysts are prepared by coprecipitation method, and the activity and stability in the catalytic wet air oxidation (CWAO) of phenol are investigated in a batch reactor and packed-bed reactor. CeO2-TiO2 mixed oxides show the higher activity than pure CeO2 and TiO2, and CeO2-TiO2 1/1 catalyst displays the highest activity in the CWAO of phenol. In a batch reactor, COD and TOC removals are about 100% and 77% after 120 min in the CWAO of phenol over CeO2-TiO2 1/1 catalyst at reaction temperature of 150 degrees C, the total pressure of 3 MPa, phenol concentration of 1000 mg/L, and catalyst dosage of 4 g/L. In a packed-bed reactor using CeO2-TiO2 1/1 particle catalyst, over 91% COD and 80% TOC removals are obtained at the reaction temperature of 140 degrees C, the air total pressure of 3.5 MPa, the phenol concentration of 1000 mg/L for 100 h continue reaction. Leaching of metal ions of CeO2-TiO2 1/1 particle catalyst is very low during the continuous reaction. CeO2-TiO2 1/1 catalyst exhibits the excellent activity and stability in the CWAO of phenol.     

Kinetics study of a palladium–nickel colloidal nanocatalyst synthesized by a wet-chemical method for reduction of nitrophenol,nitroaniline, and 4-nitrobenzo-15-crown compounds

A bimetallic, palladium–nickel (Pd–Ni) stable colloidal nanocatalyst was synthesized by a wet-chemical reduction technique using Aerosol OT (AOT) as the surfactant and hydrazine hydrate as a reducing agent. The particle size of a colloidal nanocatalyst was controlled by varying precursor concentration, reducing agent, and surfactant concentration. The particle size and morphology of the colloidal catalyst were investigated by dynamic light scattering (DLS) and transmission electron microscopy (TEM), respectively. TEM images show the actual particle size of Pd?Ni nanocatalysts to be in the range of 10?28 nm at 10 mM concentration of AOT and hydrazine hydrate. The activity of the colloidal catalyst (bimetallic) was evaluated for reduction of nitro aromatic compounds which includes 4-NP, 3-NP, 2-NP, 4-nitrobenzo-15-crown, and 4-NA. The rate constant of the Pd–Ni colloidal nanocatalyst indicates that the activity of bimetallic catalysts was higher than the monometallic catalyst for various nitro compounds. The rate constant of 4-nitrophenol was found to be ～61 × 10^?2 min^?1 at room temperature.     

Chemical fluid deposition of monometallic and bimetallic nanoparticles on ordered mesoporous silica as hydrogenation catalysts

A simple and green method of depositing monometallic (Ru, Rh, Pd) and bimetallic nanoparticles (Ru-Rh, Ru-Pd and Rh-Pd) on an ordered mesoporous silica support (MCM-41) in supercritical carbon dioxide (scCO2) is described. Metal acetylacetonates were used in the experiments as CO2-soluble metal precursors. Suitable temperature and pressure conditions for synthesizing each kind of nanoparticles were applied in this study. The characterizations of these nanocomposites were performed by transmission electron microscopy (TEM), X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDS). The nanoparticles had average sizes varying from 2 nm to 8 nm. The Ru nanoparticles were clearly shown to be inside the mesopores of MCM-41 from the TEM image. These nanocomposites used as catalysts for hydrogenation was demonstrated. The efficiency of the scCO2 prepared Ru/MCM-41 catalyst was nearly 8 times than that of a Ru/MCM-41 catalyst prepared by conventional impregnation method.     

Glycerol valorization by base-free oxidation with air using platinum–nickel nanoparticles supported on activated carbon as catalyst prepared by a simple microwave polyol method

