A comparative study of TiO2 supported noble metal catalysts for the oxidation of formaldehyde at room temperature

The TiO₂ supported noble metal (Au, Rh, Pd and Pt) catalysts were prepared by impregnation method and characterized by means of X-ray diffraction (XRD) and BET. These catalysts were tested for the catalytic oxidation of formaldehyde (HCHO). It was found that the order of activity was Pt/TiO₂  Rh/TiO₂ > Pd/TiO₂ > Au/TiO₂  TiO₂. HCHO could be completely oxidized into CO₂ and H₂O over Pt/TiO₂ in a gas hourly space velocity (GHSV) of 50,000 h⁻¹ even at room temperature. In contrast, the other catalysts were much less effective for HCHO oxidation at the same reaction conditions. HCHO conversion to CO₂ was only 20% over the Rh/TiO₂ at 20 °C. The Pd/TiO₂ and Au/TiO₂ showed no activities for HCHO oxidation at 20 °C. The different activities of the noble metals for HCHO oxidation were studied with respect to the behavior of adsorbed species on the catalysts surface at room temperature using in situ DRIFTS. The results show that the activities of the TiO₂ supported Pt, Rh, Pd and Au catalysts for HCHO oxidation are closely related to their capacities for the formation of formate species and the formate decomposition into CO species. Based on in situ DRIFTS studies, a simplified reaction scheme of HCHO oxidation was also proposed.     

Low-temperature catalytic combustion of methanol and its decomposed derivatives over supported gold catalysts

Gold can be compared favorably with Pd and Pt in the catalytic combustion of CH₃OH, HCHO and HCOOH when it is deposited on some reducible metal oxides (-Fe₂O₃, TiO₂, etc.). While the supported gold catalysts are less active in H₂ oxidation, they exhibit much higher activities in CO oxidation. For Au/TiO₂, the effect of catalyst preparation was further investigated. Since the activity for CO oxidation of the gold catalysts is not depressed but enhanced by moisture, they are practically applicable to CO removal from air at room temperature. Gold supported on manganese oxide is especially effective in the selective CO removal from hydrogen, indicating its potential applicability to polymer electrolyte fuel cells using the reformed gas of methanol.     

The nature of activity enhancement for propane oxidation over supported Pt catalysts exposed to sulphur dioxide

The nature of the active sites, the role of the support, and the mechanism by which hydrocarbons are activated over supported Pt catalysts have been investigated for the combustion of propane in the presence and absence of SO₂. A strong enhancement in the activity for propane oxidation has been confirmed either when SO₂ is introduced with the propane or with a pre-sulphated alumina-supported catalyst. No equivalent effects were found with silica-supported catalysts. Fluorination of the alumina support also leads to an increase in activity. The addition of pulses of SO₂ into the propane-containing gas stream produces a very large, but short-lived, increase in activity in addition to a more gradual and progressive activity enhancement. Reasons for these different effects are discussed. Attempts to correlate the permanent enhancement in activity with the total acidity of the support were unsuccessful. It appears that the increase in activity is due to a more subtle effect and a model is presented in which the possible role of perimeter sites at the metal–support interface is emphasised.     

Catalytic decomposition of methylene chloride by sulfated oxide catalysts

Catalytic decomposition of methylene chloride in air with a concentration of 959 ppm and temperature ranges from 160 to 275°C were studied. Three different sulfated oxide catalysts, TiO₂(SO₄), ZrO₂(SO₄), CeO₂(SO₄) were prepared and their activities and selectivities were measured. The catalytic activity decreased in the order: TiO₂(SO₄) > ZrO₂(SO₄) > CeO₂(SO₄). Complete catalytic decomposition of methylene chloride was achieved at low temperature (275°C) over a sulfated titanium dioxide catalyst. The oxygen adsorption (pick-up) and the acidity values of three catalysts showed the same trend as their activities. The presence of water (2% in volume) in the feed stream reduced the activities remarkably and raised the activation energies for the decomposition reaction. The selectivities among all three catalysts were similar, with HCl, CO and CO₂ being the products. A bifunctional catalyst comprising sulfated titanium dioxide with copper oxide was developed to improve the selectivity of catalytic oxidation of methylene chloride towards carbon dioxide.     

