Mechanism Of Catalytic Reduction Research Articles

Charge and Geometrical Effects on the Catalytic N2O Reduction by Rh6– and Rh6+ Clusters

The catalytic conversion of nitrous oxide (N2O) is of crucial environmental relevance because this chemical compound is a greenhouse gas with an important contribution to climate change, even larger than CO2, depleting the ozone layer. Recently, reduction of N2O catalyzed by rhodium subnanoclusters has been the subject of intensive research, both experimental and theoretical, finding dependencies of reaction rate on the size and geometry and electronic structure of the cluster. In this work, the catalytic reduction mechanism of N2O by Rh6– and Rh6+ ionic clusters has been studied by means of density functional theory calculations within the zero-order-regular approximation (ZORA), which explicitly includes relativistic effects. The N2O + Rh6– and N2O + Rh6+ reaction pathways were approached starting from a comprehensive search of different stable adsorption modes; transition states were determined as well. We have obtained that the Rh6– anions present the lowest activation barriers without spin selectivit...

Characterizing CO2 Reduction Mechanism Using Advanced Synchrotron-Based X-Ray Spectroscopy Techniques

Generation of solar fuels through photo-catalytic reduction of CO2 could simultaneously address both the need for sustainable and affordable energy sources and the necessity of reducing global CO2 footprint. Discovering and designing new catalysts that have both high activity and selectivity for CO2 reduction is the critical barrier to overcome in order for solar fuel generation processes to commercialize. So far, copper is the sole material known to efficiently catalyze conversion of CO2 to considerable amounts of C2 hydrocarbon products over time. However, CO2 reduction on Copper exhibits a high overpotential plus the added disadvantage of producing a mixture of several products including hydrogen.1,2 Thus, it is of vital importance to understand the so far ambiguous and poorly understood catalytic CO2 reduction mechanism on copper. In this work, a combination of various in-situ X-ray spectroscopy techniques are utilized to probe the electrode/electrolyte interface and provide insight into the reduction reaction.X-ray absorption spectroscopy (XAS) provides an element-specific probe of the conduction band via a core-level excitation into unoccupied electronic states and can reveal both oxidation state and details of the electronic structure. Operando Grazing incidence XAS at beamline 11-2 of SSRL is utilized to study the electrode surface at the metal K- and L-edges. A specially designed 3D printed flow cell maintainsa 300 micron liquid layer above the catalyst surface, enabling XAS characterization of the catalyst surface during electrochemistry in grazing incidence mode. The grazing incidence geometry provides a probe depth of about 5 nm into the catalyst, hence providing a highly surface sensitive spectroscopic tool. Preliminary measurements were conducted on the relatively inert surface of AuPd alloy catalyst. This is the first step in determining the feasibility of the study before examining the more complicated copper system. Distinct reversible shifts are observed in spectroscopic features with applied potential, which are believed to be due to H+ intercalation into Pd phase. (Figure 1) Further, operando soft X-ray spectroscopy measurements at C-edge are conducted at beamline 8.0.1 of ALS to detect intermediate carbon species. Electron yield soft XAS data are collected at a depth of 1 nm above the electrode surface in a specially designed in-situ electrochemical flow cell3. Strong 1s to π* transition in C edge spectra provides fingerprint for different products. First principles DFT calculations are conducted to characterize features obtained through these XAS measurements. (Figure 2) Combination of these spectroscopic techniques with electrochemical and theoretical calculations will paint a comprehensive picture of the exact CO2RR mechanism on Cu surface, which then can be used to design next generation photo-catalysts.

Interaction of Pyridine and Water with the Reconstructed Surfaces of GaP(111) and CdTe(111) Photoelectrodes: Implications for CO2 Reduction

Pyridine (Py) is an effective cocatalyst during the photoelectrochemical reduction of CO2 to methanol over GaP, CdTe, and CuInS2 semiconductor surfaces. Identifying the role Py plays in the catalytic reduction mechanism is essential for optimizing the design of such photocatalytic processes. The Py-enhanced mechanism, however, is under considerable debate. Recent studies suggest that the semiconductor surface itself participates in a heterogeneous mechanism, and for this reason a detailed understanding of the interaction between Py and the surface is required. Additionally, surface reconstructions occurring during operation alter the nature of adsorption sites available for interaction with the solution, therefore impacting the performance of the electrode. To address this issue, we report a density functional theory investigation of the stability of GaP(111) and CdTe(111) surface reconstructions, as well as adsorption trends of intermediate species across sites created by such reconstructions. We also de...

Is the Surface Playing a Role during Pyridine-Catalyzed CO2 Reduction on p-GaP Photoelectrodes?

