Mechanism and kinetics of Al2O3 nanoparticles formation by reaction of bulk Al with H2O and CO2 at sub- and supercritical conditions

Oxidation of bulk samples of 〈Al〉 by water and H₂O/CO₂ mixture at sub- and supercritical conditions for uniform temperature increase and at the injection of H₂O (665 K, 23.1 MPa) and H₂O/CO₂ (723 K, 38.0 MPa) fluids into the reactor has been studied. Transition of 〈Al〉 into AlOOH and Al₂O₃ nanoparticles has been found out. Aluminum samples oxidized by H₂O and H₂O/CO₂ fluids at the injection mostly consist of large particles (300-500 nm) of α-Al₂O₃. Those oxidized for uniform temperature increase contain smaller particles (20-70 nm) of γ-Al₂O₃ as well. Mechanism of this phenomenon is explained by orientation of oxygen in H₂O polar molecules to the metal in the electric field of contact voltage at Al/AlOOH and Al/Al₂O₃ boundary. Addition of CO₂ to water resulted in CO, CH₄, CH₃OH and condensed carbon, increase in size of Al₂O₃ nanoparticles and significant decrease in time delay. In pure CO₂ 〈Al〉 oxidation resulted in oxide film. Using temperature and time dependences of gaseous reactant pressure and Redlich-Kwong state equation, kinetics of H₂ formation has been described and oxidation regularities determined. At aluminum oxidation by H₂O and H₂O/CO₂ fluids, self-heating of the samples followed by oxidation rate increase has been registered. The samples of oxidized aluminum have been studied with a transmission electronic microscope, a thermal analyzer and a device for specific surface measurement. The effect of oxidation conditions on the characteristics of synthesized nanoparticles has been found out.     

Slaughterhouse wastewater treatment by combined anaerobic baffled reactor and UV/H2O2 processes

Combined processes of biological anaerobic baffled reactor (ABR) and UV/H₂O₂ at a laboratory scale were studied to treat a synthetic slaughterhouse wastewater. In this study, the total organic carbon (TOC) loadings of 0.2-1.1 g/(L day) were used. The results revealed that combined processes had a higher efficiency to treat the synthetic slaughterhouse wastewater. Up to 95% TOC removal was obtained for an influent concentration of 973.3 mgTOC/L at the hydraulic retention time (HRT) of 3.8 days in the ABR and 3.6 h in the UV photoreactor. Meanwhile, up to 97.7% and 96.6% removal of chemical oxygen demand (COD) and 5-day carbonaceous biochemical oxygen demand (CBOD₅) were observed in the ABR for the same influent concentration, respectively. Comparatively, for an influent concentration of 157.6 mgTOC/L, the UV/H₂O₂ process alone with the TOC loading of 0.06-1.9 g/(L h) was also studied, in which, up to 64.3%, 83.7%, and 84.3% of TOC, COD, and CBOD₅ removal were observed, respectively, at the HRT of 2.5 h with hydrogen peroxide (H₂O₂) concentration of 529 mg/L. It was found that individual ABR and UV/H₂O₂ processes enhanced the biodegradability of the treated effluent by an increased CBOD₅/COD ratio of 0.4 to 0.6. An optimum H₂O₂ dosage of 3.5 (mgH₂O₂)/(mgTOC_in h) was also found for the UV/H₂O₂ process.     

Low cost coal-based carbons for combined SO2 and NO removal from exhaust gas

The aim of this paper is to show how a cheap carbonaceous material such as low rank coal-based carbon (or char) can be used in the combined SO₂/NO removal from exhaust gas at the linear gas velocity used in commercial systems (0.12 m s⁻¹). Char is produced from carbonization and optionally activated with steam. This char is used in a first step to abate the SO₂ concentration at the following conditions: 100 °C, space velocity of 3600 h⁻¹, 6% O₂, 10% H₂O, 1000 ppmv SO₂, 1000 ppmv NO and N₂ as remainder. In a second step, when the SO₂ concentration in the flue gas is low, NO is reduced to N₂ and steam at the following experimental conditions: 150 °C, space velocity of 900 h⁻¹, 6% O₂, 10% H₂O, 0-500 ppmv SO₂, 1000 ppmv NO, 1000 ppmv NH₃ and N₂ as remainder.It has been shown that the presence of NO has no effect on SO₂ abatement during the first step of combined SO₂/NO removal system and that low SO₂ inlet concentration has a negligible effect on NO reduction in the second step. Moreover, this char can be thermally regenerated after use for various cycles without loss of activity. On the other hand, this regenerated char shows the highest NO removal activity (compared to parent chars, either carbonized or steam activated) which can be attributed to the activating effect of the sulfuric acid formed during the first step of the combined SO₂/NO removal system.     