Molecular structure–reactivity relationships for the oxidation of sulfur dioxide over supported metal oxide catalysts

The catalytic oxidation of sulfur dioxide to sulfur trioxide over several binary (M_xO_y/TiO₂) and ternary (V₂O₅/M_XO_Y/TiO₂) supported metal oxide catalysts was systematically investigated. The supported metal oxide components were essentially 100% dispersed as surface metal oxide species, as confirmed by Raman spectroscopy characterization. The sulfur dioxide oxidation turnover frequencies of the binary catalysts were all within an order of magnitude (V₂O₅/TiO₂>Fe₂O₃/TiO₂>Re₂O₇/TiO₂  CrO₃/TiO₂  Nb₂O₅/TiO₂>MoO₃/TiO₂  WO₃/TiO₂). An exception was the K₂O/TiO₂ catalysts, which is essentially inactive for sulfur dioxide oxidation. With the exception of K₂O, all of the surface metal oxide species present in the ternary catalysts (i.e., oxides of V, Fe, Re, Cr, Nb, Mo and W) can undergo redox cycles and oxidize SO₂ to SO₃. The turnover frequency for sulfur dioxide oxidation over all of these catalysts is approximately the same at both low and high surface coverages. This indicates that the mechanism of sulfur dioxide oxidation is not sensitive to the coordination of the surface metal oxide species. A comparison of the activities of the ternary catalysts with the corresponding binary catalysts suggests that the surface vanadium oxide and the additive surface metal oxide redox sites act independently without synergistic interactions. The V₂O₅/K₂O/TiO₂ catalyst showed a dramatic reduction in the catalytic activity in comparison to the unpromoted V₂O₅/TiO₂ catalyst. The ability of K₂O to significantly retard the redox potential of the surface vanadia species is primarily responsible for the lower catalytic activity of the ternary catalytic system. The fundamental insights generated from this research can potentially assist in the molecular design of the air pollution control catalysts: (1) the development of catalysts for low temperature oxidation of SO₂ to SO₃ during sulfuric acid manufacture (2) the design of efficient SCR DeNO_x catalysts with minimal SO₂ oxidation activity and (3) improvements in additives for the simultaneous oxidation/sorption of sulfur oxides in petroleum refinery operations.     

Pt/TiO_2催化二甲醚部分氧化重整制氢

二甲醚是一种理想的氢载体,可用于解决氢的储存和运输。以Pt/TiO_2为部分氧化催化剂,结合Ni/Al_2O_3重整催化剂,考察钛前驱体和焙烧温度对二甲醚部分氧化重整制氢反应的影响。结果表明,以Ti(C4H9O)4为原料制备的TiO_2为金红石相,Ti(SO4)2或Ti O(OH)2为原料制备的TiO_2为锐钛矿相;以Ti(C4H9O)4为原料制备的Pt/TiO_2-E催化剂催化性能略好,转化率接近100%,H2收率约90%,表明金红石相TiO_2负载的Pt催化剂略佳;以Ti(SO4)2为原料制备的Pt/TiO_2-S催化剂500℃焙烧可获得金红石相TiO_2。与Pt/Al_2O_3催化剂相比,Pt/TiO_2催化剂具有更好的催化性能,H2收率超过90%,而Pt/Al_2O_3催化剂H2收率约80%。     

A study of the abatement of VOC over V2O5–WO3–TiO2 and alternative SCR catalysts

The conversion of C3 organic compounds (propane, propene, 1- and 2-propanol, allyl alcohol, propanal, acrolein, acetone and 1- and 2-chloropropane) in the presence of excess oxygen has been investigated over two V–W–TiO₂ commercial SCR catalysts differing in the V content and over Mn–TiO₂ alternative SCR catalysts. V–W–Ti catalysts show poor activity in the oxidation of hydrocarbons and oxygenates and give significant amounts of partial oxidation products. Moreover they give rise to CO in excess of CO₂. The sample higher in V is more active. Mn–TiO₂ is definitely more active in oxidation of hydrocarbons and oxygenates, and produces, at total conversion, CO₂ as the only detectable product.

