Cl-containing Catalyst Research Articles

Unraveling promoter effect in enhancing Rh-catalyzed hydroformylation of formaldehyde

Hydroformylation is one of the most important homogenous reactions, especially formaldehyde hydroformylation is of critical importance for synthesis of ethylene glycol. Due to lack of mechanistic analysis on promoters, we have employed a combined experimental and theoretical approach to reveal the promoter effect in the Rh-catalyzed hydroformylation of formaldehyde. The base promoter was suggested to be a bifunctional promoter. The mechanism of Cl-promoted formaldehyde hydroformylation was established, which significantly different from Cl-free system, and HCHO insertion was illustrated to be the rate-determining step. The distortion/interaction-activation strain analysis and mechanism calculations of halogen types demonstrate that Cl-containing catalyst has superior promotion effects compare to F, Br, and I-containing systems. The investigation on the effect of Cl coordination number shows that one-coordinated catalyst exhibits lower energy barriers and higher reactivity.

The insights into chlorine doping effect on performance of ceria supported nickel catalysts for selective CO methanation

Selective CO methanation (CO-SMET) in the reformate gas, containing (vol.%): 1.0 CO, 65H2, 10H2O, 20 CO2 with He as balance, was investigated over a number of nickel-ceria catalysts: treated with NH4Cl before (Ni/CeO2(Cl*)) and after (Ni(Cl*)/CeO2) Ni deposition, prepared by Cl-containing Ni precursor (Ni(Cl)/CeO2) and Cl-free one (Ni/CeO2). The effect of residual chlorine, originating from the catalyst preparation procedures, on the activity and CO selectivity of the samples was demonstrated. It was shown that all Cl-containing Ni/CeO2 catalysts provided efficient CO cleanup. They provided the removal of CO from reformate gas to the level below 10ppm with a selectivity up to 90%. The catalyst characterization by BET, XRD, XPS, HAADF-STEM, EDX-mapping, FTIR in situ and CO chemisorption techniques revealed that the decrease in chlorine content in the order Ni(Cl)/CeO2≥Ni(Cl*)/CeO2>Ni/CeO2(Cl*) was accompanied by the increase of Ni dispersion that most likely provided high performance of Ni/CeO2(Cl*) in CO-SMET. The turnover frequencies of Ni surface atoms as well as activation energies in CO methanation were practically similar for all studied catalysts, indicating that Cl did not influence catalyst’s activity and CO methanation proceeded by similar ways over Ni surface in both Cl-free and Cl-containing samples. The advanced performance of Cl-containing catalysts was associated with the inhibition of undesirable side reaction of CO2 methanation. The chlorine doping effect was attributed to the blockage of surface Ce3+-coupled oxygen vacancy sites by CeOCl species that inhibited ceria-assisted CO2 activation and hydrogenation. The CeO2 treatment with NH4Cl before Ni deposition allows to prepare highly active and selective CO-SMET catalyst with high nickel dispersion and Cl-modified ceria surface.

Synergy between hydrogen and ceria in Pt-catalyzed CO oxidation: An investigation on Pt–CeO2 catalysts synthesized by solution combustion

Pt–CeO2 catalysts were prepared by solution combustion synthesis (SCS), a simple and fast one-pot method, using glycine or oxalyl dihydrazide (ODH) as the fuel and Pt chloride or nitrate as the metal precursor. The samples were characterized by ICP-OES, N2 volumetry, XRD, XPS, aberration-corrected (S)TEM, CO-DRIFTS and microRaman spectroscopy. The SCS catalysts were evaluated in CO oxidation and preferential oxidation (PROX) of CO in H2 excess, and compared to Pt/CeO2 and Pt/Al2O3 catalysts prepared by incipient wetness impregnation. The glycine fuel yields better results than ODH in terms of Pt dispersion and catalytic performance. The fresh Cl-containing catalysts consist of PtO nanoparticles poorly active in CO oxidation, while the catalysts synthesized from Pt nitrate contain highly active Ptδ+ (0<δ<2) species in interaction with ceria. Consistently, the most efficient catalyst for H2-free CO oxidation is the one synthesized from glycine and Pt nitrate. For PROX, the selectivity to CO2 is 100% at low temperature and decreases in a similar way for all the Pt–CeO2 catalysts above ca. 100°C. With respect to H2-free conditions, the CO PROX activity of the Pt–CeO2 catalysts is considerably enhanced (activity multiplied by up to 60at 110°C), while the activity gain is comparatively minor for Pt/Al2O3. We conclude that hydrogen, by increasing the mobility of oxygen species at the surface of ceria, promotes the support-assisted pathway of CO oxidation.

