The effect of Mn on the catalytic performance of V2O5/TiO2 catalyst for the selective catalytic reduction of NOx by NH3 (NH3-SCR) has been investigated in this study. It was found that the added Mn significantly enhanced the activity of V2O5/TiO2 catalyst for NH3-SCR below 400 °C. The redox cycle (V4+ + Mn4+ ↔ V5+ + Mn3+) over Mn-promoted V2O5/TiO2 catalyst plays a key role for the high catalytic deNOx performance. The redox cycle promotes the adsorption and activation of NH3 and NO, forming more reactive intermediates (NH4+, coordinated NH3, NO2, and monodentate nitrate species), thus promoting the NH3-SCR to proceed.

•The addition of CeO2 enhanced the activity of NiO catalyst for N2O decomposition.•The co-presence of NiO and CeO2 exhibited a synergetic effect.•The synergetic effect contributes to forming the stabilized Ni2+ active sites.A series of NiO-CeO2 mixed oxide catalysts prepared by the hydrothermal method were investigated for the decomposition of N2O. It was found that the addition CeO2 to NiO catalyst leads to a noticeable enhancement of the activity for the decomposition of N2O. The introduction of CeO2 inhibited the crystallization of the NiO phase, leading to the high surface area. In particular, the co-presence of NiO and CeO2 exhibited a synergetic effect, which contributes to forming the stabilized Ni2+ active sites. All of these factors, collectively, accounted for the high activity of NiO-CeO2 mixed oxide catalyst for the decomposition of N2O.Download high-res image (89KB)Download full-size image

•CuO-CeO2 catalyst is more active than pure CuO and CeO2 for N2O decomposition.•The co-existence of CuO and CeO2 exhibited a synergetic effect.•The synergetic effect contributes to the formation of Cu+ and Ce3+ active sites.•The desorption of surface adsorbed oxygen is promoted over CuO-CeO2 catalyst.The catalytic decomposition of N2O was investigated over a series of CuO-CeO2 mixed oxide catalysts prepared by the hydrothermal method. It was found that CuO-CeO2 mixed oxide catalyst exhibited higher activity than pure CuO and CeO2. The co-existence of CuO and CeO2 exhibited a synergetic effect, which inhibited the crystallization of the CuO phase, leading to the high surface area. Moreover, the two redox couples (Ce4+/Ce3+ and Cu2+/Cu+) formed over CuO-CeO2 mixed oxide contribute to the desorption of surface adsorbed oxygen species and thus the regeneration the active sites, which is favorable for the catalytic N2O decomposition to proceed.Download high-res image (115KB)Download full-size image

An environmentally benign Fe–Ce–Ti mixed oxide catalyst, which was prepared via the hydrothermal method, has been investigated for the selective catalytic reduction of NOx with NH3 (NH3-SCR). It was found that the Fe–Ce–Ti catalyst exhibited excellent NH3-SCR activity, high N2 selectivity and strong resistance against H2O and SO2 with a wide operation temperature window. XRD and Raman spectra suggest that the Fe–Ce–Ti catalyst has an amorphous structure. The co-presence of Fe and Ce induced the formation of a redox cycle (Ce4+ + Fe2+ ↔ Ce3+ + Fe3+), which promotes the activation of NO and NH3. In situ DRIFTS studies demonstrate that the synergetic effect between Fe and Ce contributes to the formation of reactive intermediate species, thus leading to the high catalytic deNOx performance of the Fe–Ce–Ti mixed oxide catalyst.