V–W–Ti catalysts are very active in dehydrochlorination of the two 2-chloropropane isomers and retain the same oxidation activity also in the presence of HCl. On the contrary, Mn-based catalysts in the presence of chlorocarbons convert into dehydrochlorination catalysts but lose their catalytic activity in oxidation. V–W–Ti catalysts can be used in Cl-containing atmospheres while Mn–TiO₂ can be proposed for DeNO_x and VOC abatement in Cl-free atmospheres such as for diesel engine exhaust gas purification.     

The CO–H2 and CO–H2O reactions over TiO2 nanotubes filled with Pt metal nanoparticles

We have investigated the catalytic behavior of Pt encapsulated TiO₂ nanotubes for the water gas shift reaction as well as the hydrogenation of CO. Pt–TiO₂ nanotube catalysts were prepared by employing fine fiber shaped crystals of [Pt(NH₃)₄](HCO₃)₂ complex as a structure determining template material. The turnover frequencies (TOF) of these nanotube catalysts were more than one order of magnitude larger than conventional impregnation Pt/TiO₂ catalysts, and the selectivity for methanol in CO–H₂ reaction was extraordinary high compared to the impregnation catalysts. The XPS and XRD analyses of the nanotubes revealed characteristic electronic state of reduced TiO₂ (Ti³⁺ in rutile structure) with zerovalent Pt even after the calcination at 773 K. In WGS reaction, electron rich Ti³⁺ on the nanotube wall may play an important role to activate water molecules for the oxidation of CO. In CO–H₂ reaction, similar promotion effect of Ti³⁺ species may be operating for selective methanol formation by supplying active OH(a).     

Effect of V2O5 on the catalytic activity of Pt-based diesel oxidation catalyst

We investigated the suppression of SO₂ oxidation activity by vanadium oxide in Pt-based diesel oxidation catalyst using reaction experiments, temperature programmed desorption (TPD), infrared (IR) and X-ray photoelectron spectroscopy (XPS). There was no interaction between Pt and S indicated by the XPS results. SO₂ was not adsorbed on Pt at room temperature indicated by the absence of peak arising from SO₂ in SO₂ TPD spectra. SO₂ molecules were adsorbed on the hydroxyl groups of TiO₂ and migrated to Pt particles to react with oxygen adsorbed on it. V₂O₅ decreased the adsorption of SO₂ on TiO₂ by the blockage of V₂O₅ on TiO₂.     

掺杂元素对Mn基催化剂SCR性能及抗硫性能的影响

采用溶胶凝胶法制备M-Mn/TiO₂催化剂,M=Fe、Ce、Ni、Sm、Cu,考察掺杂元素对催化剂SCR性能和抗硫性能的影响,并采用XRD、H₂-TPR表征手段考察掺杂组分对催化剂性质的影响。结果表明,掺杂金属元素后,催化剂的晶粒尺寸和还原性能发生变化,表现为Fe、Ni的掺杂明显降低催化剂粒径尺寸而Ce、Sm、Cu的掺杂增加了粒径尺寸;Cu和Ni的掺杂有利于提高催化剂的还原性能。SCR实验结果表明,Cu、Sm的掺杂在整个反应温度区间内都会抑制催化剂脱硝活性,而Fe、Ni、Ce的掺杂主要抑制低温活性,在高温阶段则会促进反应的进行。掺杂金属元素后催化剂T₈₀活性窗口顺序为：Fe-Mn/TiO₂＞Ni-Mn/TiO₂＞Ce-Mn/TiO₂≈Mn/TiO₂＞Sm-Mn/TiO₂＞Cu-Mn/TiO₂。Sm-Mn/TiO₂和Ce-Mn/TiO₂能保持与Mn/TiO₂基本相似的抗硫性能,但掺杂Fe、Cu、Ni会导致催化剂在较短时间内因硫中毒失活,尤其是Fe-Mn/TiO₂催化剂的活性中心受SO₂硫酸盐化影响最大,但是水洗再生之后其活性基本恢复,且Fe-Mn/TiO₂和Ni-Mn/TiO₂催化剂的抗硫中毒性能在水洗后得到提高。     

