Reason For Catalyst Deactivation Research Articles

The evaluation and mechanism study of zerovalent iron (Fe0) catalysts supported on coke for NO reduction by H2

Coke supported highly dispersed zerovalent iron Fe0 catalysts were synthesized and their catalytic activity in the reduction of NO by H2 was investigated using a fixed-bed reactor. The effect of H2 concentration on dispersion of active component Fe0 and activity of catalyst during preparation was investigated and density functional theory (DFT) simulation was used to study the reaction mechanism and the reason for catalyst deactivation. The Fe0 catalyst has a strong catalytic ability to reduce NO with H2. The activity of catalyst initially increased and then decreased in the increase of hydrogen concentration during the preparation. The carrier coke is not consumed in the reaction, which improves the dispersion of the active component Fe0. However, Fe0 catalyst is oxidized in the NO reduction, resulting in a decrease in the catalyst activity. The DFT calculations results suggest that the NO reduction follows the E-R mechanism, and the presence of Fe0 makes the N-O bond easily break to form N2. H2 has the capability to reduce oxidized Fe0 and release the catalytic active site, but the energy barrier that needs to be overcome is greater than that of oxidation of Fe0, which eventually leads to catalyst deactivation.

Selective Hydrogenolysis of Furfuryl Alcohol to Pentanediol over Pt Supported on MgO

The catalytic conversion of naturally rich and renewable biomass into high-value chemicals is of great significance for pursuing a sustainable future and a green economy. The preparation of pentanediol from furfuryl alcohol is an important means of high-value conversion of biomass. The Pt-based catalyst supported on MgO was applied to the selective hydrogenation of biomass furfuryl alcohol to prepare pentanediol. By adjusting parameters such as catalyst loading, reduction temperature, reaction temperature, and pressure, a highly active catalyst was designed and the optimal catalytic hydrogenation conditions were determined. The hydrogenation experiment results showed that the selectivity of the 2Pt/MgO-200 catalyst for 1,2-pentanediol and 1,5-pentanediol reached 59.4% and 15.2%, respectively, under 160 °C and 1 MPa hydrogen pressure. The catalyst was characterized by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectrometer (XPS), CO2-temperature programmed desorption (CO2-TPD), and other methods. The characterization results indicate that the reduction temperature has a significant impact on the metal Pt, and an appropriate reduction temperature is beneficial for the hydrogenation performance of the catalyst. In addition, the basic sites on the carrier are also another important factor affecting the activity of the catalyst. In addition, stability tests were conducted on the catalyst, and the reasons for catalyst deactivation were studied using methods such as thermogravimetric analysis (TGA) and Fourier transform infrared spectroscopy (FT-IR). The results showed that the activity of the catalyst decreased after five cycles, and the deactivation was due to the hydrolysis of the carrier, the increase in metal particle size, and the surface adsorption of organic matter.

Palladium Catalysts for Methane Oxidation: Old Materials, New Challenges.

ConspectusMethane complete oxidation is an important reaction that is part of the general scheme used for removing pollutants contained in emissions from internal combustion engines and, more generally, combustion processes. It has also recently attracted interest as an option for the removal of atmospheric methane in the context of negative emission technologies. Methane, a powerful greenhouse gas, can be converted to carbon dioxide and water via its complete oxidation. Despite burning methane being facile because the combustion sustains its complete oxidation after ignition, methane strong C-H bonds require a catalyst to perform the oxidation at low temperatures and in the absence of a flame so as to avoid the formation of nitrogen oxides, such as those produced in flares. This process allows methane removal to be obtained under conditions that usually lead to higher emissions, such as under cold start conditions in the case of internal combustion engines. Among several options that include homo- and heterogeneous catalysts, supported palladium-based catalysts are the most active heterogeneous systems for this reaction. Finely divided palladium can activate C-H bonds at temperatures as low as 150 °C, although complete conversion is usually not reached until 400-500 °C in practical applications. Major goals are to achieve catalytic methane oxidation at as low as possible temperature and to utilize this expensive metal more efficiently.Compared to any other transition metal, palladium and its oxides are orders of magnitude more reactive for methane oxidation in the absence of water. During the last few decades, much research has been devoted to unveiling the origin of the high activity of supported palladium catalysts, their active phase, the effect of support, promoters, and defects, and the effect of reaction conditions with the goal of further improving their reactivity. There is an overall agreement in trends, yet there are noticeable differences in some details of the catalytic performance of palladium, including the active phase under reaction conditions and the reasons for catalyst deactivation and poisoning. In this Account we summarize our work in this space using well-defined catalysts, especially model palladium surfaces and those prepared using colloidal nanocrystals as precursors, and spectroscopic tools to unveil important details about the chemistry of supported palladium catalysts. We describe advanced techniques aimed at elucidating the role of several parameters in the performance of palladium catalysts for methane oxidation as well as in engineering catalysts through advancing fundamental understanding and synthesis methods. We report the state of research on active phases and sites, then move to the role of supports and promoters, and finally discuss stability in catalytic performance and the role of water in the palladium active phase. Overall, we want to emphasize the importance of a fundamental understanding in designing and realizing active and stable palladium-based catalysts for methane oxidation as an example for a variety of energy and environmental applications of nanomaterials in catalysis.