The interaction of oxygen with high loaded Ru/γ-Al 2O 3 catalyst

The interaction of oxygen with the Ru/γ-Al 2O 3 catalyst comprising metal particle with sizes of 1–16 nm, was examined over a temperature range 20–400 °C. The catalyst loaded with 10.8 wt.% Ru was prepared by incipient wetness from RuCl 3 precursor. The structure of the Cl-containing catalyst and the catalyst after elimination Cl − ions was characterized using H 2 and O 2 chemisorption, O 2 uptake, BET, XRD and TEM. The Cl − ions in the catalyst decreased the H 2 and O 2 chemisorption capacity of Ru and caused large discrepancies between the mean particle size calculated from gas chemisorption and from TEM. Exposure to O 2 at 100–200 °C caused oxidation of small Ru particles, while larger particles were covered with very thin Ru x O y skin (undetected by XRD and TEM). The O/Ru ratio increased up to 200 °C implying high affinity of the small Ru particles to oxygen. Oxidation at 250 °C led to the formation of poorly crystalline RuO 2 particles with a mean size of 4 nm, and coverage of large Ru particles with 1.6 nm thick oxide layer. At 300 and 400 °C crystallization of the RuO 2 phase, as well as significant agglomeration of oxide particles was observed. However, even at 400 °C, metallic Ru was detected by XRD, TEM and SAED suggesting that large metal particles were not fully oxidized under the used conditions. Also, the O 2 uptake at 400 °C was lower than expected for oxidation of Ru metal to RuO 2. For the catalyst after elimination Cl ions the O 2 uptake (O/Ru ratio = 1.50) was higher, than for sample with large amount of Cl ions (O/Ru ratio = 1.34), indicating that the presence of Cl inhibits ruthenium oxidation in the Ru/Al 2O 3 catalyst.

Effect of the presence of chlorine in bimetallic PtZn/CeO 2 catalysts for the vapor-phase hydrogenation of crotonaldehyde

The effect of the reduction temperature has been studied on ceria-supported bimetallic platinum–zinc catalysts prepared from H 2PtCl 6 and Pt(NH 3) 4(NO 3) 2 as the platinum precursors and Zn(NO 3) 2 as the zinc precursor. The catalysts have been characterized by X-ray diffraction (XRD), temperature-programmed reduction (TPR), and X-ray photoelectron spectroscopy (XPS), and their catalytic behavior has been evaluated in the vapor-phase hydrogenation of toluene and of crotonaldehyde (2-butenal) after reduction at low (473 K) and high (773 K) temperatures. The increase in the reduction temperature produces a strong decrease in the catalytic activity for toluene hydrogenation in both systems, but an important increase of activity for crotonaldehyde hydrogenation, which is more evident for the chlorine-free catalyst. The selectivity towards the hydrogenation of the carbonyl bond to yield the unsaturated alcohol (crotyl alcohol, 2-buten-1-ol) also increases after reduction at high temperature, being somewhat higher for the Cl-containing catalyst. The results are discussed in terms of differences in surface composition of the catalysts.

