M.C.F. Oliveira / Applied Catalysis A: General 329 (2007) 7–158

such as, nitrobenzene [14], styrene, cyclohexene, cyclooctene,

acetophenone [15], furfural [16], buta-1,3-diene [17] and 1,3-

cyclopentadiene [18], using hydrogen gas supply. However,

correlation of the hydrogenation activity with the P content of the

Ni–P alloy has never been performed. A lot of experimental

results have been carried out to elucidate the promoting effect of

the alloying metalloid on the catalytic activity of the alloys but

conclusions concerning the electronic effect are very contro-

versial. Based on XPS spectra and EXAFS data, it is widely

accepted that the promoting effect of alloying is attributed to the

electronic effect of the non-metallic element on the active Ni sites

and in turn to the catalytic properties [19–21]. However, while the

higher hydrogenation activity of Ni–P amorphous alloy,

compared to Ni–B, is explained on basis that hydrogen

dissociative chemisorption is more favourable on electron-

deficient Ni centers (Ni–P) than on electron-enriched ones (Ni–

B) [17], the same argument does not account for the increase of

the catalytic activity of Ni–B amorphous alloy with the increase

of the B content (more electrons may transfer from the alloying B

to Ni with the increase of the B content) observed by Li et al. [22].

In this case, the authors advocate that the increase of the electron

density on the Ni active sites would weaken the C C bonding,

which would facilitate the hydrogenation of this group.

Concerning the structural effect there is also no agreement in

the literature regarding the P content at which the transition

from crystalline to an amorphous structure takes place. Besides,

divergences in the effect of the P content on the catalytic

activity towards the hydrogen evolution reaction (HER) are also

found. Some authors advocate that an increase of the P content

of the alloy is accomplished by an increase of its catalytic

activity [12,13] but others authors support an opposite effect

[23,24]. This incongruence seems to be related to the

amorphous/crystalline character of the alloy, i.e. whether the

catalytic activity comparison is performed between crystalline

and amorphous Ni–P alloys or just between amorphous Ni–P

alloys. On analysing the literature data, it may be concluded

that some experimental results bring out the electronic effects

of metal alloying on the catalytic activity towards HER (on

comparing amorphous alloys containing different P content),

while others make more striking the structural effects (on

comparing Ni–P alloys with different amorphous character).

This could explain the volcano type plot, which is obtained by

T. Burchardt when the rate of HER is plotted versus the P

content of the Ni alloy (maximum catalytic effect was observed

for 19 at.%) [12].

This prelude illustrates well the complexity of the overall

system and the need of considering multiple effects on

correlating the hydrogenation activity with the P content of the

Ni–P alloy.

The novel methodology will be attempted using 3-buten-1-ol

as the model molecule and Ni–P electroless alloys films

containing 7–14 at.% of P as the catalytic material. Within this

phosphorous content range, a significant effect of Ni–P alloy

composition on the HER activity has been reported [13]. Alike

the study previously performed on a Pd-black film [9], 3-buten-

1-ol was chosen as a model compound because of its high

solubility in aqueous solution and the small number of

hydrogenation products that could be formed (1-butanol is the

only product expected). Comparison of the proposed metho-

dology with the electrocatalytic hydrogenation (ECH) will be

performed, using the same organic molecule and the same

catalytic substrate.

