Characterization by temperature programmed reduction

A greener approach for synthesizing metal-decorated carbogels from alginate for emerging technologies

In the present work, a series of metal nanoparticle-decorated carbogels (M-DCs) was synthesized starting from beads of parent metal-crosslinked alginate aerogels (M-CAs). M-CAs contained Ca(ii), Ni(ii), Cu(ii), Pd(ii) and Pt(iv) ions and were converted to M-DCs by pyrolysis under a N2 atmosphere up to pyrolysis temperatures of TP = 600 °C. The textural properties of M-CAs are found to depend on the crosslinking ion, yielding fibrous pore networks with a high specific mesoporous volume and specific surface area SV (SV ∼ 480-687 m2 g-1) for M-CAs crosslinked with hard cations, Ca(ii), Ni(ii) and Cu(ii), and comparably loose networks with increased macroporosity and lower specific surface (SV ∼ 240-270 m2 g-1) for Pd(ii) and Pt(iv) crosslinked aerogels. The pyrolysis of M-CAs resulted in two simultaneously occurring processes: changes in the solid backbone and the growth of metal/metal oxide nanoparticles (NPs). The thermogravimetric analysis (TGA) showed a significant influence of the crosslinking cation on the decomposition mechanism and associated change in textural properties. Scanning electron microscopy-backscattered electron imaging (SEM-BSE) and X-ray diffraction revealed that metal ions (molecularly dispersed in the parent aerogels) formed nanoparticles composed of elementary metals and metal oxides in varying ratios over the course of pyrolytic treatment. Increasing the TP led to generally larger nanoparticles. The pyrolysis of the nickel-crosslinked aerogel (Ni-CA) preserved, to a large extent, the mesoporous structure and resulted in the evolution of fine (∼14 nm) homogeneously dispersed Ni/NiO nanoparticles. Overall, this work presents a green approach for synthesizing metal-nanoparticle containing carbon materials, useful in emerging technologies related to heterogeneous catalysis and electrocatalysis, among others.

Influence of Physicochemical Properties of Ni/Clinoptilolite Catalysts in the Hydrogenation of Acetophenone

Heterogeneous catalytic hydrogenation is an interesting alternative to conventional methods that use inorganic hydrides. The hydrogenation of acetophenone under heterogeneous conditions with the supported catalysts based on Ni is the most useful due to its redox properties and lower cost. As is well-known, catalyst support can significantly affect catalyst performance. We have investigated the influence of various physical-chemical parameters on the selective reaction of the hydrogenation of acetophenone by using different nickel catalysts on clinoptilolite supports, in four different forms: natural, previously modified with NH3 (Ni/Z+NH₄ ⁺), with HNO₃ (Ni/Z+H⁺), and thermally treated (Ni/Z 500 °C). In particular, our work focuses on determining the influence of the mentioned physical-chemical parameters on the percentages of conversion and the selectivity of the catalysis. This study aims to identify the combination of parameters that allows for obtaining the best catalytic results. The identification of the physical-chemical parameters that determine the percentages of conversion and selectivity allows us to design optimal catalysts.

NH3-NO SCR Catalysts for Engine Exhaust Gases Abatement: Replacement of Toxic V2O5 with MnOx to Improve the Environmental Sustainability

Mn-WO3/TiO2 catalysts were investigated for Selective Catalytic Reduction (SCR) of NO with NH3. The catalysts were synthesized by wetness impregnation method with different Mn loadings (1.5-3-12 wt%) on 8wt%WO3/TiO2. All three catalysts were compared with 8wt%WO3/TiO2 and bare MnOx oxide, used as references. The 1.5wt%Mn-8wt%WO3/TiO2 exhibited the highest performance in NO conversion and N2 selectivity. A commercial catalyst, based on titania supported vanadia and tungsta, (V2O5-WO3/TiO2), widely used for its high efficiency, was also investigated in the present work. The morphological, structural, redox and electronic properties of the catalysts and their thermal stability were studied by several techniques (N2 adsorption/desorption, X-ray diffraction, H2 temperature-programmed reduction, NH3 temperature programmed desorption, X-ray photoelectron spectroscopy).