Direct synthesis of H2O2 acid solutions on carbon cathode prepared from activated carbon and vapor-growing-carbon-fiber by a H2/O2 fuel cell

Direct synthesis of H₂O₂ acid solutions was studied using a gas-diffusion cathode prepared from activated carbon (AC), vapor-growing-carbon-fiber (VGCF) and poly-tetra-fluoro-ethylene (PTFE) powders, with a new H₂/O₂ fuel cell reactor. O₂ reduction to H₂O₂ was remarkably enhanced at the three-phase boundary (O₂(g)-electrode(s)-acid(l)) at the [AC + VGCF] cathode. Fast diffusion processes of O₂ to the active surface and of H₂O₂ to the bulk acid solutions were essential for H₂O₂ accumulation. Synergy of AC and VGCF was observed for the H₂O₂ formation. RRDE and cyclic voltammetry studies indicated that the surface of AC functioned as the active phase for O₂ reduction to HO₂, and VGCF functioned as an electron conductor and a promoter to convert HO₂ to H₂O₂. A maximum H₂O₂ concentration of 353 mM (1.2 wt%) was accomplished under short-circuit conditions (current density 12.7 mA cm⁻², current efficiency 40.1%, geometric area of cathode 1.3 cm², reaction time 6 h).     

SO2 and NO removal from flue gas over V2O5/AC at lower temperatures — role of V2O5 on SO2 removal

Supporting V₂O₅ onto an activated coke (AC) has been reported to significantly increase the AC's activity in simultaneous SO₂ and NO removal from flue gas. To understand the role of V₂O₅ on SO₂ removal, V₂O₅/AC is studied through SO₂ removal reaction, surface analysis, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) techniques. It is found that the main role of V₂O₅ in SO₂ removal over V₂O₅/AC is to catalyze SO₂ oxidation through a VOSO₄-like intermediate species, which reacts with O₂ to form SO₃ and V₂O₅. The SO₃ formed transfers from the V sites to AC sites and then reacts with H₂O to form H₂SO₄. At low V₂O₅ loadings, a V atom is able to catalyze as many as 8 SO₂ molecules to SO₃. At high V₂O₅ loadings, however, the number of SO₂ molecules catalyzed by a V atom is much less, due possibly to excessive amounts of V₂O₅ sites in comparison to the pores available for SO₃ and H₂SO₄ storage.     

Electrocatalyst materials for fuel cells based on the polyoxometalates—K7 or H7[(P2W17O61)Fe(H2O)] and Na12 or H12[(P2W15O56)2Fe4(H2O)2]

Free acids of the iron substituted heteropoly acids (HPA), H₇[(P₂W₁₇O₆₁)Fe^III(H₂O)] (HFe1) and H₁₈[(P₂W₁₅O₅₆)₂Fe^III₂(H₂O)₂] (HFe2) were prepared from the salts K₇[(P₂W₁₇O₆₁)Fe^III(H₂O)] (KFe1) and Na₁₂[(P₂W₁₅O₅₆)₂Fe^III₄(H₂O)₂] (NaFe4), respectively. The iron-substituted HPA were adsorbed on to XC-72 carbon based GDLs to form HPA doped GDEs after water washing with HPA loadings of ca. 1 μmol. The HPA was detected throughout the GDL by EDX. Solution electrochemistry of the free acids are reported for the first time in sulfate buffer, pH 1-3. The hydrogen oxidation reaction was catalyzed by KFe1 at 0.33 V, with an exchange current density of 38 mA/cm². Moderate activity for the oxygen reduction reaction was observed for the iron substituted HPA, which was dramatically improved by selectively removing oxygen atoms from the HPA by cycling the fuel cell cathode under N₂ followed by reoxidation to give a restructured oxide catalyst. The nanostructured oxide achieved an OCV of 0.7 V with a Tafel slope of 115 mV/decade. Cycling the same catalysts in oxygen resulted in an improved catalyst/ionomer/carbon configuration with a slightly higher Tafel slope, 128 mV/decade but a respectable current density of 100 mA/cm² at 0.2 V.     

