Desorption Rate Of CO Research Articles

A theory of phonon-induced friction on molecular adsorbates

In this manuscript, we provide a general theory for how surface phonons couple to molecular adsorbates. Our theory maps the extended dynamics of a surface's atomic vibrational motions to a generalized Langevin equation, and by doing so captures these dynamics in a single quantity: the non-Markovian friction. The different frequency components of this friction are the phonon modes of the surface slab weighted by their coupling to the adsorbate degrees of freedom. Using this formalism, we demonstrate that physisorbed species couple primarily to acoustic phonons while chemisorbed species couple to dispersionless local vibrations. We subsequently derive equations for phonon-adjusted reaction rates using transition state theory and demonstrate that these corrections improve agreement with experimental results for CO desorption rates from Pt(111).

Bond Selective Photochemistry at Metal Nanoparticle Surfaces: CO Desorption from Pt and Pd.

The use of visible photon fluxes to influence catalytic reactions on metal nanoparticle surfaces has attracted attention based on observations of reaction mechanisms and selectivity not observed under equilibrium heating. These observations suggest that photon fluxes can selectively impact the rates of certain elementary steps, creating nonequilibrium energy distributions among various reaction pathways. However, quantitative studies validating these hypotheses on metal nanoparticle surfaces are lacking. We examine the influence of continuous wave visible photon fluxes on the CO desorption rates from 1 to 2 nm diameter Pt and Pd nanoparticle surfaces supported on γ-Al2O3. Temperature-programmed desorption measurements quantified via diffuse reflectance infrared Fourier transform spectroscopy demonstrate that visible photon fluxes significantly enhanced the rate of CO desorption from Pt nanoparticles in a wavelength-dependent manner. 440 nm photons most efficiently promoted CO desorption from Pt nanoparticle surfaces, aligning with the excitation energy for the interfacial electronic transition within the Pt-CO bond. Conversely, visible photon fluxes had no measurable influence on CO desorption rates from Pd nanoparticle surfaces after accounting for photon-induced heating. Density functional theory calculations demonstrate that the Pt-CO bond exhibits a narrower LUMO resonance, stronger coupling between the photoexcitation and forces induced on the metal-C bond, and vibrational energy dissipation that more effectively couples to desorption as compared to Pd-CO. These results demonstrate the specificity photons provide in facilitating chemical reactions on metal nanoparticle surfaces and substantiate the idea that photon fluxes can steer processes and outcomes of catalytic reactions in ways not achievable by equilibrium heating.

Active Site Entropy of Atomically Dispersed Rh/Al2O3 Catalysts Dictates Activity for Ethylene Hydroformylation

The interpretation of catalytic kinetics on supported metal catalysts typically assumes that catalytic cycles occur on static active site structures apart from local rearrangement in the coordination environment. It has been reported that certain atomically dispersed metal active sites (e.g., Cu/Chabazite zeolites and Rh/γ-Al2O3) may be mobile under relevant reaction conditions, suggesting that the active sites themselves have entropy that could be relevant to apparent reaction kinetics. Here, we systematically modify the mobility (degrees of freedom or entropy) of atomically dispersed Rhodium gem-dicarbonyls, Rh(CO)2, supported on γ-Al2O3 through functionalization of the support with straight-chain alkyl-phosphonic acids of different tail lengths ranging from 1 (methyl) to 16 carbons (hexadecyl). The restricted mobility of Rh(CO)2 results in up to a 120 °C decrease in the required temperature for CO desorption from Rh(CO)2 and 1000× increase in turn over frequency for propanal formation via ethylene hydroformylation [where Rh(CO)2 is the most abundant surface intermediate] as compared to unfunctionalized Rh/γ-Al2O3. Eyring analysis suggests that the promoted rates of CO desorption and hydroformylation are due primarily to changes in apparent activation entropy [ΔΔS‡ of up to 60 J/(mol·K)], where restricted mobility of Rh(CO)2 promotes the attempt frequency of CO desorption, which is a kinetically relevant step in hydroformylation. Further, the dependence of Rh(CO)2 reactivity on alkyl phosphonic acid tail length suggests that interactions between phosphonic acid tails far from the active site modify the rigidity of the self-assembled monolayers, such that longer tails better restricted the mobility of Rh(CO)2. This work suggests that active site entropy can influence reaction kinetics on heterogeneous catalysts when changes in active site mobility are coupled to reaction coordinates and further that controlling active site entropy can be an effective design approach to increase catalytic performance.