Biodiesel is one of the most common biofuels, and its production yields a large amount of glycerol as a by-product. It is necessary to develop new technologies for the use of this by-product, adding value to the biodiesel production chain. In this work we investigated glycerol oxidation under mild reaction conditions (air as oxidizing agent and base-free medium) promoted by suitable catalysts. We prepared mono- and bimetallic catalysts of platinum, copper and nickel in the form of nanoparticles by conventional heating and by an alternative method using microwave heating. The nanoparticles were dispersed in activated carbon and tested in glycerol oxidation aiming its valorization into molecules with high added value. Copper and nickel monometallic materials were not active in glycerol oxidation. Platinum monometallic and platinum–copper and platinum–nickel bimetallic materials showed catalytic activity, with platinum–nickel prepared by microwave heating being the most active material in reactions tested. This catalyst presented glycerol conversion of approximately 20% with a turnover number of 9465 in a reaction time of 6 h and 58% of selectivity to glyceric acid, the main product obtained. The best performance of platinum–nickel prepared by microwave heating catalyst was attributed to the probable formation of a metallic alloy between Pt and Ni, as evidenced by the decrease in the lattice parameter for PtNi bimetallic nanoparticles. The results showed that it was possible to obtain an active catalyst in glycerol oxidation reaction under mild conditions via a simple methodology using microwave heating, which demands 94% less time in comparison with conventional heating.     

Enhanced CO Affinity on Cu Facilitates CO2 Electroreduction toward Multi-Carbon Products

Electrochemical CO₂ reduction reaction (CO₂RR) is a promising strategy for waste CO₂ utilization and intermittent electricity storage. Herein, it is reported that bimetallic Cu/Pd catalysts with enhanced *CO affinity show a promoted CO₂RR performance for multi-carbon (C2+) production under industry-relevant high current density. Especially, bimetallic Cu/Pd-1% catalyst shows an outstanding CO₂-to-C2+ conversion with 66.2% in Faradaic efficiency (FE) and 463.2 mA cm⁻² in partial current density. An increment in the FE ratios of C2+ products to CO  for Cu/Pd-1% catalyst further illuminates a preferable C2+ production. In situ Raman spectra reveal that the atop-bounded CO is dominated by low-frequency band CO on Cu/Pd-1% that leads to C2+ products on bimetallic catalysts, in contrast to the majority of high-frequency band CO on Cu that favors the formation of CO. Density function theory calculation confirms that bimetallic Cu/Pd catalyst enhances the *CO adsorption and reduces the Gibbs free energy of the C C coupling process, thereby favoring the formation of C2+ products.     

Effect of palladium precursor and preparation method on the catalytic performance of Pd/SiO2 catalysts for benzene hydrogenation

Supported Pd catalysts on silica were prepared by different synthesis methods using Pd(Ac)₂ and PdCl₂ as salts precursors. The obtained materials were characterized by X-Ray Diffraction (XRD), H₂ chemisorption, and temperature programmed desorption of hydrogen (H₂-TPD). The catalytic performances of these catalysts have been evaluated in the hydrogenation of benzene. The obtained results show that metal dispersion and catalytic activity are strongly dependent on the salts precursor and the method of preparation of the catalyst. The catalysts prepared by hydrazine reduction exhibit higher activity in benzene hydrogenation than that by the polyol reduction method. Moreover, the catalyst prepared with palladium acetate showed higher catalytic activity than those prepared with palladium chloride. The superior catalytic performance of this catalyst in the hydrogenation of benzene was ascribed to a significantly better dispersion of Pd particles on the silica support.     

Reduction of Np(V) with formic acid in acidic solutions in the presence of palladium catalysts

The kinetics of catalytic reduction of Np(V) with formic acid in HClO₄ solutions in the presence of Pd/SiO₂ catalysts differing in the Pd content and size of Pd nanocrystals was studied. The reaction is a structure-insenitive catalytic process, i.e., the size effect is absent. An increase in the percentage of Pd on SiO₂ leads to a decrease in the activity of the catalysis centers due to a considerable increase in the contribution of the side reaction catalytic decomposition of HCOOH with an increase in the number of active centers in the catalyst grain. The effect of the S:L ratio, concentrations of HCOOH and HClO₄, and temperature on the rate of catalytic reduction of Np(V) in the presence of palladium catalysts was examined. The suggested mechanism of the catalytic reduction of Np(V) with formic acid in the presence of Pd/SiO₂ involves a slow step of decomposition of the protonated species NpO₂H, formed by the reaction of the NpO ₂ ⁺ ion with chemisorbed hydrogen atoms Pd(H).     