SO2 promoted oxidation of ethyl acetate, ethanol and propane

The effect of SO₂ addition on the oxidation of ethyl acetate, ethanol, propane and propene, over Pt/γ-Al₂O₃ and Pt/SiO₂ has been investigated. The reactants (300–800 vol. ppm) were mixed with air and led through the catalyst bed. The conversions below and above light-off were recorded both in the absence and in the presence of 1–100 vol. ppm SO₂. For the alumina-supported catalyst, the conversion of ethyl acetate, ethanol and propane was promoted by the addition of SO₂, while the conversion of propene was inhibited. The effect of SO₂ was reversible, i.e. the conversion of the reactants returned towards the initial values when SO₂ was turned off. However, this recovery was quite slow. The oxidation of propane was inhibited by water, both in absence and presence of SO₂. For the silica-supported catalyst no significant effect of SO₂ could be observed on the conversion of ethyl acetate, ethanol or propane, whereas the conversion of propene was inhibited by the presence of SO₂. In situ FTIR measurements revealed the presence of surface sulphates on the Pt/γ-Al₂O₃ catalyst with and after SO₂ addition. It is proposed that these sulphate groups enhance the oxidation of propane, ethyl acetate and ethanol by creating additional reaction pathways to Pt on the surface of the Pt/γ-Al₂O₃ catalyst.     

TiO2-SiO2 and V2O5/TiO2-SiO2 catalyst: Physico-chemical characteristics and catalytic behavior in selective catalytic reduction of NO by NH3

TiO₂-SiO₂ with various compositions prepared by the coprecipitation method and vanadia loaded on TiO₂-SiO₂ were investigated with respect to their physico-chemical characteristics and catalytic behavior in SCR of NO by NH₃ and in the undesired oxidation of SO₂ to SO₃, using BET, XRD, XPS, NH₃-TPD, acidity measurement by the titration method and activity test. TiO₂-SiO₂, compared with pure TiO₂, exhibits a remarkably stronger acidity, a higher BET surface area, a lower crystallinity of anatase titania and results in allowing a good thermal stability and a higher vanadia dispersion on the support up to high loadings of 15 wt% V₂O₅. The SCR activity and N₂ selectivity are found to be more excellent over vanadia loaded on TiO₂-SiO₂ with 10–20 mol% of SiO₂ than over that on pure TiO₂, and this is considered to be associated with highly dispersed vanadia on the supports and large amounts of NH₃ adsorbed on the catalysts. With increasing SiO₂ content, the remarkable activity decrease in the oxidation of SO₂ to SO₃, favorable for industrial SCR catalysts, was also observed, strongly depending on the existence of vanadium species of the oxidation state close to V⁴⁺ on TiO₂-SiO₂, while V⁵⁺ exists on TiO₂, according to XPS. It is concluded that vanadia loaded on Ti-rich TiO₂-SiO₂ with low SiO₂ content is suitable as SCR catalysts for sulfur-containing exhaust gases due to showing not only the excellent de-NO_x activity but also the low SO₂ oxidation performance.     

Total oxidation of propene and propane over gold–copper oxide on alumina catalysts: Comparison with Pt/Al2O3

Oxidation of propene and propane to CO₂ and H₂O has been studied over Au/Al₂O₃ and two different Au/CuO/Al₂O₃ (4 wt.% Au and 7.4 wt.% Au) catalysts and compared with the catalytic behaviour of Au/Co₃O₄/Al₂O₃ (4.1 wt.% Au) and Pt/Al₂O₃ (4.8 wt.% Pt) catalysts. The various characterization techniques employed (XRD, HRTEM, TPR and DR-UV–vis) revealed the presence of metallic gold, along with a highly dispersed CuO (6 wt.% CuO), or more crystalline CuO phase (12 wt.% CuO).

A higher CuO loading does not significantly influence the catalytic performance of the catalyst in propene oxidation, the gold loading appears to be more important. Moreover, it was found that 7.4Au/CuO/Al₂O₃ is almost as active as Pt/Al₂O₃, whereas Au/Co₃O₄/Al₂O₃ performs less than any of the CuO-containing gold-based catalysts.

