H2 Temperature-programmed Desorption Research Articles

Catalytic partial oxidation of methanol over Au–Pd bimetallic catalysts: a comparative study of SBA-16, SBA-16-CeO2, and CeO2 as supports

Au–Pd catalysts supported on SBA-16, SBA-16-CeO2, and CeO2 had been studied for partial oxidation of methanol to produce H2. The physicochemical characteristics of the catalysts prepared by deposition–precipitation using urea hydrolysis were examined by inductively coupled plasma atomic emission spectroscopy (ICP-AES), Brunauer-Emmett-Teller (BET), X-ray powder diffraction (XRD), Temperature-programmed reduction (TPR), and H2 temperature-programmed desorption (H2-TPD) analyses. The results show that AuxPdy alloys are observed in Au–Pd/SBA-16 and Au–Pd/SBA-16-CeO2 catalysts. The catalytic results demonstrate that both Au–Pd/SBA-16 and Au–Pd/SBA-16-CeO2 catalysts exhibit higher activity and lower CO selectivity than the Au–Pd/CeO2 catalyst. This could be ascribed to the formation of AuxPdy alloys. The comparison of the Au–Pd/SBA-16 and Au–Pd/SBA-16-CeO2 catalysts reveals that the Au–Pd/SBA-16-CeO2 shows the lower CO selectivity, probably due to the presence of CeO2.

Hydrodeoxygenation of Anisole over Silica-Supported Ni2P, MoP, and NiMoP Catalysts

Ni2P/SiO2, MoP/SiO2, and NiMoP/SiO2 with different Ni/Mo molar ratios were prepared by temperature-programmed reduction (TPR). Their structural properties were characterized by N2 sorption, X-ray diffraction (XRD), CO chemisorption, X-ray photoelectron spectroscopy (XPS), H2 temperature-programmed desorption (H2-TPD), and NH3 temperature-programmed desorption (NH3-TPD). Their performances for the hydrodeoxygenation (HDO) of anisole were tested in a fixed-bed reactor. It was found that there were mainly three reactions that occurred during the HDO, i.e., the demethylation of anisole, the hydrogenolysis of phenol, and the hydrogenation of benzene. The HDO activities decreased in the sequence of Ni2P/SiO2 > NiMoP/SiO2 > MoP/SiO2. The NiMoP/SiO2 catalysts with larger Ni/Mo ratios had higher activities. In the phosphides, the Niδ+ and Moδ+ sites bearing small positive charges acted not only as Lewis acid sites for the demethylation but also as metal sites for the hydrogenolysis and hydrogenation. The Niδ+ site...

Effect of Silylation of SBA-15 on Its Supported Cobalt Catalysts for Fischer-Tropsch Synthesis

Cobalt catalysts supported on unsilylated and silylated SBA-15 were prepared by incipient wetness impregnation and characterized by elemental analysis, N2 adsorption-desorption, X-ray diffraction, H2 temperature-programmed reduction, H2 temperature-programmed desorption, and oxygen titration. Their catalytic properties for Fischer-Tropsch synthesis were evaluated in a fixed-bed reactor. The results show that the interaction between cobalt and the support was decreased, and the reducibility of the cobalt oxide species was more facile on the silylated Co/SBA-15 catalyst. With increasing (CH3)3SiCl loading, the surface coverage of the trimethylsilyl group increased, and the cobalt cluster size was larger. The higher activity of the silylated 10%Co/SBA-15 catalysts was ascribed to its improved reducibility. The increase in selectivity for C5+ hydrocarbons was attributed to the increase in cobalt cluster size.