2. Experimental

2.1. Ni–P alloy preparation and characterization

The Ni–P alloys were prepared by electroless deposition on a

10 mm � 50 mm Cu plain weave mesh with 0.38 mm nominal

aperture and 0.25 mm wire diameter (from Goodfellow). The

experimental procedure for the films deposition was adapted

from [13], on which the Ni–P alloy films deposition on carbon

steel substrate is described. The deposition of 9 and 14 at.% P

alloys was carried out from a solution of 0.1 M NiSO4 + 0.20 M

NaH2PO2 + 0.15 M CH3COONa, pH 5.5 (adjusted with

CH3COOH), at 80 8C, under solution stirring. By changing

the deposition time, variations in the P content were achieved (9

and 14 at.% P films for 20 and 40 min, respectively). A solution

with the same composition, but a lower H2PO2� concentration

(0.1 M) was used for plating the alloy containing 7 at.% P

(45 min deposition). Despite the important composition gradient

found on the deposits prepared from the 0.20 M NaH2PO2

solution, a negligible composition gradient was detected when

the films were obtained from a lower hypophosphite concentra-

tion solution (0.10 M). EDS analysis on several points of the as-

prepared films allowed to evaluate accurately the composition of

the uppermost film (surface) layer: 7.4� 0.5, 9.3� 0.5,

13.6 � 0.4. For simplicity, these Ni–P alloy films are identified,

respectively as 7, 9 and 14 at.% of P.

The thicknesses, evaluated from SEM analysis of the cross-

section of the deposits, were remarkably uniform and were

found to be 4–6 mm (Fig. 1).

The electroless deposition was induced by short-circuiting

the Cu mesh with an Al foil for 30 s in the plating bath. After

disconnecting the Al, the time deposition was monitored once

the open circuit potential reached approximately �650 mV,

which was the steady-state potential for the Ni–P/electroless

solution system at the present experimental conditions. A fresh

solution was used for each plating experiment.

The phosphorus content in the deposits and surface

morphology was determined by a Philips-FEI Quanta 400

SEM/EDS spectrometer. Structure of the alloys was assessed by

X-ray diffraction using a Panalytical, X’Pert PRO diffract-

ometer and CuKa X-ray source.

2.2. Cyclic voltammetry

A classical three-compartment cell was used for the cyclic

voltammetric experiments. The working electrode was a small

sample of the as-deposited alloy mesh (0.70 cm � 0.80 cm) with

a geometric area of 1.72 � 0.13 cm2. Calculation of the current

density relied on this geometric area. Each voltammogram was

registered on a new deposit as cycling changes the deposit

activity. All the voltammetric data was obtained by initiating the

Fig. 1. SEM images of as-prepared Ni–P films and of its cross section.

M.C.F. Oliveira / Applied Catalysis A: General 329 (2007) 7–15 9

potential scan in the negative direction. The voltammetric scan

was 10 mV s�1. Solutions were deaerated with N2.

2.3. Catalytic transfer hydrogenation and electrocatalytic

hydrogenation

The CTH and electrocatalytic hydrogenation experiments

were carried out in one compartment cell having a Pt mesh

counter electrode. The electrolyte consisted of a 10 mM 3-

buten-1-ol + 0.1 M NaOH + 0.1 M NaH2PO2 aqueous solution,

except on the ECH experiments where a hypophosphite-free

solution was used. The solution was thermostatized at 25.0 8Cand stirred with a magnetic bar. The electrode potential was

controlled by an Autolab potentiostat model 100 and is referred

to a saturated calomel electrode (SCE).

The progress of the hydrogenation reaction was monitored

by gas chromotography (GC) analysis on a Dani model 1000

gas chromatograph equipped with a FID detector and a DB-

1701 capillary column (30 m long, 0.53 diameter). 0.5 mL of

the electrolyte solution was extracted with ethyl ether and was

immediately analysed by GC. An internal standard (1-pentanol)

was used on the quantification. The reaction product was

identified by comparing its retention time with that of an

authentic sample.

The catalytic activity of the catalyst towards the hypopho-

sphite ion dehydrogenation (and consequently, towards the

organic molecule hydrogenation) was followed by the open

circuit potential evolution. ECH was carried out by passing a

charge corresponding to two electrons per molecule of 3-buten-

1-ol (2 F mol�1). On the CTH experiments, dehydrogenation of

the hypophosphite ion was initially promoted by applying

�1.1 V potential for 30 s.

The material yield (or yield of 1-butanol), selectivity and

conversion were calculated as,

Yield of1-butanolð%Þ ¼�

n1-butanol

n03-buten-1-ol

�� 100 (1)

Selectivityð%Þ ¼�

yield1-butanol

conversion3-buten-1-ol

�� 100 (2)

Conversionð%Þ ¼�ðn0

3-buten-1-ol � n3-buten-1-olÞn0

3-buten-1-ol

�� 100 (3)

Fig. 2. XRD pattern of as-deposited Ni–P films with 7, 9 and 14 at.% P content.

