In this work, titanium nitride (TiN) coating was used as a passive layer to inhibit metal catalytic coking during hydrocarbon fuel cracking on the microchannel inner surface of stainless steel 304 (SS304) tubes. In order to obtain an inert and effective passive coating, TiN coating was prepared in SS304 tubes with 2 mm inside diameter and 700 mm length by atmospheric pressure chemical vapor deposition (APCVD) using a TiCl4–H2–N2 system. The coating’s thickness, phase composition, morphology, and chemical composition were investigated by metalloscopy, X-ray diffraction, scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDX), respectively. Characterization results indicated that TiN coating had a relatively complete cubic-phase crystal form with a N/Ti ratio of 1:1, presenting small star-shaped crystals on the whole. The inhibition effects of TiN coating on the morphologies and amounts of coke were studied by SEM and EDX after n-hexane thermal cracking at 600 °C and 3.3 MPa for 20 min. Under these conditions, different contributions to carbon deposition were discussed including oxidative reactions and pyrolysis of n-hexane. Along the axial length of the bare tube, stunted and clubbed cokes formed by autoxidation near the distance of 100 mm; granular metal carbides and filamentous cokes formed by metal catalysis near the distances of 350 and 600 mm, respectively. However, no morphologies of carbon deposits on a TiN-coated tube surface were observed after thermal cracking of n-hexane at 600 °C and 3.3 MPa for 20 min. At distances of 100, 350, and 600 mm away from the tube inlet, the carbon atomic percentages of coke in these three areas were 27.28%, 58.04%, and 99.69% for the bare tube, larger than those of 5.76%, 15.73%, and 30.66% for the TiN-coated tube, respectively. The results showed that the inhibition effect of APCVD TiN coating on coke growth is superior to that of other coatings (e.g., alumina coating). The reason is that TiN coating not only creates a barrier between the hydrocarbon fuels and metal surface to inhibit related catalytic coke formation but also minimizes carbon deposits by absorbing C atoms.

•The performances of Mo-promoted composite oxide catalysts were investigated.•Surface acidity of the catalysts was modulated by MoO3 additive.•The correlation between structure, acidity and activity of catalysts were studied.•The catalyst containing 7.0 wt.% MoO3 exhibited the best catalytic activity.A series of Pt/MoO3/ZrO2-TiO2-Al2O3 composite oxides with various MoO3 contents were prepared by incipient wetness impregnation method, and coated on the inner wall of stainless-steel microchannels. Catalytic cracking of n-decane (as model molecule of hydrocarbon fuel) over such monolithic catalysts were investigated under high temperature and high pressure conditions. The catalysts were characterized by N2 adsorption-desorption, X-ray diffraction (XRD), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), Ultraviolet–visible diffuse reflectance spectroscopy (UV–vis DRS) and temperature programmed desorption of ammonia (NH3-TPD) techniques. The correlation between the catalytic performances of MoO3 modified catalysts and their structure as well as acidic property were observed. It was found that the conversion and heat sink of n-decane were obviously heightened compared with that obtained from thermal cracking. In particular, the catalyst which contains 7.0 wt.% MoO3 exhibited the best catalytic cracking activity among the prepared catalysts, which is in well agreement with its highest amount of total acid sites and strong acid centers, along with the finely dispersed isolated and octahedral molybdenum species.

A series of WO3/CeZrO2 catalysts, prepared at different calcination temperatures (400, 500, 600 and 700 °C) of cerium–zirconium mixed oxides (CeZrO2) for the selective catalytic reduction of NOx with ammonia (NH3-SCR), were investigated via various characterizations, such as N2 physisorption, XRD, Raman, NH3-TPD, DRIFTS, XPS and H2-TPR. The catalytic performance of NH3-SCR was remarkably promoted by modestly increasing the calcination temperature of CeZrO2: WO3/CeZrO2-500 possessed the lowest light-off temperature (173 °C) and complete conversion temperature (205 °C), while W/CeZrO2-600 could achieve greater than 90% NOx conversion in a broad temperature range of 220–455 °C. The characterization results indicated that modest enhancement of the calcination temperature of CeZrO2 was beneficial to stabilizing the structure of the catalysts. The largest amount of Lewis acid sites, Ce3+ and surface active oxygen species, as well as strong redox properties of WO3/CeZrO2-500 should together contribute to its better low-temperature deNOx activity. Moreover, increasing the calcination temperature of cerium–zirconium mixed oxides resulted in the enhancement of Brønsted acid sites, which was responsible for the widened operation temperature window. Therefore, WO3/CeZrO2 serial catalysts with appropriate calcination treatment of CeZrO2 would be a good choice for the removal of NOx emitted from diesel engines.

