Importance, features and uses of metal oxide catalysts in heterogeneous catalysis

This short review paper aims at assembling the present state of the art of the multiuses of metal oxides in heterogeneous catalysis, concerning liquid and gaseous phases of the reactant mixtures on solid catalysts. It includes the description of the main types of metal oxide catalysts, of their various preparation procedures and of the main reactions catalysed by them (acid-base type, selective and total oxidations, bi-functional catalysis, photocatalysis, biomass treatments, environmental catalysis and some of the numerous industrial applications). Challenges and prospectives are also discussed.

TiO_2-Supported Binary Metal Oxide Catalysts for Low-temperature Selective Catalytic Reduction of NO_x with NH_3

Binary metal oxide（MnOx-A/TiO2）catalysts were prepared by adding the second metal to manganese oxides supported on titanium dioxide（TiO2）,where,A indicates Fe2O3,WO3,MoO3,and Cr2O3.Their catalytic activity,N2 selectivity,and SO2 poisonous tolerance were investigated.The catalytic performance at low temperatures decreased in the following order：Mn-W/TiO2〉Mn-Fe/TiO2〉Mn-Cr/TiO2〉Mn-Mo/TiO2,whereas the N2 selectivity decreased in the order：Mn-Fe/TiO2〉Mn-W/TiO2〉Mn-Mo/TiO2〉Mn-Cr/TiO2.In the presence of 0.01%SO2 and 6%H2O,the NOx conversions in the presence of Mn-W/TiO2,Mn-Fe/TiO2,or Mn-Mo/TiO2 maintain 98.5%,95.8%and 94.2%, respectively,after 8 h at 120°C at GHSV 12600 h？ 1 .As effective promoters,WO3 and Fe2O3 can increase N2 selectivity and the resistance to SO2 of MnOx/TiO2 significantly.The Fourier transform infrared（FTIR）spectra of NH3 over WO3 show the presence of Lewis acid sites.The results suggest that WO3 is the best promoter of MnOx/TiO2,and Mn-W/TiO2 is one of the most active catalysts for the low temperature selective catalytic reduction of NO with NH3.

A NEW PROBE FOR IDENTIFYING PROPERTIES OF SUPPORTED METAL AND METAL OXIDE CATALYSTS

A novel molecular probe for identifying properties of supported transition metals and metal oxides catalysts was established.The catalytic mechanism of transition metals was proposed.

METHANE OXIDATIVE COUPLING OVER METAL OXYFLUORIDE CATALYSTS

The MOC reaction over ZrO_2/LaF_3, CeO_2/LaF_3 and ThO_2/LaF_3 catalysts indicated that these catalysts had high activity and high C_2 selectivity at low temperature. In the temperature range 480℃ to 650℃. The methane conversion was 24. 4% to 30. 8% and the C_2 selectivity was 40. 0% to 55. 4%. The XRD characterization of the catalysts indicated that O^2. and F exchang happened and LaOF was formed.

Progress in research on catalysts for catalytic oxidation of formaldehyde

Formaldehyde（HCHO）is carcinogenic and teratogenic,and is therefore a serious danger to human health.It also adversely affects air quality.Catalytic oxidation is an efficient technique for removing HCHO.The development of highly efficient and stable catalysts that can completely convert HCHO at low temperatures,even room temperature,is important.Supported Pt and Pd catalysts can completely convert HCHO at room temperature,but their industrial applications are limited because they are expensive.The catalytic activities in HCHO oxidation of transition-metal oxide catalysts such as manganese and cobalt oxides with unusual morphologies are better than those of traditional MnO2,Co3O4,or other metal oxides.This is attributed to their specific structures,high specific surface areas,and other factors such as active phase,reducibility,and amount of surface active oxygens.Such catalysts with various morphologies have great potential and can also be used as catalyst supports.The loading of relatively cheap Ag or Au on transition-metal oxides with special morphologies potentially improves the catalytic activity in HCHO removal at room temperature.The preparation and development of new nanocatalysts with various morphologies and structures is important for HCHO removal.In this paper,research progress on precious-metal and transition-metal oxide catalyst systems for HCHO oxidation is reviewed; topics such as oxidation properties,structure–activity relationships,and factors influencing the catalytic activity and reaction mechanism are discussed.Future prospects and directions for the development of such catalysts are also covered.