Hydrodechlorination of Chlorobenzene Over Ni2P/SiO2 Catalysts: Influence of Ni2P Loading

The influence of Ni2P loading on the properties of Ni2P/SiO2 catalysts was investigated by means of N2 adsorption–desorption, X-ray diffraction, H2 and NH3 temperature-programmed desorption, CO chemisorption, inductively coupled plasma atomic emission spectroscopy and activity evaluation for the hydrodechlorization of chlorobenzene. It was found that increasing Ni2P loading results in the decrease of Ni2P dispersion. Higher P/Ni ratio occurs in the catalysts with lower Ni2P loadings, indicating that smaller Ni2P crystallites may more strongly interact with surplus P. We suggest that the acid amount of the Ni2P/SiO2 catalyst is related to the exposed Ni sites and P-OH groups, while Bronsted acidity of P-OH groups facilitates hydrogen spillover. Increasing Ni2P loading is favorable to the catalyst activity, while this effect is not obvious when the Ni2P loading is greater than 9.6 wt%. Apart from the metallic property of Ni2P phase and spilt-over hydrogen species, we propose that the acidity and the role of P in activating reactants should be considered for the catalyst performance.

Preparation and catalytic performance of monolayer-dispersed Pd/Ni bimetallic catalysts for hydrogenation

Pd/Ni bimetallic catalysts were prepared by replacement reactions, characterized by X-ray diffraction, CO chemisorption and H2 temperature-programmed desorption, and evaluated for hydrogenation of cyclohexene, styrene and acetone. The results show that Pd atoms are monolayer-dispersed on the Ni surface in these Pd/Ni catalysts. Consequently, Pd/Ni catalysts are much more active than Pd/Ni and Pd/c-Al2O3 with the same Pd loading prepared by the conventional impregnation method.

Effect of glow discharge plasma on rhodium-based catalyst for oxygenates synthesis

Rhodium-based catalysts were prepared by impregnation, treated with glow discharge plasma, characterized by X-ray diffraction, X-ray photoelectron spectroscopy, H2 temperature-programmed reduction, H2 temperature-programmed desorption and CO temperature-programmed desorption, and investigated for oxygenate synthesis from CO hydrogenation. Based on the characterization results, plasma treatment endowed the samples with smaller particle size, higher dispersion of active components, and an enrichment of active components on the surface as well. As a result, the reducibility and adsorption properties were modified. In catalytic tests, the catalytic activity for CO hydrogenation over the samples treated by plasma was improved remarkably: the conversion of CO and the yield of oxygenates increased at most by a factor of 78.62% and 51.96%, respectively, while the selectivity of ethanol and methanol in the oxygenates was enhanced as well.

Effect of Carbon Nanotubes on Activity of Rh-Ce-Mn/SiO2 Catalyst for CO Hydrogenation to Oxygenates

Abstract The carbon nanotubes (CNTs)-promoted Rh-Ce-Mn/SiO2 catalysts were prepared by the impregnation method and characterized by X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction, N2 adsorption, X-ray diffraction (XRD), and temperature-programmed desorption of H2 or CO. The catalytic performance of Rh-Ce-Mn-CNTs/SiO2 for CO hydrogenation to oxygenates was investigated. The results of XPS and XRD showed that the introduction of CNTs improved the dispersion of Rh and caused an enrichment of active components on the catalyst surface. Meanwhile, the addition of CNTs modified the interaction between rhodium and other components, and the amount of strongly adsorbed H2 or CO on the surface of the catalyst was increased remarkably. The results of CO hydrogenation showed that the existence of CNTs enhanced the activity of the Rh-Ce-Mn/SiO2 catalyst. Under the specific reaction conditions, the CO conversion was 11.86% and the yield of oxygenates was 336.2 g/(kgh) when the CNTs content was 10%.

Effect of an Iron Oxide Precursor on the H2 Desorption Performance for an Ammonia Synthesis Catalyst

The effects of various iron oxide precursors of an ammonia synthesis catalyst on the H2 desorption performance were studied by means of H2 temperature-programmed desorption (TPD). Among the three p...