Fig. 3. Cyclic voltammograms in 0.1 M NaOH solution for different P content

on Ni–P alloy films: - - - 7 at.%, — 9 at.%, – – – 14 at.%.

M.C.F. Oliveira / Applied Catalysis A: General 329 (2007) 7–1510

where n1-butanol, n3-buten-1-ol and n03-buten-1-ol represent, respec-

tively, the amount (in mol) of 1-butanol formed, 3-buten-1-ol

not hydrogenated, and the initial amount of 3-buten-1-ol. The H-

donor efficiency of the catalytic hydrogenation was defined as,

H-donor efficiencyð%Þ ¼�

n1-butanol

nhypophosphite consumed

�� 100 (4)

Quantitative determination of the hypophosphite ion concen-

tration at the end of the hydrogenation reaction was performed

by the iodometric method [25]. Current efficiency was calcu-

lated on the basis of the amount of 1-butanol formed and the

charge passed on the cell.

3. Results

3.1. Ni–P alloy characterization

Observed by the naked eye, the composite coatings are all

smooth and semi-bright as deposited. However, on analyzing

them by SEM (Fig. 1) differences on the surface morphology

were found. Apart from the lowest P content film, spherical

particles with a diameter ranging from 3 to 10 mm, distributed

preferentially in the cross wire zone of the mesh, are detected.

The composition of these particles, determined by EDS, was

found to be approximately the same as the layer beneath them.

Apparently, the amount and average size of these particles

increases as the P content also increases. In order to confirm

whether this effect was exclusively related to the film

composition, a 14 at.% P film was prepared from a solution

presenting the same composition but a different pH (4.8 instead

of 5.5, time deposition 60 min). An identical morphology to the

one observed on the Ni–P alloy containing 7 at.% P was found.

This result suggests that the presence of spherical particles is

dependent on very particular experimental conditions and

should not be directly correlated to the P content of the film.

Underneath these microparticles the Ni–P films present a

smooth layer with nodular nature, typical of the electroless Ni–

P deposits. Apparently, this layer presents the same morphol-

ogy in the three different composition alloys, which contrasts

with the AFM data obtained by Fundo and Abrantes [13] on Ni–

P films deposited on carbon steel, containing the same

phosphorus content. These authors observed significant

morphological differences upon the P content of the film and

concluded that the surface roughness was approximately four

times higher on the 7 at.% P than on the 14 at.% P film. Further

work will be needed to characterize in more detail the

morphology of these films and to confirm if the apparent

different morphological results are related to the different metal

substrate used on the deposition process.

The XRD patterns of Ni–P coatings with different P-contents

are shown in Fig. 2. It can be seen that the full width at half

maximum of the Ni peaks increases as the phosphorus content of

the Ni–P alloy increases as well, which is indicative of a

progressive increase of the alloy amorphicity. However, it is

important to remark that contrasting to the EDS data, the XRD

analysis concerns the whole thickness of the film, which means

that a direct correlation between the XRD data and the alloy

composition may be misleading on films presenting a

concentration gradient of P on the alloy. Probably, if glancing

incidence X-ray diffraction has been used, the crystallinity

displayed by 7 and 9% at. P films would have been lower. Despite

the amorphicity exhibited by the 14 at.% P film, the possibility

that the deposit may be amorphous but incorporating micro-

crystallites, not detectable by XRD, cannot be excluded [26].

Characterization of the Ni–P deposits after the hydrogena-

tion reaction revealed that the films exhibit the same properties,

in terms of their structure, morphology, composition and

adherence to the metal substrate.

3.2. Cyclic voltammetry

Fig. 3 shows the voltammograms obtained in a 0.1 M NaOH

solution at the different prepared Ni–P electroless alloys. The

results reveal the enlargement of the anodic peak at ffi�0.78 V,

and concomitant shift of the peak potential towards more

positive potentials, with the increase of % of P on the Ni–P

alloy. Despite this peak resembles the anodic peak observed at a

Ni electrode (which is ascribed to the a-Ni(OH)2 formation),

several authors propose its assignment to the P oxidation to

hypophosphite [27].