Surface species structure of deactivated catalyst for dehydrochlorination of trichloroethane

ABSTRACT Catalytic splitting method was expected to replace saponification method in industry to produce vinylidene chloride from trichloroethane, but catalyst deactivation had become the main factor hindering its industrial application. There were many speculations based on theory about the reasons for catalyst deactivation, but there was barely any analysis of the surface substance composition of deactivated catalysts. A systematic and scientific study of the composition of deactivating substances on the surface of deactivated catalysts would contribute to further research on the causes of deactivation. Several characterizations and computing methods were combined to analyze the structure of the deactivation species on the catalyst surface. The total number of rings calculated for the extracted species was approximately 11. The number of cycloalkanes and aromatic rings was 5 and 7, respectively, and the carbon fraction of cycloalkanes was 0.38. The ratio of unsubstituted hydrogen to carbon atoms was 0.42. The results show that the molecular formula of the deactivation species on the catalyst surface was C37.72H31.51Cl15.85, which was a multi-cyclic mixed component mainly composed of aromatic hydrocarbons and aliphatic hydrocarbons.

Research Progress of a Composite Metal Oxide Catalyst for VOC Degradation

Volatile organic compounds (VOCs) are atmospheric pollutants that have been of concern for researchers in recent years because they are toxic, difficult to remove, and widely sourced and easily cause damage to the environment and human body. Most scholars use low-temperature plasma biological treatment, catalytic oxidation, adsorption, condensation, and recovery techniques to treat then effectively. Among them, catalytic oxidation technology has the advantages of a high catalytic efficiency, low energy consumption, high safety factor, high treatment efficiency, and less secondary pollution; it is currently widely used for VOC degradation technology. In this paper, the catalytic oxidation technology for the degradation of multiple types of VOCs as well as the development of a single metal oxide catalyst have been briefly introduced. We also focus on the research progress of composite metal oxide catalysts for the removal of VOCs by comparing and analyzing the metal component ratio, preparation method, and types of precursors and the catalysts' influence on the catalytic performance. In addition, the reason for catalyst deactivation and a correlation between the chemical state of the catalyst and the electron distribution are discussed. Development of a composite metal oxide catalyst for the catalytic oxidation of VOCs has been proposed.

Depolymerization Performance of Pd/Al2O3 on Ether Biomass Model Molecules under Continuous Flow Reaction

Abstract The ether model molecules BPE (phenylbenzyl ether), diphenyl ether (DPE) and phenoxyethylbenzene (PPE) of lignin were used as degradation raw materials, and Pd-Al2O3 was used as catalyst to react in batch reactor and continuous reactor, respectively. Pd-Al2O3 showed good degradation performance in batch reaction. The catalytic performance of three substances was compared in a flow reactor. The reasons of catalyst deactivation and the products adsorbed on the surface during the hydrogenolysis of BPE were studied.