Thermodynamic modeling of CO2 solubility in aqueous solutions of NaCl and Na2SO4

A thermodynamic model based on the electrolyte NRTL activity coefficient equation and PC-SAFT equation-of-state is developed for CO₂ solubility in aqueous solutions of NaCl and Na₂SO₄ with temperature up to 473.15 K, pressure up to 150 MPa, and salt concentrations up to saturation. The Henry's constant parameters of CO₂ in H₂O and the characteristic volume parameters for CO₂ required for pressure correction of Henry's constant are identified from fitting the experimental gas solubility of CO₂ in pure water with temperature up to 473.15 K and pressure up to 150 MPa. The NRTL binary parameters for the CO₂-(Na⁺, Cl⁻) pair and the CO₂-(Na⁺, SO₄²⁻) pair are regressed against the experimental VLE data for the CO₂-NaCl-H₂O ternary system up to 373.15 K and 20 MPa and the CO₂-Na₂SO₄-H₂O ternary system up to 433.15 K and 13 MPa, respectively. Model calculations on solubility and heat of solution of CO₂ in pure water and aqueous solutions of NaCl and Na₂SO₄ are compared to the available experimental data of the CO₂-H₂O binary, CO₂-NaCl-H₂O ternary and CO₂-Na₂SO₄-H₂O ternary systems with excellent results.     

Indirect cathodic electrocatalytic degradation of dimethylphthalate with PW11O39Fe(III)(H2O) and H2O2 in neutral aqueous medium

Cyclic voltammetry and degradation of dimethylphthalate (DMP) revealed that the iron-substituted heteropolytungstate anion PW₁₁O₃₉Fe(III)(H₂O)⁴⁻ is an excellent indirect cathodic oxidative electrocatalyst in the presence of H₂O₂. PW₁₁O₃₉Fe(III)(H₂O)⁴⁻ can electrocatalyze the reduction of H₂O₂ to hydroxyl radicals via an inner-sphere electron transfer mechanism, which cause oxidative decomposition of DMP. Almost complete DMP removal and ca. 30% mineralization were obtained in less than 120 min in a mixed phosphate solution at pH 6.86 containing 0.1 mM DMP. MS analyses of the intermediates and final products suggested that glyoxal, oxalic acid and acetic acid are the main ring-opening products, besides some unstable hydroxylated aromatic intermediates. The effects of added H₂O₂ concentration, applied cathodic potential and DMP initial concentration on the degradation of DMP were also investigated. A concentration of 1.0 mM H₂O₂ and cathodic potential of −0.3 V were optimal conditions for DMP degradation in our experiments. At higher initial DMP concentrations degradation also occurred, but at a slower decay rate compared to lower initial concentrations. The present system thus represents a possible method to use PW₁₁O₃₉Fe(III)(H₂O)⁴⁻ as an indirect cathodic oxidative electrocatalyst in water and wastewater treatment.     

Removal of SO2 by Activated Carbon Fibre Impregnated with Transition Metals

Phenolic resin based activated carbon fibres (ACF) impregnated with oxides of various transition metals were investigated for the catalytic oxidation of SO₂ into H₂SO₄. Oxidation was carried out in the packed bed tubular reactor in the presence of O₂ and H₂O. The activity of the various metals impregnated on ACF was observed to be in the order, Cu>Ni>Co>Cr. It was found that the metal oxides were dispersed as monolayer on ACF up to ～5% (w/w) loading, beyond which crystallites formation occurred. A kinetic mechanism for the catalytic oxidation of SO₂ into H₂SO₄ was proposed and incorporated in a transport model developed to explain the experimental breakthrough data.     

Effect of pretreatment and regeneration conditions of Ru/γ-Al2O3 catalysts for N2O decomposition and/or reduction in O2-rich atmospheres and in the presence of NOX, SO2 and H2O