The light-off temperature for C₃H₈ oxidation is significantly higher than for C₃H₆. For this reaction the particle size effect appears to prevail over the effect of gold loading. The most active catalysts are 4Au/CuO/Al₂O₃ (gold particles less than 3 nm) and 4Au/Co₃O₄/Al₂O₃ (gold particles less than 5 nm).     

The role of the oxidic support on the deactivation of Pt catalysts during the CO2 reforming of methane

Pt supported on γ-Al₂O₃, TiO₂ and ZrO₂ are active catalysts for the CO₂ reforming of methane to synthesis gas. The stability of the catalysts increased in the order Pt/γ-A1₂O₃ < Pt/TiO₂ < Pt/ZrO₂. For all catalysts, the decrease in activity with time on stream is caused by carbon formation, which blocks the active metal sites for reaction. With Pt/TiO₂ and Pt/ZrO₂, deactivation started immediately after the start of the reaction, while the Pt/γ-A1₂O₃ catalyst showed an induction period during which carbon was accumulated without affecting the catalytic activity.     

Improvement of catalytic activity and sulfur-resistance of Ag/TiO2–Al2O3 for NO reduction with propene under lean burn conditions

Ag-based catalysts supported on various metal oxides, Al₂O₃, TiO₂, and TiO₂–Al₂O₃, were prepared by the sol–gel method. The effect of SO₂ on catalytic activity was investigated for NO reduction with propene under lean burn condition. The results showed the catalytic activities were greatly enhanced on Ag/TiO₂–Al₂O₃ in comparison to Ag/Al₂O₃ and Ag/TiO₂, especially in the low temperature region. Application of different characterization techniques revealed that the activity enhancement was correlated with the properties of the support material. Silver was highly dispersed over the amorphous system of TiO₂–Al₂O₃. NO₃⁻ rather than NO₂⁻ or NOx reacted with the carboxylate species to form CN or NCO. NO₂ was the predominant desorption species in the temperature programmed desorption (TPD) of NO on Ag/TiO₂–Al₂O₃. More amount of formate (HCOO⁻) and CN were generated on the Ag/TiO₂–Al₂O₃ catalyst than the Ag/Al₂O₃ catalyst, due to an increased number of Lewis acid sites. Sulfate species, resulted from SO₂ oxidation, played dual roles on catalytic activity. On aged samples, the slow decomposition of accumulated sulfate species on catalyst surface led to poor NO conversion due to the blockage of these species on active sites. On the other hand, catalytic activity was greatly enhanced in the low temperature region because of the enhanced intensity of Lewis acid site caused by the adsorbed sulfate species. The rate of sulfate accumulation on the Ag/TiO₂–Al₂O₃ system was relatively slow. As a consequence, the system showed superior capability for selective adsorption of NO and SO₂ toleration to the Ag/Al₂O₃ catalyst.     

Synergistic catalysis of carbon black oxidation by Pt with MoO3 or V2O5

A series of SiO₂-supported MoO₃, V₂O₅, and Pt catalysts were prepared for the study of model soot oxidation with simulated diesel exhaust gas. Composite samples of Pt with the metal oxides demonstrated higher oxidation activities than the single-component SiO₂-supported MoO₃, V₂O₅ or Pt catalysts in the absence of SO₂ in the reactant gas. Based on the effects of NO₂ on carbon oxidation, a synergistic reaction mechanism was suggested to explain the effects of combining Pt with the oxides: Pt catalyzes the oxidation of NO with gas phase O₂ to NO₂, while MoO₃ and V₂O₅ catalyze the oxidation of carbon with NO₂. Finally, the effects of SO₂ on the carbon oxidation reaction were examined and discussed.     