The aim of this paper is to study the effect of different Mn loadings on 8wt%WO3/TiO2 with the ambition to obtain highly active and selective catalysts in a large window of temperature. The replacement of toxic vanadium used in the classic V2O5-WO3/TiO2 catalyst with MnOx in the best performing catalyst, 1.5wt%Mn-8wt%WO3/TiO2, represents an important achievement to improve the environmental sustainability.

Enhanced activity of vanadia supported on microporous titania for the selective catalytic reduction of NO with NH3: Effect of promoters

Vanadium oxide-based catalysts are considered a promising catalyst for selective catalytic reduction (SCR) of NO with NH₃, which is an effective NO_x removal technology. As environmental issues have garnered more attention, however, improvements to vanadium-based SCR catalysts are strongly required. In a previous study, we found that vanadium oxide on microporous titania as a support (V/MPTiO₂) has certain advantages, such as improved thermal stability and more suppressed N₂O formation, over the use of conventional nanoparticle titania (DT-51) as a support. In this study, widely used promoters, such as W, Sb, and Mo, were added to V/MPTiO₂ to investigate whether they have promoting effects on V/MPTiO₂ as well. Among these promoters added catalysts, the W and Mo were found to have significant promoting effects on the enhancement of deNO_x activities at low temperatures, while the addition of Sb to V/MPTiO₂ tended to have a negative effect on the SCR activity. Based on the characterizations, including laser Raman, H₂-temperature programmed reduction (H₂-TPR), and in situ diffuse reflectance infrared Fourier transform (in situ DRIFT) analysis, we found that the addition of W and Mo increased the degree of polymerization in V/MPTiO₂, which generated more reactive vanadia species. Hence, such changes, resulting from the addition of W and Mo promoters to V/MPTiO₂, yielded enhanced catalytic activity at low temperatures.

Silicotungstic acid modified CeO2 catalyst with high stability for the catalytic combustion of chlorobenzene

The catalysts' redox capacity and surface acidity was important during the catalytic combustion of chlorobenzene (CB). CeO₂ showed great attractiveness due to its high oxygen storage capacity. Furthermore, the increase of acidity on the catalyst surface could improve the resistance to the chlorine poisoning. In this work, the silicotungstic (HSiW) modified CeO₂ catalysts prepared by four cerium salts and exhibited the different morphologies and catalytic activity. The HSiW modified CeO₂ catalyst prepared by Ce(CH₃COO)₃ (Cat-A) exhibited the best catalytic activity due to its abundant surface weak acid sites, more Ce³⁺ species and surface adsorption oxygen. The HSiW mainly located on the CeO₂ (111) planes of the Cat-A, which was conducive to redox property of CeO₂, thus promoting the deep oxidation of CB. Meanwhile the redox ability together with the weak acidity influenced the catalytic efficiency at low temperature. And the redox ability played a major role at high temperature. In addition, the Cat-A still possessed high stability and water resistance and maintained high activity after continuous catalytic oxidation of CB at 235 and 295 °C for 100h, exhibiting the possibility of industrial application.

Comparative Studies of Effects of Vapor- and Liquid-Phase As2O3 on Catalytic Behaviors of V2O5-WO3/TiO2 Catalysts for NH3-SCR

The role of vapor- and liquid-phase As₂O₃ in deactivating commercial V₂O₅-WO₃/TiO₂ catalyst during the NH₃-selective catalytic reduction (SCR) process was explored and compared. As₂O₃ was loaded via vapor deposition (As(vap)) and the wet impregnation (As(imp)) method, respectively. Results demonstrated that the poisoning extent of vapor arsenic was much stronger than in the liquid state. Differences in As distribution on the catalyst surface was one of the main causes. Most vapor As₂O₃ could be oxidized to As₂O₅, which underwent stacking and formed a dense covering layer on the catalyst surface. In comparison, liquid As₂O₃ could also be oxidized but distributed uniformly and did not change the catalyst pore structure. Loading arsenic would destroy the V-OH and V=O active sites of the catalyst, and less reactive As⁵⁺-OH was generated. Catalyst oxidizability was also enhanced, resulting in NH₃ oxidation enhancement, decreased N₂ selectivity, and a decline in SCR activity. Importantly, the intermediate of NH₃ oxidation, NH₂-amide, also could react with NO + O₂, and more N₂O was generated on the poisoned catalyst during the SCR process, especially on As(imp). Finally, two mechanisms of arsenic poisoning were proposed, in which the role of vapor and liquid As₂O₃ over the V₂O₅-WO₃/TiO₂ catalyst was compared.

