The ionic substituted octacalcium phosphate for biomedical applications: A new pathway to follow?

Numerous studies have found that octacalcium phosphate possesses promising biological properties applicable to bone tissue regeneration. To further improve the osteogenic and regenerative properties of octacalcium phosphate, substitutions with Sr²⁺, Zn²⁺, Mg²⁺, Fe³⁺, Na⁺, F^? and CO₃^2? ions have been investigated in recent years. Despite that, hydroxyapatite is still considered the most promising calcium phosphate for bioactive bone grafts due to its biocompatibility, physicochemical similarity to biological apatite, osteoconductivity and strong bonding with the surrounding tissue. However, better biological properties of octacalcium phosphate in vivo as well as a larger volume of regenerated bone tissue, compared to hydroxyapatite, were confirmed by many studies. This review summarizes recent and relevant studies on cationic and anionic substitutions in the crystal lattice of octacalcium phosphate and its in vitro biological performance. It also discusses future challenges and prospects for the use of substituted octacalacium phosphate.     

Influence of magnesium and silver ions on rheological properties of hydroxyapatite/chitosan/calcium sulphate based bone cements

Rheology of bioceramic bone cements is usually described as properties of ceramic slurries, neglecting the self-setting character of these materials. In our studies calcium sulphate based bone cements with Ag⁺, Mg²⁺ and Mg²⁺/CO₃^2- modified hydroxyapatite were investigated. Despite of expectations, it has been proven that the presence of magnesium ions significantly influence the rheological properties of cement pastes. Changes in rheological properties were connected with (I) chemical interactions between Mg²⁺ and sulphate ions (II) chemical interaction between Mg²⁺ and chitosan. These effects were not observed for silver additive. Most of the developed calcium sulphate based pastes, except material containing MgHA and chitosan, have been categorized as thick pastes applicable with the spatula. It has been found that the chitosan present around and at the calcium sulphate grains acted as a lubricant and prolong the period of quasi-constant viscosity of the pastes.     

Drug and ion releasing tetracalcium phosphate based dual action cement for regenerative treatment of infected bone defects

Calcium phosphate cements (CPCs) are ideally suited for the local delivery of antibiotics in infected bone defects as they have multiple binding sites for loading various drugs. CPCs can also be substituted with ions such as Ag⁺, Zn²⁺, Mg²⁺, Sr²⁺, etc., to exhibit extended broad-spectrum antimicrobial activity. Strontium (Sr) in particular is known to enhance the new bone formation and decrease bone resorption. The current work aims to develop a dual action tetracalcium phosphate (TTCP) based cement which releases both the Sr²⁺ ion and ornidazole antibiotic drug for the treatment of bone infections. The TTCP with Sr²⁺ ion substitution was prepared by the solid state reaction method and it was used to form ornidazole loaded CPC. The ornidazole loaded cement prepared using 8?at% Sr substituted TTCP (8SCPC-O) showed complete hydroxyapatite (HA) formation in phosphate buffered solution at the end of 1 week. Fine needle-shaped HA crystals were observed in 8SCPC-O cement. In vitro drug release studies showed an accelerated ornidazole release from the 8SCPC-O sample when compared to samples without Sr substitution. Ornidazole releasing cements were found to be biocompatible with skeletal myoblast (L6) cells. Antibacterial activity of ornidazole releasing cement was evident from day 1 onwards against E. coli. The above results suggest 8SCPC-O as a good candidate for treating local bone infections.     

Enhancement in the osteogenesis and angiogenesis of calcium phosphate cement by incorporating magnesium-containing silicates

Based on the good osteogenic and angiogenic effects of silicon and magnesium elements, three types of micro-nano magnesium-containing silicates (MS), including akermanite (Ake, Ca₂MgSi₂O₇), diopside (Dio, CaMgSi₂O₆) and forsterite (For, Mg₂SiO₄), were incorporated into calcium phosphate cement (CPC) to improve its osteogenic and angiogenic performances for clinical application. In this present work, the physicochemical properties, osteogenesis and angiogenesis of MS/CPCs (Ake/CPCs, Dio/CPCs and For/CPCs) were investigated systematically and comparatively. The results showed that all MS/CPCs had good biomineralization and significantly stimulated the osteogenic differentiation of mBMSCs and angiogenic differentiation of HUVECs, respectively. Besides, the stimulating effects were related to not only the category of MS, but also the content of MS. The For/CPCs had a good angiogenic property but their initial setting times were beyond 60 min. The Dio/CPCs showed the lowest biological performance among the three groups of MS/CPCs due to the lower ion release (Si and Mg). The Ake was the ideal modifier that could provide CPC with appropriate physicochemical properties, better osteogenesis and angiogenesis. Simultaneously, a higher addition (10 wt%) of akermanite resulted in the best potential to bone regeneration. Taken together, this research provides an effective approach to improve the overall performance of CPC, and 10Ake/CPC is of great promising prospect in bone repair.     

