Initial Heat Of Adsorption Research Articles

Adsorbent Properties of Porous Boron Nitride and Activated Carbon: A Comparative Study.

Porous boron nitrides possess beneficial properties such as high thermal and chemical stability which are critical for applications in adsorption processes. In order to assess possible fields of applications, trace-level adsorption isotherms of different hydrocarbons on two synthesized porous boron nitrides and two commercial activated carbons are compared. By normalizing the adsorptive loadings on the micropore surface area, superior adsorption performances of the BN materials on polar and aromatic adsorptives with up to 50% higher loadings compared to the activated carbons can be shown. Nonpolar adsorptives, on the other hand, feature higher specific loadings on the activated carbon. Consequently, the size of the micropore surface appears to be decisive for nonpolar adsorptives, while the higher polarity of the boron nitrides is the dominant influencing factor for the adsorption of polar and aromatic components. For an energetic study of the adsorbents, calorimetric experiments were performed to identify and discuss adsorbent-adsorptive interactions. While the initial heat of adsorption of the nonpolar n-hexane is lower on the boron nitride than on the activated carbon due to a less favorable spatial arrangement, toluene shows comparable values on both adsorbent classes and the polar acetone shows higher values on the polar boron nitride. Considering technical applications in adsorption technology, the thermal stability of the boron nitrides is investigated using spontaneous ignition temperatures and points of initial oxidation. Here, the porous boron nitrides with oxidation temperatures above 900 °C show about 400 °C higher values and thus a significantly higher thermal stability.

Construction of Coordination Spaces with Narrow Pore Windows in Co-Based Metal-Organic Frameworks toward CO2/N2 Separation.

Carbon emission reduction is an important measure to mitigate the greenhouse effect, which has become a hotspot in global climate change research. To contribute to this, here, we fabricated two Co-based metal-organic frameworks (Co-MOFs), namely, {[Co3(NTB)2(bib)]·(DMA)2·(H2O)4}n (DZU-211) and {[Co3(NTB)2(bmip)]·(DMA)2}n (DZU-212) (H3NTB = 4,4',4″-nitrilotribenzoic acid, bib = 1,4-bis(imidazol-1-yl)-butane, bmip = 1,3-bis(2-methyl-1H-imidazol-1-yl)propane) to realize efficient CO2/N2 separation by dividing coordination spaces into suitable pores with narrow windows. DZU-211 reveals a 3D open porous framework, while DZU-212 exhibits a 3D double-fold interpenetrated structure. The two MOFs both possess large coordination spaces and small open pore sizes, via the bib ligand insertion and framework interpenetration, respectively. Comparatively, DZU-211 reveals superior selective CO2 uptake properties due to its more suitable pore characteristics. Gas sorption experiments show that DZU-211 has a CO2 uptake of 52.6 cm3 g-1 with a high simulated CO2/N2 selectivity of 101.7 (298 K, 1 atm) and a moderate initial adsorption heat of 38.1 kJ mol-1. Moreover, dynamic breakthrough experiments confirm the potential application of DZU-211 as a CO2 separation material from postcombustion flue gases.

Accurate Measurements of NH3 Differential Adsorption Heat Unveil Structural Sensitivity of Brønsted Acid and Brønsted/Lewis Acid Synergy in Zeolites.

Differential adsorption heats of NH3 on a series of zeolites, including MOR, MFI, FER, and BEA, are accurately measured to probe their acidity using flow-pulse adsorption microcalorimetry. Initial adsorption heats of NH3 at Brønsted acid sites (BAS) vary between 105 to 136 kJ/mol, depending on framework aluminum amounts and topography structures of zeolites. A Brønsted/Lewis acid synergy between BAS and proximate tricoordinated framework-associated aluminum species is identified to generate super acid sites with initial adsorption heats of NH3 around 150 kJ/mol, but occurs only in the MFI zeolites and sensitively depends on the Si/Al ratio. These accurate data of NH3 differential adsorption heats unveil structural sensitivity of BAS and Brønsted/Lewis acid synergy in zeolites and provide experimental benchmark data for fundamental understanding of acidity and acid-catalysis of zeolites.