Electronic Structure Optimization of PdZn-Graphitic Carbon Nitride Nanocomposites as Electrocatalysts for Selective CO2 to CO Conversion.

Herein, a novel PdZn/g-C3N4 nanocomposite electrocatalyst, PdZnGCN, prepared from a facile hydrothermal reduction procedure for an efficient CO2 to CO conversion has been examined. This composite catalyst reduces CO2 at a thermodynamic overpotential of 0.79 V versus RHE with a 93.6% CO Faradaic efficiency and a CO partial current density of 4.4 mA cm–2. Moreover, the turnover frequency for PdZnGCN reaches 20 974 h–1 with an average selectivity of 95.4% for CO after 1 h and an energy efficiency approaching 59%, which is superior to most reported noble metals and metal alloys as electrocatalysts. The enhanced catalytic activity of this nanocomposite is due to synergistic interactions between PdZn and g-C3N4 as evidenced by optimum work function, zeta potential, CO desorption rate, and downshifted d-band center. Furthermore, suppressed grain growth during the formation of nanocomposites also results in faster reaction kinetics, as demonstrated by a lower Tafel slope (93.6 mV/dec) and a larger electrochemically active surface, consequently enhancing the overall performance.

Effect of Ag doping on Pd/Ag-CeO2 catalysts for CO and C3H6 oxidation

To achieve high fuel efficiency and low emission in automobiles, it is necessary to develop highly active diesel oxidation catalysts (DOCs). Pd/CeO2 catalysts have been widely used as active catalysts for CO and C3H6 oxidation reactions. Additionally, Ag has been reported to enhance the oxygen storage capacity (OSC) of CeO2, which contributes to the oxidation ability of Pd/CeO2. In this study, Pd/Ag-CeO2 catalysts were used for CO and C3H6 oxidation reactions. When CeO2 was doped with appropriate amounts of Ag, reducibility and CO desorption rate were increased, which confirmed the high OSCs of Ag-doped catalysts. However, Ag particles were formed and the Ce3+/Ce4+ ratio decreased when CeO2 was doped with excess amounts of Ag. In addition, reduced Pd (Pd0), which is an active species for C3H6 oxidation, was formed and maintained even under oxidative reaction conditions. Since the removal of C3H6 is important for the oxidation of CO and C3H6, the catalyst with the highest Pd0 fraction (Pd/0.1Ag-CeO2 and Pd/0.3Ag-CeO2) presented improved catalytic activity. Consequently, the optimal amount of Ag enhanced the OSC of Pd/Ag-CeO2 catalysts and formed active Pd0 species under oxidative conditions, which resulted in the excellent catalytic activity of Pd/Ag-CeO2 for the CO and C3H6 oxidation reaction.

A Simple Angle-Resolved Thermal Molecular Beam Reactor: Applied to CO Oxidation on Pt{110}

We developed a simple form of thermal molecular beam catalytic reactor system which is capable of measuring sticking probabilities and reaction probabilities, together with angle-resolved scattering of molecules and products during catalytic reactions at the surface. This includes very fast determination of the angle dependence of the reaction product flux at steady-state. It was employed to determine the oxidation of CO on Pt{110}-(1 × 2), including individual molecular sticking and scattering. The initial sticking probability of oxygen on Pt{110} shows a small variation between 140 and 750 K surface temperature, from 0.45 to 0.28. The saturation uptake drops from 1.5 ± 0.2 ML at 140 K to 0.6 ML at 300 K and to 0.23 ± 0.02 ML at 750 K. The initial sticking probability of CO at 300 K is 0.80 and decreases to 0.62 at 470 K. Beyond that temperature, it descends steeply down to near zero at 570 K, due to the high desorption rate of CO at that temperature. Kisliuk precursor mobility parameters K were calculated from shape of the sticking curves. For 300 K, a value of 0.11 ± 0.01 was found, which increases to 0.76 ± 0.01 at 470 K, indicating a change from considerable mobility in the precursor state, to more limited mobility before desorption at high temperature. In temperature-programmed CO-O2 reaction experiments, CO2 production was observed to initiate in the temperature region 460–510 K. Using isothermal angle-resolved experiments, the CO2 flux was determined in the [11¯0] plane at temperatures of 470–620 K. Two sharp scattering lobes at positions of ±16° off the surface normal were found, with a high cosine power angle dependence, which were attributed to desorption from the {111}-like microfacets of the 1 × 2 reconstructed surface, with products evolving over a high barrier.

