Modulation of selective sites by introduction of N2O,CO2 and H2 as gaseous promoters into the feed during oxidation reactions

Results obtained by adding gaseous promoters (CO₂, N₂O and H₂) into the reaction feed are presented for two different reactions: (i) oxidative dehydrogenation of propane (ODP), and (ii) catalytic combustion of methane (CCM). The ODP is performed on a mixture of NiMoO₄ and CeO₂, by adding 3 vol.% CO₂ into the feed, and on a NiMoO₄/[Si,V]-MCM-41 mesoporous catalyst, in the presence of 1 or 5 vol.% N₂O in the feed. The CCM is carried out (i) on Pd(2 wt.%)/Ce_xZr_1−xO₂ and Pd(2 wt.%)/γ-Al₂O₃ catalysts, on pure CeO₂ and on a mixture of Pd(2 wt.%)/γ-Al₂O₃ and CeO₂ powders, by adding 3 vol.% CO₂ into the feed, and (ii) on a Pd(2 wt.%)/γ-Al₂O₃ catalyst, in the presence of various amounts of H₂ in the feed. It is shown, through all these various examples, that the activity and/or the selectivity of catalysts can be improved by tuning, in a very controlled manner, the oxidation state of active sites via the use of these gaseous promoters.     

Impact of support oxide and Ba loading on the NOx storage properties of Pt/Ba/support catalysts: CO2 and H2O effects

A series of 1 wt.%Pt/_xBa/Support (Support = Al₂O₃, SiO₂, Al₂O₃-5.5 wt.%SiO₂ and Ce_0.7Zr_0.3O₂, x = 5–30 wt.% BaO) catalysts was investigated regarding the influence of the support oxide on Ba properties for the rapid NO_x trapping (100 s). Catalysts were treated at 700 °C under wet oxidizing atmosphere. The nature of the support oxide and the Ba loading influenced the Pt–Ba proximity, the Ba dispersion and then the surface basicity of the catalysts estimated by CO₂-TPD. At high temperature (400 °C) in the absence of CO₂ and H₂O, the NO_x storage capacity increased with the catalyst basicity: Pt/20Ba/Si < Pt/20Ba/Al5.5Si < Pt/10Ba/Al < Pt/5Ba/CeZr < Pt/30Ba/Al5.5Si < Pt/20Ba/Al < Pt/10BaCeZr. Addition of CO₂ decreased catalyst performances. The inhibiting effect of CO₂ on the NO_x uptake increased generally with both the catalyst basicity and the storage temperature. Water negatively affected the NO_x storage capacity, this effect being higher on alumina containing catalysts than on ceria–zirconia samples. When both CO₂ and H₂O were present in the inlet gas, a cumulative effect was observed at low temperatures (200 °C and 300 °C) whereas mainly CO₂ was responsible for the loss of NO_x storage capacity at 400 °C. Finally, under realistic conditions (H₂O and CO₂) the Pt/20Ba/Al5.5Si catalyst showed the best performances for the rapid NO_x uptake in the 200–400 °C temperature range. It resulted mainly from: (i) enhanced dispersions of platinum and barium on the alumina–silica support, (ii) a high Pt–Ba proximity and (iii) a low basicity of the catalyst which limits the CO₂ competition for the storage sites.     

Au/Ti-SBA-15 catalysts in CO and preferential (PROX) CO oxidation

The Au/Ti-SBA-15 catalysts were found promising for CO oxidation, including preferential CO oxidation in the presence of H₂ (PROX). The catalytic performance and Au particle size depending on the Ti content: 100% selectivity to CO₂ in PROX at 90% CO conversion was found for the catalysts of the Ti content below 1.32 wt.% Ti.     

CO oxidation over CuOx-CeO2-ZrO2 catalysts: Transient behaviour and role of copper clusters in contact with ceria

The effects of ZrO₂ content on the CO oxidation activity in a series of CuO_x/Ce_xZr_1−xO₂ (x = 0, 0.15, 0.5, 0.7 and 1) catalysts were investigated, both in the absence and in the presence of H₂, i.e. preferential CO oxidation—PROX. The investigation was performed under light-off conditions to focus the effects of transients and shut-down/start-up cycles on the performance; such phenomena are expected to affect the activity of PROX catalysts in small/delocalised fuel reformers. Evidence has been obtained for a transition from an “oxidized” towards a “reduced” state of the catalyst under the simulated PROX reaction conditions as a function of the reaction temperature, leading to different active species under the reaction conditions. Both CO oxidation activity and PROX selectivity appear to be affected by this process. IR characterisation of the surface copper species suggests an important role of reduced cerium sites in close contact with copper clusters on the CO oxidation activity at low temperatures.     