Fabrication of highly interconnected porous carbonate apatite blocks based on the setting reaction of calcium sulfate hemihydrate granules

Interconnected porous carbonate apatite (CO₃Ap) blocks that emulate cancellous bone have potential as an alternative to autografts. The present study aimed to evaluate the feasibility of fabricating a block via a stepwise compositional transformation to CO₃Ap through dissolution-precipitation reactions of an interconnected porous calcium sulfate dihydrate (CSD) block, which was obtained by the setting reaction of calcium sulfate hemihydrate (CSH) granules. Exposure of the CSH granules to water resulted in a setting reaction. However, the gaps between the granules were clogged, preventing the fabrication of interconnected porous structures. Removing the water in the gaps using filter paper was beneficial in avoiding gap clogging and in fabricating interconnected porous CSD blocks. Although the CSD blocks transformed into CaCO₃ blocks, which maintained the interconnected porous structure through a dissolution-precipitation reaction in a Na₂CO₃ solution, their mechanical strength was quite low (diametral tensile strength: DTS = 75 kPa). In contrast, a CaCO₃ block with a much greater mechanical strength (DTS = 0.98 MPa) was fabricated when a calcium sulfate anhydrous block made via the heat treatment of the CSD block was used as a precursor. The CaCO₃ block transformed into a CO₃Ap block (DTS = 2.1 MPa), maintaining the interconnected porous structure through a dissolution-precipitation reaction when immersed in a Na₂HPO₄ solution. The CO₃Ap block had macropores initiated by the gaps between the granules and micropores created by the setting reaction of CSH granules and the dissolution-precipitation reactions to form CO₃Ap. The results obtained in the present study demonstrate that this method is useful for fabricating interconnected porous CO₃Ap blocks.     

Comparison of Effects of Calcium and Magnesium Doping on the Structure and Biological Properties of NaTaO3 Film on Tantalum

Tantalum is widely used in hip joint replacement and knee joint repair, but its clinical application is limited due to its poor biological activity and weak ability to promote new bone formation. Ca and Mg ions are thought to be involved in bone metabolism and play an important physiological role in the angiogenesis, growth, and mineralization of bone tissue. In this work, NaTaO₃ films doped with Ca²⁺ and Mg²⁺ were prepared by hydrothermal synthesis and molten salt method. The doping amounts of Ca²⁺ doped at 450, 550, 650 and 750 °C were 0.59, 3.44, 32.75 and 29.88 at%, and that of Mg²⁺ doped at 300, 350, 400, 450, 500, 550 and 650 °C were 0.62, 1.03, 1.54, 20.12, 21.38, 14.37 and 0.74 at%, respectively. Ca²⁺ and Mg²⁺ are evenly incorporated into NaTaO₃ and cause the change of crystal plane spacing without any significant changes of morphologies below 550 and 400 °C respectively. XPS shows that the cations are the A-site substitution of perovskite structure (ABO₃). According to the morphology and composition analysis of Ca-incorporated samples and Mg-incorporated samples, the optimal preparation temperature of them is 550?°C and 400?°C, respectively. The results show that for “550?°C-Ca” and “400?°C-Mg” the hydrophilicity is 13.9° and 96.1°, the roughness is 114.3 and 54.3?nm, the doping ion concentration of Ca and Mg is 3.44 and 1.54 at%, and the 7-day ICP results is 69.8 and 1.4?ppm, respectively. In addition, cell proliferation experiments and cell morphology related to biological activity and osteogenic properties are discussed, and it is found that the performance of “550?°C-Ca” is better than “400?°C-Mg”. Ca²⁺–NaTaO₃ is a promising implantable material that will be extensive used in bone implants, joint replacements and dental implants.

