Minhua Zhang

DFT insights into the adsorption of NH3-SCR related small gases in Mn-MOF-74

Mn-MOF-74 has great potential to catalyze selective catalytic reduction (SCR) reaction with NH3 being the reductant (NH3-SCR). However, the reaction mechanism, in particular the adsorptive properties of key reactive species in Mn-MOF-74, remains ambiguous. Besides, the effects of impurities such as H2O and SO2 on the process need further investigation. In this paper, based on density functional theory (DFT) calculations, we studied the adsorption characteristics of six NH3-SCR related small gases, namely NH3, NO2, NO, O2, H2O and SO2. DFT results show that the Mn-MOF-74 structure can bind these molecules relatively strongly in the following order: NH3 > NO2 > NO > O2, allowing for subsequent NH3-SCR reaction. In addition, a possible pathway of NO conversion to NO2 was calculated. Investigation on competitive adsorption of NH3 and H2O, NH3 and SO2 reveals that both H2O and SO2 are probable to replace NH3 under certain conditions, indicating that the two impurity gases may affect the activity of the NH3-SCR reaction. Compared with H2O, SO2 can displace NH3 more easily and should not be neglected.

Catalytic activity of transition metal doped Cu(111) surfaces for ethanol synthesis from acetic acid hydrogenation: a DFT study

Transition metal (Co, Ni, Ru, Rh, Pd and Pt) doped Cu(111) models are selected to examine the effects of transition metals on Cu surface for ethanol synthesis from acetic acid hydrogenation using density functional theory (DFT) calculations. On these surfaces, the adsorption of the main intermediates and reaction barriers of key elementary steps are investigated. The calculation results indicate that oxophilic metals are projected to be more active in acetic acid adsorption and acetaldehyde adsorption compared to less-oxophilic metals. Those metals with larger C adsorption energies generally have better C–OH bond cracking activity. Additionally, a good linear Bronsted–Evans–Polanyi (BEP) correlation is established for predicting the preferences of C–OH bond scission of acetic acid on other metals. Finally, O–H bond formation in C2-oxygenates (CH3CO, CH3CHO, CH3CH2O) hydrogenation is examined on all these surfaces. The reactions are more likely to occur on less-oxophilic metal-doped Cu surfaces. Therefore, it appears to involve an intricate balance between C–OH cracking and O–H bond formation reactions. That means those metal-doped Cu-based catalysts that are capable of preferentially activating C–OH bond without considerably inhibiting O–H bond formation of C2-oxygenates are predicted to achieve optimum catalytic activity for ethanol synthesis from acetic acid hydrogenation. The results can provide theoretical guidance for related experiments as well as the designing of Cu-based catalysts for ethanol synthesis.

DFT study of In2O3-catalyzed methanol synthesis from CO2 and CO hydrogenation on the defective site

Research on the mechanism of methanol synthesis from CO2 hydrogenation on the defective surface of In2O3 catalyst plays a pivotal role in the promotion of its catalytic performance and the catalytic conversion of CO2. Methanol synthesis from the hydrogenation of CO2 and CO on the vacancy site, consisting of Ov1 and Ov2, of the defective In2O3(110) surface (D surface) has been studied using the density functional theory method in the present work. The calculated results indicate that the HCOO route and RWGS route both are possible reaction pathways for methanol synthesis on the D surface. In the HCOO route, the reaction of p-HCOO with the surface H atom to form H2COO species is the rate-determining step, with an activation barrier of 1.25 eV. In the RWGS route, the dissociation of CO2 to CO on the D surface with a barrier of 0.99 eV is the rate-determining step for methanol synthesis. The hydrogenation of CO and HCO species on the D surface both are kinetically and energetically favorable.

Kinetic studies of the strengthening effect on liquid hot water pretreatments by organic acids

The liquid hot water (LHW) pretreatments would be accelerated by the organic acids produced from the process. In the study, the organic acids included not only acetic acid but also lactic acid during LHW hydrolysis of reeds, at 180-220°C and for 15-135min. The lactic acid was presumably produced from xylose degradation in the pretreatment process. The different organic acids, such as acetic acid, lactic acid and acetic-lactic acids, were used to strengthen the LHW pretreatments for increasing xylose production. Moreover, the work presented kinetic models of xylose and hemicellulose at different conditions, considering the generation of lactic acid. The experimental and kinetic results both indicated that acetic-lactic acids had synergistic catalytic effect on the reaction, which could not only inhibit the degradation of xylose, but also promote the hydrolysis of hemicellulose. Besides, the highest concentration of xylose of 7.323g/L was obtained at 200°C, for 45min and with 1wt% acetic-lactic acids.

