Relative Adsorption Strength Research Articles

Kinetic Monte Carlo Simulations of Low-Temperature NH3-SCR over Cu-Exchanged Chabazite.

Cu-exchanged chabazite (Cu-CHA) is widely used for ammonia assisted selective catalytic reduction of nitrogen oxides (NH3-SCR). The Cu+ ions are at low temperatures solvated by NH3 forming mobile [Cu(NH3)2]+ complexes. The dynamic behaviour of the complexes is critical as O2 adsorption requires a pair of complexes to form a [Cu2(NH3)4O2]2+ peroxo-species over which NO couples with NH3. Here we introduce a first principles-based kinetic Monte Carlo approach to explore the effect of the Al-distribution on the reaction kinetics of NH3-SCR over Cu-CHA. The method allows us to scrutinize the interplay between the pairing of [Cu(NH3)2]+ complexes and the reaction landscape for the NH3-SCR reaction over the peroxo-complex. The Al-distribution affects the stability of the [Cu(NH3)2]+ pairs as well as the kinetic parameters of the SCR-reaction. The turn-over frequency is determined by the stability of the [Cu(NH3)2]+ pairs and the relative strength of NO and NH3 adsorption once a pair is present. The results establish the hierarchy of effects that influences the performance of Cu-CHA over NH3-SCR and provide a computational basis for further development of the Cu-CHA material.

Phenol is its own selectivity promoter in low-temperature liquid-phase hydrogenation

Phenol hydrogenation is widely studied for selective production of the chemical intermediate cyclohexanone. A plethora of studies in the literature have reported catalysts aiming to achieve high selectivity compared to Pd/C. However, we demonstrate that selective and high-yielding reactions are inherent features of liquid-phase phenol hydrogenation using conventional Pd/C catalysts. We also show there is a very strong dependance of selectivity upon conversion, with high selectivity being maintained until near complete consumption of the phenol, after which subsequent reaction to the unwanted, fully hydrogenated cyclohexanol occurs rapidly. Furthermore, through competitive reactions with other aromatic molecules it is demonstrated that the phenol molecule effectively self-poisons the onwards reaction of weakly bound cyclohexanone, likely by virtue of its relative adsorption strength, and this is the source of the intrinsic selectivity. The implications of this to the reaction mechanism, and in turn to the rational design of catalysts, especially for obtaining chemicals from phenolic bio-oils, are discussed.

Relative Strength of Polycation Adsorption on Oxide Surfaces.

Polyelectrolyte adsorption onto surfaces is widely employed in water treatment and mining. However, little is known of the relative interaction strengths between surfaces and polymer. This fundamental property is assumed to be dominated by electrostatics, i.e., attractive interactions between opposite charges, which are set by the overall ionic strength ("salt concentration") of the solution, and charge densities of the surface and the polymer. A common, counterintuitive finding is a range of salt concentrations over which the amount of adsorbed polyelectrolyte increases as electrostatic interactions are tempered by the addition of salt. After an adsorption maximum, higher salt concentrations then produce the expected gradual desorption of polyelectrolyte. In this work, the salt response of the adsorption of the same narrow molecular weight distribution polycation, poly(N-methyl-4-vinylpyridinium), PM4VP, to a variety of surfaces was explored. Oxide powders for adsorption included Al2O3, SiO2, Fe2O3, Fe3O4, TiO2, ZnO, and CuO. Planar surfaces included silicon wafers, mica, calcium carbonate, and CaF2 single crystals. The PM4VP was radiolabeled with 14C so that sensitive, submonolayer amounts could be detected. The position of the peak maximum, or the lack of a peak, in response to added salt was used to rank the electrostatic component of the interaction. The importance of charge regulation, a shift in the surface pKa in response to solution species, was highlighted as a mechanism for adsorption on the "wrong" side of the isoelectric point and also as a factor contributing to the difficulty of reaching the totally desorbed state even at the highest salt concentrations.

