Uptake of Metal Ions on Humic Acids

The kinetics, the sorption capacities, pH and temperature dependence of sorption of humic acids (HAs) of Turkish brown coals with respect to Zn(II), Cu(II), Ni(II), Co(II) and Pb(II) ions were investigated, and the roles of the carboxylic and phenolic groups in the adsorption of metals ion on HAs were searched in this work. These metal ions are able to form complex compounds with carboxylic and phenolic groups of HAs. Adsorption equilibrium was achieved in between 50 and 60 min for all studied cations. HAs extracted from different brown coals have been characterized by chemical and physical methods. The chemical properties of HAs showed differences depending on the source from which they were obtained. The sorption of metals on the surface of HAs depends strongly on the pH, and sorption decreases with decreasing pH. Maximum removal of metal ions was demonstrated at pH values of 4.1–5.0. The Langmuir adsorption isotherm was used to describe observed sorption phenomena. The Δ G ⁰ became negative as the temperature increased, and so the equilibrium constant decreased slightly. The investigation proved that the HAs are suitable materials for the studied heavy metal ion removal from aqueous solution and could be considered as potential material for purification of effluent polluted with toxic metal ions.     

Poly(acrylamide-co-vinyl sulfonic acid) p(AAm-co-VSA) hydrogel templates for Co and Ni metal nanoparticle preparation and their use in hydrogen production

P(AAM-co-VSA) hydrogel was prepared at different mole ratios form the corresponding monomers and used in absorption of metal ions such as Co and Ni from aqueous environments. Then, these bound metal ions within the hydrogel matrices were reduced to their metal nanoparticles by aqueous NaBH₄ treatment. Finally, p(AAm-co-VSA)–M (M: Co or Ni) composites were used as reactor in the hydrolysis of NaBH₄ for hydrogen generation. The amounts of metal ions before and after metal nanoparticle formation were determined by Atomic Absorption Spectroscopy (AAS). P(AAm-VSA) hydrogel showed greater absorption tendency for Ni(II) ions than Co(II) ions, and the metal ion binding capacity of these hydrogels was increased with an increase in the amount of VSA in the copolymeric hydrogel. It was also found that although the amount of Ni ions loaded into the hydrogel matrices were more than Co ions, Co metal nanoparticle-containing hydrogel produced hydrogen faster than Ni metal nanoparticle-containing hydrogel composites. The activation energy for the Co nanoparticle-embedded p(AAm-co-VSA) was found as 34.505 kJ mol⁻¹k⁻¹, and other thermodynamic parameters were also calculated. The p(AAm-co-VSA)–Co hydrogel can be used up to 5 times repetitively without any loss of yield but with 55% of catalytic activity.     

Mercury(II)-complex with 5-methyl-1,3,4-thiadiazole-2-thiol: kinetic studies of hydrogen storage

Mixed ligand mercury(II) complexes of 2-meracpto-5-methyl-1,3,4-thiazdiazole (HmtzS) and phosphines or diamines having the general formulae [Hg(mtsZ)₂(diphos)] {diphos = 1,2-bis(diphenylphosphino)ethane (dppe), 1,3-bis(diphenylphosphino)propane (dppp), 1,4-bis(diphenylphosphino)butane (dppe), 1,1′-bis(diphenylphosphino)ferrocene (dppf)}, [Hg(mtsZ)₂(PPh₃)₂] and [Hg(mtsZ)₂(diamine)] {diamine = bipyridyl (Bipy) or 1,10-phenthroline (Phen), were successfully synthesized by simple mixing method. The complexes were characterized by elemental analysis, molar conductivity, IR and NMR (¹H, ¹³C and ³¹P) spectroscopic methods. The mtzS^? ligand was coordinated through the sulfur atom of thiol group, whereas the diphosphine or diamine ligands bonded as bidentate chelating ligand to afford tetrahedral environment around the Hg(II) ions. Moreover, the complex [Hg(mtsZ)₂] was used in order to study its ability to store hydrogen. The results of hydrogen isotherm at different temperatures prove that [Hg(mtsZ)₂] was able to store 0.8 wt% at a pressure of 80 bar 77 K. Furthermore, the kinetic study of hydrogen storage was studied and the kinetic study was carried out using the Langmuir. Moreover, the adsorption kinetic results revealed that hydrogen storage in [Hg(mtsZ)₂] follow the pseudo-second-order model with coefficient regression equal to 0.99.     

