Temperature-programmed Desorption Experiments Research Articles

Photothermal alternative to device fabrication using atomic precision advanced manufacturing techniques

The attachment of dopant precursor molecules to depassivated areas of hydrogen-terminated silicon templated with a scanning tunneling microscope (STM) has been used to create electronic devices with subnanometer precision, typically for quantum physics experiments. This process, which we call atomic precision advanced manufacturing (APAM), dopes silicon beyond the solid-solubility limit and produces electrical and optical characteristics that may also be useful for microelectronic and plasmonic applications. However, scanned probe lithography lacks the throughput required to develop more sophisticated applications. Here, we demonstrate and characterize an APAM device workflow where scanned probe lithography of the atomic layer resist has been replaced by photolithography. An ultraviolet laser is shown to locally and controllably heat silicon above the temperature required for hydrogen depassivation on a nanosecond timescale, a process resistant to under- and overexposure. STM images indicate a narrow range of energy density where the surface is both depassivated and undamaged. Modeling that accounts for photothermal heating and the subsequent hydrogen desorption kinetics suggests that the silicon surface temperatures reached in our patterning process exceed those required for hydrogen removal in temperature-programmed desorption experiments. A phosphorus-doped van der Pauw structure made by sequentially photodepassivating a predefined area and then exposing it to phosphine is found to have a similar mobility and higher carrier density compared with devices patterned by STM. Lastly, it is also demonstrated that photodepassivation and precursor exposure steps may be performed concomitantly, a potential route to enabling APAM outside of ultrahigh vacuum.

Hybridized Fe/Ru-SiMWCNT-ionic liquid nanofluid for CO2 conversion into carbamate using superoxide ion

Suppressing the nucleophilic susceptibility of ionic liquids based nanofluid is necessary for energy storage and superoxide ion (O2•−) utilization. This study reports the development of novel pseudocapacitive Fe/Ru-SiMWCNT nanofluid comprising of Fe3O4, RuO2, SiO2, and MWCNT hybridized phases. The Fe/Ru-SiMWCNT nanohybrid possessed CO2 and O2 sorption capability, as confirmed from the temperature-programmed desorption experiments. Detailed spectroscopy techniques characterize the Fe/Ru-SiMWCNT nanohybrid component's physicochemical and morphological properties. The novel ionic liquid (IL) based nanofluid index is Fe/Ru-SiMWCNT/1-(2-methoxyethyl)-1-methylpyrrolidinium tris(pentafluoroethyl) trifluorophosphate. Therefore, stable O2•− was generated therein at −0.445 V vs Ag/AgCl and recorded long-term stability for 2 days with 87.31 % efficiency. Moreover, the O2•− mediated CO2 conversion to C2O62− at −0.54 V vs Ag/AgCl with 97.90 % energy efficiency. Also, the normalized exchange current density in the nanofluid was 2.20 mA/cm2, which is higher than 1.94 mA/cm2 observed in the IL counterpart. The high normalized exchange current density is due to Fe/Ru-SiMWCNT nanohybrid phase's pseudocapacitance. Accordingly, this pseudocapacitive capability enables converting O2 and CO2 in the nanofluid with lower activation overpotential of −0.305 and −0.460 V vs Ag/AgCl, respectively. In contrast, the conversion of O2 and CO2 in the IL required higher activation overpotential of −0.826 and −1.013 V vs Ag/AgCl, respectively. The electrolysis of O2/CO2 in the nanofluid containing diethanolamine at −1.564 V vs Ag/AgCl, 60 °C and 1.0 h produced methyl 2-hydroxyethyl (methyl) carbamate as the primary product. The heteronuclear multiple bond correlation spectroscopy analysis finally elucidated the carbamate structure by two strong correlations between the protons and carbons in the vicinity of three and four bonds apart. Therefore, this study highlights the control design of electrochemically stable IL-based nanofluids robust for reactive oxygen species, energy storage and conversion.