Separation ofCH4/N2by an ultra‐stable metal–organic framework with the highest breakthrough selectivity

AbstractSelectively separating CH4from N2in natural gas purification is extremely important, but challenging. Herein, a copper‐based metal–organic framework (MOF) NKMOF‐8‐Me with inert pore environment was reported for efficient CH4/N2separation. Adsorption results show that this material owns the highest CH4uptake (1.76 mmol/g) and initial adsorption heat (Qst0) of CH4(28.0 kJ/mol) as well as difference inQst0(9.1 kJ/mol) among all materials with good water stability. Breakthrough experiments confirm that this MOF can completely separate the CH4/N2mixture with the highest CH4/N2breakthrough selectivity (7.8) reported so far. Theoretical calculations reveal the separation mechanism is the short average distance between CH4and pore wall, resulting in a stronger adsorption affinity for CH4. In addition, this MOF exhibits highly structural stability and regeneration. These results guarantee this MOF as a promising adsorbent for the recovery of CH4from coalbed methane.

Excellent CO2 adsorption performance of nitrogen-doped waste biocarbon prepared with different activators

A series of N-doped CO2 adsorbents were prepared by using waste walnut shell as the raw material, urea as nitrogen source, H3PO4 as a pretreatment agent and KOH, K2CO3 and ZnCl2 as the activators. Result indicates that both the pore structure and nitrogen content of the adsorbent together affect the CO2 adsorption capacity. HAC-850-KOH showed excellent CO2 adsorption capacity (5.13 mmol/g) at 0 °C and atmospheric pressure probably because of its large specific surface area (SBET 2354 m2/g). HAC-750-ZnCl2 has the higher CO2 adsorption capacity (0.6 mmol/g) at 25 °C and 0.15 bar due to its highest nitrogen content (9.32 wt%). HAC-850-K2CO3 has a good selectivity of 21.5 and cycle performance may be somewhat associated with its pore structure and nitrogen content, and the initial heat of adsorption is 44 kJ/mol. The result indicates that the pore structure of the adsorbent has a great influence on the adsorption performance under high pressure; the nitrogen content of the adsorbent has a decisive influence on the adsorption performance of the adsorbent under low pressure. At the same time, the pore structure properties of the prepared adsorbent have a more significant effect on the adsorption performance at lower temperatures. This work provides an effective guide to synthesize biomass-based adsorbent for different application conditions, especially in the selection of activators.

Ni Nanoparticles on CeO2(111): Energetics, Electron Transfer and Structure by Ni Adsorption Calorimetry, Spectroscopies and DFT

The morphology, interfacial bonding energetics and charge transfer of Ni clusters and nanoparticles on slightly-reduced CeO2-x(111) surfaces at 100 to 300 K have been studied using single crystal adsorption calorimetry (SCAC), low-energy ion scattering spectroscopy (LEIS), X-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED) and density functional theory (DFT). The initial heat of adsorption of Ni vapor decreased with the extent of pre-reduction (x) of the CeO2-x(111), showing that stoichiometric ceria adsorbs Ni more strongly than oxygen vacancies. On CeO1.95(111) at 300 K, the heat dropped quickly with coverage in the first 0.1 ML, attributed to nucleation of Ni clusters on stoichiometric steps, followed by the Ni particles spreading onto less favorable terrace sites. At 100 K, the clusters nucleate on terraces due to slower diffusion. Adsorbed Ni monomers are in the +2 oxidation state, and they bind by ~45 kJ/mol more strongly to step sites than terraces. The measured heat of adsorption versus average particle size on terraces is favorably compared to DFT calculations. The Ce 3d XPS lineshape showed an increase in Ce3+/Ce4+ ratio with Ni coverage, providing the number of electrons donated to the ceria per Ni atom. The charge transferred per Ni is initially large but strongly decreases with increasing cluster size for both experiments and DFT, and shows large differences between clusters at steps versus terraces. This charge is localized on the interfacial Ni and Ce atoms in their atomic layers closest to the interface. This knowledge is crucial to understanding the nature of the active sites on the surface of Ni-CeO2 catalysts for which metal-oxide interactions play a very important role in the activation of O-H and C-H bonds. The changes in these interactions with Ni particle size (metal loading) and the extent of reduction of the ceria help to explain how previously reported catalytic activity and selectivity change with these same structural details.

