HCOOH Decomposition Research Articles

Reduction of Np(V) with formic acid in acidic solutions in the presence of palladium catalysts

The kinetics of catalytic reduction of Np(V) with formic acid in HClO4 solutions in the presence of Pd/SiO2 catalysts differing in the Pd content and size of Pd nanocrystals was studied. The reaction is a structure-insenitive catalytic process, i.e., the size effect is absent. An increase in the percentage of Pd on SiO2 leads to a decrease in the activity of the catalysis centers due to a considerable increase in the contribution of the side reaction catalytic decomposition of HCOOH with an increase in the number of active centers in the catalyst grain. The effect of the S:L ratio, concentrations of HCOOH and HClO4, and temperature on the rate of catalytic reduction of Np(V) in the presence of palladium catalysts was examined. The suggested mechanism of the catalytic reduction of Np(V) with formic acid in the presence of Pd/SiO2 involves a slow step of decomposition of the protonated species NpO2H, formed by the reaction of the NpO 2 + ion with chemisorbed hydrogen atoms Pd(H).

Bioinspired Energy Conversion Systems for Hydrogen Production and Storage

AbstractRecent developments in photocatalytic hydrogen production by using artificial photosynthesis systems is described, together with those in hydrogen storage through the fixation of CO2 with H2. Hydrogen can be stored in the form of formic acid, which can be converted back to H2 in the presence of an appropriate catalyst. Electron donor–acceptor dyads are utilized as efficient photocatalysts to reduce methyl viologen (MV2+) by NADH (β‐nicotinamide adenine dinucleotide, reduced form) analogues to produce the methyl violgen radical cation that acts as an electron mediator for the production of hydrogen. Porphyrin‐monolayer‐protected gold clusters that enhance the light harvesting efficiency can also be used for the photocatalytic reduction of methyl viologen by NADH analogues. The use of a simple electron donor–acceptor dyad, the 9‐mesityl‐10‐methylacridinium ion (Acr+–Mes), enables the construction of a highly efficient photocatalytic hydrogen‐evolution system without an electron mediator such as MV2+, with poly(N‐vinyl‐2‐pyrrolidone)‐protected platinum nanoclusters (Pt–PVP) and NADH as a hydrogen‐evolution catalyst and an electron donor, respectively. Hydrogen thus produced can be stored in the form of formic acid (liquid) by fixation of CO2 with H2 in water by using ruthenium aqua complexes [RuII(η6‐C6Me6)(L)(OH2)]2+ [L = 2,2′‐bipyridine (bpy), 4,4′‐dimethoxy‐2,2′‐bipyridine (4,4′‐OMe‐bpy)] and iridium aqua complexes [IrIIICp*(L)(OH2)]2+ (Cp* = η5‐C5Me5, L = bpy, 4,4′‐OMe‐bpy) as catalysts at pH 3.0. Catalytic systems for the decomposition of HCOOH to H2 are also described. The combination of photocatalytic hydrogen generation with the catalytic fixation of CO2 with H2 and the decomposition of HCOOH back to H2 provides an excellent system for cutting CO2 emission.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

Catalytic reduction of U(VI) with formic acid in acid solutions on palladium catalysts

The kinetics of catalytic reduction of U(VI) with formic acid in H2SO4 solutions in the presence of Pd/SiO2 catalysts differing in the size of nanocrystallites of the active metal was studied. A decrease in the size of supported Pd particles leads to a decrease in the specific activity of the catalyst, i.e., the catalytic centers located on large crystallites exhibit higher activity. An increase in the Pd percent content on SiO2 leads to a decrease in the activity of the catalytic centers, which is caused by a considerable increase in the contribution of the side reaction of catalytic decomposition of HCOOH with an increase in the number of active centers in the catalyst grain. The results obtained are interpreted on the basis of the concepts of the energy nonuniformity of the surface atoms and of the reaction mechanism. The results show that the size of Pd nanocrystallites is an important factor of the selectivity of palladium catalysts in the preparation of U(IV) by catalytic reduction with formic acid.

