Mechanism of methanol decomposition on the Pd/WC(0001) surface unveiled by first-principles calculations

In this study, the decomposition of methanol into the CO and H species on the Pd/tungsten carbide (WC)(0001) surface is systematically investigated using periodic density functional theory (DFT) calculations. The possible reaction pathways and intermediates are determined. The results reveal that saturated molecules, i.e., methanol and formaldehyde, adsorb weakly on the Pd/ WC(0001) surface. Both CO and H prefer three-fold sites, with adsorption energies of −1.51 and −2.67 eV, respectively. On the other hand, CH₃O stably binds at three-fold and bridge sites, with an adsorption energy of −2.58 eV. However, most of the other intermediates tend to adsorb to the surface with the carbon and oxygen atoms in their sp³ and hydroxyl-like configurations, respectively. Hence, the C atom of CH₂OH preferentially attaches to the top sites, CHOH and CH₂O adsorb at the bridge sites, while COH and CHO occupy the three-fold sites. The DFT calculations indicate that the rupture of the initial C–H bond promotes the decomposition of CH₃OH and CH₂OH, whereas in the case of CHOH, O–H bond scission is favored over the C–H bond rupture. Thus, the most probable methanol decomposition pathway on the Pd/WC(0001) surface is CH₃OH → CH₂OH → trans-CHOH → CHO → CO. The present study demonstrates that the synergistic effect of WC (as carrier) and Pd (as catalyst) alters the CH₃OH decomposition pathway and reduces the noble metal utilization.     

Effect of alkyl nitrite decomposition on catalytic performance of CO coupling reaction over supported palladium catalyst

The syntheses of dimethyl oxalate (DMO) and diethyl oxalate (DEO) by CO coupling reaction in gaseous phase were investigated in a fixed bed reactor over Pd-Fe/Al₂O₃ catalyst. The catalytic performance was characterized by CO conversion, space-time yield (STY) and selectivity of DMO (or DEO). The results showed that over Pd-Fe/Al₂O₃ catalyst, the STY of DMO was higher than that of DEO under the same reaction conditions. The optimum reaction temperatures for synthesizing DMO and DEO were 403 K and 393 K, respectively, at the molar ratio 1 ∶ 1 of alkyl nitrite to CO. The difference in synthesizing DMO and DEO on the same catalyst was attributed to the decomposition performances of methyl nitrite (MN) and ethyl nitrite (EN), as density functional theory (DFT) calculation showed that EN decomposed more easily than MN.     

液相原位还原法制备钯基催化剂

采用液相原位还原法制备Pd/α-Al2O3催化剂,并应用于CO氧化偶联合成草酸二甲酯反应。对比实验发现,甲醛液相原位还原法制得的Pd基催化剂具有优异的催化活性,当Pd负载质量分数低至0.1%时,催化剂仍表现出较高的活性和稳定性。采用XRD和BET等对催化剂及载体进行表征,结果表明,催化剂活性与载体的比表面积、孔容和孔径没有必然联系。通过TEM发现,0.1%-Pd/α-Al2O3催化剂中的主要活性组分Pd具有较小的颗粒和较高的分散性,通过HRTEM发现,液相原位还原法制备的催化剂能够有效暴露出Pd(111)晶面。     

CO偶联合成草酸二甲酯催化剂研究

采用浸渍法制备CO偶联制备草酸二甲酯用负载型Pd催化剂,考察载体、浸渍方法、Pd含量、助剂对Pd催化剂性能的影响。根据傅里叶变换红外光谱研究Pd/α-Al_2O_3负载型催化剂上CO偶联制草酸二甲酯的反应机理。结果表明,采用α-Al_2O_3载体,Pd质量分数4‰,掺杂助剂Cu的蛋壳型的Pd/α-Al_2O_3催化剂上,草酸二甲酯时空收率达到735.7 g·(L·h)~(-1)。     

载体和助剂对CO气相合成草酸二甲酯的影响

采用XRD、BET等手段,考察Pd-Al2O3催化剂载体晶型和助剂种类对CO气相偶联合成草酸二甲酯的影响。结果表明,以α-Al2O3为催化剂载体,以La为助剂,草酸二甲酯的空时收率达到0.53 g/mL.h,CO转化率达到70%。     

Enhanced selectivity and formation of ethylbenzene for acetophenone hydrogenation by adsorbed oxygen on Pd/SiO2

