Decomposition Of HCN Research Articles

Simulating volatiles conversion in dense burning coal suspensions. 1. Validation of chemical reaction mechanisms

The fuels that burn in dense coal suspensions are mixtures of CH4, C2H2, H2, CO, HCN, and H2S, char, and soot. All these fuels except soot effectively compete for the available O2 throughout volatiles conversion, which significantly affects N-species transformations. This simulation study validates a collection of detailed reaction mechanisms that accurately interprets how coal quality and inlet O2 level affect all the major reactants, intermediates, and products, including N-species, within dense burning coal suspensions. Predicted flame structures are generally within measurement uncertainties for three diverse coals and inlet O2 levels from 2 to 15%, provided that the kinetics for tar decomposition into soot, CO, H2, and gaseous hydrocarbons are implemented in a particular way. Tar decomposition kinetics should always be evaluated at the particle thermal history to ensure that they keep pace with primary devolatilization so that gaseous hydrocarbons can mediate HCN conversion into NO under locally reducing atmospheres. Moreover, gaseous hydrocarbons that would otherwise add to soot during the later stages in inert atmospheres should be burned in the free stream along with other noncondensable fuel components, again, to mediate NO production under reducing conditions. However, radical scavenging and heterogeneous NO reduction on soot appear to be superfluous. These findings reveal an important qualification on the limiting scenario for complete mixing of volatiles into an entrainment stream before they are converted: Tars must coalesce into soot in the immediate vicinity of their parent particles even while noncondensable fuels are fully dispersed across an entrainment stream; otherwise, hydrocarbons, H2, and CO will be unavailable to mediate HCN decomposition into NO.

Reaction pathways for HCN on transition metal surfaces.

The adsorption and decomposition of HCN on the Pd(111) and Ru(001) surfaces have been studied with reflection absorption infrared spectroscopy and density functional theory calculations. The results are compared to earlier studies of HCN adsorption on the Pt(111) and Cu(100) surfaces. In all cases the initial adsorption at low temperatures gives rise to a ν(C-H) stretch peak at ∼3300 cm-1, which is very close to the gas phase value indicating that the triple CN bond is retained for the adsorbed molecule. When the Pd(111) surface is heated to room temperature, the HCN is converted to the aminocarbyne species, CNH2, which was also observed on the Pt(111) surface. DFT calculations confirm the high stability of CNH2 on Pd(111), and suggest a bi-molecular mechanism for its formation. When HCN on Cu(100) is heated, it desorbs without reaction. In contrast, no stable intermediates are detected on Ru(001) as the surface is heated, indicating that HCN decomposes completely to atomic species.

Catalytic decomposition of HCN on copper manganese oxide at low temperatures: Performance and mechanism

The development of innovative treatment technology for HCN at low temperatures is vital for the control of HCN pollution. Here, a copper manganese oxide (Cu-Mn-O) catalyst was prepared for the decomposition of HCN at temperatures from 80 to 200 °C. The results showed that the Cu-Mn-O catalyst exhibited excellent catalytic performance for HCN conversion. X-ray photoelectron spectroscopy analysis revealed that Mn3+ manifested highly catalytic activity and was largely responsible for HCN decomposition. The N-contained products of HCN contained NH3, NO/NO2, N2O, and N2, thereby suggesting the concurrent catalytic oxidation and hydrolysis during HCN decomposition on the catalyst. The catalytic oxidation mechanism characterized by in situ diffuse reflectance infrared Fourier transform manifested that four N-contained intermediates (i.e., -CN, -NH2, =NH and -NCO) were produced; subsequent the oxidation of these intermediates resulted in the formation of final product and/or oxidative species NO+. The reaction of NO+ with the N-contained intermediates also generated the final conversion products. Catalytic intermediate formamide plays a critical role in the hydrolysis of HCN, and its hydrolysis leads to the formation of NH3. Multiple cycle experiments demonstrate the long-term stability of the Cu-Mn-O catalyst. These results indicate that catalytic decomposition of HCN based on the Cu-Mn-O catalyst at low temperatures may be an efficient approach for the treatment of tail gases containing HCN.