Correlations of Lewis acidic sites of nickel catalysts with the properties of the coke formed in steam reforming of acetic acid

Coking is a major challenge in the reactions of steam reforming. In addition to the nature of metal species, the properties of carrier also affect coking behaviors of a reforming catalyst. In this work, we prepared nickel carried on Al2O3, SiO2 or the molecular sieves (25-H, 38-H) for assessed in steam reforming of acetic acid. The Ni/Al2O3 catalyst contained abundant Lewis acidic sites while the Ni/SiO2, Ni/38-H and Ni/25-H catalysts did not. The Lewis acidic sites in Ni/Al2O3 induced the polymerisation reactions to form the polymeric coke, which wrapped metallic nickel species and was the primary reason for deactivation of catalyst, even though the amount of coke was small. The substantial amount of coke formed over the other three catalysts did not lead to rapid catalyst deactivation. With the form of carbon nanotube or other forms was mainly the type of catalytic coke, depending on characteristic of the carrier. Comparing with the polymeric coke in the catalyst of Ni/Al2O3, the catalytic coke had a high C/H ratio, a much higher thermal stability and was more aromatic rich.

Tunable HMF hydrogenation to furan diols in a flow reactor using Ru/C as catalyst

• The hydrogenation of HMF to furan diols was studied and optimized in flow reactor. • The commercial Ru/C was an efficient catalyst for the BHMF and BHMTHF synthesis. • The highest BHMF and BHMTHF yields were ascertained only tuning the contact time. • The catalyst was stable up to 6 h and easily reactivable through acetone washing. • Internal mass transfer limitations occurred and affected the overall reaction rate. 5-hydroxymethylfurfural (HMF), accessible from various feedstocks, represents an important renewable platform-chemical, precursor for valuable biofuels and bio-based chemicals. In this work, the continuous hydrogenation of an aqueous solution of HMF to give strategic monomers, 2,5-bis(hydroxymethyl)furan (BHMF) and 2,5-bis(hydroxymethyl)tetrahydrofuran (BHMTHF) was investigated in a continuous flow reactor adopting a commercial Ru/C (5 wt%) as catalyst. The influence of the main process variables on products yield and selectivity was studied and optimized. The highest BHMF and BHMTHF yields of 87.9 and 93.7 mol%, respectively, were achieved by tuning the catalyst contact time, keeping all other variables constant (temperature, pressure, hydrogen flow rate, initial HMF concentration). Intraparticle diffusion limitation for hydrogen and HMF was shown to occur at some of the tested conditions by performing the HMF hydrogenation with different catalyst particle sizes, confirmed by calculations. Constant catalyst activity was observed up to 6 h time-on-stream and then gradually reduced. Fresh and spent catalyst characterization showed no significant sintering and negligible leaching of ruthenium during time-on-stream. A decrease of the specific surface area was observed, mainly due to humin deposition which is likely the reason for catalyst deactivation. Catalyst performance could be restored to initial values by a thorough washing of the catalyst.

Deactivation of Co/γ-Al2O3 in CO methanation studied by transient isotopic experiments: The effect of Co particle size

SSITKA coupled with transient isothermal (TIH) and temperature-programmed hydrogenation (TPH) were used to investigate the influence of Co particle size (dCo = 11–21 nm) and time-on-stream, TOS (up to 25 h) on important kinetic parameters of CO methanation at 230 °C on Co/γ-Al2O3. TOFCO and TOFCH4 were independent of dCo and decreased with TOS. The θCO and θCHx (active CO-s and CHx-s) slightly increased with dCo as opposed to θCxHy (inactive species), which remained practically constant. On the contrary, θCHx and θCxHy largely decreased and increased, respectively, while θCO stayed practically constant with TOS. An inverse D-KIE independent of dCo was estimated by a novel 13CO/D2-SSITKA experiment, reported for the first time to the best of our knowledge. The reason of catalyst deactivation was the decrease of kCHx (hydrogenation of -CHx) likely because of the influence of -CxHy on the bonding of active species and on θH (a decrease was determined).

The role of surface hydroxyl concentration on calcinated alumina in catalytic ozonation.