H2 production with ultra-low CO selectivity via photocatalytic reforming of methanol on Au/TiO2 catalyst

H2 with ultra-low CO concentration was produced via photocatalytic reforming of methanol on Au/TiO2 catalyst. The rate of H2 production is greatly increased when the gold particle size is reduced from 10 to smaller than 3nm. The concentration of CO in H2 decreases with reducing the gold particle size of the catalyst. It is suggested that the by-product CO is mostly produced via decomposition of the intermediate formic acid species derived from methanol. The smaller gold particles possibly switch the HCOOH decomposition reaction mainly to H2 and CO2 products while suppress the CO and H2O products. In addition, some CO may be oxidized to CO2 by photogenerated oxidizing species at the perimeter interface between the small gold particles and TiO2 under photocatalytic condition.

Hydrogen Production from Glucose by Partial Oxidation in High Temperature High Pressure Water -2: Reaction Mechanism and Rate of HCOOH Decomposition

By partial oxidation of glucose with ZnO in high pressure high temperature water at 300°C, glucose was converted into H2 with relatively high yield and without CH4 formation. In order to understand the reaction mechanism and the optimum condition of the process, we performed glucose partial oxidation at 200-300°C to analyse an intermediate of H2, without ZnO, which probably promotes the intermediate into H2. As a result, we detected a high yield of HCOOH. The formation of HCOOH was sensitive to reaction temperature and the yield was the highest at 200°C among the experimental conditions. It was confirmed that the role of ZnO was a promoter of HCOOH decomposition into H2 and CO2 through HCOOH conversion experiments with and without ZnO at 200-300°C. The conversion of HCOOH with ZnO was within a few minutes at 300°C. Through the study, we suggested that two-stage reaction is effective for H2 formation from glucose partial oxidation: the partial oxidation of glucose is conducted at 200°C and the conversion of HCOOH is achieved over 300°C. To confirm the idea, the two-stage reaction was performed: the partial oxidation of glucose was conducted at 200°C with ZnO followed to increase temperature up to 300°C for promotion of H2 formation. The H2 yield was less amount than expected. This was probably due that ZnO prohibited the formation of HCOOH on glucose partial oxidation at 200°C.

Switchover of Reaction Mechanism for the Catalytic Decomposition of HCOOH on a TiO2(110) Surface

Catalytic decomposition of HCOOH on a TiO2(110) surface switches over between unimolecular dehydration (HCOOH → H2O + CO) and bimolecular dehydrogenation (HCOOH → H2 + CO2), depending on the reaction conditions. As the dehydration and dehydrogenation reactions proceed on acidic and basic oxide catalyst surfaces, respectively, the aspect observed on the same single crystal surface seems to be not compatible with the conventional acid−base catalysis concept. To clarify the origin of the switchover of the acid−base catalysis, the reaction mechanism of the HCOOH dehydrogenation was studied by density functional theory (DFT) calculations. It was concluded from the DFT calculations together with the rate equation and experimentally determined activation energy that the bimolecular dehydrogenation proceeds between a strongly adsorbed bridging formate anion and a weakly adsorbed HCOOH molecule by cooperative catalysis of three adjacent surface Ti4+ ions as Lewis acidic sites on the surface. This mechanism is enti...

Trends in the First-principles Theoretical Study on Catalysis

We review recent trends in the first-principles theoretical studies on catalysis. We first see the accuracy of first-principles calculations, especially, the accuracy of generalized gradient approximation (GGA). Then we see recent developments of ab initio thermodynamics and statistical mechanics to predict surface phase diagrams as functions of chemical potentials or pressures of gas phase molecules. We also review recent development of micro-kinetic modeling for complex catalytic reactions. Finally, we describe our recent studies on HCOOH decomposition on the TiO2(110) surface and simulation of electrode surfaces.