M.C.F. Oliveira / Applied Catalysis A: General 329 (2007) 7–15 11

Comparison of the results obtained by cyclic voltammetry in a

H2PO2� solution and in a solution containing a H2PO2

� + 3-

buten-1-ol, allows to predict the effect of the organic compound

on the catalytic activity towards the hypophosphite dehydro-

genation and oxidation. The voltammograms were recorded by

initiating the potential scan from �0.70 V in the negative

direction. Despite at this potential, the surface is covered by a

nickel hydroxide film (which has been spontaneously formed as

soon as the electrode is immersed in the solution), this potential

was chosen because it is neither too anodic to oxidise irreversibly

the surface electrode (into b-Ni(OH)2), neither too cathodic to

promote the H–P cleavage of H2PO2�. The results (Fig. 4) reveal

Fig. 4. Cyclic voltammograms for different P content on Ni–P alloy films in a

solution containing 10 mM 3-buten-1-ol + 0.1 M H2PO2� + 0.1 M NaOH (- - -)

and in a 3-buten-1-ol free-solution (—). n = 10 mV s�1.

that despite the unsaturated alcohol gives rise to an important

decrease on the catalytic activity of the Ni–P films, the

voltammetric features characteristic of the hypophosphite

oxidation reaction are still depicted (an anodic peak at

ffi�1.0 V) [5,6]. It is also shown that the extension of the

inhibition effect of 3-buten-1-ol on the hypophosphite oxidation

reaction depends clearly on the phosphorous content of the Ni–P

alloys films: it is much more prominent on the Ni–P film

containing 14 at.% of P than on the 7 or 9 at.% P film. This

behaviour is indicative that 3-buten-1-ol and the H2PO2� ion

compete with each other to adsorb on the Ni–P surface and that

the adsorption ability of the unsaturated alcohol is stronger on the

highest content P film. In addition, the notable decrease of the

anodic peak atffi�0.72 Vin the 3-buten-1-ol containing solution

is indicative that this organic compound affects the Ni–P ability

to oxidize as well. This decrease is also larger on the 14% P

sample than on the less phosphorus content films.

Fig. 5 is representative of the differences in the first and

second voltammetric scan of a Ni–P electrode (9 at.% of P) in a

0.1 M H2PO2� + 0.1 M NaOH solution. It reveals that the

catalytic activity of the electrode towards the hypophosphite

dehydrogenation and oxidation is strongly reduced upon

cycling. On observing the effect of the anodic limit potential

on the second scan voltammogram, it was concluded that the

pronounced decrease on the catalytic activity only occurs when

the anodic limit potential lies below �0.80 V, i.e. within a

potential range suitable for the Ni–P oxidation. This behaviour

reminds the voltammetric data previously observed on nickel

[6,28,29] and it suggests that a pronounced modification of the

surface state occurs in a potential range below �0.80 V. This

surface modification seems to inhibit further hypophosphite

dehydrogenation and oxidation.

By comparing the anodic peak at ffi�0.72 V in the

hypophosphite solution to that obtained in a hypophosphite-

free solution (Fig. 3) it can be inferred that this peak enlarges,

its current increases and the potential peak shifts towards more

positive potentials in the presence of the hypophosphite ion.

This result is strongly indicative that the oxidation of the Ni–P

alloy is itself promoted by the presence of hypophosphite ions;

Fig. 5. Cyclic voltammograms for a Ni–P alloy film (9 at.% of P) in 0.1 M

H2PO2� + 0.1 M NaOH solution. The solid and dashed curves refer to the first

and second consecutive cycles, respectively.

