Lattice capacity-dependent activity for CO2 methanation: crafting Ni/CeO2 catalysts with outstanding performance at low temperatures.

In the pursuit of understanding lattice capacity threshold effects of oxide solid solutions for their supported Ni catalysts, a series of Ca2+-doped CeO2 solid solutions with 10 wt% Ni loading (named Ni/CaxCe1-xOy) was prepared using a sol-gel method and used for CO2 methanation. The lattice capacity of Ca2+ in the lattice of CeO2 was firstly determined by the XRD extrapolation method, corresponding to a Ca/(Ca + Ce) molar ratio of 11%. When the amount of Ca2+ in the CaxCe1-xOy supports was close to the CeO2 lattice capacity for Ca2+ incorporation, the obtained Ni/Ca0.1Ce0.9Oy catalyst possessed the optimal intrinsic activity for CO2 methanation. XPS, Raman spectroscopy, EPR and CO2-TPD analyses revealed the largest amount of highly active moderate-strength alkaline centers generated by oxygen vacancies. The catalytic reaction mechanisms were revealed using in situ IR analysis. The results clearly demonstrated that the structure and reactivity of the Ni/CaxCe1-xOy catalyst exhibited the lattice capacity threshold effect. The findings offer a new venue for developing highly efficient oxide-supported Ni catalysts for low-temperature CO2 methanation reaction and enabling efficient catalyst screening.

Decoding the Reactivity Enhancement of Ln2Ce2O7 Compounds (Ln = Yb, Y, Tb, and Gd) for Soot Combustion: The Remarkable Contribution of Variable Valence A-Sites.

The impact of variable valence A-sites on the redox property and reactivity of Ln2Ce2O7 compounds in soot particulate combustion has been investigated. It was observed that Yb2Ce2O7, Y2Ce2O7, and Gd2Ce2O7 formed a rare earth C-type phase, while Tb2Ce2O7 formed a solid solution phase. Both Tb2Ce2O7 and Yb2Ce2O7 possess dual valence state A-sites, resulting in significantly more surface vacancies. Additionally, the advantageous solid solution phase structure of Tb2Ce2O7 leads to even more surface vacancies than Yb2Ce2O7, which is crucial to generate active oxygen sites. Moreover, the introduction of NO into the reaction feed enhances combustion activity by producing active surface monodentate nitrates. A catalyst with higher numbers of surface vacancies exhibits improved NO oxidation ability and better NO2 utilization efficiency. Consequently, the Tb2Ce2O7 compound demonstrates not only the best soot combustion activity, but also an optimal NOx-assistance effect. Therefore, it is concluded that variable valence A-site is the intrinsic factor to improve the reactivity of Ln2Ce2O7 catalysts.

Electronegativity-dependent Pt anchoring and molecule adsorption for graphene-based supported Pt single atom.

CONTEXT: To unravel the effects of the C vacancy, doping N type and number, the adsorption of HCHO and O2 was investigated on the graphene (Gr)-based supported Pt single atom by density functional theory calculations. The electronegativity of the vacancy and N-doped Gr was a crucial factor both for the anchoring for a Pt and the further adsorption of HCHO and O2 on the supported Pt. The electronegativity can be tuned by the C vacancy number (1V and 2V), the doping N type (graphitic-N, pyridinic-N and pyrrolic-N) and the doping pyridinic-N number (1N ~ 4N). The high electronegativity of the vacancy and N-doped Gr favored the anchoring for a Pt compared to the Gr, while too high electronegativity was detrimental for further adsorption of adsorbates on the supported Pt. The Bader charge analysis proved that the electronegativity followed the trend as pyrrolic-N > pyridinic-N > graphitic-N, and 4N-Gr > 2V-Gr > 3N-Gr > 2N-Gr > 1N-Gr > 1V-Gr > Gr. As a result, the pyridinic-N, the 1V-Gr, 1N-Gr and 2N-Gr with the suitable electronegativity achieved both stronger anchoring for a Pt and more favorable adsorption of HCHO and O2 on the supported Pt than the pristine Gr support. METHODS: Periodic DFT calculation was performed using the VASP code. The PAW method and the GGA-PBE functionals were used. Part of work was also carried out by the DSPAW procedure of Device Studio.