Methane Adsorption on the Metal–Organic Framework Structure Al-BTC

Adsorption of natural gas (methane) on the synthesized metal–organic framework (MOF) Al-BTC with specific surface area SBET = 1422 m2/g was studied at pressures up to 40 MPa and temperatures of 303, 313, 323, and 333 K. The maximum adsorption of methane on Al-BTC reaches 10.36 mmol/g at 303 K and 40 MPa, and the initial heat of adsorption is ~14 kJ/mol. The amount of methane accumulated in a system with Al-BTC attains a value of 120–130 m3(NTP)/m3 in a range of pressures from 3.5 to 10.0 MPa, which is the most relevant for methane accumulation. The volumes of methane stored in the systems with Al-BTC and without an adsorbent differ by a factor of about 2 at 3.5 MPa, there is almost no distinction between these options of methane storage at 7.0 and 20.0 MPa, and the amount of gas in the system without an adsorbent exceeds by 25% that in Al-BTC. The pressure range from 3.5 to 6.0 MPa is most efficient for the methane adsorption accumulation in the MOF structure Al-BTC. The absolute efficiency of methane adsorption accumulation increases with lowering of temperature.

Open-Tip Carbon Nanotubes for Enhanced Methane Adsorption Performance: A Comparative Study

A series of open-tip carbon nanotubes (CNTs) were obtained by HNO3 modification with various concentrations of as-prepared carbon nanotubes via the CVD method, and this work aimed at investigating the structural features of open-tip CNTs for the methane capability. Modified CNTs had higher specific surface area and larger total pore volume, and importantly, greater micropore volume was obtained through HNO3 modification of the as-prepared CNTs. The remarkably high methane adsorption capacities were measured on the modified CNTs under pressure ranges of 0∼4.0 MPa at 298 K. The resulted H-CNT, which exhibited highest specific surface area and micropore volume, showed high methane uptake of 26.15 mg/g from the D-A model. This value was nearly as twice as the methane uptake of original CNTs (13.62 mg/g), along with an initial adsorption heat of 19.4 kJ/mol at lower coverage and 9.5 kJ/mol at higher methane coverage for H-CNT, indicating the physical nature for methane adsorption over open-tip CNTs.

Adsorption and diffusion of alkylbenzene in microspherical ZSM-5 zeolite assembled with nanocrystals

Adsorption isotherms and zero length column (ZLC) desorption curves of alkylbenzene (toluene and isopropylbenzene) in a series of microspherical ZSM-5 zeolites with inter- and intracrystalline mesopores were measured for revealing structural and surface characteristics of the hierarchical zeolites. The isotherms showed that adsorption capacities of toluene and isopropylbenzene increase with increase of mesoporosities in the range of higher relative pressure. Henry’s constants (KH) and initial heats of adsorption (Qst) turned out that introduction of mesoporous structure into the zeolites weakens interaction between alkylbenzene molecules and zeolite surface. Moreover, isopropylbenzene and toluene have different adsorption and diffusion behavior in the zeolites. The adsorption capacity, KH and Qst for isopropylbenzene to microporous and mesoporous ZSM-5 zeolites are lower than these for toluene. The effective diffusion time constants (Deff/R2) of alkylbenzene from the ZLC desorption curves increase with increase of mesoporosities in the ZSM-5 zeolites, but diffusion activation energy (Ea) expresses a contrary tendency. Due to low interaction between isopropylbenzene and zeolite surface, Deff/R2 values of isopropylbenzene with lower activation energy in all ZSM-5 samples are higher than these of toluene. The molecular sizes of two alkylbenzenes are not the key factor for their adsorption and diffusion in the mesozeolites.