Alternative Cu3Zn catalysts for enhanced reduction of CO2 to CH4: A density functional theory-based approach

The synthesis of syngas is considered to be an important area of research in the context of the energy crisis arising from the depletion of fossil fuels. CH4 is a highly potent greenhouse gas; however, it is also a renewable energy source of significant importance. CH4 can be generated via the electrochemical CO2 reduction reaction (CO2RR). Various heterogeneous and homogeneous metallic catalysts with optimal activities have been developed based on the pure Cu catalyst for enhancing reactivity for the CO2RR. However, Cu catalysts suffer from an unresolved issue involving a high overpotential for the CO2RR. Therefore, Cu3M alloy catalyst systems (M = 3d transition metals) were investigated herein for increasing the reactivity for the CO2RR. The relative adsorption strength between CO and CHO on the Cu3M surface was targeted based on the fact that the reaction step involving these species is possibly the potential-limiting step in the CO2RR. Cu3Zn was revealed to be a catalyst with tremendous potential for the reduction of CO2 to CH4. The energetic and electronic structural changes of the intermediate adsorbed state were analyzed using density functional theory, and the obtained results were used to design an effective catalyst for CH4 production via the CO2RR.

Understanding the reaction mechanism of Kolbe electrolysis on Pt anodes

Kolbe electrolysis has been proposed as an efficient electro-oxidation process to synthesize (un)symmetrical dimers from biomass-based carboxylic acids, but its mechanism remains controversial. In this work, we develop a microkinetic model based on density functional theory to study the reaction mechanism of Kolbe electrolysis of acetic acid (CH 3 COOH) on both pristine and partially oxidized Pt anodes. We show that the shift in the rate-determining step of the oxygen evolution reaction (OER) on a Pt(111)@α-PtO 2 surface from OH ∗ formation to H 2 O adsorption gives rise to large Tafel slopes, i.e., the inflection zones observed experimentally at high anodic potentials on Pt. Our simulations find that the CH 3 COO ∗ decarboxylation and CH 3 ∗ dimerization steps determine the activity of the Kolbe reaction. This work resolves major controversies in the mechanism of Kolbe electrolysis on Pt anodes: the origin of the inflection zone and the identity of the rate-limiting step. • A microkinetics model for Kolbe electrolysis of acetic acid on Pt anodes is developed • Oxidation of the Pt surface dramatically weakens the adsorption strength of Kolbe species • Suppressed activity of oxygen evolution reaction leads to the inflection zone • Decarboxylation and/or dimerization steps determine activity of Kolbe products Organic electrosynthesis has, in recent years, been revived as a promising pathway for the sustainable production of important chemicals. The advantages of electrosynthesis include mild reaction conditions, the use of green electricity, and the tunability of activity and selectivity with the applied potential. The Kolbe reaction converts biomass-derived carboxylic acids to high-value products through decarboxylation and dimerization. However, the mechanism of this reaction is not fully understood, which limits further optimization of the reaction systems. Building on previous mechanistic studies, this work identifies the key steps involved in Kolbe electrolysis of acetic acid. The mechanistic insights obtained suggest that tuning the relative adsorption strength of carboxylic species versus water in aqueous systems could promote Kolbe products. Electroorganic synthesis from biomass-derived chemicals has been revived in recent years, but reaction mechanisms remain to be established. Liu et al. perform a detailed mechanistic study of Kolbe electrolysis on oxidized platinum by developing a microkinetics model based on density functional theory calculations.