     

Multiphase zinc and magnesium mono-substituted calcium phosphates derived from cuttlefish bone: A multifunctional biomaterials

Biomimetic calcium phosphate (CaP) systems mono-substituted with zinc (Zn²⁺) and magnesium (Mg²⁺) ions were prepared from a biogenic source (cuttlefish bone) by wet precipitation method. The results revealed that the as-prepared powders were composed of calcium-deficient carbonated hydroxyapatite (HAp), octacalcium phosphate (OCP), and amorphous calcium phosphate (ACP), while the heat-treated powders consisted of HAp, α-tricalcium phosphate (α-TCP), and β-tricalcium phosphate (β-TCP). In addition to Zn²⁺ and Mg²⁺ ions, the presence of CO₃^2?, Sr²⁺ and Na ⁺ ions was detected with elemental analysis, which can be attributed to the use of cuttlefish bone as a natural precursor of Ca²⁺ ions. The data obtained by XRD study demonstrated the decrease in lattice parameters in the OCP and β-TCP phases for Zn-substitution and Mg-substitution in the HAp, OCP, and β-TCP phases. Zn²⁺ occupies the Ca(1,3,4,6,7,8) sites in OCP and Ca(1,2,3,4) sites in β-TCP, while Mg²⁺ occupies the Ca(2) sites in HAp and the Ca(4,5) sites in β-TCP. Phase transformation study under simulated physiological conditions for 7 days showed the transformation of OCP and ACP into the thermodynamically more stable HAp. Characterization of the zeta-potential showed positively charged populations for all prepared CaP powders, while all samples showed high bovine serum albumin adsorption capacity. The culture of human embryonic kidney cells showed that the prepared CaPs are non-cytotoxic and that viability of the cells increases during the culture period. All powders obtained showed antibacterial activity towards Gram-negative Escherichia coli and low antibacterial effect against Gram-positive Staphylococcus aureus, as determined by viability analysis during 48 h. Inhibition zone analysis and observation of the morphology after 24 h showed no antibacterial properties.     

Strontium-loaded magnesium phosphate bone cements and effect of polymeric additives

Magnesium phosphate-based cements (MPCs) are nowadays regarded as promising materials in the field of bone repair. The inclusion of Sr ions in the formulations may represent a valuable strategy to improve their bone regeneration performances, but the effect that such ion exerts on the physico-chemical properties of the material have not been investigated so far. In this work we describe the development of Sr-MPCs obtained including Sr ions in different forms, i.e., using Sr-substituted tri-magnesium phosphate precursor powder or including in the formulation Sr-based salts (SrCl₂ and SrHPO₄). The materials were characterized both in the form of pastes and hardened cements, finding that according to the type of Sr precursor used we can tune the setting time, the amount of binding phases in the cements, their morphology and thermal behavior. The dissolution behavior and the release kinetics of Mg²⁺ and Sr²⁺ can as well be modulated, and in particular the use of SrCl₂ in the formulation leads to a higher dissolution and a faster release of a significant amount of both Mg²⁺ and Sr²⁺, compared to the other samples. Given the unsatisfying performances obtained during the injectability and anti-washout tests, we also included two polymeric additives, namely poly(N-isopropylacrylamide) and mucin, in the Sr-MPCs formulations. The results demonstrate that it is possible to obtain Sr-MPCs with promising properties for applications as bone cements, that can be tuned according to the form under which Sr is included in the formulation. In addition, mucin markedly improves the cohesion and injectability of the Sr-MPC pastes, providing a simple but effective strategy to develop materials of interest in the orthopedic field.     

A novel bi-phase Sr-doped magnesium phosphate/calcium silicate composite scaffold and its osteogenesis promoting effect

In order to improve the osteoconductivity and the osteoinductivity of bone tissue engineering scaffold, a novel bi-phase strontium-doped magnesium phosphate/calcium silicate (Sr-MP/CSC) composite scaffold was fabricated by the self-solidifying/particulate leaching method. The bi-phase composition of the well-crystallized struvite grains wrapped by the calcium silicate floccules was propitious to the deformability and toughness of composite scaffold, and the porous structure with interconnected macropores of 100–400?µm was beneficial to supporting the tissue growth and transporting nutrients and metabolites. When the Sr-MP/CSC composite scaffolds were degraded in the simulated physiological environment, the doped strontium could be sustainably released together with Ca²⁺, Mg²⁺, PO₄^3- and silidous ions. The proliferation and osteogenic differentiation of rat bone marrow stromal cells (BMSCs) on these composite scaffolds were obviously promoted. More valuably, the Sr-doped MP/CSC scaffolds exhibited the more obvious promotion to ALP activity, Col I and OCN expression than the un-doped MP/CSC scaffold, especially in the later stage. The results suggested that the strontium combined with calcium, magnesium and silicon could synergically promote osteogenesis, and the Sr-MP/CSC composite might be one of the promising bone tissue engineering scaffold materials.     