The role of the photoelectrode surface during pyridine-catalyzed CO2 reduction on p-GaP photocathodes is currently under debate. Understanding the extent of the photoelectrode’s direct participation in the catalytic CO2 reduction mechanism is essential to improving the design of such photoelectrochemical systems. Here, we present new theoretical results demonstrating that the previously proposed pyridinyl radical intermediate is unlikely to form and that reduction of pyridinium to adsorbed pyridine and H species remains the most favorable reduction pathway, even when accounting for the aqueous environment. Furthermore, we conclude, based on recently reported experimental evidence and our new computational results reported herein, that the mechanism of CO2 reduction operating in this system is likely heterogeneous. We also introduce a new heterogeneous mechanism involving a recently proposed radical species, which we predict will be stable on the electrode surface and that may serve as the active catalytic...

Physical chemistry of catalytic reduction of nitroarenes using various nanocatalytic systems: past, present, and future

The catalytic reduction of nitroarenes under various catalytic systems has been widely reported in the flood of publications during last twenty years. This reaction has become a benchmark for testing catalytic activity of inorganic nanoparticles stabilized in various systems. This tutorial review presents design and classification of inorganic nanocatalysts along with their stabilizing agents used for catalytic reduction of nitroarenes. The techniques used for characterization of catalysts have been highlighted in this review. The mechanism of catalytic reduction has been described in a tutorial way. Factors affecting the rate of reduction of nitroarenes in the presence of metal nanoparticles stabilized in polyelectrolyte brushes, polyionic liquids, micelles, dendrimers, and microgels have been discussed for further development in this area.

Catalytic reduction of proton, oxygen and carbon dioxide with cobalt macrocyclic complexes

The conversion of solar energy into chemical energy by the reduction of small molecules provides a promising solution for the effective energy storage and transport. In this manuscript, we have highlighted our recent researches on the catalysis of cobalt-macrocycle complexes for the reduction of O2, proton and CO2. We have successfully clarified the reaction mechanisms of catalytic O2 reduction with cobalt phthalocyanine (Co[Formula: see text](Pc)) and cobalt chlorin (Co[Formula: see text](Ch)) based on detailed kinetic study under homogeneous conditions. The presence of proton-accepting moieties on these macrocyclic ligands enhances the electron-accepting ability, leading to the efficient catalytic two-electron reduction of O2 to produce hydrogen peroxide (H2O[Formula: see text] with high stability and less overpotential in acidic solutions. When Co[Formula: see text](Ch) is adsorbed on multi-walled carbon nanotubes (MWCNTs) and employed as an electrocatalyst, CO2 was successfully reduced to form CO with a Faradaic efficiency of 89% at an applied potential of -1.1 V vs. NHE in an aqueous solution. Finally, photocatalytic H2 evolution was attained from ascorbic acid with Co[Formula: see text](Ch) as a catalyst and [Ru(bpy)3][Formula: see text] (bpy [Formula: see text] 2,2[Formula: see text]-bipyridine) as a photocatalyst via a one-photon two-electron process.

Development, Application and Direction of Development of Urea-SCR

Nitrogen oxide (NOx) is one of main pollution sources of air pollution, so it need to be more and more stringent emission regulations limiting pollution. Selective catalytic reduction (SCR) technology is the mainstream technology to remove NOx at euro V and euro VI stage. By analyzing chemical reaction mechanism of selective catalytic reduction (Urea-SCR) of urea, research actuality of Urea-SCR from the urea storage device, urea supply system, catalystic converter and control unit separately was pointed out. At last development trend of SCR technology based on research that we have was analyzed.

In situ FT-IR study of highly dispersed MnOx/SAPO-34 catalyst for low-temperature selective catalytic reduction of NOx by NH3

To investigate the mechanism of low temperature selective catalytic reduction (SCR) of NOx by NH3, highly dispersed MnOx/SAPO-34 catalyst was prepared by an improved molecularly designed dispersion (MD) method and evaluated at 100–300°C. The 20wt.% MnOx/SAPO-34 showed both high SCR activity and N2 selectivity over a broad temperature window of 150–300°C. Also, the catalyst was characterized by X-ray diffraction (XRD) and field emission scanning electron microcopy (FSEM), which indicated that the active components of Mn were in a highly dispersed state on SAPO-34 support. Moreover, the possible SCR mechanism over MnOx/SAPO-34 was studied by in situ Fourier transform infrared (FT-IR). The in situ FT-IR results revealed that –NH2 species were the key intermediates for NOx removal and bridged nitrate species were the active intermediates in the SCR reaction. At 200°C, two possible reaction pathways were proposed. The first was that gas phase NO could react with –NH2 species intermediates and the formed NH2NO, producing N2 and H2O. The second was that NH3 ad-species or –NH2 species intermediates were all able to react with NOx ad-species. The possible intermediates NH4NO2 and NH2NO were unstable and would decompose into N2 and H2O.