Role of macrocyclic salen-type Schiff base ligands in one-dimensional Co(II) complexes for superior activities toward oxygen reduction/evolution reactions

Developing high activities, stable, non-precious metal based bi-functional electrocatalysts oxygen evolution/reduction reactions (OER/ORR) in rechargeable metal-air batteries and regenerative fuel cell technologies is essential for future energy conversion and storage. In this work, the potential of utilizing the synthesized one-dimensional transition metal salen-type complexes (TM-SCs) as bi-functional electrocatalysts of ORR and OER is systematically explored by computational screening approach. The results demonstrate that different types of macrocyclic ligands, including N₂O₂, N₃O and N₄ as donor groups around the active sites, govern the OER/ORR catalytic performances. Co–SCs with N₂O₂ ligands exhibit the highest bifunctional catalytic activities. In particular, low limiting overpotentials of 0.22 V for OER and 0.33 V for ORR can be observed on Co sites, which are even superior to those of noble metal catalysts. Analyzing the linear relationships between the adsorption strength of intermediates and the overpotentials shows that the origin of excellent electrocatalytic performance is the smaller slope (0.86) for OOH1 vs OH1 on TM-SCs compared to metal surfaces, resulting in strengthened binding of the OOH1 intermediate. Besides, the adsorption energies of the intermediates bound on Co–N₂O₂ are close to the ideal values, while too strong on the Co–N₃O and Co–N₄ catalysts. By applying external strains, the adsorption strengths of reaction intermediates can be further modulated due to the tunable d-band centers, and the resulting ORR/OER activities are further boosted. Considering that the Co salen-based chain has been synthesized experimentally, this work highlights the excellent electrocatalytic performances of this new material and devises novel strategy by straining for catalyst optimizations.     

Metal ion-imprinted hydrogel with magnetic properties and enhanced catalytic performances in hydrolysis of NaBH4 and NH3BH3

Metal ion-imprinted (IIH) poly(2-acrylamido-2-methyl-1-propansulfonic acid) p(AMPS) hydrogels were prepared by using a free-radical polymerization technique in the presence of metal ions (M = Co (II) or Ni (II)). Using metal ion-imprinted hydrogels (IIHs), and non-metal ion-imprinted (NIH) hydrogels as template for the preparation of Co and Ni catalyst systems, the hydrolysis kinetics of NaBH₄ and NH₃BH₃ were investigated. The catalytic performances of IIHs and NIHs were compared in terms of effect on hydrolysis kinetics of NaBH₄ and NH₃BH_3. To increase the amounts of Co nanoparticles within p(AMPS) hydrogel for better catalytic activity, several reloading and reduction cycles of Co (II) ions were carried out, and the prepared p(AMPS)-Co composite catalyst systems were tested for hydrogen generation from the hydrolysis of NaBH₄. As the number of Co (II) loading and reduction cycles increased, the amount of metal catalysts and the catalytic performance of composites increased. Kinetics studies were carried out on three times Co (II) ion loaded and reduced p(AMPS)-Co catalyst systems (containing 36.80 mg/g Co). Three time Co (II)-loaded catalyst systems provided very fast hydrolysis kinetics for NaBH₄, and provided magnetic field responsive behavior. The hydrolysis reaction of NaBH₄ was completed within 50 s, under the described conditions at 60 °C. It was demonstrated that the synthesized catalyst systems can be used ten times repetitively without significant loss of catalytic activity (86.5%).     

Immobilization of Heavy Metals and Phenol on Altered Bituminous Coals

Abstract

This article evaluates adsorption ability of the altered bituminous coals to remove heavy metals and/or phenol from aqueous solutions. As for heavy metals, copper (II), cadmium (II) and lead (II) cations were used. In addition to phenol, cyclohexanol and 2-cyclohexen-1-ol were also examined. Adsorption experiments were conducted in the batch mode at room temperature and at pH 3 and 5. To characterize the texture of coal samples, adsorption isotherms of nitrogen at ?196°C, enthalpies of the immersion in water, and pH values in aqueous dispersions were measured. Coal hydrogen aromaticities were evaluated from the infrared spectrometric examinations (DRIFTS). Based on the investigations performed, cation exchange was confirmed as the principal mechanism to immobilize heavy metallic ions on coals. However, apart from carboxylic groups, other functionalities (hydroxyl groups) were found to be involved in the adsorption process. During adsorption of phenol, π-π interactions between π-electrons of phenol and aromatic rings of coal proved to play the important role; however, no distinct correlation between adsorption capacities for phenol and hydrogen aromaticities of the coal was found. Probable involvement of oxygenated surface groups in the immobilization of phenol on coal was deduced. As a result, for waste water treatment, oxidative altered bituminous coal can be recommended as a suitable precursor, with the largest immobilization capacities both for metallic ions and phenol, as found in the studied samples.     