A series of Pt–Au/CeO2 catalysts were prepared via the impregnation deposition–precipitation (IDP) and reduction–deposition precipitation (RDP) methods. The performances of the catalysts for the simultaneous removal of carbon monoxide (CO) and formaldehyde (HCHO) at room temperature were evaluated. The results show that the Pt–Au/CeO2 catalyst, which was prepared via the RDP method, exhibited higher catalytic activity. The catalyst characterization results reveal that two factors accounted for this phenomenon. The first factor is that more negatively charged metallic Pt nanoparticles were obtained during the liquid phase NaBH4 reduction treatment preparation process and the second factor is that more Au+ species were formed using urea as the precipitant in the Au deposition–precipitation. The larger number of negatively charged metallic Pt nanoparticles and Au+ species resulted in abundant chemisorbed oxygen, which contributed to the co-oxidation of HCHO and CO. In addition, water exhibited a negative effect on the simultaneous removal of CO and HCHO. Based upon these results, a possible mechanism for the simultaneous removal of CO and HCHO at room temperature is also proposed.

Catalytic decomposition of nitrous oxide (N2O) is one of the most efficient methods for the removal of N2O which is of high greenhouse potential and ozone-depleting property. Recent progress in the decomposition of N2O has been reviewed with the focus on noble meal and metal oxide catalysts. The influence factors, such as catalyst support, preparation method, alkali metal additives and the reaction conditions (including O2, H2O, SO2, NO and CO2), on the performance of deN2O catalysts have been discussed. Finally, future research direction for the catalytic decomposition of N2O is proposed.

Mn–Ce–Ti mixed-oxide catalyst prepared by the hydrothermal method was investigated for the selective catalytic reduction (SCR) of NOx with NH3 in the presence of oxygen. It was found that the environmentally benign Mn–Ce–Ti catalyst exhibited excellent NH3-SCR activity and strong resistance against H2O and SO2 with a broad operation temperature window, which is very competitive for the practical application in controlling the NOx emission from diesel engines. On the basis of the catalyst characterization, the dual redox cycles (Mn4+ + Ce3+ ↔ Mn3+ + Ce4+, Mn4+ + Ti3+ ↔ Mn3+ + Ti4+) and the amorphous structure play key roles for the high catalytic deNOx performance. Diffuse reflectance infrared Fourier transform spectroscopy studies showed that the synergetic effect between Mn and Ce contributes to the formation of reactive intermediate species, thus promoting the NH3-SCR to proceed.Keywords: in situ drifts; manganese−cerium−titanium mixed oxide; nitrogen oxides; redox cycle; selective catalytic reduction

A series of TiO2 supported Pt–Au bimetallic catalysts were prepared by impregnation, deposition–precipitation and impregnation–deposition–precipitation, and their catalytic activities for the co-oxidation of formaldehyde (HCHO) and carbon monoxide (CO) were evaluated at room temperature. The results show that the Pt–Au/TiO2 catalyst prepared via introducing Pt by impregnation and subsequently introducing Au by deposition–precipitation, exhibits excellent catalytic performance for the co-oxidation of HCHO and CO. The characterizations of the catalyst by means of Transmission Electron Microscopy (TEM), X-ray Photoelectron Spectroscopy (XPS), Temperature Programmed Reduction (TPR) and in situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (in situ DRIFTS) revealed that the isolated Pt and Au sites are essential to the co-oxidation of HCHO and CO, because the HCHO oxidation occurring over Pt active sites while CO oxidation occurring over Au active sites can be conducted without mutual interference, thus simultaneously achieving both the high oxidation activity of HCHO and CO.

A series of Ni, Sn and Ca modified Pd/TiO2-Al2O3 catalysts were prepared by the incipient wetness impregnation method and their catalytic performance for the selective catalytic reduction of NOx by H2 was evaluated. The results showed that the NOx conversion and N2 selectivity were improved over Pd-Sn/TiO2-Al2O3 and Pd-Ni/TiO2-Al2O3 catalysts above 200 °C. More importantly, the N2 selectivity and high-temperature activity of Pd-Sn/TiO2-Al2O3 catalyst was far superior to the single Pd/TiO2-Al2O3 catalyst. The optimal Sn loading was 2 wt.%. X-ray diffraction (XRD) results showed that the interaction between Pd and Sn promotes the dispersion of Pd over TiO2-Al2O3. Temperature-programmed reduction (H2-TPR) results demonstrated that the addition of Sn contributes to the formation of Pd0 and improving the redox property of Pd/TiO2-Al2O3. The additives of Ni and Sn also facilitated the absorption of NOx and the oxidation of NO to NO2, which play important roles in the selective catalytic reduction of NOx by hydrogen.