A series of WO3/CeZrO2 catalysts, prepared at different calcination temperatures (400, 500, 600 and 700 °C) of cerium–zirconium mixed oxides (CeZrO2) for the selective catalytic reduction of NOx with ammonia (NH3-SCR), were investigated via various characterizations, such as N2 physisorption, XRD, Raman, NH3-TPD, DRIFTS, XPS and H2-TPR. The catalytic performance of NH3-SCR was remarkably promoted by modestly increasing the calcination temperature of CeZrO2: WO3/CeZrO2-500 possessed the lowest light-off temperature (173 °C) and complete conversion temperature (205 °C), while W/CeZrO2-600 could achieve greater than 90% NOx conversion in a broad temperature range of 220–455 °C. The characterization results indicated that modest enhancement of the calcination temperature of CeZrO2 was beneficial to stabilizing the structure of the catalysts. The largest amount of Lewis acid sites, Ce3+ and surface active oxygen species, as well as strong redox properties of WO3/CeZrO2-500 should together contribute to its better low-temperature deNOx activity. Moreover, increasing the calcination temperature of cerium–zirconium mixed oxides resulted in the enhancement of Brønsted acid sites, which was responsible for the widened operation temperature window. Therefore, WO3/CeZrO2 serial catalysts with appropriate calcination treatment of CeZrO2 would be a good choice for the removal of NOx emitted from diesel engines.

A series of Nb-substituted WO3/CeZrOx catalysts were prepared by the co-impregnation method and applied in the selective catalytic reduction of NOx with NH3 (NH3-SCR). NH3 oxidation, N2 sorption, XRD, Raman, UV-vis, XPS, H2-TPR, O2/NH3-TPD and in situ DRIFTS were performed to correlate the redox property and surface acidity to NH3-SCR performance of Nb-substituted catalysts. The catalyst with 5 wt% substitution amount of Nb2O5 presented excellent deNOx activity and N2 selectivity in a broad reaction temperature window of 190–434 °C at a gas space velocity of 30 000 h−1. The characterization results demonstrated that the partial substitution of WO3 by Nb2O5 not only led to strong redox properties arising from abundant surface active oxygen species, but also promoted the adsorption of NH3 and the redistribution of acid sites due to Nb–OH related to Brønsted acid sites and NbO bonded to strong Lewis acid sites. The enhancement of surface active oxygen species and Brønsted acid sites promoted the low-temperature (below 250 °C) deNOx activity. However, the preoxidation of NH3 at high temperatures slightly suppressed the NOx conversion of the catalyst with more strong Lewis acid sites at above 400 °C. Moreover, the catalyst also showed excellent sulfur tolerance and could be a promising candidate for practical applications in NOx abatement.

A series of Nb-substituted WO3/CeZrOx catalysts were prepared by the co-impregnation method and applied in the selective catalytic reduction of NOx with NH3 (NH3-SCR). NH3 oxidation, N2 sorption, XRD, Raman, UV-vis, XPS, H2-TPR, O2/NH3-TPD and in situ DRIFTS were performed to correlate the redox property and surface acidity to NH3-SCR performance of Nb-substituted catalysts. The catalyst with 5 wt% substitution amount of Nb2O5 presented excellent deNOx activity and N2 selectivity in a broad reaction temperature window of 190–434 °C at a gas space velocity of 30 000 h−1. The characterization results demonstrated that the partial substitution of WO3 by Nb2O5 not only led to strong redox properties arising from abundant surface active oxygen species, but also promoted the adsorption of NH3 and the redistribution of acid sites due to Nb–OH related to Brønsted acid sites and NbO bonded to strong Lewis acid sites. The enhancement of surface active oxygen species and Brønsted acid sites promoted the low-temperature (below 250 °C) deNOx activity. However, the preoxidation of NH3 at high temperatures slightly suppressed the NOx conversion of the catalyst with more strong Lewis acid sites at above 400 °C. Moreover, the catalyst also showed excellent sulfur tolerance and could be a promising candidate for practical applications in NOx abatement.