Formation of Hydrogen Peroxide in Photocatalytic Reactions

To characterize the formation of H2O2 in photocatalytic reactions, HCHO and HCOOH have been photocatalytically decomposed by use of TiO2 and Pd/TiO2 under irradiation with the UV light emitted from a blacklight blue fluorescent lamp (BL) or germicidal lamp (GL). An enzymatic method has been introduced to accurately measure the H2O2 concentration. The photocatalyst irradiated with the UV light emitted from the GL produces H2O2 from water at a larger rate and amount. Deposition of Pd on TiO2 increases the rate and amount of the formation of H2O2 from water. In the absence of the photocatalyst, no HCOOH is decomposed under irradiations with both the UV lights from the BL and GL, whereas the decomposition becomes possible in the presence of the photocatalyst. H2O2 is formed in parallel with the photocatalytic decomposition of HCOOH. In the presence of the photocatalyst, the concentration of the H2O2 formed during the decomposition of HCHO is smaller than that formed from water alone. In contrast, this relatio...

Infrared study of the adsorption of formic acid on clean and Ca-promoted Pd/SiO 2 catalysts

The adsorption and decomposition of formic acid on a highly dispersed supported Pd/SiO 2 catalyst (2 wt.% Pd) prepared via ion exchange (IE) of [Pd(NH 3) 4] 2+ in alkaline solution, together with two Ca-promoted preparations (Ca/Pd=2 at./at.) where calcium was added either to the prereduced Pd crystallites or to the diammine palladium complex, were studied by FTIR at 298–653 K. On the support, HCOOH is mostly adsorbed molecularly at room temperature, with partial dimerization, condensation and extensive hydrogen bonding, but readily decomposes, on the Pd crystallites of Pd/SiO 2, via decarbonylation, to give CO multicoordinated to the metal surface, and water. With heating, formic acid decomposition is accompanied by some water gas shift as well, while CO reacts to give methyl (methane) and methoxy. Calcium promotion to both the prereduced Pd and its diammine complex precursor, enhanced HCOOH decomposition onto the catalyst surface, even at 298 K. Together with sorbed HCOOH and chemisorbed CO, mono- and bidentate formates were observed on these materials, owing to the incorporation of well-dispersed CaO x H y . These formates were readily decomposed by atomic hydrogen produced by decarbonylation/WGS of formic acid on Pd. At increasing temperatures, some carbonates (polydentate and simple) were formed, but hardly any methane was detected. On Ca–Pd/SiO 2 with calcium added to prereduced Pd metal particles the extension and/or onset of all these processes was more straightforward than on the promoted Ca–Pd/SiO 2 where calcium was added to diammine palladium instead, most likely owing to the combined impact of a higher dispersion of the Pd crystallites on the former preparation, and calcium oxyhydroxide decoration (CaO x H y ) of the metal particles on the latter, which hamper H-spillover from them.

The parallel dehydrative and dehydrogenative catalytic action of γ-Al 2O 3 pure and doped by MgO : Kinetics, selectivity, time dependence of catalytic behaviour, mechanisms and interpretations

The catalytic test reaction of HCOOH decomposition on pure γ-Al 2O 3 and doped (+ MgO (1% w/w)) was studied at temperatures 275–400 °C. A double dehydrative and dehydrogenative, instead the thought single dehydrative, behaviour of γ-Al 2O 3 was disclosed. The dehydration activity is higher for pure alumina and decreases with operation time while the dehydrogenation activity is higher for doped and increases with time at the higher but decreases at the lower temperatures showing an unusual temperature dependent activation and ageing. Doping and increase of temperature reduce dehydration selectivity. It decreases with operation time at higher but increases at lower temperatures. The Arrhenius kinetic parameters of reactions differ strongly. Activation energy of dehydration seems unaffected and pre-exponential factor slightly affected by catalyst kind and operation time. Parameters concerning dehydrogenation are affected slightly by catalyst kind and strongly by operation time. Based on an atomic scale structure analysis and results, two dehydration and one dehydrogenation coexisting mechanisms stemming from identical initial step of dissociative adsorption of HCOOH as H + and HCOO − were suggested interpreting the results. The observed behaviour is due to changes of surface atomic spacing by doping Mg 2+ and of mean distances of adsorptive pair sites with operation time caused by the second mechanism.