Kinetic Simulations of Volatile Organic Compounds Decomposition by Non-thermal Plasma Treatment

Volatile organic compounds (VOCs) decomposition by non-thermal plasma (NTP) has been receiving increasing attention from the scientific communities due to its advantages of easy operation, high efficiency, energy saving, and non-secondary pollution. But most of the researches are doing experiments and existing experiment methods cannot observe the micro physical and chemical processes. In order to make up for the deficiency of the experiment, herein, a numerical method was developed to analyze the decomposition behavior of HCN, C3H3N, C3H8, C3H6, CO, and NO in the VOCs treatment by NTP. Results indicated that increasing electron density or energy was beneficial to VOCs decomposition, but when the electron density and energy was too high, the promotion would be weakened. The influences of initial concentration of O2 and H2O on different VOCs decomposition were totally different. The increase of initial concentration of oxygen was beneficial to the decomposition of HCN, C3H8, CO, and NO, but the high concentration of oxygen could promote to generate C3H6 at the initial reaction stage. The decomposition of HCN and C3H3N are not restricted by dry or wet conditions, but the increase of the concentration of water vapor is advantageous to the decomposition of C3H8, CO, and NO.

Nitrogen transformation during gasification of livestock compost over transition metal and Ca-based catalysts

Catalytic gasification of a pig compost (PC) was investigated over transition metal catalysts (TMCs, including limonite, CoMo/Al2O3, Ni/Al2O3, and nickel loaded on lignite char) and Ca-based catalysts (dolomite and CaO) in a two-stage fixed-bed reactor to understand the effects of catalyst, temperature, and steam on nitrogen distributions. Non-catalytic thermal decomposition (TD) of PC volatiles below 750°C is not effective for decomposing the entire volatile nitrogen species (VNSs) to N2. NH3 was found to be the predominant nitrogenous gas under inert conditions used in this investigation, and its yield increased with raising TD temperature. The N yield in HCN is lower than 5% below 550°C, and sharply increased to 13.9% at 750°C due to TD of volatiles. Most of VNSs were converted to N2 over TMCs, especially over Ni-based ones. The TMCs proved to be quite active not only for tar reduction, but also for VNSs decomposition at 450–650°C. On the contrary, CaO-based catalysts, especially dolomite, significantly promoted the conversion of VNSs to NH3. Ni/Al2O3 effectively promoted the conversion of NH3 and HCN to N2 at 550°C. Steam introduced mainly prevented HCN decomposition over dolomite and coke deposition over Ni/Al2O3. This study provides a basic insight into the nitrogen transformations during catalytic gasification of PC, which would benefit the clean utilization of PC as an energy source.

Decomposition of gaseous HCN in the presence of Ni-containing catalysts

This work is devoted to the decomposition of gaseous HCN by oxidation with air catalyzed by different nickel containing catalysts. The presence of metallic nickel does not cause the decomposition of cyanide at 400 °C, but NiO, electrochemical prepared nickel oxides Ni2O3·xH2O and (β,γ)-NiOOH exhibit distinctly high catalytic activity at this temperature. The effect of HCN decomposition for electrochemically prepared nickel oxides was over 90 %. Nickel deposited on activated carbon also demonstrated catalytic behavior during the destruction of HCN with an air mixture at the temperature 400 °C. Nickel oxide mixed with activated carbon showed a small increase of catalytic activity in the destruction of HCN in comparison with NiO.

Kinetics of HCN Decomposition on the Pt(111) Surface by Time-Dependent Infrared Spectroscopy

Time-dependent reflection absorption infrared spectroscopy has been used to investigate the kinetics of HCN decomposition on the Pt(111) surface over the temperature range of 120 to 135 K. At these low temperatures, HCN bonds at an atop site with the HCN axis perpendicular to the surface, which gives rise to an intense C–H stretch at ∼3300 cm–1. Further support for this HCN adsorption geometry is obtained through HCN/CO coadsorption experiments in which both molecules are seen to compete for the atop sites. The disappearance of the C–H stretch peak of HCN at low temperatures is indicative of dissociation to produce adsorbed H and CN. When the decrease in HCN coverage is followed for a sufficiently long time, the data deviate from the expected first-order rate law, and the temperature dependence of the rate constant deviates from the Arrhenius form. Over a more restricted coverage range, simpler behavior is observed, and an activation energy for HCN dissociation of 0.33 eV is obtained.

Heterogeneous radiolysis of HCN adsorbed on a solid surface

Hydrogen cyanide is a key molecule for chemical evolution studies because, when it is exposed to different sources of energy, it forms various compounds of biological importance. To understand the role of minerals in chemical evolution, a series of experiments was performed. First, the adsorption capacity of HCN on different surface minerals was studied; the results show that HCN is readily adsorbed onto the solids proposed (zeolite, serpentine, dolomite, and sodium montmorillonite), in particular zeolite and montmorillonite. Second, the radiolysis of HCN adsorbed on olivine (as an example of a mineral surface) was also followed; it was found that the rate of HCN decomposition by gamma irradiation is enhanced in the presence of the solid. The third series of studies show that organic material was produced in high abundance from HCN at high radiation doses. The radiolytic products included gases (CO 2, NH 4, and CO) and oligomeric materials that release carboxylic acids (succinic, malonic, citric, and tricarballylic acids) and amino acids upon acid hydrolysis. These experiments suggest that minerals could have participated actively in chemical evolution processes.