•The addition of TiO2 promoted structural properties and NH3-SCR activity of WO3/Ce0.68Zr0.32O2.•The WO3/Ce0.68Zr0.32Ti20O2 catalyst displayed excellent H2O/SO2 tolerance.•The amount of acid sites and reactivity of active sites were increased by doping TiO2.•Langmuir-Hinshelwood mechanism was presented at 200 °C.CeZrTixO2 mixed oxides were prepared by a co-precipitation method, and serial WO3/CeZrTixO2 catalysts were prepared to investigate the influence of doping TiO2 into CeZrO2 on the catalytic performance of selective catalytic reduction of NOx with NH3. The activity results showed that the introduction of appropriate amount of TiO2 could effectively improve the catalytic performance. WO3/CeZrTi20O2 with 20 wt.% TiO2 showed better deNOx activity and sulfur/water vapor tolerance than W/CeZrO2. Several techniques, including N2 physisorption, XRD, XPS, H2-TPR, NH3-TPD and in situ DRIFTS, were employed to characterize catalysts. The results indicated that doping TiO2 led to the formation of cerium-zirconium-titanium solid solution with larger surface area. The interactions among metal oxides could enhance the redox properties of the catalyst, which was helpful to the improvement of the low-temperature NH3-SCR activity. Moreover, the addition of TiO2 promoted the adsorption and activation of NH3 and increased the reactivity of adsorbed nitrate species with NH3 species, which significantly affected the NH3-SCR performance. Finally, the results of in situ DRIFTS demonstrated that the NH3-SCR reaction mainly followed the Langmuir-Hinshelwood mechanism over W/CeZrO2 and W/CeZrTi20O2 catalysts at 200 °C.Download high-res image (154KB)Download full-size image

•The promoted catalysts presented high hydrogen selectivity and anti-carbon property.•The Pt–Ba/ZTA showed the least carbon deposit for the moderate acidity.•The Pt–Ce/ZTA catalyst showed the highest hydrogen yield as well as the smallest particle size of Pt.In order to improve the anti-carbon property and obtain higher H2 yields, the promoters (BaO, SrO and CeO2) were introduced into Pt/ZrO2–TiO2–Al2O3 catalyst. The activity of theses catalysts were investigated in the cracking reactions of RP-3 jet fuel under high temperature and high pressure conditions. The physicochemical characteristics of the catalysts were detected by Temperature programmed oxidation, Raman spectrum, N2 adsorption–desorption, Transmission electron microscope, NH3-temperature programmed desorption and NH3-infrared spectroscopy techniques. It was found that the addition of BaO, SrO and CeO2 promoted the well dispersion of Pt, stimulated the dehydrogenation reactions, and consequently higher hydrogen yields over modified catalysts were obtained. Moreover, the acid sites have been partially neutralized by these promoters, thus the total amount of acid sites as well as the Lewis acid sites decreased over the modified catalysts. The modified acidic properties inhibited the hydrogen transfer reactions and alkenes oligomerization reactions, resulting in the obvious decrease of carbon deposit. Therefore, the catalysts exhibited remarkable anti-carbon property after modifying by BaO, SrO and CeO2. The valuable information in this work may be helpful to develop highly efficient catalysts for the advanced aircrafts.

•The introduction of promoters has a positive effect on inhibiting carbon deposit.•The Sr-PZTA showed the least carbon deposit and best cracking activity.•The performance of catalysts in inhibiting carbon deposit correlates with surface acidity closely.•The morphology and reactivity of carbon deposit over the catalysts are influenced by promoters.The study involved a Pt/ZrO2-TiO2-Al2O3 catalyst (PZTA) modified by MxOy (M = K, Ba and Sr) promoters to inhibit the formation of carbon deposit during catalytic cracking reactions. It was found that Sr-PZTA showed the least carbon deposit (5.22 mg/cm2), highest gas yield (20.05%) and heat sink (2.70 MJ/kg). Moreover, the catalysts before and after modification were characterized by N2 adsorption-desorption measurement, X-ray diffraction (XRD), NH3-Temperature programmed desorption (NH3-TPD), Pyridine-infrared spectroscopy (Py-IR), Temperature programmed oxidation (TPO), and Scanning electron microscopy (SEM). The results indicated that the addition of these promoters not only changed the strength and density of acid sites, but also regulated the amount of Brönsted and Lewis acid sites. After modification, more than 50% of the strong acid sites transformed to weak or/and medium acid sites. Meanwhile, the density of acid sites decreased from 4.5 to 4.0 μmol/m2, leading the disproportionation reactions between adjacent carbeniums which are responsible for carbon deposit inhibited. In addition, the amount of Lewis acid sites decreased seriously and only 10% of Lewis acid sites kept over K-PZTA. The dehydrogenation reactions of forming unsaturated carbenium and carbon deposit were thus hindered. By contrast, the strong Brönsted acid sites which can accelerate the rate of cracking reactions over Ba-PZTA and Sr-PZTA increased. Both the spherical and spiral filament carbon deposit were detected on the surface of PZTA. However, the deposit over the modified catalysts mainly presented as agglomerates of spherical particles.Download high-res image (104KB)Download full-size image