Surface properties of Ru(0001) electrodes interacting with formic acid

Cyclic voltammetry (CV) was used to investigate the electrocatalytic oxidation of formic acid on smooth and rough Ru(0001) electrodes in HClO4 solution. Ex-situ electron diffraction (LEED and RHEED) and Auger electron spectroscopy (AES) were applied to characterize the Ru electrode surfaces and the adsorbate structure. The CV for a smooth Ru(0001) surface in a 0.1 M HCOOH+0.1 M HClO4 solution no longer exhibits the H- and OH-adsorption peaks in the double layer region due to complete blocking of H/OH-adsorption by COad species, as has been confirmed by the observation of an ordered (2×2)-CO phase in good agreement with in-situ IR measurements. We find that the major pathway for HCOOH oxidation is via dehydration at electrode potentials below 0 V involving no faradaic current formation in agreement with the literature, since no anodic current peak occurs in this potential region. An anodic current peak appears at 0.6 V, which is ascribed to COad electrooxidation with concomitant O/OH adsorption as confirmed by a (1×1)-O phase and an increase in the Auger O-signal, which causes inhibition of HCOOH adsorption and decomposition leading to deceleration of formic acid oxidation at higher potentials, E>0.7 V.

Nanostructured platinum as an electrocatalyst for the electrooxidation of formic acid

Nanostructured platinum prepared by the chemical reduction of hexachloroplatinic acid dissolved in aqueous domains of the liquid crystalline phases of oligoethylene oxide surfactants, was examined as an electrocatalyst for the electrooxidation of formic acid. The electrocatalytic properties of the catalyst combining highly specific surface areas and a periodic mesoporous nanostructure were accessed in sulfuric acid solution containing 0.5 mol dm −3 formic acid using cyclic voltammetry (CV) and chronoamperometry. The electrocatalytic activity of the material at 60 °C, is characterised by a mass activity of 8.6 A g −1 and a specific surface area activity of 26 μA cm −2 at 0.376 V (vs. RHE). The resistance to CO poisoning was found to depend upon electrode potential. At hydrogen adsorption potentials, the material is easily poisoned, while the material shows high resistance to CO poisoning at potentials positive of the hydrogen region. These facts suggest that the decomposition of HCOOH on the mesoporous platinum is likely to proceed through a dual-path mechanism and the high surface area material is a potential electocatalyst towards the electrooxidation of small organic molecules.

IN SITU IR SPECTROSCOPIC STUDIES OF HCOOH DECOMPOSITION OVER V-TI-O CATALYST

Decomposition of formic acid over V-Ti-O catalysts was studied by in situ IR spectroscopy. Four surface compounds, among which are H-bonded acid, one mono- and two bidentate formates (BF1 and BF2), were identified in the temperature range of 100-190°C. The activation energy and rate of the BF2 decomposition were found equal to those for the CO formation. This equality points to the involvement of BF2 in the HCOOH decomposition into carbon monoxide.

Spectroscopic and Kinetic Studies of the Reaction of CO+H2O and CO+O2 and Decomposition of HCOOH on Au/H-Mordenite Catalysts

The surface species formed from the reaction of CO+H2O and CO+O2 and decomposition of HCOOH on Au incorporated into H-mordenite zeolite have been studied by means of in situ FTIR spectroscopy. On H-mordenite, a bidentate formate species (2912, 1536, and 1390 cm−1) is produced upon exposure to the CO+H2O gas mixture at 323 K, as well as different carbonate-like species (1956, 1852, 1705, and 1360 cm−1). The latter species was extensively formed in a short time and was responsible for hindering the CO2 adsorbed species. However, Au/H-mordenite presented different vibration modes of formate species with a high emphasis on the monodentate ones (2950, 2916, 2896, 1690, and 1340 cm−1). The HCOOH adsorption on Au/H-mordenite showed two bands at 1622 and 1590 cm−1 of the νas(OCO) species, suggesting the formation of two types of formate species. The decomposition rate of the formate species formed on Au moieties was faster than that formed on H-mordenite. This was consistent with the calculated activation energies of CO2 formation that showed a lower value (40.1 kJ/mol) on the former sample than on the latter one (63.3 kJ/mol). A dehydrogenation mechanism is proposed (HCOOH→H2+CO2) for the decomposition of HCOOH on the Au/H-mordenite catalyst. On the other hand, the Au/H-mordenite catalyst activated the CO oxidation reaction. This reaction proceeded mainly through the formation of carboxylate species at first, which tended to obviate with time, preferring the formate species. The latter species resulted from the interaction of CO with OH stretching of the zeolite assisted by the presence of gas phase O2. The formate species is further decomposed with time to carbonate species.