Galvanic exchange carving growth of Co–Fe LDHs with enhanced water oxidation

Preparing high-efficiency electrocatalyst through facile, low energy consumptive, and time-effective strategy is essential to the hydrogen generation via water electrolysis. Here, we report a galvanic replacement-mediated synthesis of highly active Co–Fe layered double hydroxides (Co–Fe LDHs) at room temperature as advanced electrocatalyst for water oxidation. The disc-like Co precursors were the first proposed as self-sacrificing templates for Co–Fe LDHs. By steady control of the carving of iron ions, the disc-like Co precursors are carved relaxedly and then formed to nanosheet-constructed Co–Fe LDHs. The Fe (II) ion is proved to a more preferable iron precursor than Fe (III) ion for Co–Fe LDHs nanosheets growth, owing to the “slow-growth” process. This catalyst shows a highly electrocatalytic activity towards the OER with low overpotential (η10 mA cm^-2 = 270 mV), low Tafel slope (48.7 mV dec-1), and remarkable long-time stability.     

Removal of heavy metals by adsorption on Pleurotus ostreatus

The present study explores the adsorption potential of Pleurotus ostreatus (a macro-fungus) to remove copper, nickel, zinc and chromium from water all together. Different operational parameters such as the effect of pH, biomass dose, equilibrium time, stirring intensity, temperature and initial metal ion concentrations were studied. Maximum adsorption of Ni(II), Cu(II) and Zn(II) took place in the pH range 4.5-5.0, whereas for Cr(VI) ion, best results were achieved at pH 2.5. Nearly 150 min are required to gain sorption equilibrium. Temperature has no significant effect on biosorption in the range of 20-45 °C. The maximum biosorption capacity of fungus was 8.06, 20.40, 3.22 and 10.75 mg g⁻¹ for Cu(II), Ni(II), Zn(II) and Cr(VI) in that order. FTIR analysis pointed out the involvement of amine (-NH₂) and carboxylic (-COOH) groups in the adsorption process. Simple and adjusted Langmuir and Freundlich isotherm models were used to explain the sorption phenomenon. For real effluents of electroplating, biosorption capacities were 2.73, 8.45, 0.88 and 4.45 mg g⁻¹ for Cu(II), Ni(II), Zn(II) and Cr(VI) ions, respectively. Moreover, used P. ostreatus was recycled repeatedly and used many times to evaluate the adsorption efficacy on reuse, but findings pointed out that capacity decreased, to some extent, on recycling.     

A tetranuclear nickel(II) complex for water oxidation: Meeting new challenges

Ni complexes are promising catalysts for water splitting. Herein, a tetranuclear nickel(II) complex with bis-[(E)-N′-(1-(pyridin-2-yl)ethylidene)]carbohydrazide (HL), was synthesized and characterized by spectroscopic methods and single crystal X-ray analysis. The complex is a tetranuclear complex and the ligands are coordinated to the metal ions in the mono-negative form, (L)^- to form a tetranuclear [NiL]₄⁴⁺ unit. Each Ni(II) ion is six-coordinated by pyridine nitrogen, azomethine nitrogen and oxygen atoms of two perpendicular carbohydrazone ligands in the mer configuration. The complex was studied as a water-oxidizing catalyst. In the next step, the role of the Ni-based compound for water oxidation on the surface of fluorine doped tin oxide as one of the true catalysts was investigated by scanning transmission electron microscopy, scanning transmission electron microscopy, spectroelectrochemistry, and electrochemistry. The electrode after water oxidation by the complex was studied and a relation between the decomposition of the Ni complex and water-oxidation reaction was proposed. The experiments show that under water-oxidation condition in the presence of the complex, a Ni-based compound on the electrode is a candidate as a contributor to the observed catalysis.     