Comparing BaZrO3 Perovskite and La2Zr2O7 Pyrochlore for Oxidative Coupling of Methane (OCM): The Importance of Basic Sites vs Oxygen Vacancies.

La2Zr2O7 pyrochlore with intrinsic oxygen vacancies and BaZrO3 perovskite without intrinsic oxygen vacancies were synthesized for the OCM reaction. It has been revealed that the OCM performance and surface selective oxygen species of BaZrO3 are higher than that of La2Zr2O7 under the reaction condition. This is because BaZrO3 possesses more basic sites than La2Zr2O7, and thus it can stabilize the OCM reactive oxygen species better at elevated temperature. In the structure, the A-O bond lattice oxygen of the two compounds mainly provides basic sites, but the B-O bond lattice oxygen mainly promotes deep oxidation of methane and the generated hydrocarbons. The types of the OCM reactive oxygen species are prone to be associated with the properties of the A-site metal oxides.

Doping SnO2 with metal ions of varying valence states: discerning the importance of active surface oxygen species vs. acid sites for C3H8 and CO oxidation.

To elucidate the valence state effect of doping cations, Li+, Mg2+, Cr3+, Zr4+ and Nb5+ with radii similar to Sn4+ (CN = 6) were chosen to dope tetragonal SnO2. Cr3+, Zr4+ and Nb5+ can enter the SnO2 lattice to produce solid solutions, thus creating more surface defects. However, Li+ and Mg2+ can only stay on the SnO2 surface as nitrates, thus suppressing the surface defects. The rich surface defects facilitate the generation of active O2-/Oδ- and acid sites on the solid solution catalysts, hence improving the reactivity. On the solid solution catalysts active for propane combustion, several reactive intermediates can be formed, but are negligible on those with low activity. It is confirmed that for propane combustion, surface acid sites play a more vital role than active oxygen sites. Nevertheless, for CO oxidation, the active oxygen sites play a more vital role than the acid sites.

The controlled engineering of surface oxygen defects on Bi2Zr2O7 compounds for catalytic soot combustion by adjusting the preparation methods.

The quantity of surface oxygen vacancies/defects is critical to promote the reactivity of metal oxide catalysts. Therefore, for the controlled engineering of Bi2Zr2O7 with rich surface defects for soot combustion, four different methods have been adopted. Bi2Zr2O7 compounds with a defective fluorite phase but with varied surface vacancy concentrations have been successfully synthesized by various methods. The best catalyst (Bi2Zr2O7-CP) was fabricated by a facile co-precipitation method. Both O2- and O22- were the active surface sites whose number positively correlated to the number of surface oxygen vacancies and determined the activity. Moreover, a sample with more surface vacancies usually had weaker Zr-O bonds, which could be the intrinsic factor to enhance the activity. In addition, a novel and simple method has been developed to accurately titrate the absolute amount of soot reactive oxygen sites and calculate the TOF values. In conclusion, by optimizing the preparation methods, Bi2Zr2O7 catalysts with rich surface defects can be tuned, which may help in designing more applicable soot oxidation catalysts.

Preparation of La2Zr2O7 composite oxides with fluorite/pyrochlore phases by excluding element influences for catalyzing oxidative coupling of methane.

To elucidate the effect of the A2B2O7 phase on the oxidative coupling of methane (OCM) while excluding elemental influences, La2Zr2O7 compounds with a disordered defect fluorite (La2Zr2O7-F) structure and an ordered pyrochlore phase (La2Zr2O7-P) have been synthesized. Irrespective of their element composition, the catalytic performance of La2Zr2O7-F exceeds that of La2Zr2O7-P. Furthermore, the La2Zr2O7-F surface has more oxygen vacancies/defects than the La2Zr2O7 surface because La2Zr2O7-F exhibits a higher lattice disorder degree and lower B-O bond strength, which leads to the formation of more reactive oxygen anions (O2- and O22-) and basic sites for OCM. Isotopic exchange results have testified that surface-active oxygen sites are generated due to the gaseous O2 adsorption/activation occurring on the surface vacancies via both simple and multiple hetero-exchange mechanisms. In conclusion, crystal structure is the primary factor that governs the catalytic performance of A2B2O7 compounds, with the disordered defect fluorite phase being the most optimal structure for OCM.