The effect of adsorbed oxygen for selectivity of acetophenone (AP) hydrogenation on Pd/SiO₂ catalyst at 298 K has been studied by means of gas phase acetophenone hydrogenation, infrared (IR) spectra, and temperature-programmed desorption. Acetophenone hydrogenation on reduced Pd/SiO₂ catalyst reveals a typical series reaction in which phenylethanol (PE) is the intermediate for ethylbenzene (EB) formation. The selectivity of the reaction is towards phenylethanol at low temperature. The oxidized Pd/SiO₂ catalyst exhibits very different catalytic selectivity with reduced catalyst. The selectivity of ethylbenzene can be significantly boosted to over 90%, even if the reaction approaches zero conversion, suggesting that phenylethanol needs not be an intermediate for production of ethylbenzene from acetophenone. The formation of ethylbenzene and phenylethanol on oxidized Pd may be controlled by a parallel reaction pathway. The numbers of adsorbed oxygen on Pd surface strongly dominate the rate of EB formation. The bulk Pd oxide cannot be reduced by hydrogen at 298 K, so the oxygen atoms in Pd bulk act a poison for AP hydrogenation, leading to deactivation of oxidized Pd catalyst. The adsorbed oxygen on Pd surface plays the important role that can activate the C---H bond of CH₃ group in acetophenone, leading to the formation of a new intermediate (perhaps acetophenone enolate). This intermediate is the key species that will be further hydrogenated to ethylbenzene.     

Role of active oxygen transients in selective catalytic reduction of N2O with CH4 over Fe-zeolite catalysts

Sharp NO and O₂ desorption peaks, which were caused by the decomposition of nitro and nitrate species over Fe species, were observed in the range of 520–673 K in temperature-programmed desorption (TPD) from Fe-MFI after H₂ treatment at 773 K or high-temperature (HT) treatment at 1073 K followed by N₂O treatment. The amounts of O₂ and NO desorption were dependent on the pretreatment pressure of N₂O in the H₂ and N₂O treatment. The adsorbed species could be regenerated by the H₂ and N₂O treatment after TPD, and might be considered to be active oxygen species in selective catalytic reduction (SCR) of N₂O with CH₄. However, the reaction rate of CH₄ activation by the adsorbed species formed after the H₂ and N₂O or the HT and N₂O treatment was not so high as that of the CH₄ + N₂O reaction over the catalyst after O₂ treatment. The simultaneous presence of CH₄ and N₂O is essential for the high activity of the reaction, which suggests that nascent oxygen species formed by N₂O dissociation can activate CH₄ in the SCR of N₂O with CH₄.     

First-principles analysis of the hydrogenation of carbon monoxide over palladium

The reaction paths for the hydrogenation of CO to methanol over Pd_x (x = 1–4 and 19) cluster models were examined using first-principle density functional quantum chemical calculations. The predicted adsorption energies for the most favorable binding modes for CO, H₂, HCO, H₃CO, CH₃OH, C, O and H on a Pd₁₉ model Pd(111) clusters were -147, -62, -340, -51, -195, -33, -610, -349 and -251 kJ/mol, respectively. The most favorable modes for CO, CH₃O, H, C and O on Pd(111) were all found to be the 3-fold fcc site. The most favorable modes for the formyl and formaldehyde surface intermediates at low coverage were the 3-fold (ζ²μ³), and the di-σ sites, respectively. At higher surface coverages, however, the atop ζ¹ (C) and the π modes for the formyl and formaldehyde intermediates were more likely. The computed adsorption energies were subsequently used to compute overall reaction energies for the hydrogenation of CO to methanol. The initial hydrogenation of CO to the ζ¹ (C) HCO intermediate was found to be +52 kJ/mol endothermic and has been speculated as a possible rate-limiting step. The remaining surface hydrogenation steps become increasingly more exothermic as more hydrogen was added. The elementary steps of formyl to formaldehyde, formaldehyde to methoxide and methoxide to methanol were computed to be -9, -26 and -33 kJ/mol, respectively. The overall energy for CO dissociation was found to be highly unlikely at +260 kJ/mol and a clear indication that methanation and chain growth chemistry is not very likely over Pd. The most favorable reaction coordinate for the hydrogenation of CO to the ζ¹ (C) formyl intermediate was that which proceeds over a single Pd site where there is a migratory insertion of the CO into a Pd–H bond. The barrier for this path was computed to be +78 kJ/mol on the Pd₁₉ cluster. There was a very weak dependence on cluster size. This is a likely indication that this reaction is structure insensitive. A second path which involved the coupling of H and CO over a bridge site was found to be +130 kJ/mol which is less likely, but may also occur under different conditions. This revised version was published online in June 2006 with corrections to the Cover Date.     