Specific features of the adsorption of chlorinated methanes and water on carbon nanotubes and alumina

The adsorption of CH2Cl2, CH3Cl, CCl4, and H2O vapors on the surface of multi-walled carbon nanotubes and alumina in the temperature range of 10—25 °C was studied. Dependences of the isosteric heat of adsorption on the surface coverage were plotted. The initial heat of adsorption of dichloromethane, chloroform, and CCl4 on Al2O3 was found to be equal to 71, 28, and 31 kJ mol–1, respectively. In most cases, adsorption on the oxide adsorbent was characterized by a higher heat of adsorption compared to that found on the carbon material, which is apparently related to chemical interaction of sorbed molecules with the OH groups of the alumina surface.

Adsorption and Adhesion of Au on Reduced CeO2(111) Surfaces at 300 and 100 K

The adsorption of vapor-deposited Au onto CeO2-x(111) thin films (x = 0.05 and 0.2) at 300 and 100 K was studied using single crystal adsorption calorimetry (SCAC). The morphology of Au on these films was investigated using He+ low-energy ion scattering spectroscopy (LEIS) and X-ray photoelectron spectroscopy (XPS) by monitoring the changes in substrate and adsorbate signals with Au coverage. Both techniques indicate that Au grows on CeO1.95(111) as three-dimensional particles in the approximate shape of hemispherical caps with a density of 2.8 × 1012 particles/cm2 at 300 K and 7.8 × 1012 particles/cm2 at 100 K. At 300 K, Au initially grows on CeO1.80(111) with a shape similar to hemispherical caps with a density of 5.4 × 1012 particles/cm2 until ∼1.6 ML Au coverage, above which the Au particles become thicker than hemispherical caps. At 300 K, the initial heat of adsorption of Au onto CeO1.95(111) is 259 kJ/mol, which is 37 kJ/mol lower than that on CeO1.80(111). This indicates stronger binding of Au to ...

Adsorption and diffusion of n-heptane and toluene over mesostructured ZSM-5 zeolitic materials with acidic sites

In order to understand the effects of the surface acid sites in mesoporous zeolites on the adsorption and diffusion of hydrocarbons, the adsorption and diffusion of n-heptane and toluene (two C7 hydrocarbons) over the mesoporous HZSM-5 zeolites (SiO2/Al2O3 = 36) were investigated. The adsorption isotherms of two C7 hydrocarbons over the mesoporous HZSM-5 zeolites showed the experimental data good fitting to dual-site Langmuir-Freundlich model (DSLF) similar to mesoporous NaZSM-5. Henry's constants and the initial heats of adsorption displayed stronger interactions in HZSM-5 than in NaZSM-5, especially for n-heptane, due to the presence of surface acid sites. The influence of acidity to the adsorbate-adsorbent interactions decreases with increasing of the mesoporosity. The investigation on the diffusion of n-heptane and toluene in mesoporous HZSM-5 samples by the zero-length column (ZLC) method showed the strong interaction of adsorbates and HZSM-5 samples by effective diffusivity and activation energy. Moreover, the diffusions of adsorbates in zeolites were advanced with the introduction of mesopores in the zeolitic samples, though the dramatic influence of mesopore to diffusion is not arisen in mesoporous NaZSM-5, reflecting larger influence of acid sites on diffusion in mesoporous HZSM-5 samples.

