Temperature-programmed Reaction Experiments Research Articles

Mechanistic aspects of the higher alcohol synthesis over K 2O-promoted ZnCr oxide: Temperature-programmed reaction and flow experiments of C 3, C 4, and C 5 oxygenates

Mechanistic aspects of the higher alcohol synthesis (HAS) over a K 2O-promoted ZnCrO catalyst are investigated by temperature-programmed surface reaction (TPSR) of C 3 oxygenates (1-propanol, n-propanol, and n-propanoic acid) and by flow microreactor experiments of 1-propanol, 3-pentanone, and 2-butanone. A number of chemical functions are identified by the TPSR study, including hydrogenation-dehydrogenation, “normal” and “reversal” aldolic-type condensations, ketonizations, “reversal” α-addition, dehydration, decarboxylation, and cracking. On comparing the data with those obtained during flow experiments over the same catalyst, a strict correspondence is observed between the chemical functions indicated by the TPSR study and those prevailing under steady-state conditions. However, under these conditions some of the associated chemical reactions (namely hydrogenation and ketonization) appear to be limited by chemical equilibrium and the peculiar reactivity of the different species participating in the reactions is appreciated. TPSR and continuous-flow experiments lead to the identification of a general reaction network for C N oxygenate molecules, based on the following routes; (i) hydrogenation/dehydrogenation reactions of oxygenate molecules; (ii) aldolic-type condensations of aldehydes and ketones, both in the normal and in the reversal mode, leading to the formation of higher aldehydes and ketones; (iii) ketonization reactions, leading to the formation of ketones and CO 2; (iv) reversal α-addition reactions, which result in the formation of 2-ketones; (v) dehydration of oxygenates, and particularly of secondary alcohols, leading to the formation of olefins. Olefins may also be formed by decarboxylation of surface carboxylate species. The reactivity of the species participating in the various reactions is discussed on the basis of their molecular structure, and results are compared with catalytic tests performed under HAS conditions. It is found that the reaction pattern identified in the present study basically describes the data collected under pressure.

Iodomethane decomposition on copper(110): surface reactions of C1 fragments

Methyl groups can be generated on Cu(110) surfaces under ultrahigh-vacuum conditions by dissociatively adsorbing iodomethane at temperatures above 180 K. We report here on the surface reactions of adsorbed CH3 as studied by a combination of Auger electron spectroscopy (AES), temperature-programmed reaction experiments, and isotope labeling. The reactions of methyl groups on Cu(110) are coverage-ddependent. For θ CH3 , less than 0.015 (number of methyl groups per surface copper atom), methyl groups disproportionate to produce methane and ethylene in a 2:1 ratio. At higher coverages, methyl groups also couple to form ethane

Hydrogen effects on nickel-catalyzed vapor-phase methanol carbonylation

The influence of hydrogen on the reaction orders of CO and methanol, with respect to the formation of acyl compounds, methane, and dimethyl ether, was investigated. Although hydrogen-promoted methane formation, experiments with deuterium and methanol- d 1 (MeOD) clarified that the hydrogen of the methanol hydroxyl group is the exclusive hydrogen source of the CH 4 formation and that this reaction occurs at nickel centers, which are also involved in the acyl compound formation. Temperature-programmed reaction (TPR) and X-ray diffraction (XRD) experiments suggested that the hydrogen effect is caused by changes in the number of active sites and that neither methanol carbonylation nor methane formation proceeds at the larger nickel[0] crystallites detectable by XRD. Furthermore, it has been demonstrated that under certain reaction conditions, hydrogen can induce a remarkable deactivation of the catalyst by promoting nickel aggregation. A mechanism that accounts for the hydrogen effects has been postulated.