Microkinetic Modeling of the CO2 Desorption from Supported Multifaceted Ni Catalysts

The conversion of CO2 into valuable products with hydrogenation reactions on Ni catalysts requires a fundamental understanding of the interaction of CO2 with the Ni surface. Microkinetic modeling is used to investigate the interaction of CO2 with a multifaceted Ni crystal and compared to temperature-programmed desorption (TPD) experiments from supported Ni catalysts. TPD experiments were performed for Ni/γ-Al2O3 and Ni/SiO2 catalysts. Uncertainties in the DFT-derived model parameters were accounted for by a global uncertainty analysis (GUA). On the basis of the model, the adsorption and desorption of CO2 exhibit a structure–sensitivity for the investigated Ni facets. Whereas the initial multifaceted TPD model is not in quantitative agreement with the experimentally recorded desorption profile, the GUA reveals a feasible set of model parameters that are in close agreement with the data. The comparison of the desorption profiles of the different catalysts and the multifacet simulation shows that desorption of CO2 from basic sites and desorption from the Ni facets overlap and thus have similar desorption kinetics.

Impact of defects on the decomposition of chemical warfare agent simulants in Zr‐based metal organic frameworks

AbstractMetal organic frameworks (MOFs) containing zirconium secondary building units (SBUs) in UiO‐67 and related MOFs, are highly active for neutralizing both the chemical warfare agents and simulants, such as dimethyl methylphosphonate (DMMP). However, two recent publications gave conflicting reports of DMMP reaction with UiO‐67 under ultra high vacuum (UHV) conditions, with one reporting chemisorption and reaction (Wang et al., J Phys Chem C, 2017, 121, 11261–11272) and the other reporting only physisorption and reversible desorption (Ruffley et al., J Phys Chem C, 2019, 123, 19748–19758) from very similar temperature programmed desorption experiments. We show that the discrepancy between these experiments may be explained by different levels of missing linker defects in the UiO‐67 samples. We present density functional theory calculations showing that SBU sites having two‐adjacent missing linkers exhibit reaction barriers that are about 30 kJ/mol lower than SBU sites having a single missing linker. We also show that topology of the undercoordinated sites plays an important role in the reaction barrier under UHV conditions.

How to extract adsorption energies, adsorbate-adsorbate interaction parameters and saturation coverages from temperature programmed desorption experiments.

We present a scheme to extract the adsorption energy, adsorbate interaction parameter and the saturation coverage from temperature programmed desorption (TPD) experiments. We propose that the coverage dependent adsorption energy can be fit using a functional form including the configurational entropy and linear adsorbate-adsorbate interaction terms. As one example of this scheme, we analyze TPD of CO desorption on Au(211) and Au(310) surfaces. We determine that under atmospheric CO pressure, the steps of both facets adsorb between 0.4-0.9 ML coverage of CO*. We compare this result against energies obtained from five density functionals, RPBE, PBE, PBE-D3, RPBE-D3 and BEEF-vdW. We find that the energies and equilibrium coverages from RPBE-D3 and PBE are closest to the values determined from the TPD.

Integration of an Oxidation Catalyst with Pd/Zeolite-Based Passive NOx Adsorbers: Impacts on Degradation Resistance and Desorption Characteristics