Alumina has been used as a catalyst for ozonation, surface hydroxyl on which is regarded as the active center for ozone attack, but the influences of hydroxyl generation are still vague. Here, we prepared alumina with different hydroxyl concentrations by adjusting calcination temperatures, of which the catalytic activity was evaluated with the mineralization degree of phenol, and then revealed the active sites of hydroxyl generation with characterization of XRD, Py-IR, and NH3-TPD. The results show that the greater the hydroxyl concentration, the higher the catalytic activity, demonstrating that surface hydroxyl contributes to its catalytic activity. The effect of calcination temperatures on hydroxyl concentration and catalytic activity is in accordance with the amount of weak Lewis acid sites on the surface of alumina, illustrating the surface hydroxyl derived from the decomposition of water adsorbed on weak Lewis acid sites. However, the catalytic performance of the alumina decreases slowly in a long-term reaction owing to the active center reduction resulted from the coverage by organic acids from phenol degradation. The present work reveals the influences of hydroxyl generation which are beneficial for adjusting surface hydroxyl regarded as active site for ozone attack and the reason of catalyst deactivation, which provides guideline for the rational design of catalyst.

Hydrogen production by the steam reforming of ethanol over cobalt catalysts supported on different carbon nanostructures

Abstract In this work, we studied the performance of Co catalysts supported on different carbonaceous structures (activated carbon, carbon nanofibers and carbon nanotubes) for the steam reforming of ethanol at 773 K. Co supported on activated carbon and carbon nanofiber exhibited the smallest Co particle size and the highest activity. All catalysts deactivated but the loss of activity depended on the carbon structure. Co/C and Co/CNT catalysts significantly deactivated, while only a slight decrease in activity was observed for Co/CNF. TG analysis did not detect the formation of carbon deposits. In situ XANES and EXAFS showed that Co was partially oxidized after reduction at 623 K and the catalyst was further reduced during reaction, which rules out the oxidation of the metallic particles as the reason for catalyst deactivation. TEM of the fresh and spent catalyst showed a significant growth of the particles for Co/C and Co/CNT, indicating that sintering is the main cause of catalyst deactivation. Co/CNF only slightly deactivated because Co sintering extent was low.

Catalytic amination of glycerol with dimethylamine over different type ofheteropolyacid/Zr-MCM-41 catalysts

The effect of different type of heteropolyacid/Zr-MCM-41 catalysts on the catalytic amination of glycerol with dimethylamine to produce Dimethylamino-3-propanal was researched. Under the premise of their respective optimum loading amount, the specific surface area, pore volume and pore size of the H6P2W18O62/Zr-MCM-41(Cat-2) were higher than those of the H3PW12O40/Zr-MCM-41(Cat-1). Cat-1 exhibited more total acidity than that of Cat-2, which resulted in the higher glycerol conversion of Cat-1 at the beginning of the reaction, but the coke deposited on Cat-1 was more than that of Cat-2. The amount of Brønsted acid sites and ratio of B/(B + L) on Cat-2 were higher than that of Cat-1. Therefore, the catalytic stability, selectivity and yield of Dimethylamino-3-propanal of Cat-2 were higher than those of Cat-1, which proved that the Brønsted acid sites are good for the catalytic amination of glycerol with dimethylamine to produce Dimethylamino-3-propanal. The leaching ratio of Cat-2 is slightly more than that of Cat-1, because H6P2W18O62 is more hydrophilic than H3PW12O40. However, the stability of Cat-2 is higher than that of Cat-1, therefore, the main reason for deactivation of catalyst is the formation of carbon deposit during this reaction. And the reaction pathway of catalytic amination of glycerol with dimethylamine was proposed. Furthermore, the reaction temperature, GHSV and the molar ratio of dimethylamine/glycerol were optimized on Cat-2 catalyst. Under the optimum reaction conditions: 300 °C, H6P2W18O62 loading of 20 wt%, GHSV of 3 h−1, glycerol aqueous concentration of 10 wt%, dimethylamine aqueous concentration of 30 wt%, molar ratio of dimethylamine/glycerol of 2.5, the glycerol conversion, selectivity and yield of Dimethylamino-3-propanal were 95.6%, 80.8% and 77.2%, respectively.