M.C.F. Oliveira / Applied Catalysis A: General 329 (2007) 7–1512

however, whether this oxidation gives rise to a partial leaching

of phosphorous or to a partial leaching of nickel (or both) from

the surface is not clear for the moment. Some authors support

that Ni–P anodic oxidation causes partial dissolution of

phosphorus in an alkaline medium [30,31]. However, evidence

has also been given for formation of soluble Ni(II) compounds

(Ni aquocomplexes) in a hypophosphite solution in alkaline

medium [32]. The latter phenomenon has also been reported for

a nickel electrode in the acid medium [7] and has been

explained on basis of the hypophosphite ability to complex Ni2+

ions, giving rise to a bidentate monohypophosphite complex,

Ni(H2PO2)+ [33–35]. Clearly, a better understanding of the

surface phenomena occurring on Ni–P electroless alloys will

require, in the future, the use of very sensitive surface

techniques (such as XPS or AES).

3.3. Catalytic transfer hydrogenation

The electrochemical promotion of the CTH is only required

in the first stage of the experiment (during a few seconds) in

order to induce the hypophosphite dehydrogenation. Once the

dehydrogenation reaction starts, it is self-sustained [5,7,10].

According to previous studies, the electrochemical promotion

of the hypophosphite dehydrogenation by the negative

polarization of the catalyst (�1.1 V), relies on the formation

of a free-oxide/hydroxide surface that endorses a H2PO2� ion

adsorption configuration that favours the P–H breakdown [32].

Since the OCP reflects the catalyst ability to generate atomic

hydrogen [8], it allows to follow the catalyst transfer hydro-

genation in the 10 mM 3-buten-1-ol + 0.1 M NaOH + 0.1 M

H2PO2� solution after the induction step, Fig. 6. Typically, the

OCP profile is characterized by the development of a rather

constant potential which lies between �1.3 and �1.1 V and an

abrupt potential increase (in two or three stages) to�0.3 V. Such

behaviour is similar to that observed on Pd-black [9]. However,

while in that study the potential raise observed was ascribed to a

steep pH decrease, in this system the steep potential increase

relies on the catalyst deactivation. In effect, from this point,

Fig. 6. Typical OCP profile of the Ni–P alloy films in 10 mM 3-buten-1-ol

+ 0.1 M H2PO2� + 0.1 M NaOH solution after the first electrode activation by

applying �1.1 V for 30 s.

further conversion of 3-buten-1-ol was no longer detected. The

pH of the solution was measured at the beginning and at the end

of the reaction, and for all cases, it remained nearly constant.

It can also be inferred from Fig. 6 that the time lapse for the

catalyst deactivation increases with the increase of the P content

on the Ni–P alloy. The reason for the deactivation phenomena

will be analysed in more detail in Section 3.5. Regardless the

catalyst deactivation, it was found that it could be reactivated in

the same solution by applying a �1.1 V potential for 30–90 s.

Deactivation/reactivation of the catalyst proceeded several

times till the catalyst was irreversibly deactivated. Data on the

reaction yield, selectivity and H-donor efficiency, obtained

after the irreversible deactivation of the catalyst, is summarized

in Table 1.

It is shown that the Ni alloy activity, in terms of 1-butanol

yield, increases with the P content, which seems to indicate that

the catalyst activity is related to the amorphous character of the

Ni alloy. The same trend was found by some authors for the

catalytic activity towards the HER, which was explained on the

basis that the amorphous phase presents a highest ability to

absorb hydrogen [12,13,36,37]. The hypothesis that the catalytic

activity of the Ni–P electroless alloys for the CTH may be also

correlated to the surface roughness of the films cannot also be

excluded. In effect, the highest activity is displayed by the films

presenting apparently, a higher surface area.

Despite 1-butanol was the only product detected, a low

selectivity was found. This result may be indicative that a by-

product, not detectable by GC, should have been formed. As

previously discussed [9], this behaviour may be accounted by

the formation of aqueous soluble species (sodium monoalk-

enylphosphite). Another contribution for the low selectivity

may rely on losses of 1-butanol due to evaporation during the

experiment. Even though, its evaluation has revealed that the

mass lost by evaporation does not exceed 15%. Concerning the

H-donor efficiency of the hydrogenation reaction, it is revealed

that this is three times lower on the 14 at.% P than on 9 at.% P

alloy. This result points out that a significant part of the atomic

hydrogen formed on the 14 at.% P alloy is not used on the

hydrogenation reaction, which is in agreement with the higher

intensity of gas evolved that was observed on this film,

compared to the other Ni electroless films.