Fully ion-exchanged Pd/SSZ-13 model passive NOx adsorbers (PNA) exhibit significant NOx storage capability, achieving NOx-to-Pd ratios of 1, and NOx desorption at higher temperatures, where downstream NOx reduction catalysts are active, under simulated exhaust conditions. However, CO has been found to induce PNA degradation, which limits the potential for practical application. In this study, in an attempt to understand the consequences of limiting CO exposure, we integrated a model Pt/Al2O3 diesel oxidation catalyst (DOC) with a Pd/SSZ-13 PNA and performed NOx adsorption and temperature-programmed desorption (TPD) cycles with the PNA, the DOC, and the DOC+PNA integrated system. Despite the high initial NOx-to-Pd ratio, Pd/SSZ-13 experienced significant degradation over 15 adsorption and desorption cycles when including CO, with the NOx-to-Pd ratio dropping from 0.98 to 0.75. CO oxidation over the DOC+PNA integrated system lights off at significantly lower temperature compared to the PNA, limiting the PNA CO exposure at temperatures above 200 °C. As a result, the durability of the DOC+PNA integrated system is enhanced, and only a 0.02 decrease in NOx-to-Pd ratio was observed over the 15 test cycles. A further benefit with integration of the PNA with the DOC was a lower temperature NOx release, within a more practical temperature window. This is due to the enhanced oxidation activity of the DOC+PNA integrated system and consequently an early onset of NO2 formation, which was found to trigger the low temperature NOx release. Low temperature NOx adsorption and TPD experiments with controlled exposure of CO and NO2 reveal two types of NOx storage mechanisms, one of which is destabilized by the presence of NO2, leading to the evolution of a lower temperature NOx release. Overall, integrating the PNA with a highly active low temperature CO oxidation catalyst was beneficial by lowering the NOx release temperature window and leading to significantly less NOx capacity loss.

Plasmonic Temperature-Programmed Desorption.

Temperature-programmed desorption (TPD) allows for the determination of the bonding strength and coverage of molecular mono- or multilayers on a surface and is widely used in surface science. In its traditional form using a mass spectrometric readout, this information is derived indirectly by analysis of resulting desorption peaks. This is problematic because the mass spectrometer signal not only originates from the sample surface but also potentially from other surfaces in the measurement chamber. As a complementary alternative, we introduce plasmonic TPD, which directly measures the surface coverage of molecular species adsorbed on metal nanoparticles at ultrahigh vacuum conditions. Using the examples of methanol and benzene on Au nanoparticle surfaces, the method can resolve all relevant features in the submonolayer and multilayer regimes. Furthermore, it enables the study of two types of nanoparticles simultaneously, which is challenging in a traditional TPD experiment, as we demonstrate specifically for Au and Ag.

London Dispersion Corrections to Density Functional Theory for Transition Metals Based on Fitting to Experimental Temperature-Programmed Desorption of Benzene Monolayers.

Standard implementations of generalized gradient approximation (GGA)-based density functional theory (DFT) describe well strongly bound molecules and solids but fail to describe long-range London dispersion or van der Waals (vdW) attraction interactions that are important in molecular crystals and two-dimensional solids. To provide accurate values for the vdW distance and energies for the metals Cu, Ag, Au, Ni, Pd, and Pt, we determined empirical vdW corrections to Perdew, Burke, and Ernzerhof (PBE) DFT by fitting the experimental adsorption enthalpies measured by temperature-programmed desorption (TPD) from benzene monolayers by Campbell and co-workers ( J. Phys. Chem. C 2016, 120, 25161-25172). Benzene physisorbed to these metals without chemical reaction; therefore, we consider the bonding to be vdW. We use the low gradient form for the vdW corrections, EvdW-LG = -C6LG/[R6 + RvdwLG6] with just two parameters per atom (C6LG and RvdwLG). This LG form leads to negligible changes in bond distances and angles, so adjusting the parameters should not sacrifice accuracy for the bonding interactions. We demonstrate that the parameters fitted to benzene also describe well the physisorption enthalpies for other hydrocarbons (naphthalene, cyclohexane, methane, ethane, and propane) on Pt. We also report low gradient vdW correction parameters for the noble gases that fit the equilibrium lattice parameter and heat of vaporization of the crystals.