DRIFTS study of Fe promoter effect on Rh/Al2O3 catalyst for C2 oxygenates synthesis from syngas

DRIFTS experiments such as CO adsorption, CO-TPSR and CO+H2 were designated to study the effect of Fe promoter on the key steps of C2 oxygenates formation from syngas. The CO adsorption results demonstrated that Fe weakened CO adsorption and especially the bridging adsorption. It was found in CO-TPSR experiments that the catalyst with lower Fe loading is more easily dissociated while the ones with higher Fe loading own stronger hydrogenation activity. Moreover, it was observed by CO+H2 experiments that Fe plays a role in stabilizing the lineally adsorbed CO species and decreasing the CO desorption rate. The catalytic performance results indicated that when Fe content is 4wt. %, the selectivity of total C2 oxygenates is the highest, which was in accordance with the DRIFTS results.

Anodic Electrocatalysts for Fuel Cells Based on Pt/Ti1–xRu x O2

The electrocatalytic activity of materials in the 10% Pt/Ti1–xRu x O2–δ system, where x = 0–0.3 (0 ≤ Ru ≤ 30 mol %), in the reactions of hydrogen electrooxidation in the presence of CO is studied in the liquid three-electrode cell and a model of fuel cell. It is shown that the tolerance of the electrocatalysts towards CO is determined by the crystal structure of the support: the support with the rutile structure provides a higher rate of CO desorption than the support with the anatase structure. The potential of the onset of CO oxidation decreases with increasing concentration of dopant in the support from 650 mV for 10% Pt/TiO2 to 480 mV (NHE) for 10% Pt/Ti0.91Ru0.09O2–δ (rutile). The use of these materials as the anodic catalysts of fuel cell operating with hydrogen containing 30 ppm CO enabled us to obtain a current density by 7 times higher as compared with the 20% PtRu/C E-Tec catalysts.

Methanol ice co-desorption as a mechanism to explain cold methanol in the gas-phase

Context. Methanol is formed via surface reactions on icy dust grains. Methanol is also detected in the gas-phase at temperatures below its thermal desorption temperature and at levels higher than can be explained by pure gas-phase chemistry. The process that controls the transition from solid state to gas-phase methanol in cold environments is not understood. Aims. The goal of this work is to investigate whether thermal CO desorption provides an indirect pathway for methanol to co-desorb at low temperatures. Methods. Mixed CH3OH:CO/CH4 ices were heated under ultra-high vacuum conditions and ice contents are traced using RAIRS (reflection absorption IR spectroscopy), while desorbing species were detected mass spectrometrically. An updated gas-grain chemical network was used to test the impact of the results of these experiments. The physical model used is applicable for TW Hya, a protoplanetary disk in which cold gas-phase methanol has recently been detected. Results. Methanol release together with thermal CO desorption is found to be an ineffective process in the experiments, resulting in an upper limit of ≤ 7.3 × 10−7 CH3OH molecules per CO molecule over all ice mixtures considered. Chemical modelling based on the upper limits shows that co-desorption rates as low as 10−6 CH3OH molecules per CO molecule are high enough to release substantial amounts of methanol to the gas-phase at and around the location of the CO thermal desorption front in a protoplanetary disk. The impact of thermal co-desorption of CH3OH with CO as a grain-gas bridge mechanism is compared with that of UV induced photodesorption and chemisorption.

Separation of CO<sub>2</sub> from Flue Gas via Absorption of Alcohol Amines

The separation of CO 2 from flue gas by alcohol amine method was investigated in this paper. We conducted the absorption and desorption experiments using different alcohol amines: monoethanolamine (MEA), diethanolamine (DEA), methyl diethanolamine (MDEA), triethanolamine (TEA), piperazine (PZ), and their binary mixtures. Relevant absorption rate and desorption rate of CO 2 were obtained and reported. Aspen Plus was applied to simulate the process of recovering CO 2 from flue gas via absorption of PZ-MEA solution. The influences of the lean liquid CO 2 load, lean liquid temperature and desorber pressure on the reboiler duty were investigated, and the optimal operating parameters were obtained on the basis of the minimum reboiler duty.