Activation of methane, oxygen and nitrous oxide over surface-doped MgO materials: I. Conversions in N2O, (N2O + CH4) and (O2 + CH4) reactants

The following materials have been prepared having monolayer equivalent loadings of the indicated dopants: Li⁺/MgO, Ca²⁺/MgO, Sr²⁺/MgO and Ba²⁺/MgO. A continuous-flow microcatalytic reactor system has been used to compare activities of these surface-doped materials for : (a) the catalytic dissociation of N₂O alone at 760K and 958K; (b) N₂O dissociation accompanied by methane coupling and oxidation in a flow of (N₂O + CH₄) reactants at temperatures 760–958K; (c) Methane coupling and oxidation in (O₂ + CH₄) at temperatures 760–993K. The methane-activation roles proposed for various surface sites/intermediates on MgO-based materials are reconsidered in the light of these results.     

Catalytic reduction of N2O over steam-activated FeZSM-5 zeolite: Comparison of CH4, CO, and their mixtures as reductants with or without excess O2

The catalytic reduction of N₂O by CH₄, CO, and their mixtures has been comparatively investigated over steam-activated FeZSM-5 zeolite. The influence of the molar feed ratio between N₂O and the reducing agents, the gas-hourly space velocity, and the presence of O₂ on the catalytic performance were studied in the temperature range of 475–850 K. The CH₄ is more efficient than CO for N₂O reduction, achieving the same degree of conversion at significantly lower temperatures. The apparent activation energy for N₂O reduction by CH₄ was very similar to that of direct N₂O decomposition (140 kJ mol⁻¹), being much lower for the N₂O reduction by CO (60 kJ mol⁻¹). This suggests that the reactions have a markedly different mechanism. Addition of CO using equimolar mixtures in the ternary N₂O + CH₄ + CO system did not affect the N₂O conversion with respect to the binary N₂O + CH₄ system, indicating that CO does not interfere in the low-temperature reduction of N₂O by CH₄. In the ternary system, CO contributed to N₂O reduction when methane was the limiting reactant. The conversion and selectivity of the reactions of N₂O with CH₄, CO, and their mixtures were not altered upon adding excess O₂ in the feed.     

Pd-Fe/α-Al2O3/cordierite monolithic catalysts for the synthesis of dimethyl oxalate: effects of calcination and structure

Cordierite monoliths coated with Pd-Fe/α-Al₂O₃ catalysts were prepared at various calcination temperatures and characterized by thermogravimetry, temperature-programmed reduction, transmission electron microscopy, diffuse reflectance infrared Fourier transformation spectroscopy and X-ray diffraction. The performance of the catalytic monoliths for the synthesis of dimethyl oxalate (DMO) through a CO coupling reaction was evaluated. Monolithic catalysts with calcination temperatures ranging from 473 K to 673 K exhibited excellent dispersion of Pd, good CO adsorption properties, and excellent performance for the coupling reaction. The optimized monolithic catalyst exhibited a much higher Pd efficiency (denoted as DMO (g)·Pd (g)^-1·h^-1) (733 h^-1) than that of the granular catalyst (60.2 h^-1), which can be attributed to its honeycomb structure and the large pore sizes in the α-Al₂O₃ washcoat which was accompanied with an even distribution of the active component in the coating layer along the monoliths channels.     

Aqueous N2O Reduction with H2 Over Pd-Based Catalyst: Mechanistic Insights From Experiment and Simulation

Nitrous Oxide (N₂O), an ozone depleting greenhouse gas, is an observed intermediate in aqueous nitrate/nitrite reduction mediated by both natural microbial and synthetic laboratory catalysts. Because of our interest in catalytic nitrate/nitrite remediation, we have endeavored to develop a detailed concordant experimental/theoretical picture of N₂O reduction with H₂ over a Pd catalyst in an aqueous environment. We use batch experiments in H₂ excess and limiting conditions to examine the reduction kinetics. We use density functional theory (DFT) to model the elementary steps in N₂O reduction on model Pd(100), Pd(110), Pd(111) and Pd(211) facets and including the influence of adsorbed O, H, and of H₂O. Both experiments and theory agree that hydrogen is necessary for removal of adsorbed oxygen from the catalyst surface. The dissociation of N₂O to N₂(g) and O(ads) is facile and in the absence of H proceeds until the catalyst is O-covered. Water itself is proposed to facilitate the hydrogenation of surface O by transferring absorbed hydrogen to Pd-absorbed O and OH. We measure an apparent activation energy of 41.4?kJ/mol (0.43?eV) for N₂O reduction in the presence of excess H₂, a value that is within 0.1?eV of the barriers determined theoretically.     