Interaction of HCOOH with stoichiometric and reduced SrTiO3(100) surfaces

Interaction of formic acid with stoichiometric (TiO2-terminated) and reduced SrTiO3(100) surfaces has been investigated using temperature programmed desorption (TPD), and x-ray photoelectron spectroscopy (XPS). Formic acid was dissociated to form formate and a surface proton below 250 K on both stoichiometric and reduced SrTiO3(100) surfaces. Formate was decomposed primarily through dehydration to produce CO and H2O, instead of through dehydrogenation to produce CO2 and H2, on both surfaces. Formaldehyde produced from decomposition of formate was also observed on both surfaces. On stoichiometric surfaces, formaldehyde was produced through bimolecular coupling of two formates on low-coordination Ti cation sites. However, on the reduced surface, formaldehyde formation involves the reduction of surface formates through the oxidation of reduced Ti cations. XPS results show that surface defects on reduced SrTiO3(100) surfaces were reoxidized significantly upon exposure to 30 L HCOOH at 300 K, in contrast to defects on reduced TiO2(110) surfaces where no reduction in defect intensity was observed under identical conditions. The TPD peak of formaldehyde on a reduced SrTiO3(100) surface is shifted to lower temperature and is significantly broader (down to below 300 K) compared to on the stoichiometric surface. The adsorption and decomposition of HCOOH on SrTiO3(100) surfaces are compared with those on TiO2 surfaces.

Catalysis over Porous Anodic Alumina Film Catalysts with Different Pore Surface Concentrations

The catalytic behaviour of porous anodic Al2O3 films with different surface concentrations and lengths of pores vertical to the surface was investigated in the HCOOH decomposition test reaction in which they showed an ≅100% dehydrative action. The surface density and length of pores and the position on the pore walls were found to affect strongly the kinetic parameters activation energy, preexponential factor, and total and specific activity at each constant reaction temperature, revealing a strong catalytic heterogeneity of the pore wall surface. Anodic aluminas modified by hydrothermal treatment showed a much different catalytic behaviour, a strong reduction of the pore wall surface heterogeneity, and much higher activities than those of nonmodified aluminas. The promotion factor gave a maximum at a pore length specific for each pore surface density, usually increased with reaction temperature and generally varied between ≅3 and ≅23. Methods to prepare ultra-active aluminas of designed structure are thus predicted.

Theoretical study of the decomposition of HCOOH on an MgO(100) surface

We present here a theoretical investigation of the mechanism of the decomposition of formic acid on perfect and defective MgO(100) surfaces using the ab initio molecular orbital method. The decomposition reaction does not occur on a perfect surface, but is feasible on defect surfaces: the surface with an O 2− vacancy is more favorable than the surface with an O vacancy. Although the mechanism is different from that in the gas phase, C–H and C–O bond cleavages occur at the same time in the transition state, as in the gas phase. The carbonyl O atom moves into the vacancy to form an intermediate structure. The intermediate is more stable by 41.7 kcal mol −1 at the UMP2 level than the bridging structure. The C–H interacts with the lattice O atom to form a surface OH species. The interaction between the C–H and lattice Mg atoms is repulsive. The energy barrier is 28.3 kcal mol −1 and the overall reaction is endothermic by 48.9 kcal mol −1 at the UMP2 level.