•Alkyl acids assisted co-precipitation is employed to synthesize CeO2-ZrO2-La2O3-Y2O3.•Better properties are obtained in CZ-C12 compared with the commercial material SO-2.•The supported Rh/CZ-C12 shows higher three-way catalytic activity than Rh/SO-2.A series of nanostructured CeO2-ZrO2-Y2O3-La2O3 quaternary solid solutions (CZ-C10, CZ-C12, CZ-C14 and CZ-C16) were synthesized by simultaneous co-precipitation method with the assistance of alkyl acids CnH2n+1COOH (n = 9, 11, 13 and 15). The obtained materials exhibit greatly enhanced oxygen storage capacity, textural properties and thermal stability. After aging at 1000 °C, CZ-C12-a presents a high surface area of 47 m2 g−1 and a large cumulative pore volume of 0.28 mL g−1. A comparison between CZ-C12 and the commercial material SO-2 in terms of the catalytic performance of supported Rh-only three-way catalysts indicates that Rh/CZ-C12 owns significantly higher catalytic activity than Rh/SO-2. After aging at 1050 °C for 12 h, the light-off temperatures (T50) of C3H8, CO and NO for Rh/CZ-C12-a are 314 °C, 263 °C and 322 °C, respectively, which are much lower than those (360 °C, 290 °C and 386 °C) for Rh/SO-2-a.Alkyl acids CnH2n+1COOH (n = 9, 11, 13 and 15) are applied to synthesize highly thermal stable nanostructured CeO2-ZrO2-Y2O3-La2O3 quaternary solid solutions. The results indicate that the dodecanoic acid assisted material CZ-C12 exhibits better properties than the commercial material SO-2 provided by Rhodia. The investigation of supported Rh-only three-way catalysts for gasoline engine exhaust purification show that Rh/CZ-C12 presents higher catalytic activity and better thermal stability with respect to Rh/SO-2.Download high-res image (170KB)Download full-size image

•NO and NO2 has no promotion for the soot oxidation in the tight contact.•The promotion of NOx in loose contact is mainly related to active oxygen species.•NO shows better promotion than NO2 due to formation of more surface nitrates.•Active oxygen species would play a key role in NOx-assisted soot oxidation.This study aims to compare the promoting roles of NO and NO2 additions on soot oxidation over Pt/MnOx-CeO2 under tight and loose contacts and further reveal the mechanisms for the promoted reactions by DRIFTS (diffuse reflectance infrared fourier transform spectroscopy) and GCMS (gas chromatograph mass spectrometer). The soot oxidation tests show that the promotion of NOx cannot be exhibited under tight contact and NO addition displays a greater promoting role than NO2 addition under loose contact. The mechanism investigations indicate that more surface nitrates form on the catalyst in NO and their decomposition effectively enhances the formation and desorption of more active oxygen species, which would be a key role in the accelerated soot oxidation reactions. However, the promoting role of NO can not be found in tight contact, it may be due to the inhibition of nitrate formation by soot coverage on the surface of the catalyst. This work also confirms that active oxygen would be main contributor for soot oxidation despite the presence of NO2.The improved activity of Pt/MnOx-CeO2 in NOx + O2 would be mainly attributed to the formation of active oxygen species (O∗) during surface nitrate decomposition. Furthermore, the NO in reaction gas shows greater promoting roles than NO2 due to the formation and decomposition of more surface nitrates.Download high-res image (153KB)Download full-size image