Temperature dependence of atomic hydrogen-induced surface processes on Ge(100): Thermal desorption, abstraction, and collision-induced desorption

The temperature effect on the atomic hydrogen-induced surface processes on Ge(100) has been studied from a desorption point of view. The experiments are carried out for temperature-programmed-desorption (TPD) of H2 and D2 and collision-induced desorption (CID) of D2 and abstraction of HD in the reaction system H(g)+D/Ge(100). The D2 and H2 TPD spectra exhibit two clear peaks, assigned as β1 and β2 TPD arising from a monohydride and a dihydride phase, respectively. There are isotope effects on the TPD spectra; D2 TPD peaks shift to higher surface temperature (Ts) compared to the peaks of H2, and the ratio of the β2 to the β1 TPD peak intensity is smaller for H2 than for D2. It is found that the kinetics of the abstraction and CID reactions are similar to those on Si(100), indicating that the mechanism for the abstraction and CID on Ge(100) is same to that on Si(100). The observed D2 rate curves show up a strong Ts dependence. The CID of D2 versus Ts curve exhibits a peak at Ts≃480 K corresponding to the leading edge of the β2TPD spectra. For Ts⩾530 K, CID of D2 is fully replaced by the spontaneous desorption ascribed to the β1 TPD. This fact suggests that the mechanism of CID is same to that of the β2 TPD. For Ts∼480 K, the D2 rate curves can be fitted with a fourth-order kinetics in a momentary D adatom coverage. The transiently created dihydride species that are considered to be mobile across the surface via a dihydride–monohydride isomerization reaction to exchange their sites, are invoked to explain the observed fourth-order kinetics. Probing H atoms reveals that the thermal desorption from the isolated dideuterides is not allowed, suggesting that for the β1 TPD the concerted desorption from hydrogen-prepared Ge dimers is preferred to the desorption from isolated dihydrides.

The temperature-programmed desorption of hydrogen from copper surfaces

The desorption kinetics of H2 from a Cu/ZnO/Al2O3 catalyst for methanol synthesis were studied under atmospheric pressure in a microreactor set-up by performing temperature-programmed desorption (TPD) experiments after various pretreatments of the catalyst. Complete saturation with adsorbed atomic hydrogen was obtained by dosing highly purified H2 for 1 h at 240 K and at a pressure of 15 bar. The TPD spectra showed symmetric H2 peaks centered at around 300 K caused by associative desorption of H2 from Cu metal surface sites. H2 TPD experiments performed with different initial coverages resulted in peak maxima shifting to higher temperatures with lower initial coverages indicating that the desorption of H2 from Cu is of second order. The microkinetic analysis of the TPD traces obtained with different heating rates yielded an activation energy of desorption of 78 kJ mol−1 and a corresponding frequency factor of desorption of 3×1011 s−1> in good agreement with the kinetic parameters obtained with Cu(111) under UHV conditions.

Vibrational Study of the Interaction of Acetylene and Benzene on Clean and Carbide-Modified W(211) Surfaces

As a part of a continuing investigation of acetylene reactions on tungsten surfaces covered by ultrathin metal films, the chemistry of acetylene on clean and carbide-modified W(211) surfaces has been investigated using high-resolution electron energy-loss spectroscopy (HREELS) and temperature-programmed desorption (TPD) measurements. Acetylene decomposes between 100 and 450 K on the clean W(211) surface, ultimately forming carbidic carbon and gaseous hydrogen. During this decomposition process, a mixture of different hydrocarbon fragments are formed, most likely vinylidene (CCH2) or vinyl (HCCH2) and acetylide (CCH). On the carbide-modified W(211) surface, acetylene also decomposes to produce carbon and gaseous hydrogen. However, compared to clean W(211), the CH bond activation is suppressed on the carbide-modified W surface, as evidenced by the broadening and significant shifting of the H2 TPD features toward higher temperatures as well as by the HREEL spectra monitoring the thermal decomposition of acetylene on the carbide-modified W(211) surface.