A simultaneous volumetric adsorption–isothermal titration calorimetry study of small molecules on supported metallic nanoparticles

Adsorption microcalorimetry studies can determine the energetics involved in different adsorbate–surface interactions. These measurements can be taken using various techniques such as single-crystal adsorption calorimetry, isothermal titration calorimetry and differential scanning calorimetry. The incremental dosing of a small molecule adsorbate with simultaneous calorimetric measurement enables differential heats of adsorptions to be measured at a specific temperature. In this study, an automated volumetric adsorption apparatus (Micromeritics ASAP2020) was interfaced with a Differential Scanning Calorimeter (Setaram Sensys EVO) to perform simultaneous volumetric uptake and calorimetric measurements. Heats of adsorption of small gas molecules (CO, H2) on supported metal catalysts (Pt, Pd, Au) were measured using this setup. Similar calorimetric measurements for the adsorption of CO (0.5 % Pt/SiO2; 5 % Pd/Al2O3) and H2 (0.5 % Pt/SiO2) were also taken by interfacing a Micromeritics ASAP2020 with a custom-built calorimeter. From these studies, it was observed that the initial heats of CO adsorption on supported Pt (0.5 % Pt/Al2O3, 135 kJ mol−1; 1 % Pt/SiO2, 104 kJ mol−1) and Pd (5 % Pd/Al2O3, 148 kJ mol−1) catalysts were significantly higher than the corresponding Au (1 % Au/TiO2, 57.2 kJ mol−1; 1 % Au/Al2O3, 54.1 kJ mol−1) catalysts. It was also observed that the initial heat of adsorption of H2 on the supported 0.5 % Pt/Al2O3 (~80 to 89 kJ mol−1) catalyst is higher than that on 1 % Pt/SiO2 (~61 kJ mol−1). CO and H2 adsorption studies on the 0.5 % Pt/Al2O3 and the 5 % Pd/Al2O3 catalyst using both calorimetric setups yielded similar volumetric uptakes and initial heats.

Adsorption and Diffusion of Xylene Isomers on Mesoporous Beta Zeolite

A systematic and detailed analysis of adsorption and diffusion properties of xylene isomers over Beta zeolites with different mesoporosity was conducted. Adsorption isotherms of xylene isomers over microporous and mesoporous Beta zeolites through gravimetric methods were applied to investigate the impact of mesopores inside Beta zeolites on the adsorption properties of xylene isomers in the pressure range of lower 20 mbar. It is seen that the adsorption isotherms of three xylene isomers over microporous and mesoporous Beta zeolites could be successfully described by the single-site Toth model and the dual-site Toth model, respectively. The enhanced adsorption capacities and decreased Henry’s constants (KH) and the initial heats of adsorption (Qst) for the all xylene isomers are observed after the introduction of mesopores in the zeolites. For three xylene isomers, the order of Henry’s constant is o-xylene > m-xylene > p-xylene, whereas the adsorption capacities of Beta zeolite samples for xylene isomers execute the following order of o-xylene > p-xylene > m-xylene, due to the comprehensive effects from the molecular configuration and electrostatic interaction. At the same time, the diffusion properties of xylene isomers in the mesoporous Beta zeolites were also studied through the desorption curves measured by the zero length column (ZLC) method at 333–373 K. It turned out that the effective diffusion time constant (Deff/R2) is a growing trend with the increasing mesoporosity, whereas the tendency of the activation energy is just the reverse, indicating the contribution of mesopores to facilitate molecule diffusion by shortening diffusion paths and reducing diffusion resistances. Moreover, the diffusivities of three xylene isomers in all Beta zeolites follow an order of p-xylene > m-xylene > o-xylene as opposed to KH, conforming the significant effects of adsorbate-adsorbent interaction on the diffusion.

Energetics of Cu Adsorption and Adhesion onto Reduced CeO2(111) Surfaces by Calorimetry

The morphology and interfacial energetics of vapor-deposited Cu on slightly reduced CeO2(111) surfaces at 300 K have been studied using single crystal adsorption calorimetry (SCAC), He+ low-energy ion scattering spectroscopy (ISS), X-ray photoelectron spectroscopy (XPS), and low energy electron diffraction (LEED). Copper grows as three-dimensional nanoparticles with a density of ∼1013 particles/cm2 on CeO2–x(111) (x = 0.05, 0.1, and 0.2). The initial heat of adsorption of Cu decreased with the extent of reduction, showing that stoichiometric ceria adsorbs Cu more strongly than oxygen vacancies. On CeO1.95(111), the heat dropped quickly with coverage in the first 0.1 ML, attributed to nucleation of Cu clusters on stoichiometric steps, followed by the Cu particles spreading onto less favorable sites (step vacancies and terraces). Above ∼0.1 ML (>0.8 nm in diameter), the Cu adsorption energies showed no variation with extent of ceria reduction: the heat of adsorption increased slowly with coverage (particle ...