Effects of different pollutants on the performance and deactivation of Pt–Pd catalysts in a pilot-scale waste incineration system

ABSTRACTA commercial Pt–Pd catalyst was applied in a pilot-scale waste incineration system. The characteristics of reacted catalysts and the concentrations of different pollutants were analyzed. Moreover, the effects of different pollutants on the performance and deactivation of catalyst were investigated. Experimental results showed that the activity of Pt–Pd catalysts for the removal of total hydrocarbons (THCs) was influenced by the contents of heavy metals (lead, cadmium, and mercury), chlorine, sulfur, and fly ash in the flue gas. The main pollutants that influenced the removal efficiency of THCs were lead (Pb), cadmium (Cd), and mercury (Hg). The reasons of catalyst deactivation included the deposition of carbon and fly ash on the catalyst surface, the transformation of different metals (Pb and Cd) to form metal oxides (PbO, PbO2, and CdO) to adsorb and block the active sites, and the chemical reactions of sulfur with active metals to form sulfate or sulfide (PdS2).

Mg/Ga mixed-oxide catalysts for phenol methylation: Outstanding performance in 2,4,6-trimethylphenol synthesis with co-feeding of water

In this paper, we report on a new catalyst type based on Mg/Ga/O for the gas-phase methylation of phenol with the aim of obtaining the synthesis of 2,4,6-trimethylphenol. This catalyst showed an outstanding performance compared to similar catalysts reported in the literature, with high yield to the desired product. Reasons for this excellent performance were related to the high activity in methanol dehydrogenation to formaldehyde, which is the rate-limiting step in the multi-step (but one-pot) process, as well as to the moderate acidic features due to Ga sites, which enhanced the intramolecular rearrangement of O-alkylated compounds. The surprising role for co-fed steam, which greatly enhanced the selectivity to 2,4,6-trimethylphenol, was also investigated, combining reactivity experiments, in-situ FT-IR spectroscopy, and DFT calculations. It was found that the co-adsorption of water significantly affects the chemo-selectivity of the reaction, thus decreasing the formation of O-alkylated compounds in favour of the desired C-alkylated compounds. Reasons for catalyst deactivation were also studied.

Ionic Liquid‐Encapsulated Zeolite Catalysts for the Conversion of Glucose to 5‐Hydroxymethylfurfural

AbstractIonic Liquid (1‐Butyl‐3‐methylimidazolium Bromide ([BMIM]Br) or Choline Chloride (ChCl)) encapsulated zeolites (HY and H‐MOR) have been synthesized by ship‐in‐a‐bottle strategy and tested for selective conversion of glucose to 5‐Hydroxymethylfurfural (HMF). Nitrogen sorption analysis showed that the surface area and total pore volume decreased and average pore size increased significantly when the zeolites are encapsulated with ionic liquids. The major effect was found to be on the microporosity whereas the mesoporosity is affected only marginally. From Thermogravimetric Analysis it is observed that the decomposition temperature of ionic liquids increased when encapsulated in the zeolite. The catalytic tests were carried out in presence of NaCl and an extraction phase to remove the formed HMF. The test results of BMIMBr−H‐MOR indicate that even though there is a significant decrease in the surface area with encapsulation, the yield and selectivity are higher with this catalyst as compared to its bare zeolite counterpart. The effect of the addition of NaCl and extraction phase and reaction conditions such as reaction time and temperature on activity and yield of HMF are evaluated on the best catalyst. In catalyst stability study, the reasons for deactivation of catalyst are identified by post‐reaction catalyst characterizations.

Ni–Co catalyst derived from layered double hydroxides for dry reforming of methane

Ni/Co monometallic and bimetallic layered double hydroxides (LDH) were in-situ synthesized on the surface of γ-Al2O3 by urea precipitation method. Then they were decomposed and reduced by H2/Ar atmospheric plasma jet at 400 °C for 15 min to become into catalysts. The LDH precursors were verified by XRD, SEM, FT-IR and Raman, while the catalysts were characterized by N2 adsorption-desorption, XRD, TEM and H2-TPR. After that their catalytic activity and stability were evaluated in dry reforming of methane (DRM) reaction. Both Ni and 2Ni–1Co showed better catalytic performance than other catalysts. Finally, the weight and species of carbon deposition on spent catalyst were tested by TG-DTA analysis. The deposition of inert carbon was the main reason for deactivation of catalysts in DRM. Moreover, a highly uniform dispersion and small particle size of active component are achieved by in situ co-precipitation method, and γ-Al2O3 provided large specific surface area for these Ni/Co catalysts.