Effect of Different Dopant in the Mo-V-Te-O Catalyst on the Performance of Selective Oxidation Propane to Acrolein

Several Mo-V-Te-O mixed metal oxides catalysts with different dopant were prepared and used for catalytic oxidation propane to acrolein. It was revealed that the addition of P could greatly improve the performance of the Mo-V-Te-O catalyst. The catalysts were examined by XRD and H2-TPR. The XRD characteristic of the Mo-V-Te-P-O showed that the addition of P could aggrandize the (V0.07Mo0.93)5O14 phase. H2-TPR illuminated that the MoV0.3Te0.23P0.15On catalyst took on the best redox ability.

Alloy strategy to synthesize Pt-early transition metal oxide interfacial catalysts

Metal oxide supported metal catalysts show promising catalytic performance in many industry-relevant reactions.However,the enhancement of performance is often limited by the insufficient metal/metal oxide interface.In this work,we demonstrate a general synthesis of Pt-early transition metal oxide(Pt-MO_(x),M=Ti,Zr,V,and Y)catalysts with rich interfacial sites,which is based on the air-induced surface segregation and oxidation of M in the supported Pt-M alloy catalysts.Systematic characterizations verify the dynamic structural response of Pt-M alloy catalysts to air and the formation of Pt-MO_(x) catalysts with abundant interfacial sites.The prepared Pt-TiO_(x) interfacial catalysts exhibit improved performance in hydrogenation reactions of benzaldehyde,nitrobenzene,styrene,and furfural,as a result of the heterolytic dissociation of H_(2) at Pt-metal oxide interfacial sites.

Investigation of N-（Hydroxylethyl）pyrrolidone Dehydration over REOx-doped Nano-ZrO2 Catalyst

Catalyst plays an important role in the dehydration of N-（hydroxylethyl）pyrrolidone （NHP） to prepare N-vinyl-pyrrolidone （NVP）. At present, NVP yield is only about 30% on commercial ZrO2 catalyst. A coupled precipitation and solid dispersion technique was designed to prepare the nano-ZrO2 catalyst, in which rare earth metal oxides （REOx） was used as electronic promoter. The results indicated that the catalyst doped REOx （S-1.0） exhibits the optimum performance of NHP dehydration at moderate conditions. NHP conversion and NVP selectivity are respectively 97.0%, 82.3%. Of special interest is that the indexes of the catalyst （S-1.0-1.0） are up to 98.4% and 89.2% respectively. Furthermore, this catalyst bears the good stability. It means that nano-ZrO2 doped REOx catalyst might be a potential commercial catalyst for the NHP dehydration.

Catalytic behavior of Mo-Bi-Fe-Co-K-M-O(M=Ce,Gd,CeGd)catalysts for selective oxidation of isobutene

The further improvement of methacrolein(MAL)selectivity from isobutene(IB)oxidation is crucial and challenging.In this study,based on the typical Mo-Bi-Fe-Co-K-O mixed metal oxide,the rare earth element Gd-doped,Ce-doped and CeGd co-doped catalysts were prepared by co-precipitation strategy to increase the selectivity of MAL from 47.9%to 49.8%,64.2% and 68.6%,respectively.In order to elucidate in-depth the promoting effect of Ce and/or Gd,various characterizations were utilized including X-ray diffraction patterns(XRD),Raman,X-ray fluorescence spectrometry(XRF),X-ray photoelectron spectroscopy(XPS),O_(2)-temperature programmed desorption(O_(2)-TPD),H2-temperature programmed reduction(H2-TPR),CO_(2)-temperature programmed desorption(CO_(2)-TPD),IB-temperature programmed desorption(i-C4-TPD)and in-situ IB-Fourier transform infrared spectroscopy(IB-FTIR).Both Ce and Gd finely regulate the bulk and surface structure of the catalyst,thus altering the redox ability,oxygen mobility and storage ability and basicity.Compared with Ce,Gd addition slightly regulates the variation of Co^(2+)/Co^(3+)redox couples,greatly enhances the interaction among the components on the catalyst,thus only increases the content of surface oxygen species and has little effect on their mobility.While Cecontaining catalyst performs stronger oxygen storage and migration ability,thus leading to the overproduction of surface Odefectspecies,which are proposed to be the active sites for the production of MAL and COx.The CeGd co-doped catalyst possesses the proper content of surface Odefectspecies,thus exhibits much higher MAL selectivity.Moreover,the promoting mechanism of Ce and/or Gd over IB oxidation is proposed.Therefore,this work is helpful for understanding the influence of rare earth elements on the structure of mixed metal oxides and the olefin selective oxidation reaction.