Cu/Fe双金属负载PAN非织造布催化降解甲醛气体研究

采用偕胺肟改性聚丙烯腈(PAN)非织造布作为载体材料,将其与Cu~(2+)和Fe~(3+)的混合溶液进行反应制备双金属负载PAN非织造布催化剂。采用XPS和UV-Vis对催化剂的分子结构进行了表征,然后考察了其在甲醛气体氧化降解反应中的催化作用。结果表明,Cu~(2+)和Fe~(3+)均是通过与偕胺肟基团中的羟基和氨基的配位作用负载于PAN非织造布上,而且两种金属离子可能通过配体发生了相互作用。此外,与Fe~(3+)的单金属催化剂相比,适量掺杂Cu~(2+)能够有效提高催化体系在甲醛降解反应中的催化活性,尤其有利于其在暗反应时催化降解甲醛气体,这主要归因于Cu~(2+)/Cu~+价态转化促进了强氧化性中间体Fe(Ⅳ)=O的生成。     

Catalytic reduction of U(VI) with formic acid in acid solutions on palladium catalysts

The kinetics of catalytic reduction of U(VI) with formic acid in H₂SO₄ solutions in the presence of Pd/SiO₂ catalysts differing in the size of nanocrystallites of the active metal was studied. A decrease in the size of supported Pd particles leads to a decrease in the specific activity of the catalyst, i.e., the catalytic centers located on large crystallites exhibit higher activity. An increase in the Pd percent content on SiO₂ leads to a decrease in the activity of the catalytic centers, which is caused by a considerable increase in the contribution of the side reaction of catalytic decomposition of HCOOH with an increase in the number of active centers in the catalyst grain. The results obtained are interpreted on the basis of the concepts of the energy nonuniformity of the surface atoms and of the reaction mechanism. The results show that the size of Pd nanocrystallites is an important factor of the selectivity of palladium catalysts in the preparation of U(IV) by catalytic reduction with formic acid.     

Galvanic Replacement–Mediated Synthesis of Ni‐Supported Pd Nanoparticles with Strong Metal–Support Interaction for Methanol Electro‐oxidation

Herein, well‐defined Pd nanoparticles (NPs) developed on Ni substrate (Pd NPs/Ni) are synthesized via a facile galvanic replacement reaction (GRR) route performed in ethaline‐based deep eutectic solvent (DES). For comparison, a Pd NPs/Ni composite is also prepared by the GRR method conducted in an aqueous solution. The Pd NPs/Ni obtained from the ethaline‐DES is catalytically more active and durable for the methanol electro‐oxidation reaction (MOR) than those of the counterpart derived from conventional aqueous solution and commercial Pd/C under alkaline media. Detailed kinetic analysis indicates that the unique solvent environment offered by ethaline plays vital roles in adjusting the reactivity of the active species and their mass transport properties to control over the genesis of the Pd NPs/Ni nanocomposite. The resulting Pd NPs/Ni catalyst possesses a homogeneous dispersion of Pd NPs with a strong Pd (metal)–Ni (support) interaction. This structure enhances the charge transfer between the support and the active phases, and optimizes the adsorption energy of OH^? and CO on the surface, leading to superior electrocatalytic performance. This work provides a novel GRR strategy performed in ethaline‐DES to the rational design and construction of advanced metal/support catalysts with strong interaction for improving the activity and durability for MOR.     

Structure determination of chitosan-stabilized Pt and Pd based bimetallic nanoparticles by X-ray photoelectron spectroscopy and transmission electron microscopy

Chitosan (CTS)-stabilized bimetallic nanoparticles were prepared at room temperature (rt.) in aqueous solution. Palladium (Pd) and platinum (Pt) were selected as the first metals while iron (Fe) and nickel (Ni) functioned as the second metals. In order to obtain the noble metal core-transition metal shell structures, bimetallic nanoparticles were prepared in a two-step process: the preparation of mono noble metallic (Pd or Pt) nanoparticles and the deposition of transition metals (Fe or Ni) on the surface of the monometallic nanoparticles. The structures of the nanoparticles were studied using X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM). The XPS results show that Pd and Pt exist mainly in zero valences. The presence of Fe and Ni in the bimetallic nanoparticles affects the binding energy of Pd and Pt. Moreover, the studies of O 1s spectra indicate the presence of Fe or Ni shells. The analyses of TEM micrographs give the particle size and size distributions while the high-resolution TEM (HRTEM) micrographs show the existence of noble metal core lattices. The results confirm the formation of noble metal core-transition metal shell structures.     