Efficient production of biodiesel from both esterification and transesterification over supported SO42−–MoO3–ZrO2–Nd2O3/SiO2 catalysts

SO42−–MoO3–ZrO2–Nd2O3/SiO2 (SMZN/SiO2) catalysts for the production of biodiesel via both esterification and transesterification were prepared and characterized by N2 adsorption-desorption isotherms, X-ray diffraction (XRD), scanning electron microscopy (SEM), thermogravimetry analysis (TGA), ammonia adsorption Fourier transform infrared spectra (NH3-FTIR), and ammonia adsorption temperature programmed desorption (NH3-TPD) to reveal the dependence of the stable catalytic activity on calcination time. The reason for catalyst deactivation was also studied. The calcination time remarkably affected the types of active centers on SMZN/SiO2, and 4 h was found to be the optimal calcination time. SO42− species bonded with small size ZrO2 were found to be the stable active centers, where the leaching of SO42− and the deposition of coke were inhibited. The deposition of coke was easier on large size ZrO2 than on small size ones. Calcination in air flow could eliminate the deposited coke to recover the deactivated catalysts.

Characterization for Li3PO4 catalysts toward elucidation of crystalline form and performance relationship

Alkaline lithium phosphates were prepared at different calcination temperatures and their catalytic properties were evaluated. The results showed that the 320 °C‐calcined lithium phosphate had the highest selectivity to allyl alcohol (AA) and higher propylene oxide (PO) conversion among those catalysts calcined from 320 °C to 800 °C. There were two forms of lithium phosphates, β‐form and γ‐form, and the transition temperature was 670 °C. The Brunauer‐Emmet‐Teller (BET) surface area and alkalinity of the β‐Li3PO4 was larger than that of the γ‐Li3PO4, and the change of the crystal forms affected catalytic activity a lot. Infrared spectra (IR) and X‐ray powder diffraction (XRD) results showed that the reason of catalyst deactivation might be related to the carbon deposits and the arrangement of the atoms in the two forms of lithium phosphates. Based on structural analysis for Li3PO4, a new mechanism of PO isomerization was proposed.

Deactivation of supported skeletal Ni catalyst and effect of regeneration temperature on its catalytic performance

Deactivation and regeneration of supported skeletal Ni catalyst applied to hydrogenation of indene and styrene in fixed-bed reactor were investigated. The significant aggregation of skeletal Ni and formation of coke precursors were the main reasons for deactivation of catalyst. Furthermore, TG-DTA, XRD, SEM, BET and H2-TPR were utilized to characterize the regenerated catalysts and calcining the spent catalysts in air at 550°C for 3h and then reducing in H2 at 450°C for 3h under 240h−1 (GHSV) could recover its activity according to hydrogenation evaluation results.

Effect of reduction temperature on Ru–Ir/ZnO catalyst for selective hydrogenation of crotonaldehyde

A Ru–Ir/ZnO catalyst with metal loadings of 3% Ru and 3% Ir was reduced at different temperatures (150 to 400°C) and tested for vapor-phase selective hydrogenation of crotonaldehyde at 80°C. It was found that with increasing reduction temperature, the crotonaldehyde conversion over the catalysts first increased and then decreased. A conversion of 93.5% and the selectivity to crotyl alcohol of 86.6% was observed after 10h reaction on a Ru–Ir/ZnO catalyst reduced at 200°C. Various characterizations such as X-ray photon spectroscopy (XPS) and ammonia temperature-programmed desorption (NH3-TPD) results demonstrated that moderate interaction between the CO bond and the M0 (M=Ru, Ir or Ru–Ir alloy) due to the proper charge density of M0, and surface acidity of the catalysts played decisive roles in the enhanced activity and selectivity obtained on the catalyst. In addition, the deactivation of the catalyst was due to the carbon deposit (including organic compounds) on the catalyst surface, as evidenced by Raman spectroscopy and temperature-programmed oxidation over the spent catalyst. Also, the strong adsorption of CO on the catalyst surface generated by a decarbonylation reaction could be another reason for catalyst deactivation, as evidenced by a CO poisoning experiment.