An excellent support of Pd–Fe–Ox catalyst for low temperature CO oxidation: CeO2 with rich (200) facets

Pd–Fe–O_x catalysts for low temperature CO oxidation were supported on SBA-15, CeO₂ nano-particles with rich (111) facets and CeO₂ nano-rod with rich (200) facets, and characterized by X-ray diffraction, low-temperature nitrogen adsorption, transmission electron microscopy and temperature-programmed reduction. The results showed that when CeO₂ nano-rod was used as a support, Pd–Fe–O_x catalyst exhibits higher activity (T₁₀₀ = 10 °C), resulting from the rich (200) facets of CeO₂ nano-rod, which leads to a formation of large numbers of the oxygen vacancies on the surface of Pd–Fe–O_x catalysts.     

Controllable synthesis of α-MoC1-x and β-Mo2C nanowires for highly selective CO2 reduction to CO

Transition metal carbides are attractive catalysts because of their similar properties to precious metals. Here, we report the controllable synthesis of α-MoC_1-x and β-Mo₂C nanowires as highly active and selective catalysts for CO₂ reduction to CO (CO₂ + H₂ → CO + H₂O, reverse water-gas shift reaction, RWGS). CO₂ conversion of > 60% together with nearly 100% CO selectivity was achieved at 600 °C, H₂:CO₂ molar ratio of 4:1, and space velocity of 36,000 mL g^− 1 h^− 1. A formate decomposition mechanism for the RWGS reaction was proposed based on the in-situ DRIFTS results.     

The development of a fully integrated micro-channel fuel processor using low temperature co-fired ceramic (LTCC)

A fully integrated micro-channel fuel processor system consisting of vaporizer, steam reformer, heat exchanger and preferential CO oxidation (PROX) was developed using low temperature co-fired ceramic (LTCC). To fabricate a compact all-in-one system, each substrate was stacked to build a multilayered type fuel processor. A CuO/ZnO/Al₂O₃ catalyst and Pt-based catalyst prepared by wet impregnation were deposited inside the micro-channel of steam reformer and PROX, respectively. The performance of the fully integrated micro-channel reformer was measured at various conditions such as the ratio of the feed flow rate, the ratio of H₂O/CH₃OH and the operating temperature of the reactor. In parallel with the experiments, 3-D fluid dynamics simulation (Fluent) was conducted to verify the micro-reformer performance. The fully integrated micro-channel reformer has the dimensions of W: 130 mm × D: 50 mm × H: 3 mm. The fuel processor produced the gas composition of 71% H₂ and 25% CO₂, and more than 93% of methanol conversion was achieved at 300 °C and 2 cm³/h of the feed flow rate when CO concentration was maintained below 100 ppm by PROX.     

Catalytic performance of Ag–Co/CeO2 catalyst in NO–CO and NO–CO–O2 system

A new bimetallic catalyst (Ag–Co/CeO₂) was studied for simultaneously catalytic removal of NO and CO in the absence or presence of O₂. CeO₂ prepared by homogeneous precipitation method was optimized as supports for the active components. The addition of Ag on CeO₂ greatly improved the catalytic activities in the lower temperature regions (⩽300 °C), and the introduction of Co on CeO₂ increased the activities at higher temperatures (⩾250 °C). The bimetallic Ag–Co/CeO₂ catalyst combined the advantages of the corresponding individual metal supported catalysts and showed superior activity due to the synergetic effect. The effect of support, temperature, loading amount, GHSV and oxygen on catalysis was investigated. NO and CO could be completely removed in the temperature range of 200–600 °C at a very high space velocity of 120 000 h⁻¹. No deactivation was observed over 4% Ag–0.4% Co/CeO₂ catalyst even after 50 h test.     