On the formation and bonding of a surface carbonate on Ni(100)

The formation, stability, adsorption geometry and electronic structure of a surface carbonate on Ni(100) have been investigated by photoemission (XPS, UPS) and temperature-programmed reaction (TPR). The core level binding energies of 531.2 eV for 0(1s) and 289.0 eV for C(1s) are comparable to those of bulk carbonates. The He(II) spectrum of the carbonate valence levels is not well defined because of the coexisting adsorbed and oxidic oxygen. The angular dependence of the carbonate core level intensities is characteristic of the carbonate being present as an overlayer species rather than a thicker surface phase. The XPS data and isotope labelled TPR experiments indicate the oxygen atoms of the carbonate to be electronically and chemically equivalent, and on this basis we favor a structure in which the carbonate is attached to the metal via all three oxygen atoms. This is supported by comparision with the core level binding energies of HCOO ab and chemisorbed CO 2,ad, which are similarly attached to the surface. From the core level angular behavior, the close similarity of core level binding energies and available vibrational spectroscopic data, a (nearly) planar geometry of the CO 3,ad on Ni(100) is concluded, which is comparable to the planar bulk carbonate anion and the planar carbonate species on Ag(110). The activation barrier for decomposition is estimated from the observed maximum in TPR at 420 K to be 25 ± 2 kcal/mol. CO 2 does not accumulate on the clean or O ad-precovered Ni(100) surface at 130 K. The stabilized, chemisorbed CO 2,ad species often observed on other metal surfaces therefore does not play a critical role for carbonate formation on Ni(100). Also a mechanism involving the disproportionation of a CO 2… CO 2,ad − dimer anion can be ruled out from TPR data. The evidence of the experiments discussed in this paper suggests that the carbonate is predominantly formed by reaction of CO 2,ad with a less stable, defect (disordered) O ad species rather than with isolated oxygen adatoms or with ordered O ad in the major ordered phases of O ad or NiO present on the Ni(100) surface. A comparison is made to existing literature on the CO 2/O 2 interaction and carbonate formation on other transition metal and Ag surfaces. The different reactivity for formation of carbonate on Ni as compared with Ag surfaces is explained in terms of an activated reaction, where the activation barrier is determined mainly by the energetic position of the initial state of O ad and CO 2,ad and where the much higher adsorption energy of oxygen on nickel is mainly responsible for the much lower reactivity on that surface.

Determination of reaction models from experimental measurements

AbstractIt is demonstrated through the use of transformation relations, function approximation and error limits that it is impossible to distinguish among rival gasification models using just a single temperature programmed reaction experiment. Two experiments at different heating rates must be used and the ratio between rates is important. These depend on the models to be distinguished.

On the kinetics of thermal decomposition of wood and wood components

A kinetic analysis of the thermal decomposition of eucalyptus sawdust, cellulose and eucalyptus kraft lignin has been accomplished from temperature-programmed reaction experiments, using two different approaches: the first assumes an overall reaction model, whereas in the second, pyrolysis is viewed as a process consisting of multiple reactions in parallel and a distribution of activation energies is derived. The results obtained reveal that the simple first approach, which leads to overall values of the apparent kinetic parameters allows a fairly good reproduction of the experimental curves, although in the case of wood a two-stage analysis was necessary. No improvement was noticed with the use of activation energy distribution functions resulting from the multiple reaction treatment, except for cellulose and lignin at high conversion, whereas the steep region of the weight-loss curve for cellulose is better described with the overall reaction model.

Studies on the reactivity of supported rhodium and molybdenum carbonyl complexes using temperature-programmed techniques

The reactivity of zirconia-supported RH 4(CO) 12 and Mo(CO) 6 has been studied by temperature-programmed desorption (TPD) into vacuum and temperature-programmed reaction (TPR). Thermal activation of the carbonyl precursors results in a complex reaction system including CO disproportionation, water gas shift reaction and oxidative interaction of Rh o species with hydroxyl groups on the support. The surface species formed after chemisorption of CO on the reduced samples exhibit different reactivity compared with the original metal carbonyls. K 2O added to ZrO 2 promotes the shift reaction and selectivity to methanol but suppresses CO hydrogenation; Mo(CO) 6 co-adsorbed with Rh 4(CO) 12 brings on formation of surface sites exhibiting weak adsorption properties towards CO and high hydrogenation activity. TPR experiments in flow of H 2-CO mixture show that above 500 K the thermodynamically favoured products CH 4 and CO 2 rapidly become predominant; however, with RhMo/ZrO 2, a good selectivity to oxygenated products can be obtained in a restricted temperature range by operating at high CO pressure.