Hydrolysis and oxidation of gaseous HCN over heterogeneous catalysts

The hydrolysis and oxidation of HCN, which is a potential toxic emission of automotive catalysts, were systematically examined with model gas experiments on typical hydrolysis, SCR and oxidation catalysts.TiO2-anatase showed the highest HCN hydrolysis activity among the hydrolysis catalysts, with approximately two times more activity than Al2O3. On Fe-ZSM-5, HCN was converted to NH3 to the same degree as on TiO2. In the presence of NOx, the NH3 formed from HCN reacted in the SCR reaction to form nitrogen.On Pd- and Pt-containing oxidation catalysts, which are used in SCR systems as ammonia slip catalysts, HCN is converted with very high activity above 250–300°C. The same reaction products are formed as in the oxidation of NH3, i.e., aside from nitrogen N2O and NOx appear as unwanted reaction products depending on the temperature and gas composition. Similarly high HCN conversions, but clearly better N2 selectivities, were reached on Cu-ZSM-5 and MnOx-Nb2O5-CeOx.The precise measurement of all relevant gas components allowed us to develop a reaction scheme for the HCN decomposition chemistry over a variety of heterogeneous catalysts. Over hydrolyzing catalysts water interacts with HCN, forming methanamide and then ammonium formate, which decomposes to ammonia and formic acid. The formic acid finally thermolyzes to water and CO. Catalysts with oxidizing properties oxidize HCN to HNCO in the first reaction step, which then hydrolyzes to unstable carbamic acid. Carbamic acid decomposes to CO2 and NH3, which can be further oxidized to N2, N2O or NOx. The oxidation of HCN to HNCO may also proceed with (CN)2 as an intermediate over Pd-, Pt- and Cu-containing catalysts.

A GLOBAL NOx SUBMODEL FOR PULVERIZED COAL FLAMES AT ELEVATED PRESSURES

ABSTRACT This study formulates a global NOX submodel for deployment in CFD simulations from a database on flames of three diverse coals at pressures to 3.0 MPa for broad ranges of stoichiometric ratio (S.R.). A new reaction scheme was formulated from a sensitivity analysis of simulations based on detailed reaction mechanisms for all tests. It shares many elements in common with commercial submodels, yet it correctly predicts that (1) less coal-N is converted into NO; and (2) HCN persists to higher S.R. for progressively higher pressures. Explicit dependences on O2 concentrations are responsible for the first feature, because the variations in O2 concentrations mimic the ways that the oxyhydroxyl radical pool shrinks at progressively higher pressures, which shifts HCN conversion toward N2 production. The second feature was depicted by resolving the intermediate products of HCN decomposition in the global scheme. Discrepancies surfaced when the new submodel was applied to different coals without re-adjusting rate parameters, which probably reflects a generic limitation of global NOX production submodels for coal combustion.

Formation of Hydrogen Cyanide and Ammonia during the Gasification of Sewage Sludge and Bituminous Coal

HCN and NH3 released during the gasification of sewage sludge have been measured during a program of tests with a laboratory-scale spouted-bed gasifier. The data have been compared with results from gasification tests with coal. The effect of altering the bed temperature has been investigated, and the results have been related to reactions involving gaseous N species known to occur in the gasifier. The effect of steam addition on the HCN release has been examined. It has been found that the HCN concentrations in the exit gas increase with the operating temperature, which is thought to indicate increased formation as a primary product of the decomposition of the fuel-N compounds. Increasing the height of the char bed caused a significant reduction in the HCN concentration at the exit, as this promoted the decomposition of HCN to NH3. Steam addition caused a rise in the HCN concentration during tests with sewage sludge and a similar effect had previously been reported on the NH3 concentration during tests w...

Catalytic decomposition of HCN on heated W surfaces to produce CN radicals

The catalytic decomposition processes of HCN on heated W surfaces were examined using laser spectroscopic and mass-spectrometric techniques. H atoms and CN radicals were identified as primary decomposition products on the catalyzer surfaces. The effective enthalpy for the production of CN was determined to be 370 kJ mol −1. The decomposition efficiency of HCN approaches 30% when the catalyzer temperature is over 2000 K. This efficiency is around half of that for H 2. If CN radicals produced in this manner can be doped in a-Si:H to terminate the dangling bonds, this could be a new technique to suppress the photodegradation of a-Si:H.