Rhodium(0), Ruthenium(0) and Palladium(0) nanoparticles supported on carbon-coated iron: Magnetically isolable and reusable catalysts for hydrolytic dehydrogenation of ammonia borane

We report the synthesis of magnetically isolable ruthenium(0), rhodium(0), and palladium(0) nanoparticles, supported on carbon-coated magnetic iron particles, and their employment as catalysts in hydrolysis of ammonia borane. Carbon-coated iron (C–Fe) particles are obtained by co-processing of iron powders with methane in a radio frequency thermal plasma reactor. The impregnation of ruthenium(III), rhodium(III) and palladium(II) ions on the carbon-coated iron particles followed by aqueous solution of sodium borohydride leads to the formation of respective metal(0) nanoparticles supported on carbon-coated iron, M⁰/C–Fe NP (M = Ru, Rh, and Pd) at room temperature. M⁰/C–Fe NPs are characterized using the ICP-OES, XPS, TEM, and EDX techniques and tested as catalysts for hydrolysis of ammonia borane at 298 K. The results reveal that Rh⁰/C–Fe, Ru⁰/C–Fe, Pd⁰/C–Fe catalysts provide turnover frequency of 83, 93, and 29 min^?1, respectively, in this industrially important reaction. More importantly, these magnetically separable metal(0) nanoparticles show very high reusability with no noticeable activity loss in subsequent runs of hydrolysis evolving 3.0 equivalent H₂ per mole of ammonia borane.     

Silica gel adsorbents doped with Al,Ti, and Co ions improved adsorption capacity,thermal stability and aging resistance

The metal ion doped silica gel adsorbents based on ceramic fiber were prepared by impregnation coprecipitation method. The effects of the concentration and the acidity of impregnation salt (cobalt, aluminum or titanium sulfate) solution on the adsorption capacity of the modified silica gels were investigated. The composition and surface structure of the doped adsorbents were characterized using Fourier transformation infrared spectroscopy (FTIR), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and porous medium analyzer. The desorption performance of the doped materials was evaluated by temperature programmed desorption (TPD). The thermal stability and aging resistance of the doped materials were predicted by sintering test and temperature-humidity cycling test. The results showed that the metal salt concentration had an impact on the metal ion content in the modified silica gel deposited on the surface of the ceramic fiber paper, and the solution acidity affected the amount of the modified silica gel. FTIR spectra indicated the formation of Si-O-M (M = Al, Co and Ti) linkages in the doped silica gel, which was further confirmed by sintered experiment. After metallic ion doping, the modified silica gel showed increased adsorption capacity, BET surface area, thermal stability and aging resistance. The saturated adsorption capacity and BET surface area ascended in order of Co-doped silica gel, Ti-doped silica gel and Al-doped silica gel.     

Effect of transition metal on stability and activity of La-Ca-M-(Al)-O (M = Co,Cr, Fe and Mn) perovskite oxides during partial oxidation of methane

Perovskite oxides of the type of La_xCa_1-xM_yAl_1-yO_3-δ (M = Co, Cr, Fe, Mn; x = 0.5; y = 0.7–1.0) were prepared using the polymerization methods and evaluated via N₂ adsorption, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning transmission electron microscopy (STEM), energy dispersive X-ray (EDX) spectroscopy, temperature-programmed reduction by hydrogen (TPR-H₂) and temperature-programmed oxidation by oxygen (TPO-O₂). Catalytic behaviour of the perovskite oxides during methane oxidation was studied using a tubular fixed-bed reactor. In a partial oxidation, which proceeded in two steps, there was total oxidation in the first step and CO₂ and H₂O were formed; in the second step, the total oxidation products oxidized methane by (dry and wet) reforming reactions to yield CO and H₂. Total oxidation and the two reforming reactions proceeded on two types of an active centre formed by transition metal ions, oxygen vacancies and oxide ions. The catalytic system La-Ca-Co-Al-O which contained aluminium, decomposed in partial oxidation of methane (POM) into a composite that contained firmly bonded cobalt nanoparticles in the surface of a substrate made up of La₂O₃, CaO and Al₂O₃ which catalysed POM with a high methane conversion and hydrogen selectivity.     