Preparation and desulfurization activity of nano-CeO2/γ-Al2O3 catalysts

γ-Al2O3-supported CeO2 catalysts were prepared by microemulsion and impregnation methods and characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM) techniques. At the same time, the desulfurization activity of catalysts was investigated. The results show that nanoscale active substances and a high desulfurization effect are achieved by microemulsion, exhibiting a significant dominance compared with traditional impregnation method. The optimal preparation condition is temperature of 30 °C and ratio of [H2O]/[surface active agent] of 7 with slow demulsification. The activated catalysts still keep high and stable desulfurization activity during a wide temperature range of 450–600 °C. Among a series of prepared catalysts, the desulfurization rate of 6CeO2/γ-Al2O3 is the highest, reaching up to 80 % when temperature is higher than 550 °C. The catalytic reduction mechanism of SO2 over nano-CeO2/γ-Al2O3 follows redox mechanism.

Catalytic reduction of carbon dioxide by nickel-based catalyst under atmospheric pressure

To develop a new method of CO2 resource utilization, the screening tests of catalysts were carried out, and Ni–P/Al2O3 was established as the most suitable composite catalyst for CO2 catalytic reduction. By optimizing, the main preparation conditions of Ni–P/Al2O3 were obtained with Ni/P of 1:1, impregnation time of 12h and calcination temperature of 550°C. For CO2 reduction based on Ni–P/Al2O3 catalysis, the optimal experimental conditions were determined as Ni–P/Al2O3 content of 1%, sodium borohydride (NaBH4) concentration of 0.175molL−1, reaction temperature of 55°C, pH of 8.0, ethanol concentration of 90%, and residence time of 15s. And 41.37% of the average efficiency of CO2 reduction was reached. Compared with the results of non-catalytic conditions, the NaBH4 dosage was reduced by 60.14%, which indicated the excellent catalytic ability of Ni–P/Al2O3 on CO2 reduction, from which, the innovative method of the catalytic reduction of carbon dioxide (CO2) by NaBH4 under atmospheric pressure and moderate temperature was proposed. Based on the analyses of Scanning Electron Microscope, X-ray Powder Diffraction and Ion Chromatograph, the catalytic reduction mechanism of CO2 by Ni–P/Al2O3 was revealed.

Reduction of Nitrate in Secondary Effluent of Wastewater Treatment Plants by Fe0 Reductant and Pd–Cu/Graphene Catalyst

Nitrate reduction has attracted considerable attention from wastewater in wastewater management because total nitrogen (TN), in which nitrate (NO3 −) is dominant in the effluent of most wastewater treatment plants, cannot meet the requirement of Chinese wastewater discharge standard (<15 mg/L). In this study, the novel graphene–supported palladium–copper catalyst (Pd–Cu/graphene) with Fe0 reductant was investigated in the advanced treatment of nitrate. Series of specific operational conditions (such as mass ratio of Pd:Cu, catalyst amounts, reaction time, and pH of solution) were optimized for nitrate reduction in the artificial solution, and then, the selected optimal conditions were further applied for investigating the nitrate elimination of real effluent of a wastewater treatment plant in Beijing, China. Results indicated that 82 % of nitrate removal and 66 % of N2 selectivity could be obtained under the optimum condition: 5 g/L Fe0, 3:1 mass ratio (Pd:Cu), 4 g/L catalyst, 2 h reaction time, and pH 5.1. Finally, the nitrate catalytic reduction mechanism was also analyzed and interpreted.

Study on the mechanism of NH3-selective catalytic reduction over CuCexZr1–x/TiO2

Copper-cerium-zirconium catalysts loaded on TiO2 prepared by a wet impregnation method were investigated for NH3-selective catalytic reduction (SCR) of NOx. The reaction mechanism was proposed on the basis of results from in situ diffuse reflectance infrared transform spectroscopy (DRIFT). When NH3 is introduced, ammonia bonded to Lewis acid sites is more stable over CuCe0.25Zr0.75/TiO2 at high temperature, while Bronsted acid sites are more important than Lewis acid sites at low temperature. For the NH3+NO+O2 co-adsorption, NH3 species occupy most of activity sites on CuCe0.25Zr0.75/TiO2 catalyst, and mainly exist in the forms of NH4 + (at low temperature) and NH3 coordinated (at high temperature), playing a crucial role in the NH3-SCR process. Two different reaction routes, the L-H mechanism at low temperature ( 200°C), are presented for the SCR reaction over CuCe0.25Zr0.75/TiO2 catalyst.