Fabrication of Ln2Zr2O7 Fluorite and LnAlO3 Perovskite (Ln = La, Nd, Sm) Compounds to Catalyze the OCM Reaction: On the Temperature-Induced Phase Transformation and Oxygen Vacancy.

Through synthesizing Ln2Zr2O7 and LnAlO3 (Ln = La, Nd, Sm) catalysts, the origin of active sites for oxidative coupling of methane (OCM) on A2B2O7 fluorite and ABO3 perovskite compounds has been compared and elucidated. Ln2Zr2O7 catalysts show much better reaction performance than the respective LnAlO3 catalysts at low temperatures (500-600 °C), but the difference will be mitigated significantly above 600 °C. The reaction performance ranks in the order of La2Zr2O7 > Nd2Zr2O7 > Sm2Zr2O7 > LaAlO3 > NdAlO3 > SmAlO3. It is revealed that the unit cell free volume (Vf) plays an important role in affecting the catalytic activity, and the Ln2Zr2O7 catalysts with a disordered defect fluorite phase have inherent oxygen vacancies, which can directly activate gas-phase O2 molecules to generate OCM reactive O2- anions. However, the oxygen vacancies of LnAlO3 with a perovskite structure can only be generated by lattice distortion/transformation above 600 °C. Moreover, Ln2Zr2O7 fluorites have weaker B-O bonds than LnAlO3 perovskites, thus making it easier to generate surface vacancies as well as active O2- sites. The surface alkalinity is intimately relevant to the active oxygen species, which act together to decide the OCM performance on both types of catalysts. Indeed, this explains that LnAlO3 catalysts show much worse performance than Ln2Zr2O7 catalysts below 600 °C, which will be evidently improved at elevated temperatures due to phase transformation.

Defect and strain engineered MoS2/graphene catalyst for an enhanced hydrogen evolution reaction.

Molybdenum disulfide (MoS2) has been demonstrated as a promising non-precious metal electrocatalyst for the hydrogen evolution reaction (HER). However the efficiency of the HER falls short of expectations due to the large inert basal plane and poor electrical conductivity. In order to activate the MoS2 basal plane and enhance the hydrogen evolution reaction (HER) activity, two strategies on the hybrid MoS2/graphene, including intrinsic defects and simultaneous strain engineering, have been systematically investigated based on density functional theory calculations. We firstly investigated the HER activity of a MoS2/graphene hybrid material with seven types of point defect sites, V S, VS2, V Mo, V MoS3, V MoS6, MoS2 and S2Mo. Using the hydrogen adsorption free energy (ΔG H) as the descriptor, results demonstrate that four of these seven defects (V S, V S2, MoS2, V MoS3) act as a catalytic active site for the HER and exhibited superior electrocatalytic activity. More importantly, we found that ΔG H can be further tuned to an ideal value (0 eV) with proper tensile strain, which effectively optimizes and boosts the HER activity, especially for the V S, V S2, V MoS3 defects and MoS2 antisite defects. Our results demonstrated that a proper combination of tensile strain and defect structure is an effective approach to achieve more catalytic active sites and further tune and boost the intrinsic activity of the active sites for HER performance. Furthermore, the emendatory d-band center of metal proves to be an excellent descriptor for determining H adsorption strength on defective MoS2/graphene hybrid material under different strain conditions. In addition, the low kinetic barrier of H2 evolution indicated that the defective MoS2/graphene system exhibited favorable kinetic activity in both the Volmer-Heyrovsky and the Volmer-Tafel mechanism. These results may pave a new way to design novel ultrahigh-performance MoS2-based HER catalysts.

Constructing Y2B2O7 (B = Ti, Sn, Zr, Ce) Compounds to Disclose the Effect of Surface Acidity-Basicity on Product Selectivity for Oxidative Coupling of Methane (OCM).

To investigate the influence of surface acidity and basicity on the OCM reaction, four pure-phase Y2B2O7 (B = Ti, Sn, Zr, Ce) compounds have been purposely constructed. The exquisite phase structure change results in the generation of different amounts of surface active O2- and O- sites, which affects CH4 molecule activation. Furthermore, both Lewis acidic sites and basic sites are formed on the catalysts in different amounts, which are related to the lattice disorder extent and the choice of A- and B-site elements. It is elucidated with strong evidence that the surface basic sites are favorable to C2 product selectivity, but the surface acidic sites lead to deep oxidation of CH4 and the coupling products to form COx. To design and fabricate Y2B2O7 catalysts with better C2 product selectivity for the reaction, a disordered defect fluorite structure should be engineered with A- and B- site elements having appropriate basicity.