The effect of Zr on the catalytic performance of Pd/γ-Al2O3 for the methane combustion was investigated. The results show that the addition of Zr can improve the activity and stability of Pd/γ-Al2O3 catalyst, which, based on the catalyst characterization (N2 adsorption, XRD, CO-Chemisorption, XPS, CH4-TPR and O2-TPO), is ascribed to the interaction between Pd and Zr. The active phase of methane combustion over supported palladium catalyst is the Pd0/Pd2+ mixture. Zr addition inhibits Pd aggregation and enhances the redox properties of active phase Pd0/Pd2+. H2 reduction could effectively reduce the oxidation degree of Pd species and regenerate the active sites (Pd0/Pd2+).

An environmentally benign Cu–Ce–Ti oxide catalyst exhibited excellent NH3-SCR activity, high N2 selectivity and strong resistance against H2O and SO2 with a broad operation temperature window. The dual redox cycles (Cu2+ + Ce3+ ↔ Cu+ + Ce4+, Cu2+ + Ti3+ ↔ Cu+ + Ti4+) play key roles for the superior catalytic deNOx performance.

The adsorption of NO and NO2 on Al2O3(100), SnO2(110) as well as Al2O3(100) supported SnO2 cluster has been investigated using first principle density functional theory calculations. It was found that there is a strong interaction between the SnO2 cluster and the Al2O3(100) surface. The SnO2 cluster dispersed on the Al2O3 surface provides strong binding sites for the NOx adsorption. Compared with Al2O3(100) and SnO2(110) surfaces, both NO and NO2 adsorption and activation are promoted over the Al2O3(100) supported SnO2 cluster.

Selective catalytic reduction of NOx by H2 in the presence of oxygen has been investigated over Pt/ Al2O3 catalysts pre-treated under different conditions. Catalyst preparation conditions exert significant influence on the catalytic performance, and the catalyst pre-treated by H2 or H2 then followed by O2 is much more active than that pre-treated by air. The higher surface area and the presence of metallic Pt over Pt/Al2O3 pre-treated by H2 or pretreated by H2 then followed by O2 can contribute to the formation of NO2, which then promotes the reaction to proceed at low temperatures.

A series of Co exchanged zeolites with ZSM-5, BEA, MOR and USY structures were prepared and investigated for N2O catalytic decomposition under identical reaction conditions. It is found that Co-zeolites with different structures show dramatically different catalytic activities, which could be attributed to various Co species formed in them. Co-ZSM-5, Co-BEA and Co-MOR exhibit much higher activities than Co-USY catalysts, which is attributed to the predominant formation of active isolated Co2+ ions in the ion exchange positions; while in Co-USY Co mainly exists as less active Co oxides. Moreover, it is observed that the activities of Co2+ ions in ZSM-5, BEA and MOR zeolites are quite different and are related to the specific Co ion sites presented in each zeolite structure. In Co-ZSM-5, the most active sites are the α-type Co ions, which are weakly coordinated to framework oxygens in the straight channel. On the other hand, in Co-BEA and Co-MOR, the most active sites are β-type Co ions, which are coordinated to the framework oxygens of the elongated six-membered ring of BEA and the interconnected small channel of MOR, respectively. The main factors affecting the activities of these individual Co ions are indicated to be their location in the zeolite structure, their chemical coordination and the distances between the Co ions. The highest activity of the α-type Co ions in ZSM-5 could be attributed to their favorite location in the zeolite and weak coordination to framework oxygens, which make them easily accessible and coordinated to reactants. The large number of β-sites and their structural arrangement in MOR allow the formation of two unique adjacent β-Co ions in Co–Co pairs, which cooperate in N2O splitting, consequently yielding the high activity of β-Co ions in MOR.