Robust Palladium Hydride (PdH/C) Catalyst for Electrocatalytic Formate Formation with Limited of CO Poisoning

Carbon dioxide capture, reduction, and valorization is of great importance to environmental conservation and green energy storage. Recent electrochemical CO2 reduction exhibits the capability of converting CO2 to various chemicals, which still face challenges such as high overpotential and low selectivity. The integration of electrocatalytic and biocatalytic cascade systems is able to provide substantial opportunities for CO2 conversion.1 But the selection of electrocatalysts needs more specific requirements such as low overpotential and high selectivity. Meanwhile, formate, serves as a major product of CO2 electroreduction and feedstock for subsequent biocatalytic processes, possesses highest normalized price (16.1 $ / electron) among all products2, demonstrating its high practical application value. Pd stands out from many catalysts because of its unique electrochemical CO2 reduction to formate properties such as that it can selectively produce formate at equilibrium potential.3 Inevitably produced CO, however, poisons and deactivates the surface of Pd, resulting in insufficient operating lifetime for traditional Pd.4–9 Herein, we will present our current research into Pd hydride nanoparticles (PdH/C) derived by using one-step solvothermal synthesis technique10 for formate production over potential ranges from 0V to -0.6V (vs RHE) that this large potential range selective for formate attribute to the insertion of hydride into the Pd lattice, promoting the electrohydrogenation process on the surface. This catalyst conducts a ~92% faradaic efficiency for formate at -0.4V (vs RHE) with current density as high as 5 mA/cm2 in H-cell, which provides a yield over 1600 μmol/h/mgpd. The working life at -0.4V (vs RHE) reaches a record of 4 hours, which is ~15 times longer than a commercial Pd, outperforming all previously reported Pd catalysts in electrosynthesis of formate from CO2. This high CO tolerance attribute to relative weak CO adsorption strength with the presence of α&β hydride. A practical method of removing toxic CO from the surface by applying a positive potential shock has shown promising results, avoiding the unrealistic exposure of the catalyst to air. By alloying PdH with earth abundant element Cu, the faradaic efficiency, production, and stability of formate has been further improved. In addition, a Cu-based catalyst with high selectivity to methanol (~77%) will be presented11, which can also be applied into the hybrid cascade system to convert CO2 into higher reduced products. Reference S. Guo, T. Asset, and P. Atanassov, ACS Catal., 5172–5188 (2021) https://pubs.acs.org/doi/abs/10.1021/acscatal.0c04862.M. Jouny, W. Luc, and F. Jiao, Ind. Eng. Chem. Res. (2018).M. Alfath and C. W. Lee, Catalysts (2020).X. Min and M. W. Kanan, J. Am. Chem. Soc. (2015).A. Klinkova et al., ACS Catal. (2016).D. Gao et al., Nano Res. (2017).B. Jiang, X. G. Zhang, K. Jiang, D. Y. Wu, and W. Bin Cai, J. Am. Chem. Soc. (2018).S. Chatterjee et al., ACS Catal. (2019).T. Takashima, T. Suzuki, and H. Irie, Electrochemistry (2019).Y. Qiu et al., J. Am. Chem. Soc. (2018).D. Yang et al., Nat. Commun. (2019). Figure 1

Alkaline pretreatment of walnut shells increases pore surface hydrophilicity of derived biochars

The surface chemistry and morphology of biochars produced by pyrolysis of walnut shells affects their utility for adsorption applications. Yet, little is known about surface interactions in the pores of these materials, mostly due to the challenging nature of accessing information at this length scale in a non-destructive manner. Here, for the first time, the relative adsorption strengths of solvents comprising different functional groups to internal (pore) surfaces of walnut shells and derived biochars were investigated using low-field nuclear magnetic resonance (NMR) relaxation time measurements to non-destructively probe interactions of fluids with pore surfaces. Carbon bonding state compositions of these materials with respect to distance from the particle surface were determined using X-ray photoelectron spectroscopy coupled with ion beam etching. Alkaline pretreatment was found to increase the hydrophilicity of both walnut shells and derived biochars. It was found to increase surface interactions with hydroxyl groups, and to decrease those with methyl groups. Results were contextualised by thermogravimetric analysis, scanning electron microscopy, and previous in-situ X-ray imaging results. Taken together, results showed that alkaline pretreatment may be used to modulate responses to pyrolysis temperature of several factors that affect adsorption properties including surface hydrophilicity, particle size, porosity, pore accessibility, and surface texture.