Kinetics, FTIR, and Controlled Atmosphere EXAFS Study of the Effect of Chlorine on Pt-Supported Catalysts during Oxidation Reactions

The poisoning effect of Cl on the activity of Pt-supported catalysts for CO, methane, and ethane oxidation has been investigated by kinetic studies and in situ IR and controlled atmosphere EXAFS spectroscopies. Catalysts containing 1.5% Pt/Al2O3 were prepared by incipient wetness from H2PtCl6 and Pt(NH3)4(NO3)2 precursors. The reduced catalysts have similar dispersion (0.8) as estimated by H2 chemisorption. The Cl-free catalyst was 10 times more active than the Cl-containing catalyst during CO and ethane oxidation. Addition of HCl to the Cl-free catalyst rendered its activity identical to the catalyst prepared from Cl-containing precursors. The presence of Cl also affects the activity of 2% Pt/SiO2 catalysts, but to a lower extent. On the Cl-free oxidation catalyst, Pt–Pt and Pt–O bonds were detected using EXAFS, suggesting that the reduced metal particles are not fully oxidized under the reaction conditions. Additionally, chemisorption of CO by the oxidized catalyst indicates that a portion of the reduced Pt atoms is exposed to the reactants. On the Cl-containing catalyst, there are also Pt–Cl as well as Pt–Pt and Pt–O bonds. The later catalyst, however, does not chemisorb CO, indicating that there are no reduced surface Pt atoms. The effect of Cl poisoning on the oxidation activity of Pt supported on silica is similar to that on alumina. IR results show that chlorine significantly reduces the amount of CO adsorbed on metallic Pt sites. At low temperature there is little CO adsorbed on the Cl-containing Pt/silica catalyst, while at higher temperature the amount of adsorbed CO increases, likely due to reduction of the oxidized surface. The catalyst activities correlate well with the amount of reduced surface sites, and a model is proposed to explain the mechanism of chloride poisoning, which is shown to occur mainly by site blocking.

Oxidative conversion of light alkanes to olefins over alkali promoted oxide catalysts

Alkali promoted mixed oxides were studied as catalysts for the oxidative dehydrogenation (ODH) and cracking of butane and propane. Olefin yields as high as 50% were obtained with Li/MgO-based catalysts. Magnesia-based catalysts showed higher activity for olefin production than catalysts based on zirconia and niobia. Addition of Li to magnesia increases reaction rate normalized to the specific surface area about seven times and selectivity to olefins from 40 to 70%. Li is, therefore, an essential ingredient of the catalyst in order to create the active site. Cl-containing catalysts exhibit slightly higher olefin selectivity, but chloride-free catalysts show superior stability with time on stream. Alkanes show higher conversion rates than alkenes and this surprising result explains the high selectivity to olefins. It is suggested that Li +O − defect sites are the active site for activation of the alkane via hydrogen abstraction. Production of olefins via this oxidative dehydrogenation/cracking route may be an attractive alternative for steam-cracking.

Effects of cesium and chlorine on oxygen adsorption on promoted Ag/α-Al 2O 3 catalysts

Integral heats of oxygen adsorption were measured at 443 K on unpromoted and promoted Ag/α-Al 2O 3 catalysts. Two unpromoted catalysts with average Ag particle sizes of about either 200 or 300 nm were utilized while the promoted catalysts had particle sizes in the range of 250–350 nm. The promoted catalysts were prepared with either CsNO 3 to study the effect of cesium, or CsCl to study the combined effect of cesium and chlorine. For both groups, the Cs content was around either 400 or 1200 ppm. Following a low-temperature pretreatment involving calcination at 523 K and reduction in H 2 at 473 K, the catalysts were characterized by volumetric measurements of oxygen chemisorption and subsequent hydrogen titration at 443 K. Unlike the unpromoted catalysts, the promoted samples exhibited varying amounts of reversible oxygen adsorption with a maximum of 30% reversible adsorption on a 1174 ppm Cs-only promoted catalyst; however, the presence of only Cs at either loading altered the total oxygen uptake by only ±10% compared to the unpromoted catalyst. Regardless, the H 2/(O 2) irrev titration ratio was close to the stoichiometric ratio of 2.0 except for the catalyst with the highest Cl content. With this latter sample, there was additional hydrogen uptake beyond the titration of adsorbed oxygen. Several cycles of pretreatment/adsorption measurements were needed to ‘stabilize’ these catalysts and obtain reproducible results. Energy changes were measured with samples derived from these stabilized catalysts, and an exotherm was obtained for both the initial total uptake and the reversible uptake so that the energy change associated only with the irreversible oxygen uptake could be determined. This difference exotherm frequently exhibited a long tail which could not be explained by adsorption processes alone; consequently, time-dependent uptakes and energy changes were used to calculate heats of adsorption. These apparent heat of adsorption values for O 2 were higher for the Cl-containing catalysts than for the unpromoted and Cs-only promoted catalysts. The presence of Cl decreased both the rate of oxygen adsorption and the total oxygen uptake; however, with the catalyst containing the most Cl these two parameters increased as the catalyst was given a series of low-temperature pretreatment/adsorption cycles. In contrast, a single high-temperature pretreatment at 773 K with this sample was adequate to provide uptake rates and total oxygen uptakes comparable to those with the stabilized catalyst. This stabilization process appears to be associated with the removal of Cl from the silver surface.