Comparing this data with that obtained for the CTH

performed on the Pd-black film using the same H-donor and

unsaturated organic compound [9], it can be concluded that the

CTH on the Ni–P alloys (containing 14% at. of P) gives rise to a

lower reaction yield (51% on Ni–P and 89% on Pd-black) and

selectivity (77% on Ni–P and 97% on Pd-black), but a higher

Table 1

Material yield, selectivity and H-donor efficiency in hydrogenation experiments

performed in 0.1 M NaOH + 0.05 M H2PO2� + 10 mM 3-buten-1-ol solution

with the different P content films

% at. P on the

Ni–P alloys

Yield

%

Selectivity

(%)

H-donor

efficiency (%)

7 4 59 ffi100

9 27 72 38

14 51 77 14

M.C.F. Oliveira / Applied Catalysis A: General 329 (2007) 7–15 13

H-donor efficiency (14% on Ni–P and 8% on Pd-black) and a

remarkable film stability (without peeling from the copper

substrate) is achieved.

3.4. Electrocatalytic hydrogenation

In order to better evaluate the performance of the

electrochemically promoted catalytic transfer hydrogenation

method, its comparison with another hydrogenation methodol-

ogy is required. Regarding this purpose, the ECH of the same

organic compound, on the same catalysts (which are used as

electrodes) and in the same composition of the electrolyte

(H2PO2-free) was performed. The results, Table 2, clearly

reveal that ECH is much less efficient to hydrogenate 3-buten-

1-ol than the electrochemically promoted CTH. It can also be

concluded that the relation between the films catalytic activity

(in term of the material yield) and the P content of the alloy

follow a different trend from the one observed on the CTH. The

highest catalytic activity is displayed by the 9 at.% of P alloy.

Supplementary experiments performed on a 12 at.% P alloy

(the effective composition is 11.6 � 0.2) confirmed such

unpredicted tendency.

All these results seems to point out that a poisoning effect by

the strong adsorption of 3-buten-1-ol, which would prevent the

water adsorption and consequently the generation of chemi-

sorbed hydrogen, is responsible for the low activity of the

electrodes to the ECH. This hypothesis is corroborated by the

reaction yield increase that is observed on decreasing the organic

compound concentration (performed on the 12 at.% P alloy) and

is also supported by the dependence of the films P content with

the apparent catalyst ability to adsorb 3-buten-1-ol (previously

described in Section 3.2). Hence, differences on CTH and ECH

activity could be explained qualitatively on the basis of

adsorption competition between the organic compound and

Table 2

The effect of the electrode potential and organic substrate concentration on the

electrocatalytic hydrogenation of 10 mM 3-buten-1-ol + 0.1 M NaOH solution

at different P content films

% at. P E (V) F mol�1 Yield

(%)

Selectivity

(%)

Current

efficiency (%)

Sol. 10 mM 3-buten-1-ol + 0.1 M NaOH

7 �1.2 2.0 5 69 4

�1.1 1.9 a – –

9 �1.2 2.0 30 85 22

�1.3 2.5 22 72 14

12 �1.1 2.7 a – –

�1.2 2.7 4 38 2

�1.3 1.9 8 72 8

14 �1.1 3.8 a – –

�1.2 2.0 4 25 4

�1.3 2.0 5 41 5

Sol. 3 mM 3-buten-1-ol + 0.1 M NaOH

14 �1.1 2.4 4 26 3

�1.2 2.6 10 51 7

�1.3 2.5 18 73 14

a On these experiments the reaction yield lies below data precision (�3%).

the supplier source of hydrogen. When the hydrogen source is the

hypophosphite ion, its high ability to adsorb on the catalyst would

prevent a rather high surface coverage of 3-buten-1-ol and

consequently the surface blockage by the strong adsorption of

this compound. On the ECH, in a hypophosphite-free solution,

the strong adsorption of the unsaturated alcohol compared to the

hydrogen supplier (water), would make more difficult the

approach of the water molecules to the surface of the catalyst,

leading to its partial poisoning. Nevertheless, it was found that by

regulating the Ni/P ratio, a compromise between the electrode

ability to generate chemisorbed hydrogen and to adsorb the

organic molecule can be found, making the Ni–P alloy a rather

effective catalyst for the electrocatalytic hydrogenation.