Bridge sulfur vacancies in MoS2 catalyst for reverse water gas shift: A first-principles study

Vacancies, typically exist in various types and morphologies on the surfaces of transition metal compound catalysts, have been shown to play an important role in many catalytic reactions. However, identifying the optimal type and morphology of vacancy and the key factors that control its performance remains a significant challenge. By density functional theory calculations and microkinetic modeling, we show that compared to various threefold S vacancy morphologies on the basal plane, the lower coordination number at the bridge S vacancy of Mo edge on MoS2 tunes the relative adsorption strength of key intermediates (such as O relative to OH). This not only increases CO2 hydrogenation rate by 7–11 orders of magnitude, but also leads to clearly distinct mechanism (associative & redox). The large destabilization of O and low H coverage on the Mo edge also hinder C-O scission and hydrogenation (give CH4 and CH3OH) relative to CO desorption, leading to almost 100% CO selectivity. The inexpensive MoS2 catalyst provides an alternative to the traditional noble metal catalysts for reverse water gas shift, and the coordination number of vacancy modulated catalytic performance illustrates a promising way to design transition metal compound catalysts for other important reactions of technological interest.

Rational Design of an Iridium-Tungsten Composite with an Iridium-Rich Surface for Acidic Water Oxidation.

Developing highly active and stable water oxidation catalysts with reduced cost in acidic media plays a critical role in clean energy technologies such as fuel cells and electrolyzers. Precious iridium-based oxides are still the only oxygen evolution reaction (OER) catalysts with reasonable activity and stability in acid. Herein, we design iridium-tungsten composites with a metallic tungsten-rich core and an iridium-rich surface by the sol-gel method followed by hydrogen reduction. The thus obtained iridium-tungsten catalyst shows much higher intrinsic water oxidation activity (100 mA/mgIr at an overpotential of 290 mV) and stability (100 h at 10 mA/cm2geom) together with reduced iridium content (33 wt % only) as compared with pure iridium oxide. An operando method using H2O2 as a probe molecule is developed to determine the relative adsorption strength of the reaction intermediates (*OH and *OOH) in the OER process. Detailed characterization shows that the tungsten-incorporated surface not only modulates the adsorption energy of oxygen intermediates on iridium but also enhances the stability of iridium species in acid, while the metallic tungsten core exhibits high electrical conductivity, all of which collectively give rise to the much enhanced catalytic performance of iridium-tungsten composite in acidic water oxidation. A single-membrane electrode assembly is further prepared to demonstrate the advantages and potential application of iridium-tungsten composite in practical proton exchange membrane electrolyzers.

Competitive adsorption between phosphate and arsenic in soil containing iron sulfide: XAS experiment and DFT calculation approaches

The competitive adsorption between phosphate and arsenic (As) in natural soil and mineral systems is important for controlling subsurface contaminants. Phosphate is known to adsorb more strongly than As in Fe(III) oxide-based mineral systems. Here, X-ray absorption spectroscopy (XAS) and density functional theory (DFT) were employed to understand the fundamental reaction mechanisms of As and phosphate on an FeS surface. The competitive effect of phosphate in As(V)-contaminated soil systems differed with changing FeS concentrations. In soil batches with high FeS contents, As(V) was more strongly adsorbed than phosphate, and no additional As release was observed in the aqueous phase, in the presence of phosphate. Through the XAS measurement of laboratory batch samples, the reaction mechanisms could be comprehensively understood, including the reduction of As(V) to As(III), the secondary precipitation of sulfide minerals, and the adsorption mechanism of As(V) on the surface of FeS. The DFT calculations revealed the relative adsorption strengths on the FeS surface were As(V) > phosphate > As(III). Comparing the adsorption energies revealed that As(V) reacted more strongly with FeS than phosphate and As(III), and that the effect of phosphate co-occurrence was negligible in the FeS–As system, in line with the experimental measurements.

Good's buffers have various affinities to gold nanoparticles regulating fluorescent and colorimetric DNA sensing.