Mechanistic Study of Selective Catalytic Reduction of NO with NH3 on W-Doped CeO2 Catalysts: Unraveling the Catalytic Cycle and the Role of Oxygen Vacancy

The reaction mechanism of selective catalytic reduction (SCR) of NO with NH3 on W-doped CeO2 catalysts was systematically investigated using density functional theory calculations corrected by on-site Coulomb interactions (DFT+U). A complete catalytic cycle was proposed, which consists of four steps, namely (i) Lewis acid site reaction, (ii) Bronsted acid site reaction, (iii) oxygen vacancy reaction, and (iv) catalyst regeneration. The calculated key intermediates in these four steps are in good agreement with previous experimental results, which indicates that our suggested catalytic cycle is rational. The catalytic nature of W-doped CeO2 catalysts for NH3-SCR reaction was discussed by analyzing the role of oxygen vacancy, the synergistic effect between surface acidity and reducibility, and the difference from NH3-SCR reaction on V2O5-based catalysts. Our results show that the oxygen vacancy on the surface which creates two Ce3+ cations plays a critical catalytic role in the NH3-SCR reaction, where adsor...

Theoretical Study on the Catalytic Reduction Mechanism of NO by CO on Tetrahedral Rh4 Subnanocluster.

The catalytic mechanism of 2NO + 2CO → N2 + 2CO2 on Rh4 cluster has been systematically investigated on the ground and first excited states at the B3LYP/6-311+G(2d),SDD level. For the overall reaction of 2NO + 2CO → N2 + 2CO2, the main reaction pathways take place on the facet site rather than the edge site of the Rh4 cluster. The turnover frequency (TOF) determining transition states are characteristic of the second N-O bond cleavage with rate constant k4 = 1.403 × 10(11) exp (-181 203/RT) and the N-N bond formation for the intermediate N2O formation with rate constant k2 = 3.762 × 10(12) exp (-207 817/RT). The TOF-determining intermediates of (3)N(b)Rh4NO and (3)N(b)Rh4O(b)(NO) are associated with the nitrogen-atom molecular complex, which is in agreement with the experimental observation of surface nitrogen. On the facet site of Rh4 cluster, the formation of CO2 stems solely from the recombination of CO and O atom, while N2 originates partly from the recombination of two N atoms and partly from the decomposition of N2O. For the N-O bond cleavage or the synchronous N-O bond cleavage and C-O bond formation, the neutral Rh4 cluster exhibits better catalytic performance than the cationic Rh4(+) cluster. Alternatively, for N-N bond formation, the cationic Rh4(+) cluster possesses better catalytic performance than the neutral Rh4 cluster.

Molecular Catalysis of O2 Reduction by Iron Porphyrins in Water: Heterogeneous versus Homogeneous Pathways.

Despite decades of active attention, important problems remain pending in the catalysis of dioxygen reduction by iron porphyrins in water in terms of selectivity and mechanisms. This is what happens, for example, for the distinction between heterogeneous and homogeneous catalysis for soluble porphyrins, for the estimation of H2O2/H2O product selectivity, and for the determination of the reaction mechanism in the two situations. With water-soluble iron tetrakis(N-methyl-4-pyridyl)porphyrin as an example, procedures are described that allow one to operate this distinction and determine the H2O2/H2O product ratio in each case separately. It is noteworthy that, despite the weak adsorption of the iron(II) porphyrin on the glassy carbon electrode, the contribution of the adsorbed complex to catalysis rivals that of its solution counterpart. Depending on the electrode potential, two successive catalytic pathways have been identified and characterized in terms of current-potential responses and H2O2/H2O selectivity. These observations are interpreted in the framework of the commonly accepted mechanism for catalytic reduction of dioxygen by iron porphyrins, after checking its compatibility with a change of oxygen concentration and pH. The difference in intrinsic catalytic reactivity between the catalyst in the adsorbed state and in solution is also discussed. The role of heterogeneous catalysis with iron tetrakis(N-methyl-4-pyridyl)porphyrin has been overlooked in previous studies because of its water solubility. The main objective of the present contribution is therefore to call attention, by means of this emblematic example, to such possibilities to reach a correct identification of the catalyst, its performances, and reaction mechanism. This is a question of general interest, so that reduction of dioxygen remains a topic of high importance in the context of contemporary energy challenges.