3.5. Study of the catalyst deactivation phenomena

In order to assess whether the steep increase of the OCP

observed in the 10 mM 3-buten-1-ol + 0.1 M H2PO2- + 0.1 M

NaOH solution was related to the surface poisoning by a

hydrogenation product, the OCP was recorded in a solution not

containing the unsaturated organic molecule, Fig. 7. The

obtained data allows to conclude that the abrupt OCP increase

is an intrinsic characteristic of the electrode / H2PO2� + NaOH

system. In effect, the results reveal that the potential raise

recorded in this system occurs even earlier in the presence of 3-

buten-1-ol on the 9 and 14 at.% P films. On analyzing the

curves profile, it is recognized that, excepting the region that

just precede the abrupt potential increase (a potential oscillation

is depicted), the shape of the curve resembles those obtained in

a 3-buten-1-ol containing solution.

To evaluate the relationship between the electrode potential

attained in the OCP and the modifications that may occur on the

alloy surface, cyclic voltammograms were run in 0.1 M NaOH

solution after the electrode has rested at OCP in the

hypophosphite solution until �0.30 V (Fig. 8) has been

achieved. Comparing this voltammogram with those obtained

in Fig. 3, it is clearly depicted that a new cathodic peak (A)

emerges within �0.7 to �0.8 V. A similar result was obtained

when the electrode rested at OCP in the hypophosphite solution

Fig. 7. Typical OCP profile of the Ni–P alloy films (- - - 7 at.%, — 9 at.%, – – –

14 at.%) in 0.1 M H2PO2� + 0.1 M NaOH solution after the first electrode

activation by applying �1.1 V for 30 s.

Fig. 8. Cyclic voltammograms in 0.1 M NaOH solution for the different P

content on Ni–P alloy films (- - - 7 at.%, — 9 at.%, – – – 14 at.%) after its

removal from a +0.1 M H2PO2� +0.1 M NaOH solution upon achieved�0.30 V

at OCP.

Fig. 9. The effect of the anodic limit potential on the first (—) and second (– – –)

scan of the cyclic voltammograms of a 7 at.% P alloy in 0.1 M NaOH solution

after its removal from a 0.05 M H2PO2� +0.1 M NaOH solution upon achieved

�0.30 V at OCP. Inset first voltammograms in 0.1 M NaOH solution for

different P content on Ni–P alloy films which have not been submitted to the

hypophosphite solution: - - - 7 at.%, — 9 at.%, – – – 14 at.%.

M.C.F. Oliveira / Applied Catalysis A: General 329 (2007) 7–1514

until �0.50 V was achieved. The new cathodic peak is more

discernible on the lowest P content films. The dependence of

peak A on the scan rate confirmed that this peak is related to

surface reactant species. On comparing the first and second

voltammetric scan at different anodic limit potentials (Fig. 9), it

was found that this peak was related to an anodic peak (A0) at

�0.45 V. Cyclic voltammograms recorded with electrodes that

have not been submitted to the hypophosphite solution have

also revealed the presence of this anodic peak, but with a much

less intensity (Fig. 9 inset). These data are indicative that the

OCP raise to �0.50 or �0.30 V must be allied to an oxidation

phenomenon that is allied to the appearance of peak A0.According to several authors [27,30,38] the Ni–P oxidation

leads to the hypophosphite ion or to the nickel hypophosphite

formation, but experimental evidence for the A (or A0) peak

assignment has never been given. On account of the present

experimental results, it is much more plausible than peak A is

ascribed to a surface species, such as nickel hypophosphite,

however the accurate assignment of these peaks will require the

use of surface characterization techniques which are not under

the scope of this work.