The direct synthesis of dimethyl carbonate (DMC) from CO2 and methanol is one attractive way for the reduction of greenhouse gas emission and the utilization of carbon resources. Recent progress in the direct synthesis of DMC from CO2 and methanol is reviewed with the focus on the catalyst systems, including organic metal compounds catalyst, base catalyst, acetate catalyst, metal oxide and supported metal oxide catalysts, heteropolyacid catalyst and photocatalyst. Moreover, the application of supercritical system, ionic liquid system, electrochemical system, membrane reactor and nitriles hydration in the direct synthesis of DMC are also introduced. Finally, future research direction in this area is proposed.

Al2O3 and its supported metal catalysts are widely used in deNOx catalysis, but the true nature of the catalytic sites and the structure−activity relationships are still unclear. By a set of systematic and comparative calculations, this study investigates the adsorption of NO and NO2, and nitrate formation via the oxidation of NO on Al2O3 and Ga modified Al2O3 surfaces using density functional theory. It is found that NOx gases (NO and NO2) preferentially adsorb on (110) planes, and are oriented in different configurations. While NO bonds with the (110) surfaces through an N-down orientation, the most stable mode of adsorption of NO2 on the (110) surfaces is a bidentate configuration, causing much higher net charge transfer from the surface and noticeable elongation of the N−O bond. Both the NO and NO2 adsorption and activation are promoted on the Ga modified Al2O3 (110) surface. Moreover, the activation energy barrier for nitrate formation via NO oxidation, a process crucial for the selective catalytic reduction of NOx, is about 35% less on the Ga modified Al2O3 (110) surface compared to the pristine Al2O3 (110) surface. This is one of the reasons for the high activity of Ga2O3−Al2O3 catalyst for the selective catalytic reduction of NOx.

An environmentally benign Fe–Ce–Ti mixed oxide catalyst, which was prepared via the hydrothermal method, has been investigated for the selective catalytic reduction of NOx with NH3 (NH3-SCR). It was found that the Fe–Ce–Ti catalyst exhibited excellent NH3-SCR activity, high N2 selectivity and strong resistance against H2O and SO2 with a wide operation temperature window. XRD and Raman spectra suggest that the Fe–Ce–Ti catalyst has an amorphous structure. The co-presence of Fe and Ce induced the formation of a redox cycle (Ce4+ + Fe2+ ↔ Ce3+ + Fe3+), which promotes the activation of NO and NH3. In situ DRIFTS studies demonstrate that the synergetic effect between Fe and Ce contributes to the formation of reactive intermediate species, thus leading to the high catalytic deNOx performance of the Fe–Ce–Ti mixed oxide catalyst.

A series of Co exchanged zeolites with ZSM-5, BEA, MOR and USY structures were prepared and investigated for N2O catalytic decomposition under identical reaction conditions. It is found that Co-zeolites with different structures show dramatically different catalytic activities, which could be attributed to various Co species formed in them. Co-ZSM-5, Co-BEA and Co-MOR exhibit much higher activities than Co-USY catalysts, which is attributed to the predominant formation of active isolated Co2+ ions in the ion exchange positions; while in Co-USY Co mainly exists as less active Co oxides. Moreover, it is observed that the activities of Co2+ ions in ZSM-5, BEA and MOR zeolites are quite different and are related to the specific Co ion sites presented in each zeolite structure. In Co-ZSM-5, the most active sites are the α-type Co ions, which are weakly coordinated to framework oxygens in the straight channel. On the other hand, in Co-BEA and Co-MOR, the most active sites are β-type Co ions, which are coordinated to the framework oxygens of the elongated six-membered ring of BEA and the interconnected small channel of MOR, respectively. The main factors affecting the activities of these individual Co ions are indicated to be their location in the zeolite structure, their chemical coordination and the distances between the Co ions. The highest activity of the α-type Co ions in ZSM-5 could be attributed to their favorite location in the zeolite and weak coordination to framework oxygens, which make them easily accessible and coordinated to reactants. The large number of β-sites and their structural arrangement in MOR allow the formation of two unique adjacent β-Co ions in Co–Co pairs, which cooperate in N2O splitting, consequently yielding the high activity of β-Co ions in MOR.