CO气相偶联合成草酸二甲酯催化剂的研制

考察助剂、制备方法和还原介质对CO气相偶联合成草酸二甲酯催化剂性能的影响。结果表明,以贵金属Pd为活性组分,非贵金属C为助剂,以α-Al_2O_3为载体,采用分步等体积浸渍法制备的催化剂,在水合肼还原后,具有良好的反应活性和稳定性。在反应温度130℃,空速3 000 h^(-1)时,产物中草酸二甲酯含量90%以上,时空收率平均可达813 g·(L·h)^(-1)。催化剂PC-3反应500 h后,物理性能和反应活性变化不大。     

In situ FTIR study on reaction pathways in Ni(CO)4/CH3OK catalytic system for low-temperature methanol synthesis in a liquid medium

An in situ infrared spectroscopic study was conducted to elucidate the reaction pathways for low-temperature methanol synthesis in a catalytic system composed of Ni(CO)₄ and CH₃OK (denoted as Ni(CO)₄/CH₃OK). The reaction was conducted in a liquid medium at 313–333 K with an initial pressure of 3.0 MPa. When CH₃OK was added to Ni(CO)₄ solution at 293 K, different carbonylnickelates, [Ni₅(CO)₁₂]²⁻, [Ni₆(CO)₁₂]²⁻ and [Ni(CO)₃(COOCH₃)]⁻, were immediately formed from Ni(CO)₄. The species and the composition of the carbonylnickel complexes varied with temperature. The variations in concentrations of methanol (MeOH) and methyl formate (MF) during the run, which were determined from their IR absorptions, indicated a pattern characteristic of consecutive reactions with MF as an intermediate. Thus, it was shown that methanol was produced through the carbonylation of MeOH to MF and the subsequent hydrogenation of MF to MeOH. Stable hydridocarbonylnickel anions, [HNi(CO)₃]⁻ and/or [HNi₂(CO)₆]⁻, were observed together with a small amount of Ni(CO)₄ throughout the methanol synthesis. Since Ni(CO)₄ alone showed no activity for the hydrogenation of MF, the hydridocarbonylnickel anions generated in the presence of CH₃OK must be responsible for the reaction. The dual role of CH₃OK in the catalytic system was stated.     

FTIR study of aqueous nitrate reduction over Pd/TiO2

The interactions between Pd/TiO₂ catalyst and the reactants and potential reaction intermediates present during aqueous nitrate reduction, including NO₃⁻, NO₂⁻ and NO in the presence of H₂ and H₂O were studied by infrared spectroscopy. Adsorbed forms of NO, nitrite and nitrate could all be detected in the presence of water. In the presence of water/H₂, nitrate was the most stable surface species followed by nitrite and then highly reactive NO, suggesting that the reduction of nitrate to nitrite is the rate-limiting step. High concentrations of adsorbed nitrite appear to be linked to the detection of gaseous N₂O while the formation of ammonia is related to reactions on the Pd surface and the extent of formation is linked to high levels of adsorbed NO in addition to the surface hydrogen availability and the presence of water.     

高效稳定的铜镍催化剂在草酸二甲酯加氢中的应用

为了探索高效、稳定的草酸二甲酯（DMO）加氢制乙醇酸甲酯（MG）催化剂,采用水热合成法制备Cu-Ni/SiO₂催化剂,探索了不同Cu：Ni摩尔比对于催化剂活性的影响。通过XRD、TEM和XPS等表征,结果表明：利用二氧化硅微球作载体,铜镍物种的分散更加均匀。并且调变不同的Cu：Ni摩尔比,对Cu⁺在催化剂中的比例有一定的影响,从而影响乙醇酸甲酯的收率。在氢酯比为150、反应压力2 MPa、反应温度200℃和液时空速为0.5 h^-1的反应条件下,Cu：Ni摩尔比为1：1时的催化剂Cu₁Ni₁/SiO₂表现出了最好的催化性能,草酸二甲酯的转化率达到90%,乙醇酸甲酯的选择性达到了80%,催化剂能稳定运行100 h。上述结果可为研制催化活性高、选择性强、寿命长、易于生产乙醇酸甲酯的催化剂提供一定的参考。     

Effect of the pretreatment with oxygen and/or oxygen-containing compounds on the catalytic performance of Pd-Ag/Al2O3 for acetylene hydrogenation

Catalytic performance of Pd-Ag/-Al₂O₃ was studied for the selective hydrogenation of acetylene in the presence of excess ethylene. The catalyst activation was undertaken prior to the reaction test by the pretreatment with oxygen and/or oxygen-containing compounds, i.e. O₂, NO, N₂O, CO and CO₂. The enhancement of catalytic performances by the pretreatment was a consequence of an increase in accessible Pd sites responsible for acetylene hydrogenation to ethylene. Furthermore, the sites involving direct ethane formation from acetylene could be suppressed by NO_x treatment.     