Environmentally friendly solid acid catalyst prepared by modifying TiO2 with cerium sulfate for the removal of volatile organic chemicals

An environmentally friendly solid acid catalyst, Ce(SO₄)₂/TiO₂ was prepared simply by modifying TiO₂ with Ce(SO₄)₂ for acid catalysis of volatile organic chemicals, 2-propanol and cumene. The characterization of prepared catalysts was performed using FTIR, XRD and DSC. The surface area of 7-Ce(SO₄)₂/TiO₂ calcined at 300 °C was very high (206.0 m²/g) compared to that of unmodified TiO₂ (115.2 m²/g) due to the interaction between Ce(SO₄)₂ and TiO₂. 7-Ce(SO₄)₂/TiO₂ containing 7 wt% Ce(SO₄)₂ and calcined at 300 °C exhibited maximum catalytic activities for both reactions, 2-propanol dehydration and cumene dealkylation. The catalytic activities for both reactions were correlated with the acid amounts of catalysts measured by an ammonia chemisorption method. The role of Ce results in an increase in the thermal stability of the surface sulfate species and consequently the acid amount of Ce(SO₄)₂/TiO₂ is increased. The asymmetric stretching frequency of the SO bonds for Ce(SO₄)₂/TiO₂ catalysts was related to the acidic properties and to the catalytic activity for acid catalysis to remove volatile organic chemicals, 2-propanol and cumene.     

Preferential oxidation of carbon monoxide over Pt, Au monometallic catalyst, and Pt–Au bimetallic catalyst supported on ceria in hydrogen-rich reformate

The objective of this paper was to study a preferential oxidation (PROX) of carbon monoxide over monometallic catalysts including Pt, Au and Pt–Au bimetallic catalyst supported on ceria in hydrogen-rich reformate. Single step sol–gel method (SSG) and impregnation on sol–gel method (ISG) were chosen for the preparation of the catalysts. The characteristics of these catalysts were investigated by X-ray diffractometer (XRD), Brunauer–Emmet–Teller (BET) method, transmission electron microscope (TEM), scanning electron microscope (SEM) and temperature-programmed reduction (TPR). The XRD patterns of the catalysts showed only the peaks of ceria crystallite and no metal peak appeared. From TEM images, the active components were seen to be dispersed throughout the ceria support. The TPR patterns of PtAu/CeO₂ catalyst prepared by SSG showed the reduction peaks were within a low temperature range and therefore, the catalysts prepared by SSG exhibited excellent catalytic activity for preferential oxidation of CO. Bimetallic Pt–Au catalyst improved the activity (90% conversion and 50% selectivity at 90 °C) because of the formation of a new phase. When the metal content of (1:1) PtAu/CeO₂ catalyst prepared by SSG was increased, the CO conversion did not change much while the selectivity decreased in the low temperature range (50–90 °C). The CO conversion increased with increasing W/F ratio. The presence of CO₂ and H₂O had a negative effect on CO conversion and selectivity due to blocking of carbonate and water on active sites.     

Catalytic wet air oxidation of olive mill wastewater

Au-based catalysts, known for ambient temperature CO oxidation, have to provide stable performance of up to 5000 h in order to be commercially applicable in automotive fuel cells. In this report, the on-line deactivation characteristics of Au/TiO₂ in unconventional PROX conditions are discussed. As opposed to CO removal from air, results in this report suggests that carbonates have a minor effect on deactivation of Au/TiO₂ in dry H₂-rich conditions. Also, no conclusive correlation between surface hydration and deactivation was observed. Rather, deactivation appeared to have occurred as a result of an intrinsic transformation in the oxidation state of the active species in the reducing operating conditions; a process which was reversible in an oxidizing atmosphere.     

Combinatorial investigation of Pt–Ru–M as anode electrocatalyst for direct methanol fuel cell

The addition of Au/TiO₂ and zeolites as active components to PtRu/C electrode in DMFC was investigated by using combinatorial high-throughput-screening test. Addition of Au/TiO₂ to PtRu/C electrode, especially in the ratio of PtRu/C: Au/TiO₂ 9:1, 8:2, 7:3, were effective to improve the performance of direct methanol fuel cell. The electrochemical properties of the prepared electrodes were compared using cyclic voltammetry, impedance spectroscopy and a single cell performance test of a direct methanol fuel cell (DMFC). The adsorbed CO on Pt might be easily oxidized on the surface of Au/TiO₂ by interaction between PtRu/C and Au/TiO₂. The addition of the solid acid proton conducting materials (ZSM-5) on PtRu/C anode leads to the high temperature operation. The cell performance was maintained over the cell temperature 120 °C (maximum current density was 200 mA/cm² at 160 °C) by the addition of ZSM-5 as proton conducting materials.