Catalytic Reduction of 2-Nitroaniline in Aqueous Medium Using Silver Nanoparticles Functionalized Polymer Microgels

Poly(N-isopropylacrylamide-co-methacrylic acid) [P(NIPAM-co-MAA)] microgels were prepared by free radical precipitation polymerization in aqueous medium. Silver nanoparticles were fabricated inside the microgel network by in situ reduction of silver ions. The pure and hybrid microgels were characterized by FTIR and UV–Vis spectroscopy. Dynamic light scattering was performed to investigate the pH sensitivity of the microgels. The TEM and XRD analysis was used to determine the size of silver nanoparticles. The value of λSPR was found to be red shifted with increase of pH of the medium due to change of refractive index around Ag nanoparticles as a result of swelling of microgels. The hybrid microgels were used as catalyst for reduction of 2-nitroaniline (2-NA) in aqueous medium. The progress of the reaction was monitored by UV–Vis spectrophotometry in the presence of various concentrations of NaBH4, 2-NA and hybrid microgel dose. The data was used to explore the inside mechanism of catalytic reduction of 2-NA in aqueous medium.

Concerted proton-electron transfer in electrocatalytic O2 reduction by iron porphyrin complexes: axial ligands tuning H/D isotope effect.

The electrochemical O2 reduction by thiolate- and imidazole-bound iron porphyrin complexes and H/D isotope effects on 4e(-) (determined by rotating disc electrochemistry) and 2e(-) (determined by rotating ring disc electrochemistry) O2 reduction rates are investigated. The results indicate that a thiolate axial ligand shows an H/D isotope effect greater than 18 and 47 for the 4e(-) and 2e(-) O2 reductions, respectively. Alternatively, an imidazole axial ligand results in H/D isotope effects of 1.04 and 4.7 for the 4e(-) and 2e(-) O2 reduction, respectively. The catalytic O2 reduction mechanism is investigated in situ with resonance Raman coupled with rotating disc electrochemistry. The data indicate that the rate-determining step changes from O-O bond heterolysis of Fe(III)-OOH species for a thiolate axial ligand to an O-O bond heterolysis of an Fe(II)-OOH for an imidazole axial ligand.

Investigating the mechanism of catalytic reduction of silver nitrate on the surface of barium titanate at room temperature: oxygen vacancies play a key role

The oxygen vacancies on BaTiO3 surface linked with the hydroxyl oxygen of ethylene glycol and catalyzed the reduction of AgNO3.

Catalytic chemical looping combustion of carbon with an iron-based oxygen carrier modified by K2CO3: Catalytic mechanism and multicycle tests

One of the major obstacles of chemical looping combustion (CLC) with solid fuels is the low reaction rate of solid fuels with an oxygen carrier. Alkali metal catalysts can be used to enhance the reaction rate. In this work, the catalytic reduction mechanism of a K2CO3 modified iron-based oxygen carrier by activated carbon (AC) and the multicycle CLC tests were investigated. The results confirm that the deep reduction of K2CO3 added iron-based oxygen carrier occurred at a high rate. The energy dispersive spectrometer (EDS) analysis shows that the catalytically active species of K2CO3 migrated from the oxygen carrier to the surface of AC during reduction, which could then catalyze the rate-limiting step (C–CO2 reaction) of the reduction process. Besides the catalytic effect of K2CO3, a remarkable autocatalytic effect of iron oxide itself on the reduction was also observed. The active species could also travel back to the oxygen carrier after reduction, which guaranteed the persisting catalytic effect in the multicycle tests. The loss of potassium primarily contributed to the deactivation of the oxygen carrier, but this deactivation can be partly retrieved and improved by the supplement of K2CO3.

First-Principles Study on the Mechanism of Photoselective Catalytic Reduction of NO by NH3 on Anatase TiO2(101) Surface

A promising method for NO abatement is photoselective reduction with a proper semiconductor, such as TiO2. Here we report a systematic theoretical study on NO abatement through an adsorbed NH3 molecule on the anatase TiO2(101) surface. The reaction mechanism proposed by experiments has been verified. The key process, namely, the oxidation of the adsorbed NH3 molecule by photogenerated hole, has been investigated by two different methods: one is to use the triplet state to mimic the real excited state and the other is to inject a hole to the slab by the adsorption of ·OH radical. Both methods give almost the same result, and the oxidation of the NH3 molecule is found to be a concerted proton coupled charge transfer process. The ·NH2 radical, resulting from the oxidation of NH3, can be attacked by a NO molecule from the gas phase to form a NH2NO complex spontaneously. The decomposition of this complex to N2 and H2O is the rate limiting step of the overall reaction. This multistep decomposition process consi...