Another important remark on the cyclic voltammograms of

the modified electrodes (Figs. 8 and 9) concerns the drastic

current decrease that is observed on the anodic peak assigned to

a-Ni(OH)2. Two proposals are suggested to explain this result:

(a) during the electrode rest at OCP in the hypophosphite

solution, part of the a-Ni(OH)2 formed is irreversible

transformed into b-Ni(OH)2; (b) the surface of the Ni–P alloy

film becomes partly blocked by some surface species that

inhibit further surface oxidation to a-Ni(OH)2.

The fact that the Ni–P alloy film can be re-activated by

applying �1.1 V before being irreversible deactivated allows

supporting the second hypothesis, some surface species would

be responsible for the electrode blockage inhibiting in some

extent the formation of nickel hydroxide. On account that such

specie would be reversible reduced and that would be formed

while the hypophosphite oxidation occurs, it is proposed that

such specie is a Ni(II) compound. Such compound, probably

responsible for the peak A and A0, would remain adsorbed on

the surface, acting as a protective layer which would hinder the

approach of the hypophosphite ions or the water molecules

(or hydroxyl ions) to the surface. On applying a rather width

potential step (�1.1 V/30–90 s) such specie would be reduced

which means that new vacant surface sites would be formed and

so the surface would be re-activated. This explanation is

corroborated either by the promotion effect of the hypopho-

sphite ion on the nickel oxidation, either by the inhibition effect

of the oxidized surface on the hypophosphite dehydrogenation

and oxidation, previously observed. Under the light of this

explanation, the different time lapse for the steep raise of the

OCP in a 3-buten-1-ol solution and in an alcohol-free solution

can be accounted by the inhibition effect of the adsorbed

3-buten-1-ol on the Ni oxidation. In the presence of this

unsaturated alcohol, the adsorption of the hypophosphite ion

would be less extended and consequently its effect on the Ni

dissolution would be slowed down. The same interpretation

allows to understand the dependence of the time lapse for the

steep increase of the OCP on the film P content observed in

Fig. 6.

It cannot also be disclosed the hypothesis that during the Ni–

P alloy immersion in OCP the a-Ni(OH)2 is also formed

(although in a low amount) and that it is slowly converted in the

passive form, b-Ni(OH)2 which, at a rather high coverage,

would be responsible for the non-reactivation of the electrode.

Under this scope, the potential oscillation phenomena

observed just before the potential raise can be attributed to the

concomitant H2PO2� dehydrogenation – Ni oxidation process

that occur on the electrode surface. When Ni is oxidized to

Ni(II), the potential increases in the positive direction, until the

chemisorbed hydrogen that is simultaneously formed on other

free surfaces sites reduce the Ni(II) species compelling the

potential to drop off. As new vacant surface sites are formed,

the hypophosphite ions and the water molecules are able to

interact again with the surface electrode, promoting its

oxidation and so the potential rises once again. This cycling

process will proceed until the formation of Ni(II) species is high

enough to hinder the approach of the hypophosphite ion to the

surface, then the potential increases rapidly for about 600–

800 mV in the positive direction. Certainly, a crucial point not

addressed here is the step that triggers Ni oxidation and that is

responsible for the dynamical instability. Further studies will

have to be carried out to get a better insight on this subject.

4. Conclusions

The results showed that Ni electroless alloys are effective

catalysts for the CTH. The performance of the hydrogenation,

in terms of the reaction yield, selectivity, H-donor efficiency

and catalyst deactivation hindrance were found to depend on

the P content of the Ni alloy. It was concluded that the CTH

activity, in terms of 1-butanol yield, follows the same trend

than the amorphous character of the alloy. However, a different

dependence was observed on the electrocatalytic hydrogena-

tion using the same catalysts and organic compound. This

phenomenon was attributed to the strong adsorption of the 3-

buten-1-ol on the Ni alloy surfaces which would prevent the H

generation by water electroreduction. The cyclic voltammetric

results allowed to evaluate the catalyst ability to co-adsorb the

H-donor and the organic compound and revealed that the

adsorption ability of 3-buten-1-ol is stronger on the highest P

content film.

Regardless the deactivation of the catalyst in the course of

the hydrogenation run was observed, its reactivation by

electrochemical polarization, at �1.1 V for 30–90 s allowed

a high material yield and selectivity (51% and 77%,