The influence of CO,CO2 and H2O on selective CO methanation over Ni(Cl)/CeO2 catalyst: On the way to formic acid derived CO-free hydrogen

For the first time the influence of CO, CO₂ and H₂O content on the performance of chlorinated NiCeO₂ catalyst in selective or preferential CO methanation was studied systematically. It was shown that the rate of CO methanation over Ni(Cl)/CeO₂ increases with the increasing H₂ concentration, is independent of CO₂ concentration and decreases with increasing CO and H₂O concentrations; the rate of CO₂ methanation is weakly sensitive to H₂ and CO₂ concentrations and decreases with increasing CO and H₂O concentrations. High catalyst selectivity was attributed to Ni surface blockage by strongly adsorbed CO molecules and ceria surface blockage by Cl, which both inhibit CO₂ hydrogenation.For the first time, selective CO methanation over Ni(Cl)/CeO₂ was studied for deep CO removal from formic acid derived hydrogen-rich gases characterized by high CO₂ (40–50 vol%), low CO (30–1000 ppm) content and trace amounts of water. Composite Ni(Cl)/CeO₂-η-Al₂O₃/FeCrAl wire mesh catalyst was demonstrated to be effective for this process at temperatures of 180–220°С, selectivity 30–70%, WHSV up to 200 L (STP)/(g∙h). The catalyst provides high process productivity, low pressure drop, uniform temperature distribution, and appears highly promising for the development of a compact CO cleanup reactor. Selective CO methanation was concluded to be a convenient way to CO-free hydrogen produced by formic acid decomposition.     

Selective catalytic reduction of NOx by H2 over Na modified Pd/TiO2 catalyst

The effect of alkali metal Na on Pd/TiO₂ catalyst for the selective catalytic reduction of NO_x by H₂ (H₂-SCR) has been investigated. Compared with Pd/TiO₂ and Na/TiO₂, Pd–Na/TiO₂ catalyst exhibited much higher catalytic activity and wider activity temperature window. Characterization results showed that more Pd⁰ and surface chemisorbed oxygen (O_α) existed on Pd0.5Na0.5/Ti than Pd/TiO₂. In-situ DRIFTS results showed that more NO_x adsorbed and activated on Pd0.5Na0.5/Ti, and these species are reactive. Meanwhile, NH₃ species adsorbed on Lewis acid sites was generated by the reaction between NO_x and H₂, which then reduce NO_x by the NH₃-SCR route. As a consequence, the reduction of NO_x over Pd–Na/TiO₂ was significantly improved. Our study provides new insight for developing highly H₂-SCR catalyst for the removal of NO_x.     

Effect of noble metal addition over active Ru/TiO2 catalyst for CO selective methanation from H2 rich-streams

Selective CO methanation from H₂-rich stream has been regarded as a promising route for deep removal of low CO concentration and catalytic hydrogen purification processes. This work is focused on the development of more efficient catalysts applied in practical conditions. For this purpose, we prepared a series of catalysts based on Ru supported over titania and promoted with small amounts of Rh and Pt. Characterization details revealed that Rh and Pt modify the electronic properties of Ru. The results of catalytic activity showed that Pt has a negative effect since it promotes the reverse water gas shift reaction decreasing the selectivity of methanation but Rh increases remarkably the activity and selectivity of CO methanation. The obtained results suggest that RuRh-based catalyst could become important for the treatment of industrial-volume streams.     

Influence of the supports ZrO2 on selective methanation of CO over the nickel supported catalysts

It is attempted to optimize preparation of ZrO₂ as support of the nickel catalysts for selective methanation of CO in H₂-rich gas (CO-SMET). Therefore, the supports ZrO₂ were prepared at first by thermal decomposition method from zirconium oxynitrate and zirconium oxychloride at the calcination temperature of 400 °C and 800 °C, respectively. It is illustrated that the salt kind and calcination temperature affected phase state (tetragonal, monoclinic), crystallite size and specific surface area (SSA) of the supports. The difference in property of the supports influenced catalytic performance of the catalysts Ni/ZrO₂ for CO-SMET reaction. Especially, the chlorine ion residues in the support ZrO₂ prepared from zirconium oxychloride was beneficial for CO removal selectively. Furthermore, a precipitation method was adopted to prepare ZrO₂ for comparison with the thermal decomposition method with use of the zirconium oxychloride as starting material. It is found that the supports ZrO₂ prepared by the precipitation method induced a better dispersion of metallic Ni on its surface. The catalyst Ni/ZrO₂ with use of the support ZrO₂ prepared by the precipitation method and calcination at 400 °C exhibited a good performance at the reaction temperature of 220 °C in the 100 h durability test, where CO outlet concentration was kept below 10 ppm and the selectivity remained constant at 100%. Relation of Ni crystallite size and chlorine ion residues with the catalytic performance was discussed.     