Correlation between crystal structure and modified microwave dielectric characteristics of Cu2+ substituted Li3Mg2NbO6 ceramics

Structural characteristics exert significant influences on microwave dielectric properties, and ion substitution is widely adopted to modify material performances by adjusting the crystal structure. In this work, low loss Li₃Mg_2-xCu_xNbO₆ (x?=?0–0.04) ceramics were prepared by Cu²⁺ substitution. The impacts of Cu²⁺ substitution for Mg²⁺ sites on the microwave dielectric characteristics and crystal structure were discussed in detail. Rietveld refinement results implied that a single Li₃Mg₂NbO₆ phase was formed. Additionally, a dense and homogeneous microstructure with grain sizes of 7–9?μm could be achieved, and moderate Cu²⁺ substitution could significantly promote the grain growth. The correlation between microwave dielectric characteristics and crystal structure was discussed by calculating some structural parameters. The ε_r was determined by the polarizability. The Q?×?f was influenced by the packing fraction. The τ_f value was dependent on the NbO₆ octahedron distortion, and the τ_f value could be adjusted to near zero for x?=?0.02. Typically, the Li₃Mg_2-xCu_xNbO₆ (x?=?0.02) composition exhibited remarkable microwave dielectric performances: ε_r?=?15.75, Q?×?f?=?92,134?GHz (9.86?GHz) and τ_f?=??2?ppm/°C, making it a promising candidate for temperature-stable millimeter-wave applications.     

Preparation of calcium sulfate dihydrate and calcium sulfate hemihydrate with controllable crystal morphology by using ethanol additive

Calcium sulfate hemihydrate (CSH) with controlled crystal morphology has attracted broad interests due to its superior physical and chemical properties, as well as excellent biological performance. In this study, calcium sulfate dehydrate (CSD) was firstly synthesized via the reaction of H₂SO₄ and Ca(OH)₂ using ethanol as morphology modifier. The prepared CSD was then converted to CSH through a hydrothermal method. It was found that the precipitation time of CSD powders was dramatically shortened and the morphology of CSD crystals was changed from thick tabular to short-rod with the increment in ethanol addition. The finally-obtained CSH crystals were found to have hexagonal prisms shape with smaller aspect ratios. The CSH powder with the desired crystal morphology would provide improved setting behavior and biological performance of the CSH bone cement.     

Enhanced thermoelectric properties of Na and Mg co−doped BiCuSeO

A series of Bi_0.97?xNa_0.03Mg_xCuSeO (0 ≤ x ≤ 0.12) was fabricated by a two?step solid?state reaction and spark plasma sintering (SPS), and the influence of Mg²⁺ doping on the thermoelectric properties of Bi_0.97Na_0.03CuSeO was systematically investigated. The SPS processed?Bi_0.97?xNa_0.03Mg_xCuSeO had a ZrSiCuAs?type tetragonal crystal structure (space group P4/nmm). The Mg²⁺ doping appreciably enhanced the electrical conductivity due to the increase in hole concentration. Furthermore, the Mg²⁺ doping increased the grain boundary areas and bulk porosity and induced the strain field and mass fluctuations, thereby reducing the phonon thermal conductivity. We significantly improved the thermoelectric performance of Bi_0.97?xNa_0.03Mg_xCuSeO (0 ≤ x ≤ 0.12) by enhancing the thermoelectric power factor and by reducing the thermal conductivity.     

Sol–gel synthesis and infrared radiation property of Li-substituted cordierite

The Mg_2?xAl_4+1/2xLi_1/2xSi₅O₁₈ (0.1≤x≤1) ceramics with the substitution of (Li_1/2Al_1/2)²⁺ for Mg²⁺ were synthesized by the sol–gel method. The characterization of the modified cordierite included X-ray diffraction, SEM, EDS and infrared radiation. The crystal structure of Mg₂Al₄Si₅O₁₈ with the substitution of (Li_1/2Al_1/2)²⁺ for Mg²⁺ changed and the amount of secondary phase increased with increasing the x value from 0.1 to 1. High infrared emissivity over 0.9 in the band of 8–14 μm at room temperature was obtained in Mg_2?xAl_4+1/2xLi_1/2xSi₅O₁₈ (x=0.1). The material based on cordierite with x=0.1 sintered at 1200 °C maintained a single phase, compact microstructure and good infrared emissivity with potential use in infrared heating.     