Experimental Evaluation of the Adsorption, Diffusion, and Separation of CH4/N2 and CH4/CO2 Mixtures on Al-BDC MOF

Adsorption equilibrium, thermodynamics, and kinetics of CH4, N2, and CO2 were investigated by volumetric-chromatographic and inverse gas chromatographic (IGC) methods on the Al-BDC MOF. The binary adsorption data from the volumetric-chromatographic experiments represents that the Al-BDC MOF has a high CO2/CH4 selectivity ca. 11 and a CH4/N2 selectivity ca. 4.3 at 303 K, and appears to be a good candidate for the CH4 separation. The initial adsorption heats of CH4, N2, and CO2 on the Al-BDC MOF were determined to be 15.3, 11.5, and 32.2 kJmol−1 by IGC method, respectively. Moreover, the micropore diffusivities of N2, CH4 and CO2 in the Al-BDC MOF at 303 K were also estimated to be 1.58 × 10−7 cm2/s, 7.04 × 10−8 cm2/s, and 3.95 × 10−9 cm2/s, respectively. The results indicate that micropores play a crucial role in the adsorptive separation of the CH4/N2 and CH4/CO2 mixtures, and the IGC method is a validity manner to estimate the thermodynamic and kinetic parameters of MOF adsorbents.

Adsorption and Diffusion of n-Heptane and Toluene over Mesoporous ZSM-5 Zeolites

ZSM-5 zeolites with different mesoporosities were prepared by alkaline treatment and characterized by powder XRD and nitrogen adsorption. Two C7 hydrocarbons of n-heptane and toluene were employed as probe molecules to investigate the effects of the introduction of mesopore on the adsorption and diffusion properties of ZSM-5 zeolites by comparing the experimental results of the samples treated and untreated by using NaOH. Adsorption isotherms were measured gravimetrically in the pressure range 0–32 mbar and from 293 to 338 K. The isotherms of microporous and mesoporous ZSM-5 were successfully fitted by using the Langmuir–Freundlich model and the dual-site Langmuir–Freundlich model, respectively. Henry’s constants and the initial heats of adsorption calculated from the adsorption isotherms as well as the fitting parameters displayed that the interactions between adsorbent and adsorbate were weakened after the introduction of mesopore, and the interactions between the adsorbates with microporous surface are much stronger than that between them with the mesoporous surface. Diffusion measurements were undertaken using the zero length column (ZLC) technique at partial pressure of p/p0 < 0.000 15 from 333 to 393 K. The results showed that the effective diffusion constants (Deff/R2) of the two C7 hydrocarbons increased greatly in the presence of mesopores, while the corresponding activation energy decreased due to the reduced diffusion resistance and the shortened diffusion path in the mesoporous zeolites. Also, higher and much more dramatic enhancement of the efficient diffusivities as a function of mesoporous volume for toluene relative to that for n-heptane were found, indicating that the diffusion of n-heptane is controlled by the micropore diffusion and that the diffusion of toluene is exclusively determined by mass transfer through the mesopores.