Favored hydrogenation of linear carbon monoxide over cobalt loaded on fibrous silica KCC-1

In this study, the conversion of CO into CH₄ was investigated utilizing a series of cobalt loaded on fibrous silica (KCC-1) catalysts (Co loading of 5–30 wt%), that were synthesized via microemulsion and impregnation techniques. FESEM-EDX and N₂ physisorption demonstrated that the KCC-1 possessed a spherical structure with fibrous silica dendrimeric morphology with a superior surface area of 861 m²g^-1. A significant decreased in the catalyst surface area was noticed upon the addition of Co, suggesting a possible occurrence of KCC-1 pore blockage. Inversely, the number of basic sites on KCC-1 was enhanced after the incorporation of Co, as observed by pyrrole adsorbed FTIR. At 523 K, bare KCC-1 exhibited a very low activity for CO methanation due to low basicity and the absence of surface active sites. The 20Co/KCC-1 demonstrated the best catalytic performance with 72.7% yield of CH₄ and 6.8% of CO₂. These results were plausibly attributed to the high intrinsic number of basic sites and high dispersion of Co on KCC-1 support. A detailed in-situ FTIR spectroscopy study revealed that both types of associative and dissociative mechanism pathways significantly contributed to the high catalytic methanation activity. In addition to the dissociative mechanism, the linear CO species adsorbed on the Co metal by associative mechanism was also further hydrogenated to obtain the final CH₄ products.     

Promotion effect by Mg on the catalytic behavior of MgNi/WO3 in the CO methanation

In this study, tungsten oxide with a high specific surface area was fabricated using a nanocasting technique and used to prepare support for nickel catalysts for CO methanation. Additionally, Mg was further introduced as a promoter for tuning the catalytic performance. The 25Ni/WO₃ catalyst demonstrated a relatively high CO conversion, but a poor CH₄ selectivity; however, with the addition of 7 wt% Mg to the catalyst, the CH₄ selectivity reached 92% at a temperature of 440 °C. The improved CH₄ selectivity can be attributed to the enhanced CO dissociation, which was related to the reduced Ni particle size, as well as the enhanced Ni electron cloud density. The role of a physical barrier and electron transfer of MgO induces an enhancement of the metal–support interactions, which are conducive to decreasing the Ni particle size. Meanwhile, the electron transfer performance of MgO constitutes a crucial factor in enhancing the Ni electron cloud density. Furthermore, with benefit from the inhibition of agglomeration of the Ni particles by the MgO promoter, a significantly better catalytic stability was also observed on 7Mg25Ni/WO₃ than with the 25Ni/WO₃ catalyst.     

Electrochemical investigations on CO2 reduction mechanism in molten carbonates in view of H2O/CO2 co-electrolysis

In order to reduce carbon dioxide emission, one solution is to convert into valuable chemicals or fuels, e.g. transforming CO₂ into CO by electrochemical reduction. Thus, this greenhouse gas could be re-used in particular as syngas (CO + H₂) by co-electrolysis of CO₂/H₂O. High temperature electrolysis cells can be the best energetic devices to produce such syngas. In particular, molten carbonates are known to solubilize CO₂ very significantly higher than other solvents. Therefore, it is compulsory to investigate and understand the mechanism of CO₂ reduction in such media to consider its further use and valorisation. The present study is a critical approach aiming at elucidating the mechanisms for CO₂ electroreduction, using an inert Pt electrode in the molten eutectic Li₂CO₃–K₂CO₃ (62-38 mol%), at 650 °C, under different partial pressures of CO₂. Complementary electrochemical techniques, including sweep square-wave voltammetry and relaxation chronopotentiometry, were carried out. Their combination allowed us to evidence that the electroreduction of CO₂ into CO is feasible in oxo-acidic conditions, involving a diffusion-limited quasi reversible system in a one electron-step.     