Studies on electrochemical properties of thionyl chloride reduction by Schiff base metal(II) complexes

Electrocatalytic effects associated with the reduction of thionyl chloride in a LiAlCl₄–SOCl₂ electrolyte solution containing Schiff base metal(II) (metal (M): Co, Ni, Cu and Mn) complexes are evaluated by determining the kinetic parameters for the reactions using cyclic voltammetry at a glassy carbon electrode. The charge-transfer process during the reduction of thionyl chloride is affected by the concentration of the catalyst. Catalytic effects are demonstrated from both a shift in the reduction potential for the thionyl chloride in a more positive direction and an increase in peak currents. The reduction of thionyl chloride is diffusion controlled. Catalytic effects are larger for thionyl chloride solutions containing M(II)(1,5-bis(salicylidene imino) pentane) (M(II)(SALPE)) rather than M(II)(1,3-bis(salicylidene imino) propane) (M(II)(SALPR)). Significant improvements in cell performance are found in terms of the both thermodynamics and kinetic parameters for the thionyl chloride reduction. An exchange rate constant, k⁰, of 1.89×10⁻⁸ cm s⁻¹ is found at the bare electrode, while larger values of 2.79×10⁻⁸ to 2.09×10⁻⁶ cm s⁻¹ are observed in the case of the catalyst-supported glassy carbon electrode.     

Proton conducting hybrid membrane electrolytes of sulfonated poly(ether sulfone)s and poly(ether sulfone)s containing metallophthalocyanine

A role of metallophthalocyanine (MPc) as an anti-oxidizing agent of polymer membrane and an accelerating agent of proton conductivity was discussed. The poly(ether sulfone)s bearing MPc (Ni, Co and Fe), PMPc were prepared by two-step reaction from phenolphthalein and fumaronitrile and followed reaction with metal (II) chloride (Ni, Co and Fe) and 1,2-dicyanobenzene in quinoline. The sulfonated polymer was synthesized by condensation polymerization using 1,2-bis(4-hydroxyphenyl)-1,2-diphenyl ethylene, bis(4-fluorophenyl) sulfone and followed by sulfonation reaction with concentrated sulfuric acid. A series of hybrid membranes (H–Ni, H–Co and H–Fe) were prepared from a mixture of the sulfonated copolymer and PMPcs in dimethylacetamide (DMAc). The structural properties of the synthesized polymers were studied by ¹H-NMR spectroscopy and FT-IR. The membrane properties were investigated by measurements of ion exchange capacity (IEC), water uptake, and proton conductivity, chemical degradation test, and atomic force microscopy (AFM) analysis. The cell performance of the membranes was compared with those of normal sulfonated poly(ether sulfone)s and Nafion.     

Study on the hydrogen storage performance of graphene(N)–Sc–graphene(N) structure

The electronic properties of a sandwich graphene(N)–Sc–graphene(N) structure and its average adsorption energies after the adsorption of 1, 3, 5, 7, 10, and 14H₂ molecules were investigated by first principles. The average binding energies and adsorption distances of Sc atoms at different adsorption sites in N-doped bilayer graphene (N–BLG) were calculated. It was found that Sc atoms located at different adsorption sites of BLG generated metal clusters. The binding energy of the Sc atom located at the TT position of N–BLG (5.19 eV) was higher than the experimental cohesion energy (3.90 eV), and eliminated the impact of metal clusters on adsorption properties. It was found that the G(N)–Sc–G(N) system could stably adsorb 10 hydrogen molecules with an average adsorption energy of 0.24 eV. Therefore, it can be speculated that G(N)–Sc–G(N) is an excellent hydrogen storage material.     

Stability and kinetic studies of MOF‐derived carbon‐confined ultrafine Co catalyst for sodium borohydride hydrolysis

A mesoporous carbon‐confined cobalt (Co@C) catalyst was fabricated by pyrolysis of macroscale Co‐metal–organic framework (MOF) crystals and used to catalyze NaBH₄ hydrolysis for hydrogen production. To reveal the structural changes of cobalt nanoparticles, we characterized the fresh and used Co@C catalysts using X‐ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), and N₂ adsorption. This MOF‐derived Co@C exhibits high and stable activity toward NaBH₄ hydrolysis. No obvious agglomeration of Co nanoparticles occurred after five consecutive runs, implying good resistance of Co@C composite to metal aggregation. The kinetics of NaBH₄ hydrolysis was experimentally studied by changing initial NaBH₄ concentration, NaOH concentration, and catalyst dosage, respectively. It was found that the hydrogen generation rate follows a power law: r = A exp (?45.0/RT)[NaBH₄]^0.985[cat]^1.169[NaOH]^?0.451 .     