Characterization of Carbonaceous Species Formed during Reforming of CH4with CO2over Ni/CaO–Al2O3Catalysts Studied by Various Transient Techniques

Carbon dioxide reforming of methane to synthesis gas at 750°C over 5 wt% Ni/CaO–Al2O3catalysts has been investigated with respect to effects of support composition (CaO to Al2O3ratio) on catalyst stability, amount and reactivity of carbon species formed during reaction, and relative proportion of reaction routes that lead to carbon formation (CH4vs CO2molecule). Temperature-programmed oxidation (TPO) and hydrogenation (TPH) experiments, following reforming reaction with 20% CH4/20% CO2/He and 20%13CH4/20% CO2/He mixtures, have been conducted for the aforementioned carbon characterization studies. Two kinds of carbon species (free of chemically bound hydrogen) were mainly found to accumulate on the catalyst surface, where the amount and reactivity of them are influenced by the CaO/Al2O3ratio used to deposit the nickel metal. Transient isothermal hydrogenation experiments of the carbon species formed during reforming reaction resulted in CH4responses, where the time of appearance of the CH4peak maximum in hydrogen stream as a function of hydrogenation temperature was used to obtain the intrinsic activation energy of the hydrogenation process. It was found that this activation energy is influenced by the support composition. TPO experiments conducted following reforming reaction with13CH4/CO2/He mixture have demonstrated that the relative amount of adsorbed carbon species formed via the CH4and CO2molecular routes was strongly dependent on support composition. H2temperature-programmed desorption, temperature-programmed reduction, and X-ray photoelectron spectroscopic measurements conducted over the present catalysts suggest that the nickel particle morphology and its size distribution must be influenced by the support composition, which in turn controls the origin, the kinetics, and the reactivity of carbon deposition under reforming reaction conditions.

Adsorption kinetics for ethylsilane, diethylsilane, and diethylgermane on Si(111) 7×7

The adsorption kinetics for ethylsilane (ES), diethylsilane (DES), and diethylgermane (DEG) on Si(111) 7×7 were studied using laser-induced thermal desorption (LITD) and temperature programmed desorption (TPD) techniques. The initial reactive sticking coefficients were determined as a function of surface temperature using LITD measurements. In these experiments, the ethyl coverage vs adsorption time was monitored using CH2=CH2 (ethylene) LITD signals that were produced by the β-hydride elimination of the surface ethyl groups, e.g. Si–CH2CH3(ad)→Si–H(ad)+CH2=CH2(g). The initial reactive sticking coefficients were S0≊2×10−3, 4×10−3, and 5×10−2 for DES, ES, and DEG, respectively, at 200 K. As expected from a precursor-mediated adsorption model, the initial reactive sticking coefficients were observed to decrease with increasing surface temperature. Experiments with preadsorbed hydrogen also demonstrated that the initial reactive sticking coefficients of DES and DEG were reduced as a function of hydrogen coverage. This behavior indicated that alkylsilane and alkylgermane adsorption on Si(111) 7×7 requires free dangling bond sites. LITD experiments revealed that the ethyl surface coverage saturated after large exposures. The saturation coverage corresponded to a deposited Si or Ge coverage of Θ=0.13 monolayer for DES and DEG and was independent of surface temperature between 200–400 K. DEG adsorption cycles were used to deposit increasing amounts of Ge on Si(111) 7×7. The deposited coverage was examined using H2 TPD studies which indicated that the germanium may be forming islands. LITD experiments were also used to monitor ethyl surface diffusion on Si(111) 7×7 after DES adsorption. No evidence of significant ethyl surface mobility (D≤1.0×10−10 cm2/s) was found for surface temperatures as high as 600 K.

Temperature-programmed desorption of H2 as a tool to determine metal surface areas of Cu catalysts

Temperature-programmed desorption of H2 (H2 TPD) is shown to be a useful new tool for the determination of Cu metal surface areas. The technique has been employed in on-line characterization of binary Cu/Al2O3 and ternary Cu/ZnO/Al2O3 catalysts in a combined TPD-microreactor setup. The catalysts were studied both after reduction and after water gas shift activity tests. A main H2 TPD peak is observed around 300 K which can be assigned to desorption from Cu metal surface sites. The H2 TPD method is compared with the N2O frontal chromatography method which has been extensively used in previous studies for the determination of Cu surface areas. It is found that the dissociative adsorption of N2O may induce significant changes in the catalyst structure leading to errors in the surface area determination. With the H2 TPD procedure such irreversible changes can be avoided. A further advantage of the H2 TPD method is the possibility of providing insight into the nature of the exposed Cu metal surfaces.