Influence of alkaline modification on different adsorption behavior between ZSM-5 and LSX zeolite for toluene

Abstract Adsorption as an air purification method is widely employed to remove volatile organic compounds with low concentration using zeolites. In this study, ZSM-5 and LSX zeolites modified by alkali metal cations (Li+, Na+ and K+) were used as adsorbents for the abatement of toluene. The adsorption behaviors of toluene onto adsorbents with different pore sizes and cation contents were evaluated systematically based on the experiments of adsorption equilibrium, kinetic diffusion and desorption. Besides, Langmuir and Freundlich model, constant-pattern wave propagation model and Kissinger method were applied to fit with the above experimental data. The adsorption amount followed the sequence of Li-LSX > Na-LSX > K-LSX > Li-ZSM-5 > Na-ZSM-5 > K-ZSM-5 obtained from equilibrium isotherms. LSX zeolites showed twice adsorption capacity on unit volume compared to ZSM-5. The mechanisms of steric hindrances and electrostatic interactions controlling the diffusion rate under high and low concentrations were proposed, respectively. At relatively low loadings of toluene, the diffusion resistances were primarily affected by electrostatic interaction due to preferential adsorption of toluene on cationic sites. While at relatively high loadings, the diffusion resistance was mainly restricted by steric effect because of saturated cationic sites. The strength of adsorbate-adsorbent interactions was assessed by temperature programmed desorption experiments. It showed that both adsorption sites and microporous walls contributed to the desorption energy.

Hg0 removal by palygorskite (PG) supported MnOx catalyst

The MnOx/PG catalysts were prepared by supporting Mn oxides to palygorskite (PG) and used to remove Hg0 in simulated flue gas, which were analyzed and characterized by the specific surface area analysis (BET), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The results show that the co-effect of MnOx and PG significantly enhances the Hg0 removal performance; the MnOx/PG catalyst with 8% MnOx loading has the highest Hg0 removal activity. The Hg0 removal efficiency can maintain above 95% at 210°C with a space velocity of 6000 h−1 and an inlet Hg0 concentration of 80 μg/m3 for 400 min. O2 can promote the Hg0 removal by MnOx/PG catalysts, while SO2 and H2O have inhibitory effects. In the presence of O2, the inhibitory effect of SO2 can be obviously weakened. The results of XRD, XPS and temperature programmed desorption experiments (TPD) indicate that the active components MnOx disperse well on the surface of PG. The removal process of Hg0 on the MnOx/PG catalyst includes the steps of adsorption, oxidation and reaction, and HgO and HgSO4 are generated and adsorbed on the catalyst.

Experiment and regeneration kinetic model study on CO2 adsorbent prepared from fly ash

• Loaded potassium-based CO 2 adsorbent was prepared from fly ash. • Temperature programmed desorption of adsorbent was performed. • Experiment and model fitting were combined to study desorption regeneration. • Avrami fractional kinetic model was used to simulate regeneration kinetics. • Diffusion models were used to analysis desorption mechanism. Using power plant fly ash to prepare CO 2 adsorbent can not only realize low-carbon power in power plants, but also effectively use solid waste to achieve the objective of treating the waste with waste. Regeneration characteristic is an important performance of adsorbent. Through the temperature programmed desorption experiment, the influence of different regeneration conditions on performance and regeneration kinetics can be studied, and the optimal calculation of regeneration conditions can be carried out, so as to realize the recycling of adsorbent with economic benefits. The optimal regeneration temperature and heating rate were 150 °C and 10 °C/min, respectively. Under these conditions, the desorption capacity was 1.7454 mmol/g, and the desorption efficiency was 86.38%. Avrami fraction kinetic model was used to simulate and calculate the regeneration kinetics of adsorbent. The results showed that the regeneration temperature affected the regeneration mechanism. High regeneration temperature and high heating rate could promote the desorption kinetic rate to increase. When the regeneration temperature was 90 °C and 190 °C, k a was 0.000885 min −1 and 0.001318 min −1 respectively. Three kinds of diffusion were used to study the regeneration mechanism. It was concluded that after the regeneration temperature was higher than the chemical desorption temperature, the desorption regeneration could be divided into early, middle and final stages, and the rate controlling steps were respectively: intraparticle diffusion control, joint control of surface chemical reaction and intraparticle diffusion, and membrane diffusion control. Physical desorption occured when the regeneration temperature was lower than 110 °C. The desorption time was longer and CO 2 concentration released was always low.