New insights into the deactivation mechanism of V2O5-WO3/TiO2 catalyst during selective catalytic reduction of NO with NH3: synergies between arsenic and potassium species

Synergies between arsenic (As) and potassium (K) species in the deactivation of V₂O₅-WO₃/TiO₂ catalyst were investigated. Both arsenic oxide and potassium species presented a serious poisoning impact on catalyst activities, and the extent of poisoning of (As + K) was much stronger than their single superposition. The intrinsic reasons were explored and analyzed by N₂ physisorption, XPS, H₂-TPR, NH₃-TPD, NH₃-DRIFTS and in situ FTIR. Results indicated that BET surface area decreased due to the formation of a dense arsenic coating on the catalyst surface. V–OH active sites were destroyed by arsenic and As–OH acid sites were newly generated. After potassium species were added to arsenic-poisoned catalyst, K⁺ further neutralized the As–OH acid sites, and the amount and stability of both Lewis and BrØnsted acid sites decreased more greatly. Potassium also reacted with intermediate NH₂⁻ when the temperature was elevated to higher than 250 °C, which resulted in more NH₃ consumption and NH₃-SCR reaction inhibition. The extent of deactivation was related to the potassium species when both poisons reacted on the catalyst, and the influence sequence followed AsKS < AsKN < AsKC. As₂O₃ + K₂SO₄ presented the weakest impact among these three poisoned catalysts due to the resistance of SO₄²⁻ to arsenic.

Synergies between arsenic (As) and potassium (K) species in the deactivation of V₂O₅-WO₃/TiO₂ catalyst were investigated.

Extraordinary Deactivation Offset Effect of Arsenic and Calcium on CeO2-WO3 SCR Catalysts

An extraordinary deactivation offset effect of calcium and arsenic on CeO₂-WO₃ catalyst had been found for selective catalytic reduction of NO with NH₃ (NH₃-SCR). It was discovered that the maximum NO _x conversion of As-Ca poisoned catalyst reached up to 89% at 350 °C with the gaseous hourly space velocity of 120 000 mL·(g·h)^-1. The offset effect mechanisms were explored with respect to the changes of catalyst structure, surface acidity, redox property and reaction route by XRD, XPS, H₂-TPR, O₂-TPD, NH₃-TPD and in situ Raman, in situ TG, and DRIFTS. The results manifested that Lewis acid sites and reducibility originating from CeO₂ were obviously recovered, because the strong interaction between cerium and arsenic was weakened when Ca and As coexisted. Meanwhile, the CaWO₄ phase generated on Ca poisoned catalyst almost disappeared after As doping together, which made for Brønsted acid sites reformation on catalyst surface. Furthermore, surface Ce⁴⁺ proportion and oxygen defect sites amount were also restored for two-component poisoned catalyst, which favored NH₃ activation and further reaction. Finally, the reasons for the gap of catalytic performance between fresh and As-Ca poisoned catalyst were also proposed as follows: (1) surface area decrease; (2) crystalline WO₃ particles generation; and (3) oxygen defect sites irreversible loss.

Catalytic decomposition of PCDD/Fs on a V2O5-WO3/nano-TiO2 catalyst: effect of NaCl

The effect of NaCl addition on the properties, activity, and deactivation of a V₂O₅-WO₃/nano-TiO₂ catalyst was investigated during catalytic decomposition of gas-phase polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/Fs). The extent of deactivation relates directly to the NaCl loading of the catalyst. Poisoning by sodium neutralizes acid sites, interacts strongly with active VO_x species, and reduces the redox capacity of catalysts. In addition, NaCl is also a chlorine source and may actually accelerate the synthesis of new PCDD/Fs. Washing a catalyst with dilute sulfuric acid largely restores catalytic activity, breaking the interaction of Na⁺ ions and dispersed vanadia and removing Na from the catalyst surface. Consequently, catalyst acidity and redox capacity almost recover. Furthermore, sulfate residues react with surface adsorbed water to generate Brønsted acid sites, ensuing a surge of strong acidity of the catalysts.