Effects of (Mg1/3Ta2/3)-substitution for Ti-site on the microwave dielectric properties of Li2MgTiO4 ceramics

Novel high quality factor microwave dielectric ceramics Li₂MgTi_1?x(Mg_1/3Ta_2/3)_xO₄ (0 ≤ x ≤ 0.5) were successfully prepared via a conventional solid-state ceramic route. The effects of isovalent substitutions (Mg_1/3Ta_2/3)⁴⁺ at the Ti-site on the sintering behaviors, microstructures, and microwave dielectric properties of Li₂MgTiO₄ ceramics were investigated in this paper. The sintered samples exhibited the single phase with cubic rock-salt structure belonging to Fm-3m space group in the whole composition range. Rietveld refinement which could be performed by the Fullprof program was taken to explain the effects of (Mg_1/3Ta_2/3)⁴⁺ ion substitution on the crystal structures of Li₂MgTiO₄ ceramics. With the (Mg_1/3Ta_2/3)⁴⁺ content increasing from 0 to 0.5, the quality factor Q·f firstly increased and decreased thereafter, while the dielectric constant ε_r almost linearly decreased. In addition, the τ_f values shifted to positive value with the amount of (Mg_1/3Ta_2/3)⁴⁺ increasing. The best composition appeared to be Li₂MgTi_0.6(Mg_1/3Ta_2/3)_0.4O₄, which showed excellent microwave dielectric properties of ε_r = 15.73, Q·f = 184,000 GHz and τ_f = ? 12.54 ppm/°C. This made the Li₂MgTi_0.6(Mg_1/3Ta_2/3)_0.4O₄ ceramic a very promising candidate for use as a low-loss microwave material.     

The influence of alkali-free and alkaline shotcrete accelerators within cement systems: Influence of the temperature on the sulfate attack mechanisms and damage

The resistance to sulfate attack of mixtures accelerated with alkali-free and alkaline accelerators was found to be mainly influenced by the Al³⁺ and SO₄²⁻ added via the admixtures. Microstructural observations showed decalcification and disintegration of the CSH gel, which acted as an additional Ca²⁺ supplier as compared to the CH for ettringite formation. The CSH decalcification was mainly observed with a homogeneous distribution of the alkali-free admixture. The disintegration of the CSH gel increased the porosity and allowed more sulfate solution to penetrate into the specimens. This process promoted the swelling of the specimens and directly contributed to the expansion, explaining the lack of a direct relationship between the ettringite formation and the expansion. Moreover, the CSH gel disintegration, typical for MgSO₄ attack, also occurred with Na₂SO₄ solutions and depending on the aluminate-sulfate distribution and the extent of the CSH gel disintegration, different damage types were detected. At higher temperatures (65 °C) the damage was mainly controlled by the growth, the rearrangement and the thermal stability of ettringite.     

Studies on impurity ion and radiofrequency electric field effects on calcium carbonate deposition

The influences of impurity ions (Mg²⁺, Zn²⁺, Fe²⁺, Fe³⁺, and Al³⁺ ) and radiofrequency (44 MHz) electric field (RF) on calcium carbonate deposition in glass capillaries were studied. Calcium carbonate was precipitated from 0.1 M CaCl₂ and Na₂CO₃ solutions (100+ 100 ml) just before the inlet to the capillary. It was found that except for Al ³⁺ ions at 0.001 M concentration, the rest of the ions studied decreased CaC0₃ deposition. A minor influence of the RF field on the deposition was found in some of the systems tested, but it was in the range of the standard deviation of the results. An increase in deposition was observed in systems without impurity ions when equimolar volumes (500 + 500 ml) of 0.0025 M solutions of CaCl₂ and Na₂CO₃ were used for the experiment.     