Adsorption and Reaction of Trimethyl and Triethyl Phosphite on Fe3O4 by Density Functional Theory

First-principle density functional theory (DFT) calculations are used to calculate the most stable structures and heats of adsorption of trimethyl and triethyl phosphites on an Fe3O4 surface in order to understand their behavior as lubricant additives. Previous surface science studies of phosphate and phosphite esters on iron oxide surface have shown that they react by sequentially desorbing the corresponding alcohol and aldehyde in equimolar amounts. This implies that the reactions are limited by the rate of alkoxide removal, followed by a rapid reaction to form the alcohol and aldehyde. It is found that the trialkyl phosphites, dialkyl phosphite, and monoalkyl phosphite all adsorb with the phosphorus atoms bound to surface Fe3+ ions, with the alkoxy groups close to parallel to the surface. The heat of adsorption increases as the alkoxide groups are removed. The postulate that the rate-limiting step in the decomposition of the phosphate esters is the sequential removal of the alkoxide group is tested by plotting the experimental activation energy obtained from temperature-programed desorption experiments versus the corresponding heats of adsorption calculated by DFT. A linear dependence shows that the reaction obeys the so-called Evans–Polanyi relation, thereby confirming the above postulate. The slope of the plot is close to unity and thus implies that the structure of the transition state of the reaction resembles the product.

Plasma-Enhanced Atomic Layer Deposition of Cobalt and Cobalt Nitride: What Controls the Incorporation of Nitrogen?

The present study covers the processes that govern the incorporation of nitrogen in the film during atomic layer deposition (ALD) of cobalt and cobalt nitride prepared from cobaltocene (CoCp2) and NH3 plasma. It is demonstrated that nitrogen incorporation is strongly temperature-dependent; at temperatures of 260 °C and below, the deposited films consist primarily of Co2N, whereas increasing the temperature to 300 °C leads to a mixture of Co3N and Co, and at 350 °C, nominally pure Co is obtained. The sample temperature clearly has a very strong effect on the composition of the deposited film, and in order to understand this temperature dependence, the thermal stability of the CoNx species formed during the interaction between a sample consisting of pure Co and an NH3 plasma is analyzed. X-ray photoemission spectroscopy depth profiling reveals that this plasma treatment converts the top 4 nm of the Co film into CoNx. Temperature-programmed desorption experiments show that this nitride layer starts to decompose at a temperature of approximately 290 °C, which eventually leads to complete removal of all nitrogen from the film. Based on this, a reaction scheme for ALD of Co is proposed; the interaction between the NH3 plasma and the adsorbed CoCp2-derived species leads to the formation of cobalt nitride. At temperatures below 290 °C, the resulting film primarily consists of Co2N, whereas at 300 °C, partial decomposition takes place, resulting in the formation of a mixture of Co3N and pure Co. At 350 °C, decomposition is effectively complete, leading to pure Co with nitrogen contents below 4 at %. Finally, it is argued that the aforementioned mechanism could potentially play a role in plasma-enhanced ALD of other elements that form metastable nitrides such as Ni, Cu, Re, and Ru.

Natural ferruginous manganese ore for efficient immobilization of elemental mercury from coal combustion flue gas

Mercury emission from industrial activities poses a serious threat to the ecosystem. The development of cost-effective sorbents for mercury removal is a feasible method to alleviate this problem. In this study, natural ferruginous manganese ore (NFM) was adopted as a cost-effective sorbent for elemental mercury (Hg0) removal from flue gas. Samples characterization indicated that the pore stricture of NFM was deteriorated after calcination procedure. The secondary recrystallization of manganese oxides and decomposition of MnO2 into Mn2O3 also happened during calcination process. Raw NFM exhibited high Hg0 removal efficiency (ET > 80%) over a broad temperature range (100 °C–200 °C), whereas calcination procedure had negative effects on Hg0 removal performance. The effects of calcination temperature (450 °C–650 °C) and reaction temperature (100 °C–400 °C) on Hg0 removal performance were also explored. Under the simulated flue gas condition, both equilibrium mercury adsorption capacity and adsorption rate were superior to those of a commercial activated carbon specifically used for mercury emission reduction. The Hg0 removal mechanism was revealed by combining with Hg0 adsorption/oxidization behavior and XPS analysis. It was demonstrated that chemisorption dominated Hg0 removal process, where MnO2 could serve as active components for Hg0 capture. The results were affirmed by the mercury temperature programmed desorption experiments.