Low energy electron induced decomposition on surfaces: W(CO) 6 on Si(111)-(7 × 7)

Low energy electron induced decomposition of W(CO) 6 adsorbed on Si(111)-(7 × 7) at 145 K under ultrahigh vacuum conditions is reported due to its possible importance to laser induced surface reactions. CO evolution and a minor amount of electron stimulated desorption of W(CO) 6 is observed during bombardment with 55 V electrons. Additionally, CO is evolved during temperature programmed reaction experiments following electron bombardment, indicative of the presence of partially decarbonylated fragments (W(CO) x , x < 6) on the surface. Carbon, oxygen and tungsten are detected on the surface by Auger electron spectroscopy after temperature programmed reaction indicative of strongly bound carbide and oxide. Low energy electron induced decomposition is of added importance because many surface sensitive analysis methods use or generate low energy electrons and may, therefore, induce inadvertent decomposition.

Low energy electron induced decomposition on surfaces: W(CO) 6 on Si(111)-(7 x 7)

Low energy electron induced decomposition of W(CO) 6 adsorbed on Si(111)-(7 x 7) at 145 K under ultrahigh vacuum conditions is reported due to its possible importance to laser induced surface reactions. CO evolution and a minor amount of electron stimulated desorption of W(CO) 6 is observed during bombardment with 55 V electrons. Additionally, CO is evolved during temperature programmed reaction experiments following electron bombardment, indicative of the presence of partially decarbonylated fragments (W(CO) x , x <6)on the surface. Carbon, oxygen and tungsten are detected on the surface by Auger electron spectroscopy after temperature programmed reaction indicative of strongly bound carbide and oxide. Low energy electron induced decomposition is of added importance because many surface sensitive analysis methods use or generate low energy electrons and may, therefore, induce inadvertent decomposition.

Isotopic identification of surface site transfer on nickel/alumina catalysts

Isotope labeling with temperature-programmed reaction (TPR) for CO hydrogenation was used to separate two distinct CO adsorption sites on a Ni/Al/sub 2/O/sub 3/ catalysts. One site is Ni metal and has the higher activity for CO hydrogenation. The less-reactive CO is on the Al/sub 2/O/sub 3/ support. Only the Ni metal is occupied at 300 K, and transfer for CO between the two sites occurs at higher temperatures. In the presence of adsorbed H/sub 2/, CO that was adsorbed on the Ni metal moved to the Al/sub 2/O/sub 3/ support. This is an activated process, and the only pathway to occupy the Al/sub 2/O/sub 3/ sites is by adsorption on the Ni. The reverse transfer from Al/sub 2/O/sub 3/ to Ni occurs if some of the H/sub 2/ is desorbed; surface hydrogen inhibits this reverse process. These results show that during a typical TPR experiment, transfer between sites competes with reaction.

ChemInform Abstract: Alkane Activation: Adsorption and Reaction of n‐Butane on Clean and Carbided W(100).

The adsorption and reaction of n‐butane have been investigated on clean and carbided W(100) surfaces using temperature programmed reaction spectroscopy, Auger electron spectroscopy, low energy electron diffraction, and isotopic exchange methods. For the clean surface, the initial adsorption probability is 1.0 at a crystal temperature of 130 K. At low coverages, butane adsorbed at 130 K converts to a dissociated state with unit probability below 200 K during temperature programmed reaction. Higher exposures result in a decrease in the adsorption probability and the onset of molecular butane desorption in the range of 150–200 K. A weakly adsorbed molecularly bound butane state is observed for all coverages on the [13 −10] and (5×1) carbide surfaces. Studies using n‐butane‐d10 show a dramatic isotope effect. The low coverage dissociation probability of n‐C4D10 is significantly less than unity and the saturation amount of decomposition in a temperature programmed reaction experiment is less than for n‐C4H10....