Citrate-capped gold nanoparticles (AuNPs) are highly important for sensing, drug delivery, and materials design. Many of their reactions take place in various buffers such as phosphate and Good's buffers. The effect of buffer on the surface properties of AuNPs is critical, yet this topic has not been systematically explored. Herein, we used halides such as fluoride, chloride, and bromide as probes to measure the relative adsorption strength of six common buffers. Among them, HEPES had the highest adsorption affinity, while MES, citrate and phosphate were weakly adsorbed with an overall ranking of HEPES > PIPES > MOPS > MES > citrate, phosphate. The adsorption strength was reflected from the inhibited adsorption of DNA and from the displacement of pre-adsorbed DNA. This conclusion is also supported by surface enhanced Raman spectroscopy. Furthermore, some buffer molecules did not get adsorbed instantaneously, and the MOPS buffer took up to 1 h to reach equilibrium. Finally, a classic label-free AuNP-based colorimetric sensor was tested. Its sensitivity increased by 15.7-fold when performed in a MES buffer compared to a HEPES buffer. This study has articulated the importance of buffer for AuNP-based studies and how it can improve sensors and yield more reproducible experimental systems.

Kinetics study on esterification of acrylic acid and ethanol over acidic cation-exchange resin beads Amberlyst 35

Abstract The heterogeneous kinetics and adsorption behavior for the esterification of acrylic acid and ethanol were studied by using a fixed-bed reactor over an acidic cation-exchange resin beads, Amberlyst 35. The reaction experiments were conducted at 318.15 K–348.15 K under atmospheric pressure. The influences of molar ratios of ethanol to acrylic acid in the feed (θBo) and the mass transfer resistances on the kinetic behavior were also investigated. The experimental results show that the equilibrium conversion of acrylic acid increases with increasing reaction temperature and θBo. Moreover, the adsorption experiments were also made to determine the relative adsorption strengths among the reacting species. The results reveal that the magnitude of adsorption strengths obeyed the sequence of water > acrylic acid > ethanol > ethyl acrylate. The kinetic data obtained from the present study were correlated with the ideal-quasi-homogeneous (IQH), the non-ideal-quasi-homogeneous (NIQH), the Langmuir-Hinshelwood-Hougen-Waston (LHHW), the Eley-Rideal (ER) models, the modified LHHW (M-LHHW) and the modified ER (M-ER). The best-fitted values of the kinetic parameters were reported. The correlated results indicated that the M-LHHW model provides the best representation for this reaction system.

Single Ru Sites-Embedded Rutile TiO2 Catalyst for Non-Oxidative Direct Conversion of Methane: A First-Principles Study

Non-oxidative direct methane conversion provides a potentially economic and environmental friendly route for the use of natural gas and shale gas, but this process suffers the disadvantages of low activity and selectivity and harsh operating conditions. Using density functional calculations, we develop the relations in heats of adsorption of CHx (x = 0–4) species and catalytic performance of conventional Fe, Ru, and Co-based catalysts and identify the key factors that affect the activity and selectivity as methane adsorption and the relative strength of CH2 and CH adsorption. Based on the analysis, we design the single Ru sites embedded in rutile TiO2(110) catalyst, which tunes the adsorption strength of CHx compared with the traditional Ru-based catalyst, particularly weakening CH adsorption relative to CH2 adsorption, thus leading to increased activity, improved selectivity toward ethylene, and strong resistance toward coking. This work highlights the impact of surface coordination environment, achieving fundamental insight that can be used to design and develop improved catalysts for direct methane conversion and other important reactions of technological interest.