•With the proper tungsten weight ratio (2 wt%), it showed the best stability.•The interaction of Pt-W could be favorable for the catalytic performance.•Doping tungsten generated and maintained more active Pt metallic state and sites.A series of the modified Pt/SiO2–Al2O3 diesel oxidation catalysts with different WO3 loading were prepared through co-impregnation method. With an optimum additive loading of 2 wt% WO3, the catalyst significantly improved the performance and its stability. Compared to the free-tungsten sample, the catalyst increased the maximum conversion of NO by 5–10% and decreased the reaction temperature for 30% NO conversion by 7–28 °C. It was found that there exist the electron and structure interaction between platinum and tungsten species. The interactions can generate and maintain more Pt sites in metallic state for the catalytic oxidation with enhanced activity and stability.Download high-res image (89KB)Download full-size image

•The Nd doped material owns excellent properties which could meet the requirements in practical applications.•Pd metal particles selectively adsorb onto the surface Ce sites of different supporting materials.•Pd/CZLN shows superior three-way catalytic activity with respect to Pd/CZLY and Pd/CZLP.In this work, a series of nanostructured Ce-Zr-La-RE-O (RE = Y, Nd and Pr) quaternary solid solutions (CZLY, CZLN and CZLP) were synthesized via simultaneous co-precipitation with the assistance of lauric acid. The corresponding supported Pd-only three-way catalysts (Pd/CZLY, Pd/CZLN and Pd/CZLP) were also prepared by wet-impregnation method. The effects of Y, Nd and Pr on CeO2-ZrO2-La2O3-RE2O3 system and corresponding supported Pd-only three-way catalysts were investigated by X-ray diffraction (XRD), Raman, hydrogen-temperature programmed reduction (H2-TPR), N2 adsorption-desorption and X-ray photoelectron spectroscopy (XPS) characterizations. The results show that all the fresh samples form single phase structure, however, after being thermal aged at 1000 °C, CZLN-a exhibits the best thermal stability which presents a remarkably low reduction peak temperature of 395 °C and a high surface area of 42.6 m2/g. The study of the interactions between Pd metal particles and supports indicates that Pd could selectively adsorb onto the surface Ce sites of different supporting materials by forming different chemical bonds. Pd/CZLN also exhibits superior three-way catalytic activity with respect to others, particularly, the light-off temperatures (T50) of C3H8, CO and NO for Pd/CZLN-a are as low as 339, 175 and 198 °C, respectively.Download high-res image (114KB)Download full-size image

Conventional co-precipitation combined with a urea-assisted low-temperature (90 °C) hydrothermal procedure (CZU) and the same method without urea (CZ) were used to prepare material CeO2–ZrO2–La2O3–Nd2O3. X-ray diffraction, Raman, nitrogen adsorption–desorption, transmission electron microscope, hydrogen-temperature programmed reduction (H2-TPR) and X-ray photoelectron spectroscopy were employed to study the structural, textural, reduction behavior and surface elemental composition of the materials. The results reveal that, compared to CZ, CZU exhibits more outstanding structural property, higher thermal stability and higher low-temperature reducibility. The results also show that the stability of the reduction behavior is closely related to its surface chemical properties, especially the variation of the surface atomic ratio of Ce/Zr and the surface oxygen species. The possible mechanism of urea was also discussed in this study. In addition, with regard to corresponding Pd-only three-way catalysts, remarkably boosted catalytic performance of Pd/CZU is also obtained than that of Pd/CZ, and which suggests that CZU holds better prospective applications.

To preferably catalyze the oxidation of NO to NO2 in diesel after-treatment system, a series of CeO2-MnOx composite oxides was supported on silica-alumina material by the co-impregnation method. The maximum conversion of NO of the catalyst with a Ce/Mn weight ratio of 5:5 was improved by around 40%, compared to the supported manganese-only or cerium-only sample. And its maximum reaction rate was 0.056 μmol g−1 s−1 at 250 °C at the gas hourly space velocity of 30,000 h−1. The experimental results suggested that Ce-Mn solid solution was formed, which could modulate the valence state of cerium and manganese and exhibit great redox properties. Moreover, the strong interaction between ceria and manganese resulted in the largest desorption amount of strong chemical oxygen and oxygen vacancies, leading to the maximum Oα area ratio of 62.26% from the O 1s result. These effective oxygen species could be continually transferred to the surface, leading to the best NO catalytic activity of 5Ce5Mn/SA catalyst.