Ethylene hydroformylation and carbon monoxide hydrogenation over modified and unmodified silica supported rhodium catalysts

Ethylene hydroformylation and carbon monoxide hydrogenation (leading to methanol and C₂-oxygenates) over Rh/SiO₂ catalysts share several important common mechanistic features, namely, CO insertion and metal–carbon (acyl or alkyl) bond hydrogenation. However, these processes are differentiated in that the CO hydrogenation also requires an initial CO dissociation before catalysis can proceed. In this study, the catalytic response to changes in particle size and to the addition of metal additives was studied to elucidate the differences in the two processes. In the hydroformylation process, both hydroformylation and hydrogenation of ethylene occurred concurrently. The desirable hydroformylation was enhanced over fine Rh particles with maximum activity observed at a particle diameter of 3.5 nm and hydrogenation was favored over large particles. CO hydrogenation was favored by larger particles. These results suggest that hydroformylation occurs at the edge and corner Rh sites, but that the key step in CO hydrogenation is different from that in hydroformylation and occurs on the surface. The addition of group II–VIII metal oxides, such as MoO₃, Sc₂O₃, TiO₂, V₂O₅, and Mn₂O₃, which are expected to enhance CO dissociation, leads to increased rates in CO hydrogenation, but only served to slow the hydroformylation process slightly without any effect on the selectivity. Similar comparisons using basic metals, such as the alkali and alkaline earths, which should enhance selectivity for insertion of CO over hydrogenation, increased the selectivity for the hydroformylation over hydrogenation as expected, although catalytic activity was reduced. Similarly, the selectivity toward organic oxygenates (a reflection of the degree of CO insertion) in CO hydrogenation was also increased.     

Characterization of catalysts for methane-coupling by means of temperature programmed desorption

The technique of temperature programmed desorption has been employed to study the adsorptive behaviour of CaO and Na₂O/CaO towards the reactants and products of the methane-coupling reaction. Two forms of adsorbed O₂ can exist on CaO, and they desorb at intermediate temperatures: the first corresponds to the sites for adsorption and desorption without reaction of CH₄, while the second to sites for desorption with total oxidation of CH₄ itself. At the temperatures of the activity runs for methane coupling (750–800 °C), no adsorbed species is therefore stable on the surface of CaO. 7% Na₂O/CaO shows different features, because the desorption of O₂ produces only a large broad peak at very high temperatures, corresponding to sites for adsorption and desorption with total oxidation of CH₄. (On the other hand a desorption peak of CH₄ at intermediates temperatures is also present, which seems to be due to a species, adsorbed undissociatively and desorbed without reaction). At the temperatures of the activity experiments, adsorbed species are therefore stable on the surface of Na₂O/CaO. A conclusion is therefore drawn on the necessity for the intervention of the gas phase O₂ during the methane coupling reaction in order to produce C₂-hydrocarbons, and on the greater importance of the surface reactions in the presence of Na₂O, in comparison to the case of CaO.     

草酸二甲酯加氢合成乙二醇Cu/SiO_2催化剂的稳定性试验研究

合成气经草酸二甲酯加氢制乙二醇技术在工业应用过程中存在一些技术难题,其中草酸二甲酯加氢制乙二醇催化剂的稳定性是制约该技术发展的瓶颈。以Cu/SiO_2草酸二甲酯加氢催化剂A为研究对象,通过2 600 h的稳定性实验研究,考察反应温度对草酸二甲酯转化率、乙二醇选择性以及产物中乙醇酸甲酯含量的影响,为合成气经草酸二甲酯制乙二醇技术的工业应用优化提供重要的技术支持。结果表明,当反应压力2.8 MPa、空速(0.3~0.5)h-1、氢酯物质的量比120~140和反应温度216℃时,草酸二甲酯转化率近100%,乙二醇选择性大于95.0%,产物中乙醇酸甲酯质量分数小于0.5%,Cu/SiO_2催化剂A稳定性较好。