Methanation of CO2 over Ni/Al2O3 modified with alkaline earth metals: Impacts of oxygen vacancies on catalytic activity

This study investigates the impacts of the alkaline earth metal (Mg, Ca, Sr, Ba) additives on properties and performances of nickel catalysts for CO₂ methanation. The results show that addition of Mg, Sr, and Ba creates more pores while Ca addition leads to merge of small pores. The alkalinity of the catalyst increases with the addition of Mg, Ca, Sr or Ba, however, it does not necessarily enhance the catalytic activity. The degree of reduction of nickel species is another important factor affecting catalyst activity. Mg or Ca addition promotes the reverse water gas shift reaction to form more CO but not the methanation. In converse, with the addition of Sr or Ba, the activities for methanation increased drastically, especially in the low temperature region. In situ Diffuse Reflection Infrared Fourier Transform Spectroscopy (DRIFTS) studies show that *OH, *CO₃, *CO₂, CH_x, HCOO*, *CO and H₂CO* species are main reaction intermediates. Mg or Ca promotes the carbonate formation. Sr or Ba promotes *CO and H₂CO* formation, which are the important reaction intermediates in the conversion of CO₂ to CH₄. In addition, the Electron Paramagnetic Resonance (EPR) characterization shows that the catalyst modified with Sr species generates the oxygen vacancies that prevent electrons from being paired, forming a Lewis basic position. The oxygen vacancies generated are crucial for enhancing the catalytic activities for methanation at the low reaction temperatures.     

Density functional and microkinetic study of CO2 hydrogenation to methanol on subnanometer Pd cluster doped by transition metal (M= Cu,Ni, Pt,Rh)

We study the CO₂ hydrogenation to methanol on subnanometer Pd₇ and transition metal doped Pd₆M (M = Cu, Ni, Pt, and Rh) clusters using a combination of density functional theory and microkinetic calculations. We find that, in general, the inclusion of transition metal dopants could decrease the activation energy of several important elementary reactions. This condition results in a significant improvement in the activity of the catalyst, especially for the Pd₆Ni cluster. We find that the Pd₆M clusters are more selective toward the formate pathway than the RWGS + CO hydrogenation pathway. We also compare the turnover frequency profiles of the clusters with that of the Cu(111) surface, representing the standard industrial catalyst. We find that the Pd₆Ni cluster can successfully overcome the TOF of Cu(111) surface, even at the low-pressure condition.     

A new modification on RK EOS for gaseous CO2 and gaseous mixtures of CO2 and H2O

To develop an equation of state with simple structure and reasonable accuracy for engineering application, Redlich–Kwong equation of state was modified for gaseous CO₂ and CO₂–H₂O mixtures. In the new modification, parameter ‘a’ of gaseous CO₂ was regressed as a function of temperature and pressure from recent reliable experimental data in the range: 220–750 K and 0.1–400 MPa. Moreover, a new mixing rule was proposed for gaseous CO₂–H₂O mixtures. To verify the accuracy of the new modification, densities were calculated and compared with experimental data. The average error is 1.68% for gaseous CO₂ and 0.93% for gaseous mixtures of CO₂ and H₂O. Other thermodynamic properties, such as enthalpy and heat capacities of CO₂ and excess enthalpy of gaseous CO₂–H₂O mixtures, were also calculated; results fit experimental data well, except for the critical region. Copyright © 2005 John Wiley & Sons, Ltd.     

Enhanced stability for preferential oxidation of CO in H2 under H2O and CO2 atmosphere through interaction between iridium and copper species

Preferential oxidation of CO (CO-PROX) is one of the most investigated methods for reducing residual CO in H₂-rich stream to acceptable level in proton exchange membrane fuel cells. However, development of catalyst with high stability under simulated practical conditions is still challenging. Herein, a series of Cu_xCe_1-xO₂ (x = 0, 0.05, 0.09, 0.17) supported Ir catalysts were prepared and 1 wt%Ir/Cu_0.09Ce_0.91O₂ exhibited full conversion of CO in a wide temperature window (80–180 °C), excellent stability and resistance to CO₂ and H₂O poison. Characterization results reveal that the superior performance was mainly associated with the interaction between Ir and Cu species, which resulted in that the adsorbed H₂O on Ir sites was activated to react with adsorbed CO on Cu sites to form easily decomposable bicarbonates and formate species instead of main intermediate of carbonates for 1 wt% Ir/CeO₂ and Cu_0.09Ce_0.91O₂. This work provides a new sight for developing high-performance heterogeneous catalysts.     