Expanding Pore Size of Al-BDC Metal–Organic Frameworks as a Way to Achieve High Adsorption Selectivity for CO2/CH4 Separation

The mesostructured Al-BDC metal–organic frameworks (MOFs) with an average pore size of 2.58 nm were prepared via a simplified washing and drying process and applied to the separation of CO2/CH4 mixtures. The adsorption equilibrium and thermodynamics of CH4 and CO2 were studied in the dynamic processes by the volumetric–chromatographic and inverse gas chromatographic (IGC) methods. The experiments represent that the Al-BDC MOF with large pore size has a much higher CO2/CH4 selectivity of ca. 24 at 303 K in the pressure range 0–1.0 MPa and therefore appears to be a good candidate for the separation of CH4 from CO2. The initial heats of adsorption of CH4 and CO2 on the mesostructured Al-BDC MOFs were determined to be 11.5 and 25.2 kJ mol–1 by the IGC method, respectively, which are significantly reduced by ca. 25% compared with that on the microporous Al-BDC MOFs. The results indicate that the expanded pore size not only greatly increases the selectivity of CO2 adsorption over CH4 but also reduces the adsorption heat, revealing that it should be the desired method to obtain a satisfactory absorbent for CO2/CH4 separation.

Calcium Thin Film Growth on Polyfluorenes: Interface Structure and Energetics

The adsorption of Ca vapor on two polyfluorenes, poly(9,9-di-n-hexyl-2,7-fluorene) (PDHF) and poly(9,9-di-n-hexyl-2,7-fluorene vinylene) (PDHFV), has been studied by adsorption microcalorimetry, low-energy He+ ion scattering spectroscopy (LEIS), and ultraviolet and X-ray photoelectron spectroscopies (UPS and XPS) at surface temperatures between 130 and ∼400 K. At 300 K, the initial heat of adsorption of Ca on PDHF is 250 kJ/mol and on PDHFV is 315 kJ/mol. We ascribe this initial heat to Ca reacting with impurities or defects in the polymers. The heat of adsorption for both polymers decreases nearly exponentially to the sublimation enthalpy of bulk Ca solid (178 kJ/mol) by 0.75 ML. The observed behavior is attributed to the increasing probability of Ca finding and adding to solid three-dimensional Ca islands which nucleate and grow on the polymer surface and eventually grow into a continuous solid Ca film on top of the polymer. LEIS, which shows only a slow increase of the signals related to solid Ca, supp...

Calcium Thin Film Growth on a Cyano-Substituted Poly(p-phenylene vinylene): Interface Structure and Energetics

The adsorption of Ca on poly[2-(2-ethylhexyloxy)-5-methoxy-1,4-phenylene cyanovinylene] (MEH-CN-PPV) at 130 and 300 K has been studied by adsorption microcalorimetry, X-ray photoelectron spectroscopy (XPS), and low-energy He+ ion-scattering spectroscopy (LEIS). The initial heat of adsorption of Ca on MEH-CN-PPV at 300 K is 436 kJ/mol. The heat of adsorption increases to 464 kJ/mol at ∼0.05 ML. We ascribe this initial heat to Ca abstracting cyano groups from the polymer backbone to make Ca(CN)2 clusters, plus a less important contribution due to Ca abstracting alkoxy groups to make Ca alkoxides. This is supported by XPS which shows the formation of new peaks in the N 1s and C 1s regions consistent with Ca(CN)2. There is also XPS evidence for Ca reacting with some of the ether groups to make Ca alkoxides at low coverage. Above 0.05 ML Ca coverage, the heat of adsorption decreases nearly exponentially to the sublimation enthalpy of bulk Ca solid (178 kJ/mol) by 5 ML. The observed behavior is attributed to the increasing probability of forming Ca nanoparticles and eventually a continuous solid Ca film on top of the reacted polymer. LEIS, which shows only a slow increase of the signal related to solid Ca, supports this model. Incoming Ca atoms undergo a kinetic competition between diffusing into the polymer to react with subsurface cyano or ether groups and forming or adding to three-dimensional Ca clusters on the surface. For studies done at 130 K, similar behavior is observed, with the heat of adsorption starting lower and decreasing more quickly to the heat of sublimation, already by 1.2 ML Ca coverage. This behavior along with the quicker growth of Ca on the surface of the polymer is attributed to the slower diffusion of Ca adatoms to subsurface cyano and ether groups. The effective thickness of “reacted” polymer (i.e., polymer which has lost its −CN groups and some alkoxy groups) is estimated to be 5.6 nm at 300 K but only 1.0 nm at 130 K.