Influence of inorganic and organic additives on the composition of the precipitate of synthetic hydroxylapatite and calcium oxalate monohydrate

Synthetic hydroxylapatites are prepared with additives, such as Mg²⁺, CO ₃ ^2? , and C₂O ₄ ^2? . An increase in the concentration of magnesium leads to the formation of struvite. In the Ca(NO₃)₂-(NH₄)HPO₄-Na₂CO₃-NH₄OH-H₂O system, an excess of carbonate ions leads to the formation of calcite. When the synthesis is performed using oxalate ions as additives, calcium oxalate does not form the inherent phase. Calcium oxalate monohydrate is synthesized with additives, such as CO ₃ ^2? , HPO ₄ ^2? , and SO ₄ ^2? ions and urea, glycine, and glutamic acid. X-ray powder diffraction analysis has revealed that the composition of the CaC₂O₄ · H₂O precipitate remains unchanged under these conditions and in the presence of the aforementioned additives.     

Monodisperse nanospheres of yttrium oxysulfide: Synthesis,characterization, and luminescent properties

A red long-lasting phosphorescent material, monodisperse Y₂O₂S: Eu³⁺, Mg²⁺, Ti⁴⁺ nanospheres have been prepared successfully. Y(OH)(CO₃): Eu³⁺ nanospheres were firstly synthesized via an urea-based homogeneous precipitation technique to serve as the precursor. Nanospheres long-lasting phosphors Y₂O₂S: Eu³⁺, Mg²⁺, Ti⁴⁺ were obtained by calcinating the precursor in CS₂ atmosphere. XRD investigation shows a pure phase of Y₂O₂S, indicating no other impurity phase appeared. SEM observation reveals that the structures are nanosphere. The Y₂O₂S: Eu³⁺, Mg²⁺, Ti⁴⁺ nanospheres with particle size about 100–150 nm show uniform size and well-dispersed distribution. After irradiation by ultraviolet radiation with 325 nm for 5 min, the phosphor emitted red color long-lasting phosphorescence corresponding to typical emission of Eu³⁺ ion. The main emission peaks are ascribed to Eu³⁺ ions transition from ⁵D_J (J = 0, 1, 2) to ⁷F_J (J = 0, 1, 2, 3, 4). Both the PL spectra and luminance decay revealed that this phosphor had efficient luminescent and long-lasting properties. It was considered that the red-emitting long-lasting phosphorescence was due to the persistent energy transfer from the traps to the Ti⁴⁺ and Mg²⁺ ions.     

Direct air capture of CO2 by metal cation-exchanged LTA zeolites: Effect of the charge-to-size ratio of cations

Direct air capture of CO₂ (DAC) has been increasingly recognized as a promising carbon-negative technology. The challenge in deploying energy-efficient DAC lies in effective sorbent materials. In this research, we comprehensively investigated the DAC behavior of LTA zeolites exchanged with different metal cations (Na⁺, K⁺, Mg²⁺, Ca²⁺, Mn²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺, Y³⁺, La³⁺, Ce³⁺, Eu³⁺, Tb³⁺, and Yb³⁺) by both static single-component gas adsorption and dynamic mixture gas adsorptive separation tests. We found that a large charge-to-size ratio of cations is critical to imparting a high DAC capacity of LTA zeolites, which is ascribed to the enhanced electrostatic interaction and/or π-back bonding toward CO₂. Meanwhile, a detrimental effect is associated with an excessively large charge-to-size ratio, that is, a significant “shielding effect” of (pre-) adsorbed contaminants (e.g., H₂O and CO₂) on cations (e.g., Mn²⁺ and Mg²⁺) reduce the accessible CO₂ capacity. Ca-LTA featuring Ca²⁺ with an appropriate charge-to-size ratio exhibits the highest DAC capacity (350 ppm CO₂ in the air, 1.20 mmol/g) with fast kinetics and good reusability. These results provide valuable insights for the design of zeolites-based physisorbents for DAC.     

Catalytic Features of Mg Modified Ni/SiO2/Silica Cloth Systems in the Decomposition of Methane for Making “CO x -Free” H2

The influence of the Mg addition on the catalytic performance of a 20% Ni based SiO₂/silica cloth system in the decomposition of methane for making “CO _x -free” H₂ in the T_R range 773–873 K has been evaluated. As the Mg_at/Ni_at ratio increases from 0.10 to 2.20 the incipient Ni–Mg-O interaction becomes stronger until the formation of Ni _x Mg_(1?x)O solid solution. The presence of Mg²⁺ ions allows a higher carbon capacity (C/Ni, number of CH₄ molecules converted per Ni atom until complete catalyst deactivation) and consequently a higher H₂ productivity of the Ni based systems which results in a smoothing effect of the unavoidable declining activity trend even if it does not affect the tip growth coking mechanism.