Investigation of Organic Carbon and Iron Group Elements in Bituminous Rocks

In this study, geochemical and biogeochemical features of the organic-rich bituminous shales of the Bolu-Mengen area were investigated. The deposition levels of carbon and iron group metals (Mn, Ni, V, Cr, Co, Ti, Sc) within the bituminous shales and their correlation were also examined. Samples studied contain high amount of organic carbon (Corg), and they have Type I and Type II kerogenes that can generally produce oil and gas. Corg distribution in the samples was found as 0.55–0.99% for 5 samples, 1.23–1.96% for 9 samples, 2.31–3.95% for 5 samples and 4.12–19.14% for 15 samples. 75–100% of the organic material is determined to be amorphous and algal. Corg and metal contents of the bituminous shales in the area are characteristic with cyclic changes. Samples with high Corg values show high correlation with Fe group elements. The variations in cogenetic Corg and metal contents in samples are also discussed.     

Effect of Zn/Co ratio in MOF-74 type materials containing exposed metal sites on their hydrogen adsorption behaviour and on their band gap energy

Simultaneous incorporation of Zn and Co ions into MOF-74 during the crystallization process has been studied, covering the whole Zn/Co concentration range (0-100% Co). The characterization techniques used, including X-ray diffraction, DR-UV-visible spectroscopy, N₂ adsorption isotherms and thermogravimetric analysis, strongly evidence the successful incorporation of both cations into the material framework, producing the crystallization of MOF-74 with 100% Co and MOF-74 with 100% Zn starting from Zn-free and Co-free initial mixtures, respectively, under the same conditions. H₂, CH₄ and CO₂ uptakes of MOF-74 type materials generally increase with framework Co content at any pressure, suggesting a relevant role of cobalt in the adsorption process. As expected, the comparison between the gas adsorption behavior of Co-substituted MOF-5 and Co-containing MOF-74 materials shows that exposed metal sites play a key role in MOF performance in adsorption. On the other hand, the good agreement between the evolution of the experimental band gap values as a function of the Co content and the computational-based predictions found in the literature further supports the coexistence of Zn²⁺ and Co²⁺ ions forming the MOF-74 metal clusters. Finally, we found that variations of both isosteric heat of hydrogen adsorption and band gap energy with the metal cluster composition show a parallel trend, although it is not systematic in the whole range of Zn/Co ratio. Indeed, a minimum band gap energy value and a maximum isosteric heat of H₂ adsorption value were found to appear simultaneously for Co-rich samples still having some Zn rather than for all-Co samples.     

Effects of metal additive on electrochemical performances of Mg-based hydrogen storage materials prepared by hydriding combustion synthesis and subsequent mechanical milling (HCS+MM)

Mg₂Ni-based hydride was prepared by hydriding combustion synthesis (HCS), and subsequently modified with various metal elements (Ti, Co, Cr and Y) by mechanical milling. The effects of the modifications on electrochemical properties of the hydride were investigated. Both of the maximum discharge capacity and the high rate dischargeability (HRD) are increased by the modification of Co, while the cycling stability is improved for the hydride modified with Ti, Cr and Y. The Tafel polarization shows that the addition of the metals has a positive effect on improving the anti-corrosion ability. The electrochemical kinetics was also characterized by linear polarization, electrochemical impedance spectroscopy and potentiostatic discharge. The results show that the hydrogen diffusion coefficient and the electrochemical reaction resistance are increased by the modifications, but the exchange current density decreases.     

Influence of Tb substitution on electromagnetic and microwave absorption properties of barium hexaferrites

Abstract

Rare earth element Tb substituted hexaferrite Ba_1?xTb_xCo₂Fe₁₆O₂₇ (where x?=?0·00–0·25) are synthesised by solid state reaction method. The samples present single W type phase and homogenous grain size. The substitution of Tb³⁺ decreases the lattice parameter c and cell volume of ferrite. The imaginary part of complex permittivity ?″ shows a tendency of decrease with the increasing Tb content. The imaginary part of complex permeability μ″ increases with the increasing Tb content in the frequency range of 0–12 GHz. The magnetic loss tan?δ_m of sample, with Tb content x?=?0·25, increases about 0·20 after being substituted by Tb³⁺. As a result, the microwave absorption properties of Tb substituted ferrites are enhanced in the investigated frequency. The X-ray photoelectron spectroscopy results indicate that the Fe exists as mixed valence state of Fe(II) and Fe(III), and the valence state of Co ions is unchanged as Co²⁺ after the substitution of Ba²⁺ with Tb³⁺ in hexaferrites.