Water Adsorption and Decomposition on Co(0001) Surface: A Computational Study

Water adsorption and decomposition on the Co(0001) surface has been systematically studied by spin-polarized density functional theory calculations and atomic thermodynamics. H2O adsorption mechanism has been analyzed by partial density of states. The possible structure of adsorbed H2O molecules comprised of monomer-hexamer have been investigated and the phase diagram shows that only two configurations are stable thermodynamically: clean Co(0001) surface and H2O hexamer adsorption. The competition between the ability of a H2O molecule to bond with the substrate and its ability to act as a H-bond acceptor leads to the symmetry-breaking bond alteration in the hexamer structure. In addition, the interaction among adsorbed H2O molecules can help stabilize adsorption configurations by forming H-bonds. Presence of O species has a great influence on the decomposition of water and can significantly lower the activation barrier of H–OH bond cleavage.Graphical Abstract

Investigation on the conversion of ethylene to ethylidyne on Pt(100) and Pd(100) using density functional theory

The comprehensive formation network of ethylidyne (CH3C) from ethylene (CH2CH2) is investigated on Pt(100) and Pd(100) using the density functional theory method. The structural and energetic features of all intermediate products were considered. We found that the trend of the activation barriers in each pathway on Pt(100) and Pd(100) are the same, whereas the barriers on Pt(100) are higher than that on Pd(100). The activation barriers of 1,2-H shift reactions are relatively high compared with the other reactions. We screened three possible pathways and selected the optimal route as CH2CH2(ethylene) → CH2CH(vinyl) → CH2C(vinylidene) → CH3C(ethylidyne).

Comparison of the coupling of ethylene with acetate species and ethylene dehydrogenation on Pd–Au(100): a density functional study

In this work, two key reactions in vinyl acetate monomer (VAM) synthesis, i.e., the coupling of ethylene with acetate species and ethylene dehydrogenation, on three different Pd–Au(100) surface configurations – the second nearest neighbors (denoted as PdsnAu), the first nearest neighbors (denoted as PdfnAu), and the fourth nearest-neighbor palladium island (denoted as PdislAu) – were studied. The energy barriers of the transition state of ethylene dehydrogenation to vinyl and the coupling of ethylene with acetate species on three different Pd–Au (100) surfaces were calculated. The influence of the surface properties of Pd–Au(100) on the reaction performance was analyzed and discussed at the microscopic level. The results reveal that on PdsnAu and PdfnAu surfaces where the coverage of the surface Pd atoms is relatively low, it is more likely for the coupling of ethylene with acetate species to occur, while on the PdislAu surface where the coverage of the surface Pd atoms is relatively high, it is more likely for ethylene dehydrogenation to happen. This work will improve the comprehension of the catalytic mechanism of Pd–Au(100) at the molecular and electronic levels and provide theoretical guidance for further application and development of efficient commercial catalysts, and the control of the reaction.

The deactivation of a ZnO doped ZrO2–SiO2 catalyst in the conversion of ethanol/acetaldehyde to 1,3-butadiene

A deactivation study on the ethanol/acetaldehyde conversion to 1,3-butadiene over a ZnO promoted ZrO2–SiO2 catalyst prepared by a sol–gel method was performed. The samples were characterized by N2 adsorption–desorption isotherms, scanning electron microscopy (SEM), NH3 temperature programmed desorption (NH3-TPD), X-ray powder diffraction characterization (XRD), thermogravimetric analyses (TGA), Fourier transform infrared resonance (FT-IR), 13C magic-angle spinning nuclear magnetic resonance (13C NMR) and X-ray photoelectron spectroscopy (XPS). The pore structure characteristics and surface acidity of Zn0.5–Zr–Si catalysts were largely decreased with time-on-stream and no crystal structure was formed in the used catalyst, indicating that the deactivation was caused by carbon deposition. Two main types of carbon deposition were formed, namely low-temperature carbon deposition with the oxidation temperature of around 400 °C and high-temperature carbon deposition with the oxidation temperature of 526 °C. The carbon species were mainly composed of graphitized carbon, amorphous carbon, carbon in C–O bonds and carbonyls. The deactivated catalyst could be regenerated by a simple oxidation process in air. Adding a certain amount of water into the feed had a positive effect on reducing the carbon deposition.

Conversion of Ethanol and Acetaldehyde to 1, 3-Butadiene Catalyzed by Zr–Si Materials

Series of ZrO2 supported on the different kinds of SiO2 carriers were applied in the reaction of 1, 3-butadiene formation from ethanol and acetaldehyde. ZrO2/Nano-SiO2 performed the best performance with the 1, 3-butadiene selectivity 91.43% and the total conversation 52.39%. Prepared catalysts were characterized by N2 adsorption–desorption, TEM, XRD, temperature-programmed desorption of NH3 and CO2, FTIR spectroscopy of adsorbed pyridine and CO2, Raman XPS. The results show that the weak Lewis acid and basic sites are appropriate for BD formation. Furthermore, more and balance acid-basic sites are rather important for the reaction of ethanol and acetaldehyde conversion to BD.

Selective Catalytic Reduction of NO x with NH 3 on Cu-BTC-derived Catalysts: Influence of Modulation and Thermal Treatment

In this work, copper-based metal organic frameworks Cu3(BTC)2 (BTC = 1,3,5-benzenetricarboxylatle), were applied in the conversion of toxic oxynitride into nitrogen at low temperature. Scanning electron microscope (SEM), thermal gravimetric analysis (TGA), X-ray photoelectron spectroscopy (XRD) and other characterization methods were employed to fully understand the properties of the catalysts. We introduced acetic acid into the synthesis process as the modulator of the crystal structure and morphology. The catalytic assessment indicated that compared with the prototype, modified Cu-MOFs materials obtain enhanced catalytic activity for the SCR reaction. Besides, several thermolysis experiments were conducted to explain structure–function relationship.