Effects of Oxidation–Reduction and Oxychlorination–Reduction Cycles on Pt–Ge/Al2O3 Catalysts

Two Pt(0.3%)–Ge/Al2O3 catalysts containing 0.15 and 0.45% Ge have been characterized by CO chemisorption and FTIR spectra of adsorbed CO after series of oxidation/reduction and oxychlorination/reduction cycles and have also been used for the catalytic hydroreforming of heptane. Catalysts after oxychlorination contained chloro- and oxychloro-Pt complexes which were responsible for the efficient spreading of Pt over the support surface and hence for the maintenance of a good Pt dispersion after reduction. XRD study of a more highly loaded catalyst gave no evidence for Pt–Ge alloy formation or for Ge0 species. Reduced Cl-containing catalysts contained small clustered arrays of Pt atoms dispersed possibly as mats rather than particles over the GeOx-modified alumina such that all the exposed Pt atoms were influenced by an electron withdrawing effect of both Ge2+ ions and Cl ions. Germanium did not block low-coordination Pt sites but may have partially decorated arrays of high coordination sites. Germanium partially stabilized Pt/Al2O3 against loss of activity through coking. The dominant mutual effect of Ge and Cl in catalysts was a significant loss in hydrogenolysis selectivity, but gains in the selectivities of aromatization, isomerization, and cyclization reactions. The addition of Ge to Pt/Al2O3 did not compromise the availability of Pt adsorption sites and only reduced by a small amount the turnover frequencies for heptane reforming reactions. Good activities were therefore maintained in the absence of Ge0 or alloy formation during catalyst reduction.

Hydrogenolysis of n-Hexane on Al2O3-Supported Ir Catalysts of Various Treatments

A high-dispersion 1 wt% Ir/Al2O3 with 1 wt% Cl as well as its counterparts with <0.2% Cl with or without sintering were tested in n-hexane reactions between 453 and 603 K. The Cl-containing catalyst was considerably less active in this temperature range, unfavorable for bifunctional reactions. Hydrogenolysis was the main reaction with selectivities up to 90%, along with minor amounts of nondegradative products. Single hydrogenolysis with some preference for splitting the molecule in the middle occurred at low temperatures. That was replaced gradually by multiple rupture at higher temperatures. Increasing the hydrogen pressure at 513 K led to higher hydrogenolysis selectivities up to 98% at the same time, it suppressed somewhat multiple fragmentation. With less hydrogen present (at lower H2 pressure or higher temperature), a “soft” transition from single to multiple rupture occurred. The multiplicity of rupture was most pronounced on the larger crystallites of the sintered sample, whereas a terminal rupture seemed to appear on the Cl-free high-dispersion sample. Whenever single hydrogenolysis prevailed, the fragment distribution showed a slight preference to propane formation.

Free-radical reactions involved in C 1-C 3 hydrocarbons interaction with oxide catalysts

Experimental proofs of a free radical mechanism in methane oxidative coupling, with homolytic rupture of the CH bond are given. High concentrations of free radical sites are produced by mechanical milling of SiO 2. A study of C 1C 3alkanes interaction with these sites allows to simulate the, processes of alkanes oxidation and oxidative dehydrogenation. The reactivity of ethane and propane is higher than that of methane in accordance with the Polanyi-Semenov rule. Oxidative dehydrogenation of ethane is studied on Cd-containing zeolites. CH 4, C 2H 6 and C 3H 8 oxidative dehydrogenation by O 2 or CO 2 is studied on a MNO/SiO 2 catalyst. The initiation of radical reactions of hydrocarbons on Cl-containing catalysts proceeds via chlorine atoms generation.