•ZrO2–TiO2–Al2O3 composite oxides were introduced into cracking of jet fuel.•The surface acidity provided a guarantee for activity of cracking reaction.•The centralized strong acidic sites provided a guarantee for selectivity.•The heat sink over Pt/ZrO2–TiO2–Al2O3 (1:1:3) reached by 3.88 MJ/kg at 750 °C.•A sharply deactivation is mainly caused by the quick generation of coking.A series of Pt/ZrO2–TiO2–Al2O3 (PZTA) composite oxides with different Al2O3 mass ratio (ZrO2:TiO2:Al2O3 = 1:1:x, where x = 1, 2, 3, 4, 5) were coated on the wall of stainless-steel microchannels by using a co-precipitation method. Catalytic cracking of RP-3 over such PZTA-coated catalysts were investigated under high temperature and pressure conditions. It was found that the gas yield and heat sink of catalytic cracking of RP-3 were evidently heightened compared with the thermal cracking. Furthermore, catalytic cracking activities and high-temperature stabilities of the catalysts were greatly depending on the mass ratio of Al2O3. PZTA (ZrO2:TiO2:Al2O3 with a mass ratio of 1:1:3) performed a better catalytic cracking activity and high-temperature stability, which is in accordance with its higher amount of strong acid, larger surface area and pore volume. Deactivation of the increasing rate of gas yield and heat sink over PZTA at 700 °C was possibly caused by decrement of their amount of surface acid, the collapse of the catalysts pores partly and the crystallizations of ZTA above 650 °C, and the quickly generation of carbon deposition at 700 °C. Even so, PZTA (ZrO2:TiO2:Al2O3 in the mass ratio of 1:1:3) kept a better catalytic cracking activity and high-temperature stability.

•Gas yield over Pt/WO3–ZrO2 catalysts was improved with respect to thermal cracking.•WO3 could effectively inhibit the phase transformation of ZrO2.•Cat3, the WO3 4 wt% content, performed the best catalytic cracking activity.A series of WO3–ZrO2 composite oxides were prepared by co-precipitation and their catalytic performances for the RP-3 kerosene cracking were investigated. These composite oxides were characterized by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), N2 adsorption–desorption measurement, X-ray diffraction (XRD), and NH3-temperature programmed desorption (NH3-TPD). The results showed that the addition of WO3 could effectively inhibit the phase transformation of ZrO2 from tetragonal phase to monoclinic phase. Among all as-prepared samples, the support composed of WO3 (4 wt%)–ZrO2 exhibited the largest surface area, the strongest surface acidity, and the most concentrated strong acidic density. Additionally, the gas amount of catalytic cracking over Pt/WO3 (4 wt%)–ZrO2 catalyst at the temperature region 650–700 °C was about 1.5 times higher than that of thermal cracking. After the composite oxides were calcined at 800 °C for 3 h, they almost lost their catalytic activities.

Star-shaped 800-TiN and 850-TiN coatings were deposited on the surface of 310S stainless steel foils by CVD and their oxidation behavior was investigated in ambient air, from 300 °C to 800 °C for 1800 s by XRD, SEM, EDX and Raman spectroscopy. Initial oxidation of 850-TiN coating with a partial color change occurs at 350 °C, remarkable oxidation of 850-TiN coating occurring between 400 °C and 450 °C. The EDX results show that obvious oxidation of 850-TiN starts at 400 °C with about 9 at% oxygen detected; no N atoms could be detected while the O content reaching a maximum of ca. 70% at oxidation temperature above 700 °C. The XRD and Raman results show that only rutile-TiO2 formed on the surface of oxidized TiN coating. The oxidation of star-shaped TiN coating can be divided into three stages. In the case of mild oxidation (below 500 °C), TiN coating can maintain the star-shaped microstructure although oxygen diffuses into the TiN lattice resulting in replacement of N by O atoms. For moderate oxidation (550–600 °C), the star-shaped microstructures start to crack along the (111) twin planes, and the boundary of particles remains clear with oxide and oxynitride layer coexisting on the surface of 850-TiN coating. For severe oxidation (650–750 °C), the cracks of the star-shaped microstructures start to expand and become apparent, meanwhile the boundary of particles become uncertain. After oxidizing at 800 °C, the 850-TiN coating will lose efficacy due to the bad spalling resistance.