Surface Processes in CVD: Laser- and Low Energy Electron-Induced Decomposition of W(CO)6 on Si(111)-(7×7)

ABSTRACTThe decomposition of W(CO)6 adsorbed on Si(111)-(7×7) using low energy electrons and ultraviolet photons has been investigated under ultrahigh vacuum conditions. This work is motivated by a desire to understand the mechanism for laser- and electron-assisted chemical vapor deposition (CVD) of tungsten using volatile coordination complexes and to specifically understand the role of the surface in these processes. Both electron stimulated and photo-assisted decomposition of the adsorbed W(CO)6 are observed. No thermal decomposition of the W(CO)6 occurs under the conditions of these experiments, based on independent temperature programmed reaction experiments, ruling out the possibility of laser- or electron-induced heating as the cause of decomposition. Furthermore, the interaction of the W(CO)6 with the Si(111)-(7×7) surface is shown to be exceedingly weak based on the fact that the desorption energy is 9.46 ± 0.77 kcal/mol. Desorption of CO is induced during both ultraviolet photolysis and electron bombardment. Carbon monoxide is exclusively evolved during ultraviolet photolysis: no W-containing fragments are desorbed. During electron bombardment, a small amount of the W(CO)6 is desorbed, accounting for ∼10% of the desorption. In both cases, CO-containing W fragments remain on the surface after decomposition at low surface temperature. The remaining surface fragments do not undergo further photolysis at 308 nm but do react thermally. Competing desorption and dissociation of CO are thermally induced resulting in carbide and oxide impurities in the deposited material. The fact that strongly bound W(CO)x fragments are trapped on the surface is proposed as a limiting factor in the purity of tungsten deposits using the decomposition of W(CO)6.

Effect of Catalyst Preparation on Catalytic Activity: VI. Chemical Structures on Nickel/Alumina Catalysts: their Impact on the Rate-Determining Step in the Hydrogenation of Carbon Monoxide

Titration of the carbon pool subsequent to steady-state reaction of hydrogen and carbon monoxide over Ni/Al 2O 3 catalysts in conjunction with temperature-programmed surface reaction studies has been performed to assess the impact of nickel speciation on the rate-determining step in the methanation reaction. Mass balances for the desorption products reveal significant differences that depend on the weight loading of the catalyst. Low-weight loading catalysts prepared by incipient wetness consist of NiAl 2O 4 reactive centers as revealed by electron spectroscopy for chemical analysis and carbon monoxide temperature-programmed reaction experiments. Data are presented which support the hypothesis the rate of methanation is controlled by the rate of surface-bound carbon monoxide dissociation over the catalysts. On the other hand, high-weight loading catalysts prepared by this method consist of “particle-like” nickel. Over these catalysts, the rate of methanation is found to be controlled by the rate of hydrogenation of CH x . It is proposed that variation in catalyst preparation procedures can account for the wide-spread controversy in the determination of rate-determining step for methanation over Ni/Al 2O 3 catalysts.

Reactions of pyrrole with Ni(100)

The reactions of pyrrole with a clean Ni(100) surface have been studied with temperature programmed reaction (TPR), reflection absorption infrared spectroscopy (RAIS), and Auger electron spectroscopy (AES). RAIS indicated that pyrrole adsorbs on clean Ni(100) at 200 K with its ring parallel to the surface, presumably via π -bonding with the 1A 2 orbital. During TPR experiments with a heating rate of 8 K/s, pyrrole desorbs molecularly at 235 K and dihydrogen evolves above room temperature, in accord with equilibrium thermodynamics. During TPR experiments with a heating rate of 45 K/s, new pathways emerge. Light gases, principally HCN, H 2 , and NH 3 , desorb at circa 570 K. The variation of pyrrole decomposition products with slightly different heating rates suggests a change from a decomposition reaction to a polymerization reaction with increasing heating rates.