Highly efficient Ru/Sm2O3-CeO2 catalyst for ammonia synthesis

A series of Ru/Sm₂O₃–CeO₂ catalysts were prepared by using a co-precipitation (CP) method and characterized by XRD, BET, SEM, H₂-TPD-MS, H₂-TPR and CO chemisorption. The activity test shows that ammonia concentration of the catalyst with 7% Sm is 13.4% at 10 MPa, 10,000 h^− 1, 425 °C, which is 21% higher than that of Ru/CeO₂. Such high catalytic activity was due to three effects: the morphology changes of catalyst, electrodonating property of partially reduced CeO_2 − x to Ru metal and the property of easily hydrogen desorption derived from the presence of Sm³⁺ in ceria.     

Promoting effect of Fe in oxidative dehydrogenation of ethylbenzene to styrene with CO2 (I) preparation and performance of Ce1 − xFexO2 catalyst

To improve the lattice structure of CeO₂ and the transmission capacity of oxygen, Ce_1 − xFe_xO₂(x ≤ 0.2)solid solutions were prepared by a hydrothermal method and used in oxidative dehydrogenation of ethylbenzene to styrene with CO₂. Ce_1 − xFe_xO₂ solid solutions were characterized by powder X-ray diffraction, Raman spectroscopy, N₂-adsorption, H₂ temperature-programmed reduction and H₂–O₂ titration. Results showed that approximately 20% of Fe^3 + could dissolve into the CeO₂ lattice while portions of Fe₂O₃ were highly dispersed on the surface of the Ce_1 − xFe_xO₂ solid solution. The formation of Ce–Fe solid solutions could create more oxygen vacancies to promote the absorption and activation of CO₂, which improves the activity of the catalyst and increased ethylbenzene conversion by as much as 13%.     

Hydrogen production via methanol steam reforming over Au/CuO,Au/CeO2, and Au/CuO–CeO2 catalysts prepared by deposition–precipitation

The production of hydrogen (H₂) with a low concentration of carbon monoxide (CO) via steam reforming of methanol (SRM) over Au/CuO, Au/CeO₂, (50:50)CuO–CeO₂, Au/(50:50)CuO–CeO₂, and commercial MegaMax 700 catalysts were investigated over reaction temperatures between 200 °C and 300 °C at atmospheric pressure. Au loading in the catalysts was maintained at 5 wt%. Supports were prepared by co-precipitation (CP) whilst all prepared catalysts were synthesized by deposition–precipitation (DP). The catalysts were characterized by Brunauer–Emmett–Teller (BET) surface area, X-ray diffraction (XRD), temperature-programmed reduction (TPR), and scanning electron microscopy (SEM). Au/(50:50)CuO–CeO₂ catalysts expressed a higher methanol conversion with negligible amount of CO than the others due to the integration of CuO particles into the CeO₂ lattice, as evidenced by XRD, and a interaction of Au and CuO species, as evidenced by TPR. A 50:50 Cu:Ce atomic ratio was optimal for Au supported on CuO–CeO₂ catalysts which can then promote SRM. Increasing the reaction time, by reducing the liquid feed rate from 3 to 1.5 cm³ h^?1, resulted in a catalytic activity with complete (100%) methanol conversion, and a H₂ and CO selectivity of ～82% and ～1.3%, respectively. From stability testing, Au/(50:50)CuO–CeO₂ catalysts were still active for 540 min use even though the CuO was reduced to metallic Cu, as evidenced by XRD. Therefore, it can be concluded that metallic Cu is one of active components of the catalysts for SRM.     

Preferential CO oxidation in H2-rich gas mixtures over Au/doped ceria catalysts

Nanosized gold catalysts supported on doped ceria were prepared by deposition–precipitation method. A deep characterization study by HRTEM/EDS, XRD, FT-Raman, TPR and FTIR was undergone in order to investigate the effect of ceria modification by various cations (Sm³⁺, La³⁺ and Zn²⁺) on structural and redox properties of gold catalysts. Doping of ceria affected in different way catalytic activity towards purification of H₂ via preferential CO oxidation. The following activity order was observed: Au/Zn–CeO₂ > Au/Sm–CeO₂ > Au/CeO₂ > Au/La–CeO₂. The differences in CO oxidation rates were ascribed to different concentration of metallic gold particles on the surface of Au catalysts (as confirmed by the intensity of the band at 2103 cm⁻¹ in the FTIR spectra collected during CO–O₂ interaction). Gold catalysts on modified ceria showed improved tolerance towards the presence of CO₂ and H₂O in the PROX feed. The spectroscopic experiments evidence enhanced reactivity when PROX is performed in the presence of H₂O already at 90 K.     

