Luis A. Arrúa’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.

A Co/MgAl2O4 catalyst with Ce addition was prepared and characterized by different techniques (BET, XRD, SEM-EDX, TPR and TG-TPO). Its catalytic performance was tested in the ethanol steam reforming reaction at 650 °C and a (Formula presented.) = 50 g min mol−1 using different H2O:C2H5OH molar ratios (MR) in the feed. The presence of Ce on the catalyst markedly increased selectivity of hydrogen (from 3.2 to 5.2 molH2/molC2H5OH for MR = 4.8) and carbon resistance. The increase of the H2O:C2H5OH MR led to a significant decrease in the CO/CO2 ratio (from 0.84 for RM = 4.8 to 0.5 for RM = 8) and in the carbon amount after 7 h on stream (from 22.9 % for RM = 4.8 to 6.8 % for RM = 8). MR higher than 6 did not significantly improve the catalytic performance. Taking into account previous studies about the thermal balance of this reaction, the optimum MR in the ethanol steam reforming seems to be around 6. The ethanol conversion was nearly constant after 7 h in spite of the carbon amount detected by TG-TPO and SEM-EDX. It could be inferred that an important Co fraction was exposed on the filament tips and/or a fraction of carbonaceous species was deposited on the support surface

The surface properties of Ni/MgAl2O4 catalysts doped with Ce or Pr were analyzed by XPS after treatment in an inert and reductive atmosphere at 400 °C. The Ce-promoted solids presented the Ce3+/Ce4+ couple on the surface even after treatment in a reductive atmosphere, H2(5%)/Ar. The promotion effect of Ce on these solids could be associated with their participation on the carbon deposition-removal mechanism. Pr-doped catalysts showed a very high concentration of Pr3+ under a reductive atmosphere and the redox behavior associated with the carbon removal could be partially inhibited or become slower. The size of the Ni0 particles after both an inert and a reductive atmosphere was estimated by XPS intensity ratio using the model proposed by Davis. The results obtained from the Davis model showed that an important increase occurred in Ni particle size after treatment in H2(5%)/Ar for the Pr-promoted solids . The metal sintering under reductive atmosphere could be the reason for the higher loss of activity of the Pr-doped solids under reforming conditions.

Co/MgAl2O4 catalysts modified with La, Pr or Ce were prepared, characterized by different techniques and tested in ethanol steam reforming reaction to produce hydrogen. The catalytic behavior at 650 °C depended on the nature of rare earth. The amount of carbon on promoted catalysts was significantly lower than that on unpromoted one. The Pr and La containing catalysts produced a high acetaldehyde selectivity which decreased the hydrogen production. The superior performance of the catalyst promoted with 7.8% Ce could be partially explained by a higher dispersion and a high reduction of Co species.

Hydrogen production by ethanol steam reforming was studied using Ni catalysts supported over MgAl2O4 doped with Ce or Pr. Ni/MgAl2O4–CeO2 catalyst obtained from nickel nitrate impregnation was selected as the best one (final steady conversion = 95.8 %, carbon formation = 0.4 wt% and activity decay = 6.6 %) under the experimental conditions used in this work. Graphical Abstract Product distribution and ethanol conversion in ethanol steam reforming over a MAP-Ac; b MAP-Nt; c MAC-Ac and d MAC-Nt catalysts.

The hydrogen production from ethanol steam reforming has been studied over Ni/ZnAl2O4-CeO2 catalysts. The influence of calcination atmosphere and nature of catalytic precursor was analyzed. The impregnation of Ni was carried out from two different nickel salts: nitrate (Nt) and acetate (Ac). The catalysts were obtained from the precursor calcination under reductive and oxidative atmospheres. The catalysts obtained from nitrate salt impregnation were the most carbon resistance measured as mmolC/mol reacted C2H5OH, and those obtained under a reductive atmosphere were the most stable, as evidenced by smaller activity losses. The Ni(Nt)/ZnAl2O4-CeO2 obtained under reductive atmosphere was the most active (steady state ethanol conversion=88%) and stable (activity loss=14%) under mild reforming conditions (7.8% C2H5OH inlet concentration, H2O:C2H5OH molar ratio=4.9, reaction temperature=650°C and W/F=49 g min molC2H5OH−1). The hydrogen selectivity was 49% and the main C-containing products were CO2, CO and C2H4O.

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.

Nickel catalysts supported over ZnAl2O4 modified by Ce or/and Zr addition were studied in the steam reforming of ethanol under more severe reaction conditions such as a lower extent of feed dilution and lower space velocities. The catalysts were stable at 650°C during 35h in time on stream. The main reaction products were H2, CO2, CO, C2H4O and small amounts of CH4 and C2H4. The catalyst supported over CeO2–ZnAl2O4 was the most active and showed the highest hydrogen selectivity. The addition of ZrO2 decreased the activity and favored the CO production. The lowest degree of Ni2+ reduction over Ni/ZAZr could explain the worst performance of this system. Otherwise, the highest Ni2+ dispersion and the high oxygen mobility from ceria or from Ni–Ce boundary allowed a higher residual activity and an improved coking resistance.

a b s t r a c t The ethanol steam reforming was studied at 500 and 600 • C on CoZnAl catalysts with different Co loading (9 and 25 wt.%) and a Zn:Al atomic ratio nearly constant (Zn:Al ∼ =0.6). The catalysts were prepared by the citrate sol–gel method and characterized by different techniques such as AA, TG, BET, TPR, XRD, RAMAN and SEM-EDX. They were active in the ethanol steam reforming at atmospheric pressure in the temperature range studied, but with significant differences in their performance. High hydrogen selectivities, better than 80%, were obtained on catalyst with high Co loading (25 wt.%). CO, CO 2 and minor amount of CH 4 were the only carbon products at 600 • C.