The critical roles of hydrophobicity, surface Ru0 and active O2-/O22- sites on toluene combustion on Ru/ZSM-5 with varied Si/Al ratios.

By targeting more feasible catalysts for VOC combustion, 2%Ru/ZSM-5 catalysts were fabricated by supporting RuO2, a relatively cheaper noble metal, onto HZSM-5 supports with varied Si/Al ratios for toluene combustion. The valence state distribution of Ru and the Ru/RuO2-support interaction have been explored and elucidated. It has been revealed that the catalytic activity increases with the increase of the Si/Al ratio in the order 2%Ru/ZSM-5-18 < 2%Ru/ZSM-5-40 < 2%Ru/ZSM-5-72 < 2%Ru/ZSM-5-110 < 2%Ru/ZSM-5-255 < 2%Ru/SiO2-MFI. Interestingly, the hydrophobicity of the samples improves also with the increase in the Si/Al ratio, which impedes H2O adsorption effectively and its competition for the surface-active sites with the reactants. Both RuO2 and Ru0 are detected on all the catalysts, and the Ru0 amount/ratio increases significantly with increasing the Si/Al ratio, which promotes the adsorption/activation of both toluene and O2 molecules. Furthermore, the amount of surface-active O2- and O22- is evidently improved. Therefore, the mixed interaction of higher hydrophobicity, more surface Ru0 and active oxygen sites is the major reason for the enhancement in the activity of a Ru/ZSM-5 having a higher Si/Al ratio. It is concluded that the optimal catalyst can be designed by loading Ru/RuO2 onto an MFI framework structure support with the highest Si content.

Study on the monolayer dispersion behavior of SnO2 on ZSM-5 for NOx-SCR by C3H6: the remarkable promotional effects of air plasma treatment.

Aiming to fabricate more practical catalysts for NOx-SCR with C3H6, SnO2/ZSM-5 having different SnO2 loadings was prepared and treated with DBD air plasma. The dispersion of SnO2 on the H-ZSM-5 support and their interactions were investigated with both experimental methods and DFT calculations. SnO2 displays evident monolayer dispersion behavior, getting a threshold of 0.271 mmol 100 m-2 support. Plasma treatment improves significantly the SnO2 dispersion, hence amplifying the monolayer dispersion threshold to 0.380 mmol 100 m-2. XPS and DFT calculations have testified that plasma treatment strengthens strongly the SnO2-ZSM-5 support interaction, mainly through donating electrons from Sn4+ to Al3+ in the support, thus improving the dispersion of SnO2 at the same loadings. Consequently, the catalytic performance is remarkably improved because of the generation of more abundant surface acid sites and superoxide species devoted to the reaction. The sample having a SnO2 loading near the monolayer dispersion threshold shows the optimal activity in the corresponding catalyst series, demonstrating an evident threshold effect. Over SnO2/ZSM-5, the reaction goes through a Langmuir-Hinshelwood pathway, involving the adsorption and activation of both NO and C3H6 molecules. Surface mono-dentate/bridged-nitrate and carbonate species are the main reaction intermediates.

Niobium oxide promoted with alkali metal nitrates for soot particulate combustion: elucidating the vital role of active surface nitrate groups.

With the target of developing efficient base metal oxide catalysts for soot particulate combustion, Nb2O5 catalysts promoted using different alkali metal nitrates have been prepared via an impregnation method. The activity of all the modified catalysts is better than that of the pure Nb2O5, and follows the sequence of CsNb1-9 > KNb1-9 > NaNb1-9 > LiNb1-9 > Nb2O5. It has been discovered that the original LiNO3 and NaNO3 precursors were decomposed into inert Li2O and Na2O on LiNb1-9 and NaNb1-9 during the calcination process. However, the KNO3 and CsNO3 precursors were intact on KNb1-9 and CsNb1-9 due to the strong stabilization effect of the K+ and Cs+ cations. As confirmed using different means, surface nitrates are the predominant active centers that contribute to the soot oxidation activity, through the redox cycles between nitrate (NO3-) and nitrite (NO2-) groups. Due to the existence of a large quantity of active surface NO3- groups, KNb1-9 and CsNb1-9 thus exhibit a much better reaction performance than LiNb1-9 and NaNb1-9.