Theoretical Study of Sodium Effect on the Gasification of Carbonaceous Materials with Carbon Dioxide.

The effect of sodium on the thermodynamics and kinetics of carbon gasification with carbon dioxide was studied by using quantum chemistry methods. Specifically, in the density functional context, two exchange-correlation functionals were used: B3LYP and M06. Some results obtained by these exchange-correlation functionals were contrasted with those obtained by the CCSD(T) method. It was found that density functional theory gives similar conclusions with respect to the coupled-cluster method. As one important conclusion we can mention that the thermodynamics of carbon monoxide desorption is not favored by the sodium presence. However, the presence of this metal induces: (a) an easier formation of one semiquinone group, (b) the dissociation of carbon dioxide, and

ANALYTICAL FORMULAE OF MOLECULAR ION ABUNDANCES AND THE N2H+RING IN PROTOPLANETARY DISKS

We investigate the chemistry of ion molecules in protoplanetary disks, motivated by the detection of N$_2$H$^+$ ring around TW Hya. While the ring inner radius coincides with the CO snow line, it is not apparent why N$_2$H$^+$ is abundant outside the CO snow line in spite of the similar sublimation temperatures of CO and N$_2$. Using the full gas-grain network model, we reproduced the N$_2$H$^+$ ring in a disk model with millimeter grains. The chemical conversion of CO and N$_2$ to less volatile species (sink effect hereinafter) is found to affect the N$_2$H$^+$ distribution. Since the efficiency of the sink depends on various parameters such as activation barriers of grain surface reactions, which are not well constrained, we also constructed the no-sink model; the total (gas and ice) CO and N$_2$ abundances are set constant, and their gaseous abundances are given by the balance between adsorption and desorption. Abundances of molecular ions in the no-sink model are calculated by analytical formulas, which are derived by analyzing the full-network model. The N$_2$H$^+$ ring is reproduced by the no-sink model, as well. The 2D (R-Z) distribution of N$_2$H$^+$, however, is different among the full-network model and no-sink model. The column density of N$_2$H$^+$ in the no-sink model depends sensitively on the desorption rate of CO and N$_2$, and the flux of cosmic ray. We also found that N$_2$H$^+$ abundance can peak at the temperature slightly below the CO sublimation, even if the desorption energies of CO and N$_2$ are the same.

Model for the catalytic oxidation of CO, including gas-phase impurities and CO desorption

We present results of kinetic Monte Carlo simulations of a modified Ziff-Gulari-Barshad model for the reaction CO+O → CO(2) on a catalytic surface. Our model includes impurities in the gas phase, CO desorption, and a modification known to eliminate the unphysical O poisoned phase. The impurities can adsorb and desorb on the surface, but otherwise remain inert. In a previous work that did not include CO desorption [Buendía and Rikvold, Phys. Rev. E 85, 031143 (2012)], we found that the impurities have very distinctive effects on the phase diagram and greatly diminish the reactivity of the system. If the impurities do not desorb, once the system reaches a stationary state, the CO(2) production disappears. When the impurities are allowed to desorb, there are regions where the CO(2) reaction window reappears, although greatly reduced. Following experimental evidence that indicates that temperature effects are crucial in many catalytic processes, here we further analyze these effects by including a CO desorption rate. We find that the CO desorption has the effect to smooth the transition between the reactive and the CO rich phase, and most importantly it can counteract the negative effects of the presence of impurities by widening the reactive window such that now the system remains catalytically active in the whole range of CO pressures.

Temperature-dependent CO desorption kinetics on supported gold nanoparticles: Relevance to clean hydrogen production and fuel cell systems

The influence of the temperature on the CO desorption process was studied on supported gold nanoparticles using isotopic exchange experiments and mass spectrometric detection. CO desorption kinetics data were measured for a range of temperature from 25 to 150 °C and for a fixed CO concentration of 1000 ppm. The measured temperature-dependent kinetics data were discussed with relation to the CO tolerance issue at the proton exchange membrane fuel cell anode, the water–gas-shift and preferential CO oxidation reactions. The high measured rates of CO desorption give support to the argument that the rate of CO adsorption/desorption (and not CO electrooxidation) plays the most significant role in determining the equilibrium CO coverage at the fuel cell anode and the subsequent CO tolerance. The measured desorption rate constants are believed to be also of added value for the development of elementary reaction step-based kinetics models for the water–gas-shift and preferential CO oxidation reactions. In addition, Arrhenius parameters were calculated and their variations provided insight into the underlying adsorption/desorption processes. The influence of a temperature cycle up to 150 °C on the properties of the supported gold nanoparticles was also assessed with regard to CO desorption kinetics and catalyst nanostructure.