Robust enhanced hydrogen production at acidic conditions over molybdenum oxides-stabilized ultrafine palladium electrocatalysts

Electrochemical water splitting is quite seductive for eco-friendly hydrogen fuel energy production,however,the attainment of highly efficient,durable,and cheap catalysts for the hydrogen evolution reaction(HER)remains challenging.In this study,molybdenum oxides stabilized palladium nanoparticle catalysts(MoO_x-Pd)are in situ prepared on commercial carbon cloth(CC)by the facile two-step method of dip-coating and electrochemical reduction.As a self-supported Pd-based catalyst electrode,the MoO_x-Pd/CC presents a competitive Tafel slope of 45.75 mV·dec^-1,an ultralow overpotential of 25 mV,and extremely long cycling durability(one week)in 0.5M H₂S0₄electrolyte,superior to unmodified Pd catalysts and comparable to commercial Pt mesh electrode.On the one hand,the introduction of MoO_xcan inhibit the growth of Pd particles to obtain ultrafine Pd nanoparticles,thus exposing more available active sites.On the other hand,density functional theory(DFT)calculation revealed that MoO_xon the surface of Pd metal can regulate the electronic structure of Pd metal and enhance its intrinsic catalytic activity of HER.This work suggests that transitional metal nanoparticles stabilized by molybdenum oxides are hopeful approaches for obtaining fruitful hydrogen-producing electrocatalysts.     

Synthesis and characterization of CuO/Ce 1-x Ti x O2 catalysts used for low-temperature CO oxidation

A series of CuO/Ce(1-x)Ti(x)O(2) catalysts used for low-temperature CO oxidation were prepared by impregnation with the support derived from surfactant-assisted co-precipitation. The techniques of N(2) adsorption/desorption, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and temperature-programmed reduction by H(2) (H(2)-TPR) were employed for catalyst characterization. It is found that the support CeO(2) prepared by the surfactant-assisted method possesses much larger specific surface area than the one obtained from conventional precipitation. Doping Ti in the support with Ti/Ce atomic ratio of 1:9 or 3:7 can further increase the surface area of CeO(2) and decrease its crystallite size. As a result, the active Cu species possess higher dispersion on the support Ce(1-x)Ti(x)O(2) than on pure CeO(2). The strong interaction between the dispersed Cu species and the support Ce(1-x)Ti(x)O(2) makes the catalysts possess much higher oxidation activity and thermal stability. However, when the ratio of Ti/Ce reaches 5:5, opposite effect is found, due to the highest surface concentration of Ti and the lack of surface highly dispersed copper species.     

Fe对Mn/CeO2-TiO2催化剂低温NH3选择性催化还原NO的影响

以锰氧化物为活性组分,CeO2-TiO2为载体制备了Mn/CeO2-TiO2催化剂.考察了Fe的加入对Mn/CeO2-TiO2的低温NH3-SCR活性的影响.并采用BET比表面积,H2程序升温还原(H2-TPR)和X射线光电子能谱(XPS)等手段对催化剂进行了表征.活性结果表明,Fe的引入显著改善了Mn/CeO2-TiO2的NH3-SCR活性,催化剂在113～250℃之间表现出良好的NO去除效率.表征结果表明,Fe的引入促进了锰物种在CeO2-TiO2表面的分散,降低了Mn-Fe/CeO2-TiO2中锰物种的还原温度.XPS分析指出Mn-Fe/CeO2-TiO2表面Mn以+4价存在,而Fe主要以+3价的Fe2O3存在,且Fe与载体表面间存在强相互作用.     

负载型催化剂对风化煤制备腐植酸的影响

以自制碳纳米管(CNTs)为载体,制备了负载型催化剂CeO2/CNTs、TiO2/CNTs和Ce-Ti-Ox/CNTs,并进行了TEM及XRD表征。以所得样品为催化剂用于东都风化煤降解制备腐植酸的研究,探讨了催化剂用量、反应温度及不同负载型催化剂对风化煤降解制备腐植酸的产率、分子结构及吸光度的影响。结果表明,所用负载型催化剂催化性能明显高于未负载的催化剂,能显著提高腐植酸的产率,而且所得腐植酸分子量较小,吸光度较高,其中Ce-Ti-Ox/CNTs催化效果最为显著,可使腐植酸的产率提高到65.43%,表明铈钛活性组分表现出了协同催化效应。