The effect of the pretreatment (inert, oxidative, and reducing) of Ru/γ-Al₂O₃ catalyst on its activity and stability in the decomposition of N₂O in the absence or presence of O₂, SO₂, H₂O and NO_X was studied in the present work. Decomposition of pure N₂O was slightly enhanced by the H₂-pretreated catalyst (metallic Ru) compared to the O₂- or He-pretreated ones, owing to a cyclic oxidation–reduction pathway of metallic Ru. The observed decrease of activity by O₂ or H₂O addition was reversible compared to SO₂ which caused a strong, irreversible deactivation of the catalyst, irrespective of the type of pretreatment. This was attributed to the formation of stable sulphates, mainly those on RuO₂ surface, which could only be removed by regeneration under reducing (H₂ in He) atmosphere at temperatures of ca. 500 °C. Oxidative or inert regeneration required very high temperatures (i.e. >700 °C) in order to decompose these sulphates. A method of retaining N₂O conversion activity very high (≥98%) for long reaction times is suggested and is based on frequent and short-time (ca. 10 min) regenerations of the catalyst under reducing atmosphere (ca. 5% H₂ in He). The effect of co-feeding various reducing agents, such as CO or C₃H₆, on the N₂O conversion activity in the presence of O₂, SO₂, H₂O and NO_X is negligible, mainly because they are oxidized at relatively low temperatures in the O₂-rich feeds used in this study.     

Electrochemical behavior and its electrocatalytic properties of chemically modified electrode with Keggin-type [SiNi(H2O)W11O39]

A monolayer of Keggin-type heteropolyanion [SiNi(H₂O)W₁₁O₃₉]⁶⁻ was fabricated by electrodepositing [SiNi(H₂O)W₁₁O₃₉]⁶⁻ on cysteamine modified gold electrode. The monolayer of [SiNi(H₂O)W₁₁O₃₉]⁶⁻ modified gold electrode was characterized by atomic force microscopy (AFM) and electrochemical method. AFM results showed the [SiNi(H₂O)W₁₁O₃₉]⁶⁻ uniformly deposited on the electrode surface and formed a porous monolayer. Cyclic voltammetry exhibited one oxidation peak and two reduction peaks in 1.0 M H₂SO₄ in the potential range of −0.2 to 0.7 V. The constructed electrode could exist in a large pH (0-7.6) range and showed good catalytic activity towards the reduction of bromate anion (BrO₃⁻) and nitrite (NO₂⁻), and oxidation of ascorbic acid (AA) in acidic solution. The well catalytic active of the electrode was ascribed to the porous structure of the [SiNi(H₂O)W₁₁O₃₉]6⁻ monolayer.     

Molten salt synthesis of α-Al2O3 platelets using NaAlO2 as raw material

α-Al₂O₃ platelets were prepared by a molten salt synthesis method when NaAlO₂ was used as raw material. The effects of the stirring rate during the gel preparation, heating temperature, type and addition amount of molten salts, addition of plate-like α-Al₂O₃ seeds, additives such as TiOSO₄ and Na₃PO₄·12H₂O on the morphology of α-Al₂O₃ were studied. High stirring rate during the gel preparation and high heating temperature not only help to restrain the overlapping of α-Al₂O₃ platelets, but also improve the size distribution. When the heating temperature increases to 1200 °C, most of α-Al₂O₃ platelets are hexagonal in its morphology, and the size of platelets becomes relatively uniform. When Na₂SO₄-K₂SO₄ flux is used instead of NaCl-KCl flux, it is easy to obtain α-Al₂O₃ platelets with a big size. When the molar ratio of salt to final Al₂O₃ powders increases to 4:1, most of α-Al₂O₃ platelets are hexagonal, and the overlapping of powders is inhibited. The addition of a small amount of plate-like seeds has a significant effect on the size of α-Al₂O₃ platelets. With the increase of seed amount, the diameter of α-Al₂O₃ platelets tends to decrease. The addition of 5.45 wt.% TiOSO₄ results in the formation of hexagonal α-Al₂O₃ platelets with an average diameter of 5.1 μm and an average thickness of 1.4 μm. Thin α-Al₂O₃ platelets with a discal shape are obtained owing to the co-addition of 0.51 wt.% Na₃PO₄·12H₂O and 3 wt.% TiOSO₄.     