Peroxides in ordered nanoporous silicas: clean alternatives to transition metal oxidants for the removal of toxic gases.

Ordered nano-structured MCM-48 silica containing sodium peroxydisulfate is a novel, highly effective material for the decomposition of HCN under ambient conditions.

Modified mesoporous silicates for the adsorption and decomposition of toxic gasesElectronic supplementary information (ESI) available: elemental analyses for the MCM-41 and carbon-based adsorbents. See http://www.rsc.org/suppdata/jm/b1/b110111k/

New adsorbents based on MCM-41 have been developed for the adsorption and decomposition of HCN and CNCl. Adsorption of CNCl is provided by diaminoalkylsilane tethers bound to the surface. Reactivity towards HCN is provided by Cu2+ ions complexed by the diamines. Overall reactivity towards the two gases depends on the balance between free amine and Cu2+ concentrations. The performance of these adsorbents is superior to that of carbon-based adsorbents in which alkylamine and copper(II) salt are physisorbed on the carbon surface.

Variables, kinetics and mechanisms of heterogeneous reburning

AbstractThe variables, kinetics, and mechanisms of heterogeneous reburning were studied in a flow reactor with a simulated flue gas. The efficiency of heterogeneous reburning depends on the origin of the char, char preparation history and the presence of oxidants, CO2 and O2, and the reducing agent CO, in reburning. Estimated intrinsic rate constants for surface NO reduction in various gaseous environments were compared with those published. In addition to its large surface area, the effectiveness of lignite char appears to be due to its ability to promote two consecutive reactions: (1) the gasification of char by CO2 and O2 for production of CO; (2) the removal of surface oxygen complexes, including those formed after adsorption of NO, by gaseous CO, for the regeneration of reactive sites. Moreover, lignite ash also catalyzes the decomposition of HCN, a major intermediate of NO conversion during gas reburning. These observations suggest that reburning by a mixed fuel containing natural gas and lignite char can be a potentially attractive route for the in–furnance control of nitrogen oxide.

Adsorption, Isomerization, and Decomposition of HCN on Si(100)2 × 1: A Computational Study with a Double-Dimer Cluster Model

The adsorption, isomerization, and decomposition of HCN on Si(100)-2 × 1 surface have been investigated by means of a density functional theory calculation using a double-dimer cluster model. The results revealed that both HCN and its HNC isomer can be readily adsorbed on a Si−Si dimer either dissociatively or molecularly in an end-on and a side-on configuration. Side-on adsorption occurs by the cycloaddition of the C⋮N group on to the Si−Si dimer, whereas dissociative adsorption gives rise to H(a) and CN(a) adspecies initially via the end-on configuration on the same dimer or across the two dimers. Adsorbate−adsorbate interactions and reactions have also been studied with two HCN molecules. For the end-on adsorption, the first HCN(a) exerts a significant effect on the adsorption geometry of the second HCN. In particular, a synergetic effect has been observed for the parallel adsorption of two HCNs with their CN groups bridging across the two dimers. For the side-on adsorption, the adsorbate−adsorbate interaction is negligible with minor effects on the adsorption geometry. H-migration between the two neighboring, side-on HCN admolecules can occur readily, leading to the formation of HCNH(a) and NC(a) surface species. The calculated vibrational frequencies of the HCNH(a) and NC(a) adspecies are in good agreement with the experimental HREELS data of the HCN/Si(100) system.

Catalytic Hydrolysis of HCN over H-Ferrierite

Abstract The decomposition of HCN over H-ferrierite below 670 K proceeded mainly by hydrolysis to NH3 and CO. In the presence of NO and NO2, NH3 formed further reacted to form N2 and N2O.

A Model Calculation for the Isomerization and Decomposition of Chemisorbed HCN on the Si(100)‐2×1 Surface

AbstractAb initio molecular orbital and hybrid density functional theory calculations have been performed to study the adsorption, isomerization, and decomposition of HCN on Si(100)‐2×1 using the Si9H12 cluster model of the surface. The results of our calculations indicate that the HCN can adsorb molecularly without a barrier onto the surface with both end‐on (LM1) and side‐on (LM2) positions. LM1 can isomerize to LM2 with a small barrier of 8 kcal/mol. The isomerization of LM2 by H‐migration from C to the N atom, requires 76 kcal/mol activation energy (c.f. 47.5 kcal/mol in the gas phase) because of surface stabilization. Both HCN(a) and HNC(a) end‐on adsorbates were found to dissociate readily, as concluded in our earlier experiment, to produce H and CN adspecies. The computed vibrational frequencies of HCN, CN, and also HCNH adspecies agree reasonably well with those observed experimentally. HCNH was found to be stable, with either the C or the N attaching to the surface.