Improvement of H2/O2 chemical kinetic mechanism for high pressure combustion

An updated H₂/O₂ kinetic mechanism was proposed by incorporating carefully selected reaction rate coefficient and great progress in radical chain mechanisms, in which the uncertainties of rate coefficient were discussed. The performance of the current mechanism was compared to other H₂ mechanism and validated against a wide range kinetic targets, including oxidation, decomposition in shock waves, ignition, flame speed and flame structure. Results show that the current mechanism obtains an overall improvement of performance, especially for the flame speed. By using the updated binary diffusion coefficient from ab initio calculations and the chemically termolecular reactions, the current mechanism presents better agreement with the new experimental flame speed at atmospheric pressure and obtains the improved performance with respect to the negative pressure dependence of high-pressure H₂ flame. Furthermore, the flame speed predictions are strongly sensitive to the H₂O third body efficiency in the H₂ mechanism, affecting the water-contained H₂ flame. The modeling results of rapid compression machine ignition show that present mechanism can more accurately predicts the ignition delay under engine-like conditions. However, all three mechanisms cannot accurately reproduce the negative pressure dependence behavior of mass burning rate in high-pressure H₂ flame, which may be attributed to the fact that the important reaction O + OH(+M) = HO₂(+M) that significantly affects lean high-pressure H₂ flame is not included in current mechanism. Consequently, continuous works should be emphasized on the reactions that are important but neglected in H₂ mechanism. All these not only develop an improved H₂ reaction mechanism for high-pressure combustion, but also point out the direction for refining the H₂ mechanism.     

Understanding the effect of Ni cluster size on methane activation and dehydrogenation

The cluster size of the active metal of the catalyst has great effect on the carbon deposits during the dry reforming of methane. In this paper, the effect of active metal cluster size on methane activation and dehydrogenation is specifically studied. Three cluster models of Ni₄, Ni₉, and Ni₁₆ are applied to study and simulate the activation and dehydrogenation process of CH₄ on Ni clusters of different sizes. It is found that the adsorption energy of CH₄ on Ni clusters increases with the increase of cluster size, but it is fairly close. The adsorption energies of CH₃, CH₂, CH and C are the largest on Ni₉. In this process, electrons are always transferred from Ni clusters to adsorbents, and the amount of electron transfer is proportional to the adsorption energy. In the CH₄ dehydrogenation process, Ni₁₆ showed the most excellent activity and anti-carbon performance.     

不同粒径煤粉在O2/CO2气氛下的燃烧特性

用TG研究了不同粒径煤粉在空气和O2/CO2气氛下的燃烧特性,对比分析了两种气氛下不同粒径对煤粉特征温度和特征指数的影响情况.结果表明,O2/CO2气氛下,粒径对煤粉燃烧的影响要大于空气气氛,且挥发分含量越高的煤种其影响越大.相比空气气氛,O2/CO2气氛下煤粉的燃烧特性随粒径变化会呈现较大的波动性,且随着煤种挥发分含量的增加,小粒径段煤粉的波动性增强.可以通过提高氧浓度来减少燃烧波动的影响.     

Roles and fates of K and Ca species on biochar structure during in-situ tar H2O reforming over nascent biochar

In order to obtain the catalytic effects of K and Ca species on the biochar structure during in-situ tar H₂O reforming over nascent biochar, the H-form/K-loaded/Ca-loaded rice husks were studied for the in-situ tar reforming in the two-stage fluidized bed/fixed bed reactor. The specific reaction pathway of K and Ca for tar reforming was investigated, associated with the changes of biochar structures, through the methods of ICP-AES, Raman, FTIR and XPS. The results indicate that the in-situ volatiles (tar and free radicals) H₂O reforming over nascent biochar could be conducted by three possible ways: occupying reactive sites on biochar, changing biochar structures and/or changing the total/surface concentration of AAEM species. The mechanisms of in-situ tar H₂O reforming by K and Ca species were different: tar cracking into low-quality tar or small-molecule gas may be catalyzed by K, while the combination of tar with biochar would be promoted by Ca. The volatilizations of K and Ca with the presence of volatiles were to a large extent in accordance with their valences (monovalent K⁺ and divalent Ca²⁺) and their boiling points. The subsequent transformation from the small aromatic ring systems to the larger ones occurred due to the volatile-biochar interaction. During the in-situ tar H₂O reforming over biochar, K and Ca act as the active sites on biochar surface to promote the increase of active intermediates (CO bonds and COK/Ca), which promotes the tar-biochar interactions.     

Understanding the role of K on PtCo/Al2O3 for preferential oxidation of CO in H2

CO preferential oxidation reaction (CO-PROX) can effectively eliminate CO in H₂ rich atmosphere to avoid CO poison the Pt anode of Proton Exchange Membrane Fuel Cell (PEMFC). To match the operation temperature window for PEMFC, PtCo nanoparticles supported on K modified Al₂O₃ (PtCo/K–Al₂O₃) were prepared to promote CO-PROX activity. The addition of K species weakened the interaction between PtCo nanoparticle and support, which improved the dispersion of Pt particles and redox property of PtCo/Al₂O₃. It also facilitated the formation of Pt₃Co species and active surface ?OH groups, which were involved in CO-PROX reaction. According to in situ DRIFTS spectra, HCO₃^? and HCOO^? were intermediates of PtCo/K–Al₂O₃ catalyzed CO-PROX at low temperature and high temperature, respectively. Thus, the addition of 1 wt% K to PtCo/Al₂O₃ (PtCo/1K–Al₂O₃) could completely oxidize CO in the temperature range of 127–230 °C with O₂ selectivity at 50%. The 100% CO conversion temperature window of PtCo/1K–Al₂O₃ is expanded by 100 °C in comparison of PtCo/Al₂O₃.     