Band-Gap Engineering: A New Tool for Tailoring the Activity of Semiconducting Oxide Catalysts for CO Oxidation.

Cation or anion vacancies in semiconducting oxides usually benefit activity for CO oxidation. To study the nature of vacancy engineering for a thermocatalytic reaction, we adopted lattice doping of cations with varied valence states to construct anion and cation vacancies in n-type and p-type semiconducting CeO2 and NiO, respectively. Doping cations can effectively regulate the number of the vacancies, thus tailoring the activity for CO oxidation. The strong correlation of activation energy and specific activity with a catalyst band gap verified that the nature of vacancy engineering for activity of CeO2 and NiO for CO oxidation can be attributed to tailoring of the band gap. The larger the vacancy amount, the smaller the band gap, and the lower the activation energy, thus giving a higher specific activity. Band-gap engineering, widely used for photocatalytic processes, can be a new tool for tailoring the activity of semiconducting oxide catalysts for thermocatalytic reactions.

Band Gap as a Novel Descriptor for the Reactivity of 2D Titanium Dioxide and its Supported Pt Single Atom for Methane Activation.

Ultrathin transition-metal oxides (TMOs) from nonlayered bulk structures are emerging 2D materials. Here we investigate the reactivity of a 2D TMO of varying thickness from first principles. We find that the band gap of the 2D nL-TiO2(110) shows a strong linear correlation with its surface reactivity: the smaller the band gap, the more reactive the surface oxygen; 3L-TiO2(110) has the smallest band gap and the highest reactivity. We further design Pt1 single-atom catalysts (SAC) by substituting a Pt single atom for a surface Ti atom. We find that the band gap of nL-TiO2(110) dictates both chemisorption and dissociation of CH4 on Pt1-nL-TiO2(110): the smaller the band gap, the stronger the adsorption of CH4 and the lower the barrier of heterolytic C-H activation of CH4. We propose that band gap can be a novel and direct descriptor for the reactivity of 2D TMOs and their supported SACs.

Phylogenetic analysis of two goat-origin PCV2 isolates in China.

Complete genome characterization of non-porcine origin Porcine circovirus type 2 (PCV2) was first described in 2014 in China. In the present study, we first identified PCV2 nucleotides in goat samples and the prevalence of PCV2 in goat was 6.15%. However, only two new strains, Goat2014-4 and Goat2014-5, could be completely sequenced. The genome of the strain Goat2014-4, which collected from the goat infected with PPRV, contains 1766 nt; strain Goat2014-5, which originated from a healthy goat, is comprised of 1767 nt. The results showed that they shared the highest nucleotide identity with BDH and the lowest similarity with DK1980PMWSfree strain and they belonged only to genotype PCV2d. Meanwhile, they shared higher homology with porcine-origin PCV2 strains than others. Moreover, a detailed analysis of the capsid amino acid sequences revealed that there were distinct differences for goat2014-4 (708 bp) and goat2014-5 (705 bp); strain Goat2014-4 showed an elongation of two amino acids, and strains Goat2014-5 showed an elongation of one amino acid compared with other reference strains. This is the first report of the genetic analysis of goat-origin PCV2 isolates. It also provides an additional supported evidence for cross-species transmission of PCV2.

MicroRNA-30a-5p promotes replication of porcine circovirus type 2 through enhancing autophagy by targeting 14-3-3.

Accumulating evidence demonstrates that autophagy and microRNAs (miRNAs) play key roles in regulating virus-host interactions and can restrict or facilitate viral replication. In the present study we examined whether a functional relationship exists between autophagy, miRNA and porcine circovirus type 2 (PCV2) infection, using several approaches. We demonstrated that there was a positive correlation between PCV2 infection and autophagy in 3D4/21 cells and autophagy induced by PCV2 infection triggered PCV2 replication. Four miRNA were selected by real-time PCR and further studied, but only miR-30a-5p mimic had a significant effect on PCV2 replication. Overexpression of miR-30a-5p significantly enhanced PCV2 infection and autophagy in a dose-dependent manner. Blockage of miR-30a-5p significantly decreased PCV2 replication. We provided further evidence that miR-30a-5p regulate the link between PCV2 infection and host immune system. Furthermore, miR-30a-5p targeted and regulated 14-3-3 gene, which is a regulator of autophagy. Flow cytometry data demonstrated that miR-30a-5p promotes cell cycle arrest at the G2 phase to regulate PCV2 replication and autophagy by interacting directly with 14-3-3, but not with the PCV2 genome. These data not only provide new insights into virus-host interactions during PCV2 infection but also suggest a potential new antiviral therapeutic strategy against PCV2 infection.