Catalytic pyrolysis of poplar sawdust: Excellent hydrocarbon selectivity and activity of hollow zeolites

Hollow zeolites were investigated for catalytic fast pyrolysis (CFP) of biomass to produce hydrocarbon-rich bio-oil. A series of hollow ZSM-5 catalysts were synthesized via a dissolution-recrystallization strategy. The physicochemical properties of the catalysts were investigated by high-resolution transmission electron microscopy, N2 sorption, X-ray photoelectron spectroscopy, and ammonia temperature-programmed desorption experiments. The hollow zeolite was effective for increasing the hydrocarbon fraction in bio-oil. In particular, hollow HS-ZSM-5(50) afforded the highest hydrocarbon yield (6.8 wt%), which was ~3 times of that achieved with solid ZSM-5(50). The hollowness, acidity, and the presence of secondary wall mesopores in the hollow zeolite were found to affect bio-oil production. The hollow regions stabilized more active biomass intermediates and inhibited their repolymerization to coke, while the interior acid sites continually converted these intermediates to aromatic hydrocarbons. Secondary wall mesopores compromise the hollow space and hinder consecutive catalysis, resulting in phenols as the main product.

Influence of Oxygen on Hg0 Adsorption on Non-Impregnated Activated Carbons

Both physisorptive and chemisorptive mechanisms play a role in the adsorption of mercury. The present publication investigates the influence of oxygen on the adsorption of Hg0 by breakthrough curve measurements and temperature-programmed desorption (TPD) experiments. The presence of O2 in the gas phase promotes chemisorption. Because of slow adsorption mechanisms, no equilibrium capacities of mercury chemisorption can be determined. For further investigations, coupled adsorption and desorption experiments with concentration swing adsorption and TPD experiments are performed. The results of TPD experiments are simulated and quantitatively evaluated by means of an extended transport model. From the number of desorption peaks, we obtain the number of different adsorption and desorption mechanisms. A detailed simulation of the peaks yields the reaction order, the frequency factor, and the activation energy of the desorption steps. The kinetic reaction parameters allow a mechanistic interpretation of the adsorption and desorption processes. Here, we suppose the formation of a complex between the carbon surface, mercury, and oxygen.

Morphology of Vapor-Deposited Acetonitrile Films

Crystalline acetonitrile has two polymorphs, a high-temperature (HT) phase that is stable between 217 K and its melting point at 229 K and a low-temperature (LT) phase that is stable below 217 K. Solid acetonitrile films can be prepared by vapor deposition in an ultrahigh vacuum chamber. To prevent sublimation of the film, temperatures are often kept below 150 K. While the LT phase is thermodynamically favored at these low temperatures, such preparation usually results in the formation of the metastable HT polymorph. In this work we use reflection adsorption infrared spectroscopy (RAIRS) and temperature-programmed desorption (TPD) experiments to investigate the effects of the deposition temperature and underlying substrate on the morphology of acetonitrile films prepared with molecular beam deposition. We obtained the elusive LT phase when dosing at 120 K on a graphene substrate and on a crystalline decane layer. Dosing acetonitrile on other surfaces produced the HT phase, as did annealing of amorphous films. We used TPD experiments to determine the Gibbs energy difference between the HT and the LT phases. Our ΔG values agree with extrapolation of equilibrium calorimetry data. We also observed that acetonitrile films were amorphous when dosed at temperatures ≤ 60 K and porous for temperatures ≤ 50 K.

