O. F. Gorriz’s research while affiliated with National Scientific and Technical Research Council and other places

The activity and stability of copper supported on cerium oxide have been investigated in the selective production of hydrogen via methanol steam reforming. The catalytic runs were carried out under two different operating regimes: continuous and discontinuous. The main products were H2 and CO2 and a minor amount of CO (selectivity to CO was around 6% at 300 °C). In all the cases, the catalyst was initially very active and the activity was lost with the time on stream. The catalyst under a continuous regime was easily regenerated by air at 400 °C. Under a discontinuous mode, the catalyst was self-activated after stop operation and initial conversion was almost recovered. The deactivation could be mainly attributed to the adsorption of carbonate species which were easily desorbed under an inert flow or by burning with air. The pre-reduction of catalyst should be avoided to reach higher stability.

Methanol steam reforming reaction was studied over Cu(5 wt.%)/CeO2 with and without the presence of Zn. The Zn addition decreased the Cu+2 reducibility and increased the oxygen mobility of ceria. The main products were CO2 and H2 with small amount of CO. Selectivity to CO decreased with the Zn addition and it was lower at lower reaction temperatures and lower space velocities. At 230 ºC and W/FMeOH = 648 g min mol-1 selectivities to H2 and to CO2 were 100% on Zn/Cu/Ce. The catalytic results indicated that CO was mainly a secondary product formed from reverse water gas shift reaction.

Dehydrogenation catalysts based on chromia supported on γ-alumina, containing about 3wt.% of chromium and increasing amounts of tin (up to 3wt.%), were prepared and characterized by means of several techniques, such as N2 adsorption–desorption at 77K, X-ray powder diffraction (XRPD), laser Raman spectroscopy, temperature programmed reduction (TPR) and chemical analyses. The catalytic behavior of the samples was investigated in the propane dehydrogenation reaction at 883K. The coking formation (in 150min of time of stream) was analyzed by temperature programmed oxidation (TPO). The addition of increasing amounts of the tin determined a general decrease in the coking formation. TPR profiles of the Sn-containing samples showed the presence of one reduction peaks, which decreased in intensity at high tin loading.

Cu/SiO2 catalysts were prepared via a sol–gel process in the presence and absence of a hydroxy-carboxylic acid (tartaric acid) as a non-surfactant template or pore-forming agent. These catalysts were characterized by means of nitrogen sorption, X-ray diffraction, Fourier transformed infrared spectrometry, thermal gravimetric analysis, temperature programmed reduction, N2O dissociative chemisorption, and transmission electron microscopy. They were evaluated in the partial oxidation of methanol (POM). The addition of tartaric acid showed two major and opposite effects on the final catalyst. On the one hand, tartaric acid has the ability to increase the pore size and specific surface area generating a mesoporous material without long-range ordering channel arrangements. On the other hand, the presence of tartaric acid contributes to decreasing the copper dispersion by aggregation of Cu species. The catalyst prepared in the presence of TA was much more active in POM than that prepared in the absence of TA. However, the last one showed a higher yield to hydrogen (YH2≈49%)(YH2≈49%) with a very good selectivity (SH2≈99%)(SH2≈99%).

The interaction of chromium with an alumina submitted to different pretreatments by temperature-programmed reduction (TPR) was studied. The Cr−alumina interaction is analyzed in relation to texture, structure, and surface acidity of the support, in catalysts with different chromium loads. A careful interpretation of TPR results, considering both the Tmax values and the H2/m2 consumption, may allow a better understanding of the phenomena taking place on the catalyst surface, both after preparation and after the reordering caused by the catalytic test. Speculations from TPR data were confirmed by Raman spectroscopy. The mean oxidation state of the chromium species can be accounted for by two factors:  the concentration of Cr3+ as a function of the anchoring center concentration and the influence of the Na+ concentration. Na+ elimination by washing support, which substantially modifies its acidity, does not modify the Cr−support interaction. The phase transformation of the support (from γ-Al2O3 to δ-Al2O3) caused by its thermal treatment at 1123 K results in a support with fewer chromium anchoring centers. At low values of specific surface area and low acidity, surface conditions similar to those of high loading may be attained.