A series of TiO2 supported Pt–Au bimetallic catalysts were prepared by impregnation, deposition–precipitation and impregnation–deposition–precipitation, and their catalytic activities for the co-oxidation of formaldehyde (HCHO) and carbon monoxide (CO) were evaluated at room temperature. The results show that the Pt–Au/TiO2 catalyst prepared via introducing Pt by impregnation and subsequently introducing Au by deposition–precipitation, exhibits excellent catalytic performance for the co-oxidation of HCHO and CO. The characterizations of the catalyst by means of Transmission Electron Microscopy (TEM), X-ray Photoelectron Spectroscopy (XPS), Temperature Programmed Reduction (TPR) and in situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (in situ DRIFTS) revealed that the isolated Pt and Au sites are essential to the co-oxidation of HCHO and CO, because the HCHO oxidation occurring over Pt active sites while CO oxidation occurring over Au active sites can be conducted without mutual interference, thus simultaneously achieving both the high oxidation activity of HCHO and CO.

A series of Pt–Au/CeO2 catalysts were prepared via the impregnation deposition–precipitation (IDP) and reduction–deposition precipitation (RDP) methods. The performances of the catalysts for the simultaneous removal of carbon monoxide (CO) and formaldehyde (HCHO) at room temperature were evaluated. The results show that the Pt–Au/CeO2 catalyst, which was prepared via the RDP method, exhibited higher catalytic activity. The catalyst characterization results reveal that two factors accounted for this phenomenon. The first factor is that more negatively charged metallic Pt nanoparticles were obtained during the liquid phase NaBH4 reduction treatment preparation process and the second factor is that more Au+ species were formed using urea as the precipitant in the Au deposition–precipitation. The larger number of negatively charged metallic Pt nanoparticles and Au+ species resulted in abundant chemisorbed oxygen, which contributed to the co-oxidation of HCHO and CO. In addition, water exhibited a negative effect on the simultaneous removal of CO and HCHO. Based upon these results, a possible mechanism for the simultaneous removal of CO and HCHO at room temperature is also proposed.

A series of Ce–Sb binary oxide catalysts prepared by the citric acid method have been investigated for the selective catalytic reduction (SCR) of NOx with NH3. It was found that the Ce–Sb oxide catalysts showed excellent NH3-SCR activity and a high tolerance to H2O and SO2 in a wide operation temperature window. N2-adsorption, X-ray diffraction (XRD), Raman spectroscopy, H2 temperature programmed reduction (H2-TPR), NH3 temperature programmed desorption (NH3-TPD), X-ray photoelectron spectroscopy (XPS) and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) were conducted to correlate the catalyst structure and surface properties to the catalytic performance of Ce–Sb binary oxide catalyst. The strong interaction between Sb and Ce species not only enhances the redox property of the catalyst but also increases the surface acidity, thus promoting the adsorption and activation of NH3 species, which is favorable for high NH3-SCR performance.

An environmentally benign Cu–Ce–Ti oxide catalyst exhibited excellent NH3-SCR activity, high N2 selectivity and strong resistance against H2O and SO2 with a broad operation temperature window. The dual redox cycles (Cu2+ + Ce3+ ↔ Cu+ + Ce4+, Cu2+ + Ti3+ ↔ Cu+ + Ti4+) play key roles for the superior catalytic deNOx performance.