Artificial neural network aided virtual screening of additives to a Co/SrCO3 catalyst for preferential oxidation of CO in excess hydrogen

Preferential oxidation (PROX) of 0.7–1 vol% CO was investigated using the stoichiometric amount of O₂ in excess hydrogen. Cobalt supported on SrCO₃ showed high selectivity to PROX of CO, and the new additive to the Co/SrCO₃ catalyst was investigated for the high tolerance towards CO₂ and H₂O. Representative 10 elements (B, K, Sc, Mn, Zn, Nb, Ag, Nd, Re, and Tl) were selected to represent the physicochemical properties of all elements suitable for additives of solid catalyst. A supported cobalt catalyst with one kind of the above additive was prepared for CO PROX reaction. The activities at 240 °C and the physicochemical properties of the 10 elements were used as training data of a radial basis function network (RBFN), a kind of artificial neural network. After the training, the RBFN predicted the catalytic performance of the supported catalyst containing various element X as Co–X/SrCO₃. The elements such as Bi, Ga, and In were predicted to be promising additives. Finally, the catalytic performance of these additives was experimentally verified. Sixty four percent of CO conversion and 70% selectivity for PROX at 240 °C was achieved in the presence of excess carbon dioxide and steam by Co 3.2–Bi 0.3 mol%/SrCO₃ pretreated at 345 °C.     

The CO methanation over NaY-zeolite supported Ni/Co3O4, Ni/ZrO2, Co3O4/ZrO2 and Ni/Co3O4/ZrO2 catalysts

The CO methanation was studied over zeolite NaY supported Ni, Co₃O₄, ZrO₂ catalysts. The XRD, N₂ physisorption and SEM analysis were used in order to characterize the catalysts. Catalytic activities were carried out under a feed composition of 1% CO, 50% H₂ and 49% He between the 125 °C to 375 °C. Except for the Ni/Co₃O₄/NaY catalyst, all catalysts gave high surface area because of the presence of zeolite NaY. Average pore diameter of the catalysts fell into the mesopore diameter range. The highest CO methanation activity was obtained with Ni/ZrO₂/NaY catalyst at which the CO methanation was started after 175 °C and 100% CO conversion was obtained at 275 °C using the same catalyst.     

Optimising H2 production from model biogas via combined steam reforming and CO shift reactions

H₂ production was studied through steam reforming of a clean model biogas in a fluidized-bed reactor followed by two stages of CO shift reactions (fixed-bed reactors). The steam reforming of biogas was performed over 11.5 wt.% Ni/Al₂O₃ and a molar CH₄/CO₂ ratio of 1.5 was employed as clean model biogas. Excess steam resulted in strong inhibition of carbon formation and an almost complete CH₄ (>98%) conversion was achieved.To optimise H₂ production, CO shift reactions were carried out at high (523–723 K) and low temperatures (423–523 K) using commercial catalysts, based on Cu/Fe/Cr and Cu/Zn, respectively. Increasing steam concentrations led to a lean CO, high H₂ product. The final product compositions following low temperature CO shift reaction (steam to dry gas ratio of 1.5 at 483 K) yielded H₂ at 68% and a CO concentration of 0.2% (equivalent to CO conversion of >99%).     

Hierarchically nanoporous Co–Mn–O/FeOx as a high performance catalyst for CO preferential oxidation in H2-rich stream

A novel and highly-efficient hierarchically nanoporous Co–Mn–O/FeOx catalyst fabricated by a hard/soft dual-templating and subsequent deposition–precipitation (HSDT/DP) approach demonstrates unexpectedly high catalytic activity with 100% CO conversion at 75 °C and wide temperature window of 75–200 °C with complete CO removal for CO preferential oxidation (CO PROX), ascribed to the unique microstructure and strong interaction between finely dispersed cobalt–manganese and FeO_x. The excellent catalytic performance allows it to be a practical candidate for CO elimination from H₂-rich stream.     