High purity ethane/ethylene separation by gas phase simulated moving bed using ZIF‐8 adsorbent

AbstractGrowing population and demand for plastics and synthetic fluids in climate friendly environment sets the unprecedented industrial challenge to increase production of clean chemical precursors (e.g., ethylene) with reduced energy consumption and CO2 foot print. The production of polymer‐grade ethylene (>99.9%) from steam cracked gas or oil relies on a cascade of energy intensive distillations of gases. Therefore, in this work two simulated moving bed (SMB) cycles were employed to separate mixtures containing of about 40% of ethane and 60% of ethylene on zeolitic imidazole framework‐8 granulates. The separation cycles were performed with two desorbents with different relative adsorption strength. In the experiment, with carbon dioxide as desorbent, ethylene was obtained with 99.6% purity and 94.2% recovery, while in the experiment using propane as desorbent only 77.1% of ethylene is recovered with a purity of 82.7%. Indeed, it can be concluded that when CO2 is used as desorbent, the obtained productivity is higher. The mathematical SMB model and regions of separation were determined for each desorbent based on experimental data.

Electrochemical Reduction of Carbon Dioxide at Alloy Systems: Cu-In and Cu-Bi

The ability to maintain high efficiencies while simultaneously tuning the selectivity of the electrochemical reduction of CO2 (ERC) using low cost electrodes has proven to be one of the greatest obstacles to the widespread commercialization of this technology. In this study, we prepare dendritic copper-indium and copper-bismuth alloys of various compositions and investigate their catalytic activity and selectivity towards the reduction of CO2. The Cu-In electrocatalysts are increasingly dendritic with higher In fraction and, depending on composition, consist of mixed phases of Cu, In and Cu-In intermetallic phases. ERC at these electrodes produces formate at high efficiencies (up to 62% with a 80 at% alloy, -1 V)) while also tuning the CO/H2 ratio to achieve an ideal syngas composition with a 40 at% In alloy, -1 V. The observed product distribution as a function of alloy composition and applied potential is rationalized in terms of the relative adsorption strengths of CO and COOH intermediates at Cu and In sites, and their distinct variation with applied potential may be induced by the differences in electronic structure. We also studied dendritic Cu, Bi, and Cu-Bi alloys with various compositions. The dendritic structures exhibit a high density of defect sites such as grain boundaries and twins, with these defects being increased at alloy dendrites. While the pure metals exhibited high H2 production and relatively high formate formation, the alloys exhibit a lower overall current density and a much higher selectivity towards formate, up to about 90%. Most importantly we show in this work that a combination of a d metal (Cu) with a p-block metal (In, Bi) with intimate contact between the two components is capable to tune the relative adsorption strength of the intermediates, thus steering the product distribution towards formate. The relatively purity of this product would be of interest for the direct feed of a direct formate fuel cell. Differences between In and Bi will be discussed in terms of the affinity for oxygen. This study highlights the opportunities of using alloys to enhance control over the product distribution and suggests that suitable alloys could be promising catalysts for the inexpensive and efficient production of fuels.

DFT insight into the support effect on the adsorption and activation of key species over Co catalysts for CO2 methanation

Our recent experiments for CO2 methanation reveal that Co catalysts supported on ZrO2 and Al2O3 exhibit good initial activity, but the Co/ZrO2 catalyst is catalytically more active and more stable than the Co/Al2O3 catalyst. Detailed characterization demonstrated that there are notable differences in the adsorption properties of key species between the ZrO2- and Al2O3-supported catalysts and in the metal-support interactions where Co/ZrO2 shows enhanced adsorption for key species and better reducibility of the cobalt oxide. In the present work, we performed density functional theory (DFT) calculations to provide theoretical insight into the support effect over Co/ZrO2 and Co/Al2O3 for CO2 methanation in terms of adsorption and activation of key species. The relative adsorption strengths of CO2, H2, CO, and CH4 species on Co/ZrO2 and Co/Al2O3 calculated by DFT showed qualitative agreement with experimental TPD. Importantly, DFT results uncovered an energetically more favorable H2 dissociative adsorption, faster H* surface migration, and more facile Co-O bond cleavage on Co/ZrO2 than on Co/Al2O3, thus rationalizing the experimentally observed lower temperature and higher degree of cobalt oxide reduction on the ZrO2 support.