Adsorption and decomposition of HCN on a polycrystalline Pt foil studied by TPD, MBRS, XPS and FT-RAIRS

The adsorption and decomposition of HCN on a polycrystalline Pt foil have been studied by temperature-programmed desorption (TPD), molecular beam reactive scattering (MBRS), X-ray photoelectron spectroscopy (XPS) and FT reflection–absorption IR spectroscopy (FT-RAIRS). Adsorption of HCN at 100 K yields two distinct CN-containing species for sub-monolayer coverages. The first is identified as a weakly adsorbed molecular HCN species whilst the second species is identified as CN(ads), formed by the dissociative chemisorption of HCN. Higher exposures of HCN at 100 K result in the additional formation of physisorbed multilayers of HCN, which are observed to desorb below 200 K. Adsorption of HCN at 300 K still yields CN(ads) and H(ads), with some of the latter desorbing as H2 at around 400 K, but also produces a new surface intermediate. The most likely structure for this intermediate is identified by FT-RAIRS as a trans-HCNH surface species. The near coincident desorption of HCN and H2 observed in the temperature range 400–500 K is then attributed to the decomposition of this HCNH intermediate and subsequent rapid recombination reactions. Above 650 K the desorption of C2N2 is observed owing to the surface combination of CN(ads) species.

Experimental and Theoretical Investigations of the Formation of the Diazene PhSN=C(H)N=NC(H)=NSPh from HCN(2)(SPh)(3) by a Thiyl-Radical-Catalyzed Mechanism: Identification of the HC(NSPh)(2)(*) Radical and X-ray Structures of HCN(2)(SPh)(3) and PhSN=C(H)N=NC(H)=NSPh.

The reaction of HCN(2)(SiMe(3))(3) with benzenesulfenyl chloride in a 1:3 molar ratio produces HCN(2)(SPh)(3) (4) as thermally unstable, colorless crystals. The decomposition of (4) in toluene at 95 degrees C was monitored by UV-visible, (1)H NMR and ESR spectroscopy. The major final products of the decomposition were identified as PhSN=C(H)N=NC(H)=NSPh (5) and PhSSPh. The structures of 4 and 5 were determined by X-ray crystallography. The crystals of 4 are monoclinic, space group P2(1)/a, with a = 9.874(2) Å, b = 19.133(2) Å, c = 10.280(2) Å, beta = 113.37(1) degrees, V = 1782.8(5) Å(3), and Z = 4. The final R and R(w) values were 0.042 and 0.049, respectively. The crystals of 5 are monoclinic, space group P2(1)/n, with a = 5.897(6) Å, b = 18.458(10) Å, c = 7.050(8) Å, beta = 110.97(5) degrees, V = 716(1) Å(3), and Z = 2. The final R and R(w) values were 0.075 and 0.085, respectively. The diazene 5 adopts a Z,E,Z structure with weak intramolecular S.N contacts of 2.83 Å, giving rise to four-membered NCNS rings. During the thermolysis of 4 at 95 degrees C in toluene a transient species (lambda(max) 820 nm) was detected. It decomposes with second-order kinetics to give 5 (lambda(max) 450 nm). The ESR spectrum of the reaction mixture consisted of the superposition of a three-line 1:1:1 spectrum (g = 2.0074, A(N) = 11.45 G), attributed to (PhS)(2)N(*), upon a doublet of quintets (1:2:3:2:1) with g = 2.0070, A(N) = 6.14 G, A(H) = 2.1 G assigned to the radical HCN(2)(SPh)(2)(*). Density functional theory (DFT) calculations for the models of the radical showed the E,Z isomer to have the lowest energy. Thermochemical calculations indicate that the decomposition of HCN(2)(SH)(3) into the diazene (Z,E,Z)-HSN=C(H)N=NC(H)=NSH (and 2 HSSH) is substantially more exothermic (DeltaH = -176.1 kJ mol(-)(1)) than the corresponding formation of the isomeric eight-membered ring (HC)(2)N(4)(SH)(2) (DeltaH = -40.6 kJ mol(-)(1)). These calculations also indicate that the diazene is formed by a mechanism in which the RS(*) radical acts as a catalyst.