Effect of support acidity during selective hydrogenolysis of glycerol over supported palladium-ruthenium catalysts.

We report the role of the acidity of support during the selectivity hydrogenolysis of glycerol over supported bimetallic palladium-ruthenium (PdRu) catalysts. The PdRu nanoparticles were supported on a series of metal oxides and zeolitic supports via the modified impregnation method and tested for the liquid-phase hydrogenolysis of glycerol using gaseous hydrogen. The relative acid site densities of selected catalysts were determined by ammonia temperature-programmed desorption and pyridine desorption experiments. Based on these studies, we report a direct correlation between the catalytic activity (conversion and 1,2 propane diol yield) and two different acid sites (strong acid sites and very strong acid sites). Besides zeolite-supported catalysts, TiO2 supported PdRu nanoparticles exhibit moderate catalytic activity; however, this catalyst shows high selectivity for the desired C-O bond cleavage to produce C3 products over the undesired C-C bond cleavage to produce < C3 products. This article is part of a discussion meeting issue 'Science to enable the circular economy'.

Study on sulfur migration in activated carbon adsorption-desorption cycle: Effect of alkali/alkaline earth metals

Activated carbon (AC) has been widely used in the removal of SO2 from flue gas owing to its well-developed pore structure and abundant functional groups. Herein, the effect of alkali/alkaline earth metals on sulfur migration was investigated based on the dynamic adsorption and temperature programmed desorption experiment. The adsorption and desorption properties of six types of AC (three commercial and three laboratory-made) were carried out on a fixed-bed experimental device, and the physical and chemical properties of samples were determined by X-ray fluorescence, X-ray diffraction, scanning electron microscopy/energy dispersive X-ray, and X-ray photoelectron spectroscopy analysis. The experimental results showed that the adsorbed SO2 cannot be completely desorbed by increasing the regeneration temperature (350 – 850°C), while the SO2 fixed in the AC combines with the Ca-based minerals in the ash to form a stable sulfate. For different samples, higher ash content, higher CaO content in the ash and a more developed pore structure lead to a higher SO2 fixation rate. Moreover, the multiple adsorption-desorption cycles experiment showed that the effect of SO2 fixation is mainly reflected in the first cycle, after which the adsorption and desorption amount are approximately the same. This study elucidates the effect of alkali/alkaline earth metals on the adsorption-desorption cycle of AC, which provides a deeper understanding of sulfur migration in the AC flue gas desulfurization process.

Understanding Surface Basic Sites of Catalysts: Kinetics and Mechanism of Dehydrochlorination of 1,2-Dichloroethane over N-Doped Carbon Catalysts

The production of vinyl chloride (VCM) by pyrolysis of 1,2-dichloroethane (DCE) is an important process in the ethylene-based poly(vinyl chloride) industry. The pyrolysis is performed at temperatures above 500 °C, gives low conversions, and has high energy consumption. We have shown that N-doped carbon catalysts give excellent performances in DCE dehydrochlorination at 280 °C. The current understanding of the active sites, mechanism, and kinetics of DCE dehydrochlorination over N-doped carbon catalysts is limited. Here, we showed that pyridinic-N on a N-doped carbon catalyst is the active site for catalytic production of vinyl chloride monomer from DCE. The results of CO2 and DCE temperature-programmed desorption experiments showed that the pyridinic-N catalytic sites are basic, and the mechanism of dehydrochlorination on a N-doped carbon catalyst involves a carbanion. A kinetic study of dehydrochlorination showed that the surface reaction rate on the N-doped carbon catalyst was the limiting step in the catalytic dehydrochlorination of DCE. This result enabled clarification of the dehydrochlorination mechanism and optimization of the reaction process. These findings will stimulate further studies to increase our understanding of the relationship between the base strength and catalytic performance. The results of this study provide a method for catalyst optimization, namely modification of the amount of pyridinic-N and the base strength of the catalyst, to increase the surface reaction rate of DCE dehydrochlorination on N-doped carbon catalysts.