Regeneration of sulfur-poisoned CeO2 catalyst for NH3-SCR of NOx

The effects of regeneration on the activities and structure of CeO₂ catalysts for NH₃-SCR of NO_x have been studied in this article. CeO₂ catalyst is deactivated by SO₂ for NH₃-SCR of NO_x in a 200 h long-term operation at 350 °C due to the formation of sulfates, and its NO_x conversion decreases from 100% to 83% gradually. However, sulfates can be removed from sulfur-poisoned CeO₂ catalysts under high temperature thermal treatment in air. After regeneration, NO_x conversion of sulfur-poisoned CeO₂ catalyst is recovered to about 100% at 350 °C. Moreover, the regeneration temperature is related to the nature of the sulfates formed on the sulfur-poisoned CeO₂ catalysts.     

A novel low-temperature methanol synthesis method from CO/H2/CO2 based on the synergistic effect between solid catalyst and homogeneous catalyst

The activity of a binary catalyst in alcoholic solvents for methanol synthesis from CO/H₂/CO₂ at low temperature was investigated in a concurrent synthesis course. Experiment results showed that the combination of homogeneous potassium formate catalyst and solid copper–magnesia catalyst enhanced the conversion of CO₂-containing syngas to methanol at temperature of 423–443 K and pressure of 3–5 MPa. Under a contact time of 100 g h/mol, the maximum conversion of total carbon approached the reaction equilibrium and the selectivity of methanol was 99%. A reaction pathway involving esterification and hydrogenolysis of esters was postulated based on the integrative and separate activity tests, along with the structural characterization of the catalysts. Both potassium formate for the esterification as well as Cu/MgO for the hydrogenolysis were found to be crucial to this homogeneous and heterogeneous synergistically catalytic system. CO and H₂ were involved in the recycling of potassium formate.     

The dehydrogenation of ethylbenzene with CO2 over CexZr1 − xO2 solid solutions

With the purpose of changing the lattice structure of CeO₂ and improving the transmission capacity of lattice oxygen, Ce_xZr_1 − xO₂ solid solutions with different Zr proportions were synthesized using a hydrothermal method and applied in oxidative dehydrogenation of ethylbenzene to styrene with CO₂ at 550 °C. The Ce_0.5Zr_0.5O₂ showed the highest activity with an ethylbenzene conversion of 55% and styrene selectivity above 86%. Analytical characterization showed that the lattice oxygen mobile capacity of Ce_xZr_1 − xO₂ solid solutions was enhanced, corresponding to the order as Ce_0.3Zr_0.7O₂ > Ce_0.5Zr_0.5O₂ > Ce_0.7Zr_0.3O₂ > CeO₂. The oxygen storage/release capacity, higher surface area and pore distribution of Ce–Zr mixed oxides play important roles in the activity of ethylbenzene dehydrogenation to styrene with CO₂.     

Combustion synthesis and EIS characterization of TiO2–SnO2 system

TiO₂ is an insulator, but using specific dopants, can modify sharply its electronic structure towards semiconducting behavior. This type of response is widely applied in many electrochemical and electrocatalytical devices, namely chlorine production, hydrocarbon oxidation, CO and CO₂ hydrogenation and as electroactive substrata for biological cell growth.Combustion synthesis is a very simple, rapid and clean method for material preparation, which will be used in the preparation of the (1 − x)TiO₂–xSnO₂, x = 0.05–0.3. Tin oxalate and titanium isopropoxide are used as precursors for the synthesis. The as-prepared powders are fine and homogeneous, the average particle size is in the range of 5–10 nm, powders and ceramic compact bodies are characterized by DRX, SEM–TEM–EDX, DTA–TG and EIS. The impedance spectroscopy of the sample 10 mol% of SnO₂ indicates the presence of several phases which promote a matrix composite based in an electrical TiO₂ insulator compatible with an electronic conducting phase tin rich. This could be attributed to the spinodal decomposition effect observed in TiO₂–SnO₂ system.