Alkane activation: Adsorption and reaction of n-butane on clean and carbided W(100)

The adsorption and reaction of n-butane have been investigated on clean and carbided W(100) surfaces using temperature programmed reaction spectroscopy, Auger electron spectroscopy, low energy electron diffraction, and isotopic exchange methods. For the clean surface, the initial adsorption probability is 1.0 at a crystal temperature of 130 K. At low coverages, butane adsorbed at 130 K converts to a dissociated state with unit probability below 200 K during temperature programmed reaction. Higher exposures result in a decrease in the adsorption probability and the onset of molecular butane desorption in the range of 150–200 K. A weakly adsorbed molecularly bound butane state is observed for all coverages on the [13 −10] and (5×1) carbide surfaces. Studies using n-butane-d10 show a dramatic isotope effect. The low coverage dissociation probability of n-C4D10 is significantly less than unity and the saturation amount of decomposition in a temperature programmed reaction experiment is less than for n-C4H10. This is evidence for a kinetic isotope effect in the decomposition process, with the activation barrier for C–D bond scission greater than the barrier for C–H bonds. For butane exposures made above 200 K, the initial sticking probability is lowered, and the saturation value for decomposition is also decreased. Preadsorption of deuterium suppresses greater than 90% of the butane decomposition, while postadsorption of D2 under the same conditions blocks less than 5%. These observations suggest that for low butane exposures, there is a substantial amount of dehydrogenation upon adsorption which eventually leads to complete decomposition. As the crystal temperature is increased, buildup of surface hydrogen and possibly carbon from further decomposition rapidly inhibits dissociation of additional butane and results in molecular desorption. A limited amount of butane decomposition occurs on the intermediate coverage [13 −10] carbide surface formed following temperature programmed reaction of a saturation exposure of butane. The amount of butane decomposition on the [13 −10] carbide is an order of magnitude less than on clean W(100). Likewise, no decomposition of butane is observed on the W(100)–(5×1)-C surface.

Characterization and Fourier transform infrared studies of the effects of TiO 2 crystal phases during CO oxidation on [formula omitted] catalysts

The catalytic oxidation of CO was investigated on Pt TiO 2 catalysts in an infrared cell reactor to study the influence of strong metal-support interaction effects on oxidation reactions and of using different crystal forms of TiO 2. Catalysts were prepared using rutile and anatase TiO 2 and were characterized by chemisorption, X-ray diffraction, and X-ray photoelectron spectroscopy. Hydrogen reduction at 500 °C suppresses CO and H 2 chemisorption and leads to changes in Pt 4f 7 2 electron binding energies for both phases of TiO 2. The anatase catalysts show an infrared absorption band around 2091 cm −1 which is assigned to linear-bonded CO; whereas, the rutile catalysts show an absorption doublet for adsorbed CO around 2094 and 2075 cm −1 and bands around 1840 cm −1 assigned to bridge-bonded CO. Both catalysts show the adsorption of CO 2 as a carbonate-like species which is demonstrated to be adsorbed onto the support. The effects of reduction temperature and of support material on CO oxidation activity were compared through temperature-programmed reaction experiments. The catalysts reduced at 200 °C show slightly higher activity and lower ignition temperatures than those reduced at 500 °C. The rutile-supported catalysts show much higher CO oxidation activity with lower ignition temperatures; the increased activity is speculated to result from a lower activation energy for oxygen desorption. A morphological model of metalsupport interactions involving oxygen transfer from the rutile support is proposed to coexist with the Langmuir-Hinshelwood reaction mechanism.