A novel method for sulfonation of microporous polystyrene divinyl benzene copolymer using gaseous SO2 in the waste air streams

In this study a novel sulfonation method for microporous polystyrene divinyl benzene copolymer (PSDBP) was introduced. In our sulfonation system gaseous SO₂ is used as the sulfonation agent and planned to be obtained from waste gas streams. The proposed method, therefore, combines SO₂ control and clean sulfonation technology in a single compact design.Molded polymeric monoliths of the PSDBP containing imprisoned H₂O₂ solution inside the pores (PSDBPH₂O₂) were produced in disk shapes. Dry gas mixture containing 3000 ppm SO₂ is fed into PSDBPH₂O₂ disk reactor with a flow rate of 0.8 L/min and effluent gas composition in terms of SO₂ was measured. Breakthrough curves for varied initial H₂O₂ amount were used to calculate SO₂ adsorption capacity and sulfonation degree of the PSDBPH₂O₂ disks.Successful sulfonation of PSDBPH₂O₂ was verified by the changes in its morphological structure and formed sulfone bonds determined by SEM and IR analyses, respectively. Maximum adsorption capacity for PSDBPH₂O₂ for the initial H₂O₂ volume percentage of 13% was determined as 57 mg SO₂/g polymer. It should be noted that SO₂ adsorption was observed only in H₂O₂ imprisoned polymer disks. Sulfonation degree of PSDBPH₂O₂ which attained maximum SO₂ amount is calculated as 10%.     

Effects of water vapor, CO2 and SO2 on the NO reduction by NH3 over sulfated CaO

Gas effects on NO reduction by NH₃ over sulfated CaO have been investigated in the presence of O₂ at 700–850 °C. CO₂ and SO₂ have reversible negative effects on the catalytic activity of sulfated CaO. Although H₂O alone has no obvious effect, it can depress the negative effects of CO₂ and SO₂. In the flue gas with CO₂, SO₂ and H₂O co-existing, the sulfated CaO still catalyzed the NO reduction by NH₃. The in situ DRTFTS of H₂O adsorption over sulfated CaO indicated that H₂O generated Br?nsted acid sites at high temperature, suggesting that CO₂ and SO₂ competed for only the molecularly adsorbed NH₃ over Lewis acid sites with NO, without influencing the ammonia ions adsorbed over Br?nsted acid sites. Lewis acid sites shifting to Br?nsted acid sites by H₂O adsorption at high temperature may explain the depression of the negative effect on NO reduction by CO₂ and SO₂.     

Selective oxidation of methanol to dimethoxymethane over acid-modified V2O5/TiO2 catalysts

Selective oxidation of methanol to dimethoxymethane (DMM) was conducted in a fixed-bed reactor over an acid-modified V₂O₅/TiO₂ catalyst. The influence of the acid modification on its structure, redox and acidic properties, and catalytic performance for methanol oxidation were investigated. The results indicated that the content of vanadia in the catalyst exhibits a vital influence on the dispersion of vanadium species, while the acid modification can enhance its surface acidity. Proper amounts of the acid (W() = 15%) and V₂O₅ (W(V₂O₅) = 15%) components loaded in the acid-modified V₂O₅/TiO₂ catalyst are able to build a bi-functional circumstance that is favorable for the formation of DMM with high activity and selectivity. As a result, for the selective oxidation of methanol, the H₂SO₄-modified V₂O₅/TiO₂ catalyst gives a much higher DMM yield at 150 °C than the unmodified one.     

Kinetic relationship between NO/N2O reduction and O2 consumption during flue-gas recycling coal combustion in a bubbling fluidized-bed

Flue-gas recycling combustion of a sub-bituminous coal and its rapid pyrolysis char at 1120 K has been simulated experimentally in a bubbling fluidized-bed. O₂, CO₂ and H₂O, and NO or N₂O were pre-mixed and fed into the bed together with coal/char particles with the O₂ concentration in the exit gas maintained at 3.5 vol%. Increasing the inlet O₂ concentration, thus increasing the O₂ consumption rate and decreasing the flue-gas recycling ratio, caused the once-through conversion of fuel-bound nitrogen into N₂O to decrease while the conversion to NO to remain unchanged. The in-bed reductions of NO and N₂O were both first order with respect to the respective nitrogen oxide, with the rate constants to increase linearly with the rate of O₂ consumption in the bed and thus also with that of char/volatiles consumption. This finding, which indicated linear increase in the concentrations of reactive species involved in NO/N₂O reduction with the rate of O₂ consumption, enabled consideration that the homogeneous and heterogeneous reduction rates of NO and N₂O were proportional to the consumption rates of O₂ by the volatiles and char, respectively. The rate analysis of the kinetic data revealed the relative importance of burning volatiles and char as the agents for the reduction of NO and N₂O. While the reduction in the gas phase was fully responsible for the NO-to-N₂O conversion, the reactions over the char surface governed the NO-to-N₂ reduction. The volatiles and char had comparable contributions to the reduction of N₂O to N₂. The NO-to-N₂ and N₂O-to-N₂ reductions over the char surface were, respectively, accelerated and decelerated by increasing the H₂O concentration.     