Adsorption-oxidation of hydrogen sulfide on Fe/walnut-shell activated carbon surface modified by NH3-plasma

Walnut-shell activated carbon (WSAC) supported ferric oxide was modified by non-thermal plasma (NTP), and the removal efficiency for hydrogen sulfide over Fe/WSAC modified by dielectric barrier discharge (DBD) was significantly promoted. The sample modified for 10min and 6.8kV output (30V input voltage) maintained 100% H₂S conversion over a long reaction time of 390min. The surface properties of adsorbents modified by NTP under different conditions were evaluated by the methods of X-ray photoelectron spectroscopy (XPS), Brunauer-Emmett-Teller (BET) analysis and in-situ Fourier transform infrared spectroscopy (FTIR), to help understand the effect of the NTP treatment. NTP treatment enhanced the adsorption capacity of Fe/WSAC, which could due to the formation of micro-pores with sizes of 0.4, 0.5 and 0.75nm. XPS revealed that chemisorbed oxygen changed into lattice oxygen after NTP treatment, and lattice oxygen is beneficial for H₂S oxidation. From the in-situ FTIR result, transformation of the reaction path on Fe/WSAC was observed after NTP modification. The research results indicate that NTP is an effective method to improve the surface properties of the Fe/WSAC catalyst for H₂S adsorption-oxidation.

Regeneration of Commercial SCR Catalysts: Probing the Existing Forms of Arsenic Oxide

To investigate the poisoning and regeneration of SCR catalysts, fresh and arsenic-poisoned commercial V2O5-WO3/TiO2 catalysts are researched in the context of deactivation mechanisms and regeneration technology. The results indicate that the forms of arsenic oxide on the poisoned catalyst are related to the proportion of arsenic (As) on the catalyst. When the surface coverage of (V+W+As) is lower than 1, the trivalent arsenic species (As(III)) is the major component, and this species prefers to permeate into the bulk-phase channels. However, at high As concentrations, pentavalent arsenic species (As(IV)) cover the surface of the catalyst. Although both arsenic species lower the NOx conversion, they affect the formation of N2O differently. In particular, N2O production is limited when trivalent arsenic species predominate, which may be related to As2O3 clogging the pores of the catalyst. In contrast, the pentavalent arsenic oxide species (As2O5) possess several As-OH groups. These As-OH groups could not only enhance the ability of the catalyst to become reduced, but also provide several Brønsted acid sites with weak thermal stability that promote the formation of N2O. Finally, although our novel Ca(NO3)2-based regeneration method cannot completely remove As2O3 from the micropores of the catalyst, this approach can effectively wipe off surface arsenic oxides without a significant loss of the catalyst's active components.

Alkali metal poisoning of a CeO2-WO3 catalyst used in the selective catalytic reduction of NOx with NH3: an experimental and theoretical study

The alkali metal-induced deactivation of a novel CeO(2)-WO(3) (CeW) catalyst used for selective catalytic reduction (SCR) was investigated. The CeW catalyst could resist greater amounts of alkali metals than V(2)O(5)-WO(3)/TiO(2). At the same molar concentration, the K-poisoned catalyst exhibited a greater loss in activity compared with the Na-poisoned catalyst below 200 °C. A combination of experimental and theoretical methods, including NH(3)-TPD, DRIFTS, H(2)-TPR, and density functional theory (DFT) calculations, were used to elucidate the mechanism of the alkali metal deactivation of the CeW catalyst in SCR reaction. Experiments results indicated that decreases in the reduction activity and the quantity of Brønsted acid sites rather than the acid strength were responsible for the catalyst deactivation. The DFT calculations revealed that Na and K could easily adsorb on the CeW (110) surface and that the surface oxygen could migrate to cover the active tungsten, and then inhibit the SCR of NO(x) with ammonia. Hot water washing is a convenient and effective method to regenerate alkali metal-poisoned CeW catalysts, and the catalytic activity could be recovered 90% of the fresh catalyst.