New evidence for formate mechanisms of CO hydrogenation over PdMg 2+/SiO 2 catalysts

TPD-MS, chemical trapping, and FT-1R techniques have been used to study the mechanisms of CO hydrogenation on two PdMg 2−/SiO 2 catalysts. Cl-containing and Cl-free ones. Two formate TPD-MS peaks, that are assigned to Pd −HC(O)OMg 2+ and Pd 0HC(O)OSi are observed on the Cl-containing catalyst: while only the latter one. Pd f]HC(O)OSi, is found on the Cl-free catalyst. The Pd −HC(O)OMg 2+ species and methanol formation are found to vary in concentration concurrently with reaction time. In situ chemical trapping results show that the formate species is formed much faster than it is converted. Also, the formate species is converted faster under high reaction pressure than under atmospheric pressure. All these results lead to such a reaction mechanism that the formate species is the active intermediate and its conversion is the rate-determining step for methanol formation.

Effects of chloride precursors on the palladium valency and surface structures of Pd [sbnd]Mg 2+/SiO 2catalysts for carbon monoxide hydrogenation

The effects of chloride precursors on the Pd valence states and surface structures of Pd Mg 2+/SiO 2 catalysts for carbon monoxide hydrogenation have been studied by means of electron-spin resonance (ESR) and X-ray photoelectron spectroscopy (XPS) techniques. The Cl-containing catalyst exhibits a much more positive Pd valency and a much more stable Pd surface structure than the Cl-free one. This is attributed to the stronger ability of chlorine anions to stabilize Pd structures and to transfer electrons from Pd to Mg 2+, rather than to a better Pd dispersion of the Cl-containing catalyst. Over the Cl-containing catalyst, methanol formation and a Pd + ESR signal are found to increase concurrently with reaction time. Accordingly, Pd + ions are claimed to be essential for methanol formation. However, only those Pd + ions which are located in a suitable structure are supposed to be the active center for methanol synthesis, because oxygen or hydrogen-water pretreatments are found to cause (i) a marked decrease of the turnover frequencies (TOF) for methanol formation, (ii) a probable transformation of Pd(Cl,X)Mg 2+ (X=Cl or O) bridge structures to oxychloropalladium and MgO, and (iii) the formation of more Pd + ions. Furthermore, the Pd + ions formed during the induction period are found not to be mainly due to the catalyst being modified by water or methanol, but probably due both to the high-pressure Pd reconstruction and the oxidation of an HCOO intermediate. In addition, the metal-promoter-support interactions are discussed.

The effect of chloride complexes on Pd surface structure, dispersion and CO hydrogenation over Pd-Mg(II)/SiO 2 catalysts

The effect of chloride complexes has been investigated over Cl-containing and Cl-free Pd-Mg(II)/Silica gel catalysts for CO hydrogenation. By using TPD-MS, XPS and ESR techniques, a Pd 0 x-Pd(I) (Cl, O)Mg(II) (SiO y) bridge structure has been determined on the Cl-containing catalyst. Complexing causes the Cl-containing catalyst to have about three times better Pd dispersion and much more positive Pd valency than that of the Cl-free catalyst. Moreover, the bridge structure can easily react with adsorbed CO and H , forming a Pd 0 x-Pd(I) (Cl, HCOO) Mg(II) (SiO y) species, which gives rise to a doubly-broad Pd(I) ESR signal. By using a chemical trapping method, it is also found that the bridge structure makes the Cl-containing catalyst have about twice as abundant H δ+ species as that of the Cl-free one. As a result, the turnover frequencies (TOF) and selectivity for methanol have been found to be respectively about 100% and 19% higher for the Cl-containing than those for the Cl-free one. In addition, oxygen pretreatments are found to destroy the bridge structure and hence to largely decrease the TOF and selectivity for methanol.