Mechanistic study on adsorption and reduction of NO2 over activated carbon fibers

Adsorption and reduction of NO₂ over pitch-based ACFs, both as received and calcined at 1100 °C, were studied in a range of concentrations (NO₂, 250-1000 ppm; O₂, 0-10%) and temperatures (30-70 °C). Repeated adsorption after regeneration at 300 °C, temperature-programmed desorption (TPD) and diffuse reflectance fourier transformation infrared spectroscopy (DRIFTS) were also applied to analyze the adsorbed NO₂ species. Pitch-based ACFs showed rapid NO production and adsorption at 30 °C which stayed at similar conversions until the rapid breakthrough of NO₂. A higher reaction temperature of 70 °C decreased the ratio of NO₂ adsorption to reduction in the stationary state and shortened the breakthrough time. Higher NO₂ concentration increased the rates of both adsorption and reduction to shorten breakthrough time, whereas the presence of oxygen changed the NO₂ profiles by enhancing the NO₂ adsorption rate and decreasing both the rate and the capacity of reduction. It must be noted that 10% O₂ allowed still significant production of NO. The molar O/N ratio evolved from TPD decreased and converged to a constant value according to the NO₂ adsorption time, showing that NO_x species adsorbed on the ACF changed from NO₂ to NO₃ along with the time of NO₂ adsorption. Such a trend was confirmed by DRIFTS spectra of adsorbed NO₂. These results suggest two kinds of NO₂ adsorption sites. Site 1 adsorbs NO₂ molecules strongly, transferring one oxygen to another adsorbed molecule on a similar site to form NO₃ad. Although oxygen in the gas phase oxidized adsorbed NO₂ to some extent, especially in the initial stage, disproportionation is still dominant at 10% O₂. Such disproportionation produces gaseous NO, leaving NO₃ on the surface. Site 2 adsorbs NO₂ weakly. Saturation of both sites terminates the adsorption and reduction and results in the breakthrough of NO₂. Adsorbed NO₃ produces both NO and NO₂ when heated, leaving one or two oxygen atoms on the surface, which are evolved as CO and CO₂ at the same time, restoring a stationary ability for adsorption and reduction of NO₂ through carbon loss.     

Fe2O3/Al2O3 catalysts for the N2O decomposition in the nitric acid industry

Fe₂O₃ catalysts supported on Al₂O₃ were used to remove nitrous oxide from the nitric acid plant simulated process stream (containing O₂, NO and H₂O). Catalysts were prepared by the coprecipitation method and were characterized for their physico-chemical properties by BET, XRD, AFM and TPR analysis. A strong influence of the post-preparation heating conditions on the structural and catalytic properties of the catalysts has been evidenced. Laboratory tests revealed 95% conversion of N₂O at temperature 750 °C and a slight decrease in activity in the presence of H₂O and NO. The catalysts were inert towards decomposition of NO. The pilot-plant reactor and real plant studies (up to 3300 h time-on-stream) confirmed high activity and very good mechanical stability of the catalysts as well as no decomposition of nitric oxide.     

Effects of HCl concentration on the growth and negative thermal expansion property of the ZrW2O8 nanorods

A kind of negative thermal expansion ZrW₂O₈ nanorods were synthesized using a hydrothermal method, followed with a post-annealing at 570 °C for 2 h. Effects of HCl concentration on the microstructure, morphology and negative thermal expansion property in resulting ZrW₂O₈ powders were investigated by X-ray diffraction (XRD) and transmission electron microscope (TEM). Results indicate that the formation of the precursor ZrW₂O₇(OH)₂(H₂O)₂ significantly depends on the HCl concentration, and the precursors ZrW₂O₇(OH)₂(H₂O)₂ can form in the 2-8 mol/L HCl solution. With increasing the concentration of the HCl solutions from 2 to 8 mol/L, the rod-like ZrW₂O₈ particles become more homogeneous, and the average dimension change from 10 μm × 0.5 μm to 700 nm × 50 nm. All the ZrW₂O₈ powders obtained in different conditions exhibit negative thermal expansion property, and the average negative thermal expansion coefficients from 15 °C to 600 °C decrease gradually with the increasing HCl concentration.