Design of efficient noble metal single-atom and cluster catalysts toward low-temperature preferential oxidation of CO in H2

The preferential oxidation of CO in H₂ is attractive for the removal of trace amounts of CO to meet the requirement of proton-exchange membrane fuel cells (PEMFCs) application. The key is to design highly effective catalysts that work well in a wide range of low temperatures. Here, the recent progress in Au and Pt group metal catalysts for the PROX reaction is summarized, covering those with single-atom and cluster dispersed metal species with remarkable performance. Firstly, the progress of some representative catalysts is overviewed, with an emphasis on the strategies for improving low-temperature activity, selectivity, and stability. Then, special attention is focused on the key parameters affecting performance in the PROX reaction. Moreover, the reaction mechanisms in terms of adsorption and activation of reactants are discussed. Finally, the challenges and opportunities are offered for guiding the design of advanced noble metal catalysts toward the PROX process.     

O2/CO2混合富氧燃烧技术探讨

分离处置化石燃料燃烧产生CO2的技术被认为是近期内减缓CO2排放的较为可行的措施.在众多CO2分离回收技术中,O2/CO2混合富氧燃烧技术具有明显的优势和较强的应用前景.它不仅可以使CO2的回收和利用容易进行,还可以有效减少NOx和SO2的排放,同时能提高锅炉效率,是一项高效节能的燃烧方式.对O2/CO2混合富氧燃烧技...     

Impacts of alkali or alkaline earth metals addition on reaction intermediates formed in methanation of CO2 over cobalt catalysts

Additives affect the physiochemical properties of the catalyst as well as the evolution of the reaction intermediates produced during the reaction process such as the methanation of CO₂. In this study, Co/Al₂O₃ catalysts modified with Na, K, Mg or Ca were prepared and the reaction intermediates formed during CO₂ methanation were investigated. The results showed that Na, K or Mg species reacted with alumina, forming Al(OH)₃ or MgAl₂O₄ spinel structure, leading to the re-structure of the catalysts and a remarkable decrease of the specific surface area. The increased alkalinity of the catalyst did not promote the catalytic activity for methanation but promoted CO formation. The addition of Na or K enhanced the affinity of the catalyst to the reaction intermediates of HCOO* and CO₃²⁻, slowing down their further reduction to CH₄ and leading to the lower catalytic activity. The evolution of HCOO* and CO₃²⁻ species strongly correlated with the catalytic activity, while the direct correlation between the capability for the absorbance of CO₂* as well as the C–O functionality and the catalytic activity was not found. In addition, the addition of Na or K to Co/Al₂O₃ could also induce the formation of a significant amount of the coke species in the nanotube form.     

The intrinsic reactivity of coal char conversion compared under different conditions of O2/CO2, O2/H2O and air atmospheres

The reasons for the intrinsic reactivity differences in coal char conversion under an O₂/H₂O atmosphere compared with that under an O₂/CO₂ or O₂/N₂ atmosphere have been investigated in a thermogravimetric analyzer by a simple variable activation energy (SVAE) method combined with an adsorption/desorption reaction mechanism. The results show that only CO₂ or H₂O chemisorption occurred in the non-isothermal experiments, not gasification; however, the intrinsic reaction rate (IRR) of coal char conversion at the same O₂ concentration still increases in an orderly manner under O₂/CO₂, O₂/N₂ and O₂/H₂O atmospheres. This result is due to the different chemisorption mechanisms of CO₂ and H₂O, namely, the production of C(CO), C(OH) and C(H) from CO₂ and H₂O chemisorption. At the same O₂ concentration, the trends and magnitudes of variable activation energies for coal char combustion under O₂/CO₂ and O₂/N₂ atmospheres are similar, while they are very different from those under O₂/H₂O conditions. Therefore, CO₂ has little influence on the reactivity, while H₂O changes the reactivity. In addition, according to the developed reaction mechanism, it is concluded that the SVAE method contributes to the characteristic intrinsic reactivity of coal char conversion under different atmospheres.