Oxygen-enhanced water gas shift on ceria-supported Pd–Cu and Pt–Cu bimetallic catalysts

Aiming at enhancing H 2 production in water gas shift (WGS) for fuel cell application, a small amount of oxygen was added to WGS reaction toward oxygen-enhanced water gas shift (OWGS) on ceria-supported bimetallic Pd–Cu and Pt–Cu catalysts. Both CO conversion and H 2 yield were found to increase by the oxygen addition. The remarkable enhancement of H 2 production by O 2 addition in short contact time was attributed to the enhanced shift reaction, rather than the oxidation of CO on catalyst surface. The strong dependence of H 2 production rate on CO concentration in OWGS kinetic study suggested O 2 lowers the CO surface coverage. It was proposed that O 2 breaks down the domain structure of chemisorbed CO into smaller domains to increase the chance for coreactant (H 2O) to participate in the reaction and the heat of exothermic surface reaction helping to enhance WGS kinetics. Pt–Cu and Pd–Cu bimetallic catalysts were found to be superior to monometallic catalysts for both CO conversion and H 2 production for OWGS at 300 °C or lower, while the superiority of bimetallic catalysts was not as pronounced in WGS. These catalytic properties were correlated with the structure of the bimetallic catalysts. EXAFS spectra indicated that Cu forms alloys with Pt and with Pd. TPR demonstrated the strong interaction between the two metals causing the reduction temperature of Cu to decrease upon Pd or Pt addition. The transient pulse desorption rate of CO 2 from Pd–Cu supported on CeO 2 is faster than that of Pd, suggesting the presence of Cu in Pd–Cu facilitate CO 2 desorption from Pd catalyst. The oxygen storage capacity (OSC) of CeO 2 in the bimetallic catalysts indicates that Cu is much less pyrophoric in the bimetallic catalysts due to lower O 2 uptake compared to monometallic Cu. These significant changes in structure and electronic properties of the bimetallic catalysts are the result of highly dispersed Pt or Pd in the Cu nanoparticles.

Dynamics and depletion in thermally supercritical starless cores

In previous studies, we identified two classes of starless cores, thermally subcritical and supercritical, distinguished by different dynamical behaviour and internal structure. Here, we study the evolution of the dynamically unstable, thermally supercritical cores by means of a numerical hydrodynamic simulation that includes radiative equilibrium and simple molecular chemistry. From an initial state as an unstable Bonnor–Ebert (BE) sphere, a contracting core evolves towards the configuration of a singular isothermal sphere by inside–out collapse. We follow the gas temperature and abundance of CO during the contraction. The temperature is predominantly determined by radiative equilibrium, but in the rapidly contracting centre of the core compressive heating raises the gas temperature by a few degrees over its value in static equilibrium. The time-scale for the equilibration of CO depends on the gas density and is everywhere shorter than the dynamical time-scale. The result is that the dynamics do not much affect the abundance of CO which is always close to that of a static sphere of the same density profile, and CO cannot be used as a chemical clock in starless cores. We use our non-local thermodynamic equilibrium (non-LTE) radiative transfer code mollie to predict observable CO and N2H+ line spectra, including the non-LTE hyperfine ratios of N2H+, during the contraction. These are compared against observations of the starless core L1544. The comparison indicates that the dust in L1544 has an opacity consistent with ice-covered rather than bare grains, the cosmic ray ionization rate is about 1 × 10−17 s−1 and the density structure of L1544 is approximately that of a BE sphere with a maximum central density of 2 × 107 cm−3, equivalent to an average density of 3 × 106 cm−3 within a radius of 500 au. The observed CO linewidths and intensities are reproduced if the CO desorption rate is about 30 times higher than the rate expected from cosmic ray strikes alone, indicating that other desorption processes are also active.