Adsorption and decomposition of formic acid on potassium-modified Cu(1 1 0)

Adsorption and decomposition of HCOOH on potassium-modified Cu(110) surface has been investigated by temperature-programmed desorption (TPD) and photoemission. It has been found that preadsorbed potassium enhances deprotonation of HCOOH at low temperatures. Potassium, in coverages up to one monolayer, stabilizes a formate species at 300–450K characterized by photoemission peaks at 5.8, 9.0–9.3, 10.9 and 14.3eV in the HeII spectrum. The tendency to create potassium carbonate species has been found at high temperatures for potassium coverages greater than one monolayer.

Investigation of the incorporation of electrolyte anions in porous anodic Al 2 O 3 films by employing a suitable probe catalytic reaction

A new method has been developed capable of describing the incorporation of electrolyte anions along the pore wall surface and across both the barrier layer and the pore wall oxide after the establishment of the steady state of growth of porous anodic Al2O3 where other methods cannot be applied to obtain reliable results. The knowledge of the nature/composition of anodic oxides as regards the incorporation of species like electrolyte anions is of specific importance for both the understanding of the electrochemical mechanism of oxide production and growth and the scientific and technological applications of porous anodic Al2O3 films. The method consists of the selection and use of a suitable catalytic probe reaction on porous anodic oxides at thicknesses varying from a value near zero up to the maximum limiting thickness and the treatment of the experimental reaction rate results by a properly developed mathematical formalism. This method was employed in anodic Al2O3 films prepared in H2SO4 anodizing electrolyte at a constant bath temperature and different current densities using as a probe reaction the decomposition of HCOOH on these oxides, which is almost exclusively a dehydration reaction, at relatively high reaction temperatures, 350 °C and 390 °C, where the effect of other species except SO4 2− incorporated in the oxide on the reaction rate is eliminated. It has been shown that the fraction of the intercrystallite surfaces occupied by SO4 2− follows a parabola-like distribution. It has a significant value at the pore base surface, depending on the current density, then it passes through a maximum along the pore wall surface and across both the barrier layer and the pore walls near the pore bases at positions depending on the current density and then becomes almost zero at the mouths of the pores of the oxide with the maximum limiting thickness and at both the Al2O3/Al interface and cell boundaries. The maximum value of the surface coverage is almost independent of the current density and is always near 1, showing an almost complete saturation of intercrystalline surfaces at these positions. The above distribution of surface coverage predicts a qualitatively similar distribution of the SO4 2− bulk concentration across both the barrier layer and pore wall oxide around the pore bases. The method may be improved and developed further either for a more detailed investigation of the above films or to investigate films prepared in other pore-forming electrolytes.

Preparation of ultra-active alumina of designed porous structure by successive hydrothermal and thermal treatments of porous anodic Al 2O 3 films

A porous anodic alumina film was prepared by the anodic oxidation of Al metal sheet in a thermostated and vigorously stirred bath of H 2SO 4 15% (w/v) at a temperature of 25°C and a current density of 15 mA cm −2. It had a geometric surface area of 33 cm 2, a surface density of pores ≅1.269×10 11 cm −2 and the maximum limiting thickness and porosity achieved at these conditions which are 50.3 μm and 0.42, respectively. This oxide was tried in the catalytic test reaction of the decomposition of HCOOH at temperatures 270–390°C. Then, the oxide was treated hydrothermally in H 2O at 100°C for 5 h and tried in the same test reaction. The procedure of hydrothermal treatment and catalysis experiment was repeated 40 times. In all cases the oxide showed an almost exclusively dehydrative catalytic effect, ≅98–100%. Both the total activity of the alumina film with the aforementioned constant geometric surface area and its specific activity referred to the unit of oxide mass gave a maximum in the first and a minimum about the fourth hydrothermal treatment; then, they increased strongly with the order of hydrothermal treatment. Despite the decrease of the oxide mass during hydrothermal treatment, the final promotion of the total catalytic activity of oxide was 13.7–10.6 times that of non-treated oxide for temperatures 330–390°C. The corresponding promotion of specific activity was 31.5–24.5 times that of the non-treated oxide. The results of the present study showed that the successive hydrothermal and thermal treatments of porous anodic Al 2O 3 films produce more and more active alumina catalysts. In this way ultra-active alumina catalysts or supports can be prepared.