Electrochemical Reduction of Carbon Dioxide to Syngas and Formate at Dendritic Copper–Indium Electrocatalysts

The ability to maintain high efficiencies while simultaneously tuning the selectivity of the electrochemical reduction of CO2 (ERC) using low-cost electrodes has proven to be one of the greatest obstacles to the widespread commercialization of this technology. In this study, we electrodeposit dendritic copper–indium alloys of various compositions and investigate their catalytic activity toward the reduction of CO2. These electrocatalysts are increasingly dendritic with higher In fraction and, depending on composition, consist of mixed phases of Cu, In, and Cu–In intermetallic phases. ERC at these electrodes produces formate at high efficiencies (up to 62% with a 80 at% In alloy, −1 V) while also tuning the CO/H2 ratio to achieve an ideal syngas composition with a 40 at% In alloy (−1 V). The observed product distribution as a function of alloy composition and applied potential is rationalized in terms of the relative adsorption strengths of CO and COOH intermediates at Cu and In sites and the distinct vari...

Structural Dependence of Competitive Adsorption of Water and Methanol on TiO2 Surfaces

AbstractEmploying TiO2 anatase (001)‐(1 × 4), rutile (110) and rutile (011)‐(2 × 1) single crystal surfaces, we comprehensively studied the effects of TiO2 surface structures on the competitive adsorption of water and methanol by means of low energy electron diffraction, thermal desorption spectra and X‐ray photoelectron spectroscopy. The relative adsorption strengths of chemisorbed methanol and water vary with the TiO2 surface structures and the adsorption sites. This leads to TiO2 surface structure‐dependent competitive adsorption of water and methanol. The chemisorption of CH3OH on TiO2 anatase (001)‐(1 × 4) surface is seldom affected by pre‐covered water at low coverages but is affected by pre‐covered water at high coverages; the chemisorption of CH3OH on TiO2 rutile (110) surface is seldom affected by pre‐covered water; and the chemisorption of CH3OH on TiO2 rutile (011)‐(2 × 1) surface is affected by pre‐covered water even at low coverages. These results deepen the fundamental understandings of surface chemistry on TiO2 surfaces.

Local equilibrium theory analysis of chromatographic peak shapes in the presence of adsorbing modifiers

The nature and shapes of solute and modifier peaks for a situation when the modifier and solute adsorb competitively through the Langmuir isotherm is studied using the local-equilibrium theory. The analysis yields 10 different cases depending on the relative adsorption strengths and the concentrations of the modifier and the solute. The origin of positive and negative Langmuirian/anti-Langmuirian-type peaks, even when the adsorption isotherms are strictly Langmuirian, and explicit conditions under which each combination is observed are derived. A fundamental explanation to the origin and propagation of rounded peaks reported in liquid and supercritical fluid chromatography [1–3] is offered. A rigorous theoretical basis for the rule of thumb proposed by Fornstedt and Guiochon [1,2] about the nature of peak shapes when an adsorbing modifier is present, is established.

Adsorption and Performance of Chiral Cinchona Alkaloid Modifiers over Pd/C Catalyst for Enantioselective Hydrogenation of α-Phenylcinnamic Acids

Abstract The enantioselective hydrogenation reactions of α-phenylcinnamic acid (PCA) and 4,4′-dimethoxy α-phenylcinnamic acid (DMPCA) were carried out over chiral cinchona alkaloid-modified Pd/C. Two sets of the modifiers were employed to get deeper insights into the effects of relative adsorption strength between the modifier and the substrate on the enantioselectivity; cinchonidine (CD)/cinchonine (CN) and quinine (QN)/quinidine (QD). The performances of the two sets of modifiers were compared by systematically varying the modifier concentration over a wide range. It was clearly substantiated that the origin of the low selectivity observed with QN/QD at an ordinary concentration is primarily due to its weak adsorption strength on Pd metal surface, which originates from the steric hindrance of the methoxy substituent at C6. A new modifier, 6-hydroxy CD, was found to exhibit a performance comparable to that of CD, implying that the steric hindrance of the 6-methoxy group of QN/QD is much more influential than the electronic effects.