DRIFTS study of different gas adsorption for CO selective oxidation on Cu–Zr–Ce–O catalysts

The adsorptions of different gases (CO, H 2 and O 2) in the hydrogen-rich gas on the co-precipitated Cu–Zr–Ce–O catalyst were discussed and the active sites were ascertained with infrared spectroscopy technique. It was shown that the adsorption strength of CO was stronger than that of O 2 or H 2. Hydrogen and CO were competitive adsorption and the coexistence H 2 and CO on the surface accelerated the rate of CO desorption. Adsorbed H 2 could convert into geminal OH groups on the ceria surface at high temperatures in the absence of oxygen, while it was easy to form surface hydroxyl groups at low temperatures and condensed to physical water with increasing desorption temperature in the existence of oxygen. The adsorption of CO 2 was strong and it could transform into thermal stable carbonate species even in the reaction conditions. The active sites of the Cu–Zr–Ce–O catalyst were Cu 2+ and Cu +, mainly the latter. The oxygen defect sites could be formed on the Cu–Zr–Ce–O catalyst surface through dehydration and decarboxylation.

Operando studies of desorption, reaction and carbonate formation during CO oxidation by Au/TiO 2 catalysts

Gas transient methods have been used to probe reaction pathways in catalytic CO oxidation under operando conditions. A low dead volume reactor is described, which permits monitoring of surface reaction species by repetitive FTIR while using mass spectrometry to continuously monitor reaction products and conversion throughout transient gas switching. CO desorption rates are measured from an active Au/TiO 2 catalyst under He and CO 2 and are consistent with literature values for CO desorption from Au single crystals and Au films on TiO 2. The rate of the reactive removal of adsorbed surface CO (a) was used to determine a turnover frequency of about 0.043 s −1 for CO oxidation on the Au particles, comparable to the rate obtained in a steady state flow reactor. Gas switching is used to investigate the storage of oxygen on the catalyst. It is found that following O 2 exposure, the amount of adsorbed active oxygen is either undetectably small, less than 3.1 μmoles O/g catal, or it desorbs very rapidly at 298 K. Although storage of oxygen is very ineffective on this catalyst, CO 2 is stored by its dynamic interaction with the support to form carbonates or bicarbonates causing slow removal of product CO 2 from the reactor bed. These results are summarized by a scheme describing the pathways for CO oxidation.

Decay of metastable phases in a model for the catalytic oxidation of CO

We study by kinetic Monte Carlo simulations the dynamic behavior of a Ziff-Gulari-Barshad model with CO desorption for the reaction CO + O-->CO(2) on a catalytic surface. Finite-size scaling analysis of the fluctuations and the fourth-order order-parameter cumulant show that below a critical CO desorption rate, the model exhibits a nonequilibrium first-order phase transition between low and high CO coverage phases. We calculate several points on the coexistence curve. We also measure the metastable lifetimes associated with the transition from the low CO coverage phase to the high CO coverage phase, and vice versa. Our results indicate that the transition process follows a mechanism very similar to the decay of metastable phases associated with equilibrium first-order phase transitions and can be described by the classic Kolmogorov-Johnson-Mehl-Avrami theory of phase transformation by nucleation and growth. In the present case, the desorption parameter plays the role of temperature, and the distance to the coexistence curve plays the role of an external field or supersaturation. We identify two distinct regimes, depending on whether the system is far from or close to the coexistence curve, in which the statistical properties and the system-size dependence of the lifetimes are different, corresponding to multidroplet or single-droplet decay, respectively. The crossover between the two regimes approaches the coexistence curve logarithmically with system size, analogous to the behavior of the crossover between multidroplet and single-droplet metastable decay near an equilibrium first-order phase transition.

Crossover Between Mean-Field and Ising Critical Behavior in a Lattice-Gas Reaction-Diffusion Model

Lattice-gas models for CO oxidation can exhibit a discontinuous nonequilibrium transition between reactive and inactive states, which disappears above a critical CO-desorption rate. Using finite-size-scaling analysis, we demonstrate a crossover from Ising to mean-field behavior at the critical point, with increasing surface mobility of adsorbed CO or with decreasing system size. This behavior is elucidated by analogy with that of equilibrium Ising-type systems with long-range interactions.