To inhibit the metal catalytic coking and improve the oxidation resistance of TiN coating, rutile TiO2 coating has been directly designed as an efficient anticoking coating for n-hexane pyrolysis. TiO2 coatings were prepared on the inner surface of SS304 tubes by a thermal CVD method under varied temperatures from 650 to 900 °C. The rutile TiO2 coating was obtained by annealing the as-deposited TiO2 coating, which is an alternative route for the deposition of rutile TiO2 coating. The morphology, elemental and phase composition of TiO2 coatings were characterized by SEM, EDX and XRD, respectively. The results show that deposition temperature of TiO2 coatings has a strong effect on the morphology and thickness of as-deposited TiO2 coatings. Fe, Cr and Ni at.% of the substrate gradually changes to 0 when the temperature is increased to 800 °C. The thickness of TiO2 coating is more than 6 μm and uniform by metalloscopy, and the films have a nonstoichiometric composition of Ti3O8 when the deposition temperature is above 800 °C. The anticoking tests show that the TiO2 coating at a deposition temperature of 800 °C is sufficiently thick to cover the cracks and gaps on the surface of blank substrate and cut off the catalytic coke growth effect of the metal substrate. The anticoking ratio of TiO2 coating corresponding to each 5 cm segments is above 65% and the average anticoking ratio of TiO2 coating is up to 76%. Thus, the TiO2 coating can provide a very good protective layer to prevent the substrate from severe coking efficiently.Keywords: anticoking coating; anticoking ratio; n-hexane pyrolysis; oxidation resistance; rutile TiO2

•The chemical vapor deposition TiN coating shows a good ant-coking performance.•The products distribution of the pyrolysis of fuels is influenced by TiN coating.•The influence of TiN on cracking behavior is not effect of promotion or inhibition.•The solid, liquid and gaseous products which formed extreme condition were studied.To understand the cracking behavior of hydrocarbon fuels when a passivation coating covers the metal substrate, the pyrolysis of n-hexane, cyclohexane and benzene in uncoated and titanium nitride (TiN) coated stainless steel 304 (SS304) continuous tubular reactors (3 mm diameter, 700 mm long) under high temperature and pressure were studied. The TiN coating was prepared on the inner surface of SS304 tubes by atmospheric pressure chemical vapor deposition (APCVD) using TiCl4-H2-N2 system. The solid, liquid and gaseous products distribution of the pyrolysis of n-hexane, cyclohexane and benzene for bare and TiN-coated tubes were analyzed and discussed. The results show that the influence of TiN coating on hydrocarbon fuels cracking is not a simple effect of promotion or inhibition. As a whole, the TiN coating has little effects on the distribution of the gas products for n-hexane cracking, but has great influence on that of cyclohexane and benzene cracking. For the liquid products distribution, the mass fractions of alkane for bare tubes reduce compared with those for TiN-coated tubes, while the alkene contents are higher than those for TiN-coated tubes. Moreover, the morphologies of carbon deposits are mainly filamentous and spherical on the inner surface of SS304 tubes, but no morphologies of carbon deposits are observed on TiN-coated tubes surface after thermal cracking of hydrocarbon fuels. Meanwhile, the amount and carbon deposition rate of cokes deposited in the system for TiN-coated tubes are higher than those of bare tubes, regardless of the hydrocarbon fuels are n-hexane, cyclohexane and benzene.

Cu/ZSM-5 (CZ) zeolite and Pt/CeO2–Al2O3 (PCA) composite catalysts (PCA, PCA + CZ, and CZ) were prepared using the impregnation method, the as-prepared catalysts were characterized using an automatic adsorption instrument, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and temperature-programmed desorption (TPD). Catalytic cracking of Chinese number 3 jet fuel (RP-3) was used to examine the catalytic activities and thermal stabilities of the as-prepared catalysts. It was found that PCA + CZ had better catalytic cracking activities and high-temperature stabilities compared to the other catalysts. This is in well accordance with an increase in the strong acid amount and the appearance of a bimodal structure of PCA + CZ. Rapid deactivation of the heat sink over CZ at 700 °C was possibly caused by their poor stabilities under high temperatures. The addition of CZ onto PCA to form a composite catalyst may be an effective approach to maintain both catalytic activity and thermal stability (>650 °C).