A comparative study of the different preparation conditions required to produce supported copper catalysts by the wet impregnation (WI) and the ion-exchange (IE) methods has been carried out. Two silicas with different textural characteristics, and one form of naturally occurring pumice were used as supports. In order to obtain specific chemical species of copper in solution as well as different support reactivities, three different pH values (1, 4.5 and 11.5) for the impregnating solution were tested. The catalysts were characterized by means of different techniques, such as XFS, BET, TPR, dissociative chemisorption of N2O, XRD, TEM, EXAFS and XANES and methanol (MeOH) dehydrogenation. The use of copper solutions with high pH, especially after several hours of contact with the chosen carrier, with high solution volume to pore volume ratios, modifies strongly the support texture decreasing the specific surface areas (SSA) of the final catalyst. Copper particle sizes lower than a critical limit, or that did not fulfill a certain ensemble requirement, were not active for methanol dehydrogenation indicating that below such limit, this might be a structure-sensitive reaction. A catalyst (WI-11A) prepared by a simple incipient wetness impregnation method at an alkaline pH similar to that used for the IE technique, showed high dispersion and it was simply activated under reaction conditions (MeOH/N2, 230°C) giving conversions around 50% and selectivities to methyl formate (MF) in the order of 75–80% with no deactivation observed after 3 h of time on stream.

A comparative study of the different preparation conditions required to produce supported copper catalysts by the wet impregnation (WI) and the ion-exchange (IE) methods has been carried out. Two silicas with different textural characteristics, and one form of naturally occurring pumice were used as supports. In order to obtain specific chemical species of copper in solution as well as different support reactivities, three different pH values (1, 4.5 and 11.5) for the impregnating solution were tested. The catalysts were characterized by means of different techniques, such as XFS, BET, TPR, dissociative chemisorption of N2O, XRD, TEM, EXAFS and XANES and methanol (MeOH) dehydrogenation. The use of copper solutions with high pH, especially after several hours of contact with the chosen carrier, with high solution volume to pore volume ratios, modifies strongly the support texture decreasing the specific surface areas (SSA) of the final catalyst. Copper particle sizes lower than a critical limit, or that did not fulfill a certain ensemble requirement, were not active for methanol dehydrogenation indicating that below such limit, this might be a structure-sensitive reaction. A catalyst (WI-11A) prepared by a simple incipient wetness impregnation method at an alkaline pH similar to that used for the IE technique, showed high dispersion and it was simply activated under reaction conditions (MeOH/N2, 230°C) giving conversions around 50% and selectivities to methyl formate (MF) in the order of 75–80% with no deactivation observed after 3h of time on stream.

The present paper constitutes the first stage of a systematic study of the influence of precursor nature and structure on the catalyst behavior in dehydrogenation of propane, by varying the nature of the ligands and the nuclearity of the starting compounds at fixed net surface potentials. We show results obtained by the use of different Cr carboxylates to design the Cr2O3/γ-Al2O3 catalyst at low loadings and these results are compared with those obtained for chromic acid. The following short chain chromium compounds were selected as precursors: citrate (Cr(III)), dimer monohydrate acetate (Cr(II)), acetyl acetonate (Cr(III)) and hydroxyacetate (Cr(III)). An exhaustive characterization by means of BET surface, XPS, XRD, X-ray fluorescence, laser Raman spectroscopy, EPR and catalytic test in propane dehydrogenation enabled us to draw relevant conclusions. Low metal–support interaction might be the major cause of polymerization in Carbox. The interaction between the chromia phase and the support surface, which stabilizes different oxidation stages and coordinations of the chromia species, defines the surface architecture. Part of the Cr3+ is incorporated in the vacant octahedral sites of the spinel surface. The larger extension of this phenomenon in Cral might be responsible for surface inactive species that could cause a loss of catalytic activity. Deactivation between cycles might depend on the catalyst stabilization by the incorporation of Cr3+ into the support structure. The surface array of monomer species in a polymer species environment stabilizes the catalyst architecture and confers characteristics of high stability between cycles. The above considerations permit to infer that chromium carboxylates are interesting precursors for the control of surface species in chromia/Al2O3 catalysts.

Chromium oxide was supported on different commercial aluminas. Their characteristics were assessed by different techniques (X-ray diffraction, X-ray photoelectron, laser Raman, and electron paramagnetic resonance spectroscopies, and Brunauer−Emmett−Teller surface, temperature-programmed reduction, and temperature-programmed desorption measurements). The variation of the Cr3+/Cr6+ ratio was accounted for by the interaction between the chromium phase and the support. The tendency of Cr3+ to form aggregates such as Cr2O3 was induced by the support. The features of the support exert an influence upon chromium stabilization both in its oxidation states 6+ and 3+ and in the coordination of surface chromia species. The total acid sites and the oxidized chromium have a marked effect on activity and selectivity at initial operation times.