Surface reactivity of reduced LaFeO3 as studied by TPD and IR spectroscopies of CO, CO2 and H2

CO, CO2 and H2 reactive adsorption on LaFeO3 at 298 K has been studied as a function of the reduction temperature of the perovskite oxide by means of temperature programmed desorption (TPD) and infrared (IR) spectroscopies. TPD spectra of CO after CO adsorption contained peaks at 365 to 440 K assigned to linearly adsorbed CO and at 495 to 540 K assigned to bridged CO. TPD spectra of CO2 after CO adsorption presented broad peaks centred at 570 and 715 K assigned to monodentate and bidentate carbonates, respectively. TPD spectra of CO2 obtained after CO2 adsorption contained peaks at 375 to 425 K and at 570 to 675 K. These were associated to infrared bands of monodentate and bidentate carbonates, respectively. In the CO-H2 and H2-CO successive adsorption on the reduced surface of LaFeO3 the TPD peak of H2 at 345 to 360 K is strongly inhibited and a new desorption peak appeared at 585 to 590 K. This is assumed to be due to CO adsorption on metallic Fe0 sites (CO-H2 coadsorption) or to a displacement of adsorbed hydrogen from Fe0 to a new adsorption site (H2-CO coadsorption). CO was found to interact more strongly than hydrogen with the adsorbent surface.

The influence of adsorbate–adsorbate interactions on surface structure: The coadsorption of CO and H2 on Rh(100)

The coadsorption of CO and H2 on Rh(100) at 100 K has been studied using temperature programmed desorption (TPD), low energy electron diffraction (LEED), electron energy loss spectroscopy (EELS), and temperature programmed EELS (TP-EELS). The preferred binding sites, long ranged order, and degree of segregation are dependent on the order of adsorption. When H2 is exposed to a CO preexposed surface, segregation of the surface species (atomic H and CO) is observed. The postdosed H2 causes isolated CO molecules to change from the top site to the bridge site, and compresses the c(2×2) CO islands that develop during the CO preexposure. When CO is exposed to a H2 saturated surface (one hydrogen per surface Rh atom) an intimately mixed c(2×2) CO and H structure is formed with all the CO molecules occupying the top site. Strong repulsive CO–H interactions in this mixed adlayer result in two new low temperature H2 TPD states. During the desorption of the lowest temperature H2 TPD peak, the c(2×2) LEED pattern streaks and the CO molecules shift from the top site to the bridge site. It is proposed that the preferred binding site for hydrogen in the c(2×2) bridge bonding CO structure is different from the fourfold hollow site preferred in the c(2×2) top bound CO structure. After the second H2 TPD peak, the remaining adsorbed H and CO segregate, and the CO regions are compressed. The compression is relaxed as the hydrogen desorbs. The development of the surface structures and their influence on the H2 TPD can be qualitatively understood in terms of precursor adsorption of both CO on H covered surfaces and H2 on CO covered surfaces, strong CO–H repulsions, and local poisoning of H2 dissociation by CO.

Adsorption and decomposition of ethylene and acetylene on flat Ru(001) and stepped Ru(1,1,10) surfaces

The adsorption and decomposition of ethylene and acetylene on flat Ru(001) and stepped Ru(1,1,10) surfaces have been investigated by u.p.s. and t.d.s. On the flat surface ethylene and acetylene were adsorbed molecularly at 120 K and converted into ethylidyne species, [graphic omitted]C—CH3, at 280 K. Heating to 450 K in vacuo caused the ethylidyne species to decompose, with the evolution of hydrogen and probable formation CCH(ads) species; the latter were changed to CH(ads) at 550 K and finally to C(ads) above 650 K. However, the presence of CO retarded the decomposition of the ethylidyne species. In contrast, cleavage of the C—H bond occurred at 120 K on the stepped surface, presumably with the formation of CCH(ads) or CCH2(ads) species. The formation of ethylidyne species around 280 K was not observed, which also indicates the higher reactivity of the stepped structure on the Ru surface towards the decomposition of alkenes. This is further supported by a shift of the desorption peak to lower temperature in H2 temperature-programmed desorption spectra.