Modifying the Surface of γ-Al2 O3 with Y2 Sn2 O7 Pyrochlore: Monolayer Dispersion Behaviour of Composite Oxides.

To investigate the dispersion behaviour of composite oxides on supports, and to obtain better supports for Pd for CO oxidation, a series of Y2 Sn2 O7 /Al2 O3 composite oxides with different Y2 Sn2 O7 loadings were prepared by a deposition-precipitation method. XRD and X-ray photoelectron spectroscopic extrapolation methods revealed that, similar to single-component metal oxides, composite oxides can also disperse spontaneously on support surfaces to form a monolayer with a certain capacity. The monolayer dispersion capacity/threshold for Y2 Sn2 O7 on the surface of γ-Al2 O3 is 0.109 mmol per 100 m2 γ-Al2 O3 , corresponding to 7.2 wt % Y2 Sn2 O7 loading. This is the first work to demonstrate monolayer dispersion of a composite oxide on a support. After combining Y2 Sn2 O7 with γ-Al2 O3 , active oxygen species can be introduced onto the catalyst surfaces. Thus, the interaction between Pd and the support is strengthened, the dispersion of Pd is improved in comparison with the single-component Y2 Sn2 O7 support, and a synergistic effect is induced between Pd and the composite support, which is beneficial to catalyst activity. By tuning the γ-Al2 O3 surface with different amounts of pyrochlore Y2 Sn2 O7 , CO oxidation activity on 1 % Pd/Y2 Sn2 O7 /Al2 O3 was improved. These findings may provide new insights into the design and preparation of effective supported noble metal catalysts with lower contents of noble metals.

H2 adsorption and dissociation on PdO(101) films supported on rutile TiO2 (110) facet: elucidating the support effect by DFT calculations.

To explore metal oxide-support interactions and their effect, H2 adsorption and dissociation on PdO(101)/TiO2(110) films with different film thicknesses, in comparison with that on pure PdO(101) surface without TiO2(110) support, were studied by density functional theory calculation. A monolayer PdO(101) film supported on TiO2 facet shows different properties to a pure PdO(101) surface. On the monolayer PdO(101)/TiO2(110) film, TiO2 support leads to stronger molecular adsorption of H2 on coordinatively unsaturated Pd top sites than that on a pure PdO surface. H2 dissociation with the formation of OH was preferred thermodynamically but slightly unfavorable kinetically on the monolayer PdO film due to the TiO2 support effect. Graphical abstract On the monolayer PdO(101)/TiO2(110) film, the TiO2 support effect leads to stronger H2 molecular adsorption on coordinatively unsaturated Pd top sites than on pure PdO surface. H2 dissociation with the formation of OH is preferred thermodynamically but slightly unfavorable kinetically on the film due to the TiO2 support effect.

O2 adsorption on MO2 (M=Ru, Ir, Sn) films supported on rutile TiO2(110) by DFT calculations: Probing the nature of metal oxide-support interaction.

To explore metal oxide-support interaction and its effect on O2 adsorption, periodic DFT calculations were used to explore the most preferred O2 molecular and dissociative adsorption on stoichiometric (MO2) and defective (MO2-x) (M=Ru, Ir, Sn) films supported on rutile TiO2(110), and compared with that on pure surfaces without TiO2(110) support. For defective RuO2-x films, it is revealed that the TiO2(110) support and the film thickness have an evident impact on the O2 adsorbed species. On the contrary, the two factors show little influence for defective IrO2-x and SnO2-x films. The analyses for Bader charge and density of states indicate that the reducibility change of the unsaturated surface Ru atoms, which are adjacent to the bridge oxygen vacancies, is responsible for this O2 adsorption alteration. These results provide insights into the oxide-oxide interaction, and its effect on the properties of supported oxide catalysts.