Low-temperature conversion of ammonia to nitrogen in water with ozone over composite metal oxide catalyst

As one of the most important water pollutants, ammonia nitrogen emissions have increased year by year, which has attracted people＇s attention. Catalytic ozonation technology, which involves production of ·OH radical with strong oxidation ability, is widely used in the treatment of organic-containing wastewater. In this work, MgO-Co3O4 composite metal oxide catalysts prepared with different fabrication conditions have been systematically evaluated and compared in the catalytic ozonation of ammonia（50 mg/L） in water. In terms of high catalytic activity in ammonia decomposition and high selectivity for gaseous nitrogen, the catalyst with MgO-Co3O4 molar ratio 8：2, calcined at 500°C for 3 hr, was the best one among the catalysts we tested, with an ammonia nitrogen removal rate of 85.2% and gaseous nitrogen selectivity of44.8%. In addition, the reaction mechanism of ozonation oxidative decomposition of ammonia nitrogen in water with the metal oxide catalysts was discussed. Moreover, the effect of coexisting anions on the degradation of ammonia was studied, finding that SO2-4 and HCO-3 could inhibit the catalytic activity while CO2-3 and Br-could promote it. The presence of coexisting cations had very little effect on the catalytic ozonation of ammonia nitrogen. After five successive reuses, the catalyst remained stable in the catalytic ozonation of ammonia.

Mo-modified Pd/Al_2O_3 catalysts for benzene catalytic combustion

Mo-modified Pd/Al2O3catalysts were prepared by an impregnation method and tested for the catalytic combustion of benzene. The catalysts were characterized by N2 isothermal adsorption, X-ray diffraction（XRD）, X-ray photoelectron spectroscopy（XPS）, temperatureprogrammed desorption of NH3（NH3-TPD）, H2temperature-programmed reduction（H2-TPR）, and high-angle annular dark-field scanning transmission electron microscopy（HAADF-STEM）. The results showed that the addition of Mo effectively improved the activity and stability of the Pd/Al2O3catalyst by increasing the dispersion of Pd active components, changing the partial oxidation state of palladium and increasing the oxygen species concentration on the surface of catalyst. In the case of the Pd-Mo/Al2O3catalyst,benzene conversion of 90% was obtained at temperatures as low as 190°C, which was 45°C lower than that for similar performance with the Pd/Al2O3catalyst. Moreover, the 1.0% Pd-5% Mo/Al2O3catalyst was more active than the 2.0% Pd/Al2O3catalyst. It was concluded that Pd and Mo have a synergistic effect in benzene catalytic combustion.

Tuning the local electronic structure of oxygen vacancies over copper-doped zinc oxide for efficient CO_(2) electroreduction

Oxygen vacancies in metal oxides can serve as electron trap centers to capture CO_(2) and lower energy barriers for the electrochemical CO_(2) reduction reaction(CO_(2)RR).Under aqueous electrolytes,however,such charge-enriched active sites can be occupied by adsorbed hydrogen(H∗)and lose their effectiveness for the CO_(2)RR.Here,we develop an efficient catalyst consisting of Cu-doped,defect-rich ZnO(Cu–ZnO)for the CO_(2)RR,which exhibits enhanced CO Faradaic efficiency and current density compared to pristine ZnO.The introduced Cu dopants simultaneously stabilize neighboring oxygen vacancies and modulate their local electronic structure,achieving inhibition of hydrogen evolution and acceleration of the CO_(2)RR.In a flow cell test,a current density of more than 45​mA​cm^(−2) and a CO Faradaic efficiency of>80%is obtained for a Cu–ZnO electrocatalyst in the wide potential range of−0.76​V to−1.06​V vs.Reversible Hydrogen Electrode(RHE).This work opens up great opportunities for dopant-modulated metal oxide catalysts for the CO_(2)RR.