Integrated kinetic modelling and transient FTIR studies of CO oxidation on Pt/SiO 2

The reaction dynamics and multiplicity features of CO oxidation on a Pt/SiO 2 catalyst are studied via transient experiments combining Fourier transform infrared spectroscopy (FTIR) with temperature-programmed reaction (TPR) and with concentration-programmed reaction (CPR). Bifurcation diagrams of the surface and bulk concentrations and temperatures obtained in these experiments are used to develop a reaction-reactor model to interpret the results. The model integrates non-equilibrium elementary step kinetics, based on single-crystal studies, with heat and mass transport effects and reactor modelling. The model reproduces well bifurcation cross-section diagrams at various experimental conditions and simulates well the TPR and CPR experiments. A parametric sensitivity analysis reveals how changes in the parameters affect the reaction behaviour, as well as how the rate-determining step changes with the operating conditions. The results of this work demonstrate how the many interactions between the kinetics, transport processes and reactor environment affect experimental observations, and provide a method for the proper assessment of such interactions.

Using temperature-programmed reaction for kinetics analysis of liquid-phase reactions

Temperature-programmed reaction (TPR) is shown to be a practical method for determining reaction-rate expressions and kinetics parameters for liquid-phase chemical reactions. By measuring reaction extent during a temperature rise, information normally obtained from a series of isothermal kinetics experiments can be found in a single TPR test. Using nonlinear least-squares regression eliminates the need for a constant temperature-rise rate, and enables reactions with significant heat effects to be tested. Use of experimental pressures above atmospheric can extend TPR temperature ranges so that experiments can be carried out within a reasonable time for most reactions. Simulations of TPR with random measurement error are used to assess the technique's accuracy and to identify the best values of operating parameters. Comparing TPR with isothermal methods shows the latter may be slightly more accurate for the same number of data points, but TPR is far faster and experimentally simpler, and more data points can be taken in a given time period. Two TPR experiments measuring alkaline ethyl-acetate hydrolysis show the technique's applicability.

Carbon monoxide oxidation on the kinked Pt(321) surface

Carbon monoxide oxidation has been studied using high resolution electron energy loss spectroscopy, temperature programmed reaction (TPR), and titration of adsorbed atomic oxygen on a kinked Pt(321) surface characterized using Auger electron spectroscopy and low energy electron diffraction. Two previous papers detail adsorption and dissociation of oxygen and adsorption of carbon monoxide on the Pt(321) surface. Vibrational spectra taken during interrupted reaction experiments clearly indicate that atomic oxygen adsorbed on terraces reacts with CO more rapidly than atomic oxygen adsorbed near the kinked step sites. The multiple reaction limited CO2 peaks observed in the 100–400 K region can be rationalized by proposing that the activation energy for reaction of terrace and step CO with terrace and step atomic oxygen is sufficiently different to cause discernible CO2 peaks as the various species are sequentially depleted by reaction during a TPR experiment.

Carbon monoxide oxidation on the kinked Pt(321) surface

Carbon monoxide oxidation has been studied using high resolution electron energy loss spectroscopy, temperature-programmed reaction (TPR), and titration of adsorbed atomic oxygen on a kinked Pt(321) surface characterized using Auger electron spectroscopy and low energy electron diffraction. Two previous papers detail adsorption and dissociation of oxygen and adsorption of carbon monoxide on the Pt(321) surface. Vibrational spectra taken during interrupted reaction experiments clearly indicate that atomic oxygen adsorbed on terraces reacts with CO more rapidly than atomic oxygen adsorbed near the kinked step sites. Reaction between coadsorbed atomic oxygen and carbon monoxide begins just above 100 K and persists above 400 K on a (321) surface saturated with both atomic oxygen and carbon monoxide. The multiple-reaction-limited CO2 peaks observed in the 100 to 400 K region can be rationalized by proposing that the activation energy for reaction of terrace and step CO with terrace and step atomic oxygen is sufficiently different to cause discernible CO2 peaks as the various species are sequentially depleted by reaction during the TPR experiment.